Photocatalytic Reduction of Carbon Dioxide to Methane over SiO2

Article pubs.acs.org/JPCC

Photocatalytic Reduction of Carbon Dioxide to Methane over SiO2‑Pillared HNb3O8 Xiukai Li,*,†,‡ Wei Li,†,‡,§ Zongjin Zhuang,†,‡,§ Yushu Zhong,†,‡,§ Qing Li,†,‡ and Liya Wang†,‡ †

China-Australia Joint Research Center for Functional Molecular Materials, Jiangsu University, Zhenjiang 212013, P. R. China Scientific Research Academy, Jiangsu University, Zhenjiang 212013, P. R. China § School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, P. R. China ‡

ABSTRACT: Carbon dioxide (CO2) photoreduction by gaseous water over silica-pillared lamellar niobic acid, viz. HNb3O8, was studied in this work. The physicochemical characteristics of samples were examined by techniques such as XRD, FT-IR, SEM, TEM, and UV−visible diffuse reflectance spectroscopy. Aspects that influence CO2 photoreduction, such as the layered structure, the protonic acidity, silica pillaring, and cocatalyst loading, were investigated in detail. Pt loading obvious promoted the activity for CO2 photoreduction to methane. The loading of Pt also promoted the formation of methane from catalyst associated carbon residues, although this contributes insignificantly to the overall amount of methane produced. The layered structure and the protonic acidity of the lamellar niobic acid have significant influences on CO2 photoreduction by water in gas phase. With layered structure, expanded interlayer distance, and stronger intercalation ability to water molecules, the silica pillared niobic acid showed much higher activity than the nonpillared niobic acid, Nb2O5, and TiO2. Because of the unique adsorption ability to water molecules through hydrogen bonding, the activity of silica pillared HNb3O8 increased more remarkably with elevated water content than the mostly investigated TiO2 photocatalyst. thin slices built up from metal−oxygen polyhedron units.19−25 Such layered construction is favorable for the transportation and separation of the photogenerated charge carriers (i.e., electrons and holes). In addition, lamellar materials have mesoporous character and thus could provide more reaction active sites at the interlayer space. Actually, some lamellar titanates and niobates are more efficient photocatalysts than common metal oxides (such as TiO2 and Nb2O5) for water splitting or for organic compounds degradation.22,23,26−28 Lamellar materials are also potential catalysts for CO2 photoreduction to hydrocarbons. In our previous work, we found that a lamellar niobic acid (viz. HNb3O8) showed good activity for CO2 photoreduction to methane in gas phase.29 It is very important to fabricate highly efficient photocatalysts based on lamellar materials for CO2 photoreduction, and it is very intriguing to know more about the chemistry and mechanism involved in the photocatalytic reactions over such materials. Intercalation of guest components into the interlayer space could expand the interlayer distance of lamellar materials and hence improve the catalytic activity. In the present study, lamellar HNb3O8 was purposely pillared with silica for CO2 photoreduction. The physicochemical properties of samples were characterized by various techniques such as XRD, FT-IR,

1. INTRODUCTION Green house effect caused by a large-scale emission of carbon dioxide (CO2) into the atmosphere has gained increasing concern. It is highly desired to reduce the emission level of carbon dioxide, and to convert carbon dioxide into useful substances such as alternative fuels and raw materials for chemical industry.1−3 The photocatalytic reduction of CO2 over light excited semiconductor provides a potential option for the conversion of CO2 to hydrocarbons,4−6 and this approach has drawn extensive research interest in the recent years. To date, most of the research on CO2 photoreduction has been related to TiO2.7−11 Besides TiO2 powders and films, some Ticontaining zeolites showed photocatalytic activity for CO2 photoreduction under UV light.12,13 There were also several reports on CO2 photoreduction over multiple-metal oxides, such as ZnGa2O4,3 BiVO4,4 NiO/InTaO4,5 and LiTaO3.14 The loading of noble metal (e.g., Pt, Ag) or transition metal (e.g., Fe, Ni, and Cu) as cocatalyst generally improved the activity for CO2 photoreduction.6,10−12 In contrast, photocatalysts without any cocatalyst loading were less active or were almost inactive for CO2 photoreduction.15−18 Despite great effort devoted to the work of CO2 photoreduction, there were still limited options of photocatalysts. Moreover, some issues such as the active sites and the reaction mechanism are still subjects under debate. Ti- and Nb-based lamellar materials (e.g., K2Ti4O9, HNb3O8) are one unique type of photocatalysts constructed by stacked © 2012 American Chemical Society

Received: April 9, 2012 Revised: June 19, 2012 Published: July 8, 2012 16047

dx.doi.org/10.1021/jp303365z | J. Phys. Chem. C 2012, 116, 16047−16053

The Journal of Physical Chemistry C

Article

equipped with a Porapaq Q or a 5A molecular sieve column (3 m × 3 mm) for inorganic gas determination.

SEM, TEM, and UV−visible diffuse reflectance spectroscopy. The effects of silica pillaring and cocatalyst loading on the photocatalytic activity were investigated in detail. Special attentions were paid to the influences of the protonic acidity and the layered structure on water adsorption and CO2 photoreduction. It is envisaged that the present study might provide a feasible method to fabricate materials with high activity for CO2 photoreduction and enable a more in depth understanding about the related photocatalytic reaction.

3. RESULTS AND DISCUSSION 3.1. Characterization. Figure 1 presents the XRD patterns of SiO2-pillared and nonpillared HNb3O8 samples. The pattern

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The HNb3O8 solid acid was prepared by the proton exchange reaction with KNb3O8 as a precursor.22,23 KNb3O8 was synthesized by heating stoichiometric amounts of K2CO3 and Nb2O5 at 900 °C for 10 h. SiO2 pillared HNb3O8 (SiO2−HNb3O8) was prepared by the twostep ion-exchange method as described elsewhere;24,25 ndodecylamine and tetraethyl orthosilicate (TEOS) were used as the pre-expanding reagent and silicon source, respectively. The TEOS intercalated HNb3O8 sample was calcined at 500 °C for 8 h in air for the formation of SiO2 pillared HNb3O8. The SiO2−HNb3O8 sample was stirred in an aqueous K2CO3 solution for 8 h to prepare the potassium modified sample (SiO2−KNb3O8). The molar ration of SiO2−HNb3O8 to K2CO3 was 2:1. The anatase-phased TiO2 sample was synthesized by the hydrolysis of tetrabutyl titanate in an aqueous CTAB solution. The obtained mixture was thermally treated at 100 °C for 48 h in a Teflon-lined autoclave. The white precipitate was recovered, washed thoroughly with ethanol and distilled water, dried at 80 °C for 12 h, and finally calcined at 500 °C for 8 h in air. The loading of Pt was done by photodeposition using H2PtCl6·6H2O as Pt precursor. 2.2. Sample Characterization. The phase compositions of samples were identified by X-ray Powder Diffraction (Cu Kα radiation, Bruker AXS-D8) in the 2θ range of 2−80°. The UV− visible diffuse reflectance spectra were recorded at room temperature on a Shimadzu UV-2450 UV−vis spectrometer with barium sulfate as the reference sample. Specific surface areas of samples were deduced by the BET method (N2 adsorption) with a NOVA-2000E instrument. FT-IR spectra of the samples were collected on a Nicolet Nexus 470 FT-IR spectrophotometer at room temperature by KBr method. Morphologies of samples were characterized using a scanning electron microscope (JSM-7001F, JEOL) and a high resolution transmission electron microscope (HR JEM-2100, JEOL). 2.3. Activity Evaluation. The photocatalytic reduction of CO2 by gaseous H2O was carried out in a quartz tubular reactor (length, 28.0 cm; o.d., 3.0 cm; volume, 159 mL) described previously.29 A flat quartz plate was used to hold the catalyst (typically 0.1 g). A 350 W Xe lamp (Nanshen Co., Shanghai) was used as the light source. The average irradiation intensity at the reaction point inside the reactor was 34.8 mW cm−2. Before the photocatalytic reaction, high purity CO2 (99.995%) was bubbled through deionized water and then flowed through the reactor for more than 1 h to ensure that air was eliminated. The reactor was then sealed and the light was turned on to start the reaction. The internal temperature of the reactor was controlled at 60 ± 2 °C. The products in the gas phase were analyzed with a gas chromatograph system (GC-9790A), using a flame ionization detector (FID) equipped with a FFAP capillary column (30 m × 0.32 mm × 0.5 μm) for organic compounds determination, and a thermal conductor detector (TCD)

Figure 1. XRD patterns of samples: (a) SiO2−HNb3O8; (b) HNb3O8; (c) Nb2O5. The inset shows the layered crystal structure of HNb3O8.

of Nb2O5 is also shown for comparison. As illustrated by the inset of Figure 1, HNb3O8 is a typical lamellar solid acid constructed of 2D Nb3O8− anion slices built by corner- and edge-sharing NbO6 octahedra, the H+ cations are located between the slices.30,31 The 020 diffraction peak observed at 2θ = 7.8° for HNb3O8 is characteristic of the layered structure, and the d value (ca. 1.13 nm) corresponds to the interlayer distance. The diffraction peak at 2θ = 3.1° was clearly observed for SiO2−HNb3O8, suggesting that SiO2 was successfully intercalated into HNb3O8 and that the layered configuration was well reserved. After the sample was pillared by SiO2, the interlayer distance of HNb3O8 had been remarkably expanded to 2.85 nm. Taking into account the thickness of the Nb3O8− anion slice is 0.75 nm,32 the interlayer spacing of SiO2−HNb3O8 was calculated to be 2.10 nm (2.85 nm subtract 0.75 nm), in contrast to only 0.37 nm (1.12 nm subtract 0.75 nm) of HNb3O8. Owing to the notably expanded interlayer distance, the surface area value of SiO2−HNb3O8 was as large as 197.3 m2 g−1, in contrast to 6.5 m2 g−1 of the nonpillared HNb3O8 and 6.2 m2 g−1 of the original material Nb2O5 (Table 1). Figure 2 shows the FT-IR spectra of Nb2O5 and silicapillared and nonpillared HNb3O8 samples. The IR absorption in the range of 400−1000 cm−1 for the HNb3O8 samples is assigned to the vibration of the Nb3O8− host slice.25,29,32 An additional absorption at 1077 cm−1 attributable to the vibration Table 1. Physical Characteristics and Activities of Samples sample Nb2O5 HNb3O8 SiO2− HNb3O8

d020 (nm)

interlayer spacing (nm)

SBET (m2 g−1)

band gap (eV)

rate of CH4 formationa (μmol gcat. −1 h−1)

1.12 2.85

0.37 2.10

6.3 6.5 197.3

3.1 3.5 3.3

0.15 0.47 2.90

a

0.4 wt % Pt was loaded as cocatalyst for all the samples. Reaction conditions: catalyst, 0.1 g; H2O/CO2, 0.14; temperature, 60 ± 2 °C; full Xe-arc irradiation; light intensity, 34.8 mW cm−2.

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The morphology of SiO2−HNb3O8 was investigated by the scanning electron microscopy (SEM) and transmission electron microscopy (TEM); the images are shown in Figure 4. The layered texture can be clearly observed from Figure 4A, and the interlayer spacing was approximately 2.1 nm, which is consistent with the value estimated from XRD. Figure 4B shows the SEM image of SiO2−HNb3O8, a bunch of sheetlike particles can be clearly seen from the picture. 3.2. Photocatalytic Activity. 3.2.1. Effect of SiO 2 Pillaring. The photocatalytic activities of samples for CO2 photoreduction by H2O were evaluated under UV light irradiation. All of the samples were loaded with 0.4 wt % Pt by photodeposition. It can be seen from Figure 5 that methane (CH4) evolved almost linearly with the irradiation time over the three samples. The yield of CH4 over the HNb3O8 sample is almost 3 times that achieved over Nb2O5 after 120 min of irradiation. As the two samples have similar surface area values, the improved activity for the solid acid material might be ascribed to its layered structure and the stronger adsorption ability to H2O through hydrogen bonding.29 The rate of CH4 formation was further enhanced after the HNb3O8 sample was pillared with SiO2. The yield of methane reached 2.9 μmol gcat.−1 h−1 over SiO2−HNb3O8, the data are six times that (0.47 μmol gcat.−1 h−1) over nonpillared HNb3O8 (Table 1). Depending on the reaction conditions such as Pt loading, higher water content, and higher reaction temperature, the activity of the current SiO2−HNb3O8 photocatalyst is higher than that of the previously reported HNb3O8 nanobelts.29 It has been revealed that the reaction active sites of lamellar solid acids lie at the interlayer surface of the materials.26,28 Compared to nonpillared HNb3O8, the SiO2 pillared HNb3O8 sample has notably expanded interlayer distance and much greater surface area value. Thus the reaction active sites at the interlayer space of SiO2−HNb3O8 should be more accessible to the substrates. Because the interlayer spacing of SiO2−HNb3O8 is only 2.1 nm and therefore the limit of mass transfer, the photocatalytic activity does not increase as significantly as the surface area value. The inset of Figure 5 shows the conversion of CO2 and H2O over the best performed SiO2−HNb3O8 photocatalyst. One can see that the conversion of CO2 was very low over SiO2−HNb3O8, and the data were 3.3% after 120 min of irradiation. On the contrary, about 80% of H2O was consumed in the initial 60 min, and this might account for the decreased rate of methane formation after 60 min of irradiation. As water serves as the reductant, higher water content should be favorable for a higher photocatalytic activity. However, the partial pressure of water vapor is limited by the temperature inside the reactor. 3.2.2. Origin of Methane. It is very important to confirm the photocatalytic activity of CO2 photoreduction. Controlled experiments show that in the presence of CO2 and water vapor there was trace amount of product detected under the dark condition or under UV irradiation without any photocatalyst (Figure 6a,c), indicating that the present reaction of CO2 reduction by water proceeded photocatalytically. It has been reported that the carbon residues on the sample surface could be involved in the formation of hydrocarbon products, thus the activity test in the presence of water vapor but in the absence of CO2 is needed to exclude the contribution of catalyst associated carbon residues.7 From Figure 6d one can see that methane was formed over 0.4 wt % Pt/SiO2−HNb3O8 under UV irradiation in the absence of carbon dioxide but in the presence of water vapor, nitrogen, and photocatalyst,

Figure 2. FT-IR spectra of (a) Nb2O5, (b) HNb3O8, and (c) SiO2− HNb3O8.

of the Si−O−Si linkages32,33 was also observed for SiO2− HNb3O8, indicating that SiO2 species have successfully intercalated into the host material. The absorptions of C−H stretching and bending models were not observed for SiO2− HNb3O8, signifying that the intercalated organic species were completed decomposed after the sample was calcined at 500 °C for 8 h in air. HNb3O8 has hygroscopic character and could adsorb water molecules at room temperature,30,34,35 which is evidenced by the FT-IR signal (1630 cm−1) assignable to the hydroxyl group. The optical properties of samples were investigated by UV− visible diffuse reflectance spectroscopy (UVDRS) and the results are shown in Figure 3. The absorption threshold of

Figure 3. UV−visible absorption spectra of samples: (a) SiO2− HNb3O8; (b) HNb3O8; (c) Nb2O5.

Nb2O5 was at approximately 380 nm, whereas the absorption edges of HNb3O8 and SiO2−HNb3O8 were blue-shifted by 40 and 20 nm, respectively. The band gap (BG) values were estimated according to the following equation: BG = hC/λos = 1240 eV/λos (h = Plank constant; C = the speed of light in vacuum), where λos means the onset absorption. The data are 3.1, 3.3, and 3.5 eV for Nb2O5, SiO2−HNb3O8, and HNb3O8, respectively (Table 1). HNb3O8 is thermally unstable and decomposes to Nb2O5 at temperatures above 226 °C.22,23 Due to the partial dehydration of the host part (HNb3O8) in the preparation of SiO2-pillared HNb3O8, the band gap value of SiO2−HNb3O8 is smaller than that of original HNb3O8. 16049



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Figure 4. TEM and SEM images of SiO2-pillared HNb3O8.

Figure 6. Photocatalytic reduction of CO2 over 0.4 wt % Pt loaded SiO2−HNb3O8 under different conditions: (a) 0.4 wt % Pt/SiO2− HNb3O8, CO2 + water vapor, dark condition; (b) SiO2−HNb3O8, N2 + water vapor, UV iradiation; (c) no catalyst, CO2 + water vapor, UV iradiation; (d) 0.4 wt % Pt/SiO2−HNb3O8, N2 + water vapor, UV iradiation; (e) 0.4 wt % Pt/SiO2−HNb3O8, CO2 + water vapor, UV iradiation. Overall conditions: catalyst, 0.1 g; H2O/CO2, 0.14; temperature, 60 ± 2 °C; full Xe-arc irradiation; light intensity, 34.8 mW cm−2.

Figure 5. Photocatalytic reduction of CO2 over (a) SiO2−HNb3O8, (b) HNb3O8, and (c) Nb2O5. Pt (0.4 wt %) was loaded as cocatalyst for all the samples. Reaction conditions: catalyst, 0.1 g; H2O/CO2, 0.14; temperature, 60 ± 2 °C; full Xe-arc irradiation; light intensity, 34.8 mW cm−2. The inset shows the conversion of CO2 and H2O over SiO2−HNb3O8.

indicating that methane could be produced from the catalyst associated carbon residues. However, the amount of methane produced from carbon residues (0.35 μmol gcat.−1 h−1) is much smaller compared with that (2.9 μmol gcat.−1 h−1) produced from CO2 photoreduction, suggesting that in the present study methane was formed mainly from CO2 photoreduction. It is also noted that over unloaded SiO2−HNb3O8, there was no methane detected under UV irradiation in the nitrogen atmosphere (Figure 6b). It is evident that the loading of Pt promoted the formation of methane from catalyst associated carbon residues. Thus it is highly recommended to conduct such blank reactions to confirm the origin of products in CO2 photoreduction, especially for those noble metal-loaded photocatalysts. 3.2.3. Effect of Pt Loading. Platinum is usually adopted as a cocatalyst to improve the photocatalytic activities of semiconductors. Figure 7 shows the results of CO2 photoreduction over SiO2−HNb3O8 samples loaded with various amount of Pt. The rate of CH4 evolution increased with Pt loading up to 0.4 wt % but then decreased when the loading amount exceeded 0.4 wt %. It is clear that there exists an optimum loading amount for Pt. The appropriate amount of Pt particles loaded on sample surface can trap the larger number of photoexcited

Figure 7. CO2 photoreduction over various amount of Pt loaded SiO2−HNb3O8 under UV light irradiation: (a) 0.4 wt % Pt; (b) 0.2 wt % Pt; (c) 0.6 wt % Pt; (d) non-Pt loading. Reaction conditions: catalyst, 0.1 g; H2O/CO2, 0.06; temperature, 60 ± 2 °C; full Xe-arc irradiation; light intensity, 34.8 mW cm−2.

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electrons, resulting in the enhanced photocatalytic activity. However, excess Pt particles can mask the sample surface and reduce the light absorption capability of the catalyst. The optimum loading amount of Pt for SiO2−HNb3O8 was 0.4 wt % in the present study. It should be noted that the optimum loading amount of cocatalyst usually varies with different materials and different reactions. 3.2.4. Effect of H2O Content. Figure 8 presents the relationship between the yield of CH4 and the molar ratio of

Figure 9. Photocatalytic reduction of CO2 over anatase, SiO2− KNb3O8, and SiO2−HNb3O8 under the conditions with different H2O contents. Pt (0.4 wt %) was loaded as cocatalyst for all the samples. Reaction conditions: catalyst, 0.1 g; temperature, 60 ± 2 °C; full Xearc irradiation; light intensity, 34.8 mW cm−2.

through hydrogen bonding,30,34,35 a higher water pressure should be more favorable for the adsorption of water on the surface of SiO2−HNb3O8 than on the surface of SiO2−KNb3O8 and anatase. As a result of better adsorption to the reductant molecules, the SiO2−HNb3O8 sample showed better activity than SiO2−KNb3O8 and anatase. 3.2.6. Reaction Mechanism. The separation and transportation of photogenerated carriers (i.e., electrons and holes) is crucial for the photocatalytic activity. As the HNb3O8 sample is constructed by thin Nb3O8− anion slices built by NbO6 octahedra, the photogenerated carriers in HNb3O8 could migrate to its surface/interlayer surface more quickly than in the Nb2O5 case. As a consequence, the HNb3O8 sample exhibited 3 times higher activity than Nb2O5 (Figure 5). Catalysts with larger surface area values usually could provide more reaction active sites. In the present study, the silica pillared sample possesses notably expanded interlayer distance and much greater surface area value than the nonpillared sample, enabling the active sites at the interlayer surface of the former sample more accessible to the reactants. As a consequence, expanding the interlayer distance of HNb3O8 by means of silica pillaring efficiently improved the activity for CO2 photoreduction. The current SiO2−HNb3O8 sample showed higher activity than the referenced anatase sample despite the fact that the two samples have similar surface area values (Table 1); this result further supports the deduction that the layered structure of SiO2−HNb3O8 is favorable for CO2 photoreduction. The unique adsorption behavior to water molecules should also have significant contribution to the activities of the silicapillared and nonpillared HNb3O8 samples. In CO2 photoreduction by gaseous water, the adsorbed H2O molecules reacted with photogenerated holes (h+) at the valence band to form H+, H+ then reacted with photogenerated electrons (e−) at the conduction band to form H• radicals. Subsequently, CO2 was reduced by H• to hydrocarbons.38−40 As illustrated in Figure 10, water molecules could be easily intercalated into the interlayer space of HNb3O8 samples through hydrogen bonding;30,34,35 this facilitates the trapping of photogenerated holes and the formation of H+ species and consequently contributes to the higher activity for CO2 photoreduction. With expanded interlayer spacing, the silica-pillared HNb3O8 should

Figure 8. Photocatalytic reduction of CO2 over 0.4 wt % Pt loaded SiO2−HNb3O8: (a) H2O/CO2 = 0.25; (b) H2O/CO2 = 0.14; (c) H2O/CO2 = 0.06. Reaction conditions: catalyst, 0.1 g; temperature, 60 ± 2 °C; full Xe-arc irradiation; light intensity, 34.8 mW cm−2.

H2O/CO2 in the initial feed stream. The yield of CH4 over SiO2−HNb3O8 increased appreciably with increasing concentration of H2O molecules. Such results should be ascribed to the unique protonic acidity and the intercalation property of lamellar solid acids. As aforementioned, because of the small H2O/CO2 ratio, H2O was consumed more quickly than CO2 and this resulted in the decreased rate of methane formation. It is apparent that higher water content is favorable for the photocatalytic reduction of CO2 over the present SiO2− HNb3O8 sample. However, the H2O/CO2 ratio is limited by the temperature inside the reactor. 3.2.5. Effect of the Protonic Acidity. The surface acidity and alkalinity of photocatalyst play important roles in the photocatalytic activity.36,37 It is very intriguing to know more about the protonic acidity of SiO 2 −HNb 3 O 8 on CO 2 photoreduction. For a comparison study, SiO2−HNb3O8, SiO2−KNb3O8, and an anatase sample (surface area value: 151.8 m2 g−1) were evaluated under identical reaction conditions. The H2O/CO2 molar ratio was set at 0.06 or 0.25. From Figure 9 one can see that under the same reaction conditions the SiO2−HNb3O8 sample showed higher activity than SiO2−KNb3O8 and anatase. The enhanced activity was observed for each sample when the H2O/CO2 molar ratio increased from 0.06 to 0.25. It is noteworthy that the activity of the SiO2−HNb3O8 sample enhanced more significantly than the other two samples at elevated water content. The activity improved 105.1% for SiO2−HNb3O8, and the data were 64.6% for SiO2−KNb3O8 and 12.4% for anatase, respectively. Under the condition H2O/CO2 = 0.25, the yield of methane over SiO2−HNb3O8 was almost double that achieved over SiO2− KNb3O8 and anatase. It is apparent that the SiO2−HNb3O8 sample is more sensitive to water vapor content in CO2 photoreduction. As water molecules could react easily with the protons at the interlayer space of lamellar solid acids 16051



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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 511 88797815. Fax: +86 511 88797815. E-mail: li. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (no. 21003064), the Research Foundation of Jiangsu University (No. 09JDG042), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.

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Figure 10. Schematic drawing of CO2 photoreduction by water over Pt loaded SiO2−HNb3O8 catalyst.

REFERENCES

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have better intercalation character to water molecules. When the protonic acidity at the interlayer space of SiO2−HNb3O8 was blocked by potassium, the activity should decrease, as has been revealed in Figure 8. With unique adsorption ability to water molecules, the activity of silica pillared HNb3O8 increased more remarkably with elevated water content in the feed stream than the potassium modified sample and the mostly investigated anatase photocatalyst. There was no molecular oxygen and hydrogen detected in the current study. There have been reports that the produced O2 can be physisorbed and chemisorbed on the surface of photocatalysts.41,42 H2 is an efficient reductant for CO2 photoreduction as well as H· radicals,43 the photogenerated molecular hydrogen (if there is any) might be quickly consumed by CO2 in the photocatalytic reaction and thus was hardly detected.

4. CONCLUSION In the present study, CO2 photoreduction by gaseous water over silica-pillared lamellar niobic acid, viz. HNb3O8, was investigated in detail. Because of the unique layered structure and the protonic acidity, HNb3O8 showed three times higher activity for CO2 photoreduction to CH4 than common Nb2O5. Compared to nonpillared HNb3O8, the SiO2-pillared sample has notably expanded interlayer distance, much greater surface area value, and 6 times higher photocatalytic activity as a consequence. Pt loading obvious promoted the activity for CO2 photoreduction, and the optimum loading amount of Pt for SiO2−HNb3O8 was 0.4 wt %. Controlled experiments showed that methane was produced mainly from CO2 photoreduction, and that the contribution from catalyst associated carbon residues is insignificant. The SiO2−HNb3O8 sample has unique intercalation behavior to water molecules through hydrogen bonding, and this contributed significantly to the activity of the sample. When the protonic acidity was blocked by potassium, the activity decreased. Higher water content is favorable for the photocatalytic reduction of CO2 over the present SiO2− HNb3O8 sample. Because of the unique adsorption ability to water molecules, the activity of silica-pillared HNb 3 O 8 increased more remarkably with elevated water content than the mostly investigated TiO2 photocatalyst. In summary, the unique layered crystal structure and the intercalation behavior to water molecules contribute to the overall higher photocatalytic activity of SiO2−HNb3O8. 16052



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Photocatalytic Reduction of Carbon Dioxide to Methane over SiO2

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