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Structural and surface changes of copper modified manganese oxides
Accepted Manuscript Title: Structural and surface changes of copper modified manganese oxides Author: Wojciech Gac Grzegorz Słowik Witold Zawadzki PII: DOI: Reference:
S0169-4332(16)30314-2 http://dx.doi.org/doi:10.1016/j.apsusc.2016.02.136 APSUSC 32654
To appear in:
APSUSC
Received date: Revised date: Accepted date:
17-11-2015 9-2-2016 13-2-2016
Please cite this article as: W. Gac, G. Slowik, W. Zawadzki, Structural and surface changes of copper modified manganese oxides, Applied Surface Science (2016), http://dx.doi.org/10.1016/j.apsusc.2016.02.136 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.
Structural and surface changes of copper modified manganese oxides Wojciech Gac*, Grzegorz Słowik, Witold Zawadzki Department of Chemical Technology, Faculty of Chemistry,
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Maria Curie-Skłodowska University, 3 M. Curie-Skłodowska Sq., 20-031 Lublin, Poland
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Corresponding Author
* Wojciech Gac, Department of Chemical Technology, Faculty of Chemistry,
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Maria Curie-Skłodowska University, 3 M. Curie-Skłodowska Sq., 20-031 Lublin, Poland
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Tel. +48 815377746, Fax. +48 815375565
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E-mail: [email protected].
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Abstract
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Keywords: manganese oxide, copper nanorods, methanol reforming, hydrogen
The structural and surface properties of manganese and copper-manganese oxides
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were investigated. The oxides were prepared by the redox-precipitation method. X-ray diffraction and electron microscopy studies evidenced transformation of cryptomelane-type nanoparticles with 1-D channel structure into the large MnO crystallites with regular rippledlike surface patterns under reduction conditions. The development of Cu/CuO nanorods from strongly dispersed species was evidenced. Coper-modified manganese oxides showed good catalytic performance in methanol steam reforming reaction for hydrogen production. Low selectivity to CO was observed in the wide range of temperatures.
1. Introduction Copper and manganese 1-D nanostructured materials show unique structural and
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surface properties. Manganese oxides of regular one-dimensional channels are composed of singular, double or triple chains of edge shared MnO6 octahedral units joined by the corners [1-3]. The negative charge of the framework of cryptomelane minerals with long 2×2 square
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channels is balanced by potassium cations. Surface properties of tunnel-structured manganese oxides can be modified by changing of synthesis conditions, replacement of cationic species,
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and introduction of different modifiers. 1-D nanostructured manganese materials have been proposed for versatile applications, including water purification [4-6], separation-
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transformation of radionuclides for cancer therapy [7], development of new batteries [8], and
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catalysis in carbon monoxide oxidation [9,10], removal of volatile organic compounds [11], and selective oxidation of chemical compounds [12-14]. In turn, 1-D copper nanostructures
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recently have found wide interests for preparation of microelectronic devices, including transparent conducting films, displays, solar cells, and organic light-emitting diodes, heating
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devices, sensors [15-17]. The nanowires have been prepared by different methods, such as
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hard and soft templating, pulsed laser deposition (PLD), vapour-liquid-solid (VLS) methods and chemical vapour deposition (CVD) [18-19].
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Surface and catalytic studies of modified cryptomelane-type manganese materials
have been usually focused on the properties of the systems containing modifiers in the oxide forms [6,10,20]. Recently we have evidenced that partial reduction of silver-modified crytptomelane manganese oxides strongly increased the activity of catalysts in the lowtemperature CO oxidation reaction [9]. This effect was partially ascribed to the structural changes of manganese oxides and formation of silver nanoparticles. In our opinion, other modified manganese oxide systems may show similar behavior, and this method can be used for the development of novel nanostructures and devices for various applications. Hydrogen is one of the most important products of chemical industry. It is commonly perceived as an energy carrier, especially for the production of electricity by the application of
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the fuel cells. It is widely used for large-scale production of ammonia and methanol, hydrogenation of organic compounds and hydroprocessing of hydrocarbons. Today it is mainly produced on site in the large industrial units, usually by the steam reforming of natural
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gas or partial oxidation of hydrocarbons. An application of liquid intermediate compounds, such as methanol may solve the problems of the storage and distribution of hydrogen for the
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dispersed fuel cell systems [21]. Moreover, the mixture of reaction products of the steam reforming of methanol (H2 and CO2) can be directly used for proton exchange membrane fuel
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cells, especially operated at elevated temperatures, 150-180 oC [22-24]. The best activity in
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the steam reforming of methanol and high selectivity to CO2 (low selectivity to CO) were reported for supported copper catalysts [25-26]. Good performance has been also evidenced
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for noble metal catalysts, containing intermetallic compounds, including PdZn, PdGa, PdIn or PtIn [27-29]. The improvement of the performance of copper catalysts has been often
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achieved by the increase of the active surface area of metallic copper and stabilization of the
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small crystallites over different oxides of the high specific surface area or changing metalsupport interactions, usually using pure and modified ZnO with Al2O3, CrOx, CeO2 or ZrO2
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[25]. High catalytic activity has been also reported for supported copper catalysts with CeO2, Ln2O3, ZrO2 and mixed oxides, such as CeO2-ZrO2. Studies indicated also a high catalytic performance of copper manganese systems, mostly based on CuMn2O4 spinels, prepared by the co-precipitation, soft reactive grinding or combustion methods [30-32]. The main goal of the present work was determination of structural and surface changes
of copper modified manganese oxides during reduction and their catalytic performance in the steam reforming of methanol for hydrogen production.
2. Experimental 2.1. Sample preparation. Manganese oxide of cryptomelane structure was prepared
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by the redox-precipitation technique, similar to that described in ref. [9]. A water solution of potassium permanganate (KMnO4) was introduced drop by drop to the solution containing manganese acetate (Mn(CH3COO)2·4H2O) and acetic acid (CH3COOH) while vigorous
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stirring. The mixture after precipitation was maintained at 60 oC and refluxed for 24 h. The sample after filtering and washing with distilled water was dried at 80 oC for 24 h and then
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calcined at 350 oC for 3 h. Copper-manganese oxide was prepared by the introduction of copper acetate (Cu(CH3COO)2·H2O) to the acidic solution of manganese acetate before
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precipitation with potassium permanganate. Then similar procedures as for the preparation of
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unmodified manganese oxide were applied. The samples were denoted as “MnOx” and “CuMnOx”, respectively.
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The activity of catalysts was compared to the commercial copper catalysts for methanol steam reforming HiFUEL® R120 (Alfa Aesar), denoted here as Cu-R120.
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2.2. Physicochemical properties. The composition of materials was determined by
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applying the X-ray fluorescence method (using ED-XRF Canberra 1510 spectrometer). X-ray diffraction studies (XRD) were performed with Empyrean (PANalytical) diffractometer using
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Cu-Kα radiation (λ = 1.5418 Å). Samples were studied on the two different stages: (i) after calcination and (ii) after reduction (denoted subsequently as “MnOx-red.” and “CuMnOxred.”). Samples were reduced in the quartz flow reactor under the flow of hydrogen at 250 oC for 1 h. After temperature decrease to 25 oC, hydrogen was switched for inert gas, and then for 5% O2/He mixture. The samples for transmission electron microscopic studies (TEM) were grinded in an agate mortar to a fine powder. The resulting powder was poured with 99.8% ethanol to form a slurry. The mixture was inserted into the ultrasonic homogenizer for 20 s. Then, the slurry was pipetted and applied to a copper grid covered with formvar stabilized with carbon (Ted Pella Company). The sample was left on the filter paper until ethanol was evaporated. Subsequently, the sample deposited on the grid was inserted to the holder and
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moved to the electron microscope. The electron microscope Titan G2 60-300 kV FEI Company, equipped with: field emission gun (FEG), monochromator, three condenser lenses system, the objective lens system, image correction (Cs-corrector), HAADF detector and EDS
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spectrometer (Energy Dispersive X -Ray Spectroscopy) EDAX Company with detector Si(Li) and a maximum of 20000 cps (counts per second) was used to display the prepared samples.
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Microscopic studies of the samples were carried out at an accelerating voltage of the electron beam equal to 300 kV. The mapping was carried out in the STEM mode by collecting point by
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point EDS spectrum of each of the corresponding pixels in the map. The collected maps were
corresponding to the percentage of the element.
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presented in the form of a matrix of pixels with the color mapped element and the intensity
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The nitrogen adsorption/desorption isotherms were obtained volumetrically at -196 oC using ASAP 2405N analyzer (Micromeritics Instrument Corp.). Samples were outgassed
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under low pressure (~10-2 Pa) at 130 oC. The adsorption data were used to evaluate specific
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surface area (SBET). The total pore volume (Vp) was determined on the basis of the amount of nitrogen adsorbed at the relative pressure of about 0.98. Keeping in mind some limitation of
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different models, the mean pore diameter (DBJH) was estimated from the adsorption branch of isotherms by applying the Barrett–Joyner–Halenda (BJH) method [33-34]. Reducibility of catalysts was investigated by the temperature-programmed reduction
method (TPR). Studies were carried out in the TPR apparatus Autochem II 2920 (Micromeritics) with U-tube quartz reactor. The samples (0.05 g) were reduced in the mixture of 5% H2/Ar with the total flow rate of 30 cm3/min. The rate of the temperature increase was 10 oC/min. The evolved water was removed in a cold trap maintained in the LN2-isoproyl alcohol at -89 oC. 2.3. Catalytic performance. Studies of the steam reforming of methanol were carried out in a fixed-bed flow reactor system under atmospheric pressure (Microactivity Reference,
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PID ENG&TECH). The system was equipped with HPLC pump, microchannel evaporator and mixer. The reaction products were analyzed on-line by two gas chromatographs equipped with thermal conductivity detectors (TCD). The analysis of CH3OH and H2O was carried out
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by GC-450 (Varian), whereas H2 (with argon as a carrier gas), CO, CH4, CO2 (with helium as carrier gas) by Micro GC CP-4900 (Varian). The total flow rate of methanol-water mixture
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was 50 cm3/min (molar ratio of H2O/MeOH was 3/2). The mixture was diluted with nitrogen (the flow rate was 50 cm3/min). The sample (0.1 g) was mixed with 0.9 g of quartz grains.
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The value of the volume hourly space velocity expressed as the ratio of the volume flow rate of methanol-water mixture (VHSVmix) to the weight of catalyst was 30 L h-1 gcat-1. The samples
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were reduced before catalytic tests at 250 oC for 1 h in the flow of hydrogen. The activity was
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determined at selected temperatures, from 180 oC to 420 oC. The sampling of the reaction products was carried out after 30 min after reaching of each temperature step. The individual
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points of activity (selectivity) were calculated by averaging the results from four sampling.
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The activity of catalysts was expressed in the term of methanol conversion (XMe): CCO + CCO 2 + CCH 4 + ... + ni Ci
CCO + CCO 2 + CCH 4 + ... + ni Ci + C Me
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X Me =
100%
where: Ci – concentration of n-carbons containing component in the reaction products mixture. The selectivity to carbon monoxide and carbon dioxide was calculated accordingly to the equations:
SCO =
(C
CO
SCO2 =
(C
+ CCO2
CCO 100% + CCH 4 + ... + ni Ci
)
CCO2 CO
+ CCO2 + CCH 4 + ... + ni Ci
)100%
The selectivity towards hydrogen (SH2) and hydrogen yield (Y) were calculated as follows:
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SH2 =
CH 2
(
3 ⋅ C CO + C CO 2 + C CH 4 + ... + ni C i
Y=
)100 % ,
3VmGMe X Me S H 2 104 mc
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where: GMe -initial molar flow rate of methanol (mol/h), Vm - molar volume (dm3/mol), and
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mc – sample weight.
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3. Results and discussion
3.1. Properties of materials prior reduction. The copper content in the obtained
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copper-manganese catalyst was 12.21 ± 0.36 wt. %, while commercial catalysts Cu-R120, contained 35.9 wt. % of Cu. The XRD patterns of calcined samples are presented in the
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Figure 1. The positions of the reflection peaks for MnOx sample well correspond to the cryptomelane phase with 1-D channel structure (K2-xMn8O16, JCPDS card file No. 42-1348)
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of the tetragonal crystalline structure with the I4/m space group and unit cell parameters: a =
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b = 9.766 Å, c = 2.842 Å. The broad shape of XRD peaks may indicate the presence of
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additional similar phases, including K2-xMn8O16 of monoclinic crystalline structure, the I2/m space group, and unit cell parameters: a = 9.942 Å, b = 2.866 Å, c = 9.709 Å, β = 90.84o (JCPDS card file No. 44-1386), as well as less-crystalline materials. The presence of copper in the synthesis mixture influenced the way of formation and
then final structural and surface properties of oxide materials. The XRD peaks of manganese phases for the sample containing copper are considerably wider. The peaks at 2θ = 12.7 and 18.0 degrees assigned in tetragonal K2-xMn8O16 to (110) and (200) reflection lines, are no longer visible on the XRD curve of CuMnOx sample and only wide singular peak is detectable. Similarly, the peak located at 2θ = 37.6 (the reflection line (211)) is much broader. Small peaks located at 28.7, 42.0, 49.8 and 60.2 degrees are almost undetectable. Such effects can be related to the presence of very small cryptomelane nanoparticles and may also indicate
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the formation of manganese oxides of different crystallographic structures. It is worth noting that neither separated peaks of copper-manganese nor copper oxide phases, in spite of the relatively large amount of Cu are not observed. This reveals a strong dispersion of copper
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species. The transmission electron microscopy images of calcined MnOx sample (before
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reduction) disclose the needle-like shape of nanoparticles (Figure 2a). The diameter of manganese oxide nanorods ranges from about 5 to 20 nm, whereas the length varies from
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about 20 to 200 nm. The sample contains also less ordered particles. High-resolution image
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(HRTEM), presented in the Figure 2b reveals regular arrangements of the rows of atoms along to the tunnels direction. One can also find some imperfections, mainly located on the side
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walls or on the tops of the rods. The tunnel entrances and some distortion can be observed in the bottom part of particle shown in the Figure 2b. EDS-STEM images of MnOx are
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evidenced.
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presented in the Figure 2c. Uniform distributions of manganese, oxygen and potassium are
It has been widely discussed in the literature that formation and properties of 1-D
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manganese cryptomelane structures are related to the synthesis conditions and thermal treatment. The way of arrangement of manganese octahedral units also depends on the size and oxidation state of template ions. Thus, the replacement of the large potassium cations
(rK + = 1.65 Å) in the channels by the smaller copper cations (rCu 2+ = 0.87 Å) may lead to the decrease of the size of nanoparticles, and the lack of ordering. Such changes have been confirmed in our studies. TEM images show more irregular shape and size of nanoparticles (Figures 3ab). The length of the particles is below 20 nm. The aggregates composed of small crystallites, with high-angle grain boundaries are well visible on the HRTEM image of CuMnOx sample (Figure 3b). EDS-STEM results (Figure 3c) indicate that manganese, oxygen, and copper atoms are well dispersed. It is worth mentioning, that the catalytic or
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adsorptive properties of materials are related to the number of surface active sites, and hence the small size of nanoparticles, surface heterogeneity, the presence of kinks, steps, disruption of crystal structure or impurities play crucial role.
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The shape of nitrogen adsorption/desorption isotherms of the samples confirms to some extent the structural differences (Figure 4). The isotherms of MnOx sample (after
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calcination) indicate the presence of the meso- and macroporous structure (combination of II and IV types of adsorption isotherms in the IUPAC classification) [33]. The shape of
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hysteresis loops is complex (between H2 and H3 types). A strong increase of adsorption
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values at high relative pressures and elongated shape of the hysteresis loops can be ascribed to the presence of large slit-shaped pores [35]. The specific surface area of MnOx sample (SBET)
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is equal to 136.0 m2/g. The decrease of SBET value to 103.0 m2/g for the CuMnOx sample, and decrease of the total pore volume (Vp) from 0.28 cm3/g to 0.19 cm3/g may confirm the
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presence of irregular and interconnected particles with partially collapsed cryptomelane
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channels. Mean pore diameter decreases from 8.9 to 7.4 nm. The shape of hysteresis loops in CuMnOx sample within the region of intermediate relative pressures indicates the formation
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of aggregates with bottle-ink pores.
3.2. Metal-oxygen interactions and formation of supported copper nanorods. The
reduction of cryptomelane-type oxides in hydrogen proceeds through successive decrease of oxidation states: Mn4+ → Mn3+ → Mn2+ and formation of water. This induces distortion and finally disappearance of initial tunnel-like structure. Figure 5 shows temperature-programmed reduction curves for MnOx, CuMnOx and commercial copper catalyst. The irregular shape of TPR curve for MnOx sample at low temperatures (in the range of 120 - 270 oC) points out on the removal of labile oxide species with different Mn-O bond strength, mainly regarded as the surface sites. Two well resolved and partially overlapped reduction peaks located between 270 and 310 oC, and between 310 and 350 oC are associated with the successive reduction of
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manganese oxides [36]. These reduction stages are simultaneously connected with crystallographic changes of initial structure. The reduction of the CuMnOx sample occurs between 70 oC and 300 oC. The TPR peaks correspond to the reduction of copper Cu2+ →
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Cu0 and manganese species Mn4+ → Mn3+ → Mn2+. Commercial catalyst show worse reduciblity than CuMnOx. The large singular maximum is located at around 250oC.
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The course of TPR curve confirms strong dispersion of copper species in CuMnOx catalyst. Metallic copper nuclei, resulted from the reduction of Cu2+ ions, enhance activation
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of hydrogen atoms and facilitate reduction of manganese ions located in the close proximity
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of Cu0 species. Moreover, the smaller size of manganese oxide nanoparticles and less regular arrangement of the atoms may increase reducibility. Similar effects have been often observed
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for promoted catalysts and binary oxide systems [37].
The XRD patterns of reduced samples are presented in the Figure 6. Samples after
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calcination were reduced in the flow of pure hydrogen at 250 oC for 1 h and then passivated at
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room-temperature. The main peaks, located at 35.0; 40.6; 58.7 and 70.2 of 2θ degrees well correspond to MnO phase (JCPDS card file No. 78-0424). A slight peak at 32.5 degree may
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point out on the presence of K2Mn2O3 phase (JCPDS card file No. 78-0181). The positions and width of XRD MnO peaks in the reduced MnOx and CuMnOx samples are almost the same. XRD peaks, located at 2θ = 43.3 and 50.4 degrees correspond to metallic copper phase (JCPDS card file No. 04-0836). One can also find very small and wide peaks, corresponding to CuO phase (JCPDS card file No. 74-1021). A strong decrease of the specific surface area is observed for reduced samples. The
nitrogen adsorption isotherms of the samples reduced at 250 oC in hydrogen are presented in the Figure 4. The specific surface area of MnOx sample decreases from to 136.0 to 0.8 m2/g. The decrease of SBET for CuMnOx is slightly smaller, the specific surface area decreases from 103 m2/g to 7.0 m2/g. The samples after reduction show very low porosity, the values of total
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pore volumes (Vp) are equal to 0.002 and 0.03 cm3/g for MnOx-red. and CuMnOx-red., respectively. Mean pore diameter increases to 23.2 and 23.7 nm, for MnOx-red. and CuMnOx-red. samples, respectively.
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TEM images confirm strong structural changes after reduction of the samples (Figures 7 and 8). The large, cube-like crystallites of MnO are evidenced in the Figure 7a. STEM
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image (Figure 7b) reveal the presence of well-ordered surface domains. The rippled-like surface structures are situated almost along two perpendicular directions to each other. The
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EDS-STEM images (Figure 7c) show that large crystals with the rippled-like structural
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surface heterogeneity are partially covered with amorphous phases, which are slightly embedded with potassium. The distribution of Mn and O is uniform. The intriguing question
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is how needle-like nanoparticles can be transformed to large regular crystals during reduction process, and why the surface "patterns" show such regular arrangement. In our opinion the
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growth of cube-lie species could occur via the oriented attachment mechanism of neighbor
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particles with partially reduced surfaces and distorted structural units. The presence of selforganized periodic microstructures (ripples) on the surface of MnO can be ascribed to the
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surface MnxOy species formed during high-temperature reduction and surface oxidation processes at low temperatures, stabilized due to the surface induced relaxation effects. TEM and STEM images of reduced CuMnOx sample are presented in the Figure 8.
The MnO crystals are slightly smaller and less regular than those in the MnOx sample after reduction. Similar well-ordered periodic microstructures can be observed on the larger MnO crystals. However, the arrangement is lacking near the edges of the crystals. Such imperfection may be responsible for generation of new adsorptive and catalytic sites. Studies disclose the presence of the small irregular nanoparticles and straight nanorods attached to the MnO crystals. The EDS-STEM images (Figure 8c) indicate that nanorods are composed of copper and oxygen. The diameter of nanorods in the obtained samples ranges from about 8 to
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20 nm (Figures 8 and 9), and their length approaches 200 nm. The singular nanorod is presented in Figure 9. The inset of Figure 9 shows the Fast Fourier Transform (FFT) patterns, which provide the evidence of CuO phase. The presence of slight and wide XRD peaks of
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copper oxide and more visible metallic copper phase (Figure 6), indicate that the nanorods can be composed of metallic copper core and CuO surface layers. Some imperfection of the
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arrangement of the rows of atoms, especially along the central part of nanorod are observed.
The formation of copper and coper oxide of 1-D nanostructures has been explained in
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the literature by various mechanisms, including oriented attachment, Ostwald ripening, stress
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and grain-boundary diffusion, stress-induced cracking, controlled growth mechanism [17,18]. The mechanism of the formation of copper nanorods during reduction of modified
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cryptomelane manganese oxides is complex, and further studies are necessary. In our opinion in the first stage of reduction of copper modified manganese oxides, Cu2+ ions are reduced to
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Cu0. The metallic nuclei can migrate within long channels of manganese oxide nanoparticles
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and along suitable surface rows. The simultaneous breaking of Mn-O bonds between the singular chains of octahedral units, probably at the beginning of the reduction stage, allows
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further agglomeration of copper nuclei or transformation of thin strips into the longer nanorods. The individual unzipped manganese chains are transformed in the next step under reductive conditions to the larger and regular MnO crystals. The oriented attachment of neighboring MnOx particles may expel copper nuclei or small particles from the inner parts of manganese oxide grains, which then form 1-D nanostructures. Surface copper atoms during passivation at room-temperature can react with oxygen giving CuO phases. It is interesting that some MnOx particles during reduction may retain initial elongated shape or could be partially distributed over copper nanorods. Strong mutual distribution of MnOx and Cu/CuO phases may enhance the catalytic performance of nanomaterials, via the creation of new adsorptive sites.
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3.3. Steam reforming of methanol. Strong dispersion of nanosized copper species in the heterogeneous catalysts is usually achieved by the application of the suitable supports. Catalytic activity, selectivity, and resistance towards deactivation could be also influenced by
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metal-support interactions, and structural and surface properties of the supports [25,26]. Microscopic studies of CuMnOx sample after reduction evidenced that the sample contains
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copper/copper oxide nanorods, partially covered with irregular species. Such spatial distribution of metallic and oxide species may induce similar effects as in the typically
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supported catalysts. Moreover, the active sites located on the nanorods can be easily
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accessible for molecules and the reaction rate less influenced by the diffusion effects. Figure 10 shows the conversion of methanol in the steam reforming reaction over
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reduced MnOx sample (regarded here as a support) and CuMnOx sample after reduction (described further as a copper manganese catalyst). The reaction proceeds accordingly to the
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reaction equation:
(∆H = 49.7 kJ mol-1).
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CH3OH + H2O ⇆ CO2 + 3H2
The support alone shows minor catalytic activity. Conversion of methanol over
manganese oxide at 425 oC approaches 1-2%. Conversion of methanol over CuMnOx-red. catalyst is visible at around 200 oC and gradually increases with an increase of reaction temperature. 50% conversion of methanol over CuMnOx-red. catalyst is observed at around 350 oC. Commercial copper catalyst Cu-R120 shows very high activity at low temperatures. Hydrogen yield over Cu-R120 catalysts approaches maximum at about 300 oC and then slightly decreases at higher temperatures. The content of copper in the CuMnOx catalyst is relatively small (12 wt. %) in comparison with studied commercial catalysts HiFUEL® R120
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(35.9 wt. %) and similar Cu/ZnO-Al2O3 catalysts described in the literature [25,26]. This may partially account for lower conversion of methanol in the presence of CuMnOx catalyst. It is interesting, that hydrogen yield expressed in relation to copper content (m3h-1gCu) may
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indicate higher activity of CuMnOx catalysts at high temperatures (Figure 10b). The main reaction products of the steam reforming of methanol over copper catalysts
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are carbon dioxide and hydrogen. Changes of selectivity to CO with temperature increase are presented in the Figure 10c. The opposite trend was observed for changes of selectivity to
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carbon dioxide. Other products were not produced. An increase of CO selectivity with an
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increase of reaction temperature has been often reported in the literature for different types of catalysts. The presence of CO in the product stream has been usually attributed to the direct
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methanol decomposition reaction [25]:
(∆H = 90.2 kJ mol-1).
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CH3OH ⇆ CO +2H2
This reaction simultaneously leads to the decrease of hydrogen productivity and
accounts for lower water conversion. Formation of CO has been also ascribed to the consecutive reaction – reverse-water-gas-shift reaction (RWGS) between hydrogen and carbon dioxide [38]:
CO2 + H2 ⇆ CO + H2O
(∆H = 41.2 kJ mol-1).
The support alone (MnOx-red) shows minor activity, and main detectable products are
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carbon monoxide and hydrogen. The selectivity of the support to CO is equal to 70% and slightly decreases at high temperatures. The selectivity to CO of CuMnOx catalysts is very low in the wide range of temperatures. A slight increase of CO selectivity is observed with an
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increase of the temperature and approaches only about 5% at 400 oC. The selectivity of CuR120 catalyst to CO is much higher, and approaches 20 % at 400oC, indicating beneficial
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properties of CuMnOx catalysts at high temperatures. In our opinion, the activity and selectivity of cryptomelane-based copper manganese catalysts can be further improved by the
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increase of copper content, optimization of activation procedures, introduction of additional
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modifiers, which could influence reducibility of oxide phases, formation of new copper
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nanostructures and distribution of oxide species over copper nanorods.
4. Conclusions
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Unmodified and copper modified cryptomelane-type manganese oxides were obtained
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by the redox precipitation method. The mechanism of the formation manganese and copper phases during reduction was discussed. It was found that reduction of manganese oxides of
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1-D tunnel structure led to the development of large cube-like MnO nanoparticles with unique rippled-like surface morphology. The oriented attachment mechanism was proposed for explanation of the growth of MnO nanoparticles. Reduction of copper modified manganese oxide materials resulted in the formation of MnO nanoparticles with attached copper nanorods. We have evidenced that copper containing materials showed good catalytic performance in the steam reforming of methanol. High hydrogen yield and low selectivity to carbon monoxide were observed in the wide range of temperatures. Particularly beneficial properties were evidenced at high-temperatures. In our opinion similar method, based on facile reduction of modified cryptomelane-type oxides can be used for the development of novel nanomaterials for versatile applications.
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ip t
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cr
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manganese oxide octahedral molecular sieves (OMS-2) as efficient photocatalysts in 2propanol oxidation, Appl. Catal., A 375 (2010) 295-302.
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[13] R. Hu, C. Yan L. Xie, Y. Cheng, D. Wang, Selective oxidation of CO in rich hydrogen
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stream over Ag/OMS-2 catalyst, Int. J. Hydrogen Energy 36 (2011) 64-71. [14] E.C. Njagi, H.C. Genuino, C.K. King'ondu, C.H. Chen, D. Horvath, S.L. Suib,
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Preferential oxidation of CO in H2-rich feeds over mesoporous copper manganese oxides
synthesized by a redox method, Int. J. Hydrogen Energy 36 (2011) 6768-6779.
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[16] Z. Yao, Y.W. Lu, S.G. Kandlikar, Direct growth of copper nanowires on a substrate for boiling applications, Micro Nano Lett. 6 (2011) 563-566. [17] Q. Zhang, K. Zhang, D. Xu, G. Yang, H. Huang, F. Nie, C. Liu, S. Yang, CuO nanostructures: synthesis, characterization, growth mechanisms, fundamental properties, and applications, Prog. Mater Sci. 60 (2014) 208-337.
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[18] A.R. Rathmell, S.M. Bergin, Y.L Hua, Z.Y. Li, B.J. Wiley, The growth mechanism of copper nanowires and their properties in flexible, transparent conducting films, Adv. Mater. 22 (2010) 3558-3563.
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[19] M.I. Irshad, F. Ahmad, N.M. Mohamed, M.Z. Abdullah, Preparation and structural characterization of template assisted electrodeposited copper nanowires, Int. J.
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Electrochem. Sci. 9 (2014) 2548-2555.
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over different metal-cation-doped OMS-2 materials, J. Catal. 197 (2001) 292-302.
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[21] D.R. Palo, R.A. Dagle, J.D. Holladay, Methanol steam reforming for hydrogen production, Chem. Rev. 107 (2007) 3992-4021.
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[22] C. Yanga, P. Costamagna, S. Srinivasan, J. Benziger, A.B. Bocarsly, Approaches and technical challenges to high temperature operation of proton exchange membrane fuel
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cells, J. Power Sources 103 (2001) 1-9.
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[23] A. Chandan, M. Hattenberger, A. El-Kharouf, S. Du, A. Dhir, V. Self, B.G. Pollet, A. Ingram, W. Bujalski, High temperature (HT) polymer electrolyte membrane fuel cells
Ac ce p
(PEMFC) - a review, J. Power Sources 231 (2013) 264-278.
[24] G. Avgouropoulos, T. Ioannides, J.K. Kallitsis, S. Neophytides, Development of an internal reforming alcohol fuel cell: concept, challenges and opportunities, Chem. Eng. J. 176-177 (2011) 95-101.
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[26] S.T. Yong, C.W. Ooi, S.P. Chai, X.S. Wu, Review of Methanol Reforming-Cu-based Catalysts, Surface Reaction Mechanisms, and Reaction Schemes, Int. J. Hydrogen Energy 38 (2013) 9541-9552. [27] N. Iwasa, N. Takezawa, New supported Pd and Pt alloy catalysts for steam reforming and
Page 18 of 35
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dehydrogenation of methanol, Top. Catal. 22 (2003) 215-224. [28] N. Iwasa, T. Mayanagi, W. Nomura, M. Arai, N. Takezawa, Effect of Zn addition to supported Pd catalysts in the steam reforming of methanol, Appl. Catal., A 248 (2003)
ip t
153-160. [29] C. Rameshan, W. Stadlmayr, S. Penner, H. Lorenz, L. Mayr, M. Hävecker, R. Blume, T.
cr
Rocha, D. Teschner, A. Knop-Gericke, R. Schlögl, D. Zemlyanov, N. Memmel, B. Klötzer, In situ XPS study of methanol reforming on PdGa near-surface intermetallic
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phases, J. Catal. 290 (2012) 126-137.
an
[30] G. Marbán, T. Valdés-Solís, A.B. Fuertes, High surface area CuMn2O4 prepared by silicaAquagel confined co-precipitation. Characterization and testing in steam reforming of
M
methanol (SRM), Catal. Lett. 118 (2007) 8-14.
[31] Q. Liu, L.C. Wang, M. Chen, Y.M. Liu, Y. Cao, H.Y. He, K.N. Fan, Waste-free soft
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reactive grinding synthesis of high-surface-area copper-manganese spinel oxide catalysts
te
highly effective for methanol steam reforming, Catal. Lett. 121 (2008) 144-150. [32] J. Papavasiliou, G. Avgouropoulos, T. Ioannides, CuMnOx catalysts for internal reforming
Ac ce p
methanol fuel cells: application aspects, Int. J. Hydrogen Energy 37 (2012) 16739-16747.
[33] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity. 2d ed., Academic Press: London and New York, 1982.
[34] M. Jaroniec, M. Kruk, J.P. Olivier, Standard nitrogen adsorption data for characterization of nanoporous silicas, Langmuir 15 (1999) 5410-5413.
[35] J.B. Condon, Surface Area and Porosity Determinations by Physisorption : Measurements and Theory, Elsevier, B.V. : Amsterdam, 2006; pp 1-27. [36] F. Arena, T. Torre, C. Raimondo, A. Parmaliana, Structure and redox properties of bulk and supported manganese oxide catalysts, Phys. Chem. Chem. Phys. 3 (2001) 1911-1917. [37] H. Einaga, A. Kiya, S. Yoshioka, Y. Teraoka, Catalytic properties of copper-manganese
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mixed oxides prepared by coprecipitation using tetramethylammonium hydroxide, Catal. Sci. Technol. 4 (2014) 3713-3722. [38] H. Purnama, T. Ressler, R.E. Jentoft, H. Soerijanto, R. Schlögl, R. Schomäcker, CO steam
catalyst,
reforming
Appl.
of
Catal.,
methanol A
259
with
a
commercial
(2004)
83-94.
Ac ce p
te
d
M
an
us
cr
CuO/ZnO/Al2O3
for
ip t
formation/selectivity
Page 20 of 35
20
ip t us
d
30
40
60
(541)
(521)
50
M
(411)
(301)
(310) (220)
20
70
80
90
2θ
Ac ce p
te
10
(200)
(110)
MnOx
an
(211)
Intensity (a.u.)
cr
CuMnOx
Figure 1. XRD patterns of manganese and copper-manganese cryptomelane-type materials.
Page 21 of 35
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b)
an
us
cr
ip t
a)
M
c) Mn
K
Ac ce p
te
d
O
Figure 2. TEM images of the MnOx sample before reduction (a,b); EDS-STEM images (c).
Page 22 of 35
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b)
an
us
cr
ip t
a)
K
O
Cu
Ac ce p
K
te
d
Mn
M
c)
Figure 3. TEM images of the CuMnOx sample before reduction (a,b); EDS-STEM images (c).
Page 23 of 35
23
ip t
250
cr us
150
an
MnOx
100
M
Nitrogen adsorption (cm3/g)
200
CuMnOx-red
0
0.2
MnOx-red
0.4
0.6
0.8
1
o
p/p
Ac ce p
0
te
d
50
CuMnOx
Figure 4. Nitrogen adsorption/desorption isotherms of the samples before and after reduction,
Page 24 of 35
24
lines
-
adsorption,
dashed
lines
-
desorption
branch.
Ac ce p
te
d
M
an
us
cr
ip t
solid
Page 25 of 35
25
ip t
Rate of hydrogen consumption (a.u.)
Cu-R120
0
te
d
CuMnOx
M
an
us
cr
MnOx
100
200
300
400
500
o
Ac ce p
Temperature ( C)
Figure
5.
Temperature
programmed-reduction
curves
of
the
samples.
Page 26 of 35
26
10
Figure
6. 20
XRD
te
Ac ce p 30
d
MnOx-red.
patterns
CuMnOx-red.
40
50
of
27 60
the 70
samples 80
(400) MnO
(222) MnO
(311) MnO
us
(220) MnO
an
M
(200) MnO
(111) MnO
ip t
(220) Cu
(200) Cu
(111) Cu
(-111) CuO (111) CuO
cr
Intensity (a.u.)
MnO
90
2θ
after
reduction.
Page 27 of 35
b)
an
us
cr
ip t
a)
M
c) Mn
K
Ac ce p
te
d
O
Figure 7. TEM image of the MnOx sample after reduction (a) STEM image (b); EDS-STEM
Page 28 of 35
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(c).
Ac ce p
te
d
M
an
us
cr
ip t
images
Page 29 of 35
29
b)
an
us
cr
ip t
a)
M
c) Mn
Cu
Ac ce p
te
d
O
Figure 8. TEM images of the CuMnOx sample after reduction (a,b) EDS-STEM images (c).
Page 30 of 35
30
ip t cr us an M d te Ac ce p Figure 9. HRTEM image of selected nanowire; the inset - FFT patterns.
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31
100
MnOx-red. CuMnOx-red. Cu-R120
a) Methanol conversion (%)
80
ip t
60
40
cr
20
100
200
300
Temperature ( oC) 0.2
b) 0.16
500
0.12
M
Hydrogen yield (m3 h-1gCu-1)
400
an
CuMnOx-red. Cu-R120
us
0
0.08
te
d
0.04
0
100
200
300
400
500
Temperature ( oC)
c)
Ac ce p
100
CO selectivity (%)
80
MnOx-red. CuMnOx-red. Cu-R120
60
40
20
0 100
200
300
400
500
Temperature ( oC)
Figure 10. a) Methanol conversion in the steam reforming reaction in the presence of support and catalysts. b) Hydrogen yield in relation to copper content. c) Selectivity of catalysts to
Page 32 of 35
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Ac ce p
te
d
M
an
us
cr
ip t
CO.
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Structural and surface changes of copper modified manganese oxides
ip t
Wojciech Gac*, Grzegorz Słowik, Witold Zawadzki Department of Chemical Technology, Faculty of Chemistry,
us
cr
Maria Curie-Skłodowska University, 3 M. Curie-Skłodowska Sq., 20-031 Lublin, Poland
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d
M
an
Graphical Abstract
H2O
H2 CO2
Ac ce p
CH3OH
Page 34 of 35
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Structural and surface changes of copper modified manganese oxides
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Wojciech Gac*, Grzegorz Słowik, Witold Zawadzki Department of Chemical Technology, Faculty of Chemistry,
cr
Maria Curie-Skłodowska University, 3 M. Curie-Skłodowska Sq., 20-031 Lublin, Poland
us
Highlights
Formation of MnO with regular rippled-like surface patterns
•
Synthesis of copper nanorods supported on MnO nanoparticles
•
Hydrogen production in steam methanol reforming over supported copper nanorods
Ac ce p
te
d
M
an
•
Page 35 of 35
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S0169-4332(16)30314-2 http://dx.doi.org/doi:10.1016/j.apsusc.2016.02.136 APSUSC 32654
To appear in:
APSUSC
Received date: Revised date: Accepted date:
17-11-2015 9-2-2016 13-2-2016
Please cite this article as: W. Gac, G. Slowik, W. Zawadzki, Structural and surface changes of copper modified manganese oxides, Applied Surface Science (2016), http://dx.doi.org/10.1016/j.apsusc.2016.02.136 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.
Structural and surface changes of copper modified manganese oxides Wojciech Gac*, Grzegorz Słowik, Witold Zawadzki Department of Chemical Technology, Faculty of Chemistry,
ip t
Maria Curie-Skłodowska University, 3 M. Curie-Skłodowska Sq., 20-031 Lublin, Poland
cr
Corresponding Author
* Wojciech Gac, Department of Chemical Technology, Faculty of Chemistry,
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Maria Curie-Skłodowska University, 3 M. Curie-Skłodowska Sq., 20-031 Lublin, Poland
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Tel. +48 815377746, Fax. +48 815375565
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E-mail: [email protected].
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Abstract
d
Keywords: manganese oxide, copper nanorods, methanol reforming, hydrogen
The structural and surface properties of manganese and copper-manganese oxides
Ac ce p
were investigated. The oxides were prepared by the redox-precipitation method. X-ray diffraction and electron microscopy studies evidenced transformation of cryptomelane-type nanoparticles with 1-D channel structure into the large MnO crystallites with regular rippledlike surface patterns under reduction conditions. The development of Cu/CuO nanorods from strongly dispersed species was evidenced. Coper-modified manganese oxides showed good catalytic performance in methanol steam reforming reaction for hydrogen production. Low selectivity to CO was observed in the wide range of temperatures.
1. Introduction Copper and manganese 1-D nanostructured materials show unique structural and
Page 1 of 35
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surface properties. Manganese oxides of regular one-dimensional channels are composed of singular, double or triple chains of edge shared MnO6 octahedral units joined by the corners [1-3]. The negative charge of the framework of cryptomelane minerals with long 2×2 square
ip t
channels is balanced by potassium cations. Surface properties of tunnel-structured manganese oxides can be modified by changing of synthesis conditions, replacement of cationic species,
cr
and introduction of different modifiers. 1-D nanostructured manganese materials have been proposed for versatile applications, including water purification [4-6], separation-
us
transformation of radionuclides for cancer therapy [7], development of new batteries [8], and
an
catalysis in carbon monoxide oxidation [9,10], removal of volatile organic compounds [11], and selective oxidation of chemical compounds [12-14]. In turn, 1-D copper nanostructures
M
recently have found wide interests for preparation of microelectronic devices, including transparent conducting films, displays, solar cells, and organic light-emitting diodes, heating
d
devices, sensors [15-17]. The nanowires have been prepared by different methods, such as
te
hard and soft templating, pulsed laser deposition (PLD), vapour-liquid-solid (VLS) methods and chemical vapour deposition (CVD) [18-19].
Ac ce p
Surface and catalytic studies of modified cryptomelane-type manganese materials
have been usually focused on the properties of the systems containing modifiers in the oxide forms [6,10,20]. Recently we have evidenced that partial reduction of silver-modified crytptomelane manganese oxides strongly increased the activity of catalysts in the lowtemperature CO oxidation reaction [9]. This effect was partially ascribed to the structural changes of manganese oxides and formation of silver nanoparticles. In our opinion, other modified manganese oxide systems may show similar behavior, and this method can be used for the development of novel nanostructures and devices for various applications. Hydrogen is one of the most important products of chemical industry. It is commonly perceived as an energy carrier, especially for the production of electricity by the application of
Page 2 of 35
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the fuel cells. It is widely used for large-scale production of ammonia and methanol, hydrogenation of organic compounds and hydroprocessing of hydrocarbons. Today it is mainly produced on site in the large industrial units, usually by the steam reforming of natural
ip t
gas or partial oxidation of hydrocarbons. An application of liquid intermediate compounds, such as methanol may solve the problems of the storage and distribution of hydrogen for the
cr
dispersed fuel cell systems [21]. Moreover, the mixture of reaction products of the steam reforming of methanol (H2 and CO2) can be directly used for proton exchange membrane fuel
us
cells, especially operated at elevated temperatures, 150-180 oC [22-24]. The best activity in
an
the steam reforming of methanol and high selectivity to CO2 (low selectivity to CO) were reported for supported copper catalysts [25-26]. Good performance has been also evidenced
M
for noble metal catalysts, containing intermetallic compounds, including PdZn, PdGa, PdIn or PtIn [27-29]. The improvement of the performance of copper catalysts has been often
d
achieved by the increase of the active surface area of metallic copper and stabilization of the
te
small crystallites over different oxides of the high specific surface area or changing metalsupport interactions, usually using pure and modified ZnO with Al2O3, CrOx, CeO2 or ZrO2
Ac ce p
[25]. High catalytic activity has been also reported for supported copper catalysts with CeO2, Ln2O3, ZrO2 and mixed oxides, such as CeO2-ZrO2. Studies indicated also a high catalytic performance of copper manganese systems, mostly based on CuMn2O4 spinels, prepared by the co-precipitation, soft reactive grinding or combustion methods [30-32]. The main goal of the present work was determination of structural and surface changes
of copper modified manganese oxides during reduction and their catalytic performance in the steam reforming of methanol for hydrogen production.
2. Experimental 2.1. Sample preparation. Manganese oxide of cryptomelane structure was prepared
Page 3 of 35
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by the redox-precipitation technique, similar to that described in ref. [9]. A water solution of potassium permanganate (KMnO4) was introduced drop by drop to the solution containing manganese acetate (Mn(CH3COO)2·4H2O) and acetic acid (CH3COOH) while vigorous
ip t
stirring. The mixture after precipitation was maintained at 60 oC and refluxed for 24 h. The sample after filtering and washing with distilled water was dried at 80 oC for 24 h and then
cr
calcined at 350 oC for 3 h. Copper-manganese oxide was prepared by the introduction of copper acetate (Cu(CH3COO)2·H2O) to the acidic solution of manganese acetate before
us
precipitation with potassium permanganate. Then similar procedures as for the preparation of
an
unmodified manganese oxide were applied. The samples were denoted as “MnOx” and “CuMnOx”, respectively.
M
The activity of catalysts was compared to the commercial copper catalysts for methanol steam reforming HiFUEL® R120 (Alfa Aesar), denoted here as Cu-R120.
d
2.2. Physicochemical properties. The composition of materials was determined by
te
applying the X-ray fluorescence method (using ED-XRF Canberra 1510 spectrometer). X-ray diffraction studies (XRD) were performed with Empyrean (PANalytical) diffractometer using
Ac ce p
Cu-Kα radiation (λ = 1.5418 Å). Samples were studied on the two different stages: (i) after calcination and (ii) after reduction (denoted subsequently as “MnOx-red.” and “CuMnOxred.”). Samples were reduced in the quartz flow reactor under the flow of hydrogen at 250 oC for 1 h. After temperature decrease to 25 oC, hydrogen was switched for inert gas, and then for 5% O2/He mixture. The samples for transmission electron microscopic studies (TEM) were grinded in an agate mortar to a fine powder. The resulting powder was poured with 99.8% ethanol to form a slurry. The mixture was inserted into the ultrasonic homogenizer for 20 s. Then, the slurry was pipetted and applied to a copper grid covered with formvar stabilized with carbon (Ted Pella Company). The sample was left on the filter paper until ethanol was evaporated. Subsequently, the sample deposited on the grid was inserted to the holder and
Page 4 of 35
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moved to the electron microscope. The electron microscope Titan G2 60-300 kV FEI Company, equipped with: field emission gun (FEG), monochromator, three condenser lenses system, the objective lens system, image correction (Cs-corrector), HAADF detector and EDS
ip t
spectrometer (Energy Dispersive X -Ray Spectroscopy) EDAX Company with detector Si(Li) and a maximum of 20000 cps (counts per second) was used to display the prepared samples.
cr
Microscopic studies of the samples were carried out at an accelerating voltage of the electron beam equal to 300 kV. The mapping was carried out in the STEM mode by collecting point by
us
point EDS spectrum of each of the corresponding pixels in the map. The collected maps were
corresponding to the percentage of the element.
an
presented in the form of a matrix of pixels with the color mapped element and the intensity
M
The nitrogen adsorption/desorption isotherms were obtained volumetrically at -196 oC using ASAP 2405N analyzer (Micromeritics Instrument Corp.). Samples were outgassed
d
under low pressure (~10-2 Pa) at 130 oC. The adsorption data were used to evaluate specific
te
surface area (SBET). The total pore volume (Vp) was determined on the basis of the amount of nitrogen adsorbed at the relative pressure of about 0.98. Keeping in mind some limitation of
Ac ce p
different models, the mean pore diameter (DBJH) was estimated from the adsorption branch of isotherms by applying the Barrett–Joyner–Halenda (BJH) method [33-34]. Reducibility of catalysts was investigated by the temperature-programmed reduction
method (TPR). Studies were carried out in the TPR apparatus Autochem II 2920 (Micromeritics) with U-tube quartz reactor. The samples (0.05 g) were reduced in the mixture of 5% H2/Ar with the total flow rate of 30 cm3/min. The rate of the temperature increase was 10 oC/min. The evolved water was removed in a cold trap maintained in the LN2-isoproyl alcohol at -89 oC. 2.3. Catalytic performance. Studies of the steam reforming of methanol were carried out in a fixed-bed flow reactor system under atmospheric pressure (Microactivity Reference,
Page 5 of 35
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PID ENG&TECH). The system was equipped with HPLC pump, microchannel evaporator and mixer. The reaction products were analyzed on-line by two gas chromatographs equipped with thermal conductivity detectors (TCD). The analysis of CH3OH and H2O was carried out
ip t
by GC-450 (Varian), whereas H2 (with argon as a carrier gas), CO, CH4, CO2 (with helium as carrier gas) by Micro GC CP-4900 (Varian). The total flow rate of methanol-water mixture
cr
was 50 cm3/min (molar ratio of H2O/MeOH was 3/2). The mixture was diluted with nitrogen (the flow rate was 50 cm3/min). The sample (0.1 g) was mixed with 0.9 g of quartz grains.
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The value of the volume hourly space velocity expressed as the ratio of the volume flow rate of methanol-water mixture (VHSVmix) to the weight of catalyst was 30 L h-1 gcat-1. The samples
an
were reduced before catalytic tests at 250 oC for 1 h in the flow of hydrogen. The activity was
M
determined at selected temperatures, from 180 oC to 420 oC. The sampling of the reaction products was carried out after 30 min after reaching of each temperature step. The individual
d
points of activity (selectivity) were calculated by averaging the results from four sampling.
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The activity of catalysts was expressed in the term of methanol conversion (XMe): CCO + CCO 2 + CCH 4 + ... + ni Ci
CCO + CCO 2 + CCH 4 + ... + ni Ci + C Me
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X Me =
100%
where: Ci – concentration of n-carbons containing component in the reaction products mixture. The selectivity to carbon monoxide and carbon dioxide was calculated accordingly to the equations:
SCO =
(C
CO
SCO2 =
(C
+ CCO2
CCO 100% + CCH 4 + ... + ni Ci
)
CCO2 CO
+ CCO2 + CCH 4 + ... + ni Ci
)100%
The selectivity towards hydrogen (SH2) and hydrogen yield (Y) were calculated as follows:
Page 6 of 35
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SH2 =
CH 2
(
3 ⋅ C CO + C CO 2 + C CH 4 + ... + ni C i
Y=
)100 % ,
3VmGMe X Me S H 2 104 mc
ip t
where: GMe -initial molar flow rate of methanol (mol/h), Vm - molar volume (dm3/mol), and
cr
mc – sample weight.
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3. Results and discussion
3.1. Properties of materials prior reduction. The copper content in the obtained
an
copper-manganese catalyst was 12.21 ± 0.36 wt. %, while commercial catalysts Cu-R120, contained 35.9 wt. % of Cu. The XRD patterns of calcined samples are presented in the
M
Figure 1. The positions of the reflection peaks for MnOx sample well correspond to the cryptomelane phase with 1-D channel structure (K2-xMn8O16, JCPDS card file No. 42-1348)
d
of the tetragonal crystalline structure with the I4/m space group and unit cell parameters: a =
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b = 9.766 Å, c = 2.842 Å. The broad shape of XRD peaks may indicate the presence of
Ac ce p
additional similar phases, including K2-xMn8O16 of monoclinic crystalline structure, the I2/m space group, and unit cell parameters: a = 9.942 Å, b = 2.866 Å, c = 9.709 Å, β = 90.84o (JCPDS card file No. 44-1386), as well as less-crystalline materials. The presence of copper in the synthesis mixture influenced the way of formation and
then final structural and surface properties of oxide materials. The XRD peaks of manganese phases for the sample containing copper are considerably wider. The peaks at 2θ = 12.7 and 18.0 degrees assigned in tetragonal K2-xMn8O16 to (110) and (200) reflection lines, are no longer visible on the XRD curve of CuMnOx sample and only wide singular peak is detectable. Similarly, the peak located at 2θ = 37.6 (the reflection line (211)) is much broader. Small peaks located at 28.7, 42.0, 49.8 and 60.2 degrees are almost undetectable. Such effects can be related to the presence of very small cryptomelane nanoparticles and may also indicate
Page 7 of 35
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the formation of manganese oxides of different crystallographic structures. It is worth noting that neither separated peaks of copper-manganese nor copper oxide phases, in spite of the relatively large amount of Cu are not observed. This reveals a strong dispersion of copper
ip t
species. The transmission electron microscopy images of calcined MnOx sample (before
cr
reduction) disclose the needle-like shape of nanoparticles (Figure 2a). The diameter of manganese oxide nanorods ranges from about 5 to 20 nm, whereas the length varies from
us
about 20 to 200 nm. The sample contains also less ordered particles. High-resolution image
an
(HRTEM), presented in the Figure 2b reveals regular arrangements of the rows of atoms along to the tunnels direction. One can also find some imperfections, mainly located on the side
M
walls or on the tops of the rods. The tunnel entrances and some distortion can be observed in the bottom part of particle shown in the Figure 2b. EDS-STEM images of MnOx are
te
evidenced.
d
presented in the Figure 2c. Uniform distributions of manganese, oxygen and potassium are
It has been widely discussed in the literature that formation and properties of 1-D
Ac ce p
manganese cryptomelane structures are related to the synthesis conditions and thermal treatment. The way of arrangement of manganese octahedral units also depends on the size and oxidation state of template ions. Thus, the replacement of the large potassium cations
(rK + = 1.65 Å) in the channels by the smaller copper cations (rCu 2+ = 0.87 Å) may lead to the decrease of the size of nanoparticles, and the lack of ordering. Such changes have been confirmed in our studies. TEM images show more irregular shape and size of nanoparticles (Figures 3ab). The length of the particles is below 20 nm. The aggregates composed of small crystallites, with high-angle grain boundaries are well visible on the HRTEM image of CuMnOx sample (Figure 3b). EDS-STEM results (Figure 3c) indicate that manganese, oxygen, and copper atoms are well dispersed. It is worth mentioning, that the catalytic or
Page 8 of 35
8
adsorptive properties of materials are related to the number of surface active sites, and hence the small size of nanoparticles, surface heterogeneity, the presence of kinks, steps, disruption of crystal structure or impurities play crucial role.
ip t
The shape of nitrogen adsorption/desorption isotherms of the samples confirms to some extent the structural differences (Figure 4). The isotherms of MnOx sample (after
cr
calcination) indicate the presence of the meso- and macroporous structure (combination of II and IV types of adsorption isotherms in the IUPAC classification) [33]. The shape of
us
hysteresis loops is complex (between H2 and H3 types). A strong increase of adsorption
an
values at high relative pressures and elongated shape of the hysteresis loops can be ascribed to the presence of large slit-shaped pores [35]. The specific surface area of MnOx sample (SBET)
M
is equal to 136.0 m2/g. The decrease of SBET value to 103.0 m2/g for the CuMnOx sample, and decrease of the total pore volume (Vp) from 0.28 cm3/g to 0.19 cm3/g may confirm the
d
presence of irregular and interconnected particles with partially collapsed cryptomelane
te
channels. Mean pore diameter decreases from 8.9 to 7.4 nm. The shape of hysteresis loops in CuMnOx sample within the region of intermediate relative pressures indicates the formation
Ac ce p
of aggregates with bottle-ink pores.
3.2. Metal-oxygen interactions and formation of supported copper nanorods. The
reduction of cryptomelane-type oxides in hydrogen proceeds through successive decrease of oxidation states: Mn4+ → Mn3+ → Mn2+ and formation of water. This induces distortion and finally disappearance of initial tunnel-like structure. Figure 5 shows temperature-programmed reduction curves for MnOx, CuMnOx and commercial copper catalyst. The irregular shape of TPR curve for MnOx sample at low temperatures (in the range of 120 - 270 oC) points out on the removal of labile oxide species with different Mn-O bond strength, mainly regarded as the surface sites. Two well resolved and partially overlapped reduction peaks located between 270 and 310 oC, and between 310 and 350 oC are associated with the successive reduction of
Page 9 of 35
9
manganese oxides [36]. These reduction stages are simultaneously connected with crystallographic changes of initial structure. The reduction of the CuMnOx sample occurs between 70 oC and 300 oC. The TPR peaks correspond to the reduction of copper Cu2+ →
ip t
Cu0 and manganese species Mn4+ → Mn3+ → Mn2+. Commercial catalyst show worse reduciblity than CuMnOx. The large singular maximum is located at around 250oC.
cr
The course of TPR curve confirms strong dispersion of copper species in CuMnOx catalyst. Metallic copper nuclei, resulted from the reduction of Cu2+ ions, enhance activation
us
of hydrogen atoms and facilitate reduction of manganese ions located in the close proximity
an
of Cu0 species. Moreover, the smaller size of manganese oxide nanoparticles and less regular arrangement of the atoms may increase reducibility. Similar effects have been often observed
M
for promoted catalysts and binary oxide systems [37].
The XRD patterns of reduced samples are presented in the Figure 6. Samples after
d
calcination were reduced in the flow of pure hydrogen at 250 oC for 1 h and then passivated at
te
room-temperature. The main peaks, located at 35.0; 40.6; 58.7 and 70.2 of 2θ degrees well correspond to MnO phase (JCPDS card file No. 78-0424). A slight peak at 32.5 degree may
Ac ce p
point out on the presence of K2Mn2O3 phase (JCPDS card file No. 78-0181). The positions and width of XRD MnO peaks in the reduced MnOx and CuMnOx samples are almost the same. XRD peaks, located at 2θ = 43.3 and 50.4 degrees correspond to metallic copper phase (JCPDS card file No. 04-0836). One can also find very small and wide peaks, corresponding to CuO phase (JCPDS card file No. 74-1021). A strong decrease of the specific surface area is observed for reduced samples. The
nitrogen adsorption isotherms of the samples reduced at 250 oC in hydrogen are presented in the Figure 4. The specific surface area of MnOx sample decreases from to 136.0 to 0.8 m2/g. The decrease of SBET for CuMnOx is slightly smaller, the specific surface area decreases from 103 m2/g to 7.0 m2/g. The samples after reduction show very low porosity, the values of total
Page 10 of 35
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pore volumes (Vp) are equal to 0.002 and 0.03 cm3/g for MnOx-red. and CuMnOx-red., respectively. Mean pore diameter increases to 23.2 and 23.7 nm, for MnOx-red. and CuMnOx-red. samples, respectively.
ip t
TEM images confirm strong structural changes after reduction of the samples (Figures 7 and 8). The large, cube-like crystallites of MnO are evidenced in the Figure 7a. STEM
cr
image (Figure 7b) reveal the presence of well-ordered surface domains. The rippled-like surface structures are situated almost along two perpendicular directions to each other. The
us
EDS-STEM images (Figure 7c) show that large crystals with the rippled-like structural
an
surface heterogeneity are partially covered with amorphous phases, which are slightly embedded with potassium. The distribution of Mn and O is uniform. The intriguing question
M
is how needle-like nanoparticles can be transformed to large regular crystals during reduction process, and why the surface "patterns" show such regular arrangement. In our opinion the
d
growth of cube-lie species could occur via the oriented attachment mechanism of neighbor
te
particles with partially reduced surfaces and distorted structural units. The presence of selforganized periodic microstructures (ripples) on the surface of MnO can be ascribed to the
Ac ce p
surface MnxOy species formed during high-temperature reduction and surface oxidation processes at low temperatures, stabilized due to the surface induced relaxation effects. TEM and STEM images of reduced CuMnOx sample are presented in the Figure 8.
The MnO crystals are slightly smaller and less regular than those in the MnOx sample after reduction. Similar well-ordered periodic microstructures can be observed on the larger MnO crystals. However, the arrangement is lacking near the edges of the crystals. Such imperfection may be responsible for generation of new adsorptive and catalytic sites. Studies disclose the presence of the small irregular nanoparticles and straight nanorods attached to the MnO crystals. The EDS-STEM images (Figure 8c) indicate that nanorods are composed of copper and oxygen. The diameter of nanorods in the obtained samples ranges from about 8 to
Page 11 of 35
11
20 nm (Figures 8 and 9), and their length approaches 200 nm. The singular nanorod is presented in Figure 9. The inset of Figure 9 shows the Fast Fourier Transform (FFT) patterns, which provide the evidence of CuO phase. The presence of slight and wide XRD peaks of
ip t
copper oxide and more visible metallic copper phase (Figure 6), indicate that the nanorods can be composed of metallic copper core and CuO surface layers. Some imperfection of the
cr
arrangement of the rows of atoms, especially along the central part of nanorod are observed.
The formation of copper and coper oxide of 1-D nanostructures has been explained in
us
the literature by various mechanisms, including oriented attachment, Ostwald ripening, stress
an
and grain-boundary diffusion, stress-induced cracking, controlled growth mechanism [17,18]. The mechanism of the formation of copper nanorods during reduction of modified
M
cryptomelane manganese oxides is complex, and further studies are necessary. In our opinion in the first stage of reduction of copper modified manganese oxides, Cu2+ ions are reduced to
d
Cu0. The metallic nuclei can migrate within long channels of manganese oxide nanoparticles
te
and along suitable surface rows. The simultaneous breaking of Mn-O bonds between the singular chains of octahedral units, probably at the beginning of the reduction stage, allows
Ac ce p
further agglomeration of copper nuclei or transformation of thin strips into the longer nanorods. The individual unzipped manganese chains are transformed in the next step under reductive conditions to the larger and regular MnO crystals. The oriented attachment of neighboring MnOx particles may expel copper nuclei or small particles from the inner parts of manganese oxide grains, which then form 1-D nanostructures. Surface copper atoms during passivation at room-temperature can react with oxygen giving CuO phases. It is interesting that some MnOx particles during reduction may retain initial elongated shape or could be partially distributed over copper nanorods. Strong mutual distribution of MnOx and Cu/CuO phases may enhance the catalytic performance of nanomaterials, via the creation of new adsorptive sites.
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3.3. Steam reforming of methanol. Strong dispersion of nanosized copper species in the heterogeneous catalysts is usually achieved by the application of the suitable supports. Catalytic activity, selectivity, and resistance towards deactivation could be also influenced by
ip t
metal-support interactions, and structural and surface properties of the supports [25,26]. Microscopic studies of CuMnOx sample after reduction evidenced that the sample contains
cr
copper/copper oxide nanorods, partially covered with irregular species. Such spatial distribution of metallic and oxide species may induce similar effects as in the typically
us
supported catalysts. Moreover, the active sites located on the nanorods can be easily
an
accessible for molecules and the reaction rate less influenced by the diffusion effects. Figure 10 shows the conversion of methanol in the steam reforming reaction over
M
reduced MnOx sample (regarded here as a support) and CuMnOx sample after reduction (described further as a copper manganese catalyst). The reaction proceeds accordingly to the
te
d
reaction equation:
(∆H = 49.7 kJ mol-1).
Ac ce p
CH3OH + H2O ⇆ CO2 + 3H2
The support alone shows minor catalytic activity. Conversion of methanol over
manganese oxide at 425 oC approaches 1-2%. Conversion of methanol over CuMnOx-red. catalyst is visible at around 200 oC and gradually increases with an increase of reaction temperature. 50% conversion of methanol over CuMnOx-red. catalyst is observed at around 350 oC. Commercial copper catalyst Cu-R120 shows very high activity at low temperatures. Hydrogen yield over Cu-R120 catalysts approaches maximum at about 300 oC and then slightly decreases at higher temperatures. The content of copper in the CuMnOx catalyst is relatively small (12 wt. %) in comparison with studied commercial catalysts HiFUEL® R120
Page 13 of 35
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(35.9 wt. %) and similar Cu/ZnO-Al2O3 catalysts described in the literature [25,26]. This may partially account for lower conversion of methanol in the presence of CuMnOx catalyst. It is interesting, that hydrogen yield expressed in relation to copper content (m3h-1gCu) may
ip t
indicate higher activity of CuMnOx catalysts at high temperatures (Figure 10b). The main reaction products of the steam reforming of methanol over copper catalysts
cr
are carbon dioxide and hydrogen. Changes of selectivity to CO with temperature increase are presented in the Figure 10c. The opposite trend was observed for changes of selectivity to
us
carbon dioxide. Other products were not produced. An increase of CO selectivity with an
an
increase of reaction temperature has been often reported in the literature for different types of catalysts. The presence of CO in the product stream has been usually attributed to the direct
M
methanol decomposition reaction [25]:
(∆H = 90.2 kJ mol-1).
Ac ce p
te
d
CH3OH ⇆ CO +2H2
This reaction simultaneously leads to the decrease of hydrogen productivity and
accounts for lower water conversion. Formation of CO has been also ascribed to the consecutive reaction – reverse-water-gas-shift reaction (RWGS) between hydrogen and carbon dioxide [38]:
CO2 + H2 ⇆ CO + H2O
(∆H = 41.2 kJ mol-1).
The support alone (MnOx-red) shows minor activity, and main detectable products are
Page 14 of 35
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carbon monoxide and hydrogen. The selectivity of the support to CO is equal to 70% and slightly decreases at high temperatures. The selectivity to CO of CuMnOx catalysts is very low in the wide range of temperatures. A slight increase of CO selectivity is observed with an
ip t
increase of the temperature and approaches only about 5% at 400 oC. The selectivity of CuR120 catalyst to CO is much higher, and approaches 20 % at 400oC, indicating beneficial
cr
properties of CuMnOx catalysts at high temperatures. In our opinion, the activity and selectivity of cryptomelane-based copper manganese catalysts can be further improved by the
us
increase of copper content, optimization of activation procedures, introduction of additional
an
modifiers, which could influence reducibility of oxide phases, formation of new copper
M
nanostructures and distribution of oxide species over copper nanorods.
4. Conclusions
d
Unmodified and copper modified cryptomelane-type manganese oxides were obtained
te
by the redox precipitation method. The mechanism of the formation manganese and copper phases during reduction was discussed. It was found that reduction of manganese oxides of
Ac ce p
1-D tunnel structure led to the development of large cube-like MnO nanoparticles with unique rippled-like surface morphology. The oriented attachment mechanism was proposed for explanation of the growth of MnO nanoparticles. Reduction of copper modified manganese oxide materials resulted in the formation of MnO nanoparticles with attached copper nanorods. We have evidenced that copper containing materials showed good catalytic performance in the steam reforming of methanol. High hydrogen yield and low selectivity to carbon monoxide were observed in the wide range of temperatures. Particularly beneficial properties were evidenced at high-temperatures. In our opinion similar method, based on facile reduction of modified cryptomelane-type oxides can be used for the development of novel nanomaterials for versatile applications.
Page 15 of 35
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ip t
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[10] W.Y Hernández, M.A. Centeno, S. Ivanova, P. Eloy, E.M. Gaigneaux, J.A. Odriozola, Cu-modified cryptomelane oxide as active catalyst for CO oxidation reactions, Appl.
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manganese oxide octahedral molecular sieves (OMS-2) as efficient photocatalysts in 2propanol oxidation, Appl. Catal., A 375 (2010) 295-302.
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[13] R. Hu, C. Yan L. Xie, Y. Cheng, D. Wang, Selective oxidation of CO in rich hydrogen
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Preferential oxidation of CO in H2-rich feeds over mesoporous copper manganese oxides
synthesized by a redox method, Int. J. Hydrogen Energy 36 (2011) 6768-6779.
[15] C. Mayousse, C. Celle, A. Carella, J.P. Simonato, Synthesis and purification of long copper nanowires. Application to high performance flexible transparent electrodes with and without PEDOT:PSS, Nano Res. 7 (2014) 315-324.
[16] Z. Yao, Y.W. Lu, S.G. Kandlikar, Direct growth of copper nanowires on a substrate for boiling applications, Micro Nano Lett. 6 (2011) 563-566. [17] Q. Zhang, K. Zhang, D. Xu, G. Yang, H. Huang, F. Nie, C. Liu, S. Yang, CuO nanostructures: synthesis, characterization, growth mechanisms, fundamental properties, and applications, Prog. Mater Sci. 60 (2014) 208-337.
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[18] A.R. Rathmell, S.M. Bergin, Y.L Hua, Z.Y. Li, B.J. Wiley, The growth mechanism of copper nanowires and their properties in flexible, transparent conducting films, Adv. Mater. 22 (2010) 3558-3563.
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[19] M.I. Irshad, F. Ahmad, N.M. Mohamed, M.Z. Abdullah, Preparation and structural characterization of template assisted electrodeposited copper nanowires, Int. J.
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[21] D.R. Palo, R.A. Dagle, J.D. Holladay, Methanol steam reforming for hydrogen production, Chem. Rev. 107 (2007) 3992-4021.
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(PEMFC) - a review, J. Power Sources 231 (2013) 264-278.
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[26] S.T. Yong, C.W. Ooi, S.P. Chai, X.S. Wu, Review of Methanol Reforming-Cu-based Catalysts, Surface Reaction Mechanisms, and Reaction Schemes, Int. J. Hydrogen Energy 38 (2013) 9541-9552. [27] N. Iwasa, N. Takezawa, New supported Pd and Pt alloy catalysts for steam reforming and
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dehydrogenation of methanol, Top. Catal. 22 (2003) 215-224. [28] N. Iwasa, T. Mayanagi, W. Nomura, M. Arai, N. Takezawa, Effect of Zn addition to supported Pd catalysts in the steam reforming of methanol, Appl. Catal., A 248 (2003)
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153-160. [29] C. Rameshan, W. Stadlmayr, S. Penner, H. Lorenz, L. Mayr, M. Hävecker, R. Blume, T.
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Rocha, D. Teschner, A. Knop-Gericke, R. Schlögl, D. Zemlyanov, N. Memmel, B. Klötzer, In situ XPS study of methanol reforming on PdGa near-surface intermetallic
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phases, J. Catal. 290 (2012) 126-137.
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methanol (SRM), Catal. Lett. 118 (2007) 8-14.
[31] Q. Liu, L.C. Wang, M. Chen, Y.M. Liu, Y. Cao, H.Y. He, K.N. Fan, Waste-free soft
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highly effective for methanol steam reforming, Catal. Lett. 121 (2008) 144-150. [32] J. Papavasiliou, G. Avgouropoulos, T. Ioannides, CuMnOx catalysts for internal reforming
Ac ce p
methanol fuel cells: application aspects, Int. J. Hydrogen Energy 37 (2012) 16739-16747.
[33] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity. 2d ed., Academic Press: London and New York, 1982.
[34] M. Jaroniec, M. Kruk, J.P. Olivier, Standard nitrogen adsorption data for characterization of nanoporous silicas, Langmuir 15 (1999) 5410-5413.
[35] J.B. Condon, Surface Area and Porosity Determinations by Physisorption : Measurements and Theory, Elsevier, B.V. : Amsterdam, 2006; pp 1-27. [36] F. Arena, T. Torre, C. Raimondo, A. Parmaliana, Structure and redox properties of bulk and supported manganese oxide catalysts, Phys. Chem. Chem. Phys. 3 (2001) 1911-1917. [37] H. Einaga, A. Kiya, S. Yoshioka, Y. Teraoka, Catalytic properties of copper-manganese
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mixed oxides prepared by coprecipitation using tetramethylammonium hydroxide, Catal. Sci. Technol. 4 (2014) 3713-3722. [38] H. Purnama, T. Ressler, R.E. Jentoft, H. Soerijanto, R. Schlögl, R. Schomäcker, CO steam
catalyst,
reforming
Appl.
of
Catal.,
methanol A
259
with
a
commercial
(2004)
83-94.
Ac ce p
te
d
M
an
us
cr
CuO/ZnO/Al2O3
for
ip t
formation/selectivity
Page 20 of 35
20
ip t us
d
30
40
60
(541)
(521)
50
M
(411)
(301)
(310) (220)
20
70
80
90
2θ
Ac ce p
te
10
(200)
(110)
MnOx
an
(211)
Intensity (a.u.)
cr
CuMnOx
Figure 1. XRD patterns of manganese and copper-manganese cryptomelane-type materials.
Page 21 of 35
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b)
an
us
cr
ip t
a)
M
c) Mn
K
Ac ce p
te
d
O
Figure 2. TEM images of the MnOx sample before reduction (a,b); EDS-STEM images (c).
Page 22 of 35
22
b)
an
us
cr
ip t
a)
K
O
Cu
Ac ce p
K
te
d
Mn
M
c)
Figure 3. TEM images of the CuMnOx sample before reduction (a,b); EDS-STEM images (c).
Page 23 of 35
23
ip t
250
cr us
150
an
MnOx
100
M
Nitrogen adsorption (cm3/g)
200
CuMnOx-red
0
0.2
MnOx-red
0.4
0.6
0.8
1
o
p/p
Ac ce p
0
te
d
50
CuMnOx
Figure 4. Nitrogen adsorption/desorption isotherms of the samples before and after reduction,
Page 24 of 35
24
lines
-
adsorption,
dashed
lines
-
desorption
branch.
Ac ce p
te
d
M
an
us
cr
ip t
solid
Page 25 of 35
25
ip t
Rate of hydrogen consumption (a.u.)
Cu-R120
0
te
d
CuMnOx
M
an
us
cr
MnOx
100
200
300
400
500
o
Ac ce p
Temperature ( C)
Figure
5.
Temperature
programmed-reduction
curves
of
the
samples.
Page 26 of 35
26
10
Figure
6. 20
XRD
te
Ac ce p 30
d
MnOx-red.
patterns
CuMnOx-red.
40
50
of
27 60
the 70
samples 80
(400) MnO
(222) MnO
(311) MnO
us
(220) MnO
an
M
(200) MnO
(111) MnO
ip t
(220) Cu
(200) Cu
(111) Cu
(-111) CuO (111) CuO
cr
Intensity (a.u.)
MnO
90
2θ
after
reduction.
Page 27 of 35
b)
an
us
cr
ip t
a)
M
c) Mn
K
Ac ce p
te
d
O
Figure 7. TEM image of the MnOx sample after reduction (a) STEM image (b); EDS-STEM
Page 28 of 35
28
(c).
Ac ce p
te
d
M
an
us
cr
ip t
images
Page 29 of 35
29
b)
an
us
cr
ip t
a)
M
c) Mn
Cu
Ac ce p
te
d
O
Figure 8. TEM images of the CuMnOx sample after reduction (a,b) EDS-STEM images (c).
Page 30 of 35
30
ip t cr us an M d te Ac ce p Figure 9. HRTEM image of selected nanowire; the inset - FFT patterns.
Page 31 of 35
31
100
MnOx-red. CuMnOx-red. Cu-R120
a) Methanol conversion (%)
80
ip t
60
40
cr
20
100
200
300
Temperature ( oC) 0.2
b) 0.16
500
0.12
M
Hydrogen yield (m3 h-1gCu-1)
400
an
CuMnOx-red. Cu-R120
us
0
0.08
te
d
0.04
0
100
200
300
400
500
Temperature ( oC)
c)
Ac ce p
100
CO selectivity (%)
80
MnOx-red. CuMnOx-red. Cu-R120
60
40
20
0 100
200
300
400
500
Temperature ( oC)
Figure 10. a) Methanol conversion in the steam reforming reaction in the presence of support and catalysts. b) Hydrogen yield in relation to copper content. c) Selectivity of catalysts to
Page 32 of 35
32
Ac ce p
te
d
M
an
us
cr
ip t
CO.
Page 33 of 35
33
Structural and surface changes of copper modified manganese oxides
ip t
Wojciech Gac*, Grzegorz Słowik, Witold Zawadzki Department of Chemical Technology, Faculty of Chemistry,
us
cr
Maria Curie-Skłodowska University, 3 M. Curie-Skłodowska Sq., 20-031 Lublin, Poland
te
d
M
an
Graphical Abstract
H2O
H2 CO2
Ac ce p
CH3OH
Page 34 of 35
34
Structural and surface changes of copper modified manganese oxides
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Wojciech Gac*, Grzegorz Słowik, Witold Zawadzki Department of Chemical Technology, Faculty of Chemistry,
cr
Maria Curie-Skłodowska University, 3 M. Curie-Skłodowska Sq., 20-031 Lublin, Poland
us
Highlights
Formation of MnO with regular rippled-like surface patterns
•
Synthesis of copper nanorods supported on MnO nanoparticles
•
Hydrogen production in steam methanol reforming over supported copper nanorods
Ac ce p
te
d
M
an
•
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