Reverse water gas-shift reaction product driven dynamic activation of molybdenum nitride catalyst surface

Abstract

In heterogeneous catalysis catalyst activation is often observed during the reaction process, which is mostly attributed to the induction by reactants. In this work we report that surface structure of molybdenum nitride (MoN_x) catalyst exhibits a high dependency on the partial pressure or concentration of reaction products i.e., CO and H₂O in reverse water gas-shift reaction (RWGS) (CO₂:H₂ = 1:3) but not reactants of CO₂ and H₂. Molybdenum oxide (MoO_x) overlayers formed by oxidation with H₂O are observed at reaction pressure below 10 mbar or with low partial pressure of CO/H₂O products, while CO-induced surface carbonization happens at reaction pressure above 100 mbar and with high partial pressure of CO/H₂O products. The reaction products induce restructuring of MoN_x surface into more active molybdenum carbide (MoC_x) to increase the reaction rate and make for higher partial pressure CO, which in turn promote further surface carbonization of MoN_x. We refer to this as the positive feedback between catalytic activity and catalyst activation in RWGS, which should be widely present in heterogeneous catalysis.

Catalyst activation commonly occurs during reactions. This study demonstrates that the surface’s active structure in nitride catalysts during the reverse water gas-shift reaction varies with the partial pressure of reaction products, resulting in enhanced catalytic activity through positive feedback between catalytic activity and the evolution of MoNx’s active structure.