Dry reforming of methane to test passivation stability of Ni/ Al₂O₃ catalysts

Abstract

Catalyst passivation refers to the formation of a protective oxide layer on the active metal particles that prevents their oxidation when exposed to air. Common passivation procedures, when applied to Ni/ Al2O3 catalysts, typically result in a significant decrease of the overall Ni surface area and, accordingly, the catalytic activity. Nevertheless, passivation and reactivation is an attractive pre-treatment option for this system. Ni/ Al2O3 typically requires reduction temperatures much higher than the desired reaction temperature, whereas reactivation of passivated samples is a low-temperature reduction. This can be used to avoid temperature limitations of existing systems. Thus, more insight into the passivation process of this system is desirable. In this work we analyzed the impact of passivation on the catalytic performance of a series of Ni/ Al2O3 catalysts in dry reforming of methane. This approach allows for the elimination of scale effects during passivation. We show that changes in conversion and especially of the coke content can be used to track sintering of Ni particles. These metrics allows to identify an adverse effects of catalyst passivation in excess O2, which gives rise to rapid local overheating and, accordingly, Ni sintering even when operating at tens of mg catalyst scale. Our study demonstrates that such problems are not limited to scaling issues and sufficient care must be taken even on a lab-scale when passivating Ni/ Al2O3 catalysts.