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    New Insights into the Photocatalytic Properties of RuO<sub>2</sub>/TiO<sub>2</sub> Mesoporous Heterostructures for Hydrogen Production and Organic Pollutant Photodecomposition

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    Photocatalytic activities of mesoporous RuO<sub>2</sub>/TiO<sub>2</sub> heterojunction nanocomposites for organic dye decomposition and H<sub>2</sub> production by methanol photoreforming have been studied as a function of the RuO<sub>2</sub> loading in the 1ā€“10 wt % range. An optimum RuO<sub>2</sub> loading was evidenced for both kinds of reaction, the corresponding nanocomposites showing much higher activities than pure TiO<sub>2</sub> and commercial reference P25. Thus, 1 wt % RuO<sub>2</sub>/TiO<sub>2</sub> photocatalyst led to the highest rates for the degradation of cationic (methylene blue) and anionic (methyl orange) dyes under UV light illumination. To get a better understanding of the mechanisms involved, a comprehensive investigation on the photogenerated charge carriers, detected by electron spin resonance (ESR) spectroscopy in the form of O<sup>ā€“</sup>, Ti<sup>3+</sup>, and O<sub>2</sub><sup>ā€“</sup> trapping centers, was performed. Along with the key role of superoxide paramagnetic species in the photodecomposition of organic dyes, ESR measurements revealed a higher amount of trapped holes in the case of the 1 wt % RuO<sub>2</sub>/TiO<sub>2</sub> photocatalyst that allowed rationalizing the trends observed. On the other hand, a maximum average hydrogen production rate of 618 Ī¼mol h<sup>ā€“1</sup> was reached with 5 wt % RuO<sub>2</sub>/TiO<sub>2</sub> photocatalyst to be compared with 29 Ī¼mol h<sup>ā€“1</sup> found without RuO<sub>2</sub>. Favorable band bending at the RuO<sub>2</sub>/TiO<sub>2</sub> interface and the key role of photogenerated holes have been proposed to explain the highest activity of the RuO<sub>2</sub>/TiO<sub>2</sub> photocatalysts for hydrogen production. These findings open new avenues for further design of RuO<sub>2</sub>/TiO<sub>2</sub> nanostructures with a fine-tuning of the RuO<sub>2</sub> nanoparticle distribution in order to reach optimized vectorial charge distribution and enhanced photocatalytic hydrogen production rates
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