Design of CuCs-doped Ag-based Catalyst for Ethylene Epoxidation

Our recent theoretical studies have screened out CuCs-doped Ag-based promising catalysts for ethylene epoxidation[ACS Catal.11,3371(2021)].The theoretical results were based on surface modeling,while in the actual reaction process Ag catalysts are particle shaped.In this work,we combine density functional theory(DFT),Wulff construction theory,and micro kinetic analysis to study the catalytic performance of Ag catalysts at the particle model.It demonstrates that the CuCs-doped Ag catalysts are superior to pure Ag catalysts in terms of selectivity and activity,which is further proved by experimental validation.The characterization analysis finds that both Cu and Cs dopant promote particle growth as well as particle dispersion,resulting in a grain boundary-rich Ag particle.Besides,CuCs also facilitate electrophilic atomic oxygen formation on catalyst surface,which is benefitial for ethylene oxide formation and desorption.Our work provides a case study for catalyst design by combining theory and experiment.

Metal catalyzed ethylene epoxidation:A comparative density functional theory study

Ethylene epoxidation on Ag（111）, Pt（111）, Rh（111） and Mo（100） has been studied by density functional theory （DFT） calculations. The results show that the adsorption energies of possible adsorbed species involved in the ethylene epoxidation increase in the order： Ag〈Pt〈Rh〈Mo, and the activation energies of the formation of epoxide （EtO） and acetaldehyde （Ac） follow the same order. Moreover, it is found that the smallest difference in the activation energies between EtO formation and Ac formation is shown on Ag. These results indicate that the metallic Ag shows the highest between activity and selectivity for ethylene epoxidation among the studied metal surfaces. Perhaps, the stability of OMME intermediate is the crucial factor in controlling the activity and selectivity. And the stronger the binding of OMME, the lower the activity and selectivity are. In addition, the relationships between the reaction enthalpy and activation energy on these four metal surfaces are investigated, and it is found that such a correlation is only applied for OMME（a） → EtO（a） and OMME（a） → Ac（a）, while invalid for the case of C2H4（a） ＋ O（a） → OMME（a）.

Macrokinetics of Ethylene Epoxidation over A-type Silver Catalyst

By taking the surface chemical reactions as the rate-controlling step, a possible reaction mechanism for ethylene epoxidation to synthesize ethylene oxide over the A-type silver catalyst was developed, while it was assumed that the epoxidation reaction would take place between ethylene and the un-dissociated adsorbed oxygen O2 a on the solo active sites, while the deep oxidation would occur between ethylene and the dissociated adsorbed oxygen Oa on the adjacent multi-active sites. In order to describe the effect of 1,2-C2H4Cl2(EDC) inhibitor on the ethylene epoxidation process, the reversible reactions between EDC and vinyl chloride(VC) on the active sites of silver catalyst was introduced. According to the assumed mechanism, the hyperbolic macro-kinetic model of ethylene epoxidation over the A-type silver catalyst was established, and the macrokinetic experiments were carried out in an internal-recycle gradientless reactor operating at a pressure of 2.1 MPa and a temperature in the range of 217.8—249.0 ℃, with the gas composition(molar fraction) consisting of 15.82%—34.65% C2H4, 2.55%—7.80% O2, 0.88%—6.15% CO2, 0.15—2.61 μmol/mol of 1,2-C2H4Cl2 and 0.14—1.28 μmol/mol of C2H3 Cl. By means of the Simplex Optimal Method, the parameters of the macrokinetic models were estimated. Statistical test showed that the macrokinetic models developed for the A-type silver catalyst agree well with the experimental results.

Non-noble metal single-atom catalysts with phosphotungstic acid(PTA)support:A theoretical study of ethylene epoxidation

Geometric and electronic structures of phosphotungstic acid(PTA)supported single transition metal atom(Fe,Co,Ni,Ru,Rh,Pd,Os,Ir and Pt)catalysts have been systematically investigated by using the first-principles theoretical methods.Possible reaction mechanism for ethylene epoxidation was explored.The most possible anchoring site for the single transition metal atom is the fourfold hollow site on PTA.As the non-noble metal Fe1-PTA system possesses considerable adsorption energies towards both O2 and C2H4,the strong bonding interaction between Fe1 and PTA cluster was analyzed.It is found that the electron transfers from Fe atom to PTA cluster and strong covalent metal-support interactions(CMSI)between the Fe 3 d orbitals and O 2 p orbitals of PTA lay the foundation of high stability.The proposed catalytic reaction mechanism for ethylene epoxidation on Fe1-PTA single-atom catalyst(SAC)includes three steps:the O2 adsorbs on Fe1-PTA via electron transfer;the first ethylene attacks the adsorbed O2 molecule on Fe1-PTA followed by the formation of C2H4O;finally,the O atom remained on Fe1-PTA reacts with a second ethylene to form the product and accomplish the catalytic cycle.The Fe1-PTA has high selectivity and catalytic activity for ethylene epoxidation via an Eley–Rideal mechanism with low energy barriers.A potentially competitive pathway for the formation of acetaldehyde is not kinetically favorable.These results provide insights for the development of highly efficient heterogeneous SACs for ethylene epoxidation with non-noble metals.