Structure and activity relationships of bimetallic catalysts for aqueous phase reforming and the water-gas shift reaction

As the existing deposits of petroleum and non-renewable energy resources are exhausted, become more difficult to extract, and remain in politically unstable regions of the world, the search for long term alternatives for fuels and chemicals from renewable sources such as biomass continues. One promising process for the production of renewable hydrogen to use in conversion of biomass to fuels and chemicals is aqueous phase reforming (APR) of cellulose-derived carbohydrates. In the first study Pt/MWCNT catalysts have been promoted by addition of transition metal promoters Cu, Ni, Co, Fe, Mn, Mo, W, and Re in Pt:M 1:1 atomic ratios to give up to 13 times higher glycerol reforming site time yield (STY) and 8 times higher water-gas shift (WGS) turnover rate (TOR). The bimetallic structure and oxidation state of the catalysts before and after reaction were determined by x-ray absorption spectroscopy (EXAFS and XANES spectra). While the fresh structure varied among the catalyst compositions, the used samples had similar preferences toward Pt-rich core, M-rich shell configurations, with some amount of oxidized M even after reduction in hydrogen at 450 °C. The measurement of the kinetic isotope effect (KIE) for replacement of water with D2O during glycerol APR determined that the rate determining step for APR is one which involves the O-H bond in water, not glycerol decomposition. In collaboration with Professor Jeffrey Greeley, water dissociation barriers were estimated by density function theory (DFT) calculations of OH binding energies on Pt3M (111) surfaces. A volcano plot of WGS and APR reaction rates with OH binding energy was obtained. Additionally, DFT revealed that the more oxophillic metal promoters make use of separate sites for water dissociation that otherwise would require Pt sites that are used for glycerol decomposition. This dual site advantage was explicitly shown by DFT studies of OH on Pt3M surfaces which already had CO adsorbed. The second study shows that differences in rates and selectivity between two similar catalysts PtRe/MWCNT and PtRe/C can be explained in terms of metal distribution. Comparing the reduced catalyst EXAFS and XANES to those collected in operando, we find that PtRe/MWCNT contained larger alloyed particles which sinter slightly to 3.1 ± 1.1 nm during glycerol APR. The Re metal remains primarily reduced and the most prevalent surface adsorbates are CO and H for Pt and Re sites, respectively, during both WGS and APR reactions. For the smaller metal particles in PtRe/C (1.2 ± 0.3 nm), the Re oxidizes to form acid sites to open up dehydration/hydrogenation routes to [C-O] bond scission. Electron energy loss spectroscopy (EELS) maps confirm that while for individual particles Re remains well mixed with Pt in PtRe/MWCNT, the Re metal is well dispersed on the high surface area activated carbon support of PtRe/C, allowing easy oxidation under reaction conditions. In this case where high rates of hydrogen production by APR are desired, the PtRe/MWCNT catalyst was able to obtain higher STYs and maintain higher [C-C] bond scission selectivity compared to PtRe/C due to the differences in metal distribution. In a third study, various C3 alcohol molecules were compared in their reaction selectivity towards [C-O] and [C-C] bond scission over Pt/MWCNT. Glycerol, propylene glycol, and 1-propanol were found to have a high selectivity towards [C-C] scission, in agreement with relative bond scission barriers estimated by linear scaling relationships from DFT. However, 1,3-propanediol reacted through a pathway favoring [C-O] scission. Despite having different [C-C] and [C-O] bond scission selectivity, these different alcohols had similar reactant STYs, consistent with having a common rate determining step such as WGS