First IR spectroscopic observation of the stable Mo=CH2 carbene complex on the surface of an active catalyst for olefin metathesis: Photoreduced silica-molybdena with chemisorbed cyclopropane

We report in the first IR observation of carbene complex Mo=CH₂ on the surface of Mo/SiO₂ metathesis catalyst. Mo=CH₂ species were produced by cyclopropane chemisorption on catalysts, photoreduced in CO (Mo(IV)/SiO₂), and are characterized in IR by C-H stretching vibrations at 3080 and 2945 cm^–1.     

Low-pressure chemical and photochemical reactions of oxides of nitrogen on alumina taken as a model substance for mineral dust in relation to air pollution

The interaction of γ-Al₂O₃, taken as a model substance of tropospheric mineral dust, with N₂O, NO and NO₂ has been studied using kinetic and temperature-programmed desorption (TPD) mass-spectrometry in presence and absence of UV irradiation. At low surface coverages (<0.001 ML), adsorption of N₂O and NO₂ is accompanied by dissociation and chemiluminescence, whereas adsorption of NO does not lead to appreciable dissociation. Upon UV irradiation of Al₂O₃ in a flow of N₂O, photoinduced decomposition and desorption of N₂O take place, whereas in a flow of NO, only photoinduced desorption is observed. Dark dissociative adsorption of N₂O and NO and photoinduced N₂O dissociation apparently occur by a mechanism involving electron capture from surface F- and F⁺-centers. Photoinduced desorption of N₂O and NO may be associated with decomposition of complexes of these molecules with Lewis acid sites, V-centers or OH-groups. TPD of N₂O and NO proceeds predominantly without decomposition, while NO₂ partially decomposes to NO and O₂.     

Quantitative Analysis of Photoluminescence Quenching of Silica-Supported Molybdena Catalysts. Relation to Photocatalytic Reduction of Nitric Oxide by Carbon Monoxide

The quenching effect of NO, O₂, CO, and N₂O on the photoluminescence of Mo⁶⁺/SiO₂ has been studied at room temperature as a function of gas pressure. Nonlinear plots of the relative photoluminescence intensity I ₀/I (I ₀ is the initial intensity under vacuum) vs. pressure of the quenching gases were rationalized assuming that only adsorbed molecules efficiently quench the (Mo⁵⁺–O^-)* excited state and that the fraction of adsorbed quenching molecules can be determined from the classical Langmuir-type adsorption isotherm. The ratio of the quenching rate constants for NO and CO calculated from the computer best fits of the experimental I ₀/I–pressure dependence is in agreement with earlier data on the kinetics of the photocatalytic reduction of NO by carbon monoxide on Mo⁶⁺/SiO₂.     

Iminoxyl Radical‐Based Strategy for Intermolecular C?O Bond Formation: Cross‐Dehydrogenative Coupling of 1,3‐Dicarbonyl Compounds with Oximes

Cross‐dehydrogenative C O coupling of 1,3‐diketones and 1,3‐keto esters with oximes was realized for the first time. The reaction proceeds in the presence of the oxidants [KMnO₄, Mn(OAc)₂/KMnO₄, Mn(OAc)₃⋅2 H₂O, MnO₂, Mn(acac)₃, Fe(ClO₄)₃, Cu(ClO₄)₂⋅6 H₂O, Cu(NO₃)₂⋅2.5 H₂O, and (NH₄)₂Ce(NO₃)₆]. Twenty cross‐coupling products were synthesized using potassium permanganate (KMnO₄), manganese(II) acetate dihydrate [Mn(OAc)₃⋅2 H₂O], or the manganese(II) acetate/potassium permanganate [Mn(OAc)₂/KMnO₄] system; yields are 27–92%. The synthesis can be easily scaled up to gram quantities of the target products. Apparently, the reaction proceeds via a radical mechanism in which the oxidizing agent serves to generate radicals from oximes and perform the one‐electron oxidation of 1,3‐dicarbonyl compounds. The formation of oxime radicals was confirmed quantitatively by electron spin resonance (ESR) spectroscopy. The coupling described in the present study is the first example of the selective intermolecular reaction involving unstable iminoxyl radicals generated in situ.

     

Kinetic Investigation of the Photocatalytic Reduction of Nitric Oxide by Carbon Monoxide at Low Pressure on Silica-Supported Molybdenum Oxide

The kinetics of photoinduced reactions that occur upon UV irradiation (<360 nm) of a MoO₃/SiO₂ catalyst (2.5 wt% Mo) in CO-NO mixtures and CO alone are studied at gas pressures from 0.05 to 2 torr and for CO/NO ratios from 0.3 to 3.0 in the temperature interval 20-150C. The data obtained are consistent with a previously proposed two-stage redox mechanism. In the first stage NO is reduced to N₂O through the reaction CO+2NO CO₂+N₂O, while in the second stage the N₂O formed is further reduced to N₂ via the reaction CO+N₂O CO₂+N₂. The ratio of rate constants for quenching of a transient excited state (Mo⁵⁺-O^-)* by NO and CO molecules is found to be 2.8. The reaction rates decrease with increasing temperature, apparently because of a lower concentration of adsorbed species and/or a reduction of the steady-state concentration of (Mo⁵⁺-O^-)*.