Low-temperature SCR of NO with NH 3 over noble metal promoted Fe-ZSM-5 catalysts

We have reported previously the excellent performance of Fe-exchanged ZSM-5 for selective catalytic reduction (SCR) of NO with ammonia at high temperatures (300–400 °C). In this work, we found that the reaction temperature could be decreased to 200–300 °C when a small amount of noble metal (Pt, Rh, or Pd) was added to the Fe-ZSM-5. The SCR activity follows the order Pt/Fe-ZSM-5 > Rh/Fe-ZSM-5 > Pd/Fe-ZSM-5 at 250 °C. On the Pt promoted Fe-ZSM-5, 90% NO conversion was obtained at 250 °C at GHSV  = 1.1 ×  10 5  h −1 . Moreover, the noble metal improved the resistance to H 2 O and SO 2 . The presence of H 2 O and SO 2 decreased the SCR performance only very slightly.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44250/1/10562_2004_Article_3462.pd

Environmental catalysis on iron oxide-silica aerogels: Selective oxidation of NH3 and reduction of NO by NH3

The catalytic properties of mesoporous iron oxide–silica aerogels prepared by a sol–gel process combined with ensuing supercritical extraction with CO2 was investigated in the selective oxidation (SCO) of ammonia and the selective reduction (SCR) of NO by ammonia. The main parameters changed in the aerogel preparation were the type of base used as gelation agent, the iron content, and the calcination temperature. The aerogels differed significantly in acidity and iron dispersion. Diffuse reflectance infrared Fourier transform spectroscopy studies of ammonia adsorption at different temperatures revealed that ammonia was bound to Brønsted- and Lewis-type sites, the latter being dominant at 300°C. A fraction of low coordinated Fe2+ sites were probed by NO adsorption measurements. Lewis-type sites were found to be associated with low-coordinated iron sites. Catalytic tests were performed in a continuous fixed-bed reactor in the temperature 210–550°C range and at ambient pressure. The catalytic activity of the aerogels in SCO correlated with the abundance of more strongly bound ammonia adsorbed on Lewis sites (low coordinated iron). High selectivity to nitrogen (97%) could be reached up to 500°C, whereas at higher temperature the formation of N2O and NO became significant. The apparent activation energy of N2 formation ranged from 69 to 94 kJ/mol, whereby catalysts with higher selectivity and activity showed lower activation energy. In SCR, selectivity to nitrogen was for all aerogels >98% at

Manganese oxide-silica aerogels: Synthesis and structural and catalytic properties in the selective oxidation of NH3

Manganese oxide–silica mixed oxide aerogels with different morphological and chemical properties were prepared using the sol–gel method and ensuing extraction of the solvent with supercritical CO2. Two types of manganese precursor, varying hydrolysis conditions of the silica and manganese precursors, influence of base addition for gelation, and calcination temperatures were investigated. Base addition had a strong effect on textural properties, producing high-surface-area, mesoporous aerogels, whereas these properties were only marginally affected by kind of manganese precursor used. The presence of different manganese oxide species was evidenced by X-ray diffraction, Raman and diffuse reflectance infrared Fourier transform spectroscopy, and temperature-programmed reduction. Mn4+, Mn3+, and Mn2+ oxide species were found after calcination at 600°C in air. Sol–gel processing with manganese(II) nitrate resulted in highly dispersed mixed oxides. Basic gelation of these sols strongly influenced the state of the manganese, leading to crystallites of hausmannite and to amorphous Mn5O8 in the calcined samples. Aerogels derived from the less reactive Mn(III) (acac)3 did not contain any manganese oxide crystallites when prepared under the same basic conditions. The catalytic performance of the aerogels in the selective oxidation of ammonia strongly depended on the state of the manganese. Samples containing crystalline Mn3O4 were more active than amorphous aerogels with dispersed manganese oxide species and afforded high selectivity to N2O. The presence of amorphous Mn5O8 further increased the activity and the selectivity to nitrous oxide, reaching 74% at 360°C. Nitrogen formation was found to be related to the amount of strongly Lewis-bound ammonia. The amorphous aerogels showing more Lewis-bound ammonia produced mainly nitrogen below 480°C, affording a selectivity of 78% at 360°C

Detection of magnetic circular dichroism using a transmission electron microscope

A material is said to exhibit dichroism if its photon absorption spectrum depends on the polarization of the incident radiation. In the case of X-ray magnetic circular dichroism (XMCD), the absorption cross-section of a ferromagnet or a paramagnet in a magnetic field changes when the helicity of a circularly polarized photon is reversed relative to the magnetization direction. Although similarities between X-ray absorption and electron energy-loss spectroscopy in a transmission electron microscope (TEM) have long been recognized, it has been assumed that extending such equivalence to circular dichroism would require the electron beam in the TEM to be spin-polarized. Recently, it was argued on theoretical grounds that this assumption is probably wrong(1). Here we report the direct experimental detection of magnetic circular dichroism in a TEM. We compare our measurements of electron energy-loss magnetic chiral dichroism (EMCD) with XMCD spectra obtained from the same specimen that, together with theoretical calculations, show that chiral atomic transitions in a specimen are accessible with inelastic electron scattering under particular scattering conditions. This finding could have important consequences for the study of magnetism on the nanometre and subnanometre scales, as EMCD offers the potential for such spatial resolution down to the nanometre scale while providing depth information-in contrast to X-ray methods, which are mainly surface-sensitive

Magnetization profile at the Fe/GaAs(001)-4x6 interface

The magnetization of a thin Fe film epitaxially grown on GaAs(0 0 1)-4×6 was studied at different depths from the metal/semiconductor interface using a single layer of Fe0.5Co0.5 as a marker layer through a double-wedge Fe film. By measuring the X-ray magnetic circular dichroism spectroscopy at the L2,3 of Co, the magnetic response of the film could be sensed at different distances from the interface. Data show a reduction of the magnetization at the interface though the existence of a magnetically “dead” layer is completely ruled out. Moreover, the magnetization was found to be reduced at the Fe film surface