Light-enabled selective conversion of co2 into ethanol in water

Abstract

There is provided a photocatalyst for the conversion of carbon dioxide to ethanol. The photocatalyst includes a gallium nitride powder support and gold nanoparticles deposited on the gallium nitride powder support. The gallium nitride has rod-shaped pores having a length of from 1 to 5 μm, and the gold nanoparticles have a diameter of from 5 to 20 nm.

Description

[0001] LIGHT-ENABLED SELECTIVE CONVERSION OF CO2INTO ETHANOL IN WATER

[0002] CROSS-REFERENCE TO A RELATED APPLICATION

[0003] [1] This disclosure claims priority from U.S. provisional application number 63 / 600,251 filed on November 17, 2023 which is incorporated herein by reference in its entirety.

[0004] TECHNICAL FIELD

[0005] [2] This disclosure relates to the field of photocatalytic conversion of carbon dioxide into alcohols, more specifically ethanol.

[0006] BACKGROUND OF THE ART

[0007] [3] The rapid increase of CO2levels in the atmosphere and its alarming consequences for climate change have directed research efforts to CO2capture, utilization, and conversion. Ethanol is one of the preferred target products of CO2conversion, because ethanol can serve as a clean fuel and as a chemical feedstock. Ethanol is easy to store and transport and generates more heat from combustion (28.40 MJ / kg) than methanol (20.27 MJ / kg). The conventional hydrogenation method of converting CO2to ethanol generally requires high temperature, high pressure of CO2, and high pressure of H2, which is an energy-intensive process and compromises the ethanol's environmental benefits. In contrast, artificial photosynthesis, a process to convert CO2and water to chemical fuels and O2, is benign as it uses water to replace H2and allows for conditions less harsh than hydrogenation systems.

[0008] [4] Currently, non-enzymatic efforts on photocatalytic CO2reduction to ethanol by water mainly utilize photoactive heterogeneous catalysts, such as plasmonic metal and semiconductors. In contrast to the abundant photocatalytic systems targeting C1 products and financially and societally less valuable products such as CO, the C2+ product systems not only are less studied but are also hindered by one or more of the following limitations: a requirement for a high temperature, a requirement for a high pressure, the need for energy-intensive ultraviolet light radiation, and generally a low productivity and selectivity for C2+ alcohols such as ethanol. The use of flow or circulated systems can improve productivity, while most batch systems often must compromise between selectivity and productivity. It has proven difficult to produce ethanol from CO2and water under desirable mild conditions such as under visible light, at room temperature, and at atmospheric pressure. Accordingly, improvements in catalytic systems are desired for a more efficient production of alcohols, particularly ethanol, from CO2. SUMMARY

[0009] [5] In one aspect, there is provided a photocatalyst for the conversion of carbon dioxide to ethanol, the photocatalyst comprising a gallium nitride powder support and gold nanoparticles deposited on the gallium nitride powder support, wherein the gallium nitride has rod-shaped pores having a length of from 1 to 5 μm, and wherein the gold nanoparticles have a diameter of from 5 to 20 nm, preferably from 10 to 20 nm. The gold nanoparticles can be present in a concentration of from 2.5 to 10 wt. % with respect to the total weight of the photocatalyst. In some embodiments, the photocatalyst is free of any additives or hole scavengers. In some embodiments, the photocatalyst consists of the gallium nitride powder support and the gold nanoparticles. In some embodiments, the gold nanoparticles are Au(0) characterized by a 4f7 / 2binding energy peak of from 83.95 to 84.05 eV and a 4f5 / 2binding energy peak of from 87.65 to 87.75 eV as measured by X-ray photoelectron spectroscopy (XPS).

[0010] [6] In one aspect, there is provided the use of the photocatalyst as defined in any one of claims 1 to 6, for the conversion of carbon dioxide to ethanol in an aqueous phase.

[0011] [7] There is provided a method of converting carbon dioxide to ethanol, the method comprising: providing a carbonate in an aqueous phase as a source of carbon dioxide; contacting the aqueous phase with a photocatalyst, the photocatalyst comprising a gallium nitride powder support and gold nanoparticles; and irradiating the photocatalyst with visible light to convert the carbonate into ethanol. In some embodiments, the method is performed under ambient conditions of temperature and pressure. In some embodiments, the method is performed at a temperature of from 15 to 30 °C. In some embodiments, the method is performed at a pressure of from 0.95 to 1.05 atm. In some embodiments, the carbonate is selected from the group consisting of NaHCO3, Na2CO3, KHCO3and K2CO3. In some embodiments, the carbonate is a potassium carbonate. In some embodiments, the carbon dioxide is captured into the aqueous phase from ambient air with an inorganic hydroxide base in the aqueous phase. In some embodiments, the visible light has a wavelength of 400 to 600 nm.

[0012] [8] In a further aspect, there is provided a process for producing a photocatalyst, the process comprising: providing an aqueous suspension comprising GaN and gold; adding NaBH4to the aqueous suspension dropwise; sonicating and mixing the aqueous suspension to obtain a sonicated suspension; and aging the sonicated suspension to obtain a photocatalyst comprising a gallium nitride powder support and gold nanoparticles. In some embodiments, the weight ratio of GaN to gold in the aqueous suspension is from 0.9:1.1 to 1.1 :0.9. In some embodiments, the gold is provided in the suspension as HAuCl4In some embodiments, the molar ratio of NaBH4to HAuCl4is from 150 to 180. In some embodiments the process further comprising purifying the photocatalyst with water

[0013] [9] Many further features and combinations thereof concerning the present improvements will appear to those skilled in the art following a reading of the instant disclosure.

[0014] DESCRIPTION OF THE DRAWINGS

[0015]

[0010] FIG. 1A is a photograph showing Au nanoparticles GaN catalyst (AuNP / GaN catalyst) on the left and GaN on the right.

[0016]

[0011] FIG. 1B is a transmission electron microscopy (TEM) image of AuNP / GaN catalyst (scale bar 1 μm).

[0017]

[0012] FIG. 1C is a TEM image of AuNP / GaN catalyst (scale bar 50 nm) with arrows showing the gold nanoparticles (AuNP).

[0018]

[0013] FIG. 1D is a TEM image showing the lattice spacing in AuNPs (per 10 lattices).

[0019]

[0014] FIG. 1E is an element mapping in high-angle annular dark-field imaging (HAADF)- scanning TEM.

[0020]

[0015] FIG. 1F is a diffuse ultraviolet-visible (UV-Vis) spectrum of AuNP / GaN and GaN showing the plasmonic absorption of AuNP around 536 nm.

[0021]

[0016] FIG. 1G is a powder X-ray diffraction (PXRD) spectrum of GaN powder (bottom line) and AuNP / GaN (top line).

[0022]

[0017] FIG. 1H shows an X-ray photoelectron spectroscopy (XPS) survey scan analysis of AuNP / GaN.

[0023]

[0018] FIG. 1I shows an XPS binding energy analysis of AuNP / GaN.

[0024]

[0019] FIG. 1J is a gas chromatography - thermal conductivity detector (GC-TCD) analysis of the gas phase after a reaction according to the conditions of Entry 7 of Table 4.

[0020] FIG. 2 is a combined bar graph and dot plot showing the selectivity for ethanol (EtOH) (bar graph) and the EtOH yield (dot plot).

[0025]

[0021] FIG. 3 is a bar graph showing the amount of product obtained (ethanol (EtOH), methanol (MeOH), and HCOO-).

[0026]

[0022] FIG. 4A is a nuclear magnetic resonance (NMR) spectrum of the reaction condition of Entry 1 in Table 4.

[0027]

[0023] FIG. 4B is an NMR spectrum of the reaction condition of Entry 7 in Table 4.

[0028]

[0024] FIG. 4C is an NMR spectrum of the reaction condition of Entry 8 in Table 4.

[0029]

[0025] FIG. 5A is a headspace gas chromatography mass spectrometry (GC-MS) analysis of the ethanol formed in the reaction liquid phase during the conditions of Entry 7 of Table 4.

[0030]

[0026] FIG. 5B is a close-up of the peaks of Fig. 5A.

[0031]

[0027] FIG. 6A is a GC-MS spectrum of the ambient air.

[0032]

[0028] FIG. 6B is a GC-MS spectrum of K2CO3+ H218O in the post-reaction gas phase.

[0033]

[0029] FIG. 7A is an NMR spectrum of DCOO- for the detection of DCOOK in the reaction mixture.

[0034]

[0030] FIG. 7B is an NMR spectrum of CD3in ethanol in the reaction mixture.

[0035]

[0031] FIG. 8A is an13C NMR spectrum identifying the location of13C satellite peaks of - CH3in the EtOH product.

[0036]

[0032] FIG. 8B is an NMR spectrum of 98%13C-K2CO3.

[0037]

[0033] FIG. 8C is an NMR spectrum of regular K2CO3.

[0038]

[0034] FIG. 8D is an NMR spectrum showing the EtOH - CH2signal for13C-K2CO3.

[0039]

[0035] FIG. 8E is an NMR spectrum showing the EtOH - CH2signal for regular K2CO3.

[0040]

[0036] FIG. 9 is a schematic showing the reaction mechanism of the AuNP / GaN catalyst of the present disclosure. DETAILED DESCRIPTION

[0041]

[0037] There is provided a photocatalyst for the photocatalytic reduction of CO2in aqueous phases to obtain ethanol. The photocatalyst of the present disclosure combines a plasmonic metal and a semiconductor. The plasmonic metal is gold nanoparticles (referred to herein as AuNPs) and the semiconductor is gallium nitride. The catalyst of the present disclosure advantageously operates with light in the visible range as opposed to ultra-violet (UV) light which is more energy demanding and thus more costly. More specifically, the gallium nitride (GaN) acts as a support for the AuNPs and is used in powder form. The powder form is preferred since it can be produced on a larger scale compared to other forms of GaN such as GaN nanowires. GaN nanowires are generally produced at a laboratory scale whereas powder GaN is produced at industrial scale. It has been found that the present photocatalyst is an efficient and selective photocatalyst for the conversion of CO2into ethanol in the desirable visible light region without harsh conditions (exemplary harsh conditions are a temperature above 50 °C or above 70 °C and / or a pressure below 0.5 bar or higher than 1.5 bar). Without wishing to be bound by theory, during catalysis, it is believed that gold undergoes hot hole transfer to GaN supports, while the gold functions as the active catalyst in carbon conversion. Indeed, Au catalyzes the C-C couplings in the CO2reduction.

[0042]

[0038] In some embodiments, the gallium nitride powder is porous and is characterized by rod-shaped pores. The rod-shaped pores can have a length of from 1 to 5 μm. In some embodiments, the GaN powder is characterized by a porosity of from 5 m2 / g to 20 m2 / g, from 7 m2 / g to 15 m2 / g or from 11 m2 / g to 14 m2 / g as measured by the Brunauer-Emmet-Teller (BET) method. Accordingly, the GaN is not considered mesoporous. The gold nanoparticles (AuNP) are localized on the surface of the GaN. In some embodiments, the gold nanoparticles have a diameter of from 5 nm to 20 nm and preferably from 10 nm to 20 nm. Gold nanoparticles with a diameter above 20 nm have weak localized surface plasmon resonance (LSPR) and a lower transfer efficiency of hot holes to the semiconductor. With a diameter of at least 5 nm or at least 10 nm, the gold nanoparticles achieve efficiency and selectivity for ethanol by favoring carbon- carbon couplings. In some embodiments, the gold nanoparticles are present in a concentration of from 2.5 to 10 wt. % with respect to the total weight of the photocatalyst. The gold nanoparticles can be characterized by X-ray photoelectron spectroscopy (XPS) and have a 4f7 / 2binding energy peak of from 83.95 to 84.05 eV or 83.97 to 83.01 eV and a 4f5 / 2binding energy peak of from 87.65 to 87.75 eV or 87.67 to 87.71 eV.

[0039] The photocatalyst of the present disclosure can advantageously operate at ambient conditions of temperature and pressure. For example, the catalysis can be performed at atmospheric pressure or close to atmospheric pressure (e.g. 0.95 to 1.05 atm, preferably 0.97 to 1.03 atm). Preferably, the catalysis is performed with ambient air and the CO2in the air is captured in or enters an aqueous phase as a carbonate. The capture or entry of CO2into the aqueous phase is driven by the presence of an inorganic base in the aqueous phase that leads to the formation of the carbonate. More specifically, when CO2dissolves in water, it forms a weak acid H2CO3. Through acid and base reaction, H2CO3reacts with the inorganic base and form the (bi)carbonate. The inorganic base can for example be NaOH, LiOH or KOH. An advantage of the present invention is that the catalysis can be performed at room temperature (e.g. from 15 to 30 °C or from 20 to 25 °C). The absence of heating represents a significant cost savings particularly at industrial scale. Examples of carbonates suitable for conversion of CO2into ethanol by the present photocatalyst include but are not limited to: (NH4)2CO3, NH4HCO3NaHCO3, Na2CO3, KHCO3K2CO3pentaethylenehexamine (PEHA) and ethanolamine. It was surprisingly found that potassium carbonates outperformed sodium carbonates and thus the potassium carbonates are preferred in some embodiments.

[0043]

[0040] GaN-supported AuNP was prepared by a NaBH4reduction method, which contrasts with the traditional preparation method of AuNP / GaN that utilizes physical vapour deposition. Physical vapour deposition requires specialized and expensive instruments and involves ultra high vacuum and high energy input. On the other hand, the NaBH4reduction method is facile, requires no specialized instrument, and is economical in contrast with physical vapour deposition. To produce the photocatalyst, GaN powder and a gold precursor such as a gold(lll) salt precursor (e.g. HAuCl4KAuCl4and NaAuCl)4are mixed and sonicated in an aqueous phase (e.g. water). The weight ratio of GaN powder to gold (or gold precursor) is preferably around 1 :1 , for example from 0.9: 1.1 to 1.1 :0.9 or from 0.95:1.05 to 1.05:0.95. NaBH4is then added to the aqueous phase drop-wise. Preferably, the NaBH4is in a solution that is ice cold, in other words having a temperature of less than 4 °C or less than 2 °C, for example around 0 °C. In some embodiments, the molar ratio of NaBH4to HAuCl4is from 150 to 180, preferably from 160 to 170 or 165 ±5%. After and / or during the addition of NaBH4the aqueous suspension is mixed and sonicated. Then, the resulting aqueous suspension is aged, for example overnight (i.e. for at least 6 h, preferably at least 8 h). The photocatalyst obtained can then be purified with water or other means.

[0041] As will be seen in the Example section below, the AuNP / GaN photocatalyst of the present disclosure demonstrated a high selectivity (86% or more) for ethanol and an efficient production of ethanol (192 μmol·gcat-1·h-1·mLsolution-1) from carbonates and water under visible light (400-800 nm or preferably 400-600 nm), without any additives or hole scavengers. Most importantly, the system was able to produce ethanol under ambient conditions (atmosphere air) where CO2is captured by inorganic bases with a selectivity of 61% and a productivity of 71 μmol·gcat-1·h-1·mL-1. Furthermore, the catalyst was shown to run at least 5 days for uncompromised, highly selective ethanol production, and the system was demonstrated to be scalable. In some embodiments, the present catalyst can be combined with automatic and high throughput techniques to achieve continuous industrial production of ethanol from base- captured CO2, including the various and dilute CO2sources.

[0044] EXAMPLE

[0045] Preparation of the catalysts

[0046]

[0042] All the glassware and stir bars were cleaned with fresh aqua regia, rinsed with abundant tap water, followed by abundant deionized water (DI), and oven cleaned. All the chemicals were used as received unless specified otherwise. All the water used in the following experiments was Mill i-Q™ water unless specified otherwise.

[0047]

[0043] GaN powder (99.99%, metals basis) was purchased from Alfar™ Aesar. HAuCI4·3H2O (>99.9% trace metals basis), NaBH4, CuCI2, COCI2·6H2O, K2PtCl4, K2PtCl4, imidazole, Ga2O3K2CO3-13C (98 atom %13C), water-18O (97 atom %18O) were purchased from Sigma-Aldrich™. K2CO3(American chemical society (ACS) certified), KHCO3(ACS certified), and NaHCO3(ACS certified) were purchased from Fischer™. KOH (ACS reagent) was purchased from ACP™ Chemicals, Inc. Na2CO3(reagent grade) was purchased from VWR Chemicals, D2O (99.9%) was purchased from Cambridge Isotope Laboratories, Inc.

[0048]

[0044] To prepare 5 wt. % AuNP / GaN, 3 mg GaN was added into 3 mL water, followed by 30 mins of sonication. Next, 32 μL of 24 mM HAuCl4·3H2O aqueous solution was added to the GaN suspension, followed by 30 mins of vigorous stirring. Finally, 100 μL of freshly prepared ice-cold 0.05 mg / mL NaBH4aqueous solution was added to the mixture dropwise. The mixture was immediately brought to a water-bath sonication and sonicated for 10 s, followed by 3 h of vigorous stirring. The solution was then left aging overnight without disturbance, followed by purification by rinsing with water 3 times. The resulting AuNP / GaN was pink in appearance. To prepare 5 wt. % Au / Ga2O3, 3 mg of Ga2O3was used instead of GaN. To prepare 5 wt. % AuCu / GaN (weight ratio of Au:Cu = 1:1), 16 μL of 24 mM HAuCl4.3H2O aqueous solution and 17 μL of 10 mg / mL CuCI2aqueous solution were used. To prepare 5 wt. % AU / C3N4, 3 mg of C3N4was used instead of GaN. Finally, 5 wt. % Au / SiO2was prepared following the protocol of Ni S, Zhu J, Roy R, Li C-J, Lennox RB. Catalytic hydrogenation of CO2from air via porous silica-supported Au nanoparticles in aqueous solution. Green Chemistry. 2021 ;23(10):3740-9.

[0049]

[0045] To prepare 5 wt. % Pt / GaN, 3 mg of GaN and a stir bar were added to 3 mL of water; the vial was sealed and degassed by bubbling argon for 15 mins (alternatively He, Ne Kr or Xe can be used instead of Ar). The vial was then sonicated for 30 mins. Next, 336 μL of freshly prepared 1 mg / mL K2PtCl4aqueous solution was injected into the argon-protected GaN suspension, followed by 30 mins of vigorous stirring. Next, 100 μL of freshly prepared ice-cold 0.05 mg / mL NaBH4aqueous solution was added to the mixture dropwise. The mixture was immediately brought to a water-bath sonication and sonicated for 10 s, followed by 3 h of vigorous stirring. The solution was then left aging overnight without disturbance, followed by purification by water 3 times. The resulted Pt / GaN was grey in appearance. To prepare 5 wt. % Pd / GaN, 251 μL of 2 mg / mL freshly prepared K2PdCl4aqueous solution was added instead of the K2PtCl4aqueous solution.

[0050] Comparison of GaN to other semiconductors

[0051]

[0046] GaN was the selected semiconductor based it demonstrated the capacity to produce ethanol under the following conditions: 0.5 bar CO2, 0.5 bar Ar, Xe lamp (>435 nm) at 25 °C with 1 mg of catalyst for 2 h in water. In contrast, other semiconductors showed no potential (Table 1).

[0052] Table 1. Comparison of semiconductors

[0053] Catalyst Loading

[0054]

[0047] The present example used 1 mg of catalyst because this catalyst loading was found to outperform a higher dosing of 5 mg as per Table 2. Both loadings were tested under the following conditions: 0.5 bar CO2, 0.5 bar Ar, Xe lamp (>435 nm) at 25 °C for 2 h in 0.5 mL of water.

[0055] Table 2. Catalyst loading

[0056] Photocatalysis

[0057]

[0048] Small scale reaction was performed by adding 400 μL of 0.1 M K2CO3solution, 3 mg of AuNP / GaN, and a stir bar into a quartz tube. The sealed quartz tube then underwent three cycles of freeze-pump-thaw to replace the atmosphere with argon. Next, the tube was submerged in a water bath cooled by running water underneath a Xenon lamp (300W, Excelitas™ Technologies, model #HX9) equipped with a 400 nm longpass filter 275 (Newport FSQ-GG400 Colored Glass Longpass Filter, 50.8 x 50.8 mm square). The distance between the Xenon lamp and the bottom of the tube was kept at 12 cm. The tube was then stirred under > 400 nm light radiation for 24 hr.

[0058]

[0049] For the small-scale reaction with light emitting diode (LED) sources the quartz tube containing the reaction mixture underwent the same treatment described above. For experiments using 370 nm radiation, the quartz tube was placed underneath a 370 nm LED lamp (Kessil™, PR160L-370, 43W) a distance of 5 cm. A cooling fan was placed behind the setup to avoid heating by the light source. The tube was stirred under 370 nm light radiation for 24 h. For experiments using 525 nm radiation, the glass tube was used instead of the quartz tube; otherwise, other treatments were identical. The glass tube was placed between two tilted 525 nm LED lamps (Kessil™, PR160L-525, 44W), with a cooling fan behind and a stirring plate underneath.

[0059]

[0050] For the large scale reaction, 4 - 9.2 mL 0.1 M K2CO3solution, a proper weight of AuNP / GaN (catalyst loading: 7.5 mg / mL), and a stir bar were added into a customized photo reactor. A quartz lid was placed on the chamber, and two customized clamps clamped the reactor. The reactor was frozen with liquid nitrogen and vacuumed for 5 min. The vacuumed reactor was then placed under a Xenon lamp with a 400 nm longpass filter inside a water bath cooled by running water. The distance between the Xenon lamp and the top of the reactor was kept at 5 cm. The reactor was then stirred under > 400 nm light radiation for 24 h.

[0060] Optimization and characterization of AuNP / GaN

[0061]

[0051] The size of AuNPs plays an important role in the catalytic reaction. A decreasing size generally benefits a high catalytic efficiency, increases product selectivity, and favors carbon- carbon coupling. For semiconductor-supported Au nanomaterials, overly large Au sizes not only result in weak localized surface plasmon resonance (LSPR) but also lower the transfer efficiency of hot holes to the semiconductor. Thus, the supported AuNPs were first optimized based on their catalytic activities in ethanol formation (Table 3, Scheme 1). The photocatalytic reaction conditions were as follows: 3 mg of catalyst was added into 400 μL of 0.1 M KHCO3 / H2O solution (exception: Entry 9 had 800 μL), inside a quartz tube with a stir bar. The tube undertook freeze-pump-thaw cycle three times to replace the atmosphere with argon. Then, the argon-protected tube was placed in a water bath cooled by continuously running water under a Xe lamp (no filter) for 24 hours under stirring. After the reaction, a known amount of imidazole and D2O was added to the mixture; the catalyst was removed by centrifuge, and nuclear magnetic resonance (NMR) analyzed the mixture.

[0062] Scheme 1.

[0063] Table 3: Optimization of catalyst preparation and loading

[0064]

[0052] Previously reported AuNP / GaN systems for CO2reduction required physical vapour deposition followed by annealing, resulting in AuNPs with diameters from 3 nm to 40 nm and photocatalytic (or photoelectrocatalytic, PEC) products being CO or HCOOH. In contrast, the NaBH4reduction method does not require a special instrument, and the size of AuNPs can be easily modified by adjusting the ratio of NaBH4to HAuCl4As per Table 3 Entry 2, the support concentration being 1 mg / mL, n(NaBH4):n(HAuCl4) = 165, and Au wt% = 5 was found to be the optimal catalyst preparation condition for maximum EtOH production in this experiment. Next, the catalyst loading in the photocatalytic reaction was optimized: the comparison between Table 3 Entries 2, 8, and 9 showed that although a high catalyst loading improved the selectivity, it had a negative impact on the EtOH yield. This could be due to the opacity of the catalyst (as shown in Fig. 1A): a high catalyst loading resulted in poor light penetration and thus lowered the yield.

[0065]

[0053] Transmission electron microscopy (TEM) and high-angle annular dark-field imaging (HAADF)-STEM was performed with an aqueous suspension of AuNP / GaN which was drop cast onto an SPITM200 mesh Cu grid with carbon coating and left to dry in a vacuum. The characterization was performed on Thermo Scientific Talos™ F200X G2 (S)TEM, with the beam convergence angle being 10.5 mrad and the collection angle being 58-200 mrad in HAADF imaging.

[0066]

[0054] For the diffuse UV-Vis spectra, after water removal by centrifuge, AuNP / GaN powder was dried in a vacuum chamber at room temperature. GaN powder was analyzed as received. The diffuse UV-Vis spectrum was performed on a UV-Vis-NearIR spectrophotometer Cary™ 5000 from Agilent™ with Praying™ mantis diffuse reflectance holder.

[0067]

[0055] The X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo- Scientific K-Alpha™ with an Al Kα micro-focused monochromator, an X-ray spot size of 400 μm, and the flood gun on to avoid sample charging. Spectral energies were calibrated by setting the C-C binding energy of C1s at 284.8 eV. Peak fitting was performed using the Thermo Avantage™ software (version 4.60).

[0068]

[0056] For powder X-ray diffraction (PXRD), after water removal by centrifuge, AuNP / GaN powder was dried in a vacuum chamber at room temperature. GaN powder was analyzed as received. The PXRD analysis was performed on a Bruker™ D8 Advance powder X-ray diffractometer with a Cu Kα source.

[0069]

[0057] Transmission electron microscopy (TEM) characterizations of the AuNP / GaN prepared by the optimized condition (Table 3, Entry 2) showed that the commercial GaN had rod-shaped porous structures with a length of around 2.5 μm (Figs. 1B-1C), while the AuNPs had a size distribution from 5 nm to 20 nm in diameter. This size distribution is overlayed with the optimal Au size range (10-20 nm) for hot electron generation and injection efficiency. The presence of AuNPs was further supported by element mapping in high-angle annular dark-field imaging (HAADF)-STEM (Figs. 1 D-1E) and the detection of the LSPR band in AuNP / GaN (Fig. 1F), where a 536 nm LSPR band was observed. The measurement of lattice spacing in AuNPs (Figs. 1 D-1E) showed that the predominant facets are (200) and (111), while powder X-ray diffraction (PXRD) examination of AuNP / GaN (Fig. 1G) revealed under-coordinated (311) facets, which was shown to require less energy in COOH* formation and had greater ability in CO2activation than (111). The XPS binding energy scan in the Au 4f region (Figs. 1H-1 I) showed 83.99 eV (Au 4f7 / 2) and 87.69 eV (Au 4f5 / 2) for the AuNP / GaN, corresponding to Au(0). This demonstrated that the HAuCl4had been fully reduced, and the active Au species for catalysis is Au(0).

[0070] Impact of cations and pH

[0071]

[0058] First, the impact of the cations and pH on the photocatalytic reaction was studied using four inorganic salts as model compounds: NaHCO3, Na2CO3KHCO3and K2CO3(Table 4, Scheme 2, Entries 2-5), since NaOH and KOH are the common inorganic base used in direct air capture (DAC) and the four salts are their corresponding products. It was observed that carbonates behaved better than bicarbonates in terms of alcohol formation, which could be due to (a) difference in their absorption energies and structures on Au surface; (b) difference in their reduction potentials, because alkaline conditions favor ethanol formation in CO2reduction; and (c) difference in pH, which could influence the conduction and valence band edge potentials of GaN. Scheme 2:

[0072] Table 4: Optimization of reaction conditions and control experiments a. Given the activity of GaN in the visible light is negligible (Entry 8), the yield calculations only considered Au as the actual catalyst. For comparison convenience, the yields in the brackets considered both GaN and Au into the catalyst weight. b. Ambient-pressure room-temperature air was continuously bubbled into a 2 M KOH solution overnight and was later diluted 20 times as the starting reagent.

[0073]

[0059] It was surprisingly found that the potassium cation impacted ethanol formation more significantly compared to the sodium cation. A similar potassium effect was widely observed in the thermal reduction of CO2, where potassium promoted the CO2conversion and especially the formations of C2+products by suppressing side reactions, tuning surface basicity to improve CO2adsorption and achieve desirable surface C / H ratios, and stabilizing intermediates by electrostatic Kδ+-Oδ-interaction.

[0074]

[0060] The present inventors were aware of raising concerns regarding the ultra-trace transition metal contaminations in commercial potassium and sodium (bi)carbonates, especially the Pd contamination and their disastrous consequence in “metal-free” Suzuki reactions. Thus, inductively coupled plasma-mass spectrometry (ICP-MS) was conducted on the four salts in terms of the elements Pd, Pt, and Ni, as they are known to be efficient catalysts in carbon- carbon formations. ICP-MS analysis of the four commercial (bi)carbonates (Table 5) showed that all the four commercial salts have a similar amount of Ni contamination in ppm levels. Thus, Ni could not be why potassium (bi)carbonates outperformed sodium (bi)carbonates in terms of ethanol formation. K2CO3was found to have particularly high contamination in Pd and Pt, 158 ppb and 194 ppb, respectively; however, KHCO3which was found previously to be as effective as K2CO3in ethanol formation, had Pd and Pt levels similar to that of NaHCO3, and significantly lower than Na2CO3. This comparison showed no clear relation between the potassium effect and high Pt / Pd contamination. To further examine if Pt and Pd contamination could promote ethanol formation, Pt / GaN and Pd / GaN was prepared similarly to AuNP / GaN.

[0075] Table 5: ICP-MS analysis of trace transition metals in commercial carbon sources

[0076]

[0061] Following that, AuNP / GaN, Pt / GaN, and Pd / GaN were tested under an identical reaction condition optimized to maximize ethanol production (the optimization details are explained further below, scheme 3). Results (Table 6, Entries 1, and 6-8) showed that Pt / GaN and Pd / GaN both exhibited abilities to catalyze carbonates to ethanol but were far less efficient than AuNP / GaN under the identical condition to be considered significant. Moreover, the concentrations of Pt and Pd introduced by K2CO3in the photocatalytic reaction mixture were 2 ppb and 3 ppb, respectively, while the concentration of Pt and Pd brought by the Pt or Pd catalysts in Table 6 Entries 6-8 were 395 ppm, 789 ppm, and 395 ppm 135 respectively, which are more than a thousand times higher than the Pt and Pd brought by K2CO3. Thus, the Pd and Pt contamination in commercial K2CO3is not the reason behind ethanol formation.

[0077] Scheme 3:

[0078] Table 6: Comparison to other substrate-supported metal catalysts a. The weight ratio of Au to Cu is 1:1. b. Given that the activity of GaN in the visible light is negligible (see Table4, Entry 8) and SiO2is an inert support, all the yields in Entry 1, 3, 5-8 did not consider GaN or SiO2into the catalyst weight. For comparison convenience, Entry 2 and 4 did not consider the weight of supports into the catalyst neither.

[0079] Impact of light source wavelengths

[0080]

[0062] Fig. 1F shows two absorption peaks in AuNP / GaN: the first one centered at 370 nm, attributed to GaN, and the second one centered at 536 nm, which is the LSPR band of AuNPs. With incident light exposure, plasmonic metallic nanoparticles with diameters around 10-20 nm mostly favor energy transfer to semiconductor supports via electron photocurrent (or hole photocurrent). To study the impact of wavelengths, three light sources were selected: full spectra, 370 nm, and visible light (Table 4, Entries 5-7), where the visible light was found to be the optimal light source for optimal ethanol production and selectivity. Given that GaN exhibited minimal catalytic activity under visible light (Table 4, Entry 8), AuNP was determined to be the active catalytic component in Entry 7, thus further improving the catalyst efficiency. Gas phase analysis of reactions run under Entry 7 conditions revealed that no carbon-containing side product was detected, ensuring the selectivity of the photocatalytic reaction (Fig. 1J). Moreover, O2formation was detected, which is in agreement with the mechanism of GaN (usually p-type GaN) supported AuNP proposed in the CO2reduction to CO system. Namely, under visible light radiation, AuNP transferred hot holes to GaN, and the latter involved in water oxidation reactions to O2. More specifically the comparison of the gas phase components between air and the post-reaction gas phase revealed an increase in the O2 / N2 area ratio from 1 :3.25 to 1:3.00 respectively.

[0081] Application and evaluation of the catalyst in various systems

[0082]

[0063] The compatibility of the present photocatalytic system with DAC was tested in a two- step, one-pot process: ambient-pressure room-temperature air captured by KOH solution was used as the starting material to replace the model compounds K2CO3or KHCO3. Compared to 0.1 M K2CO3as starting materials (Table 4, Entry 7), 0.1 M air-KOH in the same scale resulted in lower but enough sufficient selectivity and productivity towards EtOH: 61% instead of 86%, and 28 instead of 77 μmol·gcat-1·h-1respectively (Table 4, Entry 9). A similar decrease in selectivity and productivity was also observed when switched to DAC. The room-temperature DAC test result here suggested that the present base-assisted CO2reduction by water to EtOH system could be implemented with various carbon sources, including CO2from sources richer than room-temperature DAC such as high-temperature DAC, CO2recovery (CR) systems, and point source CO2capture (PSC). The scalability of the present system was tested by switching the reactor from tubes to chambers. Table 6 shows exemplary scale-up reactions, where the present systems can scale up without sacrificing the selectivity and production of ethanol. The catalyst loading was kept at 7.5 mg / mL. Entry 1 was run in a quartz tube with argon as atmosphere. Entries 2-3 were run in a glass chamber with a quartz lid under vacuum. Finally, the recyclability of the catalyst was tested and it was demonstrated that the catalyst able to produce EtOH with high selectivity and productivity in a relatively consistent manner for at least five rounds, equaling at least 120 hours (Fig. 2, scheme 4). The above results showed that the present AuNP / GaN applies to DAC, scale up, and recycle, demonstrating the potential of the catalytic system in continuous, large, and high production of EtOH from the air. Scheme 4:

[0083] Table 6: Scale-up reactions

[0084] Mechanistic studies

[0085]

[0064] The first step of mechanistic studies is to understand the photoelectrical behaviour of AuNP / GaN under visible light radiation. It was hypothesized that the electrons generated by AuNP plasmonic effect were involved in the reductive reaction of carbonate. The hypothesis was tested with an alternative visible light source: a 525 nm green LED, weaker than the filtered Xe lamp but almost matched the LSPR band of AuNP. Results (Table 7) showed that AuNP / GaN could catalyze the generation of EtOH from carbonate and water under green LED radiation, while GaN could not, indicating that the carbon conversion required AuNP, light sources overlayed with AuNP LSPR band, and photoelectrons generated by the AuNP plasmonic response upon radiation. Moreover, to understand the functions of GaN supports and how it impacts the catalytic efficiency of AuNP in the system, a metal / substrate comparison study was performed (Table 6). First, it was tested if AuNP alone (without semiconductor support) was an active photocatalyst enabling carbonate reduction by water (Table 6, Entry 5). By changing the support to an inert SiO2support, AuNP could still convert carbonates into alcohols and formates, demonstrating that AuNP is the deciding component in the visible-light- catalyzed carbon conversion. Secondly, it was tested if GaN exhibited any promoting effect in AuNP catalytic properties by changing the supports to other semiconductors, including Ga2O3and C3N4(Table 6, Entry 2 and 4). Although AuNP supported by the alternative semiconductors was found to be able to catalyze the conversion of carbonates to alcohols and formates, the efficiency and the selectivity were very different. Changing AuNP to other metals such as AuCuNP, Pt, and Pd (Table 6, Entry 3, 6-8) resulted in dramatic reactivity, product, and selectivity changes. Thus, it was found that the carbon conversion depends on AuNP, and the conversion efficiency depends on e--h+separation in AuNP, which is determined by the hot hole transfer process to the support. In other words, although GaN exhibited negligible catalytic effects in the tested reaction condition (Table 4, Entry 8), it was not an inert support but exhibited a crucial promoting effect on AuNP.

[0086] Table 7: Plasmonic effect study

[0087]

[0065] To understand the carbon conversion process, tendency studies were carried out (Fig. 3, scheme 5) and intermediate studies (Table 8, scheme 4). Scheme 5:

[0088]

[0066] The MeOH production peaked around 16 h, while formate production fluctuated, indicating that MeOH and formate were potential intermediates towards EtOH and were consumed as they were generated. Intermediate studies using MeOH and formate as starting material confirmed this hypothesis. MeOH can generate EtOH under photo radiation via methyl carbene; and this was confirmed by the identification of MeOH as the intermediate for EtOH formation. The formation of formate with MeOH as starting material (Table 8, Entry 2, Scheme 6), however, suggested that over oxidation side reactions existed, similar phenomena are common in photocatalytic CO2reduction systems without additional hole scavengers. The following carbon conversion process was proposed:

[0089] Scheme 6:

[0090] Table 8: Intermediate studies

[0091]

[0067] Liquid phase analysis by NMR was performed after the reaction. 100 μL of 3 mg / mL of freshly prepared imidazole / D2O solution (as internal standard) was added to the 400 μL reaction mixture, and the mixture was centrifuged to remove the AuNP / GaN. The1H NMR was carried in 5 mm NMR tubes on a Bruker™ AVIIIHD 500 MHz NMR Spectrometer, with automatic solvent suppression mode and 256 scans (Figs. 4A-4C).

[0092]

[0068] Liquid phase analysis was also performed by headspace gas chromatography - mass spectrometry (GC-MS) on a PerkinElmer™ 100 TGA-FTIR-GC-MS-Head space analyzer. 500 μL of reaction mixture was added and sealed in a headspace GC-MS vial. The oven temperature was set at 80 °C, the needle temperature was 110 °C, and the transfer temperature was 120 °C. The headspace GC-MS analysis of the ethanol obtained in the reaction liquid phase of Table 4 Entry 7 is shown in Figs. 5A-5B.

[0093]

[0069] Gas phase analysis by GC-thermal conductivity detector (GC-TCD) experiments were performed on an Agilent™ Technologies Gas Chromatograph System (Agilent™ GC 6890) equipped with a thermal conductivity detector (TCD). Two columns were used. Column 1 was an Agilent™ HP PLOT-Q column composed of bonded polystyrene-divinylbenzene. Column 2 was an Agilent™ HP-PLOT MoleSieve composed of zeolite. After the photocatalytic reaction, 1 mL of the gas phase was taken by a gas-tight sampling syringe (Hamilton™ 1700 Series Syringes). An avoidable amount of air was brought by the dead volume of the syringe needle. The GC-TCD program, the blank, and the standard gases were as described in Ni S, Zhu J, Roy R, Li C-J, Lennox RB. Catalytic hydrogenation of CO2from air via porous silica-supported Au nanoparticles in aqueous solution. Green Chemistry. 2021 ;23(10):3740-9.

[0094]

[0070] The gas phase analysis by GC-MS were performed on an Agilent™ Technologies Gas Chromatograph System equipped with a 5973 Inert Mass Selective Detector. The column Rtx-5 (30m, 0.25 112 mm ID, 0.25 μm df) from Restek Corporation was used. The gas phase was sampled in the same manner as described above.

[0095]

[0071] 13C isotope,2H isotope and18O isotope were studied. K2CO3-13C (98 atom %13C) was used instead of regular K2CO3; otherwise, other procedures were as described above. After the photocatalytic reaction, the mixture was centrifuged, and the AuNP / GaN was removed. The supernatant was transferred into a 3 mm NMR tube for characterization on a Bruker™ AVIIIHD 800 MHz NMR Spectrometer. For2H, AuNP / GaN was purified by D2O five times before being used as a catalyst; 0.1 M K2CO3 / D2O solution was used. After the photocatalytic reaction, the mixture was centrifuged, and the AuNP / GaN was removed. The supernatant was transferred into a 3 mm NMR tube for characterization on a Bruker™ AVIIIHD 800 MHz NMR Spectrometer. For18O, 0.1 M K2CO3 / H218O solution was used. After the photocatalytic reaction, 1 mL of the gas phase was taken by a gas-tight syringe and analyzed by GC-MS.

[0096]

[0072] Isotope studies were carried out utilizing H218O, D2O and K213CO3. When H218O replaced the water in the reaction, GC-MS analysis of the post-reaction gas phase revealed18O- labeled oxygen (Figs. 6A-6B), providing additional evidence for water oxidation to O2in the photocatalytic reaction, besides the GC-TCD (Fig. 1J). When D2O replaced the water in the reaction, D-labeled formate (DCOO-) and ethanol (CD3-) were observed by2H NMR spectra (Figs. 7A-7B), indicating that water was the reducing agent of carbonates. It is worthwhile mentioning that16O and H still existed in the above two reaction systems, and unlabeled or mix- labelled products were observed as well, likely due to the porous structure of GaN supports (Figs. 1 B-1C) holding residue water from the catalyst purification steps. When 98% K213CO3was used instead of K213CO3obvious13C satellites were observed in1H NMR for both CH3and CH2in EtOH, contrast to the K213CO3condition where the EtOH13C satellites due to the natural abundance of13C (1 %) were too low to be observed (Figs. 8A-8E), showing that the carbon source in EtOH is from carbonates. The12C EtOH standard was prepared by using a high concentration of12C EtOH (with a natural abundance of13C being 1 %). In Fig. 8A, compared to the standard, the EtOH formed in the K213CO3isotope study showed an obvious13C satellite in1H NMR without much zoom-in. The13C-K2CO3(Figs. 8B and 8D) showed significant13C satellite compared to the regular (Figs. 8C and 8E).

[0097]

[0073] Based on the above experiments, a mechanism was proposed and is shown in Fig. 9: upon visible light radiation which overlayed with AuNPs’ LPSR band, the electrons oscillated on AuNP due to the plasmonic effect. The hot hole transfer process from AuNP to GaN reduced the recombination of holes and electrons on AuNP, thus improving the lifetime of photoelectrons on AuNP. As a result, water oxidation occurs on GaN, while carbon reduction involving protons from water and AuNP photoelectrons occurs on the AuNP surface via key intermediates formate and MeOH, and finally, to EtOH formation. Discussion

[0098]

[0074] The present system’s productivity was in the range of 77 μmol·gcat-1·h-1EtOH production with only AuNP considered as the active catalyst. Besides the fact that flow chemistry was not utilized, it should be noted that there is an unavoidable trade-off between the productivity ability of photocatalysts and the avoidance of over oxidation of the products (namely CO2to CO and HCOOH). Thus, the high selectivity towards much more reduced C-C coupling products such as EtOH could unavoidably compromise high productivity. Compared to the existing visible-light-catalyzed photochemical CO2reduction systems by water that can generate ethanol at ambient pressure and room temperature (Table 9), the present system showed an excellent EtOH production per mL of starting material and a high EtOH selectivity; and it is the only example where a readily available, commercial, and mass-produced semiconductor was used to produce EtOH with such high selectivity and yield. Moreover, with the excellent results obtained herein in the scale-up, recycling, and DAC studies, it was demonstrated that the present system can be combined with other high throughput and automation technology such as flow system and base recycling / regeneration facility, and thus demonstrate the endeavor of selectively and efficiently converting air and water to ethanol under ambient conditions, with no requirement of additives and with renewable light energy.

[0099] Table 9: Comparison to the existing visible-light-catalyzed non-photoelectrochemical CO2reduction by water in batch reactor systems that can generate ethanol at ambient pressure and room temperature (RT) or close to these conditions

[0100] References:

[0101] 1. Liu Y, Huang B, Dai Y, Zhang X, Qin X, Jiang M, et al. Selective ethanol formation from photocatalytic reduction of carbon dioxide in water with BiVO4 photocatalyst. Catalysis Communications. 2009;11(3):210-3.

[0102] 2. An C, Wang J, Qin C, Jiang W, Wang S, Li Y, et al. Synthesis of Ag@AgBr / AgCI heterostructured nanocashews with enhanced photocatalytic performance via anion exchange. Journal of Materials Chemistry. 2012;22(26):13153-8.

[0103] 3. Mao J, Peng T, Zhang X, Li K, Ye L, Zan L. Effect of graphitic carbon nitride microstructures on the activity and selectivity of photocatalytic CO2reduction under visible light. Catalysis Science & Technology. 2013;3(5):1253-60.

[0104] 4. Pastrana-Martinez LM, Silva AMT, Fonseca NNC, Vaz JR, Figueiredo JL, Faria JL. Photocatalytic Reduction of CO2with Water into Methanol and Ethanol Using Graphene Derivative-TiO2Composites: Effect of pH and Copper(l) Oxide. Topics in Catalysis. 2016;59(15):1279-91.

[0105] 5. Zou J-P, Wu D-D, Luo J, Xing Q-J, Luo X-B, Dong W-H, et al. A Strategy for One-Pot Conversion of Organic Pollutants into Useful Hydrocarbons through Coupling Photodegradation of MB with Photoreduction of CO2. ACS Catalysis. 2016;6(10):6861-7.

[0106] 6. Bai S, Wang X, Hu C, Xie M, Jiang J, Xiong Y. Two-dimensional g-C3N4an ideal platform for examining facet selectivity of metal co-catalysts in photocatalysis. Chemical Communications. 2014;50(46):6094-7. 7. Han Q, Zhou Y, Tang L, Li P, Tu W, Li L, et al. Synthesis of single-crystalline, porous TaON microspheres toward visible-light photocatalytic conversion of CO2into liquid hydrocarbon fuels. RSC Advances. 2016;6(93):90792-6.

[0107] 8. Cai B, Wang J, Gan S, Han D, Wu Z, Niu L. A distinctive red Ag / AgCI photocatalyst with efficient photocatalytic oxidative and reductive activities. Journal of Materials Chemistry A. 2014;2(15):5280-6.

[0108] 9. Jeyalakshmi V, Mahalakshmy R, Krishnamurthy KR, Viswanathan B. Strontium titanates with perovskite structure as photo catalysts for reduction of CO2by water: Influence of co-doping with N, S & 162 Fe. Catalysis Today. 2018;300:152-9.

[0109] 10. Tang L, Kuai L, Li Y, Li H, Zhou Y, Zou Z. ZnxCd1-xS tunable band structure- di recti ng photocatalytic activity and selectivity of visible-light reduction of CO2into liquid solar fuels. Nanotechnology. 2018;29(6):064003.

[0110] 11. Zhao D, Xuan Y, Zhang K, Liu X. Highly Selective Production of Ethanol Over Hierarchical Bi@Bi2MoOe Composite via Bicarbonate-Assisted Photocatalytic CO2Reduction. ChemSusChem. 2021 ;14(16):3293-302.

[0111] 12. Ni S, Zhu J, Roy R, Li C-J, Lennox RB. Catalytic hydrogenation of CO2from air via porous silica-supported Au nanoparticles in aqueous solution. Green Chemistry. 2021 ;23(10):3740-9.

Claims

WHAT IS CLAIMED IS:

1. A photocatalyst for the conversion of carbon dioxide to ethanol, the photocatalyst comprising a gallium nitride powder support and gold nanoparticles deposited on the gallium nitride powder support, wherein the gallium nitride has rod-shaped pores having a length of from 1 to 5 μm, and wherein the gold nanoparticles have a diameter of from 5 to 20 nm.

2. The photocatalyst of claim 1 , wherein the diameter of the gold nanoparticles is of from 10 to 20 nm.

3. The photocatalyst of claim 1 or 2, wherein the gold nanoparticles are present in a concentration of from 2.5 to 10 wt. % with respect to the total weight of the photocatalyst.

4. The photocatalyst of any one of claims 1 to 3, wherein the photocatalyst is free of any additives or hole scavengers.

5. The photocatalyst of any one of claims 1 to 4, wherein the photocatalyst consists of the gallium nitride powder support and the gold nanoparticles.

6. The photocatalyst of any one of claims 1 to 5, wherein the gold nanoparticles are Au(0) characterized by a 4f7 / 2binding energy peak of from 83.95 to 84.05 eV and a 4f5 / 2binding energy peak of from 87.65 to 87.75 eV as measured by X-ray photoelectron spectroscopy (XPS).

7. Use of the photocatalyst as defined in any one of claims 1 to 6, for the conversion of carbon dioxide to ethanol in an aqueous phase.

8. A method of converting carbon dioxide to ethanol, the method comprising: providing a carbonate in an aqueous phase as a source of carbon dioxide; contacting the aqueous phase with a photocatalyst, the photocatalyst comprising a gallium nitride powder support and gold nanoparticles; and irradiating the photocatalyst with visible light to convert the carbonate into ethanol.

9. The method of claim 8, wherein the method is performed under ambient conditions of temperature and pressure.

10. The method of claim 8 or 9, wherein the method is performed at a temperature of from 15 to 30 °C.

11. The method of any one of claims 8 to 10, wherein the method is performed at a pressure of from 0.95 to 1.05 atm.

12. The method of any one of claims 8 to 11, wherein the carbonate is selected from the group consisting of NaHCO3, Na2CO3, KHCO3and K2CO3.

13. The method of any one of claims 8 to 11 , wherein the carbonate is a potassium carbonate.

14. The method of any one of claims 8 to 13, wherein the carbon dioxide is captured into the aqueous phase from ambient air with an inorganic hydroxide base in the aqueous phase.

15. The method of any one of claims 8 to 14, wherein the visible light has a wavelength of 400 to 600 nm.

16. A process for producing a photocatalyst, the process comprising: providing an aqueous suspension comprising GaN and gold; adding NaBH4to the aqueous suspension dropwise; sonicating and mixing the aqueous suspension to obtain a sonicated suspension; and aging the sonicated suspension to obtain a photocatalyst comprising a gallium nitride powder support and gold nanoparticles.

17. The process of claim 16, wherein the weight ratio of GaN to gold in the aqueous suspension is from 0.9:1.1 to 1.1 :0.9.

18. The process of claim 16 or 17, wherein the gold is provided in the suspension as HAuCl419. The process of claim 18, wherein the molar ratio of NaBH4to HAuCl4is from 150 to 180.

20. The process of any one of claims 16 to 19, further comprising purifying the photocatalyst with water.

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