Probing the Hydrodesulfurization Properties of Nickel-Rich Bimetallic Phosphides: Supported Catalysts and Encapsulated Nanoparticles

The need for improved hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) catalysts is being driven by increasingly demanding requirements for ultralow sulfur fuels and by the fact that sulfur levels in crude oil have been trending upward over recent decades. Current industrial catalysts are based on molybdenum sulfide (MoS2) and its highly anisotropic structure severely limits the number of exposed active sites. Transition metal phosphides (e.g. Ni2P/SiO2) are a new class of materials with promise to have improved properties relative to sulfided Ni-Mo/Al2O3 catalysts. The addition of a second metal can have large effects on the electronic and surface properties of a catalyst, which in turn can heavily influence its catalytic properties. Nickel-cobalt and nickel-iron phosphide catalysts (Ni2-xMxP, 0 ≤ x ≤ 2) are being investigated to determine the effect of cobalt and iron on the HDS properties of bimetallic phosphides of nickel. Catalysts prepared via conventional preparation methods using metal phosphate precursors supported on silica, and reduced by temperature programed reduction (TPR) are compared with solution-prepared metal phosphide nanoparticles that were subsequently encapsulated in a mesoporous silica shell to form core@shell nanostructures. Infrared spectra of adsorbed CO on the Ni2-xMxP catalyst surfaces show two significant peaks, one is attributed to CO bound to surface nickel atoms, while the second, smaller peak is attributed to CO bound to surface phosphorous atoms. As the second metal is added into the crystal lattice there is a shift to lower wavenumbers of the absorbance due to CO bonded to surface nickel atoms. This may indicate electron donation from cobalt to nickel, making for more electron-rich binding sites. The catalysts were tested for HDS activity and selectivity using dibenzothiophene and 4,6-dimethyldibenzothiophene. The trends in catalyst activities and turnover frequencies with Co and Fe content will be described and discussed in the context of the characterization results