The effect of weak Brönsted acids on the electrocatalytic reduction of carbon dioxide by a rhenium tricarbonyl bipyridyl complex

Abstract

The effect of four weak Brönsted acids, 2,2,2-trifluoroethanol, phenol, methanol and water on the electrocatalytic reduction of CO2 by [Re(bpy)(CO)3(py)]2+ (bpy = 2,2′-bipyridine, py = pyridine) in acetonitrile was investigated. The addition of weak Brönsted acids enhances the rate of the catalytic process and improves the lifetime of the rhenium catalyst. Under the experimental conditions employed in this study, CO was produced as the only product with faradaic efficiency close to 100%. Kinetic analysis indicated that the reaction order in acid is 2. The Brönsted acids probably stabilise the rhenium-carbon dioxide intermediate through protonation and facilitate the cleavage of one of the C-O bonds to yield CO. The efficiency of the acid increases with its acidity. Thus, trifluoroethanol and phenol are more effective in enhancing the rate of catalysis than methanol and water. Even though the acid strength of water and methanol are very similar, water is much less effective in enhancing the rate of CO2 reduction. This can be attributed to the relatively strong ligating property of water, which would compete with CO2 for the binding site on rhenium.