Mechanisms of Methanol Steam Reforming on Ni1/ZnO and Pd1/ZnO Single-Atom Catalysts: Insights from Density Functional Theory

On-demand hydrogen can be generated through methanol steam reforming. Isolated metal atoms hosted on ZnO may provide a lower-cost catalytic platform for this reaction. Density functional theory calculations were applied to examine the complete methanol steam reforming pathways over Ni1/ZnO and Pd1/ZnO single-atom catalysts, and the associated kinetics were evaluated using transition-state theory. H2O and CH3OH prefer Zn-top sites, with nearly identical adsorption energies on both catalysts, whereas most other intermediates bind more strongly to Pd1/ZnO as the Pd d-state centroid lies closer to the Fermi level. Formaldehyde (CH2O) governs product selectivity. The weaker C–3c–O orbital interaction on Ni1/ZnO favors the direct CO2-forming pathway without a CO intermediate, for which the rate-determining barrier is 0.844 eV. On Pd1/ZnO, the CO-mediated and direct CO2-forming pathways have comparable limiting barriers of 1.152 and 1.190 eV, respectively. The rate constant for the CHO rearrangement required before CO formation is only 6.704 s−1 on Ni1/ZnO, making CO formation kinetically unfavorable. Taken together, the calculations show that Ni1/ZnO provides higher intrinsic activity and CO2 selectivity than Pd1/ZnO as well as the reference Pt1/ZnO and Cu1/ZnO system. This work provides a mechanistic basis for designing efficient single-atom methanol-reforming catalysts.