Z. Schay

Acetylene hydrogenation selectivity control on PdCu/Al2O3 catalysts

Abstract Hydrogenation of acetylene in the presence of a large excess of ethylene has been investigated on 0.04 wt.% Pd/Al 2 O 3 catalyst containing 0:100, 30:70, and 80:20 atoms Cu:atoms Pd, and on 5 wt.% Pd/SiO 2 . Reaction was carried out in a back-mixed flow reactor in the presence of both 0.4 vol.% and 18 vol.% hydrogen, using 0.4–1% acetylene-in-ethylene mixtures. Ethane selectivity (moles C 2 H 6 produced per mole C 2 H 2 converted) increased from 30% to far above 100% in the course of time, due to parallel ethylene hydrogenation. Concurrently, ethylene selectivity became negative. There was no change in C 4 and higher (C 4+ ) hydrocarbon production. 14 C 2 H 2 and 14 C 2 H 4 labelling showed that the intrinsic selectivity for acetylene hydrogenation to ethylene is about 95 – 99% on all catalysts. Since ethylene is also hydrogenated at the same conditions, low overall selectivity results even when the Pd is fully covered by acetylene. Copper addition to palladium decreased the catalyst activity and increased its stability. This resulted in a significant decrease in the overall rate of C 2 H 6 formation, as well as a marginal decrease in C 4+ selectivity.

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.

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.

CO hydrogenation and methane activation over Pd-Co/SiO2 catalysts prepared by sol/gel method

Abstract Silica supported cobalt, palladium and cobalt–palladium bimetallic catalysts were prepared by sol/gel technique and characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), X-ray absorption near edge structure (XANES). The CO hydrogenation and low temperature methane activation under non-oxidative condition were used as test reactions. It was established that, in contrast to other Pd–Co systems, bimetallic particles were not formed, only a part of cobalt was reduced in the presence of palladium and the cobalt was segregated to the catalyst surface. In the CO hydrogenation over a catalyst with the ratio of Co/Pd=2, a synergism was observed, while over cobalt and palladium catalysts alone lower activities were measured. Over pure cobalt sample, which has a limited reducibility, only short chain hydrocarbons, mainly alkenes, are formed, whereas in the presence of palladium (sites for enhanced hydrogen activation), the amount of alkanes increased. The limited reducibility of the cobalt was explained by the small particle size due to the preparation technique. Palladium acts in the bimetallic system not only as a component, facilitating cobalt reduction, but as a source of hydrogen participating in the reaction.

Pd nanoparticles prepared by "controlled colloidal synthesis" in solid/liquid interfacial layer on silica. I. Particle size regulation by reduction time

Controlled colloidal synthesis (CCS) was developed to prepare monodisperse palladium particles in the nano-scale range on suspended SiO2 particles in an ethanol–toluene mixture. On colloidal SiO2 an about 1 nm thick ethanol-rich adsorption layer was produced in adsorption equilibrium with the liquid mixture. Ethanol served as a reducing agent for the Pd(II) ions diffusing from a toluene-rich liquid solution into the interfacial layer. The low reduction rate ensures the dominancy of particle growth over the nucleation of palladium during the reduction process after the initial nucleation. The relation between the reduction time and the particle size produced was studied. XRF, XPS, TEM, CO chemisorption, and benzene hydrogenation as catalytic test were employed to characterize the samples prepared using different reduction time.

Surface characterization and catalytic CO + H2 reaction on Fe82.2B17.8 amorphous alloy

Abstract Fe82.2B17.8 amorphous ribbon has been used as a catalyst for the Fischer-Tropsch-type reaction of CO+H2. Specific activity has been found to be at least an order of magnitude higher than that of either the crystallized ribbon of identical composition or the supported iron catalyst. Before and after the catalytic tests the ribbons were characterized by XRD, XPS, UPS and Mossbauer spectroscopy in transmission and in conversion electron modes. Conversion electron Mossbauer spectroscopy and UPS proved that the surface of the amorphous ribbons is being partially crystallized during 8000 min reaction time at a maximum reaction temperature of 560 K. The superior catalytic activity has been explained by stabilization of the small iron particles and Fe2O3 by boron atoms at the surface and by suppressed carbide formation.

Spectroscopic and catalytic studies on metal carbonyl clusters supported on Cab-O-Sil. II. Impregnation and decomposition of Ru3(CO)12 and the mixture of Ru3(CO)12 and Fe3(CO)12

Abstract The behavior of Ru 3 (CO) 12 (I), H 2 Ru 3 Fe(CO) 12 (II), a 1:1 Ru 3 (CO) 12 and Fe 3 (CO) 12 mixture (III), RuFe 2 (CO) 12 (IV), and Fe 3 (CO) 12 (V) deposited on Cab-O-Sil HS-5 has been compared. (III) and (V) have been studied by Mossbauer spectroscopy and by ir-spectroscopy, and (I)-(V) by temperatureprogrammed decomposition (TPDC) and temperature-programmed reduction (TPR). Decomposition, which is faster in hydrogen than in helium or in vacuum, and is reversible below 400 K, is normally faster for (V) than for (I). At low temperature, CO ligands leave the metal carbonyl cluster (MCC) in one step for (V), whereas they are decomposed stepwise via the formation of subcarbonyl species for (I). In this range the formation of Ru 3 (CO) 3 species has been verified. On decomposition of (V), there is some CO adsorption, as indicated by ir spectroscopy and low catalytic activity. This increases when decomposition occurs in helium, and is attributed to the smaller particles stabilized by the metal-carbon species, formed from CO during the decomposition. For (I), decomposition results in a slight oxidation, indicated by weak ir bands in the range of 2100–2140 cm −1 . Interaction between Fe and Ru in (III) does not occur in the impregnated phase, but develops during the decomposition, which starts with Fe 3 (CO) 12 decomposition and thereby influences the decomposition of Ru 3 (CO) 12 . However, reduction of iron is also facilitated by the presence of ruthenium, as indicated by Mossbauer spectroscopy. The general feature revealed during decomposition in helium, i.e., the increase of surface carbon, is also operative here, and thus the dispersion of the metal is higher than for decomposition in hydrogen. The mechanism of the decomposition is discussed in terms of the formation of subcarbonyl species for Ru-containing samples and the formation of surface carbon is also considered. The mechanism and possible reaction pathways are given.

Direct evidence for the correlation between surface carbon and CO + H2 selectivity on iron and iron-Ruthenium catalysts prepared from metal carbonyl clusters

Abstract In situ Mossbauer studies and simultaneous catalytic reactions of CO + H2 mixtures at 1 and 20 bars pressure to form olefins were carried out on Fe and Fe—Ru bimetallic catalysts prepared by impregnation of Cab-O-Sil® with a hexane solution of Fe3(CO)12 and a mixture of Fe3(CO)12 and Ru3(CO)12. Under reaction conditions the main component of the Mossbauer spectra(I.S. = 0.0 mm s−1 and Q.S. = 0.5 mm s–) can be assigned to the reacting species of surface carbon, which can be distinguished from χ-carbide which has also formed during the reaction. In the presence of ruthenium, the spectra are very similar to those measured on iron, but a large χ-carbide signal with hyperfine splitting cannot be detected. From the kinetic and in situ Mossbauer experiments carried out under different conditions, direct evidence was found for the formation of mobile, reactive carbon which participates in the production of olefins and higher hydrocarbons. It can be transformed either into other types of carbon, one of which is responsible for methane formation, or into that species responsible for catalyst deactivation. A possible mechanism is discussed in this paper.

On the mechanism of NO decomposition on Cu-ZSM-5 catalysts

Decomposition of NO was studied on Cu-ZSM-5 catalysts prepared by solid state ion exchange using CuCl2 (I), CuO (II) and by conventional liquid phase ion exchange with copper acetate (III). There was no difference in the catalytic activity among samples (I), (II) and (III) using the same copper loading. Treatment of the samples in argon, in air or in NO/Ar mixture at 700°C was necessary to develop optimum catalytic activity. Transient kinetic experiments using NO carried out under isothermal conditions, showed overshoots in the N2 and O2 concentration at the front and tail edge, respectively. Fourier transform-infrared studies indicated the formation of oxidized copper sites and adsorbed NO2 species during the NO decomposition. In a proposed mechanism Cu2+(O)(NO)(NO2) intermediate was suggested to play a key role in the NO decomposition.

Highly dispersed FeRu CO-conversion catalysts prepared from H2FeRu3(CO)13 and Fe2Ru(CO)12 bimetallic clusters on silicagel

Abstract Temperature programmed decomposition of H 2 FeRu 3 (CO) 12 and Fe 2 Ru(CO) 12 supported on Cab-O-Sil HS-5 reveals differences in the interaction between the complex and the support. Both in He and in H 2 flow CO evolves in several steps and hydrocarbons are formed at higher temperatures. In He flow the amount of hydrocarbons is low, about 20–40% of the carbon remains in the catalyst and stabilizes its dispersion. Mossbauer spectra show that iron is oxidized to Fe 2+ and Fe 3+ and simultaneously H 2 is formed. The final catalyst develops only during the initial hours in the CO + H 2 reaction itself. This process results in a drop in the catalytic activity. Catalysts decomposed in He are more active than those decomposed in H 2 .