Effects of pH and Potential in Kolbe Electrolysis of Acetate on Platinum

Abstract

Over many decades, reliable pathways have been invented for heating and power, as well as facilitating the complex chemical transformations in standard industrial chemical plants that enable the goods around us. Unfortunately, essentially all these processes are higher greenhouse gas (GHG)-emitting, which has led to global climate change, which is a looming threat for our world. Therefore, there is a need to find alternative processes that are able to reduce, or possibly eliminate GHG emissions.Biofuels have been, and continue to be, researched as promising alternative fuels. However, the high carboxylic acids content of the resulting bio-oil is troublesome and requires further upgrading [1]. One possible process that is able to selectively upgrade these carboxylic acids are electrochemical reactors. Electrochemical upgrading of carboxylic acids proceeds through the Kolbe reaction. In this reaction, a pair of carboxylic acids decarboxylate to two radicals which can dimerize and form alkanes (Equation 1).2 RCOOH → R–R + 2 CO2 + 2 H+ + 2 e- (Equation 1)In bio-oil upgrading studies, Kolbe electrolysis of acetic acid is typically used as a model reaction. A large number of studies on Kolbe electrolysis have published, many that focus on gaining fundamental insight in the influence of reaction parameters, like potential, pH and reactor configuration on the product yield and selectivity of the reaction [2-4]. From these studies, it is known that the Kolbe reaction proceeds through a methyl-radical pathway. Interestingly, such observations also make this reaction a potential proxy reaction for methane activation, another reaction that has received increased interest in recent years [5-7]. However, despite the previous work in this area, the work has largely been descriptive and contradictory.Therefore, this work aims to provide experimental data and optimize the methyl-radical pathway by changing pH and potential to promote selectivity of ethane, the primary Kolbe product. To do that, several studies have been performed using dilute solutions of acetate on polycrystalline platinum. Two three-electrode electrochemical cells, one batch and one flow configuration, were used for this study. The cells were equipped with a platinum foil working electrode, a platinum counter electrode and an Ag/AgCl reference electrode. The formed gas products were analyzed using in situ GC-MS. Offline liquid analysis was performed using NMR. In addition, an attempt was made to study the surface reactions on a platinum electrode under various potentials using ATR-FTIR experiments.The potential window used in this work was 2.8 V to 3.5 V vs RHE. In this range, an array of products was observed in both the gas phase (e.g., methanol, ethanol, ethylene, methane, ethane, carbon dioxide, carbon monoxide, hydrogen) and the liquid phase (e.g., methanol). It was observed that under the applied potentials high faradaic efficiencies (~90-96%) to ethane can be obtained and that this is increased with more positive potentials. Furthermore, it was observed the faradaic efficiency was maximized at pH values approaching the pKa of acetic acid. Electrolyte solutions with a bulk pH of 8 and 12 showed decreased faradaic efficiencies towards the formation of ethane over time. This occurs because the formation of bicarbonate (produced by dissolved CO2 in the electrolyte) hinders the dimerization reaction.Positively, the results of this study demonstrated high selectivity and yield of ethane from acetate on platinum for high potentials and slightly acidic acetate solutions. This finding is promising because ethane derived from the electrolysis of bio-oils can lead to the development of green chemicals – which reduces our dependence on petroleum.