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Ag-Cu catalysts for ethylene epoxidation: Selectivity and activity descriptors
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Figures

Image of FIG. 1.

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FIG. 1.

The top-view (top panel) and side-view (bottom panel) of the most stable structures at the given Cu surface content. In the top panel, the small red atoms represent oxygen on the first thin oxide layer, the large light-brown ones represent copper on the first thin oxide layer, the small dark-red ones represent sub-surface oxygen, the large dark-brown ones represent copper on the second layers, and the large white ones represent silver.

Image of FIG. 2.

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FIG. 2.

Surface phase diagram showing the surface free energy at Δμ = −0.61 eV ( = 600 K, = 1 atm), for the structures as the function of Cu surface content, while changing the O coverage.

Image of FIG. 3.

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FIG. 3.

Energy profiles for ethylene epoxidation over the selected surface structures. At the initial state (IN) ethylene is physisorbed on the surface, the intermediate state is the oxametallacycle (OMC), while the final state (FIN) of the reaction is either acetaldehyde (Ac, red solid line) or ethylene oxide (EO, blue dashed line) physisorbed on the surface. In the case of CuO/Ag(111), the formation of EO can proceed directly from the initial state without involving the OMC intermediate (black dashed line).

Image of FIG. 4.

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FIG. 4.

Transition state geometries for the formation of acetaldehyde (a)–(f) and ethylene oxide (g)–(l) for the six low energy structures considered in this work. The red, brown, gray, yellow, and green spheres represent O, Cu, Ag, C, and H atoms, respectively.

Image of FIG. 5.

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FIG. 5.

Comparison of the ability of two selectivity indicators to predict the difference in activation energy for the two reaction paths considered. Δ* are the values obtained through NEB calculations, are those obtained using the selectivity indicators. Left panel: selectivity indicator proposed by Kokalj Right panel: selectivity indicator obtained by refitting the parameters of Kokalj's indicator. Circles represent the data set used by Kokalj , squares the structures computed in this work. Red symbols indicate Cu-free structures, blue symbols Cu-containing structures.

Image of FIG. 6.

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FIG. 6.

Activation energies for the formation for Ac (left panel) and EO (right panel) obtained from NEB calculations (*), and from the indicators in Eqs. (3) and (4) , ( ). The meaning of the symbols is the same as in Fig. 5 .

Tables

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Table I.

For all the structures considered in this work we report coverages of Cu and O (in ML), activation energies and their difference (Δ*) computed using NEB, binding energies of the OMC intermediate, O, CH, and CHO radicals, and the final products Ac and EO. The last column shows the value of the selectivity indicator ( ). All energies are in eV. The (a) and (b) superscripts refer to two different geometries for the OMC intermediate.

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/content/aip/journal/jcp/138/18/10.1063/1.4803157
2013-05-10
2014-04-19

Abstract

Ag-Cu alloy catalysts for ethylene epoxidation have been shown to yield higher selectivity towards ethylene oxide compared to pure Ag, the unique catalyst employed in the industrial process. Previous studies showed that under oxidizing conditions Cu forms oxide layers on top of Ag. Using first-principles atomistic simulations based on density functional theory, we investigate the reaction mechanism on the thin oxide layer structures and establish the reasons for the improved selectivity. We extend the range of applicability of the selectivity descriptor proposed by Kokalj et al. [J. Catal.254, 304 (Year: 2008)]10.1016/j.jcat.2008.01.008, based on binding energies of reactants, intermediates, and products, by refitting its parameters so as to include thin oxide layer catalysts. We show that the selectivity is mainly controlled by the relative strength of the metal-carbon vs. metal-oxygen bonds, while the height of the reaction barriers mostly depend on the binding energy of the common oxametallacycle intermediate.

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Scitation: Ag-Cu catalysts for ethylene epoxidation: Selectivity and activity descriptors
http://aip.metastore.ingenta.com/content/aip/journal/jcp/138/18/10.1063/1.4803157
10.1063/1.4803157
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