THE MECHANISM AND KINETICS OF THIOPHENE ADSORPTION
ON NICKEL AT AMBIENT TEMPERATURES AND PRESSURES
A thesis submitted for
the degree of Doctor of Philosophy
of the University of London and for the Diploma of Imperial College
Khaliq Ahmed
Department of Chemical Engineering and Chemical Technology
Imperial College of Science and Technology London SW7
United Kingdom
January 1987 ABSTRACT
An experimental and modelling study of the kinetics of thiophene adsorption on nickel at ambient temperatures and pressures has been made. The objective of the study was to acquire a better understanding of the mechanism involved than is currently available.
The reactant (1000 ppm thiophene in hydrogen) was contacted with a nickel on gamma alumina catalyst.
Experiments were carried out in a microbalance flow reactor and the uptake of thiophene by the catalyst was recorded as a function of time. Prior to adsorption of thiophene, the catalyst was characterised in terms of its total BET surface area, pore-size distribution, active Ni-area and average crystallite size. All measurements were performed in situ except for that of the crystallite size which was determined by x-ray line broadening. The experimental study revealed that at room temperature thiophene adsorbs on nickel either directly as thiophene molecules, or as a hydrogenated species namely, thiophane ; no gaseous products eluted indicating that thiophene was not undergoing decomposition under these conditions. Overall uptake of thiophene by nickel in the catalyst showed that there were at least two possible modes of adsorption. It is postulated that these are perpendicular and coplanar. The crystallite size of nickel in the catalyst was varied by changing the loading of the catalyst, or by
2 sintering in an atmosphere of hydrogen at elevated temperatures. Adsorption of thiophene on catalysts of varying crystallite size showed that thiophene adsorption on nickel is structure sensitive, with the Ni/C.H.S adsorption b b stoichiometry ranging from a value representing a nearly coplanar mode of adsorption at large values of the crystallite size, to a value approaching a perpendicular mode at some smaller value of the crystallite size. The experimental results were consistent with a model which could be represented by two parallel rate processes, one following a two-site (presumably perpendicular) mechanism and the other following a five-site (presumably coplanar) mechanism. The rate constants for the two processes were approximated from runs on a particular sample and then used to predict the uptake vs. time behaviour of the remaining samples which varied in loading and/or crystallite size. The rate constant x"~ *he adsorption of the five-site (coplanar) species was found to be several orders of magnitude higher than that for the adsorption of the two-site (perpendicular) species, indicating that coplanar adsorption was preferential over perpendicular in the initial stages of adsorption on clean samples. The effective diffusivity parameter was calculated by using a random pore-model. Adsorption on highly sintered samples with particles in the range of .3 to .4 mm was found to be diffusion limited. For these samples a fitted diffusivity had to be used to obtain a satisfactory agreement between
3 experiment and simulation. The value of the fitted diffusivity was consistent with the pore-characteristics and crystallite size of these samples. However, adsorption on powder samples of these catalysts was found to be kinetically controlled.
The simulation studies were able to predict all the and observed results within their experimental error,Aexcept for the diffusivity in the highly sintered samples required no fitting of rate constants for individual runs.
4 ACKNOWLEDGEMENTS
I wish to express my sincere gratitude to Dr. L.S.
Kershenbaum for his constant guidance and supervision throughout the course of this research. It has been a rewarding experience for me. My deepest gratitude to Dr. D. Chadwick for his invaluable suggestions and helpful discussions on certain aspects of the present work. I wish to thank the technical and workshop staff of the
Department of Chemical Engineering and Chemical Technology for their backup assistance.
I gratefully acknowledge the Scholarship awarded by the Association of Commonwealth Universities, without which this work would not have been possible. I am indebted to Mr. Bryan Spooner of the Old Centralians Association for his generous help in obtaining a grant from the Old Centralians Trust Fund. I am also grateful to Mr. Nigel Wheatley for the award of a Lord Mountbatten Grant in the later stages of this work.
I wish to thank Dr. T. Viveros-Garcia for his friendship and for his helpful advice on this work. Thanks are also due to Mr. I. Drummond of this department for his assistance in the analytical experiments and to Mr. R. Sweeney of the Department of Metallurgy and Materials Science for his assistance in the XRD mea surements.
5 My kind est regards and gratitude to my mother and relatives for their constant encouragement and understand ing during this s tudy.
Finally, my deepest appreciation for my wife, for her patience. en couragement and understanding, throughout the course of this work.
6 TO MY LATE FATHER TO MY MOTHER Contents
1 INTRODUCTION 1 2
2 LITERATURE SURVEY 1 5
2.1 Catalyst Poisoning 1 6
2.1.1 Adsorption of the Poison Species :
Mechanistic Considerations 1 7
2.1.2 Inter- and Intra- pellet Transfer Effects 24
2.1.3 Analysis of the Poisoning Process 30
2.1.3.1 Irreversible Poisoning 30
2.1.3.2 Reversible Poisoning 37
2.1.4 Factors Affecting the Poisoning Process 38
2.2 Thiophene Poisoning of Nickel Catalysts 42
2.3 Thiophene Poisoning of Other Metal
Catalysts 55
3 MATHEMATICAL MODELS OF THIOPHENE POISONING 60
3.1 Development of the Mathematical Models 63
3.1.1 Model I 68
3.1.2 Model II 68
3.1.3 Model III 70
3.2 Model Parameters 73
3.3 Numerical Solution of the equations 74
4 EXPERIMENTAL 75
4.1 Apparatus 75
4.2 Materials 80
4.3 Catalyst Preparation 80
8 4.4 Experimental Procedure 81 4.4.1 Reduction 81
4.4.2 BET Area and PSD 83 4.4.3 Ni-Area Measurement 83
4.4.4 Crystallite Size Determination 84 4.4.5 Poisoning Studies 85
4.5 Effect Due to Variation in Room Temperature 87
4.6 Exit-gas Analysis 87
4.7 Investigations with 100ppm Thiophene 90 CHAPTER 5 CATALYST CHARACTERISATION 91 5.1 Literature Survey and Methods 91
5.1.1 Total Surface Area and PSD 91 5.1.2 Metal Area 95
5.1.3 Crystallite Size by X-Ray Methods 97 5.2 Results and Discussion 100 5.2.1 Total Surface Area 102 5.2.2 Pore-Size Distribution 108
5.2.3 Nickel Area 114 5.2.4 Mean Crystallite Size 124 5.2.5 Characterisation of the Support 129 CHAPTER 6 THIOPHENE ADSORPTION STUDIES 132
6.1 Experimental Results 132 6.1.1 Runs on Catalyst Particles 132 6.1.2 Runs on Powdered Samples 143 6.1.3 Saturation Uptake of Thiophene and the Influence of Crystallite Size 158 6.1.4 Effect of Flow Rate 160
9 6.1.5 Interparticle Diffusion 160
6.2 Simulation 164
6.3 Model Parametrs 209 6.4 Discussion 209
CHAPTER 7 CONCLUSIONS AND RECOMMENDATIONS 220 7.1 Conclusions 220
7.2 Suggestions for Further Work 223 REFERENCES 225
APPENDICES 233 A Discretization of the Models 233
B Instantaneous Adsorption Stoichiometry in Model III 237
C Estimation of Effective Diffusivity 239 0-1 Calculation of Crystallite Size from
Chemisorption Data 242 D-II Evaluation of ZD^ (Dispersion) from CO Chemisorption 245
D-III Determination of the Extent of Reduction 245 E Calculation of the Pore-size Distribution
from Type IV Isotherms 247 F Results of Different Forms of Variation
of o, the Instantaneous Stoichiometry of Adsorption 252 G Sample Calculations 254 G-I Total BET Surface Area 254
G-II Pore-Size Distribution 254 G-III Metal Area of the Catalyst 255
1 0 G-IV Extent of Reduction 255 G-V Metallic Dispersion 256
G-VI Crystallite Size Determination a By X-Ray Diffraction 256
b By CO Chemisorption 256 G-VII Effective Diffusivity 257 G-VI11 Fractions of Coplanar and Perpendicular sites in Model II 257
G-IX Thiophene uptake of Ideal Crystallites 258 CHAPTER 1
Introduction
The phenomenon of catalyst poisoning is one of the most severe problems associated with the industrial application of catalysts. The overall behaviour of catalyst poisoning has been studied extensively in the last 25 years and the results of these studies have been used to predict the life and behaviour of industrial catalysts. However, because of a lack of sufficient careful studies of the poisoning process, which is due, largely, to the complexity of the process, a quantitative understanding of the intrinsic rates and mechanism of catalyst poisoning is still not available.
According to Bartholomew et al (19B2), the poisoning effect of sulphur on metal catalysts is probably the most severe form of poisoning encountered in catalytic systems. Sulphur induced poisoning is so severe that the activity of the catalyst is reduced markedly at extremely low gas-phase concentrations of sulphur-containing compounds. In industrial practice the catalyst life may be reduced to only a few months or even weeks in the presence of only ppm concentrations of sulphur contaminants in the feedstream. The practical importance of sulphur poisoning has led to investigations of sulphur induced poisoning of metal catalysts. Recent investigations have resulted in the accumulation of fundamental information regarding the interaction of sulphur with metal surfaces. Most of these studies have been conducted with HgS as the poison and the results obtained thence constitute the majority of the literature on sulphur poisoning. Only very recently, some results have been reported on investigations with other sulphur compounds such as thiophene. However, enough studies have not been performed yet to unveil the mechanism of poisoning by thiophene. Of considerable importance is the poisoning of nickel catalysts because of their wide application e.g. steam-reforming, methanation reactions, etc. and especially because sulphur is present in ppm quantities in the feed-gas for these reactions. The present work is aimed primarily at elucidating information regarding the intrinsic rates and mechanism of thiophene adsorption on nickel catalysts from experiments conducted in a thermogravimetric system. The objective of this work is to propose a model for the adsorption of thiophene on nickel. The validity of the model is determined by a direct comparison of the experimentally observed and numerically computed thiophene adsorption uptake-time profiles.
1.1 organi sation., o.f ..the Thesis
Chapter two gives a review of previous work on catalyst poisoning. The literature on the phenomenon of catalyst poisoning is presented in the first section of chapter two.
Various aspects of poisoning in general, such as mechanism, kinetics, diffusional effects, reversibility or
13 irreversibility, pore-plugging and the distribution of the active component in the catalyst are also reviewed in this
section. In section two of chapter two, the specific cases of poisoning by thiophene are reviewed, with particular
emphasis on the aspects of modes of adsorption, mechanism and kinetics.
The theoretical basis for the models and their development are detailed in chapter three. The apparatus, materials, method of preparation of catalysts, procedures for the characterisation of the catalysts and for the
adsorption studies are presented in chapter four. Chapter five is divided into two sections : the first
section gives a review of the methods of and previous work on catalyst characterisation. In section two the results of catalyst characterisation in this study are presented along with some discussions. Thiophene adsorption results are presented in section one of chapter six and the details of the simulation results are given in section two of the same
chapter. The predictions of the models are compared with the experimental results, in the same section, to test the validity of the models. A discussion of the experimental and modelling results are presented in the last section of chapter six.
Finally, in chapter seven, the conclusions reached from the present study are summarized and some suggestions for future work are outlined.
14 CHAPTER 2
Literature Survey
Catalysts used in the chemical and process industries suffer a loss in their initial activity due to a mechanism conventionally known as catalyst deactivation. The time scale of this deactivation process may range from a few seconds to a few years, depending on the particular reaction system, properties of the catalyst and impurities in the feedstock for the reaction. The foundations for a better understanding of catalyst deactivation have been laid by general reviews of the topic presented by Butt (1972) and more recently by Hughes (1984). According to these authors , deactivation is broadly classified into :
1. Poisoning. which is the loss of activity caused by strong chemisorption of some impurity in the reacting mixture.
2. Fouling, which is the loss of activity caused by reactant or product degradation on the catalyst surface, e.g. coke deposition.
3. Sintering. which is the loss of activity caused by operation of the catalyst at high temperatures resulting in catastrophic losses in specific surface area and changes in the pore structure of the catalyst. In addition to thermal deactivation, sintering can also occur by loss of dispersion of the metal crystallites caused by coalescence.
1 5 The present review concentrates on catalyst poisoning in general and also the specific case of poisoning of
nickel catalysts by thiophene. Most of the general review in section 2.1 has been modelled after an excellent review of
catalyst poisoning presented by Hegedus and McCabe (1984).
2.1 Catalyst Poisoning
Catalyst poisoning is generally defined as the degeneration of the active sites in a catalyst by strong
chemisorption of impurities on its surface. It may include one or a combination of the following three types : (i)
poison adsorption causing mere blockage of sites, (ii) poison induced surface reconstruction and (iii) compound
formation between the poison and the catalyst. The adsorption of the poison may be irreversible or reversible.
If the activity of the catalyst is recovered, either wholly or partly, upon removal of the poison from the feed stream, the poisoning is reversible.
Maxted (1951), in an excellent review of catalyst poisoning. propounded a theory based on the electronic
properties of the poison and the metal catalyst. According to this theory, the poison is adsorbed on the active metal
sites to form a chemisorbed complex. Limitation to chemically bonded systems implies a specificity so that very low concentrations of the poison may reduce the catalyst activity markedly. The metals of the periodic table group
VIII, most of which are employed in hydrogenation and reforming reactions are most susceptible to poisoning.
16 Elements of group Vb and VIb are the principal poisons in effecting catalyst deactivation. The toxicity of the compounds of these groups was attributed by Maxted to the presence of unshared electron pairs.
2.1.1 Adsorption of the Poison Soecie_s i--hech,?ni Stic
Consider «> Li ans
Herrington and Rideal (1944) have demonstrated that the reaction rates during the poisoning of heterogeneous catalysts can be simulated if all the sites are taken to be equivalent in their energetics and geometry. In this treatment they assumed that loss of activity results from a random distribution of poison molecules. This mechanism would correspond to such strong adsorption that every molecule of poison is adsorbed when collision with the surface occurs, thus yielding an immobile film, or if evaporation does occur then readsorption is equally probable on all sites, i.e, there is no interaction between adsorbed poison molecules. This redistribution of poison due to evaporation appears possible since Maxted (1951) has found that even at room temperature a partition occurs between the catalyst surface and the fluid phase. Thus the film of poison molecules will be mobile through evaporation to the liquid or gas phase, and if there is any repulsive interaction between the poison molecules, then the sites involving lowest interaction energy (non-adjacent) will be filled first. Although they assumed that the poison merely
17 blocks a geometrically fixed number of sites , Herrington and Rideal (1944) point out that in practice a chemical interaction between the poison and the catalyst may result in modification of van der Waal's field around adjacent catalyst centres. Furthermore, the poison may modify the metallic crystal lattice, although it is believed that most poisons remain on the catalyst surface. Herrington and Rideal (1944) distinguished two different cases where an adsorbed poison interferes with the adsorption of a reactant. These are shown in Fig.2.1(a) and
2.1(b), where the lattice sites are indicated by crosses and the poisoned sites by circles ; the reactant considered is a hexagon. Conditions such as 2.1(a), where the adsorbing
Fig. 2.1 (a) Adsorption on isolated sets of sites, if one set poisoned, no effect on other. (b) Interference between two possible modes of ad sorption.CHerrington and Rideal (1944)3 sites are widely separated and no mutual interference is possible, gives a direct proportionality between activity and the number of groups of active sites e.g. oxide catalysts. The conditions in Fig.2.1(b) represent the more general case for metal catalyst, where the sets of adsorption sites are overlapping.
For case 2.1(a) the activity is proportional to the number of groups of active sites. If the poison occupies one active centre, a fraction 0 of the active sites are bare and an isolated group of n centres is required for reaction, then the number of unpoisoned groups will be proportional to
0n. With this mechanism, the plot of activity against poison concentration is linear throughout the entire poison range only for a single point poison. For case 2.1(b), they investigated the effect of a single point poison on the adsorption of a 1,2 or 7 site reactant molecule. Since most of the active hydrogenating catalysts crystalled in the face-centred cubic form, the (111) plane of an f.c.c. crystal was used in the calculations. This is shown in Fig.
2.2(a) where the ordinate is the total number of sites occupied by the reactant molecules. When reactant and poison both occupy single sites, the relationship between activity and poison is linear. The curve for two-site reactant is slightly concave to the abscissa. The curve for a seven-site reactant is nearly linear in the range of 0-337. poison, but thereafter shows a sudden curvature ; at 707 poison, the ordinate is nearly zero indicating a total loss of activity.
The catalyst is thus seen to be more readily poisoned for Fig. 2.2(a) Poisoning of a face-centred cubic (111) plane by a single-site poison for 1-, 2-, and 7- site reactants. [Herrington and Rideal (1944)]
Fig. 2.2(b) Poisoning of a face-centred cubic (111) plane by a poison molecule too large to be adsorbed on adjacent sites, but the reactant may adsorb on sites adjacent to the poison. 1-.2-, and 7- site reactants. [Herrington and Rideal ( 1 944 ) 3
20 reactions involving a multi-site reactant than those requiring fewer sites. Thus at 60Z poison, the catalyst has retained only M of its initial activity for the 7-site reactant, while it is still 31Z active for the 2-site reactant and AOt active for the single-site reactant. Fig.2.2(b) shows the results for 1,2 or 7 site reactant where the poison molecule is too large to be adsorbed on adjacent sites but where the reactant is not prevented from being adsorbed on sites adjacent to the poison. It is seen that the surface cannot be completely poisoned for a single- or a two- site reactant, but it can be poisoned completely for a seven-site reactant.
Frennet et al (1978) developed a model for the multi site adsorption of a reactant in competition with hydrogen chemisorption on the same sites. Good agreement was obtained between their computed values and experimental results in the literature and also those obtained in their work for exchange on Rh. Verma and Ruthven (1977) simulated the effect of blocking the surface with a poison which requires one, two or three sites, by randomly blocking a certain fraction of the total sites, subject to the restriction that, for multi-site poisoning the sites required must be adjacent. The fraction of the surface which is available to reactant was then calculated subject to the restriction that for an n-site reactant n adjacent sites are required. A typical plot obtained by them for a two-site poison and 1-7 site reactants is shown in Fig. 2.3. It is evident that the relationship between the fraction of
21 Fig. 2.3 Theoretical curves showing the decline in available surface with amount of poison adsorbed. A poison molecule is assumed to occupy two sites. [Verma and Ruthven (1977)] surface occupied by the reactant and the fraction of the
surface occupied by the poison is essentially linear except when the number of sites required by the reactant molecule is much greater than the number of sites occupied by a molecule of poison.
Multi-site competitive adsorption has indeed been observed experimentally. Schwarz (1979) investigated the kinetics of adsorption and desorption of hydrogen from clean and partially sulphided Ru(001) surfaces. They concluded that the adsorption kinetics for hydrogen were unaffected for sulphur coverages of less than 0.25, but at a sulphur coverage of 0.25 no hydrogen was adsorbed indicating that one sulphur atom poisons four Rh atoms. Agrawal et al (1980)
22 studied the poisoning of Co/A12o3 by H2S in CO
hydrogenation and concluded that poisoning by sulphur was
primarily due to a geometric blockage of sites with one sulphur atom adsorbed per two Co atoms. However, the
activation energy for the methanation reaction was reduced upon partial poisoning indicating the importance of
electronic effects. In summary, it may be said that much of the observed behaviour during the poisoning of heterogeneous catalysts
can be simulated by the poisoning of a uniform set of sites. This treatment probably represents an oversimplification of
conditions which may occur in reality. Thus more than one crystal face may be active for some reactions, while for
others one plane alone may be operative. Poison molecules themselves may be preferentially adsorbed on certain faces,
thus giving rise to selective poisoning by a different mechanism than those discussed here. However, in the literature "potential" sites, the density of which is usually measured by hydrogen chemisorption, and "active"
sites for chemisorption are not differentiated. Furthermore, no distinction is made between the active sites for chemisorption and the active sites for a heterogeneous reaction. Most models, based on the simplifying assumption of a uniform surface, are quite adequate in reproducing the global behaviour of adsorption and reaction of reactant and poison molecules. In some models, the sites are divided into different fractions for the chemisorption of different gases
23 [Le Goff et al (1970)] or for chemisorption and heterogeneous reaction [Weng et al (1975)].
2.1.2 Inter- and Intra- pellet Transfer Effects
A careful analysis of transport effects is an essential
part of catalytic research from the standpoint of scientific and industrial importance. Hass and heat transfer processes have a profound effect on the performance and life of catalysts.
Wheeler (1951) carried out a systematic analysis of diffusion effects in the poisoning of catalysts. He showed that relationship between catalyst activity and fraction of surface poisoned can be simulated by intra-pellet diffusion resistances. Hegedus and McCabe (1984), in an excellent review of catalyst poisoning considered a first-order, irreversible, monomolecular isothermal reaction occuring in a flat, one-dimensional porous catalyst slab. In this analysis all the sites are assumed equivalent, a fraction a of which has been irreversibly poisoned. If the poison is uniformly distributed in the pellet and finite diffusion effects on the main reaction cause intrapellet concentration gradients, then from the work of Dougharty (1970) :
F = /(1-a) tanh[hQ/(1-a)]/tanh[hQ] 2.1 where F is the ratio of the rate of reaction on deactivated pellet to that on an undeactivated pellet, hQ is the Thiele modulus for the poison-free pellet. This is represented by curve A in Fig.2.4 . If both poison and reactant are uniformly distributed i.e., for kinetic control, hQ ---> 0
24 and so F=1-cx. At the other limit where complete intrapellet diffusion control exists, h ---» co and o
Fig. 2.4 Relative reaction rate as a function of the fraction of catalyst poisoned.[Hegedus and McCabe (1984)3 so F=/(1-a) and this situation is depicted by curve B. Curve
C shows the case where the main reaction is kinetically controlled. In this case also F=1-ot i.e., the reaction rate drops linearly with increasing a independent of the manner of poison distribution in the pellet. Curve 0 represents pore-mouth poisoning. Hegedus and McCabe (1984) showed that the solution of the appropriate diffusion reaction problem in this case yields tanhCh (1-a)3 F = ------2.2 tanhCh o 3{1+ah o tanh[h o(1-a)3 Finally, the case of core-poisoning, which may occur in
25 diffusion influenced self-poisoning reactions is represented by curve E where the appropriate diffusion-reaction problem yield s
F = tanh[hQ(1 -a)]/tanhChQ] 2.3 The slope of the F vs. a curves contains important mechanistic information which can be employed as a useful diagnostic tool if a can be determined reliably.
An alternative approach to providing information on the degree of diffusion resistances is to use the single-pellet reactor [Balder and Petersen (1968)] which is capable of measuring the rate of reaction, the bulk-gas concentration and the concentration at the centre of the catalyst pellet. The simultaneous measurement of the concentrations on both sides of the catalyst pellet and of the overall rate of reaction permit evaluation of both the kinetic constant and the effective diffusivity. The order of the reaction must however be defined in advance and this is done by examining the concentration in the symmetry chamber as the concentration in the feed is varied. Examples of the application of this methodology are available in the literature [Wolf and Petersen ( 1974, 1 977 ), Hegedus and Petersen (1973,1973,1973)]. A serious problem in the control of these reactors should be mentioned here. Because of the underlying assumption that the gradient of the reactant along the axis of the pellet is zero at the centre-plane, the mixture present in the symmetry chamber must be analyzed without influencing the composition and thereby the validity of the above mentioned condition. This difficulty has been
26 overcome in various ways : by taking extremely small samples for analysis by gas chromatography, by using infrared
analysis directly in the central chamber without taking
samples [Hegedus and Petersen (1972)] or by means of thermal
conductivity measurements [Suzuki and Smith (1972)]. Balder and Petersen (1968,1968) introduced a normalized centre-
plane orientation defined by
YA(t,1) - VA (0,1) 2.4 1 - ^ ( 0 , 1 ) where ^A(1) is the centre-plane concentration of reactant A divided by the concentration of A at the pellet outer
surface. The idea is to follow F as a function of A The corresponding diffusion-reaction problem may then be easily
solved for various reaction orders and poisoning mechanisms.
o.s h
Fig. 2.5 Relative reaction rates as a function of normalized centre-plane concentration in a single-pellet diffusion reactor. [Hegedus and Petersen (1973)]
27 The resulting plot of F vs. allows one to discriminate among various poisoning mechanisms. Hegedus and Petersen
(1972) obtained the curves of Fig.2.5 for a main reaction Thiele modulus of 2.5. Curve A is for uniform poisoning,
curve B : pore-mouth poisoning and curve C : core-poisoning. The above analysis has indicated the sensitivity of the centre-plane concentration to the nature of the deactivation process. The single-pellet diffusion reactor has been employed to investigate the interactions of diffusion, reaction and deactivation for the dehydrogenation of methylcyclohexane on Pt/Al203 [Wolf and Petersen (1977)] and for the hydrogenation of cyclopropane [Hegedus and
Petersen (1973)] also on Pt/AlgO^. The fact that measurement of the centre-plane concentration vs. relative rate of reaction can provide sufficient data to discriminate among various types of deactivation mechanisms and also allows the determination of kinetic and diffusive parameters makes this technique especially valuable for catalyst deactivation studies.
In contrast to single-pellet reactors, the gravimetric system allows very precise measurements of adsorbed amounts by recording the weight of the adsorbent during the course of adsorption, by use of highly sensitive microbalances capable of recording mass changes of as little as 10 — ft g. The gravimetric method has found application in the study of fouling by coke deposition. This technique is especially valuable for examining the equilibrium uptakes of deposited
28 material by the adsorbate. However, for kinetic studies the element of diffusional effects must be separated from the intrinsic rate of adsorption. The present study has shown that the intrapellet diffusional effects can be avoided by using very fine particles of the catalyst whereas the interpellet diffusion is removed by use of shallow beds. A description of the gravimetric flow system which can be used for examining catalysed reactions can be found in Viveros-
Garcia (1985), who investigated the poisoning of nickel catalysts by H^S and thiophene. Increasing attention is now being focussed on procedures for measuring rate parameters from dynamic response systems [Dogu and Smith (1976,1977), Schneider and
Smith (1968), Suzuki and Smith (1971,1972) and Cutlip et al (1984)]. Dynamic studies of reactor performance as influenced by deactivation have been reported by several researchers. [Price and Butt (1977), Weng et al (1975),
Kershenbaum and Lopez-Isunza (1982), Bilimoira and Bailey (1978), Mukesh et al (1983)]. The dynamic response method offers a very convenient technique for measuring the amounts of reactants adsorbed on a catalyst during the actual reaction. The accurate measurement of the evolution of small quantities of product has been carried out by special techniques such as mass spectroscopy and gas chromatography, with a preference for the latter technique for catalysed reactions. In addition to the speed with which the analysis of the products can be carried out, the dynamic response technique allows the determination of kinetic orders,
29 activation energies and heats of adsorption. The system usually consists of a small catalytic reactor in a circuit
containing an analytical chromatographic column constituting what is known as the microcatalytic reactor system.
2.1.3 Analysis of the Poisoning Process
Poisoning may be reversible or irreversible depending on whether the catalyst activity is recoverable upon removal of the poison from the feedstream under actual conditions.
The recovery of activity upon changing the feedstream or the operating conditions is generally referred to as regeneration.
2.1 . 3 . 1 Irreversible Poisoning
Masamune and Smith (1966) analyzed the problem of poisoning by an impurity in the feed, following the earlier work of Wheeler (1951). In their paper, they assumed that deactivation occured by deposition of material blocking the active sites. A model was developed on the asumptions of isothermality and the absence of external film resistance.
They considered a main reaction A(g) ---> 8(g). The rate of deposition of the impurity poison was qiven by dq/8t = k (1-
YJCp, where kp and Cp are the rate constant for poisoning and gas-phase concentration of poison respectively; T represents the fraction of surface area covered by the poison. Fig.2.6 shows the intraparticle profiles for the dimensionless concentrations of the poison and reactant
30 1.0
Fig. 2.6 Radial profiles of concentration (poison and reactant) and activity for independent poisoning; hA=5, hp=10.[Masamune and Smith (1966)]
3 1 species P and A, and for the activity 1 -VP for values of 10 and 5 for the Thiele moduli for poisoning (hp) and main reaction (h^) respectively. With the relatively high value of hp the diffusion resistance is high enough to cause essentially complete deactivation of the outer layer of the catalyst pellet, even at low process time. This is observed in the bottom part of Fig.2.6, where 1-Y is nearly zero as r/rQ approaches unity. Fig.2.7 shows the decrease in effectiveness factor n with time for h =10. The dotted lines P correspond to h^ = 0, that is, no diffusion resistance for the main reaction. In these curves there is a continual decrease in n with increase in diffusion resistance for the main reaction. As the interparticle diffusion resistance for the impurity P increases, less of the interior of the catalyst pellet should be deactivated significantly due to poisoning.
Fig. 2.7 Effectiveness factor for independent poisoning ; hp=io. [Masamune and Smith (1966)]
32 The quantitative effect of this on the activity of the catalyst can be seen by comparing n at increasing hp values,
for the same t and h^. This is clearly illustrated in
Fig.2.8 where hp is shown as a parameter for hA=2. The dotted lines represent the lower ^hp = 0) and upper (hp = a>) limits of the diffusion resistance of P. For the upper
limit, there is no deactivation, and n is constant with
respect to time. This constant n depends upon hA, as given by the general Wheeler equation. These results show that in
the case of impurity poisoning, the least deactivation will occur in a catalyst for which there is a minimum diffusion
resistance for the main reactant and a maximum resistance for diffusion of the impurity into the pellet. This however,
can never be achieved in practice, so that the implication is that there is an optimum catalyst particle size for
highest activity after a certain time has elapsed.
Fig. 2.8 Effect of diffusion resistance for fouling reaction on effectiveness factor ; hA=2. [Masamune and Smith (1966)3
33 Hegedus ( 1 9 7 4 ) gave an analysis of impurity poisoning where they removed the restriction that external film
resistances were absent. The rate of the main reaction was given by :
r k ( a o - "a p 2.5 where aQ is the initial area of the active sites and ap is the poisoned area. The rate of deposition of poison was given by :
dq 2.6 k p(a o -a p )PC p< d t
The poisoning was found to be a function of time, and the
Thiele moduli (hA,hp) and Biot numbers (BiA,Bip) of the poisoning and main reactions. In this analysis hA
hA>i it will be present. A value of hp<1 implies uniform poisoning while hp>i implies strong pore-mouth poisoning. Values of the Biot numbers less than .unity indicate strong external mass transfer control, while for Biot numbers greater than unity, external mass transfer effects are less important. Hegedus (1974) investigated the cases of varying degrees of external and internal diffusion resistance.
Fig.2.9 shows the particular case of hA = hp = 10 and BiA =
Bip =10; a case of strong internal and weak external mass control. Both the main and poisoning reactions were assumed to be first order in the concentration of the respective gases and also in the dimensionless concentration of active
34 sites. The figure shows that as poisoning occurs the reactant concentration within the pellet also increases because less of reactant is consumed by the main reaction.
Fig. 2.9 Distribution of a,p and 0p in a spherical catalyst pellet ; strong internal, weak external mass transfer control. 0p=ap/ao- CHegedus (1974)]
To estimate the effect of internal and external mass transfer resistances on the poisoning behaviour of the pellet they plotted the relative overall reaction rate for the pellet at a particular time against the dimensionless time. The results shown in Fig.2.10 were obtained by Hegedus (1974) from various combinations of the parameters which have been shown in the table below
35 Table 2.1 •H CD Curve No hA hp BiA Q. 1 1 1 1 1
2 1 0 1 0 1 0 1 0 3 1 0 1 0 . 1 . 1
4 . 1 . 1 1 0 1 0 5 1 . 1 1 0 1 0
6 1 . 1 . 1 . 1
7 -1 10 . 1 . 1
8 1 1 0 1 0 1 0
Curves 1 and 2 compare the effects of increasing the
internal diffusion resistances (h goes from .1 to 10), which can be done either by increasing the rate constant or
the pellet size or by decreasing the effective diffusivity,
Fig. 2.10 Time-dependent decay of the activity in spherical catalyst pellets. [Hegedus (1974)]
36 while decreasing the external resistances (Bi goes from .1 to 10), which can be done either by increasing the mass transfer coefficient or the pellet size, or by decreasing the effective diffusivity. An increase in the internal resistance to poisoning extends the life of the catalyst pellet. The most favourable extension of catalyst life occurs in the case of curve 3 where both internal and external resistances to poisoning are increased. Curve 7, where the internal diffusion resistance to the main reaction is small and the external resistance for the poisoning reaction is high also gives an extended catalyst life. To summarize, it may be said that although there is a lack of fundamental information, considerable efforts are being made to model the mechanism of catalyst poisoning from an analysis of the kinetics of the poisoning reactions.
2.1.3.2 Reversible Poisoning
One of the earliest treatments to the case of reversible poisoning was given by Gioia (1971) and Gioia et al (1970). Their work was based on a mechanism proposed by
Innes (1951). They found excellent agreement between their modelling predictions and experimental measurements of the rate of hydrogenation of ethylene over a copper-magnesia catalyst using water as the reversible poison.
The simple case of reversible poison adsorption is represented by the Langmuir adsorption isotherm :
a C P P 2.7 ao Cpmax
37 On the assumption of the absence of external film resistances Gioia et al (1970) developed a model for the flat slab geometry. They obtained the results for the case where the main reaction was free from diffusional resistances. Valdman et al (1976) made an analysis of reversible poisoning, where the restriction of the absence of diffusional resistances were removed. Langmuir adsorption of poison and reactant was assumed. They considered a reaction A ---> products occuring by a Langmuir Hinshelwood mechanism and developed a model for the flat slab geometry.
2.1.4 Factors Affecting the Poisoning Process
The preceding section has shown that the time scale of the poisoning reaction is significantly influenced by pellet size since the Thiele modulus and the Biot number (cf. Fig.
2.8, 2.10) depend on the size of the catalyst particles. In general poisons will deposit faster on smaller particles
(curves 1,4,5 and 6 of Fig 2.10) than on larger particles (curves 2,3,7 and 8 of Fig 2.10), for a diffusion influenced reaction. On the other hand, however, with smaller particles the external diffusion resistance will tend to be higher than with larger particles. Moreover, since the Thiele modulus of the main reaction is also influenced by the particle size, deactivation reactions may involve an optimum particle size for a given catalytic activity after a certain time has elapsed, which would depend on the mechanism and kinetics of both the main and poisoning reactions. Anderson
38 et al (1965) found that the sulphur poisoning of Fischer- Tropsch catalyst decreased with decreasing particle size of the catalyst. Moseley et al (1972), on the other hand, found that the poisoning rate of steam hydrocarbon gasification decreased with increasing particle size of the catalyst. El Menshawy et al (1974) found that during the poisoning of CO oxidation over AlgC^ supported NiO by iron carbonyl there exists an optimum catalyst particle size for maximum activity .
Due to variations in surface-to-volume ratio and/or convergent or divergent geometries, the time scale of poisoning can be influenced by the shape of the catalyst particles. From an analysis of the effects of pellet geometry Sada and Wen (1967) concluded that the time required for complete poisoning increases in the sequence of sphere, cylinder and slab geometries. However, Gokarn and Doraiswamy (1973) found that the use of generalized diffusion approximations of Aris (1957) effectively cancels the shape effect. Thus the actual time scale of the poisoning process may be approximated by using a thiele modulus which involves the characteristic dimension defined by the ratio of the particle volume to the external surface area .
The degree of diffusion resistances of both the main and poisoning reactions are influenced by the pore-structure of the catalyst particle. The pore structure of the support determines the diffusivity of the gas-phase species and the
39 internal pore surface area. The observation that different
pore structures have different net effects if the poison
precursor reacts with the support, has been employed in the design of poison-resistant automobile exhaust catalysts as
discussed by Hegedus and McCabe (1984) and by Rajagopalan and Luss (1979). Rajagopalan and Luss predicted pore sizes
yielding either optimal initial activity or optimal lifetime activity for the catalytic demetallation of hydrocarbons.
Hegedus and McCabe (1984) discussed the phenomenon of the protection of the active components of a catalyst from
poisoning by subsurface impregnation. The plugging of pores during catalyst poisoning
contributes to the catastrophic loss of activity as observed by several researchers [Richardson (1972), Chen and Anderson
(1973), Bomback et al (1975), Chou and Hegedus (1978), Stanulonis et al (1976) and Newson (1975)]. The pores tend to be blocked near their entrance, reinforcing the already severe effects of pore-mouth poisoning. For pore-mouth poisoning, the activity of the partially poisoned catalyst pellets depend on the diffusivity characteristics of the poisoned shell. Pore-plugging may occur even in the absence of diffusional resistances, for complex pore structures
[Androutsopoulos and Mann (1978)]. Ramachandran and Smith (1977) performed the mathematical modelling of the pore-
structure changes during catalyst poisoning. Chou and Hegedus (1978) showed that for thin plugged crusts, transient diffusivity measurements were of significant value to quantify the extent of pore plugging ; in such cases
40 steady-state diffusivity measurements may not be sufficiently sensitive. Transient methods for the measurement of diffusivities consist in introducing a pulse of the diffusing component to one end-face of the catalyst pellet placed in a Wicke-Kallenbach type diffusion cell (1941) and measuring the concentration vs. time response peak leaving the other end face. Dynamic methods have the advantage of being very rapid, requiring small quantities of the diffusing component and can provide more information about the importance of dead-end pores. Chou and Hegedus
(1978) carried out transient pulse diffusivity measurements in a catalyst pellet whose outer skin was partially obstructed thereby giving two zones of differing diffusivities in the pellet. While steady-state counterdiffusion techniques failed to sense the diffusivity via the outer skin of the pellet, a transient technique was found to be more sensitive to the radial distribution of diffusivities in catalyst pellets.
Finally, the distribution of the active component along the characteristic dimensions of the catalyst pellet may have significant effects on the catalyst's performance. Intrapellet diffusion effects result in activity and concentration gradients in the catalyst pellets. Activity gradients may manipulate the catalyst’s activity and selectivity in the absence of poisoning and the resistance of the catalyst to poisoning. Hegedus and McCabe (1984) presented a review of the effect of catalyst impregnation profiles on catalyst poisoning.
2.2 Thiophene Poisoning of Nickel Catalysts
The poisoning effect of thiophene on nickel is well documented. However, the mechanism underlying the adsorption of thiophene on the metal surface is still not understood
completely. There have been some attempts [Lyubarskii et al (1962), Bourne et al (1965), Viveros-Garcia (1985)] to
characterise the effect of temperature on the poisoning of nickel by thiophene . These and other investigations have led to certain hypotheses regarding the mechanism of thiophene poisoning, proposed by different researchers and have a commom theme. One of the earliest reported work on thiophene poisoning is that of Lyubarskii et al (1962). They studied the
adsorption of thiophene on supported nickel catalysts. In one series of experiments, samples of reduced catalyst were discharged into a solution of thiophene in benzene at 20°C. After a 24-hr period, the solution was analyzed for residual
thiophene content. Each sample was then placed in a steady flow-circulating setup, where its activity in benzene
hydrogenation was determined in comparison to its activity when pure benzene was used. A second set of experiments were
performed at temperatures of 100-150°C. They observed an
increase in sulphur capacity of the catalyst at these elevated temperatures which they accounted for by stating that thiophene was undergoing hydrogenation at these
42 temperatures, resulting in further adsorption of thiophene. They found a linear relationship between the activity of the
catalyst and its sulphur capacity per unit surface of nickel. From their results they proposed a scheme for
thiophene poisoning, according to which, a thiophene molecule is strongly bonded with plane orientation to five
nickel atoms. At elevated temperatures thiophene is hydrogenated to thiophane which may be further hydrogenated
to butane and other decomposition products. The proposed mechanism is illustrated by the following diagram :
a
Fig.2.11 a) adsorbed thiophene ; b) adsorbed thiophane ; c) surface sulphide ; d) butane and its decomposition products. [Lyubarskii et al (1962)]
Bourne et al (1965) observed a limited degree of sulphiding of nickel catalysts by thiophene at 100°C. This
suggests that only the surface of the nickel is attacked and
that sulphiding ceases when the available surface of the nickel crystallites is used up. This hypothesis was
supported by their observations on a catalyst with large crystals of nickel which had a very low surface area and hence a small proportion of its nickel atoms exposed. When treated with thiophene this catalyst reached a S/Ni atomic
ratio of 0.002 compared with 0.06 for another catalyst which had smaller crystals of nickel and a higher surface area.
43 They suggested that at 100°c thiophene initially adsorbs flat and then decomposes, taking up hydrogen to liberate butane, leaving a sulphide ion attached to a nickel site. They claimed that their observed S/Ni ratio for thiophene poisoned catalysts indicates that the number of sulphur atoms on the nickel is only about 25Z of the number of exposed nickel atoms. They explained this partial sulphiding by a geometric model. From a consideration of the spacings in the major lattice planes of fee nickel crystals they found that if sulphiding by thiophene occurs in a random manner, maximum surface coverage by sulphur occurs when 19Z of sites on (100), 38/ on (110) and 24t on (111) have been covered. These values span the observed value of 251. They claimed that the ability of a thiophene sulphided catalyst to adsorb carbon monoxide confirms the existence of unsulphided sites which results from random poisoning. The investigations of Viveros-Garcia (1985) and of Bartholomew and Pannell (1982), however, showed that the measured uptake of carbon monoxide at 190-298K, by a S-poisoned Ni- catalyst, is increased in proportion to the sulphur present on the nickel surface due to the formation of nickel carbonyl and is therefore not representative of the free area of the catalyst. Bourne et al (1965) found that at
30°C to 150°c the S/Ni ratio ranged from 0.03 to 0.09 ; at temperatures above 150°C, the S/Ni ratio increased rapidly approaching 0.25 at 200°C, which, if sulphiding were confined to the surface only, corresponds to a fully covered
44 surface. The desulphurisation products from thiophene showed increasing amounts of butane at temperatures higher than 150°C. These catalysts retained their hydrogenation
activity and selectivity whereas catalysts treated with
thiophene above 200°C lost their activity and their sulphur
contents indicated sulphiding in depth. For high temperature
thiophene poisoning, they postulated that thiophene reacts with nickel in two ways :
i) direct reaction to give butane leaving sulphur attached to the nickel
ii) decomposition to give HgS
The former accounts for the maintained selectivity and the latter for bulk sulphiding.
Viveros-Garcia (1985) investigated the effects of temperature (25-500°C) and poison concentration (100 and
1000 ppm) on the adsorption of thiophene on a nickel-alumina
catalyst. The overall uptake of thiophene and the rate of adsorption was found to be higher at the higher concentration. At 25°C, he proposed a mechansim where
thiophen was adsorbed both coplanar to the surface and perpendicular via the sulphur atom. At higher temperatures, it was postulated that after adsorption on the nickel sites,
the thiophene molecules reacted with hydrogen to form the hydrogenation product thiophane which was eventually decomposed to butane. This hypothesis was supported by elemental analysis of the deposited material on the surface, which showed that there was less carbon on the surface than would remain if no decomposition was taking place. The
45 evolution of desulphurisation products allowed more thiophene to be adsorbed on the liberated sites. An attempt
was also made to regenerate thiophene poisoned samples by
treating in hydrogen at 600°C for a 40-hr period. Viveros
(1985) found that during regeneration C and S were removed
as decomposition products presumably in the form of H2s,
and C^H^q . Gas analysis of the product stream to support this observation was not reported. Host of the material was removed before the highest temperature of regeneration (600°C) was reached ; at 600°C sulphur was
found to be difficult to remove, confirming the observations
of Bartholomew et al (1982), Oliphant et al (1978), and McCarty and Wise (1980) CO adsorption on regenerated
samples were higher than on clean samples. Bartholomew and Pannell (1982) found that sulphur promotes the formation of
nickel carbonyl during CO chemisorption on sulphided samples resulting in higher uptakes than on S-free samples. The extent of regeneration in the investigation of Viveros- Garcia (1985), therefore, could not be determined. Viveros-
Garcia (1985) concluded that while high temperature treatment in an atmosphere of hydrogen could remove deposited carbon, the catalysts were not regenerable in the sense that sulphur still remained on the surface.
Holah et al (1979) studied the effects of thiophene poisoning in the hydrogenation of 1-octene with a partially
hydrogenated nickel boride catalyst and with Raney nickel. The general features of the poisoning curves were similar to
46 those expected for poisoning of a porous catalyst. One significant effect, they observed, was the increase in the amount of 1-alkene isomerization for a given amount of hydrogenation. This is to be expected if the hydrogenation and isomerization occur on different reaction sites or by different reaction paths as suggested by MacNab and Webb
(1968). The decreased rate of hydrogen uptake on the poisoned catalyst will permit more time for isomerization for a given degree of hydrogenation, assuming that the isomerization reaction is not poisoned. The increased isomerization activity is consistent with this view. If isomerization occurs by an "allylic-type" mechanism [MacNab and Webb (1968), (1972)] then it could proceed in a “hydrogen-deficient" region. Holah et al (1979) therefore suggested that hydrogenation occurs mainly on external surfaces and in the larger pores of the catalyst while the isomerization reaction is more likely in the interior of the small pores. Because of the rapid hydrogenation reaction at the pore mouth, the amount of hydrogen available in the interior of the pore will be limited and the pore interior will also be less accessible to catalyst poisons. The poisoning curves of Holah et al (1979), for thiophene showed that both the nickel boride and Raney nickel retain approximately 25% of their original activity even in the presence of large amount of poison. The observed stabilization of the reaction rate at high poison levels is in good agreement with the estimates of Bourne et al (1965) who pointed out that, for a nickel catalyst, about 197. of
47 the sites cannot be poisoned by thiophene adsorbed in a coplanar manner, because of geometric requirements. Previous studies of thiophene poisoning [Lyubarskii et al (1962), Bourne et al (1965), and Koppova et al (1973)] of nickel catalysts propose that the poison adsorbs coplanar to the surface. The initial portions of the thiophene poisoning curves exhibit the characteristics of a strongly adsorbed, selective poison generally referred to as pore-mouth poisoning.
Richardson (1971) studied the sulphiding of nickel catalyst beds using a moving coil permeameter. Thiophene poisoning was found to be adsorption limited and the beds so poisoned were not regenerable. From infrared studies of adsorbed thiophene, Blyholder and Bowen (1962) concluded that although adsorption of sulphur compounds by co ordination of the sulphur atom with the surface is generally proposed in the literature, their data indicate that adsorption on clean silica-supported nickel is a much more varied and complicated process involving frequent bond rupture in the adsorbing compound even at room temperature. From infrared spectrum of adsorbed thiophene they suggested that in the course of chemisorbing, the double bonds in thiophene become saturated by interaction with the surface. They proposed a structure for chemisorbed thiophene in which the double bonds are opened to form four carbon-nickel bonds with the surface. This structure was found to be consistent with their data. The spectra in the C-H stretching band,
46 in their study, revealed that another structure has a major contribution to the spectra. In this region they observed two bands which could be attributed to the presence of more than one tertiary C-H group on the surface. However, the
•fact that surface complexing does not shift CH2 and CH3 stretching frequencies by any appreciable amount negates this interpretation. In view of the low ratio of hydrogen to carbon in the parent compound, the authors favoured a CH^ assignment. Since thiophene is the only source of hydrogen atoms on the surface, the presence of CH^ group requires carbon-hydrogen bond rupture on adsorption. This means there must also be carbon atoms which are bonded only to the surface, and carbon or sulphur atoms or both. The authors pointed out that on the basis of their data, proposal of surface fragments was sheer speculation, although formation of chemisorbed carbon monoxide would release a hydrogen to form a CH^ group.
Klostermann and Hobert (1980) analyzed the reaction behaviour of sulphur compunds such as thiophene on silica- supported nickel by thermodesorption and ir transmission measurements. They concluded that all sulphur compounds investigated by them are chemisorbed dissociatively on silica-supported nickel by rupture of S-H, C-S and S-S bonds. They further suggested that surface recombination reactions take place in parallel yielding hydrocarbons and the sulphur remaining on the catalyst surface blocks the sites active for dissociation of carbon monoxide. They attributed the appearance of C-H stretching vibrations
49 characteristic of aliphatic hydrocarbons to the interaction between thiophene and freshly reduced silica-supported nickel ; addition of Hg increased the intensity of the respective absorption bands.
Rochester and Terrell (1977) questioned whether bands reported in previous infrared studies [Blyholder and Bowen
(1962)] of sulphur compounds on nickel/silica samples could be correctly attributed entirely to adsorption on the metal surface. They pointed out that some contribution to the spectra probably resulted from chemisorbed species on the oxide support. This is particularly true of spectra reported [Blyholder and Bowen (1962)] for thiophene on nickel/silica, which were attributed only to the chemisorption of thiophene on nickel. Their spectra were similar to those recorded in the work of Blyholder and Bowen (1962) and have shown that the bands observed were primarily due to chemisorptive interactions between thiophene and silica. Their contention was that spectra of species adsorbed specifically on the metal could not be distinguished in the presence of bands due to species on the surface of the oxide support.
Koppova et al (1973) investigated the influence of the poisoning of a nickel catalyst by sulphur compounds such as thiophene on the activity and selectivity of the catalyst during the course of hydrogenation of isomeric octenes in the liquid phase at 30°C and at atmospheric pressure. They found that when the amount of sulphur bound to the catalyst increased. the activity decreased monotonically, but the
50 selectivity remained unchanged. From their results they concluded that sorption occurs merely at the metallic o o surface of the catalyst. Employing a molecular area of 33 A
for thiophene after Lyubarskii et al (1962) they calculated
the nickel area of the catalyst which was in good agreement with the value determined by chemisorption of hydrogen.
Weng et al (1975) studied the poisoning kinetics of thiophene adsorption on Ni-kieselguhr catalyst. A series of
experiments were performed in a differential reactor at atmospheric total pressure for thiophene concentrations ranging from 50 to 250 ppm and at temperatures of 65-1?5°C. They found excellent agreement with a power law equation for the rate of change of activity with time, first order in catalyst activity and in thiophene concentration, with an experimental activation energy of 1080 kcal/mole. Benzene hydrogenation was used as a model reaction, the kinetics of which were correlated by Kehoe and Butt (1972). Rate parameters were determined in two series of experiments corresponding to some limiting forms of the rate equation. The first was at low temperature and high benzene concentration and the second at high temperature and low benzene concentration. The results were obtained from a non linear least squares fit of the experimental data. Kinetics of the poisoning reaction were determined in a separate experimentation. The analysis of the deactivation rate was carried out based on a separable form of the rate equation, linear in concentration of poison and availabilty of active sites. This correlation of poisoning kinetics, however, was
51 not able to predict the propagation of the zone of activity in the poisoning of an integral fixed bed reactor. Steady-
state temperature profiles were modelled satisfactory, but the rate of migration of the hot-spot was found
experimentally to be more rapid than that predicted from the correlation of poisoning kinetics. To resolve the discrepancy they proposed a semi-empirical two-site deactivation model where they considered two types of sites,
one active for thiophene chemisorption alone and the other active for hydrogenation as well. The activation energies for the two different sites were assumed equal but the rate constants differed in magnitude such that k.T = fk_, (o They established that the ratio of hydrogenation active sites to the initial number of those sites is equal to the fractional occupancy of sites active only for thiophene chemisorption. Another parameter used in the model was •y the ratio of hydrogenation active sites to the total number obtained from measurement. Using either the original model with a value of 40% of the experimentally measured thiophene capacity of the catalyst, or the two-site model with the single set of parameters f=0.015 and -*=0.35, they found reasonable fits to all the experimental runs. They claimed that the value of the parameters *y and f obtained in the fit with the two-site model are reasonable in terms of their physical significance in the model. The f value indicates that thiophene chemisorption on the sites inactive for 52 hydrogenation is slow compared to that on active sites, while the f value indicates that there are somewhat fewer hydrogenation-active sites than inactive sites and this was in qualitative agreement with the one-site results. They pointed out that although reasonable fits were obtained, this could not be a complete representation of the state of the catalyst surface without more detailed results on the intrinsic kinetics of thiophene poisoning on supported nickel. Downing et al (1979) made an experimental and modelling study of the dynamic behaviour of an industrial catalyst pellet as influenced by deactivation and dilution. The reaction system employed was benzene hydrogenation over nickel/kieselguhr catalyst pellets diluted with 10t graphite, 10Z silica or undiluted ; thiophene was employed as the poison. A one-dimensional effective transport model was developed, and the parameters for this model were determined principally from separate off-line experiments or steady-state experimental information. The adequacy of the model was determined from a direct comparison of the experimental and computed time-temperature profiles. Attempts at full a. priori simulation of spatial and temporal variation of the intraparticle temperature profiles were not successful. Investigations of the parametric response of the model revealed extreme sensitivity to the pellet intraparticle diffusivity and external heat transfer coefficient. They found that both the full model and simplified versions employing an active shell (undeactivated 53 pellet) or a dead zone (deactivated pellet) at the surface could be used for successful simulation of the experimentally observed behaviour, but only in the sense of parameter fit models. Global, averaged quantities such as steady state activity and overall rates of deactivation could . be simulated using a. priori parameter sets, but they concluded that detailed simulation of transients would require parameters to be measured or estimated to a higher degree of accuracy than was obtained in their work. Price and Butt (1977) extended prior work [Weng et al (1975)] on experimental and modelling studies of non- isothermal non-adiabatic reactor dynamics induced by catalyst poisoning, to adiabatic reactors. Thiophene poisoning of the nickel catalyzed hydrogenation of benzene was used as the experimental system. A pseudo-homogeneous one-dimensional dispersion model was used to model both steady-state and transient behaviour of the reactor on introduction of poison into the feed. Poisoning kinetics were interpreted via a shell-progressive mechanism which appeared to provide a simpler alternative to the two-site mechanism previously postulated (1975). This model was in more accord with the experimental observations on the variation of adsorption capacity with run conditions. However, the amplification of accumulated errors associated with determinations of individual parameters and the parametric sensitivity of the model ultimately limits the degree of precision which such simulations can attain. 54 Price and Butt (1977) concluded from their results that the imprecision with which reaction/reactor paramet ers are measured today coupled to the sensitivity of th e model renders a priori simulation of the tra nsient of the temperature profile in a non-isothermal pa rticle scarcely reliable. To this is added the uncertainty due to possible surface inhomogeneities which influence the effect ive value of the diffusivity, particularly in the zone of deactivation. Hence, they point out that careful kinetic studies are needed to help clarify the nature of the deactivation process from a better unders tanding of the mechanism underlying the adsorption of the poison on the metal surface. 2.3 Thiophene poisoning of other metal catalysts Oel Angel et al (1982) studied the kinetics of the deactivation by thiophene of supported rhodium catalysts. They found that the poisoning of the hydrogenation of benzene by thiophene obeys a simple kinetic model based on separability of deactivation rate laws. The deactivation rate constant obtained from this model varies when the percentage of exposed Rh changes : poisoning appears to be structure sensitive. No effect of the support is observed at low dispersions when alumina or silica are used. For particles with an average size less than 40 A, Rh/Si02 deactivates 4 times faster than Rh/Al203. For particles O greater than 50 A, Rh/SiOg deactivates at a rate comparable to that obtained for Rh/Al2°3 . This support effect is 55 attributed to a change of the morphology of the rhodium particles. Yacaman et al (1980) examined the morphology of rhodium particles on several supports by high resolution electron microscopy. Their conclusion was that rhodium particles exhibit different shapes which depend on the support : a mixture of cubooctahedra and icosahedra was O found for small particles (10-30 A) on alumina, whereas for silica the shape was icosahedral; bigger particles were cubooctahedral on all supports including alumina, silica and magnesia. As pointed out by MacKay (1962) a slight distortion of a cubooctahedron can yield an icosahedron of equal number of atoms. Such distortion may be the result of the influence of support. Ni is more refractory than rhodium and should therefore be more subject to surface reconstruction. The electronic properties of small particles have been recently examined in the case of nickel [Gordon et al (1977)]. The local density of states on fee cubooctahedral and icosahedral clusters of increasing size from 5 to 40 A were calculated for corner sites and central sites on differently oriented faces. The conclusions were : 1. the electronic properties of bulk nickel are obtained o only for particles greater than 50 A. 2. the local density of states on the icosahedron differs from that of the corresponding cubooctahedron with a higher density of states at low energies. Oel Angel et al (1982) pointed out that rhodium and nickel crystallise in the same structure and showed a comparison of 56 these theoretical computations with the experimental behaviour in deactivation : 1. for Rh/SiOg a higher sensitivity to thiophene is observed : sulphur is an electron acceptor and will react preferentially at sites with higher electron density, thus on the icosahedron. 2. particle size affects alumina : these particles are cubooctahedral, and the computation shows that increasing the size of the cluster results in a broadening of the distribution of energies, i.e., the creation of lower energy levels which explains the increased reactivity of sulphur. They, therefore concluded that the behaviour of rhodium towards sulphur deactivation may be described by assuming the main effect of the silica support is to change the morphology of the smaller particles. Richardson and Campuzano (1981) investigated the interaction of thiophene on a Cu(111) surface, at room temperature, by using angle-resolved photoemission techniques. They found that thiophene adsorbs on the (111) face of copper at room temperature associatively though relatively large exposures are required to saturate the surface. From the energies of the adsorbate induced features and their dependence on photon incidence angle and polar electron emission angle, they concluded that thiophene adsorbs on this face of copper with the aromatic ring parallel to the metal surface. Ramachandran and Massoth (1982) carried out TPD and TPR studies on thiophene poisoned Mo/A1203 and CoMo/A1203 57 catalysts. The major products of desorption were thiophene and butene, leaving a carbonaceous residue. This residue could be removed completely with the formation of H2s, thiophene and butene on subsequent TPR in The residue appeared to consist of an easily reactive thiophene polymer and a less-reactive sulphur-containing coke. From the runs on poisoned catalyst they found that the adsorption characteristics of the remaining sites were not altered significantly. In the TPO run, thiophene desorbed at a rather low temperature showing it to be weakly chemisorbed on some sites. Butene appeared at a slightly higher temperature, but no butadiene or butane were detected. The inorganic product of HDS reaction viz. HgS was absent. In the subsequent TPR run, thiophene evolved at low temperature. Butene appeared mostly at high temperature. Again, no butane was observed but considerable H2S evolved. Microbalance runs confirmed the existence of a residue remaining after the TPD which was removed completely by subsequent TPR. From these results they proposed the following scheme : C*H*Sads C4H4S + C4H8 * (polymer/coke) coke ♦ C^H^S The poisoning in their study was carried out at room temperature and their HDS scheme assumes that all thiophene adsorbed initially were in a molecular state. 58 The investigations reported in the literature have now raised questions regarding the nature of the adsorbed species when thiophene is adsorbed on the metal surface. The present work is aimed at elucidating more information on the mode of adsorption of thiophene on nickel. 59 C H A P T E R 3 Mathematical Models of Thiophene Poisoning A major objective in this study has been to see how well one can model a priori the poisoning by thiophene of nickel catalysts. The parameters required for the mathematical description of the system is determined either from experimentation or by using appropriate correlations. The rate parameters of the poisoning reaction and the surface and pore characteristics of the catalyst are determined from experiments. Additional parameters such as the effective diffusivity and Biot numbers are estimated from suitable correlations. Equations have been derived for the rate of a poisoning reaction on a porous catalyst whose active surface area decreases with time due to the deposition of an impurity poison on its surface. Three types of models have been considered. In model I, thiophene bonds to a fixed number of surface atoms. Model II assumes that thiophene can adsorb on to the metal surface with two different stoichiometries. According to this model, thiophene molecules in general tend to lie flat on the surface of the nickel atoms. However, at the edges and corners of the crystallites, there is not enough room for the thiophene molecules to be accomodated in a coplanar fashion ; they, therefore, lie perpendicular to the surface on these sites. Perpendicular adsorption also 60 occurs in the spaces in-between coplanarly adsorbed thiophene molecules, where there are no longer five simultaneous sites on the surface. As a result of the changed orientation, the stoichiometry of bonding also changes. When adsorbed coplanar, each thiophene molecule occupies five nickel sites. In the perpendicular mode, the thiophene molecule holds on to two nickel sites via the sulphur atom. Finally, model III assumes that the adsorption behaviour of thiophene on nickel can be approximated by a variable instantaneous stoichiometry of adsorption. According to this model, thiophene may adsorb coplanar or perpendicular, as with model II. Unlike model II, model III is a simplified rather than a mechanistic model. The distinctive feature of model III is that here the instantaneous stoichiometry of adsorption decreases from an initial value of 5, when the surface is bare, to some final value which is determined by the experimentally measured stoichiometry of adsorption. The basis for this model lies in the hypothesis that as more and more of the surface is poisoned, an increasingly lower number of thiophene molecules can adsorb in the coplanar mode. The extent of poisoning, in general, depends upon time and radial position within the pellet. In one of two extreme cases, where the intraparticle diffusion resistance is negligible, the deposition of poison within the pellet should be uniform. In the other case, where the diffusion resistance is large with respect to the resistance of the poisoning process, the deposition should be concentrated in 61 an outer shell of the pellet.This progressive shell model has been used by several investigators [Weng et al ( 1 9 7 5 ) , Downing et al ( 1 9 7 9 ) , Weisz and Goodwin ( 1 9 6 3 ) , Y a g i a n d K u m i i ( 1 9 5 5 ) ] to treat the deactivation or the regeneration problem when intraparticle diffusion controls the rate of reaction. In this study, rather than using the shell model the intrapellet distribution of poison has been calculated. The poisoning problem was treated in a manner where. in general, both reaction and diffusion resistances are important. The equations for concentration as a function of time and radial position within the pellet were therefore solved directly. Calculation of the mass Biot number, based on stagnant bed conditions showed that external diffusion resistance was negligible. However, experimental study indicates that external film resistance may be present due to an effect of catalyst bed-depth even when calculations from correlations show that Biot number >> 1, indicating the absence of film resistance. It is obvious, therefore, that the correlations fail to take into account an effect of bed- depth. The Biot number parameter was therefore included in the models developed here. so that adjustments to its calculated value could be made from experimental observations of the effect of bed-depth. The assumptions involved in the model are : i) the temperature within the pellet is constant. ii) the rate of poisoning is first-order with respect to the concentration of the poison within the pores of the 62 c a t a l y s t . iii) the rate of poisoning i s p r o p o r t i o n a l t o t h e concentration of the vacant sites r a i s e d t o a p o w e r n e q u a l to the stoichiometry of poison ads o r p t i o n , a c c o r d i n g t o S (g ) + n ------S . > a d s 3 . 1 iv) the extent of poisoning is a f u n c t i o n o n l y o f t h e deposition of the impurity on the ca taly st surf a c e . v) the poison acts by adsorpt i o n , preferentially and independently of other species in t h e r e a c t a n t s y s t e m . vi) since thiophene is present i n t h e f e e d i n v e r y l o w concentration the contribution o f t h e poisoning to the energy balance is negligible. 3.1 Development of the Mathematical Models Let us consider that the poisoning reaction is t h e simple irreversible reaction represented b y e q u a t i o n 3 . 1 . The intrinsic rate of deposition of S on t h e p o r e w a l l s o f the catalyst is given by d q s M k ( 1-a)a C 3.2 d t where q is the concentration of poison deposited on the catalyst in moles per unit mass of catalyst, s^ is the metal area of the catalyst per unit mass, k is expressed per unit metal area of the catalyst, a is the fraction of the surface that is deactivated and a is the instantaneous stoichiometry of adsorption which may be a constant (models I and II) or 63 it may be some function of the coverage (model III). I n equation 3.2 a can be written as a=n/nT( where n is the number of sites poisoned per unit mass of catalyst and n^ is the total number of sites per unit mass of catalyst. A relation between n and q is given by dn/dq=aN o 3.3 where N o is the Avogadro number, All sites are assumed to be equivalent and a is independent of concentration and temperature. Alternatively, it is possible to assume that there are two types of sites, one in which thiophene is chemisorbed only and the other which is active for both chemisorption and hydrogenation of thiophene. In this model, no distinction has been made between these two types of sites. The rate of depostion is linear in the concentration of poison within the pores of the catalyst which however is a function of time and radial position in the pellet as decribed by the equation for the conservation of mass of S : dC I f one assumes in equation 3.4 that the time required to reach steady-state with respect to accumulation of mass in the pores of the catalyst is negligible with respect to the time required for the catalyst activity to change significantly, the rate of accumulation of S in the mass 64 balance equation may be disregarded, thus resulting in a pseudo steady-state system. The diffusivity, D0 , is assumed to be independent of concentration and of the extent of poisoning. The pore diameters of the catalyst samples used in this study range O from 53 to 102 A ( c f . s e c t i o n 5 . 2 ) . The size of an adsorbed molecule of thiophene is calculated to be between 2.9 and * 7.2 A. depending on the stoichiometry of adsorption. The relative size of adsorbed thiophene to average pore-diameter of the catalyst samples would then lie between 0.03 and .14. The above assumption therefore seems reasonable and furthermore, pore-blockage by thiophene should also be negligible. Equation 3.4 therefore reduces to D 3 ?3C — — (r — ) - eBsMkC(1-a)a = 0 3.5 r^dr dr For the general case of combined external and internal mass transfer resistances, the initial and boundary conditions are q (0,r )-0 3.6 i . e . , the catalyst phase concentration of the poison at time t=0 is zero throughout the pellet. dc — (t,0) = 0 3.7 dr i . e . , the gradient of the gas-phase concentration is zero at the centre of the pellet at any time. 65 dC epkm tC gives the equation for the interparticle diffusion of the catalyst ; ep is the void fraction of the catalyst km is the mass transfer coefficient and C is the concentration of the o poison in the feed stream. Equations 3.2 and 3.5 to 3.8 become dimensionless if one introduces S s r / R 3 . 9 ♦ = C / C 3 . 1 0 o 4>=q/qT 3 . 1 1 where q^ is the total amount of poison t a k e n u p b y t h e catalyst in moles per unit mass of c a t a l y s t a t t h e e n d o f t h e experimental run which was terminated w h e n n o further uptake was observed. D e f i n i n g , h = R/( ksMoB/D0 ) 3 . 1 2 0=sMkCt/qT 3 . 1 3 The initial and boundary conditions become : 4>( 0 , E ) = 0 3 . 1 4 --- (0.0) = 0 3.15 as 66 64* epBi [♦ (0,R)-13 ------( 0 , R ) 3.16 where Bi is the Biot number = Rs.,k/DM e In dimensionless form the mass balance equation becomes : 32* 2d* ---- + ------h 2 ( 1 - a ) a 4> = 0 3 . 1 7 BE2 and equation 3.2 becomes 6il> — = ( 1 - a ) a 4> 3 . 1 8 60 The total poison uptake integrating the deposited poison concentration over the entire surface. Thus moles of poison deposited 2 J q . 4 i r r . d r . p = ------3 . 1 9 (4/3)ttR30 where p is the solid density of the catalyst. In dimensionless form equation 3.19 becomes : ) = 3jvl>E2dE 3.20 Equation 3.20 was solved by a NAG routine after Gill and Hiller (1972) which evaluates the integral using a quadrature formula with forward and backward interpolation 67 between successive specified points. 3.1.1 Model I For adsorption with a constant stoichiometry equation 3 . 3 y i e l d s n/nT = q/qT 3.21 which in dimensionless forms becomes a = 3 . 2 2 Equations 3.19 and 3.20 therefore reduces to 3 2 * 26* ---- ♦ ------h 2 ( 1 “4> )a + = 0 3.23 as2 EdE a n d dip — = ( 1 -il>)a* 3.24 dG respectively. Equations 3.23 and 3.24 may now be solved simultaneously, together with the initial and boundary conditions 3.14 to 3.16. The value of a is the experimentally determined overall stoichiometry of nickel to thiophene. 3.1.2 Model II According to this model a certain fraction of the sites are poisoned by thiophene molecules lying flat onto their surfaces, with each thiophene molecule occupying five 6 8 nickel sites ; in the remainder of the surface, thiophene sits perpendicular, occupying two nickel sites per molecule. The equation for the rate of deposition of the species adsorbed in the perpendicular mode may then be w r i t t e n a s 3q 1 ---- = sMk1(f1-a1)a1C 3.25 d t where f^ is the fraction of the total surface that is susceptible to perpendicular poisoning and a1 is the actual portion of the surface that has been so poisoned at time t ; cij is the Ni/C^H^S stoichiometry for the perpendicular mode of adsorption. With similar definitions for fg(si-f ), a2 and , we have for the coplanar adsorption dq 3 . 2 6 dt = sm^2 ^ 2~a2 ^ ^ Let us now consider the perpendicular mode. where the adsorption of a thiophene molecule renders two atoms of the nickel catalyst ineffective for catalysis. Since a1 is the portion of f^ fraction of the surface that is covered by the poison molecules adsorbing perpendicular, the probability of any nickel atom being bare is (f^-a^). The probablity of a 2 group of two atoms being bare is (f^-a^) . The number of unpoisoned f^ adsorption sites effective for catalysis is 2 therefore proportional to (f^-a^) . Similarly, for coplanar adsorption the corresponding term is ^2~a2^5* Now, 69 » 1 =n 1/n T = q 1/q 1T and a2=n2/n2T=q2/q2T. Therefore, introducing the dimensionless quantities (|>.| = q ^ / q ^ j a n d 4>2-q2/q2T we have in equations 3.25 and 3.26 respectively -- 1 = 3-27 de 1 1 a n d 5 i k 2 q 1 T ♦ ------3.28 90 lV * 2 > k 1 q 2 T ^ 1s w h e r e 0 = 1 T The appropriate mass conservation equation is given by r 2 aor (r A dr ) eBsMk1C(f1_ q1T ) ” 0BsMk2C(f2“ q )5 = 0 3.29 2 T and in dimensionless form : 1 d 3$ (Z2— ) )24* - h22 (f2-4>2)5^ = 0 3 . 3 0 d Z where h1= R/{k1sMp0/De ) and h2=R/(*<2sM°8^De * * Equations 3.27, 3.26 and 3.30 may now be solved simultaneously together with the appropriate initial and boundary conditions. 3.1.3 Model III The basic equations involved in models I and II also 70 apply to model III. According to this model, the instantaneous stoichiometry of adsorption Ni/C^s. varies with coverage. The model suggests that initially, when the surface is bare, the thiophene molecules adsorb coplanar. With increasing surface coverage, the oncoming thiophene molecules tend to lie perpendicular to the surface in order to accomodate themselves in the spaces between the coplanarly adsorbed thiophene molecules. As a consequence of this mechanism, the instantaneous stoichiometry of adsorption varies with the coverage. The rate of deposition of thiophene is therefore given by dq n k ( 1--)aC 3.31 dt where a, the instantaneous adsorption stoichiometry is a variable parameter, a can be postulated as a simple function of n or q. It appears unlikely that the instantaneous stoichiometry will depend on the mass of adsorbed thiophene ; it is more likely to be related to the number of available sites. In considering the possible types of variation of a with coverage, the following expressions may be written for a n a L i n e a r o n T Exponential 7 1 a a i l . H y p e r b o l i c 1 + P n / n . In all of these equations the value of aQ will have to be 5 and P will depend on the overall stoichiometry for the particular sample. All three types of variation mentioned above were applied to sample F20 (cf. sec 5.2) and the uptake of thiophene as a function time were simulated. The results are presented in Appendix F and demonstrate that poison-uptake vs. time profiles were virtually identical for the three forms. For ease of computation, the linear dependence was adopted in this study. Although a is expected to be a function of radial position, in this model it is assumed independent of r since the overall behaviour is not expected to be affected significantly by introducing this dependence . Morevever, the added complications in solving the numerical equations do not justify the approach. The equation for the mass conservation of the poison in this model is the same as that of model I except that a is a variable here. Equation 3.3 can be integrated with the appropriate boundary conditions and the particular form of variation of a t o g i v e n a _ vh O - P-*-— i. — = — ( 1 - e p 72 n 1 a 0 i i . — = - l o g ( 1 + p — 4>) Exponential 3 . 3 3 " l 0 a T n 1 5 9 i i i . — = -(-0 ♦ /(0* + 2ao*04>/aT ) H y p e r b o l i c 3 . 3 4 n T a o The above expressions of n/nT in terms of the measurable and/or calculable quantities 4>, aT , aQ and $ is required to solve equation 3.31. The derivation of equation 3.32 is given in Appendix B. Equations 3.33 and 3.34 were derived in a similar way. 3.2 Model Parameters The parameters involved in the models are the rate constant(s) and the effective diffusivity. The procedure for estimating these parameters are outlined below : i. The diffusivity parameter is calculated from the pore-size data using a random pore-model (cf. Appendix C). ii. The rate constant(s) were chosen in such a manner as to obtain the best fit between the experimental data and the model predictions for a particular sample. This value of the rate constant was then used to predict the uptake-time profile of the remaining samples. Model II has the additional parameters fj and f2 which can be calculated directly from the thiophene capacity of the catalyst measured experimentally as follows. The number of atoms of nickel in the catalyst was first determined 73 from the Ni-area of the catalyst determined experimentally ° o and by assuming a value of 6.5 for the nickel atom (cf. section 5.2). The Ni/C^H^S stoichiometry could then be calculated from the number of molecules of thiophene adsorbed, obtained from the total thiophene uptake of the catalyst. The fractions f and f£ were then calculated from the overall stoichiometry (cf.Appendix G). 3.3 Numerical Solution of the equations The equations were solved by a stepwise numerical technique with the appropriate boundary conditions . The mass conservation equation (eqn. A4 Appendix A) was discretized by employing a finite-difference method. This equation, together with the boundary conditions for the concentration of poison within the pores of the catalyst, forms a tridiagonal matrix equation. This matrix equation was solved numerically, simultaneously with the differential equations for the rates of deposition of the adsorbed species. The method of Runge-Kutta-Merson was employed to solve the differential equations. The equations were solved for every radial position simultaneously for each time-step, which was chosen by the variable time-step routine. CHAPTER 4 Experimental The adsorption studies were performed in a thermogravimetric system. In the following sections are described, in order, apparatus, materials, catalyst preparation and the procedures for reduction, BET area and PSD determination, Ni-area measurement, crystallite size determination and the adsorption of the poison. 4.1 Apparatus The system consisted of a Cahn Instruments microbalance with a sensitivity of 10 — 6 g, a BET section and a flow section. The BET section, which consisted of a rotary pump backed by an oil diffusion pump and liquid nitrogen traps, was capable of achieving a vacuum of 1.3x10 - 3 Pa(10 - 5 Torr). Figure 4.1 shows a flow diagram of the apparatus. The vacuum was measured by a McLeod gauge incorporated into the system. A manometer gave the pressure readings. Gaseous nitrogen and carbon monoxide used in the area measurements were stored in glass bulbs attached to the BET section. Poisoning studies were performed via the flow section. Hydrogen was passed through a saturator containing thiophene at a controlled temperature (-30*0.1°C). Figure 4.2 shows a plot of the vapour pressure of thiophene versus temperature. 75 -J CD N 2 ^ ^ S silica gel dryer R reactor D oil diffusion pump O oxygen trap M B microbalance X liquid N2 trap Q gas storage F rotameter MG mcleod gauge B thiophene bubbler N needle valve M manometer P rotary pump FIG. 4.1 Schematic diagram of the experimental system Fig. 4.2 Vapour pressure of thiophene as a function of temperature. The poison stream was joined by a second hydrogen stream, before entering the reactor, to obtain the desired feed concentration. Temperature control was achieved by use of a Beta-Tech refrigeration unit and monitored by a thermocouple. Flow rates of the various streams were attained by use of needle valves and measured by rotameters. An inert stream (N^) was fed to the head of the microbalance to prevent the deposition of sulphur there. The inert flowrate was maintained at 200 cm 3/mm. The poison/hydrogen mixture flowrate ranged from 100-200 77 cm 3 /mm, but was usually maintained at 150 cm 3 /min. A positive pressure differential of ~30 Pa was maintained between the inert flow to the microbalance head and the outlet stream from the reactor to prevent any sulphur deposition on the microbalance head. The degree of uncertainty in poison concentration amounted to a nominal concentration of ±50 ppm. This uncertainty was primarily due to a limitation of the temperature control unit whose effectiveness was 0.1°C The amount of catalyst used for surface-area measurements and pore-size distribution was 0.1-0.15 g. This was chosen as it was found that such a weight of the sample gave reasonably high values of adsorption which coud be read off with a high degree of precision. For poisoning experiments the amount of catalyst to be used was dictated by external mass film resistance considerations. 0.01-0.20g bed weights were tried. The apparent rate of poison uptake increased as the bed-size was progressively decreased from 0.20g to about 0.02gcat. A sample weight of about 0.02 g was found to eliminate external mass transfer effects as shown in section 6.1.5 . The catalyst was placed in a 1cm diameter silica basket which hung at the end of a silica wire suspended from one arm of the microbalance. This was contained by the reactor, whose upper part was made of pyrex and the lower of silica, and was attached to the microbalance head. The upper part of the reactor had a water jacket to prevent the Fig. 4.3 Sketch of the reactor assembly 79 dissipation of heat from the furnace to the microbalance head. Only the lower part of the reactor enclosing the catalyst basket was heated by a furnace, controlled by a Eurotherm temperature controller. The temperature inside the reactor was monitored by a chromel-alumel thermocouple placed between the reactor and the inside of the furnace. The counterweight was suspended from the other arm of the balance. A sketch of the reactor is shown in Fig. 4.3. 4.2 Materials For catalyst preparation the following materials were used : i) A Norton SA6175 "if-AlgO^ extrudate as support, ii) Analytical grade Ni(N03) . 6H20 iii) Distilled water. N2 (9 9 .9 Z) and H2(99.9/) gases supplied by BOC were passed through deoxo units and molecular-sieve/silica-gel dryers to remove oxygen and water respectively. CO of 99.95*/ purity, from BOC, was passed through molecular sieve before being stored in the glass bulbs of the BET system. Thiophene supplied by BDH chemicals was of GPR grade and had a purity of 99+Z. 4.3 Catalyst Preparation The catalyst was prepared by the method of impregnation to incipient wetness. The alumina support was crushed and sieved to obtain particles in the size range of 30-50 mesh(300-500pm). The support was calcined before impregnation of nickel nitrate solution. The treatment 80 consisted in heating in air for 2h at 393K followed by further heating in air at 673K for 16h. After cooling the support under air flow, the pore volume was determined to be .40*.05 cm g“ , using a pore filling technique [Smith (1981), Satterfield (1970)]. A solution of Ni(NO^)g•6H20 in water was added dropwise to a measured weight of *f-Al2o3 in a volume sufficient to fill the pores up to incipient wetness. The 3 preparation was dried in a flow of air (150 cm /min) at 393K for 2h and then calcined for 16h at 673K. The catalyst was cooled under air flow and then transferred to a flask to be stored in a dessicator. Catalysts were prepared in sufficient quantity to allow for use in the whole of the investigation, in order to avoid variations arising from samples prepared in different batches. 4.4 Experimental Procedure The steps involved in the poisoning study are described below : 4.4.1 Reduction Prior to adsorption study, the catalyst samples were reduced in situ by hydrogen flowing at 100ml/min. The temperature was raised to 473K, where it was left for 2h ; it was considered that all the water would be removed by this treatment. The temperature was then raised to 873K at the rate of ~10K/min. The sample was reduced at this temperature for 16h. Figure 4.4 shows a plot of weight loss against temperature during reduction. There is a sharp rise in the degree of reduction as the temperature goes above 300°c . Above 500°c the rate decreases and finally the degree of reduction reaches completion at about 600°C . At 6 0 0° C. the catalyst reaches a stable surface area and is also stable against sintering i.e.( there is no further increase in the crystallite size of the particles. This conclusion was reached from the observation that higher reduction temperatures did not increase the extent of reduction. ) Reduction. T««*ftroAurt, "C Fig. 4.4 Effect of reduction temperature on the degree of reduction of nickel oxide to nickel. 82 4.4.2 BET area and PSD The total BET area of the sample and its pore-size distribution were determined from adsorption-desorption at 77K. The pretreatment consisted in outgassing the sample - ? for 3-4h at 673K under a vacuum of 1.3x10 Pa. The outgassed sample was then brought to room temperature, kept there for 15-20 min and then surrounded by a bath of liquid nitrogen. After allowing about 45 min for the catalyst sample to reach the liquid nitrogen temperature, nitrogen gas was slowly admitted from the glass bulb through an Edwards high vacuum needle valve, until the desired pressure was reached. The first adsorption point -10 kPa(76 Torr) normally took about 20-30 min to equilibrate. The subsequent points on the isotherm took about 15 min to equilibrate. When the adsorption was completed upto a pressure close to p/pQ=1.0, the sample was evacuated through the high vacuum valve using the rotary pump, in a stepwise manner to obtain points on the desorption branch. Pressure measurements were made via a pressure transducer. The computation method used to determine the PSD is given in Appendix E. 4.4.3 Ni-area measurement Prior to metal area determination, the catalyst was outgassed following the same procedure used for the adsorption-desorption. The temperature used, 195K (cf. section 5.2.3), was attained by use of a mixture of dry ice 83 and acetone. The first reading in this case taken at around 1.3 kPa(10 Torr), required about 45 min to equilibrate. In the region of 1.5-25 kPa, the time required to reach equilibrium was about 20 min. Pressures higher than 25 kPa were not used because of the possibility of Ni(CO). formation. The system was then evacuated for 20 min at the adsorption temperature, using the rotary pump ; after which a second isotherm was determined. Points in the second isotherm were taken at the same pressure readings as in the first, by careful control of CO admission via the high vacuum needle valve. The difference between the first and second isotherm on the catalyst sample was calculated. The whole procedure was repeated on an alumina sample which was given the same pretreatment as the catalyst sample. The amount of CO adsorbed intrinsically on the support was then determined. Chemisorption on the catalyst sample was then obtained from the difference between this difference isotherm and the difference isotherm for the support. 4.4.4 Crystallite size determination Crystallite size of the samples were determined by x- ray line broadening. As an independent check, the crystallite size was also calculated from CO adsorption by using a volume-area mean diameter or by the simple assumption of equal-sized cubes or spheres (cf. Appendix 0). 84 After reduction, the catalyst sample was purged in a N2 stream containing 8-10ppm 0^ in order to form a stable layer of oxide on the surface of the catalyst and thereby prevent bulk oxidation of the reduced nickel in the catalyst when the latter is exposed to the atmosphere for XRD measurement. Powder pattern of the catalyst samples were prepared with particle size down to about 300 mesh(50pm). The diffracted peaks when 20 = 44.54 for Ni(11 1 ) and when 20 = 51.89 for Ni(200) (where 0=angle of incidence) were examined for the different samples, to determine the size of the nickel particles by line broadening calculations. The region of two or three degrees on either side of the above values of the angles were scanned. The half-width i.e., the width of the peak at half its maximum intensity, is related to the mean linear dimension 1 of the diffracting particle by the relation l=kA/BCos0, where k is a constant equal approximately to unity, A is the wave length of the O radiation (1.541A), and 0 is the angle of incidence at the maximum intensity of diffraction. Typical calculations are shown in Appendix G. 4.4.5 Poisoning Studies After reduction the sample was left to cool to the poisoning temperature while still under hydrogen flow. It was then kept at the poisoning temperature for about 2h to reach constant weight. During this time the thiophene/H2 mixture of the desired concentration was being prepared via 85 the saturator and was allowed to flow along a by-pass line to reach a steady concentration. When a steady-state was reached in terms of sample weight reading, reactor temperature and feed concentration, the poison was fed to the reactor and the corresponding increase in weight was recorded as a function of time. To avoid sulphur deposition in the microbalance head, a positive pressure differential(-3OPa), as measured by a manometer, was maintained throughout the experiment between the inert N2 stream and the poison stream. The counterweight side of the balance was kept at a constant temperature (0°C)by use of a thermostatic control to avoid spurious changes in weight reading due to changes in room temperature. After about 8-10 h, when no more increase in weight was observed, the physically adsorbed material was pumped off by use of the rotary pump, via the high vacuum needle valve. The system was then brought to atmospheric pressure by introducing hydrogen via the high vacuum valve. The thiophene/H2 mixture was then introduced for a second time and the uptake recorded as function of time. The difference between the two uptake curves was taken to be representative of the chemisorption uptake of the catalyst. An identical set of runs on the support indicated that the amount of thiophene chemisorbed on the support was insignificant. 86 4 - 5 Effect due to variation in room temperature The adsorption measurements are affected by variation in room temperature, because this has an effect on the counterbeam side of the balance ; the sample side of the balance was always held at the poisoning temperature at all times by means of a furnace and a temperature controller. To eliminate the effect due to changes in room temperature, the counterbalance side was kept at a constant temperature by use of a thermostatic control. 4.6 Exit-aas analysis Gas chromatographic analyses of the exit-stream from the poisoning experiment did not detect any butane or butene indicating that at room temperature thiophene was not undergoing any dissociation. This observation is consistent with a molecular adsorption mechanism for thiophene poisoning at room temperature [Lyubarskii et al (1962), Viveros (1985)]. Adsorption at 50 C gave results similar to that at 25 C i.e., there was no elution of butane or other decomposition products. For a run conducted at 100 C, butane was indeed detected, thereby reconfirming the observations of some previous investigators [Bourne et al (1965)], that at such temperatures thiophene hydrogenates on the nickel surface giving off butene or butane. GC results for the exit-gas stream for runs on a sample of F20 (cf. section 5.2) at 100 C and 25 C are shown in Fig. 4.5 and 4.6 respectively. 87 at 100 C on sample F20. 88 Fig 4.6 Exit-gas from run at 25 C on sample F20. 89 4.7 Investigations with 100 ppm Thiophene The present experimental set-up was capable of reaching a concentration of 100 ppm on the low side . However, thiophene uptake data employing a 100 ppm poison stream showed that adsorption in the first thirty sec was 0.04 mg thiophene for a bed size of 0.02 gcat, where the supply of thiophene during that period was 0.052 mg. The rate of thiophene uptake is thus seen to be limited by the supply of thiophene . Therefore, a 100 ppm poison feed was discarded for this study. These results were obtained for a flow-rate of 150 ml/min. Higher flow-rates were not used because this caused the initial recordings of the uptake data to fluctuate and thereby introduced large errors in measurement. Higher rates of flow also had a tendency to upset the catalyst basket at the initial thrust. 90 CHAPTER 5 Catalyst Characterisation 5.1 Literature Survey and Methods The characterisation of catalysts is aimed at providing a basis for interpreting the activity and selectivity of a catalyst in terms of its physical and chemical properties. Many techniques are used for the characterisation of catalysts, among which, the determination of the apparent and solid density by pycnometry and the determination of the pore-size distribution by nitrogen capillary condensation and mercury porosimetry have been standardized. Selective chemisorption is used to determine the metal surface-area. X-ray diffraction, magnetization measurements or electron microscopy is used for estimating the mean particle size. Baiker (1985), in an excellent review of catalyst characterisation, discussed the various methods, their applicability and the underlying physical principles. 5.1.1 Total Surface Area and PSD Determination of the number of gas molecules required to cover the surface of a solid with a monolayer of adsorbate forms the basis for measuring the total surface area of the solid. The total surface area is computed from the equation V n A m oAm BET 5 . 1 mol where Vm is the volume of the monolayer determined by the BET method, N^ is the Avogadro number, Am is the mean cross- sectional area occupied by one molecule of adsorbate gas and Vmol *s the molar volume of the gas. The cross-sectional area of an adsorbed gas molecule is usually estimated from the density of the condensed phase of the gas [Emmett and Brunauer (1967), Livingston (1949)], where certain assumptions are made about the geometrical arrangement of the molecule in the adsorbed layer. For reliable measurements small gas molecules having spherical structures are used. Nitrogen, having a cross- sectional area of 0.162 m 2 per molecule, is usually employed. Krypton or argon are used when surfaces of a few square meters must be determined accurately. Nitrogen, krypton, argon and carbon dioxide have low saturation pressures at the usual measurement temperature of 77 K. A substantially low gas pressure is thus required in the volumetric measurement apparatus in order to measure with improved accuracy the fraction of gas molecules adsorbed within the suitable range of relative pressure 0.05 < p/PQ < 0.30 , where PQ is the saturation pressure. Apparatus used for the measurement of total surface area and PSD are mainly of two types : volumetric, in which the amount of gas adsorbed is determined by measuring the decrease in pressure of the adsorbate gas in the system 92 [Gregg and Sing (1967), Innes (1968), Broekhoff and Linsen (1970)] and gravimetric, in which either quartz spring balances or commercially available electronic microbalances are used. In addition, there are the so-called dynamic flow methods which are used less frequently. In the continuous flow method [Eberly (1961), Benesi et al (1971)], a mixture of carrier and adsorbate gases is introduced, and the concentration of the adsorbate gas is measured before and after contact with the solid using a suitable detector. In the pulse method [Eberly (1961), Hansen and Gruber (1971)], pulses of the adsorbate gas are introduced into a continuous stream of carrier gas. The method of determination is the same as in the continuous flow method. The total surface area may also be determined by another method after Lippens and De Boer (1964,1965), commonly known as the "t-method". These investigators found that the thickness of the adsorbed layer is independent of the solid and depends merely on the relative pressure p/pQ and the geometrical arrangement of the molecules in the layer. This finding forms the basis for the "t-method". In addition to determining the surface area, the "t-method" affords the possibility of obtaining certain information about the types of pores in the solid. Another method, known as the as~method, proposed by Sing (1968,1970) is also used for measuring the total surface area and i s suitable for verifying the presence of micropores. 93 Brunauer et al (1970) have modified the "t-method" to determine the volume and the size distribution of micropores. The adsorption of a gas on the surface of a porous solid is frequently superimposed on condensation in the pores. A distinction is made between condensation in micropores and true capillary condensation in mesopores. The size distribution of the micropores is found from the decrease in the slope of the va(t) curve. The hysteresis part of the adsorption and desorption isotherm contains information about the mesopores. A relationship between shape and position of the isotherm and pore geometry, due to condensation and evaporation is given by Kelvin's capillary condensation equation 2*y VC o s 0 r = ------5.2 RTln(p/p ) o In calculating the pore radius by using the Kelvin equation, it is necessary to take into consideration the thickness t, of the adsorbate layer. The mesopore radius is determined using the data from either the adsorption or the desorption branch of the isotherm and the Kelvin equation. There is a disagreement in the question of whether the adsorption or the desorption branch is better suited for calculating the mesopore size. It can only be assessed if the geometry of the pores is known. If the pore-shape can be described in a suitable fashion by the Kelvin equation, then the pore-radii determined from the desorption and adsorption branch are comparable. For a symmetrical pore geometry, calculation of 94 the size distribution of the mesopores from adsorption or desorption data permits a simple determination of the mesopore surface area. The gas volumes adsorbed or desorbed upon a change of the relative pressure are taken from the isotherms, and the Kelvin equation is used to calculate the corresponding mesopore radius. Assuming a certain pore geometry, the contribution to the surface area from the pores of various sizes can be found from the pore-radius distribution. Step-wise computational methods for finding the pore-radius distribution and the mesopore surface area are described in detail elsewhere [Gregg and Sing (1967), Broekhoff and Linsen (1970), Dollimore and Heal (1970), Pierce (1953), Roberts (1967)]. 5.1.2 Metal Area The degree of dispersion is an important property of supported metal catalysts since it can affect both the selectivity and the activity of the catalyst. The degree of dispersion is defined by the ratio of surface atoms to the total number of metal atoms and can be calculated if the number of surface atoms and the weight fraction of the metal catalyst is known. The metal area of a catalyst is determined by chemisorption of a suitable gas and using the same principle as with total area determination. The amount of gas required to cover the metal with a monolayer of adsorbate is determined volumetrically, gravimetrically or by the dynamic flow method. The number of accessible surface 95 atoms of the metal is calculated from Vmol where x_m is the mean stoichiometric factor of the chemisorption process and indicates the number of surface atoms of the metal that are covered by one molecule of adsorbate in the chemisorption. The determination of the chemisorbed layer is complicated by the presence of physisorbed adsorbate, particularly when the chemisorptive bonding with the surface of the solid is weak. In practice, after the first isotherm is determined, the catalyst sample is evacuated for a given time, at a temperature which is dictated by the particular metal/adsorbate-gas combination, during which the physisorbed adsorbate is desorbed from the surface. The second isotherm which now corresponds to physisorbed adsorbate only, is determined next. The amount of chemisorbed adsorbate is then determined from the difference between the two isotherms. Various factors that influence the accuracy of the chemisorption methods include the stoichiometry of chemisorption, the crystallographic heterogeneity of the surface, interaction of the support, possible absorption of the adsorbate gas in the metal, surface reconstruction during chemisorption and adsorption of contaminants on the surface. The stoichiometric factor can be determined by comparing the particle size of the catalyst determined by x- 96 ray diffraction line broadening or electron microscopy with that determined from the measured metal surface area of the catalyst assuming a particular regular geometric shape of the metal particles. Usually H2, CO and 02 are used as adsorbate gases for chemisorption measurements ; N2o is used less frequently. That H 2 is adsorbed dissociatively i.e., with a stoichiometric factor of H2:Ni = 2:1, is evidenced by numerous studies [Schuit and van Reijen (1958), Brooks and Christopher (1968), Shephard (1969), Yates et al (1964)3. The utility of CO adsorption on alumina supported Ni was demonstrated by Hughes et al (1962). They also point out the uncertainties in measurements using CO at room temperature due to Ni(CO)^ formation. In the chemisorption of carbon monoxide there is an uncertainty with respect to the stoichiometric factor. However, the investigations of Viveros (1985) showed that at -78°C carbonyl formation did not occur and reproducible values for the metal area could be obtained with a stoichiometric factor of 1.4 (see section 5.2.3). 5.1.3 Crystallite Size bv X-rav methods Metal crystallite size is one of the most important properties of the metal catalyst, since it is a measure of metal dispersion i.e., the number of sites available in the catalyst. There are two main methods for the estimation of mean particle size which use x-ray techniques. These are : i) diffraction line broadening which makes use of 97 information contained in the peak shape of one or more diffraction lines from the metal and ii) low angle scattering in which all the particles in the divided solid contribute, but the scattering power of a component is dependent on its chemical nature. The x-ray line broadening method is based on the fact that an x-ray diffraction line due to a metal crystallite broadens when the crystallite size is decreased to a value 0 less than about 1000 A. For such small metal crystallites, the size 1 is related to the diffraction line width, by the Scherrer equation 1 = kA/BdCosG 5.4 Equation 5.4 gives the mean dimension of crystallites in a direction perpendicular to a lattice plane. The method is * e valid in the range of 50 to 500 A. Below 50 A, the lines are O too diffuse and above 500 A the lines are too sharp for accurate resolution. Various factors may contribute to the observed peak width. These range from instrumental to matters such as strain, and stacking and twin faults. For a supported metal catalyst it is probable that only the instrumental width will have any significant contribution towards the measurement of the particle size. Assuming Gaussian shapes for the lines, the line width Bd due to particle size broadening is given by 98 = B 5.5 obs - Bins t where BQbs is the observed width and B^nS£, the instrumental line width is obtained by a calibration procedure using a material consisting of large crystals of good crystalline perfection. Sodium chloride or quartz powder MOOnm < d < 1000nm) is often used. An alternative method for correction due to instrumental line broadening has been given by Jones (1938) and Klug and Alexander (1954). Having evaluated Bd , equation 5.*f may now be used to calculate the mean crystallite diameter 1. The value of k depends on how the peak width is measured. If the width is measured at half the maximum height, k takes values between 0.84 and 0.89 depending on the assumed particle shape. In the absence of detailed knowledge on this point, a spherical particle is assumed. An alternative method for the measurement of peak width is to divide the integrated peak area by the peak height. On this basis k takes values between 1.00 and 1.16 depending on particle shape, and a value of unity is commonly used. In the simple case of particle size broadening only, the simple method of using the width at half peak height is the obvious choice. There are some practical precautions which must be observed in the measurement of the peak width. In the first place, there may be sensitivity limitations with supported metal catalysts if the metal loading is low. When working with a multi-component system such as a supported catalyst it is essential to select a metal diffraction peak that is 99 well separated from peaks which may originate from the support. This is often a serious limitation on the number of metal peaks which can be used. For supported catalysts of metals such as platinum, palladium and nickel, the (111) reflection has often been used but others are also possible. However, the sensitivity is considerably lower when using, for example, the (200) line compared with that of the (111) line for nickel. Finally, sample preparation deserves care. Supported catalysts should be finely ground to about 300 mesh and the powder spread on the sample holder, care being taken that the surface of the powder sample is flat. Unsupported metal powders are spread in the same way. The absolute accuracy with which the average crystallite size can be obtained by line broadening methods should not be overestimated ; the influence of particle shape and size distribution factors probably limits this accuracy to about ±15%. The reproducibility of the measurements, however, is within 62. 5.2 Results and Discussion Ni/C^H^S stoichiometric ratio of around 3 for poisoning at room temperature, obtained on a sample of a 20 wt2 Ni/'y-AlgO^ catalyst indicates a combination of perpendicular and coplanar adsorption mechanism for thiophene adsorption on nickel, with two and five nickel sites respectively per adsorbed thiophene molecule. This hypothesis assumes that thiophene so adsorbed is in the 1 00 molecular state and that there is no decomposition or hydrogenation of thiophene. This aspect of thiophene poisoning will be discussed in detail in chapter 6. In this section are presented the results of this study which was aimed at elucidating information to further support the hypothesis presented above. With this hypothesis it is conceivable that the number of nickel sites occupied by a thiophene molecule will depend on the size of the nickel crystallites. To investigate this aspect of thiophene adsorption, poisoning studies were performed on catalysts of varying crystallite size. The crystallite size variation was effected either by sintering the catalyst at different temperatures or by varying the nickel loading of the catalyst. In this study the different catalyst samples were characterised in terms of their total surface area, pore- size distribution, nickel area and nickel crystallite size. The difference in these properties of the catalysts resulted from a difference in the temperature at which they were sintered or from a difference in the metal loading of the catalyst. Below is given a nomenclature of the samples according to their metal loading and/or their degree of sintering as determined by the different sintering temperatures. 101 Table 5.1 Sintering temperature of the different samples Sample Loading X Sintering Temperature, F 1 0 1 0 (un sintered) F 1 5 1 5 (unsintered) F2 0 20 (unsintered) S1 2 0 650 S 2 20 700 S3 20 750 S 4 20 780 All catalysts samples were reduced in situ at 600 C for 16 h, as described in section 4.4.1. Sample F20 corresponds to a 20 wtX unsintered catalyst. F10 and F15 corresponds respectively to 10 and 15 wtX unsintered catalyst. The sintered samples S1 to S4 were prepared by sintering of F20 in flowing hydrogen for 50 h at their respective sintering temperatures. 5.2.1 Total Surface Area The full adsorption-desorption isotherms for the different samples indicate a type IV behaviour. Two such isotherms are shown in figure 5.1. The hysteresis loop in figure 5.1(a) which is for sample F20, would be designated as type’A * ,after De Boer ( 1 958). The catalysts would then correspond to porous materials with cylindrical pores of 102 GO l Nitroftn tdiorbtd , of 49 3$ 39 o Maple nt i 9.1334 g .99 .19 .29 .39 .49 .59 .59 . 79 . 99 . 99 1.99 p/po Fig; 5.1(b) Nitrogen idsorptiorrdeiorption isothere Maple S3 constant cross-section. The adsorption-desorption isotherms for samples F10, F15, S1 and S2 are similar to that for F20. The isotherm 5.1(b), which is for sample S3, is of type ’E', in which case the catalyst corresponds to tubular pores with a narrow constriction or closed pores of the ink-bottle type. The isotherm for sample S4 is virtually indistinguishable from that for S3. The reason for the highly sintered samples S3 and S4 to exhibit type 'E' behaviour becomes apparent when one takes into account their crystallite sizes (cf. section 5.2.4) and their pore sizes (cf. section 5.2.2). The average pore-diameter for the unsintered sample or where substantial sintering has not © occured is ~100 A. The crystallite size of these samples are O about 30-70 A (see Table 5.7). The crystallite size of O samples S3 and S4 are about 80-90 A. It is therefore possible that the large crystallites of S3 and S4 are causing physical blockage of the pores, resulting in ink- bottle- shaped pores. This also explaines why these samples have lower average pore radii (see Table 5.^). The hysteresis loops show that in the case of samples S3 and S4 the pores begin to fill when the pressure as calculated by the Kelvin equation (5.2) reaches the value corresponding to the radius rm (cf. Fig. 5.2). It should then continue to fill up, the meniscus gradually increasing in radius and the pressure gradually rising in correspondence. The pores fail to empty until the pressure falls to the value corresponding to rmand then empties at once. The desorption branch is therefore much steeper than the adsorption branch. 105 According to De Boer (1958), in this case the adsorption branch corresponds to equilibrium and should be used for calculations of pore-size distribution. Fig. 5.2 Ink-bottle type pores The total surface area of the catalyst was determined by the BET method using N2 as the adsorbent. N2 adsorption isotherms were obtained by adsorption of the gas at 77 K on reduced samples of the different catalysts, and the BET equation in its linearized form was used to calculate the total surface area. For the determination of the monolayer uptake a linear regression analysis was applied to the sets of data of p/n(pQ-p) vs. p/pQ. Adsorption values employed for the calculation of the surface area was within the range 0.05 < p/pQ < 0.35 as recommended in the literature [Gregg and Sing (1967), Anderson (1975), Lecloux (1981), Sing (1980)]. A typical linearized plot of p/n(pQ-p) vs. p/pQ is shown in figure 5.3. The area of the nitrogen molecule was o 2 taken to be 16.2 A . The total surface area for all the 1 06 did not result in any significant change in these values. Longer reduction times than used as a standard in this study after the reduction of the samples had good reproducibility. p/n(po-p) samples are summarized in Table These 5.2 . values obtained Fig. 5.3 A linearized plot of p/n(pQ-p) vs. p/pQ ; SampleF20 107 Table 5.2 Total BET surface area Sample SBET / g F 1 0 159.2 F 1 5 155.7 F 2 0 152.8 S 1 15 1.1 S 2 149.5 S3 147.6 S 4 14 6.3 The surface area values in the above table are based on the weight of the reduced sample. Viveros (1985) has shown that for the high weight uptakes on Ni-alumina samples, such as those used in this study, corrections due to buoyancy effects were unnecessary. Prior to surface area determination, the sample was outgassed by heating in vacuum (10 - 4 Torr) at 673K for 3 to 4 h. At the end of this period the weight of outgassed sample was constant indicating that no further degassing was taking place. Such a pretreatment gave reproducible values of surface area and PSD for all samples. Shorter evacuation times or lower degassing temperatures resulted in a loss of reproducibility. 5.2.2 Pore-Size Distribution The pore-size distribution of the catalyst was calculated following the procedure outlined by Gregg and Sing (1967). The thickness of the adsorbed layer, which 108 varies according to the relative pressure was calculated from the correlation t= oC5/In(p^/p)]1^3 using a value of 3.54 A for a, the effective cross-sectional diameter of as recommended by De Boer (1958). This correlation was chosen because of its simplicity, reported accurate results obtained with it [Dollimore and Heal (1970)] and its wide use in the literature. Pore-size data were calculated using both the adsorption and the desorption branches. The PSD plots i.e., the plots of dv /dr vs p/pn and rp, for some of the samples are shown in figure 5.4. The pore-volume distribution function dvp/drp peaks at around rp = 55 A for O the first five samples and at rp = 31 A for samples S3 and S4. The calculations were carried out using the adsorption branch for samples S3 and S4 for reasons discussed below. For the first five samples the desorption branch was employed. Tables 5.3 and 5.4 give the pore-size data using the desorption and adsorption branches, respectively. Table 5.3 Pore-size Data (Desorption branch) Sample p(cum)de s Vp(cum)des rp(ave)des o m2/g cm3/g A F 1 0 166.3±2 0.417 5 1.3 F 1 5 163.1 0.395 50.6 F 2 0 160.5 0.383 50 . 1 S 1 157.4 0.368 48.7 S 2 154.4 0.356 47.6 S3 234.5 0.236 32.0 S 4 215.8 0.224 30.6 109 Fig. 5.4(a) Pore-size Distribution ; sample S1 sample ; Distribution Pore-size 5.4(a) Fig. 1 1 0 1 ol 2 ---- 1 103-3 ---- 1 ---- 1 ---- 1 ---- 1 1 ----- 1 ---- 1 r t • 1*. 0 243 352 30 4 M 523 C52 ------20*6 — i size Distribution ; sample S4 sample ; Distribution size r- 0*4 ------i 13-5 13-5 1U 0*3 Fig. 5.4(b) Pore 5.4(b) Fig. o r ! 01 0 2 3 4 o- £ 111 Table 5.4 Pore-size Data (Adsorption branch) Sample v ^(cum)ad s p(cum)ad s rp(ave)ad s O m2/g cm3/g A F 1 0 1 51 . 4±4 .421±.007 51 . 8± . 5 F 1 5 153.5 .398 5 1.1 F20 150.3 .387 50.7 S1 155.6 . 38 50 . 2 S 2 155.0 . 363 48.6 S3 126.8 . 195 26.6 S 4 123.3 . 195 26.4 In calculations of pore size distribution data from type IV isotherms by use of the Kelvin equation (5.2), the region of isotherm involved is the hysteresis loop, since it is here that capillary condensation is occuring. The formation of a liquid phase from vapour at any pressure below saturation cannot occur in the absence of a solid surface which serves to nucleate the process. Within a pore, the adsorbed film acts as a nucleus upon which condensation can take place when the relative pressure reaches the figure given by the Kelvin equation (5.2). In the converse process of evaporation, the problem of nucleation does not arise : the liquid phase is already present and evaporation can occur spontaneously from the meniscus as soon as the pressure is low enough. It is because the process of condensation and evaporation do not necessarily take place 1 1 2 as exact reverses of each other that hysteresis can arise. The question necessarily arises as to which of the two values one should use in the Kelvin equation in order to calculate the value of r corresponding to the particular adsorption value. There have been a number of attempts to explain the difference which obviously exists between the state of the adsorbate along the two sides of the loop. Since the relative pressure corresponding to a given adsorption is lower along the desorption branch, on thermodynamic grounds the chemical potential of the adsorbate is lower if one ignores changes in the solid. Accordingly the desorption branch is more likely to correspond to a condition of true equilibrium. In the case of type E isotherms, however, as recommended in the literature [de boer (1958), Gregg and Sing (1967)3 , the adsorption branch should be used for calculations of pore- size distribution. The question of whether the adsorption or the desorption branch is better suited for pore-size calculations must however be answered from a discussion of the origin of the hysteresis and must be based on actual modes of pore shape, since a purely thermodynamic approach cannot account for two positions of apparent equilibrium. In this study, the suitability of the adsorption or the desorption branch in calculating the PSD was determined by comparing the cumulative surface area calculated from PSD with the BET surface area. Tables 5.3 and 5.4 show that both the adsorption and the desorption branches give values which are comparable with the SBET values, in the case of samples F 1 0 , F15 , F 2 0 , S1 and S2. In the case of samples S3 and S4, however, the desorption branch is found to be unsuitable when one compares the S0ET values in Table 5.2 with the S(cum)des values in Table 5.4. Therefore the adsorption branch was used for samples S3 and S4. The adsorption branch was not used for the first five samples since the S, . . values are inconsistent with the BET surface area values in view of the fact that according to the adsorption branch calculations S1 has the highest area followed by S2, F15, F10, F20. The BET surface area values are in the following order : F10, F15, F20, S1. S2. 5.2.3 Nickel Area The nickel surface areas of the different catalyst samples were determined from CO chemisorption isotherms. The isotherms were obtained by adsorption of CO on the samples at 195 K. The choice of 195 K as the adsorption temperature was made from the findings of Viveros-Garcia (1985), who investigated CO adsorption on nickel/^-alumina at three different temperatures. Viveros-Garcia (1985) found that at 77 K the physisorption component of the isotherm was very high and therefore rendered the determination of the chemisorption uptake very susceptible to errors since it involved a small difference between two large quantities. At 273 K the reproducibility was poor and the intermediate evacuation between the first and the second isotherm showed a loss of weight beyond the initial value indicating the 1 1 4 formation of volatile nickel carbonyl. At 195 K, the physisorption component was small and there was no indication of nickel carbonyl formation. The magnetization measurements of Primet et al (1977) showed that at 195 K, as CO coverage increases the nature of the chemisorbed species is almost unchanged which results in a constant ratio of linear to bridged bonding. Values of 1.25-1.43 have been used by various investigators [Primet et al (1977), Klier et al (1970), Tracy (1970), Madden et al (1973), Conrad et al (1976), Bartholomew and Pannell (1980)] for high nickel loading and single crystal faces. Klier et al (1970) found that cleaned annealed surfaces of the single-crystal nickel faces exhibit well-defined reproducible adsorption and uniform binding of carbon monoxide. Both the (110) and the (100) planes were found to accomodate the same number of CO molecules (1.11-1.13 x 10 1 9 o molecules/m ) , although they have considerably different densities of nickel atoms, 1.14 x 10 19 atoms/m 7 on the (110) face and 1.61 x 1019 atoms/m2 on the (100) face. This observation indicates that the saturated value of adsorption is dictated by the lateral repulsion among the CO molecules rather than by the number of underlying nickel. This idea is clearly supported by the LEED and AES investigations of Tracy (1970) and Madden et al (1973). Conrad et al (1976) found the same density of CO molecules on the (111) face of nickel. The constant saturation coverage of 1.1 x 10 19 molecules/m 2 corresponds to 0.7 of a monolayer and is equivalent to an area of 9. 1 A° 2 per CO 1 1 5 molecule. The site density for nickel is 6.5 A‘/atom based on the arithmetic average of the planar densities of the (100), (110) and (111) planes. The Ni/CO stoichiometry is then calculated to be 1.40 . It is also important here to consider the possibility of the Ni/CO ratio varying with the crystallite size. A few studies exist in the literature where the effects of metal crystallite size on CO adsorption on nickel have been considered. Yates and Garland (1961) concluded from their infra-red and X-ray diffraction studies that CO is adsorbed as a single linear species on low loading Ni/Al2o3 and as a bridged species on high loading Ni/Al203. Their work does not provide a quantitative measure of the effects on the CO adsorption stoichiometry. The morphology of the catalysts i.e., metal crystallite size, degree of reduction,etc. were not reported. Primet et al (1977) on the other hand concluded from their infrared and magnetisation studies on Ni/Si02 catalysts that for completely reduced samples the ratio of linear to bridged species was constant, regardless of metal crystallite size. Their observation of a constant adsorption stoichiometry on a 16 wtZ catalyst reduced at temperatures of 500 650 and 950 C indicates that crystallite size variations caused by sintering does not affect the Ni/CO ratio. On partially reduced samples , however, they observed a decrease in the concentration of bridged species, and reported the presence of a third band attributed to subcarbonyl nickel species. This band was not 1 1 6 observed in the case of completely reduced samples. These observations are in agreement with the results of Yates and Garland (1961) who reported that on catalysts with 1 0 — 25Z nickel, the band due to subcarbonyl nickel was absent whereas they appeared on the lower loading catalysts. The band at 2070-2090 cm 1 due to subcarbonyl nickel was also reported by several other workers [Rochester and Terrell (1977), Primet et al (1977), Yates and Garland (1961), van Hardeveld and Hartog (1972), Derbeneva et al (1975)] on well-dispersed, low-loading catalysts. Primet et al (1977) hypothesized that the decrease in the concentration of bridged species suggests that the unreduced residues decrease the probability of having adjacent Ni atoms on the surface. Bartholomew and Pannell (1980) found that the Ni/CO values for Ni/Al2o3 ranged from 0.53 for 3l Ni/Al203 (d = 44 ° s A, f =.6 4) to 1.25 for 23 l Ni/A1203 (d = 63 A, f=.94) to 1.82 for 100Z Ni (d=20000 A), where d is the crystallite size of nickel and f is the fraction of reduction. Their results thus agree qualitatively with those of Yates and Garland (1961) and with the results of Primet et al (1977) on partially reduced samples. Their work did not include any investigation on completely reduced samples. Although not pointed out, intrinsic in these observations is a lower extent of reduction on the lower loading catalysts. This is evidenced in the results of Bartholomew and Pannell (1980). While studying the effect of weight loading on the adsorption and reaction properties of Ni/Al2o3 catalysts, Kester et al ( 19 8 <3) found that the role of the support, 1 1 7 which interacts sufficiently strongly with the nickel ions during preparation so that reduction to the metal is more difficult, is to create two distinct types of Ni sites : nickel crystallites and less reactive nickel atoms surrounded by oxygens of the alumina lattice. They found that both sites are present at Ni loadings 1 .8 — 15%, but the fraction of the less active sites is much larger at lower loadings. Because alumina has a greater capacity than silica for nickel ions during preparation by impregnation [Dzisko et al (1975)] alumina interacts more strongly with nickel ions than silica does. In studying this interaction using XPS and ISS, Wu and Hercules (1979) observed three types of Ni on impregnated and calcined Ni/Al203 catalysts, but only one type on Ni/SiOg. They also found that not all the Ni on alumina was reducible, and that the distribution among the three types of Ni varied with the weight loading and calcination temperature. In addition, the percent reduction of Ni/Al203 catalysts increased with increased Ni loading, while the percent reduction of Ni/Si02 catalysts was essentially independent of Ni loading. This explains why lower loading catalysts resulted in lower extents of reduction in the work of Bartholomew and Pannell (1980) on Ni/Al203 catalysts while Primet et al (1977) were able to obtain complete reduction of their Ni/Si02 samples. Bartholomew and Pannell (1980) considered CO adsorption on the Ni 2 + or NiO sites. If one molecule of CO adsorbed on each Ni 2 + site, CO/H ratios would increase dramatically with 1 1 8 decreasing extent of reduction, since hydrogen chemisorbs to a relatively small extent on NiO or Ni2 + compared to the metal [Pannell et al (1976)]. Such an effect could by itself explain the decreasing Ni/CO values with decreasing percentage reduction. However, two separate pieces of experimental evidence suggest that the quantity of CO adsorbing on unreduced sites in these catalysts was probably negligible relative to CO adsorption on the metal : (i) in the investigations of Bartholomew and Pannell (1980), CO adsorption on a sample of 3l Ni/Al2o3 calcined in air at 573K for 2h was determined to be the same within experimental error as on the support, and (ii) Primet et al (1977) observed that the peak at 2195 cm"^ corresponding to adsorption of CO on Ni 2 + was observed only in samples with less than 5l of the nickel reduced to the metal; moreover, the area of the peak at 2195cm 1 for the sample having a degree of reduction of 5l was approximately 2 orders of magnitude lower than the peak areas for CO adsorption on nickel. It appears from the above analysis that the reported investigations of CO adsorption on nickel has failed to separate completely the effects of metal particle size, metal loading and the extent of reduction on the Ni/CO ratio. In this study the variation of Ni/CO stoichiometry with crystallite size or loading was checked by comparing the values of the crystallite size measured by XRD and by CO chemisorption. Assumption of a Ni/CO ratio=1.4 gave constant ratios (cf. Table 5.5) of the crystallite size by XRD to the 1 1 9 crystallite size by CO chemisorption for samples F15.F20,S1 ,S2 , S3 and S4 . For sample F10 this ratio varied by Table 5.5 Ratio of Crystallite sizes bv CO to XRD Sample Ratio F 2 0 2.14 S 1 2.23 S 2 2.38 S3 2.3 S 4 2 . 28 F 1 5 2.09 F 1 0 1 . 76 an amount which was outside the limits of experimental error. For this sample a value of 1.16 for the Ni/CO ratio was found to give the same ratio of crystallite size by XRD to crystallite size by CO chemisorption as with other samples. The Ni-area of sample F10 was therefore re evaluated by taking a value of 1.16 for the Ni/CO stoichiometry. It appears therefore that the stoichiometric factor for CO adsorption on nickel varies with the crystallite size when the nickel loading of the catalyst is o low (10/. or lower) so that the crystallites are - 30 A or lower (cf. sec 5.1.4) ; but for higher loadings the Ni/CO ratio is constant. Typical CO adsorption isotherms are shown in figures 5.5 and 5.6. The intrinsic adsorption on the support was 120 FI* 5.5 CO adsfrptim «n aluaini «n adsfrptim CO 5.5 FI* 121 14 P. kPa Flf. 5.1 Typical CO adstrption isathma m Ni/Aluaina sayli Fit determined from a separate experiment on the support, which was given the same treatment in terms of reduction and/or sintering as the corresponding catalyst. The amount of adsorption on the support amounted to only 16-18 pmol/gcat. Fig. 5.5 which shows the CO isotherms on an alumina sample consists of three curves : the total adsorption (first isotherm), physisorption (second isotherm) and intrinsic adsorption or chemisorption (difference isotherm). The CO adsorption isotherms in figure 5.6 (for the catalyst) represent : the total adsorption (first isotherm) , the physisorption (second isotherm) , and the chemisorption isotherm (difference isotherm). The method for determining the chemisorption isotherm which takes into account the intrinsic adsorption on alumina was described in section 4.4.3. The difference between the first and the second isotherms for the catalyst was considerable enough so as to introduce very small errors in evaluating the chemisorption uptake. The reproduciblity was good : the error in determination was less than 6 per cent. There was no loss of weight beyond physically adsorbed material during the intermediate evacuation ; thus nickel carbonyl formation was not taking place at 195 K. Table 5.6 below lists the CO uptake, nickel area, extents of reduction and percent metallic dispersion of the various catalysts. The methods of determining the extents of reduction and the metallic dispersion are detailed in Appendix D. 123 Table 5.6 Nickel Area of the Catalysts Sample CO uptake Ni-area % Reduction l Dispersion pmol/gca t fn2/gcat F 1 0 161(133 • S)1 0.82 ( 7.3 1 ) 1 76 1 4 . 4 F 1 5 172.9 9.47 87 ,. 7 1 0 ,. 8 F 2 0 194 .6 10.66 98 .. 6 8 . 1 S 1 17 1.3 9.38 97 ,. 6 7 ,. 2 S 2 146. 3 8.01 94 6 ., 4 S3 133 .7 7.32 96,, 8 5 ., 65 S 4 116.9 6.40 96 5 ,, 0 (1) The value within brackets shows the CO uptake and Ni- area corrected for Ni/CO stoichiometry change. 5.1.4 Mean Crystallite Size Some experimental difficulties were experienced in determining the average crystallite size by XRD because of the interference of the support. It was found that the most prominent peak for Ni(111) was almost entirely obscured by a broad peak for *f-alumina. To obviate this problem the crystallite size was determined from the second most prominent peak for Ni(200), which, for the catalyst loading used in this study was of moderate intensity and most importantly, free from any support interference. Typical x- ray line broadening results are shown in figure 5.7. 124 Fig. 5.7 X-ray line broadening charts (a) Ni(111) with interference from peak due to alumina (b) Ni(200) - no interference from support. 125 Table 5.7 lists the nickel crystallite size of the different catalyst samples as measured by X-ray line broadening and by CO chemisorption. Crystallite size from CO Table 5.7 Crystallite Size Sample Crystallite size /Relative \ “1 Relative by Crystallite] Nickel XRD CO chemisorption \Size J Area (1) (2) by 0 < 9 • XRD CO A A ( 1 ) (2) F 1 0 33 58 ( 70 ) 3 3 1 ( 36 ) 3 - --- F 1 5 45 94 49 ---- F 2 0 59 126 65 1 . 0 1 . 0 1 . 0 1 . 0 CD S 1 64 143 74 . 88 CD . 88 .92 in S 2 7 1 168 83 r- .75 . 78 . 83 S3 80 184 93 . 69 .685 . 70 . 74 S 4 92 2 1 0 1 07 . 60 . 60 . 6 1 . 64 ( 1 ) Spherical equivalent volume-area mean diameter of crystallites of irregular shapes and sizes. (2) When the crystallites are spheres of the same size. (3) The values within brackets show the crystallite size by CO corrected for Ni/CO stoichiometry change. chemisorption was calculated in two ways. In the first instance, the crystallites were assumed to be of varied irregular shapes and the size was calculated as a spherical equivalent diameter (cf. Appendix D). The type of mean adopted here is the volume-area mean which, in the spherical particle equivalent approximation is related to the metallic dispersion, a quantity that could be determined from 126 catalyst characterisation. In the rare event, where the crystallites are of a specified geometric shape and are all the same size, the crystallite size is given by l=g/gs, where g is the density of the solid, s the metal area of the catalyst and g is a constant the value of which depends on the geometric shape of the crystallites. In the case of spheres g = ir. Also shown in the table 5.7 are the inverse of the relative crystallite size of each sintered sample compared to the crystallite size of the unsintered sample and the nickel area of each sintered sample relative to the nickel area of the unsintered sample. Crystallite shape and size distribution factors probably limit the accuracy of crystallite determination to about 30 per cent and also to a 6 per cent error in nickel area measurements. However, the reproducibility of crystallite size measurements by XRD was within 67.. The metal dispersion values shown in Table 5.6 were determined in order to compute the crystallite size of the samples from CO adsorption (cf. Appendix D). In Table 5.7 , the relative crystallite size is the ratio of the crystallite size of the sample to the crystallite size of the unsintered sample F20 and the relative Ni-area is the ratio of the Ni-area of the sample to the Ni-area of the unsintered sample. Values of the volume-area mean diameter and the equal sized spheres obtained from CO chemisorption ranged from 58 • O to 202 A and 31 to 107 A respectively, in comparison to the values of 33 to 92 A obtained by XRD. The ratio of the 127 crystallite size determined by XRD and by CO chemisorption was constant for the different samples with the exception of sample F-10 as explained in the previous section. The values of the volume-area mean diameter were, on average, higher by a factor of 2.18. In the case of spheres all the same size, this ratio was 1.14 again with the exception of sample F10. It is clear from the present study that although crystallite sizes determined by XRD and by CO chemisorption may not give the same values, there is a definite correspondence between the two as long as the stoichiometry of CO chemisorption does not change with the variation in crystallite size. The estimates of Brooks and Christopher (1968) of the nickel surface area obtained from CO chemisorption and by XRD varied by as much as 200-300 per cent. Absolute crystallite sizes determined from hydrogen chemisorption data by Carter et al (1966) did not agree with their line broadening results. Their XRD data gave values of 40 to 88 A, which were several-fold lower than the values of 77 to 305 A obtained from hydrogen chemisorption. However, it is not clear whether in these studies the ratio of the crystallite size by XRD and by gas chemisorption was constant. Brooks and Christopher (1968), however, found that the crystallite size estimates by hydrogen chemisorption agree well with the x-ray crystallite sizes. Their contention was that hydrogen chemisorption occurs primarily on the same nickel crystallite faces as measured by x-ray diffraction line broadening. Such may not be the case with carbon monoxide chemisorption. In this study the results 1 28 have been cited with reference to the crystallite sizes determined by XRD. 5.2.5 Characterisation of the Support Figure 5.8 shows the adsorption-desorption behaviour of nitrogen on a sample of alumina (A1S4) which was sintered at 780 C corresponding to the catalyst sample S4. The pore-size distribution of the sintered alumina sample is shown in figure 5.9 ; the adsorption-desorption and PSD features for the unsintered alumina sample (A1F) is virtually indistinguishable from that for the sintered one. The total surface area, pore-volume determined from PSD data , the mean pore-radii and the peak radii of these samples are summarized in Table 5.8 . These results show that the adsorption characteristics of the support did not change on sintering. CO adsorption isotherms on alumina sample A1F was shown in Fig. 5.5. The isotherms for A1S4 are virtually indistinguishable from those of A1F. The amount of CO adsorbed intrinsically on the support is seen to be unaltered on sintering. Table 5.8 Characterization of the support Sample BET Pore Pore Peak Intrinsic Area Volume Radius• Radius• CO m2/g cm3 /g A A pmol/g ALF 192.9 . 504 52.2 57.6 1 8 ALS4 188.0 . 489 52.0 58.2 1 8 1 29 55 Fig. 5.9 Pore-size Distribution of alumina ; sample A1S4 sample ; alumina of Distribution 5.9 Pore-size Fig. 1 3 1 CHAPTER 6 Thiophene Adsorption Studies The main objective in this study has been to obtain information on the kinetics of thiophene poisoning from adsorption studies in a microbalance reactor. The temperature chosen was 298K to minimize any possible hydrogenation and decomposition of thiophene. A concentration of 1000ppm thiophene was used. Concentrations as low as 100ppm were discarded because in such cases the rate of uptake was found to be limited by the supply of thiophene, in the present experimental setup, as has been shown in chapter A. The particle size of the catalyst samples was varied to test for internal diffusion effects. The depth of the catalyst bed was varied to test for the presence of external mass transfer resistance. As there was no flow through the bed, varying the flowrate did not serve as a check for external mass transfer limitation. In order to investigate the structure sensitivity of thiophene adsorption on nickel, the crystallite size of nickel in the catalyst was varied, by sintering in hydrogen at different temperatures or by varying the nickel loading of the catalyst. 6.1 Experimental Results 6.1.1 Runs on Catalyst Particles Experimental uptake-time relationship of thiophene on all samples with a particle size of .3-. A mm are presented in figures 6.1(a)-(g). The notations F10, F15, S2, etc. 132 mg-th i ophene/m2Ni Fig. b.1(a) Thiophene adsorption Thiophene b.1(a) Fig. sample F10 sample mg-thi ophene/m2Ni Fig. b. 1(b) Thiophene adsorption Thiophene b.Fig. 1(b) sample F15 sample mjr thioph«ne/m2Ni Fig. b. 1(c) Thiophene adsorption Thiophene b.Fig. 1(c) sample F20 sample Particle 1 ---- 1 ------1 ----- 1 ----- 1------1 ----- 1 sample SI ----- 1 (minutes) time ---- 41 41 M M 71 M M 1M til |M b. 1(d) 1(d) b. Thiopheneon adsorpti ~ 1 r ~ Fig. n ----- 1 m ----- • • ii t — i 0.78-i 8.18- B» 6 8.28- 136 mg-thi ophene/m*Ni Fig. b. 1(e) Thiophene adsorption Thiophene b.Fig. 1(e) sample S2 sample mg-thiophene/m Ni 0. 70-1 0. b0- Fig. b. Thiopheneadsorption 1(f) sampleS3 mg-thiophene/m2Ni i . .() Thiopheneadsorption Fig. b.l(g) i sampleS4 have been given in Table 5.1. These curves represent net chemisorption on nickel, as described in chapter 4 ; corrections for physical adsorption on nickel and on the support have been taken into account. The adsorption of thiophene on catalyst particles with relatively small crystallites (samples F10, F15, F20, S1 and S2) show that nearly ninety percent of the adsorption occurred in the first ten minutes with about forty to sixty percent taking place in only thirty seconds, which was the first data point on the uptake curve. Poisoning data at t < 30 sec was not measurable because of the initial fluctuations in reading when the poison is introduced. After an initial period of about twenty to thirty minutes, the rate of uptake was found to be very slow and continued for about two hours. The decrease in the rate of uptake with time presumably resulted from an increased coverage of the surface. The physical adsorption component of the isotherm continued for about six to eight hours. Fig 6.2 shows the total adsorption, physisorption and chemisorption uptake with time for a run on sample F20. In contrast, in the case of particles with large crystallites (S3 and S4), the rate of uptake was considerably lower. In the first thirty seconds the adsorption taking place accounted for only about fifteen to twenty percent of the total uptake. Ninety percent of the total adsorption was completed in twenty-five to thirty minutes. 140 thiophene uptake, mg In an attempt to explain the observed lower ' rate of adsorption in the case of the highly sintered particles (samples S3 and S 4), let us consider the pore- characteristics of these catalysts. While the average pore- diameter of samples F10, F15, F20, S1 and S2 ranged from 105 * O to 95 A (cf. Table 5.3), the pore-size for samples S3 and S4 O were found to be only 53 A . The lowering in average pore- size in these latter samples was a consequence of sintering at the high temperatures of 750 and 780 C. The average e crystallite size of nickel in these samples were 80 and 92 A respectively (cf. Table 5.7). These large crystallites would cause a restriction of some of the pores in the alumina support. This could result in the formation of ’ink-bottle* type pores (cf. section 5.2.2) so that a lower value of the mean pore-size diameter would be calculated by the BET method. This explanation is supported by the observation that samples of the support which were given the same sintering treatment as the catalyst samples S3 and S4 showed no appreciable change in their average pore-size values, as shown in section 5.2.5. Because of the lowering in pore- size, the rate of diffusion of thiophene in these samples is expected to be much lower than in samples F10, F15, F20, S1 and S2. Therefore, the apparent rate of adsorption is relatively low in the case of particles of samples S3 and S 4 . 142 6.1.2 Runs on Powdered Samples The particle size of the powdered samples was about .4 pm. The chemisorption uptake profiles on these samples are presented in Fig. 6.3(a)-(g). The adsorption features on powdered samples of F10, F15, F20, S1 and S2 are essentially the same as on the corresponding catalyst particle samples [cf. Fig. 6.4(a)- (e)]. This indicates that there was no effect of internal diffusion resistance on these samples. The rates of uptake on powdered samples S3 and S4 Ccf. Fig. 6.4 (f)-(g)] were much faster than on the corresponding catalyst particle samples, thus indicating the presence of internal diffusional resistance in the case of these samples. For the powder samples of S3 and S4, about seventy-five percent of the total adsorption occurred in the first thirty seconds. Ninety percent of the adsorption was complete in about five minutes. This rate is even faster than the rate of adsorption on samples with smaller crystallites. This apparently anomalous behaviour could be explained from the kinetics of adsorption as will be seen in section 6.2. The rate of diffusion is presumably already very high in the case of samples F10, F15, F20, S1 and S2 so that decreasing the particle size has little or no effect on the rate of poisoning. 143 mg-thi ophene/m2Ni Fig. b.3(a) Thiophene adsorption Thiophene b.3(a) Fig. sample FIB sample mg-th i ophene/m2N i Fig. 6.3(b) Thiophene adsorption Thiophene 6.3(b) Fig. sample F15 sample mg-thi ophene/m2Ni Fig. b.3(c) Thiophene adsorption Thiophene b.3(c) Fig. sample F20 sample mg-th i ophene/m2Ni Fig. b. 3(d) Thiophene adsorption Thiophene b.Fig. 3(d) sample SI sample mg-thi ophen#/m2Ni 8.70-i Fig. 6.3(e) Thiophene adsorption Thiophene 6.3(e) Fig. sample S2 sample mg-thi oph«ne/m2Ni 8 0.70-, . 60 - Fig. 6.3(f) Thiophene adsorption Thiophene 6.3(f) Fig. sample S3 sample mg~thiophene/m*Ni 0. 00^) 0 0 0.30- 0 0. 60- . . . 10 20 78-1 - - li • ----- T « a n ---- Fig. 6.3(g) Thiophene adsorption Thiophene 6.3(g) Fig. 1 ----- 1 ----- time(minutes). m 1 ------ra u sample S4 sample 4 ------1 ----- 4 1 ----- m 1 ----- m 1-----1 «- m i ----- m t u 1 ------1 Powder 1 ----- 1 Particle ------1 a ■ ■ Powder ----- 1 ----- 1 ----- 1 ----- 1 sample Fii ----- 1 time (minutes) time ---- I 49 49 99 M 79 99 99 199 119 119 b.4(a) Thiophene adsorption 99 T Fig. tl 19 T A ■ .*■ * A ► - h 89 9.29-1 0 . 1 0 - e . 0.70- 0.90 c e e* 1-0.40-# z 7 0.30-1 70.30-1 •- 0. b0—I 0. •- f0.50—I M 151 mg-thiophene/m2Ni , - 0 3 . 0 0. b0- e. 70- e. - 0 1 . 0 0 0 4 . 0 . 20 - T I M 3 4 M M 7 M M IN 11 M 1 111 N I M M 71 M M 49 39 M II • ---- r 1 ----- 1 ----- a Fig. b.4(b) Thiophene adsorption Thiophene b.4(b) Fig. m 1 ----- 1------1 a ■ time time (minutes) ----- sample F15 sample A 1 ----- 1------1 ----- 1 ----- A Powder ■ a 1 ------Particle 1 ----- ■ * 1 mg-th i ophene/m2Ni - 0 1 . 0 - 0 2 . 0 0.30- 0.40 0.50- * 0.60- 0.701 T I M M « M M n N M 1H 11 m 111 H 1 M N n M M « M M II • k ---- r 1 ----- 1 ----- * A Fig. 6.4(c) Thiophene adsorption Thiophene 6.4(c) Fig. 1 ------A 1 ----- time(minutes) 1 ----- sample F20 sample a m 1 ----- 1 ------A 1 ----- ■ 1 ---- Powder ■ a 1 ------Particle 1 ----- 1 mg-th i ophene/m2Ni 0 8.60- 0 0.30-1 0.40- 0.50- 0.70-! . . 10 20 - - « n M 4 M M W M M IN il M 1 ill N I M M W M M 41 M n « • t ----- 4 4 * 4 * ^ 1 ---- 1 ----- * Fig. 6.4(d) Thiophene adsorption Thiophene 6.4(d) Fig. 1 ----- 1 ----- time(minutes) 1 ------sample SI sample 1 ---- 1 ----- t BA B A ■ |t 1 ----- 1 ----- Powder ■ a 1------1 Particle --- 1 mg-th i ophene/m2Ni 00-f . 0 e. - 0 1 . 0 .20- 0. 0.30- 0.40 0.50- 0.60- 70 -i tl • L * ----- BA * BA B T 3 4 M l * M H 1 1M 111 1H M l t 7* bl M 41 3* M ---- Fig. 6.4(e) Thiophene adsorption Thiophene 6.4(e) Fig. 1 ----- B B A ABABA 1------1 time(minutes) ----- sample S2 sample 1 ------1 ----- 1 ----- 1 ----- B A Powder ■ a 1 ----- Particle 1 ----- 1 mg-thiophene/maNi i. .() Thiopheneadsorption Fig. b.4(f) sampleS3 mg-th i ophene/m2Ni 00-f . 0 * - 0 2 . 0 0 1 . 0 0.50- 0.60- 8 7 . 0 0.30- 0.40-| it • i A ----- A p 1 t I 1 1 • i 121 lit M 1 •• M 71 U M 41 SI ft ---- Fig. 6.4(g) Thiophene adsorption Thiophene 6.4(g) Fig. 1 ----- ▲ 1 ----- A time(minutes) 1 ----- sample S4 sample A 1 ----- 1 ----- A 1 ----- A 1 ----- 1 ----- 1 ------m a Particle Powder 1 6.1.3 Saturation Thiophene Uptake and the Influence of Crystallite Size The saturation uptake of thiophene on catalyst particles and on powdered samples were the same for corresponding samples of the different catalysts. The total thiophene uptake of these catalysts have been listed in Table 6.1 and show a marked decrease in the final uptake value in the case of samples S3 and S4. The saturation uptake values on samples F20, S1 and S2 are seen to be nearly equal. Table 6.1 Saturation Thioohene Uptake of different samples Sample Thiophene Uptake mg/gcat mg/m^Ni Particle Powder F 1 0 6.177 6.18 0.845 F 1 5 7.01 7.01 0.73 F 2 0 7.36 7.358 0.69 S1 6.367 6.38 0.68 S 2 5.22 5.207 0.65 S3 3.60 3 . 59 0.49 S 4 3.06 3.068 0.48 The results indicate a change in the mechanism of thiophene adsorption from samples F20, S1 and S2 to samples S3 and S4 on the one hand and to F10 on the other. If we consider the thiophene uptake values in the last column of the above table, the only variable factor that remains is the 158 different crystallite size of nickel in these catalysts. In this reasoning, we ignore the unquantifiable factors such as the change in the concentration of lattice defects due to sintering and the effects due to the different shapes and size-distribution of the catalysts. However, the observed effect must be at least partly due to differences in crystallite size. The crystallite size of nickel in the catalysts must have an effect on nickel/thiophene adsorption stoichiometry for the saturation uptake to change with changing crystallite size. The nickel crystallite size of the various samples and the corresponding nickel/thiophene stoichiometric ratios are shown in Table 6.2. The large crystallites of samples S3 and SA give higher nickel/thiophene ratios than the smaller crystallites of samples F20, S1 and S2. Table 6.2 Nickel/Thiophene Ratios • Sample Crystallite Size A Ni/C4H4S F1 0 33 2.5 A F1 5 A 5 2.90 F20 59 3.11 S1 6 A 3.17 S2 70 3.31 S3 81 A .38 SA 92 A .A3 To further support the view expressed above, regarding the effect of crystallite size, poisoning was conducted on 1 59 samples of 10 and 15 wtt catalysts. Because of the lower loading, these catalysts have substantially smaller Ni crystallites. The results for these samples are also included in Tables 6.1 and 6.2, and confirm the belief that crystallite size affects the nickel/thiophene adsorption stoichiometry. 6.1.4 Effect of Flow-rate To determine the optimum flow-rate for the poisoning studies, flow-rates of 50 to 200 ml/min were tried. At 50 ml/min the rate of poisoning was found to be limited by the supply of thiophene : the amount of thiophene uptake in the first thirty sec was 0.12 mg for a bed size of 0.02 gcat, where the supply of thiophene during that period was 0.17 mg. The rate of poisoning was unaffected at 100, 150 and 200 ml/min. A flowrate of 150 ml/min was chosen for the experiments. 6.1.5 Interparticle Diffusion The uptake-time relationship for three different sizes of the catalyst bed are shown in Fig. 6.5(a)-(c). The different bed sizes employed were 0.01g, 0.02g and 0.037g. Results for samples F10, F15, S1 and S2 are similar to those shown for sample F20. Results for S3 and S4 were identical. Fig. 6.5(a) (for particles and powder samples of F20 ) and Fig. 6.5(b) (for powder samples of S4) show that for the deep beds, the rate of uptake is greatly influenced by the amount of catalyst sample used. However, the profiles for 160 mg-th i ophene/m2Ni Fig. 6.5(a) Effect of EffectInterparticle Diffusion Fig. 6.5(a) time(minutes) sampleF20 mg-th i oph«ne/m2Ni Fig. 6.5(b) Effect of EffectInterparticle Diffusion Fig. 6.5(b) sample S4(Powder) sample time(minutes) 1 ---- 1 0.013g 1 i0.021g ----- 1 Bedl Bedl ---- 1 — ♦— Bed3 :0.037g Bed3 — ♦— — Bed2 ---- 1 ---- 1 ---- 1 ---- 1 time(minutes) sample S4(Particle) sample --- Effect of Interparticle Diffusion Diffusion Interparticle of Effect 41 41 M M 71 M N 1M lit in T Fig. 6.5(0Fig. i i n » n 0.70-| 0.70-| 0.50- 0.60- 0.60- 1 63 bed sizes 0.01g and 0.02g were nearly the same. It was, therefore, concluded that external mass transfer resistance is eliminated from the system by employing a bed size of about 0.02g catalyst. This was true for all the samples except, particles of samples S3 and S4 , where internal diffusion resistance is rate controlling and hence no effect of interparticle diffusion was observed. For these samples the apparent rate of uptake was the same for all bed sizes. Fig 6.5(c) shows the case for particles of S^ ; results for particles of S3 were identical. 6.2 Simulation The preceding section has given a qualitative index of the major trends observed experimentally in the adsorption of thiophene on nickel at room temperature, and some of the principal changes effected by the influence of certain parameters. The objective in simulation was to propose a model which would not only predict the uptake-time profile successfully, but would also explain various aspects of the adsorption mechanistically. Various models were examined to explain the chemistry of bonding of thiophene to nickel, to explain the difference in the overall uptake of thiophene by catalysts with different crystallite sizes and to explain the difference in the rates of uptake of thiophene by the different catalysts. In chapter three, three different models were developed. It may be recalled here that model I assumes a 164 constant Ni/C^H^S stoichiometry of adsorption of thiophene on nickel. Therefore, to explain the observed variation in overall uptake, and hence the overall stoichiometry, different constant stoichiometric ratios would be needed for catalysts with different mean crystallite size. Although such different constant stoichiometries could be assigned arbitrarily to fit the observed data, it is obvious here that to give a different overall stoichiometry for different samples, thiophene must be able to adsorb on nickel by at least two different modes with two different stoichiometries. The relative extents of these two modes of adsorption may depend on the properties of the catalyst, such as the crystallite size, giving a different overall uptake for each catalyst. Hence, it was decided to abandon model 1 ; instead, this idea is incorporated into model II which assumes that thiophene adsorbs on nickel both as a coplanar species, occupying five nickel sites per molecule, and as a perpendicular species, occupying two nickel sites per molecule. The relative fractions of these two species may well depend upon, among other things, the fractions of central, edge and corner atoms in the crystallite which in turn would depend on the size of the crystallite. The results of simulation obtained by model II are presented in figures 6.6(a)-(n). The error bars in these plots represent the degree of uncertainty in the experimental measurement of thiophene uptake. The error bars 165 thiophene uptake(dimensionless) Fig. b.b(a) Sample F10(Particle) Sample b.b(a) Fig. thiophene uptake(dimensionless) Fig. b.6(b) Sample Fll(Powder) Sample b.6(b) Fig. thiophene uptake(dimensionless) Fig. b.b(c) Semple F15(Particle) Semple b.b(c) Fig. thiophene uptake(dimensionless) i Fig. b.bCd) Sample F15(Powder) Sample b.bCd) Fig. thiophene uptake(dimensionless) Fig. b. b(e) Sample F20(Particle) Sample b.Fig. b(e) thiophene uptake(dimensionless) Fig. b.b(f) Sample F20(Powder) Sample b.b(f) Fig. thiophene uptake(dimensionless) i. fa.b(g)Sl(Particle) Fig. Sample thiophene uptake(dimension less) Fig. 6. b(h) Sample SI(Powder) Sample 6.Fig. b(h) thiophene uptake(dimensionless) time(minutes) Fig. b.b(i) Sample S2(Particle) thiophene uptake(dimens ion less) Fig. b. b(j) Sample S2(Powder) Sample b.Fig. b(j) thiophene uptake(dimensionless) Fig. b.b(k) Sample S3(Powder) Sample b.b(k) Fig. thiophene uptake(dimens ion less) Fig. b.b(l) Sample S4(Powder) Sample b.b(l) Fig. thiophene uptake(dimensionlass) Fig. b.b(m) Sample S3(Particle) Sample b.b(m) Fig. thiophene uptake(dimensionless) i. b.S4(Particle) Fig. Sample b(n) were obtained by dividing the uncertainty in microbalance reading by the total uptake for the individual samples. In applying model II, the rate constants k^ ancj were obtained by fitting the initial portion (0 < t < 30 min) of the curve of uptake vs. time for sample F20. The values of -3 3 2 the rate constants and k2 were 6.83 x 10 m /m Ni-sec 2 3 2 and 10.4 x 10 m /m Ni-sec. The same values of k1 and k2 were then used for runs on all other samples. It may be recalled here that the experimental study revealed that the adsorption rate on powdered samples of. S3 and S4 is faster than on samples F10, F15, F20, S1 or S2. This apparently anomalous behaviour is explained by the results of modelling which show that k2 >> k^. Since the relative extent of coplanar adsorption is higher in the highly sintered samples, a faster k2 leads to a faster overall rate of adsorption in these samples when compared with the rate on the mildly sintered samples. The effective diffusivity was calculated by using the random pore-model as shown in appendix C. The predictions of the model using the fitted rate constants not only agreed with the total uptake but was also found to be in excellent agreement with the uptake-time relationship over the entire poisoning period, for samples F10, F15, F20, S1 and S2, both particle and powder Ccf. Fig. 6.6(a )-(j)], and for powder samples of S3 and S4 [cf. Fig. 6.6(k)-(l)]. In the case of particle samples S3 and S4, Ccf. Fig. 6.6(m)-(n)] the model predictions were unsatisfactory : the model overpredicts the uptake over the whole period. It was decided that because of 180 the ink-bottle type pores, the diffusivity of thiophene in samples S3 and S4 may be much lower than that obtained from the random pore-model. With a fitted diffusiv ity : 2.6 x 10“9 m2/s, excellent agreement was obtained between the model and the experiment, for these sample s as shown in figures 6.7( a)-(b). The fitted value of the di ffusivity was two orders of magnitude lower than the calcu lated value. However, if one assumes that the large crystaHites (81-92 A) of nickel in these samples causes substantial blockage (about 95Z on average) of the pores (53 A) of these samples, then the value of the fitted diffusivity appears to be reasonable. In the case of powdered samples of S3 and S4 , the use of either the calculated diffusivity or the fitted value for particles gave reasonably good fits of the experimental uptake profiles. The implication is that there was no significant difference in the model predictions because, for the very small particles of these samples (.4 pm) no diffusional resistance was to be expected as is evident from the values of the Thiele moduli, which were 1.4 for coplanar adsorption and 0.004 for perpendicular. For particles of these samples the corresponding Thiele moduli were 1419 and 3.64 respectively. Simulation of powdered samples using the fitted diffusivity value are shown in Fig. 6.7 ( c) - (d ) . In order to inspect the type of poisoning that occurs when thiophene is adsorbed on nickel the radial profiles for calculated values of gaseous poison within the pores and 181 thiophene uptake(dimensionless) Fig. 6.7(a) Sample S3(Particle) Sample 6.7(a) Fig. thiophene uptake(dimensionless) Fig. 6.7(b) Sample S4(Particle) Sample 6.7(b) Fig. thiophene uptake(dimensionless) i .() SampleS3(Powder) Fig 6.7(c) S 8 l 8 S thiophene uptake(dimensionless) time(minutes) Fig. b.7(d) Sample S4(Powder) adsorbed thiophene ( coplanar and perpendicular ) are plotted in figure 6.6. The notations 4^ < ^ ( 4,4 ^ were given in chapter 3. Fig. 6.8(a) is for sample F20 ; the results for samples F10, F15, S1 and S2 and powder samples of S3 and S4 are similar. Figure 6.8(b) shows the same profiles for particles of sample S4 ; the results for particles of sample S3 are similar. The profiles in Fig. 6.8(a) resemble the case of uniform poisoning. However, figure 6.8(b) shows that for samples S3 and S4, the poisoning is severely diffusion limited and resembles the case of shell-progressive poisoning. Figure 6.9 shows the effect of diffusivity on the rate of adsorption of thiophene on particles of sample S4. The curves in figure 6.9 indicate that for large crystallites of nickel as is present in these samples, the adsorption approaches a situation which can be approximated by a shell-progressive mechanism. This result is consistent with the observations of Price and Butt (1977), who investigated the poisoning of a nickel/kieselguhr catalyst by thiophene at low temperatures and proposed a shell progressive model to simulate the steady-state and transient behaviour of the reactor upon poison introduction. Their report, however, included only the particle size and not the crystallite size. 1 8 6 concentrations of thiophene •, sample F20 sample •, thiophene of concentrations Fig. 6.0 (a) Radial profiles for gas- and adsorbed- phase adsorbed- and gas- for profiles (a) Radial 6.0 Fig. 1 87 4 (b) Radial profiles for gas- and adsorbed phase adsorbed and gas- for profiles (b) Radial 6.8 Fig. Fig. concentrations of thiophene ; sample S sample ; thiophene of concentrations 188 (dimens i onless) i (dimens 68 L 68 thiophene thiophene uptake time (minutes) Fig. b.9 Effect of Diffusivity Sample S4 (Particle) Finally, figures 6.10(a)-(n) show the results of simulation using model III, which employs an instantaneous stoichiometry of adsorption expressed as a function of coverage. The linear form of variation (see chapter 3) was used here. The rate constant k was obtained by fitting the initial portion of the curve of uptake vs. time for sample _ o F20. The value of the rate constant k was 1.36 x 10 m 3 /mcNi-sec. 2 Calculated diffusivity values were used (cf. section 6.3). This model gives good estimates of the uptake-time behaviour for both particle and powder samples of F20, S1 and S2 Ccf. Fig. 6 . 1 0(a)-(f)3. However, in the case of samples S3. S4 , F10 and F15 [cf. Fig. 6 . 10(g)-(n)], the model is found to be quite unsatisfactory. For catalyst particle samples of S3 and S4, the model grossly overpredicts the uptake of thiophene in the region of 0 < t < 30 min and underpredicts thereafter. With the same fitted value of diffusivity as in model II, the predictions [cf. Fig. 6.11(a)-(b)3 were valid only in the initial short period of 0 < t < 15 min, after which it was found to underpredict the poison uptake. Further adjustments of -the diffusivity parameter did not improve the fit between this model and experiment. For powdered samples of S3 and S4, the model grossly underpredicts the uptake [cf. Fig. 6.11(c)- (d)3. In the case of samples F10 and F15 [cf. Fig. 6.10(h)- (n)3, the model overpredicts the rate of poisoning over the entire range. It is possible that a different form of variation of the instantaneous stoichiometry than the one 190 thiophene uptake(dimensionless) Fig. b.18(a) Sample F20(Particle) Sample b.18(a) Fig. thiophene uptake(dimensionless) Fig. b.10(b) Sample F28(Powder) Sample b.10(b) Fig. thiophene uptake(dimensionless) Fig. 6.10(c) Sample SI(Particle) Sample 6.10(c) Fig. thiophene uptake(dimens ion less) Fig. b.18(d) Sample SI(Powder) Sample b.18(d) Fig. thiophene uptake(dimensionless) i . .0e SampleS2(Particle) b.l0(e)Fig. thiophene uptake(dimensionless) i. .0f SampleS2(Powder) b.10(f)Fig. thiophene uptake(dimensionless) i . .0g SampleS3(Particle) 6.10(g)Fig. thiophene uptake(diemnsionless) i . .0h SampleS3(Powder) Fig. 6.10(h) thiophene uptake(dimensionless) i . .Bi SampleS4(Particle) b.lBCi)Fig. thiophene uptake(dimensionless) Fig. b.IBCj) SampleS4(Powder) thiophene uptake(dimensionless) i . .0k SampleF10(Particle) Fig. b.10(k) thiophene uptake(dimensionless) i. .01 SampleF10(Powder) Fig. 6.10(1) thiophene uptake(dimensionless) i. .0m SampleF15(Particle) Fig. b.l0(m) thiophene uptake(dimens ion less) 0.00 0 0 0 0 1 . . . b . . 20 40 80 00 0 - - - - i. .0n SampleF15(Powder) b.10(n)Fig. 30 40 I “ time(minutes) 50 ------b 1 ------0 70 r T 90 I “ 100 ------ 110 1 ------Mdl III Model — - Error Bars Bars Error - Experiment * 120 1 thiophene uptake (dimensi i . b.ll(a)Fig. SampleS3(Particle) thiophene uptake (dimensionless) i. b.11(b)Fig. SampleS4(Particle) thiophene uptake(dimensionless) i . .1c SampleS3(Powder) b.11(c)Fig. thiophene uptake(dimensionless) i. b.Fig. 11 (d) Sample S Sample (d) 4 (Powder) used here may give better agreement between experiment and model. However, as shown in appendix F, the profiles for linear, exponential and hyperbolic forms of variation are not siginificantly different. 6.3 Model Parameters The model parameters calculated according to the procedures described in chapter3 (section 3.2) are tabulated below. Table 6.3 Model Parameters Sample f2 De x 10 7 0 (Model II ) m2/s (Model II F1 0 . 82 . 1 8 2.24 3.94 F 1 5 .70 .30 2.13 3.51 F20 .63 .37 2.10 3.23 S1 .61 .39 2.05 3.15 S2 .563 .437 2.00 2.93 S3 . 207 . 793 1.13(0.026)1 1.19 S4 . 19 .81 1.12(0.026)1 1.10 (model II) : 6 .83 x 10 - 3 m 3 /m 2Ni-sec. 2 3 2 k2 (model II) : 10.4 x 10 m /m Ni-sec. k (model III) : 1.36 x 1Q“2 m3/m2Ni-sec. 1 The values within brackets are the fitted diffusivities. 209 6.4 Discussion The temperature chosen for poisoning in this study was 298K since at higher temperatures thiophene decomposition leads to complicating interactions among deposited sulphur, deposited carbon and nickel. The saturation uptake of thiophene decreased with increasing crystallite size, to a larger extent than could be accounted for by a corresponding decrease in surface area. Since the results of this study indicate that thiophene decomposition was not taking place at room temperature, Ni/C^H^s ratios such as those obtained in this study would only be possible if one assumes that thiophene was adsorbed in two different forms ------presumably, coplanar and perpendicular. The results further indicate that the adsorption stoichiometry was varying with the crystallite size. It is postulated that a fraction of the incoming thiophene was adsorbed coplanar to the surface with a Ni/C^H^S stoichiometry of 5. The remaining fraction was adsorbed perpendicular with a stoichiometry of 2. The overall stoichiometry was then determined from the extents of the two types of adsorption, which in turn would depend on the fractions of the two different types of sites in the * catalyst. For the range of crystallite size 50 to 70 A, a Ni/C^H^S ratio of about 3 was obtained. With crystallites larger than about 80 A, this ratio was about 4.5. This would indicate that for the large crystallites the fraction of thiophene adsorbed coplanar is substantially higher than for 210 smaller crystallites These results clearly indicate an effect of crystallite size on the Ni/C^n^s ratio. Other factors like the crystallite shapes, their irregularity and their size-distribution may have influenced the observed uptake of thiophene. However, there was no suitable means to estimate the crystallite shapes or their size distributions Hence, in this study we choose to attribute the observed effects to a variation in crystallite size. This hypothesis was tested by comparing the experimental and modelling uptake of thiophene for samples of varying crystallite size. Coplanar adsorption, where one thiophene molecule takes up five nickel sites, is likely to take place at the central sites on the surface of the catalyst ; perpendicular adsorption involving two nickel sites per adsorbed thiophene molecule is more likely to prevail at the corners and edges of the crystallites. However, using catalyst data (surface area, crystallite size, etc.) and assuming ideal crystallites, the geometry indicates that there are not enough edges and corners to allow for the observed Ni/C,H,S ratios of around 3 for samples F15, F20, S1 and S2 or 2.5 for F10. Table 6.5 shows a comparison of the experimental and calculated Ni/C4H4S ratios for octahedral, cubo- octahedral and icosahedral crystallites. The calculated values are based on the assumption that all coplanar species are adsorbed on the central sites of the surface and perpendicular species are adsorbed only at the edges and corners. The possibility of perpendicular adsorption on 21 1 edge-adjacent atoms or the possibility of a coplanar species occupying one or two edge atoms has been ignored. On the other hand, for samples S3 and S4 , the difference between the computed and observed Ni/C^n^s ratios are only slightly outside the limits of experimental reproducibility. This indicates that in these samples the crystallites may not be far removed from ideal characteristics. Table 6.4 Ideal and Real Ni/C.4H^S ratios Sample Ni/C4H,S Ratio Experimental Octahedron Cubo-Octahedron Icosahedron FI 0 2.54 4.25 4.07 3.86 F1 5 2.90 4.43 4.31 4.16 F20 3.11 4.58 4.46 4.34 S1 3.17 4.61 4.51 4.39 S2 3.32 4.64 4.55 4.46 S3 4.38 4.67 4.61 4.52 S 4 4.43 4.73 4.64 4.58 The above discussion of the observed behaviour leads us to postulate various hypotheses regarding the modes of adsorption and the effect of sintering. These are presented below. ( 1 ) Coplanar and perpendicular adsorption are occurring according to the respective fractions of central sites and edge and corner sites respectively. However, to explain the 212 discrepancy between calculated and experimental Ni/C^n^s it is postulated that the crystallites in the unsintered or partially sintered catalysts may have lattice defects which give rise to internal edges and corners on which thiophene adsorbs perpendicularly ; sintering at the high temperatures of 750 and 780 C not only increases the geometric size, but also decreases the concentration of such defects so that the crystallites now approach regularity and ideal characteristics. (2) An alternative explanation lies in the following hypothesis : perpendicular adsorption is not limited to edges and corners but can also occur on the central sites as well. However, coplanar adsorption is preferential over perpendicular at the central sites. Once the thiophene molecules have adsorbed coplanar, the spaces in between, however, must be filled by perpendicularly adsorbed thiophene as these sites are no longer large enough to accommodate coplanarly adsorbed thiophene. This aspect of adsorption is accounted for in the models developed in chapter 3 by making the rate of poisoning proportional to the concentration of vacant sites raised to an index equal to the adsorption stoichiometry (see chapter 3). It should be noted here that this hypothesis is consistent with the modelling results which showed that the apparent rate constant for coplanar adsorption is several orders of magnitude higher than that for perpendicular adsorption. 2 1 3 (3) In considering other modes of adsorption, we first examine the case where the thiophene molecules lie flat on the bulk of the surface atoms, i.e., the central sites. However, at the edges and corners, the thiophene ring is hydrogenated to thiophane and eventually to butadiene, butene or butane, leaving a sulphur atom attached to one or two nickel sites. Although, gas chromatographic analyses of the exit-stream from the poisoning experiment did not show any decomposition or hydrogenation products, it is conceivable that the extent of hydrogenation might have been so small that the products would not have been detected in the analytical system. If all the perpendicular species were hydrogenated to butane, as postulated here, then the smallest possible extent of hydrogenation would correspond to that sample which has the smallest number of perpendicular species ( 0.18 fraction of the surface in sample F10 ). Under these circumstances, the size of the butane or butene peak would be about 1/10th the size of the 1000 ppm thiophene feed, a size which surely would be perceivable. Thus it may be concluded that at room temperature thiophene was not undergoing hydrogenation to butane or butene. Furthermore, such a mode of adsorption, as has been postulated above is inconsistent with the total uptake observed experimentally, i.e., there is more material on the surface than would be obtained if sulphur alone remained at the edges and corners. However, one might postulate that only a fraction of 214 the thiophene adsorbed perpendicularly undergoes hydrogenation and decomposition, so that both sulphur and thiophene remains at the edges and corners, the peak size for the hydrogenation product may be too small to detect. Furthermore, this mechanism may now be consistent with the observed uptake. However, such a mechanism would appear to be thermodynamically infeasible, since it would require an equilibrium which included the reaction of adsorbed S with butene gas for the reverse step. c4H4S(ads) + 2H2(g) C4 Hq (9 ) ♦ S(ad s ) (4) Let us now consider the possibility of thiophene undergoing hydrogenation, for coplanar or perpendicular adsorption or both, to thiophane only, but with no dissociation i.e., thiophane so formed remains adsorbed on the surface molecularly. This would affect the Ni/C.H.S 4 4 stoichiometry by less than 5t. Such a difference in the Ni/C^H^S ratio is within the limits of experimental uncertainty, and would not result in any observable difference in the modelling predictions. Thus, without direct observation of the adsorbed surface by surface sensitive techniques like infrared spectroscopy, neither the model nor the experiments are able to discriminate thiophene adsorbed molecularly from that hydrogenated to adsorbed thiophane. However, it is very unlikely that once a coplanarly adsorbed thiophene molecule is hydrogenated it will remain coplanar. Previous studies [Lyubarskii et al (1962), Bourne et al (1965)] indicate that on hydrogenation 2 1 5 to thiophane the C-Ni bonds are released and the poison molecule then a s sumes the perpendicular position via the sulphur atom. It is possible then, that the perpendicular species in the adsorbed state may well be thiophane rather than thiophene. (5) An alternative hypothesis is that initially when the thiophene/H^ stream is introduced there is a competition between the poison molecules and hydrogen for adsorption on the nickel sites. Thiophene adsorbs in the coplanar mode and then reacts irreversibly with the adsorbed hydrogen to form thiophane which then assumes the perpendicular position. However, if the perpendicular species is thiophane only, then thiophene adsorption may vary with crystallite size of nickel if there were two different types of sites for nickel ------one in which thiophene can chemisorb only, and the other in which thiophene can hydrogenate. The relative amounts of such species may vary with crystallite size. The overall uptake is expected to vary with the concentration of poison as well, if this hypothesis were true, since the extent of hydrogenation to thiophane will vary with the relative amounts of thiophene and hydrogen adsorbed initially. It may also be possible that in addition to the hydrogenation of a fraction of the thiophene adsorbed initially to thiophane, which remains in the perpendicular position, thiophene itself adsorbs on the edges and corners in the perpendicular mode as postulated earlier in this 2 1 6 study. This latter hypothesis may now be consistent with the observed thiophene uptake values (depending on the extent of hydrogenation to thiophane) in the sense that there are more perpendicular species on the surface now than would be allowed by ideal crystallites. For the purposes of this study, the question of whether thiophene or thiophane is the perpendicular species is not resolvable as this does not affect the quantitative results of modelling significantly and the qualitative conclusions remain intact. In this study. we have arbitrarily chosen to describe the mechanism as adsorption of thiophene rather than thiophane. (6) Although thiophene adsorbed intrinsically on alumina was negligible, the possibility of thiophene adsorbing on nickel aluminate cannot be discounted. If thiophene adsorption was indeed to take place on NiAl2o^, one may assume that the observed variation in Ni/C^H^S stoichiometry is due not toa variation in nickel crystallite but to the different amounts of nickel aluminate present in the catalyst. Since samples F10, F15 and F20 vary in loading, they contain different amounts of nickel aluminate. This difference in the amount of aluminate leads to a difference in the extents of reduction of these samples since nickel aluminate is very stable and not reducible to Ni. Therefore, if the variation in overall uptake was caused by adsorption on nickel aluminate, the Ni/C,H,Sb b ratios would be proportional to the fractions of reduction of individual samples. Moreover, samples F20, SI, S2, S3 and S4, which 217 have nearly the same extents of reduction, should not show any difference in overall uptake, if adsorption on aluminate was responsible for the observed variation in Ni/C^n^s stoichiometry. This however is not true suggesting that the variation in the overall uptake was not caused by chemisorption on NiAl.o.. c h Summary The experimental and modelling results have shown that thiophene adsorption on nickel is structure sensitive : the overall stoichiometry of adsorption of thiophene to nickel seem to vary with crystallite size. It is postulated that thiophene is adsorbed on nickel in two different modes : coplanar and perpendicular. In the perpendicular mode the adsorbed species is either thiophene itself or its hydrogenation product thiophane or both. The coplanar species appears to be adsorbed thiophene. The adsorption of the coplanar species is likely to prevail at the central sites of the surface, and at the edges and corners only the perpendicular species can be adsorbed because of geometric conditions. The perpendicular species may also be present in the spaces between coplanarly adsorbed thiophene. The coplanar species may be undergoing hydrogenation to thiophane which then assumes the perpendicular position and results in adsorption of more thiophene in the perpendicular mode. Since the fraction of central sites and that of edges 218 and corners varies with the crystallite size, the Ni/C^n^S ratio varies with the crystallite size . The experimental and modelling results have shown that large crystallites of nickel can block the pores in the catalyst resulting in a diffusion influenced process. The simulation also showed that the rate of adsorption of the coplanar species is faster than that of the perpendicular species indicating that coplanar adsorption is preferential over perpendicular in the initial stages of adsorption when the entire surface is available. 2 1 9 CHAPTER 7 Conclusions and Recommendations This study involved the investigation of the mechanism and kinetics of thiophen e adsorption on nickel at ambient pressures and temperatu res. The structure sensitivity of thiophene adsorption on nickel was investigated by the poisoning of samples o f different nickel loading or - of varying degree of sinter ing. The conclusions reached from the present work have been outlined below and some suggestions for future work are given in a following section. 7.1 Conclusions 1. Gas chromatographi c results have demonstrated that at low temperatures (<5 0 C), thiophene adsorbs on nickel without any decompositio n, or if hydrogenation was taking place the extent of such reaction was very small. At 100 C, there was decomposition of thiophene on the metal surface giving off butane and presumably H2s as well. 2. Experimental study h as shown that the overall uptake of thiophene by nickel va ried with the crystallite size, suggesting that thiophen e adsorption on nickel is structure sensitive. Therefore, it appears that there are at least two modes for the adsorption of thiophene on nickel, and that the relative extents of the two modes of adsorption vary 220 with the crystallite size of nickel in the catalyst. The modelling results are in agreement with this view. 3. Th e modelling results h ave shown that thiophene adsorptio n on nickel can be repre sented by two parallel rate process e s occurring on the surfa ce, corresponding to two different modes of adsorption viz., coplanar, where a thiophene molecule is bonded to five nickel atoms and perpendic ular, where the thiophe ne molecule is adsorbed via the sulphur atom and effectively poisons two nickel atoms. 4. The coplanar species appears to be adsorbed thiophene. In the perpendicular mode the adsorbed species may be thiophene alone or its hydrogenation product thiophane or both. 5. The coplanar species is likely to prevail at the central sites of the surface because of geometric requirements. The perpendicular species is likely to prevail at the edges and corners. However, it may be present at the central sites as well, in between coplanarly adsorbed species, where there are no longer five consecutive sites to accommodate a coplanar species. Furthermore, some coplanar thiophene may be hydrogenated to thiophane which would then assume the perpendicular position. This would lead to further thiophene adsorption possibly in the perpendicular mode. 6. The rate of adsorption of the coplanar species is faster than that for the perpendicular species. The rate 221 constant for the adsorption of the coplanar species is several orders of magnitude higher than that for the adsorption of the perpendicular species. This indicates that coplanar adsorption is preferential over perpendicular in the initial stages of adsorption on a freshly reduced sample. 7. The modelling results showed that the rate of uptake of thiophene is proportional to the concentration of vacant sites raised to an index equal to the stoichiometry of adsorption. 8. The experimental and modelling results have shown that large crystallites of nickel can block the pores in the catalyst resulting in lower rate of diffusion of thiophene in the pores. The rate of adsorption is then found to be diffusion limited. However, on powdered samples the rate of adsorption is kinetically controlled. 9. The ratio ^bf crystallite size measured by CO chemisorption and by x-ray line broadening was found to be constant for catalysts with a Ni-loading higher than 10Z. This indicates that the relative extents of linear and bridged CO bonding to Ni is unaltered at moderate to high loading catalysts. 10. Sintering in hydrogen at temperatures of 750 C and over changes the pore characteristics of the catalyst from a type 'A* BET isotherm to type *E*. The ink-bottle type pores in 222 the highly sintered samples result from a blockage of the pores by the large crystallites of nickel in the catalyst. 11. At ambient temperature thiophene adsorption on alumina was high, but consisted mainly of physisorption with a negligible contribution from intrinsically adsorbed thiophene. 12. Thiophene adsorption on NiAlgO^ was insignificant or not occurring at all. 7.2 Suggestions for Further work 1. In view of the uncertainty in Ni/CO ratio for low loading catalysts it is recommended to explore into the possibility of N^O for measuring Ni-area of a catalyst. In the present study the free area of poisoned samples could not be determined because CO adsorption on S-poisoned Ni is adversely affected CViveros-Garcia(1985), Bartholomew (1982)]. It is therefore suggested that the usefulness of N^O for measuring free area of S-poisoned Ni-catalysts be investigated. The stoichiometry of adsorption of N20 to nickel will need to be established first. As an alternative, the use of H2 in a volumetric apparatus may be suggested. 2. To further support the structure sensitivity of thiophene adsorption on nickel it is recommended to perform poisoning runs on samples of 107 and 157 Ni-loading sintered under the same conditions as with the 207 catalyst in this study. It is then suggested to compare the poison 223 uptake vs. time profile for these samples with the predictions of the models developed in this study to see if the mechanism suggested here is applicable for all loadings and crystallite sizes of Ni. 3. It is also suggested to perform poisoning of the samples of this study with concentrations higher and lower than that used in this study. It is suspected that for lower concentrations one may be operating in a range where thiophene adsorption is limited by the supply of thiophene as is the case with a 100ppm concentration. This would mean that the kinetics of the adsorption cannot be investigated. 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Chem., 6ji, 617 (1961) 232 Nomenclature a fraction of surface deactivated. 3 . Jj 1 . a 2 fractions of the surface deactivated by perpendicular and coplanar adsorption, respectively. Bi Biot number. C gas-phase concentration of the poison, gmol/m . 3 Co concentration of poison in the feed, gmol/m . 2 De effective diffusivity, m /s. V f2 fractions of potential sites for perpendicular and coplanar adsorption, respectively. h Thiele modulus. h 1 ' h2 Thiele moduli for perpendicular and coplanar adsorption, respectively. k rate constant for adsorption, m 3 /m 2Ni-s k1 'k2 rate constants for perpendicular and coplanar adsorption, respectively, m /m Ni-s. km mass transfer coeeficient, m/s n number of metal sites per gram catalyst, poisoned at any time t. c c CM numbers of sites poisoned by perpendicular and coplanar adsorption, respectively, at any time t. nT total number of metal sites in the catalyst, per gram of catalyst. n1T *n2T' number of sites poisoned by perpendicular and coplanar adsorption, respectively, at saturation, per gram of catalyst. 232-a No Avogadro number. q concentration of poison deposited, gmol/gcat. q 1 * q2 concentrations of adsorbed species corresponding to perpendicular and coplanar adsorption, gmol/gcat. qT concentration of deposited poison, corresponding to saturation, gmol/gcat. q1T'q2T saturation uptakes for perpendicular and coplanar adsorption respectively, gmol/gcat. r radial distance coordinate, m. R radius of catalyst particle, m. S poison species. s metal area of catalyst, m /gcat. a instantaneous adsorption stoichiometry, Ni/C.H.S. •* h a a adsorption stoichiometries for perpendicular and 1 2 coplanar adsorption respectively. ao instantaneous adsorption stoichiometry when surface is bare, for model III. aT overall adsorption stoichiometry. (3 a constant for linear, hyperbolic or exponential variation of a with n, in model III. porosity or void fraction of catalyst. = r/R. Dimensionless radius. ♦ =C/CQ. Dimensionless gas-phase concentration. 0 =s^kCt/qj. Dimensionless time. * =q/qT< Dimensionless adsorbed-phase concentration. 4v *2 = q1/q1T , q2/q2y• Dimensionless concentrations for adsorbed species in the perpendicular and 232-b coplanar modes respectively, solid density of catalyst, gcat/m 3 bulk density of catalyst, gcat/m Appendix A Discretization of the Model In order to discretize the system of equations describing the poisoning problem, the catalyst pellet is divided along the radius (0 1. Mass Conservation Equation : The discretization of the mass conservation equation has been obtained by employing a finite difference approximation to the spatial derivatives. Applying the backward finite difference method to equation 3.15 one obtains : AS i.e., ♦1 4*o A 1 Similarly, by applying the forward finite difference method, equation 3.16 becomes (*N+1 ~ <>N) EPBi[*N+1 " 13 AS or, *n+1C1 + ep ♦N + ep(AS)Bi i.e.. «►N+ 1 A2 1 + cp(AS)Bi 233 Applying the central finite difference method to equation 3.17 one obtains j -1 * 2 * 3+1 *3*1 - 2+3 ■i - h2 d a)01^^ = o A3 Equation A3 simplifies to : •e- ii n i 0 A4 where, F2+3*1 ' F1+j + Fi = 2 ( 1 ♦ AE/Ej) ♦ hZ 1 AE)2d - a )a and F2 = 1 + 2AE/Ed For model II the last term in the LHS of equation A3 is replaced by : h12(f1 - and h2?^f2 “ a2*5*j and accordingly, F1 = 2(1 ♦ AE/Ej) + Ch1 2(f^ - a,)2 + h22(f2 ” a2)53♦j(AE)2 For the Nth point, combining equations A2 and A4, the following equation is obtained : (AE)CpBi N- 1 (Fi - )*N * F2 = 0 A5 1 + (AE)SpBi 1 ♦ e0(AE)Bi Equation A4 . together with the boundary condition equations A1 and A5 forms a tridiagonal matrix equation which was solved by a NAG routine. 234 2. The rate of poison deposition The rate of impurity deposition is given by the partial differential equation (PDE) : D4» -- = ( 1 -4) )a4> A6 80 which can be written as 84> — = f (4>CE . 03 .♦CE.0] ) 80 a is either a constant (models I and II) or some function of 4> (model III). To solve the above PDE, the method of lines (MOL) has been used. The method of lines converts a general PDE into a system of ordinary differential equations (ODE). MOL is therefore useful in dealing with design and analysis of reactors and packed bed columns. MOL can be used for any number of PDE, in one or more dimensions, and any order of PDE. MOL is assisted by the capacity of the ODE solver algorithms. Since the resulting system of ODE is often stiff, stiff stable integrators such as Gear’s routine is recommended. However, when no stiffness is encountered, simpler algorithms such as Runge-Kutta can be employed. MOL usually converts the PDE into a system of coupled ODE’s by discretizing the PDE in the space dimension(s). The ODE’S are then integrated in the time domain. According to the type of piece-wise polynomials used to 235 represent the spatial derivatives, the discretization methods are classified into local or global methods. Local methods involve all the classes of finite difference approximations, whereas global methods involve the family of the methods of weighted residuals. In this work the initial estimates of il>(j=1 to N) are zero for the first time step. Applying MOL, equation A6 becomes dtj) --1 = (1-0>h )4>h d0 3 3 3=1 to N are the spatial grid points. The above system of OOE’s were solved by a variable time step routine (Runge- Kutta-Merson) simultaneously, together with the system of algebraic equations A1. A2 and A5 . 236 Appendix B Instantaneous Adsorption Stoichiometry for Model III With a linear variation of instantaneous adsorption stoichiometry for a in equation 3.3, one has dn n = °o - 0-- B1 Nodq nT In order to solve equation 3.31 appropriate values of and P must be found which are consistent with the experimental observations. Because all adsorption will initially be coplanar, according to model III a=5 for n/nT=0 and hence ao=5. P can be evaluated by integrating equation B1 between the limits of 0 and n^ for dn and 0 and q^ for dq. Thus, nT dn qT J ------■ Nq S dq 0 aQ - Pn/nT 0 n_ d(n/nT) N qT or. J = J dq 0ao -pn/nT nT0 nT or, -1 /pin (a - pn/n_)| = Nn-clT/nT 0 o i l or, 1 . ao 1 - In ------= — B2 0 a Q - 0 o T where, aT ==nT/(qT.N0) is the observed overall adsorption stoichiometry. Since aQ is known and is determined 237 experimentally, the transcendental equation B2 may now be solved for 0. An explicit relationship is needed for n/nT as a function of time in order to integrate equation 3.31. In models I and II n/n^. is simply equal to 4). In model III, however, for a = ctQ-pn/nT, equation 3.31 may be integrated to give : n d(n/n_) N q J ---- — - — 5 dq 0 aQ - pn/nT nT 0 1 o r 0n/nT , —_ In (a o a P or , o___ In . No a o 0n/nT Introducing aT, the known overall stoichiometry in the RHS of the above equation yields n — ( 1 - e"P4,/aT) nT P 238 Appendix C Estimation of Effective Diffusivitv Ordinary molecular diffusion in gases results from differences in concentration between regions of a mixture ; diffusion tends to make the concentration uniform throughout. In a stagnant binary gas mixture, the molal flux J (g- mol/sec-m*) is proportional to the concentration gradient in the direction of diffusion = “ D12(dc1/dx) = - D12 ecT (dY1/dx). The proportionality constant 012 is the diffusion coefficient for gas 1 diffusing in gas 2. This is a function of the molecular properties of the two gases, and increases with increase in temperature or reduction in pressure. The value of D12 varies little with the mole fractions Y.j or Y2 of the two gases, but does vary with changes in c^, the total molal concentration of the mixture (g-mol/cm ). To estimate the effective diffusivity of gas 1 diffusing in gas 2. the diffusion coefficient D12 is first calculated. The subscript '1* refers to thiophene and '2* to hydrogen. The theoretical equation based on the modern kinetic theory and the Lennard-Jones expression for intermolecular forces is employed. This is given by : 0.001658T3/2[ (M<| + M9)/M1M?3 1/2 C1 1 2 e°12QD 239 where T is the absolute temperature(°K) ; , Mg are the molecular weights of the two species ; P is the total pres sure(atm) ; Bp is the "collision integral”, a function of kT/e^ ; e*° are the ^orce constants in the Lennard-Jones potential function ; and k is the Boltzmann constant. Values of e and o for hydrogen were taken from Satterfield (1970). Values of the same parameters for thiophene is not reported but could be estimated from the empirical equations : kT T 1/3 -- = 1.3 0— and o=1.1 8V.b ' J e T c where Tc is the critical temperature(°K) and V^ is the molar 3 volume(cm /gmol) at the normal boiling temperature. The volume V , in turn, was estimated by Kopp's law of additive volumes using the values given in Satterfield (1970). Vb for thiophene is calculated to be 88.1 . Using the relation □12 = 1/2(o1 ♦ o2) o12 is found to be A.04. Tc for thiophene is estimated [Reid and Sherwood ( 1966 )] to be 579°K. kT/e^ then to be 4.04. Bp is then obtained from a table in Satterfield (1970), giving corresponding values of kT/e12 and 2d * This value is 0.8817. Then in equation C1, we have (0.001858)(298)3/2[(2+84)/(2x84)]1/2 D 1 2 ~ 1(4.04)^(0.8817)2 ~ ~ = 0.475 cm2/sec 240 = 4.75 X 10 5 m2/sec. The Knudsen diffusivity must now be calculated. This is given by D„ = 9.7 X 10 3 a(T/M„) 1/2 where a is the pore radius In 1 in cm. Now, may be calculated from the relation [Satterfield ( 1 970) 3 1 /D = 1 /D„+ 1 /Di0 . According to the random pore-model |J In \ i [Satterfield (1970)3.'* the effective diffusivity is given by 2 D e = e.. p D p where e p. is the porosity or void fraction of the catalyst sample. Typical values of D are of the order of l\ — fi 2 — 5 2 10 m i s and D12 is of the order of 10 m /s. Hence, Knudsen diffusion is expected to predominate. An alternative method of calculating the effective diffusivity is to use 0 = 0 e /t , where t is the e M M tortuosity. The factor x allows for both the tortuous path and the effect of varying cross section of individual pores. However. the tortuosity factor is difficult to estimate experimentally and therefore in this study, the relation 2 D e =e p D p has been used, i j 24 1 Appendix D I. Calculation of crystallite size from chemisorption data In practical situations a finely divided solid usually consists of particles the size of which spans a range of values i.e., there is a particle size distribution. Fig. D1 shows the schematic particle distribution of a solid. The mean d is the point on the diameter scale, the deviations Fig. D1 Schematic particle size distribution indicating three common measures of central tendency. from which sum to zero i.e., Ef.(d.-d )=0, and for which 1 1 a ve the sum of the squares of the deviations is minimized. The parameter d^ is not the only one which may be used to define a mean diameter. For instance, the quantity d. 2 may be used. Since d^ 2 is related to the particle surface area , we 2 have Ai = otfidJL where a is a dimensionless constant depending on the particle geometry ; if all the particles are spheres 242 diameter, a = ir ; if all are cubes and and d i. is the sphere di 2 a= where EA^ is is the cube edge 6. Thus A.=aSNEfidl the 1/2 total surface area , and d ... = ( EA . d AN 1 i 2/eV = “a n " (EA./Ef.). This defines an area-number average diameter as 11 indicated by the subscript AN. Thus there are a number of different ways of defining a mean particle diameter, and their utility is determined by the parameters that are measurable in practice. Table D1 taken from Anderson (1975) lists some of the more important definitions of mean particle size. If the particles in a sample are of varied irregular shapes, a is not defined geometrically. It is usual to assign a value to a based on an assumed spherical particle equivalent, and these are shown in Table D.1 T able PI Some definitions of mean particle size Value of a for spherical particle Type Definition of mean equivalent length-number .. area-number volume-number volume-area 2 4 3 Because dispersed metallic catalysts often have the extent of their surface characterized by gas chemisorption methods which count the number of surface metal atoms, it is convenient to define the state of subdivision of the metal in terms of the ratio of the total number of metal atoms present. Thus the metallic dispersion of the metal is defined by DM = NS/NT. This definition is related to the volume-area mean diameter. dVA. since in the spherical particle equivalent approximation EVi Vm .Nt Vm dVA * 6 ~ ' 6 ----- * 6 -- /DM EAi aM-NS aM where aM and are respectively the effective average area occupied by a metal atom in the surface, and the volume per metal atom in the bulk. Values for aM are listed in the literature, while VM is given by Mw/pNQ where Mw is the atomic weight, p the density and Nq is the Avogadro number. For nickel, Mw = 58.7 g/atom p = 8.902 x 106 g/m3 therefore, VM = 1.095 x 10~29 m3/atom 1 9 The number of surface atoms, n s, for nickel, is 1.54 x 10 2 - 2 0 2 atoms/m . Therefore, aMn = 1/n„ s = 6.5 x 10 m /atom. Then dyA is given by 6x1.095 x 10“*3 VA -/DM m 6.5 x 10-20 244 10 10 = ---- A *d h For a particle of specified geometric shape the ratio (usually defined by F^) of the number of surface atoms to the total number of atoms is well defined and calculable. Thus if an actual specimen were to contain metal crystallites of identical size and of geometrically regular shape, dm would be identical with the appropriate Fs . In practice this situation never occurs, and Fs acts as an ideal model with which to compare reality. In the rare event where the particles are spheres all the same size, the crystallite diameter is given by 1 = tt/pA, where p is the solid density and A is the surface area. 11. Evaluation of 2.DM from CO chemisorption Let the CO uptake by nickel in the catalyst be n mg/gcat. With an adsorption stoichiometry of Ni/CO = 1.4, _ 3 the number of surface metal atoms per gcat = 1.4(nx10 )/28. Let w be the Z nickel loading of the catalyst and f the extent of reduction of NiO to Ni. Then the total number of Ni atoms in the catalyst is 0.01wf/58.7. Percent metallic dispersion is then given by Dm = 1.4 n x 10"3 x 58.7 x 100/(28 x 0.01wf) = 29.35 n/wf 111 - Determination of the extent of reduction Let M be the weight in grams of the dried unreduced catalyst and x the weight of Ni in the catalyst. Then the 245 weight of 0 in the catalyst is 16x/58.7 and the weight of a fully reduced catalyst is M-16x/58.7. With w as the 1. loading of the catalyst x = 0.01w(M-16x/58.7) which gives 0.01wM x = ------9 1 + 0.16w/58.7 Then the expected weight loss of the catalyst by reduction is the weight of oxygen in the catalyst and is given by Expected weight loss = 16/58.7 x (0.0IwM/(1+0.16w/58.7)). The extent of reduction of a particular sample is then given by observed weight loss f = ------, expected weight loss where observed weight loss is the actual weight loss of the sample during reduction after accounting for the loss in weight by drying. 246 APPENDIX E Computation of Pore-size Distribution from Type IV Isotherms The Kelvin equation (5.2) reads : 2 V*y In(p/p ) = - --- Cos0 rRT It may be recalled here that p o is the saturation vapour pressure, y the surface tension, and V the molar volume of the liquid, whilst 0 is the angle of contact between the liquid and the walls of the capillary. In calculations of pore size from type IV isotherms by use of the Kelvin equation, the region of the isotherm involved is the hysteresis loop, since it is here that capillary condensation is occurring. The formation of a liquid phase from vapour at any pressure below saturation cannot occur in the absence of a solid surface which serves to nucleate the process. Within a pore, the adsorbed film acts as a nucleus upon which condensation can take place when the relative pressure reaches the figure given by the Kelvin equation. In the converse process of evaporation, the problem of nucleation does not arise : the liquid phase is already present and evaporation can occur spontaneously from the meniscus as soon as the pressure is low enough. It is because the process of condensation and evaporation do not necessarily take place as exact reverses of each other that hysteresis can arise. The question necessarily arises as to which of the two values one should use in the Kelvin equation in order to calculate the value of r corresponding 2 A 7 to the particular adsorption value. There have been a number of attempts to explain the difference which obviously exists between the state of the adsorbate along the two sides of the loop. Since the relative pressure corresponding to a given adsorption is lower along the desorption branch, on thermodynamic grounds, the chemical potential of the adsorbate is lower if one ignores changes in the solid. Accordingly, the desorption branch is more likely to correspond to a condition of true equilibrium. This, however, is not true in the case of type ’E‘ isotherms as explained in section 5.2 ; in this case the adsorption branch corresponds to equilibrium. The question of whether the adsorption or the desorption branch is better suited for pore-size calculations must however be answered from a discussion of the origin of hysteresis and must be based on actual modes of pore shape, since a purely thermodynamic approach cannot account for two positions of apparent equilibrium. Whichever branch is used for the PSD calculations, the procedures are based on imaginary emptying of the pores by step-wise lowering of relative pressure, from the point where the mesopore system is taken to be full up ; a relative pressure of 0.95pQ is frequently adopted as starting point with isotherms having a hysteresis loop of type * A * or type *E'. To apply the Kelvin equation it is necessary to assign a value to the angle of contact 0, a quantity which, unfortunately. is extremely difficult to determine directly 248 in the case of porous solids. To make progress it is usual to make the simplifying, though not always justifiable, assumption that 0=0, i.e., that the liquid wets the walls of the pores. The Kelvin equation(5.2) then simplifies to 2 V*y In(p/p ) = - --- r RT which permits of the calculation, for a given liquid, of a numerical value of r for the cylindrical radius corresponding to any desired value of p/pQ. Nitrogen at -195°C. is particularly suitable as adsorbate for reasons which have already been alluded to (cf. section 5.1). The radius r given by Kelvin equation is actually the radius r^ of the inner capillary (cf. Fig.EI) known as the ^•g- El Cross-section, parallel to theof aaxis cylindrical pore of rp.radius showing the “inner core” of radius r* and the adsorbed filmt. of thickness 249 core radius, The pore radius is given by rrp = rk + t, where t is the thic er and is calculated from t = a [ Sing (1967) 3 where o the thickness 3.54 A for nitrogen. Taking V as ; 8.85 dyne cm-1 the Kelvin equation (simplified form) becomes 4.14 rk = --=------log(pQ/p) In applying the method, the desorption or adsorption branch (as the case may be) is divided into a number of steps of relative pressure. Corresponding values of rk and t are the computed. The mean values of rp and rk# viz. rp and rk are then calculated. The diminution in film thickness 6t and the element of volume desorbed 6V are then calculated. The values of 6V^ which represents the amounts desorbed from the film on the walls, during the desorption steps, is calculated from the area of the walls covered with the film (i.e., the walls of the pores from which capillary evaporation has already occurred) and the diminution in thickness of the film during the desorption step, using the expression 6Vf = 0.064 x 6tE6Sp This calculation of 6V^ necessitates a knowledge of the value of 6Spt the decremental surface area ; the first contribution of 6Sp is zero since computation is commenced from a point which corresponds to the complete filling of 250 the pores Having calculated the amount given up by the pore walls and knowing the total decrement in the amount adsorbed, the decrement of capillary-condensed material 6Vk is obtained from = 6V - CV^. 6Vk is the volume of the inner cylinder (Fig.El) and the volume of the actual pore 6Vp is related to 6Vk by the expression 6VP * 6W rk|2 The values of 6Sp, the area of the pore walls corresponding to the decremental volumes, 6V is calculated from P 6Sp * 31.2(6Vp/rp) The cumulative surface area is the value obtained by the summation of all the values of 6S and likewise the total P pore volume Vp is calculated by summing all the individual decrements 6Vpl and then converting to liquid volume. The average pore radius is the radius of an equivalent uniform cylinder having the volume vg g5 and is given by r a = 2V0.95/SBET Typical pore-size di stribution data for samples used in this study is shown in Appendix G. The method of calculation of pore-size distribution detailed above is according to a procedure outlined by Gregg and Sing (19G7). 251 APPENDIX F Figure F1 shows a comparison of the predicted thiophene uptake vs. time profile using model III, for the three different forms of variation of the instantaneous stoichiometry a, viz., linear, exponential and hyperbolic. The curves are virtually identical although the predicted rate is lower by the hyperbolic type of variation. This however, only means a higher value of the rate constant k is needed in this case to approach the rate predicted by the other two forms. The curves presented here are for the value of k which was obtained by fitting the experimental data for sample F20 by employing the linear form [cf.section 6.21. Table F1 below shows the values of 3 for the three different forms of variation. The value of aT is 3.11 (sample F20) and the value of aQ is 5 corresponding to the initial condition that adsorption of thiophene is coplanar when the surface is bare. Table F1 Value of the parameter 6 Type of variation 0 Linear 3.93 Exponential 0.89 Hyperbolic 4.11 252 thiophene uptake (dimensionless) Fig. FI Comparison of linear, exponential and hyperbolic ochyperbolic and exponential linear, of Comparison FI Fig. APPENDIX 6 (Sample Calculations) I• Total BET Surface Area Sample F20 ; sample weight : 0.1355g Monolayer adsorption is given by nm = 1/(S+I) where, S and I are the slope and intercept of the linear regression curve (cf. Fig. 5.3). With the values for S and I from Fig. 5.3 n^/gcat is calculated to be 43.84 mg/gcat. The specific surface area is then given by nm/gcat A = ------x Avogadro number x projected surface Mol. wt. of N2 area of a molecule of N2 48.34 x 10"3 x 6.023 x 1023 x 16.2 x 10"20 28 = 152.8 m2/gcat. II. Pore-size Distribution n t r r p /pg rk P P ?k 6t 6rP © o e * e o • mg/gcat A A A A A A A 0.995 287.3 1902 35.4 1937 1 038.5 1064.1 19.4 1746.2 0.947 279.2 175 15.8 191 132.8 147.2 3.2 87.7 0.90 226.6 90.5 12.8 103.3 74.6 86.5 1 . 7 33.6 0.85 176.2 58.7 11.1 69.8 50.2 60.7 1.2 18.1 0.60 139.6 41.8 9.9 51.7 36.7 46.2 .9 11.0 0.74 120.6 31.7 9.0 40.7 27.0 35.5 1 . 0 10.4 0.65 98.7 22.3 8.0 30.3 19.9 27.6 . 6 5.4 0.58 87.7 17.5 7.4 24.9 15.7 22.8 .55 4.2 0.50 78.2 13.9 6.9 20.7 12.1 18.7 . 63 4 . 1 0.40 68.7 10.4 6.2 16.6 9.2 15.2 .52 2.9 0.30 61.3 6.0 5.7 13.7 254 v 6 v 6vf 6vk 6vp E6sp 6vp/6rp m 3 /gcat m 3 /gcat m 3 /gcat m 3 /gcat m 3 /gcat m /gcat x 1 0 1 0 229.7 6.43 0.00 6.43 6.75 0.00 0.004 223.3 42.1 0.04 42.05 51.66 0.20 0.589 18 1.2 40.3 1 .23 39.10 52.64 11.15 1 . 569 140.9 29.2 2.29 26.94 39.38 30.13 2.180 111.6 15.2 2.82 ' 12.38 19.58 50.37 1.781 96.5 17.5 4.06 13.48 23.35 63.60 2.252 78.9 8.8 3.35 5.42 10.44 84.12 1.930 70 . 1 7.6 3.39 4.21 8.91 95.91 2.131 62.5 7.6 4.34 3.26 7.72 108.09 1 . 885 55.0 5.9 4.00 1.91 5.19 120.98 1 .794 o to The above results are for a sample of S1. The plot of the above data was shown in Fig. 5.4(a) . III. Metal Area of the Catalvst Sample F20 ; From Fig. 5.6 CO uptake of Ni at zero pressure (obtained by extrapolation of the difference isotherm) = 5.45 mg/gcat. Therefore, 5.45 x 10-3 6.023 x 10 23 2 /gcatNi-area - 2/gcatNi-area 28 1.1 x 1019 m = 10.66 m2/gcat. where the saturation coverage value of 1.1 x 1019 molecules 2 • ? of CO per m , corresponds to an area of 9 . 1 A per CO molecule giving a stoichiometry of Ni/CO = 1.4. 255 IV. Extent of Reduction Sample F20 ; weight of catalyst as loaded = 0.127 g weight loss due to drying = .24 mg Therefore, weight of dried unreduced catalyst = 0.1246 g Then from appendix D-III, expected weight loss due to reduction is 16 0.01wM ---- x ------9 58.7 1 + ( 0 . 1 6/58.7 )M Here, M=0.1246 and w=20. Then expected weight loss is calculated to be 6.44 mg. Actual weight loss of the catalyst after taking into account the effect due to convective heat and loss of weight by drying was 6.35 mg. Then the extent of reduction f = 6.35/6.44 = .986 V. Metallic Dispersion From appendix D-II, metallic dispersion is given by n Dm = 29.35 — per cent wf For sample F20, n = 5.45 mg/gcat , w = 20 , f = 0.986 With these values 0M is calculated to be 8.17, VI Crystallite Size Determination a. By X-Ray Diffraction The crystallite size from x-ray line broadening is given by Scherrer equation (5.4). Sample F20 : 256 A=1.54178 A , K is taken to be 0.9 Bobs at 51.93 = 54.5 mm B inst = 8 mm Therefore, Bd = /(B obs. 2 - B. inst .2) = 53.91 mm = .0235 radian(20) Thus from Scherrer equation (5.4), 0.9x1.54178 6 1 = 59 A . 0235 x cos(.0235/2 ) b. By CO chemisorption From appendix D-I, the volume-area mean diameter is 1010 VA A ZDM Thus for sample F20 dVA = 1010/8.1 = 124.7 A VII Effective Diffusivitv From appendix C = 9.7 x 10 3 a (T/M^)1/2 O For sample F20, a = 50.1 A. The temperature T is 298 K and the molecular weight of thiophene is 84. With these values - 7 2 Dk is calculated to be 9.15 x 10 m /s. Also from appendix C, 012 = 4.75 x 10 - 5 m 2/s. Then given by 1/D^ * * 1/D1 g is calculated to be 8.98 x 10~7 m2/s and D = e 2D is 2.1 x 10 7 m2/s where e , the porosity of “ MM M the catalyst is 0.465. VIII Fractions of coplanar and perpendicular sites in model II Sample F20 ; The total thiophene uptake for this sample gives a Ni/C^H^S 257 ratio of 3.11. Then, ir> 4- ♦ 2 f 2 = 3 o r, 5 f 1 ♦ 5(1-f 1> which gives f = 0.37 and f2 = 1-f1 = 0.63 IX. Thiophene uptake of ideal crystallites Sample F20 ; 2 Ni-area : 10.66 m /gcat Diameter of crystallite, d c = 59 x 1o”10 m Area of a nickel atom, AN^=6.5 x 10 _ 2 Q m 2 /atom Then Nc, the no. of crystallites per gcat is 10.66 /it ( 59 x 1 0 “ 1 0 ) 2 = 9.75 x 1016 and the no. of surface Ni-atoms per gcat is 10.66/(6.5 x 10~20 ) = 1 . 64 x 1020 Therefore, the no. of surface atoms in a nickel crystallite, Ng is 1.64 x 1020/(9.75 x 1016) = 1682. The no. of surface atoms in an f.c.c. octahedron is given by Ng = 4m 2 - 8m + 6, where m is the no. of atoms lying on an equivalent edge. With Ng = 16 82 we have m= 22. The no. of corner atoms is 6 The no. of edge atoms = 12(m-2) = 240 The no. of central surface atoms = 4(m-3)(m-2) = 1520. Then f^ = .14 and f2 = .86 giving Ni/C^H^S = 4.58 258, at any time t, is obtained by
= ------mass of catalyst