Research Group Prof. Dr. G. Friedrichs

Shock Tube Investigations of High Temperature Reaction Kinetics

Combustion systems are characterized by a complicated interaction of flow and transport processes and a large number of elementary chemical reactions. The purpose of chemical kinetics is to unravel the underlying complex reaction mechanisms and to investigate the products and rates of selected reactions as function of temperature and pressure. In particular, the formation and the reactions of short-lived reactive intermediates (atoms and radicals) have to be well known. These intermediates maintain the combustion process and determine the end product distribution.

Fig. 1: Top: schematic setup of a shock tube. Bottom: pressure/temperature-time profile during a shock tube experiment and typical concentration-time profiles of a) thermal or b) photolytic generation of the detected species.

The shock tube technique has been proven to be a very powerful method for investigating gas phase reactions at high temperatures. With shock tubes, the experimental pressure (0.1-1000 bar) and temperature (500-15000 K) can be easily varied over a wide range. Essentially, a shock tube apparatus (Fig. 1) is just a several meters long tube, which is divided by a diaphragm into a driver (high pressure) and a driven (low pressure) section. The test gas mixture, typically a highly diluted mixture of reactants in argon, is found in the driven section. The driver section is filled with helium or hydrogen until the diaphragm bursts. A shock wave is formed, which propagates downwards the tube at supersonic speed and heats and compresses the test gas within less than 1 μs (incident shock wave). The shock wave is reflected at the end wall and the preheated testgas is heated and compressed again (reflected shock wave). The resulting pressure- and temperature-time-profile is shown in the lower left part of Fig. 1. Typically, the constant conditions behind the reflected wave last for roughly 1  - long enough for studying chemical reactions, which are mostly fast at high temperatures. In many cases, the species of interest are generated thermally by the decomposition of suitable precursor molecules (Fig. 1a), but often a photolytic production is also feasible (Fig. 1b). Finally, real-time detection of the concentration-time-profiles is accomplished through optical windows by means of a variety of sensitive spectroscopic absorption or emission techniques. A recent example is the application of the highly sensitive laser absorption based frequency modulation (FM) spectroscopy for quantitative detection of small radicals behind shock waves. With FM spectroscopy the radicals methylene (1CH2) and formyl (HCO), which are both of considerable importance in combustion chemistry, could be detected behind shock waves for the first time [1,2].
Both formaldehyde (CH2O) and formyl radicals (HCO) lie on the main oxidation pathway of hydrocarbons with CH3 as the chain center. Methane oxidation, for example, proceeds in the following steps:

CH4    =>    CH3    =>    CH2O   =>   HCO   =>   CO    =>   CO2

Under radical-rich conditions, formaldehyde is mainly formed by the reaction of methyl radicals with oxygen atoms. Subsequent abstration reactions of H, OH, O and CH3 yield formyl radicals. By reactions of HCO with H, O2 and OH, and also through its unimolecular decomposition, carbon monoxide and eventually carbon dioxide are formed. The high temperature decomposition of CH2O provides a simple system to investigate some of these reactions. The chain mechanism of the CH2O decay can be described over a wide range of temperatures and pressures by only five reactions:

CH2O        +      M           HCO       +         H        +              M        (1a)
CH2O        +      M           H2           +        CO      +              M        (1b)
CH2O        +      H           H2           +        HCO                               (2)
HCO          +      M           H              +       CO       +              M        (3)
HCO          +      H           H2            +       CO                                  (4)
HCO          +      HCO     CH2O      +       CO                                  (5)

By means of sensitive vacuum-UV-absorption detection of CH2O at 174 nm and frequency modulation detection of HCO at wavelength around 614 nm, the rate of reaction (2) was directly measured at high temperatures for the first time (CH2O detection, C2 H5I as H atom source, T=1510 - 1960 K) and the rate of reaction (3) could be measured at temperatures of 835 - 1230 K (HCO detection, photolysis of CH2O mixtures) [2, 3]. Furthermore, measurements of reactions (4) and (5) at lower temperatures (HCO detection, photolytic production of HCO) and the detection of CH2O and HCO profiles during the thermal decomposition of pure formaldehyde mixtures behind shock waves provided additional information about the rates of reactions (1a) and (3) [2,4]. Altogether, sensitive detection methods and extensive experimental data made it possible to separate the strongly coupled reactions and to obtain a consistent set of rate constants.


Fig. 2: Left: schematic potential curve of the thermal decomposition of formaldehyde (CH2O) with H2 + CO and H + HCO as reaction products, respectively. Right: calculated branching fraction as function of pressure and temperature; based on two-channel RRKM calculation.

As a final step, the rate of reaction (1b), which becomes more important at low formaldehyde concentrations, and also the branching fraction β of the initiation step (reactions (1a) and (1b)) were calculated using statistical theories of unimolecular reactions. Similar threshold energies and different energy dependencies of the decay rates of the two reaction channels with loose (1a) and tight (1b) transition states, respectively, induce a distinct pressure and temperature dependence of the branching fraction β (Fig. 2). A two-channel RRKM calculation, which takes into account rotational effects and "weak collisions" (master equation analysis) reveals that at temperatures from 1400 to 3000 K and at low pressures (1 mbar) reaction (1b) with H2 and CO as products is the main channel. However, with increasing pressure, channel (1a) eventually dominates and at very high pressures (1 kbar)
[1] G. Friedrichs, H. Gg. Wagner, Quantitative FM Spectroscopy at High Temperatures: The Detection of 1CH2 behind Shock Waves, Z. Phys. Chem. 214, 1723-1746 (2000).

[2] G. Friedrichs, J. T. Herbon, D. F. Davidson, R. K. Hanson, Quantitative Detection of HCO behind Shock Waves: The Thermal Decomposition of HCO, Phys. Chem. Chem. Phys. 4, 5778-5788 (2002).

[3] G. Friedrichs, D. F. Davidson and R. K. Hanson, Direct Measurements of the Reaction H + CH2O H2 + HCO by means of V-UV Detection of Formaldehyde behind Shock Waves, Int. J. Chem. Kinet. 34, 374-386 (2002).

[4] G. Friedrichs, D. F. Davidson, R. K. Hanson, Validation of a Thermal Decomposition Mechanism of Formaldehyde by Detection of CH2O and HCO behind Shock Waves, Int. J. Chem. Kinet. 36, 157-169 (2004).