Audrey Miles

Chemical and Biomolecular Engineering

Faculty Advisor: William Schneider

Winter 2020-21 Project: Benchmarking Ammonia Synthesis Entropies in Microkinetic Modeling

Most significant chemical processes are catalytic in nature, including those relating to emission control, sustainable energy, pollutant removal, and the fabrication of technologies.  This widespread use of catalysis has motivated research efforts to streamline industrial processes through the optimization of catalytic performance. By studying the rates and equilibrium constants for elementary reaction steps, the overall catalytic efficiency of a given process can be evaluated and improved.  Microkinetic modeling has emerged as one such tool to compute these rate constants and equilibrium constants, leading to a heightened understanding of governing reaction mechanisms. A critical component of these microkinetic models is an accurate approximation of the enthalpies and entropies of the species under consideration.  Without appropriate approximations of the entropic changes in the system, kinetic models are rendered inaccurate.  Popular approaches to compute these entropies and free energies include the lattice gas model or the ideal gas model.  The lattice gas (harmonic oscillator) and ideal gas models assume that an adsorbate’s translational degrees of freedom are either constrained or relaxed on a surface respectively. However, these models are not robust, with their individual weaknesses being exposed by various situations.  The Schneider group has developed a more sophisticated model that has been shown to accurately bridge the gap between the lattice gas and ideal gas models. This model, known as the complete potential energy sampling technique, utilizes the exact energy landscape rather than assuming some functional form of the potential energy surface.

One such significant catalytic system involves the sustainable production of ammonia.  Large-scale ammonia production is one of the most important synthetic chemical processes globally, with production plants generating over 150 million tonnes of ammonia per year.  Beyond its main use in fertilizers and the agricultural industry, ammonia is also used in water purification, plastics, fibers, pharmaceuticals, dyes, and pesticides.  The typical process used in ammonia production involves the removal of sulfur compounds from natural gas feedstock, several catalytic conversions to facilitate the removal of carbon dioxide and carbon monoxide to generate hydrogen gas, and the reaction of nitrogen gas with the hydrogen gas to form liquid ammonia.  This cycle, though, requires high pressures and temperatures, and there are relatively low single-pass conversion rates.  In addition to this, the process requires an abundance of energy and natural gas.  Ammonia synthesis is therefore vital to global industries, but inefficient in its use of energy and valued natural resources.  For this reason, a more sustainable approach to the production of ammonia is desired.  Microkinetic modeling, as discussed previously, is one method through which ammonia synthesis may be better understood and ultimately optimized. As such, accurate models to describe the translational, rotational, and vibrational degrees of freedom are needed. This project will aim to evaluate the ability of various models to accurately calculate the entropic contributions of ammonia synthesis and develop microkinetic models describing the overall catalytic process.