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The effect a 45 atom-thick overlayer of aluminum oxide, deposited by atomic layer deposition, was investigated on a 2.5 wt.% Pd/Al 2 O 3 catalyst in which the Pd nanoparticles were encapsulated by the support consequent to the preparation method. The overlayer suppressed the activity for oxidative dehydrogenation of ethane, but it also effectively prevented metal sintering and suppressed coking for reactions at high temperatures. On the catalyst with the alumina overlayer, a steady activity was observed even at 600 °C and above when the gaseous oxygen was completely consumed, while attaining 70% selectivity for ethene at 38% conversion, versus 10% selectivity at comparable conversions without the overlayer. The overlayer preferentially deposited on edge Pd atoms, as indicated by preferentially suppression of the CO chemisorption peak at 2056 cm −1 , which are responsible for coking. The high ethene selectivity at 675 °C could be due to coupled homogeneous and heterogeneous reactions at the high temperature, together with phase change of PdO to Pd.

The epoxidation of 1-hexene with titanium silicalite-1 (TS-1) catalyst was investigated to gain insight into the effect of the solvent on the observed reactivity. Three different solvents were examined: methanol, acetonitrile, and acetone. Kinetic data were obtained from batch reaction experiments, with an emphasis placed on gathering more accurate initial rates than those reported in the literature. The dependencies of the rates on the concentrations of 1-hexene, water, and hydrogen peroxide were determined. The adsorption behavior of 1-hexene in TS-1 in the three different solvents was determined independently by batch sorption experiments. Results showed that the solvent has a significant effect on the adsorption of 1-hexene, and hence on the reaction kinetics. Kinetic modeling incorporating experimental and simulated quaternary adsorption isotherms to describe quasi-equilibrated steps revealed that the differences in the observed reaction kinetics may be attributed mostly, but not entirely, to differences of the partitioning of 1-hexene between the bulk and intraporous phases among the three different solvents.

Supported Au catalysts are very active for low temperature CO oxidation. However, the reported activity from different laboratories for apparently similar catalysts can differ quite substantially. Recent progress in resolving this difficulty is summarized. Residual chloride in the sample is a very effective poison of the active site. The effect of water vapor on the catalytic activity differs depending on the support and the residual chloride content. A model of the active site, which consists of an ensemble of metallic Au atoms and Au cations with hydroxyl ligands, the reaction mechanism for CO oxidation, and the mechanism for deactivation during reaction as well as regeneration are discussed with respect to the available data.

Supported Au catalysts are generally prepared from chloride-containing Au precursors and their properties are highly sensitive to preparation procedures. To better understand and control the variables important in the preparation of these catalysts, the effect of chloride on the catalytic performances of highly active Au/Al 2 O 3 catalysts in the selective catalytic oxidation of CO (SCO) was probed. The complex manner in which Cl − affected Au catalysis was deconvoluted in a series of complementary experiments where chloride was quantitatively removed and added. The residual chloride was found to affect the activities in two different ways. It facilitates the agglomeration of Au particles during heat treatment, and it inhibits the catalytic activity by poisoning the active site. Interestingly, chloride poisoning is not only observed at the steady state, it is in fact more pronounced in the transient toward steady state. In order to better assess the poisoning effect of Cl − , phosphate was introduced to the support surface as a postsynthesis treatment before the addition of Cl − in some experiments. The results showed that activity suppression was observable at a Cl − /Au molar ratio as low as 0.0006. Thus a very small fraction of Au is associated with the active site. This implies that the activity per active site is extremely high.

An Al2O3-supported Ce0.45Zr0.55O2 was prepared with a modified sol–gel procedure designed to maximize the interaction between ceria and zirconia and promote thermal stability of alumina. Its properties, characterized in detail with XRD, TPR, and laser Raman spectroscopy, were compared with those of more conventional preparations. This sample consisted of finely dispersed CexZr1−xO2 and had a high fraction of easily reducible Ce4+. Complete insertion of Zr into the ceria fluorite structure was observed after 900°C oxidation. After reduction at 980°C and reoxidation at 900°C, the mixed oxide particles remained small and furthermore still remained single phase and homogeneous by XRD characterization. Redox treatment enhanced the reducibility of Ce4+ in this sample. One feature of the enhancement, similar to that in bulk mixed oxide of similar composition, was an increase in the ratio of Ce4+ that was reducible at ∼491°C relative to the ones at ∼577°C. A new low-temperature reduction feature appeared at 320°C. This was attributed to the segregation of a small fraction of Ce3+ from the mixed oxide upon reduction and the subsequent formation of a finely dispersed ceria-rich phase on the surface of mixed oxide crystallites upon reoxidation. This phenomenon was particular to the supported samples because Al2O3 was able to stabilize the particle size of the mixed oxide during severe redox and thermal treatments.

SnO2/Al2O3 prepared with SnCl2 impregnation onto sol–gel Al2O3 and calcined at 800°C is one of the most active catalysts for lean NOx reduction with propene under high partial pressures of O2 (15%) and H2O (10%) and space velocity (30,000 h−1). An unusual feature of this catalyst is the independence of maximum NO conversion over a wide range of Sn contents (1 to 10 wt% Sn). This catalyst is, however, sensitive to other preparation variables such as the nature of Sn precursor and the calcination temperature. X-ray photoelectron spectroscopic results indicate that Sn is present in the +4 oxidation state irrespective of Sn loadings. X-ray diffraction, temperature-programmed desorption, and temperature-programmed reduction results indicate a wide range of oxo–tin cluster sizes as well as different ratios of amorphous and crystalline SnO2 present in samples with different Sn loadings. The Sn(IV) in the amorphous SnO2 fraction can be reduced to Sn(II) below 450°C in the TPR experiments. In fact, all of the SnO2/Al2O3 catalysts have very similar H2 uptake features below 450°C. In view of the fact that they also have very similar catalytic properties, it appears likely that easily reducible amorphous SnO2 is an active phase for lean NOx catalysis in these samples.

In the reduction of NO by CH 4 in the presence of a large excess of oxygen, the conversion to N 2 over a physical mixture of Co/Al 2 O 3 and H-zeolite catalysts was found to be twice as high as that expected if these components acted independently. This synergistic effect was observed with both H-ZSM-5 and H-USY. By comparing the activities of these catalysts for NO oxidation and CH 4 reduction of NO 2 , it was concluded that the synergistic effect could be explained by the participation of Co ions and proton sites in the reaction. NO is oxidized to NO 2 over the Co ions, and NO 2 is reduced by CH 4 to N 2 over the proton sites.

Certain Co/Al2O3catalysts are active and stable for the selective catalytic reduction (SCR) of NOxby propene or propane. The addition of 30 ppm SO2to the feed only mildly affects the catalytic performance. The effectiveness of Co/Al2O3in the SCR process depends strongly on the Co loading, the calcination temperature, and the source of alumina. Characterization of the catalysts with UV-vis spectroscopy under ambient conditions, temperature-programmed reduction by H2(H2-TPR), XRD, iodometric titration, and BET surface area measurements led to the proposal that there exist four different Co species: Co2+ions in CoAl2O4, small Co3O4particles that interact strongly with Al2O3, large Co3O4particles, and dispersed surface Co2+ions in octahedral coordination. The latter Co2+species are likely the catalytically most active species. Their propensity to interact with NOxis reminiscent of that of Co ions in ZSM-5.

The catalytic oxidation of ethane, propane, 2-methylpropane, butane, pentane, and cyclohexane has been studied over Mg3(VO4)2-MgO, Mg2V2O7, and (VO)2P2O7. These reactions produced a wide range of products ranging from alkenes, dienes, anhydrides, acids, and carbon oxides. Broadly speaking, the major products were dehydrogenation products over Mg3(VO4)2, combustion products over Mg2V2O7, and oxygen-containing organic products over (VO)2P2O7, although there are exceptions to this broad generalization. From the product selectivities, the moles of oxygen consumed per mole of alkane reacted could be calculated. These were the average oxygen stoichiometries for the reactions. They had values of about unity for ethane oxidation over VPO, about two when oxidative dehydrogenation was the predominant reaction, and about four for the other reactions studied. The selectivity patterns in terms of the formation of dehydrogenation products versus oxygen-containing products including carbon oxides were explained by assuming that there existed a selectivity-determining step, which could be the reaction of the surface alkyl species or the adsorbed alkene before its desorption. The formation of oxygen-containing products would be facilitated if the surface alkyl or adsorbed alkene could interact with the vanadium ions in two adjacent VOx units such that it could easily react with the reactive oxygen in the V-O-V bridge. The results of ethane oxidation, however, could not be explained simply by this model, and other factors needed to be considered.

The catalytic properties of magnesium orthovanadate, magnesium pyrovanadate, and mixed oxides of vanadium and magnesium in the oxidative dehydrogenation of propane and butane were studied. The mixed oxides prepared by two different methods, with or without the use of K, were compared. For samples without K, Mg orthovanadate was quite selective for the oxidative dehydrogenation of both propane and butane, whereas Mg pyrovanadate was selective only for propane. However, samples that contained residual K showed decreased selectivity. The mixed oxides were selective for both hydrocarbons, although those prepared without K contained only orthovanadate as the vanadate phase, whereas those prepared with K contained both orthovanadate and pyrovanadate phases. However, judging from the CO/CO2 ratios, all mixed oxides behaved as if they contained only Mg orthovanadate on the surface. Upon heating the samples, the concentration of potassium on the surface increased. Finally, interesting correlations between the rates of consumption of oxygen and of different alkanes were observed.

The oxidative dehydrogenation of cyclohexane was studied at 440 and 484°C over NdV0 4 , Mg 3 (VO 4 ) 2 , and a mixed VMg0 catalyst. Cyclohexene was the major initial product. Benzene, carbon oxides, and a trace of cyclohexadiene were also observed. The catalytic behavior of the mixed VMg0 catalyst in this reaction was very similar to its behavior in the oxidation of butane. However, the behaviors of NdV0 4 in the two reactions were different. The difference between the two catalysts was attributed to their different rates of reoxidation.

Vanadium-magnesium oxides were found to be active and selective catalysts for the oxidative dehydrogenation of propane to propene. A selectivity of up to 65% was obtained at 10% conversion, but decreased with increasing conversion. No oxygenates were detected. The reaction rate was 0.6 ± 0.15 order in propane and zeroth order in oxygen. For comparison, the rate for butane oxidative dehydrogenation was 0.85 ± 0.15 in butane and zeroth order in oxygen. Both reactions were believed to proceed primarily with the breaking of a methylene CH bond to form an adsorbed alkyl radical species.