Sintering of Nickel Steam-Reforming Catalysts on MgAl2O4 Spinel Supports

Abstract

Sintering significantly contributes to the deactivation of Ni-based steam-reforming catalysts. We have investigated the effect of Ni loading and surface area of the support on sintering of Ni particles on MgAl2O4 spinel supports. The experiments were performed under simulated industrial pre-reforming conditions, i.e., in a 10:1 mixture of steam and hydrogen at 500°C and 30 bar total pressure. The Ni particle size of fresh and sintered catalysts was determined from sulfur chemisorption capacity, X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM). It was found that most of the sintering occurs in the first 200 h; after that period the Ni particle size changes only slightly. A remarkable result is that the average Ni particle size after sintering reaches a limiting value that depends only weakly on the Ni loading and surface area of the support. Sintering of the catalysts with a lower Ni loading is slower and they exhibit a greater loss in Ni surface area compared to those with a higher Ni loading. Because the sintered particles are polycrystalline agglomerates, the XRD estimates of particle size are lower than those obtained by the other techniques such as TEM or chemisorption. The particle size distributions derived from TEM and STEM follow a log normal distribution, suggesting that sintering occurs through crystallite migration and coalescence. The limiting size of the Ni particles after sintering, and the low sintering rate after 200 h, can both be related to the lower mobility of the larger Ni particles on the support.