B. Bachiller-Baeza

Catalytic wet air oxidation of phenol and acrylic acid over Ru/C and Ru–CeO2/C catalysts

Ru/C catalysts promoted, or not, by cerium were prepared by impregnation of an active carbon (961 m 2 g 1 ) with chlorine-free precursors of Ru and Ce. They were characterized by chemisorption of H2 and of CO and by electron microscopy. TEM and H2 chemisorption gives coherent results while CO chemisorption overestimates Ru dispersion. In Ru‐Ce/C, Ce is in close contact with Ru and decreases Ru accessibility. Catalytic wet air oxidation (CWAO) of phenol and of acrylic acid (160C and 20 bar of O2) was investigated over these catalysts and their performance (activity, selectivity to intermediate compounds) compared with that of a reference Ru/CeO 2 catalyst. Carbon-supported catalysts were very active for the CWAO of phenol but not for acrylic acid. Although high conversions were obtained, phenol was not totally mineralized after 3 h. It was shown that acrylic acid was more strongly adsorbed than phenol. Moreover, the number of contact points between Ru particles and CeO2 crystallites constitutes a key parameter in these reactions. A high surface area of ceria is required to insure O 2 activation when the organic molecule is strongly adsorbed. ©2000 Elsevier Science B.V. All rights reserved.

Influence of Mg and Ce addition to ruthenium based catalysts used in the selective hydrogenation of α, β-unsaturated aldehydes

Abstract Ce and Mg were used as promoters in two series of Ru based catalysts supported on alumina (Al 2 O 3 ) and activated carbon (AC). The catalysts were characterized by H 2 chemisorption and temperature-programmed reduction (TPR), and studied in the crotonaldehyde (gas phase) and the citral (liquid phase) hydrogenations. Addition of MgO and CeO 2 decreased the catalytic activity in crotonaldehyde and citral hydrogenations. With regard to the selectivity towards unsaturated alcohols, similar trends were observed for the two reactions. MgO did not influence the selectivity, but CeO 2 increased the selectivity to unsaturated alcohols, especially on carbon supported catalyst. Bulk CeO 2 and Ce/AC catalyst showed low activity but very high selectivity (93 and 100%, respectively) to the unsaturated alcohols. Based on these results and the calorimetric experiments of CO adsorption it was suggested that defect sites on the surface of the promoter are the active and highly selective sites for unsaturated aldehydes due to their influence on the CO bond activation.

Role of the residual chlorides in platinum and ruthenium catalysts for the hydrogenation of α,β-unsaturated aldehydes

The gas-phase hydrogenation of crotonaldehyde has been performed over platinum and ruthenium catalysts supported over graphites with a tailored content in surface oxygen groups and prepared with different metal precursors. It is found that the selectivity to the unsaturated alcohol is independent of the amount of oxygen groups on the surface of the graphite supports. Notwithstanding this, their presence has an effect when a metal chloride precursor is used in the catalyst preparation since they act as anchoring sites of the chloride ions and the latter in residual amounts promote the hydrogenation of the carbonyl group. The addition of tin to platinum enhances also the selectivity to crotyl alcohol.

Novel electrochemical sensor based on N-doped carbon nanotubes and Fe3O4 nanoparticles: simultaneous voltammetric determination of ascorbic acid, dopamine and uric acid.

A new modified electrode based on N-doped carbon nanotubes functionalized with Fe3O4 nanoparticles (Fe3O4@CNT-N) has been prepared and applied on the simultaneous electrochemical determination of small biomolecules such as dopamine (DA), uric acid (UA) and ascorbic acid (AA) using voltammetric methods. The unique properties of CNT-N and Fe3O4 nanoparticles individually and the synergetic effect between them led to an improved electrocatalytic activity toward the oxidation of AA, DA and UA. The overlapping anodic peaks of these three biomolecules could be resolved from each other due to their lower oxidation potentials and enhanced oxidation currents when using the Fe3O4@CNT-N modified electrode. The linear response ranges for the square wave voltammetric determination of AA, DA and UA were 5-235, 2.5-65 and 2.5-85μmoldm(-3) with detection limit (S/N=3) of 0.24, 0.050 and 0.047μmoldm(-3), respectively. These results show that Fe3O4@CNT-N nanocomposite is a promising candidate of cutting-edge electrode materials for electrocatalytic applications.

Surface Properties of Supported Metallic Clusters as Determined by Microcalorimetry of CO Chemisorption

Recent results achieved by microcalorimetry of CO adsorption over supported metal catalysts will be reviewed by considering some parameters that control the surface properties of the supported metallic particles. In particular, the effects induced by the nature and the particle size of supported metallic clusters, the conditions of pretreatment, the support materials, the addition of promoters and the presence of a second metal will be shown.

Catalytic properties of carbon-supported ruthenium catalysts for n-hexane conversion

Abstract The effect of the support (graphite, activated carbon, silica and alumina) has been investigated using the conversion of n-hexane as model reaction. The catalysts were characterized by H2 adsorption and transmission electron microscopy. Also, the CO adsorption was studied by microcalorimetry. The higher initial heat of CO adsorption observed on ruthenium/graphite indicates an enhanced electron density of the ruthenium particles caused by electron transfer from the graphite. The catalytic results show that ruthenium particles with an increased electron density have a higher activity for the n-hexane conversion accompanied by a preference for hydrogenolysis reaction, with the hydrogenolysis pattern corresponding to a multiple fragmentation. The higher resistance of ruthenium/graphite catalysts to deactivation by carbonaceous deposits was also observed.

Correlation between metal oxidation state and catalytic activity : hydrogenation of crotonaldehyde over Rh catalysts

The effects of the rhodium (oxidation) state on the activity and selectivity for the crotonaldehyde hydrogenation reaction over Rh/Al2O3 and Rh/SiO2 catalysts were examined using the techniques of temperature-programmed reduction, hydrogen chemisorption and X-ray absorption near-edge structure (XANES). In the alumina-supported system, the active phase-support interaction is shown to affect the chemical behavior of rhodium under the influence of a reductive atmosphere by stabilizing Rh3+ species. This behavior is not observed (as expected) for Rh/SiO2 catalysts. The structural and electronic bases of the active phase-support interaction and the effect of the latter phenomenon on the hydrogenation of crotonaldehyde are discussed.

Diastereoselective hydrogenation of o-toluic acid coupled with (S)-proline and (S)-pyroglutamic acid methyl esters on ruthenium catalysts

Abstract The diastereoselective hydrogenation of two o -toluic acid derivatives, N -(2-methyl-benzoyl)-( S )-proline methyl ester here named substrate 1 , and N -(2-methyl-benzoyl)-( S )-pyroglutamic acid methyl ester abbreviated as substrate 1 ′, was studied on metallic ruthenium particles deposited on two oxidic supports, Al 2 O 3 and SiO 2 and on different carbon materials, such as an activated carbon, a carbon molecular sieve and two high surface area graphites. The asymmetric induction was dependent on the chiral auxiliary employed: cis isomers were formed preferentially, with configuration (1 R , 2 S , 2′ S ) and (1 S , 2 R , 2′ S ) for substrates 1 and 1 ′, respectively. The diastereoselectivity also depended on the nature of the support and higher values of diastereomeric excess (d.e.) were obtained on catalysts supported on metal oxides, 35% for substrate 1 and 82% for 1 ′, than on catalyst supported on carbon materials, 23% for substrate 1 and 61% for 1 ′. No relevant effects were found among the various allotropic forms of carbon. The introduction of an additive, ethyldicyclohexylamine (EDCA), in the hydrogenation solution of substrate 1 reduced the d.e. from 21 to 10% for high surface area graphite catalyst, while for alumina supported catalyst the d.e. was inversed from 35% (1 R , 2 S , 2′ S ) isomer to 2% in favour of isomer (1 S , 2 R , 2′ S ). All these findings confirm the importance of catalytic supports in modifying the adsorption properties of ruthenium particles deposited on them.

The effect of Cu loading on Ni/carbon nanotubes catalysts for hydrodeoxygenation of guaiacol

Commercial carbon nanotubes (CNT), were used as supports to prepare Ni/CNT catalysts with a 15 wt% Ni loading and NiCux/CNT catalysts with Cu loadings of x: 1.5, 2.25, 3.0 and 3.75 wt% were prepared. The catalysts were characterized by N2 physisorption, H2-temperature programmed reduction (TPR), transmission electron microscopy (TEM), STEM-HAADF and X-ray photoelectron spectra (XPS), and they were evaluated in the conversion of guaiacol at 573 K and 5 MPa H2 pressure for 4 h in a batch reactor. The characterization showed that Ni–Cu alloys were formed which existed along with Ni, Cu and NiO entities. The Ni metallic particle size was lower upon increasing Cu content, however the increased proportion of NiO species and the coverage of Ni sites with Cu diminished the intrinsic activity. The addition of Cu also decreased the hydrogenolysis and deoxygenation ability for the catalysts up to 3 wt% of Cu, which can be related to the lower size of Ni ensembles. Beyond that percentage, the selectivity was modified and it was attributed to changes in the Ni dispersion.

Hydrogenation of crotonaldehyde over carbon-supported molybdenum nitrides

The gas‐phase hydrogenation of crotonaldehyde has been performed over unsupported Mo2N and over Mo2N supported on two carbonaceous materials, an activated carbon (AC) and a high surface area graphite (G). These catalysts were in situ prepared by the temperature‐programmed reaction of MoO3 and carbon‐supported Mo precursors with NH3. It is found that selectivity to the unsaturated alcohol is maximum for the Mo2N/G sample, where crotylalcohol selectivities exceed the 60% during all time in reaction. Higher selectivity surface sites are associated with the (200) planes of the γ‐Mo2N crystallites.