The infrared (IR) spectra of CO adsorbed on 10,20, and 30 wt % nickel phosphide-containing reduced SBA-15 and KIT-6 mesoporous silica-supported catalysts have been studied at 300-473 K. On the catalysts containing a stoichiometric amount of phosphorus with 20 wt % loading, the most intense IR absorption band was observed at 2097-2099 cm-1, which was assigned to CO terminally bonded to coordinatively unsaturated Niδ+ (0 < δ < 1) sites. The frequency of this band was 15 cm-1, higher than that in the spectrum of a reduced Ni2P/SiO2 catalyst, indicating a modified Ni-P charge distribution. This band shifted to lower wavenumbers, and its intensity decreased, while the relative intensity of another band at 2191-2194 cm-1 assigned to CO terminally bonded to P increased going to catalytically less active, excess-P-containing SBA-15-supported catalysts. CO also adsorbed as a bridged carbonyl (1910 cm -1) and as Ni(CO)4 (2050 cm-1) species, and the formation of surface carbonates was also identified. The nature of the surface acidity was studied by temperature-programmed desorption of ammonia (NH 3-TPD). Weak and strong acid sites were revealed, and the high excess-P-containing catalyst released the highest amount of ammonia, indicating that a high concentration of strong acidity can be disadvantageous for reaching high hydrotreating catalytic activity. The modified Ni-P charge distribution, the mode of CO adsorption on surface nickel phosphide sites, as well as the acidity can be directly connected to the catalytic activity of these mesoporous silica-supported catalysts.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry