Rate Coefficients and Branching Ratios for the Reaction of the OH Radical with Formic Acid under Low-Temperature Combustion Conditions and the Fate of the HOCO Product

Formic acid (FA, HC(O)OH) is of interest as an e-fuel
and hydrogen
carrier, as well as a significant component of atmospheric acidification.
Reaction with the OH radical is a major removal process for FA. We
have measured the overall rate coefficient, k1, for the reaction of OH and FA over the temperature range
of 300–850 K using laser flash photolysis to generate OH and
monitoring the disappearance of OH under pseudo-first-order conditions
using laser-induced fluorescence. The rate coefficient can be parameterized
as k1(T) = 9.8 ×
10–15 × (T/298 K)5.1 × exp(−14200/RT) cm3 molecule–1 s–1 where we estimate that the
uncertainty increases from ∼ ±20% in the range 300–600
K to ±50% above 600 K. We have also determined the branching
ratio to H2O + HCO2 (abstraction at the O–H
site) by observing the H atoms produced by the fast decomposition
of HCO2. H atom yields drop from ∼0.9 at 300 K to
∼0.5 at 600 K. The kinetics and branching ratios of k1 are in good agreement with theoretical calculations.
Above 600 K, the observed H atom yield increases, and we show that
this is due to the decomposition of the HOCO product. The implications
of these studies on FA and HOCO in the combustion and pyrolysis of
FA are considered.