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

The surface chemistry of CO on reduced and sulfided Ni2P/SiO2 catalysts has been investigated with infrared (IR) spectroscopy. There are four different νCO absorbances observed for adsorbed CO on reduced and sulfided Ni2P/SiO2 catalysts with the following assignments: (1) CO terminally bonded to cus Niδ+ (0 < δ < 1) sites (2083−2093 cm-1); (2) CO adsorbed on Niδ+ (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 Niδ+ (0 < δ < 1) sites on reduced Ni2P/SiO2 catalysts exhibited a characteristic νCO absorbance at 2083 cm-1, while treatment of Ni2P/SiO2 catalysts with a 3.03 mol % H2S/H2 mixture shifts this νCO absorbance to 2093 cm-1. The enthalpy of adsorption (ΔHads) of CO on the Ni2P/SiO2 catalysts was estimated by using a van't Hoff plot: ΔHads = −31.4 ± 1.5 and −30.5 ± 1.5 kJ/mol for reduced and sulfided Ni2P/SiO2 catalysts, respectively. The IR spectroscopic data for the adsorption of CO on reduced and sulfided Ni2P/SiO2 and Ni/SiO2 catalysts correlate well with the trend of hydrodesulfurization (HDS) activity of catalysts. Specifically, we observe a higher site density and higher ΔHads for CO adsorption on the Ni2P/SiO2 catalysts compared to both reduced and sulfided Ni/SiO2 catalysts. The IR spectroscopic and HDS catalytic results are consistent with the formation of a surface phosphosulfide phase (i.e. NiPxSy) on the Ni2P/SiO2 catalysts under sulfiding conditions.