Photo-induced reforming of alcohols with improved hydrogen apparent quantum yield on TiO2 nanotubes loaded with ultra-small Pt nanoparticles
Graphical abstract
Introduction
Photocatalytic hydrogen production via photocatalysis has great potential for solving environmental and energy issues and has been the focus of increasing research in the last decade. In particular, heterogeneous photocatalytic reactions on TiO2 semiconductors have been attracting much attention because of their potential applications in hydrogen production by water splitting (WS) and environmental clean-up by the so-called advanced oxidative processes (AOP) [1], [2]. The WS reaction was discovered in 1972 [3], and after more than 40 years, no real breakthroughs have been observed in the field. The WS reaction, referred to as one of the “holy grail” [4] of chemistry, remains elusive in spite of the exponential growth of scientific articles that have been published in the last decade. In spite of promising results that have been reported, a commercially viable catalyst system for this transformation is still absent, even for the high surface area TiO2 nanotubes (NTs). This difficulty is mainly due to the rapid recombination of photogenerated conduction-band electrons and valence band holes summed to the efficient surface recombination back-reaction of oxygen and hydrogen to produce water [5].
Photogeneration of hydrogen from water is usually investigated, contrary to AOPs, in unaerated conditions with simultaneous hydrogen production rate enhanced by the use of the so called sacrificial agents [5]. These agents increase the life time of the photoelectrons, decreasing the rate of electron–hole recombination and increasing consequently the hydrogen generation yield. In this sense, if the sacrificial agent is a pollutant present in the wastewater there is a possibility of the organic pollutant degradation together with hydrogen generation. In the last decade there have been used a large variety of electron donors for photocatalytic hydrogen production, such as alcohols [6], [7], [8], [9], polyalcohols [10], [11], sugars [12], n-heptane cracking [13] and chloroacetic acids [14]. Within all the above series of sacrificial agents, alcohols were found to be very efficient as hole scavengers increasing the hydrogen production rate one or two orders of magnitude compared to the WS reaction carried out in pure water. For this reason a number of research groups have recently moved their attention to the study of hydrogen production over TiO2 nanoparticles together with pollutant degradation [8], [10], [14], [15]. The results were promising, especially because the overall process can be described as a photo-induced reform of alcohols at room temperature representing an environmentally friendly and low cost method for wastewater treatment with simultaneous production of clean and renewable energy source, i.e., hydrogen.
Photocatalytic activity can be increased, for example, by the presence of Pt nanoparticles (NPs) [7], [10], [16], [17], [18] or Gold NPs [19], [20], [21], [22] as co-catalyst deposited on TiO2 surface. Several works have reported new Pt-loaded photocatalysts with improve efficiency towards water splitting using TiO2 NTs structures [23], [24]. The photocatalytic reaction over Pt/TiO2 systems has been already studied from the point of view of pollutant degradation [8] and also for the production of hydrogen [7], [8], [10], [25], [26], [27], [28]. After the absorption of the photon and charge separation, the photocatalytic reaction requires that the photogenerated electrons in the conduction band of TiO2 migrate to the Pt NPs through the Pt/TiO2 interfaces. The electrons finally are trapped by the Pt particles and available to participate in reduction reactions. The holes generated in the valence band remain for oxidation reactions [7]. The low-charge separation and transfer efficiency constitutes the bottle neck of the process where an intense electron–hole recombination takes place. If a sacrificial agent is present in the water and the electron transfer at the Pt/TiO2 interface is efficient, high production rates should be expected. The photocatalytic hydrogen production from aqueous methanol solutions over Pt-loaded laboratory prepared TiO2, non nonstoichiometric titania oxide and commercial TiO2 photocatalysts has been recently studied [26], [29]. In particular, Kandiel et al. [26] obtained interesting results using methanol as hole scavenger, indicating that this system should be described as a methanol dehydrogenation reaction when the photocatalytic reaction is stopped at the first step of photocatalytic oxidation (formaldehyde formation). Then when carbon dioxide is detected the photocatalytic reaction would be described better as methanol reforming but not as a real water splitting system.
The amount of Pt can play a key role in deciding whether a particular catalyst has a positive, negative or even null photocatalytic effect. Several authors have observed that an optimal loading of Pt existed for promoting photocatalytic rates on TiO2 surfaces and Pt loading over about 1 wt% resulted in attenuation of photochemical rates as a result of blocking of the TiO2 surface [8], [10], [16], [30], [31]. Several ways of Pt deposition have been used over the years; among them photoassisted Pt deposition [16], [32], [33] and in particular the wet impregnation method were employed due to its simplicity [7], [16], [25], [34]. Wet impregnation methodology, the more common approach used, has a poor control on the amount and type of Pt nanoparticles deposited on the TiO2 structures. For this reason, a number of groups starting to use sputtering deposition methods to control the amount of Pt loading with precision [30], [31], [35]. In spite Platinum has a high price and limited world-wide supply, recently it was shown that the sputtering method allow to use a minimum metal loading without compromise efficiency [36].
Herein we extend our previous work on hydrogen production by NTs photocatalysts [37], [38], [39] loading ultra-small Pt NPs on TiO2 NTs by DC-magnetron sputtering deposition (DC-MS) method. Photocatalytic hydrogen generation under UV irradiation using methanol, ethanol, glycerol and phenol as sacrificial species was investigated. Hydrogen apparent quantum efficiencies (Фapp), using methanol and ethanol as typical hole scavengers to enhance hydrogen production rate, were also measured. The prepared photocatalysts showed Фapp of 16% at an excitation wavelength of 313 nm for thin 1.5 μm length TiO2 NTs. The Pt NPs loaded on TiO2 NTs reported here represents also a true WS catalyst under UV irradiation and pure distilled water.
Section snippets
Preparation and characterization of TiO2 photocatalysts
TiO2 NTs were prepared by anodization of a Ti foil with constant applied voltage at room temperature, using an electrolyte containing ethylene glycol + 0.25 wt% NH4F + 10 wt% H2O with ultrasonic bath following a methodology already described [37], [38], [39], [40]. After the anodization the TiO2 NTs were annealed at 400 °C for 3 h in air atmosphere in order to crystallize the oxide nanotubes layer. The crystal structure analyses of TiO2 NTs after thermal treatment was performed by Rietveld
Photocatalyst characterization
The TiO2 NTs substrates after annealed at 400 °C for 3 h in air atmosphere showed a typical crystalline anatase phase. Fig. 1 shows the GAXRD patterns of the as-anodized TiO2 NTs and annealed TiO2 NTs, respectively. The results show that as-anodized sample present an amorphous TiO2 structure (Fig. 1a) and after the annealing process only tetragonal anatase structure was formed (Fig. 1b). The Rietveld refinement indicated that the average size of the anatase crystallites structure presented
Conclusions
In summary, the results presented here are very promising because hydrogen can be produced at ambient conditions via an efficient, technologically simple, ecologically benign and potentially very low-cost process. Indeed, the system uses simple Pt-loaded TiO2 NTs photocatalyst and abundant and renewable sources: alcohols and water. Pt loading on TiO2 NTs was highly efficiently in hydrogen generation. The apparent quantum yield in hydrogen formation measured at 313 nm was about 16%. The clear
Acknowledgment
This work was partially sponsored by CNPq, CAPES and ANEEL-CEEE GT (no. 9945481). Thanks to CME (UFRGS), LNLS – National Synchrotron Light Laboratory, Brazil (TGM beam line), INMETRO and Dr. Heberton Wender for his collaboration on GAXRD measurements in XRD-2 beam line.
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