Selective Methane Photooxidation into Methanol under Mild Conditions Promoted by Highly Dispersed Cu Atoms on Crystalline Carbon Nitrides

Here we report a photocatalytic system based on crystalline carbon nitrides (PHI) and highly dispersed transition metals (Fe, Co and Cu) for controlled methane photooxidation to methanol under mild conditions. The Cu-PHI catalyst showed a remarkable methanol production (2900 μmol g-1) in 4 hours, with a turnover number of 51 moles of oxygenated liquid product per mole of Cu. To date, this result is the highest value for methane oxidation under mild conditions (1 bar, 25 °C).

For transmission (TEM) and scanning transmission electron microscopy (STEM) observations, a suspension of the sample in ethanol was sonicated for 10 min and then drop-casted to a Cu grid with a lacey carbon support and dried for 5 min.The (S)TEM study was performed using a double Cs corrected JEOL JEM-ARM200F (S)TEM operated at 80 kV and equipped with a cold-field emission gun and a high-angle silicon drift energy dispersive X-ray (EDX) detector (solid angle up to 0.98 steradians with a detection area of 100 mm 2 ).Annular dark-field scanning transmission electron microscopy (ADF -STEM) images were collected at a probe convergence semi-angle of 25 mrad.The "beam shower" procedure was performed for 30 min to reduce hydrocarbon contamination during subsequent imaging at high magnification.
Raman spectra were recorded using a confocal Raman microscope alpha300 (WITec, Germany) coupled with a laser excitation at wavelength of 785 nm.The laser beam of was focused through a Nikon 20 × microscope objective lens.The Raman spectra have been measured with an integration time of 10 s under excitation laser powers 60 mW, respectively.The spectra were acquired with a thermoelectrically cooled Andor CCD detector DU401A-BV placed behind the spectrometer UHTS 300 from WITec with a spectral resolution of 3 cm -1 .The Raman band of a silicon wafer at 520 cm -1 was used to calibrate the spectrometer.The experiments under the reaction condition were performed by dropping H2O2 on Cu-PHI, deposited over a glass microscope slide, the analysis was carried in the same way described above.
The Mott-Schottky measurements were performed in a Biologic MPG-2 system using a 3 electrode set up consisting of a Pt wire working as counter electrode, an Ag/AgCl as reference electrode F-doped tin oxide (FTO) glass coated with the material as working electrode.The working electrode was prepared on FTO glass that was cleaned by sonication in ethanol for 30 min and dried at 353 K.The boundary of FTO glass was protected using Scotch tape.The 3 mg sample was dispersed in 0.2 mL of water by sonication to get a slurry mixture with 20 μL of Nafion.The slurry was spread onto pretreated FTO glass.After air-drying, the Scotch tape was removed and the working electrode was further dried at 393 K for 2 h to improve adhesion.X-ray photoelectron spectroscopy (XPS) measurements were performed on a ThermoScientific Escalab 250 Xi.A microfocused, monochromated Al Kα X-ray source (1486.68eV) and a 400 μm spot size were used in the analysis.Samples were prepared using carbon tape.LiCl was added to each sample in order to calibrate the binding energies towards Li.ThermoScientific Avantage software was used to analyze the resulting spectra.Time-resolved fluorescence measurements were performed by using a single photon counting setup (TCSPC) with a Becker&Hickl PML-spectrometer (modified Oriel MS-125) with a laser repetition rate of 2 MHz.The detector comprises a Becker&Hickl PML-16-C-1 (modified Hamamatsu) multi-alkaline photomultiplier.The excitation wavelength was 405 nm.The excitation was carried out using a pulsed laser diode at ~30 nJ/cm² (LDH-P-C405, PicoQuant GmbH).The emission was recorded in the range of 460-600 nm, while blocking the secondary detection of the excitation pulses with a 450 nm cut-off-filter.Raw decay data presented as logarithm of photon counts versus time were analyzed with data analysis software of PicoQuant GmbH (Germany)

Photocatalytic Tests
Methane photo-oxidation tests was carried out in a quartz tube (140 mL) illuminated with 6 visible light lamps (15 W) (Fig S15).The reaction temperature was maintained at 25ºC using a thermostatic bath.In each test, 50 mg of photocatalyst was added to a hydrogen peroxide solution (0.8 mM) in deionized water.In order to saturate the reactor, CH4 (99.9%) was bubbled into the suspension with a constant flow for 15 minutes.The production of CO2 and CO was analyzed at the end of the reaction (4 h) in a gas chromatograph (Thermo CP-3800) equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD) with a packed HayeSep N column (0.5 m x 1.8") and a 13X molecular sieve column (1.5 m x 1.8").Argon was used as the carrier gas, and the methanizer temperature was 350 °C.
Liquid products were quantified by 1 H nuclear magnetic resonance (NMR) (600 MHz, Ascend™ 600 Bruker) at 25 °C.For each test, 540 μL of the sample was mixed with 60 μL of D2O solution containing 5.0 mM dimethyl sulfoxide (DMSO) as a standard and 0.21 mM TSPd4 as a reference.
A WET procedure suppressed the water peak.Nuclear magnetic resonance data were processed using the MestReNova software.Representative The concentration of liquid formaldehyde (HCHO) was quantified by the colorimetric method described elsewhere. 3An aqueous solution (100 mL) was prepared by dissolving 15 g of ammonium acetate, 0.3 mL of acetic acid, and 0.2 mL of pentane-2,4-dione.Then, 0.5 mL of reaction liquid product was mixed with 2.0 mL of water and 0.5 mL of reagent solution.The Methane conversion was calculated based on the sum of methane in liquid (dissolved in water) and gaseous phases.Liquid methane was extracted from Duan and Mao's study 4 , which predicts that solubility of methane in pure H2O is 0.00126 mol.kg -1 .Since 100 mL of water was used at methane oxidation reactions, we can assume that 126 µmols of CH4 is present in liquid phase.
Methane in gaseous phase was calculated based on STP conditions, i.e. 1 mole of gas at 25 L.
Therefore, 1600 µmols of CH4 was considered in 40 mL of headspace.Methane conversion was calculated as follows: CH4 Conversion = Sum of all gaseous and liquid products (μmols) 1600+126 μmols x 100%
For the spin trapping experiments with DMPO, 20 mg of this spin trap were solubilized in 1 mL of solvent: (i) deionized water to detect .OH radicals or (ii) acetonitrile saturated with oxygen to observe O2 .radicals.For the spin trapping tests involving PBN, 25 mg of this compound was solubilized in 1 mL of a 1:1 solution (ethanol/ water) to evaluate the kinetics of hydroxyl radicals (indirectly) or in methanol to evaluate the formation of alcohol radicals.In these solutions, 5 mg of photocatalyst was suspended, 6 µL of H2O2 (30 % V/V) were added and the systems was illuminated with a white LED lamp with irradiance of 16 mW.cm - .Aliquots were removed with the aid of a glass capillary (~ 50 µL) and placed in a quartz tube (Wilmad Labglass, United States,) which was then inserted into the cavity of the EPR spectrometer.The adducts were simulated using Easyspin 5 .

SUPPLEMENTARY TABLES
Table S1 Vibrational modes and corresponding Raman shifts for M-PHIs materials.

Raman Shift (cm
1 H NMR spectra used to determine and calculate the concentration of liquid products are shown in Fig S16.The quantification of liquid products by NMR 1 H for all compounds follows the equation described below: µmols = Compound Area x 6 (number of H of DMSO) x 5 (Concentration of DMSO in µmols) DMSO Area x Number of H of the compound To confirm the quantification of NMR experiments, methanol and ethanol were also quantified by GC-FID (using a DB-WAX column and He as carrier gas).For each test, 150 µL of reaction sample was mixed with 50 µL of a 1-Octanol solution (1.5 mM) in high purity CH3CN (99.99%).To ensure that methanol was only present in the reaction sample, blanks with deionized water and the external standard were injected.The chromatogram of a methanol and ethanol solution is exhibit in Fig S17.Calibration curve of methanol using GC-FID is shown at Fig S18.
mixed solution was maintained at 35 °C and measured by UV−Vis absorption spectroscopy at 412 nm.The concentration of HCHO in the liquid product was determined by the calibration curve (Fig S21).

Fig
Fig S4 FTIR spectra for Cu-PHI and Na-PHI showing slighter shifts in wavenumbers, indicating coordination of metal atoms in PHI structure.

Fig
Fig S14 EPR powder spectra of (a) Cu-PHI 0.5% at different temperatures and (b) the Fe-PHI powder spectrum measured at room temperature.

Fig
Fig S15 Schematic representation of photocatalytic system for methane oxidation reactions.

Fig S19
Fig S19 Calibration curve of CO2 calculated by GC-TCD.

Fig
Fig S21 Calibration curve for formaldehyde quantification by colorimetric method.

Fig
Fig S22 XRD of Cu-PHI 0.5% after reaction.The strong broad peak between 20-30° indicated that 2D distance is distorted after the break of symmetry promoted by reaction conditions (see Raman).However, the structural integrity of the Cu-PHI is mostly preserved.

Table S2
Methane oxidation results from literature.

Table S3
Methane photo-oxidation results of the catalysts synthesized in this work.Productions below 20 µmol.g -1 are considered negligible.

Table S4
Metal loading measured by ICP-OES for the catalysts.

Table S5
Time-resolved photoluminescence fluorescence lifetimes for the catalysts and with the addition of hydrogen peroxide.

Table S6
Quantities of copper chloride added to Na-PHI suspension during the cation exchange procedure and its respective yields.