Ru^III-edta (edta⁴⁻ = ethylenediaminetetraacetate) mediated photocatalytic conversion of bicarbonate to formate over visible light irradiated non-metal doped TiO₂ semiconductor photocatalysts

Abstract

Reported in this paper is the first example of a ruthenium(III) complex, [Ru^III(edta)] (edta⁴⁻ = ethylenediaminetetraacetate), that catalyzes the conversion of bicarbonate to formate selectively over a visible light irradiated surface of carbon, nitrogen and sulfur doped TiO₂ (represented as C–TiO₂, N–TiO₂ and S–TiO₂, respectively) semiconductor photocatalyst particles. Formation of formate, the only reduction product, was identified by ¹³C NMR analysis of the reaction mixture. Based on the experimental findings a working mechanism involving activation of bicarbonate (HCO₃⁻) through the formation of a [Ru^III(edta)(HCO₃)]²⁻ intermediate complex followed by the photochemical reduction of the coordinated bicarbonate to produce formate at the surface of non-metal (C, N, S) doped TiO₂ semiconductor particles is proposed. The efficacy of bicarbonate reduction over such photocatalysts decreased in the following order C–TiO₂ > N–TiO₂ > S–TiO₂.

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Introduction

Carbon dioxide (CO₂), a putative greenhouse gas discharged into the environment by natural sources as well as by human activities, could be considered one of the simplest alternative feed stocks. The production of chemicals by reduction of CO₂ has thus been a subject of continued research interest not only because of its low cost and worldwide availability, but for environmental protection also. However, reduction of CO₂ remains a challenging task due its thermodynamic and kinetic stability. To circumvent the energetically uphill pathway for CO₂ reduction use of transition metal complexes that allow binding of CO₂ to the metal centre, thus reducing the activation barrier of the reduction of CO₂ is well documented in the literature concerning catalytic,^1–3 electrocatalytic^4–6 and photocatalytic^7,8 reduction of CO₂. In this context, use of ruthenium(II) complexes as catalyst seems to be of continued interest.^9–20 However, potential of ruthenium(III) complex containing edta type ligand (edta⁴⁻ = ethylenediaminetetraacetate) towards catalyzing electrochemical reduction²¹ or hydrogenation²² of bicarbonate to formate has recently been explored. The feature that dominates the chemistry of [Ru^III(edta)(H₂O)]⁻ complex is its lability towards substitution reaction²³ for which binding of substrate molecule, CO₂/HCO₃⁻ (ref. 21) to the Ru(III) centre is facilitated. Being inspired by such ability of [Ru^III(edta)(H₂O)]⁻ in mediating the electrochemical reduction²¹ or hydrogenation²² of bicarbonate to formate, we set out to examine the catalytic ability of [Ru^III(edta)(H₂O)]⁻ towards conversion of bicarbonate to formate over visible light illuminated semiconductor photocatalyst. In the present work we have studied the reduction of HCO₃⁻ over visible light irradiated non-metal doped TiO₂ semiconductor particles in presence of [Ru^III(edta)(H₂O)]⁻. We have chosen non-metal doped TiO₂ because doping with non-metallic species causes the photosensitization of TiO₂ semiconductor in the visible light illumination.²⁴

Experimental

Materials

Anatase grade titanium dioxide (TiO₂), powder (Evonik Corp. P-25; particle size of 20 nm and BET surface area 50 m² g⁻¹) was used in the present studies. All other chemicals were of the highest available purity and used as received otherwise noted. ¹³C-enriched (99 atom% ¹³C) sodium bicarbonate was procured from Sigma-Aldrich (CAS: 87081-58-1) and used as received. Double-distilled water was used throughout the experiments.

K[Ru^III(Hedta)Cl] was prepared by following the published procedure.²⁵ The micro-analysis and spectral data are in good agreement with those reported in the literature.²⁵ Anal. calculated for K[Ru^III(Hedta)Cl]·2H₂O: C 24.0, H 3.42, N 5.59; found C 23.8, H 3.45, N 5.63. IR, ν/cm⁻¹: 1720 (free –COOH), 1650 (coordinated –COO⁻). UV-Vis in H₂O: λ_max/nm (ε_max/M⁻¹ cm⁻¹): 283 (2800 ± 50), 350 sh (680 ± 10).

Carbon-doped TiO₂ (C–TiO₂),²⁶ nitrogen doped TiO₂ (N–TiO₂)²⁷ and sulfur-doped (S–TiO₂)²⁸ photocatalysts were prepared by following literature procedure and characterized. Average particle size of the photocatalysts estimated was found to be 20 nm. BET surface area of C–TiO₂, N–TiO₂ and S–TiO₂ are 54 m² g⁻¹, 57 m² g⁻¹ and 55 m² g⁻¹, respectively.

Instrumentation

UV-visible spectral measurements were performed using a Varian Model Cary 100 Bio spectrophotometer equipped with an Diffuse Reflectance Spectral attachment. A Perkin Elmer Model Lambda 783 spectrophotometer was used for obtaining IR spectra (using KBr pellets). A Perkin-Elmer 240C elemental analyzer was used to collect microanalytical (C, H, N) data. The pH of the reaction mixture was measured using a Mettler Delta 350 pH meter.

Photocatalytic experiments

In a typical photocatalytic experiment, 50 mL of an aqueous solution of catalyst complex [Ru^III(edta)] (1.0 mM) and ¹³C-enriched NaHCO₃ (10.0 mM) was taken in a flat-surfaced glass reactor and 100 mg of the photocatalyst (non-metal doped TiO₂) and 10.0 mM of triethanolamine (TEOA), were added prior to the photolysis. The pH of the solution was adjusted (using 0.1 M HCl/0.1 M NaOH) to 6.4. The temperature of the reaction mixture was kept constant (at room temperature) by circulating water through the outer jacket of the reactor. The reaction mixture pre-purged with Ar (for 30 min to remove atmospheric oxygen) was illuminated with a 500 W xenon lamp (Oriel Illuminator) under continuous magnetic stirring. A filter was used to eliminate light <420 nm. The distance between the lamp and the glass reactor containing the reaction mixture was fixed at approximately 20 cm. After irradiation (8 h), the reaction mixture was centrifuged and then filtered by using a membrane filter (0.1 μm), and the filtrate was subjected to ¹³C NMR analysis for identification of the product(s) of the photocatalytic reaction. ¹³C NMR spectra were collected with a Bruker DPX 300 MHz NMR spectrometer at room temperature. 10% D₂O was used for instrument inter locking. The peak at 160.2 ppm is assigned to HCO₃⁻ (bicarbonate), and the peak at 170.9 ppm is assigned to HCO₂⁻ (formate).²⁹

Results and discussion

The starting complex K[Ru^III(Hedta)Cl] rapidly converts into the [Ru^III(Hedta)(H₂O)] when dissolved in water.^30,31 The ‘Hedta³⁻’ ligand in [Ru^III(Hedta)(H₂O)] complex acts as a pentadentate ligand. The sixth coordination site of the catalyst complex is occupied either at low pH by a water molecule or at high pH by an hydroxide ion. The pK_a values corresponding to the proton-dissociation of the pendant carboxylic acid arm and the coordinated water molecule are 2.4 and 7.6, respectively at 25 °C.^30,31

Evidence in favour of the adsorption of Ru(III)–catalyst complex onto the surface of TiO₂ was obtained from IR spectral studies of [Ru^III(edta)] and [Ru^III(edta)]–TiO₂ samples. The IR spectral features (Fig. S1†) of [Ru^III(edta)] catalyst complex are essentially retained in the spectrum of [Ru^III(edta)]–TiO₂, except for the peak at 1720 cm⁻¹ (ν_COOH) corresponding to the uncoordinated –COOH group (Fig. S1†). Above observation suggests that the [Ru(edta)] catalyst interacts with TiO₂ semiconductor particles through its dangling –COOH group as pictorially shown in Fig. 1. A similar observation on the interaction of ruthenium complex with TiO₂ through –COOH was reported previously in the literature.³²

Fig. 1 Pictorial representation of the surface adsorbed [Ru^III(edta)(H₂O)] complex at the surface of TiO₂ semiconductor photocatalyst.

Prolonged photolysis of the reaction mixture containing bicarbonate (HCO₃⁻), Ru^III(edta) catalyst complex, non-metal doped TiO₂ semiconductor photocatalyst, and TEOA (sacrificial electron donor) yielded appreciable amount of formate (HCO₂⁻) in the reacting system. Initially, the ¹³C NMR spectrum (Fig. 2a) of the reaction mixture recorded prior to the photolysis revealed a sharp signal at 160.2 ppm (singlet) which was attributed to the HCO₃⁻.²⁹ However, the ¹³C NMR spectrum (Fig. 2b) of the resultant solution obtained 8 h of photolysis displayed a clear peak (singlet at 170.7 ppm) confirms the presence of formate (HCO₂⁻)²⁹ in the reacting system. In situ IR spectrum of the resultant solution obtained at the end of the catalytic reaction exhibiting a band at 1610 cm⁻¹ (characteristic of HCO₂⁻) further supports the formation of formate in the photocatalytic reduction of bicarbonate. Gas chromatographic analysis of headspace gas of the reactor did not confirm the formation of carbon monoxide (CO) or methane (CH₄) in the catalytic process.

Fig. 2 ¹³C NMR spectra of the reaction mixture containing [Ru^III(edta)(H₂O)]⁻ (1 mM) and ¹³C-enriched NaHCO₃ (10 mM), TEOA (10 mM) and C–TiO₂ photocatalyst (a) recorded prior to the photolysis and (b) after 8 h of visible light illumination. pH 6.4 (bicarbonate buffer).

Results of the catalytic studies along with the yield of formate assessed by ¹³C NMR analysis of the resultant solution are summarized in Table 1. From successive blank experiments it was concluded that each component is essential for a effective reduction of bicarbonate to formate (Table 1). In the absence of either light or semiconductor photocatalysts no formate was detected (entry 1 in Table 1) under the employed conditions. Preliminary photocatalytic experiments also revealed that no HCO₂⁻ production occurred while visible light irradiated pure TiO₂ photocatalyst was used even in the presence of Ru(edta) complex (entry 2 and 3 in Table 1) under specified conditions. In the absence of Ru^III(edta) catalyst only a low yield (<2%) of formate, a desired product was obtained (Table 1, entries 4, 8 and 10) after 8 h. However, a significant increase in product yield was noticed when the reaction was performed in the presence of Ru^III(edta) catalyst (Table 1, entries 5, 6, 7, 9 and 11). The activity of non-metal doped TiO₂ semiconductor photocatalysts slightly varied in the following order C–TiO₂ > N–TiO₂ > S–TiO₂ (Table 1). This is probably associated (admittedly speculative) with the difference in the visible-light absorption properties of non-metal doped TiO₂ semiconductor photocatalysts having the additional electronic states above the valence band edge of pure TiO₂ for C-, N- and S-doped TiO₂,³³ however, influence of other factors could not be ruled out.

Table 1 Results of photocatalytic reduction of bicarbonate (HCO₃⁻) over visible light illuminated non-metal doped TiO₂ semiconductor photocatalyst in presence of [Ru^III(edta)] and TEOA (10 mM) at pH 6.4

Entry	Photocatalyst	[Ru^III]/(mM)	[HCO3^−]/(mM)	Yield (%) of formate^a
a Based on initial concentration of bicarbonate.b Reaction time 8 h.c Reaction time 16 h.d pH 8.2 (without buffer).
1	—	1	10	0^b
2	TiO2	0	10	0^b
3	TiO2	1.0	10	0^b
4	C–TiO2	0	10	<2^b
5	C–TiO2	1.0	10	29^b
6	C–TiO2	1.0	10	47^c
7	C–TiO2	1.0	10	35^d
8	N–TiO2	0	10	<2^b
9	N–TiO2	1.0	10	25^b
10	S–TiO2	0	10	<2^b
11	S–TiO2	1.0	10	23^b

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Possibility of the further reduction of formate was examined separately using ¹³C enriched HCO₂⁻ as substrate under identical experimental conditions employed for the reduction of bicarbonate and found no significant change in the ¹³C NMR spectra of the reaction mixture before and at the end of the reaction. This essentially suggests that the formate ion did not undergo any further reduction under the employed conditions. By taking into account the results of the present study (Table 1) indicating a catalytic role of [Ru^III(edta)H₂O]⁻ in the reduction of HCO₃⁻ and considering our previous report on the electrochemical reduction of bicarbonate to formate,³³ a following working mechanism, admittedly speculative is proposed (Scheme 1) for the photocatalytic reduction of bicarbonate to formate over non-metal doped TiO₂ semiconductor photocatalysts.

Scheme 1 Possible mechanism for the [Ru^III(edta)] mediated photocatalytic reduction of bicarbonate to formate in aqueous solution.

In the proposed mechanism (Scheme 1), surface adsorbed [Ru^III(edta)(H₂O)]⁻ reacts with HCO₃⁻ to form catalytic active intermediate complex [Ru^III(edta)(HCO₃)]²⁻ through a aquo-substitution reaction (eqn (1)). Noteworthy here that the reaction of [Ru^III(edta)(H₂O)]⁻ with HCO₃⁻ was studied in aqueous solution, and we found that the formation of [Ru^III(edta)(HCO₃)]²⁻ was considerably fast.²¹ The value of the second-order rate constant we reported earlier is 82 ± 7 M⁻¹ s⁻¹ at 25 °C.²¹ It is well documented in the literature that upon visible light illumination electrons are excited to the conduction band (CB) of the non-metal doped TiO₂ semiconductor (eqn (2)).^26–28 In the present case it is presumed that the reduction of coordinated bicarbonate (HCO₃⁻) involved interfacial electron transfer from the conduction band (CB) of the non-metal doped TiO₂ to the surface adsorbed [Ru^III(edta)(HCO₃)]²⁻ to produce formate (eqn (3)), the reduction product of bicarbonate in the reacting system. At the end of the catalytic reaction the IR spectrum of the solid mass (collected by filtration of the resultant reaction mixture) was recorded, and spectral features similar to that of the surface adsorbed [Ru^III(edta)(H₂O)]⁻ (as shown typically in Fig. S1†) were noticed. Diffuse reflectance spectrum (Fig. S2 in ESI†) of the same sample revealed spectral pattern characteristic of [Ru^III(edta)(H₂O)]⁻. Above spectral observations essentially suggest that the surface adsorbed catalyst complex, [Ru^III(edta)(H₂O)]⁻ does not undergo degradation under the employed conditions. Furthermore, we have checked the reusability of the solid mass by performing successive catalytic runs under conditions specified for entry 6 in Table 1, and found formate yields were 45% and 41% for 2nd and 3rd catalytic run, respectively. It is noteworthy here that bicarbonate could be converted into carbon dioxide in equilibrium reaction at the experimental pH 6.4.^34,35 At higher pH, formation of formate in significant amount (entry 7 in Table 1) conclusively support the fact that the bicarbonate (not carbon dioxide) is the reacting species that undergoes photocatalytic reduction to yield formate under the specified reaction condition.

In order to ascertain the possibility of the interfacial electron transfer, a separate experiment using [Ru^III(edta)(pz)]⁻ (pz = pyrazine) complex was performed. Diffuse reflectance spectral studies of the pale-yellow sample of [Ru^III(edta)(pz)]⁻ (pz = pyrazine) adsorbed on the surface of TiO₂ revealed no spectral feature in the visible region (Fig. S3a†). However, the colour of the sample turned red when the it was exposed to light for longer period under anaerobic conditions. The diffuse reflectance spectrum (Fig. S3b†) of the sample recorded after 2 h of illumination clearly reveals a peak at 465 nm characteristic of [Ru^II(edta)(pz)]²⁻ complex.³⁰ It had been reported earlier³⁰ that the absorption spectrum of [Ru^III(edta)(pz)]⁻ is almost featureless above 390 nm, whereas the corresponding ruthenium(II) analogue exhibits a strong characteristic metal-to-ligand charge transfer band in the visible region (λ_max = 462 nm, ε_max = 11 000 M⁻¹ cm⁻¹). Above findings clearly suggest the occurrence of the interfacial electron transfer which resulted in the reduction of the surface adsorbed [Ru^III(edta)(pz)]⁻ to [Ru^II(edta)(pz)]²⁻.

Conclusion

In summary, this work conclusively demonstrates the selective reduction of bicarbonate to formate over visible light irradiated non-metal doped TiO₂ semiconductor photocatalyst in presence of a Ru(III)-complex. To our knowledge, it is the first time that a Ru(III)-complex is successfully used as mediator in the photocatalytic reduction of bicarbonate to formate. Ru(edta) catalyst complex plausibly lowers the energy barrier of bicarbonate reduction through coordination of bicarbonate. The present system featuring negligible leaching, the mild conditions, the utilization of visible light, and the environmental sustainability make it altogether attractive for further research to develop other hybrid photocatalysts, effective for photocatalytic reactions, to reduce CO₂ to formic acid. Results reported in the present studies could be useful to broaden the present study further in developing more improved ruthenium(III) catalysts for bicarbonate reduction with high efficiency and selectivity.

Acknowledgements

This work is supported by a research grant (No. SR/S1/IC-04/2011) from the Department of Science and Technology, Government of India. DC is thankful to the Director of the CSIR-Central Mechanical Engineering Research Institute, for his encouragements.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: IR and DRSs spectra. See DOI: 10.1039/c6ra11464d

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