REVERSIBLE POISONING OF PALLADIUM CATALYSTS FOR METHANE OXIDATION

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 precursor 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.