Three‐Phase Photocatalysis for the Enhanced Selectivity and Activity of CO2 Reduction on a Hydrophobic Surface

Abstract The photocatalytic CO2 reduction reaction (CRR) represents a promising route for the clean utilization of stranded renewable resources, but poor selectivity resulting from the competing hydrogen evolution reaction (HER) in aqueous solution limits its practical applicability. In the present contribution a photocatalyst with hydrophobic surfaces was fabricated. It facilitates an efficient three‐phase contact of CO2 (gas), H2O (liquid), and catalyst (solid). Thus, concentrated CO2 molecules in the gas phase contact the catalyst surface directly, and can overcome the mass‐transfer limitations of CO2, inhibit the HER because of lowering proton contacts, and overall enhance the CRR. Even when loaded with platinum nanoparticles, one of the most efficient HER promotion cocatalysts, the three‐phase photocatalyst maintains a selectivity of 87.9 %. Overall, three‐phase photocatalysis provides a general and reliable method to enhance the competitiveness of the CRR.


Section 4. Pt loading on o-PCN to synthesize Pt/o-PCN. 0.3 g o-PCN
was dispersed in the mixture of 20 mL acetone and 10 mL ethanol under sonication. Then 0.03 g Pt(acac) 2 was added to the mixture under sonication. After stirring for 30 min, the mixture was moved to a sealed transparent container which was set under a pressure of 0.13 bar. Subsequently, the mixture under 0.13 bar was irradiated under visible light supplied by the visible light source for 18 h. Then the as-prepared sample was separated by centrifugation and washed three times with acetone. Then the samples were vacuum dried at 80 °C overnight to finally form Pt/o-PCN as a powder.  Figure S9, in which the Cu or Pd particles loaded on o-PCN can be clearly observed, and the lattice spacing can prove the composition. Section 6. Photocatalytic CO2 reduction. The home-made photocatalytic reactor consisted of a sealed chamber, an embedded window made by quartz glass and a liquid sampling port sealed by silicone pad. The reactor was connected to a gas circulation system with a ten-port valve (VICI) for on-line sampling to a gas chromatograph (Ruimin GC 2060, Shanghai). The gas circulation system was primarily made of stainless steel tubing and a home-made gas pump for the circulation of the gas. A mechanical pump was connected into the system to exhaust the carrier gas of the gas chromatograph when switch back the ten-port value. A pressure gauge was also connected into the system to monitor the gas pressure. The total volume of the gas in the circulation system after filling the reactor with solvent was 80 mL.
At the start of this reaction, 0.01 g catalysts were mixed with 40 mL KHCO 3 (0.1 M), Na 2 S (0.1 M) and Na 2 SO 3 (0.1 M) aqueous solution in the reactor. KHCO 3 is used to solution to enhance the solubility of CO 2 during photocatalytic reactions. Upon the introduction of CO 2 , a chemical equilibrium (HCO 3 -+ H 2 O ⇌ H 2 CO 3 + OH -⇌ CO 2 +H 2 O+OH -) will be established. [4] With the consumption of CO 2 , this chemical equilibrium will shift right, while if CO 2 is introduced again, this chemical equilibrium will shift left. [4][5] Thus, even without CO 2 , there is also a few of CO 2 molecules existing in KHCO 3 solution, which can be used to produce carbon derivatives ( Figure S7). However, without CO 2 , the photoreaction is unsustainable, because CO 2 and KHCO 3 in the solution will be used up soon. This chemical equilibrium can be proved by the isotope experiment reported by Dunwell et al. [5] They use 13 CO 2 purged NaH 12 CO 3 for the reaction of CO 2 reduction, and results showed that 13 C atoms exist in both dissolved CO 2 and KHCO 3 : 13 CO 2 (aq) + H 12 CO 3 -⇌ 12 CO 2 (aq) + H 13 CO 3 -. [5] The system was evacuated to remove air firstly. Subsequently, the suspension in the reactor was purged with CO 2 (≥ 99.995%) for 1 h to achieve CO 2 saturation and the initial CO 2 pressure was kept atmospheric. There was no more CO 2 purged into the closed system during the reaction. The gases in the closed circulation system were continuously circulated through the suspension for the entire reaction period. The incident light source was supplied by a 300 W xenon lamp and a visible light filter (Beijing PerfectLight Co.), which can provide the visible light with the wavelength longer than 400 nm (λ > 400 nm). After the reaction of 1 h, the gas and liquid products was detected by the analysis system.
The analysis of the gaseous reaction mixtures containing CO, CH 4 , H 2 , was carried out using a gas chromatograph, which was equipped with a TCD, FID and a methanizer which contained a Ni catalyst. Argon (≥ 99.999%) was used as the carrier gas. The back channel of GC was equipped with two packed columns, TDX-01 and Molsieve 5 Å, and two gas switch valves. During the analysis, 1.0 mL of gas sample in the sample loop of ten-port value was introduced to the TDX-01 column where CO 2 was separated from the other gases due to its longer retention time. The rest of the gases after the TDX-01 column was further separated by the Molsieve 5 Å column. The gas product of H 2 was detected by TCD and CH 4 , CO were further detected by FID with higher sensitivities. The role of methanizer was to convert CO to CH 4 for FID analysis. The liquid product of CH 3 OH was injected into HP-5 capillary column (30 m × 0.32 mm ID × 0.5 μm, Agilent) and detected by FID after the reaction. No CH 3 OH can be detected in this work. Other liquid products such as HCOOH and HCHO possibly formed during the reaction were also analyzed by high-performance liquid chromatography (HPLC 7080B, Agilent Co., USA), but no such products were detected under our reaction conditions. Cycling test was operated using the same method, and each cycle continued 3 h. after each cycle, the catalysts were separated by centrifugation, dried under vacuum at 80 °C for 6 h and then reused in the next cycle.

Section 8. in situ IR test.
A diffuse reflection in situ IR was used to investigate the effective CO 2 adsorption on the surface. Samples are loaded in the sample holder with a flat surface. A cover was fixed on the sample holder to form a reaction space, which was then cleaned by argon. Then the IR data was collected as background. Subsequently, visible light was introduced to the reaction space by observation window, and CO 2 was introduced through container filled with water which make the gas bring water into the reaction space. Then IR data was collected every 20 min, and the change based on IR can be recorded. Section 9. Stability of the polymer-modified surface during visible irradiation and during the reaction. The polymer modified surface of o-PCN is very stable under visible light irradiation and during the photoreduction, which can be proved by several evidences. (1) As shown in insert of Figure S7, after the photoreaction under visible light, the contact angle of water on o-PCN is 91.5°, which is very close to that before photoreaction (89.7°), almost remaining unchanged. Because the hydrophobic properties of o-PCN are provided by the polymer, it can be deduced that the polymers remain stable under visible irradiation and during the photoreduction. (2) It has been explained in details in section 6 of methods and Figure S7 that the carbon source of carbon derivatives is CO 2 , which means the carbon skeleton of polymer did not react during photoreaction. (3) As described in section 2 of methods, the polymer was connected to PCN by visible light with 48 h, and after that the hydrophobicity of surface was greatly improved. It can be deduced that if the polymer is unstable under visible light, it will be decomposed totally in 48 h and would not improve the hydrophobicity of the surface. Section 10. The calculation of turnover number (TON) over Pt/o-PCN. The turnover number (TON) was calculated using the following equation: [8] TON= moles of products evolved moles of active components on photocatalyst (1) Moles of products evolved: In this work, only CO and CH 4 can be detected as the products of CO 2 reduction. We detected the amount of both CO and CH 4 base on the method described in section 6 of methods in the SI. Section 11. Investigation of carbon source with isotope labeled CO2. The reaction system of the isotope experiment is the sealed circulatory system mentioned in section 6 of methods in SI. 1 g NaH 13 CO 3 (98 atom% 13 C, Sigma Aldrich) was added into 32.2 mL Na 2 S (0.1 M) and Na 2 SO 3 (0.1 M) aqueous solution in the reactor to provide H 13 CO 3 -. Then after adding 0.01 g Pt/o-PCN, the system was sealed. The system was evacuated to remove air firstly, and then the suspension in the reactor was purged with argon for 1 h to remove air. Then 7.8 mL hydrochloric acid (1 M) was injected through the liquid sampling port with silica gel pad for the in situ generation of 13 CO 2 via the reaction H + + H 13 CO 3 -= H 2 O + 13 CO 2 . According to the stoichiometric ratio, the amount of hydrochloric acid added can ensure the remaining of 4 mmol H 13 CO 3in these 40 mL mixture, making the concentration of H 13 CO 3to be 0.1 M. Then after a circulation for 1 h, visible light was provided for the photocatalytic reaction. After the reaction of 5 h, the gas sample was detected by a gas chromatography-mass spectrometer (GC-MS) instrument.
The gas chromatograph (GC) is the same one that we used to detect reaction activity (described in section 6 of methods in SI) with the columns of TDX-01 and Molsieve 5 Å. The difference is that the gas products separated by GC was introduced into a mass spectrometer (HIDEN QIC-20) instead of Flame Ionization Detector (FID). We monitored the characteristic mass-charge ratios (m/z) of 28, 29, 16 and 17 to detect the 12 CO, 13 CO, 12 CH 4 and 13 CH 4 in the product, respectively, and results are shown in Figure S6. According to the results of GC recorded by FID in the former experiment when detecting activity (section 6 of methods in SI), the retention time of CO and CH 4 is 4.990 and 9.234 min, respectively. Thus, because the same GC was used for GC-MS, the signal appeared in similar retention recorded by MS were the time is reasonable to be recognized as the signal of CO and CH 4 . The retention time of carbon derivatives with 13 C is slightly larger than that with 12 C, which also have been observed by other researchers. [10] Figure S6b shows that 12 CO can be detected when 12 CO 2 and H 12 CO 3are provided, and 13 CO can be detected when 13 CO 2 and H 13 CO 3are used as the carbon source. [11] Figure S6c shows that 12 CH 4 can be detected when 12 CO 2 and H 12 CO 3are provided, and 13 CH 4 can be detected when 13 CO 2 and H 13 CO 3are used as the carbon source. [10] These results proved that the photocatalytic products indeed originate from CO 2 reduction. [10] Section 12. The action spectra of photocatalytic CO2 reduction reaction (CRR). The action spectra is a graph of the rate of an activity plotted against the wavelength of light. [12] To detect the activity of CRR with Pt/o-PCN under the irradiations with different wavelength, we use a similar method which was described in section 6 of methods in the SI. The only difference is the incident light was changed from visible light (λ > 400 nm) to monochromatic light with a specific wavelengths, which were provided by a 300 W xenon lamp (Beijing PerfectLight Co.) and various bandpass filters (Beijing China Education Au-light Co., Ltd). The CRR activities under monochromatic irradiations with wavelengths of 365, 380, 420, 435, 525 and 700 nm are listed in Table S1, According to which we can get the action spectra ( Figure S8). Section 13. Apparent quantum efficiency (AQE) for photocatalytic CRR. The AQE is defined by the equation: [13] where d[x]/dt is the rate of change of the concentration of the reactant (or product) and d[hv] inc /dt is the total optical power impinging on the sample. Generally, for the convenient of measurement and calculation, researchers usually use the integral form of equation (2): [14] AQE(%)= number of the electrons taking part in reaction number of incident photons ×100% (3) The number of electrons taking part in the reaction is calculated according to the action spectra, which was described in section 12 of methods in the SI. In this work, the products of CRR are only CO and CH 4  The numerator and denominator of equation (4) are divided by unit time simultaneously, we can get: [15] AQE(%)= (2 × number of evolved CO molecules + 8 × number of evolved CH 4 molecules)/unit time number of incident photons/unit time ×100% (5) We denote the numerator of equation (5) as N e , and the denominator of equation (5) as N p . Thus, the N e means the amounts of electrons taking part in CRR during the unit time, and N p means the amounts of incident photons during the unit time, and equation (5) can be written as: [15] AQE(%)= N e N p ×100% Thus, we can calculate the AQE as long as we can get the value of N e and N p under the incident light with a specific wavelength.
Here we demonstrate the process with the example of AQE calculation under irradiation with the wavelength of 420 nm.
N e : The amount of CO and CH 4 generated by unit catalyst at the unit time was detected under the irradiation with the wavelength of 420 nm (as described in section 12 of methods in the SI). As shown in Table S1, the generation rate of CO and CH 4 under 420 nm is 0.531 and 0.229 μmol h -1 , which can be transferred to be 1.475 × 10 -10 and 6.361 × 10 -11 mol s -1 , respectively. Thus, N e = 2 × 1.475 × 10 -10 + 8 × 6.361 × 10 -11 = 8.039 × 10 -10 mol s -1 .
N p : The energy of single photon (denoted as E s ) at λ = 420 nm can be calculated to be 4.736 × 10 -19 J according to E s = (hc) / λ, where h and c are the Planck constant (6.626 × 10 -34 J· s) and the speed of light (2.998 × 10 8 m s -1 ), respectively. The light intensity (denoted as I in ) was measured to be 0.35 mW cm −2 by an irradiatometer (Photoelectric Instrument Factory of Beijing Normal University). The diameter (denoted as d) of illumination window of the reactor was measured to be 5 cm, and thus the irradiation area (denoted as S in ) can be calculated to be 19.63 cm 2 according to the equation S in =π × (d/2) 2 . Thus, the N p can be calculated by divide the total energy provided in unit time by the energy of single photon: N p = (I in × S in ) / E s = 2.408 × 10 -08 mol s -1 . [16] Thus, according to equation (6), AQE (%) = N e / N p × 100%= 8.039 × 10 -10 mol s -1 / 2.408 × 10 -08 mol s -1 × 100% = 3.337%.
In addition, the AQE of CRR at λ = 365, 380, 435, 525 and 700 nm can be calculated by the same process. Results are listed in Table S1, according to which we can get the AQE spectra as shown in Figure S8.      Figure S6. Signal of GC-FID and GC-MS. a) the raw data of the signal of GC-FID which was used to investigation the activity using the method described in section 6 of methods in the SI. b) The GC-MS signal monitored by m/z = 28 and m/z = 29 for the detection of 12 CO and 13 CO, respectively. [11] c) The GC-MS signal monitored by m/z = 16 and m/z = 17 for the detection of 12 CH4 and 13 CH4, respectively. [10] Figure S7. To investigate the carbon source of CRR with o-PCN, control experiment was operated by replacing CO2 to N2. Result shown that without CO2 there are almost no carbon derivatives can be detected. A few carbon derivatives in products can be attribute to the KHCO3 solution, which is described in detail in section 6 of methods. Insert: Photographs of contact angles of water on o-PCN before and after photoreaction.  Table S1. Under the irradiation with different wavelengths, the ration of CO and CH4 is variable. The highest AQE within the visible region is 3.337, which is achieved under the irradiation of 420 nm.  Because the adsorption of CO2 as the form of COOH* is a result of the effective mass transfer of gas phase to the surface, the rate of increase of the main IR signal peaks of COOH* may be used as the quantitative parameter to reflect the effective mass transfer. The main IR peak of COOH* is the one at the wavenumber of 1728 cm -1 (Figure 4d). The average increasing rates can be calculated based on the slope of fitting lines, which determined to be 7.040 × 10 -3 min -1 and 1.859 × 10 -3 min -1 for Pt/o-PCN and Pt/PCN, respectively.