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Degradation of air pollutants from waste burning using photocatalyst TiO2 With Co(NO3)2 doped under ultraviolet irradiation

1 , 2 ORCID , 2 *

Vol. 1 No. 1 |pp. 25–31 |Received Mar 4, 2024 |Revised Mar 11, 2024 |Accepted Mar 13, 2024

Copyright © 2024 The Authors. This publication is licensed under CC BY 4.0 .
Published Jul 1, 2024 | DOI: https://doi.org/10.62755/greensusmater.2024.1.1.25-31
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Abstract

Air pollutants, such as carbon dioxide (CO2), hydrogen cyanide (HCN), and methane (CH4), can harm the respiratory organs of humans and cause several diseases. This study successfully utilized a photocatalyst from TiO2 with Co(NO3) doped to degrade these air pollutants from waste burning. The photocatalyst layer was produced by dissolving TiO2 and Co(NO3)2 in distilled water, and then the solution was coated on a mica surface using a spray coating method. The coated mica was then dried in an oven. The crystallite structure of TiO2/Co(NO3)2 was analyzed by X-ray diffraction. The obtained crystallite size was (15.38 ± 0.03) nm with lattice parameters a and c were (3.8 ± 0.3) Å and (9.3 ± 0.3) Å, respectively, which shows that it is an anatase phase. The band gap energy was measured by diffuse reflectance UV-visible spectroscopy and analyzed using Tauc's plot method. The measured band gap energy of the photocatalyst was 2.81 eV, which can be easily activated by ultraviolet (UV) light. The photocatalyst sheets successfully degraded air pollutants from waste burning, including 53.139\% CO2 for 4 hours, 100\% HCN for 10 minutes, and 72.381\% CH4 for 40 minutes. Therefore, the fabricated photocatalyst in this study can potentially be an alternative to degrading air pollutants, especially CO2, HCN, and CH4.

Keywords:
Photocatalyst TiO2 Co(NO3)2 Air Pollutants UV Light
Subjects:
Water & Wastewater Treatment Photocatalysis & Advanced Oxidation Functional & Nanostructured Materials Metal Oxides & Semiconductors Environmental Assessment & Management Air Quality & Particulate Control
Graphical Abstract
Graphical abstract

1. INTRODUCTION

Fresh air is a very important element for living things. However, the fresh air issue has recently taken center stage because of air pollution. Air pollution is a mixture of gases and particles produced by many sources, such as the smoke of cigarettes, vehicles, industry, agriculture, combustion, and natural sources 1,2,3. Several gases widely known as air pollutants include SO2, NOx, NH3, and CO, as well as greenhouse gases such as CO2, CH4, and N2O 4,5. These pollutants pose significant risks to human health, particularly when they infiltrate indoor spaces through air vents. Confined within walls and roofs, these gases are impeded from dispersing naturally with the wind. Consequently, occupants inhale the pollution readily, leading to many health issues and compromising the overall comfort of the indoor environment 6.

Researchers have made various efforts to reduce air pollutants. Photocatalysis, an environmentally friendly and sustainable technique, holds great promise for degrading these pollutants 7,8,9. Photocatalyst is a catalytic process whose success depends on light absorption capability, active site density, redox capacity, and the rate of light-triggered electron-hole recombination 10. Titanium dioxide (TiO2) is one of the most excellent and popular semiconductor photocatalysts, with good photosensitivity and chemical stability, non-toxicity, and low cost 11,12. TiO2 has a crystallite size of 19.82 nm 13. This crystal is divided into three polymorphic forms: rutile, anatase, and brookite 14,15. TiO2 is used exclusively as a white pigment and has been widely used as a base material for producing sunscreen, photocatalysts, and solar cells 16. However, this photocatalyst has limited optical and electronic properties that are active only when exposed to UV light 17, which is only ± 5% of solar energy 18. The band gap energy on TiO2 is about 3.23 eV 19.

Modifying the surface of TiO2 by incorporating other materials is recognized as an effective technique to enhance the efficiency of sunlight utilization and broaden its photocatalytic activity 13. Doping of metal ions (such as Fe, Co, Ag, Ni) and non-metallic elements (such as C, N, F, S) or carbon-containing materials (such as carbon nanotubes, flora, and graphene oxide) can improve the photocatalytic properties of TiO213,17. Among these materials, Co can effectively enhance the photocatalytic activity of TiO220. When Co is introduced into the TiO2 material, it acts as an impurity within the conduction band, leading to an electronic transition, namely the band gap transition from the valence band consisting of O 2p orbitals to the Co 3d and Ti 3d conduction bands. This intensity indicates that TiO2/Co has good photocatalytic activity. After electron excitation, the photogenerated electrons are reserved in the conduction band, which are further transferred to the surface of catalyst particles to be captured by O2 molecules adsorbed on the surface. The photogenerated holes in the valence band are trapped by hydroxyl molecules and water as active radicals to oxidize organic compounds 20.

Hosseini et al. found that a fabricated photocatalytic mixed matrix membrane containing Co and TiO2 nanoparticles was suitable for the photocatalytic separation of 2,4-dichlorophenol 13. Khaki et al. also researched Co and TiO2, which resulted in the bandgap energy of the transition metal Co-doped with TiO2 being lower than that of TiO2, which is 2.96 eV 17. In addition to its utilization in liquid samples, TiO2 can also be used in gas samples. Mamaghani et al. carried out hydrothermal/solvothermal synthesis and processing of TiO2 as an effective photocatalyst for the degradation of air pollutants 21. Therefore, research on photocatalysts for the degradation of air pollutants is possible.

In this study, TiO2 with a Co-doped photocatalyst was fabricated. To assess the impact of Co doping on the crystallite structure of TiO2, X-ray diffraction (XRD) patterns were conducted. Additionally, to confirm that Co doping reduces the band gap energy of TiO2, testing was performed using diffuse reflection UV-visible (DR UV-Vis) spectroscopy. The photocatalyst was tested to degrade the air pollutants, namely CO2, HCN, and CH4, from waste burning under UV light irradiation. Gas concentrations were detected by a gas chromatography thermal conductivity detector (GC-TCD) and gas chromatography-mass spectroscopy (GC-MS).

2. MATERIAL AND METHODS

2.1 Materials and Instruments

The titanium dioxide (TiO2) (anatase-type) powder is produced by Sigma Aldrich. Cobalt nitrate powder (Co(NO3)2) was purchased from Nitra Kimia, Indonesia. UV lamp (220V, 50 Hz, 6W) was used as light sources. X-ray diffraction (Shimadzu XRD-6000) and diffuse reflectance UV-visible spectroscopy (Shimadzu UV-1700) were used to characterize the photocatalyst doped. Contaminated smoke is generated from burned waste such as plastic, paper, cardboard, and dry leaves. The 5-mL venoject tube was used to collect the smoke samples. Gas chromatography thermal conductivity detector (GC-TCD, Shimadzu GC 8A) and gas chromatography-mass spectroscopy (GC-MS, Shimadzu QP 2010 SE) were utilized to measure the concentration of the molecules of the samples.

2.2 Method and Procedure

The testing chamber in this study was constructed using a modified hexagonal prismatic glass container with a hole for smoke and a hole for sample collection. UV lamps were installed inside the chamber as UV light sources. A fan was installed in one of the holes to introduce smoke so that the smoke would be attracted and enter the chamber. An illustration of the photocatalyst fabrication process and the test chamber are shown in Fig. 1. In this study, the TiO2 photocatalyst was doped with Co derived from Co(NO3)2. 40 mg of TiO2 and 10 mg of Co(NO3)2 were dissolved in 5 mL of distilled water and stirred with a stirring speed of 570 rpm for 2 hours at room temperature to produce a homogenous photocatalyst solution. The solution was then sprayed on two sheets of mica (13 × 29 cm) coated with aluminum foil for photocatalyst test under UV light. These sheets aim to cover the glass in the test room so that UV light does not radiate out. After spraying on the mica, the solution was dried in a furnace at 90 °C for 1 hour. Then, the mica was mounted on the inner side of the test chamber.

Figure 1. The chamber of photocatalyst test. The smoke sample flows from the burning box to the chamber. UV lamps are used as light sources.

The photocatalyst was synthesized in solid phase (powder) for material characterization and then tested using XRD to determine the type of crystal. Analysis was carried out with diffraction angles (2θ) in the range of 3°C–80°C and scanned continuously at a scan rate of 4 degrees·s-1. XRD was conditioned at a voltage of 40 kV with a current of 30 mA. The resulting data are 2θ and intensity, then presented in a graph to determine the intensity peaks. The values calculated in XRD are crystallite size (t) and lattice parameters (a and c) with the following equations (Eq. 1 and Eq. 2).

t=Kλβcos⁡θ(1)

1d2=h2a2+k2b2+l2c2(2)

where K is Scherrer constant (0.9), λ is X-ray wavelength (0.154 nm), β is the full width at half the maximum intensity of the reflection peak, θ is diffraction angle, d is the distance between Bragg planes, and (hkl) is miller index 16. Photocatalysts synthesized in the solid phase (powder) are also tested using DR UV-vis spectroscopy to ensure photocatalyst band gap energy changes after doping with Co(NO3)2. Data acquisition is performed at a wavelength of 200–800 nm every 1 nm. The data results were then analyzed using Tauc’s Plot method, which is the relationship between absorption coefficient (α) and incident photon energy hf in the form of (Eq. 3):

(αhf)1m=c(hf−Eg)(3)

where α is the absorbance coefficient used in the Tauc Plot method, h is the photon energy (eV), c is a constant, and Eg is the band gap energy. The value of m depends on the type of transition: m = 1/2 for direct and permitted transitions, m = 2 for indirect transitions, and m = 3/2 for forbidden transitions 22. OriginLab software was used to determine the bandgap energy of the photocatalyst.

The photocatalyst test begins by filling the chamber to the brim with polluted smoke/gas by capturing the combustion exhaust using a funnel and hose connected to the vent hole in the chamber. The UV lamp is then turned on. Gas samples were taken with a syringe directly into a 5 mL venoject tube to preserve the gas. In the first test to observe the degradation of CO2, samples were taken every 2 hours from before the reaction occurred (t = 0 s) to 6 hours. In the second test to observe the degradation of HCN and CH4, samples were collected every 10 minutes from pre-reaction (t = 0 s) to 40 minutes.

Gas samples stored in venoject tubes were analyzed by GC-TCD and GC-MS to determine changes in the CO2, HCN, and CH4 concentrations after the photocatalyst test. The concentration of the molecules in the gas is indicated by the percentage of the area of the peaks on the graph produced by the detector. The percentage of CH4 and CO2 degradation is calculated using the following equation (see Eq. 4).

Degradation(%)=(1−CtC0)×100%(4)

where Ct is the concentration of the molecule at time t, and C0 is the initial concentration of the molecule 13

3. RESULTS AND DISCUSSION

3.1 Crystral Size Analysis of TiO2/Co(NO3)2 Photocatalyst

The crystal structure of the photocatalyst layer used in this study was analyzed using an X-ray diffractometer (XDR). The data obtained was further analyzed using OriginLab software to determine the diffraction peaks. The peaks are the scattering of light hitting the TiO2/Co crystal, which interferes constructively. The XRD patterns of TiO2/Co(NO3)2 photocatalyst is shown in Fig 2.

Figure 2. XRD pattern of TiO2/Co(NO3)2 photocatalyst

Based on Fig. 2, diffraction peaks indicate that X-rays hit the crystal plane. Four diffraction peaks are visible on the TiO2/Co(NO3)2 , namely at 2θ of 22.526°C, 25.013°C, 37.552°C, and 47.761°C. These diffraction peaks were compared with the TiO2 standard diffractogram from the Joint Committee on Powder Diffraction Standars-International Center for Diffraction Data (JCPDS-ICDD) No. 21–1272. The first peak did not find its Miller index from these data, so further analysis could not be performed. Meanwhile, the Miller indices at 2θ of 25.013°C, 37.552°C, and 47.761°C are (101), (004), and (200), respectively. The results of this crystal field analysis indicate that the detected material is anatase-type TiO214,15,19. The highest diffraction peak of the TiO2/Co(NO3)2 photocatalyst is located at 2θ of 25.013°C with plane (101). The peak appears due to the effect of the addition of Co(NO3)2 on TiO2, which then characterizes the material we produce. Through the calculations in Eq. 1 and Eq. 2, the values in Table 1 are obtained.

Table 1. Crystallite structure analysis results of TiO2/Co(NO3)2
2θ (°) FWHM (rad) Miller Index (hkl) Crystallite size (nm) Lattice parameter a (Å) Lattice parameter c (Å)
25.013 0.00923 -101 15.38 ± 0.03 3.8 ± 0.3 9.3 ± 0.3

Based on the calculations performed, the crystallite size of the TiO2/Co(NO3)2 is (15.38 ± 0.03) nm. The crystallite size is smaller than pure TiO2, which is 19.82 nm 13. Co ions in the lattice seem to form a complex with oxygen on TiO2, thus suppressing the growth of TiO2 crystals 13. This small crystallite size makes the surface of the material high, allowing the crystals to be evenly distributed. Meanwhile, the obtained lattice parameters (a and c) belong to anatase type TiO215. Although the value is different, it is still within the calculation error range. Small differences from the calculation with the reference (a = 3.784 and c = 9.515 15,23) result from the addition of Co(NO3)2 on TiO2. This indicates that the doping used does not significantly change the crystal structure of TiO2. Therefore, Co(NO3)2 doping does not inhibit the catalytic function of TiO2.

3.2 Band Gap Energy of TiO2/Co(NO3)2 Photocatalyst

The band gap energy of TiO2/Co(NO3)2 photocatalyst was analyzed using DR UV-vis spectroscopy in a wavelength range of 200–800 nm. The increasing absorbance value indicates that the intensity of the absorbed light is becoming greater, resulting in many free electrons that will fill the conduction band and produce an electric current. The absorbance of TiO2 and TiO2/Co(NO3)2 is shown in Fig. 3.

Figure 3. Absorbance spectrum of TiO2/Co(NO3)2

From the absorbance spectra, it can be seen that the highest absorbance on TiO2/Co(NO3)2 occurs when exposed to a wavelength of 283 nm, then drops and rises again at 363 nm, then drops dramatically and rises slowly to peak at 583 nm (see Fig. 3). When compared with the absorption spectrum of pure TiO224, there are differences in the wavelength of maximum absorption. In pure TiO2, there is only one maximum absorbance peak at a wavelength of 302 nm 24. The difference in absorbance spectra was believed to be an impact of the addition of Co(NO3)2 and affects the photocatalyst’s ability to absorb light. The value of the band gap energy is very important because it affects the ability of the material to form electrons and holes. This energy determines how much photon energy is required to activate the photocatalytic process. The lower the band gap energy of a material, the lower the photon energy required to initiate photocatalysis. The band gap energy of the materials used in this study was calculated using Tauc’s Plot method. The data obtained from the DR UV-vis spectroscopy was processed in OriginLab software to obtain a graph absorption coefficient (αhf)2 vs the photon energy (hf). Then, extrapolation was performed in the region of the curve that increased sharply, as shown in Fig 4.

Figure 4. Tauc’s plot on TiO2/Co(NO3)2
Table 2. Results of GC-TCD analysis of the degradation of CO2 under UV light irradiation
Time (hours) Retention time (min) Area (%) Compound name Degradation (%)
0 8.723 1.513 CO2 0
2 8.719 0.894 CO2 40.912
4 8.718 0.709 CO2 53.139
6 8.746 1.349 CO2 10.839
Table 3. Results of GC-MS Analysis of the degradation of HCN and CH4 under UV light irradiation
Time (min) Retention time (min) Area (%) Similarity Index Compound name Degradation (%)
0 2.211 1.18 50 HCN 0
2.615 1.05 52 CH4-D4 0
10 - - - HCN 100
2.031 0.41 51 CH4 60.952
20 - - - HCN 100
- - - CH4 100
40 - - - HCN 100
0.25 0.29 47 CH4 72.381

The red line in Fig. 4 is an extrapolation line made using Tauc’s plot method, which shows the material’s band gap. The band gap energy of TiO2/Co(NO3)2 was obtained from the extrapolation line, which is 2.81 eV. The anatase band gap energy of TiO2, which was initially 3.2 eV 19, becomes smaller after adding Co(NO3)2. The reduction in band gap energy, attributed to the presence of Co as a transition metal, induces the formation of an impurity state between the conduction band and the valence band in TiO2. As the band gap is smaller due to the incorporation of Co into the TiO2 lattice, the absorption edge shifts towards longer wavelengths (redshift). In this investigation, to surpass the band gap energy of 2.81 eV, light with a wavelength of 441.52 nm was necessary to activate the photocatalyst.

3.3 The Mechanism Of Degradation Of CO2 and CH4 Air Pollutants with TiO2/Co(NO3)2 Photocatalyst

The degradation process of air pollutants can be carried out by the photocatalytic process on TiO2/Co(NO3)2. Photocatalyst materials typically exist in semiconductors, capable of initiating chemical reactions upon exposure to photon energy surpassing the band gap energy. Initially, TiO2 has a band gap energy of 3.2 eV 19. However, after adding Co(NO3)2, the band gap energy becomes 2.81 eV due to Co2+ replacing Ti4+ in the TiO2 crystal structure. The size of Co2+ ions (radius 0.074 nm) is almost similar to Ti4+ (radius 0.068 nm), allowing them to enter the TiO2 crystal to decrease the band gap and increase the activity of the photocatalyst 13.

Photons with energy greater than 2.81 eV, such as UV light, can cause electronic transitions from the valence band to the conduction band. Electrons in the O 2p orbitals in the valence band are excited to the Co 3d and Ti 3d orbitals in the minimum conduction band. The presence of valence band electrons creates holes (hvb+) due to electron donors. Meanwhile, the conduction band produces electrons (ecb-) as electron acceptors. This process is where the decomposition begins. The holes will interact with the water molecules in the air, while the electrons will interact with the oxygen in the air. The reaction of electrons and holes with oxygen and water produces hydroxyl radicals, which can break down organic polymers such as CH4. On the other hand, CO2 can be degraded due to adsorption by the photocatalyst layer but cannot be chemically decomposed. The chemical reaction that occurs is as follows 25.

Co(NO3)2−TiO2+hf→Co(NO3)2−TiO2(ecb++hvb−)(5)

hvb++H2O→H++HO⋅(oxidation)(6)

ecb++O2→O2⋅−(reduction)(7)

O2⋅−+H+→HO2⋅(8)

HO2⋅+HO2⋅→H2O2+O2(9)

H2O2+ecb+→HO⋅+OH−(10)

H2O2+hv→2HO⋅(11)

CH4+4HO⋅+O2→CO2+4H2O(12)

3.4 Degradation Of Air Pollutants With Photocatalysts Under Ultraviolet Light Irradiation

The first energy source used is ultraviolet light (λ < 400 nm), which has energy greater than 2.81 eV, the bandgap energy of TiO2/Co(NO3)2 photocatalyst. This energy can activate the photocatalytic process on the photocatalyst layer. In the first observation (i.e., degradation of CO2), time variations of 0, 2, 4, and 6 hours were used (see Table 2). GC-TCD detected the concentration of the CO2. The GC-TCD instrument can only make semi-quantitative measurements, which is the amount of molecular concentration expressed in the percentage range based on the compound’s retention time. Retention time is the required time for the analyte to be injected until the detector captures the signal in the form of a peak plot generated by the instrument. The peaks of the graph have an area whose percentage represents the concentration of the compounds detected in the gas being analyzed. The degradation percentage is calculated using Eq. 4 based on these concentrations.

The calculation result shows increased CO2 degradation at 2 and 4 hours (see Fig. 5). The increase in the degradation percentage with the irradiation time is caused by the number of electrons excited from the valence band to the conduction band to produce free electrons on the surface of the photocatalyst layer, which interacts with CO2 in the air. The ability of the photocatalyst to degrade CO2 pollutants reaches a maximum of 53.14% for 4 hours (see Table 2). However, the percentage decreased drastically to 10.84% at 6 hours, and there was a possibility of the photocatalyst experiencing saturation, so its degradation ability decreased. Saturation may occur because CO2 previously adsorbed on the photocatalyst’s surface blocks the photocatalytic process. Another possibility is that CO2 is desorbed, increasing the measured concentration.

Figure 5. Percentage degradation of CO2 pollutant of waste burning using TiO2/Co(NO3)2 photocatalyst under UV light irradiation

Furthermore, the degradation ability of the TiO2/Co(NO3)2 photocatalyst under UV light irradiation was observed with time variations of 0, 10, 20, 30, and 40 minutes using GC-MS to observe the degradation of HCN and CH4. Similar to GC-TCD, GC-MS can only make semiquantitative measurements by area percentage. GC-MS uses the similarity index (the degree of similarity of the detector data to the instrument’s reference) to determine which molecules are detected in the peaks of the graph produced by the detector.

In the second observation (i.e., degradation of HCN and CH4 under UV light), Table 3 shows no HCN and CH4 molecules initially (t = 0 s). After the photocatalyst was activated with UV light energy, the HCN molecule no longer appeared during the observation. The phenomenon shows that the photocatalyst can degrade HCN within 10 minutes (see Fig. 6). Meanwhile, the CH4 molecule experienced a degradation of 60.95% in the first 10 minutes and an increase in the percentage of degradation to 100% in the first 20 minutes. However, the CH4 molecule suddenly reappeared in 40 minutes with a degraded percentage of 72.38%. This phenomenon is possibly due to the photocatalyst experiencing saturation, which decreases the photocatalyst’s ability to degrade.

Figure 6. Percentage degradation of CH4 and HCN pollutants from waste burning using TiO2/Co(NO3)2 photocatalyst under UV irradiation

4. CONCLUSION

The doping of Co(NO3)2 onto the TiO2 photocatalyst was successfully carried out. The crystallite size of the photocatalyst after doping is (15.38 ± 0.03) nm, while the lattice parameters a and c are (3.8 ± 0.3) Å and (9.3 ± 0.3) Å, respectively. These results indicate that Co doping does not change the crystal structure of TiO2, namely the anatase phase. Furthermore, the band gap energy decreased after doping to 2.81 eV, which UV light can easily transmit. As a result, the TiO2/Co(NO3)2 photocatalyst successfully degraded CO2 gas pollutants by 53.139% for 4 hours, HCN by 100% for 10 minutes, and CH4 by 72.381% for 40 minutes under UV light irradiation.

Acknowledgments

Kamila Munna acknowledges an undergraduate scholarship from the Ministry of Education, Culture, Research, and Technology of the Republic of Indonesia through the Bidikmisi program and a master scholarship from the Ministry of Finance through the Indonesian Endowment Fund for Education (LPDP). Rizky Aflaha acknowledges a Ph.D. scholarship from the Ministry of Education, Culture, Research, and Technology of the Republic of Indonesia through the PMDSU program. The authors thank Mr. Dhe, Eka Dwi Lestari, and Nurul Imani Istiqamah, who helped greatly in the Materials Physics Laboratory at Gadjah Mada University.

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