Detection of radical species produced by catalytic decomposition of H2, O2 and their mixtures on heated Ir surfaces

Abstract

a b s t r a c tEfficient production of H atoms, O atoms and OH radicals was confirmed by laser spectroscopic measurements in the catalytic decomposition of H 2 , O 2 and their mixtures on a heated Ir filament. No change in electric resistivity was observed when the filament was kept at 2350 K in the presence of 1.0 Pa of pure O 2 , showing that oxidation is minor. Arrhenius-type temperature dependences were observed for the densities of H (O) atoms in pure H 2 (O 2 ) systems. In the H 2 /O 2 mixed system, the H-atom density was almost independent of the O 2 partial pressure, although the O-atom and OH-radical densities increased with the O 2 pressure. These O 2 pressure dependences are completely different from those observed for W. Ir is less poisoned by O 2 compared to W. In addition, direct production of H 2 O molecules on Ir surfaces must take place besides the production of radical species.

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

“…H-atom ( ), O-atom ( ), and OH-radical ( ) densities calculated by the CHEMKIN software package. The preexponential factors for reactions (4) and (5) The OH-radical density obtained in the present system is higher than those obtained in the catalytic decomposition of H 2 /O 2 mixed systems [6,8]. However, that is less than those reported in plasma processes [32][33][34][35][36].…”

“…When 1.00 sccm of O 2 was added to 1.50 sccm of H 2 O, the OH density increased from 3. 7 10 10 to 1.6 10 11 cm -3 when the catalyst temperature was 2100 K. This value is much larger than those observed in the decomposition of H 2 /O 2 mixtures [6,8]. The decrease in the OH density with the increase in the catalyst temperature over 2100 K could also be observed in the presence of O 2 , but the decrease was more gradual compared to that in the absence of O 2 .…”

“…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.…”

“…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.…”

“…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]. It was also shown that Ir is not oxidized by these species when heated up to 2350 K and that metal contamination on the deposited films is minor.…”