Formation of Labile Surface Compounds and Catalysis

Abstract

As mentioned in Chap. 5, various unusual compounds are formed by the reaction of adsorbed atoms and molecules with metal atoms, but these compounds are not necessarily active as catalyst. In the methanation of CO with H2, CO + 3H2 → CH4 + H2O, on the Ni catalyst, a steady amount of active Ni4C and Ni–CH2 are detectable on the Ni catalyst during steady reaction. The surface of Fe catalyst in ammonia synthesis reaction is similar. The dissociation of N2 is the rate-determining step in the ammonia synthesis reaction. That is, the reaction rate is controlled by dissociation rate of N2, but the activity depends on a steady amount of labile FexN on the Fe catalysts. Therefore, the role of promoting materials is to increase the density of labile FexN on the surface. A steady amount of FexN, NH, and NH2 observed in the ammonia decomposition reaction and in ammonia synthesis reaction suggests a significant potential barrier between NH(a) and NH2(a) on Fe catalyst and also on other metals.