Ag–Cu alloy surfaces in an oxidizing environment: A first-principles study

Piccinin, Simone; Stampfl, Catherine; Scheffler, Matthias

doi:10.1016/j.susc.2008.10.050

Cited by 39 publications

(52 citation statements)

References 55 publications

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“…The formation of the CuO-like layer is in agreement with theoretical work that has predicted bulk copper oxides are stable at high copper concentrations. 10 This finding also explains the plateau in selectivity observed by Linic et. al.…”

Section: Discussionsupporting

confidence: 55%

“…If the surface were truly to remain metallic under epoxidation conditions, with Cu atoms distributed over the surface, the Cu concentration and O chemical potentials would need to be extremely low. 10 It is therefore likely that either the surface is completely coated with some form of oxidized copper, or all the Cu has segregated to Cu 2 O/CuO regions, leaving regions of pure Ag behind. This scenario is depicted in Figure 9 D. If this were the case, then what role would copper play?…”

Section: Discussionmentioning

confidence: 99%

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Phase Coexistence of Multiple Copper Oxides on AgCu Catalysts during Ethylene Epoxidation

et al. 2018

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Alloy catalysts under reaction conditions are complex entities. In oxidizing atmospheres multiple phases can coexist on a catalyst's surface as a result of phase segregation and preferential oxidation. Such a scenario can result in unusual sub-stoichiometric and meta-stable phases that could play important roles in catalytic processes. For instance, AgCu alloys-known to exhibit enhanced epoxide selectivity in partial oxidation of ethylene-form an oxide-like surface structure under reaction conditions. Under these conditions, copper oxides are stable, while silver oxides are not. Consequently, copper segregates to the alloy's surface and forms an oxide overlayer. Little is known about the structure or function of such overlayers, and it is unknown whether they play an active role in the catalyst's enhanced selectivity. In order to develop a clearer picture of such catalysts, the current work utilizes several in-situ spectroscopic and microscopic techniques to examine the copper oxide phases that form when AgCu is exposed to epoxidation conditions. It is found that several forms of oxidic Cu coexist simultaneously on the active catalyst's surface, namely CuO, Cu 2 O and some previously unreported form of oxidized Cu, referred to here as Cu x O y. On-line product analysis, performed during the in-situ spectroscopic measurements, shows that increased epoxide selectivity is correlated with the presence of mixed copper oxidation states and the presence of the Cu x O y species. These results support previous theoretical predictions that oxidic copper overlayers on silver play an active role in epoxidation. These results furthermore emphasize the need for in-situ spectromicroscopic methods to understand the complexity of alloy catalysts.

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Section: Discussionsupporting

confidence: 55%

Section: Discussionmentioning

confidence: 99%

Phase Coexistence of Multiple Copper Oxides on AgCu Catalysts during Ethylene Epoxidation

et al. 2018

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“…The on average rather Ir-rich surface composition observed in the as-prepared sample reects the preferential surface conguration of this alloy under vacuum conditions, which can be rationalised by the lower surface energy of Ir as compared to Ru. 48,49 In the presence of a gaseous adsorbate, it is known that the surface compositions of alloys can substantially change, or be inverted, 35,[50][51][52] as discussed below.…”

Section: Validation Of Proposed Experimental Protocolmentioning

confidence: 99%

Probing catalytic surfaces by correlative scanning photoemission electron microscopy and atom probe tomography

et al. 2020

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The chemical composition and the electronic state of the surface of alloys or mixed oxides with enhanced electrocatalytic properties are usually heterogeneous at the nanoscale. The non-uniform distribution of the potential across their surface affects both activity and stability. Studying such heterogeneities at the relevant length scale is crucial for understanding the relationships between structure and catalytic behaviour. Here, we demonstrate an experimental approach combining scanning photoemission electron microscopy and atom probe tomography performed at identical locations to characterise the surface's structure and oxidation states, and the chemical composition of the surface and sub-surface regions. Showcased on an Ir-Ru thermally grown oxide, an efficient catalyst for the anodic oxygen evolution reaction, the complementary techniques yield consistent results in terms of the determined surface oxidation states and local oxide stoichiometry. Significant chemical heterogeneities in the sputter-deposited Ir-Ru alloy thin films govern the oxide's chemistry, observed after thermal oxidation both laterally and vertically. While the oxide grains have a composition of Ir 0.94 Ru 0.06 O 2 , the composition in the grain boundary region varies from Ir 0.70 Ru 0.30 O 2 to Ir 0.40 Ru 0.60 O 2 and eventually to Ir 0.75 Ru 0.25 O 2 from the top surface into the depth. The influence of such compositional non-uniformities on the catalytic performance of the material is discussed, along with possible engineering levers for the synthesis of more stable and reactive mixed oxides. The proposed method provides a framework for investigating materials of interest in the field of electrocatalysis and beyond.

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“…Using the ab initio atomistic thermodynamics approach, Piccinin et al have studied the surface structure of the Ag-Cu alloy in the presence of oxygen. [40][41][42] They have shown that the strength of Cu-O bond relative to the Ag-O bond drives the segregation of Cu to the surface. Their main finding is that under conditions of pressure and temperature relevant for ethylene epoxidation, a thin copper-oxide-like layer is predicted to form at the surface.…”

Section: Selected Theoretical Studies Of Bi-metallic Alloys In An Oxymentioning

confidence: 99%

Alloys in catalysis: phase separation and surface segregation phenomena in response to the reactive environment

Zafeiratos

Piccinin

Teschner

2012

Catal. Sci. Technol.

Self Cite

213

182

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Alloys play a crucial role in several heterogeneous catalytic processes, and their applications are expected to rise rapidly. This is essentially related to the vast number of configurations and type of surface sites that multi-component materials can afford. It is well established that the chemical composition at the surface of an alloy usually differs from that in the bulk. This phenomenon, referred to as surface segregation, is largely controlled by the surface free energy. However, surface energy alone cannot safely predict the active surface state of a solid catalyst, since the contribution of other parameters such as size and support effects, as well as influence of the adsorbates, play a major role. This can lead to numerous surface configurations as for example over the length of a catalytic reactor, as the chemical potential of the gas phase changes continuously over the catalyst bed and hence different reactions may prevail at different catalyst bed segments. Thanks to the rapid improvement of the analytical surface science characterization techniques and theoretical methodologies, the potential effects induced by alloyed catalysts are better understood. For catalysis, the relevance of measurements performed on well-defined surfaces under idealized ultrahigh vacuum conditions has been questioned and studies in environments that closely resemble conditions of working alloy catalysts are needed. In this review we focus on experimental and theoretical studies related to in situ (operando) observations of surface segregation and phase separation phenomena taking place on the outermost surface layers of alloy catalysts. The combination of first principles theoretical treatment and in situ observation opens up new perspectives of designing alloy catalysts with tailored properties

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Ag–Cu alloy surfaces in an oxidizing environment: A first-principles study

Cited by 39 publications

References 55 publications

Phase Coexistence of Multiple Copper Oxides on AgCu Catalysts during Ethylene Epoxidation

Phase Coexistence of Multiple Copper Oxides on AgCu Catalysts during Ethylene Epoxidation

Probing catalytic surfaces by correlative scanning photoemission electron microscopy and atom probe tomography

Alloys in catalysis: phase separation and surface segregation phenomena in response to the reactive environment

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