4.2.9. NH3-TPD Temperature-Programmed Desorption

Quantity and distribution of acid sites were performed by temperature-programmed desorption of ammonia (NH3-TPD) technique with the same apparatus mentioned above. Before the measurement, the catalyst was saturated using 200 µL injected pulse NH<sup>3</sup> gas at 50 ◦C. The NH3-TPD experiment was carried out in a flow system with thermal conductivity detectors (TCD) by heating with 10 K/min (from room temperature up to 1073 K). This was used as a carrier gas (70 mL·min−<sup>1</sup> ). The NH<sup>3</sup> desorption was estimated from the area of the recorded peaks. The calibration of the TCD signal was performed by injecting a known quantity of NH3. The surface of the as-obtained peak (mV·s) was converted into micromoles of ammonia.

#### *4.3. Catalytic Ozonation and Photocatalytic Tests*

Ozonation of ammonia in water was performed by using a semi-batch reactor connected to a gas flow line. Aside from the reactor, the catalytic setup contained a pH meter, stirring system, and a recirculator to maintain a constant temperature (25 ◦C) and an ozonegenerating source. Typically, 0.015 g of catalyst was added into 100 mL of the ammonia

solution containing 20 ppm NH<sup>4</sup> + . The suspension was vigorously stirred in a stream of O3/O<sup>2</sup> (10 cm<sup>3</sup> ·min−<sup>1</sup> ). O<sup>3</sup> was generated from O<sup>2</sup> using an ozone generator (OzoneFIX, Mures, , Romania).

Photocatalytic tests of aqueous ammonia oxidation were carried out in a photoreactor (120 mL) provided with a quartz window and thermostated at 18 ◦C. Catalyst (0.015 g) was suspended in ammonia solution containing 20 ppm NH<sup>4</sup> <sup>+</sup> obtained by adding of ammonia solution (NH4OH 25%) in ultrapure water. Light irradiation of the reaction medium (AM 1.5) was realised by exposing it to AM 1.5 light provided by a solar simulator (Peccell-L01) with a xenon short-arc lamp (150 W). A gaseous mixture of argon (10 cm<sup>3</sup> ·min−<sup>1</sup> ) and oxygen (1 cm<sup>3</sup> ·min−<sup>1</sup> ) was bubbled into the aqueous suspension. The liquid aliquots were collected every 30 min and analysed with ion chromatographs (Dionex ICS 900, Sunnyvale, CA, USA) for the monitoring of NH<sup>4</sup> <sup>+</sup> and the resulting anions (NO<sup>3</sup> −).

Ammonia oxidation with ozone, assisted by solar light irradiation, was performed using the above-mentioned set-up, also equipped with an ozone generator:

$$\chi\_{\text{NH}\_4^+}\%=\frac{\left[\text{NH}\_4^+\right]\_\text{t}}{\left[\text{NH}\_4^+\right]\_\text{i}}\ast 100\tag{13}$$

$$\mathbf{S}\_{\rm N\_2} - \mathbf{g} \mathbf{as} \,\%= 100 - \mathbf{S}\_{\rm NO\_3^-} \,\% \tag{14}$$

$$\text{S}\_{\text{NO}\_3^-}\% = \frac{\left[\text{NO}\_3^-\right]\_\text{t}}{\left[\text{NH}\_4^+\right]\_\text{c}} \ast 100\tag{15}$$

$$\text{Y}\_{\text{NO}\_3^-}\% = \frac{\left[\text{NO}\_3^-\right]\_\text{t}}{\left[\text{NH}\_4^+\right]\_\text{i}} \ast 100\tag{16}$$

where: XNH<sup>+</sup> 4 is the NH<sup>4</sup> + conversion, YNO<sup>−</sup> 3 Yis the yield of NO<sup>3</sup> <sup>−</sup>, [NH<sup>4</sup> + ]t is the ammonium ion concentration consumed at time t, [NH<sup>4</sup> + ]i is initial ammonium ion concentration, and [NO<sup>3</sup> −]<sup>t</sup> is the nitrate concentration formed at time t.

#### **5. Conclusions**

Hydrothermally synthesized titanate nanorods were successfully modified by the addition of Fe precursor.

α-Fe2O<sup>3</sup> with a well-defined morphology (nanocubes) was obtained by similar hydrothermal treatment and tested for comparative evaluation.

The comparison of the structural and functional characterization data of both titanatebased samples showed that the modification of titanate nanorods with iron did not affect the nanorod morphology and significantly improved optical and photocatalytical properties.

Catalytic investigations for aqueous ammonia oxidation showed the best reactivity for the Fe-modified sample (FeTiR) under solar light irradiation and ozonation. The NH<sup>4</sup> + conversion and selectivity to gaseous end products was further improved by combining ozonation with the photo-assisted catalytic oxidation of aqueous ammonia.

An environmentally friendly, non-expensive and innovative depollution technology can be developed based on these materials.

**Author Contributions:** Conceptualization, S.P., M.Z., C.A. and F.P.; methodology, C.A., I.B. and F.P.; formal analysis, investigation, S.P., P.U., C.A., F.P., S.V.P., C.G. and R.S.; data curation, D.-I.E. I.B. and C.A.; writing—original draft preparation, C.A., F.P., S.V.P. and R.S.; writing—review and editing, C.A., R.S. and F.P.; supervision, I.B.; funding acquisition, C.A. and F.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Unitatea Executiva pentru Finantarea Invatamantului Superior, a Cercetarii, Dezvoltarii si Inovarii (UEFISCDI), grant number: PN-III-P2-2.1-PTE-2019-0222, 26PTE/2020 DENOX.

**Conflicts of Interest:** The authors declare no conflict of interest.
