Abstract
Photoassisted synthesis is as a highly appealing green procedure for controlled decoration of semiconductor catalysts with co-catalyst nanoparticles, which can be carried out without the concourse of elevated temperatures, external chemical reducing agents or applied bias potential and in a simple slurry reactor. The aim of this study is to evaluate the control that such a photoassisted method can exert on the properties of ruthenium nanoparticles supported on TiO2 by means of the variation of the incident irradiance and hence of the photodeposition rate. For that purpose, different Ru/TiO2 systems with the same metal load have been prepared under varying irradiance and characterized by means of elemental analysis, transmission electron microscopy and X-ray photoelectron spectroscopy. The photocatalytic activity of the so-obtained materials has been evaluated by using the degradation of formic acid in water under UV-A light. Particles with size around or below one nanometer were obtained, depending on the irradiance employed in the synthesis, with narrow size distribution and homogeneous dispersion over the titania support. The relation between neutral and positive oxidation states of ruthenium could also be controlled by the variation of the irradiance. The obtained photocatalytic activities for formic acid oxidation were in all cases higher than that of undecorated titania, with the sample obtained with the lowest irradiation giving rise to the highest oxidation rate. According to the catalysts characterization, photocatalytic activity is influenced by both Ru size and Ru0/Ruδ+ ratio.
Highlights
Heterogeneous photocatalysis has gained renewed interest in the last years as a sustainable, solar-driven process with applications in the elimination of a plethora of hazardous substances in water and air, as well as in the production of solar fuels and chemicals by means of water splitting, photoreforming, CO2 reduction or nitrogen fixation [1,2]
In addition to the photocatalyst itself, the concourse of co-catalysts in heterogeneous photocatalysis, composed of metal or metal oxide nanoparticles deposited on the semiconductor photocatalyst, can significantly enhance critical aspects of the photocatalytic process such as light absorption and charge transfer; activation of reactant molecules; selectivity of the reaction and catalyst stability [3]
Photoassisted synthesis or photodeposition is based on the light-induced electrochemistry that occurs at the surface of a semiconductor upon irradiation
Summary
Heterogeneous photocatalysis has gained renewed interest in the last years as a sustainable, solar-driven process with applications in the elimination of a plethora of hazardous substances in water and air, as well as in the production of solar fuels and chemicals by means of water splitting, photoreforming, CO2 reduction or nitrogen fixation [1,2]. Metal cations adsorbed on the semiconductor surface can be reduced (oxidized) by reactive electrons (holes) photo-generated by band-gap excitation, provided that the reduction potential of the conduction (valence) band is adequate. Among different alternatives, this is considered as a green and elegant preparation method for the obtainment of well-defined, small-sized supported nanoparticles with control over size, distribution and oxidation state [4]. As in the case of any metal and any catalytic application, control over size, distribution and oxidation state is highly desirable, and in this respect photodeposition appears as a highly appealing synthetic procedure, which can be carried out without the concourse of elevated temperatures, strong chemical reducing agents such as hydrogen or NaBH4 or applied bias potential and in a simple slurry reactor. In the particular case of Ru, it has been reported that the size of the nanoparticles can be controlled by varying the irradiation time [13], to the case of photodeposited platinum [14,15]
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