Photochemical composition and use thereof for producing CH.SUB.4 .from CO.SUB.2 .and/or CO

Abstract

The present invention relates to photochemical compositions comprising: a solution comprising an organic solvent, preferably selected from dimethylformamide, acetonitrile, and mixtures thereof with water, a sacrificial electron donor; a proton donor having a pKa in acetonitrile greater than or equal to 28; a photosensitizer whose reduced state has a standard redox potential more negative than 1.45 V vs SCE; and a metal porphyrin complex of formula (I) as defined in claim 1,
useful in the production of CH.sub.4 from CO.sub.2 or CO by photochemical catalysis, to a photochemical cell comprising same and to a method for producing CH.sub.4 from CO.sub.2 or CO by photochemical catalysis using same.

Claims

1. A photochemical composition for producing methane by reducing at least one of CO.sub.2 and CO at visible light, comprising: a solution comprising an organic solvent, a sacrificial electron donor; a proton donor having a pKa in acetonitrile greater than or equal to 28; a photosensitizer whose reduced state has a standard redox potential more negative than 1.45 V vs Saturated Calomel Electrode, wherein the photosensitizer is a complex of a transition metal with at least two 2-phenylpyridine (ppy) ligand; and a metal porphyrin complex of formula (I): ##STR00019## wherein: M represents a transition metal ion, R.sub.1 to R.sub.10 and R.sub.1 to R.sub.10 are independently selected from the group consisting of H, OH, F, C.sub.1-C.sub.6 alcohol, and N.sup.+(C.sub.1-C.sub.4 alkyl).sub.3, and wherein: at least one of R.sub.1 to R.sub.5 is OH and at least one of R.sub.1 to R.sub.5 is OH, or at least one of R.sub.1 to R.sub.5 is N.sup.+(C.sub.1-C.sub.4 alkyl).sub.3, and at least one of R.sub.1 to R.sub.5 is N.sup.+(C.sub.1-C.sub.4 alkyl).sub.3, and salts thereof.

2. The photochemical composition of claim 1, wherein the photosensitizer having a standard redox potential more negative than 1.35 V vs Saturated Calomel Electrode in the reduced state is a metal complex of formula (III): ##STR00020## wherein Y.sub.1, Y.sub.2 and Y.sub.3 are CH.sub.2, and M represents a transition metal.

3. The photochemical composition of claim 1, wherein the concentration of photosensitizer in the photochemical composition is of between 50 mM and 1 mM.

4. The photochemical composition of claim 1, wherein the concentration of sacrificial electron donor in the photochemical composition is of between 10 mM and 500 mM.

5. The photochemical composition of claim 1, wherein the sacrificial electron donor is a tertiary amine.

6. The photochemical composition of claim 5, wherein the tertiary amine is: of formula NR.sub.1R.sub.2R.sub.3, in which R.sub.1, R.sub.2 and R.sub.3 are identical or different and each independently selected from a C.sub.1-C.sub.6 alkyl group optionally substituted with OH, OC.sub.1-C.sub.6 alkyl, or COOH; or of formula (IV): ##STR00021## wherein X is O or NR, with R representing a (C.sub.1-C.sub.4)alkyl, R is a (C.sub.1-C.sub.4) alkyl, R is a hydrogen, halogen, CN, or a (C.sub.1-C.sub.4) alkyl, and Ar.sub.4 is selected from a monocyclic or bicyclic 5- to 10-membered aromatic or heteroaromatic ring, optionally substituted by a halogen, CN, OH, a biaryl group or a monocyclic 5- or 6-membered aromatic or heteroaromatic ring, said monocyclic 5- or 6-membered aromatic or heteroaromatic ring being optionally substituted by a halogen, CN, OH.

7. The photochemical composition of claim 1, wherein the proton donor is phenol or trifluoroethanol.

8. The photochemical composition of claim 1, wherein the metal porphyrin complex of formula (I) comprises at least two N.sup.+(C.sub.1-C.sub.4 alkyl).sub.3 groups.

9. The photochemical composition of claim 1, wherein, in the metal porphyrin complex of formula (I): at least one of R.sub.6 to R.sub.10 is OH and at least one of R.sub.6 to R.sub.10 is OH, or at least one of R.sub.6 to R.sub.10 is N.sup.+(C.sub.1-C.sub.4 alkyl).sub.3, and at least one of R.sub.6 to R.sub.10 is N.sup.+(C.sub.1-C.sub.4 alkyl).sub.3.

10. The photochemical composition of claim 1, wherein in the metal porphyrin complex of formula (I): R.sub.1 to R.sub.10 and R.sub.1 to R.sub.10 are independently H or N.sup.+(C.sub.1-C.sub.4 alkyl).sub.3, at least one and at most two of R.sub.1 to R.sub.5 represent N.sup.+(C.sub.1-C.sub.4 alkyl).sub.3, and at least one and at most two of R.sub.1 to R.sub.5 represent N.sup.+(C.sub.1-C.sub.4 alkyl).sub.3.

11. The photochemical composition of claim 1, wherein the metal porphyrin complex of formula (I) is selected from: ##STR00022## ##STR00023## ##STR00024## ##STR00025## ##STR00026## ##STR00027## and salts thereof.

12. The photochemical composition of claim 1, wherein: the sacrificial electron donor is the tertiary amine of formula NR.sub.1R.sub.2R.sub.3, in which R.sub.1, R.sub.2 and R.sub.3 are identical or different and each independently selected from a C.sub.1-C.sub.6 alkyl group optionally substituted with OH, OC.sub.1-C.sub.6 alkyl, or COOH; the photosensitizer is the following complex: ##STR00028## the metal porphyrin complex is ##STR00029## the solvent is acetonitrile; phenol or trifluoroethanol as a proton donor.

13. The photochemical composition of claim 1, wherein the concentration of the metal porphyrin complex of formula (I) as defined in claim 1 is between 1 mM and 50 mM.

14. A method for producing methane from CO.sub.2 or CO, said method comprising: a) contacting gaseous CO.sub.2 or CO, with a photochemical composition as claimed in claim 1 to obtain a solution comprising dissolved at least one of CO.sub.2 and CO; b) irradiation of said solution with visible light; and c) collecting methane.

15. The method of claim 14, wherein the pressure of CO.sub.2 or CO of less than 1 bar.

16. The method of claim 14, wherein the pressure of CO.sub.2 or CO is of 1 bar or more.

17. The method of claim 14, wherein: the concentration of a sacrificial electron donor is between 10 mM and 500 mM, the concentration of the photosensitizer is between 50 mM and 1 mM, the concentration of the metal porphyrin complex of formula (I) is between 1 mM and 50 mM, the concentration of a proton donor is between 1 mM and 1 M.

18. The method of claim 14, wherein the irradiation lasts several days with wavelength of 400 nm or more, the reaction being stable.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1. Photochemical reduction of CO.sub.2 under visible light irradiation. Shown is the formation of gaseous products (in terms of turnover numbers, TONs) as a function of irradiation time, using an acetonitrile solution saturated with 1 atm CO.sub.2 (a, b, c) or 1 atm CO (d) and containing 2 M of catalyst chloro Fe-p-TMA and 50 mM of TEA. a, With no sensitizer, only CO is produced. b, When 0.2 mM of sensitizer Ir(ppy).sub.3 is present, Hz, CO and CH.sub.4 are produced (filled symbols); adding 0.1 M of TFE increases their production rate (open symbols). c, Hz, CO and CH.sub.4 product evolution over an extended irradiation time in the presence of 0.2 mM of Ir(ppy).sub.3. d, Under a CO atmosphere and with 0.2 mM of sensitizer Ir(ppy).sub.3 present, H.sub.2 and CH.sub.4 are produced (filled symbols); adding 0.1 M of TFE increases their production rate (open symbols). Data points in FIG. 2 are the results of at least two individual experiments. Typical uncertainty on TON values is ca. 5%, corresponding to the size of data points.

(2) FIG. 2. Methane detection. Typical gas chromatogram observed during long-term irradiation of a solution containing 2 M of catalyst chloro Fe-p-TMA, 50 mM of TEA and 0.2 mM of sensitizer Ir(ppy).sub.3, under .sup.12CO.sub.2 or .sup.13CO.sub.2 atmosphere. Inset, Mass spectra of methane generated under a .sup.12CO.sub.2 or .sup.13CO.sub.2 atmosphere.

(3) FIG. 3. Sketch of the proposed mechanism for CO.sub.2 reduction to CH.sub.4 by catalyst Fe-p-TMA. Initially, the starting Fe.sup.III porphyrin (shown at top left) is reduced with three electrons to the catalytically active Fe.sup.0 species (top part of the scheme). The Fe.sup.0 species reduces CO.sub.2, with the resultant Fe.sup.I regenerated through electron transfer from the excited photosensitizer (right-hand side cycle). The CO produced binds to Fe.sup.II and is further reduced with a total of six electrons (transferred from the excited sensitizer) and six protons to generate methane, via a postulated Fe.sup.I-formyl (Fe.sup.ICHO) intermediate (left-hand side cycle).

(4) FIG. 4. Evolution of the absorption spectrum with time. The absorption spectrum of a CO.sub.2-saturated ACN solution containing 2 M of chloro Fe-p-TMA, 0.2 mM of Ir(ppy).sub.3, 0.05 M of TEA upon visible (>420 nm) light irradiation remains stable over the course of experiments, highlighting the stability of the system. Inset, absorption spectrum of 2 M of catalyst Fe-p-TMA in ACN (no sensitizer Ir(ppy).sub.3), showing that in the photochemical mix, >90% of photons above 420 nm are absorbed by Ir(ppy).sub.3.

(5) FIG. 5. Sensitizer emission quenching after excitation at 420 nm. a, Upon increasing concentration of TEA in a 0.1 mM ACN solution of Ir(ppy).sub.3, no emission quenching is observed, as confirmed by the Stern-Volmer analysis (inset). b, Upon increasing concentration of chloro Fe-p-TMA in a 0.2 mM ACN solution of Ir(ppy).sub.3, emission quenching is observed corresponding to a diffusion-controlled quenching rate of (1.70.1)10.sup.10 M.sup.1 s.sup.1 as determined by Stern-Volmer analysis (inset).

(6) FIG. 6. Evolution of the TON with time. The reaction conditions are those described for experiment 16 (see Table 2 below).

(7) FIG. 7. Evolution of the TON with time. The reaction conditions are those described for experiment 17(see Table 2 below).

(8) FIG. 8. Evolution of the TON with time. The reaction conditions are those described for experiment 18 (see Table 2 below).

(9) FIG. 9. Evolution of the TON with time. The reaction conditions are those described for experiment 19 (see Table 2 below).

(10) FIG. 10. Evolution of the TON with time. The reaction conditions are those described for experiment 20 (see Table 2 below).

(11) FIG. 11. Evolution of the TON with time. The reaction conditions are those described for experiment 21 (see Table 2 below).

EXAMPLES

(12) The following examples are meant for illustrative purposes only, and shall not be construed as limitative in any way.

(13) Materials and Methods

(14) Synthesis of Catalysts Chloro Fe-p-TMA, Chloro Fe-o-TMA and Chloro Fe-o-OH.

(15) The synthesis of chloro iron(III) 5,10,15,20-tetra(4-N,N,N-trimethylanilinium)porphyrin (chloro Fe-p-TMA), chloro iron(III) 5,10,15,20-tetra(2-N,N,N-trimethylanilinium)porphyrin (chloro Fe-o-TMA) and chloro iron(III) 5,10,15,20-tetrakis(2,6-dihydroxyphenyl) porphyrin (chloro Fe-o-OH) have been described (see respectively Costentin et al. Proc. Natl. Acad. Sci. U.S.A. 112, 6882-6886 (2015) and Costentin et al. Science 338, 90-94 (2012)).

Synthesis of Catalysts Chloro Co-p-TMA, Chloro Cu-o-TMA and Chloro Cu-o-OH

(16) chloro cobalt(III) 5,10,15,20-tetra(4-N,N,N-trimethylanilinium)porphyrin (chloro Co-p-TMA), chloro copper(II) 5,10,15,20-tetra(2-N,N,N-trimethylanilinium)porphyrin (chloro Cu-o-TMA) and chloro copper(II) 5,10,15,20-tetrakis(2,6-dihydroxyphenyl) porphyrin (chloro Cu-o-OH) were synthesized using the protocols described in Costentin et al. Proc. Natl. Acad. Sci. U.S.A. 112, 6882-6886 (2015) and Costentin et al. Science 338, 90-94 (2012), replacing FeCl.sub.3 respectively by CoCl.sub.3 and CuCl.sub.2.

(17) Photochemical Measurements.

(18) Irradiations of acetonitrile (99.9% extra-dry, Acros Organics) solutions containing triethylamine (99% pure, Acros Organics) as sacrificial electron donor, and fac-(tris-(2-phenylpyridine))iridium(III) (Ir(ppy).sub.3, 99%, Aldrich) as sensitizer were realized in a closed 11 cm quartz suprasil cuvette (Helima 117.100F-QS) equipped with a home-designed headspace glassware. Solutions were saturated with argon (>99.998%, Air Liquide), .sup.12CO.sub.2 (>99.7%, Air Liquide), .sup.13CO.sub.2 (99% atom .sup.13C, Aldrich) or .sup.12C.sub.0 (>99.997%, Air Liquide) for 20 minutes before irradiation. A Newport LCS-100 solar simulator, equipped with an AM1.5 G standard filter allowing 1 Sun irradiance, was used as the light source combined with a Schott GG420 longpass filter and 2 cm long glass OS cell filled with deionized water to prevent catalyst absorbance and to cut off IR and low UV.

(19) Spectrophotometric Measurements.

(20) UV-Visible absorption data were collected with an Analytik Jena Specord 600 UV/Vis spectrophotometer. Emission quenching measurements were conducted with a Cary Eclipse fluorescence spectrophotometer (Agilent Technologies), with the excitation wavelength set at 420 nm and the emission spectrum measured between 430 and 700 nm. Emission intensities used for the Stern-Volmer analysis were taken at 517 nm, i.e. the emission maximum of Ir(ppy).sub.3. The lifetime of the emissive excited state of Ir(ppy).sub.3 was taken as 1.9 s as reported by Dedeian et al. (Inorg. Chem. 30, 1685-1687 (1991)).

(21) Reduction Products Analysis.

(22) Gaseous products analysis was performed with an Agilent Technology 7820A GC system equipped with a capillary column (CarboPLOT P7, length 25 m, inner diameter 25 mm) and a thermal conductivity detector. Calibration curves for H.sub.2, CO and CH.sub.4 were established separately. Control experiments, with no catalyst, no CO.sub.2 or no light were conducted in the same conditions than the full system. Ionic chromatography measurements were performed with a Thermo Scientific Dionex ICS-1100 system. Mass spectra were obtained by a ThermoFisher Scientific TRACE Ultra gas chromatograph equipped with a CP 7514 column (Agilent Technologies) and coupled to a DSQ II mass spectrometer in positive ionisation mode, using a TriPlus headspace autosampler.

(23) Ton Calculation.

(24) Turnover number is practically defined as the number of catalytic cycles per catalyst amount. Mol number of H.sub.2, CO and CH.sub.4 were determined by converting peak integrations from GC measurements into moles in the sample headspace thanks to individual calibration curves taking into account the irradiated sample volume (3.5 mL).

(25) Quantum Yield Calculation.

(26) Using the method described above, based on three independent measurements, the number of incident photons to the sample was determined to be (2.180.17)10.sup.19 photons per hour. Taking 195 as the highest TON number for CH.sub.4 (Table 1, entry 13), a quantum yield of ca. 0.22% after 102 hours of irradiation is obtained.

(27) Results

(28) The results are summarized in table 1 below.

(29) TABLE-US-00001 TABLE 1 Summary of the reaction conditions used for evaluating the catalytic performance of catalysts chloro Fe-p-TMA and chloro Fe-o-OH. The solvent is acetonitrile. Experiment [Fe-p-TMA] [Ir(ppy).sub.3] [TEA] TONs n M Gas mM mM nm Time CO CH.sub.4 H.sub.2 1 2 CO.sub.2 50 >420 47 33 2 2 CO.sub.2 0.2 50 >420 47 198 31 24 3 2 CO.sub.2 0.2 50 >420 47 240 66 73 (in the presence of 0.1M TFE) 4 2 CO.sub.2 0.2 50 >420 102 367 79 26 5 2 Argon 0.2 50 >420 47 43 6 CO.sub.2 0.2 50 >420 47 3 1 7 2 CO.sub.2 0.2 >420 23 5 8 2 CO.sub.2 0.2 50 dark 23 9 2 CO.sub.2 0.2 50 >420 47 139 26 15 (Fe-o-OH used instead of Fe-p-TMA) 10 2 CO 0.2 50 >420 47 89 18 11 2 CO 0.2 50 >420 102 140 28 12 2 CO 0.2 50 >420 102 159 34 (in the presence of 0.1M TFE) 13 2 CO 0.2 50 >420 102 195 45 (in the presence of 0.5M TFE) 14 CO 0.2 50 >420 47 15 2 CO 0.2 50 dark 23

(30) Methane formation was also observed under the conditions of entry 2 or 10, replacing catalyst chloro Fe-p-TMA by chloro Fe-o-TMA, chloro Co-p-TMA, and Cu-o-OH.

(31) Finally, 3 experiments were performed using compound:

(32) ##STR00018##

(33) The experimental conditions are summarized in Table 2, and in FIG. 6 (experiment 16), FIG. 7 (experiment 17), FIG. 8 (experiment 18), FIG. 9 (experiment 19), FIG. 10 (experiment 20), FIG. 11 (experiment 21).

(34) TABLE-US-00002 TABLE 2 Summary of the reaction conditions used for evaluating the catalytic performance of catalysts chloro Fe-p-TMA and chloro Fe-p-OH. The solvent is dimethylformamide (DMF). Experiment [Fe-p-TMA] [phen4] Time TONs n M Gas mM mM nm h CO CH.sub.4 H.sub.2 16 10 CO.sub.2 1 100 >435 47 50 8 8 17 10 CO.sub.2 1 100 >435 47 71 14 10 (in the presence of 0.1M TFE) 18 10 CO 1 100 >435 47 17 37 (in the presence of 0.5M TFE) 19 10 CO 1 100 >435 47 27 17 (in the presence of 0.25M TFE) 20 10 CO 1 100 >435 47 45 7 (in the presence of 0.1M TFE) 21 10 CO 1 100 >435 47 10 20

(35) Discussion

(36) Chloro Fe-p-TMA was firstly used as a photocatalyst without a photosensitizer under visible light irradiation (>420 nm) with triethylamine (TEA, 50 mM) as sacrificial electron donor. Illumination of a 1 atm CO.sub.2-saturated solution of acetonitrile (ACN) containing 2 M of Fe-p-TMA at room temperature for 47 h selectively produced CO, with a turnover number (TON) in CO relative to catalyst concentration of 33. No side products were observed, and the linear production of CO with time indicates good stability of the catalytic system.

(37) A factor that can potentially limit the catalytic rate of this system is the 3-electron reduction of the initial Fe.sup.III porphyrin species to generate the active Fe.sup.0 state. Using electron donors with high reducing ability was envisioned to be favourable, and adding 0.2 mM of Ir(ppy).sub.3 (Table 1) as photosensitizer (E.sup.0(Ir(ppy).sub.3.sup.+/Ir(ppy).sub.3*1.73 V vs. SCE and e(Ir(ppy).sub.3/Ir(ppy).sub.3.sup.2.19 V vs. SCE) to the solution indeed enhanced the photochemical CO.sub.2 reduction, so that 47 h of irradiation gave a TON in CO relative to chloro Fe-p-TMA of 198 (Table 1, entry 2 and FIG. 1b). Adding 0.1 M trifluoroethanol (TFE, Table 1, entry 3) slightly increased the TON further to 240, likely due to TFE facilitating the CO bond cleavage step. With the photosensitizer, products included not only CO but also 10% hydrogen and 12% methane that correspond to TONs of 24 and 31 (Table 1, entry 2 and FIG. 1b). No other gaseous product was formed, and analysis of the liquid phase failed to detect methanol or formaldehyde by .sup.1H NMR or formate (HCOO.sup.) by ion chromatography. The presence of 0.1 M of TFE increased the selectivities (and TONs) for H.sub.2 and CH.sub.4 to 19% (73) and 18% (66), respectively (Table 1, entry 3 and FIG. 1b). Blank experiments (Table 1, entries 1, 5-8) confirmed that no methane is formed in the absence of sensitizer, CO.sub.2, catalyst, electron donor or light.

(38) In isotope labelling experiments conducted under a .sup.12CO.sub.2 or a .sup.13CO.sub.2 atmosphere, GC-MS analysis (FIG. 2) identified as reaction product .sup.12CH.sub.4 (m/z=16) or .sup.13CH.sub.4 (m/z=17), respectively, confirming that methane originates from CO.sub.2 reduction. Increasing the irradiation time increased the amount of CO.sub.2 reduction products generated (FIG. 1c). The longest irradiation time of 102 h produced CO, CH.sub.4 and H.sub.2 with TONs (and selectivities) of 367 (78%), 79 (17%) and 26 (5%) respectively (Table 1, entry 4 and FIG. 1c). These values correspond to a methane production rate of 763 mol per hour and per gram of catalyst (mol/h/g), which exceeds the rate of many other catalysts that generate methane from CO.sub.2. The linear evolution of both CO and CH.sub.4 over more than 80 h and the stable absorption spectrum of the system under irradiation (FIG. 4), with no evidence for degradation of the sensitizer Ir(ppy).sub.3 or catalyst chloro Fe-p-TMA, illustrate the stability of the catalytic system.

(39) Evolution of the different products (FIG. 1c) shows that methane production only starts after a significant amount of CO has built up, suggesting that CO is an intermediate in the methane formation process. It was previously shown by UV-Vis spectroscopy that irradiation of a CO.sub.2 saturated solution of chloro Fe-p-TMA without a sensitizer (in that case only CO is obtained) led to the formation of detectable amount of Fe.sup.IICO species. It may thus be hypothesized that this iron-carbonyl adduct is an intermediate for further reduction towards methane in the presence of a strong reducing agent. Experiments were thus conducted in a 1 atmosphere CO-saturated acetonitrile solution under visible light irradiation (>420 nm), with Ir(ppy).sub.3 as sensitizer and TEA as sacrificial electron donor (FIG. 1d). In a 47 h irradiation experiment, this slightly lowered H.sub.2 production and increased by almost a factor of 3 CH.sub.4 production compared to the experiment using a CO.sub.2-saturated solution: 83% of product was CH.sub.4 and 17% H.sub.2 (Table 1, entry 10), with the CH.sub.4 formation rate of 1865 mol/h/g. Blank experiments in the absence of Fe-p-TMA or in the absence of light did not give any reduction product (Table 1, entries 13 and 14), while a longer irradiation time of 102 h enhanced the selectivity for methane further to 87% (Table 1, entry 11 and FIG. 1d). Addition of a weak acid in moderate concentration (TFE 0.1 M) slightly increased the methane formation rate (from a TON of 140 to 159) with some loss of selectivity (from 87% to 82%, Table 1, entry 12). The successful methane evolution under these conditions over 102 h with an average rate of 1467 mol/h/g illustrate the robustness, activity and selectivity of the photochemical system. Optimized concentration for TFE (0.5 M) led to a TON of 195 for methane with 81% selectivity (Table 1, entry 13).

(40) When replacing chloro Fe-p-TMA by chloro Fe-o-OH, methane was also evolved although in slightly smaller amounts (26 TON after 47 h irradiation and 14% CS, Table 1, entry 9). The standard redox potential E.sup.0(Fe.sup.I/Fe.sup.0)=1.575 V vs. SCE.sup.26 in DMF (dimethylformamide) for chloro Fe-o-OH is only 75 mV more negative than that of Fe-p-TMA, and as in the latter case, the substituents on the phenyls may help stabilizing reaction intermediates (through internal H bonds involving the OH groups). In contrast, the non-substituted tetraphenyl Fe porphyrin (chloro FeTPP, Table 1) only gives CO and H.sub.2 (with TONs/selectivities of 84/79% and 22/21%, respectively) under the same irradiation conditions, likely due to its significantly more negative standard redox potentials (e.g. E.sup.0(Fe.sup.I/Fe.sup.0)=1.67 V vs. SCE in DMF) and the absence of phenyl ring substituents for stabilizing intermediate species involved in hydrocarbon production. The ability to produce methane is thus not restricted to catalyst chloro Fe-p-TMA, but is likely a more general property of Fe porphyrins that have a sufficiently positive standard redox potential and are functionalized with substituents that can stabilize intermediates involved in the catalytic cycle.

(41) Another parameter for CO.sub.2 reduction beyond the two-electrons production of CO is the driving force for charge transfer from the excited state of the sensitizer. When replacing Ir(ppy).sub.3 by the less reducing ruthenium complex Ru(bpy).sub.3.sup.2+ (E.sup.0(Ru(bpy).sub.3.sup.2+/Ru(bpy).sub.3.sup.+)1.33 V vs. SCE and E.sup.0(Ru(bpy).sub.3.sup.3+/Ru(bpy).sub.3.sup.2+*)=0.81 V vs. SCE), only CO and H.sub.2 and no CH.sub.4 were obtained, possibly because the Ru excited state or its reduced form are not able to trigger the carbonyl reduction from the Fe.sup.IICO adduct. Emission quenching experiments between Ir(ppy).sub.3* and chloro Fe-p-TMA on one hand and Ir(ppy).sub.3* and TEA on the other hand revealed very weak quenching in the latter case while it is very efficient, diffusion-controlled, in the former case (k.sub.q1.710.sup.10 M.sup.1 s.sup.1, FIG. 5), suggesting that direct electron transfer occurs from the excited sensitizer Ir(ppy).sub.3 to the Fe porphyrin. This is in line with the standard redox potential value of the excited iridium complex (E.sup.0(Ir(ppy).sub.3.sup.+/Ir(ppy).sub.3*)1.73 V vs. SCE), which is more negative than all three redox couples related to the Fe porphyrin (Fe.sup.III/Fe.sup.II, Fe.sup.II/Fe.sup.I and Fe.sup.I/Fe.sup.0). After electron transfer, the oxidized Ir(ppy).sub.3.sup.+ is reduced by the sacrificial electron donor TEA upon irradiation, thereby closing the catalytic cycle and generating the protonated triethylamine TEAH.sup.+ that could then act as proton donor as seen before. FIG. 3 sketches a plausible mechanism based on these considerations, which involves a postulated formyl intermediate that may be stabilized by through-space interactions between the positive charges of the trimethylammonio groups and the partial negative charge on the CHO species bound to the metal. With complete reduction of the Fe.sup.IICO adduct necessitating six electrons, the quantum yield for CH.sub.4 formation is =0.22%.

(42) The generality of the reaction to other metal (Cobalt and Copper) porphyrins was also demonstrated. It was also shown that the photosensitizer is not restricted to metal complexes, and that organic photosensitizers may also be used. Finally, it has been showed that aprotic solvent could be mixed with high content water (up to 70% in acetonitrile) for producing CH.sub.4.