Coffee-waste templated CeO_x/TiO₂ nanostructured materials for selective photocatalytic oxidations

An environmentally friendly solvent-free approach was tested using spent coffee as a biomass sacrificial template for the preparation of TiO₂ modified with CeO_x. The use of coffee as a template pursues the preparation of a nanostructured heterojunction without the need for a solvent. Two variables were optimized in the synthesis process, i.e. calcination temperature and proportion of CeO_x. Firstly, bare coffee-template titania was prepared to explore the effect of the calcination temperature, within 500-650 °C. The anatase phase was obtained up to 600 °C. Higher temperatures, i.e. 650 °C, led to the appearance of rutile (10%) and efficient removal of the sacrificial agent (0.6% residue). The maximum photocatalytic activity in terms of conversion, in the oxidation of benzyl alcohol, was achieved employing the bare coffee-template TiO₂ at 650 °C, and it was found comparable to the benchmarked P25. The incorporation of ceria in the solvent-free approach considerably improved photocatalytic benzaldehyde production. No changes in the XRD pattern of TiO₂ were appreciated in the presence of ceria due to the low amount added, within 1.5-6.0%, confirmed by XPS as superficial Ce³⁺/Ce⁴⁺. The UV-visible absorption spectra were considerably redshifted in the presence of Ce, reducing the bandgap values of bare titania. An optimum amount of ceria in the structure within 3-0% was found. In this case, the selectivity towards benzaldehyde was ca. 75%, 3 times higher than the selectivity value registered for the benchmarked P25 or the bare prepared TiO₂.