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Enhanced Visible Light Photocatalytic Hydrogenation of CO into Methane over Pd/Ce-TiO Nanocomposition 2

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Naixu Li, Xiaoyue Zou, Ming Liu, Lingfei Wei, Quanhao Shen, Rehana Bibi, Chongjiu Xu, Quanhong Ma, and Jiancheng Zhou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07298 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 25, 2017

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The Journal of Physical Chemistry

Enhanced Visible Light Photocatalytic Hydrogenation of CO2 into Methane over Pd/Ce–TiO2 Nanocomposition Naixu Li*, †, Xiaoyue Zou†, Ming Liu†, Lingfei Wei†, Quanhao Shen†, Rehana Bibi†, Chongjiu Xu†, Quanhong Ma†, 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.

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Abstract A series of Ce–doped TiO2 nanoparticles were prepared by a sol–gel process and characterized by XRD, SEM, TEM, EDX mapping, UV–vis DRS, Raman spectroscopy, N2 adsorption–desorption, PL spectra, CO2–TPD, and XPS. It is found that Ce ions can enter the lattice matrix of TiO2 and occupy of Ti sites. This atom replacement leads the formation of impurity energy levels in the band–gap of TiO2, extending light absorption into visible–light region. Since Ce has a more flexible valence states, both Ce3+ and Ce4+ could be formed in the composites. The preference facilitates the photo–induced charge separation inside the crystals. Moreover, Pd nanoparticles were then loaded as a cocatalyst on the surface of doped composites. As the trapping center of electrons, it can efficiently adsorb and activate CO2 molecules, promoting their transformation into CH4. These composites were then evaluated as photocatalysts for CO2 hydrogenation. While all of them could efficiently catalyze the reaction, 1.0% Pd/0.5% Ce–TiO2 catalysts show the best photocatalytic performance, with CH4 and CO yields up to 220.61 and 27.36 μmol/g, respectively, under visible light irradiation of 3 hours. The improved photocatalytic behavior could be possibly induced by the synergistic effect between Ce and Pd. A probable mechanism was thus proposed based on above characterizations and experimental results.

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1、Introduction The increasing amounts of carbon dioxide (CO2), a greenhouse gas, mainly caused by the widespread use of fossil fuels in industry and transportation, has brought a great threat to the environment.1–3 At the same time, the excessive dependence on fossil energy makes the problem of energy shortage more serious.4–8 The photocatalytic reduction of CO2 into high value–added fuels (CH4, CH3OH, HCOOH, CO, etc) is considered to be a promising approach to reduce the CO2 level and solve the energy problems to a certain degree. 9,10,11,12 Since the pioneering work of Inoue et al. on photoelectrocatalytic reduction of CO2 in 1972, photocatalytic reduction of CO2 has attracted many researchers' attention.13 Traditionally, the reaction between CO2 and H2 is carried out under high temperature and pressure, which limits the larger scale application in the field of energy utilization.14,15,16 Photocatalytic of CO2 with H2 over semiconductor materials, not only overcomes traditional harsh conditions, but also makes full use of solar energy, an inexhaustible energy sources. Photocatalytic CO2 hydrogenation has attracted an increasing interest of many researchers.17–22 In 1987, Thampi et al. firstly studied photo–methanation of CO2 with H2 over Ru/TiO2 catalyst at the room temperature and atmospheric pressure.18 Kohno et al. confirmed that the photoreduction of CO2 to CO using hydrogen as a reductant over MgO showed certain activity and a reaction intermediate (surface formate ion) was observed to improve the selectivity of CO.19 A nickel

supported

on

silica–alumina

photocatalyst

was

fabricated

for

the

photoreduction of CO2 to CH4 by H2 with the CH4 selectivity of 95% and CO2 3



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conversion of 90%.20 Our group have previously reported that photocatalytic CO2 hydrogenation over nanosized TiO2 loaded Pd showed an enhanced performance, resulting from a synergistic effect between the palladium nanoparticles and the TiO2 support.21 In most studies, significant efforts have been taken to find better and more efficient photocatalysts to promote photocatalytic reduction process of CO2. Oxide semiconductors have immeasurable superiority due to their increased activity, a large surface–to–volume ratio, special optical and electrical properties.23,24,25 Among a series of semiconductor materials, titanium dioxide (TiO2), an n–type semiconductor, was widely researched in photocatalytic field because of its high physical and chemical stability, low cost, easy availability, low toxicity, and excellent photoactivity.26,27 However, the photocatalytic efficiency of pure TiO2 is relatively low, especially under visible light. The wide band–gap of TiO2 (Eg=3.3 eV) allows it to be excited only under UV light and the rapid recombination of electron–hole pairs limits its photocatalytic activity.26,28,29 To solve this problem, some strategies have been proposed, including compounding semiconductor oxides,30,34 doping elements into semiconductors,31,32,33 loading noble metal.21,35–37 Cerium (Ce), as a rare earth element, has been widely used as a dopant to enhance the photocatalytic activity of semiconductors.31,32,38–44 Miyauchi’s group synthesized a novel Ce–doped ZnO photocatalyst by co–catalyst (Cu+) grafting and claimed that impurity states formed by Ce doping in the band–gap were responsible for the visible–light absorption.31 Aman et al. found that the photocatalytic performance of Ce–doped TiO2 depended on 4


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the presence of Ce4+/Ce3+ redox couple rather than only visible light absorption.42 Ce was selected as a doping element in our study considering that the shift of Ce4+/Ce3+ can accelerate charge separation and the impurity levels caused by Ce doping enable TiO2 to be excited in the visible region. The effect of cocatalyst can not only improve the charge separation by capturing electrons but also favor the catalytic reaction of surface.45 Palladium nanoparticles as the cocatalyst can gather electrons to activate CO2

molecules

and

provide

the

active

sites

for

photocatalytic

CO2

hydrogenation.21,47–49 In our previous work, palladium nanoparticles has been discussed in promoting photocatalytic CO2 hydrogenation.21 Now, our interest is exploring the synergistic effect between Ce and Pd in photocatalytic CO2 hydrogenation over Ce–doped TiO2 by co–catalyst Pd modification. In the present study, we succesfully fabricated a series of Ce–doped TiO2 photocatalysts by the sol–gel method, Pd nanoparticles were loaded on samples by the method of glucose reduction as a cocatalyst. The performance of photocatalysts was evaluated for photocatalytic CO2 hydrogenation under visible light irradiation in the tank reactor. Additionally, the synergistic effect between Ce and Pd for the reaction was investigated. The enhanced activity is attributed to the shift of Ce4+/Ce3+ and red shift of absorption edge. Finally, a possible mechanism was also proposed to account for the enhancement of photocatalytic activity of the Pd/Ce–TiO2 photocatalysts.

2、Experimental 2.1 Chemicals and Materials

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Tetrabutyl titanate (C16H36O4Ti), acetic acid (CH3COOH), isopropanol (C3H8O), cerium nitrate (Ce(NO3)3•6H2O), sodium tetrachloropalladate (II) (Na2PdCl4, 36.4% Pd), sodium hydroxide (NaOH) and D–(+)–glucose (C6H12O6•H2O) were purchased from the Shanghai Chemical Reagent Company. All reagents were AR grade and used directly. Carbon dioxide (CO2, 99.999%), hydrogen (H2, 99.999%) and nitrogen (N2, 99.99%) were purchased from Nanjing shangyuan industrial gas plant. Deionized water was used throughout the studies. 2.2 Synthesis of Ce–doped TiO2 Catalysts The bare TiO2 and a series of cerium doped TiO2 (Ce–TiO2) catalysts in the range of 0.1–1.0% were prepared via a simple sol–gel method. Typically, 10 mL of tetrabutyl titanate (C16H36O4Ti) was fully dissolved in 30 mL of isopropanol by stirring for 30 min, called solution A. While a certain amount of acetic acid (1.0 M) was also added into 10 mL of isopropanol and stirred for 30 minutes, called sollution B. Then, sollution B was slowly added dropwise to sollution A and stirred for 24 h at the constant temperature of 30 oC. Next an appropriate amount of cerium nitrate was dispersed in isopropanol under stirring for 30 min and put it into the TiO2–sol. The mixed solution was kept stirring for 12 h at room temperature. Finally, the suspension was then centrifuged, washed by distilled water repeatedly and dried at 80 oC. The obtained powders were calcined in a muffle furnace at 500 oC for 5 h at a heating rate of 5 oC/min. The bare TiO2 was prepared using the same method without the addition of cerium nitrate. 2.3 Synthesis of Pd/Ce–TiO2 Composition 6


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The catalysts of Pd/Ce–TiO2 were synthesized by the previous method.21 In a typical procedure, 0.5 g of catalyst prepared above was dispersed in 60 mL of distilled water by sonicating for 10 min. A proper amount of Na2PdCl4 solution (1.0% Pd: theoretical amount) was slowly added dropwise into the above solution under constant stirring for 30 min. Next, the NaOH solution (0.1 M) was used to adjust the pH value of solution to 8.5. A certain amount of D–(+)–glucose as a reductant was dissolved in the suspension and the mixture was vigorously stirred at 80 oC for 1 h in a water bath. Finally, the resulting suspension was centrifuged, washed by deionized water repeatedly and vacuum dried at 60 oC for 12 h. 2.4 Characterization Methods X–ray powder diffraction patterns (XRD) of all samples were measured under ambient atmosphere on the Bruker D8–Discover with Cu–Kα radiation (λ=0.1542 nm) in the 2θ range from 10o to 90o. The scanning electron microscope (SEM) was performed to observe the structure and morphology of samples. The particle size of catalysts and the distribution of metal particles were obtained by transmission electron microscopy (TEM, Hitachi H–600). In order to investigate the range of photoexcitation, UV–visible diffuse reflectance spectra were collected on Shimadzu UV 3600 in the wavelength range of 200–800 nm. Raman spectra was recorded on a Lab RAM HR800 confocal microscope Raman system (Horiba Jobin Yvon) using 633 nm He–Ne laser with an integration time of 1 s and laser power of 1 mW. The specific surface area (SBET) and pore size were estimated by a Mieromerities ASAP 2010 BET apparatus. All samples were outgassed at 180 oC for 10 h and the temperature of 7



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adsorption–desorption was –196 oC. Temperature–programmed desorption (TPD) of carbon dioxide (CO2–TPD) was used to investigate the basicity and properties of catalysts with a linear heating rate of 10 oC/min in a gas stream of 4% Ar at a flow rate of 30 mL/min. To study the valance state and chemical state of the photocatalysts, we received the X–ray photoelectron spectroscopy (XPS) results with 2000 XPS system equipped with a monochromatic Al–Ka source. All binding energies were calibrated according to the C 1s peak at 284.6 eV. 2.5 Photocatalytic Activity Measurement The reaction of photocatalytic CO2 hydrogenation was carried out in a micro visual autoclave, as shown in Figure 1. The internal volume of reactor is 250 cm3 and a quartz glass window of 10 mm thickness was equipped to pass the light. The provided light source was Xe lamp, which was also the only heat throughout the experiment. Before the reaction, it was necessary to put a layer of silver paper surrounded inside of polytetrafluoroethylene liner for focusing the light. Then, 0.5 g of quartz wool was used as the catalyst carrier in the lining, leaving the bottom a certain space to stir for gas flow. Additionally, 0.1 g of the catalyst was dispersed uniformly on the quart wool. Nitrogen gas was imported to the reactor for 30 minutes with the air outlet valve open to remove residual air. CO2 and H2 with the ratio of 1:4 were added into the reactor chamber until the total pressure reached to 2.5 MPa. The photocatalytic activity was conducted with the Xe lamp turning on. The reaction products were detected at a 30 min interval by an online gas chromatograph (GC–9860–5C) equipped with flame ionized detector (FID) for 8


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hydrocarbons and thermal conductivity detector (TCD) for CO and H2. In order to decrease the error, we detected the gas three times at each time point and the average values of them were calculated in our analysis. The quantification of the products was obtained on the basis of the external standard curve.

Figure 1. The schematic diagram of the whole experimental process

3、Results and Discussion 3.1 XRD Analysis To investigate the crystal structure of the samples, we employed XRD for pure TiO2 and Ce–doped TiO2 samples, as shown in Figure 2. All of the diffraction peaks of pure TiO2 prepared by sol–gel method can be matched to the anatase TiO2 (JCPDS No. 21–1272). The main peaks at 25.4o, 37.8o, 48.2o, 54.1o, 55.2o, 62.9o, 69.0o, 70.5o and 75.4o are corresponding to (101), (004), (200), (105), (211), (204), (116), (220) and (215) crystal planes of the anatase TiO2, respectively. It is clearly seen the intensity of the character diffraction peaks of anatase TiO2 decreases gradually with the increasing of cerium doping, suggesting the crystal structures of anatase TiO2 was destroyed. 9



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Theoretically, the ionic radius of Ti4+ (0.068 nm) is significantly smaller than the ionic radii of Ce3+ (0.111 nm) and Ce4+ (0.101 nm).

41

Thus, Ce dopants cause the lattice

contraction of TiO2, further hindering the crystallization process. The bump at about 21o indicates the formation of amorphous state by cerium doping. After cerium doping, the characteristic peak of (101) crystal plane shows a certain deviation, illustrating that the cerium is successfully incorporated into the TiO2 lattice, which is consistent with previous report.39 Additionally, the characteristic diffraction peaks of cerium oxides are not found in the XRD images, which could be caused by the low doping amount of cerium or the most of the Ce doped into the TiO2 lattice.

Figure 2. XRD patterns of TiO2, 0.1% Ce–TiO2, 0.5% Ce–TiO2 and 1.0% Ce–TiO2

The crystallite sizes are calculated based on our analysis and the average crystallite sizes of pure TiO2, 0.1% Ce–TiO2, 0.5% Ce–TiO2 and 1.0% Ce–TiO2 are estimated to be 24, 11.1, 7.6 and 5.7 nm, respectively. The decreasing trend of crystallite size with the increase of Ce doping can be explained by the reason that lanthanide–cerium is not as active as TiO2 precursor species and the doping of Ce ions slow down the condensation and crystallization process of TiO2. In the XRD spectra, the weaken

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intensity and increased width of diffraction peaks also evidence that the crystallinity decreases gradually. 3.2 SEM and TEM The scanning electron microscope (SEM) and the transmission electron microscopy (TEM) analysis were carried out to investigate surface morphology and particle sizes of samples (Figure 3). It is obviously observed that Ce–doped TiO2 is composed of well–shaped mesoporous spherical particles (Figure 3a) and the average particle size of nanoparticles in mesoporous spherical particles was estimated to be about 10 nm (Figure 3c). Figure 3b shows the SEM image of Ce–doped TiO2 samples with Pd loading. Compared to Ce–doped TiO2 catalyst, the same morphology indicates that Pd loading by the glucose reduction method has no effect on the morphology of samples. Besides, the palladium nanoparticles in Figure 3b are seldom visible, possibly due to its low amount and high dispersion. The HRTEM image of the 1.0% Pd/0.5% Ce–TiO2 sample in Figure 3d exhibits that the lattice spacing of 0.352 and 0.194 nm are corresponding to the (101) plane of anatase TiO2 and (200) plane of palladium nanoparticle, respectively. The presence of palladium nanoparticle of 4 nm diameter confirms the successful loading of palladium nanoparticle.

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Figure 3. (a, b) SEM images of (a) 0.5% Ce–TiO2 and (b) 1.0% Pd/0.5% Ce–TiO2; (c) TEM images of 1.0 % Pd/0.5% Ce–TiO2; (d) HRTEM images of 1.0% Pd/0.5% Ce–TiO2

Figure 4 shows the STEM and mapping images of 1.0% Pd/0.5% Ce–TiO2. The energy dispersive X–ray spectroscopy (EDX) mapping analysis was used to investigate the elemental distribution. As displayed in Figure 4a, mesoporous spherical particles of selected 1.0% Pd/0.5% Ce–TiO2 are observed. Panels b–e of Figure 4 show the homogeneous distributions of Ti, O, Ce and Pd elements, respectively. The analysis of EDX in Figure 4f also reveals the presence of all elements.

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Figure 4. (a) STEM image of 1.0% Pd/0.5% Ce–TiO2 and (b−e) corresponding EDX mappings of 1.0% Pd/0.5% Ce–TiO2 in the region shown in panel (a), indicating the particular distributions of (b) Ti, (c) O, (d) Ce, and (e) Pd. (f) EDX pattern of 1.0% Pd/0.5% Ce–TiO2 catalyst

3.3 UV–Visible DRS The ultraviolet–visible absorption spectrum of pure TiO2 and Ce–doped TiO2 samples are shown in Figure 5 within the wavelength range of 200nm to 800nm. Compared to the commercial anatase TiO2 with the maximum absorption wavelength of 380 nm, the pure TiO2 samples (398 nm) prepared by sol–gel method shows a certain red–shift arising from a smaller particle size of TiO2. Ce ions doping extends the absorption edge of TiO2 from UV region to visible region. With the increasing of Ce doping content, the color of catalysts gradually change from white to yellow and 13



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the absorption edge gradually moves towards the long wavelength region accordingly. The calculated band–gap energies of pure TiO2 and Ce–doped TiO2 samples are shown in the Figure 5. The optical band–gap energies are estimated by the following equation for semiconductors: αhν = A (hν – Eg)

n/2

. Where α, h, ν, A and Eg

represent the absorption coefficient, planck constant, light frequency, proportionality constant and band–gap energy, respectively. The value of n depends on the optical transition way of semiconductor materials and n is equal to 1 for an allowed direct transition of TiO2.50 The Eg value of pure TiO2 prepared by sol–gel method is estimated to be 3.13 eV. After Ce doping, the absorption edge exhibits a increasing red shift and the Eg value of 0.1%, 0.5% and 1.0% Ce content are extended to 3.0, 2.95 and 2.87 eV, respectively. These above results indicate that the modification of TiO2 with cerium can affect the energy band structure, shifting the spectral response of photocatalysts to visible light.

Figure 5. UV−vis absorption spectra of TiO2 and Ce–doped TiO2 samples

3.4 Raman Spectroscopy

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The Raman spectroscopy is an effective tool to evaluate the structural properties of Ce–doped TiO2 catalysts. It can be seen from Figure 6 that the main bands of TiO2 prepared by sol–gel method at 147, 198, 396, 515, 519 (with 515 cm–1 peak overlap) and 638 cm–1 are corresponding to Eg(1), Eg(2), B1g(1), A1g, B1g(2) and Eg(3) modes of anatase TiO2, respectively.29 All of the Raman characteristic peaks are closely related to the stretching and bending vibrations of the Ti–O bond. The Eg, B1g and A1g peaks are due to symmetrical stretching vibration, symmetric bending vibration and antisymmetric bending vibration of O–Ti–O, respectively.29 With increasing of Ce content in the range of 0.1–1.0%, no any peaks of Ce species are found and the peaks of anatase TiO2 move to higher wavelengths. In fact, the ionic radii of Ce 3+ and Ce4+ are much bigger than that of Ti4+, so Ce doping can lead to the contortion and distortion of lattice structure. The substitution of Ce for Ti element forms the new Ce–O–Ti or Ce–O–Ce key. During the process of doping, the destruction of the Ti–O–Ti keys and the formation of Ce–O–Ce keys will affect the Raman active mode, further causing deviation of the Raman peaks.

Figure 6. Raman spectra of TiO2 and a series of Ce–TiO2 samples 15



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3.5 N2 Adsorption–Desorption Figure 7 shows the N2 adsorption–desorption isotherms (Figure 7a) and pore size distribution curves (Figure 7b) of pure TiO2 and Ce–doped TiO2 samples. All photocatalysts exhibit a type–Ⅳ adsorption with a hysteresis loop, indicating the existence of the mesopore structure.52,53 The pore size distributions of pure TiO2, 0.1% Ce–TiO2, 0.5% Ce–TiO2, 1.0% Ce–TiO2，and 1.0% Pd/0.5% Ce–TiO2 are similar, with a uniform diameter in the range of 5–8 nm.

Figure 7. (a) N2 adsorption–desorption isotherms (b) pore size distribution curves

Table 1 lists the BET surface areas, pore size and pore volume of different catalysts. It can be seen that Ce doping has a remarkable influence on the BET surface areas, pore size and pore volume. The BET surface area of pure TiO2 is 35.974 m2/g and increased to 45.573, 52.408 and 63.877 m2/g with the Ce doping content of 0.1%, 0.5% and 1.0%, respectively. However, when 1.0% Pd was included, the BET surface area of 0.5% Ce–TiO2 was decreased to 49.322 m2/g. This slight reduction was possibly as a result of partial coverage of the pores by Pd particles. A volcano–type relationship between the pore size and the concentration of Ce doping was noticed, with the maximum value of 68.11 Å obtained on the 0.5% Ce–TiO2 sample. Generally, 16


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the pore volume increases with the increase of Ce doping. The behavior indicates the inhibition effect of Ce doping towards TiO2 crystal growth. Table 1. Specific surface area, pore size and pore volume of the different photocatalysts

Catalyst

SBET (m2/g)

Dpore (Å)

Vpore (cm3/g)

TiO2

35.974

59.51

0.063

0.1% Ce–TiO2

45.573

64.25

0.149

0.5% Ce–TiO2

52.408

68.11

0.166

1.0% Ce–TiO2

63.877

62.25

0.189

1.0%Pd/ 0.5% Ce–TiO2

49.322

69.71

0.159

3.6 Photoluminescence Spectra Figure 8 shows the PL spectra of the catalysts with the exciting wavelength of 325nm. The lowered PL intensity because of Ce doping indicates that the recombination rate of photogenerated electron–hole is decreased (Figure 8a). With the increasing Ce content, the PL intensity shows a tendency of decreasing firstly and then increasing, and the 0.5% Ce–doping TiO2 photocatalyst exhibits the lowest PL intensity. The phenomenon of PL intensity increasing again may be because excessive Ce amount form a new recombination center, accelerating charge recombination. Pd nanoparticle (1.0%) loaded on the surface of catalysts can aggregate electron, promote the separation of electron–hole, and decrease PL intensity (Figure 8b). For Pd/TiO2 catalyst, the electrons on the conduction band of TiO2 transfer to the Pd nanoparticle.21 Similarly, Pd nanoparticles on the surface of Ce–doped TiO2 absorb the electrons from Ce impurity levels and TiO2 conduction band. The strong 17



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electron–withdrawing property of Pd nanoparticles can improve the shift of Ce3+ and Ce4+, and the lowest PL intensity of Pd/0.5% Ce–TiO2 is attributed to the synergistic effect of Pd and Ce.

Figure 8. PL spectra of different catalysts measured at an excitation wavelength of 325 nm

3.7 CO2–TPD To investigate the active site of CO2 adsorption, we performed CO2–TPD test (see Figure 9). It has been reported that CO2 has different adsorption modes, but mainly in the forms of molecularly adsorbed CO2 and carbonates, on the surface of metal oxide.52 The monodentate (m–CO32–) and bidentate carbonates (b–CO32–) will be formed if CO2 molecule combines directly with oxygen atoms or simultaneously with both oxygen and metal atoms from Pd/Ce–TiO2. For the generation of HCO3–, it is due to the reaction of CO2 molecules and surface hydroxyls. The characteristic peaks can be attributed to the decomposition of molecularly adsorbed CO2 (75–180oC), HCO3– (180–380oC), b–CO32– (380–550oC), and m–CO32– (550–760oC), respectively.52,57 Obviously, loading of Pd nanoparticles will increase the adsorption efficiency of CO2 on the catalyst, indicating an increased base position as well as the increased number of active site for the reaction over Pd nanoparticles. 18


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Figure 9. CO2–TPD profiles for TiO2, 0.5%Ce–TiO2, 1.0%Pd/TiO2, 1.0%Pd/0.5%Ce–TiO2

3.8 XPS Analysis The XPS analysis was performed to investigate the chemical composition and the element state of as–prepared samples. Figure 10a shows the survey spectrum of Pd/Ce–TiO2 catalyst, which indicates the presence of O, Ti, Ce and Pd elements. As displayed in Figure 10b, the two peaks at binding energies of 530.1 and 531.9 eV are assigned to the lattice oxygen of TiO2 and the oxygen of hydroxide ions.47 In the XPS analysis, all the binding energies were referenced to the C 1s peak at 284.6 eV (Figure 10c). The two multiplets at 458.5 and 464.4 eV (Figure 10d) in the Ti 2p spectrum are corresponded to Ti 2p1/2 and Ti 2p2/3 respectively, suggesting that the main existent form of Ti is the chemical state of Ti4+.29 The weak characterized peaks of Ce 3d spectrum were caused by low Ce doping content (Figure 10e). Generally, the Ce 3d spectrum could be deconvoluted into four pairs of spin−orbital bands. Specifically, the peaks located at 916.38, 906.73, 904.27, and 902.56 eV, correspond to the β1, β2, β3, and β4 components of Ce 3d3/2, respectively; While the peaks of Ce 3d5/2 at 899.6, 19



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890.781, 885.8, and 882.0 eV can be indexed to α1, α2, α3, and α4 constituents, respectively.55 The results indicate the coexistence of Ce3+ and Ce4+, which is helpful for improving the photocatalytic efficiency.25 For Pd 3d spectrum (Figure 10f), the peaks at 335.1 and 340.2 eV are related to Pd 3d5/2 and 3d3/2 states of metallic Pd, respectively. The shoulder peak at about 336.2 eV implies the oxidation state of Pd, probably in the form of PdO from surface oxidation.56

Figure 10. XPS spectra of Pd/Ce–TiO2 catalyst, (a) survey spectrum, (b) O 1s, (c) C 1s, (d) Ti 2p, (e) Ce 3d, (f ) Pd 3d

3.9 Photocatalytic Activity towards CO2 Hydrogenation under Visible Light The photocatalytic performances of as–prepared samples were evaluated by the reaction of photocatalytic CO2 hydrogenation under visible light. The experiments were carried out in a miniature visual autoclave under the total pressure of 2.5 MPa (H2:CO2=4:1) and the Xe lamp was provided as the source of visible light. It can be observed from Figure 11a and Figure 11b that CH4 are the main products of CO2 20


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hydrogenation with a small amount of CO. The yields of CH4 and CO increase as the extension of the illuminating time within 3 h. Noticeably, the catalysts without Pd loading were also used for the reaction and only a trace of CH4 was detected. The poor activities of Ce doping catalysts without Pd loading are possibly due to the absence of active sites for activation between CO2 and H2 on the surface. The role of palladium nanoparticle is collecting electrons to activate CO2 and H2, further to promote the reaction. The 1.0% Pd/TiO2 catalysts exhibit a weak photocatalytic activity with 92.39 μmol/g of CH4 and 9.43 μmol/g of CO under visible light irradiation for 3h. This phenomenon can be explained by the reason that the 1% Pd/TiO2 can only be excited by a small amount of visible light. The Pd/Ce–TiO2 catalysts exhibit higher photocatalytic activities compared to the Pd/TiO2 catalysts, indicating that Ce plays a crucial role for photocatalytic CO2 hydrogenetion. With the increase of Ce doping content, the photocatalytic activity shows a tendency of first increased and then decreased. The highest activity is performed with 1.0% Pd/0.5% Ce–TiO2 catalyst and the yields of CH4, CO are reached 220.61 and 27.36 μmol/g respectively under visible light irradiation for 3 h, which is about 2.39 and 2.90 times than that of pure 1.0% Pd/TiO2. These results are attributed to the impurity levels formed by Ce doping, which extend the wavelength response to the visible region. In addition, the conversion between the two valence states of Ce3+ and Ce4+ can separate the electron–hole pairs and promote the reaction. During the illumination, no other heating devices were used and the change curve of the reaction temperature with the illumination time is shown in Figure 11c. The 21



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temperature measured in the reaction was no more than 70 oC under visible light for 3 h. The control experiment (Figure 11d) under the dark with the temperature of 70 oC was carried out to investigate the photocatalytic process of CO2 hydrogenation. Compared to the photocatalytic process (CH4: 220.61 μmol/g), the yield of CH4 in thermo–catalytic process at 70 oC was 40.9 μmol/g over 1.0% Pd/0.5% Ce–TiO2 catalyst for 3 h. The greatly enhanced performance under illumination indicates that the CO2 hygrogenation is a photocatalytic process, not simply a thermal process.

Figure 11. Effect of irradiation time on products of CO2 hydrogenation with a series of Pd/Ce–TiO2 catalysts, (a) CH4, (b) CO. (c) Relationship between temperature and irradiation time. (d) Yield of CH4 under visible light and dark 70 oC

The blank experiment under the absence of catalyst with other identical conditions was operated. A small amount of CH4 was produced, which indicates that the CO2 hydrogenation reaction can occur without the catalysts and the role of catalysts here 22


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are great enhancement for the reaction activity. To verify carbon source of all products, N2 was chosen to replace CO2 with other conditions unchanged in the control experiment. Absence of CH4 in detection suggests that the carbon source in CH4 comes from CO2 and the catalyst surface is clean. To investigate the synergistic effect of Pd and Ce for the enhanced activity, a series of experiments and researches were conducted. Note blank experiments with the sole use of Al2O3 and TiO2 indicated trace yield of CH4. As shown in Figure 12, a CH4 yield of 32.3 μmol/g under visible light for 3 h over the 1.0% Pd/Al2O3 catalyst was gained. Since the inert material Al2O3 is not sensitive to light, this activity can be considered as the effect of Pd independently. On the other hand, the yield of CH4 over 0.5% Ce–TiO2 is the smallest (< 13.86 μmol/g). However, when 1.0% Pd was introduced, the activity was significantly improved with a CH4 yield of 220.61 μmol/g. These results demonstrated the crucial role of Pd towards photocatalytic CO2 hydrogenation and for selective CH4 production. It is worth pointing out that using a Pd contained catalyst yet without Ce, such as 1.0% Pd/TiO2, this improvement is also limited. The synergistic effect of Pd and Ce on the surface of TiO2 for enhanced photocatalytic activity is thus clearly elucidated. Although thermodynamically, Ce–TiO2 is capable for selective CO2/CH4 conversion (conduction band potential (ECB) ≈ –0.3 V, EoCO2/CH4 = –0.24 V at pH = 7), 59

only a very small amount of CH4 (< 13.86 μmol/g) was detected by the sole use of

Ce–TiO2 catalyst. Basically, CO2/CH4 is a reduction process containing 8–electron transfer. This multiple electron transfer process will lead to increased electron–hole 23



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recombination possibility thus reduced electron concentration at the surface of Ce–TiO2 particles. The CO2/CH4 process is sequentially restricted. However, electron transfer can be accelerated by trapping electrons at a metal cocatalyst such as Pd. On the other hand, Pd nanoparticles have strong adsorption ability to CO2,

58

which can

be also confirmed by the CO2–TPD results shown in Figure 9. As a result, the yield of CH4 increased from 13.86 μmol/g to 220.61 μmol/g by loading Pd nanoparticles onto the Ce–TiO2 surface.

Figure 12. Yield of CH4 over 1.0% Pd/Al2O3, 1.0% Pd/TiO2, 0.5% Ce–TiO2 and 1.0% Pd/0.5% Ce–TiO2 catalysts

3.10 Mechanism of Photocatalytic CO2 Hydrogenation Based on the above analysis, a possible mechanism of photocatalytic CO2 hydrogenation is speculated (Figure 13). For the pure TiO2, only a small amount of visible light can be absorbed. After Ce doping, the impurity levels formed extend the photoresponse range of TiO2 to visible light region. Typically, the photogenerated electron–hole pairs are produced under the excitation of visible light and the electrons enriched on the VB of TiO2 transmit to the new impurity levels of cerium, leaving the 24


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holes on the VB to oxide H2 molecules to H+. The d–d transition inside the impurity states ensure the transfer of electrons. At the same time, a part of the electrons on the impurity states are transferred to the CB of TiO2. The adjacent Pd nanoparticles attract the electrons from the CB of TiO2 and impurity states of Ce to provide active sites to promote the reaction.

Figure 13. Schematic illustration of charges transfer

The detailed reaction mechanism over cerium doped 1.0% Pd/TiO2 towards photocatalytic CO2 hydrogenation is displayed in Figure 14. Pd nanoparticles, as the co–catalyst, can aggregate electrons to activate CO2 molecules. A strong adsorption of CO2 on the surface of catalysts is formed under visible light due to the effect of palladium and Lewis acid sites. Meanwhile, the electrons transferred to impurity levels and a part of Ce4+ are reduced to Ce3+. However, the electrons gathering effect of Pd nanoparticles can take a electron from Ce3+, causing a new Ce4+. The shift between the two valence states of Ce3+ and Ce4+ contributes the separate of charges, further promoting the reaction. The electrons obtained on Pd nanoparticles are injected into CO2 molecule to yield CO2–, which then reacts with the oxidized H+ and e– to form a intermediate Pd–C=O. On the one hand, CO can be generated from the 25



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desorption of Pd–C=O. On the other hand, Pd–C=O interacts with Pd–H which from the reaction of H+ and e–, to produce Pd–C species. Finally, the Pd–C species continue to react with the Pd–H species for obtaining the product CH4.

Figure 14. The surface reaction mechanism of photocatalytic CO2 hydrogenation

The Figure 15 depicts the stability of the catalysts. As shown in the five–reaction cycles, the yield of CH4 and CO is slightly decreased with the extension of illumination time. The reason is possibly attributed to the carbon deposition induced by Pd nanoparticles at the reaction stage Ⅵ (see Figure 14). More specifically, at the beginning of the reaction, the deposition of carbon does not occur severely. In this case, the initial efficiency for adsorption of CO2 and subsequent reduction reaction is relatively high. However, the amount of carbon deposition increases gradually with the reaction further proceeded. The number of active site of CO2 is thus decreased, which leads to the slightly decreased yield of CH4 and CO. However, it will be eventually reached a stable point as the amount of carbon deposition will be balanced finally.

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Figure 15.The photocatalytic stability of 1.0% Pd/0.5% Ce–TiO2 after five cycles

4、Conclusions The overall photocatalytic activity towards CO2 hydrogenation under visible light irradiation is significantly enhanced by Ce doping. Various characterizations show that Ce atoms can be doped into the lattice matrix of TiO2 as a substitution of Ti sites, leading to the formation of impurity energy levels in the band–gap of TiO2. The light response property of TiO2 is thus extended to the visible light region. It is found that the catalysts in the form of 1.0% Pd/0.5% Ce–TiO2 exhibited the best photocatalytic activity under the visible–light irradiation for 3 h. The yields of CH4 and CO reached 220.61 and 27.36 μmol/g, respectively. In addition to the extended light absorption property, our experimental analysis indicates that the synergistic effect taken by Pd and Ce is also of crucial importance to the enhanced photocatalytic performance. Specifically, the charge separation inside the crystal taken by the Ce3+ and Ce4+ ions, subsequently the surface electron capture and CO2 activation given by Pd nanoparticles synergistically promote the formation of CH4.

AUTHOR INFORMATION Corresponding Author 27



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* Phone: (+86)025–52090621. Fax: (+86)025–52090620. E–mail: [email protected] * Phone: (+86)025–52090621. Fax: (+86)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, 3207045409, 3207046414), National Natural Science Foundation of China (No. 21576050 and No. 51602052), Jiangsu Provincial Natural Science Foundation of China (BK20150604), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and Zhongying Young Scholars of Southeast University.

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