CO HYDROGENATION ON A NICKEL-CATALYST .1. KINETICS AND MODELING OF A LOW-TEMPERATURE SINTERING PROCESS

Sintering of a Ni/SiO2 catalyst was studied under methanation conditio ns. The loss of metallic surface, accounting for the catalyst deactiva tion, was shown to come from the migration of nickel subcarbonyl adspe cies formed during the reaction. The catalyst, presenting initially a monomodal Ni particle size distribution, evolved towards a bimodal sys tem consisting of small spherical and large faceted crystals, with a s elective development of [111] planes after several hours under reactio n conditions. The formation of a nickel carbide monolayer could interf ere with the formation of the large particles, possibly by decreasing the concentration of the mobile subcarbonyl adspecies. The best descri ption of the observed sintering process was provided by considering a model with two distinct species, evolving according to the Ostwald-rip ening mechanism, and being coupled by mass transport through the mobil e nickel subcarbonyl intermediates. Long-term simulations allowed for the prediction of the ultimate state of the catalyst, and solutions ai med at slowing down the loss of nickel surface were proposed. (C) 1994 Academic Press. Inc.