Carbon dioxide (CO2) is notorious for being a greenhouse gas and is the most important cause of global warming. However, it can be converted into useful products as it is a source of carbon. Reduction of CO2 is therefore an attractive research topic for many chemists. Different methods of electrocatalytic reduction of CO2 have been reported previously. Since CO2 is very stable, the direct electroreduction of CO2 into CO requires high potential at −2.2 V versus Ag/AgCl. In this work, CO2 reduction was carried out by the photoelectrocatalysis of CO2 in the presence of cobalt(III)tetraphenylporphyrin [Co(TPP)Cl] at −1.85 V with a current efficiency of 71%. At illuminated p-type silicon photocathode, the reduction of CO2 into CO was performed at a potential of 300 mV which is positive. However, at the same conditions, potential of −1.55 V with a current efficiency of ca 65% is required for the carbon electrode.

1. Introduction

The increasing amount of carbon dioxide (CO2) over the past years may affect the environment adversely due to the fact that it is a greenhouse gas and can lead to global warming. In the air, CO2 is a final product of combustion of carbon-containing compounds and represents fully oxidized carbon. It is thermodynamically very stable and it requires a lot of energy to break its C-O bonds. However, there are four different methods for CO2 splitting: enzymatic [1, 2], electrochemical (electrocatalytic) reduction [3, 4], photoreduction [5, 6], and abstraction of an oxygen atom from a CO2 molecule by coordination complexes [7]. The conversion of CO2 into valuable products is challenging as it requires a lot of energy [810]. Photosynthesis, photocatalytic, and electrochemical reduction of CO2 are the effective processes to use CO2 as a carbon source and convert it into useful products. Energy is required for all these processes, such as requirement of electricity in electrochemical reduction process [11]. CO2 has been converted into chemicals (such as formic acid) and fuels (such as methanol, methane, and carbon monoxide (CO)) previously [1214]. Photochemical conversion of CO2 to fuels or valuable chemicals using renewable solar energy can decrease the amount of CO2 in the atmosphere [15].

Recycling atmospheric CO2, by its capture and subsequent reduction to valuable products and liquid fuels, is an increasingly important research area. It is possible to drive the half-cell reaction (CO2 + 2H+ + + H2O) under visible light illumination at a p-type silicon (p-Si) photocathode using a catalyst [16, 17].

A light assisted generation of syngas (H2 : CO = 2 : 1) from CO2 and water can be achieved by using p-Si/catalyst. In the system, water is reduced heterogeneously on p-Si surface and CO2 is reduced homogeneously by the catalyst (Scheme 1) [17, 18].

The field of electrocatalytic reduction of CO2 by metal complexes has been studied extensively by many researchers [1922]. Homogeneous transition-metal catalysts, such as Ru, Re, Co, and Ni complexes, have been used to reduce the CO2 molecule by a multielectron reduction process [2325]. CO2 reduction by tricarbonyl rhenium(I) complex was first reported by Hawecker and coworkers [26], and the mechanism of such reactions have been studied previously [2630]. The most efficient catalyst [Re(bpy)(CO)3]+ was used for the selective photoreduction of CO2 to CO in a homogeneous system [27]. The drawback of using rhenium complex as a catalyst is that it reacts with CO2 very slowly [2832]. Binuclear rhenium(I) complexes with saturated bridging ligands have been synthesized for the photocatalytic CO2 reduction [33]. Recently, the synthesis of dinuclear rhenium complexes of the 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz) ligand and catalytic activity of tptz, as a bis(bidentate) ligand or as a bi- and tridentate chelating ligand, has been reported [34].

The reduction of CO2 to CO using visible light can be used to cycle CO2 gas [8, 17, 3538]. Hawecker and coworkers used Re(I) pyridyl complex as a catalyst for CO2 reduction into CO [26]. In addition, the same reaction has been carried out by using sacrificial amine as an electron source [27, 30]. Re(4,4-But2bpy)(CO)3Cl has been used as a catalyst for the same purpose at p-type silicon photocathodes when illuminated with visible light giving a photo-voltage of ca 600 mV. Thus, it makes the reduction of CO2 possible at potentials of −1.2 V to −1.4 V versus saturated calomel electrode (SCE). A current efficiency of 97% was achieved under illumination. The duration of electrolysis was 3 h. However, attempts have been made to replace rhenium with more abundant metals for organometallic complexes due to the fact that rhenium is a rare metal found on earth [10, 39, 40].

Tetraphenylporphyrin iron chloride complex (Fe(TPP)Cl) was previously used for CO2 reduction to CO and was claimed to be an efficient electrocatalyst for CO2 reduction. 95% of current efficiency was reported and CO was reported to have high selectivity compared to H2 at a mercury pool cathode when 2,2,2-trifluoroethanol (CF3CH2OH) was used as a proton source. Reduction of CO2 to CO was carried out in a visible light at illuminated p-type Si photocathode using Fe(TPP)Cl in the presence of CF3CH2OH as a proton source gave a current efficiency >90% and a high selectivity over H2 formation. The potential of ca −1.2 V versus SCE was required under illumination. In the dark, on vitreous carbon, a potential of −1.85 V versus SCE was required. Iron(0)pentaflourotetraphenylporphyrin (Fe(PFTPP)Cl) was reported to be more positive potential than Fe(TPP)Cl, because the strong electron withdrawing fluorogroups shifted the potential for electrocatalysis about 400 mV positive compared to that of Fe(TPP)Cl [41].

The economic and efficient conversion of overabundant CO2 into sources of fuel by means of renewable solar energy is one of the important objectives [4245]. This work presents the electrochemical catalysis of CO2 reduction into CO by cobalt(III)tetraphenylporphyrin [Co(TPP)Cl] at carbon and illuminated p-type Si electrodes.

2. Materials and Methods

Chemicals, solvents, Co(TPP)Cl, and CF3CH2OH were purchased from Aldrich and used as received. Methyl cyanide (CH3CN) was purified by distillation over calcium hydride. The single crystal B-doped p-type Si (1–10 cm−1, (111) face, thickness 500–550 μm) was supplied by Silicon Materials (Germany). The ohmic contacts were made using Ga-In eutectic and silver epoxy resin by the method of Tamaki et al. [43]. The photoelectrochemical cell was described earlier [45].

Cyclic voltammetric experiments were carried out using an Autolab PGSTAT 30 potentiostat. A conventional three-electrode arrangement was employed, consisting of a vitreous carbon working electrode, a platinum wire as the auxiliary electrode, and Ag+/AgCl as a reference electrode.

The electrolysis cells were degassed with argon gas to remove oxygen. The cell was filled with an electrolyte (a solvent containing 0.2 M [Bu4N][BF4]). The volume of electrolyte was 14 mL, out of which 5 mL occupied the working electrode compartment. About 9-10 mL gas phase took place at the working electrode part. 0.35 mM catalyst Co(TPP)Cl was added and dissolved in 5 mL dry CH3CN and stirred under Ar in electrochemical cell which was under Ar. Cyclic voltammetric measurements of Co(TPP)Cl were carried out under Ar; then, the solution was bubbled with CO2 (saturated with CO2). The cyclic voltammetry of Co(TPP)Cl was done under CO2 atmosphere to know the reduction catalytic CO2. CF3CH2OH was added to improve both efficiency and catalyst life time, without any significant formation of H2.

The electrolysis was carried out at the fitting potential and the current was recorded during the course of electrolysis verses the time. In addition, the charge passed was recorded. The electrolysis was stopped when the current decayed after 1.4 h.

3. Results and Discussion

The electrocatalytic behavior of Co(TPP)Cl was tested by cyclic voltammetry in the absence of CO2.

Figure 1 shows the typical cyclic voltammetry of Co(TPP)Cl at carbon electrode in the absence of CO2. The cyclic voltammetry of the complex exhibits two successive reduction waves. The first wave is reversible corresponding to Co(II)/Co(I) (one electron) at  V, and the second wave is irreversible corresponding to Co(I)/Co(0)2− (two electrons, one electron for second process of Co(TPP)Cl, versus V, and maybe the farther electron related to reduction of complex); potentials cited are versus Ag/AgCl.

Figure 2 shows the different scan rate in 0.1 M [Bu4N][BF4]-95% MeCN+ 5% DMF at vitreous carbon and Figure 3 shows the plots of versus of the first and second processes of Co(TPP)Cl, which are diffusion-controlled and involve an electrochemically reversible one-electron transfer. The plot of peak current versus is linear which means no complicated mass transfer control of one electron-transfer rate. The diffusion of first wave is calculated according to the following:

3.1. Electrocatalytic Reduction of CO2 by Co(TPP)Cl on Vitreous Carbon and p-Type Si Electrodes

Figure 4(a) shows the electrocatalytic reduction of CO2 by Co(TPP)Cl on vitreous carbon, under argon. The current density of second reduction of Co(TPP)Cl is around 2.24 × 10−4 A·cm−2. In the same figure, the cyclic voltammetry of the catalyst shows that CO2 interacts with the reduced catalyst. The second wave process increased in height and became irreversible under CO2, and the catalytic current density increased to 1.76 × 10−3 A·cm−2.

As known, the direct reduction of CO2 at vitreous carbon electrode is around −2.2 V versus Ag/AgCl. In the presence of Co(TPP)Cl, the carbon CO2 shifted to −1.85 V which is more positive in comparison with the direct reduction. At p-type Si electrode, the second wave reduction of Co(TPP)Cl shifted to a more positive value ca 300 mV under illumination of light (Figure 4(b) in the absence and presence of CO2).

Figure 5 shows the comparison of cyclic voltammetry of Co(TPP)Cl at p-type Si electrode in dark and light which proves the shifting of potential in the presence of light around 300 mV.

3.2. Preparative-Scale Electrolysis

Preparative bulk photoelectrosynthesis of CO on the p-type Si photocathode was performed in 1 M [Bu4N][BF4]-95% MeCN+ 5% DMF at room temperature in an H-type in the presence of 0.2 mM Co(TPP)Cl and 0.28 mM CF3CH2OH. The gas chromatography (GC-TCD) confirmed the formation of CO with a current efficiency of ca 65%. During the course of 1.4 h (−1.55 V versus Ag/AgCl), the charge passed was 4.18 C, the yield of CO was 14 μmoles, and ca 10% amount of hydrogen was produced as a by-product (Figure 6).

In a separate experiment at the same conditions, the bulk electrolysis at carbon electrode was carried out at −1.85 V versus Ag/AgCl. CO2 was converted into CO with a current efficiency of ca 71.6%, where the yield of CO was 19 μmoles and the charge passed was 5.3 C. Also, a small amount of hydrogen was obtained which can be ignored. The current efficiency to produce CO at both carbon and p-type Si electrodes is smaller but the amount of hydrogen is different which may be because of coupling of homogeneous catalysts for the reduction of CO2 with heterogeneity of small amount of H2O. Reduction of CO2 by photocathode at p-type Si electrode is shown in Figure 7 and the course of electrolysis was 1.4 h.

Table 1 summarises the results of both electrocatalysis and photoelectrocatalysis of CO2 reduction by Co(TPP)Cl at carbon and p-type Si electrodes.

4. Conclusion

It has been shown that the CO2 reduction can be achieved by using a simple cobalt porphyrin complex as a catalyst in the electroreduction that is carried out in dark (carbon electrode) and under illumination (p-type Si electrode). At carbon electrode, cobalt porphyrin catalyzes conversion of CO2 to CO with the current efficiency of 72%. On the other hand, the current efficiency of CO2 to CO reduction was only 65% at p-type Si electrode in the presence of cobalt porphyrin. However, CF3CH2OH was added to improve the catalysis of CO2 reduction.

Under illumination at p-type Si electrode (boron-doped p-type H-terminated silicon), and in presence of cobalt porphyrin, the reduction of CO2 to CO can be achieved at a potential ca 300 mV positive to that of an inert vitreous carbon electrode. The reduction of CO2 to CO catalyzed by cobalt porphyrin was carried out at 1.85 V, but at p-type Si electrode it shifts more positive at −1.55 V versus Ag/AgCl.

Conflict of Interests

The author declares that there is no conflict of interests regarding the publication of this paper.


The author would like to thank Research Deanship, University of Hail, Kingdom of Saudi Arabia, for providing research fund (SC14012). The author also thanks Dr. Saravanan Rajendrasozhan, University of Hail, Saudi Arabia, for his assistance in paper preparation.