We have prepared and characterized atomically well-defined model systems for ceria-supported Pt-Co core-shell catalysts. Pt@Co and Co@Pt core-shell nanostructures were grown on well-ordered CeO2(111) films on Cu(111) ...We have prepared and characterized atomically well-defined model systems for ceria-supported Pt-Co core-shell catalysts. Pt@Co and Co@Pt core-shell nanostructures were grown on well-ordered CeO2(111) films on Cu(111) by physical vapour deposition of Pt and Co metals in ultrahigh vacuum and investigated by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy. The deposition of Co onto CeO2(111) yields CoCeO2(111) solid solution at low Co coverage(0.5 ML), followed by the growth of metallic Co nanoparticles at higher Co coverages. Both Pt@Co and Co@Pt model structures are stable against sintering in the temperature range between 300 and 500 K. After annealing at 500 K, the Pt@Co nanostructure contains nearly pure Co-shell while the Pt-shell in the Co@Pt is partially covered by metallic Co. Above 550 K, the re-ordering in the near surface regions yields a subsurface Pt-Co alloy and Pt-rich shells in both Pt@Co and Co@Pt nanostructures. In the case of Co@Pt nanoparticles, the chemical ordering in the near surface region depends on the initial thickness of the deposited Pt-shell. Annealing of the Co@Pt nanostructures in the presence of O2 triggers the decomposition of Pt-Co alloy along with the oxidation of Co, regardless of the thickness of the initial Pt-shell. Progressive oxidation of Co coupled with adsorbate-induced Co segregation leads to the formation of thick CoO layers on the surfaces of the supported Co@Pt nanostructures. This process is accompanied by the disintegration of the CeO2(111) film and encapsulation of oxidized Co@Pt nanostructures by CeO2 upon annealing in O2 above 550 K. Notably, during oxidation and reduction cycles with O2 and H2 at different temperatures, the changes in the structure and chemical composition of supported Co@Pt nanostructures were driven mainly by oxidation while reduction treatments had little effect regardless of the initial thickness of the Pt-shell.展开更多
Electrocatalytic activity is influenced by the surface charge on the solid catalyst.Conventionally,our attention has been focused on how the surface charge shapes the electric potential and concentration of ionic reac...Electrocatalytic activity is influenced by the surface charge on the solid catalyst.Conventionally,our attention has been focused on how the surface charge shapes the electric potential and concentration of ionic reactant(s)in the local reaction zone.Taking H_(2)O_(2)redox reactions at Pt(111)as a model system,we reveal a peculiar surface charge effect using ab initio molecular dynamics simulations of electrified Pt(111)-water interfaces.In this scenario,the negative surface charge on Pt(111)repels the O-O bond of the reactant(H_(2)O_(2))farther away from the electrode surface.This leads to a higher activation barrier for breaking the O-O bond.Incorporating this microscopic mechanism into a microkinetic-double-layer model,we are able to semi-quantitatively interpret the pH-dependent activity of H_(2)O_(2)redox reactions at Pt(111),especially the anomalously suppressed activity of H_(2)O_(2)reduction with decreasing electrode potential.The relevance of the present surface charge effect is also examined in wider scenarios with different electrolyte cations,solution pHs,crystal facets of the catalyst,and model parameters.In contrast with previous mechanisms focusing on how surface charge influences the local reaction condition at a fixed reaction plane,the present work gives an example in which the location of the reaction plane is adjusted by the surface charge.展开更多
基金funded by the European Community(FP7-NMP.2012.1.1-1 project chip CAT,Reference No.310191)by the Deutsche Forschungsgemeinschaft(DFG)within the Excellence Cluster“Engineering of Advanced Materials”in the framework of the excellence initiative+2 种基金support by the DFG is acknowledged through the Priority Program SPP 1708 and the Research Unit FOR 1878supported by structural funds under project CZ.02.1.01/0.0/0.0/16_025/0007414by the Czech Ministry of Education(grant LM2015057)。
文摘We have prepared and characterized atomically well-defined model systems for ceria-supported Pt-Co core-shell catalysts. Pt@Co and Co@Pt core-shell nanostructures were grown on well-ordered CeO2(111) films on Cu(111) by physical vapour deposition of Pt and Co metals in ultrahigh vacuum and investigated by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy. The deposition of Co onto CeO2(111) yields CoCeO2(111) solid solution at low Co coverage(0.5 ML), followed by the growth of metallic Co nanoparticles at higher Co coverages. Both Pt@Co and Co@Pt model structures are stable against sintering in the temperature range between 300 and 500 K. After annealing at 500 K, the Pt@Co nanostructure contains nearly pure Co-shell while the Pt-shell in the Co@Pt is partially covered by metallic Co. Above 550 K, the re-ordering in the near surface regions yields a subsurface Pt-Co alloy and Pt-rich shells in both Pt@Co and Co@Pt nanostructures. In the case of Co@Pt nanoparticles, the chemical ordering in the near surface region depends on the initial thickness of the deposited Pt-shell. Annealing of the Co@Pt nanostructures in the presence of O2 triggers the decomposition of Pt-Co alloy along with the oxidation of Co, regardless of the thickness of the initial Pt-shell. Progressive oxidation of Co coupled with adsorbate-induced Co segregation leads to the formation of thick CoO layers on the surfaces of the supported Co@Pt nanostructures. This process is accompanied by the disintegration of the CeO2(111) film and encapsulation of oxidized Co@Pt nanostructures by CeO2 upon annealing in O2 above 550 K. Notably, during oxidation and reduction cycles with O2 and H2 at different temperatures, the changes in the structure and chemical composition of supported Co@Pt nanostructures were driven mainly by oxidation while reduction treatments had little effect regardless of the initial thickness of the Pt-shell.
文摘Electrocatalytic activity is influenced by the surface charge on the solid catalyst.Conventionally,our attention has been focused on how the surface charge shapes the electric potential and concentration of ionic reactant(s)in the local reaction zone.Taking H_(2)O_(2)redox reactions at Pt(111)as a model system,we reveal a peculiar surface charge effect using ab initio molecular dynamics simulations of electrified Pt(111)-water interfaces.In this scenario,the negative surface charge on Pt(111)repels the O-O bond of the reactant(H_(2)O_(2))farther away from the electrode surface.This leads to a higher activation barrier for breaking the O-O bond.Incorporating this microscopic mechanism into a microkinetic-double-layer model,we are able to semi-quantitatively interpret the pH-dependent activity of H_(2)O_(2)redox reactions at Pt(111),especially the anomalously suppressed activity of H_(2)O_(2)reduction with decreasing electrode potential.The relevance of the present surface charge effect is also examined in wider scenarios with different electrolyte cations,solution pHs,crystal facets of the catalyst,and model parameters.In contrast with previous mechanisms focusing on how surface charge influences the local reaction condition at a fixed reaction plane,the present work gives an example in which the location of the reaction plane is adjusted by the surface charge.
