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Sulfided αand β-NiMoO4 have been employed as model HDS catalysts, aiming to investigate the effect of phase composition of the precursor mixed oxide on the physicochemical characteristics and activity of the sulfides. Both sulfides are highly amorphous or microcrystalline, and could not be differentiated by means of XRD. The differences in BET area were also found to be minimal. However, it was found by means of TPR that the stable α-isomorph is reduced at lower temperatures than the unstable β-phase in both the oxidic and sulfided states. Sulfided β-NiMoO4 was found to be a better catalyst for the HDS of thiophene than the sulfided α-isomorph. This could be related to the higher stability of the former in H2, as decomposition of the active, amorphous Ni-Mo-S structures results in less active and more crystalline phases, as found by XRD.

Temperature-programmed reduction (TPR) has been employed to study the reducibility of Ni-Mo/γ-Al2O3 catalysts (0-5 wt% NiO; 12 wt% MoO3) calcined at temperatures (TC) between 400 and 800°C. Catalysts were further characterized by XRD, BET area, and ammonia adsorption measurements. Ni/Al2O3 samples showed two broad TPR bands: NiI, at lower temperature (Tm), assigned to a surface phase with similar stoichiometry to NiAl2O4, and the high Tm NiII, assigned to bulk-like NiAl2O4. On increasing TC, NiII area increases at the expense of NiI. Mo/Al2O3 catalysts showed spectra with two peaks which were almost insensitive to varying TC up to 660°C. The low Tm MoI peak was assigned to (partial) reduction of polymolybdates, and the high-Tm MoII to further reduction of polymolybdates plus that of more refractory tetrahedrally coordinated and/or monomeric species. In Ni-Mo catalysts, XRD and NH3 adsorption measurements, together with TPR data, indicate surface interaction between Ni and Mo, probably through the formation of Ni-Mo-O surface phases precursors of β-NiMoO4. Ni facilitated the reduction of Mo, as the Tm of the MoI peak decreased on increasing Ni content between 0 and 5 wt% NiO. This effect was less apparent upon increasing TC, indicating that it is related to the presence of Ni in or near the surface. However, quantitative TPR suggested that no appreciable amount of Ni was reduced simultaneously with MoI, and that the extent of MoI reduction was not affected by varying Ni concentration. In a similar way, Mo facilitated reduction of Ni, as reflected in the lower Tm of the NiI band in presence of Mo. Thus synergistic effects between Ni and Mo may be opperative not only in the sulfided state but also in the oxidic precursor of the catalysts. It is speculated that Ni ions responsible for NiI might be involved in the generation of active Ni-Mo-S phases.

Quantum chemistry calculations were carried out, using ONIOM2 methodology, to investigate the CO adsorption and oxidation on gold supported on Silicoaluminophospates (SAPO) molecular sieves Au/SAPO-11 catalysts. Two models were studied, one containing one Au atom per site (AuSAPO-11), and the other with two Au atoms per site (Au2SAPO-11). The results reveal that the CO adsorption and oxidation are exothermic on Au/SAPO11 with an ΔE of −41.0 kcal/mol and ΔE = −52.0 kcal/mol, for the adsorption and oxidation, respectively. On the Au2SAPO-11 model, the CO adsorption and oxidation reaction occur, with a ΔE of −29.7 kcal/mol and −52 kcal/mol, respectively. According to our results, the oxidation reaction exhibits an Eley-Rideal type mechanism with adsorbed CO. The theoretical calculations reveal that this type of material could be interesting to disperse Au and consequently to strengthen its catalytic use for different reactions. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110:2573–2582, 2010

The thermal stability of a series of cobalt and nickel molybdates (AMoO4·nH2O, α-AMoO4, and β-AMoO4; A = Co or Ni) was examined using synchrotron-based time-resolved X-ray powder diffraction (XRD). The results of X-ray absorption near-edge spectroscopy (XANES) indicate that the Co and Ni atoms are in octahedral sites in all these compounds, while the coordination of Mo varies from octahedral in the α-phases to tetrahedral in the β-phases and hydrates. Upon heating of AMoO4·nH2O, evolution of gaseous water was seen at two different temperature ranges: 100−200 °C for reversibly bound H2O; 200−400 °C for H2O from the crystal structure. The results of time-resolved XRD show a direct transformation of the hydrates into the β-AMoO4 compounds (following a kinetics of first order) without any intermediate phase. This is probably facilitated by the similarities that AMoO4·nH2O and β-AMoO4 have in their structural and electronic properties. The XRD experiments show that the α-AMoO4 → β-AMoO4 transitions occur at m...

The combination of two metals in an oxide matrix can produce materials with novel physical and chemical properties. The reactivity of a series of cobalt and nickel molybdates (α-AMoO4, β-AMoO4, and...

Silver nanoparticles (AgNPs) were prepared by means of the polyol method in the absence of stabilizing polymers. To accomplish this objective, AgNO3 was added to ethylene glycol in the presence of NaOH (1 mol.L-1), the suspension formed was irradiated with a microwave source for 60 seconds at a power of 465 watts. It was found that under these conditions AgNPs of sizes between 4-18 nm are formed. Also the results indicate that part of the ethylene glycol is oxidized to carbonyl compounds that reduce the Ag+. These organic compounds are adsorbed on the surfaces of AgNPs, forming a protective film that prevents their aggregation.

Time-resolved X-ray diffraction (XRD) has emerged as a powerful technique for studying the behavior of heterogeneous catalysts (metal oxides, sulfides, carbides, phosphides, zeolites, etc.) in-situ during reaction conditions. The technique can identify the active phase of a heterogeneous catalyst and how its structure changes after interacting with the reactants and products (80 K

A series of NiCoMo hydrodesulfurization (HDS) catalysts supported on silica was prepared varying the ratio r = Co /( Co + Ni ) between 0 and 1, maintaining Mo and (Co + Ni) concentrations constant. Calcination temperatures employed were 500 and 600°C. The promoter (Co and/or Ni) was present in the precursor as a hydrated molybdate phase that suppressed both anhydrous MoO 3 crystallite growth and decrease in surface area resulting from sintering of the support. Both phenomena occurred in the nonpromoted catalyst after calcining at 600°C, probably by a dehydroxylation process resulting from dislinking of polymolybdates from the silica support. A correlation between TPR and dispersion given by XRD was established, confirming that the dispersion of molybdenum on silica was lower than that on the aluminas previously studied. According to the proposed correlation, sulfidation caused an increase in dispersion in all the catalysts. The presence of anhydrous MoO 3 was held responsible for the formation of amorphous MoO 2 observed by TPR after sulfidation. The increase in dispersion after sulfidation was accompanied by a remarkable increase in the initial HDS activity of both nonpromoted and promoted catalysts. However, the nonpromoted sample showed a pronounced initial deactivation during use. Compared with another similar, but supported on alumina, series of catalysts, a minimum instead of a maximum steadystate HDS activity was found for an intermediate value of r .

The effect of prereduction on CO oxidation activity of unsupported copper-chromite oxide catalysts was examined. Results were found to be in good agreement with two mechanisms for a surface copper enrichment due to CO prereduction which produced an activity increase in the copper-chromite catalyst.

International audience; The photocatalytic degradation of phenol was performed at room temperature in aqueous suspended mixtures of TiO2 and activated carbon (AC). The main objective of the present work was to verify the potential of TiO2/AC system in the photocatalytic degradation of phenol and the principal intermediate products after performing three consecutive runs. The phenol disappearance follows a first-order kinetics. Therefore, the apparent first-order rate constant of phenol and total organic carbon photodegradations were selected to evaluate the photoefficiency of the system. From the present results it can be concluded that there is a synergistic effect between both solids which is determined by the numbers of photocatalytic runs. From a practical point of view, TiO2/AC is able to photomineralize phenol and total organic carbon for three and two, respectively, consecutive runs more efficiently than TiO2 alone.

En el presente estudio se investiga la Deshidrogenación Oxidativa (DHOX) de n-butano utilizando catalizadores de VMgO soportados en SiO2 o promovidos con diferentes cantidades de galio o molibdeno. La relación atómica Mg/V usada fue de cuatro para todos los catalizadores. Los experimentos se realizaron en un reactor de lecho fijo a temperaturas dentro del rango de 480 a 540ºC y con un tiempo de residencia que se varió entre 6 y 50 (g cat min mol total-1). Se demuestra que para evitar cambios de comportamiento es necesario trabajar con catalizadores previamente estabilizados. La fase VMgO no soportada ni promovida presentó un buen nivel de actividad (conversiones de ±30% a un tiempo de contacto de 50 [g cat min mol total-1] y una temperatura de 540ºC) con selectividades hacia hidrocarburos no saturados del orden del 90%. Con el objeto de aumentar la resistencia a la atrición del catalizador, esta fase activa se soportó sobre SiO2 (30% en peso). La actividad y la selectividad de este catalizador fueron algo menores a las observadas sobre la fase activa no soportada pero todavía interesante. La adición de óxidos de molibdeno o galio con relaciones atómicas (Mo/V o Ga/V) en el rango de 0.1a 1.0 mostró efectos variados. El molibdeno promovió la selectividad hacia butenos pero con reducción de la actividad. En cambio el galio conduce tan solo a cambios menores. The present study investigates the n-butane oxidative dehydrogenation (ODH) using VMgO catalysts supported on SiO2 or promoted with different amounts of Gallium or Molybdenum. The Mg/V atomic ratio used for all catalysts was four. Experiments were performed in a fixed bed reactor within the temperature range of 480 to 540ºC and a residence time the range of 6 to 50 (g cat min mol total-1). The need for catalysts previously stabilized in order to avoid changes in behavior with time on run was demonstrated. The VMgO unsupported and unpromoted phase showed a good level of activity (conversions of ±30% at a contact time of 50 [g cat min mol total-1] and a temperature of 540°C) with selectivities toward unsaturated hydrocarbons in the order of 90%. In order to increase the catalyst resistance to attrition, this active phase was supported on SiO2 (30% by weight). The activity and selectivity of this catalyst was somewhat lower than the observed on the unsupported phase but still interesting. The additions of molybdenum or gallium oxides with an atomic ratio (Mo/V or Ga/V) in the range 0.1 to 1.0 show instead to have mixed effects. Molybdenum promotes the selectivity toward butylenes but with a reduction in activity. Instead, gallium introduces a rather small effect.

The phase transitions in a series of CoMoO4 and NiMoO4 catalysts have been studied using time-resolved x-ray diffraction and calculations based on density functional theory. Important differences are observed in the temperatures and energetics for these transitions. X-ray absorption spectroscopy was used to study the sulfidation of the molybdates with H2S and SO2. For small exposures of H2S and moderate temperatures (50–150°C), one sees the formation of sulfides and sulfates on the oxide catalysts. Exposure to large amounts of H2S at elevated temperatures (350–400°C) leads to formation of pure NiMoSx and CoMoSx compounds (no SOx species) that are active as HDS catalysts. CoMoO4 and NiMoO4 are good sorbents for sulfur dioxide. The adsorption of SO2 on the molybdates at room temperature produces SO3 and SO4 species without dissociation of SO2. Most of the SO3 and SO4 species decompose after heating to 400°C. The sulfided CoMoO4 and NiMoO4 catalysts can be “regenerated” by heating in O2 at 500°C.

The XRD signal at interplanar spacing around 3.33Ais now attributed to be diagnostic forf βl-NiMoO 4 in Ni Mo catalysts. Due to an error in the JCPDS Powder Diffraction File Card 12–348, it has sometimes been assigned to y NiO·MoO 3 · z H 2 O. Previously published data on oxidic Ni Mo/Al 2 O 3 points to a role of the support in stabilizing β-NiMoO 4 .

Experiments are described showing the utility of synchrotron-based time-resolved x-ray diffraction (TR-XRD) and x-ray absorption near-edge spectroscopy (XANES) for characterizing the physical and chemical properties of mixed-metal oxides that contain Mo and a second transition metal (Fe, Co or Ni). TR-XRD was used to study the transformations that occur during the heating of a FeMoO4/Fe2(MoO4)3 mixture and the α⇒β phase transitions in CoMoO4 and NiMoO4. The Mo LII- and O K-edges in XANES are very useful for probing the local symmetry of Mo atoms in mixed-metal oxides. The results of XANES and density-functional calculations (DMo13, DFT-GGA) show large changes in the splitting of the empty Mo 4d levels when going from tetrahedral to octahedral coordinations. XANES is very useful for studying the reaction of H2, H2S and SO2 with the mixed-metal oxides. Measurements at the S K-edge allow a clear identification of S, SO2, SO3 or SO4 on the oxide surfaces. Changes in the oxidation state of molybdenum produce substantial shifts in the position of the Mo LII- and MIII-edges.

X-ray absorption near-edge spectroscopy (XANES) was used to characterize the structural and electronic properties of a series of cobalt- and nickel-molybdate catalysts (AMoO4.nH20, α-AMoO4, β-AMoO4; A= Co or Ni). The results of XANES indicate that the Co and Ni atoms are in octahedral sites in all these compounds, while the coordination of Mo varies from octahedral in the a-phases to tetrahedral in the β-phases and hydrate. Time-resolved x-ray diffraction shows a direct transformation of the hydrates into the β-AMoO4 compounds (following a kinetics of first order) at temperatures between 200 and 350 °C. This is facilitated by the similarities that the AMoO4.nH20 and β-AMoO4 compounds have in their structural and electronic properties. The molybdates react with H 2 at temperatures between 400 and 600 °C, forming gaseous water and oxides in which the oxidation state of Co and Ni remains +2 while that of Mo is reduced to +5 or +4. After exposing α-NiMoO4 and P-NiMoO4 to H2S, both metals get sulfided and a NiMoSx phase is formed. For the β phase of NiMoO4 the sulfidation of Mo is more extensive than for the a phase, making the former a better precursor for catalysts of hydrodesulfurization reactions.

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

The initial hydrodesulfurization activity behavior of supported nickel molybdate catalysts has been found to be markedly influenced by the presence of the promoter (nickel). The promoter diminished both the rate and the extent of deactivation of the fresh catalyst when exposed to a mixture of hydrogen and thiophene at atmospheric pressure. Presulfidation of the catalysts with pure H 2 S did not affect the optimum nickel concentration but increased the optimum molybdenum concentration. These results together with those obtained on surface acidity, carbon deposition, and sulfidation extent seems to support the idea that the positive influence of the promoter on the initial transformation of the catalyst is to maintain the dispersion of molybdenum during the sulfidation of the catalyst and to diminish carbon deposition.

Initial rates of carbon deposition from 1,3-butadiene on Al2O3, MoO3/Al2O3 and NiOMoO3/Al203 were examined using He or H2 as diluting gas. The promoter (nickel) is found to have a hydrogenation role that suppresses carbon formation on the molybdate catalysts. Catalytic activity for thiophene hydrodesulfurization and carbon and sulfur content of aged catalysts are found to be related to this proposed role.

Temperature-programmed reduction (TPR) was employed for the characterization of molybdena catalysts. The effect of several variables on reduction of the supported catalysts was determined. The TPR spectra changed significantly with both the amount of MoO 3 , from 5 to 80 wt %, and the type of support, several silica-alumina carriers, of compositions ranging from pure SiO 2 to pure Al 2 O 3 . The peak assigned to buildup of multilayers of molybdena on the γ-Al 2 O 3 supports was affected by the calcination temperature and by the amount of promoter (0–5 wt % NiO). Nickel-promoted catalysts activated hydrogen easier than both cobalt-promoted and unpromoted catalysts.

Alumina-supported CuO and CuCr2O4 catalysts of various compositions were prepared and their activity for CO oxidation measured. Fresh, pretreated with CO, and reoxidized catalysts were studied. In general, the activity increased with the CO pretreatment. The extent of the activation depended on the catalyst composition. Thus, copper catalysts were more active than copper chromite at low metal concentrations (

A sample of solvent-extracted sand from the Cerro Negro tar-sand deposit was found to have catalytic activity in the gas-phase desulphurization of tert.-butanethiol. The activity of the sand was compared with those of silica and silica-alumina standards. Pretreatment effect on the thiol conversion were attributed to hydrocarbons remaining on the surface after solvent extraction. The activity was related to ammonia adsorption and increased on introducing water vapour with the feed. A carbonium-ion mechanism for the surface reaction is proposed. It is suggested that underground generation of hydrogen sulphide during steam injection could be catalytically assisted if thiols are formed prior to hydrogen sulphide formation from the complex sulphur compounds in the tar.

A comparison of n-butylamine titrations with color indicators and gravimetric adsorption of ammonia on alumina, 12% molybdenum trioxide/alumina, 75Vertical Bar3< silica-alumina, and 12% molybdenum trioxide/75% silica-alumina gave the following relationship: pK/sub a/ = (11-1/x)/2 for ammonia adsorption at 200/sup 0/C, where x is the molar fraction of ammonia. The ammonia adsorption isotherm at 200/sup 0/C can thus be used for acid distribution determinations of colored catalysts.

Copper-chromium catalysts employed for CO oxidation were found to be affected by composition and pretreatment parameters. CuCr 2 O 4 , was more active than CuO only if prereduction was carried out and if metal concentration on alumina support was larger than 12 wt%. The presence of Cr with Cu in the oxide limited the extent of catalyst reduction leading also to less deactivation as compared to Cu on alumina. The presence of Cr also decreased an activity inhibition effected by water. A supported Cu-Cr catalyst used in an automobile ran with leaded petrol was deactivated by lead deposition. Deposits were mainly lead sulphate located on pellet periphery. Also, lead was preferentially distributed on the alumina instead of on the active metal-rich zones of the catalysts.

Alumina-supported CuO and CuCr2O4 catalysts of various compositions were prepared and their activity for CO oxidation measured. Fresh, pretreated with CO, and reoxidized catalysts were studied. In general, the activity increased with the CO pretreatment. The extent of the activation depended on the catalyst composition. Thus, copper catalysts were more active than copper chromite at low metal concentrations (

The effect of passivation and presulfidation after carbiding of activated carbon-supported Fe–Mo, Co–Mo and Ni–Mo catalysts on their thiophene HDS activity was evaluated. Catalytic precursors were prepared by co-impregnation of the support with solutions of ammonium heptamolybdate and the promotor nitrates or sulfates. Carbiding was achieved by means of the carbothermal method, employing pure H 2 as reductant and the support as the carbon source. Carbided samples were submitted to one out of three types of procedures before HDS tests: (a) passivation at room temperature followed by presulfiding; (b) presulfiding (no passivation); and (c) neither passivation nor sulfiding before HDS. Samples of passivated catalysts prepared from the sulfates of Fe, Co or Ni contained variable amounts of sulfur, as shown by XPS and elemental analysis, while XRD showed only metals and mixed Fe 3 Mo 3 C or η-M 6 Mo 6 C 2 (M Co, or Ni) phases. The nitrate-derived catalysts only presented β-Mo 2 C and metals (XRD). Sulfur containing catalysts showed high initial activities although deactivate strongly during the first 40 min on the reaction stream, while the unsulfided nitrate-derived samples showed a more stable behavior and lower activities during the 2–3 h of testing. In general, samples submitted to passivation followed by presulfiding showed the higher steady state activities and those neither passivated nor sulfided were the less active. The results show the benefits of a passivating treatment on these carbon-supported catalysts, and point out to the importance of sulfided surface phases in HDS on carbides of transition metal catalysts.

The purpose of this work is to elucidate the effect of acid sites density on the catalytic behaviour of a series of bifunctional catalysts prepared by supporting platinum (about 1.00 wt.%) on MFI-type gallosilicates (H[Ga]/ZSM5 with Si/Ga atomic ratio = 16, 46 and 145). The XPS characterization studies of the bifunctional catalysts allowed to detect the presence of framework as well as extraframework gallium species for the catalyst with highest density of acid sites (Si/Ga = 16), while only framework Ga species could be detected for catalysts with lower acid site density. It was also verified in all the bifunctional catalysts the presence of three types of metal sites: Pt (0), Pt–O ads and PtO species. The results obtained in the synthesis of MIBK from acetone with the different bifunctional solids, indicate that as the density of the acid sites increases, the solids start to behave solely as acid catalysts. This phenomenon is probably related to the presence of extraframework gallium species in the catalyst with higher surface acidity, capable to interact with the platinum species and thus provoking the passivation of the metallic phase, while in the solids with lower surface acidity it seems to exist a better balance between the acidic–metallic functionalities.

Mixed-metal oxides play a relevant role in many areas of chemistry, physics, and materials science. We have examined the structural and electronic properties of NiMoO4 and MgMoO4 by means of synchrotron-based time-resolved x-ray diffraction (XRD), x-ray absorption near-edge spectroscopy (XANES), and first-principles density functional theory (DFT) calculations. Nickel molybdate can exist in two phases (α and β). Mo is in a near tetrahedral environment in the β-phase, whereas in the α-phase the metal exhibits a pseudo-octahedral coordination with two very long Mo–O distances (2.3–2.4 A). The results of DFT calculations indicate that the α-phase of NiMoO4 is ∼9 kcal/mol more stable than the β-phase. On the other hand, in the case of magnesium molybdate, an α-NiMoO4-type phase is ∼13 kcal/mol less stable than β-MgMoO4. These trends in stability probably result from variations in the metal–metal repulsion within the α-phases of the compounds. For the α→β transition in NiMoO4, the DFT calculations predict an e...

The SiO2 anchored Mo(h 3 -C3H5)4 catalyst was modeled by means of the CNDO-UHF parametrical method. Two models (two separated centers (TSC) and two adjacent centers (TAC)) were proposed for the silica pre-treated at relatively low temperature. An analysis of total energies, bond indexes, diatomic energies, and parametric diatomic binding energies for selected bonds suggests the existence of both models on the surface. However, TSC is favored with respect to TAC. Furthermore, it is observed an allyl group interaction with the support through a C‐O bond that leads to a C‐C bond activation. Also, C‐H activation is found due to Mo‐H interaction. These results support several features of the generally accepted mechanism for olefin metathesis in this type of catalyst. Finally, experimental results of NMR, obtained in previous work by some of the authors, are interpreted by inequivalency of different allyl ligands. © 2001 Elsevier Science B.V. All rights reserved.

Temperature-programmed reduction (TPR) has been employed to study Ni-Mo mixed oxides which were previously used as model hydrodesulphurization (HDS) catalysts, using compositions ranging from pure MoO3 to pure NiO. An assignment of TPR signals to the different bulk phases was attempted. Good agreement between TPR spectra and structural data obtained previously from X-ray and electron diffraction was observed. TPR traces were consistent with proposed mechanisms of reduction of the bulk oxides MoO3, MoO2, NiO and NiMoO4. The variations in TPR spectra were interpreted in terms of effects such as crystallite size, ageing of the samples, hydrous state and chemical interactions between the different species. The significance of these reducibility results for HDS catalysis is discussed.

Unsupported copper-aluminum and copper-chromium oxides were prepared by mixing solutions of the nitrates and calcining at 1223 K, followed by treatment with washing solutions of either ammonium carbonate or nitric acid in order to extract uncombined copper oxide. Ammonium carbonate was only effective for removal of dispersed CuO, whereas nitric acid removed both dispersed and crystalline CuO. Catalyst activity for CO oxidation increased with the extraction treatments, particularly with the nitric acid washing solution, indicating that the active sites are copper species derived from CuAl2O4and CuCr2O4spinels rather than from CuO. The CuCr2O4catalyst exhibited higher activity, suggesting that Cu species in tetrahedral coordination lead to higher activity. Also, prereduction with H2produced higher activity than prereduction with CO, which can be attributed to a higher surface concentration of active species or their precursors after the former treatment. Activity results, together with TPR and XPS characterization point out that both Cu0and Cu+species are involved in the mechanism of CO oxidation.

Zeolites FAU, EMT and their intergrowths were synthesized using 15-crown-5, 18-crown-6 and their equimolar mixture, respectively. The synthesized products were characterized by XRD, SEM-EDX, N 2 adsorption and TPD-NH 3 . The solids obtained were all highly crystalline. The FAU samples were formed by octahedral submicrometric crystallites, EMT samples had hexagonal plate morphology of 2–5 μm. The intergrowth crystals were micrometric hexagonal plates through whose hexagonal faces, the octahedral FAU crystallites intergrow. The intergrowth proportion was evaluated by means of DIFFaX, resulting in different intergrowth proportions, depending on the molar ratio of template/Al 2 O 3 and on the relative template proportion used in the synthesis gel. For a template/Al 2 O 3 ratio of 0.70, a 50%FAU/50%EMT intergrowth proportion was obtained, with cluster-type stacking and for template/Al 2 O 3 of 0.30 the intergrowth proportion was 12%FAU/88%EMT with two stacking arrangements: clusters and random. Platinum was incorporated to these zeolites and their intergrowths by solid ion exchange; the metal dispersion was evaluated by TEM. For most catalysts the platinum particles were between 4 and 10 nm. All the catalysts were active for n -pentane conversion. The activity was found to be a function of acidity. The intergrowth catalysts were the most active materials. The iso -pentane selectivity, at 350 °C and carrier gas composition of H 2 :N 2 = 2:1, was 82% independent of time on stream, acidity, Pt/Al ratio and Pt dispersion. The selectivity increased with decreasing temperature and as carrier gas composition became richer in H 2 . The catalytic remaining activity (at 10 min) decreased in the following order: FAU > EMT > intergrowth.

Resumen en: This paper studies the effect of the presulfiding agent and the synthesis method on the catalytic activity of thiophene hydrodesulfurization (HDS), using...

The synthesis and definition of the topological organization of polygonal macro-networks of carbon in the form of films have been performed by controlled pyrolysis of saccharose. The pentagon was found to be the most abundant polygon in the film networks, which presented ring areas in the range of 4-12 mm 2 . In addition to the bi-dimensional films, irregular sponge balls were also obtained featuring a similar random network structure to that obtained in the films. The sponges developed into a continuous net of tri-dimensional random network in the form of tubes with coiled shapes. The formation of carbon macro-coils was attributed to the presence of potassium particles inside the original carbonaceous matrix. A study of textural changes of the macro-networks of carbon by activation under flow of N 2 and CO 2 has shown that it is possible to obtain activated carbons from the macro-networks with BET surface areas as high as 850 and 981 m 2 g -1 under flow of N 2 and CO 2 , respectively. From the study of textural changes of the macro-networks of carbon, a detailed mechanism that involves the role of KOH as a catalyst for the activation of the macro-networks has been proposed.

The thermal stability of a series of cobalt and nickel molybdates (AMoO4·nH2O, α-AMoO4, and β-AMoO4; A = Co or Ni) was examined using synchrotron-based time-resolved X-ray powder diffraction (XRD). The results of X-ray absorption near-edge spectroscopy (XANES) indicate that the Co and Ni atoms are in octahedral sites in all these compounds, while the coordination of Mo varies from octahedral in the α-phases to tetrahedral in the β-phases and hydrates. Upon heating of AMoO4·nH2O, evolution of gaseous water was seen at two different temperature ranges: 100−200 °C for reversibly bound H2O; 200−400 °C for H2O from the crystal structure. The results of time-resolved XRD show a direct transformation of the hydrates into the β-AMoO4 compounds (following a kinetics of first order) without any intermediate phase. This is probably facilitated by the similarities that AMoO4·nH2O and β-AMoO4 have in their structural and electronic properties. The XRD experiments show that the α-AMoO4 → β-AMoO4 transitions occur at m...

In this work it is studied the effect of the type of precursor (sulfate vs. nitrate of promotor) and the synthesis method (conventional vs. carbothermal carbiding) on the thiophene hydrodesulfurization (HDS) activity of activated carbon supported Fe-Mo, Co-Mo and Ni-Mo carbides. Catalytic precursors were prepared by co-impregnation of the support with solutions of ammonium heptamolybdate and the promotor salt. Conventional carbiding consists of a temperature-programmed treatment under a CH4/H2 (1:4) atmosphere, while the carbothermal method employs pure H2. The passivated carbided solids were characterized by XRD and XPS. The presence of metals and Fe3Mo3C or M6Mo6C2 phases (M = Co or Ni) in samples from sulfate precursors and of β-Mo2C and metals in those from nitrate precursors was verified by XRD, whereas XPS showed the presence at the surface of Moδ+(0 ≤ δ ≤ 2), Mo4+, Mo6+, Fe3+, Co2+, Ni0 and Ni2+ species. The bimetallic carbides obtained from sulfate precursors retained sulfur on the surface, as shown by signals in the S 2p region, at 169 and 162 eV, assigned to S2− and SO42−, respectively. Prior to the catalytic reaction, the passivated carburized forms of carbon supported catalysts were presulfided in situ. The catalytic activity of carbides was strongly influenced by the type of precursor and slightly by the synthesis method. The activity of carbides obtained by the carbothermal method and with sulfate precursors was greater than that of the solids obtained by the conventional method and with nitrate precursors. The carbides obtained from nitrates showed increased catalytic activity when presulfided, suggesting that the carbides with sulfided surfaces or mixed carbo-sulfide species could be the active phase in HDS on carbide catalysts. The catalysts derived from sulfates showed higher activity than those obtained from nitrates, even without presulfiding, suggesting that sulfide from the precursor sulfate results in more active species than those obtained after presulfiding.

The present work is part of an ongoing project aiming to a better understanding of the mechanisms of atomization on graphite furnace platforms used for electrothermal atomic absorption spectrometry (ETAAS). It reports the study of unused pyrolytic graphite coated platforms of commercial origin, as well as platforms thermally or thermo-chemically treated under simulated ETAAS analysis conditions. X-ray photoelectron spectroscopy (XPS) was employed to study the elements present at the surfaces of the platforms. New, unused platforms showed the presence of molybdenum, of unknown origin, in concentrations up to 1 at.%. Species in two different oxidations states (Mo6+ and Mo2+) were detected by analyzing the Mo 3d spectral region with high resolution XPS. The analysis of the C 1s region demonstrated the presence of several signals, one of these at 283.3 eV related to the presence of Mo carbide. The O 1s region showed also various peaks, including a signal that can be attributed to the presence of MoO3. Some carbon and oxygen signals were consistent with the presence of C O and C–O– (probably C–OH) groups on the platforms surfaces. Upon thermal treatment up to 2900 °C, the intensity of the Mo signal decreased, but peaks due to Mo oxides (Mo6+ and Mo5+) and carbide (Mo2+) were still apparent. Thermo-chemical treatment with 3 vol.% HCl solutions and heating up to 2900 °C resulted in further diminution of the Mo signal, with complete disappearance of Mo carbide species. Depth profiling of unused platforms by Ar+ ion etching at increasing time periods demonstrated that, upon removal of several layers of carbonaceous material, the Mo signal disappears suggesting that this contamination is present only at the surface of the pyrolytic graphite platform.