Converting of CO2 into low-molecular-weight organic compounds with the TiO2/ZrO2 composites under solar irradiation

The preparation of a specially modified titanium dioxide/zirconium oxide (TiO2/ZrO2) composite and its subsequent application using a unique method are described. Specifically, after the whole surface of the composite was covered with a very thin layer of water, solar light was irradiated onto it. This method is unique because the reduction of CO2 was performed in air (gas phase). The light source was real solar light. In this study, novel composites comprising nanometre-sized TiO2 and micrometre-sized zirconium oxide (ZrO2) increased the amount of reduced CO2. And, suitable weight ratio of TiO2/ZrO2 was 6/4-5/5. Thin water layer on the composite offered catalytic-reaction medium, and, catalytic-reaction cite existed at interface of TiO2 and ZrO2 particles, and, this reaction was catalytic reaction enhanced by photocatalytic effect. A large amount of reduced products (maximum: approximately 300 μmol/(g·300 s) of formaldehyde and methanol)was obtained under only 300 s of irradiation of solar light.

. Moreover, they demonstrated that H 2 and O 2 were generated by the splitting of H 2 O added. Author also started this study to obtain desirable result by the photocatalytic reaction.
In this report, following items were studied.
First: Ascertaining whether the thin water layer on the surface of composites is essential. Second: Consideration of preparing method (Effect of press on preparing method of composites) Third: Dependence of quantity of reduced products on TiO 2 /ZrO 2 ratios. (TiO 2 /ZrO 2 ratios were changed between 10/0 and 0/10). Forth: Measurements of quantity of reduced products under irradiation of real solar light (at TiO 2 /ZrO 2 = 1/1). Fifth: Proving the water layer on the surface of composites. Sixth: Discussion regarding with macro mechanism (hypothesis) through which the large quantity reduced product can be obtained.

Experiments
The author devised composites consisting of nanometre-sized TiO 2 particles and micrometre-sized zirconium oxide (ZrO 2 ) particles and applied them using an original method. First, particles or molecules adsorbed onto the composite surface were removed by the elimination of static electricity using an ion blower. Next, after the composites were cooled in a refrigerator for more than 20 h, they were placed into a transparent gas-barrier plastic bag with high-temperature and high-humidity room air. Cooling in a refrigerator enabled the whole surface of the composite to be covered with a very thin layer of water via the condensation of water vapour in the air to reproduce Sato's condition (wet condition). The reduction of CO 2 did not occur until the thin water layer condensed onto the composite's surface. This method is unique because the reduction of CO 2 is performed in air (or more precisely, in the air phase via a very thin water layer). Moreover, this method increases the efficiency of the light irradiation because real solar light is directly irradiated onto the composites.
In this study, the TiO 2 /ZrO 2 composites greatly increased the amount of reduced CO 2 product. The composite exhibited a particulate morphology. In this report, CO 2 was reduced under real solar light without the use of platinum as a co-catalyst.
The new photo-catalyst composites are composed of two types of inorganic compounds (a,b) that were combined and pressed together. The first inorganic compound (a) is characterized by nanometre-sized anatase TiO 2 photo-catalyst particles, and the other inorganic compound (b) is composed of micrometre-sized ZrO 2 particles. The weight ratio (TiO 2 /ZrO 2 ) of the composites is mainly 1:1. The use of such a large amount of additive (ZrO 2 ) is unique. After (a) and (b) were pressed to form the composites (c), the composites (c) were scattered onto an electric conducting material such as a copper plate. The copper plate can facilitate the transfer of electrons and holes from one composite to the other. Then, the composites and Cu plate were held in a glass laboratory dish (the glass laboratory dish containing the composites and Cu plate is hereafter referred to as the test unit). Figure 1 shows SEM image of the composite with water vapour condensed on its surface and with the thin water layer evaporated away (SEM: JAPAN ELECTRONIC Co. Ltd., JSM-5310LV). Image "a" shows that numerous nanometre-sized TiO 2 particles are present on the top layer of the composite, and the core of the composite is composed of micrometre-sized ZrO 2 particle. Image "b" shows a higher-magnification (20,000×) SEM image of the surface of the composites shown in image "a" of Fig. 1. Pores of various sizes are present in the nanometre-sized TiO 2 layer. The CO 2 in the air can penetrate the pores and reach the water layer existing at the surface of the ZrO 2 particles. Figure 2 shows a photograph of the experiment under irradiation by real solar light. Experiments were performed during the Japanese summer season, which provides high temperatures and high humidity. During irradiation, the test unit was placed into a transparent gas-barrier plastic bag with 1000 ml of air. The bag transfers solar light and contains the produced organic compounds. The bag allows both the concentration of organic compounds and the inner volume to be measured after solar-light irradiation. After irradiation, the concentration of formaldehyde and methanol in the bag was measured using gas-detecting tubes.
A gas-barrier plastic bag through which CO 2 molecules can penetrate but not oxygen (O 2 ) or nitrogen molecules (N 2 ) was used. When the amount of CO 2 in the bag is decreased by the reduction of CO 2 , the CO 2 in the air outside of the bag diffuses into the bag because of the concentration gradient. Therefore, the concentration (in ppm) of the total reduced products exceeding the CO 2 concentration in the atmosphere (approximately 400 ppm) is not contradictory.
The experiments were conducted in Funabashi in the Chiba prefecture of Japan (lat. 35° 70′ N and long. 140° 02′ E).
Additionally, the author performed several weight measurements 5-8 min after removing the test unit from the refrigerator. The weight of the test unit increased by 70-110 mg because of the condensation of water vapour (ambient air conditions: 30.9 °C in temperature and 65% in humidity −30.0 °C in temperature and 90% in humidity).

Results
In the subsequent description, the term "dry conditions" refers to experiments in which the test unit was not cooled in a refrigerator, whereas the term "wet conditions" refers to experiments in which the test unit was cooled in a refrigerator, resulting in the formation of a thin water layer on the composites. The term "the dry conditions" refers to condition that the formation of a thin water layer on the composites was not generated. Table 1 show the experimental conditions and Table 2 show the experimental results of experimental No. 1-7. And, Table 3 show the experimental conditions and Table 4 show the experimental results of experimental No. 8-25. Hence, Tables 1 and 2 make a pair, and, Tables 3 and 4 also make a pair.
(a) Group A of Tables 1 and 2 • The results in Group A of Tables 1 and 2 show that no reduced product was obtained under dry conditions. By contrast, reduced products were obtained under wet conditions. Therefore, wet conditions are essential. Moreover, Group A shows the following results • Under dry conditions with TiO 2 only, no products reduced by the photo-catalyst were detected (Tables 1  and 2, No. 1). That is, reduction driving force of TiO 2 photo-catalyst is not enough to reduce the CO 2 . • Under wet conditions with TiO 2 only, reduced products were probably obtained by a catalytic-reaction (Tables 1 and 2, No. 2). • Under dark (no light irradiation) and wet conditions, the TiO 2 /ZrO 2 composite produced methanol, which is clearly a catalytic-reaction (Tables 1 and 2 Tables 1 and 2 On the amount of reduction products, very large deference was showed between No. 6 and No. 7. No. 6 was prepared with strong press during mixing, whereas, No. 7 was prepared with no press (only mixing). On the surface of composites with press during mixing, large amount of interface between TiO 2 particles and ZrO 2 ones are formed, whereas, in the case of no press, the interface exist at only contact point between TiO 2 particles and ZrO 2 ones. Therefore, possible explanation is that reaction cites exist at the interface. And, this result supports that reaction of this research is catalytic one. Incidentally, standard method of this report is strong press during mixing. (c) Dependence of quantity of reduced products on TiO 2 /ZrO 2 ratios Figure 3 shows plots of formaldehyde and methanol products (μmol/(g·300 s)) against ZrO 2 weight % in the TiO 2 /ZrO 2 composite that correspond to the results in Group C of Tables 3 and 4. Figure 3 shows volcano plots, and, peak was at TiO 2 /ZrO 2 = 6/4−5/5. In left side of the peak, reduced products increased with increasing of ZrO 2 , and, in right side of the peak, decreased with decreasing of TiO 2 . This shows that activating effect of ZrO 2 increased in left side of the peak, by contrast, photocatalytic effort of TiO 2 decreased in right side of the peak. (Band gap of ZrO 2 is 5.0 eV, whereas, maximum energy of solar light is equal to 4.1 eV (1240/300 nm in wavelength). Band gap of ZrO 2 is larger than maximum energy of solar light. Therefore, photocatalytic effect of ZrO 2 does not occur). Moreover, interface of TiO 2 particle and ZrO 2 one increases with increasing of ZrO 2 . Increasing of interface increased the reaction products. This result agrees with the description of b) in this section (d) Group D of Tables 3 and 4 Group D of Tables 3 and 4 show the results obtained when the composites were used. The results indicate that CO 2 was reduced under real solar light and that formaldehyde and methanol were obtained in every experiment. A large amount of reduced products (maximum: approximately 300 μmol/(g·300 s) of formaldehyde and methanol)was obtained. These reaction also is catalytic one at the interface of TiO 2 particle and ZrO 2 one, because composites were prepared with author's standard method (mix and press). And, Turnover number (reduction products (mole/h)/catalyst (mole)) was equal to 1.7. Figure 4 shows plots of formaldehyde and methanol products (μmol/(g·300 s)) as a function of the irradiation intensity of solar light (mW/cm 2 ). These plots reveal that the formaldehyde yield increased rapidly  Table 2. Experimental results † . † Samples of 0.2 g were irradiated with only real solar light. And, sign "−" shows that color change of gas-detecting-tube was not detected (reduced product is equal to almost 0). ‡ There is inaccuracy from 30 ml to +10 ml.
with increasing irradiation energy of solar light. By contrast, the amount of methanol gradually increased with increasing irradiation intensity of solar light. This shows that CO 2 reducing reaction was enhanced strongly by the photocatalytic effort. (e) Evidence of the generation of the thin water layer In this report, thin water layer on the surface of composites is most important. As this water layer is very thin, transparent, liquid and not-flat, visualization of water layer was difficult. Thus, below mentioned considerations were performed. • As described at the end part of the Experiments section, the weight of the test unit increased 70-110 mg, 5-8 min after being removed from the refrigerator. This weight gain certainly includes water-vapour condensation on the surfaces of the composites, the Cu plate and the glass laboratory dish. However, this weight gain also provides indirect evidence of the formation of a thin condensed water layer on the composites because the surface area of the composites is markedly larger than the surface areas of the other materials; i.e., this weight gain would be dominated by the weight gain on the composite surface. • Electric conductivities of two kinds of composites were measured under dry and wet conditions. The composites were TiO 2 /ZrO 2 /NaCl (weight ratio) = 4.5/4.5/1 and 4/4/2. In the case of wet condition, each composite (0.2 g) was scattered in glass laboratory dish, held in the refrigerator for 20-24 h. They were measured in atmosphere 5 min after being removed from refrigerator.
Atmosphere conditions were 26-28 °C in temperature and 73-77% in humidity, respectively. They were measured by HORIBA COMPACT CONDUCTIVITY METER B-771.
The measuring results were In contrast to these, in the dry condition, no conductivities of two kinds of composites were detected (electric conductivities were 0 μS).
Therefore, in the case of wet condition, surface of composites were covered with continuous water layer by condensation of water vapour, NaCl dissolved and dissociated in the water layer. Then, the conductivity was generated. This result shows existence of the water layer on the surface of composites.
• If the water layer is not formed, the TiO 2 /ZrO 2 composite's surface is dry. As dry TiO 2 photo-catalyst particles exhibit only strong oxidation activity, no reduced products are produced under UV light irradiation.
(Author dispose gas in the bag after the formaldehyde and methanol were returned to CO 2 by the oxidation effect of dry TiO 2 particles.) But large amount of reduced products actually generated. This also evidences the existing of water layer on the surface of the composites by the condensation of water vapour.  Table 3. Experimental conditions.
(f) The role of ZrO 2 ZrO 2 have affinity to both acid and base 21 . Further, ZrO 2 strongly support for hydrogenation of CO and CO 2 as a carrier [22][23][24][25] . Therefore, ZrO 2 provably have moderate adsorption force with carbonyl group. And, ZrO 2 probably adsorbs carbonyl group on the surface physically and transport adsorbing species to the interface of TiO 2 particle and ZrO 2 one.
It was shown from Fig. 3 that adding of ZrO 2 largely increased the reduced products. From these results, the use of ZrO 2 was essential for obtaining large amounts of reduced products.  Table 4. Experimental results † . † Samples of 0.2 g were irradiated with only real solar light. And, sign "−" shows that color change of gas-detecting-tube was not detected (reduced product is equal to almost 0). ‡ There is inaccuracy from 30 ml to +10 ml. The measuring results were. Discussion CO 2 molecules dissolve in the thin layer of water on the composites. Part of the dissolved CO 2 in the thin water layer generates carbonic acid 26 (equation (1)): The equilibrium constant (K h ) for this reaction is small (1.7 × 10 −3 at 25 °C). Furthermore, H 2 CO 3 dissociates, as described by equations (2)  where pK* a1 and pK a2 are the apparent acid-dissociation constant and acid-dissociation constant, respectively. The reaction formulas that include CO 3 2− , HCHO and CH 3 OH are shown as equations (4) and (5); the standard electrode potentials E 0 are also shown for these equations 29 The standard electrode potential E 0 is related to the standard Gibbs energy transition by following equation: In equation (6), E 0 is positive; therefore, ∆G 0 is negative because nF is positive. As ∆G 0 is negative, the reactions in equations (4) and (5) proceed towards the right to achieve equilibrium if the energy exceeds the activation energy. Catalysts are known to remarkably decrease the activation energy. Thus, equations (4) and (5) through which carbonic ions are reduced are not photocatalytic but catalytic reaction. CO 3 2− is converted into HCHO and CH 3 OH; then, HCHO and CH 3 OH volatilize from the thin water layer. As the equilibrium (equations (4) and (5)) is disrupted, equations (4) and (5) proceed towards the right to maintain equilibrium, thereby decreasing the concentration of CO 3 2− . To maintain the equilibria of equations (1), (2) and (3), CO 2 dissolves and dissociates and new CO 3 2− is generated. The generation of HCHO and CH 3 OH therefore continues.
These reactions together form a chain reaction, and the reaction from volatilization to the next volatilization is cyclic. Moreover, the thin water layer accelerates this cycle by the facile volatilization of HCHO and CH 3 OH. After the cycle completes, the next cycle continues until the water layer disappears. A large amount of reduced products are obtained. In equation (3) 30 These results show a relation of formaldehyde > methanol > water. This result (formaldehyde > ethanol) was in agreement with order of the quantity of reduced products (formaldehyde > ethanol).
The solar light that reaches ground is greater than 300 nm in wavelength. The TiO 2 photo-catalyst absorbs light less than 380 nm in wavelength. Therefore, the available wavelength range is only 300-380 nm. The energy of the light in this range is estimated to be approximately 3-4% of the entire solar energy.
However, solar energy of 3-4% is useful. Both e − and h + are generated by the TiO 2 photo-catalyst. In this report, the author used only TiO 2 , not Pt/TiO 2 . Therefore, the reaction schemes are based on the report of Fujishima et al. 17 :  (4) and (5). Reduction driving force of electron generated by TiO 2 photo-catalyst is not enough to reduce the CO 2 directly. However, as the reduction driving force (−0.52 eV with respect to SHE 31,32 ) is stronger than that of normal electrons, and, high energy e − ions can move to the interface of TiO 2 particle and ZrO 2 one. And, the catalytic reducing reaction can be enhanced by the strong reduction driving force of e − ions at the interface. Figure 4 showed that products yield of formaldehyde and methanol largely increased with increasing irradiation intensity of solar light. Increasing of irradiation intensity offers increasing numbers of high energy electron, thus, catalytic reducing reaction was enhanced.
From results and discussion, possible macro mechanism is following 1. Dissolve and dissociate of CO 2 in air into the thin water layer on the surface of composite, and generation of CO 3 2− ions in the thin water layer. 2. Physical adsorbing of CO 3 2− ion to the ZrO 2 surface. 3. Movement of adsorbing species to the interface of TiO 2 particle and ZrO 2 one. 4. Catalytic reducing reaction of adsorbing species at the interface.
High energy e − generated by TiO 2 photocatalytic effect strongly accelerates the catalytic reducing reaction. 5. Desorption of reduced products (low-molecular-weight organic compounds) from the interface into thin water layer. 6. Evaporation of reduced products (low-molecular-weight organic compounds) from the water thin layer into air. 7. Repeating of 1-6.

Conclusion
In this study, the thin water layer noted by Sato et al. splits to provide H + ions because of the TiO 2 photo-catalyst and provides a reaction medium for the catalytic reduction of CO 2 .
Thin water layer on the composite surface (wet condition) is essential, and, author's method is catalytic reaction enhanced by the TiO 2 photo-catalyst. This method is new approach upon which nobody could hit. Moreover, the fact that a large amount of reduced products was obtained from CO 2 and H 2 O in air under irradiation of only real solar light at room temperature and atmosphere pressure is a novel and important finding.
Experimental procedure. Next, 0.2 g of the composites (c) was scattered uniformly onto a copper plate, which was placed in a glass laboratory dish (inner diameter: 56 mm). The assembly with (c) scattered on the Cu plate in a glass laboratory dish is referred to as the test unit.
Initially, particles or molecules adsorbed onto the composites surface were removed by eliminating static electricity using an ion blower (AZ ONE Co. Ltd., Ion Blower AS-18). The blowing time was 1 h in the strong-wind mode. (Without the use of the ion blower, the test unit must be kept in a refrigerator for 2-3 days).
After the test unit was kept for more than 20 h in a refrigerator (inner temperature of the refrigerator: 1-5 °C), it was removed and placed into a transparent gas-barrier plastic bag (OKURA KOGYO Co. Ltd., OE-4, 200 × 300 mm 2 ). The bag was heat-sealed at its entrance. Urethane tape (1 × 1 cm 2 ) was adhered to the outside surface of the bag. After the air in the bag was removed using a plastic syringe, new air was injected into the bag. The needle of the syringe was inserted though the urethane tape to avoid leaving a small hole after the needle was removed. The inner volume of the bag was 1000 ml. The bag was then placed on the ground facing the sun.
The injected air was at a sufficient temperature and humidity. With the using of 0.2 g of the composites (c), the desired temperature and humidity were approximately 30 °C and 60-80%, respectively. Too much quantity of water layer by condensation of water vapour considerably decreased the reduced products, for instance, in the case of 95-100% in humidity.
After irradiation by real solar light through the bag for 0.5 h or 10 min or 300 s, the concentrations (ppm) of formaldehyde and methanol were measured by a gas-detecting tube (GASTEC Co. Ltd., tube No. 91 M (measuring range: 8-20 ppm, 20-2000 ppm, and 2000-6400 ppm; three measuring ranges were available by altering the suction volume) and Tube No. 111L (measuring range: 20-40 ppm and 40-1000 ppm; two measuring ranges are available by altering the suction volume)). The tubes were inserted one by one thorough the plastic tape adhered to the outside surface of the bag, and the concentrations were measured; after the tubes were removed, the remaining hole was immediately sealed with another piece of tape. The inner volume of the bag was measured by extracting the gas with the plastic syringe. The measured concentrations (ppm) of both formaldehyde and methanol were then corrected for temperature and expressed in terms of μmol/g: = × Micromole/g cV/ (22, 400 g) where, c is the gas concentration (ppm), V is the inner volume after irradiation (ml), and g is the mass of composite (c) used (g).
The intensity of the UV light (mW/cm 2 ) with a wavelength of 365 nm was measured using a UNIMETER (USHIO Co. Ltd., UIT-201 and UVD-365PD) during the irradiation of solar light during each experiment; the measurements were performed near the point where the composites were placed in the bag. The intensity was measured every 5 min (Experiment Nos 1-4 and Nos 17-25) or every 1 min (Experiment Nos 5-16). In the case of experiment Nos 17-25, the intensity was measured at the start and the end of irradiation. And, average of each experiment was calculated.