Interaction between the active components and support in the Co-MO-Al2O3 system. II. The Role of Sodium and Cobalt

Abstract

The kinetics of reduction of MOO3 molybdenum-alumina, cobalt-alumina and cobaltmolybdenum- alumina in He has been investigated. The equation of Avrami and Erofeev assuming the formation and growth of nuclei describes mathematically most appropriately the rate of reduction for all these samples. The presence of sodium in the alumina support profoundly affects the reducibility curves. In its absence, cobalt has an inhibitory influence on the reduction of molybdenum-alumina. However, in the presence of sodium, cobalt accelerates the reduction process. The extent of reduction of all these samples is limited by the water evolved during the process. For 14 different cobalt-molybdenum-alumina catalysts, as their reducibility increases, the catalytic activity passes through a maximum and falls at higher values of reducibility. It is postulated that in the presence of sodium, most of the cobalt occurs in the form of Co,Ol which on reduction forms the metal. Once formed, cobalt metal accelerates the reduction of supported MOO3. In the absence of sodium, cobalt, now occurring mostly as well-dispersed Co”+ ions which are not reduced during the reduction, strongly retains the water evolved during the process and thereby suppresses the reduction of MOO3. A model of the catalyst surface and the probable role of cobalt and sodium in the catalyst formulation are advanced to account for the observed dependence of hydrodesulfurization activity on the reducibility of the samples.