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

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-H-2. 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-H-2 curve. Kinetics of reduction of CuO, NiO and CuO-NiO oxides by H-2 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. (c) 2020 Elsevier B.V. All rights reserved.