Pt and Au/TiO2 photocatalysts for methanol reforming: Role of metal nanoparticles in tuning charge trapping properties and photoefficiency

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Abstract

Metal-loaded TiO2 is, by far, one of the most important class of photocatalysts in hydrogen production through photoreforming of organics and water photosplitting. In this study anatase loaded with Au and Pt nanoparticles (Au/TiO2 and Pt/TiO2) by an impregnation-reduction method was investigated as for morphological, electronic (XPS) and photocatalytic properties in hydrogen production by methanol photoreforming. The electron and hole trapping centers, Ti3+ and O, respectively, formed under UV–vis irradiation of the photocatalysts, were studied by in situ electron spin resonance (ESR) spectroscopy. The nature of the loaded metal affected both the H2 evolution rate and the distribution of the methanol oxidation products. The better performance of Pt/TiO2 is attributable to the greater ability of Pt with respect to Au to act as electron sink, slowering the recombination of photoproduced electron–hole couples. Direct evidence of this effect was obtained by ESR analysis, showing that the amount of Ti3+ active sites follows the order TiO2 > Au/TiO2  Pt/TiO2, thus confirming easier electron transfer from Ti3+ to Pt, where the H+ reduction to H2 occurs.

Highlights

► Pt nanoparticles loaded on TiO2 highly active in hydrogen production. ► Pt nanoparticles loaded on TiO2 highly active in the methanol complete oxidation. ► The amount of electron traps (Ti3+ centers) follows the order TiO2  Au/TiO2 > Pt/TiO2. ► The amount of hole traps (O centers) is highest in Pt/TiO2. ► Easier transfer of photopromoted electrons to Pt NPs accounts for higher performance.

Introduction

Hydrogen generation from biomass and/or water represents a sustainable option for the future energy economy [1], [2]. Photocatalytic and photoelectrochemical water splitting as well as photoreforming of organics are viable solutions for small and medium-size scale hydrogen production [3], [4], [5]. Although a large number of materials potentially suitable for photo(electro)catalytic hydrogen production have been recently explored, mainly based on semiconductor oxides [6], [7], sulphides [8], nitrides [9], oxysulphides [10], and oxynitrides [11], titanium dioxide (TiO2) is still the benchmark photocatalytic material due to its large availability, cheapness, stability, and non-toxicity. Nevertheless, its wide band gap allows appreciable photoactivity only under UV light irradiation and the fast electron (e)–hole (h+) recombination, common to all semiconducting materials, lowers the photocatalytic efficiency.

The availability and separation of photogenerated charges in the photocatalytic material are indeed key issues, which can be improved either (i) by cocatalyst loading (e.g. noble metal, transition-metal oxide, nonmetal-oxide), (ii) by using combinations of semiconductors, or (iii) by modifying the crystal structure and the morphology of the material [12]. Furthermore, the rate of photocatalytic hydrogen production increases also in the presence of sacrificial organic compounds (e.g. methanol [13], [14], [15], [16], [17], ethanol, and glycerol [18]), which are able to combine with photogenerated h+ more readily than water itself in the photocatalytic splitting of pure water. Although interesting results have been obtained in recent years using approaches (ii) and (iii) [19], [20], [21], TiO2 modified by noble metal nanoparticles (M/TiO2) still remains the model system for photocatalytic H2 production [15], [16], [17], [22], [23].

When noble metal nanoparticles (NPs) are loaded onto the surface of TiO2, photopromoted electrons preferentially migrate from the conduction band of the oxide to the noble metal cocatalyst [24], [25], thus decreasing e–h+ recombination. Up to now, several metals have been used as titania cocatalyst [26], [27], [28], [29] albeit the best performances in the photocatalytic production of H2 have been obtained with Pt/TiO2 and Au/TiO2. The main reason of this behavior relies on the separation of photogenerated charge carriers generally attributed to the formation of a Schottky barrier at the metal/TiO2 interface. The higher is the Schottky barrier, the lower is the recombination rate between the e transferred to the metal and the h+, and the greater the H2 production [13], [15], [16], [17], [30].

In their early work on the photocatalytic effects of noble metal NPs deposition on TiO2, Bamwenda et al. [31] reported that the hydrogen yield from water/ethanol solution was greatly improved by Pt deposition on TiO2, rather than by Au deposition on TiO2. The higher overall activity of Pt loaded catalysts was ascribed to more effective trapping and pooling of photogenerated electrons on Pt and/or to the higher intrinsic platinum activity in reduction processes.

The literature reports several different methods to load metal NPs onto TiO2, such as deposition precipitation, impregnation, photoreduction and chemical reduction (with H2 or NaBH4) [32]. Among all these methods, the adsorption of noble metal ions onto the titania surface followed by reduction with NaBH4 produces highly dispersed metal NPs in intimate contact with TiO2 [32]. Moreover, the obtained M/TiO2 products exhibit better photocatalytic properties than bare titania, confirming that this technique is suitable to deliver highly active photocatalysts.

Whereas the enhancing effect of loaded noble metals on the photocatalytic properties of TiO2 in reduction processes is well-established, their action mechanism in the parallel oxidation reactions is still under debate. Furthermore, there is hardly any investigation which directly relates the morphological and electronic properties of the metal co-catalysts to the type, amount and location of the electronic defects and the photoproduced charge carriers active in photocatalytic processes.

Aiming at addressing these points, in the present study the photocatalytic efficiency of Au/TiO2 and Pt/TiO2 in hydrogen production from methanol photoreforming has been investigated in relation to the morphological and electronic properties of the photocatalysts, as well as to the charge carriers, detected by electron spin resonance (ESR) spectroscopy in the form of Ti3+, O and O2 trapping centers, which are photogenerated in the early stage of the photocatalytic process. The direct investigation, performed by ESR analysis, of the paramagnetic species formed under irradiation on TiO2 has recently provided relevant insight into the mechanism of photocatalytic processes and suggested ways to improve photoefficiency through the manipulation of defect states [33], [34], [35].

Section snippets

Photocatalysts preparation

A commercial amorphous TiO2 powder (NanoActive®, NanoScale Co., USA) was used as starting material. It was first kept at 473 K under flowing O2 for 1 h. After cooling down to room temperature, it underwent further oxidation in flowing O2 at 773 K for 1 h. The sample obtained by this way was labeled TiO2-A.

Au- and Pt-modified TiO2 photocatalysts were prepared as it follows. 500 mg of TiO2-A were suspended in 40 mL of H2O and sonicated for 20 min. Then, a HAuCl4 or H2PtCl6 water solution was added and

XRPD analysis

The XRPD pattern of TiO2-A, reported in Fig. S1 (Supporting Information), shows that the sample was crystalline and contained 100% anatase phase, with 15 nm average crystallite size, evaluated using the Scherrer equation based on the [1 0 1] anatase reflection. Almost identical XRPD patterns were recorded with TiO2-Rf and with the two metal-loaded samples (not reported), indicating that metal deposition followed by reduction with NaBH4 did not affect the phase and the crystallite size of the

Conclusions

In the present study, the catalytic efficiency of Au/TiO2 and Pt/TiO2 in H2 production through photoreforming of methanol was related to their morphological and electronic properties, as well as to the trapping and the reactivity of the charge carriers photogenerated in the early stage of the process. Pt/TiO2 displays higher activity than Au/TiO2 and bare TiO2, in both hydrogen production and complete methanol oxidation.

ESR investigation evidenced that interfacial transfer and trapping of

Acknowledgements

M.D. gratefully acknowledges Dr. Ulrich Roll and Omicron Nanotechnology for the experimental support in the XPS characterization of the samples. A.N. and M.M gratefully acknowledge grants from the Italian Ministry of Education, University and research (MIUR) through the FIRB project “ItalNanoNet” (RBPR05JH2P). V.D.S. gratefully acknowledges financial support from the Italian Ministry of Education, University and research (MIUR) through the FIRB project “Oxides at the nanoscale:

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