Theoretical Study of NO Conversion on Ag/TiO₂ Systems. I. Anatase (100) Surface

Abstract

A theoretical study of nitric oxide (NO) conversion on the anatase (100) surface covered with silver clusters has been performed. Two complementary approaches based on density functional theory (DFT) have been applied, in which the electron density was expanded in plane waves and in atom-centered Gaussian-type orbitals, respectively. It was observed that the NO interaction with the surface occurs mainly via the N atom. Adsorption of NO on silver clusters or at the border between silver and the TiO2 surface is more exothermic than at the uncovered anatase surface. Therefore, all stages of NO degradation proceed mainly on these active sites. Further adsorption of NO molecules leads to the formation of dimer species with previously adsorbed ones. Only acyclic cis-isomers of ONNO are formed according to the calculated energies. An analysis of electron density shows that the LUMO of adsorbed (NO)2 becomes partially occupied so that the adsorbed nitric oxide dimers are negatively charged. As a result of this charge transfer, the (NO)2 species are decomposed by breaking one or two N–O bonds, followed by the formation and desorption of N2 or N2O. In the case of decomposition at silver-surface boundaries, the main gas-phase product is N2O, whereas on the silver cluster both N2 and N2O are formed. After the (NO)2 decomposition, oxygen atoms remain on the surface and can further react with NO molecules from the gas phase, leading to the formation of rather tightly bound nitrogen dioxide molecules.

A First-Principle Study of Synergized O2 Activation and CO Oxidation by Ag Nanoparticles on TiO2(101) Support

“…The structure of Ag 8 clusters was optimized in a unit cell of 15 Å × 15 Å × 15 Å, and only the Γ-point was used. Because the Ag(111) surface was a common exposed surface for Ag clusters on TiO 2 support, , the initial bond lengths and bond angles of Ag 8 clusters were all referred to this surface. For geometry optimization, the adsorbates (Ag 8 , CO, and O 2 ) and the three topmost trilayers of TiO 2 (101) surface were allowed to relax, whereas the bottom three trilayers were frozen to simulate the bulk effects.…”

Removal of NO with silicene: a DFT investigation

“…Moreover, more electron donated by silicene are occupied the 2p*-2p* orbital of (NO) 2 , leading to the increasing of the N-N bond distance while the decreasing of the N-O distance and nally facilitating the decomposition of the dimer. 20,21,25,32 Accordingly, the N-N bond is greatly shortened in D4, D5 and D6 compared to 2.048 Å of the gas phase (NO) 2 and the key N-O bond becomes signicantly longer in D4, D5 and D6 compared to 1.164 Å of the free NO molecule. The lengths of N-N bond and N-O bond in D4, D5 and D6 have been described in Fig.…”

“…21,23 In previous theoretical and experimental studies, NO dimer, (NO) 2 , has also been proposed to play an important role in the reactions involving NO. 2,20,21,23,[29][30][31][32] For example, the reduction of NO to N 2 O on Ag{111} surface at very low temperature ($80 K) was found to go through a dimer intermediate by RAIRS 2 and STM 31 in experiments and further a mechanism via an inverted (NO) 2 dimer was identied by theoretical investigations. 25 (NO) 2 dimer was also detected inside single-walled carbon nanotubes 29 and in the process of NO adsorption on Au 3D hemispherical crystals.…”

“…The occupation of the LUMO of (NO) 2 increased the N-N bonding and decreased the N-O bonding and nally facilitated the decomposition of the dimer. 20,21,25,32 Since only physisorption is found on pristine graphene, doping Si in graphene is proposed to be an active site for NO capture and reduction. Considering the role of doping Si atom in graphene, silicene comes into view of us for investigating the behavior of NO together with considering the dimerization of NO on silicene.…”

“…Therefore, an investigation on the probability of NO reduction on silicene is presented in this paper and we will further detail the mechanism of this process since the importance of (NO) 2 dimer formation in NO reduction demonstrated by previous investigations. 2,20,21,[29][30][31][32] Moreover, calculations on NO reduction on silicene with silicane as substrate are carried out to interpret the substrate effect since DFT investigation has revealed silicane is promising for producing free-standing silicene due to the successful synthesis of silicane in experiments, [49][50][51] the preserved chemical reactivity of silicene by the weak interaction between silicene and silicane and the predicted ability for silicane to stabilize silicene. 52,53…”