Operando X-ray absorption spectroscopy for elucidating photocatalytic reaction mechanism
Résumé
1Introduction
Photocatalysis for solar energy storage is a growing research field. In particular, H2 production by photocatalytic reforming or water splitting are widely studied. This needs the development of photocatalyst materials by adding co-catalysts permitting either proton reduction and water oxidation. It is also necessary to understand processes occurring during reactions to optimized photocatalysts. In order to have a better knowledge of the water splitting reaction, the characterization of platinum nanoparticles supported on TiO2 has been performed, by in-situ XAS during the photocatalytic reaction. Furthermore, the photoreduction of platinum on TiO2 has been studied to improve the understanding of the mechanism of the photodeposition.
2Experimental
The experiments were carried out on the French beamline (FAME) of the European Synchroton Resarch Facility of Grenoble (France).
XANES (X-ray Absorption Near Edge Structure) and EXAFS (Extended X-ray Absorption Fine Structure) spectra were obtained in a fluorescence mode (low metal loading) at the platinum L3-edge (11564 eV) and L2-edge (13276eV). On one hand, the oscillation part of EXAFS as a function of the X-ray photon energy was extracted as described elsewhere[2]. Analysis of the data has been done using the software IFFEFIT [3]. Phase shift and back-scattering amplitudes were given by the reference compound: platinum foil for Pt-Pt (2.77Å) [3]. On the other hand, the XANES spectra were decomposed into one arctangent contribution (constant) and one Lorentzian contribution (variable). The area of the lorentzian curve was adjusted to fit the experimental curve. This parameter is assumed to represent the variation of the number of electrons on d orbitals of platinum. To determinate this relationship, three references compounds have been analyzed: Pt0, Pt+II and Pt+IV.
3Results and discussion
The X-ray absorption spectra were performed on different amount of H2PtCl6 (1% wt.) impregnated on different TiO2.
The first spectra on the different photocatalysts obtained under helium flow in the dark do not show any modification of the state of the platinum due to beam damage. The exploitation of EXAFS data highlights that the initial state of platinum species adsorbed is clearly dependent on the nature of the support and the crystalline structure of TiO2.
Then, in the dark, a helium flow saturated in methanol (3.8 kPa) is introduced in the cell. The first oscillations are modified without any shift of the edge. The difference observed may be due to the variation of the nature of the ligand (-Cl to –OH).
Under UV light and methanol flow, the height of the white line of platinum decreases slightly as a function of irradiation time, and its position shifts to lower energy. After about one hour, a stationary state is obtained. The increase of the photon flux leads to a decrease of the area of the Lorentzian curve. This modification is assumed to be a variation of the electronic density of platinum.
To support the hypothesis of the modification of the electronic density, the oscillations of the EXAFS spectra were analyzed to evaluate the neighbourhood of the platinum. Coordination number (CN), distance (R), Debye-Waller factor (s) and E0 correction are adjusted to fit with the experimental curve in the R space between 1 and 3.5 Å. Whatever the photon flux,, the fitting curve was obtained using only Pt-Pt bond with a coordination number equal to 7.8 with a distance Pt-Pt equal to 2.75 Å. Whatever the conditions, the particles of platinum have the same size.
Using the relationship between the area of the Lorentzian curve and the number of electrons in the d orbitals, their filling can be followed as a function of irradiation time (Fig 1.A). The final stationary state is also dependent on the photon flux of UV light used during the photoreduction (Fig 1.B).
Fig 1 : Evolution of the number of electrons in layer “d” of Pt as a function of irradiation time (A) and photon flux (B)
4Conclusions
In-situ XAS characterization has been performed on TiO2 impregnated with H2PtCl6 (1% wt. of Pt), during the photocatalytic reaction. Operando experiments allowed to better understand the mechanism of the photoreduction of platinum. It has been observed a dependence of the platinum precursor state adsorbed on TiO2 with crystalline structure. Then the modification of the platinum d orbital filling is related to experimental conditions permitting to better understand electron transfer from the semiconductor to the metal. This innovative set up appears as powerful tool to understand photocatalytic processes.
Acknowledgements
We would like to thank the King Abdullah University of Science and Technologies (KAUST) for their support (award n° UKC0017) on this project. We also thank Dr. Denis Testemale and Dr. Olivier Proux from FAME-ESRF for their help and for their technical advice during and after the EXAFS experiments.
References
1D.Yamasita et al., Solid State Ionics, 172 (2004) 591
[2]O. Alexeev et al., J. Catal. 164, (1996) 1.
[3]W.A.Spieker et al., Applied Catalysis A, 232 (2002) 219
[4]Kip, B. J., et al., J. Catal. 105, (1987) 26.