Candida Milone

X-ray photoelectron spectroscopy of Au/Fe2O3 catalysts

Gold catalysts supported on iron oxide have been characterized by X-ray photoelectron spectroscopy (XPS). Depending on the method of preparation and pretreatment, different gold species have been detected on the catalyst surface. In the absence of thermal treatment, the XPS spectra of the impregnated samples show the formation of gold(III) (oxy)chloride species. No metallic gold is formed. Both metallic and oxidized gold species are instead present on the surface of the samples prepared by coprecipitation. Oxidized gold(III) species are predominant on the uncalcinated catalysts. After treatment at high temperature and/or under reaction conditions the amount of metallic gold increases. The XPS data have been correlated with the activity of the investigated catalysts in the oxidation of carbon monoxide at low temperature.

Hydrogenation of Phenol to Cyclohexanone over Palladium and Alkali-Doped Palladium Catalysts

Abstract Palladium catalysts supported on Al2O3 and MgO were prepared and used in the gas-phase hydrogenation of phenol to cyclohexanone. In all range of conditions investigated, selectivity to cyclohexanone was higher on Pd/MgO than on Pd/Al2O3. On Pd/Al2O3 the reaction rates were found to be of first order with respect to the phenol partial pressure and second order with respect to the hydrogen pressure. On Pd/MgO, instead, the order of reaction with respect to phenol was −1 and the order with respect to hydrogen was about 1. Addition of alkali or alkaline earth metals (Ca, K, Cs) to Pd/Al2O3 increased the activity and the selectivity to cyclohexanone without modifying the orders of reaction. On the basis of the results of this study, a reaction mechanism involving adsorption of phenol on the sites of the support at the interface of the palladium metal particles is proposed. The higher selectivity to cyclohexanone found on Pd/MgO compared to Pd/Al2O3, is explained by two different forms of adsorbed phenol.

Particle size effect in the catalytic hydrogenation of 2,4-dinitrotoluene over Pd/C catalysts

Abstract The liquid phase hydrogenation of 2,4-dinitrotoluene (2,4-DNT) to 2,4-diaminotoluene (2,4-DAT) has been studied over palladium supported on carbon. Catalytic activity and selectivity to the intermediate products were found to depend on the metal particle size. The larger metal particles were the most active and selective towards the formation of the 2,4-nitrohydroxyaminotoluene isomers. The catalysts have been characterized by TPR, XRD, TEM and CO chemisorption. TPR analysis has shown that no formation of a palladium β-hydride phase occurs on these systems irrespectively of the metal particle size. This apparently anomalous behavior has been attributed to the contamination of the Pd metal particles by impurities and/or C atoms migrating from the support. On the basis of the reported results the relationships between the catalytic activity and the metal particle size in the hydrogenation of 2,4-DNT is discussed.

Catalytic combustion of diesel soot over metal oxide catalysts

Abstract The activity of different metal oxides in the catalytic combustion of a diesel soot having a high amount of adsorbed hydrocarbons has been investigated and tested in a TPO apparatus. Two different steps have been observed. The first is related to the combustion of hydrocarbons adsorbed on soot and the second one to the combustion of the graphitic solid fraction. A high surface area Fe2O3 was found to be the most active catalyst for the oxidation of hydrocarbons, whereas V2O5 was also able to promote the combustion of graphitic carbon. The oxidation of the hydrocarbon fraction has been correlated with the surface area and the strength of the metal-oxygen bond of the metal oxide. The combustion of graphitic carbon is favoured instead on metal oxides having a low melting point. The effect of platinum addition to γ-alumina has been also investigated. Spillover of activated oxygen from Pt to the support has been postulated to take into account the promotion of the oxidation of graphitic carbon.

Influence of metal particle size in the hydrogenation of citral over Ru/C

Selective hydrogenation of citral has been studied under mild conditions over Ru catalysts supported on activated carbon. Geraniol, nerol, citronellal and isopulegol were the main reaction products. Isopulegol is obtained by isomerization of citronellal. Small amounts of citronellol are formed through the hydrogenation of citronellal. The specific catalytic activity per Ru surface atom as well as the products distribution remains constant with changing metal particle size. A comparison with results obtained on the hydrogenation of cinnamaldehyde on the same Ru/C catalysts is reported.

Hydrogenation of α,β-unsaturated aldehydes over Ru/Al2O3 catalysts

Abstract The hydrogenation of citral and cinnamaldehyde has been investigated over Ru/Al2O3 catalysts. The effect of metal particle size on the catalytic activity and selectivity has been studied by using catalysts having a metal dispersion ranging from 0.05 to 0.88. It has been observed that the overall rate of hydrogenation of both the unsaturated aldehydes is not influenced by the Ru particle size. In the hydrogenation of cinnamaldehyde a higher selectivity to cinnamyl alcohol has been observed on catalysts with larger metal particle size. No variations have been found in the hydrogenation of citral. It is suggested that steric effects do not influence significantly the product selectivity. A repulsive interaction between the aromatic ring and the catalyst surface would explain the higher selectivity to cinnamyl alcohol. On all the investigated catalysts the isolated C C double bond of citral shows a low reactivity. It is suggested that the unsaturated aldehyde is adsorbed through the carbonyl group. This strong adsorption prevents the hydrogenation of the isolated olefinic bond which is located far away from the adsorption centers.

Hydrogenation of citral over Ru-Sn/C

Citral has been hydrogenated over Ru-Sn/C catalysts. The Ru-Sn/C system has been studied as a function of the Ru/Sn ratio. Under the conditions used the carbonyl group and the conjugated double bond are hydrogenated. No products obtained from the hydrogenation of the isolated double bond are formed. Addition of tin on the Ru/C decreases the number of Ru active sites on the surface with a corresponding increase in the specific catalytic activity. A higher selectivity (>80%) to geraniol + nerol is observed on the samples with the highest Sn/Ru ratio. The higher selectivity and higher specific activity are attributed to an activation of the C≐O bond by tin ions.

Characterization of Pt-Sn/carbon hydrogenation catalysts

Abstract Bimetallic Pt-Sn/C catalysts have been characterized by TPR, CO chemisorption, X-ray photoelectron spectroscopy (XPS) and electron microscopy techniques (TEM and HRTEM). Characterization has been carried out on the samples after reduction and after exposure to air at room temperature. On the surface of the reduced catalysts platinum is in the metallic state whereas tin is mainly in an oxidized state and only a minor fraction is present in a zero-valent state, likely in combination with platinum. On the most tin-rich catalysts, PtSn alloy particles have been identified. Electron microscopy has revealed that the size and the morphology of the metal particles are dependent on the tin loading. Moreover, the combination of the techniques used has shown that, after exposure to air, surface oxidation of zero-valent metal species occurs but their bulk structure is retained. On the basis of a comparison between the characterization data here reported and the catalytic activity measured in the hydrogenation of α,β-unsaturated aldehydes over these catalysts it is suggested that the active sites for the selective hydrogenation reaction are made of metallic platinum decorated by tin, most likely in an ionic form.

Catalytic oxidation of carbon monoxide over Au/Fe2O3 preparations

Low temperature catalytic oxidation of carbon monoxide has been studied over Au/Fe2O3. On the coprecipitated samples the catalytic activity shows a maximum at a gold content of about 5%. Calcination of Au/Fe2O3 decreases the catalytic activity and the stability. The catalytic activity measured is the highest ever reported in the literature on Au/Fe2O3 preparations.

Hydrogenation of citral and cinnamaldehyde over bimetallic Ru-Me/Al2O3 catalysts

Abstract Hydrogenations of citral and cinnamaldehyde have been investigated over Ru-Me/Al 2 O 3 (Me = Ge, Sn, Pb). Two series of catalysts have been prepared by using the controlled surface reaction (CSR) technique (RuEC series) and the more conventional co-impregnation method (RuNI series). Addition of Ge increases the catalytic activity on the RuEC series whereas no modifications are found on the samples prepared by co-impregnation. The selectivity to unsaturated alcohols is not influenced by the presence of Ge. Addition of tin increases both the activity and the selectivity regardless of the preparation method. The presence of lead does not modify the selectivity and a slight decrease in the catalytic activity is observed on the sample of the RuNI series. On the basis of the characterization data available on the investigated catalysts, it is suggested that Ru and Ge interacts when the samples are obtained through the CSR technique. Such an interaction increases the catalytic activity likely through a decrease of the strength of adsorption of one of the reaction components. The positive effect of tin on the catalytic activity and selectivity is discussed.