The BiOCl and BiOBr oxyhalides were prepared by a simple coprecipitation route in a rich polyol medium at 100oC, where glycerol was used as solvent and mannitol as additive to increase the concentration of -OH groups in the reaction medium. The characterization of the samples was performed by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), diffusion reflectance spectroscopy (DRS), and the specific surface area was revised by N2 adsorption-desorption isotherms following BET protocol. The photocatalytic activity of BiOCl and BiOBr was determined in the photooxidation of nitric oxide (NO) in air, obtaining values of nitric oxide conversion degree of 77 and 90%, respectively. The origin of the photocatalytic activity was associated to the higher concentration of -OH groups in the medium of reaction that induce a preferential orientation of the crystalline plane (110) in BiOCl, and in the formation of the heterojunction BiOBr/B24O31Br10 in BiOBr. The high selectivity in the reaction of NO photooxidation to innocuous NO3− ions was confirmed with values of 96% (BiOCl) and 93% (BiOBr). Electron paramagnetic resonance (EPR) measurements determined that the hydroxyl (•OH) and the superoxide (•O2¯) radicals are the highly oxygen reactive species that rule the NO oxidation photocatalyzed by BiOCl and BiOBr oxyhalides.
Research Article
Photocatalytic characterization of BiOCl and BiOBr prepared by polyol-assisted coprecipitation for the elimination of NO x from air
https://doi.org/10.21203/rs.3.rs-4283217/v1
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BiOCl
BiOBr
photocatalysis
NOx
Heterogeneous photocatalysis is a promising technology to solve environmental problems associated with the pollution of soil, water and air, especially in places with a high population density [1]. By using a semiconductor irradiated with an appropriate energy, it is possible to promote electrons from its valence band to the conduction band generating a charge separation. If the separation of the hole-electron pair generated is successful, they can migrate separately to the surface of the photocatalyst to induce the formation of highly reactive radicals with high oxidation-reduction potentials to catalyze the conversion of organic and inorganic pollutants into innocuous chemical compounds.
Aside of using TiO2 anatase as photocatalyst, several compounds have been proposed to reach higher conversion percentages of pollutants, deep oxidation or reduction to final innocuous products, cyclability of the photocatalyst, as well as the use of visible light irradiation to activate the photocatalyst. In this direction, simple binary oxides as ZnO, WO3, Fe2O3, and more complex ternary oxides of the type BiVO4, Bi2WO6, Bi2MoO6, BiOI, among others, were successfully tested as photocatalysts for the elimination of chemical pollutants from water [2–7].
Various strategies have been employed to achieve high photocatalytic activity, beginning with the chemical synthesis route to develop specific physicochemical properties. In this sense, the most effective methods typically involve a liquid medium, such as water or an organic solvent, for synthesizing semiconductor materials using sol-gel, coprecipitation, hydrothermal, sonochemical, and other advanced techniques [8–11]. The reaction medium significantly impacts the final properties of the photocatalyst, including morphology, particle size, and specific surface area. The photocatalytic properties of the synthesized semiconductor can be directly affected using polar or nonpolar solvents with large or branched carbon chains in their structure, as well as the incorporation of organic additives as structuring agents. However, the use of heterojunction systems to prevent electron-hole recombination during the initial stages of the photocatalytic process is often mentioned [12–13].
In the last decade, bismuth oxyhalides BiOX (X = Cl, Br or I) were proposed as attractive photocatalysts over the basis of the existence of an internal electric field in their layered structure, which promotes an effective charge separation [14–15]. Among this family of compounds, BiOI has been extensively studied for the photocatalytic degradation of organic dyes in water and for NOx abatement in gaseous phase. The formation of macrostructures with spherical morphology, high specific surface area, preferential crystalline orientation of planes, and the formation of secondary phases have been crucial factors to enhance the photocatalytic activity of BiOI [16–17]. Recently, BiOI was prepared by polyol-assisted coprecipitation in a medium reaction of glycerol and mannitol, obtaining a conversion degree of nitric oxide of 96% with a selectivity to complete the oxidation to nitrate ions of 97% when was used as photocatalyst [18].
In this work, the synthesis of BiOCl and BiOBr by co-precipitation of salts of the metallic elements involved in an -OH enriched medium with glycerol and a mixture of glycerol/mannitol is revised. The capacity of the samples to act as photocatalysts is presented in the NO photooxidation reaction in gas phase. Some aspects of the reaction mechanism are elucidated by following the formation of deep products of NO photooxidation and by electron paramagnetic resonance (EPR) to detect the contribution of the most common radicals to the photocatalytic process.
2.1 Synthesis of BiOCl and BiOBr
BiOCl and BiOBr samples were synthesized by a simple coprecipitation method. First, 18 mL of a 0.2 M solution of bismuth nitrate pentahydrate (Bi(NO3)3.5H2O, ≥ 98%) in glycerol (C3H8O3, ≥ 99.5%) was placed in a glass beaker and heated to 100oC. Then 4 mL of a 0.25 M solution of mannitol (C6H14O6, > 98%) in glycerol was added dropwise over a period of 15 minutes. When the solution reached 100°C again, 18 mL of a 0.2 M precursor solution of halide (KCl or KBr) in glycerol was added dropwise to the solution and the system was maintained at 100°C with continuous stirring for 12 or 48 h. After this period, the precipitate was separated from the liquid by centrifugation at 9,000 rpm, washed several times with distilled water and a last time with ethanol. Finally, the resulting powder was put in an electric furnace at 70°C during 12 h. Following the same procedure, samples of BiOCl and BiOBr were prepared without mannitol.
2.2 Characterization of BiOCl and BiOBr
The formation of the crystalline structure of BiOCl and BiOBr was investigated by XRD using a PANalytical Empyrean X-ray diffractometer operated at 45 kV and 40 mA with Cu Kα radiation. Data were measured in the 2θ range from 10 to 70° at a scan rate of 0.016° every 59 s. The morphology of the prepared photocatalysts was analyzed using a field emission scanning electron microscope (FEI Nova NanoSEM 200). The samples were prepared by depositing a small amount of powder in a double-sided graphite tape and removing the excess with compressed air.
The specific surface area of the synthesized photocatalysts was evaluated by BET protocol from the measurements at 77 K of N2 adsorption-desorption isotherms on a BEL-Japan Minisorp II analyzer. Before the measurements, the samples were conditioned by thermal treatment at 100oC for 24 h. The optical properties of the BiOCl and BiOBr samples were evaluated by UV-Vis diffuse reflectance spectroscopy (DRS) by using an Agilent Technologies UV-Vis NIR spectrophotometer model Cary 5000 series equipped with an integrating sphere. The mathematical treatment of the data by the Kubelka-Munk function allowed the calculation of the energy band gap (Eg) value of the materials.
2.3 Photocatalytic experiments
The photooxidation of nitric oxide (NO) in gaseous phase was used as the reaction model to determine the photocatalytic activity of BiOCl and BiOBr samples. The experiments were carried out at room temperature in a stainless-steel photocatalytic reactor constructed based on ISO 22197-1. The photocatalytic reactor, with a capacity of 50 mL, had a window of tempered glass to allow the passage of the irradiation coming from the lamp. In a typical run, 0.1 g of BiOCl or BiOBr were dispersed in ethyl alcohol and then deposited on an area of 0.005 m2 of a glass substrate that was put inside of the photocatalytic reactor. Before introducing the gas into the system, a gas mixture of 100 ppm nitric oxide stabilized in N2 was diluted to 1 ppm with synthetic air (volume relation N2/O2 = 3.88). A 100 watts LED lamp was employed as the irradiation source and was turned on when the adsorption-desorption equilibrium between the gases and the photocatalyst surface was reached. During the photocatalytic experiment, the evolution of NO concentration was followed using a nitric oxide analyzer that operates by chemiluminescence (ECO Physics CLD 88p). To ensure the formation of harmless compounds as end products of the complete photooxidation of nitric oxide, nitrate (NO3−) and nitrite (NO2−) ions were analyzed from the water used to wash the photocatalyst before and after the photocatalytic reaction. To eliminate any interference in the quantification of NO3−and NO2− ions due to the presence of both chemical species in the photocatalyst prior to the reaction, the photocatalyst were washed several times by dispersing in 50 mL of deionized water and applying 30 min of sonication. The washing of the samples was repeated several times until the accumulated mass of NO2− and nitrite NO3− ions reached a constant value, and then the washed photocatalyst was used in the photocatalytic experiments. The concentration of NO2− and NO3− ions was measured through the reactions of the cadmium reduction and diazotization, respectively, using a Hach colorimeter model DR/890.
To identify the main highly reactive species that participle in the photocatalytic reaction, experiments by the technique of electron paramagnetic resonance (EPR) spin-trapping were performed by using 5, 5-dimethyl-1-pyrrolineN-oxide (DMPO) as a spin trap. For this purpose, 5 mg of the BiOCl or BiOBr were dispersed in water (DMPO−•OH) or methanol (DMPO−•O2−) containing 40 mmol L− 1 of DMPO. A 400 W lamp of the metal halide type was used as the excitation source. The measurements were performed on an EMX micro 6/1 Bruker ESR spectrometer equipped with Bruker Super High QE cavity resonator.
3.1 Characterization of BiOCl and BiOBr.
Table 1 lists all BiOCl and BiOBr samples synthesized via coprecipitation at 100°C for 12 and 48 h using glycerol and glycerol/mannitol as reaction media. The XRD patterns of BiOCl and BiOBr samples synthesized under the conditions described in Table I are shown in Figures 1a and 2a.
Table 1. Summary of BiOCl and BiOBr samples synthesized and their physicochemical properties.
When the medium of reaction was glycerol, a reaction time of 48 h was required for the appearance of diffraction lines of the BiOCl (CCl-48) and BiOBr (CBr-48) crystal structures. Thus, diffraction lines located in 2q= 11.9o, 25.9o, 32.6o, 41.0o, 46.8o, 54.3o, and 58.6o in figure 1a were indexed respectively with (001), (101), (110), (112), (200), (211), and (212) crystalline planes of BiOCl according with ICDD Card No. 04-002-3608. Diffraction lines with minor intensity reported for BiOCl were not clearly identified due to the low crystallinity of the sample. The diffraction patterns in Figure 2a showed the main diffraction lines associated with (110), (200), and (212) crystalline planes of BiOBr (ICDD Card. No, 01-085-0862). Both sets of samples had broad diffraction lines, indicating low crystallinity related to small particle size of BiOCl and BiOBr samples. These prepared samples showed a remarkable preferential orientation along the (110) crystalline plane. However, the commonly stronger diffraction line (102) was absent in both samples (CCl-48 and CBr-48). This absence can be associated to the width of the diffraction peak indexed as (110),
which could mask the presence of the diffraction line (102). The absence of the (102) diffraction line in BiOCl and BiOBr samples can also be attributed to the high concentration of hydroxyl groups in the reaction medium, as previously described by Wang et al. [19]. Hydroxyl groups can be adsorbed in the (102) crystalline plane through strong coordination with Bi3+ions, thereby limiting growth in this direction. While previous research has highlighted the high photocatalytic activity of the BiOX system (X = Cl, Br, and I) when exposed to the (001) crystalline planes, it has also been found that growth along the (110) direction can contribute to the development of high photocatalytic activity [20].
The use of mannitol as additive to the reaction medium reduced the reaction time to form the crystalline structures of BiOCl (CCl-M12) and BiOBr (CBr-M12) to 12 hours, as shown in Figures 1a and 2a. Additionally, extra diffraction lines not identified were observed in both systems around 2q=15o. To enhance crystallinity and remove organic matter, BiOCl and BiOBr samples were calcined at 250oC and 400oC, respectively, for 24 h. As a product of thermal treatment, weight loss of 11.6% (CCl-48), 12.7% (CCl-M12), 20.9% (CBr-48), and 25.9% (CBr-M12) was detected, indicating the significance of this process in eliminating organic compounds from the reaction medium.
In addition to weight loss resulting from calcination, the crystallinity of the samples noticeably increased, as shown by the narrower diffraction lines in Figures 1b and 2b. Specifically, all diffraction lines in the XRD patterns of the BiOCl calcinated samples (CCl-48C and CCl-M12C) were indexed based on the BiOCl crystalline structure. Moreover, the extra diffraction lines observed in CCl-M12 disappeared, obtaining thus, in both samples of BiOCl, only a single phase according to the resolution of the X-ray diffraction technique. As was observed in BiOCl samples without thermal treatment, the diffraction line associated with the (110) crystalline plane for CCl-48C and CCl-M12C showed a higher relative intensity regarding the reported in the ICDD Card No. 04-002-3608.
The thermal treatment of BiOBr samples led to the elimination of the extra diffraction lines in the samples prepared with glycerol/mannitol (CBr-M12), and the confirmation of crystalline structure of BiOBr, as is shown in Figure 2b for CBr-48C and CBr-M12C
samples. Additionally, strong diffraction lines at 2q=28.8o, 29.8o, 39.9o, and 45.7o appeared, which were associated with the formation of the crystalline phase of Bi24O31Br10 (ICDD Card No. 98-001-3105). The presence of Bi24O31Br10 as secondary crystalline phase during the synthesis of BiOBr was previously reported in samples thermally treated in the range of 400-600oC [21].
Figure 3 displays SEM representative images of BiOCl synthesized under various experimental conditions. Overall, the samples exhibited primary particle agglomeration with irregular shapes. The addition of mannitol to the medium of reaction introduced in the system a higher agglomeration of the nanosheets (CCl-M12 and CCl-M12C). The thermal treatment of the samples led to the thickening of the nanosheets, for example, in the sample prepared without mannitol (CCl-48), the thickness of the nanosheets increased from 4-9 nm to 30-40 nm in average (CCl-48C). A similar situation was observed when mannitol was incorporate in the medium of reaction, although probably the shorter time of reaction employed influenced the formation of thinner nanosheets, around 15-30 nm (CCl-M12C).
The BiOBr samples (CBr-48 and CBr-M12) that were not thermally treated showed the presence of large agglomerates of nanosheets with a thickness ranging from 3-10 nm, as shown in Figure 4. When the samples were calcinated at 400oC, they underwent a change in morphology. Firstly, thickness of the nanosheets was increased to 10-25 nm, but a more significant change was observed with the apparition of plates with larger dimensions. This may be related to the formation of Bi24O31Br10 detected in the X-ray diffraction characterization, as described previously in the synthesis of BiOBr at 550oC, where plates associated with the secondary phase were reported [22].
Textural properties of BiOCl and BiOBr samples were analyzed following BET protocol through the N2 adsorption-desorption isotherms. Figure 5 shows that the samples match a type II isotherm, which is typical for non-porous or possibly macroporous materials with high energy of adsorption [23]. A moderate increase in the BET surface area values was detected for samples obtained in presence of mannitol, which was of 16% for BiOCl (CCl-48, SBET = 88 m2g-1; CCl-M12, SBET = 102 m2g-1) and 130% for BiOBr (CBr-48, SBET = 39 m2g-1; CCl-M12, SBET = 90 m2g-1).
Although these values are higher than the previously reported for BiOCl and BiOBr in presence of different alcohols as medium of reaction [24-27], a decrease to values of 28 m2g-1; 28 m2g-1; 17 m2g-1; and 18 m2g-1 was observed after the thermal treatment for the samples CCl-48C, CCl-M12C, CBr-48C and CBr-M12C, respectively, as is showed in Table 1.
Figure 6a shows the diffusion reflectance spectra of BiOCl samples, indicating an absorption edge limit of 350-360 nm. This suggests that UV irradiation is required to activate this photocatalyst. In contrast, BiOBr samples have absorption edge limits in the visible region, with thermally treated samples being active around 450 nm, as shown in Figure 6b. The Kubelka-Munk function was applied to the data, revealing an Eg = 3.5 eV for the BiOCl samples. However, for the BiOBr samples, a range of values between 2.7-3.0 eV was observed, with a shift towards lower values in the calcined samples, i.e. CBr-48C (2.7 eV) and CBr-M12C (2.8 V). The samples that underwent thermal treatment (CBr-48C and CBr-M12C) showed a lower Eg. This could be due to the heterogeneity of these samples caused by the presence of Bi24O31Br10, as revealed by X-ray diffraction. These results agree with Li et al [28] who reported lower Eg values in BiOBr samples heated due to the formation of BiOBr/Bi24O31Br10 heterojunction. All Eg values for BiOCl and BiOBr are summarized in Table 1.
3.2 Photocatalytic activity of BiOCl and BiOBr.
Figure 7 shows the variation of the nitric oxide conversion degree (%) with the UV lamp irradiation when BiOCl samples were tested as photocatalysts. For CCl-48 sample, synthesized in glycerol medium, a maximum conversion degree of 90% was reached in the first 5 minutes of lamp irradiation, then this value decreases, resting at a semi-constant value of 62% at end of the experiment. On the other hand a notable decrease in the activity was observed for the sample prepared with mannitol (CCl.M12), resting at around 33% of conversion. As previously described, due to the synthesis process, all samples required a thermal treatment to eliminate organic matter. In this sense, when both photocatalysts were heated at 250oC, the respective NO conversion degrees in the semi steady state for CCl-48C and CCl-M12C were 77 and 57%. Although the photocatalytic activity improved, the best NO conversion degrees were achieved in samples prepared in a glycerol medium, specifically CCl-48 and CCl-48C. The higher intensity of the (110) diffraction line of BiOCl observed in the DRX data provides an explanation for this. Previously, it was reported that increasing the exposure of 110 planes can reduce charge recombination during photocatalyst activation. This is due to the internal electrical field between the positive layer of [Bi-Cl] and negative layers of [O] atoms in the crystalline structure of BiOCl [29].
The effect of adding mannitol to the reaction medium differed in the BiOBr system. Figure 8 shows the behavior of nitric oxide conversion degree for different BiOBr samples tested as photocatalysts, in relation to lamp irradiation time. Among this series of samples, the lowest NO conversion degree was observed when using sample CBr-48 as photocatalyst, with a conversion degree of 59% after 30 minutes of photocatalytic reaction. In contrast, using BiOBr prepared in a glycerol/mannitol reaction medium (CBr-M12) resulted in a superior photocatalytic activity of 90%, compared to the BiOCl system. This differs from what was observed in the BiOCl system, where the addition of the reaction medium improved the photocatalytic activity. Moreover, after thermal treatment at 400°C, the CBr-48C and CBr-M12C samples achieved a NO conversion degree above 90%. This suggests that interpreting the improvement in photocatalytic activity in the BiOBr system is complex.
The high degree of NO conversion achieved by sample CBr-M12 can be attributed to its physicochemical properties. This sample, which was prepared in the presence of mannitol, had the smallest crystal size, allowing for better charge transfer to the photocatalyst surface [28]. Additionally, its specific surface area was 2 to 5 times higher than that of the other BiOBr samples.
Although the BiOBr samples that were thermally treated, CBr-48C and CBr-M12C, had lower specific surface areas, their photocatalytic activities were similar to that of the untreated sample CBr-M12. The origin of the NO conversion degrees up to 90% for both samples could be associated with the presence of the B24O31Br10 crystalline phase, which was detected by X-ray diffraction. Previously, the heterojunction BiOBr/B24O31Br10 was reported as a successful photocatalyst due to its ability to reduce charge recombination [22]. Furthermore, heterojunctions such as BiOBr/BiOI and BiOI/BiOCl systems were reported to have a significantly increased photocatalytic activity compared to their pristine materials [29-30].
The photocatalysts with the best performance in both systems, CCl-48C and CBr-M12, were selected to undergo successive cycles of turning the lamp on and off to assess the stability of BiOCl and BiOBr systems during the photocatalytic process. Figure 9 shows a decrease in the degree of NO conversion by 5% and 10% after three successive cycles when CCl-48C and CBr-M12 were used as photocatalysts. Of course, further study is required to use these synthesized samples as active components in building materials, however, their stability during photocatalytic cycles is promising. The final products of the photooxidation reaction of nitric oxide were analyzed by quantifying the formation of nitrates (NO3-) and nitrites (NO2-) ions on the surface of the photocatalyst. The quantification was performed before and after using CCl-48C and CBr-M12 as photocatalysts. Figure 10 shows that 8 and 7 washes with deionized water were necessary to remove NO3- and NO2- ions from the surface of CCl-48C and CBr-M12, respectively, before using them as photocatalysts. The presence of these ions on the surface of samples can be associated with impurities of chemical reagents and, due to the low concentration produced during the photocatalytic process, its elimination is necessary to avoid interference when interpreting the results. After removing nitrate and nitrite ions from the surface, the samples underwent photocatalytic reaction. They were then washed with deionized water and the concentration of ions in the washing liquid was quantified. A notable increase in the accumulation of NO3- and NO2- ions were observed for the washed number 9 and number 8 of the CCl-48C and CBr-M12 samples, respectively. The increase in the concentration of nitrate and nitrite ions compared to the last washed in each case was related with the oxidation of NO to the innocuous chemical species (NO3- and NO2-). Based on these results, the selectivity of the conversion from NO to NO3- ions was of 96% for CCl-48C and 93% for CBr-M12, and from NO to NO2- ions the selectivity was of 4% in both samples. Therefore, it can be concluded that BiOCl and BiOBr are effective photocatalysts for completely oxidizing nitric oxide, primarily to nitrate ions. EPR spectroscopy was used to investigate the highly reactive species generated during NO photocatalytic oxidation by BiOCl and BiOBr. This provides insight into the conversion mechanism of NO. Taking this into consideration, DMPO spin trapping adducts were used for the detection of •OH radicals (DMPO-•OH) and •O2¯ radicals (DMPO-•O2¯) in aqueous and methanol dispersions, respectively. Figure 11 shows the corresponding spectra of CCl48C and CBr-M12 samples for DMPO adducts. For both samples, the four peaks of the characteristic peaks of the DMPO-•OH (Figure 11a-b) and DMPO-•O2¯ (Figure 11c-d) adducts were detected, similar to what was referenced previously [31-32]. It can be inferred from these results, that •OH and •O2¯ radicals play significant role in the nitric oxide photooxidation process.
Then, a general mechanism for the photooxidation reaction of by BiOCl or BiOBr photocatalysts can be proposed. The semiconductor is excited by promoting electrons from the valence band to the conduction band when UV (BiOCl) or Vis (BiOBr) irradiation is incident on the photocatalyst surface. The photogenerated holes and electrons promote the formation of hydroxyl (•OH) and superoxide (•O2¯) radicals through the oxidation of H2O and by the reduction of molecular O2, respectively. These radicals primarily react with nitric oxide to produce harmless nitrate ions (NO3¯). The proposed mechanism is consistent with the EPR analysis presented in Figure 11. The reactions [33] can follow the afore mentioned process:
BiOCl and BiOBr phases with high photocatalytic activity were prepared by coprecipitation route in glycerol medium. Both oxyhalides were effective for the elimination of NO from air, with high selectivity to form innocuous species as final products of reaction. The incorporation of mannitol to the medium of reaction reduced the time of formation of oxyhalides and enhanced its photocatalytic activity. An additional thermal treatment was necessary to reach the maximum NO conversion degree using this type of photocatalysts. Samples of BiOBr were benefited by a thermal treatment at 400oC due to the formation of the heterojunction BiOBr/B24O31Br10, which was reported in the past as a better photocatalyst than the NO to NO3- with a 96% for BiOCl and 93% for BiOBr with UV and visible light irradiation, respectively. EPR analysis determined the predominant role of the hydroxyl (•OH) and superoxide (•O2¯) radicals as the highly oxygen reactive species that rule the photocatalytic process of NO photooxidation to innocuous NO3- nitrates.
Author Contribution
1. A.M.C. wrote the main manuscript and revise photocatalytic experiments.2. K.A.R.C. is the student who carried out the experiments, this paper is based in her work for phD degree.3.E.LC. he is the one who carried out the characterization by SEM and contributed to the writing of the article.4. R.M.I. he is the one who carried out the characterization of the optical properties and the analysis of the reactive species in the reaction mechanism.
ACKNOWLEDGMENTS
The authors want to thank to the CONAHCYT for its appreciable support through the Projects 15762 and 552274 (FRONTERAS DE LA CIENCIA).
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No competing interests reported.