Eu-doped TiO2 nanoparticles with enhanced activity for CO2 phpotcatalytic reduction

Journal of CO₂ Utilization 26 (2018) 487–495

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Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou

Eu-doped TiO2 nanoparticles with enhanced activity for CO2 phpotcatalytic reduction ⁎

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Chun-ying Huanga, Rui-tang Guoa, , Wei-guo Pana, , Jun-ying Tangb, Wei-guo Zhoub, , Hao Qina, Xing-yu Liua, Peng-yao Jiaa a b

College of Energy and Mechanical Engineering, Shanghai University of Electric Power, Shanghai, China College of Mechanical Engineering, Tongji University, Shanghai, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Eu-doped TiO2 Nanoparticles Europium oxide CO2reduction

The Eu-doped TiO2 nanoparticles were successfully prepared by a simple sol-gel method. The properties of the catalysts were characterized by means of X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive spectrometer (EDS), N2 adsorption-desorption, UV–vis diffuse reflectance and photoluminescence (PL) spectroscopy. The results indicated that Eu-TiO2 catalysts had a high specific surface area, enhanced visible light response and low recombination rate of electron-hole pairs. The photocatalytic performances of Eu-TiO2 catalysts for CO2 reduction were investigated under 9 h irradiation of a 300 W Xe arc lamp. The maximum yields of CH4 and CO over the optimized Eu-TiO2 catalyst were 65.53 μmol/ g.cat and 42.91 μmol /g.cat, respectively, which were approximately 13 times and 1 times higher than that of pure TiO2. The enhanced photocatalytic performance could be ascribed to the high-efficiency separation of photo-generated electron-hole pairs.

1. Introduction The atmospheric concentrations of carbon dioxide have increased significantly since the industrial revolution, which is considered as the major cause of the global warming [1–3]. In addition, the depletion of fossil fuels should be another serious problem that humanity is facing [4]. Therefore, converting carbon dioxide emitted from the combustion of fossil fuels into hydrocarbon fuels is one of the most effective ways to mitigate global warming and energy crisis [5]. During the recent decades, various semiconductors materials have been investigated in photocatalytic reduction of CO2 such as g-C3N4, TiO2, CdS, W2O3, ZnO, and GaP [6–8]. Among these materials, titanium dioxide (TiO2) is favored by many researchers due to its low cost, non-toxicity, high photostability and strong reduction properties [9–11]. However, TiO2 displays a poor photo-activity under simulated sunlight irradiation, which is due to its wide band energy (3.2 eV of anatase phase TiO2) and high electron-hole pairs recombination rate [12,13]. In order to improve the photocatalytic efficiency of TiO2, various modification techniques including doping with transition metals [14], noble metals [15] and non-metals [16] have been investigated in recent years. Transition metal doping has been extensively adopted to modify the nanostructure and optimize optical properties of TiO2 [17]. These dopants could enhance the absorption of the visible light, but the

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photocatalytic activity decreases significantly under the UV irradiation [18]. As for noble metal dopants, although they could reduce the recombination rate of photo-generated electron–hole pairs, however, the light absorption is still hardly extended into the visible light region [19]. It has been reported that TiO2 doped with non-metallic element could generate the intra-band-gap state situated closely to the edges of conduction or valence band, which is beneficial to visible light absorption [20]. Besides, these dopants are unlikely to form recombination centers and facilitate photocatalytic performance [21]. However, such valence band holes excited by visible light have lower oxidative properties than those excited by UV light, which may cause a lower photocatalytic performance [20]. Therefore, it is significant to find an effective dopant to overcome these shortcomings mentioned above. Doping TiO2 with rare earth metals has attracted wide attention in recent years due to the 4f electron configurations of rare earth metals, which could form complexes with various Lewis bases [22]. Furthermore, lanthanide ions can also prolong the life of electron-hole pairs by trapping the photo-generate electrons and its f-orbitals in oxides could provide high conductivity and thermal stability [23,24]. In recent years, rare earth metals have been successively used for increasing the catalytic activity of photocatalysts. It has been reported that incorporation of lanthanide ions such as europium (Eu) could improve the photocatalytic activity of TiO2 [25]. G. V. Khade et al. [26]

Corresponding authors. E-mail addresses: [email protected] (R.-t. Guo), [email protected] (W.-g. Pan), [email protected] (W.-g. Zhou).

https://doi.org/10.1016/j.jcou.2018.06.004 Received 8 April 2018; Received in revised form 10 May 2018; Accepted 8 June 2018 2212-9820/ © 2018 Elsevier Ltd. All rights reserved.

Journal of CO₂ Utilization 26 (2018) 487–495

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AG instrument. In order to analyze the optical absorption properties, ultraviolet visible (UV–vis) diffuse reflection spectra was conducted at room temperature using BaSO4 as a reflectance standard material within the range of 250–800 nm on a spectrophotometer (SHIMADZU UV-3600, Japan). Photoluminescence (PL) characteristics were analyzed with an excitation wavelength of 350 nm via a fluorescence spectrophotometer (Hitachi F-4600).

prepared Eu-TiO2 catalyst by microwave assisted sol–gel method. The photocatalytic activity for methyl orange degradation was improved significantly due to the higher absorption of the visible light and 4f electron transition of rare earth ions. The study conducted by Shi et al. [27] indicated that the increased photocatalytic activity of La-Eu/TiO2 catalyst in MB degradation could be ascribed to the increase of the BET surface area, the decreased crystallite size and the inhibited recombination of the electron-hole pairs. Moreover, previous studies also revealed that Eu-TiO2 catalysts prepared by a sol–gel method could greatly improve the degradation of salicylic acid owing to the presence of trap states, the enhanced absorption of visible light and the inhibited formation of rutile phase [26,28]. However, to our best knowledge, rare earth metal doped TiO2 is still not applied in CO2 photocatalytic reaction. In present study, the Eu doped TiO2 samples were prepared by a simple sol–gel method and used as photocatalyst for the reduction CO2 with H2O under irradiation of a 300 W Xe arc lamp. The surface morphological structure, structural properties, optical properties of Eu-TiO2 had been systematically studied. The possible reaction mechanism for CO2 photocatalytic reduction over Eu-TiO2 catalyst under illumination was also proposed.

2.4. Photoelectrochemical measurements Photoelectrochemical measurements, including Mott-Schottky tests, transient photo-current responses and electrochemical impedance spectroscopy analysis (EIS), were performed on a CHI 660E electrochemical workstation (CHI 660E, ShanghaiChenhua Instrument Co. Ltd. China) with a standard three-electrode system in a 0.5 M Na2SO4 aqueous solution, using Ag/AgCl and Pt plate electrodes as the reference and counter electrode, respectively. The working electrode was prepared by dipping the catalyst slurry on FTO glass (1 × 1 cm): 20 mg prepared photocatalyst was dispersed in 2 ml ethanol, then the suspension was directly added dropwise onto the FTO glass substrate, followed by air-drying. Photocurrent and EIS tests were performed under visible light irradiation using a xenon lamp of 300 W. The MottSchottky measurement was performed at two frequencies of 500 Hz and 1000 Hz respectively.

2. Experimental 2.1. Materials

2.5. Photocatalytic activity Europium nitrate (Eu(NO3)3.6H2O) was purchased from Acros Chemical Company, USA. Tetrabutyl titanate (TBT), absolute ethanol and citric acid were purchased from Sinopharm Chemical Reagent Corp, P. R. China. All these reagents were of analytical pure grade and used without further purification.

The activity of CO2 photoreduction was tested in a gas phase continuous flow reactor (Labsolar-6 A, Beijing Perfect Light Company, China). The reactor is consisted of a cylindrical container (volume: 500 cm3) and a quartz glass window (thickness: 8 mm) for irradiation. A 300 W Xenon-arc lamp served as the light source for the photoreaction progress. In a typical process, 50 mg catalyst was suspended uniformly in 100 ml deionized water at room temperature under adequate stirring. Ahead of illumination, the suspension was vacuum-treated and then the reactor was purged with 101 kPa high-purity CO2 (99.99%) controlled with a mass flow controller for 25 min to reach the adsorption-desorption equilibrium. Then the experiment was started when the light was on. The reactor temperature is maintained at 25 °C via a cooling sink. During the reaction, the products were sampled at given time intervals (one hour) and analyzed by a gas chromatograph (GC2010 Plus, SHIMADZU, Japan).

2.2. Synthesis of Eu-TiO2 The Eu-TiO2 catalysts were synthesized using the sol-gel method. In a typical preparation process, 5 ml tetrabutyl titanate was dissolved in 10 ml absolute ethanol at room temperature firstly, then 5 ml citric acid was added dropwise into the solution under vigorous stirring for 2 h to get solution A. Simultaneously, a certain amount of europium nitrate was dissolved in 10 ml absolute ethanol, followed by the addition of 2 ml distilled water and 5 ml citric acid under stirring for 2 h to get solution B. Then the solution B was slowly and uniformly added to solution A under constant stirring and aged for 2 h to hydrolyze completely. Afterwards, the obtained gel was aged at room temperature for 36 h and dried in an oven at 60 °C overnight. The white Eu-TiO2 nanoparticles were obtained by calcining the gel in air at 500 °C for 5 h. The pure TiO2 were also synthesized by a similar procedure and used as a reference catalyst. The molar ratios of Eu/Ti were set as 0, 0.15%, 0.25%, 0.4% respectively. Correspondingly, the obtained samples were named as TiO2, 0.15% Eu-TiO2, 0.25% Eu-TiO2 and 0.4% Eu-TiO2.

3. Results and discussion XRD provides information pertaining to the crystallinity and crystallite sizes of the phase for the samples. As shown in Fig. 1, all samples show sharp peaks of high diffraction, which is characteristic of small grains of the anatase crystal phase of TiO2 (JCPDS 21–1272). Besides, the XRD patterns exhibit three peaks at 2 θ = 25.2°, 37.55° and 48.1°, corresponding to (101), (004) and (200) diffraction planes of TiO2 (JCPDS Card NO.21–1272), respectively. No characteristic diffraction peaks of Eu2O3 are observed, implying the low content or good dispersion of Eu on TiO2 surface. It is also found that the diffraction peak of Eu doped TiO2 becomes broader and weaker with the increase of Eu doping content, indicating a decrease in crystallite size and the weakened agglomeration of TiO2 particles [29,30]. It is accepted that smaller crystallite size may facilitate the migration of photoelectrons to photocatalyst surface and could provide more active sites (oxygen cavities or defects), resulting in the enhanced photocatalytic activity [31]. The crystal parameters of all catalysts were calculated by DebyeScherrer equation, the results are listed in Table. 1 [32]. It can be noted that the grain size decreases gradually with the increase of Eu content, which proves that Eu could obstruct the crystallization of TiO2 [33]. The ion radius of Eu3+ (0.095 nm) is much larger than that of Ti4+

2.3. Characterization The crystallographic phase of the as-prepared catalysts was analyzed by powder X-ray diffraction patterns (XRD) on Bruker D 8 diffractometer with Cu Kα radiation. The measurement was conducted at room temperature in the 2θ range of 5-80°. X-ray photoelectron spectroscopy (XPS, ESCALAB 250 xi, USA) with Al Kα radiation source was used to analyze the surface chemical compositions. The morphologies of the prepared samples were observed by a scanning electron microscopy (SEM, Phillips XL-30 FEG/NEW) and a transmission electron microscopy (TEM, Phillips Model CM 200). The energy dispersive X-ray spectrometer (EDS) attached to the SEM was used to detect the components of samples. The Brunauer–Emmett–Teller (BET) specific surface areas, pore structures of the samples were measured by N2 adsorption-desorption analysis at 77 K on a Quantachrome Autosorb-iQ488

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position in the XPS spectra of pure TiO2. As shown in Fig. 2a, for the 0.25% Eu-TiO2 sample, the position of the characteristic doublet of Ti4+ shifted slightly to a higher binding energy about 0.3 eV. This may be due to the changes in the electron cloud density and the surrounding chemical environment of the Ti atom caused by the Eu doping, resulting in the changes of the electron binding energy [44–47]. The structure and morphology of 0.25% Eu-TiO2 catalyst were further characterized by SEM and HRTEM. Fig. 3a shows the spherical shape particles with diameters ranging in 6∼25 nm and most of the particles are evenly distributed with an average size of 15 nm. The TEM image of the 0.25% Eu-TiO2 sample (Fig. 3b) shows that the particle size is relatively uniform and the average particle size is around 12 nm, which is well consistent with the XRD results. The high distribution of Eu2O3 in the bright and dark parts of TiO2 is confirmed by the presence of electronic black spots [26,48]. The HRTEM (Fig. 3c) shows that the interplanar spacing values are 0.352 nm and 0.245 nm, which match well with the (101) and (004) planes of TiO2, respectively. Furthermore, the EDX spectrum and the element mapping of 0.25%Eu-TiO2 catalyst also were recorded and the results are presented in Fig. 3d. It can be easily found that the signals of Ti, O, Eu and C appear, where the C signal might be caused by carbon dioxide (CO2) in the air during the calcination process, which is in good accordance with the XPS analysis results. The surface characteristics of Eu-TiO2 photocatalysts were investigated using nitrogen adsorption-desorption measurement. As shown in Fig. 4, all the samples exhibit the type IV isotherms with a hysteresis loop (IUPAC), reflecting the mesoporous structure of the composites. The values of the specific surface areas, pore sizes of for all the catalysts are summarized in Table 1. Obviously, the specific surface area is greatly enhanced after Eu doping, and the maximum is 86.01 m2/g for 0.25% Eu-TiO2 catalyst, while the minimum value is only 46.28 m2/g for pure TiO2. The increase of specific surface area may provide more active sites for photocatalytic reaction. However, the specific surface area decreases with further increase of Eu content, which may be due to the blocking effect of Eu on TiO2 pores [49]. The BJH pore size distribution of all samples is displayed in Fig. 4b, revealing that all catalysts samples show a relatively narrow pore size distribution centered at 6.55 nm. The optical absorption properties of the Eu-TiO2 photocatalysts were investigated in the range of 300–600 nm. As can be seen from Fig. 5a, the absorption edges of all Eu-doped TiO2 catalysts slightly shift to the visible light region, due to the charge transfer transition between 4f electrons of Eu and the valence or conduction band of TiO2 [50]. The corresponding Tauc plots of all samples (Fig. 5b) indicate the presence of Eu has a significant effect on the band gap of TiO2 photocatalyst [51]. The band gap of all samples was calculated using Kubelka–Munk equation and listed in Table 1. It is clear that the Eu-TiO2 catalysts exhibit lower band gap energies ranging from 3.00 eV to 3.12 eV compared with the pure TiO2. To investigate the electronic properties of the samples, PL was performed under the excitation of 350 nm (Fig. 6). It is observed that all catalysts show obvious PL peaks around 420 nm due to the band-edge free exciton in TiO2 [52,53]. Noticeably, the difference in intensity of prepared samples may be ascribed to the amount of defect sites, which can trap the photoelectrons and recombine them [26]. Moreover, a new peak at 624 nm appears in the spectra of the Eu-doped samples, suggesting that the introduction of Eu into TiO2 could cause new luminescence phenomena [54,55]. The emission intensity of the peak at 624 nm seems to be proportional to the amount of Eu3+ ions in the samples, revealing that efficient energy can be transferred from TiO2 to Eu3+ ions [26]. Furthermore, the weakest photoluminescence intensity in the range of 375–550 nm is detected in the spectrum of 0.25% EuTiO2 catalyst. This indicates that Eu doping can reduce the recombination rate of electrons and holes and prolong the life of electronhole pairs [56], which is also consistent with its photocatalytic performance as described below.

Fig. 1. XRD patterns of the samples. Table 1 Textural and optical properties for pure and Eu doped TiO2 photocatalysts. Catalyst

TiO2 0.15%EuTiO2 0.25%EuTiO2 0.4% EuTiO2

Textural

Crystallite size(nm)

Band gap (eV)

BET surface area (m2g−1)

Average pore diameter (nm)

pore volume (cm3g−1)

46.28 80.07

6.55 6.56

0.0883 0.1681

16.2 12.9

3.22 3.12

86.01

6.54

0.1676

11.2

3.00

75.48

6.56

0.1431

9.0

3.05

(0.061 nm), therefore, it is difficult for the replacement Ti4+ in the TiO2 lattice by Eu3+ [34,35]. It can be inferred that most Eu3+ ions are dispersed on the surface of TiO2, which is consistent with the XRD results. The chemical states of component elements in TiO2 and 0.25% EuTiO2 samples were investigated by XPS. The XPS survey spectra of TiO2 and 0.25% Eu-TiO2 composite are depicted in Fig. 2a. It can be clearly seen that the Ti 2p, O 1 s and C 1 s signals are detected in both TiO2 and 0.25% Eu-TiO2 samples, while Eu 3d is only observed in the spectra of 0.25% Eu-TiO2 sample, indicating that the Eu-doped TiO2 catalyst consists of Ti, O, C, and Eu. The corresponding high resolution spectra of Ti 2p, O 1 s, C 1 s and Eu 3d are also presented in Fig. 2b-e. The Ti 2p spectrum of TiO2 in Fig. 2b shows two peaks at 463.9 eV and 458.2 eV, which could be attributed to Ti 2p1/2 and Ti 2p3/2 respectively, indicating the presence of Ti4+. A small peak at 471.6 eV is due to the existence of Ti3+. As shown in Fig. 2c, the spectrum of O 1 s in TiO2 reveals two peaks centered at 529.4 eV and 531.1 eV, which belong to lattice oxygen (O2−) and metal OH bond or free hydroxyl group (OH) on TiO2 surface, respectively [36,37]. The C1 s peaks for pure TiO2 and 0.25% Eu-TiO2 samples are presented in Fig. 2d. The dominated peak at 284.5 eV is ascribed to CeC and C–H hydrocarbon bonds, while the peak at 288.3 eV is possibly ascribed to the carbon atoms bound to oxygen(C]O) [38–41]. Fig. 2e shows the spectra of Eu 3d for 0.25% Eu-TiO2 catalyst, which contains four distinct peaks. The two peaks at 1134.6 eV (Eu 3d5/2) and 1164.1 eV (Eu 3d3/2) indicate the presence of Eu3+ or europium oxide (Eu2O3). The other two peaks at around 1125.2 eV (Eu 3d5/2) and 1154.3 eV (Eu 3d3/2) could be ascribed to a Eu2+ oxidation state, which may be owing to the photoreduction of Eu3+ to Eu2+ during the XPS measurements [42,43]. Furthermore, the Ti 2p, O 1 s and C 1 s peaks of 0.25% Eu-TiO2 catalyst become broader and shift to higher binding energies compared with the reference 489

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Fig. 2. (a) XPS survey spectra of TiO2 and 0.25% Eu-TiO2, (b) high-resolution Ti 2p, and (c) O 1 s, and (d) C 1 s, and (e) high-resolution Eu 3d of 0.25% Eu-TiO2.

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Fig. 3. (a) SEM images of 0.25% Eu-TiO2 (b) TEM image (c) HRTEM image (d) EDS spectrum (e) the element mapping of 0.25%Eu-TiO2.

conduction band is very close to the flat-band potentials. The conductivity of pure TiO2 were not measured, so the energy gap between the bottom of the conduction band (ECB) and the Fermi level (Ef) in the bulk oxide is assumed to be 0.2 V [59,60]. Consequently, the CB potential for TiO2 is equal to -0.56 V versus the normal hydrogen electrode at pH 7(NHE, pH = 7). Therefore, the edge of the valence band (VB) of TiO2 is calculated to be 2.66 V based on its band gap determined from the UV–vis absorption spectrum to be 3.22 V. The reduction potential for the formations of CH4 and CO are -0.24 V and -0.52 V, respectively

In order to better comprehend the electronic properties of TiO2 and 0.25% Eu–TiO2 samples, the band-edge position of TiO2 has been calculated by the combination of the band-gap and flat-band potential [57]. The flat-band potential of pure TiO2, evaluated by Mott–Schottky method was displayed in Fig. 7. The flat potential of TiO2 is calculated to be -0.56 V versus the Ag/AgCl electrode at pH 7, which is equal to -0.36 V versus the normal hydrogen electrode at pH 7 (NHE, PH = 7). Obviously, the slopes of the plots are positive, indicating that TiO2 is ntype semiconductor [58,59]. For the n-type semiconductors, the

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Fig. 5. (a) UV–vis diffuse reflectance spectra and (b) corresponding Tauc plots of the samples. Fig. 4. (a) N2 isotherms and (b) pore size distribution plots of the samples.

[61]. This indicates that the separation of electrons and holes in TiO2 could photoreduce CO2 with H2O under simulated sunlight. The excitation and transfer of photo-generated charge carriers in TiO2 and 0.25% Eu–TiO2 samples was further studied by photo-electrochemical measurements. Fig. 8a shows the transient photocurrent responses under simulated solar irradiation. The maximum photocurrent of the 0.25% Eu–TiO2 sample is 2.075 μA/cm2 when the light is turned on, which is about 2.23 times as high as that of pure TiO2 (0.915 μA/cm2). Obviously, the 0.25% Eu–TiO2 sample exhibits a higher photocurrent than pure TiO2 sample, indicating the longer lifetime and higher efficient separation of photo-generated electron-hole pairs than pure TiO2 [62–64]. In order to verify this hypothesis, the EIS experiment was also conducted. Fig. 8b shows the semicircular Nyquist plots of TiO2 and 0.25% Eu–TiO2 samples. It is clearly seen that the semicircle diameter of 0.25% Eu–TiO2 catalyst is much smaller compared with that of the pure TiO2, implying the introduction of Eu ions could accelerate the electron transfer in 0.25% Eu–TiO2 catalyst. The performances of the samples were tested for the photocatalytic reduction of CO2 under simulated sunlight irradiation for 9 h. Two groups of controlled trials were carried out: (1) in the darkness with the photocatalyst, and (2) under light irradiation in the absence of photocatalyst. No hydrocarbon products were discovered in both the cases, indicating that the photocatalytic process could not proceed without catalysts or sunlight.

Fig. 6. PL emission spectra of the samples under the excitation of 350 nm.

Fig. 9(a–b) shows the production of CH4 and CO over all photocatalysts along with the irradiation time. Obviously, CH4 and CO are the main products in CO2 photoreduction reaction. The product yields of CH4 and CO increase with irradiation time and reach maximum 492

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Fig. 7. Mott-Schottky plots of pure TiO2 at the frequencies of 500 Hz and 1000 Hz.

Fig. 9. The yields of (a) CO (b) CH4 along with the irradiation time and (c) after 9 h irradiation over the samples.

Fig. 8. (a) The transient photocurrent responses, and (b) the electrochemical impedance spectra of TiO2 and 0.25% Eu–TiO2 samples.

pure TiO2. It is obvious that the photoactivity increases distinctly with the increase of Eu doping amount and achieves the maximum on 0.25% Eu-TiO2 catalyst, which are 65.53 μmol /g.cat for CH4 and 42.91 μmol /g.cat for CO after 9 h irradiation. It is amazing that it is 13 times and 1 times higher than pure TiO2, respectively. This improvement of photoactivity of the 0.25% Eu-TiO2 catalysts could be ascribed to the enhanced absorption of visible light and the hindered recombination of

values after 9 h irradiation. The pure TiO2 shows a very poor photocatalytic activity under the irradiation, only trace amount of CH4 and CO could be observed. This may be attributed to its small surface area, poor visible light response and rapid recombination rate of electrons and holes pairs during the reaction. As shown in Fig. 9c, all the Eudoped TiO2 catalysts demonstrate higher activity compared with the 493

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Scheme 1. Proposed reaction mechanism of CO2 photoreduction with H2O on Eu-TiO2 catalyst: (a) oxidation and reduction process over Eu-doped TiO2, and (b) recombination and separation of charges.

photogenerated electrons and holes, resulting in its excellent photocatalytic performance.

charge carriers. However, a decrease in photoactivity is observed when Eu content is more than 0.25%, which may be due to the excessive Eu could act as the recombination centers for charge carriers over TiO2 surface.

5. Conclusions In summary, the Eu-doped TiO2 samples were successfully synthesized by a sol-gel method and tested for photoreduction of CO2 with H2O under the visible-light irradiation. Experimental results showed that 0.25% Eu-TiO2 catalyst exhibited the highest photocatalytic activity for CO2 reduction with the yields of 65.53 μmol/g.cat CH4 and 42.91 μmol/g.cat CO, which were approximately 13 times and 1 times higher than that of pure TiO2 catalyst. The significantly enhanced photocatalytic activity of Eu-doped TiO2 is mainly owing to the efficient separation of photogenerated electron-hole pairs and the strong visible light response. Besides, the high specific surface area is another reason for its high photoactivity. This study implies that rare earth modification has great potential on improving the photocatalytic performance of TiO2 catalyst.

4. Proposed reaction mechanism According to the results of characterizations and the evaluation of photocatalytic activity mentioned above, the mechanism of CO2 photoreduction with H2O over the samples could be proposed. The possible reaction pathways during the CO2 photoreduction progress in this work could be written as follows [65–67].

TiO2 + hν → h+ + e− (1) 2H2 O + 4h+ → 4H+ + O2 (2) CO2 + 2H+ + 2e− → CO + H2 O (3) CO2 + 8H+ + 8e− → CH 4 + 2H2 O (4)

CO + 2H+ + 2e− → ∙C + H2 O (5)

Acknowledgments

∙C + 4H+ + 4e− → CH 4 (6)

This work was supported by the Natural Science Foundation of China (21546014) and the Natural Science Foundation of Shanghai, China (14ZR1417800).

Reactions (1)-(4) suggest that CO and CH4 are formed during the photoreduction of CO2 with H2O, which requires two and eight electrons, respectively. Therefore, CH4 will be favorably formed if there are sufficient electrons and H+ [67]. As shown in Fig. 6, compared with other catalysts, the yield of CH4 over 0.25% Eu-TiO2 catalyst increases rapidly along with the irradiation time. This may be due to the sufficient electrons generated by light excitation, which promotes the reaction progress in equation (4). In addition, the yield of CH4 is much higher than that of CO, inferring the existence of another pathway. The product of CO may be further reduced to %C radicals and then form CH4 (reactions (5)-(6)), indicating that CO is possibly the precursor in this reaction [68–70]. The dispersion of small Eu2O3 on the TiO2 surface is a key factor in photoreduction of CO2, which affects the efficiency of CO2 photocatalytic conversion. As shown in Scheme 1, when the photon energy absorbed by the catalyst is greater or equal to the band-gap of TiO2 (hν≥Ebg), a large number electrons and holes are generated in its VB and CB, respectively. Without Eu doping, these electron-hole pairs recombine fast and only a trace of electron- hole pairs could participate in the reduction and the oxidation processes. Therefore, TiO2 exhibits a very poor photoactivity [71]. When Eu is doped on TiO2, Eu2O3 on TiO2 surface could capture the electrons and prevent the recombination of

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