Electrospun fibers as catalytic converters

Small diameter of electrospun fiber gives it a very high surface area for catalytic reaction. In fibrous form, the material is self-supporting and does not need an additional carrier or supporting substrate. Electrospinning may be used to produce fibers from polymers and inorganic precursors and this allows it to be used either as a standalone catalytic material or as carrier for catalyst.

Schematic illustration of catalytic conversion of CO to CO2.

Platinum and palladium are commonly used in catalytic converters and there are several studies on the fabrication of electrospun fibers containing Pt or Pd nanoparticles. Shahreen et al (2013, 2015) fabricated titania nanofibers loaded with palladium nanoparticles by electrospinning a solution of titanium isopropoxide and PVP loaded with PdCl₂ salt. As a salt, PdCl₂ aids in the formation of beads-free nanofibers by increasing the conductivity of the solution [Shahreen et al 2015]. Titania fiber containing PdO was formed by sintering and the PdO was reduced to Pd nanoparticles using hydrazine reaction. The resultant composite fibers showed significant conversion of NO and CO gases to NO₂ and CO₂ respectively. Ebert et al (2008) constructed a catalytically active poly(amideimide) nanofibre mat containing palladium nanoparticles on the surface of the nanofibers. Poly(amideimide) nanofibre mat doped with citric acid was first constructed by electrospinning. The resultant mat was dipped in an organic solution containing PdAc₂ and high concentration of citric acid. Heat treatment was carried out to convert the PdAc₂ into Pd nanoparticles. Performance of the catalytically active poly(amideimide) nanofibre mat was tested using methyl oleate as the model reaction and compared with commercial palladium catalyst supported on alumina. Their tests showed that the nanofibers yielded seven times higher hydrogenation rate than the commercial counterpart.

Between palladium and platinum, the latter is more commonly known to be a catalytic material. Dai et al (2010) fabricated a layered composite inorganic fibrous membrane containing Pt nanoparticles. First, TiO₂ nanofibers were formed by sintering its electrospun precursor fibers. This is then coated with polyvinyl pyrrolidone (PVP) stabilised Pt nanoparticles followed by another layer of SiO₂ nanoparticles. The presence of SiO₂ nanoparticles was found to stabilise the Pt nanoparticles and prevent their aggregation during sintering at temperatures above 350 °C. Catalytic activity of the composite was determined using methyl red for the model reaction. It was shown that even with the SiO₂ sheath over the Pt nanoparticles layer, methyl red molecules were able to penetrate through the porous sheath layer for catalytic reaction.

For many catalytic applications, research are being carried out to replace platinum with other less expensive materials. Yan et al (2018) developed Fe-N-C mesoporous nanofibers with low-cost urea and FeCl₃ as the nitride and iron source respectively for use as electrocatalysis in oxygen reduction reaction (ORR). The electrospinning solution was prepared with polyacrylonitrile (PAN), urea and FeCl₃ in N,N-dimethyl formamide (DMF). The resultant electrospun fibers were carbonized and treated with acid to remove excess Fe. Examination of the carbonised fibers showed presence of more ordered graphitic carbon, which is helpful for stability and charge transfer, residual Fe and N. Optimised Fe-N-C mesoporous nanofibers exhibited polarization curve at an onset potential of 0.93 V and a half-wave potential of 0.82 which was close to Pt/C (onset potential of 0.96 V and half-wave potential 0.8 V). Without N or Fe, ORR ability was markedly reduced. Durability of the Fe-N-C mesoporous nanofibers was demonstrated by half-wave potential decreases of only ??18 mV after 5000 cycles, with no appreciable variation in the onset potential.