Carbon monoxide and oxygen chemisorption and functionalities of sulphided Ni-W/Al 2O 3 hydrotreating catalysts

Abstract

Carbon monoxide and oxygen chemisorption was used to investigate the nature and number of surface sites present over sulphided Ni-W/Al 2O 3 catalysts with compositions varying over a wide range (0–36% WO 3; 0–8% NiO). The gas uptakes were measured by the pulse method at 273 K (for CO) and 333 K (for O 2). The binding of CO on the surface was further studied by FTIR spectroscopy. The significance of the results in terms of the different functions of the catalysts, namely hydrogenolysis of C-S and C-N bonds and hydrogenation, is discussed. In the pulse experiments, the amounts of CO chemisorbed on the single-component Ni or W/Al 2O 3 catalysts are negligible. On the promoted catalysts, the amounts chemisorbed are much more important: they can be related to the activities for both C-S and C-N hydrogenolysis, and thus follow the promoter effect. The correlation accounts for the effects of sulphiding, calcination temperature and deactivation with running time, all of which modify the number of active sites, but not their intrinsic activity. The IR spectra of CO adsorbed on the promoted catalysts are characterized by a band at 2090 cm −1, which persists after evacuation, whereas the bands recorded over unpromoted Ni or W sulphide do not persist under these conditions. The 2090 cm −1 band is assigned to CO adsorbed on highly reduces W ions influenced by neighbouring Ni ions, and the intensity of the signal is a function of both catalyst composition and sulphiding procedure. In addition, a new band at 1830 cm −1 is recorded over the tungsten-rich promoted catalysts. Oxygen chemisorption measurements provide complementary data. The oxygen uptake measures the number of promoter (Ni) ions in the Ni-W phase, but does not account for all of the HDS sites as CO adsorption is not completely hampered by pre-adsorbed oxygen. However, it appears that oxygen is able to reach the sites responsible for hydrogenation of the pyridine ring. Thus, combining CO and O 2 chemisorption measurements provides a satisfactory correlation with the hydrogenation activity. The nature of the sites remains unclear, however.