Enhanced Simultaneous Photocatalytic Removal of SO2 and CO2 Using Powder and Coated Zeolites-Supported TiO2 Under Concentrated Sunlight Irradiation


 Single-step process for simultaneous removal of gaseous pollutants is more advantageous than multi-steps one. In this study, the efficiency of a novel synthetic zeolite (Ze) prepared from stone cutting sludge and a natural zeolite (clinoptilolite, Cp) as the supports of TiO2 photocatalyst were examined for the separate and simultaneous removal of SO2 and CO2 under solar irradiation using a parabolic trough collector (PTC). The composites exhibited a higher efficiency than raw zeolites and TiO2 for the removal of both gases. The maximum removal of SO2 by TiO2-Ze and TiO2-Cp under sunlight was 41.9 % and 56.2 % that enhanced to 53.4 % and 78.8 %, respectively in the presence of CO2. Correspondingly, it was 61.8 % and 68.7 % for single CO2 removal that increased to 74.2 % and 79.0 % in the binary gas stream. This behavior could be due to the enhanced simultaneous SO2 oxidation and CO2 reduction. The performance of coated composite for SO2 was higher than of powder one (54.3% vs. 41.9%) and for CO2 removal was almost close together (58.9% vs. 61.2%). This work promises the application of photocatalytic co-removal of SO2 and CO2 by using synthetic and natural zeolite under solar irradiation.


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Combustion of fossil fuels including petroleum, coal, and natural gas results in the significant 21 emissions of acidic gases such as carbon dioxide (CO2) and sulfur dioxide (SO2) that are 22 harmful to human health and the environment 1,2 . SO2 is an extremely toxic gas and a major 23 contributor to acid rain 3 . Also, CO2 is a key greenhouse gas that promotes global climate 24 change via global warming 4 . 25 Although the flue gas desulfurization (FGD) technology especially wet scrubbing has been 26 widely used for SO2 treatment, the high operational costs and secondary pollution are its main 27 disadvantages 5 . The main technologies for CO2 reduction are CO2 capture and storage (CCS) 28 as well as CO2 capture and utilization (CCU). The major concerns associated with CCS are 29 high cost and the risk of leakage from the storage of the gas. Among the methods, the CCU 30 approach is more economically feasible and helps the conversion of CO2 into a wide range of 31 end products providing hydrocarbon recovery 6 . 32 Separate technologies (i.e single-step processes) can remove pollutants from exhaust gases 33 efficiently 7 . However, these technologies are often multi-step processes, complex, and costly. 34 Therefore, it is suitable to develop an integrated, economic, easy implementation and high 35 efficient process for the purification of flue gas 7,8 . 36 Simultaneous removal of SO2 and CO2 as the main components in the flue gas has been 37 greatly concerned in the air pollution control 2,9 . Adsorption and photocatalytic oxidation 38 (PCO) can offer promising approaches for the simultaneous removal of gases because of low 39 cost, relatively simple design and minimum secondary pollution 6,8 . During the past decade, 40 PCO is emerged as an environmentally friendly and economically profitable technology for 41 pollutants removal through their converting into harmless compounds or value-added 42 chemicals 10 . Besides, it can be more attractive by using of solar energy as a clean and 43 sustainable source 11 . 44 Among various photocatalysts, TiO2 has received tremendous attention for the 45 photocatalysis process because of availability, non-toxicity, low cost, high photostability, 46 abundance and environmentally friendly nature 12 . However, it suffers from several drawbacks 47 such as low surface area, activity only under ultraviolet (UV) light (about 5% of the sunlight) 48 and high recombination of photo-generated electron-hole pairs 13, 14 . To overcome these 49 limitations, many researches have been focused on the TiO2 modification via doping elements, 50 coupling of semiconductors and composites synthesis of photocatalyst and adsorbent support 14,15 . 51 Zeolites, one of the most preferred supports for photocatalystes, are crystalline 52 aluminosilicates that have many desirable properties including uniform-shaped pores, high 53 surface area, thermal stability, hydrophilic /hydrophobic nature and polarizability 2,11 . These 54 properties can improve the potential of pollutants adsorption by TiO2-zeolite composites.

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The adsorption-photocatalytsis efficiency of TiO2-zeolite composites for removal of SO2

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The results showed that the composites had greater efficiency than the zeolite and TiO2  The adsorption behavior of SO2 and CO2 depends on the nature of adsorbates such as acidity, 117 polarity and kinetic diameter as well as the nature of adsorbents like polarity and pore size 2,36 .

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Both SO2 and CO2 are acidic molecules and show a high affinity for alkaline surfaces like 119 TiO2 1 . As the polarizability of SO2 (4.3 × 10 -24 cm 3 ) is more than CO2 (1.9 × 10 -24 cm 3 ), 120 it has a stronger tendency to bind to polar adsorbents like TiO2 (5.0 × 10 -24 cm 3 ) and zeolites 121 8,47 . The SiO2/Al2O3 ratio can determine the polarity of the zeolites. The smaller ratio, the 122 greater the polarity 48 . The Ze with a SiO2/Al2O3 ratio of 2.7 is more polar than Cp with a ratio 123 of 6.0 19 . However, the lower polarity of CO2 compared to SO2 showed a greater adsorption 124 tendency towards zeolites than TiO2. The kinetic diameter of SO2 and CO2 is 0.4 and 0.3 nm, 125 respectively 23 . The pore diameters of TiO2, Ze and Cp are 3.0, 11.0 and 23.2 nm, respectively 126 19 . Therefore, the kinetic diameter can not be an obstacle to the adsorption of the studied gases 127 by these adsorbents. The higher adsorption of both SO2 and CO2 gases by Cp may be due to 128 its larger pores that can provide more adsorption sites for gas molecules 2 129 130

Comparison of the composites performance in the removal of SO2 and CO2 131
The TiO2-Cp composite with a large pore size distribution has a greater tendency to remove 132 the studied gases through the adsorption-photocatalysis processes because the large particles revealed that the reaction of TiO2 with Cp was better performed leading to higher performance 137 in removing SO2 and CO2 gases.

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Although the amount of TiO2 added to the both zeolites was the same, but based on the 139 results of energy dispersive X-ray Spectrometer (EDS) and X-ray photoelectron spectroscopy 140 (XPS) analysis 19 , it was found that the amount of TiO2 in the surface of the TiO2-Cp sample 141 was higher than TiO2-Ze. This could be due to the presence of more oxygen groups in TiO2-

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Cp composite. In this composite, the presence of more surface TiO2 species could increase the 143 active sites for adsorption and photocatalysis reactions 51 . Moreover, the increase of photo 144 excited electron-hole pairs, followed by the increase of oxidative species such as radical OH 145 52 improved the performance of TiO2-Cp composite for SO2 and CO2 photodegradation.

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As mentioned earlier, the polarity of SO2 is higher than CO2. Besides, it is expected that 147 TiO2-Ze has more polarity compared to TiO2-Cp. Hence, it can be inferred that TiO2-Ze 148 composite must have more tendency for SO2 adsorption than TiO2-Cp, whereas the TiO2-Cp 149 composite showed better performance in the removal of both gases. This indicates that the 150 polarity was not the only parameter affecting the removal of these gases.

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The TiO2-Cp composite with a higher SiO2/Al2O3 ratio than TiO2-Ze has higher  whereas the CO2 adsorption on the TiO2-Ze was slightly reduced.

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The difference in the polarity of SO2 and CO2 molecules and their adsorption tendency on  The coating has the greatest effect on the contact surface of gases and composites, so it 224 increases the adsorption amount and consequently improves the photocatalysis removal of gases.

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The results showed that the dip-coating method influence more on the SO2 removal than CO2.

Langmuir-Hinshelwood (L-H) kinetic 250
As shown in Figure 4, the experimental data was in good agreement with a pseudo-first-order 251 kinetic model (R 2 = 0.92 for SO2 and R 2 = 1.0 for CO2). The applicability of the Langmuir-252 Hinshelwood (L-H) kinetic model for the photocatalysis removal of SO2 and CO2 pollutants 253 has been confirmed by the linearity in the plot of 1/r against 1/Ce (R 2 =1 for both gases). The 254 value of k was found to be 0.007 min -1 for SO2 and 0.03 min -1 for CO2, respectively (Table 3).    The synthetic zeolite X with 80 % purity hereafter named as Ze, was previously synthesized 279 from the stone cutting sludge using the alkali fusion method 30 . The natural zeolite, 280 clinoptilolite (Cp), with 73 % purity was purchased from Aria Tamadon Company, Tehran, 281 Iran. Titanium tetraisopropoxide (Ti(OC3H7)4, 97%) was purchased from Sigma-Aldrich 282 (Germany).

Preparation of zeolites 284
Synthetic zeolite (Ze) with a particle size of less than 500 μm was used without any treatment.

Synthesis of TiO2 particles and zeolite-supported TiO2 composites 291
TiO2 particles and TiO2-based composites containing TiO2-Cp and TiO2-Ze (with 50 wt % 292 TiO2 loading to zeolites) were prepared by the sol-gel method 19 . Then, the samples were 293 calcined at 300ºC for 3 hours to obtain the powder form. The characterization of the composites 294 were also described in our previous study 19 . 295 The coated composite was prepared using a sol-gel-based dip-coating method. Before

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The prepared beads were transferred to a filter and kept after 1 min in contact with the 303 composite sol. Then, they were allowed to drain freely. The coated beads were dried in the 304 oven at 100°C and weighted 34 . The difference in the weight of beads before and after coating 305 indicated the coated composite amount on the supports. Finally, the coated composite was 306 calcinated at 300°C for 3 h.

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The performance of TiO2, zeolites and their composites for the removal of SO2 and CO2 was 341 done in the optimized conditions that were already obtained in the preliminary experiments.

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The concentrations of SO2 and CO2 were 8.0 and 943.8 ppm, respectively. The relative humidity 343 (10 %) was adjusted by injection of water to the surface of composites using the micropipette.