Thermal unimolecular decomposition of formic acid

Abstract

The initial steps involved in the thermal decomposition of formic acid in the temperature range of 1370–2000 K have been investigated by monitoring the IR emission intensities at 3.4 and 4.6 μm, corresponding to the reactant and carbon monoxide, respectively, behind reflected shock waves in mixtures diluted in Ar (0.1–1.5 mol%, total densities 5.3×10−6–3.2×10−5 mol cm−3). It was found that the decomposition proceeded via channel (1) HCOOH+Ar→CO+H2O+Ar dominantly and the contribution of channel (2) HCOOH+Ar→CO2+H2+Ar was very small. An Arrhenius expression of the second order rate constant was obtained as k1,0 =1014.32 exp(−40.4 kcal mol−1/RT) cm3 mol−1 s−1. Ab initio calculations were performed for probable transition states (TS) of channels (1) and (2). The results showed that the potential energy for a TS of channel (1) was lower than that of channel (2) by 20–30 kcal mol−1. On the basis of a RRKM weak collision, k2,0 values were estimated which was smaller than k1,0 by about two orders, being consistent with the experimental results.

Unimolecular decomposition of formic and acetic acids: A shock tube/laser absorption study

“…4 for comparison with our data. For the rate constant of dehydration reaction (R1), there is good agreement between our 1 atm data and Saito et al [7] data at temperatures less than 1430 K, while at higher temperatures our measured rate constants are faster by a factor of 2 -4. The dehydration rate constant measured by Hsu et al [6] is lower at all temperatures, though their activation energy is quite similar to our data.…”

“…The current study exhibits less uncertainty because of the lower reactant concentration, and thus lower influence from secondary reactions, that was possible because of the sensitive laser absorption diagnostics for CO, CO 2 and H 2 O. The rate constant for decarboxylation reaction (R2) provided by Saito et al [7] is lower than that of Hsu et al [6] by approximately two orders of magnitude. Our measurements fall between these two previous determinations.…”

“…Best-fit lines to the experimental data of Hsu et al [6] and Saito et al [7] are also shown in Fig. 4 for comparison with our data.…”

“…! , was ~10 at low temperatures and approaches 1 at 2000 K. Saito et al [7] studied the thermal decomposition of formic acid in a shock tube using formic acid emission near 3.4 µm, CO emission near 4. [7].…”

“…A number of experimental [6][7][8][9][10][11] and theoretical [12][13][14][15] studies have been carried out on the decomposition of formic acid, including three shock tube studies. Hsu et al [6] studied the decomposition of formic acid using infrared (IR) laser absorption of CO.…”