Cu/Zn-based catalysts improved by adding magnesium for water-gas shift reaction

Ternary Cu/MeO/ZnO (Me: alkaline-earth metal, Mg, Ca, Sr and Ba) 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 of the catalysts prepared by coprecipitation (cp). The highest activity was obtained over hp-Cu/MgO/ZnO among the catalysts tested. The catalyst precursors after the precipitation contained mainly aurichalcite, (Cu,Zn)(5)(CO3)(2)(OH)(16), while the decomposed products after the calcination contained apparently CuO and ZnO as crystalline phases, since the amount of Mg actually included in the catalyst was less than 1.0 at.%. The Cu metal surface area was larger and the particle size of Cu metal was smaller on the hp-catalysts than those on the cp-catalysts; nonetheless the BET surface area was sometimes larger on the latter than on the former. The addition of similar to 0.1 at.% of Mg was the most effective, resulting in the highest activity as well as the lowest activation energy. A good correlation was observed between the amount of Cu+ species and the activation energy of the shift reaction, suggesting that MgO significantly enhanced the formation of Cu+ species as the active sites. Even after the pre-reduction at the high temperature, 250 degrees C, hp-Cu/MgO/ZnO catalyst showed no significant decrease in the activity as well as no detectable sintering in the Cu metal particles during 50 h of the reaction. It was supposed that the shift reaction proceeds by a reduction-oxidation mechanism between Cu-0 <-> Cu+. (c) 2006 Elsevier B.V. All rights reserved.