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

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 su-percritical water are HF, CO2, CO and H2. In supercritical water, center dot F radicals can easily combine with H2O molecules and then rapidly decompose to generate HF molecules and center dot OH radicals. The hydrogen extraction reaction of center dot H radicals with H atoms in HFC-245fa molecules and H2O molecules has relatively low energy barrier. center dot 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 ox-ygen facilitates the generation of center dot 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 con-centration of 13.0%, the calculated formation apparent activation energies of HF, CO and CO2, H2 are 196.3, 368.5 and 249.0 kJ center dot mol-1, respectively.(c) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.