R. Burch

Selective reduction of nitrogen oxides by hydrocarbons under lean-burn conditions using supported platinum group metal catalysts

The performance of supported platinum-group metals (Pt, Pd, Rh, Ir and Ru) for the selective reduction of nitrogen oxides by hydrocarbons under oxidising conditions is reviewed. Kinetic and mechanistic studies on these catalysts are assessed. Many of the factors influencing de-NOx behavior are considered. These include the choice of metal, the metal loading and the metal particle size, the carbon number and hydrocarbon type of the reductant used and the type of support material. It is found that highest de-NOx activity is obtained with platinum-based catalysts although large quantities of nitrous oxide are formed in addition to nitrogen. Rhodium also exhibits a significant but lower activity although much less N2O is produced. It is seen that NOx reduction is coincident with hydrocarbon combustion. Although metal particle size has little effect on overall de-NOx activity, large particles have higher turnover frequencies. For a given carbon number, it is found that the de-NOx efficiency increases in the order i-paraffins < aromatics < n-paraffins < olefins ≈ alcohols and that, in general, activity increases with increasing carbon number. When using olefins as the reductant, varying the support material has little effect on activity although large differences can be seen when using saturated hydrocarbons. Mechanistic studies, including FT-IR and TAP experiments, are reviewed and the various mechanisms that have been proposed for this reaction are discussed.

Catalytic combustion of methane over supported palladium catalysts. I, Alumina supported catalysts

Abstract A series of Pd/Al2O3 catalysts were tested, under continuous-flow conditions, for their activity towards methane combustion in an oxygen rich atmosphere. It was found that, under reaction conditions, catalysts became more active with time-on-stream with rate constants increasing by up to 100 fold. This activation effect could be observed over a short period or for many tens of hours. Both γ-and δ-alumina-supported catalysts exhibited activation and the effect was not eliminated by vigorous calcination of the catalysts. The enhanced activities were not due to a redispersion of the active phase or thermal effects in the catalyst bed. No clear relationship between palladium particle size and rate of reaction was dicernible.

C-H bond activation in hydrocarbon oxidation on heterogeneous catalysts

Abstract The activation of C–H bonds in different hydrocarbons on the surfaces of metal oxide and metal catalysts is considered. On oxides, it appears that the initial activation may occur through either homolytic or heterolytic scission of the C–H bond, but the reaction is surface-catalysed. The activation of methane requires highly basic sites which are susceptible to severe poisoning by carbon dioxide. With metal surfaces, the extent of oxidation of the surface can strongly affect the oxidation activity. For rhodium catalysts, it is shown that the intrinsic activity for methane combustion is high. However, rhodium is strongly deactivated under oxidising conditions when alumina is used as the support: deactivation is not observed when the support is zirconia. Transient effects on the activity of an alumina-supported palladium catalyst are reported and these show that the steady state is not easily established. Water severely inhibits the methane combustion reaction on palladium, and chlorine and sulphur dioxide are strong poisons. In contrast, for the combustion of propane on alumina-supported platinum catalysts, sulphur dioxide is a significant promoter of the reaction.

The role of copper and zinc oxide in methanol synthesis catalysts

The addition of Al2O3, ZrO2, Ga2O3 or ZnO to Cu/SiO2 catalysts results in a definite increase in the rate of methanol synthesis per unit area of copper. Most apparent is the synergy between ZnO and Cu, which occurs even when particles of ZnO/SiO2 and Cu/SiO2 are physically mixed. If, after high-pressure testing, the Cu component of a physical mixture is isolated, its activity on re-testing is found to return to a value expected for Cu/SiO2. The synergy is not due to ‘classical’ bifunctional catalysis, because no enhancement in activity is observed when the two phases are loaded as separate beds. However, a correlation is observed between the incidence of synergy and the spillover of hydrogen, which is detected during the temperature-programmed dissociation of intermediates on the Cu surface. It is proposed that ZnO can act as a reservoir for spillover hydrogen, and that reverse spillover may account for the higher rate of methanol synthesis on Cu when ZnO is present in the catalyst. The ZnO is perceived as playing the role of an enabling support, acting as a reservoir for atomic hydrogen and promoting hydrogen spillover.

Catalytic combustion of methane over supported palladium catalysts: II. Support and possible morphological effects

Abstract The effect of using various precursor palladium salts and different support materials in the preparation of palladium catalysts for methane combustion has been investigated. Catalyst activities were found to increase dramatically after heating in a 1% CH4/air reaction mixture. These activation effects were altered according to the precursor salt and support material used. The activation of silica samples occurred over short time periods (minutes-hours) whilst the activation over alumina was more prolonged (up to 8 days). Many experimental factors which could be responsible for the activation were investigated and are discussed, but none were found to be of paramount importance in initiating catalyst activation. Reconstruction of the palladium oxide crystallites is proposed as a cause of catalyst activation. Possible mechanisms for this reconstruction are considered. It appears that only a small fraction of the palladium surface is active for methane combustion.

The role of hydrogen in methanol synthesis over copper catalysts

The experimental observation that the activity of a series of zinc oxide supported copper catalysts is related to the surface area of the copper is examined in the light of recent evidence that hydrogen spillover from copper to zinc oxide is facile. A model is proposed which explains the effect of zinc oxide on copper in terms of a reversible transfer of hydrogen between copper and zinc oxide. It is suggested that therate determining step normally occurs on the surface of the copper but that therate of reaction can be dependent on the availability of spillover hydrogen from the zinc oxide.

Oxidative dehydrogenation of ethane on vanadium-molybdenum oxide and vanadium-niobium-molybdenum oxide catalysts

Abstract The oxidative dehydrogenation of ethane has been investigated on a range of mixed Mo/V/Nb oxide catalysts. A maximum in activity was observed for Mo/V oxide catalysts at ca. 70% Mo. Addition of niobium oxide to this mixed oxide resulted in a more active and selective catalyst. The orders of reaction with respect to the ethane and oxygen partial pressures have been determined and found to be close to first order in ethane pressure but almost independent of oxygen pressure, except at higher temperatures and lower oxygen pressures. The role of lattice oxygen in the reaction has been explored by examining the reducibility of the catalysts, the thermal desorption of oxygen, and the activity in pulse experiments in the absence of gas-phase oxygen. A correlation is observed between the ease of reduction of the catalyst and the activity and selectivity, with the most easily reduced catalyst being the most active and selective for the formation of ethene. Possible mechanisms to account for the results are discussed and the role of niobium is considered.

Support and morphological effects in the synthesis of methanol over Cu/ZnO, Cu/ZrO2 and Cu/SiO2 catalysts

Copper catalysts supported on ZnO, SiO2 and ZrO2 have been prepared and the copper surface areas determined using the technique of frontal chromatography. The activities of the catalysts for the synthesis of methanol from CO/CO2/H2 and CO/H2 mixtures have been determined at 10 and 40 bar pressure and in the temperature range 250–270°C under conditions of continuous flow. The results show a clear support effect in both reaction mixtures and at both pressures. Under our experimental conditions there is no correlation between the copper surface area and the activity for the synthesis of methanol. Instead we observe that the activity of a Cu/ZnO catalyst is between 10 and 30 times higher per unit copper area than is the activity of a Cu/SiO2 catalyst. Cu/ZrO2 catalysts are intermediate in activity. It is concluded that support effects can strongly influence the activity of copper in the methanol synthesis reaction at elevated pressures. Possible explanations in terms of the morphology of the copper particles and the structure sensitivity of the reaction are discussed.

Homogeneous and heterogeneous contributions to the oxidative dehydrogenation of propane on oxide catalysts

Abstract The pyrolysis and the oxidative dehydrogenation of propane have been investigated in a quartz reactor. It has been found that by optimising the propane/air ratio it has been possible to obtain very high yields of propene and ethene. A variety of oxide-based catalysts have been prepared, characterised, and tested in the oxidative dehydrogenation reaction. It has been found that with these catalysts, the maximum selectivity to propene at a specific conversion of propane is never higher than can be obtained in the absence of a catalyst. The results also show no obvious correlation between catalytic properties and the presence of specific crystalline phases. It is suggested that lower than optimum selectivities to propene may arise because of the presence in the catalysts of small, but undetected, amounts of pure V 2 O 5 . Comparison with other published work suggests that there is an upper limit on the yield of propene which can be obtained for a purely heterogeneously catalysed reaction. A combination of homogeneous and heterogeneous contributions to the oxidative dehydrogenation reaction may provide a means of obtaining even higher yields of propene.

Selective reduction of NOx by hydrocarbons in excess oxygen by alumina- and silica-supported catalysts

A range of alumina- and silica-supported metal catalysts have been investigated for the selective reduction of NOx by propene in excess oxygen. Platinum, in particular, has been found to have a high activity and selectivity. There is a close correlation between the activity for NOx reduction and propene combustion. For platinum group metals, it is found that silica-supported catalysts are more active than alumina-based materials. For Pt/SiO2 catalysts, it is found that the specific activity for NOx reduction decreases with decreasing metal dispersion. NO is found to inhibit the oxidation of propene on Pt-based catalysts