Metal-Catalysed Reactions of Hydrocarbons / 02-Small Metal Particles and Supported Metal Catalysts
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surface, and deposits something that may be decomposed to a metal; the method has been reviewed.110 These complexes often react with partial decomposition with hydroxylated support surfaces, and the conversion to metal is completed by heating. The processes occurring have been scrutinised using spectroscopic techniques, and with care the original architecture of the complex can be preserved; thus for example the Ir4 cluster has been studied in detail.111 Where particle growth occurs, it is frequently limited so that very high dispersions can be obtained: the use of Ru(acac)3 with alumina gives metal particles containing only 12–15 atoms.112
Although gold is still only of limited interest as a catalyst for hydrocarbon reactions,113,114 it exhibits outstanding activity for the oxidation of carbon monoxide and similar processes, and for this purpose it has to be made as 2 to 4 nm particles supported on an oxide of the first row Transition Metals; this is done by co-precipitation or deposition-precipitation.115
Unless the metal is introduced as such, e.g. as a colloid or by metal-atom- vapour deposition116,117 (see later), the final and critical step is inevitably a reduction, performed either ex situ or in situ (or both). Molecular hydrogen is most often used, although carbon monoxide has a thermodynamic advantage, which is useful for less easily reducible species because the carbon dioxide produced is less effective than water in reversing the process. Reduction of a base metal oxide can be effected by hydrogen atoms spilling over (see Section 3.34) from reduced noble metal particles.118 More exotic reductants (e.g. CrII ions,119 oxirane120 and hydrogen atoms121) have been tried. Particle size122 and surface morphology123 are affected by reduction conditions (i.e. temperature ramping rate, time at maximum temperature, hydrogen pressure, flow-rate and purity etc). One very simple but useful technique to monitor the process is temperature-programmed reduction124,125 (TPR; see Further Reading section), which can help to determine the temperature at which precursors become reduced, the number of different steps or species involved, and the stoichiometry of the process. Exposure of metal particles on ceramic oxides to hydrogen at high temperatures (>773 K) creates a strongly held form of hydrogen that is inimical to catalytic activity.126,127 It can also lead to the partial formation of intermetallic compounds such as PtAlx128 and PtSix, and with reducible oxides to the Strong Metal-Support Interaction (see Sections 2.6 and 3.35). Reduction is probably the most important (and capricious) step in making a supported metal catalyst.
It is now possible to delve deeper into the chemical processes occurring during all stagesof catalyst preparation by employing the whole panoply of techniques now available.129−139However it cannot be stressed too strongly that very great care is needed to control each step in the preparation if reproducible results are to be obtained. This warning seems to apply particularly to preparations made on a small scale ( 5 g). Lack of such control means that it is very difficult to compare results from different laboratories, and the tendency for each
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group to make its own catalysts has materially hindered progress. In contrast the availability of a number of standard supported metal catalysts in the USA, in Europe (e.g. EUROPT-1, 6% Pt/SiO2), in Japan, and in Russia, has proved beneficial.10,140,141
The term model catalyst142−144(see Further Reading section) is applied to a material prepared in UHV conditions by condensing metal atoms formed by evaporating from a heated source onto the flat surface of an oxide either as a single crystal (e.g. MgO) or as a cleavage plane of crystal (e.g. mica, quartz, diamond) or as a thin oxide layer formed on the surface of amother metal.145 This simulates a supported metal catalyst in a way that facilitates examination by physical methods, especially electron microscopy (see Section 2.42), and thus allows direct study of matters such as effect of gas atmosphere on particle shape, and the mechanism and kinetics of particle growth by sintering, and dependence of catalytic rates on particle size. In a similar way, small particles of oxide can be created on the surface of a metal single crystal; this is a useful way of exploring the possible role of sites at the metal-support interface.
We turn now to the problem of preparing supported bimetallic catalysts146−147(see Further Reading section). Catalysts having two metallic components have found great use in industrial processing, especially in petroleum reforming, and this has lead to extensive academic research especially on PtRe/A12O1033 ,148,149 and PtSn/A12O1503 ,151 catalysts. Information on bulk alloys is of marginal relevance except when large particles exist on the support, when their structure can be determined by X-ray diffraction. In small bimetallic particles, the normal constraints on solubility no longer apply, and very useful bimetallic catalysts can be made using metals that have little tendency to interact in the bulk.152 A further system that has attracted much attention is ruthenium-copper,72,85,152,153 where copper segregates to the surface of the particle, preferentially occupying sites of low co-ordination number: the effect of ensemble size of the active metal can then be studied by changing the composition.
Two types of method are commonly used to make supported bimetallic catalysts: (i) impregnation, usually by simultaneous introduction of both precursor ions, although sequential addition with an intervening calcination is sometimes used; (ii) impregnation with solutions of organo-bimetallic complexes,155 especially carbonyls in an organic solvent.146 Choice of support is important in achieving conjunction of the compounds; silica is preferred to alumina because metal ions and atoms are more freely mobile on it,156 and calcination before reduction is generally to be avoided, because, if separate oxide particles are formed and these make separate metal particles on reduction, a high temperature treatment will be needed to homogenise the system. Techniques for answering the vexed question157−159 of whether your procedure has or has not succeeded in bringing the two metals into the desired degree of intimacy will be described later. (Section 2.4.2)
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Esoteric methods of forming supported metals or their analogues include the use of aerogels and xerogels,160 photolithography161 and electron-beam lithography,162 and a ‘solvated metal-atom dispersion’ method.163
Two final words of caution are necessary. (i) It is essential to analyse the finished catalyst chemically, because not all of the active components will have ended up where you would like – on the support. (ii) It is desirable to measure the total surface area and porosity of the finished catalyst, because they may have been changed by the preparation. This is particularly necessary when silica is the support, because it readily undergoes hydrothermal sintering during drying, calcination and reduction, leading to the sealing off of internal pores.
2.4.MEASUREMENT OF THE SIZE AND SHAPE OF SMALL METAL
PARTICLES1,2,6,46,73,164−169
2.4.1. Introduction: Sites, Models, and Size Distributions
Understanding the fundamental connections between the chemical composition and physical structure of a catalyst and its ability to perform chemical reactions is the central problem of catalysts: it has exercised the minds of scientists over a long period of years. In the field of our immediate concern, exploration of the links between them requires amongst other things the fullest possible knowledge of the sizes and shapes of the metal particles responsible. Each of the many available physical techniques addresses a somewhat different aspect of the problem. Sometimes good luck or skill ensures that several different methods give convergent answers; at other times, conflicting answers are obtained, but this can be just as informative if the reason for it is discovered.
Selection of the techniques for investigation is conditioned by the information required. There is a general need to be able to express the catalytic rate under defined experimental conditions in as meaningful a way as possible. We may start with the rate per unit mass of catalyst; then, with knowledge of the analysis, per unit mass of metal. An estimate of the dispersion allows the specific or areal rate to be stated, i.e. the rate per unit area of metal. The dispersion may have been derived from some technique giving the average particle size or the size distribution: assuming we know how the atoms are packed on the surface, we can guess the total number of exposed atoms, a figure which is also obtainable with some further assumptions from a chemical titration (Section 2.43). We can then quote a rate per exposed atom, which is termed the turnover frequency. The number of active sites may however be less than the number of surface atoms, if each site is as ensemble of more than one atom: if each site comprises NB atoms, (the Balandin number), then the Taylor fraction, which is the fraction of sites in a given number of surface atoms, equals NB −1 (see also Section 5.4). Estimates of
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metal area by physical techniques may exceed those by chemical titration if the surface is partly obscured by some strongly held but unreactive species. In such cases the lower values are the more reliable.
A great deal of enjoyment, and some enlightenment, is to be had by constructing models of small metal particles with plastic spheres (e.g. table tennis balls170) or by simulating them with computer based molecular graphics or even simply by performing calculations.171–173 These exercises are usually conducted using the close-packed fcc structure,5,9,168,174 although the cph and bcc structures have also been studied. Using perfect geometric forms, such as cubes, octahedra, tetrahedra or cubo-octahedra (Figure 2.3A), the surfaces of which contain atoms having only a few different co-ordination numbers (CN), it is at once clear that the fraction of surface atoms having low CN decreases quickly, while that of atoms having high CN rises, with increasing particle size.5–7,9,175 These changes occur within what Poltorak called171 the mitohedrical region, although the term has not caught on. For complete regular fcc octahedra having m atom along each side, the fractions of atoms having CN four (apex), seven (edge) and nine (plane) are shown in Figure 2.3B. These model studies also allow attention to be given to multi-atom sites, which may have importance as active sites in catalytic reactions; the B5 site (of which there are two kinds) occurs on the incomplete surfaces of fcc particles174 (Figure 2.3C), and has been implicated in certain unusual chemisorptions and reactions.8 All possible low CN fcc surface atoms have been identified, codified and illustrated.175–178
Work of this kind is easily (and cheaply) performed, but it has serious limitations: (1) as noted above (Section 2.1.1), real catalysts will have a distribution of particle sizes, and not contain simply particles of all the same size; (2) real particles will only rarely have a complete outer shell, because this requires a very specific number of atoms. Almost always the surfaces will be rough, and contain atoms of various CN. While this situation can be modelled to a limited extent, the use of random procedures to add atoms to a perfect shape shows that the number of atoms of a given CN changes irrationally and irreproducibly. Only a time-average over all possible configurations of surface atoms has any significance, but the average CN of surface atoms (defined as all whose CN is less than the bulk value), for arrangements that maximise contacts, changes more smoothly with total number of atoms. In the case of octahedral and tetrahedral, exposing only (111) facets, for example, this will tend to a value of nine for large sizes, and to a mean over all atoms of 12 (Figure 2.4). Considerations based on co-ordination numbers also avoid the problem, mentioned above (Section 2.11), of deciding what atoms are on the surface and what are not. For this reason a ‘free–valence’ dispersion D f v defined for particles having fcc structure as
D f v = (12 − CN)/12 NT |
(2.1) |
Figure 2.3. (A) Model of a small cubo-octahedron; (B) fractions of surface atoms on perfect fcc octahedra having CN four (corner, circle), seven (edge, triangle) and nine (face, square), together with the corresponding dispersions (NS /NT , filled circle) and sizes (—);5 (C) model of large cubo-octahedron having an incomplete outer layer.
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Figure 2.3. (Cont.)
where NT is the total number of atoms (Figure 2.5), would perhaps be more significant than that based on an arbitrary definition of ‘surface atom’. (3) A further reason caution in using surface models is that small particles will be in a state of constant agitation, with surface atoms in particular in rapid motion (Sections 1.2.2 and 2.5). Models create an unwarranted impression of rigidity and indeed the fluid nature of surfaces of small crystals calls into question the whole concept of geometric factors in catalysis. It also appears that quite different structures can have similar stabilities, and oscillation between, for example, cubic and pentagonal forms (e.g. cubo-octahedra and icosahedra) can happen easily (see also Section 2.5): indeed the order of stability may depend upon the strength of the metal support interaction (Section 2.6).
The purpose of studying the physical parameters of supported metal particles is not only to refer the measured rate to unit area of active surface, but also to see whether the rate so expressed (or TOF, see Section 5.2.3) is itself a function
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Figure 2.4. Mean CN of surface (CNs ) and all atoms (CNT ) as a function of size as given by (i)
1
NT /3 where NT = total number of atoms, and (ii) number of atoms per side m, for perfect octahedral particles.
of particle size; the resulting enquiries as to whether there is, in any particular case, a particle-size effect have occupied the minds of numerous researchers,179 and the conclusions reached will require attention in all chapters from Chapter 6 onwards. The inference of ‘particle size’ from a physical measurement is not straightforward. To start with, theoretical treatments assume that the inevitable size distribution is mononodal (that is, it has a single maximum), but this is not always the case. A mononodal distribution will be skewed and not symmetrical, because there is no upper limit to size, so the mean size is taken as the central tendency.180 There are several ways of defining the mean, depending on the type of measurement
1
Figure 2.5. ‘Free-valence’ dispersion as a function of size (NT /3 ) for perfect octahedral particles.
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made;164,165 gas chemisorption (Section 2.4.3), for example, measures surface area, and this provides a volume-area (or volume-surface181) mean diameter dvs . Initially this may be taken as the reciprocal of the dispersion D, but the proportionality constant contains the average area of each surface atom, which as we have seen is hard to define. Theoretical considerations based on fractal analysis have led to the relation
ln drel = ln kc − (3 − Dc )−1 ln D |
(2.2) |
where drel is dvs/dat (dat = atomic diameter) and Dc is the ‘chemisorption dimension’. A plot of ln drel vs. ln D for various approximately spherical crystal forms is satisfactorily linear with Dc = 2 up to drel = 0.2, but for higher dispersions (up to 0.92) the size of the atom relative to that of the particle becomes significant, and the slope of the plot becomes more negative (Dc = 2.19). Above 0.92, the concept of size loses much of its meaning, but nevertheless the logarithmic plot can be used as a universal curve for obtaining an estimate of mean size from a measurement of dispersion, irrespective of the exact crystal form. The study by Borodzinski´ and Bonarowska181 is strongly recommended for a penetrating discussion of this subject; other arguments also lead to the expectation that the logarithmic plot should not be linear over the entire range.182 The task of divining the number of multiatomic sites that may constitute active centres is even more demanding,182,183 and for very small particles is almost impossible.
2.4.2.Physical Methods for Characterising Small Metal
Particles164,168,169,184
Half a century ago there was simple no way of knowing the size of metal particles in a supported metal catalyst. X-ray diffraction had been used in the 1940s to obtain information on particles larger than about 5 nm, and the BET method could be used on metal powders. Questions concerning size effects were therefore rarely considered. Two developments in the 1950s and 1960s changed that: Transmission Electron Microscopy (TEM) began to be applied,185 and the use of the hydrogen chemisorption titration was developed186 (Section 3.2).
Transmission Electron Microscopy187 (TEM; see also Further Reading section) is probably the single most informative technique for the study of catalytic materials. In this method, electrons having energies between 20 and 1000 keV are fired at a thin specimen mounted in ultrahigh vacuum; transmitted electrons are converted into visual images, the proper interpretation of which calls for much skill and experience. In the low-energy, low-resolution mode, scanning transmission EM (STEM) simply reveals gross morphology, e.g., the shapes and sizes of
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support particles. At higher energy and better resolution, small metal particles become visible, and, at the highest electron energy, high-resolution TEM (HRTEM) affords rewarding detail of the structure of metal particles,188,189 with resolution of lattice planes and detection of surface contamination (Section 2.5). Work of this nature requires the instrument to be firmly located in a vibration free situation, and is now routinely carried out. Various methods are available for making thin layers of supported metal catalysts.
The technique, which is capable of much refinement, can supply other information: examination of the diffraction pattern identifies the phase, space group and unit cell dimension;190,191 scattered X-rays observed in energy dispersive spectroscopy (EDS) provide information on chemical composition, this enabling concentration gradients of metal within support particles to be seen; and the electroenergy loss spectrum reveals the electronic structure and atomic environment of the material. The most common use is however just to obtain a particle size distribution; the form of the distribution is found if a sufficient number of particles (typically 1000) is measured. This tedious procedure can be automated, and the results may reveal a binodal or even trinodal distribution, which other techniques might miss. It is important to view images taken at different points in the sample in order to get a representative answer; lack of uniformity in the density of particles at different places is often seen, and may indicate a failure in the preparative method. Observation of metal particles on the surface and within the cavities of zeolites helps to explain catalytic behaviour. Somewhat large particles can be formed inside the zeolite particle by disruption of the lattice.67
When X-ray quanta hit a crystalline material, at certain angles of incidence θ those that are reflected reinforce one another, while at other angles (due to their wave character) they interfere and cancel: this creates the phenomenon of X-ray diffraction,73,168,192,193 described by the equation
nλ = 2d sin θ |
(2.3) |
where λ is the wavelength of the X-rays, d the distance between adjacent layers of atoms and n is the order of the reflection. This leads readily to identification of the phase causing the diffraction. If however the size of the particle responsible is less than about 100 nm, the diffraction line is broadened, and for particles between about 5 and 50 nm this effect can be used to give a volume-averaged particle diameter d with the help of the Scherrer equation:
d = 0.9λ/βcos θ |
(2.4) |
where β is the peak width at half-height in radians. This approximate relation has to be corrected for instrumental broadening and the non-monochromaticity of
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the X-ray beam. Analysis of the diffraction peak profile can yield a particle size distribution.
X-ray diffraction (see Further Reading section) may now be performed in controlled gas atmospheres and at elevated temperatures (up to 1270 K) permitting the identification of phases after chemisorption or during catalysis: in this way, for example, the conversion of palladium to the β-hydride phase during hydrogenation reactions has been followed. X-rays may be scattered by the surface as well as reflected, and their analysis is the basis of Small Angle X-ray Scattering7,194,195 (SAXS), which can also give size estimates, provided any pores of the support are the first eliminated by filling or compression. The use of anomalous SAXS196−199 (ASAXS) has also been described. Interaction of X-rays with amorphous materials leads to scattering at wide angles, from which structural information is extracted by recording intensity as a function of angle: this is Wide Angle X-ray Scattering (WAXS), and use of an equation due to Debye (i.e., Debye function analysis200) gives a radial distribution function (RDF). This method has been applied to metal particles in zeolites, revealing changes in their structure with conditions of treatment. Debye function analysis has also led to determination of the structure of platinum particles in the standard Pt/SiO2 catalyst EUROPT-1.201,202 Anomalous XRD190 is informative, but not yet widely practised; diffuse anomalous X-ray scattering203 and wide-angle anomalous scattering199 are claimed to be superior to EXAFS (see below) for studying small metal particles.
The majority of X-rays impinging on a solid surface result in the formation of a photoelectron, and a plot of the dependence of absorption coefficient against X-ray photon energy shows an extended fine structure above each absorption edge, K-, L-, M-, etc. The K- shell edge is due to the onset of ejection of 1s electrons, and the three L edges to the start of ejection of 2s (LI) and 2 p1/2 (LII) and 2 p3/2 (LIII). The K edges of most of the 3d and 4d metals are accessible as are the LIII of the 5d metals. In a solid the emerging photoelectron is scattered by interference with adjacent atoms or ions, and constructive interference between the outgoing and back–scattered waves creates a diffraction effect, which shows itself as the fine structure above the absorption edge. The technique is therefore known as
Extended X-ray Absorption Fine Structure (EXAFS; see Further Reading section).
The method provides information on the atoms or ions surrounding the source of the electron, but extraction of quantitative information from the fine structure is a complex process, and the reader wishing to do this will need to consult specialised texts; numerous summaries are also available63,73,165,193,204,207 (see also Further Reading section).
The outgoing wave is spherical and is scattered by all species adjacent to the absorbing atom; unlike X-ray diffraction, long-range order is not a prerequisite, so amorphous samples can be studied. The method does not directly identify the nature of the neighbours, which can only be inferred from the interatomic distances.