Catalysis on evaporated metal films III. The efficiency of different metals for the reaction between ethane and deuterium

Abstract

The catalyzed exchange reaction between ethane and deuterium has been studied over evaporated films of tungsten, molybdenum, tantalum, zirconium, chromium, vanadium, nickel, platinum, palladium, rhodium and ruthenium. Manganese, silver and iron films were inactive up to 370° C, but the latter produced some highly deuterated methane. Cobalt showed simultaneous cracking and exchange at 300° C and cracking was observed on nickel above 160° C. The initial distribution of the deutero-ethanes fell into three classes which are exemplified by molybdenum, palladium and chromium. With molybdenum the main product was C 2 H 5 D, with palladium C 2 D 6 , and both were major products with chromium. A theory is developed which accounts for the observed distributions in terms of the relative probability of an adsorbed ethyl radical either desorbing as ethane or adsorbing further to ethylene— the highly deuterated products being formed by repeated transitions between ethyl radical and ethylene residue. The energies of activation ranged from 7 kcal/mole for molybdenum to 21 kcal/mole for palladium, and high-frequency factors were associated with high energies of activation. The orders of reaction with respect to both gases were determined on tungsten, palladium and rhodium, and possible mechanisms are formulated. Some evidence that different crystal faces had different catalytic activities was obtained by comparing the distribution of products formed on oriented and unoriented nickel films. Nickel films showed a falling energy of activation in the temperature range 0 to 75° C, and evidence was obtained that this was due to a reversible shift in the equilibrium amounts of different adsorbed species.