The effect of the suppport on oxidative dehydrogenation activity for vanadia/ceria systems is examined for the oxidation of methanol to formaldehyde by use of well-defined VO(x)/CeO(2)(111) model catalysts. Temperature-programmed desorption at low vanadia loadings revealed reactivity at much lower temperature (370 K) as compared to pure ceria and vanadia on inert supports such as silica. Density functional theory is applied and the energies of hydrogenation and oxygen vacancy formation also predict an enhanced reactivity of the vanadia/ceria system. At the origin of this support effect is the ability of ceria to stabilize reduced states by accommodating electrons in localized f-states.

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...

We model monomeric vanadia adspecies on the CeO 2 (111) surface of composition VO n 3 Ce 12 O 24 (n = -1, 0, ..., 4) using the DFTþU approach and statistical thermodynamics. At low oxygen pressure (10 -9 atm), VO 4 is the most stable species below 400 K; in the 400-900 K range, VO 2 is stable; and above 900 K, VO becomes stable. In all of these systems, vanadium is stabilized in the þ5 oxidation state. Using the energies of hydrogenation and oxygen vacancy formation as reactivity descriptors, we predict an enhanced reactivity of the vanadia/ceria system in Mars-van Krevelen-type oxidation reactions. At the origin of this support effect is the ability of ceria to stabilize reduced states by accommodating electrons in localized f states. We also calculate the frequencies of the normal vibrational modes of the supported VO n species and their infrared intensity.

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...