Mechanistic studies of water-gas-shift reaction on transition metals

A density functional theory (DFT) calculation has been carried out to investigate a water-gas-shift reaction (WGSR) on a series of chemical related materials of Co, Ni, and Cu (from the 3d row); Rh, Pd, and Ag (from the 4d row); and Ir, Pt, and Au (from the 5d row). The result shows that WGSR mechanism involves the redox, carboxyl, and formate pathways, which correspond to CO* + O* → CO_2(g), CO* + OH* → COOH* → CO_2(g) + H*, and CO* + H* + O* → CHO* + O* → HCOO** → CO _2(g) + H*, respectively. The reaction barriers in the three pathways are competitive and have a similar trend that groups 9 > 10 > 11 and 3d > 4d >5d. Thus, the bottom-right d-block metals (Cu, Pt, and Au) show better WGSR activity. The experimentally most observed formate can be attributed to its lower formation and higher dissociation barriers. Furthermore, the catalytic behavior on these active metal surfaces has been examined. The result shows that WGSR is mostly follows the redox pathway on Au(111) surface due to the negligible CO* oxidation barriers; on the other hand, all the three pathways contribute similarly in WGSR on Cu(111) and Pt(111) surfaces. Finally, the feasible steps of formyl in Fischer-Tropsch synthesis (FTS), the combustion reaction, and formate pathway, CHO* → CH* + O*, CHO* → CO* + H*, and CHO* + O* → HCOO**, respectively, have also been studied. The result shows that activities of FTS and the WGSR have opposite trends on these metal surfaces.