[Show abstract][Hide abstract] ABSTRACT: A functionalized Ti-containing polyhedral oligomeric silsesquioxane was anchored on the surface of an ordered SBA-15 and a non-ordered amorphous mesoporous silica. The physico-chemical characterization of the textural, thermal and electronic properties of the final anchored materials led to the evaluation of the distribution of Ti-POSS moieties on the silica surface, of the oxidation state and of the coordination geometry of the titanium centres. It was discovered that Ti-POSS are accommodated mainly as dinuclear dimeric species on the external surface of ordered mesoporous silica or in the large mesopores of non-ordered silica supports. Epoxidation tests on unsaturated terpenes (limonene, carveol and a-pinene) with TBHP as oxidant were performed to monitor the activity and accessibility of Ti(IV) sites in the anchored samples and com-pared with conventional titanocene-grafted Ti/SBA-15 and Ti/SiO 2 systems. The different chemoselectivity recorded over anchored catalysts, with respect to the grafted ones, can be a potential diagnostic tool to shed light on the mechanism or the molecular intermediates occurring at the surface during the oxidation reaction.
[Show abstract][Hide abstract] ABSTRACT: 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.
Chemistry - An Asian Journal 03/2011; 6(3):914-21. · 4.57 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The impregnation of Ketjen Black (C) with iron and cobalt phthalocyanines (MPc) taken one by one or as a 1:1 stoichiometric mixture, followed by heat treatment at 600 °C under inert atmosphere, gave materials containing arrays of single metal ions coordinated by four nitrogen atoms (M-N4 units). Increasing the pyrolysis temperature to 800° resulted in the formation of carbon-supported, nanosized metal particles. A key role of the carbon support in determining the material structure at either temperature investigated was demonstrated by TPD, EXAFS, XANES and XRPD studies. These also showed that a Fe–Co alloy is obtained at 800 °C when the impregnation of Ketjen Black involves a mixture of FePc and CoPc. Electrodes coated with the different Fe, Co and Fe–Co materials, containing ca. 3 wt% metal loadings, were scrutinized for the oxygen reduction reaction (ORR) in alkaline media by linear sweep voltammetry. For comparative purposes, two Pt electrocatalysts containing 3 and 20 wt% metal were investigated. The electrochemical activity of all materials was analyzed by Tafel and Koutecky–Levich plots as well as chronopotentiometry. The Fe-containing electrocatalysts have been found to be highly active for the ORR in alkaline media with convective limiting currents as high as 600 A g Fe−1 at room temperature and onset potentials as high as 1.02 V vs. RHE. It has been found that (i) the ORR mass activity of the Pc-derived electrocatalysts is superior to that of the Pt catalysts investigated; (ii) the activity of FePc and FePc–CoPc/C, heat treated at either 600 or 800 °C, is superior to that of the corresponding Co materials; (iii) the electrocatalysts obtained at 600 °C are fairly more active than those obtained at 800 °C.Research highlights▶ Effective cobalt(II)–iron(II) single site electrocatalysts for the ORR. ▶ Nanosized Co–Fe alloy catalyzes efficiently the ORR. ▶ EXAFS study of pyrolized Fe and Co phthalocyanines supported on Ketjen Black. ▶ Highly stable non-noble metal electrocatalysts for the ORR.
Journal of Power Sources 01/2011; 196(5):2519-2529. · 4.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Copper catalysts supported on silica or silica–alumina are used to promote the insertion of carbenes, coming from methyl phenyldiazoacetate and ethyl diazoacetate, into one C–H bond of THF, constituting the first example of this reaction promoted by a purely inorganic catalyst. Cu/SiO2–Al2O3 leads to better results, regarding yield and catalyst recovery, than with Cu/SiO2. With the former, yields are similar or even slightly better than those reached in solution using Cu (OTf)2 as the catalyst. These yields are further improved by the addition of an external bis(oxazoline) ligand. The reaction is promoted by Cu(I), obtained by in situ reduction of Cu(II) with the diazocompound; therefore, different behavior of the catalysts, depending on the support, is related to the different redox properties of the supported Cu phase. Isolated Cu(II) species on the surface of Cu/ SiO2–Al2O3 are easily reduced to Cu(I), whereas supported CuO is much more easily reduced to Cu(0) as shown by EXAFS analysis.
Journal of Catalysis 01/2011; 281:273. · 5.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Styrene oxide can be effectively isomerized to phenyl-acetaldehyde (98%) over amorphous silica alumina catalysts under very mild liquid phase conditions. On the other hand, a copper catalyst prepared using a silica zirconia support gave up to 80% yield in the hydrogenation of styrene oxide to 2-phenyl-ethanol.
[Show abstract][Hide abstract] ABSTRACT: The organometallics chemical vapor deposition (OM-CVD) technique, using Pd(hfac)2 as a precursor, was employed for the preparation of a series of heterogeneous catalysts based on Pd nanoparticles supported on nanometric oxides (CeO2, ZnO and TiO2). The properties of metal nanoparticles were tuned by proper pretreatment of the oxidic support.Complete decomposition of Pd precursor during formation of the metal phase by hydrogen reduction was investigated by TPRD-mass spectrometry analysis, and the obtained nanoparticles were characterized by CO-DRIFT spectroscopy and HR-TEM microscopy.Butadiene selective hydrogenation to butene, a well known structure-sensitive reaction, was used to test the performances of the systems prepared.Pd/CeO2 catalyst showed the best selectivity to butene with the lowest amount of undesired butane, result ascribable to the morphology of palladium nanoparticles.
[Show abstract][Hide abstract] ABSTRACT: The synthesis and catalytic activity of some new titanium(IV) complexes with incompletely condensed Cy6Si6O7(OH)4 and endo-Cy8Si8O11(OH)2 (Cy = c-C6H11) silsesquioxane cages have been carried out with the aim of investigating new Ti silsesquioxane frameworks as molecular models of titanium active sites of catalysts prepared by grafting Ti(IV) precursors on the surface or in a matrix of mesoporous silica. The silsesquioxane Cy6Si6O7(OH)4, which bears four potentially reactive OH groups, when reacted with TiCpCl3 (Cp = η5-C5H5) affords the new species [Cy6Si6O12Ti2(Cp)2], with two titanium atoms connected through one μ-oxygen bridge, as confirmed by a XANES and EXAFS spectroscopic investigation. By reaction of endo-Cy8Si8O11(OH)2 with [Ti(o-C6H4O2)(OCH2CH3)2] the new complex [Cy8Si8O11O2]2Ti was obtained, which exhibits a tetrapodal coordination of Ti, as shown by a X-ray diffraction investigation. The catalytic activity of these two new Ti silsesquioxane complexes was investigated for the catalytic epoxidation of cyclohexene with tert-butyl hydroperoxide and compared with the catalytic activity of the known [Cy7Si7O12TiCp], widely used to model very active tripodal titanium single sites. It appears that a tripodal coordination of Ti sites facilitates the catalytic activity that is in contrast quite low for the two new complexes investigated in this work, probably due to the higher stability toward dissociation to generate a tripodal coordination of the Ti−O−Ti bond and to the steric hindrance of Ti tetrapodal coordination.
[Show abstract][Hide abstract] ABSTRACT: 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)).
[Show abstract][Hide abstract] ABSTRACT: A rotary-bed OMCVD device was developed and successfully applied to the scale-up of the preparation of supported rhodiumnanoparticles. 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 Rhnanoparticles showed outstanding performance in the hydrogen production by methanecatalytic partial oxidation at low contact times. Materials produced were fully characterized by a pool of techniques including HRTEM, CO-DRIFTS, TPRD, etc.
Journal of Materials Chemistry 01/2009; 19(47). · 5.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The complex [Rh(cod)(dppp)]OTf (Rh(cod)) has been immobilized onto silica-supported palladium nanoparticles (Pd/SiO2) via a dual H-bond/ionic interaction (dppp = 1,3-bis(diphenylphosphino)propane; cod = cycloocta-1,5-diene). The product obtained, Rh(cod)-Pd/SiO2, has been employed to catalyze the hydrogenation of benzene to cyclohexane, showing much higher activity as compared to Pd/SiO2, while Rh(cod) grafted on bare silica (Rh(cod)/SiO2) is totally inactive. The catalyst generated by Rh(cod)-Pd/SiO2 exhibits a remarkable stability and can be recycled several times with no loss of activity, even if exposed to air. In situ and ex situ EXAFS and DRIFTS measurements, batch catalytic reactions under different conditions, deuterium labeling experiments, and model organometallic studies, taken altogether, have provided valuable mechanistic information. The reduction of benzene to cyclohexa-1,3-diene occurs with the cooperation of the two metals, while the rhodium single sites are more effective than the palladium nanoparticles in the hydrogenation of cyclohexa-1,3-diene to cyclohexane.
[Show abstract][Hide abstract] ABSTRACT: The organometallics chemical vapour deposition (OM-CVD) technique, using Rh(acac)(CO)2 as a precursor, was employed for the preparation of heterogeneous Rh catalysts supported on low surface area refractory oxides (α-Al2O3, ZrO2, MgO and La2O3). Prepared systems were tested in the methane catalytic partial oxidation (CH4-CPO) reaction in a fixed bed reactor and compared to a reference catalyst prepared from impregnation of Rh4(CO)12.Catalysts supported on Al2O3, ZrO2 and MgO show better or comparable performances with respect to the reference system.Complete decomposition of Rh precursor during formation of the metal phase under reductive conditions was investigated by TPRD and confirmed by infrared and mass spectrometry data.Supported Rh phase was characterized by CO and H2 chemisorption, CO-DRIFT spectroscopy and HRTEM microscopy in fresh and aged selected samples. Rh(I) isolated sites and Rh(0) metal particles were found on fresh catalysts; after ageing an extensive reconstruction occurs mainly consisting in a sintering of Rh isolate sites to metal particles but without large increase in mean particles size.Catalytic performances and Rh species balance were found to be dependent on the support material.
[Show abstract][Hide abstract] ABSTRACT: This work extends a previous study on the conditioning of Rh/Al2O3 during CH4-CPO experiments at high space velocity. The effect of the preparation procedure was investigated by comparing well-known preparation techniques such as incipient wetness impregnation and grafting, and a novel CVD technique. Catalysts prepared by impregnation of the support with a solution of Rh(NO3)3 showed a very slow activation process, starting from an initially poor syngas activity and ending up to outstanding performances. Catalysts prepared by grafting of Rh4(CO)12 onto alumina showed a similar behaviour, but the initial activity was closer to final activity (similarly high). Catalysts prepared by CVD of Rh(acac)(CO)2 showed very good and stable performances from the very initial CPO run. To better understand the morphological changes which occur during reaction conditions, H2 chemisorption, CO chemisorption, CO-DRIFT and HRTEM were applied in concert to analyze the surface of samples prior and after catalytic testing. Characterization results support the hypothesis that the conditioning process is related to a reconstruction of the Rh particles which tends to eliminate small defective clusters and particles, wherein C-forming reactions are presumably highly favoured. The extent of reconstruction is thus strictly related to the heterogeneity of the surface, which is affected by the preparation procedure; heterogeneity is high over samples prepared via impregnation and grafting, low over sample prepared by CVD.
[Show abstract][Hide abstract] ABSTRACT: The catalytic hydrogenation of 1,10-phenanthroline (Phen) or 2,9-dimethyl-1,10-phenanthroline (DMPhen) has been achieved using silica-supported palladium nanoparticles (Pd/SiO2) with metal contents of 1.98 or 9.95wt%. With either catalyst, the hydrogenation regiochemistry has been effectively controlled by the reaction temperature. The catalyst with the higher metal content was selective for the hydrogenation of one heterocyclic ring of either substrate at 80°C and for both external rings at 130°C for Phen and at 160°C for DMPhen. The catalyst with the lower metal content was more active and exhibited comparable selectivity.
[Show abstract][Hide abstract] ABSTRACT: The dehydrogenation of propane was studied in gas-phase at 773K over two series of silica-deposited Ir–Sn systems: the bimetallic
catalysts obtained from Ir–Sn carbonyl clusters precursors and the ones prepared by deposition of a metallorganic Sn precursor
onto preformed Ir nanoparticles. In the comparison, cluster-derived catalysts showed good propane conversion, optimal selectivity
to propene and high stability under the severe reaction conditions.
[Show abstract][Hide abstract] ABSTRACT: The complex Rh(cod)(sulfos) (Rh(I); sulfos = (-)O(3)S(C(6)H(4))CH(2)C(CH(2)PPh(2))(3); cod = cycloocta-1,5-diene), either free or supported on silica, does not catalyze the hydrogenation of benzene in either homogeneous or heterogeneous phase. However, when silica contains supported Pd metal nanoparticles (Pd(0)/SiO(2)), a hybrid catalyst (Rh(I)-Pd(0)/SiO(2)) is formed that hydrogenates benzene 4 times faster than does Pd(0)/SiO(2) alone. EXAFS and DRIFT measurements of in situ and ex situ prepared samples, batch catalytic reactions under different conditions, deuterium labeling experiments, and model organometallic studies, taken together, have shown that the rhodium single sites and the palladium nanoparticles cooperate with each other in promoting the hydrogenation of benzene through the formation of a unique entity throughout the catalytic cycle. Besides decreasing the extent of cyclohexa-1,3-diene disproportionation at palladium, the combined action of the two metals activates the arene so as to allow the rhodium sites to enter the catalytic cycle and speed up the overall hydrogenation process by rapidly reducing benzene to cyclohexa-1,3-diene.
Journal of the American Chemical Society 06/2006; 128(21):7065-76. · 10.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Copper catalysts prepared by chemisorption-hydrolysis technique over silica (Cu/Si) and silica-alumina (Cu/SiAl) supports were studied to understand the role of the support on the nature and surface properties of the copper species stabilized on their surfaces. The morphological and surface properties of the copper phases have been characterized by complementary techniques, such as HRTEM, EXAFS-XANES, EPR, XPS, and FTIR. For the FTIR investigation, molecular probes (CO and NO) were also adsorbed on the surfaces to test the reactivity of the copper species. Moreover, the catalytic performances of the two catalysts have been compared in the HC-SCR reaction (NO reduction by C(2)H(4)) performed in highly oxidant conditions. The superior activity and selectivity of the supported silica-alumina catalyst with respect to that supported on silica could be related with the different nature of the copper species stabilized on the two supports, as emerged from the results obtained from the spectroscopic investigations. Small and well-dispersed CuO particles were present on silica, whereas isolated copper ions predominated on silica-alumina, likely in regions rich in alumina that made some exchangeable sites available, as indicated by FTIR spectra of adsorbed CO. The less positive global charge of copper species on Cu/SiAl than in Cu/Si has been confirmed by EPR, XPS, and EXAFS-XANES analyses.
The Journal of Physical Chemistry B 05/2006; 110(15):7851-61. · 3.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A parent acidic H-BEA with crystallites very small in size and high external surface area was used to prepare a series of materials loaded with increasing Cs+ contents by firstly ion-exchange and then impregnation with CsOH solutions. The monitoring of the ion-exchange process by chemical analysis and by IR spectroscopy in presence of CO or NH3 reveals that a relevant amount of Brønsted acid sites in dehydrated H-BEA is related to framework Al sites that, in aqueous solution, turn into partially extraframework Al species unable to act any longer as sites of cationic exchange. This limits the exchange capacity in solution and higher levels of ion-exchange are attained by subsequent impregnation and calcination. A possible explanation for such a behaviour is proposed. The formation of carbonates by adsorption of CO2, monitored by IR, confirms that the basic character induced on framework oxygen atoms by exchange of H+ with Cs+ is significantly weaker than that reached upon Cs-overloading. For the latter, the strong basicity is related to the presence of Cs2O-like nanoparticles (also detected by EXAFS), dispersed within the zeolite pores (as shown by pore volume and TEM/EDX measurements). IR spectroscopy of adsorbed CO shows that Cs+ as countercations or as surface sites of occluded Cs2O-like species exhibit a similar Lewis acid strength. Noticeably, in Cs-overloaded BEA, pairs of Cs+ sites (formed by two countercations and/or one countercation and a Cs+ at the surface of Cs2O-like particles) are present, where CO can be adsorbed in a head–tail form, producing a distinct νCO band at 2145cm−1.
Microporous and Mesoporous Materials - MICROPOROUS MESOPOROUS MAT. 01/2006; 90(1):175-187.