Mechanistic investigations of photo-driven processes over TiO2 by in-situ DRIFTS-MS: Part 1. Platinization and methanol reforming

J. G. Highfield, M. H. Chen, P. T. Nguyen, Z. Chen
2009 Energy & Environmental Science  
There is growing interest in the reforming of methanol and other bio-oxygenates as highdensity, CO 2 -neutral, renewable sources of H 2 . Photocatalysis is worthy of investigation as a potentially economic means to drive such endothermic processes. In this study, in-situ DRIFTS, adapted for optical pumping and coupled to on-line MS, was used to observe the surface of TiO 2 (Degussa P25) during photo-metallization from pre-sorbed hexachloroplatinate, at a nominal Pt loading of 1 wt%, and to
more » ... 1 wt%, and to evaluate photo-reforming of methanol over the resulting Pt/TiO 2 composite. The irreversible growth of a quasi-continuum absorption, characteristic of the surface plasmon resonance of zerovalent Pt nanoparticles, along with bands at 2050 and 1830 cm -1 typical of metal-adsorbed CO, indicated that photometallization was complete typically within 2 hours. Methanol reforming was photocatalyzed at room temperature but in low quantum efficiency, ø ≈ 0.01. However, this was raised substantially, to ø ≈ 0.07, simply by the application of mild heating (T ≤ 70 ºC). Photoreforming proceeded at a fixed rate but the H 2 /CO 2 ratio generally exceeded that of the reforming stoichiometry, suggesting some retention of CO 2 . The photo-thermal synergy was rationalized by model DRIFTS studies, starting from formalin (hydrated formaldehyde), which revealed key features of the mechanism. TiO 2 promoted the Cannizzaro disproportionation in the dark, yielding formate and methoxy species already at 40 ºC. While methoxy was effectively cycled back to the initial photo-dehydrogenation stage, the slow step was identified as formate decomposition to H 2 and CO 2 . The low value measured for the apparent activation energy (~40 kJ mol -1 ) was taken as supporting evidence for 'waterassisted destabilization' of formate, as originally reported by Shido and Iwasawa. No evidence was found for an alternative thermal or photo-reforming mechanism involving the Pt-CO ad species. Broader context Bio-alcohols will be important fuels in bio-refineries. As energy-dense liquids, they offer the handling advantages of gasoline while also being renewable and CO 2 -neutral. Furthermore, they are amenable to "fuel processing" by steam reforming, which yields two extra benefits. The H 2 -rich gas product is renowned for its clean combustion properties and/or for efficient electricity generation in fuel cells (FC). The reforming process itself results in a substantial increase in exergy, which is being exploited as "chemical recuperation" in advanced turbine technology. This work shows that methanol can be photo-reformed under mild conditions, rendering it more compatible with low temperature FCs. A simple field test might use a cheap, low-power solar concentrator to provide both the photon flux and the modest heating required to create a H 2 buffer, in a suitably-adapted stationary unit, for start-up purposes. The photo/thermal synergy should impact on various photocatalytic processes that are currently too slow for technical application based solely on UV light activation. Any benefits accruing in the more complex and energy-demanding process of ethanol reforming are under study. Ultimately, the key breakthrough needed in photocatalysis is sensitization of stable, wideband-gap, oxide semiconductors like TiO 2 to visible light without compromising their redox properties
doi:10.1039/b907781m fatcat:a4jjtcn3snhstiqnvgiejtyf3u