Alan Brenner

Studies of the oxygen release reaction in the platinum-ceria-zirconia system

Abstract The relative rates of the CO+oxygen storage material (OSM)⇒CO2 reaction (RCO2) and the amounts of rate enhancement obtained upon Pt promotion were examined for ceria and ceria–zirconia OSM. The effect of Pt surface area on RCO2 was de-coupled from metal–oxide surface area by pre-sintering the oxides prior to Pt deposition. We find that RCO2 is linearly dependent on Pt area over Pt/CeO2, but over Pt/Ce0.75Zr0.25O2, the rate is independent of Pt surface area above a threshold surface area. Although Pt sinters more readily on the Ce0.75Zr0.25O2 support, the dispersion effect is more than compensated by the enhanced availability of “bulk” O2− within the Ce0.75Zr0.25O2 particles. Furthermore, this study demonstrates that, on a unit surface area basis, the ceria–zirconia support is at least two times more active for the oxygen release reaction than the pure ceria support when the materials are slightly reduced. It is also shown that O2− diffusion is not rate-limiting for the ceria reduction reaction when the reaction is carried out in the kinetically limited regime at 500°C with CO as the reductant.

The synthesis and nature of heterogeneous catalysts of low-valent tungsten supported on alumina

Abstract Temperature-programmed decomposition has been used to study the nature of the bonding to Al 2 O 3 of the zero-valent catalyst precursor W(CO) 6 . The pretreatment conditions and activation temperatures which lead to the formation of supported, low-valent W (which cannot be prepared by the standard techniques of catalyst synthesis) have been identified. At temperatures below 200 °C highly dispersed, zero-valent, subcarbonyl species are formed. W(CO) 3 ads is readily isolated and the decomposition can be reversed by exposure to a flow of CO. This is only the second system for which the stoichiometry of supported subcarbonyl species has been identified. Decomposition at high temperatures is sensitive to the ratio of hydroxyl groups on the alumina divided by the number of complexes on the surface. At high ratios the primary product of the complete decarbonylation is W(VI), whereas at low ratios W(0) is the primary product. Both of these products are also well dispersed and the complete decomposition is not reversible.

Temperature-programmed reduction and oxidation of metals supported on γ-alumina

Quantitative measurements of temperature-programmed reduction and oxidation (TPRTPO) at temperatures up to 1200 °C were used to characterize 2% metal on γ-alumina catalysts. The TPRTPO results were also compared with the redox behavior of the bulk compounds. It was found that supported Fe2O3, MoO3, WO3, and NiO could be reduced to metal only at temperatures near 1100 °C. In all cases reduction of supported oxides occurred at temperatures higher than those for the bulk oxides, indicating that interaction of the metal oxides with the support inhibited reduction. However, TPO of the supported metals usually occurred at a temperature lower than those for the bulk metals. This suggests that oxidation diminishes metal-support interaction and the higher rate of oxidation of supported metals reflects their smaller crystallite size. Chemisorption data are also given for the supported oxides after reduction at both moderate and high temperatures.

Active sites of H-ZSM5 catalysts for the oxidation of nitric oxide by oxygen

The catalytic activities of H-ZSM5-18, H-ZSM5-150, Li-ZSM5-18, and H-Mag (numerical suffixes mark the Si/Al ratios of zeolites, H-Mag is the proton exchanged form of the layered sodium silicate, magadiite) were compared for the oxidation of NO by O2 at different ratios of reactants at reaction temperatures from 25 to 600°C. H-ZSM5-18 typically has activity maxima near 25 and 400°C at most O2/NO reactant ratios. Regardless of the partial pressures of reactants, NO±1/2O2 ⇋ NO2 equilibria are attained at 400°C and above. The H-ZSM5-150 and Li-ZSM5-18 zeolites are only active at temperatures near 25°C. H-Mag is practically inactive at the reaction conditions used. Results indicate that Lewis acidic lattice aluminium ions and silanol hydroxyls are not active in the oxidation of NO to NO2 over H-ZSM5 zeolites. Brønsted acidic bridging hydroxyls are probably active sites for this reaction at temperatures above 200°C.

Catalytic reduction of nitric oxide on PdO—MoO3/γ-Al2O3

Abstract The activity and selectivity of PdO—MoO 3 /γ-Al 2 O 3 catalysts containing 2% Pd and 20% Mo for the reduction of nitric oxide by carbon monoxide, by hydrogen, and by a mixture of CO + H 2 was investigated in the presence of different amounts of oxygen at reaction temperatures from 25 to 550°C. Results are compared with PdO/γ-Al 2 O 3 , MoO 3 /γ-Al 2 O 3 , and a commercial Pt—Rh catalyst. The palladium determines the performance of PdO—MoO 3 /γ-Al 2 O 3 under oxygen free reaction conditions up to 400°C, notwithstanding the high loading of the molybdena. At higher temperatures the molybdena can improve the performance of the catalyst, especially at slightly oxidizing conditions. Neither the activity nor the selectivity of PdO—MoO 3 /γ-Al 2 O 3 resemble those of the monometallic catalysts in the presence of oxygen when CO + H 2 is used for reduction. In the case of the present catalyst in fresh condition, nitric oxide is reduced selectively to nitrogen and nitrous oxide with a conversion > 70% at temperatures from 300 to 550°C under either slightly reducing or slightly oxidizing conditions.

Preparation and characterization of PdO-MoO3/γ-Al2O3 catalysts

Three different procedures have been compared for the preparation of PdO-MoO3/ /gg-Al2O3 catalysts for the purpose of achieving close proximity of the palladium and molybdenum oxides. The methods are: 1. adsorption of [PdCl2(OH)2]2− anions on MoO3/γ-Al2O3; 2. coadsorption of [Pd(NH3)4]2+ cations and MoO42− anions on γ-Al2O3; 3. adsorption of Mo(CO)6 on PdO/γ-Al2O3. Twenty catalysts were prepared and characterized by temperature-programmed reduction and X-ray powder diffraction. In general, the first method resulted in large, weakly bou oxide, whereas the other two methods resulted in a strong interaction between finely divided palladium and molybdenum oxides.

Comparison of oxidation of carbon monoxide on superconducting and insulating Y-Ba-Cu-O catalysts

Investigation of the catalytic activity of Y{sub 2}BaCuO{sub 5} has shown high and stable activity for CO oxidation when the catalyst is slightly reduced, comparable with the activity of the best catalysts for CO oxidation. This oxide was found to be about two orders of magnitude more active than the superconducting perovskite YBa{sub 2}Cu{sub 3}O{sub 7-x}. Furthermore, in contrast to the perovskite catalyst, surface reduction increased the activity of Y{sub 2}BaCuO{sub 5}. Zero-order kinetics were observed for O{sub 2} on both the oxidized and reduced forms of the catalyst, but the reaction order for CO was in the range of 1.1 to 1.5. Differences between the oxidized and reduced forms of the catalyst and between the two types of Y-Ba-Cu-O catalysts are discussed.

Well-dispersed catalysts of supported chromium, molybdenum and tungsten carbonyls which are highly active for alkene hydrogenation

Abstract Traditional catalysts (prepared by the reduction of a salt) of supported Cr, Mo, and W compounds have low activities for the hydrogenation of alkenes. This low activity is partly due to the difficulty in fully reducing these catalysts. The use of the zero-valent complexes Cr(CO)6, Mo(CO)6, and W(CO)6 for catalyst synthesis is described. Proper activation can yield well-dispersed and low-valent catalysts primarily containing subcarbonyl complexes which are far more active than traditional catalysts of these elements for the hydrogenation of alkenes.

Fischer-Tropsch synthesis over supported molybdenum hexacarbonyl

The first study of Mo(CO/sub 6//Al/sub 2/O/sub 3/ as a catalyst for Fischer-Tropsch synthesis is presented. This catalyst demonstrates improved activity and selectivity when compared to the traditional MoO/sub 3//Al/sub 2/O/sub 3/ catalyst. Activity and selectivity of the hexacarbonyl catalyst are also compared using standard alumina and dehydroxylated alumina as catalyst supports. The Mo(CO)/sub 6//dehydroxylated alumina catalyst has the highest activity. This result is attributed to the ability to initially prepare the Mo in an oxidation state close to zero. 26 references, 2 tables.

Determination of the oxygen content in superconducting and related cuprates using temperature-programmed reduction

The oxygen content and reducibilities of four base metal oxides, six nonconducting cuprates, and three superconducting cuprates were investigated by temperature-programmed reduction (TPR) from 30 to 900C. It was shown that the copper, bismuth, and thallium ions of these oxides were reduced to the zero valence state. Thus, TPR is a useful technique for determining the actual oxygen stoichiometry of cuprates. For multiphase Bi{sub 2}CaSr{sub 2}Cu{sub 2}O{sub 8+x} and Tl{sub 2}CaBa{sub 2}Cu{sub 2}O{sub 8+x} superconductors pretreated in air, x was found to be 2 {plus minus} 0.2 and 1 {plus minus} 0.2, respectively. Both values are significantly higher than previously reported. The reduction of superconducting and related cuprates required higher temperatures than CuO, but the reducibility of CuO decreased when it was calcined in air at 940C. The Bi- and Y-based superconducting cuprates exhibited the lowest reducibility among the samples investigated.