Research Group Prof. Dr. G. Friedrichs

Glyoxal - An Efficient HCO Source

The formyl radical (HCO) is a key intermediate along the direct oxidation pathway of hydrocarbons. The rate constants of the bimolecular reactions of HCO with the most important oxygen species O, OH, and O2 as well as the HCO thermal decomposition and the reaction HCO + H need to be precisely known in order to model the overall combustion process.

Most of the high-temperature rate constants have been determined in shock tube experiments. But often, due to the high reactivity and low thermal stability of the HCO radical, these experiments turned out to be difficult and inaccurate. Therefore, our group developed the 193 nm glyoxal photolysis as an effective and quantitative source of HCO:

(CHO)2 + hν → (H, HCO, CO, H2, CH2O)

H + (CHO)2 → HCO + CO + H2

In combination with a sensitive frequency modulation (FM) scheme for time-resolved HCO radical detection, we were able to directly measure and validate the rate constants of many HCO reactions. In the meantime, the recommended rate constant expressions have been implemented in many broadly used combustion mechanisms.

Glyoxal Thermal Decomposition


Interestingly and despite the fact that the HCO forming channel of the unimolecular decomposition is energetically less favorable, also the thermal decomposition of glyoxal yields high amounts of HCO radicals:

(CHO)2 + M → HCO + HCO + M        E0 = 291 kJ/mol

(CHO)2 + M → CO + CO + H2 + M    E0 = 256 kJ/mol

(CHO)2 + M → HCOH + CO + M        E0 = 254 kJ/mol

(CHO)2 + M → CH2O + CO + M         E0 = 232 kJ/mol

Experimentally measured HCO radical, (CHO)2, and H atom concentration-time profiles could be very well modeled with the help of unimolecular rate theory (RRKM/SACM/ME calculations). Actually, it turned out that the gyloxal thermal decomposition system is a suitable textbook example for a multi-channel unimolecular decomposition reaction with pronounced weak-collision and rotational effects. At high temperatures and pressures, HCO formations becomes the main product channel.

HCO Detection
HCO concentration-time profiles have been detected by FM spectroscopy at a wavelength of λ = 614.752 nm. Absorption cross sections and line shape data for quantitative FM measurements of HCO have been determined for the Q(6)P(1) absorption line of the A2A''-X2A'(0900 ←  0010) transition.

[1] Room Temperature and Shock Tube Study of the Reaction HCO + O2 using the Photolysis of Glyoxal as an Efficient HCO Source, M. Colberg, G. Friedrichs, J. Phys. Chem. A 110 (2006) 160-170.

[2] Wide Temperature Range (T = 295 K and 770-1305 K) Study of the Kinetics of the Reactions HCO + NO and HCO + NO2 using Frequency Modulation Spectroscopy, J. Dammeier, M. Colberg, G. Friedrichs, Phys. Chem. Chem. Phys. 9 (2007) 4177-4188.

[3] HCO Formation in the Thermal Unimolecular Decomposition of Glyoxal: Rotational and Weak Collision Effects, G. Friedrichs, M. Colberg, J. Dammeier, T. Bentz, M. Olzmann, Phys. Chem. Chem. Phys. 10 (2008) 6520-6533.

[4] Glyoxal Oxidation Mechanism: Implications for the reactions HCO + O2 and (CHO)2 + HO2, N. Faßheber, G. Friedrichs, P. Marshall, P. Glarborg, J. Phys. Chem. A 119 (2015) 7305 – 7315.

Contributing Researchers: N. Faßheber, G. Friedrichs and (formerly) M. Colberg, J. Dammeier