ARID ZONE JOURNAL OF ENGINEERING, TECHNOLOGY & ENVIRONMENT AZOJETE March 2022. Vol. 18(1):121-134 Published by the Faculty of Engineering, University of Maiduguri, Maiduguri, Nigeria. Print ISSN: 1596-2490, Electronic ISSN: 2545-5818 www.azojete.com.ng Corresponding author’s e-mail address: silaskiman@unimaid.edu.ng 121 ORIGINAL RESEARCH ARTICLE PERFORMANCES OF METAL OXIDES SUPPORTED IN MONOLITH FOR COMBINED SO2/NOx REMOVAL FROM FLUE GAS S. Kiman*, H. D. Mohammed, N. B. Aliyu, T. H. Usman and D. B. Kayode Department of Chemical Engineering, University of Maiduguri, Bama Road, PMB 1069 Maiduguri, Borno State, Nigeria *Corresponding author’s email address: silaskiman@unimaid.edu.ng 1.0 Introduction Nitrogen oxides (NOx) and sulfur dioxide (SO2) are pollutants that are produced from fossil fuels which may cause acid rain, depletion of the ozone layer, urban smog, several illnesses, acceleration of the degradation of buildings, monuments, and structures (Rosas et al., 2017; Wu et al., 2016). The apprehension of such pollutants on the environment and human health has motivated a global campaign on strategies to abate their emission (Abdulrasheed et al., 2018). Wet flue gas desulfurization (WFGD) being the evolved matured technology used for SO2 removal is incapable of eliminating NOx. Alternatively, Alternatively, selective catalytic reduction (SCR) is widely utilized for the NOx control but due to catalyst fouling, operating and capital cost, it is limited (Wu et al., 2016). Therefore, the development of a cost- effective method for combined NOx/SO2 removal is desirable. It was reported that the removal of acidic gases in flue gas composition by adsorption has advantages such as high removal efficiency, selectivity, regeneration potential with the simplicity of design and operation (Abdulrasheed et al., 2018). Also, adsorption is one of the most promising methods for pollutants abatement, however, however, the adsorbent and supported active species are critical to the adsorptive performance (Yin et al., 2018). The supporting material must be characterized by high surface area, pore structure, and abrasion resistance moreover, the dispersion of the metallic components can promote the catalytic properties on the support (El-Khouly et al., 2017). Kiman et al. (2018) found that monolith has the required surface area, attrition, and fly ash plugging resistance, therefore, it can be used in heterogeneous catalysis as support. However, Mo et al. (2018) stated that the dispersion of active components on the monolith is always in-homogeneous and easy to aggregate during impregnation and therefore has a negative effect on practical use. Therefore, some efforts must be devoted to growing nanoarray-based catalysts on channeled monolithic substrates to promote catalytic activity. ARTICLE INFORMATION ABSTRACT The emission of acid gases such SO2 and NOx have detrimental effects on human beings and the environment hence, removing them from flue gas in a dry adsorption process is desirable. In this work, carbon monolith was synthesized with various metals consisting of CeO2, Co3O4, V2O5, and CuO by deposition precipitation, hydrothermal and wet pore volume impregnation synthesis techniques. The fixed bed adsorption activity test was carried out with flue gas production by coal burning. Furthermore, the breakthrough curves of the adsorbents were investigated where the adsorbent developed via hydrothermal technique with Co3O4 metal oxide termed as HM-Co3O4 /CM displayed higher adsorption capacity for both SO2 (103.9 mg/g) and NOx and (106.6 mg/g). The Langmuir model can best describe the experimental data based on the correlation coefficient and separation factor results. These results implied that the developed adsorbent can be potentially used in the industry for flue gas cleaning. © 2022 Faculty of Engineering, University of Maiduguri, Nigeria. All rights reserved. Submitted 30 August, 2021 Revised 2 December, 2021 Accepted 2 December, 2021 Keywords: Adsorption capacity Fossil fuels Breakthrough curve Monolith adsorbent http://www.azojete.com.ng/ file:///C:/Users/HP/Downloads/AZOJETE%20VOL%2018%20NO%201/silaskiman@unimaid.edu.ng silaskiman@unimaid.edu.ng Arid Zone Journal of Engineering, Technology and Environment, March, 2022; Vol. 18(1):121-134. ISSN 1596-2490; e-ISSN 2545-5818; www.azojete.com.ng Corresponding author’s e-mail address: silaskiman@unimaid.edu.ng 122 The surface chemistry of an adsorbent can affect its adsorption and catalytic abilities (Sousa et al., 2018) while the surface functional groups of the support played a significant role in determining the extent of interaction with the active precursor during adsorbent preparation (Ibraheem et al., 2014). Hence, the introduction of hetero elements such as Co3O4, Ni V2O5, Ce, and CuO on the adsorbents can significantly improve the catalytic activity (Cui et al., 2018; Ibraheem et al., 2014). The objectives of this study are to synthesize monolith adsorbents by deposition precipitation, hydrothermal and wet pore volume impregnation synthesis techniques with various metal oxides, and the breakthrough study of the adsorbents through the combined NOx/SO2 removal from flue gas. The adsorbent with highest adsorption capacity was investigated by FTIR, BET surface area, SEM and EDX analysis, isotherm study, optimization of independent variables in adsorption with RSM software and the regeneration potential of the adsorbent. 2. Materials and Methods 2. 1. Materials The monolith used has a channel of 1.02mm and a wall thickness of 0.25mm. Urea (CH4N2O), nitrate hexahydrates (X(NO3)2.6H2O), and nitric acid (HNO3, 65%), were purchased from Beihai Huijuang Chemical China. The chemicals and reagents used were all of analytical grade. 2.2 Preparation and synthesis of the adsorbent The monolith structure was carbonized in a horizontal furnace (SA-A35014, Malaysia) by heating at a rate of 5 ºC min-1 up to 800 ºC for 4 h and in inert condition following activation in CO2 flow at 350 ºC for 2 h to obtain carbon monolith (CM). The CM is immersed in HNO3 for 24 h next it was filtered and washed with distilled water before being placed in a vacuum oven at 70 ºC and 24 h to dry. 250 mL of deionized water, CM support metal nitrate, and substrate were charged into a reactor vessel of 350 mL capacity while the pH of the solution was made to 3.5 with HNO3 and heated to 90 ºC then a solution of urea was added and further heated for 18 h while stirring at 300 rpm. The substrate was filtered, severally washed, and dried at 120 ºC for 24 h. Finally, it was calcinated in inert at 500 ºC for 4 h using a heating rate of 5 ºC min-1. The adsorbent using the deposition precipitation technique is termed DP/CM. Initially, the metal nitrate hexahydrates precursor and urea (CH4N2O) were dissolved into a homogeneous solution with 80 mL deionized water under and stirred with the CM. the mixture was charged into 100mL Teflon stainless steel. The autoclave was sealed and reacted at 100°C for 5 h, then it was allowed to cool to room temperature. The samples were washed and ultrasonication before drying at 80 ºC for 2 h. The active metal component catalysts with the CM were heated to 300°C for 2 h at a heating rate of 3 °C min-1. The hydrothermally synthesized technique adsorbent is termed HM/CM. An aqueous solution with the metal nitrate precursor was mixed with the CM and heated at 70 ºC with constant stirring (300 rpm) to allow the entire solution to completely evaporate, it was washed and dried at 110 ºC for 24 h. The CM was annealed at 500 ºC for 4 h and a heating rate of 5 ºC min-1. The adsorbent from the pore volume wetness impregnation technique is termed IM/CM. 2.3 Fixed bed adsorptive test In the experiment, 1g of the adsorbent was charged into the fixed bed reactor (Figure 1) and firstly preheated in inert for 1h at 100 °C. Flue gas was produced in a vertical furnace at 850 °C by coal combustion since the industrial flue gas is produced in a power plant furnace at an operating temperature of >800 °C (Li et al., 2008). The generated flue gas was channeled to the adsorbent at 450 mL/min and a file:///C:/user/Downloads/azojete143/www.azojete.com.ng file:///C:/Users/HP/Downloads/AZOJETE%20VOL%2018%20NO%201/silaskiman@unimaid.edu.ng Kiman et al: Performances of Metal Oxides Supported in Monolith for Combined SO2/NOx removal from Flue Gas. AZOJETE, 18(1):121- 134. ISSN 1596-2490; e-ISSN 2545-5818, www.azojete.com.ng Corresponding author’s e-mail address: silaskiman@unimaid.edu.ng 123 temperature of 150 °C. The inlet and outlet concentrations of NOx and SO2 were measured with a T- 350 flue gas analyzer. The adsorbent activity test was evaluated through the adsorption capacity as defined by the weight of NOx and SO2 captured from the flue gas per gram of adsorbent. The experimental run was performed in duplicate and only the average value was used. Figure 1: Schematic of the fixed bed adsorption system 2.4 Adsorption isotherm studies In this work, the equilibrium adsorption data were fitted in Langmuir and Freundlich models. The Langmuir model describes the monolayer adsorption that occurs on the adsorbent surface without interaction among the adsorbed molecules while the Freundlich isotherm is defining multilayer adsorption where the non-uniform distribution of adsorption heat and affinities on heterogeneous adsorbent surfaces occurs (Singh et al., 2018). The adsorption capacity (qe) in mg/g is given by Langmuir isotherm and presented as (Yi et al., 2012): 𝑞𝑒 = 𝑞𝑚 . 𝐾𝐿. 𝐶𝑒 1+𝐾𝐿 . 𝐶𝑒 (1) Where Ce is concentration, qm is the maximum adsorption capacity in mg/g and 𝐾𝐿 is the Langmuir constant in L/g, respectively. The separation factor (RL) indicates Langmuir isotherm suitability (favorable; 0 < RL< 1): 𝑅𝐿 = 1 1+ 𝐾𝐿 . 𝐶𝑜 (2) where Co initial concentration in mg/L. The Freundlich model is (Yi et al., 2012): 𝑞e= 𝑘f𝐶e 1/n (3) where, kf and n are Freundlich constants (intercept and slope) and the heterogeneity factor. The linearized form of Langmuir and Freundlich isotherms are (Sharma et al., 2017): Ce qe = 1 qmKL + Ce qm (4) and logqe= logkf+1/nCe (5) http://www.azojete.com.ng/ file:///C:/Users/HP/Downloads/AZOJETE%20VOL%2018%20NO%201/silaskiman@unimaid.edu.ng Arid Zone Journal of Engineering, Technology and Environment, March, 2022; Vol. 18(1):121-134. ISSN 1596-2490; e-ISSN 2545-5818; www.azojete.com.ng Corresponding author’s e-mail address: silaskiman@unimaid.edu.ng 124 2.4 Adsorbent characterization The N2 adsorption/desorption isotherm was measured with Micrometrics ASAP2020 instrument. Initially, the samples were outgassed at 200 °C for 4 h in the presence of N2. The isotherms were utilized in calculating the specific surface area SBET. The Fourier transform infrared spectroscopy (FTIR) was used to investigate the functional group attached to the surface of the adsorbent. The infrared spectra of synthesized adsorbent were mixed with KBr and the transmittance mode was verified between 4000 cm−1and 400 cm−1 with a resolution of 4 cm−1, using a Thermo NicoletAES0200682. Scanning electron microscope with energy dispersive X-ray micro-analyzer (SEM/EDX) (Hitachi Co., Japan, Model No. S3400N) was used to study the morphology of the adsorbent. 3. Results and Discussion 3.1. Breakthrough curves for various adsorbents The breakthrough curves were plotted from average experimental values while the performance in terms of adsorption capacity for each adsorbate was evaluated. Figure 2(a-d) shows the breakthrough curves for the combined NOx/SO2 removal for the various adsorbents developed. The CeO2-based adsorbents in Figure 2a demonstrate 100% initial NOx/SO2 adsorption while the hydrothermal synthetic method exhibit a longer breakthrough time about both NOx and SO2. file:///C:/user/Downloads/azojete143/www.azojete.com.ng file:///C:/Users/HP/Downloads/AZOJETE%20VOL%2018%20NO%201/silaskiman@unimaid.edu.ng Kiman et al: Performances of Metal Oxides Supported in Monolith for Combined SO2/NOx removal from Flue Gas. AZOJETE, 18(1):121- 134. ISSN 1596-2490; e-ISSN 2545-5818, www.azojete.com.ng Corresponding author’s e-mail address: silaskiman@unimaid.edu.ng 125 http://www.azojete.com.ng/ file:///C:/Users/HP/Downloads/AZOJETE%20VOL%2018%20NO%201/silaskiman@unimaid.edu.ng Arid Zone Journal of Engineering, Technology and Environment, March, 2022; Vol. 18(1):121-134. ISSN 1596-2490; e-ISSN 2545-5818; www.azojete.com.ng Corresponding author’s e-mail address: silaskiman@unimaid.edu.ng 126 Figure 2: Breakthrough curves of (a) CeO2-based (b) CuO-based (c) V2O5-based and (d) Co3O4-based adsorbents. However, the breakthrough and the saturation times were longer for SO2 over NOx and the CeO2 plays an important role in the NOx/SO2 removal. The presence of oxygen surface groups might have increased the tb because CeO2 possessed oxidizing property and this fact may indirectly lower the conversion of NOx (Sumathi et al., 2010a). The hierarchy of breakthrough time displayed by the CuO-based adsorbent follows as: HM- CeO2/SO2>HM-CeO2/NOx>DP-CeO2/NOx>IM-CeO2/SO2>IM-CeO2/NOx>DP-CeO2/SO2. Copper based catalysts are known to have high reactivity for SO2 removal and high catalytic activity for NOx reduction in its oxide form (Hu et al., 2001). It is assumed that the simultaneous SO2/NOx removal occurred for these reasons (Figure 2b). Meanwhile, pores may be blocked which leads to reduced adsorbent performance depending on the synthesis method and compatibility with the metal oxide catalyst. The presence of SO2 in the flue gas reduces the adsorbent performance and longevity Garci-Bordeje et al., (2004) but from Figure 1c, V2O5 has shown high SO2 uptake at stack temperatures and the overall performance of the V2O5 based adsorbent showed higher SO2 adsorption compared to NOx. The role of V2O5 in the higher SO2 removal consists of four steps as follows: the adsorption of SO2 at sites adjacent to V2O5, the interaction of SO2 with V2O5 to form an intermediate with a structure like VOSO4, the reactions of O2 with the VOSO4-like species to yield V2O5 and SO3 and finally, the migration of SO3 or H2SO4 formed between SO3 and H2O to the adjacent ACM pores. The former authors reported similar findings (Ma et al., 2008). Tricobalt tetraoxide (Co3O4) has attracted lots of concern because it is cost-effective, has environmental benignity and has good catalytic activity at low temperatures (Mo et al., 2018). The noticeable difference between the HM, DP, and IM methods of synthesis with the Co3O4 is shown by the Co3O4-based adsorbents of Figure 2d. The HM method demonstrates outstanding performance in the combined file:///C:/user/Downloads/azojete143/www.azojete.com.ng file:///C:/Users/HP/Downloads/AZOJETE%20VOL%2018%20NO%201/silaskiman@unimaid.edu.ng Kiman et al: Performances of Metal Oxides Supported in Monolith for Combined SO2/NOx removal from Flue Gas. AZOJETE, 18(1):121- 134. ISSN 1596-2490; e-ISSN 2545-5818, www.azojete.com.ng Corresponding author’s e-mail address: silaskiman@unimaid.edu.ng 127 NOx/SO2 removal. This performance suggested that there is uniform dispersion of the Co3O4 on the CM. Further, it implies that the hydrothermal treatment and the monolith support used with respect to Co3O4 catalyst have a great effect on the adsorption performance. The HM synthesis method also influences the catalytic activity according to previous authors (Radwan et al., 2007). Initially, there was no 100% SO2 removal by the IM/CM adsorbent in all of Figure 2, similar findings are reported in previous works (Ghorai and Pant, 2005; Zhou et al., 2012). The highest performance is shown by the HM/CM adsorbent regarding q, tb and tsat when compared to the Co3O4-based adsorbents. Figure 3a illustrates the adsorption capacity of the adsorbents and Figure 3b shows the breakthrough and saturation times of the adsorbents. It can be observed that the highest point is at 130.2 mg/g for q-NOx and for the HM-Co3O4/CM adsorbent while the lowest point is shown at 3.1 mg/g for q-SO2 and for HM- V2O5/CM adsorbent. Figure 3: (a) Adsorption capacity measurement (b) Breakthrough and saturation times of the adsorbents. The demonstration of the HM-Co3O4/CM adsorbent to perform in practical application beyond the other adsorbents is because of the compatibility of the adsorbent surface with the Co3O4 metal oxide. Furthermore, the result shows how well the hydrothermal synthesis disperses the Co3O4 on the support. Meanwhile, the adsorption capacity can be seen to be much favorable towards NOx adsorbate. The adsorbent’s demonstration of high NOx adsorption affinity is majorly due to the NOx concentration in the flue gas (Sumathi et al., 2010b; Galvez et al., 2005), the preparation route (Lazaro et al., 2008; Hosseini et al., 2016; Kreutzer et al., 2001), temperature (Sumathi et al., 2010a; Zhao et al., 2011), the oxygen http://www.azojete.com.ng/ file:///C:/Users/HP/Downloads/AZOJETE%20VOL%2018%20NO%201/silaskiman@unimaid.edu.ng Arid Zone Journal of Engineering, Technology and Environment, March, 2022; Vol. 18(1):121-134. ISSN 1596-2490; e-ISSN 2545-5818; www.azojete.com.ng Corresponding author’s e-mail address: silaskiman@unimaid.edu.ng 128 present (Zhang et al., 2006; Liu and Shih, 2006) and relative humidity (Sumathi et al., 2010b; Dahlan et al., 2009) inactivity test. Figure 3b displayed the time taken for the breakthrough to happen and the saturation time for each adsorbate respectively. The HM-Co3O4/CM adsorbent is picked for further studies including characterization, isotherm and RSM optimization. 3.2 HM- Co3O4/CM surface area and functional groups It is revealed in Figure 4a that the adsorbent is of IV isotherm type based on IUPAC classification, suggesting the dominance of mesoporous in the sample. A similar sample of carbon-coated monolith was reported previously (Hosseini et al., 2015). From the BET analysis of the surface area, the surface area is 47.9 m2/g. Figure 4: The (a) N2 adsorption-desorption plot (b) FTIR spectrum of HM-Co3O4/CM adsorbent. The FTIR spectrum in Figure 4b showed several bands belonging to the function groups that are attached on the adsorbent’s surface. At 2627 cm-1 there is band ascribed to O-H of carboxylic acid while 2335- 2361cm−1 is ascribed to C-N triple bond. Also, at 1741 cm-1 the C=O of ester stretch was observed, 1370 cm-1 (C-O of a phenolic stretch). Also, assigned to C-O, C-H, CH=CH2 stretches are the absorption bands at 1175-440 cm-1 (Raghunath and Mondal, 2017; Malkoc et al., 2006). The absorption peaks at 675cm−1-576cm−1 are ascribed to Si–O stretching bands and indicate the presence of Co3O4 (Dahlan et al., 2009; Li et al., 2011; Teoh et al., 2013). file:///C:/user/Downloads/azojete143/www.azojete.com.ng file:///C:/Users/HP/Downloads/AZOJETE%20VOL%2018%20NO%201/silaskiman@unimaid.edu.ng Kiman et al: Performances of Metal Oxides Supported in Monolith for Combined SO2/NOx removal from Flue Gas. AZOJETE, 18(1):121- 134. ISSN 1596-2490; e-ISSN 2545-5818, www.azojete.com.ng Corresponding author’s e-mail address: silaskiman@unimaid.edu.ng 129 3.3 HM- Co3O4/CM morphology results The surface morphology and elements of the HM-Co3O4/CM adsorbent are presented in Figure 5. Figure 5: (a) SEM microphotographs (b) Elemental result and (c) EDX peaks In Figure 5a, the surface structures by SEM illustrates the uniform dispersion of the Co3O4 catalyst on the CM surface area in a rich spider-like web, similar microphotograph was previously presented (Sumathi et al., 2009). Furthermore, the cordierite has a surface with many large pores which are available for the adsorption to take place. The EDX spectra in Figure 5b showed the presence of Co3O4 (1.1%) in the http://www.azojete.com.ng/ file:///C:/Users/HP/Downloads/AZOJETE%20VOL%2018%20NO%201/silaskiman@unimaid.edu.ng Arid Zone Journal of Engineering, Technology and Environment, March, 2022; Vol. 18(1):121-134. ISSN 1596-2490; e-ISSN 2545-5818; www.azojete.com.ng Corresponding author’s e-mail address: silaskiman@unimaid.edu.ng 130 impregnated CM by EDX techniques. The bare cordierite is made up of Si, Al, O and MgO however, the carbonization process is responsible for the presence of carbon of up to 41.4% by weight which results in the improvement of highly porous surface (Hosseini et al., 2011). 3.4 The isotherms From Figures 6 a-d, the higher correlation coefficient (r2) was obtained from the Langmuir model which suggested that the experimental data can be best described by the Langmuir model when compared to the Freundlich model which displayed the r2 of 0.6682 and 0.8734 for SO2 and NOx respectively. Figure 6: (a) Langmuir isotherm for SO2 (b) Langmuir isotherm for NOx (c) Freundlich isotherm for SO2 and (d) Freundlich isotherm for NOx. The r2 and the parameters for Langmuir and Freundlich models are presented in Table 1. Moreover, the RL value further indicates the suitability of the Langmuir model (0.3377 and 0.0973 for SO2 and NOx). The range of favorable adsorption based on the Freundlich model is within the range 1 < n < 10 (Hosseini et al., 2011). The values for heterogeneity factor for both SO2 and NOx are well below the range of favorable adsorption as shown in Table 1. file:///C:/user/Downloads/azojete143/www.azojete.com.ng file:///C:/Users/HP/Downloads/AZOJETE%20VOL%2018%20NO%201/silaskiman@unimaid.edu.ng Kiman et al: Performances of Metal Oxides Supported in Monolith for Combined SO2/NOx removal from Flue Gas. AZOJETE, 18(1):121- 134. ISSN 1596-2490; e-ISSN 2545-5818, www.azojete.com.ng Corresponding author’s e-mail address: silaskiman@unimaid.edu.ng 131 Table 1. The Langmuir and Freundlich parameters results. q (mg/g) SO2/NOx Langmuir parameter SO2 NOx Freundlich parameter SO2 NOx 121.1 and 130.2 qm kL r2 20.0 0.344 0.9509 20.4 3.45 0.9300 kf n r2 1.26 0.7936 0.6682 -1.42 0.7042 0.8734 4. Conclusion The adsorption capacity of various adsorbents that were impregnated with metal oxides and using three different synthesis methods was studied. By coal combustion in a vertical furnace, flue gas was produced to simulate the generation of flue gas from power plants and the activity test was performed. Meanwhile, Meanwhile, the breakthrough curve of combined NOx/SO2 was reported. Based on the regression coefficient and the favorable separation factor, the Langmuir model can best describe the experimental data. According to characterization results of the adsorbent with the highest adsorption capacity (HM- Co3O4/CM), there is an available active site for the adsorption to occur, the presence of Co3O4 was shown. References Abdulrasheed, AA., Jalil, AA., Triwahyono, S., Zaini, MAA., Gambo, Y. and Ibrahim, M. 2018. Surface Modification of Activated Carbon for Adsorption of SO2 and NOx: A Review of Existing and Emerging Technologies. 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