Hydrogen production from bioethanol using cobalt hydrotalcites
Authors:   , Espinal Bustos Raúl Uziel 
Published: October 2023

Hydrogen constitutes a promising alternative to manage our energy supply more efficiently. Hydrogen can be stored and used in fuel cells to produce electricity, where it combines with the oxygen present in the air and generates solely water as by-product. Of the different methods available to produce hydrogen, the catalytic reaction of ethanol and water (reforming) is one of the most advantageous alternatives, since ethanol can be produced easily from biomass (bioethanol), is liquid and simple to manipulate. This doctoral thesis studies the behavior of a family of cobalt catalysts to produce hydrogen from ethanol and water; to be more precise, catalysts based on cobalt hydrotalcites. The same process could be triggered by other types of catalyst, but many of them are far more expensive due to the noble metals they contain, and others - those based on nickel and cobalt - desactivate after a short amount of time because their surface accumulate carbon. This thesis demonstrates that with the help of a precise method of preparation, one can create inexpensive catalysts from cobalt hydrotalcites, which remain quite stable under realistic operating conditions.
Chapter 1 introduces the reader to the key aspects of this doctoral thesis. It explains the objectives pursued and gives an overview of the state of art and the groundwork on which the experimental work is based. Besides explaining the general characteristics of the catalysts and the reactions that will be studied, chapter 1 also informs about cordierite monoliths: what exactly are they and why are they used in this work to physically stabilize the catalysts and catalytic membrane reactors. In this way, the aim of this doctoral thesis is to acquire new scientific knowledge on the one hand and on the other, to apply this knowledge in the development of devices that can be applied in practice.
The four chapters following thereafter form a compound of papers that have been published in notable international journals (three of them) and one article in process of revision.
Chapter 2 describes the preparation of a family of cobalt hydrotalcites with different ratios of cobalt, magnesia and aluminum, and how these cobalt hydrotalcites behave in the ethanol steam reforming reaction to produce hydrogen. Starting from a detailed characterization using different techniques like TEM, XRD, IR, TGA, In situ XPS, magnetism, etc., the different chemical elements present are identified, and their structure in the catalysts before, during, and after reaction is analyzed. It becomes evident that the best formula (with the greatest yield of hydrogen and the least amount of coke residual) is a hydrotalcite with a relation of Co:Mg:Al=1:2:1. It is concluded that during the reaction, the hydrotalcite-based catalyst transforms itself to a mix of cobalt spinel, strongly interacting with MgO on a nanometric scale. Nevertheless, if the reaction is repeated using only cobalt spinel (synthesized specifically for this purpose), the outcome is a smaller amount of hydrogen. This shows that cobalt hydrotalcite used as a catalyst precursor plays a crucial part in the final structure of the catalyst.
Hydrotalcite Co:Mg:Al=1:2:1 doped with Pt and Rh is studied in chapter 3. For this, two families of catalysts with different ratios of Pt and Rh were prepared. They were analyzed under the same conditions as explained in chapter 2 and were tested in the reaction. The objective of doping the cobalt hydrotalcite with noble metals was to facilitate the reaction of cobalt, given the fact that metallic cobalt is the active element in ethanol steam reforming. Besides this key function of metallic cobalt, chapter 2 also reveals, however, that metallic cobalt speeds up the catalyst deactivation by causing severe coke accumulation. Hydrotalcite Co:Mg:Al=1:2:1 doped with Pt and Rh is studied in chapter 3. For this, two families of catalysts with different ratios of Pt and Rh were prepared.