I. Rodríguez-Ramos

Comparative study at low and medium reaction temperatures of syngas production by methane reforming with carbon dioxide over silica and alumina supported catalysts

Two series of transition metals (Co, Ni, Ru, Rh, Ir, Pt) based catalysts have been prepared using silica and γ-alumina. Their activity and stability for the dry reforming of methane in the temperature interval from 673 up to 1023 K have been examined and compared. The obtained results show that the support exerts a great influence on the turnover frequency of a given metal but deactivation occurring under reaction conditions mainly depends on the nature of the active metallic phase. Some of the catalysts suffer deactivation processes at temperatures close to 1023 K, that can be ascribed to sintering, and in only some cases, also to carbon deposition. At low reaction temperatures, i.e. 723 K, deactivation was only observed over supported iridium. Among all tested catalysts, those based on nickel, cobalt and rhodium appear to be the most resistant to deactivation processes under our experimental conditions and for the whole temperature range up to 1023 K. Among them, supported rhodium catalysts show an excellent stability, though alumina supported rhodium exhibits a much higher turnover frequency.

Study of some factors affecting the Ru and Pt dispersions over high surface area graphite-supported catalysts

Abstract In a study of the preparation of carbon-supported Pt and Ru catalysts, three high surface area graphites with similar textural characteristics, but differing in surface chemistry, have been used as carriers. Four series of metallic catalysts (two based on Pt and two on Ru) have been prepared by using different metal precursors and preparation methods. The catalysts were characterized by temperature-programmed reduction, H2 and CO chemisorption and microcalorimetry of CO chemisorption. The achieved metallic dispersions mainly depend on the reduction-decomposition treatments and on specific interactions between the metal particles and surface defects at the graphite. Microcalorimetry of CO adsorption evidences that the presence of oxygen surface groups diminishes the metal–support interaction.

Methane combustion over supported palladium catalysts I.Reactivity and active phase

The performance of Al(2)O(3), ZrO(2) and ZrO(2) stabilized with SiO(2) (ZrO(2)-s) supported palladium catalysts for the methane combustion was studied between 473 and 873 K. The nature of the surface species of palladium catalysts under reaction conditions were detected by FT-IR and microcalorimetry of CO adsorbed. The different behavior of palladium catalysts under reaction conditions is attributed to support effects associated to differences in thermal conductivity and oxygen mobility of supports. Prereduction of the catalysts enhances their activity. Under reaction conditions, the prereduced sample becomes partially oxidized by preferential adsorption/reaction of oxygen both on Pd ( 1 1 1) planes and on the sites that can multibondedly adsorb CO. The reconstruction of the metallic particles and the formation of PdO(x) (0 1) phase were directly observed by FT-IR and microcalorimetry of adsorbed CO. Combination of different characterization techniques with reaction results suggests that a mixed phase, Pd(0)/PdO(x), is the most active phase for methane combustion, and that a redox mechanism may occur on this phase

Mechanistic aspects of the dry reforming of methane over ruthenium catalysts

Abstract Carbon dioxide reforming of methane has been studied over two ruthenium catalysts supported on silica and on γ-alumina. Catalytic activity measurements, infrared spectroscopic analysis and isotopic tracing experiments applied to the study of the surface hydroxyl groups of the supports have allowed different reaction mechanisms to be proposed on the bases of the detected surface species, their mobility, stability and reactivity. Activation of both reactants takes place on the ruthenium surface for Ru/SiO 2 catalyst. The accumulation of carbon adspecies formed from methane decomposition on the metallic particles finally impedes carbon dioxide dissociation and induces rapid deactivation of this catalyst. The alumina support provides an alternate route for CO 2 activation by producing formate intermediates on its surface that subsequently decompose releasing CO. This bifunctional mechanism, in which the hydroxyl groups of the support play a key role, induces greater stability on the Ru/Al 2 O 3 catalyst by significantly decreasing the rate of carbon deposition on the metal. The proposed reaction pathway requires continuous surface mobility of species from the metal to the support and vice versa.

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.

Surface chemical modifications induced on high surface area graphite and carbon nanofibers using different oxidation and functionalization treatments

Two graphitic carbon materials with different edge to basal plane ratio, high surface area graphite (HSAG) and graphitized carbon nanofibers (CNFs), were oxidized by two methods, aqueous-HNO(3) wet oxidation and oxygen plasma oxidation. Characterization of the materials by temperature-programmed desorption, thermogravimetry and X-ray photoelectron and Raman spectroscopies indicated that the amount and nature of oxygen surface groups introduced depended on the oxidation method and on the structure of the original material. While surface sites within the layers were only oxidized by oxygen plasma, surface sites at the edges of graphene layers were oxidized by both treatments being the wet oxidation more effective. Modification of the oxidized materials with a diamine or a triamine molecule resulted in the formation of ammonium carboxylate salt species on the carbon surface.

Dehydrogenation of methanol to methyl formate over supported copper catalysts

Abstract The catalytic dehydrogenation of methanol over copper-supported (Cu/MOχ,M=Si, Zr, Mg, Zn and graphite) catalysts has been studied in a fixed bed microreactor in the temperature range of 180–260°C. The copper/silica and copper/zirconium oxide catalysts were found to be very active and selective toward methyl formate (MF) formation. However, the copper/zinc oxide catalyst was less active and the copper/magnesium oxide one much less selective. The exception was the copper/graphite catalyst whose performance approached that of massive copper. The reaction has also been studied on CuM/SiO2 (M=Na2O, ZnO, Cr2O3) promoted catalysts. Only in the case of copper promotion by 0.1% sodium (copper/sodium=11.2 atomic) was the specific activity found to increase, while the MF selectivity remained unchanged. The state of copper in the catalysts used in the reaction was determined by X-ray photoelectron spectroscopy. With the sole exception of the copper/magnesium oxide catalyst, which was partially oxidised as Cu2+, the other catalysts showed only reduced copper species. For the used 15.1% copper/silica catalyst the position of the copper L3VV X-ray induced Auger transition at 913.4 eV confirmed the existence of reduced Cu0. Additional catalytic experiments carried out on unreduced silica-supported copper catalysts, using carbon dioxide as a carrier feed, showed that a longer time onstream was required before a steady state was achieved, which in turn is typical of catalyst activation by the hydrogen reaction byproduct (eqn. 4). Carbon monoxide poisoning experiments led to the conclusion that metallic copper is very likely the active copper species in this reaction. Furthermore, the infrared spectra of methanol adsorption on the poorly selective 5.7% copper/magnesium oxide catalyst revealed that the MF product undergoes decomposition through carbonate structures at basic sites.

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.

Methane interaction with silica and alumina supported metal catalysts

Abstract Six different transition metals — cobalt, nickel, ruthenium, rhodium, iridium and platinum — have been supported on silica or alumina by means of the incipient wetness method. After drying and calcination at 773 K, they have been characterized by the hydrogen pulse chemisorption technique and by transmission electron microscopy. The interaction of CH4 with the catalyst surface has been studied by a temperature programmed surface reaction of methane and by examining the reactivity with hydrogen of the carbon deposits formed for three hours of reaction at 623 K in a flow of diluted methane. It has been found that the support exerts a great influence in the activity of dehydrogenation of methane and in the kind of carbonaceous species generated. It has also been proved that rhodium catalysts have a special ability to stabilize reactive carbon species (Cβ) hydrogenable between 423 K and 498 K.

Oxydehydrogenation of ethylbenzene to styrene catalyzed by graphites and activated carbons

Abstract The catalytic activity and selectivity of carbon materials (graphites and activated carbons) in the oxidative dehydrogenation of ethylbenzene have been studied as a function of time on stream. The changes in activity and selectivity are explained as due to the formation of carbonaceous deposits on the sample surfaces. The surface area, the porous structure, the presence of surface groups, and the reactivity in air of the different carbon samples have been determined and related to their catalytic performance. The ability of hydrogen present at the carbon surfaces to be exchanged with deuterium atoms of deuterated benzene have been measured before and after the use as catalyst. The chemical nature of the surface coke is found to be conditioned by that of the carbon substrate. The selectivity for styrene is concluded to be associated with the capacity to form surface groups with labile hydrogen atoms.