Geoffrey C. Bond

Gold-catalysed oxidation of carbon monoxide

Of the many important reactions that are known to be catalysed by gold particles supported on transition metal oxides, the oxidation of carbon monoxide is of outstanding significance: the current state of understanding of its mechanism is reviewed, and discordant observations reported in the literature are noted and analysed. A general mechanism involving reaction at the edge of a particle containing both gold atoms and ions, and involving the support, is suggested, although not all features of it are necessarily always operative. Possible practical uses for gold in catalysing this reaction include pollution control, fuel cells, and gas sensing.

Vanadium oxide monolayer catalysts Preparation, characterization and catalytic activity

Abstract The purpose of this review is to provide a guide to the recent literature on the preparation, structure and catalytic properties of vanadium oxide monolayer catalysts. Monolayers of vanadium oxide, represented as VO χ , may be formed on the surface of a supporting oxide by impregnation with a suitable vanadium source, followed by calcination, by grafting methods using VOCl 3 or a vanadyl alkoxide, by heating mechanical mixtures or by coprecipitation. Information on the chemical structures present when there is less than a monolayer of VO χ can be obtained from vibrational and electron spectroscopies (Raman, FT-IR, XPS). Temperature-programmed reduction (TPR), XRD, ESR and 51 V NMR provide additional characterisation of the materials. VO χ monolayers are particularly stable on TiO 2 and on Al 2 O 3 , but are not readily formed on SiO 2 , where aggregation to V 2 O 5 occurs. At low vanadium contents, isolated VO 4 tetrahedra have been detected, and polymeric octahedral VO 6 species at higher loadings: various disordered and ‘paracrystalline’ V 2 O 5 phases have been seen when more than the monolayer loading is present. Species in the monolayer are effective catalysts for oxidation of o-xylene to phthalic anhydride, for ammoxidation of aromatics and for selective reduction of nitric oxide by ammonia, but not for oxidation of butadiene to maleic anhydride.

Structure and reactivity of titania-supported oxides. Part 1: vanadium oxide on titania in the sub- and super-monolayer regions

Vanadium oxide has been deposited on TiO2 (washed anatase, 10 m2g−1; Degussa P-25, 55 ±3 m2g−1; Eurotitania, 46 m2g−1) by aqueous impregnation of (NH4)2[VO(C2O4)2] and by reaction with VOCl3, VO(OR)3 (R=iBu) and VO(acac)2 in organic solvents. Single applications of the last tree reagents form not more than a monolayer of vanadium oxide VOx, a monolayer being defined as 0.10 wt.% V2O5 per m2 of surface. When less than about four monolayers of VOx are present, there is in most cases only a single TPR peak: Tmax values, which increase with V2O5 content, are almost independent of the method used but vary slightly with the support (P-25 < Eurotitania < washed anatase). The 995 cm−1 band, characteristic of VzO in V2O5, only appears when more than a monolayer of VOx is present. In the sub-monolayer region, VOx is best formulated as an oxohydroxy species bonded to two surface oxygens. As the V2O5 content is increased, layers of disordered V2O5 are formed on limited areas of the surface, but crystalline V2O5 only occurs, probably on top of the disordered V2O5, when the V2O5 content exceeds about four monolayers, and takes the form of acicular crystals exposing only planes perpendicular to the a and b axes.

The origins of particle size effects in heterogeneous catalysis

Abstract Model calculations are presented to show how the fraction of atoms at the surface of small metal particles increases as their size diminishes in the range 10 to 2 nm. Such particles are prepared either by condensing atoms or aggregates from the vapour phase onto a support, or by chemical methods in the liquid phase, i.e. the traditional routes for preparing supported metal catalysts. The first group of methods leads to artificially pure materials in which the contact between metal and support is poor. The second group of methods leads to the introduction of impurities, to a greater variety of forms of particle, but to a generally firmer binding of metal to support: this permits electronic interactions between the components to occur. Recent literature on the chemisorptive and catalytic properties of metal particles, usually less than 10 nm in size, suggests that certain classes of reaction may be designated as “structure-insensitive” in that their rates depend only minimally on particle size, whereas others, denoted as “structure-sensitive”, have rates which either increase or decrease with size. After discounting trivial effects, a hard core of results remains, demanding explanation. Although certain hydrocarbon transformations appear to need sites comprising more than a certain minimum number of atoms, it is thought that the electronic character of surface atoms plays a greater role than their geometric disposition.

The vanadium pentoxide-titanium dioxide system. Structural investigation and activity for the oxidation of butadiene

The interactions of various anatase and rutile samples with V2O5, and their products, have been studied by thermal analysis, by ir and ESR spectroscopy and by X-ray diffraction. The V2O5 was introduced either by mechanical admixture or by impregnation of the TiO2 with NH4VO3 and subsequent calcination. With anatase samples containing only low levels of P2O5 and K2O (< 0.6%) or containing SO4 = (6.3%), heating to 450 to 600 °C leads only to supported V2O5: but heating at 750 °C produces (i) a change in color to black, (ii) an irreversible loss of oxygen, (iii) a transformation of the anatase into rutile, and (iv) a substantial decrease in surface area. The results of experiments performed with various V2O5 concentrations (1 to 60% ww) suggest that about 8% ww V2O5 becomes incorporated as V4+ ions into the rutile lattice during the transformation, through the formation of a compound having the composition V0.04Ti0.96O2. Corresponding processes with rutile take place only at higher temperatures. Anatase containing Na+ (1.4%) behaves quite differently, and there is evidence for the formation of sodium vanadium bronzes on heating to 450 or 750 °C. Catalysts containing supported V2O5 oxidize butadiene to maleic anhydride at 260 °C with a selectivity which increases with V2O5 contents between 1 and 10%, and thereafter remains constant (S ∼- 0.45). Those containing the compound V0.04Ti0.96O2 are less active but more selective (S ∼- 0.57 for 10 to 30% V2O5). With both types, the selective reaction is zero order in butadiene and in O2, and both show activation energies for selective and for nonselective oxidation of 20 to 25 kcal · mol−1. Selective oxidation is believed to require lattice oxygen, and selectivity correlates with difficulty of reduction by CO.

Compensation Phenomena in Heterogeneous Catalysis: General Principles and a Possible Explanation

The compensation phenomenon in heterogeneous catalysis takes the form of a sympathetic linear correlation between the observed parameters of the Arrhenius equation E app and In A app for a series of related reactions or catalysts: Exact obedience to this Cremer-Constable relation requires all Arrhenius plots in the set to intersect at the isokinetic temperature (Ti ). When this is established with statistical rigor (which is rare), the term “isokinetic relationship” (IKR) is used, the term “compensation” being reserved for cases in which this has not been done. Early work suggested that Ti equated to that at which the catalysts were prepared, but this observation has not been generally confirmed. Experimental error in the Arrhenius plots and other causes, such as a change in mechanism or the onset of diffusion limitation, can give rise to false “apparent” compensation which is of no real significance. The scope of this review is strictly limited to heterogeneously catalyzed reactions; thermal desorption is excluded. Explanations advanced include [1] distribution of active-site energy and [2] an enthalpy–entropy relation originating either in the thermodynamics of chemisorption, or in the activation parameters of the Transition State Theory, or in the process of energy transfer between initial and final states. The distinction between apparent (E app and ln A app) and true (Et and In At ) Arrhenius parameters is clearly drawn: The latter are observed only where the catalyst exhibits a high affinity for the adsorbate(s) and where, therefore, the coverage is high and independent of temperature within the range of measurement; zero-order kinetics then apply. Structure insensitivity is often observed under these conditions. Compensation is only seen with apparent Arrhenius parameters, which occur when chemisorption is weaker: The resulting lower coverages are temperature dependent and sensitive to many variables such as catalyst composition and the nature of the reactant(s). Reaction orders are then greater than zero, and under these conditions, structure sensitivity is often found. Values of Et then exceed those of E app by the appropriate adsorption enthalpy terms. Model calculations based on Langmuir–Hinshelwood bimolecular kinetics show that compensation may occur within a single system, due simply to a change of coverage with reactant pressure: Consequential changes in the exponential term exp(—E app/RT) are greater than changes in rate, so that compensation is inevitable. These concepts are illustrated by reference to three systems: [1] acid-catalyzed hydrocarbon transformations, where independent measurements of adsorption enthalpies are available; [2] metal-catalyzed hydrogenation of benzene and its homologs, where mathematical modeling extracts values for Et and adsorption enthalpies; and [3] metal-catalyzed hydrogenolysis of alkanes, where the endothermic initial C—H bond splitting causes the value of E app to be larger than Et , thus explaining inter alia the variation of E app with chain length. If, in a set of reaction systems, conditions for measuring the temperature dependence of rate are chosen such that adsorbed reactant concentrations vary to an appreciable degree, values of E app will necessarily differ, and compensation will appear if accompanying changes in rate are small (i.e., if Ti lies within or close to the temperature range used).

The vanadium pentoxide-titanium dioxide system: Part 2. Oxidation of o-xylene on a monolayer catalyst☆

Hydroxyl groups on the surface of anatase react with the vapor of VOCl3 at room temperature to give a partial monolayer of a vanadium species which after heating to 670 K is active for the oxidation of o-xylene. Rehydroxylation by H2O followed by outgassing at 410 K and further treatment with VOCl3 leads to improved catalytic properties, and after six such cycles a monolayer catalyst is produced which contains 1.7% V2O5 and which is superior even to a doubly promoted catalyst prepared by conventional impregnation. Below about 600 K, however, the carbon mass balance shows that not all the o-xylene fed to the reactor is recovered as volatile products. The missing carbon is at least partly in the form of a surface deposit which is thought to be responsible for initial losses of activity: these have been followed at various temperatures and flow rates. The carbon in this surface deposit has been quantitatively estimated by oxidation to CO2, the results indicating that below about 560 K the deposit covers most of the surface. The kinetics of the reaction have been studied at 582 and 608–610 K. At both temperatures the process of carbon loss increases with o-xylene pressure, causing maxima in the rate of o-xylene conversion, and is suppressed by increasing oxygen pressure: the conversion varies as the square root of the oxygen pressure. A model is developed in which chemisorbed oxygen atoms doubly bonded to V5+ ions are the oxidizing species. If o-xylene adsorbs by dissociation of a hydrogen atom from a single methyl group at an uncovered site, its further oxidation to phthalic anhydride proceeds smoothly. If, however, by interaction with another uncovered site a hydrogen atom is lost from the second methyl group, a strongly adsorbed species is formed which constitutes or is the precursor to the surface deposit.

Preparation and properties of vanadia/titania monolayer catalysts

Abstract Work performed on vanadia/titania catalysts at Brunel University over the past 20 years is summarised, and the thinking that led to the concept of reactive oxide monolayers is described. Both impregnation and grafting techniques lead to monolayers, and their structure, and that of the supramonolayer region is revealed through characterisation by a number of physical techniques. Factors responsible for the stability of monolayer systems are considered, and the basis of their activity in selective oxidations is discussed.

Gold catalysts for olefin hydrogenation

Gold catalysts active for the hydrogenation and isomerisation of olefins have been prepared by the thermal decomposition of chloroauric acid supported on silica and alumina. The activity depends critically on the concentration of gold in the catalyst. This article considers the implications of these results and their possible industrial applications.

Structure and reactivity of titania-supported oxides. Part 2: characterisation of various vanadium oxide on titania catalysts by x-ray photoelectron spectroscopy

Quantitative X-ray photoelectron spectroscopy (XPS) has been used to characterise V2O5/TiO2 catalysts prepared in a variety of ways, as described in Part I of this series. As the V2O5 concentration is raised, the V/Ti ratio at first increases, but after the equivalent of about two monolayers has been deposited the ratio remains approximately constant. This shows unequivocally that V2O5 present in amounts exceeding that required to form the first monolayer is not uniformly spread over it. A model, outlined in Part 1, in which ‘towers’ of V2O5 cover only a small fraction of the first monolayer, is shown to provide a quantitative interpretation of the results. They appear to cover 5 to 12% of the monolayer, depending upon the preparation method.