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The Photocatalytic Oxidative Dehydrogenation of Ethane using CO2 as a Soft Oxidant over Pd/TiO2 Catalysts to C2H4 and Syngas Ronghao Zhang, Hong Wang, Siyang Tang, Changjun Liu, Fan Dong, Hairong Yue, and Bin Liang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02441 • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018
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The Photocatalytic Oxidative Dehydrogenation of Ethane using CO2 as a Soft Oxidant over Pd/TiO2 Catalysts to C2H4 and Syngas Ronghao Zhang,1 Hong Wang,3 Siyang Tang,1* Changjun Liu,1,2 Fan Dong,3 Hairong Yue,1,2* Bin Liang1,2 1
Low-carbon Technology and Chemical Reaction Engineering Laboratory, School of Chemical Engineering, Sichuan University, Chengdu 610065, China
2
Institute of New Energy and Low-Carbon Technology, Sichuan University, Chengdu 610207, China
3
Chongqing Key Laboratory of Catalysis and New Environmental Materials, College of Environment and Resources, Chongqing Technology and Business University, Chongqing 400067, China
ABSTRACT: Using CO2 as the soft oxidant for the oxidative dehydrogenation of ethane (ODE) is a potential advancement for ethylene production from ethane. However, the current ODE reaction is primarily operated at high temperatures (e.g., 873K), and the development of an alternative approaches for the ODE reaction under the mild conditions is still a challenge. Herein, we report a photocatalytic ODE using CO2 as the oxidant over Pd/TiO2 catalysts at room temperature. The presence of CO2 significantly promoted the production of C2H4 and syngas, and the 1% Pd/TiO2 catalyst exhibited a C2H4 production rate of 230.5 μmol/gcat·h and syngas of 282.6 μmol/gcat·h. Density functional theory (DFT) calculation verified that the intermediate energy level provided by Pd and the covalent bond in Pd-O stimulated the electron transfer, excitation and separation. The photoinduced electron, hole and isolate OH on the surface of TiO2 played essential roles during the whole process. In addition, the possible reaction pathways of photocatalytic ODE with CO2 were proposed on the basis of the experimental data and DFT calculation results. KEYWORDS: Oxidative dehydrogenation of ethane, photocatalysis, Pd/TiO2, ethylene, in situ FTIR. Exploitation of shale gas is one of the most meaningful energy revolutions of the 21st century. Ethane, the secondary constituent (approximately 10%) of shale gas, is also required for effective and economical utilization.1 Oxidative dehydrogenation of ethane (ODE) is an energyefficient and environmentally promising approach to convert ethane into ethylene, which is a major feedstock for the synthesis of a wide variety of commercial products, such as polyethylene, ethylene oxide, ethylene glycol, styrene, etc.2 The current ODE reaction uses several oxidants, such as dry air, O2, SO2 and N2O. Among them, the use of CO2 as a soft oxidant could effectively enhance the ethane conversion and ethylene selectivity by inhibiting the deep oxidation of ethane to avoid coke and changing the chemical equilibrium of ethane dehydrogenation.3 Current studies on performing ODE with CO2 mainly focus on thermocatalytic systems, using Cr, Mo, Ce, and Ga-based catalysts and operating at relatively high temperatures (e.g., 873 K).2b, 4 However, the high-temperature reaction induces coke and carbon deposition, while the reduction of the reaction temperature leads to low ethane conversion, due to the high active energy (a Gibbs free energy of 101 kJ/mol for ethane dehydrogenation, 121 kJ/mol for ODE with CO2 and 750 kJ/mol for C=O activation of CO2) and thermodynamic constraints.5 Therefore, the development of novel catalytic systems in mild conditions are desirable for the ODE process.
Since the ODE with CO2 process includes both the ethane dehydrogenation and CO2 conversion reactions, the efficient activation of the C-H and C=O bonds in mild conditions is crucial for the reduction of the active energy and the temperature in the ODE reaction. Photocatalysis is an alternative approach using clean solar energy to activate the C-H and C=O groups in mild conditions and is widely applied in the dehydrogenation and CO2 conversion reaction. For instance, in methanol dehydrogenation reactions, the C-H of CH3OH could be activated by a photoinduced hole and transformed to •CH2OH and H+ in room temperature.6 In addition, the reduction of CO2 to CO, CH4, CH3OH and HCOOH, 7 and the CO2-based reforming of CH4 to CO and H2 were also reported using the photocatalytic systems.8 The CO2 activation could be facilitated by the photogenerated electron donating from the photocatalyst, which could transfer to the π* orbital of CO2 to form asymmetric CO2δ•- and lower the lowest unoccupied molecular orbital (LUMO) level as the molecule bends. Therefore, the utilization of photoinduced electrons and holes in photocatalysis for the two coupling catalytic reactions may facilitate the efficient separation and adequate utilization of photoinduced electrons and holes and thus improve the reaction efficiency. TiO2 was frequently used as the catalyst support in the photocatalytic system since it possesses photo-stability, and suitable valence band and conduction band positions. Since bulk TiO2 often exhibits poor activities due to the high recombination rate for photoinduced electron and
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hole pairs, the deposition of noble metals (such as Pd, Pt and Au) onto TiO2 could enhance the lifetime of photogenerated electron and holes.9 Herein, a novel ODE with CO2 reaction was explored in a photocatalytic system under mild conditions (approximately 308 K) for the production of ethylene and syngas. Pd-, Pt- and Au-doped TiO2 have been investigated in this reaction. The catalytic activities were tested in a batch stirred tank reactor,
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shown in Figure S1, at a temperature of 308 K and pressure of 0.2 MPa. The band structures, electronic structures and electron transfer pathways of the Pd/TiO2 catalysts were illustrated using density functional theory (DFT) calculations. The possible photocatalytic reaction mechanisms of ODE with CO2 were proposed on the basis of the in situ spectra and theoretical calculations.
Figure 1. The photocatalytic activities of the catalysts: (a) in dark, Ar and CO2 conditions, (b) with different metals and supports, and (c) different fractions of the Pd-doped TiO2. Reaction condition: CO2 (Ar):C2H6=1:1, 25mg of catalyst, a 1h irradiation time, 0.2MPa, and at ambient temperature. Figure 1 shows the photocatalytic activities of the ODE with CO2 reaction under several crucial reaction conditions. Obviously, the reaction did not proceed without illumination. The existence of CO2 remarkably promoted the C2H6 conversion and the C2H4 selectivity compared with the argon atmosphere (Figure 1a). Moreover, the Pd/TiO2 catalyst showed much higher C2H4, H2 and CO production rates than that of the Pt/TiO2, Au/TiO2 and Pd/SiO2 catalysts (Figure 1b). The difference of catalytic performance may be attributed to the following evidence: (a) SiO2 has a large band gap and is not easily activated by light; (b) Noble metals of VIII B (e.g., Pt and Pd) are active for alkanes and facilitate the C2H6 dehydrogenation and CO2 hydrogenation; (c) The Pd-based catalysts possess lower C-C cleavage activity and CO adsorption abilities than Pt.10 The photocatalytic activity of the Pd/TiO2 catalysts with varying amounts of Pd was shown in Figure 1c and Table S1. The bulk TiO2 exhibited poor photocatalytic activity while the Pd addition induced a dramatic promotion of the amount of all the products. A steady increase of the syngas (CO &H2) and C2H4 production rate was observed with the increment of Pd loading, reached a maximum over the 1% Pd/TiO2 catalyst. The highest production rate (C2H4 of 230.5 μmol/gcat·h, H2 and CO total amount of 282.6 μmol/gcat·h), selectivity of C2H4 (95.42%) and quantum yield (0.93%, Table S2) over the 1% Pd/TiO2 catalyst may result from the high separation efficiency of charge carriers and the synergistic effects of Pd and TiO2. To reflect the benefit of the present ODE reaction system, the catalytic activity of the similar typical systems and their reaction conditions are summarized in Table S3. In the thermo-catalytic ODE systems, the C2H4 selectivity (70% and 38% using O2 and CO2 as the oxidant, respectively) is limited due to the thermocracking or hydrogenolysis of
ethane at high reaction temperatures (600~700ºC). The present photocatalytic ODE reaction exhibited a C2H4 selectivity of 95.42% (Table S1) at ambient temperature. The present work obtained a C2H6 conversion of 0.14%, which is a very low value, but it is thousands of times higher than the equilibrium conversion of C2H6 (Table S4) at ambient temperature. If the use of thermo-catalytic reaction, it requires a reaction temperature of about 225 ºC to obtain C2H6 conversion of 0.14%. When compared with other photocatalytic CO2 and alkanes conversion reaction systems, this process could not only produce C2H4 and CO, but also generate extra hydrogen. To explore the reason for the excellent activity for the photocatalytic ODE with CO2 process over the Pd/TiO2 catalysts, the physicochemical properties were characterized via several frequently used techniques. Figure 2a shows the powder X-ray Diffraction (XRD) patterns of the x%-Pd/TiO2 catalysts (x=0, 0.2, 0.5, 1, 1.5, 2). The diffraction peaks labeled as ★ and ▲ are characteristic of anatase TiO2 (PDF no. 21-1272) and rutile TiO2 (PDF no. 211276), respectively, which showed no significant changes in all the samples. The diffraction peaks of metallic Pd (labeled as ◆, PDF no. 46-104 3) were not observed when the Pd loading was below 1.5 %wt and became apparent when the Pd loading was higher than 1.5 %wt. The XRD results indicated a high dispersion of Pd nanoparticles on TiO2 loaded with less than 1.5 % Pd, and high loading will lead to an aggregation. Figure 2b shows the scanning electron microscope (SEM) images of the 1% Pd/TiO2 catalyst. The TiO2 nanoparticles were distributed uniformly with an average diameter of 30-40 nm. The high-resolution transmission electron microscopy (HRTEM) image of 1% Pd/TiO2 in
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Figure 2c revealed that Pd nanoparticles had an average diameter of around 3 nm. Lattice fringe spacings of 0.353 nm, 0.325 nm, 0.23 nm and 0.195 nm were indexed as the (101) plane of anatase TiO2, (110) plane of rutile, and the (111) and (200) planes of palladium, respectively. X-ray photoelectron spectroscopy (XPS) was used to identify the surface composition and chemical state of pure TiO2 and 1% Pd/TiO2 samples. Figure 2d and figure 2e show the Ti 2p and O 1s spectra of the two samples. The peaks at 458.5 eV and 464.4 eV in the Ti 2p spectrum could be assigned to Ti 2p1/2 and Ti 2p3/2, indicating the presence of Ti4+.11 In the O 1s spectrum, the main peak at 529.5 eV was assigned to the lattice oxygen of TiO2, while the minor peak at 531.5 eV was assigned to the oxygen of the surface hydroxyl of TiO2.12 However, for the 1% Pd/TiO2 catalyst, the binding energy of Ti 2p and O 1s was shifted to a higher value, which indicates the strong interaction between Pd and TiO2.13 Figure 2f shows the Pd 3d spectrum of 1% Pd/TiO2. The peaks centered at 336.2 eV and 341.8 eV indicated Pd species with oxide states. The banding energy at 335.1 eV and 334.0 eV, corresponding to the Pd 3d5/2 and Pd 3d3/2 peaks, were assigned to metallic Pd.14 Therefore, the primary Pd species over the catalysts were
in the form of the Pd0 state. Figure S2 and Figure S3 showed the H2-temperature programmed desorption patterns of x%-Pd/TiO2 and the H2-temperature programmed reduction profiles of TiO2, fresh PdO/TiO2 and photo-reduction Pd/TiO2 samples. There was no obvious relationship between the surface area of Pd and the catalytic performance (Table S5), indicating the activity may be not only determined by the Pd surface area, but also the excitation electron of TiO2 support.6 The PdO/TiO2 showed a sharp reduction peak at around 30ºC, while the Pd/TiO2 showed no reduction peak at this temperature, which means the Pd species over the catalyst could be reduced to Pd0 using a photo-reduction method. The negative peaks observed in the region of 60-100 ºC were the decomposition of the palladium hydride. A peak at around 600 ºC in the profile of bulk TiO2 sample shifted to around 300 ºC in the Pd/TiO2 and PdO/TiO2 samples, which could be attributed to the reduction of surfacecapping oxygen. This result has verified the strong interaction of Pd with TiO2, and the Pd could promote the reduction of the lattice oxygen on the TiO2 through the spillover effect.15
Figure 2. Morphology and structures of Pd/TiO2 catalysts: (a) The XRD patterns of the x%-Pd/TiO2(x=0, 0.2, 0.5, 1, 1.5, 2), (b-f) SEM, HRTEM and XPS of 1%Pd/TiO2. Based on the above characterization results, a typical model of a Pd/TiO2 catalyst was established for DFT calculation to further reveal the synergistic effects of Pd and TiO2. Figure 3a shows the density of state (DOS) of TiO2 (green line), the single Pd atom loading on TiO2 (blue line) and the partial density of state (PDOS) of the Pd atom (purple line). Compared to pure TiO2, the lower valence band and conductive band energy of Pd/TiO2 indicated the stronger oxidization ability of photogenerated holes. Due to the orbital of Pd atoms, an intermediate level formed in Pd/TiO2, which contributed to the electron excitation, transformation and separation. The results were also illustrated by ultraviolet-visible absorption spectra of x%-Pd/TiO2 (Figure S4), indicating that the
electrons in TiO2 were more easily promoted in samples with the Pd loading. Moreover, the electronic location function (ELF) of the Pd (111) plane exposed on TiO2 is shown in Figure 3b. The green contours around the Pd and O atoms indicated the existence of covalent bonding between the Pd and O layer and a channel for hot electron transfer in Pd-O. To further illustrate the direction of electron transport in the Pd-O layer, the charge difference distribution of elemental Pd and TiO2 was calculated and is shown in Figure 3c, with the electron gathering region shown in blue and the depletion zone in yellow. It is notable that the charges, which deviated from Pd and accumulated in the O layer of TiO2, were transported to the photoactive sites.
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Figure 3. DFT calculation results: (a) DOS of TiO2, single Pd onTiO2 and PDOS of single Pd atom; (b) the ELF of Pd (111) plane on TiO2; (c) the charge difference distribution between Pd (111) plane and TiO2. ESR spectra were measured to detect the separation efficiency of charge carriers and the involved active species during the photocatalytic process of ODE with CO2. An ESR spin-trapping technique with 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) and 5,5-dimethyl-1-pyrroline Noxide (DMPO) was conducted to investigate the amount of photoinduced electrons, holes and hydroxyl radicals under irradiation using the x%-Pd/TiO2 catalysts. Figure 4a shows the signals of the residual electron sacrificial agent in the solution, which could provide the amount of photogenerated electrons produced over the catalysts.16 The TEMPO depleted rapidly after loading Pd on pure TiO2, illustrating that more free electrons were motivated and separated, and the electron mobility could be significantly boosted in Pd/TiO2 catalysts than in pure TiO2, which is in accordance with the results of the DFT calculation. The 1% Pd/TiO2 sample exhibited a relatively weak
TEMPO signal and generated more active electrons than the other catalysts, thus leading to a maximum gross product of CO and H2 (Figure 1b). Moreover, the stronger h+ signal of the Pd/TiO2 catalyst in Figure 4c further verified that Pd boosted the separation of the photoinduced electron and hole. The photo-generated hole, regarded as an oxidizing species, could directly oxidize the adsorbed reactant and the OH at the surface of the TiO2 (illustrated in the XPS spectra, Figure 2e) to form ·OH.17 Therefore, the amount of h+ could be used to estimate the production of ·OH. As shown in Figure 4c, the signal of DMPO·OH became stronger along with the increase in the Pd loading and reached a maximum when the Pd fraction was 1 %wt. These results were consistent with the production of C2H4 (Figure 1b), indicating that the ·OH was a crucial species for the activation of the ethane molecule.
Figure 4. Spin-trapping ESR spectra under UV light irradiation for 10 min: (a) e- and (b)•OH and (c) solid state for h+ (d) active species trapping experiments with various scavengers, with the following reaction conditions: a CO2:C2H6 ratio of 1:1, 25 mg of catalyst, 3 mg of scavenger, 1 h of irradiation time, at a pressure of 0.2 MPa, and at ambient temperature.
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Figure 5. In-situ FTIR spectra of photocatalytic ODE with CO2 over 1% Pd/TiO2 under UV-light irradiation. Active species capture experiments with various scavengers were also carried out to further demonstrate the results in ESR analysis. Ammonium oxalate, potassium dichromate and tertiary butanol were used as hole, electron and hydroxyl radical scavengers via grinding the original catalyst and trapping the agents together.18 The results in Figure 4d showed that the trapping of holes induced a significant decrease in the C2H4 production rate from 230.5 to 77.9μmol/gcat·h, while it caused an increase in the H2 production rate from 162.2 to 526.8μmol/gcat·h. In contrast, when an electron scavenger was added to the system, the production rate of H2 was inhibited from 162.2 to 87.4μmol/gcat·h, while the C2H4 production rate was promoted from 230.5 to 314.7μmol/gcat·h. All the obtained products were reduced with the mixing of the hydroxyl radical scavenger. These results indicated that the photoinduced electron was crucial to the reduction of H+, while the photoinduced hole and hydroxyl radical played essential roles in the process of ODE to C2H4. In addition, the electron (hole) scavenger may facilitate the mobility and availability of the hole (electron), and the hydroxyl radical acted as an important intermediate oxidant to motivate the reaction. To further reveal the reaction mechanism, the in situ FTIR spectra of ethane and CO2 adsorption and reaction processes on 1%Pd/TiO2 in dark/under UV-light irradiation are recorded in Figure 5. The bands with apparent changes were marked with dotted line, the red lines represented to the decreasing or the disappearing bands, and the green line denoted the new appearance or increased bands. Table S6 summarized the assignments of labelled bands and the corresponding species. Under dark condition, the bans at 1455, 1403 and 1377cm-1 were assigned to the deformation vibrations of methyl groups, the bands at 2970, 2930, 2900 and 2878cm-1 were assigned to C-H stretching vibrations, and the band at 1252cm-1 was caused by the OH stretching vibration. The groups of bands mentioned above were generated by the interaction of ethane with the lattice oxygen and surface OH of TiO2.19 The band at 1677 cm-1 suggested the formation of CO2-, which requires the presence of a metal ionic site (e.g., Ti3+
in the current study) and surface OH.20 Meanwhile, the bands at 3727, 3703 and 3630cm-1 were assigned to the isolate OH on the surface of the TiO2, which is crucial to the whole reaction. After UV-light irradiation, all the adsorption species were depleted gradually, while several new species appeared. For instance, the band at 1444cm-1 belonged to the bending vibration of CH2=CH2, indicating the formation of ethylene.21 The surface carboxylate species were detected with bands at 1558 and 1351cm-1, which could be attributed to the νCOO asymmetric and symmetric stretching vibrations, respectively.22 In addition, the strong band at 1723cm-1 was attributed to the bending vibration of C=O, which was derived from the accumulation of CO. On the basis of the theoretical calculation and experimental results, a possible mechanism for the photocatalytic ODE with CO2 over the Pd/TiO2 catalyst is shown in Figure 6. At the first step, according to the results of ELF and charge difference distribution, the electron was transferred from Pd to TiO2 through the Pd-O covalent bond. Subsequently, the electrons were easily promoted from the valence band (VB) to the conduction band (CB) of electron enriched TiO2 under the excitation of UV light. The photoinduced holes and electrons of TiO2 were consequently separated to the VB and CB, respectively. The holes in the VB of TiO2 were captured by the free OH on the surface of TiO2 to form ·OH. Specifically, the C-H bond of ethane was activated through the formation of the alkoxides and adsorbed ethyl, which were then oxidized by h+ or ·OH to an intermediate radical. This unstable radical could rapidly form a lower-energy C=C bond. On the other hand, the photogenerated electron in the CB of TiO2 continuously transferred to the adjacent Pd atoms to maintain the neutrality of the Pd atoms. This phenomenon was demonstrated by the Schottky barrier theory between noble metals and metal oxide nanoparticles.23 Moreover, the Ti3+ and surface OH of TiO2 provided the sites for CO2 adsorption sites and conversion of the possible intermediates (CO2- and HCOO) to CO. Finally, the photoinduced electrons at the Pd sites combined with H+ and CO2 to form H2 and CO.
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Materials, experimental procedures, methods, and part of the catalyst characterization. This material is available free of charge via the Internet at http://pubs.acs.org.
ACKNOWLEDGMENT The authors are grateful for the support from the National Natural Science Foundation of China (21506128, 21576169), Outstanding young scholar fund of Sichuan University (2015SCU04A10), Qingdao Benzo New Materials Co., Ltd. (QDBC) (15H0829). This work is also supported by the Fundamental Research Funds for the central Universities (2015SCU11018).
REFERENCES
Figure 6. The possible reaction pathway and mechanism for the photocatalytic ODE with CO2 over Pd/TiO2 catalyst. In summary, we developed a novel photocatalytic process for the ODE reaction with CO2 as a soft oxidant under mild conditions. Pd/TiO2 catalyst exhibited a good photocatalytic activity in ODE reaction, and the highest production rate of C2H4 and syngas (H2 and CO) were obtained on 1% Pd/TiO2. The theoretical calculation results showed that the intermediate energy level (provided by the 3d orbital of Pd) and the electron channel (formed via the Pd-O covalent bonding) over the Pd/TiO2 catalyst facilitated the electron transfer, excitation and separation. The possible photocatalysis mechanism of ODE with CO2 on Pd/TiO2 was proposed on the basis of the ESR, FTIR characterizations and DFT calculation. The photoinduced e-, h+ and hydroxyl radical played primary roles in the ODE reaction. This work has demonstrated the feasibility of the mild condition for the ODE reaction, and we believe that the results will deliver a more attractive insight into both the conventional catalysis and the photocatalysis for the ODE reaction.
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected] (Siyang Tang); *E-mail:
[email protected] (Hairong Yue).
Author Contributions R.H.Z, H. W, S.Y.T, H.R.Y. and B.L. conceived and designed the experiments, analyzed the results and participated in writing the manuscript. All authors contributed to the discussions of the results in this manuscript.
Notes The authors declare no competing financial interest.
ASSOCIATED CONTENT Supporting Information.
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Reactions. Int. J. Photoenergy 2014, 9, 1-9. (7) (a) Fu, Y. H.; Sun, D. R.; Chen, Y. J.; Huang, R. K.; Ding, Z. X.; Fu, X. Z.; Li, Z. H., An Amine-Functionalized Titanium Metal-Organic Framework Photocatalyst with Visible-LightInduced Activity for CO2 Reduction. Angew. Chem.-Int. Edit. 2012, 51 (14), 3364-3367; (b) Zhang, L.; Zhao, Z.-J.; Wang, T.; Gong, J., Nano-designed semiconductors for electro- and photoelectro-catalytic conversion of carbon dioxide. Chem. Soc. Rev. 2018, 47(14), 5423-5443. (8) (a) Han, B.; Wei, W.; Chang, L.; Cheng, P. F.; Hu, Y. H., Efficient Visible Light Photocatalytic CO2 Reforming of CH4. ACS Catal, 2016, 6(2), 494-497; (b) Laszlo, B.; Baan, K.; Varga, E.; Oszko, A.; Erdohelyi, A.; Konya, Z.; Kiss, J., Photo-induced reactions in the CO2-methane system on titanate nanotubes modified with Au and Rh nanoparticles. Appl. Catal. BEnviron. 2016, 199, 473-484. (9) (a) Maicu, M.; Hidalgo, M. C.; Colon, G.; Navio, J. A., Comparative study of the photodeposition of Pt, Au and Pd on pre-sulphated TiO2 for the photocatalytic decomposition of phenol. J. Photochem. Photobiol., A , 2011, 217 (2-3), 275-283; (b) Yu, Y. L.; Zheng, W. J.; Cao, Y. A., TiO2-Pd/C composited photocatalyst with improved photocatalytic activity for photoreduction of CO2 into CH4. New J. Chem. 2017, 41 (8), 3204-3210; (c) Long, R.; Li, Y.; Liu, Y.; Chen, S. M.; Zheng, X. S.; Gao, C.; He, C. H.; Chen, N. S.; Qi, Z. M.; Song, L.; Jiang, J.; Zhu, J. F.; Xiong, Y. J., Isolation of Cu Atoms in Pd Lattice: Forming Highly Selective Sites for Photocatalytic Conversion of CO2 to CH4. J. Am. Chem. Soc. 2017, 139 (12), 4486-4492; (d) Kim, J.; Zhang, P. Y.; Li, J. G.; Wang, J. L.; Fu, P. F., Photocatalytic degradation of gaseous toluene and ozone under UV254+185 (nm) irradiation using a Pd-deposited TiO2 film. Chem. Eng. J. 2014, 252, 337-345; (e) Fujiwara, K.; Pratsinis, S. E., Single Pd atoms on TiO2 dominate photocatalytic NOx removal. Appl. Catal. B-Environ. 2018, 226, 127-134; (f) Long, R.; Rao, Z. L.; Mao, K. K.; Li, Y.; Zhang, C.; Liu, Q. L.; Wang, C. M.; Li, Z. Y.; Wu, X. J.; Xiong, Y. J., Efficient Coupling of Solar Energy to Catalytic Hydrogenation by Using Well-Designed Palladium Nanostructures. Angew. Chem.-Int. Edit. 2015, 54 (8), 2425-2430. (10) (a) Sattler, J.; Ruiz-Martinez, J.; Santillan-Jimenez, E.; Weckhuysen, B. M., Catalytic Dehydrogenation of Light Alkanes on Metals and Metal Oxides. Chem. Rev. 2014, 114 (20), 10613-10653; (b) Li, N.; Liu, M.; Yang, B.; Shu, W.; Shen, Q.; Liu, M.; Zhou, J., Enhanced Photocatalytic Performance toward CO2 Hydrogenation over Nanosized TiO2-Loaded Pd under UV Irradiation. J. Phys. Chem. C, 2017, 121 (5), 29232932; (c) Ji, Z. H.; Lv, H. F.; Pan, X. L.; Bao, X. H., Enhanced ethylene selectivity and stability of Mo/ZSM5 upon modification with phosphorus in ethane dehydrogenation. J. Catal. 2018, 361, 94-104; (d) Wanbayor, R.; Ruangpornvisuti, V., A periodic DFT study on binding of Pd, Pt and Au on the anatase TiO2 (001) surface and adsorption of CO on the TiO2 surface-supported Pd, Pt and Au. Appl. Surf. Sci. 2012, 258 (7), 3298-3301. (11) Erdem, B.; Hunsicker, R. A.; Simmons, G. W.; Sudol, E. D.; Dimonie, V. L.; El-Aasser, M. S., XPS and FTIR surface characterization of TiO2 particles used in polymer encapsulation. Langmuir, 2001, 17 (9), 2664-2669. (12) Nie, L. H.; Yu, J. G.; Li, X. Y.; Cheng, B.; Liu, G.; Jaroniec, M., Enhanced Performance of NaOH-Modified Pt/TiO2 toward Room Temperature Selective Oxidation of Formaldehyde. Environ. Sci. Technol. 2013, 47 (6), 2777-2783.
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Figure 1. The photocatalytic activities of the catalysts: (a) in dark, Ar and CO2 conditions, (b) with different metals and supports, and (c) different fractions of the Pd-doped TiO2. Reaction condition: CO2 (Ar):C2H6=1:1, 25mg of catalyst, a 1h irradiation time, 0.2MPa, and at ambient temperature. 329x123mm (150 x 150 DPI)
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Figure 2. Morphology and structures of Pd/TiO2 catalysts: (a) The XRD patterns of the x%-Pd/TiO2(x=0, 0.2, 0.5, 1, 1.5, 2), (b-f) SEM, HRTEM and XPS of 1%Pd/TiO2. 329x195mm (150 x 150 DPI)
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Figure 3. DFT calculation results: (a) DOS of TiO2, single Pd onTiO2 and PDOS of single Pd atom; (b) the ELF of Pd (111) plane on TiO2; (c) the charge difference distribution between Pd (111) plane and TiO2. 335x99mm (150 x 150 DPI)
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Figure 4. Spin-trapping ESR spectra under UV light irradiation for 10 min: (a) e- and (b)•OH and (c) solid state for h+ (d) active species trapping experiments with various scavengers, with the following reaction conditions: a CO2:C2H6 ratio of 1:1, 25 mg of catalyst, 3 mg of scavenger, 1 h of irradiation time, at a pressure of 0.2 MPa, and at ambient tempera-ture. 295x202mm (150 x 150 DPI)
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Figure 5. In-situ FTIR spectra of photocatalytic ODE with CO2 over 1% Pd/TiO2 under UV-light irradiation. 323x191mm (150 x 150 DPI)
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Figure 6. The possible reaction pathway and mechanism for the photocatalytic ODE with CO2 over Pd/TiO2 cata-lyst. 174x147mm (150 x 150 DPI)
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