Electrochemical Lignin Oxidation Reaction on CuO: In Situ Spectroelectrochemical Point of View

Lignins are macromolecules present in biomass tissues, the third most prevalent component. This compound is frequently used as a thermoelectric fuel generating cheap energy; however, it burns the most abundant renewable source of aromatic structures in the world. For obtaining useful low-molecular weight compounds, depolymerization needs to be considered. One way of accomplishing this is the lignin oxidation reaction, which can be catalytic or noncatalytic. The noncatalytic type uses hard chemicals, temperature, and pressure and can present lower selectivity, further oxidizing monoaromatic products. The use of catalysts, especially electrocatalysts, makes the conditions mild and increases the selectivity; however, the reaction pathway followed is still under debate. It is difficult to find proposals in the literature that consider the heterogeneous catalyst-lignin interaction, so in this work, we applied in situ spectroelectrochemical techniques in the infrared region to check this, using commercial CuO catalyst powder as the catalyst and sugar cane lignin in 2.0 mol L^-1 KOH. The spectra were registered in external reflection-absorption configuration and deconvoluted, and the intensities and position of the bands were analyzed. For a deeper understanding of the chemical adsorption, the first step of lignin electro-oxidation, computational simulations were performed for a model lignin molecule with C_γ as the C atom closest to the CuO surface. The C_α-C_β breaking linkages could be followed, and some improved mechanistic steps were proposed.