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Preparation and Evaluation of Cu-Mn-Oxide as High Efficiency Catalyst for CO Oxidation and NO Reduction by CO Tangkang Liu, Yanyan Yao, Longqing Wei, Zhangfu Shi, Liying Han, Haoxuan Yuan, Bin Li, Lihui Dong, Fan Wang, and Chuan-Zhi Sun J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 22 May 2017 Downloaded from http://pubs.acs.org on May 23, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Preparation and Evaluation of Cu-Mn-Oxide as High Efficiency Catalyst for CO Oxidation and NO Reduction by CO

Tangkang Liu,† Yanyan Yao,† Longqing Wei,† Zhangfu Shi,† Liying Han,† Haoxuan Yuan,† Bin Li,† Lihui Dong,*,† Fan Wang,† and Chuanzhi Sun‡

†

Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification

Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, PR. China ‡

Department of Chemistry, College of Chemistry, Chemical Engineering and Materials Science,

Shandong Normal University, Jinan 250014, PR. China

ACS Paragon Plus Environment


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ABSTRACT: In this work, highly efficient Cux-Mn composite catalysts (0 ≤ x ≤ 0.20) were synthesized by the improving hydrothermal-citrate complex method and tested in the catalytic total oxidation of CO and the removal of NO by CO. The influence of Cu on Mn-oxide materials was characterized by several characterization techniques such as: FESEM, HRTEM, XRD, BET, H2-TPR, O2-TPD, XPS, and DRIFTS, respectively. Possible reaction mechanisms for NO + CO model reaction and CO oxidation were also tentatively proposed. The Cu-modified manganese oxide materials showed the higher catalytic activity in CO oxidation and selective catalytic reduction (SCR) of NO with CO than pure MnOx materials. The improved catalytic activity of CO oxidation observed in Cu-Mn-oxide catalyst was associated to more adsorbed oxygen species and high lattice oxygen mobility due to the formation of Cu1.5Mn1.5O4 spinel active phase (Cux2+-Mnx3+-[O(y-z)Ͼz] species). Furthermore, in terms of the CO-SCR model reaction, the surface dispersed Cux+-O2--Mny+ active species could be reduced to Cu+-□-Mn(4-x)+ active species, which was considered to be the primary active component in the reduction of NO by CO. The results of catalytic performance testing indicated that Cu0.075Mn had the highest catalytic activity in CO oxidation, whereas, Cu0.15Mn exhibited the most excellent CO-SCR catalytic performance.


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1. INTRODUCTION Nitrogen oxides (NOx) and carbon monoxide are one of the most harmful air pollutants, because they will lead to undesirable severe damage for the human health as well as ecological environment. As we all know, catalytic eliminations are considered to be a most efficient and cost effective method for the eliminating of nitrogen oxides (NOx) and carbon monoxide.1,2 In the past decades, the excellent catalytic reactive catalysts are supported noble metals nanoparticles (e.g., Pt, Pd, Ru and Au) in NO + CO model reaction and CO oxidation.2-5 However, the expensive and scarce resources and the lower thermal stability of the precious noble-metals restrict the broad application of supported noble-metal catalysts.6,7 Consequently, some alternatives to the noble metals catalysts are increasingly attracted attention toward the design of low-cost and high-efficiency catalysts, in which transition metal Cu and Mn are the more attractive one.4-7,8 Recently, reducible metal oxides (such as MnOx, CoOx, FeOx, CeO2, CuOx) have been widely studied in heterogeneous catalysis reactions, which is mainly because of which its variable valence states connected with the variable occupancy of 3d or 4f orbitals.7,9,10 Among them, manganese oxides show a possible double-exchange theory between Mn3+ and Mn4+ and possess the unique catalytic redox performance and thus Mn-based materials exhibits a large amounts of surface active oxygen species, which can attract much attention on the field of NO + CO model reaction or CO oxidation.7,8,11-13 Thus for example, Zou et al.14 demonstrated that the good performance of their catalysts which was closely associated with Mn contents for CO oxidation. It is widely accepted that the Cu1.5Mn1.5O4 spinel phase is considered to be one of the main active phase in mixed Mn-Cu type oxides for the catalytic oxidation reaction.15,16 Lu et al.15 mentioned that the Cu1.5Mn1.5O4 structure was the main active site and the MnOx species was the center of oxygen supply and transmission in Cu-Mn-based catalysts. Therefore, two species can play an important key in the CO oxidation. For NO + CO model reaction, the existence of the redox equilibrium with Cu2+ + Mn3+ ↔ Cu+ + Mn4+ can promote the electron transfer and exchange between the various active components, which can enhance the catalytic activity for NO reduction by CO.7,16-18 Cu- and/or Mn-based composite oxides are very promising catalytic materials in CO oxidation or NO reduction by CO. Nevertheless, in previous works,13,18-21 most Cu- and/or Mn-based catalysts are only studied in the field of NO + CO model reaction or CO oxidation. Hence, the innovative synthesis of Cu-Mn-oxide catalysts may incarnate the scientific significance for both NO + CO model reaction and CO oxidation. Herein, this paper develops a new improving hydrothermal-citrate complex method for the preparation of Cu-Mn and Mn-oxides catalysts, to study the substituted effect of copper on the



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physicochemical properties and catalytic properties for NO + CO model reaction as well as CO oxidation. These powder catalysts are characterized by FESEM, HRTEM, XRD, BET, H2-TPR, O2-TPD, XPS, and in situ DRIFTS. Reaction performances of the Cu-Mn and Mn-oxide catalytic materials are compared. 2. EXPERIMENTAL 2.1. Catalyst Preparation. The mixed-oxide catalysts are prepared by an improving hydrothermal-citrate complexation method with a appropriate amounts of Cu and Mn nitrates as precursors. Aqueous solutions of 3.0 mol•L-1 Mn(NO3)2, 3.0 mol•L-1 citric acid, 3.0 mol•L-1 urea, and 3.0 mol•L-1 Cu(NO3)2•3H2O are premixed on the basis of the desired appropriate molar ratio, and then 0.3 mol•L-1 KMnO4 solution is slowly dripped into the above mixed solution ([Mn(NO3)2]/[KMnO4] = 1 : 2 and [urea]/[citric acid]/[Mn]all/[Cu] = 1.5 : 1.5 : 1 : 0 ~ 0.2). Afterwards, an aqueous solution of ammonia (25 wt%) is added dropwise to the pre-mixed solution under vigorous stirring until pH = 6 ~ 7. The resulting solution is stirred for 4 h, and then the solution is transferred into a Teflon-lined stainless steel autoclave (100 mL) and the autoclave is sealed and heated at 180 °C for 12 h. The obtained precipitate is filtered off and repeatedly washed with double distilled water until pH = 7, and subsequently is washed with ethanol (about three times). All powder samples are dried at 80 °C overnight, and calcined at 350 °C in the air for 4 h. The catalyst is denoted as CuxMn, herein x represent the molar ratio of Cu/Mn (x = nCu/nMn, 0 ≤ x ≤ 0.20). For comparison, CuO and MnOx are also prepared by the same method. 2.2. Catalyst Characterization. Field emission scanning electron microscope (FESEM) images are performed using a Hitachi SU-8020, which can obtain the surface morphology of Cu-Mn composite-oxide catalysts with different Cu contents. High resolution transmission electron microscopy (HRTEM) images are recorded on a Tecnai G2 F20 S-TWIN microscope (FEI Company, America) with an accelerating voltage of 200 kV, and the data are collected at room temperature. The X-ray powder diffraction patterns of the as-prepared catalysts are obtained by a PANalytical X’pert PRO powder diffractometer equipped with Cu Kα (40 kV, 40 mA) radiation. And the BET surface area is measured by Micrometrics TriStar Ⅱ 3020 instrument. The temperature-programmed reduction (H2-TPR, 7.03 vol% H2/Ar mixture) analysis and the temperature-programmed desorption (O2-TPD, pure oxygen) analysis are performed on a Pantech instruments company’s Finesorb-3010 chemisorption analyzer. Among them, the thermal conductivity detector (TCD) is used to record the effluent gas. X-ray photoelectron spectroscopy (XPS) experiments are carried on a Thermo ESCALAB 250Xi spectrometer (America). The X-ray source is Al Kα X-rays (hν = 1486.6 eV) radiation, and working power is 150 W. In


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addition, adventitious carbon at 284.8 eV (C1s) is used as a reference for binding energy calibration. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) are collected from 650 to 4000 cm-1 at a spectral resolution of 4 cm-1 (number of scans, 32) on a Nicolet iS50 Fourier transform infrared spectrometer equipped with a MCT detector cooled by liquid nitrogen. The fine samples powder placed on an in situ chamber are carefully flattened to enhance IR reflection. The catalysts (~ 10 mg) are mounted in a quarts IR cell and pretreated in flowing N2 atmosphere from 25 to 350 °C at a rate of 5 °C•min-1 (or pretreated for 1 h at 200 °C in flowing N2 atmosphere). After cooled to room temperature, the samples are conducted to a controlled stream of CO-He (10 % of CO by volume) and/or NO-He (5 % of NO by volume) at a rate of 10.0 mL•min-1 (or CO-N2 (2 % of CO by volume) and/or dry air at a rate of 10.4 and 8.2 mL•min-1, respectively) for 30~40 min. Subsequently, reaction/desorption studies are measured by heating the adsorbed species and the DRIFTS spectra are obtained at various target temperatures by subtraction of the corresponding background reference. 2.3. Catalytic Performance Evaluation 2.3.1. NO + CO Model Reaction. The catalytic performances of these catalysts for NO reduction by CO are obtained under the simulated condition, involving a feed steam with a fixed composition of NO 5 Vol%, CO 10 Vol% and He 85 Vol% as diluents with a space velocity of 24,000 mL•(g•h)-1. The sample (50 mg) is sieved with a 40 ~ 60 mesh and fitted in a quartz tube, and pretreated in a high purified N2 stream at 110 °C for 1 h. And then cooled to room temperature, after that, the reaction mixed gases are switched. The reactions are carried at different target temperatures. Analysis of the catalytic reaction products using two thermal conductivity detectors (T = 100 °C) and two chromatographic columns (Length: 1.75 m; Diameter: 3 mm). The NO conversion and N2 selectivity are calculated as follows:

 2([Ν2 ]out + [Ν2Ο]out )   ×100 % NO conversion (%)=  [ΝΟ]in     2[Ν 2 ]  ×100 % N2 selectivity (%)=   [ ] [ ] ΝΟ − ΝΟ in out   Herein, the subscripts in and out represent the inlet and outlet concentration in a flow micro-reactor device, respectively. 2.3.2. CO Oxidation. The catalytic activities of these catalysts in CO + O2 model reaction are measured in a flow micro-reactor device with a fixed composition of 1.6 % CO, 20.8 % O2 and 77.6 % N2 by volume as diluents. For all of the tests, the powder samples (50 mg) are sieved with



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a 40 ~ 60 mesh and fitted in a quartz tube, and pretreated in a high purified N2 stream at 100 °C for 1 h. And then cooled to room temperature, after that, the reaction mixed gases are switched. Reactions are carried at various target temperatures with a space velocity of 30,000 mL•(g•h)-1. Effluent gases are tested with a gas chromatograph (GC-7890T, Shanghai Scientific Instrument Co., LTD) and a thermal conductivity detector. The CO conversion is defined as follows: CO conversion (%)= ([CΟ ]in − [CΟ ]οut ) ×100 % Which is similar to that the above formula definition of NO conversion and N2 selectivity. 3. RESULTS AND DISCUSSION 3.1. Texture Characterizations. The morphologies of these samples are revealed by FESEM, and the corresponding images are shown in Figure 1. Figure 1(a) and (b) shows the FESEM images of the pure MnOx with the different magnifications, the samples are mainly composed of a large and longerity-peach-like microparticles with regular morphology. Interestingly, it can be found from Figure 1(b) that some peach appears a narrow crack, which may suggest that the existence of hollow peach-like particles related to the single MnOx. In addition, it is found that the peach-like structures of Cu-Mn catalysts are broken with the increasing of Cu contents, the particle size of the mentioned samples is decreased due to the reason that the peach-like microparticles are smashed into irregular fragment resulted from the doping of copper in MnOx crystal lattice which would inhibit the development of peach-like nanoparticles. Figure 2 shows the morphologies and microstructures of Cu-Mn catalysts with the different magnification through TEM images. In combination with the SEM results, the single MnOx samples have a typical copulative-double-peach like morphology. For the catalyst Cu0.075Mn (shown in Figure 2(b) and (c)), the lattice fringes of 0.49 nm and 0.29 nm are indexed to the (1 1 1) and (2 2 0) crystal planes of the spinel structure Cu1.5Mn1.5O4 in accordance with the standard Cu1.5Mn1.5O4 card (JCPDS35-1172), respectively. In addition, Figure 2(b) and (c) suggest that an the amorphous crystal structure of catalyst Cu0.075Mn are identified, which are closed to the amorphous phases MnOx. As shown in Figure 2((d), (e) and (f)), HRTEM micrographs of catalyst Cu0.15Mn shows a lattice fringes of 0.25 nm which can be assigned to the (1 1 1) crystal planes of crystalline CuO as the standard CuO card (JCPDS01-1117), which indicates that copper oxides have agglomerated on the sample surfaces. Similarly, the lattice spacing of 0.29 nm should be ascribed to the (2 2 0) crystal planes of the spinel Cu1.5Mn1.5O4 (JCPDS35-1172). Interestingly, catalyst Cu0.15Mn show the lattice spacing of 0.22 nm which may suggest that the active components (CuOx) are interacted obviously with manganese oxides on the catalysts surface, i. e.,


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the active components CuOx are highly dispersed on the catalyst Cu0.15Mn surface and there exists the strong synergistic interaction between active components and manganese oxides. The XRD patterns of the as-synthesized composites oxides are shown in Figure 3. Obviously, the main diffraction peaks at 2θ of 37.3 °, 42.5 ° and 56.4 ° are attributed to (101), (200) and (211) planes, respectively, which can be indexed to β-MnO2.22 Additionally, the existence of the inherently amorphous structure related to Mn-oxides can lead to the phenomenon that the catalysts possess the lager BET surface area, more mesoporous structure or wider pores distribution range. With increasing of the Cu contents, the XRD patterns shows a weak X-ray diffraction peaks that are assigned to a particular composition Cu1.5Mn1.5O4.23 It is worth noting that the intensity of Cu1.5Mn1.5O4 diffraction peak is the strongest on Cu0.075Mn sample than other samples, indicating that this sample exists more Cu1.5Mn1.5O4 spinel active structure, which is in good agreement with HRTEM results. More Cu1.5Mn1.5O4 spinel active phase and the poor crystallinity related to the sample MnOx provides a larger amount of oxygen vacancies,24 probably improving the catalytic permanence of Cu-Mn catalysts in CO oxidation. Otherwise, the characteristic peaks of CuO crystalline phases appear and the peak’s intensities increases with the rise of Cu contents when the Cu contents is more than 10 mol%, suggesting the agglomeration of copper oxides over the Cu-Mn catalysts, whereas the intensities of Cu1.5Mn1.5O4 active phase diffraction peaks decrease. Combined with the HRTEM analysis, these results would illustrate that the addition of more copper can enhance the dispersion of Cu oxides on the catalyst surfaces. The textural properties of Cu-Mn catalysts and pure metal oxides are summarized in Table 1. Interestingly, the addition of Cu can cause the emergence of the phenomenon that the average crystallite size gradually decreases and the X-ray diffraction peaks clearly shift towards lower Bragg angles, which can be associated with the incorporation of Cu2+ into the crystal lattice of β-MnO2 (the ionic radii of Cu2+ and Mn4+ are 0.072 nm and 0.054 nm, respectively) causing the shrinking with the sample cell size, which is in good agreement with the calculated value of interplanar distance. Figure 4 provides the N2 adsorption-desorption isotherms and the pore-size distribution of the as-prepared composites. According to the IUPAC classification, the isotherms (Figure 4 (A)) of the catalytic materials show the IV-type isotherms with the H3-type hysteresis loops (P/P0 ≥ 0.6) which is corresponded to the capillary condensation of N2 gas, indicating the formation of the mesoporous structure, an additional appreciable amount of larger pores as well as the interconnected mesopores or macropores loop networks.25,26 Furthermore, the existence of a H3-type hysteresis loops are ascribed to the slit-shaped pores formed by the aggregation of



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nanoparticles.27,28 Meanwhile, the corresponding BJH pore-size distribution curves (Figure 4 (B)) show that the above samples possess mesoporous and macroporous structures and its pore-size distribution range is wider. With the increasing of Cu contents, partial mesoporous structure gradually transforms into macroporous structure which are related to the diminishing of BET surface area. The BET surface area of Cu-Mn composite oxides with different Cu/Mn ratio are given in Table 1. It is obvious that Cu0.075Mn sample shows the largest BET surface area than other Cu-doped Mn oxides, which is the result that the doping of Cu can result in the formation of Cu1.5Mn1.5O4 spinel structure and thus causing the change with cell size. This is consistent with the above discussions. 3.2. H2-TPR Measurements. H2-TPR experiments are employed to investigate the effect of Cu on the reducibility of the Cu-Mn catalysts, and the corresponding results are shown in Figure 5. According to the relevant literatures, the reduction processes of pure MnOx or Mn-based H] H] materials usually take place through a distinct three-step process of MnO2 [→ Mn2O3 [→ H] H] H] Mn3O4 [→ MnO29,30 or a two-step process for MnO2 [→ Mn2O3 [→ MnO31,32. Consequently, two broad H2 consumption peaks can be observed for pure MnOx and Cu0.01Mn

located at 294 (287) and 426 (406) °C, corresponding to the stepwise reduction of Mn oxides H] H] with different chemical valence states: MnO2 [→ Mn2O3 [→ MnO, respectively, whereas the reduction with copper oxides for Cu0.01Mn is not detected. When the Cu contents is higher

than 0.05, two reduction peaks appearing at relatively low reduction temperature are characteristic for dispersed copper oxides (labeled as α and β, respectively), α peak is assigned to the reduction of the CuOx species that have strong interaction with Mn-oxides and β peak belongs to the reduction of highly dispersed CuOx species on the catalysts surface which have weak interaction with surface Mn oxides or the Cu clusters with small size for two- and three-dimensional in structure;33,34 three reduction peaks detecting above the temperature for H] Mn2O3 280 °C are attributed to the three reduction processes related to Mn oxides: MnO2 [→ H] H] Mn3O4 [→ MnO. On the other hand, the reduction peak at about 266 °C can be due to [→ the reduction of crystal CuO when C(Cu) ≥ 0.1 (C(Cu) represent the relative contents for the doping

of Cu),35 indicating that the relatively higher Cu contents can promote the agglomeration of Cu oxides on the samples surface rather than the interaction between Cu and Mn, which can cause the formation of Cu1.5Mn1.5O4 phase. Remarkably, the mentioned catalysts have more excellent reducibility than that of CuO and Mn oxides because of which the collaborative effect between Cu and Mn components as the Cux+-O2--Mny+ structure could enhance the redox potential of metal-oxide species. The reduction temperature of the each peaks are also summarized in Table 2. In combination


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with the XRD results, when the Cu content is related higher (x ≥ 0.05), the doping of Cu can cause a larger change of the microstructure for Mn-based materials which can improve the performance of Cu-doped Mn oxides dramatically, and it can also make that the reduction process of Mn species with various valence states is significantly detected. In comparison to pure CuO sample, the reduction peaks in the profiles of CuOx species move to lower temperature, therefore, the strong interaction with Mn and Cu makes the Cu species more relatively unstable and easily reducible.29,36 Similarly, the presence of Cu oxides (Cu2+ or Cu+) can depress the reduction temperature of Mn oxides greatly, thereby effectively improving the reducibility with Mn-based materials. Consequently, these peaks at lower reduction temperature are assigned to the reduction of the chemical-adsorbed oxygen species which have a significant influence on the catalytic performances.37 Otherwise, it is remarkable from Table 1 that the low-temperature reducibility of Cu0.15Mn catalyst is better than that of other Cu-doped samples, suggesting it can have optimal reducing capacity. It is considered to be favorable for its high catalytic performance for the removal of NO by CO. 3.3. O2-TPD Measurements. TPD experiments are devoted to investigate the reversibility of O2 adsorption. The TPD results of O2 on the Cu-Mn catalysts with different Cu-doped contents are within the temperature range from 50 to 650 °C, and the profiles are depicted in Figure 6, and the corresponding adsorption capacity of β and δ O species are shown in Table 2. For all catalysts, there are four O species desorption peaks (marked as α, β, δ and γ, respectively) as the temperature increased: α peak could be assigned to the desorption related to the surface physically adsorbed oxygen species (O2(ad)) at low temperature below 200 °C; β and δ peaks are attributed to the desorption with respect to the chemically adsorbed species (O2-(ad) and O-(ad)), which can be associated with the surface oxygen defects; γ peak located at the high temperature belongs to the desorption for the surface lattice oxygen (O2-(ad/lattice)).38,39 In comparison to pure MnOx sample, Cux-Mn catalysts show the larger deconvolution peak areas of β and δ O species (from Table 2) and the relevant desorption temperature move to the lower temperature region. The results indicate that the doping of Cu-oxide can give rise to an apparent increase of the amount of chemically adsorption oxygen species (O2-(ad) and O-(ad)) and oxygen defect and improve the mobility of surface O species and the adsorption capacity for oxygen on its surface, suggesting the existence of the strong synergy effect between Cu and Mn, which can clearly make the catalysts oxidation properties more excellent.40 However, the desorption temperature of the surface lattice oxygen (O2-(ad/lattice)) for Cu-Mn catalysts shifts to the higher temperature after doping Cu-oxides, which can be attributed to the fact that the formation of more stable structure



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between Mn and Cu such as Cu1.5Mn1.5O4 spinel phase or a strong synergy interaction between binary metal oxides. This can availably promote the stability of the microcosmic crystal structure for Cu-Mn mixed-oxides, which has significantly contributed to its catalytic performance. Besides, it is a valuable discovery from Table 2 that Cu0.075Mn has the largest deconvolution peak areas of chemically adsorption oxygen (β and δ O species) than other Cu-doped samples, illustrating that the sample have most chemically adsorption oxygen species (O2-(ad) and O-(ad)) and oxygen vacancies as well as best mobility of the surface O species, which suggesting that it can have optimal oxidation capacity. It is considered to be favorable for its high catalytic performance in CO oxidation. 3.4. XPS Analysis. The XPS study is performed on Cu-Mn samples with different Cu contents and the results are recorded in Figure 7 corresponding to Mn 2p, Cu 2p and O 1s, respectively. The obtained Mn 2p curves are fitted with four peaks that consist of a spin-orbit doublet with Mn 2p1/2 spectrum associated with a binding energy of 660 ~ 650 eV and Mn 2p3/2 spectrum related to a binding energy of 647.5 ~ 638 eV.30 For the Mn 2p3/2 spectrum with the typical catalysts, it is clearly observed that the binding energies at around 643.6 and 641.9 eV could be ascribed to the presence of Mn4+ and Mn3+ species, respectively.41,42 This is to support the TPR results. As observed, Mn3+ species are dominant manganese oxide species in Cu-Mn catalysts with a relatively small amounts of Mn4+ species. A study has shown that the existence of Cu1.5Mn1.5O4 cubic spinel active phase is advantageous to facilitate the catalytic oxidation reaction, where an illustration of this point is that the presence for two Jahn-Teller ion (Mn3+ and Cu2+) in the spinel structure resulting in more oxygen defect and chemical adsorption O species.43 As calculated in Table 3, Mn3+/Mn4+ molar ratio is largest for Cu0.075Mn than other Cu-doped samples which probably proves that the solid state charge transfer redox couple, i.e., the redox cycle of Cu+ + Mn4+ ↔ Cu2+ + Mn3+ may enhance the electron interaction between Cu2+ and Mn4+. In contrast to pure MnO2 and Mn2O3, the binding energies of Mn 2p1/2 and 2p3/2 shift to high binding energy direction slightly, also indicating that there are some electron interactions between Mn4+ and Cu2+ due to the strong interaction between manganese and copper oxides (Cu-Mn-O bridge).5,44 To investigate the various O species on the surface, the XP spectra of O1s level for the Cu-Mn samples are shown in Figure 7(B). The deconvoluted peaks in Figure 7(B) illustrate that the existence of two different O species on the catalysts surface: the splitting peak (labeled as Oα) appearing at higher binding energy of about 531.4 eV is attributed to the adsorbed oxygen (O2-/O-); the dominant peak (denoted as Oβ) arising at higher binding energy of about 529.5 eV have been regarded as characteristic of the lattice oxygen (O2-), potentially including surface hydroxyl, oxygen vacancies and/or carbonate species.45,46 Compared to the pure MnOx, the


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doping of Cu can make more adsorbed O species (O2-/O-) for Cu-Mn catalysts (Table 3) which indicates the formation of more surface lattice defect, thus causing more oxygen adsorption/storage capacity associated with Cu-modified materials. In terms of Cu-Mn catalysts (Figure 7(B)), the binding energies of Oβ species (O2-) shift to the high binding energy region along with the increasing of Cu contents (x ≤ 0.075), suggesting that the doping of Cu can make partial Cu atom replaces O atom in －O－Mn－ connection (O → Cu) and can enhance the instability related to O species so as to form more active O species (O•, O2- and O-), this is similar with the explanation in the literatures.5,47 Interestingly, catalyst Cu0.075Mn has a largest Oα/(Oα + Oβ) molar ratio and it can be regarded as possessing more adsorbed O species, which is in good agreement with the previous O2-TPD results. The XP spectrum of Cu 2p is numerically fitted with four components for these Cu-based catalysts, and the corresponding spectra are displayed in Figure 7(C). It is of note that a characteristic Cu 2p3/2 peaks at around 935.1 eV with a shake-up satellite peaks located at the binding energy of 949.5 ~ 939.5 eV are corresponded to Cu2+ species and the absorption peaks located at around 933.7 eV should be corresponded to Cu+ and/or Cu0 species.48-50 Figure 7(D) presents the Cu-LMM Auger spectra for catalysts. From Figure 7(D), it is remarkable that the broad and asymmetry peaks are resolved into three asymmetrical peaks located at about 568.3 eV, 569.2 eV, and 570.1 eV, indicating the main existence of Cu0, Cu2+, and Cu+ species, respectively.51 The Cu2+ species are main Cu species which is mainly due to the redox cycles, i.e., Cu+ + Mn4+ ↔ Cu2+ + Mn3+ shifting to right, suggesting that the electron interaction between Cu and Mn is very strong. In combination with Figure 7(C) and (D), the amount of the reduced copper species (Cu+/Cu0 species) add gradually with the increasing of Cu contents. In general, the amount of the reduced copper species (Cu+/Cu0 species) can be positively related with the catalytic performance for the reductive catalysts.49,52,53 Hence, the most reduced copper species (Cu+/Cu0 species) means the optimal SCR catalytic performance for Cu0.15Mn than other Cu-modified catalysts. As listed in Table 3, the surface Cu/Mn molar ratio is less than the actual Cu/Mn molar ratio, which may be an evidence that Cu atom into the bulk phase of manganese oxides. Besides, the Cu0.075Mn sample possesses the minimum surface Cu/Mn ratio that is much less than its actual Cu/Mn ratio, which would surely be proof positive that more Cu1.5Mn1.5O4 spinel active phase is existed in structure for Cu0.075Mn. 3.5. Catalytic Activity Measurements (CO Oxidation and NO + CO Model Reaction). The catalytic performance results for CO oxidation as a function of temperature related to the synthesized catalysts are shown in Figure 8. All Mn-based catalysts show a certain activities in the room temperature, especially for the Cu-modified catalysts that show higher activities. It can



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be seen that Cu-Mn catalysts show enhanced activity as compared to pure MnOx sample, herein the catalytic activity of the Cu-doped catalysts grow greatly with the increasing of Cu contents that is less than 0.075. Obviously, Cu0.05Mn and Cu0.075Mn embodies more excellent catalytic activity than other catalysts. As displayed in Figure 8, the Cu0.075Mn catalyst shows most excellent catalytic activity in which its CO catalytic oxidation activity reaches 100 % in the temperature of 65 °C. However, when the Cu contents is more than 0.075, the CO catalytic activity of the catalysts decrease memorably accompanied by the increasing of Cu contents. Notably, the CO catalytic activity of the Cu0.20Mn sample is the worst than other Cu-doped catalysts, in which its activity is almost as bad as the sample MnOx. Figure 9 depicts the catalytic performance for NO reduction by CO as a function of temperature of the synthesized materials. As shown in Figure 9, pure MnOx gives a well CO-SCR catalytic activities that its NO conversion reaches 99 % located at the temperature about 300 °C. However, the catalytic performance of sample MnOx is unstable which acts as a phenomenon that its NO conversion exhibits a tendency of slow decline above 300 °C. In addition, all Cu-doped catalysts show higher activities with increasing Cu concentration, which are much higher than that of Mn-oxide catalyst in the whole temperature range. Notably, the SCR activities of Cu-Mn catalysts exhibit a big growth trend when temperature is more than 150 °C. With a totally different from the activities testing results of CO oxidation, catalyst Cu0.15Mn reveals the finest CO-SCR catalytic performance instead of catalyst Cu0.075Mn, and the NO conversion reaches 100 % and N2 selectivity embodies nearly 100 % at 275 °C. Furthermore, it can be found from Figure 9(C) that the CO conversion data are in line to that of NO. Interestingly, the CO conversion curves of MnOx and Cu0.01Mn showing suddenly a larger downtrend when the reaction temperature above 350 (or 325) °C, which illustrates that the crystal structure for the samples is unstable and the O derived from these unstable structure are easy to irreversibly reduce, in other words, the doping of Cu-oxides is beneficial to the stability of Cu-Mn catalysts and the improvement of catalytic properties heads from the strong synergistic effect between binary metal oxides. Generally, it is accepted that Cu1.5Mn1.5O4 spinels active phase is a high reaction activity center in the catalytic oxidoreaction process.7,8,13,16,21 Obviously, Cu1.5Mn1.5O4 spinels active phase play the decisive role in CO oxidation and the catalytic performance of the catalyst with the spinels structure is much better than other catalysts. An illustration of this point from the CO catalytic activity results is that the Cu-doped catalysts exhibit more excellent catalytic activity than the pure MnOx sample, herein sample Cu0.075Mn show the best catalytic activity for CO oxidation, which suggests that it has the largest number of Cu1.5Mn1.5O4 spinels phase in structure via the


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redox cycle of Cu+ + Mn4+ ↔ Cu2+ + Mn3+. What is more, the presence of Cu1.5Mn1.5O4 spinels phase in structure for Cu-modified catalysts is confirmed by the XRD patterns and the XPS results. Interestingly, the excellent order of CO catalytic oxidation performance positively corresponding to the scalar sequence chemically adsorbed O species in the light of the characteristic of XPS and TPD. According to the previous study,15,43 Cu1.5Mn1.5O4 spinels structure can promote effectively the formation of surface oxygen defects in the oxidizing atmosphere, and surface oxygen defects can give impetus to the adsorption of O2 on the catalyst surfaces so as to benefit the generation of the reactive O species (O•, O2- and O-) that can combine with CO molecule to forming CO2 gas, which can provide a key factor related to the improvement of CO catalytic oxidation activity. Similarly, it is of no doubt that highly dispersed Cu-oxide active species that of the interaction between Cu-oxides and Mn-oxides (Cu-O-Mn species) can form surface synergy oxygen vacancies activity center (Cu-□-Mn species) effectually with the catalytic temperature increasing, thus clearly promoting the CO-SCR performance of the as-prepared materials.54,55 On the basis of the CO-SCR activity results, it is clearly obvious that the doping of Cu can improve significantly the catalytic performance for the composite materials, however, which is different from the CO catalytic activity results. In combination with the XRD, TPR and XPS results, a potentially reasonable explanation for which a considerable portion of Cu species are highly dispersed on the catalyst surfaces and it exists in the form of Cu-O-Mn active species and CuO species when Cu contents is more than 0.1, which can make the gratified improvement for CO-SCR catalytic activity. Nevertheless, this fact can give rise to covering and decreasing of the active sites for CO catalytic oxidoreaction, which is main reason with the significantly decreasing of CO catalytic activity. Also interestingly, the excellent order of CO-SCR catalytic performance is positively compatible with the reduction ability and the amount of the reduced copper species (Cu+/Cu0 species) based on the TPR and XPS results. Therefore, it can be reached the following conclusion that the reduction ability play a vital role in the CO-SCR activity under the reducing atmosphere. 3.6. In situ DRIFTS Spectra Results 3.6.1. CO and/or O2 Adsorption for CO Oxidation. In order to further understand the surface adsorbed species and probe the active factors on the reactivity of Cu-Mn catalyst for CO oxidation, CO and/or O2 adsorption in situ DRIFTS spectra of Cu-Mn catalysts are measured under the simulative reaction conditions. Figure 10 shows the in situ DRIFTS spectra of CO adsorption on MnOx and Cu-Mn catalysts. As shown in Figure 10(A), the peaks at around 1495, 1306, and 1070 cm-1 are attributed to



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monodentate carbonate species (i.e., ν(C-O), νs(CO32-) and νas(CO32-)); the bands at around 1548 and 1396 cm-1 are ascribed to the vibration modes of bidentate carbonate (ν(C=O)) and carboxylate (ν(O-C-O (sym))), respectively.7,56,57 Additionally, the bands at around 1638 and 1442 cm-1 corresponds to bicarbonate and monodentate formate, respectively; the weak peak at around 1790 cm-1 can belong to the combination band for the interaction between divalent metal ions and carbonate groups.58,59 Obviously, the Cu0.075Mn sample is similar with the MnOx for the DRIFTS spectra of CO adsorption. Furthermore, it is notable that Cu0.075Mn reveal a unique broad peak in the range of 1830 ~ 2060 cm-1 related to the stretching vibration of bridgebonded CO species on the surface lewis acid sites.60 This can be attributed to the reason that the O species from Cu1.5Mn1.5O4 spinel active phase can oxidize gradually CO molecules formed carbonate species, thus making that two Jahn-Teller ions in Cu1.5Mn1.5O4 spinel phase can be exposed on the surface and it interacts with CO molecules formed linearly bridgebonded CO species. As can be seen from Figure 10, the bands at around 2119 and 2171 cm-1, 2360 cm-1 can be attribute to gaseous CO and CO2, respectively.57,60 The generated species for the adsorption of CO molecules are very stable under the higher simulative temperature and the characteristic peaks for CO2 gas and surface carbonate species appears at low temperature, indicating that the catalysts own strong adsorption capacity and reactivity for CO molecules. Much to our pleasure, the intensities of the surface carbonate species related to the Cu0.075Mn sample is stronger than the MnOx sample, suggesting that CO molecule can be easily oxidized on the Cu0.075Mn sample surface and manifesting that the doping of copper oxides can improve the oxidizability of Mn-based catalysts. The in situ DRIFTS spectra of CO and O2 co-adsorption over MnOx and Cu-Mn catalysts are shown in Figure 11. Similar with the above results (Figure 10), for the MnOx and Cu0.075Mn samples, the peaks of surface carbonate and carboxylate species are formed by CO adsorption on the surfaces, as indicated by the range of 1000~1700 cm-1 peaks. However, these peaks intensity are much weaker than that of CO-adsorption especially for the higher temperature region (T ≥ 75 °C), in contrast, the peak intensity of CO2 gas is stronger than that of CO-adsorption. This illustrates that oxygen molecules are preferentially adsorbed on the catalysts surface under the oxygen-enriched atmosphere, which can facilitate the formation of surface active O species (labeled as O*, such as O2-, O- or O•) and thereby hold the surface O vacancies, making that the adsorption behavior of CO and CO2 molecules is inhibited under the simulated condition.61 For sample Cu0.075Mn, linear Cu+-CO species can not be detected at the simulated temperature, suggesting that the presence of Cu+ species is impossible on the sample surfaces, that is, the reducing copper species (Cu+/Cu0 species) are not essential during the CO oxidation reaction.


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This fact proves the above speculation. 3.6.2. NO + CO Co-adsorption for NO + CO Model Reaction. In order to further probe the effect of the Cu doping on the variation of manganese species and comprehend the interaction of active factors with the catalysts, the in-situ DRIFTS spectra for NO + CO co-adsorption are performed at different temperatures in the simulated conditions, and the corresponding spectra are shown in Figure 12. From Figure 12, the bands at around 1630 and 1030 cm-1 should be attributed to the bridge bidentate NO3-1; the bands at 1503 (1519), 1348, and 1297 (1277) cm-1 should be ascribed to the monodentate NO3-1, cis-N2O22- species, and linear nitrite NO2-1 (ν(O-N-O)), respectively; the band at around 1596 cm-1 corresponds to the chelating nitrate.54,56,62 Interestingly, with increasing of the desorption temperature, the band for the adsorption of linear nitrite NO2-1 (ν(O-N-O)) has shifted from 1297 to 1277 cm-1 and its intensity has gradually decreased and eventually disappeared. What’s more, the nitrate and nitrite species have also gradually disappeared with the increasing of temperature. These clearly indicate that the thermolability of the nitrate and nitrite species formed by the adsorption process. In addition, with respect to the temperature region below 200 °C, the bands of surface carbonate and carboxylate species formed by the CO molecules adsorption are not detected. A reasonable explanation for this phenomenon is that NO molecules and CO molecules would share a similar adsorption sites, and NO molecules can be preferentially adsorbed on the catalyst surfaces in the reduction atmosphere and thereby subduing the adsorbed process for CO molecules.53,56 In line with the in situ DRIFTS spectra of the above CO adsorption, for the temperature region above 200 °C, the bands in the wavenumber range of 1000 ~ 1700 cm-1 are also attribute to surface carbonate and carboxylate species formed by the adsorption of carbon monoxide; the bands at about 2171 and 2119 cm-1 are corresponded to gaseous CO just as the band at about 2360 cm-1 can be corresponded to gaseous CO2. For the NO + CO co-adsorption DRIFTS spectra, a new peak appearing at around 1791 cm-1 is assigned to the interaction between divalent metal and surface COx species, which can be caused by the CO molecules adsorption related to Mn2+ ions.59 Furthermore, the characteristic peaks N2O molecules are detected at about 2237 and 2206 cm-1,35 which indicates the formation of gaseous N2O as an intermediate products in the NO dissociation process. It seems reasonable to explain that NO molecules are induced to form [N] and [O] under the catalytic effect of surface oxygen vacancies, subsequently, the dissociated [N] species can recombine with NO molecules, i.e.,

[N ] + NO → N 2O .

When the temperature

increases gradually, the peak with N2O molecules disappears gradually due to a process for the dissociation and recombination of NO → N 2 . Simultaneously, as illustrated in Figure 12(B), the band at 2113 cm-1 is assigned to linear copper carbonyl (Cu+-CO) species,48,63 herein this species



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peak appear at 150 °C and then disappear 300 °C, which can be ascribed to a fact that copper CO CO oxides are reduced step by step, i.e., Cu 2 + → Cu + → Cu 0 . Therefore, N2O species and

linear copper carbonyl (Cu+-CO) species are intimately associated with the catalytic performance. This is a powerful evidence for which the doping of Cu can improve convincingly the CO-SCR catalytic performance with regard to Mn-based catalysts. 3.7. Catalytic Mechanisms 3.7.1. Proposed Mechanism for CO Oxidation Reaction. Based on the more important information of catalytic routes obtained from DRIFTS analysis, a plausible mechanism (Schematic diagram) for CO oxidation reaction over Cu-Mn catalysts is tentatively proposed. As shown in Scheme 1, we have considered the proposed mechanism including two processes as follows: (I) O2 molecules are preferentially adsorbed on the catalyst surfaces under the oxygen-rich atmosphere, and subsequently formed surface active O species (labeled as O*, such as O2-(ads), O-(ads)) by the catalytic effect in the Cu1.5Mn1.5O4 spinel active phase. In this process, −

−

e ] e ] the adsorption reaction of O2 molecules are formulated as O2 [ → O2− ( ads ) [ → 2O(−ads ) . (II)

CO molecules are oxidized by surface active O species and forms gaseous CO2, which are formulated as CO + O(−ads ) → CO2 + e − and CO + 2O(−ads ) → CO32− ( ads ) , leading to a catalytic cycle. Herein, the cooperative effect of the dual metal oxide components (the redox cycle of Cu+ + Mn4+ ↔ Cu2+ + Mn3+) on the distorted spinel structure of Cu2+ and Mn3+ can promote the generation of more surface Lewis acid sites (Cux2+-Mnx3+-[O(y-z)Ͼz] species) in Cu0.075Mn sample, which is beneficial to the activation of two reactants to form more active species and to the enhancement of catalytic performance for CO oxidation, and this is supported by XPS and catalytic activity testing as well as CO and O2 co-adsorption in situ DRIFTS. In the proposed catalytic cycle, O2 gas is activated at the catalysts surface. By the time, the Cu1.5Mn1.5O4 spinel active phase is to act as a transportation function of the surface active O species. For the first round, at the lower temperature, the Cux2+-Mnx3+-Oy in Cu1.5Mn1.5O4 spinel structure loses some oxygen: Cux2+-Mnx3+-Oy + zCO → Cux2+-Mnx3+-[O(y-z)Ͼz] + zCO2 Herein, Ͼ is labeled as oxygen vacancies in Cu1.5Mn1.5O4 spinel structure. Whereafter, Cu1.5Mn1.5O4 spinel phase are rapidly activated in line with increased temperatures. Cux2+-Mnx3+-[O(y-z)Ͼz] species in Cu1.5Mn1.5O4 spinel phase captures some O molecules from the


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flue gas and leading to the formation of surface active O species (O*, including O2-, O- and O•). Cux2+-Mnx3+-[O(y-z)Ͼz] + nO2 → Cux2+-Mnx3+-[O(y-z)Ͼ2n-z(O*)2n] Cux2+-Mnx3+-[O(y-z)Ͼ2n-z(O*)2n] + mCO → Cux2+-Mnx3+-[O(y-z)Ͼz] + mCO2 Therefore, Cux2+-Mnx3+-Oy species are reduced to Cux2+-Mnx3+-[O(y-z)Ͼz] species, and Cux2+-Mnx3+-[O(y-z)Ͼz] species should be a Lewis acid that is to act as an active center in the catalytic cycle. 3.7.2. Proposed Mechanism for NO+CO Model Reaction. In line with the detailed information for intermediate products recorded from DRIFTS analysis, a possible reaction mechanism for NO+CO model reaction under the simulated conditions over Cu-Mn catalysts can also be tentatively proposed, and corresponding schematic illustration is illustrated in Scheme 2. It is universally accepted that the formation of surface synergetic oxygen vacancies between binary metal oxides are contributed to a fact that the activation of the reactant gas to form more high-active species for NO+CO model reaction, which can promote the catalytic performance signally for CO-SCR of NO.56 Firstly, at normal temperature, NO molecules are preferentially adsorbed on the catalysts surface formed surface NOx species, and a small amount of CO molecules are oxidized by surface active O species possibly derived from Cu-O-Mn species. With the increasing of the temperature, surface NOx species and NO molecules are gradually dissociated into [N] and [O], and the high-state metal species (Mn4+ or Mn3+, Cu2+) are partially reduced by CO molecules generated the low-state metal species (Mn3+ or Mn2+, Cu+ or Cu0), which can lead to the phenomenon that the formation of the bands for Mn2+-COx species (defined as the interaction between divalent metal and surface COx species) and Cu+-CO species in DRIFTS spectra. In the meantime, intermediate products N2O are detected in DRIFTS spectra and the surface synergetic O vacancies (SSOV), i.e, Cu+-□-Mn(4-x)+ active species, are produced through the synergistic effect between Cu and Mn. More SSOV species are generated as the adsorbed temperature continues to rise, resulting to the remarkable enhancement of the catalytic performance, and CO molecules and the intermediate product N2O turn into gaseous CO2 and the final product N2. This is a catalytic cycle. 4. CONCLUSIONS In this work, a high-efficiency and multifunctional catalyst Cu-Mn composite has been successfully fabricated via an innovative improving hydrothermal-citrate complexation protocol. Simultaneously, pure MnOx sample is also synthesized by the same method for purpose of comparison. As demonstrated by the above-integrated characterizations, the different



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physicochemical properties of the as-prepared materials have been investigated systematically, respectively, and further work proves that the existence of intermediate active species and the catalytic pathway in CO oxidation and NO + CO model reaction. Hence, we can draw several conclusions as following: (i) The doping of Cu species can elevate redox properties and catalytic activity through the movement of the redox equilibrium, i.e., Cu+ + Mn4+ ↔ Cu2+ + Mn3+, and may promote the grain formation and growth of Cu1.5Mn1.5O4 spinel phase rather than manganese oxides resulting in increasing the surface area and particle size. (ii) Doped Cu-Mn catalysts are much easier to be oxidized and form more activating O species on the surface than that of pure MnOx, and the adsorbability of oxygen molecules are improved. An illustration of this phenomenon is that the formation of Cu1.5Mn1.5O4 spinel active phase in structure leads to more Lewis acid site (Cux2+-Mnx3+-[O(y-z)Ͼz] species) on the catalysts surface and the helpful changing of ECD (Electron Charge Density) properties based on the existence of two Jahn-Teller ions, and it is beneficial to the activity in low-temperature oxidation of CO. (iii) The Cu-oxides doped Mn-based catalysts are more favorable to adsorbing NOx species, in which these possess the linear Cu+-CO active species is other than pure MnOx. An interesting process is that the surface dispersed Cux+-O2--Mny+ active species could be reduced to Cu+-□-Mn(4-x)+ active species with the raising of adsorbed temperature, and the synergy effect between Cu and Mn (Cux+-O2--Mny+ species) is important for NO + CO model reaction. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: DRIFT sections, Additional in situ DRIFT spectra of NO adsorption, it can be seen from Figure S1 that the NO adsorption spectra results are similar to that of CO + NO co-adsorption spectra. (PDF) AUTHOR INFORMATION Corresponding author: *E-mail address: [email protected]


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ACKNOWLEDGMENTS This work was supported by National Nature Science Foundation of China (No. 21507014,21663006)

and

Nature

Science

Foundation

of

Guangxi

Province

(No.

2014GXNSFBA118036) and Program for Science and Technology Development Plan of Nanning (No. 20163146).



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Highly

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Pretreatment on CuO/CeO2 Catalyst for Catalytic Reduction of NO by CO. J. Rare Earth. 2014, 32, 139-145. (53) Zou, W. X.; Liu, L. C.; Zhang, L.; Li, L. L.; Cao, Y.; Wang, X. B.; Tang, C. J.; Gao, F.; Dong, L. Crystal-Plane Effects on Surface and Catalytic Properties of Cu2O Nanocrystals for NO Reduction by CO. Appl. Catal. A: Gen. 2015, 505, 334-343. (54) Liu, L. J.; Chen, Y.; Dong, L. H.; Zhu, J.; Wan, H. Q.; Liu, B.; Zhao, B.; Zhu, H. Y.; Sun, K. Q.; Dong, L.; Chen, Y. Investigation of the NO Removal by CO on CuO-CoOx Binary Metal Oxides Supported on Ce0.67Zr0.33O2. Appl. Catal. B: Environ. 2009, 90, 105-114. (55) Lv, Y. Y.; Liu, L. C.; Zhang, H. L.; Yao, X. J.; Gao, F.; Yao, K. A.; Dong, L.; Chen, Y. Investigation of Surface Synergetic Oxygen Vacancy in CuO-CoO Binary Metal Oxides Supported on c-Al2O3 for NO Removal by CO. J. Colloid. Interf. Sci. 2013, 390, 158-169. (56) Hadjiivanov, K. I.; Vayssilov, G. N. Characterization of Oxide Surfaces and Zeolites by Carbon Monoxide as an IR Probe Molecule. Adv. Catal. 2002, 47, 307. (57) Qi, L.; Yu, Q.; Dai, Y.; Tang, C. J.; Liu, L. J.; Zhang, H. L.; Gao, F.; Dong, L.; Chen, Y. Influence of Cerium Precursors on the Structure and Reducibility of Mesoporous CuO-CeO2 Catalysts for CO Oxidation. Appl. Catal. B: Environ. 2012, 119-120, 308-320. (58) Li, S. Y.; Jia, M. J.; Gao, J.; Wu, P.; Yang, M. L.; Huang, S. H.; Dou, X. W.; Yang, Y.; Zhang, W. X. Infrared Studies of the Promoting Role of Water on the Reactivity of Pt/FeOx Catalyst in Low-Temperature Oxidation of Carbon Monoxide. J. Phys. Chem. C 2015, 119, 2483-2490. (59) Liu, L.; Zhang, X. J.; Wang, R. Y.; Liu, J. Z. Facile Synthesis of Mn2O3 Hollow and Core-Shell Cube-Like Nanostructures and Their Catalytic Properties. Superlattice. Microst. 2014, 72, 219-229. (60) Morales, F.; Smit, E. D.; Groot, F. M. F. D.; Visser, T.; Weckhuysen, B. M. Effects of Manganese Oxide Promoter on the CO and H2 Adsorption Properties of Titania-Supported Cobalt Fischer-Tropsch Catalysts. J. Catal. 2007, 246, 91-99. (61) Tang, Y. S.; Dong, L. H.; Deng, C. S.; Huang, M. N.; Li, B.; Zhang, H. L. In situ FT-IR Investigation of CO Oxidation on CuO/TiO2 Catalysts. Catal. Commun. 2016, 78, 33-36. (62) Yu, Q.; Yao, X. J.; Zhang, H. L.; Gao, F.; Dong, L. Effect of ZrO2 Addition Method on the Activity of Al2O3-Supported CuO for NO Reduction with CO: Impregnation vs.



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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure captions Figure 1. FESEM images of the representative samples ((a) and (b)) MnOx; (c) Cu0.075Mn sample; (d) Cu0.15Mn sample. Figure 2. TEM and HRTEM images of Cu-Mn catalysts. (a) The single MnOx at lower magnification (TEM). ((b) and (c)) Cu0.075Mn composite oxides at high magnification (HRTEM). ((d), (e) and (f)) Cu0.15Mn composite oxides with HRTEM. Figure 3. XRD patterns of Cu-Mn catalysts with various Cu contents. Figure 4. The (A) N2 adsorption-desorption isotherms and (B) BJH pore-size distribution of the Cu-Mn composite oxides. Figure 5. The H2-TPR profiles for Cu-Mn catalysts. Figure 6. The O2-TPD profiles of Cu-Mn catalysts. Figure 7. The XPS spectra of Cu-Mn catalysts with different Cu contents. Figure 8. The catalytic oxidation of CO (average of three measurements) over the Cu-Mn catalysts with various Cu contents. Figure 9. The (A) NO conversions, (B) N2 selectivity and (C) CO conversion (%) (average of three measurements) as a function of temperature over the Cu-Mn catalysts with various Cu contents. Figure 10. In situ DRIFT spectra of Cu-Mn catalysts when exposed to CO stream at different temperatures. Figure 11. In situ DRIFT spectra of Cu-Mn catalysts when exposed to CO and O2 (Dry air) mixed stream at different temperatures. Figure 12. In situ DRIFT spectra of adsorbed species under CO (10 vol%) and NO (5 vol%) mixed steam over the samples. Scheme 1. Proposed Mechanism (Schematic Illustrations) for the Catalytic Oxidation of CO over Cu-Mn Catalysts. Scheme 2. Proposed Mechanism (Schematic Illustrations) of NO Reduction by CO over Cu-Mn Mixed Oxides. (□: Surface Oxygen Vacancies; □: Surface Synergetic Oxygen Vacancies)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Textural Properties of Cu-Mn Mixed-Oxide Catalysts and Pure Metal Oxides Molar ratio Cu/Mn

Average crystallite size Da/nm

(101) Interplanar distanceb/nm

BET surface area/(m2/g)

-

-

-

3.5

Cu0.20Mn

0.20

5.8

0.2439

112

Cu0.15Mn

0.15

6.2

0.2431

114

Cu0.10Mn

0.10

6.3

0.2423

124

Cu0.075Mn

0.075

7.0

0.2410

126

Cu0.05Mn

0.05

8.2

0.2415

117

Cu0.01Mn

0.01

10.1

0.2411

118

0

10.5

0.2409

129

Sample CuO

MnOx a

Average crystallite size is calculated from the (101) peak of β-MnO2.

b

(101) Interplanar distance is calculated from the diffraction angle.



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Table 2. The Analysis Data of H2-TPR and O2-TPD Profiles Over the Cu-Mn Catalysts H2-TPR results

O2-TPD results

Samples α/(°C)

β/(°C)

δ/(°C)

γ/(°C)

λ/(°C)

ν/(°C)

β peaks (~357 °C) areas

δ peaks (~459 °C) areas

Cu0.20Mn

200

240

270

300

341

373

5229.93

6948.93

Cu0.15Mn

198

238

268

298

335

373

6098.73

8239.39

Cu0.10Mn

204

243

267

296

342

380

7223.85

6062.60

Cu0.075Mn

208

254

-

281

322

366

9291.05

8552.03

Cu0.05Mn

213

260

-

292

343

397

8350.91

9160.89

Cu0.01Mn

-

-

-

-

-

-

6706.49

7130.38

MnOx

-

-

-

-

-

-

6437.99

7723.20


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3. Surface Atomic Concentrations of Cu, Mn and O Over the Synthesized Catalysts Surface atomic concentration (%)a Samples

Cu/Mn molar ratiob

Cu/Mn surface ratioc

Mn3+/Mn4+

Oα/(Oα+Oβ) (%)

C

O

Cu

Mn

Cu0.15Mn

38.76

45.61

1.49

14.14

0.15

0.105

2.46

44.47

Cu0.075Mn

35.88

49.63

0.43

14.06

0.075

0.031

2.85

49.65

Cu0.05Mn

35.25

50.19

0.62

13.94

0.05

0.044

2.77

47.12

MnOx

31.76

53.42

-

14.81

-

-

2.69

43.35

a: The data were obtained by XPS spectrometer. b: The actual ratio of Cu and Mn in the samples. c: The ratio of Cu and Mn in the surface were obtained by the data of XPS measurement.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. FESEM images of the representative samples ((a) and (b)) MnOx; (c) Cu0.075Mn sample; (d) Cu0.15Mn sample.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. TEM and HRTEM images of Cu-Mn catalysts. (a) The single MnOx at lower magnification (TEM). ((b) and (c)) Cu0.075Mn composite oxides at high magnification (HRTEM). ((d), (e) and (f)) Cu0.15Mn composite oxides with HRTEM.



Figure 3. XRD patterns of Cu-Mn catalysts with various Cu contents.

♥ ♥

♦ - CuO ♥ - Cu1.5Mn1.5O4

(211)

♥ ♦ ♦

(200)

♥

(101)

250

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Cu0.20Mn

♦ ♦

Cu0.15Mn

♦ ♦

Cu0.10Mn

♥

Cu0.075Mn

♥

Cu0.05Mn

♥

Cu0.01Mn MnOx

10

20

30

40

50

60

70

52

80

56

60

2 Theta(θ)/degree


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Figure 4. The (A) N2 adsorption-desorption isotherms and (B) BJH pore-size distribution of the Cu-Mn composite oxides.

(A)

volume adsorbed (a.u.)

50

Cu0.20-Mn-O Cu0.15-Mn-O Cu0.10-Mn-O Cu0.075-Mn-O Cu0.05-Mn-O Cu0.01-Mn-O MnOx 0.0

0.2

0.4

0.6

Relative Pressure/(P/P0)

0.8

1.0

(B) 0.002

Cu0.20Mn Cu0.15Mn dV/dD(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cu0.10Mn Cu0.075Mn Cu0.05Mn Cu0.01Mn MnOx 20

40

60

80

100

Pore Diameter/nm

120

140



Figure 5. The H2-TPR profiles for Cu-Mn catalysts.

250

α

H2 Consumption(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

β δ γ

λ ν

Cu0.200-Mn-O Cu0.150-Mn-O Cu0.100-Mn-O Cu0.075-Mn-O Cu0.050-Mn-O

287°C

406°C

Cu0.010-Mn-O

294°C 426°C

100

200

300

400

MnOx CuO 500

600

700

Temperature/°C


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Figure 6. The O2-TPD profiles of Cu-Mn catalysts.

100

α=118 °C

β=357 °C

γ=522 °C δ=459 °C

Cu0.20Mn Cu0.15Mn

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cu0.10Mn Cu0.075Mn Cu0.05Mn Cu0.01Mn MnOx 100

200

300

400

500

600

Temperature/°C



Figure 7. The XPS spectra of Cu-Mn catalysts with different Cu contents.

(A) Mn2p

50000

641.9 eV 643.6 eV

Intensity(a.u.)

Cu0.15Mn

Cu0.075Mn Cu0.05Mn

MnOx 665

660

655

650

645

640

635

Binding energy/eV

(B) 50000

Oβ

Oα

O1s

Cu0.15Mn Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Cu0.075Mn

Cu0.05Mn

MnOx 537.5

535.0

532.5

530.0

Binding energy/(eV)

527.5

525.0


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(C) 5000

Cu0.15Mn

Cu 2p1/2

Cu2p

Shake-up 933.7 eV

Intensity(a.u.)

935.1eV

Cu0.075Mn Cu0.05Mn

960

955

950

945

940

935

930

Binding energy/eV

(D) 1000

Cu-LMM

569.2 eV 568.3 eV

570.1 eV

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cu0.15Mn Cu0.075Mn Cu0.05Mn

578

576

574

572

570

568

566

564

562

Binding energy/eV



Figure 8. The catalytic oxidation of CO (average of three measurements) over the Cu-Mn catalysts with various Cu contents.

110 100 90 80

CO Conversion/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

70

Cu0.20Mn

60

Cu0.15Mn

50

Cu0.10Mn

40

Cu0.075Mn

30

Cu0.05Mn

20

Cu0.01Mn

10

MnOx

0 -10 20

30

40

50

60

70

80

90

100

Temperature/°C


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Figure 9. The (A) NO conversions, (B) N2 selectivity and (C) CO conversion (%) (average of three measurements) as a function of temperature over the Cu-Mn catalysts with various Cu contents.

110

(A)

100

NO Conversion/%

90 80 70

Cu0.20Mn

60

Cu0.15Mn

50

Cu0.10Mn Cu0.075Mn

40

Cu0.05Mn

30

Cu0.01Mn

20

MnOx

10 100

150

200

250

Temperature/°C

300

350

400

(B) 110 100 90 80

N2 Selectivity/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70 60

Cu0.20Mn

50

Cu0.15Mn

40

Cu0.10Mn

30

Cu0.075Mn Cu0.05Mn

20

Cu0.01Mn MnOx

10 0 100

150

200

250

300

350

400

Temperature/°C



(C) 60

50

CO Conversion/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

40

Cu 0.20Mn

30

Cu 0.15Mn Cu 0.10Mn

20

Cu 0.075Mn Cu 0.05Mn

10

Cu 0.01Mn MnO x

0 100

150

200

250

Temperature/° C

300

350

400


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Figure 10. In situ DRIFT spectra of Cu-Mn catalysts when exposed to CO stream at different temperatures.

(A) 1396

Kubelka-Munk(a.u.)

1070

1000

1306

1548

1205

1790

1638

1200

200 °C 175 °C 150 °C 125 °C 100 °C 75 °C 50 °C 25 °C

MnOx

1442 1495

0.0125

1400

1600

2119 2171

1800

2000

CO2 gas

2200

2400

2600

-1

Wavenumber/cm

(B) 1452 1395 1499

0.0125 1073

Kubelka-Munk(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1000

1312

Cu0.075Mn

1551

1199 1640

1200

1400

1600

1793

1800

2112 2173

2000 -1

2200

CO2 gas

200 °C 175 °C 150 °C 125 °C 100 °C 75 °C 50 °C 25 °C

2400

2600

Wavenumber/cm



Figure 11. In situ DRIFT spectra of Cu-Mn catalysts when exposed to CO and O2 (Dry air) mixed stream at different temperatures.

(A)

MnOx

0.025

gaseous CO

CO2 gas 200 °C

Kubelka-Munk(a.u.)

175 °C 150 °C 125 °C 100 °C 75 °C 50 °C 25 °C

1000

1200

1400

1600

1800

2000 -1

2200

2400

2600

Wavenumber/cm

(B) 0.025

CO2 gas

Cu0.075Mn gaseous CO

200 °C 175 °C

Kubelka-Munk(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

150 °C 125 °C 100 °C 75 °C 50 °C 25 °C

1200

1600

2000

2400

-1

Wavenumber/cm


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Page 45 of 48

Figure 12. In situ DRIFT spectra of adsorbed species under CO (10 vol%) and NO (5 vol%) mixed steam over the samples.

(A) 0.05

MnOX

Kubelka-Munk(a.u.)

1058

CO2 gas

1297 1277

350 °C 300 °C 250 °C 200 °C 150 °C 100 °C 50 °C 25 °C

2206 2172 2237 2113 1352

1030

1503

1000

1200

1400

1596 1630

1600

1800

2000

2200

-1

2400

2600

Wavenumber(cm )

(B) 0.05

Cu0.15Mn CO2 gas 1060

Kubelka-Munk(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2175 1791

2113

2098

350 °C 300 °C 250 °C 200 °C 150 °C 100 °C 50 °C 25 °C

2206 2240

1296 1274 1026

1348 1519 1601 1627

1000

1200

1400

1600

1800

2000

2200

2400

2600

-1

Wavenumber(cm )



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Scheme 1. Proposed Mechanism (Schematic Illustrations) for the Catalytic Oxidation of CO over Cu-Mn Catalysts.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 2. Proposed Mechanism (Schematic Illustrations) of NO Reduction by CO over Cu-Mn Mixed Oxides. (□: Surface Oxygen Vacancies; □: Surface Synergetic Oxygen Vacancies)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TOC Graphic

Figure 1 of 13

In the Cu-Mn catalysts, the Cux2+-Mnx3+-Oy could be changed into Cux2+-Mnx3+-[O(y-z)Ͼz] species in Cu1.5Mn1.5O4 spinel active phase for which takes up the major role in the CO oxidation. Furthermore, the surface dispersed Cux+-O2--Mny+ species could be reduced to Cu+-□-Mn(4-x)+ active species for which play an essential role in the CO-SCR model reaction.


Page 48 of 48

Preparation and Evaluation of Copper ... - ACS Publications

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