Vitalii Petranovskii

Preparation of MoS2 and WS2 catalysts by in situ decomposition of ammonium thiosalts

The in situ decomposition of ammonium thiometallates during the hydrodesulfurization (HDS) of dibenzothiophene (DBT), to obtain molybdenum disulfide and tungsten disulfide catalysts, was investigated. It was found that very efficient catalysts for the HDS of DBT were obtained by in situ decomposition. Mechanical uniaxial pressing of the precursors (ammonium thiometallates) affected both textural and catalytic properties of the catalysts. Surface areas of molybdenum and tungsten disulfides increased as a function of uniaxial pressing, while catalytic activities went through a maximum. For MoS2, the hydrogenation selectivity was much higher for in situ catalysts than for ex situ ones. For WS2 catalysts, the hydrogenation selectivity was less sensitive to the condition of decomposition (ex situ/in situ). The surface S/M (M = Mo, W) atomic ratio from the Auger signal decreased as a function of uniaxial pressing, while the C/M ratio remained almost constant at 1.6. The best catalyst showed an experimental S/Mo ratio that is slightly higher than the stoichiometric value. The effect of in situ decomposition and mechanical deformation of thiometallate precursors is discussed.

Selective oxidation of alcohols over foam-metal catalysts

Abstract Catalysts based on the foam metals (silver and copper) were studied in the processes of partial oxidation of methanol, ethanol and ethylene glycol. The experiments showed that the foam catalysts have high gas permeability, mechanical strength, thermostability and catalytic properties exceeding the ones of the traditional crystal and granular metal catalysts. Electronic states of silver and copper on the surface of the catalysts were studied by the method of diffuse reflectance electron spectroscopy in UV–VIS range.

Preparation of WS2 catalysts by in situ decomposition of tetraalkylammonium thiotungstates

Tungsten tetraalkylammonium thiosalts are used as precursors for the in situ formation of WS2 catalysts in dibenzothiophene (DBT) hydrodesulfurization. The thermal decomposition of alkyl-ammonium thiosalts proceeds directly to WS2 without WS3 formation, as in the case of ammonium thiotungstate (ATT), allowing good control of the catalyst’s stoichiometry. The alkyl-ammonium thiosalts give WS2 particles with different characteristic morphologies. The hydrodesulfurization (HDS) activities of WS2 catalysts derived from alkylthiosalts are higher than those of catalysts derived from the ammonium thiosalt. The reaction rate increases with the size of the cation in the precursor. No correlation of catalytic activities with surface areas is found. The S/W and C/W surface ratios determined by Auger electron spectroscopy decrease with increasing cation size. Surface composition is WS2:25C1:7 ,W S 1:7 C0:9 and WS1:3 C0:7 for the in situ catalysts derived from ammonium, methylammonium, and butylammonium precursors, respectively. The improved catalytic properties of WS2 catalysts derived from alkylammonium thiosalts in the HDS of DBT are attributed to the formation of carbon-containing tungsten sulfide phases on the catalyst’s surface.

Stability of silver clusters in mordenites with different SiO2/Al2O3 molar ratio

Abstract The stability and decay of silver clusters characterized by absorption bands 320 and 285 nm incorporated in mordenites with different SiO2/Al2O3 molar ratios were studied under ambient conditions. Significantly different rates of disappearance of these two bands were the basis for assigning them to different silver species. Oxidation converts the clusters peaking at 320 and 285 nm into other silver clusters characterized by absorption bands at 310 and 240 nm, respectively. The oxidation of silver clusters peaking at 285 nm is significantly faster than that at 320 nm. The stability of both original clusters decreases with acid site strength that, in its turn, depends on Si02/Al2O3 molar ratio. The minimum lifetime of clusters peaking at 320 and 285 nm equal to ca. 40 and 20 days, respectively, was observed for mordenite characterized by the strongest acid sites. For mordenites possessing weaker acid sites, the lifetime of these clusters is found to be more than 50 months. The oxidation of silver clusters is reversible, and re-reduced silver clusters were revealed to possess the same electron structure as original ones.

Metal clusters and nanoparticles assembled in zeolites: an example of stable materials with controllable particle size

Abstract An ion-exchangeable zeolite (mordenite) is used to control the formation of nanoparticles and clusters within the solid matrix by the hydrogen reduction of metal ions (Ag+, Cu2+, and Ni2+). SiO2/Al2O3 molar ratio in mordenite appears to be an efficient tool to manage the reducibility of the metal ions. Few-atomic silver clusters in line with the larger silver nanoparticles were observed with DRS for the reduced Ag+-exchanged mordenites. Cu2+-exchanged ones produce the copper nanoparticles with different optical appearance, and Ni2+-exchanged mordenites are reduced up to complicated species with no explicit assignment of metal particles under the conditions studied.

Role of mordenite acid properties in silver cluster stabilization

Abstract In the present work, the role of SiO 2 /Al 2 O 3 molar ratio in silver cluster stabilization inside mordenite pores was studied. It was found that SiO 2 /Al 2 O 3 molar ratio regulates silver cluster stabilization by means of change of mordenite acid properties. The high concentration and high strength of Bronsted acid sites favor the stabilization of silver in the form of clusters inside the pores. In contrast, low concentration and low strength of Bronsted acid sites and high concentration of Lewis sites inside the channels can hinder stabilization of silver clusters. In this case, large particles on the external surface of zeolite are preferably formed. It was revealed that silver cluster introduction in its turn influences acid properties of mordenites, changing the concentration and structure of acid sites. As a result of silver incorporation, four types of Lewis acid sites presented in H-mordenites disappear and a new type of Lewis acid site appears. For the majority of samples, it is accompanied by enhancement of concentration of Lewis sites so that all Al becomes ‘IR visible’.

The effect of SiO2/Al2O3 molar ratio in mordenite upon the optical appearance of reduced copper

Abstract A series of protonated copper-containing mordenites with variable SiO 2 /Al 2 O 3 molar ratio (MR) in the range 10⩽MR⩽206 was prepared by ion exchange in copper nitrate aqueous solution. The reduction of copper ions incorporated into the mordenites by hydrogen was shown to lead to different reduced copper species including small metallic particles inter alia . The optical appearance of the copper particles depends strongly on the value of MR, however, in a non-monotonic manner in line with the variation of acidity of this series of mordenites. Copper particles formation in the mordenite with MR=15 is almost non-existent, while an efficient reduction proceeds for MR=10 and 20⩽MR⩽206. The optical appearance of copper particles is different and consists of a contribution of the pronounced plasmon resonance maximum at 550–600 nm for MR=10 and MR=206, while for MR=20 and MR=31, it has the shape of shoulder in the same range. These features are simulated using Mie theory, taking into account the size-dependence of the imaginary part of the dielectric constant and the properties of the host medium. The differences in size and particle localization are considered as the main reasons for the difference of the optical features in the visible range.

Influence of preparation conditions on formation of crystalline phases of nickel sulfide

Abstract Nickel sulfide catalysts obtained by homogeneous precipitation were prepared by varying the homogenization time and the sulfiding temperature. The surface area with respect to preparation conditions was studied by the BET method. Structure characterization studies by X-ray diffraction showed the formation of two main crystalline phases (NiS 1.03 and NiS–millerite). Their appearance depends on preparation conditions. Formation of millerite or NiS 1.03 occurs at the constant sulfiding temperature, when time of homogenization of precursors increases.

Growth and characterization of ZnS and ZnCdS nanoclusters in mordenite zeolite host

Abstract The growth of semiconductor nanoclusters of ZnS and the ternary ZnCdS system embedded in synthetic mordenites by ionic exchange using different mixtures of CdCl 2 and ZnCl 2 as starting sources, followed by treatment in hydrogen sulphide flow, is reported. Compositional, morphological and structural studies were realized by XRD, SEM, TEM and energy dispersive spectroscopy (EDS). XRD shows well-crystallized mordenite and the absence of secondary phases; EDS gives us the chemical composition of the matrix host and the relative weight percent of Cd, Zn and S. TEM and SEM show the morphology, packing and sizes of mordenite crystals (bundled needles of 3 μm long and 0.07 μm width). Measurements of the absorption spectra by diffuse reflectance spectroscopy in the UV-visible range allow us an analysis of the optical behavior as a function of the relative concentration of Zn and Cd. Samples show blue shifts in the absorption edges when the Cd concentration decreases and correspondingly, the Zn concentration increases. The optical band gap is determined and its behavior with concentration changes is discussed. Previously, ZnCdS clusters supported on mordenite were prepared in our laboratory by hydrothermal growth. A comparative discussion of the results obtained through both synthetic processes is presented.

Self-assembling of silver and copper small clusters within the zeolite cavities: prediction of geometry

Abstract The factors affecting a self-assembling of certain kind of silver and copper clusters during reduction of ion-exchanged forms of zeolites in hydrogen were studied. Conditions leading to the preferable M8 cluster formation were revealed. A matching of zeolite void and cluster sizes was found to be the key factor facilitating this process. Ag8 clusters can be easily stabilized in the zeolites with certain pore size (a cross-section in the range of 0.63–0.67 nm). Zeolites possessing voids wider than 0.70 nm stabilize these clusters only for low reduction temperatures. Small pores possessing at least one axis of pore cross-section ≤0.63 nm do not stabilize Ag8 clusters. Cu clusters assigned to Cu8 were predicted to be formed in narrow pores of erionite much easier than in mordenite voids. Comparison of experimental UV–Vis spectra with the results of ab initio calculations for selected M8 isomers was performed. The difference between Ag8 and Cu8 cluster models is discussed.