Zs. Koppány

Activity of SiO2 supported gold-palladium catalysts in CO oxidation

Abstract Bimetallic Au-Pd catalysts supported on silica with different Au/Pd atomic ratios were prepared by simultaneous reduction of palladium and gold precursors by ethanol in the presence of the polymer, polyvinylpyrrolidone (PVP). Formation of alloyed particles was detected by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and CO chemisorption measurements. The catalysts were tested in the catalytic oxidation of CO using a plug–flow reactor. The CO conversion was determined as a function of temperature. The monometallic palladium and the palladium-rich catalysts behaved quite similarly and were the most active catalysts. A drastic reduction of the CO oxidation activity was observed for the 50/50 Au/Pd catalyst and for samples with increasing amount of gold.

Structure of Pt–Co/Al2O3 and Pt–Co/NaY Bimetallic Catalysts: Characterization by In Situ EXAFS, TPR, XPS and by Activity in Co (Carbon Monoxide) Hydrogenation

Temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and in situ extended X-ray absorption fine structure (EXAFS) studies were performed to investigate Pt-Co/NaY and Pt-Co/Al2O3 bimetallic catalysts. The EXAFS experiments were carried out at the Pt LIII and Co K edges of the same sample. This particular approach allows a precise determination of the electronic and structural characteristics of the metallic part of the catalyst. For both systems in situ reduction under pure H2 results in the formation of nanometer-scale metallic clusters. For both Co and Pt, nearest neighbors are Co atoms. The complete set of parameters implies the presence of two families of nanometer-scale metallic clusters: monometallic Co nanosized particles and Pt-Co bimetallic clusters, in which only Pt-Co bonds exist (no Pt-Pt bonds). TPR and XPS results indicating a reduction of Co2+ ions in Pt-Co/NaY to a greater extent than in Pt-Co/Al2O3 give evidence of a facilitated reduction. XRD also shows the presence of nanometer-scale particles with only a very small fraction of larger bimetallic particles. In subsequent mild oxidation of the reduced systems the Co nanoparticles are still present inside the supercage of NaY zeolite in bimetallic form and the oxidation of the metallic particles is slowed down. Catalytic behavior is in good agreement with the structure of the Pt-Co bimetallic system.

Re-Co/NaY and Re-Co/Al2O3 bimetallic catalysts: In situ EXAFS study and catalytic activity

To reveal possible relations between the structure and catalytic activity, in situ EXAFS and catalytic studies complemented with XRD, XPS, and TPR measurements have been performed on the promotion of cobalt catalysts by rhenium prepared by the incipient-wetness technique on Al2O3 and NaY zeolite.In situ EXAFS data collected at the Co K-edge and at the Re LIII-edge provided direct evidence of the rhenium-cobalt bond formation. The degree of reducibility depends on the support. There are two structural features, that is, on Re-Co/NaY nearly all rhenium atoms are in contact with Co atoms, whereas the cobalt atoms are surrounded by cobalt atoms in the first coordination sphere. In the case of Re-Co/Al2O3 samples the rhenium in oxide form may prevent the development of the “cobalt surface phase” (CSP), which is hardly reducible.The rate, α value and olefin/paraffin ratio showing special features in the CO hydrogenation and CH4 conversion to higher hydrocarbons are in line with the structural architecture of the catalysts. Despite the difference in the degree of reducibility, the various activity of Re-Co/NaY and Re-Co/Al2O3 may be interpreted by the formation of mixed oxide on alumina preventing the deactivation and agglomeration of small metal particles. Furthermore, the rhenium promotes cobalt activity and the selective formation of higher hydrocarbon. In the mechanism the rhenium also prevents fast deactivation of cobalt.

Influence of pretreatment conditions on acidity of cobalt-based bimetallic systems in NaY zeolite

Co/NaY, Pt/NaY, Ru/NaY, Ru-Co/NaY and Pt-Co/NaY samples were pretreated in different ways and their Brønsted and Lewis acidities were measured by means of pyridine adsorption and by double-bond isomerization of 1-butene. It is established that during nitrogen/hydrogen pretreatment small metal or bimetallic particles are formed and the system possesses Brønsted and Lewis acidity, whereas by pretreatment in oxygen/hydrogen only Lewis acidity is developed (large metal particles segregated to the zeolite surface). The difference is due to the formation of proton-metal adducts inside the zeolite supercage when nitrogen/hydrogen pretreatment is applied and this is ceased when oxygen/hydrogen pretreatment is used. This is fully supported by the double-bond isomerization of 1-butene to cis- and trans 2-butenes.

Sol-derived Pd/SiO2 catalyst: characterization and activity in benzene hydrogenation

Abstract Pd/SiO2 catalysts were prepared by adsorption of Pd sols on support Aerosil 200 and characterized by transmission electron microscopy (TEM), CO chemisorption and temperature programmed oxidation (TPO). The Pd hydrosols were generated from PdCl2 solution and ethanol as reducing agent in the presence of poly(diallyldimethylammonium chloride), PDDA. The pH of the solution was increased to ensure adsorption of the Pd sol with high concentration of PDDA. The catalytic activity was tested in benzene hydrogenation. The presence of organic impurities and the role of morphology affecting the activity have been discussed in terms of the preparation procedure applied.

Effect of different treatments on Aerosil silica-supported Pd nanoparticles produced by ‘controlled colloidal synthesis’

Abstract Pd nanoparticles were prepared by ‘controlled colloidal synthesis’ (CCS) using aerosil silica support. On SiO 2 , 1–2 nm thick ethanol-rich adsorption layer in ethanol–toluene and ethanol–water mixtures was formed into which Pd(II) ions diffused and were reduced there by ethanol. Using Pd(II)-acetate precursor in an ethanol–toluene mixture (sample G1) and PdCl 2 precursor in an ethanol–water mixture (sample G2), palladium particles of 7.0 and 8.8 nm size, respectively, were produced. Effects of treatments in air and hydrogen were studied on the Pd/SiO 2 samples. The Pd particles showed a twofold/threefold increase in the catalytic activity in benzene hydrogenation after treatment in air at 573 K. The activity increase cannot be explained by a change in the particle size as indicated by transmission electron microscopy (TEM) and CO chemisorption data. Augmentation of turnover number is interpreted by the removal of carbonaceous residues and by the restructuring of the surface caused by carbon removal followed by PdO/Pd transition in the subsequent hydrogen treatment.

Oxidative conversion of isobutane to isobutene over V-Sb-Ni oxide catalysts

Insertion of proper amounts of nickel oxide into alumina-supported V-Sb oxide catalyst for the oxidative dehydrogenation of isobutane substantially increases isobutane conversion (from 36 to 42-44%) at selectivity to isobutene similar to70%. Fresh and used catalysts (including reference bulk V-Sb-O and V-Sb-Ni-O systems) were characterised by BET, XRD, XPS and H-2-TPR. Formation of new phase of nickel vanadate NiV2O6 at the expense of free VOx-phase leads to more reducible catalyst with increased amounts of mobile lattice oxygen. The facile redox cycle of vanadium species is considered to improve the catalyst activity

Structure and catalytic activity of Co-based bimetallic systems in NaY zeolite: Low temperature methane activation

Dissociative chemisorption of methane over cobalt, platinum, ruthenium particles and over their bimetallic combinations inserted into NaY by ion exchange, as well as sequential hydrogenation of the CHx species formed have been investigated. Temperature programmed reduction (TPR), X-ray diffraction (TRD), X-ray photoelectron spectroscopy (XPS) and CO chemisorption indicated that reduction of Co2+ ions in Co/NaY can significantly be facilitated by the presence of platinum and ruthenium. In Co-Pt/NaY bimetallic Co-Pt particles were formed, whereas in Co-Ru/NaY no direct evidence is available for the formation of Co-Ru bimetallic particles. It is established that Co/NaY sample modified by platinum and ruthenium exhibits a superior behavior in methane activation and in its sequential hydrogenation to higher hydrocarbon. This is interpreted by two factors: (i) enhanced reduction of cobalt and (ii) stabilization of small bimetallic particles inside the zeolite supercage. Calcination in oxygen promotes migration of ruthenium particles to the external surface, but in the case of cobaltruthenium bimetallic particles this process is hampered by cobalt. Nevertheless, in both systems a synergistic effect exists and the amount of CHx species considerably increase at the temperature lower than those applied for e.g. Co-Ru/Al2O3. For the formation of higher hydrocarbons the CHx surface species with a value of 1

New features of sol/gel prepared ruthenium catalysts in CO hydrogenation and CH4 coupling

Abstract CO hydrogenation and the low temperature non-oxidative methane coupling to form higher hydrocarbons over Ru/SiO 2 and Ru/Al 2 O 3 prepared by incipient wetness method and Ru/Al 2 O 3 prepared by sol/gel method (denoted by catalysts I, II and III, respectively) have been compared. Catalysts II and III possess selectivity for the formation of C 5+ hydrocarbons higher than sample I and the energy of activation is 85.7 kJ/mol on the catalyst III, which is about 29–33 kJ/mol less than those measured for catalysts I and II. The striking feature of the Anderson-Schulz-Flory distribution (ASF) points to the existence of an umbilical cord mechanism and at lower temperature the reinsertion of α-olefins in concurrence with hydrogenolysis results in a sigmoid distribution. With increase in temperature insertion becomes more and more prevalent process and the sigmoid curve is transformed into two straight lines with different slopes. Similar behavior is valid for the “two-step reaction” in the methane coupling carried out over different ruthenium catalysts, of which sample III has the highest selectivity for C 2+ hydrocarbons.

Comparative study on low temperature methane activation over cobalt and ruthenium supported on Al2O3, SiO2 and NaY

Abstract Structure of the alumina, silica and NaY zeolites supported Co and Ru catalysts determined by XRD, XPS and TPR, and its activity in low temperature methane activation measured by deuterium uptake were investigated. Formation of CH X species is affected by the nature of metal (e.g. methane dissociation occurs on Pt to a lesser extent than on Co and Ru), by the metal/support interaction causing difficulty in reduction of metals and the metal particle size.