Synthesis of Catalytic Nanoporous Metallic Thin Films on Polymer Membranes

Abstract

Composite membranes were produced with a metallic thin film forming the upper layer of the composite on a porous polymer support. Commercially available membranes were used as supports with both micron and nanometer scale pores. Alloy films of ~110 nm thickness were deposited via magnetron sputtering to produce the top layer of the composite. Dealloying the film with sulfuric acid allowed the creation of a nanoporous film structure with a ligament size of 7.7 ± 2.5 nm. Resulting composite membranes were permeable to water at all stages of production, and a UF PSf membrane with 90 nm of nanoporous Fe/Pd on top showed a flux of 183 LHM/bar. The films were evaluated for dechlorination of toxic polychlorinated biphenyls from water. At a loading of 6.6 mg/L of Pd attached to 13.2 cm² support in a 2.5 ppm PCB-1 solution with 1.5 ppm dissolved H₂, over 90% of PCB-1 was removed from solution in 30 minutes, which produced the expected product biphenyl from the dechlorination reaction.

Keywords: composite membranes; dealloying; dechlorination; nanoporous metals.

Figure 1.

Schematic of the three stages of fabrication. A.) is the first stage, a bare membrane substrate. B.) shows the MTFC stage, when a metallic film has been deposited onto the membrane substrate by sputtering. C.) is the npMTFC stage. Here, after dealloying, the film is nanoporous and anchored to the membrane.

Figure 2.

Schematic of dead-end permeation cell used for flux measurements in determination of permeability of membranes.

Figure 3.

SEM images of Fe/Pd films sputtered on various substrates: (A) PVDF MF membrane (B) Polysulfone UF membrane.

Figure 4.

Cross-sectional SEM micrographs of as-deposited Fe/Pd film on membrane substrate. (A) Low magnification view of the MF PVDF composite structure, with (B) showing a higher magnification image of the red outlined section of (A). (C) Is a micrograph showing a cross-section of the polysulfone UF based composite after FIB milling. The darker regions correspond to the polymer membrane; the membrane is coated with a Fe/Pd layer approximately 110 nm thick; the top coating of protective platinum (added for FIB work) is visible near the top of the image.

Figure 5.

Cross-sectional images of composite membrane consisting of the nanoporous Fe/Pd layer on UF polysulfone substrate. (A) HAADF-STEM image of a FIB milled lamella of npMTFC sample shows a strong Z contrast. This region is a magnified image of the area noted in red box of (B). Red arrows point out a single ligament of the nanoporous structure. (B) A low magnification STEM image of the lamella produced via FIB milling. The brighter regions of (B) and (C) correspond to the high atomic number Fe/Pd alloy and the Pt layer, and the darker region corresponds to polymer membrane region. C) SEM of a sample cross-sectioned using FIB. Designations: Above the blue line is a protective Pt layer deposited using the FIB; the nanoporous Fe/Pd layer is between the blue and green lines; polymer membrane is below the green line. Image taken at an angle of 54º for FIB milling. This tilt results in a shortening effect of the film thickness in images that does not correspond to true film thickness. This effect explains the seeming different thickness of the film when viewed in the SEM mode in the Helios FIB (at an angle of 54˚) and the HAADF-STEM images where the sample is not tilted.

Figure 6.

SEM images of npMTFC microfiltration membrane (PVDF substrate). (A) Shows a large view cross-section of the npMTFC membrane after FIB milling. (B) Depicts a surface view of the membrane at an angle of 60°, showing the roughness of the membrane and the porosity of the metallic layer. (C) Shows a magnified view of the cross-section as in (A). The film at this depth is thinner because of shadowing effects as discussed in the text. As a result, only a single layer of ligaments is formed on the polymer here. The darker regions correspond to the polymer substrate (PVDF), whereas the brighter, porous layer is the nanoporous Fe/Pd layer; this is covered with a thick protective platinum layer for FIB milling.

Figure 7.

Variation of pure water flux with applied pressure for base polymer membrane, sulfuric acid treated PSf membrane, and unleached and leached metal-polymer membranes. Linear fit corresponds to Eq 1 and the slope provides water permeability for the first set of membrane replicates. Inset shows magnified plot of MTFC flux.

Figure 8.

Dechlorination of PCB-1 by commercial Pd alumina particles. 6.6 mg/L Pd loading in solution. 1 bar H₂ pressure was used. The solid points show the result of a control experiment without catalyst of a solution containing initial amounts of PCB-1 and biphenyl.

Figure 9.

Dechlorination of PCB-1 by nanoporous Fe/Pd in the presence of hydrogen gas of 1 bar for three different membrane batches. Pd loading is determined to be 6.6 mg/L.