The project investigated the reactivity of different surface sites on Ag catalysts during ethylene epoxidation. The reaction was studied under near-ambient pressure conditions but with well-defined catalyst surfaces.
At the outset of the project, a comprehensive characterization of Ag surfaces was carried out, using techniques such as Scanning Tunneling Microscopy (STM) and X-ray Photoelectron Spectroscopy (XPS). These methods allowed for detailed analysis of various Ag surfaces, including flat and curved crystals with different step types, and provided insights into surface species under both ultra-high vacuum and near-ambient pressure conditions.
One of the key findings of the project was the differentiation between nucleophilic and electrophilic oxygen species on different Ag surfaces, while simultaneously tracking product formation. It was found that steps exhibited higher selectivity for electrophilic oxygen, which is crucial for EO production. However, stepped surfaces were more prone to ethylene cracking and surface poisoning, while flat (111) surfaces provided more stable conditions for consistent product formation. These findings underscore the importance of surface stability for optimal catalytic performance.
Efforts to fabricate stable Ag nanoparticle arrays on oxide supports encountered unforeseen challenges, particularly due to the instability of Al2O3 films in reaction EO-selective conditions. As a result, the project maintained a focus on experiments with Ag surfaces. The use of near-ambient pressure XPS (NAP-XPS) proved successful in observing reactions under near-realistic conditions. Various oxygen- and carbon-containing species were identified, including electrophilic and nucleophilic oxygen, carbonates, and reaction products such as CO2 and EO. We gained new insights on the interplay of these species in different pressures, gas mixes, temperatures, and on different surfaces.
Throughout the project, the researcher gained extensive expertise in advanced spectroscopy techniques and project management skills. During the two years of this project, the researcher established active collbaorations with five international research groups. She has contributed six talks at conferences (three invited).
