Sintering of nickel catalysts in the presence of NH3/H2
Supported metal catalysts are used in many important industrial processes, where amination of alcohols is one example. The exposure of the supported metal catalyst to reaction conditions results usually to a decrease in catalytic activity, by for instance thermal degradation. One of the aspects of thermal degradation is growth of the metal particles with simultaneous loss of active metal surface area. This phenomenon, which is referred to as sintering, is normally a problem at temperatures above 500°C. This thesis focuses on the sintering phenomenon of nickel particles supported on -alumina, in view of the industrial application as an amination catalyst.
In the present study, model catalysts of nickel on -alumina, were heat-treated at 210 - 250°C and 200-250 bar under different ammonia and hydrogen containing atmospheres. The sintering of the nickel crystallites was monitored by H2-chemisorption, N2-physisorption, X-ray diffraction and transmission electron microscopy. The results indicate that a significant loss of active metal surface area of reduced nickel particles occurs as a result of the heat-treatment when ammonia is present. The results suggest that the sintering proceeds via particle migration.
In order to enhance the catalyst stability towards sintering, the catalyst preparation procedure was modified by either co-impregnation of the nickel with promoters (i.e. oxides of Li, Na, K, Mg, Ca, La and Ce) or formation of a nickel aluminate layer between the nickel particles and the -alumina support or exchanging the -alumina by other alumina supports. The most pronounced effect was achieved with co-impregnation of alkaline earths. Co-impregnation of calcium gave up to 50 % more remaining active surface area after the heat-treatment as compared with the un-promoted reference sample.
The stability and diffusivity of different adsorbed species on nickel surfaces were studied with density functional theory calculations with the aim to elucidate the mechanism behind the high sintering rate during amination conditions. The results suggest that sintering is not increased by higher mobility of nickel-ammonia complexes.