Facile synthesis of Mo-doped TiO2 for selective photocatalytic CO2 reduction to methane: Promoted H2O dissociation by Mo doping

Journal of CO₂ Utilization 38 (2020) 1–9

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Facile synthesis of Mo-doped TiO2 for selective photocatalytic CO2 reduction to methane: Promoted H2O dissociation by Mo doping

T

Shuaijun Feng, Jie Zhao*, Yujie Bai, Xinxin Liang, Ting Wang, Chuanyi Wang School of Environmental Sciences and Engineering, Shaanxi University of Science & Technology, Xian, Shaanxi, 710021, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Photocatalytic CO2 reduction CH4 selectivity Mo doping H2O dissociation

Noble-metal co-catalysis could increase the CH4 product selectivity for photocatalytic CO2 reduction by TiO2. However, the high cost and scarcity of noble metals are key obstacles for their extensive use. Thus, it is necessary to explore cheap alternatives. Herein, Mo-doped TiO2 with different Mo concentrations were successfully prepared via a one-pot hydrothermal method at 473 K. The activity measurements of the photocatalysts show that the CH4 selectivity is increased with the Mo concentrations increasing and reaches 54.1 % at a Mo concentration of about 0.3 wt%. With further increasing the Mo concentrations, the CH4 selectivity begins to decrease. Transient photocurrent and fluorescence emission spectroscopy measurements, IR spectra for adsorbed D2O and ·OH trapping suggest that the promoted electron-hole separation and proton supply are together responsible for the enhanced CH4 selectivity. Moreover, photocatalytic CO, formaldehyde, and methanol reductions with H2O and ·CH3 trapping show that the enhanced CH4 selectivity is not directly related with the interaction of CO2 with catalyst surfaces. These reaction results also imply that photocatalytic CO2 reduction might follow the fastdeoxygenation pathway over TiO2-based catalysts. The present work supplies a guide to explore cheap modifiers for selective photocatalytic CO2 reduction to CH4.

1. Introduction Photocatalytic CO2 reduction with H2O vapor on chemically stable and cheap TiO2 has gained extensive attention because it is a promising “green chemistry” strategy for direct conversion of CO2 to fuels or value-added chemicals driven by sunlight. [1,2] Both liquid-(HCOOH, CH3OH, etc.) and gas-phase (CH4) products have been monitored in liquid-solid systems where TiO2 is dispersed in CO2-saturated aqueous solutions, [3–5] while CO and CH4 are commonly produced at the solid–gas interface of TiO2 and reactants. [6–8] The solid-gas reaction mode obviously facilitates the separation of TiO2 and products and industrial amplification. However, it suffers from low selectivity to the CH4 product that has quite high heat of combustion, owing to the fact that CH4 generation requires eight photoinduced electrons and eight protons, involving multiple reaction steps. [9,10] Several strategies were applied to enhance the CH4 selectivity over TiO2, such as surface modification, [11] heterojunction constructing [5], metal co-catalysis [12], and so on [13–15]. Among them, metal cocatalysis has been a popular technique since Kraeutler et al. first introduced Pt on TiO2 for photocatalytic CO2 reduction in 1978. So far, the employment of metal co-catalysts mainly focused on noble metals (such as Pt, Pd, Ru, Au, Ag and Ir) because of high chemical stability in ⁎

oxidizing atmospheres. [16–19] Nevertheless, the high cost and scarcity of noble metals are key obstacles for their extensive use [20]. As such, it is necessary to explore cheap alternatives to noble metals for selective photocatalytic CO2 reduction to CH4 [21,22], which requires understanding of the mechanism of promoting CH4 production by noble metals. Wang and co-workers research suggested that the efficient electron − hole separation by ultrafine Pt nanoparticles was the main reason attributable for the CH4-yield enhancement over Pt/TiO2; [23] it has been also reported by Dong and co-workers that the terrace sites of Pt nanoparticles are served as the active sites for methane generation, while the low-coordinated sites are more favorable in the competing hydrogen evolution reaction [24]. In our previous study, it was found that low H2O dissociation barrier on Pt play a key role in the formation of CH4. H2O dissociation on Pt nanoparticles supplies sufficient and readily available protons for CH4 formation in the photocatalytic CO2 reduction. [25] Thus, the substance capable of strongly dissociating adsorbed water molecules might be a good alternative to previous metals. Recently, it was reported that ultra-small MoOx clusters as a cocatalyst of CdS nanowires can effectively activate the adsorbed water molecules, which is one of the key reasons for enhancing the photocatalytic hydrogen evolution activity, [26] so it was speculated that

Corresponding author. E-mail address: [email protected] (J. Zhao).

https://doi.org/10.1016/j.jcou.2019.12.019 Received 1 September 2019; Received in revised form 30 November 2019; Accepted 24 December 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.

Journal of CO₂ Utilization 38 (2020) 1–9

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infrared (FTIR) spectra were recorded with a Brucker (Verte70 V) spectrophotometer (MCT detector) in the range 4000–400 cm−1 with a resolution of 4 cm−1. Electron paramagnetic resonance (EPR) spectra were obtained by an E-500 CW EPR (293 K, 9.866 GHz, X-band). EPR measurements for the photocatalysts were carried out by using a homemade cell. Prior to the measurements, the photocatalysts were treated at 473 K under vacuum for 1 h to remove adsorbates. Spin trapping experiments were standardized by the amounts of the photocatalyst, dispersant and trapping agent.

MoOx is a suitable modifier for selective photocatalytic CO2 reduction to CH4. Additionally, doping is a good strategy to obtain single MoOx or ultra-small MoOx clusters, being able to enhance the utilization efficiency of MoOx. Up to now, Mo-doped TiO2 samples, usually prepared at high temperatures and/or with multistep process, have been reported to apply to photocatalysis. [27–33] However, to the best of our knowledge, very few studies focused on photocatalytic CO2 reduction over Mo-doped TiO2. More importantly, it is unclear how Mo doping impacts the photocatalytic CO2 reduction over TiO2. In this work, we report the successful fabrication of Mo-doped TiO2 photocatalysts via a one-pot hydrothermal method at a relatively low temperature (200 ℃). The nature of MoOx is determined via X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The surface structure of photocatalysts and the adsorption of reactants were probed by electron paramagnetic resonance and Fourier transform infrared spectroscopy. Reasons for the enhanced CH4 selectivity over Mo-doped TiO2 were elucidated, supported by optical characterization, surface-reactant interaction investigation and activity texts.

2.3. Photoactivity tests Photocatalytic reduction of gas-phase CO2 with H2O vapor was carried out in a vacuum reactor equipped with a cooling system and a 300 W Xe lamp, while the whole reaction process was maintained at 298 K. Generally, TiO2 or Mo-doped TiO2 powder was sprinkled on a circular quartz plate with a diameter of 50 mm. Prior to light irradiation, the vacuum reactor system was injected with CO2 (20 ml) and H2O (20 μl). The reaction products were monitored at a 60 min interval by an online gas chromatograph (GC, SP-3420A, BFRL) equipped with a TDX-01 column and a flame ionization detector (FID) for CO and CH4 products and a thermal conductivity detector (TCD) for O2 analysis. CH4 selectivity was obtained from equation 1, where ni is the yield of each product. Si is the selectivity of each product.

2. Experimental section 2.1. Synthesis of the photocatalysts Bare TiO2 and Mo-doped TiO2 were prepared via a one-step hydrothermal method using dihydroxy bis (ammonium lactato) titanium (IV) (TALH; AR, Alfa Aesar) and (NH4)6Mo7O24·4H2O (AR; Sinopharm Chemical Reagent Co, Ltd) as Ti source and Mo source, respectively. In details, 0.5 mL of TALH and a desired amount of (NH4)6Mo7O24·4H2O were dissolved in 35 mL of deionized water under stirring, then the solutions were transferred into 50 ml Teflon-lined stainless steel autoclaves, which were sealed and placed in an electric oven at 200 ℃ for 12 h with a heating rate of 8 ℃/min. After being cooled to room temperature naturally, the as-formed precipitates were collected by centrifugation, washed thoroughly with deionized water, and then dried at 80 °C. The as-prepared photocatalysts with Mo concentrations 0, 0.112 %, 0.287 % and 0.536 %, determined by an inductively coupled plasma-atomic emission spectrometer (ICP-AES), were denoted as TiO2, 0.1 %Mo/TiO2, 0.3 %Mo/TiO2 and 0.5 %Mo/TiO2, respectively.

Si= ni/∑ni

(1)

3. Results and discussion 3.1. Structure and surface N2 adsorption-desorption isotherms and pore size distribution curves of the TiO2, 0.1 %Mo/TiO2, 0.3 %Mo/TiO2 and 0.5 %Mo/TiO2 samples are shown in Fig. 1 A and B. All the isotherms are of the type-IV adsorption with a loop ring of type-H3 and there are no obvious saturation adsorption platforms, indicating the presence of irregular mesopores in the samples. [35] The pore size distributions of the photocatalysts are relatively concentrated with the most probable pore sizes at around 10 nm (Fig. 1B). The BET surface area and pore volume of TiO2 were measured to be 119 m2/g and 0.32 cm3/g. The addition of Mo increases the surface area and pore volume but has no influence on the average pore size of the samples (Table 1). TEM and EDS mapping images of a representative photocatalyst 0.3 %Mo/TiO2 are shown in Fig. 2. Spherical and square nanocrystals are observed with a particle size of around 10 nm (Fig. 2A). EDS mapping analysis shows that element Mo was uniformly dispersed on the surface of TiO2. (Fig. 2 B–D) Fig. 1C shows the characteristic powder-XRD patterns of the samples. The diffraction peaks around 2θ of 25.3°, 38.0°, 47.9°, 53.9°, 55.1°, 62.8°, 68.7°, 70.4° and 75.2° correspond to the crystal planes (101), (004), (200), (105), (211), (204), (116), (215) of anatase TiO2 (JCPDS Card No. 21-1272), respectively. [14,36] No other phases were detected for all the samples. Based on the Scherrer equation and the broadening of the (101) peak, the average crystallite sizes of anatase TiO2 were calculated to be 13.8, 12.8, 12.7 and 12.8 nm for TiO2, 0.1 %Mo/TiO2, 0.3 %Mo/TiO2 and 0.5 %Mo/TiO2, respectively (shown in Table 1). Clearly, the addition of element Mo slightly decreases the crystallite sizes, in accord with the specific surface area measurements, which can be explained by the reason that the external Mo ions retard the growth of TiO2 crystallites during the preparation. [30] The lattice parameters, an important indicator of the structural response to the dopant dissolution, are derived from XRD patterns and listed in Table 1. The a and c axial lengths of the TiO2 sample are 3.7864 and 9.5047 Å, respectively. The a-axial lengths of TiO2 in the Mo/TiO2 samples are comparable to that of the TiO2 sample, while the c-axial lengths and c/a ratios have been reduced, reflecting the lattice contraction of TiO2.

2.2. Photocatalyst characterization The surface area and pore size were determined using a Micromeritics ASAP 2460 automatic adsorption analyzer. The pore size distribution curves were derived from the adsorption branch of isotherms. X-ray diffraction (XRD) patterns of the samples were collected under ambient atmosphere by a Bruker D8 powder diffractometer with an operating voltage of 40 kV and current of 30 mA, while the wavelength of monochromatized Cu Kα radiation was 0.15406 nm. X-ray photoelectron spectroscopy (XPS) measurements were performed using a VG Microtech MT500 with a Mg-K _ X-ray source. The adventitious C 1s peak at 284.6 eV was used as an internal standard to compensate for the charging effect. Transmission electron microscopy (TEM) characterizations were performed on a JEOL-JEM 2100 electron microscope. UV–vis diffuse reflectance spectroscopy (DRS) over a range of 200–800 nm was recorded on a UV–vis spectrophotometer (Cary 5000) with an integration sphere diffuse reflectance attachment. Raman and fluorescence spectroscopy was measured with HORIBA Raman LabRAM HR Evolution (at 532 nm, grating 600 grooves/mm, spectral resolution of 0.35 cm−1). Electrochemical measurements were performed in three electrode quartz cells with a 0.1 M Na2SO4 electrolyte solution. A platinum electrode was used as the counter electrode, and a saturated calomel electrode (SCE) was used as the reference electrode. The working electrodes were prepared according to a previously reported protocol. [34] A 300 W xenon lamp was used as a light source, and it was 15 cm away from the working electrodes. Fourier transform 2

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Fig. 1. Adsorption isotherms (A), the corresponding pore size distributions (B) and XRD patterns (C) for the TiO2 (a), 0.1 %Mo/TiO2 (b), 0.3 %Mo/TiO2 (c) and 0.5 % Mo/TiO2 (d) photocatalysts. (D) Mo 3d core level XPS spectrum of 0.3 %Mo/TiO2.

increasing the Mo concentrations, accompanied by the broadening of full width at the half-maximum (HWHM). Several effects could cause the variation of position and width of this peak, such as defects in the stoichiometry, [43] phonon confinement in nanoparticles, extrinsic doping [44], pressure and temperature effects. A research reported by Li Bassi and co-worker suggested that the stoichiometry defects in anatase TiO2 contributed to the peak blue-shift, while the peak width seems to be less affected by the oxygen concentration, [43] which is inconsistent in the case of the present work. Based on the fact that the crystallite sizes of the three Mo-doped samples are almost equal, it can be speculated that the incessant blue-shift and broadening of the Eg(1) peak with the Mo concentrations increasing should not be assigned to phonon confinement. Additionally, the pressure and temperature effects can also be ruled out due to the same conditions for Raman analysis. Thus, Mo doping is most likely responsible for the blue-shift and broadening of the Eg(1) peak. Mo doping causes the lattice construction, evidenced by XRD analysis, thereby reinforcing the O-Ti-O bending vibration. Based on the results above, it can be concluded that Mo-doped TiO2 photocatalysts were successfully fabricated by a one-pot hydrothermal method.

Table 1 Structural and Lattice parameters of the TiO2, 0.1 %Mo/TiO2, 0.3 %Mo/TiO2, and 0.5 %Mo/TiO2 photocatalysts. Samples

a b c d

d

Surface area (m2/g)

Pore volume (cm3/g)

Average pore size (nm)

Anatase

Lattice parameters

(nm)

aÅ

cÅ

c/a

119 140 145 143

0.32 0.38 0.40 0.36

10.6 10.8 11.1 10.1

13.8 12.8 12.7 12.8

3.7864 3.7901 3.7854 3.7866

9.5047 9.4624 9.4748 9.4316

2.510 2.496 2.503 2.491

a

a Particle size of anatase TiO2 as calculated according to the Sherrer’s equation.

According to Hume-Rothery’s rules for metallic substitutional doping, substitutional doping is favored when the dopant radius varies by < 15 % of that of the matrix cation. [37] Mo 3d core level XPS spectrum of 0.3 %Mo/TiO2 shows that there are Mo6+ and Mo5+ near the surface (Fig.1D). [38] Mo6+ radius (0.073 nm) is slightly smaller while Mo5+ (0.075 nm) is slightly larger than that of Ti4+ (0.0745 nm). [37] This suggests that Mo substitutional doping is highly likely in the Mo/TiO2 samples. From size considerations alone, Mo5+ substitution for Ti4+ would be expected to cause lattice expansion of TiO2. Mo6+ substitution would give rise to lattice construction. From valence considerations alone, Mo6+ and Mo5+ with a relatively higher electronegativity would be expected to reduce both the a − b plane and c-axis. Because the a − b plane is tightly packed, the a-axial is hardly effected. [31,37,39] Whereas the c-axis is shortened by the valence effect. The Raman data is shown in Fig. 3. Five typical vibrational modes at around 144 cm−1 (Eg(1)), 197 cm−1 (Eg(2)), 399 cm−1 (B1g), 516 cm−1 (A1g+B1g(2)) and 639 cm−1 (Eg(3)) assigned to anatase TiO2 are observed for all the photocatalysts. [40] Among them, the Eg(1) peak is the most intense and the most investigated in the literature, [41] which corresponds to the O-Ti-O bending type vibration [42]. The Eg(1) peak appears to be increasingly shifted toward higher wavenumbers with

3.2. Optical properties The UV–vis absorption spectra of the TiO2 and Mo-doped TiO2 photocatalysts are shown in Fig. 4A. The TiO2 photocatalyst only absorbs the light with wavelength lower than 400 nm, in agreement with previous reports. [45] After Mo doping, the photocatalysts appear to absorb visible light and the visible-light absorption slightly increases with the Mo concentrations increasing, which can be attributed to the transition from O 2p to Mo 4d and the intra-band transition of between Mo 4d and Ti 3d states [30]. The transient photocurrent measurements and fluorescence emission spectroscopy were used to probe the efficiency of photoinduced charge-carrier separation of the photocatalysts. As shown in Fig. 4B. Each electrode exhibits a current response to light, 3

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Fig. 2. TEM and EDS mapping images of a representative photocatalyst 0.3 %Mo/TiO2.

the lowest for 0.1 %Mo/TiO2, further confirming the highest efficiency of the photoinduced charge-carrier separation. Theoretical calculation indicates that Mo doping induces some splits in VB and CB because of the breaking of symmetry of TiO2 lattice, thereby facilitating the separation of electrons and holes, [30] whereas the excessive Mo6+ ions would be served as a carrier recombination center, [29,47] which explain the observation that the efficiency of the electron-hole separation is the highest at the Mo doping concentration of about 0.1 wt% but begins to decrease with further increasing the Mo concentrations. The improved charge transport of the Mo-doped TiO2 photocatalysts can be very classically confirmed by the decreased hemicycle radius measured by using electrochemical impedance spectroscopy (EIS), due to the presence of electron donor Mo5+ (Fig. 4D). 3.3. Photocatalytic CO2 reduction with H2O vapor Photocatalytic CO2 reduction was carried out in a reactor equipped with an online gas chromatograph to monitor products. In addition to oxygen, CO and CH4 were found to be the main products for CO2 reduction, no other carbon-containing products were detected by GC, in agreement with previous reports. [35] The averaged activities for three independent measurements of the samples are provided in Fig. 5. CO and CH4 yields increase with irradiation time. At the first hour, CO yield is the lowest for TiO2, but it is dramatically increasing. At the fourth and fifth hours CO yield for TiO2 is already greater than for 0.1 %Mo/ TiO2 and 0.5 %Mo/TiO2 photocatalysts. CH4 yield for TiO2 is always lower than for Mo-doped photocatalysts in the whole tests. It is known that the reduction of CO2 to CO requires two photoinduced electrons and two protons and that CH4 production needs eight photoinduced electrons and eight protons, involving multiple reaction steps. Because the photoinduced electrons of TiO2 mainly serve for CO production, the CO yield growth is relatively faster with irradiation time. At the fifth hours, the mean CO yields were detected to be 29, 26, 41 and 23 μmol/ g and the mean CH4 yields were 4, 18, 49 and 11 μmol/g for TiO2, 0.1 % Mo/TiO2, 0.3 %Mo/TiO2 and 0.5 %Mo/TiO2, respectively. Based on carbon conservation, the conversion of CO2 is calculated to be about 33,

Fig. 3. Raman spectra of TiO2, 0.1 %Mo/TiO2, 0.3 %Mo/TiO2, and 0.5 %Mo/ TiO2.

and the current rapidly decreases as soon as the light is turned off. The photocurrent density is the lowest for the TiO2 photocatalyst. 0.1 %Mo/ TiO2 exhibits the highest photocurrent density, indicating the highest photoinduced charge-carrier separation efficiency. With further increasing the Mo concentrations, the photocurrent density gradually decreases. Fluorescence emission spectra of the photocatalysts are shown in Fig. 4C. The weaker fluorescence emission peaks, the higher photoinduced charge-carrier separation efficiency. The peak centered at around 396 cm−1 is assigned to the electron transfer between the conduction band and the valence band. [46] The peak height first decreases and then increases with the Mo concentrations increasing. It is 4

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Fig. 4. UV–vis diffuse reflectance spectra (A), transient photocurrent (B) conducted at 0.2 V vs. a saturated calomel electrode (SCE), room temperature steady-state PL spectra (C) and electrochemical impedance spectroscopy (EIS) Nyquist plots (D) for the photocatalysts.

Fig. 5. Catalytic performance of the photocatalysts under light irradiation with a Xe lamp: A) CO yield with irradiation time, B) CH4 yield with irradiation time, C) CH4 selectivity after 5 h of light irradiation, D) CH4 selectivity at the CO2 conversion of 33 μmol. 5

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44, 90 and 34 μmol after 5 h of irradiation. Fig.5C shows the CH4 selectivity over the photocatalysts after 5 h of light irradiation. it is found that the CH4 selectivity is increased with the Mo concentrations increasing and reaches 54.1 % at a Mo concentration of about 0.3 wt%. With further increasing the Mo concentrations, the CH4 selectivity begins to decrease. It is better to compare a product selectivity at the same level of the transformation of reactants. Therefore, the CH4 selectivity at the CO2 conversion of 33 μmol over the photocatalysts is supplied in Fig. 5D. Here, 33 μmol of CO2 transformation is derived from CO and CH4 yields according to carbon conservation. The CH4 selectivity was calculated to be 12.8 %, 40.6 %, 46.2 % and 30.3 % for TiO2, 0.1 %Mo/ TiO2, 0.3 %Mo/TiO2 and 0.5 %Mo/TiO2, respectively. Wang and coworkers reported that the CH4 selectivity can exceed 95 % over ultrafine Pt Nanoparticles on TiO2 single crystals. [23] Therefore, there is room for CH4 selectivity improvement over Mo-doped TiO2 by material structure and surface regulation. It is well known that the photoinduced electron-hole separation is a prerequisite for photocatalysis. The 0.1 %Mo/TiO2 photocatalyst exhibits the highest efficiency of photoinduced charge carrier separation, but the activity and selectivity to CH4 are not the highest, indicating that there should be other factors controlling photocatalytic CO2 reduction to CH4 over Mo-doped TiO2.

Fig. 7. CH4 yields for photocatalytic CH3OH, HCHO, and CO reductions over TiO2 and 0.3 %Mo/TiO2 after 5 h of light irradiation.

leading to the transform of Ti3+ to Ti4+ according to the Eq. 2. In sum, Mo doping overtly changes the surface structure of TiO2 and adjusts the adsorption of reactants on the surface. Mo6+ + Ti3+→Mo5+ + Ti4+

(2)

To identify whether Mo doping adjusts the interaction of CO2 with catalyst surfaces leading to the higher selectivity to CH4 over Mo-doped TiO2, Photocatalytic CO, formaldehyde, and methanol reductions with H2O vapor were carried out over TiO2 and 0.3 %Mo/TiO2 under the same conditions with CO2 photoreduction. These reactions were chosen because it was considered by many theoretical calculations that these C1 molecules were intermediates for photocatalytic CO2 reduction. [54] As shown in Fig. 7, CH4 yields over 0.3 %Mo/TiO2 are higher than that over TiO2 regardless of which C1 molecule is served as the reactant, implying that the enhanced CH4 selectivity over Mo-doped TiO2 might not relate with the surface−CO2 interaction. So far, two pathways for photocatalytic CO2 reduction were proposed according to whether the hydrogenation or the deoxygenation process is faster. [54–56] In the fast-hydrogenation pathway, CO2 is reduced along the path CO2→ HCOOH→CH2O→CH3OH→CH4; and in the fast-deoxygenation pathway, it follows the path CO2→CO→C·→CH2·→CH3·→CH4. Very little CH4 was produced when CH3OH and CH2O were served as reactants, whereas CH4 yields derived from CO reductions are observably higher than those from CO2 reductions. The results show that photocatalytic CO2 reduction appears to follow the fast-deoxygenation pathway over TiO2 and Mo-doped TiO2, which will be further evidenced by EPR spin trapping of CH3· in a NaCO3 aqueous solution. In the liquid-solid photocatalytic CO2 reduction system, where CO2 was dissolved in photocatalyst-dispersed aqueous solutions, carbonates/bicarbonates are the actual reactants in the overall reaction mechanism. [57] Thus, sodium carbonate solution was used to simulate the liquid-solid CO2 reduction system in the work, and the spin-trapping technique was applied to monitor possible radicals. Fig. 8 presents EPR spectra recorded after 1 min of illumination of the photocatalyst dispersions containing DMPO as a spin trap. The signals for DMPO−CH3 adducts were observed in the presence of sodium carbonate, [57] proving that CH3· is crucial intermediate for CH4 generation. Note that the signals for DMPO−OH adducts were also detected, suggesting that H2O competes with carbonate for photogenerated holes. According to signal intensity for DMPO−CH3 adducts, the amount of ·CH3 generated first increases and then decreases with the Mo doping concentrations increasing, in the same order as CH4 yield and selectivity. The result further implied that the enhanced CH4 did not depend on the interaction between CO2 molecules and catalyst surfaces. 0.5 %Mo/TiO2 exhibits the lower ·CH3 production than 0.3 %Mo/TiO2, due to the decreased electron-hole separation. To examine the interaction of H2O molecules with catalyst surfaces, the Fourier infrared spectra of absorbed D2O on the surface of the

3.4. Promoting mechanism for CH4 production In order to explore other possible reasons for the enhanced CH4 selectivity over Mo-doped TiO2, EPR measurements of TiO2 and 0.3 % Mo/TiO2 before and after the adsorption of CO2 and H2O were carried out at 300 K. As shown in Fig. 6, a relatively stronger signal at g = 1.964 is observed for TiO2 before the adsorption of reactants, which can be assigned to the Ti3+ ions. [48,49] A weak signal at g = 2.001 corresponds to oxygen vacancies in the lattice of TiO2. [49,50] After the adsorption of reactants, the Ti3+ signal almost disappears, indicating that Ti3+ spin centers locate at the surface. Whereas a new signal at g = 2.08 appear, corresponding to oxygen vacancies at the surface of TiO2 [51]. These results suggest that Ti3+ spin centers transform to oxygen vacancy spin centers after the adsorption of reactants. Note that the reactant adsorption has no influence on oxygen vacancies in the lattice. As for 0.3 %Mo/TiO2 before the adsorption of reactants, two differentiable signals can be assigned to Mo5+ with the set of g values g⊥ = 1.899 and g∥ = 1.928, [52,53] which are observably weakened after the adsorption of reactants. At the same time, a new signal at g = 1.941 is observed, probably due to the formation of Ti3+ spin centers near Mo ions. It should be note that there is no Ti3+ at the native surface of 0.3 %Mo/TiO2, implying that the presence of Mo suppresses the Ti3+ generation during the preparation. This can be attributed to the higher electronegativity of Mo (Ⅵ) than Ti (Ⅲ),

Fig. 6. EPR spectra of TiO2 and 0.3 %Mo/TiO2 before (a, c) and after (b, d) the adsorption of reactants (H2O and CO2). 6

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Fig. 8. EPR spectra recorded after 1 min of illumination of TiO2 (a), 0.1 %Mo/ TiO2 (b), 0.3 %Mo/TiO2 (c), and 0.5 %Mo/TiO2 (d) in 0.5 M aqueous Na2CO3 using DMPO as a spin trap.

Fig. 10. EPR spectra recorded after 1 min of illumination of TiO2 (a), 0.1 %Mo/ TiO2 (b), 0.3 %Mo/TiO2 (c), and 0.5 %Mo/TiO2(d) in water using DMPO as a spin trap.

depends on the water oxidation step. Thus, the amount of ·OH can be also used to assess the proton supply for CO2 reduction. Herein, DMPO was used as the trapping agent for the detection of •OH in water and the results are displayed in Fig. 10. The ESR signals—a 1:2:2:1 quartet pattern, with a splitting of about 1.5 m T characteristic of a DMPO−•OH adduct—are clearly observed. [52] According to the signal intensity, it can be inferred that the production of •OH were promoted by Mo doping, and 0.3 %Mo/TiO2, whose efficiency of photoinduced charge-carrier separation is not the highest, exhibits the largest •OH production, being consistent with the case of CH4 production from CO2 reduction. This result also indicates that H2O activation by Mo doping plays the key role of H2O oxidation that is the semireaction for photocatalytic CO2 reduction with H2O. On the other hand, 0.5 %Mo/TiO2 has a higher ability to H2O activation, but •OH production is lower than 0.3 %Mo/TiO2. This is attributed to the decreased photoinduced electron-hole separation.

Fig. 9. FTIR spectra for D2O adsorbed on the surface of the TiO2, 0.3 %Mo/TiO2 and 0.5 %Mo/TiO2 photocatalysts at room temperature.

H2O + h+→H+ + ·OH

(3)

H2O + 2h →2H

(4)

+

photocatalysts were conducted, where D2O was selected to eliminate interference from H2O vapor in air and OH groups at the catalyst surface. The self-supporting wafers (15 mg) were first annealed in an IR cell at 473 K under vacuum for 2 h to remove the adsorbed impurities. After the IR cell was cooled naturally, D2O vapor were introduced into the cell. Prior to the measurements, the IR cell was evacuated for 15 min to remove gaseous- and physically adsorbed-D2O molecules. Fig. 9 shows IR spectra for D2O adsorbed on the photocatalysts; the band of the photocatalysts were used as the background. A peak observed at 2720 cm−1 can be assigned to DO groups derived from the dissociative adsorption of D2O. [58–61] The peak of DO group was not observed for bare TiO2, and the peak height increases with increasing the Mo concentrations. The results show a weak ability to dissociate H2O molecules over bare TiO2, in agreements with previous reports. [62] In contrast, Mo doping promotes the H2O dissociation and strengthens the interaction of H2O with catalyst surfaces. The peak height of DO group for 0.5 %Mo/TiO2 is higher than that for 0.3 %Mo/ TiO2, suggesting the more amount of D2O dissociated. Note that the peak of DO group was not observed for 0.1 %Mo/TiO2(not shown in Fig. 9). This is because the amount of DO group generated on 0.1 %Mo/ TiO2 goes beyond the detection limit of the infrared spectrometer. Theoretical calculation shows that the adsorption free energy of H2O over MoOx clusters exceed -2.0 eV, implying the high ability to activate H2O molecules, [26] which supports the results of this work. In the photocatalytic CO2 reduction with H2O, H2O supply protons for CH4 generation, accompanying water oxidation to produce hydroxyl radicals or O2 (equ. 3–4). As such, the efficiency of proton generation

+

+ 1/2O2

In the CH4 formation, CO2 molecules are first adsorbed on the surface of catalysts and react with photoinduced electrons, forming activated adducts. The adducts deoxygenate and were bonded with protons to generate CH3· and then CH4, illustrated in Scheme 1. The adduct generation depends on the photoinduced electron-hole separation. The higher electron-hole separation efficiency, the more adducts

Scheme 1. possible pathway for photocatalytic CO2 reduction over Mo-doped TiO2. 7

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generated. On the other hand, if the adducts cannot be bound to protons in time, they will deactivate via vibration relaxation or heat inactivation. In other words, the CH4 selectivity and yield are at least dominated by the two factors of the proton supply and the electron-hole separation. The ability of Mo-doped TiO2 to H2O activation and dissociation increases with increasing the Mo doping concentrations, whereas the electron-hole separation efficiency first increases and then decreases with the Mo doping concentrations increasing. As such, there is an optimal Mo doping concentration for CH4 generation. In the work, 0.3 wt% of the Mo doping concertation is the best for CH4 generation. Although 0.1 %Mo/TiO2 has the highest electron-hole separation efficiency, the ability to H2O activation and dissociation is relatively weaker, leading to a relatively lower CH4 selectivity. Whereas the lower electron-hole separation efficiency of 0.5 %Mo/TiO2 results in a decrease in CH4 selectivity.

[5]

[6]

[7]

[8]

[9]

[10]

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4. Conclusion [12]

In summary, Mo-doped TiO2 photocatalysts were successfully prepared via a one-pot hydrothermal method at 473 K, characterized by XRD and Raman spectroscopy techniques revealing the lattice contraction of anatase TiO2. Element Mo exists in the form of Mo (Ⅵ) and Mo (Ⅴ), which suppresses the formation of Ti3+ at the surface of Modoped TiO2. The CH4 selectivity is increased with the Mo concentrations increasing and reaches 54.1 % at a Mo concentration of about 0.3 wt%, which can be attributed to the promoted electron-hole separation and proton supply. With further increasing the Mo concentrations, the CH4 selectivity begins to decrease due to a decrease in the electron-hole separation efficiency. Photocatalytic CO, formaldehyde, and methanol reductions with H2O and ·CH3 trapping indicate that the enhanced CH4 selectivity over Mo-doped TiO2 is not directly related with the interaction of CO2 with catalyst surfaces. These reaction results also imply that photocatalytic CO2 reduction over TiO2based catalysts might follow the fast-deoxygenation pathway (eg. CO2→CO→C·→CH2·→CH3·→CH4). The present work supplies a guide to explore cheap modifiers for selective photocatalytic CO2 reduction to CH4.

[13] [14] [15]

[16]

[17]

[18] [19]

[20]

[21]

Author contribution statement

[22]

Shuaijun Feng & Jie Zhao conceived the idea and co-wrote the paper. Shuaijun Feng, Yujie Bai & Ting Wang carried out the sample synthesis, characterization and CO2 reduction meansurement. Xinxin Liang and Chuanyi Wang interpreted the data.

[23]

[24]

Declaration of Competing Interest [25]

The authors declare no competing interests. [26]

Acknowledgments

[27]

Financial support by the National Nature Science Foundation of China (Grant No. 21976116), and the Youth Talent Support Program of Shaanxi University of Science & Technology is gratefully appreciated.

[28]

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