CO Dissociation on Face-Centered Cubic and Hexagonal Close-Packed Nickel Catalysts: A First-Principles Study

摘要

Exploring the dependence of the structure activity relationship of catalysts is important for improving the activity and selectivity in heterogeneous catalysis. Among other factors, the influence of the crystal phases, face-centered cubic (FCC) and hexagonal close packed (HCP), of Ni catalysts on CO dissociation is studied by density functional theory (DFT). Surface energies of numerous FCC and HCP facets are calculated to construct the corresponding morphologies, and the exposed facets (six facets for FCC Ni and five facets for HCP Ni) are used to investigate the CO dissociation. For FCC Ni, (311) is the most active facet with the least barrier of 1.58 eV, followed by (100) and (211) with barriers of 1.63 and 1.75 eV, respectively. For HCP Ni, (10 (1) over bar2) is the most active facet with the least barrier of 1.73 eV, followed by (10 (1) over bar1) with a barrier of 1.86 eV. On both FCC and HCP Ni, CO dissociation shows a dramatic structural sensitivity irrespective of direct or H-assisted pathway. Compared to;the direct dissociation, the H-assisted dissociation is kinetically favorable on both FCC and HCP Ni. With increase of dissociation barrier, the preferred dissociation pathway changes from COH intermediate to CHO intermediate. FCC Ni can expose abundant facets with low barrier. The result is compared with more active cobalt catalysts showing an opposite dependence on the crystal phases. The revealed insights regarding the crystal phase and the composition of catalysts upon activation of the diatomic molecules provide a new perspective for rational design of catalysts to expose more active sites for a higher specific activity.