Synergistic Effect of Co/La in Oxygen Vacancy Rich Ternary CoAlLa Layered Double Hydroxide with Enhanced Reductive Sites for Selective Photoreduction of CO2 to CH4

Abstract

Developing a convenient ternary layered double hydroxide (LDH) semiconductor for highly active photocatalytic reduction is necessary for solving the energy crisis and environmental pollution. In this work, a highly active ternary CoAlLa-LDH was formed through inducing La³⁺ to enhance coordinatively unsaturated metals centers for development of reductive sites that resulted in improvement of photogenerated charge carrier separation. It was demonstrated that the ratio of cations between the Co/Al/La-LDHs greatly determined the LDH structure for photocatalytic activity toward CO₂ reduction. The increase of La content resulted in destruction of LDH structure with lower photoactivity for CO₂ reduction. The rate of CO and CH₄ evolution in Co₂Al₁-LDH was about 15.3 and 9.2 μmol gcₐₜ–¹ h–¹, much higher than Co₃Al₁-LDH and Co₄Al₁-LDH, which indicates that the Co₂Al₁-LDH among CoAl-LDHs is a promising photocatalyst with high photocatalytic activity in CO₂ reduction to CO and CH₄. The Co/Al/La molar ratios were optimized to be 2:0.95:0.05 with a production rate of 25.5 μmol gcₐₜ–¹ h–¹ and 21.80 μmol gcₐₜ–¹ h–¹ for CH₄ and CO, respectively. A more interesting finding is that the selectivity of CH₄ was raised to 53.93% as compared to 46.10% for CO. Besides, good stability was observed due to the presence of the intact hexagonal nanosheet structure, which was sustained in multiple cycles without obvious deactivation. The highest activity was due to the proper weaving of lanthanum into the framework of LDH. This work demonstrates a facile synthetic method of rationally doping La transition-metal to develop new photocatalyst with remarkable performance in CO₂ reduction.