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

TABLE 2.1. References to EXAFS Studies of the Metal-Support Interface

Neutral and acidic

The number of neighbours of a given type (i.e. the CN of the absorber) contributes to the intensity of the scattered wave at a particular energy, and this enables the structure, and for the small enough particles also the size, to be determined. The method has been applied to many pure supported metals (Table 2.1), as well as to bimetallic and promoted or modified catalysts(see Further Reading section). Its limitation lies in its sensing all atoms of a given kind in the sample, and it only shows a degree of surface sensitivity when the particle size is so small that surface atoms predominate. Pitfalls encountered in extracting particle size estimates from EXAFS results have been discussed.208 Interpretation is greatly helped comparison with compounds of known structure, and the combination of EXAFS with XRD is particularly powerful.72 The important information arising from the analysis of peaks in the Fourier transform of EXAFS spectra at very short distances209 (AXAFS) is mentioned in a later section.

Further significant developments of the EXAFS technique are envisaged.210 Energy-dispersive X-ray absorption fine structure (DXAFS) allows collection of a spectrum in much less than 1 s, and its extension to the µs or even the ns time scale is not impossible. Polarisation-dependent total-reflection fluorescence X-ray absorption fine structure (PTRF-XAFS) permits high spatial resolution and has been applied to the copper trimer Cu3 on a Ti(110) surface. The use of ‘timeand space-resolved XAFS observation for studies of dynamic aspects of the local structure at catalyst surfaces under working conditions’ is forseen210. Note that the acronym XAFS is used, rather than EXAFS.

Ejected electrons of low kinetic energy (<30 eV) interact with the valence electrons of other atoms and often show complex scattering paths. This leads to an additional peak near the absorption edge; it is known as a white line, and the X-ray Absorption Near-Edge Structure (NEXAFS or XANES) is caused by transitions of (for example) 2 p electrons to unoccupied 5d levels.65,211 The white line intensity can therefore sense the occupancy of d-levels in certain transition metals (particularly platinum; see Further Reading section), so no line appears with the Group 11 metals, which have filled d-shells. Changes in its intensity have sometimes been connected with the presence of chemisorbed hydrogen atoms,212−214 and

56 CHAPTER 2

sometimes not.215 Its possible extension to recognising the positions of chemisorbed hydrogen atoms will be noted in the next chapter.

A further manifestation of the power in X-rays in the analysis of solids is X-ray Photoelectron Spectroscopy73,193,216,217 (XPS; see Further Reading section). Originally used purely to identify elements present in a sample, as its first name (Electron Spectroscopy for Chemical Analysis, ESCA) indicates, it identifies the binding energies of various electron levels for each element after correction for sample charging. For many elements the technique is almost surface specific, since the escape depth of electrons for many transitions is only a few nm, and the ability to etch the sample by ion bombardment enables depth profiling to be performed. The binding energy is somewhat sensitive to the oxidation state of the element, and peak shape analysis may reveal the presence of more than one state. It also appears to be sensitive to particle size when this is small enough, but the interpretation of the observed effect, which is small, has been disputed,218−220 and it has not become a routine method for particle size estimation. Auger transitions are readily observed particularly for the lighter elements, and can give helpful information.

Mossbauer¨ spectroscopy193,221 is based on the observation that nuclei held rigidly in a lattice can undergo recoil-free emission and absorption of X-radiation; the separation of nuclear energy levels can be measured with great accuracy, and it is possible to detect weak interactions between a nucleus and its electronic environment. This may reveal the chemical state of the atom or ion, but only a few nuclei are susceptible to the effect, most work having been done with iron (57Fe) and tin (119 Sn) and a little with ruthenium (99Ru).222,223

Atomic nuclei having an odd number of protons and neutrons have a non-

-

zero nuclear spin I , and thus a magnetic moment µ which equals γ hI, where γ is the gyromagnetic ratio, which is a quantity specific to each nucleus. When such a nucleus is placed in an external magnetic field B0 of strength typically 2–14 T, the Zeeman interaction leads to quantised orientation of the nuclear magnetic moments, and the nucleus adopts (2I + 1) magnetic eigenstates having energies Em given by:

±1) can be effected by electromagnetic radiation having the Larmor frequency V0, where

These are in the radio frequency range of 1–600 MHz. The usefulness of this nuclear magnetic resonance (NMR; see Further Reading section), which is central to determining the structures of organic molecules including chemisorbed species

(see Chapter 4), hinges on the phenomenon of the chemical shift σ, which is due to the perturbation of the Larmor frequency by the influence on the specified nucleus of the diamagnetic field about it. For a variety of reasons the number of metals of interest in catalysis and having nuclei suitable for study by NMR is small: they include hydrogen, carbon, silicon and aluminium, the last two being much used in work on zeolites, but of the metals only platinum (195Pt) is really relevant to our interests. Little has been done with other possible nuclei (103Rh, 109Ag, 61Ni, 63Cu), but effective use has been made of proton NMR in the study of chemisorbed hydrogen (see Chapter 3). The theory and scope of the method has been reviewed on a number of occasions (see Further Reading section).

The spectral lines in the NMR of solids are very broad, due to static anisotropic interactions to which nuclei are subjected, whereas molecular motion in liquids averages these out, and narrow lines result. The same effect is produced in solids by rotating the sample at high speed at an angle θ to the direction of Bo of 54◦ 44 since the term (3 cos2θ − 1), which appears in equations for dipolar interaction, chemical shift anisotropy and quadrupolar interactions of first order, is zero for this angle. This value of θ is termed the ‘magic angle’ and the technique is therefore named Magic Angle Spinning NMR (MASNMR).

In the case of metals, an additional effect on the Larmor frequency arises because of the polarisation of conduction electrons by the magnetic field of the nucleus; this creates the Knight shift, which can be much greater than the chemical shift.224 The spin-lattice relaxation rate T1−1 and the associated Korringa constant (T1 T , where T is the absolute temperature), also provide useful information. Marked changes in NMR line shape of platinum in Pt/A12O3 catalysts with dispersion have been observed225–228(see also Further Reading setion). The use of spin-echo double resonance169,229 (SEDOR) gives a high degree of surface specificity, and the combination of 195Pt on the surface of a particle with a nucleus such as 13C in a chemisorbed species allows determination of its structure (See Chapter 4). Chemical shifts on 129Xe NMR caused by interaction with 1H nuclei has led to estimation of the size of metal particles in zeolite cavities,230 and even on conventional supports.231,232

The intensity of magnetisation M of a substance is proportional to the magnetic field strength H :7,164,168

where χ is the magnetic susceptibility. If this is positive, the material is paramagnetic; if negative, it is diamagnetic. Many metals have weak temperatureindependent paramagnetism, but iron, cobalt and nickel (and their alloys) are ferromagnetic, that is, they can be permanently magnetised below the Curie temperature. Small particles (<20 nm) of a ferromagnetic material consist of single magnetic domains with a magnetic moment µ proportional to the volume. In the

absence of a magnetic field, these moments are randomly oriented, but they align themselves in the direction of an applied field, the largest particles responding most quickly. The saturation moment Ms is reached when all particles are so oriented: the rate of increase of M/Ms with H is thus characteristic of a specific particle size, and the experimentally measured curve can be manipulated to give a particle-size distribution.168 Experimental procedures for examining the magnetic properties of small metal particles have been described.164,168

Cyclic voltammetry applied to Pt/graphite allows an estimate of the relative amounts of the exposed low-index planes of the metal particles through comparison with results found with single-crystal surfaces;233 the method is of course limited to conducting supports. Time-differential perturbed angular correlation234

(TDPAC) is a nuclear spectroscopy that permits characterisation of materials on an atomic scale through hyperfine interactions due to interactions between the nuclear electrical quadruole moment of a suitable radioactive probe isotope and the electric field gradients originating in its neighbourhood: characteristic parameters obtained through detecting emitted γ -rays originating in the nuclear cascade of the isotope are (i) the nuclear quadrupole frequency interaction and (ii) asymmetry parameters. The method has been used to examine the structure of the PtIn/Nb2O5 system, using the 111In isotope.

A number of other techniques have been used in the study of metal catalysts, but some of them only rarely by reason of their difficulty, cost or other limitation.193 Scanning-tunnelling and atomic-force microscoies (STM/AFM), which permit the visual representation of small particles are however quite widely available.235–240 Other methods meriting note include Rutherford back-scattering (RBS),220,236 inelastic neutron scattering (INS),77 electron holography,241,242 electron paramagnetic resonance (EPR or ESR),243,244 secondary-ion mass-spectroscopy (SIMS),245 and plasma atomic emission spectroscopy.246

The advantages of using two or more methods on the same material are almost self-evident,72,153 as each method fails on occasion or has constraints that are not immediately apparent. When different methods have been compared there has often been a pleasing consistency between the results; major discrepancies sometime find a logical explanation.

2.4.3.Measurement of Dispersion by Selective Gas-Chemisorption7,164,169,180,247

Unlike the physical methods of characterisation outlined above, the theory underlying this procedure is very straightforward and the equipment needed is relatively cheap,247−250 but the practice is surrounded by pitfalls for the unwary and the interpretation is fraught with difficulties. Starting with a clean surface, one measures the number of molecules needed to form a monolayer just on the metal, and from this number, which is taken to be equal or proportional to the number

of surface metal atoms, and a knowledge of the total amount of metal present from chemical analysis, a figure for the dispersion is at once obtained; and hence also, knowing the size of the metal atom, the surface area and the mean size (if the particle shape is assumed). This seemingly simple procedure is however capable of much elaboration.251,252 This section offers a short review of the technique; a more detailed coverage of methods involving hydrogen will be found in Section 3.3.

The first problem is to clean the surface of the metal particles. They will usually start with a layer of chemisorbed oxygen on them, and this cannot be removed merely by heating and evacuation, because the metal-oxygen bond is too strong. The best course is to treat with hydrogen, which reduces the oxygen to water and leaves chemisorbed hydrogen atoms, which are more easily persuaded off. High temperatures are not usually needed, and indeed are to be avoided with reducible supports such as titania. Evacuation of gases from the pores of a microporous solid can however be tedious, and getting a good dynamic vacuum is no guarantee that it will hold when pumping ceases. Other contaminants (carbon, sulfur) less easily removed even than oxygen may also be present, and should be eliminated: chloride ion remaining from a preparation using a chloride salt may hold fast to metal and especially to support and can only be eliminated by prior washing or steam treatment in situ.

Selection of the adsorbate molecule requires care. Hydrogen247 and carbon monoxide are by far the most often used, but both have some disadvantages. Hydrogen taken up is not always used only to form atoms on the surface metal atoms; it may ‘spill over’ onto or into the support, or it may break metal-oxygen bonds at the metal-support interface (Section 3.3), or it may form weakly-held species above the monolayer point, or in the case of palladium may dissolve into the metal (Section 3.1). Most of these difficulties can be avoided by suitable choice of experimental conditions, but it is by no means unusual to find that the number of atoms taken up exceeds the total number of metal atoms present (i.e. H/Mtσ t >1).253–256 Use of carbon monoxide avoids most of these difficulties, but it may chemisorb either in a linear or bridged form, and may even on some metals dissociate entirely; parallel use of IR spectroscopy will reveal what has happened. Other molecules are rarely used: oxygen atoms are prone to dissolve into metals except at low temperatures,257,258 although nitrous oxide is successfully used as a source of oxygen atoms to chemisorb on copper and ruthenium, its use for other metals has not been explored.

There are various ways in which the monolayer volume can be measured. In the static method, successive small doses of the adsorbate are admitted, and the number of adsorbed molecules after equilibrium has been reached are deduced either gravimetrically (possible with carbon monoxide, difficult with hydrogen) or volumetrically from the residual pressure or by some other technique such as NMR or XANES.211 The procedure is repeated until no further uptake occurs, when the monolayer capacity will be known. If chemisorption on the metal is strong and

specific to the metal, the adsorption isotherm will show an initial rapid increase of uptake with pressure, followed by a linear region of low slope. The monolayer capacity is often reported (when anything at all is said) as the intercept at zero pressure obtained by extrapolation of the linear part. Alternatively, and perhaps better, the results are plotted according to the one of the linearised forms of the Langmuir equation from which the monolayer volume can be obtained.

Metal dispersion is also measurable in a dynamic system, in which the surface is initially cleansed after reduction by flowing pure helium or other inert gas. Injection of small doses of adsorbate is started and the non-retained part measured (e.g., by a thermal conductivity or other detector); this is continued until no more adsorbate is retained, and the aggregate of the adsorbed gas is found. This may be taken as a measure of monolayer capacity, but it is a function of the time gap between injections: increasing the time allows more weakly adsorbed species to desorb, and by varying the interval information is obtained on their amount. Although this procedure can be used with hydrogen, it is difficult to exclude oxygen entirely, and spuriously high retention may result from the reaction to form water. A safer method is to inject carbon monoxide into a stream of hydrogen, which does not compete with its chemisorption, and maintains a fully reduced surface.

It is claimed that measurement of the saturation amounts of hydrogen, oxygen and carbon monoxide can quantify the relative contributions of the three low index planes exposed by palladium particles: this however assumes that all adsorbing atoms belong to one or other of these planes.141,259,260

2.5. PROPERTIES OF SMALL METAL PARTICLES2,6,164

2.5.1. Variation of Physical Properties with Size: Introduction

Since chemisorption and catalysis are surface phenomena, their importance and usefulness increase with dispersion; great interest has therefore be shown in the physical and chemical properties of small metal particles (see Further Reading section). Since the limit of dispersion is the single atom, there must come a point at which, as dispersion is increased, metallic character is lost (Section 2.5.4). It may of course disappear slowly rather than suddenly, like the Cheshire cat, and it is necessary to explore several parameters that might characterise the metallic state, since not all may very with size in the same way. For example metallic behaviour may be manifested by Pauli-like paramagnetic susceptibility and NMR Knight shift even when the separation of energy levels is greater than kT, and there is no periodic-in-space Bloch wave.224 Although single metal atoms (and ions) can undoubtedly act as catalytic centres, workers in the field of catalysis have often wondered how important metallic character (however defined) is to catalytic activity, and whether there comes a point at which further efforts to

improve dispersion become self-defeating. They have therefore made attempts to relate activity (best expressed per unit area or per surface atom, Section 2.4.1) to particle size.

This work, conducted on a variety of catalytic systems, has revealed clear trends, which have enabled systems to be broadly classified as either sizedependent or size-independent. Quantification of these trends has however proved difficult for two reasons: (1) it is almost impossible to produce monodisperse metal particles on a support, so the best that can be done is to relate activity to average size; (2) those means that are deployed to change the size (e.g. altering metal concentration, thermal treatment) may introduce other factors that will affect activity (e.g. concentration of impurities, surface structure); and (3) the reaction may not proceed on all or indeed any of the metal surface, because specific sites at the metal-support interface of the support itself, by ‘spillover catalysis’, may be the site of catalytic activity.56,261 Notwithstanding the improbability of small particles remaining rigid under catalytic conditions (Section 2.4.1) there have been many proposals to attribute activity to specific types of atom (e.g. edge, corner) or of site (e.g. the B5 site). Because of the ease with which structural models can be created and analysed, the geometric factor in catalysis has featured more prominently in the literature than the energetic factor, which is harder to evaluate.

There is one further important general point to make. It is almost impossible to investigate the size dependence of any physical or chemical parameter in the absence of support, although certain properties of small vapour-phase ‘clusters’ are susceptible to study.262 If metal particles are formed by metal evaporation or ion implantation onto an oxide support, we may suppose that the effect of the support on the metal particle is minimal. Particles may grow epitaxially on singlecrystal oxide surfaces, but this method is as near as we can come to looking at intrinsic size effects. Certainly, as we shall see shortly, when metal particles are created by chemical means on high area supports their properties and behaviour may be considerably, indeed sometimes profoundly, affected by interaction with the support. Although such interactions may be of small importance with supports that are neutral (e.g. carbon) or are of low acidity (e.g. silica), the old idea that the support was simply an inert vehicle for the metal has long since been rejected as having universal validity. Understanding the extent and nature of the metal-support interaction is essential for rationalising the catalysis of hydrocarbon reactions by metals. The essence of the problem is this: supported metal particles may be expected to show intrinsic size effects, but superimposed on them are support interaction effects which themselves may be size-dependent, i.e. small particles will be more liable than large ones to experience the consequences of propinquity to the support.

Although it is widely accepted that it is difficult to prepare metal particles in a narrow size range, and while several techniques are known for determining size distribution, it is often assumed that sizes are distributed about a single mean value.

is clearly at work, where the act of observation distorts what is being examined. The harder you look, the less you see. The stabilities of the various forms are therefore probably not greatly different; exposure to different gas atmospheres can also affect particle morphology, because the adsorbate may change the relative surface free energies of crystal planes, and the heat released by chemisorption can trigger change from a metastable to a stable structure.6,271 The equilibrium shape of a free-floating single crystal is expressed by the Wulff construction;6 it defines the proportions of different crystallographic planes that minimise the total energy. It is not clear however how such a crystal can be formed and studied in practice.

Anomalous structures (e.g. bcc gold, fcc lithium) have sometimes been found; these seem to occur most frequently with metals of low sublimation enthalpy, and less often with palladium and platinum.6 Their formation may be linked to an epitaxial effect of the support on which they are formed and grow. Clearly developed crystal planes were only shown by particles larger than about 2 nm;70 Mossbauer¨ spectroscopy showed a platinum particle with 309 atoms to have the normal fcc structure,222 but palladium-platinum particles suffered electron-beam- induced change from fcc to cph.272

In parallel with the usually observed contraction in interatomic spacing at the surface of single crystals (Section 1.2.2), it is commonly found that interatomic distances are less in small metal particles than in bulk metals.6,232,273−275 Very accurate estimation of the lattice perimeter of aluminium particles on magnesia using the moir´e fringes of individual particles showed that contraction increases as size decreases, and that the effect (which is usual only a few percent at most) is greatest at the surface; particles larger than about 20 nm gave the bulk value. Atomic separation in platinum particles deposited on Al2O3/NiAl(100) began to decrease at 3 nm size, and at 1 nm was only 90% of the bulk value.276 EXAFS measurements on supported ruthenium and rhodium particles also revealed static disorder, as well as decreased bond length, which was increased to values greater than the bulk value after chemisorption of hydrogen.275 The Debye temperature of Rh/TiO2 rose from 241 K to 341 K on passing into the SMSI state (see Section 3.3.5). The relevance of shorter bond length to catalysis may however only be slight, because it is usually observed under UHV and is apparently negated by chemisorption.6 Indeed the driving force for this is the surface energy residing in atoms having less than the bulk CN, and when this is utilised by chemisorption the surface atoms relax towards their bulk positions, or may (as noted above) move even further outwards if bonding to the adsorbate is strong enough.

Platinum ‘nanowires’ have been formed in the channels of NaY and FSM-16 (wide pore) zeolites,277 and ‘nanosheets’ between the layers of graphite.278

There seems to be comparatively little information available on the structures of bimetallic particles,147 and although certain systems of particular industrial relevance (e.g. Pt-Re and Pt-Sn), and others having great scientific interest (e.g. Ni-Cu, Ru-Cu), have been intensively studied, the emphasis has been on chemical