Abstract
This paper used an equal volume impregnation method to produce egg yolk-egg white bimetallic-supported catalyst. Phase structure and crystal morphologies were characterized by X-ray diffraction, scanning electron microscopy, temperature programmed reduction (TPR), and inductive coupling plasma methods, and the paper discussed the effect of different metal loading, calcination temperature, and calcination time on the desulfurization activity. The experimental results showed that desulfurization performance increased first and then gradually decreased as metal loading, calcination temperature, and calcination time increased. The optimum preparation conditions of bimetallic catalyst were 8% NiO and 5% MnO2, the most suitable calcination temperature was 400°C, and the most suitable calcination time was 4 h. Finally, the eggshell, the egg white, and the egg yolk catalysts had its own different characteristics in the desulfurization processing: the eggshell catalyst showed a rapid reaction rate but lasted for a short time, the egg yolk catalyst showed a slow reaction rate but lasted for a long time, and the reaction rate and lasting time of the egg white were between the eggshell and the egg yolk.
1 Introduction
China is one of the largest countries in the world, with a large amount of consumption and production of coal. The proportion of coal in China’s energy structure reaches to 76.2%, and the proportion of high sulfur coal (including S>2%) is larger. There is a generation of a large number of sulfur dioxide and nitrogen oxide by coal combustion each year [1]. Thus, the major task of our country is to reduce emissions of sulfur dioxide and prevent sulfur dioxide pollution in the present and in the future.
A high concentration of sulfur dioxide is usually converted into sulfuric acid recycling, and the processing method for the low-concentration sulfur dioxide based on the desulfurizer form can be divided into dry, half-dry, and wet desulfurization process technology. A dry flue gas desulfurization system is gradually favored by people because of the advantages of its lowest investment, installation, and operation cost; high desulfurization efficiency, simple technological process, and small floor area; low energy consumption and high utilization efficiency of absorbent; low water consumption, no wastewater, and no corrosion; and dry desulfurization slag solid and convenience for manage [2, 3]. However, the expected effect of desulfurization is frequently difficult to achieve without using any catalyst in the desulfurization process. Thus, the study of the dry system of the metal oxide catalyst for flue gas desulfurization is a hot issue.
In the early stage, the effect of distribution form for supported catalyst basically was not considered. With the development of production and the increasingly large industrial equipment, the problems of internal surface utilization of catalyst, macro selectivity, and resistance drop increasingly attract people’s attention [4]. In order to improve the utilization rate of catalysts, people begin to pay close attention to the catalytic activity with inhomogeneous distribution. In recent years, there are a lot of theoretic discussions and practical applications about the effect of the inhomogeneous distribution of active component on the catalyst particles (macrodistribution) for the catalytic reaction [5, 6]. The inhomogeneous distribution of the active component on the carrier refers to the regular changes of the concentration of active component from the carrier center to the carrier surface and can be divided into three kinds of typical characteristics: eggshell type, egg white type, and egg yolk type. The interaction between active component and carrier decides what kinds of distribution form of active component on the carrier. Impregnation is generally given priority to preparation methods of the catalyst with inhomogeneous distribution, and the inhomogeneous distribution of catalyst is the result of the internal particle flow, diffusion, and interaction of interface phenomenon.
By controlling the amount of citric acid in this experiment and by using an equal volume impregnation method, two kinds of metal oxides, MnO2 and NiO, were supported on the carrier γ-Al2O3. Acid oxide MnO2 dipped in the outer layer mainly distributes in the position of the egg white, and alkaline oxides NiO dipped into the inner layer mainly distributes in the position of the egg yolk. The structure of two circulars was formed to decrease the probability of catalyst poisoning and to improve the performance of catalyst in order to achieve better desulfurization effect.
2 Materials and methods
2.1 MnO2-NiO/γ-Al2O3 catalyst preparation
This experiment selected γ-Al2O3 as raw material and catalyst prepared with equivalent volume impregnation method. Catalysts had different structures (types of eggshell, egg white, and egg yolk) by changing the different concentrations of citric acid. The effects of the different catalyst structure and desulfurization performance were investigated at different loadings of NiO, calcination temperature, and calcination time, and the optimum preparation technology of NiO/γ-Al2O3 was screened out.
On the basis of the optimum condition of γ-Al2O3 carrier, the second active component MnO2 continued to be loaded to prepare the catalyst of circular structure. To improve the desulfurization performance and to consider the antipoisoning performance of the catalyst, the different loadings of MnO2, calcination temperature, and calcination time were chosen to discuss the effect of different preparation conditions on the desulfurization performance of MnO2- NiO/γ-Al2O3 bimetallic catalyst [7, 8].
2.2 The testing device and method of catalyst performance
2.2.1 The testing device of catalyst performance
The experiments were conducted in the fixed adsorption column at constant temperature, and the adsorption column was in a quartz tube 20 mm in diameter and 50 cm in height. The testing device of catalyst performance is shown in Figure 1.
The testing device of catalyst performance: (1) gas flow meter, (2) relief valve, (3) high pressure steel, (4) oxygen, (5) nitrogen, (6) nitrogen monoxide, (7) ammonia, (8) mixed gas cylinders, (9) tube furnace control instrument, (10) gas emission, (11) hand-held gas analyzer, (12) tube furnace.
2.2.2 Detection method of catalyst
Catalysts with different conditions were prepared using the single-factor experiment method, and using the tests of catalyst performance, the optimum preparation conditions of catalysts are screened out. The testing steps of catalyst performance are shown as follows:
Gas compositions for simulation are chosen as 6% O2 concentration, 8% H2O concentration, and 0.2% SO2 concentration; the rest is N2.
After adjustment for the gas concentration and flow rate, the temperature controller was adjusted to 180°C. When the temperature was constant, the experimental gas was joined into the adsorption column, and a stopwatch was pressed at the same time.
The terminal pipe was connected to the flue gas analyzer. When numerical values began to change from 0, the data were recorded every 1 min, and the experiment was finally stopped until the concentration of sulfur dioxide reached more than 1000 mg/m3. The desulfurization efficiency and the effective time of catalyst were calculated to draw charts for the following analysis [9, 10].
3 Results and discussion
3.1 Factor experiment of best preparation conditions of MnO2-NiO/γ-Al2O3
3.1.1 The effect of different MnO2 loading on the flue-gas desulfurization performance
The mixed gas flow rate was kept as 0.8 m/s in the desulfurization system. Egg yolk-egg white bimetallic-supported catalyst MnO2-NiO/γ-Al2O3 was prepared on the egg yolk NiO/γ-Al2O3 with 5%, 8%, and 10% MnO2 loading to discuss the effect of the different loadings of MnO2 on the desulfurization performance.
Figure 2 shows that the catalytic activity of MnO2-NiO/ γ-Al2O3 increased first and then decreased as MnO2 loading increased. In this experiment, catalytic activity increased with MnO2 loading from 0% to 5% and decreased with MnO2 loading from 5% to 10%; among them, the catalyst with 5% MnO2 has the best catalytic effect. These relevant references are the most suitable ratios of the two metals with bimetallic-supported catalyst [11, 12]. The conclusion obtained from effective data in the experiment was that with the decrease in the proportion of nickel and manganese, the desulfurization effect of MnO2-NiO/γ-Al2O3 catalyst increased first and then decreased, and the catalytic activity could be enhanced with the introduction of a little amount of manganese. With the continuing increase in manganese content, the catalytic activity reduced, and the effect of 5% MnO2–8% NiO/γ-Al2O3 catalyst was the best.
Effect of different MnO2 loading on the catalyst desulfurization performance.
When the reaction time was <10 min and the desulfurization rate was kept for more than 80%, the catalytic activity of the monometallic-supported catalyst NiO/γ-Al2O3 was better than that of 8% MnO2-NiO/γ-Al2O3 catalyst and 10% MnO2-NiO/γ-Al2O3 catalyst. When the desulfurization rate was kept for more than 75%, the catalytic activity of the monometallic-supported catalyst NiO/γ-Al2O3 was better than that of 10% MnO2-NiO/γ-Al2O3. The reason that the desulfurization effect of monometallic oxide-supported catalyst was better than that of bimetallic oxide-supported catalyst was within a few minutes at the beginning of desulfurization. MnO2 distributed in the position of egg white in bimetallic oxide-supported catalyst plays a major role on the desulfurization reaction, whereas NiO distributed in the position of egg yolk in monometallic oxide-supported catalyst plays a major role in the desulfurization reaction, and the desulfurization effect of alkaline oxide NiO in the whole reaction process was better than that of acidic oxide MnO2 [13, 14].
After the reaction time was more than 10 min, the catalytic activity of MnO2-NiO/γ-Al2O3 catalyst decreased as MnO2 loading increased. Specifically in this experiment, catalytic activity decreased with loading from 5% to 10%, and the catalyst with the 5% MnO2 has the best catalytic effect. The reasons that these catalytic effects of catalyst-supported MnO2 were all better than the monometallic-supported catalysts may be that the addition of Mn does not change the structural characteristics of the catalyst, but it significantly promoted the dispersion of the active component in the catalyst and weakens the interaction between the metal and the carrier and the formation probability of nickel aluminum spinel without catalytic activity to increase the number of catalytic active species and catalytic ability for SO2 significantly. The reason that the desulfurization effect of MnO2-NiO/γ-Al2O3 catalyst gradually decreased with the increase in MnO2 loading may be a considerable part of finite activity positions on γ-Al2O3 carrier, which were occupied by NiO supported in the first step and only part of MnO2 supported in the second step work (probably loading was less than or equal to 5%), and if the MnO2 loading was too much (loading was 8% or 10%), the excess loaded materials have a hindrance function to contraction between the mixed gas and the catalyst. This hindrance function decreased the effective specific surface area of the catalyst to a certain extent and the catalytic effect of bimetallic oxide-supported catalyst MnO2-NiO/γ-Al2O3.
3.1.2 The effect of different calcination temperature on the flue-gas desulfurization performance
The mixed gas flow rate was kept as 0.8 m/s in the desulfurization system. On the basis of the optimal 8% MnO2 loading, calcination temperatures of 300°C, 400°C, and 500°C were selected to discuss the effect of the different calcination temperature on the desulfurization performance.
Figure 3 showed that the catalytic activity of MnO2-NiO/γ-Al2O3 catalyst increased first and then decreased as calcination temperature increased. In this experiment, catalytic activity increased with calcination temperature from 300°C to 400°C and decreased with calcination temperature from 400°C to 500°C; among them, the catalyst with calcination temperatures of 400°C had the best catalytic effect. This was because the active component of the supported catalyst exists on the carrier of high melting point in the form of highly dispersion. The reason that the surface area of active component changed in the calcination process for this type of catalyst was that the change of metal grain size leads to the change of active surface area. Nickel nitrate could not be completely decomposed and form the appropriate active phase structure at temperatures of 300°C, and the catalytic activity of calcination decreases more obviously at temperatures of 500°C. The possible reasons are two factors: one is that excessively high calcination temperature can cause sintering phenomenon result, which can result in a significant decrease in surface free energy and a change in the crystal form of the carrier and the active component, and the other is that the generation of the nucleation rate can increase in the calcination process by excessively high calcination temperature, and this increase is beneficial to obtain the metal microcrystalline in the form of high dispersion. Therefore, the catalyst activity decreases. From this figure, when the loading was the same, the different calcination temperature had a significant influence on the activity of catalyst. Thus, the calcination temperature maintained under a certain range should not change the crystal form of activated alumina (sintering) but should completely decompose the impregnating nitrates.
Effect of different calcination temperature on the catalyst desulfurization performance.
3.1.3 The effect of different calcination time on the flue-gas desulfurization performance
The mixed gas flow rate was kept as 0.8 m/s in the desulfurization system. Calcination times of 2, 3, 4, and 5 h were selected to discuss the effect of the different calcination time on the desulfurization performance.
Figure 4 shows that the catalytic activity of MnO2-NiO/γ-Al2O3 catalyst increased first and then decreased as the calcination time increased. In this experiment, catalytic activity increased with calcination time from 3 to 4 h and decreased with calcination time from 4 to 5 h; among them, the catalyst with a calcination time of 4 h had the best catalytic effect. From this figure, a calcination time of 3 and 4 h had more obvious influence on the catalytic activity of catalysts than loading and calcination temperature, but the calcination time had some influence on the catalytic activity. The catalyst had high stable activity when the calcination time takes more than 3 h at temperatures of 400°C. Within the time range, the reason that the catalyst has the best catalytic effect at 4 h, and the catalytic effect at 5 h decreased significantly compared with 4 h, was because the interaction between carrier and active component could form more nickel aluminum spinel without catalytic activity at a high temperature for a long calcination time. Thus, the appropriate calcination temperature and the time chosen in the calcination process were favorable for the formation of the active center, and on the carrier, there were many microspores that can increase the surface area of the carrier, thus being beneficial to improve the catalytic activity.
Effect of calcination temperature of different time on the catalyst desulfurization performance.
3.2 Characterization of catalyst
3.2.1 XRD characterization
The X-ray diffraction (XRD) analyzer XRD-7000 adopted in XRD test is produced by Shimadzu International Trading (Shanghai) Colimited. With the incident light source for Cu target, the test method is that powder is pressed into flake on glass slide, and the scanning range 2θ is from 10° to 70° and from 10° to 90° and scanning rate is 8 (°/min).
It can be seen in Figure 5 that the XRD diffraction peak shape is less shrill, and the peak width is wider, so the conclusion that crystallization degree of this catalyst is not good is obtained.
The XRD spectrogram of the bimetallic catalyst MnO2-NiO/γ-Al2O3.
Diffraction peaks of crystal NiO are not obvious, and only a few extremely weak and very small amounts of NiO diffraction peak exist, as shown in Figure 5, showing that metal oxide NiO on MnO2-NiO/γ-Al2O3 with the form of microcrystalline disperses highly and uniformly on the surface of the catalyst. At the same time, we also observe that the principal phase of XRD spectra is NiAl2O4, and the main component except NiAl2O4 is the carrier γ-Al2O3.
Nickel aluminum spinel NiAl2O4 formed by the interaction between NiO and Al2O3 in oxidizing atmosphere is very difficult to be reduced, and its catalytic activity is quite lower than that of MnO2 and NiO. The concrete reaction is as follows:
The reason that these catalytic effects of catalyst-supported MnO2 were all better than monometallic-supported catalysts may be that the addition of Mn does not change the structural characteristics of the catalyst, but it significantly promoted the dispersion of active component in catalyst.
3.2.2 SEM characterization
The scanning electron microscope (SEM) JSM-6460LV used in SEM test is produced in Japan Electron Optics Laboratory Co., Ltd. γ-Al2O3, 8%NiO/γ-Al2O3, and 5%MnO2-8% NiO/γ-Al2O3 can be scanned by SEM in front of the desulfurization. The working voltage is 20 kV, and the amplification of sample is 1000 times. The surface shape appearance figures of these three materials by SEM are obtained in this figure.
From Figure 6, on the γ-Al2O3 loaded without oxides (Figure 6A), the quantity of surface particles is less and the amount of void space is more and uneven. The catalyst surface is smooth and has no grains; the crystal phenomenon can be barely seen. Compared with Figure 6A, there were only a small number of particles, which can be seen on the surface of γ-Al2O3 carrier from Figure 6B, and the size and distribution of particles with a state of layered distribution are more uniform, just like a layer of thing spread on surface, which shows matters with a state of dispersion appear.
The SEM chart of the bimetallic catalyst MnO2-NiO/γ-Al2O3.
Compared with Figure 6B, the catalyst particles are bigger and overlap each other, and the layer structure on the carrier surface with the addition of another substance appears as gobbets, as shown in Figure 6C. Combined with the results of XRD characterization, it can be concluded that NiO and MnO2 have a dispersed state account for a major portion on the structure, which appears as gobbets. This conclusion is consistent with the effect of catalyst, further indicating that the addition of loaded metal makes the desulfurization effect of γ-Al2O3 better.
3.2.3 ICP characterization
Inductive coupling plasma emission spectrograph is abbreviated as ICP. The strength of emission spectra of atoms or ions is directly determined by atomic emission spectrometry to qualitatively or quantitatively analyze elements.
Testing results are shown in Table 1. It can be seen from Table 1 that the theoretical NiO loading is 8.00%, the actual loading is 6.17%, the theoretical MnO2 loading is 5.00%, and the actual loading is 4.83%, which is relatively close to theoretical value.
The testing result of ICP.
| Test sample | Cu 327.395 | Fe 238.204 | K 766.491 | Mn 257.610 | Ni 231.604 | Zn 213.857 |
|---|---|---|---|---|---|---|
| Blank sample (ppm) | 0 | 0 | 0 | 0 | 0 | 0 |
| Standard sample (ppm) | 100 | 100 | 100 | 100 | 100 | 100 |
| Testing value (ppm) | 0.404673 | 87.0642 | 4.41438 | 0.448328 | 0.220089 | 7.61204 |
| Theory of NiO capacity | 8.00% | |||||
| Actual of NiO capacity | 6.16% | |||||
| Theory of MnO2 capacity | 5.00% | |||||
| Actual of MnO2 capacity | 4.83% |
3.2.4 TPR characterization
TPR testing instrument is the chemical adsorption equipment of Micromeritics 2720 type, using TCD to detect the consumption of H2. MnO2-NiO/γ-Al2O3 catalyst weighed as 0.1 g is placed in the glass reactor and purged for 1 h at the temperature of 400°C in nitrogen atmosphere (nitrogen flow is 25 ml/min) and then reduced the temperature to 30°C and switched N2 to 10% H2/N2 (gas flow is 25 ml/min). Moreover, the temperature increased from 30°C to 800°C at 10°C/min.
Figure 7 shows three reduction peaks in the H2-TPR curve at 315°C, 519°C, and 730°C appear. On the basis of the correlative references, there were reduction peaks of dispersed state MnO2 and highly decentralized NiO and NiAl2O4, which was on the surface [15]. The TPR result was in agreement with the XRD result, which showed that the vast majority of Ni in the MnO2-NiO/γ-Al2O3 highly disperse in the carrier Al2O3 in the form of NiO, and only a minuscule portion exists in the form of NiO crystal. Moreover, the NiAl2O4 spinal with low catalytic activity formed by the interaction between NiO and γ-Al2O3 carrier also is found in the TPR result.
The TRP spectrogram of the bimetallic catalyst MnO2-NiO/γ-Al2O3.
4 Conclusions
Egg shell type, egg white type, and egg yolk type catalysts have different characteristics in the desulfurization process.
The desulfurization of bimetallic-supported catalyst MnO2-NiO/γ-Al2O3 increased first and then gradually decreased as loading increased, and the optimum loading is 5% MnO2. Desulfurization increased first and then gradually decreased as calcination temperature increased, and the most suitable calcination temperature was 400°C. Desulfurization increased first and then gradually decreased as calcination time increased, and the optimum calcination time was 4 h.
The optimum preparation conditions of bimetallic-supported catalyst MnO2-NiO/γ-Al2O3 screened in this experiment were as follows: the amount of oxide loading was 5%, calcination temperature was 400°C, and calcination time was 4 h.
The modified catalysts were characterized using XRD, SEM, and TPR methods to study the phase transformation of catalysts before and after the desulfurization and the desulfurization mechanism of catalytic reduction, which provides the reasonable theory basis for the application of catalytic technology of oxide-supported catalyst for SO2.
Funding source: National Natural Science Foundation of China
Award Identifier / Grant number: 41202176
Funding statement: The financial support of this research is from the Scientific Research Program Funded by the Shanxi Provincial Education Department (Program No. 2013JK0869) in PR China, the Shanxi —Province Innovation of Science and Technology Project Plan (2012KTZ03-01-02-01), the National Natural Science Foundation of China (41202176), and the Key Laboratory of Coal Resources Exploration and Comprehensive Utilization, Ministry of Land and Resources (Program No. KF2015-1) in PR China were gratefully acknowledged.
Acknowledgments:
The financial support of this research is from the Scientific Research Program Funded by the Shanxi Provincial Education Department (Program No. 2013JK0869) in PR China, the Shanxi —Province Innovation of Science and Technology Project Plan (2012KTZ03-01-02-01), the National Natural Science Foundation of China (41202176), and the Key Laboratory of Coal Resources Exploration and Comprehensive Utilization, Ministry of Land and Resources (Program No. KF2015-1) in PR China were gratefully acknowledged.
References
[1] Yunwei X. Min. Saf. Environ. Prot. 2000, 27, 7–40.10.1016/S1074-9098(00)00110-6Search in Google Scholar
[2] Dongming G. Textbook of Sulfur Nitrogen Pollution Prevention and Control Engineering Technology and Application, 1st ed., Chemical Industry Press, Beijing, 2001.Search in Google Scholar
[3] Yi Q, Jiming H. Textbook of Industrial Pollution Prevention and Control, 1st ed., China Science and Technology Press, Beijing, 1989.Search in Google Scholar
[4] Lowell PS, Schwitzgebel K, Parsons TB, Sladek KJ. Ind. Eng, Chem. Proc. Des. Dev. 1971, 10, 384–390.10.1021/i260039a018Search in Google Scholar
[5] Qi J, Guocai D, Rongti C. Chin. J. Catal. 1997, 18, 5–8.Search in Google Scholar
[6] Jitao L, Weide Z, Lizhen G. Nat. Gas Chem. Ind. 1998, 23, 18–21.Search in Google Scholar
[7] Yuan Y, Rui Q. Petrochem. Technol. 2008, 37, 198–202.10.1007/978-3-540-75662-0_15Search in Google Scholar
[8] Hui H, Shengli L, Shunxi Z. Chin. J. Catal. 2004, 25, 115–119.10.1007/s11284-004-0020-ySearch in Google Scholar
[9] Muzio LJ, Offen GR. J. Air Pollut. Control Assoc. 1987, 37, 642–654.10.1080/08940630.1987.10466251Search in Google Scholar
[10] Faltsi-Saravelou O, Vasalos IA. Ind. Eng. Chem. Res. 1990, 29, 251–256.10.1021/ie00098a015Search in Google Scholar
[11] Yue X. Textbook of Catalyst Design and Manufacturing Process, 1st ed., Chemical Industry Press, Beijing, 2003.Search in Google Scholar
[12] Zhongtao H, Weiming L, Xianshen P. Design and Development of Industry Catalyst, South China University of Technology Press, Guang Zhou, 1991.Search in Google Scholar
[13] Sufang Y. J. Zhejiang Normal Univ. (Natoral Sciences), 2000, 23, 150–154.Search in Google Scholar
[14] Kaidong C, Qijie Y, Baicheng S. J. Fuel Chem. Technol. 1997, 25, 175–17.Search in Google Scholar
[15] Qi J. Nat. Gas Chem. Ind. 2000, 25, 9–14.Search in Google Scholar
©2017 Walter de Gruyter GmbH, Berlin/Boston
This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
