Reforma eletroquímica de glicerol em sistema ácido-alcalino com catalisadores de paládio

Abstract

In the incessant search for change in the energy model by reducing the dependence on fossil fuels, hydrogen is gaining importance as an energy vector and lever to replace combustion engines with fuel cells. Nevertheless, producing hydrogen from non-fossil fuel sources is still a challenge, with an emphasis on electrolytic hydrogen. To close the green cycle, it is possible to connect hydrogen production to renewable sources to supply the required energy. A shortcoming of this process is the high price of the electrolytic hydrogen, between 3.4 and 12 USD per kg, according to the International Energy Agency in its 2023 annual report about the global status of hydrogen technologies (INTERNATIONAL ENERGY AGENCY, 2023), three times higher than “fossil” H2. To boost green hydrogen development, alternatives are mandatory to reduce the cost of electrolytic hydrogen. This dissertation presents an alternative by developing and optimizing an acid-alkaline glycerol electrochemical reformed to cogenerate hydrogen and electricity by applying a transmembrane pH gradient through a Cation Exchange Membrane. Moreover, this work proposes valorizing the endocarp and shell of the macauba wastes to obtain carbon support by hydrothermal treatment. The prepared materials were characterized by measuring the surface area, pore size distribution, functional groups, and the degree of defects and crystallinity. All the used supports were chemically functionalized by treating them with sulfuric, nitric, chlorohydric acid, and hydrogen peroxide to add oxygenated surface groups. With these supports, Pd/C electrocatalysts were prepared by chemical reduction with sodium formate in alkaline medium. The Pd/C electrocatalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Transmission Electron Microscopy, and quantifying the electrochemically active surface area by CO stripping. Vulcan XC-72 carbon black was used as a reference support, following the same protocol as the macauba carbon supports. The preliminary electrochemical results evidenced the potential of the alternative carbon supports, stimulating their ulterior application in the acid-alkaline electrochemical reformer. The temperature and the flow rate were optimized, taking the hydrogen produced as the output variable. Preliminary electroreforming experiments under the optimum conditions were galvanostatically carried out, evaluating the amount of hydrogen, electricity, and glycerol oxidation products obtained. With the best Pd/C material (Vulcan XC-72 treated with hydrogen peroxide), it was possible to produce 0.528 Nm3 m-2 h -1 of H2 with a concomitant energy of 0.725 kWh m-2 . In addition, it was possible to obtain high-added-value products such as glyceric, tartronic, and oxalic acid as the most abundant glycerol electro-oxidation products. The system demonstrated to be an alternative for glycerine valorization, the main by-product of biodiesel synthesis, capable of diversifying the portfolio (hydrogen and fine chemicals) of this industry with direct implications on the cash flow and the process sustainability. Finally, xi this work also evidences the feasibility of using activated carbon from the thermochemical valorization of lignocellulosic wastes from the macauba processing (endocarp and shell).