Teresa Kennelly

Catalytic chemistry of supported palladium for combustion of methane

The high-temperature catalytic chemistry of supported palladium for methane oxidation has been studied. Palladium oxide supported on alumina decomposes in two distinct steps in air at one atmosphere. The first step occurs between 750 and 800 ° C and is believed to be a decomposition of palladium-oxygen species dispersed on bulk palladium metal designated (PdOx/Pd). The second decomposition is between 800 and 850 ° C and behaves like crystalline palladium oxide designated (PdO). To reform the oxide, the temperature must be decreased well below 650 ° C. Thus, there is a significant hysteresis between decomposition to palladium and re-formation of the oxide. Above 500 ° C, methane oxidation occurs readily when the catalyst contains PdO. However, when only palladium metal is present no oxygen adsorption occurs and no methane activity exists. One may conclude that the high temperature (> 500 ° C) activity of a supported palladium containing catalyst is due to the ability of palladium oxide to chemisorb oxygen. Palladium, as a metal, does not chemisorb oxygen above 650 ° C and thus, is completely inactive toward methane oxidation.

Reversible poisoning of palladium catalysts for methane oxidation

Abstract the observation that impurities such as halogens and alkali poison heterogeneous catalysts is well documented in the literature. the precise mechanism, however, is not always clear. chloride containing precursor salts produce catalysts inferior in performance to others made without chloride, leading one to conclude any or all of the potential mechanisms listed below are operative. (1) charge effects on the precursor complex are incompatible with the carrier surface charge, resulting in poor distribution of the salt. (2) the presence of certain impurities promotes metal sintering when the catalyst precursor is decomposed, resulting in lower catalytic surface areas. (3) impurities irreversibly react with the catalytic species. (4) impurities either block or distort the surface electronic properties of the catalytic species leading to poor performance. the current study addresses the effects of chloride, from the pre-cursor salts and other impurities from the alumina, on the activity of palladium on alumina for the complete oxidation of methane for combustor applications. results strongly suggest that localized site blockage and/or inductive effects are responsible for poor performance. methods for regeneration are proposed. characterization techniques such as xps analysis, carbon monoxide, chemisorption, and activity testing were used to support the conclusions of this work.