The degree of substitution of Si(4+) by Al(3+) in the oxygen-terminated tetrahedra (Al T-sites) of zeolites determines the concentration of ion-exchange and Brønsted acid sites. Because the location of the tetrahedra and the associated subtle variations in bond angles influence the acid strength, quantitative information about Al T-sites in the framework is critical to rationalize catalytic properties and to design new catalysts. A quantitative analysis is reported that uses a combination of extended X-ray absorption fine structure (EXAFS) analysis and (27)Al MAS NMR spectroscopy supported by DFT-based molecular dynamics simulations. To discriminate individual Al atoms, sets of ab initio EXAFS spectra for various T-sites are generated from DFT-based molecular dynamics simulations, allowing quantitative treatment of the EXAFS single- and multiple-photoelectron scattering processes out to 3-4 atom shells surrounding the Al absorption center. It is observed that identical zeolite types show dramatically different Al distributions. A preference of Al for T-sites that are part of one or more 4-member rings in the framework over those T-sites that are part of only 5- and 6-member rings in an HBEA150 zeolite has been determined using this analysis.

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We report on the successful installation and operation of a scanning transmission x-ray microspectroscope (STXM) at the PolLux facility at the Swiss Light Source. This integration of an advanced STXM with improved sample handling capabilities and a novel beamline provides unique capabilities. PolLux uses linearly or circularly polarized x-rays from a bending magnet with an extended photon energy range (200-1400 eV). It is therefore well suited to determine a sample's quantitative chemical composition, molecular orientation, or thickness of organic as well as condensed matter materials. The local magnetic state of magnetic thin films is accessible through fast helicity switching by steering the electron beam off axis through the bending magnet. Ex vacuo girder movers allow fast and highly reproducible (<1 microm) alignment of the instrument with respect to the photon beam. The present spatial resolution is approximately 20 nm, limited by the zone plates utilized. The instrument has the stability and positional resolution to operate with much higher resolution optics as it becomes available. In addition to characterization experiments, we present several typical examples from materials research and environmental science to exemplify the capabilities.

Rapid temperature fluctuations are shown to cause liquid H2SO4/HNO3/H2O stratospheric aerosols to depart considerably from thermodynamic equilibrium. While HNO3 uptake by larger droplets is diffusively hindered, small droplets can approach the composition of a pure binary HNO3/H2O solution with up to 52 wt% HNO3, 48 wt% H2O and very small amounts of H2SO4. The stoichiometry of these droplets is close to that of nitric acid trihydrate (NAT) and freezing experiments suggest that this could be a suitable pathway for the formation of frozen polar stratospheric clouds (PSCs) of type‐Ia.