Kinetic Measurements Using Flow Tubes The Journal of Physical Chemistry, Vol. 83, No. 1, 1979 3 for diatomics substantial progress has been achieved, as bring experimenitalists and theorists together and to show evidenced by some of the present papers. that the field of thermal elementary reaction kinetics is Lastly, the symposium did achieve its major goal: to alive and well. Kinetic Measurements Using Flow Tubes Carleton J. Howard Aeronomy Laboratory, NOAA Environmental Research Laboratory, Boulder, Colorado 80303 (Received October 6, 1978) Publication costs assisted by the NOAA Environmental Research Laboratory This paper is a selective survey of the chemical kinetic literature involving flow tube measurements of elementary reaction rate constants. It describes the origins of the flow tube method, the experimental technique, the measurement of rate constants, and an analysis of the inherent errors. Emphasis is placed on the discussion of the strengths and limitations of the method as a source of kinetic data. I. Introduction TABLE I: Comparison of Flow Tube and Flash-Photo- In recent years there has been an increasing demand for lysis Kinetic Tecliniques gas phase reaction rate data. Laser development,l at- flow tube flash photolysis mospheric chemi~try,~,~and combustion4 are examples of temperature 200-600 K 100-600 K fields of application of elementary reaction studies. range Committees and organizations have been created to collect, pressure 1-10 torr 5 torr-several evaluate, and disseminate kinetic information. In at- range atmospheres mospheric chemistry, for example, there are serious rate constant (1O-lo-lO-l ') cm3 (10~'o-10~'8)cm3 economic and social implications derived from the ap- range molecule-' s-' molecule-' s-l detection excellent requires fast plication of kinetic data in computer models that assess versatility detector the impact of anthropogenic chemicals on stratospheric reactant excellent limited ozone. Thui;, increased concern for the accuracy of rate versatility constant measurements has developed concurrently with heterogeneous can be serious none the demand for more data. reactions The flow tube technique has been the most prolific expense low moderate source of kinetic data near 300 K. In evaluating the usefulness of this method for obtaining rate constant data work deals with reactions of metals and metal oxides in it is instructive to make a comparison with the flash- a special application of the flow tube technique and is photolysis technique. This comparison on the basis of described in detail elsewhere in this volume.1° seven different criteria is summarized in Table I. The B. Pressure Range. The flow tube technique is basically emphasis of this discussion is not to demonstrate the a low pressure technique as will be discussed later. Flash superiority of one method in all categories but rather to photolysis, on the other hand, can be used to very high show the complementary nature and strengths of both pressures with the main limitation being the detection of methods. In making such a comparison it is necessary to reactants. If resonance fluorescence is used,for detection, make some generalizations that are not accurate for every some species such as OH are quenched by the buffer gas. study. In this respect the discussion is influenced by the In this case resonance absorption may be usedll to extend experiences we have had in the NOAA Aeronomy Labo- the pressure range. ratory using both techniques. C. Rate Constant Range. Both flow tube and flash- A. Ternpcrature Range. The useful temperature range photolysis techniques are used to measure fast reactions is nearly the same for both techniques. The upper with rates up to gas kinetic collision rates. The greater temperature limit is established by the onset of problems pressure range of the photolysis method also allows larger with the thermal stability of reactants and the selection reactant concentrations to be used and hence smaller rate of materials for fabricating the apparatus. At the low constants to be measured. Thus photolysis systems have temperature extreme the flow tube method is somewhat a significant advantage for studying slow reactions and more restricted than the flash-photolysis method because termolecular reactions at high pressures. of heterogeneous reactions. It is often observed that the D. Detector Versatility. One of the two major ad- rate of destruction of radicals such as C1, OH, and H02on vantages of the flow tube technique is the immense variety the reactor surface increases significantly at temperatures of methods that can be used to detect the reactants and below about 250 Ke5Nevertheless, there have been several products. This advantage is derived from the steady-state studies using flow itube techniques beyond these limits. nature of the flow system in which the progress of the For example, Trairior et aL6 have studied the recombi- reaction is frozen at any fixed observation point along the nation of atomic hydrogen down to 77 K, Westenberg and tube. Since the concentrations of the reactants are deHaas7s8 have routinely studied reactions of 0 and OH constant at that point, there are no constraints on the up to 1000 K, and Fontijn et ale9have developed a flow detector speed. A flash-photolysis experiment, on the reactor designed for operation up to 2000 K. The latter other hand, is studied in real time and requires a detector This article not subject to US. Copyright. Published 1979 by the American Chemical Society 4 The Journal of Physical Chemistry, Vol. 83, No. 1, 1979 Carleton J. Howard with a time resolution that is at least 1/10 the period of powerful kinetic tools, particularly through the use of laser the experiment, i.e., in the millisecond range. Low signal light sources. levels can be overcome using signal averaging Wood also discovered that certain materials became technique^'^,^^ but some detectors such as magnetic res- incandescent when exposed to the products of a hydrogen onance methods cannot be applied to flash-photolysis discharge.2fi He deduced that the glow was due to energy experiments. released by the surface-catalyzed recombination of radicals E. Reactant Versatility. The second major advantage produced in the discharge. This discovery was a primitive of the flow tube method is the great versatility it provides ancestor of catalytic probes used to measure atom con- for working with a wide variety of reactants. With the flow centrations in numerous flow tube kinetic st~dies.~~~~-~~ tube method it is possible to generate two different labile The first flow tube kinetic measurements were made in reactants in isolation and to study their reactions under the late 1920’s. In 1929 Smallw~od~~reported measure- carefully controlled conditions. For example, reactions ments of the rate of recombination of hydrogen atoms. He such as HOz + C10 - HOC1 + 02,140 + C10 - C1+ 02,15 employed a Wood-Bonhoeffer type discharge tube as a and N + OH - NO + HI6 have been studied in flow tubes. source of atomic hydrogen and a moveable catalytic probe Titration reactions, which will be described later, play an inspired by Wood’s “incandescent wire” experiment (both important role in the reactant versatility of the flow tube, Wood and Smallwood were at The Johns Hopkins since they make it possible to produce accurately known University). The atom concentration in the flow tube was concentrations of labile reactants. measured using a calorimeter attached to the outside of The flash-photolysis technique is limited by the re- the flow tube. Smallwood calculated both homogeneous quirement of photolytic generation of the radical reactant. and heterogeneous recombination rate constants using an This factor is a restriction on both reactants because some analysis similar to the modern method described later. gases such as NOz and O3 are also dissociated by the flash At about the same time in Germany, chemists following radiation and may produce unwanted reactive fragments. the leadership of BonhoeffeF were also making kinetic F. Heterogeneous Reactions. An important advantage measurements in discharge-flow systems. An important of the flash-photolysis experiment is that it can be con- step in this development was the invention of the Wre- ducted at the center of a larger reactor, far removed from de-Harteck gauge32 a simple device for measuring the the walls and the possibility of heterogeneous reactions. partial pressure of atoms in a discharged gas mixture. Heterogeneous chemistry is observed to interfere in the Harteck and Kops~h~~studied the reactions of atomic study of both bimolecular17 and termolecular18 reactions. oxygen with 22 different compounds including H2, CO, The reactor surface can also be an impediment to studying H2S, CSz, NH,, HC1, and numerous hydrocarbons. The reactions of vibrationally or electronically excited reactants atomic oxygen was produced by an ac discharge and its because of its high deactivation efficiency. Although concentration was estimated using a Wrede-Harteck gauge innovative studies of reactions of vibrationally excited and a hot platinum wire. The relative reactivities of the OH19 and metastable N(2D)20have been made in flow reactants were determined qualitatively by spectroscopic tubes, the photolysis technique is generally superior for measurements of chemiluminescent emissions from the this type of study. reaction zone and by trapping and analyzing the final G. Expense. Although this consideration is seldom products. examined, there can be a significant difference between In the late 1950’s and early 1960’s several important a flow tube system and a flash-photolysis system in initial advances were made in flow tube techniques and in- expenditure. A major component that contributes to this strumentation. Much of our present technology can be difference is the multichannel analyzer that is normally traced to these developments. used to do time-resolved signal averaging. By using a A paper by Ka~fman~~in 1958 made two major con- simple detection scheme such as chemiluminescence, a flow tributions to flow tube kinetics.