Mechanism of ammonia decomposition and oxidation on Ir(110): a first-principles study.

The mechanism of ammonia decomposition and oxidation on Ir(110) was studied on the basis of periodic density functional theory calculations and microkinetic modeling. The results indicate that NH3 dissociation is more favorable than desorption at atop site, while at top site NH3 desorption and dissociation are competitive. On the other hand, when O or OH is co-adsorbed, the NH3 dehydrogenation is slightly inhibited and mainly via hydrogen abstraction reaction rather than thermal decomposition, while it is reversed for NH2 dehydrogenation. The former mechanism is favored for O assisted NH dehydrogenation, while it changed to latter one for OH. On clean Ir(110), N + NH → N2 + H pathway is the major N2 formation pathway and N + N is also involved but less competitive, while N + N becomes the predominant one and is enhanced on O-predosed Ir(110). NO formation occurs only at higher temperature when N2 is desorbed from the surface. The microkinetic analysis further confirms that the dominant product is N2 at low temperature while becomes NO as temperature increases, and the temperature of NO formation decreases when O2 partial pressure increases. The present calculation results are in good agreement with the experimental observations.

Theoretical mechanistic study on the reaction of CN radical with HNCN.

The mechanism for the reaction of the cyanogen radical (CN) with the cyanomidyl radical (HNCN) has been investigated theoretically. The electronic structure information of the singlet and triplet potential energy surfaces (PESs) is obtained at the B3LYP/6-311+G(3df,2p) level, and the single-point energies are refined at the CCSD(T)/6-311+G(3df,2p) level as well as by multilevel MCG3-MPWB method. The calculations show that the C atom of CN additions to middle- and end-N atoms of HNCN are two barrierless association processes leading to the energy-rich intermediates IM1 HN(CN)CN and IM2 HNCNCN, respectively, on the singlet PES. The higher barriers of the subsequent isomerization and dissociation channels from IM1 and IM2 indicate that these two intermediates, which have considerably thermodynamic and kinetic stability, are the dominant product at high pressure. While at low pressure, the most favorable product is P(2) H + NCNCN, which will be formed from both IM1 and IM2 via direct dissociation processes by the H-N bond rupture, and the secondary feasible product is P(4) HCN + (1) NCN, while P(5) HCCN + N(2) and P(6) HCNC + N(2) are the least competitive products. On the triplet PES, P(14) NCNC + HN may be a comparable competitive product at high temperature. In addition, the comparison between the mechanisms of the CN + HNCN and OH + HNCN reactions is made. The present results will enrich our understanding of the chemistry of the HNCN radical in combustion processes and interstellar space.

Adsorption and dissociation of NO on Ir(100): a first-principles study.

Density functional theory (DFT) and periodic slab model have been used to systemically study the adsorption and dissociation of NO and the formation of N(2) on the Ir(100) surface. The results show that NO prefers the bridge site with the N-end down and NO bond-axis perpendicular to the Ir surface, and adsorption to the top site is only 0.05 eV less favorable, whereas the hollow adsorption is the least stable. Two dissociation pathways for the adsorbed NO on bridge or top site are located: One is a direct decomposition of NO and the other is diffusion of NO from the initial state to the hollow site followed by dissociation into N and O atoms. The latter pathway is more favorable than the former one due to the lower energy barrier and is the primary pathway for NO dissociation. Based on the DFT results, microkinetic analysis suggests that the recombination of two N adatoms on the di-bridge sites is the predominant pathway for N(2) formation, whereas the formation of N(2)O or NO(2) is unlikely to occur during NO reduction. The high selectivity of Ir(100) toward N(2) is in good agreement with the experimental observations.

Ag (111) surface for ambient electrolysis of nitrogen to ammonia.

In this paper, the reaction process of N2 convert to NH3 catalyzed by Ag (111) surface was obtained through the construction of Ag (111) surface and computational simulation. The charge transfer in the reaction process and the change of N≡N bond length are described. Since the N2 reduction reaction (NRR) usually occurs under alkaline solution conditions, we calculated and described the coexistence of OH* and N2. At the same time, the co-adsorption structure of OH* and N2 at different adsorption sites was studied.