The Underlying Chemistry to the Formation of PO2 Radicals from Organophosphorus Compounds: A Missing Puzzle Piece in Flame Chemistry.

Reactive species, such as . PO2 and HOPO, are considered of upmost importance in flame inhibition and catalytic combustion processes of fuels. However, the underlying chemistry of their formation remains speculative due to the unavailability of suitable analytical techniques that can be used to identify the transient species which lead to their formation. This study elucidates the reaction mechanisms of the formation of phosphoryl species from dimethyl methyl phosphonate (DMMP) and dimethyl methyl phosphoramidate (DMPR) under well-defined oxidative conditions. Photoelectron photoion coincidence techniques that utilized vacuum ultraviolet synchrotron radiation were applied to isomer-selectively detect the elusive key intermediates and stable products. With the help of in situ recorded spectral fingerprints, different transient species, such as PO2 and triplet O radicals, have been exclusively identified from their isomeric components, which has helped to piece together the formation mechanisms of phosphoryl species under various conditions. It was found that . PO2 formation required oxidative conditions above 1070 K. The combined presence of O2 and H2 led to significant changes in the decomposition chemistry of both model phosphorus compounds, leading to the formation of . PO2 . The reaction . PO+O2 →. PO2 +O: was identified as the key step in the formation of . PO2 . Interestingly, the presence of O2 in DMPR thermolysis suppresses the formation of PN-containing species. In a previous study, PN species were identified as the major species formed during the pyrolysis of DMPR. Thus, the findings of this study has shed light onto the decomposition pathways of organophosphorus compounds, which are beneficial for their fuel additive and fire suppressant applications.