THE EVOLUTION OF SURFACE SPECIES IN NIW AL2O3 CATALYSTS IN VARIOUS STAGES OF SULFIDATION - A QUASI IN-SITU HIGH-RESOLUTION TRANSMISSION ELECTRON-MICROSCOPIC INVESTIGATION/

The effect of the calcination temperature on the evolution of the acti ve phase in NiW/gamma-Al2O3 catalysts during sulfidation has been stud ied using high resolution transmission electron microscopy (HREM) and temperature programmed sulfidation (TPS). The evolution of the active phase is discussed in view of the performance of the catalysts in the hydrodesulfurization (HDS) of thiophene. Dependent on the pretreatment , three different species were observed with HREM: (i) subnanometer cl usters (about 0.5 nm diameter), (ii) nanometer size particles (1 to 2 mn diameter), and (iii) WS2-like slabs (2-3 nm length). The clusters a nd particles predominantly appear after low temperature sulfidation of catalysts calcined at 673 and 823 K. Hence, it is concluded that they are initially formed from the oxidic catalyst precursor. WS2-like sla bs are predominantly formed during high temperature sulfidation, indep endent of the calcination temperature. Activity measurements for the H DS of thiophene show that all phases exhibit a significant catalytic a ctivity. HREM and activity measurements on various freshly sulfided an d spent catalysts showed no significant differences in the catalysts p rior to and after the reaction. (C) 1998 Academic Press.