Enhanced Photocatalytic Performance toward CO2 Hydrogenation

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Enhanced Photocatalytic Performance Towards CO Hydrogenation over Nanosized TiO Loaded Pd under UV Irradiation 2

Naixu Li, Ming Liu, Bin Yang, Weixin Shu, Quanhao Shen, Maochang Liu, and Jiancheng Zhou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12683 • Publication Date (Web): 19 Jan 2017 Downloaded from http://pubs.acs.org on January 26, 2017

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The Journal of Physical Chemistry

Enhanced Photocatalytic Performance Towards CO2 Hydrogenation over Nanosized TiO2 Loaded Pd under UV Irradiation Naixu Li*,†, Ming Liu†, Bin Yang†, Weixin Shu†, Quanhao Shen†, Maochang Liu|| and Jiancheng Zhou*,†,‡,§ †

School of Chemistry and Chemical Engineering, Southeast University, Nanjing

211189, P.R. China, E-mail: [email protected] (Dr. N. Li), [email protected] (Prof. J. Zhou). ‡

Department of Chemical and Pharmaceutical Engineering, Southeast University

Chengxian College, Nanjing 210088, Jiangsu, P.R. China. §

Jiangsu Province Hi-Tech Key Laboratory for Bio-medical Research, Southeast

University, Nanjing 211189, P.R. China. ||

International Research Center for Renewable Energy, State Key Laboratory of

Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, Shanxi 710049, P.R. China.

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Abstract A series of Pd/TiO2 photocatalysts were synthesized by a simple glucose reduction method and their photocatalysis was evaluated in an array of CO2 hydrogenations. The samples were characterized by XRD, SEM, TEM, EDX, EDX mapping, UV-vis DRS, Raman spectroscopy, PL spectroscopy, XPS and N2 adsorption. The 1.0 wt% Pd/TiO2 (CH4: 355.62, CO: 46.35, C2H6: 39.69 μmol/g-cat) was above pristine TiO2 (CH4: 42.65, CO: 4.73, C2H6: 2.7 μmol/g-cat) and other composites under UV irradiation for 3 h in the rank of yield to products, possibly resulting from the synergy effect between palladium nanoparticles and TiO2. The palladium nanoparticles on the surface of TiO2 substantially accelerate electron transfer and act as active sites for adsorption and activation of CO2 molecules, to promote CO2 hydrogenation. During the photocatalytic CO2 hydrogenation, dissociated hydrogen reacts with CO2activated on Pd/TiO2 photocatalysts to form a new surface species Pd-C, which is stable in the reaction, and further transform to generate methane. A detailed mechanism of photocatalytic CO2 hydrogenation was then discussed to account for the performance of Pd/TiO2 photocatalysts in the reaction.

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1、Introduction The greenhouse effect mainly from an excess of CO2 emission via substantial combustion of fossil fuel and severe destruction of forest cover has triggered environmental and energy-related concerns towards humans.1-3 The photocatalytic CO2 reduction is considered one of the most promising technology that can address the energy crisis effectively by converting CO2 to high value added products, such as CO, CH4, CH3OH, HCOOH, etc.4-7 The photoelectrocatalytic CO2 reduction was firstly reported by Inoue et al..8 For decades, many researchers devote themselves to the studies of the photocatalytic reduction of CO2 with hydrogen and make a great progress. Photocatalytic CO2 hydrogenation to produce methane, which is the main component of natural gas and can be transported safely, can not only convert hydrogen energy into more stable energy, but also reduce levels of atmospheric carbon dioxide, and is a very significant research direction.9 As we all know, the reaction conditions are very harsh between hydrogen and carbon dioxide, such as high temperature and pressure, specified catalyst, etc.10-16 Hydrogen as a reducing agent, photocatalytic CO2 reduction is significant for the utilization of solar energy and has been studied by many researchers.17-26 Thampi et al. firstly reported that photoreducing CO2 with Ru-loading TiO2 showed a high activity by using hydrogen as the reducing agent.17 Sastre et al. discussed the photocatalytic CO2 reduction to methane with existence of H2 under solar light irradiation and asserted that CH4 formation is a photoactivated and not simply a thermal process.23 Ye’s group utilized

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the surface plasmon resonance effect characteristic of Group VIII metal nanoparticles to develop a new type of non-semiconductor photocatalyst, and the photothermal CO2 reaction rates can reach the order of mol•h-1•g-1.25 Photocatalytic hydrogenation of carbon dioxide is of great significance and will have a bright future. In the photocatalytic reaction, various semiconductors have been reported, such as TiO2, ZnO, WO3, C3N4.27-30 However, due to the high recombination rate for electron-hole pairs, these semiconductors show poor activity. To solve this problem, some strategies have been proposed, including loading noble metals, doping elements into semiconductors,31-34 or compounding different semiconductors.7,34 Among them, the noble metal loading is a simple and effective method to improve the catalyst. Titanium dioxide (TiO2), an n-type semiconductor, has an appropriate band gap (Eg=3.3 eV), and for this reason, photoresponse range of TiO2 belongs to the UV light region.35 Due to the good characteristics, such as high redox potential, high electron mobility, good stability and biocompatibility, TiO2 has been widely used in the photocatalytic field.36,37 It has been reported that Group VIII metals are very active for CO2 hydrogenation.17,18,21,25,38,39 Xiong’s team reported the unique Pd concave nanostructures with numbers of atoms at corners and edges can provide reactive sites and promote the photocatalytic hydrogenation reaction.39 The nature of the metal and support is an important factor in affecting the photocatalytic activity and the photo enhancement of the methanation reaction should be attributed to metal-support interactions.38 Among the various candidates, TiO2-supported palladium have been shown to play a crucial role in promoting the photocatalytic reduction of CO2.39-41 To

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the best of our knowledge, it has not been reported that synthesizing the TiO2-supported palladium with glucose reduction method and applied it to the reaction of CO2 hydrogenation to produce methane. In the present study, we successfully synthesized a series of TiO2-supported palladium photocatalysts by glucose reduction method. Photocatalytic CO2 hydrogenation by TiO2-supported palladium was carried out in a tank reactor and the effect of each active part was also investigated, which has been proved to have the high activity for photocatalytic CO2 hydrogenation and high selectivity toward CH4 formation. Additionally, the mechanism of photocatalytic CO2 hydrogenation to form CH4 on the TiO2-supported palladium was proposed.

2、Experimental 2.1 Chemicals and materials Titanium dioxide (TiO2, anatase, 99.8% metal basis), sodium tetrachloropalladate (II) (Na2PdCl4, 36.4%Pd), sodium hydroxide (NaOH), D-(+)-glucose (C6H12O6•H2O), aluminium oxide (Al2O3), sodium chloroplatinate (Na2PtCl6•6H2O), ruthenium chloride (RuCl3), trisodium hexachlororhodate (Cl6Na3Rh), iridium chloride (IrCl3•xH2O) and nickel chloride (NiCl2•6H2O) were purchased from the Shanghai Chemical Reagent Company. All reagents were used directly without further purification. Carbon dioxide (CO2, 99.999%), hydrogen (H2, 99.999%) and nitrogen (N2, 99.99%) were obtained from Nanjing shangyuan industrial gas plant. Deionized water was used for all the studies. 2.2 Synthesis of Pd loaded on TiO2 catalysts

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TiO2-supported palladium catalysts were fabricated via a facile reduction by glucose. Typically, 0.5 g TiO2 was dispersed in distilled water (60 mL) and sonicated for 10 min. An appropriate amount of Na2PdCl4 solution was slowly added into the above solution with continuous magnetic stirring. The pH of the solution was adjusted to 8.5 by 0.1 M sodium hydroxide solution. Then a certain amount of D-(+)-glucose as a reducing agent was added and the suspension was kept in a water bath at 80 oC for 1 h. After cooling to room temperature, the suspension was separated by centrifugation, washed with distilled water repeatedly and then dried in a vacuum at 60 oC for 12 h. 2.3 Characterization methods X-ray diffraction patterns (XRD) of all samples were obtained on the Bruker D8-Discover with Cu-Ka radiation (λ=0.1542 nm) in the 2θ range of 10o to 90o. The structure and morphology of samples were observed by the scanning electron microscope (SEM). The catalysts in powder form were scattered in the conductive tape and coated with gold before the examination. The particle size of catalysts and the dispersion of metal particles were measured using transmission electron microscopy (TEM, Hitachi H-600, FEI Tecnai G2 F30). UV-visible Diffuse reflectance spectra of the samples were collected on a Shimadzu UV 3600 spectrometer with the wavelength range of 200-700 nm. Raman spectra were recorded in Lab RAM HR800 confocal microscope Raman system (Horiba Jobin Yvon) equipped with 633 nm He-Ne laser with an integration time of 1 s and laser power of 1 mW. The charge recombination rate was analyzed using a F-4600 PL

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spectrophotometer with the excitation wavelength of 325 nm. Specific surface area and pore size were collected in a Mieromerities ASAP 2010 BET apparatus. All samples should be outgassed at 180 oC for 10 h and the adsorption-desorption was at -196

C. The chemical composition of samples was determined by X-ray

o

photoelectron spectroscopy (XPS, 2000 XPS system with a monochromatic Al-Ka source). All binding energies were normalized to the C 1s peak at 284.6 eV. 2.4 Photocatalytic reaction testing The photocatalytic reaction was carried out in a miniature visual autoclave with the total volume of 250 cm3 and the schematic illustration of the experimental setup is shown in Fig. 1. The reactor was equipped with a quartz glass window of 10 mm thickness to pass light irradiation from the reflector lamp. No other heating devices have been used in the whole experiment and a 150 W mercury lamp serves as the only heat and UV-light supplier. Prior to use, the catalyst should be dried in a vacuum at 60 C for 2 h and fully ground to ensure an excellent dispersion. A layer of silver paper

o

was employed inside the lining of reactor in order to focus the light. Typically, 0.5 g of silica wool as catalyst support, was distributed inside the reactor and the bottom of the reactor was set aside a certain space to magnetically stir for gas flow. Furthermore, 0.1 g of powdered photocatalyst was uniformly dispersed onto the silica wool to ensure that light was used efficiently. The reactor was purged by nitrogen for 30 min with the gas outlet valve open to remove the air completely. After that, the hydrogen and carbon dioxide with the ratio of 4:1 were introduced until the reactor reached a

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pressure of 2.5 MPa. The photoreactor was illuminated under stirring and the temperature increased with the illumination time. During the reaction, the products were detected by using an online gas chromatograph (GC-9860-5C) equipped with flame ionized detector (FID) and thermal conductivity detector (TCD). The FID detector was connected with a HP PLOT Q column (length 30 m ID 0.53 mm, film 20 μm) while the TCD detector was connected to a TDX-01column. Every detection was repeated three times and their average values were analyzed in our study. The quantification of the production was calculated according to the external standard curve.

Fig. 1. Schematic illustration of photocatalytic reaction system for CO2 hydrogenation under UV light irradiation.

3、Results and Discussion 3.1 Catalysis characterization

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The X-ray diffraction (XRD) spectra of the TiO2 and various Pd/TiO2 powders are exhibited in Fig. 2. For the pure TiO2 sample (Fig. 2a), the peaks at 25.4o , 37.8o , 48.2o , 54.1o , 55.2o , 62.9o , 69.0o , 70.5o and 75.4o can be indexed to (101), (004), (200), (105), (211), (204), (116), (220) and (215) crystal planes of anatase structure TiO2 (JCPDS No. 73-1764), respectively. Besides, similar diffraction lines of modified TiO2 to those of the bare one indicate its structural independence of Pd loading. In all cases, diffraction peaks assigned to Pd are seldom visible possibly associated with its high dispersion and low content.

Fig. 2. XRD patterns of TiO2 and Pd/TiO2: (a) TiO2, (b) 0.5 wt% Pd/TiO2, (c) 0.8 wt% Pd/TiO2, (d) 1.0 wt% Pd/TiO2, (e) 1.2 wt% Pd/TiO2, (f) 1.5 wt% Pd/TiO2. The morphology and particle size of the TiO2 and 1.0 wt% Pd/TiO2 are investigated via SEM and TEM, as shown in Fig. 3. It can be clearly seen that pure TiO2 is consist of spherical aggregates of small sub-particles, which are homogeneous in size and shape (Fig. 3a). The size of TiO2 is estimated to have an average diameter of 30-40 nm (Fig. 3c). In Fig. 3b, palladium nanoparticles loaded on the surface of TiO2 are clearly observed. Noticeably, a comparative observation between Fig. 3a and Fig. 3b disclose morphological maintenance by Pd loading. The additional information

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regarding the microstructure of Pd/TiO2 nanoparticles is obtained in Fig. 3d. The dark spots of TEM image indicate that the palladium nanoparticles are dispersed uniformly on the surface of TiO2, which consists well with the result of XRD. The HRTEM image Fig. 3e reveals palladium nanoparticle with a diameter of 3 nm and lattice spacing of 0.237 nm and 0.194 nm respectively indexed as (004) plane of anatase TiO2 and Pd (200) plane. More importantly, intimate interface contact suggests Pd loaded on the surface of TiO2. EDX analysis is a powerful tool in determining the chemical composition of 1.0 wt% Pd/TiO2. The presence of Pd nanoparticles in 1.0 wt% Pd/TiO2 is confirmed in Fig. 3f. Fig. 4 shows the STEM and mapping images of 1.0 wt% Pd/TiO2. The EDX mapping was used to investigate the distribution of Ti, O, Pd elements in the catalyst. The white dots in the selected area (Fig. 4a) reveal homogeneous dispersion of palladium nanoparticles. Fig. 4b-d shows the presence of Ti, O and Pd elements in 1.0 wt% Pd/TiO2 sample, which is in good agreement with the result of EDX.

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Fig. 3. (a) SEM images of TiO2, (b) SEM images of 1.0 wt% Pd/TiO2, (c) TEM images of TiO2, (d) TEM images of Pd/TiO2, (e) HRTEM images of Pd/TiO2, (f) EDX patterns of 1.0 wt% Pd/TiO2 catalyst.

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Fig. 4. (a) STEM images of 1.0 wt% Pd/TiO2 and (b-d) the corresponding EDX mapping of 1.0 wt% Pd/TiO2 at the region shown in (a), indicating special distribution of Ti, O, Pd, respectively. Fig. 5a. shows the ultraviolet-visible absorption spectrum of TiO2 and a series of Pd-loaded TiO2. All samples exhibit an intense optical absorption ranging from 200 to 380 nm, indicating the catalysts can be photoactived under UV-light illumination. With the increasing of Pd loading, the samples exhibit better optical response ability and the absorbance at the range of 380-700 nm is much higher than the pure TiO2. This phenomenon can be attributed to the surface plasmon resonance (SPR) of Pd nanoparticles, which will have an influence on the surface charge distribution of TiO2, further causing the change in the particle energy gap. The band gap of the samples can be calculated on the basis of the following empirical formulas:

hv  A(hv  E g ) n / 2 where α, h, ν, A, Eg are the absorption coefficient, planck constant, light frequency, proportionality constant and band gap energy, respectively. For TiO2, n=1 for direct transition.42 The band gap energies of TiO2 and Pd-loaded TiO2 are calculated in the Fig. 5(b). Eg of pure TiO2 is estimated to be 3.27 eV and after the Pd loaded, the absorption edge shows a red shift, extending the Eg of 1.5 wt% Pd/TiO2 to 3.06 eV. This result indicates that the absorption intensity of TiO2 in the region of visible light can be increased by loading palladium.

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Fig. 5. (a) UV-vis absorption spectra of TiO2 and Pd-loaded TiO2, (b) Plot of (αhv)2 vs. photon energy (hv) for the direct optical band gap of TiO2 and Pd-loaded TiO2. The structural properties of metal oxide can be sensitively identified by the Raman spectrum. Fig. 6 provides Raman spectra of the TiO2 and Pd/TiO2 samples. For the TiO2 sample (Fig. 6a), the major Raman bands at 147, 198, 396, 515,519 (overlapped with the band of 515 cm-1) and 638 cm-1 can be assigned to the Eg(1), Eg(2), B1g(1), A1g, B1g(2), and Eg(3) modes of TiO2 anatase phase, respectively.43 The result of Raman spectra shows that both TiO2 and Pd/TiO2 sample are in the anatase phase, corresponding to the XRD results. The palladium-modified TiO2 (Fig. 6b) has the same Raman bands with those of the pure TiO2, indicating that palladium was loaded

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on TiO2 by glucose reduction method without any structural changes in the catalyst. For the Pd/TiO2 after reaction (Fig. 6c), the unchanged peak position reveals that the catalyst remains original structure after reaction.

Fig. 6. Raman spectra of TiO2 and Pd/TiO2: (a) TiO2, (b) 1.0 wt% Pd/TiO2, (c) 1.0 wt% Pd/TiO2 (after reaction). Photoluminescence (PL) spectra provide a convincing indicator of recombination rate of photogenerated excitons, thus leading to a deeper insight into photocatalytic process. The photoluminescence spectra of pure TiO2 and Pd-loaded TiO2 have the same photoluminescence peaks with an excited wavelength of 325 nm (Fig. 7). It is obvious that the photoluminescence intensity gradually decreases with the increase in the palladium loading (Pd loading ≤ 1.0 wt%), demonstrating Pd-loaded on TiO2 enables to accelerate separation efficiency and hinder photogenerated charges recombination rate on the Pd-loaded TiO2 surface. However, with Pd loading above 1.0 wt%, the photoluminescence intensity increases again. This phenomenon is possible due to the formation of new recombination center by an excess of palladium.

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Fig. 7. PL spectra of TiO2 and Pd/TiO2 measured at excitation wavelength of 325 nm. To further investigate the element state and interaction between palladium nanoparticles and TiO2, XPS spectra were obtained and showed in Fig. 8. O, Ti, Pd and C elements are determined in 1.0 wt% Pd/TiO2 catalyst by the XPS survey spectrum (Fig. 8a). The two peaks at 530.1 and 531.6 eV of O 1s spectrum in Fig. 8b are assigned to the lattice oxygen of TiO2 and the oxygen of hydroxide ions, respectively.44 The doublet at 458.5 and 464.4 eV respectively related to Ti 2p3/2 and Ti 2p1/2 supports Ti4+.45 There are no peak shift and valence state changes for O 1s and Ti 2p spectrum between fresh and used catalyst. In Fig. 8d, the peaks at 335.1 and 340.2 eV corresponding to Pd 3d5/2 and Pd 3d3/2 respectively are attributed to metallic Pd and the rest at 336.2 and 341.8 eV ascribed to PdO.46,47 The presence of PdO may be due to the surface oxidation in catalyst preparation and storage. Compared with the fresh catalyst, the peaks of PdO for the used sample are shifted down by 0.1 and 0.3 eV, respectively. This phenomenon indicates the generation of a new surface species Pd-C and the increased peak value at 284.6 eV of C 1s in Fig. 8a can also support this conclusion.48,49 The formation of Pd-C surface species is helpful to explain the reaction mechanism of photocatalytic CO2 hydrogenation. 15



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Fig. 8. X-ray photoelectron spectra of (a) survey spectrum, (b) O 1s, (c) Ti 2p, (d) Pd 3d. 3.2 Photocatalytic activity towards CO2 hydrogenation The photocatalytic function of catalysts was evaluated in a miniature visual autoclave under the pressure of 2.5 MPa with high-purity H2 and CO2 at a ratio of 4:1 sealed in the system. Typically, we found that the main product of CO2 hydrogenation is CH4 with a small amount of CO and C2H6, as shown in Fig. 9. The time courses of CH4, CO and C2H6 yields over Pd/TiO2 are presented in Fig. 9a-c respectively. The pure TiO2 show a poor photocatalytic activity towards CO2 hydrogenation throughout reaction course. However, for every composite, prolonged time allows a continual and fast growth of yields for each product. More importantly, Pd loading dependence of yield proves to be double-edged. Obviously, for all products, yields are on the increase to their 16


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optimal values below 1.0 wt% loading (Fig. 9d). The highest value of CH4, CO, C2H6 can reach 355.62, 46.35 and 39.69 μmol/g-cat respectively with the palladium content of 1.0 wt% after irradiation for 3 h. This trend is closely related to remarkably boosted separation efficiency of charge carriers as PL result implies (Fig. 7), leaving more photons to run targeted reaction. On the other hand, excessive Pd are harmful to further increase in yields possibly due to formation of new recombination center or shielding effect of noble metal on TiO2 to inhibit light harvesting.

Fig. 9. Effect of irradiation time on products of CO2 hydrogenation with a series of Pd-loaded TiO2 catalysts, (a) CH4, (b) CO, (c) C2H6. (d) The the relationship between the products and the loading amount of Pd after irradiation for 3 h.

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In order to investigate the synergy effect between palladium nanoparticles and TiO2 during the photocatalytic CO2 hydrogenation, aluminium oxide was substituted for TiO2. The BET information of support and Pd-loaded support are summarized in Table 1. The specific surface area of TiO2 (83.45 m2/g) is slightly less than that of Al2O3 (97.92 m2/g),

which

can

eliminate

effects

of

specific

surface

area

for

the

adsorption/concentration of CO2 molecules. There is a small decrease in specific surface area after loading palladium both for TiO2 and Al2O3, 61.82 m2/g for 1.0 wt% Pd/TiO2 and 94.86 m2/g for 1.0 wt% Pd/Al2O3, which can be attributed to covering effect of palladium nanoparticles on partial pores of supports. In order to investigate the dispersion of palladium nanoparticles on TiO2 and Al2O3, TEM images of 1.0 wt% Pd/TiO2 and 1.0 wt% Pd/Al2O3 are illustrated in Fig. S1. There is no much difference about the dispersion of palladium nanoparticles between 1.0 wt% Pd/TiO2 and 1.0 wt% Pd/Al2O3, ruling out that the enhanced photocatalytic process is not caused by better dispersion of palladium nanoparticles on TiO2. Table 1 Specific surface area (BET), pore parameters of support and Pd-modified support. Catalyst

Pd (wt%)

SBET (m2/g)

Dpore (Å)

Vpore (cm3/g)

TiO2

-

83.45

131.4

0.2742

Pd/TiO2

1.0

61.82

171.8

0.2655

Al2O3

-

97.92

80.17

0.1963

Pd/Al2O3

1.0

94.86

80.27

0.1903

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Aluminium oxide, a photocatalytic inert material, almost did not show any photocatalytic activity in our study, as shown in Fig. 10a.25 However, the yield of CH4 over 1.0 wt% Pd/Al2O3 after irradiation for 3 h can reach 252.79 μmol/g, which is about 13.72 times higher than that of pure Al2O3. This result can almost be seen as the performance of metal palladium independently. Pure TiO2 has a certain photocatalytic activity. It is obvious that 1.0 wt% Pd/TiO2 exhibits the highest photocatalytic activity with 355.62 μmol/g of CH4 after irradiation for 3 h, which is about 8.34 and 1.41 times higher than over pure TiO2 and palladium, respectively. It can be concluded from the above comparison that the photocatalytic activity of TiO2 supported Pd is beneficial from synergy effect between TiO2 and palladium instead of a simple superposition of their individual effects. The control experiment (Fig. 10b red line) was conducted in the absence of catalyst with otherwise identical conditions, which shows that a very small amount of CH4 was found, suggesting that the reaction of CO2 hydrogenation can occur without any catalysts. Photocatalytic CO2 hydrogenation over Pd-loading TiO2 is an enhanced reaction. In order to confirm CO2 as a carbon source of CH4, a blank experiment (Fig. 10b black line) was operated by replacing CO2 with N2 under other conditions being equal and no CH4 was detected, which can indicate that the carbon of CH4 is derived from CO2 and the surface of catalyst is clean. In our study, there is no other heating device except the lamp and the temperature measured in the reactor was always lower than 130 oC within three hours (Fig. 10c). To discuss the effect of the light, we carried out a dark experiment under 130 oC and a visible light experiment for comparison

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(Fig. 10d). Only a small amount of CH4 was found in this two processes, revealing that Pd-loading TiO2 is active in UV light, which is also evidenced by the UV-vis spectra.

Fig. 10. Effect of irradiation time on the yield of CH4 of CO2 hydrogenation with different catalysts (a) and different conditions (b). (c) The change of temperature with irradiation time. (d) The relationship between the yield of CH4 and reaction time using 1.0 wt% Pd/TiO2 catalyst with different light source. The structural stability of catalyst after reaction was investigated by characterizing the XRD (Fig. S2) and Raman spectrum (Fig. 6). No significant changes are found in the structure of used catalyst, meaning that the structure is highly stable. It has been demonstrated by above instructions that photocatalytic CO2 hydrogenation is a synergy between metal and support. There has been some reoprts that various metals will have different influence on the reaction of CO2 20


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hydrogenation.14,18,25 Herein, six different metals (Pd, Pt, Ru, Rh, Ir, Ni) loaded on TiO2 were prepared with the same method (Fig. S3) to evaluate the photocatalytic activity for CO2 hydrogenation (Table 2). Compared with pure TiO2, in addition to the Ni-loaded TiO2, other samples prepared show different levels of activity increased. The adverse performance of Ni-loaded on TiO2 is possibly due to the decrease in specific surface area. We can see clearly that 1.0 wt% Pd/TiO2 exhibits the top performance with the CH4 of 355.62 μmol/g, the CO of 46.35 μmol/g and the C2H6 of 39.69 μmol/g after irradiation for 3 h. It may be explained by the adsorption of carbon dioxide,

hydrogen

evolution

and

plasma

resonance

effect

of

palladium

nanoparticles.25,39,50 Table 2 Distribution and yield of each product from CO2 hydrogenation over different catalysts.[a] Products (μmol/g)[c] #

Catalyst[b]

CH4

CO

C2H6

Other hydrocarbons

1

-

14.89

Trace

Trace

Trace

2

TiO2

42.65

4.73

2.70

Trace

3

Al2O3

18.43

Trace

Trace

Trace

4

1.0 wt%Pd/Al2O3

252.79

24.61

17.90

35.64

5

1.0 wt%Pd/TiO2

355.62

46.35

39.69

8.32

6

1.0 wt%Pt/TiO2

180.64

25.77

18.47

3.97

7

1.0 wt%Ru/TiO2

130.75

20.32

21.09

10.46

8

1.0 wt%Rh/TiO2

124.94

14.90

8.89

2.79

21



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9

1.0 wt%Ir/TiO2

54.65

9.79

8.04

1.56

10

1.0 wt%Ni/TiO2

29.60

3.18

2.67

Trace

[a] Typical reaction conditions: the photocatalyst amount, 0.1 g; the silica wool amount, 0.5 g; irradiation time, 3 h; mixture of CO2 (0.5 MPa) and H2 (2 MPa), irradiation source, 150 W mercury lamp; miniature visual autoclave volume, 250 cm3; magneton speed, 400 r/min; absence of additional heat suppliers except the light. [b] Metal loading (wt%) refers to theoretical amount of noble metals loaded on the support. [c] Other hydrocarbons contain ethylene, propane and propylene and the yield of each product was calculated according to their standard curve respectively. 3.3 Mechanism of photocatalytic CO2 hydrogenation Based on the above analysis, a possible mechanism of photocatalytic CO2 hydrogenation was proposed (Fig. 11). Palladium plays a very important role in effective separation of photo generated charges. Electron transition from the valence band (VB) to the conduction band (CB) of TiO2 under its exposure to UV and the photoelectrons will be further transferred to the palladium nanoparticles, promoting the reduction of carbon dioxide. The detailed process of reaction is as follows.

Fig. 11. Schematic illustration of changes transfer over Pd/TiO2 during the photoreaction. 22


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Firstly, TiO2 absorb energy higher than or equal to the band gap energy to produce photogenerated electron-hole pairs under the excitation of UV light. Photoinduced electrons and holes are enriched on CB and VB of TiO2 since their birth. A part of charges will recombine in bulk and the other part will transfer to the surface of TiO2 to recombine or carry out the next reaction. Subsequently, the electrons on the surface of TiO2 are transferred to adjacent Pd to combine with chemsorbed CO2 to yield CO2-, leading to prompt separation and retarded recombination of excitons and thus to enhanced catalytic function. Also, the holes in the valence band of TiO2 are captured by highly reactive atomic hydrogen to obtain H+. Then, the H was dissociated at Pd sites by combine the H+ oxidized and electrons on palladium. The carbon dioxide molecules activated at Pd sites react with hydrogen ions (H+) and the electrons to produce intermediate Pd-C=O. On the one hand, a small amount of CO is generated by C=O desorption, on the other Pd-C=O further interact with the dissociated H to form a Pd-C species. Finally, the carbon species generated continue to react with the H species at Pd sites to obtain the product CH4. During CH4 formation, some intermediates (such as •CH, •CH2, •CH3) are produced and C2H6 is obtained when two •CH3 are interacted with each other. The aforementioned process is formulized as follows: 



TiO 2  hv  TiO 2 (ecb , h vb ) 

Pd  TiO 2 (ecb )  TiO 2  Pd(e ) 

H 2  TiO 2 (h vb )  TiO 2 — H  CO2  Pd(e )  Pd — CO2



23



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TiO 2 — H   Pd(e )  TiO 2  Pd — H Pd — CO2



 TiO 2 — H   Pd(e )  Pd — C  O  H 2O

Pd — C  O  Pd — H  Pd — C  H 2O Pd — C  4Pd — H  Pd  CH4

4、Conclusions Pd/TiO2 has been developed by a simple glucose reduction method to promote photocatalytic CO2 hydrogenation. It is found that palladium nanoparticles were uniformly dispersed on the surface of TiO2. The synergy effect between palladium nanoparticles and TiO2 during the photocatalytic CO2 hydrogenation was well-confirmed. Palladium supported on TiO2 has a significant influence on the photocatalytic activity and the top photocatalytic performance is obtained at 1.0 wt% Pd loading in UV window for 3 h. The corresponding yields of CH4, CO, C2H6 was 355.62, 46.35, 39.69 μmol/g-cat, respectively. The result outperforms that of pure TiO2, resulting from the synergy effect between palladium nanoparticles and TiO2. The palladium species loaded on TiO2 can not only migration electrons to separate the electron-hole pairs, but also promote the adsorption and activation of CO2 molecule, resulting an active site for CH4 formation. It is promising for photocatalytic CO2 hydrogenation, since the carbon dioxide in the air can be reduced to produce useful fuel. Future work will focus on the improvement of experimental device and selection of catalysts to increase the yields of the products.

ASSOCIATED CONTENT Supporting Information 24


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TEM images of 1.0 wt% Pd/TiO2 and 1.0 wt% Pd/Al2O3 (Figure S1); XRD patterns of Pd/TiO2 (Figure S2); Photograph of different catalyst powder (Figure S3); Reaction equipment (Figure S4).

AUTHOR INFORMATION Corresponding Author * J. Zhou. Phone: (+86)025-52090621. Fax: (+086)025-52090620. E-mail: [email protected]. Notes The authors declare no competing financial interest.

Acknowledgements This work was financially supported by the Fundamental Research Funds for the Central Universities of China (No. 3207045403 and 3207045409), National Natural Science Foundation of China (No. 21576050 and 51602052), Jiangsu Provincial Natural Science Foundation of China (BK20150604) and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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TOC/ABSTRACT ART

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Enhanced Photocatalytic Performance toward CO2 Hydrogenation

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