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Metal-Catalysed Reactions of Hydrocarbons / 02-Small Metal Particles and Supported Metal Catalysts

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SMALL METAL PARTICLES AND SUPPORTED METAL CATALYSTS

PREFACE

Studies of chemisorption and catalysis on metal surfaces fall into two categories: (i) those made with massive or macroscopic metals, either monocrystalline or polycrystalline, and (ii) those using very small metal particles containing from a few tens to a few thousands of atoms. The first category affords access to a much wider range of investigational techniques, many of which are unfortunately only applicable to massive metals because of the necessity of working under ultrahigh vacuum (UHV) conditions: such surfaces are however of limited practical utility. A few techniques are only usable when the extended surface area provided by highly dispersed metals is available; thus it is difficult to obtain an adsorption isotherm on a single crystal, although heats of adsorption can now be measured calorimetrically. Some procedures (particularly spectroscopic ones) are of course applied to both types of surface. Small metal particles are extremely useful, but are difficult to characterise, and because they are inherently unstable it is usually necessary to affix them to a support, typically a high surface area oxide, in order to prevent their aggregation. The desired adherence of metal support does however lead to a number of ambiguities in the interpretation of what is observed. The relevance of information obtained with massive metals to the behaviour of small metal particles has been hotly debated, but techniques such as X-ray absorption spectroscopy and nuclear magnetic resonance will however provide detailed descriptions of even the smallest particle.

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2.1. INTRODUCTION

2.1.1. Microscopic Metals1,2

Chemisorption and catalysis are surface phenomena, and to optimise the catalytic activity of a given mass of a metal it is necessary to increase its surface area to the greatest possible extent, by forming it into very small particles. Terms used to describe what has been achieved by doing this are (i) the dispersion (or more properly the degree of dispersion), (ii) the dispersity (favoured by European scientists) and (iii) the fraction exposed (a term which has not found widespread acceptance). All three terms express the ratio of surface atoms to total atoms in the particle, but there are of course other ways of expressing the degree of subdivision. As this is increased, the surface area of unit mass will increase, as will the numbers of particles per unit mass, while the number of atoms per particle decreases. These statements apply to the somewhat artificial situation when all particles have the same size or shape. Approximate relations between all these quantities are easily calculated by assuming the particles are all spheres of uniform size3,4 or are uniform cubes exposing five faces. Some results for palladium and platinum are shown in Figures 2.1 and 2.2. We may note that while the area per unit mass for a given

Figure 2.1. Calculations based on the Uniform Sphere Model for palladium and platinum: dependence of (i) number of particles per g of metal, (ii) number of atoms per particle, and (iii) surface area per particle, on particle size.3,4 Note: (ii) and (iii) are the same for both metals (atoms of each are about the same size).

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Figure 2.2. Further calculations based on the Uniform Sphere Model: dependence of (i) surface area per g metal and (ii) degree of dispersion, on particle size.3,4

particle size increases as atomic mass decreases, the dispersions shown by a sphere and by a five sided cube are about the same for bodies of the same diameter or length of side, although the cube contains almost twice as many atoms. Indeed it hardly matters whether the pyramid, the hemisphere, the octahedron or any other shape is considered; the dependence of dispersion on size is much the same.59 A useful benchmark is that a spherical particle of platinum having 230 atoms will be 2 nm in diameter and will have a dispersion of about 60%.

The conclusions of these rough calculations should not however be pressed too far, for several reasons: (i) they pay no regard to the way in which atoms are packed in the solid; (ii) for small particles in particular, the uncertainty of knowing which atoms to count as being on the surface affects the estimation of dispersion, (unless all the faces are close packed); and (iii) for the same reason the estimation of surface area becomes difficult for very small particles. Nevertheless they have some qualitative value in directing our thinking towards the size of particles likely to be found useful in catalysis.

Such model calculations are in addition divorced from the real world for other reasons. In practice the members of a small collection of small particles are most unlikely all to have exactly the same size, i.e. the set will not be monodisperse nor

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will their shape be a single geometrical form.5 Practical methods for determining mean size, size distribution and shape will be treated in Section 2.4.

Microscopic bimetallic particles have been the subject of much interest and attention because of their superiority over pure metals in a number of catalysed hydrocarbon reactions. The term ‘alloy’ is not applied to them, because it implies a very intimate mixing of the components, in the manner discussed in Chapter 1; this is not always the case with very small supported bimetallic particles. It has been necessary to try to establish the surface concentration of each component by physical and theoretical methods. Pairs of metals forming a continuous range of solid solutions, (e.g. Pd-Ag and Pd-Au) are readily formed into small bimetallic particles, although their surfaces are expected10 and found11 to be enriched with the Group 11 metal, having the lower surface energy and sublimation enthalpy. Those pairs showing a miscibility gap in some range of temperature (e.g. Ni-Cu, Pt-Au) are more problematic and the Ni-Cu system in particular had to be carefully examined before the truth emerged. Useful bimetallic particles can however also be fabricated with pairs of metals for which the mutual solubility is very low, (e.g. Ru-Cu, Os-Cu), and here again the Group 11 metal is found at the surface as if it was chemisorbed on the surface of the Group 8 metal. Efforts to analyse the surface composition of small bimetallic particles are partly vitiated by the observation that, due to the flexibility of the structure, the component interacting most strongly with a chemisorbed species can draw it to the surface against the thermodynamic force that would apply in a vacuum. Methods of preparing small bimetallic particles are considered in Sections 2.2 and 2.3.2; theoretical studies are mentioned in Section 2.5.5.

2.1.2. Instability of Small Metal Particles

The self-evident characteristic of small metal particles, which determines most of their properties, is that many of the atoms are on the surface and are therefore atypical. Their total free energy is greater than that of the same amount of macroscopic metal because of free energy (see Sections 1.2.2 and 2.5.3) is additional to the other forms. Another way of visualising the greater energy of small particles is to remember how much energy would have to be used to break a large lump of metal into tiny particles. Very many strong chemical bonds would have to be broken, and the energy used to do this remains in them. The total free energy would therefore decrease if particles were to grow again, and the proportion of surface atoms (i.e. the dispersion) becomes smaller. This indeed happens in an exothermic process, which releases the energy used in the subdivision: this is known as sintering. It is a process that is important in powder metallurgy, where metal powders are compacted by pressure and heated to form the desired strong shape. The ease with which this occurs depends on the atmosphere in which it is conducted. A layer of chemisorbed oxygen atoms both stabilises small particles by

SMALL METAL PARTICLES AND SUPPORTED METAL CATALYSTS

39

lowering the surface energy, and inhibits the migration of surface metal atoms that is needed to form links between particles at the start of the growth process. Removal of the oxide layer by a reducing gas facilitates sintering. Sometimes it is desirable to prevent the further growth of metal crystallites; this can be done by coating them with a thin layer of an oxide such as zirconia, which even more effectively than oxygen cuts down the surface movement of metal atoms. This effect is called grain stabilisation. It is apparent that the stronger the bond between the metal atoms, the greater will be the surface energy of small particles, and the greater their tendency to sinter. Thus ease of sintering increases with the enthalpy of sublimation of the metal.

The above remarks are couched in very general terms, and apply to some extent to most forms of small metal particle. The available forms are: (i) powders, a term which embraces a set of particles of any size, as long as it flows freely (Section 2.2), (ii) aerosols, (iii) colloidal dispersions (Section 2.2), and (iv) supported metals (Section 2.3), including particles formed by condensation of metal atoms onto a flat surface: this leads ultimately to a condensed metal film.

2.2. PREPARATION OF UNSUPPORTED METAL PARTICLES

We may return to consider briefly the methods available for making small metal particles in an unsupported form. Because of handling problems and difficulty in restraining their movement, they are chiefly used in liquid media: even so, their separation by filtration may be difficult or indeed impossible. Colloidal dispersions containing metal particles between 2 and 20 nm in size may be made by careful reduction of a dilute aqueous solution of an appropriate salt12–16 (see also Further Reading section at the end of the chapter). Colloidal gold has been very much studied;17 stable platinum sols can be made by citrate ion reduction, but not all metals are capable of forming stable dispersions, the base metals of Groups 8 to 10 being particularly intractable.18 Sols of platinum, palladium, rhodium and ruthenium can be made by reducing solutions of their salts with silanes such as (EtO)3SiH.19 Stabilisation is helped by the introduction of protective agents,16 but their presence is likely to inhibit catalytic properties. Recent advances in colloid chemistry include the use of non-aqueous solvents19 and reverse micelles20,21 (i.e. a microemulsion in which very small droplets of aqueous solution are dispersed in a hydrophobic solvent); sonochemical reduction has also been used.22 Gold and gold-silver bimetallic colloidal particles form ordered arrays by selforganisation on removal of the dispersant, especially when stabilised by organic thiols.23−28

Less well dispersed metal powders are made by reducing solutions of salts with reducing agents such as formaldehyde (methanal), formic (methanoic) acid,29 hydrazine and the tetrahydridoborate (BH4) ion,30–32 although in this last case

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some boron may be incorporated in the product.13 The term metal black is often applied to powders made in this way:33,34 they are used for basic research on gasphase reactions but they sinter easily. Metal powders are also made by reduction of oxides (or sulfides35) by hydrogen or by thermal decomposition of metal formates. There are two routes to well dispersed metal that are particularly useful for three phase reactions. Raney or skeletal metals36−39 are made by forming an alloy of the active metal with aluminium, and then treating it with strong base so as to dissolve most of the aluminium, leaving the desired metal as a sponge like material.40 Nickel has been much used in this form, but many other metals can be treated similarly.41 Finally, Adams oxides of the noble metals are made by treating a metal salt in molten sodium nitrate; they are reduced in situ under ambient conditions by hydrogen, thus avoiding the need to form the metal outside the reactor, with the attendant risks of sintering and poisoning. All the above procedures can be equally applied to mixtures of two or more metal compounds, leading to bimetallic or multimetallic products in which the elements are intimately mixed.20

Metallic aerosols can be made by passing a massive amount of electricity through a thin wire by discharging a bank of capacitors, whereupon the wire explodes.42 The technique, to which the name deflagration has been applied, was pioneered (as were so many things) by Michael Faraday,43 who converted a gold wire into powder by using the current produced by a Leiden battery.

2.3. SUPPORTED METAL CATALYSTS1,44–46

2.3.1. Scope

The need to employ very small particles to secure high dispersion, with their irritating habit of sintering, poses a major problem in the design of a technically usable metal catalyst. Fortunately it has been resolved by the simple expedient of affixing the particles to a thermally stable material usually known as a support, but sometimes a carrier. The following statement has been offered45 as a definition and statement of the scope of the resulting substance.

Supported metal catalysts comprise 0.1–20% by weight of a metal of Groups 8-11 dispersed over the surface of a support, which is typically a high-surface-area oxide. They are widely used on an industrial scale and in research laboratories. These materials are effective as catalysts because the metallic phase is present as extremely small particles, having a degree of dispersion of 10 to 100%. They are firmly anchored to the surface and are widely separated from each other, and hence do not readily coalesce or sinter.

Like all attempts at generalisations, the above statements, while reasonably accurate, provide only the barest outline of the wealth of information available in

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the patent and the open literature on these materials. Their study, and descriptions of the science (or art) of their preparation, has engendered an enormous literature, as numerous books and review articles attest. We shall have to content ourselves now with a quite short discussion of their most relevant features to the matter in hand.

The earliest material to resemble a supported metal catalyst was made by Dobereiner,¨ who mixed platinum black with clay in order to dilute its catalytic action. This remains a significant objective, since for most purposes it would be quite impractical to employ undiluted metal. The use of supported metals facilitates handling and minimises metal loss, a particularly important consideration with the noble metals: by appropriate choice of the physical form of the support, they can be used in various types of catalytic reactor, such as fixed-bed or fluidised-bed configurations. The support has often been regarded as catalytically inert, but in addition to those cases such as bifunctional catalysts, where the acidic support has long been known to play a vital role, there is a growing number of examples of participation by the support in catalytic processes. The support surface also facilitates the incorporation of modifiers, such as promoters or selective poisons.

Much attention has been given in recent years to the general question of how the composition and structure of a supported metal affects its ability as a catalyst, in terms of activity, stability and product specificity. The need to address this question has stimulated very fundamental studies of the processes happening in preparation, and the size, shape and location of within the support body of the metal particles,47 to which matters the most refined (and costly) physical techniques have been applied. If progress is measured by the precision of the questions that we can now ask, we are indeed making rapid progress.

2.3.2. Methods of Preparation

This has been the subject of a number of recent surveys48−57(see also Further Reading section), and this fact combined with the need to be brief means that the following account will have to be in the nature of a framework that readers must fill in according to their needs, using the cited references, of which there is a generous number (see the ‘Further Reading’ sections at the end of the chapter).

For catalyst supports that are to be used in industrial processes, the principal considerations58 are: (i) chemical stability, (ii) mechanical strength and stability, (iii) surface area and porosity, (iv) cost, (v) physical form and (vi) cooperation¨ (if any) with the active phase. These apply equally to reactions in the gas phase, the liquid phase and to three phase systems, but some of them are of small importance in fundamental research (e.g., i, iii, and vi). Nevertheless because of the general desire (for obvious reasons) to perform basic work that has a detectable relevance to industrial problems, those supports that feature most commonly in large-scale operation also appear most often in academic laboratories.

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For many purposes it is very desirable to use a support59 having a high surface area, which we may take to mean greater than 100 m2g−1. This ensures that the metal particles can be well separated from each other, to minimise sintering, but it implies the presence of microporosity, i.e. of pores less than 2 nm in diameter. Areas greater than about 50 m2g−1 cannot generally be obtained by lowering the size of non-porous particles. Mesoporosity (pore diameter 2–50 nm) is due to cracks between the primary particles, and macroporosity to voids between larger aggregates (pore size greater than 50 nm). Surface area and pore structure are assessed by the physisorption of nitrogen or other inert gas,60−62 or by mercury porosimetry. The methodology is very well documented, so it is only necessary to consider the results.

Only two simple oxides have the ability to become microporous: alumina and silica. Alumina exists in a number of different phases, derivable by calcination from different oxyhydroxides, the nature of which depends upon the procedure for precipitation form solution of an aluminium salt.63 The most commonly used forms of alumina are the γ (area 100 m2 g−1) and the low-area α(1–10 m2 g−1),formed by high-temperature calcination. The chemistry of silica64 is simpler: amorphous silica can be made in a number of ways, with areas in the range 500–600 m2g−1. Several low-area forms occur naturally, or may be obtained by high-temperature calcination (quartz, cristobalite).

The introduction of about 13% alumina into the silica lattice generates new acidic centres, which have Lewis character in the absence of water but Brønsted character when water is around. The high surface area of silica is maintained, and amorphous materials of this type have been much used as catalysts in their own right, where carbocationic species are involved, or as supports for bifunctional petroleum reforming catalysts.

This leads us directly to zeolites, which are crystalline aluminosilicates: some occur naturally and many others have been made artificially, showing a large and bewildering variety of crystal habits. All have microporous channels permeating the crystals, with cavities of various sizes at regular intervals, where are found the cations, which balance the negative charge on the lattice, caused by the partial substitution of silicon by aluminium. Protonic (i.e. Brønsted) acidity also occurs when alkali metal cations are replaced by protons, so in summary we have materials having molecular sieve properties, with a controllable level of acidity or alkalinity.65 They have found numerous applications (e.g., as drying agents and water softeners), and are catalysts in their own right as well as being supports for metals.66−71

Other substances exhibit the characterisations of zeolites. Silicalite is the zeolitic form of silica, and an hexagonal mesoporous silica (HI-SiO2) has been prepared and characterised.65 Aluminium phosphate gives rise to a family of ALPO zeolites, but they lack useful acidity. Much interest has been shown in the

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incorporation of small amounts of other elements into aluminosilicate and ALPO structures in order either to increase acidity or to generate new catalytic capabilities (e.g. in oxidation). We may expect further significant developments in the design of microporous structures having utility as catalyst supports: the creation of widepore zeolites such as MCM-4172 indicates the possibilities.73 The modification of the properties of small metal particles by interaction with the zeolite framework and its ions will be discussed in Section 2.6.

This brief survey concludes with mention of other classes, support of particular relevance to the catalysis of hydrocarbon reactions. Basic supports such as magnesia, ferroelectrics such as CaTiO3 and BaTiO3, and hydrotalcites74,75 have interesting and useful properties. Carbon76−79 is available in a number of physical forms: amorphous carbon (activated charcoal) has a very high surface area (up to ca.1000 m2g−1) and is particularly suitable for use in liquid media. The preparation of carbon as ‘nanofibres’80,81 and as ‘nanotubes’82,83 has also been reported. Graphite is less useful because of its very low area, but C60 buckminsterfullerene has been used in laboratory work.83−86 Organic polymers containing functional groups (silk, Nylon and other polyamides) have also been used in three-phase systems. Oxides other than silica and alumina, and their combinations, do not form microporous structures, although low levels of various elements have been introduced into zeolitic structures. Titania has been made as very small particles with an area of 180 m2g−1;87 it has been the focus of much basic research in recent years, for reasons that will appear later. It exists in two main polymorphic forms, viz., anatase and rutile, the former changing into the latter at high temperatures; the third form (brookite) is only rarely used. It is important to know how much of each form is present: pigmentary anatase ( 10 m2g−1) contains additives (e.g. K+ and PO43−) and trace impurities (e.g. Clor SO42−) that may have disastrous consequences for a supported metal catalyst made from it. The commonly used Degussa P-25 ( 50 m2g−1) is purer, but contains both anatase and rutile as separate particles in the ratio of about 3:1. Quite generally it is helpful to characterise a support as fully as possible, both chemically and physically, before using it to make a catalyst, if the process of preparation and the structure of the product is to be understood.88

The physical form of the support has to be chosen with a view to the type of reactor in which its use is intended. Silica and alumina are available as coarse granules or fine powders, and may be formed into various shapes with the aid of a binder (stearic acid, graphite): they can then be used in fixed bed reactors. For fluidised beds, or for use in liquid media, fine powders are required.89 Ceramic monoliths having structures resembling a honeycomb are used where (as in vehicle exhaust treatment) very high space velocities have to be used, but they are made of a non-porous material (α-alumina, mullite) and have to have a thin wash-coat of high area alumina applied, so that the metal can be firmly affixed.

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Methods of preparation fall into two classes: (i) the support and precursor to the metal are formed at the same time, or (ii) the metal precursor is applied to the pre-formed support. The first, less used category embraces co-precipitation, which works well, for example, for Ni/Al2O3, where Ni(OH)2 and Al(OH)3 can be coprecipitated from a mixed solution, but it is somewhat limited in scope. Similar precursors can be made by the sol-gel method90−96 and the active metal ion protected against premature reduction by complexation. Mixed aqueous solutions can also be sprayed into a hot zone (873-1273 K) to form a useful precursor. The second, more widely used method has a number of manifestations, choice of which is conditioned by the kind of support, and the desired size and location of the metal particles at the end.97 With impregnation, an aqueous or organic98 solution containing the metal salt or complex99−101 is drawn by osmosis into the pores of the support, which acts like a sponge. With ion exchange,102 protons of acidic hydroxyl groups are exchanged for cationic precursors of the metal. In deposition-precipitation, a metal precursor such as a hydroxide is precipitated onto the support, which nucleates its formation. In each case, the solvent (which is usually but not necessarily water) is removed by heating and/or evacuation, after washing when appropriate. The dried material may be calcined to convert the precursor to its oxide103 or it may be reduced directly with hydrogen, to form the metal. All of these operations, so briefly described, are capable of infinite variation and refinement, of which the following are simply examples. (i) The location of the metal within the pore system is controllable by the rate of drying or use of competitive adsorbates47 in the case of impregnation. (ii) With ion exchange, a better dispersion of the precursor ion through the support particle is achieved if a competitive cation such as NH4+ is also present. (iii) Smaller metal particles are formed by ion exchange than by impregnation but the loading is limited by the number of surface hydroxyl groups. (iv) Direct reduction of the adsorbed precursor sometimes gives smaller metal particles than reduction after calcination, but if the precursor is a metal chloride it may leave chloride ions on the support,104−106 whereas calcination will eliminate them. (v) The use of microwave radiation for heating supported catalyst precursors has produced interesting and unexpected results.107 Metal particles so formed differ form those made conventionally with respect to selectivities shown in certain hydrocarbon reactions (see Chapter 14). Commonly used precursor salts are the chlorides of the noble metals, (H2PtCl6,97 RhCl3, PdCl2, HAuCl4) and nitrates of the base metals.

Much use has also been made of the zero-valent metal complexes as metal precursors, where it is thought advisable to avoid chloride or other possible harmful ions: the carbonyls of ruthenium, rhodium, osmium, and iridium (of which there are many) have been used either as vapour if that is possible, or more often as solutions in organic solvents.68,100,108,109 Acetylacetonate (acac) complexes and π -allylic complexes have also been used. The term chemical vapour deposition (CVD) is used when the vapour of a volatile complex reacts with a support