Promoter Location on NiW/Al2O3 Sulfide Catalysts: Parallel Study by IR/CO Spectroscopy and High-Resolution STEM-HAADF Microscopy

Abstract

In this work, the effect of citric acid (CA) on the structure and activity of Ni-promoted WS2 particles supported on γ-Al2O3 were investigated by HRTEM (high resolution transmission electron microscopy), HR STEM-HAADF (high resolution scanning transmission electron microscopy equipped with high angular annular dark field detector), CO adsorption followed by infrared (IR) spectroscopy, and thiophene hydrodesulfurization (HDS) test. It is shown that CA addition increases the thiophene HDS activity. This may be directly ascribed to the increase in NiWS site concentration detected by IR spectroscopy. HRTEM emphasizes that the addition of CA to NiW catalysts decreases the average size of the particles. This is confirmed by HR STEM-HAADF that even evidences the presence of nanoclusters on NiW catalysts prepared with CA. Hence, local and global analysis of the NiW samples are in good agreement to account for the increase of NiWS site concentration by CA addition. Then the contrast and interatomic distances were analyzed in the atomic-scale images obtained by HR STEM-HAADF. These measurements allow identification of the nature of the atoms in the core and edge of the sulfided particles. The metal atoms of particle core are only W ones, whereas the edges of the particles could be either fully promoted with detection of only Ni atoms or partially promoted with detection of both Ni and W atoms. Accordingly, IR spectroscopy evidenced edge sites with different promotion degrees.