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

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Figure 3.17. Configurations for the observations of hydrogen spillover.

(I) (a) Supported metal; (b) physical mixture of metal + support; (c) physical mixture of supported metal + support.

(II) (a) Physical mixture of metal + acceptor; (b) physical mixture of supported metal + acceptor; (c) metal supported on acceptor; (d) physical mixture of supported metal + supported acceptor.

For each class, one or more of the following questions needs to be asked, and if possible answered.

(1)Under what conditions does the process occur?

(2)To what extent and over what distance?

(3)At what rate? With what Arrhenius parameters?

(4)How is the process to be detected?

(5)By what mechanism does it occur?

(6)Is it reversible?

(7)What are the consequences especially for catalysis?

This framework should be helpful in the following survey of the five classes of behaviour.

When the support contains cations that are not easily reducible (i.e. those of Al, Si, Mg, Zr), hydrogen spillover occurs above 573 K without observable chemical reaction (Class A). However if it contains ferric ions as impurities, as is often the case with alumina, reduction to ferrous ion is detectable by EPR; and if it contains sulfate ion, as may be the case with titania, reduction of the precursor with hydrogen automatically generates hydrogen sulfide which poisons the metal (Class B). If deuterium is used in place of hydrogen, support hydroxyls

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Figure 3.18. A possible mechanism for the exchange of protons on the support by deuterium atoms on a metal particle: the small open circles are H atoms and the small filled circles are D atoms.

near metal particles are exchanged266,267 (Figure 3.18), and the process which can be followed spectroscopically, provides estimates of the diffusion coefficient D. Unfortunately, measured values of D vary over a very large range (10−7 to 10−20 m2 s−1), as do the associated activation energies,14,262,268 even for Class A supports, showing the sensitivity of the process to the precise material used, and certainly also the method of measurement. An intermediate value (7 × 10−10 m2s−1) has been determined by 1H NMR for the Pt/WO3 system.269 One simple manifestation of spillover is the H/Mtot attained at high pressures; even allowing for the values above unity shown by very small particles (Section 3.3.1), the cause of which is unlikely to be spillover, ratios greater than three have often be observed and can only be the result of spillover. The amount of spiltover hydrogen can also be detected by alkene titration, in which the slow diffusion of hydrogen back to the metal is monitored as it is subsequently picked up by the alkene, and appears as alkane.270 Reverse spillover is also responsible for the catalytic effect of platinum on the decomposition of germanium hydride;271 desorption of hydrogen atoms left on the surface of the germanium film is accelerated by their diffusion to and recombination at adjacent platinum.272 Spillover is also recognised as a slow uptake of hydrogen, following fast chemisorption on the metal, in a volumetric apparatus.273 High temperature peaks in TPD spectra have been attributed131,132 to spillover hydrogen, which has also been identified by 1H NMR spectroscopy, electrical conductivity and calorimetry.268 The occurrence of spillover is a potential threat to the use of hydrogen chemisorption to estimate metal dispersion, but for measurements at room temperature it is rarely very significant.274

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There was a time when much interest was shown in the possible catalytic activity of spillover hydrogen.260,268,275,276 Questions of the following kind were posed. (i) Can this form of hydrogen react with acceptors (e.g. alkenes) arriving through the gas phase, or adsorbed alongside? (ii) Does the spillover process generate active centres at which pairs of reactants (e.g. alkenes and hydrogen) can chemisorb and react? The answer to both questions is a qualified ‘yes’ but the concentration of spiltover species on ceramic oxides is at best very low (<1% coverage), and the contribution of this ‘spillover catalysis’ to the whole is usually very small. That is not to say that the possibility of its occurrence should be ignored, as in certain reactions (not involving hydrocarbons) it assumes a dominant role.277 As to the second question, elegant experimentation268,275−279 has shown that removal of the catalyst from pure support with which it was admixed allows, after appropriate treatment, a continuing mild catalytic activity.

In a recent and perhaps more significant development, the classical mechanism for the bifunctional cracking of alkanes on acidic zeolites has had to be revised to provide a role for the involvement of spiltover hydrogen at the acidic centres of a Pt/erionite catalyst.280

With carbon as support, very high H/Mtot ratios have been reported,13,268 and much higher concentrations of hydrogen on the support are suspected. Activated charcoal of course contains a variety of reactive surface groupings (>CO, CHO etc) with which hydrogen could react, and spillover to clean graphite has been studied by thermal analysis,281 where attachment to ‘dangling bonds’ at the edges of the graphite sheets is possible.

The chemical nature of the migrating species has been much discussed,13,262,268,275,276,282,283 without definitive conclusions emerging. The presence of hydroxyl groups on ceramic supports is clearly beneficial, and their partial replacement by chlorine, or neutralisation by potassium ion, is detrimental. Migrants evidently have reducing properties, so they are most probably atoms (H·) or hydride ions (H−). Protons (H+) have also been considered,283 and could move across an hydroxylated support surface by a kind of Grotthuss conduction; it is conceivable that hydrogen atoms could move in a similar way,262 and this is the simplest and most probable explanation for spillover at low and moderate temperatures.268,284 However the use of 18O and deuterium as tracers show that on Rh/γ -Al2O3 free of chloride at 473 K the rates of oxygen and hydrogen migration are the same, so that spillover by motion of hydroxyl groups is a further possibility.262

The reducing power of splitover hydrogen is very clearly shown in classes C, D and E. In class C, there are many examples276,285−295 of the catalysis by a noble metal of Group 10 of the hydrogen reduction of an oxide not easily reduced, in one of the configurations shown in Figure 3.18; Group 11 metals have low activity.268 Examination of a number of oxides showed that the catalytic effect was usually greater when the cation had an accessible oxidation state one less than the initial

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value296 (e.g., CoIII, FeIII, MoVI, WVI etc). This suggested that the active species was a one electron reductant, and there were indications that unusual oxides (e.g. of MoV and CrV) could be synthesised in this way, being stable at near ambient temperature but unstable at higher temperature.292,296 Gaseous hydrogen atoms are effective and useful reductants of metal oxides.297

If the activating metal is placed on or very close to the reducible oxide, careful reduction may lead to mobile atoms which attack the activating metal particles: thus reduction of Pd/ZnO leads to PdZn alloy particles. Reduction may cease at an intermediate stage, or be limited to the neighbourhood of the metal particle, or to the surface as with Pd/CeO2.298 Interesting consequences can follow; these are recognised by the somewhat misleading title of ‘Strong Metal-Support Interaction’ (SMSI), which is the subject of the next section.

Perhaps the most fascinating exemplar of hydrogen spillover, and certainly the most visually convincing is the process whereby it reacts with certain oxides to be incorporated in their structures without the elimination of water. The products, which are known collectively as hydrogen bronzes, are formed especially by the oxides of tungsten, molybdenum, vanadium and rhenium (see Further Reading section). The highly coloured products (W blue; Mo violet) contain ions of lower oxidation state (e.g. WV, MoV,VIV) formed for example as

WVIO2− → WVOH− ←→ WVO2−H+

(3.E)

but the electron is itinerant and is not fixed to any specific cation. The products are therefore analogous to the better known alkali metal bronzes (e.g. NaxWO3), which in fact resemble bronze in colour; the name has stuck, although the colours are different. It was in fact with Pt/WO3 that the phenomenon of hydrogen spillover was first encountered;299 the extents of hydrogen uptake make it an easy system to study, and it has been used to elucidate the spillover mechanism, and show the importance of water and other polar molecules in enabling the process to occur.300−302 Quite enormous amounts of hydrogen can be accommodated in these lattices (for Hx XOy , X = W, x < 0.6; X = Mo, x = 0.3−2.0, X = V, x = 1.5–1.9): when x 2, the concentration of hydrogen approaches that in the liquid state. These materials have many interesting solid-state properties;270,303 the tungsten and molybdenum bronzes are for example metallic conductors, and all are able to release their hydrogen to acceptors such as alkenes270,303,304 (Section 7.2.7). The small size and electronic simplicity of the hydrogen atom permits it to undertake numerous missions denied to larger and more complex species: these enrich its chemistry but frustrate the aims of the scientific investigator intent on finding definitive answers to simple questions. Hydrogen spillover is a prime example of the mobility of the atom (or ion or whatever), and this irritating habit is elegantly expressed in a sentence to be found in the Proceedings of the 1993 International

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Conference:260 We have such a view that the hydrogen sneaked out from palladium to carbon.

3.3.5. The “Strong Metal-Support Interaction”

Before the late 1970’s there was comparatively little research on supported metal catalysts, except on methods for estimating particle size. There then occurred an apocalyptic event in the form of two papers305,306 by scientists of the Exxon Research and Engineering Laboratory (Linden, New Jersey), following an earlier patent.307 This work attracted enormous attention rather because of its unexpected and inexplicable nature than its practical importance. It had the effect however of drawing attention to interesting but difficult questions concerning supported metals simply as materials, and it unleashed a flood of publications in which all manner of investigational methods were applied, and theoretical explanations suggested. The flood has now subsided and it is possible to regard it in perspective. The mass of publications on the subject is however so great that only a few original works can be cited, and many of the references given are to books and reviews of which there are plenty. References 277, 308 and 309 are particularly rich in information, as are the reports of conference proceedings259−262 (see also Further Reading section).

The basic observations were simple enough. It was observed that when a metal of Groups 8–10 supported on certain oxides of Transition Metals was heated in hydrogen above about 473 K, it progressively lost its ability to chemisorb hydrogen at ambient temperature.273,277 The original work was performed with iridium,306 but was later extended to the other noble metals,304 and by others to the base metals of these Groups: on titania, on which most work has been done, all six noble metals lost almost all their chemisorption capacity for hydrogen (somewhat less so with osmium and ruthenium than the others, but their initial dispersions were lower309,310). Ability to chemisorb carbon monoxide was also diminished. The supports showing this effect included the oxides of tantalum (Ta2O5), niobium (Nb2O5), vanadium (V2O3311), titanium (TiO2), and manganese (MnO); others (SiO2, Al2O3, ZrO2, HfO2, Sc2O3, MgO) did not show it to any degree not explained by a little sintering.306 The possibility that the major effect was due to catastrophic sintering was eliminated by TEM observations, which showed that it did not occur, and by the fact that it was readily reversed by an oxidative treatment. Metal particles that had suffered this loss of chemisorptive virility were said to have experienced a ‘Strong Metal-Support Interaction’ or SMSI; the term has stuck although it may not describe accurately what has taken place. One consequence of the obstruction to hydrogen chemisorption was that its dissolution into palladium was inhibited.15

Not surprisingly, it was soon found that metals in this state had also lost much if not all their activity for reactions performed under strictly reducing conditions;273,277 the only class of reaction which seemed to benefit was

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that involving reduction of carbon monoxide277 (methanation, Fischer-Tropsch synthesis312,313) and the water-gas shift,314,315 and much of the subsequent literature was focused on this area. It seems strange with hindsight that a phenomenon so lacking in practical utility should have commanded such attention, but enquiring minds needed to understand the cause, and the hope (ever springing eternal) was that there might be some great benefit still to be discovered. We should briefly review some of the other pertinent observations, and try to assess the several explanations that have been advanced. Our main interest will lie, not in the many simple observations of how entry into the SMSI state decreases catalytic activity, but rather in the changes produced in the selectivities shown of hydrocarbon reactions, since this can illuminate the dependence of reaction path on the structure and composition of the active centre.

Inspection of the above lists of oxides that furnish the SMSI and of those that do not at once reveals that the former have one or more readily accessible lower oxidation states, the cations of which (Ti3+, Nb4+, Ta4+, V2+ etc) can be formed by spiltover hydrogen. The titanous ion has frequently been detected273,316,317 by EPR spectroscopy in catalysts in the SMSI condition, and its presence has been confirmed or deduced by many other techniques (e.g. electrical conductivity318 and EXAFS319,320). By electron diffraction it has been shown that the Ti4O7 phase is formed around platinum particles,273 and model studies indicate that entry into the SMSI state is easier when the monoxide (TiO) or the sequioxide (Ti2O3) is used as support. There is less information about the other reduced cations. The necessity for having lower valent cations is reinforced by the reversal of the effect under oxidising conditions,321 and even with some metals during reactions that generate water (e.g. CO methanation, Fischer-Tropsch synthesis), but the ease of reversal varies considerably from one metal to the another (Ru > Rh > Pt). A further early observation, often confirmed, was that passage into the SMSI state led to a change in metal particle morphology, from three-dimensional to thin rafts;273,310 this may result from a change in the interfacial energy caused by reduction of the support.

The initial reduction of a support cation may be represented as

Pt0 Ti4+ O2− → Pt0 Ti3+ OH− ←→ Pt−1 Ti4+ OH−

(3.F)

and the thought that in this way the metal particles might become electron rich, and thus more gold-like, was the first attempted explanation, inevitably backed by theoretical analysis.320,322,323 However, XPS measurements324−326 have shown little change in binding energy, and it is now thought that any slight tendency for the metal to acquire an electron would immediately create a Schottky barrier to the transfer of further charge.273 In fact the effects of SMSI are uncannily like those produced when the active metal is admixed with a Group 11 metal,273 where the arguments against electron transfer are very strong. There is much more evidence, some of a very direct kind (e.g. TEM327−329) for the migration of species such as TiO or TiO+ from the support onto the metal, leading ultimately to the burial of the

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metal particles within the support.324 This would be aided by the creation of anion vacancies by loss of water, although in the SMSI state only 0.008 oxygens per titanium ion have been lost.330 Many of the observations are well explained by a simple “site blocking” model, although it is not always thought quite adequate.212 A variant on this is the idea of TiO species dissolving in the metal.313,332 There are several claims that intermetallic phases (e.g. Pt3Ti) are formed,277 and there is the additional possibility noted in the last Section of strongly held hydrogen being retained within the metal. Magnetic studies on Ni/TiO2 have shown that the SMSI is a purely surface matter, and although the explanation may not be the same for all systems and all circumstances the ‘site blocking’ picture remains the most attractive, together with a possible rehybridisation of the surface metal orbitals of the kind advanced to explain the characteristics of metal particles in zeolites, and which is indicated by changes in the shapes of LII and LIII absorption edges.212,308

There have been a number of studies in which small amounts of reducible oxides have been added as ‘promoters’ to metals supported on ‘inert’ oxides (SiO2, Al2O3), e.g. Rh/Nb2O5-SiO2,262,333,334 Ru/TiO2-SiO2,335,336 Ni/TiO2-Al2O3.260 In this way the extent of the SMSI effect can be controlled by varying the amount of ‘promoter’, and indeed SMSI phenomena are produced in this way. There have been many studies of model systems, mainly formed by evaporating the oxide or its parent metal onto the active metal;335,337 nickel has also been deposited on the rutile (100) surface to generate an SMSI.338 This paper reminds us that little attention has been paid to the relevance of the titania phase (anatase, rutile) to the onset of the SMSI. Frequent use is made of Degussa P-25, which contains both phases, but a systematic study of this matter has not been undertaken.

One of the consequences of hydrogen spillover briefly noted in the last section (see also Further Reading section) is the partial reduction at high temperatures (773–1273 K) of ceramic oxides (Al2O3, SiO2, MgO, BeO, CaO, La2O3, Y2O3, ZrO2), leading to effects resembling the SMSI, namely, loss of inclination to chemisorb hydrogen and a fortiori diminution of catalytic activity. It is not always clear what has happened; with alumina and silica there is definite evidence for the formation of stoichiometric aluminides and silicides,273 the driving force being the very exothermic nature of the process: the energy barrier for this is high, so high temperatures are needed. With magnesia and possibly also alumina, the effect may be caused by reduction of sulfate impurity, giving a partial coating of the metal by sulfur.339 In other cases (including titania), there may be intermetallic phases formed, or simply surface decoration. There is also a considerable literature to show that treatment with hydrogen at about 770 K can create some strongly retained hydrogen on the catalyst, and that this also has a toxic effect, by entering sub-surface sites.110,340

This brief survey must suffice to introduce a pervasive phenomenon which must often affect the structure and composition of small metal particles, and hence their catalytic behaviour: this will be a recurring item in later chapters. Nunc est bibendum.

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TABLE 3.2. Catalysed Reactions of Hydrogen and its Analogues

	Designation	Reaction
1	Dissociation	H2 + 2 → 2H → 2H· + 2
2	Atom recombination	2H· + 2 → 2H → H2 + 2
3	Isotopic equilibration	H2 + D2 + 4 → 2H + 2D → 2HD + 4
4	Spin-isomer equilibration	p-H2 + 2 → 2H → o-H2 + 2
5	Isotopic exchange	H + D2 → D + HD
6	Hydrogen ion discharge	H+ + + e− → H → 1/2 H2 +

· Gaseous H atomAdsorption site

3.4. REACTIONS OF HYDROGEN

The concern of this section is with incestuous reactions of hydrogen, that is to say, with reactions between the various distinguishable forms of the molecule rather than of the molecule with the catalyst or anything else. They will be treated only briefly (see Further Reading section), because they do not bear importantly on reactions of hydrocarbons although they may sometimes occur in parallel with them.

The reactions that hydrogen can undergo are listed in Table 3.2. The dissociation of molecules on metal surfaces has already been discussed at length; at temperatures above about 1270 K, hydrogen atoms desorb from the surface of metal wires and can be trapped (e.g. by MoO324,341). Rates are proportional to PH0.5 and activation energies are about 200 kJ mol−1. The reverse process of atom recombination has been a popular reaction to study, as it was thought to sense directly the bonding propensity of the surface responsible, and the relative efficiencies of a number of metals,24 and alloys (Pd-Ag, Pd-Au,342 Cu-Ni) have been determined. High recombination efficiency correlates with low heat of adsorption. A related process is the discharge of hydrogen ions in solution,343 where acquisition of an electron from the metal gives adsorbed atoms, which then recombine. Gaseous hydrogen atoms reacted with chemisorbed deuterium atoms on Pt(111) at 110 K to give gaseous hydrogen deuteride.344

The processes of isotopic and spin isomer equilibration have also been much studied in the past. The latter reaction can be followed simply by changes in thermal conductivity, and was attractive for this reason,345 but it does not necessarily require the molecule to be dissociated, as nuclear spins can reverse in the presence of strong paramagnetic centres. The hydrogen-deuterium reaction (homomolecular exchange) is however diagnostic for dissociation and recombination of atoms.346 The mechanism of these deceptively simple reactions is however not at all straightforward. The early work has been fully reviewed,24,49,342,337 and is unnecessary to rehearse this at length; two mechanisms have been considered, the first of which is that shown in Table 3.2 and is associated with the names of Bonhoeffer and

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Farkas, while the second is a chain mechanism:

(3.G)

proposed by Eley and Rideal. It has proved extraordinarily difficult to distinguish between these mechanisms on kinetic grounds, but it seems likely that the former applies on platinum below 110 K and the latter in the range 110–200 K.348 There is however also a contribution from a paramagnetic mechanism due to unpaired electrons at the surface, because the conversions of para-hydrogen and of orthodeuterium are faster than the isotopic equilibration.349−351 Analogous findings have been reported sfor nickel.

A particular objective of the early studies was to establish a connection between electronic structure and catalytic activity,345 and they have been extensively reviewed.24,49 In the nickel-copper system,352,353 the activation energy was independent of composition, but the pre-exponential factor decreased with increasing copper concentration, in line with the AES-determined surface concentration. With palladium-silver, the activation energy increased with silver content in the range 48–100% from exceedingly low values ( 1.7 kJ mol−1) to about 25 kJ mol−1, in line with the heat of adsorption derived from the kinetics.354 An excellent compensation plot was obtained, except for one point (50% Ag) where the film may have been poisoned. These well-intentioned but ultimately misguided studies failed because the results were interpreted by a simplistic model of the electronic structures of alloys, which as we have seen (Section 1.3.2) was incorrect. A re-interpretation based on a knowledge of surface composition and structure is still awaited. Parahydrogen conversion is hardly ever used now, but the hydrogen-deuterium reaction is still used as a rapid means of showing that reversible dissociation of the molecules occurs.355,356 “Model” studies with Pd/C catalysts suggested357 that the reaction was particle-size dependent with this system, the rate being maximal at a mean size of 1.3 nm; the activation energy was also size dependent, falling from 50 kJ mol−1 at 1.1 nm to 20 kJ mol−1 at 1.9 nm, in parallel with the decrease in the 3d5/2 binding energy. This reaction is particularly useful with the Group 11 metals,253,358−360 the reaction on Au/MgO and Au/SiO2 being accelerated by adsorbed oxygen atoms.359 Similar promotion occurs with sulfur on Pt(111).360,361 It is a classic case of a structure-insensitive reaction over a wide range of particle size.

Isotopic exchange (see Table 3.2) in which an atom one isotope is replaced by another is a useful way of assessing the available pool of atoms, either to determine the dispersion of metal particles or the extent of hydrogen spillover (e.g. into hydrogen bronzes) or of dissolution into palladium.362 The rate of hydrogen evolution from the discharge of solvated protons is also determined by the strength of the H M bond. Extensive results for rates of para-hydrogen conversion, hydrogen atom recombination and hydrogen evolution show a pleasing parallelism,363,364

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and especially highlight the low activity of manganese, on which hydrogen is only weakly adsorbed. They also emphasise the superior activity of iron, cobalt and nickel in the first Transition Series.

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