Z. Zsoldos

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.

XPS study on the effects of treatments on Pt2+ and Co2+ exchanged into NaY zeolite

Abstract X-ray photoelectron spectroscopy was applied to investigate Pt/NaY, Co/NaY and Pt-Co/NaY zeolites prepared by ion exchange (IE) and impregnation (IM). Changes in the electronic states and surface distribution of sodium and silicon were followed in NaY and in Pt and Co containing NaY in the as prepared (AP), calcined (OX) and reduced (RE) states. Decoration of the zeolite surface by alumina detected, is attributed to the local processes during ion exchange. Introduction of Co 2+ ions into the Pt-containing samples influences the electronic states of platinum both in calcined and reduced state. Pt° facilitates reduction of Co 2+ ions and formation of bimetallic Pt-Co particles within the zeolite supercages could also be evidenced. The results support earlier TPR measurements indicating the presence of very small Pt-Co bimetallic particles for (IE) samples. On the other hand, in the impregnated NaY no sodium exchange occurs and the transition-metal species are located at the external surface of NaY zeolites.

Bimetallic catalysts: Structure and reactivity

Abstract The effect of the support in constraining the formation and defining the structure of bimetallic particles has been considered. Alumina and NaY zeolite supported platinum and cobalt bimetallic catalysts are illustrated to rationalize this effect. The surface composition and dispersion of the bimetallic particles are further correlated with the catalytic activity of bimetallic catalysts in CO hydrogenation.

On the development of metallic particles and their initial catalytic properties in the CO + H2 reaction over Co/Al2O3 catalyst

Formation of Coo phases with different surface structure over 10 wt% Co/Al2O3 and their catalytic properties were induced by pretreatments in H2 at 570 K for 1 h or 20 h. Electronic behaviour of the Coo phase, which consists of small (after 1 h reduction) or large bulk-like particles (after 20 h reduction), did not change during the CO hydrogenation after 5 h on stream as was determined by XPS. On the basis of the measured C2+ hydrocarbon selectivities the CO molecules are suggested to dissociate on small Co particles to a larger extent than on large cobalt particles. The slight decrease in the catalytic activity with increasing time on stream obtained for the long-term reduced sample is explained by the change in the surface Coo content detected by XPS. The increase in the catalytic activity along with the change in olefin selectivity, measured for the sample reduced for 1 h, is interpreted by the change of a reaction path involving the Coo-support interface during the initial period of the reaction.

Stability of Pt-Co bimetallic particles: effect of the support induced morphology on reactivity in CO hydrogenation

The structure and the stability of bimetallic particles are controlled by their interaction with surface oxide species on support or by support geometry. Location and size of the bimetallic particles estimated by XRD and XPS, depends on the environment and treatment. Thus, the Pt-Co bimetallic particles entrapped in NaY zeolite cages become unstable upon mild oxidation by O2 and/or reaction with surface protons causing the Co2+ ions formed to migrate irreversibly from the supercages into the sodalite cages as determined by TPR and TPD. Differences in the selectivity pattern observed for the CO hydrogenation over alumina and NaY zeolite supported Pt-Co catalysts, can be interpreted by the interaction between bimetallic particles with surface cobalt oxide species and the zeolite cage wall, respectively.

Heterogeneous catalysis: retrospect and prospectsBimetallic catalysts: structure and reactivity

The effect of the support in constraining the formation and defining the structure of bimetallic particles has been considered. Alumina and NaY zeolite supported platinum and cobalt bimetallic catalysts are illustrated to rationalize this effect. The surface composition and dispersion of the bimetallic particles are further correlated with the catalytic activity of bimetallic catalysts in CO hydrogenation.

Genesis of cobalt oxide-induced surface structure in PtCoxAl2O3 catalysts

Molecular approach to surface structure of the PtCoxAl2O3 (x = 0–0.67) samples prepared by co-impregnation with aqueous solution of H2[PtCl6] and Co(NO3)2 as precursors, has been investigated by means of X-ray photoelectron spectroscopy during the drying-calcination-reduction sequence of preparation. Ion exchange between the chloride ligands and oxygen ions during calcination in oxygen of the samples containing constant loading of Pt and increasing amount of Co, was induced by cobalt oxide at high cobalt content. Simultaneously, CoO was anchored to alumina surface, whereby, after subsequent reduction, a novel CoO-modified-support interface was developed and metallic platinum and intermetallic PtCo species were formed. As a result, the estimated dispersion of Pt, DPt passed through a shallow minimum over the various composition. Similar trend was observed for the reaction rate in the CO hydrogenation suggesting that the catalytic activity of the PtCoxAl2O3 samples is related to platinum, while product selectivity is primarily controlled in a complex manner by the platinum particles on the CoO-modified support. The present work represented how molecular approach in the genesis of a bimetallic catalyst helps in understanding the mechanism of the catalytic reaction.

Rhenium tricarbonyl as a common intermediate in the CO+H2 reaction over H3Re3(CO)12/Al2O3 and NH4ReO4/Al2O3 samples

Abstract A comprehensive mechanism is provided for surface transformations in the H 3 Re 3 (CO) 12 /Al 2 O 3 and NH 4 ReO 4 /Al 2 O 3 induced by treatment in vacuo , hydrogen and CO + H 2 mixtures. Both inorganic and organometallic precursors are partly transformed to the same, rather stable Re 4+ species during pretreatment. Regardless of the nature of the starting materials, a common intermediate species, Re(CO) 3 (OAl)(HOAl) 2 , is shown to be formed under the reaction conditions employed, provided that the Re 0 or Re 4+ species produced possess ultra-high dispersion.

Promoter effect of niobia on Pt/Al2O3 catalysts.: Part II. Effect of chlorine and Nb2O5 content on the structure and reactivity in CH3OH/D2 exchange

Abstract The effect of chlorine using precursors like H 2 PtCl 6 (0.55,2 and 10 wt.% Pt) and Pt(NH 3 ) 4 (NO 3 ) 2 (2 wt.% Pt) and of the Nb 2 O 5 loading on the structure of Pt/Al 2 O 3 and its catalytic activity in methanol/deuterium exchange has been investigated. In presence of niobia the BET surface of the support increases but niobia itself remains amorphous and its presence in increasing quantities results in the easier reduction of both chlorine-containing and chlorine-free Pt samples measured by TPR. XPS provides evidence for the effect of chlorine and niobia on the reduction of Pt. After reduction Nb 2 O 5 is converted into NbO x species which may migrate on top of the Pt particles measured by XRD, CO chemisorption and O/H titration. The increased perimeter of the Pt/NbO x /Al 2 O 3 interface raises the deuterium availability across this interface, which in turn, gives rise to the interaction of methyl groups in the anchored species with Pt for multiple exchange as was assumed in Part I.

CO chemisorption on small cobalt particles deposited on Si(111)

Abstract “Island-like”, highly dispersed, cobalt particles on a Si(111) single crystal have been prepared by means of an evaporation-sputtering technique. A unique electron structure of the Co islands measured by UPS, was developed. On adsorption of the CO molecules in l L dose a strong decrease in the density of states (DOS) of cobalt at 1.6 eV BE was observed which did not occur on a thick evaporated Co Si(111) sample. Comparison of the photoemission spectra revealed the dissociation of the CO molecules as a prevailing process on small cobalt islands, whereas CO retains its molecular form on thick Co layers. Here UPS evidence is given for the chemical interaction between highly reactive Co and CO molecules as compared to that measured for thick Co Si(111) and Si(111).