CO2 adsorption and its visible-light-driven reduction using CuO synthesized by an eco-friendly sonochemical method

Abstract

One alternative to mitigate the high CO2 concentration in the atmosphere is its conversion into value added-products or solar fuels by heterogeneous photocatalysis. However, in these reactions, the CO2 fixation remains a challenge due to the inert nature of greenhouse gas. Thus, it is necessary to propose materials that presented good performances in both processes: CO2 adsorption and its photocatalytic conversion to solar fuels. Copper oxide is a potential material that could exhibit high efficiencies for both processes if it is prepared by adequate synthesis methods. In this work, CuO was obtained by a sonochemical method using an eco-friendly surfactant (Na3C6H5O7), low sonochemical time (15 min) and low temperatures (80 °C). The materials were evaluated in a combined process that involves CO2 adsorption and its photocatalytic conversion to liquid fuels (CH3OH) under LED visible-light at 25 °C. CuO particles were characterized by X-ray powder diffraction, Scanning Electronic Microscopy, N2 physisorption, FTIR spectroscopy, Diffuse Reflectance Spectroscopy, and X-ray photoelectron spectroscopy. CuO pure showed the best efficiencies for CO2 adsorption: 318 mgCO₂ g−1 and CH3OH production: 3.7 μmol g-1 h-1, which was associated with it’s the high number of active sites. Bohem titrations were used to calculate the active: acid/basic sites in the samples, which results demonstrated a higher contribution of basic sites (2.4 mmol). These active sites favored the formation of monodentate bonds between CO2 and CuO, which promoted its photoconversion to solar fuels such as CH3OH. Additionally, CuO/Cu(OH)2 mixtures with the high surface area were identified at higher concentrations of Na3C6H5O7. However, these mixtures did not show high efficiencies for both processes. A mechanism of combined CO2 adsorption and its photocatalytic conversion to methanol under visible light was proposed. The stability and recyclability of CuO as CO2 adsorbent and as a photocatalyst was demonstrated after three consecutive cycles without compromise its efficiency in the combined process.