Nitrous acid in high-pressure oxidation of CH4 doped with nitric oxide: Challenges in the isomer-selective detection and quantification of an elusive intermediate

Abstract

The nitrous acid combustion intermediate has recently been detected in several reaction conditions using hydrocarbon fuels with different analytical techniques. Three of the several isomers of nitrous acid are expected to be produced during combustion: trans-HONO, cis-HONO and HNO2. It has recently been shown that cis-HONO dissociates upon ionization, rendering isomer selective quantification with methods requiring photoionization prior to detection impossible. This fact is of importance, since cis-HONO is produced at a ten times higher rate than trans-HONO according to recently published isomer branching ratios, possibly leading to sensitivity issues when a detection of the isomer mix is attempted with photoionization methods. We provide a quantitative glimpse at the trans-HONO isomer in a systematic set of measurements of NO doped methane oxidized in a plug-flow reactor covering three reaction conditions in the lean and rich regimes. Reactions take place at equivalence ratios of 0.7 and 1.2 with 1000 ppm NO and at an equivalence ratio of 2.1 doped with 1% NO. Double imaging photoelectron photo ion coincidence spectroscopy, i2PEPICO, was used to selectively and assuredly detect and assign trans-HONO. We touch on the difficulties encountered when attempting to detect cis-HONO. HNO2 remained undetectable despite recently published reaction rates for HNO2 decomposition suggesting modelled concentrations of this species two orders of magnitude larger than previously believed, yet 10 times lower than the reported isomer branching ratio. The recent reaction rates add a new path for HNO2 decomposition leading to formation of OH and NO which in turn influences the remaining decomposition kinetics of HNO2. A literature model is modified to include the recently published reaction rates for HONO and HNO2 decomposition and isomerization and compared to the measurements. Despite the higher predicted concentration of HNO2, that should be sufficient for detection, no HNO2 is detected in the experiment. Other nitrogen containing species, such as nitromethane and NO2, the precursor of both HONO and nitromethane, have also been detected. Interestingly, ammonia was also present in significant concentration, albeit exclusively in the fuel-rich conditions, despite the relatively low maximum temperature of 923 K at which the experiments have been performed. We conclude that, facing the unfavorable photoionization properties of cis-HONO as well as the decomposition and formation kinetics of HNO2, a measurement of isomer branching fractions by means of selective and sensitive photoionization methods may remain unattainable.