Methane Dissociation on Bimetallic AuNi3, Au2Ni2 and Au3Ni Clusters‐A DFT Study

Abstract

AbstractThe effect of gold atom for the dissociation of methane on tetrahedral bimetallic AuxNiy (x+y=4; x,y=1‐3) clusters are introduced using density functional theory method. The binding energy and HOMO‐LUMO gap fluctuate with odd‐even number of gold atoms. The clusters, AuNi3 and Au3Ni, containing odd number of doped gold atoms, which causes open shell configuration, are exceptionally stable and highly resistance to carbon deposition, whereas closed shell Au2Ni2 cluster is less stable. As the number of doped Au atom increases in the clusters, adsorption or binding energy of methane increases. The dissociation of methane is a four elementary steps process, and first step of dissociation, CH4→CH3, is not the rate determining step, and thermodynamically favorable at room temperature and pressure for the clusters due to adsorption. Whereas, the dissociation of CH→C, coke formation step 4, is the rate determining step due to highest energy barrier for all the clusters. The calculated activation energies for the rate determining step are 0.92 eV, 0.41 eV and 1.64 eV for AuNi3, Au2Ni2 and Au3Ni clusters, respectively. In addition, the dissociation temperature for all the intermediates, e. g., CH3, CH2 and CH are 450 K(CH3 and CH2) and 650 K(CH) on AuNi3 cluster; 450 K(CH3) and 550 K(CH2 and CH) on Au2Ni2 cluster; and 650 K(CH3), 750 K(CH2) and 950 K(CH) on Au3Ni cluster. Thus, the coke resistivity decreases as Au3Ni >AuNi3>Au2Ni2 and corresponding coke resistance temperatures are 950 K, 650 K and 550 K, respectively. The charge tranfer to the clusters during adsorption is negative, and is positive in the dissociation steps. Thus, the number of doped gold atom not only influences the stability of bimetallic Au−Ni clusters, but also their reactivity towards methane, which can provide some new insight for the design of coke resistant catalysts for methane reforming.