Redox Catalysts for Electrochemical CO2 Reduction: the Role of Local Proton Source

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12th_{European Congress on Catalysis – EuropaCat-XII, Kazan, Russia, 30 August – 4 September,} 2015

Redox Catalysts for Electrochemical CO

2

Reduction: the Role

of Local Proton Source

C. Nervi,*_{R. Gobetto, C. Minero, F. Franco, C. Cometto, C. Sun, L. Nencini, F. Sordello} 1_{Department of Chemistry, via P. Giuria 7, 10125, Torino, Italy}

(*) corresponding author: carlo.nervi@unito.it

Keywords: Carbon dioxide, Redox Catalyst, Electrochemical Reduction, Organometallic complexes.

1 Introduction

The reduction of CO2 emissions and the need for a green and sustainable energy source are

among the most important world’s strategic research topics. The conversion of CO2 into “Solar

Fuels” [1] via the so-called artificial photosynthesis proceed by the photoactivation of a dye that generates a electron–hole pairs, which provides the thermodynamic driving force for the subsequent electron transfer reactions. A convenient approach is to optimize separately the catalytic and light-harvesting properties. This is possible because the photocatalytic mechanism usually proceed via a reductive quenching pathway [2], i.e. the real catalyst is the reduced form produced in situ; thus the search of an efficient catalyst for CO2 reduction and its study can be

pursued using electro-chemical techniques. To reduce the large overpotentials required for CO2

reduction, the use of catalyst able to take advantage of proton-assisted processes [2] is mandatory [1]. In this communication we would like to show our recent findings on the use of Re and earth-abundant transition metals (Mn, Mo, W) organometallic complexes as redox catalyst for CO2 electrochemical reduction.

2 Experimental/methodology

The complexes were synthesized and characterized in our laboratory [3]. Cyclic Voltammetry experiments were performed normally in 1 mM MeCN solutions, containing tetrabutylammonium hexafluorophosphate (TBAPF6) as supporting electrolyte (0.1 M). Brønsted acids with different

strength (MeOH, H2O, CF3CH2OH) were added to test the proton-assisted ability of the catalysts.

Photocatalytic measurements were performed in a Pyrex cell filled with a mixture of dimethylformamide and triethanolamine (DMF/TEOA 5:1 v/v; 6 mL) with the catalyst (2 mM). Experiments were carried out whilst stirring and with a constant flow of CO2. The cell was irradiated

with a Philips PLS 9 W/52 lamp. CO and H2 were detected and quantified by GC. A Smart Trak 100

(Sierra) flow controller C100L was used during the electrochemical and photocatalytic experiments. Formic acid production was evaluated with ion chromatography.

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The effect of an antenna on ReI_{complex catalytic properties has been explored and}

contrasting results have been observed. For example, the covalently linked non-conjugated RuII_–

ReI_{dyad is a superior catalyst to the corresponding mixture of Ru}II_{and Re}I_{complexes; however,}

the Zn–porphyrin and ReI_{complex mixture seems to be a better catalyst than the corresponding}

covalently linked dyad [4]. We herein compare the redox and photochemical catalytic properties of a series of ReI_{carbonyl complexes that contain α,α-diimine ligands covalently linked to a}

highly delocalized 4-piperidinyl-1,8-naphthalimide (PNI) moiety for the CO2 reduction process.

The photochemical data parallel reasonably well the electrochemical one. But the most interesting catalytic features has been obtained in homogeneous solutions with a MnI_catalyst

carrying a local proton source. While the ReI_{catalysts can work in CO}

2 reduction also in

absence of Brønsted acids, the MnI_{catalysts normally cannot. However, the Mn}I_{catalysts 1,}

characterized also by single crystal X-ray diffraction, showed a substantial catalytic activity in anhydrous media induced by the presence of a local proton source [3]. Bulk electrolysis at the end of the catalytic plateau (–1.8 V) under CO2 showed an unusual change in selectivity for CO2

reduction by the Mn(I) catalyst, giving a mixture of CO and HCOOH.

Fig. 1. CO production against the charge passed during bulk electrolysis at –1.8 V vs. SCE of a 1 mM solution of 1. The slope of about 0.35 corresponds to a faradic efficiency of 70%.

The catalytic activity is inhibited in 2, where the two local protons are substituted by methyl groups. We also performed a series of controlled potential electrolysis with different Brønsted acids, showing that their strengths can drive the selectivity of the catalyst towards the formation of CO or formic acid.

4 Conclusions

The introduction of a PNI group in certain ReI_{complexes allowed a second electron transfer}

to occur at much less negative potentials than the corresponding parent derivative. For these redox catalysts, the two-electron reduction mechanism might also proceed as the formal [PNI]–

group worked as an electron reservoir. The importance of local factors is evidenced by the strong catalytic activity of MnI_{complex 1. This represents the first reported experimental}

evidence of an intramolecular proton-assisted catalytic process for a MnI_{catalyst, in which the}

key factor is the spatial closeness of protons to the active site rather than their bulk concentration in solution.

Acknowledgements

The organizing committee would like to thank you in advance for your contribution(s) and to wish you a fruitful conference. (font style: Times New Roman 12pt)

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