Sintering of nickel steam-reforming catalysts on MgAl2O4 spinel supports

Sintering significantly contributes to the deactivation of Ni-based steam-r eforming catalysts. We have investigated the effect of Ni loading and surfa ce area of the support on sintering of Ni particles on MgAl2O4 spinel suppo rts. The experiments were performed under simulated industrial pre-reformin g 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 w as 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 fi rst 200 h; after that period the Ni particle size changes only slightly. A remarkable result is that the average Ni particle size after sintering reac hes 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 thos e with a higher Ni loading. Because the sintered particles are polycrystall ine agglomerates, the XRD estimates of particle size are lower than those o btained by the other techniques such as TEM or chemisorption. The particle size distributions derived from TEM and STEM follow a log normal distributi on, suggesting that sintering occurs through crystallite migration and coal escence. 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 t he larger Ni particles on the support, (C) 2001 Academic Press.