Department of Chemistry, University of New Hampshire, Durham, New Hampshire 03824, United

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1 Supporting Information Selective CO2 Reduction Catalyzed by Single Cobalt Sites on Carbon Nitride under Visible-Light Irradiation Peipei Huang, Jiahao Huang, Sebastian A. Pantovich, Alexander D. Carl, Thomas G. Fenton, Christine A. Caputo, Ronald L. Grimm, Anatoly I. Frenkel,*,, ǁ and Gonghu Li *, Department of Chemistry, University of New Hampshire, Durham, New Hampshire 03824, United States Department of Materials Science and Chemical Engineering, Stony Brook University, Stony Brook, New York 11794, United States ǁ Division of Chemistry, Brookhaven National Laboratory, Upton, New York 11973, United States Department of Chemistry & Biochemistry, Worcester Polytechnic Institute, Worcester, Massachusetts 01609, United States * Corresponding authors. anatoly.frenkel@stonybrook.edu (A.I.F.); gonghu.li@unh.edu (G.L.) Content 1. Experimental Details 2. Estimated Quantum Yields 3. Supporting Figures References S1

2 1. Experimental Details Materials: Acetonitrile (99.999%), 1,4,8,11-tetraazacyclotetradecane (cyclam, 98%), and cobalt(ii) chloride were obtained from Sigma-Aldrich. Methanol and chloroform (99.8%) were purchased from Pharmco-Aaper. Triethylamine (TEA, 99%) was obtained from Acros. Triethanolamine (TEOA, 99%) and urea (98+%) were obtained from Alfa Aesar. Hydrophilic fumed silica (Aerosil 200, SiO 2 ) was purchased from Evonik Industries. All reagents were used without further purification. Catalyst Synthesis: Graphitic carbon nitride (C 3 N 4 ) was prepared by pyrolysis of urea. In a typical synthesis, 20 g urea was put into a covered crucible and calcined in a muffle furnace at 600 C for 4 hours (ramp rate 5 C/min). In order to deposite Co 2+ on C 3 N 4, 100 mg C 3 N 4 were mixed with a specific amount of CoCl 2 in 7.5 ml acetonitrile. Then 65 µl TEA was added to the mixture, which was subsequently stirred for 1 h. The mixture in a capped reaction vessel was placed in a CEM Discover single-mode microwave reactor and was heated to 80 C for 120 min. After the microwave reaction, the resulting precipitate was recovered by centrifugation, and washed twice with chloroform, methanol and acetonitrile, respectively. After drying at room temperature, a hybrid photocatalyst, denoted Co 3 N 4, was obtained. Following the same method, Co 3 N 4 samples were also synthesized in the absence of TEA. A control sample, CoO x /SiO 2, was synthesized using 100 mg SiO 2 and 5.0 mg CoCl 2 in the presence of TEA. A molecular catalyst, Co-cyclam, was synthesized following an established method. 1 Materials Characterization: Elemental analysis was conducted by acid digestion, followed by quantification using a Varian Vista AX induced coupled plasma atomic emission spectrometer. X-ray diffraction (XRD) patterns of powder samples were collected on a Rigaku XDS 2000 diffractometer using nickel-filtered Cu Kα radiation (λ = Ǻ). Scanning electron microscopy (SEM) images and energy dispersive spectroscopy (EDS) were collected on an Amray 3300FE field emission SEM with PGT Imix-PC microanalysis system. Transmission electron microscopy (TEM) images were obtained on a Zeiss/LEO 922 Omega system. X-ray photoelectron spectra (XPS) were collected on a Kratos Axis HS XPS system. UV-visible spectra were obtained on a Cary 50 Bio spectrophotometer. A Barrelino diffuse reflectance probe was used to collect UV-visible spectra of powder samples using BaSO 4 as a standard. Transmission FTIR spectra were collected on a Thermo Nicolet is10 FTIR spectrometer. X-ray Absorption Spectroscopy: X-ray absorption spectra at Co K-edge were taken at the beamline 7-BM (QAS) of NSLS-II at Brookhaven National Laboratory. Si (111) double crystal was used as monochromator and detuned 30% to reduce harmonics. The 15-cm long ion chambers, which were filled with 100% N 2, were used for detection of incident and transmitted beams, and passivated implanted planar silicon (PIPS) detector was used for detection of fluorescence from the sample. The beam size was 1.4 mm (vertical) 6 mm (horizontal). Co-cyclam, high-co 3 N 4 and CoO x /SiO 2 samples were measured in transmission mode, and the low-co 3 N 4 was measured in fluorescence mode. CoO x /SiO 2 was deposited on tape and the other samples were made into 13 mm S2

3 diameter pellets. At least three scans were measured for each sample. All measurements were performed in ambient atmosphere at room temperature and the samples were held in 45 geometry. A Co foil was placed between the two detectors downstream from the sample and measured simultaneously with the sample as reference for energy alignment. The existing data for Co oxides were aligned with the samples spectra using their respective reference foil spectra. Photocatalytic Testing: In photocatalytic CO 2 reduction, a specific amount (~1 mg) of catalyst was dispersed in a 4.0 ml acetonitrile solution containing TEOA (acetonitrile : TEOA= 4:1 v/v) in a quartz test tube. Prior to photocatalytic testing, the reaction solution was bubbled with CO 2 (99.999%, Airgas) in the dark for 20 min. The reaction solution was then irradiated with a halogen lamp equipped with a water filter (see Figure S3 for output spectrum). In some tests, a long-pass (λ > 420 nm) optical filter was used with the lamp. Light intensity on the reaction solution was fixed at 200 mw/cm 2. The head space above the reaction solution was sampled with a gas-tight syringe at different time intervals for product analysis using an Agilent 7820 GC equipped with a TCD detector and a 60/80 Carboxen-1000 packed column (Supelco). In isotope experiments, 13CO 2 (Sigma-Aldrich, carbon-13 99%) was introduced into a reaction solution containing 4 mg Co 3 N 4 after degassing the solution through freeze-pump-thaw cycling. The head space above the reaction solution was sampled and analyzed in a gas cell with the Thermo Nicolet is10 FTIR spectrometer. S3

4 2. Estimated Quantum Yields Quantum yields for photocatalytic CO 2 reduction were estimated based on the amounts of produced CO and the amounts of photons absorbed by the reaction solutions. The change in light intensity at 400 nm was measured after passing through a reaction solution with a cross section area of 4.95 cm 2. This following equation was used to calculate quantum yields because CO 2 -to-co conversion is a two-electron process, Quantum Yield = 2n(CO)/n(photon), where n(co) and n(photon) are the amounts of CO molecules produced and photons absorbed, respectively. Table S1 summarizes calculated quantum yields for CO production using different Co 3 N 4 samples prepared in the absence or presence of TEA. The quantum yield was further optimized by varying the mass of Co 3 N 4 and light intensity in photocatalysis using Co 3 N 4 with a cobalt loading of µmol/mg (Table S2). Table S1. Quantum yields for CO production after CO 2 reduction for 2 h using 1 mg Co 3 N 4. Light intensity was kept at 200 mw/cm 2. Table S2. Optimized quantum yields for CO production after CO 2 reduction for 2 h using Co 3 N 4 with a cobalt loading of µmol/mg. S4

5 3. Supplementary Figures Figure S1. Actual cobalt loadings in Co 3 N 4 prepared in the presence (solid cycle) and absence (open cycle) of TEA using different amounts of CoCl 2 in the synthesis. Figure S2. (a) XRD and (b) optical spectra of C-doped (red) and O-doped (blue) C 3 N 4 ; (c) CO production upon CO 2 reduction using Co 2+ deposited on doped C 3 N 4. The doped C 3 N 4 samples were synthesized following literature methods. 2-3 Cobalt loadings were determined to be µmol/mg and µmol/mg for Co 3 N 4 and Co 3 N 4, respectively. The Co 3 N 4 samples reported in this work are likely C-doped since their properties are similar to those of Co 3 N 4. Photocatalysis conditions: 1 mg photocatalyst in 4.0 ml acetonitrile containing TEOA, light intensity 200 mw/cm 2. S5

6 Figure S3. (a) TEM and (b) SEM images of bare C 3 N 4 ; (c) TEM image, (d) SEM image, and elemental distribution of (e) N and (f) Co of Co 3 N 4 with a cobalt loading of µmol/mg. Figure S4. Output spectra of the halogen lamp used in this study without (red trace) and with (black trace) a 420-nm long-pass optical filter. S6

7 Figure S5. The amounts of CO produced in CO 2 reduction using (a) 1 mg Co 3 N 4 with a cobalt loading of µmol/mg; (b) a mixture of µmol CoCl 2 and 1 mg C 3 N 4 ; and (c) 1 mg bare C 3 N 4 (light intensity 200 mw/cm 2 ). Figure S6. TONs of CO produced in CO 2 reduction under visible-light irradiation (λ > 420 nm, light intensity 200 mw/cm 2 ) using 1 mg Co 3 N 4 with a cobalt loading of µmol/mg prepared in the absence of TEA. Insert: infrared spectrum of gaseous 13 CO produced in the reduction of 13 CO 2. S7

8 Figure S7. CO production upon CO 2 reduction for five 2-hour cycles using 10 mg Co 3 N 4 with a cobalt loading of µmol/mg in 4.0 ml acetonitrile containing TEOA (light intensity 60 mw/cm 2 ). After reaction for 2 h, the photocatalyst was collected by centrifugation, rinsed with acetonitrile once, and re-dispersed in 4.0 ml fresh acetonitrile containing TEOA. Photocatalysis was then repeated under the same conditions after purging with CO 2. Figure S8. SEM images of Co 3 N 4 with a cobalt loading of µmol/mg: (a) before and (b) after photocatalysis for 30 h (see Figure S6 for experimental conditions). S8

9 Figure S9. SEM images of Co 3 N 4 with a cobalt loading of µmol/mg: (a) before and (b) after photocatalysis for five 2-hour cycles (see Figure S7 for experimental conditions). Figure S10. Optical spectrum of CoO x /SiO 2 in the powder form. The peaks between 500 nm and 700 nm are associated with cobalt oxides. 4 S9

10 Figure S11. Close comparison of normalized Co K-edge XANES spectra shown in Figure 4. Figure S12. O 1s XPS of bare C 3 N 4 and different Co 3 N 4 samples. In the spectra of Co 3 N 4 samples, O 1s peaks associated with O 2- are seen between 528 ev and 530 ev, 5 likely due to the presence of OH - bound to Co 2+ as well as CoO in the sample with a cobalt loading of µmol/mg. S10

11 References (1) Jin, T.; Liu, C.; Li, G. Photocatalytic CO 2 reduction using a molecular cobalt complex deposited on TiO 2 nanoparticles, Chem. Commun. 2014, 50, (2) Che, W.; Cheng, W.; Yao, T.; Tang, F.; Liu, W.; Su, H.; Huang, Y.; Liu, Q.; Liu, J.; Hu, F.; Pan, Z.; Sun, Z.; Wei, S. Fast Photoelectron Transfer in (C ring ) C 3 N 4 Plane Heterostructural Nanosheets for Overall Water Splitting, J. Am. Chem. Soc. 2017, 139, (3) Liu, C.; Huang, H.; Cui, W.; Dong, F.; Zhang, Y. Band structure engineering and efficient charge transport in oxygen substituted g-c 3 N 4 for superior photocatalytic hydrogen evolution, Applied Catal. B: Environ. 2018, 230, (4) Katsoulidis, A.P.; Petrakis, D.E.; Armatas, G.S.; Trikalitis, P.N.; Pomonis, P.J. Ordered mesoporous CoOx/MCM-41 materials exhibiting long-range self-organized nanostructured morphology, Microporous Mesoporous Mater. 2006, 92, (5) Wang, X.; Yang, W.; Li, F.; Xue, Y.; Liu, R.; Hao, Y. In Situ Microwave-Assisted Synthesis of Porous N-TiO 2 /g-c 3 N 4 Heterojunctions with Enhanced Visible-Light Photocatalytic Properties, Ind. Eng. Chem. Res. 2013, 52, S11