Synergy of Ag and AgBr in a Pressurized Flow Reactor for Selective Photocatalytic Oxidative Coupling of Methane

Oxidation of methane into valuable chemicals, such as C2+ molecules, has been long sought after but the dilemma between high yield and high selectivity of desired products remains. Herein, methane is upgraded through the photocatalytic oxidative coupling of methane (OCM) over a ternary Ag–AgBr/TiO2 catalyst in a pressurized flow reactor. The ethane yield of 35.4 μmol/h with a high C2+ selectivity of 79% has been obtained under 6 bar pressure. These are much better than most of the previous benchmark performance in photocatalytic OCM processes. These results are attributed to the synergy between Ag and AgBr, where Ag serves as an electron acceptor and promotes the charge transfer and AgBr forms a heterostructure with TiO2 not only to facilitate charge separation but also to avoid the overoxidation process. This work thus demonstrates an efficient strategy for photocatalytic methane conversion by both the rational design of the catalyst for the high selectivity and reactor engineering for the high conversion.


■ INTRODUCTION
Large reserves of natural gas and shale gas, especially those in remote areas, have raised incentives for the on-site and largescale conversion of methane (CH 4 ) to high-value chemicals, which also avoids adverse environmental impact due to the nearly 30-time more potent greenhouse gas effect of methane than CO 2 . 1,2 However, the low polarisability and high C−H bond energy (439 kJ/mol) of CH 4 make its economic conversion extremely challenging. 3 Methane conversion, including the nonoxidative coupling of methane, oxidative coupling of methane (OCM), and partial oxidation of methane, has been developed in thermocatalysis for the production of value-added products, such as C 2+ hydrocarbons and alcohols. 4−6 However, most of the processes require strong oxidants (H 2 O 2 or H 2 SO 4 ) and/or harsh reaction conditions (e.g., high temperature and pressure). 4,7,8 Photocatalysis uses the energy of photons instead of heat to drive thermodynamically nonspontaneous reactions, such as water splitting, carbon dioxide reduction, etc. Since photons are the main energy source, photocatalytic reactions can be conducted under very mild conditions. Methane oxidation by oxygen gas to C 1 oxygenates (e.g., CH 3 OH, CH 3 OOH, and HCHO) in the presence of water has been well-studied using oxide-based photocatalysts. The selectivity of products can be manipulated via the modification of different co-catalysts. 9 For instance, a high primary products (CH 3 OOH and CH 3 OH) yield of 25.4 μmol/h and a selectivity of 95% were achieved over TiO 2 modified by Au-CoO x dual co-catalyst. 10 Up to now, photocatalytic methane conversion has already been achieved over TiO 2 , ZnO, WO 3 , etc. 11−17 However the upgrade of methane into C 2 products is still one of the most challenging pathways as it is difficult to minimize overoxidation while maintaining a high conversion rate. 18−21 Recently, CH 4 was successfully converted into C 2 H 6 at a selectivity of 90% in a photochemical looping process by an Ag-HPW/TiO 2 photocatalyst. 22 However, the ethane yield (2.3 μmol/h) was very moderate, and a subsequent catalyst recovery process was required to regenerate the active silver species on TiO 2 . More importantly, when O 2 was introduced into the reaction atmosphere, only overoxidation products (CO x ) were obtained. 23 Apart from the selection of photocatalysts, the reaction system is equally important for an efficient photochemical process. Most of the reported reactors used in photocatalytic methane conversion were batch reactors. 24 However, the products in a batch reactor easily undergo overoxidation in the presence of oxidants because all products from methane conversion are more reactive than methane itself. Thus, the use of flow reactors in photocatalytic methane conversion is crucial to manipulate the mass transfer, thus minimizing the drawback of batch reactors and improving the selectivity of the less stable valuable chemicals. Our group reported the first photocatalytic OCM in a flow reaction system, an improved ethane (C 2 H 6 ) yield of 6.8 μmol/h was achieved, but it was still quite moderate. 19 Very recently, Au-ZnO/TiO 2 was also reported for photocatalytic OCM in a flow reactor under atmospheric pressure. 25 A high C 2 H 6 yield of 100 μmol/h was obtained without external heating, although the temperature of the catalyst reached 413 K due to Xe lamp irradiation. These results indicate it is still challenging to achieve a high yield of C 2 products at low temperatures. Moreover, the reaction pressure, as a crucial factor in gas phase reactions, has not been investigated in flow systems for photocatalytic methane conversion. Considering the high pressure of natural gas in both production sites and transportation pipelines, it is economical to convert methane in pressurized reactors.
Herein, we report the selective photocatalytic OCM in a pressurized flow reactor over an Ag−AgBr/TiO 2 catalyst. The ethane production rate of 35.4 μmol/h was achieved, together with an excellent C 2+ selectivity of 79% operated at a low temperature of 40°C. The utilization of a pressurized flow reactor likely enhanced the mass transfer of both reactants and products. Ag serves as an electron sink, while AgBr forms a heterostructure with TiO 2 , which improves charge separation and migration, and more importantly avoids overoxidation. Overall, the results suggest that both the photocatalyst and the reaction system play important roles in photocatalytic methane conversion.

■ RESULTS AND DISCUSSION
Ag and AgBr were loaded on anatase TiO 2 by a two-step precipitation−photodeposition method as detailed in the Materials Synthesis (Supporting Information) and denoted Ag−AgBr/TiO 2 . The same amount of Ag was also loaded on TiO 2 by photodeposition as a reference and denoted Ag/TiO 2 . The photocatalysts were tested in a pressurized flow reaction system (Scheme S1). The control experiment shows that the photocatalyst, CH 4 , and light irradiation are all indispensable to converting methane at low temperatures ( Figure S1). Then, CH 4 conversion was evaluated on TiO 2 . Bare TiO 2 produces CO 2 with a selectivity of 91% at a rate of 45.1 μmol/h ( Figures  1a and S2). With 2 wt % Ag deposited on TiO 2 , the C 2 H 6 production rate increases from 2.1 μmol/h to 36.7 μmol/h. Additionally, C 3 H 8 is also produced at a rate of 1.9 μmol/h. Ag loading can facilitate the formation of C 2+ products. However, severe overoxidation is also observed as the CO 2 production rate surges to 99.5 μmol/h, corresponding to a selectivity of 52%. When Ag and AgBr were co-loaded on TiO 2 , the C 2 H 6 and C 3 H 8 production rates slightly reduce to 35.4 and 1.1 μmol/h, respectively, while CO 2 production is substantially suppressed. A high C 2+ selectivity of 79% has been achieved over Ag−AgBr/TiO 2 in contrast to 8% over TiO 2 and 44% over Ag/TiO 2 .
Following this, a series of Ag−AgBr/TiO 2 photocatalysts with various Ag loading amounts were synthesized to optimize the photocatalytic performance under the reaction pressure of 6 bar (Figures 1b and S2). The selectivity shifts toward C 2 H 6 (70%) even with a small AgBr amount of 0.5 wt %. Increasing the amount of AgBr to 2 wt % has little effect on the yield of C 2 H 6 but effectively decreases the production of CO 2 . Although further increasing the AgBr amount can improve C 2+ selectivity to as high as 90%, a decrease in the yield of all products is observed. The results indicate that AgBr plays an important role in controlling overoxidation during methane conversion. TiO 2 decorated with 2 wt % AgBr was then chosen for further study to improve both the yield and selectivity of C 2 H 6 . The effect of reaction pressure on the photocatalytic OCM performance was investigated (Figures 1c and S3). The yield of C 2 H 6 increases from 18.2 to 35.4 μmol/h as the reaction pressure elevates from 1 to 6 bar. An apparent quantum efficiency under 6 bar based on methane conversion was calculated to be 3% at 365 nm. The photocatalytic performance at 7 bar stops increasing, which is possibly caused by the limited photo-induced carriers generated by the photocatalyst. The enhanced performance under elevated pressures is mainly due to the enhanced mass transfer, which increases the adsorption of CH 4 on the surface of photocatalysts. On the other hand, the selectivity of CO 2 gradually increases from 9 to 16% as pressure increases. The partial pressures of both CH 4 and O 2 in the reaction atmosphere increase as the total pressure increases. Considering the activation of O 2 is much easier than that of CH 4 , 26 O 2 reduction is improved more significantly than CH 4 under higher pressures, resulting in the formation of excessive O 2 − radicals, which contributes to overoxidation. Thus, the selectivity toward C 2+ products decreases under higher pressures. The effect of CH 4 to air ratio on the photocatalytic performance was next investigated at a total flow rate of 400 mL/min (Figures 1d and S4). When changing the ratio of CH 4 /air ratio from 40:1 to 1:1, the C 2 H 6 production rate first increases from 35.4 to 58.1 μmol/h at the CH 4 /air of 2:1, and finally drops to 52.1 μmol/h when the CH 4 /O 2 ratio reaches 1:1. The yield of CO 2 is greatly accelerated with the increase of O 2 proportion.
The optimized Ag−AgBr/TiO 2 photocatalyst was further tested under 6 bar pressure to examine its long-term durability (Figures 1e and S5). The yield of all products increases in the first 3 h and becomes stable afterward. It suggests that there is an in situ activation process. It is well known that AgBr is lightsensitive and can decompose into Ag and Br 2 upon exposure to irradiation. 27 The enhancement of plasmonic Ag signal in the UV−vis diffuse reflectance spectrum (DRS) of Ag−AgBr/TiO 2 after reaction for 3 h suggests that the amount of Ag in the photocatalyst increases, indicating AgBr underwent partial decomposition at this stage ( Figure S6). The reduced Br 3d Xray photoelectron spectroscopy (XPS) signal after the photocatalytic OCM reaction for 3 h and the similar intensity of the XPS peaks after running for 3 and 12 h suggest that AgBr is not fully decomposed even after the long-term irradiation ( Figure S7). The main Ag species in AgBr/TiO 2 before the photocatalytic reaction are positively charged silver ( Figure S8i, e.g., AgBr), with only a small amount of metallic Ag. After the reaction for 3 h, the portion of metallic Ag increases while Ag + decreases. This results from the fact that AgBr is partially decomposed into metallic Ag at the first 3 h of irradiation. The ratio of Ag to AgBr is calculated to be 1.34:1 based on the integrated area of the corresponding band after the catalyst is run for 3 h. Further prolonging the reaction time to 12 h results in a slight increase of Ag with an Ag to AgBr ratio of 1.57:1. The XPS results further confirm that AgBr almost remains and the chemical state of Ag in Ag−AgBr/TiO 2 is hardly changed during the photocatalytic OCM reaction from 3 to 12 h. X-ray diffraction (XRD) also confirms the partial decomposition of AgBr after 3 h and the amount of AgBr is relatively stable in the subsequent 9 h ( Figure S9), consistent with the reported. 28 Photoluminescence (PL) spectra display improved separation of charge carriers in Ag−AgBr/TiO 2 after 3 h of reaction ( Figure S10). A similar luminescent property was observed in the photocatalyst after 3 and 12 h of methane conversion. Combined with the longterm photocatalytic performance, the above results reveal that AgBr is rather stable after the initial in situ activation process. Table S1 shows the performance of different reported photocatalysts for photocatalytic C 2 H 6 production from methane, and it is clear that Ag−AgBr/TiO 2 shows a high C 2 H 6 production rates of 34.5 μmol/h in photocatalytic methane oxidation by air with a C 2+ selectivity of 79% in a low reaction temperature of 40°C.
XRD patterns of the photocatalysts display the main component of anatase (Figure 2a). Ag was not detected in either Ag/TiO 2 or Ag−AgBr/TiO 2 , possibly due to its small particle size and/or high dispersity. 29  with the Br 3d spectrum ( Figure S7), XPS analysis reveals the co-existence of Ag and AgBr on Ag−AgBr/TiO 2 . The band at 460 nm of UV−vis DRS spectra of Ag/TiO 2 and Ag−AgBr/ TiO 2 is attributed to the plasmonic effect of metallic Ag (Figure 2c). 33 Absorption of AgBr is not observed in the DRS spectrum of Ag−AgBr/TiO 2 , possibly due to the low loading amount. Then, the absorption spectrum of pure AgBr was measured ( Figure S11), representing a visible absorption when the amount of AgBr is large enough. PL spectroscopy was used to investigate the charge separation and recombination process of the photocatalysts. TiO 2 shows the highest PL emission, implying an intense recombination process. Ag loading causes a reduction in the PL intensity, and co-modification of TiO 2 with Ag and AgBr results in the lowest PL signals. Considering the similar absorption of three catalysts in the UV region, the most efficient charge separation is achieved over Ag−AgBr/ TiO 2 . To further study the effect of AgBr on charge separation and migration, the open circuit photovoltage decay spectra of three catalysts were measured ( Figure S12a−c). The average lifetimes of the charges in TiO 2 , Ag/TiO 2 , and Ag−AgBr/TiO 2 are determined to be 6.4, 12.1, and 52.9 s, respectively ( Figure  S12d). The eightfold increased lifetime indicates that the formation of a heterojunction between TiO 2 and AgBr significantly prolongs the charge lifetime. The longest lifetime of charge carriers is resulted from the efficient separation of electrons and holes and could improve photon efficiency in photocatalysis.
Transmission electron microscopy (TEM) images show that TiO 2 consists of nanoparticles of 20−30 nm ( Figure S13a). High-resolution TEM (HRTEM) displays the (101) plane of anatase TiO 2 with an interplanar spacing of 0.346 nm ( Figure  S13b). Metallic Ag nanoparticles can be confirmed by the line scan of Ag/TiO 2 ( Figure S14). Nanoparticles of AgBr with diameters of 100−200 nm were observed in Ag−AgBr/TiO 2 (Figure 2e). Small Ag nanoparticles of 5 nm are also found to co-exist with AgBr on TiO 2 in Ag−AgBr/TiO 2 (Figure 2f). The HRTEM ( Figure S15) further verifies the existence of AgBr in Ag−AgBr/TiO 2 , the interplanar distances of 0.206 and 0.290 nm are ascribed to the (220) and (200) crystal facets of AgBr, which is in accordance with the XRD analysis. In the scanning transmission electron microscopy energy-dispersive spectrometry (EDS) mapping images ( Figure S16), the Ti element from TiO 2 is detected in the selected area. Ag and Br elements are consistent with large particles of AgBr. The quality of Br mapping is slightly lower than Ag. This is mainly because that AgBr is partially decomposed under the long-term irradiation of the electron beam. Overall, Ti, O, Ag, and Br are detected in Ag−AgBr/TiO 2 as observed from the EDS sum spectrum ( Figure S16e).
The reduction of oxygen gas by electrons and oxidation of methane by holes are two crucial steps during photocatalytic OCM. The oxygen reduction capability of the three photocatalysts was tested via linear sweep voltammetry (LSV) in a three-electrode cell at a potential ranging from 0.4 to −1.2 V vs Ag/AgCl ( Figure S17). In the absence of air, little current is generated until the applied voltage reaches −1.0 V due to hydrogen evolution. On the contrary, a negative current is generated at the onset potential of −0.4 V in the presence of air, which is attributed to the oxygen reduction reaction. Therefore, the signal obtained in the presence of air is contributed by both oxygen reduction and hydrogen evolution. To reflect the actual oxygen reduction ability of the catalysts, the difference between the LSV spectra obtained with and without air is replotted (Figure 3a). The results show that Ag nanoparticles play a major role in oxygen reduction, as both Ag/TiO 2 and Ag−AgBr/TiO 2 show improved current density compared with TiO 2 when the bias is more negative than −0.6 V. Ag/TiO 2 exhibits the highest current density due to the highest metallic Ag amount of 2 wt %. This is consistent with the previous report that metallic Ag could promote oxygen adsorption on TiO 2 . 34 Ag acts as an electron sink and can promote charge separation by accepting electrons from the conduction band (CB) of TiO 2 . Therefore, more photogenerated holes in the Ag-containing photocatalysts are available to activate methane molecules. As a result, the improved conversion rate of CH 4 is achieved after the loading of Ag. O 2 can then be reduced by electrons on the surface of Ag to produce superoxide radicals (O 2 − ). To confirm this, the formation of O 2 − radicals was monitored by electron paramagnetic resonance (EPR) using 5,5-dimethyl-1-pyrroline N-oxide as the spin-trapping reagent (Figure 3b). No EPR signal is generated in dark conditions ( Figure S18), suggesting that the formation of O 2 − resulted from the combination of O 2 and photoinduced electrons. Ag/TiO 2 and Ag−AgBr/TiO 2 generate a higher level of O 2 − radicals than TiO 2 , which remains in the same order as the LSV oxygen reduction results when the bias is more negative than −0.6 V. The highest amount of O 2 − radicals are generated over Ag/TiO 2 . O 2 − radicals clean the surface of the photocatalyst by combining with H + to produce H 2 O. Ag serves as an electron acceptor and catalyzes O 2 reduction, which contributes to charge separation and photon efficiency, thus resulting in improved methane conversion. However, a high level of O 2 − radicals also encourage the complete mineralization of organic compounds or overoxidation to produce CO 2 . 35 Thus, a large amount of CO 2 (99.5 μmol/h) was detected in the photocatalytic OCM over Ag/TiO 2 . Ag−AgBr/TiO 2 shows intermediate oxygen reduction properties among three photocatalysts, which is beneficial for reducing the selectivity of CO 2 while maintaining a relatively high CH 4 conversion. XPS valence band (VB) spectra were measured, which shows that the relative VB potentials of AgBr and TiO 2 are 1.9 and 3 eV, respectively (Figure 3c). Taking into account of the reported VB potential of anatase TiO 2 is 2.9 V vs NHE, 36 the VB potential of AgBr should be 1.8 V vs NHE. Thus, photoholes can potentially transfer from the VB of TiO 2 to AgBr in Ag−AgBr/TiO 2 upon light irradiation, resulting in a reduced oxidation potential. Thus, the overoxidation is suppressed and a high selectivity toward C 2 H 6 is reasonable after the introduction of AgBr. To further evaluate the oxidation capability of the photocatalysts, the transient photocurrents of TiO 2 , Ag/TiO 2 , and Ag−AgBr/TiO 2 were measured in a 0.5 M NaSO 4 aqueous solution containing 10 vol % methanol. A bias potential of 0.25 V was applied in the test (Figures S19 and 3d). TiO 2 shows a high photocurrent density of 40 μA/cm 2 , suggesting a fast electron transfer from the working electrode to the counter electrode and intensive methanol oxidation. When adding Ag to TiO 2 , the photocurrent is slightly reduced somehow. It is probably because the electrons are trapped by metallic Ag, which then reduces some intermediates from methanol oxidation. When methanol is removed from the electrolyte, Ag/TiO 2 displays the highest photocurrent for water oxidation among the three catalysts ( Figure S20). The lowest photocurrent is generated by Ag− AgBr/TiO 2 . After photogenerated holes transfer from the VB of TiO 2 to AgBr, the oxidation potential is reduced, leading to a slow methanol oxidation process. This also mitigates the overoxidation of the produced C 2+ to CO 2 . A much higher selectivity toward C 2+ products has thus been achieved over Ag−AgBr/TiO 2 compared with TiO 2 and Ag/TiO 2 .
To provide insights into the reaction mechanism and reaction pathway, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was performed on Ag/TiO 2 and Ag−AgBr/TiO 2 (Figure 4). The infrared (IR) signal at 2875/2880 cm −1 under light irradiation is ascribed to the stretching vibration of C−H in CH 3 · radicals adsorbed on the oxide surface. This band is stronger in the spectrum of Ag/ TiO 2 than Ag−AgBr/TiO 2 , which shows the same trend as the methane conversion performance over the two photocatalysts. The peaks at 2358 and 2331/2326 cm −1 are typical signals originating from CO 2 generated due to overoxidation in the photocatalytic OCM process. The CO 2 peaks over Ag/TiO 2 are much stronger than Ag−AgBr/TiO 2 and the intensity keeps increasing with prolonged irradiation time. In contrast, Ag−AgBr/TiO 2 generates a moderate amount of CO 2 under identical reaction conditions. This result is in accordance with the product selectivity of Ag/TiO 2 and Ag−AgBr/TiO 2 . An additional band at 1552/1558 cm −1 is ascribed to the HCOO· species, which is an important intermediate and finally results in the formation of CO 2 in the methane oxidation process. The strong band at 1558 cm −1 indicates that the consumption of HCOO· is slower than its formation over Ag−AgBr/TiO 2 , suggesting a mild overoxidation process. However, the HCOO· species on Ag/TiO 2 can be facilely converted to CO 2 , which is deduced from the low IR band intensity of HCOO· species at 1552 cm −1 . The IR spectra over the two photocatalysts show different features when the irradiation time reaches 70 to 120 min, as displayed in Figure S21. An increased absorption across the whole spectrum is observed over Ag/TiO 2 with the increased irradiation time. Due to the fast oxidation of methane by Ag/TiO 2 , the amount of O 2 gas decreases rapidly in the reaction chamber. Therefore, photogenerated electrons could not be consumed due to the low level of oxygen and electrons start to accumulate on the CB of TiO 2 , consistent with the report that the photogenerated electrons display an IR absorption from 4000 to 1500 cm −1 . 37 Nevertheless, this phenomenon is not observed in the IR spectra of Ag−AgBr/ TiO 2 . The spectra almost overlap at the irradiation time from 70 to 120 min. This indicates that a slow oxygen consumption process and a mild methane oxidation process are achieved over Ag−AgBr/TiO 2 , compared with Ag/TiO 2 . This result is in accordance with the LSV oxygen reduction and EPR O 2 − trapping analysis.
To provide evidence on the universal synergy effect of Ag and AgBr on semiconductors for photocatalytic methane conversion, an Ag−AgBr/ZnO photocatalyst was synthesized by the same method, and its performance is compared with the pristine ZnO. ZnO, with a similar band structure as TiO 2 , should display similar behavior as TiO 2 in photocatalytic methane conversion. The results are shown in Figure S22. When pure ZnO is applied as the photocatalyst, CH 3 OH and CO 2 are generated at production rates of 6.1 and 7.5 μmol/h, respectively. It suggests that the methyl radicals formed from the reaction between methane and photoholes are mostly overoxidised into CO 2 . The production of CH 3 OH could probably be due to the unique surface features of ZnO, which might facilitate CH 3 OH desorption. Overall, coupling of methyl radicals is not encouraged on ZnO surfaces as C 2 H 6 is not detected in the products. After modification with Ag− AgBr, C 2 H 6 with a high production rate of 22.8 μmol/h is detected. A trace amount of C 3 H 8 (0.4 μmol/h) is also produced, which is the product of further C 2 H 6 activation. CH 3 OH is not detectable in the products. Most importantly, the production rate of CO 2 is reduced from 7.5 to 4.2 μmol/h after the modification of ZnO with AgBr. Combined with the methane oxidation performance of the Ag−AgBr/TiO 2 photocatalyst, direct evidence of the function of AgBr in facilitating C 2 H 6 production and reducing overoxidation is obtained.
The band gaps of TiO 2 and AgBr were determined to be 3.2 and 2.5 eV, respectively, from the Kubelka−Munk conversion plots ( Figure S23), consistent with the reported. 36 Combined with the XPS VB analysis (Figure 3c), the CB potentials of AgBr and TiO 2 are determined to be −0.7 and −0.3 V vs NHE, respectively. In Ag−AgBr/TiO 2 , AgBr forms a type II heterojunction with TiO 2 (Scheme 1). Upon light irradiation, photoelectrons tend to transport from the CB of AgBr to TiO 2 , and further to Ag, while photoholes transfer from the VB of TiO 2 to AgBr. Considering the potentials required for O 2 − formation from O 2 reduction and ·CH 3 production from CH 4 oxidation are −0.16 and 1.75 V vs NHE, 1,22,38 such a structure is not only beneficial for charge separation but also capable of driving the photocatalytic OCM reaction, therefore resulting in improved photon utilization efficiency and a high methane conversion. Although the enhanced charge separation is also achieved in Ag/TiO 2 , the highly oxidative holes at the VB of TiO 2 and the large number of O 2 − radicals formed on the metallic Ag cause severe overoxidation and significantly deteriorate the selectivity toward C 2+ products (Scheme S2 in the Supporting Information). After the introduction of AgBr, photogenerated holes at the VB of TiO 2 transfer to that of AgBr and then oxidize methane into methyl radicals and protons. A mild methane oxidation process is thus achieved by the less oxidative holes at the VB of AgBr. Next, the formed methyl radicals are prone to couple into C 2 H 6 and are less likely to undergo overoxidation due to the relatively weak oxidation potential of photoholes in the VB of AgBr. In parallel, O 2 − radicals react with protons to generate water. Some C 2 H 6 molecules are activated again by photoholes to form ·C 2 H 5 radicals, which couple with ·CH 3 radicals and form C 3 H 8 .

■ CONCLUSIONS
In summary, an efficient and selective photocatalytic OCM process has been realized with a ternary Ag−AgBr/TiO 2 photocatalyst in a pressurized flow reactor. The production rate of C 2 H 6 achieved is as high as 35.4 μmol/h, with a C 2+ selectivity of 74−90% depending on the pressures used, together with an apparent quantum efficiency of 3% at 365 nm. These results suggest that both the reaction system and the photocatalyst play important roles in the performance of photocatalytic methane conversion as detailed by a series of characterizations. The utilization of a pressurized flow reactor enhances the mass transfer of reactants and products, contributing to the high methane conversion and selectivity toward C 2+ products. A series of electrochemical tests and EPR results proved that Ag nanoparticles serve as an electron acceptor to improve charge separation, while the reactive holes from TiO 2 transfer to AgBr and become less oxidative to avoid overoxidation. Therefore, both the high yield and high selectivity of C 2+ products have been obtained. The findings demonstrate a potential to realize the efficient and selective conversion of methane to C 2+ by the synergy of Ag and AgBr driven by photocatalysis.
Experimental procedures and characterization data (PDF)