Ab initio theoretical studies of reaction kinetics in mixtures

Our group seeks to understand how reaction kinetics occurs differently in gas mixtures than in pure gases, with important implications for practical mixtures in combustion and planetary atmospheres. For context, the collisions between energized intermediates and the surrounding gas are responsible for pressure dependence in gas-phase kinetics. Current frameworks for pressure-dependent reactions are largely based on theories and data for pure, inert gases as collision partners. In most practical mixtures, however, energized molecules collide with many different inert and reactive collision partners.

The effect of many inert collision partners is essentially always handled via a mixture "rule," which is embedded in reacting flow codes (e.g. CHEMKIN, Cantera) and used to derive collision partner efficiencies from experiments. Our group has found that the most common mixture rule, on which codes and experimental interpretations are based, fails for exactly the mixtures in combustion – with errors up to an order of magnitude. These errors significantly influence predictions of flame speeds, ignition delay times, and other combustion properties as well as experimental interpretations used to derive collision efficiencies. To address these deficiencies, we have been developing new mixture rules that can reliably predict kinetics in mixtures. Once completed, these new mixture rules will enable more accurate treatment of mixture composition effects on kinetics in future reacting flow codes.

The effect of reactive collision partners has been largely ignored in combustion and other gas-phase kinetic systems – equivalent to assuming that bimolecular reactions only occur between species with Boltzmann distributions. As a result, kinetic mechanisms traditionally have only included three types of reactions: unimolecular reactions, bimolecular reactions, and termolecular association reactions. "Chemically termolecular" reactions, in which the collision partner reacts instead of simply transferring energy, were hypothesized a century ago by Hinshelwood and Semenov, but were later thought to be either unphysical or unimportant. However, we recently revealed (in collaboration with Klippenstein) that reactions of this type (e.g. H + O2 + H) can be major chemical pathways influencing global system kinetics. There are, in principle, countless possible reactions of this type that could be important pathways, with equally important implications, that are yet to be discovered. Our group has created an automated procedure to aid in discovering them.  We have been using this code to identify other chemically termolecular reactions that could potentially impact predictions of combustion phenomena and energetic materials and, therefore, warrant further investigation.

Our recent ab initio theoretical study of one of these reactions (H + C2H2 + O2) confirm the reaction impacts predictions – accelerating autoignition by an order of magnitude.  Notably, these calculations were performed using an entirely automated procedure for calculating rate constants for non-Boltzmann kinetic sequences – which will enable more tractable, and even high-throughput, calculations of many chemically termolecular reaction rate constants, for which almost no information is available at present.

In reality, the effects of many inert and reactive collision partners are coupled. We are now crafting mixture rules and rate laws that reflect this coupling. Our ultimate goal is to represent composition dependence at the same level of fidelity as current treatments of temperature and pressure dependence in reacting flow simulation software.

Columbia Affiliations
Mechanical Engineering
Chemical Engineering
Data Science Institute