Effective MgO surface doping of Cu/Zn/Al oxides as water–gas shift catalysts

Trace amounts of MgO were doped on Cu/ZnO/Al2O3 catalysts with the Cu/Zn/Al molar ratio of 45/45/10 and tested for the water–gas shift (WGS) reaction. A mixture of Zn(Cu)–Al hydrotalcite (HT) and Cu/Zn aurichalcite was prepared by co-precipitation (cp) of the metal nitrates and calcined at 300 °C to form the catalyst precursor. When the precursor was dispersed in an aqueous solution of Mg(II) nitrate, HT was reconstituted by the “memory effect.” During this procedure, the catalyst particle surface was modified by MgO-doping, leading to a high sustainability. Contrarily, cp-Mg/Cu/Zn/Al prepared by Mg2+, Cu2+, Zn2+ and Al3+ co-precipitation as a control exhibited high activity but low sustainability. Mg2+ ions were enriched in the surface layer of m-Mg–Cu/Zn/Al, whereas Mg2+ ions were homogeneously distributed throughout the particles of cp-Mg/Cu/Zn/Al. CuO particles were significantly sintered on the m-catalyst during the dispersion, whereas CuO particles were highly dispersed on the cp-catalyst. However, the m-catalyst was more sustainable against sintering than the cp-catalyst. Judging from TOF, the surface doping of MgO more efficiently enhanced an intrinsic activity of the m-catalyst than the cp-catalyst. Trace amounts of MgO on the catalyst surface were enough to enhance both activity and sustainability of the m-catalyst by accelerating the reduction–oxidation between Cu0 and Cu+ and by suppressing Cu0 (or Cu+) oxidation to Cu2+.