XANES and EXAFS studies on metal nanoparticle growth and bimetallic interaction of Ni-based catalysts for CO2 reforming of CH4

Two groups of catalysts containing Ni and/or Co metals and MgAlOx as support material were made using coprecipitation and incipient wetness impregnation methods, respectively. The mechanism of metal particle growth during reduction of the monometallic Ni or Co and bimetallic Ni–Co catalysts was studied using X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure spectroscopy (EXAFS). The results show that the bimetallic Ni–Co catalysts made by coprecipitation method resulted in smaller metal nanoparticles upon reduction. TEM confirmed this observation and revealed that this catalyst has well-dispersed, more uniform metal particles. The metal reduction extent of Ni monometallic catalysts and the bimetallic catalysts made by impregnation was similar with respect to each metal. For the bimetallic Ni–Co catalysts made by coprecipitation, the interactions between two metals in reduction were observed such that Ni reduction was mitigated by Co, and Co reduction was promoted by Ni. And the first derivative of XANES of such made catalysts indicated stronger interaction between Ni and Co atoms and, perhaps, alloy formation. Time resolved analysis found that Ni reduction followed the second order kinetics at early stage and Co proceeded through the zero order all the way in catalyst reduction. The catalyst stability enhancement by Co atoms and the mechanism of Ni and Co interaction during reduction are explained.