Hydrogen temperature-programmed desorptions in platinum catalysts: Decomposition and isotopic exchange by spillover hydrogen of chemisorbed ammonia

H-2-TPD of Pt/alumina catalysts display multiple hydrogen desorptions, In addition to chemisorbed hydrogen (Peak I) at approximately 175 degrees C, there is a small hydrogen desorption (Peak II) at about 250 degrees C and a large, irreversible hydrogen desorption (Peak III) at 450 degrees C. The quantity of hydrogen associated with Peak III decreases with increasing precalcination temperature of the alumina. It is shown that the hydrogen from Peak III is caused by decomposition of ammonia chemisorbed on acid sites of the alumina, which are introduced during the catalyst's preparation, In the presence of Pt, the chemisorbed ammonia undergoes isotopic exchange with D-2 by a spillover hydrogen process, even in mixtures where chemisorbed ammonia and Pt are located on separate support particles. Although spillover hydrogen species affect deuterium exchange of ammonium ions, they do not contribute to the catalytic activity of platinum for neopentane hydrogenolysis or isomerization. (C) 1996 Academic Press, Inc.