Effect of ionic liquid in a pressurized reactor to enhance CO2 photocatalytic reduction at TiO2 modified by gold nanoparticles
Graphical abstract
Introduction
Global warming caused by emissions of carbon dioxide (CO2) to the atmosphere is widely considered to be one of the most urgent current environmental issues [1]. The concentration of CO2 in the atmosphere has increased greatly since the beginning of the Industrial Revolution, mainly due to the burning of fossil fuels in power stations and combustion engine vehicles. Energy consumption was approximately 13.5 × 1012 W in 2001 and is expected to double to 27 × 1012 W by 2050 [2]. Therefore, finding an effective solution to reduce the emissions of CO2, combined with clean energy production, has attracted the interest of many researchers. The sun is one of the most attractive sources for production of useable energy, by means of solar heating [3], [4], solar photovoltaics [5], [6], solar thermal electricity [7], [8], solar hydrogen production [9], [10], and artificial photosynthesis [11], [12], [13].
One promising technology is the conversion of CO2 by photocatalysis, using semiconductors to promote the reactions in the presence of irradiation (sunlight), at low temperature and pressure. The process essentially consists of three steps: (i) generation of electron–hole pairs by absorption of photons with energy higher than the band gap, (ii) electron–hole pair separation, and (iii) reactions between surface species and electron–hole pairs. The pioneering work of Inoue and Fujishima in 1979 [14] demonstrated that titanium dioxide powder can reduce CO2 to organic compounds by photocatalysis in aqueous solution.
The mechanism by which catalytic CO2 photoreduction occurs in semiconductors is still controversial and is influenced by the materials, reaction conditions, and irradiation, among other parameters. In addition, achieving selectivity and efficiency of the reactions remains a major challenge. The CO2 molecule is inert and stable, so its reduction by an electron to generate the CO2 anion radical (CO2•–) requires an electrochemical potential of −1.9 V vs. NHE (normal hydrogen electrode). In practice, no feasible semiconductor reaches the potential for transferring a photogenerated electron to a free CO2 molecule, which makes this step highly implausible. However, proton-assisted multi-electron transfer favors the reaction. Under certain reaction conditions, CO2 reduction can occur, leading to the formation of products including formic acid, carbon monoxide, formaldehyde, methanol, and methane, among others [15], [16], [17].
The photocatalytic reduction of CO2 over a TiO2 semiconductor is an attractive technique, since TiO2 is inexpensive, abundant, nontoxic, presents high activity, and is chemically inert, with these properties enabling its application in a broad range of fields [18]. On the other hand, the efficiency of TiO2 is limited due to fast recombination of the photo-generated charge carriers [19], [20]. TiO2 with nanostructured morphologies, such as TiO2 nanotubes (TNTs) and nanowires, are considered superior in photocatalysis applications, compared to amorphous TiO2 nanoparticles [21]. TNT materials with regular structures can provide direct pathways for photo-generated electron transfer along the longitudinal dimensions, with a low rate of recombination [22], [23], and can be easily grown aligned with a titanium substrate. However, the large band gap of the material means that it is only functional under UV irradiation [24].
In order to expand its applicability, new approaches have been used, such as the introduction of single-atom co-catalysts [25] and co-catalysts with smart engineering in molecular level [26] as well as band gap engineering [27], [28], [29]. Among these methods, the photoactivity response of TNTs to irradiation in the visible region can be enhanced by modification of the semiconductor with nanoparticles of noble metals such as Au. This can improve the catalyst efficiency, due to surface plasmon resonance (SPR), with the nanoparticles acting as co-catalysts and favoring CO2 activation [30], [31]. In addition, Schottky barrier formation at the metal–semiconductor interface reduces fast recombination of the charge carriers, due to the difference in Fermi energy levels between the materials [19]. The photoreduction of CO2 at gold/silver-supported TiO2 nanowire catalysts improves the reduction of CO2 to CH4 and C2 products, due to the increased charge carrier separation between the TiO2 and Au nanoparticles (NPs) [32]. Jiao et al. [33] showed that in photoreduction of CO2 to CH4 over macroporous TiO2-supported Au-NPs, improved photoactivity was due to SPR effects of the Au-NPs.
Zeng et al. [34] have used TiO2 decorated with gold NPs to form a plasmonic photonic z-scheme photocatalyst. They showed that the selectivity of CO2 reduction could be designed by changing the illumination source. When the AM 1.5G filter (solar light simulator) was used, methane was the main product (302 μmol h−1, 89 % selectivity). On the other hand, using a UV light source the main products were formaldehyde (420 μmol h−1) and CO (323 μmol h−1), with practically no methane formation. In another study, Au@TiO2 yolk–shell hollow spheres created an electric field that enhanced the generation of electron-hole pair. Local SPR moderated the local electric field close to Au NPs and also promoted the chemical reaction involving electrons and protons, enabling the production of higher carbon species [35].
In all the studies reported so far, one of the limitations of the photocatalytic method has been the low solubility of CO2 (only around 0.033 M) in water at room temperature and pressure. Given that the reaction occurs on the catalyst surface, which must be in contact with the CO2, this low water solubility substantially decreases the amount of CO2 available for the reduction. Therefore, one option is to accelerate the reaction by increasing the pressure. Some studies of photocatalytic reduction of CO2 have been carried out under high pressure, using several types of catalyst in aqueous media [36], [37], [38], [39], and in all cases there was an increase in the proportion of reduction products. The main products found in the processes were CO, H2, and formic acid, together with traces of methane and C2 hydrocarbons.
Another strategy to improve the solubilization of CO2 in an aqueous medium is to use ionic liquids (ILs), which can improve the catalytic reactivity of CO2. ILs are conceptually similar to strong acids, since they are completely dissociated into ions. However, they present negligible vapor pressure at room temperature, resulting in a thermally, chemically, and electrically stable liquid phase, composed entirely by ions. The solubilities and thermophysical properties of mixture of various ILs and CO2 have been studied in recent years [40], [41].
Understanding of the solubility of CO2 in ILs remains incomplete, despite the growth of knowledge in this field, so further studies are needed in order to extend the applications of the processes. It is known that the functionalities of anions and cations can improve the solubility of CO2 in ILs. For example, by increasing the fluorination of the Tf2N− anion, concomitantly obtaining cations with longer alkyl chains, the “CO2-philic” nature of the IL increases, which promotes an increase in CO2 solubility [42]. The use of ILs based on the cationic imidazole group can increase CO2 solubility, as reported by Cadena et al. [43], who found that the main factor controlling CO2 solubility was the nature of the anion in ILs based on alkyl imidazole groups. The uses of tetrabutylphosphonium citrazinate IL during the photoreduction of CO2 with anatase TiO2 under visible light, produces 3.52 μmol g−1h−1 methane with 96% of selectivity. In this case, The IL-CO2 complex have a visible light absorption due to the delocalization effect in the conjugated anion complex [44]. In another work, Nie et al. [45] have used the 1-aminopropyl-3-methylimidazolium bromide IL as a good CO2 absorber with wide potential window throughout the CO2 photoelectroreduction. Yu and Jain [46] have synthetized higher C1-C3 hydrocarbons from CO2 photoreduction using plasmonic Au NPs and aqueous solution of 1-Ethyl-3-methylimidazolium tetrafluoroborate IL irradiated with a laser of 532 nm. In this instance, the IL can complex with CO2, allowing the CO2 molecule to accept electrons from Au NPS.
The present work investigates the combination of several parameters, such as operation of a photocatalytic reactor at 5 atm, use of IL, and TNT catalyst modified with gold nanoparticles (NPs), in order to enhance the photocatalytic reduction of CO2 to valued-added products, in an aqueous medium under a solar simulator. The easy deposition of Au NPs onto a TNT catalyst was performed using citric acid from lime juice, resulting in alteration of the photoexcitation of the new material. The catalysts were fully characterized by multiple spectroscopic techniques (XPS, XRD, EDS, EIS, Raman, IR, DRS, and ICP-OES), microscopy (SEM) and chronoamperometry (on–off curves). The photoconversion of CO2 was evaluated using multiple chromatographic techniques, enabling the proposal of a mechanism for the enhanced efficiency of CO2 conversion.
Section snippets
TNT catalyst preparation and decoration with Au NPs
TNT catalysts were prepared electrochemically by anodizing titanium foils (Realum, São Paulo, Brazil), as described previously [11], [47]. The decoration of the TNTs with Au NPs was performed by the chemical reduction of a chloroauric acid solution with citric acid, adapted from the method of Turkevich et al. [48]. Firstly, the TNT catalyst was immersed for 5 min in a solution of HAuCl4 (1 mM, Sigma-Aldrich) heated at 100 °C, under constant agitation. After this period, the heating was turned
Characteristics of the TNT/AuNP catalyst
Scanning electron microscopy images of the TNT catalyst with and without Au NPs are shown in Fig. 1. The micrograph shown in Fig. 1A revealed that the TiO2 nanotubes deposited on the Ti substrate by the electrochemical anodizing method were highly ordered, with average diameter of around 100 nm, tube thickness of 30 nm and length of 2.0 μm, as reported previously [11]. The TNT/AuNP catalyst prepared by the chemical deposition of gold using citric acid showed a uniform distribution of Au NPs on
Conclusions
The findings of this work constitute the first demonstration of photocatalytic reduction of CO2 to methanol and methane, with high efficiency, using TiO2 modified by gold nanoparticles, a high-pressured reactor, and aqueous solution containing ionic liquid at a low ratio (up to 4% v/v), under solar simulator irradiation. The gold nanoparticles were deposited according to a simple and green method, using Persian lime as a citric acid source. The use of a pressure of around 5 atm, together with
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are grateful to FAPESP (grant number #2014/50945-4) and CNPq (grant number #465571/2014-0) for support of this work. J.A.L. Perini, K. Irikura, and L.D.M. Torquato received scholarships from FAPESP (grant numbers #2014/50945-4, #2016/18057-7, #2017/12790-7, and #2019/00463-7). J.B.S. Flor received scholarships from CNPq (grant number #465571/2014-0, and #168961/2017-2). The authors would like to thank Professor Edenir R.P. Filho for the ICP-OES analyses.
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