- Email: [email protected]
Development of LaFeO3 modified with potassium as catalyst for coal char CO2 gasification
Journal of CO₂ Utilization 32 (2019) 163–169
Contents lists available at ScienceDirect
Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Development of LaFeO3 modified with potassium as catalyst for coal char CO2 gasification
T
⁎
Qiyao Wang1, Cong Luo1, Xiaoshan Li , Haoran Ding, Cheng Shen, Dingshan Cao, Liqi Zhang State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Coal char CO2 gasification Perovskite Potassium
Coal catalytic gasification is an effective way to realize clean coal utilization. The core of this technology is the preparation of highly efficient and stable catalysts. At present, perovskite-type oxides have attracted increasing attention as a promising catalyst because of its good thermal stability and redox activity. In this study, perovskite-type oxides were proposed to act as catalyst for coal char CO2 gasification, and K was further used for LaFeO3 catalyst modification to improve its catalytic activity. The catalytic performances of La1-xKxFeO3 (x=00.9) were tested by thermogravimetric analysis and fixed-bed tube reactor. The crystal structure was determined by X-ray diffraction (XRD), the percentage content and valence state of surface elements were examined by X-ray photoelectron spectroscopy (XPS), the surface morphology of perovskite samples was observed by scanning electron microscopy (SEM). Besides, the specific surface area and average particle diameter were analyzed by N2 absorption/desorption technology (BET) and laser particle size analyzer, respectively. Results suggested that the perovskite-type catalyst can effectively lowered the initial reaction temperature and exhibited improved catalytic activity after a part of La3+ at A-site was replaced by K+. The time required to achieve 50% carbon conversion in the initial reaction was reduced by 87.3% when the molar ratio of K+ increased from 0 to 0.7. The La0.3K0.7FeO3 perovskite also exhibited relatively stable catalytic activity during cyclic process, and its optimal temperature in coal char CO2 gasification was approximately 850 °C. The XRD patterns indicated that the perovskite structure of catalyst can remain unchanged when K+ doping ratio was lower than 0.8. Therefore, 0.7 is a suitable doping ratio. The perovskite La0.3K0.7FeO3 still exhibited perovskite crystal structure after 10 cycles of experiment. The XPS results indicated that additional oxygen vacancies and Fe4+ formed after K+ doping, which contributed to gas adsorption and electron transfer and improved oxidation performance. SEM and BET showed that the pore structure of catalyst was destroyed during cyclic gasification, resulted in the enlargement of particle size detected by laser particle size analyzer. Leading to the decrease in effective contact area and catalytic performance. Thus, K+-modified perovskite La1-xKxFeO3 is a kind of equipment-friendly, efficient and recyclable catalyst and can be used as a promising catalyst for coal char CO2 gasification.
1. Introduction Coal is the main fossil resource and energy in China, but pollutants originating from coal combustion have considerably harmed the ecological environment and human health. Developing clean coal technology becomes an inevitable choice. Considering the influences of CO2 on global climate change, how to utilize CO2 after it was captured from flue gas is a hot research topic. Coal gasification with CO2 can realize the utilization of CO2 and then the CO product can be further used for chemical industry [1–4]. Therefore, Carbon dioxide gasification of coal char that could achieve the clean utilization of coal and CO2 utilization at the same time has received increasing attention. Conventional
gasification techniques do not use catalysts and the operating temperature is generally higher than 1000 °C. The gas composition is complicated and difficult to purify, thereby restricting its development and utilization. However, the use of catalyst in coal gasification can effectively reduce the reaction temperature, shorten the gasification time, and regulate the components of gaseous production, thereby lowering the energy consumption and equipment requirements of the gasification reaction [5–7]. Many studies have been carried out to date and found that alkali and alkaline earth metals (K, Na, Ca, etc.) are the most commonly used as catalysts for coal char CO2 gasification due to their relatively low cost and acceptable catalytic performance. However, these types of catalysts are easy to vaporize, corrode equipment
⁎
Corresponding author. E-mail address: [email protected] (X. Li). 1 Authors Q.W. and C.L. contributed equally. https://doi.org/10.1016/j.jcou.2019.04.010 Received 11 January 2019; Received in revised form 2 March 2019; Accepted 16 April 2019 Available online 29 April 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.
Journal of CO₂ Utilization 32 (2019) 163–169
Q. Wang, et al.
during high temperature reactions, and are difficult recycle [8–10]. Transition metals (Fe, Ni, etc.) are also considered as good catalysts because of their advantages of noncorrosive to equipment, low consumption, good catalytic activity, and the ability to regulate the composition of product gas. However, the vulnerability of transition metals to poisoning and inactivation in gasification limits their wide used in coal gasification process [6,11]. The exploration of new catalysts for coal gasification with high efficiency and good cyclic performance is still a topic receiving considerable attention. Perovskite-type oxides have been widely studied and used as a catalyst previously because of their high thermal stability, good redox property, and strong lattice O transportation ability. The structural formula of perovskite oxides is ABO3, where A-cations are located on the edge of the crystal structure and play a major role in stabilization, and B-cations are located in the center of structure and mainly affect the catalytic activity because of their variable valence states. The elements at A- and B-sites can be completely or partially replaced by other metallic elements without destroying the perovskite-type crystal structure, which indicated that catalytic performances of the materials can easily be controlled by adjusting the constituent elements [12–15]. Among all these studies, La-Fe perovskite has received considerable attention due to its low price, easy molding, and good thermal stability, and many efforts have been devoted to improve the catalytic properties of LaFeO3 by doping at the A/B site. Runduo et al. [16,17]investigated the effects of Cu2+ and Pb2+ ion doping on LaFeO3 for catalytic reduction of NO by propene and found that N2 yields were improved significantly after partial Fe3+ was substituted. Sun et al. [18] reported that the high Ce content is unconducive to the formation of perovskite structure, but moderate doping can provide active O and considerably improve catalytic performance without destroying the crystal structure. Khine et al. [19] studied the capabilities of (La0.7A0.3)BO3 (A = Ba, Ca, Mg, Sr, and B = Cr or Fe) for the partial oxidation of CH4 and found that Sr doping improves the mobility of lattice O, and (La0.75Sr0.25)FeO3 can achieve 65% CH4 conversion and obtain the highest H yield. We have previously investigated the properties of perovskite-type oxides as O carrier for chemical looping reforming of CH4 and studied the characteristics, especially the catalytic performance and O desorption [20,21], in detail. Perovskite-type oxides exhibit excellent catalytic performance for high-temperature gas–solid reactions. Thus, whether perovskite can be further used in catalytic gasification of coal char with CO2 is an attractive research topic. This work aimed to use LaFe-based perovskite oxides as a new catalyst for coal char CO2 gasification. K+ ions, which demonstrate low valence, high activity, and mobility and contribute to the formation of O vacancies and the increase in valence state of B-site ions [22–24],were selected for A-site doping to improve the catalytic properties of La-Fe-based perovskite oxides. New La1-xKxFeO3 (x = 0-0.9) catalysts were prepared via sol-gel method. The catalytic activity and cycle performance were tested using a thermogravimetric analyzer and fixed-bed reactor, and the proper doping ratio of K+ and reaction temperature for coal char CO2 gasification were investigated. The crystal structure, surface states of O and Fe, and surface topography after K+ doping and gasification reaction were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM), respectively.
Table 1 Zhundong demineral coal properties. proximate analysis (wt.%,air-dry)
M 2.78 a
A 0.52
V 2.7
Ultimate analysis (wt.%,dry and ash free) FC 94
C 69.86
H 0.81
Oa 28.89
N 0.44
Calculated by difference method.
ambient condition, and then deionized water was used to wash the coal until the solution was neutral. After filtration, the coal was added to a solution of 2 L hydrofluoric acid (3 mol/L) and 2 L deionized water and repeat the stirring and washing process as described above. The samples after filtration were dried at 105℃ for 24 h to obtain the de-mineral coal. In the end the de-mineral coal were heated from room temperature to 950℃ at the rate of 10℃/min and stay at 950℃ for 30 min to obtain the de-mineral char. The analyses data of ultimate de-mineral char were shown in Table 1 The perovskite-type oxides La1-xKxFeO3 were prepared by employing citric acid-aided sol-gel method. The starting materials were analytical purity, metal nitrate La(NO3)3·6H2O, KNO3 and Fe (NO3)3·9H2O were prepared according to stoichiometric ratio and dissolved in citric acid aqueous solvent and stirred at 70℃ for 4 h until sticky gel was formed, then the gel was placed in an air-circulating oven at 105℃ for 12 h. The dried gel was heated from ambient temperature to 400℃ at a rate of 10℃/min and kept at 400℃ for 1 h to burn out the organic compounds. The final remains were calcined at 750℃ for 5 h for nitrate decomposition and the perovskite-type catalysts were obtained after grinding, the particle size of grinded samples was less than 1 μm. 2.2. Gasification tests The experiment samples were prepared by mixing perovskite and de-mineral coal char (50:50wt.%) mechanically for 10 min in agate mortar, then put it in an oven for drying. The catalytic activity of catalyst for de-mineral coal CO2 gasification was investigated in the thermogravimetric analysis (TGA). About 18 mg of experiment samples were heated from 200℃ to 950℃ at 10℃/min heating rate in a flow of CO2 (100 ml/min). The initial reaction temperature (Ti) and the temperature at maximum weight loss rate (Tm) were used as an evaluation index of catalytic performance. The cyclic performance of catalysts was investigated in a fixed-bed tubular quartz reactor system (Φ = 20 mm, L = 600 mm). Approximately 0.5 ± 0.005 g reaction samples were added into quartz boat and placed into a quartz tube that was in the mixed flow of CO2/N2 (100:400 ml/min flow rate). The quartz tube was placed into the tube furnace at a predetermined temperature, and the composition of gas productions was monitored continuously by an online gas analyzer (Cubic Inc., Gasboard-3100). The feeding gas was switched to air (200 ml/min) until the CO concentration dropped to 0. Then, the quartz tube was removed from the furnace to cool the reactor to room temperature. The reaction catalyst sample remaining in the quartz boat was recovered and weighed for the next cycle. The number of cycles (3–10) varied with the catalyst. t50%, which was defined as the time for carbon conversions to reach 50%, was used to evaluate the catalytic activities of the catalyst; the lower the t50% value was, the better the catalytic performance was.
2. Experimental 2.1. Material preparation and characterization Zhundong(ZD) coal from Xinjiang region in China was selected for this study. The coal was ground below 48 μm. To avoid the potential confusion from minerals in coal, de-mineral coal was prepared by HCLHF method. Detailed steps are as follows: 100 g zhundong raw coal was added to a solution of 1 L hydrochloric acid (6 mol/L) and 3 L deionized water, the mixture was stirred for 24 h and placed for 12 h under
2.3. Characterization The perovskite-type phase of as-prepared catalysts was measured by X-ray diffusion (XRD) on X’Pert PRO (PANalytical B.V.) with Cu Kα radiation (λ = 0.1542 nm). X-ray tube voltage and current were 40 kV and 30 mA respectively. The diffractogram in the 2θ range of 20°–80° 164
Journal of CO₂ Utilization 32 (2019) 163–169
Q. Wang, et al.
Fig. 1. DTG profiles of CO2 catalytic gasification over La1-xKxFeO3.
Fig. 2. Carbon conversion of catalytic ZD-dem char CO2 gasification with La0.7K0.3FeO3 under different temperatures.
was recorded with a scanning step of 0.02°.The surface content and valence of oxygen and iron were carried out via X-ray photoelectron spectroscopy (XPS), performed with AXIS-ULTRA DLD X-ray Photoelectron Spectrometer and an Al-Kα anode. Binding energies were calibrated by using C1s at 284.8 eV as internal standard. The microstructures of particles were obtained by scanning electron microscopy (SEM) on FEI Inspect F50 operated at 10 kV. The Brunauer, Emmett and Teller (BET) specific surface area analysis were tested by N2 at 77 K on a 3H-2000PS instrument (Beishide Instrument Technology Co., Ltd.). The average size of the samples were determined by Laser particle size analyzer (ZS90, Malvern Instruments Ltd., UK).
to these results, the effect of reaction temperatures and cyclic performance of catalyst on char CO2 gasification experiments were carried out using La0.7K0.3FeO3 as catalyst. To investigate the influence of reaction temperature on the catalytic activity of La0.7K0.3FeO3 further, we carried out CO2 gasification experiments at the temperature ranging from 700 to °C 900 °C. As shown in Fig. 2, the reaction rates increased with temperature, especially ranging from 700 °C 850 °C. Coal char CO2 gasification is an endothermic reaction, and the increase in temperature is conducive to the gasification process. Because the adsorption process is an exothermic process, high reaction temperature contributed to the desorption of adsorbed oxygen species. Our previous research [20,28] also suggested that perovskite-type oxides exhibited better catalytic activity at higher temperature, promoting the consumption of adsorbed oxygen. Thus, more lattice oxygen would travel through oxygen vacancies to supply the oxygen species on the surface and be released, which leads to improved redox performance. When the temperature reaches 850 °C, the catalytic activity is relatively high, and the carbon conversion rate of gasification is mainly dependent upon the redox performance of the catalyst. Excessively high temperature has a negative effect on the surface morphology of catalyst. Thus, 850 °C is considered as the optimal temperature for the catalytic CO2 gasification of ZD-dem char with perovskite.
3. Results and discussions 3.1. Catalytic performance of different perovskites The CO2 catalytic activity of ZD dem-char with different perovskites evaluated by TGA is shown in Fig. 1. The figure indicates that LaFeO3 catalyst exhibited the lowest activities and has insignificant effect on the increase in the coal char CO2 gasification rate. The catalytic performance of La-Fe-based perovskite was enhanced when 10% (molar fraction) of La3+ at A-site was replaced by K+, but it still can only react at high temperatures. When the doping ratio x increased from 0.1 to 0.3, a significant improvement in the catalytic performance was observed. The carbon conversion rate did not only increased significantly, but the Ti value also decreased from 821 °C to 713 °C. Afterward, the Ti and Tm values showed a decreasing trend with the increase of K+ ions at the A-site, which indicated that the catalytic performance was further improved, and the requirement of temperature was reduced. One possible reason for the increased catalytic activity of K+-doped perovskite is the change in Fe valence state and oxygen vacancy formation to maintain electron neutrality when some parts of trivalent La3+ are substituted by monovalent K+ [23,24,24]. Another reason may due to the high catalytic activity of K+ for coal gasification processes [25,26]. Therefore, the K+ doped at the A-site positively affects catalytic performance improvement. When the molar ratios of K+ at the A-site was > 0.7, the reaction curves showed that further K+ doping at A-site only slightly contributed to the improvement of the catalyzing performance of the modified perovskite-type catalyst. This result was possibly observed because the degree of substitution of the A-site ions was limited. Excessive K+ ions can only attached to the surface of catalyst after saturation. Although those K+ can complement the lattice K+ ions during cyclic reactions. Surface K+ ions were also easy to melt in gasification reaction because of its low melting point, leading to the increase of agglomerate strength of catalyst particles [14,27]. According
3.2. Cyclic performance of recovered catalysts To study the cyclic stability of K+-doped perovskite catalysts, we reused La1-xKxFeO3 for coal-char gasification in fixed-bed reactor at 850 °C, and the results are presented in Fig. 3. The results showed that LaFeO3 perovskite without K+ doping required the longest reaction time to achieve 50% carbon conversion. After a small amount of K+ was doped at the A-site of perovskite, the catalytic activity of La0.9K0.1FeO3 was enhanced significantly, which was in agreement with the TGA results. However, the catalytic performance of perovskite catalyst with low K+ amounts showed significant reduction in cycle experiments. With the introduction of additional K+, the catalysts exhibited increased catalytic activity and cycling performance stability. Compared with undoped sample perovskite, the t50% value of La0.3K0.7FeO3 in the first cycle decreased by 87.3% and only increased by 2.6 min after reusing 7 times. A significant decrease in the reaction time was observed between the seventh and eighth cycle, which was still less than that of La0.3K0.7FeO3 during the first cycle. The detailed data of the catalytic coal char CO2 gasification by using La0.3K0.7FeO3 catalyst at 850 °C on fixed bed are shown in Fig. 4. The 165
Journal of CO₂ Utilization 32 (2019) 163–169
Q. Wang, et al.
Fig. 3. t50% value of CO2 catalytic gasification over La1-xKxFeO3 for different cycle times.
Fig. 4. Reaction rates of La0.7K0.3FeO3 for cyclic ZD-dem char CO2 gasification.
undoped perovskite was included as a reference. The figure indicates that the carbon conversion increased to 90% in a straight line over time and exhibited stable catalytic performance in the first seven cycles. Then, the carbon conversion rate decreased gradually, which was possibly due to the fact that sintering on the perovskite surface reduced the contact efficiency between coal char and catalyst, and the sintering phenomenon became increasingly serious with the progress of the cyclic reaction. Thus, the carbon conversion rate began to decline when the carbon conversion ratio reached 80%.
Fig. 5. XRD patterns of La1-xKxFeO3(x = 0–0.9); (a) the whole range; (b) an enlarged screenshot of La1-xKxFeO3(x = 0.7–0.9, 2θ = 29°–44°).
3.3. Catalyst characterization Fig. 5 displays the XRD patterns of La1-xKxFeO3(x = 0-0.9). All the characteristic peaks of perovskite weakened with the increase in K+ at the A-site but still exhibited the perovskite crystal structure when the molar ratio of K+ was < 0.8. The peaks of other impurities were undetected, which indicated that K+ ions entered into the lattice of perovskite [29,30]. Fig. 5(b) shows the enlarged XRD patterns (2θ = 29°–44°) of the modified perovskite (x = 0.7–0.9) after the excess K + was incorporated into the perovskite oxides. Some byproduct peaks were observed. A weak diffraction peak at 30.1° corresponded to the cubic structure of Fe3O4 (JCPDS 87–2334), and the peak at 32.9° that contributed to the distortion of the main characteristic peak of perovskite was ascribed to K3FeO2 (JCPDS 48-0957). The changes in the crystal structure of La0.3K0.7FeO3 after different
Fig. 6. XRD patterns of La0.3K0.7FeO3 before and after different cycles at 850℃. 166
Journal of CO₂ Utilization 32 (2019) 163–169
Q. Wang, et al.
Fig. 7. O1s and Fe2p XPS spectra of perovskite samples; (a, d) fresh LaFeO3, (b) (e) fresh La0.3K0.7FeO3, and (c) (f) La0.3K0.7FeO3 after the first gasification.
529.15–529.21 eV was ascribed to lattice oxygen and marked as Olat. The second peak at 530.99–531.35 eV possibly originated from carbonate species and C1s and was marked as Ocar. Finally, the third peak at 532.02–532.32 eV was ascribed to physical adsorbed oxygen and marked as Oads [21,31]. The comparison results of Fig. 7(a) and (b) suggested that the modified perovskite-type catalyst exhibited high Oads peak and low Olat due to oxygen vacancy formation caused by the partial substitution of monovalent K+ for trivalent La3+, which has been proven to increase oxygen fluidity, facilitate electron transfer, and enhance catalytic activity [32]. The binding energy of modified perovskite was also lower than that of undoped perovskite, which indicated that the lattice O activity was enhanced by K+ doping [33]. Thus, modified perovskite catalysts exhibit improved redox performance. After catalytic gasification reaction, the Olat and Oads contents decreased, and that of Ocar increased, as shown in Fig. 7(c). This result
reaction times were investigated, as shown in Fig. 6. Although the peaks of Fe3O4, which was produced by the reaction of perovskite with CO2, were detected, the modified perovskite still exhibited the characteristic peaks of the perovskite after the first cycle. After nine additional gasification cycles, the perovskite characteristic peaks were still detected, which were only slightly weakened. These results suggested that the modified perovskite La0.3K0.7FeO3 had high crystal structure stability. XPS characterization was performed to investigate the surface state of undoped perovskite and modified perovskite samples before and after gasification. The fitting results of O1s and Fe2p XPS spectra are shown in Fig. 7, and the relative percentages that were calculated from the integrated area of these subpeaks are listed in Table 2. The O1s spectrum for all the three samples showed two peaks in the range of 526–537 eV. After peak separation, three small peaks representing the three different types of oxygen species were obtained. The first peak at
Table 2 Binding energies (BEs) of the Fe2p3/2 and O1s and calculated perovskite sample percentages. Samples
O1s
Fe2p3/2
Olat
LaFeO3 La0.3K0.7FeO3 La0.3K0.7FeO3 1 st
Ocar
Fe2+
Oads
Fe3+
Fe4+
BE (eV)
%
BE (eV)
%
BE (eV)
%
BE (eV)
%
BE (eV)
%
BE (eV)
%
529.2 528.9 528.8
60 47 23
531.3 531.5 531.0
28 27 61
532.2 532.1 532.5
12 26 16
709.8 709.6 709.6
45 32 38
711.3 711.0 710.9
29 32 35
713.1 713.3 713.0
26 36 27
167
Journal of CO₂ Utilization 32 (2019) 163–169
Q. Wang, et al.
Fig. 8. SEM images of La0.3K0.7FeO3 sample; (a) fresh sample, (b) after 1 st gasification, and (c) after 10th gasification.
indicated that the lattice and surface oxygen were involved in catalytic coal char gasification. The high Ocar peak may due to CO2 adsorption. As shown in Fig. 7, the Fe2p3/2 spectra deconvoluted into three subpeaks. The peaks at 709.6–709.9, 710.9–711.9, and 712–714 eV were assigned to Fe2+, Fe3+, and Fe4+, respectively [34,35].A comparison of Fig. 7(d) and (e) suggested that additional Fe3+ and Fe4+ existed in modified perovskite to maintain electrical neutrality. Fe4+ showed strong redox activity and was beneficial to the gasification reaction of coal char. After catalytic gasification, the perovskite sample showed low Fe4+ peak in Fig. 7(f), which indicated that some Fe4+ in the lattice were converted to Fe3+ and Fe2+ during the catalytic reaction. This result agreed with the XRD analysis shown in Fig. 6. The reduction reaction of Fe can also promote the charge transfer from adsorbed oxygen to the adsorbed oxygen and O2. The morphological changes in La0.3K0.7FeO3 during cyclic reactions obtained by SEM were shown in Fig. 8. The fresh sample had uniform spherical crystalline grain distribution and porous structure, as shown in Fig. 8(a). Such structure was beneficial for the improved contact of the catalyst with coke particles. The crystalline grains structure was maintained and showed slight agglomeration after the first cycle, as shown in Fig. 8(b). However, after 10 cycles of the experiment, the crystalline grains melted and fused with other grains, which destroyed the pore structure of the catalyst, as presented in Fig. 8(c). The decreased contact efficiency between coal char and catalyst decreased the carbon conversion rate, as shown in Fig. 4. The average size of the samples was determined by Laser particle size analyzer. It can be seen from Table 3 that the particle size of modified perovskite La0.3K0.7FeO3 changed a little after first cycle of gasification. But the average size of La0.3K0.7FeO3 after 10 cycles increased significantly, which is due to the agglomeration of perovskite particles according to the SEM characterization results. Therefore, the agglomeration of perovskite will resulted in the increase of particle size, which reduced the effective contact area between coal char and perovskite. And eventually lead to a decline in carbon conversion rate during cyclic experiments. The specific surface areas of La1−xKxFeO3 catalysts tested by N2 absorption/desorption method were listed in Table 4. The BET specific surface of K-modified perovskite were between 1 and 9 m2/g, which is in agreement with previous study [36]. As can be seen, K+-doping had a significant effect on the specific surface area of the catalyst, but the change was not linear with doping ratio at A-site. The undoped perovskite had a relatively low specific surface area, 2.6 m2/g, and La0.7K0.3FeO3 had the largest specific area, 8.7 m2/g. It was reported
Table 4 Specific surface area of La1-xKxFeO3 (x = 0–0.9) catalysts.
Particle size(nm)
La0.3K0.7FeO3 fresh La0.3K0.7FeO3 1 st La0.3K0.7FeO3 10th
680 743 1234
SBET(m2/g)
LaFeO3 La0.9K0.1FeO3 La0.7K0.3FeO3 La0.5K0.5FeO3 La0.3K0.7FeO3 La0.2K0.8FeO3 La0.3K0.7FeO3 1 st
2.6 1.5 8.7 4.3 3.3 2.0 1.7
that the changes of specific surface area related to the expansion of unit cells due to the radius of K+ is slightly higher than La3+ [37]. However, the formation of oxygen vacancies resulted in the shrank of unit cell when partial of trivalent La3+ was replaced by K+. Thus, the specific surface area decreased after more K+ ions were doped at A-site. It was also can be seen that the specific surface area of modified perovskite La0.3K0.7FeO3 decreased from 3.3 to 1.7 m2/g after one times of gasification, which proved that the pore structure of the catalyst was damaged, the results of BET analysis were consistent with the results of SEM. 4. Conclusions A series of K+-modified perovskite catalysts La1-xKxFeO3 (x = 0-0.9) was synthesized by the sol-gel method, and their catalytic and cycle performances for coal char CO2 gasification were investigated by TGA and fixed-bed reactor, respectively. The catalytic and cycle performances of perovskite-type catalysts were significantly improved by the partial substitution of La3+ by K+ until the molar ratio of K + reached > 0.7. For La0.3K0.7FeO3, the Ti value of first reaction dropped from 821 °C to 643 °C, and the t50% value only increased by 2.6 min after seven cycles of reaction. The further increase in K+ ions at the A-site did not decrease the reaction time and temperature. The increase in temperature was also beneficial to the improvement of catalytic performance. When the adverse effect of further heating on the surface morphology was considered, 850 °C was the optimal gasification temperature. The XRD patterns showed that the pure perovskite phase was obtained for all catalyst samples when the molar ratio was < 0.7. Thus, 0.7 is the best x value of La1-xKxFeO3. The XRD results also suggested that La0.3K0.7FeO3 had good structural stability because its characteristic perovskite-type crystal structure was maintained after 10 cycles of gasification. The XPS results showed that additional oxygen vacancies and Fe4+ formed after partial La3+ were replaced by K+ to preserve electroneutrality. Thus, the modified perovskite exhibits increased catalytic activity in coal char CO2 gasification. SEM analysis indicated that the spherical particles were uniformly distributed on the surface of fresh catalyst samples, but the La0.3K0.7FeO3 particles melted and merged together in CO2 gasification reaction processing. Besides, the results of BET analysis and laser particle size analyser showed that the catalyst was sintered and the particle size increased during cyclic
Table 3 Average particle diameter of La0.3K0.7FeO3 during cylic experiments. Perovskite
Perovskite
168
Journal of CO₂ Utilization 32 (2019) 163–169
Q. Wang, et al.
reactions, which was not conducive to the effective contact between perovskite and coal char and cycling performance of modified perovskite.
Alloys Compd. 682 (2016) 686–694. [18] Y. Sun, S.S. Hla, G.J. Duffy, A.J. Cousins, D. French, L.D. Morpeth, J.H. Edwards, D.G. Roberts, Effect of Ce on the structural features and catalytic properties of La(0.9−x)CexFeO3 perovskite-like catalysts for the high temperature water–gas shift reaction, Int. J. Hydrogen Energy 36 (1) (2011) 79–86. [19] M.S.S. Khine, L. Chen, S. Zhang, J. Lin, S.P. Jiang, Syngas production by catalytic partial oxidation of methane over (La0.7A0.3)BO3 (A = Ba, Ca, Mg, Sr, and B = Cr or Fe) perovskite oxides for portable fuel cell applications, Int. J. Hydrogen Energy 38 (30) (2013) 13300–13308. [20] H. Ding, Y. Xu, C. Luo, Q. Wang, S. Li, G. Cai, L. Zhang, Y. Zheng, Q. Shen, Oxygen desorption behavior of sol-gel derived perovskite-type oxides in a pressurized fixed bed reactor, Chem. Eng. J. 323 (2017) 340–346. [21] H. Ding, Y. Xu, C. Luo, Q. Wang, C. Shen, J. Xu, L. Zhang, A novel composite perovskite-based material for chemical-looping steam methane reforming to hydrogen and syngas, Energy Convers. Manage. 171 (2018) 12–19. [22] L. Xu, F. Wang, M. Chen, X. Fan, H. Yang, D. Nie, L. Qi, Alkaline-promoted Co-Ni bimetal ordered mesoporous catalysts with enhanced coke-resistant performance toward CO2 reforming of CH4, J. CO2 Util. 18 (2017) 1–14. [23] H. Shimokawa, Y. Kurihara, H. Kusaba, H. Einaga, Y. Teraoka, Comparison of catalytic performance of Ag- and K-based catalysts for diesel soot combustion, Catal. Today 185 (1) (2012) 99–103. [24] H. Liang, Y. Mou, H. Zhang, S. Li, C. Yao, X. Yu, Sulfur resistance and soot combustion for La0.8K0.2Co1-yMnyO3 catalyst, Catal. Today 281 (2017) 477–481. [25] H. Ge, L. Shen, H. Gu, S. Jiang, Effect of co-precipitation and impregnation on Kdecorated Fe2O3/Al2O3 oxygen carrier in Chemical Looping Combustion of bituminous coal, Chem. Eng. J. 262 (2015) 1065–1076. [26] Y.-K. Kim, L.-f. Hao, J.-I. Park, J. Miyawaki, I. Mochida, S.-H. Yoon, Catalytic activity and activation mechanism of potassium carbonate supported on perovskite oxide for coal char combustion, Fuel 94 (2012) 516–522. [27] Y. Teraoka, K. Nakano, W. Shangguan, S. Kagawa, Simultaneous catalytic removal of nitrogen oxides and diesel soot particulate over perovskite-related oxides, Catal. Today 27 (1) (1996) 107–113. [28] Q. Shen, Y. Zheng, S. Li, H. Ding, Y. Xu, C. Zheng, M. Thern, Optimize process parameters of microwave-assisted EDTA method using orthogonal experiment for novel BaCoO3−δ perovskite, J. Alloys Compd. 658 (2016) 125–131. [29] J. Xu, J. Liu, Z. Zhao, C. Xu, J. Zheng, A. Duan, G. Jiang, Easy synthesis of threedimensionally ordered macroporous La1−xKxCoO3 catalysts and their high activities for the catalytic combustion of soot, J. Catal. 282 (1) (2011) 1–12. [30] K. Taniguchi, N. Okinaka, T. Akiyama, Preparation and characterization of La1−xKxFeO3 (x=0–1) by self-propagating high-temperature synthesis for use as soot combustion catalyst, J. Alloys Compd. 509 (10) (2011) 4084–4088. [31] M. Ghaffari, M. Shannon, H. Hui, O.K. Tan, A. Irannejad, Preparation, surface state and band structure studies of SrTi(1− x)Fe(x)O(3− δ)(x= 0–1) perovskite-type nano structure by X-ray and ultraviolet photoelectron spectroscopy, Surf. Sci. 606 (5-6) (2012) 670–677. [32] Y. Teraoka, K. Kanada, S. Kagawa, Synthesis of La-K-M-O perovskite-type oxides and their catalytic property for simultaneous removal of NOx and diesel soot particulates, Appl. Catal. B 34 (1) (2001) 73–78 78. [33] K. Sutthiumporn, T. Maneerung, Y. Kathiraser, S. Kawi, CO2 dry-reforming of methane over La0.8Sr0.2Ni0.8M0.2O3 perovskite (M= Bi, Co, Cr, Cu, Fe): roles of lattice oxygen on C–H activation and carbon suppression, Int. J. Hydrogen Energy 37 (15) (2012) 11195–11207. [34] W. Cheng, X. Ma, Structural, optical and magnetic properties of Fe-doped ZnO, IOP Publishing, 2019, p. 012039. [35] Y. Bu, Q. Zhong, D. Xu, W. Tan, Redox stability and sulfur resistance of Sm0.9Sr0.1CrxFe1−xO3−δ perovskite materials, J. Alloys Compd. 578 (2013) 60–66. [36] Z. Wang, P. Zhao, C. Liang, A. Xu, G. Duan, J. Jiang, J. Xu, Liu, Highly active La1−xKxCoO3 perovskite-type complex oxide catalysts for the simultaneous removal of diesel soot and nitrogen oxides under loose contact conditions, Catal. Lett. 124 (1-2) (2008) 91–99. [37] S. Rousseau, S. Loridant, P. Delichere, A. Boreave, J. Deloume, P. Vernoux, La (1− x) SrxCo1− yFeyO3 perovskites prepared by sol–gel method: characterization and relationships with catalytic properties for total oxidation of toluene, Appl. Catal. B 88 (3-4) (2009) 438–447.
Acknowledgements This work was supported by National Key R&D Program of China (2016YFB0600801), National Natural Science Foundation of China (No.51606076) and Analytical and Testing Center of HUST for XRD measurements. References [1] H. Yang, Z. Xu, M. Fan, R. Gupta, R.B. Slimane, A.E. Bland, I. Wright, Progress in carbon dioxide separation and capture: a review, J. Environ. Sci. 20 (1) (2008) 14–27. [2] X. Li, Q. Huang, C. Luo, D. Zhou, Y. Xu, L. Zhang, C. Zheng, Effect of acid gases on elemental mercury removal in an oxy-fuel CO2 compression process, Energy Fuels 32 (4) (2018) 4334–4340. [3] G. Kumari, P. Vairakannu, Laboratory scale studies on CO2 oxy-fuel combustion in the context of underground coal gasification, J. CO2 Util. 21 (2017) 177–190. [4] B. Hu, Q. Huang, A. Buekens, Y. Chi, J. Yan, Co-gasification of municipal solid waste with high alkali coal char in a three-stage gasifier, Energy Convers. Manage. 153 (2017) 473–481. [5] K.-C. Xie, Coal gasification, Structure and Reactivity of Coal, Springer, 2015, pp. 181–241. [6] B. Bhui, P. Vairakannu, Experimental and kinetic studies on in-situ CO2 gasification based chemical looping combustion of low ash coal using Fe2O3as the oxygen carrier, J. CO2 Util. 29 (2019) 103–116. [7] D.M. Quyn, H. Wu, J.-i. Hayashi, C.-Z. Li, Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part IV. Catalytic effects of NaCl and ion-exchangeable Na in coal on char reactivity☆, Fuel 82 (5) (2003) 587–593. [8] A. Nzihou, B. Stanmore, P. Sharrock, A review of catalysts for the gasification of biomass char, with some reference to coal, Energy 58 (2013) 305–317. [9] K. Matsuoka, T. Yamashita, K. Kuramoto, Y. Suzuki, A. Takaya, A. Tomita, Transformation of alkali and alkaline earth metals in low rank coal during gasification, Fuel 87 (6) (2008) 885–893. [10] P. Lahijani, Z.A. Zainal, A.R. Mohamed, M. Mohammadi, CO2 gasification reactivity of biomass char: catalytic influence of alkali, alkaline earth and transition metal salts, Bioresour. Technol. 144 (2013) 288–295. [11] J. Yu, F.-J. Tian, M.C. Chow, L.J. McKenzie, C.-Z. Li, Effect of iron on the gasification of Victorian brown coal with steam: enhancement of hydrogen production, Fuel 85 (2) (2006) 127–133. [12] F. Rahbar Shamskar, F. Meshkani, M. Rezaei, Preparation and characterization of ultrasound-assisted co-precipitated nanocrystalline La-, Ce-, Zr –promoted Ni-Al2O3 catalysts for dry reforming reaction, J. CO2 Util. 22 (2017) 124–134. [13] P.A. Shaikh, Y.D. Kolekar, Study of microstructural, electrical and dielectric properties of perovskite(0.7)PMN–(0.3)PT ferroelectric at different sintering temperatures, J. Anal. Appl. Pyrolysis 93 (2012) 41–46. [14] H. Wang, J. Liu, Z. Zhao, Y. Wei, C. Xu, Comparative study of nanometric Co-, Mnand Fe-based perovskite-type complex oxide catalysts for the simultaneous elimination of soot and NOx from diesel engine exhaust, Catal. Today 184 (1) (2012) 288–300. [15] H. Tanaka, M. Misono, Advances in designing perovskite catalysts, Curr. Opin. Solid State Mater. Sci. 5 (5) (2001) 381–387. [16] R. Zhang, A. Villanueva, H. Alamdari, S. Kaliaguine, Cu- and Pd-substituted nanoscale Fe-based perovskites for selective catalytic reduction of NO by propene, J. Catal. 237 (2) (2006) 368–380. [17] F. Parrino, E. García-López, G. Marcì, L. Palmisano, V. Felice, I.N. Sora, L. Armelao, Cu-substituted lanthanum ferrite perovskites: preparation, characterization and photocatalytic activity in gas-solid regime under simulated solar light irradiation, J.
169
Contents lists available at ScienceDirect
Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Development of LaFeO3 modified with potassium as catalyst for coal char CO2 gasification
T
⁎
Qiyao Wang1, Cong Luo1, Xiaoshan Li , Haoran Ding, Cheng Shen, Dingshan Cao, Liqi Zhang State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Coal char CO2 gasification Perovskite Potassium
Coal catalytic gasification is an effective way to realize clean coal utilization. The core of this technology is the preparation of highly efficient and stable catalysts. At present, perovskite-type oxides have attracted increasing attention as a promising catalyst because of its good thermal stability and redox activity. In this study, perovskite-type oxides were proposed to act as catalyst for coal char CO2 gasification, and K was further used for LaFeO3 catalyst modification to improve its catalytic activity. The catalytic performances of La1-xKxFeO3 (x=00.9) were tested by thermogravimetric analysis and fixed-bed tube reactor. The crystal structure was determined by X-ray diffraction (XRD), the percentage content and valence state of surface elements were examined by X-ray photoelectron spectroscopy (XPS), the surface morphology of perovskite samples was observed by scanning electron microscopy (SEM). Besides, the specific surface area and average particle diameter were analyzed by N2 absorption/desorption technology (BET) and laser particle size analyzer, respectively. Results suggested that the perovskite-type catalyst can effectively lowered the initial reaction temperature and exhibited improved catalytic activity after a part of La3+ at A-site was replaced by K+. The time required to achieve 50% carbon conversion in the initial reaction was reduced by 87.3% when the molar ratio of K+ increased from 0 to 0.7. The La0.3K0.7FeO3 perovskite also exhibited relatively stable catalytic activity during cyclic process, and its optimal temperature in coal char CO2 gasification was approximately 850 °C. The XRD patterns indicated that the perovskite structure of catalyst can remain unchanged when K+ doping ratio was lower than 0.8. Therefore, 0.7 is a suitable doping ratio. The perovskite La0.3K0.7FeO3 still exhibited perovskite crystal structure after 10 cycles of experiment. The XPS results indicated that additional oxygen vacancies and Fe4+ formed after K+ doping, which contributed to gas adsorption and electron transfer and improved oxidation performance. SEM and BET showed that the pore structure of catalyst was destroyed during cyclic gasification, resulted in the enlargement of particle size detected by laser particle size analyzer. Leading to the decrease in effective contact area and catalytic performance. Thus, K+-modified perovskite La1-xKxFeO3 is a kind of equipment-friendly, efficient and recyclable catalyst and can be used as a promising catalyst for coal char CO2 gasification.
1. Introduction Coal is the main fossil resource and energy in China, but pollutants originating from coal combustion have considerably harmed the ecological environment and human health. Developing clean coal technology becomes an inevitable choice. Considering the influences of CO2 on global climate change, how to utilize CO2 after it was captured from flue gas is a hot research topic. Coal gasification with CO2 can realize the utilization of CO2 and then the CO product can be further used for chemical industry [1–4]. Therefore, Carbon dioxide gasification of coal char that could achieve the clean utilization of coal and CO2 utilization at the same time has received increasing attention. Conventional
gasification techniques do not use catalysts and the operating temperature is generally higher than 1000 °C. The gas composition is complicated and difficult to purify, thereby restricting its development and utilization. However, the use of catalyst in coal gasification can effectively reduce the reaction temperature, shorten the gasification time, and regulate the components of gaseous production, thereby lowering the energy consumption and equipment requirements of the gasification reaction [5–7]. Many studies have been carried out to date and found that alkali and alkaline earth metals (K, Na, Ca, etc.) are the most commonly used as catalysts for coal char CO2 gasification due to their relatively low cost and acceptable catalytic performance. However, these types of catalysts are easy to vaporize, corrode equipment
⁎
Corresponding author. E-mail address: [email protected] (X. Li). 1 Authors Q.W. and C.L. contributed equally. https://doi.org/10.1016/j.jcou.2019.04.010 Received 11 January 2019; Received in revised form 2 March 2019; Accepted 16 April 2019 Available online 29 April 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.
Journal of CO₂ Utilization 32 (2019) 163–169
Q. Wang, et al.
during high temperature reactions, and are difficult recycle [8–10]. Transition metals (Fe, Ni, etc.) are also considered as good catalysts because of their advantages of noncorrosive to equipment, low consumption, good catalytic activity, and the ability to regulate the composition of product gas. However, the vulnerability of transition metals to poisoning and inactivation in gasification limits their wide used in coal gasification process [6,11]. The exploration of new catalysts for coal gasification with high efficiency and good cyclic performance is still a topic receiving considerable attention. Perovskite-type oxides have been widely studied and used as a catalyst previously because of their high thermal stability, good redox property, and strong lattice O transportation ability. The structural formula of perovskite oxides is ABO3, where A-cations are located on the edge of the crystal structure and play a major role in stabilization, and B-cations are located in the center of structure and mainly affect the catalytic activity because of their variable valence states. The elements at A- and B-sites can be completely or partially replaced by other metallic elements without destroying the perovskite-type crystal structure, which indicated that catalytic performances of the materials can easily be controlled by adjusting the constituent elements [12–15]. Among all these studies, La-Fe perovskite has received considerable attention due to its low price, easy molding, and good thermal stability, and many efforts have been devoted to improve the catalytic properties of LaFeO3 by doping at the A/B site. Runduo et al. [16,17]investigated the effects of Cu2+ and Pb2+ ion doping on LaFeO3 for catalytic reduction of NO by propene and found that N2 yields were improved significantly after partial Fe3+ was substituted. Sun et al. [18] reported that the high Ce content is unconducive to the formation of perovskite structure, but moderate doping can provide active O and considerably improve catalytic performance without destroying the crystal structure. Khine et al. [19] studied the capabilities of (La0.7A0.3)BO3 (A = Ba, Ca, Mg, Sr, and B = Cr or Fe) for the partial oxidation of CH4 and found that Sr doping improves the mobility of lattice O, and (La0.75Sr0.25)FeO3 can achieve 65% CH4 conversion and obtain the highest H yield. We have previously investigated the properties of perovskite-type oxides as O carrier for chemical looping reforming of CH4 and studied the characteristics, especially the catalytic performance and O desorption [20,21], in detail. Perovskite-type oxides exhibit excellent catalytic performance for high-temperature gas–solid reactions. Thus, whether perovskite can be further used in catalytic gasification of coal char with CO2 is an attractive research topic. This work aimed to use LaFe-based perovskite oxides as a new catalyst for coal char CO2 gasification. K+ ions, which demonstrate low valence, high activity, and mobility and contribute to the formation of O vacancies and the increase in valence state of B-site ions [22–24],were selected for A-site doping to improve the catalytic properties of La-Fe-based perovskite oxides. New La1-xKxFeO3 (x = 0-0.9) catalysts were prepared via sol-gel method. The catalytic activity and cycle performance were tested using a thermogravimetric analyzer and fixed-bed reactor, and the proper doping ratio of K+ and reaction temperature for coal char CO2 gasification were investigated. The crystal structure, surface states of O and Fe, and surface topography after K+ doping and gasification reaction were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM), respectively.
Table 1 Zhundong demineral coal properties. proximate analysis (wt.%,air-dry)
M 2.78 a
A 0.52
V 2.7
Ultimate analysis (wt.%,dry and ash free) FC 94
C 69.86
H 0.81
Oa 28.89
N 0.44
Calculated by difference method.
ambient condition, and then deionized water was used to wash the coal until the solution was neutral. After filtration, the coal was added to a solution of 2 L hydrofluoric acid (3 mol/L) and 2 L deionized water and repeat the stirring and washing process as described above. The samples after filtration were dried at 105℃ for 24 h to obtain the de-mineral coal. In the end the de-mineral coal were heated from room temperature to 950℃ at the rate of 10℃/min and stay at 950℃ for 30 min to obtain the de-mineral char. The analyses data of ultimate de-mineral char were shown in Table 1 The perovskite-type oxides La1-xKxFeO3 were prepared by employing citric acid-aided sol-gel method. The starting materials were analytical purity, metal nitrate La(NO3)3·6H2O, KNO3 and Fe (NO3)3·9H2O were prepared according to stoichiometric ratio and dissolved in citric acid aqueous solvent and stirred at 70℃ for 4 h until sticky gel was formed, then the gel was placed in an air-circulating oven at 105℃ for 12 h. The dried gel was heated from ambient temperature to 400℃ at a rate of 10℃/min and kept at 400℃ for 1 h to burn out the organic compounds. The final remains were calcined at 750℃ for 5 h for nitrate decomposition and the perovskite-type catalysts were obtained after grinding, the particle size of grinded samples was less than 1 μm. 2.2. Gasification tests The experiment samples were prepared by mixing perovskite and de-mineral coal char (50:50wt.%) mechanically for 10 min in agate mortar, then put it in an oven for drying. The catalytic activity of catalyst for de-mineral coal CO2 gasification was investigated in the thermogravimetric analysis (TGA). About 18 mg of experiment samples were heated from 200℃ to 950℃ at 10℃/min heating rate in a flow of CO2 (100 ml/min). The initial reaction temperature (Ti) and the temperature at maximum weight loss rate (Tm) were used as an evaluation index of catalytic performance. The cyclic performance of catalysts was investigated in a fixed-bed tubular quartz reactor system (Φ = 20 mm, L = 600 mm). Approximately 0.5 ± 0.005 g reaction samples were added into quartz boat and placed into a quartz tube that was in the mixed flow of CO2/N2 (100:400 ml/min flow rate). The quartz tube was placed into the tube furnace at a predetermined temperature, and the composition of gas productions was monitored continuously by an online gas analyzer (Cubic Inc., Gasboard-3100). The feeding gas was switched to air (200 ml/min) until the CO concentration dropped to 0. Then, the quartz tube was removed from the furnace to cool the reactor to room temperature. The reaction catalyst sample remaining in the quartz boat was recovered and weighed for the next cycle. The number of cycles (3–10) varied with the catalyst. t50%, which was defined as the time for carbon conversions to reach 50%, was used to evaluate the catalytic activities of the catalyst; the lower the t50% value was, the better the catalytic performance was.
2. Experimental 2.1. Material preparation and characterization Zhundong(ZD) coal from Xinjiang region in China was selected for this study. The coal was ground below 48 μm. To avoid the potential confusion from minerals in coal, de-mineral coal was prepared by HCLHF method. Detailed steps are as follows: 100 g zhundong raw coal was added to a solution of 1 L hydrochloric acid (6 mol/L) and 3 L deionized water, the mixture was stirred for 24 h and placed for 12 h under
2.3. Characterization The perovskite-type phase of as-prepared catalysts was measured by X-ray diffusion (XRD) on X’Pert PRO (PANalytical B.V.) with Cu Kα radiation (λ = 0.1542 nm). X-ray tube voltage and current were 40 kV and 30 mA respectively. The diffractogram in the 2θ range of 20°–80° 164
Journal of CO₂ Utilization 32 (2019) 163–169
Q. Wang, et al.
Fig. 1. DTG profiles of CO2 catalytic gasification over La1-xKxFeO3.
Fig. 2. Carbon conversion of catalytic ZD-dem char CO2 gasification with La0.7K0.3FeO3 under different temperatures.
was recorded with a scanning step of 0.02°.The surface content and valence of oxygen and iron were carried out via X-ray photoelectron spectroscopy (XPS), performed with AXIS-ULTRA DLD X-ray Photoelectron Spectrometer and an Al-Kα anode. Binding energies were calibrated by using C1s at 284.8 eV as internal standard. The microstructures of particles were obtained by scanning electron microscopy (SEM) on FEI Inspect F50 operated at 10 kV. The Brunauer, Emmett and Teller (BET) specific surface area analysis were tested by N2 at 77 K on a 3H-2000PS instrument (Beishide Instrument Technology Co., Ltd.). The average size of the samples were determined by Laser particle size analyzer (ZS90, Malvern Instruments Ltd., UK).
to these results, the effect of reaction temperatures and cyclic performance of catalyst on char CO2 gasification experiments were carried out using La0.7K0.3FeO3 as catalyst. To investigate the influence of reaction temperature on the catalytic activity of La0.7K0.3FeO3 further, we carried out CO2 gasification experiments at the temperature ranging from 700 to °C 900 °C. As shown in Fig. 2, the reaction rates increased with temperature, especially ranging from 700 °C 850 °C. Coal char CO2 gasification is an endothermic reaction, and the increase in temperature is conducive to the gasification process. Because the adsorption process is an exothermic process, high reaction temperature contributed to the desorption of adsorbed oxygen species. Our previous research [20,28] also suggested that perovskite-type oxides exhibited better catalytic activity at higher temperature, promoting the consumption of adsorbed oxygen. Thus, more lattice oxygen would travel through oxygen vacancies to supply the oxygen species on the surface and be released, which leads to improved redox performance. When the temperature reaches 850 °C, the catalytic activity is relatively high, and the carbon conversion rate of gasification is mainly dependent upon the redox performance of the catalyst. Excessively high temperature has a negative effect on the surface morphology of catalyst. Thus, 850 °C is considered as the optimal temperature for the catalytic CO2 gasification of ZD-dem char with perovskite.
3. Results and discussions 3.1. Catalytic performance of different perovskites The CO2 catalytic activity of ZD dem-char with different perovskites evaluated by TGA is shown in Fig. 1. The figure indicates that LaFeO3 catalyst exhibited the lowest activities and has insignificant effect on the increase in the coal char CO2 gasification rate. The catalytic performance of La-Fe-based perovskite was enhanced when 10% (molar fraction) of La3+ at A-site was replaced by K+, but it still can only react at high temperatures. When the doping ratio x increased from 0.1 to 0.3, a significant improvement in the catalytic performance was observed. The carbon conversion rate did not only increased significantly, but the Ti value also decreased from 821 °C to 713 °C. Afterward, the Ti and Tm values showed a decreasing trend with the increase of K+ ions at the A-site, which indicated that the catalytic performance was further improved, and the requirement of temperature was reduced. One possible reason for the increased catalytic activity of K+-doped perovskite is the change in Fe valence state and oxygen vacancy formation to maintain electron neutrality when some parts of trivalent La3+ are substituted by monovalent K+ [23,24,24]. Another reason may due to the high catalytic activity of K+ for coal gasification processes [25,26]. Therefore, the K+ doped at the A-site positively affects catalytic performance improvement. When the molar ratios of K+ at the A-site was > 0.7, the reaction curves showed that further K+ doping at A-site only slightly contributed to the improvement of the catalyzing performance of the modified perovskite-type catalyst. This result was possibly observed because the degree of substitution of the A-site ions was limited. Excessive K+ ions can only attached to the surface of catalyst after saturation. Although those K+ can complement the lattice K+ ions during cyclic reactions. Surface K+ ions were also easy to melt in gasification reaction because of its low melting point, leading to the increase of agglomerate strength of catalyst particles [14,27]. According
3.2. Cyclic performance of recovered catalysts To study the cyclic stability of K+-doped perovskite catalysts, we reused La1-xKxFeO3 for coal-char gasification in fixed-bed reactor at 850 °C, and the results are presented in Fig. 3. The results showed that LaFeO3 perovskite without K+ doping required the longest reaction time to achieve 50% carbon conversion. After a small amount of K+ was doped at the A-site of perovskite, the catalytic activity of La0.9K0.1FeO3 was enhanced significantly, which was in agreement with the TGA results. However, the catalytic performance of perovskite catalyst with low K+ amounts showed significant reduction in cycle experiments. With the introduction of additional K+, the catalysts exhibited increased catalytic activity and cycling performance stability. Compared with undoped sample perovskite, the t50% value of La0.3K0.7FeO3 in the first cycle decreased by 87.3% and only increased by 2.6 min after reusing 7 times. A significant decrease in the reaction time was observed between the seventh and eighth cycle, which was still less than that of La0.3K0.7FeO3 during the first cycle. The detailed data of the catalytic coal char CO2 gasification by using La0.3K0.7FeO3 catalyst at 850 °C on fixed bed are shown in Fig. 4. The 165
Journal of CO₂ Utilization 32 (2019) 163–169
Q. Wang, et al.
Fig. 3. t50% value of CO2 catalytic gasification over La1-xKxFeO3 for different cycle times.
Fig. 4. Reaction rates of La0.7K0.3FeO3 for cyclic ZD-dem char CO2 gasification.
undoped perovskite was included as a reference. The figure indicates that the carbon conversion increased to 90% in a straight line over time and exhibited stable catalytic performance in the first seven cycles. Then, the carbon conversion rate decreased gradually, which was possibly due to the fact that sintering on the perovskite surface reduced the contact efficiency between coal char and catalyst, and the sintering phenomenon became increasingly serious with the progress of the cyclic reaction. Thus, the carbon conversion rate began to decline when the carbon conversion ratio reached 80%.
Fig. 5. XRD patterns of La1-xKxFeO3(x = 0–0.9); (a) the whole range; (b) an enlarged screenshot of La1-xKxFeO3(x = 0.7–0.9, 2θ = 29°–44°).
3.3. Catalyst characterization Fig. 5 displays the XRD patterns of La1-xKxFeO3(x = 0-0.9). All the characteristic peaks of perovskite weakened with the increase in K+ at the A-site but still exhibited the perovskite crystal structure when the molar ratio of K+ was < 0.8. The peaks of other impurities were undetected, which indicated that K+ ions entered into the lattice of perovskite [29,30]. Fig. 5(b) shows the enlarged XRD patterns (2θ = 29°–44°) of the modified perovskite (x = 0.7–0.9) after the excess K + was incorporated into the perovskite oxides. Some byproduct peaks were observed. A weak diffraction peak at 30.1° corresponded to the cubic structure of Fe3O4 (JCPDS 87–2334), and the peak at 32.9° that contributed to the distortion of the main characteristic peak of perovskite was ascribed to K3FeO2 (JCPDS 48-0957). The changes in the crystal structure of La0.3K0.7FeO3 after different
Fig. 6. XRD patterns of La0.3K0.7FeO3 before and after different cycles at 850℃. 166
Journal of CO₂ Utilization 32 (2019) 163–169
Q. Wang, et al.
Fig. 7. O1s and Fe2p XPS spectra of perovskite samples; (a, d) fresh LaFeO3, (b) (e) fresh La0.3K0.7FeO3, and (c) (f) La0.3K0.7FeO3 after the first gasification.
529.15–529.21 eV was ascribed to lattice oxygen and marked as Olat. The second peak at 530.99–531.35 eV possibly originated from carbonate species and C1s and was marked as Ocar. Finally, the third peak at 532.02–532.32 eV was ascribed to physical adsorbed oxygen and marked as Oads [21,31]. The comparison results of Fig. 7(a) and (b) suggested that the modified perovskite-type catalyst exhibited high Oads peak and low Olat due to oxygen vacancy formation caused by the partial substitution of monovalent K+ for trivalent La3+, which has been proven to increase oxygen fluidity, facilitate electron transfer, and enhance catalytic activity [32]. The binding energy of modified perovskite was also lower than that of undoped perovskite, which indicated that the lattice O activity was enhanced by K+ doping [33]. Thus, modified perovskite catalysts exhibit improved redox performance. After catalytic gasification reaction, the Olat and Oads contents decreased, and that of Ocar increased, as shown in Fig. 7(c). This result
reaction times were investigated, as shown in Fig. 6. Although the peaks of Fe3O4, which was produced by the reaction of perovskite with CO2, were detected, the modified perovskite still exhibited the characteristic peaks of the perovskite after the first cycle. After nine additional gasification cycles, the perovskite characteristic peaks were still detected, which were only slightly weakened. These results suggested that the modified perovskite La0.3K0.7FeO3 had high crystal structure stability. XPS characterization was performed to investigate the surface state of undoped perovskite and modified perovskite samples before and after gasification. The fitting results of O1s and Fe2p XPS spectra are shown in Fig. 7, and the relative percentages that were calculated from the integrated area of these subpeaks are listed in Table 2. The O1s spectrum for all the three samples showed two peaks in the range of 526–537 eV. After peak separation, three small peaks representing the three different types of oxygen species were obtained. The first peak at
Table 2 Binding energies (BEs) of the Fe2p3/2 and O1s and calculated perovskite sample percentages. Samples
O1s
Fe2p3/2
Olat
LaFeO3 La0.3K0.7FeO3 La0.3K0.7FeO3 1 st
Ocar
Fe2+
Oads
Fe3+
Fe4+
BE (eV)
%
BE (eV)
%
BE (eV)
%
BE (eV)
%
BE (eV)
%
BE (eV)
%
529.2 528.9 528.8
60 47 23
531.3 531.5 531.0
28 27 61
532.2 532.1 532.5
12 26 16
709.8 709.6 709.6
45 32 38
711.3 711.0 710.9
29 32 35
713.1 713.3 713.0
26 36 27
167
Journal of CO₂ Utilization 32 (2019) 163–169
Q. Wang, et al.
Fig. 8. SEM images of La0.3K0.7FeO3 sample; (a) fresh sample, (b) after 1 st gasification, and (c) after 10th gasification.
indicated that the lattice and surface oxygen were involved in catalytic coal char gasification. The high Ocar peak may due to CO2 adsorption. As shown in Fig. 7, the Fe2p3/2 spectra deconvoluted into three subpeaks. The peaks at 709.6–709.9, 710.9–711.9, and 712–714 eV were assigned to Fe2+, Fe3+, and Fe4+, respectively [34,35].A comparison of Fig. 7(d) and (e) suggested that additional Fe3+ and Fe4+ existed in modified perovskite to maintain electrical neutrality. Fe4+ showed strong redox activity and was beneficial to the gasification reaction of coal char. After catalytic gasification, the perovskite sample showed low Fe4+ peak in Fig. 7(f), which indicated that some Fe4+ in the lattice were converted to Fe3+ and Fe2+ during the catalytic reaction. This result agreed with the XRD analysis shown in Fig. 6. The reduction reaction of Fe can also promote the charge transfer from adsorbed oxygen to the adsorbed oxygen and O2. The morphological changes in La0.3K0.7FeO3 during cyclic reactions obtained by SEM were shown in Fig. 8. The fresh sample had uniform spherical crystalline grain distribution and porous structure, as shown in Fig. 8(a). Such structure was beneficial for the improved contact of the catalyst with coke particles. The crystalline grains structure was maintained and showed slight agglomeration after the first cycle, as shown in Fig. 8(b). However, after 10 cycles of the experiment, the crystalline grains melted and fused with other grains, which destroyed the pore structure of the catalyst, as presented in Fig. 8(c). The decreased contact efficiency between coal char and catalyst decreased the carbon conversion rate, as shown in Fig. 4. The average size of the samples was determined by Laser particle size analyzer. It can be seen from Table 3 that the particle size of modified perovskite La0.3K0.7FeO3 changed a little after first cycle of gasification. But the average size of La0.3K0.7FeO3 after 10 cycles increased significantly, which is due to the agglomeration of perovskite particles according to the SEM characterization results. Therefore, the agglomeration of perovskite will resulted in the increase of particle size, which reduced the effective contact area between coal char and perovskite. And eventually lead to a decline in carbon conversion rate during cyclic experiments. The specific surface areas of La1−xKxFeO3 catalysts tested by N2 absorption/desorption method were listed in Table 4. The BET specific surface of K-modified perovskite were between 1 and 9 m2/g, which is in agreement with previous study [36]. As can be seen, K+-doping had a significant effect on the specific surface area of the catalyst, but the change was not linear with doping ratio at A-site. The undoped perovskite had a relatively low specific surface area, 2.6 m2/g, and La0.7K0.3FeO3 had the largest specific area, 8.7 m2/g. It was reported
Table 4 Specific surface area of La1-xKxFeO3 (x = 0–0.9) catalysts.
Particle size(nm)
La0.3K0.7FeO3 fresh La0.3K0.7FeO3 1 st La0.3K0.7FeO3 10th
680 743 1234
SBET(m2/g)
LaFeO3 La0.9K0.1FeO3 La0.7K0.3FeO3 La0.5K0.5FeO3 La0.3K0.7FeO3 La0.2K0.8FeO3 La0.3K0.7FeO3 1 st
2.6 1.5 8.7 4.3 3.3 2.0 1.7
that the changes of specific surface area related to the expansion of unit cells due to the radius of K+ is slightly higher than La3+ [37]. However, the formation of oxygen vacancies resulted in the shrank of unit cell when partial of trivalent La3+ was replaced by K+. Thus, the specific surface area decreased after more K+ ions were doped at A-site. It was also can be seen that the specific surface area of modified perovskite La0.3K0.7FeO3 decreased from 3.3 to 1.7 m2/g after one times of gasification, which proved that the pore structure of the catalyst was damaged, the results of BET analysis were consistent with the results of SEM. 4. Conclusions A series of K+-modified perovskite catalysts La1-xKxFeO3 (x = 0-0.9) was synthesized by the sol-gel method, and their catalytic and cycle performances for coal char CO2 gasification were investigated by TGA and fixed-bed reactor, respectively. The catalytic and cycle performances of perovskite-type catalysts were significantly improved by the partial substitution of La3+ by K+ until the molar ratio of K + reached > 0.7. For La0.3K0.7FeO3, the Ti value of first reaction dropped from 821 °C to 643 °C, and the t50% value only increased by 2.6 min after seven cycles of reaction. The further increase in K+ ions at the A-site did not decrease the reaction time and temperature. The increase in temperature was also beneficial to the improvement of catalytic performance. When the adverse effect of further heating on the surface morphology was considered, 850 °C was the optimal gasification temperature. The XRD patterns showed that the pure perovskite phase was obtained for all catalyst samples when the molar ratio was < 0.7. Thus, 0.7 is the best x value of La1-xKxFeO3. The XRD results also suggested that La0.3K0.7FeO3 had good structural stability because its characteristic perovskite-type crystal structure was maintained after 10 cycles of gasification. The XPS results showed that additional oxygen vacancies and Fe4+ formed after partial La3+ were replaced by K+ to preserve electroneutrality. Thus, the modified perovskite exhibits increased catalytic activity in coal char CO2 gasification. SEM analysis indicated that the spherical particles were uniformly distributed on the surface of fresh catalyst samples, but the La0.3K0.7FeO3 particles melted and merged together in CO2 gasification reaction processing. Besides, the results of BET analysis and laser particle size analyser showed that the catalyst was sintered and the particle size increased during cyclic
Table 3 Average particle diameter of La0.3K0.7FeO3 during cylic experiments. Perovskite
Perovskite
168
Journal of CO₂ Utilization 32 (2019) 163–169
Q. Wang, et al.
reactions, which was not conducive to the effective contact between perovskite and coal char and cycling performance of modified perovskite.
Alloys Compd. 682 (2016) 686–694. [18] Y. Sun, S.S. Hla, G.J. Duffy, A.J. Cousins, D. French, L.D. Morpeth, J.H. Edwards, D.G. Roberts, Effect of Ce on the structural features and catalytic properties of La(0.9−x)CexFeO3 perovskite-like catalysts for the high temperature water–gas shift reaction, Int. J. Hydrogen Energy 36 (1) (2011) 79–86. [19] M.S.S. Khine, L. Chen, S. Zhang, J. Lin, S.P. Jiang, Syngas production by catalytic partial oxidation of methane over (La0.7A0.3)BO3 (A = Ba, Ca, Mg, Sr, and B = Cr or Fe) perovskite oxides for portable fuel cell applications, Int. J. Hydrogen Energy 38 (30) (2013) 13300–13308. [20] H. Ding, Y. Xu, C. Luo, Q. Wang, S. Li, G. Cai, L. Zhang, Y. Zheng, Q. Shen, Oxygen desorption behavior of sol-gel derived perovskite-type oxides in a pressurized fixed bed reactor, Chem. Eng. J. 323 (2017) 340–346. [21] H. Ding, Y. Xu, C. Luo, Q. Wang, C. Shen, J. Xu, L. Zhang, A novel composite perovskite-based material for chemical-looping steam methane reforming to hydrogen and syngas, Energy Convers. Manage. 171 (2018) 12–19. [22] L. Xu, F. Wang, M. Chen, X. Fan, H. Yang, D. Nie, L. Qi, Alkaline-promoted Co-Ni bimetal ordered mesoporous catalysts with enhanced coke-resistant performance toward CO2 reforming of CH4, J. CO2 Util. 18 (2017) 1–14. [23] H. Shimokawa, Y. Kurihara, H. Kusaba, H. Einaga, Y. Teraoka, Comparison of catalytic performance of Ag- and K-based catalysts for diesel soot combustion, Catal. Today 185 (1) (2012) 99–103. [24] H. Liang, Y. Mou, H. Zhang, S. Li, C. Yao, X. Yu, Sulfur resistance and soot combustion for La0.8K0.2Co1-yMnyO3 catalyst, Catal. Today 281 (2017) 477–481. [25] H. Ge, L. Shen, H. Gu, S. Jiang, Effect of co-precipitation and impregnation on Kdecorated Fe2O3/Al2O3 oxygen carrier in Chemical Looping Combustion of bituminous coal, Chem. Eng. J. 262 (2015) 1065–1076. [26] Y.-K. Kim, L.-f. Hao, J.-I. Park, J. Miyawaki, I. Mochida, S.-H. Yoon, Catalytic activity and activation mechanism of potassium carbonate supported on perovskite oxide for coal char combustion, Fuel 94 (2012) 516–522. [27] Y. Teraoka, K. Nakano, W. Shangguan, S. Kagawa, Simultaneous catalytic removal of nitrogen oxides and diesel soot particulate over perovskite-related oxides, Catal. Today 27 (1) (1996) 107–113. [28] Q. Shen, Y. Zheng, S. Li, H. Ding, Y. Xu, C. Zheng, M. Thern, Optimize process parameters of microwave-assisted EDTA method using orthogonal experiment for novel BaCoO3−δ perovskite, J. Alloys Compd. 658 (2016) 125–131. [29] J. Xu, J. Liu, Z. Zhao, C. Xu, J. Zheng, A. Duan, G. Jiang, Easy synthesis of threedimensionally ordered macroporous La1−xKxCoO3 catalysts and their high activities for the catalytic combustion of soot, J. Catal. 282 (1) (2011) 1–12. [30] K. Taniguchi, N. Okinaka, T. Akiyama, Preparation and characterization of La1−xKxFeO3 (x=0–1) by self-propagating high-temperature synthesis for use as soot combustion catalyst, J. Alloys Compd. 509 (10) (2011) 4084–4088. [31] M. Ghaffari, M. Shannon, H. Hui, O.K. Tan, A. Irannejad, Preparation, surface state and band structure studies of SrTi(1− x)Fe(x)O(3− δ)(x= 0–1) perovskite-type nano structure by X-ray and ultraviolet photoelectron spectroscopy, Surf. Sci. 606 (5-6) (2012) 670–677. [32] Y. Teraoka, K. Kanada, S. Kagawa, Synthesis of La-K-M-O perovskite-type oxides and their catalytic property for simultaneous removal of NOx and diesel soot particulates, Appl. Catal. B 34 (1) (2001) 73–78 78. [33] K. Sutthiumporn, T. Maneerung, Y. Kathiraser, S. Kawi, CO2 dry-reforming of methane over La0.8Sr0.2Ni0.8M0.2O3 perovskite (M= Bi, Co, Cr, Cu, Fe): roles of lattice oxygen on C–H activation and carbon suppression, Int. J. Hydrogen Energy 37 (15) (2012) 11195–11207. [34] W. Cheng, X. Ma, Structural, optical and magnetic properties of Fe-doped ZnO, IOP Publishing, 2019, p. 012039. [35] Y. Bu, Q. Zhong, D. Xu, W. Tan, Redox stability and sulfur resistance of Sm0.9Sr0.1CrxFe1−xO3−δ perovskite materials, J. Alloys Compd. 578 (2013) 60–66. [36] Z. Wang, P. Zhao, C. Liang, A. Xu, G. Duan, J. Jiang, J. Xu, Liu, Highly active La1−xKxCoO3 perovskite-type complex oxide catalysts for the simultaneous removal of diesel soot and nitrogen oxides under loose contact conditions, Catal. Lett. 124 (1-2) (2008) 91–99. [37] S. Rousseau, S. Loridant, P. Delichere, A. Boreave, J. Deloume, P. Vernoux, La (1− x) SrxCo1− yFeyO3 perovskites prepared by sol–gel method: characterization and relationships with catalytic properties for total oxidation of toluene, Appl. Catal. B 88 (3-4) (2009) 438–447.
Acknowledgements This work was supported by National Key R&D Program of China (2016YFB0600801), National Natural Science Foundation of China (No.51606076) and Analytical and Testing Center of HUST for XRD measurements. References [1] H. Yang, Z. Xu, M. Fan, R. Gupta, R.B. Slimane, A.E. Bland, I. Wright, Progress in carbon dioxide separation and capture: a review, J. Environ. Sci. 20 (1) (2008) 14–27. [2] X. Li, Q. Huang, C. Luo, D. Zhou, Y. Xu, L. Zhang, C. Zheng, Effect of acid gases on elemental mercury removal in an oxy-fuel CO2 compression process, Energy Fuels 32 (4) (2018) 4334–4340. [3] G. Kumari, P. Vairakannu, Laboratory scale studies on CO2 oxy-fuel combustion in the context of underground coal gasification, J. CO2 Util. 21 (2017) 177–190. [4] B. Hu, Q. Huang, A. Buekens, Y. Chi, J. Yan, Co-gasification of municipal solid waste with high alkali coal char in a three-stage gasifier, Energy Convers. Manage. 153 (2017) 473–481. [5] K.-C. Xie, Coal gasification, Structure and Reactivity of Coal, Springer, 2015, pp. 181–241. [6] B. Bhui, P. Vairakannu, Experimental and kinetic studies on in-situ CO2 gasification based chemical looping combustion of low ash coal using Fe2O3as the oxygen carrier, J. CO2 Util. 29 (2019) 103–116. [7] D.M. Quyn, H. Wu, J.-i. Hayashi, C.-Z. Li, Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part IV. Catalytic effects of NaCl and ion-exchangeable Na in coal on char reactivity☆, Fuel 82 (5) (2003) 587–593. [8] A. Nzihou, B. Stanmore, P. Sharrock, A review of catalysts for the gasification of biomass char, with some reference to coal, Energy 58 (2013) 305–317. [9] K. Matsuoka, T. Yamashita, K. Kuramoto, Y. Suzuki, A. Takaya, A. Tomita, Transformation of alkali and alkaline earth metals in low rank coal during gasification, Fuel 87 (6) (2008) 885–893. [10] P. Lahijani, Z.A. Zainal, A.R. Mohamed, M. Mohammadi, CO2 gasification reactivity of biomass char: catalytic influence of alkali, alkaline earth and transition metal salts, Bioresour. Technol. 144 (2013) 288–295. [11] J. Yu, F.-J. Tian, M.C. Chow, L.J. McKenzie, C.-Z. Li, Effect of iron on the gasification of Victorian brown coal with steam: enhancement of hydrogen production, Fuel 85 (2) (2006) 127–133. [12] F. Rahbar Shamskar, F. Meshkani, M. Rezaei, Preparation and characterization of ultrasound-assisted co-precipitated nanocrystalline La-, Ce-, Zr –promoted Ni-Al2O3 catalysts for dry reforming reaction, J. CO2 Util. 22 (2017) 124–134. [13] P.A. Shaikh, Y.D. Kolekar, Study of microstructural, electrical and dielectric properties of perovskite(0.7)PMN–(0.3)PT ferroelectric at different sintering temperatures, J. Anal. Appl. Pyrolysis 93 (2012) 41–46. [14] H. Wang, J. Liu, Z. Zhao, Y. Wei, C. Xu, Comparative study of nanometric Co-, Mnand Fe-based perovskite-type complex oxide catalysts for the simultaneous elimination of soot and NOx from diesel engine exhaust, Catal. Today 184 (1) (2012) 288–300. [15] H. Tanaka, M. Misono, Advances in designing perovskite catalysts, Curr. Opin. Solid State Mater. Sci. 5 (5) (2001) 381–387. [16] R. Zhang, A. Villanueva, H. Alamdari, S. Kaliaguine, Cu- and Pd-substituted nanoscale Fe-based perovskites for selective catalytic reduction of NO by propene, J. Catal. 237 (2) (2006) 368–380. [17] F. Parrino, E. García-López, G. Marcì, L. Palmisano, V. Felice, I.N. Sora, L. Armelao, Cu-substituted lanthanum ferrite perovskites: preparation, characterization and photocatalytic activity in gas-solid regime under simulated solar light irradiation, J.
169