Nanostructured Metal Thin Films as Components of Composite Membranes for Separations and Catalysis

Novel metallic thin film composite membranes are synthesized and evaluated in this work for improved separations and catalysis capabilities. Advances in technology that allow for improved membrane performance in solvent separations are desirable for low molecular weight organic separation applications such as those in pharmaceutical industries. Additionally, the introduction of catalytic materials into membrane systems allow for optimization of complex processes in a single step. By adding a nanostructured metallic thin film to its surface, a polymer membrane may be modified to exhibit these improved properties. Using magnetron sputtering, thin metal films may be deposited on commercially available membranes to modify separations properties. Alloy films may then be deposited onto the membrane surface and dealloyed to produce a porous structure with a small feature size for catalysis. Multiple composites were studied in this research. Metallic thin film composites (MTFCs) of 10 nm Ta films deposited on top of commercially available ultrafiltration polysulfone (UF PSf) membranes were fabricated and characterized to study the film’s effect on effective pore size of the membrane. A significant water flux drop from the UF PSf (168 LMH/bar) to the resulting MTFC (8.8 LMH/bar) was found. Effective pore size was studied using rejection experiments with molecules of known sizes as markers. The UF PSf rejected about 90% of the 70 kDa while all smaller molecules were rejected minimally. The MTFC, however, rejected down to 5 kDa dextran indicating a reduction in effective pore size through the addition of 10 nm of Ta. Further experiments with IPA and water indicated that the structure was stable in this solvent. Two different alloy systems were studied as precursors to nanoporous films for further catalysis. Both were Fe/Pd (80/20 at. %) and Mg/Pd (75/25 at.%) precursors were used to produce nanoporous Pd. In all cases the alloy films were anchored to the membrane substrate with a thin Ta film, then dealloyed to produce a nanoporous metal thin film composite (npMTFC). In both cases the npMTFC was produced to catalyze a dechlorination reaction using hydrogen gas. Chlorinated organic compounds were the target compound for this system, as they are a persistent pollutant. Fe/Pd alloy films were dealloyed using a solution of 25% sulfuric acid to etch away the iron and generate porosity. The Fe/Pd npMTFCs were then tested for both batch mode dechlorination and permeation testing with a model COC, chlorobiphenyl (PCB-1). Permeation of a 5 ppm solution of PCB-1 through a similar membrane degraded 28% of PCB-1 from solution with a single pass under H2 pressurization. For the Mg/Pd precursor system only water was needed in order to etch magnesium, preserving the pore structure of the underlying UF PSf substrate with little deformation from dealloying. Under convective flow, the membrane removed over 70% of PCB-1 from solution with a single pass at 4 bar H2 pressurization. Successful fabrication of a novel composite membrane type has been demonstrated with applications for improved separations in solvents and in catalysis. The catalytic applications here may be easily modified for a variety of reactions

Composite membrane fabrication with nanoporous metallic films

Magnetron sputtering is a physical vapor deposition method widely used for deposition of thin films of different materials on a variety of substrate materials. Sputtering allows fine control of the film thickness and composition through co-sputtering from multiple target materials. As part of this study thin films have been sputtered on top of membrane substrates. Microfiltration, ultrafiltration, and nanofiltration membranes have been investigated as substrates for thin film deposition. The resulting composite membranes have remained permeable under testing with deionized water. The base nanofiltration membrane showed permeability of 9.75 LMH/bar, while the membrane-film composite had a permeability of 2.76 LMH/bar. Thin films of metallic alloys deposited in this way can be made nanoporous through a process called dealloying. The process involves the removal of the less noble component of an alloy by an etchant creating an open nanoporous structure. The pores created by this method commonly vary from a few nanometers to a few hundred nanometers. This research focuses on using magnetron sputtering to deposit precursor metallic alloy films from 100 to 250nm thick on top of porous membrane substrates. These dense precursor films are then dealloyed to produce pore/ligament structures of approximately 10nm characteristic size. In these studies iron and palladium were chosen as a precursor alloy. A portion of the iron is etched away with sulfuric acid to generate an open nanoporous structure. Fe/Pd nanoparticles have been used with success to dechlorinate various chlorinated organic compounds (COCs) for wastewater treatment purposes. Nanoporous Fe/Pd films have shown similar activity in batch testing towards PCB degradation as nanoparticles. Taken together this means the composite membrane produced by fabricating a high surface area, porous Fe/Pd film on top of a membrane substrate shows promise both as a catalyst and as a platform for separations. This project is funded by NIH-NIEHS-SRC and by NSF KY EPSCOR at the University of Kentucky

Composition and Work Function Relationship in Os–Ru–W Ternary Alloys

Os–Ru thin films with varying concentrations of W were sputter deposited in order to investigate their structure–property relationships. The films were analyzed with x-ray diffraction to investigate their crystal structures, and a Kelvin probe to investigate their work functions. An Os–Ru–W film with ∼30 at. % W yielded a work function maximum of approximately 5.38 eV. These results align well with other studies that found work function minima from thermionic emission data on M-type cathodes with varying amounts of W in the coatings. Furthermore, the results are consistent with other work explaining energy-level alignment and charge transfer of molecules on metal oxides. This may shed light on the mechanism behind the “anomalous effect” first reported by Zalm et al., whereby a high work function coating results in a low work function for emitting cathode surfaces. An important implication of this work is the potential for the Kelvin probe to evaluate the effectiveness of dispenser cathode coatings