Density functional theory calculations have been performed to elucidate the mechanism of N2O formation over the Au(111) surface during NO reduction. It is shown that the dissociation of NO into an N atom and an O atom involves a barrier as high as 3.9 eV, implying that the formation of N2O does not occur via the direct dissociation mechanism of NO. Alternatively, we find that the reaction may occur via a dimer mechanism; i.e., two NO molecules initially associate into a dimeric (NO)2, which then dissociates into a N2O molecule and a N atom. We have scanned the potential energy surface forming N2O along different pathways, which involve a trapezoid OadNNOad dimer, an inverted trapezoid ONadNadO dimer, a zigzag ONadNOad dimer, or a rhombus ONadOadN dimer. The trapezoid dimer, OadNNOad, is found to be a necessary intermediate for the formation of N2O, and the calculated barrier for the rate-determining step along the energetically most favorable pathway is only 0.34 eV. The present results rationalize the early experimental findings well and enrich our understanding of the reduction of NO on the Au surface.

No abstract

By using electrochemical measurement techniques and based on the point-defect model ͑PDM͒, this work studied the point-defectconductive property of the n-type semiconducting anodic oxide film formed on titanium under steady-state condition within the passive potential region of ca. 0-7 V in 1.0 M HClO 4 solution. The point-defect transport property was characterized by the steady-state current density ͑i ss ͒ through the film and the diffusion coefficient ͑D O ͒ of the point defects in the film. It was demonstrated that with the increase of the film formation potentials, i ss ͑and D O ͒ show an exponentially dependent increase on the potential. This behavior of the oxide films formed on titanium is to some extent different from that of some other n-type anodic oxide films. The influence of potential on the changes in structure and crystallization of the oxide film on titanium was proposed to be taken as a reason for the potential-dependence of i ss ͑and D O ͒ observed in this work. It was also shown that within the passive potential region most of any change in the applied potential does appear predominantly as a change in the potential drop within the oxide film ͑only ϳ6% of that drops across the film/solution interface͒. Additionally, a question about the values of the polarizability of the oxide film/solution interface ͑␣͒ used and estimated within the frames of the PDM was raised and discussed at the end of this paper.The titanium/TiO 2 film/electrolyte interface system has received more and more attention because it has a wide variety of scientific interest and technological applications, including corrosion inhibition, 1,2 dye-sensitized solar energy conversion cells, 3 electrocatalysis ͑or in terms of dimensionally stable anodes͒, 4,5 and the use in photoelectrocatalytical degradation of certain categories of organic pollutants in wastewater, 5-7 etc. In close relation with these applications of titanium-based electrodes, the charge transfer through the oxide film formed on titanium is the most fundamental and essential issue, which has been intensively studied, but has not yet been completely understood. Although there have been considerable experimental study reports about the electron conductive property of the oxide film on titanium, 5,8-12 only very few papers have dealt with the ion-conductive property of the oxide films on titanium. 5 The point-defect model ͑PDM͒ ͑which was developed by Macdonald and Macdonald, 13 Macdonald et al.,14 and Macdonald 15 ͒ and the related treatments such as the mixed-conduction model and the surface charge approach ͑which were developed by Bojinov, 16 Metikos-Hukovic and Grubac, 17 and Bojinov et al. 18 ͒, provide a microscopic description of the migration of the Schottky point defects under the influence of the electrostatic field in the passive oxide film during the film growth under steady-state conditions. Although the validity of the PDM was tested and verified by experimental results for passive films on many metals such as Fe

No abstract