Electronic processes in NO dimerization on Ag and Cu clusters: DFT and MRMP2 studies

Area of Science:

• Computational Chemistry
• Surface Science
• Inorganic Chemistry

Background:

• Nitric oxide (NO) dimerization on copper (Cu) and silver (Ag) surfaces is experimentally observed, contrasting with its weak binding energy in the gas phase.
• Understanding the electronic and chemical factors governing NO dimerization on metal surfaces is crucial for catalysis and materials science.

Purpose of the Study:

• To investigate the mechanism and energetics of NO dimerization on small Ag and Cu clusters using advanced computational methods.
• To identify suitable theoretical methods for accurately predicting the activation barriers and reaction energies of this surface-confined reaction.

Main Methods:

• High-level quantum chemical calculations, including coupled cluster with singles, doubles, and perturbative triples (CCSD(T)) and various density functional theory (DFT) functionals.
• Study of NO dimerization on Ag2, Cu2, Ag5, Cu5, Ag7, and Ag38 clusters to probe size-dependent effects.
• Analysis of charge transfer (CT) between metal clusters and NO molecules as a key factor in N-N bond formation.

Main Results:

• Charge transfer from Ag2 and Cu2 clusters to NO moieties significantly lowers the activation barrier for N-N bond formation, reducing nondynamical electron correlation effects.
• The ωB97X DFT functional accurately predicted the activation barrier, unlike other tested functionals that underestimated it.
• NO dimerization on Cu clusters requires a moderately larger activation barrier than on Ag clusters due to lower-lying frontier orbitals in Cu.
• Activation barriers decrease and exothermicity increases with increasing size of Ag clusters (Ag2 < Ag38 < Ag7 ≈ Ag5), with Ag clusters larger than Ag2 being effective for NO dimerization.

Conclusions:

• Charge transfer is a critical factor enabling NO dimerization on Cu and Ag surfaces, making it computationally tractable with methods like CCSD(T) and specific DFT functionals.
• The choice of DFT functional is crucial for accurate barrier predictions, with ωB97X showing good agreement with CCSD(T) results.
• Silver clusters, particularly larger ones, are more effective for NO dimerization than small Ag2 clusters, offering potential for catalytic applications.