Robert Wojcieszak

Magnetic nanomaterials in catalysis: advanced catalysts for magnetic separation and beyond

While magnetic separation techniques have long been in use, intensive research into superparamagnetic nanomaterials has accelerated the development of magnetically recoverable catalysts. Preparation techniques are currently undergoing rapid development and magnetic separation has been studied to facilitate the handling and recovery of enzyme, organo-, metal complex-, and nanoparticle-catalysts. In this article, we emphasize the preparation of support materials, because the choice of the correct support and the immobilization strategy are of primary importance in the development of high-quality magnetically recoverable catalysts. We summarize the representative methods for the synthesis of well-defined uncoated and coated magnetic nanomaterials. Recent scientific progress on the preparation of surface-modified magnetic nanomaterials and the most common synthetic approaches to attach or immobilize non-magnetic catalytic active phases onto magnetic nanomaterials were discussed. Moreover, better control and understanding of the magnetic properties is now an essential tool not only in selecting the best preparation route for recoverable catalysts, but also for designing reactors (e.g., magnetic fluidized-bed reactors) and for developing magnetic field-driven technologies (e.g., changes in the catalytic output operating under an applied magnetic field).

Insights into the active surface species formed on Ta2O5 nanotubes in the catalytic oxidation of CO

Freestanding Ta2O5 nanotubes were prepared by an anodizing method. As-anodized amorphous nanotubes were calcined at high temperature to obtain a crystalline phase. All materials were studied by means of BET analysis, XRD, TEM, SEM, XPS, and FTIR and were evaluated in the catalytic oxidation of CO. An XPS study confirmed the formation of different tantalum surface species after high temperature treatment of amorphous Ta2O5 nanotubes. Calcination at 800 °C generated Ta(4+) while higher temperature (1000 °C) treatment led to the formation of Ta(3+) species. These materials also showed significant differences in catalytic activity. Higher activity was observed for samples calcined at 800 °C than at 1000 °C, suggesting that Ta(4+) species are active sites for CO oxidation.

Easy Access to Metallic Copper Nanoparticles with High Activity and Stability for CO Oxidation.

Copper catalysts are very promising, affordable alternatives for noble metals in CO oxidation; however, the nature of the active species remains unclear and differs throughout previous reports. Here, we report the preparation of 8 nm copper nanoparticles (Cu NPs), with high metallic content, directly deposited onto the surface of silica nanopowders by magnetron sputtering deposition. The as-prepared Cu/SiO2 contains 85% Cu0 and 15% Cu2+ and was enriched in the Cu0 phase by H2 soft pretreatment (96% Cu0 and 4% Cu2+) or further oxidized after treatment with O2 (33% Cu0 and 67% Cu2+). These catalysts were studied in the catalytic oxidation of CO under dry and humid conditions. Higher activity was observed for the sample previously reduced with H2, suggesting that the presence of Cu-metal species enhances CO oxidation performance. Inversely, a poorer performance was observed for the sample previously oxidized with O2. The presence of water vapor caused only a small increase in the temperature require for the reaction to reach 100% conversion. Under dry conditions, the Cu NP catalyst was able to maintain full conversion for up to 45 h at 350 °C, but it deactivated with time on stream in the presence of water vapor.

Study of mesoporous CdS-quantum-dot-sensitized TiO2 films by using X-ray photoelectron spectroscopy and AFM

Summary CdS quantum dots were grown on mesoporous TiO2 films by successive ionic layer adsorption and reaction processes in order to obtain CdS particles of various sizes. AFM analysis shows that the growth of the CdS particles is a two-step process. The first step is the formation of new crystallites at each deposition cycle. In the next step the pre-deposited crystallites grow to form larger aggregates. Special attention is paid to the estimation of the CdS particle size by X-ray photoelectron spectroscopy (XPS). Among the classical methods of characterization the XPS model is described in detail. In order to make an attempt to validate the XPS model, the results are compared to those obtained from AFM analysis and to the evolution of the band gap energy of the CdS nanoparticles as obtained by UV–vis spectroscopy. The results showed that XPS technique is a powerful tool in the estimation of the CdS particle size. In conjunction with these results, a very good correlation has been found between the number of deposition cycles and the particle size.

Low temperature oxidation of methanol to methyl formate over Pd nanoparticles supported on γ-Fe2O3

Pd nanoparticles supported on γ-Fe2O3 (2 wt.%) were synthesized using the water-in-oil microemulsion method (using hydrazine as a reductant agent). Materials were characterized by N2-BET at low temperature, XRD, XPS, Raman, and FTIR and tested in the gas phase reaction of oxidation of methanol. The direct formation of methyl formate (MF) from methanol was observed. Supported palladium catalysts produced methyl formate at low temperature (<150 °C) with a relatively high selectivity depending on the Fe+2/Fe+3 ratio (2:1, 1:1, 1:2) used for the preparation of the supports. Methyl formate is already formed at 50 °C with the maximum at about 80 °C. At higher temperature, methyl formate is no longer formed and the oxidation to CO2 and CO occurs. Raman studies indicated the changes in the structure of the Fe2O3 support in the case of the 1:2 sample after chemical reduction with hydrazine.

NiAg catalysts prepared by reduction of Ni2+ ions in aqueous hydrazine: II. Support effect

A series of bimetallic NiAg (Ni + Ag = 1% wt) catalysts supported on amorphous silica was synthesized via chemical reduction using hydrazine as the reducing agent at 353 K. Catalysts were prepared via impregnation or precipitation technique. It was found that the reduction of the Ni(2+) ions occurred only in the presence of silver, otherwise a stable blue [Ni(N(2)H(4))(3)](2+) complex was formed. Comparisons with similar NiAg catalysts supported on crystallized silica as prepared in our previous work indicated that the Ni(2+) ions weakly interacted with acidic crystallized silica on which they were readily reduced. For both supports, the combination of silver and nickel gave rise to a synergistic effect due to the existence of NiAg groupings. The surface and catalytic properties of the metal particles formed depended on the Ni:Ag ratio, method of preparation, and acidity of the support.

Oxidation of methanol to methyl formate over supported Pd nanoparticles: insights into the reaction mechanism at low temperature

Pd nanoparticles supported on TiO2 and SiO2 (2 wt.%) were synthesized by the water-in-oil microemulsion method. The materials were characterized by standard physico-chemical methods (XRD, ICP, TEM, BET, XPS) and DRIFT in operando mode and tested in the gas-phase reaction of methanol oxidation. The direct formation of methyl formate (MF) from methanol was observed. Supported palladium catalysts produced methyl formate at low temperature (<100 °C) with high selectivity. At higher temperatures methyl formate is no longer formed and the total oxidation to CO2 occurred. The DRIFT-operando study confirmed that methanol is adsorbed mainly in two forms, the undissociated gaseous methanol (via H bond) and dissociatively adsorbed methoxy species (CH3O−) on the surface. Methyl formate is formed already at RT with the maximum at about 80 °C. The mechanism of the formation of methyl formate from methanol at low temperature is discussed

Direct dehydration of 1,3-butanediol into butadiene over aluminosilicate catalysts

The catalytic dehydration of 1,3-butanediol into butadiene was investigated over various aluminosilicates with different SiO2/Al2O3 ratios and pore architectures. A correlation between the catalytic performance and the total number of acid sites and acid strength was established, with a better performance for lower acid site densities as inferred from combined NH3-TPD, pyridine adsorption and 27Al-NMR MAS spectroscopy. The presence of native Bronsted acid sites of medium strength was correlated to the formation of butadiene. A maximum butadiene yield of 60% was achieved at 300 °C over H-ZSM-5 with a SiO2/Al2O3 ratio of 260 with the simultaneous formation of propylene at a BD/propylene selectivity ratio of 2.5. This catalyst further exhibited a slight deactivation during a 102 h run with a decrease in the conversion from 100% to 80% due to coke deposition as evidenced by XPS and TGA-MS, resulting in a 36% loss of the specific surface area.

Supported Pd nanoparticles prepared by a modified water-in-oil microemulsion method

Supported Pd nanoparticles (1.6 wt. %) with different diameters were synthesized by the modified water-in-oil microemulsion method using hydrazine as reducing agent. The size of palladium nanoparticles was investigated by varying the nature of the organic surfactant and solvent. The catalysts were characterized by XRD, XPS, ICP, and TEM. Supported palladium nanoparticles (1–8 nm) were obtained. The results confirmed the dependence of the particle size on the nature of organic surfactants. Smaller particles were obtained with organic solvents and anionic surfactants.

Incorporation of group five elements into the faujasite structure

Niobium and tantalum were incorporated into the faujasite aluminosilicate structure in one-pot synthesis and also by post-synthesis method, i.e. solid-state ion exchange. One-pot synthesized zeolites exhibit Nb and Ta incorporated into the zeolite skeleton as evidenced by different methods. The efficiency of metal incorporation examined by XPS measurements was found to be higher for tantalum than for niobium.