Al/Cu-pillared clay catalyzes CO2 valorization driven by UV and Vis light

Pillared interlayer clay (PILC) is a low-cost material with great specific surface area, large pore volume and surface acidity that ensures its effectiveness in catalytic reactions. In this work, Al pillars and Cu ions were inserted between layers of a natural silicate clay mineral (bentonite) by cation exchange to constitute the PILC (Al/Cu-PILC) and characterized by atomic absorption spectrometry (AAS), X-ray diffraction (XRD), ammonia adsorption, infrared spectroscopy (IR), TEM, scanning electron microscopy and energy-dispersive spectrometry (SEM-EDS), N₂ adsorption–desorption and X-Ray photoelectron spectroscopy (XPS). Then Al/Cu-PILC was evaluated as photocatalyst for CO₂ reduction. The characterization results confirmed a successful pillaring process that increased the basal space and specific surface area of initial bentonite. The catalyst showed activity for CO₂ photoreduction. The reaction products were identified by gas and liquid chromatography and formic acid was identified as the major product. The studied variables were the effect of the catalyst loading, wavelength, and reactor type; for this purpose, four catalyst concentrations were tested (0.03, 0.055, 0.11 and 0.5 g L⁻¹), UV light and visible light were also studied, the performance of a typical semi-batch stirred tank reactor (STR) and capillary reactor was compared. The highest specific productivity corresponded to the catalyst loading of 0.055 g L⁻¹ (3926 μmol⋅g_cat⁻¹ h⁻¹). The highly efficient photocatalytic activities of Al/Cu-PILC for the reduction process of CO₂ were ascribed to the synergistic effect by incorporation of copper ions and the use of a capillary reactor. By probe molecules addition, photo-generated holes and hydroxyl radicals were proven to exert an important positive effect over formic acid production.