Research into new, efficient catalysts for the electrochemical reduction of CO_2­ to fuels is a major challenge in the field of renewable energy. Study of the mechanisms of electrocatalysis is key in order to rationally modify catalysts to improve their performance, however the complicated nature of the double layer and the surface interactions involved during catalysis make these studies challenging.

Vibrational sum frequency generation (VSFG) spectroscopy is a powerful technique, as it is intrinsically surface specific, and allows detection of surface species in submonolayer concentrations. We have successfully applied this technique to the study of a few well-known group 6 and 7 molecular catalysts for electrochemical CO₂ reduction.

Mo(bpy)(CO)₄ is an efficient CO₂ reduction homogeneous electrocatalyst.^1,2 It has been found that the onset for catalysis is highly dependent on the working electrode material, with CO₂ reduction on Au occurring ca. 300 mV earlier than on Pt and GCE. We have investigated the mechanism of CO₂ reduction by Mo(bpy)(CO)₄ on Au and Pt electrodes using in situ VSFG spectroelectrochemistry (SEC-VSFG).³ The technique is highly surface selective, allowing the identification of species forming at the electrochemical interface during CV measurements. Our results indicate that on Au the active catalyst is formed via a second pathway following the formation of the intermediate [Mo(bpy)(CO)₄]•^-, and that the degree of interaction of [Mo(bpy)(CO)₄]•^- and the electrode material is the controlling factor for enabling the lower energy pathway. Our study demonstrates the viability of Mo(bpy)(CO)₄ and analogue group 6 complexes as efficient CO₂ reduction catalysts, by careful modification of the reaction conditions such as electrode material, solvent and electrolyte salt.

Mn(bpy)(CO)₃Br and its derivatives is one of the most widely studied class of catalysts due to their high selectivity for CO, high turnover frequency, ease of synthesis and low cost of the metal centre. Most studies involving these type of complexes use aprotic solvents with the addition of an acid.

The mechanism of Mn(bpy)(CO)₃Br has been studied both experimentally and computationally, and two main pathways have been identified, however some of the key intermediates have not been experimentally observed. We have used SEC-VSFG to study the catalyst under operating conditions. We have studied the catalyst with a range of added acids and were able to: 1)rationalize the high selectivity of Mn(bpy)(CO)₃Br towards CO, 2)observe a previously undetected reaction intermediate,⁴ and 3)find evidence for the much debated “dimer mechanism”.⁵