Heterogeneous uptake and reactivity of formic acid on calcium carbonate particles: a Knudsen cell reactor, FTIR and SEM study.

The heterogeneous uptake and reactivity of formic acid (HCOOH), a common gas-phase org. acid found in the environment, on calcium carbonate (CaCO3) particles have been investigated using a Knudsen cell reactor, Fourier transform IR (FTIR) spectroscopy and SEM. FTIR measurements show that the adsorption of formic acid on the surface of calcium carbonate results in the formation of calcium formate. Besides calcium formate, carbonic acid is also a reaction product under dry conditions (<1% RH). Under dry conditions and at low pressures, the initial uptake coeff. of formic acid on CaCO3 particles is measured to be 3 ± 1 × 10-3 and decreases as the surface sats. with adsorbed products. The max. surface coverage of formic acid under dry conditions is detd. to be (3 ± 1) × 1014 mols. cm-2. Under humidified conditions (RH >10%), adsorbed water on the surface of the carbonate particles participates in the surface reactivity of these particles, which results in the enhanced uptake kinetics and extent of reaction of this org. acid on CaCO3 as well as opens up several new reaction pathways. These reaction pathways include: (i) the water-assisted dissocn. of carbonic acid to CO2 and H2O and (ii) the formation of calcium formate islands and crystallites, as evident by SEM images. The results presented here show that adsorbed water plays a potentially important role in the surface chem. of gas-phase org. acids on calcium carbonate particles. [on SciFinder(R)]