Determining accurate kinetic parameters of potentially important heterogeneous atmospheric reactions on solid particle surfaces with a Knudsen cell reactor

One of the most important applications of the Knudsen cell reactor is in de termining heterogeneous reaction kinetics of potentially important atmosphe ric reactions. Knudsen cell measurements involving gas reactions on atmosph erically relevant particle surfaces, including salt, carbon black, soot, an d mineral dust, are often obtained using powdered samples. In this study, w e have investigated the importance of gas diffusion into the underlying lay ers of powdered samples when determining kinetic parameters from Knudsen ce ll experiments. In particular, we show that the use of the geometric surfac e area of the sample holder is, in general, not justified in determining in itial uptake coefficients or reaction probabilities because the interrogati on or probe depth of gas-phase molecules into the bulk powder can be anywhe re from tens to thousands of layers deep. One problem encountered by curren t models used to account for gas diffusion into the underlying layers is th at the diffusion constant of the gas through the powdered sample must be kn own. Typically, diffusion constants for gases into powdered samples are unk nown and are difficult to measure or accurately calculate. One way to circu mvent this problem is to use thin samples such that the thickness of the sa mple is less than the interrogation depth of the gas-phase molecules. Under these conditions, the observed initial uptake coefficient is directly prop ortional to the surface area of the entire sample. This region is termed th e linear mass-dependent regime and can be experimentally accessed for many, but not all, heterogeneous reactions. Several examples discussed here incl ude heterogeneous reaction of NO2 on gamma- and alpha-Al2O3, alpha-Fe2O3, c arbon black; HNO3 on CaCO3; and acetone on TiO2.