FT-IR study of NO adsorption on MoS2/Al2O3 hydrodesulfurization catalysts: effect of catalyst preparation

The effect of catalyst preparation on structure and performance of MoS2/Al2O3 hydrodesulfurization (HDS) catalysts was studied by NO adsorption, monitored by infrared spectroscopy (NO-IR) and complemented with EXAFS. The main bands at 1785 cm−1 and 1690 cm−1 were assigned to the ν(NO) stretching vibrations of coupled mono- or dinitrosyl surface complexes. NO adsorption on sulfided catalysts prepared with nitrilotriacetic acid (NTA) or citric acid (CA) gave rise to additional bands in the IR spectra at 1655 cm−1, 1630 cm−1 and 1615 cm−1, assigned to mononitrosyl adsorption modes. Catalysts prepared with NTA and CA were more active in thiophene HDS than calcined or dried catalysts, despite comparable NO uptake (0.10-0.12 NO/Mo). This was attributed to a changed MoS2 edge structure as suggested by the NO-IR data, as well as to an improved degree of sulfidation as observed by EXAFS. Correlating NO uptake with MoS2 dispersion shows that not more than 30–40% of edge and corner sites were probed with NO. This implies that only a small portion of edge sites may act as active sites in HDS catalysis.