(580f) Catalytic Fundamentals of Renewable Hydrogen Production from Aqueous Phase Reforming: Active Sites, Solvent Effects, and Deactivation

Hydrogen is an essential commodity for a sustainable chemical industry. Aqueous phase reforming (APR) is a catalytic process that converts polyols (C_xH_2x+2O_x) derived from biomass into H₂ and CO₂. However, H₂ yields severely decrease with increasing oxygenate size: methanol > glycerol > sorbitol > glucose. Significant yields are accomplished through sequential dehydrogenation, decarbonylation, and water-gas shift. Our goal is to isolate constituent reactions and surface chemical phenomena occurring in APR to improve mechanistic understanding. This includes determining active sites (i.e. metal particle terraces, edges), effects of co-adsorbed H₂O (e.g. solvation, inhibition), and chemical poisons originating from side reactions involving C_≥3 oxygenates.

Surface chemistry on Pt/γ-Al₂O₃ was probed using infrared spectroscopy and reagent vapors in a high vacuum cell. Methanol was used to isolate the dehydrogenation reaction, resulting in a strong IR band within 1900 – 2100 cm^-1 representing linearly adsorbed carbon monoxide (CO_L). The time- and temperature-dependent features of the CO_L band (integrals, frequency, etc.) reveal a kinetic preference for Pt terrace sites. Various di/ketones were used to isolate decarbonylation reactions and replicate surface species we suspect to deactivate Pt. Poisoning extents were gauged by integration of the CO_L band after methanol dehydrogenation on poisoned Pt/γ-Al₂O₃. Both strongly bound di/ketones and alkyl fragments from decarbonylation are believed to poison Pt surfaces. Inelastic neutron scattering spectra showed evidence of alkyl fragments on a Pt sponge up to 250 °C. Density functional theory was used to calculate binding energies and configurations of these poisons on Pt(111).

The results herein have furthered understanding of essential APR catalytic fundamentals, such as higher reactivity of Pt terrace sites, greater resilience of larger metal particles to transport limitations caused by H₂O multilayers, and severe poisoning effects of di/ketones and related surface species; each of which may facilitate improvements in catalyst design and sustainable H₂ production.