Density functional studies of the adsorption and dissociation of NO _x (x = 1, 2) molecules on the W(111) surface

Adsorption and dissociation of NO _x (x ) 1, 2) molecules on the W(111) surface have been investigated by using the density functional theory (DFT) with the projector-augmented wave (PAW) approach in periodic boundary conditions. The adsorption structures, vibrational frequencies, and binding energies of NO ₂, NO, N, and O on the W(111) surface were predicted. It was shown that the most favorable adsorption geometry of W(111)/NO ₂ is the WNO ₂(IV-μ ³-N2, O1, O1) configuration with NO2 at the 3-fold-shallow site of the surface and has an adsorption energy of 79.4 kcal/mol. For W(111)/NO, WNO(II-μ ²-N1, O1) with NO at the bridge site is energetically the most favorable one and has an adsorption energy of 74.4 kcal/mol. The N and O atoms are bound preferentially at the bridge and top sites, respectively. The potential energy profiles for the decomposition of NO ₂ on W(111) were constructed using the nudged elastic band (NEB) method. The barriers for the stepwise NO ₂-deoxygenation process to N _(ads) + 2O _(ads) are calculated to be only 0.5-3.7 (for the first step of the reaction, ON-O bond activation) and 0.2-3.0 (for the second step of the reaction, N-O bond activation of the coordinated NO molecule) kcal/mol, with an overall exothermicity of 177.0-185.2 kcal/mol. These findings show that NO ₂ can easily decompose on the W(111) surface. The rate constants for NO ₂ and NO dissociation on the surface were also predicted.