Water-gas shift reaction over Cu/ZnO and Cu/ZnO/Al2O3 catalysts prepared by homogeneous precipitation

Both binary Cu/ZnO and ternary Cu/ZnO/Al2O3 catalysts were prepared by homogeneous precipitation (hp) using urea hydrolysis. The structure and the activity for the water-gas shift reaction of these catalysts were studied compared with those prepared by coprecipitation (cp). The binary precursors contained hydroxycarbonates such as malachite and aurichalcite phases, whereas the ternary precursors were composed of hydrotalcite, malachite and aurichalcite phases depending on the metal composition. After thermal decomposition, both catalysts contained apparently CuO and ZnO as crystalline phase. No phase derived from Al was observed, since the amount of Al was small as 10 at.% in the ternary catalysts. After reduction pretreatment with hydrogen, the catalysts were tested for the shift reaction between 150 and 300 °C. The activity of hp-catalysts was higher than that of cp-catalysts; binary hp-Cu/ZnO showed higher activity than ternary hp-Cu/ZnO/Al2O3 catalysts none the less the surface area was larger for the latter than for the former. The activity apparently depended on the surface area of Cu metal formed on the surface of hp-catalysts and a good correlation was observed between the Cu metal particle size and the activation energy of the shift reaction. However, more precise evaluation of the activity based on turn-over frequency strongly suggested the formation of Cu+ species as the active sites at the boundary between Cu metal particles and ZnO particles. Even after the pre-reduction at the high temperature, 250 °C, hp-Cu/ZnO catalyst showed no significant deactivation as well as no detectable sintering of the Cu metal particles during 50 h of the reaction, indicating that the hp-preparation method afforded the Cu catalysts with high sustainability in the shift reaction.