Core-level binding-energy shifts at surfaces and in solids

Clusters that contain only a small number of atoms can exhibit unique and often unexpected properties. The clusters are of particular interest in catalysis because they can act as individual active sites, and minor changes in size and composition--such as the addition or removal of a single atom--can have a substantial influence on the activity and selectivity of a reaction. Here, we review recent progress in the synthesis and characterization of well-defined subnanometre clusters, and the understanding and exploitation of their catalytic properties. We examine work on size-selected supported clusters in ultrahigh-vacuum environments and under realistic reaction conditions, and explore the use of computational methods to provide a mechanistic understanding of their catalytic properties. We also highlight the potential of size-selected clusters to provide insights into important catalytic processes and their use in the development of novel catalytic systems.

Catalysis by clusters with precise numbers of atoms

The goal of this review is to present some aspects of the use of a test reaction, i.e., methanol oxidation, as a tool to characterize oxidation catalysts. The selectivity pattern and the formation rates of the reaction products are used to characterize both structural (dispersion) as well as chemical properties (acid-base and redox) on supported oxide catalysts, especially for molybdenum-based systems supported on silica and vanadia on titanium oxide. This highly sensitive technique which gives information on the catalytically active sites at the molecular level characterizes a catalyst at work and is particularly well-adapted to the study of supported catalysts.

Methanol oxidation as a catalytic surface probe

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Lecture notes in Chemistry

Two gas-phase catalytic cycles for the two-electron oxidation of primary and secondary alcohols were detected by multistage mass spectrometry experiments. A binuclear dimolybdate center [Mo(2)O(6)(OCHR(2))](-) acts as the catalyst in both these cycles. The first cycle proceeds via three steps: (1) reaction of [Mo(2)O(6)(OH)](-) with alcohol R(2)HCOH and elimination of water to form [Mo(2)O(6)(OCHR(2))](-); (2) oxidation of the alkoxo ligand and its elimination as aldehyde or ketone in the rate-determining step; and (3) regeneration of the catalyst via oxidation by nitromethane. Step 2 does not occur at room temperature and requires the use of collisional activation to proceed. The second cycle is similar but differs in the order of reaction with alcohol and nitromethane. The nature of each of these reactions was probed by kinetic measurements and by variation of the substrate alcohols (structure and isotope labeling). The role of the binuclear molybdenum center was assessed by examination of the relative reactivities of the mononuclear [MO(3)(OH)](-) and binuclear [M(2)O(6)(OH)](-) ions (M = Cr, Mo, W). The molybdenum and tungsten binuclear centers [M(2)O(6)(OH)](-) (M = Mo, W) were reactive toward alcohol but the chromium center [Cr(2)O(6)(OH)](-) was not. This is consistent with the expected order of basicity of the hydroxo ligand in these species. The chromium and molybdenum centers [M(2)O(6)(OCHR(2))](-) (M = Cr, Mo) oxidized the alkoxo ligand to aldehyde, while the tungsten center [W(2)O(6)(OCHR(2))](-) did not, instead preferring the non-redox elimination of alkene. This is consistent with the expected order of oxidizing power of the anions. Each of the mononuclear anions [MO(3)(OH)](-) (M = Cr, Mo, W) was inert to reaction with methanol, highlighting the importance of the second MoO(3) unit in these catalytic cycles. Only the dimolybdate center has the mix of properties that allow it to participate in each of the three steps of the two catalytic cycles. The three reactions of these cycles are equivalent to the three essential steps proposed to occur in the industrial oxidation of gaseous methanol to formaldehyde at 300-400 degrees C over solid-state catalysts based upon molybdenum(VI)-trioxide. The new gas-phase catalytic data is compared with those for the heterogeneous process.

Catalytic Gas Phase Oxidation of Methanol to Formaldehyde

Oxidative dehydrogenation of methanol to formaldehyde

The ground state of CsO is 2Σ+, whereas the ground state of LiO is 2Π. Using purely ionic wave functions, we explain the origin of this effect in terms of a balance between Pauli repulsion (favoring 2Σ) and quadrupole attractions (favoring 2Π) and provide spectroscopic parameters (Re, ωe, De) on LiO, NaO, KO, RbO, and CsO from ab initio correlated wave functions.

Alkali oxide diatomics: Explanation of the change in ground state symmetry from LiO(2Π) CsO(2Σ+)

Theoretical studies of CO/Ni(100): Geometry, vibrational frequencies and ionization potentials for the on-top site

Etude de la liaison dans MO (M=Li, Na, K, Rb, Cs) a l'aide de fonctions d'onde correlees ab initio: etat fondamental 2 Π pour M=Li, Na, K et 2 Σ + pour M=Rb, Cs. Interpretation

Janet N. Allison

Papers

Oxidative dehydrogenation of methanol to formaldehyde

Alkali oxide diatomics: Explanation of the change in ground state symmetry from LiO(2Π) CsO(2Σ+)

Theoretical studies of CO/Ni(100): Geometry, vibrational frequencies and ionization potentials for the on-top site

Alkali oxides. Analysis of bonding and explanation of the reversal in ordering of the 2.PI. and 2.SIGMA. states

The lower electronic states of MoN