Heterogeneous kinetics of the uptake of HOBr on solid alkali metal halides at ambient temperature

The heterogeneous reactions of HOBr with solid crystalline NaCl [HOBr(g) + NaCl(s) --> BrCl(g) + NaOH(s)] and KBr [HOBr(g) + KBr(s) --> Br-2(g) + KOH(s)] substrates at ambient temperature have been investigated using a Teflon coated Knudsen cell reactor. Powder, grain, and spray-deposited salt substrates were used for the measurement of the HOBr reactivity. The observed uptake probability depends on the total external surface area of the salt substrates. For NaCl substrates, Br-2 and BrCl are observed as products; for KBr substrates, Br-2 is observed as the sole product. In both cases, a dependence of the initial uptake probability gamma(0) on HOBr flow rate has been observed. The initial uptake is large at low flow rate and 10 times smaller at high flow rate. Values of gamma(0) less than or equal to (6.5 +/- 2.5) x 10(-3) for NaCl and gamma(0) less than or equal to 0.18 +/- 0.03 for KBr are obtained under our experimental conditions of limiting low flow rates akin to atmospheric conditions. The production of Br-2 is observed even for HOBr interacting on solid NaNO3, a non-halogen containing substrate. The yield measurements imply that a HOBr self-reaction occurs on salt surfaces according to 2HOBr --> Br-2 + H2O + 1/2O(2). The decrease in Br-2 yield with increasing HOBr flow rate from 100 to 50% indicates that a competition between the heterogeneous reaction of HOBr with NaCl or KBr and the self-reaction of HOBr takes place on the solid salt surface under laboratory experimental conditions. The decrease of gamma(0) with time indicates that approximately 5-10% of the Br atoms on a KBr surface interact with HOBr.