Anomalous Surface Compositions of Stoichiometric Mixed Oxide Compounds

Abstract

Surface-oxide films are present in many types of oxidecontaining materials, such as grain boundaries in ceramics, [1] interfaces in ceramic-ceramic [2] and metal-oxide systems, [3] and affect their materials and transport properties. In heterogeneous catalysis, the properties of the outermost surface layer are of prime importance because they control the catalytic performance. Although bulk mixed-metal oxide catalysts are widely used in industrial selective oxidation processes, [4,5] not much is known about their outermost surface composition. Models based on surfaces derived from a truncation of the bulk structure have dominated discussion on catalytic reaction mechanisms and active sites (reviewed, for example, in Ref. [6]). This view has been questioned by several recent studies reporting the surface enrichment and depletion phenomena in solid-oxide solutions (e.g., Co x Ni 1Àx O [7] ), the identification of TiO 2 -rich overlayers on reconstructed SrTiO 3 (001) model surfaces, [8] and evidence for the formation of amorphous oxide overlayers in which there is surface enrichment of one of the components under selective oxidation reaction conditions. [9,10] However, the development of realistic concepts on reactant activation, surface reaction mechanisms, and the design of advanced catalytic materials are still hampered by the lack of detailed knowledge of the surface composition and structure of bulk mixed-metal oxides.For such studies, X-ray photoelectron spectroscopy (XPS) with laboratory sources is of limited value because its average sampling depth of 1-3 nm results in a signal where the outermost surface layer only contributes on the order of 30 %. Synchrotron radiation allows for increasing the surface sensitivity of XPS by decreasing excitation and, hence, photoelectron kinetic energies. Exclusive information on the outermost surface layer, however, is only given by low-energy ion scattering (LEIS) because ions penetrating below the surface become largely neutralized.[11]The surfaces of stoichiometric bulk mixed-metal molybdates and vanadates have also been characterized through their interactions with probe molecules, for example, CH 3 OH, [12][13][14][15] which allows CH 3 O* and intact CH 3 OH* intermediates on different surface cations to be discriminated by IR spectroscopy. For such materials, combined methanol chemisorption and oxidation kinetic studies suggested a strong surface enrichment of MoO x or VO x . [12,14,15] In methanol oxidation studies, similar catalytic turnover frequencies were found over bulk mixed-metal oxides and related supported metal oxides (e.g., Fe 2 (MoO 4 ) 3 and MoO 3 /Fe 2 O 3 ), which supports the idea of surface MoO x enrichment of the bulk phases. [16][17][18][19] These observations, however, are qualitative as exposed metal oxide ions of low catalytic activity would not be detected by the test reaction. Thus, we have undertaken a study of the outermost surface compositions of such compounds by LEIS and excitation-energy resolved XPS (ERXPS). The LEIS was applied in sp...

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