CAREER: Understanding Effects of Surface Coverage and Catalyst Composition on Vinyl Acetate Synthesis

The majority of chemical processes worldwide utilize catalysts that promote the efficient manufacturing of chemicals and fuels, while also lowering energy costs and environmental footprint. Advances in catalyst technology depend, in turn, on understanding how a catalyst’s performance is determined by the properties of its surface and of the reacting molecules.

This project focuses on developing such understanding for the catalytic reaction involved in the synthesis of vinyl acetate (VA). VA is a high-value chemical produced in large volumes by the coupling of ethylene and acetic acid molecules on expensive palladium-based catalysts. It is an important component of many polymers and consumer products.

However, current VA manufacturing processes have several limitations. The project addresses those limitations through research aimed at developing more efficient catalysts consisting of atomically dispersed precious metal atoms on inexpensive copper matrices. The project integrates research with educational and outreach activities aimed at middle- and high-school students.

High surface coverages prevail in many catalytic reactions at practical conditions. Rigorous incorporation of the effects of coverage in the analysis of experimental data, as well as incorporation in simulation models, is challenging, however. Thus, few studies exist focusing on the design of catalysts to operate efficiently under high-coverage conditions.

Coverage plays an important role in VA synthesis via oxidative coupling of acetic acid and ethylene. In this reaction, crowding of the surface with acetate species promotes C-O coupling and inhibits dissociative steps in ways that influence both reactivity and selectivity. Mechanistic understanding of this reaction has emerged from surface science and density functional theory (DFT), but essentially all past work has focused solely on supported palladium (Pd) or palladium-gold (PdAu) catalysts that are unstable at practical conditions without alkali stabilizers that tend to deplete over long times.

This project is aimed at, (i) understanding (using kinetics and DFT) the mechanistic aspects pertaining to steady-state catalysis, (ii) assessing how the very different strengths of lateral interactions for active atoms dispersed in an Au versus Cu matrix influence rate and selectivity, and (iii) leveraging such insights to design more efficient VA synthesis catalysts. The project builds on preliminary experimental and computational results for monometallic Pd catalysts obtained by the investigator in collaboration with colleagues at his institution.

The Pd work has provided significant insights on how coverages impose changes in rate liming steps when reactant pressures change in steady-state catalysis. DFT studies will build on these insights and probe, on a range of single atoms dispersed on Cu and Au matrices, how (i) enhanced lateral interactions imposed by the smaller lattice constant of Cu can promote C-O coupling, and (ii) how combined effects of coverage and composition influence catalytic performance.

These results will further guide the preparation and structural-functional characterization of single atom alloy catalysts with potential for better activity, selectivity, and stability in VA synthesis catalysis. The data generation, catalyst characterization, and analysis methods employed in the study – combined with the resulting insights from VA synthesis - will carry over to other catalytic reactions that operate under conditions of high surface coverage of adsorbed species.

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.