Quasi in situ sequential sulfidation of CoMo/Al2O3 studied using high-resolution electron microscopy

Using a specially developed, quasi in situ vacuum-sample-transfer system, the stepwise sequential sulfidation of CoMo/gamma-Al2O3 was studied with transmission electron microscopy without exposing the catalyst to air. In a first. step, the sulfidation was performed at 400 degreesC at either 1 or 10 bar pressure. In a second step, the sulfidation was performed at 600 degreesC and 1 bar. As the second sulfidation step was performed on the sample on the transmission electron microscopy grid, the same areas of the samples that had been studied for the first sulfidation step could be studied once more, providing detailed data on the changes in the sulfided phase. The slab length and degree of stacking have been studied in detail from the TEM/HREM images. Sulfidation at higher pressure or higher temperature results in a higher average slab size. Detailed study of the same areas of the catalysts before and after the second, more severe, sulfidation step shows that although the dispersion of the sulfided phase decreases, the total amount visible in the TEM images remains unchanged. This indicates that practically all the sulfide present is actually imaged on the micrographs. Additionally, detailed HRTEM study shows increased crystallinity of the slabs after the second sulfidation step. XPS analysis shows that after sulfidation at 400 degreesC and 1 bar, the degree of sulfidation is 95%, whereas after sulfidation at 600 degreesC and 1 bar, sulfidation is complete. The catalytic performance for thiophene hydrodesulfurization is much higher after sulfidation at 10 bar and 400 degreesC as compared to 1 bar and 400 degreesC, and decreases after the second sulfidation step (1 bar 600 degreesC). The latter effect is more pronounced for the sample sulfided at 10 bar in the first step, indicating that a distorted yet fully sulfided structure is the most active phase for thiophene HDS.