(673d) Elucidation of the Mechanism of Ethylene Epoxidation on Promoted Supported Silver Catalysts Using Density Functional Theory and Microkinetic Modeling

The ubiquity of ethylene oxide (EO) as a precursor for a wide range of commercial products has led to an extensive body of literature studying its catalytic synthesis in the past several decades. Through surface science and reaction kinetic studies on well-defined silver (Ag) catalyst analogs, EO synthesis on un-promoted single-crystal or polycrystalline Ag catalysts have been significantly explored, with an additional focus on the oxametallacycle (OMC) intermediate as a pivotal intermediate in the reaction mechanism. In contrast, however, industrial EO production are done on supported Ag catalysts that are heavily promoted. This leads us to the following questions: 1) How do the detailed reaction mechanisms differ between supported and unsupported Ag catalysts? 2) How applicable is the currently accepted OMC intermediate, especially when evidence of a stable OMC structure is limited to conditions far from industrial operating conditions? 3) How do the promoter(s), individually as well as a combination thereof, affect the reaction kinetics and mechanism? Answering these questions would lead to an improved understanding of the mechanism of industrial EO synthesis.

The current study proposes a Microkinetic Model (MKM)-driven Density Functional Theory (DFT) approach that will be used to assess the credibility of the OMC reaction pathway, effects of catalyst promotion, and to gain an improved general understanding of the kinetics and mechanism of industrial EO synthesis. Preliminary MKM optimization based on experimental kinetics data suggests that the OMC may not be the only valid key intermediate for EO synthesis, and that sub-oxide Ag surfaces may be of significance in the reaction. This serves as the basis for a DFT exploration of relevant hydrocarbon chemistries on reconstructed Ag surfaces. This talk highlights the results of the initial MKM optimization, as well as DFT results highlighting the mechanistic complexity of oxidic-reconstructed Ag surfaces.