CO2 photoreduction with H2O vapor on highly dispersed CeO2/TiO2 catalysts: Surface species and their reactivity
Graphical abstract
Introduction
Photocatalytic reduction of CO2 with H2O vapor on chemically stable and environmentally benign TiO2 is gaining increased interest because it is a promising “green chemistry” approach for the direct conversion of CO2 to value-added fuels (CO, methane, methanol, etc.) driven by sunlight [1], [2], [3]. However, TiO2 photocatalyst suffers from several disadvantages that ultimately lead to low reaction efficiency [4], including (1) low solar energy utilization due to its large band gap (3.2 eV for anatase TiO2 and 3.0 for rutile TiO2), (2) fast recombination of photogenerated electron–hole pairs, and (3) weak interaction between CO2 molecules and TiO2 surfaces, leading to low coverage of reactive adsorbed species and difficult displacement of the reaction products and/or inactive intermediates by CO2 molecules [3]. The former two limitations have been extensively addressed in the literature by metal (e.g., Pt, Au) coupling or nonmetal doping, mixed phase TiO2 [5], [6], p–n heterojunction construction, photosensitizer decoration, and defect production [7], [8]. However, limited attention has been focused on the last one, which is also a key factor in CO2 photoreduction efficiency [3], [9], [10], [11], [12].
The surface chemistry of CO2 suggests that two types of surface species for the adsorption of CO2 exist on the surface of TiO2, molecularly adsorbed CO2 and surface carbonates [13], [14]. They are easily desorbed from the clean and hydrated TiO2 surfaces at room temperature due to low adsorption energy [13], [15]. Recently, the use of basic additives to improve CO2 adsorption has attracted some attention due to the fact that CO2 is an acidic molecule [4]. Indeed, it has been found that alkali and alkaline earth metal additives such as MgO, Na2CO3, and NaOH exhibit positive effects on the photoreduction of CO2 [3], [9], [10], [16]. Meng et al. pointed out the modification of TiO2 with NaOH can promote CO2 chemisorption and subsequent activation, thereby resulting in highly effective conversion of CO2 to CH4 [9]. Rare earth metal oxides have been widely investigated as basic promoters. Besides promoting the adsorption of CO2 [4], [17], their addition could also provide several other benefits: (1) extending the light absorption of TiO2-based catalysts to the visible region [18], [19], (2) promoting photogenerated electron–hole pair separation at ceria–titania interfaces [20], [21], [22], (3) increasing the redox capability [23], and (4) tailoring surface states of TiO2 [24] Considering these factors, rare earth metal oxides could be promising promoters for CO2 photoreduction on TiO2.
Ceria is an important representative of rare earth metal oxides, and a lot of effort has been made to apply CeO2–TiO2 composites for photocatalytic oxidation of various organic pollutants such as dyes, toluene [25], [26], [27], pesticides, acetaldehyde, or 4-chlorophenol in condensed phase [28], [29]. For example, Muñoz-Batista et al. have done systematic studies on photocatalytic degeneration of toluene over ceria–titania composites, revealing degeneration kinetics, the role of CeO2–TiO2 interface contact, and g-C3N4 modification effect [24], [25], [26]. Based on previous studies, it can be concluded that ceria–titania catalysts are advantageous in photocatalytic oxidation of organic pollutants, as compared to bare TiO2. Unfortunately, little research regarding the photocatalytic reduction of CO2 over CeO2–TiO2 photocatalysts has been reported, except for studies by Wang et al. [18], Matějová et al. [28], and Jiao et al. [30]. Wang et al. [18] and Jiao et al. [30] mainly focused on the preparation of ordered macro- and meso-porous CeO2–TiO2 materials and attributed enhanced CO2 photoreduction performance to their special composition and structure. The work by Matějová et al. [28] indicated that the introduction of ceria to TiO2 adjusted the energies of electrons and holes of the catalysts, thereby enhancing the CO2 photoreduction activity. These three studies focused on the relationship between the structural/electronic properties of the CeO2–TiO2 catalyst and its CO2 photoreduction activity. To the best of our knowledge, however, the fundamental role of ceria-tuned CO2 adsorption in its photoreduction has not yet been investigated.
Interface plays an important role in catalytic reactions [22], [24], [31]; e.g., the presence of interfaces obviously facilitates photogenerated charge separation in photocatalysis. Reducing particle size can effectively increase the interfacial areas, thus achieving desirable catalytic activity [24]. In view of this, highly dispersed CeO2 on TiO2 catalysts were prepared by a one-pot hydrothermal method in this work. Their structural, surface, and optical properties and activity for CO2 photoreduction were systematically studied. Microcalorimetric measurement and in situ infrared spectroscopy (IR) were used to reveal the strengths and states of CO2 adsorption and the course of photoreduction of CO2 with H2O vapor. The presence of CeO2 tuned adsorptive states of CO2 on catalyst surfaces in the presence of H2O, resulting in increased production of bidentate carbonate (b-CO32−) and bidentate bicarbonate (b-HCO3−) relative to monodentate carbonate (m-CO32−) (shown in Scheme 1). The two surface species could be readily transformed to surface CO2− under simulated sunlight irradiation, which is a key intermediate for CO2 photoreduction. The present work deepens the understanding of the role of ceria in CO2 photoreduction at TiO2 catalysts and the course of catalysis of CO2 photoreduction in the presence of H2O vapor.
Section snippets
Synthesis of photocatalysts
Highly dispersed CeO2 on TiO2 photocatalysts was prepared through a one-pot hydrothermal method using titanium (IV) bis (ammonium lactate) dihydroxides (TALH; AR, Alfa Aesar) as Ti source and Ce(NO3)3·6H2O (AR; Aladdin) as Ce source, respectively. In detail, a desired amount of TALH and Ce(NO3)3·6H2O was dissolved into 120 mL of distilled water in the presence of 0.1 g polyethylene glycol (PEG, Mw = 6000; AR, Aladdin). The solution was transferred to a 175 mL Teflon-lined stainless steel autoclave,
Textural and structural properties
Fig. 1 shows the N2 adsorption–desorption isotherms and pore size distribution curves of as-prepared TiO2, 0.1 CeO2/TiO2, 0.2 CeO2/TiO2, and 0.4 CeO2/TiO2 photocatalysts. Their compositions, BET surface areas, and pore structure parameters are summarized in Table 1. All of the photocatalysts exhibit type-IV adsorption with a hysteresis loop of type H1, indicating the existence of mesopore structure. A relatively broad pore size distribution is observed for bare TiO2, and it narrows down
Conclusions
Highly dispersed CeO2-on-TiO2 photocatalysts were facilely prepared by a one-pot hydrothermal method. The addition of CeO2 extends the light absorption of the resultant photocatalyst to the visible region and facilitates the photogenerated charge separation, which could be attributed to the presence of Ce3+ in CeO2/TiO2 hybrids.
Monodentate carbonate (m-CO32−), bidentate carbonate (b-CO32−), and bidentate bicarbonate (b-HCO3−) are found to be the main surface species for the coadsorption of CO2
Acknowledgments
Financial support by the National Nature Science Foundation of China (Grant No. 21473248), the CAS Hundred Talents Program, the CAS-SAFEA International Partnership Program for Creative Research Teams, and the CAS “Western Light” Program (XBBS201408) is gratefully acknowledged.
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