Reaction mechanism and kinetics of the important but neglected reaction of Hg with NO2 at low temperature

Abstract

Reaction of Hg and NO2 significantly contributes to mercury removal in CO2 compression and purification unit. However, the reaction mechanism remains unclear and the product species is difficult to collect and identify experimentally. To gain insight into this reaction, we studied it using quantum chemical and kinetic calculations. A two-step reaction mechanism of Hg and NO2 was investigated. Initially, Hg could react with NO2 without a barrier to form two isomeric HgNO2 complexes (i.e. HgO2N and HgNO2) that achieve equilibrium very quickly. Then both HgNO2 isomers can add another NO2 to form a variety of HgN2O4 compounds, among which the most stable is trans-syn, syn-Hg(ONO)2. Kinetics were investigated using variational transition state theory (VTST) and RRKM theory to calculate the rate constants with respect to both temperature and pressure. The only effective reaction pathway is the one described above leading to trans-syn, syn-Hg(ONO)2. The temperature- and pressure-dependent rate constants of these relevant reactions were presented. The calculated third-order rate constant is 7.4 × 10-35 cm6 molecule-2 s−1 at 298 K and 1 atm, which is of the same order of magnitude as the experimental value. This reaction pathway shows a negative temperature dependence from 250 to 400 K, which is in accordance with the existing experimental results. The gas phase reaction mechanism of Hg with NO2 could well explain the observations in the transformation of mercury in the CO2 compression and purification unit.