Reaction of neopentane with hydrogen over Pd, Pt, Ir and Rh

Abstract

The skeletal reaction of neopentane with dihydrogen has been studied over powders and (111) oriented films of palladium, platinum, iridium and rhodium. Various pretreatments of the metals have been used for changing the surface morphology (O₂–H₂ pretreatment, very high temperature reduction etc). The effect of O₂–H₂ pretreatment on the course of the reaction was most pronounced in case of palladium (selectivity change) and platinum (overall activity change). The catalytic behaviour of rhodium and iridium suffers only minor changes after these pretreatments: demethylation always predominates. In contrast to unsintered palladium, heavily annealed Pd samples should have a lower density of surface imperfections. This fact seems to be responsible for the negligible activity of sintered Pd surfaces in hydrogenolysis and, in effect, for the higher selectivity toward isomerization. XRD analysis of variously pretreated metal powders shows that the increase in isomerization selectivity may be attributed to the sintering of palladium. At ca. 200 °C iridium powder was only slightly selective toward isomerization (2–3 %). This selectivity is limited to the initial stage of the reaction only and to clean, but not carbided, surfaces of iridium. A carbided Ir surface produces unusually large amounts of ethane among the hydrogenolysis products. The comparison between catalytic behaviour of platinum and other Group VIII metals allows us to rationalize the results in terms of surface morphology and the reaction mechanism.