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Study of low temperature combustion performance for composite metal catalysts prepared via rotating packed bed
Energy 179 (2019) 431e441
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Energy journal homepage: www.elsevier.com/locate/energy
Study of low temperature combustion performance for composite metal catalysts prepared via rotating packed bed Qiang Guo, Youzhi Liu*, Guisheng Qi, Weizhou Jiao Shanxi Province Key Laboratory of Higee-Oriented Chemical Engineering, North University of China, Taiyuan, Shanxi, 030051, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 July 2018 Received in revised form 3 March 2019 Accepted 4 May 2019 Available online 7 May 2019
The study considered the preparation of a Mn/Cu catalyst using an RPB (rotating packed bed) and assessed the feasibility of the catalytic combustion of toluene gas. AC (activated carbon) was selected as the carrier material while copper and manganese were used as the catalytically active phases. The catalyst was characterised using BET, XRD, SEM, ICP-MS, XPS and TPG. The RPB-prepared catalyst exhibited a good dispersion for the metal particles and had a higher loading, which effectively increased the catalytic conversion of toluene. It was observed that the catalyst prepared under a high rotation speed and the same mass ratio of Mn and Cu had a superior catalytic activity. The results further show that RPB can successfully prepare metal catalysts for the catalytic combustion of VOCs (volatile organic compounds). © 2019 Elsevier Ltd. All rights reserved.
Keywords: Composite metal catalyst Rotating packed bed Catalytic combustion Impregnation Activation
1. Introduction VOCs (Volatile organic compounds) are irritating and toxic organic compounds [1] that have become one of the main components of air pollutants [2]. There are diverse types and concentrations of VOCs for different emission systems. Toluene, a common volatile organic compound contaminant, is primarily produced in the industrial, automotive and printing industries [3]. It forms aerosols that generate photochemical smog, which endangers human health and undermines environmental quality. This study seeks an environmentally friendly and cheap method to remove toluene because of its toxicity. It has been previously shown that catalytic combustion is the most effective and economical way to regulate and remove VOCs [4e6]. Of fundamental importance in catalytic combustion is the selection and preparation of the catalysts [7]. Good catalysts can increase the reactivity and decrease the combustion temperature [8,9]. In addition, the carrier and active component are important factors that can depend on the choice of catalyst. The carrier generally requires a high mechanical strength, good thermal stability, has a suitable pore structure and specific surface area, and is inexpensive and readily available, which are all met by AC
* Corresponding author. E-mail address: [email protected] (Y. Liu). https://doi.org/10.1016/j.energy.2019.05.033 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
(activated carbon). Zhou [10] found that activated carbon as a carrier composite metal catalyst is high in active oxygen clusters and has a loose porous structure, which provide a rich catalytic active centre and promotes good catalytic activity. The loading and dispersion of the active components affect the reactivity of the catalyst. Common active components are the noble metals [11], transition metals [12] and composite metal phases [13]. Precious metals have a lower light-off temperature but are of limited availability, expensive and unstable at high temperatures, which has led to their sparse usage in industrial production. Therefore, researchers have turned to transition metal catalysts for these reactions. A large number of studies have shown that the catalyst can have an enhanced catalytic activity when its preparation method is optimised. Manganese-based catalysts have been widely studied, which is mainly attributed to the fact that manganese-based oxides have more surface-active oxygen and can achieve low-temperature catalysis. To further enhance the activity of manganese-based catalysts, it has been found that doping the manganese with some transition metals can effectively improve its thermal stability and electron mobility. Thus, the catalytic activity of manganese-based oxides can be further improved, with copper-manganese composite catalysts having been favoured by researchers because of its wide availability, reasonable price and good activity. He [14] found that interactions between copper and manganese produce a large number of oxygen acupoints and effectively improve the dispersion of metal
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oxides in the carrier. Morales [15] found that the coppermanganese phase has a higher crystallinity. At the same time, the copper species doping can greatly reduce the crystallisation of manganese oxide and provide abundant surface and hole oxygen, which enhances the catalytic oxidation activity. Therefore, this paper used AC as the carrier and the manganese-copper composite phase as the active component to prepare the composite transition metal catalyst. It has been shown that the catalyst preparation method impacts the active component loading and the supported crystal dispersion, which directly affects the catalyst activity. Common preparation methods for the supported catalyst include the impregnation method [16], hydrothermal method [17], co-precipitation method [18] and sol-gel method [19]. The precipitation method can horizontally mix the active components and effectively control the crystal phase distribution, but the coprecipitation method for multiple materials is cumbersome to perform. Since the difference in precipitation rates has a large influence on the structure and gives a poor reproducibility, it is not suitable for mass production. An important research topic is to find a preparation method that shows a good dispersibility and high load and is simultaneously simple and cheap. The impregnation process is simple and is commonly used in industry. The traditional impregnation equipment consists of a fluidized bed and a stirring tank. The fluidized bed falls off during processing due to the mutual friction between the carriers, which affects the activity of the catalyst to some extent. The stirring kettle has the disadvantage of uneven stirring, which results in an uneven loading of the active components. Wang [20] found that surface metal oxide structures prepared using the conventional stirring impregnation method have a poor disorder and dispersibility. Therefore, there is an urgent need to develop an impregnation method with high a loading capacity that also disperses the active component well. The high-gravity technology is a new concept that enhances the interface mass transfer and relies on an RPB (rotating packed bed) for efficient mixing. A phase-to-phase contact reaction is achieved in the high-speed rotating environment. The liquid is divided into droplets, liquid filaments and a liquid film in the RPB using a distributor, which is then efficiently dispersed on the surface and inside the other phase. At the same time, the load of the liquid is effectively increased due to the high-gravity action. To date, the technology has been successfully applied to absorption [21], dust removal [22], the preparation of nanomaterials [23], desulfurization [24], distillation [25] and adsorption [26]. The preparation process of composite metal catalysts using the high-gravity method is a type of liquid-solid system. It has been reported that the high-gravity technology can effectively strengthen the liquid-solid mass transfer process. Lin [27] showed that AC in a high-gravity environment has good adsorption mass transfer and diffusion effects on yellow dyes. In addition, the liquidsolid mass transfer coefficient is 3e6 times that of a fixed bed. This is mainly due to the fact that the high-gravity technology can provide more adsorption sites and promote the external solid phase diffusion and internal mass transfer. Therefore, the application of the high-gravity technology to prepare composite metal catalysts has improved effects and more application prospects. The high-gravity technology is used in the process of preparing the catalyst through impregnation. A novel catalyst is synthesised in combination with the calcination technology, which not only increases the loading of the active components and adsorbs the oxygen, but also maintains a good dispersibility of the metal components. Therefore, the composite Mn/Cu catalyst was prepared using the high-gravity impregnation method and applied to the catalytic combustion of toluene gas to study the performance of the catalyst.
2. Experiments and methods 2.1. Materials The AC was provided by Shanxi Xinhua Chemical Co., Ltd (Taiyuan, China) with an average particle size of 1 mm. Manganese nitrate (Mn(NO3)2), copper nitrate (Cu(NO3)2), hydrochloric acid (HCl) and nitric acid (HNO3) were supplied by Tianjin Guangfu Reagent Co., Ltd (Tianjin, China). Toluene was provided by Tianjin Shentai Chemical Reagent Co., Ltd (Tianjin, China), and the N2 and O2 were provided by Shanxi Taineng Company (Taiyuan, China). 2.2. AC pre-treatment The original AC was washed and then dried in an oven at 100 C for 10 h. The pre-processed AC allows it to maintain a rich pore structure and increase its loading as a carrier for a longer duration. 2.3. Preparation of supported catalysts Three kinds of composite metal catalysts were synthesised using the impregnation method with AC as the carrier and Mn(NO3)2 and Cu(NO3)2 as the active components. The atomic numbers were varied as Mn/Cu ¼ 1, 1/3, and 3 in the RPB and fixed bed. The metal salt solution was passed through a liquid distributor into the RPB with the AC for 30 min to complete the impregnation process. During the entire impregnation process, the liquid flow rate should generally be sufficient to ensure that the AC is completely wetted. The loading of the AC in the RPB was set to two-thirds. The N2 at 180 C was passed through the RPB and dried in a rotating state for 20 min after the immersion was complete. Then, the sample was calcined at 650 C for 240 min at a heating rate of 5 C$min-1 to obtain the corresponding mixed oxide catalyst, which is labelled as Mn/Cu-AC-RPB. To study the effects of the rotational speed on the catalytic performance of the composite oxide, a series of catalysts were prepared at different rotating speeds of 750, 1050 and 1350 r/ min with the same Mn/Cu ratio. The composite oxide catalyst is represented as Mn/Cu(X)-AC-RPB(Y), where X represents the Mn/ Cu atomic ratio, and Y represents the rotation speed of the RPB. Fig. 1 shows the process flow chart for the preparation the composite metal catalyst in the RPB. 2.4. Characterisation The metal oxide formation and distribution were characterised
Flowmeter
RPB
Valve
Liquid Tank
Pump
Fig. 1. Preparation of composite metal catalyst flow chart in rotating.
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using SEM(scanning electron microscopy) (HITACHI MI4050), where the samples were pre-coated with gold. The metal ion content was detected using ICP-MS (inductively coupled plasma mass spectrometry). Five mg of the catalyst and 4 mL of aqua regia were subjected to high temperature digestion in a microwave digestion apparatus (HACH DRB200) and then diluted to 250 mL. The supernatant was then taken for ICP-MS detection. The surface area and pore size distribution were determined via BET (BrunauereEmmetteTeller) method (Quantachrome NOVA-4000e). The sample was disgust at 150 C for 300 min. Powder XRD (Xray diffraction) patterns of the catalysts were recorded using a D8 Discover Plus with the monochromatic Cu-Ka radiation (l ¼ 1.542 Å). The spectra were recorded with 0.02 (2q) steps over a 3e70 2q angular range and 1 s counting time per step. XPS (X-ray photoelectron spectroscopy) (Thermofisher EscaLab Xiþ) was used to determine the chemical state of the elements on the catalyst surface with a SPECS spectrometer using a monochromatic Al Ka radiation source (1487eV). The acquisition was operated in a pass energy of 20 eV for the individual spectral lines and 50 eV for the extended spectra. H2-TPR (Hydrogen temperature programmed reduction) (FineSorb 3010) was used to characterise the catalytic redox performance. A 0.1 g sample of the catalyst was placed in a quartz tube reactor and the temperature was raised to 400 C for 2 h to remove any gas that was adsorbed on the surface of the catalyst. The temperature was then lowered to 50 C and switched to a 10% H2eHe gas mixture. The sample was treated in an environment with a gas flow rate of 30 ml/min for 2 h, which was then purged under He for 30 min. After the system stabilised, the hydrogen signal was detected with TCD (thermal conductivity detector) at a rate of 15 C/min to 800e900 C. TGA (thermal gravimetric analysis) was then used to characterise the thermal stability of the catalyst at elevated temperatures. The TGA analysis was performed with a thermogravimetric analyser (NETZSCH STA449) in air with a temperature range of 0e1000 C at a heating rate of 10 C/min.
2.5. Catalytic combustion experiment Fig. 2 shows a flow chart for the catalytic combustion of toluene using a metal catalyst. The catalytic activity test for toluene was performed in a flow reactor with a fixed quartz tube operated at atmospheric pressures. The toluene gas was produced using a VOC
Flowmeter
433
gas generator (SPG, China)) mixed with air and was passed through a fixed bed at a total flow rate of 100 ml/min. The activity was detected by periodically increasing the reaction temperature from 180 to 300 C using 30 g of the catalyst. The toluene concentration was analysed online throughout the reaction system using a gas chromatograph (SIGNAL2012, United Kingdom) equipped with a thermal conductivity detector. The catalytic activity was characterised and labelled as T10, T50 and T90 corresponding to toluene conversions of 10%, 50% and 90%, respectively. 3. Results and discussion 3.1. SEM analysis An SEM micrograph showing the surface morphology of the sample is given in Fig. 3. The results indicate that the original AC surface is smooth and free of metal oxides. However, metal oxide is present on the AC surface after impregnation, which indicates that the supported metal catalyst was successfully prepared using the RPB. There is significant accumulation of Mn/Cu(1∶1)-AC-RPB (0) on the surface, which shows that the conventional impregnation method is not conducive for a uniform dispersion of the metal oxide on the AC. At the same time, the surface oxide dispersion of the catalyst increases with the rotation speed giving a better dispersion with higher RPB speeds. This shows that a high rotation speed is favourable to uniformly disperse the active component, so that the metal oxide forms a film on the AC surface instead of oxide accumulation. In high-speed RPB, the active component can be efficiently sheared and fully atomised, where the dispersion improves with the rotational speed. 3.2. ICP-MS measurement of the metal loading The SEM results demonstrate that the metal oxide of the catalyst prepared with RPB is uniformly dispersed on the AC surface. To illustrate the loading for the metal oxide of the catalyst, the metal content of different catalysts was determined using ICP-MS, with the results presented in Table 1. The table shows that RPB can effectively increase the loading of metal oxides. This is mainly due to the fact that the active component is sheared during high-speed RPB, which is not only uniformly loaded onto the AC, but also effectively increases the load. The AC is a porous material with a
Flowmeter Valve
Toluene Tank
VOCs Gas Generator
Tube Furnace
Flowmeter
VOCs Detector
T
Temperature Deterctor Oxygen Cylinder
Absorption Tank Fig. 2. Metal catalyst catalytic combustion process flow chart.
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A
B
C
E
D
Fig. 3. The SEM images of different catalysts. A.AC; B.Mn/Cu(1∶1)-AC-RPB(1350); C. Mn/Cu (1∶1) -AC-RPB (0); D. Mn/Cu(1∶1)-AC-RPB(1050); E. . Mn/Cu (1∶1) -AC-RPB (750).
Table 1 Metal loading of various impregnation methods from ICP-MS.
AC Mn/Cu Mn/Cu Mn/Cu Mn/Cu Mn/Cu Mn/Cu
(3∶1) (1∶1) (1∶3) (1∶1) (1∶1) (1∶1)
-AC-RPB -AC-RPB -AC-RPB -AC-RPB -AC-RPB -AC-RPB
(1050) (1050) (1050) (0) (750) (1350)
Mn (wt.%)
Cu (wt.%)
0 0.163 0.127 0.032 0.098 0.119 0.151
0 0.045 0.108 0.157 0.083 0.115 0.129
well-developed pore structure. The high-shear active component of the RPB is in contact with AC pores of similar sizes, which effectively enters the interior of the AC due to drag forces. However, the conventional impregnation method can only accomplish a loading of the active component for the interaction force between molecules. The loading of the active component is an important factor that affects the catalytic activity of the metal catalyst. Therefore, the catalyst prepared using RPB can effectively increase the metal oxide loading and thereby increase the catalytic activity. RPB is an impregnation device where the liquid flow rate and rotation speed affect the catalyst loading. Figs. 4 and 5 show the effect of the liquid flow rate and rotational speed of the Mn/ Cu(1:1)-AC-RPB loading. The results indicate that the loading increases initially, and then decreases as the liquid flow rate increased, while the loading increases with the rotational speed. This is because smaller liquid flow rates cannot sufficiently contact the AC to cause a drop in the load. At larger liquid flow rates, the AC is completely immersed in the active component, which then cannot be sheared by the RPB and will not sufficiently penetrate into the interior of the AC. Therefore, a suitable liquid flow rate enables the liquid to be efficiently sheared by the RPB for improved atomisation. These liquid filaments and liquid membranes can fully contact the AC to complete the impregnation process under a strong drag force. A high rotational speed provides a good liquid
Mn ion 200
Mn ion (ppm)
Catalyst type
160
120
80
120
160
200
240
Flow (L/h) Fig. 4. The effect of the liquid flow rate on the Mn loading.
shearing environment that enhances the mixing strength between the AC and the active component, which consequently increases the loading. 3.3. XRD analysis The XRD analysis results can explain the dispersion and the crystal size for the metal active phase of the AC. Fig. 6 shows the XRD patterns of the catalysts under different conditions. The XRD patterns of the surfactant show that the catalyst has obvious diffraction peaks at 24 and 42.6 , which are the typical carbon characteristic peaks. The XRD pattern of the Mn/Cu-AC-RPB shows no obvious characteristic peaks except for those from the carbon, which indicates that the metal active phase is well dispersed on the
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Mn ion
Mn ion
200
160
120
80 0
200
400
600
800
1000
1200
speed (r/min) Fig. 5. The effect of the rotation speed on the Mn loading.
435
from diffraction off the MnOx, which indicates that the metal oxide is enriched on the AC. The XRD testing results are consistent with the SEM imagery. The XRD spectra of the Mn/Cu-AC composite catalysts (Mn:Cu ¼ 1:1) at different rotational speeds are shown in Fig. 6A. The results indicate that Cu exists in the form of CuO and Cu2O while manganese is in the form of MnO2 and Mn3O4 for all the composite metal catalysts. The intensity of the MnO2 diffraction peak for the calcined catalyst prepared with the rotary bed rotation is significantly lower than that of the catalyst prepared without rotation, which means that the rotation can reduce the crystallinity and grain size of the active component on the AC. The diffraction peak intensities of the copper and manganese oxides slightly weaken with the increasing rotational speed. This shows that the active components are uniformly distributed on the surface and inside of the AC and are not likely to cause crystallisation or deposition from the high-speed rotating environment of the RPB. At the same time, the strong interactions between the Cu and Mn mutually suppress the oxide grains, where the oxides exist in an amorphous phase or in a highly dispersed state. Fig. 6B shows the XRD spectra for different molar ratios of the CueMn catalyst. The results show that the intensity of the diffraction peaks for the CuO and CuO2 gradually decrease with the increase of the Mn content. The composite metal catalyst prepared using RPB consists of both MnOx and CuOx. The diffraction peak of the entire catalysts is relatively low, which indicates that the catalyst has a higher degree of amorphization and a minimal crystal size, which is favourable for the catalytic combustion of VOCs. In addition, the highly dispersed CueMn oxides have a synergistic effect and the complex metal catalysts exhibit improved properties compared with the single components of the reaction. 3.4. XPS analysis
Fig. 6. The XRD patterns of different AC conditions and catalysts.
AC after the RPB process. However, the metal catalyst was confirmed to contain a metal component based on the ICP-MS and SEM. The SEM image shows that the catalyst before loading has a smooth layer, while it has an obvious metal film on the surface after loading. The XRD pattern of the Mn/Cu-AC-RPB shows a peak at 36
The XPS technique is focused on determining the elemental composition, metal oxidation state and reactive oxygen species at the sample surface. Fig. 7A and C shows the spectra for different samples. The binding energy of the Mn 2p2/3 for Mn/Cu-AC-RPB is 642 eV. However, the Mn 2p2/3 spectrum can be divided into three peaks (Mn2þ, Mn3þ and Mn4þ) with binding energies of 641e641.5, 642.3e642.7 and 639.7e644 eV, respectively. These ions are essential for the catalytic combustion performance of the catalyst. After integrating the fit peaks, it was found that the manganese ion content in the Mn/Cu-AC-RPB catalyst gradually decreases as the Mn/Cu ratio decreases, while the copper ion concentration gradually increases. The content of Mn3þ and Mn4þ in the Mn/Cu-AC-RPB sample increases with the rotation speed of the RPB when the ratio of manganese to copper is 1. The Mn3þ and Mn4þ lead to a higher activity by participating in the redox reaction: Cu2þþMn3þ4CuþþMn4þ. The high-speed RPB increases the highvalent manganese ion content in the sample and promotes further redox reactions. Fig. 7B and D shows the Cu 2p spectra for different samples. The Cu 2p signal shows the two copper species of Cuþ (932.7 eV) and Cu2þ (953.3 eV). The integral of the peak fit indicates that the Cu2þ content in the sample prepared using the high rotational speed RPB is greater than that when using the conventional impregnation method. The experiment also determined the XPS spectrum of the O 1s peak. The O 1s peak fitting found both a high binding energy (Ohigh) (533.5 eV) and a low binding energy (Olow) (531.5 eV) for the chemisorbed oxygen. The Ohigh of the high-speed RPB prepared sample increases while the Olow decreases. Due to the higher mobility of oxygen, the chemisorbed oxygen contributes more to the catalytic combustion of the catalyst. The excellent catalytic performance of the Mn/Cu composite
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Fig. 7. XPS spectra of the Mn 2p and Cu 2p for different catalysts.
metal catalyst is due to the redox reaction between the two metals as given by Cu2þþMn3þ4CuþþMn4þ. The reaction occurs on the surface of the catalyst as Ohigh þ Cuþ4Cu2þþOad, where the Oad reacts directly with the toluene. The catalyst prepared using RPB has a higher concentration of high-valent metal cations of Cu2þ, Mn3þ and Mn4þ, which increases the catalyst activity and promotes the combustion of the catalyst. At the same time, the relative
proportion of Ohigh increases due to the RPB preparation, which in turn increases the Oad. Therefore, the preparation of the Mn/Cu catalyst using RPB produces an improved activity. The XPS results show that high concentrations of metal cations and a high binding energy of the chemisorbed oxygen help to enhance the catalytic activity of the composite catalyst.
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Table 2 Specific surface area, Pore volume and Average pore of various catalysts. Catalyst type
Specific surface area (m2/g)
Pore volume (ml/g)
Average pore (nm)
AC Mn/Cu Mn/Cu Mn/Cu Mn/Cu Mn/Cu Mn/Cu
1174.043 887.621 963.057 871.026 807.43 931.586 1005.289
0.4627 0.4785 0.501 0.464 0.408 0.4897 0.5095
0.545 0.484 0.539 0.524 0.321 0.524 0.484
(3∶1) (1∶1) (1∶3) (1∶1) (1∶1) (1∶1)
-AC-RPB -AC-RPB -AC-RPB -AC-RPB -AC-RPB -AC-RPB
(1050) (1050) (1050) (0) (750) (1350)
Fig. 8. Pore size distribution of the AC and catalyst.
3.5. BET surface area measurement To confirm the dispersion of the catalyst from the supported active phase, we analysed the specific surface area, pore size and pore volume of the different catalysts. Table 2 provides the structural characteristics of the MneCu catalysts prepared under different conditions. The specific surface area, pore volume and pore size of the catalysts are all reduced compared to the AC support, indicating that some amount of the metal oxide was grafted onto the AC. In the active component with equal Mn and Cu contents, the higher rotational speed of the RPB gives a larger specific surface area for the catalyst. In addition, the ICP-MS results show that larger rotational speeds lead to larger metal loadings.
Theoretically, the specific surface area of the catalyst decreases with an increased metal loading, meaning the experimental results are inconsistent with the theoretical analysis. This is primarily due to the fact that the catalyst prepared in the high-speed RPB increases the dispersion of the active component on the surface and inside of the AC. The active component then forms a uniform metal oxide film rather than performing an enrichment, which clogs the microporous channels of the AC. These results are in agreement with the SEM analysis. We know that the activity of the catalyst depends on a good pore structure and abundant active components. From the results of the BET and ICP-MS, it can be seen that the catalyst prepared using RPB both increases the loading of the metal oxide and ensures the pore structure of the catalyst, which are ideal catalyst properties. To further investigate the pore structure of the RPB-prepared catalyst, we determined the pore size distribution of the Mn/ Cu(1:1)-AC-RPB (1050) and the original AC, as shown in Fig. 8. The results reveal that the average pore diameters for both samples are less than 1 nm, which are described as microporous structures. The Mn/Cu(1:1)-AC-RPB (1050) does not seriously damage the developed pore structure of the AC. This means that the RPB allows the active component to be highly dispersed into the interior and on the surface of the AC. 3.6. TPR analysis
Fig. 9. TPR profiles for the different catalysts.
Fig. 9 shows the TPG spectrum for different catalysts. The Mn/ Cu(1:3)-AC-RPB (1050) has a reduced peak at a temperature of 220 C, which is due to a reduction of the CuO on the catalyst surface. Two reduction peaks appear in the Mn/Cu(3:1)-AC-RPB (1050) at 350 and 425 C. The lower temperature reduction peak corresponds to the reduction of Mn3O4 by Mn2O3 on the catalyst surface, and the higher temperature reduction peak corresponds to the reduction of Mn3O4 to MnO. There is only one peak that appears in the Mn/Cu(1:1)-AC-RPB (1050), which is due to the simultaneous
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Fig. 10. TGA profiles of different catalysts.
3.7. TGA analysis Heat resistance is an important indicator when measuring the stability of a catalyst. The TGA curves for the different catalysts are shown in Fig. 10. The AC has an obvious weight loss process at 450 C, which is attributed to a high-temperature carbonisation reaction through decomposition into CO and CO2 [29]. The weight loss of the composite metal catalyst is significantly reduced compared to the AC. The weight loss (5%) of the composite metal catalyst in the range of 450e950 C is significantly smaller than that of AC (50%). This is because the metal active component covers the AC surface and prevents the carbonisation reactions. The experimental results show that the composite metal catalyst is stable from 0 to 1000 C. The weight loss of the Mn/Cu(1:1)-AC-RPB (1050) is slightly lower than that of the Mn/Cu(1:1)-AC-RPB (0). The active component in the Mn/Cu(1:1)-AC-RPB (1050) has a good dispersibility and a rich pore structure, which facilitate the gas mass transfer and prevent carbon deposition. The TGA results indicate that the composite metal catalyst exhibits a good thermal performance at elevated temperatures. Fig. 11 shows the relationship between specific surface area and thermal properties(weight loss between 50 and 980 C) of the catalyst. The larger specific surface area of catalyst, the better thermal properties. Well-developed tunnels provide a good place for gas circulation and prevent local carbon deposits in catalyst. The results show that the thermal performance of the prepared catalyst in the rotating packed bed is better than the traditional impregnation method, and the higher the rotational speed, the better the thermal performance. It can be seen that the rotating packed bed is an excellent catalyst preparation device.
3.8. Tests of the catalyst activity 3.8.1. Catalytic activity of different toluene catalysts Fig. 12 shows the catalytic combustion activity of different catalysts with respect to toluene. The results indicate that the catalytic combustion activity for toluene is related to the preparation method and the active components. The temperatures at which 10%, 50% and 90% methane conversion is reached (T10, T50 and T90) are used to compare the catalytic activity of the solids (Table 3). The activity of the catalyst prepared using RPB is higher than that of the conventional impregnation method, and a higher rotational speed during preparation leads to higher activities. This is because the RPB process can effectively increase the metal oxide
6.4
6.0
weightlessness (%)
reductions of Cu and Mn. These results are consistent with the previous XRD analyses. The Mn temperature in the composite oxide is low due to the presence of Cu species that accelerate the reduction of Mn [28].
5.6
5.2
4.8 800
850
900
950
1000
2
Specific surface area (m /g) Fig. 11. The relationship between specific surface area and thermal properties(weight loss between 50 and 980 C) of the catalysts.
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439
＞Mn/Cu (1∶1) -AC-RPB (1050) ＞Mn/Cu (1∶1) -AC-RPB (750) ＞ Mn/Cu (3∶1) -AC-RPB (1050) ＞Mn/Cu (1∶3) -AC-RPB (1050) ＞Mn/ Cu (1∶1) -AC-RPB (0).
3.8.2. The kinetics of the catalytic combustion of toluene To investigate the reaction kinetics of toluene on different catalysts, the MVK (Mars-Van Krevelen) model was used to fit the experimental data. The MVK model is based on the oxidationreduction mechanism and is formulated as follows:
1 1 ¼A þ ri ki ci
(1)
vi koi coi
(2)
A¼
Fig. 12. The effect of different catalysts on the catalytic activity of toluene.
Table 3 Catalytic performance of different catalysts for catalytic combustion of toluene. Catalyst Mn/Cu Mn/Cu Mn/Cu Mn/Cu Mn/Cu Mn/Cu
(1∶1) (1∶1) (1∶1) (3∶1) (1∶3) (1∶1)
-AC-RPB -AC-RPB -AC-RPB -AC-RPB -AC-RPB -AC-RPB
(1350) (1050) (750) (1050) (1050) (0)
T10 ( C)
T50 ( C)
T90 ( C)
175 205 230 250 260 278
261 265 270 292 e e
283 292 e e e e
content on the AC while maintaining its good structure. The experimental results also show the activity of the catalyst is the highest when the Mn to Cu ratio is 1. This is because there is a synergistic effect between these metals that increases the catalytic activity and lowers the catalytic combustion temperature of toluene. The catalytic activities for the different catalysts in the combustion of toluene are ranked as Mn/Cu (1∶1) -AC-RPB (1350)
where, -ri (mol/g/s) is the reaction speed, ki (s-1) is the reaction rate constant for the different components, ci (mol/cm3) is the export concentration of the different components, а (mol) is the oxygen content required to oxidise 1 mol of toluene, koi (s-1) and coi (mol) are, respectively, the reaction rate constant and concentration of oxygen. The oxygen concentration was nearly constant during the catalytic reaction. Therefore, A can be considered to be constant. The expression for the reaction rate is then simplified as follows:
ri ¼
Fi X Wcat A
(3)
where, Fi (mol/s) is the molar concentration of the different components, Wcat (g) is the catalyst quality and XA is the conversion rate. The fitting results show that the MVK model can describe the reaction kinetics of the catalytic combustion of toluene with Mn/ Cu(1:1)-AC-RPB (1050) and Mn/Cu(1:1)-AC-RPB (0) (Fig. 13). At the same time, the fitting results were used to find the rate constants for the oxidation of toluene from these two samples as 0.03845 and 0.0203 s-1, respectively, at the same temperature. This
Fig. 13. The MVK model fit for the Cu/Mn-AC-RPB and Cu/Mn-AC-MS samples.
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Science Foundation of China (U1610106), the Excellent Youth Science and Technology Foundation of Province Shanxi of China (2014021007), and the financial support from National Key P&D program of China (2016YFC0204103).
Conversion rate (%)
94
92 References
90
88
86 0
100
200
300
400
500
600
t (min) Fig. 14. Test results for the catalyst stability performance.
indicates that the Mn/Cu(1:1)-AC-RPB (1050) sample can accelerate the oxidation rate with toluene, which is related to the Cu and Mn loading of the catalyst. The MVK model also showed that the catalytic combustion reaction of toluene on the Cu/Mn-AC is based on the oxidation-reduction mechanisms. 3.8.3. Test for the catalyst stability Fig. 14 shows the stability for the catalytic combustion of toluene with Mn/Cu(1:1)-AC-RPB (1050) with a toluene inlet concentration of 500 ppm, a catalytic combustion temperature of 270 C and a gas flow rate of 0.36 m3 h-1. The results reveal that the conversion of toluene is greater than 90% and the catalytic combustion performance is maintained at a high level over the longterm catalytic reaction for 600 min. This indicates that the stability of the Mn/Cu(1:1)-AC-RPB (1050) sample is the best. 4. Conclusions Composite Mn/Cu metal catalysts were prepared using RPB as the impregnation process. The dispersibility of the metal oxides on the AC surface was analysed via SEM and BET. It was proven that the catalyst prepared with RPB has a good dispersibility and abundant pore structure. It was confirmed using ICP-MS that the RPB preparation gave a higher metal oxide loading. The XRD and XPS results showed that the Mn and Cu interactions led to an enhanced metal reducibility, and that a Mn/Cu ratio of 1 could increase the intrinsic activity of the catalyst. The loading and activity of the catalystsupported metal oxide increased as the rotational speed increased, and the catalyst exhibited a good thermal stability at higher temperatures. The metal oxide loading and developed pore structure were found to be major factors for the increased catalytic activity. The Mn/Cu(1∶1)-AC-RPB (1050) had the highest reaction rate in the redox process based on the kinetic studies, and the catalytic combustion of toluene revealed that it also possessed the best low temperature combustion performance. Conflicts of interest There are no conflicts of interest. Acknowledgement Authors thank to the financial support from National Natural
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Study of low temperature combustion performance for composite metal catalysts prepared via rotating packed bed Qiang Guo, Youzhi Liu*, Guisheng Qi, Weizhou Jiao Shanxi Province Key Laboratory of Higee-Oriented Chemical Engineering, North University of China, Taiyuan, Shanxi, 030051, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 July 2018 Received in revised form 3 March 2019 Accepted 4 May 2019 Available online 7 May 2019
The study considered the preparation of a Mn/Cu catalyst using an RPB (rotating packed bed) and assessed the feasibility of the catalytic combustion of toluene gas. AC (activated carbon) was selected as the carrier material while copper and manganese were used as the catalytically active phases. The catalyst was characterised using BET, XRD, SEM, ICP-MS, XPS and TPG. The RPB-prepared catalyst exhibited a good dispersion for the metal particles and had a higher loading, which effectively increased the catalytic conversion of toluene. It was observed that the catalyst prepared under a high rotation speed and the same mass ratio of Mn and Cu had a superior catalytic activity. The results further show that RPB can successfully prepare metal catalysts for the catalytic combustion of VOCs (volatile organic compounds). © 2019 Elsevier Ltd. All rights reserved.
Keywords: Composite metal catalyst Rotating packed bed Catalytic combustion Impregnation Activation
1. Introduction VOCs (Volatile organic compounds) are irritating and toxic organic compounds [1] that have become one of the main components of air pollutants [2]. There are diverse types and concentrations of VOCs for different emission systems. Toluene, a common volatile organic compound contaminant, is primarily produced in the industrial, automotive and printing industries [3]. It forms aerosols that generate photochemical smog, which endangers human health and undermines environmental quality. This study seeks an environmentally friendly and cheap method to remove toluene because of its toxicity. It has been previously shown that catalytic combustion is the most effective and economical way to regulate and remove VOCs [4e6]. Of fundamental importance in catalytic combustion is the selection and preparation of the catalysts [7]. Good catalysts can increase the reactivity and decrease the combustion temperature [8,9]. In addition, the carrier and active component are important factors that can depend on the choice of catalyst. The carrier generally requires a high mechanical strength, good thermal stability, has a suitable pore structure and specific surface area, and is inexpensive and readily available, which are all met by AC
* Corresponding author. E-mail address: [email protected] (Y. Liu). https://doi.org/10.1016/j.energy.2019.05.033 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
(activated carbon). Zhou [10] found that activated carbon as a carrier composite metal catalyst is high in active oxygen clusters and has a loose porous structure, which provide a rich catalytic active centre and promotes good catalytic activity. The loading and dispersion of the active components affect the reactivity of the catalyst. Common active components are the noble metals [11], transition metals [12] and composite metal phases [13]. Precious metals have a lower light-off temperature but are of limited availability, expensive and unstable at high temperatures, which has led to their sparse usage in industrial production. Therefore, researchers have turned to transition metal catalysts for these reactions. A large number of studies have shown that the catalyst can have an enhanced catalytic activity when its preparation method is optimised. Manganese-based catalysts have been widely studied, which is mainly attributed to the fact that manganese-based oxides have more surface-active oxygen and can achieve low-temperature catalysis. To further enhance the activity of manganese-based catalysts, it has been found that doping the manganese with some transition metals can effectively improve its thermal stability and electron mobility. Thus, the catalytic activity of manganese-based oxides can be further improved, with copper-manganese composite catalysts having been favoured by researchers because of its wide availability, reasonable price and good activity. He [14] found that interactions between copper and manganese produce a large number of oxygen acupoints and effectively improve the dispersion of metal
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oxides in the carrier. Morales [15] found that the coppermanganese phase has a higher crystallinity. At the same time, the copper species doping can greatly reduce the crystallisation of manganese oxide and provide abundant surface and hole oxygen, which enhances the catalytic oxidation activity. Therefore, this paper used AC as the carrier and the manganese-copper composite phase as the active component to prepare the composite transition metal catalyst. It has been shown that the catalyst preparation method impacts the active component loading and the supported crystal dispersion, which directly affects the catalyst activity. Common preparation methods for the supported catalyst include the impregnation method [16], hydrothermal method [17], co-precipitation method [18] and sol-gel method [19]. The precipitation method can horizontally mix the active components and effectively control the crystal phase distribution, but the coprecipitation method for multiple materials is cumbersome to perform. Since the difference in precipitation rates has a large influence on the structure and gives a poor reproducibility, it is not suitable for mass production. An important research topic is to find a preparation method that shows a good dispersibility and high load and is simultaneously simple and cheap. The impregnation process is simple and is commonly used in industry. The traditional impregnation equipment consists of a fluidized bed and a stirring tank. The fluidized bed falls off during processing due to the mutual friction between the carriers, which affects the activity of the catalyst to some extent. The stirring kettle has the disadvantage of uneven stirring, which results in an uneven loading of the active components. Wang [20] found that surface metal oxide structures prepared using the conventional stirring impregnation method have a poor disorder and dispersibility. Therefore, there is an urgent need to develop an impregnation method with high a loading capacity that also disperses the active component well. The high-gravity technology is a new concept that enhances the interface mass transfer and relies on an RPB (rotating packed bed) for efficient mixing. A phase-to-phase contact reaction is achieved in the high-speed rotating environment. The liquid is divided into droplets, liquid filaments and a liquid film in the RPB using a distributor, which is then efficiently dispersed on the surface and inside the other phase. At the same time, the load of the liquid is effectively increased due to the high-gravity action. To date, the technology has been successfully applied to absorption [21], dust removal [22], the preparation of nanomaterials [23], desulfurization [24], distillation [25] and adsorption [26]. The preparation process of composite metal catalysts using the high-gravity method is a type of liquid-solid system. It has been reported that the high-gravity technology can effectively strengthen the liquid-solid mass transfer process. Lin [27] showed that AC in a high-gravity environment has good adsorption mass transfer and diffusion effects on yellow dyes. In addition, the liquidsolid mass transfer coefficient is 3e6 times that of a fixed bed. This is mainly due to the fact that the high-gravity technology can provide more adsorption sites and promote the external solid phase diffusion and internal mass transfer. Therefore, the application of the high-gravity technology to prepare composite metal catalysts has improved effects and more application prospects. The high-gravity technology is used in the process of preparing the catalyst through impregnation. A novel catalyst is synthesised in combination with the calcination technology, which not only increases the loading of the active components and adsorbs the oxygen, but also maintains a good dispersibility of the metal components. Therefore, the composite Mn/Cu catalyst was prepared using the high-gravity impregnation method and applied to the catalytic combustion of toluene gas to study the performance of the catalyst.
2. Experiments and methods 2.1. Materials The AC was provided by Shanxi Xinhua Chemical Co., Ltd (Taiyuan, China) with an average particle size of 1 mm. Manganese nitrate (Mn(NO3)2), copper nitrate (Cu(NO3)2), hydrochloric acid (HCl) and nitric acid (HNO3) were supplied by Tianjin Guangfu Reagent Co., Ltd (Tianjin, China). Toluene was provided by Tianjin Shentai Chemical Reagent Co., Ltd (Tianjin, China), and the N2 and O2 were provided by Shanxi Taineng Company (Taiyuan, China). 2.2. AC pre-treatment The original AC was washed and then dried in an oven at 100 C for 10 h. The pre-processed AC allows it to maintain a rich pore structure and increase its loading as a carrier for a longer duration. 2.3. Preparation of supported catalysts Three kinds of composite metal catalysts were synthesised using the impregnation method with AC as the carrier and Mn(NO3)2 and Cu(NO3)2 as the active components. The atomic numbers were varied as Mn/Cu ¼ 1, 1/3, and 3 in the RPB and fixed bed. The metal salt solution was passed through a liquid distributor into the RPB with the AC for 30 min to complete the impregnation process. During the entire impregnation process, the liquid flow rate should generally be sufficient to ensure that the AC is completely wetted. The loading of the AC in the RPB was set to two-thirds. The N2 at 180 C was passed through the RPB and dried in a rotating state for 20 min after the immersion was complete. Then, the sample was calcined at 650 C for 240 min at a heating rate of 5 C$min-1 to obtain the corresponding mixed oxide catalyst, which is labelled as Mn/Cu-AC-RPB. To study the effects of the rotational speed on the catalytic performance of the composite oxide, a series of catalysts were prepared at different rotating speeds of 750, 1050 and 1350 r/ min with the same Mn/Cu ratio. The composite oxide catalyst is represented as Mn/Cu(X)-AC-RPB(Y), where X represents the Mn/ Cu atomic ratio, and Y represents the rotation speed of the RPB. Fig. 1 shows the process flow chart for the preparation the composite metal catalyst in the RPB. 2.4. Characterisation The metal oxide formation and distribution were characterised
Flowmeter
RPB
Valve
Liquid Tank
Pump
Fig. 1. Preparation of composite metal catalyst flow chart in rotating.
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using SEM(scanning electron microscopy) (HITACHI MI4050), where the samples were pre-coated with gold. The metal ion content was detected using ICP-MS (inductively coupled plasma mass spectrometry). Five mg of the catalyst and 4 mL of aqua regia were subjected to high temperature digestion in a microwave digestion apparatus (HACH DRB200) and then diluted to 250 mL. The supernatant was then taken for ICP-MS detection. The surface area and pore size distribution were determined via BET (BrunauereEmmetteTeller) method (Quantachrome NOVA-4000e). The sample was disgust at 150 C for 300 min. Powder XRD (Xray diffraction) patterns of the catalysts were recorded using a D8 Discover Plus with the monochromatic Cu-Ka radiation (l ¼ 1.542 Å). The spectra were recorded with 0.02 (2q) steps over a 3e70 2q angular range and 1 s counting time per step. XPS (X-ray photoelectron spectroscopy) (Thermofisher EscaLab Xiþ) was used to determine the chemical state of the elements on the catalyst surface with a SPECS spectrometer using a monochromatic Al Ka radiation source (1487eV). The acquisition was operated in a pass energy of 20 eV for the individual spectral lines and 50 eV for the extended spectra. H2-TPR (Hydrogen temperature programmed reduction) (FineSorb 3010) was used to characterise the catalytic redox performance. A 0.1 g sample of the catalyst was placed in a quartz tube reactor and the temperature was raised to 400 C for 2 h to remove any gas that was adsorbed on the surface of the catalyst. The temperature was then lowered to 50 C and switched to a 10% H2eHe gas mixture. The sample was treated in an environment with a gas flow rate of 30 ml/min for 2 h, which was then purged under He for 30 min. After the system stabilised, the hydrogen signal was detected with TCD (thermal conductivity detector) at a rate of 15 C/min to 800e900 C. TGA (thermal gravimetric analysis) was then used to characterise the thermal stability of the catalyst at elevated temperatures. The TGA analysis was performed with a thermogravimetric analyser (NETZSCH STA449) in air with a temperature range of 0e1000 C at a heating rate of 10 C/min.
2.5. Catalytic combustion experiment Fig. 2 shows a flow chart for the catalytic combustion of toluene using a metal catalyst. The catalytic activity test for toluene was performed in a flow reactor with a fixed quartz tube operated at atmospheric pressures. The toluene gas was produced using a VOC
Flowmeter
433
gas generator (SPG, China)) mixed with air and was passed through a fixed bed at a total flow rate of 100 ml/min. The activity was detected by periodically increasing the reaction temperature from 180 to 300 C using 30 g of the catalyst. The toluene concentration was analysed online throughout the reaction system using a gas chromatograph (SIGNAL2012, United Kingdom) equipped with a thermal conductivity detector. The catalytic activity was characterised and labelled as T10, T50 and T90 corresponding to toluene conversions of 10%, 50% and 90%, respectively. 3. Results and discussion 3.1. SEM analysis An SEM micrograph showing the surface morphology of the sample is given in Fig. 3. The results indicate that the original AC surface is smooth and free of metal oxides. However, metal oxide is present on the AC surface after impregnation, which indicates that the supported metal catalyst was successfully prepared using the RPB. There is significant accumulation of Mn/Cu(1∶1)-AC-RPB (0) on the surface, which shows that the conventional impregnation method is not conducive for a uniform dispersion of the metal oxide on the AC. At the same time, the surface oxide dispersion of the catalyst increases with the rotation speed giving a better dispersion with higher RPB speeds. This shows that a high rotation speed is favourable to uniformly disperse the active component, so that the metal oxide forms a film on the AC surface instead of oxide accumulation. In high-speed RPB, the active component can be efficiently sheared and fully atomised, where the dispersion improves with the rotational speed. 3.2. ICP-MS measurement of the metal loading The SEM results demonstrate that the metal oxide of the catalyst prepared with RPB is uniformly dispersed on the AC surface. To illustrate the loading for the metal oxide of the catalyst, the metal content of different catalysts was determined using ICP-MS, with the results presented in Table 1. The table shows that RPB can effectively increase the loading of metal oxides. This is mainly due to the fact that the active component is sheared during high-speed RPB, which is not only uniformly loaded onto the AC, but also effectively increases the load. The AC is a porous material with a
Flowmeter Valve
Toluene Tank
VOCs Gas Generator
Tube Furnace
Flowmeter
VOCs Detector
T
Temperature Deterctor Oxygen Cylinder
Absorption Tank Fig. 2. Metal catalyst catalytic combustion process flow chart.
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A
B
C
E
D
Fig. 3. The SEM images of different catalysts. A.AC; B.Mn/Cu(1∶1)-AC-RPB(1350); C. Mn/Cu (1∶1) -AC-RPB (0); D. Mn/Cu(1∶1)-AC-RPB(1050); E. . Mn/Cu (1∶1) -AC-RPB (750).
Table 1 Metal loading of various impregnation methods from ICP-MS.
AC Mn/Cu Mn/Cu Mn/Cu Mn/Cu Mn/Cu Mn/Cu
(3∶1) (1∶1) (1∶3) (1∶1) (1∶1) (1∶1)
-AC-RPB -AC-RPB -AC-RPB -AC-RPB -AC-RPB -AC-RPB
(1050) (1050) (1050) (0) (750) (1350)
Mn (wt.%)
Cu (wt.%)
0 0.163 0.127 0.032 0.098 0.119 0.151
0 0.045 0.108 0.157 0.083 0.115 0.129
well-developed pore structure. The high-shear active component of the RPB is in contact with AC pores of similar sizes, which effectively enters the interior of the AC due to drag forces. However, the conventional impregnation method can only accomplish a loading of the active component for the interaction force between molecules. The loading of the active component is an important factor that affects the catalytic activity of the metal catalyst. Therefore, the catalyst prepared using RPB can effectively increase the metal oxide loading and thereby increase the catalytic activity. RPB is an impregnation device where the liquid flow rate and rotation speed affect the catalyst loading. Figs. 4 and 5 show the effect of the liquid flow rate and rotational speed of the Mn/ Cu(1:1)-AC-RPB loading. The results indicate that the loading increases initially, and then decreases as the liquid flow rate increased, while the loading increases with the rotational speed. This is because smaller liquid flow rates cannot sufficiently contact the AC to cause a drop in the load. At larger liquid flow rates, the AC is completely immersed in the active component, which then cannot be sheared by the RPB and will not sufficiently penetrate into the interior of the AC. Therefore, a suitable liquid flow rate enables the liquid to be efficiently sheared by the RPB for improved atomisation. These liquid filaments and liquid membranes can fully contact the AC to complete the impregnation process under a strong drag force. A high rotational speed provides a good liquid
Mn ion 200
Mn ion (ppm)
Catalyst type
160
120
80
120
160
200
240
Flow (L/h) Fig. 4. The effect of the liquid flow rate on the Mn loading.
shearing environment that enhances the mixing strength between the AC and the active component, which consequently increases the loading. 3.3. XRD analysis The XRD analysis results can explain the dispersion and the crystal size for the metal active phase of the AC. Fig. 6 shows the XRD patterns of the catalysts under different conditions. The XRD patterns of the surfactant show that the catalyst has obvious diffraction peaks at 24 and 42.6 , which are the typical carbon characteristic peaks. The XRD pattern of the Mn/Cu-AC-RPB shows no obvious characteristic peaks except for those from the carbon, which indicates that the metal active phase is well dispersed on the
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Mn ion
Mn ion
200
160
120
80 0
200
400
600
800
1000
1200
speed (r/min) Fig. 5. The effect of the rotation speed on the Mn loading.
435
from diffraction off the MnOx, which indicates that the metal oxide is enriched on the AC. The XRD testing results are consistent with the SEM imagery. The XRD spectra of the Mn/Cu-AC composite catalysts (Mn:Cu ¼ 1:1) at different rotational speeds are shown in Fig. 6A. The results indicate that Cu exists in the form of CuO and Cu2O while manganese is in the form of MnO2 and Mn3O4 for all the composite metal catalysts. The intensity of the MnO2 diffraction peak for the calcined catalyst prepared with the rotary bed rotation is significantly lower than that of the catalyst prepared without rotation, which means that the rotation can reduce the crystallinity and grain size of the active component on the AC. The diffraction peak intensities of the copper and manganese oxides slightly weaken with the increasing rotational speed. This shows that the active components are uniformly distributed on the surface and inside of the AC and are not likely to cause crystallisation or deposition from the high-speed rotating environment of the RPB. At the same time, the strong interactions between the Cu and Mn mutually suppress the oxide grains, where the oxides exist in an amorphous phase or in a highly dispersed state. Fig. 6B shows the XRD spectra for different molar ratios of the CueMn catalyst. The results show that the intensity of the diffraction peaks for the CuO and CuO2 gradually decrease with the increase of the Mn content. The composite metal catalyst prepared using RPB consists of both MnOx and CuOx. The diffraction peak of the entire catalysts is relatively low, which indicates that the catalyst has a higher degree of amorphization and a minimal crystal size, which is favourable for the catalytic combustion of VOCs. In addition, the highly dispersed CueMn oxides have a synergistic effect and the complex metal catalysts exhibit improved properties compared with the single components of the reaction. 3.4. XPS analysis
Fig. 6. The XRD patterns of different AC conditions and catalysts.
AC after the RPB process. However, the metal catalyst was confirmed to contain a metal component based on the ICP-MS and SEM. The SEM image shows that the catalyst before loading has a smooth layer, while it has an obvious metal film on the surface after loading. The XRD pattern of the Mn/Cu-AC-RPB shows a peak at 36
The XPS technique is focused on determining the elemental composition, metal oxidation state and reactive oxygen species at the sample surface. Fig. 7A and C shows the spectra for different samples. The binding energy of the Mn 2p2/3 for Mn/Cu-AC-RPB is 642 eV. However, the Mn 2p2/3 spectrum can be divided into three peaks (Mn2þ, Mn3þ and Mn4þ) with binding energies of 641e641.5, 642.3e642.7 and 639.7e644 eV, respectively. These ions are essential for the catalytic combustion performance of the catalyst. After integrating the fit peaks, it was found that the manganese ion content in the Mn/Cu-AC-RPB catalyst gradually decreases as the Mn/Cu ratio decreases, while the copper ion concentration gradually increases. The content of Mn3þ and Mn4þ in the Mn/Cu-AC-RPB sample increases with the rotation speed of the RPB when the ratio of manganese to copper is 1. The Mn3þ and Mn4þ lead to a higher activity by participating in the redox reaction: Cu2þþMn3þ4CuþþMn4þ. The high-speed RPB increases the highvalent manganese ion content in the sample and promotes further redox reactions. Fig. 7B and D shows the Cu 2p spectra for different samples. The Cu 2p signal shows the two copper species of Cuþ (932.7 eV) and Cu2þ (953.3 eV). The integral of the peak fit indicates that the Cu2þ content in the sample prepared using the high rotational speed RPB is greater than that when using the conventional impregnation method. The experiment also determined the XPS spectrum of the O 1s peak. The O 1s peak fitting found both a high binding energy (Ohigh) (533.5 eV) and a low binding energy (Olow) (531.5 eV) for the chemisorbed oxygen. The Ohigh of the high-speed RPB prepared sample increases while the Olow decreases. Due to the higher mobility of oxygen, the chemisorbed oxygen contributes more to the catalytic combustion of the catalyst. The excellent catalytic performance of the Mn/Cu composite
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Fig. 7. XPS spectra of the Mn 2p and Cu 2p for different catalysts.
metal catalyst is due to the redox reaction between the two metals as given by Cu2þþMn3þ4CuþþMn4þ. The reaction occurs on the surface of the catalyst as Ohigh þ Cuþ4Cu2þþOad, where the Oad reacts directly with the toluene. The catalyst prepared using RPB has a higher concentration of high-valent metal cations of Cu2þ, Mn3þ and Mn4þ, which increases the catalyst activity and promotes the combustion of the catalyst. At the same time, the relative
proportion of Ohigh increases due to the RPB preparation, which in turn increases the Oad. Therefore, the preparation of the Mn/Cu catalyst using RPB produces an improved activity. The XPS results show that high concentrations of metal cations and a high binding energy of the chemisorbed oxygen help to enhance the catalytic activity of the composite catalyst.
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Table 2 Specific surface area, Pore volume and Average pore of various catalysts. Catalyst type
Specific surface area (m2/g)
Pore volume (ml/g)
Average pore (nm)
AC Mn/Cu Mn/Cu Mn/Cu Mn/Cu Mn/Cu Mn/Cu
1174.043 887.621 963.057 871.026 807.43 931.586 1005.289
0.4627 0.4785 0.501 0.464 0.408 0.4897 0.5095
0.545 0.484 0.539 0.524 0.321 0.524 0.484
(3∶1) (1∶1) (1∶3) (1∶1) (1∶1) (1∶1)
-AC-RPB -AC-RPB -AC-RPB -AC-RPB -AC-RPB -AC-RPB
(1050) (1050) (1050) (0) (750) (1350)
Fig. 8. Pore size distribution of the AC and catalyst.
3.5. BET surface area measurement To confirm the dispersion of the catalyst from the supported active phase, we analysed the specific surface area, pore size and pore volume of the different catalysts. Table 2 provides the structural characteristics of the MneCu catalysts prepared under different conditions. The specific surface area, pore volume and pore size of the catalysts are all reduced compared to the AC support, indicating that some amount of the metal oxide was grafted onto the AC. In the active component with equal Mn and Cu contents, the higher rotational speed of the RPB gives a larger specific surface area for the catalyst. In addition, the ICP-MS results show that larger rotational speeds lead to larger metal loadings.
Theoretically, the specific surface area of the catalyst decreases with an increased metal loading, meaning the experimental results are inconsistent with the theoretical analysis. This is primarily due to the fact that the catalyst prepared in the high-speed RPB increases the dispersion of the active component on the surface and inside of the AC. The active component then forms a uniform metal oxide film rather than performing an enrichment, which clogs the microporous channels of the AC. These results are in agreement with the SEM analysis. We know that the activity of the catalyst depends on a good pore structure and abundant active components. From the results of the BET and ICP-MS, it can be seen that the catalyst prepared using RPB both increases the loading of the metal oxide and ensures the pore structure of the catalyst, which are ideal catalyst properties. To further investigate the pore structure of the RPB-prepared catalyst, we determined the pore size distribution of the Mn/ Cu(1:1)-AC-RPB (1050) and the original AC, as shown in Fig. 8. The results reveal that the average pore diameters for both samples are less than 1 nm, which are described as microporous structures. The Mn/Cu(1:1)-AC-RPB (1050) does not seriously damage the developed pore structure of the AC. This means that the RPB allows the active component to be highly dispersed into the interior and on the surface of the AC. 3.6. TPR analysis
Fig. 9. TPR profiles for the different catalysts.
Fig. 9 shows the TPG spectrum for different catalysts. The Mn/ Cu(1:3)-AC-RPB (1050) has a reduced peak at a temperature of 220 C, which is due to a reduction of the CuO on the catalyst surface. Two reduction peaks appear in the Mn/Cu(3:1)-AC-RPB (1050) at 350 and 425 C. The lower temperature reduction peak corresponds to the reduction of Mn3O4 by Mn2O3 on the catalyst surface, and the higher temperature reduction peak corresponds to the reduction of Mn3O4 to MnO. There is only one peak that appears in the Mn/Cu(1:1)-AC-RPB (1050), which is due to the simultaneous
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Fig. 10. TGA profiles of different catalysts.
3.7. TGA analysis Heat resistance is an important indicator when measuring the stability of a catalyst. The TGA curves for the different catalysts are shown in Fig. 10. The AC has an obvious weight loss process at 450 C, which is attributed to a high-temperature carbonisation reaction through decomposition into CO and CO2 [29]. The weight loss of the composite metal catalyst is significantly reduced compared to the AC. The weight loss (5%) of the composite metal catalyst in the range of 450e950 C is significantly smaller than that of AC (50%). This is because the metal active component covers the AC surface and prevents the carbonisation reactions. The experimental results show that the composite metal catalyst is stable from 0 to 1000 C. The weight loss of the Mn/Cu(1:1)-AC-RPB (1050) is slightly lower than that of the Mn/Cu(1:1)-AC-RPB (0). The active component in the Mn/Cu(1:1)-AC-RPB (1050) has a good dispersibility and a rich pore structure, which facilitate the gas mass transfer and prevent carbon deposition. The TGA results indicate that the composite metal catalyst exhibits a good thermal performance at elevated temperatures. Fig. 11 shows the relationship between specific surface area and thermal properties(weight loss between 50 and 980 C) of the catalyst. The larger specific surface area of catalyst, the better thermal properties. Well-developed tunnels provide a good place for gas circulation and prevent local carbon deposits in catalyst. The results show that the thermal performance of the prepared catalyst in the rotating packed bed is better than the traditional impregnation method, and the higher the rotational speed, the better the thermal performance. It can be seen that the rotating packed bed is an excellent catalyst preparation device.
3.8. Tests of the catalyst activity 3.8.1. Catalytic activity of different toluene catalysts Fig. 12 shows the catalytic combustion activity of different catalysts with respect to toluene. The results indicate that the catalytic combustion activity for toluene is related to the preparation method and the active components. The temperatures at which 10%, 50% and 90% methane conversion is reached (T10, T50 and T90) are used to compare the catalytic activity of the solids (Table 3). The activity of the catalyst prepared using RPB is higher than that of the conventional impregnation method, and a higher rotational speed during preparation leads to higher activities. This is because the RPB process can effectively increase the metal oxide
6.4
6.0
weightlessness (%)
reductions of Cu and Mn. These results are consistent with the previous XRD analyses. The Mn temperature in the composite oxide is low due to the presence of Cu species that accelerate the reduction of Mn [28].
5.6
5.2
4.8 800
850
900
950
1000
2
Specific surface area (m /g) Fig. 11. The relationship between specific surface area and thermal properties(weight loss between 50 and 980 C) of the catalysts.
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439
＞Mn/Cu (1∶1) -AC-RPB (1050) ＞Mn/Cu (1∶1) -AC-RPB (750) ＞ Mn/Cu (3∶1) -AC-RPB (1050) ＞Mn/Cu (1∶3) -AC-RPB (1050) ＞Mn/ Cu (1∶1) -AC-RPB (0).
3.8.2. The kinetics of the catalytic combustion of toluene To investigate the reaction kinetics of toluene on different catalysts, the MVK (Mars-Van Krevelen) model was used to fit the experimental data. The MVK model is based on the oxidationreduction mechanism and is formulated as follows:
1 1 ¼A þ ri ki ci
(1)
vi koi coi
(2)
A¼
Fig. 12. The effect of different catalysts on the catalytic activity of toluene.
Table 3 Catalytic performance of different catalysts for catalytic combustion of toluene. Catalyst Mn/Cu Mn/Cu Mn/Cu Mn/Cu Mn/Cu Mn/Cu
(1∶1) (1∶1) (1∶1) (3∶1) (1∶3) (1∶1)
-AC-RPB -AC-RPB -AC-RPB -AC-RPB -AC-RPB -AC-RPB
(1350) (1050) (750) (1050) (1050) (0)
T10 ( C)
T50 ( C)
T90 ( C)
175 205 230 250 260 278
261 265 270 292 e e
283 292 e e e e
content on the AC while maintaining its good structure. The experimental results also show the activity of the catalyst is the highest when the Mn to Cu ratio is 1. This is because there is a synergistic effect between these metals that increases the catalytic activity and lowers the catalytic combustion temperature of toluene. The catalytic activities for the different catalysts in the combustion of toluene are ranked as Mn/Cu (1∶1) -AC-RPB (1350)
where, -ri (mol/g/s) is the reaction speed, ki (s-1) is the reaction rate constant for the different components, ci (mol/cm3) is the export concentration of the different components, а (mol) is the oxygen content required to oxidise 1 mol of toluene, koi (s-1) and coi (mol) are, respectively, the reaction rate constant and concentration of oxygen. The oxygen concentration was nearly constant during the catalytic reaction. Therefore, A can be considered to be constant. The expression for the reaction rate is then simplified as follows:
ri ¼
Fi X Wcat A
(3)
where, Fi (mol/s) is the molar concentration of the different components, Wcat (g) is the catalyst quality and XA is the conversion rate. The fitting results show that the MVK model can describe the reaction kinetics of the catalytic combustion of toluene with Mn/ Cu(1:1)-AC-RPB (1050) and Mn/Cu(1:1)-AC-RPB (0) (Fig. 13). At the same time, the fitting results were used to find the rate constants for the oxidation of toluene from these two samples as 0.03845 and 0.0203 s-1, respectively, at the same temperature. This
Fig. 13. The MVK model fit for the Cu/Mn-AC-RPB and Cu/Mn-AC-MS samples.
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Science Foundation of China (U1610106), the Excellent Youth Science and Technology Foundation of Province Shanxi of China (2014021007), and the financial support from National Key P&D program of China (2016YFC0204103).
Conversion rate (%)
94
92 References
90
88
86 0
100
200
300
400
500
600
t (min) Fig. 14. Test results for the catalyst stability performance.
indicates that the Mn/Cu(1:1)-AC-RPB (1050) sample can accelerate the oxidation rate with toluene, which is related to the Cu and Mn loading of the catalyst. The MVK model also showed that the catalytic combustion reaction of toluene on the Cu/Mn-AC is based on the oxidation-reduction mechanisms. 3.8.3. Test for the catalyst stability Fig. 14 shows the stability for the catalytic combustion of toluene with Mn/Cu(1:1)-AC-RPB (1050) with a toluene inlet concentration of 500 ppm, a catalytic combustion temperature of 270 C and a gas flow rate of 0.36 m3 h-1. The results reveal that the conversion of toluene is greater than 90% and the catalytic combustion performance is maintained at a high level over the longterm catalytic reaction for 600 min. This indicates that the stability of the Mn/Cu(1:1)-AC-RPB (1050) sample is the best. 4. Conclusions Composite Mn/Cu metal catalysts were prepared using RPB as the impregnation process. The dispersibility of the metal oxides on the AC surface was analysed via SEM and BET. It was proven that the catalyst prepared with RPB has a good dispersibility and abundant pore structure. It was confirmed using ICP-MS that the RPB preparation gave a higher metal oxide loading. The XRD and XPS results showed that the Mn and Cu interactions led to an enhanced metal reducibility, and that a Mn/Cu ratio of 1 could increase the intrinsic activity of the catalyst. The loading and activity of the catalystsupported metal oxide increased as the rotational speed increased, and the catalyst exhibited a good thermal stability at higher temperatures. The metal oxide loading and developed pore structure were found to be major factors for the increased catalytic activity. The Mn/Cu(1∶1)-AC-RPB (1050) had the highest reaction rate in the redox process based on the kinetic studies, and the catalytic combustion of toluene revealed that it also possessed the best low temperature combustion performance. Conflicts of interest There are no conflicts of interest. Acknowledgement Authors thank to the financial support from National Natural
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