Kinetic study of ethylene hydrogenation

Abstract

Turnover frequencies and kinetic reaction orders were determined for ethylene hydrogenation over platinum catalysts at temperatures from 223 to 336 K, hydrogen pressures between 50 and 650 Torr, and ethylene pressures from 5 to 600 Torr. The hydrogen kinetic order was observed to decrease continuously from first order at 336 K to half order at 248 K. Zero-order ethylene kinetics were found at ethylene pressures above 75 Torr, and the ethylene order became negative at lower ethylene pressures. The half-order hydrogen kinetics at low temperatures and the observed zero-order ethylene kinetics at higher ethylene pressures can be explained by a Horiuti-Polanyi mechanism in which hydrogen is adsorbed noncompetitively on a surface essentially covered with adsorbed hydrocarbon species. Hydrogen competes with ethylene adsorption at higher temperatures and lower ethylene pressures. A combination of the noncompetitive and competitive pathways explains the steady-state kinetics over the entire range of temperatures and pressures studied.