Electrocatalytic Study of Carbon Dioxide Reduction By Co ( TPP ) Cl Complex

Carbon dioxide (CO 2 ) 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 CO 2 is therefore an attractive research topic for many chemists. Different methods of electrocatalytic reduction of CO 2 have been reported previously. Since CO 2 is very stable, the direct electroreduction of CO 2 into CO requires high potential at −2.2 V versus Ag/AgCl. In this work, CO 2 reduction was carried out by the photoelectrocatalysis of CO 2 in the presence of cobalt(III)tetraphenylporphyrin [Co(TPP)Cl] at −1.85V with a current efficiency of 71%. At illuminated p-type silicon photocathode, the reduction of CO 2 into CO was performed at a potential of 300mV 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.


Introduction
The increasing amount of carbon dioxide (CO 2 ) 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, CO 2 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 CO 2 splitting: enzymatic [1,2], electrochemical (electrocatalytic) reduction [3,4], photoreduction [5,6], and abstraction of an oxygen atom from a CO 2 molecule by coordination complexes [7].The conversion of CO 2 into valuable products is challenging as it requires a lot of energy [8][9][10].Photosynthesis, photocatalytic, and electrochemical reduction of CO 2 are the effective processes to use CO 2 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].CO 2 has been converted into chemicals (such as formic acid) and fuels (such as methanol, methane, and carbon monoxide (CO)) previously [12][13][14].Photochemical conversion of CO 2 to fuels or valuable chemicals using renewable solar energy can decrease the amount of CO 2 in the atmosphere [15].
Recycling atmospheric CO 2 , 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 (CO 2 + 2H + + 2e − → CO + H 2 O) under visible light illumination at a p-type silicon (p-Si) photocathode using a catalyst [16,17].
A light assisted generation of syngas (H 2 : CO = 2 : 1) from CO 2 and water can be achieved by using p-Si/catalyst.In the system, water is reduced heterogeneously on p-Si surface and CO 2 is reduced homogeneously by the catalyst (Scheme 1) [17,18].
The reduction of CO 2 to CO using visible light can be used to cycle CO 2 gas [8,17,[35][36][37][38].Hawecker and coworkers used Re(I) pyridyl complex as a catalyst for CO 2 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-But 2 bpy)(CO) 3 Cl 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 CO 2 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 CO 2 reduction to CO and was claimed to be an efficient electrocatalyst for CO 2 reduction.95% of current efficiency was reported and CO was reported to have high selectivity compared to H 2 at a mercury pool cathode when 2,2,2-trifluoroethanol (CF 3 CH 2 OH) was used as a proton source.Reduction of CO 2 to CO was carried out in a visible light at illuminated p-type Si photocathode using Fe(TPP)Cl in the presence of CF 3 CH 2 OH as a proton source gave a current efficiency >90% and a high selectivity over H 2 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 CO 2 into sources of fuel by means of renewable solar energy is one of the important objectives [42][43][44][45].This work presents the electrochemical catalysis of CO 2 reduction into CO by cobalt(III)tetraphenylporphyrin [Co(TPP)Cl] at carbon and illuminated p-type Si electrodes.

Materials and Methods
Chemicals, solvents, Co(TPP)Cl, and CF 3 CH 2 OH were purchased from Aldrich and used as received.Methyl cyanide (CH 3 CN) was purified by distillation over calcium hydride.The single crystal B-doped p-type Si (1-10 cm −1 , (111) face, thickness 500-550 m) 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 threeelectrode 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 [Bu 4 N][BF 4 ]).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 CH 3 CN 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 CO 2 (saturated with CO 2 ).The cyclic voltammetry of Co(TPP)Cl was done under CO 2 atmosphere to know the reduction catalytic CO 2 .CF 3 CH 2 OH was added to improve both efficiency and catalyst life time, without any significant formation of H 2 .
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.

Results and Discussion
The electrocatalytic behavior of Co(TPP)Cl was tested by cyclic voltammetry in the absence of CO 2 .
Figure 1 shows the typical cyclic voltammetry of Co(TPP)Cl at carbon electrode in the absence of CO 2 .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  1/2 = −0.76V, and the second wave is irreversible corresponding to Co(I) − /Co(0) 2− (two electrons, one electron for second process of Co(TPP)Cl,   = −1.98 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 [Bu 4 N][BF 4 ]-95% MeCN+ 5% DMF at vitreous carbon and Figure 3 shows the plots of   red versus  1/2 of the first and second processes of Co(TPP)Cl, which are diffusioncontrolled and involve an electrochemically reversible one-electron transfer.The plot of peak current    red versus  1/2 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: Area of electrode is 0.071 cm 2 (1) Randles-Sevcik Equation ( 2) (3) As known, the direct reduction of CO 2 at vitreous carbon electrode is around −2.2 V versus Ag/AgCl.In the presence of Co(TPP)Cl, the carbon CO 2 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 CO 2 ).

Electrocatalytic Reduction of CO 2 by Co(TPP)Cl on Vitreous Carbon and p-Type Si Electrodes
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.

Preparative-Scale Electrolysis.
Preparative bulk photoelectrosynthesis of CO on the p-type Si photocathode was performed in 1 M [Bu 4 N][BF 4 ]-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 CF 3 CH 2 OH.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 moles, 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.CO 2 was converted into CO with a current efficiency of ca 71.6%, where the yield of CO was 19 moles 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 CO

Conclusion
It has been shown that the CO 2 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 CO 2 to CO with the current efficiency of 72%.On the other hand, the current efficiency of CO 2 to CO reduction was only 65% at p-type Si electrode in the presence of cobalt porphyrin.However, CF 3 CH 2 OH was added to improve the catalysis of CO 2 reduction.Under illumination at p-type Si electrode (boron-doped p-type H-terminated silicon), and in presence of cobalt porphyrin, the reduction of CO 2 to CO can be achieved at a potential ca 300 mV positive to that of an inert vitreous carbon electrode.The reduction of CO 2 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.

Figure 4 (
a) shows the electrocatalytic reduction of CO 2 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 CO 2 interacts with the reduced catalyst.The second wave process increased in height and became irreversible under CO 2 , and the catalytic current density increased to 1.76 × 10 −3 A⋅cm −2 .
2 with heterogeneity of small amount of H 2 O. Reduction of CO 2 by photocathode at p-type Si electrode is shown in Figure7and the course of electrolysis was 1.4 h.

Table 1
summarises the results of both electrocatalysis and photoelectrocatalysis of CO 2 reduction by Co(TPP)Cl at carbon and p-type Si electrodes.

Table 1 :
[42]ent efficiencies and turnover numbers of electrocatalytic reduction of CO 2 catalyzed by Co(TPP)Cl at both carbon and p-type Si electrodes.T.N. = moles of product/moles of catalyst.The rest of current efficiency was unknown which might have been consumed by conversion of CO 2 into oxalate or formate.Saveant and coworker reported that CO 2 will be converted to oxalate or formate in the presence of a weak acid[42], or by decomposition of catalyst. *