Copper-doped titania photocatalysts for simultaneous reduction of CO2 and production of H2 from aqueous sulfide

https://doi.org/10.1016/j.apcatb.2015.06.019Get rights and content

Highlights

  • Copper is incorporated as a dopant in the lattice of TiO2 by solvo-thermal methods.

  • Cu-TiO2 enables CO2 reduction and H2 production under irradiation.

  • Cu-doped photocatalysts are more active than Cu-impregnated counterparts.

  • Concomitant sulfide pollutant removal improves process efficiencies.

Abstract

Copper-doped titanium dioxide materials with anatase phase (Cu-TiO2, atomic Cu contents ranging from 0 to 3% relative to the sum of Cu and Ti), and particle sizes of 12–15 nm, were synthesised by a solvo-thermal method using ethanol as the solvent and small amounts of water to promote the hydrolysis-condensation processes. Diffuse reflectance UV–vis spectroscopy show that the edges of absorption of the titania materials are somewhat shifted to higher wavelengths due to the presence of Cu. X-ray photoelectron spectroscopy (XPS) indicate that Cu(II) is predominant. Photocatalytic CO2 reduction experiments were performed in aqueous Cu-TiO2 suspensions under UV-rich light and in the presence of different solutes. Sulfide was found to promote the efficient production of H2 from water and formic acid from CO2. The effect of the Cu content on the photoactivity of Cu-TiO2 was also studied, showing that copper plays a role on the photocatalytic reduction of CO2.

Introduction

Carbon dioxide is an inexpensive and widely available feedstock. Its use for the production of chemicals is doubly beneficial, since in addition to its availability, it would represent a decrease in the carbon footprint for the manufacturing of the particular chemical. Catalytic processes are crucial for improving efficiency and conversion in the use of CO2 for the manufacture of a wide range of substances, including methanol [1], [2], [3], [4], carboxylic acids [1], [4], polycarbonates [1], [4], [5], polylactones [1], [6] or hydrocarbons [1], [4], [7], [8]. As a particularly important matter, the catalytic production of hydrocarbon fuels from CO2 constitutes a highly desirable strategy due to its potential of closing the carbon cycle in future energy schemes. This would in turn contribute to alleviate both our dependence on fossil fuels and the greenhouse effect. Unfortunately, CO2 is a very stable molecule and its reduction into hydrocarbons is an energy-consuming process. From a sustainability point of view, a source of renewable energy would be required. Thus, one direction towards the conversion of CO2 to fuels could be the use of light, most preferably sunlight, as the energy source to promote the reaction. Reduction of CO2 by H2 into methane, i.e. the Sabatier reaction [9], activated by solar light can be achieved at very high efficiencies on nanoparticulate nickel materials [10] or on various supported metal (especially Ru or Ni) catalysts [11], [12]. A further step in this sense would be the use of H2O instead of H2 as the reducing agent, in essence mimicking the activation route of natural photosynthesis. Although more desirable, the latter involves surmounting a considerably high energetic barrier and a complex mechanistic pathway. In recent times, a number of research teams around the world are tackling this issue by designing photocatalysts for the process. The most frequently investigated photocatalysts for these transformations are based on titanium dioxide as the light absorbing semiconductor [13], [14], [15], [16], and include one or more co-catalysts, although the focus on other semiconducting materials cannot be neglected [14], [17].

Copper has received attention as a metal with activity in photocatalytic TiO2 systems for the reduction of CO2 [13]. By introducing Cu into TiO2 by various methods, the production of hydrocarbons (such as methanol or methane) from CO2 at different levels of success has been reported. For example, Cu/TiO2 materials prepared by impregnation of copper onto a titania support and subsequent calcination have been used to generate methanol photocatalytically from aqueous CO2 solutions under black light, at production rates as high as 443 μmol gcat−1 h−1 [18]. Other researchers have reported lower but noticeable methanol yields using similarly prepared photocatalysts and Hg lamp irradiation (10–23 μmol gcat−1 h−1) [19], [20]. In gas-phase reactions, the formation of methanol was also reported, but in considerably lower yields (2 nmol gcat−1 h−1), being methane the major product formed by Cu/TiO2 photocatalysis [21]. In contrast, similar materials were found to promote the formation of several hydrocarbons including methane, ethane and ethylene in conjunction with larger amounts of H2 [22], whereas CO and methane were major products found in a more recent study [23]. A significant degree of discrepancy regarding the identity of the reaction products (in addition to their selectivities and yields) is apparent by considering the aforementioned literature data, although this could be in part due to the different synthetic procedures used for the preparation of the photocatalysts and to the varied irradiation conditions used.

Despite the notable progress in this field of research, there is no clear trend which may allow to conclude on the actual mechanistic route and to gain knowledge on the role of copper on the reaction. Thorough kinetic and in situ spectroscopic studies would provide valuable information on the reaction at the surface of the photocatalyst and may guide future design of more active copper-titania materials. An important issue to focus on is the possible back-reactions of the reduction products on the photocatalyst. For example, it was observed that methanol and formaldehyde, initially formed by a photocatalytic process using Cu(0) powder and TiO2, were rapidly consumed by in situ formed Cu/TiO2 [24]. The simultaneous occurrence of CO2 reduction and decomposition of the products should result in stationary concentrations reflecting the relative reaction rates. Furthermore, any residual carbon contamination on the photocatalysts, even at low levels, has been shown to seriously interfere with the quantification of CO2 reduction products and may thus, lead to overestimated activities [25].

In this work, photocatalysts based on titanium dioxide containing small amounts of copper (<3% by weight) have been prepared by a solvo-thermal method. This procedure leads to Cu-doped anatase TiO2. Copper is mostly within the TiO2 framework, thus, forming isolated copper sites, as opposed to the most commonly used Cu-impregnated materials. The use of these materials for the photocatalytic reduction of CO2 in aqueous suspensions has been investigated. The effect of copper content and of additional solutes on the efficiency of the process has been studied. Sulfide, which is a common and undesirable water pollutant, especially in sewage effluents [26], [27], has been selected as an efficient sacrificial electron-donating solute. In addition to the sulfide anion, its conjugated acid hydrogen sulfide is a large scale by-product in petrochemical processing [28], [29] (where it is converted to elemental sulfur for disposal), and uses thereof are intensively sought as a desirable strategy for its valorisation as a feedstock [30]. The possibility of using sulfide species as sacrificial electron donors for the photocatalytic production of fuels has been extensively studied mostly on metal sulfide semiconductors, although these processes often require additional solutes such as sulfite to prevent photocorrosion [27], [28], [29]. In contrast, the system presented here relies on a stable copper-titania photocatalyst which can remove aqueous sulfide, without the need of additives, with simultaneous production of H2 and reduction of CO2.

Section snippets

Materials

Copper(II) acetate monohydrate (≥99%), copper(II) nitrate hemi(pentahydrate) (≥98%), titanium(IV) isopropoxide, acetylacetone (≥99%), sodium sulfide (99%) and tetrachloroethylene (99%) were supplied by Sigma–Aldrich. Sodium hydroxide (pellets, 98%) was supplied by VWR. Titanium dioxide (Aeroxide® P25) was kindly supplied by Evonik Degussa. Carbon dioxide (≥99,995%) and argon (≥99.995%) were supplied by Abelló Linde. Absolute ethanol (Multisolvent® HPLC grade) was supplied by Scharlau.

Synthesis of the photocatalysts

The

Synthesis of the photocatalysts

Nanoparticulate copper-doped TiO2 materials have been prepared by a solvo-thermal method based on the hydrolysis-condensation of a titanium alkoxyde (2-propoxide) using ethanol as the solvent and a small amount of water in order to promote the hydrolysis reaction. Copper acetate was used as the Cu precursor, and acetylacetone was added in order to form the corresponding complex. Then, the solubility of the metallic cation increases and the availability of copper to be incorporated into the

Conclusions

The synthesis of nanoparticulate copper-doped titania photocatalysts can be achieved by a solvo-thermal method using ethanol as the solvent. The addition of small amounts of water induced the controlled hydrolysis-condensation of titanium isopropoxide in the presence of Cu2+, thus, giving rise to Cu-TiO2 solids composed of small anatase crystallites (12–15 nm in diameter). Copper is incorporated the bulk of the crystalline titania matrix, thus, resulting in the substitution of Ti(IV) atoms with

Acknowledgments

Financial support by the Spanish Ministry of Economy and Competitiveness (Severo Ochoa and CTQ2012-32315) is gratefully acknowledged. F.G. and B.J.-L. are thankful for financial support from Spanish Government (AP2010-2748 PhD grant and MAT2011-27008 project) and Jaume I University (P1 1B2014-21 project). SCIC from Jaume I University and Servicio de Microscopía Electrónica at Universitat Politècnica de València are also acknowledged for instrumental facilities. A.V.P. is grateful to both the

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