Impact of copper on activity, selectivity, and deactivation in the photocatalytic reduction of CO2 over TiO2

Abstract

The photocatalytic reduction of CO₂ to hydrocarbons may be a promising mechanistic route to reduce greenhouse gas CO₂, convert it into useful products, and limit the direct emission of CO₂. The photocatalytic reduction of CO₂ is a reaction in which photons activate the photocatalyst by generating reduced and oxidized sites, that are re-oxidized and re-reduced by the reactants. The photocatalytic reduction of CO₂ can produce various products, including CO, formic acid, formaldehyde, methanol, methane, etc.

Here, the activity/selectivity of produced hydrocarbons in the presence of oxygen on various photocatalysts (TiO₂, 5 wt%Cu-TiO₂, 5 wt%Cu-C-TiO₂) was determined. The reaction rate increased with increasing reaction time up to the first half an hour of the reaction but then started to decrease with photocatalysts, indicating photocatalyst deactivation, a rate-limiting step by hydroxyl radicals and adsorption of intermediates on TiO₂. Carbon deposition as the origin of photocatalyst deactivation was confirmed using the TGA of the spent photocatalyst. Additionally, absorption of intermediates on spent catalysts were confirmed by FTIR. On the other hand, the conversion increased with time when copper was used as a promoter on a TiO₂ compared with TiO₂ due to its larger surface area and having more active sites.

The photocatalysts were characterized using BET, ICP-OES, XRD, TGA, XPS, Fourier-Transform Infrared Spectroscopy (FTIR), and UV–Vis. The focus of this study was to determine the activity and efficacy of different photocatalysts (TiO₂, 5 wt%Cu-TiO₂, 5 wt%Cu-C-TiO₂) by gas phase measurement, with a particular emphasis on obtaining reproducible data on conversion and selectivity as a function of irradiation time.

The copper on TiO₂ was found to be more selective towards sodium formate/formic acid with a maximum selectivity of ∼90 % in 4 h and had higher activity (74.3 µmol g_cat^-1 h-1). A maximum CO with a selectivity of 86 % was found when TiO₂ was used in 4 h.

Copper transfer electrons on the TiO₂ surface enhance CO₂ adsorption on the catalytic surface and is the reason for having higher valuable product on Cu-C-TiO₂ and Cu-TiO₂ compared with TiO₂. Carbon in this experiment did not have a role and its effect was negligible.