Voltammetric Study of Relationship between Metal-Hydrogen Bonding Energy, pK\(_{a}\), and Pyridine Reduction

Abstract

The reduction of CO\(_{2}\) to methanol proceeds more easily in the presence of pyridinium, but the mechanism by which pyridinium and then CO\(_{2}\) are reduced is unknown at this point. A proposed mechanism involving the reduction of the protonated nitrogen in the pyridinium, a transfer of that hydrogen molecule to the metal surface, and then a reaction of that surface hydride with CO\(_{2}\) has been proposed and verified by calculations but has yet to be verified by experiments. For this mechanism to be correct, the varying of the metal-hydrogen bonding energy must result in the activity of the electrochemical cell changing as well. Along with this, this mechanism implies that any change in the amount of protonated species will result in a lower amount of activity. As such, this thesis focuses on creating a relationship between both the metal-hydrogen bonding energy and the activity of the system and pK\(_{a}\) and the activity of the system. This is accomplished through the use of multiple non-noble and noble metals and multiple pyridine derivatives tested using cyclic voltammetry. Both yielded inconclusive results. Most of the metals did not reduce pyridinium the way that platinum does, though that effect was predicted by their metal-hydrogen bonding energy. A few metals did show some results, the metals that were closer in energy to platinum. However, with very few results to draw from, it was impossible to create a relationship. The pK\(_{a}\) experiment yielded the expected relationship barring one possible outlier, though it is difficult to eliminate that result without more data, making that result inconclusive as well. Both experiments point toward the mechanism being correct, but more experimentation is necessary.