Aqueous Phase Glycerol Reforming with Pt and PtMo Bimetallic Nanoparticle Catalysts: The Role of the Mo Promoter

The turnover rate (TOR, normalized to sites measured by CO chemisorption before reaction) and selectivity for the aqueous phase reforming of glycerol have been determined for Pt/C and PtMo/C catalysts. While the TOR of PtMo/C is higher than that of Pt/C by about 4 times at comparable conversion, the selectivity to C-O bond cleavage is higher, thus reducing the H-2 yield at high conversion. Under reaction conditions on Pt/C, CO is observed as the most abundant Pt surface species with a fractional coverage of about 0.6 using operando X-ray absorption spectroscopy. Since there is little CO in the effluent (CO2:CO ratios > 100:1, when CO is detected), it is thought that surface CO is converted to H-2 and CO2 by the water gas shift reaction. DFT calculations suggest that the role of metallic Mo is to alter the electronic properties of Pt lowering the binding energy of CO and reducing the activation energies of dehydrogenation and C-O bond cleavage. Because the activation energy for C-O cleavage is lowered more than for dehydrogenation, the selectivity for C-O bond cleavage is increased, ultimately lowering the H-2 yield compared to Pt/C.