Identification of Intermediates in the Reaction Pathway of SO2 on the CaO Surface: From Physisorption to Sulfite to Sulfate.

The interaction of CaO and Ca(OH)2 with solvated or gaseous SO2 plays a crucial role in the corrosion of urban infrastructure by acid rain or in the removal of SO2 from flue gas. We carried out a combined spectroscopic and theoretical investigation on the interaction of SO2 with a CaO(001) single crystal. First, the surface chemistry of SO2 was investigated at different temperatures using polarization-resolved IR reflection absorption spectroscopy. Three species were identified, and an in-depth density functional theory study was carried out, which allowed deriving a consistent picture. Unexpectedly, low temperature exposure to SO2 solely yields a physisorbed species. Only above 100 K, the transformation of this weakly bound adsorbate first to a chemisorbed sulfite and then to a sulfate occurs, effectively passivatating the surface. Our results provide the basis for more efficient strategies in corrosion protection of urban infrastructure and in lime-based desulfurization of flue gas.

Antibacterial Inorganic Coating of Calcium Silicate Hydrate Substrates by Copper Incorporation.

Under environmental conditions, biofilms can oftentimes be found on different surfaces, accompanied by the structural degradation of the substrate. Since high-copper-content paints were banned in the EU, a solution for the protection of these surfaces has to be found. In addition to hydrophobation, making the surfaces inherently biofilm-repellent is a valid strategy. We want to accomplish this via the metal exchange in calcium silicate hydrate (CSH) substrates with transition metals. As has been shown with Europium, even small amounts of metal can have a great influence on the material properties. To effectively model CSH surfaces, ultrathin CSH films were grown on silicon wafers using Ca(OH)2 solutions. Subsequently, copper was incorporated as an active component via ion exchange. Biofilm development is quantified using a multiple-resistant Pseudomonas aeruginosa strain described as a strong biofilm former cultivated in the culture medium for 24 h. Comprehensive structural and chemical analyses of the substrates are done by environmental scanning electron microscopy (ESEM), transmission Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Results do not show any structural deformation of the substrates by the incorporation of the Cu combined with three-dimensional (3D) homogeneous distribution. While the copper-free CSH phase shows a completely random distribution of the bacteria in biofilms, the samples with copper incorporation reveal lower bacterial colonization of the modified surfaces with an enhanced cluster formation.