3.2.1. Characterization

samples.

with diameters ranging in 350–80 nm span (Table 2).

SEM micrographs, recorded for all the obtained nanofibrous samples (Figure 3), show thin fibers with diameters ranging in 350–80 nm span (Table 2). *Processes* **2020**, *8*, 45 8 of 15 SEM micrographs, recorded for all the obtained nanofibrous samples (Figure 3), show thin fibers

**Figure 3.** SEM images acquired at different magnifications of: (**A**) NYL; (**B**) NYL+TiO2; (**C**) NYL + AuPd/TiO2; (**D**) NYL + AuPd and (**E**) NYL + AuPd + TiO2. Scale bars: 10 µm (first column); 2 µm (second column) and 1 µm (third column). **Figure 3.** SEM images acquired at different magnifications of: (**A**) NYL; (**B**) NYL+TiO<sup>2</sup> ; (**C**) NYL + AuPd/TiO<sup>2</sup> ; (**D**) NYL + AuPd and (**E**) NYL + AuPd + TiO<sup>2</sup> . Scale bars: 10 µm (first column); 2 µm (second column) and 1 µm (third column).

**Table 2.** NYL-based membranes: fiber average diameter and specific surface area of the electrospun

**Sample Average Diameter (nm) Specific Surface Area (m2 g−1)** 

NYL + Au/PdTiO2 300 ± 80 32 NYL + AuPd 100 ± 20 <10 NYL + AuPd + TiO2 80 ± 10 36


**Table 2.** NYL-based membranes: fiber average diameter and specific surface area of the electrospun samples.

While the fibers are all thin and smooth, it was observed that inorganic component promoted fiber diameter thinning, and in particular, the presence of free unsupported AuPd NPs, decreased the average diameter around, or even below, 100 nm. Moreover, the addition of TiO<sup>2</sup> (both as plain titania or when NPs are supported on it) led to aggregates formation whose morphology well compared with the previously analyzed PAN based fibers. The dimension of such aggregates exceeded the fibers diameters, which by the way were way thinner than the pristine NYL counterpart, with some extroversion outside the smooth profile of the single filament. However, aggregates belonged to the fibers bulk, and were not simply leaning on the surface. The lower concentration of particles, due to the previously highlighted high-concentration suspension stability issues with the solvent system to be used for NYL fibers, provoked, in turn, minor aggregation phenomena with respect to the highly loaded PAN fibers, with no excess particle entrapment within the membrane pores. On the other side no aggregate was detected with just unsupported NPs, whose diameters were well below the fibers average size.

TGA measurements (Table S5) confirmed the trend observations with PAN fibers and the composition of the starting solution, meaning that no significant inorganic phase separation occurred during electrospinning, which might result in fibers with significant depletion of inorganic content in the fibers.

An additional important feature was also highlighted for nanofibrous membranes inorganic fillers such as titania, or NPs (either supported on titania or unsupported). Indeed, all of the "loaded fibers" are characterized by a higher T<sup>g</sup> (about <sup>+</sup><sup>10</sup> ◦C) and a higher degree of crystallinity with respect to plain nylon nanofibers. The first observation accounts for a good dispersion of NP and TiO<sup>2</sup> supported NP within the polymer, so much that the mobility of the macromolecules was hindered by the interaction with the inorganic components, with a significant increase in glass transition. As far as the crystal phase is concerned, electrospinning is well renown to discourage crystal formation, while the presence of such fillers seems to help promoting crystallization during spinning, acting as nucleants [51]. This observation, while possibly not relevant for the mere catalytic activity, can be of paramount importance when considering the fibers mechanical properties, which are strongly influenced by their crystallinity, and is an important factor when dealing with membranes that should be able to withstand continuous flow conditions.
