Hydrogenated Anatase: Strong Photocatalytic Dihydrogen Evolution without the Use of a Co‐Catalyst

Abstract

Abstract:In the present work we show, how a high pressure hydrogenation of commercial anatase or anatase/rutile powder can create a photocatalyst for hydrogen evolution that is highly effective and stable without the need of any additional co-catalyst. This activation effect can not be observed for rutile. For anatase/rutile mixtures, however, a strong synergistic effect is found (similar to findings commonly observed for noble metal decorated TiO 2 ). ESR measurements indicate the intrinsic co-catalytic activation of anatase TiO 2 to be due to specific defect centers formed during hydrogenation. 2Ever since the groundbreaking work of Fujishima and Honda in 1972 [1], TiO 2 is considered as a promising photocatalyst for the splitting of water into H 2 and O 2 . In the original experiment, Fujishima et al. used a TiO 2 photoanode, connected via an external circuit to a platinum counter electrode -the latter was needed to successfully evolve H 2 from water. Due to the simplicity of the concept, illumination of a cheap and abundant semiconductor to create photoexcited charge carriers that can be transferred directly to water to form a high density energy fuel (H 2 ), the report found a tremendous scientific resonance. Meanwhile, more than 10000 papers have been published on using TiO 2 in a large palette of morphologies and modifications, to trigger a wide range of photocatalytic ractions (for overviews see e.g. refs.[2-8]). While numerous photoelectrochemical studies (i.e., using an illuminated TiO 2 electrode in an electrochemical circuit) where performed, still the most direct and economic approach is the use of TiO 2 in the form of particle suspensions -thus using the photocatalytic system without an external applied voltage. However, under these so called open-circuit conditions (OCP), TiO 2 alone is not efficient for the photoproduction of hydrogen without the use of a co-catalyst -mostly this is a noble metal (M), such as Pt, Pd or Au -for overviews see e.g. refs. [9][10][11]. These combined photocatalytic M@TiO 2 systems have therefore been widely investigated in view of optimizing their efficiency towards H 2 generation from water (with or without using sacrificial agents such as ethanol) [9,12].In general, the function of the noble metal co-catalyst has been described in terms of i) providing an electron acceptor that mediates electron transfer to the electrolyte, ii) forming solid state junctions (metal/semiconductor), or iii) acting as a hydrogen recombination center that strongly promotes H 2 formation [9][10][11]. In M@TiO 2 catalysts it has been generally observed that the crystalline phase of TiO 2 is a very important factor for the performance of such photocatalytic H 2 -generation systems [6-9, 13, 14]. Anatase and rutile are the most commonly used polymorphs in photoactivated TiO 2 applications. In photocatalytic water splitting, generally M@anatase combinations are found to be more efficient than M@rutile. 3This difference in photocatalytic activity is commonly attributed to a higher charge recom...

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