Metal-Catalysed Reactions of Hydrocarbons / 03-Chemisorption and Reactions of Hydrogen

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these circumstances is easily calculated. This amount may however comprise both ‘strong’ (Hs) and ‘weak’ forms (Hw) forms; the two can be resolved by altering the time between doses, since the longer the interval the more of the ‘weak’ form will be lost between doses. Thus from a plot of amount retained versus time interval, (Hs + Hw) is obtained by extrapolation to zero time while Hs alone is found at long time ( 1/2 h). It is necessary to use very pure carrier gas to avoid error due to traces of oxygen.

There are two variants of this method. If the carrier gas is suddenly replaced by hydrogen, the manner in which it is eluted (i.e., the change in its concentration with time at the outlet) allows estimation of the amount adsorbed at whatever hydrogen pressure is used: this is the frontal elution method.135,190 In the chromatographic method,191,192 a long column is packed with catalyst and pre-treated as before: if small doses of hydrogen are admitted, it is found that after the surface is saturated each further dose passes quickly, because it does not interact with the surface. Then at higher temperatures ( 500 K for Pt/SiO2) the retention time increases because the surface is now partly bare, and the dosed molecules adsorb and desorb as they pass. Finally retention time falls again as the adsorption equilibrium moves more and more towards the gas side. These results can be manipulated to give a value for the heat of adsorption.191

The final procedure requiring attention is the titration method.41,103,110 If a catalyst sample is pre-treated with oxygen in a standard way, so that the O/Ms ratio is known or can be safely assumed, then admission of hydrogen leads quickly to the formation of water, which is adsorbed on the support, and to a layer of chemisorbed hydrogen atoms, e.g.

3	H2	→ Pts	H + H2O	(3.B:HT)
Pts O + /2

The amount of hydrogen used is therefore thrice the amount that would be used just for chemisorption, and this can be measured more accurately. The cycle is completed by titrating the chemisorbed hydrogen with oxygen:

3	O2	→ 2 Pts	O + H2O	(3.C:OT)
2Pts H + /2

If then the usual volumetric uptake for hydrogen (HC) is also measured, the relative amounts of HC: OC: HT should be as 1:1:3. These ratios are in fact observed,7,41,110 but they depend of course on the assumed Pts−H and Pts−O stoichiometry. We have seen how hard it is to obtain a reliable value for the hydrogen monolayer amount, and with oxygen it is even harder because the stoichiometry is size-dependent and partial oxidation of the metal can occur. Very careful work with platinum catalysts41 has shown that the HC:OC:HT ratios should be taken as 1.1:0.71:2.42. This method, which has been widely used,135,162,190,193−196 has the advantage that it is not necessary to start with a totally clean surface, so that catalysts that

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are sensitive to high temperature reduction and evacuation can be examined. Also it appears that only the hydrogen in the exposed metal reacts quickly with oxygen; that trapped at the interface or elsewhere is not sensed. Care must be taken to ensure that the support is able to adsorb the water formed.190 Titrations may be performed either volumetrically or dynamically (using frontal elution).135

Variants on this procedure are possible. Carbon monoxide has been used in place of hydrogen,197 but although this eliminates some problems, it often creates others.198 Chemisorbed hydrogen has been titrated with alkenes:53,199,200 these reactions may be followed using frontal elution and gas chromatography to detect products, or pulse-wise.199 Chemisorbed hydrogen also reacts almost instantly with excess deuterium:24,201

Pts		H + D2 → Pts		D + HD	(3.D)

and this leads to a simple but effective means for estimating the adsorbed amount;201 spillover hydrogen exchanges more slowly (see below). This method, developed many years ago, has not enjoyed the popularity it deserves.

Our conclusion must be that although hydrogen chemisorption is a powerful tool for assessing the degree of metal dispersion in supported metal catalysts, the results obtained by any single method may not be reliable unless they are cross-checked by comparing them with purely physical methods. A profound understanding of the changes that can occur during reduction and outgassing is also needed if the results are to be related to valid models of the catalyst structure.

3.3.2. Characterisation of Chemisorbed Hydrogen

Much has already been learned about the nature of chemisorbed hydrogen on supported metals from the foregoing discussion of its quantitative use to determine metal area and particle size, but in the main the techniques used have been relatively simple and straightforward, although requiring good experimental skills. The chief exception is the use of EXAFS to monitor the structural changes occurring at the metal-support interface when hydrogen is introduced or removed; these studies have added importantly to our understanding of the metal-support interaction. It remains to see what confirmation or extension of ideas presented above are derived from the use of other sophisticated techniques such as vibrational spectroscopies and nuclear magnetic resonance spectroscopy, and to enquire what information is there is concerning the strengths of M H bonds on supported metals.

It might have been logical to start a disquisition on the interaction of hydrogen with supported metals with a paragraph or two on the rates of chemisorption, as was the case with unsupported metals (Section 3.22), but the quantitative measurement of rates of adsorption (and of desorption) on highly porous materials presents formidable difficulties, and the general opinion seems to be that the reward does not justify the effort needed. It is true that some years ago it was discovered that the

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rate of chemisorption on many supported metal catalysts was readily measurable, the rate decreasing with the progress of the adsorption and being described by the Elovich equation:24,26,202,203

+dθ /dt = a exp (−αθ /RT )

(3.12)

where a and α are constants, the intergrated form being

αθ = RT ln ((t + to)/to)

(3.13)

where to is RT/aα. Theses studies are particularly associated with the names of H. Austin Taylor204,205 (a brother of H.S. Taylor) and Manfred Low,206,207 and a model consistent with the Elovich equation was developed by Nathaniel Thon.204 It is however now thought doubtful to what physical process the rate limiting step should be assigned, and this approach has therefore been largely abandoned. While there is much qualitative and anecdotal evidence for the importance of foreign substances in determining rate (some of it mentioned earlier in this chapter), there is only the occasional mention in the recent literature on the qualitative effect of inactive metals on rates (e.g. of Ag on Ru/SiO2). The use of temperatureprogrammed desorption has however generated values for heats of adsorption (see following text).

Proton (1H) nuclear magnetic resonance (NMR) has been extensively applied to characterising chemisorbed hydrogen,208−211 because of the high NMR

¯ =

sensitivity of this element. With Pt/SiO2 (homemade, d 10 nm) the NMR spectrum showed129,208 an α peak at 0 ppm due to the support hydroxyl groups, and a β peak Knight-shifted to higher frequencies, the size of which correlated with hydrogen coverage. At pressures greater than 50 Torr, gaseous hydrogen (oNH2) made a growing contribution, but there was no evidence of hydrogen spillover. The results in the low-pressure region were interpreted in terms of stronglyand weakly-bonded atoms, the former being 3-coordinate and the latter, arising later, occupying atop positions; chemical shifts (resonance frequencies) were respectively about −48 and +37 ppm. A further (γ ) peak was also detected at −20 ppm and ascribed to hydrogen atoms at the metal-support interface, the presence of which has also been implied by EXAFS studies (see above). EUROPT-1 gave essentially similar behaviour,212 with two adsorbed states showing rapid exchange at all temperatures; the smaller particle size (dav = 1.8 nm) caused a change in chemical shifts, which were now respectively −31 and +45 ppm. The greater change for the strongly held species was thought to be due to its being more deeply embedded in the surface, and therefore more influenced by the electronic structure of the surface metal atoms, than were the weaker species. The apparent independence of chemical shift upon coverage in the low-pressure region has been ascribed209 to an inhomogeneous distribution of hydrogen within the sample, but this explanation has been strongly criticised.213 The NMR spectrum of 129Xe is affected by whether

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it is physically adsorbed on bare metal or on hydrogen-covered metal; this effect has been used to measure the hydrogen monolayer amount, and hence particle size, particularly for zeolite supports.209

Use has also been made of the NMR behaviour of the 195Pt nucleus in following the consequences of hydrogen chemisorption on platinum catalysts.2,214−216

There have been a number of proton or deuteron NMR studies involving ruthenium catalysts, especially Ru/SiO2, and Ru/TiO2(see Further Reading section). With the former, in the pressure range 10−4 to 600 Torr, three hydrogen species designated αI (I = immobile), αM (M = mobile) and β appeared progressively as pressure was increased. With silica as support, the technique has been used to observe the effect of chloride ion on the metal; it affects the 1H resonance frequency and the spin-lattice relaxation time, weakening the Ru−H bond by suppressing the amount of strongly held hydrogen through site blocking and local electronic effects. Sulfur from hydrogen sulfide has the opposite effect,175 perhaps by interacting selectively with low CN atoms: sulfur coverages of about 0.5 inhibit hydrogen adsorption entirely. Studies have also been made with bimetallic ruthenium-silver187,188 and –copper217 catalysts, and with potassium-promoted materials.187,188

The 2H NMR spectrum for deuterium on Pd/A12O3 showed181 a chemical shift moving from −78 to −20 ppm as coverage was raised to 0.6, with a further movement to −12 ppm at saturation. Strong and weak forms, rapidly exchanging, were invoked to explain the results. With Rh/Al2O3 the chemical shift rose from −180 to −100 with increasing coverage, the difference enabling the surface composition of a bimetallic Rh-Pd/Al2O3 catalyst to be estimated. Hydrogen chemisorptions on Pd/NaY zeolite209 and on the Cu/MgO208 and Cu/Al2O1443 have also been reported.

1H NMR is clearly an informative and sensitive technique for observing the forms of hydrogen on and around supported metals; although in the main it has only served to confirm existing concepts, it deserves further development, as the interpretation of what is seen is not always obvious, and extension to other systems is desirable.

There are three vibrational spectroscopies for studying chemisorption of hydrogen on supported metals: (1) infrared spectroscopy, (2) Raman spectroscopy5,110,206 and (3) Incoherent Inelastic Neutron Scattering (IINS)9,110. For the first of these, the equipment is relatively cheap and readily available, but unfortunately the small dynamic dipole moment and the small polarisability of metal-hydrogen bonds renders recording of their vibrational spectrum difficult, and in the case of infrared spectroscopy it is necessary to use the transmission mode. Nevertheless, in spite of such difficulty, spectra have been published for a number of supported metals, and these show bands in a high-frequency region (1850–2100 cm−1) associated with the weaker form on atop positions, and others at lower frequencies (700-950 cm−1) due to the more strongly-held form occupying multi-atom sites.10 In a given system, the band frequencies do not change with

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increasing coverage. The great advantage of infrared spectroscopy is that it may be used with high hydrogen pressures as the molecule itself is not infrared active.

IINS is less widely used because it requires a source of thermal neutrons, but its advantage lies in hydrogen’s very high incoherent inelastic cross-section; it has however been mainly applied to metal powders9,218 (Raney Ni,219 Pt and Pd blacks), but the adsorption of hydrogen on Ru/C220 and Pt/C141 catalysts has been studied. Adsorption sites were determined by comparison with known structures (e.g. hydridocarbonyls218). HREELS cannot be used with supported metals.

Some attention was given in Section 2.4.2 to the use of magnetic characteristics for determining metal particle size, the technique being limited to paramagnetic metals. The chemisorption of hydrogen induces a loss of saturation magnetisation with ferromagnetic nickel due to the coupling of unpaired spins on surface atoms with the hydrogen’s 1s electron. In times past these effects were intensively investigated,11 using supported nickel catalysts and films, and more limited work was done with iron and cobalt catalysts because of the difficulty of obtaining them in a completely reduced state. The technique has however commanded little interest in recent years, perhaps, because of the limited range of metals having catalytic interest to which it can be applied. (See however references 11and 182).

The strength of the metal-hydrogen bond DMH as derived from the heat of adsorption by the equation

− Ha = 2DHM − DHH

(3.14)

is an important characterising parameter, and the latter may be obtained with supported metals using either (i) TPD (somewhat unsatisfactorily) or (ii) (better) calorimetry,221,222 a technique not yet applied to single crystals with hydrogen, or (iii) temperature-dependence of isotherms. There is an extensive literature on the results obtained, particularly by calorimetry, and this has been reviewed:15,25,61,71 values obtained, even for a single type of catalyst are disconcertingly variable, often for unknown causes. Some very general statements can however be made.

Method (iii) yields isosteric heats from the temperature dependence of the pressure needed to produce a constant coverage (provided this can be measured): it can certainly reveal the presence of strong and weak forms, adsorption on the support, and, in the case of palladium, dissolved hydrogen also. Indeed it is stated that this information can all be extracted by analysis of the adsorption isochore.223 Calorimetry provides either an integral value, i.e., the average value to saturation or whatever coverage is reached, or a differential value, obtained by sensing the heat released as small doses are admitted: the latter procedure permits extrapolation to zero coverage and may also reveal steps and plateaux indicative of different states of adsorption.224 Care is however necessary to ensure uniform distribution of the gas through the catalyst sample and this may not be achieved at low temperature.224

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Integral values are normally less than initial differential values to an extent that depends on the coverage dependence in the range examined.

The most striking and surprising observation is that heats of adsorption of comparable type vary so little either with the type of metal, the nature of the support (if any) and the degree of dispersion. Much of the literature225,226 concerns palladium and platinum, for which integral values have been summarised71 as, respectively, 63 (± 4) and 56 (±10) kJ mol−1: the latter range is validated by values of 59 kJ mol−1 for platinum powder and 56 kJ mol−1 for Pt/Al2O3.227 For 20% Ni/SiO2, the adsorption heat for the strong (irreversible) form was 78 kJ mol−1, and the value falling to 29 kJ mol−1, characteristic of the weak (reversible) form, at high pressure. The Hw/Hs ratio decreased with rising temperature and increasing particle size, and was greater for Ni/Al2O3 than Ni/SiO2.159 The initial heat on NiCu/SiO2 catalysts (105 kJ mol−1) started to fall when the copper content rose above 15%, due it was thought to the hydrogen atom being forced to occupy energetically less favourable sites such as bridge sites, or three-fold sites where one of the atoms was copper.228

The differential heat for Pt/SiO2 (EUROPT-1) was229 110 kJ mol−1, independent of coverage below the isotherm knee; inclusion of gold,221 or of tin230,231 and/or potassium,232 into Pt/SiO2 did not alter the initial value233 ( 110 kJ mol−1), but the rate of decrease with increasing coverage was faster the higher the concentration of the additive, due mainly to blockage of surface sites rather than electronic effects. There was however much less electronic interaction in the case of PtAu/SiO2 than with PtSn/SiO2. Integral values for Pt/SiO2 showed no systematic dependence on particle size,234 and were similar to those quoted above for powder and Pt/Al2O3. For platinum in various zeolites the heat of adsorption varied somewhat with structure and with cation71 (Pt/NaY, 95 kJ mol−1; Pt/KL, 84 kJ mol−1; Pt/BaL, 105 kJ mol−1).

With Pd/SiO2 the heat fell from 84 kJ mol−1 initially to 12 kJ mol−1 for the weak form, rising again to 33 kJ mol−1 at higher pressures as the hydride phase was formed.223 Very similar heats were recorded222 for palladium powder and for palladium on various supports ( 63 kJ mol−1), but this value rose sharply to100 kJ mol−1 when the particle size fell below 3 nm; at this point the heat of formation of the hydride phase ( 36 kJ mol−1) also increased. For Pd/Al2O3, it was higher at 97% dispersion than at 26% dispersion140 (125 compared to 110 kJ mol−1), in qualitative agreement with the previous study. The activation energy for hydrogen desorption from Pt/SiO2 increased from 34 to 50 kJ mol−1 as the particle size increased from 1.6 to 4.3 nm.235

Some of the clearest quantitative evidence for the different strengths of adsorption of the various forms of hydrogen has been shown by Ru/SiO2.188,236 The heat of adsorption for the strong αI form was 80–90 kJ mol−1, for the weaker αM form 40–43 kJ mol−1 and for the weakest β form at high pressure about 10 kJ mol−1. Very high ratios (>5) of H/Rus were noted at high pressure (>500 Torr), and the state of adsorbed hydrogen was then described graphically as constituting a cloud

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or fog.236 A lower value (57 kJ mol−1) has been given165 for Ru/TiO2, and much lower values (26–29 kJ mol−1) for Cu/MgO.

Thus comparable values for the differential heats shown by supported metals of Groups 8 to 10 are remarkably uniform in the range 100 ± 25 kJ mol−1(for SiO2 as support at low coverage). This range is almost identical that reported many years ago for silica-supported nickel, ruthenium, rhodium, iridium, platinum and copper (100–117 kJ mol−1). Plus c¸a change, plus c’est la memeˆ chose. Values for unsupported powders also fall in this area. Many of the values reported in Table 3.1 for single crystals and films are also in the same region, although films tend to show higher values and single crystals (especially iridium and platinum) lower values, perhaps reflecting the greater perfection of their surfaces. The strength of the metal-hydrogen bond formed at low coverage is quite strikingly insensitive to the nature of the metal, and it is perhaps only at higher coverages that larger differences occur: it is of course here that the forms likely to be active in catalysis occur, but where also it is more difficult to obtain reliable experimental results. The most useful information comes when the isotherm is measured as well as the heat released, and it is here that the isosteric method scores. There are unfortunately few cases where energetic measurements are accompanied by those of other techniques (e.g. spectroscopic, NMR etc, see also reference 236).

We noted in Section 2.6 that alteration of the acidity/basicity of the support produced changes, recognised by atomic XAFS (AXAFS), in the population of the valence orbitals of the surface atoms, through a charge rearrangement between the 6s orbital and the adjacent oxide ions; the effect of this on surface reactivity was not however developed. It is well known that XANES is sensitive to the presence of hydrogen atoms on the surface, and a recent development claims to analyse the change that they produce in the spectrum to give information not only on their number, but also on the types of site they occupy. The allocation is also sensitive to the nature of the support, and has been used to attempt a better understanding of the mechanism of alkane hydrogenolysis. This interesting thought will be considered again in Chapter 14.

Electrochemical methods have also been applied to characterising adsorbed hydrogen on supported metals.12,237−239

3.3.3. Theoretical Approaches24,240–246

The earliest theoretical exercises attempted to calculate the initial heat of adsorption of hydrogen and the corresponding bond energies DMH in order to compare them with observed values, using an adaptation of the Pauling equation:

DMH = 1/2(DMM − DHH) + (xM − xH)2

(3.15)

where DMM is the energy of the metal-metal bond assumed to be broken, taken as one-sixth of the heat of sublimation, and xM and xH are the electronegativities

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of the metal and hydrogen atoms respectively in appropriate units. Several methods for estimating the electronegativity difference were tried, and more by luck than judgement, answers approximating to the observed values (sometimes surprisingly closely) were obtained. In an extension of this approach, Tanaka and Tamaru observed247 a close parallelism between the heats of sublimation of metals and the heats of formation of oxides (usually in the highest oxidation state), and regarding the chemisorbed state as a surface compound then showed that adsorption heats for hydrogen (and other molecules) were also linear functions of the M M bond strength. These effects suggested that the H M bond was qualitatively similar to the M M bond. Other empirical methods have been described.248

There have been many publications describing theoretical analyses of the potential energy surface for hydrogen chemisorption, with particular attention to the size and location of the energy barrier, and the importance of precursor states. Density functional theory (DFT) has been widely applied,240,244,249,250 and shows clearly the distinction between the metals of Groups 10 and 11; however the conclusion244 that chemisorption on gold is impossible may require modification as it does happen to some extent under some circumstances.251 Other recent applications have been to the interaction of hydrogen with palladium67,252 and nickel67,253 and their alloys,67 and to fcc(100) planes in general,254 where the place at which the molecule is presumed to sit first has been identified (see Figure 3.8). M H bond energies for all Transition Series metals have been calculated with various degrees of sophistication,244,250 and show at least the same trends as the measured values. A statistical mechanical calculation of the kinetic parameters for the forward and reverse reactions of hydrogen on the metals of Group 10 has been described.255

The prior question, which has only recently received attention,78 is why the hydrogen molecule desires to chemisorb at all, since the atom’s valencies are already mutually satisfied. Its truly remarkable propensity for chemisorbing on metals having unfilled d-orbitals and its reluctance to do so on metals having either no d-electrons (Mg,Al) or filled d-shells (Groups 11, 12 etc) imply a controlling role for vacant d-orbitals. Two alternative models have been suggested to explain this. The first, which originated almost half a century ago, took note of the extensive evidence then available for a form of chemisorption of hydrogen on d-metals intermediate between the physically adsorbed and dissociated states; this was referred to as Type C chemisorption.24 It was then supposed that the potential energy curves for both classes of metal were essentially similar, since hydrogen atoms chemisorb quite strongly on sp-metals, but that the intervention of an additional curve for the Type C state overcame the barrier imposed by the intersection of the curves for physical adsorption and chemisorption well above the zero of potential energy, thus effectively removing the activation energy that is shown in Figure 3.3. The absence of Type C chemisorption on sp-metals allowed this barrier to remain, thus accounting for the sizable activation energies they display. The nature

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of the Type C state was considered, and it was thought it might be molecular in form, and be due to some weak electronic interaction between the undissociated molecule and the metal’s surface orbitals. Although the molecular states observed on stepped surfaces saturated with hydrogen atoms appear to be sui generis,21,22 a molecular precursor state has been identified on Ni(111),39 and the evidence for the Type C state cannot be ignored (although the term itself is no longer used).

The difficulty with this model is that the filled 1sg binding level of the hydrogen molecule lies well below that of the typical Fermi level for d-metals, so that transfer of an electron from molecule to metal cannot happen. The vacant antibonding 1su lies above the Fermi level, so that transfer from metal to molecule, which has in any event to precede dissociation, is however feasible. Electronic charge extending into space beyond the cores of the surface atoms is rich in electrons having energies close to the Fermi level, so that the difficulty cannot be evaded by arguments based on local densities of states (LDOS; see Section 2.54). In one of the models proposed by Harris,256,257 as a hydrogen molecule approaches the surface, the two sets of levels move to facilitate electron transfer in both directions, leading to dissociative chemisorption. The process of chemisorption can therefore be regarded as ‘push’ and ‘pull’, and thus shows some resemblance to the Chatt model for alkene coordination and chemisorption (see Section 4.5). Clearly if there are no d-band vacancies, the ‘pull’ element cannot operate; thus with sp metals it is necessary to create them by thermal excitation, or to find them at defects or atoms of unusual CN, and this is the origin of the activation energy. With d-metals it was suggested that transfer of the metal’s sp-electrons into vacant d-levels lowered the Pauli repulsion between them and the approaching 1sg orbital of the molecule, thus allowing the potential energy curve for physical adsorption to get closer to the surface, with the consequent almost complete removal of the activation energy.257 The curve for physical adsorption (Figure 3.2) then becomes indistinguishable from that which would represent Type C chemisorption. It may however be that the mere existence of d-band vacancies is sufficient to provide the ‘push’ element necessary for completion of the process. This model does not include a molecular precursor state, but does not necessarily rule it out. The extensive evidence for its existence, and for the weak states observed at high coverage on supported metals (Section 3.3.1), suggest that the last word has not yet been written on this formally simple process.

Theoretical work on the adsorption and desorption of hydrogen is perhaps of greatest interests to theoreticians who are honing their skills to devise ever more complex and possibly accurate procedures to describe the behaviour of physical systems. To experimentalists in heterogeneous catalysis however their work can seem arcane and remote from the realities of the natural world, and only rarely does it tackle questions about small metal particles, such as their interaction with the support and the dependence of shape on the reduction protocol. Theoretical insights into matters such as these would be really worthwhile.

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3.3.4. Hydrogen Spillover

It has been known for some years that hydrogen in some form or other can under favourable circumstances migrate from the metal particle on which it is chemisorbed onto the support, where it may initiate one or more of several possible consequences; the process is known as spillover258 (in French epandage)´ . This subject has been deeply researched (see Further Reading section), and a number of substantial reviews have appeared; indeed, so wide and various are the implications of this and cognate processes that there have been a series of international conferences devoted to it, the first in 1983.259−262 It is not easy to summarise this large body of work in a page or so, especially since the phenomenon is hard to study, and so many different and conflicting explanations have been offered; but the attempt must be made, because of its probable importance to the proper understanding of metal-catalysed reactions. A large number of factors can affect the rate and extent of the process: these include (i) the type and composition of the metallic phase, (ii) its dispersion, (iii) the kind of support, (iv) impurities thereon,

(v) temperature, (vi) hydrogen pressure and coverage of the metal by hydrogen, and (vii) water content. Once again, almost every study employs only one technique and a unique material, making comparisons difficult, and the mechanism is also likely to vary from system to system, so that the distillation of any universal truths from the available literature is unlikely to happen.

We may construct the taxonomy of hydrogen spillover in the following way.

(A)Spillover to chemically inert supports, including the ceramic oxides (Al2O3, SiO2, MgO etc), acidic oxides (including zeolites), and carbon (activated charcoal, graphite etc).

(B)Spillover leading to reduction of impurities in or on the support (e.g. Fe 3+ in Al2O3, SO4 2− in anything).

(C)Spillover from a metal (supported or unsupported) to a reducible oxide (which may itself be supported).

(D)Spillover to a support that contains cations that are partially reducible, the former leading very often to the ‘Strong Metal-Support Interaction’ to be discussed in the next section.

(E)Spillover at high temperature, leading to the partial reduction of ceramic oxides (see the following section).

(F)Spillover leading to a solid state reaction in which hydrogen atoms are

retained in some form in the support or in the acceptor phase; the product is generically called a hydrogen bronze.263−265 This process is distinct from those in B, C and D, where the hydrogen appears as water.

The various configurations in which spillover can be observed are illustrated in Figure 3.17.