Pd on low-sureace-area α-alumina system: Is metallic Pd active for methane combustion?

Abstract

Palladium (Pd) supported on alumina with various additives is the catalyst of choice for the catalytic combustion of methane. This catalyst is interesting in that it can change dramatically under reaction conditions and can chemically interact withhigh-surface-area (HSA) y -alumina supports. Much has been made of the observation that the apparent activity of these catalysts decreases in certain regimes as temperature increases, leading to the assertion that Pd metal is not active for methane oxidation, at least under fuel-leam conditons. This phenomenon has been suggested as a means to keep a catalyst from overheating in practical combustor designs. In this paper, we show that for Pd supported on low-surface-area (LSA) α-alumina, the methane oxidation activity increases as PdO is reduced to Pd at a fixed operating temperature, even for fuel-lean gas mixtures. This higher activity is significantly greater than the highest turnover frequencies reported by previous investigators when extrapolated back to 550 K and 2% methane in air. We propose that the differences in relative activity for methane oxidation between catalysts supported on different alminase are likely related to differences in morphology, the state of the Pd phases, differences in catalyst poisoning by water, and interactions between the Pd phases and the support. Most of these parameters were not directly monitored in previous studies.