Johannes Schwank

Improvement of Activity and SO2 Tolerance of Sn-Modified MnOx–CeO2 Catalysts for NH3-SCR at Low Temperatures

The performances of fresh and sulfated MnOx-CeO₂ catalysts for selective catalytic reduction of NOx by NH₃ (NH₃-SCR) in a low-temperature range (T < 300 °C) were investigated. Characterization of these catalysts aimed at elucidating the role of additive and the effect of sulfation. The catalyst having a Sn:Mn:Ce = 1:4:5 molar ratio showed the widest SCR activity improvement with near 100% NOx conversion at 110-230 °C. Raman and X-ray photoelectron spectroscopy (XPS) indicated that Sn modification significantly increases the concentration of oxygen vacancies that may facilitate NO oxidation to NO₂. NH₃-TPD characterization showed that the low-temperature NH₃-SCR activity is well correlated with surface acidity for NH3 adsorption, which is also enhanced by Sn modification. Furthermore, as compared to MnOx-CeO₂, Sn-modified MnOx-CeO₂ showed remarkably improved tolerance to SO₂ sulfation and to the combined effect of SO₂ and H₂O. In the presence of SO₂ and H₂O, the Sn-modified MnOx-CeO₂ catalyst gave 62% and 94% NOx conversions as compared to 18% and 56% over MnOx-CeO₂ at temperatures of 110 and 220 °C, respectively. Sulfation of SnO₂-modified MnOx-CeO₂ may form Ce(III) sulfate that could enhance the Lewis acidity and improve NO oxidation to NO₂ during NH₃-SCR at T > 200 °C.

Preparation, structure, properties and thermal behavior of rigid-rod polyimide/montmorillonite nanocomposites

Polyimide/montmorillonite nanocomposites were prepared from solution of poly(amic acid) precursors and the dodecyl-montmorillonite (DMONT) using N-methyl-2-pyrrolidone as a solvent. The cured films of the rigid-rod polyimide/DMONT hybrids as characterized by FTIR, TEM and WAXD were exfoliated nanocomposites at low DMONT content (<1 wt.%) and partially exfoliated nanocomposites at high DMONT content (containing aggregates of DMONT). The nanocomposite showed optimum properties at 3 wt.% DMONT with low water absorption, improved mechanical strength and modulus, and high dielectric strength. The clay content significantly influences thermal behavior of the nanocomposite films, such as thermal expansion coefficient (CTE), glass transition and yielding temperatures of the rigid-rod and flexible polyimide nanocomposites. It was found that the rigid polyimide matrix gave a superior thermal behavior; i.e. lower CTE, higher glass transition and yielding temperatures than the flexible ones. Effect of DMONT content was significant at high temperature operation where a side reaction of dodecylamine and a poly(amic acid) should be aware to cause a reduction in degree of imidisation and glass transition temperature and the formation of silicate layer aggregates. High DMONT content nanocomposites (aggregates were formed) showed high yielding temperatures indicating the suppression of polymer flow by the rigidity of the clay. The flexible polyimide nanocomposites showed higher thermal deformation than the rigid ones. Moreover, the rigid polyimide nanocomposite with 9 wt.% DMONT showed a thermally reversible behavior indicating that the silicate layers efficiently suppressed the deformation of the rigid polyimide.

A chemisorption and XPS study of bimetallic Pt-Sn/Al2O3 catalysts

Bimetallic Pt-Sn/AlzO3 catalysts with nominally 1 wt% Pt and varying tin contents (0-3.25 wt%) prepared by coimpregnation of nonporous Degussa alumina were characterized by chemisorption of H2, 02, and CO at room temperature. The surface compositions and oxidation states of the reduced catalysts were tracked by XPS. Addition of tin to Pt resulted in significant differences in the gas uptake characteristics of the three adsorbates. Both H2 and CO showed an initial increase in gas uptake with addition of small amounts of tin, and then the chemisorbed amount tended to drop off with further addition of tin. In the case of 02 adsorption, there was a steady increase in gas uptake with increasing tin content. XPS of the reduced catalysts showed that in aluminasupported samples most of the tin was in a valence state of either Sn(II) or Sn(IV). On the other hand, large amounts of zero-valent tin were found in a SiO2-supported Pt-Sn catalyst which had been prepared from the same precursors and reduced under identical conditions. This supports the notion that interactions between the alumina support and tin prevent the complete reduction of tin.

Influence of surface Pd doping on gas sensing characteristics of SnO2 thin films deposited by spray pirolysis

Abstract In this article the analysis of steady state and transient gas sensing characteristics of undoped and Pd surface doped SnO2 films, deposited by spray pyrolysis, is described. The influence of parameters such as air humidity (2–50% RH), operation temperature (25–500 °C) and Pd surface concentration (0–1% ML Pd) on gas response to CO and H2 (0.1–0.5%), response time, shape of sensitivity S(T) curves and activation energy of τ(1/kT) dependencies are discussed. A mechanism based on a chemisorption model is proposed to explain how Pd influences the gas sensing characteristics of SnO2 films.

Catalytic gold - Applications of elemental gold in heterogeneous catalysis

Although gold as been little used in heterogeneous catalysis hitherto, the metal shows some very interesting properties which ensure continued study of it for this field. In particular its remarkably high selectivity in some reactions and striking activity in oxygen transfer catalysis have received significant attention.

Surface degradation of α-naphthalene sulfonate-doped polypyrrole during XPS characterization

The surface compositions of non-aging chemically synthesized polypyrrole samples doped with a-naphthalene sulfonate (PPy/ a-NS � ) were investigated by X-ray photoelectron spectroscopy (XPS). The power of the X-ray source used was 300 W. Employing liquid nitrogen cooling of the sample holder, the spectra in the regions of C 1s, O 1s, N 1s and S 2p were found to be reproducible with a gradual change in S 2p spectrum when the accumulated X-ray exposure time was less than 1050 min. Beyond this accumulated X-ray exposure time, dramatic changes were observed in all spectral regions. The decreases in the amount of dopant and HSO4 � or SO4 2� co-dopant species correspond to the decrease in the amount of polaron charge carrier species of PPy (–NH � þ –). We found increases in the amount of the imine-like nitrogen (=N–) and hydrocarbon, and the presence of new sulfurcontaining species. These results suggested the deprotonation of the polaron by the dopant and the desulfonation of the protonated dopant. After desulfonation, these new sulfur-containing species were removed whereas the carbons of the naphthalene rings were still adhering to the surface of the PPy pellet. This degradation became more severe when the liquid nitrogen cooling system of the spectrometer was not used. The results indicate that the degradation was mainly induced by heat from the X-ray beam. # 2002 Elsevier Science B.V. All rights reserved.

Measuring and relating the electronic structures of nonmodel supported catalytic materials to their performance.

Identifying structure-performance relationships is critical for the discovery and optimization of heterogeneous catalysts. Recent theoretical contributions have led to the development of d-band theory, which relates the calculated electronic structure of a metal to its chemical and catalytic activity. While there are many contributions where quantum-chemical calculations have been utilized to validate the d-band theory, experimental examples relating the electronic structures of commercially relevant nonmodel catalysts to their performance are lacking. We show that even small changes in the near-Fermi-level electronic structures of nonmodel supported catalysts, induced by the formation of surface alloys, can be measured and related to the chemical and catalytic performance of these materials. We demonstrate that critical shifts in the d-band center in alloys are related to the formation of new electronic states in response to alloying rather than to charge redistribution among constitutive alloy elements, i.e., the number of d holes and d electrons localized on the constitutive alloy elements is constant. On the basis of the presented results, we provide a simple, physically transparent framework for predicting shifts in the d-band center in response to alloying and relating these shifts to the chemical characteristics of the alloys.

A micromachined ultra-thin-film gas detector

This paper reports a second-generation gas detector developed for eventual use in a multi-element gas analyzer. The detector utilizes an ultra-thin metal sensing film supported on a selectively micromachined dielectric window, although the basic structure is also suitable for use with more conventional sensing films. A 5 /spl mu/m-thick boron-diffused silicon heater under the window permits the window temperature to be varied between ambient and over 1200/spl deg/C with heating efficiencies in air and in vacuum of 6/spl deg/C/mW and 20/spl deg/C/mW, respectively. The total window area is 1 mm/sup 2/, with an active sensing area of 0.12 mm/sup 2/. The circuit simulator SPICE is used to optimize the coupled thermal and electrical characteristics of the window simultaneously, resulting in a simulated temperature uniformity over the sensing area of better than /spl plusmn/0.5%. Two boron-diffused silicon resistors having TCRs of 1800 ppm//spl deg/C are interleaved with the heater to allow the average temperature over the active area to be determined to within about /spl plusmn/0.1/spl deg/C. The detectors are realized using a six-mask process in a die size of 2.8 mm/spl times/2.8 mm. A subset of the same process is also used to produce wafer-level shadow masks to permit the detectors to be used with any sensing films capable of being vacuum deposited. >

Possibilities of aerosol technology for deposition of SnO2-based films with improved gas sensing characteristics

Abstract Advantages of aerosol technology for deposition of nano-scaled (5–50 nm) undoped and Pd-doped metal oxide films for gas sensor applications are discussed in this report. Using SnO2-based film deposition as a case study, it has been shown that this technology has great potential for controlling the structure, the electrophysical, and gas sensing properties of metal oxides.

Bimetallic Ru-Au catalysts: Effect of the support

Abstract Ru-Au catalysts supported on SiO 2 were characterized by using H 2 and O 2 chemisorption, wide-angle X-ray scattering, diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy. Catalytic activity was measured for the hydrogenolysis of propane and ethane. The hydrogenolysis activity of ruthenium decreased by two orders of magnitude with addition of gold. This suggested that Ru and Au did not exist as separate particles but formed bimetallic aggregates. Chemisorption and XPS experiments showed a surface composition similar to the bulk. A comparison was made with a previously studied Ru-Au-on-MgO system, on which an enrichment of Ru on the surface of bimetallic Ru-Au clusters was discovered. It is suggested that the strength of the metal-support interaction can affect the surface composition of multimetallic supported systems.