Which is an Intermediate Species for Photocatalytic Conversion of

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Which is an Intermediate Species for Photocatalytic Conversion of CO by HO as the Electron Donor: CO Molecule, Carbonic Acid, Bicarbonate, or Carbonate Ions? 2

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Kentaro Teramura, Kazutaka Hori, Yosuke Terao, Zeai Huang, Shoji Iguchi, Zheng Wang, Hiroyuki Asakura, Saburo Hosokawa, and Tsunehiro Tanaka J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12809 • Publication Date (Web): 11 Apr 2017 Downloaded from http://pubs.acs.org on April 16, 2017

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The Journal of Physical Chemistry

Which is an Intermediate Species for Photocatalytic Conversion of CO2 by H2O as the Electron Donor: CO2 Molecule, Carbonic Acid, Bicarbonate, or Carbonate Ions? Kentaro Teramura†,‡*, Kazutaka Hori†, Yosuke Terao†, Zeai Huang†, Shoji Iguchi†, Zheng Wang†, Hiroyuki Asakura†,‡, Saburo Hosokawa†,‡, Tsunehiro Tanaka†,‡* Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyotodaigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan

†

Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, 1-30 Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan ‡

ABSTRACT: In the photocatalytic conversion of CO2 using H2O as the electron donor, there are four candidates of the intermediate species in the solution: hydrated CO2 molecule (CO2(aq)), carbonic acid (H2CO3), bicarbonate (HCO3−), and carbonate (CO32−) ions. The concentrations of all four species in the system at equilibrium can be controlled by controlling the temperature, pH value, and concentration of the counter cation. According to our experiments conducted under different conditions, we narrowed the possible intermediates down to two: CO2(aq) and HCO3−. The isotopic experiment using 13CO2 and in situ infrared spectroscopy revealed that CO2(aq) reacts with the hydroxyl group anchored on the Ga atom on the surface of ZnGa2O4/Ga2O3 to form bidentate HCO3-Ga, followed by the formation of bidentate HCOO-Ga via monodentate HCO3-Ga. We conclude that bidentate HCOO-Ga is the real intermediate species for the photocatalytic conversion of CO2 by H2O as the electron donor over Ag/ZnGa2O4/Ga2O3 catalyst, and it decomposes into CO as a product.

towards the reduction products. In addition, the stoichiometry of O2 is one of the strongest pieces of evidence that H2O functions as the electron donor and undergoes 4-electron oxidation reaction (eqn. 3).15

Introduction In order to counter global climate change, many countries have taken measures to reduce the emission of CO2. Artificial photosynthesis, especially the photocatalytic conversion of CO2 into valuable compounds using water as the electron donor, is a chemical process much sought after for energy conversion and chemical feedstock production.1-5 Since the early reports by Herrman et al.6 and Honda et al.7, many research groups have observed the conversion of CO2 into reduction products such as carbon monoxide (CO), formic acid (HCOOH), formaldehyde (HCHO), methanol (CH3OH), and methane (CH4). In many cases, only the reduction products of CO2 were focused upon, meaning that all electrons generated by charge transfer are used for the reduction of CO2, although the overall water splitting can occur with many kinds of photocatalysts.8-13 That is to say, when H2O is used as the electron donor, the reduction of CO2 (eqn. 1 in the case of producing CO) invariably competes with the production of H2 from the overall water splitting (eqn. 2).14 CO 2 + 2H + + 2e − → CO + H 2 O 2H + + 2e − → H 2

(3)

2H 2 O → O 2 + 4H + + 4e − (h+)

The number of holes generated and consumed is consistent with that of electrons (e−), thereby essentially achieving the photocatalytic conversion of CO2 using H2O as the electron donor. If CO is also evolved as a reduction product, the selectivity toward it and the balance between the consumed electrons and holes can be expressed as follows: Selectivity toward CO evolution (%)= 2RCO/(2RCO+2RH2)×100 Consumed e−/h+ = (2RCO + 2RH2) /4RO2

(4) (5)

where Rx is the formation rate of species x. Recently, Kudo and co-workers opened the gate to the next stage of the photocatalytic conversion of CO2 by H2O as the electron donor. The photocatalyst ALa4Ti4O15 (A = Ca, Sr, and Ba) with Ag nanoparticles exhibited good activity with a higher rate of CO formation than H2 evolution, together with stoichiometric rate of O2 formation.16 Therefore, all the requirements mentioned above for the photocatalytic

(1) (2)

It is therefore necessary to monitor the evolution of H2, and determine the selectivity of the generated electrons

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It is well-known that K1 and K2 depend on the solution temperature, with the following empirical expressions according to Plummer and Busenberg27

conversion of CO2 using H2O as the electron donor were satisfied. Their work inspired our group to switch the electron donor from H2 to H2O. Kudo et al. also reported that KCaSrTa5O15 with Ag nanoparticles functions as a photocatalyst for the photocatalytic conversion of CO2 by H2O.17-18 Their isotope experiment using 13CO2 revealed that CO was generated from the introduced CO2. Meanwhile, we succeeded in designing highly selective photocatalytic conversion of CO2 by H2O as the electron donor, by the simultaneous use of an inhibitor against the production of H2 and a material for CO2 capture and storage, such as ZnGa2O4/Ga2O3,19-20 La2Ti2O7,21 SrO/Ta2O5,22 ZnGa2O4,23 ZnTa2O6,24 and Sr2KTa5O1525 with the modification of Ag cocatalyst. Ag/ZnGa2O4/Ga2O319-20 is remarkable for suppressing the H2 evolution by forming ZnGa2O4 on the surface of Ga2O3, resulting in high selectivity toward CO evolution.23

log K1 = −356.3094 − 0.06091964T +

HCO (aq) ⇌ CO (aq) + H (aq) − 3

2− 3

+

+

(12)

K2 =

[H + ][CO32 − ] [HCO3− ]

(13)

[CO32 − ] =

K1K 2 ⋅S [H + ]2 + K1[H + ] + K1K 2

(17)

[H + ]2 ⋅S [H ] + K1[H + ] + K1K 2 + 2

(18)

[ Na + ] + [H + ] = [HCO3− ] + 2[CO32 − ] + [OH − ]

(19)

K w = [H + ][OH − ]

(20)

 K  [H + ]2 + K1[H + ] + K1K 2   S =  [ Na + ] + [H + ] − w+  [H ]  K1[H + ] + 2 K1K 2  

(21)

Therefore, we can estimate the concentrations of all CO2-related species in the aqueous solution from the amount of buffer, the temperature, and pH of the solution. Figure 1 shows [CO2(aq)], [HCO3−], and [CO32−] under the typical reaction conditions in this study (303 K, 0.10 M added NaHCO3). Based on this figure, the fractions of CO2-related species at equilibrium (the so-called Bjerrum plot) are often used as an example in textbooks of analytical chemistry (Figure S1). In this study, the measured pH values were 4.0–8.0, a range in which [CO32−] is vanishingly small as shown in Figure 1. On the other hand, [HCO3−] changes very little within this pH range. [CO2(aq)] dramatically decreases with increasing pH, vanishing at around 8.0 under our typical experimental conditions (pressure of CO2: 101.325 kPa, [NaHCO3]: 0.10 M,

On the basis of eqns. (9) and (10), the first and second dissociation constants of carbonic acid can be denoted by the following equations, respectively:

[H + ][HCO3− ] [H 2CO3 *]

(16)

where Kw = 1.0×10−14 is the self-ionization constant of water. When eqns. (16), (17), and (20) are introduced into (19), we obtain the following equation:

We define S as the total concentration of all CO2related species (H2CO3* (i.e., CO2(aq) + H2CO3), HCO3−, and CO32−):

K1 =

K1[H + ] ⋅S [H ] + K1[H + ] + K1K 2 + 2

On the other hand, S is constrained by the electrical neutrality when CO2 dissociates in a solution containing Na+ ions from a buffer such as NaOH, NaHCO3, and Na2CO3:

From the low hydration equilibrium constant of carbonic acid in pure water (eqn. (7), Kh = [H2CO3]/[CO2(aq)] ≈ 1.7×10−3), the dissolved CO2 consists of mostly CO2(aq) together with a small amount of H2CO3; therefore, H2CO3* is used to represent the two species of CO2(aq) and H2CO3 in writing the aqueous chemical equilibrium equation. Eqns. (7) and (8) are rewritten as follows

(11)

(15)

[HCO3− ] =

[CO 2 (aq)] =

(9)

S = [H 2 CO 3 *] + [HCO 3− ] + [CO 32− ]

5151.79 563713.9 + 38.92561 log T − T T2

Eventually, [HCO3−] and [CO32−] are represented using S and [H+]. [H+] is easily determined from the pH value recorded during the reaction. Because [H2CO3*] is almost equal to [CO2(aq)], [CO2(aq)] can be estimated as follows:

(8)

(10)

(14)

where T is the temperature of the solution. Substituting eqns. (12) and (13) into (11) results in

(6) (7)

H 2CO3 * ⇌ HCO3− + H +

21834.37 1684915 + 126.8339 log T − T T2

log K 2 = −107.8871 − 0.03252849T

To enhance the CO formation rate, it is very important to identify the intermediate species with multiple characterization methods, as well as the reaction mechanism. In this study, we examine the intermediate species of CO2 reduction in aqueous solutions. It is known that gaseous CO2 can dissolve in H2O.26-27 The hydrated CO2 molecule (CO2(aq)) reacts with H2O to form carbonic acid (H2CO3). Dissociation of H2CO3 produces bicarbonate (HCO3−) and carbonate ions (CO32−) in two stages, depending on the pH value. These four reactions reach equilibria during the photocatalytic reaction:

CO 2 (g) ⇌ CO 2 (aq) CO 2 (aq) + H 2 O(l) ⇌ H 2 CO 3 (aq) H 2 CO 3 (aq) ⇌ HCO 3− (aq) + H + (aq)

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The Journal of Physical Chemistry integrating sphere. Spectralon® (Labsphere Inc.) was used as a standard reflection sample like BaSO4. The X-ray photoelectron spectra (XPS) were acquired using an ESCA 3400 X-ray photoelectron spectrometer (Shimadzu Corporation) using Mg Kα source in a vacuum chamber in 0.1 eV steps. A pellet sample was mounted on the sample holder by using conductive carbon tape. The position of the C1s carbon peak at 284.6 eV was used to calibrate the binding energies for all the samples. The surface atomic ratio was estimated from the areas of Ga 2p and Zn 2p peaks using the corresponding relative sensitivity factors. Fourier transform infrared (FT-IR) spectra of the ZnGa2O4/Ga2O3 sample and adsorbed CO2 species were observed with an FT/IR-4200 spectrometer (JASCO Corporation) equipped with a mercury-cadmium-tellurium (MCT) detector cooled by liquid N2 in a transmission mode at 303 K. Approximately 50 mg of ZnGa2O4/Ga2O3 sample was pressed into a wafer 10 mm in diameter at the pressure of 15 MPa. The wafer was placed in a cylindrical glass cell equipped with CaF2 windows. The cell allowed us to perform heating, O2 treatment, introduction of various gases, photoirradiation, and in situ spectral measurement. The wafer was evacuated at 673 K for 3 h before measurement, followed by treatment with 5 kPa of O2 for 30 min and evacuation for 10 min at 673 K. For each spectrum, data from 100 scans were accumulated at a resolution of 4 cm−1.

T: 303 K), although it is dominant in acidic conditions. As mentioned above, [H2CO3] is also negligible, because the CO2 hydration equilibrium constant is very small. As a result, we will focus on CO2(aq) and HCO3− as candidate intermediate species for the reaction (i.e. photocatalytic conversion of CO2 by H2O as the electron donor) with Ag/ZnGa2O4/Ga2O3, and discuss the reaction mechanism including the determination of the intermediate species.

Figure 1 Calculated concentrations of CO2(aq) (circle), HCO3− (triangle), and CO32− (square) in 0.10 M aqueous solution of NaHCO3 at 303 K under 101.325 kPa of CO2.

Experimental Section Ga2O3 was fabricated by a typical precipitation method using a gallium nitrate precursor (Ga(NO3)·nH2O).19-20 An aqueous solution of ammonia (28%) was added dropwise into an aqueous solution of Ga(NO3)·nH2O until the pH value reached approximately 8.9. The gallium hydroxide (Ga(OH)3) was centrifuged, washed with milli-Q water, and dried aerially at 353 K in an oven, followed by calcination at 1273 K for 6 h to obtain β-Ga2O3. ZnGa2O4modified Ga2O3 (ZnGa2O4/Ga2O3) was fabricated by an impregnation method, as reported by us elsewhere.19-20 The as-prepared β-Ga2O3 was dispersed in an aqueous solution of Zn(NO3)2·6H2O by ultrasonication. After impregnation, the sample was dried under vacuum with ultrasonication at 313–323 K, and then calcined at 1223 K for 6 h. Ag-loading onto the ZnGa2O4-modified Ga2O3 to form Ag/ZnGa2O4/Ga2O3 was carried out by a well-known photodeposition method.19-20 The amount of loaded Ag cocatalyst was fixed at 1.0 wt% in this study. Specifically, the prepared ZnGa2O4/Ga2O3 was dispersed in an aqueous solution of AgNO3. The suspension was deaerated using CO2 gas (not inert gases like N2 or Ar) for 30 min, and then illuminated under a 400 W high-pressure mercury lamp (SEN LIGHTS Co, Ltd.) through a quartz filter equipped with a water cooling system. After 2 h, the suspension was filtered, and the filtrate washed with distilled water several times and dried in an oven at 353 K overnight. The X-ray diffraction (XRD) patterns of Ag/ZnGa2O4/Ga2O3 were recorded using an Ultima IV powder diffractometer (Rigaku Corporation). The UV-vis diffuse reflectance spectra (DRS) were measured with a V670 spectrometer (JASCO Corporation) equipped with an

The photocatalytic conversion of CO2 by H2O was carried out in a quartz inner-irradiation-type reaction vessel (1.0 L) in a quasi-flowing batch system. The synthesized Ag/ZnGa2O4/Ga2O3 was dispersed in an aqueous solution of NaHCO3 or other buffers. CO2 gas (99.999%) was bubbled into the solution at a flow rate of 30 ml min−1. The suspension was irradiated using the same mercury lamp system as that used in the photodeposition process. The produced H2 and O2 were analyzed by using a GC-8A gas chromatograph (GC) (Shimadzu Corporation) equipped with thermal conductivity detector (TCD) and a Molecular Sieve 5A column (carrier gas: Ar). On the other hand, the main product CO and the substrate CO2 were monitored by using a GC-8A system equipped with flame ionization detector (FID), a methanizer, and a SHINCARBON ST column (carrier gas: N2). The initial formation rate of each product was estimated every hour by linear fitting during the 5 hours of photoirradiation. In the isotopic experiment with 13CO2, the formed 13CO and 12CO were observed by using a quadrupole-type mass spectrometer (MS, MicrotracBEL, BEL Mass).

Results and Discussion The prepared Ga2O3, ZnGa2O4/Ga2O3, and Ag/ZnGa2O4/Ga2O3 samples were characterized by XRD and UV-vis DRS. Their crystal phase and estimated band gap energy were confirmed to be similar to those we reported before. 19-20 The as-prepared Ga2O3 exhibited the typical XRD pattern of β-Ga2O3 (Figure S2). A peak at 35.7° appeared after the modification by Zn(NO3)2 and calcination at high temperature, and was assigned to the (311) facet of the ZnGa2O4 structure. On the other hand,



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indicates that the total number of electrons and holes generated by charge transfer remains the same in the entire temperature range. The selectivity toward CO evolution clearly depends on p[CO2]. In general, x2, the mole fraction of CO2 in H2O (mole of solute per mole of solvent), which is related to its solubility, can be calculated by the following formula:26

there was no peak assignable to Ag after the photodeposition of Ag cocatalyst, indicating that Ag existed on the surface of ZnGa2O4/Ga2O3 as relatively small nanoparticles. The absorption edge of bare Ga2O3 was observed at 265 nm, and the band gap energy was estimated as 4.6 eV, according to the Davis-Mott equation (Figure S3). The edge was not changed by the modification of ZnGa2O4. However, the loading of Ag afforded a new broad band at 270–330 nm, which was derived from surface plasmon resonance (SPR) of the Ag nanoparticles.28-30 This result agrees well with the absence of Ag peaks in the XRD measurement. In this study, the ratio of Zn/(Zn+Ga) in the starting material was fixed to 3.0 mol%, because the selectivity toward CO evolution becomes saturated when the amount of added Zn(NO3)2 precursor exceeds this ratio (Figure S4).

ln x2 = −60.069 +

8742.4  T  0.1103 × T (22) + 21.671 × ln − 100 T  100 

The value of x2 decreases with increasing solution temperature (T). In our case, the concentration of HCO3− was much higher than that of CO2 for the entire temperature range. Note that the concentration of CO2 is temperaturedependent. Therefore, p[CO2] decreased with decreasing temperature of the solution. On the other hand, the pH value remained constant (6.7−6.8) in this temperature range, as shown in Figure 2. The equilibrium in eqn. (7) between CO2 and H2CO3 in aqueous solutions goes strongly towards to the reactant. Therefore, most of CO2 in the solution exists as CO2(aq), and the pH value should not change even though p[CO2] becomes lower when the solution temperature is lowered. In order to encourage the reduction of CO2 and suppress the production of H2 (i.e., high selectivity toward CO evolution), the photocatalytic reaction should be carried out at a lower temperature.

Figure 2 Temperature-dependent (A) formation rates of CO (circle), H2 (triangle), and O2 (square), and selectivity toward CO evolution (open diamond), and (B) p[CO2] (diamond) and pH (open circle) values for the photocatalytic conversion of CO2 by H2O. Catalyst: Ag (1.0 wt%)/ZnGa2O4 (3.0 mol%)/Ga2O3 (0.5 g), solution: H2O (1.0 L), buffer: 0.10 M NaHCO3, flow rate of CO2: 30 ml min−1.

Figure 2 shows the temperature-dependent formation rates of CO and H2 as reduction products, the evolved O2 as an oxidation product, the selectivity toward CO evolution, p[CO2] which is defined as −log[CO2(aq)], and pH value when using Ag (1.0 wt%)/ZnGa2O4 (3.0 mol%)/Ga2O3. The formation rate of CO decreased with increasing reaction temperature. However, that of H2 evolution showed the opposite trend, which means that a higher reaction temperature promotes H2 evolution and inhibits CO evolution. Interestingly, the number of electrons involved in the photocatalytic reaction, as estimated by the formation rates of CO and H2, remained approximately constant with temperature. Additionally, the formation rate of O2, which was monitored stoichiometrically, was also constant at all reaction temperatures. This

Figure 3 (A) Formation rates of CO (circle), H2 (triangle), and O2 (square), and selectivity toward CO evolution (open diamond), and (B) p[CO2] (diamond) and pH (open circle) values with various partial pressures of CO2, for the photocatalytic conversion of CO2 by H2O. Catalyst: Ag (1.0 wt%)/ZnGa2O4(3.0 mol%)/Ga2O3 (0.5 g), solution: H2O (1.0 L), buffer: 0.10 M NaHCO3, flow rate of gases (CO2 + Ar): 30 ml min−1, reaction temperature: 303 K.

Figure 3 shows that the formation rate of CO increases remarkably with the partial pressure of CO2, even though the rate of H2 evolution is also slightly decreased at higher


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CO2 pressure. As a result, the selectivity toward CO evolution steadily increased with the partial pressure of CO2. The behavior observed in Figure 3 is a bit different from that in Figure 2. As expected, the stoichiometric formation rate of O2 could be also observed in this case. According to Henry’s law, the concentration of CO2 in an aqueous solution increases with the CO2 partial pressure in the gas phase. Therefore, the formation rate of CO is also increased.

rate of H2 also increases with [H+]. However, the pH value depends on p[CO2] in the solution, and the production of H2 competes with the reduction of CO2. Overall, the relationship between H+ and CO2 in this system is very complex. We conclude that with Ag/ZnGa2O4/Ga2O3 the 2electron reduction of CO2 to CO with two protons is preferred over the 2-electron reduction of H+ to H2. Therefore, the formation rate of H2 in the presence of CO2 depends not on the pH value, but rather on p[CO2], which also affects the formation rate of CO. It is well-known that the pH value of a solution can be adjusted by using a buffer. Figure 4 displays the effect of NaHCO3 as a buffer on the photocatalytic conversion of CO2 by H2O. The formation rates of H2, O2, and CO, the selectivity toward CO evolution, and the pH and p[CO2] values all depend exponentially on [NaHCO3]. As we have already reported,19 the evolution of CO was hardly observed without NaHCO3, although the amount of CO2 dissolved in the aqueous solution (0.03 M as calculated by eqn. (19) at 303 K) was high enough for the reduction reaction. The formation rate of CO increased with [NaHCO3], due to the decrement of p[CO2]. We obtained the same results in this case as those achieved by changing the reaction temperature and the partial pressure of CO2. Note that the formation rate of H2 does not depend on pH, as explained earlier. Therefore, a high rate of H2 formation could be obtained at relatively low [H+]. This tendency is the same as when the partial pressure of CO2 is increased. The formation of H2 as a by-product in the reduction of H+ is not pH-dependent in the photocatalytic conversion of CO2 by H2O, especially when using a photocatalyst with sufficiently high activity for the evolution of CO. Rather, it is very important to maintain high pH and low p[CO2] values to obtain high photocatalytic activity and good selectivity toward CO evolution.

Figure 4 (A) Formation rates of CO (circle), H2 (triangle), and O2 (square), and selectivity toward CO evolution (open diamond), and (B) p[CO2] (diamond) and pH (open circle) values at different concentrations of NaHCO3, for the photocatalytic conversion of CO2 by H2O. Catalyst: Ag (1.0 wt%)/ZnGa2O4 (3.0 mol%)/Ga2O3 (0.5 g), solution: H2O (1.0 L), flow rate of CO2: 30 ml min−1, reaction temperature: 303 K.

On the other hand, we speculate that H2 evolution depends not on the pH value, but on the concentration of CO2 in the liquid phase instead. The reason is that the formation rate of H2 changed very little with CO2 partial pressure compared to that of CO (Figure 3A), but it decreased with decreasing reaction temperature (Figure 2A) which did not affect the pH value (Figure 2B). It is somewhat counterintuitive that the rate of H2 formation does not depend on [H+]. Indeed, the evolution of H2 is generally enhanced by very low pH values for overall water splitting. For this reason, in many reports the overall water splitting is carried out in the presence of strong acids like H2SO4. In the current reaction (photocatalytic conversion of CO2 by H2O as the electron donor over Ag/ZnGa2O4/Ga2O3), however, the generated electrons are mainly used for the reduction of CO2. Because p[CO2] is much lower than the pH value in the solution, it is expected that the formation of H2 is regulated by the generation of electrons that are not used for the reduction of CO2. When p[CO2] is relatively high, overall water splitting preferentially takes place. Naturally, the formation

Figure 5 Time courses of evolved CO (circle), H2 (triangle), and O2 (square) for the photocatalytic conversion of CO2 by H2O. Catalyst: Ag (1.0 wt%)/ZnGa2O4 (3.0 mol%)/Ga2O3 (0.5 g), solution: H2O (1.0 L), buffer: 1.2 M NaHCO3, flow rate of CO2: 30 ml min−1, reaction temperature: 303 K.

The results discussed so far show that a low reaction temperature, high partial pressure of CO2, and high concentration of NaHCO3 are important for achieving highly selective photocatalytic conversion of CO2 by H2O. Figure 5 shows the time course of CO, H2, and O2 evolutions for



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this reaction in 1.2 M aqueous NaHCO3 solution over Ag/ZnGa2O4/Ga2O3. The formation rate of CO (165 µmol h−1) was clearly higher than that of H2 (31.5 µmol h−1) with stoichiometric formation rate of O2 (101 µmol h−1) at the initial stage. These observations indicate that the charge separation took place smoothly, the generated electrons reduced not H+ into H2 but preferentially CO2 into CO, and the generated holes oxidized H2O into O2. The selectivity toward CO evolution reached 84.0%. The concentration of CO evolved in the gas phase was estimated to be approximately 2300 ppm.

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tor for the reduction of CO2. CO was generated as a reduction product together with H2 in the presence of NaCl, indicating that Cl− ion does not inhibit the reduction of CO2. Note that the total formation rate of CO and H2 is almost at the same order of magnitude when using NaOH, NaHCO3, or Na2CO3; even though a portion of the base sites should be poisoned by these buffers. On the other hand, we did not monitor the evolution of O2 as an oxidation product, indicating that the generated holes do not oxidize H2O to O2, but Cl− to chlorine (Cl2) instead. Our research group has already reported that the Cl− ion works as a hole scavenger for the photocatalytic conversion of CO2 in H2O over various layered double hydroxide photocatalysts.31-32 Cl− ion should be oxidized into Cl2 by the generated holes, and then Cl2 decomposes by disproportionation into Cl− and hypochlorite (ClO−) ions in the aqueous solution. In the cases of Na2SO4 and NaNO3 (Figures 6(g) and (h)), CO evolution was not observed, and relatively tiny amounts of H2 and O2 were generated stoichiometrically from overall water splitting due to the low pH value (~4.0). We expect the reduction of CO2 to be more affected by sulfide and nitrate ions than the pH value, because the observed behavior was different from that with NaCl even at the same pH (Figure 6(f)). In general, Na2SO4 is used as a pH conditioning agent for overall water splitting to maintain high [H+] (low pH value). On the other hand, the SO42− ion behaves as a strong acid and easily poisons the base sites where the CO2-derived species should be adsorbed and then reduced to CO. Therefore, overall water splitting takes place stoichiometrically. We conclude that, for highly selective photocatalytic conversion of CO2 by H2O, it is very important to maintain a high concentration of counter cations in the aqueous solution, and not to add anions other than CO2-related species.

Figure 6 Formation rates of evolved H2 (blue), O2 (green), CO (red), and pH value (black circle) in the photocatalytic conversion of CO2 by H2O, using different buffers. (a) NaOH, (b) NaHCO3, (c) Na2CO3, (d) Li2CO3, (e) K2CO3 (f) NaCl, (g) Na2SO4, and (h) NaNO3. Catalyst: Ag (1.0 wt%)/ZnGa2O4 (3.0 mol%)/Ga2O3 (0.5 g), solution: H2O (1.0 L), concentration of counter cation: 0.10 M, flow rate of CO2: 30 ml min−1, reaction temperature: 303 K.

Interestingly, the photocatalytic activity depends on the kind of buffer. There was no visible difference among NaOH, NaHCO3, and Na2CO3 (Figure 6(a), (b), and (c), respectively) in terms of the formation rates of H2, O2, and CO, and the pH value. This means that the equilibrium of CO2 dissolved in the solution remains unchanged with these 3 buffers, because the pH, the concentration of Na+ counter cation, and the partial pressure of CO2 are all the same. As discussed previously, the conversion of CO2 and selectivity toward CO evolution strongly depend on the equilibrium amount of dissolved CO2, which is defined in terms of eqns. (6)−(9). This equilibrium is only affected by the kind of anions, not that of the counter cations. For example, when using Na2CO3, Li2CO3, or K2CO3, the same profiles were observed in the formation rates of H2, O2, and CO (Figure 6(c), (d), and (e), respectively). The pH value was not changed because of the same concentration of the buffer. Hence, the concentration of the counter cation, not its type, is important for obtaining high conversion of CO2 and good selectivity toward CO evolution.

Figure 7 IR spectra of ZnGa2O4 (3.0 mol%)/Ga2O3 (a) before and (b) after introduction of CO2 (2.0 kPa).

At the end of the introduction section, we raised the question of whether CO2, HCO3−, or both of them in the aqueous solution can function as intermediate species in the photocatalytic conversion of CO2 by H2O as the electron donor. As shown in Figure 1, [CO2(aq)] and [HCO3−] are almost the same at pH ≈ 6.7, where a relatively high formation rate of CO was observed. FT-IR spectroscopy is known to be a very powerful tool for investigating several

On the other hand, there was a huge change when anions other than (bi)carbonates were present in the aqueous solution. With NaCl (Figure 6(f)), the Cl− ion is expected to act as a hole scavenger rather than as an inhibi-


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species adsorbed on the surface of heterogeneous catalysts and photocatalysts. Previously, we have used FT-IR spectroscopy to reveal that bicarbonate and formate as intermediate species are generated on the surfaces of ZrO2,33 MgO,34-35 and Ga2O3,36 which exhibit relatively good CO evolution for the photoreduction of CO2 in the presence of H2.

mean that CO2 could be initially adsorbed on the surface of ZnGa2O4/Ga2O3, and then desorbed into the gas phase during photoirradiation due to the formation of H2O. Generally, the information of surface hydroxyl groups is obtained in the spectral range from 3200 to 3800 cm−1.43 In our case, it was difficult to observe the change after the introduction of CO2 and photoirradiation, because the band assigned to OH stretching [ν(OH)] of isolated surface hydroxyl group on ZnGa2O4/Ga2O3 (3684 cm−1) was very weak, as shown in Figure 7. To clarify the interaction between the isolated surface hydroxyl group and CO2, the OH group on ZnGa2O4/Ga2O3 was converted into OD by using D2O. ZnGa2O4/Ga2O3 was pretreated in this manner, followed by deuteration with 1.8 kPa of D2O for 30 min. Figure 8 shows the difference IR spectra of surface OD stretching on ZnGa2O4/Ga2O3 after introducing CO2 under photoirradiation. The band at 2691 cm−1, which is assigned to OD stretching [ν(OD)] of the isolated surface hydroxyl group on ZnGa2O4/Ga2O3, became weaker upon the introduction of CO2. This indicates that CO2 gas interacts with the isolated surface OD group. As a result, the band assigned to OD stretching of bicarbonate appeared at 2669 cm−1. These two bands gradually became weaker during the photoirradiation, due to the generation of CO and H2O. The broad band at 3300–3800 cm−1 derived from OH stretching increased after photoirradiation, which indicates the generation of H2O by the photocatalytic reduction of CO2 with the isolated surface hydroxyl group.

Figure 8 Difference IR spectra of the stretching mode of surface OD group of ZnGa2O4 (3.0 mol%)/Ga2O3 (a) before and (b) after introducing CO2 (1.0 kPa) under photoirradiation for (c) 15 min and (d) 30 min.

We measured the IR spectrum of ZnGa2O4/Ga2O3 in the presence of CO2 as shown in Figure 7. The spectrum of pretreated ZnGa2O4/Ga2O3 (Figure 7(a)) was used as the background. A peak assigned to OH stretching [ν(OH)] of isolated surface hydroxyl group on ZnGa2O4/Ga2O3 was observed at 3684 cm−1 after the pretreatment, which was similar for that of Ga2O3 at 3685 cm−1 (Figure S5). Many bands related to CO2 were observed in the IR spectra (Figure S6) and the difference IR spectra (Figure S7) after the introduction of CO2 over ZnGa2O4/Ga2O3. This is consistent with the results of Knoezinger et al.37, Morterra et al.38-39, Busca et al.40, and Lavalley et al.41-42 who reported adsorption of CO2 on the surface by in situ FT-IR spectroscopy. Four bands between 1000 and 1800 cm−1 were derived from bicarbonate on Ga2O3. The bands at 1624 and 1228 cm−1 are assigned to asymmetric CO3 stretching [νas(CO3)] and OH deformation [δ(OH)], respectively. The symmetric CO3 stretching band [νs(CO3)] from 1350 to 1500 cm−1 will be discussed later.

Figure 9 Difference IR spectra of adsorbed CO2 species on ZnGa2O4 (3.0 mol%)/Ga2O3 under photoirradiation for (a) 0 h, (b) 0.5 h, and (c) 1 h.

On the other hand, the typical bands assigned to molecular CO2 adsorbed on surfaces were monitored between 2200 and 2600 cm−1. The band with peak located at 2342 cm−1 is slightly shifted in frequency from the standard vibration of molecular CO2 at 2349 cm−1. In fact, the IR spectra of adsorbed CO2 is not consistent with that of CO2 gas at the same partial pressure (Figure S8). Therefore, the CO2 molecules must interact with surface Lewis sites, as mentioned by Lavalley et al.41 Interestingly, the band at 2342 cm−1 was decreased by the photoirradiation and then the bands between 2200 and 2600 cm−1 became similar to those assigned to molecular CO2. These observations

Figure 9 shows selected difference IR spectra from 1350 to 1500 cm−1 of adsorbed CO2 species on ZnGa2O4/Ga2O3 after introducing CO2 under photoirradiation. Two typical bands were observed at 1460 and 1430 cm−1, and are respectively assigned to the symmetric CO3 stretching [νs(CO3)] of bidentate bicarbonate and monodentate bicarbonate species, by comparison to a Ga2O3 reference.4445 A new shoulder peak appeared at 1412 cm−1 in the case



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of ZnGa2O4/Ga2O3, although it was not observed on the surface of bare Ga2O3 (Figure S9). This band, which can be better understood after deconvolution, is derived from monodentate bicarbonate species anchored on the Zn atom; because ZnO produces a νs(CO3) band of monodentate bicarbonate species at the same position.41, 46-47 In short, there are three kinds of bicarbonate species on ZnGa2O4/Ga2O3 after introducing CO2 gas: (i) bidentate bicarbonate species anchored on Ga (b-HCO3-Ga), (ii) monodentate bicarbonate species anchored on Ga (mHCO3-Ga), and (iii) monodentate bicarbonate species anchored on Zn (m-HCO3-Zn). It is reasonable to assume that the bidentate bicarbonate species anchored on Zn (bHCO3-Zn) is also present on ZnGa2O4/Ga2O3. However, it could not be observed in the spectra, because the corresponding band is expected to be buried within the two bands at 1460 and 1430 cm−1. The behaviors of the three bands under photoirradiation are totally different from each other, as shown in Figure 9. The band at 1430 cm−1 (m-HCO3-Ga) became stronger with the photoirradiation time, in association with the decreasing intensity of the 1460 cm−1 band (b-HCO3-Ga), indicating that bidentate bicarbonates should be converted to monodentate bicarbonates on the Ga atom. On the other hand, the band at 1412 cm−1 (m-HCO3-Zn) was not changed by photoirradiation at all.

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bands at 1579, 1390, and 1356 cm−1 (asymmetric CO2 stretching [νas(CO2)], CH deformation [δ(CH)], and symmetric CO2 stretching [νs(CO2)] of bidentate formate (bHCOO-Ga), respectively) gradually appeared at the same time when the band of m-HCO3-Ga at 1430 cm−1 was reduced.36, 44-45 When D2 was used for exchange of the OH group, only the band assigned to δ(CH) was shifted, although the νas(CO2) and νs(CO2) bands remained in the range from 1200 to 1600 cm−1 (Figure S11). These bands completely disappeared after 180 h of photoirradiation. According to the time course of the integrated absorbance of the characteristic bands, both long-term (Figure 11) and in the early stages (inset), the amount of m-HCO3Ga increased while that of b-HCO3-Ga decreased, as mentioned above. When b-HCO3-Ga vanished, the amount of m-HCO3-Ga was also reduced. The amount of b-HCOOGa first increased and eventually declined, indicating that it acted as an intermediate species in the reaction.

Figure 11 Time course of integrated absorbance of bands at (a) 1430 cm−1 (m-HCO3-Ga, circle), 1460 cm−1 (b-HCO3-Ga, triangle), and 1579 cm−1 (b-HCOO-Ga, square) in the difference IR spectra of the adsorbed CO2 species on ZnGa2O4 (3.0 mol%)/Ga2O3. Inset: time course during 0–360 min.

In the above discussion, we monitored the decrement of formate during the reaction, as indicated in Figures 10 and 11. One obvious question here is: why did the band assigned to the reaction intermediate b-HCOO-Ga decrease after photoirradiation (Figure 11, green squares), even though the conversion of CO2 by H2O proceeded catalytically? This would mean that b-HCOO-Ga was not provided from m-HCO3-Ga continually. There are two reasons for this. (1) The absence of Ag cocatalyst. We have previously reported the importance of surface Ag cocatalyst on ZnGa2O4/Ga2O3 for obtaining a high rate of CO formation. 19-20 Without it, overall water splitting dominantly proceeds over ZnGa2O4/Ga2O3. Here, we chose to use unmodified ZnGa2O4/Ga2O3 in order to observe changes in the intermediate species (such as b-HCO3-Ga, m-HCO3-Ga, and b-HCOO-Ga). These intermediate species will persist in the presence of Ag cocatalyst, which indicates that formate is generated as the intermediate species stably, and the reaction proceeds catalytically. Further investigations are needed to better understand this. (2) It is well-known that bicarbonate on the surface easily desorbs into the gas phase in the presence of H2O.

Figure 10 Difference IR spectra of adsorbed CO2 species on ZnGa2O4 (3.0 mol%)/Ga2O3 under photoirradiation for (a) 1 h, (b) 3 h, (c) 6 h, (d) 12 h, (e) 56 h, (f) 152 h, and (g) 180 h.

As the photoirradiation continues, the band at 1430 cm−1 (m-HCO3-Ga) decreased, shifted to a much lower wavenumber, and then vanished after 56 h, as shown in Figure 10. The same behavior was observed in the case of Ga2O3 (Figure S10). However, the band of Ga2O3 decreased with photoirradiation time at a slower rate than that of ZnGa2O4/Ga2O3. The band at 1228 cm−1, assigned to the deformation of bicarbonate [δ(OH)], also became weaker with photoirradiation time, meaning that m-HCO3-Ga was converted into another species. Note that the new


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The Journal of Physical Chemistry that 13CO was initially generated as the main product. However, the formation rate of 12CO steadily rose and surged past that of 13CO after 90 min. Finally, 13CO vanished and only 12CO was produced. This result strongly suggests that the formation rate of CO (12CO + 13CO) depends not on the concentration of HCO3− but on that of CO2(aq). If HCO3− acts as an intermediate species, the highest formation rate of 13CO would be achieved as soon as the lamp was turned on. In fact, we have observed that H2 and O2 derived from the overall water splitting were generated in aqueous NaHCO3 solution, together with a tiny amount of CO without the bubbling of CO2 gas.19 At the initial stage where a large fraction of CO2 gas molecules was not dissolved the solution, 13CO2(aq) was produced from NaH13CO3 on the basis of the equilibrium in eqn. (10), causing 13CO to be preferentially evolved. With the continuous bubbling of 12CO2, the concentration of CO2(aq) reached equilibrium with that of HCO3− after 60 min. At this point, the formation rate of 13CO decreased while that of 12CO increased. Finally, only 12CO was generated as the main product as 12CO2 was continuously supplied from the gas to the liquid phase. On the other hand, CO2 in the gas phase displayed the same tendency in isotope composition as CO evolved. The concentration of 13CO increased until 60 min and then tapered off, while 2 that of 12CO2 gradually increased with photoirradiation time. In this study, 12CO2 was flowed at the rate of 30 ml min−1, which is approximately 1.3 × 103 µmol min−1, a value consistent with the rate of CO2 detected by GC and the total rate of 12CO2 and 13CO2 estimated by MS. In addition, the integrated amount of 13CO2 from 0 to 600 min was 0.093 mol, which is very similar to the initial amount of NaH13CO3 (0.1 M in 1.0 L of aqueous solution). Therefore, the 13C-labeled species in the solution, derived from NaH13CO3, was gradually released into the gas phase as 13CO . Our results indicate that this reaction is in a state 2 of dynamic equilibrium, with the rapid interconversion of CO2-related molecules (e.g., CO2(aq), H2CO3, HCO3−, and CO32−) in the solution. When CO2 was bubbled in the dark for 120 min, and then the suspension was illuminated, the steady formation rate of CO was initially monitored and the formation rates of 12CO and 13CO depended on the concentrations of 12CO2 and 13CO2 in the gas phase, respectively (Figure S12). The time course completely agrees with that in Figure 12. Therefore, the ratio of evolved 13CO/12CO is similar to that of 13CO (aq)/12CO (aq). Alt2 2 hough we could not rule out the possibility that CO2(aq) and HCO3− work cooperatively as the intermediate species, it is clear that CO2(aq) is reduced to CO in the photocatalytic conversion of CO2, when using H2O as the electron donor in the aqueous solution of NaHCO3 with Ag/ZnGa2O4/Ga2O3.

Thus, at the beginning of the photoirradiation, a part of bicarbonate could be converted into formate as the real intermediate species. Afterwards, a large amount of bicarbonate would be desorbed due to the formation of H2O under photoirradiation (as discussed earlier). Hence the amount of b-HCOO-Ga is lowered later.

Figure 12 Time course of (A) 13CO (closed circle) and 12CO (closed triangle) determined by MS, CO (closed square) determined by FID-GC, and pH value (open diamond) in the solution; and (B) 13CO2 (open circle) and 12CO2 (open triangle) determined by MS, CO2 (open square) determined by FID-GC, and pH value (cross) in the solution for the photocatalytic conversion of CO2 by H2O. Catalyst: Ag (1.0 wt%)/ZnGa2O4 (3.0 mol%)/Ga2O3 (1.0 g), solution: H2O (1.0 L), buffer: 0.10 M NaH13CO3, flow rate of CO2: 30 ml min−1.

As mentioned earlier, we have four candidates of intermediate species in the aqueous solution of NaHCO3 for the photocatalytic conversion of CO2 by H2O as the electron donor. They are: hydrated CO2 molecule (CO2(aq)), carbonic acid (H2CO3), bicarbonate (HCO3−), and carbonate ions (CO32−). In the earlier discussion, we suppose that CO2(aq) is an intermediate species for the reaction. Thus, bicarbonate species derived from CO2(aq) is generated on the surface of ZnGa2O4/Ga2O3, and then reduced into CO via formate upon photoirradiation. For a more concrete discussion of the intermediate species, we carried out the photocatalytic conversion of CO2 by H2O using isotopic NaH13CO3. Figure 12 represents the time course of carbon-containing species detected by MS (12CO, 13CO, 12CO2, and 13CO2) and GC (CO and CO2). The amount of CO estimated by GC is consistent with the total amount of 12CO and 13CO determined by MS, the same for CO2. The CO formation rate steadily increased with decreasing solution pH and became constant after 60 min, because CO2 gas gradually dissolved in the solution and increased the concentration of CO2(aq). Note

Under the current circumstances, we propose the formation mechanism of several intermediate species as shown in Scheme 1. Dissolved CO2 molecules (CO2(aq)) react with the hydroxyl group anchored with Ga atom on the surface of ZnGa2O4/Ga2O3 to form b-HCO3-Ga. bHCO3-Ga is further converted to b-HCOO-Ga via mHCO3-Ga. Finally, b-HCOO-Ga is decomposed into CO.



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Interestingly, photoirradiation is not exactly necessary in this process, except for the formation of b-HCO3-Ga. The following charge transfer takes place during the conversion of m-HCO3-Ga to b-HCOO-Ga: − HCO3− + 2H + + 2e − ⇌ HCOO + H 2 O

the conversion of CO2 and selectivity toward CO evolution, it is very important to adjust the pH to ensure a sufficient quantity of CO2(aq) in the solution, and to produce suitable CO2 adsorption sites such as the hydroxyl group, which functions as a Brønsted solid base.

E 0 = −0.41　 eV (23)

Conclusion CO2 dissolved in aqueous solution of NaHCO3 is determined to be the intermediate species for the photocatalytic conversion of CO2 by H2O as the electron donor. At the typical pH values in this study, H2CO3 and CO32− are present only at very low concentrations in the solution. HCO3− derived from NaHCO3 does not contribute to the reduction of CO2, but functions as a buffer to enable the dissolution of much more CO2 into the solution, even though the concentration of HCO3− is higher than that of the CO2 molecule. The conversion of HCO3− into dissolved CO2 does not occur in equilibrium, due to the extremely low equilibrium constant (K1) and the continuous feeding of CO2 from the gas phase. The dissolved CO2 molecules bind to the hydroxyl groups on the surface of ZnGa2O4/Ga2O3 to form several bicarbonate species. Among them, the monodentate bicarbonate is reduced by the electrons generated under photoirradiation into a bidentate formate intermediate via the bidentate bicarbonate. The main product CO is evolved from the introduced CO2 gas through the decomposition of formate.

Scheme 1 Proposed mechanism of the formation of intermediate species in the photocatalytic conversion of CO2 by H2O as the electron donor over ZnGa2O4/Ga2O3. On the other hand, the other two processes, namely the transformation of b-HCO3-Ga to m-HCO3-Ga and the decomposition of b-HCOO-Ga, do not involve any generated electrons. The former is either a kind of light switching of molecules on the surface or photoabsorption. We have already reported that b-HCO3-Ga is converted to mHCO3-Ga when the pressure of introduced CO2 is increased.36 The reaction is irreversible, so there is no conversion from m-HCO3-Ga back to b-HCO3-Ga. This means that the transformation from the quasi-stable b-HCO3-Ga to the more stable m-HCO3-Ga occurs by fractional driving force, such as the pressure and photoirradiation. The latter is the so-called photolysis. It is well known that formate is an intermediate species in the water gas-shift (WGS) reaction:

CO + H 2O ⇌ CO 2 + H 2

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ASSOCIATED CONTENT Supporting Information. Bjerrum plot, XRD patterns, UVVis. DRS, XP spectra, several IR spectra, and time course of 12CO and 13CO The following file is available free of charge on the ACS Publications website at DOI:

AUTHOR INFORMATION Corresponding Author * Profs. Kentaro Teramura ([email protected]) and Tsunehiro Tanaka (tanakat@ moleng.kyoto-u.ac.jp)

(24)

Generally, the equilibrium in eqn. (24) strongly leans towards the reactant side, meaning that formate is easily decomposed into CO and H2O upon heating (the socalled backward decomposition of formate intermediate in WGS). Hirose et al.48 found that the decomposition of formate on NiO(111) film grown on Ni(111) surface can produce both H2 and CO2 via dehydrogenation, and CO and H2O by dehydration. In our case, b-HCOO-Ga, which is a formate intermediate on the surface of ZnGa2O4/Ga2O3, should decompose into the product CO to leave Ga-OH on the surface. Calatayud et al.49 reported that very stable formate species are generated by CO insertion into the surface hydroxyl groups on a wide range of hydroxylated metal oxides (including Ga2O3) at high temperatures. Therefore, the backward reaction (formate decomposition) can easily take place on Ga2O3, as suggested in our proposed reaction mechanism. To enhance

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript

ACKNOWLEDGMENT This study was partially supported by a Grant-in-Aid for Scientific Research(B) (KAKENHI Grant Number 15H04187) of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan; a Grant-in-Aid for Scientific Research on Innovative Areas, “All Nippon Artificial Photosynthesis Project for Living Earth” (No. 2406) of the MEXT of Japan; the Precursory Research for Embryonic Science and Technology (PRESTO) program, supported by the Japan Science and Technology Agency


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(JST); and the Program for Element Strategy Initiative for Catalysts & Batteries (ESICB), commissioned by the MEXT of Japan.

16. Iizuka, K.; Wato, T.; Miseki, Y.; Saito, K.; Kudo, A., Photocatalytic Reduction of Carbon Dioxide over Ag Cocatalyst-Loaded ALa4Ti4O15 (a = Ca, Sr, and Ba) Using Water as a Reducing Reagent. J. Am. Chem. Soc. 2011, 133, 20863-20868. 17. Takayama, T.; Tanabe, K.; Saito, K.; Iwase, A.; Kudo, A., The KCaSrTa5O15 Photocatalyst with Tungsten Bronze Structure for Water Splitting and CO2 Reduction. Phys. Chem. Chem. Phys. 2014, 16, 24417-24422. 18. Takayama, T.; Iwase, A.; Kudo, A., Photocatalytic Water Splitting and CO2 Reduction over KCaSrTa5O15 Nanorod Prepared by a Polymerized Complex Method. Bull. Chem. Soc. Jpn. 2015, 88, 538-543. 19. Teramura, K.; Wang, Z.; Hosokawa, S.; Sakata, Y.; Tanaka, T., A Doping Technique That Suppresses Undesirable H2 Evolution Derived from Overall Water Splitting in the Highly Selective Photocatalytic Conversion of CO2 in and by Water. Chem. - Eur. J. 2014, 20, 9906-9909. 20. Wang, Z.; Teramura, K.; Huang, Z.; Hosokawa, S.; Sakata, Y.; Tanaka, T., Tuning the Selectivity toward CO Evolution in the Photocatalytic Conversion of CO2 with H2O through the Modification of Ag-Loaded Ga2o3 with a Znga2o4 Layer. Catal. Sci. Technol. 2016, 6, 1025-1032. 21. Wang, Z.; Teramura, K.; Hosokawa, S.; Tanaka, T., Photocatalytic Conversion of CO2 in Water over Ag-Modified La2ti2o7. Appl. Catal., B 2015, 163, 241-247. 22. Teramura, K.; Tatsumi, H.; Wang, Z.; Hosokawa, S.; Tanaka, T., Photocatalytic Conversion of CO2 by H2O over Ag-Loaded Sro-Modified Ta2o5. Bull. Chem. Soc. Jpn. 2015, 88, 431-437. 23. Wang, Z.; Teramura, K.; Hosokawa, S.; Tanaka, T., Highly Efficient Photocatalytic Conversion of CO2 into Solid CO Using H2O as a Reductant over Ag-Modified Znga2o4. J. Mater. Chem. A 2015, 3, 11313-11319. 24. Iguchi, S.; Teramura, K.; Hosokawa, S.; Tanaka, T., A Znta2o6 Photocatalyst Synthesized Via Solid State Reaction for Conversion of CO2 into CO in Water. Catal. Sci. Technol. 2016, 6, 4978-4985. 25. Huang, Z.; Teramura, K.; Hosokawa, S.; Tanaka, T., Fabrication of Well-Shaped Sr2KTa5O15 Nanorods with a Tetragonal Tungsten Bronze Structure by a Flux Method for Artificial Photosynthesis. Appl. Catal., B 2016, 199, 272-281. 26. Wilhelm, E.; Battino, R.; Wilcock, R. J., LowPressure Solubility of Gases in Liquid Water. Chem. Rev. 1977, 77, 219-62. 27. Plummer, L. N.; Busenberg, E., The Solubilities of Calcite, Aragonite, and Vaterite in Carbon Dioxide-Water Solutions between 0 and 90°C, and an Evaluation of the Aqueous Model for the System Calcium Carbonate-Carbon Dioxide-Water. Geochim. Cosmochim. Acta 1982, 46, 1011-40. 28. Jensen, T. R.; Malinsky, M. D.; Haynes, C. L.; van Duyne, R. P., Nanosphere Lithography: Tunable Localized Surface Plasmon Resonance Spectra of Silver Nanoparticles. J. Phys. Chem. B 2000, 104, 10549-10556. 29. Malinsky, M. D.; Kelly, K. L.; Schatz, G. C.; Van Duyne, R. P., Chain Length Dependence and Sensing Capabilities of the Localized Surface Plasmon Resonance of Silver Nanoparticles Chemically Modified with Alkanethiol SelfAssembled Monolayers. J. Am. Chem. Soc. 2001, 123, 1471-1482. 30. Wiley, B. J.; Im, S. H.; Li, Z.-Y.; McLellan, J.; Siekkinen, A.; Xia, Y., Maneuvering the Surface Plasmon Resonance of Silver Nanostructures through Shape-Controlled Synthesis. J. Phys. Chem. B 2006, 110, 15666-15675.

REFERENCES 1. Morris, A. J.; Meyer, G. J.; Fujita, E., Molecular Approaches to the Photocatalytic Reduction of Carbon Dioxide for Solar Fuels. Acc. Chem. Res. 2009, 42, 1983-1994. 2. Teramura, K.; Tanaka, T., Photocatalytic Reduction of CO2 Using H2 as Reductant over Solid Base Photocatalysts. ACS Symp. Ser. 2010, 1056, 15-24. 3. Kobayashi, K.; Tanaka, K., Approach to MultiElectron Reduction Beyond Two-Electron Reduction of CO2. Phys. Chem. Chem. Phys. 2014, 16, 2240-2250. 4. Zhao, J.; Wang, X.; Xu, Z.; Loo, J. S. C., Hybrid Catalysts for Photoelectrochemical Reduction of Carbon Dioxide: A Prospective Review on Semiconductor/Metal Complex CoCatalyst Systems. J. Mater. Chem. A 2014, 2, 15228-15233. 5. Sahara, G.; Ishitani, O., Efficient Photocatalysts for CO2 Reduction. Inorg. Chem. 2015, 54, 5096-5104. 6. Halmann, M., Photoelectrochemical Reduction of Aqueous Carbon Dioxide on p-Type Gallium Phosphide in Liquid Junction Solar Cells. Nature (London, United Kingdom) 1978, 275, 115-16. 7. Inoue, T.; Fujishima, A.; Konishi, S.; Honda, K., Photoelectrocatalytic Reduction of Carbon Dioxide in Aqueous Suspensions of Semiconductor Powders. Nature (London, United Kingdom) 1979, 277, 637-8. 8. Dhakshinamoorthy, A.; Navalon, S.; Corma, A.; Garcia, H., Photocatalytic CO2 Reduction by Tio2 and Related Titanium Containing Solids. Energy Environ. Sci. 2012, 5, 9217-9233. 9. Navalon, S.; Dhakshinamoorthy, A.; Alvaro, M.; Garcia, H., Photocatalytic CO2 Reduction Using NonTitanium Metal Oxides and Sulfides. ChemSusChem 2013, 6, 562-577. 10. Mao, J.; Li, K.; Peng, T., Recent Advances in the Photocatalytic CO2 Reduction over Semiconductors. Catal. Sci. Technol. 2013, 3, 2481-2498. 11. Das, S.; Wan Daud, W. M. A., A Review on Advances in Photocatalysts Towards CO2 Conversion. RSC Adv. 2014, 4, 20856-20893. 12. Tu, W.; Zhou, Y.; Zou, Z., Photocatalytic Conversion of CO2 into Renewable Hydrocarbon Fuels: State-of-the-Art Accomplishment, Challenges, and Prospects. Adv. Mater. (Weinheim, Ger.) 2014, 26, 4607-4626. 13. Xie, S.; Zhang, Q.; Liu, G.; Wang, Y., Photocatalytic and Photoelectrocatalytic Reduction of CO2 Using Heterogeneous Catalysts with Controlled Nanostructures. Chem. Commun. (Cambridge, U. K.) 2016, 52, 35-59. 14. Yamamoto, M.; Yoshida, T.; Yamamoto, N.; Nomoto, T.; Yamamoto, Y.; Yagi, S.; Yoshida, H., Photocatalytic Reduction of CO2 with Water Promoted by Ag Clusters in Ag/Ga2O3 Photocatalysts. J. Mater. Chem. A 2015, 3, 1681016816. 15. Lehn, J. M.; Ziessel, R., Photochemical Generation of Carbon Monoxide and Hydrogen by Reduction of Carbon Dioxide and Water under Visible Light Irradiation [Artificial Photosynthesis/Solar-Energy Conversion/Metal Complex Catalysis/Tris(2,2'-Bipyridine)Ruthenium(II)/Cobalt(II) System]. Proc. Natl. Acad. Sci. U. S. A. 1982, 79, 701-4.



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

31. Iguchi, S.; Teramura, K.; Hosokawa, S.; Tanaka, T., Effect of the Chloride Ion as a Hole Scavenger on the Photocatalytic Conversion of CO2 in an Aqueous Solution over NiAl Layered Double Hydroxides. Phys. Chem. Chem. Phys. 2015, 17, 17995-18003. 32. Iguchi, S.; Hasegawa, Y.; Teramura, K.; Hosokawa, S.; Tanaka, T., Preparation of Transition Metal-Containing Layered Double Hydroxides and Application to the Photocatalytic Conversion of CO2 in Water. Journal of CO2 Utilization 2016, 15, 6-14. 33. Kohno, Y.; Tanaka, T.; Funabiki, T.; Yoshida, S., Identification and Reactivity of a Surface Intermediate in the Photoreduction of CO2 with H2 over ZrO2. J. Chem. Soc., Faraday Trans. 1998, 94, 1875-1880. 34. Kohno, Y.; Ishikawa, H.; Tanaka, T.; Funabiki, T.; Yoshida, S., Photoreduction of Carbon Dioxide by Hydrogen over Magnesium Oxide. Phys. Chem. Chem. Phys. 2001, 3, 1108-1113. 35. Teramura, K.; Tanaka, T.; Ishikawa, H.; Kohno, Y.; Funabiki, T., Photocatalytic Reduction of CO2 to CO in the Presence of H2 or CH4 as a Reductant over MgO. J. Phys. Chem. B 2004, 108, 346-354. 36. Tsuneoka, H.; Teramura, K.; Shishido, T.; Tanaka, T., Adsorbed Species of CO2 and H2 on Ga2O3 for the Photocatalytic Reduction of CO2. J. Phys. Chem. C 2010, 114, 8892-8898. 37. Knoezinger, H., Specific Poisoning and Characterization of Catalytically Active Oxide Surfaces. Advances in Catalysis 1976, 25, 184-271. 38. Morterra, C.; Zecchina, A.; Coluccia, S.; Chiorino, A., Ir Spectroscopic Study of Carbon Dioxide Adsorption onto Η-Alumina. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases 1977, 73, 1544-60. 39. Morterra, C.; Magnacca, G.; Cerrato, G.; Del Favero, N.; Filippi, F.; Folonari, C. V., Surface Characterization of Modified Aluminas. 2. X-Ray Diffraction, High-Resolution Transmission Electron Microscopy and Fourier-Transform Infrared Study of Calcium Doped Alumina. J. Chem. Soc., Faraday Trans. 1993, 89, 135-50. 40. Busca, G.; Saussey, H.; Saur, O.; Lavalley, J. C.; Lorenzelli, V., Ft-Ir Characterization of the Surface Acidity of Different Titanium Dioxide Anatase Preparations. Applied Catalysis 1985, 14, 245-60. 41. Saussey, J.; Lavalley, J. C.; Bovet, C., Infrared Study of Carbon Dioxide Adsorption on Zinc Oxide. Adsorption Sites. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases 1982, 78, 1457-63. 42. Lamotte, J.; Lavalley, J. C.; Druet, E.; Freund, E., Infrared Study of Acid-Base Properties of Thorium Dioxide. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases 1983, 79, 2219-27. 43. Ballinger, T. H.; Yates, J. T., Jr., Ir Spectroscopic Detection of Lewis Acid Sites on Alumina Using Adsorbed Carbon Monoxide. Correlation with Aluminum-Hydroxyl Group Removal. Langmuir 1991, 7, 3041-5. 44. Collins, S. E.; Baltanas, M. A.; Bonivardi, A. L., An Infrared Study of the Intermediates of Methanol Synthesis from Carbon Dioxide over Pd/β-Ga2O3. J. Catal. 2004, 226, 410-421. 45. Collins, S. E.; Baltanas, M. A.; Bonivardi, A. L., Infrared Spectroscopic Study of the Carbon Dioxide Adsorption on the Surface of Ga2O3 Polymorphs. J. Phys. Chem. B 2006, 110, 5498-5507.

Page 12 of 13

46. Taylor, J. H.; Amberg, C. H., Infrared Spectra of Gases Chemisorbed on Zinc Oxide. I. CO and CO2. Can. J. Chem. 1961, 39, 535-9. 47. Boehm, H. P., et al., General Discussion. Discussions of the Faraday Society 1971, 52, 44-54. 48. Bandara, A.; Kubota, J.; Wada, A.; Domen, K.; Hirose, C., Adsorption and Reactions of Formic Acid on (2×2)NiO(111)/Ni(111) Surface: Part 2. Iras Study under Catalytic Steady-State Conditions. J. Phys. Chem. B 1997, 101, 361-368. 49. Calatayud, M.; Collins, S. E.; Baltanas, M. A.; Bonivardi, A. L., Stability of Formate Species on β-Ga2O3. Phys. Chem. Chem. Phys. 2009, 11, 1397-1405.


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Which is an Intermediate Species for Photocatalytic Conversion of

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