K2. Real-Time Measurement of Residence Times of Gas Molecules on Solid Surfaces: Relevance for Heterogeneous Chemical Kinetics

Michel J. Rossi

Laboratory of Air and Soil Pollution Studies (LPAS), Institute of Environmental Engineering

Swiss Federal Institute of Technology

CH-1015 Lausanne, Switzerland

The residence time ts of a molecule on a surface which may be expressed by the inverse of the rate constant for desorption (kd) is an important parameter controlling the efficiency of a heterogeneous chemical reaction occurring on a solid surface such as the interaction of NO2 on amorphous carbon or soot, the reaction HNO3 + NaCl ==> HCl + NaNO3 or "bimolecular" heterogeneous reactions such as ClONO2 + HCl on an ice surface. In view of the complex nature of most heterogeneous reactions we may further our understanding by separately studying adsorption and desorption processes of molecules on surfaces.

We will present a new technique for measuring ts of gases under molecular flow conditions. A pulse of molecules is passed through a cylindrical tube where the molecules undergo multiple collisions with the wall according to the kinetic theory of gases. The transit time or arrival time spectrum, measured using a mass spectrometer in the transient mode, is governed by the number of collisions as well as by the magnitude of the gas-surface interaction per collision expressed in terms of ts. Monte Carlo trajectory calculations of molecules diffusing across a Pyrex tube were in excellent agreement with experimental results on non-interacting gases such as the rare gases He, Ne, Ar, Kr, Xe as well as CO2, N2, O2 and SF6. "Sticky" gases, such as H2O, HCl and NO2 as well as a number of ideal gases were investigated on three materials commonly used in vacuum science: Pyrex glass, stainless steel and PTFE-Teflon. No deviation from ideal gas behavior was observed for H2O, HCl and NO2 on Teflon, whereas we had to invoke a two-site adsorption model for both H2O and HCl interacting with Pyrex and stainless steel in order to interprete the double-exponential decaying portion of the arrival time spectrum. For H2O/Pyrex the arrival time of a small pulse, on the order of 1015 H2O molecules per pulse, is late leading to ts on the order of a fraction of a ms at ambient temperature, whereas ts for a pulse larger by a factor of was of the order of tens of ms once the strongly interacting sites were saturated by the first part of the H2O pulse propagating across the test tube. Similar results have been found for H2O and HCl interacting with stainless steel as well as for HCl on Pyrex. We will also report on the temperature dependence of these processes as well as on additional results including HBr, HI and HNO3 on a range of materials.