*3.1. Physicochemical Characterization*

The route selected in this paper, i.e., in situ reduction and subsequent fusion, is one of the two common strategies found in the literature for the preparation of Pd aerogels [28–31]. This route is essentially a one-pot synthesis that significantly shortens and simplifies the metal nanoparticles aerogels. The aerogels are thus synthetized by mixing noble metal salts (PdCl2 in our case) with a strong reducing agent, typically NaBH4, LiAlH4, hydrazine, sodium citrate, tannic acid or, in our case, glyoxylic acid monohydrate (combined with a base). The two reactants lead to the Pd2+ to Pd<sup>0</sup> reduction at moderate temperatures, typically around 60 ◦C. Under these conditions, the metal nanoparticles in the sol state tend to aggregate until the stability of the solution is compromised and turned into a gel state (Figure 1). The hydrogel obtained after the Pd aggregation is finally washed several times using deionized water by carefully exchanging the supernatant to ensure the integrity of the formed hydrogel but minimizing the presence of impurities (remnant salts and organic matter) and dried either using supercritical CO2 extraction or, in our case, lyophilization.

Glyoxylic acid monohydrate (combined with a base) is a popular reducing agent in the electroless copper plating [32], and it has been used before in the synthesis of metal aerogels [31]. Although the mechanism of the Pd reduction is not fully understood, it is plausible that the contribution of the glyoxylic acid is twofold according to the following equations. First, the disproportionation of the glyoxylic acid in a basic medium occurs:

$$2\text{OCHCOOH} + \text{H}\_2\text{O} \rightarrow \text{HOCH}\_2\text{COOH} + \text{HOOC-COOH} \tag{3}$$

The oxalic acid would then react with the PdCl2:

$$\text{HONOC-COOH} + \text{PdCl}\_2 + \text{Na}\_2\text{CO}\_3 \rightarrow \text{Pd(COO)}\_2 + 2\text{NaCl} + \text{H}\_2\text{CO}\_3 \tag{4}$$

Finally, the palladium oxalate would be reduced by the glyoxylic acid [32]:

$$\text{Pd(COO)}\_{2} + \text{OCHCOOH} + 2\text{OH}^{-} \rightarrow \text{Pd} + 2\text{C}\_{2}\text{O}\_{4}^{2-} + \text{H}\_{2}\text{O} + 2\text{H}^{+} \tag{5}$$

In this work, the yield of the Pd aerogel synthesis was 67%.

The XPS analysis was performed to evaluate the composition and electronic structure of the four Pd aerogels obtained. Figure 3 displays their core-level binding energy Pd 3d5/2 (336.8–337.0 eV) and Pd 3d3/2 (340.0–341.0 eV) XP spectra [21,33]. For all samples, each peak can be deconvoluted into two contributions: 335.0 and 340.2 eV for metallic Pd; and 337.0 and 342.4 eV for Pd2+ [34]. XPS data revealed that Pd0 is the main species on the Pd aerogels surface for PdA-MMT. The high-resolution Pd XPS profiles of samples PdA-CC and PdA-MM are very similar pointing out that the microwave heating produces an analogous reduction process but with a remarkable saving of time (i.e., 7 h vs. 24 h, see Table 1). In addition, it can be also observed that the Pd metallic phase on PdA-MMT aerogel is higher than that on the other samples. This analysis shows that the ratio between Pd0 and Pd2+ favors Pd0 when the aerogels were prepared by microwave heating, and this would probably be one of the main reasons for high stability and performance of that catalyst, as it will be shown below. Therefore, increasing the temperature from 45 ◦C to 67 ◦C during the reduction stage would be preferred because it brings about a greater quantity of metallic Pd0.

**Figure 3.** XPS of the Pd aerogels obtained.

Powder X-ray diffraction (XRD) measurements were performed to evaluate the crystallinity of Pd aerogels samples. Figure 4 illustrates the XRD patterns of the synthesized catalysts. In all XRD patterns, two major diffraction peaks appear at about 40.1◦ 2θ and 46.6◦ 2θ, which are ascribed to the (111) and (200) reflection planes of metallic Pd, respectively [35]. These peaks agree with a face-centered cubic crystal structure of Pd (JCPDS# 46-1043) [36]. In addition, the crystallite size was calculated using the Scherrer equation:

$$\mathbf{d}\_{111} = \frac{\mathbf{K}\lambda}{\mathbb{B}\_{111}\cos\theta}$$

**Figure 4.** XRD of Pd aerogels samples.

The crystallite sizes for the PdA-CC, PdA-MC, PdA-MM, and PdA-MMT samples were 10.3, 7.2, 6.9, and 7.6 nm, respectively.

The use of microwaves reduces the size of the Pd crystallites, the minimum size being obtained when microwave was used as the heating method in the two stages of the synthesis. The size of the crystal is significantly smaller when using microwaves as a heating method because this heating process is volumetric and heat gradients are minimized. Thus, the reaction occurs uniformly in the precursor mixture. This means that under microwave heating, there are multiple crystallization spots in the precursor solution, whereas in conventional heating, the temperature gradient produces less crystallization spots that grow to form larger particles.

The morphologies of the Pd aerogels were characterized by SEM and the images are presented in Figure 5c–f.

**Figure 5.** Physicochemical analysis. BET analysis of Pd aerogels: (**a**) N2 adsorption isotherms; and (**b**) pore size distribution. SEM of Pd aerogels: (**c**) PdA-CC; (**d**) PdA-MC; (**e**) PdA-MM; and (**f**) PdA-MMT.

The aerogels obtained present a three-dimensional porous network anchored with nanochains which are extremely thin and make the material look like a sponge with a wide pore size distribution in the range of mesopores and macropores, i.e., Figure 5a,b.

All samples present a similar morphology, although sample PdA-MM (i.e., Figure 5c) seems to present the most open 3D structure. Slight differences in morphology between the different metallic Pd aerogels are attributed, again, to the heating technology applied in their synthesis. Thus, a finer distribution of particles appears in the material synthesized and reduced by microwave heating. This can be explained in Figure 6f where it is observed that the distribution of heat within the solution, by the radiation of microwaves, allows the reaction to occur homogeneously. Instead, larger particles are formed by conventional heating due to the fact that the heat distribution begins at the edges and causes the reaction to occur in an inhomogeneous way, resulting in larger particles that decrease the BET surface area in the aerogels.

TEM images at different magnifications show chains of particles surrounding pores of different sizes. The spherical nanoparticles in those chain structures present different lengths depending on the treatment performed during the synthesis of the aerogels. Particle diameters of up to 30 nm for the aerogels were synthesized in the conventional oven, i.e., Figure 6c, whereas particles less than 9 nm in diameter are characteristic of samples synthesized in MW (Figure 6d–f). To understand the effect of heating on the particle size, the normal distribution of the widths was analyzed, the trend of the means for each sample analyzed is observed in the histograms, being 15.3 nm for PdA-CC, 6.2 nm for PdA-MC, and 5 nm for PdA-MM and Pd-MMT. Therefore, a smaller particle size is attributed to the effect of microwave heating.

**Figure 6.** TEM micrographs of Pd aerogels. (**a**) PdA-CC, (**b**) PdA-MC, (**c**) PdA-MM, (**d**) PdA-MMT, (**e**,**f**) crystallographic patterns observed in PdA-MMT aerogel, and (**g**) heating effect on the particle size of Pd-CC and Pd-MMT aerogels.

According to the different structures observed by SEM and TEM, different porosity and therefore availability of reactive surface area of the aerogels studied was expected, which could be relevant for their further use in electrochemistry. Porous properties of samples were investigated by nitrogen adsorption–desorption isotherm at −196 ◦C (see Figure 5a). The isotherms of the aerogels are of type II according to the IUPAC classification, which are characteristic of meso–macroporous materials [36] according to the low volume of adsorption at low relative pressures, the sharp increase in the adsorption at high relative pressures, and the absence of a hysteresis loop. Furthermore, the pore size distribution reveals a high volume of mesopores (i.e., 2–50 nm) and macropores (i.e., >50 nm) as can be seen in Figure 5b. The surface area of the samples was determined using the BET equation, giving relatively low values due to the lack of microporosity (i.e., pores < 2 nm) in these samples: 45, 65, 75, and 77 m2g−<sup>1</sup> for PdA-CC, PdA-MC, PdA-MM, and PdA-MMT, respectively. Nevertheless, a trend to increase the BET surface area is observed if microwave heating is used in the different steps during the synthesis.
