Wojciech Gac

Complete Oxidation of Methane over Palladium Supported on Alumina Modified with Calcium, Lanthanum, and Cerium Ions

Abstract The activity and thermal stability of Pd/Al 2 O 3 and Pd/(Al 2 O 3 +MO x ) (M=Ca, La, Ce) palladium catalysts in the reaction of complete oxidation of methane are presented in this study. The catalyst supports were prepared by sol-gel method and they were dried either conventionally or with supercritical carbon dioxide. Then they were impregnated with palladium nitrate solution. The catalysts with unmodified alumina had a high surface area. The activity and thermal stability of the alumina- supported catalyst was also very high. The introduction of calcium, lanthanum, or cerium oxide into alumina support caused a decrease of the surface area in the way dependent on the support precursor drying method. These modifiers decreased the activity of palladium catalysts, and they required higher temperatures for the complete oxidation of methane than unmodified Pd/Al 2 O 3 . The improvement of the palladium activity by lanthanum and cerium support modifier was observed only at low temperatures of the reaction.

Thermodesorption studies of energetic properties of nickel and nickel-molybdenum catalysts based on the statistical rate theory of interfacial transport

The new theoretical approach to adsorption/desorption kinetics, called the statistical rate theory of interfacial transport, is used to analyze quantitatively spectra of hydrogen thermodesorption from alumina-supported nickel and nickel-molybdenum catalysts. A new more accurate method is developed to calculate the distribution of the chemisorption energies on the surfaces of these two catalysts. The comparison of their adsorption energy distributions shows that the addition of molybdenum, changes relative mutual proportions of the four kinds of chemisorption sites existing on these catalysts surfaces. Changing these proportions is related to changing tendencies to coking formation and changing activities for methane steam reforming.

Surface State and Catalytic Performance of Ceria‐Supported Cobalt Catalysts in the Steam Reforming of Ethanol

The effect of the particle size and the addition of a K promoter on the oxidation state of Co/CeO2 was investigated and correlated with selectivity for ethanol steam reforming performed with various H2O/EtOH molar ratios. Spectroscopic studies showed that the oxidation state of the catalyst components depends on the H2O/EtOH molar ratio. Surface oxygen‐containing species (OH and Kδ+−Oδ−) are necessary to achieve a good catalyst selectivity, to decrease the amount of coke deposited and to change the nature of the product from fully dehydrogenated C=C to CHx. Besides the surface concentration of oxygen‐containing species, the catalyst morphology and the location at which oxygen‐containing species are chemisorbed may be equally important to their abundance; the selective ceria‐supported Co catalyst should have well‐dispersed Co particles deposited on a well‐dispersed support.

Nickel-promoted Catalysts in the Reforming of n-Butane with CO2 or H2O

Studies of the reforming of n-butane by steam and carbon dioxide have been undertaken employing commercial catalysts such as Ni/α-Al2O3 modified by small additions of promoters such as K, Ba, Ce, W and Mo compounds. It was found that promoters improved the resistance of the catalyst towards coking, with this effect being smaller where CO2 was employed in the reforming reaction than when steam was used. The dynamics of the growth of the carbon deposit on the catalyst surface depended on the promoters employed as well as on the operating conditions; however, in the CO2 reforming of n-butane, such coking was more pronounced than in steam reforming. Use of the TPO method with oxygen or CO2 did not reveal considerable differences between the coke deposits formed during the reforming of n-butane with CO2 or H2O on the catalysts examined.

Influence of annealing temperature on structural and magnetic properties of MnFe2O4 nanoparticles

Abstract Nanoparticles of manganese ferrite were obtained by the impregnation of highly ordered mesoporous MCM-41 silica support. The investigated sample contained 20% wt. Fe. The obtained nanocrystallites were strongly dispersed in silica matrix and their size was about 2 nm. The sample annealing at 500°C led to increase of particle size to about 5 nm. The Mössbauer spectroscopy investigations performed at room temperature show on occurrence of MnFe2O4 nanoparticle in superparamagnetic state for the sample annealed in all temperatures. The coexistence of superparamagnetic and ferromagnetic phase was observed at liquid nitrogen temperature. The sample annealed at 400°C and 500°C has bigger manganese ferrite particle and better crystallized structure. One can assign them the discrete hyperfine magnetic field components.

P-Arylation of secondary phosphine oxides catalyzed by nickel-supported nanoparticles

A protocol for the cross coupling of aryl halides with secondary phosphine oxides over Ni/CeO2 or Ni/Al2O3 catalysts has been developed for the first time. The advantages offered using this protocol are operational simplicity and the use of an inexpensive heterogeneous nickel catalyst in the absence of any additional ligand. A range of aryl bromides and iodides are coupled with secondary phosphine oxides using low loadings of a catalyst (1 mol%) via this procedure, with good to high yield (up to 86%). Moreover, the influence of nickel particle size and catalyst support on P-arylation of secondary phosphine oxides has also been established.

The vibrational spectrum of 1,4-dioxane in aqueous solution – theory and experiment

Continuum solvation models become unreliable when the dissolved molecule has specific interactions with the solvent. The present work focuses on comparison between a representative continuum solvation model, the conductor-like screening model (COSMO) and the explicit solvation model (ESM). Other continuum solvation models behave similarly to COSMO. Vibrational spectroscopy is a sensitive test of these models. Their performance for reproducing vibrational frequencies is examined by comparison with our experimental results on 1,4-dioxane dissolved in water. COSMO reproduces the frequency shifts only in the CH stretching vibration region but fails in the lower frequency range dominated by CC and CO stretching and deformations. It often predicts the wrong signs for the frequency shifts and the root-mean-square (RMS) deviation between the calculated and observed shifts is large (nearly 9 cm−1). The explicit solvation model, in which representative dioxane–water clusters are considered, provides accurate solvent frequency shifts, with the correct sign in the overwhelming majority of cases and the RMS deviation is below 2 cm−1 in the low-frequency range. However, it performs slightly worse than COSMO for CH stretching. Surprisingly it was also found that the combined COSMO + ESM model performs worst of all three models considered in this work.

Study on the effect of atmospheric gases adsorbed in MnFe2O4/MCM-41 nanocomposite on ortho-positronium annihilation

Abstract In this paper, results of positron annihilation lifetime spectroscopy (PALS) studies of MnFe2O4/MCM-41 nanocomposites in N2 and O2 atmosphere have been presented. In particular, the influence of manganese ferrite loading and gas filling on pick-off ortho-positronium (o-Ps) annihilation processes in the investigated samples was a point of interest. Disappearance of the longest-lived o-Ps component with τ5 present in the PAL spectrum of initial MCM-41 mesoporous material in the PAL spectra of MnFe2O4-impregnated MCM-41 measured in vacuum is a result of either a strong chemical o-Ps quenching or the Ps inhibition effects. The intensity I4 of the medium-lived component initially increases, reaching a maximum value for the sample with minimum manganese ferrite content, and then decreases monotonically. Analogous dependence for the intensity I3 of the shortest-lived component shows a maximum at higher MnFe2O4 content. Filling of open pores present in the studied nanocomposites by N2 or O2 at ambient pressure causes partial reappearance of the τ4 and τ5 components, except a sample with maximum ferrite content. The lifetimes of these components measured in O2 are shortened in comparison to that observed in N2 because of paramagnetic quenching. Anti-inhibition and anti-quenching effects of atmospheric gases observed in the MnFe2O4/MCM-41 samples are a result of neutralization of some surface active centers acting as inhibitors and weakening of pick-off annihilation mechanism, respectively.

An application of microemulsion method for synthesis of copper-zinc materials

Abstract Microemulsion method was used for preparation of copperzinc mixed oxides. Samples were prepared from the solutions containing zinc and copper nitrates. Sodium carbonate was used as a precipitant and hydrazine as reducing agent. Water-in-oil (W/O) microemulsions were formed by the application of cyclohexane, isopropyl alcohol, and hexadecyltrimethylammonium bromide (CTAB) as surfactant. The aim of the studies was determination of the influence of the sequence of synthesis stages on the formation of materials, their surface and structural properties. Thermal decomposition studies of materials were carried out by infrared spectroscopy. The physicochemical properties were characterized by nitrogen adsorption-desorption method, X-ray diffraction (XRD) and temperature-programmed reduction (TPR).

The influence of gold on the properties of silica mesoporous materials

The influence of gold on the properties of silica mesoporous materials The properties of gold modified silica mesoporous materials were investigated on the successive preparation stages. Samples were obtained by the direct introduction of HAuCl4 aqueous solution to the synthesis mixture of silica. It was stated that partial substitution of -Cl with -OH ligands occurred in the diluted solutions of HAuCl4. The replacement of -Cl with -Br groups in gold complexes took place in the synthesis mixture containing hexadecyltrimethylammonium bromide surfactant molecules. It was found that gold species influenced the removal of organic templates from the channels of silica materials. Pore arrangement was not strongly distorted by the presence of small amounts of gold. Heat treatment led to the pronounced changes of the nature of gold species.