Promoting the mechanism of OMS-2 for gas adsorption in different K+ concentrations.

Abstract

Catalytic combustion technology is an efficient and green method to deal with low concentration methane. Gas adsorption over the catalyst surface is a key step in the catalytic combustion process, which has attracted much interest. In this work, the first-principles density functional theory calculation method has been applied to explore the adsorption processes of CH4 and O2 molecules on the surface of cryptomelane type manganese oxide octahedral molecular sieves (OMS-2). In addition, the effect of K+ concentration in the OMS-2 tunnel on the adsorption of the two gaseous molecules has also been investigated. The results of adsorption energy and structural characteristics show that the adsorption energies of CH4 and O2 molecules over the catalyst surface are favorable. Adsorption sites of CH4 are the K+ and O sites, among which the K+ site is the most stable adsorption site. In addition, Mn sites are favorable for adsorbing O2 molecules. The interactions between the catalyst and the adsorbed CH4 and O2 are enhanced with the increasing tunnel potassium ions. It should be noted that with the increasing strength of the adsorption energies, equilibrium distances from the two gaseous molecules to the active sites become shorter and the bond lengths of C-H and O-O bonds become longer. Moreover, the adsorption sites of CH4 on the catalyst surface increase with the increasing K+ concentration. Bader charge and cohesive energy calculations reveal that the tunnel K+ can balance charges and help strengthen the structural stability of OMS-2. Interestingly, the electronegativity of the catalyst has been altered after introducing K+, which leads to better adsorption of gaseous CH4 and O2. The microscopic mechanism of the effect of K+ concentration on the adsorption of CH4 and O2 over the catalyst surface paves the way for further deciphering the mechanism underlying the catalytic oxidation process and helps design more efficient catalysts for methane utilization.