The role of exposed silver in CO oxidation over MgO(0 0 1)/Ag(0 0 1) thin films

Abstract

The reactivity of MgO(001) films deposited on Ag(001) and Mo(001) in CO oxidation as a function of oxide film thickness was investigated experimentally at ambient pressure reaction conditions. MgO films grown on Mo(001) were found to be inactive in CO oxidation, whereas activity enhancement with decreasing oxide film thickness was observed for MgO(001)/Ag(001). In-situ infrared and post-reaction X-ray photoemission data showed that ultra-thin MgO films interact much more strongly with the reactants and residual water than bulk-like MgO. Poisoning of the MgO surfaces by the resultant accumulation of carbonate and hydroxyl species is suggested to inhibit the reactivity in CO oxidation. Detailed investigations of the surface structure of MgO(001)/Ag(001) films indicate that silver adislands on Ag(001), which are formed during MgO growth, are responsible for the enhancement of CO oxidation activity over ultrathin MgO(001)/Ag(001) films.

Structure and Reactivity of a Model Oxide Supported Silver Nanocluster Catalyst Studied by Near Ambient Pressure X-ray Photoelectron Spectroscopy

“…These higher energy features were not observed in spectra recorded from an Ag(001) exposed to 5 mbar CO and subjected to "post-mortem" analysis. 49 Figure 4b shows that removal of the CO/H 2 O gas in order to return to UHV conditions leads to the disappearance of both sets of doublets introduced by the exposure to the gas mixture. These observations suggest that these higher binding energy peaks are associated with the interaction of CO with the Ag clusters, but their origin cannot be unambiguously determined from these data.…”

“…The lower binding energy peaks in the Ag spectra are consistent with the oxidation of some Ag moieties 48 and have been observed following CO oxidation on an Ag(001) surface. 49 This downward shift appears anomalous, since an increase in oxidation state is usually associated with an increase in binding energy for metal cations in ionic systems. Although the downward shift in binding energy of oxidized silver was first reported over 40 years ago by Schon et al, 50 the origin of the shift is still unclear.…”

“…48 The precise oxidation state of the Ag can not be determined from these measurements, but the binding energy agrees well with the formation of Ag δ+ . 49 The origin of the peaks at 371.4 and 377.5 eV is less clear. The binding energies are consistent with Ag plasmon loss peaks, but if this were the case then one would also expect to observe them on the Ag clusters before exposure to H 2 O/CO.…”

Reactive metal–oxide interfaces: A microscopic view

“…These films are often exploited for the study of complex chemical processes involved in heterogeneous catalysis. Phenomena like water interaction with oxide surfaces [164][165][166][167] and CO oxidation [168,169] have been deeply investigated thanks to the use of these model systems. A prerequisite to successfully rationalize the complex chemical reactions occurring at oxides surfaces relies on the possibility to obtain highly ordered films, allowing their investigation by means of scanning probe techniques [64,170] and a direct comparison with theoretical models [171].…”

A DFT study of (WO3)3nanoclusters adsorption on defective MgO ultrathin films on Ag(001)

“…20 Furthermore, except that introducing diverse nature of support defects, a continuously increasing interest has also been devoted to another form of the support material, ultrathin oxide lms grown on metal substrates, to control the congurations and electronic properties of the monodispersed oxide clusters. [21][22][23][24][25][26] These systems exhibit unusual chemical and electronic properties with respect to thicker lms or single crystal oxide surfaces, due to the presence of the dielectric boundary and the reduced dimensionality of the insulating lms, offering new opportunities for the design of new functional materials. [27][28][29][30] One specic and crucial property of ultrathin oxide lms is the occurrence of a spontaneous charge transfer from the metal support to an adsorbed species through the thin insulating layer (or vice versa).…”