Production and detection of reducing and oxidizing radicals in the catalytic decomposition of H2∕O2 mixtures on heated tungsten surfaces

Abstract

H atoms, O atoms, and OH radicals were identified in the catalytic decomposition of H2∕O2 mixtures on heated polycrystalline tungsten surfaces. In order to suppress the oxidization of the tungsten catalyzer surfaces, the H2∕O2 pressure ratio was kept more than 83, while the catalyzer temperature was kept below 2000K. The absolute density of H atoms was determined by a vacuum-ultraviolet laser absorption technique, while one-photon and two-photon laser-induced fluorescence techniques were employed to extend the dynamic range. Since the O-atom density was much smaller, only a vacuum-ultraviolet laser-induced fluorescence technique could be used for the detection. The absolute density could be estimated by comparing the induced fluorescence intensity with that for H atoms. OH radicals could be identified by a laser-induced fluorescence technique in the ultraviolet region. The absolute density was determined by comparing the induced fluorescence intensity with that of Rayleigh scattering caused by Ar. The H-atom density decreased with the increase in the O2 partial pressure stepwisely. The O-atom density increased with the O2 partial pressure monotonously, but the increase was rather slow at low O2 pressures. The OH-radical density showed saturation against the O2 partial pressure. These results can be explained by the change in the coverage conditions of the catalyzer surfaces.

Catalytic Decomposition of H2O(D2O) on a Heated Ir Filament to Produce O and OH(OD) Radicals

“…The rotational transition probabilities were assumed to be the same as those of OH(A 2 + -X 2 ) transitions [21,22]. The LIF spectrum of OH produced from H 2 O was very similar to those reported previously for the catalytic decomposition of H 2 /O 2 mixtures [6,8]. The rotational distributions of OH and OD radicals could be fitted by a Boltzmann distribution at 350 K. This result shows that the rotational relaxation is rapid even when the total pressure is as low as 0.8 Pa. Vibrationally excited OH(OD) radicals were not identified.…”

“…By employing these techniques, it is possible to determine the absolute densities under wide conditions [6]. Atomic deuterium was detected similarly.…”

“…The experimental apparatus and the procedure were similar to those described elsewhere [6][7][8]. A cylindrical reaction chamber, 10 cm in internal diameter, made of stainless steel was used.…”

“…OH(OD) radicals were detected by employing an LIF technique in the ultraviolet region. The absolute densities of OH were evaluated by comparing the time integrated LIF intensity for the P 1 (N"=2, J"=3/2) transition at 308.6 nm under saturated conditions with the intensity of Rayleigh scattering caused by Ar [6]. The absolute OD densities were evaluated from the LIF intensity of the P 1 (N"=4, J"=7/2) transition at 308.2 nm.…”

“…In this technique, radical species are produced from material gases on heated metal surfaces without co-producing ionic or metastable excited species. In order to make clear the decomposition and ejection mechanisms, many studies have been carried out to identify the radical species produced [3][4][5][6][7][8][9][10][11][12][13][14][15][16]. Although it has been rather difficult to produce oxidizing radicals, we have recently shown that atomic oxygen can be produced efficiently by the catalytic decomposition of O 2 , NO, N 2 O, and NO 2 on a heated Ir filament [7,8].…”