Enhanced TiO2 photocatalysis by Cu in hydrogen production from aqueous methanol solution

https://doi.org/10.1016/j.ijhydene.2004.02.013Get rights and content

Abstract

Cu particles have been deposited on TiO2 by incipient-wetness impregnation followed by low-temperature (400°C) calcination/reduction, and the metallization process leads to significant enhancement in photocatalytic activity of TiO2 for H2 production from aqueous methanol solution. The activity exhibits up to 10-fold enhancement at the optimum loading of ∼1.2wt% Cu. Spectroscopic studies indicated that the Cu particles were oxidized during reaction to have a valence lower those of thermally oxidized particles, which showed inferior activities. Dissolution of Cu ion in TiO2 lattice, in contrast, resulted in reduction in photocatalytic activity.

Introduction

Photocatalytic production of hydrogen using sunlight as the energy input is a valuable sustainable-energy technology. In this case, solar energy is stored by driving chemical reactions “up-hill” toward chemicals, such as H2, of higher chemical potentials. Due to its high stability against photo-corrosion and its favorite electronic energy band structure, TiO2 has drawn tremendous attention for such applications. For H2 production from water, many studies have concluded that direct splitting of water into H2 and O2 has a very low efficiency due to rapid reverse reaction. A much higher hydrogen production rate can be obtained by addition of the “so-called “sacrificial reagents,” such as alcohols and other organics [1], [2], [3], [4], which are oxidized to products that are less reactive toward hydrogen. For H2 production from a water/methanol (MeOH) solution, depending on reaction conditions and on whether metal catalyst used, the reaction could proceed either stepwise, involving stable intermediates, such as aldehydes and acids:MeOH(l)hν,catal.HCHO(g)+H2(g)ΔG1°=64.1kJ/mol[5],HCHO(g)+H2O(l)hν,catal.HCO2H(l)+H2(g)ΔG2°=47.8kJ/mol,HCO2H(l)hν,catal.CO2(g)+H2(g)ΔG3°=−95.8kJ/mol,as suggested by Sakata et al. [1], [2], [3], [4], or in one-step on catalyst surface to give the overall reaction:MeOH(l)+H2O(l)hν,catal.CO2(g)+3H2(g)ΔG°=16.1kJ/mol,as suggested by Chen et al. [6]. H2 is produced in all of these steps.

Deposition of Pt-group metals, including Ni, Pd and Pt, on TiO2 has been shown to greatly enhance the photocatalytic production of H2 from either pure water or water/sacrificial reagent solutions. Sakata et al. [4] attributed the enhancement to the catalytic effects of the metal particles on H2 evolution. Bowker et al. [7], on the other hand, suggested that the presence of Pd provides a reaction pathway which involves chemisorption and dehydrogenation of MeOH on Pd to produce chemisorbed CO and subsequent oxidation of the chemisorbed CO to CO2.

Cu-containing TiO2 catalysts are well known for their photocatalytic activity toward CO2 reduction [8], [9], [10], [11] but much less is known for its performance in H2 production from water/alcohol solution, except in one case where a Cu–TiO2 catalyst was reported to exhibit enhanced H2 production from a water/MeOH solution with photon energies within the visible-light region [12]. In that particular study, the photocatalyst was synthesized by calcining a mixture of Ti(SO4)2 and Cu(NO3)2 (1 wt% of CuO) at 930K, and it exhibited enhanced absorption within the visible light range, in accompanied with increased catalytic activity, upon Xe lamp irradiation. Cu could be either dissolved in TiO2 lattice or deposited as individual particles on the TiO2 surface. No attempt was made therein to distinguish their roles in the enhancement effect.

In this work, Cu particles were deposited on TiO2 via an incipient-wetness impregnation method followed by low-temperature (400°C) calcination in order to minimize Cu dissolution into the oxide lattice, and its photocatalytic activity toward H2 production from a water/MeOH solution was investigated. It is shown that the deposited Cu particles, which are in the oxidized state during reaction, cause up to 10-fold enhancement in H2 production. In contrast, dissolution of Cu ion in TiO2 lattice exhibited negative effect.

Section snippets

Experimental

TiO2 powder was synthesized by a conventional sol–gel process. TiCl4 was first dissolved in an ethanol/water (volume ratio=4:1) solution, and ammonia was then introduced into the solution to induce condensation until pH reached 7.5. The resulted gelatinous precipitate was filtered and washed to reduce [Cl] to below 5×10−4M, as determined by ion chromatography, and then dried at 65°C in air. Cu was loaded by an incipient-wetness method, in which an aqueous solution containing CuCl2·2H2O was

Results and discussion

The blank TiO2 catalyst (Cu loading=0 wt%) appeared yellowish, and XRD analysis indicated the presence of only the anatase phase with an average grain size of 9.0 nm, based on the (1 0 1) reflection. The yellowish color, which can be removed by annealing in oxygen above 500°C, is believed to be caused by oxygen vacancies. The BET surface area is 110m/g. With increasing Cu deposition up to a maximum loading of 1.9wt% Cu, the catalysts turned increasing reddish. XRD, however, did not detect

Acknowledgements

The work is supported by the Ministry of Economic Affair under grant 92-EC-17-A-09-S1-019.

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