Effect of water on the reduction of nickel/alumina catalysts Catalyst characterization by temperature-programmed reduction

Abstract

The effect of water on the reduction of nickel/alumina catalysts has been studied in relation to characterization of the catalysts by temperature-programmed reduction (TPR). It was demonstrated that the TPR spectra are strongly perturbed by water vapour, and special precautions must be taken to obtain results reflecting the structure of the catalysts. TPR studies of NiO/Al₂O₃ showed that: (i) TPR spectra recorded in the absence of water indicate bulk structure in agreement with XRD data and (ii) spectra recorded in the presence of water reflect a complex surface structure. The results have been summarised in a model relating structure and reducibility of calcined NiO/Al₂O₃ precursors. The precursors contain nickel oxide in three forms: F1, crystalline nickel oxide fully exposed to the gas phase; F2, crystalline nickel oxide covered with nickel aluminate species and F3, amorphous nickel aluminate species. F1 is reduced at 200–240°C, similarly to NiO powder, and F3 is reduced at ca. 450°C, similarly to nickel aluminate. The reduction of F2 depends strongly on the concentration of water vapour in the reducing gas; it is reduced similarly to NiO powder in the absence of water and similarly to nickel aluminate in the presence of water. The reduction of F2 and F3 leads to the formation of crystalline nickel decorated with nickel alumina species.