Sodium Titanate Synthesis For The Photocatalytic Degradation of NO

The synthesis and characterization of sodium titanates (ST) and its evaluation in the photocatalytic reduction of nitric oxide (NO) is described in this contribution. The materials were synthesized by an hydrothermal route using the following parameters; 5 M NaOH concentration used as TiO 2 mineralizer agent, under 170 °C for 48 hours, and a dose of TiO 2 of 0.06 mg/mL (expressed as the mass ratio of TiO 2 /mL with respect to NaOH); resulting in tri- and hexa- ST. A nanotubular morphology was observed for the ST as proved by scanning electron microscopy (SEM) and a subsequent heat-treatment at 400 °C allowed a complete transformation of tri- to hexa- sodium titanates to modify the bandgap. The obtained ST were impregnated with Ag + and Zn + cations, respectively (ST-Ag, ST-Zn), to tune the bandgap of the materials. XPS analysis of the ST-Ag materials showed evidence of metallic Ag pointing to the formation of silver nanoparticles, whereas for ST-Zn oxide phases were mainly spotted. The materials were evaluated for the photocatalytic reduction of NO using a reactor fed with a continuous ow rate of NO, generated in situ, at a ow rate of 280 ml/min using nitrogen and a 253 nm wavelength UV irradiation source. The photocatalytic tests showed that pristine ST (tri and hexa-titanate) was the photocatalyst that displayed the best performance in the reduction of NO with respect to the impregnated samples (ST-Ag, ST-Zn). Maximum eciencies of 80% degradation were reached when using 1 g of photocatalyst with a ow of 280 ml/min and a lamp of 253 nm.


Introduction
Photocatalysis is a process with important applications in environmental remediation and energy production. Nowadays, TiO 2 is the photocatalyst more applied in several process for treating wastewater, generation of hydrogen, degradation of pollutants in the atmosphere and in the generation of electricity by photoelectron-chemical conversion of solar energy (Carbajo et al., 2017;Chiarello et al., 2017;Yan et al., 2013;). However, the re-combination of (e − -h + ) pair in the surface of TiO 2 when the photocatalytic process occurs and the bang gap of 3.2 eV, limits the use of a small fraction of solar irradiation (approximately 4%) in form of UV light, this has motivated the quest of novel photocatalysts or strategies to modify TiO 2 to overcome these limitations (Carbajo et al., 2017). For instance, Fe-doped TiO 2 material has been prepared avoiding charge recombination (Wu and van de Krol, 2012). The Fe-doped TiO 2 photocatalyst was evaluated for photo-oxidation and photo-reduction of NO by the presence of Fe and oxygen vacancies that suppresses the formation of NO 2 . In the mentioned work, three issues were concluded: NO 3 − generation via photo-oxidation, suppression of NO 2 and the NO photo-reduction to obtain N 2 and O 2 . However, the poisoning of the catalysts was observed as ascribed to the formation of NO 3 − in the surface of the material blocking the oxygen vacancies. Other attempts include the use of titanium-based inorganic perovskites, such as SrTiO 3 decorated with SrCO 3, which was tested to degrade NO by photocatalysis (Jin et al. 2018). Charge recombination was avoided in these materials by using a co-photocatalyst strategy (SrTiO 3 -SrCO 3 ). In the same line, Parayil et al., (2015) employed nitrogen-doped sodium titanate nanotubes (STT) in the photocatalytic conversion of CO 2 . Doped and undoped STT prepared by hydrothermal treatment showed a promising performance to reduce CO 2 to CH 4 . ST can be modi ed by acid washing and thermal treatments to enhance their photocatalytic activity (Nguyen and Bai, 2015). Other modi cations involve doping with noble metals to degrade organic matter (El Rouby et al., 2017). Their morphology is appropriate to increase the catalytic and photocatalytic activity due to tubular structure and surface areas > 200 m 2 /g (Machorro López et al., 2021). Then, the use of ST in the degradation of NOx is a potential option for NOx photo-reduction with the subsequent generation of N 2 and O 2 in comparison with the photo-oxidation obtained in other studies (Nguyen and Bai, 2015).
Alternative strategies have been implemented to modify the band gap and avoid the recombination of (e −h + ) pair. Ibukun and Jeong (2020) achieved the TiO 2 modi cation with silver to avoid the abovementioned issues. These authors reported that the modi ed TiO 2 was active in the visible region; moreover, degradation kinetic of methyl blue was increased. Turkten

Characterization
The crystalline structure of the samples was analyzed by a Bruker D8 Advance X-ray Diffractometer (XRD) with a Cu-λ radiation (λ = 0.15406 Ǻ) with a Nickel lter operating at 40 kV and 40 mA. The data were collected in the 2θ range from 5° to 50° with a scan rate of 0.02˚. The morphology of the samples was observed by SEM (Jeol, JSM-6510 LV), while elemental identi cation was carried out by energydispersive X-ray analysis (EDX). Transmission Electron Microscopy (Jeol, JEM-1400Plus) was also used for the morphological studies. The surface chemical bonding was analyzed by X-ray photoelectron spectroscopy (XPS) (Thermo Scienti c, K-Alpha) equipped with Al Kα X-ray source at energy of 1486.6 eV measuring points of 200 x 200 mm. Calibration was con rmed by adventitious carbon C1s at 285 eV. Spectrum deconvolution was carried out using a Gaussian model. The thermal stability of the samples was investigated by thermogravimetric analysis (TGA) in a Netzsch STA 449 F5 Jupiter from room temperature to 900°C. The samples were heated with 10°C/min from room temperature to 900°C, under a nitrogen ow of 50 mL/min. The optical characterization of the samples was carried out by UV-Vis diffuse re ectance with a Shimadzu UV-2600 spectrophotometer. The diffuse re ectance spectra (DRS) were recorded in the wavelength range of from 200 to 800 nm.

Photocatalysts evaluation
NOx generation was carried out using HNO 3 and metallic copper according to reaction 1 and 2: The NO generation was carried in the absence of oxygen in the reaction 2. Cu and HNO 3 were mixed in a Buchner ask to generate NO. N 2 was used as carrier gas using a ow of 280 mL/min. Gaseous ow passed through a photocatalysis reactor that consists of a ask with a monochromatic pencil lamp of 253 nm, where 1000 milligrams of the photocatalysis in powder form was loaded. The photocatalytic activity of the samples was evaluated in an experimental arrangement shown in the Fig. 1, where a continuous ow of N 2 (used as carrier of in-situ produced NO) was fed in the photocatalytic reactor.
Mixture of gases were transported to the abatement chamber passing through the surface of the photocatalyst and irradiated at 253 nm (UV) from monochromatic source inside the chamber for 10 minutes. An AMPRO analyzer measured the concentration of the gas products.
Previous to photocatalytic evaluation, a statistical analysis of variance (ANOVA) of initial NO concentration was carried out (  (Fig. 1(d)). There is a modi cation in the low angle, 10° 2θ corresponding to a partial collapse in the tubular structure from thermal treatment (Lee et al., 2007). The deterioration of the tubular structure may be attributed to the dehydration of inter-layered OH radical which led to contraction and breaking of tubes structures. During the annealing treatment, the chemical bonds such as H EDX analysis is showed in the  (Nguyen and Bai, 2015), that could be related to the loss of Na atoms from the ionic substitution of Na + by H + when the washing is carried out to reach a pH of 7; although, the washing was conducted only with distilled water. When Ag is impregnated in the ST material, Ti/Na increased while O/Ti decreased pointing to an ionic exchange of Na from Ag. When ST is modi ed with Zn, Ti/Na increased possibly from the ionic exchange of Na by Zn; however, O/Ti kept indicates the formation of ZnO in the ST. XPS analysis was conducted to verify this hypothesis (Fig. 4).  Table 3.  Table 3: ionic exchange of Na + by Ag + and ZnO formation. As expected, the values of elemental composition of Table 3 are different with respect to Table 2 due to the difference in the volume of analysis in each technique. Analysis from XPS guarantees the surface inspection of the samples. Surface composition change in the calcined and modi ed ST was expected; however, in CST composition practically this change did not occur. In the modi ed ST a slight decrease in the Ti/Na ratio was observed due to migration of Na atoms from the inner part of the sample to the surface originating an increase of Na composition in the surface and a change in the surrounding chemical (Fig. 5).
Characteristic signals of Ti at 459 and 465 eV that corresponds to 2p 3/2 and 2p 1/2 are showed in Fig. 5. Additionally, the ionic exchange of Na by Zn and Ag occurs. In TEM images are appreciated the morphology of the samples prepared herein (Fig. 6).
TEM images shows the tubular structure of ST (Fig. 5a) although this morphology collapses when is treated at 400°C (Fig. 6b). In the case of modi ed ST, small particles are appreciated in each case demonstrating the formation of nano particles from the ionic exchange. In order to explain the collapse of morphology thermogravimetric (TGA) analysis was employed. In Fig. 7 the TGA/DSC analysis of the samples is showed.
Thermogravimetric curves of the obtained ST materials are displayed in Fig. 7. The greater part of the weight loss occurs at lower temperatures, where the dehydration of physisorbed water takes place. The single stage weight loss of roughly 10% occurs up to 200°C. This process is re ected as an endothermic peak on a DSC curve. After this temperature the weight slowly and continuously decreases nearly up to 700°C. At T ≤ 300°C the dehydration of interlayered OH groups could reduce the interlayer distance but does not destroy the tubular shape. When temperature > 300°C, the dehydration of interlayered OH groups induced the change of crystalline form and, at the same time, the nanotube morphology is destroyed. A broad exothermic peak in a temperature range from 300 to 800 C on the DSC curve could indicate that the synthesized ST, loose interlayered OH groups in a broader range while interlayered OH groups remain in the structure up to 600°C or the cleavage of both type of OH groups occurs simultaneously. Between 200-300°C an exothermic reaction occurs attributing this process to hexa-tri-titanate transformation (Lee et al. 2007). At 400-600°C an endothermic process is observed that means the collapse of tubular structure forming spherical particles.
A comparison of prepared ST using UV-vis spectroscopy demonstrates the absorbance of ST with and without modi cations in a range of 600 − 200 nm (Figure not showed). Figure 8 shows  Table 3.   The most e cient synthesized photocatalyst was ST using 1.0 g in the experiments. NO degradation occurred in stages and is relatively similar to other photocatalyst used (65% percentage of degradation). Compared with the same material load, CST degraded 62% of NO, while TS + Ag5 and TS + Zn5 showed a degradation percentage of 45% and 40%, respectively. Major percentage of NO degradation is attributable at higher surface area in ST than other photocatalysts. Possibly, the thermal treatment in the modi ed ST affected the performance reducing the surface area and the active sites to achieve the photocatalytic process. This is better related to the band gap of the ST; it is consistent to say that the band gap, together with the surface area of the sample, have a greater in uence on the photocatalytic effect of the synthesized materials. Then, the thermal treatment in the ST was not suitable to degrade NO.
One of the possible routes for NO photoreduction is described as catalytic sites which tetrahedrally coordinated Ti is found on the surface. This species has been reported by Anpo

Conclusions
This work demonstrated the preparation of sodium titanate from hydrothermal treatment as well as the modi cation of this photocatalysts by thermal treatment and impregnation of zinc and silver to modify their photocatalytic properties. Mixture of tri and hexa titanates were obtained with low concentration of alkali being important to generate an environmental friendly process. Moreover, photocatalytic activity of sodium titanates synthesized from hydrothermal treatment was tested to degrade NO by reduction. Sodium titanate was modi ed with Ag and Zn to increase the photocatalytic activity; however, the major activity to degrade NO was reported with the sodium titanate without modi cation. Likely, this high activity to degrade NO occurred from the number of active sites in the photocatalyst in comparison with the modi ed sodium titanates. Photo reduction of NOx occurred in comparison with other works to degrade this pollutant. This means a advantage due to not-generation of subproducts in the reaction being the nitrogen and oxygen only generated.

Declarations
Ethic approval and consenting to participate: not applicable.
Consent to publication: not applicable.
Availability of data and materials: no applicable.
Competing interest: there are not competing interests.      Normalized UV-vis re ectance diffuse spectra (DRS) of ST prepared and TiO2.