Catalytic Fundamentals of Aqueous Phase Reforming: A Spectroscopic Approach

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Hare, Bryan J.
Sievers, Carsten
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Hydrogen is an essential commodity for a sustainable chemical industry. Aqueous phase reforming (APR) is a catalytic process that converts polyols (CxH2x+2Ox) 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 (COL). The time- and temperature-dependent features of the COL 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 catalysts. Reaction pathways of adsorbed di/ketones on γ-Al2O3 up to 250 °C were first deduced through deconvoluting the 1500 – 1800 cm-1 region unique to the ν(C=O) and ν(C=C) modes of adsorbed species. It was concluded that most of the di/ketones studied underwent aldol self-condensation on Lewis acid sites to form larger conjugated di/ketones. Pt Poisoning extents were gauged by integration of the COL 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. The conjugated species formed on Lewis acidic γ-Al2O3 were also hypothesized to play a major role in poisoning the interfacial sites of Pt particles of any size. 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 including those on the γ-Al2O3; each of which may facilitate improvements in catalyst design and sustainable H2 production.
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