Molecular mechanism of formation of supported platinum catalysts of the Pt/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> family

Certain stages of the classical procedure for preparing aluminum-platinum catalysts, whose understanding is important from both the basic and practical viewpoints, are considered on the molecular level. Properties of both participants (active component precursor solution and oxide surface) are shown to be inhomogeneous, which can affect interaction of the components at the fixing stage. Existing views (and their evolution) on the mechanism of adsorption of platinum complexes from aqueous solutions on aluminum oxide surface are discussed. The role of chemical processes on the stage of fixing precursor on the electronic and structural properties of the supported metal is demonstrates. Strong metal-support interaction is shown to endow platinum with specific adsorption and catalytic properties in hydrocarbon transformation reactions.     

Kinetic studies on the hydrogenation of nitrate in water using Rh/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Rh-Cu/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts

Liquid-phase reduction NO ₃ ⁻ using monometallic and bimetallic catalysts (5% Rh/Al₂O₃, 5% Rh-0.5% Cu/Al₂O₃, 5% Rh-1.5% Cu/Al₂O₃, 5% Rh-5% Cu/Al₂O₃ and a physical mixture of 5% Rh/Al₂O₃ and 1.5% Cu/Al₂O₃) was studied in a slurry reactor operating at atmospheric pressure. Kinetic measurements were performed for a low concentration of nitrate (0.4 × 10⁻³−3.2 × 10⁻³ mol dm⁻³) and the temperature range 293–313 K. From the experimental data, it was found that the reduction of nitrate is first order with respect to nitrate. On the basis of the rate constants, the apparent activation energy was established using a graphic method. Published in Russian in Kinetika i Kataliz, 2007, Vol. 48, No. 6, pp. 881–886. This article was submitted by the authors in English.     

One-pot synthesis of Nb-modified Al<Subscript>2</Subscript>O<Subscript>3</Subscript> support for NiMo hydrodesulfurization catalysts

The effect of Nb as a support modifier on the NiMo₆/Al₂O₃–Nb₂O₅(x) (x?=?0, 1, 4, and 8?wt% Nb) catalysts was studied. The supports were prepared by one-pot coprecipitation from soluble precursors. The XRF analysis of the catalysts showed that the contents of Mo and Ni increased slightly with the presence of Nb. Micropore area and pore volume augmented importantly with Nb content, resulting in pore diameters between 5.3 and 9.3?nm. XPS analysis showed that the presence of Nb decreases the active metal–support interaction, improving the Mo and Ni sulfidation degree. The Raman spectra of sulfided catalysts suggested an increase in the number of layers of MoS₂ in the presence of Nb. Generally, the thiophene HDS activity at normal pressure of sulfided NiMo₆/Al₂O₃–Nb₂O₅(8) was greater than that of the sulfided catalysts with x?=?0, 1, and 4?wt% Nb, which can be attributed to the Nb promotion that would have an effect on the type of active site (Brønsted or Lewis acidic sites), since the number of sites by CO chemisorption for sulfided NiMo₆/Al₂O₃–Nb₂O₅(x) did not show correlation with the catalytic activity. The high-pressure HDS activity of dibenzothiophene was also greater in the presence of Nb, and the hydrogenation route was preferred for the Nb-promoted solid, while the unpromoted one showed a larger yield of direct desulfurization products.

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Effect of cerium dioxide on the properties of monolithic CuO-ZnO-CeO<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/cordierite catalysts in methanol decomposition

Cerium dioxide as a component of CuO-ZnO-CeO₂/Al₂O₃/cordierite catalysts stabilizes their action in the decomposition of methanol by preventing carbon deposition on the surface and facilitating hydrogen formation with selectivity and yield in the range 85–96%. The optimal indices for this reaction are obtained for a CeO₂-CuO/Al₂O₃/cordierite sample prepared using an ammonium precursor for cerium, (NH₄)₂Ce(NO₃)₆. This catalyst displays enhanced reductive capacity relative to the analogous CeO₂-CuO composition prepared using Ce(NO₃)₃·6H₂O.     

Co-Mo/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Ni-W/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts: Potential and prospects for use in hydrotreating of light cycle oil from catalytic cracking

Со-Мо/Al₂O₃ and Ni-W/Al₂O₃ catalysts were tested in hydrotreating of light cycle oil from catalytic cracking, of the straight-run gasoil, and of their mixture under typical hydrotreating conditions used in industry. The catalysts prepared using PMo₁₂ and PW₁₂ heteropoly acids exhibit high catalytic activity. The Со-Мо/Al₂O₃ catalyst is more active in hydrodesulfurization and hydrogenation of olefin and diene hydrocarbons, whereas the Ni-W/Al₂O₃ catalysts are more active in hydrogenation of mono- and polycyclic aromatic hydrocarbons. Comparison of the quality characteristics of the hydrogenizates obtained with the requirements of the technical regulations shows that the required levels of the sulfur content and cetane number of the hydrogenizates at practically accessible process parameters can be reached for mixtures of the straight-run gasoil and light cycle oil from catalytic cracking with high content of the latter component only when the process with the Со-Мо/Al₂O₃ system and Ni-W/Al₂O₃ catalysts is performed in two steps.     

Reactivation of CoMo/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> hydrotreating catalysts with chelating agents

Effect of various chelating components, multibasic carboxylic acids and glycols, used to prepare hydrotreating catalysts on the activity regeneration of calcined hydrotreating catalysts was studied. Reactivated catalyst samples were tested in a model reaction of hydrodesulfurization of dibenzothiophene. It was shown that the treatment of calcined catalysts with the chelating components leads to an increase in the catalytic activity. The best catalytic characteristics are observed for the catalyst reactivated with a solution containing citric acid and triethylene glycol.     

Effect of the alkoxides addition order on the properties of Pd/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–ZrO<Subscript>2</Subscript> catalysts used for methane combustion

A series of Pd/Al₂O₃–ZrO₂ catalysts were prepared to be used in methane oxidation. The effect of the addition order of metal alkoxides on the texture, structure and catalytic properties of the solids is studied. The control of the preparation parameters is achieved via sol gel way as an attractive route of the preparation of these catalysts. N₂ physisorption, XRD, Scanning Electronic Microscopy (SEM) and H₂ chemisorption are the main techniques used to characterize the prepared Pd/Al₂O₃–ZrO₂ catalysts. Textural analysis reveals the mesoporosity of all the catalysts independently of the addition order of alkoxides while surface area is more pronounced when the aluminium alkoxide is added before or with the zirconium precursor. XRD patterns show the development of the zirconia tetragonal phase for all the catalysts. Better metallic dispersion is obtained when aluminium alkoxide is added first which can be justified by the high homogeneity observed on the corresponding catalyst as revealed by SEM technique.     

A comparative study of Ir/Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>, Pt/Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>, and Ru/Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts in selective hydrogenation of crotonaldehyde

Catalytic performance of gallia-supported iridium catalysts in the reaction of selective hydrogenation of crotonaldehyde in the gas phase was studied and compared to that of platinum and ruthenium catalysts. The best catalytic properties in terms of the selectivity to crotyl alcohol are shown by 5 wt % Pt/α-Ga₂O₃ and 5 wt % Ir/α-Ga₂O₃ catalysts prepared from nonchlorine precursors: Pt(acac)₂ and Ir(acac)₃, but for the 5 wt % Pt/α-Ga₂O₃ a very high selectivity of 75% at the high conversion (ca. 60%) is observed. A high selectivity of galia-supported iridium and platinum catalysts was explained by the surface reducibility of gallium oxide leading to covering (decoration) of platinum and iridium by gallium suboxides and the promoting effect of gallium.     

Gold catalysts supported on ZnO/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> for low-temperature CO oxidation

Gold catalysts with loadings ranging from 0.5 to 7.0 wt% on a ZnO/Al₂O₃ support were prepared by the deposition–precipitation method (Au/ZnO/Al₂O₃) with ammonium bicarbonate as the precipitation agent and were evaluated for performance in CO oxidation. These catalysts were characterized by inductively coupled plasma-atom emission spectrometry, temperature programmed reduction, and scanning transmission electron microscopy. The catalytic activity for CO oxidation was measured using a flow reactor under atmospheric pressure. Catalytic activity was found to be strongly dependent on the reduction property of oxygen adsorbed on the gold surface, which related to gold particle size. Higher catalytic activity was found when the gold particles had an average diameter of 3–5 nm; in this range, gold catalysts were more active than the Pt/ZnO/Al₂O₃ catalyst in CO oxidation. Au/ZnO/Al₂O₃ catalyst with small amount of ZnO is more active than Au/Al₂O₃ catalyst due to higher dispersion of gold particles.     

Hydrogenolysis of glycerol to propanediols over heteropolyacids promoted AgCu/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts

Various amounts and different types of heteropolyacids promoted 5Ag15Cu/Al₂O₃ catalysts were prepared by impregnation method and analyzed through many techniques. The synthesized catalysts were evaluated for hydrogenolysis of glycerol to propanediols. The results demonstrated that heteropolyacids loading facilitated the reduction, promoted the dispersion, enhanced the acidity, and increased Broensted acid sites of the AgCu catalysts, which benefited the generation of 1,3-propanediol. Compared with phosphotungstic acid and phosphomolybdic acid, silicotungstic acid promoted AgCu catalyst had a better performance for 1,3-propanediol due to the better Cu dispersion and higher Broensted acidity. In addition, when the reaction was performed at 220 °C under 3.5 MPa for 8 h, the chosen 5Ag15Cu-10HSiW/Al₂O₃ achieved a 69.6% glycerol conversion with 35.6% 1,2-propanediol selectivity and 21.5% 1,3-propanediol selectivity.     

Comparative Study of the Oxygen Storage Capacity of Pt/M<Subscript>x</Subscript>O<Subscript>y</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Systems

The oxygen storage capacity of 1% Pt/15% M_xO_y/Al₂O₃ systems containing a rare-earth or an alkaline-earth metal oxide or TiO₂ as the oxygen-storing component was studied. Oxygen storage capacity was evaluated as the amount of C₃H₈ reacting at 400°C with oxygen that was taken up by the catalyst during oxidative treatment. The systems containing a rare-earth metal oxide or TiO₂ possess the highest oxygen storage capacity among the catalysts examined (80 and 75 µmol C₃H₈/g Cat, respectively). Of the BaO and SrO systems, the latter is of interest, although its oxygen storage capacity (∼27 µmol C₃H₈/g Cat) is somewhat lower than the oxygen storage capacity of any rare-earth metal oxide or the TiO₂ system.__________Translated from Kinetika i Kataliz, Vol. 46, No. 4, 2005, pp. 585–589.Original Russian Text Copyright © 2005 by Sinel’nikov, Tolkachev, Stakheev.     

Effect of hydrothermal treatment on the composition and structure of Pt(IV) hydroxo complexes as model precursors of the active component in Pt/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts

The influence of hydrothermal treatment conditions (temperature, duration, acidity of the medium) on the structural and chemical transformations of the complex H₂[Pt(OH)₆] was studied. The composition and structure of the resulting compounds were determined by several physicochemical methods. Thermal analysis coupled with mass spectrometry showed that, as the hydrothermal treatment temperature is raised from 25 to 120 and 150°C, the product composition in terms of empirical formulas changes as follows: PtO₂ · 4H₂O → PtO₂ · 3.5H₂O → PtO₂ · 1.5H₂O. X-ray diffraction and UV and IR spectroscopy demonstrated that the changes in the chemical composition are accompanied by the amorphization of the structure and Pt-O bond strengthening. X-ray structure determination using the radial electron density distribution method showed that polynuclear species ～10–15 Å in size with a structure similar to that of orthorhombic PtO₂ form in the complexes subjected to “hard” hydrothermal treatment (T ≥ 150°C).     

Composition of the gas phase over Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and the enthalpies of atomization of AlO,Al<Subscript>2</Subscript>O,and Al<Subscript>2</Subscript>O<Subscript>2</Subscript>

The A1, O, AlO, A1₂O, Al₂O₂, WO₂, and WO₃, partial pressures in the vapor over Al₂O₃ in a tungsten Knudsen effusion cell between 2300 and 2600 K were derived from A1⁺, O⁺, AlO⁺, A1₂O⁺, Al₂O₂⁺, WO₂⁺, and WO₃⁺, ion intensities. The mass spectrometer was calibrated against the equilibrium constant of the WO₃(g) = WO₂(g) + O(g) reaction. Refined values of the ionization cross sections of AlO and A1₂O₂ were used in the partial pressure calculations. The enthalpies of atomization of aluminum suboxides were determined to be Δ_at H ^o(AlO, g, 0) = 510.7 ± 3.3 kJ mol⁻¹, Δ_at H ^o(Al₂O, g, 0) = 1067.2 ± 6.9 kJ mol⁻¹, and Δ_at H ^o(Al₂O₂, g, 0) = 1556.7 ± 9.9 kJ mol⁻¹.     

Effect of boron on acid and catalytic properties of Pd-ZSM-23/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts in the reaction of diesel fuel hydroisomerization

Effect of the concentration and introduction method of boron on the acid characteristics and catalytic properties of Pd-ZSM-23/Al₂O₃ catalysts in hydroisomerization of diesel fuel was studied. It was found that raising the concentration of introduced boron in a support results in that the concentration of acid centers, specific surface area, and volume of micropores in the catalysts nonlinearly decrease. To obtain the optimal acidity it is primarily necessary to reduce the concentration of excessively strong Lewis acid centers in zeolites and also to reach a moderate concentration of Brønsted acid centers. The best catalytic properties are observed for the catalyst containing 0.5 wt % boron.     

Genesis of the active-component precursor in the synthesis of Pt/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts: III. Transformations of adsorbed platinum complexes during drying

The transformations of platinum(IV) complexes subsequent to their sorption on the support are considered. As the Pt(IV)/Al₂O₃ systems are dried at 25°C in daylight, their dehydration is accompanied by the replacement of inner sphere ligands of Pt(IV) by OH groups and the Coulombic bonding between the adsorbed metal complexes and the support turns into coordination bonding. Drying at a higher temperature of 120°C does not increase the extent of hydrolysis of the bound complexes in the predried samples. The observed increase in the proportion of unreadily reducible platinum species is likely due to the multisite adsorption of platinum complexes taking place.     

Thermal stability and surface structure of Mo/CeO<Subscript>2</Subscript> and Ce-doped Mo/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts

In order to explore the influence of CeO₂ on the structure and surface characteristics of molybdena, an investigation was undertaken by using N₂ adsorption (BET method), thermal analysis and in-situ diffuse reflectance infrared (DRIFT) techniques. In this work, the Mo/CeO₂ and Ce-Mo/Al₂O₃ samples were prepared by impregnation and co-precipitation methods with high Mo loadings. Combining the results one may notice that the presence of ceria led to the increase of polymerized surface Mo species so as to forming Mo-O-Ce linkages besides the formation of coupled O=Mo=O bonds indicative of polymeric MoO₃. From thermal analysis, it can be inferred that Mo/Al₂O₃ is the thermally most stable material in the temperature range used in the experiment (up to 900°C), whereas Ce-Mo/Al₂O₃ and Mo/CeO₂ samples undergo morphological modifications above 700°C resulting in lattice defects, which motivate the mobility of Mo and Ce ions and thus enhance the possibility of interaction between them. Additionally, their activity towards CO adsorption needs reduced ceria and molybdena containing coordinatively unsaturated sites (CUS), oxygen vacancies and hydroxyl groups to form various carbonate species.     

Competitive adsorption in the system: carboxymethylcellulose/surfactant/electrolyte/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

The adsorption of carboxymethylcellulose (CMC) in the presence or absence of the surfactants: anionic SDS, nonionic Triton X-100 and their mixture SDS/TX-100 from the electrolyte solutions (NaCl, CaCl₂) on the alumina surface (Al₂O₃) was studied. In each measured system the increase of CMC adsorption in the presence of surfactants was observed. This increase was the smallest in the presence of SDS, a bit larger in the presence of Triton X-100 and the largest when the mixture of SDS/Triton X-100 was used. These results are a consequence of formation of complexes between the CMC and the surfactant particles. Moreover, the dependence between the amount of surfactants’ adsorption and the CMC initial concentration was measured. It comes out that the surfactants’ adsorption amount is not dependent on the CMC initial concentration and moreover, it is unchanged in the whole measured concentration range. The influence of kind of electrolyte, its ionic strength as well as pH of a solution on the amount of the CMC adsorption at alumina surface was also measured. The amount of CMC adsorption is larger in the presence of NaCl than in the presence of CaCl₂ as the background electrolyte. It is a result of the complexation reaction between Ca²⁺ ions and the functional groups of CMC belonging to the same macromolecule. As far as the electrolyte ionic strength is concerned the increase of CMC adsorption amount accompanying the increase of electrolyte ionic strength is observed. The reason for that is the ability of electrolyte cations to screen every electrostatic repulsion in the adsorption system. Another observation is that the increase of pH caused the decrease of CMC adsorption. The explanation of this phenomenon is connected with the influence of pH on both dissociation degree of polyelectrolyte and kind and concentration of surface active groups of the adsorbent.     

Investigation of macro-/mesoporous Re<Subscript>2</Subscript>O<Subscript>7</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts prepared by a particle-template method for butylene metathesis

Macro-/mesoporous Al₂O₃ supports were prepared by using monodisperse polystyrene (PS) microspheres as a template. The pore volume and BET surface area of the Al₂O₃ supports increased considerably with increasing amounts of the PS microspheres; further investigation showed that PS template only increased the volume of macro-pores but did not change the volume of meso-pores or micro-pores. Macro-/mesoporous Re₂O₇/Al₂O₃ metathesis catalysts were prepared through loading Re₂O₇ onto the as-prepared macro-/mesoporous Al₂O₃ supports, and their catalytic performance was tested in a fixed-bed tubular reactor using the metathesis of normal butylenes as a probe reaction. The results showed that the prepared macro-/mesoporous Re₂O₇/Al₂O₃ catalyst had high activity with consistent selectivity; propylene and pentene accounted for more than 90 wt% of the metathesis products, while the amount of ethylene plus hexane was less than 10 wt%, the majority of which was hexane. These Re₂O₇/Al₂O₃ catalysts had not only higher activity, but also longer working life span and higher tolerance to carbon residues than conventional Re₂O₇/Al₂O₃ catalysts.     

Study of <Emphasis Type="Italic">n</Emphasis>-hexane isomerization on Pt/SO<Subscript>4</Subscript>/ZrO<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts: Effect of the state of platinum on catalytic and adsorption properties

The state of surface Pt atoms in the Pt/SO₄/ZrO₂/Al₂O₃ catalyst and the effect of the state of platinum on its adsorption and catalytic properties in the reaction of n-hexane isomerization were studied. The Pt-X/Al₂O₃ alumina-platinum catalysts modified with various halogens (X = Br, Cl, and F) and their mechanical mixtures with the SO₄/ZrO₂/Al₂O₃ superacid catalyst were used in this study. With the use of IR spectroscopy (CO_ads), oxygen chemisorption, and oxygen-hydrogen titration, it was found that ionic platinum species were present on the reduced form of the catalysts. These species can adsorb to three hydrogen atoms per each surface platinum atom. The specific properties of ionic platinum manifested themselves in the formation of a hydride form of adsorbed hydrogen. It is believed that the catalytic activity and operational stability of the superacid system based on sulfated zirconium dioxide were due to the participation of ionic and metallic platinum in the activation of hydrogen for the reaction of n-hexane isomerization.     

Oxidative desulfurization of thiophene derivatives with H<Subscript>2</Subscript>O<Subscript>2</Subscript> in the presence of catalysts based on MoO<Subscript>3</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> under mild conditions

The catalysts based on MoO₃/Al₂O₃ were synthesized and tested using aqueous hydrogen peroxide as the oxidant in the oxidative desulfurization of thiophene, benzothiophene (BT) and dibenzothiophene (DBT) into the corresponding sulfones. Among catalysts tested, 15%(MoO₃–WO₃)/Al₂O₃ prepared by a conventional impregnation method was considerably active for the oxidation of thiophene, BT and DBT, which could achieve higher than 99.2% conversions at lower reaction temperature (≤338 K). The use of hexadecyltrimethyl ammonium bromide as the phase-transfer reagent in small amounts could promote the reaction efficiently.