New HFO Refrigerants Transform in the Atmosphere to Ultimately Produce Problematic Old HFCs
Tuesday, 15 December 2020
Since the early-20th century various haloalkanes have been phased in for use as refrigerants, foam blowing agents, and propellants. They have been widely used in these applications until a negative environmental impact is discovered and they are phased out under international legislation. This began with the chlorofluorocarbons (CFCs) which transform in the stratosphere to produce reactive chlorine which depletes ozone, leading the hole in the ozone layer. To halt this environmental impact, they were phased out in accordance with the Montreal Protocol (1987) and replaced by a new class of molecule, the hydrofluorocarbons (HFCs). These molecules do not produce reactive halogens in the upper atmosphere, so they did not pose a threat to the ozone layer, however they have significant global warming potentials (GWPs). As their role in global warming was recognized, the HFCs were added to the Montreal Protocol in a 2016 amendment. The next generation of refrigerants are the hydrofluoroolefins (HFOs), which are based on HFCs but incorporate a double bond to shorten their atmospheric lifetimes, resulting in near zero GWPs. The immediate environmental risk from the HFOs and some decomposition products is well determined, however the ultimate impact of higher order decomposition products is less well known and thus not considered. Many of the HFOs produce CF3CHO at some yield up to 100%. Various studies over the last ~50 years show that photolysis of CF3CHO at tropospheric wavelengths produces the radicals CF3 and CHO, which are both considered environmentally benign. However, a channel producing CO and CHF3 has occasionally been observed and its analogue is an established channel in CH3CHO photochemistry at similar wavelengths.
This presentation will report on laboratory velocity-mapped ion imaging (VMI) and FT-IR experiments investigating gas-phase photochemistry following λ = 308 nm excitation. We identify CHF3 as a primary photoproduct with a quantum yield of several percent leading to effective GWPs in the hundreds for the ‘zero GWP’ HFO progenitor.