Decomposition mechanism of hydrofluorocarbon (HFC-245fa) in supercritical water: A ReaxFF-MD and DFT study

Abstract

In this work, the concept of using supercritical water to treat Hydrofluorocarbons (HFCs) refrigerants waste is proposed. Taking 1,1,1,3,3-Pentafluoropropane (HFC-245fa) as the research object, based on the ReaxFF reactive force field molecular dynamics method and density function theory, the decomposition mechanism of HFC-245fa in supercritical water, the effects of temperature, HFC-245fa mass concentration and oxygen addition on the decomposition of HFC-245fa in supercritical water and decomposition mechanism were studied. The results show that the main stable products of HFC-245fa decomposition in supercritical water are HF, CO2, CO and H2. In supercritical water, ·F radicals can easily combine with H2O molecules and then rapidly decompose to generate HF molecules and ·OH radicals. The hydrogen extraction reaction of ·H radicals with H atoms in HFC-245fa molecules and H2O molecules has relatively low energy barrier. ·OH radicals can also participate in the dehydrogenation reaction, promoting the further dehydrogenation of HFC-245fa molecules and their molecular fragments. Moreover, it can combine with carbon-containing molecular fragments to promote the conversion of C element to CO and CO2. The lower concentration of HFC-245fa promotes the conversion of C element and generation of H2. The addition of oxygen facilitates the generation of ·OH radicals and promotes the conversion of C element to CO2, but reduces the generation of CO and H2. Under the condition of HFC-245fa mass concentration of 13.0%, the calculated formation apparent activation energies of HF, CO and CO2, H2 are 196.3, 368.5 and 249.0 kJ·mol−1, respectively.