Influence of the precursors on the properties of alumina supported rhodium particles

The effect of chlorine on the properties of Rh particles supported on alumina was studied using the hydroconversion of alkanes as a molecular probe. Chloride ions on the alumina have little influence on the Rh particles. Chlorine adsorbed on the Rh particles decreases the chemisorption capacity and increases the metal-support interaction, which results in modifications of their catalytic properties. Depending on the alkane processed, specific activity can be depressed up to four orders of magnitude.     

Hydrogenolysis of hexane on supported rhodium catalysts

Hexane was reacted in mixtures with excess hydrogen on 5% Rh on different supports: Al₂O₃ and SiO₂, the latter catalyst in two states: after reduction at 603 K (LT) and after a prereduction at 1253 K (HT). The main reaction was hydrogenolysis. The catalysts were characterized with the fragment composition at different temperatures and hydrogen pressures. Two surface states could be distinguished: one with more hydrogen favored single rupture, the other with less hydrogen preferred multiple fragmentation. The transition between these states could be rather abrupt, as the surface hydrogen availability changed. The tendency to produce multiple fragments increased in the order Rh/Al₂O₃< Rh/SiO₂-LT < Rh/SiO₂-HT.     

A study of metals supported on active carbon with a temperature programmed reduction (TPR) technique

The influence of active carbon as support on the reducibility of supported metals (Ni, Cu, Cd, Zn) has been studied by means of a temperature programmed reduction (TPR) technique. The TPR profiles indicate that active carbon behaves as a dispersion agent and the supported metal is dispersed in a disordered phase rather than as a stoichiometric compound. The hydrogen consumed in the reduction indicates that the metal residues are present in monovalent and divalent forms. High-temperature reduction peaks were also observed and are explained on the basis of metal-surface interactions and the porosity of the active carbon. Nitrogen adsorption reveals that the active carbon porosity decreases due to progressive closure of the pores when metals are supported on the surface of active carbon.     

Electrochemical activity of rhodium complexes supported on fibrous-carbon material

The redox properties of heterogenized Rh^I, Rh^II, and Rh^III complexes with different, particularly organophosphorus, ligands were studied by cyclic voltammetry (CVA). The support is a carbon-paste electrode based on a fibrous-carbon material and activated carbon. The electrochemical reduction of Rh^III produces Rh metal, which further catalyzes hydrogen evolution. After the reduction of water-soluble binuclear Rh^II complexes, the CVA curves exhibit peaks of electrocatalytic hydrogen evolution and irreversible Rh^I→Rh^II oxidation. The Rh^II complexes with organophosphorus ligands are characterized only by the peak of Rh^I→Rh^II oxidation. After reduction, the Rh^I complexes behave as a pseudo-reverse Rh⁰/Rh^I pair. The electron-donating properties of the ligand determine the reversibility of the system. The degree of structurization of the carbon matrix and the presence of phosphorus(v) atoms in it affect the electrochemical activity of the Rh^II and Rh^I complexes. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 908–914, May, 1999.     

Characterization of titania and zirconia supported molybdenum oxide catalysts by low temperature oxygen chemisorption

Low temperature oxygen chemisorption (LTOC) has been applied to characterize a series of TiO₂ and ZrO₂ supported Mo-oxide catalysts. The monolayer coverage of the surface is completed when the Mo loading reaches 6% and 4% on the TiO₂ and ZrO₂ supports, respectively. The results are explained with the help of a Patch model of the Mo-oxide phase.

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Mo- TiO₂ ZrO₂ . Mo TiO₂ ZrO₂ 6% 4%, . Patch Mo- .

     

Calorimetric study on oxidation of alumina supported rhodium by dioxygen

Rhodium particles in nanometer size were prepared by impregnating alumina powders with aqueous solutions containing rhodium salts. The dispersion (D) of rhodium crystallites on the prepared samples was estimated by dioxygen adsorption measured at 300 K. Phenomena of oxidizing the supported crystallites with 2.5 × 10⁴ Pa O₂ in a temperature range between 280 and 870 K were calorimetrically studied. Extent of oxidation may be distinguished into three stages, i.e., adsorption on surface (T < 300 K), progressive penetration into bulk, and formation of a stable bulk oxide (T> 700 K), on raising the oxidation temperature. Heat of dioxygen adsorption varies only slightly with the dispersion (D) of rhodium and has a value of 294 ± 6 kJ (mol O₂)⁻¹. Chemical stoichiometry of the bulk oxide formed, however, varies with the dispersion of rhodium crystallites. A dioxide (RhO₂) (_f H = 225 ± 3 kJ (mol O₂)⁻¹) and a sesquioxide (Rh₂O₃) (_f H = 273 ± 3 kJ (mol O₂)⁻¹) was formed at D < 60% and D> 80%,     

Radiation-assisted synthesis of iridium and rhodium nanoparticles supported on polyvinylpyrrolidone

The controlled synthesis of rhodium (Rh) and iridium (Ir) nanoparticles was carried out by gamma irradiation of aqueous solutions containing the metal precursor salt and polyvinylpyrrolidone (PVP). The nanoparticles were synthesized at various PVP and precursor concentrations with absorbed doses between 20 and 60 kGy. Nanoparticles with average sizes of 2.4 and 2.6 nm and narrow particle size distributions were obtained at metal precursor/PVP concentrations of 6/0.3 and 6/3 mM for Ir and Rh when irradiated at 60 kGy. The interaction of the nanoparticles surfaces with the PVP was studied.     

惰性载气对TPR动力学的影响

对于一些气-固相反应,同样条件下的动力学行为随惰性载气不同有所差异^[1,2]。为从实验上进一步揭示这一问题,并从理论上给出圆满的解释,我们对程序升温还原(TPR)^[8]过程进行了系统的考察,同时研究了不同惰性载气条件下铜氧化物还原反应的动力学问题,结合有关反应机理和统计热力学的概念,讨论惰性载气对气-固相还原反应动力学的影响。     

Influence of the experimental parameters on the TPR profiles

Some corrections are introduced into relations used both to compute TPR curves and to determine kinetic parameters. New experimental parameters which have an influence on the TPR curve profiles are described.     

Formation of supported size-controlled nanoparticles of sulfated zirconia

Methods of the preparation of catalysts for alkane skeletal isomerization based on uniform nanoparticles of sulfated zirconia anchored to different supports were investigated. These catalysts were characterized by using the ICP, HRTEM and BET techniques. The activities of the catalysts in the reaction of n-butane isomerization were measured and compared with those of bulk catalysts.     

Gas phase conversion of chlorobenzene over supported rhodium catalysts

The catalytic conversion of chlorobenzene has been investigated over Rh/Al₂O₃ and Rh/SiO₂ with metal dispersities varying from 7 to 80%. The rates of hydrogenation to chlorocyclohexane and hydrodechlorination to benzene decrease when the dispersity increases, therefore, both reactions appear as structure sensitive. This behavior is interpreted by a weak interaction between the aromatic ring and the metal surface. At similar particle sizes the activity is the same on Rh/Al₂O₃ and Rh/SiO₂, however, the selectivity for dechlorination exhibits a support effect.

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Rh/Al₂O₃ Rh/SiO₂ 7 80%. . , . . Rh/Al₂O₃ Rh/SiO₂ , .

     

Catalytic hydrogenation of propene over polymer supported rhodium complexes

Hydrogenation of propene and other substrates has been studied in flow and batch reactors using various rhodium catalysts. The results show that in some cases rhodium metal is probably formed, but only if solvent is present. A possible explanation is given.     

Characterization and catalytic functionalities of copper oxide catalysts supported on zirconia

A series of zirconia supported copper oxide catalysts with varying copper loadings (1.2-19.1 wt %) were prepared by impregnation method. The catalysts were characterized by X-ray diffraction, UV-visible diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), and temperature-programmed desorption of CO2. Copper dispersion and metal area were determined by N2O decomposition method. X-ray diffraction patterns indicate the presence of crystalline CuO phase beyond 2.7 wt % of Cu on zirconia. UV-visible diffuse reflectance spectra suggest the presence of two types of copper species on the ZrO2 support. XPS peaks intensity ratio of Cu 2p3/2 and Zr 3d5/2 was compared with Cu dispersion calculated from N2O decomposition. TPR patterns reveal the presence of highly dispersed copper oxide at lower temperatures and bulk CuO at higher temperatures. The basicity of the catalysts was found to increase with Cu loading, and the activity of the catalysts was also found to increase with the increase in Cu loading up to 2.7 wt % Cu loading. The catalytic properties were evaluated for the dehydrogenation of cyclohexanol to cyclohexanone and were related to surface properties of the copper species supported on zirconia.     

Influence of electronic state of rhodium in supported bimetallic catalysts on catalytic activity and selectivity in carbon monoxide hydrogenation reactions

Data on the mechanism and kinetics of the hydrogenation of carbon monoxide to saturated hydrocarbons and alcohols over supported bimetallic rhodium-containing catalysts are summarized and correlated. Rankings of specific catalytic activity and selectivity of Rh-M′/Al₂O₃ catalysts in relation to the chemical nature of M′, and hence in relation to the electronic state of the rhodium, are interpreted from a common point of view. On the basis of the interrelations that were found between the physicochemical and catalytic properties of these bimetallic systems, ground rules were formulated for selecting monotypical catalysts for the selective hydrogenation of carbon monoxide. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 33, No. 4, pp. 199–218, July–August, 1997.     

Facile synthesis of zirconia supported nanomaterials for efficient photocatalytic applications

The recent study enlightens the synthesis and characterization of zirconia (ZrO₂), Fe-doped ZrO₂ (Fe@ZrO₂), and Ni-doped ZrO₂ (Ni@ZrO₂) catalysts having new-fangled morphology with tuned band gap as photocatalysts for degradation of textile wastewater dyes methylene blue (MB), malachite green (MG), and their mixtures. SEM imaging of Fe@ZrO₂ and Ni@ZrO₂ is interestingly varied from ZrO₂ as well as doping of transition metals greatly affects the morphology of composites. The optimization study depicts that the pH, time (min) and catalyst amount (g) have a direct relation with degradation efficiency, while the dye concentration (mg/L) has an indirect relation as well. The photocatalytic studies depict that the degradation of MB and MG follows oxidation pathway via hydroxyl radicals (OH^•) and holes. Reusability of catalysts corresponds to a little decrease in degradation efficiency in the first two cycles and decreases to about ∼10% (Ni@ZrO₂) and ∼12% (Fe@ZrO₂) in next three cycles.     

Oxidative dehydrogenation of ethane over sufated zirconia supported oxides catalysts

The oxidative dehydrogenation of ethane into ethylene has been investigated on metal oxide-based sulfated zirconia catalysts at temperatures of 400–600°C. It is found that the activity and selectivity toward ethylene depend on the nature of metal oxide and temperature and that Ni and V oxides supported on sulfated zirconia exhibited higher ethylene yields.     

Catalysts containing supported rhodium complexes. A new support: polyphenylsiloxane on silica

When silica is coated with an organosilicon ladder polymer, after chloromethylation and phosphination it is possible to obtain a support for catalysts containing rhodium complexes which have proved much more active towards olefin hydrogenation than their homogeneous analogues, although they seem to follow the same general mechanism. The reasons for this new but expected result are discussed and a comparison is made with other similar supported catalysts.     

TPR Investigation of bimetallic Ru-Cu samples supported on SiO2, Al2O3 and MgO

Journal of Thermal Analysis and Calorimetry - Bimetallic Ru-Cu samples supported on SiO2, Al2O3 and MgO were studied by the temperature-programmed reduction (TPR) technique. Experiments were...     

Effect of benzothiophene on tetralin hydrogenation over supported rhodium catalyst in supercritical carbon dioxide solvent

The hydrogenation behavior of tetralin in the presence and absence of benzothiophene was studied over a charcoal-supported rhodium catalyst in supercritical carbon dioxide solvent. The catalyst activity, as well ascis-decalin selectivity, decreased in the presence of benzothiophene.