Infrared Spectroscopic Investigation of CO Adsorption on Silica-Supported Nickel Phosphide Catalysts

The surface chemistry of CO on reduced and sulfided Ni 2 P/SiO 2 catalysts has been investigated with infrared (IR) spectroscopy. There are four different ν CO absorbances observed for adsorbed CO on reduced and sulfided Ni 2 P/SiO 2 catalysts with the following assignments: (1) CO terminally bonded to cus Ni δ+ (0 < δ < 1) sites (2083−2093 cm -1 ); (2) CO adsorbed on Ni δ+ (0 < δ < 1) bridge sites (1914 cm -1 ); (3) CO terminally bonded to P (∼2200 cm -1 ); and (4) formation of Ni(CO) 4 (2056 cm -1 ). At 298 K, CO terminally bonded to cus Ni δ+ (0 < δ < 1) sites on reduced Ni 2 P/SiO 2 catalysts exhibited a characteristic ν CO absorbance at 2083 cm -1, while treatment of Ni 2 P/SiO 2 catalysts with a 3.03 mol % H 2 S/H 2 mixture shifts this ν CO absorbance to 2093 cm -1 . The enthalpy of adsorption (Δ H ads ) of CO on the Ni 2 P/SiO 2 catalysts was estimated by using a van't Hoff plot: Δ H ads = −31.4 ± 1.5 and −30.5 ± 1.5 kJ/mol for reduced and sulfided Ni 2 P/SiO 2 catalysts, respectively. The IR spectroscopic data for the adsorption of CO on reduced and sulfided Ni 2 P/SiO 2 and Ni/SiO 2 catalysts correlate well with the trend of hydrodesulfurization (HDS) activity of catalysts. Specifically, we observe a higher site density and higher Δ H ads for CO adsorption on the Ni 2 P/SiO 2 catalysts compared to both reduced and sulfided Ni/SiO 2 catalysts. The IR spectroscopic and HDS catalytic results are consistent with the formation of a surface phosphosulfide phase (i.e. NiP x S y ) on the Ni 2 P/SiO 2 catalysts under sulfiding conditions.