Noble Metal Catalysts for Total Oxidation of Volatile Organic Compounds and Carbon Monoxide
Mixed Metal Oxide - Noble Metal Catalysts for Total Oxidation of Volatile Organic Compounds and Carbon Monoxide
Magali Ferrandon
Department of Chemical Engineering and Technology Chemical Reaction Engineering Royal Institute of Technology Stockholm, 2001
ABSTRACT CO, volatile organic compounds, and polyaromatics are ubiquitous air pollutants that give rise to deleterious health and environmental effects. Such compounds are emitted, for instance, by the combustion of wood, particularly from small-scale heating appliances. Total catalytic oxidation is considered to be an effective approach in controlling these emissions, however, some problems remain such as the non-availability of catalysts with low-cost, high activity and stability in prevailing conditions. Hence, this thesis aims at the development of oxidation catalysts and improved understanding of their behaviour. The catalytic activity was evaluated for the oxidation of a mixture of CO, naphthalene (or ethylene), and methane in presence of carbon dioxide, water, oxygen and nitrogen. Various characterisation techniques, including Temperature-Programmed Reduction and Oxidation, BET- Surface Area Analysis, X-Ray Diffraction, X-Ray Photoelectron Spectroscopy, Raman Spectroscopy and Scanning and Transmission Electron Microscopy were used. In the first part of this thesis, catalysts based on metal oxides (MnOx, CuO) and/or a low amount of noble metals (Pt, Pd) supported on alumina washcoat were selected. It was shown that Pt and Pd possessed a superior catalytic activity to that of CuO and MnOx for the oxidation of CO, C10H8 and C2H4, while for the oxidation of CH4, CuO was largely more active than noble metals, and MnOx as active as Pd and Pt. Some mixed metal oxide-noble metal catalysts showed decreased activity compared to that of noble metals, however, a higher noble metal loading or a successive impregnation with noble metals led to positive synergetic effects for oxidation. Deactivation of the catalysts by thermal damage and sulphur poisoning is addressed in the second part of the dissertation. An alumina washcoat was found to be well anchored to the metallic support after thermal treatment at 900°C due to the growth of alumina whiskers. The sintering of the washcoat was accelerated after high temperature treatments in the presence of metal catalysts. In addition, alumina was found to react with CuO, particularly in presence of noble metals at 900°C, to form inactive CuAl2O4. However, MnOx catalyst benefits from the more active Mn3O4 phase at high temperature, which makes it a suitable active catalyst for the difficult oxidation of CH4.Ptsintering was delayed when mixed with CuO, thus giving more thermally resistant catalyst. The mixed metal oxide-noble metal catalysts showed higher activity after pre-sulphation of the catalysts with 1000 ppm SO2 in air at 600°C or during activity measurement in presence of 20 ppm SO2 in the gas mixture, compared to single component catalysts. In some cases, the activities of the mixed catalysts were promoted by pre-sulphation due to the presence of sulphate species. Thermal stabilisation of the catalytic components and the alumina by promotion of La in the washcoat is discussed in the third section. The stabilising effect of La at high temperature is also compared to that of Ce added in the catalysts for other purposes. Due to its better dispersion, La contributed to the thermal stabilisation of the alumina washcoat and its active components to a higher extent than Ce did. La provided a better dispersion and a higher saturation of metal oxides in the alumina support, and at the same time stabilised the activity of the catalysts by preventing undesirable solid-phase reactions between metal oxide and alumina. In addition, La was found to enhance the dispersion and the oxygen mobility of CeO2. Cu-Ce interactions were found to promote substantially the CO oxidation due to an increase of the stability and reducibility of Cu species. Synergetic effects were also found between Ce and La in the washcoat of CuO-Pt catalyst, which facilitated the formation of reduced Pt and CeO2, thus enhancing significantly the catalytic activity compared to that of a Pt only catalyst. The last part was an attempt to demonstrate the potential of a catalyst equipped with a pre-heating device in a full-scale wood-fired boiler for minimising the high emissions during the start-up phase. During the first ten minutes of the burning cycle a significant reduction of CO and hydrocarbons were achieved. Keywords: wood combustion, catalysts, total oxidation, manganese, copper, platinum, palladium, lanthanum, cerium, CO, VOC, methane, deactivation, thermal stability, sulphur dioxide.
Dedicatedtomyfamily
PREFACE
Via a Diploma work at the Royal Institute of Technology Stockholm, thanks to an “ERASMUS” exchange programme, I ended up as a PhD student at the department of Chemical Technology. I became rapidly under the spell of Stockholm. It gave me the chance to discover the beautiful nature in Sweden and to learn about the Swedish people, and their traditions, art and culture, as well as to experience the long Swedish winters and the wonderful summers. During the few years I spent at the Royal Institute of Technology, I began to be aware of the fascinating jungle of research, and more particularly in the field of environmental catalysis. Indeed, it has been an enriching experience and, during these years, I had the opportunity to meet people who have contributed to my professional and spiritual evolution. Among all the special people I would like to mention some of them. Especially, I am very thankful to Johanna Carnö for her support, close collaboration, encouragement and fruitful discussions. Despite your short stay, I learned a lot with you. I would like to thank Professor Sven Järås for accepting me in his division, Chemical Technology, and for giving me the opportunity to start a PhD. I wish to express my sincere gratitude to Professor Pehr Björnbom head of the Chemical Reaction Engineering division, for letting me the opportunity to achieve my PhD and for tremendous support. I would like to thank my supervisor Docent Emilia Björnbom for help with the financial applications and for her support and encouragement, as well as for improving my manuscripts. I am indebted to Docent Ahmad Kalanthar Neyestanaki for inspiring me, for great co-operation and scientific comments on my work and my thesis. Special acknowledgements are forwarded to Professor Govind Menon, Dr. Marco Zwinkels and Dr. Magnus Johansson for valuable advises during the course of this project. I am very grateful to Christina Hörnell for reviewing and improving the linguistic quality, as well as other useful comments, of my manuscripts. Thanks to Philippe Thevenin for his friendship and for providing such a source of positivism! Special thanks to Sandrine Ringler for her friendship and for valuable comments on my thesis. I would like to express many thanks to Liam Good for scrutinising the English in this thesis. Thanks to Eloise Heginuz for having been a pleasant roommate and for her enjoyable and comforting chat. I am grateful to Massoud Pirjamali for being so enthusiastic and a genius at fixing things from scratch. Thanks also for the help with the gas chromatograph. I would like to thank Inga Groth for her confidence in me and for her endeavour concerning the artistic micrographs taken with the scanning electron microscope. I would like to thank Magnus Berg for being a very dynamic co-ordinator in our project. Thank you also for fruitful co-operation! Thanks to Lars Pettersson for being available and an outstanding source of enthusiasm, as well as for sharing his wide knowledge about science and many other fields with us. Financial support to this work given by the Swedish National Board for Technical Developments (NUTEK), the Swedish National Energy Administration (STEM), and the European Commission, the FAIR-program (CT95-0682) is greatly acknowledged. I would like to acknowledge the discussions and meetings with the following persons who have contributed in the European and Swedish projects: Lennart Gustavsson (Swedish National Testing and Research Institute), Björn Gustavsson and Irène Wrande (Swedish National Energy Administration), Sven-Erik Gustavsson (Vedsol AB), Gisela Köthnig (Swedish Environmental Protection Agency), Tihamer Hargitai (Catator AB), Niklas Berge (Termiska Processer AB), Daisy Hagman (Swedish Consumer Agency), Bengt-Erik Löfgren (ÄFAB), Björn Björkman (Skorstensfejarmästarnas Riksförbund), Harald Raupenstrauch (AMVT, University of Technology Graz, Austria), Francoise Duprat (ENSSPICAM, France), Hannu Karhu, Frederik Klingstedt and Professor Lars-Erik Lindfors (Åbo Akademi, Finland). I would like also to thank all my diploma work students for their enthusiasm and hard work: Frederic Pouly, Adam Delattre, Sylvain Derrey, Benedicte Ferrand and María Sanz Soria. I hope you learned as much as I did! Thanks to assistant research Michel Bellais for his help in the project. Thank you also, to all of you that made my stay at KTH very pleasant as well as during the conference trips: Anders, Annika, Baback, Bagher, Benny, Cecile, Henrik, Jeroen, Johan, Jonny, Mostafa, Peter, Susanna and Winnie. Dennis you are the best! Thank you for your love and support. Last but not least, I would like to thank all my friends and my parents for considerable support and encouragement throughout the years. PUBLICATIONS REFERRED TO IN THIS THESIS
The work presented in this thesis is based on the following publications, referred to in the text using the following assigned Roman numerals:
I. Carnö, J., Ferrandon, M., Björnbom, E., and Järås, S., Mixed manganese oxide/platinum catalysts for total oxidation of model gas from wood boilers, Appl. Catal. A 155, 265-281 (1997).
II. Ferrandon, M., Carnö, J., Järås, S., and Björnbom, E., Total oxidation catalysts based on manganese or copper oxides and platinum or palladium, I. Characterisation, Appl. Catal. A 180, 141-151 (1999).
III. Ferrandon, M., Carnö, J., Järås, S., and Björnbom, E., Total oxidation catalysts based on manganese or copper oxides and platinum or palladium, II. Activity, hydrothermal stability and sulphur resistance, Appl. Catal. A 180, 153-161 (1999).
IV. Ferrandon, M., Berg, M., and Björnbom, E., Thermal stability of metal- supported catalysts for reduction of cold-start emissions in a wood-fired domestic boiler, Catal. Today 53, 647-659 (1999).
V. Ferrandon, M. and Björnbom, E., Hydrothermal stabilization by lanthanum of mixed metal oxides and noble metals catalysts for volatile organic compound removal, accepted for publication in Journal of Catalysis, 2001.
VI. Ferrandon, M., Ferrand, B., Björnbom, E., Klingstedt, F., Kalantar Neyestanaki, A., Karhu, H., and Väyrynen, I.J., Copper oxide- platinum/alumina catalysts for volatile organic compounds and carbon monoxide oxidation: synergetic effect of cerium and lanthanum, submitted to Journal of Catalysis, 2001.
OTHER DISSEMINATIONS
Some other publications, reports and conferences papers not included in this thesis.
Ferrandon, M. and Björnbom, E., Deactivation in a wood stove of catalysts for total oxidation. Stud. Surf. Sci. Catal., Catalyst Deactivation, 126 (1999) 426. Ferrandon,M.andThevenin,P.,Low temperature catalytic systems for indoor and outdoor removal of odours and smells, in ”Environmental Catalysis” (L. Pettersson, Ed.), ISSN 1104-3466, Stockholm, 1999, p. 117. Ferrandon, M., Carnö, J., Björnbom, E., and Järås, S., Catalytic abatement of emissions in small-scale combustion of wood. Poster presentation. In proceedings, 8th International Symposium on Heterogeneous Catalysis, Varna, Bulgaria, October 5-9, 1996. Ferrandon, M., Carnö, J., Järås, S., and Björnbom, E., Sulphur and thermal resistance of manganese oxide/platinum catalysts for total oxidation. Oral presentation. In book of abstracts, 1st European Congress on Chemical Engineering, Florence, Italy, May 4- 7, 1997. Ferrandon, M., Pouly, F., Carnö, J., Björnbom, E., and Järås, S., Poisoning effects of catalysts for total oxidation in wood-stoves. Poster presentation. In book of abstracts, 3rd European Congress on Catalysis, EUROPACAT, Krakow, Poland, August 31- September 6, 1997. Ferrandon, M. and Björnbom, E., Effect of the mixture of combustibles on the activity of a Pd catalyst for total oxidation. Poster presentation. In proceedings, Survey of Combustion Research in Sweden, Göteborg, Sweden, October 21-22, 1998, p. 221. Ferrandon, M., Berg, M., Björnbom, E., and Järås, S., Metal-supported catalysts for reduction of cold-start emissions in a wood stove. Oral presentation. In book of abstracts, 2nd World Congress on Environmental Catalysis, Miami Beach, USA, November 15-20, 1998. Ferrandon, M. and Björnbom, E., Småskalig vedeldning, Skorstensfejarmästare, 4 (1999). In Swedish. Ferrandon, M. and Björnbom, E., Effect of sulphur dioxide on the activity of deep oxidation catalysts. Poster presentation. In book of abstracts, 4th European Congress on Catalysis, EUROPACAT, Remini, Italy, September 5-10, 1999. Ferrandon, M., Delattre, A., and Björnbom, E., Sulphur dioxide poisoning of catalysts for VOC abatement. Poster presentation. In book of abstracts, 16th Canadian Symposium on Catalysis, Banff, Canada, Maj 23-26, 2000. Ferrandon, M., Nilsson Ebers, A., Jilborg, M., Würtzel, P., and Björnbom, E., Manganese oxide catalysts for VOCs oxidation. Oral presentation. In book of abstract, 9th Nordic Symposium on Catalysis, Lidingö, Sweden, June 4-6, 2000. Ferrandon, M. and Björnbom, E., Katalytisk minskning av utsläpp från småskalig vedeldning, in “Småskalig Förbränning av Biobränslen”, The Swedish National Energy Administration, Report EI 7:2000, 2000. In Swedish. Ferrandon, M., Carnö, J., and Björnbom, E., Med katalysator kan smustig rökgas bli ren, VVS-Forum, nr 9, september 2000. In Swedish.
CONTENTS
CHAPTER 1. INTRODUCTION 1.1 Background 1.1.1 The Power of Biomass ...... 1 1.1.2 Emissions from Small-Scale Combustion of Wood ...... 2 1.1.3 Environmental Targets and Legislation ...... 4 1.1.4 Actions to Reduce the Emissions ...... 4 1.1.5 Catalytic Oxidation ...... 6 1.2ScopeoftheThesis...... 7
CHAPTER 2. CATALYSTS FOR TOTAL OXIDATION 2.1 Noble Metal Catalysts 2.1.1 General...... 9 2.1.2 Results and Discussion ...... 11 2.2 Metal Oxide Catalysts 2.2.1 General ...... 13 2.2.2 Results and Discussion Choiceofthemetaloxides...... 15 Manganeseoxidescatalysts...... 16 Copperoxidecatalysts...... 22 2.3 Combination of Metal Oxides and Noble Metals 2.3.1 General ...... 25 2.3.2 Results and Discussion Effectsofmetaloxidesonnoblemetals...... 26 Effects of noble metals on metal oxides ...... 31
Combination of Pt and MnOx ...... 34 2.4ConcludingRemarks...... 39
CHAPTER 3. CATALYST DEACTIVATION 3.1 Thermal Deactivation 3.1.1 General ...... 41 3.1.2 Results and Discussion Adherence of washcoat onto metallic monoliths ...... 43 Characterisation of thermally-treated catalysts ...... 45 Effectsofmetalsinthewashcoat...... 48 Catalytic activity of thermally-treated catalysts ...... 50
Optimisation of thermally-stable MnOx/Al2O3 catalysts . . . . 52 3.2 Sulphur Poisoning 3.2.1 General ...... 54 3.2.2 Results and Discussion Pre-sulphationofthecatalysts...... 56 Sulphurpoisoningonstream...... 60 3.3ConcludingRemarks...... 62
CHAPTER 4. ADDITIVES:LANTHANUM AND CERIUM 4.1 Stabilisers ...... 63 4.2 Lanthanum 4.2.1 Preparation Method ...... 64 4.2.2 Effect of the Loading ...... 64 4.2.3 Effect of Steam ...... 65 4.2.4 Mechanism of Stabilisation ...... 66 4.2.5 Additional Effects of Lanthanum ...... 67 4.3 Cerium in Catalysis 4.3.1 Oxygen Storage Capacity ...... 68 4.3.2 Noble Metal-Ceria Interactions ...... 69 4.3.3 Metal Oxide-Ceria Interactions ...... 70 4.3.4 Additional Effects of Ceria ...... 71 4.3.5 Deactivation of Ceria ...... 72 4.3.6 Ceria Promoters ...... 73 4.3.7 Synergetic Effect between La and Ce ...... 74 4.4 Results and Discussion 4.4.1 Characteristics of the La- and/or Ce- Doped Washcoat ...... 75 4.4.2 Effects of La on the Stability of Manganese Oxides Catalysts . . . . 79 4.4.3 Effects of La on the Reducibility of Copper Oxide Catalysts . . . . 81 4.4.4 Effects of La on the Stability of Copper Oxide Catalysts ...... 82 4.4.5 Synergetic Effects in CuO-Ce and CuO-La-Ce...... 86 4.4.6 Synergetic Effects in CuO-Pt-La-Ce ...... 89 4.5ConcludingRemarks...... 92
CHAPTER 5. FIELD APPLICATION ...... 95 CHAPTER 6. CONCLUSIONS...... 99 REFERENCES ...... 103 APPENDICES:PAPER I TO VI
1 INTRODUCTION
1.1 Background
Coal, oil and natural gas now account for more than 85% of the world’s industrial generation of energy and constitute the main driving force in all industrialised countries [Herzog et al., 2000]. Primarily as a result of burning fossil fuels, the concentration of CO2 in the atmosphere has risen by almost one third, from 280 to 370 ppm, since the beginning of the industrial age, 150 years ago. There are risks for a long term climate change due to the increase of CO2 in the atmosphere, because gases that reflect the infrared radiation from the earth are believed to contribute to surface warming, thereby seriously affecting the conditions of life on earth [Degobert, 1995]. CO2, produced by combustion of biofuels is naturally recycled and consumed in photosynthesis. This means that there is no increase of CO2 in the atmosphere when burning biomass for production of energy. Efforts to develop ways of producing and using renewable and domestic resources such as biomass for heat and power generation are currently supported by various national and international programs. Governments of developed countries are searching for ways to reduce the emissions, especially CO2, produced by combustion of traditional fuels, whereas developing countries face pressures to build energy systems that supply heat and power to rural areas.
1.1.1 The Power of Biomass
The total energy content of biomass reserves equals the proven oil, coal and gas reserves combined; markedly, 90% of this biomass energy is held in trees. There are indications that bioenergy is catching on as a feasible energy alternative. For example, 15% of the world’s energy requirements are met with biomass fuels; 35% in the developing countries and 3% in the industrialised countries [Kendall et al., 1997]. In Europe, and especially in the Nordic countries and the Alps regions, biofuels are easily available from agricultural and forestry products. In Sweden, a large supply of bioenergy is potentially available. Indeed, approximately 200 TWh could be utilised for production of energy while only 93 TWH is now being consumed of the total Swedish energy supply of 582 TWh [Löfgren, 1998a; Swedish National Energy Administration, 2000]. The
1 utilisation of bioenergy in Northern Europe has increased appreciably under the last 30 years, particularly after the oil crisis in the beginning of the 70’s. During the 80’s there was inexpensive electricity on the market, and electricity became a popular heating alternative. However, environmental concerns led to a renaissance of the idea of using biomass for energy production during the 90’s. Due to the high cost for transportation of bulky fuels, large amounts of biofuels are used in residential small-scale heating appliances. In Sweden, the consumption of biofuels in small-scale wood appliances for house heating is around 12 TWh which is approximately 22% of the total energy utilisation for heating single family homes. Approximately 600 000 boilers, out of a total of 747 000, are thought to be capable of burning wood for home heating in Sweden. Wood is burned regularly in 270 000 of these. Furthermore, there are 965 000 local wood appliances (for example stoves, tiled-stoves) of which 298 000 are used for heating [Askensten, 2000]. In total, more than one third of Swedish homes are able to use wood for heating. Besides the zero CO2 net-contribution of wood combustion, there are other advantages of using wood as fuel in Northern Europe. It is a cheap domestic fuel and many people have free access to it. The oil or electricity costs of a Swedish home can be lowered significantly by using wood [Krögerström, 1994]. Using biomass limits the dependence on foreign energy sources, such as coal and petroleum and the risks of sudden increase in their prices. Biomass contains less impurities, as sulphur and heavy metals compared to petroleum and coal. It differs from hydroelectric power, nuclear plants and transportation of fossil fuels with a low environmental impact risk profile. Although biomass is CO2-neutral, its combustion is a serious environmental problem. Harmful emissions are present in the flue gases, largely caused by incomplete combustion, particularly in small-scale combustion appliances, mainly in the range up to 100 kW. Indeed, combustion in small-scale appliances is unstable; the inhomogeneous fuel, lack of a proper control system and irregular fuelling are some of the causes that lead to increased emissions relative to larger installations.
1.1.2 Emissions from Small-Scale Combustion of Wood
In some areas wood combustion is regarded as the main contributor to air quality problems. The harmful emissions from combustion of wood consist mainly of Volatile Organic Compounds (VOC), tars including Polycyclic Organic Matter (POM), carbon monoxide (CO) and particulates. VOC refers to the organic compounds which are present in the atmosphere as gases, but under normal temperatures and pressure may also be liquids or solids. Polyaromatic hydrocarbons (PAH) constitute a sublevel to POM and include
2 all compounds with more than two aromatic rings composed only of carbon and hydrogen with boiling points around 200°Corhigher.Undercertain conditions there is also a significant emission of nitrogen oxides (NOx)dueto high concentrations of nitrogen in the fuel and high excess of air. In Sweden, the emission of VOC from electricity and heat production is 146 000 tons/year (29% of the total amount of VOC in Sweden) and the dominant part (94%; 136 000 tons/year) comes from the small-scale combustion of biofuels [Swedish Environmental Protection Agency, 1992], as seen in Figure 1. In addition, small-scale combustion units contribute to about 50% of the emissions of PAH although it represents only 5% of the total fuel energy [Köthnig, 2000]. During the initial stage of a wood burning cycle, termed cold-start phase, 60% of the total emissions are released as a result of high volatilisation and low combustion temperature in the fired bed [Axell et al., 1997; Pettersson, 2000]. In the final stage of the combustion cycle, when only a small amount of fuel remains, the excess air ratio increases and the combustion temperature decreases. This is because the heat generation is lower than the heat conveyed bytheair.ThisleadstoahigheremissionofCO,however,thelevelof unburned hydrocarbons (HCs) is relatively low, because at this stage the fuel is almost fully devolatilised.
Household Electricity and 8% heat Industry production 4% Industry 29% 22% Electricity, gas and heating plants 2%
Transport 41%
Combustion of wood 94% Figure 1. Distribution of VOC emissions between various sources in Sweden (left) and VOC from electricity and heat production (right) [Swedish Environmental Protection Agency, 1992].
These emissions can give rise to deleterious health effects such as cancer, weakened immune defence, allergic reactions as well as odour problems. Also, such emissions may lead to local and global environmental impacts, such as ground level photochemical ozone formation, acidification, stratospheric ozone depletion and greenhouse effect [Erngren & Annerberg, 1993].
3 1.1.3 Environmental Targets and Legislation
The efficient removal of emissions that contribute to atmospheric pollution is an environmental issue of paramount importance. Increased environmental awareness coupled with European governmental regulations make it necessary to reduce such emissions. At the European level, emissions of VOC and nitrogen oxides, which are involved in the formation of ground level ozone and photochemical smog, must be reduced by 75% if harmful ground-level ozone and photochemical oxidants levels are to be avoided. The Swedish government has proposed national environmentally quality objectives. One objective is that by 2020 the emission of carcinogenic substances in urban areas should not exceed the low-risk levels for the protection of human health. This means that levels of benzene and ethene should be lower than 1 µg/m3 as an annual mean value. By 2020 the concentrations of particulates in the air must not exceed levels that may damage human health, cultural values and materials. This means that levels of inhalable particulates, are less than 15 µg/m3 as an annual mean value (health). Another objective set by Parliament is a 50% reduction in total emissions of VOC by the year 2010 as compared with 1995 levels (to 219 000 tonnes) [Swedish Environmental Protection Agency, 2001]. Concerning wood combustion in Sweden, regulations from the 80’s are among the strongest in Europe. According to National Board of Housing, Building and Planning regulations all new wood-burning units installed in urban areas must be “environmentally approved”, from 1st January 1999. However, the Swedish Environmental Protection Agency proposes that the regulation must also be applied outside urban areas. Environmentally approved wood boilers are allowed to emit a maximum of 30 mg tars/MJ energy produced, and 40 mg/MJ for wood fired-stoves (except open fireplaces which are not included). The regulation is applied on installations that are used on a regular basis, therefore some local heating appliances, used only occasionally, are not included [Krögerström, 1994].
1.1.4 Actions to Reduce the Emissions
There are two principal approaches for decreasing emission from combustion: optimisation of the combustion process and cleaning of flue gases. To achieve efficient combustion several conditions must be met. Oxygen must be brought in a sufficient amount and must be mixed properly with the fuel. Also, there is an optimum combustion temperature, i.e. 900–1000°C which is based on the conversion of unburned compounds and the formation
4 of fuel NOx. Finally, the residence time has to be long enough for complete reactions. The high emission levels are largely due to existing out-of-date units. The average age of wood-fired boilers in Sweden is thought to be 20-25 years. Many existing boilers are of an old type constructed according to the natural draught burning principle with a chamber cooled by water. In these boilers, the combustion temperature is low, which leads to low efficiency, not more than 70%, and higher emission levels than in modern units (Table 1). Efficiency and low emission levels may be improved by fitting supplementary equipment to old boilers or by replacing them with new “environmentally approved” wood-burning boilers. The proportion of units meeting the emission standards set in the National Board of Housing, Building and Planning regulations (so called “environmentally approved units”) varies in Sweden, but it is estimated to be around 17% [Askensten, 2000]. Modern boilers are in theory very efficient with a well-designed ceramic insulated combustion chamber. However, these boilers are generally constructed for a higher output power than the immediate need. Indeed if the size of the combustion chamber is too small, the surfaces of the walls are too large in relation to the volume, leading to a great heat loss through the walls and short gas and particle residence times. Normally, to avoid over-heating of the surroundings, the amount of oxygen is decreased manually to minimise combustion, and this leads to high emission levels. In order to optimise the utilisation of the boilers, they may be equipped with a hot water storage tank [Krögerström, 1994]. This technique gives a substantial improvement even in combination with traditional boilers. A storage tank allows the boiler to work at full load for shorter periods since the boiler is then being used at its full design capacity. Hot water is then stored in a tank and is available for the whole day. It results in much cleaner flue gases and more efficient boilers (Table 1). For example, the emissions of VOC and tar from traditional boilers equipped with a storage tank are reduced by around 60-70%. Also, this implies wood and time savings as the fuel is added only once or twice a day during wintertime. On the basis of surveys, it is estimated that approximately 30% of existing wood-burning boilers in Sweden are equipped with a hot water storage tank. By the 1st of January 2005, in urban areas all existing wood boilers will have to install a hot water storage tank, or equivalent equipment. A further requirement is that heat storage equipment should be large enough to store the heat generated by a full load of wood inserted in the unit [Köthnig, 2000]. The use of a proper fuel of a relatively small size, low moisture and ash content and of homogeneous composition may also contribute to improved and more even combustion. Pellets for instance have a high potential energy and boilers with pellets burners have low emissions (Table 1).
5 In some appliances, it is also possible to feed the fuel automatically. This can provide a more stable and efficient combustion, because the temperature, mixing, and residence time are better balanced. Moreover, starting and finishing phases are also decreased due to a more even combustion. The supply of air that feeds the combustion can also be optimised. A too large supply of air may result in low residence time and low combustion temperatureandatthesametime,oxygenmustbesuppliedinasufficient amount to oxidise the products from the pyrolysis. The addition of air and its mixing with gases may be provided by using fans. A further improvement is the use of a sensor (lambda sond) to control the air supply, similar to that used in automotive exhaust systems, thus decreasing emissions during the starting and finishing phases.
Table 1. Comparison of the emissions from different burning units determined for a house with an annual consumption of ca 25 000 kWh [Löfgren, 1998b]. Burning Emissions (kg/year) units VOC Tars Particulates SO2 NOx CO2 Traditonal oil boiler 2 ca. 0 3 7 8 10 000 Traditional wood boiler 720 270 200 5 9 0 Traditional wood boiler with hot water storage tank 225 45 8 5 11 0 Modern wood boiler with hot water storage tank 28 0.5 2 5 13 0 Pellets burner 4 0.5 4 4 5 0
1.1.5 Catalytic Oxidation
Optimised combustion techniques can lead to emissions below the limits discussed above, however the costs engendered are sometimes prohibitive. In addition, it is likely that the emission legislation will be more stringent in the near future, since the current limits lead to unacceptable emissions from a health point of view [Viktorin, 1993]. An alternative or supplementary solution is to incorporate a catalytic system to oxidise the unburned compounds to CO2 and water at moderate temperatures. However, the integration of a catalyst should be considered as a final step in the design of wood burning appliances, because the main objective is to achieve high combustion efficiency. By using a catalyst the oxidation reactions in the flue gas can proceed although the temperature is lower and the residence time shorter than needed for homogeneous oxidation. It may be possible to install catalysts in traditional installations, but further studies are needed to evaluate this possibility. In addition, the utilisation of a catalytic system lowers the formation of NOx and
6 the material costs for boiler construction because catalytic oxidation is carried out at low temperature. The use of a properly designed and constructed catalytic system would also reduce the deposition of soot on the walls of the flue duct and hence limit the risk of fire. Catalysts intended for abatement of emissions from wood combustion are found among those which are being developed for other applications such as oxidation in lean-burn engines and removal of industrial solvents, mainly based on noble metals. Catalysts are already used in American, Norwegian and Austrian wood stoves. High conversion of unburned compounds over the catalyst and thus very low emissions for wood-fired boilers equipped with such catalysts have already been demonstrated [Carnö et al., 1996; Berg, 2001]. Nevertheless, the implementation of catalysts in the hostile environment of small-scale wood burning appliances (which can include harsh treatment by user) poses some special problems and challenges, such as:
- Varying temperature conditions (thermal deactivation of the catalyst), - Ash and particulates deposition on the catalytic surface (mechanical and chemical deactivation), - Catalyst inefficiency during the cold start-up phase. - Requirement for a low-cost catalytic system,
1.2 Scope of the Thesis
The present study was part of the activities within the framework of the EC FAIR-CT95–0682 project (1996-1998) “Abatement of emissions from small- scale combustion of biofuels” [Berg & Berge, 1999]. The work at the Royal Institute of Technology was focused on the development of total oxidation catalysts. In parallel with and as a continuation of this work, but outside the scope of this thesis, field tests were performed in collaboration with boiler manufacturers. The objective of the work, presented here, is the development of catalysts for total oxidation of VOC, CO and CH4, with particular emphasis on the utilisation of low-cost and environment-friendly raw materials, resistant to thermal and sulphur deactivation and high durability. Monolithic catalysts based on a mixture of metal oxides and noble metals supported on alumina are of particular interest here. Also, improving the understanding of the structural and chemical properties of the catalysts by various characterisation techniques has been attempted using Temperature-Programmed Reduction and Oxidation (TPR and TPO), BET-Surface Area Analysis, X-Ray Diffraction (XRD), X-Ray Photoelectron Spectroscopy (XPS), Raman Spectroscopy and Scanning and Transmission Electron Microscopy (SEM and TEM).
7 The present thesis consists of 6 papers and a main section where the results have been restructured in 4 chapters: “Catalysts for Total Oxidation”, “Catalyst Deactivation”, “Additives: Lanthanum and Cerium” and “Field Application”. Besides the results from the papers, a few additional experimental results are also included in the main section. The details concerning the preparation methods, characterisation techniques, reaction conditions and apparatus are described in the papers. It should be noted that the composition of the synthetic gas mixture used for catalytic activity measurements was chosen to represent some of the most essential compounds emitted from wood combustion. A mixture containing CO (ca 2500 ppm), naphthalene (ca 50 ppm), methane (ca 200 ppm), CO2 (12%), H2O (12%), O2 (10%) and N2 (balance, 66%) was chosen and denoted gas mixture 1. Naphthalene has been replaced in some of the activity measurements by the same amount of ethylene. In that case, the mixture was denoted gas mixture 2. Paper I describes the influence of the Pt content (0.01 to 1mol%/alumina) and the calcination temperature (500°C and 800°C) on the reduction behaviour of mixed MnOx-Pt/alumina catalysts and on its activity for the oxidation of CO, C10H8 and CH4 in comparison with single component catalysts, i.e., Pt and MnOx catalysts. Papers II and III discuss other combinations of metal oxides and noble metals. More specifically MnOx and CuO mixed with low amounts of Pt and Pd are investigated with emphasis on the thermal and sulphur resistance of mixed catalysts compared to metal oxide or noble metal catalysts. Paper IV presents the development of a well-adhered washcoat deposited on a metallic support upon high temperature treatments and discusses the influence of the amount of washcoat as well as the content of MnOx on the oxidation of CO, C10H8 and CH4. Finally, the possibility of minimising the cold-start emissions in a commercial wood boiler by pre-heating a full-scale catalyst based on MnOx-Pt/Al2O3 supported on a metallic monolith is demonstrated. Paper V focuses on the interactions between metal oxides (MnOx,CuO)and alumina doped with lanthanum. The stabilisation of the washcoat by lanthanum is examined, and more particularly the inhibition of undesirable solid-phase reactions between the active phases and the support during thermal treatment at high temperatures. Paper VI deals with the promoting effect of Ce and/or La in alumina- supported CuO, Pt and mixed CuO-Pt catalysts. The study investigates the synergism between Cu-Ce and Pt-Ce.
8 2 CATALYSTS FOR TOTAL OXIDATION
Comparison of different studies concerning the active components in catalysts is usually very difficult because of the divergence in concentrations, supports, preparation techniques, catalyst history and test conditions. The aim of this study is to select catalysts suitable for total oxidation in our reaction medium while at the same time providing high activity and stability at low cost.
2.1 Noble Metal Catalysts
2.1.1 General Noble metals are well-known oxidation catalysts with high activities, and are widely used for controlling exhaust gas emissions such as VOC, HCs and CO. Apart from the higher specific activities, noble metals are preferred because they are less liable to sulphur poisoning than metal oxide catalysts [Shelef et al., 1978]. Pt and Pd catalysts are the most commonly used for total oxidation [Kummer, 1980]. Pd was less used than Pt until the early 1990s partly because it is more sensitive to lead and sulphur compounds usually present in car exhaust gases. However, sharp decreases in lead levels in fuel led to increased interest in Pd-supported catalysts. The oxides of Pt and Pd formed during reaction cycles are not as volatile in contrast to RuO2,OsO4 or Ir2O3 which are also poisonous [Cotton & Wilkinson, 1988]. Other noble metals, such as Ag and Au, are not appropriate for high temperature and high space velocity applications. Also, the required use of Rh compared to Pt in TWCs far exceeds the natural ratio occurring in mines. In addition Rh2O3 is known to react with alumina [Yao et al., 1980; Shelef & Graham, 1994]. Information concerning the activities of Pt and Pd catalysts varies in the literature. Pt catalysts are well known to be the most active for the combustion of HCs containing more than one carbon atom whereas Pd is the most active catalyst for CO and CH4 oxidation [Kummer, 1980; Satterfield, 1991; Kang et al., 1994; Burch & Hayes, 1995]. However, Ball & Stack reported that Pt had higher activity than Pd for both the oxidation of HCs and CO [Ball & Stack, 1991]. Oxidation over noble metals is generally considered to be a structure- sensitive reaction [Briot & Auroux, 1990; Briot & Primet, 1991; Hicks et al., 1990a; Baldwin & Burch, 1990], albeit there are some controversy in literature [Chin & Resasco, 1999]. It is an accepted view that whenever the surface
9 reaction involves the scissions of a C-C bond, structure-sensitivity is to be expected [Gandhi & Shelef, 1987]. Oxidation of saturated hydrocarbons, especially those of short chain length, does not proceed readily on noble metal catalysts with very high dispersion, but rather on larger crystallites of Pt [Gandhi & Shelef, 1987; Otto, 1989; Briot & Auroux, 1990] or Pd [Hicks et al., 1990a; Hicks et al., 1990b; Briot & Primet, 1991; Carstens et al., 1998]. In general, the specific catalytic activity per noble metal surface atom for emission control is larger for the metallic crystallites than for the dispersed metal oxides [Yu Yao, 1984]. However, for Pd, the high thermal stability of the dispersed oxide in particular when CeO2 is present [Yu Yao, 1984; Groppi et al., 1999] makes it attractive for the oxidation of CO and olefinic or aromatic hydrocarbons. The turnover frequency (TOF) of the Pd for CH4 oxidation has been reported to increase with the size of the Pd-particles [Hicks et al., 1990a; Chin & Resasco, 1999]. In addition, the activity is strongly influenced by the interaction between Pd and the support [Sekizawa et al., 1993]. Two kinds of Pd oxide has been postulated: dispersed Pd oxide on alumina and Pd oxide deposited on metallic Pd with the latter being very active [Hicks et al., 1990a; Carstens et al., 1998; Chin & Resasco, 1999]. The degree of Pd-oxidation depends on the Pd particle size with small particles being oxidised easily [Hicks et al., 1990a; Chin & Resasco, 1999], while for Pt the formation of dispersed or crystalline phases depends more on the support composition and the method of preparation [Hicks et al., 1990a]. At high temperatures (> 500°C) the activity of supported Pd catalyst for CH4 oxidation might be due to the ability of the Pd oxide to chemisorb oxygen [Farrauto et al., 1992]. Pd as metal does not chemisorb oxygen above 650°C and is thus inactive toward CH4 oxidation [Farrauto et al., 1992]. However, CH4 can dissociatively adsorb on metallic Pd [Solymosi et al., 1994]. Pd was said to be more resistant to thermal sintering in an oxidising environment than Pt [Hegedus et al., 1979; Spivey & Butt, 1992; Heck & Farrauto, 1995]. Indeed, Pt does not penetrate into the alumina support but volatilise under oxidising conditions [Gandhi & Shelef, 1987]. This volatility of Pt when dispersed as an oxide on alumina under oxidising conditions results in a growth of the Pt crystallites. When Pt oxide is completely dispersed, it starts to decompose in oxygen at about 475°C [Kummer, 1986], while larger crystallites of Pt oxide may remained oxidised up to 700°C[Hickset al., 1990a; Cotton & Wilkinson, 1988]. Pd, however, can be dispersed as oxides on Al2O3 at higher temperature (750-850°C) than does Pt [Kummer, 1986]. This interaction between PdO and Al2O3 gives considerable activity to Pd-Al2O3 catalysts in an oxidising atmosphere. Pt was found to have higher sulphur resistance than Pd [Hegedus et al., 1979; Deng et al., 1993; Kang et al., 1994] and a quicker recovery once sulphur was removed from the gas stream [Monroe et al., 1991; Beck & Sommers, 1995]. Pt is more active for the oxidation of SO2 to SO3 [Kummer, 1980; Ball & Stack,
10 1991; Heck & Farrauto, 1995], which is regarded as the first step for the formation of sulphate on the catalytic surface. However, the influence of sulphur on the oxidation of HCs was said to be insignificant on both Pt and Pd, especially at high temperatures [Musialik-Piotrowska et al., 1987; Beck & Sommers, 1995]. 2.1.2 Results and Discussion [Paper III]
The Pd catalyst was slightly more active than the Pt (0.1 mol%) catalyst for the oxidation of CO, C10H8 and CH4, as seen in Figure 2. The activity loss above 500°C of the Pd catalyst for the oxidation of CH4 with increasing temperature, termed “v” shape, is attributed to the decomposition of PdO to Pd metal, which is less active for the oxidation of CH4 [Farrauto et al., 1992; Sekizawa et al., 1993; Chin & Resasco, 1999; Forzatti & Groppi, 1999].
100
CO 80 CH4 C10H8 Pd Pt 60
40 Pd Pt Conversion (%)
20
0 100 200 300 400 500 600 700 800 Catalyst temperature (oC)
Figure 2. CO, C10H8 and CH4 conversion for Al2O3-supported Pt and Pd (0.1 mol%/Al2O3), calcined at 800°C for 4 h in air. Gas mixture 1.
When C10H8 was replaced by C2H4 in the gas mixture and with an amount of H2O of 4% instead of 12%, a peculiar C2H4 conversion for Pd was observed as can be seen in Figure 3 [Ferrandon & Björnbom, 1998]. Namely, the conversion of C2H4 occurred readily together with CO until CO was completely converted, above that temperature the oxidation of C2H4 was slowed. This behaviour of C2H4 was seen on the Pd catalyst but not on other catalysts such as Pt or metal oxides. When CO was removed from the gas mixture, the oxidation of C2H4 occurred much slower, suggesting that the presence of CO has a positive effect on the oxidation of C2H4 for a Pd catalyst. Similarly, the presence of CO in the gas mixture had a beneficial effect on the oxidation of CH4 (Figure 3). Indeed, the temperature required for 50% conversion of CH4 was lowered by around 140°C, in the presence of CO. In all tests where both CO and CH4 were present, i.e., experiments 1,4 and 7 (Table 2), the conversion of CH4 followed the ”v” shape, while in the absence of CO,
11 no such behaviour was observed. An improvement for the oxidation of C10H8 was also seen in presence of CO (Table 2, tests 4 and 5), however, the effect was not as great as for C2H4 and CH4.
100
80 CO (a)
60 C2H4 (a) C2H4 (b)
CH (a) 40 4 CH4 (b) Conversion (%) 20
0 100 200 300 400 500 600 700 800 Catalyst temperature (oC)
Figure 3. Conversion of the combustibles for a Pd/Al2O3 (0.1 mol%/Al2O3)ingas mixture 2 (a) and without CO (b). 4% instead of 12% H2O.
At this stage it is difficult to draw conclusions about the role of CO on the Pd catalyst. However, Carstens et al. have reported that oxidation of Pd under reaction conditions with 3% CH4 formed large PdO crystallites while in the absence of CH4 amorphous PdO was formed, the former being more active for CH4 oxidation [Carstens et al., 1998]. It is probable that the presence of 2500 ppm CO in our reaction medium induces a similar change at the PdO surface.
Table 2. Temperature (°C) for 50% conversion of the combustibles in different mixtures for a Pd/Al2O3 (0.1 mol%/Al2O3). 4% instead of 12% H2O.
Tests T50% (°C)
No. Combustibles CO C10H8 C2H4 CH4
1CO,C2H4,CH4 181 - 270 504 2-,C2H4,CH4 - - 364 652 3CO,C2H4, - 192 - 202 - 4CO,C10H8,CH4 216 236 - 501 5-,C10H8,CH4 - 273 - 642 6CO,C10H8, - 225 246 - - 7CO,-,CH4 184 - - 496
Some important results from Section 2.1:
) The oxidation activity of the Al2O3-supported catalysts (0.1 mol%) is as follows: CO, C10H8,CH4:Pd>Pt C2H4: Pt>Pd
) C10H8,C2H4 and CH4 oxidation on Pd/Al2O3 are enhanced in presence of CO in the gas mixture.
12 2.2 Metal Oxide Catalysts
2.2.1 General
The high cost of precious metals, their limited availability and their sensitivity to high temperatures have long motivated the search for substitute catalysts. Metal oxides are an alternative to noble metals as catalysts for total oxidation. They have sufficient activity, although they are less active than noble metals at low temperatures. However, at high temperatures the activities are similar. Some metal oxides deteriorate when exposed alternately to oxidising/ reducing atmospheres [Satterfied, 1991]. They may also react with Al2O3 to form metal aluminates, MeAl2O4, of low activity [Taylor, 1984; Zwinkels et al., 1993; Bolt et al., 1998]. However, some combinations of oxides may have high catalytic performance and high thermal stability as compared to single components. Such catalysts include Cu-Mn [Puckhaber et al., 1989; Agarwal & Spivey, 1992; Wang et al., 1999; Mehandjiev et al., 2000], Cu-Cr [Yu Yao, 1975; Heyes et al., 1982b; Severino et al., 1986; Laine et al., 1987; Terlecki- Baričević et al., 1989; Laine et al., 1991; Stegenga et al., 1991; López Agudo et al., 1992; Kapteijn et al., 1993; Murthy & Ghose, 1994; Chien et al., 1995; Vass & Georgescu, 1996], Cu-V [Ahlström & Odenbrand, 1990], Mn-Ni [Mehandjiev et al., 1998], Ag-Mn [Haruta & Sano, 1983; Watanabe et al., 1996; Luo et al., 1998], Ag-Co [Haruta & Sano, 1983; Luo et al., 1998], Cr-Co [Prasad et al., 1980; Vass & Georgescu, 1996] and Co-Zn [Klissurski & Uzunova, 1993]. Metal oxides are also more susceptible to poisoning by sulphur compounds than noble metals [Ball & Stack, 1991]. However, reports in literature claim some metal oxides have good sulphur poisoning resistance. For some applications, the higher overall loading of metal oxides in the catalysts makes them more tolerant to poisons than noble metals, since some compounds even at low concentrations may quickly poison the limited number of noble metal oxidation sites present [Hegedus et al., 1979]. It has been shown that oxides of Co could act both as catalysts for total oxidation of CO and at the same time they can act as sorbents for sulphur [Pope et al., 1976]. Zarkov and Mehandjiev [Zarkov & Mehandjiev, 1993] found that CoAl2O4 is stable towards SO2 and has activity for CO oxidation similar to that of Pt catalysts. A catalyst containing mostly CuO on Al2O3 was found to have high activity for the oxidation of CO and HCs and high sulphur and lead poisoning resistance [Peiyan et al., 1987]. Terlecki-Baričević et al. [Terlecki-Baričević et al., 1989] reported that copper chromite was sulphur resistant because only SOx chemisorption occurs, rather than formation of sulphates [Farrauto & Wedding, 1974]. In an investigation of CH4 and CO oxidation over metal oxides, Yu Yao [Yu Yao, 1975] studied CuO, CuCr2O4,Co3O4,Fe2O3,MnO2,
13 SnO2 and ZrO2 andreportedthatCuOandCuCr2O4 had the highest activity and best sulphur tolerance. The most active single metal oxide catalysts for complete oxidation for a variety of oxidation reactions are oxides of Ag, V, Cr, Mn, Fe, Co, Ni and Cu [Dmuchovsky et al., 1965; Moro-Oka et al., 1967; Shelef et al., 1968; Heyes et al., 1982a; Spivey, 1989; McCarty et al., 1997; Tahir & Koh, 1997]. However, vanadia is known to convert sulphur into sulphur oxides [Dunn et al., 1998], which can pose a problem when using Al2O3 asasupport,sinceitmayreact and form sulphate. Chromium is toxic and thereby should be avoided. Among the oxides mentioned in the literature, a few seems particularly promising as follows. CoOx is known to be an effective catalyst for total oxidation reactions [Pope et al., 1976; Boreskov, 1982; Sinha & Shankar, 1993; Zarkov & Mehandjiev, 1993; Luo et al., 1998; Ji et al., 2000]. CuO is also a well-known component of oxidation catalysts [Yu Yao, 1975; Kummer, 1980; Severino et al., 1986; Huang & Yu, 1991; Boon et al., 1992; Cordi et al., 1997; Park et al., 1998b], and has been considered as a substitute for noble metal catalysts in emission control applications due to its high activity, tolerance to sulphur [Yu Yao, 1975; Peiyan et al., 1987] and refractory nature [Prasad et al., 1984]. CuO-based catalysts show similar activity to noble metal catalysts for CO oxidation [Kummer, 1980; Larsson et al., 1996] and exhibit the greatest ability amongst Co3O4,MnO2 and Pt to maintain oxidative capacity of butanal under the sulphating effect of mercaptan [Heyes et al., 1982a]. Also, CuO/TiO2 was found to be more active than oxides of Co, Mn and Fe for both CO and toluene oxidation [Larsson et al., 1996]. Among the transition metal oxides, Mn oxides are recognised as being very active for total oxidation of CO and HCs [Kummer, 1980; van de Kleut, 1994; Kalantar Neyestanaki, 1995; Baldi et al., 1998; Lahousse et al., 1998; Tsyrulnikov et al., 1998; Parida & Samal, 1999; Zaki et al., 1999] and they are considered to be environment-friendly materials [Reidies, 1986]. Mn oxides assume a wide range of simple and mixed compositions with Mn atoms in different oxidation states such as β-MnO2, γ-MnO2,Mn5O8, α-Mn2O3, γ-Mn2O3 and α-Mn3O4, which can, according to Zener [Zener, 1951], establish the necessary electron- mobile environment for optimal surface redox catalysts. Also Mn oxides, compared with other metal oxides for example CuO, present a lower volatility at high temperatures in presence of steam [van de Kleut, 1994] and react to a lower extent with Al2O3 to form spinel aluminate, MnAl2O4, of low activity [Strohmeier & Hercules, 1984]. According to the phase diagrams, Mn oxides react with Al2O3 to form MnAl2O4 only from 1000°C upwards [Ranganathan et al., 1962], while from 800°C upwards, CuO reacts more readily with Al2O3 to form CuAl2O4 [Misra & Chaklader, 1963]. Commercial catalysts based on oxides of Mn are available and used in self-cleaning oven walls [Nishino et al., 1981; Tsyrulnikov et al., 1998]
14 2.2.2 Results and Discussion
Choice of the metal oxides
Al2O3-supported oxides of Cu, Mn, Fe, Co and Ni (prepared by incipient wetness technique and calcined at 800°C for 4 h in air) were compared for their ability to oxidise CO, C10H8 and CH4 in the gas mixture, as seen in Figures 4a, 4b and 4c respectively. Oxides of Cu and Mn seem to be the most active oxidation catalysts in our reaction medium. For the oxidation of CO and CH4, CuO had a better activity compared to MnOx, while for C10H8, the opposite was observed. Grisel & Nieuwenhuys, also found that CuO was more active than MnOx for the oxidation of CO and CH4 [Grisel & Nieuwenhuys, 2001]. Similar results were also found for unsupported metal oxides [Yu Yao, 1975; Boreskov, 1982]. The activities of oxides of Co and Ni supported on Al2O3 were very low, certainly due to the reaction with Al2O3 to form spinel at 500°C[Boltet al., 1999]. It was reported that CoAl2O4 has activity for CO oxidation similar to that of Pt catalysts [Zarkov & Mehandjiev, 1993]. On the other hand, it is reported that spinel, such as cobalt and nickel aluminates, has low activity, in agreement with our present results [Garbowski et al., 1990; Schmieg & Belton, 1995; Ji et al., 2000]. Accordingly, further studies have focused on oxides of Mn and Cu. 100
80
60 CuMn Fe Co Ni
40 Conversion (%)
20
0 200 300 400 500 600 700 800 Catalyst temperature (oC)
Figure 4a. CO conversion for Al2O3-supported oxides of Cu, Mn, Fe, Co and Ni (Metal:10mol%/Al2O3), calcined at 800°C for 4 h in air. Gas mixture 1.
15 100
80
60 Mn Cu Fe Co Ni
40 Conversion (%)
20
0 200 300 400 500 600 700 800 Catalyst temperature (oC)
Figure 4b. C10H8 conversion for Al2O3-supported oxides of Cu, Mn, Fe, Co and Ni (Metal:10mol%/Al2O3), calcined at 800°C for 4 h in air. Gas mixture 1.
100
80
60 Cu MnFe Co Ni
40 Conversion (%) 20
0 500 600 700 800 Catalyst temperature (oC)
Figure 4c. CH4 conversion for Al2O3-supported oxides of Cu, Mn, Fe, Co and Ni (Metal:10mol%/Al2O3), calcined at 800°C for 4 h in air. Gas mixture 1.
Manganese oxides catalysts
In the following section, the effects of the calcination temperature and the type of Al2O3 support on the oxidation activity of MnOx/Al2O3 were investigated. Furthermore, the influence of the amount of washcoat that contained MnOx as well as the influence of MnOx concentration was studied.
Calcination temperature [Papers I, II, III]
As mentioned earlier, MnOx is known to have many oxidation states, redox reactions are thus of great importance. When heated in air, MnOx undergo phase transitions; according to literature data, between 500-600°CMnO2 is converted into Mn2O3 and above 890°CintoMn3O4 [Reidies, 1986]. Figure 5 presents the results from the activity measurements at 50% conversion, performed in gas mixture 1, on MnOx/Al2O3 (10 mol%/Al2O3)
16 catalysts calcined at different temperatures between 500°C and 900°C. The preparation method used for these catalysts was the deposition-precipitation technique, as described in Paper I. A calcination temperature of 500°C results in the formation of a more active MnOx for the oxidation of CO and C10H8, compared to those calcined at 650°C or 800°C, provided that the catalysts were not exposed to higher temperature than that of the calcination. CH4 oxidation occurs at a higher temperature than that of the calcination, consequently the activities were equal for catalysts calcined at 500°C and 650°C (Figure 5). According to characterisation using temperature-programmed reduction (TPR) (Table 3), Raman spectroscopy (Figure 6) and X-ray diffraction (XRD) (not shown here), Mn2O3 was prevalent in catalysts calcined between 500°C and 800°C, and this in line with literature data [Reidies, 1986]. After treatment at higher temperature, viz. 900°C for 60 h in air with 12% steam [Papers II, III], most of the MnOx catalysts were activated for the oxidation of CO and C10H8 and only slightly deactivated for the oxidation of CH4, compared to the catalysts calcined at 800°C (Figure 5). The enhancement in activity of the hydrothermally treated MnOx catalyst for the oxidation of C10H8 and CO was probably due to the appearance of a new more active MnOx phase, Mn3O4 formed during the treatment at 900°Casobservedinthe characterisation using Raman (Figure 6) [Paper II]. It should be noted that this enhancement is obtained despite the much lower BET-surface area of MnOx/Al2O3 treated at 900°C (59 m2/g). Similar improvements have also been discussed by Tsyrulnikov et al. [Tsyrulnikov et al., 1991; Tsyrulnikov et al., 1998], who observed an increase in catalytic activity of MnOx with a lower average valence state for the oxidation of butane, benzene, and particularly, CO. They attributed the increase of activity to the formation of Mn3O4.2 which has a defective structure similar to the structure of Mn3O4 spinel. Therefore it may be concluded that for the oxidation of all the compounds studied here the activity of alumina-supported Mn3O4 is superior to that of Mn2O3.
Table 3. TPR and BET data of MnOx/Al2O3 (Mn: 10 mol%/Al2O3) calcined at various treatments. Calcination TPR data BET-surface area treatments O/Mn ratio Predominant species (m2/g)
500°C, 4 h, air 1.60 Mn2O3 182 800°C, 4 h, air 1.60 Mn2O3 144
17 700
C) CH4 o 600
500
400 CO
300 C10H8 Temperature for 50% conversion ( 200 500 650 800 900 o MnOx calcination temperature ( C)
Figure 5. Temperature for 50% conversion of CO, C10H8 and CH4 for MnOx/Al2O3 (Mn: 10 mol%/Al2O3) calcined at 500°C, 650°C, 800°C for 4 h in air and at 900°Cfor 60 h in air with 12% steam. Gas mixture 1.
O O:Mn3O4 * :Mn2O3
O O O
MnOx A Intensity (a.u.) * * MnOx F
1100 900 700 500 300 100 Ramanshift+(cm-1)
Figure 6. Raman spectra of MnOx/Al2O3 (Mn: 10 mol%/Al2O3) calcined at 800°Cfor 4 h in air (F) and after thermal treatment at 900°C for 60 h in air with 12% steam (A).
Influence of the support
Minimising the undesirable interactions between the active components and the washcoat is a prerequisite for practical applications of the catalysts. MnOx was supported on either γ-Al2O3 or α-Al2O3 and tested for the oxidation of a mixture containing CO, C2H4 and CH4. α-Al2O3 was prepared by calcining γ-Al2O3 at 1200°Cfor4h,whichgavea BET-surface area of 5 m2/g, compared to γ-Al2O3 which has a surface area between 150-250 m2/g (Table 4). α-Al2O3 has a hexagonal close-packed structure that cannot accommodate foreign ions to be in a defective spinel structure such as γ-Al2O3, thus the former may interact to a lesser extent with the catalytic components. α-Al2O3 is the most thermodynamically stable Al2O3, hence encapsulation of the active phases during high-temperature treatment
18 and thus loss of activity could be avoided, since final phase transformation has already occurred. The results from the activity measurements, carried out in gas mixture 2, at 50% conversion are presented in Figures 7a, 7b and 7c. Both the oxidations of CO and C2H4 occurred at lower temperature on MnOx/α-Al2O3, while for the oxidation of CH4,MnOx/γ-Al2O3 led to a higher activity. Remarkably, after a calcination temperature of 1000°C, MnOx/α-Al2O3 shows a significant increase in activity for the oxidation of CH4 (Figure 7c). XRD measurements on samples deposited on α-Al2O3 demonstrated the presence of Mn3O4 already at a calcination temperature of 600°C(Table4), whereas on samples deposited on γ-Al2O3,noMn3O4 was found at a calcination temperature lower than 900°C, as shown previously (Table 3). Thereby, the early transformation into Mn3O4 results in a higher activity of the catalysts deposited on α-Al2O3,comparedtoγ-Al2O3. The change in oxidation state of MnOx with temperature differs on the two supports, probably due to a higher dispersion of MnOx on γ-Al2O3 that is maintained on the larger surface area and more amorphous support. Because a large dispersion of active components is required for the oxidation of CH4, γ-Al2O3 seems to be the appropriate support for MnOx catalysts.
Table 4. Characteristics of the MnOx supported on α-Al2O3 and γ-Al2O3 (Mn: 10 mol%/Al2O3) calcined at various temperatures for 4 h in air. Calcination BET data (m2/g) XRD data
Temperature (°C) γ-Al2O3 α-Al2O3 500 243 - 600 229 Mn2O3/Mn3O4 700 201 Mn2O3/Mn3O4 800 178 Mn2O3/Mn3O4 900 143 Mn2O3/Mn3O4 1000 - Mn3O4
19 C)
o 500
γ γ-Al2O3 400
300
α-Al2O3 Temperature for 50% conversion ( 200 500 600 700 800 900 1000 Calcination Temperature (oC)
Figure 7a. Temperature for 50% CO conversion for MnOx deposited on γ-Al2O3 and α-Al2O3 (Mn: 10 mol%/Al2O3), treated at various temperatures for 4 h in air. Gas mixture 2. C) o 500
γ γ-Al2O3
400
α α-Al2O3 300 Temperature for 50% conversion ( 200 500 600 700 800 900 1000 Calcination temperature (oC)
Figure 7b. Temperature for 50% C2H4 conversion for MnOx deposited on γ-Al2O3 and α-Al2O3 (Mn: 10 mol%/Al2O3), treated at various temperatures for 4 h in air. Gas mixture 2.
800 C) o
α-Al2O3
700
Temperature for 50% conversion ( γ γ-Al2O3 600 500 600 700 800 900 1000 Calcination temperature (oC)
Figure 7c. Temperature for 50% CH4 conversion for MnOx deposited on γ-Al2O3 and α-Al2O3 (Mn: 10 mol%/Al2O3), treated at various temperatures for 4 h in air. Gas mixture 2.
20 Concentration of active components [Paper IV]
The influence of the amount of washcoat (up to 20 wt%/catalyst weight), as well as the influence of the concentration of MnOx in it (Mn: 5 to 20 mol%/Al2O3) on the activity of catalysts calcined at 800°C for 4 h were studied. The results are presented in Figures 8 and 9. For the catalysts containing the same total amount of Mn but different amount of washcoat (Figure 8), a high concentration of MnOx in the washcoat, i.e. low amount of washcoat, favoured the oxidation of CO and C10H8,whereas a lower concentration of MnOx in the washcoat, i.e. a high amount of washcoat, gave higher activity for the oxidation of CH4. Because the reaction of CO and C10H8 are fast reactions, an increase dispersion of Mn would increase the internal diffusion and thereby limit the reaction. In the catalysts, which had the same amount of washcoat (Figure 9), an increased total amount of MnOx resulted in an increase in activity for the oxidation of the three combustibles. 800 C) o CH4 700
600
500
CO 400 C10H8 Temperature for 50% conversion ( 300 0 5 10 15 20 25 washcoat (wt%/catalyst)
Figure 8. Temperature for 50% conversion of CO, C10H8 and CH4 for MnOx/Al2O3 with different amounts of washcoat, but the same total amount of Mn (ca 3.4 10-4 mol) calcined at 800°C for 4 h in air. Gas mixture 1. 800 C) o 700 CH4
600 CO 500
400 C10H8
300 Temperature for 50% conversion ( 200 0 5 10 15 20 25 Mn (mol%/Al2O3)
Figure 9. Temperature for 50% conversion of CO, C10H8 and CH4 for MnOx/Al2O3 with different amounts of Mn, but the same total amount of washcoat (20 wt%) calcined at 800°C for 4 h in air. Gas mixture 1.
21 Copper oxide catalysts [Paper V]
According to reports in the literature, CuO/Al2O3 is present under various species that differ in activity towards oxidation of CO and HCs [Marion et al., 1991; Park & Ledford, 1998b]. Therefore it was important to identify the Cu species present in our catalyst samples. TPR is a very useful method that gives information about the species produced during calcination, especially through analysis of the reduction temperature [Hurst et al., 1982], and it further shows the influence of the support on the metal oxides [Gentry & Walsh, 1982; Solcova et al., 1993]. Dumas et al. [Dumas et al., 1989] showed that H2 TPR of CuO/Al2O3 exhibits different reduction peaks. One corresponds to the reduction at 200-220°Cof the surface Cu species for Cu loading under saturation of Al2O3, i.e. < 6.4 - 8 mol% Cu / 100 m2/g Al2O3 [Friedman et al., 1978]. According to several authors [Wolberg & Roth, 1969; Lo Jacono & Schiavello, 1976; Friedman et al., 1978; Strohmeier et al., 1985], these species, in strong interaction with Al2O3, would consist of Cu2+ ions, forming on the surface of the Al2O3, a structure related to that of a spinel compound. Also, Garbowski and Primet [Garbowski & Primet, 1991] reported that Cu2+ ions may be anchored by the hydroxyl groups of Al2O3. It is believed that a “surface spinel” compound is an unusual structure (different to bulk spinel compounds), occupying tetragonal distorted octahedral sites (>90%) with only a small fraction located in the tetrahedral sites [Freeman & Friedman, 1978; Friedman et al., 1978]. Two surface Cu species can be distinguished by ESR and magnetic susceptibility: isolated and interacting or clustered ions [Berger & Roth, 1967; Centi et al., 1995; Park & Ledford, 1998b]. As the Cu loading increases, the ratio between the isolated and the interacting species decreased, however Centi et al. reported the latter to be predominant even at very low loading [Centi et al., 1995]. To understand the nature of the Cu species occurring in this study, some TPR experiments on CuO/Al2O3 (1.6 to 8 mol% Cu / 100 m2/g Al2O3), were conducted. At 1.6 mol% Cu, the TPR profile showed a single peak at 250°C which appeared at a lower temperature with increasing amounts of Cu (Figure 10). It can be seen that above 4.8 mol% Cu, a second peak appeared at 250°C and grew with increasing content of Cu, which also corresponded to the detection of CuO by XRD analyses (data not shown here). Furthermore, the colour of the samples changed from green to grey with an increasing amount of Cu, indicating the progressive formation of CuO. It is known that with higher Cu loading, segregation of CuO appears. Reduction of bulk CuO appears at a higher temperature (ca. 300-350°C) than the reduction of surface Cu2+ [Dumas et al., 1989; Marion et al., 1991], since surface ions are more reactive towards hydrogen [Marion et al., 1991]. Moreover, on a large CuO aggregate, H2 first reduces the surface, generating a skin of metallic copper on the top of CuO. This may in turn reduce the H2-diffusion rate to the bulk CuO,
22 hence longer time and consequently temperatures are required to completely reduce the aggregate. It can be observed (Figure 10) that with a higher concentration of Cu, the temperature for reduction of the 2nd step increased indicating that the larger the particle size, the higher is the reduction temperature which tended to be close to that of the bulk CuO. These observations would be helpful concerning the activities in the next sections of this thesis.
8 mol% Cu
6.4 mol% Cu
4.8 mol% Cu consumption (a.u.) 2 H 3.2 mol% Cu
1.6 mol% Cu
bulk CuO
150 250 350 450 Temperature (oC)
Figure 10. TPR profiles of CuO/Al2O3 (1.6 to 8 mol% Cu / 100 m2/g Al2O3) calcined at 300°C for 4 h. TPR of bulk CuO is also shown. TPR experimental conditions: 40 ml/min,5%H2 in Ar.
Some important results from Section 2.2:
) The oxidation activity of the Al2O3-supported oxide catalysts (10 mol%) is as follows: CO, CH4: Cu > Mn >Fe > Co = Ni C10H8: Mn>Cu=Fe>Co=Ni
) Between 500°C and 800°CAl2O3-supported MnOx is in the form of Mn2O3, while above 900°CasMn3O4 which is more active for the oxidation of CO, C10H8 and CH4.
) The oxidation activity of the γ or α-Al2O3-supported MnOx catalysts (Mn: 10 mol%/Al2O3) calcined at different temperature is as follows: CO, C2H4:MnOx/α-Al2O3 >MnOx/γ-Al2O3 (500-1000°C) CH4:MnOx/γ-Al2O3 >MnOx/α-Al2O3 (500-950°C) CH4:MnOx/α-Al2O3 >MnOx/γ-Al2O3 (950-1000°C)
23 ) The oxidation activity of the Al2O3-supported MnOx catalysts depends on both the amount of washcoat and the Mn content in the washcoat. For the same total amount of Mn in the washcoat a low amount of washcoat is preferred for the oxidation of CO and C10H8, conversely to CH4. For the same amount of washcoat, an increased amount of Mn enhances the oxidation of all combustibles studied here.
) The Al2O3 washcoat can accommodate a Cu concentration between 3.2 and 4.8 mol% Cu / 100 m2 Al2O3. Under this saturation value, Cu species are in the form of surface Cu2+ species. Above this saturation value, CuO crystallites are formed.
24 2.3 Combination of Metal Oxides and Noble Metals
2.3.1 General
As a single component, base metal oxide catalyst cannot rival a precious metal catalyst, hence improvement in their activities has been attempted by combining base metal oxide catalysts and small amounts of noble metals. Mixtures of noble metal and metal oxide merit more attention as catalysts for total oxidation, since both components have different good properties by themselves. Namely, noble metals have moderate oxygen affinity and its release proceeds smoothly [Haruta & Sano, 1983]. So the activity of noble metals may be further increased by combining them with other elements, such as metal oxides, which modify the mobility of oxygen. The role of metal oxides is also to diminish the CO inhibition which is typical of Pt catalysts at low temperatures [Mergler et al., 1996]. On the other hand, Pt helps metal oxides with multioxidation states to accelerate the oxygen transfer from the gas phase to the catalyst [Ménézo et al., 1993]. Pt and Pd can also be incorporated in catalytic systems containing metal oxides in order to increase the selectivity towards CO2 [Ménézo et al., 1993; Khairulin et al., 1997]. Some studies reported the particular performance of Pt and Pd mixed with metal oxides when pre- reduced [Mergler et al., 1996; Törncrona et al., 1997; Holmgren et al., 1999]. However, in the conditions studied here, with an excess of oxygen, pre- reducing treatment would not be useful for long-term use of the catalysts. A famous example of synergism is shown by a Au - Fe oxides composite catalysts developed by Haruta et al. for low-temperature oxidation of CO [Haruta et al., 1993]. Au/MnOx has also been regarded as a promising catalyst for such reaction with relatively high durability [Gardner et al., 1991]. Grisel & Nieuwenhuys studied the effect of various combinations of metal oxides with Au for the oxidation of CO and concluded that the activities were related to the average particle size whereas the identity of the metal oxides was less important [Grisel & Nieuwenhuys, 2001]. They found also that the oxidation of CH4 occurred more swiftly on Au/MnOx/Al2O3 compared to single component catalysts. They concluded that both CO and CH4 were adsorbed at Au/MnOx interface, while the role of MnOx was to dissociate O2 and supply oxygen since Au itself is not active in O-O bond scission. Another example is a catalyst containing Mn2O3 and Pd that showed higher activity for the oxidation of CO than a Pd catalyst and other combinations such as Pd/Fe2O3 and Pd/Co3O4 catalysts [Tsuji & Imamura, 1993; Imamura et al., 1995]. Imamura et al. attributed the increase of CO oxidation, for unsupported Pd/Mn2O3,toanadsorptionofCOontooxidisedPdincontact with Mn2O3, and then oxidation of CO into CO2 which migrates and desorbs from Mn2O3 [Imamura et al., 1995]. Also, addition of CoOx to Pd causes a
25 significant decrease in the temperature for oxidation of CO and C3H8 under stoichiometric conditions [Skoglundh et al., 1996]. It was observed that CoOx could be reduced and oxidised and that the redox Co sites are active for oxidation [Skoglundh et al., 1996; Törncrona et al., 1997]. Some authors also noted that doping of Pd with oxides of Co or Cr decreases the amount of bonded sulphate and thus restrains the poisoning effect on these catalysts [Khairulin et al., 1997]. Mixtures of metal oxides with Pd were reported to be highly active for the oxidation of CH4. Particularly oxides of Ni have been reported to decrease the Pd particles size and, by stabilising the PdO phase, enhance its activity [Ishihara et al., 1993; Kalantar Neyestanaki & Lindfors, 1998; Widjaja et al., 1999]. The doping of Pt with Al, Cr, Cu, Mn and Co has been reported to result in catalysts with high activity for the oxidation of CO and stability with respect to SO2 poisoning [Osmanov, 1986]. Compared with unpromoted Pt, promotion with pre-oxidised CoOx enhances the low-temperature activity for both CO and C3H6 in lean conditions [Törncrona et al., 1997]. Mergler et al. discussed a number of models which could account for the observed improvement in CO oxidation in stoichiometric or rich conditions over a CoOx-Pt catalyst [Mergler et al., 1997]. One is based on the reaction between weakly bound O on CoOx andCOonPtattheCo-Ptinterface.
2.3.2 Results and Discussion
Combinations of MnOx or CuO and Pt or Pd supported on Al2O3 were studied in this thesis. Their activities were compared to single component catalysts to identify any synergetic effects, as found for other combinations in the literature reports.
Effects of metal oxides on noble metals [Paper II, III, VI]
Table 5 and Figures 11 and 12 present the results from the activity measurements carried out with the gas mixture 1, while Table 6 and Figure 13 present the results from activity tests performed with the gas mixture 2 (see Section 1.2). Addition of MnOx (10 mol%/Al2O3) to the noble metals (up to 0.1 mol%), particularly Pd, decreased the activity of the corresponding noble metals for the oxidation of CO and C10H8 (Table 5). Increasing the amount of Pt (0.5 mol%), as seen in Figure 11 and Table 6, dampened the negative effect of mixing Pt and MnOx. Indeed, the temperature difference for 50% CO conversion between Pt and MnOx-Pt was 80°C at 0.05 mol% Pt, 45°C at 0.1mol% Pt, and only 10°C at 0.5 mol% Pt. In
26 addition, as seen in Figure 11, MnOx-Pt (10-0.5) had a higher activity than Pt catalyst at low conversion levels. For the oxidation of CH4, the mixed MnOx-Pt and MnOx-Pd catalysts had an activity similar to that of the MnOx catalyst (Table 5 and Figure 12). The activity of the mixed CuO-Pt (10-0.1) catalyst was similar, but very slightly decreased, compared to Pt catalyst for the oxidation of CO and C10H8, and similar to the CuO catalyst for the oxidation of CH4, as can be seen in Table 5. A similar behaviour was observed for mixed CuO-Pd for the oxidation of CO, C10H8 (=Pd) and CH4 (= CuO). A clear synergetic effect between CuO and Pt was observed for the CuO-Pt catalyst for CO and C2H4 at increased amount Pt (0.5 mol%), as seen in Table 6 and Figure 13.
Table 5. Temperature for 50% conversion of CO, C10H8 and CH4 for Al2O3-supported MnOx, CuO, Pd, Pt and mixed catalysts calcined at 800°Cfor4h.Mn,Cu,PtandPd mol%/Al2O3 are indicated in parentheses. Gas mixture 1.
Catalysts CO C10H8 CH4 MnOx (10) 425 365 645 CuO (10) 344 420 590 Pt(0.05) 265 270 690 Pt (0.1) 245 250 640 Pd (0.1) 217 220 640
MnOx-Pt (10-0.05) 345 335 640 MnOx-Pt (10-0.1) 290 290 640 MnOx-Pd (10-0.1) 290 290 640 CuO-Pt (10-0.1) 255 265 593 CuO-Pd (10-0.1) 220 229 595
Table 6. Temperature for 50% conversion of CO, C2H4 and CH4 for Al2O3-supported MnOx, CuO, Pt and mixed catalysts calcined at 800°Cfor4h.Mn,Cu:10;Pt:0.5 mol%/Al2O3. Gas mixture 2.
Catalysts CO C2H4 CH4
MnOx 406 450 619 CuO 327 471 578 Pt 200 206 629 MnOx-Pt 210 218 620 CuO-Pt 175 182 576
It should be noted that in the concentrations used in this study, Pt and Pd possess a superior catalytic activity to that of CuO and MnOx for the oxidation of CO, C10H8 and C2H4, while for the oxidation of CH4, CuO is largely more active than Pt and Pd, while MnOx is as active as Pd and Pt.
27 100 Pt (0.5) 80 MnOx-Pt (10-0.5) 60
40 Pt (0.1) MnOx-Pt
Conversion (%) (10-0.1) 20
0 150 250 350 Catalyst temperature (oC)
Figure 11. CO conversion for Al2O3-supported MnOx, Pt and mixed MnOx-Pt catalysts calcined at 800°C for 4 h in air. Mn and Pt mol%/Al2O3 are indicated in parentheses. Gas mixture 1.
100
80 CuO (10) = CuO-Pd 60 (10-0.1)
40
Conversion (%) Pd (0.1) MnOx (10) 20 =
MnOx-Pd (10-0.1) 0 400 500 600 700 800 Catalyst temperature (oC)
Figure 12. CH4 conversion for Al2O3-supported CuO, MnOx, Pd, mixed CuO-Pd and MnOx-Pd catalysts calcined at 800°C for 4 h in air. Mn, Cu and Pd mol%/Al2O3 are indicated in parentheses. Gas mixture 1.
100
80
60 CuO-Pt Pt CuO
40 Conversion (%) 20
0 100 200 300 400 Catalyst temperature (oC)
Figure 13. CO conversion for Al2O3-supported CuO, Pt and mixed CuO-Pt catalysts calcined at 800°C for 4 h. Cu: 10; Pt: 0.5 mol%/Al2O3. Gas mixture 2.
28 The different effects of metal oxides on noble metals can explain the differences in the activity behaviour of the mixed metal oxide-noble metal catalysts, with either MnOx or CuO, compared to the activity of the noble metals. SEM analyses were conducted on some of the samples. Back-scattered electron (BSE) detection, based on composition contrast was used to distinguish the heavy platinum (white spots) from the lighter components in the washcoat (darker background). It revealed a better dispersion and smaller size of Pt particles in CuO-Pt (Figure 14b), compared to Pt alone (Figure 14a), whereas in MnOx-Pt catalyst (Figure 14c), Pt seemed to disappear in the washcoat. Encapsulation of the noble metals by the metal oxides could explain a lower activity of MnOx-Pt and MnOx-Pd catalysts relative to the noble metal catalysts, as observed by SEM measurements. With increasing amounts of noble metal but similar amounts of metal oxides, the effect of the encapsulating phenomenon is dampened. The encapsulating effect is also stronger from MnOx than for CuO. The effect is more obvious on mixed-Pd than on mixed-Pt catalysts. The fact that the oxidation of CH4 is inhibited at low conversion on mixed CuO-Pd compared to that of Pd may be due to encapsulation of Pd that is covered, since at low conversion the reaction occurs on the surface. Since the oxidation of CH4 requires large numbers of active sites, the effect of the encapsulation phenomenon prevails, hence the activity of the mixed catalysts is similar to that of metal oxides.
a
b c
Figure 14: SEM micrographs of Pt/Al2O3 (a), CuO-Pt/Al2O3 (b) and MnOx-Pt/Al2O3 (c) calcined at 800°C for 4 h in air. Mn, Cu: 10; Pt: 0.5 mol%/Al2O3. White spots correspond to Pt particles.
29 There are maybe also other effects than mechanical encapsulation of the noble metals by metal oxides. Hurst et al., 1982 have studied the interaction between noble metal and metal oxide by TPR and reported the inclusion of small amounts of noble metals in the lattice of metal oxides [Hurst et al., 1982]. They observed also that at high noble metal loading, a metal oxide lattice is less able to tolerate noble metal ions and phase separation occurs, producing a noble metal-rich phase that can result in a shoulder in the TPR profile. TPR experiments performed on MnOx-Pt with two different loadings of Pt (0.1 and 1 mol%/Al2O3)indicated the formation at 1 mol% Pt of a low-temperature peak in the reduction profile, as seen in Figure 15. This confirms the formation of a mixed Pt-MnOx phase at a low Pt loading and a Pt-rich phase at a higher Pt loading. This is also in agreement with the activity tests which show that at low Pt loadings due to strong interactions between metal oxides and Pt, the mixed MnOx-Pt catalysts are less active than Pt catalysts, while at higher Pt loadings, the activity of the mixed catalysts are approaching the activities of Pt catalysts.
1 mol% Pt consumption (a.u.) 2 H 0.1 mol% Pt
0 100 200 300 400 500 600 Temperature (oC)
Figure 15. TPR profiles of Al2O3-supported MnOx-Pt calcined at 800°Cfor4h.Mn, 10; Pt 0.1 and 1 mol%/Al2O3. TPR experimental conditions: 17 ml/min, 10% H2 in Ar.
Another reason for the decrease in activity when a noble metal is mixed with a less electronegative cation has been discussed by several authors. Sugaya et al. found that the oxidation of Pt is favoured by the addition of basic MgO, and that PtOx is less active than Pt metal for the oxidation of some alkanes [Yu Yao, 1980; Sugaya et al., 1994]. However, due to the small amounts of noble metals, it is difficult to characterise them, therefore we cannot conclude on this last point.
30 Effects of noble metals on metal oxides [Papers I, II, III]
In the previous sub-section, metal oxides were found to affect the activity of noble metals. Here some contradictory effects are presented, i.e. noble metals, Pd or Pt, are shown to affect the reduction beh aviour of oxides of Mn and Cu, as well as their oxidation state. The reduction behaviour of CuO and MnOx was studied by means of TPR. Mn and Cu-based samples containing noble metals, and more particularly Pd showed lower reduction peak temperatures compared to MnOx or CuO alone, as seen in Figures 16a and 16b. The effect of noble metals on the reduction temperature of metal oxides has been reported [Gentry et al., 1981; Hurst et al., 1982] with Pd having a stronger effect than Pt. The presence of group VIII metals promotes the reduction of metal oxide. Gentry et al. [Gentry et al., 1981] studied the effect of various amounts of Pt and Pd on the reduction of unsupported CuO. They proposed the following mechanism: Hydrogen is dissociatively adsorbed on metal islands of Pd and Pt and transferred by a spillover process to CuO, which then is easily reduced. According to Gentry et al., Pd has a stronger effect on the reduction of CuO relative to Pt due to the ability of Pd2+ to substitute Cu2+ within the CuO lattice without phase separation.
a 210 b 186
MnOx-Pd
260 CuO-Pd
209 consumption (a.u.) consumption (a.u.) 344 2 2 390 H H CuO-Pt MnOx-Pt
424 207 328
190 MnOx CuO
0 200 400 600 800 0 200 400 600 Temperature (oC) Temperature (oC)
Figure 16. TPR profiles of Al2O3-supported MnOx,MnOx-Pt and MnOx-Pd, (a); CuO, CuO-Pt and CuO-Pd, (b). All samples calcined at 800°Cfor4h.Mn,Cu:10;Pt,Pd: 0.1 mol%/Al2O3. TPR experimental conditions: 17 ml/min, 10% H2 in Ar.
31 When increasing the amount of noble metals, the reducibility of MnOx was enhanced as seen in Figure 17. For the mixed MnOx-Pt catalysts, the reduction rate was high and resulted in narrow single peaks at lower temperatures than for MnOx alone. The presence of noble metals not only affected the reducibility of metal oxides but also the oxidation state that is fixed during preparation of the catalysts. This was determined according to the hydrogen consumed during TPR measurements carried out on MnOx/Al2O3 and mixed MnOx-Pt/Al2O3. The amount of hydrogen consumed remained constant for all the catalysts calcined at 800°C and gave a O/Mn molar ratio close to 1.6, which corresponds to the level of Mn2O3 present. When the catalysts were calcined at 500°C, the ratio was also 1.6 for the Mn sample but increased with increasing amount of Pt and reached 1.95, which corresponded to the presence of MnO2, whenthePtamountwas1mol%/Al2O3. This increase of the O/Mn ratio in the presence of 1 mol% Pt and this could not be attributed to an oxygen increase caused by Pt oxide, since its reduction causes low hydrogen consumption. Thus, Pt at high concentration increased the oxidation state of MnOx.
0.5 mol% Pt
0.1 mol% Pt
0.05 mol% Pt consumption (a.u.) 2 H
0.01 mol% Pt
MnOx
0 200 400 600 800 Temperature (oC)
Figure 17. TPR profiles of Al2O3-supported MnOx and mixed MnOx-Pt with different concentrations of Pt calcined at 800°C for 4 h. Mn: 10; Pt: 0.01 to 0.5 mol%/Al2O3. TPR experimental conditions: 50 ml/min, 10% H2 in Ar.
With TPR it is somehow difficult to evaluate the proportions of different oxides, since the hydrogen consumption during TPR gives the total oxidation
32 state. Moreover, in presence of noble metals, the reduction steps particular to each oxidation state of MnOx, are not so well defined. Therefore, the use of a more accurate technique was necessary. Raman spectroscopy was conducted [Paper II] on reference compounds (Figure 18a) and on MnOx alone and mixed with noble metals after different thermal treatments (Figure 18b). When comparing the composition of the catalysts calcined at 800°C (F), an increase in oxidation state can be observed in the presence of noble metals, i.e. towards MnO2, while MnOx alone contains mostly Mn2O3. A similar tendency towards higher oxidation state of MnOx in presence of noble metals was observed after thermal treatments at 900°C for 60 h in air with 12% steam (A), especially for MnOx-Pd. Indeed, the addition of noble metals decreased the amount of Mn3O4 (Figure 18b). This is in agreement with the activity measurements of thermally treated catalysts. Whereas, on MnOx alone, the formation of Mn3O4 induced a significant increase in activity, as seen in Figure 19, it could not be observed on MnOx-Pd (10-0.1) catalyst, since the latter contained a lower amount of Mn3O4. However for a lower amount of Pt (0.05) in MnOx-Pt, the boost of activity could be observed (Figure 20), because a lower amount of noble metals influences the oxidation state of MnOx to a lower extent. The possible reason for an increase of oxidation state of MnOx in the presence of noble metals, is an activation of oxygen in the gas phase during calcination that adsorbs preferentially on noble metals and is spilled over MnOx.
a 514 b 666
385 330
664 MnOx-Pd A 382 328 1010 298 150 MnOx-Pt A 666 MnO
MnOx A Intensity (a.u.) Intensity (a.u.)
384 MnO -Pd F x 530 329 665 320 300 710 486 MnOx-Pt F Mn3O4 665 710 382 Mn2O3 318 MnOx F 650588532 MnO2
1100 900 700 500 300 100 1100 900 700 500 300 100 Raman Shift +(cm-1) Raman Shift +(cm-1)
Figure 18. Raman spectra of bulk MnO2,Mn2O3,Mn3O4 and MnO reference compounds (a), Al2O3-supported MnOx,mixedMnOx-Pt and MnOx-Pd samples calcined at 800°C for 4 h in air (F) and after thermal treatment at 900°Cfor60hinair with 12% steam (A). Mn: 10; Pt, Pd: 0.1 mol%/Al2O3.
33 100
80
MnOx-Pd F MnOx-Pd A
60
MnO A 40 x MnOx F Conversion (%)
20
0 200 250 300 350 400 450 Catalyst temperature (oC)
Figure 19. C10H8 conversion for Al2O3-supported MnOx and mixed MnOx-Pd calcined at 800°C for 4 h in air (F) and after thermal treatments at 900°Cfor60hinairwith 12% steam. Mn: 10; Pd: 0.1 mol%/Al2O3. Gas mixture 1.
100
MnOx-Pt A MnOx-Pt F 80
60
40
Conversion (%) MnO A x MnOx F 20
0 200 250 300 350 400 450 Catalyst temperature (oC)
Figure 20. C10H8 conversion for Al2O3-supported MnOx, Pt and mixed MnOx-Pt calcined at 800°C for 4 h in air (F) and after thermal treatments at 900°Cfor60hinair with 12% steam (A). Mn: 10; Pt: 0.05 mol%/Al2O3. Gas mixture 1.
Combination of Pt and MnOx
Among the combinations of metal oxide and noble metal, our interest has been focused on the mixture of MnOx and Pt. Some results concerning the deposition method of Pt and its concentration into MnOx-Pt/Al2O3,aswellas the effect of the calcination temperature on the activity of the mixed MnOx- Pt/Al2O3 catalysts are presented here.
Impregnation method of Pt on MnOx/Al2O3
Two methods of impregnation of Pt into MnOx-Pt (Mn, 10; Pt, 0.1 mol%/Al2O3) catalysts were investigated. The aim is to see if there is a
34 possibility to avoid encapsulation of Pt by MnOx and if one method tends to stabilise Pt against thermal damages. The first consisted of mixing the Pt precursor in the aqueous slurry together with Mn salt, Al2O3 and urea [Paper IV] before precipitating the active components. The second method was carried out by impregnating the Pt precursor after Mn has been deposited onto Al2O3 and calcined at 500°C for 4 h. Thus, the first method was denoted as co- impregnation (CI) and the second as successive impregnation (SI). In both cases, the catalysts were finally calcined at 800°C for 4 h. As seen in Figure 21 and in Table 7, the SI method used for the mixed catalysts calcined at 800°C for 4 h resulted in a higher activity for both CO and C2H4 oxidation, while for CH4, there was no difference (not shown here). This was probably due to the inhibition of the encapsulation of Pt into MnOx by employing the SI technique, conversely to the CI one. Interestingly, it can be seen that MnOx-Pt (SI) had a larger activity for the oxidation of CO and C2H4 compared to the Pt catalyst (Figure 21). Thus not only the encapsulation of the noble metals was avoided, but the activity was enhanced, probably due to the synergetic effect of metal oxides that provided oxygen and noble metals that adsorbed CO or C2H4, while the reaction occurred at the interface, as discussed in the introduction of this chapter. However, after thermal treatments at either 900°C or 1000°C, the activity seemed to be better stabilised when using the CI method, as seen in Figure 22 and Table 7. It can be concluded that encapsulation of Pt by MnOx may lower the activity of fresh Pt catalyst, but at higher temperatures the CI method may result in a slightly more thermally stable catalyst.
100
MnOx-Pt/Al2O3 80 MnOx-Pt/Al2O3 Pt /Al2O3 SI CI
60
40 Conversion (%)
20
0 150 200 250 300 Catalyst temperature (oC)
Figure 21. Conversion of CO for MnOx-Pt/Al2O3, prepared by co-impregnation (CI) or successive impregnation (SI) calcined at 800°C for 4 h in air. Pt/Al2O3 calcined at 800°C for 4 h in air is also added. Mn: 10; Pt: 0.1 mol%/Al2O3. Gas mixture 2.
35 100
80 SI = CI 900oC 60
40 CI SI 1000oC 1000oC Conversion (%)
20
0 200 250 300 350 Catalyst temperature (oC)
Figure 22. Conversion of CO for MnOx-Pt/Al2O3, prepared by co-impregnation (CI) or successive impregnation (SI) calcined at 900°C (70 h) and 1000°C(4h)inair.Gas mixture 2.
Table 7. Temperature for 50% conversion of C2H4 for mixed MnOx-Pt/Al2O3 catalysts calcined under various treatments in air. Mn: 10; Pt: 0.1 mol%Al2O3. Gas mixture 2. Preparation Calcination treatments method 800°C, 4h 900°C, 70h 1000°C, 4h Co-impregnation 253 267 285 Successive-impregnation 194 262 279
Effect of the amount of Pt on MnOx /Al2O3 catalysts[PaperI]
Figures 23a and 23b show the conversion of CO and C10H8, respectively, for combined MnOx-Pt/Al2O3 catalysts with varying amounts of Pt, calcined at 800°C. It clearly shows that the higher the concentration of Pt, the higher the activity. Even very low amounts of Pt, such as 0.05 mol% of the washcoat amount, had effects on the activity for oxidation of CO and C10H8.Compared to the catalyst with only MnOx, the temperature for 50% conversion was decreased by 90°C for CO and by 25°C for C10H8 oxidation, respectively. The activity for CH4 oxidation was not as affected by the addition of Pt at low concentrations (not shown here). This is consistent with the fact that metal oxides and noble metals have similar activities at high temperatures. Namely the temperatures for 50% CH4 conversion for MnOx and MnOx-Pt, with an amount of Pt up to 0.5 mol%, were similar and equal to ca 640°C. An amount of 1 mol% Pt was necessary to enhance the activity to T50% = 612°C. Thus, the superiorityofPttoMnOx seemed to be less pronounced for CH4 oxidation than for oxidation of the other compounds.
36 100
80
60 1 0.5 0.1 0.05 0.01 none
40 Conversion (%)
20
0 0 100 200 300 400 500 600 Catalyst temperature (oC)
Figure 23a. CO conversion for Al2O3-supported MnOx and MnOx-Pt calcined at 800°C for 4 h in air. Mn: 10; Pt: 0.01 to 1 mol%/Al2O3. Gas mixture 1.
100
80
60 1 0.5 0.1 0.05 0.01 none
40 Conversion (%)
20
0 100 200 300 400 500 Catalyst temperature (oC)
Figure 23b. C10H8 conversion for Al2O3-supported MnOx and MnOx-Pt calcined at 800°C for 4 h in air. Mn: 10; Pt: 0.01 to 1 mol%/Al2O3. Gas mixture 1.
Calcination temperature of MnOx-Pt/Al2O3 [Paper I]
The influence of the calcination temperature on MnOx-Pt/Al2O3 catalysts with various concentrations of Pt in it was studied. The results are presented in Figure 24 and Table 8. With increasing Pt content in the mixed catalysts, from 0.05 to 0.5 mol%, the activities for CO and C10H8 oxidation of the catalysts calcined at 800°C gained over those calcined at 500°C, as seen in Figure 24 for CO. A possible explanation is that higher calcination temperatures may result in the migration of “trapped” Pt to the surface, increasing the activity of the mixed catalysts calcined at 800°C relative to those calcined at 500°C. However, when the concentration of Pt was 0.01 or 1 mol% the mixed catalysts calcined at 500°C were more active. MnOx as well as Pt alone catalysts were more active when calcined at 500°C instead of 800°C as seen in Figure 24 for MnOx catalysts and in Table 8 for Pt
37 catalysts for the oxidation of CO and C10H8. For the oxidation of CH4,Pt calcined at 500°C has higher activity compared to that calcined at 800°C, however for MnOx, the temperatures for oxidation are similar for both calcination. Thus it may be assumed that the higher activity for the mixed catalysts exposed to the higher temperature is due to some favourable synergetic effects.
Table 8. Temperature for 50% conversion of CO, C10H8 and CH4 for Pt/Al2O3 (0.05 mol%/Al2O3) calcined at various temperatures for 4 h in air. Gas mixture 1.
Calcination T50% (°C)
Temperature (°C) CO C10H8 CH4 500 226 241 665 800 265 270 686
700
C) CH
o 4 600
500
400 CO
300
o 200 500 C o
Temperature for 50% conversion ( 800 C 100 0 0.01 0.05 0.1 0.5 1
Concentration of Pt in MnOx-Pt (mol%/Al2O3)
Figure 24. Temperature for 50% conversion of CO and CH4 for MnOx/Al2O3 and MnOx-Pt/Al2O3 calcined at 500°C and 800°C for 4 h in air. Mn: 10; Pt: 0.01 to 1 mol%/Al2O3. Gas mixture 1.
Some important results from Section 2.3:
) When 0.1 mol% Pt or Pd is mixed with metal oxides (Mn, Cu: 10 mol%/Al2O3), it results in either an alteration (with MnOx)orasimilarity(withCuO)oftheactivity of the noble metal catalysts for the oxidation at CO and C10H8. The order of activity of the catalysts, calcined at 800°C, is as follows: Pd = CuO-Pd > Pt = CuO-Pt > MnOx-Pd > MnOx-Pt At higher noble metal concentration (Pt: 0.5 mol%/Al2O3) the effect is dampened: MnOx-Pt=Pt>MnOx and for CuO-Pt catalysts there is a synergetic effect: CuO-Pt>Pt>CuO
) For the oxidation of CH4, the mixed catalysts have a similar activity to that of the metal oxides: CH4: CuO=CuO-Pd=CuO-Pt>Pd=MnOx-Pd = MnOx-Pt=MnOx =Pt
38 ) The presence of noble metals in the mixed catalysts induces an increase of reducibility for CuO and MnOx and a higher oxidation state of MnOx. ) When Pt is successively impregnated (SI) in the preparation of the mixed MnOx-Pt catalysts calcined at 800°C, it results in synergetic effect for the oxidation of CO and C2H4: MnOx-Pt(SI)>Pt>MnOx-Pt (CI) (800°C,4h) At higher calcination temperature the co-impregnation method (CI) is catching on: MnOx-Pt (SI) = MnOx-Pt (CI) (900°C70h) MnOx-Pt (CI) > MnOx-Pt (SI) (1000°C,4h)
2.4 Concluding Remarks
In our reaction medium, the activity of 0.1 mol% Pd/Al2O3 was found to be superior to that of the same amount of Pt for the oxidation of CO, C10H8 and CH4, whereas the opposite was observed for the oxidation of C2H4.WhenCO (2500 ppm) is present in the synthetic gas mixture, it was found to enhance appreciably the activity of the Pd/Al2O3 catalyst for the oxidation of the other combustibles,i.e.C2H4,C10H8 and CH4. Al2O3-supported MnOx andCuO(Mn,Cu:10mol%/Al2O3) were selected, among the oxides of Fe, Co and Ni, as high active catalysts for the oxidation of all the combustibles studied here. It was found that a calcination temperature of 900°ConMnOx/Al2O3 results in an increase in the oxidation state of MnOx towards Mn3O4, yielding, despite a surface area loss, a more active catalyst. Pt and Pd possess superior catalytic activity relative to CuO and MnOx for the oxidation of CO, C10H8 and C2H4, however, for the oxidation of CH4,CuO is largely more active than Pt and Pd, while MnOx is as active as Pd and Pt. In mixed catalysts, MnOx tends to form a mixed phase with Pt at low noble metal loadings that encapsulate noble metals, hence inhibiting activity. The encapsulation is avoided by using a successive impregnation of Pt which in addition leads to enhanced catalytic activities for mixed MnOx-Pt compared to that of Pt. Furthermore, the inhibiting effect is dampened by using a higher amount of Pt, i.e. 0.5 mol%. Mixed CuO-Pt and CuO-Pd catalysts (Cu: 10; Pt, Pd: 0.1 mol%) preserve the individual activities of each active components for the respective combustible. In addition, at higher Pt loading (0.5 mol%), there is a synergetic effect between CuO and Pt which yields a higher conversion of CO and C2H4, relative to single component catalysts. All mixed metal oxide-noble metal catalysts benefit from the presence of the metal oxides for the oxidation of CH4. The synergetic catalytic effects between metal oxides and noble metals can be related to the increase of reducibility of the metal oxides in the presence of
39 noble metals which may enhance the oxygen transfer from the gas phase to the noble metals.
40 3 CATALYST DEACTIVATION
The practical application of a particular catalyst depends not only on its activity but also on its thermal stability and resistance to poisons. Increasing the lifetime of catalysts for the removal of VOC is one of the biggest challenges of catalyst development today. The deactivation of catalysts falls into four general categories: chemical, fouling, mechanical and thermal [Carol et al., 1989]. In this chapter, thermal treatments in the presence of steam, pre- sulphation and sulphur poisoning on stream are discussed.
3.1 Thermal Deactivation
3.1.1 General
The thermal degradation of supported catalysts in wood boilers is caused notonlybyhightemperaturebutalsobysuddentemperaturechanges. Usually the maximum temperature within a flue duct is around 700°C, however, peaks at higher temperatures have also been registered [Berg, 2001]. Moreover, the sintering of catalysts could be accelerated due to the oxidising [Wanke & Flynn, 1975] and steam-containing atmosphere [Wanke et al., 1987] in wood boilers as well as in many industrial applications [Brey & Krieger, 1949; Aldcroft et al., 1968]. Thermal deactivation, in contrast to other types of deactivation, is a more serious problem as it is irreversible. Thermal deactivation of the catalysts may result in thermal damage of the monolith substrate, washcoat and the active phases as well, as the formation of compounds of lower or negligible activities by reactions of the active material with the support [Trimm, 1991]. Monolith substrates may be made of either metallic or ceramic materials. Metallic monoliths present some advantages over ceramic monoliths, such as lower heat capacities and greater thermal shock resistance. Metallic monoliths are used in pre-heated catalytic converters for minimising the cold-start phase. Any flow maldistribution concentrates the thermal ageing to certain areas of the monolith. Compared with the ceramic monolith, the metallic monolith, with its higher thermal conductivity, suffers less from thermal ageing [Twigg & Webster, 1998]. The lower heat resistance of metallic supports compared with that of ceramic supports does not have to be a limitation in wood boiler
41 applications since the maximum temperatures in the flue gas duct can be kept below 900°C. Nevertheless, metallic monoliths present some disadvantages. For example, the deposition of washcoats on metallic monoliths is not as well developed as for ceramic monoliths. The non-porous nature of the metal foil, coupled with the mismatch in the thermal expansion between the foil and the ceramic washcoat, contributes to a washcoat adhesion problem during thermal cycling. Many important industrial processes are based on high-surface-area (150- 200 m2/g) γ-Al2O3 -supported catalysts. γ-Al2O3 looses surface area by two processes: sintering and phase transformation to α-Al2O3 at about 1000- 1150°C, depending on the starting material [Trimm, 1991]. α-Al2O3 is the most thermodynamically stable phase with a surface area of ca 2 m2/g [Trimm, 1991]. Sintering is the redistribution of material in the solid state in order to decrease the surface energy. With a high temperature, and in the presence of water vapour, the surface defects become very mobile and reactive, allowing interactions between Al2O3 particles (hydrogen bonded hydroxyl groups for instance), and by subsequent water elimination, a “neck” is formed [Johnson, 1990]. This process can continue, resulting in a low surface area Al2O3 and finally its transformation into the α form. Sintering occurs before the phase transformation due to very different kinetic rates between both processes [Schaper et al., 1985; Burtin et al., 1987b]. Moreover, when γ-Al2O3 is transformed to the α state this is accompanied by a decrease in the mechanical strength of the catalyst [Shkrabina et al., 1995]. Because the transformation of the metastable α-phase is irreversible, loss of activity is permanent. Such morphological changes in a catalyst support are accompanied by a loss of activity occasioned by an encapsulation of the active component, known as the “earthquake phenomena” [Tucci, 1982]. Moreover, thermal sintering of the Al2O3 washcoat causes loss of the metal surface area (crystallite sintering) due to macroscopic movements of the substrate during phase transformation of the Al2O3 [Dalla Betta et al., 1976; Chu & Ruckenstein, 1978; Miyoshi et al., 1989]. On the other hand, it has been observed that metals such as Pt [Kozlov et al., 1973; Vereschagin et al., 1982; Burtin et al., 1987a], as well as oxides of metals such as Mn, Fe, Ni, Mo, Co, V and Cu [Fink, 1968; Bye & Simpkin, 1974; Gauguin et al., 1975; Young et al., 1980; Vereschagin et al., 1982; Peiyan et al., 1995; Ozawa et al., 1996a], accelerated the phase transformation of Al2O3 into the stable α form. Fink proposed that the impurities act as mineralisers, destroying the metastable Al-O-Al bond in γ-orθ-Al2O3 [Fink, 1968]. This can be explained by cation migration of some transition metals, which occurs at temperatures of ca 1000°C at rates of 100-1000 times faster than those of Al2O3 [McCarty et al., 1999]. In addition, metal oxides generally have a stronger effect on sintering than noble metals, because of their higher concentration.
42 3.1.2 Results and Discussion
In this part, the effects of thermal treatments on i) the adherence of washcoat onto metallic substrate, ii) the properties of the active components, and iii) the influence of active phases on the Al2O3-phase transformation are discussed. Finally the activity of mixed noble metal-metal oxide and single component catalysts are presented.
Adherence of washcoat onto metallic monoliths [Paper IV]
The effect of hydrothermal treatment at 900°C for 270 h in air with 10% steam on the adherence of the washcoat onto a Fecralloy metallic substrate (Emitec Gmbh, Germany) was studied using SEM. To obtain a better adherence of the washcoat onto the substrate, the latter was first subjected to a special treatment in our laboratory. This led to the migration of Al2O3 to the surface, formed by the oxidation of bulk aluminium, thus providing a textured whisker structure, as seen in Figure 25a. The mean size of the whiskers formed, which completely covered the metal surface, varied between 1 and 2 µm. The advantage associated with the Al2O3 film is its compatibility with Al2O3-containing washcoat, which substantially eases application and ensures its adhesion to the substrate. Also, the Al2O3 layer completely covered the metal surface, thus rendering the alloy highly oxidation-resistant.
Figure 25 a. SEM micrographs of the alumina whiskers on bare metal. Side view (left) and view from above (right).
43 SEM pictures revealed the presence of cracks in the washcoat after calcination at 800°C for 4 h, probably due to the mismatch in the thermal expansion between the metal foil and the washcoat (not shown here). Figure 25b shows a layer of washcoat deposited on the Al2O3 whiskers from the metallic surface. The whiskers act as anchors for the washcoat when deposited onto the substrate. Figure 25c shows the layer of washcoat deposited on the metallic substrate after hydrothermal treatment at 900°C for 270 h in air with 10% steam. It can be seen that the whiskers become bigger when exposed to higher temperature, thus well-adhered washcoat onto the metallic support can be achieved through the growth of the Al2O3 whiskers.
Figure 25 b. SEM micrographs of the alumina washcoat deposited on metal (left) and enlargement of a section (right), after calcination at 800°Cfor4hinair.
Figure 25 c. Same as Figure 25b after hydrothermal treatment at 900°Cfor270hinair with 10% steam.
44 Characterisation of thermally-treated catalysts [Paper II]
The present study concerns characterisation, by Raman, TPR and XPS, of active components (MnOx or CuO mixed with Pt or Pd) supported on γ-Al2O3 after a hydrothermal treatment at 900°C for 60 h in air with 12% steam. The aim is to observe any interaction metal-supports.
Manganese oxides-containing catalysts
The TPR profiles of the fresh and hydrothermally treated samples of MnOx alone and mixed with Pt or Pd are shown in Figure 26. After hydrothermal treatment the reduction profile of the MnOx sample is shifted to higher temperatures and the peaks are less well defined than for the fresh sample. For the mixed MnOx-Pt sample the range of reduction temperature is almost the same as for the fresh sample. The H2/Mn ratios for all the samples are equal to ca 0.5, thus MnOx can be composed of either mainly Mn2O3 or a mixture of oxides that lead to the same hydrogen consumption, and hence it is difficult to comment on the valency of MnOx in this system. Mn/Al ratios, as determined by XPS, are strongly increased after thermal treatment as seen in Table 9. These results are in conformity with an increased surface MnOx concentration which is probably due to the decrease in surface area and/or to an increased metal oxide particle size due to sintering. The B.E. of the principal Mn 2p3/2 peak for all the samples containing MnOx alone and mixed with noble metals were between 642.6-642.8 and 642.7-642.9 eV for fresh and aged samples, respectively (Table 9). References samples from literature are also shown in this table. There is respectively a complete and large overlapping of the Mn 2p3/2 peak position domain between MnO2 and Mn2O3 on the one hand and Mn2O3 and Mn3O4 on the other hand [Wagner et al., 1979]. Therefore, determination of the MnOx oxidation-state on the basis of the Mn 2p3/2 peak position is difficult. There was no indication that MnAl2O4 existed in our samples. It has been reported that the formation of MnAl2O4 occurs only when samples are calcined in reducing atmosphere, because of the instability of Mn2+ ions in air at temperatures above 150°C[LoJacono& Schiavello, 1976; Strohmeier & Hercules, 1984]. On the other hand, the use of Raman Spectroscopy could reveal some discrepancies between the composition of the fresh and treated samples. Mn2O3 and Mn3O4 were found in samples calcined at 800°C for 4 h and after thermal treatments at 900°C, respectively (see Section 2.2.2, Figure 6).
45 MnOx-Pd consumption (a.u.) 2 H
MnOx-Pt
MnOx
0 200 400 600 800 Temperature (oC)
Figure 26. TPR profiles of Al2O3-supported MnOx,MnOx-Pt and MnOx-Pd, calcined at 800°C for 4 h in air (thin line) and after thermal treatment at 900°Cfor60hinair with 12% steam (thick line). Mn: 10; Pt, Pd: 0.1 mol%/Al2O3.TPRexperimental conditions: 17 ml/min, 10% H2/Ar.
Table 9. Data from XPS analyses for Al2O3-supported MnOx,mixedMnOx-Pt and MnOx-Pd calcined at 800°C for 4 h (F) and after thermal treatment at 900°C for 60 h in air with 12% steam (A). Mn: 10; Pt, Pd: 0.1 mol%/Al2O3. Binding energies of reference compounds are included. Catalysts and Mn2p3/2 binding energies (eV) a Atomic ratio Mn/Al reference compounds Ref. F A F A MnOx/Al2O3 642.6 642.9 0.084 0.26 MnOx-Pt/Al2O3 642.8 642.7 0.10 0.35 MnOx-Pd/Al2O3 642.8 642.8 0.10 0.29 MnO2 641.7 [Baltanás et al., 1987] 642.1 [Strohmeier & Hercules, 1984] 642.2 [Wagner et al., 1979; Baltanás et al., 1987] Mn2O3 641.4 [Strohmeier & Hercules, 1984] 641.5 [Di Castro & Polzonetti, 1989] Mn3O4 640.9 [Strohmeier & Hercules, 1984] 641.1 [Di Castro & Polzonetti, 1989] MnO 640.1 [Baltanás et al., 1987] 640.6 [Di Castro & Polzonetti, 1989] 641.2 [Strohmeier & Hercules, 1984] MnAl2O4 641.1 [Strohmeier & Hercules, 1984] a All B.E.’s (eV) referenced to C1s=284.6 eV
46 Copper oxide-containing catalysts
The TPR profiles of the fresh and hydrothermally treated samples of CuO alone and mixed with Pt or Pd are shown in Figure 27b. Reference compounds of bulk CuO and bulk copper aluminate, i.e. CuAl2O4, are included for comparison (Figure 27a). The profiles of the treated samples show two separate peaks with the first one at a lower temperature and the second peak at a higher temperature than the single peak in the fresh samples. The temperature of the first reduction peak for both mixed samples is lower than for the CuO/Al2O3 alone. A similar effect of Pd on the reduction behaviour of CuO was observed for the fresh sample (Figure 16b). TPR suggested the presence of CuAl2O4 in the samples by comparison with the reduction profile of a reference CuAl2O4 compound (Figure 27a). It is known that bulk copper aluminate (60% tetrahedral and 40% octahedral Cu2+) can only be formed at high calcination temperatures, i.e. above ca 800°C [Misra & Chaklader, 1963; Wolberg & Roth, 1969; Friedman et al., 1978], and is reduced at temperatures of approximately 500°C[Dumaset al., 1989; Marion et al., 1991]. The formation of the copper aluminate seems to be increased when noble metals are present in the catalysts.
a 432 b
CuO-Pd
Bulk CuAl2O4
321 consumption (a.u.) 2 consumption (a.u.) H 2 H CuO-Pt
Bulk CuO CuO
0 200 400 600 0 200 400 600 o Temperature (oC) Temperature ( C)
Figure 27. TPR profiles of bulk CuO and CuAl2O4 reference compounds (a), Al2O3- supported CuO, CuO-Pt and CuO-Pd calcined at 800°C for 4 h in air (thin line) and after treatment at 900°C for 60 h in air with 12% steam (thick line) (b). Cu: 10; Pt, Pd: 0.1 mol%/Al2O3. TPR experimental conditions: 40 ml/min, 5% H2/Ar (a) and 17 ml/min, 10% H2/Ar (b).
47 Effects of metals in the washcoat [Paper III, IV, V]
Tables 10 and 11 show the results from characterisation studies using BET and XRD concerning the effects of metal oxides (CuO or MnOx) and noble metals (Pt or Pd) on the properties of washcoat. The presence of the active components in the washcoat increased the sintering, as seen by a decrease of the BET-surface area, transformation of γ-to α-Al2O3 phases at lower temperatures, increase of the Al2O3 particle size, and reactions of the active phases, notably CuO, with Al2O3, especially after thermal treatment at 900°C for 60 h in air with 12% steam Table 10 [Paper III] and for 300 h in air with 10% steam, Table 11 and Figure 28 [Paper V]. The effects of metal oxides, particularly Cu, are stronger than those of noble metals, and this can be seen in both Tables and in Figure 28, with a higher formation of α-Al2O3 and spinel compound CuAl2O4. The effect of Pd in mixed catalysts on the loss of surface area is also stronger than that of Pt, as seen after a treatment for a longer period of time, i.e., 300 h (Table 11).
Table 10. Surface area of catalysts after hydrothermal treatment at 900°C for 60 h in air with 12% steam. Mn, Cu: 10; Pt, Pd: 0.1 mol%/Al2O3. Samples Surface area (m2/g) Al2O3 115 Pt/Al2O3 111 Pd/Al2O3 110 MnOx/Al2O3 59 CuO/Al2O3 39 MnOx-Pt/Al2O3 15 MnOx-Pd/Al2O3 18 CuO-Pt/Al2O3 10 CuO-Pd/Al2O3 15
Table 11. Surface area, α-Al2O3 content and α-Al2O3 particle size in the catalysts determined on samples calcined at 800°C for 4 h in air (F) and hydrothermally treated at 900°C for 300 h in air with 10% steam (A). Mn, Cu: 10; Pt, Pd: 0.1 mol%/Al2O3.
BET data XRD data on α-Al2O3 surface area (m2/g) % a Particle size (nm) b Samples F A A A
Al2O3 169 97 none - MnOx-Pt/Al2O3 156 27 50 70 MnOx-Pd/Al2O3 149 10 58 100 CuO-Pt/Al2O3 133 15 53 90 CuO-Pd/Al2O3 131 7 76 100 a Determined from the intensity of the peak 2θ = 35.2 b Determined by Scherrer’s formula at 2θ = 35.2
48 In general terms, the role of foreign ions can be explained. Thermal reorganisation involves diffusion via defects and vacancies [Trimm, 1991]. The presence of both MnOx and CuO accelerated the sintering, with CuO having a stronger effect than MnOx. In both studies, after calcination at 800°C, Cu2+ was found to be present on Cu-containing samples and Mn2O3 was present in the Mn-containing samples. After ageing at 900°C, Cu2+ (copper aluminate) and Mn3O4 were found on Cu and Mn-containing catalysts, respectively, as discussed previously. The size of the ionic radius and the charge also play an important role in the mobility of the ion into the Al2O3. A larger charge and ionic radius hampers the mobility [Burtin et al., 1987a; Miyoshi et al., 1989; Church et al., 1993]. The ionic radii of these metal ions are 0.72 Å and 0.66 Å for Cu(+II) and Mn(+III), respectively [Weast & Astle, 1979]. According to the model described by Burtin et al., which takes in account charge and ionic radius, it seems that Cu(+II) has a more pronounced effect as an accelerator than Mn(+III). Also Tijburg suggested that growth of copper aluminate observed during treatment at high temperature results in a disordered Al2O3 layer at the interface with copper aluminate, where α-Al2O3 can nucleate more easily [Tijburg, 1989]. That could explain the higher mobility of Cu in the Al2O3 structure compared to Mn, leading to the higher level of Al2O3 sintering.
α α α α α sα α αα s s α CuO-Pd/Al2O3 s s s α α
CuO-Pt/Al2O3
MnOx-Pd/Al2O3
MnOx-Pt/Al2O3
Al2O3 20 30 40 50 60 70 80 2θ
Figure 28. XRD patterns of Al2O3,MnOx-Pt, MnOx-Pd, CuO-Pt and CuO-Pd deposited on Al2O3 after hydrothermal treatment at 900°C for 300 h in air with 10% steam. Mn, Cu: 10; Pt, Pd: 0.1 mol%/Al2O3. α: α-Al2O3,s:CuAl2O4.
The effect of metals on washcoat properties depends not only on the type of active phases but also on their concentration. For instance with increasing concentrations of Mn in the washcoat, a decrease in BET-surface area was observed after thermal treatment, as can be seen in Figure 29 [Paper IV].
49 100
80 /g) 2
60
40
20 BET-surface area (m
0 056.6710 20
MnOx (mol% Mn / Al2O3)
Figure 29. BET-surface area for MnOx/γ-Al2O3 (Mn:5to20mol%/Al2O3) catalysts treated at 900°C for 270 h in air with 10% steam.
Catalytic activity of thermally-treated catalysts [Paper III]
Mixtures of MnOx or CuO and Pt or Pd subjected to thermal treatments at 900°C for 60 h in air with 12% steam were tested for the catalytic oxidation of CO, C10H8 and CH4 in the gas mixture 1. Comparisons were made with single component catalysts. After the treatment, the MnOx catalyst shows higher activity than the fresh catalyst for the oxidation of C10H8 and CO and almost no change in activity for the oxidation of CH4, as seen in Table 12. This enhancement, discussed in Section 2.2.2, correlates with the change in MnOx oxidation state towards Mn3O4. For the treated CuO catalyst, the temperatures for 50% conversion of CO and CH4 were increased by 60°C and 130°C, respectively, whereas the oxidation of C10H8 was almost not affected. In the thermally treated catalysts, the formation of CuAl2O4, as mentioned in the characterisation study, could have contributed to decreased activity. Indeed Cu in CuAl2O4 cannot change oxidation state, but remains as Cu(+II). Thus Cu cannot be an active redox centre for oxidation of VOC´s, which generally proceed via the Mars-van Krevelen type mechanism (i.e. redox mechanism) [Spivey, 1989]. After hydrothermal treatment of the Pd catalyst, the oxidation of CO and C10H8 was only slightly decreased, and the oxidation of CH4 was decreased to a higher extent, as seen in Table 12. The Pt catalyst showed a larger decrease in activity for the oxidation of CO and C10H8 than the Pd catalyst, as can be seen in Table 12, while the Pt catalyst showed less decrease in activity for the oxidation of CH4. The treated and mixed MnOx-Pt catalyst showed a slight deactivation for all components, shown in Table 12. It can also be seen that the thermal deactivation, i.e. ∆T, of the mixed MnOx-Pt catalyst was lower than for the Pt
50 catalyst. After hydrothermal treatment, the Pt catalyst was still slightly more active than the mixed MnOx-Pt catalysts for the oxidation of CO and C10H8 oxidation, but the opposite was observed for CH4. The hydrothermally treated mixed CuO-Pt catalyst had a higher activity than the treated Pt catalyst for the oxidation of all the combustibles studied here, as seen in Figure 30 and Table 12. It seems that there was a stabilising effect between CuO and Pt which made the mixed CuO-Pt catalyst more resistant to thermal deactivation than CuO or Pt catalysts alone, despite the lower surface area of the mixed catalyst compared to the single-component catalysts (Table 10). The treated mixed MnOx-Pd and CuO-Pd catalysts showed a higher decrease in activity than the treated Pd catalyst for the oxidation of CO and C10H8. However, for the oxidation of CH4, the activities of the treated mixed MnOx-Pd and CuO-Pd catalysts were better than the activity of the treated Pd catalyst. As discussed in Section 2.3.2, the mixed MnOx-Pd catalyst benefited from the presence of the new active Mn3O4 phase, which was formed during the thermal treatment, to a lesser extent than MnOx catalyst. Therefore, MnOx-Pd catalyst had a lower activity than the treated MnOx catalyst for the oxidation of C10H8 and CH4.
Table 12. Temperature for 50% conversion of CO, C10H8 and CH4 for Al2O3-supported MnOx, CuO, Pd, Pt and mixed catalysts calcined at 800°C for 4 h (F) and after treatment at 900°C for 60 h in air with 12% steam (A). Mn, Cu: 10; Pt, Pd: 0.1 mol%/Al2O3. Gas mixture 1. CO C10H8 CH4 Catalysts F A ∆TFA∆TFA∆T
MnOx/Al2O3 425 360 -65 365 315 -50 645 655 10 CuO/Al2O3 344 407 63 420 430 10 590 720 130
Pt /Al2O3 245 287 42 250 284 34 640 720 80 Pd/Al2O3 217 230 13 220 237 17 640 745 105
MnOx-Pt/Al2O3 290 317 27 290 310 20 640 690 50 MnOx-Pd/Al2O3 290 340 50 290 332 42 640 715 75
CuO-Pt /Al2O3 255 270 15 265 275 10 593 705 112 CuO-Pd/Al2O3 220 330 110 229 365 136 595 695 100
∆T= T50%(A) – T50%(F)
51 100
80 Pt F Pt A = CuO-Pt F CuO-Pt A 60
CuO F CuO A 40 Conversion (%)
20
0 200 300 400 500 Catalyst temperature (oC)
Figure 30. CO conversion for Al2O3-supported Pt, CuO and mixed CuO-Pt calcined at 800°C for 4 h (F) and after hydrothermal treatment at 900°C for 60 h in air with 12% steam (A). Cu: 10; Pt: 0.1 mol%/Al2O3. Gas mixture 1.
Optimisation of thermally-stable MnOx/Al2O3 catalysts [Paper IV]
As discussed previously, a high amount of metal oxides may accelerate the sintering of the Al2O3 support. On the other hand, MnOx has interesting properties when subjected to high temperatures, i.e., phase transformation towards a more active state. Therefore it seems to be important to optimise the amount of washcoat and the concentration of MnOx to obtain the maximum activity after ageing. Thus, the influence of the amount of washcoat (up to 20 wt%/catalyst weight), as well as the influence of the concentration of MnOx (Mn:5to20 mol%/Al2O3) on the activity of catalysts after hydrothermal treatment were studied.
800
C) CH4 o 700
600
CO 500
400 C H
Temperature for 50% conversion ( 10 8 300 0 5 10 15 20 25 washcoat (wt%/catalyst)
Figure 31. Temperature for 50% conversion of CO, C10H8 and CH4 for MnOx/Al2O3 with different amounts of washcoat, but the same total amount of Mn (ca 3.4 10-4 mol) calcined at 900°C for 270 h in air with 10% steam. Gas mixture 1.
52 After hydrothermal treatment, the catalyst with a concentration of MnOx of 10 mol%/Al2O3 seems to be optimum (Figures 31 and 32), showing a high activity due to the presence of the new Mn3O4 phase and stability, i.e. relatively high surface area (Figure 29).
800 C) o CH 700 4
600
500
400 CO C H 300 10 8 Temperature for 50% conversion ( 200 0 5 10 15 20 25 Mn (mol%/Al2O3)
Figure 32. Temperature for 50% conversion of CO, C10H8 and CH4 for MnOx/Al2O3 with different amounts of Mn, but the same total amount of washcoat (20 wt%) calcined at 900°C for 270 h in air with 10% steam. Gas mixture 1.
Some important results from Section 3.1:
) After hydrothermal treatment at 900°C, Al2O3 whiskers grew from the metallic substrate thus yielding to well-adhered washcoat. ) Hydrothermal treatment at 900°C on catalysts induces a surface enrichment of Mn and Cu and the formation of CuAl2O4. ) Metals increase the sintering of alumina, i.e.: - increase the α-Al2O3 particle size, - lower the temperature for γ-Al2O3 →α-Al2O3 transformation, which is accompanied by a decrease of surface area. ) The sequence of activity of the hydrothermally treated catalysts (900°C, 60 h in air with 12% steam), with an amount of noble metal of 0.1 mol%, for the oxidation of the combustibles is as follows: CO: Pd>CuO-Pt>Pt>MnOx-Pt > CuO-Pd > MnOx-Pd > MnOx >CuO C10H8: Pd>CuO-Pt>Pt>MnOx-Pt>MnOx >MnOx-Pd > CuO-Pd >CuO CH4:MnOx >MnOx-Pt = CuO-Pd > CuO-Pt > MnOx-Pd=CuO=Pt>Pd
) An amount of 10 mol% Mn/Al2O3 in MnOx/Al2O3 hydrotreated at high temperature was found to be the optimum for the oxidation of CO, C10H8 and CH4.
53 3.2 Sulphur Poisoning
3.2.1 General
Sulphur poisoning is a serious cause for deactivation of catalysts. Usually the sulphur level in the flue gases from wood combustion is in the range of 20 to 200 ppm, mainly in the form of SO2. It is extremely difficult to simulate the way in which the catalysts are subjected to poisons in realistic conditions. Indeed, both fuel composition and operating conditions affect the poisoning process. Wood containing bark or residues from agriculture contain higher amount of inorganic materials [Linsmeyer & Hofbauer, 1994]. Sulphur poisoning is less of a problem at higher operating temperatures, approximately 600-800°C, and regeneration of poisoned catalysts is possible with similarly elevated temperatures [Yao et al., 1981; Ball & Stack, 1991; Deng et al., 1993; Beck & Sommers, 1995]. On the other hand, higher operating temperatures have additional energy penalties, and risk of deactivation by sintering of active components or washcoat, as discussed previously. Under oxidising conditions there are several basic means by which sulphur poisoning of catalysts occurs. The first occurs at temperatures above 300-350°C and involves the conversion of SO2 to SO3, which then reacts directly with the catalytic components or the washcoat. If Al2O3 is present, the compound formed will be Al2(SO4)3, a large volume, low density material that blocks active sites [Heck & Farrauto, 1995]. The second mechanism is the chemisorption of SO2 or SO3 onto catalytic sites at lower temperatures, which prevents those sites from further catalytic action by either inaccessibility of active surface sites due to geometric blockage or changes in the structure of the catalytic surface. The sulphur species that accumulate on catalysts tend to be sulphates or sulphite species [Zwinkels, 1994]. Sulphur deactivates both noble metals and metal oxides, but apparently through different mechanisms. On noble metals, the sulphur retained on the catalyst is desorbed at elevated temperatures. On metal oxides, however, the sulphur is incorporated within the catalyst or support as a sulphate or stable adsorbates (H2SO4 or SO3)andtheSO3 remains bonded to the surface even at temperatures at which the sulphate decomposes [Sultanov et al., 1987]. Metal oxides are therefore more susceptible to poisoning by sulphur compared to noble metal catalysts [Shelef et al., 1978]. In most cases sulphur components in the gas stream have been shown to decrease the activity of catalysts. However, a chemisorbed species can also have electronic effects on the surrounding catalytic surface that maybe beneficial, in which case it is termed a promoter [Satterfield, 1991]. SO2 has been reported to have a promoting effect on Pt/γ-Al2O3 catalyst for the oxidation of propane [Yao et al., 1981; Monroe et al., 1991; Hubbard et al., 1993;
54 Ishikawa et al., 1994; Marecot et al., 1994; Sugaya et al., 1994]. Burch et al. noticed that there is both a transient effect which causes a temporay sharp increase in activity and a more-lasting effect which may be associated with accumulation of sulphate species on the alumina support [Burch et al., 1998]. The reason for the enhancing effect was suggested to be the promotion of the dissociative adsorption of propane on the Pt, which is the rate determining step in C-H bond activation. Wilson et al. proposed that pre-adsorption of oxygenreactswithSO2 to yield co-adsorbed sulphate species on Pt that lead to the initial dissociative adsorption of propane by abstraction of H by SOx, which then forms an intermediate that is more easily oxidised [Wilson et al., 1996]. Burch et al. suggested also that the oxidation of SO2 to SO3 by Pt causes the removal of surface oxygen thus facilitating the adsorption of propane [Burch et al., 1998]. The promotional effect of SO2 onto Pt is usually lower for hydrocarbons higher than propane, since the activation of the C-H bond in these molecules would be less susceptible to additives [Burch & Hayes, 1995]. Wilson et al. also reported that the reaction is not only support-mediated as the enhancement is also observed on a Pt(111) surface in the complete absence of a support phase [Wilson et al., 1996]. In addition, early work carried out by Yao et al. demonstrated that the extent of the promotional effect of SO2 was directly related to the Pt dispersion onto the support [Yao et al., 1981]. However, some studies show that the support may affect the promotion by SO2. Sugaya et al. [Sugaya et al., 1994] claimed that increased acidity of the catalysts led to increased activity for propane oxidation, because the role of the acidic support is to prevent the oxidation of Pt into less active Pt oxides. However, Hubbard et al. observed that alumina-supported Pt is promoted while silica-supported Pt was not promoted by SO2 and concluded that the changes in the acid-strength does not account for the activity of a Pt catalyst for propane oxidation [Hubbard et al., 1993]. Apart from the possibility of discrepancies between the morphological effects of the Pt deposited on the two different supports, the alumina may also contribute to the promotion by SO2. Sulphate is much more stable on alumina than on silica and the probability to find a sulphate species adjacent to a Pt-particle is more likely on the former. Also, Pt is known to catalyse the formation of SO2 to SO3 which will proceed at a Pt particle and SO3 can react with alumina to form Al2(SO4)3 and this reaction is further catalysed by Pt. The role of the support would also be to provide sulphate species that are more stable on the alumina than on Pt at high temperatures. Thus the promoting effect could be due to the presence of both sulphate on, or adjacent to, a Pt particle. The phenomenon is unique for Pt catalysts since it forms relatively unstable bonds to sulphur oxides. Conversely, the effect of SO2 onto Pd is inhibitory since PdO can react strongly with SO2 andthesurfacePdsulphateisinactivefortheC-Hbond breaking [Hubbard et al., 1995; Burch & Hayes, 1995].
55 3.2.2 Results and Discussion
In this part, two types of sulphur poisoning are presented. The first one consist in treating the catalysts in a SO2-containing stream prior to any activity measurements or characterisation studies, while the second one consist on introducing a small amount of SO2 in the feed gas during the activity measurements.
Pre-sulphation of the catalysts [Paper II, III]
Catalysts based on a combination of metal oxides and noble metals, calcined at 800°C for 4 h in air, were subjected to SO2 treatments with 1000 ppm SO2 (2 l/min) in air at 600°C for 16 h.
Characterisation [Paper II]
Characterisation using TPR and XPS was carried out on fresh and sulphur- treated samples of MnOx and CuO. The reduction profile of sulphur-treated samples of MnOx has changed drastically compared to that of fresh samples; most of the species are reduced at much higher temperatures at around 480-560°C, as seen in Table 13 and Figure 33. There is an increase in the reduction temperature and in the hydrogen consumption for all the sulphur-treated samples compared to the fresh ones, which is probably due to the reduction of formed sulphate, MnSO4. The reduction temperature for the mixed samples is lower than for the MnOx alone. A similar effect of Pt and Pd on the reduction behaviour of MnOx was observed for the fresh samples, with Pd having stronger influence than Pt. As in the fresh samples, noble metals seem to catalyse the reduction, in this case of manganese sulphate. Table 13 shows a larger increase in the hydrogen consumption when MnOx is mixed with noble metals and this can be attributed to a higher amount of sulphate which formation is catalysed by noble metals. XPS measurements were conducted on Mn-containing samples which revealed the presence of adsorbed sulphate species, as seen by the B.E. of S 2p which is about 169.4 eV for all the samples (Table 13) [Briggs & Seah, 1983]. We cannot discriminate between Mn and Al bonded species. The increasing uptake of sulphur by MnOx with the addition of noble metal, which is suggested by the increasing ratios S/Al and S/Mn, was also confirmed by TPR.
56 Table 13. Maximum reduction temperature, Tm, and H2/Mn ratio from TPR experiments, S 2p binding energies (B.E.), S/Al and S/Mn ratios, determined by XPS, on pre-sulphated samples. Mn: 10; Pt, Pd: 0.1 mol%/Al2O3. TPR data a XPS data b
Samples Tm (°C) H2/Mn S 2p B.E. (eV) S/Al S/Mn MnOx/Al2O3 562 1.33 169.4 0.015 0.106 MnOx-Pt/Al2O3 501, 522 1.55 169.5 0.013 0.13 MnOx-Pd/Al2O3 482 1.43 169.4 0.028 0.20 a TPR conditions: 10% H2 inAr,17ml/min,5°C/min b All B.E.’s (eV) referenced to Al2p=74.6 eV
MnOx/Al2O3 consumption (a.u.) 2 H
CuO/Al2O3
100 200 300 400 500 600 700 800 Temperature (oC)
Figure 33. TPR profiles of Al2O3-supported MnOx and CuO calcined at 800°C for 4 h in air, before (thin line) and after pre-sulphation (thick line). Mn, Cu: 10 mol%/Al2O3. TPR experimental conditions: 17 ml/min, 10% H2/Ar.
All the samples of CuO alone (and mixed with noble metals, not shown) treated by sulphur show one main peak occurring at slightly higher temperatures than for the fresh ones, and a “hill” in the range 350-700°C (Figure 33). The small increase in the reduction temperature of the CuO samples is probably due to the treatment at 600°C for 16 h which has induced stronger interaction with the support and causes the reduction to occur at higher temperatures. The hill observed at 350-700°C probably corresponds to reduction of the sulphate. The change in the TPR profile for Cu-containing samples is nevertheless very small compared to that of Mn-containing samples. Thus this indicates the larger stability of manganese sulphate
57 compared to copper sulphate. Namely, it is known that CuSO4 has a lower decomposition temperature than MnSO4 [Ostroff & Sanderson, 1959].
Catalytic activity [Paper III]
Table 14 and Figure 34 present the results from the activity measurements performed after the pre-sulphation performed with 1000 ppm SO2 (2 l/min) in air at 600°C for 16 h. For the MnOx catalyst, the activity for the oxidation of C10H8 and CH4 was almost the same before and after sulphur treatment, whereas the oxidation of CO was more inhibited, as seen in Figure 34 and Table 14.
100
80 F SFSS F
60 C10H8 CO CH4
40 Conversion (%)
20
0 200 300 400 500 600 700 800 Catalyst temperature (oC)
Figure 34. CO, C10H8 and CH4 conversion for MnOx/Al2O3 calcined at 800°Cfor4h in air, before (F) and after the pre-sulphation (S). Mn: 10 mol%/Al2O3. Gas mixture 1.
The activity of the CuO catalyst was affected for the oxidation of CO in a similar way by the sulphur treatment as the MnOx catalyst, but with a lesser deactivation, as is shown in Table 14. However, there was an appreciable promoting effect for the oxidation of C10H8. The specific impediment in the oxidation of CO, which affected both MnOx and CuO catalysts by sulphur treatment, could be attributed to an inhibiting effect of SO2 on the adsorption sites of CO, whereas the adsorption sites of HCs were not affected to the same extent [Farrauto & Wedding, 1973; Yu Yao, 1975]. Farrauto and Wedding attributed this difference to a fast poisoning of the carbonyl sites (responsible for CO oxidation) and a slow poisoning of the carbonate sites (responsible for the HC oxidation) and that SO2 selectively adsorbs on the carbonyl sites at low temperatures and on carbonate sites at higher temperatures.
58 Table 14. Temperature for 50% conversion of CO, C10H8 and CH4 for Al2O3- supported MnOx, CuO, Pd, Pt and mixed catalysts, calcined at 800°Cfor4hinair, before (F) and after the pre-sulphation (S). Mn, Cu, Pt and Pd mol%/Al2O3 are indicated in parentheses. Gas mixture 1. CO C10H8 CH4 Samples F S ∆TFS∆TFS∆T MnOx (10) 425 535 110 365 375 10 645 666 21 CuO (10) 344 395 51 420 382 -38 590 628 38 Pt (0.1) 245 276 31 250 277 17 640 655 15 Pd (0.1) 217 242 25 221 248 27 640 681 41
MnOx-Pt (10-0.05) 344 317 -27 334 317 -17 640 665 25 MnOx-Pt (10-0.1) 290 300 10 291 300 9 638 654 16 MnOx-Pd (10-0.1) 290 274 -16 290 282 -8 641 681 40 CuO-Pt (10-0.1) 254 273 19 264 278 14 593 630 37 CuO-Pd (10-0.1) 220 216 -4 230 219 -11 595 630 35
∆T= T50%(S) – T50%(F)
As seen in Table 14, the noble metals Pt and Pd seem to be more sulphur- resistant than the transition metal oxides, in agreement with reports in the literature [Shelef et al., 1978; Kummer, 1980]. The sulphur deactivation of the Pt catalyst slightly affected the oxidation of all the combustibles (Table 14). The Pd catalyst had a higher sulphur sensitivity than the Pt catalyst which also is in agreement with literature data [Hegedus et al., 1979; Deng et al., 1993; Kang et al., 1994]. Several different effects were observed on the mixed catalysts. After exposure to SO2, the activities of the mixed MnOx-Pt (10-0.05) and MnOx-Pd (10-0.1) catalysts were enhanced for the oxidation of C10H8 and CO, and almost no deactivation was observed with a higher amount of Pt (0.1) in the catalyst. The conversion of CH4 was slightly decreased for MnOx-Pd and both mixed MnOx-Pt catalysts. However, the activity of the mixed MnOx-Pd catalyst for the oxidation of CO and C10H8 remained slightly lower than the activity of the Pd catalyst after sulphur treatment. The extent of the decrease in activity for the oxidation of CH4 was the same for both Pd and mixed MnOx-Pd catalysts, which is seen with 40°C increase of the temperature for 50% conversion. The mixed CuOx-Pt catalyst was slightly deactivated by the sulphur treatment. It is deactivated to the same extent as the Pt catalyst for the oxidation of CO and C10H8 and also to the same extent as the CuO catalyst for the oxidation of CH4. The CuO-Pd catalyst showed an interesting behaviour. After sulphur treatment, the activity was slightly enhanced for the oxidation of CO and C10H8, and decreased for the oxidation of CH4. However, the activity of the sulphur treated mixed CuO-Pd catalyst for the oxidation of all the combustibles studied here was better than the activity of the Pd catalyst.
59 A reason for the promotional effect by sulphur treatment in the activity of the mixed MnOx-Pt (Pt: 0.05 mol%/Al2O3) catalyst for the oxidation of C10H8 can be attributed to formation of sulphate species, that activates C-H bond, as mentioned in the introduction part of this section [Sekizawa et al., 1993; Burch et al., 1998; Zaki et al., 1999]. This effect was not observed at higher Pt concentration and this may reflect a difference in Pt dispersion or interaction with manganese or aluminium sulphate that may change the effect of SO2 [Burch et al., 1998]. On mixed catalysts containing Pd, there are also some promoting effect in contradiction with the literature concerning Pd only catalysts [Hubbard et al., 1995]. Because metal oxides readily react with SO2 to form stable sulphate, metal oxides can serve as a sink and prevent the formation of strongly adsorbed sulphate on the active Pd catalyst. In addition, compared to the metal oxide catalysts, the sulphate species in the Pt or Pd catalysts are not as stable, thus metal oxides provide sulphate species that are stable at higher temperatures.
Sulphur poisoning on stream
The behaviour of pre-sulphated catalysts may differ from that of catalysts subjected to SO2 on stream. Table 15 compares results from activity tests without and with the presence of 20 ppm SO2 inthegasmixture2.ForthePt catalyst, in presence of 20 ppm SO2, the conversion of CO and C2H4 is slightly hampered whereas CH4 was unaffected. For the mixed MnOx-Pt, only the temperature for CH4 oxidation was increased during the test in the presence of 20 ppm SO2, whereas the oxidation of CO and C2H4 was similar to that in the absence of SO2. The oxidation of all the combustibles on the CuO-Pt catalyst was hampered. However, the CuO-Pt catalyst was more active for all the combustibles studied here than Pt or MnOx-Pt catalysts both in the absence and in the presence of 20 ppm SO2. Interestingly, it can be seen that the activity of a Pd catalyst in the presence of 20 ppm SO2 differed greatly from that of a Pt catalyst. The oxidation was enhanced for C2H4, while the oxidations of CO and CH4 were inhibited (Figure 35). This enhancement correlated with reduced CO conversion, which allowed the oxidation of C2H4 to occur readily, as discussed in Section 2.1.2 (Figure 3). Mixed catalysts with Pd were, despite a negative effect by SO2, more active for the oxidation of some combustibles and more sulphur resistant than the Pd catalyst.
60 Table 15. Temperature for 50% conversion of CO, C2H4 and CH4 for Al2O3-supported Pd, Pt and mixed catalysts, calcined at 800°C for 4 h in air, without (a) and with 20 ppm SO2 in the gas stream (b). Mn, Cu: 10; Pt, Pd: 0.1 mol%/Al2O3. Gas mixture 2. CO C2H4 CH4 ab∆Tab∆Tab∆T Pt /Al2O3 208 244 36 225 251 26 634 634 0 MnOx-Pt/Al2O3 296 296 0 306 306 0 631 678 47 CuO-Pt/Al2O3 204 226 22 215 236 21 583 624 41
Pd/Al2O3 180 214 34 275 222 -53 640 670 30 MnOx-Pd/Al2O3 189 194 5 290 290 0 628 682 54 CuO-Pd/Al2O3 184 203 19 202 218 16 594 653 59
∆T= T50%(b) – T50%(a)
100
80 C2H4 (b) C2H4 (a)
60 CO (a) CO (b)
40 Conversion (%)
20
0 100 200 300 400 Catalyst temperature (oC)
Figure 35. Conversion of CO and C2H4 for a Pd (0.1 mol%/Al2O3) catalyst, calcined at 800°C for 4 h in air, without (a) and with 20 ppm SO2 (b). Gas mixture 2.
Some important results from Section 3.2:
) The sequence of activity of the sulphur treated catalysts (1000 ppm SO2 in air at 600°C for 16 h), with an amount of noble metal of 0.1 mol%, for the oxidation of the combustibles is as follows: CO: CuO-Pd > Pd > CuO-Pt = Pt > MnOx-Pd > MnOx-Pt>MnOx >CuOx C10H8: CuO-Pd > Pd > CuO-Pt = Pt > MnOx-Pd > MnOx-Pt>CuO>MnOx CH4: CuO-Pd>CuO-Pt=CuO>MnOx =MnOx-Pt=Pt>MnOx-Pd = Pd
) The sequence of activity of catalysts in presence of 20 ppm SO2,withanamount of noble metal of 0.1 mol%, for the oxidation of the combustibles is as follows: CO: MnOx-Pd>CuO-Pd>Pd>CuO-Pt>Pt>MnOx-Pt C10H8: CuO-Pd = Pd > CuO-Pt > Pt > MnOx-Pd > MnOx-Pt CH4: CuO-Pt > Pt > CuO-Pd > Pd > MnOx-Pt>MnOx-Pd
61 3.3. Concluding Remarks
The effect of the hydrothermal treatment at 900°C on the adherence of the washcoat onto a metallic substrate was studied using SEM. It revealed a well- anchored washcoat onto the metallic support due to the growth of the alumina whiskers during the treatment. The sintering of the washcoat, viz. decrease of the BET-surface area, transformation of γ-toα-Al2O3 phases at lower temperatures and increase of the Al2O3 particle size, is accelerated after high temperature treatments in the presence of metal catalysts. This effect is more pronounced in the presence of Cu and Pd, compared to Mn and Pt, respectively, and increases with an increasing metal loading. In addition, alumina was found to react with CuO, particularly in the presence of noble metals at 900°C, to form inactive CuAl2O4. However, MnOx catalyst benefits from the more active Mn3O4 phase at high temperature and an optimum loading of 10 mol% Mn/Al2O3 was found for the oxidation of all combustibles. Pt sintering was delayed when mixed with CuO, thus giving more thermal resistant catalyst. The mixed MnOx-Pd and CuO-Pd catalysts were less active than Pd catalyst for the oxidation of CO and C10H8 however the opposite was observed for CH4. After a thermal treatment at 900°C for 60 h in air with 12% steam, Pd and CuO-Pt were found to be the most active for CO and C10H8 oxidation, while for CH4,MnOx had the highest activity. Pre-sulphating the catalysts with 1000 ppm SO2 (2 l/min) in air at 600°C for 16 h results in the formation of sulphate on MnOx-containing catalysts to a higher extent than on CuO-containing catalysts, and particularly in the presence of the noble metals. The oxidation of CO was largely inhibited by the pre-sulphation of the MnOx and CuO catalysts while the oxidation of C10H8 and CH4 was less inhibited with the MnOx catalyst, and the oxidation of C10H8 substantially improved with the CuO catalyst. Sulphur treatment was found to enhance the activity of some mixed catalysts while noble metal catalysts were not promoted by sulphation. CuO-Pd/Al2O3 catalyst was found to be the most active catalyst after pre-sulphation treatment for the oxidation of all combustibles. 20 ppm SO2 in the synthetic gas mixture results in a significant enhancement of the catalytic activity of Pd/Al2O3 catalyst for the oxidation of C2H4. However, the mixed catalysts were more active than the noble metal catalysts. Namely, in presence of 20 ppm SO2 in the feed, MnOx-Pd was found the most active for the oxidation of CO, CuO-Pd and Pd for the oxidation of C2H4 and CuO-Pt for the oxidation of CH4.
62 4 ADDITIVES:LANTHANUM AND CERIUM
In order to cope with thermal damage, a catalyst and all its components may be exposed to during elevated temperature and thermal fluctuations, the promotion by a stabiliser seems necessary. Furthermore it is important to develop low-temperature active catalysts, as this is one of the most promising ways to minimise the cold-start period.
4.1 Stabilisers
The addition of a stabiliser may inhibit the sintering effects. The transition aluminas are found to be stabilised by: Be2+,Cr3+,Sc3+ [Vereschagin et al., 1982], K+,Cs+,Na+ [Miyoshi et al., 1989], Yb3+,Y3+ [Ozawa et al., 1990b], Cr6+ [Bye & Simpkin, 1974; Tsuchida et al., 1983], Th4+ [Vereschagin et al., 1982; Burtin et al., 1987a], Sm3+,Gd3+,Dy3+ [Kato et al., 1989; Ozawa et al., 1990b], Mg2+ [Gauguin et al., 1975; Schaper et al., 1982; Miyoshi et al., 1989], Si4+ [Gauguin et al., 1975; Church et al., 1993; Huuska & Maunula, 1993; Ismagilov et al., 1995], Ca2+ [Vereschagin et al., 1982; Burtin et al., 1987a; Miyoshi et al., 1989], Sr2+ [Vereschagin et al., 1982; Miyoshi et al., 1989; Mizukami et al., 1991; Church et al., 1993], Nd3+ [Oudet et al., 1988; Kato et al., 1989; Ozawa et al., 1990b], Pr3+ [Oudet et al., 1988; Kato et al., 1989; Church et al., 1993], Ba2+ [Miyoshi et al., 1989; Mizukami et al., 1991; Church et al., 1993], and Zr4+ [Schaper et al., 1982; Burtin et al., 1987a; Mizukami et al., 1991]. There is a body of literature on studies concerning the stabilisation by La3+ [Schaper et al., 1982; Sauvion & Ducros, 1985; Burtin et al., 1987a; Oudet et al., 1988; Kato et al., 1989; Miyoshi et al., 1989; Ozawa et al., 1990a; Ozawa et al., 1990b; Mizukami et al., 1991; Church et al., 1993; Ismagilov et al., 1995; Groppi et al., 2000] and Ce4+ [Gauguin et al., 1975; Vereschagin et al., 1982; Sauvion & Ducros, 1985; Kato et al., 1989; Ozawa et al., 1990b; Ismagilov et al., 1995]. Some contradictory effects have been found with: Na+ [Gauguin et al., 1975], Mg2+ [Vereschagin et al., 1982; Burtin et al., 1987a], Sr2+ [Mizukami et al., 1991], Ba2+ [Mizukami et al., 1991; Huuska & Maunula, 1993], Yb3+ [Kato et al., 1989], Cr3+ [Tsuchida et al., 1983], Y3+ [Vereschagin et al., 1982; Kato et al., 1989], and La3+ [Vereschagin et al., 1982; Huuska & Maunula, 1993]. Discrepancies may be attributed to a non- optimised loading of stabiliser for the temperature of the treatment. Substitution of stabilisers in the surface of Al2O3 may be beneficial in several ways. Johnson suggests that stabilising elements slow phase transformations by replacing surface hydroxyls and as a result, slow the formation of Al-O-Al
63 bridges [Johnson, 1990]. Silica has been found to interact with surface hydroxyls on Al2O3 and this is assumed to reduce sintering. Also the foreign ion may occupy a vacancy in the Al2O3 lattice, thereby reducing diffusion [Schaper et al., 1985]. The fact that foreign ion reacts with a host lattice to form a new compound that stabilises Al2O3 is however certain [Young et al., 1980; Beguin et al., 1991]. Some authors noted a relationship between stabilisation and the radius and/or the charge of the added cation and attributed this to a reduction in mobility with increase in cation size and larger charge [Wakao & Hibino, 1962; Burtin et al., 1987a; Miyoshi et al., 1989; Mizukami et al., 1991; Church et al., 1993]. A stabiliser must remain at the surface in order to decrease the rate of surface diffusion. Large ions are therefore required to prevent dissolution into the bulk. It is also preferable that the additive may form a compound with the Al2O3 surface. Based on these considerations, the lanthanides are a logical choice.
4.2 Lanthanum
La appears to be one of the best additives for inhibiting the sintering of high surface-area Al2O3 [Burtin et al., 1987a; Church et al., 1993; Peiyan et al., 1995], especially when active species are deposited on it [Shkrabina et al., 1995; Ozawa et al., 1996b]. The stabilisation of Al2O3 appears to be a rather complex process and is dependent upon several factors such as preparation method, La loading, temperature and presence of water.
4.2.1 Preparation Method
A good dispersion of La throughout the Al2O3 depends on the methods used to insert the La. Mizukami et al., reported that the sol-gel aluminas show higher surface areas between 550 and 1000°C than the corresponding precipitated ones, although the differences between the two procedures are not existent at calcination temperatures of 1000°C and above [Mizukami et al., 1991]. Also, compared with incipient wetness impregnation and deposition- precipitation, the specific adsorption of a La(EDTA)- complex on Al2O3 has been found to result in supports that were more homogeneously covered with La and thereby require lower loading of La to stabilise Al2O3 at 1050°C [Tijburg et al., 1991].
4.2.2 Effect of the Loading
Depending on the preparation method and on the treatment conditions used, the optimum concentration of La to be added into the Al2O3 may vary
64 [Schaper et al., 1983; Oudet et al., 1987; Beguin et al., 1991; Tijburg et al., 1991]. Usually, a low La content is sufficient to preserve the Al2O3 against thermal sintering at temperatures below 1050°C[Beguinet al., 1991]. A higher temperature or the presence of steam usually requires a higher loading of La [Beguin et al., 1991]. However, the amount of La necessary to ensure such a stabilising effect depends also on the properties of the starting Al2O3 material [Mizukami et al., 1991] and also the deposition method of La. Therefore many studies in the literature may lead to some discrepancies as follows. For ageing at temperatures of 1000°C, an amount of 1 and 2 mol% La2O3 was found to be best [Schaper et al., 1983; Matsuda et al., 1984; Kato et al., 1989]. At 1100°C, an amount of 2 mol% La2O3 [Ozawa et al., 1990b] was found to have a better retarding effect on sintering than 1 mol% [Schaper et al., 1983] or 5 mol% [Church et al., 1993]. However, after an ageing at 1200°Cahigher amount (5.6 mol% La2O3) was found to give the highest surface area [Matsuda et al., 1984; Kato et al., 1989] whereas an optimum amount of 0.5-1 mol% La2O3 was also reported [Miyoshi et al., 1989; Ozawa et al., 1990a]. There is an optimum of La content in the Al2O3 washcoat that is shifted to higher loadings as the calcination temperature increases. An excess of La loading decreases the surface area of Al2O3 due to the solid state reaction of La with Al2O3 and formation of the perovskite structure LaAlO3 [Matsuda et al., 1984; Kato et al., 1989; Miyoshi et al., 1989; Beguin et al., 1991], that is able for instance to include Pd in the crystal lattice and therefore impedes the catalytic activity [Matsuda et al., 1984].
4.2.3 Effect of Steam
La is particularly adapted for atmosphere which contains water [Schaper et al., 1984; Beguin et al., 1991]. Beguin et al. investigated response to ageing of low and high loaded La-Al2O3 samples (2.65 and 11wt%La/Al2O3) at 1050°C and 1220°C in 20% humidity-containing atmosphere which corresponds to the content of water during the combustion of natural gas [Beguin et al., 1991]. Schaper et al. also aged samples of γ-Al2O3 promoted with 0-5 mol% La2O3 under 12 bar partial steam pressure at 840°C for 40 h [Schaper et al., 1984]. In both studies, the influence of water in the sintering test performed at high temperature becomes less pronounced as the amount of La introduced increases. Such behaviour was connected with the increase in the amounts of the LaAlO3 perovskite phase found in both studies which lacks hydroxyl groups, in contrast to Al2O3, and as such cannot so readily react with water.
65 4.2.4 Mechanism of Stabilisation
The main role of La is to stabilise Al2O3 against thermal damages. The mechanism of stabilisation by La is not yet fully established. The role of La, according to Schaper et al. is to decrease the rate of surface diffusion that prevents sintering and loss of surface area [Schaper et al., 1985]. According to Ozawa et al., at low La loadings and low temperatures, La3+ substitutes in the Al2O3 lattice and hinders both bulk and surface diffusion [Ozawa et al., 1990]. There is a concentration limit, termed saturation value, to which the Al2O3 support can accommodate dispersed La in the form of a two- dimensional overlayer invisible by XRD. Bettman et al. and Scheitauer et al. concluded, by Auger electron spectroscopy and CO2 chemisorption, and by Raman spectroscopy, respectively, a saturation value equal to 8-9 µmol La/m2 [Bettman et al., 1989; Scheithauer et al., 1998]. However Xie et al. and Yang and Swartz used XRD methods and found ca 17-18 µmol La/m2 [Yang & Swartz, 1984; Xie et al., 1984]. Haack et al. found a value between 35 and 61 µmol La/m2 by XPS measurements [Haack et al., 1992a]. The exaggerated values found may be due to the low detection limit of the XRD since small invisible crystallites may be formed. Also, Haack et al. calcined their samples at higher temperatures. Matsuda et al. found that below a ratio of La/Al=1, La is in dispersed phase for samples calcined at 800°C[Matsudaet al., 1984]. Compared to the other studies, Matsuda et al. used the co-precipitation method (against incipient wetness technique with lanthanum nitrate) that allows a better homogeneity, and thus a higher saturation value. Bettman et al. have demonstrated that above this critical concentration La would only form crystalline oxides which in general at calcination temperatures below 800°C, exist as La2O3, whereas at higher temperatures LaAlO3 could be observed. Indeed at higher temperatures or higher La concentration the formation of LaAlO3 phase has been reported [Schaper et al., 1983; Schaper et al., 1984; Ledford et al., 1987; Oudet et al., 1988; Bettman et al., 1989; Béguin et al., 1991; Haack et al., 1992a; Haack et al., 1992b]. LaAlO3 has a perovskite structure involving only octahedral aluminium and according to Beguin et al. [Beguin et al., 1991], a strongly bound surface layer is formed which protects tetrahedral aluminium in the underlying Al2O3 against transformation. However, work by Haack et al. [Haack et al., 1992b] shows that after thermal treatment at 1500°C for low La/Al ratios (<0.1), the predominant La- containing phase is lanthanum-β-alumina (La2O3.11Al2O3)aspreviously claimed by Matsuda et al. [Matsuda et al., 1984]. These authors attributed the ability of added La to retard sintering as a consequence of lanthanum-β- alumina and not LaAlO3 formation, because at low concentration the formation of LaAlO3 is unlikely [Haack et al., 1992b]. However, Beguin et al.
66 claimed that the loss of stabilisation is associated with the solid reaction of LaAlO3 and Al2O3 to form lanthanum-β-alumina [Béguin et al., 1991]. The formation of lanthanum-β-alumina requires both a sufficiently low La concentration and a high calcination temperature. Dexpert-Ghys et al. [Dexpert-Ghys et al., 1976] reported that the lanthanum-β-alumina phase exists over the composition range La/Al = 1/11 to 1/14. They synthesised lanthanum-β-alumina by firing La2O3 and Al2O3 powder. Ropp and Caroll [Ropp & Caroll, 1980] have observed that lanthanum-β-alumina is formed via LaAlO3 at temperatures above 1400°C by firing La2O3-Al2O3 (8.3-91.7) mixture for 24 h. The formation rate of lanthanum-β-alumina is extremely slow below 1500°C. However, in some studies, formation of lanthanum-β-alumina at a lower temperature, i.e. 1000°C,couldbeobserved[Matsudaet al., 1984; Kato et al., 1989]. This formation may result from the difference in the preparation method, i.e. co-precipitation compared to mixing of two oxides by Ropp and Caroll. The co-precipitation method gave a larger surface area for the two oxides, which resulted in the formation of lanthanum-β-alumina at the relatively low temperature. Thus it is clear that La has a positive effect on the stabilisation of Al2O3 at high temperatures. Nevertheless the mechanism of the stabilising effect is unclear, because the dispersed La phase, the perovskite LaAlO3 and the lanthanum-β-alumina were all found to be responsible for the stabilisation.
4.2.5 Additional Effects of Lanthanum
Besides the thermal stabilisation effect on the support, La has been proven to be very effective for increasing the dispersion and stabilising the particle size of Pt [Yang & Swartz, 1984; Drozdov et al., 1986; Oudet et al., 1987; Oudet et al., 1989; Härkönen et al., 1991] and Pd [Matsuda et al., 1984; Chou et al., 1995]. Oudet et al. reported that the presence of La strongly modifies the morphological aspect of the Al2O3 support, which appears poorly crystallised and composed of particles of undefined shape [Oudet et al., 1989]. This provides an increased number of nucleation sites for metals during the first steps of deposition, leading to an enhancement of the initial repartition and dispersion of the metallic phase on the doped samples. Xie et al. suggested that the promotion by La changes the surface property of the support and the interface energy between Ni and the support causes the crystallite of Ni to become smaller [Xie et al., 1982]. It is certain that the role of La is also to decrease the movements of the washcoat and thus restrain the coalescence or encapsulation of the metal particles. La has also been reported to be effective against the undesirable reactions between Al2O3 and CuO that lead to copper aluminate [Tijburg, 1989].
67 The durability of Pd catalyst modified by La for the oxidation of CH4 has been shown by several authors [Matsuda et al., 1984; Kato et al., 1989; Chou et al., 1995]. A 4 h test performed at 860°C for the combustion of CH4 showed that the activity of a Pd/Al2O3 with 6% La2O3 could be retained. TPO studies demonstrated that the retardation in Pd sintering resulted from an increase in the bond strength Pd-O [Chou et al., 1995]. Kato et al. studied the durability of combustion catalysts using a mixture containing 3 vol% CH4 at the inlet temperature of 500°C and at about 1200°C in the catalyst bed temperature after CH4 combustion. The CH4 conversion was above 99.5% for the Pd/La2O3- Al2O3 (La/Al = 5/95) during 1000 h, while it decrease from 99.3 to 90% for the Pd/Al2O3 only after 250 h [Kato et al., 1989].
4.3 Cerium in Catalysis
It is an ambitious task to define the role of ceria (CeO2) in catalysis, particularly for abatement of pollutants, since many effects have been attributed to CeO2. The different applications and roles of Ce have been reviewed by several authors [Kummer, 1986; Trovarelli, 1996; Kašpar et al., 1999]. Ceria is already used in three-way catalysts [Kim, 1982; Kummer, 1986; Gandhi & Shelef, 1987], diesel oxidation catalysts [Farrauto & Voss, 1996] and combustion catalysts [Groppi et al., 1999]. Its use in catalysts was implemented in the beginning of the 1980s following the use of FeO and NiO as oxygen storage components [Funabiki et al., 1991].
4.3.1 Oxygen Storage Capacity (OSC)
The most crucial role of ceria is its ability to exchange oxygen with the environment. Ceria is a non-stoichiometric compound [Yao & Yu Yao, 1984; Harrisson et al., 1988] which exhibits two valences, Ce(+III) and Ce(+IV), with low redox potential. The oxygen storage on the catalyst is simply described as a cyclic reduction and oxidation of CeO2. The process of oxygen storage and transport in ceria can be described by the defect mechanism and there are two types of defects: intrinsic and extrinsic [Cho, 1991; Mogensen et al., 2000]. The former is due to the oxygen anion vacancies created upon the reduction of ceria according to the redox process involving ceria:
+ δ ⇔ + δ + δ CeO2 R CeO2−δ RO Vo where R is a reductant, RO is a gaseous product, and Vo is an oxygen anion vacancy which may be charged singly or doubly [Cho, 1991]. The extrinsic defects are due to the oxygen anion vacancies created by the charge compensation effect of foreign cations, which have a valence, lower
68 than that of the host Ce ions they substitute. For example, the doping reaction of ceria with bi- or tri-valent cations can be written as:
+ → + + 2MO CeCe 2M Ce CeO2 2Vo
+ → + + 2M 2O3 3CeCe 4M Ce 3CeO2 2Vo where M is the bi- or tri-valent cation, CeCe istheCecationontheCesiteof the ceria lattice, MCe is the foreign cation on the Ce cation site [Cho, 1991]. Trivalent dopants generally produce a higher ionic conductivity, i.e. oxygen mobility, than divalent ones [Cho, 1991]. Both of these vacancies were believed to provide a practical way to increase the OSC of CeO2.
4.3.2 Noble metal-Ceria Interactions
In TWC catalysts the air/fuel ratio in TWC oscillates around the stoichiometric value, due to the lag between the oxygen sensor and the carburettor. Hence, CeO2 dominates in the oxidative atmosphere, while in reducing atmosphere Ce2O3 becomes predominant. Thus according to a cyclic rich-lean composition fluctuation in the automotive exhaust gas, the cerium oxide can either provide oxygen for the oxidation of CO and HC, for instance, or remove oxygen from the gas phase for the reduction of NOx to N2 [Gandhi et al., 1976]. The formation of lattice oxygen vacancies in CeO2, which plays an important part in the oxidation of CO [Jin et al., 1987; Sanchez & Gasquez, 1987] is associated with the reduction of Ce(+IV) to Ce(+III) [Yao & Yu Yao 1984]. The interaction of CeO2 with precious metals (Pt, Pd and Rh) and its effect on catalytic activity has been intensively studied [Yao & Yu Yao, 1984; Crucq et al., 1991]. CeO2 itself does not have good O2 adsorption. Yao and Yu Yao found no OSC on CeO2 alone at 300°C and very little at 400°C. This is consistent with the fact that surface capping oxygen (SCO) in CeO2 which contributes to catalytic reactions [Funabiki et al., 1997] cannot be reduced below 300-350°C [Yao & Yu Yao, 1984; Harrison et al., 1988]. However, when it coexists with noble metals, the total amount of O2 is greatly increased even at 300°C [Yao & Yu Yao, 1984] and the presence of Pt, Pd or Rh lowers the temperature of the surface CeO2 reduction [Yao & Yu Yao, 1984; Harrison et al., 1988; Bernal et al., 1993; Serre et al., 1993; Bouly et al., 1995; Funabiki et al., 1997]. Yu Yao also showed that Pd, Pt and Rh metals promote the reduction of Ce4+ to Ce3+ and thus facilitate the charge transfer from metal to Ce, which results in the higher oxidation states of the metals and hence increases the oxygen storage capacity of CeO2 [Yu Yao, 1984].
69 There is significant evidence that oxygen storage property in noble metal catalysts is correlated with the degree of interaction between a group VIII metal and CeO2 [Nunan et al., 1992; Funabiki et al., 1997]. Noble metals/CeO2 catalysts have been found to have considerable activity at very low temperatures after certain pre-treatments. The noble metals in the order of Rh, Pd, Pt helped to enhance the O2 uptake and maintain the SCO after thermal ageing due to intimate contact with noble metals [Funabiki et al., 1997]. Yu Yao observed an improved activity after a reduction in CO at 300°C for the oxidation of CO and HC with either Pt, Pd or Rh supported on CeO2 [Yu Yao, 1984]. Kubsh et al. observed higher activity of Pt/Rh/CeO2 catalysts after reduction at 500°C[Kubshet al., 1991]. Nunan et al. observed an improved low-temperature activity of Pt/CeO2 for both oxidation of CO and HC and reduction of NOx, after a reductive pre-treatment in a rich synthetic exhaust gas mixture at 450°C[Nunanet al., 1992]. The same observation has been made with other reducing agents: H2 at 200°C[Holmgrenet al., 1999], 300°C[Diwell et al., 1991], 500°C[Kubshet al., 1991] and CO at 490°C[Serreet al., 1993b]. The high activity is discussed in terms of reduction of CeO2 in the vicinity of Pt particles [Nunan et al., 1992; Serre et al., 1993b; Holmgren et al., 1999] and that decreasing CeO2 crystallite size leads to higher Pt/Ce interaction and hence greater activity [Nunan et al., 1991]. Unfortunately, without any pre-reducing treatment and in lean conditions CeO2 is reported to convert Pt and Pd metals into an oxidised and less active state for the oxidation of alkanes [Yu Yao, 1980; Kummer, 1986; Gandhi & Shelef, 1987; Shyu & Otto, 1989]. However, for the oxidation of CH4,the promotion of Pd by CeO2 may be beneficial. According to several authors, CeO2 retards the thermal decomposition of PdO and promotes Pd oxidation through Pd-Ce interactions [Shyu et al., 1988b; Ishihara et al., 1993; Hu et al., 1996; Groppi et al., 1999; Ciuparu et al., 2000]. However, some authors also reported negative effects of CeO2 onto Pd for oxidation of CH4 [Hicks et al., 1990c; Ahlström-Silversand & Odenbrand, 1997].
4.3.3 Metal Oxide-Ceria Interactions
Previous studies of Ce promoters have focused primarily on its effects on the structure and activity of noble metal catalysts. Studies of the promoter effect on supported transition metal oxide catalysts have been limited. However, in some studies CeO2 has been examined as a promoter for oxidation reactions. The catalytic activity of CeO2 for the oxidation of CO is reported to be enhanced in the presence of Zr [De Leitenburg et al., 1996; Terribile et al., 1999], Ag [Luo et al., 1998] Mn [Imamura et al., 1996; Terribile et al., 1999] and Mn-Cu [Agarwal & Spivey, 1982; Terribile et al., 1999]. Cu catalysts prepared on CeO2/Al2O3 supports are particularly effective for
70 oxidation reactions of CO and CH4 [Peiyan et al., 1987; Peiyan et al., 1995; Liu & Flytzani-Stephanopoulos, 1995a; Luo et al., 1997; Xavier et al., 1998; Martínez-Arias et al., 1998; Terribile et al., 1999; Larsson & Andersson, 2000; Radwan et al., 2001] compared to any other base metal catalysts found in the literature and to a 0.5 wt% Pt catalyst [Liu & Flytzani-Stephanopoulos, 1995c]. Oxidation of CO at room temperature has been reported to occur on CuO deposited on pure CeO2 [Liu & Flytzani-Stephanopoulos, 1995a]. In addition to an enhanced activity, the CuO/Ce(La)O2/Al2O3 catalyst also showed a much better resistance to water vapour than other Cu catalysts [Liu & Flytzani-Stephanopoulos, 1995a; Xavier et al., 1998]. What makes doped CeO2 more active is the increase of oxygen mobility, which is the result of introduction of defect sites by addition of dopants into CeO2 lattice (extrinsic defects). The reason for this behaviour can be found in the efficiency of the Ce(+IV)-Ce(+III) redox couple, which is strongly enhanced in solid solutions due to the introduction of the smaller cation into the fluorite lattice of CeO2. This generates defects throughout the matter which in turn, brings about an increase in oxygen mobility and diffusion in the lattice [Pijolat et al., 1995; Lamonier et al., 1996]. Thus this increases the total amount of oxygen that can be reversibly exchanged between the solid and the surrounding atmosphere, and affects the kinetics of the redox processes by lowering the activation energy for “hopping” of oxygen, which in turns results in a higher oxygen storage capacity. The synergetic effect between Cu and Ce, that leads to an increase in activity, has also been attributed to the presence of reduced Cu [Liu & Flytzani-Stephanopoulos, 1995b] particularly active for the oxidation of CO [Jernigan & Somorjai, 1994], and an increase of CO adsorption on CuO-CeO2 compared to CuO or CeO2 alone [Luo et al., 1997].
4.3.4 Additional Effects of Ceria
One of the main functions of CeO2 in TWC catalysts is to promote the water-gas shift reaction [Gandhi et al., 1976; Schlatter & Mitchell, 1980; Kim, 1982; Herz & Sell, 1985; Harrison et al., 1988]:
+ → + CO H 2O CO2 H 2 and the steam reforming reaction [Diwell et al., 1991]:
+ → + C3 H 8 3H 2O 3CO 7H 2
Ce which is included in the catalysts for automotive emission control for other purposes than stabilisation of the washcoat has also been claimed to be an effective stabiliser to a small extent [Rogemond et al., 1997], but less than
71 other rare earth oxides such as La [Harrison et al., 1988; Kato et al., 1989; Ozawa et al., 1990; Church et al., 1993; Ismagilov et al., 1995; Shkrabina et al., 1995; Ahlström-Silversand & Odenbrand, 1997; Groppi et al., 1999]. CeO2 is also used to improve the dispersion of noble metals, Pt [Yao, 1984; Drozdov et al., 1986; Gandhi & Shelef, 1987; Diwell et al., 1991; Gonzales- Velasco et al., 1993], Pd [Duplan & Praliaud, 1991], and Rh [Yu Yao, 1980]. The addition of ceria leads to an increase of the resistance to sintering of precious metals [Sanchez & Gazquez, 1987; Diwell et al., 1991]. The deactivation of Rh, which is a crucial component of TWC particularly with regard to CO and to NO to N2 conversion, is thought to be due to a strong Rh-Al2O3 interaction, which fixes Rh in a high oxidation state and may inhibit the redox capability of Rh under transient conditions. This effect can be retarded by the incorporation of CeO2 into the catalyst [Harrison et al., 1988]. However, some studies have shown negative effects using Ce in combination with noble metals, as follows. It was reported that the addition of CeO2 in Pt (0.02 wt% Pt) catalysts decreased the dispersion and increased the particle size with increasing Ce content, whereas for Pd (0.05 wt%), the content of Ce did not affect the dispersion. However, CeO2 did not keep either Pt or Pd in a highly dispersed state during exposure at 900°C [Summers & Ausen, 1979]. These results are consistent with those of Hicks et al. who studied the thermal stability of Pd in a range of temperature from 500°C to 900°C[Hickset al., 1990c] and found that with high loadings of Pd (2.5 wt%), CeO2 did not alter the thermal stability of the Pd, whereas at lower loadings (0.2 wt%) and temperatures above 600°C, the stability of Pd is greatly reduced by CeO2 [Hicks et al., 1990c]. It was concluded that CeO2 might preferentially cover sites on Al2O3 that bond tightly to oxidised Pd at high temperatures, and thereby promote the agglomeration of the oxidised Pd. However, on Cu/Al2O3 catalysts the promotion by CeO2 was reported to stabilise Cu against sintering, by decreasing the particle size and the formation of copper aluminate due to Cu-Ce interactions [Fernández-García et al., 1997]. Besides these effects, Ce in catalysts has been reported to reduce the inhibition effect of CO, and makes the oxygen partial-pressure dependence less pronounced [Yu Yao, 1984; Oh & Eickel, 1988].
4.3.5 Deactivation of Ceria
The deactivation of the oxygen storage capacity is a large problem. The deactivation has been found to be mainly due to thermal sintering. Indeed, the increasing restrictions for the automotive emissions led to the development of so-called closed-coupled catalysts (CCC). These catalysts, being manifold mounted, experience high temperatures (1000-1100ºC), and thus require extremely high thermal resistance [Cuif et al., 1998; Kašpar et al., 1999].
72 Thermal ageing of CeO2 resulted in a significant decrease in the surface capping oxygen (SCO), but caused no change in bulk oxygen [Funabiki et al., 1997]. The suppression of the redox capacity of CeO2 in aged catalysts is associated with the crystallisation of CeO2 [Engler et al., 1989; Nunan et al., 1991; Zhang et al., 1995; Funabiki et al., 1997; Rogemond et al., 1997], decrease of surface area [Yao & Yu Yao, 1984; Funabiki & Yamada, 1988; Kašpar et al., 1999], loss of metal-CeO2 interactions [Nunan et al., 1992; Bouly et al., 1995; Schmieg & Belton, 1995] and formation of CeAlO3 that fixes Ce in one oxidation state [Geller & Raccah, 1970]. Other deactivation mechanisms have been suggested such as CeO2 poisoning by P [Smedler et al., 1993], and S to form sulphate [Harrison et al., 1988; Trimm, 1991; Boaro et al., 2000] thus inhibiting the water-gas shift reaction [Schlatter & Mitchell, 1980; Su & Rotschild, 1986].
4.3.6 Ceria Promoters
Structural doping of CeO2 may provide an efficient route to stabilise and to enhance the oxygen storage and hence catalytic activity. The effects of doping with foreign cations, at relatively low concentrations, on the thermal stability of CeO2 has been extensively investigated by Pijolat et al. [Pijolat et al., 1995]. These authors tried to rationalise the ability of foreign cations to stabilise CeO2 against sintering by developing a complete set of equations based on the diffusion of Ce vacancies as the limiting step in the sintering process. Among the different cations investigated (Th4+,Zr4+,Si4+, La3+,Y3+,Sc3+,Al3+,Ca2+ and Mg2+), those with ionic radii smaller than that of Ce4+ effectively stabilised the CeO2 against sintering [Pijolat et al., 1995]. As a general trend, an increase in the amount of added cation decreased sintering. Thermal stabilisation of a CeO2 surface area was observed also in mixed oxides prepared by co-precipitation method [Kubsch et al., 1991]. Conversely, all the dopants whose radii are larger than that of Ce4+, e.g. La, Nd and Y significantly stabilised CeO2. The discrepancy between the results found by the studies was due to the preparation method, according to Kaspar et al., because Pijolat et al. used the incipient wetness method that gave large particle diameter, thus only the small cations had the ability to migrate into CeO2 lattice and form a solid solution. Numerous studies concern mixed CeO2-ZrO2 systems which were already employed in the 4th generation TWCs, in close coupled catalysts by the mid- 1990s [Heck & Farrauto, 1995; De Leintenburg et al., 1996; Nunan et al., 1996; Cuif et al., 1997; Fornasiero et al., 2000]. Ce-Zr solutions were reported to be effective catalysts for the total oxidation of CH4 [Zamar et al., 1995]. The improved OSC is due to the higher lability of oxygen atoms compared to pure CeO2. Some oxygen atoms are located at long distances from the Zr atoms, providing higher mobility. Abundant literature is available on the study of
73 interactions between CeO2–ZrO2 and Pt or Rh [Nunan et al., 1991; Trovarelli, 1996]. The addition of 10% Zr has been found to enhance significantly the O2 uptake of Pt/CeO2 catalysts [Funabiki et al., 1997]. In the fuel-rich conditions, theroleofZrO2 which is to strongly enhance the reducibility of CeO2 with or without noble metals, led to an enhanced NO removal at lower temperatures [Fornasiero et al., 1995]. In addition, the presence of ZrO2 facilitates removal of sulphate species under reductive atmospheres, thus restoring most of the OSC of Rh/CeO2–ZrO2 catalyst compared to Rh/CeO2 after sulphur poisoning [Boaro et al., 2000]. Also, higher thermal stabilities were reported [Balducci et al., 1995; Nunan et al., 1996; Permana et al., 1997; Cuif et al., 1998]. For instance, Pd/CeO2-ZrO2 catalyst showed higher oxygen storage characteristics after 1050°CageingthanPd/CeO2 catalyst [Cuif et al., 1998].
4.3.7 Synergetic Effect between La and Ce
The initial studies concerning the use of solid solution CeO2-ZrO2 to enhance OSC have stimulated interest on investigating the effects of other doping agents, for example La. The addition of La to CeO2 has been suggested to have several positive effects. It is known that rare earths have high solubility in fluorite oxides [Kim, 1982] and dissolution of La3+ ions into CeO2 lattice to form La2O2-CeO2 solid solution has been reported by several authors [Miyoshi et al., 1989; Miki et al., 1990; Bernal et al., 1997; Groppi et al., 1999]. This leads to an improvement of CeO2 dispersion [Graham et al., 1993], a decrease of CeO2 crystallite growth [Pijolat et al., 1995; Groppi et al., 1999] and an inhibiting effect of sintering of CeO2 with or without Pt/Rh at high temperatures in oxidising environments [Miyoshi et al., 1989; Kubsh et al., 1991; Ozawa et al., 1991]. XPS measurements carried out on La/CeO2 indicated that stabilisation of surface area with this tri-valent dopant is a surface La3+ enrichment that impedes CeO2 crystallite growth under oxidising conditions [Kubsh et al., 1991; Harrison et al., 1996]. Also, inhibition of the formation of CeAlO3 could be achieved by incorporating La3+ [Graham et al., 1993]. La3+ has to be deposited before CeO2 is added because surface segregation of La3+ onto CeO2, impedes the reducibility of CeO2, apparently due to an effective blocking of the reducible- oxides surfaces [Kubsh et al., 1991]. However, in the presence of La the reducibility of CeO2 is reported to increase [Miyoshi et al., 1989; Bernal et al., 1997; Groppi et al., 1999] and the degree of oxidation of CeO2 to be decreased [Talo et al., 1995; Harrison et al., 1996]. Thus La accelerates the diffusion of oxygen from the bulk to surface and the total amount of O2 adsorbed is much larger than that in undoped CeO2 [Miyoshi et al., 1989; Cho, 1991; Logan & Shelef, 1994; Bernal et al., 1997]. Many studies have reported the impact of La on TWC performance. Combination of La and Ce have been found to improve the light-off
74 performance of the Pt-Rh/CeO2/La2O3/Al2O3 catalysts for abatements of CO, NO and HC at low temperatures [Miyoshi et al., 1989; Miki et al., 1990; Nunan et al., 1992]. Miki et al. [Miki et al., 1990] found that the oxygen storage capacity of CeO2 was increased in presence of La2O3 but only in the presence of noble metals, due to the greater potential of precious metals over Ce to activate hydrogen and oxygen in the gas phase by adsorbing them on the surfaces. However, at high temperatures, the diffusion rates of both the lattice oxygen and oxygen vacancies in CeO2 are facilitated leading to the enhanced activities with hydrogen and oxygen in the gas phase without the aid of the precious metals [Miki et al., 1990]. The maximum increase in the oxygen storage capacity is reached when the amount of La in CeO2 is 25% [Miki et al., 1990]. However, Kubsh et al. observed a decreased TWC catalytic activity in the presence of La. The discrepancy in the results may be due to the preparation methods used and the fact that in the other studies Al2O3 was used, whereas Kubsh et al. studied the addition of La on Pt-Rh/CeO2 catalysts [Kubsh et al., 1991].
4.4 Results and Discussion
According to reports in the literature, many studies concern the interaction between La and Al2O3, but little information is available on the interaction of La with the active phases in the washcoat, particularly for metal oxides. The utilisation of CeO2 in catalysis other than TWC catalysts seems to be very promising. However, the oxygen storage capacity of CeO2 is insufficient to meet future requirements [Cuif et al., 1998]. Therefore, it seems important to develop catalytic systems that can stabilise CeO2 against thermal damages and can be utilised in oxidising conditions. Only a few studies concern the promotion of Ce in metal oxide catalysts and almost none deal with a combination of metal oxide and noble metals. The following studies aims to tackle some of these problems.
4.4.1 Characteristics of the La- and/or Ce- Doped Washcoat [Paper VI]
The addition of either 3 mol% La, 3 mol% Ce, or a mixture of 1.5 mol% La and 1.5 mol% Ce onto the Al2O3 was done by specific adsorption of a Me(EDTA)- (Me=La, Ce) complex on Al2O3 [Tijburg et al., 1991]. This method was originally applied only for the deposition of La. We could observe some differences between La- and Ce-modified washcoat by characterisation with XRD, BET, TPR, XPS and SEM.
75 XRD did not indicate any La-containing phase on all samples even after high-temperature treatments up to 1000°C and XPS measurements evidenced the formation of a dispersed “La” phase. Indeed, the La 3d5/2 B.E. values for the samples containing La are equal to 835.5 eV (Table 16), which are substantially higher than the values observed for B.E. observed for La2O3, i.e. 833.2-833.8 eV, but they correspond to the values found for the dispersed "La" phase, i.e. 835.0-836.1 eV (Table 16). This La species gives a La XP line similar to that reported for La2O3, La(OH)3 or LaAlO3 which has two doublets [Haack et al., 1992; Siegmann et al., 1978]. However, La2O3 or LaAlO3 would be formed at higher concentrations [Bettman et al., 1989; Haack et al., 1992]. According to Bettman et al., under saturation concentration, i.e. 8.5 µmol La/m2,Laisina two-dimensional lanthanum aluminate [Bettman et al., 1989]. Therefore the La, present at less than 1.7 µmol La/m2, was probably present as a dispersed phase. In addition the atomic ratios of La/Al determined for CuO-Pt/La-Al2O3 and CuO-Pt/La-CeAl2O3 samples were close to that of theoretical ratios, as seen in Table 16, thus a good dispersion was obtained. No CeO2 was detected on samples containing Ce when calcined below 900°Cfor4h,asobservedbyXRDandXPSintheCe-Al2O3, La-Ce-Al2O3 samples, but only in samples without active phases, as seen by XRD in Table 17. Also TPR results, as seen in Figure 36, indicated the presence of three reducible species, i.e., at 100°C the removable oxygen anions on the bare amorphous alumina support in the presence of ceria [Yao & Yu Yao, 1984], at 200-500°C the surface capping oxygen anions attached to a surface Ce4+ ion in octahedral co-ordination [Rosynek, 1977] and at 600°CtheCeAlO3 precursor that is reduced to CeAlO3 [Shyu et al., 1998a], but no peak that corresponds to the reduction of bulk ceria, at usually 750-900°C, was detected. This is in agreement with Yao and Yu Yao who conducted TPR and chemisorption studies [Yao & Yu Yao, 1984] and Shyu et al. [Shyu et al., 1988a] who did not observe the formation of CeO2 bulk below a concentration of Ce of ca. 2.5 µmol CeO2/m2 Al2O3, which is much higher than our values (1.7 µmol CeO2/m2 Al2O3 for 3 mol%Ce/Al2O3). These well-dispersed Ce species were reported to be CeAlO3 precursors, in agreement with our results from TPR and XPS measurements [Shyu et al., 1988a]. Che et al. proposed that CeAlO3 precursors were stabilised in the cation vacancies of the Al2O3 surface [Che et al., 1973]. However, in presence of Cu, already after pre-treatment at 800°C, some CeO2 crystallites could be detected by XRD and also by XPS, as seen in Table 18, probably due to the sintering of CeO2 accelerated by the addition of Cu [Park & Ledford, 1998a].
76 Table 16. Data from XPS analyses of samples calcined at 800°C for 4 h in air. Binding energies of reference compounds are included. La: 3; Ce: 3; La-Ce: 1.5-1.5; Cu: 10; Pt: 0.5 mol%/Al2O3. La 3d5/2 binding energies (eV)a Surface atomic ratiob Samples Ref. Exp. La/Al Ce/Al CuO-Pt/La-Al2O3 835.5 0.018 (0.015) - CuO-Pt/Ce-Al2O3 - - 0.0059(0.015) CuO-Pt/La-Ce-Al2O3 835.5 0.0054(0.0075) 0.0054(0.0075)
La2O3 833.2 [Haack et al., 1992a] 833.5 [Ledford et al., 1989] 833.8 [Alvero et al., 1987] LaAlO3 833.8 [Haack et al., 1992a] 835.7 [Ledford et al., 1989] La(OH)3 834.8 [Alvero et al., 1987] Dispersed “La” 835.0 [Haack et al., 1992a; Talo et al., 1995] 835.5 [Alvero et al., 1987] 836.1 [Ledford et al., 1989] a All B.E.’s (eV) referenced to C1s=284.6 eV b theoretical ratios are indicated in parentheses
La-CeAl2O3 consumption (a.u.) 2 H
Ce-Al2O3
0 100 200 300 400 500 600 700 Temperature (oC)
Figure 36. TPR of Ce-Al2O3 and La-Ce-Al2O3 calcined at 800°C for 4 h in air. Ce: 3; La- Ce: 1.5-1.5 mol%/Al2O3. TPR experimental conditions: 5 ml/min, 5% H2/Ar.
When La was added, a Ce-La solid solution could be detected by XRD as observed by a shift of the 2θ position of the diffraction peaks of La-Ce-Al2O3 to lower angles with respect to the position of the same peaks in Ce-Al2O3 (not shown here) [Paper VI]. The insertion of La into the CeO2 lattice resulted into
77 the stabilisation of CeO2, as seen by an increase of Ce dispersion (Table 16), and a lower formation of CeO2 crystallites (Table 17). Although the amount of Ce in the La-Ce-Al2O3 samplewashalfofthatinCe-Al2O3, the intensity corresponding to the CeO2 diffraction peak was much smaller when La is present, thus La appeared to stabilise CeO2. Remarkably, dopants whose radii are larger than that of Ce4+, e.g. La, were reported to significantly stabilise CeO2 [Kubsh et al., 1991]. These interactions between La and Ce also promote the reducibility of CeO2, as observed by a increase by 40% of the H2 consumption during TPR experiments in presence of La in Ce-doped alumina sample [Paper VI]. This is in agreement with literature and indicates that La increases the OSC of CeO2 because when La3+ is partly substituted for Ce4+,the charge is compensated by the formation of oxygen vacancies which promote the diffusion of oxygen in the bulk [Miyoshi et al., 1989; Cho, 1991].
Table 17. BET-surface area of Al2O3 alone and modified after calcination in air at 800°C, 900°C and 1000°C (4 and 200 h). Weight % of corundum and peak intensities of CeO2 crystallites (2θ = 28.546) determined by XRD are indicated in parentheses. La: 3; Ce: 3; La-Ce: 1.5-1.5 mol%/Al2O3. BET-surface area (m2/g) XRD data 800°C 900°C 1000°C 900°C 1000°C Samples 4 h 200 h 4 h 200 h 4 h 200 h 200 h 4 h 200 h Al2O3 184 148 155 119 144 75 - - - La-Al2O3 176 138 148 117 141 96 - - - Ce-Al2O3 169 133 138 105 117 37 Ce(169) Ce(72) Ce(906), α(35%) La-Ce-Al2O3 174 138 147 116 131 86 - - Ce(140), α(2%)
The results from the BET-surface area and XRD experiments obtained for washcoat with or without active phases are reported in Tables 17 and 18. It can be noted that La was a more effective inhibitor to washcoat sintering than Ce. Notably no copper aluminate was found in samples containing La, indicating an effective inhibition by La in the reaction between CuO and Al2O3.The superiority of La over Ce in the thermal stabilisation of washcoats has also been reported by other researchers [Kato et al., 1988; Ozawa et al., 1990; Church et al., 1993; Ismagilov et al., 1995; Shkrabina et al., 1995; Groppi et al., 1999]. Some authors maintained that La is inserted into the crystal lattice of Al2O3 with a spinel structure, while Ce remains on the surface in the form of CeO2 [Church et al., 1993; Groppi et al., 1999]. This characteristic difference explains why the modification by La was more effective than that of Ce in improving the thermal stability of Al2O3. Also this higher dispersion of La led to a more efficient restraint of the undesirable reaction between Al2O3 and CuO, which forms bulk copper aluminate.
78 Table 18. BET-surface area of CuO deposited on Al2O3 alone and modified after calcination in air at 800°C, 900°C and 1000°C, for 4 h in air. Weight % of corundum, peak intensities of CeO2 (2θ = 28.546) and CuAl2O4 (2θ = 36.858) crystallites determined by XRD are indicated in parentheses. La: 3; Ce: 3; La-Ce: 1.5-1.5; Cu: 10 mol%/Al2O3. BET-surface area (m2/g) XRD data Samples 800°C 900°C 1000°C 800°C 900°C 1000°C
CuO/Al2O3 148 94 9 - θ, α(12%) α(82%), Cu(1614) CuO/La-Al2O3 143 112 57 - - α(9%) CuO/Ce-Al2O3 144 107 9 Ce(117) Ce(146) α(55%),Ce(991), Cu(891) CuO/La-Ce-Al2O3 140 108 46 Ce(53) Ce(72) α(11%), Ce(177)
4.4.2 Effects of La on the Stability of Manganese Oxides catalysts [Paper V]
The stabilising effect of La on MnOx catalysts in mixtures with small amounts of Pt or Pd was investigated by means of XPS, EDX, XRD and TPR after calcination at 800°C for 4 h in air and after thermal treatments in air with 10% steam at 900°C for 300 h. Catalytic activity was evaluated by oxidation of CO, C10H8 and CH4 (gas mixture 1). The dispersion of Mn was determined by XPS measurements (Table 19). It can be observed that the ratios Mn/Al measured for the fresh samples were higher than the theoretical one (0.05), due to a surface enrichment. In presence of La in the MnOx-Pt catalysts, the measured Mn/Al ratio was higher, as seen in Table 19. This indicates that La increased the dispersion of MnOx,orthat Mn particles were smaller in the presence of La. This can be attributed to interactions between La and Mn during insertion of the Mn salt. Indeed, the La/Al atomic ratio increased following the deposition of Mn and calcination at 800C for 4 h in air (0.024 for La-Al2O3 and 0.034 for MnOx-Pt/La-Al2O3), which suggests that La phases dissolved in the solution containing Mn salt. However, after hydrothermal treatment at 900°C for 300 h, the samples deposited on Al2O3 alone become enriched in Mn at the surface as seen by an increasing Mn/Al ratio, whereas, for the MnOx-Pt/La-Al2O3 sample the Mn/Al ratio was similar to that of the fresh sample. EDX was also used to determine the atomic ratio of Mn/Al in the thermally treated samples. The Mn/Al ratio for the aged MnOx-Pt/La-Al2O3 sample (0.058) was close to the theoretical ratio (0.05), while it was higher in the undoped sample (0.07), as seen in Table 19. This could be explained by the fact that La stabilises the washcoat and the dispersion of the metal oxides and prevents coalescence of particles after hydrothermal treatment. The discrepancy between the two analytical methods is due to the lower analysis depth of XPS compared to EDX.
79 Table 19. Mn/Al atomic ratio determined by EDX (HRTEM) and by XPS for catalysts calcined at 800°C for 4 h in air (F) and hydrothermally treated at 900°C for 300 h in air with 10% steam (A). La: 3; Mn: 10; Pt: 0.1 mol%/Al2O3. XPS data a EDX data b Samples F A A MnOx-Pt/Al2O3 0.078 0.15 0.07 MnOx-Pt/La-Al2O3 0.13 0.12 0.058 a 2-nm depth b 14-nm depth
XRD revealed no Mn phases before and after thermal treatment, probably because of small sized particles in the samples, thus a good dispersion of Mn was obtained. Neither was MnAl2O4 detected, in agreement with data in the literature [van de Kleut, 1994]. The TPR profiles of the samples containing manganese oxides (MnOx- Pt/Al2O3,MnOx-Pt/La-Al2O3,MnOx-Pd/Al2O3 and MnOx-Pd/La-Al2O3)were similar and not well defined, therefore these will not be discussed here. However the initial composition of the samples could be determined. The addition of La did not affect the oxidation state of MnOx/Al2O3,sinceMn2O3 was found to be the predominant species in all the samples (doped and undoped) calcined at 800°C for 4 h in air, and Mn3O4 for all the thermally treated samples. The results of the activity measurements performed for the mixed MnOx- noble metal catalysts in gas mixture 1 are given in Table 20. For the oxidation of CH4, the addition of La decreased the temperature for 50% conversion by ca. 20°C for both fresh MnOx-Pd and MnOx-Pt catalysts. This enhancement in activity for CH4 oxidation can be explained by a better dispersion of Mn in the presence of La, as determined by characterisation studies. After hydrothermal treatment, the activities of the modified catalysts are larger than that of unmodified ones for the oxidation of all combustibles and more particularly for CH4, certainly due to the stabilisation effect of La on the washcoat and MnOx.
Table 20. Temperature (°C) for 50% conversion CO, C10H8 and CH4 for catalysts calcined at 800°C for 4 h in air (F) and hydrothermally treated at 900°C for 300 h in air with 10% steam (A). La: 3; Mn: 10; Pt, Pd: 0.1 mol%/Al2O3. Gas mixture 1.
CO C10H8 CH4 Samples F A ∆TF A∆TF A∆T MnOx-Pt/Al2O3 290 330 40 287 320 33 639 706 67 MnOx-Pt/La-Al2O3 288 327 39 284 316 32 620 688 68
MnOx-Pd/Al2O3 283 342 59 284 332 48 636 716 80 MnOx-Pd/La-Al2O3 287 310 23 287 314 27 618 674 56
∆T=T50%(A) – T50%(F)
80 4.4.3 Effects of La on the Reducibility of Copper Oxide Catalysts [Paper V]
In chapter 2, Section 2.2.2, Cu species in CuO/Al2O3 catalysts with various amounts of Cu, were identified by means of TPR. The aim of the present study is to compare the influence of La on the formation of reducible Cu-species in CuO/La-Al2O3 samples calcined at 300°C for 4 h in air. Thus TPR experiments on CuO/La-Al2O3 (from 1.6 to 8 mol% Cu / 100 m2/g Al2O3) were performed (Figure 37). The results show that, for each Cu content, the reduction of CuO on doped Al2O3 occurs at a higher temperature than that with Al2O3 alone. This could be a consequence of stronger interactions between Cu and Al2O3,[Gentry& Walsh, 1982] as the support could be less crystalline in the presence of La. Indeed, Solcova et al. [Solcova et al., 1993] found that more crystalline supports helped increase the reducibility of NiO due to weaker interactions between NiO and the support, as compared to that on an amorphous support [Mile et al., 1988; Kadkhodayan & Brenner, 1989]. In addition, the formation of CuO appeared at a Cu concentration of 4.8-6.4 mol% for the sample containing La, whereas in the case of Al2O3 alone, CuO was already detectable at a concentration of 3.2-4.8 mol%. This indicates that La increased the dispersion of Cu onto Al2O3, thus leading to a higher saturation value to Cu in Al2O3.
8mol%Cu
6.4 mol% Cu
4.8 mol% Cu consumption (a.u.) 2 H 3.2 mol% Cu
1.6 mol% Cu
bulk CuO
150 250 350 450 Temperature (oC)
Figure 37. TPR profiles of CuO/Al2O3 (1.6 to 8 mol% Cu/100 m2/g Al2O3) alone (thin line) and stabilised with La (thick line), calcined at 300°C for 4 h in air. TPR of bulk CuO is added. TPR experimental conditions: 40 ml/min, 5% H2/Ar.
81 4.4.4 Effects of La on the Stability of Copper Oxide Catalysts [Paper V]
The stabilising effect of La on Cu species in mixture with small amounts of Pt or Pd was investigated by means of TPR, XRD and XPS after calcination at 800°C for 4 h and after thermal treatments in air with 10% steam at 900°C for 300 h. Catalytic activity was evaluated by oxidation of CO, C10H8 and CH4. On samples calcined at 800°C for 4 h, surface Cu2+ species were present in all samples (CuO-Pt/Al2O3, CuO-Pt/La-Al2O3,CuO-Pd/Al2O3 and CuO- Pd/La-Al2O3). Indeed the hydrogen consumption during TPR measurements indicated Cu2+ species while according to the analysis of the TPR reduction temperature no reduction peak that corresponds to bulk CuO was found, but only the reduction of surface Cu2+ species [Dumas et al., 1989], as discussed above and in Section 2.2.2 (Figure 10). The sample containing La had a higher reduction peak temperature compared to that without La (Figure 38, CuO-Pt/Al2O3, CuO-Pt/La-Al2O3 samples only are shown). XPS measurements evidenced also the presence of Cu species different from bulk CuO (Table 21). Indeed the B.E. of the principal Cu 2p3/2 peak was lower than the values usually found for CuO, as seen in Table 21. Also the satellite intensity of Cu 2p3/2 to that of the main photoelectron line for all the samples calcined at 800°C for 4 h was smaller (ca 0.20) than those found for bulk CuO (0.45) and bulk CuAl2O4 (1.10) [Strohmeier et al., 1985]. Thus the Cu species present maybe surface Cu2+.
After hydrothermal treatment at 900°C for 300 h, copper aluminate, i.e. CuAl2O4, was detected by XRD analyses (Figure 39) and TPR measurements (Figure 38) on Cu-containing samples without La (CuO-PtAl2O3 and CuO- Pd/Al2O3), as seen by the peak observed at ca 400°C (see also Section 3.1.2, Figure 27a). According to the hydrogen consumption corresponding to the “CuAl2O4 peak”, ca. 64 and 81% of the initial CuO had reacted with Al2O3 to form bulk aluminate in both CuO-Pt and CuO-Pd, respectively. The hydrogen consumption in the second step for both CuO-Pt and CuO-Pd samples were proportionally related to the XRD intensities of CuAl2O4 in these samples. Hence, CuAl2O4 would seem to be the major species after hydrothermal treatment. The presence of bulk spinel compound after hydrothermal treatment at 900°C was detected only in the Cu-containing undoped samples, but not in those containing La. In addition, in the hydrothermal treated samples, bulk CuO appeared on the unpromoted Cu-based catalysts, whereas, in the presence of La, the surface Cu2+ species were still present. This was clearly seen by the formation of a new reduction step during TPR analyses of
82 hydrothermally treated CuO-Pt/Al2O3, (Figure 38) and by the highest B.E. value obtained for the same sample from XPS analyses (Table 21).
217
212
CuO-Pt/La-Al2O3
189 consumption (a.u.) 2 H
422
CuO-Pt/Al2O3 150 179
100 200 300 400 500 Temperature (oC)
Figure 38. TPR profiles of CuO-Pt deposited on Al2O3 alone and stabilised with La calcined at 800°C for 4 h (thin line) and after hydrothermal treatment at 900°C for 300 h in air with 10% steam (thick line). La: 3; Cu: 10; Pt: 0.1 mol%/Al2O3.TPR experimental conditions: 40 ml/min, 5% H2/Ar.
Table 21. XPS data determined for catalysts calcined at 800°C for 4 h in air (F) and after hydrothermal treatment at 900°C for 300 h in air with 10% steam (A). Binding energies of reference compounds are included. La: 3; Cu: 10; Pt: 0.1 mol%/Al2O3. Cu 2p3/2 binding energies (eV)a Surface composition Cu/Al Samples Ref. F A F A CuO-Pt/Al2O3 933.4(3.6) 933.6(3.7) 0.13 0.23 CuO-Pt/La-Al2O3 932.8(3.2) 933.2(3.4) 0.13 0.12 Bulk CuO 933.6 [Friedman et al., 1978; Strohmeier et al., 1985] 933.7 [Ertl et al., 1980] 933.9 [Wolberg et al., 1970; Park & Ledford, 1998b] bulk CuAl2O4 934.5 [Strohmeier et al., 1985] 934.6 [Friedman et al., 1978] 934.8 [Wolberg et al., 1970] 935.0 [Park & Ledford, 1998b] a All B.E.’s (eV) referenced to C1s=284.6 eV. FWHM indicated in parentheses
83 α α α α α s α α α α s s CuO-Pd/Al O sα α 2 3 ss s α
CuO-Pd/La-Al2O3
CuO-Pt/Al2O3
CuO-Pt/La-Al2O3 20 35 2θ 50 65 80
Figure 39. XRD patterns of CuO-Pt and CuO-Pd deposited on Al2O3 alone and stabilised with La after hydrothermal treatment at 900°C for 300 h in air with 10% steam. La: 3; Cu: 10; Pt, Pd: 0.1 mol%/Al2O3. α : α-Al2O3,s:CuAl2O4.
Moreover, the ratio of the satellite intensities (relative to the main Cu 2p3/2 photoelectron line), determined by XPS, was ca. 0.42 (= bulk CuO) for the hydrothermally-treated CuO-Pt/Al2O3, while lower ratios were found for the fresh samples of CuO-Pt/Al2O3 and for the fresh and aged samples of CuO- Pt/La-Al2O3. This meant that bulk CuO was present only in the hydrothermally treated CuO-Pt/Al2O3, as confirmed by TPR. It is likely that the surface of the aged CuO-Pt/Al2O3 sample was covered by a layer of CuO, which could explain why bulk copper aluminate could not be detected by XPS. Also, the presence of La in the samples was found to stabilise the surface composition, as observed by similar Cu/Al ratios before and after hydrothermal treatment at 900°C for 300 h in presence of La (Table 21). Thus, it can be summarised that the role of La was to to prevent coalescence of particles, by stabilising the washcoat and the dispersion of the metal oxides, and to restrain solid-phase reactions between alumina and metal oxides after hydrothermal treatment.
For the Cu-based catalysts, calcined at 800°C for 4 h, the presence of La did not influence the activity for the conversion of C10H8 and CO (Table 22). This is probably due to the fact that La affects the dispersion of metal oxides more than noble metals, the latter being the “active phases” for the oxidation of CO and C10H8 [Paper III]. Characterisation using XPS and TPR indicated the presence of surface Cu2+ species in all fresh samples (doped and undoped), thus no significant difference in activity between the doped and undoped catalysts could be observed. These isolated Cu2+ species, and not CuO clusters, were reported to be the active species for the oxidation of CH4 [Marion et al., 1991; Jiang et al., 1997]. The CuO-Pd (on Al2O3 and La-Al2O3) catalysts had the best activity for the oxidation of the combustibles studied here, followed by the CuO-Pt catalysts.
84 The activities of the catalysts containing La were better after hydrothermal treatment at 900°C for 300 h in air with 10% steam, particularly for the oxidation of CH4 (Table 22). This is mainly due to the stabilisation effect of La on the washcoat against sintering. Also, the formation of copper aluminate in absence of La may have contributed to a decrease in activity, as discussed in Section 3.1.2 due to the fact that in CuAl2O4, the oxidation state of Cu is fixed to one valence. It could be also due to the formation of CuO crystallites that are less active than surface Cu2+ for the oxidation of CH4 [Marion et al., 1991]. The CuO-Pd/La-Al2O3 catalyst (Figure 40) remained the best catalyst for the oxidation of CO, C10H8 and CH4 after hydrothermal treatment, and this was also true when compared to mixed catalysts with Mn (see Section 4.4.2).
Table 22. Temperature (°C) for 50% conversion CO, C10H8 and CH4 for catalysts calcined at 800°C for 4 h in air (F) and hydrothermally treated at 900°C for 300 h in air with 10% steam (A). La: 3; Cu: 10; Pt, Pd: 0.1 mol%/Al2O3. Gas mixture 1. CO C10H8 CH4 Samples F A ∆TF A∆TF A∆T
CuO-Pt/Al2O3 266 284 18 271 289 18 602 724 122 CuO-Pt/La-Al2O3 266 276 10 273 281 8 599 686 87
CuO-Pd/Al2O3 216 392 176 221 406 185 578 748 170 CuO-Pd/La-Al2O3 214 259 45 221 268 47 577 652 75
∆T=T50(A)-T50(F)
100
80 CO CH4
60 F ALa ALa A F A
40 Conversion (%)
20
0 100 200 300 400 500 600 700 800 900 Catalyst temperature (oC)
Figure 40. Conversion of CO and CH4 over CuO-Pd catalysts deposited on Al2O3 alone and stabilised with La calcined at 800°C for 4 hours in air (F) and after hydrothermal treatment at 900°C for 300 hours in air with 10% steam (A). La: 3; Cu: 10; Pd: 0.1 mol%/Al2O3. Gas mixture 1.
85 4.4.5 Synergetic Effects in CuO-Ce and CuO-La-Ce [Paper VI]
In this study, the effects of Ce, La or both in Al2O3 support on CuO catalysts are presented. Characterisation techniques such as XRD, TPR, TPO and XPS have shed some light on the chemical and physical changes of the samples in the absence or presence of these additives. Catalytic activity was evaluated using a mixture containing CO, C2H4 and CH4 (gas mixture 2). XRD measurements determined the formation of a solid solution between Ce and Cu, as seen by a shift towards higher angles of the peak positions of CeO2 in CuO/Ce-Al2O3 with respect to those of Ce-Al2O3 [Paper VI]. These interactions may be reached during the deposition of Cu that preferentially deposited on CeO2, as observed from TEM studies by Fernandez-Garcia et al. [Fernández-García et al., 1997]. Namely, in the present work, hydrogen TPR on CuO/Ce-Al2O3 indicated the presence of two Cu species, whereas on CuO/Al2O3 only one was detected (Figure 41). The first species reduced at lower temperature corresponds to Cu2+ in interactions with Ce, whereas the second corresponds to Cu2+ deposited on Al2O3.
220 239
248 CuO/Ce-Al2O3
250 CuO/La-Ce-Al2O3
257 CuO/Al2O3
consumption (a.u.) CuO/La-Al2O3 2
H 345 Bulk CuO
100 200 300 400 500 Temperature (oC)
Figure 41. TPR profiles of CuO catalysts deposited on Al2O3 alone and modified with La and/or Ce, after calcination in air at 800°C for 4 h in air. TPR profile of bulk CuO is also shown. La: 3; Ce: 3; La-Ce: 1.5-1.5; Cu: 10 mol%/Al2O3. TPR conditions: 5 ml/min, 5% H2 in Ar.
As seen by XPS means, the surface concentration of Ce was affected by the subsequent deposition of Cu, whereas the surface concentration of La was not, as seen by the atomic ratios Ce/Al and La/Al (Table 23). In addition, a higher fraction of Ce4+ in the presence of Cu was determined as calculated from the XPS measurements. Contacts between Cu and Ce resulted in the stabilisation of Cu, as seen by a lesser formation (Table 18) and a lower average particle size of bulk copper aluminate, i.e. CuAl2O4,inCuO/Ce-Al2O3 compared to CuO/Al2O3 after ageing at 1000°C for 4 h (Table 23). The stabilisation of CuO by CeO2 can also be seen by the slower re-oxidation and lower oxygen
86 consumption of reduced Cu in the presence of CeO2 during TPO after reduction of the samples up to 700°C (Table 23).
Table 23. XPS, XRD and TPO data. La: 3; Ce: 3; La-Ce: 1.5-1.5; Cu: 10 mol%/Al2O3. XPS data a XRD data TPO data
Samples La/Al Ce/Al % Ce4+ CuAl2O4 size (Å) b O/Cuc
La-Ce-Al2O3 0.012 0.011 6 - - CuO/Al2O3 - - - 55 1.00 CuO/Ce-Al2O3 - n.d. n.d. 26 0.47 CuO/La-Ce-Al2O3 0.010 0.0059 20 n.d. 0.08 a determined on samples calcined at 800°C for 4h in air; % Ce4+ obtained from u’’’ and v’’’ relative amounts in Ce 3d [Paper VI] b determined by Scherrer’s formula at 2θ = 36.858°, samples calcined at 1000°Cfor4hinair. c TPO experimental conditions: 42 ml/min, 5% O2 in N2, samples calcined at 800°Cfor4hin air and reduced in 5% H2 in Ar up to 700°C. n.d.: not determined.
The promotion of CeO2 into CuO catalyst led to a decreased in the temperature for 50% conversion of CO by more than 80°C, as seen in Figure 42. No enhancement was seen for the oxidation of C2H4 and CH4.
100
80
60 Ce La-Ce none La
40 Conversion (%) 20
0 150 200 250 300 350 400 Catalyst temperature (oC)
Figure 42. CO conversion on CuO catalysts deposited on Al2O3 alone and modified after calcination at 800°C for 4 h in air. La: 3; Ce: 3; La-Ce: 1.5-1.5; Cu: 10; Pt: 0.5 mol%/Al2O3. Gas mixture 2.
We showed that this enhancement is seen even without any steam in the feed, thus it cannot be attributed to the water-gas shift reaction, despite the fact that it has been reported that CuO enhances this reaction when deposited on pure CeO2 [Li et al., 2000]. Some authors [Lamonier et al., 1996; Xavier et al., 1998] reported the creation of anionic vacancies due to the insertion of Cu2+ into the CeO2 lattice. The insertion of Cu into a CeO2 lattice led to the creation of vacancies that increases the CuO reducibility, i.e., lowers the CuO reduction temperature, as seen by
87 Figure 41. This enhancement in oxygen mobility in Cu catalysts doped with CeO2 favours a higher redox process, according to the Mars-van Krevelen type of mechanism, and significantly promotes the CO oxidation. Luo et al. claimed that the enhancement in CO oxidation was due to CO adsorption, which occurs only for Cu interacting with CeO2 [Luo et al., 1997]. Despite the fact that in this study, B.E. determined from XPS and H2 consumption from TPR experiments indicated the presence of mainly Cu2+, an Auger peak that can be assigned to Cu+ was also found in Cu catalyst containing only Ce as additive [Paper VI]. The stabilisation of reduced Cu, as Cu+, was also clearer after reduction and re-oxidation during TPO of CuO/Ce-Al2O3 (Table 23). Liu et al. [Liu & Flytzani-Stephanopoulos, 1995a] also reported that CeO2 enhanced the oxidation of CO on CuO catalysts. The authors attributed these results to the presence of small, dispersed Cu+ clusters stabilised by interaction with the CeO2 [Liu & Flytzani-Stephanopoulos, 1995b], and proposed a model according which Cu clusters provide surface sites for CO adsorption and the CeO2 provides the oxygen source which is then transferred to CO. Furthermore, Jernigan and Somorjai compared the CO oxidation over CuO, Cu2O and Cu, and concluded that the apparent activation energy increases with increasing CuO oxidation state [Jernigan & Somorjai, 1994]. Therefore, both effects (from Cu2+ and Cu+) maybe present in the CuO/Ce-Al2O3 catalyst. Inhibition of CO poisoning by the presence of CeO2 is unlikely to be the cause of the enhancement due to the net oxidising conditions and of the high working temperature [Oh & Eickel, 1988].
The oxidation of C2H4 and CH4 was, however, not boosted by the addition of CeO2 in CuO/Al2O3 catalyst.ItseemsthattheCu-Cesystemhasa propensity to activate the oxidation of CO while oxidation of HCs are not affected [Park & Ledford, 1998a; Larsson & Andersson, 2000]. We demonstrated that this peculiar effect could not be attributed to the WGS reaction that involves only CO and no HCs, as discussed earlier. The slow step of alkane oxidation has been postulated to be the dissociative chemisorption of the alkane on the metal with the breakage of the weakest C-H bond [Cullis et al., 1970].
Despite the lower amount of Ce in CuO/La-Ce-Al2O3 compared to CuO/Ce-Al2O3, the CO oxidation is also favoured, probably because the presence of La promotes the effect of CeO2 on CuO, as observed by an increase of CeO2 reducibility during TPR experiments. Thus La and Ce do have a synergetic effect on the catalytic oxidation of CuO catalyst.
88 4.4.6 Synergetic Effects in CuO-Pt-La-Ce [Paper VI]
In this study, the effects of Ce, La or both in Al2O3 support on Pt and CuO- Pt catalysts are presented. Characterisation techniques such as XRD and XPS were used. Catalytic activity was evaluated using a mixture containing CO, C2H4 and CH4 (gas mixture 2). Characterisation studies and activity measurements confirmed the slight influence of the additives (La, Ce or both of them) on Pt alone catalysts. Markedly, the effect of the additives on Pt catalysts was much clearer when Cu was also present. Namely, in CuO-Pt/Al2O3 catalysts Ce induced a lower fraction of metallic Pt whereas mixed La-Ce brought about a higher fraction of metallic Pt and reduced Ce, compared to CuO-Pt/Al2O3 without additives, determined from XRD and XPS measurements. Figure 43 shows the XRD profiles of all CuO-Pt samples calcined at 800°C for 4 h. Pt crystallites were detected in all CuO-Pt samples, as well as in all Pt catalysts (not shown here), despite the low concentration of Pt loading. No Pt oxides were detected, probably due to their high dispersion and/or too small a crystallite size to be made visible by XRD. Pt peak intensities were found to be the highest in the CuO-Pt/La-Ce-Al2O3 sample, followed by CuO-Pt/Al2O3 and the lowest in CuO-Pt/La-Al2O3 and CuO-Pt/Ce-Al2O3 samples. This indicates that the fraction of Pt in metallic form was the highest in CuO-Pt/La- Ce-Al2O3.
Pt
Pt Pt La-Ce CeO2 CeO2 CeO2 Ce La
none
20 30 40 50 60 70 80 2θ
Figure 43. XRD profiles of CuO-Pt deposited on Al2O3 alone and modified calcined at 800°C for 4 h in air. La: 3; Ce: 3; La-Ce: 1.5-1.5; Cu: 10; Pt: 0.5 mol%/Al2O3.
Concerning the XPS data, when comparing the relative amounts of Ce4+ in the CuO-Pt/Ce-Al2O3 (21%) and CuO-Pt/La-Ce-Al2O3 (13%), it could be observed that the presence of La lowers the Ce4+ fraction by 8% which is in agreement with the results reported by Talo et al. [Talo et al., 1995]. Moreover, it was observed that the addition of Pt in CuO/La-Ce-Al2O3 sample, when comparing CuO/La-Ce-Al2O3 (20%) and CuO-Pt/La-Ce-Al2O3 (13%), also
89 lowers the Ce4+ fraction, which suggests the presence of interactions between Pt and Ce. In the present study, CuO-Pt/Ce-Al2O3 and Pt/Ce-Al2O3 catalysts had slightly lower activities than CuO-Pt/Al2O3 and Pt/Al2O3 catalysts, respectively, as seen by the results in Table 24. Several authors reported that after pre-treatment of Pt/CeO2–containing catalysts in an oxidising atmosphere, the Pt at the interface with CeO2 was partially oxidised thus leading to a worsened or similar activity compared to Pt/Al2O3 catalyst [Yu Yao, 1980; Serre et al., 1993b]. However, when Ce was mixed with La in CuO-Pt/La-Ce-Al2O3 catalyst, more reduced Ce and Pt were found, as determined by XPS and XRD, respectively, as discussed above, compared to CuO-Pt/Ce-Al2O3 catalyst, as well as Pt-CeO2 interactions were detected. These resulted in a very active CuO-Pt/La-Ce-Al2O3 catalyst compared to CuO-Pt/Al2O3, CuO-Pt/Ce-Al2O3 or any Pt catalysts for all combustibles, as seen in Table 24, particularly for the oxidation of C2H4 presented also in Figure 44.
Table 24. Temperature (°C) for 50% conversion of the catalysts calcined at 800°Cfor4 h in air. La: 3; Ce: 3; La-Ce: 1.5-1.5; Cu: 10; Pt: 0.5 mol%/Al2O3. Gas mixture 2.
Samples CO C2H4 CH4
Pt/Al2O3 200 206 629 Pt/La-Al2O3 212 212 627 Pt/Ce-Al2O3 216 216 633 Pt/La-Ce-Al2O3 223 223 645
CuO-Pt/Al2O3 175 182 576 CuO-Pt/La-Al2O3 173 179 581 CuO-Pt/Ce-Al2O3 186 190 600 CuO-Pt/La-Ce-Al2O3 156 146 573
Pt/CeO2-Al2O3 catalysts have been found to have considerable activity for CO oxidation at low temperatures under stoichiometric fuel-conditions, but this improvement was present only after a high-temperature reduction with H2 [Diwel et al., 1991; Golunski et al., 1995; Holmgren et al., 1999], and CO [Serre et al., 1993b]. Serre et al. proposed that the high temperature was needed to reduce PtO2 and PtO at the interface between Pt and CeO2, and that it was this reduced Pt in contact with Ce that was responsible for the high activity. Reduced Ce in the vicinity of reduced Pt atoms on the surface were also proposed by other authors to be the most active sites of the catalyst, especially for CO oxidation [Nunan et al., 1992; Holmgren et al., 1999]. Serre et al. [Serre et al., 1993a] gave two interpretations of the Pt-CeO2 interactions: a C-O bond weakening when CO is adsorbed on Pt particles, thus a CO adsorbed near the Pt-CeO2 interface will more easily insert an oxygen atom from CeO2 into its structure, and will quickly desorb as CO2; a decrease of the Ce-O bond
90 strength for CeO2 localised near Pt. This interpretation is supported by results from Yu Yao [Yu Yao, 1984] who pointed a charge transfer from metal to CeO2, leading to a small increase in the oxidation state of metal, indicating that Ce is also slightly reduced, leading to a decrease of the Ce-O bond strength [Yao, 1984]. Also, Golunski et al. proposed that the improvement might be due to strong metal-support interaction (SMSI) obtained during the high- temperature reduction, which could led to migration of CeO2 to cover Pt thus diminishing the CO adsorption capacity of Pt [Golunski et al., 1995]. In this study, the promotion of Pt/CeO2-Al2O3 catalyst with both Cu and La could improve significantly the oxidation for CO, C2H4 and CH4 in oxidising atmosphere. Moreover this result was achieved without any prior reducing treatment of the samples, which is usually needed to improve the activity of Pt/CeO2 catalysts [Diwell et al., 1991; Kubsh et al., 1991; Serre et al., 1993b; Holmgren et al., 1999]. This is an advantage since pre-reducing treatment would have only a short time effect on the activity of the catalysts, and hence be useless for total oxidation applications.
100
80 none 60 La-Ce = Ce Pt/Al2O3 La
40 Conversion (%)
20
0 100 150 200 250 Catalyst temperature (oC)
Figure 44. C2H4 conversion over Pt/Al2O3 catalyst and CuO-Pt catalysts deposited on Al2O3 alone and modified after calcination in air at 800°Cfor4hinair.La:3;Ce:3; La-Ce: 1.5-1.5; Cu: 10; Pt: 0.5 mol%/Al2O3. Gas mixture 2.
Interestingly, after calcination at 900°C, the synergetic effects are enhanced as seen by a decrease of the temperature for 50% conversion for CuO- Pt/Al2O3 and particularly for CuO-Pt/La-Ce-Al2O3, while the activity of Pt/Al2O3 is hampered, as seen in Figure 45. The activity of CuO-Pt/La-Al2O3 was similar before and after calcination at 900°C, thus the improvement cannot be provided by La alone. The enhancement in activity can be attributed to enrichment in Pt that migrated at the surface with a higher calcination temperature and hence the synergetic effects that were already observed for the CuO-Pt samples calcined at 800°C are emphasised. Therefore the combined CuO-Pt/La-Ce-Al2O3 catalyst was concluded to have the highest activity and thermal stability of all the catalysts tested.
91 100
CuO-Pt/ 80 La-Ce-Al2O3
T F Pt/Al2O3 60 CuO-Pt/ F T Al2O3 40 T F Conversion (%)
20
0 100 150 200 250 Catalyst temperature (oC)
Figure 45. CO conversion for Pt/Al2O3, CuO-Pt/Al2O3, CuO-Pt/La-Ce-Al2O3 calcined at 800°C for 4 h in air (F) and 900°C for 4 h in air (T). La-Ce: 1.5-1.5; Cu: 10; Pt: 0.5 mol%/Al2O3. Gas mixture 2.
4.5 Concluding Remarks
Due to its dispersed form, La contributes to the thermal stabilisation of the alumina washcoat, thus allowing it to retain a high surface area and its amorphous structure under high temperature conditions. Indeed, to a higher extent than Ce, because Ce sinters and remains on the surface in the form of CeO2. This La stabilisation of the alumina structure results in a better dispersion and higher saturation of metal oxides onto an alumina support, and at the same time stabilised the oxidation activity of catalysts by preventing the interaction of metal oxide with alumina to form inactive bulk spinelcompoundssuchasCuAl2O4. After calcination at 800°C and after hydrothermal treatments at 900°C for 300 hours in 10% steam performed on a series of mixed catalysts doped and undoped with La, the CuO-Pd/La-Al2O3 catalyst was found to be very active for the oxidation of CO, C10H8 and CH4. Moreover, due to the formation of a La-Ce solid solution, the promotion of the Ce-doped washcoat by La stabilised CeO2 and increased its dispersion, as well as enhanced the oxygen mobility of CeO2. In CuO/Ce-Al2O3, the formation of a Cu-Ce solid solution, which was detected by XRD and by TPR, led to a higher thermal stability of Cu by producing smaller amounts of CuAl2O4 and of lower particle size than on CuO/Al2O3 after thermal treatments at 1000°C. CeO2 also promoted the formation of some reduced copper (Cu+) in catalysts calcined in air, and particularly after a redox cycle. In addition, this interaction brought down a significant increase in oxygen mobility that resulted in an increase of CuO
92 reducibility. Thus, these two effects led to a substantial enhancement of CO oxidation. In the mixed CuO-Pt/Al2O3 catalysts, the addition of Ce led to a decrease in the fraction of metallic Pt, thus decreasing the activity of the catalysts. However, this effect was countered by the addition of La, which facilitated the reductionofbothPtandCeO2, thus enhancing greatly the catalytic activity.
93 94 5 FIELD APPLICATION
To demonstrate the possibility of using a full-scale catalytic system, a metallic monolith equipped with a pre-heating device was inserted in a commercial 30 kW wood-fired boiler (model CTC V30, CTC Parca AB, Sweden), as seen in Figure 46 [Paper IV]. The catalyst prepared was based on an 100 cpsi metallic monolithic substrate, Fecralloy (Emitec GmbH), of the same kind as described in Paper IV, which contained a mixture of MnOx (Mn: 10 mol%/Al2O3) and Pt (0.5 mol%/Al2O3) deposited on Al2O3 stabilised with 3% La [Paper IV]. The shape of the catalyst was half a cylinder divided into 4 segments (Figure 46), each with a length of ca 50 mm, separated by approximately 10 mm in order to increase mass transfer within the catalyst sections as a result of increased turbulence at the inlet sections [Berg & Berge, 1999; Berg, 2001]. In these experiments, an electric hot air heater introduced through the ash pit door pre-heated the catalyst. The heater had to be switched off and the ash pit door closed before ignition of the fuel, at which point the catalyst started to cool down, as seen in Figure 47. The flue gas flow was approximately 100 Nm3/h, corresponding to a space velocity of 32 000 h-1.
1. Primary air 2. Wood fuel storage and primary combustion zone 3. Grate with addition of secondary air 4. Catalyst (see picture) 5. Convection section 6. Flue gas fan
Figure 46. Commercial boiler with the MnOx-Pt/La-Al2O3 catalyst (reprinted with permission from Termiska Processer AB).
95 For experiments with and without preheating, the temperature and the emission of CO and total HC (THC) during the first 10 minutes after ignition of the first small batch of wood was measured. The results for the oxidation of CO and THC are presented in Figures 48a and 48b, respectively. The catalyst temperature during the start-up phase was above 250°C, therefore a good effect on the conversion of CO over the pre-heated catalysts could be expected when comparing with results obtained in laboratory [Paper IV]. Since the pre- heating of the catalyst was interrupted just before the fuel is ignited, the temperature of the pre-heated catalyst decreased during the first 3 minutes (Figure 48). After this period, the hot flue gases increased the temperature of the pre-heated catalyst and after 8 to 10 minutes no significant difference in temperature between the different experiments remained. It is, however, clear that the pre-heated catalyst results in lower emissions during the period from 1 up to 8 or 9 minutes, the latter part of the period being the time when the effect of pre-heating can no longer be seen on the temperature measurements. The reduction of CO during these 10 minutes with the pre-heated catalyst compared with the boiler operating without any catalyst is 67%. Higher conversion could be expected if the space-velocity is lowered or if the pre- heating of the catalyst is increased or not interrupted. Also lack of oxygen could contribute to insufficient oxidation. Other solutions could of course be chosen if the catalyst was installed in a commercial unit but this study is a first attempt to demonstrate the potential of this method. Another advantage with pre-heating the catalyst is that the deactivation may be reduced, especially for a boiler designed without any possibility to by-pass the catalyst during start- up, since the condensation of tars on the cold catalyst surface contributes significantly to deactivation.
600 no pre-heating 500 with pre-heating
400
300
200 Temperature (°C)
100
0 0 24 6810 Time (min)
Figure 47. Temperature measured downstream the MnOx-Pt/La-Al2O3 catalyst calcined at 800°C for 4 h in air, with and without pre-heating. La: 3; Mn: 10; Pt: 0.5 mol%/Al2O3.
96 8000 no pre-heating with pre-heating 6000
4000 CO ppm
2000
0 0 2 46810 Time (min)
Figure 48a. CO concentration measured downstream the MnOx-Pt/La-Al2O3 catalyst, calcined at 800°C for 4 h in air, with and without pre-heating. La: 3; Mn: 10; Pt: 0.5 mol%/Al2O3.
2500 no pre-heating
2000 with pre-heating
1500
1000 THC ppm
500
0 0 246810 Time (min)
Figure 48b. Total hydrocarbons concentration measured downstream the MnOx- Pt/La-Al2O3 catalyst, calcined at 800°C for 4 h in air, with and without pre-heating. La: 3; Mn: 10; Pt: 0.5 mol%/Al2O3.
97 98 6 CONCLUSIONS
The general conclusion from this work is that the mixed metal oxide (CuO, MnOx) - noble metal (Pt, Pd) catalysts selected for this study are superior to single component catalysts, for complete oxidation of volatile organic compounds, carbon monoxide and methane.
The combined utilisation of characterisation techniques have shed some light on the properties of the catalytic components and the washcoat and allowed a better understanding of some catalytic phenomena.
In our reaction medium, the activity of 0.1 mol% Pd/Al2O3 is higher compared to that of the catalyst with the same amount of Pt for the oxidation of CO, C10H8 and CH4, whereas the opposite is observed for the oxidation of C2H4. CO enhances appreciably the activity of the Pd/Al2O3 catalyst for the oxidation of C2H4,C10H8 and CH4. Al2O3-supported MnOx andCuO(Mn,Cu:10mol%/Al2O3) have been selected, among the oxides of Fe, Co and Ni, as high active catalysts for the oxidation of all the combustibles studied here. Pt and Pd possess superior catalytic activity compared to CuO and MnOx for the oxidation of CO, C10H8 and C2H4, however, for the oxidation of CH4,CuO is much more active than Pt and Pd, while MnOx is as active as the noble metals.
In mixed MnOx-Pt catalysts with low noble metal loading (Pt, Pd: 0.1 mol%/Al2O3), MnOx tends to encapsulate the noble metal, hence hampering its activity. The encapsulation can be avoided by using a successive impregnation with Pt which, in addition, leads to improved catalytic activities for the mixed MnOx-Pt compared to that of Pt alone. Higher amount of Pt, i.e. 0.5 mol%, also dampens the inhibiting effect.
Mixed CuO-Pt and CuO-Pd catalysts (Cu: 10; Pt, Pd: 0.1 mol%/Al2O3) preserve the activities of the most active components in the catalysts for the oxidation of the combustible. At higher Pt loading (0.5 mol%), a synergetic effect between CuO and Pt yields a higher conversion of CO and C2H4, relative to single component catalysts. All mixed metal oxide-noble metal catalysts benefit from the presence of the metal oxides which are active for the oxidation of CH4.
99 The synergetic effects between metal oxides and noble metals can be related to increased reducibility of the metal oxides in the presence of noble metals which may enhance the oxygen transfer from the gas phase to the noble metals.
An investigation concerning the effect of hydrothermal treatment at 900°C of the catalysts based on alumina-supported MnOx or CuO and/or Pt or Pd reveals that the presence of active components significantly accelerates the sintering of the washcoat support. In addition, at 900°C alumina reacts with CuO, particularly in the presence of noble metals, to form inactive bulk spinel CuAl2O4. However, MnOx catalyst benefits from the more active Mn3O4 phase obtained at high temperature which makes it suitable for the high temperature oxidation of CH4. A loading of 10 mol% Mn/Al2O3 is a good compromise between conversion and extent of washcoat sintering. When mixed with CuO, the sintering of Pt is delayed thus giving more thermal resistant catalyst. After a thermal treatment at 900°C for 60 h in air with 12% steam, Pd and CuO-Pt are the most active for CO and C10H8 oxidation, while MnOx has the highest activity for CH4 oxidation.
Further attempt to increase the thermal stability of the washcoat and the active components by promotion with La and/or Ce indicates that due to better dispersion, La is more efficient than Ce for stabilising the high-surface area and the amorphous structure of the alumina washcoat. In addition, La contributes to a better dispersion and a higher saturation of metal oxides in the alumina, and at the same time it stabilises the oxidation activity of the catalysts by preventing the solid-phase reactions between metal oxides and alumina. After calcination at 800°C and after hydrothermal treatments at 900°C for 300 hours in 10% steam, the CuO-Pd/La-Al2O3 catalyst was very active for oxidation of CO, C10H8 and CH4.
After pre-sulphation of the catalysts (1000 ppm SO2 for 16 h at 600°C) prior to activity tests or in the presence of SO2 (20 ppm) in the synthetic gas stream, the mixed catalysts, particularly CuO-Pd/Al2O3 and CuO-Pt/Al2O3 catalysts, are more resistant to sulphur poisoning compared to single component catalysts.
CeO2 provides a substantial enhancement of the low-temperature CO oxidation with CuO/Al2O3 catalysts due to beneficial Cu-Ce interaction. Moreover, in combination with La, the stabilisation and the dispersion of CeO2 as well as its oxygen mobility are appreciably increased. Positive synergetic effect is observed also in CuO-Pt/La-Ce-Al2O3 catalyst that leads to enhanced catalytic activity and thermal stability compared to Pt alone catalyst.
100 Scanning Electron Microscope revealed a well-anchored washcoat onto the metallic support, which is employed for fast pre-heating of the catalysts, due to the growth of whiskers from the metallic substrate during a treatment at high temperature.
Experiments in a commercial wood-fired boiler with pre-heating of a catalyst with a metallic monolith have shown significant decrease of the emissions of CO and unburned hydrocarbons during the start-up phase.
The potential of highly active and thermally stable catalysts of low cost for total oxidation has been clearly demonstrated in this thesis. Application of stabilised washcoat supports could provide long-term and cost-effective utilisation of mixed metal oxides and noble metals catalysts for the removal of VOC, CO and CH4, under high temperature conditions and sulphur- containing atmosphere.
Large numbers of catalysts, prepared in parallel to this study, have been subjected to ageing in wood-fired boilers. The results concerning the stability are encouraging, however, in wood-burning appliances due to the complexity of the flue gases composition, the diversity of the existing installations and the placement of the catalyst in them, it is still difficult to evaluate sufficiently well their lifetime.
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