280712 Activity Relationships Between Aqueous Phase Reforming and the Water Gas Shift Reaction
As the limited petroleum resources dwindle in the future, renewable processes will play an increasingly important role in the world economies. Reactions converting biomass to useful fuels and chemicals require significant hydrotreating, and thus will necessitate the development of renewable hydrogen sources. Cortright and coworkers (R.D. Cortright et al., Nature 2002, 44, 964) have demonstrated that the aqueous phase reforming (APR) of biomass derived molecules, such as carbohydrates and polyols, on Pt based catalysts is an effective route for hydrogen generation.
Previous work has demonstrated that the addition of a second ‘promoter’ metal to the Pt system increases the rate of hydrogen generation (G.W. Huber et al., App. Cat. B., 2006, 62, 226). In this study, a series of Pt and PtMe bimetallic (where Me = Mo, Co, Ni, Fe) catalysts supported on nitric acid treated multiwalled carbon nanotubes (MWCNT) were tested in the aqueous phase reforming of glycerol and water-gas shift (WGS) reaction in both the liquid and gas phases. While it is impossible to directly compare rates due to the vastly different conditions for each reaction, it was observed that rate promotion trends were consistent across all catalysts. For both reactions, a PtCo bimetallic showed the best rate promotion: for glycerol reforming, the rate of hydrogen generation increased 10 times, and the WGS reaction showed an 8 fold increase. X-ray absorption spectroscopy (XAS) indicated that these bimetallic catalysts form alloys of the secondary metals and Pt, as the fits for these materials contained Pt-Me scattering. Operando XAS showed the presence of CO on the surface of Pt catalysts under the APR reaction, but less on the surface of bimetallic PtMe catalysts under the same conditions, suggesting that the WGS reaction may be clearing CO species from the surface of reforming catalysts. Density functional theory (DFT) studies show that this alloying decreases the CO binding energy on Pt clusters, while the literature has shown CO binding energy (BE) to be a key descriptor for WGS activity. It is proposed that CO BE might also be an effective descriptor for aqueous phase reforming activity.
While most of the catalysts studied followed similar trends in reactivity, Pt supported on MWCNT-n showed much lower than expected WGS rates, while still maintaining a reforming rate consistent with the trends. There was also an activation period observed over the first few days on stream, which is consistent with results from the Haller group (X. Wang et al., J. Phys. Chem. C, 2010, 114, 16996) for ethylene glycol reforming on Pt supported on acid washed single wall carbon nanotubes. This suggests that the acid treatment modifies the support in a way that inhibits reactivity, but is reversible upon treatment with high pressure and temperature liquid water. Acid treated MWCNT catalysts used for liquid phase WGS (120°C, 40 bar) showed improved activity when subsequently used for gas phase WGS.