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Catalysts for Complete Oxidation of Gaseous Fuels
Ahmad Kalantar Neyestanaki
Academic Dissertation
Thesis for the Degree of Doctor of Technology to be presented with due permission of the Department of Chemical Engineering at Abo Akademi for public criticism in the Stina Auditorium of the Axelia Building, Biskopsgatan 8, on August 25, 1995 at 12 noon.
The opponent appointed by the Department of Chemical Engineering is Professor Zinfer R. Ismagilov from the Department of Environmental Catalysis, Boreskov Institute of Catalysis, Novosibirsk, Russia.
Laboratory of Industrial Chemistry Department of Chemical Engineering Abo Akademi University Abo 1995
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Catalysts for Complete Oxidation of Gaseous Fuels
Catalysts for Complete Oxidation of Gaseous Fuels
Ahmad Kalantar Neyestanaki
Laboratory of Industrial Chemistry Faculty of Chemical Engineering Abo Akademi University Abo 1995 ISBN 951-650-584-8
Abo Akademis tryckeri Abo 1995 1
Preface
This research work was carried out at the Laboratory of Industrial Chemistry at Abo Akademi University during the years 1991-1995, under the guidance of Professor Lars-Eric Lindfors.
First and foremost, I wish to express my gratitude to Professor Lars-Eric Lindfors, head of the Laboratory of Industrial Chemistry, for fruitful discussions and inspiring advice. His constructive criticism and scientific ideas have been of great help. I am also extremely thankful to my colleagues from the Laboratory for their help and understanding. My special thanks go to our skillful Laboratory Engineer, DI Kari Eranen, whose technical assistance was of great help. I would like to express my sincere thanks to TkL Narendra Kumar, for fruitful discussions and valuable advice. I am also grateful to the collective of theKemira Oulu Research Laboratory, Catalyst Section for their assistance.
The financial support from Kemira Ltd, Abo Akademi University, the Foundation of Abo Akademi University Research Institute, Finnish Technology Development Centre (TEKES) and the Graduate School in Chemical Engineering is gratefully acknowledged.
Last, but not the least, I would like to thank my family for continuous support throughout the years.
Turku, April 1995
Ahmad Kalantar Neyestanaki
11
Abstract
Catalytic combustion is considered to be an effective approach in controlling the emissions of hydrocarbons, carbon monoxide and nitric oxides. This thesis presents a study on the complete oxidation of propane, natural gas and the conversion of car exhaust gases over two types of catalysts: a) knitted silica-fibre supported catalysts and b) metal-modified ZSM zeolite catalysts.
A hybrid textile made up of an organic-inorganic hybrid fibre containing 70% cellulose and 30% silicic acid was used as the raw material for preparation of the fibre support for combustion catalysts. The hybrid textile was burnt to obtain a knitted silica-fibre. The changes in the surface area, pore volume and the crystallinity of the obtained support were studied as a function of burning temperature. The stability of the support in steam-rich atmospheres was tested. Theknitted silica-fibre obtained by burning the hybrid textile at 1223 K was found to have sufficient strength and high BET specific surface area (140 m2/g) to be used as a catalyst support
A series of knitted silica-fibre supported metal oxides (oxides of Co, Ni, Mn, Cr and Cu) and combinations of them, platinum-activated metal oxides (Pt-Co 304, Pt-NiO, Pt-Mn0 2 and Pt-Cr203) as well as noble metal (Pt, Pd) catalysts were prepared. The location of the metal oxides on the catalyst was studied by SEM equipped with EDXA. The metal oxide was found to be located mostly inside the pores rather than on the exterior surface of the silica-fibre. The catalysts were characterized by XRD, N2-physisorption, 02-TPD and the chemisorption of propane, carbon monoxide and hydrogen.
The activity of the catalysts was tested in the combustion of propane, natural gas and in the conversion of automobile exhaust gases. The effect of residence time and stoichiometry on the conversion behaviour of the catalysts was studied. The results indicated that high combustion efficiencies can be achieved by using base metal oxides. In each series, an increase in the metal oxide content of the catalysts resulted in an improved combustion efficiency and reduced light-off temperatures. The activity pattern in terms of light-off temperatures and final conversions was established. Co 304 was found to be the most active single metal oxide in propane combustion whereas NiO was the most active in natural gas combustion. Combination of metal oxides generally resulted in improved final conversions but not the light-off temperatures. Platinum-activated metal oxides, with the exception of Cr203, exhibited a better light-off and conversion behaviour as compared to the component catalysts. The Pd-catalysts were found to be more active in natural gas than in propane combustion, as compared to the Pt and Pt-activated metal oxide (Pt-Co and Pt-Ni) catalysts. The propane oxidation over Pd/Si0 2 was found to be structure sensitive and the reaction was found to take place over the PdO phase. The dependency of the rate of propane iii combustion over the catalysts containing noble metals was found to be zero with respect to oxygen and one with respect to propane. The reaction orders for metal oxides were fractional.
Different Cu-ZSM (Cu-ZSM-5, -11 and -48) catalysts as well as Pt- and Pd-ZSM-5 catalysts were prepared and tested for their activities in propane and natural gas combustion. The zeolites were modified by the ion-exchange technique and by introduction of the metal during the process of zeolite synthesis. The effect of basicity of the ion- exchange mixture on the metal uptake was studied. The prepared catalysts were characterised by SEM, XRD, N2-physisorption, TPD of oxygen, carbon monoxide and ammonia. The redox capacity of the copper-modified zeolites was determined by subjecting the catalysts to redox cycles in a microbalance.
The prepared metal-modified zeolites were tested for their activity in propane and natural gas combustion. The effect of stoichiometry and contact time on the light-off and conversion behaviour of these catalysts was studied. The kinetic parameters (reaction orders, activation energies and the pre-exponential factors) of propane combustion over the prepared zeolites were determined. The modified zeolites exhibited higher activities in oxidation reactions as compared to the corresponding Cu-, Pt- and Pd/alumina catalysts. Among the Cu-ZSM zeolites, ion-exchanged Cu-ZSM-5 was the most active in complete oxidation reactions. Increase in the pH of the ion-exchange mixture results in increased copper uptake and, consequently, in increased catalytic activity.
Temperature programmed desorption of oxygen and carbon monoxide as well as microbalance measurements of the redox cycles indicated the important influence of the extra lattice oxygen on these catalysts. Cu-ZSM-5 tends to stabilise the copper as Cu2+ and Cu-O-Cu species while on the ZSM-11 and ZSM-48, the copper is present as CuO. The ion-exchanged Pd-ZSM-5 exhibited lower, low-temperature activities in propane combustion, compared to the Cu-ZSM-5 catalysts, while the final conversions were close. The Pd-ZSM-5 catalysts, on the other hand, were found to be most active in natural gas combustion. The activity of these catalysts was correlated to theiroxygen carrier capacity.
The method of introducing the metal into the zeolite was found to be very important in determining the oxidation activities of the ZSM-5 catalysts. The introduction of Cu and Pd into the zeolite during the process of the hydrothermal zeolite synthesis, although very convenient, resulted in a less active catalyst. The Pt-ZSM-5 catalysts in which the platinum was introduced to the zeolite during the process of zeolite synthesis, however, exhibited high activity in propane combustion. The high activity of the Pt-ZSM-5 in propane combustion is most probably due to the activity of the platinum itself, rather than the mutual effect of the platinum and zeolitic structure. IV
List of Publications
This thesis is a summary of the following papers, referred to in the text by their corresponding Roman numerals.
I Kalantar Neyestanaki, A. and Lindfors, L.-E., "Catalytic Combustion of Propane Over Silica Fibre Supported Pt-Co Oxide Catalysts", 7th International Symposium on Heterogeneous Catalysis, Bourgas, Bulgaria, Part 2, 673 (1991).
II Neyestanaki, A. K. and Lindfors, L.-E. "Catalytic Combustion Over Transition Metal Oxides and Platinum-Transition Metal Oxides Supported on Knitted Silica Fibre", Combustion Science and Technology , 121, 97 (1994). m Kalantar Neyestanaki, A. and Lindfors, L.-E. "Catalytic Combustion of Propane and Natural Gas over Silica-Fibre Supported Catalysts", Combustion Science and Technology (submitted, 1995).
VI Kalantar Neyestanaki, A., Kumar N. and Lindfors, L.-E. "Catalytic Combustion of Propane over Pt and Cu Modified ZSM-5 Zeolite Catalysts", Fuel, 690, 74, 5 (1995).
V Kalantar Neyestanaki, A., Kumar N. and L.-E. Lindfors, "Catalytic Combustion of Propane and Natural Gas over Cu and Pd Modified ZSM Zeolite Catalysts", Applied Catalysis B: Environmental (accepted, 1995). V
The topic of the present thesis has also been presented in the following international conferences, report and patent:
1 Kalantar Neyestanaki, A. and Lindfors, L.-E., "Catalytic Combustion of Propane Over Cobalt Oxide Catalysts Supported on Knitted Silica-Fibre", Oral presentation at the 4th Nordic Symposium on Catalysis, Trondheim, Norway, October 3-4,1991.
2 Kalantar Neyestanaki, A., Paren, A. and Heidari, S., "New Development in Catalyst Supports", Technical Information, Kemira Ltd., January 1992.
3 Lindfors, L.-E. and Kalantar Neyestanaki, A., "Catalytic Combustion of Natural Gas", Poster presentation at the Twenty-Fourth International Symposium on Combustion", Sydney, Australia. July 5-10, 1992.
4 Salanne, S., Kalantar Neyestanaki, A., Lindfors, L.-E., and Vapaaoksa, P. "Catalyst Support ", Finnish Patent No 89012, 1993, Finland.
5 Kalantar Neyestanaki, A. and Lindfors, L.-E., "Catalytic Combustion of Propane", Oral presentation at the First International and 8thNational Congress of Chemistry and Chemical Engineering, Teheran, Iran, September 1-3, 1993.
6 Kalantar Neyestanaki, A. and Lindfors, L.-E., "Catalytic Combustion of Propane by Transition Metal Oxides", Poster presentation at EUROPACAT-I, Montpellier, France, September 12-17, 1993.
7 Kalantar Neyestanaki, A. and Lindfors, L.-E., "Catalytic Combustion of Natural Gas Over Silica Fibre Supported Catalysts", Oral presentation at the 6th Nordic Symposium on Catalysis, Hombaek, Denmark, June 1-3, 1994. VI
Contents
Preface .... i Abstract ....ii List of Publications ....iv Contents ....vi
1. INTRODUCTION .... 1 2. CATALYSTS AND COMBUSTION REACTIONS .... 3 2.1 Metal Catalysts .... 3 2.2 Metal Oxide Catalysts .... 7 2.3 Metal-Modified Zeolites ....14 3. SUPPORTS ....19 3.1 Monolithic Honeycomb Supports ....19 3.2 Fibrous Supports ....22 4. EXPERIMENTAL ....24 4.1 Silica-Fibre Supported Catalysts ....24 4.1.1 Support Preparation ....24 4.1.2 Catalyst Preparation ....29 4.1.3 Catalyst Testing Results ....30 4.1.4 Discussion ....43 4.2 Metal-Modified ZSM Zeolite Catalysts ....48 4.2.1 Zeolite Synthesis and Catalyst Preparation ....48 4.2.2 Catalyst Testing Results ....51 4.2.3 Discussion ....60 5. CONCLUDING REMARKS ....66 6. REFERENCES ....69
1 INTRODUCTION
With rapid industrialisation and motorisation throughout the world, the air pollution problem has been aggravated. The major pollutants from today ’s combustion systems are carbon monoxide, particulate matter, unbumed hydrocarbons, sulphur and nitrogen oxides. The formation of carbon monoxide, particulate matter and hydrocarbons is due to the incomplete combustion of fuels whereas the SOx emissions are due to combustion of sulphur-containing fuels. The NOx formation is due to the oxidation of fuel-bond nitrogen (fuel-NOx) or as a result of direct formation of NOx from atmospheric nitrogen (thermal- NOx). Thermal-NOx formation is governed by the atom-shuttle reactions of the Zeldovich mechanisms [1], in which the formation rate is controlled by the reaction:
N2 + O —> NO + N k = 7.6 x 1013 exp (-75.4 kcal/RT) (1)
The high activation energy of this reaction leads to the strong temperature dependence of the NO formation. In most cases, the conventional flame combustors operate at near the adiabatic flame temperature (> 1920 K) which results in thermal-NOx emissions of several hundred ppm. Thus, by holding the operation temperature within certain limits (below 1600 K), one can minimise its formation. Although low temperatures are acceptable for many practical purposes, the equivalence ratios that they correspond to are too lean for the conventional flame to be stable. On the other hand, the conventional combustors, in fuel- lean operation, tend to oxidise the fuel-N to NOx, resulting in high emission levels of fuel- NOx. To settle these global environmental and energy issues, the combustion systems need many improvements to render high efficiency and low emissions.
Catalytic combustion is generally considered to be an effective approach in controlling the emissions of hydrocarbons, carbon monoxide and nitric oxides. The aim of introducing a catalyst is to carry out heterogeneous oxidation on the catalyst surface. By choosing a suitable catalyst one can significantly reduce the activation energy of the oxidation reaction. Consequently, appreciable heterogeneous oxidation rates can be achieved for fuel concentrations and temperatures much lower than those required for homogeneous reactions to proceed. The advantages of the catalytic combustion can be summarised as:
- The ability to burn fuel to completion which results in negligible emissions of carbon monoxide and hydrocarbons.
- The ability to bum combustibles in very low concentrations. The catalytic combustion is
1 effectively applied even when the fuel concentration is insufficient to sustain stable homogeneous combustion. For example, the fuel-air mixture at an equivalence ratio of 0.3, not flammable under ordinary conditions, is easily ignited in a catalytic combustor.
- The ability to carry out stable combustion of fuels in a comparatively low concentration of oxygen.
- The ability to bum fuels to completion at lower temperatures than the non-catalytic flame combustion. Since the combustion is carried out at lower temperatures, the thermal-NOx formation is minimised.
- The ability to control the fuel-NOx formation. Catalytic combustion possesses the ability to control the fuel-NOx formation either by favouring the oxidation of N-containing fuels to nitrogen or by producing NOx and promote its removal by reactions of the type:
3NO + 2 NH3 = 5/2 N2 + 3 H20 2NO + 2CO -> N2 + 2C02 (2) 2NO + 2H2 —) N2 + 2H20
High combustion efficiency and lower temperatures result in minimum fuel consumption, lower emissions and reduced material costs (less temperature-resistant materials are required). The performance of the catalytic combustor very much depends on the premixed reactant flow conditions such as stoichiometry, pressure, velocity and inlet temperature as well as on catalyst activity and configuration. Increase in flow velocity generally decreases the combustion efficiency due to the decreased residence time of the reactants. At very high velocities, the reaction rate may not be able to keep up with the reactant mass fed to the catalyst surface, resulting in blow-out. This condition determines the upper practical energy release rate per unit of catalyst volume. Increase in inlet temperature and pressure results in increasing combustion efficiency by increasing the reaction rate.
Catalysts for catalytic combustion are usually noble metals such as Ft, Pd and Rh or metal oxides such as Co 304, Cr203, NiO, etc. or mixed oxides such as Cr203-Mn0 2, Cr203- Co 304, Cr203-Fe203 and Cr203-Mg0. In order to increase the thermal stability and the surface area of the active metal(s) or metal oxide(s), the catalysts are usually deposited on supports. Different supports such as monolithic honeycomb, ceramics, fibres and pellets have been used. Washcoat substrate materials are used for monolithic structures which strongly adhere to the support and provide a large surface area.
2 The scope of this thesis was to develop a new type of knitted silica-fibre. The knitted silica-fibre obtained was further used as support for combustion catalysts which were tested in the combustion of propane, natural gas and a mixture simulating the car exhaust gases. Different metal-modified (Cu, Pt, Pd) ZSM zeolites were also prepared by different techniques and were tested for propane and natural gas combustion. The nature of the active centres was investigated with different surface characterisation methods.
2 CATALYSTS AND COMBUSTION REACTIONS
Deep oxidation is carried out on highly active noble metals or on oxides of base metals. The choice of catalyst for catalytic combustion is based on three main considerations [2,3]: a) It must promote the total oxidation of hydrocarbons at low temperatures, b) It should not favour the formation of nitric oxides, c) It should have good thermal stability.
2.1 Metal Catalysts
Metals of interest as oxidation catalysts are mostly from group Vm and group IB, although in a high-temperature oxidising environment they are prone to form oxides. Of the noble metals platinum and palladium are the most active catalysts for fuel oxidation. The high activity of these metals is related to their ability to activate H2, P2, C-H and O-H bonds [4], The problems with theoperation of noble metals are due to the high volatility of noble metals or their oxides at high temperatures. Since these noble metals are used as highly dispersed particles and are exposed to a gas with high velocity, volatilisation will occur more rapidly. This volatility of e.g. platinum under oxidising conditions results in a growth of the platinum crystallite. Therefore, at high temperatures high platinum dispersion can not be maintained but this loss of platinum surface area may not affect the light-off characteristics to a great extent
The use of other noble metals is limited for catalytic combustion applications. Ruthenium forms a volatile oxide (RuO^) under oxidising conditions which is readily removed from the catalyst supports. Osmium is even more volatile and its oxide is poisonous. Iridium and Rhodium are also very costly and their availability is limited. Silver melts at a low temperature and gold is not so active for the oxidation reactions. Noble metals are less prone to sulphur poisoning than oxide catalysts [5].
3
2 A general sequence of activity of metals for oxidation reactions can be given as [6]: Rh, Pd, Os, Ir, Pt > Fe, Ni > Ta, W, Cr, Cu but, as mentioned previously, in high temperature oxygen environments the metal catalysts are prone to undergo either bulk oxidation or become covered with a thick oxide layer on their surfaces.
The rate of hydrocarbon oxidation has been correlated with the metal-oxygen bond strength [7,8]. Therate was found to increase as AH0 decreased, and platinum and palladium were shown to be two of the most active catalysts. Palladium has been found to be more active than platinum for CO oxidation and to be less active than platinum for the oxidation of saturated hydrocarbons. Combinations of noble metals, e.g. Pt, Pd and Rh, with one or two others exhibit synergism. Rhodium is highly active in NOx conversion but the rhodium itself is not able to convert the hydrocarbons to the desired levels. Palladium is the most active of these metals in conversion of all three pollutants (CO, HC, NOx) but it is sensitive to lead and sulphur poisoning, it sinters in reducing atmospheres and it also decreases the rhodium activity due to alloy formation with rhodium. The combination of platinum and rhodium is found to be able to convert all three pollutants from car exhaust gases to the desired levels.
Table 1 presents the kinetic data of propane combustion [8]. As can be seen platinum is much more active than the most active metal oxide catalysts (Co 304 and Cr203). Under common conditions for hydrocarbon oxidation, the metal surface is fully covered by oxygen. Therefore, generally no (or a slightly negative) influence of the oxygen partial pressure on the reaction rate is found. The reaction order with respect to hydrocarbons [9- 14], e.g. methane, is around one. The reaction orders, m and n, for platinum in propane oxidation are also close to that of the oxidation of methane (m=l and n=0).
Arai et al. [15] have reported a first-order kinetic with respect to methane and a negative value in reaction order in oxygen over a Pt/alumina catalyst. A Langmuir-Hinshelwood type equation, in which the surface reaction between adsorbed methane and adsorbed oxygen was proposed to be the rate-determining step, was considered. The dissociatively adsorbed oxygen was assumed to be the active species. The rate was expressed as:
k Kcnfa,. r (3) a * v™. * «W1B)2 where and KCH4 are the adsorption equilibria of oxygen and methane and k is the rate
4 Table 1 Kinetic parameters for propane combustion [8].
Catalyst Order in Temp, range ( °C) E, kcal/mol LogA* LogV**
£3^8 o 2 Pt 0.83 -0.09 220-260 17.0 1.39 -5.09
Pd 1.30 -1.60 337-368 36.3 6.89 -6.99
CuO 0.54 0.16 309-363 28.7 3.83 -7.13
C03O4 0.94 0.30 275-341 24.5 2.58 -6.76
NiO 0.89 0.46 350-416 26.2 1.68 -8.33
Mn0 2 1.01 0.00 294-358 28.3 3.80 -7.01 Fe203 0.68 0.22 395-425 35.9 4.87 -8.83 Cr203 0.78 0.17 283-341 21.9 1.47 -6.88 Ce02 0.67 0.25 383-450 27.8 1.41 -9.21
Th02 - - 373-418 36.8 4.65 -9.26
* -A in mollm 2s. ** - at 300 °C with 2% C3H8, 50% 02 and 48% N2. V in mol/m 2s.
constant. The decomposition of the oxygen-methane complexes at the catalyst surface has also been proposed to be the rate-determining step [16]. Comparison of different studies is usually very difficult because of the differences in the catalyst preparation techniques, test conditions and methods used to present the kinetic data.
The oxidation over noble metals is considered to be a structure-sensitive reaction. Hicks et al. [17,18] in their study of methane oxidation on supported platinum and palladium catalysts concluded that the rate of methane oxidation is affected by the structure of the exposed platinum. The catalytic activity of platinum was found to depend on the distribution of the metal between a dispersed and a crystalline phase. When platinum is completely dispersed, it oxidises to Pt02 starting at 573 K [19-21], However, when platinum is in the form of crystallites, only the surface of the crystallites is oxidised during the heating up to 873 K [13,19, 20 ].
5
•v The activity of the palladium depends on the metal particle size. The turnover frequencies in methane combustion have been found to increase with increase of the Pd-particle diameter [17,18]. The presence of two kinds of palladium oxide has been postulated: dispersed palladium oxide on alumina and palladium oxide deposited on metallic palladium, with the latter being very active. The degree of Pd-oxidation depends on the palladium particle size: small palladium particles are oxidised easily [17,18]. The oxidation over Pd- catalysts is assumed to take place over the palladium oxide phase, even if the catalyst is initially reduced, i.e. an oxidation of the catalyst takes place during the reaction [22]. At high temperatures (>773 K), the activity of the supported palladium catalyst might be due to the ability of the palladium oxide to chemisorb oxygen. Palladium, as metal, does not chemisorb oxygen above 923 K and is thus inactive toward hydrocarbon oxidation [23].
The activity of the palladium catalysts is also found to increase with time with the activation being faster on Pd/silica than on Pd/alumina [24,25]. The low-temperature titration experiments have shown that the oxygen adsorbed on aged palladium (aged under reactant mixture flow for 14 h at 873 K) is much more reactive to hydrogen than the fresh catalyst A reconstruction of the catalyst under reactant mixture has been postulated to be the reason for this observation with the restructuring being faster on silica-supported samples [24,25]. Chemisorption of molecules or atoms on a metal may result in the localised restructuring of surface atoms around the adsorption sites [26]. Collision of small molecules with high kinetic energies with the surface can transfer kinetic energy to surface atoms, thus inducing the restructuring of the surface, which facilitates bond breaking in the incident molecule [27]. Recently Gabrovski et al. [28], in theirstudy of catalytic oxidation of methane over Pd/Al 203 catalysts, presented evidence for the reconstruction of the Pd- particles. Their nanodiffraction and FITR measurements showed that fresh catalysts exhibited mainly Pd(lll) surface crystal planes while after the reaction with oxygen- methane mixtures the Pd(200) surface crystal planes were developed. Here also the dispersion of the metallic phase decreased by the catalyst treatment with the reactants mixture. Nanodifraction and TEM studies have also shown that platinum particles grow epitaxially on alumina with Pt(110) planes parallel to y-Al203 (110) planes and Pt(lll) axes parallel to y-Al203 (111) axes. The preferential exposition of Pt(110) planes in the aged sample as compared to the freshly reduced catalyst is considered to be responsible for the increase in the catalytic activity by time [29].
Olefin oxidation has been studied on supported and unsupported Pt, Pd and Rh catalysts [30]. The rate of complete oxidation over platinum and palladium wires was found to depend strongly on the oxygen partial pressure. The reaction orders with respect to oxygen
6 were 1.3 to 2 and the reaction was inhibited by hydrocarbon. Orders of -0.6 to -2 were found with respect to hydrocarbons. The negative orders indicate strong adsorption of unsaturated hydrocarbons on the platinum and palladium surface through the %-bonds. The kinetics of olefin oxidation on supported catalysts exhibit different features. Here, the reaction is less inhibited by the hydrocarbon (for supported Pt- and Pd-catalysts) or by oxygen (for supported Rh-catalyst).
2.2 Metal Oxide Catalysts
Transition metal oxides are generally effective as combustion catalysts [2, 3, 7, 8, 31-40], Oxides exhibit lower catalytic activity at low temperatures compared to metals, but at temperatures above 670 K their activities are about thesame. Many of the transition metal oxides can withstand very high temperatures which is a very important criterion for the combustion catalysts. Their lower price compared to the noble metals also makes them very attractive for use as catalysts for catalytic combustion. The combination of oxides often results in greater thermal stability and combustion activity than their separate components [33, 40]. The NOx formation from the fuel-bond nitrogen can also be suppressed by the metal oxides [31, 35].
Thecatalytic activity of the transition metal oxides is determined by their d-shell electron configuration. The maxima in activity are usually observed for the cations with three, six and eight d-electrons while minima occur for zero, five and ten d-electrons. The relationship between the catalyst activity and the d-shell electron has been explained by applying crystal field theory to the problem [41]. The basis of the theory lies in the fact that d-orbitals have directional properties and if a transition metal ion is associated with a ligand, the energies associated with these orbitals can vary. The nature of the ligand and the nature of the complex (high or low spin) can affect the energies, but the geometry of the complex is very important The chemisorption of a reactant on a metal ion can also be described as the formation of a complex. The formation of these complexes changes the energy. The energy changes depend on many factors, of which one is the crystal field energy. The five degenerate d-orbitals of the free ion are split by crystal fields of different symmetry and the degree of splitting is measured in terms of an energy parameter, lODq, which can be obtained from optical data. Chemisorption, the addition of a ligand to a complex, results in a change in geometry of the complex, for example from a square pyramid to octahedron. This changes the crystal field stabilisation energy, and calculations have shown that a characteristic twin peak pattern is obtained [42]. This pattern is similar
7 -7 -
a Ho ( Real/ 0 atom )
Figure 1: Correlation of the catalytic activity (rate at 573 K) with AH0 in the oxidation of ethylene [8],
to the pattern of chemisorption and catalytic activity for many metal oxides. The twin peak pattern is also observed for H2-D2 exchange reactions on oxides of the fourth period transition metals [43]. Peaks in activity are observed for Co 304 and Cr203.
Moro-oka et al. [7, 8] have correlated the catalytic activity of metal oxides and the reaction orders in the oxidation of propylene, isobutene, acetylene, ethylene and propane with the heat of formation of the catalyst oxides divided by the number of oxygen atoms in the oxide molecule (AH,,). It was found that the larger AH0, the less active the catalyst (Figure 1) and the higher the reaction order in hydrocarbon and the lower the reaction order in oxygen. From their results of competitive oxidation of hydrocarbons the authors categorised the surface of the catalyst as:
Group I, the catalysts having low AH0 values, such as platinum and palladium, is characterised by the negative order in hydrocarbon and nearly first order in oxygen. This means that the surface is completely covered by hydrocarbon and the slow step of the oxidation is the oxygen chemisorption on the surface covered by hydrocarbon.
8 Group n, catalysts with medium AH0 values, such as Co 304 and Fe203, is characterised by a nearly zero or positive low order in hydrocarbon and a nearly halforder in 02. This means that the surface is occupied by oxygen and hydrocarbon and the rate-determining step is the surface reaction between the adsorbed reactants. An increase in AH,, results in a decreased surface coverage of hydrocarbon and increased surface coverage of oxygen.
Group m, catalysts with higher AH0 values, is characterised by the first order in hydrocarbon and zero order in oxygen. Here, the surface is completely covered by oxygen and the rate-determining step of the oxidation over this type of catalyst would be either the chemisorption or the reaction of the hydrocarbon on the oxygen-covered surface. This variation of reaction order with AH0 of the catalysts was found to be less significant in the oxidation of ethylene and propane. The kinetics of propane oxidation on all catalysts was approximately first order in hydrocarbon and independent of oxygen. The weakness of hydrocarbon adsorption seems to favour an oxygen-covered surface.
The activity pattern for the oxidation of carbon monoxide at 423 K is found [44] to be:
Mn0 2 > CoO > Co 304 > MnO > CdO > Ag 20 > CuO > NiO > Sn0 2 > Cu20 > Co 203 > ZnO > Ti02 > Fe203 > Zr02 > Cr203 > Ce02 > V205 > HgO > W03 > Th02 > BeO > MgO > Ge02 > A1203 > Si02
The most active catalysts for the oxidation of carbon monoxide are the oxides of Mn, Co Cu and Ni. The oxides of the other transition metals such as Fe, Cr and Zn are moderately active while the higher oxides of polyvalent metals such as Ti, Zr, Ce, Th, V and W and most of the non-transition metal oxides are relatively inactive. The sequence of specific activity of metal oxides for methane oxidation can be given as:
Co 304 > NiO > Mn0 2 > Cr203 > Fe203 > CuO > ZnO > Ti02 > V205
The kinetics of methane oxidation over simple metal oxide catalysts is given in Table 2 [45]. The reaction order in CH4 was between 0.5 and 1.0. The rate was found not to be affected by water vapour and Co 304 and Mn0 2 were found to be the most active catalysts. The reaction mechanism for low conversions can be written as [46]: () () 1) 02 —>(02) —> 2(0)
rt fast (4)
9 Table 2 The kinetic parameters of methane oxidation over simple metal oxides [45]:
Reaction rate* E Order in
Catalyst CE4 molecules cm"2 s'1 kcal mol" 1 cb4 C02
C03O4 3.9 x 1012 18 0.9 0 NiO 2.4 x 1011 25 0.5 -0.2 MnOz 1.2 x 1011 19 0.7 0 Cr203 4.7 x 1010 27 1.0 0 ^s203 3.5 x 1010 29 0.6 0 CuO 3.4 x 1010 25 0.6 0 ZnO 7.5 x 10 8 31 1.0 0
* rate at 573 K and 1% CH4.
ECHO (4) (O) (O) tr3 (O) (O) 2) CH4 —> (CH2) —> (CH20) —> (HCOOH) —> C02 + H20 r2 r4
The methylene radicals, (CH2), are first formed and are then transformed into (CH20) species. The (CH20) species can either desorb resulting in the mild oxidation product, ECHO, or can be further oxidised to surface formate complexes and finally oxidise to the products C02 and E20. The rate equations are:
&1 ^2 Po2 PcnA r ~ ^2 Pcha 8 - (5) h Po2+v h Pcha
h n ■ h Pc"< (6)
h pcha
kojk4 (7)
10 0 (8) ■ C°2 kojkA + 0
where r is the overall rate and r- is the rate of formation of ECHO and C02. With 0 =1 Equation 5 reduces to:
, 1 o r - k Pcha Po2 (9)
The mechanism also allows one to explain a decrease in the rates of methane oxidation in the presence of the more reactive molecules of propane, formaldehyde and methanol. These substances interact more rapidly than methane with adsorbed oxygen resulting in decreased 0; this gives rise to a lower rate of methane oxidation. The more detailed heterogeneous catalytic oxidation of methane can be expressed as [46]:
la) 02 + ( ) -» (Og) ,1b) (02) + ( ) -» 2(0) 2a) CH4 + ( ) -» (CH*) 2b) (CH4) + (0) -> (CH3) + (OH) 3) (CH3) + (OH) -> CH3OH + 2( ) 4) (CH3) + (0) -> (CH30) + ( ) 5) (CH30) + (O) -* (CH20)+ (OH) 6) (CH20) ECHO + ( ) 7) 2(0H) -> H20 + (O) +( ) (10) 8) (CH20) + (O) -> (HCOOH) (CO) + H20 + ( ) 8a) (CO) -> CO + ( ) 9) (CO) + (O) -» (COj) + ( ) 10) (COa) -» 02 + ( ) 11) (COa) + (0) -> (C03) + ( ) 12) CH3OH + (O) + ( ) -» (CH30) + (OH) 13) HCHO + (0) (HCOOH) -> (CO) + H20 14) CO + (0) -> (CO^
Here the reaction proceeds by seven independent routes and it is assumed that methane has been adsorbed (Step 2a) before its interaction with adsorbed oxygen. Equation 5 is still valid, but the rate constant, k2, becomes equal to k2bbCH4 where k2b is the rate constant for Step 2b and bCH4 is the adsorption coefficient of CH4. The adsorption of CH4 is assumed to take place in the Henry region so the surface coverage with CH4 is small.
11 In general, high oxidation activity requires metal ions that can assume more than one valence state and can participate in redox cycles in this way. Thed-character of the metals is the single factor determining the ability of the metal to break the C-H bond. Anderson [47,48] has shown that d z2 or dx 2-y2 orbital of metal atoms or cations, relatively little involved in bonding, are responsible for methane activation on the surface of transition metals or transition metal compounds.
The surface oxygen is usually considered to be the important species in complete oxidation reactions. The oxygen molecule can be activated by interacting with the surface of the oxide catalyst. Theactivation process involves coordination, electron transfer, dissociation and incorporation into the oxide lattice. The oxygen interaction with metal oxides and its participation in catalytic oxidation can be represented by the following scheme [49]:
Or
k4 R Y R°r k? Y co2
where Os is a partially reduced electrophilic form of oxygen possessing increased reactivity. In particular, they can be ion-radical species [50]. These species must possess high oxidative activity with respect to organic electron-donating compounds and must lead to the complete oxidation of organic compounds by non-specific attack of the sites having the maximum electron density in oxidisable molecules (e.g. multiple bonds). Or is thetwo electron-reduced lattice oxygen providing selective oxidation products (RO). Part of this oxygen can also participate in the formation of deep oxidation products. In the absence of the combustible substances the 02 concentration will be controlled by the ratio of Step 1 and 2 in the scheme. The formation of Os must be promoted by the presence of surface donor centres having low ionisable'potentials. In Step 2, an additional transfer of electrons to oxygen and their insertion into an oxygen vacancy takes place. Thus the high concentration of Os can be achieved on the oxides containing electron-donor centres having low ionization potentials and low degrees of surface reduction, when the concentration of
12 vacancies is low. This situation can be predicted for transition metal oxides with partially filled d-shells in media with excess oxygen [49]. The concentration of the surface oxidised structures, ROr and ROs, is determined by theratio of their formation and decomposition. If the formation rate of these structures is relatively low and the rate of their spontaneous decomposition is high, i.e. kg » k3 ; k7 » k4 , then their steady state concentration is low. At higher formation rates which is probable for the reaction of oxidisable reactants with Os , then k3 » k6 and their concentrations is high. The total rate for the concerted mechanism can be written as [51]: ^ = W (ID by neglecting the reverse reactions in Steps 1 and 2 and having the steady state with respect to Os, the reaction rate for the concerted mechanism is expressed as:
(kl +k5R0 s).0 2k^R (12) k-fi 2+ k£l+ k^R assuming that at low temperatures k3R » k102 + k2CI, then :
= *i+ (^o;o 2 d3) The reaction rate of the concerted mechanism is determined by the step of oxygen interaction with catalyst. The opposite is observed for the stepwise mechanism. Under these conditions, deep oxidation catalysts operate at a high degree of surface oxidation and the rate is limited by the step of catalyst interaction with oxidisable reactants (Steps 4 and 7). The rate for the stepwise mechanism is [51]:
^ = W (W)
Theactivation energy for the stepwise mechanism is usually higher than for the concerted mechanism [52]. At lower temperatures the oxidation reaction follows a concerted mechanism. The rate-determining step is the decomposition of oxidised surface structures. The presence of the gas phase oxygen increases the rate of the decomposition of these structures. Thehigher the mobility of catalyst oxygen, the lower the temperature at which the concerted mechanism transforms into the stepwise one.
Therefore, in the region of the concerted mechanism, the reaction rate is controlled not by oxygen detachment from the catalyst, but by the catalyst’s ability to bind oxygen. It is promoted by the presence of a large number of surface donor centres with low ionisation energies at relatively low conversion rates of active forms Os in two-electron-reduced Or. In the region of the stepwise mechanism, two cases can be observed: a) The decomposition
13 of partially oxidised surface structures occurs slowly. During their decomposition the oxygen catalyst bonds dissociate and the reaction rate can depend on the energy of these bonds, b) The decomposition of surface structures is fast. This situation can be realised on active catalysts having low oxygen bond energies and also in media with excess oxygen. Here the reaction rate is controlled by the catalyst interaction with oxidisable substances rather than by the oxygen bond energy.
Thus, in any case, an active catalyst for complete oxidation first has to bind oxygen at a high rate, generate active states, and slowly transform these states into the oxide lattice ions to provide their considerable concentration on the catalyst surface. According to the data in the literature, of the simple oxides of 3d elements, Co 304 possesses the highest rate of oxygen bonding and the lowest heat of oxygen bonding.
2.3 Metal-Modified Zeolites
The zeolites are hydrated, crystalline tectoaluminosilicates with the general formula:
where Mn+ is the cation which balances the negative charge associated with the framework aluminium ions. Zeolites are constructed from T04 tetrahedra (T=tetrahedral atom, e.g. Si, Al); each epical oxygen atom is shared with an adjacent tetrahedron. Thus the frame work ratio of O/T is always equal to two. From the valency of silicon it follows that silicon atoms generally prefer to form bonds with four neighbouring atoms in a tetrahedral geometry. If a Si04 entity could be isolated, its charge would be -4 , however a Si04 unit in a framework is neutral since an oxygen atom bridges two T atoms and shares electron density with each (Figure 2) [53]. O 0 o o o |.+4 .Si O o/ 'C O o -1
Figure 2: The T02 units in zeolites [53]
14 Therefor a defect-free pure Si02 framework will not contain any charge. If aluminium is tetrahedrally coordinated to four oxygen atoms in a framework, the net charge is -1. When tetrahedra containing silicon and aluminium are connected to form an aluminosilicate framework, there is a negative charge associated with each aluminium atom and is balanced by a positive ion (M+ in Figure 2) to give an electrical neutrality. Typical cations are alkali metals, e.g. Na+, K+; alkaline earth metals, e.g. Ca2+, Ba2+ and the proton, H+. Tectoaluminosilicates do not in general have ratio Si/Al The framework of ZSM-5 contains a novel configuration of linked tetrahedra (Figure 3a) and consisting of eight five-membered rings. These ZSM-5 units join through edges to form chains as shown in Figure 3b. The chains can be connected to form sheets and the linking of the sheets leads to a three-dimensional framework structure (Figure 3c). The chains extend along the z-axis. The ZSM-5 framework can be generated by linking the sheets (Figure 3c) across mirror planes forming four and six-membered rings. The unit cell content of the Na form is NanAlnSi96.n.01g 2.~16 H20, where n < 27 and is typically about 3. (b) (c) Figure 3: The characteristic configuration (a) and its linkage within chains (b) in ZSM-5 . (c) the (010) face of the ZSM-5 unit cell [55, 56]. 15 t- In ZSM-5 two types of intersecting channels exist that are both defined by 10-membered rings (Figure 4). These channels are sinusoidal and parallel to the (100) axis and straight and parallel to the (010) axis, respectively. The sinusoidal channels are nearly circular and 0.54 x 0.56 nm in dimension, while the straight channels are elliptical with 0.51 x 0.55 nm in dimension. This two dimensional channel system has pore intersections where the cross- section is much larger [57]. ZSM-11 is closely related to ZSM-5 and also belongs to the pentasil family of zeolites, however its channel structure is different from ZSM-5, as it consists of perpendicularly intersecting straight channels (Figure 4) with dimensions of 0.51 x 0.55 nm. ZSM-11 has tetragonal symmetry and its crystallographic structure is more symmetric than that of ZSM-5. ZSM-11 has two types of channel intersections one comparable to the intersections in ZSM-5 and another having 30 % more free space [58]. ZSM-48 belongs to a family of zeolites with disordered ferrierite structure. The framework of ZSM-48 is composed of ferrierite-type sheets connected in such a way as to generate 10-membered ring channels. ZSM-48 zeolites have a typical morphology of needles or agglomerates of them. The ideal channel dimensions of ZSM-48 are 0.53x0.56 nm [59]. The typical morphology of the ZSM-5, -11 and -48 zeolites is given in Figure 5. 0.51 X 0.54 nm 0.54 X 0.56 nm ZSM-5 intersection 1 . 0.51 X 0.55 nm ZSM-11 Figure 4: The channel systems of ZSM-5 and ZSM-11 [60, 61]. 16 Figure 5: The SEM images of a) ZSM-5 , b) ZSM-11 and c) ZSM-48. 17 Cu' Cu" Cu* (AlO)* Cu* ws/wwA \ a A A /\ A pooopoooooooooo t i i i i i i T i i i i i i i i (II) (HI) ♦1/20,%\CO, ' Cu” Cu” ♦CO ♦H, ooooooooo -CO, -H,Q H,0 ??99?9^?999?99? +1/20, (I) Cu* (CuOCu)” (AlO)' o O o o o o o o o o o ' AI Si Al Si W Ai V a / X ^0^000(^000000^00 9999,9999999999999 (V) (IV) Figure 6: Interactions of CO and 02 with Cu-Y zeolites [62]. Metal-modified zeolites are of interest as oxidation catalysts [62,63]. It is known that FeY zeolites are oxygen carriers. The Fe2+Y can be oxidised with oxygen, forming Fe3+, i.e. addition of one oxygen atom for every two Fe2+ oxidised. The process was found to be reversible i.e., the Fe3+Y can be reduced by H2 or CO to Fe2+ with theformation of C02 and H20. The infrared, mossbauer and stoichiometric gas consumption measurements of these Fe-Y catalysts showed that the oxygen is held bridged between two Fe ions as an extra lattice oxygen [62,63]. Cu-exchanged zeolites have similar oxygen carrier behaviour, however, here Cu2+ are exchanged into the zeolite and they must be reduced before Cu+/Cu2+ redox can occur. Cu-Y zeolites can be reduced with CO to produce C02, thus removing oxygen from the lattice. Jacobs and Beyer [64] described the process as partial dealumination of the lattice. Cu+ and A10+ are formed by the reduction of Cu2+-Y with CO (Figure 6). In this way the lattice becomes charge-balanced by equal amounts of Cu+ and A10+. By re-oxidation, the Cu+ ions are converted into Cu2+-0-Cu2+ species, while the A10+ concentrations remain unchanged, meaning that one aluminium T-site must be eliminated for every two Cu2+ that are reduced to Cu+. TheCu-Y zeolite now becomes an oxygen carrier analogous to the Fe-Y zeolite. It has been shown that upon evacuation or flashing at high temperatures with He, Cu-ZSM-5 can undergo self-reduction with the spontaneous desorption of oxygen [65, 66]. It might be difficult to explain the 02- desorption through dealumination. 18 The redox capacity of the excessively ion-exchanged Cu-ZSM-5 and Cu-Y has been found to be "0.5 O/Cu, i.e. le'/Cu, and this could be attributed to reduction of Cu2+ to Cu+ [65,67-69]. Petunchi et al. [68] in their XRD, Si and A1 MAS NMR of Cu-Y zeolites showed that the Cu-Y, and possibly ZSM-5, were not extensively dealuminated on reduction with either CO or H2. The detected amount of dealumination was found to be much less than that required to charge-balance the lattice on the conversion of Cu2+ to Cu+. Evidence was presented that the extra lattice oxygen (ELO) is introduced into the zeolite during the preparation step especially with over-exchanged zeolite (Cu/Al > 0.5) and an important part of it is held bridged between two Cu2+ ions. The ELO is carried into the zeolite during the Cu-exchange process and the lattice oxygen is not removed as Cu2+ is converted to Cu+. This means that the catalyst as prepared is an oxygen carrier which can be reduced by CO, HC or by spontaneous desorption of oxygen above 623 K [65,70,71]. The [Cu-0-Cu]2+ is therefore the most active site in hydrocarbon oxidation and its concentration increases by increasing the copper loading. However there are questions still to be answered. If the ELO is held bridged between two Cu2+ ions — thus, two such sites (or four Cu2+ions) are required to hold a single pair of oxygen atoms necessary to form an oxygen molecule released by spontaneous desorption of oxygen — then how do these two oxygen atoms in high silica zeolites, such as ZSM-5, where Al T sites are relatively far from each other can get together to recombine and spontaneously eliminate oxygen? 3 SUPPORTS The catalysts for catalytic combustion are usually deposited on supports in order to increase the surface area of the active components) by providing a matrix that stabilises the formation of very small particles. The thermal stability of these small particles is increased, thus preventing the agglomeration and sintering which results in loss of active surface. In some cases the support itself exhibits some catalytic activity due to its own special properties. Deposition of an active catalyst on the supports is usually done by impregnation and special techniques have been developed to obtain a good distribution of finely dispersed particles. Depending on the applications, different kinds of supports such as monolithic honeycomb, ceramics, fibres and pellets are used. 3.1 Monolithic Honeycomb Supports The monolithic honeycomb is technologically the most advanced support for high 19 3 throughput catalytic combustion. They consist of small parallel channels and are available in different shapes (triangular, square or circular channels) and sizes. The monoliths can be made of ceramics extruded in one piece, or metals. The monolithic honeycomb supports are used for automobile exhaust, pollution clean up and gas turbine applications. Because of high operation temperatures, it is necessary to maintain the catalyst dispersion. For monoliths, the best way to do this is by using a washcoat which adheres strongly to the monolith ’s surface and increases the monolith ’s surface area. The most common washcoat material is y-Al203, however at temperatures above 1170 K, y-Al203 undergoes a phase change to a-Al203. This phase change results in a drastic decrease in its surface area (from approx. 300 m2/g to approx. 5 m2/g) and results in closure of the pores and burying of the active sites. So, usually stabilisers such as Ba, Zr, Sr, La, Nd, Sn and Si02 are added to the A1203 washcoat [72, 73]. Thus, the role of the washcoat is to provide a good distribution of the catalyst and to inhibit the sintering. Since the monolith is coated with a washcoat, thermal expansion coefficients are important when selecting a washcoat-support system. The support must withstand high temperatures and must have high thermal shock resistance. It is found that the lower the thermal expansion, the higher the thermal shock resistance. A general light-off curve is given in Figure 7a. The curve can be divided into three regions. In region A the reaction rate is controlled by the chemical kinetics of the surface reaction. The catalyst performance in this region depends on the activity and the dispersion of the catalyst. As the rate is increased, heat increases to the point where the catalyst lights-off (region B). The faster the monolith heats up, the faster will be the catalytic reaction and the faster the light-off. Rapid oxidation continues to the point where mass transfer of gases to the catalyst becomes rate determining (region C). This point can be affected by the turbulence in the monolith channels or by the dimensions of the channel [74]. Thus the cell density of the monolith is very important. The catalytic combustion is controlled not only by surface phenomena but also by gas- phase chemical reactions and transport processes of mass, momentum and energy. While working with honeycomb monolithic substrates, different processes dominate under different conditions. At the entrance to a honeycomb catalyst cell, the reactant’s temperature is too low for homogeneous reaction to take place however heterogeneous reaction does occur at the catalyst wall. At very low inlet temperatures the surface reaction rate is negligible and almost no oxidation takes place. As the inlet temperature moderately increases, the surface reaction rate increases rapidly and some oxidation takes place in the bed. The reaction is kinetically controlled and thecombustion efficiency strongly depends 20 on the inlet temperature. Here the temperature is too low for homogeneous gas-phase reactions to take place. At higher inlet temperatures the combustor enters the regime of mass-transfer control. Here the reaction at the initial section of the bed is controlled by the kinetics of the surface reaction (kinetically controlled). The heat released in this part of the bed increases the gas and surface temperatures. Since the surface reaction rate constant increases exponentially with increasing surface temperature it will soon become so large (the fuel concentration at the surface will become practically zero) that the rate of transport of reactants to the surface becomes the rate-controlling step. Thus, in this regime of operation, in the initial part of the bed the reaction is kinetically controlled while the rest is mass transfer controlled. The combustion efficiency becomes relatively independent of the temperature (the mass transfer coefficient is a weak function of temperature). Here also the gas temperature in the bed does not become sufficiently high for the homogeneous gas- phase reactions to take place. If the inlet temperature is increased sufficiently, the axial temperature increases, and the gas temperature at the lower part of the bed becomes high and, consequently, the gas phase combustion will be initiated and the reaction goes to completion. Temperatures at which transitions from the surface kinetically controlled region to the mass transfer controlled region and from the mass transfer controlled region to where the homogeneous reactions take place decrease with increasing the equivalence ratio of the inlet mixture (for constant residence time) [33]. Most conventional catalytic combustors operate under mass transfer controlled conditions. As a result of gas-phase combustion the characteristic light-off curve presented in Figure 7a is changed (Figure 7b). This is the case for high-temperature catalytic combustion. Here, the catalytic combustion reaction contains the surface reaction, mass transfer and gas phase homogeneous reaction as rate-determining processes [75]. Homogeneous combustion is a free-radical process and the overall process is affected by the process of initiation, propagation and termination of the free radical chain reactions. The smaller monolithic channels give more exposed surface which in turn results in higher catalytic activity. An increase in exposed surface means a high surface to volume ratio and this increases the probability of the free-radical chain termination. However, small monolithic channels result in an increased pressure drop which under high throughput and high temperatures is very undesirable. The highest possible surface to volume ratio with a pressure drop of less than 6% gives the most efficient overall performance [76]. The overall performance increases with increasing surface to volume ratio; the pressure drop must be small and a monolith with thin walls which is thermally stable is desired. 21 c g (a) cr2 TEMPERATURE TEMPERATURE ------Figure 7: a) A typical light-off temperature curve of an automobile catalyst, b) Effect of homogeneous combustion on the temperature dependence of the overall reaction rate in catalytic combustion. 3.2 Fibrous Supports Fibrous substrates are used in low-temperature combustors for domestic, agricultural and industrial heaters. The catalyst is usually deposited on fibres which are woven into a pad. The unit can be used for space heating in industrial, agricultural and domestic applications. The infra-red emission produced is suitable for paint drying and the temperature of the burner is not high enough to cause any explosion of the emitted vapours. Because of the high tendency of the reactants to blow off the surface, the applications of the fibre pads are restricted to relatively low flow rate conditions. Pressure drop limitations are of less importance while working with fibrous materials. The low thermal conductivity of the fibre materials results in little heat conduction in the upstream direction of fibre layers, hence the plenum side of the fibre pad remains below the ignition temperature of the premixed air/fuel mixture. This prevents flash-back from occurring, allowing safe operation. The 22 main advantage of the fibre pads is that they can be constructed in different geometries suitable for any given application. The fibres also have better warm-up characteristics as compared to the monolithic honeycomb or pellet supports [77,78]. The disadvantage is that the combustion may not be complete and some slippage of the combustible may occur. Different fibres such as ceramic fibres, fibre glass and alumina/zirconia fibres, have been used. Asbestos wool is not used any more because of health hazards. Commercial combustor pads (mats) have been manufactured mainly from silica glass but because of production costs and the low specific surface area of these fibres, attention has been focused on alumina fibres. Washcoats can also be used but since the working conditions are not extreme the use of washcoats is not very common. Metal sintering can also be observed but massive agglomeration, or melting, is not common. The fibres are stable under operating condition, however they tend to sinter under steam-rich atmospheres [74]. Sintering has more effect on the light-off characteristics than on the steady state operations. A schematic design of the fibre burner is given in Figure 8a [4], A premixed airrfuel mixture passes through the catalyst fibre pad where it combusts. Energy is emitted mainly by radiation. Radiation from the burner surface causes the peak temperature achieved below the burner surface to decrease slightly. The burner is highly non-adiabatic and can be operated at near stoichiometric conditions with low pollutant emissions. The convective diffusive combustor is widely used as a domestic heater. In this device, as is evident from its name, the combustion occurs in the diffusive mode. Fuel is passed through the catalyst pad by forced convection from the rear and the air is supplied to the front vertical face by natural convection induced by the transpiration of hot products from this face into ambient air (Figure 8b). The system has been studied by using a platinum catalyst supported on alumina fibre [79]. The method of loading the catalyst, the thickness of the pad, the packing density of the fibre pad, the rate of the fuel supply as well as the nature of the catalyst are all factors that must be considered while working with catalytic combustors based on catalysts supported on fibre materials. Chemical reactions increase with increase in the rate of the fuel supply, as does heat generation. However, this increase in the fuel-supply rate results in a decreased contact time as well as counter diffusion of oxygen, leading to increased fuel slippage. The temperature profile measurements of the catalyst pad have shown the existence of a hot reaction zone which moves toward the front surface as the rate of the fuel supply is increased [79]. The increase in the fuel supply can also affect the radiation efficiency. At low fuel input rates, the reaction takes place at the back of the catalyst pad (at a low rate) and the heat generated transfers to the front of the pad. At higher fuel flow rates, the reaction zone moves toward the front of the catalyst 23 Support screen Fibre 3/ Vacuum-formed matrix 'fibre layer Balance -Conduction Premixed — 1150K - 1375K (a) reactants Radiation Metal Casing •Gas Distributor Fibre , Catalyst nod •Riel Inlet Electric NNN 7Z Heating Element — Heating Element Figure 8: a) Schematic design of fibrous burner b ) Convective-diffusive burner [4,79]. pad, but less reaction takes place and less heatgenerates due to the reduced contact time. Additional thickness of the pad will make it serve as an insulant. An increase in the packing density of the catalyst pad results in decreased combustion efficiency since the diffusion of gases through the pad decreases, but it also results in more surface area from which radiation could occur. Therefore, the rate of the fuel supply, the packing density and the pad thickness must be optimised. 4. EXPERIMENTAL 4.1 Silica-Fibre Supported Catalysts 4.1.1 Support Preparation In this work a hybrid textile received from Kemira Ltd. was used as a raw material for the support preparation. This textile was made of a hybrid fibre (organic - inorganic hybrid fibre) which is a cellulosic fibre containing molecular chains of silicic acid: 24 Cellulose Polysilicic acid ™'n These fibres (VISIL) were produced via a modified viscose process in which the cellulosic component was regenerated simultaneously with the polymerisation of the silicic acid. The fibre exhibits a kidney-bean shaped cross-section and a smooth, semi-crenellated exterior surface. The chemical composition of the cellulosic fibre was: Cellulose 67 - 70 % Silica ( Si02 . xH 20 ) 30-33 % Impurities: Na, Zn, S < 1 % P < 200 ppm By heating the fibre in the presence of oxygen at temperatures of 533 K to 573 K the cellulosic component carbonises. The carbonisation is accelerated at temperatures of 573 K - 653 K. By 873 K the pyrolysis of the cellulosic component is completed. The carbonisation and burning of the carbon as well as continuous heat treatment up to the temperature range of 873 K - 973 K, dissipates large amounts of heat which initiate a sintering condition for polysilicic acid conversion into coherent filament of silica (Figure 9a) without an actual fusing of the particles. The hybrid fibre was burnt at different temperatures (with a heating rate of 10 K/min and the temperature was kept at the final temperature for lh, followed by slow cooling to room temperature) and the BET specific surface area, pore volume and the crystallinity of the obtained silica-fibres were measured by nitrogen adsorption and X-ray powder diffraction. By increasing the burning temperature from 773 K to 1273 K, the BET specific surface area decreased from 220 m2/g to 10 m2/g. The specific pore volume was also decreased (in the temperature range of 923 K - 1273 K) by increasing the burning temperature (Figure 10a). The loss on ignition was 69% and the shrinkage of the fibre during the firing in the fibre length was 25%. The chemical composition of the obtained silica-fibre was: 25 Figure 9: a) SEM image of a single silica-fibre, b) SEM image of the catalyst support. Si02 97-99% Na < 1% Zn < 500 ppm P < 600 ppm 26 SPECIFIC SURFACE AREA SPECIFIC PORE VOLUME BET (m2/g) Vp (ml/g) 250 x x 0.6 * - 0.5 200 - * - 0.4 150- - - 0.3 (a) 100-- - - 0.2 50 - - - - 0.1 BURNING TEMPERATURE, K 2 3 BET SPECIFIC SURFACE AREA, m /g SPECIFIC PORE VOLUME, cm /g (b) Figure 10: BET surface area of: a) the silica-fibre as a function of burning temperature, b) the knitted silica-fibre as a function of burning temperature. At temperatures below 1223 K the silica-fibre, stretched and unstretched, retained its amorphous structure (Figure 11a). Crystallisation appeared at the temperature range of 1273 K - 1373 K (the alkali content accelerates the crystallisation of the silica-fibre). At 1373 K - 1673 K the crystals of cristobalite began to form. A 3.5 decitex hybrid textile, weighing 570 g/m 2, was washed with methanol and boiled in distilled water followed by rinsing with hot distilled water to wash away the impurities from the spinning bath and the finishing agents (mostly phosphorous compounds). The washed textile was burned to obtain a pure (99.5% Si02) knitted silica-fibre (Figure 9b). 27 30 -r 1273 K Stretched 25 -- 1273 K No Stretching 20 -- 1233 K Stretched 1233 KNo Stretching (a) i i:8e\ 1.2t' '•. 65 5.56 6.75 ■ "%ti (b) t-.t-y 6.36 >>r 6.15 5.8 16.6 15.6 26.6 36.6 25.6 26 Figure 11: a) XRD pattern of the silica-fibre; b) XRD pattern of the catalyst support. The hybrid textile was burned at different temperatures (under the same conditions as with the hybrid fibre) and the BET surface area of the obtained knitted silica-fibre was measured. The BET surface area of the obtained knitted silica-fibre decreased by increasing the burning temperature of the hybrid textile (Figure 10b). However, this decrease in the surface area was less extreme as compared to the decrease in the BET surface area of the obtained silica-fibre by increasing the burning temperature of thehybrid fibre. This is due to the absence of impurities on the hybrid textile which accelerates the partial crystallisation of the silica at higher temperatures. Most of the impurities are water soluble and after washing the hybrid textile and burning it, the total amount of impurities was less than 0.5 wt.%. 28 Pores size distribution (Des) (SUPPORT.SOR) Radius Figure 12: The pore size distribution curve of the catalyst support. The knitted silica-fibre obtained by burning the hybrid textile at 1223 K was selected to be used as a support for catalysts for catalytic combustion. The support had a thickness of 1 mm and weighed 190 g/m 2, with a BET specific surface area of 140 m2/g and a specific pore volume of 0.469 cm3/g. XRD analysis of the obtained knitted silica-fibre exhibited an amorphous structure (Figure lib). The pore size distribution curve of the catalyst support showed that 82% of the pore volume is located in pores of 20-1000 A in radius (Figure 12). The stability of the silica-fibre support in steam-rich atmosphere was tested at 973 K with 40% and 60% moisturised air for six hours. The absence of any change in the BET surface area indicated the absence of fibre sintering under these conditions. 4.1.2 Catalyst Preparation The knitted silica-fibre obtained by burning the 3.5 decitex hybrid textile was used as a support for combustion catalysts. A series of silica-fibre supported metal oxide catalysts (C03O4, Cr203, NiO, Mn0 2, CuO, Co 304-Cr203, Co 304-NiO, Co 304-NiO-Cr203), and Pt- metal oxide catalysts (Pt-Co 304, Pt-NiO, Pt-MnOz Pt-Cr203, Pt-Co 304-NiO and Pt-Co 304- Ni0-Cr203) as well as Pd and Pd-Pt catalysts was prepared [I-3H]. The catalysts were prepared by impregnation (for the single component catalysts), subsequent impregnation with intermediate drying and calcination (for Pt-activated metal oxide catalysts) and co - impregnation (for the multi-component catalysts) of the support with aqueous solutions of metal salts. Thecatalysts were then dried and calcined in air at the required temperatures. 29 4.1.3 Catalyst Testing Results The prepared catalysts were tested for their light-off and conversion behaviour in the conversion of car exhaust gases [II] and the complete oxidation of propane [I, 1H] and natural gas [HI]. The feed gas mixture simulating the car exhaust gases used in the light-off temperature testing was composed of 0.0125% CgHg, 0.0375% CgHg, 1.00% CO, 10.00% C02, 0.15% NO, 1.02% 02, 10.00% water (gas), and balanced by N2 (A. = 1.010). The total flow rate (at standard conditions) was 8900 ml/min (GHSV=53000 h"1). A part of the air (187 ml/min) was pulsed into the reactor at a frequency of 1 Hz. The experimental setup for the catalyst testing in the conversion of car exhaust gases is given in detail in Paper EL For the light-off testing, the temperature of the preheater was kept at 673 K and the tube furnace temperature was raised from 433 K to 773 K at the rate of 12 K/min. During the light-off testing the gas content and the conversions of HC, CO and NO as well as temperatures before and after the catalyst sample were registered. After the light-off testing the tube furnace was cooled to 623 K for the A-window measurements. After stabilisation of the temperature, the gas mixtures presented in Table IH, Paper II (the total flow rate was 8900 ml/min and all the gas flows except the 02 and N2 were kept constant) were led to the reactor and the conversions were measured. The gas mixture in propane combustion consisted of 0.285 vol.% propane (99.5 % pure), 6.8 vol.% synthetic air (dry) and 92.92 vol.% nitrogen (A=1.05). The total flow rate was 2500 and 5000 ml/min and GHSV = 23900 h"1, 47800 h"1, respectively. The gas mixture for natural gas combustion consisted of 1.44 vol.% natural gas, 14.64 vol.% air and 83.92 vol.% nitrogen (the total flow rate was 2500 ml/min, GHSV = 23900 h"1 and A = 1.05). The hydrocarbon content of the natural gas was 92.7 vol.% CH4, 5.1 vol.% C^Hg, 2.1 vol.% CgHg and 0.1 vol.% C4H10. The experimental setup for the testing of the silica-fibre supported catalysts in propane and natural gas combustion is given in detail in Paper IQ. For catalyst testing the preheater temperature was kept constant at 623 K and the tube furnace temperature was raised from 423 K to 873 K at a heating rate of 12 K/min and the propane/natural gas conversion (to C02 and H20) and temperatures before and after the catalyst were measured continuously. As a result of the catalyst testing, curves of conversion as a function of temperature (light- off curves) were drawn which enabled the measurement of the temperature at 50% conversion (light-off temperature) and the final conversion at 673 K or 823 K. 30 a) Mixture simulating car exhaust gases Tables 3 and 4 present the results of the temperature and conversion measurements for various catalysts at two lambda values (GHSV = 53000 h"1). The temperatures mentioned for the catalyst testing represent the temperatures before the catalyst. As mentioned previously, three reactions are considered: oxidation of carbon monoxide, hydrocarbons and the reduction of nitric oxide. The catalysts in the text and tables are subsequently denoted as the values in the parentheses corresponding to the metal content of the catalyst in wt.%, e.g. the (7.0, 6.8, 3.8)wt.% Co-Ni-Cr describes the Co 304-Ni0-Cr203 catalyst containing 6.5 wt.% cobalt, 6.8 wt.% nickel and 3.8 wt.% chromium. The light-off temperature is denoted by T50% and is given for each reactant and the given catalyst. Also listed in Tables 3 and 4 are the corresponding values of the conversions at 673 K. As is presented in Table 3, by increasing the cobalt content of the Co 304 catalysts from 5% to 15 wt.% cobalt (reduced basis), the conversions (at 673 K and X = 0.981) of carbon monoxide and hydrocarbons increase. Conversions of 57.4% for carbon monoxide and 30.7% for hydrocarbons were achieved with the catalyst containing (15.5)wt.% Co. This increase in the Co 304 content of the catalyst significantly lowered the light-off temperature for carbon monoxide oxidation from above 673 K to 560 K, but the light-off temperature for hydrocarbons remained above 673 K. In order to study the effect of stoichiometry on the light-off temperatures, the (14.9)wt.% Co catalyst was tested at X = 1.010 (mixture number 9 in Table m Paper II). This increase in the oxygen content of the feed resulted in a considerable decrease in the light-off temperatures for both carbon monoxide and hydrocarbons as well as in an increase in their conversion at 673 K. It should be noted, however, that the NO conversions over Co 304 catalysts remained at a low level throughout Combinations of Co 304 and NiO catalysts, tested at X = 1.010, deteriorates the conversions of both carbon monoxide and hydrocarbons, compared to the catalysts containing the same amounts of Co 304. Chromium (ID) oxide appeared to be more active in HC combustion than in CO oxidation, judging from its conversion behaviour (Table 3). Combining Co 304 and Cr203 resulted in a decrease in carbon monoxide conversion (from 43.9% to 7.9%), but increased the conversion of hydrocarbons (from 13.9% to 41.7%) in comparison to the Co 304 catalyst (9.1 wt.%Co-catalyst in Table 3). Combination of Cr203 and NiO also resulted in an improved HC conversion but only slightly increased the conversion of CO, as compared to the chromium (HI) oxide catalyst. 31 Table 3 The light-off temperatures and the conversions at 673 K of the metal oxide catalysts, space velocity = 53000 h"1. Catalysts (wt.%) X* (%) Conversion (%) at 673 K for: CO HC NO CO HC NO (5.5)wt.% Co 0.981 >673 > 673 >673 38.6 9.9 2.0 (9.1)wt.% Co 0.981 >673 > 673 >673 43.9 13.9 ' 2.0 (15.5)wt.% Co 0.981 560 >673 > 673 57.4 30.7 1.0 (14.9)wt.% Co 1.010 532 660 > 673 62.5 52.3 1.0 (7.2, 7.1)wt.% Co-Ni 1.010 >673 > 673 >673 25.7 10.0 0.0 (14.3, 3.4)wt.% Co-Ni 1.010 >673 > 673 >673 30.0 9.4 1.0 (6.2)wt.% Cr 1.010 >673 >673 >673 4.4 13.2 0.0 (8.6, 4.7) wt.% Co-Cr 1.010 >673 > 673 > 673 7.9 41.7 0.0 (7.2, 4.0) wt.% Ni-Cr 1.010 >673 > 673 >673 6.1 23.7 0.0 (7.0,6.8,3.8)wt.% Co-Ni-Cr 1.010 661 683 > 673 51.8 45.5 0.0 (8.9)wt% Mn 1.010 > 673 >673 > 673 39.8 39.3 2.0 (14.6)wt% Mn 1.010 >673 > 673 > 673 42.1 40.8 0.0 **- Light-off temperature, Tso%, defined as the temperature (before the catalyst) at which 50% conversion takes place. 32 The (7.0, 6.8, 3.8)wt.% Co-Ni-Cr catalyst also exhibited increases in both CO and HC conversions at 673 K, compared to the catalysts consisting of any of these metal oxides (with the same metal content) alone or combined with one of the other two. Mn0 2 catalysts exhibited a relatively good conversion behaviour. Conversions of 40% for HC and CO were achieved over the (14.6)wt.% Mn-catalyst (Table 3). Here, increase in the manganese content of the catalyst, from 8.9 to 14.6%, only slightly improved the final conversions at 673 K. The light-off temperature measurements of the Pt and Pt-metal oxide catalysts were carried out at X = 1.010 and are presented in Table 4. The combination of platinum and Co 304 increased the final conversions at 673 K, but did not alter the light-off temperatures with respect to the platinum catalyst. The final conversions of 98.2% for carbon monoxide, 99.8% for hydrocarbons and 41% for NO were achieved by using the (1.2, 13.6)wt.% Pt- Co catalyst (Table 4, Fig. 13). These resultant conversions are remarkably higher than those obtained with the platinum catalyst (75.2% for CO, 72.3% for HC and 36% for NO), indicating a strong synergistic effect between platinum and Co 304 in catalytic oxidation reactions. Here also an increase in the cobalt content of the catalyst resulted in increases in the final conversions at 673 K. The results from the light-off temperature testings of the Pt-Ni catalysts (at 1=1.010), showed that this combination (Pt, NiO) results in reduced light-off temperatures for both CO and HC, compared to those of the platinum and Pt-Co 304 catalysts. The light-off temperatures of the Pt-Ni catalysts were 493 K-503 K for carbon monoxide and 503 K-513 K for hydrocarbons. These values are approximately 40 K lower than the light-off temperatures of the platinum catalyst. Pt-Ni catalysts followed the same trends in catalytic activity — i.e increase in the conversions of CO, HC and NO by increasing the metal oxide content of the catalyst — as with the Co 304, Pt-Co 304 and Mn0 2 catalysts. A decreased catalytic performance of the Pt-Co 304 catalysts was observed in the fuel-rich regions. The decreased activity seemed to be a consequence of the catalyst reduction as was also evident from the partial change in the catalyst’s colour. The reduced catalyst could be regenerated by heating it in air at 773 K - 873 K. Reductions of this type were not observed for the Pt-Ni catalysts. This can possibly be explained by the oxygen storage capability of nickel, which minimises the effect of variation in the air/fuel ratios on the catalyst. The combination of platinum and nickel was also found to result in the partial formation of Ni203 (see below), which probably acts as an oxygen pool under oxygen-lean conditions. 33 Table 4 The light-off temperatures and the conversions at 673 K of the Pt-activated metal oxide catalysts, space velocity = 53000 h'1. Catalysts (wt.%) X T%% (K) for: Conversion (%) at 673 K for: CO HC NO CO HC NO (1.3)wt.% Pt 1.010 548 541 > 673 75.2 72.3 36.0 (1.3, 5.6)wt.% Pt-Co 1.010 528 538 > 673 74.6 70.5 29.7 (1.0, 8.8)wt.% Pt-Co 1.010 553 563 > 673 81.6 77.0 38.9 (1.2, 13.6)wt.% Pt-Co 1.010 542 550 583 98.2 99.8 41.1 (1.4, 4.0)wt.% Pt-Ni 1.010 494 513 >673 74.1 70.5 35.0 (1.5, 7.2)wt.% Pt-Ni 1.010 508 511 > 673 78.6 77.5 36.4 (1.3, 13.5)wt.% Pt-Ni 1.010 503 505 > 673 92.9 92.3 43.7 (l.l,6.9,6.7)wt.% Pt-Co-Ni 1.010 520 535 > 673 73.2 69.3 38.0 (1.2, 5.7)wt.% Pt-Cr 1.010 558 588 > 673 62.3 58.9 38.1 (1.0,7.1,6.6)wt.% Pt-Co-Ni-Cr 1.010 561 566 683 85.1 84.1 48.5 (1.2, 13.2)wt.% Pt-Mn 1.05 542 545 > 673 93.0 82.0 3.0 34 Temperature before the catalyst, K Figure 13: The light-off curves of the (1.2, 13.6)wL% Pt-Co catalyst (A^l.010, GHSV = 53000 h"1)- The deteriorating effect of the Co 304 and NiO combination in the final conversions was also obvious for the Pt-Co 304-NiO catalyst. The conversions at 673 K of the Pt-Co 304-NiO catalyst were lower than the final conversions at 673 of the catalysts containing the same amounts of any of these metal oxides and platinum. However, its light-off temperatures were between those obtained with the Pt-NiO and Pt-Co 304 catalysts. Combination of Cr203 and platinum also resulted in decreased conversions at 673 K as compared to the Pt-catalyst. Conversions of 62.3% for CO and 58.9% for HC were achieved with the (1.2, 5.7) wt.% Pt-Cr catalyst. These conversions are much lower than those obtained with the platinum catalyst. The light-off behaviour of the Pt-Cr203 catalyst was also greater than that of the Pt-catalyst. ThePt-activated Co-Ni-Cr oxide catalyst (Table 4) exhibited higher conversions at 673 K compared to the catalysts containing the same amounts of any of these metal oxides and platinum and its light-off behaviour was close to that of the (1.0, 8.8) wt.% Pt-Co catalyst. 35 4 As was the case with the Pt-Co 304 catalysts, a catalyst reduction was also observed with the Pt-Mn0 2 catalysts. The Pt-Mn0 2 catalysts were reduced even at X = 1.010 which strongly affected their light-off and conversion behaviour. At higher excess air level (5% 02 in the feed gas) this catalyst reduction did not take place and high conversions of CO and HC were achieved (93% for CO, 82% for HC). The NO conversion was very low due to the high excess air used. The light-off temperatures of the Pt-Mn0 2 catalyst were close to those of the platinum catalyst. In order to obtain the relationship between the degree of conversion and the oxygen content of the feed mixture, lambda window measurements were performed at 623 K for Pt-NiO catalysts, platinum and the (15.5)wt.% Co catalysts. Theresults of such measurements for the (1.3, 13.5)wt.% Pt-Ni catalyst are depicted in Figure 14. As can be seen, an increase in the oxygen content of the introduced mixture increases the conversions of both carbon monoxide and hydrocarbons, but decreases the NO conversion which might be due to the reduced competitiveness of NO for CO oxidation caused by inhibition of NO adsorption by oxygen. The triple point, where the conversions of all three reactants are the same (87%), was achieved by working with the fuel-rich mixture (0.59% 02, i.e. X = 0.983). Similar trends were obtained for the (15.5)wt.% Co-catalyst and the (1.3)wt.% Pt-catalyst, with the exception that the NO conversions over the Co-catalyst were low at all X-values. 0.97 0.98 0.99 1.00 1.01 1.02 Lambda Figure 14: The effect of the oxygen content of the feed on the conversion behaviourof the (1.3, 13.5)wt.% Pt-Ni catalyst (T = 623 K, GHSV = 53000 h"1). 36 From the obtained results the following activity patterns can be discerned: - The final conversions (at 673 K) of HC and CO for the Pt-metal oxide catalysts containing =13% transition metal (reduced basis) decreased in the order : Pt-Co 304 > Pt-NiO > Pt-MnOz (1=1.05) > Pt-Co 304-NiO-Cr203 > Pt > Pt-Co 304-Ni0 and the light-off temperatures in HC and CO combustion was found to increase as: Pt-NiO < Pt-Co 304-Ni0 < Pt-Co 304 = Pt-Mn0 2 < Pt < Pt-Co 304-Ni0-Cr203 - The catalytic activity of the metal oxide catalysts (with equal metal content) for the oxidation of CO decreased as: Co 304 > Co 304-NiO-Cr203 > Mn0 2 > Co 304-NiO > Co 304-Cr203 > Ni0-Cr203 - The catalytic activity of the metal oxide catalysts (with equal metal content) for HC combustion decreased as: Co 304 > Co 304-NiO-Cr203 > Co 304-Cr203 >Mn0 2 > NiO-Cr203 > Co 304-Ni0 b) Propane combustion. The results from the light-off testing in propane combustion are given in Tables 5 and 6. The light-off temperature is denoted by T50% and the conversions (X, %) are given at 523 K, 623 K, 723 K and 823 K (temperatures before the catalyst bed). The experimental setup and the procedures for the catalyst testing are given in detail in Paper HI. As shown in Table 5, an increase in the metal content of the catalysts was found to result in improved light-off and conversion behaviour in propane combustion (Table 5). An increase in the metal content, e.g. of the Co 304 catalyst from 4.5 to 15.2 wt.%, increased the final conversion by 13% and lowered the light-off temperature by 45 K. A decrease in the residence time from GHSV = 23900 h"1 to 47800 h"1, resulted in an increased light-off temperature and a decrease in the final propane conversion. Of the single metal oxide catalysts, Co 304/Si02 was found to exhibit the highest activity at lower temperatures followed by Mn0 2, Cr203 and the Ni0/Si02 was found to be the least active (Table 5) in propane combustion under our experimental conditions. 37 Table 5 Results from the catalyst testing in propane combustion. Propane conversion, % Catalyst GHSV = 23900 h"1, X = 1.05 GHSV = 47800 h"1, X = 1.05 T50% X523 K X623 K X823 K T50% X623 K X823 K X?23 K XS23 K X723 % (4.5)wt.% Co 591 3.5 64.8 80.0 82.2 620 0.4 52.0 76.1 78.9 (8.8).wt.% Co 565 12.7 72.8 86.4 90.9 597 2.8 65.7 83.9 87.5 (15.2).wt.% Co 547 24.1 83.9 92.5 95.1 575 4.5 77.0 89.1 92.2 (3.9)wt.% Mn 644 0.0 25.6 80.6 87.0 677 0.0 12.1 78.5 83.5 (7.6)wt.% Mn 621 0.9 50.8 87.2 90.3 649 0.0 28.1 83.7 87.2 (12.9)wL% Mn 612 1.6 57.8 88.1 92.4 641 0.0 32.0 83.9 89.1 (5.2)wL% Cr 711 0.0 6.1 53.9 76.5 735 0.0 4.7 44.6 71.0 (9.1)wt.% Cr 699 0.0 10.5 60.3 84.7 726 0.0 7.0 48.3 80.6 (15.1)wt.% Ni 723 0.0 4.9 50.0 86.5 748 0.0 2.9 27.2 82.8 (8.7, 8.8)wt.% Co-Ni 591 4.1 68.2 89.1 94.2 619 1.2 57.6 85.7 90.1 (7.9, 4.1)wt.% Co-Cr 625 1.2 48.9 88.7 93.7 654 0.0 20.8 84.4 89.6 (8.0, 4.2)wt.% Ni-Cr 634 0.0 36.1 84.8 88.9 661 0.0 13.2 81.7 85.6 (6.5,6.8,3.8)wt.% Co-Ni-Cr 609 0.8 64.2 92.9 96.3 636 0.0 34.7 89.8 93.1 38 Temperature before the catalyst, K Figure 15: The light-off curves of the metal oxides andPt-metal oxide catalyst, o- Co 304, □- NiO, A-Pt, v-R-Co, O-R-Ni (X=1.05, GHSV=23900 h"‘). Combinations of Co 304 and NiO (i.e. the Co-Ni catalyst), Co 304 and Cr203 (i.e. the Co-Cr catalyst) NiO and Cr203 (i.e. the Ni-Cr catalyst) resulted in improved final conversion with respect to the single component oxide catalysts (with equal metal content). However, the light- off temperatures of these combined catalysts were always higher than that of the Co 304 catalyst. Combinations of Co 304, NiO and Cr203 (i.e. the Co-Ni-Cr catalyst) also resulted in increased final conversion (at 823 K) as compared to any of these metal oxides alone or in combination with one another (with equal metal content). However, in this case the light-off temperature was again higher than that of the Co 304 catalyst (Table 5). Platinum-activated Co 304 and NiO catalysts (i.e. Pt-Co and Pt-Ni catalysts) again exhibited higher final conversion and reduced light-off temperature in propane combustion (Table 5, Figure 15) as compared to the Pt, NiO, and Co 304 catalysts having the same metal content The propane combustion efficiency over the Pt-Co catalysts was also found to improve by increasing the platinum content of the catalysts [I, Tables 2-4] 39 Table 6 Results from the catalyst testing in propane combustion. Propane conversion, % Catalyst GHSV = 23900 h'1, X= 1.05 GHSV = 47800 h"1, X = 1.05 T50% XS23 K X623 K X723 K X823 K T50% X523 K X623 K X723 K X823 K (0.3)wt.% Pt 548 3.3 86.2 92.7 92.9 565 1.4 84.0 90.8 90.8 (0.95)wt.% Pt 479 82.5 91.2 94.8 94.8 497 79.1 89.9 93.3 93.3 Pt/quartz* 514 63.9 93.5 94.1 94.1 (0.59, 9.1)wt.% Pt-Co 465 92.2 95.4 96.1 96.1 482 87.7 93.5 94.8 94.9 (0.71, 13.9)wt.% Pt-Co 463 93.5 98.1 99.1 99.1 481 90.4 96.5 97.7 98.1 (0.68, 8.9)wt.% Pt-Ni 474 91.4 96.7 97.8 97.8 490 85.5 94.9 96.5 96.7 (0.73, 14.3)wt.% Pt-Ni 468 93.7 97.1 97.7 98.0 482 90.6 95.7 96.6 96.9 (3.9)wt.% Pd 554 2.1 93.9 97.1 97.1 569 1.1 92.6 95.9 95.9 (3.9)wt.% Pd** 660 0.0 11.8 89.2 92.3 668 0.0 7.1 86.1 91.2 (3.3, 0.27) wt.% Pd-Pt 541 5.9 96.0 97.8 97.8 559 1.2 94.8 96.5 96.6 (3.3, 0.27)wt.%Pd-Pt** 667 0.40 12.5 88.1 94.9 674 0.00 10.2 86.0 93.5 *- Commercial Ftlquartz mat catalyst (Heraeus). Ft-content=7% **- Reduced catalysts 40 The activity of the knitted silica-fibre supported Pt and Pt-activated metal oxides were compared to that of a commercial (Heraeus) Pt/quartz mat catalyst (containing 7% Pt supported on a quartz mat). The knitted silica-fibre supported 0.95 wti% Pt-catalyst was found to exhibit much lower light-off temperature (Table 6) than the commercial catalyst (the mass of the catalyst mat was adjusted so the Pt-content was equal) which might be due to the better packing density of the knitted silica-fibre support. The activity of the metal oxides and Pt-metal oxide catalysts, containing =15 wt.% metal (it should be noted thatall the catalysts in this class have approximately the same amount of metal and the differences in the weight percentages are due to the differences in the oxygen/metal ratio of the component oxides as well as in the slight difference in the support mass) measured as the propane conversion at 823 K was found to decrease as: Pt-Co 304 > Pt-NiO > Co 304-NiO-Cr203 > Co 304 > Co 304-NiO > Oo 304-Cr203 > tvln0 2 > Ni0-Cr203 > NiO. The light-off temperatures increased as: Pt-Co 304 = Pt-NiO < Co 304 < Co 304-Ni0 < Co 304-NiO-Cr203 < Mn0 2 < Co 304-Cr203 < Ni0-Cr203 < NiO. The activity pattern for the metal oxide and Pt-metal oxide catalysts, containing =9 wt% metal, measured as theconversion at 823 K, was found to decrease as: Pt-NiO > Pt-Co 304 > Co 304 > Mn0 2 > NiO > Cr203 and the light-off temperatures for this class of catalysts were found to follow the same order with the exception that the Pt-Co 304 exhibited better light-off behaviour than the Pt-NiO catalyst Results of the catalyst testings for the Pd and Pd-Pt catalysts are given in Table 6. The Pd- containing catalysts were found to be less active in propane combustion as compared to the platinum-activated Co 304 and NiO catalysts. Combinations of Pt and Pd also resulted in improved light-off and conversion behaviour in propane combustion. Pd-containing catalysts were found to exhibit higher activities in the oxide form than in the reduced form. The light-off temperatures over Pd/Si0 2 and Pd-Pt/Si0 2 were increased by 99 and 115 K with this reduction of the catalysts (2h at 773 K in 100 ml/min hydrogen flow). Oxygen treatment of the reduced Pd-containing catalysts resulted in the restored catalytic activity. The response of the single component metal oxide catalysts to variations in the air/fuel 41 ratio was studied by varying the amount of the feed air (GHSV = 47800 h"1 while the molar flow of propane was kept constant) from X = 0.90 to X = 2.00 at 723 K. For all the catalysts this increase in the oxygen content of the feed mixture, up to 20% excess air, increased the final propane conversion at 723 K [HI, Figure 2]. Further increases in the oxygen concentration did not affect the propane conversion strongly. The reaction orders (based on the mole fractions) in propane combustion were found to be zero with respect to oxygen and one with respect to propane over the noble metal containing catalysts, while the orders were fractional over the base oxide catalysts [HI, Table 3]. Considering the activation energies and the pre-exponential factors, the Pt-Co 304 and C03O4 were found to be the most active among the noble metal containing and metal oxide catalysts. c) Natural gas combustion The light-off testing for the noble metals, metal oxides and Pt-metal oxide catalysts in natural gas combustion was performed (Table 7). The procedure for the catalyst testing was as with the propane combustion tests. As can be seen from Table 7, of the base metal oxide catalysts, NiO was found to be the most active in natural gas combustion. Here, as in the previous cases, a synergistic effect was observed with the combination of Pt and C03O4 resulting in improved catalyst light-off and conversion behaviour, as compared to the component catalysts (with the same metal content). However, this was not the case with the NiO and platinum combination, where the resultant catalyst exhibited a lower activity than the NiO/Si02 catalyst. Combination of PdO and Pt also resulted in improved light-off behaviour with respect to the PdO/Si0 2 catalyst. The activity of the prepared catalysts, in terms of T50%, was found to decrease as: Pt-PdO > PdO > NiO > Pt-Co 304 > Pt-NiO > Co 304 > Pt, and the activity, in terms of the final natural gas conversion at 823 K, decreased in the order of: Pt-PdO > PdO = NiO > Pt- C03O4 > Pt-NiO > C03O4 > Mn0 2 > Pt > Cr203 > CuO. The effect of increase in the 02-content of the feed mixture on the conversion behaviour of the catalysts was studied by increasing the stoichiometric ratio (X) from 0.90 to 2.00 at constant GHSV of 23900 h"1 (T = 823 K). With the exception of the Pt-Co 304 catalyst, the combustion efficiencies increased with increasing 02-content of the feed [HI, Figure 6 ]. 42 Table 7 Results from the light-off testings in natural gas combustion (GHSV = 23900 h"1, X = 1.05) Natural Gas Conversion, (X, %) Catalyst Tso%> K %673 K X723 K Xg23 K (9.8)wt.% Cu >823 1.6 1.4 4.0 (9.1)wt.% Cr >823 4.0 5.0 10.3 (12.9)wt.% Mn >823 9.7 13.2 34.7 (15.2)wt.% Co 799 12.8 20.0 60.3 (15.1)wt.% Ni 756 8.0 19.0 88.5 (0.95)wt.% Pt 830 5.1 8.0 34.3 (3.9)wt.% Pd 717 21.0 53.1 88.6 (0.73, 14.3)wt.% Pt-Ni 796 7.8 11.0 74.0 (0.71, 13.9)wt.% Pt-Co 779 11.0 19.4 84.7 (3.3 , 0.27)wt.% Pd-Pt 688 37.0 71.1 89.9 4.1.4 Discussion The high surface area and the flexibility of the prepared knitted silica-fibre make it very suitable for use as a catalyst carrier for systems where the pressure is not high. The support also exhibited very good temperature response as well as resistance under steam. By choosing the burning temperature of the hybrid textile, the specific surface area of the obtained knitted silica-fibre can be adjusted in an appropriate manner. The SEM-EDXA studies of the cross-section of a single fibre of the catalyst [U Figure 6] revealed that the metal is distributed everywhere inside the fibre or concentrated in the strip inside the fibre but not much on the edge. The metal was found to be located =500 nm from the edges [II Figure 6b]. This means that the support has a complex pore structure which is the consequence of the hybrid fibre’s composition. The burning of the cellulosic component of the hybrid fibre results in the formation of gaps between the silicic acid particles and the condensation of the two neighbouring silicic acid particles results in a 43 formation of pores. Measurements of the BET specific surface areas of the catalysts [II Table 1] indicate that high metal loadings have not resulted in a strong pore blockage of the catalysts with the exception of Mn, Pt-Mn and Pt-Co catalysts where the BET specific surface areas were decreased by as much as 50 m2/g. This was also followed by a decreased specific pore volume. The CgHg and CO adsorption as well as the oxygen-TPD measurements were used to explain the catalytic activities. The results of the chemisorption measurements of propane and carbon monoxide on Co, Ni, Pt-Co and Pt-Ni catalysts [HI Table 4] are in agreement with the catalyst activity, the uptake being higher on the more active catalysts. The combinations of Co and Ni with platinum were found to result in increased uptake of propane and carbon monoxide, as compared to the Co 304 and NiO catalysts. From the chemisorption measurements of C^Hg and CO on Pd and PdO as well as Pd-Pt and PdO- Pt02 catalysts, it was not possible to explain the higher activities of the oxidised palladium containing catalyst. As expected the uptakes were much higher on Pd/Si0 2 than on PdO/Si0 2 catalysts. The amount of oxygen evolved at temperatures below 873 K obtained from the 02-TPD measurements are given in paper III (for the Mn and Pt/Si02 catalyst, the high evolution of the oxygen is the result of oxide decomposition, i.e. Mn0 2 to Mn 203 at 773-873 K and PtOx to Pt at T > 823 K). The amount of desorbed oxygen from the Pt- Co 304 was higher than that of the Co 304/Si02 catalyst. However, the Pt-Ni catalyst desorbed much less oxygen than the Ni-catalyst. The 02-TPD profile of the Pt-Ni catalyst exhibited a major oxygen evolution at 903 - 1073 K, [HI Figure 5] corresponding to the decomposition of Ni203 to NiO. This 02-desorption peak did not appear from the NiO/Si02 catalyst The interaction of platinum with nickel has evidently resulted in the partial formation of nickel (HI) oxide. Interaction of oxygen with metal oxides can be represented by activation of oxygen resulting in the formation of highly reactive surface states (incompletely reduced active states of oxygen) which have high oxidative activity with respect to electron donating compounds. This reactive oxygen is assumed to be responsible for complete oxidation of organic compounds. This can be followed by transition of these states into oxygen of the catalyst lattice, i.e. an additional transfer of electron to oxygen and their insertion into an - oxygen vacancy. This oxygen is believed mostly to participate in the formation of selective oxidation products. In oxidation over noble metals the active species is the dissociatively adsorbed oxygen, while in oxidation over e.g. Co 304 at low temperatures, oxygen is weakly adsorbed. At higher temperatures the lattice oxygen of Co 304 participates in the oxidation reaction. During the reaction, the consumed lattice oxygen is substituted by 44 oxygen from the gas phase. Therefore, on Pt-metal oxide (Pt-Co 304, Pt-Ni and Pt-MnO%) the combined effect of dissociatively adsorbed oxygen on Pt and the weakly adsorbed oxygen on metal oxide may result in lower light-off temperatures of these catalysts, while at higher temperatures the combined effect of adsorbed 02 on platinum and the lattice oxygen from the metal oxides might be responsible for theobserved higher conversions at 823 K as compared to the Pt-catalyst. As was mentioned earlier, the combination of Pt and Ni has resulted in the formation of nickel (HI) oxide which is believed to be less active in hydrocarbon oxidation. However, the Pt-Ni/Si02 catalyst appeared to be more active in propane than in natural gas combustion. The reason for this might be that the activation of the C-H bonds in methane (the major component of the natural gas) is more difficult than the C-H and C-C bond activation of propane by Ni203. It was observed that by re-testings of the initially reduced Pd-catalysts, their light-off and conversion behaviour continuously improved. Therefore, the effect of catalyst aging on the reduced palladium containing catalysts was studied as a fresh (oxidised) Pd/Si0 2 catalyst was reduced (2h at 773 K in hydrogen flow) and further tested for its light-off behaviour (Figure 16a, curve a), after which the catalyst was aged for six hours at 823 K under continuous flow of the reactants. The reactor was then cooled to 373 K and the aged catalyst was tested for its light-off behaviour (Figure 16a, curve b). As is depicted, the light-off temperature and conversion at 823 K were altered by 45 K and 4 % as a result of this catalyst aging. It was also observed that the catalyst colour was changed from black (in reduced form) to a partially amber colour, characteristic for palladium (II) oxide. This increase in the activity must be a result of partial oxidation of the pre-reduced catalyst during the light-off testings. The results indicate that the propane oxidation, under our experimental conditions (02-rich reactant mixture), over the silica-fibre supported palladium catalyst occurs on the palladium oxide phase, even if the catalyst is initially reduced. The re-oxidation of the reduced catalyst takes place during the reaction. The reason for the higher activity of the oxidised PdO/Si0 2 catalyst might be in the nature of the surface rather than in the activity of the palladium. Palladium can form bulk oxide which is highly porous [80-82], Oxidation breaks apart the palladium crystallites and results in emergence of new active sites and therefore increased rate of hydrocarbon oxidation. The hydrocarbons oxidation over palladium is considered to proceed according to two steps. The first one involves strong adsorption of gaseous oxygen on the metallic palladium particle, resulting in surface PdO. This is followed by hydrocarbon adsorption on the oxidic surface where it becomes oxidised leading to the reduced (metallic) surface. On pre oxidised Pd/Si0 2, the oxidation of palladium to PdO is no longer a prerequisite, in other 45 473 523 573 623 673 723 773 823 Temperature before the catalyst, K 473 523 573 623 673 723 773 823 Temperature before the catalyst, K Figure 16: a) The light-off curves of the (3.2)wt.% Pd/Si0 2 catalyst: O- fresh catalyst (oxidised), #- aged catalyst b) The light-off curves of the (3.1)wt% Pd/Si0 2 catalyst: O- fresh catalyst (reduced), •-aged catalyst (total flow rate=5000 ml/min, GHSV=47800 h'1 , X = 1.05). 46 words, the PdO acts as an oxygen pool for the reaction and therefore the catalyst is temporarily more active. The low temperature activity of the aged-reduced catalysts (Figure 16a, curve b) was, however, still lower than the oxidised palladium catalysts (Figure 16b, curve a). It might be that only a fraction of the bulk palladium is oxidised during this aging and the palladium particles are probably covered with a layer of oxide while the direct oxygen treatment of the catalyst results in the full oxidation of the bulk palladium and therefore larger number of active centres are exposed to the reactants. Hicks et al. [17,18], in their study of methane oxidation on supported platinum and palladium catalysts, postulated the presence of two kinds of palladium oxide: dispersed palladium oxide on alumina and palladium oxide deposited on metallic palladium, with the latter being very active. The degree of Pd oxidation depends on the palladium particle size: small palladium particles are oxidised easily. In contrast to the propane combustion, no difference between the activity of the reduced and oxidised Pd-catalysts in natural gas combustion was observed here. The reason for this might be that the temperatures at which the catalyst exhibits considerable activity are high enough to re-oxidise the palladium particles and the reaction still occurs on the PdO phase. Another possible reason for the decreased activity of the reduced Pd-catalysts might be due to the formation of Pd-Si compound during the high-temperature reduction. Therefore, in order to study the possible formation of the Pd-Si compounds, X-ray powder diffraction measurements were carried out for the reduced Pd/Si0 2 catalyst. The XRD spectra of the catalyst indicated the presence of the palladium silicide (Pd 5Si). However, the peak intensity was very low and the majority of the palladium was as Pd(l 11) crystals. It is, therefore, possible to draw the conclusion that the formation of the palladium silicide species is not the main (if any) reason for the decreased catalytic activity of the reduced Pd/Si0 2 catalyst. As an activation of the Pd-catalysts over time was observed during the catalyst testings, a fresh Pd/Si0 2 (oxidised) catalyst was tested for the light-off temperature and was aged as in the previous case under the reactants mixture flow at 823 K for six hours. After cooling the catalyst temperature to 373 K, the catalyst was tested again (Figure 16b, curve b). It was observed that the light-off temperature decreased by 19 K, while the conversion levels (at 823 K) of the fresh and aged catalyst were very close. The hydrogen chemisorption measurement of the fresh Pd/Si0 2 indicated a palladium dispersion of 19.5 % and a mean metal particle diameter of 57.7 A (a spherical geometry was assumed). The aged catalyst had a dispersion of 15.7 % and the metal particle size increased to 71.8 A. This slight increase in the palladium particle size can not be easily correlated to the 47 improved catalytic activity of the catalyst. The increased catalyst activity can as well be a result of the slow release of chlorinated species from the catalyst (PdCl 2 was used as the metal precursor). Chlorine is known to inhibit the hydrocarbon oxidation on palladium [83]. However, when the exhaust gases were analysed by the quadrupole mass spectrometer (a fresh oxidised catalyst was employed), no chlorinated species were detected. Therefore, although in this case the change in the Pd particle size was small (and theaging time was also relatively short) one can conclude that the propane oxidation, under these experimental conditions, over silica-supported palladium is to be considered as a structure-sensitive reaction. C03O4 and Pt-Co 304 catalysts were found to be the most active of the silica-fibre supported single metal oxides and Pt-metal oxides in car exhaust gases and propane combustion, whereas in natural gas combustion, the NiO and Pt-PdO catalysts were most active. The catalysts exhibited a higher light-off behaviour in natural gas than in the propane combustion. This is due to the fact that methane (major component of natural gas) is the most difficult gas to combust. The conversion behaviour of the NiO catalyst in natural gas combustion was close to those of the PdO and Pt-PdO catalysts, however, the possible nickel emissions should be taken into account while working with Ni-catalysts. For the conversion of exhaust gases, the final conversions at 673 K of the Pt-Co 304 catalysts were higher than those obtained with the Pt-NiO and Pt-Mn0 2 catalysts (with equal amounts of metal oxide and platinum), but the lower light-off behaviour and resistance in reducing atmospheres of these supported Pt-NiO catalysts makes them more suitable for systems where the oxygen content of the feed gas varies. However, here also the possible nickel emissions should be considered. 4.2 Metal-Modified ZSM Zeolite Catalysts 4.2.1 Zeolite synthesis and catalyst preparation The parent Na-ZSM zeolites, i.e. the Na-ZSM-5, Na-ZSM-11 and Na-ZSM-48 zeolites used for ion-exchange, were synthesized in the laboratory as in References 84-87 with some modifications. The reagents used for the synthesis are given in Table 8. After completion of the zeolite synthesis, the synthesized zeolites were washed, filtered and dried at 383 K, followed by calcination at 813 K for 8 h. The Si/Al ratio determined by NMR spectroscopy for Na-ZSM-5, Na-ZSM-11 and Na-ZSM-48 was 39, 32 and 50, respectively. 48 Platinum, palladium and copper-modified ZSM-5 zeolite catalysts for propane and natural gas oxidation were prepared by the following methods: a) Ion-exchange of Na-ZSM with copper or palladium. b) Introduction of copper, platinum and palladium into the zeolite during the process of hydrothermal zeolite synthesis. Copper-modified ZSM-5, ZSM-11 and ZSM-48 catalysts were prepared by ion-exchange of the parent Na-ZSM zeolites with an aqueous solution of copper nitrate. Thesecatalysts will be referred to as Cu-ZSM-5-IE, Cu-ZSM-11-EE and Cu-ZSM-48-EE, respectively. The ion-exchange for Na-ZSM-5, Na-ZSM-11 and Na-ZSM-48 was also carried out at increased pH. For this purpose the pH of the ion-exchange mixture was kept at thedesired levels by adding 0.1 M NH4OH to the ion exchange mixture. These catalysts will be referred to as Cu-ZSM-5-BE, Cu-ZSM-11-BE and Cu-ZSM-48-BE, where BE stands for ion-exchange under basic conditions. The ion-exchanged Cu-ZSM catalysts were filtered, dried at 353 K for twelve hours and finally calcined in air at 773 K for three hours. The Pd-ZSM-5-IE catalysts were prepared by the ion-exchange of Na-ZSM-5 with aqueous solutions of PdCNH^Cl^ The prepared Pd-ZSM-5 catalysts were dried at 353 K for twelve hours and calcined at 773 K for three hours. Alternatively, copper, platinum and palladium were introduced to the Na-ZSM-5 zeolite by adding required amounts of Cu(N03)2.6H20, H2PtCl6.xH20 and PdCl 2 directly during the process of zeolite synthesis. After the completion of the hydrothermal synthesis, the zeolites were washed with deionized water, dried at 383 K followed by calcination at 813K for 8 h. Thesecatalysts will be denoted Cu-ZSM-5-DS, Pt-ZSM-5-DS and Pd-ZSM-5-DS. Table 8 The reagents used for the zeolite synthesis. Zeolite Reagents: silica source alumina source organic template ZSM-5 sodium silicate* aluminum sulphate TPABr ZSM-11 sodium silicate* sodium aluminate TBPC1 ZSM-48 sodium silicate* aluminum sulphate HMDBr 28.5 wt.% Si02, 8.8 wt.% Na20 and 62.7 wt.% H20 49 \ * 3.80 g 2.SO y 2.so I I I Figure 17: XRD patterns of a) Na-ZSM-5, b) Na-ZSM-11, c) Na-ZSM-48, and d) Na-Cu-ZSM-5-DS. 50 The SEM and XRD analysis (Figure 5 and 17) of the prepared catalysts exhibited the typical shape and morphology of the corresponding zeolites given in the relevant literature. It was also demonstrated that the framework of the ZSM-5 was not affected by introduction of the Pt, Pd, and Cu to the zeolites during the process of zeolite synthesis. 4.2.2 Catalyst Testing Results The prepared metal-modified zeolites were tested for their light-off and conversion behaviour in propane and natural gas combustion [TV, V]. The composition and the flows of the feed gas mixtures were identical with the testing of the silica-fibre supported catalysts (see page 30). Before the light-off tests, the catalysts were pretreated in a flow of dry air at 623 K for 30 minutes and for one hour at 813 K. The catalyst was then slowly cooled down (in the air flow) to 423 K. The reaction mixture was introduced and the oven temperature increased from 423 to 873 K at a heating rate of 12 K/min, and the propane/natural gas conversion (to C02 and H20) as well as the temperatures before and after the catalyst were measured continuously. The experimental system for the catalyst testings is described in detail in Papers IV and V. a) Propane combustion The results of the zeolite catalysts testing in propane combustion are given in Tables 9 and 10. The conversions (X, %) are given at WHSV of 82.5 and 165 h"1 coresponding to GHSV = 69600 and 34800 h"1 (total flow rate = 2500 and 5000 ml/min) and temperatures of 523 K, 623 K, 723 K and 823 K. The light-off temperature is denoted T50%, while the values in the parentheses represent the metal content in wt.%. The results indicated that for the Cu-ZSM-5, Cu-ZSM-11 and Cu-ZSM-48 zeolite catalysts, increase in the pH of the ion-exchange mixture resulted in increased metal uptake [IV Table 1 and V Table 1]. This increase in the metal content of the catalysts resulted in lower light-off temperature and higher final conversions at 823 K (Table 9). Cu-ZSM-5 catalysts were found to be the most active copper-modified ZSM zeolites in propane combustion. The light-off temperatures of the Cu-ZSM-11 catalysts in propane combustion were much lower than those of the corresponding Cu-ZSM-48 catalysts (692 and 644 K v.s. 741 and 670 K, respectively) while the latter catalysts exhibited higher final conversions. The Cu-ZSM-5 catalyst prepared during the process of zeolite synthesis exhibited a lower catalytic activity as compared to the ion-exchanged ZSM-5 catalysts. Cu- 51 5 Table 9 Results from the ZSM catalyst testing in propane combustion. Propane conversion, % Catalyst WHSV = 82.5 h"1, X = 1.05 WHSV = 165 h-\ X = 1.05 T50% X523 K X623K X723 K X823K X523 X623K X723 K X823 K (1.3)Cu-ZSM-5-IE 615 1.4 59.5 96.7 97.6 639 0.6 33.2 90.1 95.4 (2.8)Cu-ZSM-5-BE 606 5.6 65.0 97.1 97.9 633 1.2 44.1 90.7 96.1 (3.6)Cu-ZSM-5-BE 553 11.2 87.3 97.7 98.3 583 8.2 80.2 94.2 97.0 (5.2)Cu-ZSM-5-BE 520 56.8 97.2 98.8 99.6 541 14.5 96.9 98.7 98.8 (2.1)Cu-ZSM-5-DS 703 0.00 18.1 59.5 84.8 718 0.0 12.5 50.9 78.3 (2.8)Cu-ZSM-ll-IE 692 0.0 34.1 56.1 81.5 723 0.0 21.2 50.0 73.4 (7.4)Cu-ZSM-l 1-BE 644 0.3 39.4 78.8 92.7 659 0.0 24.0 68.5 86.4 (3.4)Cu-ZSM-48-IE 741 0.0 14.7 42.5 90.9 784 0.0 7.1 32.6 82.1 (8.5)Cu-ZSM-48-BE 670 0.2 26.1 75.2 95.9 715 0.0 9.1 54.0 89.7 (10.0)Cu /A1203 724 0.0 6.0 49.4 92.8 52 423 523 623 723 823 Temperature before the catalyst, K Figure 18: The light-off curves of the Cu-ZSM and Cu/AI203 catalysts (WHSV = 82.5 h"1, X^1.05), O- (5.2)Cu-ZSM-5-BE, •-(1.3)Cu-ZSM-5-IE, D- (7.4)Cu-ZSM-5-IE, ■- (8.5)Cu-ZSM-48-BE, a- (10)Cu /A12O 3 . ZSM catalysts also exhibited higher activities at lower temperatures as compared to a 10wt.% Cu /A1203 catalyst prepared for comparison purposes (Table 9, Figure 18). The (5.2)Cu-ZSM-5-BE catalyst exhibited a light-off temperature of 520 K and propane conversion at 823 K of 99.6%. These values are remarkably higher than those obtained over the (10.0)Cu/alumina catalyst. The parent Na-ZSM and H-ZSM zeolite tested in propane oxidation exhibited very low catalytic activity. A decrease in the residence time resulted in decreased catalytic activity (Table 9 and 10) of all the prepared catalysts. The effect of the residence time on the light-off and conversion behaviour of the catalysts was studied with the most active of the Cu-containing catalysts i.e. the (5.2)Cu-ZSM-5-BE catalyst. As presented in Table 2 and Figure 2 in Paper IV, the increase in the space velocity resulted in a considerable increase in the light- off temperatures and a decrease in final propane conversion. The light-off temperature of the catalyst for propane combustion increased from 548 K (WHSV = 165 IT1) to 674 K (WHSV=660 h'1). The final propane conversions over the catalyst tested at WHSV = 165 and 330 h"1 were quite close to each other. Further decrease in the residence time (WHSV = 660 h"1) resulted in considerable slippage of propane and the final combustion 53 efficiencies are therefore much lower. Increase in the 02-content of the feed mixture (from 1=1.00 to 1=1.05) was found to result in improved catalytic performance of the catalysts, i.e. decreased light-off temperatures and increased final conversions [TV Table 2], The effect of the oxygen content of the feed mixture on the conversion behaviour of the catalysts was studied at 773 K [V Figure 2], The propane conversion over the catalysts increased with the oxygen content of the feed. This improvement in the activity was more significant up to 25% increase in the oxygen content, with the exception that the conversion behaviour of the Cu-ZSM-5 in propane combustion was not affected by the oxygen content On the other hand, the propane conversion over the (3.4)Cu-ZSM-48-IE strongly increased with the increasing 02-content of the feed. The reaction orders, the activation energies and the pre-exponential factors in propane combustion is given in Table 11. The Cu-ZSM-5-IE and Cu-ZSM-11-BE exhibited similar kinetics under our experimental conditions whereas the Cu-ZSM-5-DS and Cu-ZSM-48-BE exhibited different kinetics. These catalysts exhibited a much lower activation energies (74.8 and 97.1 kJ/mol) which were strongly compensated by the pre-exponential factor. The Cu-ZSM-5-DS and Cu-ZSM-48-BE also exhibited a higher dependency on the propane partial pressure. The results from the testing of the noble metal modified ZSM-5 catalysts in propane combustion is presented in Table 10. Here as in the case of the copper containing catalysts, decrease in the residence time (at 1=1.05) resulted in increased light-off temperature and a decreased final propane conversion. The (0.2)Pt-ZSM-5-DS catalyst was found to exhibit the highest activity at lower temperatures. The catalyst exhibited a light-off temperature of =30 K lower than that obtained with the (5.2)Cu-ZSM-5-BE, while the latter catalyst exhibited a higher final conversion at 823 K. However, the Pt-ZSM-5-DS was found to be only slightly more active than the Pt/alumina catalyst (T50%= 497 vs. T50%= 509 K). Theion-exchanged Pd-ZSM-5-IE catalysts exhibited higher activities compared to the Pd- ZSM-5-DS catalyst where the Pd was introduced into the zeolite during the process of hydrothermal zeolite synthesis. The latter catalyst also exhibited a different kinetic behaviour than the ion-exchanged Pd-ZSM-5 catalyst, exhibiting a zero order dependency on the oxygen partial pressure (Table 11) which is often reported for oxidation over silica or alumina supported palladium catalysts. The former catalyst also exhibited much lower light-off temperatures than the (0.95)Pd/Al 2O3 catalysts while the final conversions (at 823 K) of both catalysts were close (Table 10). 54 Table 10 Results from the ZSM catalyst testing in propane combustion. Propane conversion, % Catalyst WHSV = 82.5 h"1, X= 1.05 WHSV = 165 h'1, X = 1.05 Tso% X523 K X623K X723 K X823 K Tso% X723 K X623 K X723K X823K (0.2)Pt-ZSM-5-DS 497 92.1 96.9 97.9 98.2 503 87.1 96.4 97.6 97.6 (0.2)Pd-ZSM-5-DS 649 0.2 27.1 77.5 77.5 674 0.00 23.6 53.0 55.9 (0.2)Pd-ZSM-5-IE 601 0.5 92.0 94.4 95.3 615 0.00 89.6 90.1 90.2 (0.9)Pd-ZSM-5-IE 573 3.7 99.8 100 100 590 1.2 96.8 96.9 96.9 (0.95) Pd-AI 203 645 0.0 26.2 91.1 98.6 (0.2)Pt-Al2O3 509 70.9 96.0 98.1 98.2 55 From the results in CgHg combustion the following activity patterns can be discerned: The activity of the Cu- ZSM catalysts in terms of T50% was found to decrease as: (1.3/5.2)Cu-ZSM-5-IE/BE > (2.8/7.4)Cu-ZSM-l 1-IE/BE > (3.4/8.5)Cu-ZSM-48 -IE /BE > Cu-ZSM-5-DS Theactivity of the Cu-ZSM catalysts in terms of the final conversions at 823 K decreased as: (1.3/5.2)Cu-ZSM-5-IE/BE > (3.4/8.5)Cu-ZSM-48-IE/BE > (2.8/7.4)Cu-ZSM-l 1-IE/BE > Cu-ZSM-5-DS The general activity pattern of the prepared catalysts in terms of the light-off temperature can be given as: (0.2)Pt-ZSM-5-DS > (0.2)Pt/Al2O3 > (5.2)Cu-ZSM-5-BE > (0.9)Pd-ZSM-5-IE > (0.2)Pd-ZSM-5-IE > (7.4)Cu-ZSM-l 1-BE > (0.2)Pd-ZSM-5-DS > (0,9)Pd/Al 2O3 > (8.5)Cu-ZSM-48-BE > (10.0)Cu /A1203 The general activity pattern of the prepared ZSM catalysts in terms of the final propane conversions at 823 K can be given as: (0.9)Pd-ZSM-5-IE > (5.2)Cu-ZSM-5-BE > (0.2)Pt-ZSM-5-DS > (0.9)Pd/Al 2O3 = (0.2)Pt/Al2O3 > (8.5)Cu-ZSM-48-BE > (0.2)Pd-ZSM-5-IE > (10.0)Cu/Al203 > (7.4)Cu-ZSM-l 1-BE > (0.2)Pd-ZSM-5-DS Table 3 in [TV] and Table 5 in [V] present the results from the light-off testings of the hydrogen treated catalysts. For the Cu-ZSM-5 catalysts the light-off temperatures strongly deteriorated with hydrogen treatment of the catalysts. However, the final conversions of propane were close to those obtained with the air treated catalysts. It was found that re oxidation of the catalysts by reactants takes place during the light-off testings. The reduced catalysts after being tested for their light-off behaviour were cooled down in a 99.999% N2 flow and tested again. The resultant light-off temperatures and conversions were quite close to those obtained with air treated catalysts, indicating the re-oxidation of the catalysts during the light-off measurements. On the other hand, the hydrogen treatment of the Cu- ZSM-11 and Cu-ZSM-48 zeolite catalysts did not affect their light-off and conversion behaviour strongly [V Table 5], The decrease in the light-off behaviour of the catalysts by hydrogen treatment was also obvious in the case of the Pt- and Pd-ZSM-5 catalysts. Here also a re-oxidation of the catalysts during the light-off testings was observed. 56 Table 11 Kinetic parameters for propane combustion. Conditions: C3H8, % 02, % o2ic3h8 a) variable 1.496 5.25 -14.96 b) 0.285 variable 5.25 - 8.75 total flow rate = 2500 mllmin, WHSV = 82.5 li1 Ea, Temperature Range, Temperature*, In A, Catalyst Reaction Orders kJ mol" 1 K K kmol kg-cat" 1 s"1 C3H8 (1.3)Cu-ZSM-5-IE 0.66 0.22 115.5 558-583 573 13.3 (7.4)Cu-ZSM- 11 -BE 0.81 0.21 119.1 563-593 573 13.1 (8.5)Cu-ZSM-48-BE 0.58 0.78 74.8 563-593 573 6.5 (2. l)Cu-ZSM-5-DS 0.42 0.67 97.1 563-603 573 9.9 (0.2)Pd-ZSM-DS 0.02 0.55 104.6 563-593 573 9.3 (0.2)Pd-ZSM-5-IE 0.18 0.69 110.9 538-568 548 13.4 (0.9)Pd-ZSM-5-IE 0.22 0.65 110.0 543-553 548 13.6 (0.2)Pt-ZSM-5-DS -0.05 1.01 194.5 463-483 468 39.7 *- Temperature at which the reaction orders were measured. 57 b) Natural gas combustion The results from the testing of the metal-modified zeolite catalysts in natural gas combustion are given in Table 12. The final conversions for natural gas combustion over the prepared catalysts were relatively lower than those obtained for the propane combustion. The catalysts also exhibited higher light-off temperatures in natural gas than in propane combustion which is due to the fact that methane is the most difficult gas to combust catalytically. Here also the same activity pattern was observed as in the case of propane combustion: the (5.2)Cu-ZSM-5-BE catalyst was the most active of the Cu- containing catalysts, exhibiting a combustion efficiency of 96.7% and a light-off temperature of 649 K. The Cu-ZSM-5-DS exhibited a lower catalytic activity as compared to the Cu-exchanged ZSM-5 catalysts (Table 12). The light-off temperatures of the Cu- ZSM-11 catalysts in natural gas combustion were again lower than those of the corresponding Cu-ZSM-48 catalysts while the latter catalysts exhibited higher final conversions. As compared to the Cu-ZSM-5 catalysts, the Pd-exchanged ZSM-5 zeolites were found to exhibit a higher activity in natural gas combustion than in the propane combustion. The Pd- exchanged ZSM-5 catalysts were again found to be superior to the Pd-ZSM-5-DS. The performance of the Pt-ZSM-5-DS catalyst was found to be most interesting. The catalyst, in contrast to the propane combustion, exhibited a very low activity in natural gas combustion (T50%= >823, X823 K = 13.1%). The effect of the oxygen content of the feed mixture on the conversion behaviour of the catalysts was studied [V Figure 2]. The conversions of natural gas over the catalysts increased with the oxygen content of the feed mixture (T = 773 K). This improvement in the activity was more significant up to 25% increase in the oxygen content; further increases in the oxygen content of the feed did not affect the natural gas conversion strongly. The effect of the oxygen content of the feed on the conversion behaviour of the Cu-catalysts was more significant with the (1.3)Cu-ZSM-5-IE catalyst [V Figure 2b]. The activity of the Cu-containing ZSM catalysts in terms of the light-off temperature in natural gas combustion was found to follow the same pattern as in propane combustion. The general activity pattern of the prepared catalysts in natural gas combustion in terms of the light-off temperature can be given as: 58 Table 12 Results from the catalyst testing in natural gas combustion (WHSV=82.5 h'1, X = 1.05). Natural gas conversion, % Catalysts T50% X623 K X723 K X823 K (1.3)Cu-ZSM-5-IE 756 12.4 27.1 94.8 (2.8)Cu-ZSM-l 1-IE >823 2.7 4.8 19.8 (3.4)Cu-ZSM-48-IE >823 2.4 11.9 27.5 (5.2)Cu-ZSM-5-BE 649 21.7 95.9 96.7 (7.4)Cu-ZSM-l 1-BE 808 3.4 13.0 51.8 (8.5)Cu-ZSM-48-BE 812 3.3 12.9 68.5 (2.1 )Cu-ZSM-5-DS >823 1.6 3.9 16.5 (0.2)Pt-ZSM-5-DS >823 5.6 7.9 13.1 (0.2)Pd-ZSM-5-DS 676 16.4 73.5 85.8 (0.2)Pd-ZSM-5-IE 629 44.9 93.7 94.1 (0.9)Pd-ZSM-5-IE 608 82.0 98.1 98.1 (0.9)Pd-ZSM-5-IE > (0.2)Pd-ZSM-5-IE > (5.2)Cu-ZSM-5-BE > (0.2)Pd-ZSM-5-DS >(1.3)Cu-ZSM-5-IE > (7.4)Cu-ZSM-l 1-BE > (8.5)Cu-ZSM-48-BE > (2.1)Cu-ZSM-5-DS > (0.2)Pt-ZSM-5-DS The general activity pattern of the prepared ZSM catalysts in terms of the final natural gas conversions at 823 K can be given as: (0.9)Pd-ZSM-5-IE > (5.2)Cu-ZSM-5-BE > (1.3)Cu-ZSM-5-IE > (0.2)Pd-ZSM-5-IE > (0.2)Pd-ZSM-5-DS > (8.5)Cu-ZSM-48-BE > (7.4)Cu-ZSM-ll-BE > (2.1)'Cu-ZSM-5-DS > (0.2)Pt-ZSM-5-DS 59 4.2.3 Discussion The catalyst testing results indicated that increase in the metal content of the zeolites results in increased catalytic activity. The metal content is certainly an important factor affecting the catalytic activity. However, not just the metal uptake in the ZSM zeolite is responsible for catalytic activity but also the position of metal in the zeolite matrix and its coordination geometry within the zeolite structure are of great significance. The results from the 02 and CO-TPD and microbalance studies were used to acquire a better understanding of the nature of the active sites in Cu-ZSM catalysts. The results of the 02 and CO-TPD for some of the Cu-ZSM catalysts are given in Table 13. The procedures for the TPD measurement are given in Paper V. The O/Cu ratios were calculated from the oxygen desorbed from the catalyst during the 02-TPD as well as the amount of oxygen required to produce the amount of C02 formed during the CO-TPD. The highest 02- desorption (< 823 K) was from the (1.3)Cu-ZSM-5-IE and (5.2)Cu-ZSM-5-BE. These catalysts also exhibited a much higher O/Cu ratio (both from the 02 and CO-TPD). The (2.1 )Cu-ZSM-5-DS was found to desorbed much less oxygen as compared to the ZSM-5 catalysts where the copper was introduced into the ZSM-5 by ion-exchange. The 02-TPD pattern of the (1.3)Cu-ZSM-5-IE [V Figure 3] exhibited three different oxygen desorption/evolution peaks: one at lower temperatures (= 343 K) which is assumed to be the remainder of the physically adsorbed oxygen, the second peak (583-773 K) is the extra lattice oxygen (ELO), while the last peak (>773 K) has been attributed to the oxygen from the lattice. The0 2-desorption peak at = 623 K did not appear in the 02-TPD pattern of the Cu-ZSM-5-DS catalyst [IV Figure 3], which is an indication for the absence of ELO on this particular catalyst The catalytic activity can be related to the oxygen desorption (fraction of ELO released) and the reducibility of the catalyst as well as to the nature and the structure of the zeolite itself. The higher the amount of oxygen desorbed below 823 K per gram copper, the higher the catalytic activity, i.e. the (1.3)Cu-ZSM-5 and (5.2)Cu-ZSM-5-BE catalysts are the most active. It appears thatthe extraction of the ELO by carbon monoxide is also much easier on these catalysts. The 02-desoiption from the Cu-ZSM-11 was the lowest and therefore a lower catalytic activity is to be expected. The (10.0)Cu /A1203 catalyst exhibited a different 02-desoiption pattern as compared to the Cu-ZSM-5-IE catalyst [V Figure 3]. The catalyst desorbs much less oxygen at 573-673 K and therefore exhibits lower activities at lower temperatures. The amount of the oxygen released per gram copper of the Cu /A1203 catalyst was close to that of the Cu-ZSM-11 catalyst (0.283 mmol 02/g-Cu). 60 Table 13 Results from the 02, CO-TPD and microbalance studies. o 2--TPD CO-TPD Microbalance Catalysts O/Cu a mmol 02/g-Cu . O/Cu b mmol CO oxidized/g-Cu O/Cu (1.3)Cu-ZSM-5-IE 0.128 1.006 0.241 3.789 0.26 (5.2)Cu-ZSM-5-BE 0.153 1.207 0.269 4.237 0.61 (2. l)Cu-ZSM-5-DS 3.882 xlO"2 0.306 4.48 IxlO 2 0.705 0.93 ' (3.4)Cu-ZSM-48-IE 5.260xl0" 2 0.428 7.415xl0' 2 1.167 0.92 (8.5)Cu-ZSM-48-BE 5.454xl0"2 0.429 7.484xl0" 2 1.177 0.96 (2.8)Cu-ZSM-l 1-IE 3.106xl0"2 0.244 3.736xl0" 2 0.588 0.86 (7.4)Cu-ZSM-l 1-BE 3.303xl0"2 0.260 3.827xl0 2 0.602 0.88 a - Oxygen atoms desorbed per number of catalyst’s copper atoms b - Oxygen atoms required for CO oxidation equivalent to the CO2 formed during the CO-TPD per number of catalyst's copper atoms 61 The results of the catalyst testing (Table 9) indicated that the Cu-ZSM-48 catalysts exhibited higher light-off temperatures than Cu-ZSM-11 while the final conversions over the former catalysts were higher. The Cu-ZSM-48 catalysts desorbed more oxygen than Cu- ZSM-11 (0.42 vs. 0.26 mmol 02/g-Cu) and therefore exhibits higher final conversions. The reason for the lower light-off temperatures of the Cu-ZSM-5 and Cu-ZSM-11 catalysts, besides theiroxygen desorption and their reducibility, might also be in the nature and the geometry of the zeolites themselves. The geometry of the zeolitic channels seems to be an important factor in determining the catalytic activities of these zeolites in the complete oxidation of hydrocarbons. The ZSM-5 and ZSM-11 catalysts due to their bi-directional channels provide a better coordinative environment for copper, thereby making it catalydcally more active and thus exhibiting higher activities at lower temperatures. In ZSM-48 with a one-dimensional micropore system, the accessibility of the catalytic active sites is a potential hindrance. Thepolymeric chains or other stable forms of the Cu-oxide may form in the zeolite pores [88, 89]. For the excessively ion-exchanged Cu-ZSM-5 zeolite it has been proposed that the excess copper is present mostly as Cu-O-Cu bridges which participate in the redox cycle (see Chapter 2.3). Therefor the redox capacity of the prepared Cu-ZSM catalysts was studied by subjecting the catalysts to redox cycles at 773 K in a microbalance. The calculated O/Cu ratios are given in Table 13. The O/Cu ratio of the (1.3)Cu-ZSM-5-IE calculated from the redox cycle was close to that obtained from the CO-TPD measurements (O/Cu = 0.25). The (5.2)Cu-ZSM-5-BE catalyst, on the other hand, exhibited a O/Cu ratio of 0.61. TheO/Cu ratio of the other catalysts, i.e. the Cu-ZSM-5-DS, Cu-ZSM-11 and Cu- ZSM-48 zeolite catalysts, was close to one. This indicates that the majority of the copper on Cu-ZSM-5-DS, Cu-ZSM-11 and Cu-ZSM-48 is present in the form of CuO whereas on the(1.3)Cu-ZSM-5-IE and (5.2)Cu-ZSM-5-BE the copper most probably exists in the form of Cu2+ and [Cu-0-Cu] 2+ species, with the latter being more active and easily reduced by carbon monoxide. The state of copper in a calcined ZSM zeolite is dependent on the zeolitic structure. On calcined ion-exchanged ZSM-11 and ZSM-48 the copper is present as CuO while the ZSM-5 tends to stabilise the copper as Cu2+ or Cu-O-Cu species. The hydrogen treatment of the Cu-ZSM-11 and -48 zeolite catalysts did not affect their light-off and conversion behaviour strongly [V Table 5]. However, this was not the case with the Cu-ZSM-5-IE/BE [TV Table 3 and V Table 5] catalysts where the catalysts light- off temperatures were decreased by 196 K and 84 K, while the final conversions of the oxidised and pre-reduced catalysts were close. The re-testing of the catalysts resulted in a light-off and conversion behaviour similar to those obtained over the initially oxidised 62 catalysts. The reason for these observations might be that with the pre-reduced catalysts a re-oxidation of the catalyst takes place during the light-off testings and therefore the final conversions are close. Here it is assumed that under the performed hydrogen treatment condition (2h at 803 Kin a 150 ml/min hydrogen flow), all the copper is converted to Cu°; therefore the re-oxidation of Cu° should take place before the redox couple, Cu2+/Cu+, can be established. In the case of the Cu-ZSM-5-IE/BE catalysts (which we believe to contain most ELO), at lower temperatures, the catalyst is not fully oxidised (i.e. the Cu-O-Cu species are absent) and therefore lower activities are found at lower temperatures. In the case of the Cu-ZSM-11 and Cu-ZSM-48 zeolite catalysts, the temperatures at which the catalysts exhibit activities are sufficiently high to fully oxidise the catalyst, and therefore the light-off and the conversion behaviour of the catalysts were not affected by hydrogen treatment. The decrease in the oxidative activity of the Cu-H-ZSM-5 catalyst by the hydrogen treatment has also been explained as the changes in the oxidation state of copper (from Cu2+ to Cu4) are accompanied by the migration of the Cu(II) isolated ions from zeolite channels to the outer surface of the zeolite. In the re-oxidation of the reduced catalyst, the Cu(II) ions from the outer surface gradually migrate back into the zeolite channels and stabilize in the cationic positions, resulting in restored catalytic activity [90]. Zeolites in the acid form can activate molecular oxygen, forming peroxide species [91-93]. Molecular oxygen has been proposed to adsorb on weak Lewis sites of H-Y [94, 95] increasing the electron deficiency of the site, giving rise to species able to withdraw electrons from the hydrocarbon and oxidise them. In zeolites, the localised surface defects formed in the oxygen interaction with Brpnsted acid sites can act as active centres for the hydrocarbon activation. In an ESR spectroscopy of ZSM-5, Shih [91] observed the presence of localised electrons in ZSM-5. These solid state paramagnetic defects act as very energetic free radicals able to activate oxygen, forming oxygen free radical species. He suggested an oxygen role in creation of these localised free electrons through a mechanism in which two Brpnsted acid sites interact with oxygen forming solid state defects. The localised nature of the electron on the solid state defects will interact with the ionisable organic molecule adsorbed on the zeolite surface. The ion-exchange of metals can result in incorporation of the protonic sites to the zeolites. In our case, when the ion-exchange was carried out at increased pH, the acidity can be introduced to the zeolite by some NH4+ exchange (and subsequent heattreatments). The exchange of copper into Na-ZSM-5 has also been reported to result in Brpnsted acid sites [96]. Therefore, in order to study the possible effect of the acidity on the catalytic activity of the prepared Cu-ZSM catalysts, NH3-TPD experiments were carried out [V Table 7], 63 However, from the NH3-TPD of the prepared catalysts it was not possible to give an accurate estimate of the acidity of the catalysts since a significant oxidation of ammonia was observed to take place over all of the prepared catalysts. The ammonia oxidation was more significant over the catalysts with higher copper loadings, however, the TPD pattern of all the catalysts still indicated the presence of the acidic sites. Therefore, in order to give a comparative degree of acidity, the ammonia-TPD was carried out on the protonated zeolite catalyst, prepared by theion-exchange of the parent Na-ZSM catalysts with 0.5 M NH4C1 solution, followed by washing, drying and calcination at 813 K. The acidity pattern was found to be: H-ZSM-5 > H-ZSM-1 l>H-ZSM-48. The acidity pattern also follows the low-temperature activity pattern in propane and natural gas combustion. As mentioned earlier the Pd-catalysts prepared by the ion-exchange method exhibited higher activities than when the palladium was introduced into the zeolites during the process of hydrothermal zeolite synthesis, i.e. the (0.2)Pd-ZSM-5-DS catalyst. Here also reduction of the catalysts resulted in decreased low-temperature activities [V Table 5] while the re-oxidation of the Pd-catalyst during the light-off testing was observed. On supported Pd-catalysts it has been proposed that a re-oxidation of the palladium particle takes place during the methane oxidation [17,18] and the reaction is assumed to take place over the palladium oxide phase even if the catalyst is initially reduced. The PdO lattice obtained from the metallic palladium has a porous character [81,82], therefore during the oxidation process new surfaces of PdO develop, i.e. more active centres. The (0.9)Pd-ZSM-5-IE exhibited much higher activities at lower temperatures as compared to the Pd/AI 203 catalysts although the final propane conversions of the two catalysts were quite close (Table 7). In a recent study of the catalytic combustion of methane, Li et al. [97] reported a comparative study of Pd-ZSM-5 and Pd0/Al 203. ThePd-ZSM-5 catalyst exhibited a light-off temperature of =100 K lower than the corresponding Pd/Al 203 catalyst which is in good agreement with our results. The authors related the catalytic activity to the reducibility of the catalysts where the Pd-ZSM-5 catalyst was found to be reduced much more easily than the Pd/alumina catalyst. Another reason for the higher activities of the Pd-ZSM-5-IE catalysts as compared to the Pd/alumina catalyst might be their oxygen carrier capacities. During the 02 -TPD measurements, the (0.2)Pd-ZSM-5-DS and (0.9)Pd- ZSM-5-EE catalysts desorbed 3.78 and 3.88 mmol/g Pd, whereas the (0.95)Pd/alumina catalyst desorbed 1.87 mmol/g-Pd. As has been reviewed by Gallezot [98], the electronic and catalytic properties of metals introduced in zeolite can be modified. One of the reasons for higher catalytic activity of 64 the Pd-exchanged ZSM-5 catalysts, compared to the Pd/alumina catalyst, could be the electron-deficiency caused by .transfer of electrons from palladium to the Brpnsted and Lewis acid sites. It seems as if the zeolitic structure provides a better environment for electron donating properties of the exchanged palladium as compared to conventional alumina support. The results also indicated that the method of metal introduction to the zeolite is of importance as was indicated by the higher activities of the ion-exchanged Cu-ZSM-5-IE/BE and Pd-ZSM-5-IE catalysts, compared to the Cu-ZSM-5-DS and Pd-ZSM-5-DS catalyst. The state and accessibility of the copper on the Cu-ZSM-5-DS catalyst is probably the main reason for the lower activity of this catalyst compared to the ion-exchanged Cu-ZSM- 5-IE/BE catalysts. The microbalance studies indicated thaton the Cu-ZSM-5-DS catalyst, the copper is most probably present as CuO. The absence of the 02-desorption peak at =623 K as well as the low O/Cu ratios from the CO and 02-TPD measurements is another indication for the low concentration or absence of the [Cu-0-Cu] 2+ species which is believed to be the most active site in oxidation reactions. The higher activity of the Pd-ZSM-5-IE as compared to the Pd-ZSM-5-DS catalyst might be due to the better Pd distribution as well as the better coordinative environment of the palladium in the ion-exchange sites. The Pd-ZSM-5-DS catalyst also exhibited a different kinetic behaviour than the ion-exchanged Pd-ZSM-5 catalyst, exhibiting a zero order dependency on the oxygen partial pressure (Table 9) which is often reported for the oxidation over silica or alumina supported palladium catalysts. The behaviour of the Pt-ZSM-5-DS catalyst is quite interesting. While being the most active in propane combustion, the catalyst exhibited poor catalytic activity in natural gas combustion. The catalyst also exhibited higher activity in oxidised form than in reduced form [IV, Table 3]. This is not quite usual for the platinum catalysts, since they are known to be more active in reduced form than in oxidised (Pt02) form [99]. We assume that the PtOx particles are located in the ZSM-5 zeolite channels and intersections and act as the active centres for propane oxidation —it should be noted thatcalcination at 813 K can be sufficient to reduce some platinum to the metallic state —. Upon thereduction, platinum atoms gradually migrate to the external surface of the ZSM-5 zeolite where they form large platinum particles, resulting in decreased catalytic activity. Although the Pt-ZSM-5-DS exhibited the highest activities in propane combustion, its light-off temperature was close to that of the Pt/alumina catalyst (Table 10). 65 5 CONCLUDING REMARKS A method was developed for the preparation of a knitted silica-fibre, to be used as a catalyst carrier for combustion catalysts. The prepared knitted silica-fibre support was found to be an excellent support for combustion catalysts. Its good temperature response and high surface area make it a very suitable catalyst carrier in the low/medium throughput catalytic combustor/heater. Thesupport was found to have a complex pore structure which is formed during the preparation procedure, and as a consequence, the active metals are located in the pores rather than on the exterior surface of the silica-fibre, as one may anticipate. The support can also be prepared in different shapes and geometries to suit different applications. The specific surface area of the catalyst support can be easily manipulated simply by changing the burning temperatures of the hybrid textile. The silica-fibre supported catalysts were tested for their activity (in terms of light-off and conversion behaviour) in the conversion of car exhaust gases and combustion of propane and natural gas. The results indicated that high combustion efficiency can be achieved by using the transition metal oxides and their combinations. Of the single base metal oxides, supported Co 304 were found to be most active in the conversion of car exhaust gases and in propane combustion, while NiO exhibited higher activity in methane combustion. Combinations of the metal oxides improved the final conversions but did not improve the light-off temperatures with respect to the component base oxide catalysts. The metal oxides generally exhibit lower activities at lower temperatures as compared to noble metals. The low-temperature activities of the metal oxide can be strongly improved by using platinum as the activator, as was evident from the combination of platinum and metal oxides (Co 304, NiO and Mn0 2) which resulted in an improved light-off and conversion behaviour of the catalyst in hydrocarbon oxidation as compared to the component catalysts. Thus in this way one can lower the use of expensive noble metals while still achieving high activities at low temperatures. The Pt-Co 304 was found to be the most active catalyst in conversion of car exhaust gases as well as in propane oxidation in the fuel-lean region (up to 100% excess air). However, the catalyst was not able to operate effectively under oxygen-lean conditions as a catalyst reduction took place. The reduced catalytic activity can be regenerated by heating the catalyst to 773-873 K. Reductions of these types were not observed over the Pt-Ni catalysts, and e.g. a conversion of 87% for HC, CO and NO was achieved under the net oxidising-reducing character (X) of 0.983. The interaction of Pt and Ni was found to result in the partial formation of nickel (HI) oxide as was evidenced by the 02-TPD 66 measurements. Nickel (IQ) oxide probably acts as an oxygen pool under fuel-rich conditions. However the possible nickel emissions should be considered while working with Ni-containing catalysts.. Propane combustion over the supported Pd-catalysts was found to be structure sensitive under our experimental conditions as was evident from increased particle size determined from the hydrogen chemisorption measurements. The oxidation reaction over the Pd- catalysts takes place over the PdO phase. In general, the light-off temperatures in natural gas combustion were higher than the corresponding light-off temperatures in propane combustion. The Pd-catalysts were found to be the most active catalysts in natural gas combustion. The performance of the NiO/Si02 catalyst in natural gas combustion was very close to that of the PdO/Si0 2. However, as in the case of platinum-activated nickel catalysts, possible nickel emissions cannot be excluded. Different Cu-ZSM catalysts as well as Pt- and Pd-ZSM-5 catalysts were prepared and tested for their activities in propane and natural gas combustion. The modified zeolites exhibited very high activity in oxidation reactions. For the Cu-ZSM catalysts, an increase in the pH of the ion-exchange mixture results in increased copper uptake and consequently in increased catalytic activity. Of the Cu-containing ZSM catalysts, ion-exchanged Cu- ZSM-5 were most active in complete oxidation reactions. The temperature programmed desorption of oxygen and carbon monoxide as well as the microbalance measurements of the redox cycles indicated the important role of the extra lattice oxygen on these catalysts. Cu-ZSM-5 tend to stabilise the copper as Cu2+ and Cu-O-Cu species while on the ZSM-11 and ZSM-48, the copper is present as CuO. The Pd-ZSM-5-HE catalysts exhibited lower, low-temperature activities in propane combustion as compared to the Cu-ZSM-DE/BE catalysts, though the final conversions were close. The ion-exchanged Pd-ZSM-5, on the other hand, were found to be most active in natural gas combustion. The activity of these catalysts depends on their reducibility and on their oxygen carrier capacity. Here also as in the case of Pd/Si0 2, the reaction is believed to take place over the PdO phase, stabilised by the zeolite matrix. The method of metal introduction into the zeolite was found to be very important in determining the oxidation activities of the ZSM-5 catalysts. The introduction of Cu and Pd to the zeolite during the process of the hydrothermal zeolite synthesis, although very 67 6 convenient, results in a less active catalyst. 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Laboratory of Industrial Chemistry, Abo Akademi, SF-20500 Abo F inland INTRODUCTION Catalytic heaters are mainly based on a catalyst supported on fibrous pads to avoid pres sure drop limitations. The system is highly non-adiabatic and can thus be operated with near stoichiometric mixtures. The low thermal conductivity of the fibres prevents flash back from occuring. The use of such equipment in place of conventional systems in some stationary and mobile combustion systems has shown significant advantages in the control of emissions. High combustion efficiency results in both minimum fuel consumption and low pollutant emission. Major goals in the development of catalytic combustion are the ability to bum the fuel to completion at a low combustion temperature, the complete oxida tion of CO to CO2, thecombustion of fuel-lean mixtures, and the control of the formation of nitrogen oxides from atmospheric nitrogen (thermal N0r). Catalytic combustion has even been found to possess a unique ability to control N0r formation from the fuel-bound nitrogen (fuel NOr) under some conditions ( CO + NO —► 1/2 N2 + C02 ) (1). Metal or metal oxides can promote the combustion. Metals of interest as catalysts are mostly noble metals. Some of the simple oxides, such as Co oxides, have an oxidation activity comparable to that of the noble metals. Oxides typically have light-off tempera tures significantly higher than do noble metals (with the same fuel) (2). In this work we have tried to use metal oxides such as cobalt oxides with small amounts of a noble metal (platinum in this case) as a promotor and another subtarget was to use a new type of support in place of commercially available quartz supports. THE CATALYTIC REACTOR SYSTEM The experimental set-up is shown schematically in fig.l. The flow system is a series of regulators and ducts to provide the reactor with a homogeneous mixture of air and fuel. Air was supplied through an air filter/dryer in order to minimize the moisture and the dust particle level. It then passes through a pressure regulator where any residual pressure Proc. 7th Int. Symp. Heterogeneous Catalysis, Bourgas, 1991 674 fluctuations from the compressor were essentially damped down. A back-pressure regulator kept the pressure in the rotameters at 3 bars. The premixed gas/air entered the reactor (6 cm in diameter) in which the catalyst mat was superimposed over a fenestrated plate, and passed through the catalyst mat where it sustained combustion. An electrical element, inserted through the exhaust pipe which reached the mat surface, was used for initial heating of the catalyst and was removed after the combustion started. mb»u UDucti I Cp[ [P!)( Ft Fig. 1.The flow system of the catalytic reactor. Three thermocouples were inserted into the reactor, monitoring the instant temperature on a PC screen. The analysis of unbumt carbon in the flue gas was performed using a CO-CO2 infrared instrument. CATALYST PREPARATION A hybride textile (67% cellulose, 33% silicic acid) recieved from Ivemira Ltd. was used to provide a knitted silica support for catalyst for catalytic combustion. Since the finish ing agent (lubricant) used during the preparation of the hybride textile contained certain amounts of phosphorous compounds, the first task was to wash away the finishing agent. The hybride textiles (570 g/m 2) were washed with dioxan, and then were heated at differ ent temperatures in order to bum off cellulose and to obtain pure knitted silica support with optimal strength and surface area. The textile burnt at 950°C was found to have a good strength and surface area having a thickness of 1 mm and 190 g/m 2 weight which 675 was used as support (fig.2). The support obtained was cut in' pieces ( 6 cm diameter; weighing 0.6-0.8 g) which were impregnated with cobalt using alcoholic solutions of cobalt nitrate with different concen trations. The impregnated supports were dried at room temperature for 12 hours and then heated at 600°C in the presence of air. These supported cobalt oxide catalysts were then soaked in alcoholic solutions of HoPtCle with different concentrations, dried at room temperature followed by heatig at 500°C in the presense of air. •v Fig.2. The SEM image of the catalyst support. Three classes of catalysts were prepared which differed in cobalt content, class I, II, III containing ~ 20%, —10%, ~5%, respectively. The catalysts in each class differed in their Pt content as is shown in table 1: Catalysts Co, wt% Pt, wt% la 22.9 1.00 lb 21.5 1.25 Ic 20.7 1.40 Ha 10.5 1.20 lib 11.0 1.25 lie 11.1 1.30 ffla 5.2 0.70 Illb 5.8 0.90 IIIc 5.1 1.20 676 RESULTS AND DISCUSSION The calculation of combustion efficiency was based on the measurement of the amount of unburnt hydrocarbons in the exhaust gas. The efficiency of the combustion can be calculated as follows: 9 = (Xy-X./Xy) Xy = Fuel part of the mixture; ppm Xu = Unburnt hydrocarbons in the exhaust gas; ppm A calculation was made for combustion efficiency at different equivalence ratios. The results were then compared with the results obtained using a commercial catalyst mat received from Heraeus containing ~7 wt% Pt supported on a quartz fiber (SiO% purity ~99.9%) having 1 cm thickness and weight of 0.5 g (for the same reactor diameter). Propane and air flow rates were 0.11 and 2.64 1/min, respectively. The results are tabu lated in tables 2-4 and flg.3. Table 2: Combustion efficiency (%) of the class I catalysts and of the commercial catalyst: Equivalence Ratio Catalyst la Catalyst lb Catalyst Ic Com. Cat 1.000 98.60 98.70 98.90 98.60 0.914 98.50 98.80 99.20 0.845 98.50 98.56 99.30 99.60 0.783 97.90 98.00 99.00 99.50 0.689 97.10 97.30 98.60 99.40 0.634 96.80 96.40 98.20 99.10 Table 3: Combustion efficiency (%) of the class II catalysts: Equivalence Ratio Catalyst Ha Catalyst lib Catalyst lie 1.000 97.80 98.70 98.80 0.914 97.80 98.90 98.90 0.845 97.70 99.00 98.90 0.783 97.20 98.70 98.30 0.689 96.20 98.20 97.80 0.634 95.10 97.80 97.30 677 Table 4: Combustion efficiency (%) of the class III catalysts: Equivalence Ratio Catalyst Ilia Catalyst Mb Catalyst IIIc 1.000 93.20 96.80 97.10 0.914 92.20 96.60 96.80 0.845 88.50 96.10 96.30 0.783 85.90 95.00 85.00 0.689 93.30 93.10 0.634 81.70 92.10 91.90 Conversion, % Equivalence Ratio ----- Catalyst Ic ~Catalyst lib —Catalyst lllc Fig.3. Combustion efficiency as a function of equivalence ratio. The results showed an increase in combustion efficiency by increasing the cobalt content (fig.4) and also by increasing the platinum content. The best performance was that of catalyst Ic (99.3%) at equivalence ratio of 0.845 (containing 1/3 of the platinum in the commercial catalyst), which had an operation temperature of 450°C (fig.5). It was found that when working with fuel-rich mixtures some gray spots appeared on the mats and their combustion efficiencies dropped with 1-2%. SEM analysis equipped with EDXA showed that the gray spots contained the same amount of cobalt as existed in the other parts of the catalyst mat which did not turn to gray. The catalysts with gray spots were heated to 600°C in air and the gray spots disappeared and their catalytic activity restored. It seems that under reducing condition C03O4 is reduced to other forms of cobalt oxides with lower catalytic activity, however this conclusion is still under investigation. A long term experiment was performed using the catalyst Ic at 20% excess air, after 8 hours the combustion efficiency was still 99.2%. 678 Combustion efficiency,% Cobalt content, wt% —— 0% excess air —9.4% excess air 18.3% excess air Fig.4. Combustion efficiency as a function of cobalt content at different fuel/air ratios (Pt content is constant). 8 Channel lenperature Logging Sgsten CH VALUE 8 23.0469 >1 145.4761 >2 456.5874 ) 3 268.9356 4 ****** vvvvyv 5 XAAA aa 6 ****** WWW 7 AAAAAA Fig.5. The temperature profile inside the catalytic combustor using catalyst Ic. 1, 2, 3 are the temperatures of hue gas, catalyst surface and close to the reactor body, respectively. REFERENCES ^ 1. Trimm, D.L. Catalytic Combustion; Appl.Cat.7(1983)249-282. 2. Weinberg, F.J. Advanced Combustion Methods. Academic Press, London, pp 237-272. I / Combust. Scl and Tech., 1994, Vo!. 97, pp. 121-136 ©Gordon and Breach Science Publishers SA. Photocopying permitted by license only Printed in the United States of America Catalytic Combustion Over Transition Metal Oxides and Platinum-Transition Metal Oxides Supported on Knitted Silica Fibre A. K. NEYESTANAKI AND L.-E. LINDFORS, Laboratory of Industrial Chemistry, Abo Akademi University, Biskopsgatan 8, FIN-20500 Turku, Finland (Received December 15, 1992) Abstract—A textile made up of an organic-inorganic hybrid fibre containing 70% cellulose and 30% silicic acid was used as a raw material for the preparation of the thermostable support for combustion catalysts. The hybrid textile was burnt to obtain a pure knitted silica fibre. The surface area and crystallinity of this fibre was measured as a function of burning temperature. The knitted silica fibre obtained by burning the hybrid textile at 950° C was found to have sufficient strength and a high BET specific surface area (rs 140m2/g) to be used as a support for the catalysts for catalytic combustion. A series of Co 304, Pt-Co 304, Pt-NiO, Co 304-NiO, Pt-Co 304-NiO, Cr203, Cr203-Co 304. Cr203- NiO, Cr203-Co 304-NiO, Pt-Cr203-Co 304-NiO, Mn02, Pt-Mn0 2 and Pt02 catalysts were prepared by impregnation of the obtained support with solutions of metal salts. The catalytic activities of the catalysts were tested by burning a gas mixture consisting of HC, CO, C02, NO, water vapour and N2. In each series an increase in the metal oxide content of the catalyst resulted in an improved combustion efficiency. The combination of platinum and metal oxides (Co 304, NiO, Mn02) increased the conversions of hydrocarbons, carbon monoxide and nitric oxide. Conversions of 98.2% for CO, 99.8% for hydrocarbons and 41% for NO were achieved with a Pt-Co 304 catalyst at A=1.010. The combination of Pt and metal oxides, with the exception of NiO, did not alter the light-off temperatures with respect to the platinum catalyst. The light-off temperatures of the Pt-NiO catalysts were some 40°C lower than those of the platinum, Pt-C03O4 and Pt-Mn0 2 catalysts. The activity patterns for HC and CO oxidation were established. Keywords—Catalytic combustion, transition metal oxides, silica fibre support. INTRODUCTION Major advantages of catalytic combustion are the ability to bum the fuel to completion at a low combustion temperature, thecomplete oxidation of CO, thecombustion of fuel- lean "mixtures, the control of NO, formation from atmospheric nitrogen (thermal-NO,) and from fuel bounded nitrogen (fuel-NO,). Catalysts for catalytic combustion are mostly noble metals and transition metal oxides. Noble metals exhibit activity at lower temperature than transition metal oxides, but at temperatures above 400°C the activities are about the same (Blazowski and Walsh, 1975). The catalytic activity of the transition metal oxides is determined by their d-shell electron configuration (Dowden and Wells, 1961). The maxima in activity are usually observed for cations with 3, 6 and 8 d-electrons while minima occur for 0, 5 and 10 d-electrons. This relationship between d-shell electrons and the catalytic activity has been explained by applying the crystal field theory to the problem;:.Peaks in activity occur for C03O4 and Cr203 (Dixon et al., 1964). The potential metal oxide catalysts for hydrocarbon oxidation are C03O4, CraOs, CuO, NiO, Mn0 2 and V2Os (Blazowski and Walsh, 1975; Prasad et al, 1980; Prasad et al., 1982; Gabrovski et al., 1990). Catalytic heaters are used to generate heat and are based mainly on catalysts supported on fibrous pad. Pads made of alumina or silica fibres are usually used, although they sinter rapidly in the steam-rich atmosphere (Trimm, 1983). The low thermal conductivity of the fibre materials results in little heat conduction in the upstream direction of the fibre, hence the plenum side of the fibre remains below the ignition temperature of the premixed fuel/air mixture, this prevents flash back from occurring, allowing safe 121 7 122 A. K. NEYESTANAKI AND L.-E. LINDFORS operation (Weinberg, 1986). The advantage of the fibrous supports is that they can be constructed in different shapes and geometries suitable for any application. In this work we have tried to develop a new type of knitted silica fibre support and to study the catalytic activity of the supported transition metal oxides (C03O4, NiO, Mn0 2 and Q2O3) and Pt-transition metal oxides catalysts in combustion of a mixture simulating the automobile exhaust gases. It should be noted, however, that the ultimate goal is to use these catalysts for energy generation by catalytic combustion. EXPERIMENTAL Support Preparation and Characterization A commercial hybrid textile (Kemira Ltd.) was used as a raw material for the support preparation. This textile was made of a hybrid fibre (organic-inorganic hybrid fibre), which is a cellulosic fibre containing molecular chains of polysilicic acid (Heidari, 1991): The fibres were produced via a modified viscose process, in which the cellulosic component was regenerated simultaneously with the polymerization of the silicic acid. The fibre exhibits a kidney-bean shaped cross section and a smooth, semi-crenellated exterior surface. The chemical composition of the hybrid fibre was: Cellulose 67-70% Silica (Si02.xH20) 30-33% Impurities: Na, Zn, S <1% P <200 ppm By heating the hybrid fibre in the presence of oxygen at temperatures of 260 to 300° C, the cellulosic component carbonizes. The carbonization is accelerated at temperatures of 300-380° C. By 600° C the pyrolysis of the cellulosic component is completed. The carbonization and the burning of the carbon as well as continuous heat treatment up to the temperature range of 600-700° C, dissipate large amounts of heat, which initiate a sintering condition for polysilicic acid conversion into coherent filament of silica without an actual fusing of the particles. The hybrid fibre was burnt at different temperatures and the BET specific surface area, pore volume and the crystallinity of the obtained silica fibre were measured. The BET specific surface area and specific pore volume were measured by N2 adsorption using a Sorptomatic 1900 (Carlo Erba Instruments). By increasing the burning temperature from 500° C to 1000° C the BET specific surface area decreased from 220 CATALYTIC COMBUSTION 123 SPECIFIC SURFACE AREA SPECIFIC PORE VOLUME BET (m2/g) Vp (ml/g) 250 T T 0.6 -- 0.5 200 - - - - 0.4 150 -- -- 0.3 X— BET -- 0.2 50 -- -- 0.1 1000 BURNING TEMPERATURE,°C FIGURE 1 BET specific surface area and specific pore volume of the obtained silica fibre as a function of the burning temperature of the hybrid fibre. 30 -r 1000 C Stretched 25 -- 1000 C No stretching 960 C Stretched 960 C no stretching 20 -- 15 -- 10 -- FIGURE 2 XRD pattern of the silica fibre. 124 A. K. NEYESTANAKI AND L.-E. LINDFORS m2/g to 10 m2/g. The specific pore volume was also decreased (in the temperature range of 650-1000°C) by increasing the burning temperature (Fig. 1). The loss on ignition was 69% and the shrinkage of the fibre during the firing in the fibre length was 25%. The chemical composition of the obtained silica fibre was: SiOz 97-99% Na <1% Zn <500 ppm P <600 ppm At temperatures below 950° C the silica fibre retained its amorphous structure (stretched and non stretched fibre). Crystallization appeared at the temperature range of 1000- 1100°C (the alkali content accelerates the crystallization of the silica fibre). At 1100- 1400°C the crystals of cristobalite begin to form (Fig. 2). A 3.5 decitex hybrid textile, weighing 570 g/m 2, was washed with methanol and boiled in distilled water followed by rinsing with hot distilled water to wash away theimpurities from the spinning bath and the finishing agents (mostly phosphorous compounds). The washed textile was burned to obtain a pure (99.5% SiOa) knitted silica fibre (Fig. 3). The hybrid textile was burned at different temperatures (with the heating rate of 10°C/min and the final temperature kept constant for one hour followed by slow cooling to room temperature) and the BET specific surface area of the obtained knitted silica fibre was measured. The BET specific surface area of the obtained knitted silica fibre decreased by increasing the burning temperature of the hybrid textile (Fig. 4). However, this decrease in the surface area was less extreme as compared to the decrease in the BET specific surface area of the obtained silica fibre by increasing the burning temperature of the hybrid fibre. This is due to the absence of impurities on the hybrid textile which accelerates the partial crystallisation of the silica at higher temperatures. Most of the impurities are water soluble and after washing thehybrid textile and burning it, the total amount of impurities was less than 0.5 wt.-%. The knitted silica fibre obtained by burning the hybrid textile at 950°C was selected to be used as a support for catalysts for catalytic combustion. The support had a thickness of 1mm and weighed 190 g/m 2, with a BET specific surface area of « 140m2/g and a specific pore volume of 0.469 cm3/g. XRD analysis of the obtained knitted silica fibre exhibited an amorphous structure. The pore size distribution curve of the catalyst support showed that 82% of the pore volume is located in pores of 20-1000 A radius. Catalyst Preparation and Characterization The obtained support was cut into pieces of 7 cm x 10 cm, weighing 1.7-1.9 g, which were impregnated with cobalt using alcoholic solutions of Co(N03)2.6HzO (Merck) with different concentrations. The samples were dried at room temperature for 12 hours followed by heating at 550° C for one hour in the presence of air. These supported C03O4 catalysts were soaked in alcoholic solutions of HzPtClg.xHzO (Aldrich Chem.) for the platinum impregnation. The platinum impregnated C03O4 catalysts were dried at room temperature for 12 hours followed by heating at 500° C for one hour in the presence of air (Neyestanaki et al., 1991). The catalysts containing nickel oxide and platinum were prepared by impregnation of the supports (7 cm x 10 cm) with alcoholic solutions of Ni(N03)z.6H20 (Merck) with different concentrations. The supports were dried at room temperature for 12 hours followed by heating at 700° C for one hour in the presence of air. The catalysts were then impregnated with platinum using alcoholic solutions of H2PtCl6.xH20, dried at room temperature for 12 hours followed by heating in air at 550° C for one hour. CATALYTIC COMBUSTION 125 FIGURE 3 SEM image of the catalyst support. The catalysts containing nickel and cobalt oxides were prepared by co-impregnation of the supports (7cm x 10cm) with alcoholic solutions of cobalt nitrate and nickel nitrate hexahydrate. The catalysts were dried at room temperature for 12 hours and heated in air at 700° C for one hour. The CraOa catalyst was prepared by impregnation of the support with aqueous solution of ammoniumbichromate (Merck), dried at room temperature for 48 hours followed by heating in air at 550°C for one hour. The CoaO^CrzO; catalyst was prepared by co-impregnation of the support with aqueous solution of cobalt nitrate hexahydrate and ammoniumbichromate, dried at room temperature for 48 hours followed by heating in air at 550° C for one hour. The NiO-CrzOa catalyst was prepared by co impregnation of the support with aqueous solution of nickel nitrate hexahydrate and ammoniumbichromate, dried at room temperature and calcined at 700° C for one hour. The C03O4 — NiO — Cr%0^ was prepared by co-impregnation of the support with aqueous solution of cobalt nitrate, nickel nitrate and ammoniumbichromate, dried at room temperature for 48 hours followed by heating in air at 700° C. The catalysts containing MnOa were prepared by impregnation of the supports with alcoholic solutions of manganese(II) nitrate tetrahydrate (Merck), dried at room temper ature for 12 hours followed by heating in air at 500° C for one hour. The platinum content of all the prepared catalysts was about the same. After all measurements the catalysts were dissolved in acids (HF and aqua regia) and their metal content was measured by Direct Current Plasma (DCP) emission technique. The BET specific surface areas of the prepared catalysts were measured by Carlo Erba Sorptomatic 1900. The catalysts were degassed at 300° C prior to their surface 126 A. K. NEYESTANAKI AND L.-E. LINDFORS BET SPECIFIC SURFACE AREA, m% SPECIFIC PORE VOLUME, cnVg —— BET surface area ------specific pore volume 800 BURNING TEMPERATURE,°C FIGURE 4 BET specific surface area and specific pore volume of the obtained knitted silica fibre as a function of the burning temperature of the hybrid textile. area measurement by nitrogen adsorption; the results are shown in Table I. The average particle diameter of the metal oxides was measured by X-Ray Powder Diffraction method using a Philips pw 1820 with Xe-proportional detector, Cu-anode (50 kV, 40 A) according to Sherrer’s equation. The results are listed in Table I. The Flow System for the Light-Off Temperature and Lambda Window Testings The experimental set-up for the light-off temperature and lambda window testing (Harkonen et al., 1991) consists of a gas blending system, a preheater and a tube furnace (Fig. 5). The gas mixture used was a mixture of CsHg, CgHg, CO, COz, NO, N2, air and water vapour to simulate thegases of theautomobile exhaust (with the exception of H2). The gas flows were measured by mass flow meters which can be controlled either manually or by a computer. One part of the air was pulsed into the reactor with a timer and magnetic valve (1 Hz frequency). Before reaching the tube furnace all the flows except the pulse air were connected. Nitrogen was preheated and the pumped water was evaporated in the preheater. The catalyst sample was inserted in a tubular continuous flow reactor, with a diameter of 1.35 cm, which was placed in a tubular furnace. The temperature was measured before and after thecatalyst sample. Thepulse air was introduced just before the catalyst sample. The exhaust gases from the tubular reactor were led through a magnetic valve (bypass/sample) to a heated gas sample line (180-190° C), from where the hot gases were led to HC and NO* analyzers and to the gas cooling unit. One part of the cooled gases was led to CO2, CO analyzers, another part to theoxygen analyzer and the rest to the outlet. The analyses of the exhaust gases were performed using the analyzers listed in Table II. CATALYTIC COMBUSTION 127 TABLE I The BET specific surface area, specific pore volume and the average particle size (diameter) of the metal oxide catalysts. CATALYSTS BET SURFACE SPECIFIC AVERAGE AREA PORE VOLUME PARTICLE SIZE (m2/g) (cm3/g) W°) SUPPORT 139.88 0.469 5.5%Co 141.80 0.511 110 9.1% Co 128.39 0.446 120 14.9% Co 126.20 0.361 140 1.3% Pt 135.52 0.529 - 5.6% Co + 1.3% Pt 141.30 0.438 8.8% Co + 1.0% Pt 103.94 0.406 13.6% Co + 1.2% Pt 97.74 0.307 200 4%Ni + 1.4% Pt 118.69 0.465 120 7.2% Ni + 1.5% Pt 123.15 0.421 110 13.5%Ni + 1.3% Pt 110.42 0.368 150 7.2%Co + 7.1% Ni 95.51 0.410 6.2% Cr 129.04 0.426 190 8.6%Co + 4.7% Cr 100.90 0.348 7.2%Ni + 4% Cr 135.04 0.405 7%Co+6.8%Ni+3.8%Cr 110.00 0.629 80 8.9% Mn 109.90 0.445 14.6% Mn 97.00 0.416 90 13.2%Mn + 1.2%Pt 82.12 0.267 The control of the mass flow meters, the bypass/sample magnetic valve, the data acquisition from the analyzers and the thermocouples as well as the handling of the collected data was performed by a personal computer. The results were saved in the computer memory in the form needed for LOTUS Worksheet/Graphics/Database program. The standard feed gas mixture used in the light-off temperature testing composed of 0.0125% C3H8 , 0.0375% C3H6,1.00% CO, 10.00% C02, 0.15% NO, 1.02% 02,10.00% water (gas), and the rest N2 (A = 1.010). The total flow rate (at standard conditions) was 8900 ml/min (space velocity = 53000 h-1) and the pulse air flow rate was 187 ml/min. For the light-off testing the temperature of the preheater was kept at 400° C and the tube furnace temperature was raised from 160 to 500° C at the rate of 12°C/min. During the light-off testing the gas content and the conversions of HC, CO and NO as well as temperatures before and after the catalyst sample were registered. As a result of the light-off testing the light-off curves for HC, CO and NO were drawn which enabled the measurements of the temperatures at 50% conversion (light-off temperature) and the end conversions at 400° C. After the light-off testing the tube furnace was cooled to 350° C for the lambda-window measurements. After stabilization of the temperature the gas mixtures presented in Table III (the total flow rate was 8900 ml/min and all the gas flows except the oxygen and nitrogen were kept constant) were led to the reactor and theconversions were measured and saved in the computer memory. As a result of lambda window testing, curves of conversions as a function of lambda —calculated according to Horiba equations —or as a function of oxygen content of the feed gas were drawn (lambda window curves). 128 A. K. NEYESTANAKI AND L.-E. LINDFORS TUBE FURNACE WITH PREHEATING THE TUBE REACTOR FURNACE • GAS SAMPLES WATER Srsm GAS ANALYZERS PULSE AIR MASS FLOW METERS | NOx Micro computer Control unit FIGURE 5 The experimental set-up for the light-off and lambda window measurements. TABLE II The Gas analyzers used and their measurement principles and ranges. Component Analyzer Measurement Principle Ranges Hydrocarbons JUM 3-300 FID P § 500, 1000, 5000, 10000, 50000 ppm Nitric oxides IMES PN-1B Chemilum. 0...10, 25, 100, 1000, 2500, 10000 ppm Carbon monoxide BINOS 100 NDIR 0...1000 ppm 0...1, 20% Carbon dioxide BINGS 100 NDIR 0—20% Oxygen MAGNOS 3 Paramag. 0...1, 3, 10, 30, 100% RESULTS AND DISCUSSION The support obtained was found to be very versatile and it can be made in different shapes and geometries. By varying the burning temperature of the hybrid textile, the CATALYTIC COMBUSTION 129 TABLE III The oxygen content of the feed gas flow and the calculated * lambda values in the lambda window testing (total flow rate = 8900 ml/min, pulse air = 187 ml/min). MIXTURE OXYGEN CONTENT (%) LAMBDA 1 0.44 0.974 db 0.028 2 0.50 0.978 ± 0.028 3 0.54 0.980 ± 0.028 4 0.59 0.983 ± 0.028 5 0.65 0.987 ± 0.028 6 0.73 0.992 ± 0.028 7 0.82 0.997 ± 0.028 8 0.93 1.004 ± 0.028 9 1.02 1.010 ± 0.028 * -Lambda values were calculated according to Horiba equations. surface area of the obtained knitted silica fibre support can easily be manipulated. Its high surface area allows it to be employed as a catalyst carrier without any washcoat being needed. Most of the pore volume (82%) is in pores with 20-1000 A radius. The stability of the silica fibre support in steam-rich atmosphere was tested at 700° C with 40 and 60 vol.-% moisturised air for she hours. The absence of any change in the BET specific surface area indicates that there is no effect of sintering under these conditions. While trying to determine the oxidation state of cobalt on the catalyst samples with ESCA no cobalt was detected, but when the ESCA analysis was carried out on the powdered catalyst it became possible to detect cobalt on the catalyst sample. Considering this and taking into account the high specific surface area (140 m2/g) and the specific pore volume (0.469) cm3/g) of the catalyst support and also the analyzed depth by ESCA (<10 nm) the question arose whether the metal oxide is located mostly inside thepores or on the exterior surface of the silica fibre. In order to find out the exact location of metal oxide, the supported C03O4 catalyst was immersed in an epoxy glue and dried for 24 hours. The obtained rigid material was broken and the cross-section of the catalyst fibre was studied by SEM (Stereoscan 360, Leica Cambridge) equipped with EDXA using the back scattered electron detector. The cobalt scan line for the supported C03O4 catalyst showed that the cobalt is distributed everywhere inside the fibre (Fig 6a) or concentrated in strips inside the fibre (Fig. 6b), but not much on the edges (the bark opaque region). The X-ray mapping of the cross-section also confirmed this distribution of the cobalt. The spot scan of the cross-section of a single fibre presented in Figure 6b was performed and theresults from standardless analysis of the bulk sample for the bright region (region b) and the inner edge opaque region (region a) showed that the cobalt content of the bright region is three times higher than the cobalt content of the opaque (bark, edge) region. Thecobalt is located at about 500 nm from the edges (Fig. 6b) and this explains why it was not possible to detect any cobalt on the non-powdered catalyst, since only a small fraction of the cobalt on the catalyst was detected. From these results it can be concluded that the obtained silica fibre has a complex pore structure which is formed when the cellulose component of the hybrid fibre is burnt off. As a consequence of the fibre morphology, the active metal oxide is located mostly inside the pores rather than on the exterior surface of the catalyst support. Tables IV and V present the results of the temperature and conversion measurements for various catalysts at two lambda-values (space velocity = 53000 h-1). As mentioned 130 A. K. NEYESTANAKI AND L.-E L1NDFORS a b FIGURE 6 SEM images of the cross-sections of single fibres of the CojOVsilica-fibrc catalyst. CATALYTIC COMBUSTION 131 TABLE IV The light-off temperatures* and the conversions at 400° C of the transition metal oxide catalysts, space velocity = 53000 h-1. LIGHT-OFF TEMP. CONVERSION CATALYSTS A T>o%»° C at 400 °C, % wt.-% CO HC NO CO HC NO 5.5% Co 0.981 >400 >400 >400 38.6 9.9 2.0 9.1% Co 0.981 >400 >400 >400 43.9 13.9 2.0 15.5% Co 0.981 287 >400 >400 57.4 30.7 1.0 14.9% Co 1.010 259 387 >400 62.5 52.3 1.0 7.2%Co + 7.1% Ni 1.010 >400 >400 >400 25.7 10.0 0.0 14.3% Co + 3.4% Ni 1.010 >400 >400 >400 30.0 9.4 1.0 6.2% Cr 1.010 >400 >400 >400 4.4 13.2 0.0 8.6% Co + 4.7% Cr 1.010 >400 >400 >400 7.9 41.7 0.0 7.2% Ni + 4% Cr 1.010 >400 >400 >400 6.1 23.7 0.0 7% Co + 6.8% Ni + 3.8% Cr 1.010 388 410 >400 51.8 45.5 0.0 8.9% Mn 1.010 >400 >400 >400 39.8 39.3 2.0 14.6% Mn 1.010 >400 >400 >400 42.1 40.8 0.0 * - measured as the temperature before the catalyst. previously three reactions are considered: oxidation of carbon monoxide and hydrocar bons and thereduction of nitric oxide. Thelight-off temperature is denoted by Tso% and is given for each reactant and the given catalyst. Also listed in Tables IV and V are the corresponding values of the conversions at 400° C. As can be seen in Table IV, by increasing the cobalt content of the C03O4 catalysts from «5% to «15 wt.-% cobalt (reduced basis), the conversions (at 400 °C and A = 0.981) of CO and HC increase. Conversions of 57.4% for carbon monoxide and 30.7% for hydrocarbons were achieved with the catalyst containing 15.5 wt.-% Co. This increase in the C03O4 content of the catalyst significantly lowered the light-off temperature for carbon monoxide oxidation from above 400° C to 287° C, but the light-off temperature for hydrocarbons remained above 400° C. In order to study the effect of stoichiometry on the light-off temperatures, the catalyst containing 14.9wt.-%Co was tested with the fuel-lean mixture number 9 (see Table III). This increase in the oxygen content of the feed mixture resulted in a considerable decrease in the light-off temperatures for both carbon monoxide and hydrocarbons as well as in an increase in their conversion at 400°C. It should be mentioned, however, that the NO conversions over C03O4 catalysts remained at a low level throughout. The conversion behaviour of the Co304-NiO catalysts, tested at A = 1.010, indicated that the combination of C03O4 and NiO deteriorates the conversions of both carbon monoxide and hydrocarbons, compared to the catalysts containing thesame amounts of C03O4. The results of the light-off testings of the CT2O3 catalyst and its combinations are also given in Table IV (A=1.010). It seems that chromium (III) oxide is more active in HC combustion than in CO oxidation, judged by its conversion behaviour. Combination of C03O4 and CraOs resulted in a decrease in carbon monoxide conversion (from 43.9 to 7.9%), but increased the conversion of hydrocarbons (from 13.9 to 41.7%) in comparison to the catalyst containing the same amount of C03O4 (9.1 wt.-%Co-catalyst in Table IV). Combination of Cr%03 and NiO also resulted in an improved HC conversion but only slightly increased the conversion of CO, as compared to the chromium (III) oxide catalyst. 132 A. K. NEYESTANAKI AND L-E. LINDFORS TABLE V The light-off temperatures* and the converstions at 400° C of the platinum activated transition metal oxide catalysts, space velocity = 53000 h-1. LIGHT-OFF TEMP. CONVERSION CATALYSTS A T50%,o C at 400 °C, % wt.-% CO HC NO CO HC NO 1.3% Pt 1.010 275 268 >400 75.2 72.3 36.0 5.6% Co + 1.3% Pt 1.010 255 265 >400 74.6 70.5 29.7 8.8% Co + 1.0% Pt 1.010 280 290 >400 81.6 77.0 38.9 13.6% Co + 1.2% Pt 1.010 269 277 310 98.2 99.8 41.1 4% Ni + 1.4% Pt 1.010 221 240 >400 74.1 70.5 35.0 7.2% Ni + 1.5% Pt 1.010 235 238 >400 78.6 77.5 36.4 13.5% Ni + 1.3%. Pt 1.010 230 232 >400 92.9 92.3 43.7 6.9%Co + 6.7% Ni + 1.1% Pt 1.010 247 262 >400 73.2 69.3 38.0 5.7% Cr + 1.2% Pt 1.010 285 315 >400 62.3 58.9 38.1 7.1%Co+6.6%Ni+3.8%Cr+l%Pt 1.010 288 293 410 85.1 84.1 48.5 13.2%Mn + 1.2% Pt 5%02 269 272 >400 93.0 82.0 3.0 * - measured as the temperature before the catalyst. The catalyst containing 7%Co+6.8%Ni+3.8%Cr, exhibited increases both in CO and HC conversions at 400° C, compared to the catalysts consisting of any of these metal oxides alone or combined by one of the other two. MnO% catalysts exhibited a good catalytic activity. Conversions of ~40% for HC and CO were achieved with MnO% catalysts (Table IV). Here, increase in the Mn content of the catalyst, from 8.9 to 14.6%, only slightly improved the final conversions at 400° C. The light-off temperature measurements of the Pt-Co304 catalysts were carried out at A = 1.010 (mixture 9 in Table III) and are presented in Table V. This combination of platinum and C03O4 increased the conversions at 400°C, but did not alter the light-off temperatures with respect to the platinum catalyst. The light-off temperatures of thePt- C03O4 catalysts were 255-280° C for carbon monoxide and 265-290°C for hydrocarbons. Here also an increase in the cobalt content of the catalyst resulted in increases in the final conversions at 400°C. The final conversions of 98.2% for carbon monoxide, 99.8% for hydrocarbons and 41% for NO were achieved by using the catalyst containing 13.6% Co + 1.2% Pt (Table V, Fig. 7). These resultant conversions are remarkably higher than those obtained with the platinum catalyst (75.2% for CO, 72.3% for HC and 36% for NO), indicating strong synergistic effect between platinum and C03O4 in catalytic oxidation reactions. The results from the light-off temperature testings of the Pt-NiO catalysts (at A=1.010), showed that this combination (Pt, NiO) results in reduced light-off temperatures for both CO and HC, compared to those of the platinum and Pt-Co304 catalysts. The light-off-temperatures of the Pt-NiO catalysts were 220-230° C for carbon monoxide and 230-240° C for hydrocarbons. These values are approximately 40° C lower than thelight- off temperatures of the platinum catalyst. Pt-NiO catalysts followed the same trends in catalytic activity— i.e. increase in the conversions of CO, HC and NO by increasing the metal oxide content of the catalyst—as with the C03O4, Pt-Co304 and MnOa catalysts. The final conversions of 92.9% for CO, 92.3% for HC and 43.7% for NO were achieved with the catalyst containing 13.5%Ni + 1.3% Pt (Table V, Fig. 8). It is worth mentioning, that a decreased catalytic performance of the Pt-Co304 catalysts—but not the catalysts which contained only C03O4—was observed in the fuel- rich region (A=0.981), but not in the fuel-lean region (A = 1.010). The decreased activity CATALYTIC COMBUSTION 133 CONVERSION ■*' HC TEMPERATURE BEFORE THE CATALYST,(°C) FIGURE 7 The light-off temperature curves of the catalyst containing 13.6% Co + 1.2% Pt (A = 1.010, space velocity = 53000 h-1). seems to be a consequence of the catalyst reduction as was also evident by the partial change in the catalyst’s colour. The reduced catalyst could be regenerated by heating in air at 500-600° C. Reductions of this type were not observed for the Pt-NiO catalysts. This can possibly be explained by the oxygen storage capability of nickel, which minimizes the effect of variation in the fuel/air ratios on the catalyst. Further studies on the Oz adsorption and TPD measurements of the catalysts are being conducted. The deteriorating effect of the C03O4 and NiO combination was also obvious for the Pt-Co304-NiO catalyst. The conversions at 400°C of the Pt-Co304-NiO catalyst were lower than the final conversions at 400° C of the catalysts containing the same amounts of any of these metal oxides and platinum. However, its light-off temperatures were between those obtained with Pt-NiO and Pt-Co3C>4 catalysts. Combination of Q2O3 and platinum also resulted in decreased conversions at 400° C as compared to the Pt- catalyst. Conversions of 62.3% for CO and 58.9% for HC were achieved with the catalyst containing 5.7% Cr + 1.2%Pt (Table V). These conversions are much lower than those 134 A. K. NEYESTANAKI AND L.-E. UNDFORS CONVERSION, TEMPERATURE BEFORE THE CATALYST,(°C) FIGURE 8 The light-off temperature curves of the catalyst containing 13.5% Ni + 1.3% PI (A = 1.010, space velocity = 53000 h-1). obtained with the platinum catalyst. The light-off behaviour of this Pt-CrzOz catalyst was also higher than that of the Pt-catalyst. The platinum activated Co-Ni-Cr oxide catalyst (Table V) exhibited higher conversions at 400° C compared to the catalysts containing the same amounts of any of these metal oxides and platinum and its light-off behaviour was close to that of the C03O4 catalyst (8.8%Co+l%Pt). As with the Pt-Co3C>4 catalysts, a catalyst reduction was also observed with the Pt- MnC>2 catalysts. The Pt-MnOz catalysts were reduced even at X = 1.010 which strongly affected their light-off and conversion behaviour. At higher excess air level (5% O2 in the feed gas) this catalyst reduction did not happen and high conversions of CO and HC were achieved (93% for CO, 82% for HC). TheNO conversion was very low due to the high excess air used. The light-off temperatures of the Pt-MnOz catalyst were close to those of the platinum catalyst (Table V). In order to obtain the relationship between the degree of conversion and the oxygen content of the feed mixture, lambda window measurements were performed at 350° C for CATALYTIC COMBUSTION 135 CONVERSION, (% 100 ------—------V 90 1—Jlk1 —4* ------7fi------> !2jb=* X 80 Xf X 70 60 X — CO 50 'Tx HG \ -O- NO 40 30 20 10 0 0.4 0.6 0.8 1 OXYGEN % IN THE FEED GAS FIGURE 9 The lambda window curves of the catalyst containing 13.5% Ni + 1.3% Pt (T = 350° C, space velocity = 53000 h~'). Pt-NiO catalysts, platinum and the C03O4 (15.5 wt.-% Co) catalysts. The results of such measurements for the 13.5%Ni+1.3%Pt catalyst are depicted in Figure 9. As can be seen, an increase in theoxygen content of the introduced mixture increases theconversions of both carbon monoxide and hydrocarbons, but decreases theNO conversion which might be due to the reduced competitiveness of NO for CO oxidation caused by inhibition of NO adsorption by oxygen. The triple point, where the conversions of all three reactants are the same (87%), was achieved by working with the fuel-rich mixture number 4 (A=0.983). Similar trends were obtained for the 15.5wt.-%Co-catalyst and the 1.3wt.-% Pt-catalyst, with the exception that the NO conversions over thecobalt catalyst were low at all A- values. From the obtained results the following activity patterns can be drawn: a). The final conversions (at 400° C) of HC and CO for the Pt-metal oxide catalysts containing «13% transition metal (reduced basis) decreased in the order: Pt-Coa04 > Pt-NiO > Pt-MnOz > Pt (Pt-Mn0 2 catalyst was tested at 5% excess air) 136 A. K. NEYESTANAKI AND L-E. LINDFORS or 13.6%Co + 1.2%Pt > 13.5%Ni + 1.3%Pt > 13.2%Mn + 1.2%Pt > 1.3%Pt b). The final conversions at 400° C of HC and CO for the Pt-metal oxide catalysts containing «7% transition metal decreased in the order: Pt - C03O4 - NiO - Cr203 > Pt - C03O4 > Pt - NiO > Pt > Pt - C03O4 - NiO or 7.1%Cb + 6.6%Ni + 3.8%Cr + l%Pt > 8.8%Co + l%Pt > 7.2%Ni + 1.5%Pt > 1.3%Pt > 6.9%Co + 6.7%Ni u%Pt c) . The catalytic activity of the transition metal oxide catalysts for the oxidation of carbon monoxide decreased in the order: 14.9%Co > 7%Co + 6.8%Ni + 3.8% Cr > 9.1%Co (A=0.981) > 14.6%Mn > 8.9% Mn > 5.5%Co (A=0.981) > 14.3%Co + 3.4%Ni > 7.2%Co + 7.1%Ni > 8.6%Co + 4.7% Cr > 7.2%Ni + 4%Cr > 6.2% Cr d) . The catalytic activity of the transition metal oxide catalysts for HC combustion decreased in the order: 14.9%Co > 7%Co + 6.8%Ni + 3.8%Cr > 8.6%Co + 4.7% Cr > 14.6% Mn > 8.97.2%Ni + 4%Cr > 9.1%Co (A =0.981) > 6.2%Cr > 14.3% Co + 3.4%Ni ~ 7.2%Co + 7.1%Ni ~ 5.5% Co (A =0.981) The final conversions at 400° C of the Pt-Co 304 catalysts were higher than those obtained with the Pt-NiO and Pt-Mn0 2 catalysts (with equal amounts of metal oxide and platinum), but the lower light-off behaviour and resistance in reducing atmospheres of these supported Pt-NiO catalysts makes them more suitable for systems where the oxygen content of the feed gas is varying. REFERENCES Blazowski, W.S. and Walsh, D.E., (1975), Catalytic Combustion: An Important Consideration for Future Applications, Combust. Sci. Tech. 10, 233. Dixon, G.M., Nicholls, D. and Steiner, H., (1964), Proc. Third Intern. Congr. Catalysis, 2, 815 Amsterdam. Dowden, D.A. and Wells, D., (1961), Actes 2e Cong. Inter. Catalyse, 2, 1489, Technip, Paris. Garbowski, E., Guenin, M., Marion, M.-G and Primet, M., (1990), Catalytic Properties and Surface States of Cobalt Containing Oxidation Catalysts, Appl. CataL 64, 209. Heidari, S., (1991), V1SIL A New Hybrid Technical Fibre, Kemira Ltd. Harkonen, M.A., Aitta, E., Lahti, A., Luoma, M., and Maunula, T., (1991), SAE Paper 910846. Salanne, SJ., Neyestanaki, A. K., Lindfors, L-E., Vapaaoksa, P.J., (1993), Finnish patent No. 89012. Catalyst Carrier, Patent application (patenltihakemus). Prasad, R., Kennedy, LA. and Ruckenstein, E., (1980), Catalytic Combustion of Propane Using Transition Metal Oxides, Combust. Sci. Tech. 22, 271. Prasad, R., Kennedy, LA. and Ruckenstein, E, (1982), Kinetics of Catalytic Combustion of Propane on Transition Metal Oxides, Combust. Set Tech. 27, 171. Trimm, D.L, (1983), Catalytic Combustion (Review), Appl. CataL 7, 249. Weinberg, F.G., (1986), Advanced Combustion Methods, Academic Press, London, chap. 4, pp. 237-275. I / 8 Catalytic Combustion of Propane and Natural Gas over Silica-Fibre Supported Catalysts A. Kalantar Neyestanaki and L.-E. Lindfors Laboratory of Industrial Chemistry, Abo Akademi University, Biskopsgatan 8, FIN- 20500 ABO, Finland ABSTRACT- Different knitted silica-fibre supported metal oxides (oxides of Co, Ni, Mn, Cr) and various combinations of them, platinum-activated cobalt and nickel oxide catalysts as well as noble metal (Pt, Pd) catalysts were prepared. The catalysts were tested for their activity in propane and natural gas combustion. Co 304 was found to be the most active single metal oxide catalyst in propane combustion. Combinations of platinum with cobalt or nickel oxide improved the light-off and conversion behaviour of the catalysts in propane combustion. The propane combustion over Pd/Si0 2 was found to be structure sensitive under our experimental conditions. The dependence of the rate of propane combustion over noble metal containing catalysts was found to be zero order with respect to oxygen and one with respect to propane. The reaction orders over metal oxides were fractional (0 < n,m < 1). NiO and Pd were found to be the most active catalysts in natural gas combustion. The activity pattern of the prepared catalysts in complete oxidation of propane and natural gas is reported. The catalysts were characterised by 02-TPD and the chemisorption of propane, carbon monoxide and hydrogen. Key Words - Catalytic combustion, noble metals, transition metal oxides, propane, natural gas. 1 INTRODUCTION Application of catalytic combustion to some stationary and mobile combustion systems has shown significant advantages in controlling the emissions of hydrocarbons, carbon monoxide and nitric oxides, usually accompanying conventional combustion systems. Deep oxidation of hydrocarbons is carried out using highly active, non-selective 1 platinum group metal catalysts (Pt, Pd, Rh). Due to the high cost and limited availability of noble metals attention has been focused on metal oxides, such as oxides of Co, Cr, Mn and Cu (Sinha and Shankar 1993, Haber et al. 1987, Rajesh and Ozkan 1993, Kapteijn et al. 1993), which can assume more than one valence state and in this way participate in the redox cycle. Oxides generally exhibit activities at higher temperatures than noble metals. Efficient catalysts of complete oxidation must provide a high rate of the primary oxygen activation, i.e. they must have a large number of sites that can coordinate the molecules of oxygen and must quite easily donate and accept electrons. They should also provide a slow transformation of active oxygen into lattice oxygen (Sokolovskii, 1990). Thed- character of the metals is found to be the major factor in determining the ability of the metals to break the C-H bond (Pitachi and Klier, 1986). It has been shown that dz 2 or dx^y 2 orbitals of metal atoms or cations are responsible for methane activation on transition metal surfaces (Anderson and Maloney, 1988) or on transition metal compounds (Anderson et al., 1988). Depending on the application, the catalysts are usually supported on monolithic honeycomb or fibrous substrates. Catalytic heaters are based on catalysts supported on fibres, the system is highly non-adiabatic and can be operated at near stoichiometric conditions (Kesserling, 1986). The packing density of the fibres has been found to be of importance in determining the performance of the fibre-supported catalysts. The aim of the present work was to prepare different silica-fibre supported base metal oxides, Pd and Pd-Pt catalysts and to study the activity and the behaviour of these catalysts in the complete oxidation of propane and natural gas. 2 EXPERIMENTAL A. Support and Catalyst Preparation A knitted silica fibre obtained from a hybrid textile (organic-inorganic hybrid textile) was used as a catalyst support for combustion catalysts. The procedure for the support preparation and characterisation is given elsewhere (Neyestanaki and Lindfors, 1994). The support used had the following characteristics: 2 Si02 purity: 99.5% BET surface area: 138 m2/g Specific pore volume: 0.435 ml/g Pore size distribution: 25 / (10-100) vol%/nm (pore volume / radius) 30 / (5-10) vol%/nm 26 / (2-5) vol%/nm 19 / < 2 vol%/nm A series of silica-fibre supported metal oxide catalysts (Co 304, Cr203, NiO, Mn0 2, CuO, Co 304-Cr203, Co 304-NiO, Co 304-NiO-Cr203), and Pt-metal oxide catalysts (Pt- Co 304, Pt-NiO) as well as Pd and Pd-Pt catalysts were prepared. The catalysts were prepared by impregnation (for the single component catalysts), subsequent impregnation with intermediate drying and calcination (for the Pt-activated cobalt and nickel oxide catalysts) and co-impregnation (for the multi component catalysts) of the support with aqueous solutions of metal salts. The catalysts were then dried and calcined in air at the required temperatures. The reagents used were Co(N0 3)2.6H20 (Merck), Ni(N03)2.6.H20 (Merck), Mn(N0 3)2.4H20 (Merck), (NH^G^O-? (Merck), Cu (N03)2.3H20 (Merck), H2PtCl6.xH20 (Aldrich), and PdCl 2 (Aldrich). B. Catalyst Testing The experimental setup for catalyst testing, Figure 1, consisted of pressure regulators, filters, mass flow controllers, a preheater and a tube furnace. Catalyst samples were packed in a continuous flow tube reactor, with a diameter of 4 cm, which was placed in the tubular furnace. Immediately after the mass flow controllers, the air and nitrogen flows were connected and heated in the preheater while the fuel was introduced just before the tube furnace. A ceramic bed was placed into the reactor in order to improve the gas mixing. The temperature was measured before and after the catalyst bed. The analyses of the hydrocarbons, CO and C02 were performed by a TEN3 NDIR gas analyser. The data acquisition from the analyser and the temperature measurements were carried out and recorded by a PC. The catalysts were tested for their activity in propane and natural gas combustion. The gas mixture for propane combustion consisted of 0.285 vol.% propane, 6.8 vol.% air, and 92.92 vol.% N2. The total flow rates at standard conditions were 2500 and 5000 ml/min (space velocity, GHSV, of 23900 and 47800 h"1,1=1.05). The gas mixture for 3 natural gas combustion consisted of 1.44 vol.% natural gas, 14.64 vol.% air and 83.92 vol.% N2 (total flow rate at standard conditions = 2500 ml/min, GHSV = 23900 h"1, X = 1.05). For catalyst testing, the tube furnace temperature was raised from 423 K to 873 K at a heating rate of 12 K/min (the preheater temperature was kept constant at 623 K) and the hydrocarbon conversion (to C02 and H20) and temperatures before and after the catalyst were measured continuously. Based on the catalyst testing, curves of conversion as a function of temperature before the catalyst (light-off curves) were drawn which enabled the measurement of the temperature at 50% conversion (light-off temperature) and the final conversion at 823 K. For the measurement of the kinetic parameters of the catalytic oxidation of propane over the prepared catalysts, the propane conversions were kept below 10% and a power rate law (r = k [02]"[C2Hg]'") was assumed. The reaction orders were determined by varying the concentration of the reactants in the feed while keeping the total flow rate constant. The apparent activation energies and the pre-exponential factors were determined from the Arrhenius plots. C. Catalyst characterisation - Metal content. The metal content of the prepared catalysts was determined by DCP (Spectraspan IDA, Spectrometries) and X-ray fluorescence (X-MET 880, Outokumpu). - Chemisorption measurement. The measurements of CO, CgHg and H2 chemisorption were carried out using a Sorptomatic 1900, Carlo Erba Instruments. The adsorption isotherms were obtained at 298 K and pressures of 1-100 torr (1 torr = 133.32 Pa). Extrapolation of the adsorption isotherms to zero pressure was used for the determination of the irreversibly adsorbed gases. The amount of reversibly adsorbed gases was determined by the back-sorption method. Prior to the chemisorption measurements of CO or CjHg, the catalyst samples were treated with oxygen at 773 K, slowly cooled down to room temperature, purged with nitrogen (99.999% purity) and finally outgassed at 723 K and 10"4 Pa for two hours. For the determination of the metal dispersion and particle size by H2 chemisorption, the catalysts were first reduced at 773 K for two hours in 100 ml/min hydrogen flow, followed by purging with nitrogen and evacuation at 723 K and 10"4 Pa for two hours. In order to minimise the H2 absorption by palladium, the hydrogen adsorption was performed at 363 K for Pd-containing catalysts. 4 - Temperature Programmed Desorption. TPD of oxygen was carried out on a volumetric flow apparatus. After being treated with oxygen (99.99% purity) at 773 K the catalysts were cooled down in an oxygen flow to 298 K, pre-treated with oxygen at 298 K for 30 minutes followed by helium (dry, 99.999% pure) purging for 30 minutes. The catalyst temperature was then linearly increased from 298 K to 1103 K at a heating rate of 8 K/min where the helium was used as the carrier gas. The desorbed oxygen was detected by a Quadmpole Thermal-Programmed Mass Spectrometer, Carlo Erba Instruments. For the quantitative measurements, the mass spectrometer was calibrated for oxygen. The flows were controlled by means of mass flow controllers. 3 RESULTS AND DISCUSSION A. Propane Combustion The results of the catalyst testing in propane combustion are given in Tables 1-2 and Figures 2-4. The light-off temperature is denoted T50% and the conversions (X, %) are given at 523 K, 623 K, 723 K and 823 K (temperatures before the catalyst bed). The catalysts in the text and tables are subsequently denoted as the values in the parentheses corresponding to the metal content of the catalyst, e.g. the (6.5, 6.8, 3.8)wL% Co-Ni-Cr describes the Co 304-Ni0-Cr203 catalyst containing 6.5 wt% cobalt, 6.8 wt% nickel and 3.8 wL% chromium. As can be seen, an increase in the metal content of the catalysts improved their light-off and conversion behaviour (Table 1). An increase in the metal content, e.g. of the Co 304 catalyst from 4.5 to 15.2 wt.%, increased the final conversion at 823 K by =13% and lowered the light-off temperature by =45 K. Of the single metal oxide catalysts, Co 304/Si02 was found to exhibit the highest activity at lower temperatures followed by Mn0 2 and Cr203 while NiO/Si02 was found to be the least active (Table 1) in propane combustion under our experimental conditions. Decrease in the residence time from GHSV=23900 h"1 to GHSV=47800 h'1, resulted in an increased light-off temperature and decreased the final propane conversions over all the catalysts. Combinations of Co 304 and NiO (i.e. the Co-Ni catalyst), Co 304 and Cr203 (i.e. the Co- Cr catalyst), NiO and Cr203 (i.e. the Ni-Cr catalyst) resulted in improved final conversion at 823 K with respect to the component oxide catalysts (with the same metal content). However, the light-off temperatures of these mixed oxide catalysts were always 5 higher than that of the Co 304 catalyst (Table 1). Combinations of Co 304, NiO and Cr203 (i.e. the Co-Ni-Cr catalyst) also resulted in increased final conversion (at 823 K) as compared to any of these metal oxides alone or in combination with one another (with the same metal content). However, in this case the light-off temperature was again higher than that of the Co 304 catalyst (Table 1). Platinum-activated Co 304 and NiO catalysts (i.e. Pt-Co and Pt-Ni catalysts) exhibited higher final conversion and reduced light-off temperature (Table 2) as compared to the Pt-catalyst having the same Pt-content ((0.95)wt.% Pt/Si02 catalyst). Propane conversion of 97.7% was achieved with a (0.71, 13.9)wt.% Pt-Co 304 catalyst. The activity of the metal oxides and Pt-metal oxide catalysts, containing =15 wt.% metal, - it should be noted that all the catalysts in this class have approximately the same amount of metal and the differences in the weight percentages are due to the differences in the oxygen/metal ratio of the component oxides as well as to the slight difference in the support mass - measured as the propane conversion at 823 K was found to decrease as: Pt-Co 304 > Pt-NiO > Co 304-NiO-Cr203 > Co 304 > Co 304-Ni0 > Co 304-Cr203 > Mn0 2 > Cr203-Ni0 > NiO. The light-off temperatures increased as: Pt-Co 304 < Pt-NiO < Co 304 < Co 304-NiO < Co 304-Ni0-Cr203 < Mn0 2 < Co 304- Cr203 < Cr203-Ni0 < NiO. The activity pattern of the metal oxide and Pt-metal oxide catalysts, containing =9 wt.% metal, measured as the conversion at 823 K, was found to decrease as: Pt-NiO > Pt- Co 304 > Co 304 > Mn0 2 > Cr203 > NiO, and the light-off temperatures for this class of catalysts were found to follow the same pattern with the exception that the Pt-Co 304 catalyst exhibited better light-off behaviour than the Pt-NiO catalyst. The results of the kinetic study of the propane oxidation over some of the prepared catalysts are given in Table 3. The reaction orders (based on the mole fractions) were found to be zero with respect to oxygen and one with respect to propane over the noble metal containing catalysts, while the orders were fractional over the base oxide catalysts. Considering the activation energies and the pre-exponential factors it can be concluded that the Pt-Co 304 and Co 304 are the most active among the noble metal containing and 6 metal oxide catalysts, respectively. Theresponse of the single component metal oxide catalysts to variations in the air/fuel ratio was studied by varying the amount of the feed air (total flow rate = 5000 ml/min and the fuel flow rate was kept constant) from X = 0.90 to X = 2.00 at 723 K. For all the catalysts this increase in the oxygen content of the feed mixture, up to 20% excess air, increased the final propane conversion at 723 K (Figure 2). Further increase in the oxygen concentration did not affect the propane conversion. Results of the catalyst testing of Pd and Pd-Pt catalysts are given in Table 2. As can be seen, the Pd-containing catalysts exhibited higher activities in oxide form than in the reduced form. The light-off temperatures over Pd/Si0 2 and Pd-Pt/Si0 2 were increased by 99 K and 115 K with this reduction of the catalysts (two hours at 773 K in 100 ml/min hydrogen flow). Oxygen treatment of the reduced Pd-containing catalyst resulted in restored catalytic activity. It was also observed that by retesting of the initially reduced Pd-catalysts, their light-off and conversion behaviour continuously improved. Therefore, the effect of catalyst aging on the reduced Pd-catalyst was studied as a fresh Pd/Si0 2 catalyst was reduced (two hours at 773 K in hydrogen flow) and tested for the light-off behaviour (Figure 3, curve a), after which the catalyst temperature was raised to 823 K and the catalyst was aged at this temperature for six hours under continuous flow of the reactants. The reactor was then cooled to 373 K and the aged catalyst was tested for the light-off behaviour (Figure 3, curve b). As is depicted in Figure 3, the light-off temperature and conversion at 823 K were altered by =45 K and =4 % as a result of this catalyst aging. It was also observed that the catalyst colour changed from black (in reduced form) to a partially amber colour, characteristic of palladium (II) oxide. This increase in the activity must be a result of partial oxidation of the pre reduced catalyst during the light-off testing. The results indicate that the propane oxidation, under our experimental conditions ((Zu rich reactant mixture), over the silica-fibre supported palladium catalyst occurs on the palladium oxide phase, even if the catalyst is initially reduced. The re-oxidation of the reduced catalyst takes place during the reaction. The reason for the higher activity of the oxidised PdO/Si0 2 catalyst might be in the nature of the surface rather than in the activity of the palladium. Palladium can form bulk oxide which is highly porous (Rucktenstein and Chen 1981 and 1982, Chen and Rucktenstein 1981). Oxidation breaks apart the palladium crystallites and results in emergence of new active sites and therefore increased rate of hydrocarbon oxidation. The oxidation of hydrocarbons over 7 palladium is considered to proceed according to two steps, the first one involving strong adsorption of gaseous oxygen on the metallic palladium particle, resulting in surface PdO. This is followed by the hydrocarbon adsorption on this oxidic surface where it becomes oxidised leading to the reduced (metallic) surface. On pre-oxidised Pd/Si0 2 the oxidation of palladium to PdO is no longer a prerequisite, in other words the PdO acts as an oxygen pool for the reaction and therefore the catalyst is temporarily more active. The activity of the aged-reduced catalysts (Figure 3, curve b) was, however, still lower than the oxidised palladium catalysts (Figure 4, curve a). It might be that only a fraction of bulk palladium is oxidised during this aging and the palladium particles are probably covered with a layer of oxide while the direct oxygen treatment of the catalyst results in the full oxidation of the bulk palladium and therefore a larger number of active centres are exposed to the reactants. Hicks et al. (1990,1990), in theirstudy of methane oxidation on supported Pt and Pd catalysts, postulated the presence of two kinds of palladium oxide: dispersed palladium oxide on alumina and palladium oxide deposited on metallic palladium, the latter being very active. The degree of palladium oxidation depends on the palladium particle size: small palladium particles are oxidised easily. Hydrogen treatment of the oxidised catalyst may result in the modification of the active site. Juszczyk and Karpinski (1989) demonstrated that the Pd/Si0 2 catalyst subjected to high temperature reduction results in the formation of Pd-Si compounds (most probably Pd 3Si) through the incorporation of Pd atoms (or ions) into the silica support via oxygen vacancies. They demonstrated that the re-oxidation of the catalyst restores the Pd particles. Therefore, in order to study the possible formation of the Pd-Si compounds, X-ray powder diffraction measurements were carried out for the reduced Pd/Si0 2 catalyst. The XRD spectra of the catalyst indicated the presence of the palladium silicide (Pd 5Si). However, the peak intensity was very low and the majority of the palladium was in the form of Pd(l 11) crystals. It is, therefore, possible to draw the conclusion that the formation of the palladium silicide species is not the main (if any) reason for the decreased catalytic activity of the reduced Pd/Si0 2 catalyst As an activation of the Pd-catalysts over time was observed during the testings, a fresh Pd/Si0 2 (oxidised) catalyst was tested for the light-off temperature and aged as in the previous case under the reactant mixture flow at 823 K for six hours. After cooling the catalyst to 373 K, the catalyst was tested again (Figure 4). It was observed that the light- off temperature decreased by 15 K, while the conversion levels (at 823 K) of the fresh and aged catalyst were very close. The hydrogen chemisorption measurement of the fresh Pd/Si0 2 indicated a palladium dispersion of 19.5 % and a mean metal particle 8 diameter of 57.7 A (assuming a spherical geometry), whereas the aged catalyst had a dispersion of 15.7 % and the metal particle size increased to 71.8 A. This slight increase in the palladium particle size can not be easily correlated to the improved catalytic activity of the catalyst. The increased catalyst activity can as well be a result of the slow release of chlorinated species from the catalyst (PdCl 2 was used as the metal precursor). Chlorine is known to inhibit hydrocarbon oxidation on palladium (O’Simone et al., 1991). However, when the exhaust gases were analysed by the quadrupole thermal- programmed mass spectrometer (a fresh oxidised catalyst was employed) no chlorinated species were detected. Therefore, although in this case the change in the Pd-pardcle size was small (and the aging time was relatively short) one can conclude thatthe propane oxidation, under these experimental conditions, over silica-supported palladium is to be considered as a structure-sensitive reaction. Briot and Primet (1991), in their study of methane oxidation over alumina-supported palladium catalysts, concluded that the reaction is structure-sensitive and in their low- temperature titration experiments they showed that the oxygen adsorbed on aged palladium (aged under reactant mixture flow for 14 h at 873 K) is much more reactive to hydrogen than the fresh catalyst. The restructuring of the surface during the reaction can also account for the increased catalytic activity over time. Baldwin and Burch (1990, 1990), in their study of methane oxidation over palladium catalysts, found that the catalyst activation is very fast on silica-supported palladium catalysts. They postulated that the reconstruction of the catalyst under reactant mixture might be the reason for this observation and the restructuring being faster on silica-supported samples. Somoijai and Van Hove (1988) have shown that chemisorption of molecules or atoms on a metal results in localised restructuring of the surface atoms around the adsorption sites. Collision of small molecules with high kinetic energies with the surface can transfer kinetic energy to surface atoms, thus inducing restructuring of the surface, which facilitates bond breaking in the incident molecules (Lee et al. 1987). Recently, Gabrovski et al. (1994), in their study of catalytic oxidation of methane over Pd/Al 203 catalysts, presented evidence for the reconstruction of the Pd-particles. Their nanodiffraction and bilk measurements showed that fresh catalysts exhibited mainly Pd(lll) surface crystal planes while after the reaction with oxygen-methane mixtures the Pd (200) surface crystal planes were developed. Here also the dispersion of the metallic phase decreased by the catalyst treatment with the reactants mixture. The results of the chemisorption and TPD measurements were used to correlate the catalytic activities. Table 4 presents the results of the chemisorption measurements for 9 some of the prepared knitted silica-fibre supported catalysts. The results for CgHg and CO adsorption on Co, Ni, Pt-Co and Pt-Ni catalysts are in agreement with the catalyst activity, the uptake being higher on the more active catalyst The combinations of Co and Ni with platinum resulted in increased uptake of propane and carbon monoxide, as compared to the Co 304 and NiO catalysts. From the chemisorption measurements (Table 4) of CjHg and CO on Pd and PdO as well as Pd-Pt and Pd0-Pt0 2 catalysts, it was not possible to explain the higher activities of the oxidized palladium-containing catalysts. The uptakes were much higher on Pd/Si0 2 than on PdO/Si0 2 catalyst, as would be expected. The amount of oxygen evolved at temperatures below 873 K obtained from the 02-TPD measurements are given in Table 5. For the Mn and Pt/Si02 catalysts, the high evolution of the oxygen is the result of oxide decompositions (Mn0 2 to Mn 203 at 773-873 K and PtOx to Pt at T > 823 K). The amount of desorbed oxygen from Pt- Co 304 was higher than that of the Co 304/Si02 catalyst However, the Pt-Ni catalyst desorbed much less oxygen than the Ni-catalyst. The 02-TPD profile of the Pt-Ni catalyst exhibited a major oxygen evolution at 903 - 1073 K (Figure 5), corresponding to the decomposition of Ni203 to NiO. This 02-desorption peak did not appear in the case of NiO/Si02 catalyst. The interaction of platinum with nickel has evidently resulted in the partial formation of nickel (HI) oxide. The state of oxygen in oxidative catalysis is of great importance. Interaction of oxygen with metal oxides can be represented by activation of oxygen resulting in the formation of highly reactive surface states (incompletely reduced active states of oxygen) which have high oxidative activity with respect to electron donating compounds. This reactive oxygen is assumed to be responsible for the complete oxidation of organic compounds. This can be followed by transition of these states into oxygen of the catalyst lattice, i.e. an additional transfer of electron to oxygen and theirinsertion into an oxygen vacancy. This oxygen is believed mostly to participate in the formation of selective oxidation products (Sokolovskii, 1987). In oxidation over noble metals, the active species is the dissociatively adsorbed oxygen. In oxidation over e.g. Co 304 at low temperatures, oxygen is weakly adsorbed. At higher temperatures, the lattice oxygen of Co 304 participates in the oxidation reaction. During the reaction, the consumed lattice oxygen is substituted by oxygen from the gas phase. Therefore, on Pt-Co 304 and Pt-Ni the combined effect of dissociatively adsorbed oxygen on Pt and the weakly adsorbed oxygen on Co 304 or NiO may result in lower light-off temperatures of these catalysts, while at higher temperatures the combined effect of adsorbed 02 on platinum and the lattice oxygen from the metal oxides might be responsible for the observed higher propane conversions at 823 K as compared to the Pt-catalyst. 10 B. Natural Gas Combustion Theresults of the light-off testing for natural gas are given in Table 6. As can be seen from Table 6, of the base metal oxide catalysts, NiO was found to be the most active in natural gas combustion. Here, as in the case of propane, a synergistic effect was observed with the combination of Pt and Co 304 resulting in improved catalyst light-off and conversion behaviour, as compared to the component catalysts (with the same metal content). However, this was not the case with the NiO and platinum combination, where the resultant catalyst exhibited a lower activity than the Ni0/Si02 catalyst As was mentioned earlier, this combination of Pt and Ni has resulted in the formation of nickel (ID) oxide which is less active in hydrocarbon oxidation. Pt-Ni203/Si02 catalyst appeared to be more active in propane than in natural gas combustion. The reason for this might be that the activation of the C-H bond in methane is more difficult than the C-H or the C-C bond activation of propane by Ni203. Combination of PdO and Pt also resulted in improved light-off behaviour with respect to the PdO/Si0 2 catalyst. The Pd-catalysts were also found to be more active in natural gas than in propane combustion, as compared to the Pt- and Pt-Co catalysts. The activity of the prepared catalysts, in terms of T50%, was found to decrease as: Pt-PdO > PdO > NiO > Pt-Co 304 > Pt-NiO > Co 304 > Pt, and the activity, in terms of the final natural gas conversion at 823 K, decreased in the order: Pt-PdO > PdO = NiO > Pt-Co 304 > Pt-NiO > Co 304 > Mn0 2 > Pt > Cr203 > CuO. The effect of increase in the 02 content of the feed mixture on the conversion behaviour of the catalysts was studied by increasing the stoichiometric ratio (X) from 0.90 to 2.00 at a constant GHSV of 23900 h"1 (T = 823 K and the natural gas flow rate was kept constant at 0.285 vol.%). With the exception of the Pt-Co 304 catalyst, the combustion efficiencies increased with increasing 02-content of the feed mixture (Figure 6). In contrast to the propane combustion, no big difference between the activity of the reduced and oxidised Pd-catalysts was observed here. The reason for this might be that the temperatures at which the catalysts exhibit considerable activity are high enough to re-oxidise the palladium and the reaction still occurs on the PdO phase. 11 4 SUMMARY High combustion efficiencies can be achieved by metal oxide catalysts, and Co 304 was found to be the most active base oxide catalyst for propane combustion while NiO was most active in natural gas combustion. In general, the light-off temperatures in natural gas combustion were higher than the corresponding light-off temperatures in propane combustion. This is due to the fact that methane (the major component of the natural gas) is the most difficult gas to combust catalytically. Combinations of the metal oxides improved the final conversions but did not improve the light-off temperatures with respect to the component base oxide catalysts. The metal oxides generally exhibit lower activities at lower temperatures as compared to the noble metals. The low-temperature activities of the metal oxide can be strongly improved by using platinum as the activator, as was evident from the combination of platinum and metal oxides. Thus one can reduce the use of the expensive noble metals in this way while still achieving high activities at low temperatures. Catalysts containing palladium exhibit higher activities in oxidised form. A re-oxidation of the initially reduced Pd-catalyst takes place during the reaction. The propane oxidation over Pd/Si0 2 catalysts is considered to be a structure-sensitive reaction. The PdO and Pt-PdO catalysts were found to be the most active catalysts in natural gas combustion. The performance of the NiO/Si02 catalyst in natural gas was very close to that of the PdO/Si0 2. However, as with the platinum-activated nickel catalysts, possible nickel emissions should be taken into account while working with Ni-containing catalysts. Increase in the oxygen content of the feed mixture, up to 20% excess air, improved the propane conversion over all the catalysts. The conversion behaviour of the catalysts in natural gas combustion was more dependent on the oxygen content of the feed mixture. REFERENCES Sinha, A. S. K and Shankar, V., (1993), Ind. Eng. Chem. Res. 32, 1061. Haber, J., Mielczarska, E., and Turek, W.,(1987), Reac. Kinet. Catal. Lett. 34, 45. Rajesh, H. and Ozkan, U. S., (1993), Ind. Eng. Chem. Res., 32, 1622,. Kapteijn, F., Stegenga, S., Dekker, N. J. J, Bijsterbosch, J. W., and Moulijn, J. A., (1993), Cat. Today, 16, 273. 12 Sokolovskii, V. D., (1990), Catal. Rev.-Sci. Eng., 32(1&2), 1. Sokolovskii, V. D., (1987), React. Kinet. Catal. Lett., 35,1-2, 337. Pitachi, R. and Klier, K., (1986), Catal. Rev.-Sci. Eng., 28, 13. Anderson, A. and Maloney, J., (1988), J. Phys. Chem., 92, 809. Anderson, A., Maloney, J., and Yu, J., (1988), J. Catal., 112, 392. Kesserling, J. P., (1986), Catalytic Combustion. In Weinberg F. J. (Ed.) Advanced Combustion Methods, Academic Press, London, p.237. Neyestanaki, A. K. and Lindfors, L.-E., (1994), Combust. Sci. and Tech. 97, 121. Ruckenstein, E. and Chen, J. J., (1981), J. Catal., 70, 233. Ruckenstein, E. and Chen, J. J., (1982), Colloid Interface Sci., 86, 1. Chen, J. J. and Ruckenstein, E., (1981), J. Phys. Chem., 85, 1606. Hicks, R. F., Qi, H., Young, M. L., and Lee, R. G., (1990), /. Catal., 122, 280. Hicks, R. F., Qi, H., Young, M. L., and Lee, R. G. , (1990), J. Catal., 122, 295. Juszczyk, W. and Karpinski, Z., (1989), J. Catal., 117, 519. O’Simone, D., Kennely, T., Brungard, N. L., and Farrauto, R.J., (1991), Appl. Catal. 70, 87. Briot, P. and Primet, M., (1991), Appl. Catal., 68, 301. Beldwin, T. R. and Burch, R., (1990), Appl.Catal., 66, 337. Beldwin, T. R. and Burch, R., (1990), Appl.Catal., 66, 359. Somorjai, G. A. and Van Hove, M. A., (1988), Catal. Lett., 1, 433. Lee, M. B., Yang, Q. Y., and Ceyer, S. T., (1987), J. Chem. Phys., 87, 2724. Gabrovski, E., Feumi-Jantou, C., Mouaddib, N., and Primet, M., (1994), Appl.Catal. A, 109, 277. 13 Table I Results of the catalyst testing in propane combustion. Propane conversion, % Catalyst GHSV = 23900 h'1, XT = 1.05 GHSV = 47800 h'1, XT = 1.05 X523 K X623 K X723 K X823 K T*50% X523 K X623 K X723 K X823 K (4.5)wt.% Co 591 3.5 64.8 80.0 82.2 620 0.4 52.0 76.1 78.9 (8.8)wt.% Co 565 12.7 72.8 86.4 90.9 597 2.8 65.7 83.9 87.5 (15.2)wt.% Co 547 24.1 83.9 92.5 95.1 575 4.5 77.0 89.1. 92.2 (3.9)wt.% Mn 644 0.0 25.6 80.6 87.0 677 0.0 12.1 78.5 83.5 (7.6) wL% Mn 621 0.9 50.8 87.2 90.3 649 0.0 28.1 83.7 87.2 (12.9)wt.% Mn 612 1.6 57.8 88.1 92.4 641 0.0 32.0 83.9 89.1 (5.2)Wt.% Cr 711 0.0 6.1 53.9 76.5 735 0.0 4.7 44.6 71.0 (9.1)wt.% Cr 699 0.0 10.5 60.3 84.7 726 0.0 7.0 48.3 80.6 (15.1)wt.% Ni 723 0.0 4.9 50.0 86.5 748 0.0 2.9 27.2 82.8 (8.7, 8.8)wt.% Co-Ni 591 4.1 68.2 89.1 94.2 619 1.2 57.6 85.7 90.1 (7.9, 4.1)wt.% Co-Cr 625 1.2 48.9 88.7 93.7 654 0.0 20.8 84.4 89.6 (8.0, 4.2)wt% Ni-Cr 634 0.0 36.1 84.8 88.9 661 0.0 13.2 81.7 85.6 (6.5,6.8,3.8)wt%Co-Ni-Cr 609 0.8 64.2 92.9 96.3 636 0.0 34.7 89.8 93.1 t Stoichiometric ratio, X, defined as the airlfuel ratio normalised to the stoichiometric airlfuel ratio, f Light-off temperature, Tso%, defined as the temperature before the catalyst at which 50% fuel conversion take place. 14 Table H Results of the catalyst testing in propane combustion. Propane conversion, % Catalyst GHSV = 23900 h"1, X= 1.05 GHSV = 47800 h’1, X = 1.05 Tso% X523 K X623 K X723 K X823 K T50% X523 K X623 K X723 K X823 K (0.3)wL% Pt 548 3.3 86.2 92.7 92.9 565 1.4 84.0 90.8 90.8 (0.95)wL% Pt 479 82.5 91.2 94.8 94.8 497 79.1 89.9 93.3 93.3 (0.59, 9.1)wt% Pt-Co 465 92.2 95.4 96.1 96.1 482 87.7 93.5 94.8 94.9 (0.71, 13.9)wt.% Pt-Co 463 93.5 98.1 99.1 99.1 481 90.4 96.5 97.7 98.1- (0.68, 8.9)wt% Pt-Ni 474 91.4 96.7 97.8 97.8 490 85.5 94.9 96.5 96.7 (0.73, 14.3)wt.% Pt-Ni 468 93.7 97.1 97.7 98.0 482 90.6 95.7 96.6 96.9 (3.9)wt.% Pd 554 2.1 93.9 97.1 97.1 569 1.1 92.6 95.9 95.9 (3.9)wt.% Pd* 660 0.0 11.8 89.2 92.3 668 0.0 7.1 86.1 91.2 (3.3, 0.27) wL% Pd-Pt 541 5.9 96.0 97.8 97.8 559 1.2 94.8 96.5 96.6 (3.3, 0.27)wt.% Pd-Pt* 667 0.4 12.5 88.1 94.9 674 0.00 10.2 86.0 93.5 *- Reduced catalyst. 15 TABLE m The kinetic parameters for the catalytic oxidation of propane Conditions: C3HS,% 02, %0 2/Cy/5 a) variable 1.49 5.25 - 14.96 b) 0.285 variable 5.25 - 8.75 Total flow rate = 5000 mllmin, GHSV = 47800 h'1 Ea, Temperature Range, Temperature*, In A, Catalysts Reaction Orders kJ.mol" 1 K K kmol.kg-cat^.s" 1 0% qHg (15.0)wt.% Co 0.19 0.69 92.35 513 - 543 513 11.33 (13.2)wt.% Mn 0.44 0.75 93.49 548 - 583 548 11.27 (9.1)wt.% Cr 0.17 0.88 86.24 603 - 658 603 7.65 (15.2)wt.% Ni 0.43 0.71 75.49 618 - 658 618 5.64 (3.8)wt.% Pd 0.00 1.00 86.88 538 - 553 538 10.24 (0.91)wt.% Pt -0.01 1.02 84.02 468 - 483 468 12.55 (0.73 , 14.2)wt.% Pt-Co 0.00 0.99 84.88 453 - 473 453 13.03 (0.75, 14.4)wt.% Pt-Ni 0.00 1.03 72.91 458 - 473 458 9.92 * - Temperature at which the reaction orders were measured. 16 TABLE IV Results of the chemisorption measurements Uptake of Catalysts CsHg CO Dispersion cm3/g metal cm3/g metal % (15.2)wt.% Co 0.306 2.410 (15.1)wt.% Ni 0.232 0.640 (0.71, 13.9)wt.% Pt-Co 0.464 2.466 (0.73, 14.3)wt.% Pt-Ni 0.363 0.708 (3.9)wt.% Pd 0.613 2.672 (3.9)wt.% Pd** 1.690 13.65 13.7 (3.3, 0.27)wt.% Pd-Pt 0.640 1.790 (3.3, 0.27)wt.% Pd-Pt** 0.941 8.482 12.2 (0.95) wt.% Pt** 29.6 *- Determined from the quantity of H2 adsorbed on reduced catalysts, assuming the dissociative adsorption ofH2. **- Reduced catalyst. 17 TABLE V Results of the 0%-TPD measurements 02 evolved below 873 K Catalyst pmol/g metal pmol/g catalyst (13.2)wt.% Mn 624.6* 82.45 (0.91)wt.% Pt 1109.9** 10.10 (3.8)wt.% Pd 75.3 2.86 (0.73, 14.2)wt.% Pt-Co 53.7 8.01 (15.0)wt.% Co 46.8 7.02 (15.2)wt.% Ni 39.4 6.00 (0.75, 14.4)wt.% Pt-Ni 21.7 3.29 * - Mn02 decomposes to Mn203. ** - PtOx decomposes to Pt. 18 TABLE VI Results of the light-off testing in natural gas combustion (GHSV=23900 h'\ 1=1.05). Natural Gas Conversion, (X, %) Catalyst Tso%, K X673 K ^723 K ^823 K (9.8)wt.% Cu >823 1.6 1.4 4.0 (9.1)wt.% Cr >823 4.0 5.0 10.3 (12.9)wt.% Mn >823 9.7 13.2 34.7 (15.2)wt.% Co 799 12.8 20.0 60.3 (15.1)wt.% Ni 756 8.0 19.0 88.5 (0.95)wt.% Pt 830 5.1 8.0 34.3 (3.9)wt.% Pd 717 21.0 53.1 88.6 (0.73, 14.3)wt.% Pt-Ni 796 7.8 11.0 74.0 (0.71, 13.9)wt.% R-Co 779 11.0 19.4 84.7 (3.3, 0.27)wt.% Pd-Pt 688 37.0 71.1 89.9 19 t* Outlet FIGURE 1. The experimental setup for catalyst testings. FIGURE Propane conversion, % 2. propane ; Dependence oxygen - 0.7 a - (12.9)wt% content flow 0.9 of rate the Stoichiometric of 1.1 Mn was the combustion ; kept -v- feed 1.3 (9.1)wt% constant): mixture 1.5 efficiencies (T= Cr. -o-(15.2)wt% ratio, 1.7 723 in K, 1.9 X propane GHSV=47800 Co; 2.1 combustion -D- 2.3 (15.1)wt.% h" 1 and 2.5 on the the Ni 100 T T Temperature before the catalyst, K FIGURE 3. The light-off curves of the (3.1) wt.% Pd/Si0 2 catalyst: -o- fresh catalyst (reduced); -D- aged catalyst, aged under the reactant flow at 823 K for six hours (total flow rate = 5000 ml/min, GHSV=47800 h'1,1=1.05). I FIGURE Propane conversion, % 100 20 40 60 80 0 473 4. L - The hours (oxidised); 523 light-off (total T Temperature -D- flow curves 573 aged rate catalyst, of = the 5000 623 before (3.2) aged ml/min, wt.% under 673 the T GHSV=47800 Pd/Si0 the catalyst, reactant 723 2 catalyst: h' flow 1 K , 773 A, -o- at = 823 fresh 1.05). K cataiyst 823 for six I ! Pt-Ni/Si02 Temperature, K FIGURE 5. The 02-TPD pattern of the Ni and Pt-Ni catalysts. 0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 2.3 2.5 Stoichiometric ratio, X FIGURE 6. Dependence of the combustion efficiencies in natural gas combustion on the oxygen content of the feed mixture (T = 823 K, GHSV = 23900 h"1 and the natural gas flow rate was kept contant): -o- (15.2)wt.% Co ; -D-(15.1)wt.%Ni ; -a- (12.9)wt.% Mn; -v- (0.95)wt.% Pt; -0- (3.9)wt% Pd ; -•-(0.71, 13.9)wt.% Pt-Co ; -■- (0.73, 14.3)wt.% Pt-Ni. Lutterworth Fuel Vol 74 No. 5, pp. 690-696, 1995 %jEIN E~M~~A N N Copyright © 1995 Elsevier Science Ltd 0016-2361(94)00015-8 Printed in Great Britain. All rights reserved 0016-2361/95/S 10.00+0.00 Catalytic combustion of propane over Ft and Cu modified ZSM-5 zeolite catalysts Ahmad Kalantar Neyestanaki, Narendra Kumar and Lars-Eric Lindfors Abo Akademi University, Laboratory of Industrial Chemistry, Biskopsgatan 8, FIN-20500 Abo, Finland (Received 9 March 1994; revised 7 June 1994) ZSM-5 zeolites were modified with copper and platinum by ion exchange and during the process of hydrothermal zeolite synthesis. The catalytic activity of the prepared Pt and Cu ZSM-5 zeolite catalysts was tested for complete oxidation of propane. It was observed that the mode of introduction of metals into the zeolite and the metal content influenced the catalytic activity. Copper introduced into the zeolite by ion exchange under basic conditions exhibited a high catalytic activity. Combustion efficiencies of 98.9 and 97.6% were achieved with the Cu ZSM-5 and Pt ZSM-5 catalysts containing 5.2 wt% Cu and 0.27 wt% Pt, respectively. The light-off temperature of the platinum containing catalyst was about 30 K lower than that of the 5.2 Cu ZSM-5 catalyst. The Cu ZSM-5 prepared during the process of hydrothermal synthesis exhibited a lower catalytic activity as compared to the ion exchanged Cu ZSM-5 catalysts. Temperature programmed desorption (TPD) measurements of CO and 02 were used to interpret the catalytic activity. The results indicated the important role of the extra lattice oxygen on these catalysts. (Keywords; catalytic combustion; propane; zeolites) There has been considerable academic and industrial cations. This is followed by a secondary step in which research in zeolite catalysis in the last decades. Most of the reduction occurs: the extra lattice oxygen is extracted, this research is related to the acid-base functions of thus reducing the cation. COz and H20 are formed in zeolites and less attention has been paid to their redox the process 1 *. functions, which also seem to be very important. Surface In this paper a study of copper and platinum modified modification of the zeolites by different metals enables ZSM-5 type zeolite catalysts for complete oxidation of them to be used in several other non-acid catalysis propane is reported. reactions, such as hydrocarbon oxidation and reduction, olefin oligomerization, carbonylation, hydroformulation and synthesis gas conversion 1,2. EXPERIMENTAL The use of catalysts for total oxidation of hydrocarbons offers several advantages, including the fact that the Catalyst preparation emissions of CO, NO* and unburned hydrocarbons can Platinum and copper modified ZSM-5 zeolite catalysts be almost completely prevented by using a well for propane oxidation were prepared by the following tailored catalyst formulation. Catalysts used are mostly methods: alumina or silica-fibre supported noble metals and 1. ion exchange of Na ZSM-5 with copper, and transition metal oxides. Noble metals exhibit activities 2. introduction of copper and platinum into the zeolite at lower temperatures than metal oxides. However, at during the process of synthesis. temperatures above 673 K their activities are about the same3. There has been some publications in which The parent Na ZSM-5 zeolite used for copper ion transition and noble metal exchanged zeolite catalysts exchange was synthesized in the laboratory as in were used for oxidation reactions. These studies cover Reference 14, with some modifications. The reagents used the investigation of redox properties of cation exchanged were sodium silicate solution (Merck, 28.5 wt% Si02, zeolites for carbon monoxide and hydrocarbon oxidation 8.8 wt% Na20, 62.7 wt% H20) as a source for silica, over X and Y type zeolites 4-13. However, not much aluminum sulfate (Merck) as a source for alumina and attention has been paid to the redox properties of cation tetrapropylammoniumbromide (Merck) as the organic exchanged high silica zeolites in complete hydrocarbon template. The synthesized zeolite was washed, filtered oxidation reactions. and dried at 383 K, followed by calcination at 813 K for In zeolite catalysis, redox reactions are believed to 8 h. occur via a mechanism involving separate oxidation and Copper was introduced into Na ZSM-5 by ion reduction steps. According to this mechanism, oxidizing exchange under basic conditions at pH 7.2, 8.0 and 8.8. gases such as Oz react with the exchanged cations in An aqueous solution of copper nitrate was used for the zeolite, oxidize them and deposit extra lattice 02 on the ion exchange and the pH of the zeolite-copper nitrate 690 Fuel 1995 Volume 74 Number 5 Catalytic combustion of propane: A. K. Neyestanaki et al. Table 1 Effect of pH of the aqueous Cu(II) nitrate solution on metal Temperature programmed desorption (TPD). For TPD uptake measurements of Oz and CO, the samples, after initial oxidation or reduction in 02 or H2 flows at 773 K with Copper content subsequent outgassing at 773 K and «10-4 Pa for 1 h, Catalyst pH wt% mmolg' 1 zeolite were treated with 02 (99.998% purity) or CO (99.97% purity) at 298 K for 1 h. The TPD measurements were Cu ZSM-5 7.2 2.80 0.453 carried out under vacuum («2.1 x 10-4 Pa) and the Cu ZSM-5 8.0 3.60 0.588 Cu ZSM-5 8.8 5.20 0.863 temperature was linearly increased from 298 to 823 K at a heating rate of 10 K min-1. The desorbed gases were analysed using a Carlo Erba QTMD mass spectrometer (m.s.). In all the runs the m.s. was conditioned for 45 min suspension was kept constant at the desired level by in order to desorb physically adsorbed gases. adding 3 M NH^OH solution. After ion exchange, the sample was washed thoroughly and filtered, followed by Catalyst performance testings drying at 353 K for 12 h. The experimental setup for catalyst testings consisted In the second method copper and platinum were of pressure regulators, filters, mass flow controllers, a introduced into the Na ZSM-5 zeolite by adding required preheater, a tube furnace and a tube reactor in which the amounts of Cu(N03)2-6H20 and H2PtCl6-xH20 directly catalyst sample was placed. A ceramic bed was placed during the process of zeolite synthesis. After completion into the reactor in order to improve gas mixing. The of the hydrothermal synthesis, the zeolites were washed catalysts were tested by burning a mixture containing with deionized water, dried at 383 K, followed by 0.285 vol% propane (99.5% purity), 92.92 vol% N2 calcination at 813 K for 8h. These catalysts will be (99.9% purity) and 6.8 vol% air. The total flow rate (at denoted Cu ZSM-5-DS and Pt ZSM-5-DS. The Si/Al standard conditions) was 5000 ml min-1 and the space ratio of all the prepared zeolite catalysts was %40. velocity, WHSV, was 165 h- *. In some of the experiments the catalysts were tested at space velocities of 330 and Catalyst characterization 660 h-1 (total flow rate=5000 ml min - *). Air and The prepared zeolite catalysts were characterized by nitrogen flows were connected after the mass flow meters the following techniques: and heated in the preheater. Catalyst granules were packed in a tube with a diameter of 1.4 cm, which in turn X-ray powder diffraction (XRD). These measurements was placed in the reactor. Temperatures were measured were carried out for determining the phase purity of the before and after the catalyst bed. The analyses of prepared zeolites, using a Philips PW 1830 with Cu Ka hydrocarbons, CO and C02 were performed using a radiation. XRD of all the prepared zeolite catalysts TEN3 nqn-dispersive infrared (NDIR) gas analyser. Data exhibited very high percentage of crystallinity and it was acquisition from the analyser and the temperature also observed that the framework of the ZSM-5 zeolites measurements were carried out by a personal computer. was not affected by the introduction of metal during the Prior to the light-off testings, the catalyst samples were process of hydrothermal synthesis. heated in a dry air flow of 1000mlmin-1 for 30min at 623 K and for 120 min at 813 K. The catalysts were then Scanning electron microscopy (SEM). The morphology cooled down to 423 K in air flow. After stabilization of of the ZSM-5 zeolite catalysts was studied using a Leica the temperature, the gas mixture was fed to the reactor Cambridge Stereoscan 360. SEM micrographs of the and the tube furnace temperature was raised from parent Na ZSM-5 and metal modified ZSM-5 (either by 423 to 873 K at a heating rate of 12 K min-1 (the ion exchange or during the process of synthesis) exhibited preheater temperature was kept constant at 623 K) the typical crystal size and form of ZSM-5 zeolites. and the temperatures and conversions were registered continuously. The combustion efficiency was calculated X-ray fluorescence (XRF) analysis. The bulk metal by carbon balance across the reactor. As a result of these content of the catalysts was measured by an X-MET 880 experiments, curves of propane conversion as a function (Outokumpu) XRF analyser. The catalysts were dissolved of temperature (light-off curves) were drawn which in acids (HF and aqua regia) prior to their metal content enabled measurement of the temperature at 50% propane measurements. It was found that for copper ion conversion (light-off temperature) and the final propane exchanged zeolites, an increase in the pH of the conversions at either 773 or 823 K. The catalysts were ion exchange mixture resulted in increased copper uptake tested at stoichiometric ainfuel ratio, A = 1.00, and at 5% (Table 1). excess air, A =1.05. The light-off testings were also performed for the initially reduced catalysts. The 02 chemisorption. Oxygen chemisorption measure reduction of the catalysts was carried out at 813 K for ments were carried out using a Carlo Erba Sorptomatic 2.5 h in a 150 ml min-1 hydrogen flow. 1900. The adsorption isotherms were obtained at 298 K and pressures of 0.133-13.33 kPa. Extrapolation of the adsorption isotherms to zero pressure was used for RESULTS AND DISCUSSION the determination of the irreversibly adsorbed gases. The amounts of the reversibly adsorbed gases were The results from the light-off testings are presented in determined by backsorption. Prior to the chemisorption Tables 2 and 3 and in Figures 1 and 2. Conversions (X, %) measurements the samples were outgassed at 723 K for are given at temperatures, T, of 623, 723, 773 and 823 K 1 h at 10-4 Pa. The reduction of the catalysts was (reproducibility wl%). As can be seen in Tables 2 performed at 773 K in H2 flow for 2 h. and 3, an increase in the copper content of the catalysts Fuel 1995 Volume 74 Number 5 691 Catalytic combustion of propane: A. K. Neyestanaki et al. Tabic 2 The light-off and conversion behaviour of the oxidized catalysts, WHSV = 165 h-1 Propane conversion, X (%) Metal content 1.00 1.05 Catalyst wt% mmolg -1 zeolite w -^623K ■X723K ^773K W •*623K ■^723 K ^773K Pi ZSM-5-DS' 0.27 0.014 521 96.6 97.3 503 96.4 97.6 Cu ZSM-5-DS' 2.10 0.337 724 10.7 49.5 66.4 718 12.5 50.9 67.0 Cu ZSM-5 2.80 0.453 644 35.5 85.0 92.7 633 44.1 90.7 95.3 Cu ZSM-5 3.60 0.588 611 59.5 88.8 93.1 583 80.2 94.2 96.8 Cu ZSM-5 5.20 0.863 548 97.0 98.1 98.3 541 96.9 98.7 98.9 Cu ZSM-5' 5.20 0.863 585 95.1 96.8 96.9 583 95.0 97.2 97.2 Cu ZSM-5“ 5.20 0.863 674 18.6 72.0 85.0 649 26.0 81.8 90.1 “Stoichiometric ratio, z, defined as the air:fuel ratio normalized with respect to the stoichiometric air:fuel ratio “ Light-off temperature, rs0.z. (K), defined as the temperature at which 50% propane conversion takes place ’ Metal was introduced into zeolite during the process of hydrothermal synthesis 'WHSV=330h-1 “ WHSV=660 h"‘ Table 3 The light-off and conversion behaviour of the H2 treated catalysts, WHSV = 165 h-1 Metal content Propane conversion, X (%) at zfl=I.OO Catalyst wt% mmolg -1 zeolite 7so% b •^623K •X?23K -X773K ^823K Pt ZSM-5-&S' 0.27 0.014 578 93.1 95.5 97.4 Cu ZSM-5-DSf 2.10 0.337 817 0.5 14.8 30.0 51.9 Cu ZSM-5 2.80 0.453 744 5.5 41.2 68.2 88.9 Cu ZSM-5 3.60 0.588 721 11.5 51.2 73.0 90.4 Cu ZSM-5' 5.20 0.863 688 10.1 84.5 96.8 96.9 “Stoichiometric ratio, 7, defined as the air:fue! ratio normalized with respect to the stoichiometric airtfuel ratio 6 Light-off temperature, TJ0% (K), defined as the temperature at which 50% propane conversion takes place ’ Metal was introduced into zeolite during the process of hydrothermal synthesis 'WHSV=330h-1 Temperature before the catalyst (K) Temperature before the catalyst (K) Figure 1 Light-off curves of the oxidized catalysts tested at stoichiometric airtfucl ratio and WHSV = 165 h"(-Q-) 2.8Cu ZSM-5, (-#-) 3.6Cu Figure 2 Effect of residence time on the light-off and conversion ZSM-5. (-V-I 5.2Cu ZSM-5, (-▼-) 0.27Pt ZSM-5-DS, (-□-) 2.1Cu behaviour of the 5.2Cu ZSM-5 catalyst tested at 7.= 1.05. WHSV: ZSM-5-DS (-0-1 165 h' 330 h-'. (-V-) 660 h-1 692 Fuel 1995 Volume 74 Number 5 10 Catalytic combustion of propane: A. K. Neyestanaki et al. resulted in increased propane conversion (to COz Table 4 The apparent activation energies, E, and K0m/F of the and H,0) and reduced light-off temperature. The prepared catalysts at 7.= 1.00 (the K0m/F values were calculated at 623 K) highest propane conversion, at T= 723 K, was obtained with the ion exchanged and oxidized Cu ZSM-5 Oxidized catalysts Reduced catalysts catalyst containing 5.2 wt% Cu, i.e. 5.2Cu ZSM-5, exhibiting conversions of 98.1 and 98.7% at 7.= 1.00 and E E Catalyst 1.05, respectively. The propane conversions over the (kJ mol -1) K ju/F (kJmor 1) K„m/F Pt ZSM-5-DS catalyst containing 0.27 wt% Pt, i.e. 2.lCu ZSM-5-DS 57 0.1144 78 0.0051 0.27 Pt ZSM-5-DS, were slightly lower than those 2.8Cu ZSM-5 80 0.4453 72 0.0574 obtained with the 5.2Cu ZSM-5 catalyst, although its 3.6Cu ZSM-5 94 0.9204 53 0.1234 light-off temperature was approximately 30 K lower. 5.2Cu ZSM-5 143 1.1585" 106 0.1087' 0.27PI ZSM-5-DS 196 1.6309" 207 0.0238" An increase in the oxygen content of the feed mixture (from 7. = 1.00 to 1.05) resulted in improved light-off "At 553K behaviour of all the catalysts, as well as their final propane "At 533 K conversions (Table 2). Thiseffect on the final conversions 'At \VHSV = 330h-‘ was more significant with the ion exchanged catalysts having lower copper content (catalysts containing 2.8 and 3.6 wt% Cu) and also with the high Cu-containing catalyst tested at decreased residence time (5.2Cu ZSM-5 rate of the reactants, respectively, and R is the gas tested at WHSV = 660 h-1). constant. The activation energies, £, were calculated from The effect of the residence time on the light-off and the linear part of the Arrhenius plot. At high propane conversion behaviour of the catalysts was studied with conversions (>75%) deviations were observed which can be attributed to diffusional phenomena. K0m/F is a the 5.2Cu ZSM-5 catalyst. As presented in Table 2 and Figure 2, the increase of the space velocity resulted in a dimensionless constant which is proportional to the considerable increase of the light-off temperatures and a number of active sites in the catalyst as suggested by Hoy os et al.15 . The E (kj mol -1) and the K 0m/F values decrease of propane conversion at 773 K. The light-off temperature of the catalyst for propane combustion are given in Table 4. The K0m/F values were calculated increased from 548 K (WHSV = 165 h-1) to 674 K from the Arrhenius equation at the reference temperature (WHSV = 660 h-1). The propane conversions at 773 K of 623 K and were used for comparing the catalytic over the catalyst tested at WHSV = 165 and 330 h”1 were activity of the prepared catalysts. The K0m/F values quite close to each other. Further decrease of the residence indicate an increase in the number of active sites with time (WHSV = 660 h-1) resulted in considerable slippage increasing Cu loadings of the ion exchanged zeolite of propane and the final combustion efficiencies were catalysts. Hydrogen treatment of the catalysts, on the therefore much lower. other hand, strongly decreased the number of active The Cu ZSM-5 catalyst prepared during the process centres in the catalysts, as was indicated by the sharp of zeolite synthesis exhibited a lower catalytic activity as decrease in the K0m/F values. The 2.1Cu ZSM-5-DS was compared to the ion exchanged catalysts. The parent Na found to contain the least number of active centres. The ZSM-5 and H ZSM-5 zeolite tested in propane oxidation observed wide variations in the activation energies were exhibited a very poor catalytic activity. strongly compensated by the Kam/F values. Copper ion exchanged zeolites exhibit oxygen carrier Table 3 presents the results from the light-off testings of the hydrogen treated catalysts. A comparison of behaviour. The Cu-zeolites can be reduced with CO to Tables 2 and 3 reveals that the light-off temperatures produce C02, thus removing oxygen from the lattice. Jacobs and Beyer16 described the process as partial strongly deteriorated by hydrogen treatment of the catalysts. However, the final conversions (at 823 K) of dealumination of the lattice, resulting in the formation of Cu+ and A10+ by the reduction of the Cu2+-zeolite propane were close to those obtained with the air treated catalysts (at 773 K). It was found that reoxidation of the withCO. In this way the lattice becomes charge balanced catalysts by reactants takes place during the light-off by equal amounts of Cu+ and A10+. By reoxidation, the Cu+-ions are converted into Cu2+-0-Cu2+ species, testings. The reduced catalysts after being tested at stoichiometric ainfuel ratio (7.= 1.00) were cooled down while the A10+ concentration remains unchanged. The in 99.999% N2 flow and tested again. The resultant redox capacity of Cu ZSM-5 and Cu-Y is found to be light-off temperatures and conversions were quite close *0.5 O/Cu, i.e. 1 e/Cu, and this could be attributed to reduction of Cu2+ to Cu+ 12-17,18 . It has been shown to those obtained with air treated catalysts, indicating the reoxidation of the catalysts during the light-off that upon evacuation or flashing at high temperatures measurements. with He, the initially oxidized Cu ZSM-5 catalyst can The activation energies of the reaction were calculated undergo self reduction with spontaneous desorption of O,18 '19. Petunchi et al.20 in their XRD and Si, Al magic from the fractional propane conversion, X, by assuming the reaction to be first-order with respect to propane and angle spinning nuclear magnetic resonance (MAS n.m.r.) zero-order with respect to oxygen. Due to the high study of the Cu-Y zeolites showed that the Cu-Y (and conversion levels the reactor was considered an integral possibly ZSM-5) zeolite catalysts were not extensively plug flow reactor which yields the following equation 15: dealuminated on reduction with either CO or H2 at 773 K. Evidence was presented that the extra lattice ln[ — ln( 1 — A")] = — E/RT+ In (K „m/F) oxygen (ELO) is introduced into the copper ion exchanged ZSM-5 zeolites during the preparation step where X, m and F are the fractional propane conversion, and that an important part of it is held bridged between mass of the zeolite catalyst and the volumetric feed flow two Cu2+ ions ([Cu-O-Cu] 24-)18 -21-22. Fuel 1995 Volume 74 Number 5 693 Catalytic combustion of propane: A. K. Neyestanaki et al. Table 5 Results of 02-chemisorption measurements for oxidized and reduced catalysts Oxidized catalyst Reduced catalyst Catalyst cm3g 11 metal cm3 g_l zeolite cm3g 11 metal cm3g “‘ zeolite 5.2Cu ZSM-5 2.45 0.134 13.32 0.731 2.1Cu ZSM-5-DS" 1.38 2.90x10“ 2 8.39 0.180 “ Metal was introduced into zeolite during the process of hydrothermal synthesis The authors’ results (Tables 1 and 2) showed that an increase in the pH of the ion exchange mixture by addition of NH4OH resulted in increased copper uptake as well as improved catalytic behaviour. An increase in the pH results in cation hydrolysis producing Cu24(OH) species, upon dehydration of which Cu-O-Cu bridges are formed. The ionization property of thezeolite can be responsible for the Cu-ion hydrolysis in zeolites. Therefore, by increasing pH (up to a certain level) the concentration of bridged Cu-O-Cu species can be increased, which 718 K besides the copper content is a reason for the increase in 623 K the catalytic activity of the ion exchanged Cu ZSM-5 catalysts. The formation of copper-ammonia complexes, [Cu (NH 4)4]2+, can also take place at increased pH. Precipitation of Cu(OH)2 in the zeolite channels and on the exterior surface, the thermal decomposition of which will result in the formation of CuO, cannot either be excluded. The metal content is an important factor affecting the catalytic activity. However, not just the metal uptake in the ZSM-5 zeolite is responsible for catalytic activity, but also the position of the metal in the zeolite matrix and Temperature (K) its coordination geometry within the zeolite structure are Figure 3 TPD of oxygen from the oxidized Cu ZSM-5 catalysts: curve of great significance. It has been proposed that the most a, 5.2Cu ZSM-5, curve b, 2.1Cu ZSM-5-DS coordinatively unsaturated square planar Cu2+ cations are more active in oxidation reactions than the square pyramidal coordination of Cu(II) ions in the zeolite reoxidation of Cu4 to Cu24 with oxygen, establishing structure23. the Cu24/Cu4 redox cycle; C02 and HzO are formed in The Oj-chemisorption measurement results are given the process. in Table 5. The results showed that the 2.1Cu ZSM-5-DS CO-TPD patterns of the oxidized and reduced catalysts adsorbed *40% less oxygen g -1 Cu, in both the are presented in Figure 4a-d. With oxidized catalysts a oxidized and reduced state, than the ion exchanged (at considerable amount of C02 was formed, indicating the pH = 8.8) 5.2Cu ZSM-5 catalyst. A possible reason for importance of the lattice oxygen on these catalysts. The this might be that not all the copper on the 2.1Cu maximum C02 formation was observed with the ion ZSM-5-DS catalyst is exposed to the oxygen. The 02 exchanged 5.2Cu ZSM-5 (Figure 4a), exhibiting a major TPD pattern of this catalyst (Figure 3, curve b) was also C02 peak in the range 432-673 K, corresponding to the different from that of the ion exchanged 5.2Cu ZSM-5 second 02-desorption peak (Figure 3, curve a), and a catalyst (Figure 3, curve a). The oxygen desorption from smaller C02 peak in the range 343-423 K, corresponding theCu ZSM-5-DS catalyst took place at T> 653 K, while to the first Oz-desorption. With all the oxidized catalysts the ion exchanged catalyst exhibited 02 desorption peaks no induction period for C02 formation was observed. In with maxima at 343, 623, 718, and one still increasing order to compare the relative degree of participation of above 823 K (Figure 3, curve a). The low temperature the ELO in CO oxidation in the authors’ CO-TPD peak is molecularly chemisorbed oxygen, and those measurements, the ratio of the areas under the C02 and desorbed at higher temperatures are atomically bound CO peaks was calculated. The area ratios, AC0JAC0, were extra lattice oxygen (it must be noted that a fraction of found to be 1.031, 0.252 and 0.438 for 5.2Cu ZSM-5, ELO might have been desorbed during the evacuation 2.1Cu ZSM-5-DS and 0.27Pt ZSM-5-DS catalysts, of the sample prior to TPD measurement). The oxygen respectively. The ion exchanged Cu ZSM-5 catalyst, desorbed at T> 823 K is probably due to loss of therefore, contains most ELO. The C02 formation oxygen from the lattice24. The results of the Q2-TPD on the reduced catalysts was very low due to the measurements of these catalysts can be related to their prior reduction of ELO sites (Figure 4d). Since the catalytic behaviour; in both catalysts the oxygen desorption Cu ZSM-5-DS catalyst exhibited much lower activity in starts in the catalyst’s light-off region (Figure 1). Here, propane combustion, as well as in reduction by CO the extra lattice oxygen is extracted by propane, thus (Figure4b), one may assume that very few [Cu-O-Cu]24 reducing Cu2+ to Cu4. This is followed by the species are available on this catalyst, and the redox cycle 694 Fuel 1995 Volume 74 Number 5 Catalytic combustion of propane: A. K. Neyestanaki et al. .= 40!- .=? 30 • Temperature (K) Temperature (K) Temperature (K) Temperature (K) Figure 4 TPD of carbon monoxide: (a) oxidized 5.2Cu ZSM-5. (b) oxidized 2.1Cu ZSM-5-DS, (c) oxidized 0.27Pt ZSM-5-DS. (d) reduced 5.2Cu ZSM-5 will occur with CuO rather than with [Cu-0-Cu]2+. and Slinkin 25, who suggested that the change in the The absence of 02 desorption at 573 Fuel 1995 Volume 74 Number 5 695 Catalytic combustion of propane: A. K. Neyestanaki et al. to reach the conversion levels exhibited in its oxidized that zeolite dealumination, formation of extra lattice state (at T= 773 K). The reoxidation of the [Cu-O-Cu] 2+ oxygen, as well as migration of the cations, are all species is very fast, therefore on ion exchanged catalysts necessary for complete hydrocarbon oxidation. which contain more [Cu-O-Cu] 2 + species, the catalyst reoxidation takes place at much lower temperatures. However, in the case of Cu ZSM-5-DS, the authors believe that most of the copper is present as CuO which REFERENCES is strongly held by the zeolite and its reduction-oxidation 1 Elcy. D. D.. Pines, H. and Weisz, P. B. Adc. in Calal. 1982.31.2 is more difficult. Further temperature programmed 2 Kallo, D. and Minachev, Kh. M. "Catalysis on Zeolites ’. reduction and temperature programmed oxidation of Akademiai Kiad, Budapest. 1988, p. 489 these catalysts might give more information concerning 3 Blazowski. W. S. and Walsh. D. E. Combust. Sci. Tec/mol. 1975, 10. 233 their oxidation-reduction behaviour. 4 Firth, J. G. and Holland. H. B. Trans. Faraday Soc. 1969.65,1891 The behaviour of the Pt ZSM-5-DS catalyst is quite 5 Mochida. I.. Hayata, S.. Kato. A. and Seiyama. T. J. Calal. 1970. interesting. The catalysts exhibited a higher activity in 19.405 oxidized form than in reduced form. This is not quite 6 Mochida, I., Hayata. S.. Kato. A. and Seiyama. T. J. Calal. 1971. 23.31 usual for the platinum catalysts, since they are known to 7 Rudham, R. and Sanders. M. K. J. Calal. 1972,27, 287 be more active in reduced form than in oxidized (PtOz) 8 Leith,I. R. and Leach. H. F. Proc. Row Soc. London 1972, A330. form 26. The authors assume that the PtOx particles are 247 located in the ZSM-5 zeolite channels and intersections, 9 Benn, F. B., Dwyer, J., Esfahani, A_ Evmerides, N. P. and Szczepura, A. K. J. Calal. 1977,48, 60 and act as active centres for propane oxidation (it should 10 Fletcher, P„ Lower, P. R. and Townsend, R. P. Spec. Publ. be noted that calcination at 813 K can be sufficient to Client. Soc. 1980,33, 353 reduce some platinum to the metallic state). Upon 11 Petunchi. J. O. and Hall, W. K. J. Calal. 1982,78, 327 reduction, platinum atoms gradually migrate to the 12 Petunchi, J. O. and Hall, W. K. J. Calal. 1983,80,403 external surface of the ZSM-5 zeolite where they form 13 Takahashi, N.. Saito, M.. Nagumo. M. and Mijin, A. Zeolites 1986. 6. 420 large platinum particles, resulting in decreased catalytic 14 Plank, C. J.. Rosinski. E. J. and Schwartz. A. B. British Patent activity. The behaviour of the Pt ZSM-5 catalyst is still No. 1402981. 1974 under investigation. 15 Hoyos, L. J.. Praliaud, H. and Prime!. M. Appl. Calal. A 1993. 98, 125 16 Jacobs, P. A. and Beyer, H. K. J. Pltrs. Client. 1979, 83, 1174 CONCLUSIONS 17 Valyon, J. and Hall, W. K. J. Plus. Client. 1993.97, 1204 18 Li, Y. and Hall, W. K. J. Calal. 1991, 129, 202 The results in this study show that high combustion 19 Iwamoto, M„ Yashiro, H„ Ooe, K„ Banno, Y. and Okamoto, F. efficiencies can be achieved with copper and platinum Shokubai 1991,32 (2), 91 modified ZSM-5 catalysts. For the Cu ZSM-5 catalysts, 20 Petunchi, J. O., Marcelin, G. and Hall, W. K. J. Plus. Client. 1992, 96.9967 the mode of introduction of copper to the zeolite was 21 Valyon, J. and Hall, W. K. Calal. Leu. 1993, 19, 109 found to be very important in determining their catalytic 22 Sarkany, J., d'ltri. J. L. and Sachtler. W. M. H. Calal. Leu. 1992, activity. The results indicate the important role of the 16, 241 extra lattice oxygen on the oxidation properties of these 23 Kucherov, A. V., Slinkin, A. A., Goryashenko, S. S. and catalysts. However, the exact type and function of the Slovetskaja. K. I. J. Calal. 1989, 118, 459 24 Valyon, J. and Hall, W. K. J. Calal. 1993, 143, 520 active centres is still not very well understood and requires 25 Kucherov. A. V. and Slinkin. A. A. Zeolites 1986, 6. 175 further investigation. The possibility cannot be excluded 26 Serre. C.. Garin. F„ Beiot. G. and Maire. G. J. Calal. 1993.141.9 696 Fuel 1995 Volume 74 Number 5 Catalytic Combustion of Propane and Natural Gas over Cu and Pd Modified ZSM Zeolite Catalysts A. Kalantar Neyestanaki, N. Kumar and L.-E. Lindfors Laboratory of Industrial Chemistry, Abo Akademi University, Biskopsgatan 8, FIN- 20500 Abo, Finland Abstract Copper-exchanged ZSM-5, ZSM-11 and ZSM-48 as well as Pd-ZSM-5 zeolite catalysts were prepared and tested for their light-off and _ conversion behaviour in complete oxidation of propane and natural gas. Metal modified ZSM zeolite catalysts were found to be superior to the Pd and Cu/alumina catalysts prepared for comparison. Cu-ZSM-5 (Cu: 1.3 wt.-%) appeared to be the most active of the Cu-containing ZSM catalysts exhibiting combustion efficiencies of 97.6% and 94.8% for propane and natural gas, respectively. The method of introducing Pd into the zeolite was found to affect the catalyst’s activity strongly. Pd-ZSM-5 catalysts prepared by ion-exchange were more active than when the Pd was introduced into the zeolite during the process of hydrothermal zeolite synthesis. Combustion efficiencies of 100% and 98.1% for propane and natural gas combustion were achieved with Pd-ZSM-5 (Pd: 0.9 wt.-%) prepared by the ion-exchange method. The light-off temperatures were found to increase as: Pd- ZSM-5 < Cu-ZSM-5 < Cu-ZSM-11 < Cu-ZSM-48. The redox capacities of the Cu-ZSM catalysts were determined by subjecting the catalysts to redox cycles in a micro-balance. The TPD of oxygen, carbon monoxide and ammonia was used to interpret the catalytic activities. The catalytic activity was related to the oxygen carrier capacities, the reducibility and the acidity of the catalysts. The state of copper was found to be of great importance in determining the catalytic activities in oxidation reactions. Keywords: (Catalytic combustion; propane; natural gas; Cu-ZSM; Pd-ZSM-5) 1. Introduction Catalytic combustion is considered to be an effective approach for controlling the emissions of hydrocarbons, carbon monoxide and nitric oxides. High combustion 1 efficiencies can be achieved at temperatures and concentrations much lower than those required for a stable homogeneous gas phase reaction to proceed. Complete oxidation at low combustion temperatures results in minimum thermal-NOx formation and better fuel efficiency. The noble metals (Pt, Pd, Rh) and metal oxides (oxides of Ni, Cu, Co, Cr, Mn) supported on alumina and silica have been studied intensively for the catalytic combustion of hydrocarbons [1-7]. Metal modified X ,Y and mordenite zeolite catalysts have been studied for the complete oxidation of hydrocarbons [8-11]. However, there are only few investigations about this type of reaction over metal-modified ZSM series of zeolites. Modified ZSM series zeolite catalysts are of interest as possible oxidation catalysts. The catalytic activity of these catalysts depends on the type of zeolite structure, method of catalyst preparation and also on the pretreatment of the catalysts. However, there are still many unresolved questions such as the location of the metal, the role of the zeolite structure, the effect of particle size of metal and salt solutions used for the catalyst preparation. As compared to the other supports such as alumina and silica, the ZSM zeolite has the following advantages: ZSM zeolites with the same structures but different alumina concentrations can be prepared and theresulting zeolite with a given structure will have different acidic properties. ZSM zeolites have a high internal surface area (250-450 m2/g) and different types of channels and channel intersections where the catalytic active sites are possibly located and in the process of reaction the reactants are adsorbed and the reaction takes place. Since ZSM zeolites have fixed dimensions the metal particle size distribution can be controlled in an appropriate way. Cu-exchanged zeolites have oxygen carrier behaviour in a redox cycle Cu+ /Cu2+. The Cu2+ ions are exchanged into the zeolite and they must be reduced before the redox cycle can commence. It is known that the Cu-Y zeolites can be reduced by CO and produce C02. The process has been described as the partial dealumination of the lattice. Cu+ and A10+ are formed by the reduction of Cu2+-Y with CO. In this way the lattice becomes charged-balanced by equal amounts of Cu+ and A10+. By reoxidation, the Cu+ ions are converted into Cu2+-0-Cu2+ species, while the A10+ concentrations remain unchanged and the zeolite becomes an oxygen carrier [12]. Petunchi et al. [13] in their XRD, Si and A1 MAS NMR studies of the Cu-Y zeolites showed that Cu-Y and possibly ZSM-5 were not extensively dealuminated on reduction with either CO or with H2 at 773 K. The detected dealumination was found to be much less than that required to charge-balance the lattice on the conversion of Cu2+ to Cu+. 2 Evidence was presented that extra lattice oxygen (ELO) is introduced into the zeolite during the preparation step, especially with over-exchanged zeolites (Cu/Al > 0.5), and is bound to the Cu2+sites and is independent of the lattice oxygen. This means that the catalyst as prepared is an oxygen carrier which can be reduced by CO, hydrocarbons or by the spontaneous desorption of oxygen above 623 K [14-16], Sarkany et al. [17] in their TPR/TPD and FTIR study of excessively ion-exchanged Cu/Na-ZSM-5, concluded that in a calcined sample (at 773 K) the excess copper is primarily present as CuO and [Cu-0-Cu] 2+. The [Cu-0-Cu]2+ is transformed to Cu+ during outgassing in He or Ar. The activity of the Pd-exchanged ZSM-5 catalyst has been studied in the complete oxidation of methane and was compared to that of a Pd0/Al 203 catalyst [18]. The Pd- ZSM-5 was found to exhibit higher activities at lower temperatures as compared to the Pd0/Al 203 catalyst. From their TPR experiments with carbon monoxide, the authors related the higher activity of the Pd-ZSM-5 catalyst to its reducibility, as the reduction temperature of the Pd-ZSM-5 was ca. 70 K lower than that of the PdO/Al 2Os catalyst. Furthermore, the authors concluded that due to the very high metal dispersion of the Pd- exchanged zeolites and very high intrinsic reaction rates, the Pd-ZSM-5 are suitable for low temperature methane emission control. The objective of the present study was to make a comparative investigation of the catalytic behaviour of the different Cu- and Pd-modified ZSM zeolite catalysts in complete oxidation of propane and natural gas. 2. Experimental 2.1 Zeolite synthesis and catalyst preparation The Na-ZSM-5, Na-ZSM-11 and Na-ZSM-48 zeolites were synthesized in the laboratory as mentioned in the references [19-21] with some modifications. The Si/Al ratio determined by NMR spectroscopy for Na-ZSM-5, Na-ZSM-11 and Na-ZSM-48 was 39, 32 and 50, respectively. Copper-modified ZSM-5, ZSM-11 and ZSM-48 catalysts were prepared by ion- exchange of the parent Na-ZSM zeolites with an aqueous solution of copper nitrate. These catalysts will be refereed to as Cu-ZSM-5-EE, Cu-ZSM-ll-IE and Cu-ZSM-48-IE, respectively. The ion-exchange for Na-ZSM-11 and Na-ZSM-48 was also carried out at increased pH. For this purpose the pH of the ion-exchange mixture was kept at 7.5 by means of adding 0.1 M NH4OH to the ion exchange mixture 3 (copper nitrate was used as the copper precursor). These catalysts will be referred to as Cu-ZSM-ll-BE and Cu-ZSM-48-BE, where BE stands for ion-exchange under basic conditions. All the ion-exchanges were carried out at 298 K and for 72 hours. The ion- exchanged Cu-ZSM catalysts were then filtered, dried at 353 K for 12 hours and finally calcined in air at 773 K for three hours. The Pd-ZSM-5-IE catalysts were prepared by ion-exchange of Na-ZSM-5 with aqueous solutions of Pd(NH4) 2Cl2 (72 hours at 298 K). The prepared Pd-ZSM-5 were dried at 353 K for 12 hours and calcined at 773 K for three hours. Alternatively, palladium was introduced into the ZSM-5 zeolite during the process of hydrothermal zeolite synthesis [22]. This catalyst will be referred to as Pd-ZSM-5-DS. 2.2 Catalyst testing The prepared catalysts were tested for their activity in propane and natural gas combustion. The experimental set-up for the catalyst testing consisted of pressure regulators, filters, mass flow controllers, a preheater, a tube furnace and a tube reactor in which the catalyst sample was placed. The gas mixture in propane combustion consisted of 0.285 vol.-% propane (99.5 % pure), 6.8 vol.-% synthetic air (dry) and 92.92 vol.-% nitrogen (1=1.05). The total flow rate (at standard conditions) was 2500 and 5000 tnl/min i.e. WHSV = 82.5 and 165 h"1 corresponding to GHSV=69600 and 34800 h'1, respectively. The gas mixture for natural gas combustion consisted of 1.44 voL-% natural gas, 14.64 vol.-% air and 83.92 vol.-% nitrogen (1=1.05). The total flow rate at standard conditions was 2500 ml/min i.e. WHSV = 82.5 h"1 corresponding to GHSV=34800 h"1. Air and nitrogen were connected after themass flow controllers and heated in a preheater. Catalyst samples were packed in a tube with an inner diameter of 14 mm which in turn was placed in the tube reactor. The temperatures were measured before and after the catalyst bed. The analyses of hydrocarbons, CO and C02 were carried out continuously using a TEN3 NDER. gas analyzer. Before the light-off tests the catalysts were pretreated in a flow of 150 ml/min of dry air at 623 K for 30 minutes and for one hour at 813 K. The catalyst was then cooled down slowly (in the air flow) to 423 K. The reaction mixture was introduced and the oven temperature increased from 423 to 873 K at a heating rate of 12 K/min, and the propane/natural gas conversion (to C02 and H20) as well as the temperatures before and after the catalyst were measured continuously. As a result of the catalyst testing, curves of conversion as a function of the temperature before the catalyst (light-off curves) were 4 drawn which enabled the measurement of the temperature at 50% conversion (light-off temperature) and the final conversion at 823 K. For the measurement of the kinetic parameters of the catalytic oxidation of propane over the prepared catalysts, the conversions were kept below 10% and a power rate law: r = k[02]n [C^Hg]" 1 was assumed. The reaction orders were determined by varying the concentration of the reactants in the feed while keeping the total flow rate constant. The apparent activation energies and the pre-exponential factors were determined from Arrhenius plots. 2.3 Catalyst characterization a) SEM, XRD and Specific surface area. The morphology and the crystallinity of the prepared ZSM catalysts were studied by SEM and XRD using a Leica Cambridge stereoscan 360 and a Philips pw 1830 Cu-Ka diffractometer. The specific surface area was measured by nitrogen adsorption at 77 K using a Sorptomatic 1900, Carlo Erba Instruments. b) The metal content of the prepared catalysts was determined by DCP (Spectro Span HI) and X-ray fluorescence (X-MET Outokumpu). c) Temperature programmed desorption of oxygen, carbon monoxide and ammonia was performed in a volumetric flow apparatus with helium (dry) as the carrier gas. The TPD of oxygen was carried out after the catalyst had been treated at 773 K with a dry oxygen/helium mixture (10 vol.-% 02 in helium) for one hour, cooled to 298 K and treated again with oxygen for 30 minutes. The catalyst sample was then flushed with helium for one hour in order to desorb the physically adsorbed oxygen. The catalyst bed temperature was then increased linearly from 298 to 823 Kata heating rate of 8 K/min and kept constant at 823 K for one hour. For the CO-TPD, the catalysts after the oxygen treatment at 773 K were cooled down slowly to 298 K, He-flushed for one hour followed by CO adsorption (10 vol.-% CO in He) and finally flushed again with helium for 30 minutes. The catalyst bed temperature was then increased linearly (8 K/min) to 823 K and the desorbed CO and the formed C02 were detected. For the NH3-TPD the catalysts were first pretreated with He (dry) at 773 K, cooled down to 373 K followed by ammonia treatment (5 vol.-% in He) for 60 minutes. The catalysts were then flushed with helium for one hour and the temperature was increased linearly to 773 K at a heating rate of 8 K/min. The analyses of the desorbed gases were carried out continuously using a quadrupole thermalprogrammed mass detector (QTMD-mass spectrometer), Carlo-Erba Instruments. For the quantitative measurements the mass 5 spectrometer was calibrated for 02, CO, C02 and NH3. The amount of the gases desorbed or formed was determined by time integration of the TPD-curves. The gases used were of high purity and were additionally dried and de-oxygenated. The flows were controlled by means of mass flow controllers. d) Thermogravimetric analysis. The redox capacities of theCu-containing catalysts were determined by subjecting the catalyst to redox cycles at 773 K using a Cahn-D200 microbalance. Oxygen (10 vol.-% in He) was used as the oxidizing agent and carbon monoxide (10 vol.-% in He) was used as the reducing agent. The microbalance was calibrated for the flows in use. After each CO treatment, the system was flushed with helium for 30 minutes in order to remove the adsorbed carbon monoxide. The gases were dry and were of high purity (99.999 vol.-%). The flows were controlled by means of mass flow controllers. 3. Results and discussion The phase purity and the crystallinity of the prepared zeolites were examined by X-ray diffraction and SEM analysis.The SEM images and the XRD patterns of the synthesised zeolites exhibited the typical shape and pattern of the respective ZSM zeolites given in the relevant literature. It also demonstrated that the structure of the ZSM-5 was not affected by the introduction of palladium to ZSM-5 during the process of hydrothermal zeolite synthesis. The metal content, the specific surface area and the micro pore volumes of the prepared catalysts are given in Table 1. The values in the parentheses represent the metal content in wt.-% of the catalyst. The surface area and the pore volume of the catalysts decreased as ZSM-5 > ZSM-11 > ZSM-48 which is a consequence of the geometric structure of these three different zeolites. The surface area of the (3.4)Cu-ZSM-48-IE was =50 m2/g greater than that of the (8.5)Cu-ZSM-48-BE catalyst, suggesting thatsome pore blockage might have occurred due to the high metal loadings. Increase in the pH of the ion-exchange mixture was found to result in increased copper uptake. The results indicated that the copper was exchanged in excess (Cu/Al>0.5) on the ZSM zeolites (Table 1). The results of the catalyst testing in propane and natural gas combustion are given in Tables 2-5. The conversions (X, %) are given at WHSV of 82.5, 165 h'1 and temperatures of 623 K, 723 K and 823 K. The light-off temperature is denoted by T50%. The results of the catalyst testing of Cu-ZSM-11 and Cu-ZSM-48 zeolite catalysts in 6 propane combustion (Table 2) indicate that the increase in the pH of the ion-exchange mixture resulted in increased metal uptake and an increase in the catalyst activity. In our previous study [23] we demonstrated thatthe same is valid for the Cu-ZSM-5 catalysts where also the catalytic performance increased by increasing the metal content of the prepared zeolites caused by an increase in the pH of the ion-exchange mixture. The (1,3)Cu-ZSM-5-IE was found to be the most active copper-modified ZSM zeolite in propane combustion. The Cu-ZSM-5 catalyst exhibited higher activities at lower temperatures (Table 2, Fig. 1) as compared to a 10wL-% Cu /A1203 catalyst, (the Cu /A1203 catalyst was prepared by incipient wetness impregnation of a y-alumina support with an aqueous solution of copper nitrate, with the subsequent drying and calcination for 3h at 773 K). A decrease in the residence time resulted in an increase in the light-off temperature and a decrease in the final propane conversions at 823 K. The final conversions for natural gas combustion over the prepared catalysts were relatively lower than those obtained for the propane combustion (Table 3). The catalysts also exhibited higher light-off temperatures in natural gas combustion which is due to the fact that methane is the most difficult gas to combust catalytically. Here also the same activity pattern was observed as in the case of propane combustion: the (1.3)Cu-ZSM-5- EE catalyst was the most active of the Cu-containing catalysts, exhibiting a combustion efficiency and a light-off temperature of 94.8 % and 756 K, respectively. The light-off temperatures of the Cu-ZSM-11 catalysts in propane and natural gas combustion were lower than those of the corresponding Cu-ZSM-48 catalysts while the latter exhibited higher final conversions. The effect of the oxygen content of the feed mixture on the conversion behaviour of the catalysts is given in Fig. 2. The conversions of propane and natural gas over the catalysts increased with the 02-content of the feed mixture (T = 773 K). This improvement in the activity was more significant up to 25% increase in the oxygen content, with the exception that the conversion behaviour of the Cu-ZSM-5-IE catalyst in propane combustion was not affected by the 02 content. On the other hand the propane conversion over the (3.4)Cu-ZSM-48-IE strongly increased with increasing oxygen content of the feed. The reaction orders (based on mole fractions), the activation energies and the pre- exponential factors in propane combustion are given in Table 4. TheCu-ZSM-5 and Cu- ZSM-11 exhibited similar kinetics under our experimental conditions whereas the Cu- ZSM-48 exhibited different kinetics. The latter catalyst exhibited a much lower activation energy (74.8 Kj/mol) which was strongly compensated by the pre-exponential 7 factor. The Cu-ZSM-48 also exhibited a higher dependency on the propane partial pressure. The results of the 02 and CO-TPD for some of the prepared catalysts are given in Table 6. The O/Cu ratios were calculated from the oxygen desorbed from the catalyst during the 02-TPD as well as the amount of oxygen required to produce the amount of C02 formed during the CO-TPD. The highest oxygen desorption (< 823 K) was from the (1.3)Cu-ZSM-5-IE. The catalyst also exhibited a much higher O/Cu ratio (both from the 02 and CO-TPD). The 02-TPD pattern of the (1.3)Cu-ZSM-5-IE catalyst is given in Fig. 3 and is compared to that of a 10wt.-% Cu /A1203 catalyst.Three different oxygen desorption/evolution peaks were observed from the (1.3)Cu-ZSM-5-IE catalyst: one at lower temperatures (= 343 K) which is assumed to be the remainder of the physically adsorbed oxygen, the second peak (583-773 K) is the extra lattice oxygen (ELO), while the last peak (>773 K) has been attributed to the oxygen from the lattice. The catalytic activity can be related to the oxygen desorption (fraction of ELO released) and the reducibility of the catalyst as well as to the nature and the structure of the zeolite itself. The higher the amount of oxygen desorbed below 823 K per gram copper, the higher the catalytic activity, i.e. the (1.3)Cu-ZSM-5 is the most active. It appears that the extraction of the ELO by carbon monoxide is also much easier on this catalyst The0 2- desorption from the Cu-ZSM-11 was the lowest and therefore a lower catalytic activity is to be expected. The (10.0)Cu /A1203 catalyst exhibited a different 02-desorpdon pattern as compared to the Cu-ZSM-5-IE catalyst (Fig. 3). The catalyst desorbed much less oxygen at 573-673 K and therefore exhibits lower activities at lower temperatures. Theamount of the oxygen released per gram copper of the Cu /A1203 catalyst was close to that of the Cu-ZSM-11 catalyst (0.283 mmol 02/g-Cu). The results of the catalyst testing indicated that the Cu-ZSM-48 catalysts exhibited higher light-off temperatures than Cu-ZSM-11 while the final conversions over the former catalysts were higher. The Cu-ZSM-48 catalysts desorbed more oxygen than Cu- ZSM-11 (0.42 vs. 0.26 mmol0 2/g-Cu) and therefore exhibits higher final conversions. The reason for the lower light-off temperatures of the Cu-ZSM-5 and Cu-ZSM-11 catalysts, besides their oxygen desorption and their reducibility, might also be found in the nature and the geometry of the zeolites themselves. In spite of the fact that these zeolites (ZSM-5, 11 and 48) belong to the group of medium pore zeolites, they differ substantially in their structure and catalytic properties. In ZSM-5 two types of intersecting channels exist that are both defined by 10-membered 8 rings. These channels are sinusoidal and parallel to the [100] axis , and straight and parallel to the [010] axis, respectively. The sinusoidal channels are nearly circular with the dimensions 0.54 x 0.56 nm, while the straight channels are elliptical with dimensions of 0.51 x 0.55 nm. This two-dimensional channel system has pore intersections with much larger cross-sections [19]. ZSM-11 is closely related to ZSM-5 and also belongs to the pentasil family of zeolites, however its channel structure is very different from that of ZSM-5 and consists of perpendicularly intersecting straight channels with dimensions of 0.51 x 0.55 nm. ZSM-11 has tetragonal symmetry and its crystallographic structure is more symmetric than that of ZSM-5. ZSM-11 has two types of channel intersections where one is comparable to the intersections in ZSM-5 and the other has 30% more free space [20]. ZSM-48 belongs to the family of EUO, and in contrast to ZSM-11 and ZSM-5, it has no bidirectional channels. The framework of ZSM-48 is composed of ferrierite type sheets, connected in such a specific way as to generate linear 10-membered ring channels. ZSM-48 has a typical morphology consisting of needles or agglomerates of them. The ideal channel dimensions of ZSM-48 are 0.53 x 0.56 nm [21]. The geometry of the zeolitic channels seems to be an important factor in determining the catalytic activities of these zeolites in the complete oxidation of hydrocarbons. The ZSM-5 and ZSM-11 catalysts due to their bi-directional channels provide a better coordinative environment for copper, thereby making it catalytically more active and exhibiting higher activities at lower temperatures. In ZSM-48 with a one-dimensional micropore system the accessibility of the catalytic active sites is a potential hindrance. For Cu-ZSM-48 -IE and Cu-ZSM-48-BE under reaction conditions there is a probability that the pores of the catalysts are filled with physisorbed molecules or that the reaction products formed deep inside the pores are unable to escape to the surface thus blocking the pores of the catalyst. The hydrogen treatment of the Cu-ZSM-11 and 48 zeolite catalysts did not affect their light-off and conversion behaviour strongly (Table 5). However, this was not the case with the Cu-ZSM-5-IE catalyst where the catalyst’s light-off temperature strongly increased (from 615 to 699 K) while the final conversions at 823 K of the oxidized and pre-reduced catalysts were close. The re-testing of the catalysts resulted in a light-off and conversion behaviours similar to those of the initially oxidized catalysts. The reason for these observations might be that with the pre-reduced catalysts a reoxidation of the catalyst takes place during the light-off testing and therefore the final conversions are close. In the case of the (1.3)Cu-ZSM-5-IE at lower temperatures, the catalyst is not 9 n fully oxidized and therefore lower activities are found at lower temperatures. In the case of the Cu-ZSM-11 and Cu-ZSM-48 zeolite catalysts, the temperatures at which the catalysts exhibit activities are sufficiently high to fully oxidize the catalyst, and therefore the light-off and the conversion behaviour of the catalysts were not affected by hydrogen treatment. The decrease in the oxidative activity of the Cu-H-ZSM-5 catalyst by the hydrogen treatment has been explained as the changes in the oxidation state of copper (from Cu2+ to Cu+) is accompanied by the migration of the Cu(II) isolated ions from zeolite channels to the outer surface of the zeolite. In the reoxidation of the reduced catalyst, the Cu(II) ions from the outer surface gradually migrate back into the zeolite channels and stabilize in the cationic positions, resulting in a restored catalytic activity [24]. For the excessively ion-exchanged Cu-ZSM-5 zeolite it has been proposed that the excess copper is present mostly as Cu-O-Cu bridges which participate in the redox cycle. In order to determine the redox capacity of the different catalysts, the catalysts were subjected to redox cycles at 773 K in a microbalance. The calculated O/Cu ratios are given in Table 6. The O/Cu ratio of the (1.3)Cu-ZSM-5-IE calculated from the redox cycle was close to that obtained from the CO-TPD measurements (O/Cu = 0.2). The O/Cu ratio of the other catalysts, i.e. Cu-ZSM-11 and Cu-ZSM-48 zeolite catalysts, was close to one. This indicates that the majority of the copper on Cu-ZSM-11 and Cu-ZSM- 48 (Fig. 4) is present in the form of CuO whereas on the (1.3)Cu-ZSM-5-IE thecopper most probably exists as Cu2+ and [Cu-0-Cu] 2+species, with the latter being more active and easily reduced by carbon monoxide. Thepolymeric chains or other stable forms of the Cu-oxide may form in the zeolite pores [17,25], The exchange of copper to the ZSM-11 and ZSM-48 at increased pH (7.5) was found to result in a high Cu/Al ratios. It is assumed that beside the copper being present at ion-exchange sites, a large amount of copper are also present on the zeolite surface. An increase in the pH of the ion- exchange mixture results in the formation of [Cu(NH4)2]2+complexes, the thermal decomposition (in air at 773 K) of which will result in the formation of CuO. The precipitation of the copper hydroxide (although not likely) and its further thermal decomposition to CuO during the calcination cannot be excluded either. Li et al. [15] in their micro balance study of an ion-exchanged Cu-ZSM-5 catalyst subjected to similar redox cycles, found a O/Cu ratio of 0.5 (i.e. le"/Cu). This difference might be a consequence of the higher Cu-content of their catalyst (0.49 vs. 0.21 mmol/g zeolite) since the excess exchanged copper is assumed to be present as [Cu-0-Cu]2+. This was further confirmed when a (5.2)Cu-ZSM-5 zeolite catalyst prepared by ion-exchange under basic conditions exhibited an O/Cu ratio of 0.6 (this catalyst is not going to be 10 discussed here). The state of copper in a calcined ZSM zeolite is dependent on the zeolitic structure. On calcined ZSM-11 and ZSM-48 the copper is present as CuO while the ZSM-5 tends to stabilize the copper as Cu2+ or Cu-O-Cu species. Zeolites in the acid form can activate molecular oxygen, forming peroxide species [26- 28]. Molecular oxygen has been proposed to adsorb on weak Lewis sites of H-Y [29, 30] increasing the electron deficiency of the site, giving rise to species able to withdraw electrons from the hydrocarbon and oxidize them. In zeolites the localized surface defects formed in the oxygen interaction with Brpnsted acid sites can act as active centres for the hydrocarbon activation. In an ESR spectroscopy of ZSM-5, Shih [26] observed the presence of localized electrons in ZSM-5. These solid state paramagnetic defects act as very energetic free radicals able to activate oxygen, forming oxygen free radical species. He suggested an oxygen role in creation of these localized free electrons through a mechanism in which two Brpnsted acid sites interact with oxygen forming solid state defects. The localized nature of the electron on the solid state defects will interact with the ionisable organic molecule adsorbed on the zeolite surface. The ion- exchange of metals can result in incorporation of the protonic sites to the zeolites. In our case, when the ion-exchange was carried out at increased pH, the acidity can be introduced to the zeolite by some NH4+exchange (and the subsequent heat treatments). The exchange of copper into Na-ZSM-5 has also been reported to result in Brpnsted acid sites [31]. In order to study the possible effect of the acidity on the catalytic activity of the prepared Cu-ZSM catalysts, NH3-TPD experiments were carried out. However, from the NH3-TPD of the prepared catalysts it was not possible to give an accurate estimate of the acidity of the catalysts since a significant oxidation of ammonia was observed to take place over all the prepared catalysts. The ammonia oxidation was more significant over the catalysts with higher copper loadings, however, the TPD pattern of all the catalysts still indicated the presence of the acidic sites. Therefore, in order to give a comparative degree of acidity, the ammonia-TPD was carried out on the protonated zeolite catalyst, prepared by the ion-exchange of the parent Na-ZSM catalysts with 0.5 M NH4C1 solution, followed by washing, drying and calcination at 813 K. The amount of the desorbed ammonia at different temperature ranges is given in Table 7. The acidity pattern was found to be: H-ZSM-5 > H-ZSM-1 l>H-ZSM-48. The acidity pattern also follows the low temperature activity pattern in propane and natural gas combustion. Theresults from the catalyst testing of the Pd-containing ZSM-5 catalysts are given in Tables 2-5. As can be seen the Pd-catalysts prepared by the ion-exchange method exhibited higher activities than when the palladium was introduced into the zeolites 11 during the process of hydrothermal zeolite synthesis, i.e. the (0.2)Pd-ZSM-5-DS catalyst. The (0.9)Pd-ZSM-5-IE was found to be the most active catalyst both in propane and natural gas combustion exhibiting a 100% conversion of propane and 98% conversion of natural gas. Here as in the case of the Cu-containing catalysts an increase in the residence time resulted in decreased conversions and increased light-off temperatures in propane combustion (Table 2). An increase in the 02-content of the feed mixture strongly improved the propane conversion over the (0.2)Pd-ZSM-5-DS catalyst, whereas the natural gas conversion over the same catalyst was less affected by this increase (Fig. 2a, b). The combustion efficiencies of propane and natural gas over the (0.2)Pd-ZSM-IE were not affected strongly by this increase in the oxygen content of the feed (Fig. 2a,b). Reduction of the catalysts also decreased the low temperature activities of the Pd-containing ZSM-5 catalysts in propane combustion (Table 5). However, the catalysts exhibited the final conversions close to those obtained over the initially oxidized Pd-catalysts. Here also the re-tested catalysts exhibited the conversion and light-off behaviour similar to those over the oxidized Pd-ZSM catalysts, indicating that as in the case of Cu-ZSM catalysts, a reoxidation of the catalyst also takes place during the light-off testing. On alumina supported Pd-catalysts it has been proposed that a reoxidation of the palladium particle takes place during the methane oxidation [32, 33] and the reaction is supposed to take place over the palladium oxide phase even if the catalyst is initially reduced [4], The PdO lattice obtained from the metallic palladium has a porous character, therefore during the oxidation process new surfaces of PdO develop, i.e. more active centres [34,35], In order to compare the activity of the Pd-ZSM-5 zeolites with other supported Pd- catalysts, a (0.95)Pd/Al 2O3 catalyst was prepared by impregnation of a y-Al203 support with aqueous solution of PdCl 2 (followed by drying and calcination at 773 K for 3h). The prepared Pd/Al 203 was tested for its light-off and conversion behaviour in propane combustion. The (0.9)Pd-ZSM-5-IE exhibited much higher activities at lower temperatures although the final conversion of the two catalysts were quite close (Fig. 5). In a recent study of the catalytic combustion of methane, Li et al. [18] reported a comparative study of Pd-ZSM-5 and Pd0/Al 203. The Pd-ZSM-5 catalyst exhibited a light-off temperature of =100 K lower than the corresponding Pd/Al 203 catalyst which is in good agreement with our results. The authors related the catalytic activity to the reducibility of the catalysts where the Pd-ZSM-5 catalyst was found to be reduced much more easily than the Pd/alumina catalyst. Another reason for the higher activities of the Pd-ZSM5-IE catalysts as compared to the Pd/alumina catalyst might be their oxygen 12 carrier capacities. During the 02 -TPD measurements the (0.2)Pd-ZSM-5-DS and (0.9)Pd-ZSM-5-DE catalysts desorbed 3.78 and 3.88 mmol/g Pd , whereas the (0.95)Pd/alumina catalyst desorbed 1.87 mmol/g-Pd. As it has been reviewed by Gallezot [36] , the electronic and catalytic properties of metals introduced in zeolite can be modified. One of the reasons for higher catalytic activity of the Pd-exchanged ZSM-5 catalysts could be the electron-deficiency caused by transfer of electrons from palladium to the Brpnsted and Lewis acid sites. It seems as if the zeolitic structure provides a better environment in electron donating properties of the exchanged palladium as compared to conventional alumina support. The results also indicated that the method of metal introduction to the zeolite is of importance as was indicated by the higher activities of the ion-exchanged Pd-ZSM-5-EE catalysts, compared to the Pd-ZSM-5-DS catalyst. The latter catalyst also exhibited a different kinetic behaviour than the ion- exchanged Pd-ZSM-5 catalyst, exhibiting a zero order dependency on the oxygen partial pressure (Table 4) which is often reported for the oxidation over silica or alumina supported palladium catalysts. 4. Conclusions High combustion efficiencies in complete oxidation of propane and natural gas can be achieved by using Cu-ZSM and Pd-ZSM-5 zeolite catalysts. The Cu-ZSM-5 was found to be the most active of the Cu-containing ZSM zeolite catalysts. The higher activity of the Cu-ZSM-5 as compared to the Cu-ZSM-11 and 48 catalysts, might be due to its higher ELO content For the Pd-ZSM-5, the method of palladium introduction into the zeolite was found to be of importance. The Pd-catalysts prepared by the ion-exchange technique exhibited much higher activities in both propane and natural gas combustion than when the Pd was introduced into the ZSM-5 during the process of zeolite synthesis. In general, the light-off temperatures in natural gas combustion were higher than the corresponding light-off temperatures in propane combustion. This is due to the fact that methane (the major component of the natural gas) is the most difficult gas to combust catalytically. 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Catalyst Metal content Cu/Alb Pd/Al b Na/Alb Surface area0, Pore volume 0, wt.-%a mmol/g-zeolite m2/g cm3/g (1.3)Cu-ZSM-5-IE 1.3 0.207 0.57 0.13 460.3 0.163 (2.8)Cu-ZSM-ll-IE 2.8 0.453 1.09 0.05 375.7 0.133 (3.4)Cu-ZSM-48-IE 3.4 0.553 1.55 0.03 301.8 0.107 (7.4)Cu-ZSM-l 1-BE 7.4 1.257 2.59 0.02 382.4 0.136 (8.5)Cu-ZSM-48-BE 8.5 1.461 3.61 0.02 251.2 0.089 (0.2)Pd-ZSM-5-DS 0.23 0.022 0.06 1.09 418.4 0.149 (0.2)Pd-ZSM-5-IE 0.21 0.020 0.05 1.05 458.6 0.166 (0.9)Pd-ZSM-5-IE 0.90 0.085 0.19 0.74 453.7 0.161 a - wt.-% of the catalyst b - molar ratios c - Calculated from the Dubinin-Radushkevich plot 16 Table 2 Results from the catalyst testing in propane combustion. Propane conversion, % £ Catalyst WHSV = 82.5 h'1, s WHSV = 165 h"\, X* = 1.05 Tt50% X623 K X723 K X823 K Tt50% X623 K X723 K X823 K (1.3)Cu-ZSM-5-E 615 59.5 96.7 97.6 639 33.2 90.1 95.4 (2.8)Cu-ZSM-l 1-IE 692 34.1 56.1 81.5 723 21.2 50.0 73.4 (3.4)Cu-ZSM-48-DE 741 14.7 42.5 90.9 784 7.1 32.6 82.1 (7.4)Cu-ZSM- 11-BE 644 39.4 78.8 92.7 659 24.0 68.5 86.4 (8.5)Cu-ZSM-48-BE 670 26.1 75.2 95.9 715 9.1 54.0 89.7 (0.2)Pd-ZSM-5-DS 649 27.1 77.5 77.5 674 23.6 54.5 55.9 (0.2)Pd-ZSM-5-IE 601 92.0 94.4 95.3 615 89.6 90.1 90.2 (0.9)Pd-ZSM-5-IE 573 99.8 100 100 590 96.8 96.9 96.9 (10.0) Cu /A1203 724 6.0 49.4 92.8 (0.95) Pd/Al 203 645 26.2 91.1 98.6 * - Stoichiometric ratio, X, defined, as the air/fuel ratio normalized with respect to the stoichiometric air/fuel ratio t - Light-off temperature, T50% , K, defined as the temperature at which 50% conversion takes place 17 Table 3 Results from the catalyst testing in natural gas combustion (WHSV = 82.5 h"1, X* = 1.05). Natural gas conversion, % Catalysts Tt50% X623 K X723 K X823 K (1.3)Cu-ZSM-5-IE 756 12.4 27.1 94.8 (2.8)Cu-ZSM-l 1-EE >823 2.7 4.8 19.8 (3.4)Cu-ZSM-48-IE >823 2.4 11.9 27.5 (7.4)Cu-ZSM-l 1-BE 808 3.4 13.0 51.8 (8.5)Cu-ZSM-48-BE 812 3.3 12.9 68.5 (0.2)Pd-ZSM-5-DS 676 16.4 73.5 85.8 (0.2)Pd-ZSM-5-IE 629 44.9 93.7 94.1 (0.9)Pd-ZSM-5-IE 608 82.0 98.1 98.1 * - Stoichiometric ratio, X, defined as the airlfuel ratio normalized with respect to the stoichiometric airlfuel ratio t - Light-off temperature, Tso% , K, defined as the temperature at which 50% conversion takes place 18 Table 4 Kinetic parameters for propane combustion. Conditions: % % 0;, % a) variable 1.496 5.25 -14.96 b) 0285 variable 5.25 - 8.75 total flow rate = 2500 ml/miti, WHSV 82.5 h'1 Ea, Temperature Range, Temperature*, In A, Catalyst Reaction Orders KJ.mol" 1 K K kmol.kg-cat'^s" 1 O2 CgHg (1.3)Cu-ZSM-5-IE 0.66 0.22 115.5 558-583 573 13.3 (7.4)Cu-ZSM-ll-BE 0.81 0.21 119.1 563-593 573 13.1 (8.5)Cu-ZSM-48-BE 0.58 0.78 74.8 563-593 573 6.5 (0.2)Pd-ZSM-DS 0.02 0.55 104.6 563-593 573 9.3 (0.2)Pd-ZSM-5-IE 0.18 0.69 110.9 538-568 548 13.4 (0.9)Pd-ZSM-5-IE 0.22 0.65 110.0 543-553 548 13.6 *- Temperature at which the reaction orders were measured 19 Table 5 Results from the catalyst testing of the reduced catalysts in propane combustion (WHSV = 82.5 h'1, X* = 1.05). r ?• Propane conversion, % Catalyst Tt50% X623 k X723 K X823 K (1.3)Cu-ZSM-5-IE 699 20.1 66.1 97.5 (7.4)Cu-ZSM-l 1-BE 651 28.8 78.1 92.5 (8.5)Cu-ZSM-48-BE 681 19.9 75.4 95.5 (0.2)Pd-ZSM-5-DS 668 17.9 68.8 74.4 (0.2)Pd-ZSM-5-IE 647 11.1 92.5 92.5 (0.9)Pd-ZSM-5-IE 643 15.0 98.4 98.6 * - Stoichiometric ratio, X, defined as the air/fuel ratio normalized with respect to the stoichiometric air/fuel ratio t - Light-off temperature, Tsm , K, defined as the temperature at which 50% conversion takes place ' ri 20 N — Table 6 Results from the 02 , CO-TPD and microbalance studies. o 2-tpd CO-TPD Microbalance Catalysts O/Cu a mmol 02/g-Cu O/Cu b mmol CO oxidized/g-Cu O/Cu (1.3)Cu-ZSM-5-IE 0.128 1.006 0.241 3.789 0.26 (3.4)Cu-ZSM-48-IE 5.260xl0" 2 0.428 7.415xl0' 2 1.167 0.92 (8.5)Cu-ZSM-48-BE 5.454x10"2 0.429 7.484x1 O'2 1.177 0.96 • (2.8)Cu-ZSM-l 1-EE 3.106xl0"2 0.244 3.736xl0" 2 0.588 0.86 (7 .4)Cu-ZSM-11 -BE 3.303xl0 ‘2 0.260 3.827xl0" 2 0.602 0.88 a - Oxygen atoms desorbed per catalyst’s copper atoms b - Oxygen atoms required for CO oxidation equivalent to the CO 2 formed during the CO-TPD per catalyst’s copper atoms 21 Table 7 Amount of desorbed ammonia and the temperatures of the peak maximum (Tm): Amount of NH3 desorbed, (mmol/g-zeolite) Catalyst Temperature range ; Tm (K) 373 K-573 K, (Tm) 573 K- 823 K, (Tm) 373 K- 823 K H-ZSM-5 0.558 (466) 0.415 (670) 0.974 H-ZSM-11 0.294 (446) 0.182 (672) 0.476 H-ZSM-48 0.272 (437) 0.167 (663) 0.439 22 z f Figure Propane conversion, % 473 1. The IE, 1=1.05); Temperature light-off ■- 523 (10.0 0-(1.3)Cu-ZSM-5-IE, curves )C 573 u /A1 of 2 0 the before 3 . 623 Cu-ZSM •- the (7.4)Cu-ZSM-ll-BE, and 673 catalyst, C u /A1 2 0 723 3 catalysts K □- 773 (WHSV=82.5h" (8.5)Cu-ZSM-48- 823 1 , •v. 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 Stoichiometric ratio, X 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 Stoichiometric ratio, X Figure 2. Effect of the oxygen content of the feed mixture on th conversion behaviour of the catalysts in: a) propane, and b) natural gas combustion (T=773 K, WHSV=82.5 h"\ n fuel = constant); O- (1.3)Cu-ZSM-5-IE, □- (2.8)Cu-ZSM-l 1-IE, ■- (7.4)Cu-ZSM- 11-BE, A-(3.4)Cu-ZSM-48-IE, A- (8.5)Cu-ZSM-48-BE, 0- (0.2)Pd-ZSM-5-DS, ♦- (0.21)Pd-ZSM-5-IE. V «•» 373 473 573 673 773 6 18 30 42 TEMPERATURE (K)/TIME (min) Figure 3. 02-TPD pattern of the (1.3)Cu-ZSM-5-IE (#) and (10)Cu /A1 2O3 (O) catalysts (heating rate=8 K/min). Figure W eight, m g 4. microbalance The and the redox durations, cycles (the texts ON at 773 = and over K the of night) values the (8.5)Cu-ZSM-48-BE on the figure represent catalyst the gas in used the Figure Propane conversion, % 5. The h\ DS, ?i=1.05). Temperature light-off ■- (0.95)Pd/Al O-(0.9)Pd-ZSM-5-IE, curves 2 of O before 3 the . Pd-ZSM-5 the •- (0.2)Pd-ZSM-5-IE, and catalyst, Pd/Al 2 0 3 catalysts K □- (0.2)Pd-ZSM-5- (WHSV=82.5