[Show abstract][Hide abstract] ABSTRACT: PtNi nanoparticle catalysts supported on oxygen functionalized carbon nanotubes were prepared by microwave-assisted polyol reduction using two different modes of irradiation, namely, continuous or pulsed irradiation. The influence of irradiation time or pulse number on catalyst structure and activity in methanol electrooxidation has been studied. Characterization was done with ICP-OES, XRD, TEM, XPS, and XAS to determine composition, morphology, crystal structural and chemical state. The electrocatalytic activity has been evaluated by cyclic voltammetry (CV) and chronoamperometry (CA). PtNi nanoparticles are present in alloy form and are well dispersed on the carbon nanotubes. Pt is in its metallic state, whereas Ni is present in metallic and oxidized form depending on the preparation conditions. The electrocatalytic activity both in terms of surface and mass specific activity is higher than that of the state-of-the-art-catalyst Pt/C (E-TEK). The enhancement of the electrocatalytic activity is discussed with respect to PtNi alloy formation and the resulting modification of the electronic properties of Pt by Ni in the alloy structure. The microwave assisted polyol method with continuous irradiation is more effective in the preparation of PtNi electrocatalysts both in terms of reaction time and activity than the pulsed microwave method.
[Show abstract][Hide abstract] ABSTRACT: Au/TiO2 catalysts prepared by a deposition-precipitation process and used for CO oxidation without previous calcination exhibited high, largely temperature-independent conversions at low temperatures, with apparent activation energies of about zero. Thermal treatments, such as He at 623 K, changed the conversion-temperature characteristics to the well-known S-shape, with activation energies slightly below 30 kJ mol(-1) . Sample characterization by XAFS and electron microscopy and a low-temperature IR study of CO adsorption and oxidation showed that CO can be oxidized by gas-phase O2 at 90 K already over the freeze-dried catalyst in the initial state that contained Au exclusively in the +3 oxidation state. CO conversion after activation in the feed at 303 K is due to Au(III) -containing sites at low temperatures, while Au(0) dominates conversion at higher temperatures. After thermal treatments, CO conversion in the whole investigated temperature range results from sites containing exclusively Au(0) .
Angewandte Chemie International Edition 02/2014; · 11.34 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Au/TiO2-Katalysatoren, die durch das “Deposition-Precipitation”-Verfahren hergestellt und ohne Kalzinierung eingesetzt wurden, erreichten in der CO-Oxidation hohe, weitgehend temperaturunabhängige Umsätze. Dagegen erschien nach thermischen Vorbehandlungen, z. B. in He bei 623 K, die Umsatz-Temperatur-Charakteristik in der bekannten S-Form, mit Aktivierungsenergien nahe 30 kJ mol−1. Charakterisierung der Proben durch XAFS und HAADF-STEM sowie eine Tieftemperatur-IR-Studie von Adsorption und Oxidation des CO zeigten, dass letzteres am frisch präparierten (gefriergetrockneten) Katalysator, der Gold ausschließlich als Au3+ enthielt, bereits bei 90 K durch Gasphasensauerstoff oxidiert wurde. Nach Aktivierung im Reaktantenstrom geht der CO-Umsatz bei niedrigen Reaktionstemperaturen auf Zentren zurück, die AuIII enthalten, bei höheren Temperaturen wird er von Au0 getragen. Nach thermischen Behandlungen wird CO im ganzen Temperaturbereich an Zentren umgesetzt, die ausschließlich metallisches Gold enthalten.
[Show abstract][Hide abstract] ABSTRACT: α-Pinene oxide, an oxygenated derivative of α-pinene, can be converted into various valuable substances useful as flavour, fragrance and pharmaceutical compounds. Campholenic aldehyde is one of the most desired products of α-pinene oxide isomerization being a valuable intermediate for the production of sandalwood-like fragrances. Iron modified zeolites Beta-75 and ZSM-5, mesoporous material MCM-41, silica and alumina were prepared by two methods (impregnation and solid-state ion exchange) and tested for selective preparation of campholenic aldehyde by isomerization of α-pinene oxide. The characterization of tested catalyst was carried out using scanning electron microscope analysis, nitrogen adsorption measurements, pyridine adsorption–desorption with FTIR, X-ray absorption spectroscopy measurements, XPS-analysis, 29Si MAS NMR and 27Al MAS NMR and X-ray diffraction. The isomerization of α-pinene oxide was carried out in toluene as a solvent at 70 °C. The main properties influencing the activity and the selectivity are the acidic and structural properties of the tested catalysts. The highest selectivity of 66% was achieved at complete conversion of α-pinene oxide with Fe-MCM-41.
Applied Catalysis A General 01/2014; 470:162–176. · 3.41 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Fe-ZSM-5 catalysts were prepared by different techniques, including some with additional inert cations such as Na+ or Ca2+ blocking between 25% and 80% of the exchange capacity of the zeolite. Their catalytic behavior in NO oxidation, standard SCR, and fast SCR was studied, with their site structure in different catalyst states investigated by UV–vis and EPR spectroscopy. Their activity for oxidation of NO to NO2 was greatly boosted by previous contact with a feed containing a reductant, e.g. NH3, at elevated temperatures. Therefore, NO2 formation rates measured after mere calcination of freshly prepared samples are irrelevant for mechanistic discussions related to NOx abatement reactions. The rates of NO2 formation and standard SCR were demonstrated to be uncorrelated over a wide range of catalysts and reaction conditions. Depending on catalyst and reaction conditions, the rate of NO2 formation exceeded, equaled or fell short of the rate of standard SCR. Our results strongly suggest that NO2 formation is inhibited by NH3 in the reaction environment of standard SCR. As a result, NO2 formation is slower than standard SCR under many different reaction conditions, and therefore, it cannot be a part of the reaction mechanism of standard SCR. Our results favor earlier mechanistic concepts of standard SCR being initiated by oxidation of NO to nitrite, while oxidation to NO2 seems to require specific sites.
[Show abstract][Hide abstract] ABSTRACT: Structural disintegration or the loss of accessible surfaces of functional nanostructures due to processes involving mass transport (e.g. sintering) is a serious problem for any application of these materials at elevated temperatures, like in heterogeneous catalysis or chemical sensing. Phases with low sintering temperatures, e.g. some metals or metal oxides like zinc oxide (ZnO), are very sensitive in this respect. Therefore, it is not only relevant to prepare important materials with refined morphologies, but the desired features need to be stable under real conditions. In this study, we describe the preparation of mesoporous ZnO nano-/microspheres by means of a template-assisted aerosol technique. Furthermore, by intentional introduction of impurity elements as dopants, specific surface areas and porosities of the prepared materials can be increased significantly. The impurities also strongly improve the thermal stability of the described ZnO nanostructures against thermal sintering. Although the pure ZnO material suffers from a complete loss of porosity, the structures of the impure ("dirty") materials change only negligibly. Even at 500 °C morphology and porosity are preserved. The latter advantageous property was used for testing the novel nanocatalysts in heterogeneous catalysis.
[Show abstract][Hide abstract] ABSTRACT: Lanthanide complexes LnL3 (Ln = Sm, Eu, Tb, Dy, Tm, Yb, Lu) with aromatic o-phosphorylated ligands (HL(1) and HL(2)) have been synthesized and identified. Their molecular structure was proposed on the basis of a new complex approach, including DFT calculations, Sparkle/PM3 modelling, EXAFS spectroscopy and luminescent probing. The photophysical properties of all of the complexes were investigated in detail to obtain a deeper insight into the energy transfer processes.
[Show abstract][Hide abstract] ABSTRACT: The structural properties of copper complexes with (2-hydroxyphenyl)di-p-tolylphosphane oxide (CuL12) and 3-(2-hydroxyphenyl)-3,4-dihydro-2H-benzo[f][1,5,3]dioxaphosphepine 3-oxide (CuL22) are studied in detail. The o-phosphorylated phenols demonstrate both bridging and chelating behaviour, and the main reaction product is a coordination polymer containing dimeric Cu2O8 fragments. The structural peculiarities of the o-phosphorylated phenols were revealed by single-crystal X-ray diffraction. The copper complexes are studied by single-crystal and powder X-ray diffraction, and IR, Raman and X-ray photoelectron spectroscopy (XPS). The applicability of XPS spectroscopy to structural studies of coordination compounds with complex organic ligands is demonstrated with CuL12 and CuL22. The results of ab initio structure solution and Rietveld refinement of the CuL22 structure from powder data are augmented by PW-PBE-D calculations.
European Journal of Inorganic Chemistry. 09/2013; 2013(27):4823-4831.
[Show abstract][Hide abstract] ABSTRACT: Mild-condition nitric acid functionalized carbon nanotubes were used as support for PtNi electrocatalysts with variation of Pt to Ni atomic ratio prepared by a polyol method in which ethylene glycol was used as solvent and reducing agent under conventional reflux conditions. TPD, TGA, N2 adsorption and cyclic voltammetry confirmed the presence of functional groups in the functionalized carbon nanotubes. Supported PtNi nanocatalysts were characterized with ICP-OES, TEM, XRD, XPS and XAFS. Well dispersed particles on the supporting material with particle sizes in the range of 2-3 nm were obtained. PtNi alloy formation was concluded from XRD, XPS and XAFS results, while the latter two methods point to the formation of Ni oxides as well. The elemental distribution within the catalyst nanoparticles is inhomogeneous with Ni enrichment close to the support. Heat treatment in inert gas up to 400 °C results in restructuring of the catalysts surface which changes the active sites arrangements. CV and CO stripping measurements showed that the PtNi catalysts have a higher electrocatalytic activity toward methanol oxidation in comparison to a commercial Pt/C E-TEK catalyst and the highest activity was found for a Pt to Ni atomic ratio of 3. The prepared catalysts show highly stable mass specific activity over 200 potential cycles. The catalysts treated at higher temperature (400 °C) show a surface enriched in Pt and exhibit lower activity for methanol oxidation reaction but higher stability over 200 cycles. The high catalytic activity and durability of the prepared PtNi electrocatalysts render them possible candidate catalysts for methanol oxidation in direct methanol fuel cells (DMFCs).
[Show abstract][Hide abstract] ABSTRACT: An overview of work with model systems designed to study metal–support interactions in heterogeneous catalysts is given. In these models, metal and support are both miniaturized by introduction as guests into a mesoporous host. The use of such models is demonstrated with AuTiO2 clusters encaged in MCM-48, and CuZnO clusters encapsulated in siliceous mesopore systems and in carbon nanotubes. The models promise a better opportunity to track changes in the support component during catalyst activation and catalysis, including the action of poisons that may at first be trapped on the support surface. Challenges to be met are the stabilization of the mesoporous matrix during synthesis and catalysis, possible reactivity of the matrix surface towards any of the catalyst components, as well as clustering and segregation of the latter from the matrix. The challenges were encountered as pore damage during preparation of AuTiO2/MCM-48 catalysts, as deactivating interactions of siliceous walls with zinc ions during deposition of zinc species from aqueous media, and as clustering of the Cu component during calcination and reduction. Among the conclusions drawn from the studies are the irrelevance of order at the AuTiO2 interface (and, hence, of epitaxy and of crystal strain in gold) for high activity of Au/TiO2 catalysts in CO oxidation. In the models for CuZnO methanol synthesis catalysts, two different types of CuZn interaction could be observed: a direct contact between Zn2+ and Cu(0) under strong reducing conditions, and the formation of alloy nanoparticles (nano-brass). A discussion of the relevance of these interactions for the methanol synthesis reaction is given.
physica status solidi (b) 06/2013; 250(6):1081-1093. · 1.49 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Simple test reactions as ethene hydrogenation, 2-butene cis-trans isomeri¬zation and H2/D2 scrambling were shown to be catalyzed by MoS2 and WS2 in surface states which did not chemisorb oxygen and were, according to XPS analysis, saturated by sulfide species. This is a clear experimental disproof of classical concepts that require coordinative unsaturation for catalytic reactions to occur on such surfaces. It supports new concepts developed on model catalysts and by theoretical calculations so far, which have been in need of confirmation from real catalysis.
Journal of the American Chemical Society 11/2012; · 10.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Two versions of Pd-doped perovskite based catalysts (Pd impregnated onto or integrated into perovskite: Pd–LaFe0.65Co0.35O3 and LaFe0.65Co0.3Pd0.05O3) were synthesized by applying a modified citrate route and analyzed by XRD, TEM and XPS. The catalytic properties of the perovskites were measured for NO reduction reactions occurring in three way catalysis, including lean, stoichiometric and rich conditions. The N2O concentrations were measured and compared to the N2O formation of a commercial three way catalyst. The perovskite-based catalysts produced significantly less N2O than the TWC – ca. 75% under lean conditions at the temperatures of maximum N2O-formation and ca. 60% less N2O under stoichiometric conditions. Under rich conditions the Pd–LaFe0.65Co0.35O3 catalyst produced ca. 58% less N2O than the TWC, while no N2O was detected at all on the Pd-integrated perovskite LaFe0.65Co0.3Pd0.05O3 under these conditions.
[Show abstract][Hide abstract] ABSTRACT: Titania supported ceria–lanthana solid solutions (CexLa1−xO2−δ/TiO2; CLT) have been synthesized by a facile and economical route. Existence of synergism between ceria–lanthana (CL) solid solutions and titania-anatase phase, which leads to decrease in the crystallite size, retarded titania phase transformation, and improved redox properties, has been thoroughly investigated by various techniques, namely, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), UV-visible diffuse reflectance spectroscopy (UV–vis DRS), Raman spectroscopy (UV–RS and Vis–RS), BET surface area analysis, and temperature programmed reduction (TPR). Two key observations made from the whole exercise were (i) mutual interaction of Ce and Ti ions could impose typical Ce–O–Ti modes at the interfacial region and (ii) the La3+ ion as a dopant provokes a large number of oxygen vacancies via a charge compensation mechanism. The promising role of these factors in the CO oxidation (one of the most formidable challenges) has been comprehensively described. The observed enhanced activity for the CLT sample is primarily attributed to an apparent specific orientation of the active component over the support, which is endorsed by the interfacial interaction. This specific mode could facilitate the CO adsorption with simultaneous bulk oxygen diffusion for more consumption and in turn better activity.
[Show abstract][Hide abstract] ABSTRACT: A new binuclear cobalt complex with Schiff base ligand and amino acid was synthesized and characterized using XRD, elemental analysis, IR spectroscopy, XPS and electrochemical measurements. The presence of cobalt in the oxidation state 3+ in this complex was proved. The catalytic activity of this complex was investigated in the reaction of partial cyclohexane oxidation with air, and high activity and selectivity for cyclohexanol and cyclohexanone formation were demonstrated. The possibility of complex reuse in cyclohexane oxidation was studied and it was shown that the activity slightly decreases in a second oxidation cycle along with changes in main product distribution due to partial transfer of Co3+ to Co2+.