Oxidation of NO Into NO₂ by Surface Adsorbed O Atoms

Abstract

Plasma-surface interactions are increasingly recognised as a key factor in explaining molecule production and conversion processes. In order to scrutinise such effects well-defined surface and gas phase conditions are required. Hence, the inner surface of a Pyrex tube was treated by a capacitively coupled RF plasma at low pressure. The post-plasma oxidation of gas mixtures containing 1 % NO into NO2 has been studied by means of quantum cascade laser absorption spectroscopy in the mid-infrared spectral range. The plasma pre-treatment experiments with oxygen containing precursors suggest O atom adsorption on the Pyrex tube with a surface density of 2.7x10 14 cm −2 . A simple kinetic model was developed and shows good agreement with the measured NO and NO2 gas phase concentrations. In the model a fraction of the surface is considered to be covered with chemisorption sites where atoms and molecules can be adsorbed, whereas they can be removed only by recombination.

Dynamical Monte Carlo methods for plasma-surface reactions

“…The first one was NO oxidation on pre-treated pyrex to form NO 2 , on a surface that exhibits a distribution of reactivity among the chemisorption sites. This system was previously addressed both experimentally and theoretically [27][28][29], constituting the perfect ground to test and validate the DMC simulations. The n-fold way / Bortz-Kalos-Lebowitz (BKL) [56] dynamical Monte Carlo algorithm proved to be very efficient, coping with elementary processes with rates spanning over several orders of magnitude.…”

“…where ν D can be associated with a frequency of vibration parallel to the surface and E D is the energy barrier for diffusion, typically E D 0.5E d [41]. Expression (28) is all that is necessary to simulate the mechanisms (R5) and (R6) within a dynamic Monte Carlo approach. On the deterministic description some additional assumptions have to be made [41,45].…”

“…For the dynamical Monte Carlo simulations in this work the surface is represented as a square lattice and a diffusing atom may hop in one of fourth equally likely directions. The hopping of diffusing physisobed atoms is directly described by (28). The corresponding event and rate are included in the sets E and R defined by (2) and (3) and the rate is used in the calculation of the time increment (5).…”

“…Herein we take as case studies NO oxidation on Pyrex pre-treated with an oxygen discharge and O recombination on silica forming O 2 . NO oxidation on Pyrex was recently investigated both experimentally [27][28][29] and theoretically [28,29], providing a reliable benchmark for the new calculations. The experiments were designed to ensure a good reproducibility and allow an unambiguous interpretation of the results.…”

NO2dynamics of an Ar/Air plasma jet investigated byin situquantum cascade laser spectroscopy at atmospheric pressure

“…The scientific and technological interest in quantum cascade lasers (QCLs) for plasma diagnostic has increased over the last few years [1][2][3][4][5]. These types of lasers emit monochromatic mid-infrared (mid-IR) radiation with appropriate power for absorption spectroscopy.…”