O. P. Tkachenko

Methanol synthesis over ZnO: A structure-sensitive reaction?

In order to identify active sites on ZnO powdered catalysts in methanol synthesis a total of five ZnO samples with different degrees of crystallinity were characterized by means of N2 physisorption, XRD, TEM, and EXAFS. With respect to catalysis, high-pressure methanol synthesis was performed as a test reaction. A linear area-activity relationship for the highly crystalline materials was obtained, but at high BET surface areas a strong deviation from linearity was found. The observed phenomena provide evidence for a structure-sensitivity, suggesting that a specific active site is favored for methanol formation. Based on earlier work with polycrystalline ZnO powder and with respect to the current theoretical and experimental work with ZnO single crystals, the polar ZnO faces are assumed to be highly relevant for the catalytic activity under methanol synthesis conditions.

X.p.s. investigation of methylene blue incorporated into faujasites and AIPO family molecular sieves

Abstract The distribution of methylene blue (MB) between shell and core domains of molecular sieve crystalsand the extent of aggregation of the molecules in the molecular sieve hosts is studied using X-ray photoelectron spectroscopy. It follows (i) from differences of the binding energies of the N1s electrons of solid MB and of molecular sieve-supported MB and (ii) from N/Si atomic ratios that MB is encapsulated inside molecular sieve cages and channels as isolated singular species or small aggregate if added as dissolved dye to the batch composition for the hydrothermal crystallization of molecular sieves (faujasite Y, SAPO-5, SAPO-34, AIPO-5). Ion-exchange of the MB cation into zeolites results also in high dispersions of the dye, but can produce a local destruction of the zeolite framework at the crystal surface. The structure-directing role of the dye was recognized for MB-loaded SAPO-5 and SAPO-34, exhibiting an improved phase purity.

Synthesis and characterization of silica MCM-48 as carrier of size-confined nanocrystalline metal oxides particles inside the pore system

Abstract The interpenetrating 3-dimensional channel system of silica MCM-48 has been selected for the deposition of Cu/Zn/O mixed metal oxide particles. With the wet impregnation technique aqueous solutions of metal acetates have been used to load the calcined form of the mesoporous silica. Successive impregnation yielded metal contents of ca. 9 wt.%. Calcination of the composite transformed the acetates to the metal oxides. X-ray powder diffraction and solid-state MAS NMR showed the uptake of the metal salt inside the pore system. N 2 -adsorption, X-ray diffraction and TEM confirmed the mesoporous structure. XPS measurements and EXAFS analysis (Cu K- and Zn K-edges) confirmed the metal uptake. Whereas nano-disperse CuO particles have been obtained ZnO shows no regular structure and seems to have reacted with the silicate channel surface by coating the channel wall.

Platinum promoting effects in Pt/Ga zeolite Catalysts of lower alkane aromatization. I. Ga and Pt electronic states, dispersion and distribution in zeolite crystals in dependence of preparation techniques. Dynamic effects caused by reaction mixture

Abstract X-ray photoelectron spectroscopy (XPS), infrared (IR), electron microscopy (EM) and energy dispersive X-ray (EDAX) studies of in various ways prepared Ga/H-ZSM-5, Pt-Ga/H-ZSM-5 as well as Ga-silicate and Pt/Ga-silicate have been performed. The external surface of the zeolite crystal was originally strongly enriched with a Ga2O3 phase the dispersion of which depends on the preparation as well. Ga reduction, evidenced by XPS Ga 3d line shifts, occurred during reduction and/or the reaction mixture treatment and it was accompanied by migration of GaI cations into the zeolite channels. The migration of unreduced GaIII species via solid-state reactions is less pronounced. The gallium reduction and migration is dramatically promoted by platinum introduced into both Ga/H-ZSM-5 and Ga-silicate; the reduction degree in the last sample is limited by the concentration of the extra-framework GaIII ions. Platinum was shown to be finely dispersed in highly acidic Pt-Ga/H-ZSM-5 while most of the platinum formed aggregates (30–50A˚) on the external surface of Ga-silicate. IR of adsorbed hydrogen and pyridine confirmed that reduction results in the formation of new strong Lewis acidic sites. Along with GaI-HZ and GaIIHZ structures mixed Pt-Ga particles located within the zeolite structure are most likely to be responsible for the dramatic enhancement of the aromatic selectivity found with Pt-Ga/zeolite catalysts (see Part II).

Pd-Fe nanoparticles stabilized by chitosan derivatives for perchloroethene dechlorination.

A series of chitosan-stabilized Pd-NZVI (nano-zero-valent-iron) catalysts for dechlorination with variation in their composition and in the nature of the polymer has been prepared. The synthesis procedures and palladium and chitosan contents were optimized. It was demonstrated by the XPS method that Fe and Pd in Fe-Pd/chitosan samples exist in the metallic state. The positive shift of the binding energy as compared with the bulk metal shows that the iron metal in the surface layers exists as very small nanoparticles. The prepared materials were characterized also by the XAS method. The presence of O and N atoms in the first coordination shell of the central Fe atom in the Fe-Pd/chitosan samples certifies the binding of the Fe metal particles with the chitosan surface via OH and NH(2) groups. The samples are characterized by the high stability of the nanoparticles as compared to unstabilized Pd-NZVI. The materials were tested to evaluate their catalytic activity in the perchloroethene (PCE) dechlorination reaction. Some samples of chitosan-stabilized Pd-NZVI revealed a good performance in PCE degradation as compared to unstabilized Pd-NZVI.

Strong metal-support interaction: The role of electronic and geometric factors in real Me/TiO2 catalysts

Abstract XPS, X-ray-induced AES (XAES), AES, SIMS, and ISS studies of metals (Pt, Rh) supported on TiO 2 powder catalysts combine to give new evidence in favor of electronic nature of SMSI. When reduced at high temperatures (HTR), the XPS spectra, of Me TiO 2 , corrected for extra-atomic relaxation, exhibit negative chemical shifts of core levels along with a gain in intensity of the density of states near the Fermi edge, indicating an increase of electron density on metal clusters. Tausters model of SMSI is further confirmed by the appearance of mixed RhTi + and RhTiO + clusters in SIMS spectra of Rh TiO 2 (HTR), which reflect direct interaction between metal atoms and Ti 3+ cations. The latter were detected by Ti L 3 M 23 V Auger peaks. The Rh + Ti + depth profile and the ISS Rh Ti ratio reveal some migration of TiO x entities onto metal surface, but no strong encapsulation of metallic particles is observed in real TiO 2 -supported catalysts reduced at temperatures as high as 500 °C. Thus, the observed decrease in the activities of Me TiO 2 in the C 2 H 6 hydrogenolysis and C 6 H 6 hydrogenation reactions seems to be accounted for by both geometric and electronic factors. The kinetics of 16 O 18 O homomolecular isotopic exchange measured for the first time on these catalysts is in accordance with the proposed SMSI model and explains its degradation during catalyst exposure to O 2 at low or elevated temperatures.

New high-selectivity hydrogenation catalysts prepared from bimetallic acetate complexes

The catalytic properties of new Pd-Zn/Al2O3 catalysts in selective acetylene hydrogenation in an acetylene-ethylene mixture at 30–120°C and atmospheric pressure are reported. The catalysts prepared from the bimetallic complex Pd-Zn(OOCMe)4(OH2) are much more selective than the catalysts prepared by simultaneously supporting the homonuclear complexes Pd3(OOCMe)6 and Zn(OOCMe)2 · 2H2O. It is demonstrated by diffuse reflectance IR spectroscopy of adsorbed CO that the heat treatment of the supported bimetallic complex at 250°C in flowing H2 yields a Pd-Zn alloy on the surface. It is this alloy that ensures the high selectivity of the Pd-Zn/Al2O3 catalysts.

X-ray photoelectron/X-ray excited auger electron spectroscopic study of highly dispersed semiconductor CdS and CdO species in zeolites

The identification and study of the electronic properties and location of CdSx(x < 1) and CdO particles in faujasites have been performed using X-ray photoelectron spectroscopy (XPS), X-ray excited Auger electron spectroscopy (XAES) and fluorescence spectroscopy. These data were compared with results obtained by transmission electron microscopy (TEM), UV–VIS reflectance spectroscopy and adsorption measurements reported earlier for the same samples. Depending on the preparation procedure, CdS aggregates have different dispersions and can be located either inside the zeolite structure or on the external surface. Small CdS particles (1.5–2 nm) exhibit both XPS and fluorescence band shifts indicating a semiconductor size quantization (Q-size) effect. In addition, the non-stoichiometry of these species was also displayed in the fluorescence spectra. Larger CdS particles exhibit electronic and relaxation (polarization) properties rather close to those of bulk CdS. Strong surface enrichment with these particles was also observed. The CdO aggregates are formed during CdY treatment with NaOH and calcination. The identification of CdO is based on an abnormally low binding energy of Cd, which is characteristic for cadmium oxide only.

Xps Study of Transition Metal Complexes in Zeolites

Abstract Transition metal complexes (TMC) fixed in Y zeolites by “surface assembling” [Ni, Co, Cu, Ru phthalocyanines (Pc)] or by exchanging Na + for [RuNO(NH 3 ) 4 OH] 2+ and [Rh(NH 3 ) 5 Cl] 2+ cations have been investigated by XPS. MePc were established to be located in supercages as isolated complexes which interact with zeolitic OH groups via meso N atoms of the porphyrin ring. The stability of both MePc and complex cations under various conditions of zeolites pretreatment was compared. MePc were found to be quite stable in inert and reducing atmosphere while Ru and Rh complexes decomposed causing the stabilization of metals in intermediate and zero-valence states. XPS combined with a catalytic study showed that zeolites with TMC are of great interest as promising metal-complex catalysts or precursors of zeolite supported catalysts with high dispersion of metals.

Pd–Cu catalysts from acetate complexes in liquid-phase diphenylacetylene hydrogenation

Properties of Pd–Cu/Al2O3 catalysts prepared using PdCu(CH3CO2)4 acetate heteronuclear complexes as precursors in the liquid-phase diphenylacetylene (DPA) hydrogenation have been studied. It has been established that the reaction over the Pd–Cu/Al2O3 catalyst proceeds more selectively than over the commercial Lindlar catalyst; in addition, high activity is achieved at a substantially lower palladium content. The maximum selectivity of DPA hydrogenation is observed with the catalyst reduced in a hydrogen atmosphere without any intermediate calcination that can result in the destruction of the bimetallic acetate complex. FTIR spectroscopy data for adsorbed CO show that the high selectivity of hydrogenation is due to the formation of homogeneous Pd–Cu particles and to the absence of monometallic palladium particles. This can be explained by the retention of the initial complex structure at all of the catalyst preparation stages until the formation of bimetallic particles during hydrogenation.