Theoretical study of syngas hydrogenation to methanol on the polar Zn-terminated ZnO(0001) surface

Methanol synthesis from syngas (CO/CO ₂/H ₂) hydrogenation on the perfect Zn-terminated polar ZnO(0001) surface has been investigated using periodic density functional theory calculations. Our results show that direct CO ₂ hydrogenation to methanol is unlikely because, in the presence of surface atomic H and O, the highly stable formate (HCOO) and carbonate (CO ₃) readily produced from CO ₂ with low barriers of 0.11 and 0.09 eV will eventually accumulate and block the active sites of the ZnO(0001) surface. In contrast, methanol synthesis from CO hydrogenation is thermodynamically and kinetically feasible on the perfect ZnO(0001) surface. CO can be consecutively hydrogenated into formyl (HCO), formaldehyde (H ₂CO), and methoxy (H ₃CO) intermediates, leading to the final formation of methanol (H ₃COH). The reaction route via hydroxymethyl (H ₂COH) intermediate, a previously proposed species on the defective O-terminated ZnO(0001̄) surface, is kinetically inhibited on the perfect ZnO(0001) surface. The rate-determining step in the consecutive CO hydrogenation route is the hydrogenation of H ₃CO to H ₃COH. We also find that this final hydrogenation step is pronouncedly facilitated in the presence of water by lowering the activation barrier from 1.02 to 0.55 eV.