Design, Demonstration and Characterization of NiAg Dilute Atom Alloy Catalysts for Selective Ethylene Oxidation

Abstract

Ethylene epoxidation is a critical $40B/year industrial process that produces ethylene oxide— an intermediate for many chemical products. Ethylene oxide (EO) is a thermodynamically unstable molecule that shows high propensity toward combustion to carbon dioxide, which makes it challenging to achieve high selectivity and high conversion in the catalytic reaction. The process typically relies on a heavily promoted Ag/a-Al₂O₃ catalyst and Cl co-flow to achieve and sustain high ethylene oxide selectivity (>90%) at reasonable ethylene conversions (10-15%). Cl has been the ubiquitous promoter of this reaction, providing a ~25% selectivity increase over unpromoted Ag/a-Al₂O₃ catalysts (~55%). Promoters like Cs, Re, and Mo each add a few percent of selectivity enhancements to achieve 90% overall, but their co-dependence on each other and Cl makes understanding their function complex.

We took a theory-guided single-atom alloy approach to identify dopants in Ag that can facilitate selective oxidation by activating O₂ without binding O too strongly. Ni was found to be an outlier in both respects, and surface science experiments confirmed the facile adsorption/desorption of O₂ on NiAg, as well as demonstrating that Ni serves to stabilize unselective nucleophilic oxygen.  This dissertation presents the synthesis, characterization, and identification of Ni promoters, as effective independent promoters for EO production. We developed a robust colloidal synthesis strategy to produce Ag nanoparticles with precisely controlled size and morphology, then incorporated ppm-level Ni dopants at target Ni:Ag atomic ratios (1:500 to 1:50). These methods enabled the preparation of catalysts with uniform size distributions and reproducible Ni loadings, addressing a key challenge in the synthesis of bimetallic colloids in the epoxidation-relevant size regime. Supported Ag catalyst studies revealed that a 1:200 Ni:Ag atomic ratio provides a ~25% selectivity increase to unpromoted Ag (~55% selectivity) without the need for Cl co-flow. Further, Ni acts cooperatively with Cl resulting in a further 10% initial increase in selectivity.

To elucidate the electronic and structural role of Ni in these materials, we employed high-energy resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XANES) and used multivariate statistics to quantify Ni speciation during epoxidation. NiAg exists in an electronically unique environment distinct from either bulk NiO or Ni, and dynamically evolves under reducing and reactive conditions. Scarcely demonstrated quantitatively in the dilute alloy literature, we conclude that enthalpic surface segregation dominates entropic driving forces for this dilute alloy system.  

We also assessed reaction orders while accounting for catalyst deactivation in NiAg and Ag materials. The kinetic behavior of NiAg catalysts was found to differ from that of Ag, with distinct oxygen and ethylene reaction orders, thereby supporting a modified surface mechanism resulting from Ni incorporation. NiAg catalysts additionally demonstrated significantly decreased deactivation compared to Ag, suggesting that alloying with Ni not only alters surface reactivity but also enhances catalyst stability under reaction conditions.

The use of alloyed Ni in Ag ethylene epoxidation catalysts has been previously unreported in academic and patent literature. Our results demonstrate that Ni holds substantial promise for the development of next generation Ag based epoxidation catalysts. This highlights the potential of theory-led exploration in dilute alloy materials space and the utility of our multifaceted approach for the design of selective oxidation catalysts.