FTIR Studies of Mo/Al2O3-Based Catalysts

FTIR has been used to study the surface hydroxyl structures, the metal sulfide structures, and the acidic properties of Mo/Al2O3-based hydrotreating catalysts. The investigations of the functional group of the catalyst have been complemented by probe molecule studies using NO and pyridine. The different studies have provided evidence for the presence of SH groups in the sulfided state of both promoted and unpromoted catalyst. These SH groups are found to be associated with the Mo sulfide phase and they are involved in hydrogen bonding with surface alumina hydroxyl groups. It is concluded from a comparison with NO adsorption results that the SH groups are mainly associated with the edges of the MoS2 phase. Furthermore, pyridine adsorption experiments have shown that Brønsted acidity is associated with the SH species. It is suggested that the SH groups may not only play a role determining the catalytic activity of reactions proceeding via Brønsted acid sites, but they may also be the source of hydrogen during a catalytic cycle.