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FeOx as a high performance catalyst for CO preferential oxidation in H2-rich stream
Hierarchicsally nanoporous Co-Mn-O/FeO x as a high performance catalyst for CO preferential oxidation in H 2 -rich stream Zhongkui Zhao, Yu Li, Ting Bao, Guiru Wang, Turghun Muhammad PII: DOI: Reference:
S1566-7367(13)00448-2 doi: 10.1016/j.catcom.2013.11.019 CATCOM 3722
To appear in:
Catalysis Communications
Received date: Revised date: Accepted date:
16 October 2013 14 November 2013 20 November 2013
Please cite this article as: Zhongkui Zhao, Yu Li, Ting Bao, Guiru Wang, Turghun Muhammad, Hierarchicsally nanoporous Co-Mn-O/FeOx as a high performance catalyst for CO preferential oxidation in H2 -rich stream, Catalysis Communications (2013), doi: 10.1016/j.catcom.2013.11.019
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ACCEPTED MANUSCRIPT Hierarchically nanoporous Co-Mn-O/FeOx as a high performance catalyst for CO
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preferential oxidation in H2-rich stream
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Zhongkui Zhaoa,b,*, Yu Lia, Ting Baoa, Guiru Wanga and Turghun Muhammadb
State Key Laboratory of Fine Chemicals, Department of Catalysis Chemistry and
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Engineering, School of Chemical Engineering, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China; E-mail: [email protected] Key Laboratory of Oil & Gas Fine Chemicals, Ministry of Education & Xinjiang Uyghur
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Autonomous Region, Xinjiang University, Urumqi, Xinjiang 830046, China.
* Corresponding Author Tel. +86-411-84986354 Fax. +86-411-84986354 E-mail: [email protected]
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ACCEPTED MANUSCRIPT Abstract A novel and highly-efficient hierarchically nanoporous Co-Mn-O/FeOx catalyst fabricated by
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a hard/soft dual-templating and subsequent deposition-precipitation (HSDT/DP) approach
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demonstrates unexpectedly high catalytic activity with 100% CO conversion at 75 oC and wide temperature window of 75-200 oC with complete CO removal for CO preferential oxidation (CO PROX), ascribed to the unique microstructure and strong interaction between
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finely dispersed cobalt-manganese and FeOx. The excellent catalytic performance allows it to
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be a practical candidate for CO elimination from H2-rich stream. Keywords: cobalt-based catalyst; CO preferential oxidation; support structure; unexpected
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catalytic activity; temperature-operating window
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1. Introduction
Hydrogen as one of the cleanest and most efficient new energy source for fuel cell is
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generally obtained by catalytic reforming of hydrocarbons and followed by water gas shift (WGS) reaction [1]. However, because of the limited activity of current WGS catalysts for complete CO conversion, a thermodynamically favored reaction at a low temperature, approximately 0.5-1.0 vol% of residual CO still remains in syngas and requires to be decreased to less than 100 ppm to avoid poisoning the anode of proton-exchange membrane fuel cell (PEMFC) [2,3], one of the most efficient candidates for full use of hydrogen energy [4]. The CO PROX reaction has been considered to be a straightforward and effective protocol for eliminating trace CO to purify hydrogen [5-7]. The CO PROX reaction unit can be attached to a PEMFC (typically working at 80-100 oC) or to a WGS
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ACCEPTED MANUSCRIPT reactor (typically working at 200-250 oC), and it can also be a medium unit between a Fuel Cell and a WGS reactor for immobile electricity-production station [8]. Therefore, to
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development highly active and selective CO PROX catalysts with a wide temperature
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window is highly desirable.
Among a number of active catalysts reported for CO PROX, the precious metal-free copper [9-11] and cobalt [12-17] based catalysts have been considered to be promising
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and interesting alternatives in view of their high catalytic performance besides their good
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availability and low cost. Generally, copper-based catalysts exhibited high activity but narrow temperature window and also not satisfactory selectivity, and many efforts are
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being made to further improve activity and selectivity and also to widen temperature
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window for full CO removal [9-20]. The 100% CO conversion (40-60 oC) of the highest
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activity towards copper based catalyst has been reported recenly [18], which is most efficient catalyst for single CO PROX unit, but the narrow temperature window limits its industrial
application [8]. Through fabricating inverse CeO 2/CuO
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flexility of
configuration, the operation temperature window can be broadened to 60 oC (90-150 oC) [9], but the activity was depressed. As the other candidate for CO PROX, the cobalt-based catalysts have illustrated a wider temperature window for complete CO removal, but their catalytic activity at low temperature is definitely required to be improved [12-17]. In our “Nano Catalysis and Energy Storage” research group, the continuing efforts are being made to improve catalytic performance especially catalytic activity at low temperature of cobalt based catalysts [21-26], and the Co-Mn based catalysts have been considered to be a promising candidate [12-17,21-26]. However, the further improvement in catalytic
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ACCEPTED MANUSCRIPT activity at low temperature is essential for practical applications in PEMFC. The catalytic performance of supported-type catalysts is strongly dependent on their
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element content and microstrucutre of support [18-20,27-29]. Iron oxides or hydroxides,
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as novel and effective supports, have been used in CO low-temperature oxidation and CO PROX reactions [30-32]. The reported results illustrate that the hierarchically mesoporous macroporous catalysts, due to the unique microstructure and the metal-support interaction,
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exhibit excellent catalytic performance in various reactions [33,34]. The crystal plane of
performance [35].
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supported active phase dependent on the support nature may strongly affect the catalytic We envision that the hierarchically porous FeOx supported Co-Mn-O
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catalyst may be a promising candidate for CO PROX. However, to the best of knowledges,
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no report on supported Co-Mn based catalysts on iron oxides can be found. In this paper,
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we report a facilely prepared but high performance Co-Mn-O/FeOx catalyst by HSDT/DP method (Scheme S1 in supporting information), exhibiting significantly superior catalytic
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performance.
2. Experimental
2.1. Preparation of catalysts The hierarchically porous Co-Mn-O/FeOx catalyst was prepared by a HSDT/DP route. The typical experimental procedure as follows: firstly, monodispersed SiO2 microsphere was synthesized by using classical stöber method, and then 0.2 g SiO2 was re-dispersed into 10 ml ethanol assisted by ultrasound. 0.819 g cetyl trimethylammonium bromide (CTAB) was dissolved into 22.5 ml ethanol. Secondly, the dispersed SiO2 ethanol solution and CTAB ethanol solution were combined into one dual-template system. Thirdly, 4.83 g iron nitrate 4
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was transferred into autoclave, and suffering from a hydrothermal process at 180 oC for 6 h.
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After filtering, washing, drying, and removing CTAB by calcinations at 500 oC for 5 h via a step of 50 oC from 200 to 500 oC, 2.0 M NaOH solution was used to remove SiO2 template. The obtained FeOx was denoted as FeOx (Si+CTAB). By only using one kind of templates,
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FeOx (Si) and FeOx (CTAB) were prepared, respectively. The FeOx was also prepared by
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traditional precipitation method, and denoted as FeOx (traditional). The various iron oxides (FeOx) were used as supports, The supported Co-Mn-O catalysts on diverse FeOx with an
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optimized 50% loading (from SEM, Co-Mn-O layer deposited on nanoporous FeOx) were
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prepared by our previously reported deposition-precipitation (DP) method (300 oC for 2.5 h)
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[23], and denoted as Co-Mn-O/FeOx (Si+CTAB), Co-Mn-O/FeOx (Si), Co-Mn-O/FeOx (CTAB) and Co-Mn-O/FeOx (traditional), respectively.
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2.2. Characterization of catalysts X-ray diffraction (XRD) profiles were collected from 10o to 80o at a step width of 0.02o using Rigaku Automatic X-ray Diffractometer (D/Max 2400) equipped with a CuKa source (λ=1.5406 Å). The average crystalline particle size estimation was performed according to the
Scherrer
equation
over
multiple
characteristic
diffraction
peaks.
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temperature-programmed reduction (H2-TPR) experiments were performed in an in-house constructed system equipped with a thermal conductivity detector (TCD) to measure H2 consumption. A quartz tube was loaded with 50 mg of catalyst which was pretreated by calcination in Ar at 300 oC for 30 min and then was cooled to ambient temperature in Ar.
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ACCEPTED MANUSCRIPT After that, it was reduced with a 10 vol.% H2/Ar mixture (30 ml min-1) by heating up to 800 o
C at a ramp rate of 10 oC min-1. Nitrogen adsorption and desorption isotherms were
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determined on a Micromeritics apparatus of model ASAP-2050 system at -196 °C. The
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specific surface areas were calculated by the BET method and the pore size distributions were calculated from desorption branch of the isotherm by BJH model. Scanning electron microscope (SEM) experiments were performed on JEOL JSM-5600LV SEM/EDX
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instrument. High-resolution transmission electron microscopy (HRTEM) images were
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obtained by using Tecnai F30 HRTEM instrument (FEI Corp.). 2.3. Catalytic performance measurement
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Catalytic reaction experiments were performed in a stainless steel, fixed bed flow
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reactor (6 mm O.D.) with 200 mg of catalyst held between quartz wool plugs. Typically, the
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reaction feed consisted of 1.0 vol.% CO, 1.0 vol.% O2, 50 vol.% H2 and Ar balance (with or without 10 vol.% CO2 and 10 vol.% H2O) and Ar balance). Samples were pretreated in Ar at
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300 oC for 30 min. Temperatures were measured using K-type thermocouples and controlled by a PID controller. The analysis of the effluent gas was performed using a gas chromatograph on-line with a molecular sieve column and a Porapaq Q column. The CO and CO2 signals were detected by the FID detector after the gas passing through a methanizer. CO conversion and CO2 selectivity were calculated on the basis of the equations as follows: CO conversion CO (%) [CO ]in - [CO]out 100 [CO ]in
O2 conversion O (%) 2
[O2 ]in - [O2 ]out 100 [O2 ]in
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CO 100 2 O 2
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3. Results and Discussion
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Fig. 1 illustrates the reaction results of the diverse Co-Mn-O/FeOx catalysts for CO PROX reaction in H2-rich gas. As illustrated in Fig. 1, in comparison of the other three, the developed Co-Mn-O/FeOx (Si+CTAB) catalyst by a HSDT/DP approach in this work
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demonstrates the remarkably superior catalytic performance, and the CO can be complete
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eliminated in a wide temperature range of 75-200 oC. The catalytic performance of the supported Co-Mn catalyst on FeOx is significantly affected by the microstructure of supports.
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The catalytic activity of the developed catalyst has been near to those of precious metal ones
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reported in literatures [6,36]. From previous literatures, the supported Ni-Co catalysts on
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activated carbon could provide 100% CO conversion only at narrow temperature range of 130-150 oC [16]. The CO conversion on MnOx-promoted Co3O4 catalyst can reach 100%
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only at 125-175 oC [15,25,26]. The initial temperature of 100% CO conversion is obviously lower than those obtained over the previously reported Co-based catalysts in references [12-17,21-26]. The newly reported Cu-based catalyst exhibited very high activity [18], however, as a bottle-neck of Cu-based catalysts, the only 40-60 oC of too narrow temperature window for full CO removal limits its industrially applications. Although 100% CO conversion over the developed catalyst in this work only can be reached at 75 oC, the obtained 75-200 oC of unexpectedly wide temperature window allows it to be a practical candidate. A wider temperature range for complete CO oxidation is considered to improve the operation flexibility for practical applications. Moreover, among all the investigated catalysts,
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ACCEPTED MANUSCRIPT the Co-Mn-O/FeOx (Si+CTAB) catalyst exhibits the highest selectivity for CO PROX reaction at the temperature of initial 100% CO conversion (Fig. S1). Thus, the developed
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catalyst could be considered as a promising candidate for eliminating CO from H2-rich gases.
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To clarify the reason behind the unexpectedly high activity of Co-Mn-O/FeOx (Si+CTAB), the nitrogen adsorption/desorption, XRD, HRTEM, and H2-TPR characterizations were performed.
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Fig. 2 top and bottom present nitrogen adsorption/desorption isotherms and the BJH
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pore size distributions, respectively, and the corresponding quantitative analysis data are given in Table S1 in Supporting Information. It can be found that the adsorption amount
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increases very slowly at low relative pressure, demonstrating not existence of too many
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micropores; and there exists a hysteresis loop after P/P0=0.6 but no adsorption plateau near
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P/P0=1.0, illustrating the presence of mesopores and macropores. The BET surface area decreases from 248.6 to 154.8 m2 g-1 after supporting Co-Mn-O on FeOx, ascribed to the
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overlay of pores with Co-Mn-O. It can be confirmed by the pore size distribution in bottom figure. As illustrated in Fig. 1b, the pore volume for the pore size less than 10 nm or larger than 25 nm decreases but the pore amount with pore size of 10-25 nm increase, implying the formation of accumulating mesopores while the Co-Mn-O being supported on the carrier [23]. As illustrated in Fig. S2 and S3, disordered meso or macropores appear on the samples. From Table S1, the specific surface areas of the Co-Mn-O/FeOx (Si), Co-Mn-O/FeOx (CTAB) and the developed Co-Mn-O/FeOx (Si+CTAB) catalysts are 233.7, 68.2 and 154.8 m2 g-1, showing that the larger surface area of Co-Mn-O/FeOx (Si+CTAB) is not the main reason for
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ACCEPTED MANUSCRIPT its higher activity, since the Co-Mn-O/FeOx (Si) has larger surface area but worse activity, but the larger volume of pores is essential for CO PROX reaction (Fig. 2 bottom and Table S1).
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The XRD patterns of the various Co-Mn-O/FeOx catalysts are demonstrated in Fig. 3, and
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the data for crystallite size are listed in Table S2. The XRD peaks of the four samples can be indexed to cubic Co3O4 (pdf # 42-1467), cubic Fe3O4 (pdf # 85-1436) and rhombohedral Fe2O3 (pdf # 86-0550) [30,31]. But no characteristic diffraction peaks corresponding to MnO2
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can be observed, ascribed to the insertion into Co3O4 matrix [23]. The detectable shift of
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diffraction peaks belongings to Co3O4 over the different supported Co-Mn-O catalysts further suggests the insertion of Mn into Co3O4 matrix, resulting in the strong interaction of Co and
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Mn, which improves the catalytic performance of Co-based catalyst for CO PROX [21-26].
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Of course the presence of partial MnO2 in long-range disorder or possible high dispersion or
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existence in amorphous state cannot be definitely excluded [21-23]. The decrease in diffraction peak intensity of Co-Mn-O/FeOx (Si+CTAB) and expansion in peak width can be
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observed, suggesting smaller average crystallite size (calculated from the half-width of (311) plane using Scherrer’s formula, 8.44 nm of average crystallite size for cubic Co 3O4, smaller than those over other samples, Table S2), which is consistent with the results from HRTEM (Fig. S4). Furthermore, the sharp and strong peaks corresponding to Fe2O3 can be clearly seen on Co-Mn-O/FeOx (CTAB) and Co-Mn-O/FeOx (traditional), but the existence of iron oxide mainly in Fe3O4 on the Co-Mn-O/FeOx (Si) and Co-Mn-O/FeOx (Si+CTAB). From Fig. S5, the larger bulk can be observed on Co-Mn-O/FeOx (CTAB) and Co-Mn-O/FeOx (traditional) but loosely arranged sphere on Co-Mn-O/FeOx (Si+CTAB) and Co-Mn-O/FeOx (Si), correlated to reaction results, the smaller loosely arranged Co-Mn-O microsphere on
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ACCEPTED MANUSCRIPT supported Co-Mn-O catalyst is also favorable for CO PROX reaction. Moreover, the accumulated disordered pore can be observed, consistent with the results from nitrogen
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adsorption/desorption analysis. The CO conversion is consistent with the order of crystalline
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size (Table S2). The remarkably superior catalytic activity of the developed Co-Mn-O/FeOx (Si+CTAB) catalyst to the other supported Co-Mn-O catalysts on FeOx for CO PROX reaction can be ascribed to the high dispersity of Co-Mn-O on support significantly affected
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by the unique microstructure of FeOx supports.
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In order to investigate the redox behavior and the interaction between Co-Mn and Fe in the as-prepared four supported Co-Mn-O catalysts, the H2-TPR experiments were performed,
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and the H2-TPR profiles are presented in Fig. 4. From Fig. 4, the different support structure
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leads to significantly different redox of the supported Co-Mn-O catalysts, ascribed to the
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interaction between Co-Mn-O and FeOx. The -peak can be assigned to the reduction of highly dispersed Co3O4/MnO2 [23]. -peak corresponding to the reduction of Fe2O3 to Fe3O4
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can be observed on Co-Mn-O/FeOx (CTAB) and Co-Mn-O/FeOx (traditional) but not on the other two [30], suggesting there are reducible Fe2O3 on Co-Mn-O/FeOx (CTAB) and Co-Mn-O/FeOx (traditional) but not Co-Mn-O/FeOx (Si+CTAB) and Co-Mn-O/FeOx (Si), which is consistent with the XRD results. From above analysis, by using the diverse FeOx as supports, even if the same loading of Co-Mn (Co/Mn=8:1, and the total loading is 50%) is employed, the supported Co-Mn-O catalysts may demonstrate obviously different redox behaviors, resulted from the dispersity of Co-Mn-O and the interaction between Co-Mn-O and FeOx. The appropriate but not high reducibility is favorable for the selective CO oxidation.
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4. Conclusions
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In summary, we firstly prepared a novel and highly-efficient hierarchically nanoporous
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Co-Mn-O/FeOx (Si+CTAB) catalyst by a HSDT/DP approach, which demonstrates
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unexpectedly superior catalytic performance with 100% conversion in the temperature range of 75-200 oC for CO PROX reaction, ascribed to its unique support microstructure and
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interaction between Co-Mn-O and FeOx as well as the smaller crystalline size of active phase on support, besides the previously confirmed Co-Mn interaction in our previous
Acknowledgements
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CO removal from H2-rich stream.
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report [23]. Thus it allows the developed non-precious catalyst to be a practical candidate for
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This work is financially supported by the National Natural Science Foundation of China (20803006), also sponsored by the Chinese Ministry of Education via the Program
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for New Century Excellent Talents in University (NCET-12-0079), and the Key Laboratory of Oil & Gas Fine Chemicals, Ministry of Education & Xinjiang Uyghur Autonomous Region, Xinjiang University (XJDX0908-2011-10) and the Fundamental Research Funds for the Central Universities (DUT12LK51).
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ACCEPTED MANUSCRIPT List of Figure Captions: Fig. 1. Catalytic activity curves of the various supported Co-Mn-O catalysts for CO
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preferential oxidation in H2-rich stream. Operation conditions: GHSV = 15,000 ml h-1 g-1,
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1.0 % CO, 1.0 % O2, 50 % H2 and Ar balance.
Fig. 2. N2 adsorption isotherms (top) and BJH pore diameter distribution curves (bottom) of the supported Co-Mn-O catalysts on different FeOx supports prepared with diverse templates,
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and the FeOx (Si+CTAB) support is included for comparison. The magnified pore diameter
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distribution in a range of 0-25 nm are given as inset in the bottom figures, for clarity. Fig. 3. XRD patterns of the various supported Co-Mn-O catalysts on FeOx.
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Fig. 4. H2-TPR profiles of the various supported Co-Mn-O catalysts on FeOx.
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Figures:
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Fig. 1. Catalytic activity curves of the various supported Co-Mn-O catalysts for CO preferential oxidation in H2-rich stream. Operation conditions: GHSV = 15,000 ml h-1 g-1,
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1.0 % CO, 1.0 % O2, 50 % H2 and Ar balance.
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Fig. 2. N2 adsorption isotherms (top) and BJH pore diameter distribution curves (bottom) of the supported Co-Mn-O catalysts on different FeOx supports prepared with diverse templates, and the FeOx (Si+CTAB) support is included for comparison. The magnified pore diameter distribution in a range of 0-25 nm are given as inset in the bottom figures, for clarity.
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Fig. 3. XRD patterns of the various supported Co-Mn-O catalysts on FeOx.
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Fig. 4. H2-TPR profiles of the various supported Co-Mn-O catalysts on FeOx.
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ACCEPTED MANUSCRIPT Hierarchically nanoporous Co-Mn-O/FeOx as a high performance catalyst for CO preferential oxidation in H2-rich stream† Zhongkui Zhaoa,b,*, Yu Lia, Ting Baoa, Guiru Wanga and Turghun Muhammadb a
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State Key Laboratory of Fine Chemicals, Department of Catalysis Chemistry and Engineering, School of Chemical Engineering, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China; E-mail: [email protected] b Key Laboratory of Oil & Gas Fine Chemicals, Ministry of Education & Xinjiang Uyghur Autonomous Region, Xinjiang University, Urumqi, Xinjiang 830046, China.
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Graphic Abstract
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ACCEPTED MANUSCRIPT Highlights
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Hierarchically porous Co-Mn-O/FeOx composite shows superior catalytic properties. Full CO removal at 75-200 oC can be obtained over the nonprecious metal catalyst. The developed catalyst can be a practical candidate for CO preferential oxidation. Strong interaction of highly-dispersed Co-Mn with Fe results in high activity. Co-Mn-O dispersity and the interaction are notably affected by support structure.
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S1566-7367(13)00448-2 doi: 10.1016/j.catcom.2013.11.019 CATCOM 3722
To appear in:
Catalysis Communications
Received date: Revised date: Accepted date:
16 October 2013 14 November 2013 20 November 2013
Please cite this article as: Zhongkui Zhao, Yu Li, Ting Bao, Guiru Wang, Turghun Muhammad, Hierarchicsally nanoporous Co-Mn-O/FeOx as a high performance catalyst for CO preferential oxidation in H2 -rich stream, Catalysis Communications (2013), doi: 10.1016/j.catcom.2013.11.019
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Hierarchically nanoporous Co-Mn-O/FeOx as a high performance catalyst for CO
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preferential oxidation in H2-rich stream
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Zhongkui Zhaoa,b,*, Yu Lia, Ting Baoa, Guiru Wanga and Turghun Muhammadb
State Key Laboratory of Fine Chemicals, Department of Catalysis Chemistry and
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Engineering, School of Chemical Engineering, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China; E-mail: [email protected] Key Laboratory of Oil & Gas Fine Chemicals, Ministry of Education & Xinjiang Uyghur
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b
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Autonomous Region, Xinjiang University, Urumqi, Xinjiang 830046, China.
* Corresponding Author Tel. +86-411-84986354 Fax. +86-411-84986354 E-mail: [email protected]
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ACCEPTED MANUSCRIPT Abstract A novel and highly-efficient hierarchically nanoporous Co-Mn-O/FeOx catalyst fabricated by
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a hard/soft dual-templating and subsequent deposition-precipitation (HSDT/DP) approach
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demonstrates unexpectedly high catalytic activity with 100% CO conversion at 75 oC and wide temperature window of 75-200 oC with complete CO removal for CO preferential oxidation (CO PROX), ascribed to the unique microstructure and strong interaction between
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finely dispersed cobalt-manganese and FeOx. The excellent catalytic performance allows it to
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be a practical candidate for CO elimination from H2-rich stream. Keywords: cobalt-based catalyst; CO preferential oxidation; support structure; unexpected
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catalytic activity; temperature-operating window
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1. Introduction
Hydrogen as one of the cleanest and most efficient new energy source for fuel cell is
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generally obtained by catalytic reforming of hydrocarbons and followed by water gas shift (WGS) reaction [1]. However, because of the limited activity of current WGS catalysts for complete CO conversion, a thermodynamically favored reaction at a low temperature, approximately 0.5-1.0 vol% of residual CO still remains in syngas and requires to be decreased to less than 100 ppm to avoid poisoning the anode of proton-exchange membrane fuel cell (PEMFC) [2,3], one of the most efficient candidates for full use of hydrogen energy [4]. The CO PROX reaction has been considered to be a straightforward and effective protocol for eliminating trace CO to purify hydrogen [5-7]. The CO PROX reaction unit can be attached to a PEMFC (typically working at 80-100 oC) or to a WGS
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ACCEPTED MANUSCRIPT reactor (typically working at 200-250 oC), and it can also be a medium unit between a Fuel Cell and a WGS reactor for immobile electricity-production station [8]. Therefore, to
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development highly active and selective CO PROX catalysts with a wide temperature
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window is highly desirable.
Among a number of active catalysts reported for CO PROX, the precious metal-free copper [9-11] and cobalt [12-17] based catalysts have been considered to be promising
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and interesting alternatives in view of their high catalytic performance besides their good
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availability and low cost. Generally, copper-based catalysts exhibited high activity but narrow temperature window and also not satisfactory selectivity, and many efforts are
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being made to further improve activity and selectivity and also to widen temperature
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window for full CO removal [9-20]. The 100% CO conversion (40-60 oC) of the highest
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activity towards copper based catalyst has been reported recenly [18], which is most efficient catalyst for single CO PROX unit, but the narrow temperature window limits its industrial
application [8]. Through fabricating inverse CeO 2/CuO
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flexility of
configuration, the operation temperature window can be broadened to 60 oC (90-150 oC) [9], but the activity was depressed. As the other candidate for CO PROX, the cobalt-based catalysts have illustrated a wider temperature window for complete CO removal, but their catalytic activity at low temperature is definitely required to be improved [12-17]. In our “Nano Catalysis and Energy Storage” research group, the continuing efforts are being made to improve catalytic performance especially catalytic activity at low temperature of cobalt based catalysts [21-26], and the Co-Mn based catalysts have been considered to be a promising candidate [12-17,21-26]. However, the further improvement in catalytic
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ACCEPTED MANUSCRIPT activity at low temperature is essential for practical applications in PEMFC. The catalytic performance of supported-type catalysts is strongly dependent on their
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element content and microstrucutre of support [18-20,27-29]. Iron oxides or hydroxides,
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as novel and effective supports, have been used in CO low-temperature oxidation and CO PROX reactions [30-32]. The reported results illustrate that the hierarchically mesoporous macroporous catalysts, due to the unique microstructure and the metal-support interaction,
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exhibit excellent catalytic performance in various reactions [33,34]. The crystal plane of
performance [35].
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supported active phase dependent on the support nature may strongly affect the catalytic We envision that the hierarchically porous FeOx supported Co-Mn-O
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catalyst may be a promising candidate for CO PROX. However, to the best of knowledges,
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no report on supported Co-Mn based catalysts on iron oxides can be found. In this paper,
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we report a facilely prepared but high performance Co-Mn-O/FeOx catalyst by HSDT/DP method (Scheme S1 in supporting information), exhibiting significantly superior catalytic
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performance.
2. Experimental
2.1. Preparation of catalysts The hierarchically porous Co-Mn-O/FeOx catalyst was prepared by a HSDT/DP route. The typical experimental procedure as follows: firstly, monodispersed SiO2 microsphere was synthesized by using classical stöber method, and then 0.2 g SiO2 was re-dispersed into 10 ml ethanol assisted by ultrasound. 0.819 g cetyl trimethylammonium bromide (CTAB) was dissolved into 22.5 ml ethanol. Secondly, the dispersed SiO2 ethanol solution and CTAB ethanol solution were combined into one dual-template system. Thirdly, 4.83 g iron nitrate 4
ACCEPTED MANUSCRIPT was dissolved into 20 ml deioned water, and gradually dropped into the above dual-template system under strong stirring, subsequently gelled by dropping ammonia. Then the resulted gel
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was transferred into autoclave, and suffering from a hydrothermal process at 180 oC for 6 h.
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After filtering, washing, drying, and removing CTAB by calcinations at 500 oC for 5 h via a step of 50 oC from 200 to 500 oC, 2.0 M NaOH solution was used to remove SiO2 template. The obtained FeOx was denoted as FeOx (Si+CTAB). By only using one kind of templates,
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FeOx (Si) and FeOx (CTAB) were prepared, respectively. The FeOx was also prepared by
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traditional precipitation method, and denoted as FeOx (traditional). The various iron oxides (FeOx) were used as supports, The supported Co-Mn-O catalysts on diverse FeOx with an
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optimized 50% loading (from SEM, Co-Mn-O layer deposited on nanoporous FeOx) were
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prepared by our previously reported deposition-precipitation (DP) method (300 oC for 2.5 h)
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[23], and denoted as Co-Mn-O/FeOx (Si+CTAB), Co-Mn-O/FeOx (Si), Co-Mn-O/FeOx (CTAB) and Co-Mn-O/FeOx (traditional), respectively.
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2.2. Characterization of catalysts X-ray diffraction (XRD) profiles were collected from 10o to 80o at a step width of 0.02o using Rigaku Automatic X-ray Diffractometer (D/Max 2400) equipped with a CuKa source (λ=1.5406 Å). The average crystalline particle size estimation was performed according to the
Scherrer
equation
over
multiple
characteristic
diffraction
peaks.
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temperature-programmed reduction (H2-TPR) experiments were performed in an in-house constructed system equipped with a thermal conductivity detector (TCD) to measure H2 consumption. A quartz tube was loaded with 50 mg of catalyst which was pretreated by calcination in Ar at 300 oC for 30 min and then was cooled to ambient temperature in Ar.
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ACCEPTED MANUSCRIPT After that, it was reduced with a 10 vol.% H2/Ar mixture (30 ml min-1) by heating up to 800 o
C at a ramp rate of 10 oC min-1. Nitrogen adsorption and desorption isotherms were
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determined on a Micromeritics apparatus of model ASAP-2050 system at -196 °C. The
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specific surface areas were calculated by the BET method and the pore size distributions were calculated from desorption branch of the isotherm by BJH model. Scanning electron microscope (SEM) experiments were performed on JEOL JSM-5600LV SEM/EDX
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instrument. High-resolution transmission electron microscopy (HRTEM) images were
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obtained by using Tecnai F30 HRTEM instrument (FEI Corp.). 2.3. Catalytic performance measurement
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Catalytic reaction experiments were performed in a stainless steel, fixed bed flow
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reactor (6 mm O.D.) with 200 mg of catalyst held between quartz wool plugs. Typically, the
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reaction feed consisted of 1.0 vol.% CO, 1.0 vol.% O2, 50 vol.% H2 and Ar balance (with or without 10 vol.% CO2 and 10 vol.% H2O) and Ar balance). Samples were pretreated in Ar at
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300 oC for 30 min. Temperatures were measured using K-type thermocouples and controlled by a PID controller. The analysis of the effluent gas was performed using a gas chromatograph on-line with a molecular sieve column and a Porapaq Q column. The CO and CO2 signals were detected by the FID detector after the gas passing through a methanizer. CO conversion and CO2 selectivity were calculated on the basis of the equations as follows: CO conversion CO (%) [CO ]in - [CO]out 100 [CO ]in
O2 conversion O (%) 2
[O2 ]in - [O2 ]out 100 [O2 ]in
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CO 100 2 O 2
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3. Results and Discussion
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Fig. 1 illustrates the reaction results of the diverse Co-Mn-O/FeOx catalysts for CO PROX reaction in H2-rich gas. As illustrated in Fig. 1, in comparison of the other three, the developed Co-Mn-O/FeOx (Si+CTAB) catalyst by a HSDT/DP approach in this work
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demonstrates the remarkably superior catalytic performance, and the CO can be complete
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eliminated in a wide temperature range of 75-200 oC. The catalytic performance of the supported Co-Mn catalyst on FeOx is significantly affected by the microstructure of supports.
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The catalytic activity of the developed catalyst has been near to those of precious metal ones
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reported in literatures [6,36]. From previous literatures, the supported Ni-Co catalysts on
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activated carbon could provide 100% CO conversion only at narrow temperature range of 130-150 oC [16]. The CO conversion on MnOx-promoted Co3O4 catalyst can reach 100%
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only at 125-175 oC [15,25,26]. The initial temperature of 100% CO conversion is obviously lower than those obtained over the previously reported Co-based catalysts in references [12-17,21-26]. The newly reported Cu-based catalyst exhibited very high activity [18], however, as a bottle-neck of Cu-based catalysts, the only 40-60 oC of too narrow temperature window for full CO removal limits its industrially applications. Although 100% CO conversion over the developed catalyst in this work only can be reached at 75 oC, the obtained 75-200 oC of unexpectedly wide temperature window allows it to be a practical candidate. A wider temperature range for complete CO oxidation is considered to improve the operation flexibility for practical applications. Moreover, among all the investigated catalysts,
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ACCEPTED MANUSCRIPT the Co-Mn-O/FeOx (Si+CTAB) catalyst exhibits the highest selectivity for CO PROX reaction at the temperature of initial 100% CO conversion (Fig. S1). Thus, the developed
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catalyst could be considered as a promising candidate for eliminating CO from H2-rich gases.
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To clarify the reason behind the unexpectedly high activity of Co-Mn-O/FeOx (Si+CTAB), the nitrogen adsorption/desorption, XRD, HRTEM, and H2-TPR characterizations were performed.
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Fig. 2 top and bottom present nitrogen adsorption/desorption isotherms and the BJH
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pore size distributions, respectively, and the corresponding quantitative analysis data are given in Table S1 in Supporting Information. It can be found that the adsorption amount
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increases very slowly at low relative pressure, demonstrating not existence of too many
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micropores; and there exists a hysteresis loop after P/P0=0.6 but no adsorption plateau near
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P/P0=1.0, illustrating the presence of mesopores and macropores. The BET surface area decreases from 248.6 to 154.8 m2 g-1 after supporting Co-Mn-O on FeOx, ascribed to the
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overlay of pores with Co-Mn-O. It can be confirmed by the pore size distribution in bottom figure. As illustrated in Fig. 1b, the pore volume for the pore size less than 10 nm or larger than 25 nm decreases but the pore amount with pore size of 10-25 nm increase, implying the formation of accumulating mesopores while the Co-Mn-O being supported on the carrier [23]. As illustrated in Fig. S2 and S3, disordered meso or macropores appear on the samples. From Table S1, the specific surface areas of the Co-Mn-O/FeOx (Si), Co-Mn-O/FeOx (CTAB) and the developed Co-Mn-O/FeOx (Si+CTAB) catalysts are 233.7, 68.2 and 154.8 m2 g-1, showing that the larger surface area of Co-Mn-O/FeOx (Si+CTAB) is not the main reason for
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ACCEPTED MANUSCRIPT its higher activity, since the Co-Mn-O/FeOx (Si) has larger surface area but worse activity, but the larger volume of pores is essential for CO PROX reaction (Fig. 2 bottom and Table S1).
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The XRD patterns of the various Co-Mn-O/FeOx catalysts are demonstrated in Fig. 3, and
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the data for crystallite size are listed in Table S2. The XRD peaks of the four samples can be indexed to cubic Co3O4 (pdf # 42-1467), cubic Fe3O4 (pdf # 85-1436) and rhombohedral Fe2O3 (pdf # 86-0550) [30,31]. But no characteristic diffraction peaks corresponding to MnO2
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can be observed, ascribed to the insertion into Co3O4 matrix [23]. The detectable shift of
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diffraction peaks belongings to Co3O4 over the different supported Co-Mn-O catalysts further suggests the insertion of Mn into Co3O4 matrix, resulting in the strong interaction of Co and
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Mn, which improves the catalytic performance of Co-based catalyst for CO PROX [21-26].
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Of course the presence of partial MnO2 in long-range disorder or possible high dispersion or
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existence in amorphous state cannot be definitely excluded [21-23]. The decrease in diffraction peak intensity of Co-Mn-O/FeOx (Si+CTAB) and expansion in peak width can be
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observed, suggesting smaller average crystallite size (calculated from the half-width of (311) plane using Scherrer’s formula, 8.44 nm of average crystallite size for cubic Co 3O4, smaller than those over other samples, Table S2), which is consistent with the results from HRTEM (Fig. S4). Furthermore, the sharp and strong peaks corresponding to Fe2O3 can be clearly seen on Co-Mn-O/FeOx (CTAB) and Co-Mn-O/FeOx (traditional), but the existence of iron oxide mainly in Fe3O4 on the Co-Mn-O/FeOx (Si) and Co-Mn-O/FeOx (Si+CTAB). From Fig. S5, the larger bulk can be observed on Co-Mn-O/FeOx (CTAB) and Co-Mn-O/FeOx (traditional) but loosely arranged sphere on Co-Mn-O/FeOx (Si+CTAB) and Co-Mn-O/FeOx (Si), correlated to reaction results, the smaller loosely arranged Co-Mn-O microsphere on
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ACCEPTED MANUSCRIPT supported Co-Mn-O catalyst is also favorable for CO PROX reaction. Moreover, the accumulated disordered pore can be observed, consistent with the results from nitrogen
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adsorption/desorption analysis. The CO conversion is consistent with the order of crystalline
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size (Table S2). The remarkably superior catalytic activity of the developed Co-Mn-O/FeOx (Si+CTAB) catalyst to the other supported Co-Mn-O catalysts on FeOx for CO PROX reaction can be ascribed to the high dispersity of Co-Mn-O on support significantly affected
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by the unique microstructure of FeOx supports.
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In order to investigate the redox behavior and the interaction between Co-Mn and Fe in the as-prepared four supported Co-Mn-O catalysts, the H2-TPR experiments were performed,
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and the H2-TPR profiles are presented in Fig. 4. From Fig. 4, the different support structure
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leads to significantly different redox of the supported Co-Mn-O catalysts, ascribed to the
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interaction between Co-Mn-O and FeOx. The -peak can be assigned to the reduction of highly dispersed Co3O4/MnO2 [23]. -peak corresponding to the reduction of Fe2O3 to Fe3O4
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can be observed on Co-Mn-O/FeOx (CTAB) and Co-Mn-O/FeOx (traditional) but not on the other two [30], suggesting there are reducible Fe2O3 on Co-Mn-O/FeOx (CTAB) and Co-Mn-O/FeOx (traditional) but not Co-Mn-O/FeOx (Si+CTAB) and Co-Mn-O/FeOx (Si), which is consistent with the XRD results. From above analysis, by using the diverse FeOx as supports, even if the same loading of Co-Mn (Co/Mn=8:1, and the total loading is 50%) is employed, the supported Co-Mn-O catalysts may demonstrate obviously different redox behaviors, resulted from the dispersity of Co-Mn-O and the interaction between Co-Mn-O and FeOx. The appropriate but not high reducibility is favorable for the selective CO oxidation.
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4. Conclusions
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In summary, we firstly prepared a novel and highly-efficient hierarchically nanoporous
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Co-Mn-O/FeOx (Si+CTAB) catalyst by a HSDT/DP approach, which demonstrates
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unexpectedly superior catalytic performance with 100% conversion in the temperature range of 75-200 oC for CO PROX reaction, ascribed to its unique support microstructure and
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interaction between Co-Mn-O and FeOx as well as the smaller crystalline size of active phase on support, besides the previously confirmed Co-Mn interaction in our previous
Acknowledgements
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CO removal from H2-rich stream.
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report [23]. Thus it allows the developed non-precious catalyst to be a practical candidate for
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This work is financially supported by the National Natural Science Foundation of China (20803006), also sponsored by the Chinese Ministry of Education via the Program
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for New Century Excellent Talents in University (NCET-12-0079), and the Key Laboratory of Oil & Gas Fine Chemicals, Ministry of Education & Xinjiang Uyghur Autonomous Region, Xinjiang University (XJDX0908-2011-10) and the Fundamental Research Funds for the Central Universities (DUT12LK51).
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ACCEPTED MANUSCRIPT List of Figure Captions: Fig. 1. Catalytic activity curves of the various supported Co-Mn-O catalysts for CO
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preferential oxidation in H2-rich stream. Operation conditions: GHSV = 15,000 ml h-1 g-1,
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1.0 % CO, 1.0 % O2, 50 % H2 and Ar balance.
Fig. 2. N2 adsorption isotherms (top) and BJH pore diameter distribution curves (bottom) of the supported Co-Mn-O catalysts on different FeOx supports prepared with diverse templates,
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and the FeOx (Si+CTAB) support is included for comparison. The magnified pore diameter
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distribution in a range of 0-25 nm are given as inset in the bottom figures, for clarity. Fig. 3. XRD patterns of the various supported Co-Mn-O catalysts on FeOx.
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Fig. 4. H2-TPR profiles of the various supported Co-Mn-O catalysts on FeOx.
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Figures:
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Fig. 1. Catalytic activity curves of the various supported Co-Mn-O catalysts for CO preferential oxidation in H2-rich stream. Operation conditions: GHSV = 15,000 ml h-1 g-1,
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1.0 % CO, 1.0 % O2, 50 % H2 and Ar balance.
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Fig. 2. N2 adsorption isotherms (top) and BJH pore diameter distribution curves (bottom) of the supported Co-Mn-O catalysts on different FeOx supports prepared with diverse templates, and the FeOx (Si+CTAB) support is included for comparison. The magnified pore diameter distribution in a range of 0-25 nm are given as inset in the bottom figures, for clarity.
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Fig. 3. XRD patterns of the various supported Co-Mn-O catalysts on FeOx.
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Fig. 4. H2-TPR profiles of the various supported Co-Mn-O catalysts on FeOx.
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ACCEPTED MANUSCRIPT Hierarchically nanoporous Co-Mn-O/FeOx as a high performance catalyst for CO preferential oxidation in H2-rich stream† Zhongkui Zhaoa,b,*, Yu Lia, Ting Baoa, Guiru Wanga and Turghun Muhammadb a
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State Key Laboratory of Fine Chemicals, Department of Catalysis Chemistry and Engineering, School of Chemical Engineering, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China; E-mail: [email protected] b Key Laboratory of Oil & Gas Fine Chemicals, Ministry of Education & Xinjiang Uyghur Autonomous Region, Xinjiang University, Urumqi, Xinjiang 830046, China.
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Graphic Abstract
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ACCEPTED MANUSCRIPT Highlights
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Hierarchically porous Co-Mn-O/FeOx composite shows superior catalytic properties. Full CO removal at 75-200 oC can be obtained over the nonprecious metal catalyst. The developed catalyst can be a practical candidate for CO preferential oxidation. Strong interaction of highly-dispersed Co-Mn with Fe results in high activity. Co-Mn-O dispersity and the interaction are notably affected by support structure.
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