Role of Ceria In Ti0.9Ce0.1O2 Materials for Adsorptive Desulfurization

Fuel cell systems efficiently convert chemical energy to electrical energy. The reforming and water-gas shift catalysts used to produce hydrogen and the fuel cell electrodes are all sensitive to the sulfur compounds in the fuels. Conventional desulfurization methods, i.e. hydrodesulfurization (HDS), consume large amounts of energy to reach sulfur concentrations low enough for fuel cells. Adsorptive desulfurization (ADS) is a promising method being developed to produce ultra-low sulfur fuels.  A Ti_xCe_1-xO₂ material has been reported by the Song group at Penn State to be a potential adsorbent for ADS due to its good selectivity and regenerability. The ratios of Ti:Ce were optimized to 9:1 for the maximum adsorption capacity. Comparing Ti_0.9Ce_0.1O₂ to TiO₂, the 10% CeO₂ addition significantly increases the capacity, despite only a slight surface area increase.  We have applied density functional theory (DFT) calculations to examine the role of Ce addition to TiO₂ in promoting the adsorption of thiophenic molecules.  A number of possible contributions of Ce addition were evaluated, including 1) stabilization of TiO₂ surfaces that adsorb thiophene strongly, 2) stronger adsorption to Ce-doped TiO₂ surfaces, 3) stabilization of reduced or over-oxidized sites on Ce-doped TiO₂, and 4) increased oxygen storage capacity to promote formation of adsorbed sulfone-like complexes.  We find that Ce-doping of the surface can offer adsorption sites with stronger binding of thiophene, and can stabilize the anatase (100) surface which demonstrates strong adsorption affinity. Ceria may also serve as an oxygen reservoir to promote formation of adsorbed sulfone species to the TiO₂ surface.  Thiophene adsorption energies are compared with cyclopentene to probe adsorption selectivity, and substitution of both thiophene and cyclopentene is considered to evaluate variations in selectivity.  Calculations using DFT and dispersion corrected DFT, DFT+D, are compared.