Letter pubs.acs.org/JPCL
Economic Hydrophobicity Triggering of CO2 Photoreduction for Selective CH4 Generation on Noble-Metal-Free TiO2−SiO2 Chunyang Dong, Mingyang Xing,* and Jinlong Zhang* Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry & Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, P.R. China
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S Supporting Information *
ABSTRACT: On the basis of the fact that the competitive adsorption between CO2 and H2O on the catalyst plays an important role in the CO2 photoreduction process, here we develop an economic NH4F-induced hydrophobic modification strategy to enhance the CO2 competitive adsorption on the mesoporous TiO2−SiO2 composite surface via a simple solvothermal method. After the hydrophobic modification, the CO2 photoreduction for the selective generation of CH4 over the noble-metal-free TiO2−SiO2 composite can be greatly enhanced (2.42 vs 0.10 μmol/g in 4h). The enhanced CO2 photoreduction efficiency is assigned to the rational hydrophobic modification on TiO2−SiO2 surface by replacing Si− OH to hydrophobic Si−F bonds, which will improve the CO2 competitive adsorption and trigger the eight-electron CO2 photoreduction on the reaction kinetics.
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adsorption of CO2 on the catalyst surface in the presence of H2O vapor, and more active sites will be exposed and facilitate the photoreduction of CO2 to CH4. Mesoporous TiO2−SiO2 composites (MASC, A stands for anatase phase TiO2) have been successfully synthesized by using an EISA (evaporation induced self-assemble) method,15 which displays a large surface area (Figure S1 and Table S1) to facilitate the further surface modification. Variation of Ti/Si ratio 8:2 and 6:4 was denoted as MASC (8:2) and MASC (6:4), respectively. Highly dispersed anatase nanocrystals were clearly embedded in the matrix of the pore walls of SiO2 with random orientation (Figure 1a and red cycles in Figure 1b). The hydrophobic modification on MASC has been done according to our previous work22 by using NH4F as the hydrophobic modifier and isopropanol (IPA) as the solvent via a solvothermal treatment, which was denoted as F-MASC. Compared with other silica−titania hybrid materials like Ti− HMS and Ti−MCM-41,16,17 MASC exhibits relative high crystallinity of TiO2, which is beneficial to simultaneously produce Ti−F and Si−F bonds on the catalysts surface during the NH4F modification. Recently, Eaton et al.18,19 have characterized and quantified the available sites and undercoordinated Ti sites on the TiO2−SiO2 catalysts. The surface Ti−OH bonds and the under-coordinated Ti sites play an important role in the formation of active sites.20 Hence, these Si−F and Ti−F bonds produced by the substitution of F for hydroxyl groups (−OH) with a certain degree of hydro-
ith the development of human society, the discharge amount of greenhouse gas like CO2 into the atmosphere is increasing year by year.1 As one of the most popular technologies for the CO2 reduction, photocatalysis displays some advantages such as low cost, mild reaction conditions, simplified operational procedure, and the raw materials easy to get.2,3 Although enormous efforts have been done on improving the efficiency of CO2 photoreduction,4−11 its quantum yield for the CH4 is still very low,3,12 owing to the fact that reduction of CO2 to CH4 is an eight-electron reduction reaction. The noble metals of Pt,7 Au,13 Pt@Cu2O,6 and so on are always employed as the active sites to enhance the CH4 generation efficiency due to their excellent capability for the trapping of photoelectrons; however, the adsorption of some intermediate products such as CO on the surface of Pt makes the active sites inactive, leading to the gradual decrease in CH4 yield during the CO2 photoreduction.7 Thereby, some other modification technologies which can be used instead of expensive noble metals should be developed to improve the CH4 yield in the CO2 photoreduction. For instance, the surface modification based on the hydrophobicity to enhance the CO2 competitive adsorption to H2O on the catalysts is an ideal strategy to trigger the eight-electron CO2 photoreduction according to the reaction kinetics. Although, Liu et al.14 have demonstrated the dynamic hydrophobic hindrance effect of zeolite@zeolitic imidazolate framework composites for the CO2 capture in the presence of water, there are still fewer reports on hydrophobicity triggering photoreduction of CO2 up to now. Hence, in present work we proposed a new strategy of hydrophobic modification on the TiO2−SiO2 composite to regulate the surface adsorption amount of H2O. In this way we could improve the competitive © 2016 American Chemical Society
Received: June 11, 2016 Accepted: July 14, 2016 Published: July 14, 2016 2962
DOI: 10.1021/acs.jpclett.6b01287 J. Phys. Chem. Lett. 2016, 7, 2962−2966
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The Journal of Physical Chemistry Letters
Figure 1. (a) Elemental mapping images of MASC (6:4); red dots denote element Ti and yellow dots denote element Si. HRTEM image of (b) MASC (8:2) and (c) F-MASC (6:4). (d) XRD patterns of MASC (8:2) and F-MASC(8:2).
Figure 2. (a) F 1s XPS spectra of F-MASC (8:2). CO2 adsorption isotherms of the MASC and F-MASC in the absence (b) and presence (c) of water vapor (0.1 g catalysts).
0.46 after the hydrophobic treatment, implying that a large number of hydroxyl groups have been replaced.21,22 F 1s XPS spectra of F-MASC (8:2) showed that the surface fluorine species were existing in two forms (Figure 2a): The 688.3 eV peak was attributed to surface Si−F bonds, which are responsible for the hydrophobic properties,22 and another peak at 685.4 eV was assigned to the surface Ti−F bonds.22 The Ti−F/Si−F in a ratio of 5.25 suggests that an appropriate proportion of Ti−F bonds are beneficial to control the hydrophobic degree on the catalyst surface, owing to its relative lower hydrophobicity than Si−F. The UV-DRS spectra for the catalysts before and after NH4F modification did not change (Figure S6), indicating the fluorination treatment is just a surface modification.23 Therefore, we can eliminate the impact of F doping on the photocatalytic activity of F-MASC. To highlight the importance of hydrophobic surface in the CO2 photoreduction, we carefully carried out the CO2 adsorption tests under different conditions (details in the SI). When the adsorbent was pure CO2, there is no adsorption difference between the MASC (8:2) and F-MASC (8:2), as shown in Figure 2b. Interestingly, when the gas mixture of CO2 and H2O vapor was introduced into the reactor simultaneously, the F-MASC (8:2 and 6:4) presented a strong CO2 adsorption than the blank MASC (Figure 2c), which confirms that the
phobicity are expected to be the active sites for the CO2 photoreduction. Figure 1c and Figure S2 give the TEM images for the FMASC (6:4) and F-MASC (8:2). The lattice fringes of d = 0.34 nm and d = 0.20 nm are ascribed to the (101) and (100) facets of anatase, respectively. TEM images alone the [001] and [110] direction confirms the F-MASC (6:4) possessed a highly ordered 2-D hexagonal regularity (Figure 1c). The surface area of F-MASC (8:2) and F-MASC (6:4) is 259.5 and 211.2 m2/g respectively, which are very close to the area of blank MASCs (Figure S1 and Table S1), indicating the solvothermal stability of the composite. Additionally, the wide- and small-angle XRD patterns also suggest the consistent anatase crystallinity and the ordering mesoporous structure of F-MASC and MASC (Figure 1d and Figure S3). Overall, these structure characterization results indicate that the surface hydrophobic modification cannot destroy the microstructure of MASC. FT-IR and XPS analysis were conducted to confirm the occurring of surface fluorine modification. The decrease in peaks at 3430 and 1630 cm−1 in the FTIR spectra (Figure S4) indicates that the number of surface hydroxyl groups was reduced, leading to the decrease in surface water adsorption of the composite.22 Furthermore, seen from the O 1s XPS spectra (Figure S5), the ratio of −O−/−OH decreased from 1.05 to 2963
DOI: 10.1021/acs.jpclett.6b01287 J. Phys. Chem. Lett. 2016, 7, 2962−2966
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Figure 3. (a,b) Evaluation of photocatalytic CO2 conversion to CH4 over MASC and F-MASC with different Ti/Si ratios. (c) CH4 yield comparison between different catalysts in the solar light irradiation for 4 h (“B-MASC” was prepared in the presence of IPA but absence of NH4F). (d) TEM and HRTEM images of Pt/MASC (8:2). (e) Photocatalytic CO2 conversion to CH4 over different samples.
metal-free composite of F-MASC (6:4) with hydrophobic sites exhibits superior CO2 photoreduction for the selective generation of CH4 (2.42 μmol/g vs 0.10 μmol/g in 4 h). To highlight the important role of fluorination in the photocatalytic activity, the activity of B-MASC was prepared by a solvothermal treatment in the presence of IPA but absence of NH4F, as shown in Figure 3c. Not surprisingly, the CH4 yield of F-MASC is obviously higher than that of B-MASC. A little higher activity of B-MASC than MASC implies that maybe some generated CH4 is coming from IPA. Reasonable explanations should be that IPA remains on the surface during the washing process and is hard to be removed completely.25,26 To prove the generated CH4 over F-MASC not only coming from IPA photoreduction but also coming from the CO2 photoreduction, we give the CH4 yield obtained in different atmosphere of CO2 and Ar in Figure S11. A higher yield of both CH4 and CO obtained in CO2 than the yield obtained in Ar indicates the occurrence of CO2 photoreduction on the F-MASC surface. After ruling out the influence of IPA, the real evaluation of CH4 and CO yields over F-MASC is presented in Figure S12. For instance, the real CH4 yield of F-MASC (8:2) of 2.48 μmol/g is obtained by the total yield of 6.70 μmol/g minus the yield of IPA photoreduction of 4.22 μmol/g, which is much higher than the yield of blank MASC (1.21 μmol/g). To emphasize the potential of metal-free hydrophobicity for the replacement of noble metals (Pt) on the CO2 photoreduction, we show the CH4 and CO yield comparison between them in Figure 3d,e and Figure S12. The Pt content of about 0.8 to 0.9 wt % on Pt/MASC has been proved to exhibit a high CH4 yield.6,7 Unsurprisingly, the Pt nanocrystals with the size of 3.0 to 4.0 nm are highly dispersed on the surface of MASC (Figure 3d), and the lattice spacings of 0.220 nm are ascribed to the (111) plane of Pt. Interestingly, the yields of CH4 and CO on F-MASC(8:2) are all higher than that of Pt/MASC (Figure S12). Importantly, even though the irradiation time prolongs to 4 h, F-MASC (8:2) still has a linearity formation rate of CH4 evolution; however, the Pt/MASC displays an obvious decreasing trend for the CH4 generation at the beginning of 2 h of irradiation (Figure 3e). That is because the adsorption of CO or other intermediates on the Pt surface makes the active sites passive.7 In addition to the mesoporous MASC, the
hydrophobic surface could indeed enhance the competitive adsorption of CO2 to H2O on the F-MASC surface. Additionally, compared with the blank MASC (6:4 or 8:2), the increasing rate of CO2 adsorption of F-MASC (6:4) is much higher than that of F-MASC (8:2) (54 vs 18%), owing to the greater number of Si−F bonds on the F-MASC (6:4) surface (Ti−F/Si−F ratios of 1.98 vs 5.25, Figure S7), which further determines that the ratio of Ti−F/Si−F could control the hydrophobic degree of catalyst surface. Although Liu et al.14 have demonstrated that the surface hydrophobicity is indeed beneficial to the adsorption of CO2 on the catalyst surface, there is some research reported that the oxygen-deficient surface also favors the CO2 activation and adsorption.8,24 Hence, the EPR spectra of MASC (8:2) before and after hydrophobic modification are shown in Figure S8. The absence of any peaks ascribed to Ti3+ and oxygen vacancies can eliminate the influence of oxygen deficient surfaces on the adsorption of CO2. Photocatalytic CO2 conversion in the presence of H2O vapor was measured under the irradiation of 300W Xe lamp (with AM 1.5 filter, setup diagram in Scheme S1), and the reaction products were monitored by GC (details in the SI). Figure 3 and Figure S9 present the CH4 and CO formation rates of various samples in 4 h of irradiation. Linearity formation rates on CH4 and CO evolution of each sample can be observed (Figure 3a,b and Figure S9), which indicates the stability of our CO2 photoreduction setup and the photostable hydrophobic modification on the catalysts. For F-MASC (8:2), the CH4 and CO formation rates are about 5.5 and 2.1 times higher than that of MASC (8:2), respectively. Conversely, the lower H2 yield of F-MASC (8:2) than MASC (8:2) further confirms that the hydrophobic surface indeed contributes to the CO2 photoreduction and goes against to the water splitting (Figure S10). On the contrary, compared with MASC (8:2) and F-MASC (8:2), the F-MASC (6:4) with smaller TiO2 amount also displays high yield rates in both CH4 and CO, indicating that the hydrophobic surface modification plays a key role in rational regulation of the adsorption amount of CO2 and H2O molecules on the catalysts surface. Most importantly, considering that the MASC (6:4) can only generate a trace amount of CH4 (0.10 μmol/g), the noble2964
DOI: 10.1021/acs.jpclett.6b01287 J. Phys. Chem. Lett. 2016, 7, 2962−2966
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The Journal of Physical Chemistry Letters NH4F-modified TiO2 (F-P25) particles were also prepared by using the same method with F-MASC. The significant enhancement of CH4 and CO yields of F-P25 compared with the yields of pure P25 further demonstrates the critical role of NH4F induced hydrophobicity in the CO2 photoreduction (Figure S13). All of the above results imply that the metal-free hydrophobic strategy is an ideal strategy to replace the expensive noble metals in the CO2 photoreduction. On the basis of the above-mentioned results, the explanations for the superior efficiency of F-MASC on CO2 photoreduction are illustrated in Scheme 1. After 550 °C
photoreduction for CH4 generation over the noble-metal-free photocatalyst is always considered to be one of the most difficult reactions. The selective generation of CH4 just triggered by a simple hydrophobic modification is reported for the first time, which is very significant for the development of economic CO2 photoreduction technology.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b01287. Detailed experimental procedures; CO2 photoreduction setup diagram; and characterization methods and figures of BET, XRD, FT-IR, XPS, UV-DRS. (PDF)
Scheme 1. Scheme Illustration of the Competitive Adsorption Mechanism of the CO2 and H2O Molecules on the Surface of MASC and F-MASC
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AUTHOR INFORMATION
Corresponding Authors
*M.X.: E-mail:
[email protected]. *J.Z.: E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work has been supported by National Nature Science Foundation of China (21577036, 21377038, 21237003) and State Key Research Development Program of China (2016YFA0204200) and sponsored by “Chenguang Program” supported by Shanghai Education Development Foundation and Shanghai Municipal Education Commission (14CG30, 16JC1401400) and the Fundamental Research Funds for the Central Universities (22A201514021).
calcination, titanium precursor gel transformed into anatase nanoparticles and embedded uniformly in the amorphous silica skeleton; however, owing to abundant hydrophilic hydroxyl groups on the outer surface of the catalysts, H2O molecules could be attached to the surface through many hydrogen bonds. Therefore, less surface active sites will be reserved for the CO2 molecules adsorption and reduction. Compared with the fluorine-containing silylation organic modifier, NH4F is lowcost and has low toxicity and strong photostability. During the solvothermal process, HF will be released and F− could be adsorbed into the mesoporous anatase-silica composites channels. Part of the hydroxyl groups will be exchanged to F− and formed Ti−F and hydrophobic Si−F bonds, which will be exposed as the active sites for the CO2 adsorption and photoreduction. Deservedly, there are still some hydroxyl groups remaining after the solvothermal treatment based on the XPS results, which suggests that the hydrophobic modification is controllable and incomplete. Moderate H2O molecule adsorption is necessary for the CO2 photoreduction, which can be regulated through the surface hydrophobicity adjustment. From this standpoint we can easily control the CO2 and H2O molecules adsorption in a more desirable proportional amount on the catalysts surface by a simple hydrophobic modification. Importantly, this facile strategy can be easily spread to many other kinds of catalysts. In conclusion, a surface partial hydrophobic modified TiO2− SiO2 composite with enhanced CO2 photoreduction efficiency was successfully synthesized via a simple solvothermal process, using NH4F as the hydrophobic modifier. After hydrophobic treatment, part of the hydroxyl groups can be replaced to the hydrophobic Si−F bonds. The enhancement of the CH4 and CO yield can be attributed to the enhancing competitive adsorption of CO2 induced by the hydrophobic modification on MASC composites. The eight-electron-involved CO2
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