Plasma-assisted Photocatalysis of CH4 and CO2 into Ethylene | ACS

Subscriber access provided by UNIV AUTONOMA DE COAHUILA UADEC

Article

Plasma-assisted Photocatalysis of CH4 and CO2 into Ethylene Naixu Li, Rumeng Jiang, Yao Li, Jiancheng Zhou, Quanhong Ma, Shaohua Shen, and Maochang Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b01284 • Publication Date (Web): 30 May 2019 Downloaded from http://pubs.acs.org on May 31, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

Revised MS No. sc-2019-012843.R1, 2019.05

Plasma-assisted Photocatalysis of CH4 and CO2 into Ethylene

Naixu Li,*,†,§ Rumeng Jiang,† Yao Li,† Jiancheng Zhou,†,§ Quanhong Ma,† Shaohua Shen,‡,# Maochang Liu*,‡,#

†School

of Chemistry and Chemical Engineering, Southeast University, No.2 Dongnandaxue Road,

Nanjing 211189, P.R. China ‡International

Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow

in Power Engineering, Xi'an Jiaotong University, No.28 Xianning West Road, Xi'an, Shaanxi 710049, P. R. China §Jiangsu Key Laboratory for Biomass Energy and Material, No.16 Suojin Wucun, Nanjing 210042,

P. R. China #Suzhou

Academy of Xi’an Jiaotong University, No.99 Renai Road, Suzhou, Jiangsu 215123, P. R.

China

*To whom correspondence should be addressed. Email: [email protected] (M.L.) and [email protected] (N.L.)

1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Oxidative coupling of methane (OCM) using CO2 as the oxidant is a potential advancement for methane conversion and greenhouse gases reduction. However, the reaction is usually limited by both high energy consumption and strict reaction condition. Here, we report, for the first time, photo-induced efficient ethylene (C2H4) and CO production by OCM over TiO2-supported Ag nanoparticles at mild conditions. The success relies on a synergy coupling visible-light-induced strong surface plasma resonance (SPR) effect localized on Ag(0), ultraviolet-light-induced photoelectric effect on TiO2, and the separated adsorption of CO2 and CH4 on TiO2 and Ag. The yields of CO and C2H4 reach 1149 μmol·g-1·h-1 and 686 μmol·g-1·h-1, respectively, under simulated solar irradiation. The origination of carbon-based gas products from CO2 and CH4 is also certified by a 13C isotope-labeled experiment. This work presents a new and ideal route of photocatalytic OCM reaction for C2H4 and CO production by coupling both SPR and photoelectric effects.

Keywords: Photocatalysis, plasma effect, oxidative coupling of methane, CO2 reduction, ethylene

2


Page 2 of 30

Page 3 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION The wide depletion of globe crude oil has brought about an urgent demand for efficient production of sustainable fuels and value-added chemicals. Methane conversion into fuels and value-added chemicals, to this end, has received intense interests,1 which yet can only be achieved by traditional petrochemical methods such as catalytic steam cracking and autothermal reforming reaction.2 Ethylene (C2H4) has been regarded as one of the most important light olefins for the modern chemical industry, as both fuel and highly valued chemicals. However, industrial production of ethylene usually contains multi-steps and requires high reaction temperature (750950 oC). In this context, one-step oxidative coupling of methane (OCM) to ethylene in the presence of O2 has been highly desired and attracted the attention. It is usually a thermocatalytic process that involves the use of, for example, La,2 Ce,3 Zn,4 Mg,5 and Ti-based thermocatalysts.6 However, the tetrahedral arrangement of strong C-H bonds (439.3 kJ·mol-1) in methane without functional group, magnetic moments, and small polarizability makes methane extremely stable from either electrophilic or nucleophilic attack. Since the pioneering work taken by Keller and Bhasin in 1982,7 hundreds of thermocatalysts have been developed, yet with limited success in achieving good property for effective C-H activation at mild reaction condition. As a result, OCM usually operates at relatively high temperatures (e.g., 750-950 oC).2 However, high reaction temperature not only leads to high energy consumption, but also induces unexpected reaction pathways instead of ethylene production. Both the high reaction temperature and the poor selectivity involved in the reported thermocatalytic OCM systems prevent the commercial progress of the technology. Therefore, development of novel catalytic OCM process with high selectivity but low reaction temperature is highly desired. It has been reported that both CH4 and CO2 can be activated by photocatalysis at room temperature. Specifically, in a photocatalytic reaction contains CH4 and CO2, C-H bonds could be broken by accepting photoinduced holes, generating •CH3 and H+, while O=C=O bonds could be activated by photoelectrons, producing OCO-, and finally converted to synthetic bidentate carbonate.8 It has thus been a great advancement that O2 can be replaced by CO2 in the OCM 3



reaction. But similarly, this process relies on well-designed photocatalysts, which generally are semiconductors such as MgO,8 Al2O3,9 TiO2,10-13 ZnO,14 Ga2O3,15 Cu/CdS-TiO2/SiO2,16 C3N417, and TiO2 nanotube.18 In addition, plasmonic metal catalysts may also assist these processes. For example, Au was selected as a promotor for Rh/SBA-15 in photo-thermal conversion of dry reforming of methane (DRM).19 Particularly, the catalytic performance of Rh/SBA-15 under visible-light irradiation was significantly enhanced with CH4 and CO2 conversion rates reached 1850 and 2350 mmol g-1s-1, respectively. Despite these efforts and results successes, none of the reported system has demonstrated the possibility of ethylene production in this reaction. In principle, the OCM reaction usually initiates with the activation of CH4 by an oxidant (e.g., O2 and CO2) to generate •CH3 radicals at a catalyst surface. The subsequent step is a homogeneous gas-phase step involving the coupling of •CH3 into C2H6. Clearly, to produce C2H4, one needs to further have the C2H6 oxidatively dehydrogenated, which is yet also high-temperature guided. Nevertheless, if methylene can be further obtained, mild OCM reaction condition for ethylene shall be expected. Herein, we reported that Ag/TiO2 can be used as catalyst for selectively converting a gas mixture containing CH4, CO2, and Ar (with molar ratio of 7.5/7.5/85) into ethylene and CO at room temperature under light irradiation. The success to the synthesis relies on a synergic effect taken by photoinduced charge excitation and plasmonic hot charge injection. Proof-of-concept experiments have demonstrated that the type of semiconductor and metal nanocrystals can impact both the selectivity and activity.

EXPERIMENTAL SECTION Reagents. Titanium dioxide (Degussa P25), aluminum oxide (Al2O3), magnesium oxide (MgO), zinc oxide (ZnO), silicon dioxide (SiO2), silver nitrate (AgNO3), ruthenium trichloride (RuCl3), iridous chloride (IrCl3), palladium dichloride (PdCl2), rhodium chloride (RhCl3), and sodium borohydride (NaBH4) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All reagents and solvents were analytical grade and were used as received without further purification. Mixed reactant gas containing methane, carbon dioxide and nitrogen (CH4: CO2: N2= 7.5: 7.5: 85) were purchased from Nanjing Shangyuan industrial gas plant. 4


Page 4 of 30

Page 5 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Preparation of the mental/semiconductor nanocomposites. A simple room-temperature chemical reduction method was used to synthesize the Ag/TiO2 composite photocatalyst. Specifically, titanium dioxide (Degussa P25) was selected as the substrate material. In the typical synthesis of 1 wt% Ag/TiO2 photocatalyst, 2 g of TiO2 and 6.4 mL AgNO3 (5 mg/mL) were mixed in a flask containing 30 mL deionized water. Subsequently, 14 mL frozen NaBH4 (0.5 mg/mL) was added dropwise into above white suspension. The white liquor became tan abruptly. The reaction was allowed to proceed for 3 hours at room temperature. Note the entire process was in the presence of magnetic stirring. The obtained product was washed with water and ethanol several times, and dried at 60 ℃ in vacuum overnight. To produce 0.3 wt%, 0.5 wt%, and 1.5 wt% Ag/TiO2 composites, only different volumes of AgNO3 and NaBH4 solution was used. The Ag/Al2O3, Ag/MgO, Ag/ZnO, Ag/SiO2, Ru/TiO2, Ir/TiO2, Pd/TiO2, and Rh/TiO2 catalyst were prepared by the same method, except that the corresponding metal precursor or metal oxide was used instead. Characterization of Samples. The amount of Ag in the 1 wt% Ag/TiO2 catalysis was determined by X-ray fluorescence analysis (XRF, S4 PIONEER). Scanning electron microscopy (SEM, FEI INSPECT F 50) and transmission electron microscopy (TEM, Hitachi H-600, FEI Tecnai G2 F30) were used to measure the morphology, particle size, and dispersibility of Ag. Xray powder diffraction patterns (XRD, Bruker D8-Discover) of all samples were measured under ambient condition on a X-ray diffractometer equipped with a Cu-Kα source (λ = 0.1542 nm). UVVisible diffuse reflectance spectra (UV-Vis DRS, Shimadzu UV-3600) were obtained by a spectrophotometer equipped with an integrating sphere and with BaSO4 as the reference. Photoluminescence spectroscopy (PL, Fluoromax-4) was performed from 340 to 600 nm with an excitation wavelength of 325nm. X-ray photoelectron spectroscope (XPS, Thermo ESCALAB 250XI) measurement was calibrated with respect to the C 1s signal at 284.6 eV. Temperatureprogrammed desorption (TPD) was carried out by the automated chemisorption analyzer (Quantachrome Autosorb-iQ-C). Photocatalytic measurements. Photocatalytic reformation of CO2 and CH4 mixture was carried out in a 100-mL micro autoclave as shown in Figure S1. A xenon lamp with a power of 84.2 5



mW/cm2 was employed to serve as light source. In a typical photocatalytic reaction, 100 mg of the as-prepared composite photocatalyst was dispersed on 400 mg quartz cotton. Besides, the reactor was first flushed with N2 0.5 h. The mixed reaction gas (CO2/CH4/Ar = 7.5/7.5/85 in terms of mole fraction) was the injected into the aforementioned reactor until the pressure reached 2 MPa. The gas in the autoclave was sampled every 0.5 h. A gas chromatography (GC-9860-5C) equipped with a flame ionized detector FID detector after Porapak Q (60-80 mesh) packed column and an in-situ Fourier transform infrared spectroscopy (FTIR, Thermo fisher Nicolet iz10) were used to analyze the gas composition and capture the intermediate adsorbed species. Gas chromatography-mass spectrometry (GC-MS, 7890A and 5975C, Agilent) was adopted for the isotope-labeled experiments with the 12CO2 replaced by 13CO2.

RESULTS AND DISCUSSION Taking 1 wt% Ag/TiO2 as an example, we firstly examined the morphology, structure, and particle size of the as-prepared composite by SEM and TEM. Figure 1a shows the typical SEM image of the 1 wt% Ag/TiO2 composite. Clearly, the original morphology of P25 nanoparticles was well maintained after loading Ag. TiO2 particles with an average size of 25 nm agglomerated into larger collectives and generally homogeneous distributed. It is hard to resolve silver from the surface of TiO2 nanoparticles from the SEM image properly due to the minuscule particle size of silver. This structure behavior was further demonstrated by TEM bright-field image as shown in Figure 1b. While the nonuniform size distribution of TiO2 nanoparticles is obvious, the integration of silver nanoparticles is still invisible. In principle, since Ag has a much larger atomic weight compared to Ti, we may expect to have Ag resolved by TEM dark-field image. As shown in Figure 1c, bright silver nanoparticles were uniformly anchored on the surface of TiO2 nanoparticles. The average size of Ag is about 4 nm (see statistic result in the set of Figure 1c). Such small nanoparticles were also evidenced by EDX mapping images. As shown in Figure S2, uniform and highly dispersive mapping dots without notable aggregation was only observed for the scan of Ag. Figure 1d shows the representative HRTEM image of the composite. The well-resolved, continuous lattice fringes in the same orientation, indicates that Ag was in the form of single6


Page 6 of 30

Page 7 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


crystal nanospheres. The lattice spacing of 0.207 nm can be indexed to the planes of Ag with a face-centered cubic structure. In addition, introduction of Ag did not impact the specific surface area, crystal structure, and chemical state of the P25 substrate, as demonstrated by BET results (Table S1), XRD patterns (Figure S3), and XPS spectra (Figure S4) of TiO2 and Ag/TiO2 samples. It is also worth pointing out that the most Ag was well-preserved in the Ag/TiO2 during the synthetic process. Taking 1 wt% Ag/TiO2 as an example, X-ray fluorescence analysis showed that the exact content of Ag in the composite was 0.92 wt%. The as-prepared composites were further examined by UV-Vis spectra. As shown in Figure 2a, all of the as-prepared samples showed two notable absorption edges at around 380 and 400 nm. Since commercial P25 usually contains both rutile and anatase structures, the two cutting edges should be attributed to the characteristic band-gap transitions of the two phases. When incorporated with silver, a notable absorption peak at 450 nm appears. The intensity of the peak gets stronger with the increment of Ag content. In principle, metal nanostructures can induce a strong interaction between incident light and free electrons through an effect called surface plasma resonance (SPR).20 Particularly, this interaction can be tightly manipulated by controlling the nanostructures in terms of both size and shape. In this regard, the absorption peak, or so-called SPR peak, also provides the certain information of a given metal nanostructure. The single SPR peak at 450 nm indicates that silver is in the form spherical nanoparticles and has an average size of ~4 nm. The result is in accordance with the TEM observation. We next tried to investigate the charge separation property by using photoluminescence (PL) spectrum. Figure 2b shows the typical PL spectra of TiO2 and Ag/TiO2 photocatalysts that were excited at 325 nm. The emission peaks appear at about 390~420 nm wavelengths are ascribed to the emission of band gap transition of anatase and rutile TiO2. The peaks in the low energy region, for example, the sharp ones located at ~450 nm and ~466 nm, are ascribed to the oxygen vacancies and defects on the catalyst surface.21,22 Normally, PL emission originates from the recombination of photogenerated electrons and holes. The high fluorescence intensity of the bara TiO2 indicated rapid recombination of the photogenerated electrons and holes. Incorporation of Ag significantly quenched the PL intensity, 7



which was on account of the formation of Schottky barrier between Ag and TiO2,23 for the reduction of radiative recombination. While too heavy loading of Ag not only weakened the ability of the Schottky barrier, but also shielded the incident light, an optimal content of 1 wt% for Ag was needed as indicated in the PL spectra. In principle, hot electrons can be generated by visible-light-induced SPR effect over Ag. The resonance frequency depends on the shape and size of Ag.24 In our case, this energy input should be around 2.76 eV according to the SPR peak position. Once irradiated by visible light containing this band waves, as shown in Figure 2c, energetic hot electrons will overcome the Schottky barrier and inject into a semiconductor. It is worth pointing out that this transportation highly relies on the density of the hot electrons. Significantly, electron injection to the adjacent semiconductors would leave plenty of positive charges on the plasma, leading to the formation of Ag(I). While Ag(I) is beneficial for rapid oxidation of a reductant such as CH4, it also leads to the disappearance of plasma resonance of Ag, and this ability could be recovered by accepting electrons from the reductant or the adjacent semiconductor, or by UV-light-induced self-reduction of Ag(I).25,26 This notion indicates that oxidation of CH4 only happens on the surface of Ag under visible light irradiation. On the other hand, as shown in Figure 2e, if the Ag/TiO2 composite is in the presence of UV light (λ ≤ 420 nm), photoexcitation occurs on TiO2 instead of Ag. The generated electrons are quickly transferred to Ag according to the band theory. In the case, on the contrary, oxidation of CH4 will take place on TiO2 while reduction of CO2 will be localized on Ag. Similar arguments can be applied to the situation of a combination of UV and visible light irradiation where both Ag and TiO2 can be excited. There are two mechanisms could be involved in the charge transfer process: One is hot electron injection from Ag(0) to TiO2, while the other is the band-gap excited photoelectron transfer from TiO2 to Ag(0). Clearly, the two opposite mechanisms shall be balanced in a certain reaction condition, with only one process can be observed. For example, if stronger SPR effect is induced, only directed electron transfer from Ag(0) to TiO2 will occur (see Figure 2d). Significantly, this transfer allows the rapidly capture of photogenerated holes on TiO2, giving rise to the largely improved ability of TiO2 toward CO2 reduction. More importantly, UV-light 8


Page 8 of 30

Page 9 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


irradiation could also assist the recovery Ag(0) by a UV-light-induced self-reduction process. Obviously, in this case, if CH4 and CO2 are selectively adsorbed on Ag and TiO2, both photocatalytic oxidation and reduction reactions could be promoted under simulated sunlight irradiation (containing both UV and visible light). Consequently, it is necessary to investigate the adsorption ability of Ag/TiO2 to CO2 and CH4. TPD has been regarded as a versatile characterization to this end. Generally, chemical adsorption of CO2 on the surface of a given catalyst includes two typical steps, i.e., molecular adsorption and subsequently carbonates formation. Notably, if adsorption happens by bonding the carbon atom, the monodentate (m-CO32-) will be produced. In case of both carbon and oxygen atoms are connected, the bidentate carbonate (b-CO32-) instead will be formed. As a third situation, the bicarbonate (HCO3-) will be fabricated if the surface of the catalyst contains hydroxyl group for linking CO2. The results of CO2-TPD over TiO2 and Ag/TiO2 photocatalysts are shown in Figure S5a. While the two samples presented similar TPD peaks of HCO3- (180-380 ℃), b-CO32- (380570 ℃), and m-CO32- (550-780 ℃), only Ag/TiO2 showed an additional peak representing to the decomposition of molecularly adsorbed CO2 (75-180℃).27-29 The quantitative adsorption behaviors are also summarized in Table S2. Clearly, the addition of Ag reduced by almost half of the adsorption capacity (from 7.32 to 4.89 mmol/g). Basically, the emergence of the desorption peak of molecular CO2 over the Ag/TiO2 indicated the weakened interaction induced by Ag coating, and this also resulted in an adsorption of molecular CO2 of 0.72 mmol/g. On the other hand, the high-temperature shift of the peak positions should be a result of the increased of coordination numbers after the addition of silver. These results demonstrate that Ti should serve as the active sites for CO2 adsorption and subsequent reduction. At the same time, CH4-TPD characterizations over the same samples were also carried out. The results are shown in Figure S5b and Table S3. Basically, the TPD peak at the range of 300-400 ℃ can be assigned to CH4-TiO2,30-32 while that at around 500 ℃ originates from CH4-Ag.33 Significantly, strong selectivity toward the adsorption of CH4 was observed over Ag/TiO2. The adsorption capacity was increased by nearly 8 times (2.473 mmol/g) than that of bare TiO2 (0.284 mmol/g). It indicates that Ag is main species to adsorb the 9



methane. Taken all, it could be concluded that the adsorption of CO2 and CH4 were largely separated on the surface of TiO2 and Ag, respectively. These unique properties of Ag/TiO2 allowed us to expect some special performance of the composite toward OCM reaction in the presence of light irradiation. The photocatalytic reactions were conducted in a micro autoclave equipped with an on-line GC system. Figure 3a shows gas yields over 1% Ag/TiO2 catalyst under simulate solar irradiation. Surprisingly, there are only CO and C2H4 emerged were generated. To the best of our knowledge, this is the first report that C2H4 was produced via this mild photocatalytic reaction. Moreover, the high yields of CO (2298 μmol·g1)

and C2H4 (1372 μmol·g-1) indicate that Ag/TiO2 is a suitable catalyst for this photocatalytic

OCM reaction. We next sought to determine the mass activity taken by Ag addition. As shown in Figure 3b, while only incorporation with Ag, the catalyst could show activity toward C2H4 generation, the optimal loading amount of Ag is 1% in terms of mass concentration. The result indicates that to achieve the best performance, the exposed Ag and TiO2 on the surface need to be balanced. It is worth pointing out that the origin of produced CO and C2H4 is of great significance for the photocatalytic reaction. To this end, this photocatalytic transformation was also tracked by a

13CO

2-labeled

isotopic experiment, coupling with a gas chromatography-mass spectrometry

(GC-MS) for product analysis.34-36 The GC spectrum was shown at Figure 3c. The peaks located at 1.78, 1.94, 2.65, and 3.96 min corresponded to typical signal of the CO, CH4, CO2, and C2H4 respectively. Significantly, the peaks at 1.78 and 3.96 min with the identified maximum m/z values of 29 and 28 could be indexed to 13CO and 12C2H4, respectively, as shown in the MS spectra (Figure 3, d and e). It indicates that CO was originated from the CO2 while the C2H4 was obtained from CH4. The result also excluded the possible decomposition of carbon species from the sample. To demonstrate the synergy of CH4 and CO2 in the photocatalytic process, the single CO2 conversion or CH4 conversion experiment was also operated. As shown in Figure S6, the sole use of either CH4 or CO2 has shown neglectable photocatalytic activity. The amounts of the products were at the level of only several μmol·h-1·g-1, resulted from the disproportionation of either CH4 and CO2. However, the result also demonstrated the possibility of activation of both CH4 and CO2 10


Page 10 of 30

Page 11 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


over the surface of Ag/TiO2 catalyst for the generation of methyl, methylene, carbonyl, and so on. In addition, the Ag/TiO2 photocatalyst also should good photostability. As shown in Figure S7, Ag/TiO2 remained highly active after five photocatalytic reactions. Although slight decrement also appeared, this photocatalyst still showed a great potential for this conversion and thus deserved to future investigation. To elucidate the SPR effect, we further investigated the role of the light source. Figure 4a, left column shows the reaction that was conducted under the visible light irradiation. Although the performance has been largely weakened, reaction pathway has been well-reserved with CO (467 μmol·g-1) and ethylene (278 μmol·g-1) as the final products. On the contrary, when the reaction is activated by UV light, ethane instead of ethylene was produced. The yields of CO and ethane were 330 μmol·g-1 and 178 μmol·g-1, respectively (Figure 4a, right column). As aforementioned, Ag only can be excited by visible light. The distinct products under visible light and UV light irradiation imply the essential role of Ag(I) for ethylene formation. It also indicates that ethylene was generated on the surface of Ag. The synergy taken by Ag SPR effect and selective adsorption of CH4 on Ag (see TPD characterization) was thus verified. Interestingly, as a mixed irradiation source containing UV and visible light, simulated sun light did not give the mixed reaction routes to generate both C2H4 and C2H6 (see Figure 4a, middle column or Figure 3a). Similar arguments of the visible light case can be applied, while a much larger amount of gas products was produced in this situation. In principle, both Ag and TiO2 are excited in this case. The fate of photogenerated electrons determines the reaction pathway. Since CO2 was selectively adsorbed on TiO2 rather than Ag, UV-light-motivated photoelectrons from TiO2 would preferentially transfer to CO2 to conduct reduction reaction. This process should be further promoted if hot electrons from Ag were injected to TiO2. The entire reaction thus worked in a Z-scheme way. Although the utilization efficiency of electrons could be reduced by half, the reaction rate would be remarkably improved. Again, the rapid consumption of hot electrons generated by SPR effect on Ag is crucial to largeamount generation of ethylene. We next tried to better understand this photocatalytic process. To figure out the role of each 11



component of the composite photocatalyst, reactions by using different designed catalysts were carried out. The results were shown in Figure 4b. Blank experiment without adding catalyst or in absence of light showed no products, indicating the photocatalytic nature of the reaction. Sole addition of TiO2 only gave a small amount of CO (158 μmol·g-1·h–1). The role of Ag for ethylene formation further demonstrated by using Ag/SiO2 as the photocatalyst, in spite of the reduced activity. Except SiO2 and TiO2, other materials such as Al2O3, ZnO, and MgO, did not present any synergistic effect with Ag for ethylene production. These results also suggested that the critical role of an active supporting material for the improvement of reactivity by forming finely-divided interfacial states such as Schottky barrier and CO2 adsorption sites. We also explored the effect taken by using different precious metals, including Ru, Ir, Pd, and Rh. As shown in Figure 4c, while all the metals showed ability toward ethylene generation, the poor SPR effect of Ru, Ir, and Pd led to an inferior performance of their corresponding composite photocatalysts. The higher activity of Rh/TiO2 probably arose from slight stronger SPR effect.37 The comparison between Ag/TiO2 and other similar catalysts that were used for similar photocatalytic reactions is also carried out. As shown in Table S4, it is clear that C2H4 was only produced in our system and the yields of the products should be among the highest reported values. To further demonstrate the role of Ag, the chemical states of Ag in the composite photocatalysts before and after reaction under different light sources were examined by XPS. Clearly, the binding energies of 374.1 and 368.1 eV for Ag 3d demonstrate the zero metallic state of silver before the reaction (Figure S8a), implying that Ag(I) has been fully reduced on the surface of TiO2 during the synthesis. Both Ag(0) and Ag(I) were found after reaction in the presence of either visiblelight (Figure S8b) or UV-visible-light irradiation (Figure S8c), although their proportion was of a little difference. As mentioned, this transformation from Ag(0) to Ag(I) indicates the strong plasmon resonance when the composite photocatalyst is exposed to the visible light. Significantly, this strong SPR effect will have most of the Ag(0) oxidized if the electron injection from, e.g., the adsorbed CH4, is restricted by reaction dynamics. As a result, the SPR effect would be remarkably weakened by the largely generated Ag(I) (see Figure S8b), leading to a gradually reduced 12


Page 12 of 30

Page 13 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


photocatalytic activity under visible light irradiation (see Figure 4a, left column). Interestingly, when ultraviolet light was introduced, the oxidized Ag(I) species could be reduced back to Ag(0) by a self-decomposition process, and maintain the strong SPR effect continuously. Particularly, given the large consumption of photogenerated holes on TiO2 by the SPR hot electrons (in a Zscheme way), the reaction dynamics of CO2 reduction on TiO2 and CH4 oxidation on Ag could be remarkably promoted. Obviously, a synergy could be created with improved photocatalytic performance when the two light-induced effects are well-balanced by using both UV and visible light.38,39 This notion explains why simulated sun light is more favorable for this OCM reaction (see Figure 4a, middle column) and the observed larger amount of surface Ag(0) relative to Ag(I) after reaction (Figure S8c). Under sole UV light irradiation, Ag(0) was kept unchanged after the reaction (Figure S8d). No binding energies represented to Ag(I) emerged. This phenomenon again demonstrates the significance of visible-light-induced SPR effect for the formation of Ag(I) and ethylene generation as well. The reaction mechanism was further investigated by in-situ FT-IR spectra. Figure 5a depicts the time-course FT-IR spectra of the gas mixture containing CO2, CH4, and the carrier gas Ar (Ar/CO2/CH4: 85/7.5/7.5) for the initial 15 min before light irradiation. While the bands at 16001800 cm-1 and ~2350 cm-1 correspond well to the physical and chemical adsorption states of CO2, the peaks at 3016 cm-1 and 1304 cm-1 are identified as the typical vibration modes of adsorbed CH4.40 The time-dependent increment of the peak intensity indicates the necessity of pretreatment to reach a saturation adsorption. Similar measurements were then applied to the simulated-sunlight-induced reaction process as shown Figure 5b. Although notable decrease of the IR bands for CO2 and CH4 adsorption was observed, some new peaks also appeared at the wavenumber ranging from 1800-1390 cm-1. To get a closer assessment of this broad band, we further magnified the spectra in this range (see Figure 5c). The clear vibration peaks at 1611, 1508, and 1444 cm-1 should be assigned to adsorption of CO2, in the modes of [b-CO32-], [m-CO32-], and [HCO3-], respectively.41 The new emerging peaks of 1715, 1555, and 1397 cm-1 could be attributed to stretching vibration band of [ν(C=O)], asymmetrical stretching vibration band of [νas(OCO-)], and 13



symmetrical stretching vibration band of [νs(OCO-)] from intermediates carboxylate [•HOCO-*], respectively.42-44 On the other hand, the peaks at 1694, 1508, 1457, and 1444 cm-1 were tested for stretching vibration band of [ν(C=C)], bending vibration bands of [δ(C=H)], [δ(CH2)], and [δ(C=C)], respectively.45 It is also worth noting that the bending vibration band of [δ(H2O)] at ~1525 cm-1 should be attributed to the surface adsorption of catalyst and OCM reaction. A parallel analysis was also implemented to the IR spectra at the wavenumber from 2400 to 1800 cm-1 (see Figure 5d). The lower intensity of these bands should owe to the bluntness of the detector to CO2 and CO. While the vibrations represented to CO2 (2360 cm-1) and Ag-H (1868 cm-1) were weakened with the reaction proceeded, the band of CO (2170 cm-1) increased gradually. Consequently, •HOCO-*, H, and CO should be the main intermediates in this photocatalytic OCM reaction. Taken all, we can clearly summarize the mechanism involved in the photocatalytic reaction. Generally, silver can be stimulated by visible light and generate hot electrons and holes, leading to a hot-electron injection to TiO2. The hot holes on Ag will and should be captured by CH4. Otherwise, it will lead to accumulation of Ag(I) and this oxidized species can be reduced back to Ag(0) under UV light irradiation.46 At the same time, TiO2 can be motivated by UV light to produce photoexcited electrons and holes. While these photogenerated holes are combined with the hot electrons from Ag, the photogenerated electrons can present larger capability to reduce CO2 adsorbed on TiO2. This improved reduction process also promotes the oxidation of CH4 on Ag. In short, the synergy created by the SPR and UV photoelectric effects not only enhances the photocatalytic activity, but also contributes to the high stability. Figure 6 illustrates the details involved in the reaction. Specifically, the process can be divided into five major courses, including (1) Adsorption: CO2 and CH4 are selectively adsorbed on Ag and TiO2, respectively (step 1 and 2); (2) Photoexcitation: Ag and TiO2 are simultaneously excited by simulated sun light due to visible-light-induced SPR (step 3) and UV-light-driven photoelectric effects (step 4 and 5), in which Ag(0) coverts to Ag(I) (step 3) and the photoelectrons have partial Ag(I) reduced back to Ag(0) (step 5), respectively; (3) Oxidation of CH4: the hot electrons produced on the silver surface 14


Page 14 of 30

Page 15 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


are injected into TiO2, while the left holes are captured by methane to generate the methylene by removing two hydrogen atoms (step 6 and 7). Two methylene species further combine together to form ethylene (step 8); (4) Reduction of CO2: CO2 firstly converts into •CO2- after receiving the electrons from TiO2 (step 9) and further turns into •HOCO-* with the existence of H+ (step 10). The reactive intermediate •HOCO-* is decomposed into CO and OH- rapidly (step 11) and water is formed due to the combination H+ and OH- (step 12); and (5) Desorption: the resulting ethylene, carbon monoxide, and water are desorbed from the surface of catalyst (step 13-15).47,48

CONCLUSION In summary, we put forward a novel concept of photocatalytic oxidative coupling of methane (OCM) with high selectivity of ethylene generation, which is operated under mild conditions, in the presence of sun light, with CO2 as an oxidant and a Ag/TiO2 composite as photocatalyst. The rates of CO and C2H4 reach 1149 and 686 μmol g-1·h-1, respectively. Experimental analyses indicate that the success relies upon the synergy of visible-light-induced Ag SPR effect and UVlight-induced TiO2 photoelectric effect, as well as the preferential adsorption of CO2 and CH4 on the surface of TiO2 and Ag. This work demonstrates the feasibility of photocatalytic OCM reaction at mild conditions, thus may open up a new door for developing alternative strategies for C2H4 or solar fuel production.

ASSOCIATED CONTENT Supporting Information BET and TPD results; Schematic illustration of the reaction system; STEM mapping images, XRD patterns, TPD spectra, photocatalytic stability tests, and XPS spectra of the as-prepared catalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] (M. Liu); [email protected] (N. Li). 15



Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No. 21576050, No. 51602052, and No. 51672210), Jiangsu Provincial Natural Science Foundation of China (BK20150604), Fundamental Research Funds for the Central Universities of China (No. 3207045403, 3207045409, 3207046414), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and Zhongying Young Scholar of Southeast University.

REFERENCES (1) Schwach, P.; Pan, X.; Bao, X. Direct Conversion of Methane to Value-added Chemicals over Heterogeneous Catalysts: Challenges and Prospects. Chem. Rev. 2017, 117, 8497-8520, DOI org/10.1021/acs.chemrev.6b00715. (2) Kondratenko, E. V.; Schlüter, M.; Baerns, M.; Linke, D.; Holena, M. Developing Catalytic Materials for the Oxidative Coupling of Methane Through Statistical Analysis of Literature Data. Catal. Sci. Technol. 2015, 5, 1668-1677, DOI 10.1039/C4CY01443J. (3) Sugiura, K.; Ogo, S.; Iwasaki, K.; Yabe, T.; Sekine, Y. Low-Temperature Catalytic Oxidative Coupling of Methane in an Electric Field over a Ce-W-O Catalyst System. Sci. Rep. 2016, 6, 25154, DOI 10.1038/srep25154. (4) Kumar, G.; Lau, S. L. J.; Krcha, M. D.; Janik, M. J. Correlation of Methane Activation and Oxide Catalyst Reducibility and Its Implications for Oxidative Coupling. ACS Catal. 2016, 6, 1812-1821, DOI org/10.1021/acscatal.5b02657. (5) Luo, L.; Tang, X.; Wang, W.; Wang, Y.; Sun, S.; Qi, F.; Huang, W. Methyl Radicals in Oxidative Coupling of Methane Directly Confirmed by Synchrotron VUV Photoionization 16


Page 16 of 30

Page 17 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Mass Spectroscopy. Sci. Rep. 2013, 3, 1625, DOI 10.1038/srep01625. (6) Farrell, B. L.; Igenegbai, V. O.; Linic, S. A Viewpoint on Direct Methane Conversion to Ethane and Ethylene Using Oxidative Coupling on Solid Catalysts. ACS Catal. 2016, 6, 43404346, DOI org/10.1021/acscatal.6b01087. (7) Keller, G.; Bhasin, M. Synthesis of Ethylene via Oxidative Coupling of Methane: I. Determination of Active Catalysts. J. Catal. 1982, 73, 9-19, DOI org/10.1016/00219517(82)90075-6. (8) Teramura, K.; Tanaka, T.; Ishikawa, H.; Kohno, Y.; Funabiki, T. Photocatalytic Reduction of CO2 to CO in the Presence of H2 or CH4 as a Reductant over MgO. J. Phy. Chem. B. 2004, 108, 346-354, DOI org/10.1021/jp0362943. (9) Liu, H.; Dao, T. D.; Liu, L.; Meng, X.; Nagao, T.; Ye, J. Light Assisted CO2 Reduction with Methane over Group VIII Metals: Universality of Metal Localized Surface Plasmon Resonance in Reactant Activation. Appl. Catal. B: Environ. 2017, 209, 183-189, DOI org/10.1016/j.apcatb.2017.02.080. (10) Merajin, M. T.; Sharifnia, S.; Hosseini, S.; Yazdanpour, N. Photocatalytic Conversion of Greenhouse Gases (CO2 and CH4) to High Value Products Using TiO2 Nanoparticles Supported on Stainless Steel Webnet. J. Taiwan Inst. Chem. E. 2013, 44, 239-246, DOI org/10.1016/j.jtice.2012.11.007. (11) Tahir, M.; Tahir, B.; Zakaria, Z. Y.; Muhammad, A. Enhanced Photocatalytic Carbon Dioxide Reforming of Methane to Fuels over Nickel and Montmorillonite Supported TiO2 Nanocomposite under UV-light Using Monolith Photoreactor. J. Clean. Prod. 2019, 213, 451461, DOI org/10.1016/j.jclepro.2018.12.169. (12)Han, B.; Wei, W.; Chang, L.; Cheng, P.; Hu, Y. H. Efficient Visible Light Photocatalytic CO2 Reforming of CH4. ACS Catal. 2016, 6, 494-497, DOI org/10.1021/acscatal.5b02653. (13)Delavari, S.; Amin, N. A. S. Photocatalytic Conversion of CO2 and CH4 Over Immobilized Titania Nanoparticles Coated on Mesh: Optimization and Kinetic Study. Appl. Energy 2016, 162, 1171-1185, DOI org/10.1016/j.apenergy.2015.03.125. 17



(14) Mahmodi, G.; Sharifnia, S.; Rahimpour, F.; Hosseini, S. Photocatalytic Conversion of CO2 and CH4 Using ZnO Coated Mesh: Effect of Operational Parameters and Optimization. Sol. Energy Mater Sol. Cells 2013, 111, 31-40, DOI org/10.1016/j.solmat.2012.12.017. (15) Yuliati, L.; Itoh, H.; Yoshida, H. Photocatalytic Conversion of Methane and Carbon Dioxide over Gallium Oxide. Chem. Phy. Lett. 2008, 452, 178-182, DOI org/10.1016/j.cplett.2007.12. 051. (16) Shi, D.; Feng, Y.; Zhong, S. Photocatalytic Conversion of CH4 and CO2 to Oxygenated Compounds over Cu/CdS-TiO2/SiO2 Catalyst. Catal. Today 2004, 98, 505-509, DOI org/10. 1016/j.cattod.2004.09.004. (17) Tahir, B.; Tahir, M.; Amin, N. A. S. Photo-Induced CO2 Reduction by CH4/H2O to Fuels over Cu-modified g-C3N4 Nanorods under Simulated Solar Energy. Appl. Surf. Sci. 2017, 419, 875885, DOI org/10.1016/j.apsusc.2017.05.117. (18) László, B.; Baán, K.; Varga, E.; Oszkó, A.; Erdőhelyi, A.; Kónya, Z.; Kiss, J. Photo-Induced Reactions in the CO2-Methane System on Titanate Nanotubes Modified with Au and Rh Nanoparticles. Appl. Catal. B: Environ. 2016, 199, 473-484, DOI org/10.1016/j.apcatb.2016. 06.057. (19) Liu, H.; Meng, X.; Dao, T. D.; Zhang, H.; Li, P.; Chang, K.; Wang, T.; Li, M.; Nagao, T.; Ye, J. Conversion of Carbon Dioxide by Methane Reforming under Visible-Light Irradiation: Surface-Plasmon-Mediated Nonpolar Molecule Activation. Angew. Chem. Int. Ed. 2015, 54, 11545-11549, DOI org/10.1002/anie.201504933. (20) Rycenga, M.; Cobley, C. M.; Zeng, J.; Li, W.; Moran, C. H.; Zhang, Q.; Qin, D.; Xia, Y. Controlling the Synthesis and Assembly of Silver Nanostructures for Plasmonic Applications. Chem. Rev. 2011, 111, 3669-3712, DOI org/10.1021/cr100275d. (21)Yu, J.; Yu, H.; Cheng, B.; Zhao, X.; Yu, J. C.; Ho, W. The Effect of Calcination Temperature on the Surface Microstructure and Photocatalytic Activity of TiO2 Thin Films Prepared by Liquid Phase Deposition. J. Phys. Chem. B 2003, 107, 13871-13879, DOI org/10.1021/jp 036158y 18


Page 18 of 30

Page 19 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(22) Bhavani, K., Naresh, G., Srinivas, B., Venugopal, A. Plasmonic resonance nature of AgCu/TiO2 photocatalyst under solar and artificial light: Synthesis, characterization and evaluation of H2O splitting activity. Appl. Catal. B: Environ. 2016, 199, 282-291, DOI org/10.1016/j.apcatb.2016.06.050. (23) Zhao, G.; Kozuka, H.; Yoko, T. Sol-Gel Preparation and Photoelectro Chemical Properties of TiO2 Films Containing Au and Ag Metal Particles. Thin Solid Films 1996, 277, 147-154, DOI org/10.1016/0040-6090(95)08006-6. (24) Clavero, C. Plasmon-Induced Hot-Electron Generation at Nanoparticle/Metal-Oxide Interfaces for Photovoltaic and Photocatalytic Devices. Nat. Photonics 2014, 8, 95-103, DOI 10.1038/NPHOTON.2013.238. (25) Bowering, N.; Croston, D.; Harrison, P. G.; Walker, G. S. Silver Modified Degussa P25 for the Photocatalytic Removal of Nitric Oxide. Int. J. Photoenergy 2007, 9, 90752, DOI org/10.1155/2007/90752. (26) Zhang, H.; Wang, G.; Chen, D.; Lv, X.; Li, J. Tuning Photoelectrochemical Performances of Ag-TiO2 Nanocomposites via Reduction/Oxidation of Ag. J. Chem.Mater. 2008, 20, 65436549, DOI org/10.1021/cm801796q. (27) Zhao, J.; Wang, Y.; Li, Y.; Yue, X.; Wang, C. Phase-Dependent Enhancement for CO2 Photocatalytic Reduction over CeO2/TiO2 Catalysts. Catal. Sci. Technol. 2016, 6, 7967-7975, DOI 10.1039/C6CY01365A. (28) Mao, J.; Ye, L.; Li, K.; Zhang, X.; Liu, J.; Peng, T.; Zan, L. Pt-Loading Reverses the Photocatalytic Activity Order of Anatase TiO2 {001} and {010} Facets for Photoreduction of CO2 to CH4. Appl. Catal. B: Environ. 2014, 144, 855-862, DOI org/10.1016/j.apcatb.2013.08. 027. (29) Pokrovski, K.; Jung, K. T.; Bell, A. T. Investigation of CO and CO2 Adsorption on Tetragonal and Monoclinic Zirconia. Langmuir 2001, 17, 4297-4303, DOI org/10.1021/la001723z. (30) Bang, Y.; Park, S.; Han, S. J.; Yoo, J.; Song, J. H.; Choi, J. H.; Kang, K. H.; Song, I. K. Hydrogen Production by Steam Reforming of Liquefied Natural Gas (LNG) over Mesoporous 19



Ni/Al2O3 Catalyst Prepared by an EDTA-Assisted Impregnation Method. Appl. Catal. B: Environ. 2016, 180, 179-188, DOI org/10.1016/j.apcatb.2015.06.023. (31) Liang, Z.; Li, T.; Kim, M.; Asthagiri, A.; Weaver, J. F. Low-Temperature Activation of Methane on the IrO2 (110) Surface. Science 2017, 356, 299-303, DOI 10.1126/science. aam9147. (32) Kwak, B. S.; Park, N. K.; Ryu, S. O.; Baek, J. I.; Ryu, H. J.; Kang, M. Improved Reversible Redox Cycles on MTiOx (M= Fe, Co, Ni, and Cu) Particles Afforded by Rapid and Stable Oxygen Carrier Capacity for Use in Chemical Looping Combustion of Methane. Chem. Eng. J. 2017, 309, 617-627, DOI org/10.1016/j.cej.2016.10.040. (33) Foo, S. Y.; Cheng, C. K.; Nguyen, T. H.; Adesina, A. A. Evaluation of Lanthanide-group Promoters on Co–Ni/Al2O3 Catalysts for CH4 Dry Reforming. J. Mol. Catal. A: Chem. 2011, 344, 28-36, DOI org/10.1016/j.molcata.2011.04.018. (34) Gao, C.; Chen, S.; Wang, Y.; Wang, J.; Zheng, X.; Zhu, J.; Song, L.; Zhang, W.; Xiong, Y. Heterogeneous Single Atom Catalyst for Visible Light Driven High Turnover CO2 Reduction: The Role of Electron Transfer. Adv. Mater. 2018, 30, 1704624, DOI org/10.1002/adma. 201704624. (35) Zhou, M.; Wang, S.; Yang, P.; Huang, C.; Wang, X. Boron Carbon Nitride Semiconductors Decorated with CdS Nanoparticles for Photocatalytic Reduction of CO2. ACS Catal. 2018, 8, 4928-4936, DOI org/10.1021/acscatal.8b00104. (36) Zhu, Y.; Gao, C.; Bai, S.; Chen, S.; Long, R.; Song, L.; Li, Z.; Xiong, Y. Hydriding Pd Cocatalysts: An Approach to Giant Enhancement on Photocatalytic CO2 Reduction into CH4. Nano Res. 2017, 10, 3396-3406, DOI 10.1007/s12274-017-1552-0. (37) Song, H.; Meng, X.; Wang, Z.; Wang, Z.; Chen, H.; Weng, Y.; Ichihara, F.; Oshikiri, M.; Kako, T.; Ye, J. Visible Light Mediated Methane Activation for Steam Methane Reforming under Mild Conditions: A Case Study of Rh/TiO2 Catalysts. ACS Catal. 2018, 8, 7556-7565, DOI org/10.1021/acscatal.8b01787. (38) Priya, R.; Baiju, K.; Shukla, S.; Biju, S.; Reddy, M.; Patil, K.; Warrier, K. Comparing 20


Page 20 of 30

Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ultraviolet and Chemical Reduction Techniques for Enhancing Photocatalytic Activity of Silver Oxide/Silver Deposited Nanocrystalline Anatase Titania. J. Phy. Chem. C 2009, 113, 6243-6255, DOI org/10.1021/jp8105343. (39) You, X.; Chen, F.; Zhang, J.; Anpo, M. A Novel Deposition Precipitation Method for Preparation of Ag-Loaded Titanium Dioxide. Catal. Lett. 2005, 102, 247-250, DOI 10.1007/ s10562-005-5863-5. (40) Chen, X.; Li, Y.; Pan, X.; Cortie, D.; Huang, X.; Yi, Z. Photocatalytic Oxidation of Methane over Silver Decorated Zinc Oxide Nanocatalysts. Nat. Commun. 2016, 7, 12273, DOI 10.1038/ncomms12273. (41) Li, K.; Peng, T.; Ying, Z.; Song, S.; Zhang, J. Ag-Loading on Brookite TiO2 Quasi Nanocubes with Exposed {210} and {001} Facets: Activity and Selectivity of CO2 Photoreduction to CO/CH4.

Appl.

Catal.

B:

Environ.

2016,

180,

130-138,

DOI

org/10.1016/j.apcatb.2015.06.022. (42) Firet, N. J.; Smith, W. A. Probing the Reaction Mechanism of CO2 Electroreduction over Ag Films via Operando Infrared Spectroscopy. ACS Catal. 2016, 7, 606-612, DOI org/10.1021/ acscatal.6b02382. (43) Liu, L.; Zhao, H.; Andino, J. M.; Li, Y. Photocatalytic CO2 Reduction with H2O on TiO2 Nanocrystals: Comparison of Anatase, Rutile, and Brookite Polymorphs and Exploration of Surface Chemistry. Acs Catal. 2012, 2, 1817-1828, DOI org/10.1021/cs300273q. (44) Zhang, W.; Wang, L.; Liu, H.; Hao, Y.; Li, H.; Khan, M. U.; Zeng, J. Integration of Quantum Confinement and Alloy Effect to Modulate Electronic Properties of RhW Nanocrystals for Improved Catalytic Performance Toward CO2 Hydrogenation. Nano Lett. 2017, 17, 788-793, DOI org/10.1021/acs.nanolett.6b03967. (45) Hauchecorne, B.; Tytgat, T.; Verbruggen, S. W.; Hauchecorne, D.; Terrens, D.; Smits, M.; Vinken, K.; Lenaerts, S. Photocatalytic Degradation of Ethylene: an FTIR in situ Study under Atmospheric Conditions. Appl. Catal. B: Environ. 2011, 105, 111-116, DOI org/10.1016/j. apcatb.2011.03.041. 21



(46) Liu, M.; Yang, Y.; Li, N.; Du, Y.; Song, D.; Ma, L.; Wang, Y.; Zheng, Y.; Jing, D. Controlled Formation of Intense Hot Spots in Pd@Ag Core-Shell Nanooctapods for Efficient Photothermal Conversion. Appl. Phys. Lett. 2017, 111, 073903. 7, DOI org/10.1063/1. 4999305. (47) Zhang, Y.; Zong, S.; Cheng, C.; Shi, J.; Guo, P.; Guan, X.; Luo, B.; Shen, S.; Guo, L. Rapid High-Temperature Treatment on Graphitic Carbon Nitride for Excellent Photocatalytic H2Evolution Performance. Appl. Catal. B: Environ. 2018, 233, 80-87, DOI org/10.1016/j.apcatb. 2018.03.104. (48) Zhang, Y.; Shi, J.; Cheng, C.; Zong, S.; Geng, J.; Guan, X.; Guo, L. Hydrothermal Growth of Co3 (OH)2(HPO4)2 Nano-Needles on LaTiO2N for Enhanced Water Oxidation under VisibleLight Irradiation. Appl. Catal. B: Environ. 2018, 232, 268-274, DOI org/10.1016/j.apcatb. 2018.03.067.

22


Page 22 of 30

Page 23 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. (a) SEM, (b) TEM, (c) STEM, and (d) HRTEM images of 1 wt% Ag/TiO2. Inset in (c) shows the statistic results of the size distribution of Ag nanoparticles in the 1 wt% Ag/TiO2 composite.

23



Figure 2. (a) UV-visible spectra and (b) PL spectra of different Ag contained TiO2 photocatalysts. (c-e) Schematic illustration of the proposed electron transfer process involved in Ag/TiO2 photocatalyst under the irradiation of (c) visible light, (d) simulated sunlight, and (e) ultraviolet light. Ec is the conduction band. φM is the work function of the Ag and χS is the electron affinity of the TiO2.

24


Page 24 of 30

Page 25 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. (a) Time-course photocatalytic CO and C2H4 production from CO2 and CH4 over the 1 wt% Ag/TiO2 catalyst and (b) the specific activities over different amount of Ag contained composites under simulated solar irradiation. (c-e) Photocatalytic isotope-labeled measurement using 13CO2 instead of 12CO2 with the products analyzed by GC-MS: (c) GC, (d) CO MS, and (e) C2H4 MS spectra.

25



Figure 4. Photocatalytic performance for CO2 and CH4 reformation: (a) the effect of different light source (visible light, simulated sun light, and ultraviolet light) over 1%Ag/TiO2 catalyst, and (b, c) effect the type of (b) primary photocatalyst and (c) metal cocatalyst, under simulated solar irradiation.

26


Page 26 of 30

Page 27 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. In-situ IR spectra of (a) the pre-adsorption process without light irradiation and (b) reaction process in the presence of solar irradiation. Magnified IR spectra of (b) in the range of (c) 1390-1800 cm-1 and (d) 2100-2400 cm-1.

27



Figure 6. A schematic illustration showing the reaction process and mechanism.

28


Page 28 of 30

Page 29 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For Table of Contents Only

Synopsis Selective conversion of CO2 and CH4 into ethylene at mile condition was achieved by coupling photocatalytic and plasma effects.

29



For Table of Contents Only

Synopsis Selective conversion of CO2 and CH4 into ethylene at mile condition was achieved by coupling photocatalytic and plasma effects.

1


Page 30 of 30

Plasma-assisted Photocatalysis of CH4 and CO2 into Ethylene | ACS

Recommend Documents