Low-temperature oxidation of methane mediated by Al-doped ZnO cluster and nanowire: a first-principles investigation

Context

First-principles calculations are performed to investigate the catalytic oxidation of methane by using N₂O as an oxidizing agent over aluminum (Al)-doped Zn₁₂O₁₂ cluster and (Zn₁₂O₁₂)₂ nanowire. The impact of Al impurity on the geometry, electronic structure, and surface reactivity of Zn₁₂O₁₂ and (Zn₁₂O₁₂)₂ is thoroughly studied. Our study demonstrates that Al-doped ZnO systems have a better adsorption ability than the corresponding pristine counterparts. It is found that N₂O molecule is initially decomposed on the Al site to provide the N₂ molecule, and an Al–O intermediate which is an active species for the CH₄ oxidation. The conversion of CH₄ into CH₃OH over AlZn₁₁O₁₂ and (AlZn₁₁O₁₂)₂ requires an activation energy of 0.45 and 0.29 eV, respectively, indicating it can be easily performed at normal temperatures. Besides, the overoxidation of methanol into formaldehyde cannot take place over the AlZn₁₁O₁₂ and (AlZn₁₁O₁₂)₂, due to the high energy barrier needed to dissociate C–H bond of the CH₃O intermediate.

Method

Dispersion-corrected density functional theory calculations were performed through GGA-PBE exchange–correlation functional combined with a numerical double-ζ plus polarization (DNP) basis set as implemented in DMol³. To include the relativistic effects of core electrons of Zn atoms, DFT-semicore pseudopotentials were adopted. The DFT + D scheme proposed by Grimme was used to involve weak dispersion interactions within the DFT calculations. The reaction energy paths were generated by the minimum energy path calculations using the NEB method.