[Show abstract][Hide abstract] ABSTRACT: Metal vanadates are promising catalysts for the ammoxidation of 2-methylpyrazine to 2-cyanopyrazine, an important intermediate in the production of pyrazinamide as an effective anti-tubercular drug. Low-Energy Ion Scattering (LEIS) investigations supplementing X-ray Photoelectron Spectroscopy (XPS) measurements showed the variation of the composition in the near-surface region. Only with this experimental approach it was possible to elucidate, how important are stable vanadium-enriched or pure vanadia outermost surface layers for a selective catalyst. In contrast, mixed oxide outermost layers are detrimental for the catalytic performance.
[Show abstract][Hide abstract] ABSTRACT: The selective catalytic reduction (SCR) of NO by NH3 was investigated over physical mixtures of an oxidation catalyst with an SCR catalyst ("hybrid catalyst") in which the components were arranged in variable distance to each other: pressed in mixed particles after grinding the mixture, mixing particles of individual components, and in two-layer and four-layer beds. Using Fe-ZSM-5 or V2O5-WO3/TiO2 as SCR components and Mn2O3, Mn-Cu and Ce-Zr mixed oxides as oxidation components, it was observed that the pronounced synergies described in literature are observed only when the components are mixed within the particles. The synergy is strongly attenuated in beds of component particles and fades away completely in the layered structures. This observation is at variance with earlier ideas according to which the oxidation component catalyses the formation of NO2 which opens a fast SCR reaction path over the SCR component. Instead, a more labile critical intermediate is more likely, e.g., HNO2, which may be formed over the oxidation component and proceeds to the SCR sites for further reaction along routes discussed in recent literature. The new understanding is in accordance with earlier observations that correlation between NO oxidation activities of oxidation components and performace of the resulting hybrids was observed to be coarse and to include remarkable outliers.
[Show abstract][Hide abstract] ABSTRACT: Herein an entirely new and simple room temperature synthesis of an amorphous molybdenum sulfide stabilized by complexing ammonia and hydrazine is reported. The resulting material exhibits an outstanding activity for the photocatalytic hydrogen evolution driven by visible light. It is chemically stable during the reaction conditions of the photocatalysis and shows unusual thermal stability up to 350 °C without crystallization. The new material is obtained by a reaction of solid ammonium tetrathiomolybdate and gaseous hydrazine. In the as-prepared state Mo atoms are surrounded by μ2-briding S2−, NH3 and hydrazine, the latter being coordinated to Mo(IV) in a bridging or side-on mode. Heating at 450 °C or irradiation with an electron beam generates nanosized crystalline MoS2 slabs. The two modes for crystallization are characterized by distinct mechanisms for crystal growth. The stacking of the slabs is low and the material exhibits a pronounced turbostratic disorder. Heat treatment at 900 °C yields more ordered MoS2 but structural disorder is still present. The visible-light driven hydrogen evolution experiments evidence an outstanding performance of the as-prepared sample. The materials were thoroughly characterized by optical spectroscopy, chemical analysis, in situ HRTEM, XRD, 1H and 15N solid-state NMR, XPS, and thermal analysis.
[Show abstract][Hide abstract] ABSTRACT: The selective catalytic reduction (SCR) of NO by NH3 was investigated over mechanical mixtures consisting of an oxidation catalyst and Fe-ZSM-5 as a component active for SCR. Oxidation components used included several metal oxides MOx, where M is Mn, Mn-Ce, Mn-Cr, Mn-Cu, Ce-Zr, Mn/Ce-Zr. By comparing SCR rates and selectivities obtained with these mixtures ("hybrid catalysts") with those of their individual components, significant, sometimes drastic synergistic effects between both components could be established. Mn-based oxidation components, which provide high SCR activity on their own, were improved with respect to selectivity towards N-2 by the presence of Fe-ZSM-5. A strong synergy with clearly improved N-2 selectivity remained after the SCR activity of the Mn-containing phases was suppressed by thermal ageing. With the Ce-Zr oxidation component, lower activities were achieved, however at very high selectivity. The measurement of NO oxidation rates could not prove the basic idea of the oxidation catalyst providing NO2 and enabling Fe-ZSM-5 to react the remaining NO via a fast SCR path. While the trends concerning synergy and NO oxidation activity were generally parallel, the NO oxidation activity of Fe-ZSM-5 in the absence of NH3 exceeded or equaled that of most oxidation components which provided significant synergetic effects. For proving the mechanism, data on NO2 formation in the presence of NH3 are needed, which can be made available only by kinetic modeling as the NO/NH3/O-2 system prefers the SCR route over NO oxidation. With Cu- and Cr-containing systems, unexpected changes in selectivity (N2O formation) were noted upon thermal stress, the reasons of which remain unclear.
[Show abstract][Hide abstract] ABSTRACT: Methanol synthesis was studied with catalysts containing Cu and ZnO in the interior space or on the exterior surface of multi-walled carbon nanotubes (CNTs) and were activated by different procedures–reduction in dilute H2 at 513 K with or without subsequent exposure to 10% CO/H2 at 673 K for 30 min. Characterization of the transition-metal species after these treatments by XRD and XAFS revealed striking differences. After mild reduction, XRD reflections related to Cu were missing or were of weak intensity, which could be assigned to very low primary particle sizes as detected by EXAFS. After treatment in CO/H2, reflections arising from alloy phases were obtained for all samples while non-alloyed Cu, although observed by EXAFS, escaped detection by XRD due to small primary particle sizes. Reduction of Zn2+ to Zn(0) was revealed by ZnK XANES only for some samples, which qualifies Zn(0) as a minority oxidation state for the remaining ones. Based on the XANES evidence, a new feature in ZnK EXAFS developing after CO/H2 treatment was interpreted as arising from either alloying or from an SMSI-type interaction of ZnO1-x entities with Cu nanoparticle surfaces. The catalysts exhibited very different productivities and responses to the CO/H2 treatment. In terms of specific activity (related to m2 Cu), the better samples achieved a multiple of the performance shown by a commercial reference, but suffered from insufficient stability. By contrast, such stability was demonstrated for a catalyst containing Cu/ZnO hosted in SBA-15. A catalyst series based on narrow CNTs previously functionalized by thermal shocks in flowing air stood out due to poor performance for unknown reasons. Due to these uncertainties, the data does not permit clear conclusions on the oxidation state of Zn in the promoting interaction with Cu although the general trends favor Zn2+ over Zn(0).
Applied Catalysis A General 02/2015; 504. DOI:10.1016/j.apcata.2015.02.031 · 3.94 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Fe-ZSM-5 catalysts (Si/Al in ZSM-5–14) were prepared by different routes and used for an investigation of relations between normalized reaction rates (rates per Fe atom present) and Fe content in reactions relevant for NOx abatement by selective catalytic reduction (SCR) with NH3: standard SCR, fast SCR, and NO oxidation. Samples were characterized by XRD, ex situ UV–vis and EPR spectroscopy. In standard SCR, normalized reaction rates increased with Fe content to a maximum around 0.5 wt.% (Fe/Al ≈ 0.07) and decreased at further growing Fe loading. This suggests the existence of Fe sites of different activity, which we identify with isolated Fe3+ sites (on α and/or β cation positions, lower activity) and Fe3+ in small oligomeric structures (higher activity) on the basis of our characterization data and recent results from operando-EPR experiments (Pérez Vélez et al., 2014). The decay with further increasing Fe loading is assigned to Fe being included in more aggregated structures. Normalized reaction rates of NO oxidation to NO2 decrease from very small Fe contents on, which indicates that NO oxidation is catalyzed by a small minority of Fe sites, most likely by a variety of isolated sites, in strong contrast to standard SCR. Normalized reaction rates for fast SCR are difficult to establish due to complexity and high rate of the reaction. The results suggest, however, that fast SCR proceeds on a small minority of Fe sites as well, in agreement with our recent operando EPR and Moessbauer studies (Pérez Vélez et al., 2014). Fast SCR proceeds also in absence of Fe, but rates are inferior and easily exceeded even by contributions arising from Fe impurities usually present in technical zeolites.
[Show abstract][Hide abstract] ABSTRACT: Copper and zinc oxide nanoparticles have been reproducibly deposited into carbon nanotubes (CNT) of 6-7 nm internal diameter via simple impregnation techniques with different metal salts followed by thermal decomposition of the precursors and reduction in H-2 in case of Cu. Oxygen functionalization via a gas-phase method involving thermal shocks was a critical step while traditional functionalization with nitric acid resulted in failures. Intra-CNT location of CuO particles could be proven by STEM images, and was examined by TEM for materials prepared by various routes. It was found that Cu and Zn oxide nanoparticles could be deposited throughout the whole interior CNT space. The filling capacity depended on the preparation conditions, on conditions of subsequent precursor decomposition, and on the inner diameter of the CNTs. After the reduction of the CuO nanoparticles, XRD, XAFS, and N2O reactive frontal chromatography indicated a bimodal particle size distribution due to the presence of agglomerates outside the CNTs. To enhance selectivity for endohedral location, a washing step with HNO3 with the inner CNT space blocked by xylene was applied to selectively remove aggregates in the outer space. Based on the best procedures for introduction of CuO and ZnO, a bimetallic CuZnO@CNT sample was prepared via a consecutive preparation route.
[Show abstract][Hide abstract] ABSTRACT: CeO2 nanoparticles were synthesized via a one-step ultrasound synthesis in different kinds of ionic liquids based on bis(trifluoromethanesulfonylamide, [Tf2N](-),in combination with various cations including 1-butyl-3-methylimidazolium ([C(4)mim](+)), 1-ethyl-2,3-dimethylimidazolium ([Edimim](+)), butyl-pyridinium([Py-4](+)), 1-butyl-1-methyl-pyrrolidinium ([Pyrr(14)](+)), and 2-hydroxyethyl-trimethylammonium ([N1112OH](+)). Depending on synthetic parameters, such as ionic liquid, Ce(IV) precursor, heating method, and precipitator, formed ceria exhibits different morphologies, varying from nanospheres, nanorods, nanoribbons, and nanoflowers. The morphology, crystallinity, and chemical composition of the obtained materials were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy XPS), energy dispersive X-ray spectroscopy (EDX), Raman spectroscopy, and N-2 adsorption. The structural and electronic properties of the as-prepared CeO2 samples were probed by CO adsorption using IR spectroscopy under ultrahigh vacuum conditions. The catalytic activities of CeO2 nanoparticles were investigated in the oxidation of CO. CeO2 nanospheres obtained sonochemically in [C(4)mim] [Tf2N] exhibit the best performance for low-temperature CO oxidation. The superior catalytic performance of this material can be related to its mesoporous structure, small particle size, large surface area, and high number of surface oxygen vacancy sites.
[Show abstract][Hide abstract] ABSTRACT: Fe-ZSM-5 was prepared via solid-state exchange method using ferrocene as iron precursor and applied as a model catalyst to investigate the reaction and deactivation mechanisms of the oxidative dehydrogenation of propane (ODHP) with nitrous oxide. Characterization results reveal that after severe calcination highly isolated Fe-O-Al species are the only exposed iron sites detectable, which account for ca. 60% of the total iron species in Fe-ZSM-5. Results from temperature-programmed experiments and in situ DRIFT spectroscopy suggest that the chemisorption of nitrous oxide on Fe-O-Al species leads to the formation of stable mono-oxygen species, which can react with gaseous propane to produce propylene with high selectivity. The accumulation of organic species in the catalyst is observed during the reaction, and the major organic species are determined to be alkylbenzenes. The accumulation rate and the specific constitution of alkylbenzenes are found to depend on the relative partial pressures of propane and nitrous oxide: lower N2O/C3H8 ratios result in formation of aromatics with smaller kinetic diameter, which are accumulated at a lower rate. This leads to lower deactivation rates and longer catalyst lifetimes. Remarkably, a superior stable propane conversion rate of ca. 13 mmol g(cat)(-1) h(-1) and a propylene production rate of ca. 6 mmol g(cat)(-1) h(-1) can be kept for >40 h with a N2O/C3H8 ratio of 1:2.
[Show abstract][Hide abstract] ABSTRACT: TiO2- supported gold species were prepared via the deposition-precipitation route, with conservation of the initial speciation by freeze-drying. The structural and electronic properties of the Au species were investigated by X-ray absorption spectroscopy, electron microscopy, and IR spectroscopy of adsorbed CO in four states. Exclusively Au-III was deposited on the TiO2 surface in patches ranging from isolated Au ions to three-dimensional clusters. This paper illustrates in detail the unique contributions of all characterization techniques to this structural model.
[Show abstract][Hide abstract] ABSTRACT: Three Fe-ZSM-5 catalysts (0.15-0.46 wt.% Fe) prepared via different routes with Fe ions present in extra-framework single sites as well as in oxidic clusters have been catalytically tested and monitored by operando EPR and UV-vis spectroscopy under standard and fast SCR conditions. In both cases, Fe ions in single alpha positions and oxidic clusters remain essentially trivalent while significant differences are evident for the remaining sites. During standard SCR, Fe sites in beta positions are completely, and those in gamma positions are partly reduced to inactive Fe-II, which is not able to catalyze the oxidative activation of NO being essential for its subsequent reduction to N-2. During fast SCR, the same beta and gamma sites are effectively reoxidized by NO2 and are thus kept in a redox-active state. The high reaction rates in fast SCR already at low temperatures are therefore assigned to those sites.
Journal of Catalysis 07/2014; 316:103–111. DOI:10.1016/j.jcat.2014.05.001 · 6.92 Impact Factor
[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: 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.
Journal of Catalysis 03/2014; 311:199–211. DOI:10.1016/j.jcat.2013.11.024 · 6.92 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: 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) .
[Show abstract][Hide abstract] ABSTRACT: The present investigation was undertaken to know the influence of different dopants on the physicochemical properties and catalytic behavior of nano-Au/CeO2 catalyst for CO oxidation. Accordingly, various metal ions namely, Fe3+, La3+ and Zr4+ were incorporated into the ceria lattice by a facile coprecipitation approach using ultra-high dilute aqueous solutions. An anion adsorption method was used to prepare the Au/doped-CeO2 catalysts in the absence of any base, reducing and protective agents. The physicochemical characterization was performed by XRD, BET surface area, ICP-AES, TG-DTA, FT-IR, TEM, UV-vis DRS, Raman, XPS and TPD techniques. Doped CeO2 exhibited smaller crystallite size, higher BET surface area and larger amount of oxygen vacancies than the pure CeO2. These remarkable properties showed a beneficial effect toward gold particle size as confirmed by XRD and TEM studies. XPS results revealed that Au is present in the metallic state and Ce in both +3 and +4 oxidation states. Incorporation of Zr into the Au/CeO2 resulted in high CO oxidation activity attributed to the presence of more Ce3+ ions and oxygen vacancies. In contrast, the La-incorporation caused an opposite effect due to the presence of carbonate species on the surface of Au/CeO2-La2O3 catalyst, which blocked the active sites essential for CO oxidation. It was shown that accumulation of carbonate species strongly depends on the acid-base properties of the supports. The catalytic performance of Au catalysts is highly dependent on the nature of the support.
[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. DOI:10.1016/j.apcata.2013.10.044 · 3.94 Impact Factor
[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.