(5aj) Experimental, Mechanistic, and Molecular Investigations Involving Water for Environmental Applications

Water is an important fluid for both scientific and engineering applications. This body of work presents results from investigations of pure water and water-based reaction systems for environmental applications, through a combination of experimentation and mechanistic and molecular modeling.

Reactions in Supercritical Water

Near- and supercritical water have received much attention over the past few decades as environmentally benign solvents for use in green chemistry applications. The most popular application is supercritical water oxidation (SCWO), a waste treatment process by which organic compounds are oxidized in a supercritical water (SCW) solvent. In work completed at the University of Michigan, we expanded the existing SCWO kinetics database to include nitrogen chemistry by investigating pyrolysis, hydrolysis, and oxidation of methylamine, a model nitrogen-containing compound, in SCW. From experimentally determined global reaction kinetics, we have discerned different regions of reaction conditions where different mechanisms (pyrolytic, hydrolytic, oxidative) dominate. Also, the results implicate a heterogeneous, wall-catalyzed reaction mechanism for reactions conducted in reactors made of Nickel-based materials. Additionally, we constructed a detailed chemical kinetic model (DCKM) to describe methylamine oxidation and pyrolysis in SCW, as well as ammonia SCWO. Reaction pathway and sensitivity analyses indicate the most important elementary reactions in each system. These mechanistic modeling results highlight the need for improved free-radical reaction kinetic parameters for nitrogen chemistry.

Molecular Clustering from Higher Order Virial Coefficients

The formation of water clusters are important for several environmental phenomena, including atmospheric chemistry of acid rain formation, nucleation of water droplets, absorption of infrared radiation by water in the atmosphere (global warming), and reactions in supercritical water. One main research tool for investigating clustering behavior is molecular simulation. Mayer Sampling molecular simulation is a free energy perturbation method for calculating cluster integrals. In ongoing work at SUNY-Buffalo, we employ the Mayer Sampling method to determine higher order virial coefficients for various non-polarizable and polarizable water models. We use these virial coefficients to predict the pressure-volume-temperature behavior of sub- and super-critical water, as well as critical properties. Additionally, these virial coefficients provide a framework for examining gas-phase molecular clustering in water under a range of sub- and super-critical conditions.