Catalysis Volume 3

A Review of the Literature Published up to late 1978

By C. Kemball, D. A. Dowden

The Royal Society of Chemistry

Copyright © 1980 The Chemical Society
All rights reserved.
ISBN: 978-0-85186-720-5

Contents

Chapter 1 Catalysis on Well-defined Metal Surfaces and Non-metallic Substrates By S. J. Thomson, 1,
Chapter 2 The Steam Reforming of Hydrocarbons By G. W. Bridger, 39,
Chapter 3 Oxidation over Copper, Silver and Gold Catalysts By R. W. Clayton and S. V. Norval, 70,
Chapter 4 High-temperature Oxidation by Metals By M. H. Stacey, 98,
Chapter 5 The Catalytic Oxidation of Sulphur Dioxide By C. N. Kenney, 123,
Chapter 6 The Spillover of Chemisorbed Species By D. A. Dowden, 136,
Chapter 7 Catalysis by Zinc Oxide By C. S. John, 169,
Chapter 8 Catalysis on Non-faujasitic Zeolites and Other Strongly Acidic Oxides By M. S. Spencer and T. V. Whittam, 189,
Chapter 9 Polymerization By Carboanions and Carbocations By D. C. Sherrington, 228,
Author Index, 275,

CHAPTER 1

Catalysis on Well-defined Metal Surfaces and Non-metallic Substrates

BY S. J.THOMSON

1 Introduction

The previous review in this series was written with more emphasis on adsorption and techniques than on catalysis. With the passing of two years the number of papers devoted to catalysis has increased and this is reflected in the material in this review and in its arrangement. The papers chosen illustrate the direction in which the applications are proceeding; with limited space it cannot be exhaustive. It is hoped that the references given to books and to reviews will be of value to readers.

2 Changes in Surfaces Accompanying Adsorption and Catalysis

A surprising variety of changes occur in the surfaces of catalysts. There are intentional changes brought about through the use of modifying agents or poisons and there are inevitable changes which occur during the use of catalysts. Chemical intuition would also lead to the expectation of segregation of surface species, corrosive chemisorption, and adsorption-induced surface enrichments. There is in addition a range of more subtle and unexpected changes which might be collected under the heading of surface rearrangements.

Modifying Agents and Poisons. — Marbrow and Lambert have studied O2 adsorption on Na-modified Ag(110) surfaces. Group I and Group II metals modify catalytic properties of metal surfaces and are particularly important in the selective oxidation of ethylene to ethylene oxide over silver. LEED, Auger, and TPD were used in the study. Na could be adsorbed on Ag(110) to θ ~ 1 to form a (1 x 1) monolayer; dosing to the two-monolayer level produced a (1 x 2) LEED pattern. Thermal desorptions were studied and it was noted that some Na diffused into sub-surface layers. The main feature of the work with relevance to catalysis is that whereas O2 has a low sticking probability on Ag(110), if surface Na is present there is an enormously enhanced probability of O2 adsorption. There was also evidence for diffusion of O into the lattice, in that thermal desorption failed to show release of O2. Na atoms in the (1 x 1) structure were probably in simple register with Ag atoms. When sub-surface Na was present, added Na below the monolayer level produced a (1 x 2) LEED pattern. When Na-dosed specimens were exposed to O2, a (4 x 1) LEED pattern developed where it was likely that both O and Na were present in the surface as an ordered layer; a c(4 x 2) structure was also observed with a Na: O ratio of 1 : 1. What has not yet been resolved is the relationship between these results and the possibility that it is a molecularly adsorbed O2 which is active in ethylene oxidation.

Riassian et aL have used ESCA to investigate the changes in activity of supported Ag for ethylene oxidation; the origin of the change lies in the accumulation of organic impurities on the Ag/SiO2 or Ag/Al2O3 catalysts. Persistence of an unchanged Ag doublet peak meant that there was no change in the Ag: changes in the C 1s signal supported the model of organic deposition.

The next example is also concerned with modification of catalysts, again in an industrially important process. X-ray diffraction, Mössbauer spectroscopy, and SIMS have all been utilized to examine promoted iron catalysts for ammonia synthesis. After reviewing the problems associated with defining the states of the promoters, Ludwiczek et al. proceeded to establish that in Al2O3-promoted Fe catalysts the aluminium may appear, for example, as Al2FeO4 units endo-tactically built into the a-Fe surface; this accounted for the stability of the Fe surface.

A paper of significance in considering catalyst poisoning is that by Fisher, who propounds interesting views. He uses UPS information to interpret bonding of adatoms adsorbed on Ni(100). Taking the surface structures shown in Figure 1, he then distinguishes between two features of the system: (a) atoms may be adsorbed at sites with a number of nearest neighbours, i.e. the site co-ordination Cs, and (6) they may be bonded to only some or all of the neighbouring atoms; this is the bond co-ordination Cb.

In considering this situation the author chooses to examine UPS spectra for adsorbed S, Se, and Te and to compare spectra with solid-state spectra for these chalcogens. He found compounds and elemental solids in which the S, Se, or Te were bonded covalently to two neighbours. The main feature observed from known standards was a double-peaked spectrum, and the general result that the shape of the density of valence states is largely determined by the number of nearest neighbours to which the atom is bonded. Applying these ideas to the Ni(100) one half monolayer c(2 x 2) surface, the author finds the double peaks characteristic of 2-co-ordination. For one quarter coverage of the surface, from his own and other work, the author concludes that bonding is four-fold.

The overall conclusion reached, which is of importance to catalysis, is the possibility that the strong adatom–adatom interactions may alter electronic structure for the catalyst system and that this may be more important than a simple site-blocking mechanism for poisoning.

The subject of ISS has been considered in some detail by Baun. The technique is a powerful one for identification of atoms adsorbed on surfaces. It is best illustrated by an example. In catalysts where ionic solids are doped with foreign ions it must be of importance to know the distribution of the foreign ions. Though not used here in a catalytic study, ISS has been used to establish the way in which Pb concentrates in the surface of AgBr crystals. In the experiment Ne+ ions were used as projectiles with energy E1 scattered (through 90°) with energy E2. E2/E1 = (M2 – M1)/( M2 + M1), where M2 = mass of substrate atom and M1 = mass of neon atom.

As an example of the results it was found that for 1000 p.p.m. Pb impurity the Pb/Br ratio on the surface reached 13. For 300 p.p.m. Pb, the ratio was 4. The author proposes that the concentration of Pb2+ on the surface arises from the negative charge produced there by the movement of Ag+ ions into interstitial positions in the lattice.

Segregation. — In a number of catalytic systems in which a poison is present or in which C deposition takes place on a metal it is of importance to know the surface concentration of these species. Thus Grabke et al. have studied the equilibria of surface segregation of X (= C, N, or S) in the system X(disso1ved) = X(adsorbed) for Fe single crystals on (100) faces. They used LEED and AES.

At the equilibrium temperature within the solid solution range LEED showed c(2 x 2) structures for C, N, and Son Fe(100) surfaces. For sulphur, experiments were conducted in which the S content of two samples was 10 and 27 p.p.m. and the temperature range 650–850 °C. The ratio of S to Fe atoms in the surface was found to be constant(0.5) and to correspond to the c (2 x2) structure; the S formed a saturated layer at both concentrations. The behaviour was different for carbon, there being a wide range of values of the C/Fe ratio on the surface for changes in bulk concentration of C and temperatures between 550 and 850 °C.

N to Fe ratios on the surface showed constant coverage in the range 300–550 °C as measured by AES; thereafter surface coverage fell steeply. These results were obtained for 150 and 530 p.p.m. N.

There is a relationship between surface segregation and surface reaction kinetics. Thus for the carburization and decarburization of Fe, Fe, CH4 [??] C(dis-solved) + 2H2, the rate is given by

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

where nc/A = number of carbon atoms per unit area, k and k’ are rate constants for forward and back reactions, p represents partial pressure, and [C] is concentration of carbon in solid solution. It has been shown that this equation is associated with the rate-determining step

CH3(ads) = CH2(ads) + H(ads) (2)

According to equation (1) the forward rate is independent of [C] and with increasing [C] no retardation occurs. This has been proved for the temperature range 500–800 °C where, according to the segregation experiment of Grabke et al., the surface would be nearly saturated with C at a bulk concentration of [C] = 100 p.p.m. Thus the authors make the important deduction that the rate-determining step (2) does not take place on sites where C is chemisorbed, viz.in the quasi-internal (1/2 1/2) site on the (100) face; it must occur on ‘the outer surface’.

In contrast, the rate-determining step of the nitrogenation reaction N2 [??] 2N(dissolved) occurs on interstitial sites on the surface where N atoms are adsorbed: this is in accord with the rate equation (3), where K is the equilibrium

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

constant for the segregation reaction. In this case the reaction is inhibited by bulk nitrogen and the rate-determining step N2 (g) = 2N(ads) occurs on sites where one of the N atoms can enter the lattice.

The carburization reaction and the nitrogenation reaction are strongly inhibited by small amounts of sulphur. This is in accordance with the observed surface saturation by S at low concentrations in the solid.

The paper by Schouten et al. on an AES LEED ellipsometry study of the reaction of methane with Ni(110) might have been expected to have close similarity with the study of CH4-Fe by Grabke et al. This is not so, for the temperatures used by Grabke et al., >550 °C, were chosen to bring about the equilibration of bulk and surface C. Schouten et al. used surfaces with temperatures between 25 and 320 °C, where C diffusion was shown by them to be less important.

The authors begin by pointing out the large range of sticking coefficients for CH4, on Ni, viz. 10-3 to <10-12 for single crystals, films, and powder. They show that C deposition is affected by electron sources and they therefore studied adsorption without the operation of electron gun, ionization gauge, or ion pump. They conclude that excited CH4 or dissociated CH4, is readily adsorbed. Without excitation, CH4, does not undergo adsorption at room temperature on Ni(110). Between 200 and 327 °C, carbon is deposited in amounts which rise linearly with exposure to a saturation value, θc ~ 1/3. The authors discuss the kinetics of the deposition and use their LEED information to support the second of the possible mechanisms: (1) two-dimensional carbide islands all nucleate at the start of the process, then grow with constant radial rate; (2) one-dimensional carbide islands nucleate and grow during the entire exposure.

Surface Enrichments. — Alloy catalysts may or may not have the same composition on their surfaces as they do in the bulk. In the case of 22 atom % Pd-Au alloy the surface and bulk compositions, as measured by Maire et al using AES, are identical for clean surfaces. Now pretreatments in O have an effect on catalytic activities for Pd-Au and Pt-Au alloys. In the case of Pd-Au the O2 pretreatment induces surface enrichment of Pd.

The paper by Watanabe et al. is important for alloy catalysis; they examined the possible enrichment of Cu in the surface layers of Cu-Ni alloys. They used Auger spectroscopy to study the escape of low- and high-energy electrons (around 100 and 700–1000 eV) and were able to make quantitative in-depth profiles, of which an example is shown in Figure 2. This enrichment was also confirmed by AES measurements for films by Benndorf et al.

These examples of quantitative AES raise the question of the reliability of the method and the results. This topic is dealt with admirably by Hall et al. They conclude that the accuracy of quantitative analysis is <30% using peak heights and relative sensitivity factors. Improvement can result from the use of standards in the same concentration range as the unknown.

Moss et al. have used AES as their source of information on surface composition in alloy catalysts in their study of ethylene hydrogenation over Ni-Pd films. Mössbauer spectroscopy has also been used by Lam and Boudart to examine Au in Pd-Au particles on SiO2. Alloying and uniform Au distributions were observed.

Although at first sight Auger spectrometry might be expected to give fruitful information on surfaces, there are unexpected problems. In their summary of the Ni-Cu alloy system Harberts et al. point out that analysis based on high energy peaks (0.7–0.9 keV) suggested a surface composition equal to that of the bulk; low energy peaks (~0.1 keV) revealed Cu-enrichment of the surface over a wide range of composition. In addition they find that CO can bring about a gas-enrichment of Ni in the surface. They state that the ‘corrosive’ character of CO adsorption leads to Ni atoms of the sub-surface layer becoming accessible to CO and that these Ni atoms are irreversibly attracted to the surface. Surface segregation has been further explored in that hydrogen-induced segregation of S has been observed for Pt(100) and Cu(100); the authors, Szymerska et al., conclude that it is a common phenomenon.

Rearrangement of Surfaces. — LEED continues to be used in establishing whether or not substrates and adsorbed species are regular in their geometrical arrangements. Together with electron spectroscopy for the examination of surfaces for cleanliness, or for the extent and nature of adsorption, the techniques provide information on well-defined surfaces. One of the striking features to emerge from such studies is the phenomenon of rearrangement of surface atoms. This process can occur on well-defined surfaces when chemisorption takes place. Good examples are provided by Lambert and his co-workers and references to their work appear in Bridge and Lambert’ on the (1 x 2) [right arrow] (1 x 1) change in Pt(110) upon chemisorption. They were interested in adsorption studies which might be of use in suggesting models for catalytic reactions such as C2N2 on Pt(ll0) and C2N2 + CO on Pt(ll0).

Their work was extended further than most in this field by using UPS to examine 0-containing organic molecules on clean Pd. In this latter study at 120–300 K of CH3OH, CH3COCH3, H2CO,CH3CHO, and CH3OCH3, chemisorption occurred primarily through the lone-pair orbitals associated with the O atoms. The evidence was the shift to larger binding energies relative to the adsorbate valence levels. Decomposition of these molecules to CO occurred at 300 K.

To return to the theme of rearrangement, the effect of the presence of foreign atoms on the arrangement of a surface has been successfully demonstrated by Haas et al. They used various methods to produce different amounts of O and C on an Ir(100) surface:

(a) flashing for 2 min at 1650 °C gave a surface with 9% C and 0.5% O (Ca and K <0.2%);

(b) BaO deposition and flashing gave 3% C, 0.2% O;

(c) flashing after (b) gave 7% C;

(d) O2 treatment gave up to 1% O, which may lie below the surface.

The results were elegantly summarized by the authors as shown in Figure 3.

Adsorbed Species. — Now that some features of the substrate have been examined we can turn our attention to adsorbed species. As Saijo et al. say in introducing their elegant study, information on orientated adsorption of organic molecules on solid surfaces is an essential requirement for understanding catalysis. They point out that LEED experiments, while producing information on regularity or otherwise of adsorbed arrays, do not give information on correlation between molecular positions and substrate atoms. In what seems to be a successful attempt to overcome this problem the authors used LEED to establish the maintenance of surface regularity after adsorption of TCNE (tetracyano-ethylene) on KCl cleavage faces. Then AES was used to study changes in the K and Cl signals as adsorption took place. A significant fall in the signal from K+ by a factor of ten was interpreted as evidence for CN adsorption on this ion, with C=C groups adsorbed on Cl- sites. The Cl signal fell by a half and the authors point out the consistency of these changes in signal strength with the model for the adsorbed state shown in Figure 4.

In interpretation of LEED patterns there seems to be conflict between protagonists of

(i) corrosion models in which surface compounds are formed, and

(ii) ‘on-top’ models where the adsorbed layer lies above the metal surface: (a) with the adsorbed atoms all lying above the same kind of sites, but with vacancies in the network, and (b) with the adsorbed layer having its own periodicity which is distinct from that of its underlying metal layer, with site coincidence occurring after say 6 or 11 atomic distances in the adsorbed layer.

(Continues…)Excerpted from Catalysis Volume 3 by C. Kemball, D. A. Dowden. Copyright © 1980 The Chemical Society. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Catalysis Volume 7

A Review of the Recent Literature Published up to end-1983

By G. C. Bond, G. Webb

The Royal Society of Chemistry

Copyright © 1985 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85186-584-3

Contents

Chapter 1 Metal Catalysed Methanation and Steam Reforming By J. R. H. Ross,
Chapter 2 Catalyst Characterization with Neutron Techniques By C. J. Wright,
Chapter 3 High Resolution Solid State N.M.R. – Theory and Applications By A. D. H. Clague,
Chapter 4 Oxidation by Catalysts Containing Vanadium By P. J. Gellings,
Chapter 5 Hydrodenitrogenation By M. J. Ledoux,
Chapter 6 Structure and Properties of Supported Noble Metal Catalysts By R. Burch,

CHAPTER 1

Metal Catalysed Methanation and Steam Reforming

BY J. R. H. ROSS

1 Introduction

The methanation and steam reforming reactions are closely inter-related and, in general, catalysts used for one reaction will be usable, with some limitations, for the other. This similarity arises from the fact that both reactions occur under reducing conditions over metallic (most commonly, nickel) catalysts and, more importantly, that the types of reactive surface intermediate found during one reaction are also found during the other. Both reactions also suffer from the same constraints, for example, carbon deposition and susceptibility to sulphur poisoning, and hence similar approaches are adopted in both cases in attempts to overcome these constraints. The conditions under which the reactions are carried out depend to a large extent on the composition of the reactant mixture and, in the case of steam reforming, on the desired product distribution. The two reactions have another very different factor in common: there has been a considerable resurgence of commercial interesting variants of both processes. For example, although the methanation reaction has been known since the beginning of the century and it has been practiced commercially to remove traces of carbon monoxide prior to the synthesis reactor in ammonia plants, there has recently been considerable activity on the subject of the methanation of synthesis gas; this activity has arisen because of a resurgence of interest (if only transient) in coal gasification. Prior to the increase in oil prices that sparked these renewed efforts in coal gasification, there was also an increased interest in processes and catalysts for the production of synthetic natural gas (SNG) by the steam reforming of the then cheap naphtha fractions of crude oil. These developments have led to considerable research on the catalysts for these processes and also on the reactions themselves. For example, a total of 26 reviews were published on the subject of methanation in the first six months of 1982 and this puts the subject in the top fifteen ‘research fronts’ in the physical, chemical, and earth sciences. Under the index terms ‘methanation’ and ‘methanation catalysts’ alone, there were approximately 200 articles listed in the 1972–1976 cumulative index of Chemical Abstracts. Under the corresponding headings of the 1977–1981 index, there were about 650 references and there were 155 references in 1982 alone. Clearly, with such an enormous literature and with such an extensive coverage by reviews, it would be unreasonable to attempt to give a comprehensive description of all the work in the subject area embraced by the title of this review. Instead, an attempt will be made to draw a general picture of progress in steam reforming and methanation, with particular emphasis on the catalysts used. Most of the literature covered will be that from the last few years but, of necessity, some earlier work will also receive mention. The structure of the review will be such that a number of the processes themselves will be described in rather general terms in order to establish the requirements for the catalysts; some of the catalysts used for the processes themselves, particularly those based on nickel, will then be described, with particular emphasis on improvements in knowledge of the structure of these materials; finally, a brief description will be given-of some of the more relevant academic publications on the steam reforming and methanation reactions over these catalysts.

2 The Processes

Steam Reforming. – The steam reforming reaction may be described by the general equation:

CnH2n+2 + nH2O [right arrow] nCO + (2n + 1)H2 (1)

The CO formed may take part in two further reactions, the water-gas shift reaction:

CO + H2O [??] CO2 + H2 (2)

and the methanation reaction:

CO + 3H2 [??] CH4 + H2O (3)

Both of these reactions are exothermic and are favoured by reduction in temperature. Hence, while the products of the steam reforming reaction at higher temperatures (~800 °C) are CO and H2, lower temperatures are used to produce methane-rich gases; in this case, the overall reaction can be approximated by:

Cn + H2n+x + (n – 1)/2 H2O [right arrow] (3n + 1)/4 CH4 + (n -1)/4 CO2 (4)

The thermodynamics of these reactions have been discussed in some detail elsewhere.

High-temperature Steam Reforming. The high-temperature steam-reforming reaction is one of the most commonly occurring industrial processes. The major use of steam reforming is in ammonia plants, when the feedstock is most generally natural gas, but other feedstocks such as naphtha or LPG (liquefied petroleum gas) may be used if there is an economic advantage to be gained. The modern generation of ammonia plants have capacities of ~1000 tons per day of ammonia and utilize some 20 m of catalyst in the primary steam-reformer tubes. The service life of a primary steam-reforming catalyst is generally of the order of 2–3 years; however, the catalyst can still have adequate activity at this stage, the replacements being timed to coincide with routine maintenance of the plant. The secondary steam reformer contains a similar quantity of catalyst but here the duty is somewhat less and lives of around 5 years are normal. The secondary reformer in an ammonia plant brings about the complete conversion of the hydrocarbon feedstock by the injection of air to the process-steam prior to the reactor, the amount of air being adjusted to give the required amount of nitrogen for ammonia synthesis. However, although undoubtedly some of the reaction occurring in this bed is steam reforming according to equation (1), the main reaction can be thought of as that of the oxygen of the air with some of the product H2 and CO to form H2O and CO2. There are currently more than 100 plants in the world with capacities of the order of 1000 tons per day and it has been argued that in order to keep up with the fertiliser requirements for the production of food for an expanding world population, a new large-scale plant will need to be constructed each month.

The requirements of the primary reforming catalyst are generally thought to be greater than those of the secondary reformer. The predominant reaction is that given by equation (1), with n = 1 or higher, depending on the availability of fuel. For any value of n, the reaction is highly endothermic and so considerable heat has to be supplied to the reactor; this is generally achieved by burning a proportion of the feedstock, the flame being played directly on the exterior of the reactor tubes. However, in the steam-reforming reactor of the so-called Adam and Eve system (to be discussed further below), the heating is achieved by using a flow of preheated helium (<950 °C). In order to achieve the desired conversions, the exit temperature of the catalyst bed is generally of the order of 820 °C. The inlet temperature achieved will depend to a large extent on the activity of the catalyst. If the catalyst is relatively active and the majority of the conversion occurs near the beginning of the bed, then the inlet temperature may drop to values of the order of 450 °C, as shown schematically in Figure 1. The function of the remainder of the bed is then largely to shift the product distribution towards that corresponding to the exit temperature, i.e., with reactions (2) and (3) as far as possible to the left-hand side. In conventional hydrogen plants, it is common practice to direct most of the heat at the beginning of the reactor tubes to encourage as large a conversion as possible at that point. The effectiveness of the catalyst in the primary reformer is often expressed by the approach to equilibrium of the exit gas. This quantity is computed by working out the temperature required to give an equilibrium gas mixture corresponding to the exit-gas composition and comparing this with the measured bed temperature at the exit; an approach of 0°C corresponds to complete equilibration of the gas mixture while an approach of greater than 10 °C will indicate that the catalyst is not as effective as it should be. Operating on methane as feedstock, an active catalyst can give an exit gas containing of the order of 0.1% CH at a bed exit temperature of 850 °C, but higher proportions are common. As the catalyst ages, for example by sintering, the temperature profile will gradually move down the bed, as is shown schematically in Figure 1, and the approach to equilibrium will deteriorate.

In ammonia plants, the secondary reformer is included to decrease further the proportion of methane in the final gas and also to introduce the required amount of nitrogen for ammonia synthesis. The bed temperature is maintained at ~ 1000 °C and this is achieved by adding air to the gas stream, the oxygen of the air reacting with the hydrogen of the gas stream to form water. The reactor consists of a packed bed and no additional heating is required. The exit gas contains less than 0.1% CH4. The catalyst for this reactor does not require to have very high activity but it must be stable under these reaction conditions.

A variant of the continuous steam reforming process for hydrogen production is the cyclic reforming process which is used largely for the production of towns’ gas by the steam reforming of naphtha feedstocks. In these plants, the catalyst is maintained in a wide, relatively shallow bed which is heated by a flame fueled by the feedstock being used. When the upper part of the bed has reached a temperature of about 725 °C, the reactor is purged with steam and then steam reforming is begun, reaction being continued until the temperature drops considerably. The system is then purged once more and the bed is again heated with a flame. During steam reforming, carbon is deposited on the catalyst and this is burnt off again, exothermically, during the heating phase. In typical plants, cycle times are of the order of 4 min and steam reforming occurs for approximately half of that time. A typical exit gas contains 56% H, 15% CO, 6% CO2, 19% CH4, 4% N2, and a trace of oxygen. The catalyst for these purposes must be mechanically very stable and be able to resist the stresses caused by carbon deposition and by rapid cycles in bed temperature. As a result, the catalysts are often supported on refractory oxides such as α-Al2O3 (see later section dealing with the catalysts for these processes). Recent modifications of the cyclic reforming process include air injection during the steam reforming process; this apparently gives an improvement in the efficiency of the process. The process has also been used to steam reform methane to towns’ gas in situations where conversion of gas mains and appliances is not economical.

Low-temperature Steam Reforming. The steam reforming of naphthas at lower temperatures is used to produce methane for use as substitute natural gas (SNG) particularly in situations where there is a shortage of natural gas or for supplementing supplies at peak-load periods. By operating at temperatures of the order of 450 °C, the methanation reaction is favoured and the all-over process can be represented by equation (3). The early developments in this area were carried out by the British Gas Corporation, whose Catalytic Rich Gas (CRG) process is in wide-spread use. The latest variant of the process, entitled the CRG Hydrogasification Process, has been described in some detail by Gray. In this, several CRG reactors are used. After the first, further naphtha is added to the product gas and gasified in another CRG reactor operated at lower temperature; the use of the unreacted steam from the first reactor to convert more naphtha improves the overall efficiency of the process. Another variant of the CRG process recirculates some of the product gas from the first CRG reactor back to its inlet. This is claimed to reduce the speed of catalyst deactivation and to enable heavier feedstocks to be gasified successfully.

A number of other commercial processes have been described which are similar to the CRG processes discussed above. For example, Skov has claimed a process in which half the reactant stream is fed to the first reactor. The product of this reactor is combined with the remainder of the reactant stream and fed to a second reactor in which a methane content of greater than 95% is achieved. Similarly, Nikki has claimed a process in which, after steam reforming at 350 – 550 °C, a CH content of 98% is achieved by methanation at a temperature of 220 °C.

Methanation of Coal-derived Synthesis Gas. – The majority of the energy requirements of the world are supplied by fossil fuels, i.e., natural gas, oil, and coal. Which is the preferred feedstock at any time and in any geographical situation depends on a complex inter-relationship between political, economical, and environmental factors. At the present time, oil is still the preferred feedstock in most developed nations because of its price and because well-developed technology exists to utilize most of the factions of the oil. The lighter factions are used as chemical feedstocks and for petroleum and domestic heating purposes while the heavier fractions are used, e.g., in electricity generation. Whenever natural gas is available, it is used as an alternative to oil, both as a fuel and as a chemical feedstock. Coal is generally, however, used as a fuel and only in places where there is a lack of oil and gas is it used as a chemical feedstock. The gasification of coal and the Fischer –Tropsch process for hydrocarbon production were developed in Germany in the period prior to the second World War. This technology is now practiced in a number of plants operated by SASOL in South Africa. The gasification in these SASOL plants is carried out in Lurgi Gasifiers. In such as gasifier, a fixed bed of graded coal is exposed, under pressure, to a mixture of steam and oxygen, the ash produced being discharged by a rotating grate as an unfused granular solid. The Lurgi process was, at least until recently,’ the only commercially proven process in the world suitable for the manufacture of SNG from coal. The greatest problem with such a gasifier is the requirements to supply sufficient excess steam to keep temperatures in the fuel bed below that at which the ash, which forms a substantial proportion of the coal, melts or ‘clinkers’ and causes problems in the grate of the gasifier. The addition of excess steam reduces the efficiency of the plant and also increases the cost of treatment of the effluent from the plant. However, if only enough steam is added to the gasifier to ensure complete reaction, bed temperatures around 2000 °C are produced and this is sufficient to melt the ash which can then be removed as a slag at the bottom of the gasifier. The so-called ‘Slagging Gasifier’ was initially developed by British Gas between 1955 and 1964, when the project was closed down. The work was recommenced in 1974 and has resulted in the British Gas ‘High Carbon Monoxide’ (HCM) process, which incorporates water-gas shift and methanation reactors and produces methane (SNG) with good efficiency. Some of the work was carried out in collaboration with Conoco and details of part of it have been published.

The future of coal gasification for SNG production is uncertain as it depends upon a number of economical and political factors as much as upon technical achievements. It is felt in some quarters that the solution to the current problem of ‘Acid Rain’ may lie not in the use of SO2 (and NOx) removal processes in the flue gases of oil- and coal-burning power stations but in the prior gasification of the coal or oil, desulphurization of the syngas produced, followed either by the combustion of the syngas or methanation to give SNG. The SNG can then either be distributed to consumers or itself used for electricity generation; the heat liberated in the methantion process would also make an important contribution to the energy balance (see also the following section). At the time of the energy crisis, a number of government and industrially sponsored coal-gasification projects were started but, with the stabilization of oil prices, a proportion of these appear to have been cancelled.

A number of papers have described the design of reactors for methantion of coal- and naphtha-derived synthesis gas; Frohning and Hammer have reviewed some of these and Cornils has described research in West Germany on methanation (and on the Fischer–Tropsch reaction) and the reactors used in this work. A number of different conformations have been described. In one arrangement, a high recycle ratio of product gas is passed back through the methanator to moderate the temperature rise. (A rough rule of thumb is that there will be an 80 °C temperature rise for the conversion of each 1% CO in the feed gas). An alternative arrangement, which seems to be more generally favoured’ (see also the next section) is that using three consecutive reactors. The inlet temperature of the first may be ~ 400 °C and there may be a temperature rise of more than 300 °C, even with control of temperature by the injection of some steam; the inlet temperatures and temperature rises of the subsequent beds are somewhat lower. A Union Carbide patent describes a two-bed system in which half the feed is water-gas shifted and then combined with the remainder of the stream and fed to the methanation reactor, this feed-gas now having a CO content of 3–6 vol %. The Forster Wheeler Energy Corporation has described the use of a reactor which includes a twisted nickel ribbon as catalyst and which operates with an outlet temperature of 785 °C and gives a gas containing 54.5% CH4, 41.1% H2O, 0.4% CO, 3.5% CO2, and 0.5% N2, i.e., almost complete conversion. Pennline and his colleagues from the Pittsburg Energy Technology Center have reviewed the use of catalyst-sprayed tube-wall reactors for methanation, while a patent to the French Institute of Petroleum (IFP) describes the use of slurry reactors operating at 200–350 °C. Details of some of the pilot methanation plants in current operation were summarized in the review by Hohlein, Menger, and Range.

(Continues…)Excerpted from Catalysis Volume 7 by G. C. Bond, G. Webb. Copyright © 1985 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

Catalysis Volume 6

A Review of the Recent Literature Published up to mid–1982

By G. C. Bond, G. Webb

The Royal Society of Chemistry

Copyright © 1983 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85186-574-4

Contents

Chapter 1 Oscillatory Phenomena in Heterogeneous Catalysed Oxidation Reactions By D. Mukesh, M. Goodman, C. N. Kenney, and W. Morton, 1,
Chapter 2 Strong Metal–Support Interactions By G. C. Bond and R. Burch, 27,
Chapter 3 The Catalytic Hydrogenation of Organic Compounds – A Comparison between the Gas-phase, Liquid-phase, and Electrochemical Routes By M. D. Birkett, A. T. Kuhn, and G. C. Bond, 61,
Chapter 4 Structural Characterization of Surface Species and Surface Sites by Conventional Optical Spectroscopies By A. Zecchina, E. Garrone, and E. Guglielminotti, 90,
Chapter 5 Use of Radiotracers in the Study of Surface Catalysed Processes By G. F. Berndt, 144,
Chapter 6 Hydroformylation By B. A. Murrer and M. J. H. Russell, 169,
Chapter 7 Formation of Oxygenated Products from Synthesis Gas By E. K. Poets and V. Ponec, 196,

CHAPTER 1

Oscillatory Phenomena in Heterogeneous Catalysed Oxidation Reactions Oxidation Reactions

BY D. MUKESH, M. GOODMAN, C. N. KENNEY, AND W. MORTON

1 Introduction

The observation of oscillations in heterogeneous catalytic reactions is an indication of the complexity of catalyst kinetics and makes considerable demands on the theories of the rates of surface processes. In experimental studies the observed fluctuations may be in catalyst temperature, surface species concentrations, or most commonly because of its accessibility, in the time variation of the concentrations of reactants and products in contact with the catalyst. It is now clear that spontaneous oscillations are primarily due to non-linearities associated with the rates of surface reactions as influenced by adsorbed reactants and products, and the large number of experimental studies of the last decade have stimulated a considerable amount of theoretical kinetic modelling to attempt to account for the wide range of oscillatory behaviour observed.

Several homogeneous gas- and liquid-phase reactions are now also known to exhibit self oscillations and it is clear that many living organisms depend on coupled oscillatory reactions catalysed by enzymes to control biological functions. However, only heterogeneous oxidation reactions catalysed by noble metals are reviewed here. Experimental studies are first described, followed by a discussion of kinetic analyses which have been put forward to account for them. Particular attention is given to the most extensively studied system to date, the oxidation of CO over Pt catalysts.

2 CO Oxidation

Sheintuch and Schmitz have thoroughly reviewed oscillatory oxidation reactions up to 1977. A year later another review was published by Slinko and Slinko. Varghese et al. reported oscillations in the rate of CO production during the oxidation of CO over Pt supported on γ-[Al.sub.2][O.sub.3] catalyst between 100–150 °C. They also observed multiple peak limit cycles in gas-phase concentrations in the presence of hydrocarbon impurities. Oscillations have been observed by Plichta and Schmitz and by Sheintuch over Pt foil over a temperature range of 150–250 °C. The former authors also detected simultaneous oscillations in the catalyst temperature. The time period of oscillation observed by Sheintuch was of the order of 25 min. The oscillations were of single and multiple peak type. The feed gas concentration was varied between 0–10% CO and 10–20 0. Turner et al. observed rate oscillations on Pt wire over a wide range of temperature from 150–300°C. The oscillations were observed when the ratio of partial pressures of CO to 0 was in the range of 0.001 to 0.045. Dauchot and Van Cakenberghe have also observed oscillations on Pt wire. Gray et al. and Barkowski et al. have found oscillations in the production of CO2 when a feed of 5% CO in O2 was passed over polycrystalline Pt at 250°C. Hugo and Jakubith reported oscillations when a mixture of CO and air was passed over Pt gauze at 120°C. The time period of these oscillations was around 2min. Beusch et al., McCarthy et al., and Rathousky et al. have observed oscillations at 180 °C over a Pt catalyst supported on A10. Beusch et al. also found oscillations in catalyst temperature of the order of 2–3 °C. The oscillations observed by Rathousky et al. could be spikes or quasi-sinusoid al with a period of oscillations as high as 8h. Turner et al. found oscillations in C0 production under similar conditions even when the reaction was carried out over polycrystalline Pd or Ir wire. Sales et al. observed formation of an ‘oxide’ layer on the Pt wire during the reaction and they suggested this fact as the cause of the observed oscillations.

3 H2 Oxidation

The oxidation reaction of H2 has also been shown to exhibit relaxation and sinusoidal oscillations over noble metal catalysts for a wide range of temperatures. Thus Wicke et al. observed oscillations during the oxidation of H2 over 0.4% Pt on SiO2–Al2O3 support over a temperature range of 95–200 °C. The variation in the catalyst temperature was of the order of [+ or -] 20 °C. Rajagopalan and Luus observed oscillations over Pt wire in the presence of impurities. Beusch et al. observed similar oscillations when a mixture of 3.14% H2 in air was passed over Pt catalyst at 80 °C. Horak and Jiracekand Zuniga and Luus have also observed oscillations over Pt catalyst. The former authors observed that there was a very large difference between the catalyst and the gas temperature when oscillations occur. Kurtanjek et al. observed oscillations on Ni plate over a temperature range of 160–400 °C. Belyaev et al. found oscillations over Ni foil at 180°C in the presence of excess H. The period of oscillation varied between 6 and 120 s. Schmitz et al. observed multi-peak oscillations and chaotic behaviour when H2 oxidation was carried out on Ni catalyst in the presence of excess H2.

4 Hydrocarbon Oxidation

Vayenas et al. observed multiple peak oscillations during the oxidation of ethene over polycrystalline Pt film between 200–400 °C. Formation and disintegration of an oxide film on the catalyst was given as the reason for the observed periodic behaviour. Sheintuch and Luus recently observed oscillations during the oxidation of propene when 1% propene in 0 was passed over Pt wire over a temperature range of 175–228 °C. Krylov and his co-workers have also reported oscillations in cyclohexane oxidation on Zeolite NaX and in propene oxidation on the surface of CaO–MgO solid solutions.

5 NH3 Oxidation

Stephanopoulos et al, have reported oscillations in studies on the oxidation of NH3 over Pt wire or foils. In this case there were temperature fluctuations on the catalyst of the order of 20 °C. The feed consisted of 20&dnash;40% NHin air. The period of oscillation varied from 1–200s. The oxidation products consisted of NO, N2, and H2O.

6 Oxidation of CO/H2 or CO/Hydrocarbon Mixtures

Oscillations have been observed in this department when mixtures of gases were oxidized over supported Pt catalyst, although such behaviour was not found during the oxidation of CO alone. Multi-peak oscillations have been reported during the oxidation of mixtures of CO and but-1-ene above 150°C when the feed consisted of 2% CO, 3% O2, and 1% but-1-ene. The period of oscillation varied from 1.5 to 90 min. Goodman has observed sinusoidal oscillations during the oxidation of a mixture of CO and H2 or CO and trans-but-2-ene at similar conditions. Cutlip and Kenney have also observed relaxation-type oscillations during the oxidation of a mixture of CO and propene.

In contrast to other studies, oxidation carried out in this department on a Pt/γ-Al2O3 catalyst has not uncovered any oscillatory behaviour in the temperature range of 100–185 °C. Addition of a hydrocarbon like but-1-ene, but-2-ene, or propene induces sinusoidal or relaxation type oscillations at temperatures above 150°C. The experimental set-up used consists of a continuous recycle reactor system. The catalyst is packed in the cylindrical tubes. The gas flow rates are precisely measured with a bubble flow-meter. The reactor outlet is connected to a magnetic deflection mass spectrometer. An electronic peak select unit allows up to four mass numbers to be continuously monitored. The output data are connected to a PDP 11/45 computer for automatic and fast data logging. The data thus stored in the computer can be analysed later. The line diagram of the experimental set up is given in Figure 1.

Limit cycles observed when mixtures containing 2% CO, 3% O2, and 1% propene at 68 cm3 min-1 and 44 cm3 min-1 are shown in Figures 2 and 3 and mixtures containing 2% CO, 3% O2, and 1% but-1-ene at 50cm3 min-1 and 68.9 cm3min-1 are shown in Figures 4a and 4b. The but-1-ene system exhibits multi-peak relaxation oscillations at higher flow rates with very long time period of oscillation. A feature of this catalyst system is that the CO2 product is adsorbed on the A12O3 support and desorbs somewhat slowly relative to the other rate processes.

7 Forced Periodic Oscillations

A related oscillatory phenomenon is that in which the concentration of one or more reactants, fed to a flow reactor, is varied in time. Such forced periodic feed oscillations during oxidation reactions have now been studied by a number of authors. It is found that not only can conversion be increased but the selectivity of certain parallel reactions can be improved, which may be of value in industrial applications. Cutlip and Abdul-Karem and Jain have observed increased conversion during the oxidation of CO over both Pt and V2O5 catalysts. Hegedus et al. have also observed an improvement in conversion during the oxidation of CO and NO over Pt/γ-Al2O3 at 500°C. The feed was switched between NO and a mixture of CO and O2. Unni et al. observed a 30% improvement in selectivity during the oxidation of SO2 over V2O5. Renken et al. showed experimentally that improvement in selectivity could be achieved during the oxidation of ethene over Ag catalyst. Horn et al. and Bailey et al. have demonstrated theoretically that the selectivity where parallel reactions occur could be improved by varying feed composition in periodic manner.

Another advantage of forced periodic feed experiments, which has not been fully exploited so far, is that the technique could be used for kinetic model discrimination, a technique in which large deviations could be induced into calculated reponses between rival models under consideration. Hawkins has carried out experiments on oxidation of CO for discriminating between several Hougen and Watson rival models. Cutlip et al. have compared experimental forced periodic feed CO oxidation experimental transients with simulations using an elementary step model and compared theory with experiment in studies of the variation of the conversion as a function of time period of the forced oscillation.

8 Steady State and Dynamic Models

In considering kinetic models which can display oscillatory behaviour, it is useful to recall the Langmuir–Hinshelwood approach to a simple reaction such as the oxidation of CO, taking place in a closed system and consider the commonly adopted assumptions:

CO + 1/2O2 -> CO2

The adsorption of the gases on surface metal sites followed by reaction could be written as:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

The rate of adsorption of CO from the gas phase is

d (CO)/dt = -k1(CO)θv + k-1 (CO-S) (2)

whilst on the catalyst surface:

dθ CO/Dt = d(CO – S)/dt = k1(CO)θ v – k-1(CO – S) – k3(CO – S)(O – S) (3)

where θv is the number of vacant sites = 1 – θCO – θ0, and θCO is the surface coverage of CO. Similar expressions exist for O2 adsorption but allow for dissociative adsorption. The surface reaction rate term is usually written as first order in both adsorbed species, that is second order in surface concentrations. Thus:

d(CO2)/dt = k3θCO θ 0 – k-3(CO2) (4)

where θCO and θ0 are the surface coverages of CO and O2, respectively, on the catalyst. The following 5 assumptions are then usually made.

1. The adsorption–desorption steps are fast compared with surface reaction so the term in k3 can be neglected in equation (3).

2. A steady state is established between gas-phase and surface concentrations so the time derivatives in equations (2) and (3), etc. are effectively zero.

3. Surface concentrations such as θCO may be written in terms of k1/k-1 = KCO, the equilibrium adsorption constant, and gas-phase concentrations.

4. In this reacion the reactants compete for the same type of surface site.

5. CO2 is not adsorbed on the metal surface sites giving k-3 = 0.

These assumptions lead to an expression for the rate of formation of CO2 in terms of reactant gas-phase pressures.

d(CO2])/dt = k3θCO θ0

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

Such equations have some success, albeit often qualitative, in describing the variation of reaction rate with CO and O2 partial pressures. In particular if [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], then at low pCO the rate is proportional to pCO, and at sufficiently high pCO the rate falls due to high coverage of surface sites with CO, the rate becoming inversely dependent on CO pressure as first observed by Langmuir. It will also be noted that in this very simple formulation all rate constants are assumed to be independent of surface coverage.

In an open system, such as a well mixed flow reactor (CSTR), the convective transport of reactants and products must be allowed for; the analogue of equation (2) is:

Vd(CO)out/dt = vin(CO)in – vout (CO)out – k1(CO)outθ v + k-(CO-S) (2′)

Here V is the reactor gas-phase volume, v the volumetric flow rate to and leaving the reactor, and the concentration defined by the subscript ‘out’ applied throughout the reactor since it is well mixed.

If the set of assumptions (1–3) apply, then the steady state concentration of CO leaving a CSTR for a given (CO)in is obtained by solving the non-linear algebraic equation:

vin(CO)in – vout(CO) out = rx = 0 (5′)

where rx, the rate of CO oxidation, is given by (5′). We shall call equations (2), (3), and (2′) with finite derivative terms ‘elementary step’ equations and equations of the form (5) and (5′) ‘steady state’ equations.

Equation (5′) has a number of important and well known features which follow from inspection of Figure 5, where the O2 concentration is calculated from an analogous coupled mass balance equation for O2.

(a) For appropriate values of v, V, and (CO)in, the system can have one or two stable steady states, one of high conversion and one of low conversion. These stable states can be on either side of a third ‘unstable state’, that is unstable with respect to concentration perturbations in (CO).

(b) The flow reactor shows quite different features from those displayed by the corresponding batch reactor; for which the rate-concentration behaviour would have to be much more non-linear than (5) to show multiples states.

(c) Alterations in the relative positions of the rate-concentration curve (by altering the temperature) or the mass balance line (by changing the inlet concentration or flow rate) can induce transitions from one stable state to the other.

(d) A theoretical mechanism which might produce (non-linear) oscillations is the existence of an additional ‘slow’ variable which periodically alters the relative position of the rate envelope relative to the mass balance line so producing periodic transitions between high and low conversion states. This picture is conceptually relatively simple but unfortunately of limited utility in accounting for experimental observations.

In an attempt to describe what are clearly time dependent phenomena, recourse has been made to solving equations of the forms (2), (3), and (3′). The systematic study of the dynamics of a heterogeneous reacting system of n reactants unfortunately involves the solution of at least (2n + 1) coupled non-linear ordinary differential equations; n for gas-phase reactants, as in equation (2′), n for each surface concentration, as in equation (3), and at least one surface reaction equation like equation (4). The absence of a tractable theory of the stability and behaviour of sets of coupled non-linear differential equations, which are also ‘stiff’, has necessitated the use of simplifying assumptions wherever possible. A widely adopted approach is to reduce the dynamical variables in the problem preferably to two, and apply a standard linearized stability analysis to the equilibrium points of the differential equations.

9 Modelling of Oscillations in CO Oxidation

The oxidation of CO over Pt/Al2O3 catalyst can be represented with an elementary step model described in equation (1), together with an extra equation for the adsorption of CO2 on the support:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1a)

where S’ represents a vacant site on alumina. The simpler assumptions that can be made for describing the reaction in a CSTR are given below.

1. Neglect reaction between adsorbed O2 and gaseous CO (Eley–Rideal step).

2. Rate constants are independent of surface coverages.

3. Oxygen dissociatively adsorbs on the catalyst.

4. All gases compete for the same type of sites on the Pt.

The model equations describing the response of a well stirred reactor catalysing the above reaction system are:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (6)

The rate constants are in dimensionless form. The differential equations for the two gases (C1 and C2) and their corresponding surface concentrations (C4 and C5) can be represented in a simplified form as:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (7)

The modified rate constants α1, α 2, β1, β2, β3, β4, and ε are defined in the symbols table. Figure 6 shows the transient behaviour of the system defined in (2) for the set of rate constant values given in Table 1. The system exhibits sinusoidal type oscillation with a time period of 0.87 residence time. It is seen that excess CO is required to simulate these oscillations. However, it is found from our work and that of others that simple elementary step models alone cannot generate oscillations with excess O2 in the feed, which is a condition under which many experimentally observed oscillations occur.

(Continues…)Excerpted from Catalysis Volume 6 by G. C. Bond, G. Webb. Copyright © 1983 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

Catalysis Volume 4

A Review of the Literature Published up to mid-1980

By C. Kemball, D. A. Dowden

The Royal Society of Chemistry

Copyright © 1981 The Chemical Society
All rights reserved.
ISBN: 978-0-85186-554-6

Contents

Chapter 1 The Design and Preparation of Supported Catalysts By G. J. K. Acres, A. J. Bird, J. W. Jenkins, and F. King, 1,
Chapter 2 Aspects of the Characterization and Activity of Supported Metal and Bimetallic Catalysts By R. L. Moss, 31,
Chapter 3 Metal Clusters and Cluster Catalysis By S.D. Jackson, P. B. Wells, R. Whyrnan, and P. Worthington, 75,
Chapter 4 Olefin Metathesis By R. L. Banks, 100,
Chapter 5 Superbasic Heterogeneous Catalysts By S. Malinowski and J. Kijenski, 130,
Chapter 6 Hydration and Dehydration by Heterogeneous Catalysts By J. M. Winterbottom, 141,
Chapter 7 Sulphide Catalysts: Characterization and Reactions Including Hydrodesulphurization By P. C. H. Mitchell, 175,
Chapter 8 Carbon as a Catalyst and Reactions of Carbon By D. L. Trimm, 210,
Author Index, 243,

CHAPTER 1

The Design and Preparation of Supported Catalysts

BY G. J. K. ACRES, A. J. BIRD, J. W. JENKINS AND F. KING

1 Introduction

In this Report of catalyst-preparation technology we have placed particular emphasis on catalyst design as opposed to preparation. A properly designed catalyst should have the essential attributes of activity, stability, selectivity, and regenerability. These can be related to the physical and chemical properties of the catalyst, which in turn can be related to the variable parameters inherent in the method used for the preparation of the catalyst. In the past much of the literature on supported catalysts has not included this information. In part this was due to the lack of techniques for physically and chemically characterizing supported catalysts. Many advances have been made in recent years in this area, as described in Chapter 2, so that the design of supported catalysts has become a feasible activity.

In addition to a wide range of techniques for the preparation of supported catalysts a substantial number of supports are available for such systems. In this Chapter we highlight the technology of catalyst preparation and the role of the support in its application. In Table 1 are listed the total U.S. sales of catalyst support materials for 1977.

The predominence of alumina and zeolites is reflected in the literature on the preparation of supported catalysts and hence in the contents of this Chapter.

2 General Methods of Preparation for Supported Catalyst Systems

The principal catalyst-preparation technique involves two stages. First, rendering a metal-salt component into a finely divided form on a support and secondly; conversion of the supported metal salt to a metallic or oxide state.

The first stage is known as dispersion and is achieved by impregnation, adsorption from solution, co-precipation, or deposition, while the second stage is variously called calcination or reduction. It is brought about by a thermal treatment in either an inert atmosphere or an active atmosphere of either oxygen or hydrogen. When the active atmosphere is hydrogen the process is known as reduction. Although calcination/reduction can cause major problems in catalyst preparation on a large scale, it is a generalization to say that once the metal species has been bound to the support surface its degree of dispersion and location will be retained during subsequent treatments. This Chapter therefore concentrates on the dispersion stage of catalyst preparation rather than the thermal treatment stage, although where this is known to cause difficulty it is discussed.

The primary aim of applying a catalytically active component to a support is to obtain the catalyst in a highly dispersed form and hence in a highly active form when expressed as a function of the weight of the active component. This feature of supported catalysts is especially important with regard to precious-metal catalysts, because it allows more effective utilization of the metal than can be achieved in bulk-metal systems. However, in the case of base-metal catalysts the use of the support is often primarily aimed at improving the catalyst stability. This can be achieved by suitable interaction between the active material and the support. For example unsupported copper oxide is a very active oxidation catalyst but suffers from thermal instability at high temperatures. However, when copper oxide is supported on a high-surface-area alumina, its thermal stability is improved.

A wide range of techniques has been employed for the incorporation of a catalytically active species onto a support material. A summary of the most widely used techniques is given below as an introduction to later Sections in this Chapter, which describe the more important chemical and physical factors involved in the dispersion of metal salts onto supports and their influence on the activity, selectivity, and durability of the catalyst system.

Impregnation. – Impregnation as a means of supported catalyst preparation is achieved by filling the pores of a support with a solution of the metal salt from which the solvent is subsequently evaporated. The catalyst is prepared either by spraying the support with a solution of the metal compound or by adding the support material to a solution of a suitable metal salt, such that the required weight of the active component is incorporated into the support without the use of excess of solution. This is then followed by drying and subsequent decomposition of the salt at an elevated temperature, either by thermal decomposition or reduction. With this method of preparation it is essential to have an understanding of both chemical and physical properties of the support and the chemistry of the impregnating solution in order to control the physical properties of the finished catalyst. Comment on these factors is reserved for discussion in a later Section of this Chapter. When used for the preparation of mixed metal catalysts, care has to be taken to confirm that a component in an impregnating solution of metal salts is not selectively adsorbed, resulting in an unexpectedly different and undesirable concentration of metals in a mixed-metal catalyst. This technique has been widely used for the preparation of small amounts of catalyst for basic studies,

Adsorption from Solution. – Adsorption is defined as the selective removal of metal salts or metal ion species from their solution by a process of either physisorption or chemical bonding with active sites on the support. Depending upon the strength of adsorption of the adsorbing species, the concentration of the active material through the catalyst particle may be varied and controlled. This technique is widely used in the preparation of industrial catalysts as it permits a greater degree of control over the dispersion and distribution of the active species on the support. In some systems, however, the weight of the active component that can be incorporated into the support is limited. Although multiple adsorption is often possible it is not recommended when close control of physical parameters is required.

Co-precipitation. – The preparation of supported catalysts by the co-precipitation of metal ions with the support ions usually produces an intimate mixing of catalysts and support. An example of this technique is the co-precipitation of metal ions with aluminium ions to produce a precipitated alumina gel containing the metal hydroxide. This precipitate when calcined produces a refractory support with active component dispersed throughout the bulk as well as at the surface. However, in the preparation of multi-component catalysts, it is possible under improper conditions to obtain a heterogeneous product because of the different solubility products of the constituents. Care should be taken therefore to avoid this undesirable situation by appropriate forethought.

Deposition. – Deposition, as used in preparing supported catalysts, is the laying down or placing of the active components on the exterior surface of a support. One means by which this may be achieved is the preparation of catalysts by sputtering, which involves condensing the metal vapour onto an agitated finely dispersed support. However, as this process is performed under a high vacuum, the technique is probably only useful for the preparation of ‘model’ catalysts. Alternatively, the process may be performed in the liquid phase by the deposition of a metal sol onto a suspended support.

Chemical Vapour Deposition (CVD). – Another example of deposition is the vapour plating of the support with a volatile inorganic or organometallic compound. The process requires only a moderate vacuum and is currently one of the methods under research in industry as a means of preparing catalysts with a purely surface deposition.

Also included in this preparation category is the addition of a precipitating agent for the metal ion to a suspension of the support in an impregnating solution. A layer of precipitated metal ion adheres to the support material, which can be thermally decomposed as before.

In the case of vapour-phase processes for metal deposition on the support, only limited control of dispersion and distribution of the metal crystallites is possible. In the case of liquid-phase systems, they do not provide as wide a range of catalysts as is possible with techniques based on adsorption from solution. However, the technique does provide a means of preparing well characterized surface-impregnated supports.

3 Catalyst Design Parameters

For catalyst design purposes it is first necessary to translate the catalyst performance parameters into a physical picture of catalyst structure. As we shall see, different performance parameters can give rise to different structural features and so a compromise is generally required. For example it is commonly found in industrial applications that initial catalyst activity may be sacrificed in favour of improved catalyst stability, since a lower activity and a prolonged operating catalyst life is in general preferable to a higher initial activity that decays rapidly. First, we should therefore discuss some of the relationships between the catalyst performance parameters and physical structure.

Activity. – In general activity arises from maximizing both the dispersion and availability of the active catalytic material. Ideally, from an activity viewpoint, the catalyst material should be highly dispersed and concentrated on the external surface of the support. Already, however, there is an inherent conflict as high concentrations of active material become progressively more difficult to disperse.

Stability. – By stability we refer to the loss in activity with time. This is due to one or several of four main causes; fouling of the active surface with involatile reaction by-products, sintering or crystal growth of the active material, poisoning of the active surface by feed impurities, and blockage of the support pore structure.

Sintering during catalyst use is usually not a problem if catalysts are properly designed for their end use, although it is perhaps an important problem during catalyst preparation, activation, and reduction if the impregnated metal is not bound to the support surface. It also becomes an important factor under the more severe conditions imposed during catalyst regeneration.

Fouling of the active surface by reaction by-products is a real problem, which typically can be partially met by selective poisoning of the active ingredient. In a general sense the use of bimetallic supported catalysts would also commonly fall into this category, since selective poisoning implies a close control over the ratio of poison to active material. In this case a severe constraint is imposed upon catalyst design in that both active and moderating components should ideally be in a constant ratio throughout the catalyst support, that is to say, the placement of both should be the same.

Poisoning of the catalyst by impurities introduced with the reactants can often be minimized by placing the active material deep within the catalyst support structure, and since most catalyst supports are also good absorbents, poisons frequently can be selectively removed by such absorption before reaching the active surface. An example would be the removal of traces of lead and phosphorous from a car exhaust by the surface of the catalyst support. A catalyst design modification of this same technique would be the deposition of a poison-resistant catalyst component close to the surface and a poison-sensitive component deep within the support. This technique can be taken even further; an inert material can be used as a poison trap close to the support’s external surface. In this way each catalyst support particle can be viewed as coming complete with its own catalyst guard bed. Once again for poison resistance the location of the active component becomes a critical factor in proper catalyst design.

Finally, blockage of the support-pore structure is critically dependent upon the pore-size distribution of the support. Normally a correct balance of large and small pores is required; the former to aid reactant transport and the latter to provide the large surface necessary for the optimal dispersion of the active components. Whereas one might intuitively expect that small pores would block more readily, an important exception has only recently been recognized in the case of ZSM 5 type zeolites. In these the structure is small enough to prevent the formation of the high molecular weight involatile by-products that normally are the pore blocking agents, and yet is still large enough to allow for the transport of reactants and products to and away from the catalytically active sites.

Selectivity. – Catalyst selectivity can change due either to physical or chemical reasons. For sequential reactions diffusivity and mass transport through the pore structure can lead to apparent loss in selectivity in the formation of intermediate products. Location of active ingredients and pore-size distributions are therefore again of importance. Changes in selectivity can also arise from changes in intrinsic chemical activity of the active component. Typically this can be affected by use of multicomponent catalysts in which case, as we saw earlier for stability improvement, the location of the difference components ideally should be the same. A specific example of this type of selectivity arises in the case of multifunctional catalysts in which a hydro-genation function is combined with an acid function. Since the latter is typically provided by the support and the former by the impregnated material, a uniform impregnation is required.

Regenerability. – Regenerability refers to the reactivation of a catalyst, which typically will involve an air calcination followed in some cases by a redispersion of the active components. From the catalyst design viewpoint this will generally imply enhanced thermal-hydrothermal stability of the support itself, combined with stability of the active components under the high temperature oxidizing environments required for the oxidation of the deactivating carbonaceous deposits. It is now generally recognized that many metals sinter more readily under oxidizing conditions and in extreme cases may even dissolve in the underlying support and become effectively removed from the reaction system. A further complication arises with multicomponent catalysts in which the combination ratio is all important, since such combinations frequently are destroyed under oxidizing conditions.

Summarizing this Section, the activity, stability, and selectivity are determined by the correct dispersion and location of the active ingredients. Dispersion, location, and regenerability are each in their turn determined by the interaction of the active components with the support surface and with each other during preparation, activation, use, and regeneration. It is the purpose of this Report to examine in greater detail the extent to which our knowledge of these matters has progressed in the past few years. It is our thesis that substantial progress has been made and although much remains to be done, the present status is such as to justify our claim that we can now talk of catalyst design rather than catalyst preparation.

4 The Control of Metal Dispersion and Location during Catalyst Preparation

It is not our purpose in this Section to give an extended list of catalyst-preparation recipes. Neither is it our intent to give an exhaustive and complete review of all the published papers on catalysis in recent years that may have some aspect of catalysis preparation. The former has been amply covered in recent reviews and the proceedings of two recent symposia are devoted to this subject. The latter would be a virtual impossibility. Rather we intend to try and identify those factors that contribute most to the preparation of viable catalysts and the recent papers that exemplify these requirements and contribute to what we feel have been some of the major significant advances in this field.

Techniques used for Characterization. – As is true in other fields of scientific endeavour, much of this advance has been due to the introduction and wider use of new analytical techniques for catalyst characterization. These are discussed elsewhere in this Volume, but since these form an integral part of much of the work to be discussed, some recapitulation is appropriate. This recent period has been particularly fortunate in the introduction and dissemination of these newer techniques, and this has done much to put a firmer foundation to catalyst preparation and augers well for the immediate future.

(Continues…)Excerpted from Catalysis Volume 4 by C. Kemball, D. A. Dowden. Copyright © 1981 The Chemical Society. Excerpted by permission of The Royal Society of Chemistry.
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