Understanding Structure-Function Relationships and Dynamical Restructuring in Near Surface Alloy Catalysts for Selective Oxidations

Tufts University, Medford MA

Investigators

Abstract

The partial oxidation of hydrocarbons can generate compounds that contain oxygen atoms (oxygenates). Oxygenates are high-value chemicals that are essential intermediates in industrial processes and important ingredients in many consumer products. Partial oxidation reactions require efficient catalysts that selectively yield the desired oxygenate without completely oxidizing the compound to carbon dioxide. The development of more efficient and selective catalysts than those used in current chemical manufacturing is essential for decreasing manufacturing costs while reducing energy consumption and environmental impacts. Improved catalysts and catalytic processes also will enhance the competitiveness and sustainability of the U.S. chemical industry. This research project will study a new class of catalysts comprised of a single layer of silver atoms on inexpensive copper nanoparticles. Such catalysts hold significant promise for the selective partial oxidation of unsaturated hydrocarbons to epoxide-type oxygenates. The researchers will use experimental and computational methods to prepare the catalysts, to evaluate their performance, and to understand how the structure and electronic properties of the catalyst influence its reactivity and selectivity. The knowledge gained from this research will pave the way for the design of a next generation of partial oxidation catalysts for chemical manufacturing processes. The project also includes an outreach program to a local high school in which graduate students will assist high-school students in preparing their presentations for a local science fair. The project participants also will collaborate with a neighboring university on an Upward Bound Math Science program to provide early research experiences to low-income and first-generation college bound students. Near-surface alloys have been predicted by theory to have unique electronic structures that lead to deviations from scaling relationships that limit traditional catalysts. However, such catalysts exhibit intrinsic disorder and inhomogeneity introduced by segregation and restructuring that occur under relevant reaction conditions. This research project aims to develop an atomic-level understanding of AgCu near-surface alloys using surface science model studies, density functional theory simulations, and steady-state catalysis kinetics. A silver -terminated, AgCu near-surface alloy has been chosen for study due to its promising catalytic properties for partial oxidation reactions, which require both efficient activation of molecular oxygen and weak surface binding of intermediates. Preliminary data shows that these near-surface alloys enhance molecular oxygen activation by electronic modification of the silver layer by the underlying copper. Under reaction conditions isolated copper atoms are dynamically exposed, which further increases the molecular oxygen activation rate while maintaining a majority of silver sites at the surface. The silver sites promote selective oxidation via a bifunctional mechanism. The preliminary data also includes evidence for dynamic restructuring of AgCu near-surface alloys in response to surface oxidation. This restructuring yields highly active and selective model catalytic surfaces displaying 100% selective butadiene epoxidation. Building on these preliminary data, this research project will study the structural evolution and epoxidation reactions on model AgCu near-surface alloys and use density functional theory to enhance understanding of the structure, energetics, and electronic properties of the near-surface alloys. New AgCu near-surface alloy epoxidation catalysts will be synthesized and tested. 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.