Gabriela Díaz

Hydrodesulphurization activity and characterization of sulphided molybdenum and cobalt—molybdenum catalysts : Comparison of Alumina-, Silica-Alumina- and Titania-Supported Catalysts

The nature of the support effect in unpromoted (Mo) and promoted (CoMo) catalysts was examined by comparing the thiophene hydrodesulphurization activities of the catalysts supported on Al2O3, TiO2 and SiO2Al2O3. Catalyst samples were prepared by the incipient wetness impregnation method and characterized by photoelectron spectroscopy (XPS) and high-resolution electron microscopy (HREM). The activity per atom of molybdenum was higher for the TiO2-unpromoted and -promoted catalysts, but the synergistic effect was higher for the Al2O3, -supported catalysts. The absence of a significant shift in the titanium and molybdenum binding energies for catalyst supported on TiO2 and Al2O3 with respect to unsupported MoS2 catalyst does not support an electronic effect as the main cause of the great differences in activity between TiO2- and Al2O3- supported catalysts. The HREM results show clear evidence of the existence of smaller MoS2 particles on the TiO2-supported catalysts. Also, the addition of the promoter leads to the formation of smaller particles on the surface of the TiO2-supported catalysts. The differences in activity and synergistic effect for the different catalysts could be explained in terms of different activities for the smaller and larger particles. It is proposed that the difference in activity between the smaller and larger particles is related to the MoS2 crystallite orientation on the surface of the support.

Desorption and catalytic properties of palladium, supported on Al2O3-La2O3, prepared by the sol-gel method

Desorption and catalytic properties of 0.3 wt% Pd catalysts supported on alumina-lanthana, prepared by the sol-gel method, were studied. A large excess of consumed hydrogen was observed during TPR experiments on these catalysts due to the partial reduction of lanthana. The enhanced hydrogen adsorption (H/Pd varies from 8 to 15) detected by TPD for these catalysts is supposed to be a consequence of spillover. Spillover of hydrogen is favored by a high dispersion of palladium on the support surface and the presence of lanthana reduced species. Palladium on alumina-lanthana catalysts show a higher catalytic activity than palladium on alumina catalysts because reduced species of lanthana stabilize palladium particles. Similarity of ammonia selectivity at high temperatures allows one to suggest that reduced lanthana is involved in the reaction on pure alumina-lanthana support and palladium on alumina-lanthana catalysts at these temperatures. A synergetic effect between small palladium particles and reduced species of lanthana is suggested to be responsible for observed behavior of the lanthana-promoted palladium catalysts.

Structural and catalytic properties of Pd/Al2O3–La2O3 catalysts

Abstract A new Pd/Al2O3–La2O3 catalyst has been synthesized for the reduction of NO with hydrogen. This catalyst is more active than coprecipitated Pd/Al2O3 catalysts. The revealed effect of the improvement of the catalytic activity at medium temperature and the increase of NH3 formation at high temperatures for Pd catalyst supported on alumina–lanthana prepared by the sol–gel method are ascribed to a new lanthanum-containing phase observed by X-ray powder diffraction and high resolution electron microscopy.

Hydrogenolysis of methylcyclopentane and isomerization of 2-methylpentane over well characterized silica-supported platinum-ruthenium catalysts

Abstract The hydrogenolysis of methylcyclopentane and the isomerization of 2-methylpentane were studied over Pt-Ru bimetallic catalysts prepared using either H 2 PtCl 6 and RuCl 3 (Series A) or from Pt(NH 3 ) 4 (NO 3 ) 2 and Ru(NH 3 ) 6 Cl 3 (series B). Energy-dispersive X-ray spectroscopy (EDXS) studies showed that the catalysts prepared using the amine precursors were 100% bimetallic. Image defocussing studies suggest that approximately 20% of the particles prepared using the chloride precursors were monometallic. At temperatures at which only ruthenium is active (413 K), the formation of C 1 to C 5 products was depressed over Pt-Ru bimetallic clusters. Tertiary-secondary carbon-carbon bond cleavage approaches that observed for ruthenium while secondary-secondary carbon-carbon bond cleavage is closer to that observed for platinum. The isomerization of 2MP at 493 K is representative of Pt-Ru activated dual sites since platinum is inactive at this temperature and ruthenium leads to extensive degradation of the molecule. The contribution of the cyclic mechanism was obtained using 13 C labelled species. A synergistic effect was observed for the catalysts prepared using amine precursors. This is explained by considering an increase in the number of Pt-Ru dual surface sites.

Carbon Nanotubes Prepared by Catalytic Decomposition of Benzene Over Silica Supported Cobalt Catalysts

Abstract Carbon nanostructures were prepared by catalytic decomposition of benzene on Co/SiO2 catalysts at 873 K. As a function of particle size and reactive mixture, carbon filaments and nanotubes were obtained and various shapes observed such as straight, curved and helically-coiled nanotubes. Particularly, it was shown that the helically-coiled nanotubes do not present an ordered atomic structure as it was predicted theoretically. However, the active catalytic particle as well as the reactive environment may play an important role in the growth of such nanostructures.

Catalytic activity and X-ray photoelectron spectroscopy performance of Bi2MoxW(1−x)O6 solid-solutions

The present paper describes the results of the study of solid solutions of Bi2MoxW(1−x)O6, which were prepared by the route of solid-state reaction and tested for catalytic activity in the oxidation of CO into CO2. X-Ray photoelectron spectroscopy (XPS) was conducted to obtain information on the chemical composition, and with the use of Bi2MoO6 and Bi2WO6 synthesized standards, quantitative measures were obtained. Catalytic studies of the samples show that they possess high activity, as a change in the temperature of activation as a function of the sintering temperature with the Mo/W ratio was detected. Bi–Mo mixed oxides have synergistic effects on the oxidation of CO, making possible the full conversion at lower temperatures than compounds that contain only single phases. The work described here demonstrates that a solid solution can be more efficient in oxidation, compared with single oxides studied separately.

NO + H2 reaction on Pt-Ru/SiO2 catalysts: correlation between nanostructure and catalytic activity and selectivity

Nitric oxide reduction by hydrogen has been studied on Pt-Ru/SiO2 catalysts of various Pt/Ru atomic compositions in the temperature range 298–673 K. Physical characterization showed the presence of bimetallic particles which tend to be Pt-rich. The overall activity of the bimetallic catalysts suggests a dilution of the active component (Pt) in the range 373–523 K. The addition of Ru results in a general improvement of the N2 selectivity and significant modifications in the product distribution are observed as a function of the catalyst composition. A bimetallic particle model is proposed in which various types of surfaces are exposed including those with pure Pt atoms and/or Pt-Ru mixture. This model allows to explain the overall activity and selectivity in the whole series of catalysts.

High oxygen storage capacity and enhanced catalytic performance of NiO/NixCe1−xO2−δ nanorods: synergy between Ni-doping and 1D morphology

A series of Ni-doped ceria nanorods with low Ni content was synthesized via a hydrothermal approach and characterized by DRX, SEM, HRTEM, Raman and TPR techniques. Simultaneous tuning of morphology and composition, i.e. exposure of reactive (100) and (110) facets together with Ni substitution, provide great improvements in their physical chemical properties. Segregated NiO as a secondary phase was observed as Ni content increases affecting the growth of the nanorods. The system is described as NiO dispersed on NixCe1−xO2−δ nanorods. The materials showed a remarkably high OSC at 400 °C and good activity for CO oxidation. Results corroborate the synergic effects between Ni-doping and one-dimensional morphology on ceria structure.

Hydrodesulphurization activity and characterization of sulphided molybdenum and cobalt—molybdenum catalysts

The nature of the support effect in unpromoted (Mo) and promoted (CoMo) catalysts was examined by comparing the thiophene hydrodesulphurization activities of the catalysts supported on Al2O3, TiO2 and SiO2Al2O3. Catalyst samples were prepared by the incipient wetness impregnation method and characterized by photoelectron spectroscopy (XPS) and high-resolution electron microscopy (HREM). The activity per atom of molybdenum was higher for the TiO2-unpromoted and -promoted catalysts, but the synergistic effect was higher for the Al2O3, -supported catalysts. The absence of a significant shift in the titanium and molybdenum binding energies for catalyst supported on TiO2 and Al2O3 with respect to unsupported MoS2 catalyst does not support an electronic effect as the main cause of the great differences in activity between TiO2- and Al2O3- supported catalysts. The HREM results show clear evidence of the existence of smaller MoS2 particles on the TiO2-supported catalysts. Also, the addition of the promoter leads to the formation of smaller particles on the surface of the TiO2-supported catalysts. The differences in activity and synergistic effect for the different catalysts could be explained in terms of different activities for the smaller and larger particles. It is proposed that the difference in activity between the smaller and larger particles is related to the MoS2 crystallite orientation on the surface of the support.

On the atomic structure of Fe/Mn promoted sulfated zirconia

Abstract The aim of the present work is to shed some light on the surface atomic structure of an Fe Mn-promoted sulfated zirconia catalyst using high-resolution transmission electron microscopy (HRTEM) imaging. The presence of Fe and Mn species, which were found to be dispersed at a molecular level, were apparent on the {{001}} and {{110}} surfaces of zirconia particles. A combination of the present results with additional detailed studies using other physicochemical characterization techniques is needed to elucidate the atomic configuration of the active sites responsible for the enhanced catalytic activity of the Fe Mn-promoted sulfated zirconia catalyst.