CuO Monolayer Dispersion on ZnAl2O4 Spinel: Design Catalysts with High H2 Generation Efficiency and Extremely Low CO...

To unravel the intrinsic reasons accounting for the spontaneous monolayer dispersion behavior of metal oxides, CuO/ZnAl2O4 catalysts with different CuO loadings were synthesized. As quantified by XRD and XPS extrapolation methods, CuO can spontaneously disperse on the ZnAl2O4 surface with a monolayer threshold of 1.299 mmol/100 m2 (7.2 wt %). DFT calculations were adopted to interpret theoretically the monolayer dispersion phenomenon. It is disclosed that the competition between adsorption energy (Eads) and cohesive energy (Ecohe) dictates the thermodynamic state of CuO on the support. The catalyst near the dispersion capacity exerts the strongest SMOSI/SMSI effect, which leads to the generation of the largest amount of Cu+ and Cu0 sites, the optimum Cu+/Cu0 ratio, and the most abundant active oxygen sites, thus getting the best catalytic performance. In situ DRIFTS and in situ operando pulse tests have confirmed that the reaction follows an HCOO* pathway, with monodentate formate as the key intermediate. The monolayer-dispersed CuO can facilitate the formation of this intermediate effectively, but the agglomerated CuO grains above the capacity suppress its formation. It is concluded that regulating CuO loading at the monolayer dispersion threshold enables the fabrication of methanol steam reforming (MSR) catalysts with high H2 production efficiency and low CO selectivity.