Xenophon E. Verykios

Dopant-induced metal-support interactions: 1. Influence on chemisorptive behavior

Abstract Titania supports doped with cations of valence lower than, equal to, and higher than the parent cation were used as carriers for Pt crystallites, and the concept of dopant-induced metal-support interactions (DIMSI) was investigated. Platinized titania doped with cations of lower or equal valence (K + , Mg 2+ , Ge 4+ ) exhibited normal H 2 and O 2 chemisorption behavior. Platinized TiO 2 doped with cations of higher valence (Ta 5+ , Sb 5+ , W 6+ ) exhibited drastically reduced H 2 and O 2 chemisorption capacity, especially at high Pt dispersions. Transmission electron microscopy analysis showed that these results cannot be attributed to reduced Pt dispersion of the doped catalysts, while electron spectroscopy for chemical analysis showed no surface enrichment of the catalyst supports with the dopant. Results are then interpreted in terms of electronic interactions at the metal-support interface, involving electron transfer from the doped support to the metal particles. Significant changes in the electronic structure of the doped carriers were detected by electrical conductivity measurements.

Effects of dopants on performance of metal crystallites: 2. Further characterization of doped supports and catalysts

Abstract The effects of dopants on chemisorptive and catalytic properties of metal crystallites are further investigated by additional characterization of supports and catalysts. Alterations of the Fermi energy level of TiO2 doped with altervalent cations are detected by measurements of specific electrical conductivity and activation energy of electron conduction. It is established that under any environment the specific electrical conductivity of higher-valence doped TiO2 is significantly higher than that of undoped, equal-, and lower-valence doped TiO2. The CO chemisorption capacity of Pt supported on higher-valence doped carriers is found to be significantly reduced and to follow the same general trends as those of O2 and H2. Shifts in the vibrational frequency of CO adsorbed on these catalysts indicate that the metal-carbon bond is weakened, probably due to increased saturation of Pt d orbitals as a result of electronic interactions at the metal-support interface.

Nonuniformly activated catalysts

Abstract In most theoretical and experimental studies in the area of heterogeneous catalysis, the distribution of catalytically active material within three-dimensional carriers has been assumed to be uniform. An extensive literature exists [1, 2] dealing with interactions between chemical and physical transport phenomena within such porous catalysts. The effects of intraparticle mass and heat diffusional resistances in catalysts exhibiting uniform activity distributions have been extensively studied in terms of point and overall effectiveness, selectivity, and yield, as well as in terms of concentration and temperature profiles within catalyst particles and chemical reactors.

Effects of altervalent cation doping on electrical conductivity of platinized titania

Abstract Electrical conductivity measurements of pure and doped platinized titania were conducted to obtain a qualitative estimate of alterations of the Fermi energy level of electrons of the carrier upon doping. It is established that, under any environment, the specific electrical conductivity of TiO 2 doped with cations of higher valence (Sb 5+ , Ta 5+ , W 6+ ) is significantly higher than that of undoped or equal valence (Ge 4+ )-doped TiO 2 which, in turn, is higher than that of lower valence (Mg 2+ )-doped TiO 2 . The activation energy of electron conduction follows the opposite trend. Differences in electrical conductivity are a function of temperature and of the environment in which the sample exists. Electrical conductivity was found to decrease in the order: H 2 > CO > vacuum.

Dispersion and support effects in carbon monoxide oxidation over platinum

Abstract The effects of Pt dispersion on its specific activity under CO oxidation were investigated using a zeolite Y as the carrier. At both, low (0.3%) and hi

Isomerization and hydrolysis reactions of important disaccharides over inorganic heterogeneous catalysts

Abstract Isomerization and hydrolysis reactions of cellobiose, maltose, and lactose were investigated over various minerals and synthetic zeolite catalysts. Zeolites of type A, X, and Y are the most active for such reactions. Product distributions were determined from batch experiments and are compared with those obtained under homogeneous alkaline conditions. Product distributions indicate that reaction routes consist of parallel hydrolysis and isomerization of the disaccharides to their corresponding ketoses, followed by hydrolysis of the ketoses. Approximately 10–13% of the disaccharide reacted is not accounted for in the product distribution, indicating that degradation reactions occur, probably in the alkaline broth of the mixtures.

Support participation in chemistry of ethylene oxidation on silver catalysts

Abstract Activity and selectivity of supported silver catalysts in ethylene epoxidation and combustion is shown to be a strong function of the carrier employed. All supports used in this study, except for α-alumina, were found to exhibit significant activity in ethylene oxide isomerization and oxidation. Their activity was found to be proportional to their surface acidity. Poisoning of surface acid sites by alkali and alkaline earth metal impregnation resulted in suppression of isomerization activity and reduction of oxidation activity. These phenomena are explained evoking the concept of acid-base bifunctional catalysis.

Support and crystallite size effects in ethylene oxidation catalysis

Abstract The influence of the support and metal crystallite size of silver catalysts under ethylene oxidation conditions is investigated. It is shown that the specific activity of silver can vary by as much as a factor of 50 depending on the carrier employed. Among the supports investigated, only α-Al 2 O 3 , ZrO 2 , SiC, and TiO 2 of the rutile structure, were found to offer non-zero apparent selectivities towards epoxide formation. The influence of metal crystallite size on the kinetic parameters was also found to depend on the carrier employed. Thus, the specific activity of Ag/α-Al 2 O 3 was found to exhibit a maximum at an average crystallite size of approximately 400A˚while that of Ag/SiO 2 increases with crystallite size and appears to level off at an average crystallite size of approximately 500A˚.

Selectivity enhancement in ethylene oxidation employing partially impregnated catalysts

Abstract The effects of radially nonuniform distributions of catalytic activity on the performance of spherical catalyst pellets under ethylene oxidation conditions are investigated. It is shown that partially impregnated catalysts exhibit higher effectiveness factors than uniformly activated ones, in a large range of values of the Thiele modulus. The opposite is observed for Thiele modulus values close to zero. Significantly enhanced selectivity to ethylene oxide formation is also shown to result from nonuniform activity distributions for all values of the Thiele modulus. Multiple steady states in effectiveness and selectivity are observed under sufficiently high values of the heat-of-reaction parameter. This phenomenon occurs in a narrow range of Thiele modulus values which is a strong function of the activity distribution within the catalyst pellet.

The oxidation of ethylene over silver-based alloy catalysts: I. Silver-palladium alloys

Abstract Silver-cadmium-supported alloy catalysts were prepared by impregnation of α-alumina with mixed silver nitrate and cadmium nitrate solutions. They were characterized in terms of exposed metallic area and surface composition employing selective chemisorption of oxygen and hydrogen. Surface enrichment in cadmium was observed which increased after the catalysts had been exposed to reaction conditions. Turnover frequencies of ethylene epoxidation and combustion reactions were determined. The former was found to increase significantly with increasing cadmium content of the surface while the latter was found to remain constant or slightly increase, resulting in improved selectivities. Activation energies were found to decrease with increasing cadmium content. The results are discussed in terms of geometric and electronic considerations of the alloy surfaces and it is concluded that electronic interactions between silver and cadmium atoms, affecting the rate-controlling step of the reactions, are primarily responsible for their observation.