The Photolysis of Water Vapor and Reactions of Hydroxyl Radicals

Abstract

The photolysis of water vapor a t 1849 A has been investigated as a possible source of hydroxyl radicals for kinetic studies. At temperatures from 23 to 350' C and pressures from 1.3 t o 28 mm, H2 and H402 were the only detectable products. Experiments with added oxygen indicated that 0 2 ma37 have been present as a n intermediate a t a very low steady-state concentration, although this is not certain. Possible mechanisms are discussed. Ea2 -E C O = 4.0Zk0.3 kcal/mole, AH,/Aco = 3 1 f 5 , At temperatures below 200" C, hydroxyl radicals were not completely converted to COs, as the yield of Cop increased to a maximum, then decreased again, with increasing pressure of CO. The mechanism of this system is complex, but probably involves secondary reactions of HCO or COOH radicals.

The reaction of OH with CO

“…However, Smith and Zellner [4] failed to observe any rate enhancement for O H removal, even a t 250 torr He. Ung and Back [8] in a competitive study with H2 at 250 torr total pressure (mostly Hz) found k2/kl to be somewhat greater than the present consensus value for this ratio.…”

“…where a: is a small correction factor due to reactions (4) and (6) The effect due to reaction (5) was neglected because it is very small (<5Y0) and is further reduced because of reactions (7) and (8). a can be computed from the known rate coefficients given in Figure 1, and at 250°K and a single total pressure of about 600 torr in Figure 2.…”

Reactions of OH radicals with inorganic compounds in the gas phase

“…In 1974 it was reported [57], and later confirmed by both indirect [58][59][60] and direct measurements [61,62], that the rate constant at ambient temperatures increases with total pressure, although indications of a potential pressure dependence could be inferred from an early competitive study of Ung and Back [63]. It was also established that various gases have different efficiencies in increasing the rate constant; SF6 is very efficient whereas He and Ar have little or no effect [61,62].…”

Detection of H2O2 Released from TiO2 Photocatalyst to Air

“…(7) → (6)). 12 H2O + hν → ·H + ·OH (7) The excitation wavelength is 200 nm or shorter. Although light beams of such short wavelengths were mostly cut off by the cold mirror, the photolysis might have been driven by stray UV light.…”