Figure 6 Energy diagram of R123 decomposition pathway.
In order to unravel the fire extinguishing mechanism of R123, the
reaction pathways of R123 with H and OH radicals and the direct
decomposition pathway of R123, were separately explored. the
B3LYP/6-311++G(d,p) method was used for structural optimization of
transition states, and the CCSD/aug-cc-pVDZ method was used for
correlation energy calculations.
The decomposition pathway of the ground state of R123 should be first
considered. As revealed in Figure 6, there are three main pyrolysis
paths for R123, and path 4 and 5 are the subsequent reactions of the
pyrolysis products. In path 1, the break of the C-Cl bond occurred, the
products CF3CHCl· and
Cl· (P1) are obtained by overcoming
an energy barrier of 343.78 kj·mol-1. Path 2
predominantly undergoes the elimination reaction, the path eliminates
from R123 to form a HF molecule and
CF2=CCl2 by overcoming an energy barrier
of 348.68 kj·mol-1. There are two paths for the
decomposition of CF2=CCl2. The
CF2=CCl2 can consume 656.02
kj·mol-1 energy to produce CF=CHCl2·
and F· via the transition state of TSa4 in path 4, or consume 431.56
kj·mol-1 energy to produce
CF2=CHCl· and Cl· via
the transition state of TSa5 in path 5. In addition, it is found that
the above paths are endothermic reactions, indicating that R123 can play
a physical fire extinguishing method of cooling in fire extinguishing.
In path 3, the products
CF3H and
CCl2 are obtained by overcoming an energy barrier of
420.5 kj·mol-1. As
shown in path 6~8, CF3H has three paths
to generate plentiful
CF3· free radicals,
where CF3H decomposition produces CF3·
without transition state generation. It is worth noted that
CF3· is one of the free radical groups that will play an
important role in the process of fire suppression. According to the
literature[34, 35],
CF3· free radicals
can effectively capture both H· and
OH· in combustion, thereby interrupting the combustion chain reaction.
R123 is capable of producing enough CF3·, indicating
that R123 has the potential to extinguish fire as an alternative
extinguishing agent. According to the literature, R125 mainly occurs C-C
fracture reactions reaction and elimination reaction, wherein the
product contains
CF3· and
CF2·. Like the pyrolysis products of R123, these
pyrolysis products are very easy to react with H· and OH· free radicals,
interrupting the combustion chain reaction.