Hydrogen Temperature-Programmed Desorption in Platinum Catalysts: Decomposition and Isotopic Exchange by Spillover Hydrogen of Chemisorbed Ammonia.

Abstract

H{2}-TPD of Pt/alumina catalysts display multiple hydrogendesorptions. In addition to chemisorbed hydrogen (Peak I) atapproximately 175}o{C, there is a small hydrogen desorption (PeakII) at about 250}o{C and a large, irreversible hydrogen desorption(Peak III) at 450}o{C. The quantity of hydrogen associated withPeak III decreases with increasing precalcination temperature ofthe alumina. It is shown that the hydrogen from Peak III iscaused by decomposition of ammonia chemisorbed on acid sites ofthe alumina, which are introduced during the catalyst'spreparation. In the presence of Pt, the chemisorbed ammoniaundergoes isotopic exchange with D{2}by a spillover hydrogenprocess, even in mixtures where chemisorbed ammonia and Pt arelocated on separate support particles. Although spilloverhydrogen species affect deuterium exchange of ammonium ions, they do not contribute to the catalytic activity of platinum forneopentane hydrogenolysis or isomerization.