Na3[Ru2(µ-CO3)4] as a Homogeneous Catalyst for Water Oxidation; HCO3− as a Co-Catalyst

Shanti Gopal Patra,Dan Meyerstein,Totan Mondal,Haya Kornweitz,Amir Mizrahi,Krishnamoorthy Sathiyan

doi:10.3390/catal11020281

Shanti Gopal Patra, Dan Meyerstein + Show 4 more

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https://doi.org/10.3390/catal11020281

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Journal: Catalysts	Publication Date: Feb 21, 2021
Citations: 9	License type: CC BY 4.0

Affiliation: Ariel University, Ben-Gurion University of the Negev

Abstract

In neutral medium (pH 7.0) [RuIIIRuII(µ-CO3)4(OH)]4− undergoes one electron oxidation to form [RuIIIRuIII(µ-CO3)4(OH)2]4− at an E1/2 of 0.85 V vs. NHE followed by electro-catalytic water oxidation at a potential ≥1.5 V. When the same electrochemical measurements are performed in bicarbonate medium (pH 8.3), the complex first undergoes one electron oxidation at an Epa of 0.86 V to form [RuIIIRuIII(µ-CO3)4(OH)2]4−. This complex further undergoes two step one electron oxidations to form RuIVRuIII and RuIVRuIV species at potentials (Epa) 1.18 and 1.35 V, respectively. The RuIVRuIII and RuIVRuIV species in bicarbonate solutions are [RuIVRuIII(µ-CO3)4(OH)(CO3)]4− and [RuIVRuIV(µ-CO3)4(O)(CO3)]4− based on density functional theory (DFT) calculations. The formation of HCO4− in the course of the oxidation has been demonstrated by DFT. The catalyst acts as homogeneous water oxidation catalyst, and after long term chronoamperometry, the absorption spectra does not change significantly. Each step has been found to follow a proton coupled electron transfer process (PCET) as obtained from the pH dependent studies. The catalytic current is found to follow linear relation with the concentration of the catalyst and bicarbonate. Thus, bicarbonate is involved in the catalytic process that is also evident from the generation of higher oxidation peaks in cyclic voltammetry. The detailed mechanism has been derived by DFT. A catalyst with no organic ligands has the advantage of long-time stability.

Highlights

IntroductionWith the decrease in the abundance of fossil fuels (coal, petroleum, and natural gas), the search for alternative energy sources is an immense challenge for mankind [1]
With the decrease in the abundance of fossil fuels, the search for alternative energy sources is an immense challenge for mankind [1]
The first oxidation peak can be seen, but the other intermediate oxidation steps are not seen before the electrocatalytic water oxidation step

Summary

Introduction

With the decrease in the abundance of fossil fuels (coal, petroleum, and natural gas), the search for alternative energy sources is an immense challenge for mankind [1]. In the past few decades, solar energy and electricity have been considered as the source of alternative energies [2]. It is important to note that in nature, plants employ the highly sophisticated machinery called Photosystem II to convert sunlight into fuel [3]. It uses the calcium-manganese based oxo-cluster (CaMn4 O5 core) as the catalyst to split water via a series of proton-coupled electron transfer (PCET) processes [4]. Because of pollution free combustion and high energy density, hydrogen is considered a green and sustainable source of energy that can be produced by splitting water using solar photocatalysis [5] or electrocatalysis [6]. An efficient and durable catalyst is in high demand for promoting proton coupled evolution of oxygen with the removal of four electrons (Equation (1))

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Conclusion