Iron and copper pillared clay photo-catalyzes carbon dioxide chemical reduction in aqueous medium

Abstract

This work aimed to assess the catalytic activity of iron and copper pillared bentonite in the photoreduction of carbon dioxide towards organic compounds that are considered fuels, hydrogen carriers and brick molecules: methanol (M), formic acid (FA) and acetic acid (AA). The assessed variables were copper content, temperature, NaOH concentration and activation wavelength (254 nm and > 400 nm). Fe and Cu content was quantified by atomic absorption: 13.62 % Fe (Fe-PILC), 2.51 % Cu (Cu/Fe-PILC-I), 3.86 % Cu (Cu/Fe-PILC-II). The addition of Fe and Cu increased specific surface area of bentonite, from 36 to 245, 137 and 98 m² g⁻¹ for Fe-PILC, Cu/Fe-PILC-I and Cu/Fe-PILC-II, respectively. FeO, Fe₃O₄, Fe₂O₃, CuO and Cu₂O were identified by X-ray photoelectron spectroscopy, XRD and TEM. By UV–Vis diffuse reflectance spectroscopy the band-gap energy of the synthesized materials was found to be ca. 2 eV. Pillaring increased total surface acidity, with Lewis acid sites rising by up to 63 % for Fe-PILC, and 395 % for Cu/Fe-PILC-II materials, respectively, in comparison to Bentonite. The optimization of the CO₂ photo-reduction process was conducted by a Face Centered Central Composite Design (FCCD) and the optimal conditions maximizing FA (289.6 $\mathrm{\mu }\mathrm{m}\mathrm{o}\mathrm{l}{\mathrm{g}}_{\mathrm{C}\mathrm{A}\mathrm{T}}^{-1}$$\mathrm{\mu }\mathrm{m}\mathrm{o}\mathrm{l}{\mathrm{g}}_{\mathrm{C}\mathrm{A}\mathrm{T}}^{-1}$) and AA (28.7 $\mathrm{\mu }\mathrm{m}\mathrm{o}\mathrm{l}{\mathrm{g}}_{\mathrm{C}\mathrm{A}\mathrm{T}}^{-1}$$\mathrm{\mu }\mathrm{m}\mathrm{o}\mathrm{l}{\mathrm{g}}_{\mathrm{C}\mathrm{A}\mathrm{T}}^{-1}$) were: 20 °C, 0.25 M [NaOH]₀ and 3.86 % of Cu (C); Cu/Fe-PILC-II, under UV light. At this point, acrylic acid and methanol concentrations were 0.8 $\mathrm{\mu }\mathrm{m}\mathrm{o}\mathrm{l}{\mathrm{g}}_{\mathrm{C}\mathrm{A}\mathrm{T}}^{-1}$$\mathrm{\mu }\mathrm{m}\mathrm{o}\mathrm{l}{\mathrm{g}}_{\mathrm{C}\mathrm{A}\mathrm{T}}^{-1}$ and 3572.08 $\mathrm{\mu }\mathrm{m}\mathrm{o}\mathrm{l}{\mathrm{g}}_{\mathrm{C}\mathrm{A}\mathrm{T}}^{-1}$$\mathrm{\mu }\mathrm{m}\mathrm{o}\mathrm{l}{\mathrm{g}}_{\mathrm{C}\mathrm{A}\mathrm{T}}^{-1}$, respectively. CO₂ photoreduction driven by Visible light was also tested, requiring moderate experimental conditions to accomplish a nearby value of carboxylic acids formation rate in comparison to the process assisted by UV light.