Abstract
ABSTRACT The ethylene epoxidation is a challenging catalytic process, and development of active and selective catalyst requires profound understanding of its chemical behaviour under reaction conditions. The systematic study on intermetallic compounds in the Ca–Ag system under ethylene epoxidation conditions clearly shows that the character of the oxidation processes on the surface originates from the atomic interactions in the pristine compound. The Ag-rich compounds Ca2Ag7 and CaAg2 undergo oxidation towards fcc Ag and a complex Ca-based support, whereas equiatomic CaAg and the Ca-rich compounds Ca5Ag3 and Ca3Ag in bulk remain stable under harsh ethylene epoxidation conditions. For the latter presence of water vapour in the gas stream leads to noticeable corrosion. Combining the experimental results with the chemical bonding analysis and first-principles calculations, the relationships among the chemical nature of the compounds, their reactivity and catalytic performance towards epoxidation of ethylene are investigated.
Highlights
Every industrial chemical process is an extremely complex and dynamic system in terms of physical, chemical and engineering parameters
The assessment of intermetallic compounds as catalysts requires the careful characterization of the as-synthesized material to relate the catalytic performance to either the chemical nature of IMCs or the changes occurring under reactive atmosphere
The divergent behaviour of Ca–Ag compounds under exposure to air and under ethylene epoxidation conditions provoked thinking about other components of the system, which can act as oxidants, namely water vapour and carbon dioxide
Summary
Every industrial chemical process is an extremely complex and dynamic system in terms of physical, chemical and engineering parameters. The catalyst should be considered as a dynamic part of the catalytic system, and its completely different behaviour under operation as opposed to under steady-state conditions needs to be taken into account. These observations act as driving forces for development of advanced in situ and operando techniques [2,3,4,5,6,7,8,9], which give an insight into the crystal structure, surface morphology and composition of the catalyst. The experimental results, supported by chemical bonding analysis and first-principles calculations, strengthen experience on intermetallic compounds as catalyst precursors for the ethylene epoxidation and contribute to the fundamental understanding of their chemical properties
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