L. Guczi

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.

Structure sensitivity of acetylene-ethylene hydrogenation over Pd catalysts

Pd-Based catalysts are used for the selective hydrogenation of alkynes and alkadienes to the corresponding alkenes on a large scale. Addition of Pb and Zn (Lindlar type catalysts) to Pd, and alloying of Pd with Ag, Cu, Pb improve the selectivity of alkyne hydrogenation. The presence of organic bases organic sulfides, or CO in the reaction mixture is known to inhibit hydrogenation of the alkene formed. Although structure sensitivity in alkene hydrogenation has been systematically investigated, only a few studies have been devoted to the effect of metal dispersion on the selectively of hydrogenation of an alkyne-alkene mixture. The work described in this article aimed to investigate the effect of Pd dispersion on the competitive hydrogenation of a mixture containing 0.3% C/sub 2/H/sub 2/, 0.5% H/sub 2/ balanced by C/sub 2/H/sub 4/ over Pd/alumina as catalyst. In particular, attempts have been made to clarify the extent of ethane formation produced directly from acetylene (intrinsic selectivity of the ethane formation) and that from ethylene in the presence of acetylene with the use of (/sup 14/C)C/sub 2/H/sub 2/. Isotopic labeling techniques have unambiguously demonstrated that ethane is generally formed from both acetylene and ethylene. 30 references.

On the aging phenomenon in palladium catalysed acetylene hydrogenation

Abstract The effect of aging on the hydrogenation of a tail-end mixture (0.29 mol % C2H2, 0.44 mol % H2 balanced by C2H4) has been investigated on Pd-black and on various Pd/Al2O3 catalysts. Over supported catalysts the selectivity of ethylene formation decreases with time on stream in the first 40–60 hours of operation whereas the C2H2 conversion is not, or only slightly, influenced. With Pd-black the selectivity of ethylene formation changes in the opposite direction.14C-C2H2 experiments have revealed that the aging has only a minute effect on the intrinsic selectivity of C2H2 hydrogenation. The effect of the surface polymer and the nature of the ethylene hydrogenating sites are discussed. The results with supported catalysts have been interpreted by a polymer promoted hydrogen spillover.

Reactions of acetylene during hydrogenation on Pd black catalyst

Deuteration of trace amounts of acetylene in the presence and absence of ethylene has been studied on Pd black catalyst in a continuous flow reactor. At nonsteady state the Pd black catalysts showed very low selectivities, whereas higher selectivities were obtained at steady state. Pretreatment with air and H2 yielded a catalyst with higher selectivity than the same catalyst treated only with H2. The order in acetylene pressure passed from positive to negative with increasing pC2H2 both in the presence and absence of ethylene. The deuterium distribution of the ethylene formed showed that hydrogen originating from acetylene also took part in the hydrogenation reaction; thus, part of the acetylene dissociatively adsorbed on the surface. Using [14C]C2H2 it was demonstrated that at low acetylene partial pressure the main route of acetylene hydrogenation was the formation of ethane, ethylene, and C4 hydrocarbons. Using [14C]C2H4 it was also shown that at certain level of acetylene partial pressure the formation of ethane from ethylene was completely terminated. The observed kinetic data are discussed and it is suggested that different surface species such as dissociatively and associatively adsorbed acetylene as well as ethylidyne species are present on the palladium surface. Experimental conditions, such as catalyst pretreatment and partial pressure of the reactants, can influence the relative concentrations of these surface species and can also change the routes of surface reactions, leading to different reaction products from acetylene. The reaction mechanism proposed is discussed based on different organometallic and spectroscopic evidence published recently.

Double promotion of palladium/silica catalysts by iron and magnesium oxide in the synthesis of methanol from carbon monoxide and hydrogen

Abstract The structure and catalytic properties of 2 wt.-% Pd/SiO2 catalysts promoted with 2 wt.-% MgO and 0.2-5 wt.- % iron have been investigated. Catalysts were characterized by TPR, hydrogen chemisorption, and Mossbauer spectroscopy and their catalytic performance in methanol formation was also measured. At 16 at.-% iron a sharp maximum was observed in the activity which can be interpreted as being due to simultaneous promotion with iron and MgO. Independent mechanisms of promotion were found for the two modifiers: iron influenced the metallic component by forming bimetallic PdFe particles while MgO changed the properties of silica in a favourable way.

Kinetics and mechanism of ethane hydrogenolysis on silica-supported platinum and platinum-iron catalysts

A kinetic analysis of ethane hydrogenolysis has been carried out on PtSiO2 and PtFeSiO2 catalysts with a wide range of concentration of the components, and with an excess of ethane. With the help of the theory of stationary reactions on heterogeneous surfaces, kinetic equations were obtained describing the reaction rate both in excess hydrogen and excess ethane which are in full agreement with the experimental observations. On platinum in excess hydrogen the rate of hydrogenolysis is determined by the CC bond rupture of ethane adsorbed in a mildly dissociated C2Hx form, while in excess hydrocarbon it is determined by the CC bond rupture of ethane adsorbed in the deeply dissociated form of C2H2. The reaction rate passes through a maximum vs ethane pressure at constant hydrogen pressure and vice versa. The formal reaction order in hydrogen and hydrocarbon can be either positive or negative depending on the conditions; the formal power-rate equations may be considered as approximations of the more complicated equations presented in this work.

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.

Green oil poisoning of a Pd/a1203 acetylene hydrogenation catalyst

The composition and deactivation effect of green oil on a Pd/A1203 catalyst (ICI 38-1) in the hydrogenation of a tail-end acetylene-ethylene mixture are discussed. Infrared and NMR measurements confirmed the presence of carboxylic acids in the oil. The green oil poisoned the acetylene hydrogenation sites but the product distribution pattern was not greatly affected.

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.

Anomalous wide angle X-ray scattering (AWAXS) and heterogeneous catalysts

Abstract This review paper will be focused on anomalous wide angle X-ray scattering (AWAXS) and more precisely on the numerous breakthroughs, which have been achieved using this technique in heterogeneous catalysis. We present some basic elements of classical X-ray diffraction (XRD) and underline the limitation of this technique when we consider the structural characterisation of nanometer scale metallic clusters supported on supports such silica, zeolite or γ-alumina. Then we introduce the theoretical formalism of the anomalous diffraction and more precisely the f ′ and f ″ dispersive terms, including other physical processes such Compton scattering and fluorescence. A brief presentation of experimental set-ups dedicated to anomalous scattering implemented on different synchrotron radiation centres is given. We conclude with a review of the different studies already published and with some selected examples related to nanometer scale mono and bimetallic, sulfide and oxide clusters to illustrate the nature of information obtained through this technique and discuss the advantages and the limits of this approach.