Theoretical study of the mechanism for photocatalytic CO2 reduction to methanol over layered double hydroxides

Abstract

Converting the greenhouse gas CO₂ to high value-added chemicals by photocatalysis has attracted much attention from scientists in recent decades. The performance of the photocatalytic CO₂-reduction reaction is largely related to the properties of the photocatalyst. Layered double hydroxides (LDHs) have been widely used as photocatalysts in this reaction since Izumi, Teramura, and Tanaka's findings in the 2010s. Although much experimental work has been done, reaction mechanisms for CO₂ reduction over LDHs have not been completely illustrated because of the complexity of the possible reaction pathways and products, limiting the selectivity of methanol. In this work, we first constructed a series of models representing M₂M′–NO₃–LDHs (M = Mg, Co, Ni, Zn; M′ = Al, Ga). After calculating their electronic properties (band structures, density of states, work functions, and band edge placements), together with their Gibbs free energy diagrams of CO₂ reduction to the target product CH₃OH and side products CO, HCOOH, HCHO, and CH₄, it was found that the most favorable reaction pathway for the calculated LDHs is * + CO₂ → CO₂* → OCOH* → CO*/HCOOH* → CHO* → HCHO*/HCOH* → CH₂OH* → CH₃OH* → CH₃* → CH₄* → * + CH₄. The main products for the calculated LDHs are CO and CH₄. Although HCHO* and CH₃OH* could be generated over most of the calculated LDHs, they could not facilely desorb while they could be easily further reduced, which may be the reason that HCHO and CH₃OH are seldom found as products of photocatalytic CO₂ reduction over LDHs without Cu. After that, we constructed a Cu atom-substituted model denoted as (ZnCu)₂Ga–NO₃–LDH. According to its Gibbs free energy diagram of photocatalytic CO₂ reduction to CH₃OH, the doping of Cu in the active site resulted in an improved Gibbs free energy change (ΔG) for the further reduction of CH₃OH* to CH₃*, which was deduced to be derived from the Jahn–Teller effect of divalent Cu. In general, this work proposes a reasonable reaction mechanism explaining the reaction selectivities to CO and CH₄ for photocatalytic CO₂ reduction over LDHs without Cu, and reveals the reason why Cu doping benefits producing CH₃OH, facilitating the rational design of LDH-based photocatalysts towards selective CO₂ reduction to CH₃OH.