A TPD and XPS investigation of palladium on modified alumina supports used for the catalytic decomposition of methanol

Abstract

A series of modified palladium catalysts on alumina supports previously have been prepared and evaluated for selectivity, activity, and thermal stability in decomposing methanol into CO and H 2. In this investigation the modified palladium catalysts are characterized by X ray photoelectron spectroscopy (XPS) and CO and NH 3 temperature-programmed desorption (TPD). Average particle sizes of the palladium crystallites were also determined using chemisorption techniques. The XPS results indicate that the lithium-modified catalyst, which deactivated substantially during methanol decomposition testing, exhibited a significant build-up of carbon on the catalyst surface, while a similar accumulation was not observed for the lanthanum-modified catalyst under identical conditions. The CO TPD results show that the lithium-, sodium-, and barium-modified catalysts, which deactivate during methanol decomposition testing, produce a much greater amount of CO 2 relative to the amount of CO desorbed than do the potassium-, rubidium-, cesium-, and lanthanum-modified and the unmodified catalysts, which do not deactivate considerably under identical conditions. Ammonia TPD and chemisorption results indicate that no correlation exists between support acidity or particle size and the degree of CO dissociation. Finally, a correlation between the charge density of the modifier and the degree of CO disproportionation is presented which fits for all cases except one. This appears to support an electrostatic model for the promotion of CO bond cleavage by the alkali modifier.