Re-Co alloys and single-atom Re catalysts in ammonia synthesis: A DFT study

Abstract

• BE of the N atom adsorbed on Co, Re, Re 3 Co 1 is -5.61, -6.71, -7.05 eV, respectively. • Re-Co alloys exhibit a variable synergy depending on the rate-determining step. • Re centers on Co split nitrogen atoms in electronic and thermodynamic properties. • Weakly bound N atoms work well on a surface modified by strongly bound N atoms. In this study, density functional theory calculations and microkinetic modeling were used to estimate the catalytic activity of Re δ Co 1-δ (0001) alloys (δ = 0; 0.25; 0.5; 0.75, 1), and Co(0001)-supported single- Re 1 and dual-atom Re 2 centers in the synthesis of ammonia. Nitrogen atoms (N ad ) form an ordered structure, and the bridge state is most active among others on the Re(0001) plane; while N ad are concentrated around the Re 1 and Re 2 centers, splitting in electronic and thermodynamic properties. The specific catalytic activity of the centers Re 1 , Re 2 and the Re(0001) plane changes in the order of 8.0·10 3 , 32.0, and 1.0, respectively, due to weakly bound but highly active N ad , which are stabilized by strongly bound and low-active N ad . The calculations revealed the NRe 3 state on the Re 3 Co 1 alloy with an abnormally high binding energy (BE) of -7.05 eV, unequal Re-N distances and Re5d fillings, in contrast to the similar NRe 3 state on Re(0001) with BE = -6.71 eV. A variable synergism of Re-Co alloys was established: the Re 3 Co 1 or Re 1 Co 3 alloy exhibits maximum catalytic activity if the rate-determining step is the formation or hydrogenation of N ad under appropriate experimental conditions, respectively.