A microkinetic analysis of the reverse water gas shift reaction

Abstract

The previously published model of the forward and reverse water gas shift reaction on copper (1) has been made more realistic by the incorporation of an energy barrier for the adsorption of hydrogen on to the copper. An activation energy barrier for the adsorption of hydrogen of 12 k cal mol−1 reduces the steady state hydrogen atom coverage of the copper from 90% of a monolayer to <10% of a monolayer. Paradoxically this model predicts an increase in the rate of the reverse water gas shift reaction at all temperatures studied in the temperature range 300 K to 470 K. This results from the lower hydrogen atom coverage producing a greater amound of free copper area on which the CO2 can decompose. Reducing the activation energy barrier to hydrogen atom adsorption to 9 k cal mol−1 increases the steady state hydrogen atom coverage of the copper to ∼30% of a monolayer, a value which is more consistent with the H:Cu ratio of 4 found by Topsoe and co-workers (13). This overall potential energy diagram, incorporating this 9 k cal mol−1 activation energy barrier now predicts rates of the reverse water gas shift reaction in good agreement with experiment.