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

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) catalyst exhibits a high dependency on the partial pressure or concentration of reaction products i.e., CO and HO in reverse water gas-shift reaction (RWGS) (CO:H = 1:3) but not reactants of CO and H. Molybdenum oxide (MoO) overlayers formed by oxidation with HO are observed at reaction pressure below 10 mbar or with low partial pressure of CO/HO products, while CO-induced surface carbonization happens at reaction pressure above 100 mbar and with high partial pressure of CO/HO products. The reaction products induce restructuring of MoN surface into more active molybdenum carbide (MoC) to increase the reaction rate and make for higher partial pressure CO, which in turn promote further surface carbonization of MoN. We refer to this as the positive feedback between catalytic activity and catalyst activation in RWGS, which should be widely present in heterogeneous catalysis.