The results of a systematic XPS study, under high controlled conditions, of different basic oxides of transition metals, alkali and alkaline-earth metals are presented; the XPS data of some hydroxides and peroxides are also reported. Variations of the O 1s binding energies are analysed and one point of interest is the large
 binding energy scale obtained for O 1s peaks all associated with a ‘‘2−
’’ formal charge. Through extended
 Huckel theory-tight binding (EHT-TB) calculations, attempts are made to rationalize the observed variations.
 The results illustrate
 the significant
 differences
 between real
 charges on oxygen atoms in transition metal and alkaline-earth oxides.

Systematic XPS studies of metal oxides, hydroxides and peroxides

A study is conducted to demonstrate catalytic dehydrogenation of light alkanes on metals and metal oxides. The study provides a complete overview of the materials used to catalyze this reaction, as dehydrogenation for the production of light olefins has become extremely relevant. Relevant factors, such as the specific nature of the active sites, as well as the effect of support, promoters, and reaction feed on catalyst performance and lifetime, are discussed for each catalytic Material. The study compares different catalysts in terms of the reaction mechanism and deactivation pathways and catalytic performance. The duration of the dehydrogenation step depends on the heat content of the catalyst bed, which decreases rapidly due to the endothermic nature of the reaction. Part of the heat required for the reaction is introduced to the reactors by preheating the reaction feed, additional heat being provided by adjacent reactors that are regenerating the coked catalysts.

Catalytic dehydrogenation of light alkanes on metals and metal oxides.

Focuses on the surface chemistry and spectroscopy of chromium in inorganic oxides. Characterization of the molecular structures of chromium; Mechanics of hydrogenation-dehydrogenation reactions; Mobility and reactivity on oxidic surfaces.

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Surface Chemistry and Spectroscopy of Chromium in Inorganic Oxides.

Transition metal oxides are capable of a wide range of work functions. This quality allows them to be used in many applications that involve charge transfer with adsorbed molecules, for example as heterogeneous catalysts, as charge-injection layers in organic electronics, and as electrodes in fuel cells. Chemical and structural factors can alter transition-metal oxide work functions, often making their work functions difficult to control. Little is known about the effects of the cation oxidation state and point defects on the oxide work function. It is necessary to understand how such chemical and structural factors affect work functions in order to controllably tune transition metal oxides for desired applications. Here, a correlation between the oxide work function and cation oxidation state is demonstrated. This correlation is attributed to the change in cation electronegativity with oxidation state. A model is presented that relates the work function to the oxygen deficiency for d0 oxides in the limit of dilute oxygen vacancies. It is proposed that the rapid initial decrease in work function, observed for d0 oxides, is caused by an increase in the density of donor-like defect states. It is also shown that oxides tend to have decreased work functions near a metal/metal-oxide interface as a consequence of the relationship between defects and work function. These insights provide guidelines for tuning transition metal oxide work functions.

Transition Metal Oxide Work Functions: The Influence of Cation Oxidation State and Oxygen Vacancies

Molecularly well-defined homogeneous catalysts are known for a wide variety of chemical transformations. The effect of small changes in molecular structure can be studied in detail and used to optimize many processes. However, many industrial processes require heterogeneous catalysts because of their stability, ease of separation and recyclability, but these are more difficult to control on a molecular level. Here, we describe the conversion of homogeneous cobalt complexes into heterogeneous cobalt oxide catalysts via immobilization and pyrolysis on activated carbon. The catalysts thus produced are useful for the industrially important reduction of nitroarenes to anilines. The ligand indirectly controls the selectivity and activity of the recyclable catalyst and catalyst optimization can be performed at the level of the solution-phase precursor before conversion into the active heterogeneous catalyst.

Heterogenized cobalt oxide catalysts for nitroarene reduction by pyrolysis of molecularly defined complexes

A new interpretation of the Mo(3d) XPS spectra of reduced supported Mo catalysts is proposed. It is based on the assumption of a linear relation between the binding energies and the oxidation numbers of Mo states, with a slope of {approx equal}0.8 eV per oxidation state. The proposal is substantiated by the results of an XPS investigation of the thermal decomposition of MoO{sub 3} and by a survey of evidence published in the literature. It implies that two different Mo(IV) states have to be considered, one with a binding energy obeying the linear relation with the oxidation state (isolated Mo(IV)) and a second one deviating for structural reasons (paired double-bonded Mo(IV), e.g., in MoO{sub 2}). The application of this approach to the fitting of Mo(3d) signal shapes in the spectra of reduced supported Mo catalysts yields estimates for the amounts of Mo(VI), Mo(O), and a state that may be Mo(II) or paired double-bonded Mo(IV) ions. For Mo(V) and isolated Mo(IV), ranges for possible amounts may be given.

R. Feldhaus

Papers

Analysis of molybdenum(3d) XPS spectra of supported molybdenum catalysts: an alternative approach

Reduction behavior and metathesis activity of WO3Al2O3 catalysts: I. An XPS investigation of WO3Al2O3 catalysts

Reduction and aromatization activity of chromia-alumina catalysts: II. An XPS investigation of chromia-alumina catalysts

Reduction and metathesis activity of MoO3/Al2O3 catalysts: I. An XPS investigation of MoO3/AI2O3 catalysts

Reduction behavior and metathesis activity of WO3Al2O3 catalysts: III. The activation of WO3Al2O3 catalysts