H2 production by renewables photoreforming on Pt-Au/TiO2 catalysts: the effect of reduction treatment.

Introduction Hydrogen production starting from renewables is one of the most pursued topics in the development of a future "hydrogen economy". Photoreforming reactions of biomasses can successfully cope with environmental and energetic requirements by the use of renewable raw materials together with a renewable energy source. Titania is a benchmark material widely used in photocatalysts but several factors limit the its activity. The deposition of metal NPs on bare TiO2 [1,2] and the self-doping of TiO2 [3] seem to be the most promising and viable ways for practical applications. Here we will show how the use of bimetallic Pt-Au NPs and the self-doping of TiO2 allows to obtain systems active under UV-A irradiation and simulated sunlight, with an appreciable activity under visible irradiation [4]. Experimental The samples were prepared by impregnation of (NH4)2PtCl6 and/or HAuCl4 aqueous solutions on Nanoactive TiO2 (Nanoscale, USA). After drying, the powder underwent oxidation in flowing O2 at 200°C and reduction in flowing H2 at 250°C or at 500°C. Structure, morphology, phases, composition, light absorption properties of the samples were characterized by ICP-OES; HRTEM; XRD; DR UV-Vis; CO-probe molecule DRIFTS and ciclo-voltammetry. Photocatalytic tests were performed using, as sacrificial reagents, EtOH or glycerol. For UV-A experiment, a suspension of the photocatalysts was irradiated, at 20°C, using a 125W medium pressure Hg lamp. For simulated sunlight experiments a Solar Simulator (150 W Xe lamp and an Atmospheric filter, cut-off 320 nm) was used working at 25°C. The effluent (Ar purging) stream was analyzed by GC-FID-TCD. Selected experiments at wavelength close to the SPR band of Au were performed (same equipment of UV-A experiments using NaNO2 in the cooling bath to remove all the spectral lines below 420 nm). Results and Discussion The H2 evolution performances strongly depend on the nature of the photocatalyst, the type of sacrificial agent and irradiation type. Remarkably, the pre-reduction treatment has been found to be one of the key parameter in determining catalysts final properties, affecting the formation of alloyed metal nanoparticles in the bimetallic samples, the crystallization degree of the TiO2 support and the concentration of Ti3+/O2- vacancies sites. The reduction at 500°C is essential to obtain of alloyed Au-Pt NPs, to completely crystallize TiO2 into anatase and to induce a significant visible light absorption due to high concentration of Ti3+ sites/O2- vacancies. Under UV-A, the light is mainly absorbed by reduced TiO2 and electrons are transferred to the metal NPs. The activity order is Pt-Au>Pt>Au and could be related with the alloyed nature of Pt-Au nanoparticles, favoring in some way H2 desorption. Under simulated sunlight, the reduced state of TiO2 plays the key role in determining the activity. Pt0.5-Au0.5 shows the higher activity due to the best combination of alloyed nanoparticles and high reduction degree of TiO2. When using the less reactive glycerol, our systems show remarkably good performances, both under UV-A and simulated sunlight. Finally, under visible light irradiation samples show low but appreciable activity and, again, light absorption induced by reduced Ti3+ sites / O2- vacancies plays the major role. Figure 1. H2 evolution during ethanol photoreforming over Pt1-Au1, Pt0.5-Au0.5, Pt1 and Au1 reduced at 500 °C under simulated sunlight (A) and under irradiation with NaNO2-filtered Hg lamp (B). Acknowledgements Financial support from Regione Lombardia through the project "ACCORDO QUADRO Regione Lombardia e CNR", from the Italian MIUR through the FIRB Projects "ItalNanoNet" (RBPR05JH2P) and "Oxides at the nanoscale: multifunctionality and applications" (RBAP115AYN), from Regione Friuli Venezia Giulia through the project "IDRO-BIM", from Fondazione CRTrieste and from Fondazione Benefica Kathleen Foreman Casali is gratefully acknowledged. References 1. K. Shimura, H. Yoshida, Energ. Environ. Sci. 4 (2011) 2467. 2. T. Montini, V. Gombac, L. Sordelli, J. J. Delgado, X. Chen, G. Adami, P. Fornasiero, ChemCatChem 3 (2011) 574. 3. X. Chen, L. Liu, P. Y. Yu, S. S. Mao, Science 331 (2011) 746. 4. A. Gallo, M. Marelli, R. Psaro, V. Gombac, T. Montini, P. Fornasiero, R. Pievo, and V. Dal Santo, Green Chem., 14 (2012) 330.