In-Situ Pump-Probe DRIFTS Studies of Photocatalytic Reforming of Simple Alcohols

  • James Highfield, Institute of Chemical & Engineering Sciences, Singapore
  • Ms Huijun Chen, Nanyang Polytechnic, Singapore
  • Ms Clarice Chong, National Junior College, Singapore
  • Professor Zhong Chen, Nanyang Technological University, Singapore

    • Oxygenates are good candidates for fuel processing to a hydrogen-rich fuel gas via endothermic catalytic steam-reforming (SR):-
    CnHmOk + (2n-k) H2O = n CO2 + [2n + m/2 – k] H2
    Band-gap (UV) photons are sufficiently energetic to drive full reforming, but photo-dehydrogenation of alcohols, although a highly efficient process over Pt/TiO2, yields only one H2 molecule per alcohol. Hence, photo-reforming of alcohols is an attractive objective.
    Here, pump-probe DRIFTS spectroscopy is used to elucidate mechanistic aspects of alcohols SR over TiO2 (Degussa P25) containing 1 wt.% Pt as H2PtCl6. During photo-metallization under band-gap irradiation (15 mW cm-2) in the presence of a few % ethanol/water in N2, the most striking observation is the rapid development of a strong IR band at 2030 cm-1, characteristic of CO adsorbed on Pt nanoparticles. The implication is that photocatalysis not only drives complete dehydrogenation, but even C-C bond cleavage. However, build-up of adsorbed CO and the observed slow evolution of CO2 indicates a kinetic botleneck in which the water-gas-shift(WGS)process is rate-determining. This also explains why photo-dehydrogenation is effectively arrested at the aldehyde product. Weak bands of adsorbed formate, a probable WGS intermediate, also appear at 1580 and 1360 cm-1.
    Future work will attempt to identify which stage in the WGS reaction is slower, viz., formate creation or decomposition. Water-activating additives such as Cu and Ru will be tested, while modest heating may be explored to promote rate-determining dark reaction(s). A mass spectrometer will be incorporated to measure H2 levels.