Reduction of CoMoO₄ and NiMoO₄: In situ time-resolved XRD studies

One method frequently employed for the preparation of active oxide catalysts consists of partial reduction under hydrogen at elevated temperatures. In this process, it is important to identify well-defined suboxides that can have high catalytic activity and are stable at the elevated temperatures typical of many catalytic reactions. Our results for the reaction of H₂ with α-NiMoO₄ and β-CoMoO₄ show that in situ time-resolved X-ray diffraction is a powerful technique to study the reduction/activation of mixed-metal oxides. It is clearly shown that the mechanism for the reduction of a mixed-metal oxide catalyst can exhibit drastic changes with respect to that observed for simple metal oxide catalysts. The generation of stable suboxides is difficult to predict. Thus, the reaction of H₂ with α-NiMoO₄ does not lead to formation of a well-ordered NiMoO_x intermediate. On the other hand, during the reduction of β-CoMoO₄, Co₂Mo₃O₈ and/or CoMoO₃ are formed. These chemical transformations are accompanied by changes in the line shape and position of the Mo Ln-edge in XANES and affect the behavior of reduced NiMoO₄ and CoMoO₄ catalysts. Induction times were detected in the reduction process of CoMoO₄. From the present results and data previously reported for NiO, it is clear that this phenomenon should be taken into consideration when aiming at the activation of oxide catalysts via reduction in H₂.