Marco Conte

Spin trapping of Au-H intermediate in the alcohol oxidation by supported and unsupported gold catalysts.

Electron paramagnetic resonance (EPR) spectroscopy and spin trapping were used to explore the mechanism of alcohol oxidation over gold catalysts. Reaction of secondary alcohols with supported and unsupported gold catalysts (e.g., Au/CeO(2), polymer-incarcerated Au nanoparticles, PPh(3)-protected Au nanoparticles) in the presence of spin traps led to the formation of a hydrogen spin adduct. Using isotope labeling, we confirmed that the hydrogen in the spin adduct originates from the cleavage of the C-H bond in the alcohol molecule. The formation of the hydrogen spin adduct most likely results from the abstraction of hydrogen from the Au surface by a spin trap. These results thus strongly suggest intermediate formation of Au-H species during alcohol oxidation. The role of oxygen in this mechanism is to restore the catalytic activity rather than oxidize alcohol. This was further confirmed by carrying out gold-catalyzed alcohol oxidation in the absence of oxygen, with nitroxides as hydrogen abstractors. The support (e.g., metal oxides) can activate oxygen and act as an H abstractor from the gold surface and hence lead to a faster recovery of the activity. Peroxyl radicals were also observed during alcohol oxidation, consistent with a free-radical autoxidation mechanism. However, this mechanism is likely to be a minor side reaction, which does not lead to the formation of an appreciable amount of oxidation products.

Selective Oxidation of Glycerol by Highly Active Bimetallic Catalysts at Ambient Temperature under Base-Free Conditions†

Au–Pt alloy nanoparticles deposited on Mg(OH)2 (see STEM-HAADF image) show high activity in the selective oxidation of polyols using molecular oxygen as oxidant at mild and base-free conditions.

Energy dispersive X-ray spectroscopy of bimetallic nanoparticles in an aberration corrected scanning transmission electron microscope

The technique of X-ray energy dispersive spectroscopy (XEDS) spectrum imaging in a dedicated scanning transmission electron microscope (STEM) is discussed in relation to its applicability to bimetallic nanoparticles. It is shown that the recent availability of aberration corrected microscopes and multivariate statistical analysis (MSA) techniques has allowed us to overcome many of the intrinsic limitations previously encountered when attempting STEM-XEDS spectrum imaging on nanoscopic volumes of material. We demonstrate through a variety of applications to Au-Ag and Au-Pd bimetallic nanoparticle systems, that STEM-XEDS can provide invaluable high spatial resolution compositional information on (i) alloy homogeneity and phase segregation effects within individual nanoparticles, (ii) particle size-alloy composition correlations, (iii) the detection of trace amounts of alloying element and (iv) metal component distribution in extremely highly dispersed catalyst systems.

Modifications of the metal and support during the deactivation and regeneration of Au/C catalysts for the hydrochlorination of acetylene

The effect of the gold oxidation state and carbon structure on the activity of Au/C catalysts for the hydrochlorination of acetylene was investigated by a combined approach using TPR, XPS and porosimetry determinations. The activity of the catalyst in the synthesis of vinyl chloride monomer was found to be dependent on the presence of Au3+ species in the catalyst. However, by preparing catalysts with different Au3+ content it was possible to determine the existence of a threshold Au3+ amount, beyond which the excess of Au3+ was not active for the reaction. This was explained by the existence of active sites at the Au/C interface, and not just by the presence of Au3+ species on top of Au nanoparticles, as explained by current models for these catalysts. It was also possible to determine the existence of a subset of Au nanoclusters which do not take part in the reaction, as well as changes in the textural properties of the carbon that can affect its long term reusability.

Modified zeolite ZSM-5 for the methanol to aromatics reaction

Catalysts comprising zeolite ZSM-5 impregnated with precious metals including Ag, Cu, Ni, Pd, Ir and Ru, have been tested for the methanol to hydrocarbons reaction in a continuous flow fixed bed reactor. Comparison with the activity of unmodified ZSM-5 showed that Ag, Cu and Ni enhanced the selectivity to C6–C11 aromatic products by a factor of two or higher. Moreover, Ag/ZSM-5 showed improved selectivity for the C6–C7 fraction of aromatic products. Ni/ZSM-5 was found to be selective to naphthalene, while Cu/ZSM-5 was selective for C9–C11 aromatic products. It was ascertained that all the impregnated metals were present as metal oxides in the starting materials. It is therefore proposed that the enhanced selectivity to aromatic products is due to the interaction of the acid sites of the zeolite with the basic sites of the metal oxide at the edge of the zeolite crystals, as well as the possible coordination of propene molecules formed during the reaction, that are likely to be the building blocks for the formation of aromatics.

Chemically Induced Fast Solid-State Transitions of ω-VOPO4 in Vanadium Phosphate Catalysts

Vanadium phosphates are important catalysts for the oxidation of alkanes, and commercial catalysts comprise a complex range of V4+ and V5+ phosphates. We used three complementary in situ characterization methodologies—powder x-ray diffraction and laser Raman and electron paramagnetic resonance spectroscopies—to show that the metastable phase ω-VOPO4 is very sensitive to many of the reactants and products of butane oxidation. A rapid transformation from ω-VOPO4 to δ-VOPO4 occurs on exposure to butane at the reaction temperature, and hence the metastable ω-VOPO4 may play a role in the formation of commercial catalysts.

Gold nanoparticle-initiated free radical oxidations and halogen abstractions

We report on the use of EPR spectroscopy and spin trapping technique to detect free radical intermediates formed in the presence of gold nanoparticles. Phosphine- and amine-protected gold nanoparticles were found to initiate air oxidation of organic substrates containing active hydrogen atoms, such as amines and phosphine oxides. Nanoparticles protected by stronger bound ligands (e.g., thiols) were inactive in these reactions. We also found that gold nanoparticles are able to abstract a halogen atom from the halogenated compounds, presumably due to the high affinity of gold metal for halogens. Reaction of Au nanoparticles with chloroform showed an unusual inverse isotope effect. The trichloromethyl spin adduct was observed when Au nanoparticles were mixed with CDCl(3) but not with CHCl(3). This unexpected behaviour suggests that C-H bond breaking is not the rate-determining step in Au-initiated hydrogen abstraction.

Cyclohexane oxidation using Au/MgO: an investigation of the reaction mechanism

The liquid phase oxidation of cyclohexane was undertaken using Au/MgO and the reaction mechanism was investigated by means of continuous wave (CW) EPR spectroscopy employing the spin trapping technique. Activity tests aimed to determine the conversion and selectivity of Au/MgO catalyst showed that Au was capable of selectivity control to cyclohexanol formation up to 70%, but this was accompanied by a limited enhancement in conversion when compared with the reaction in the absence of catalyst. In contrast, when radical initiators were used, in combination with Au/MgO, an activity comparable to that observed in industrial processes at ca. 5% conversion was found, with retained high selectivity. By studying the free radical autoxidation of cyclohexane and the cyclohexyl hydroperoxide decomposition in the presence of spin traps, we show that Au nanoparticles are capable of an enhanced generation of cyclohexyl alkoxy radicals, and the role of Au is identified as a promoter of the catalytic autoxidation processes, therefore demonstrating that the reaction proceeds via a radical chain mechanism.

Oxidation of Benzyl Alcohol and Carbon Monoxide Using Gold Nanoparticles Supported on MnO2 Nanowire Microspheres

MnO2 was synthesised as a catalyst support material using a hydrothermal method. This involved reacting MnSO4⋅H2O and (NH4)2S2O8 at 120 °C for a range of crystallisation times, which affords control over the morphology and phase composition of the MnO2 formed. Gold was deposited on these supports using sol-immobilisation, impregnation and deposition precipitation methods, and the resultant materials were used for the oxidation of benzyl alcohol and carbon monoxide. The effect of the support morphology on the dispersion of the gold nanoparticles and the consequent effect on the catalytic performance is described and discussed.

Reactivity of Ga2O3 Clusters on Zeolite ZSM-5 for the Conversion of Methanol to Aromatics

Composites of Ga2O3 clusters and zeolite ZSM-5 were evaluated for the transformation of methanol to hydrocarbons. Comparison of the activity with ZSM-5 showed that the Ga2O3 clusters are responsible for the enhanced selectivity to aromatics via contact synergy, thus showing the importance of non framework gallium species for this reaction. TEM analysis of fresh and spent catalysts allowed the identification of the formation of carbonaceous products at the Ga2O3/zeolite interface region, and this interface is also the probable location of the catalyst active sites.Graphical Abstract