Laura Sordelli

Ethanol Oxidation on Electrocatalysts Obtained by Spontaneous Deposition of Palladium onto Nickel-Zinc Materials

Ni-Zn and Ni-Zn-P alloys supported on Vulcan XC-72 are effective materials for the spontaneous deposition of palladium through redox transmetalation with Pd(IV) salts. The materials obtained, Pd-(Ni-Zn)/C and Pd-(Ni-Zn-P)/C, have been characterized by a variety of techniques. The analytical and spectroscopic data show that the surface of Pd-(Ni-Zn)/C and Pd-(Ni-Zn-P)/C contain very small, highly dispersed, and highly crystalline palladium clusters as well as single palladium sites, likely stabilized by interaction with oxygen atoms from Ni--O moieties. As a reference material, a nanostructured Pd/C material was prepared by reduction of an aqueous solution of PdCl(2)/HCl with ethylene glycol in the presence of Vulcan XC-72. In Pd/C, the Pd particles are larger, less dispersed, and much less crystalline. Glassy carbon electrodes coated with the Pd-(Ni-Zn)/C and Pd-(Ni-Zn-P)/C materials, containing very low Pd loadings (22-25 microg cm(-2)), were studied for the oxidation of ethanol in alkaline media in half cells and provided excellent results in terms of both specific current (as high as 3600 A g(Pd)(-1) at room temperature) and onset potential (as low as -0.6 V vs Ag/AgCl/KCl(sat)).

A comparison between silica-immobilized ruthenium(II) single sites and silica-supported ruthenium nanoparticles in the catalytic hydrogenation of model hetero- and polyaromatics contained in raw oil materials

A comparative study of the hydrogenation of various heterocycles, model compounds in raw oil materials, by either Ru(II) complex immobilized on mesoporous silica or Ru(0) nanoparticles deposited on the same support has been performed. The single-site catalyst contains the molecular precursor [Ru(NCMe)3(sulphos)](OSO2CF3) tethered to partially dehydroxylated high-surface-area silica through hydrogen bonds between silanol groups of the support and SO3− groups from both the sulphos ligand [−O3S(C6H4)CH2C(CH2PPh2)3] and the triflate counter anion. Highly dispersed ruthenium nanoparticles were prepared by calcination/reduction of silica-supported Ru3(CO)12. The heterocycles (benzo[b]thiophene, quinoline, indole, acridine) are hydrogenated to cyclic thioethers or amines. The Ru(II) single-site catalyst is active for both benzo[b]thiophene and the N-heterocycles, while the Ru(0) catalyst does not hydrogenate the S-heterocycle, yet is efficient for the reduction of the N-heterocycles and simple aromatic hydrocarbons. The surface silanols promote the hydrogenation of indole via NH⋯ O(H)Si hydrogen bonds and can interact with the π-electron density of all substrates.

Zeolite-supported metals by design: organometallic-based tin-promoted rhodium NaY catalysts

Abstract Rhodium–tin bimetallic particles entrapped in NaY cages were used to study the mechanism of tin-promotion in the selective hydrogenation of α,β-unsaturated aldehydes. These model materials were obtained by chemical vapour deposition (CVD) and subsequent H 2 reduction of Sn(R) 4 (R=C 2 H 5 ; C 6 H 5 ) onto reduced Rh/NaY samples that were prepared by ion-exchange (IE) or by chemical vapour deposition (CVD). In the former case, we have catalysts containing appreciable amounts of proton, while non-acidic metal-in-zeolite samples are obtained with CVD. TPRD studies indicate that the decomposition of tin precursors takes place on the surface of the rhodium particles only if the monolayer capacity is not exceeded. In addition, the mechanism of decomposition is influenced by protons and by the tin precursor used. Carbonyl DRIFT spectra reveal clear evidences of a surface tin-enriched Rh–Sn phase only for proton-free CVD-based samples. In this respect, Sn(C 6 H 5 ) 4 leads to the formation of a higher tin coverage than that obtained from Sn(C 2 H 5 ) 4 . In the selective hydrogenation of citral (3,7-dimethyl-2,6-octadienal), the presence of protons was highly detrimental leading to the acetal formed by reaction with the solvent (ethanol). With proton-free catalysts, the formation of the saturated aldehyde and of the two unsaturated alcohols is observed. Selectivities could be influenced by both monolayer and multilayer deposits of tin on Rh/NaY. The promotion effect under running catalytic conditions is ascribed to the presence of non-ionic oxidised SnO x phases.

Tailored supported metal nanoparticles by CVD: an easy and efficient scale-up by a rotary bed OMCVD device

A rotary-bed OMCVD device was developed and successfully applied to the scale-up of the preparation of supported rhodium nanoparticles. The device revealed suitability for the preparation of up to ten grams of material. A multistep CVD protocol was established and allowed a catalytic material with tailored structural/morphological properties of the supported nanoparticles to be obtained. As the result of this kind of catalyst design the supported Rh nanoparticles showed outstanding performance in the hydrogen production by methane catalytic partial oxidation at low contact times. Materials produced were fully characterized by a pool of techniques including HRTEM, CO-DRIFTS, TPRD, etc.

Thermochemical mass-spectrometric investigation under reducing conditions of [Pd(acac)(2)] adsorbed on magnesium oxide

Abstract The thermochemical behavior of [Pd(acac) 2 ] adsorbed onto different magnesium oxides has been studied under reducing conditions by means of the TPRD (temperature-programmed reductive decomposition) technique coupled to mass spectrometry. The thermal profiles of the different decomposition products have been rationalized in terms of chemical interactions with the support, and correlated with the catalytic properties in the dehydrocyclization reaction of n -heptane. The technique is then proposed to be a valid screening methodology for routine testing of catalytic batches without the need of running costly and time-consuming catalytic tests.

Surface organometallic chemistry: physisorption and thermal decomposition of Ir4(CO)12 on silica

The surface reactivity of Ir4(CO)12 supported on silica has been investigated by means of Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy and temperature-programmed decomposition techniques. The initial stage of physisorption from a solution results in the deposition of large crystallites (ϕ > 2µ) of Ir4(CO)12, unevenly distributed on the surface. By thermal treatment in vacuum at 100 °C the complete sublimation of Ir4(CO)12 occurs. By heating under Ar or O2 at 100 °C a disaggregation of the crystallites to single Ir4(CO)12 units evenly distributed on the surface is observed. Admission of water vapour at 25 °C promotes a high surface mobility of the Ir4(CO)12 units towards reaggregation. The formation of metallic particles is achieved by thermal decomposition of well dispersed Ir4(CO)12 in Ar at 150–200 °C or at lower temperatures in H2. The adsorption of CO at 100 °C leads to disruption of large metallic particles. This surface process is enhanced by a preliminary study under O2 at 25 °C.

Organic–Inorganic Hybrid Saponites Obtained by Intercalation of Titano-Silsesquioxane

The synthesis and characterization of two bifunctional composite materials based on synthetic saponite clays is here presented. These materials were prepared by intercalation of a Ti-containing aminopropylisobutyl polyhedral oligomeric silsesquioxane (Ti-NH(2) POSS) in synthetic saponite samples containing interlayer sodium (Na-SAP) or protons (H-SAP). Hybrid organic-inorganic materials, Ti-NHM-1 and Ti-NHM-2, were obtained upon ion exchange. Structural, spectroscopic, and thermal properties of both hybrid materials were investigated in detail along with their catalytic activity in cyclohexene oxidation.

Intrazeolite Pd clusters: particle location

A chemical vapour deposition (CVD) procedure has been adopted for the preparation of Pd/NaY, resulting in a high dispersion of the metal phase, characterised by a bimodal distribution of particle size. The most abundant particles (80%) are about 25 Å in size, corresponding to almost twice the dimension of the zeolitic supercages. To identify their location inside or outside the zeolite matrix, the IR spectra of adsorbed CO, obtained before and after admission of NH3, have been compared. The results obtained are in sharp contrast with those for a Pd/SiO2 system, where Pd particles of comparable size are exclusively located on the external surface of the carrier. These differences support the conclusion that Pd particles in Pd/NaY are indeed located in the zeolitic cavities.

Intrazeolite large palladium clusters prepared by organometallic chemical vapour deposition

Allyl(cyclopentadienyl)palladium was selectively introduced inside the cages of the zeolite NaY via chemical vapour deposition in Ar at 25 °C. Palladium clusters were then formed by thermal removal of the volatile ligands under H2 at 300 °C. High-resolution transmission electron microscopy measurements indicated the formation of large palladium clusters with diameters ranging from 7 to 25 A. Indirect characterization by IR spectroscopy confirmed that most of the metal is located inside the cages. The existence of entrapped clusters of 25 A, i.e. larger than the zeolite cages, can be explained by a local disruption of the zeolite lattice. The sintering process observed in H2 at 500 °C is caused by the coalescence of clusters in adjacent cavities, together with a limited migration of palladium atoms resulting in larger external particles. The electronic state and location of the large palladium clusters were studied by Fourier-transform infrared spectroscopy of adsorbed CO molecules. The marked downward shift of the vCO bands is attributed to the increase in electron density on these clusters. This electron-rich character is due to charge-transfer interactions with the negatively charged oxygen atoms of the cage walls.

Application of XAFS to Catalytic Materials

EXAFS spectroscopy is a powerful technique to determine local structure of a selected atomic species in a sample in any aggregation state and under any kind of atmosphere; for these reasons it is widely used in catalysis. A brief phenomenological explanation of the EXAFS signal and a short description of data collection and data analysis methods are presented. A recent example of its application to a heterogeneous catalytic system is illustrated.