Evaluation of Nb 2 O 5 ceramic nanoﬁbers efﬁcacy to promote CO 2 photoconversion.

: The increase in global warming due to NO x , CO 2 , and CH 4 harmfully different ecosys- 1 tems and signiﬁcantly prejudice world life. A promising methodology in this sense is the pollutant 2 conversion into valuable chemicals from photocatalytic processes by reusable photocatalyst. In 3 this way, the present work aimed to produce a Nb 2 O 5 photocatalyst nanoﬁbers system to convert 4 CO 2 by the electrospinning method. Based on the collected data, the nanoﬁbers calcination at 5 600°C for 2 h resulted in the best condition to obtain a homogeneous surface with an average 6 diameter of 84 nm. As a result, the Nb 2 O 5 nanoﬁbers converted CO 2 mostly into CO and CH 4 , 7 reaching values around 8.5 mol g − 1 and 0.55 mol g − 1 , respectively. 8


Introduction
Diverse methodologies have been developed to remove pollutants from different 11 media to address the growing concern with environmental quality. In this vein, hetero-12 geneous photocatalysis has gained attention due to its capacity to remove and degrade 13 contaminants, photocatalytic recovery, and reuse in new cycles [1,2] CO 2 photoreduction 14 is especially emerging because of the need to decrease greenhouse gases and diminish 15 their consequent harmful environmental impacts [3].Photocatalysis is based on com- 16 plex catalytic reactions that enable the formation of specific products such as methanol, 17 methane, carbon monoxide, formic acid, etc [4].In order to improve the photocatalytic 18 stability, production rate, and selectivity, different catalysts systems have been studied, 19 such as TiO 2 [5],ZnO [6], and WO 3 [7]. 20 21 Zeng et al. [8] investigated the incorporation of Cu 2 O nanowires in titanium car-22 bide (Ti 3 C 2 ). These authors confirmed the enhancement of the material efficiency to 23 convert Cu 2 O into methanol, inducing an increase in the production around 8.25 times 24 compared to the isolated Cu 2 O nanowires. This performance improved the charge 25 carrier's transport and reduced the bandgap (from 2.2 to 2.02 eV) that optimizes the 26 light absorption capacity and modifies the charge recombination processes. Ye et al. [9] 27 proposed the design of nano-tubes bismuth-based heterostructure photocatalysts for 28 CO 2 photoconversion. Among the tested BiOX type heterostructures (with X equal to Cl, 29 Br, or I), BiOI showed the best performance under visible irradiation, as its bandgap was 30 the smallest one (1.7 -1.8 eV), favoring the CO 2 photoreduction in CO and CH 4 (19.82 31 mol g −1 h −1 and 0.22 mol g −1 h −1 , respectively). However, even though this narrow 32 bandgap range facilitates the recombination of the photo-generated pairs, such a feature 33 may also lead to photocatalytic activity loss.

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Niobium pentoxide (Nb 2 O 5 ) is a semiconductor that has received attention due to 36 its similar features to TiO 2 and promising performance when applied in CO 2 photocon-37 version [10][11][12]. Silva et al. [12] studied the photocatalytic activity of Nb 2 O 5 particles 38 after modifying their surface with peroxo groups. The authors observed that selectivity 39 and photocatalytic activity were related to the surface acidity of the nanoparticles. Thus, 40 high surface acidity led to CO 2 conversion into CO, HCOOH, and CH 3 COOH, whereas 41 low acidity induced mainly CH 4 formation. Although niobium-based materials are 42 promising alternatives to be applied in CO 2 photoconversion, it was not found in the 43 literature the use of Nb 2 O 5 ceramic nanofibers for this application. However, photo-44 catalysis using nanofibers has been employed to degrade pollutants [13,14] due to their 45 porous control and homogeneous diameter distribution that enhance the availability 46 of catalytic sites aside from the improvement in the photocatalyst activity due to its 47 preferentially directed form.

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To the best of our knowledge, no investigations have been carried out considering 50 the use of polyvinyl alcohol (PVA, which is highly soluble in water) as a polymeric 51 precursor for the Nb 2 O 5 fibers synthesis. Based on these aspects, the present work    The immobilized photocatalysts were inserted into the cavity of a stainless steel 84 reactor. The system was purged with ultra-pure gas containing CO 2 and water vapor 85 for 20 min before the experiment. Afterward, the reactor was sealed and exposed to 86 UV-C irradiation (TUV Philips 18 W mercury lamp, 254 nm) for 6 h at room temperature.  phology is observed at temperatures from 400ºC to 700ºC, and agglomerates at higher 111 temperatures. Additionally, the fibers average diameter obtained from 400°C to 700°C was 170 nm, 140 nm, 150 nm, and 120 nm, respectively. However, when the temperature 113 was raised to 800ºC, the formation of a more significant number of shapeless particles 114 with an average diameter of 135 nm was verified. Furthermore, the fiber loss format was 115 observed after 900ºC that led to the particulate material formation. The mentioned morphological characteristics are due to the increasing temperature.

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Consequently, the energy supplied favors the Nb 2 O 5 fibers coalescence effect, resulting 118 in a fiber losing initial shape and introducing new particles, inducing growth until ther-119 modynamic equilibrium [18,19]. Furthermore, according to Figure 2, samples treated at 120 400°C, 500°C, and 600°C using a heating rate of 1°C min −1 showed fiber-like morphology 121 with mean diameters of 280 nm, 130 nm, and 84 nm, respectively. Thus, fibers obtained 122 at 600°C showed greater homogeneity and smaller diameter when compared to those 123 obtained at 10°C min −1 . The obtained results are similar to the literature [20,21], which 124 also verified morphological differences related to the heating rate during the annealing 125 process; this step is essential for morphology formulation. Besides that, to the lowest 126 heating rate, a more regular aspect was defined for fibers without pores, attributed to 127 greater control of polymeric matrix degradation.

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In order to verify the heating rate effects, the range of 400 -600°C was chosen to 129 observe the structural changes due to PVA matrix degradation and Nb 2 O 5 calcination.   was observed that the different materials prepared based on niobium tend to produce 216 CO preferentially. This behavior is justified because the CO 2 reduction mechanism 217 requires two electrons to produce CO molecules and eight electrons for CH 4 formation.

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For this reason, the generation of CH 4 is much more complex [29]]. Furthermore, CO 219 can strongly adsorb on the photocatalyst surface, and desorption of the process can 220 significantly affect the yield of the CH 4 formation [30].

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Studies reveal that the surface acidity of the photocatalyst can influence the selectiv-222 ity of the products generated from CO 2 photoreduction [11,12]. The size of nanoparticles, 223 composition of the photocatalysts can also influence the selectivity during the photore-224 duction reaction [31]. Nogueira et al. [4] proposed an interaction mechanism between  Figure 7a. Thus, although the secondary NbO 2 phase in the Nb 2 O 5 system decreases 253 its overall photocatalytic performance, the presence of NbO 2 resulted in an enhanced 254 capacity in the production of CH 4 . Therefore, the production rate, selectivity of the 255 products, and the maintenance of the photocatalytic performance in the reuse cycles are 256 directly related to the niobium oxide phases.