CeO2 and La- or Pr-doped CeO2 were prepared at calcination temperatures of 600, 800, and 1000 degrees C. The samples were characterized by Raman, XRD, and N2 adsorption and tested as Rh2O3 support ...for the catalytic decomposition of N2O at low temperature (starting at 200 degrees C). Catalyst characterization was done by XPS analysis of fresh and in situ treated catalysts, TPR, and TEM. As deduced by XPS analysis and catalytic tests, Rh2O3 is more active than Rh0 for N2O decomposition. Pure CeO2 calcined at low temperature (600 degrees C) and La or Pr-doped CeO2 calcined at 600, 800 or 1000 degrees C can keep Rh2O3 stable under reaction conditions. In contrast, Rh2O3 supported on pure CeO2 calcined at high temperature (800 or 1000 degrees C) is reduced to Rh0 under reaction conditions. The redox properties of the support are decisive for Rh2O3 stabilization and catalytic performance; the larger the reducible surface CeO2 (determined by TPR), the better the catalytic activity. In addition, structural and textural features of the support (crystallinity, BET surface area, and particle size) affect Rh2O3 dispersion, the smaller the support particle size (the higher the surface area), the better the dispersion and the catalytic activity. The redox properties and particle size of supports depend on calcination temperature. Doping improves thermal stability with regard to pure CeO2. La and Pr form solid solutions with CeO2, preventing sintering and maintaining a high percentage of reducible CeO2 at high calcination temperatures (800 and 1000 degree C). PUBLICATION ABSTRACT
•Ceria doping with 10% Pr had a positive effect on RhOx-support interaction.•The benefit on the catalytic activity was only obtained for water impregnation.•Ethanol and acetone promoted Ce0.9Pr0.1O2 ...and RhOx sintering.•The RhOx and Ce0.5Pr0.5O2 interaction was not as good as that with Ce0.9Pr0.1O2.
The effect of the solvent (water, ethanol or acetone) used to impregnate CeyPr1−yO2 (y=1, 0.9 or 0.5) supports with rhodium nitrate, in order to prepare N2O decomposition catalysts, has been studied. RhOx/CeyPr1−yO2 catalysts were prepared and characterized by N2 adsorption at −196°C, XRD, Raman spectroscopy, TEM, XPS and H2-TPR. The activity for N2O decomposition of the catalysts studied was related with the RhOx-support interaction, and both the nature of the ceria support and of the solvent used for rhodium impregnation affected such interaction. Ceria doping with 10% praseodymium had a positive effect in the RhOx-support interaction, but the benefit on the catalytic activity was only obtained for water impregnation because the temperature peaks created during calcination of ethanol and acetone-impregnated catalysts promoted Ce0.9Pr0.1O2 and RhOx sintering. The interaction between RhOx and Ce0.5Pr0.5O2 was not as good as that with Ce0.9Pr0.1O2. The best catalyst was obtained by impregnating Ce0.9Pr0.1O2 with a water solution of rhodium. However, if acetone or ethanol must be used for any reason the pure ceria support is more suitable (under the calcination conditions of this study; 250 to 500°C at 10°C/min) because do not sinters during solvents combustion.
•The chiral catalyst RhDuphos has been anchored on carbon nanotubes and xerogels.•Non covalent anchoring strategies were used: electrostatic and π–π interactions.•Supports have been chemically ...modified to promote desired interactions.•RhDuphos on an oxidized carbon xerogel is active, enantioselective and reusable.
The immobilization of the chiral complex RhDuphos, by electrostatic or π–π (adsorption) interactions, on carbon nanotubes and carbon xerogels is investigated. To promote such interactions, the supports were either oxidized or heat treated to create carboxylic type surface groups or an apolar surface, respectively. The catalysts were tested in the hydrogenation of methyl 2-acetamidoacrylate.
The prepared hybrid catalysts are less active than the homogeneous RhDuphos, but most of them show a high enantioselectivity and the one prepared with the oxidized carbon xerogel is also reusable, being able to give a high substrate conversion, keeping as well a high enantioselectivity. The anchorage by electrostatic interactions is more interesting than the anchorage by π–π interactions, as the π–π adsorption method produces a modification of the metal complex structure leading to an active hybrid catalyst but without enantioselectivity.
The creation of carboxylic groups on the support surface has led to some hindering of the complex leaching.
Two phosphoric acid activation procedures; Activation after Hydrothermal Impregnation (recently published) and Activation after Incipient Wetness Impregnation instead of conventional impregnation are ...analyzed in two natural bio-fiber precursors: banana pseudostem and coconut fiber matting. Both procedures are compared analyzing, in both precursors, the influence that variables such as H3PO4/precursor ratio, activation temperature and impregnation time have on the resulting activated carbons (ACs) properties. The work also pays special attention to the mesoporosity development and the application of these ACs to adsorb gasoline vapors.
Both H3PO4 activation procedures develop activated carbons having suitable activation yields and porosity developments, giving the Activation after Incipient Wetness Impregnation method better results than the Activation after Hydrothermal Impregnation. Both natural bio-fibers are good precursors, rendering the coconut fiber matting better results than the banana pseudostem. The variables studied affect the porosity development, being precursor and H3PO4/precursor ratio the variables that most affect. By a suitable selection of these variables, activated carbons having high adsorption capacities (BET above 2500m2g−1 and micropore volume above 1.00cm3g−1) and well developed mesoporosity (reaching 1.41cm3g−1), can be prepared. Most of the samples prepared perform very well for adsorbing gasoline vapors, showing a linear relationship with their resulting volumes.
Hybrid catalysts, consisting of the rhodium complex Rh(COD)NH
2CH
2CH
2NH(CH
2)
3Si(OCH
3)
3
+BF
4
−, anchored on activated carbon have been prepared. The anchorage takes place by reaction of the
...Si(OCH
3)
3 ligand functionality with the surface oxygen complexes of the support. This work analyzes the effect of the extent and type of surface oxidation of the support on the catalytic properties of the obtained catalysts.
▪
Hybrid catalysts, consisting of the rhodium complex Rh(COD)NH
2CH
2CH
2NH(CH
2)
3Si(OCH
3)
3
+BF
4
−, (abbreviated as Rh(NN)Si), anchored on activated carbon have been prepared. The anchorage takes place by reaction of the
Si(OCH
3)
3 ligand functionality with the surface oxygen complexes of the support. This work analyzes the effect of the extent and type of surface oxidation of the support on the catalytic properties of the obtained catalysts. The original activated carbon (ROX0.8, NORIT) was oxidized using HNO
3 and (NH
4)
2S
2O
8 solutions and synthetic air. The supports were characterized by gas adsorption (N
2 at 77
K and CO
2 at 273
K), temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). The hybrid catalysts were characterized by XPS and their catalytic activity tested in the hydrogenation of cyclohexene. The results show that the catalysts prepared with extensively oxidized supports are less active. Surface oxidation has been considered to affect the location of the metal complex on the support surface. The metal complex is not deactivated in consecutive reaction cycles, but in most cases some leaching takes place. The catalyst prepared with the air oxidized support is the most active and stable one.
We present a novel and facile synthesis methodology for obtaining graphitic carbon structures from Fe(II) and Co(II) gluconates. The formation of graphitic carbon can be carried out in only one step ...by means of heat treatment of these organic salts at a temperature of 900 degrees C or 1000 degrees C under inert atmosphere. This process consists of the following steps: (a) pyrolysis of the organic gluconate and its transformation to amorphous carbon, (b) conversion of Fe(2+) and Co(2+) ions to Fe(2)O(3) and CoO and their subsequent reduction to metallic nanoparticles by the carbon and (c) conversion of a fraction of formed amorphous carbon to graphitic structures by Fe and Co nanoparticles that act as catalysts in the graphitization process. The removal of the amorphous carbon and metallic nanoparticles by means of oxidative treatment (KMnO(4) in an acid solution) allows graphitic carbon nanostructures (GCNs) to be selectively recovered. The GCNs thus obtained (i.e. nanocapsules and nanopipes) have a high crystallinity as evidenced by TEM/SAED, XRD and Raman analysis. In addition, we used these GCNs as supports for platinum nanoparticles, which were well dispersed (mean Pt size approximately 2.5-3.2 nm). Most electrocatalysts prepared in this way have a high electrocatalytical surface area, up to 90 m(2) g(-1) Pt, and exhibit high catalytic activities toward methanol electrooxidation.
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► Activated carbon and carbon nanotubes are suitable supports for the Wilkinson's catalyst. ► The immobilized Wilkinson's complex is active in hydrogenation and reusable. ► The ...catalyst prepared with carbon nanotubes is more active than the homogeneous complex. ► The positive effect of the support is explained by a confinement effect.
A Rh complex, derived from the Wilkinson's catalyst RhCl(PPh
3)
3, has been immobilized on activated carbon and carbon nanotubes through a linear organic molecule (6-amino-1-hexanol), covalently bonded to the support. This molecule acts as a linker in order to keep the complex apart of the support surface and to mimic its environment in homogeneous phase. The catalysts have been tested in the hydrogenation of cyclohexene. The hybrid catalyst prepared with the activated carbon is as active as the homogeneous Wilkinson's catalyst, while the catalyst prepared with nanotubes is noticeably more active. Negligible leaching is observed and the catalysts are recyclable. The positive effect of the support has been attributed to the confinement of the active species inside the pores, and thus, the structure and the pore size of the support have an outstanding role in the catalytic behaviour of the hybrid catalyst. The structure of the carbon nanotubes leads to hybrid catalysts with a noticeably enhanced catalytic activity.
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