Au nanoparticles supported on nanorod-like TiO2 as catalysts in the CO-PROX reaction under dark and light irradiation: Effect of acidic and alkaline synthesis conditions
Graphical abstract
Introduction
During the last decades, titania and titania-based materials have been used in a broad and large spectrum of processes connected to heterogeneous photocatalysis. Titania-based materials display a strong oxidation ability, super-hydrophilicity, chemical stability, non-toxicity and transparency to visible light [1], [2]. Normally, the photocatalytic performance of pure titania (TiO2) is limited to less than 3% of the incoming solar spectrum due to its large band gap (3.0 eV for rutile and 3.2 eV for anatase). Therefore, the current effort in photocatalysis is shifting the absorption of TiO2 to include a greater portion of the solar spectrum (about 42%). In recent years, solid state nanomaterials like nanostructured semiconductors, nanometals, nanowires and nanotubes materials are studied and utilized as catalysts in the areas of energy and environment [3], [4].
In comparison to the single-phase titania photocatalyst, ordered TiO2-based heterostructures open the light-response range up to the visible region. Decorating titania with nanoparticles of noble metals improve furthermore the charge separation and transfer properties thus promoting photocatalytic efficiency. This type of catalysts has been used as a photocatalyst in a wide range of applications, including conversion of solar energy to oxidize or reduce materials, production of materials and environmental purification [5], [6], [7], [8]. These processes present the main advantage of being conducted at ambient temperatures by using an inexhaustible and clean energy source, such as the sunlight. Since TiO2 shows good ability to oxidize organic and inorganic substrates in air and water through redox processes, it has been recently applied as catalyst support in the preferential oxidation of CO (CO-PROX) in excess of H2, which is an important reaction for pollution control and energy production [9], [10], [11], [12], [13].The CO-PROX process is at the end of the reforming process to reduce the CO level in the feed to 10 ppm to avoid the poisoning of the platinum-containing anode of the Polymer Electrolyte Membrane Fuel Cells (PEMFCs), the most promising candidates for electric power generator in transportation applications. In such a way the competitive oxidation of CO in the presence of an excess of H2 occurs and the selection of the adequate catalyst becomes very relevant. Loading gold on the surface of titania or titania-based materials is proved to improve the efficiency of CO oxidation at low temperatures [14], [15], [16], [17], [18]. Au deposited on TiO2 surface can act as a trap aiding electron-hole separation improving the quantum yield. In addition, when excited by electromagnetic radiation in the visible range gold nanoparticles undergo the localized surface plasmon resonance (LSPR) phenomenon, which occurs in metallic nanostructures such as rough surfaces and nanoparticles and is a collective electron density oscillation at the interface of metallic structures caused by the electromagnetic interaction of the incident light of a specific wavelength with a metal smaller than the incident λ. This effect can enhance the localized electric field in the proximity of the gold particles, while the interaction of localized electric fields with a neighbouring semiconductor such as titania allows for the facile formation of electron-hole pairs [19] in the near surface region of TiO2. For that purpose, the deposition-precipitation method with NaOH has been successfully used as a common method for preparing Au/TiO2 materials with very small gold particles, by controlling the pH and the calcination temperature, which is a key factor to ensure high activity for CO oxidation [20], [21]. On the other hand, structural, textural and morphological properties of titania support play an important role in the catalytic response of the resultant material [22]. Thus, 1D TiO2 nanostructures, such as nanowires, nanosheets [23], nanorods and nanotubes [10] can combine unusual and versatile physico-chemical properties with the conventional advantages of titania [3]. As reported in recent works [24], [25], nanorods have relatively large surface area with high length to diameter ratio, as well as fast charge separation efficiency with short radial distance, which are supposed to be beneficial to enhancing the photocatalytic activity.
Nanostructured materials have shown remarkable photocatalytic behaviour, which is associated to their novel physical and chemical properties originated from their distinctive geometries [24], [26], [27]. Their open structure leads to low resistance to mass transport and ability to be easily recovered. In this work, Au nanoparticles highly dispersed on titania nanorods surface have been prepared. Supports have been synthesized by a hydrothermal method [23], both under acid and basic conditions, giving rise to different titania polymorphs, and the incorporation of gold nanoparticles has been carried out by a deposition-precipitation method. The nanostructured photocatalysts have been tested in the preferential oxidation of CO in excess of H2 at room temperature and atmospheric pressure, both under dark and simulated solar light conditions.
Section snippets
Synthesis of TiO2 nanorods by acidic and basic hydrothermal conditions
The TiO2 materials submitted to the acid treatment (denoted as TNRa) were prepared by dispersing 10 g of titanium dioxide P25 (Degussa) into 100 mL of 15 wt% H2SO4 solution and transferred into a sealed Teflon stainless steel autoclave and kept at 200 °C for 15 days [28], [29]. The obtained product was filtered and washed several times: firstly with deionized water, secondly with NH4NO3 0.1 M and finally with hot deionized water until pH = 7. The absence of sulfate ions was verified by the BaCl2
Characterization results
The textural properties of the bare supports and the corresponding Au catalysts are included in Table 1, where two uneven trends are revealed after the hydrothermal treatments. The titania nanorods resulting from the acidic synthetic approach show a much lower specific surface area than the titania P25 precursor (about 50 m2 g–1) and this parameter decreases with the thermal treatment. However, after the basic treatment, the supports show an increase on their surface area, which also slightly
Conclusions
We have reported a simple wet chemical method for the preparation of AuNPs (1.0 wt% nominal loading) on nanorod-like TiO2 structures to be used as very active catalysts in the preferential CO oxidation in excess of H2. Titania supports, synthesized by acidic or basic hydrothermal methods, to give rutile or anatase nanorods respectively, seem to play a different role in dispersing and growing the Au nanoparticles. Gold particle size was found to depend on both the surface area and the type of
Acknowledgements
The authors would like to acknowledge the Spanish MINECO Project CTQ2015-68951-C3-3-R and FEDER funds, for financial support. A.I.M. thanks the Ministry of Economy and Competitiveness for a Ramón y Cajal contract (RyC-2015-17870).
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