Source: Lehigh Preserve (article, 2022-08-01)
Field: Materials Science — Materials Chemistry

Selective Oxidation of Ethylene to Ethylene Oxide by Silver Catalysts: Active Sites, Reaction Mechanism and Role of Promoters The present thesis is motivated from the insightful recognition of major inconsistencies in the literature of ethylene oxidation over Ag based catalysts, a simple yet complicated model reaction with long-lasting history since 1940s, that possesses large economic potential in the present and future. The following fundamental questions were raised, which is embeded in the entire thesis research, namely i) What is the nature of active sites on Ag based catalysts? ii) How to utilize the current available characterization techniques under reaction condition to advance our fundamental understanding? and iii) How to establish a proper structure � activity relationship?We first contributed a critical review of the ethyelene epoxidation by Ag catalysts literature (Chapter 1), which covers what is known and what issues still need to be resolved. Major gaps are found in the understandings of the selective oxygen species for ethylene epoxidation to EO. The reaction studies were conducted over single crystals, powder/foils and supported Ag catalysts under distinct conditions (ultrahigh vacuum � tens of bar, various oxygen coverage, etc). The assignments of oxygen species in spectroscopic studies are often empirical and lack the rigorous proof from density functional theory (DFT) calculations. The computational modelings adopted the metallic Ag surface which does not reflect the actual surface oxidized state of Ag under ethylene oxidation reaction condition. The above issues, as well as others, have hindered the correct understanding of the reaction mechanism for decades. The application of Surface Enhanced Raman Spectroscopy (SERS) under controlled environmental conditions and coupled with DFT calculations holds great promise in resolving the fundamental issues. Our first published experimental work (Chapter 2) started with a systematic study of the nature of the oxygen species on high purity bulk Ag powder under ethylene oxidation reaction conditions. Three important findings were drawn from this work that i) The Ag surface is covered by a thin oxide layer (1-3 nm) during ethylene oxidation. ii) DFT calculations suggest the oxidic reconstructed p(4 � 4)?O?Ag(111) surfaces is best representative of Ag surface under reaction conditions. iii) The selective oxygen species for EO formation over bulk Ag is surface Ag4-O2*, which vibrates at 650-1000 cm-1. The studies of oxygen species on supported Ag/?-Al2O3 catalysts is reported in Chapter 3. This work aims to resolve the conflicts in the literature regarding the reaction mechanism (L-H? E-R? MvK?) and selective oxygen species (atomic, molecular, other types of oxygen?) responsible for EO formation. Multiple spectroscopic techniques, including SERS and high sensitivity � low energy ion scattering (HS-LEIS) were utilized in combination with isotopic labelling studies and temperature programmed surface reaction (TPSR). The findings suggest that ethylene oxidation by supporetd Ag/?-Al2O3 catalysts follows a combined L-H and MvK mechanism, with Ag4-O2* species responsible for epoxidation of ethylene. With the above knowledge of the supported Ag/?-Al2O3 catalyst, the promotional effect of Cl was investigated in Chapter 4 since Cl is arguably the most important promoter in the industrial process for high EO selectivity. Previous literature studies proposed both geometric (site blocking) as well as electronic effect of the electon-withdrawing Cl on the Ag catalyst in ethylene oxidation, which has been confirmed with combined experimental and theoretical approaches. In this chapter, we exhibit the generation of surface Ag?+ species with Cl addition using near ambient pressure � X-ray photoelectron spectroscopy (NAP-XPS), as well as measurements of number of active sites (Nsite) and turnover frequencies (TOF) for ethylene oxidation over promoted and unpromoted catalysts. The dispersion and location of ppm levels of the Cs and Re promoter were investigated in Chapter 5 with the aid of in situ HAADF-STEM-EDS as well as quasi in situ HS-LEIS. For the first time in the literature, we demonstrate the dynamics of promoted and unpromoted Ag particles on the ?-Al2O3 support under ethylene oxidation reaction environments with in situ HAADF-STEM. Combination of Cs and Re promotion stabilized the Ag particles against sintering during ethylene oxidation. Corresponding EDS mapping revealed that the Re and Cs promoters are present on both the Ag particles and ?-Al2O3 support, but more concentrated on Ag. Combined HS-LEIS and SERS measurements showed thate the Cs and Re promoters are present as CsxReO4 and CsxCO3 nanoparticles on the surface of the Ag particles. The Cs and Re promoters suppress EO combustion while also suppressing ethylene oxidation activity and CO2 formation through selective site blocking. The combined Cs+Re promoters increase the concentration of surface Ag4-O2 species, which is suggested to be responsible for EO formation. The synergestic effect between Cl and Cs, Re promoters in the highly selective Ag(CsReCl)/?-Al2O3 catalyst is lastly investigated (Chapter 6). It is observed that Cl promotes dispersion of Cs and Re species, thus optimizing the ensemble effect in improving EO selectivity We drew the conclusions from the observations and methodologies developed in the present investigation of this thesis work that i) The surface of Ag is oxidized under ethylene oxidation reaction conditions (both unsupported and supported Ag catalysts). In the case of the promoted Ag/?-Al2O3 catalyst, promoters are mostly concentrated on surface with the outermost surface layer of the supported catalyst containing only a minor amount of Ag. ii) The Ag4-O2* species is responsible for the formation of EO. iii) Cl, Cs and Re have both geometric (poisoning) and electronic effects on the promoted Ag/?-Al2O3 catalysts for ethylene oxidation. Several advanced techniques, including in situ XAS, TAP as well as machine learning � DFT algorithms are suggested for future studies to further advance the understandings of the Ag-EO catalytic system. Ethylene ; Catalysis ; Density functional theory ; Reaction mechanism ; Ethylene oxide ; Oxidation state ; Oxide ; Oxygen ; Reaction conditions Chemical and Physical Properties of Materials ; Catalysis and Oxidation Reactions ; Catalytic Processes in Materials Science