Studies on CO2 Adsorption and Desorption Properties from Various

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Studies on CO Adsorption and Desorption Properties from Various Type Iron Oxides (FeO, FeO and FeO) 2

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Azizul Hakim, Tengku Sharifah Marliza, Najiha Maratun Abu Tahari, Wan Nor Roslam Wan Isahak, Muhammad Rahimi Yusop, Mohamad Wahab Mohamad Hisham, and Ambar Mohd. Yarmo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04091 • Publication Date (Web): 16 Mar 2016 Downloaded from http://pubs.acs.org on March 17, 2016

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Industrial & Engineering Chemistry Research

Studies on CO2 Adsorption and Desorption Properties from Various Type Iron Oxides (FeO, Fe2O3 and Fe3O4) Azizul Hakim*,Tengku S. Marliza*§, Najiha M. Abu Tahari*, Roslam W. N. Wan Isahak†, Rahimi M. Yusop*, Wahab M. Mohamed Hisham*, Ambar M. Yarmo* *Catalysis Research Group, School of Chemical Sciences and Food Technology, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM, Bangi, Selangor, Malaysia. §

Catalysis Science and Technology Research Centre, Faculty of Science, Universiti Putra

Malaysia, 43400 UPM Serdang, Selangor, Malaysia. †

Department of Chemical and Process Engineering, Faculty of Engineering and Built

Environment, Universiti Kebangsaan Malaysia, 43600 UKM, Bangi, Selangor, Malaysia.

ABSTRACT. Various type iron oxides of FeO, Fe2O3 and Fe3O4 were used for carbon dioxide (CO2) capture at room temperature and pressure by studying its adsorption-desorption properties. Several interactions of carbonate species were detected on its surface. The morphology of carbonate formation shows different structures on FeO (grooves-like), Fe2O3 (fine sharp

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particles) and Fe3O4 (aggregated nano particles). CO2 chemisorption discovered potential adsorbent of Fe2O3 with adsorption capacity of 3.95 mgCO2/gadsorbent. The adsorption capacity increased up to 62.8 % by using concentrated 99.9 % CO2 for adsorption. At higher concentration of CO2 exposure, it partially turns to red color which indicated less stability of Fe3O4 was easily oxidized to Fe2O3 after CO2 regenerated with thermal up to 500 °C. Hence, Fe2O3 possessed highest basicity strength (1.26 cm3/g) with adsorption capacity after four cycles was not significantly reduced by 8.6 % indicating effectiveness in chemical nor physical adsorption in CO2 capturing.

KEYWORDS. Iron oxides, adsorbent, CO2 capture, desorption, basicity 1.0 Introduction The main concern worldwide recently is renewable energy which could generate for the rest of decades without worrying the depletion of sources in the earth. Human population and energy consumption are directly proportional with addition of technology provided and yet growing explosively. This progressive situation made human realized with considerable attentions were paid to green technology by applying anthropocentric environmental ethics which is people centered as approach, to reduces environmental pollutions, saving the earth and future generation.1 Rapid development growth resulting harmful air pollutants releases into atmosphere which become major contribution in global warming and several issues on urban smog, acid rain, health problems contributed from emission of anthropogenic CO2 gas.2 The CO2 in atmosphere, one of the major greenhouse gas, is now higher than the safety limit for atmospheric CO2 level of 350 ppm. According to National Oceanic and Atmospheric Administration (NOAA) 3 in U.S, the CO2


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level was higher than safety limit since 80’s and to date on July 2015 was approximate 401.30 ppm. Furthermore, the CO2 growth rate increased recently approximate 2.0 ppm/year give rises of mitigating measure by capture, sequestrate and utilize it.3 CO2 capture has draw attention in research studies during middle of 2000’s and studies were urged by the industrial since abundances of CO2 produced as well as mitigating measure purpose.

Single metal oxide has been used as adsorbent in CO2 adsorption for ages and

preliminary studies were done prior to modification its properties, hence enhanced the adsorption capacity. For instance, a numbers of classical studies of CaO in CO2 adsorption with different scope includes kinetic4, regeneration by desorption with thermal exposure5-6, adsorption capacity7 and others. Instead of alkali metal and alkali earth metal oxides, an alternative metal oxides such as lanthanum sesquioxide (La2O3)8, silver oxide (AgO)9, titanium dioxide (TiO2)10, copper oxides (CuO and Cu2O)11, nickel oxide (NiO)12, iron oxide (Fe2O3)13 and cerium oxide (CeO2)14 were studied its capability in CO2 adsorption.8-14 Iron oxide’s contribution in catalytic activity of CO2 adsorption-desorption still lack of supportive evidence. Vibrational spectroscopy study on iron oxide was among earliest research that discovered carbonate interaction on the surface. Progressively, several carbonate species monodentate carbonate, bidentate carbonate, bicarbonate and carboxylate were discovered by the interaction on the iron oxide surface. The reason of cheap catalyst, easy to synthesize and abundance of source should not under estimate its potential in CO2 capture. For further understanding the activity of various type iron oxides toward its CO2 adsorption trends, this research focused on infrared spectroscopy of carbonate adsorbed, while it’s morphological carbonate formation were observed under field emission scanning electron microscope (FESEM). The chemically adsorbed carbonate on iron oxides surface were measured by


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temperature programmed desorption using CO2 (CO2-TPD) analysis. Adsorption capacity for recyclability of adsorption-desorption process were done with textural properties for each cycle. 2.0 Experiment 2.1 Sources of iron oxides adsorbents and gases Commercially available iron oxides used in this experiment were purchased from several sources. The iron oxide adsorbent of Fe2O3 was purchased from BDH whereas FeO, and Fe3O4 were obtained from Sigma Aldrich. These iron oxides were used without any treatment except Fe2O3 was heated to 150 °C to remove moisture content and humidity gas.

99.9 % CO2

(minimum level) tank was supplied by Mox-Linde Gases Sdn Bhd, Malaysia with 20 ppm of moisture (maximum level) and 20 ppm of hydrocarbon such as methane (maximum level). The mixture gas of 5% CO2 in He (v/v) and 5% CO in He (v/v) were purchased from Southern Industrial Gas Sdn Bhd, Malaysia. 2.2 Nitrogen adsorption-desorption and carbon dioxide adsorption at 25 °C N2 adsorption–desorption isotherms of FeO, Fe2O3 and Fe3O4 were measured on a static volumetric technique instrument (gas sorption analyzer, Micromeritics ASAP 2020) for determination of the surface area, total pore volume and the average pore diameter. Prior to measurement, approximate 50 mg of the iron oxides were degassed at 200 °C for 6 hours under vacuum in order to eliminate humidity and trapped gasses. The N2 adsorption–desorption data were recorded at liquid nitrogen temperature, 77 K and applied in a relative pressure (P/Pο) range from 0 to 1.0. The surface areas (SBET) were calculated using the BET equation by Brunauer, Emmett and Teller, which is the most widely used model for determining the specific surface area. All surface area measurements were calculated from the nitrogen adsorption


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isotherms by assuming the area of the nitrogen molecule to be 0.162 nm2. The total pore volume (Vtot) was assessed by converting the amount of N2 gas adsorbed (expressed in cm3/g at STP) at relative pressure to the volume of liquid adsorbate. Multilayer CO2 adsorbed by physisorption at 25 °C and room pressure was measured using above mentioned instrument with step of degassed prior to analysis. The CO2 analysis was conducted and water circulating bath was used to control the temperatures. 2.3 IR studies The infrared (IR) spectra of various iron oxides were recorded between 4000 and 400 cm -1 wave number using FTIR Perkin Elmer, model GX with KBR pellets method for sample preparation technique. In order to study the formation of carbonate on iron oxides surface, approximate 3 g of iron oxides were exposed with 99.9 % CO2 for 24 hours. The fresh iron oxides and after exposed were examined using FTIR spectroscopy. It is important to remove moisture content and humidity gas before exposed with any gases to have better and accurate adsorption. Therefore, the iron oxides were purged with N2 and heated at 150 °C prior to CO2 exposure at 25 °C and room pressure.

2.4 Surface morphology using FESEM The fresh iron oxides and after exposed were observed the differences growth of carbonate on adsorbent’s surface. Approximate 3.0 g of fresh iron oxides were cleaned by purged with N2 at 150 °C for 30 minutes to remove moisture content prior to expose with 99.9 % CO2 for 4, 24 and 48 hours at 25 °C and room pressure. The fresh iron oxides used for analyze were cleaned too with same conditions. The carbonate formation on these iron oxides surface were observed by examined the fresh iron oxides before and after exposed with CO2 using field emission scanning


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electron microscope (FESEM) ZEISS Supra VP55. This instrument has a common tungsten electron gun with operational volt is ranging of 0.1–3.0 kV. The surface micrograph of adsorbents with formation of carbonate was studied by observed the differences of its morphology. 2.5 Basicity properties by CO-TPD analysis The surface basicity/base strength distributions of iron oxides were determined by the CO-TPD in a chemisorption analyzer model Micrometrics 2920 Chemisorb. The measurement was carried out in a quartz tube using helium (He) as a carrier gas. The samples (approximate 50 mg) were initially purged with flow of He at temperature 150 °C for 30 minutes to remove traces moisture and humidity gases. The chemisorption of CO on the adsorbents surface was carried out at 30 °C by pulse chemisorption method where He as carrier gas flow continuously with 20 times pulse of mixture gas of 5 % CO in He (v/v). The excess weakly physisorption of CO desorbed at 50 °C in a flow of He (30 ml/min) for 30 minutes. After this step, TPD was carried out. The investigated sample was heated up to 900 °C at 10 °C/min to observe the distribution of CO adsorbed on the surface of sample wherein effluent gas stream of desorbed CO was monitored quantitatively by thermal conductivity detector (TCD). The CO2 adsorption capacity was evaluated by CO2-TPD analysis using steps as mentioned above except CO2 chemisorption was carried out by saturated flow with two different concentration CO2 gas of mixture gas 5 % CO2 in He and 99.9 % CO2 as probe molecule. 2.6 CO2 regeneration process and recyclability Recyclability test carried out by using fluedized-bed reactor with glass column contained 4 gram of adsorbent as shown in Figure 1. The selected adsorbent was pre-treated with N2 atmosphere at 150° C for 30 minutes. The adsorption process performed at 30 °C by using synthetic flue gas


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mixture atmosphere containing 15 % CO2 and 85 % N2 flowing at 100 ml/min for 1 hour. The N2 was again purged to removed excess mixture gas in the column. Regeneration process performed by desorbed up to 650 °C at 20 °C/min with N2 atmosphere and the gas analyzed with gas chromatography that using TCD. Figure 1. Schematic diagram of fluidized-bed reactor. 3.0 Results and discussion 3.1 Physical properties of iron oxides N2 adsorption-desorption isotherms for all iron oxides presented in Figure 2 exhibited type III profile according to IUPAC isotherms of classification with typical sloping adsorption and from the H3 type desorption branches covering a large range of relative pressure (P/Po) where the adsorbent-adsorbate interaction is weak, but could obtained with certain porous adsorbents.15 It was proven hysteresis based on the isotherms ascribed from plate-like particles giving rise to slitshaped pores from the adsorbent with large ranges of pore diameters which does not exhibit any limiting adsorption at high P/Po is observed.16 The specific BET surface area, average pore diameter and total pore volume calculated from a N2 adsorption-desorption isotherm using the BJH model, were compared for the textural properties of various iron oxides in Table 1. The BET surface area of Fe2O3 was highest (6.60 m2/g) among all samples with its total pore volume of 0.03 cm3/g. Average pore diameter for all iron oxides were considered in mesopore range of 2-50 nm except FeO shows macropore size of 62.5 nm. Larger pore diameter of FeO contributed to lowest surface area (0.02 m2/g) which indicative of poor in gas adsorption according to N2 adsorption-desorption isotherms in Figure 2. These features derived an estimation of pores image for iron oxides as shown in the inset Figure 2.


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Figure 2. N2 adsorption-desorption isotherms of FeO, Fe2O3 and Fe3O4 with the inset indicating estimated pores image according to the isotherms. Table 1 Textural parameters of iron oxides on the basis of N2 adsorption-desorption and CO2 adsorption isotherms. 3.2 Carbonate formation on iron oxides surface using IR spectroscopy The nature of CO2 adsorption products on oxide surfaces was long been investigated using infrared spectroscopy.17 As the vibrational spectra reflects both the properties of the whole molecule and the separations of chemical bonds characteristic, IR spectroscopy offers the fullest possible information on the perturbation experienced by a molecule on contact with the solid surface, and often determines the structure of adsorption complexes of surface compounds. Previous studies reported that KBr pellets method was used for FTIR to determine the formation of two major surface complexes between hematite and carbonate ions.17 Transmission of IR spectra for various type iron oxides were described in Figure 3, 4 and 5. These spectra represents before and after CO2 adsorption wherein fresh iron oxides were cleaned by purging with N2 and heated at 150 °C prior to adsorption. All of the absorption bands were tabulated in the Table 2. Sorption of carbonate ions on iron oxides was found to be relatively weak, since it is strongly influenced by ionic strength and type of electrolyte anion.18 However, its effect on the electrophoretic mobility is well known.19 Several species adsorbed on FeO, Fe2O3, and Fe3O4 surface in the presence of gas-phase CO2 have been identified including linear and bend CO2, bicarbonate, and varies form of carbonate species. The spectral regions between 1000 to 1700cm−1 were assignment for carbonate species. CO2 adsorption on FeO surface at 25 °C and 1 atm found that it may forms several types of carbonate (Figure 3). The position of the bands at 1716, 1475, 1218 cm−1 were indicative


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structures of bicarbonate for vibrational modes of asymmetry O-C-O, symmetry O-C-O and COH stretching.19-24 The assignment of O-H vibrational mode for bicarbonate for FeO was shown at 3639 cm−1. The presence of adsorbed bicarbonate can be unambiguously identified by the presence this peak due to this OH stretch.21 Another peaks observed at 1043 and 1394, cm−1 were associated with C-O stretching and symmetry O-C-O vibrational modes of assignment for monodentate carbonate species.19-24 Absorbed bidentate carbonate has results of C-O stretching and symmetry O-C-O vibrational modes which can be seen at absorption bands 1013 and 1245 cm−1. Hydroxy functional group information can also be obtained from Figure 3. Typical signal for O-H stretch with broad adsorption was located at 3248 cm-1. However, another broad absorption band at 3382 cm−1 ascribed to the increasing hydrogen bonding between the CO2 adsorbed products and hydroxy groups on FeO surface.21,28 This shows the complexity of surface product interactions where adsorbed hydroxy groups are not only involved in the formation of surface bicarbonate, but also in proton donating interactions via hydrogen bond induced proton delocalization. Several peaks at absorptions region of 2500-3200 cm-1 were mostly consist of chelated hydroxy band.18 CO2 stretching with presence of interfering gas H2O which contain in the tank, was assigned for absorption band at 2232 cm-1.19,20 An absorption band appeared at 2423 cm-1 could be assigned to linearly adsorbed CO2 species, while bending CO2 species was not observed.27,32,33 Absorption bands associated in the region below 1000 cm-1, obscured by some metal oxides. These features do not seem compatible with carbonate-like species from the spectroscopic point of view.25 Also their behaviour does not seem compatible with this assignment. During CO2 adsorption, the source of gas contained traces amount (20 ppm) of moisture that led to hydroxide compound formation on metal oxide surface. Thus, evidences presents of Fe-O-H deformation were measured at 743 and 918 cm−1.17,34 The broad signal for


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hydroxyl group that appeared before adsorption at 3246 cm−1 became distinct after adsorption due to additive of traces amount moisture (20 ppm) from feeding gas of CO2. After CO2 adsorption, other evidence that shown before adsorption for Fe-O-Fe vibrational modes at 469 and 567 cm−1 were disappeared indicative of high intensity of carbonate compound formed.35 Figure 3. IR spectra of FeO where a) CO2 24 hours and b) before adsorption. CO2 adsorption at 25 °C and 1 atm for 24 hours on Fe2O3 surface shows none of monodentate and bidentate carbonate are forms since both spectra of before and after CO2 adsorption were not much different at spectra region 1000-2200 cm−1 in Figure 4. However, it was noticed that the absorption due to the O-H stretch at 3627 cm−1 was significant evidence for the presence of adsorbed bicarbonate.21 The contribution of the bands at 1635 and 1496 cm−1 were indicative structures of bicarbonate for asymmetry O-C-O and symmetry O-C-O vibrational modes.20-24 The absorptions region of 2500-3200 cm-1 were associated chelated hydroxy due to interference by the H2O during CO2 adsorption.29 Typical signal for O-H stretch with broad absorption at 3285 cm-1 became distinct after CO2 adsorption compared to before adsorption (3403 cm−1) due to moisture content (20 ppm) from feeding gas. Broad absorption at 3337 cm−1 ascribed from hydrogen bonding between the CO2 adsorption products and hydroxy groups.21,28 CO2 stretching with presence of interfering gas H2O which contained in the tank, was assigned for absorption at 2169 cm-1.30,31 It was noticed that bending CO2 from functional group of C=O that contributed to 656 cm-1.27,31 Whereas linear CO2 species was assigned to 2425 cm-1.27,32,33 Traces amount on moisture in CO2 tank lead to hydroxide compound formation on metal oxide surface with the presence of of Fe-O-H deformation at 796 and 930 cm−1.17,34 Lower regions of Fe-O-Fe vibrational modes at 451 and 559 cm−1 before CO2 adsorption were vanished after the exposure due to intensity of oxide reduced by forming carbonate compound.35


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Figure 4. IR spectra of Fe2O3 where a) CO2 24 hours and b) before adsorption. The IR spectra for Fe3O4 and after CO2 adsorption described significant difference where carbonate adsorbed easily at 25 °C and 1 atm (Figure 5). The contribution of bicarbonate which associated with symmetry O-C-O was at 1470 cm−1.20-24 Meanwhile, presence of adsorbed bicarbonate was identified by the absorption at 3636 cm−1 that assigned for O-H vibrational mode.21 Carbonate that adsorbed in the form of monodentate carbonate with the features at 1394, and 1453 cm−1 were associated with symmetry O-C-O and asymmetry O-C-O of vibrational modes respectively.19-24 The weak contribution around 1586 cm−1 agree with the formation of surface bidentate carbonate species for asymmetry (O-C-O) stretching.20-23 Whereas symmetry O-C-O and C-O stretching vibrational modes can be seen at absorption bands 1243 and 1127 cm−1 respectively.20-23 Since the CO2 tank contained traces amount of moisture (20 ppm), hence high possibilities of interference by the H2O that appeared at absorption region of 2500-3200 cm1

which were mostly chelated hydroxy band.29 Typical signal for O-H stretch with broad

adsorption was at 3084 cm-1. The strong absorption band for hydroxyls group present before adsorption at 3374 cm−1 became sharp (3084 cm−1) due to exposed with moisture content from feeding gas during CO2 adsorption. Another contribution of broad absorption band at 3336 cm−1 indicative of the hydrogen bonding between the CO2 adsorption products and hydroxy groups.21,28 This explained the complexity of surface product interactions where adsorbed hydroxy groups are not only involved in the formation of surface bicarbonate, but also in proton donating interactions via hydrogen bond induced proton delocalization. Interference of H2O during CO2 adsorption (approximate 20 ppm) resulting CO2 form hydrogen bond, and the CO2 stretching shows peak at 2171 cm-1. Another feature at 2449 cm-1 was attributed to linear CO2 species adsorbed.27,32,33 Moreover, two O-C-O deformation modes are also expected at lower


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frequencies. Accordingly, spectra similar to ours have been recorded for CO2, adducts with metal centers, where bent structures are well established.26 Functional group of C=O from bending CO2 was observed at 624 cm-1.27,31 Absorption bands associated in the region below 1000 cm-1 was expected for inorganic interactions. Traces amount of moisture (20 ppm) that contained in the CO2 tank led to hydroxide compound formation on metal oxide surface. Thus, absorption bands at 754 and 912 cm−1 were evidences presents of Fe-O-H deformation.17,34 The assignment of FeO-Fe vibrational modes at 560 cm−1 for Fe3O4 before CO2 adsorption diminished after the adsorption35, as well as other bands (416, 605, 690, and 768 cm−1) that agree with reference correlated with formation of carbonate compound.36 Figure 5. IR spectra of Fe3O4 where a) CO2 24 hours and b) before adsorption. Table 2. Vibrational frequencies IR spectroscopy of iron oxides adsorbed product by 24 hours CO2 exposure. From these observations, strong evidences for various type of FeO, Fe2O3, and Fe3O4 consisted similar to the hematite/carbonate complexes reported by Bargar et al.20 It shares almost same regions of carbonate formation on its surface. Furthermore, surface hydroxyl groups formed easily that interacted with CO2 to generate bicarbonate species. Traces amount of moisture content approximately 20 ppm from feeding gas CO2 has a potential as precursor and enhanced to form more surface hydroxyl groups on the ready presence of hydroxyl before adsorption. In order to promote formation of bicarbonate species, hydroxyl groups can be increased by treating iron oxides with water vapour.37, 38 However, adsorption at room temperature and pressure shows no adsorption bands for monodentate and bidentate carbonate on Fe2O3. These carbonate species forms by interaction between surface oxygen of Fe atom and CO2 to produce monodentate carbonate and bidentate


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carbonate. In this regard, thermal supply or pressure should be considered to activate the iron oxides which sufficient energy to form carbonate bonding. 3.3 Basicity strength using CO-TPD analysis While absorption band assignments for CO2 adsorbed as different species of carbonate have been explored, the adsorption capacity deserves additional attention. In order to study the CO2 adsorption-desorption, the properties of its surface basicity strength/distributions was identified. The application of CO as probe molecule provides both the concentration and the basic strength of adsorbents on metal oxide and other systems. The adsorbent that having a broad signal of distribution attributed to the adsorbate adsorbed on weaker sites is desorbed at lower temperatures and that adsorbed on stronger sites is desorbed at higher temperature. Hence, the strength of a particular group of basic sites can be expressed in terms of the temperature interval in which the CO chemisorbed on the basic sites is desorbed. The size and proportions of the peak areas varied depends on the catalysts and adsorbents.39-41 From the basicity distribution curves of CO-TPD analysis for various type iron oxides (Figure 6), the temperature intervals was next estimated for the weak, intermediate, strong and very strong sites. The temperature range were assigned to 45 - 250 °C, 251 - 480 °C, 481 - 700 °C and 701 - 900 °C wherein the basic sites were weak, intermediate, strong and very strong respectively. The data showing quantitative distribution of basic sites for different strengths with maximum desorption temperatures are included in Tables 3. Among all iron oxides, Fe2O3 exhibited all of the basic sites with strong basic site adsorbed highest (0.43 cm3/g). Whereas the CO-TPD curves for FeO and Fe3O4, both appeared peaks only at weak basic (0.20 cm3/g) and strong basic (0.23 cm3/g) sites respectively. The total basicity of Fe2O3 is highest than those other samples (Table 3). These results provide


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extensive prediction of Fe2O3 has potentially in capturing CO2 the most since it has high basic active sites on its surface to attract CO2, hence a chemical interaction occurring with bonding. Figure 6. Basicity distribution by CO-TPD analysis on various iron oxides. Table 3. The basicity distribution and amount of weak, medium, strong and very strong basic sites. 3.4 Morphology of carbonate formation on iron oxides surface The surface morphology of the adsorbents performed by FESEM provides advance view of its textural structure. The fresh adsorbents before and after CO2 exposure for 4, 24 and 48 hours were observed the differences of carbonate growth on adsorbent’s surface. It was noted that various type iron oxides having different shapes and structures nor indicative of carbonates presence were totally different. The BET surface area measured on iron oxides was confirmed by the morphological differences observed under SEM. The rhombohedral lattice structure for Fe2O3 (JCPDS file 33-0664) from XRD was convinced by SEM images which described as nano coral reefs like texture42 (Figure 7 a (ii)) that exhibited highest BET surface area among iron oxides. Fine sharp particles aggregated on the Fe2O3 began to appear after 24 hours of CO2 adsorption, and even more after 48 hours (Figure 7 d (ii)). Fresh Fe3O4 have clear cubical structure (JCPDS file 79-0416) as shown in Figure 7 a (iii). Nano particles aggregated on these cubical structures were indicative of carbonates formation that started to be seen after exposed with CO2 for 24 hours (Figure 7 c (iii)). Meanwhile, the cubic lattice of fresh FeO (JCPDS file 89-0686) from XRD was not seen since it was agglomerated which support the result of low in BET surface area. The rough surface on honeycomb structure of fresh FeO was diminished which became smooth and grooves-like structure appeared after 4 hours of CO2 exposure (Figure


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7 b (i)). The grooves-like structure were even more and clearer for longer CO2 exposure (Figure 7 d (i)). This alteration was ascribed from chemical interaction forms several carbonate species. Figure 7. SEM images for a) fresh iron oxides, b) Purged with CO2 for 4 hours c) Purged with CO2 for 24 hours and d) Purged with CO2 for 48 hours where (i) FeO (ii) Fe2O3 and (iii) Fe3O4. 3.5 CO2 adsorption-desorption studies Physically adsorbed CO2 at 25 °C and 1 atm was measured using isotherm adsorption technique. The main feature in physical adsorption is adsorption layer by layer, from the surface of adsorbent until it forms several layers covering the adsorbent structure as shown in the inset Figure 8. The CO2 adsorption isotherms at 25 °C and 1 atm were described in Figure 8. The physical adsorption behaviour is mainly contributed from its physical and morphology. Therefore, Fe2O3 with highest surface area recorded largest adsorption (3.01 mgCO2/gadsorbent) among all iron oxides, whereas FeO and Fe3O4 adsorbed 1.36 and 1.23 mgCO2/gadsorbent respectively (Table 1). These results are in total agreement with BET surface area shows different pore structures that confirmed by SEM images provides the space that can be occupied by physical adsorption of CO2. The CO2 adsorption isotherms were correlated with pore structure condition whereby less steep initial region isotherm for FeO due to big and shallow slit-shaped pores. Hence, CO2 easily attracted forming monolayer adsorption compared to Fe3O4 which exhibited deeper pores. As for Fe2O3, the steep initial region isotherm indicated more CO2 required to covering on it deepest pore structures that confirmed by pore volume (Table 1) and SEM images (Figure 7 a (ii)). Consequently, modification of the Fe2O3 may possibly enhance the physisorption which nano particles with high porosity and surface area properties desirable for physical adsorption.


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Figure 8. CO2 adsorption isotherms at 25 °C and 1 atm using isotherm adsorption technique with multilayer of CO2 adsorbed on iron oxides surface diagram in the inset. The CO2 adsorption-desorption studies performed using CO2-TPD analysis that provides desorption curves of which CO2 chemically adsorbed and interact with adsorbents surface. The amounts of desorbed CO2 that give peaks during thermal exposure under N2 flow were recorded in cm3/g and mgCO2/gadsorbent (Table 4). FeO shows similar amount of adsorption capacity for both 5 % and 99.9 % CO2 (Figure 9) at 0.22 mgCO2/gadsorbent with lowest desorption temperature among iron oxides. Adsorption of Fe3O4 was increased (14.6 mgCO2/gadsorbent) by using 99.9 % CO2 with maximum temperature desorption (MaxDT) at 444 °C. Both FeO and Fe3O4 shows weak desorption peaks ascribed to the low basic sites which in the agreement from the CO-TPD results. Moreover, the low surface area for both FeO and Fe3O4 provides less active site for CO2 to be trapped, hence reduces the capability in adsorption. However, adsorption by higher concentration adsorbate of 99.9 % CO2 on Fe3O4 adsorbent made it oxidized and partially turns to red powder indicative of easily oxidized to Fe2O3 after TPD at 500 °C. The stability of the adsorbent itself considered as one of the factor in concerning a good adsorbent. Especially consideration of the stability after modification with other metals and support materials. Fe2O3 in Figure 9 shows distinct peak of MaxDT at 575 °C with adsorption capacity of 3.83 mgCO2/gadsorbent, while another small peak appeared at 208 °C (0.12 mgCO2/gadsorbent). At higher concentration of 99.9 % CO2, Fe2O3 still has the capability to capture CO2 up to 62.8 % with total adsorption capacity of 6.43 mgCO2/gadsorbent. It is interesting to note that Fe2O3 having all regions of basic sites indicative of better in CO2 adsorption. Nonetheless, the distinct peak was appeared at temperature range which higher desorption temperature required to dissociate carbonate formed. This results proved that iron oxides has lower MaxDT compared to other common used adsorbent


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of CaO which required higher desorption temperature of 900 °C.4-6 Furthermore, Fe2O3 was used in the recyclability test by performed CO2 adsorption at 30 °C and 1 atm using fluedized-bed reactor scale for several cycles. The adsorption capacity and texture properties for every cycle by desorption were tabulated in Table 5. The desorption temperature of 650 °C was used according to the CO2-TPD curve wherein no signal shows after reach this temperature. It was found that adsorption capacity for the fourth cycle was 21.86 gCO2/gadsorbent which is not significantly decreased by 8.6 %. The adsorption-desorption cycle on Fe2O3 was diminishing its pore structure and scavenged the pores by leaving larger pore diameter after every cycle (Table 5). Consequently, the main reason of reduces it adsorption capacity ascribed from the active sites that chiefly responsible to attract CO2 was depleted after each cycle. Figure 9. CO2 adsorption-desorption curves for CO2-TPD analysis with different concentrations of 5 % and 99.9 % CO2 on various type iron oxides. Table 4. The CO2 adsorption capacity of FeO, Fe2O3 and Fe3O4 using CO2-TPD analysis. Table 5. Adsorption capacity for recyclability of Fe2O3 at desorption temperature 650 °C by using mixture gas 15 % CO2 and N2 (balance) during adsorption at 30 °C. 4.0 Conclusion Various type iron oxides product following CO2 adsorption-desorption mainly contributed by its basicity and morphology porosity. Formation of carbonate on iron oxides was identified by IR spectra with formation of linear and bending CO2, bicarbonate, monodentate carbonate and bidententate carbonate on its surface. The basicity distribution of Fe2O3 shows presence all basic sites of weak, medium, strong and very strong phase. Although Fe3O4 indicated potential in CO2 adsorption, it stability during adsorbed higher CO2 concentration is the constraint where it oxidized turns to Fe2O3. Fe2O3 exhibited highest BET surface area with nano coral reefs like


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structure by SEM images was highly desirable for CO2 adsorption. It shows fine sharp particles formed after CO2 adsorbed on its surface. Furthermore, MaxDT 575 °C is lower than common material used such as CaO (900 °C). The physical adsorption of CO2 on Fe2O3 3.01 mgCO2/gadsorbent was highest among all iron oxides. For chemisorption, Fe2O3 able to desorbed 3.95 mgCO2/gadsorbent by using 5% CO2 for adsorption. The adsorption capacity increased to 62.8 % by using concentrated 99.9 % CO2 for adsorption. After four cycle of adsorption-desorption by using reactor, its adsorption capacity was 21.86 gCO2/gadsorbent which is not significantly reduced by 8.6 %. Thus, improvement can be done by chemical modification on Fe2O3 surface through increase its basicity by additive of basic metal oxide or increase the surface area by using support materials.

AUTHOR INFORMATION Corresponding Author Azizul Hakim Email: [email protected] Prof. Dr. Mohd. Ambar Yarmo Email: [email protected] Notes The authors declare no completing financial interest. Author Contributions


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Tengku Sharifah Marliza, Maratun Najiha Abu Tahari, Muhammad Rahimi Yusop, Mohamed Wahab Mohamed Hisham, Wan Nor Roslam Wan Isahak. ACKNOWLEDGEMENT The authors wish to thanks Universiti Kebangsaan Malaysia (UKM) for funding this project under research grant number LRGS/BU/2011/USM-UKM/PG/02, BKBP-FST-K003323, ETP2013-066 and TD-2014-024 from Ministry of Higher Education (MOHE) Malaysia and Centre of Research and Innovation Management (CRIM) UKM for the instruments facilities. ABBREVIATIONS CO2-carbon dioxide, FESEM-field emission scanning electron microscope, iR-infrared, COTPD-temperature programmed desorption using carbon monoxide, CO2-TPD-temperature programmed desorption using carbon dioxide, NOAA-National Oceanographic Atmospheric Administration, U.S-United State, ppm- part per million, CaO-calcim oxide, La2O3-lanthanum sesquioxide, AgO-silver oxide, TiO2-titanium oxide, CuO-copper (II) oxide, Cu2O-copper (I) oxide, NiO-nickel oxide, Fe2O3-iron (III) oxide, CeO2-cerium oxide, nm-nano meter, Vtot-total pore volume, TCD-thermal conductivity detecteor, BET-Brunauer-Emmet-Teller, STP-standard temperature and pressure, SBET-surface area by Brunauer-Emmet-Teller method, KBr-potassium bromide, JCPDS-Joint Committee on Powder Diffraction Standards. REFERENCES (1) Thompson, S. C. G.; Barton, M. A. Ecocentric and anthropocentric attitudes toward the environment. J. Environ. Psycol. 1994, 14: 149-157.


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(2) Song, C. S. Global challenges and strategies for control, conversion and utilization of CO2 for sustainable development involving energy, catalysis, adsorption and chemical processing. Catal. Today. 2006, 115, 2-32. (3) Tans, P.; Keeling, R. Trends in Atmospheric Carbon Dioxide. Earth System Research Laboratory Global Monitoring Division, National Oceanic and Atmospheric Administration (NOAA). U.S. Department of Commerce [Online]. http://www.esrl.noaa.gov/gmd/ccgg/trends/ (accessed July 28, 2015) (4) Zhong, Q.; Bjerle, I. Calcination kinetics of limestone and the microstructure of nascent CaO. Thermochim. Acta. 1993, 223, 109-120. (5) Abanades, J. C. The maximum capture efficiency of CO2 using a carbonation/calcination cycle of CaO/CaCO3. Chem. Eng. J. 2002, 90, 303-306. (6) Abanades, J. C.; Oakey, J. E.; Alvarez, D.; Hamalaimen, J. Novel combustion cycles incorporating capture of CO2 with CaO. Greenhouse Gas Control Technol. 2003, 1, 181-186. (7) Salvador, C.; Lu, D.; Anthony, E. J.; Abanades, J. C. Enhancement of CaO for CO2 capture in an FBC environment. Chem. Eng. J. 2003, 96, 187-195. (8) Rosynek, M. P.; Magnuson, D. T. Infrared study of CO2 adsorption on lanthanum sesquioxide and trihydroxide. J. Catal. 1977, 48, 417-421. (9) Okawa, Y.; Tanaka, K. STM investigation of the reaction of Ag-O added rows with CO2 on a Ag(1 1 0) surface. Surf. Sci. Lett. 1995, 344, L1207-L1212.


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For Table of Contents Only


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Table of contents 83x58mm (96 x 96 DPI)



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List of Figures: Figure 1. Schematic diagram of fluidized-bed reactor. Figure 2. N2 adsorption-desorption isotherms of FeO, Fe2O3 and Fe3O4 with the inset indicating estimated pores image according to the isotherms. Figure 3. IR spectra of FeO where a) after CO2 adsorption for 24 hours and b) before adsorption. Figure 4. IR spectra of Fe2O3 where a) after CO2 adsorption for 24 hours and b) before adsorption. Figure 5. IR spectra of Fe3O4 where a) after CO2 adsorption for 24 hours and b) before adsorption. Figure 6. Basicity distribution by CO-TPD analysis on various iron oxides. Figure 7. FESEM images for a) fresh iron oxides, b) Purged with CO2 for 4 hours and c) Purged with CO2 for 24 hours d) Purged with CO2 for 48 hours where (i) FeO (ii) Fe2O3 and (iii) Fe3O4. Figure 8. CO2 adsorption isotherms at 25 °C using isotherm adsorption technique technique with multilayer of CO2 adsorbed on iron oxides surface diagram in the inset. Figure 9. CO2 adsorption-desorption curves for CO2-TPD analysis with different concentrations of 5 % and 99.9 % CO2 on various type iron oxides.


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Figure 1. Schematic diagram of fluidized-bed reactor.


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Figure 2. N2 adsorption-desorption isotherms of FeO, Fe2O3 and Fe3O4 and estimated pores image according to the isotherms.


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Figure 3. IR spectra of FeO where a) after CO2 adsorption for 24 hours and b) before adsorption.


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Figure 4. IR spectra of Fe2O3 where a) after CO2 adsorption for 24 hours and b) before adsorption.


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Figure 5. IR spectra of Fe3O4 where a) after CO2 adsorption for 24 hours and b) before adsorption.


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Figure 6. Basicity distribution by CO-TPD analysis on various iron oxides.


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a.i

2 μm 10.00kx b.i

2 μm 50.00kx c.i

2 μm 10.00kx d.i

1 μm 10.00kx

ii

200 nm 50.00kx ii

200 nm 50.00kx ii

1 μm 20.00kx ii

1 μm 20.00kx

iii

100 nm 50.00kx iii

200 nm 50.00 kx iii

100 nm 50.00kx iii

100 nm 50.00kx

Figure 7. FESEM images for a) fresh iron oxides, b) Purged with CO2 for 4 hours and c) Purged with CO2 for 24 hours d) Purged with CO2 for 48 hours where (i) FeO (ii) Fe2O3 and (iii) Fe3O4.


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Figure 8. CO2 adsorption isotherms at 25 °C using isotherm adsorption technique with multilayer of CO2 adsorbed on iron oxides surface.


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Figure 9. CO2 adsorption-desorption curves for CO2-TPD analysis with different concentrations of 5 % and 99.9 % CO2 on various type iron oxides.


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Studies on CO2 Adsorption and Desorption Properties from Various

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