Nina Bogdanchikova

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.

Nanostructured iron oxide catalysts with gold for the oxidation of carbon monoxide

A commercial iron oxide support is compared with Fe2O3 samples prepared by decomposition of iron nitrate, at 300 °C and 500 °C and heating times varying from 30 min to 96 h in N2. Different methods were used for gold deposition, namely double impregnation (DIM), liquid phase reductive deposition (LPRD) and ultrasonication (US). Samples were characterised by N2 adsorption at −196 °C, high-resolution transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and temperature programmed reduction. CO oxidation was used as a test reaction to compare the catalytic activities. The best results were obtained for the sample produced by decomposition of nitrate at 300 °C for 1 h, which showed the largest surface area and highest amount of hydroxylated iron species. Increasing the calcination time and/or the temperature produced less active samples. Although LPRD materials showed the smallest gold nanoparticle sizes (1–12 nm), the best catalytic results were obtained for the DIM materials. This is most likely related to the oxidation state of gold (Au+) found in the DIM catalysts, in contrast with LPRD and US materials, where metallic gold (Au0) is present. Other samples showed lower activities. Nevertheless, all samples prepared by DIM had better catalytic activity than the Au/Fe2O3 World Gold Council reference catalyst.

Effect of the pretreatment with oxygen and/or oxygen-containing compounds on the catalytic performance of Pd-Ag/Al2O3 for acetylene hydrogenation

Abstract Catalytic performance of Pd-Ag/α-Al 2 O 3 was studied for the selective hydrogenation of acetylene in the presence of excess ethylene. The catalyst activation was undertaken prior to the reaction test by the pretreatment with oxygen and/or oxygen-containing compounds, i.e. O 2 , NO, N 2 O, CO and CO 2 . The enhancement of catalytic performances by the pretreatment was a consequence of an increase in accessible Pd sites responsible for acetylene hydrogenation to ethylene. Furthermore, the sites involving direct ethane formation from acetylene could be suppressed by NO x treatment.

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.

FORMATION OF GOLD NANOPARTICLES IN ZEOLITES

The formation of gold nanoparticles in mordenites has been studied by the methods of FTIR spectroscopy of adsorbed CO and diffuse reflectance UV-visible spectroscopy. Different states of ionic and metallic gold were detected in the zeolite cavities and on the external surface of the zeolite – Au+ and Au3+ ions, charged clusters Aunδ+, and neutral nanoparticles Aum. The relative amount of these states depends on the method of sample preparation, type of redox treatment and the SiO2/Al2O3 molar ratio.

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.

Gold nanoparticles supported on magnesium oxide for CO oxidation

Au was loaded (1 wt%) on a commercial MgO support by three different methods: double impregnation, liquid-phase reductive deposition and ultrasonication. Samples were characterised by adsorption of N2 at -96°C, temperature-programmed reduction, high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction. Upon loading with Au, MgO changed into Mg(OH)2 (the hydroxide was most likely formed by reaction with water, in which the gold precursor was dissolved). The size range for gold nanoparticles was 2-12 nm for the DIM method and 3-15 nm for LPRD and US. The average size of gold particles was 5.4 nm for DIM and larger than 6.5 for the other methods. CO oxidation was used as a test reaction to compare the catalytic activity. The best results were obtained with the DIM method, followed by LPRD and US. This can be explained in terms of the nanoparticle size, well known to determine the catalytic activity of gold catalysts.

Structural and catalytic properties of Pd/Al2O3–La2O3 catalysts

Abstract A new Pd/Al2O3–La2O3 catalyst has been synthesized for the reduction of NO with hydrogen. This catalyst is more active than coprecipitated Pd/Al2O3 catalysts. The revealed effect of the improvement of the catalytic activity at medium temperature and the increase of NH3 formation at high temperatures for Pd catalyst supported on alumina–lanthana prepared by the sol–gel method are ascribed to a new lanthanum-containing phase observed by X-ray powder diffraction and high resolution electron microscopy.