Catalytic behavior of ternary Cu/ZnO/Al2O3 systems prepared by homogeneous precipitation in water-gas shift reaction

Ternary Cu/ZnO/Al2O3 catalysts have been prepared by homogeneous precipitation (hp) using urea hydrolysis and tested for the water-gas shift reaction. The Cu/Zn ratio was fixed at 1/1, and the effects of the Al addition on the precipitation procedure, the precursor structure and the catalytic activity have been studied.The precipitation proceeded stepwise; Cu(II) nitrate was first hydrolyzed, followed by the hydrolysis of Zn(II) nitrate, but the final compounds consist mainly of aurichalcite. It is likely that amorophous Cu(OH)2 formed first was converted to aurichalcite via a dissolution-reprecipitation mechanism assisted by Zn(II). A significant leaching of Cu took place with increasing Al content during the precipitation at 90 °C. The Cu leaching was effectively suppressed by lowering the temperature to 80 °C, but resulting in a slight decrease in the catalytic activity. In the hp-catalyst precursors, aurichalcite was always observed as the main component, whereas hydrotalcite and malachite appeared with increasing Al component. The catalytic activity increased by the addition of 5 mol% of Al and decreased with further addition of Al. The activity apparently depended on the Cu metal surface area on the catalyst, but the turn over frequency calculated based on the surface Cu metal significantly varied depending on the Al content. Moreover, the intensity of the reduction peak around 225 °C assigned to Cu2+ → Cu+ in the TPR well correlated with the catalytic activity. It is suggested that Cu/Zn bimetallic aurichalcite has an important role as the catalyst precursor and the reduction–oxidation between Cu+ and Cu0 plays in the catalytic mechanism of the shift reaction.

Graphical abstractTernary Cu/ZnO/Al2O3 catalysts were prepared by urea-assisted homogeneous precipitation and the precipitation procedures were studied. Aurichalcite as the catalyst precursor played an important role in the activity for the water-gas shift reaction. Figure optionsDownload full-size imageDownload as PowerPoint slide