280283 An Integrated Approach for Characterization of Pt Based Aqueous Phase Reforming Catalysts
Cortright et al. (R.D. Cortright et al., Nature. 2002, 44 (6901), 964) have identified aqueous phase reforming (APR) of carbohydrate feedstocks as an effective route for the generation of renewable hydrogen. Further work by Huber et al. (G.W. Huber et al., App. Cat. B: Environ. 2006, 62, 226) demonstrated that adding a second metal to the Pt system can increase the rates of hydrogen generation. In this work, we utilize a characterization approach integrating kinetic experiments, density functional theory (DFT), and spectroscopy, both operando and ex-situ, to investigate the active state of the catalyst and determine how the composition, structure, and morphology of the particles impact the activity and selectivity for glycerol reforming.
In this study, Mo and Co were added to a Pt catalyst supported on acid functionalized multiwall carbon nanotubes (MWCNT). Kinetic experiments indicate that the addition of a second metal enhances the rates of hydrogen generation by up to a factor of 10. However, the addition of Co and Mo affected selectivity differently. Experiments at low conversion indicate that the PtMo bimetallic had a higher selectivity to products consistent with a deoxygenation pathway, which significantly reduces hydrogen yield at higher conversions. By contrast, the PtCo catalyst reduced selectivity to deoxygenation products, resulting in significant improvement in hydrogen selectivity at high conversion from 50% (PtMo) to 90% (PtCo).
Operando X-ray absorption spectroscopy (o-XAS) indicates a difference in structure of PtMo versus PtCo. Extended x-ray absorption fine structure (EXAFS) indicates the PtMo forms particles with a Pt rich core and Mo rich surface phase. X-ray absorption near edge results show mixed oxidation states (Mox+, 0<x<4), suggesting the presence of zero valent Mo. By contrast, PtCo showed a higher degree of mixing between Pt and Co and a Pt rich surface/Co rich core structure, with most of the Co fully reduced. DFT results corroborated these structures. Both bimetallics had Pt-Me coordination consistent with alloying, which decreases CO binding energy (BE) according to DFT calculations. CO BE is a possible reaction descriptor for H2 generation, and thus could be used as a reactivity probe for future catalysts. Preliminary DFT results on possible surface sites on the catalyst lend insight into how surface composition of the active particles affects the chemistry.
This material is based upon work supported as part of the Institute for Atom-efficient Chemical Transformations (IACT), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.