On the “possible” synergism of the different phases of TiO2 in photo-catalysis for hydrogen production

- The effect of TiO2 phase composition on the photocatalytic H2 production is studied.
- Highest H2 production rate was observed for catalysts composed of 80% anatase.
- The c-dimension of the anatase phase at this maximum reaction rate is 9.510-9.515 Å.
- Changes in structural parameters might be the cause; unrelated to synergism.

The effect of TiO2 phase composition on the photocatalytic hydrogen production of water/ethanol (95/5 volume ratio) has been studied in order to understand the structural effect (and associated electronic properties) on the reaction within the so called “synergistic effect” concept. Within the investigated series of 1 wt.% Pt/TiO2 with initial particle dimension of ca. 15 nm the highest hydrogen production rate per unit area was observed for catalysts composed of 80% anatase, 18% rutile and 2% brookite. The associated particle sizes for this catalyst were 44, 82 and 33 nm for the three phases, respectively. XRD patterns analyzed by the Rietveld method as well as X-ray absorption near-edge spectra (XANES) of the Ti K-edge mapped the phase transformation from anatase/brookite to rutile where it appears that the brookite phase is initially transformed to anatase phase. XRD patterns and Raman shift were found to be more sensitive to subtle changes in phase composition when compared to UV-vis absorbance or XANES of the Ti K-edge. The photocatalytic reaction for the complete series was conducted in identical condition and with excess photon flux in order to extract accurate reaction rates. In addition to the observed multi-phase effects on the reaction rate, other parameters extracted from the Rietveld refinement of the X-ray diffraction patterns were found to be useful. In particular, at the narrow window where the reaction rate was found to be maximum, the c-dimension of the anatase phase had values in the range 9.510-9.515 Å. These c-dimension values are between those observed for nanoparticles with less than 20 nm in size and those for larger particles with a size above 50 nm. Results from this work indicate that the synergism between anatase and rutile on the photocatalytic reaction for hydrogen production, often attributed to increase the charge carrier life time, may be linked to the lattice expansion of the anatase phase which in turn would affect its electronic/catalytic properties.

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