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Photocatalytic CO2 Transformation to CH4 by Ag/ Pd Bimetals Supported on N-Doped TiO2 Nanosheet Dongxing Tan, Jianling Zhang, Jinbiao Shi, Shaopeng Li, Bingxing Zhang, Xiuniang Tan, Fanyu Zhang, Lifei Liu, Dan Shao, and Buxing Han ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06320 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 4, 2018

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ACS Applied Materials & Interfaces

Photocatalytic CO2 Transformation to CH4 by Ag/Pd Bimetals Supported on N-Doped TiO2 Nanosheet Dongxing Tan, Jianling Zhang*, Jinbiao Shi, Shaopeng Li, Bingxing Zhang, Xiuniang Tan, Fanyu Zhang, Lifei Liu, Dan Shao and Buxing Han Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid and Interface and Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, University of Chinese Academy of Sciences (China). E-mail: [email protected] KEYWORDS: Photocatalyst; CO2 transformation; CH4 production; Ag/Pd bimetal; TiO2 nanosheet

ABSTRACT: To develop photocatalysts with desirable compositions and structures for improving the efficiency and selectivity of CO2 conversion to CH4 under mild conditions is of great importance. Here, we design an effective photocatalyst that bimetal (Ag/Pd) nanoalloys are supported on nitrogen doped TiO2 nanosheet for CO2 conversion. Such a novel photocatalyst combines multiple advantages of abundant Ti3+ ions, oxygen vacancies and substitutional nitrogen that are favorable for catalyzing CO2 reduction. It was found that CO2 could be efficiently transformed to CH4 under mild conditions, i.e., in aqueous solution, at atmospheric pressure and room temperature. The maximum production rate of CH4 can reach 79.0 µmol g-1 h1

. Moreover, the Ag/Pd bimetals supported on N-doped TiO2 nanosheet exhibit high selectivity

to CH4. The as-synthesized photocatalyst can be well recycled for CO2 reduction.

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1. INTRODUCTION The transformation of carbon dioxide (CO2) into hydrocarbon compounds has attracted more and more attentions in recent years, because it will not only mitigate the greenhouse effect but also reduce the consumption of fossil fuels.1-5 Among the diverse kinds of transformation reactions, the conversion of CO2 into methane (CH4) is of great importance because CH4 is a common fuel and important chemical.6-8 Particularly, it is much promising to transform CO2 into CH4 by harvesting solar energy owing to low energy input.9,10 However, CO2 is a molecule with ultrahigh stability and the conversion of CO2 with H2O into CH4 is uphill reaction (ΔG= 818.3 kJ mol-1). Therefore, the CO2 reduction into CH4 remains quite a big challenge.11 On the other hand, many by-products are generally produced during the CO2 reduction to CH4, such as carbon monoxide6,12-18, methanol8,14,15 or acetaldehyde19. It is very interesting to develop photocatalysts with desirable compositions and structures for improving both the efficiency and selectivity of CO2 conversion to CH4 under mild conditions. TiO2

is

a

wide-bandgap semiconductor material,

which

has

been

explored

for

the photocatalytic reduction of CO2.3,5-9 Nevertheless, due to the lack of visible-light photoresponse and fast electron-hole recombination of the photogenerated charge at TiO2 surface and inefficient CO2 capture, its photocatalytic efficiency to CO2 transformation is still far from optimum. As is recognized, the combination of TiO2 with other active component is an promising way to overcome the limitations of pure TiO2 for photocatalytic CO2 conversion. Diverse kinds of TiO2-based composites have been fabricated as photocatalysts for CO2 transformation, including noble metal (e.g. Pt, Au, Ag, or Cu) loaded TiO2,3,6-8 semiconductor13 or metal-organic framework17 loaded TiO2. Despite the largely improved activity of the TiO2-


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based composite photocatalysts as compared with pure TiO2, the catalytic efficiency and selectivity to reduction products are still restricted. Here, we design a photocatalyst for the high-efficient CO2 conversion, i.e., the Ag/Pd nanoalloys are supported on nitrogen doped TiO2 nanosheets. To get such a novel architecture, hydrazine hydrate was utilized both as reducing reagent for metal salts and as nitrogen source to dope TiO2 nanosheets. The as-synthesized Ag/Pd/TiO2 photocatalyst has the following advantages. First, the Ag/Pd bimetallic nanoparticles with small size and narrow distribution range (e.g. 7.8-9.2 nm) are immobilized on TiO2 nanosheets, which can improve the visible-light photoresponse and reduce electron-hole recombination of the photogenerated charge on TiO2 surface. Second, nitrogen is introduced to TiO2 nanosheets, which may improve the charge separation efficiency and enhance the absorption of the catalyst in visible light region. Third, there are abundant surface defects and oxygen vacancies in the Ag/Pd/TiO2 photocatalyst, which are favorable for photoexcited charge transfer and CO2 activation. Owing to the combined advantages, the as-synthesized Ag/Pd/TiO2 photocatalyst shows high catalytic efficiency and selectivity for the CO2 transformation to CH4 under mild conditions (i.e., in aqueous solution, at atmospheric pressure and room temperature). 2. RESULTS AND DISCUSSION To get the Ag/Pd/TiO2 composite, the TiO2 nanosheets with an average size of 40 nm and thickness of 5 nm were first prepared (Figure S1). Then the TiO2 nanosheets were dispersed in the aqueous solution dissolved with silver nitrate (AgNO3), potassium palladium (II) chloride (K2PdCl4) and sodium citrate, which was used to prevent the aggregation of alloy particles. Then hydrazine hydrate was added into the above solution dropwise, which played multiple functions of reducing reagent for metal salts, nitrogen source for doping TiO2 nanosheet and generating a


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large number of surface defects of TiO2. The obtained product was denoted as AgxPdy/TiO2, where x/y presents the molar ratio of AgNO3 to K2PdCl4. For comparison, the binary systems of Ag/TiO2 and Pd/TiO2 were synthesized by the similar route (see experimental details and characterizations in Figure S2). As shown in Figure 1, the X-ray diffraction (XRD) patterns of AgxPdy/TiO2 show a combination of diffractions from pure TiO2 nanosheets and Ag/Pd alloy. The diffractions at 25.3°, 48.1° and 55.1° correspond to the (101), (200) and (211) planes of anatase phase TiO2 (JCPDS, No. 21-1272), while those at 38.3° and 44.5° correspond to the (111) and (200) planes of Ag/Pd alloy nanoparticles with a face-centered cubic (fcc) structure,20,21 respectively. As compared with the diffractions of the binary systems, the diffractions of Ag/Pd alloy in the AgxPdy/TiO2 ternary system are broadened, which may result from their smaller particles. Moreover, it is evident that the diffractions of Pd shift towards lower diffraction degree with increasing Ag content, implying the mergence of more Ag atoms into Pd lattice.6,22


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Figure 1. XRD patterns of bare TiO2 nanosheets, Ag/TiO2, Pd/TiO2 and AgxPdy/TiO2 composites. The standard diffraction patterns for anatase TiO2 (JCPDS, No. 21-1272), fcc Ag (JCPDS, No. 04-0783) and fcc Pd (JCPDS No. 46-1043) are provided as references. Figure 2 shows the scanning electronic microscopy (SEM) image of the as-synthesized Ag2Pd1/TiO2 composite as an example. The Ag2Pd1/TiO2 maintains the nanosheet structure of original TiO2 (Figure 2a). The nanoparticles with an average diameter of 8.5 nm are highly dispersed on TiO2 nanosheet (Figures 2b and c). The high-resolution transmission electronic microscopy (HRTEM) image reveals that the lattice spacing of TiO2 nanosheet is 0.235 nm (Figure 2d), which corresponds to the (001) plane of anatase phase TiO2.23 The HRTEM image of the nanoparticle shows the lattice fringes with a spacing of 0.233 nm, which can be assigned to the (111) plane of fcc Ag2Pd1 alloys (Figure 2e).21 These results are in agreement with the XRD results. As seen from energy-dispersive X-ray spectroscopy (EDS) line scans on nanoparticles (Figure 2f), Ag and Pd elements distribute nearly at the same locations, further proving the formation of Ag2Pd1 alloys. The energy dispersive X-ray elemental mappings reveal that Ti, O, N, Ag and Pd elements distribute in the sample (Figure 2g). The Ag and Pd contents in Ag2Pd1/TiO2 composite were determined to be 1.26 wt% and 0.64 wt% respectively by inductively coupled plasma-mass spectrometry (ICP-MS), with Ag/Pd molar ratio of 1.9. The nitrogen content is 0.56 wt% in Ag2Pd1/TiO2 composite as determined by elemental analysis. The other AgxPdy/TiO2 composites show similar morphologies to that of Ag2Pd1/TiO2 composite, i.e., the Ag/Pd nanoparticles smaller than 10 nm are highly dispersed on TiO2 nanosheet (Figure S3). The Ag/Pd contents and molar ratio in the AgxPdy/TiO2 composites are listed in Table S1.


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Figure 2. (a) SEM image, TEM image (b), particle size distribution (c), HRTEM images (d, e), EDS line scans (f) and elemental mappings (g) of Ag2Pd1/TiO2 photocatalyst. Scale bars, 100 nm in (a), 20 nm in (b), 1 nm in (d), 1 nm in (e). The chemical composition and bonding configuration of Ag2Pd1/TiO2 photocatalyst were characterized by X-ray photoelectron spectroscopy (XPS) analysis. The high-resolution Ag 3d (Figure 3a) and Pd 3d (Figure 3b) XPS spectra show the binding energies for Ag 3d3/2, Ag 3d5/2, Pd 3d3/2 and Pd 3d5/2 at 373.5 eV, 367.5 eV, 340.1 eV and 334.7 eV, which are characteristic for Ag0 and Pd0, respectively. In the high-resolution Ti 2p spectrum (Figure 3c), the binding energies at 464.1 eV and 458.3 eV correspond to Ti 2p1/2 and Ti 2p3/2, respectively.24 Compared to pure anatase TiO2 nanosheets (Figure S4), the Ti 2p peak of Ag2Pd1/TiO2 photocatalyst is broadened and shifts towards lower binding energy, indicating the presence of Ti3+ in Ag2Pd1/TiO2 photocatalyst.24,25 Meanwhile, in the high-resolution O1s spectrum, the lattice oxygen (Ti-O-Ti)


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and non-lattice oxygen (Ti-OH) (Figure 3d) shift to lower binding energies compared to those of pure TiO2 nanosheets (Figure S4). It also implies the presence of Ti3+ in Ag2Pd1/TiO2 photocatalyst,25 which can be attributed to the TiO2 reduction by hydrazine during synthesis process.26 Furthermore, the peak area of non-lattice oxygen in Ag2Pd1/TiO2 photocatalyst possesses a larger area than that in pure TiO2, indicating more oxygen vacancies in the assynthesized photocatalyst.27 The presence of abundant Ti3+ ions and oxygen vacancies in AgxPdy/TiO2 photocatalyst may favor CO2 activation and promote electron transfer toward CO2.28-30 For the high-resolution N1s XPS spectrum (Figure 3e), a peak at 399.8 eV corresponding to O-Ti-N was observed.31,32 It implies that N atom was successfully introduced into TiO2, forming substitutional nitrogen state.31,32 The presence of nitrogen in the Ag2Pd1/TiO2 photocatalyst may improve the charge separation efficiency and enhance the absorption of the catalyst in visible light region.32


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Figure 3. High-resolution XPS spectra of Ag 3d (a), Pd 3d (b), Ti 2p (c), O1s (d), and N 1s (e) of Ag2Pd1/TiO2 photocatalyst. The performance of the as-synthesized AgxPdy/TiO2 photocatalyst for CO2 reduction was investigated under simulate sunlight irradiation. The reaction was carried out in CO2-saturated aqueous solution33 at atmospheric pressure and 298.2 K, which was thermostated by a water bath. As analyzed by gas chromatograph, CH4 was produced and no other carbon products were detected (Figure S5 and Table S2). The obtained CH4 originates from the used CO2, which was confirmed by isotope experiment using

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C isotopic label and a series of control experiments

(Figure S6 and Table S3). As shown in Figure 4a, all the AgxPdy/TiO2 photocatalysts show improved activity to CH4 production as compared with the pure TiO2 catalyst (no detectable CH4), Ag/TiO2 (19.2 µmol g-1 h-1) and Pd/TiO2 (24.5 µmol g-1 h-1). In particular, the CH4 production rate as catalyzed by Ag2Pd1/TiO2 photocatalyst reaches a maximum value 79.0 µmol g-1 h-1. As compared with the reported photocatalysts for CO2 reduction in aqueous solution (Table S4),14,15,34-37 the AgxPdy/TiO2 photocatalyst is promising owing to both high CH4 production rate and high selectivity to CH4. The hourly progression of CO2 conversion shows that the yield of CH4 increases linearly with reaction time during 20 h test (Figure 4b), indicating the stability of the Ag2Pd1/TiO2 photocatalyst for the durative reaction process. It was further proved by the cycle experiment that the production rate of CH4 is basically stable after five cycle test (Figure 4c). Furthermore, the XRD pattern of the Ag2Pd1/TiO2 catalyst after use for five cycles is simialr to that of the pristine Ag2Pd1/TiO2 (Figure 4d). The re-used Ag2Pd1/TiO2 catalyst remains the initial morphology and no aggregation for the alloy nanoparticles was observed (Figure S7).


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Figure 4. (a) Production rates of CH4 in CO2 reduction by Ag/TiO2, Pd/TiO2 and AgxPdy/TiO2 photocatalysts. (b) Time-dependent production of CH4 by Ag2Pd1/TiO2. (c) Stability test of Ag2Pd1/TiO2 photocatalyst for five cycles. (d) XRD patterns of the pristine Ag2Pd1/TiO2 and the Ag2Pd1/TiO2 photocatalyst after use for five cycles. The mechanism for the high performance of the as-synthesized AgxPdy/TiO2 photocatalysts was investigated. First, the UV-vis-NIR absorption spectra of the AgxPdy/TiO2 photocatalysts were determined (Figure 5a). One absorption band corresponding to TiO2 appears at about 300 nm; the other absorption band increases into IR region with no indication of leveling off, resulting from the nitrogen doping and metal loading.29,38,39 As compared with pure TiO2 and binary catalysts, the absorption of the AgxPdy/TiO2 photocatalysts in visible light region is obviously enhanced. Particularly, the Ag2Pd1/TiO2 catalyst has the strongest absorption in visible light region. Therefore, the AgxPdy/TiO2 photocatalysts can be more effective in the use of sunlight, so as to achieve higher photocatalytic efficiency. Second, electrochemical impedance


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spectroscopy (EIS) was employed to evaluate the photoexcited charge transfer property of the photocatalysts. Nyquist plots of the different photocatalysts show a similar shape (Figure 5b). The arc radius of the EIS Nyquist plot of the Ag2Pd1/TiO2 photocatalyst is much smaller than other photocatalysts, implying the smallest resistance for charge transfer and superior charge separation efficiency.37,40 Third, photoluminescence spectra (PL) were used to evaluate the separation efficiency of photoexcited charge. The TiO2 nanosheets show an very strong main emission profile in the range from 360 to 430 nm (Figure 5c). Although the hybrid Ag2Pd1/TiO2 materials show similar PL profiles to TiO2 nanosheets, the peak intensity is drastically lowered. The Ag2Pd1/TiO2 photocatalyst presents extremely low peak intensity, indicating effectively reduced recombination rate of the photoexcited electron-hole pairs.41,42 These evidences reveal that the Ag2Pd1/TiO2 photocatalyst remarkably promotes the photoexcited charge transfer and separation, which would improve the photocatalytic performance of the as-synthesized AgxPdy/TiO2 photocatalyst for the production of CH4 from CO2.

Figure 5. (a) UV-vis-NIR absorption spectra of TiO2 nanosheets, Ag/TiO2, Pd/TiO2 and AgxPdy/TiO2 photocatalysts. (b) Electrochemical impedance spectroscopy of TiO2 nanosheets, Ag/TiO2, Pd/TiO2 and Ag2Pd1/TiO2 photocatalysts. (c) Photoluminescence spectra of TiO2 nanosheets, Ag/TiO2, Pd/TiO2 and Ag2Pd1/TiO2 photocatalysts. 3. CONCLUSIONS


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In conclusion, we prepared a novel photocatalyst that the Ag/Pd bimetal nanoalloys are highly dispersed on the nitrogen doped TiO2 nanosheet by a simple route using hydrazine as reducing reagent. The AgxPdy/TiO2 photocatalyst combines the advantages of alloy nanoparticles, TiO2 nanosheet, nitrogen doping and abundant Ti3+ ions and oxygen vacancies. It can efficiently reduce CO2 to CH4 under mild conditions without the production of any other carbon-containing products. The maximum production rate of CH4 can reach 79.0 µmol g-1 h-1. Moreover, the AgxPdy/TiO2 photocatalyst can be well recycled. We anticipate that the as-synthesized AgxPdy/TiO2 photocatalyst would find more applications in catalyzing different chemical reactions owing to the high activity, selectivity and recyclability. 4. EXPERIMENTAL SECTION 4.1. Materials CO2 (>99.9999% purity) was provided by Beijing Analysis Instrument Factory. AgNO3 (>99% purity) was purchased from Alfa Aesar. K2PdCl4 (>99% purity) was provided by Adamas Reagent Co., Ltd. Triethylamine (TEA, >99% purity), Deuterium oxide-d2 (99.9 atom % D) and hydrofluoric acid (HF, 48 to 51% solution in water) were purchased from Beijing InnoChem Science & Technology Co., Ltd. Titanium (IV) butoxide (>98.5% purity) was provided by Beijing Xingjin Chemical Works. Hydrazine hydrate (80% purity) was bought from Aladdin Industrial Corporation. 4. 2. Preparation of TiO2 nanosheet In a typical synthesis process, 25 mL of titanium (IV) butoxide (>98.5% purity) was mixed with 4 mL of HF and stirred for 30 min at room temperature. Then the mixed solution was transferred into a 100 mL Teflon-lined stainless steel autoclave which was kept at 180 oC for 24


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h. After reaction, the precipitate was centrifuged and washed with ethanol and distilled water. Finally, the product was dried under vacuum at 80 oC for 12 h. 4.3. Preparation of AgxPdy/TiO2 Firstly, 6.0 g sodium citrate was dissolved in 60 mL ultrapure water in a 250 mL glass vial. Secondly, a certain amount of AgNO3 and K2PdCl4 were added to the glass vial and stirred at room temperature for 10 min. Thirdly, 40 mg TiO2 nanosheet was added to the solution and with ultra-sonication for 30 min. Then the glass vial was transferred into ice bath and 40 mL hydrazine hydrate was added dropwise under vigorous magnetic stirring. The suspension was stirred for 2 h, followed by filtration, washing with distilled water and drying in a vacuum oven at 60 oC for 24 h. 4.5. Characterizations The morphologies of the photocatalysts and EDS mapping were characterized by SEM (HITACHI S-4800). Transmission electron microscopy (TEM) images and energy-dispersive spectroscopy (EDS) line scan profiles were taken on a JEOL JEM-2100F field-emission highresolution transmission electron microscope operated at 200 kV. X-ray diffraction (XRD) patterns were conducted on a Rigaku D/max 2400 diffractometer with Cu Ka radiation (λ= 0.15418 nm) at a scanning rate of 4o min-1. The X-ray photoemission spectroscopy (XPS) was carried out with a multipurpose X-ray photoemission spectroscope (Thermo Scientific ESCALAB 250Xi). The UV-Vis-NIR absorption spectra were taken with a UV-2600 spectrometer within the wavelength range between 220 and 800 nm, and BaSO4 was used as the reference material. The elemental analysis of N was performed on FLASH EA1112 elemental analysis instrument. The element contents of Ag and Pd were determined by ICP-MS. The Electrochemical impedance spectra were measured on a CHI 660D electrochemical station in 0.2


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M Na2SO4 solution under ambient conditions. The photoluminescence spectra were measured by F-4600 FL Spectrophotometer at room temperature with a 325 nm Xe laser as an excitation light source. 4.6. Photocatalytic CO2 conversion The CO2 photoreduction experiment was performed in a 100 mL gastight reactor. In a typical process, 5 mg AgxPdy/TiO2 and 100 µL TEA were dispersed in 5 mL ultrapure water. TEA acted as sacrificial electron donor, providing the electrons needed for the reduction of CO2 into CH4.43,44 Water worked as both solvent and hydrogen source for photocatalytic CO2 reduction.28,45 Then CO2 (1 atm) was charged into the reactor, which was placed in a water bath of 25 oC. A 300 W Xe lamp was used as simulated sunlight to start the photoreaction. After reaction for 10 h, the gaseous products were analyzed by gas chromatograph (GC, HP 4890D), which was equipped with TCD and FID detectors using helium as the internal standard. The liquid was analyzed by 1H NMR spectroscopy (AVANCE III HD 400 MHz), in deuterium oxided2 with TMS as an internal standard. For the stability test, the catalyst after photocatalytic reaction was centrifugally separated and vacuum dried. Then, the next photocatalytic reaction was carried out with the recovered catalyst. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. SEM and TEM images; XPS spectra; 1H NMR spectra; Mass spectra and so on (PDF) AUTHOR INFORMATION


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Corresponding Author *

E-mail: [email protected]. Tel./.Fax: +86-010-62528953.

Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGEMENTS We thank the financial supports from National Natural Science Foundation of China (21525316, 21673254), Ministry of Science and Technology of China (2017YFA0403003) and Chinese Academy of Sciences (QYZDY-SSW-SLH013). REFERENCES (1) Lu, W.; Jia, B.; Cui, B.; Zhang, Y.; Yao, K.; Zhao, Y.; Wang, J. J. Efficient Photoelectrochemical Reduction of CO2 to Formic Acid with Functionalized Ionic Liquid as Absorbent and Electrolyte. Angew. Chem. Int. Ed. 2017, 56, 11851-11854. (2) Kuehnel, M. F.; Orchard, K. L.; Dalle, K. E.; Reisner, E. Selective Photocatalytic CO2 Reduction in Water through Anchoring of a Molecular Ni Catalyst on Cds Nanocrystals. J. Am. Chem. Soc. 2017, 139, 7217-7223. (3) Xiong, Z.; Lei, Z.; Kuang, C. C.; Chen, X.; Gong, B.; Zhao, Y.; Zhang, J.; Zheng, C.; Wu, J. C. S. Selective Photocatalytic Reduction of CO2 into CH4 over Pt-Cu2O TiO2 Nanocrystals: The Interaction between Pt and Cu2O Cocatalysts. Appl. Catal., B 2017, 202, 695-703. (4) Wang, S.; Yao, W.; Lin, J.; Ding, Z.; Wang, X. Cobalt Imidazolate Metal-Organic Frameworks Photosplit CO2 under Mild Reaction Conditions. Angew. Chem. Int. Ed. 2014, 53, 1034-1038.


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Photocatalytic CO2 Transformation to CH4 by Ag ... - ACS Publications

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