The highly selective catalytic hydrogenation of CO2 to CO over transition metal nitrides

Three transition metal-like facet centered cubic structured transition metal nitrides, γ-Mo2N, β-W2N and δ-NbN, are synthesized and applied in the reaction of CO2 hydrogenation to CO. Among the three nitride catalysts, the γ-Mo2N exhibits superior activity to target product CO, which is 4.6 and 76 times higher than the other two counterparts of β-W2N and δ-NbN at 600 °C, respectively. Additionally, γ-Mo2N exhibits excellent stability on both cyclic heating–cooling and high space velocity steady state operation. The deactivation degree of cyclic heating–cooling evaluation after 5 cycles and long-term stability performance at 773 and 873 K in 50 h are all less than 10%. In-situ XRD and kinetic studies suggest that the γ-Mo2N itself is able to activate both of the reactants CO2 and H2. Below 400 °C, the reaction mainly occurs at the surface of γ-Mo2N catalyst. CO2 and H2 competitively adsorbe on the surface of catalyst and CO2 is the relatively stronger surface adsorbate. At a higher temperature, the interstitial vacancies of the γ-Mo2N can be reversibly filled with the oxygen from CO2 dissociation. Both of the surface and bulk phase sites of γ-Mo2N participate in the high temperature CO2 hydrogenation pathway.