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Article Cite This: ACS Omega 2017, 2, 8187-8197

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Sodium Cation Substitution in Sr2KTa5O15 toward Enhancement of Photocatalytic Conversion of CO2 Using H2O as an Electron Donor Zeai Huang,† Sumika Yoshizawa,† Kentaro Teramura,*,†,‡ Hiroyuki Asakura,†,‡ Saburo Hosokawa,†,‡ and Tsunehiro Tanaka*,†,‡ †

Department of Molecular Engineering, Graduate School of Engineering and ‡Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, Kyoto 615-8510, Japan S Supporting Information *

ABSTRACT: The K and Sr cations (K+ and Sr2+) in a Sr2KTa5O15 photocatalyst were found to be easily substituted by Na cations (Na+) to form SrxKyNazTa5O15 by a facile onepot flux method using a mixture of potassium chloride (KCl) and sodium chloride (NaCl). SrxKyNazTa5O15 fabricated using a mixture of KCl and NaCl with a Ag cocatalyst showed enhanced photocatalytic activity without apparent change in selectivity toward CO for the photocatalytic conversion of CO2 using H2O. The present study demonstrates that the flux treatment significantly affected the phase, morphology, band gap, and surface Sr composition of the catalyst owing to the substitution of K+ and Sr2+ for Na+. The stability and durability of the catalyst were also enhanced as compared to those of the photocatalyst fabricated using only KCl flux due to more stable Ag on the surface of SrxKyNazTa5O15.

1. INTRODUCTION The amount of CO2 produced from the oxidation of carbon compounds has increased dramatically in recent decades owing to the heavy use of fossil fuels to produce energy. The large amount of CO2 emitted into the atmosphere from fuel combustion is considered one of the main reasons for global warming.1,2 Therefore, reducing the concentration of CO2 in the atmosphere is important. Considering CO2 as a potential source for carbon feedstocks, fuels, and chemicals, many researchers have explored the conversion of CO2 into other feedstocks and hydrocarbon fuels, such as carbon monoxide (CO), formic acid (HCOOH), formaldehyde (HCHO), methanol (CH3OH), methane (CH4), ethylene (CH2CH2), and ethanol (CH3CH2OH).3−5 However, substantial energy input is necessary to convert CO2 into other carbon sources due to its thermodynamic stability under natural conditions. In this context, converting CO2 into fuels and chemicals using solar light as the energy source directly is one of the most promising ways to achieve high-density storage of solar energy.6,7 Photocatalytic conversion of CO2 into other carbon sources by heterogeneous catalysts using H2O as an electron donor is believed to be one of the best methods to mitigate CO2 emissions.8 Ensuring the conversion of CO2 into carbon sources as the main products is important because water splitting to form H2 and O2 is thermodynamically more favorable than CO2 conversion when using H2O as a reductant over heterogeneous photocatalysts. However, only a few oxides, including BaLa 4Ti4 O15 ,9 ZnGa2O4 /Ga2 O3,10 ZnGa 2 O4,11 La2Ti2O7,12 CaTiO3,13 ZnTa2O6,14 and SrNb2O6,15,16 have been clearly demonstrated as suitable photocatalysts for the © 2017 American Chemical Society

stoichiometric production of CO, H2, and O2 from the conversion of CO2 using H2O as an electron donor. Very recently, tetragonal tungsten bronze (TTB) materials, such as Sr2KTa5O15,17 KCaSrTa5O15,18,19 and K2RETa5O15 (RE = rare-earth element)20 have been reported to show good photocatalytic activity and high selectivity for CO evolution in the conversion of CO2 using H2O as an electron donor. Figure 1 shows the typical TTB structure along the [001] direction. The TTB structure is a derivative of the classical perovskite

Figure 1. Schematic illustration of the tetragonal tungsten bronze crystal structure along the [001] direction. Received: September 4, 2017 Accepted: October 17, 2017 Published: November 20, 2017 8187

DOI: 10.1021/acsomega.7b01305 ACS Omega 2017, 2, 8187−8197

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treatment revealed that a large amount of Sr species was present in the solution. However, this was not the case for (KCl)100(NaCl)0 (Table S1), confirming that Sr species are not completely consumed in the mixture flux during the treatment process. We calculated the chemical formulae for the as-fabricated catalysts (Table 1). It is clearly shown that substitution of K+ and Sr2+ for Na+ occurs to form SrxKyNazTa5O15. Many researchers have reported that larger cations prefer to occupy A1 sites and smaller cations sit in A2 sites because A1 tunnels are larger than A2 tunnels.25−27 Using the extensively investigated TTB niobates as examples, in Ba2NaNb5O15, 85% Ba and 15% K cations occupy the A1 sites, and 30% Ba and 70% K cations occupy the A2 sites. However, in the case of Ba2NaNb5O15, only Ba cations occupy the A1 sites and all the Na cations occupy the A2 sites.28 The ionic radius of Na+ (1.39 Å, coordination number (CN) 12) is smaller than that of K+ (1.64 Å, CN 12) and Sr2+ (1.44 Å, CN 12).27 Moreover, Na+ can only occupy the A2 sites.22 Therefore, it is reasonable that K+ and Sr2+ are substituted for Na+ during the flux treatment. Figure 2A shows the X-ray diffraction (XRD) patterns of the photocatalysts fabricated using different kinds of flux. All of the peaks assigned to Sr2KTa5O15 are observed after using only KCl as the flux material (Figure 2A(a)). Interestingly, all of the main peaks show the same shifts toward higher angles with an increasing NaCl ratio in the flux mixture (Figure 2A(b)−(e)). Figure 2C clearly shows that both the a and c axes decrease linearly with an increasing molar ratio of NaCl in the flux mixture, indicating that NaCl flux affects the phase structure of the TTB material. This phenomenon is due to the substitution of K+ and Sr2+ for the smaller Na+ in the TTB structure, which was verified by the ICP analysis (Table 1). The ionic radius of Na+ (1.39 Å, CN 12) is smaller than that of Sr2+ (1.44 Å, CN 12) and much smaller than that of K+ (1.64 Å, CN 12); thus, the substitution leads to structural distortion of the TTB structure. It is noted that the intensity of the main peaks at 2θ ranging from 25 to 31.5° decreases drastically when larger ratios ((KCl)75(NaCl)25 and (KCl)0(NaCl)100) of NaCl are used in the flux (Figure 2A). Specifically, the intensities of the peaks attributed to crystal planes (hk0) parallel to the [001] direction decrease by more than half,29,30 indicating inhibited growth in the [001] direction, such as that for the (320), (400), (410), and (630) planes. However, changing the molar ratio of NaCl in the flux has no obvious influence on the crystallinities and Brunauer−Emmett−Teller (BET) specific surface areas of the catalysts (Figure 2D). The crystallite diameter determined from the full width at half-maximum of the (320) plane using the Scherrer equation is around 40 nm, and the BET specific surface area is around 6 m2 g−1 for all of the catalysts. The results of XRD pattern analysis suggest that the molar ratio of NaCl in the flux mixture has a significant effect on the TTB structure, especially the growth directions of SrxKyNazTa5O15. The effect of flux treatment on the morphology of the asfabricated catalysts was investigated using scanning electron microscopy (SEM) (Figure 3). The catalyst fabricated by KCl flux shows only rodlike structures with diameters of 100−300 nm and lengths of 0.5−2 μm (Figure 3a). Two kinds of morphologies were found in the catalysts fabricated with different molar ratios of NaCl, i.e., nanorods and nanoparticles (Figure 3b−f). The nanorods show diameters and lengths similar to those of the catalyst fabricated using only KCl. The nanoparticles have sizes less than 1 μm. The high-resolution SEM image of (KCl)55(NaCl)45 shows that some of the

structure and has three different types of tunnels, i.e., pentagonal (A1), square (A2), and triangular tunnels. Unlike the perovskite structure, which only has one type of square tunnel, this unique property of the TTB structure facilitates a wide variety of cation substitutions owing to the presence of several interstices.21 Alkali metal, alkaline-earth metal, and rareearth metal cations can easily occupy the 15 coordination sites of pentagonal A1 tunnels and the 12 coordination sites of square A2 tunnels.22 These cations have been reported to have a strong effect on CO2 adsorption and are believed to play important roles during the photocatalytic conversion of CO2.23 Therefore, TTB-structured materials show excellent potential for the photocatalytic conversion of CO2. In this work, we found that the K+ and Sr2+ in a Sr2KTa5O15 photocatalyst could be easily substituted by Na+ using a mixture of NaCl and KCl as flux reactants. The effect of the substitution on the phase, morphology, band gap, surface composition, and photocatalytic activity for CO2 conversion were studied and discussed.

2. RESULTS AND DISCUSSION Table 1 shows the inductively coupled plasma (ICP) analysis of all the catalysts fabricated using different kinds of flux at 1173 K Table 1. Chemical Formulae of the Fabricated Samples As Derived by ICP Analysis element analysis sample

K/Ta

Na/Ta

Sr/Ta

calculated chemical formula

(KCl)100(NaCl)0 (KCl)75(NaCl)25 (KCl)55(NaCl)45 (KCl)25(NaCl)75 (KCl)0(NaCl)100

0.19 0.100 0.072 0.034 0.007

0 0.27 0.30 0.39 0.43

0.40 0.33 0.32 0.32 0.31

Sr2KTa5O15 Sr1.6K0.5Na1.3Ta5O15 Sr1.6K0.37Na1.43Ta5O15 Sr1.6K0.17Na1.63Ta5O15 Sr1.5Na2Ta5O15

for 3 h. The K/Ta ratio of the products decreases gradually with increasing molar ratio of NaCl in the flux, and almost all the K element disappears when only NaCl is used as flux. Conversely, the Na/Ta ratio increases with increasing ratio of NaCl in the flux. Interestingly, the ratio of Sr/Ta decreases by ∼25% when the mixed flux and NaCl flux are used, indicating that the Sr content in the prepared catalysts decreases. Note that the amount of SrCO3 precursor was the same for all fabrication processes, suggesting that Sr species are changed to dissolvable species and then dissolve into the solution during the washing process. We propose reaction processes for SrCO3, K2CO3, and Ta2O5 during the flux treatment as follows SrCO3 → SrO + CO2

(1)

K 2CO3 → K 2O + CO2

(2)

SrO + 2NaCl → SrCl 2 + Na 2O

(3)

4xSrO + 2y K 2O + 2(5 − 2x − y)Na 2O + 10Ta 2O5 → 4SrxK yNa5 − 2x − yTa5O15

(4)

At the beginning of calcination, SrCO3 and K2CO3 are decomposed to their oxides (eqs 1 and 2);24 then, some of the SrO reacts with NaCl to form Na2O and SrCl2 (eq 3), which then reacted further with other oxides to form the product SrxKyNa5−2x−yTa5O15 (eq 4). ICP analysis of the solution used to wash (KCl)55(NaCl)45 catalyst after the flux 8188


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Figure 2. (A) XRD patterns of samples calcined at 1173 K for 3 h with flux reagent ratios of (KCl)100(NaCl)0 (a), (KCl)75(NaCl)25 (b), (KCl)55(NaCl)45 (c), (KCl)25(NaCl)75 (d), and (KCl)0(NaCl)100 (e); (B) enlarged XRD patterns of the related samples at 2θ from 34 to 35.5°; (C) lattice parameters of the samples; (D) crystallite diameter of the (321) facet and BET surface areas of samples prepared with different flux reagent ratios.

nanoparticles have many characteristic steps and are surrounded by the nanorods (Figure 3d). Similar phenomena have been observed for Sr-doped NaTaO3 owing to its Sr-rich surface.31 The Sr-rich surface was believed to have advantages for inhibiting the recombination of photogenerated electrons and holes. The aspect ratio (length/diameter) of the nanorods in the catalysts decrease with an increasing molar ratio of NaCl in the flux treatment (Figure 3b−f). Because these nanorods grow along the [001] direction, the decrease in the aspect ratio of the nanorods leads to a decrease in the peak intensities of the crystal planes (hk0) parallel to the [001] direction. This result is consistent with the results of XRD analysis (Figure 2A). Conversely, the number of nanorods decreases and the number

of nanoparticles increases upon increasing the ratio of NaCl in the flux treatment (Figure 3b−f). The dominant morphology changes from nanorods to nanoparticles when using only NaCl in the flux treatment (Figure 3a,e). These results can be easily understood: the substitution of cations causes distortion of the TTB structure; thus, the change in the morphology of these catalysts is due to the restricted growth of these specific planes. The UV−vis diffuse reflectance (DR) spectra of the catalysts are shown in Figure 4A. The absorption edges of the asfabricated catalysts first decrease to shorter wavelengths and then increase slightly with an increasing molar ratio of NaCl in the flux treatment, indicating an expansion and subsequent contraction of the band gaps. The band gaps of (KCl) 100 (NaCl) 0 , (KCl) 75 (NaCl) 25 , (KCl) 55 (NaCl) 45 , 8189


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Figure 3. SEM images of samples calcined at 1173 K for 3 h with flux reagent ratios of (KCl)100(NaCl)0 (a), (KCl)75(NaCl)25 (b), (KCl)55(NaCl)45 (c, d), (KCl)25(NaCl)75 (e), and (KCl)0(NaCl)100 (f).

ratio of NaCl is higher than 50 mol %, almost all of the K on the surface of the photocatalysts disappears (Figure 5A(d),(e)), even though K2CO3 is used as a precursor during all of the fabrication processes. The peaks for K 2p show no obvious shifts with upon changing the ratio of KCl and NaCl in the flux, indicating that the environment of the K species is not changed. Conversely, Na is not found in the catalyst fabricated using only KCl. However, clear peaks at 1072.6 eV with no obvious shifts attributed to Na 1s are observed in all of the other catalysts fabricated using different ratios of NaCl. Because the Sr cations are changed during the flux treatment, careful analysis of the Sr 3d X-ray photoelectron spectrum was performed (Figure 6). The Sr 3d spectrum can be fitted by two doublets (Sr 3d5/2 and 3d3/2) with an energy separation of 1.80 eV, and the branching ratio of 3d5/2/3d3/2 = 1.5.37 Moreover, the “surface” component doublet at high BE is associated with surface terminal species, such as SrO, Sr(OH)2, and SrCO3. The “lattice” component doublet at low BE is assigned to the near-surface regions.38−40 Table 2 summarizes the K, Na, Sr, and Ta composition ratios. Although the X-ray photoelectron spectroscopy (XPS) results indicate different K and Na to Ta ratios compared to the ICP results, the two elements show

(KCl)25(NaCl)75, and (KCl)0(NaCl)100 are estimated to be 3.89, 3.93, 3.95, 3.92, and 3.89 eV, respectively (Figure 4B) using the Davis−Mott method.32 The (KCl)55(NaCl)45 catalyst shows the largest band gap of the catalysts fabricated by using the flux method. The enlarged band gap is due to the substitution of K+ and Sr2+ for Na+. Large band gaps in tantalates have been reported to be beneficial for photocatalytic water splitting and CO2 conversion by H2.33,34 Because the TTB structure of these catalysts still remains after the substitution, the valence band levels of these catalysts should be similar.35 The alkali and alkaline-earth metals were found to affect the conduction band levels in NaTaO3; thus, a larger band gap means a larger conduction band level, which directly dictates the water splitting and CO2 conversion ability of a material. Figure 5A shows the K 2p X-ray photoelectron spectra. The two peak components at binding energies (BEs) of 292.6 and 295.6 eV correspond to K 2p3/2 and K 2p1/2, respectively. The two peaks indicate a spin−orbit split doublet and can be ascribed to the K oxides and cations, respectively.36 It can be clearly seen that the intensity of the K 2p peak decreases, increasing the ratio of NaCl in the flux treatment. When the 8190


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Figure 6. Sr 3d X-ray photoelectron spectra of samples calcined at 1173 K for 3 h with flux reagent ratios of (KCl)100(NaCl)0 (a), (KCl)75(NaCl)25 (b), (KCl)55(NaCl)45 (c), (KCl)25(NaCl)75 (d), and (KCl)0(NaCl)100 (e). Figure 4. (A) UV−vis DR spectra and (B) energy band gap of samples calcined at 1173 K for 3 h with flux reagent ratios of (KCl)100(NaCl)0 (a), (KCl)75(NaCl)25 (b), (KCl)55(NaCl)45 (c), (KCl)25(NaCl)75 (d), and (KCl)0(NaCl)100 (e).

those derived by ICP upon increasing the NaCl molar ratio in the flux treatment. The XPS-derived Sr/Ta values are much higher than the ICP-derived values in all of these cases, indicating that the samples present Sr-rich surfaces. The corresponding surface and lattice Sr/Ta values calculated using the fitting results are shown in Table 2, and it is clear that the surface Sr/Ta decreases with increasing NaCl in the flux treatment. It has been previously reported that Sr-doped

similar trends with an increasing NaCl molar ratio in the flux treatment (Table 1). Interestingly, the Sr/Ta ratios of the samples, as derived by XPS, show obvious differences from

Figure 5. (A) K 2p and (B) Na 1s X-ray photoelectron spectrum of samples calcined at 1173 K for 3 h with flux reagent ratios of (KCl)100(NaCl)0 (a), (KCl)75(NaCl)25 (b), (KCl)55(NaCl)45 (c), (KCl)25(NaCl)75 (d), and (KCl)0(NaCl)100 (e). 8191


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the photocatalytic activity toward CO without any change in selectivity toward CO evolution. Note that the catalyst fabricated with the KCl/NaCl molar ratio of 55/45 shows the highest photocatalytic activity toward CO evolution (94.6 μmol h−1) and good selectivity toward CO (85.3%) (Figure 7c). We summarized other recently reported materials that used similar systems for photocatalytic conversion of CO2 using H2O as an electron donor (Table S2). Our catalyst showed good photocatalytic activity and selectivity toward CO evolution among all these reported catalysts. The enhanced photocatalytic activity is due to the enlarged band gap with the higher conduction band level and the maintained Sr-rich surface. The higher conduction band level is beneficial for CO2 conversion because higher conduction band levels possess stronger reduction potentials.43 The maintained Sr-rich surface is beneficial for CO2 capture because Sr as an alkaline-earth metal can be used for the adsorption of CO2.44 The adsorption of CO2 species on the surface of catalysts is shown in Figure S3. CO2 showed strong adsorption on the surface of catalysts; after introducing CO2, various kinds of carbonates and bicarbonates have been formed (1200−1800 cm−1). The intergrade intensity of these species is much stronger in (KCl) 75 (NaCl) 25 , (KCl) 55 (NaCl) 45 , and (KCl)25(NaCl)75 as compared to that in (KCl)100(NaCl)0 and (KCl)0(NaCl)100. We have reported that the adsorption of these species on the surface of a photocatalyst is important to achieve high activity for the conversion of CO2.45 Thus, catalysts fabricated using mixture flux enhanced CO 2 adsorption and photocatalytic activity. Further increasing the ratio of NaCl in the flux treatment leads to a decrease in the photocatalytic activity toward CO (Figure 7d,e); the samples with lower conduction band levels and Sr-poor surface are not good for CO2 capture and reduction. This means that too much NaCl in the flux mixture destroys the TTB structure, affecting the conduction band level and surface Sr/Ta ratio, leading to a decrease in the photocatalytic activity for CO2 conversion. Blank tests suggest that all of the components of the system, i.e., photocatalyst, photoirradiation, CO2 bubbling, the Ag

Table 2. Surface Composition of the Fabricated Samples Estimated from XPS Measurements surface

lattice

sample

K/Ta

Na/Ta

Sr/Ta

Sr/Ta

Sr/Ta

(KCl)100(NaCl)0 (KCl)75(NaCl)25 (KCl)55(NaCl)45 (KCl)25(NaCl)75 (KCl)0(NaCl)100

0.13 0.04 0.03 0.003 0

0 0.18 0.32 0.35 0.41

0.49 0.46 0.39 0.38 0.44

0.27 0.24 0.22 0.17 0.18

0.22 0.22 0.17 0.21 0.26

NaTaO3 easily forms Sr-rich shells covering Sr-poor cores and that electron−hole recombination is restricted during photoirradiation, enhancing photocatalytic activity.41,42 In the current work, a Sr-rich shell with a Sr-poor core is also observed when the molar ratio of NaCl/KCl is less than 50%. Conversely, a Srpoor shell with a Sr-rich core is formed when the molar ratio of NaCl/KCl is higher than 50% (Table 2). Thus, changing the flux conditions not only affects the cation occupation but also changes the surface Sr/Ta composition. We have previously reported that TTB Sr2KTa5O1517 and K2RETa5O15 (RE = rare-earth element)20 fabricated using KCl exhibit good photocatalytic activity and selectivity toward CO for CO2 conversion using H2O as an electron donor.17,20 In the current study, the effect of substituting K+ and Sr2+ for Na+ in Sr2KTa5O15 using different ratios of KCl and NaCl as flux materials on its photocatalytic activity was investigated. Figure 7 presents the formation rates of evolved gas products using catalysts fabricated using different ratios of NaCl. The photocatalytic activity of Sr2KTa5O15 (i.e., the catalyst fabricated using only KCl) toward CO was 60.2 μmol h−1, which is consistent with our previous report (Figure 7a).17 Other kinds of reduction products, such as CH4 or HCOOH, HCHO, and CH3OH, were not detected. The catalysts fabricated in flux mixtures show enhanced photocatalytic activities for CO evolution (Figure 7b−d) without any obvious change in H2 generation during photoirradiation. This result indicates that the substitution of K+ and Sr2+ for Na+ enhances

Figure 7. Rates of CO (red filled), O2 (green dotted), and H2 (blue slashed) evolution and related selectivity toward CO evolution over samples calcined at 1173 K for 3 h with flux reagent ratios of (KCl)100(NaCl)0 (a), (KCl)75(NaCl)25 (b), (KCl)55(NaCl)45 (c), (KCl)25(NaCl)75 (d), and (KCl)0(NaCl)100 (e). Conditions: 1.0 wt % Ag cocatalyst loaded by the chemical reduction method, 1.0 g of catalyst, 400 W high-pressure mercury lamp, 1.0 L of water, CO2 flow rate: 30 mL min−1, additive: NaHCO3 (0.1 mol L−1). 8192


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Figure 8. Time courses of CO (circle), O2 (square), and H2 (triangle) evolutions for the photocatalytic conversion of CO2 by H2O over the Agloaded sample calcined at 1173 K for 3 h with a flux reagent ratio of (KCl)55(NaCl)45. Amount of catalyst: 1.0 g, loading amount of cocatalyst: 1.0 wt % Ag cocatalyst loaded by the chemical reduction method, light source: 400 W high-pressure mercury lamp, volume of water: 1.0 L, CO2 flow rate: 30 mL min−1, additive: NaHCO3 (0.1 mol L−1).

cocatalyst, and the NaHCO3 additive, are necessary to achieve the highly selective conversion of CO2 into CO (Figure S2), as we have previously reported.10 In the photocatalytic conversion of the CO2 reaction, the photocatalyst absorbs photons in bulk to generate electrons and holes. The photoexcited electrons subsequently migrate to the surface active sites to reduce CO2 and H+. On the other hand, the holes are used to oxidize H2O. It is well known that the electrochemical reduction of CO2 to CO occurs selectively on a Ag metal electrode in aqueous solution.46,47 Similarly, the Ag cocatalysts function as active sites to capture the photoexcited electrons on the surface of catalysts for the reduction of CO2. The loading amount dependence of Ag on the photocatalytic activity is shown in Figure S4. Ag (1.0 wt %) showed the highest photocatalytic activity toward CO evolution among these catalysts with different amounts of Ag loading. The recycle test was performed to confirm the stability and durability of our catalyst and system using the Ag-modified (KCl)55(NaCl)45 catalyst (Figure 8). CO was evolved as the main reduction product, and a small amount of H2 was generated during the three runs. Simultaneously, the evolution of O2 increased linearly during the reaction. The evolution of CO, H2, and O2 are in an approximately stoichiometric ratio, indicating that the number of photoelectrons used for the conversion of CO2 and protons is equivalent to the number of holes consumed for H2O oxidation. The formation rate of CO slightly decreases together with slight increase in the formation rate of H2 during the third run. However, as compared to the decline in the formation rate of CO using the (KCl)100(NaCl)0 catalyst reported previously,17 the decline in CO evolution over the (KCl)55(NaCl)45 catalyst is much slower. This result means that the substitution of K+ and Sr2+ for Na+ in Sr2KTa5O15 not only enhances the photocatalytic activity but also enhances the stability toward CO evolution. Figure 9 shows the DR spectra before the reaction, after the first run, and after the third run for the Ag-loaded catalyst fabricated using the (KCl)55(NaCl)45 catalyst. A Ag surface plasmon absorption around 350−600 nm is observed in the before-reaction catalyst. After the first reaction cycle, the shape and intensity of the Ag plasmon absorption is similar to that of

Figure 9. UV−vis DR spectra of 1.0 wt % Ag-loaded sample calcined at 1173 K for 3 h with a flux reagent ratio of (KCl)55(NaCl)45 before reaction (a), after first run (b), and after third run (c).

the before-reaction catalyst, suggesting that the situation of Ag cocatalyst is not significantly changed during the first run. However, the intensity after the third run is smaller than that after the first run, indicating that the amount of Ag cocatalyst decreases.9 This result means that the decrease of Ag cocatalyst has an obvious effect on the photocatalytic conversion of CO2 to CO by H2O, leading to a decrease of the photocatalytic activity toward CO and selectivity toward CO evolution, which is consistent with our previous reports.11,44 In contrast, the catalyst prepared by using only KCl shows an obvious decrease in the intensity of the Ag plasmon absorption, even after the first run (Figure S5). This means that the Ag deposited on the surface of the (KCl)55(NaCl)45 catalyst is much more stable than that deposited on the (KCl)100(NaCl)0 catalyst. To further investigate the change of Ag cocatalyst on the (KCl)55(NaCl)45 catalyst, we used SEM to observe the morphology change of the Ag cocatalyst. It can be clearly seen that small Ag particles are well arched on the surface of both the nanorods and nanoparticles before the reaction (Figure 10a,b). The average size of the Ag particles before the reaction is ∼5.6 nm (Figure S6a). We have previously reported 8193


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Figure 10. SEM images of 1.0 wt % Ag-loaded sample calcined at 1173 K for 3 h with a flux reagent ratio of (KCl)55(NaCl)45 before the reaction (a, b), after the first run (c, d), and after the third run (e, f).

evolution, the formation of H2 increases with a decrease in the number of active sites for CO evolution. The analysis above clearly demonstrates that it is necessary to maintain the small particle size of the Ag cocatalyst to ensure a stable rate of CO formation during the photocatalytic conversion of CO2 by H2O in aqueous NaHCO3 solution.

that small Ag cocatalyst particles are more efficient than large Ag particles during photocatalytic conversion of CO2 using H2O as the electron donor in aqueous NaHCO3 solution.11,12,44 For the same amount of Ag, smaller Ag particles present more active sites than larger Ag particles, leading to high photocatalytic activity for CO evolution. After the first run, some aggregation of Ag is observed in the SEM images (Figure 10c,d). The sizes of some Ag particles increase to 10−20 nm (Figure S6b), which may be due to the dissolutionrephotodeposition of Ag particles.9 It is also noted that some small Ag particles still remain on the surface of the photocatalyst, which is consistent with the DR spectra. The aggregation and size growth of Ag on the (KCl)55(NaCl)45 catalyst is much slower than that on the (KCl)100(NaCl)0 catalyst.17 However, obvious aggregation is observed in the SEM images (Figure 10e,f), with the size of the Ag particles reaching 20−40 nm after the third run (Figure S6c). The increase in the size of the Ag particles leads to a decrease in the number of active sites for CO evolution, leading to a decrease in the amount of CO formed. We also noted that the amount of H2 formed increases for the second and third runs. Because the Ag particles are not the active sites for H2 evolution but for CO

3. CONCLUSIONS Substitution of K+ and Sr2+ for Na+ was easily achieved during the preparation of SrxKyNazTa5O15 using different ratios of KCl and NaCl as flux reagents. This substitution not only showed effects on the phase and morphology of the photocatalysts but also changed their band gaps and surface compositions. The photocatalytic activity for CO2 conversion using H2O as an electron donor was enhanced by the substitution without obviously changing the selectivity toward CO evolution. More importantly, the stability of the formation rate of CO was also enhanced by using the (KCl)55(NaCl)45 catalyst as compared to that of the photocatalyst fabricated using only KCl. This is because the Ag cocatalyst was more stable on the surface of the (KCl)55(NaCl)45 photocatalyst. 8194


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bubbled into the solution at a flow rate of 30 mL min−1 for 1 h before the photoirradiation and continuously flowed during the photoirradiation. The suspension was irradiated under a 400 W high-pressure mercury lamp, with a quartz filter connected to a water cooling system. The gaseous products generated, such as H2, O2, and CO, were analyzed by thermal conductivity detector-gas chromatography using a GC-8A chromatographer (Shimadzu Corp.) equipped with a 5A molecular sieve column, with Ar as the carrier gas, and by flame ionization detector-gas chromatography with a methanizer using a ShinCarbon ST column, with N2 as the carrier gas. The selectivity toward CO evolution compared with H2 evolution and the balance between consumed electrons (e−) and holes (h+) is generally described using the formulae shown below

4. EXPERIMENTAL SECTION Catalysts were fabricated by a flux method using pure KCl (99.8%, Wako) or NaCl (99.9%, Wako) or mixtures with different molar ratios of KCl and NaCl. A stoichiometric mixture of K2CO3 (99.5%, Wako), SrCO3 (99.9%, Wako), and Ta2O5 (99.9%, Kojundo) was ground with the flux reagents in an Al2O3 mortar for 5 min. The total mass ratio of the flux reagents to the precursors (K2CO3, SrCO3, and Ta2O5) was fixed at 1.0. The mixture was then transferred to a 50 mL Al2O3 crucible calcined at 1173 K for 3 h in air. After cooling down to room temperature, the mixture was thoroughly washed with distilled water at 358 K three times to remove any residual salts and then dried in air at 80 °C. The same procedure was employed in all catalyst fabrication processes. The catalysts were labeled as (KCl)x(NaCl)100−x, where x represents the mol % of KCl and (100−x) represents the mol % of NaCl. For instance, (KCl)55(NaCl)45 denotes a catalyst fabricated using 55 mol % KCl and 45 mol % NaCl as the flux reactants. The Ag cocatalyst was deposited on the as-fabricated catalysts by a chemical reduction method. Specifically, 1.8 mL of an aqueous NaPH2O2 solution (0.33 mol L−1) was added to 50 mL of a suspension of the photocatalyst (1.5 g) containing 1.4 mL of AgNO3 (0.1 mol L−1). After stirring at 358 K for 1.5 h, the suspension was filtered and dried in air at room temperature prior to further use. Elemental analysis using inductively coupled plasma optical emission spectrometry (ICP-OES, iCAP7400 Duo, Thermo Fisher Scientific, Inc.) has been used for determining the real chemical formula. The structure and crystallinity of the catalysts were characterized by X-ray diffraction (XRD) using a Rigaku Multiflex powder X-ray diffractometer. The Brunauer− Emmett−Teller (BET) surface area was measured by N2 adsorption at 77 K using a volumetric gas adsorption apparatus (BELmini, Bel Japan, Inc.). The UV−vis diffuse reflectance (UV−vis DR) spectra were measured using a JASCO Corporation V-670 spectrometer equipped with an integrating sphere. A spectralon filled with BaSO4, supplied by Labsphere Inc., was used as a reflection standard. The XPS measurement was acquired using an X-ray photoelectron spectrometer (ESCA 3400, Shimadzu Corp.). SEM images were obtained from field emission scanning electron microscopy (SU-8220, Hitachi High-Technologies) equipped with an energy-dispersive X-ray spectroscopy apparatus. Fourier transform infrared (FT-IR) spectra of the catalysts and adsorbed CO2 species were observed with an FT/IR-4700 spectrometer (JASCO Corporation) equipped with a mercury−cadmium− tellurium detector cooled by liquid N2 in a transmission mode at 303 K. About 50 mg of each sample was pressed into a wafer 10 mm in diameter at the pressure of 10 MPa. The wafer was then placed in a cylindrical glass cell equipped with CaF2 windows. The cell allowed us to perform heating, O2 treatment, introduction of CO2 gas, and in situ spectral measurements. The wafer was evacuated at 873 K for 30 min before measurement, followed by treatment under about 4.67 KPa of O2 for 30 min, and subsequently it was evacuated and cooled to 303 K. For each spectrum, data from 128 scans were accumulated at a resolution of 4 cm−1. The photocatalytic conversion of CO2 by H2O was carried out in a flow system using an inner-irradiation-type reaction vessel at room temperature and ambient pressure (Figure S1). The photocatalyst (1.0 g) was dispersed in ultrapure water (1.0 L) containing 0.1 mol NaHCO3. CO2 gas (99.999%) was

selectivity toward CO evolution (%) = 100 × 2R CO/(2R CO + 2R H2) Consumed e−/h+ = (2R CO + 2R H2)/(4×R O2)

where RCO, RH2, and RO2 represent the rates of formation of CO, H2, and O2, respectively.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01305. ICP test for washing solution, lists of reported catalysts, scheme of reactor, blank tests, optimizing of Ag cocatalyst and UV−vis DR spectra of (KCl)100(NaCl)0 during the recycle test (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +81-75-3832559. Fax: +81-75-383-2561 (K.T.). *E-mail: [email protected]. Tel: +81-75-383-2558. Fax: +81-75-383-2561 (T.T.). ORCID

Zeai Huang: 0000-0002-7079-2504 Kentaro Teramura: 0000-0003-2916-4597 Hiroyuki Asakura: 0000-0001-6451-4738 Saburo Hosokawa: 0000-0003-1251-3543 Tsunehiro Tanaka: 0000-0002-1371-5836 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was partially supported by a Grant-in-Aid for Scientific Research on Innovative Areas “All Nippon Artificial Photosynthesis Project for Living Earth” (No. 2406) of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan, the Precursory Research for Embryonic Science and Technology (PRESTO), supported by the Japan Science and Technology Agency (JST), and the Program for Elements Strategy Initiative for Catalysts & Batteries (ESICB), commissioned by the MEXT of Japan. Z.H. is thankful to the State Scholarship of China Scholarship Council, affiliated with the Ministry of Education of the P. R. China. 8195


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