Unraveling the role of a Cu dopant in formaldehyde catalytic oxidation over a La0.8Sr0.2Mn1−xCuxO3 perovskite: an experimental and theoretical study

Abstract

A series of La_0.8Sr_0.2Mn_1−xCu_xO₃ perovskite-type catalysts were prepared through a sol–gel method and evaluated for formaldehyde catalytic oxidation. Experimental and DFT studies were performed to reveal the role of the Cu dopant on formaldehyde oxidation over La_0.8Sr_0.2Mn_1−xCu_xO₃ catalysts and determine the optimal doping amount of Cu. The perovskite with a Cu substitution content of 0.2 exhibited the highest catalytic activity and good thermal stability for formaldehyde oxidation. The degree of Cu substitution significantly influenced the textural properties of the catalysts. The La_0.8Sr_0.2Mn_0.8Cu_0.2O₃ catalyst exhibited the highest specific area, pore volume, and crystalline degree, which enabled the availability of more active sites for formaldehyde adsorption. The introduction of bivalent Cu²⁺ resulted in a charge imbalance that was compensated by the increased Mn⁴⁺/Mn³⁺ ratio of the perovskite. Partial substitution of Mn by Cu cations enhanced the oxygen mobility of perovskites, which was ascribed to a synergy between surface Cu and Mn atoms. The La_0.8Sr_0.2Mn_0.8Cu_0.2O₃ catalyst presented excellent oxygen mobility and thus promoted formaldehyde catalytic oxidation. DFT calculation results indicated that the absolute value of the formaldehyde adsorption energy on the surface Cu–O site was higher than that on the Mn–O site. The Cu dopant facilitated formaldehyde adsorption and promoted the transfer of more electrons from formaldehyde to the catalyst, which was beneficial for formaldehyde activation and subsequent oxidation. Finally, combining the in situ FTIR measurements with DFT calculations revealed the reaction mechanism of formaldehyde oxidation on the La_0.8Sr_0.2Mn_1−xCu_xO₃ perovskite. Based on the experimental and theoretical methods, herein, the corresponding reaction cycle of formaldehyde oxidation is proposed. The reaction cycle contained seven elementary reaction steps, in which O₂ dissociation was the rate-limiting step with the highest energy barrier of 1.47 eV.