G. Stefler

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.

Silica-supported Au nanoparticles decorated by TiO2 : Formation, morphology, and CO oxidation activity

Au-TiO(2) interface on silica support was aimed to be produced in a controlled way by use of Au hydrosol. In method A, the Au colloids were modified by hydrolysis of the water-soluble Ti(IV) bis(ammoniumlactato)dihydroxide (TALH) precursor and then adsorbed on Aerosil SiO(2) surface. In method B, Au sol was first deposited onto the SiO(2) surface and then TALH was adsorbed on it. Regular and high-resolution transmission electron microscopy (TEM and HRTEM) and energy dispersive spectrometry (EDS) analysis allowed us to conclude that, in method A, gold particles were able to retain the precursor of TiO(2) at 1.5 wt % TiO(2) loading, but at 4 wt % TiO(2) content the promoter oxide appeared over the silica surface as well. With method B, titania was detected on silica at each TiO(2) concentration. In Au-TiO(2)/SiO(2) samples, the stability of Au particles against sintering was much higher than in Au/TiO(2). The formation of an active Au-TiO(2) perimeter was proven by the greatly increased CO oxidation activity compared to that of the reference Au/SiO(2).

Bimetallic catalysis : CO hydrogenation over palladium-cobalt catalysts prepared by sol/gel method

Abstract Silica supported cobalt, palladium and cobalt–palladium bimetallic catalysts prepared by sol/gel technique and characterized by X-ray photoelectron spectroscopy, have been investigated in the CO hydrogenation reaction at 1 bar pressure in the temperature range between 200 and 300°C. It has been established that palladium facilitates reduction of cobalt which segregates to the catalyst surface to some extent. In the catalytic hydrogenation of CO over a catalyst with the ratio of Co/Pd=2 a synergism is observed, while over cobalt and palladium catalysts alone much lower activities are measured. Over the pure cobalt sample which has a limited reducibility, only short chain hydrocarbons, mainly alkenes, are formed, whereas when palladium sites activating hydrogen, are introduced the amount of alkanes is enhanced and the chain length increases up to C 8 –C 9 . The limited reducibility of the cobalt is explained by the small particle size due to the preparation technique. Palladium acts in the bimetallic system not only as a component which helps cobalt reduction, but as sites activating hydrogen participating in the reaction and causes synergism.

Silica-Supported Au Nanoparticles Decorated by CeO2: Formation, Morphology, and CO Oxidation Activity

Silica-Supported Au Nanoparticles Decorated by CeO2: Formation, Morphology, and CO Oxidation Activity Anita Horv ath,* Andrea Beck, Gy€orgyi Stefler, Tímea Benk o, Gy€orgy S afr an, Zsolt Varga, Jen 00 o Gubicza, and L aszl o Guczi Department of Surface Chemistry and Catalysis and Department of Radiation Safety, Institute of Isotopes of HAS, P.O. Box 77, H-1525 Budapest, Hungary Research Institute for Technical Physics and Materials Science of HAS, P.O. Box 49, H-1525 Budapest, Hungary Department of Materials Physics, E€otv€os Lor and University, Budapest, P.O. Box 32, H-1518, Hungary

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.

CO Hydrogenation over Re-Co Bimetallic Catalysts Supported over SiO2, Al2O3 and NaY Zeolite

The promotion of rhenium on the reduction of Co2+ ions in cobalt based samples prepared by various methods, such as, sol/gel technique, incipient wetness impregnationof Al2O3 and NaY, has been investigated in the CO hydrogenation. Among the samples the reducibility of the sol/gel, Re-Co/Al2O3 and Re-Co/NaY(IM) were the highest compared to Re-Co/NaY (IE) [8]. In addition to facilitating reduction of Co2+ ions, the rhenium increased the rate of reaction and the formation ofC5+ hydrocarbons at 10 bar pressures. The samples pretreated in various manner, revealed an about five fold increase in activity and an increase in the chain length in the NaYsupported sample while the activity and selectivity changed in various way in the sol/gel prepared samples. Simultaneously, rhenium increased the amount of olefins of low carbon numbers.In the mechanism the rhenium provides an action preventing fast deactivation of cobalt.

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.

Structure and catalytic properties Of cobalt-rhenium bimetallic catalysts prepared by sol/gel method and by ion exchange in NaY zeolite

Abstract The promotion of rhenium on cobalt catalysts prepared by various methods has been investigated in the CO hydrogenation. 10 at.% rhenium was added to Co/NaY sample and 10 at.% with 90 at.% Co bimetallic samples were prepared by sol/gel technique. The samples pretreated in various manner revealed an about five fold increase in activity and an increase in the chain length in the NaY supported sample while the activity and selectivity changed in various way in the sol/gel prepared samples. In the mechanism the rhenium provides an action preventing fast deactivation of cobalt.

Stability and deactivation of Ru/NaY and Ru-Co/NaY catalysts in CO hydrogenation

Abstract Ru/NaY and Ru-Co/NaY samples pretreated by O 2 /H 2 (a), He/H 2 (b) and He/H 2 /O 2 /H 2 (c) have been investigated in the CO hydrogenation. The kinetic parameters (rate, energy of activation and the selectivity values) have been determined. The Anderson-Schulz-Flory distribution characterized by α-values are more or less independent of the temperature. This behaviour of the differently treated samples is supported by the olefin/paraffin ratios. XPS measurements [1] indicated formation of the reduced bimetallic particles along with some minute amount of Co 2+ ions. As a result, the major difference between mono- and bimetallic samples is mainly controlled by the location of the metal particles. The rate of reaction is significantly lower for Ru/NaY sample treated by (a) and (c). This is likely due to the location of the metal particles and the residence time of the species which are present during the chain growth.