Mid-infrared Frequency Modulation Detection of HCN and Its Reaction with O Atoms behind Shock Waves

Hydrogen cyanide (HCN) is the primary cyanide species in combustion processes and plays a key role in the formation of NO in the combustion of fossil fuels, nitrogen-containing biofuels, and blended hydrocarbon-ammonia mixtures. Robust, sensitive, and time-resolved in situ laser diagnostic methods are needed to gain insight into the combustion chemistry of HCN. Mid-infrared frequency modulation spectroscopy (MIR-FMS) has recently been established as such a quantitative technique for HCN detection behind shock waves. With a minimum detectable fractional absorption of 2 × 10^-4 at a time resolution of 5 μs, an improved spectrometer design enabled the detection of HCN behind shock waves down to the ppm mole fraction level. An Allan noise analysis revealed that a further improvement toward shot-noise limited detection should be possible when fast mid-infrared photodetectors with a higher saturation limit will become available. HCN kinetic profiles in the presence of O(³P) atoms from thermal N₂O decomposition have been measured in the temperature range 1448 K < T < 1954 K. The determined total rate constants for the key HCN oxidation reaction HCN + O, k/(cm³ mol^-1 s^-1) = 1.88 × 10¹⁴ exp(-64.5 kJ mol^-1/RT)(+28%, -37%), turn out to be largely consistent with previous measurements. These data complete the set of available rate constant studies, by now covering the temperature range 450 K < T < 2500 K and relying on the detection of almost all feasible reactant and product species.