In-situ FTIR spectroelectrochemical studies on the electrocatalytic reduction of carbon dioxide by some ruthenium and iron complexes

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In-situ FTIR spectroelectrochemical studies on the electrocatalytic reduction of carbon dioxide by some ruthenium and iron complexes

 

Author: Pun, So-ngan
Title: In-situ FTIR spectroelectrochemical studies on the electrocatalytic reduction of carbon dioxide by some ruthenium and iron complexes
Degree: M.Phil.
Year: 2001
Subject: Carbon dioxide -- Recycling
Reduction (Chemistry)
Electrocatalysis
Hong Kong Polytechnic University -- Dissertations
Electronic dissertations
E-thesis
Department: Dept. of Applied Biology and Chemical Technology
Pages: xii, 181 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1566420
URI: http://theses.lib.polyu.edu.hk/handle/200/2169
Abstract: The excessive production of CO2 is a major environmental issue. The removal of CO2 by reducing it to energetically rich raw materials is one way to recycle the carbon source. In the past two decades, much attention has been paid to the eletrocatalytic reduction of CO2. In order to develope an efficient system to recycle CO2, an in-depth understanding of the mechanism of CO2 reduction is necessary. In this study, we have synthesized and characterized two new ruthenium carbonyl complexes, namely [Ru(bdmpp)(bpy)CO]2+ [1] and [Ru(tpm)(bpy)CO]2+ [2] (bdmpp = 2,6'-bis(3,5-dimethylpyrazol)pyridine; tpm = tris(1-pyrazoyl)methane; bpy = 2,2'-bipyridine) that are electrocatalysts for CO2 reduction. The mechanism for CO2 reduction was investigated by cyclic voltammetry, constant potential electrolysis and in-situ ETIR spectroelectrochemistry. It was found that addition of protic sources significantly enhanced the rate of CO2 reduction. In the presence of H20 as protonsource, CO was produced exclusively with current efficiency close to 98%. In the presence of protonated amine salts such as Et3NH+Cl-, selective production of formate could be achieved with a current efficiency as high as 90%. Mechanistic studies by in-situ FTIR spectroelectrochemistry suggested that CO was afforded via a Ru-COOH intermediate in the presence of H2O as proton source. CO was produced by protonation of the coordinated CO2 by H2O followed by the cleavage of one C-O bond and subsequent release of CO from the metal center. On the contrary, a Ru-H species was detected in the IR spectrum prior to the formation of the metalloformate species in the presence of EtsNH+Cl-, which supported that the ruthenium formato species was formed via the insertion of CO2 into a Ru-H moiety. The effect of weak Bronsted acid on the electrocatalytic reduction of CO2 by [Fe(dophen)(N-MeIm)2]+ [3] was also studied. Addition of weak Bronsted acids such as trifluoroethanol or methanol can enhance the rate of catalysis to yield a mixture of carbon monoxide, formate and oxalate. Monitoring the reduction process by in-situ FTIR showed the existence of both an iron carbonyl and an iron fonnato species. While CO and HCOO- were suggested to be produced by similar mechanisms as the ruthenium catalysts, the homalytic cleavage of the Fe-C bond of Fe-COO- will lead to theformation of CO2- which will subsequently dimerize to yield oxalate. This is probably because the carbenoid character of Fe-COO- is weaker than that of Ru-COO- which leads to the release of CO2- in an early stage of the catalytic process.

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