Collaborative Research: Catalyst Structure, Reaction Mechanism, and Roles of Chlorine for Ethylene Epoxidation

点击翻译按钮获取中文摘要

Ethylene oxide (EO) is a key intermediate in the production of a wide array of advanced materials, industrial solutions, and surfactants. Worldwide production of EO exceeds 27 billion kilograms, and the primary source involves selective reaction of the raw feedstock, ethylene, with oxygen over silver (Ag) catalysts. The Ag catalysts are promoted with complex combinations of five or more elements, each with different roles in catalysis. Chlorine (Cl) is one of the main promoters used to enhance the selectivity of ethylene conversion to EO. Although the EO selectivity of current industrial process technology is above 90%, opportunity exists to further increase the selectivity while simultaneously improving process efficiency and decreasing CO2 emissions. Yet, the interactions by which Cl increases selectivity are not fully understood, thus limiting further improvements in selectivity. The project combines advanced experimental, computational, and machine learning methods to better understand the catalytic mechanism of EO manufacture and the role played by the Cl promoter. The project will identify the steady-state active phase, surface structure, and surface intermediates present on Cl-promoted Ag nanoparticles at pressures and temperatures relevant for EO catalysis. Deeper understanding of the operation and design principles for EO catalysts, and increases in EO selectivity, could lead to transformative advances in the U.S. chemicals industry while simultaneously decreasing carbon emissions. In addition, the project will enhance graduate and undergraduate student education at the partner institutions, through cross-training of students between theory and experimental research groups, K-12 outreach via mentorship programs, and integration with educational opportunities that target high school women in STEM. The project explores the likelihood that surface structures for active Ag catalysts in many prior studies do not represent catalysts operating at practical conditions. Consequently, the dominant reaction mechanisms and the ways in which Cl improves selectivity remain unclear, particularly at industrially relevant reaction conditions and Cl coverages. The requirement for Cl as a promoter has long been associated with either induced reactivity of surface oxygen atoms or the selective titration of oxygen vacancies responsible for combustion present on oxide surfaces, yet those explanations presume specific mechanistic pathways that are dependent on catalyst surface structure. This project will establish the mechanism and reaction pathways for epoxidation and combustion, and the ways by which those pathways involve specific oxygen intermediates and oxygen vacancies upon oxygen-induced surface reconstructions. Additional insight will be obtained regarding how adsorbed Cl atoms affect surface reconstructions, the distribution and coverages of reactive oxygen species and vacancies, and the associated changes in reaction barriers, kinetics and selectivity. The inherent complexity of the catalytic system under reaction conditions is addressed through a combination of experimental (time-resolved in situ Raman spectroscopy) and computational methodologies (grand canonical Monte Carlo and molecular dynamics simulations employing machine learning-based potentials). More broadly, this project will provide opportunities for creating and validating methods (both experimental and computational) that address changes in surface structure during catalysis, a major challenge in the field.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.