Temperature-programmed reduction of model CuO, NiO and mixed CuO–NiO catalysts with hydrogen

Abstract

Samples of model CuO, NiO and CuO–NiO catalysts were synthesized by calcination of individual copper and nickel hydrated nitrates, and according to a technique of their co-calcination. They were characterized by XRD, Raman spectroscopy, TEM and XPS. Solid solutions of Cu2+ in NiO and Ni2+ in CuO in the CuO–NiO sample were shown to be formed. Reduction of CuO, NiO and CuO–NiO oxides as model catalytic systems was studied by in situ XRD and TPR-H2. Reduction of CuO–NiO system was found to take place at notably lower temperatures than in the case of individual CuO and NiO. In situ XRD studies showed that the NiO–CuO reduction begins from copper oxide reduction evidenced by a sharp hydrogen uptake peak in the TPR-H2 curve. Kinetics of reduction of CuO, NiO and CuO–NiO oxides by H2 was investigated in detail. It was shown that apparent activation energy and pre-exponential factor obtained by Kissinger method for copper oxide reduction in CuO–NiO, compared to pure copper oxide, increased from 38 to 54 kJ mol−1 and from 6.4 to 13.0 (ln A) correspondingly. This is connected with formation of solid solution of Ni2+ in CuO that results in a significant growth of nucleation sites. As compared to individual nickel oxide, reduction of NiO in CuO–NiO is characterized by considerably lower activation energy. This is most probably caused by changes of chemical state of Ni2+ as a result of copper introduction into nickel oxide structure and formation of solid solution of Cu2+ in NiO.