Modification of nanostructured tin oxide-based catalysts for CO2 valorization

In the present energetic scenario, the world primary energy supply is mainly provided by fossil fuels. As a consequence, the global increasing energy demand causes an increase in carbon dioxide concentration in the atmosphere, that boosts the greenhouse effect. Electrochemical CO2 reduction represents a way to mitigate this effect since it converts this pollutant into value-added products, such as methane, methanol, formic acid, etc… which otherwise are derived from fossil resources. In this thesis work, developed in the laboratories of the Italian Institute of Technology (Center for Sustainable Future Technology@PoliTo), SnO2 is proposed as catalyst for the CO2 electroreduction, since it is an environmentally friendly and inexpensive material. Even though, to date, tin oxide has been widely studied for CO2 reduction into formic acid, its selectivity and conversion efficiency need to be improved in order to apply it at the industrial scale. The goal is to improve its electrochemical performances by introducing dopant elements in the catalyst, according to the theoretical calculations proposed in the literature [J. Mater. Chem. A, 2017, 5, 11756]. The methods and technological approaches used in this work to prepare the catalyst (doped and undoped), and the electrodes, follow up from previously optimized procedures. The thesis is divided into six sections. The first is the introduction to the scientific context on the CO2-related climate change. The following two chapters describe 1) the methods and procedures used to prepare and characterize the catalyst; 2) the electrochemical reaction, its characterization set-up and methods to quantify the products. In the fourth chapter, the obtained results on undoped SnO2 are commented, and in the following the role of the dopant is discussed. Finally, in the sixth chapter, some conclusions are drawn. In brief, the catalyst is prepared by anodic oxidation of Sn in a NaOH solution. This creates an amorphous porous layer composed by SnO and SnO2. This layer is later detached from the substrate and annealed with or without the dopant (Zn and Ti) precursors, to obtain doped or undoped crystalline SnO2, respectively. The annealing temperature of 600°C is selected to promote the dopant diffusion in the crystalline structure. The electrodes are then prepared by coating a conducting support (carbon paper gas diffusion layer) with a paste obtained by mixing the catalyst (undoped or doped SnO2) with Nafion 117 solution, acetylene carbon black and isopropanol. The electrochemical characterization of the undoped electrodes put in evidence their reproducibility, with a faradic efficiency towards formic acid at around 70%. Which is in line with those of the same material prepared in the same laboratory [ACS Appl. Energy Mater. 2019, 2, 3081−3091]. Some of the doped samples show an increase in the average current density. And even though the selectivity towards formic acid is slightly reduced, an increase of the quantity of valuable products was observed. Some examples: the undoped sample has an average current density for formic acid of 2.9 mA cm-2 at -1.06V vs. RHE and 4 mA cm-2 at -1.15V vs. RHE. While the sample doped with titanium has 3.8 mA cm-2 at -1.06V vs. RHE and 4.2 mA cm-2 at -1.15V vs. RHE, and the one doped with zinc reached 1.7 mA cm-2 at -1.06V vs. RHE and 5.9 mA cm-2 at -1.15V vs. RHE. This suggests that doping is a possible way to improve the performances of the SnO2-based electrodes.