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PSAC for hydrogen sulphide removal from biogas
Journal of Environmental Chemical Engineering 3 (2015) 1522–1529
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Selection of better synthesis route of CeO2/NaOH/PSAC for hydrogen sulphide removal from biogas Lee Chung Laua,* , Norhusna Mohamad Norb , Keat Teong Leeb , Abdul Rahman Mohamedb a Department of Petrochemical Engineering, Faculty of Engineering and Green Technology, Perak Campus, Universiti Tunku Abdul Rahman, Jalan Universiti, Bandar Barat, 31900 Kampar, Perak, Malaysia b School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal, Pulau Pinang, Malaysia
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 March 2015 Accepted 29 May 2015
Biogas contains hydrogen sulphide (H2S), which is an acidic gas that would causes severe corrosion to the mechanical parts of combustion engines. Therefore, hydrogen sulphide must be separated from biogas prior to combustion. Among the conventional methods, the use of a cheap adsorbent appears to be one of the most promising methods for H2S removal. CeO2/NaOH/PSAC was successfully synthesized and used to remove H2S from a simulated biogas stream. A better synthesis method was found by applying a soaking method to impregnate cerium oxide into palm shell activated carbon (PSAC). Cerium oxide (CeO2) was obtained via a reaction between cerium nitrate hydrate (Ce(NO3)36H2O) and sodium hydroxide (NaOH). The calcination step was crucial for this synthetic route to enhance the H2S adsorption capacity of the adsorbent. Selected adsorbents were characterized using several techniques and provided further information about the surface properties, functional groups, morphologies/composition and chemical states of the selected adsorbents. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Hydrogen sulphide Activated carbon Cerium oxide Adsorption Biogas Palm shell
Introduction The palm oil industry is one of the most important sectors of the national economy Malaysia. In 2012, an estimated 64 million metric tons of palm oil mill effluent (POME) was produced [1]. To achieve the POME discharge standard, POME is treated using anaerobic digestion method. In anaerobic digestion, a combustible biogas, is produced from the decomposition of organic matter. The composition of a typical biogas is shown in Table 1 [2]. Biogas generally consists of CH4, CO2 and H2O. Trace amounts of other components such as N2, O2, NH3, CO and H2S are also found in biogas. Generally, the pond in which the digestion occurs is not covered so the biogas is directly emitted into the atmosphere. Biogas contains CH4 which is a gas that has a high global warming potential. Therefore, the open release of biogas contributes to global warming. Additionally, biogas contains a high heating value so it can serve as a good source of fuel. However, biogas contains hydrogen sulphide, an acidic gas that could cause severe corrosion
* Corresponding author. Tel.: +60 5 4688888; fax: +60 5 4667449. E-mail addresses: [email protected], [email protected] (L.C. Lau). http://dx.doi.org/10.1016/j.jece.2015.05.027 2213-3437/ ã 2015 Elsevier Ltd. All rights reserved.
to the mechanical parts of a combustion engine. Therefore, hydrogen sulphide must be separated from biogas prior to combustion. Conventional removal methods for H2S include wet scrubbing, adsorption, biological methods and catalytic oxidation. Wet scrubbing uses basic chemicals such as an amine solution, because H2S is an acidic gas [3,4]. Adsorption by an adsorbent, such as activated carbon, is able to trap H2S in its porous structure [5–7]. Biological methods use bacteria to consume H2S for growth [8,9]. Catalytic oxidation employs a catalyst to convert H2S into elemental sulphur [10–14]. Among these methods, adsorption using a cheap adsorbent appears to be one of the most promising methods for H2S removal. In most cases, these methods can only remove a certain portion of the pollutant H2S. For a low pollutant concentration at the parts per million (ppm) level, these methods become less efficient. If stringent regulations require higher removal efficiency, adsorption can provide the solution. Adsorption is a better choice for targeting pollutants at low concentrations. This is because the porous structure of activated carbon has a large surface area allowing all of the pollutant molecules to react, producing a purified biogas with a hydrogen sulphide content below 1 ppm. A precursor of activated carbon can
L.C. Lau et al. / Journal of Environmental Chemical Engineering 3 (2015) 1522–1529 Table 1 Composition of typical biogas.
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Experimental
Component
Composition (%)
CH4 CO2 H2O N2 O2 NH3 CO H2S
40–75 15–60 5–10 0–2 0–1 <1 <0.6 0.005–2
be obtained from agricultural biomass which is a cheap and economical source. Palm shell is a waste product generated from the palm oil industry that appears to be a suitable precursor for activated carbon. In fact, palm shell activated carbon (PSAC) has been successfully utilized to remove several types of gaseous pollutants such as SO2, NO2 and NH3 [15–17]. PSAC can be used for H2S removal but un-impregnated PSAC is effective only for the removal of H2S at a low space velocity [18]. The space velocity can be quantified via the ratio of flow rate to adsorbent volume. A high space velocity is related to a low contact time of H2S with the adsorbent. With a low residence time, H2S has insufficient time to be adsorbed onto the PSAC surface. Therefore, the PSAC must be modified to improve its adsorption capacity and to achieve faster reaction kinetics. Modifying activated carbon using metal oxides has been a popular method in previous studies. Metal oxides such as aluminium oxide, iron oxide, zinc oxide, titanium oxide, manganese oxide, tin oxide, copper oxide, cobalt oxide and vanadium oxide have been impregnated as active species for the catalytic oxidation of pollutants [19–23]. Regardless of the vast amount of research that has been conducted in adsorption technology, these adsorbents are not considered to be highly suitable for industrial application due to their low adsorption capacity, high cost and potential hazard. In addition, to the best of our knowledge, cerium oxide-impregnated activated carbon has not been used for H2S removal. Therefore, in this study, we concentrated on cerium oxide-impregnated activated carbon. Cerium oxide is a good oxygen storage material for oxidation reactions because of the Ce4+/Ce3+ redox couple. The redox couple allows an oxidation reaction to occur between CeO2 and Ce2O3 [24,25]. Therefore, it is highly suitable for use in the oxidation of hydrogen sulphide. Highly dispersed cerium oxide can be achieved by impregnating cerium on a porous support, such as activated carbon. Activated carbon generally has a large surface area that increases the dispersion of cerium oxide. This enhances the reaction kinetics of H2S removal and leads to a larger biogas cleaning capacity. In addition, activated carbon also serves as a medium to bind reactants by adsorption on the surface to improve catalytic performance. Therefore, a synergistic effect between adsorption and catalytic oxidation can be obtained. Cerium oxide can be impregnated on activated carbon by various means [26–34]. Precursors of cerium are normally in the nitrate form because they are soluble. The nitrate can either be decomposed using a thermal treatment or by reaction with an alkaline compound such as sodium hydroxide. Thermal treatment at temperatures higher than 250 C decomposes the nitrate to an oxide with an inert atmosphere. In the adsorption of hydrogen sulphide, which is acidic, the utilization of alkaline compound has a synergistic effect on the reaction. In this paper, the effects of the impregnation method, the use of a cerium precursor and sodium hydroxide, and the thermal calcination on the H2S adsorption capacity were studied.
Chemicals The palm shell activated carbon (PSAC) used in this study was steam activated and purchased from Victory Element Sdn Bhd, Malaysia. Upon receipt, the PSAC was sieved to 1–2 mm and dried at 80 C in an oven. From an N2 adsorption desorption study, the PSAC surface properties were determined as a BET surface area of 729 m2/g, a total pore volume of 0.37 cm3/g and a pore size of 20.6 Å. During the PSAC impregnation, extra pure cerium(III) nitrate hexahydrate (Ce(NO3)36H2O) and analytical reagent grade sodium hydroxide (NaOH) in pellet form supplied by Merck Sdn Bhd were used. Purified nitrogen (99.99% N2) was used in the calcination of the impregnated activated carbon. In the H2S adsorption test, three gases (1) 99.99% CH4, (2) 99.99% CO2 and (3) 1% H2S, balance with CH4 were supplied by Wellgas Sdn Bhd, Malaysia were used to simulate an industrial biogas. PSAC impregnation In total, a 25 ml of a 0.0375 M cerium nitrate solution was added to 2.50 g of PSAC in a conical flask. The amount of cerium added was 5 wt% of the PSAC weight. Then, 25 ml of 2.0 M sodium hydroxide was slowly added to the flask. Subsequently, the mixture was shaken in a water bath at 30 C for 3 h. After the incubation, the impregnated activated carbon was filtered and dried in an oven at 80 C. This process is termed the soaking method. For incipient wetness impregnation (IWI), the impregnated activated carbon was not filtered, but only dried in an oven at 80 C. The dried impregnated activated carbon was then subjected to calcination at 400 C under a flow of N2 at 50 ml/min for 3 h. Adsorption test An H2S adsorption test was performed using a packed bed reactor test rig as depicted in Fig. 1. The composition of simulated biogas was adjusted by controlling the flow rate of gases using Aalborg AFC26 mass flow controllers, supplied by Bioclear Sdn Bhd, Malaysia. CO2 and CH4 were passed through a humidification system to provide approximately 25% relative humidity to the biogas inlet stream. H2S was not passed through the humidification system because H2S is soluble in water. The path of the biogas between the humidification system and reactor was insulated to avoid moisture condensation. The diameter of the stainless steel tubular reactor used was half an inch. The flow rate of the biogas was fixed at 500 ml/min with 40% CO2, 3000 ppm H2S and balance with CH4. This biogas composition was simulated to mimic the data obtained from a collection pond located at the FELDA Besout Palm Oil Mill that is owned by FELDA Palm Industries Sdn Bhd, Malaysia. In all runs, 1.0 g of PSAC was placed in the middle of the tubular reactor and supported with approximately 0.05 g of glass wool. The adsorption temperature was controlled at 30 C using a Linberg/Blue M tube furnace. The concentration of H2S at the outlet stream was analysed using an IMR 6000 gas analyser equipped with an electrochemical sensor calibrated for 0–5000 ppm H2S. The test rig workspace was ventilated as a safety measure in case a leakage of hazardous H2S occurred. Characterizations Several characterizations, such as N2 adsorption–desorption, scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM–EDX), thermogravimetric analysis (TGA), X-ray fluorescence spectroscopy (XRF), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS)
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Fig. 1. Schematic diagram of a H2S adsorption test rig.
were performed on selected adsorbents to analyse the surface properties, functional groups and morphology/composition. N2 adsorption–desorption was carried out with an Autosorb IC Quantachrome analyser. The adorbent was degassed for at least 5 h at 120 C before the analysis was conducted. Degassed adsorbent was cooled in liquid nitrogen and analysed by computer software (Micropore v2.46), which allowed for rapid numerical results from the adsorption–desorption isotherm. SEM–EDX analysis was performed by a Quanta FEG 450 at an accelerating voltage of 3 kV to obtain the adsorbent surface morphology and to determine the localized chemical composition. TGA was performed using a PerkinElmer TGA7 with a heating rate of 10 C/min at a 100 ml/min flow rate of purified air. XRF was performed using a Rikagu RIX 3000. FTIR was performed using a Thermo Scientific Nicolet iS10 FT-IR Spectrometer. XPS was performed using a High Resolution Multi Technique X-Ray Spectrometer (Axis Ultra DLD XPS, Kratos) to study the chemical state of the selected sorbents. The pass energy used was 25 eV and the charging effect was corrected using the C 1s peak at 284.5 eV. Results and discussion The H2S adsorption capacity for the adsorbent prepared under different conditions is shown in Table 2. The tick in the table indicates the condition that was used in the adsorbent synthesis route. The hyphen in the table denotes that the condition was not used in the adsorbent synthesis route. For example, the Adsorbent N5 was prepared using the soaking method to impregnate cerium
and sodium hydroxide onto its porous structure. Subsequently, a calcination step was employed before the adsorbent was tested for its H2S adsorption capacity. Fig. 2 shows the breakthrough curve for H2S removal using Adsorbent N5. The calculation of the adsorption capacity, Q, is shown in Eq. (1). Z t c0 M w q c 1 dt (1) Q¼ 1000wV m 0 c0 where c0 is the inlet H2S concentration (ppm), Mw is the molecular weight of H2S (g/mol), q is the total gas flow rate (l/min), w is weight of the adsorbent (g), Vm is the molar volume (l/mol), and c (ppm) is the outlet H2S concentration at time t (min). The adsorption capacity is reported in units of mg H2S/g of adsorbent at c/c0 = 0.05 and 0.50. In an adsorption study, the adsorption capacity is generally calculated at a c/c0 between 0.01 and 0.05 and is termed the breakthrough capacity. In practice, an adsorbent at the top of the adsorption column is saturated with H2S. Therefore, the saturated adsorption capacity of the activated carbon must be determined also. Theoretically, the breakthrough curve is sigmoid and is symmetrical at c/c0 = 0.50. Therefore, the saturated capacity can be calculated by taking the corresponding time at c/c0 = 0.50 as the saturated time. Selection between the incipient wetness impregnation and soaking method Impregnation is generally performed using two methods: incipient wetness impregnation and soaking method. Incipient
Table 2 H2S adsorption capacity for adsorbents prepared at different condition. Adsorbent
Breakthrough capacity (mg/g)
Saturated capacity (mg/g)
IWI/soak
Cerium
NaOH
Calcination
N1 N2 N3 N4 N5 N6 N7 N8 N9 N10 N11
1.83 1.83 51.50 1.315 54.57 0.86 9.04 0.80 0.66 67.10 0.70
4.20 3.60 171.82 13.49 187.08 4.07 128.67 1.73 1.06 153.79 1.06
– Soak Soak Soak Soak IWI IWI IWI IWI Soak IWI
– U U U U U U U U – –
– – U – U – – U U U U
– – – U U – U – U – –
L.C. Lau et al. / Journal of Environmental Chemical Engineering 3 (2015) 1522–1529
Fig. 2. Breakthrough curve for H2S removal using adsorbent N5.
wetness impregnation is performed by adding the minimum amount of aqueous solution containing the compound to be impregnated to the support. Subsequently, the mixture is dried in an oven. By contrast, the soaking method is performed by placing the activated carbon in an aqueous solution and allowing the material to soak for a certain period of time (3 h in this study). Then, the activated carbon is filtered and dried. In incipient wetness impregnation, the amount of metal impregnation into the activated carbon is identical to that which is initially added to the solution because water alone is assumed to evaporate. However, in the soaking method, the amount of metal impregnation is not the same as the amount of metal amount initially added. The amount of metal impregnated depends on the soaking time and temperature, which affects the diffusion of the metal species into the porous structure of the activated carbon. The incipient wetness impregnation method is simpler but requires a higher heat input to evaporate the added water. In addition, the distribution of the impregnated metal might not be uneven. The soaking method, however, is wasteful because a portion of the chemical is filtered. Nevertheless, both methods have advantages and disadvantages. Therefore, a suitable method must be identified in this study. From Table 2, most adsorbents prepared with incipient wetness impregnation did not have a good H2S adsorption capacity compared to those prepared with the soaking method. This lower capacity could be because of the high concentration of impregnates used in this study. Regarding the physical appearance of the adsorbent, a lump of cerium oxide with a pale blue colour was observed to cover the surface of the adsorbent. Thus, the surface area of the adsorbent was severely reduced, resulting in a poor adsorption capacity. Conversely, adsorbents prepared by the soaking method generally had a good adsorption capacity. Therefore, for the range of the parameters used in this study, we concluded that the soaking method was better than incipient wetness impregnation.
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oxide. However, Adsorbent N4 was still not a good H2S adsorbent. Between Adsorbents N4 and N7, Adsorbent N7 was actually a special case because it had a higher saturated adsorption capacity compared to Adsorbent N4. In this case, the incipient wetness method might be more suitable because of the amount of cerium that could be added to the activated carbon. For both Adsorbents N4 and N7, an identical amount of cerium nitrate was added to the aqueous solution. However, in the soaking method, not all of the cerium nitrate molecules will diffuse into the porous structure of the activated carbon. Thus, cerium nitrate will be removed by subsequent filtration in an aqueous solution. Therefore, the amount of cerium added to the activated carbon was much less compared to the incipient wetness method. Conversely, the phenomenon of a low breakthrough capacity but high saturated capacity showed that H2S adsorption on Adsorbent N7 was not rapid enough to maintain a high removal rate of H2S from the simulated biogas stream. Cerium nitrate can be converted to cerium oxide by adding sodium hydroxide or be decomposed using a heat treatment called calcination. The former method was exemplified by Adsorbent N3 and the latter by Adsorbent N7. The breakthrough capacity for Adsorbent N7 was as low as that for unmodified activated carbon. However, the saturation capacity was more than 30-fold higher than that for unmodified activated carbon which indicated that cerium oxide participated in H2S adsorption but at a slower rate. Adsorbent N3, in which cerium oxide was prepared from the reaction of sodium hydroxide and cerium nitrate but without calcination, had a much higher breakthrough and saturated capacity that was sufficient for industrial application. This enhanced adsorption capacity probably arose from the synergistic effect of cerium oxide and sodium hydroxide. Sodium hydroxide played an important role in H2S adsorption because H2S is acidic. Therefore, using a basic material for acid gas removal is a reasonable method. Adsorbent N10, prepared by the impregnation of sodium hydroxide, showed a high breakthrough and saturated capacity without the addition of cerium. No cerium addition was necessary, because cerium would have been a waste product in this route. However, coupling with cerium caused an increase in the saturated capacity from 153.79 mg/g (Adsorbent N10) to 171.82 mg/g (Adsorbent N3), a very significant 11.7% increase of the saturated capacity. The synergistic effect of cerium oxide and sodium hydroxide showed that a fast kinetic reaction could be achieved. Sodium hydroxide, together with the humidity in the biogas stream, enhanced the solubility of H2S on the porous surface of PSAC [35]. This provided sufficient time for cerium oxide to further oxidize H2S into products. A further increase to 187.08 mg/g (Adsorbent N5) was also observed when calcination was applied. Calcination enabled better dispersion of the active species on the activated carbon and converted the inactive cerium nitrate into active cerium oxide. From these findings, the better route to prepare CeO2/NaOH/PSAC was the production of cerium oxide from the reaction of cerium nitrate and sodium hydroxide using the soaking method further enhanced by calcination.
Role of cerium oxide, calcination and sodium hydroxide Characterizations of the selected adsorbents In this study, cerium in the nitrate form was added to activated carbon. Neither NaOH nor calcination was added to the Adsorbent N2. Therefore, this adsorbent can be considered as doping cerium nitrate on activated carbon. The adsorption capacity indicated that Adsorbent N2 was not a good adsorbent and that it was even worse than unmodified activated carbon. Therefore, the actual active species for the H2S adsorption reaction was cerium oxide instead of cerium nitrate. Adsorbent N4, which was prepared using the soaking method and calcination after cerium nitrate impregnation, showed a better result than Adsorbent N2, indicating the necessity to convert inactive cerium nitrate into more highly active cerium
Fig. 3 shows the surface morphology of the raw PSAC used in this study. Three different surface structures were found: (a) a rough terrain without regular structures, (b) a cratered surface with pores smaller than 1 mm and (c) canyon-like structures. These structures formed because of the decomposition of the organic components during the activation process. Among these structures, the porous cratered surface (further magnified in Fig. 3d) was estimated to contribute to a larger portion of the surface area and the pore volume of the raw activated carbon. Regardless of the large surface area of the unimpregnated PSAC, the
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Fig. 3. SEM micrograph of Adsorbent N1: raw PSAC at various surfaces and magnifications, (a) 1500, (b) 1000, (c) 1000, and (d) 8000.
H2S adsorption capacity obtained in this study was low. PSAC should be a significant adsorbent for H2S. The low H2S adsorption capacity could be due to the high flow rate, a low amount of adsorbent and the high H2S concentration used in this study. Therefore, the adsorption capacity reported for raw PSAC was insufficient for industrial application. Additionally, because of the nature of the experimental setting used, the adsorption capacity of other adsorbents synthesized in this study should be generally be higher because further optimization of the experimental setup is necessary. The XRF results for the raw PSAC (Adsorbent N1) are shown in Table 3. The results show that SiO2 led to the highest amount of oxides in PSAC. These oxides were more precisely located on the cratered surface using SEM–EDX techniques as shown in Fig. 4 and Table 4. These oxides should be the main components of the ash content obtained from the proximate analysis of the raw PSAC because the total amount obtained from XRF was similar. The proximate analysis showed that the moisture and the content of volatiles, fixed carbon and ash of the raw PSAC were 21.2, 4.2, 72.4 and 2.2%, respectively. In addition, these oxides could theoretically assist or catalyse the H2S removal reaction to a
certain extent. Nevertheless, further examination of the effect of these oxides on H2S removal was not performed because that analysis was out of the scope of this study. Fig. 5 shows the TGA of the raw PSAC in changes of weight and derivative weight with temperature. The first peak in the derivative weight change at approximately 100 C should be attributed to the moisture adsorbed by the raw PSAC. Another peak in the derivative weight change was identified at approximately 700 C, which should be
Table 3 XRF result of Adsorbent N1: raw PSAC. Composition
Amount (wt%)
MgO SiO2 P2 O 5 SO3 Cl K2O Fe2O3
0.1586 2.6444 0.0598 0.1112 0.0436 0.4888 0.0380 Fig. 4. SEM micrograph of SiO2 spheres on Adsorbent N1: raw PSAC.
L.C. Lau et al. / Journal of Environmental Chemical Engineering 3 (2015) 1522–1529 Table 4 EDX of SiO2 spheres on Adsorbent N1: raw PSAC (refer to Fig. 4).
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Table 5 EDX of Adsorbent N5 at the tiles of impregnate (refer to Fig. 6).
Element
Atomic%
Element
Atomic%
C O Na Mg Si K Ca Mn Fe
29.86 47.77 0.16 0.30 14.62 1.81 4.88 0.11 0.49
C O Na Si K Ca Ce
60.16 26.31 1.63 1.91 0.42 1.58 8.00
Fig. 5. TGA of Adsorbent N1: raw PSAC.
the oxidation of fixed carbon into carbon dioxide. The burning of carbon in air began at approximately 500 C and ended approximately at 800 C [35]. The small amount of material retained was ash and likely consisted of metal oxides as shown in Table 4. Upon impregnation of cerium oxides, impregnate tiles formed at the surface of PSAC as shown in Fig. 6. The size of these impregnates was approximately 10 mm, which could not be considered well-dispersed. Nevertheless, after calcination at 400 C, the lumps of impregnates were found to crack into smaller pieces and produce a higher surface area for subsequent H2S adsorption. These impregnates were determined using EDX techniques and the results are shown in Table 5. The EDX result
Fig. 6. SEM image for Adsorbent N5.
showed that these impregnates consist of a detectable amount of elemental cerium and sodium in atomic percentage, which indicated that these impregnates were aggregates of cerium oxide and sodium hydroxide. FTIR spectra of the selected adsorbents are shown in Fig. 7. From the FTIR spectra, the functional groups detected were roughly similar, but with different intensities. Unmodified PSAC show a high-intensity band at approximately 3400 cm1, which indicates phenolic or hydroxyl groups [25,36]. However, after the impregnation of cerium, the intensity of these functional groups was reduced. This could be because, during the calcination step used in the preparation of these adsorbents, these functional groups decomposed at the calcination temperature. In addition, calcination could also eliminate all of the alkyl groups, such as CH3,¼CH2, CH2CH3, which were detected only in raw PSAC at 2900 cm1 [37]. Other functional groups were also detected for all of the adsorbents at 3600–3900 cm1 corresponding to OH stretching [27]; 1400–1680 cm1 corresponding to lactonic and carbonyl groups [38]; 1020–1220 cm1 corresponding to the carboxylic group [39]; and 400–700 cm1 corresponding to C H out of plane bending [27]. For the adsorbents impregnated with cerium, the carbonyl C¼O stretch because of CeO2 was also detected [27]. In the analysis of chemical states using the XPS technique, Ce4+ should result in six peaks at binding energies of 882.2, 888.6, 898, 900.7, 907.2 and 916.15 eV. Ce3+ should, on the other hand, show peaks at 880.6, 884.4, 899.3 and 903.9 eV [40]. The concentration of Ce4+ or Ce3+ on the sorbent surface can be calculated [41]. Ce 3d XPS spectra for Sorbents N5 and N7 are shown in Fig. 8. The Ce4+ concentration for Sorbents N5 and N7 was found to be 50.5 and 52.6%, respectively. In addition, the O 1s XPS spectra for Sorbents N5 and N7 are shown in Fig. 9. O2 was indicated in the oxide bonds. The analysis showed that the cerium species were CeO2 and Ce2O3. CeO2 in Sorbent N5 was derived from cerium nitrate using sodium hydroxide and the calcination technique. On the other
Fig. 7. FTIR spectra of the selected adsorbents.
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adsorption capacity. Characterization studies showed that cerium species in the form of cerium oxides were the active species for H2S removal. Acknowledgments The authors would like to acknowledge the financial supports from MyBrain 15 Program, Knowledge Transfer Program (grant no. 203/PJKIMIA/6750045), FELDA Palm Industries Sdn Bhd, and Universiti Sains Malaysia RU-PRGS (grant no. 1001/PJKIMIA/ 8045042). References
Fig. 8. Ce 3d XPS spectra for selected sorbents.
Fig. 9. O 1s XPS spectra for selected sorbents.
hand, CeO2 in Sorbent N7 was formed using the calcination technique only. The small difference of Ce4+ concentration in both samples showed that the addition of sodium hydroxide in the sorbent preparation step would not affect the formation of cerium oxide significantly. From the XPS analysis of Ce 3d, it was found that the amount of Ce4+ and Ce3+ on the sorbent surface was almost the same. However, the notation of the adsorbent name is CeO2 instead of Ce2O3 because hydrogen sulphide sorption actually involves the oxidation of hydrogen sulphide to elemental sulphur by CeO2. This indicated that CeO2 was the active species in the sorption process. By varying the cerium loading, the NaOH concentration and the calcination process, it is necessary to optimize the process to obtain a higher concentration of Ce4+. Conclusions CeO2/NaOH/PSAC was successfully synthesized and used for the removal of H2S from a simulated biogas stream. The better synthesis method found for the catalyst was the impregnation of both cerium oxide and sodium into the PSAC with the soaking method. Incipient wetness impregnation was not appropriate because it reduced the surface area of the adsorbent and therefore the adsorption capacity. A synergistic effect of cerium oxide and sodium hydroxide was also identified in this study. In the synthesis route, the calcination step was crucial to further enhance the
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Contents lists available at ScienceDirect
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Selection of better synthesis route of CeO2/NaOH/PSAC for hydrogen sulphide removal from biogas Lee Chung Laua,* , Norhusna Mohamad Norb , Keat Teong Leeb , Abdul Rahman Mohamedb a Department of Petrochemical Engineering, Faculty of Engineering and Green Technology, Perak Campus, Universiti Tunku Abdul Rahman, Jalan Universiti, Bandar Barat, 31900 Kampar, Perak, Malaysia b School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal, Pulau Pinang, Malaysia
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 March 2015 Accepted 29 May 2015
Biogas contains hydrogen sulphide (H2S), which is an acidic gas that would causes severe corrosion to the mechanical parts of combustion engines. Therefore, hydrogen sulphide must be separated from biogas prior to combustion. Among the conventional methods, the use of a cheap adsorbent appears to be one of the most promising methods for H2S removal. CeO2/NaOH/PSAC was successfully synthesized and used to remove H2S from a simulated biogas stream. A better synthesis method was found by applying a soaking method to impregnate cerium oxide into palm shell activated carbon (PSAC). Cerium oxide (CeO2) was obtained via a reaction between cerium nitrate hydrate (Ce(NO3)36H2O) and sodium hydroxide (NaOH). The calcination step was crucial for this synthetic route to enhance the H2S adsorption capacity of the adsorbent. Selected adsorbents were characterized using several techniques and provided further information about the surface properties, functional groups, morphologies/composition and chemical states of the selected adsorbents. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Hydrogen sulphide Activated carbon Cerium oxide Adsorption Biogas Palm shell
Introduction The palm oil industry is one of the most important sectors of the national economy Malaysia. In 2012, an estimated 64 million metric tons of palm oil mill effluent (POME) was produced [1]. To achieve the POME discharge standard, POME is treated using anaerobic digestion method. In anaerobic digestion, a combustible biogas, is produced from the decomposition of organic matter. The composition of a typical biogas is shown in Table 1 [2]. Biogas generally consists of CH4, CO2 and H2O. Trace amounts of other components such as N2, O2, NH3, CO and H2S are also found in biogas. Generally, the pond in which the digestion occurs is not covered so the biogas is directly emitted into the atmosphere. Biogas contains CH4 which is a gas that has a high global warming potential. Therefore, the open release of biogas contributes to global warming. Additionally, biogas contains a high heating value so it can serve as a good source of fuel. However, biogas contains hydrogen sulphide, an acidic gas that could cause severe corrosion
* Corresponding author. Tel.: +60 5 4688888; fax: +60 5 4667449. E-mail addresses: [email protected], [email protected] (L.C. Lau). http://dx.doi.org/10.1016/j.jece.2015.05.027 2213-3437/ ã 2015 Elsevier Ltd. All rights reserved.
to the mechanical parts of a combustion engine. Therefore, hydrogen sulphide must be separated from biogas prior to combustion. Conventional removal methods for H2S include wet scrubbing, adsorption, biological methods and catalytic oxidation. Wet scrubbing uses basic chemicals such as an amine solution, because H2S is an acidic gas [3,4]. Adsorption by an adsorbent, such as activated carbon, is able to trap H2S in its porous structure [5–7]. Biological methods use bacteria to consume H2S for growth [8,9]. Catalytic oxidation employs a catalyst to convert H2S into elemental sulphur [10–14]. Among these methods, adsorption using a cheap adsorbent appears to be one of the most promising methods for H2S removal. In most cases, these methods can only remove a certain portion of the pollutant H2S. For a low pollutant concentration at the parts per million (ppm) level, these methods become less efficient. If stringent regulations require higher removal efficiency, adsorption can provide the solution. Adsorption is a better choice for targeting pollutants at low concentrations. This is because the porous structure of activated carbon has a large surface area allowing all of the pollutant molecules to react, producing a purified biogas with a hydrogen sulphide content below 1 ppm. A precursor of activated carbon can
L.C. Lau et al. / Journal of Environmental Chemical Engineering 3 (2015) 1522–1529 Table 1 Composition of typical biogas.
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Experimental
Component
Composition (%)
CH4 CO2 H2O N2 O2 NH3 CO H2S
40–75 15–60 5–10 0–2 0–1 <1 <0.6 0.005–2
be obtained from agricultural biomass which is a cheap and economical source. Palm shell is a waste product generated from the palm oil industry that appears to be a suitable precursor for activated carbon. In fact, palm shell activated carbon (PSAC) has been successfully utilized to remove several types of gaseous pollutants such as SO2, NO2 and NH3 [15–17]. PSAC can be used for H2S removal but un-impregnated PSAC is effective only for the removal of H2S at a low space velocity [18]. The space velocity can be quantified via the ratio of flow rate to adsorbent volume. A high space velocity is related to a low contact time of H2S with the adsorbent. With a low residence time, H2S has insufficient time to be adsorbed onto the PSAC surface. Therefore, the PSAC must be modified to improve its adsorption capacity and to achieve faster reaction kinetics. Modifying activated carbon using metal oxides has been a popular method in previous studies. Metal oxides such as aluminium oxide, iron oxide, zinc oxide, titanium oxide, manganese oxide, tin oxide, copper oxide, cobalt oxide and vanadium oxide have been impregnated as active species for the catalytic oxidation of pollutants [19–23]. Regardless of the vast amount of research that has been conducted in adsorption technology, these adsorbents are not considered to be highly suitable for industrial application due to their low adsorption capacity, high cost and potential hazard. In addition, to the best of our knowledge, cerium oxide-impregnated activated carbon has not been used for H2S removal. Therefore, in this study, we concentrated on cerium oxide-impregnated activated carbon. Cerium oxide is a good oxygen storage material for oxidation reactions because of the Ce4+/Ce3+ redox couple. The redox couple allows an oxidation reaction to occur between CeO2 and Ce2O3 [24,25]. Therefore, it is highly suitable for use in the oxidation of hydrogen sulphide. Highly dispersed cerium oxide can be achieved by impregnating cerium on a porous support, such as activated carbon. Activated carbon generally has a large surface area that increases the dispersion of cerium oxide. This enhances the reaction kinetics of H2S removal and leads to a larger biogas cleaning capacity. In addition, activated carbon also serves as a medium to bind reactants by adsorption on the surface to improve catalytic performance. Therefore, a synergistic effect between adsorption and catalytic oxidation can be obtained. Cerium oxide can be impregnated on activated carbon by various means [26–34]. Precursors of cerium are normally in the nitrate form because they are soluble. The nitrate can either be decomposed using a thermal treatment or by reaction with an alkaline compound such as sodium hydroxide. Thermal treatment at temperatures higher than 250 C decomposes the nitrate to an oxide with an inert atmosphere. In the adsorption of hydrogen sulphide, which is acidic, the utilization of alkaline compound has a synergistic effect on the reaction. In this paper, the effects of the impregnation method, the use of a cerium precursor and sodium hydroxide, and the thermal calcination on the H2S adsorption capacity were studied.
Chemicals The palm shell activated carbon (PSAC) used in this study was steam activated and purchased from Victory Element Sdn Bhd, Malaysia. Upon receipt, the PSAC was sieved to 1–2 mm and dried at 80 C in an oven. From an N2 adsorption desorption study, the PSAC surface properties were determined as a BET surface area of 729 m2/g, a total pore volume of 0.37 cm3/g and a pore size of 20.6 Å. During the PSAC impregnation, extra pure cerium(III) nitrate hexahydrate (Ce(NO3)36H2O) and analytical reagent grade sodium hydroxide (NaOH) in pellet form supplied by Merck Sdn Bhd were used. Purified nitrogen (99.99% N2) was used in the calcination of the impregnated activated carbon. In the H2S adsorption test, three gases (1) 99.99% CH4, (2) 99.99% CO2 and (3) 1% H2S, balance with CH4 were supplied by Wellgas Sdn Bhd, Malaysia were used to simulate an industrial biogas. PSAC impregnation In total, a 25 ml of a 0.0375 M cerium nitrate solution was added to 2.50 g of PSAC in a conical flask. The amount of cerium added was 5 wt% of the PSAC weight. Then, 25 ml of 2.0 M sodium hydroxide was slowly added to the flask. Subsequently, the mixture was shaken in a water bath at 30 C for 3 h. After the incubation, the impregnated activated carbon was filtered and dried in an oven at 80 C. This process is termed the soaking method. For incipient wetness impregnation (IWI), the impregnated activated carbon was not filtered, but only dried in an oven at 80 C. The dried impregnated activated carbon was then subjected to calcination at 400 C under a flow of N2 at 50 ml/min for 3 h. Adsorption test An H2S adsorption test was performed using a packed bed reactor test rig as depicted in Fig. 1. The composition of simulated biogas was adjusted by controlling the flow rate of gases using Aalborg AFC26 mass flow controllers, supplied by Bioclear Sdn Bhd, Malaysia. CO2 and CH4 were passed through a humidification system to provide approximately 25% relative humidity to the biogas inlet stream. H2S was not passed through the humidification system because H2S is soluble in water. The path of the biogas between the humidification system and reactor was insulated to avoid moisture condensation. The diameter of the stainless steel tubular reactor used was half an inch. The flow rate of the biogas was fixed at 500 ml/min with 40% CO2, 3000 ppm H2S and balance with CH4. This biogas composition was simulated to mimic the data obtained from a collection pond located at the FELDA Besout Palm Oil Mill that is owned by FELDA Palm Industries Sdn Bhd, Malaysia. In all runs, 1.0 g of PSAC was placed in the middle of the tubular reactor and supported with approximately 0.05 g of glass wool. The adsorption temperature was controlled at 30 C using a Linberg/Blue M tube furnace. The concentration of H2S at the outlet stream was analysed using an IMR 6000 gas analyser equipped with an electrochemical sensor calibrated for 0–5000 ppm H2S. The test rig workspace was ventilated as a safety measure in case a leakage of hazardous H2S occurred. Characterizations Several characterizations, such as N2 adsorption–desorption, scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM–EDX), thermogravimetric analysis (TGA), X-ray fluorescence spectroscopy (XRF), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS)
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L.C. Lau et al. / Journal of Environmental Chemical Engineering 3 (2015) 1522–1529
Fig. 1. Schematic diagram of a H2S adsorption test rig.
were performed on selected adsorbents to analyse the surface properties, functional groups and morphology/composition. N2 adsorption–desorption was carried out with an Autosorb IC Quantachrome analyser. The adorbent was degassed for at least 5 h at 120 C before the analysis was conducted. Degassed adsorbent was cooled in liquid nitrogen and analysed by computer software (Micropore v2.46), which allowed for rapid numerical results from the adsorption–desorption isotherm. SEM–EDX analysis was performed by a Quanta FEG 450 at an accelerating voltage of 3 kV to obtain the adsorbent surface morphology and to determine the localized chemical composition. TGA was performed using a PerkinElmer TGA7 with a heating rate of 10 C/min at a 100 ml/min flow rate of purified air. XRF was performed using a Rikagu RIX 3000. FTIR was performed using a Thermo Scientific Nicolet iS10 FT-IR Spectrometer. XPS was performed using a High Resolution Multi Technique X-Ray Spectrometer (Axis Ultra DLD XPS, Kratos) to study the chemical state of the selected sorbents. The pass energy used was 25 eV and the charging effect was corrected using the C 1s peak at 284.5 eV. Results and discussion The H2S adsorption capacity for the adsorbent prepared under different conditions is shown in Table 2. The tick in the table indicates the condition that was used in the adsorbent synthesis route. The hyphen in the table denotes that the condition was not used in the adsorbent synthesis route. For example, the Adsorbent N5 was prepared using the soaking method to impregnate cerium
and sodium hydroxide onto its porous structure. Subsequently, a calcination step was employed before the adsorbent was tested for its H2S adsorption capacity. Fig. 2 shows the breakthrough curve for H2S removal using Adsorbent N5. The calculation of the adsorption capacity, Q, is shown in Eq. (1). Z t c0 M w q c 1 dt (1) Q¼ 1000wV m 0 c0 where c0 is the inlet H2S concentration (ppm), Mw is the molecular weight of H2S (g/mol), q is the total gas flow rate (l/min), w is weight of the adsorbent (g), Vm is the molar volume (l/mol), and c (ppm) is the outlet H2S concentration at time t (min). The adsorption capacity is reported in units of mg H2S/g of adsorbent at c/c0 = 0.05 and 0.50. In an adsorption study, the adsorption capacity is generally calculated at a c/c0 between 0.01 and 0.05 and is termed the breakthrough capacity. In practice, an adsorbent at the top of the adsorption column is saturated with H2S. Therefore, the saturated adsorption capacity of the activated carbon must be determined also. Theoretically, the breakthrough curve is sigmoid and is symmetrical at c/c0 = 0.50. Therefore, the saturated capacity can be calculated by taking the corresponding time at c/c0 = 0.50 as the saturated time. Selection between the incipient wetness impregnation and soaking method Impregnation is generally performed using two methods: incipient wetness impregnation and soaking method. Incipient
Table 2 H2S adsorption capacity for adsorbents prepared at different condition. Adsorbent
Breakthrough capacity (mg/g)
Saturated capacity (mg/g)
IWI/soak
Cerium
NaOH
Calcination
N1 N2 N3 N4 N5 N6 N7 N8 N9 N10 N11
1.83 1.83 51.50 1.315 54.57 0.86 9.04 0.80 0.66 67.10 0.70
4.20 3.60 171.82 13.49 187.08 4.07 128.67 1.73 1.06 153.79 1.06
– Soak Soak Soak Soak IWI IWI IWI IWI Soak IWI
– U U U U U U U U – –
– – U – U – – U U U U
– – – U U – U – U – –
L.C. Lau et al. / Journal of Environmental Chemical Engineering 3 (2015) 1522–1529
Fig. 2. Breakthrough curve for H2S removal using adsorbent N5.
wetness impregnation is performed by adding the minimum amount of aqueous solution containing the compound to be impregnated to the support. Subsequently, the mixture is dried in an oven. By contrast, the soaking method is performed by placing the activated carbon in an aqueous solution and allowing the material to soak for a certain period of time (3 h in this study). Then, the activated carbon is filtered and dried. In incipient wetness impregnation, the amount of metal impregnation into the activated carbon is identical to that which is initially added to the solution because water alone is assumed to evaporate. However, in the soaking method, the amount of metal impregnation is not the same as the amount of metal amount initially added. The amount of metal impregnated depends on the soaking time and temperature, which affects the diffusion of the metal species into the porous structure of the activated carbon. The incipient wetness impregnation method is simpler but requires a higher heat input to evaporate the added water. In addition, the distribution of the impregnated metal might not be uneven. The soaking method, however, is wasteful because a portion of the chemical is filtered. Nevertheless, both methods have advantages and disadvantages. Therefore, a suitable method must be identified in this study. From Table 2, most adsorbents prepared with incipient wetness impregnation did not have a good H2S adsorption capacity compared to those prepared with the soaking method. This lower capacity could be because of the high concentration of impregnates used in this study. Regarding the physical appearance of the adsorbent, a lump of cerium oxide with a pale blue colour was observed to cover the surface of the adsorbent. Thus, the surface area of the adsorbent was severely reduced, resulting in a poor adsorption capacity. Conversely, adsorbents prepared by the soaking method generally had a good adsorption capacity. Therefore, for the range of the parameters used in this study, we concluded that the soaking method was better than incipient wetness impregnation.
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oxide. However, Adsorbent N4 was still not a good H2S adsorbent. Between Adsorbents N4 and N7, Adsorbent N7 was actually a special case because it had a higher saturated adsorption capacity compared to Adsorbent N4. In this case, the incipient wetness method might be more suitable because of the amount of cerium that could be added to the activated carbon. For both Adsorbents N4 and N7, an identical amount of cerium nitrate was added to the aqueous solution. However, in the soaking method, not all of the cerium nitrate molecules will diffuse into the porous structure of the activated carbon. Thus, cerium nitrate will be removed by subsequent filtration in an aqueous solution. Therefore, the amount of cerium added to the activated carbon was much less compared to the incipient wetness method. Conversely, the phenomenon of a low breakthrough capacity but high saturated capacity showed that H2S adsorption on Adsorbent N7 was not rapid enough to maintain a high removal rate of H2S from the simulated biogas stream. Cerium nitrate can be converted to cerium oxide by adding sodium hydroxide or be decomposed using a heat treatment called calcination. The former method was exemplified by Adsorbent N3 and the latter by Adsorbent N7. The breakthrough capacity for Adsorbent N7 was as low as that for unmodified activated carbon. However, the saturation capacity was more than 30-fold higher than that for unmodified activated carbon which indicated that cerium oxide participated in H2S adsorption but at a slower rate. Adsorbent N3, in which cerium oxide was prepared from the reaction of sodium hydroxide and cerium nitrate but without calcination, had a much higher breakthrough and saturated capacity that was sufficient for industrial application. This enhanced adsorption capacity probably arose from the synergistic effect of cerium oxide and sodium hydroxide. Sodium hydroxide played an important role in H2S adsorption because H2S is acidic. Therefore, using a basic material for acid gas removal is a reasonable method. Adsorbent N10, prepared by the impregnation of sodium hydroxide, showed a high breakthrough and saturated capacity without the addition of cerium. No cerium addition was necessary, because cerium would have been a waste product in this route. However, coupling with cerium caused an increase in the saturated capacity from 153.79 mg/g (Adsorbent N10) to 171.82 mg/g (Adsorbent N3), a very significant 11.7% increase of the saturated capacity. The synergistic effect of cerium oxide and sodium hydroxide showed that a fast kinetic reaction could be achieved. Sodium hydroxide, together with the humidity in the biogas stream, enhanced the solubility of H2S on the porous surface of PSAC [35]. This provided sufficient time for cerium oxide to further oxidize H2S into products. A further increase to 187.08 mg/g (Adsorbent N5) was also observed when calcination was applied. Calcination enabled better dispersion of the active species on the activated carbon and converted the inactive cerium nitrate into active cerium oxide. From these findings, the better route to prepare CeO2/NaOH/PSAC was the production of cerium oxide from the reaction of cerium nitrate and sodium hydroxide using the soaking method further enhanced by calcination.
Role of cerium oxide, calcination and sodium hydroxide Characterizations of the selected adsorbents In this study, cerium in the nitrate form was added to activated carbon. Neither NaOH nor calcination was added to the Adsorbent N2. Therefore, this adsorbent can be considered as doping cerium nitrate on activated carbon. The adsorption capacity indicated that Adsorbent N2 was not a good adsorbent and that it was even worse than unmodified activated carbon. Therefore, the actual active species for the H2S adsorption reaction was cerium oxide instead of cerium nitrate. Adsorbent N4, which was prepared using the soaking method and calcination after cerium nitrate impregnation, showed a better result than Adsorbent N2, indicating the necessity to convert inactive cerium nitrate into more highly active cerium
Fig. 3 shows the surface morphology of the raw PSAC used in this study. Three different surface structures were found: (a) a rough terrain without regular structures, (b) a cratered surface with pores smaller than 1 mm and (c) canyon-like structures. These structures formed because of the decomposition of the organic components during the activation process. Among these structures, the porous cratered surface (further magnified in Fig. 3d) was estimated to contribute to a larger portion of the surface area and the pore volume of the raw activated carbon. Regardless of the large surface area of the unimpregnated PSAC, the
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Fig. 3. SEM micrograph of Adsorbent N1: raw PSAC at various surfaces and magnifications, (a) 1500, (b) 1000, (c) 1000, and (d) 8000.
H2S adsorption capacity obtained in this study was low. PSAC should be a significant adsorbent for H2S. The low H2S adsorption capacity could be due to the high flow rate, a low amount of adsorbent and the high H2S concentration used in this study. Therefore, the adsorption capacity reported for raw PSAC was insufficient for industrial application. Additionally, because of the nature of the experimental setting used, the adsorption capacity of other adsorbents synthesized in this study should be generally be higher because further optimization of the experimental setup is necessary. The XRF results for the raw PSAC (Adsorbent N1) are shown in Table 3. The results show that SiO2 led to the highest amount of oxides in PSAC. These oxides were more precisely located on the cratered surface using SEM–EDX techniques as shown in Fig. 4 and Table 4. These oxides should be the main components of the ash content obtained from the proximate analysis of the raw PSAC because the total amount obtained from XRF was similar. The proximate analysis showed that the moisture and the content of volatiles, fixed carbon and ash of the raw PSAC were 21.2, 4.2, 72.4 and 2.2%, respectively. In addition, these oxides could theoretically assist or catalyse the H2S removal reaction to a
certain extent. Nevertheless, further examination of the effect of these oxides on H2S removal was not performed because that analysis was out of the scope of this study. Fig. 5 shows the TGA of the raw PSAC in changes of weight and derivative weight with temperature. The first peak in the derivative weight change at approximately 100 C should be attributed to the moisture adsorbed by the raw PSAC. Another peak in the derivative weight change was identified at approximately 700 C, which should be
Table 3 XRF result of Adsorbent N1: raw PSAC. Composition
Amount (wt%)
MgO SiO2 P2 O 5 SO3 Cl K2O Fe2O3
0.1586 2.6444 0.0598 0.1112 0.0436 0.4888 0.0380 Fig. 4. SEM micrograph of SiO2 spheres on Adsorbent N1: raw PSAC.
L.C. Lau et al. / Journal of Environmental Chemical Engineering 3 (2015) 1522–1529 Table 4 EDX of SiO2 spheres on Adsorbent N1: raw PSAC (refer to Fig. 4).
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Table 5 EDX of Adsorbent N5 at the tiles of impregnate (refer to Fig. 6).
Element
Atomic%
Element
Atomic%
C O Na Mg Si K Ca Mn Fe
29.86 47.77 0.16 0.30 14.62 1.81 4.88 0.11 0.49
C O Na Si K Ca Ce
60.16 26.31 1.63 1.91 0.42 1.58 8.00
Fig. 5. TGA of Adsorbent N1: raw PSAC.
the oxidation of fixed carbon into carbon dioxide. The burning of carbon in air began at approximately 500 C and ended approximately at 800 C [35]. The small amount of material retained was ash and likely consisted of metal oxides as shown in Table 4. Upon impregnation of cerium oxides, impregnate tiles formed at the surface of PSAC as shown in Fig. 6. The size of these impregnates was approximately 10 mm, which could not be considered well-dispersed. Nevertheless, after calcination at 400 C, the lumps of impregnates were found to crack into smaller pieces and produce a higher surface area for subsequent H2S adsorption. These impregnates were determined using EDX techniques and the results are shown in Table 5. The EDX result
Fig. 6. SEM image for Adsorbent N5.
showed that these impregnates consist of a detectable amount of elemental cerium and sodium in atomic percentage, which indicated that these impregnates were aggregates of cerium oxide and sodium hydroxide. FTIR spectra of the selected adsorbents are shown in Fig. 7. From the FTIR spectra, the functional groups detected were roughly similar, but with different intensities. Unmodified PSAC show a high-intensity band at approximately 3400 cm1, which indicates phenolic or hydroxyl groups [25,36]. However, after the impregnation of cerium, the intensity of these functional groups was reduced. This could be because, during the calcination step used in the preparation of these adsorbents, these functional groups decomposed at the calcination temperature. In addition, calcination could also eliminate all of the alkyl groups, such as CH3,¼CH2, CH2CH3, which were detected only in raw PSAC at 2900 cm1 [37]. Other functional groups were also detected for all of the adsorbents at 3600–3900 cm1 corresponding to OH stretching [27]; 1400–1680 cm1 corresponding to lactonic and carbonyl groups [38]; 1020–1220 cm1 corresponding to the carboxylic group [39]; and 400–700 cm1 corresponding to C H out of plane bending [27]. For the adsorbents impregnated with cerium, the carbonyl C¼O stretch because of CeO2 was also detected [27]. In the analysis of chemical states using the XPS technique, Ce4+ should result in six peaks at binding energies of 882.2, 888.6, 898, 900.7, 907.2 and 916.15 eV. Ce3+ should, on the other hand, show peaks at 880.6, 884.4, 899.3 and 903.9 eV [40]. The concentration of Ce4+ or Ce3+ on the sorbent surface can be calculated [41]. Ce 3d XPS spectra for Sorbents N5 and N7 are shown in Fig. 8. The Ce4+ concentration for Sorbents N5 and N7 was found to be 50.5 and 52.6%, respectively. In addition, the O 1s XPS spectra for Sorbents N5 and N7 are shown in Fig. 9. O2 was indicated in the oxide bonds. The analysis showed that the cerium species were CeO2 and Ce2O3. CeO2 in Sorbent N5 was derived from cerium nitrate using sodium hydroxide and the calcination technique. On the other
Fig. 7. FTIR spectra of the selected adsorbents.
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adsorption capacity. Characterization studies showed that cerium species in the form of cerium oxides were the active species for H2S removal. Acknowledgments The authors would like to acknowledge the financial supports from MyBrain 15 Program, Knowledge Transfer Program (grant no. 203/PJKIMIA/6750045), FELDA Palm Industries Sdn Bhd, and Universiti Sains Malaysia RU-PRGS (grant no. 1001/PJKIMIA/ 8045042). References
Fig. 8. Ce 3d XPS spectra for selected sorbents.
Fig. 9. O 1s XPS spectra for selected sorbents.
hand, CeO2 in Sorbent N7 was formed using the calcination technique only. The small difference of Ce4+ concentration in both samples showed that the addition of sodium hydroxide in the sorbent preparation step would not affect the formation of cerium oxide significantly. From the XPS analysis of Ce 3d, it was found that the amount of Ce4+ and Ce3+ on the sorbent surface was almost the same. However, the notation of the adsorbent name is CeO2 instead of Ce2O3 because hydrogen sulphide sorption actually involves the oxidation of hydrogen sulphide to elemental sulphur by CeO2. This indicated that CeO2 was the active species in the sorption process. By varying the cerium loading, the NaOH concentration and the calcination process, it is necessary to optimize the process to obtain a higher concentration of Ce4+. Conclusions CeO2/NaOH/PSAC was successfully synthesized and used for the removal of H2S from a simulated biogas stream. The better synthesis method found for the catalyst was the impregnation of both cerium oxide and sodium into the PSAC with the soaking method. Incipient wetness impregnation was not appropriate because it reduced the surface area of the adsorbent and therefore the adsorption capacity. A synergistic effect of cerium oxide and sodium hydroxide was also identified in this study. In the synthesis route, the calcination step was crucial to further enhance the
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