Influence of the textural characteristics and the preparation conditions of the support on coking resistance and performance of Ni/MgAl2O4 catalysts

Abstract

Listen

Translate article

This study aimed at establishing the influence of the textural properties, interparticle pores in particular, of the support in Ni/MgAl2O4 catalyst on other physicochemical properties and on coke formation during partial oxidation of methane. Ni catalysts were prepared over four different MgAl2O4 supports with different surface area and pore characteristics to establish their impact. Mesoporous MgAl2O4 supports were prepared by modified sol-gel and co-precipitation methods, and were compared with a commercial nano-powder counterpart. The catalysts were characterized by various techniques including powder XRD, XRF, N2 sorption, TEM, CO2-TPD, H2-TPR, DRIFTS, TGA, and Raman spectroscopy. The catalysts over the sol-gel prepared supports showed CH4 conversion around 89%, H2 selectivity close to 100%, H2/CO ratio ∼2.05, and negligible carbon deposits. While the catalyst over the support prepared by coprecipitation showed comparable conversion and products selectivity, it showed a slight decrease in conversion that was referred to the formation of a small amount of carbon deposit. On the other hand, the catalyst based on the commercial MgAl2O4 nano-powder showed conversion ∼84%, H2/CO ratio of 2.2, and large amount of coke deposit that lead to unstable conversion and gradual deactivation. The significantly enhanced coking resistance of the catalysts based on the prepared supports was referred to their larger volume of wider mesopores and their higher density of basic sites. On the other hand, the considerable and rapid coke formation over commercial support-based catalyst was referred to the narrow pores as well as lower pore volume, in addition to the lower density of basic sites, of its support. Narrow pores are expected to get rapidly filled with carbonaceous species blocking some active sites, hindering diffusion of products as well as O2 and CO2 access, and limiting oxidation and removal of these species as they form. The results indicate that the dominance of wide interparticle mesopores (>5 nm) of the support are essential to avoid coke accumulation.