The effect of inert gases (Xe, Ar, Ne, He) on decomposition reactions of N₂O, CH₄ and CO₂ at high pressures

In a pulsed compression reactor (PCR) experiments were done with N₂O (4%_mol), CH₄ (1%_mol) and CO₂ (2%_mol) diluted in the inert gases Xe, Ar, Ne and He. The mixtures were compressed up to 250 bar, reaching temperatures of up to 4000 K. At equal temperature, pressure and volume, significant differences (up to 20%) were measured in the conversion of the three species in different noble gases. The measurements with N₂O decomposition showed that the reaction is the fastest in the most heavy noble gas tested. The conversion decreased as the molar mass of the noble gas decreased. Likewise, methane pyrolysis was measured to be the fastest in xenon and slowed down in accordance with the mass of the inert molecule. A reverse trend was measured for the decomposition of CO₂ to CO and O⋅, which is explained by the dominant role of the reverse reaction. As a result, the CO₂ data is also explained by conversion rates that are higher in heavier gases. This paper provides a first attempt to understand the observed influence of the molar mass of the inert bathing gas on the reaction rate in the high pressure domain. A theory is proposed based on a Newtonian description of reactant activation by the inert bathing gas.