Development of Cu-Sn-based photo-electrocatalysts for the CO2 valorization

The scope of this work is the finding of important information on copper (I) oxide and its use in the development of Cu-Sn-based photo-electrochemical catalysts for the valorization of carbon dioxide. Initially, the activities are focused on a bibliographic research on the most active crystalline phase of Cu2O for the photoelectrochemical CO2 reduction and its synthesis. From literature, it appears that the rhombic dodecahedral Cu2O is the most photoactive form, while the cubic is the least photoactive. Subsequently, the attention was turned to the synthesis, characterization and testing of Cu2O-SnO2 catalysts prepared by two synthesis methods: co-precipitation and precipitation-impregnation two-steps strategy. For the co-precipitation method, the study has started from a synthesis already implemented by the research team of Prof. Hernández at Politecnico di Torino, in which Cu(NO3)2 * 3H2O and SnCl4 * 5H2O are used as precursors, Na2CO3 is used as a precipitating agent, while NaBH4 has the function of reducing agent. This synthesis strategy has been modified by applying ultrasounds throughout the reaction time. Moreover, The solution pH was also modified to favor the formation of rhombic dodecahedral Cu2O and SnO2 simultaneously. XRD and BET analysis show how the synthesis at pH 7, in addition to being replicable, presents samples with Cu2O and SnO2 as only chemical species, lower energy bandgap, higher surface area and porosity. On the other hand, the precipitation-impregnation two-steps strategy, already developed by Zhang and co-workers, aimed to synthesize Cu2O-SnO2 core-shell catalyst with a controlled thickness of the tin (IV) oxide. This method is subdivided into two steps; the first step is a wet precipitation synthesis to obtain copper(I) oxide nanocubes, using CuCl2, NaOH and C6H8O6 as reagents; the second one is the formation of a SnO2 layer by impregnation on Cu2O, utilizing SnCl4 *5H2O. Since nanocubes are the least photo-active crystalline form of copper(I) oxide, the aim is to obtain Cu2O with a crystal structure similar to rhombic dodecahedral. Cu2O yield is raised from 86% to 99%, increasing by 100% the concentration of the CuCl2 and 200 % that of precipitant and reducing agents. An attempt to modify the crystalline form is tried through an additional acid etching step. However, the result is the synthesis of mixed Cu-Cu2O catalyst, which is an interesting material for the electrochemical CO2 conversion to alcohols. XRD spectra of all synthesized copper(I) oxide samples showed typical characteristics of the octahedral crystalline form, such as the peak of maximum intensity at 36.5 and intensity ratio between the peaks {220} and {200} facets in the range between 0.6 and 0.7.