(729e) Strong Interactions Between Molybdenum Carbide and Metal Catalysts: The Source of Enhanced Dispersion and Catalytic Activity
We have developed a method for loading metal catalysts onto active, high surface area carbide and nitride supports. Using this technique, we have shown that Pt/Mo2C catalysts exhibit much higher rates for water-gas shift (normalized to BET surface area) than traditional oxide supported Pt catalysts (Pt/TiO2, Pt/CeO2, Pt/Al2O3, with similar molPt/m2 loadings), and can exhibit rates even higher than the industrial standard Cu/Zn/Al2O3 catalysts. We used a wide variety of characterization tools, both experimental and theoretical, to describe the Mo2C surface under reaction conditions and evaluate the WGS reaction mechanism. XPS results and DFT calculations indicate that the surface is saturated with oxygen. However, it is thermodynamically unfavorable for the surface oxygen to diffuse subsurface, so the nature of the bulk carbide is preserved under reaction conditions. This oxygen is very reactive towards CO oxidation. Using steady-state kinetic analysis, DFT calculations, and pulse chemisorption experiments, we confirmed that the red-ox mechanism dominates the activity of the catalyst. Pt serves as and adsorption site for CO, which is then easily oxidized by the oxygen adsorbed on the support. Additionally, the support easily dissociates water and desorbs hydrogen to reoxidize the surface. Studying the structure of Pt on the Mo2C support has proven to be a difficult task. Since Mo2C is an active support, CO chemisorption measurements grossly overestimate the Pt dispersion. Furthermore, identification of Pt particles size using TEM or STEM is equally difficult because Mo is such a heavy atom. However, in this contribution we show that structure of the Pt particles can be deduced by combining the WGS reaction rates with EXAFS results. EXAFS results show that for a wide range of catalyst loadings (from 0.5wt% up to 12wt%), the Pt-Pt and Pt-Mo coordination number do not change, and remain relatively low for both. Further, WGS rates normalized to BET surface area indicate that the rate increases linearly with weight loading, but WGS rates normalized to mass of Pt decrease inversely with Pt loading. This implies that the number of active sites of the Pt structure are increasing with loading, but the dispersion of Pt is decreasing. Using this information, we conclude that the Pt domains are raft structures (i.e. the structures are essentially very wide circles of very few atoms thickness) . Using a simple model to calculate the average Pt-Pt and Pt-Mo coordination number for different raft structures, we have determined that the structures are likely 2 - 3 monolayers thick. This results are supported by DFT calculations which indicate that the binding energy of Pt to an Mo2C surface is extremely strong, strong enough to even overcome the cohesive energy of bulk Pt.