Resolving the Atomic Structure of Vanadia Monolayer Catalysts: Monomers, Trimers, and Oligomers on Ceria

Abstract

Supported vanadium oxide catalysts have received considerable attention owing to their high activity for selective oxidation reactions. [1][2][3][4][5] The reactivity has been shown to depend strongly on the oxide support, [2][3][4][5] with reducible oxides (e.g., ceria, titania, and zirconia) exhibiting much higher turnover frequencies for oxidative dehydrogenation (ODH) reactions than irreducible oxides (e.g., silica and alumina). [3,5] Structural characterization of the catalysts has been performed primarily using Raman and UV/Vis spectroscopy (see Ref. [4,6,7] and references therein), as well as X-ray absorption spectroscopy.[8] These results have been used to postulate that vanadia catalysts consist of isolated and polymer structures that wet the supporting oxide (so-called "monolayer catalysts"). To elucidate the surface chemistry of vanadia, different model systems, such as vanadia single crystals [9] and thin films [10] as well as vanadia clusters supported on planar metal oxide substrates, [11][12][13][14][15] have been studied experimentally by surface-science techniques and computational means. [16,17] To rationalize structure-reactivity relationships, welldefined systems are required. Of the reducible metal oxide supports that are known to be highly active in ODH reactions, ceria is particularly suited, because well-ordered thin films can be grown with a known surface termination. [18,19] Previously, the structure and reactivity of vanadia supported on CeO 2 (111) has been studied using photoelectron spectroscopy (PES) and temperature-programmed desorption (TPD). [14,15] However, the atomic structure of ceria-supported vanadia monolayer catalysts has not been resolved.Herein, using a combination of high-resolution scanning tunneling microscopy (STM), infrared reflection absorption spectroscopy (IRAS), and PES with synchrotron radiation, we unambiguously demonstrate the formation of monomeric O=V 5+ O 3 species on the CeO 2 (111) surface at low vanadia loadings. For the first time, we show a direct relationship between the nuclearity of vanadia species (monomeric vs. polymeric) as observed by STM and their vibrational properties. We show that ceria stabilizes the vanadium + 5 oxidation state, leading to partially reduced ceria upon vanadium deposition. These experimental results are fully supported by density functional theory (DFT) calculations. The results indicate that ceria surfaces stabilize small vanadia species, such as monomers and trimers, that sinter into two-dimensional, monolayer islands. Such stabilization probably plays a crucial role in the enhanced activity observed for ceriasupported vanadia in ODH reactions. Indeed, low-nuclearity species revealed reactivities at much lower temperatures [20] than those with higher nuclearity, which in turn show strong similarities to the reactivity of vanadia clusters supported on alumina and silica. [11,13] Figure 1 presents compelling evidence for the presence of vanadia monomers on ceria at low coverage (ca. 0.3 V atoms nm À2 ). The STM image in Fi...

Model Studies on Heterogeneous Catalysts at the Atomic Scale

“…The Sauer group has theoretically described the binding of vanadia monomers on the ceria surface using density functional calculations [17]. Their results are in very good agreement with the microscopic and spectroscopic information, and allow, beyond the possibilities of the experiment, predictions on the stability of vanadia species on the surface.…”

“…The photoelectron spectra recorded for the same set of preparations exhibits further interesting structure-property relations: The V2p spectra of the vanadia deposits reveal an oxidation state of V 5+ [27], compatible with a fourfold coordinated vanadyl containing species and indicating that the support is actively involved in determining the electronic structure through redox processes. As the coverage increases V2p intensities increase but the local electronic structure does not change until the second layer starts to be populated as shown by STM [17], and V2p bands characteristic of V 3+ appear in the spectra, revealing considerable changes in the electronic structure. The valence band photoemission features are only modified moderately, except the feature between the valence band and the Fermi-energy, which increases as the vanadia loading increases.…”

“…Fig. 3 shows a wide scan STM image of a CeO 2 (111) surface grown as a thin film (ca 3 nm in thickness) on a Ru(0001) single crystal [17]. The inset reveals the atomic structure of the surface.…”

“…Their concentration as well as their charge state may be controlled by the concentration of dopants [16]. In the present paper we will discuss two examples for the influence of dopants by studying the activity of very small vanadia particles deposited onto a ceria surface with respect to methanol oxidation [17], and the influence of Mo dopants within CaO onto adsorption of oxygen from the gas phase [18].…”

Oxygen activation on oxide surfaces: A perspective at the atomic level

“…Recently, comprehensive studies of the structure of vanadia clusters on CeO 2 (1 1 1) thin films as a function of vanadia coverage have been performed [3]. Fig.…”