Study of the formation and stability of the Pd and Pt metallic nanoparticles on carbon support

Catalysts containing Pd and Pt on a Sibunit carbon support were studied by the temperature-programmed reduction, in situ X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy (XAFS). The reduction of Pd and Pt species in samples 2%Pd/C and 2%Pt/C calcined in an air flow at 370°C was studied. Reduction of the 2%Pd/C sample begins at 50—60 °C and is completed at 250—300°C. Particles of various dispersion are formed during reduction. Long-distance peaks observed in the EXAFS spectra point to the presence of a fraction of relatively large crystallites. The average Pd—Pd coordination number (5) at 200 °C gives evidence that a number of very small Pd nanoparticles, oligomeric clusters, is present. Reduction at T > 200°C results in sintering of a small fraction of the Pd particles. Reduction of Pt in 2%Pt/C sample begins at 120—150 °C and is completed at 300—350°C. The sintering-resistant monodispersed Pt particles are formed under these conditions.     

Catalytic conversions of coumarans

The dehydrogenation of 2-methylcoumaran on platinum, palladium, and rhodium catalysts deposited on carbon (content of the metal 5%), has been studied. The maximum yields of 2-methylbenzofuran at 300–350° C were 89, 95, and 81%, respectively, for Pt, Rh, and Pd, the yield of catalyzates being 98–99%.     

Isomerization ofn-hexane overplatinum loaded zeolites

The effect of the calcination temperature of bifunctional Pt/zeolites on the isomerization ofn-hexane has been investigated. The catalyst calcined at 350°C showed the highest metal dispersity and the best activity. The higher selectivity of dimethylbutanes over Pt/H- than over Pt/H-MOR might be attributed to the combined effect of acidity, channel structure and pore size of zeolites.     

Physico-Chemical and Catalytic Study of the Co/SiO2 Catalysts

This paper is focused on the physico-chemical and catalytic properties of Co/SiO₂ catalysts. Silica-supported cobalt catalysts were prepared by sol-gel and impregnation methods and characterized by BET measurements, temperature programmed reduction (TPR_H2), X-ray diffraction (XRD), and thermogravimetry-mass spectroscopy (TG-DTA-MS). The sol-gel method of preparation leads to metal/support catalyst precursor with a homogenous distribution of metal ions into bulk silica network or on its surface. After drying the catalysts were calcined at 500, 700, and 900°C. The reducibility of the supported metal oxide phases in hydrogen was determined by TPR measurements. The influence of high temperature—atmosphere treatment on the phase composition of Co/SiO₂ catalysts was investigated by XRD and TG-DTA-MS methods. At least five crystallographic cobalt phases may exist on silica: metallic Co, CoO, Co₃O₄, and two different forms of Co₂SiO₄ cobalt silicate. Those catalysts in which cobalt was chemically bonded with silica show worse reducibility as a result of strongly bonded Co-O-Si species formed during high-temperature oxidation. The TPR measurements show that a gradual increase in the oxidation temperature (500–900°C) leads to a decrease in low-temperature hydrogen reduction effects (<600°C). The decrease of cobalt oxide reduction degree is caused by cobalt silicate formation during the oxidation at high temperature (T 1000°C). The catalysts were tested by the reforming of methane by carbon dioxide and methanation of CO₂ reactions.     

State of supported rhodium nanoparticles for methane catalytic partial oxidation (CPO): FT-IR studies

The effect of pretreatments as well as of rhodium precursor and of the support over the morphology of Rh nanoparticles were investigated by Fourier transform infrared (FT-IR) spectroscopy of adsorbed CO. Over a Rh/alumina catalyst, both metallic Rh particles, characterized by IR bands in the range 2070-2060 cm-1 and 1820-1850 cm-1, and highly dispersed rhodium species, characterized by symmetric and asymmetric stretching bands of RhI(CO)2 gem-dicarbonyl species, are present. Their relative amount changes following pretreatments with gaseous mixtures, representative of the catalytic partial oxidation (CPO) reaction process. The Rh metal particle fraction decreases with respect to the Rh highly dispersed fraction in the order CO approximately CO/H2 > CH4/H2O, CH4/O2 > CH4 > H2. The metal particle dimensions decrease in the order CH4/O2 > H2 > CH4/H2O > CO > CO/H2. Grafting from a carbonyl rhodium complex also increases the amount and the dimensions of Rh0 particles at the catalyst surface. Increasing the ratio (extended rhodium metal particles/highly dispersed Rh species) allows a shorter conditioning process. The surface reconstruction phenomena going on during catalytic activity are related to this effect.     

Structural Characterization of Alumina‐Supported Rh Catalysts: Effects of Ceriation and Zirconiation by using Metal–Organic Precursors

The effects of the addition of ceria and zirconia on the structural properties of supported rhodium catalysts (1.6 and 4 wt % Rh/γ‐Al₂O₃) are studied. Ceria and zirconia are deposited by using two preparation methods. Method I involves the deposition of ceria on γ‐Al₂O₃ from Ce(acac)₃, and the rhodium metal is subsequently added, whereas method II is based on a controlled surface reaction technique, that is, the decomposition of metal–organic M(acac)_x (in which M=Ce, x=3 and M=Zr, x=4) on Rh/γ‐Al₂O₃. The structures of the prepared catalyst materials are characterized ex situ by using N₂ physisorption, transmission electron microscopy, high‐angle annular dark‐field scanning transmission election microscopy, energy‐dispersive X‐ray spectroscopy, X‐ray photoelectron spectroscopy (XPS), and X‐ray absorption fine structure spectroscopy (XAFS). All supported rhodium systems readily oxidize in air at room temperature. By using ceriated and zirconiated precursors, a larger rhodium‐based metallic core fraction is obtained in comparison to the undoped rhodium catalysts, suggesting that ceria and zirconia protect the rhodium particles against extensive oxidation. XPS results indicate that after the calcination and reduction treatments, a small amount of chlorine is retained on the support of all rhodium catalysts. EXAFS analysis shows significant Rh? Cl interactions for Rh/Al₂O₃ and Rh/CeO_x/Al₂O₃ (method I) catalysts. After reaction with H₂/He in situ, for series of samples with 1.6 wt % Rh, the EXAFS first shell analysis affords a mean size of approximately 30 atoms. A broader spread is evident with a 4 wt % rhodium loading (ca. 30–110 atoms), with the incorporation of zirconium providing the largest particle sizes.     

Reaction of CO oxidation on platinum,rhodium, a platinum-rhodium alloy,and a heterophase bimetallic platinum/rhodium surface

The reaction of CO oxidation on thin metal films of platinum, rhodium, and their alloy and on a heterophase bimetallic Pt/Rh surface that consisted of platinum particles of size 10–20 nm on the surface of rhodium was studied in the region of low reactant pressures (lower than 2 × 10^?5 mbar). At low temperatures (T < 200°C), the activity of samples increased in the order Rh > Pt/Rh > Pt-Rh alloy > Pt. Above 200°C, the rate of reaction on the heterophase Pt/Rh surface was almost twice as high as the sum of the rates of reaction on the individual metals; this fact is indicative of a synergistic effect. The nature of this effect is considered.     

Catalytic Reduction of NOx by CO and Hydrocarbons Over Different Catalysts in the Presence or Absence of O2

The performances of a non-noble metal (catalyst A), a non-noble metal catalyst containing a smaller amount of a noble metal (catalyst B) and noble metal (catalyst C) for NOx reduction at 400 - 600°C and space velocity of 16×10^-1h^-1 have been studied by means of a fixed-bed continuous flow system. In both the absence and presence of oxygen, the following activity orders of catalysts for the reduction of NOx to N₂ by CO and C₃H₆(HC) can be given: C > B > A and B > A C, respectively. Meanwhile, there was CO formation in the reduction reactions of NOx by HC on the three catalysts. The amount of CO produced on catalyst C was the largest of the three catalysts.     

Mechanism of C[bond]H bond activation/C[bond]C bond formation reaction between diazo compound and alkane catalyzed by dirhodium tetracarboxylate

The B3LYP density functional studies on the dirhodium tetracarboxylate-catalyzed C-H bond activation/C-C bond formation reaction of a diazo compound with an alkane revealed the energetics and the geometry of important intermediates and transition states in the catalytic cycle. The reaction is initiated by complexation between the rhodium catalyst and the diazo compound. Driven by the back-donation from the Rh 4d(xz) orbital to the C[bond]N sigma*-orbital, nitrogen extrusion takes place to afford a rhodium[bond]carbene complex. The carbene carbon of the complex is strongly electrophilic because of its vacant 2p orbital. The C[bond]H activation/C[bond]C formation proceeds in a single step through a three-centered hydride transfer-like transition state with a small activation energy. Only one of the two rhodium atoms works as a carbene binding site throughout the reaction, and the other rhodium atom assists the C[bond]H insertion reaction. The second Rh atom acts as a mobile ligand for the first one to enhance the electrophilicity of the carbene moiety and to facilitate the cleavage of the rhodium[bond]carbon bond. The calculations reproduce experimental data including the activation enthalpy of the nitrogen extrusion, the kinetic isotope effect of the C[bond]H insertion, and the reactivity order of the C[bond]H bond.     

The surface structure of sulfated zirconia: Studies of XPS and thermal analysis

Sulfated zirconias were prepared using two kinds of amorphous zirconia gels, XZO 631 and 632 supplied by MEL Chemicals, and their thermal gravimetrical analyses were carried out. DTG of the former sample showed two peaks based on decomposition of the sulfate species on the surface, the first peak at 680 °C and the second broad one centered at 850 °C. The latter sample indicated only broad peak at 850 °C in the range from 700 to >1000 °C. The first peak for the former sample was ascribed to the decomposition of Zr(SO₄)₂ remained on the surface, and the broad one at 700 to >1000 °C for the both samples was attributed to the catalytically active species. The acidic character of sulfated zirconia calcined at 1000 °C was examined in acid-catalyzed reactions of cumene, ethylbenzene, and butane together with the adsorption heat of Ar, showing a solid acid with acidity higher than that of silica-alumina. It was indicated from the XPS analysis that the S species are composed of SO₄²⁻. The results led to a structural model of the active surface to be polysulfate species containing mainly three or four S atoms with two ionic bonds of SOZr in addition to coordination bonds of SO with Zr, the active site being Lewis sites on the S atoms.     

Structural evolution of a recoverable rhodium hydrogenation catalyst

A recoverable, water soluble, hydrogenation catalyst was synthesized by reacting poly-N-isopropylacrylamide containing a terminal amino group (H₂N-CH₂CH₂-S-pNIPAAm) with [Rh(CO)₂Cl]₂ in organic solvents to form the square planar rhodium complex (Rh(CO)₂Cl(H₂N-CH₂CH₂-S-pNIPAAm)). The catalyst-ligand structure was characterized using in situ multinuclear NMR, XAFS and IR spectroscopic methods. Model complexes containing glycine (H₂NCH₂COOH), cysteamine (H₂NCH₂CH₂SH) and methionine methyl ester (H₂NCH(CH₂CH₂SCH₃)COOCH₃) ligands were studied to aid in the interpretation of the coordination sphere of the rhodium catalyst. The spectroscopic data revealed a switch in ligation from the amine bound (Rh-NH₂-CH₂CH₂-S-pNIPAAm) to the thioether bound (Rh-S(-CH₂CH₂NH₂)(-pNIPAAm)) rhodium when the complex was dissolved in water. The evolution of the structure of the rhodium complex dissolved in water was followed by XAFS. The structure changed from the expected monomeric complex to form a rhodium cluster of up to four rhodium atoms containing one SRR′ ligand and one CO ligand per rhodium center. No metallic rhodium was observed during this transformation. The rhodium-rhodium interactions were disrupted when an alkene (3-butenol) was added to the aqueous solution. The kinetics of the hydrogenation reaction were measured using a novel high-pressure flow-through NMR system and the catalyst was found to have a TOF of 3000/Rh/h at 25 °C for the hydrogenation of 3-butenol in water.     

Reductive dehydration of alcohols: A route to alkanes

The reductive dehydration of cyclic (C₅, C₆) and aliphatic (C₂–C₄) alcohols into polycycloalkanes and alkanes whose carbon skeleton has at least twice as many atoms as that of the original substrate is reported for the first time. This reaction takes place at 250–350°C and 7–50 atm in the presence of a polymetallic catalytic system based on an iron-containing fused catalyst or a hydrided iron- and titanium-containing intermetallide mixed with a small amount of Pt/Al₂O₃. The mechanism of this new reaction is discussed in terms of the mechanisms of known reactions of alcohols and aldehydes enlarging the carbon skeleton.Translated from Kinetika i Kataliz, Vol. 45, No. 6, 2004, pp. 904–916.Original Russian Text Copyright © 2004 by Tsodikov, Kugel, Yandieva, Kliger, Glebov, Mikaya, Zaikin, Slivinskii, Plate, Gekhman, Moiseev.     

Influence of a Fe/activated carbon catalyst and reaction parameters on methane decomposition during the synthesis of carbon nanotubes

In our experimental work on carbon nanotubes synthesis, the influence of pre-treatment and reaction temperature conditions over Fe catalyst loaded on low-cost activated carbon (AC) in the catalytic chemical vapor deposition of methane was studied. Catalyst with the metal concentration of 5 mass % calcined at 350°C and reduced at 450°C was effective in CH₄ decomposition giving 98 % conversions. TEM images showed that thin multi-walled carbon nanotubes (MWNTs) with the average internal diameter of ∼ 8 nm and the wall thickness of ∼ 2.5 nm were obtained over unreduced Fe/AC catalyst at the reaction temperature of 850°C. On the other hand, broader filamentous nanostructures with the diameter of ∼ 22 nm and the wall thickness of ∼ 3.72 nm were observed over reduced catalyst.     

Rhodium(I) complexes with 1′-(diphenylphosphino)ferrocenecarboxylic acid as active and recyclable catalysts for 1-hexene hydroformylation

The rhodium complex trans-[Rh(CO)(Hdpf-κP)(dpf-κ²O,P)] (1), (Hdpf = 1′-(diphenylphosphino)ferrocenecarboxylic acid) was used as an efficient and recyclable catalyst for 1-hexene hydroformylation producing ca. 80% of aldehydes at 10 atm CO/H₂ and 80 °C. After the reaction, unchanged complex 1 was separated from the reaction mixture and used again three times with the same catalytic activity. The effect of modifying ligands, phosphines and phosphites, on the reactivity of 1 was investigated. The active catalytic systems containing 1 or trans-[Rh(CO)(L)(dpf-κ²O,P)] (2) were formed in situ from acetylacetonato rhodium(I) precursors [Rh(CO)₂(acac)] (3) or [RhL(CO)(acac)] (4) and Hdpf or Medpf (L = phosphine, Medpf = methyl ester of Hdpf).     

Structure, magnetic properties and Sn Mössbauer spectroscopy of PrRhSn

PrRhSn was synthesized in polycrystalline form by a reaction of praseodymium, rhodium, and tin in an arc-melting furnace. The sample was investigated by powder and single crystal X-ray diffraction: ZrNiAl type, space group a=742.49(7), c=415.05(5) pm, wR₂=0.0737, 353F² values and 14 variables. The PrRhSn structure has two crystallographically independent rhodium sites with a tricapped trigonal prismatic coordination, i.e. [Rh1Sn₃Pr₆] and [Rh2Sn₆Pr₃]. The rhodium and tin atoms build up a three-dimensional [RhSn] network with short Rh-Sn contacts (278 and 285 pm), in which the praseodymium atoms fill distorted hexagonal channels. The magnetic and electronic properties of PrRhSn have been studied by means of AC and DC magnetic susceptibility measurements as well as ¹¹⁹Sn Mössbauer spectroscopy. A transition from a paramagnetic to a ferromagnetic state was found at .     

Reactions of 1-vinyl-4,5,6,7-tetrahydroindole and its mixtures with 4,5,6,7-tetrahydroindole in the presence of Cr- and Pd-coated catalysts

In the presence of Cr- and Pd-coated -alumina catalysts, 1-vinyl-4,5,6,7-tetrahydroindole (VTHI) and its mixtures with 4,5,6,7-tetrahydroindole (THI) are converted into 1-ethyl-4,5,6,7-tetrahydroindole (1-ETHI), indole, and 2-ethylindole, in proportions dependent on the reaction conditions and the catalyst. Over a sulfided 1% Pd--alumina catalyst in the presence of hydrogen at 200°C, VTHI is converted into 1-ETHI and THI. When the temperature is raised to 300–350°C, indole is formed in addition to these products. A 11 mixture of VTHI and THI over 1% Pd--alumina at 300°C gives indole and 2-ethylindole, over a sulfided 1% Pd --alumina catalyst at 200°C, 1-ETHI, and over a Cr oxide catalyst at 500°C, indole.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1417–1422, June, 1991.     

The RhX n (X = S,Se, Te) Coordination Polyhedra in Crystal Structures

The Voronoi–Dirichlet polyhedra (VDP) and the method of intersecting spheres were applied to crystal-chemical analysis of all known compounds whose structures contain rhodium atoms surrounded by chalcogen atoms. The influence of the rhodium valence state and the nature of the chalcogen on the main features of Rh stereochemistry are discussed. Rhodium atoms exhibit coordination numbers of 6, 5, or 4 with respect to S, Se, or Te atoms; in addition to the bonds with chalcogens, rhodium can form 1 to 4 bonds with metal atoms. The VDP volume for Rh(III), Rh(2.67), and Rh(II) atoms in selenides and tellurides very weakly depends on the valence state, whereas in the case of sulfides, the volume increases rather regularly with a decrease in the metal oxidation number from Rh(III) to Rh(I).     

Structure, magnetic properties and Mössbauer spectroscopy of GdRhSn

Polycrystalline GdRhSn was obtained by a reaction of the elements in a sealed tantalum tube in a high-frequency furnace. The sample was investigated by X-ray diffraction on powder and a single crystal: ZrNiAl type, space group , a=752.6(1),c=386.38(6) pm, wR₂=0.0353, 454 F² values and 14 variables. Both crystallographically independent rhodium atoms have a tricapped trigonal prismatic coordination, i.e. [Rh1Sn₃Gd₆] and [Rh2Sn₆Gd₃]. The shortest distances occur for the Rh-Sn contacts (274 and 283 pm). Together the rhodium and tin atoms build up a three-dimensional [RhSn] network in which the gadolinium atoms fill distorted hexagonal channels. The magnetic and electronic properties of GdRhSn have been studied by means of magnetic AC and DC susceptibility measurements as well as ¹¹⁹Sn and ¹⁵⁵Gd Mössbauer spectroscopy. A transition from a paramagnetic to an antiferromagnetic state with non-collinear magnetic ordering takes place at .     

Pathways to Better Stability of Catalysts for Reforming of Gasoline Fractions

Adsorbents AS-31 and AKD-981 based on zinc oxide and alumina are described. They are used in the purification of hydrogen-containing gas of reforming from hydrogen sulfide at 20–120°C and pressures up to 4 MPa. The second of these adsorbents is capable of forming weak adsorption complexes with hydrogen sulfide and can be recovered 2–3 more times than the first one. To remove sulfur-containing compounds from the gaseous feed of reforming at 350–450°C, the KAS-50 catalyst/adsorbent is proposed, which is prepared by mixing manganese dioxide and aluminum hydroxide. The sulfur capacity of this catalyst is 20%. New platinum–rhenium reforming catalysts KR-200 and KR-201 are proposed, which have higher stability when they work with purified feedstock. The concentration of the active catalyst is the same or lower, and these catalysts show better activity than their predecessors. All catalysts and adsorbents are tested and work in industry.     

Conversion of carbon dioxide to propionaldehyde over cobalt and rhodium nanoparticles supported on MIL-53 (Al) metal–organic framework

The two-step conversion of carbon dioxide to propionic acid and propionaldehyde has been studied in the presence of novel catalysts, cobalt and rhodium nanoparticles supported on MIL-53(Al) microporous metal–organic framework. The first step is hydrogenation of carbon dioxide with formation of synthesis gas over cobalt-containing catalyst Co/MIL-53(Al) (500°C, 1 atm), and the second step is continuous (without separation) Rh/MIL-53 (Al)-catalyzed hydroformylation of ethylene with the synthesis gas formed in the first step.