Search

Layer-by-layer(LbL) deposition using primarily inorganic silicananoparticles is employed for surface modification of polymeric micro-and ultrafiltration (MF/UF) membranes to produce novel thin film composite(TFC) membranes intended for nanofiltration (NF) and reverse osmosis(RO) applications. A wide variety of porous substrate membranes withdifferent surface characteristics are successfully employed. Thisreport gives detailed results for polycarbonate track etched (PCTE),polyethersulfone (PES), and sulfonated PES (SPEES) MF/UF substrates.Both spherical (cationic/anionic) and eccentric elongated (anionic)silica nanoparticles are deposited using conditions similar to thosein prior works for solid substrates (e.g., Lee et al.1). Appropriate selection of the pH for anionic and cationicparticle deposition enables construction of nanoparticle-only layers100–1200 nm in thickness atop the original porous membranesubstrates. The surface layer thickness appears to vary linearly withthe number of bilayers deposited, i.e., with the number of anionic/cationicdeposition cycles. The deposition process is optimized to eliminatedrying-induced cracking and improve mechanical durability via thicknesscontrol and postdeposition hydrothermal treatment. “Dead-end”permeation tests using dextran standards reveal the hydraulic characteristicsand separations capability for the PCTE-based TFC membranes. The resultsshow that nanoparticle-based LbL surface modification of MF and UFrated media can produce TFC membranes with NF capabilities. [ABSTRACT FROM AUTHOR]

Microcantilever stress measurements are examined to contrast and compare their attributes with those from in situ X-ray absorption spectroscopy to elucidate bonding dynamics during the oxygen reduction reaction (ORR) on a Pt catalyst. The present work explores multiple atomistic catalyst properties that notably include features of the Pt-Pt bonding and changes in bond strains that occur upon exposure to O2 in the electrochemical environment. The alteration of the Pt electronic and physical structures due to O2 exposure occurs over a wide potential range (1.2 to 0.4 V vs normal hydrogen electrode), a range spanning potentials where Pt catalyzes the ORR to those where Pt-oxide forms and all ORR activity ceases. We show that Pt-Pt surface bond strains due to oxygen interactions with Pt-Pt bonds are discernible at macroscopic scales in cantilever-based bending measurements of Pt thin films under O2 and Ar. Complementary extended X-ray absorption fine structure (EXAFS) measurements of nanoscale Pt clusters supported on carbon provide an estimate of the magnitude and direction of the in-operando bond strains. The data show that under O2 the M-M bonds elongate as compared to an N2 atmosphere across a broad range of potentials and ORR rates, an interfacial bond expansion that falls within a range of 0.23 (±0.15)% to 0.40 (±0.20)%. The EXAFS-measured Pt-Pt bond strains correspond to a stress thickness and magnitude that is well matched to the predictions of a mechanics mode applied to experimentally determined data obtained via the cantilever bending method. The data provide new quantitative understandings of bonding dynamics that will need to be considered in theoretical treatments of ORR catalysis and substantiate the subpicometer resolution of electrochemically mediated bond strains detected on the macroscale.