I-doped BiOCl Boosting Photocatalytic CO2 Methanation Activity of TiO2 Nanotube Arrays

Abstract

Photocatalytic CO₂ methanation provides a sustainable approach for massive CO₂ conversion, where the protonation of carbon intermediates and electron transfer (eight electrons and four protons required from CO₂ to CH₄) kinetics are essential. In this work, I-doped BiOCl on TiO₂ nanotube arrays (BiOCl-I/TNTs) are prepared, which gives a methane production rate of 2.8 × 10⁻³ µmol·cm⁻²·h⁻¹, that is nearly 5 times higher than TiO₂ (5.5 × 10⁻⁴ µmol·cm⁻²·h⁻¹). The selectivity towards methane production is 84.4% for BiOCl-I/TNTs, competing with hydrogen evolution reaction, showing nearly twice the increase compared to TiO₂. The enhancement on the activity and selectivity is ascribed to high solar light absorption, and excited energetic band of I-BiOCl (-1.75 eV vs. NHE) that offers high reductive potential of electrons for CO₂ activation and reduction thermodynamically. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) analysis demonstrates a Mixed pathway for methanation is favored on I-BiOCl with *COOH as activated states of CO₂, where the reaction barrier is lower compared to conversion of CO₂ to bidentate adsorbed CO₂ (b-CO₃²⁻) or monodentate adsorbed CO₂ (m-CO₃²⁻) on bare TiO₂, that can be stemmed from the water dissociation and oxidation ability of I-BiOCl, providing high density of protons to react with carbon intermediates. This work thus provides new insights on the methanation of CO₂ using BiOX heterojunctions.

Graphical Abstract

I-BiOCl/TNTs significantly enhances CO₂ methanation activity by high solar light absorption and excited virtual states under illumination that offers high reductive potential of electrons for CO₂ activation. The lowered energy barrier for CO₂ reduction pathway via *COOH intermediates, leading to high CH₄ selectivity and yield.