Oxide Catalysts for Low-Temperature - American Chemical Society

Subscriber access provided by GAZI UNIV

Article

Reaction characteristics of precious metal-free ternary Mn-Cu-M (M = Ce, Co, Cr, and Fe) oxide catalysts for low-temperature CO oxidation Ki-Hwan Choi, Dong Hee Lee, Hyo-Sub Kim, Young-Chan Yoon, Chu-Sik Park, and Young Ho Kim Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04985 • Publication Date (Web): 29 Mar 2016 Downloaded from http://pubs.acs.org on March 31, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research 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.

Page 1 of 23

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

Industrial & Engineering Chemistry Research

Reaction characteristics of precious metal-free ternary Mn-Cu-M (M = Ce, Co, Cr, and Fe) oxide catalysts for low temperature CO oxidation

Ki-Hwan Choi1, Dong-Hee Lee1, Hyo-Sub Kim1, Young-Chan Yoon1, Chu-Sik Park2 and Young Ho Kim1 1

Department of Chemical Engineering and Applied Chemistry, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon 34134, Republic of Korea

2

Hydrogen Energy Research Center, Korea Institute of Energy Research,71-2 Jang-dong, Yuseong-gu, Daejeon 34129, Republic of Korea

1

ACS Paragon Plus Environment


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

In this paper, we prepared binary Mn-Cu and M (M = Ce, Co, Cr, and Fe) added ternary Mn-Cu-M oxide catalysts using the redox precipitation method, and their catalytic performances in lowtemperature CO oxidation were investigated. The prepared catalysts were characterized using XRD, SEM, N2 adsorption/desorption isotherms, XPS, and H2-TPR techniques. It was observed that the CO oxidation performance at low temperature improved significantly when Cu and other dopants were added. The optimum molar ratio of Mn to Cu in a binary oxide catalyst was 1.0/0.1. The Mn-Cu-Co oxide catalyst had the highest activity, which was converted 90% of the CO at the relatively low temperature of 75 °C, among the prepared catalysts. It was concluded that the improvement of oxygen mobility and textural properties for Mn-Cu-Co catalyst are more favorably influenced on the enhanced catalytic activity.

Keywords: Low temperature CO oxidation, MnO2, Metal doping, Mn-Cu-Co mixed oxide

2


Page 2 of 23

Page 3 of 23

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


1. INTRODUCTION

CO is easily generated by the incomplete combustion of carbon compounds, and exposure to high concentrations of CO for long periods is fatal to humans because the bonds formed between CO and hemoglobin in blood inhibit the supply of oxygen.1,2 Catalytic carbon monoxide (CO) oxidation is a common method of removing toxic CO by converting it to innocuous CO2. The most effective catalysts for CO oxidation are novel metal catalysts such as Pt, Pd, Au, and Ru due to their excellent catalytic activity, even at low temperatures and ambient conditions.3-7 Compared to these novel metal oxide catalysts, transition metal-based catalysts have many practical advantages due to their cost efficiency, high availability, and high catalytic activity that results from to their ability to provide active oxygen species by changing oxidation states. Among the transition metal catalysts, manganese-based mixed oxide catalysts have recently been proposed as good candidates for oxidative CO removal without the use of precious metals. Manganese oxide catalysts have high oxygen storage capacities and exhibit high levels of activity in catalytic reactions that result in the elimination of CO by catalytic oxidation. Particular interest has been paid to hopcalite catalysts composed mainly of Mn and Cu. It has been reported that these catalysts’ high catalytic activity in CO oxidation could be attributed to the resonance system Cu2+ + Mn3+ ⇆ Cu+ + Mn4+ and the high adsorption of CO onto Cu2+/Mn4+ and of O2 onto Cu+/Mn3+.8-10 However, Mn-Cu oxide catalysts (hopcalite) have suffered from low catalytic activity under certain ambient conditions.11-13 To improve the catalytic activity of a Mn-Cu mixed oxide catalyst, it is generally agreed that controlling the preparation conditions, which include the precipitation method, ageing time, pH and temperature, is crucial because the catalyst’s structural, morphological and catalytic properties are strongly affected by the preparation process.14-18 Doping a transition metal oxide into a Mn-Cu oxide catalyst is also one of the common approaches to improving its CO oxidation reactivity by modifying its oxygen mobility. Peng et al.19 reported that the catalytic activity of 7% CuO/Ce1-xMnxO2 catalysts prepared by impregnation showed higher CO selectivity than a 5% Pt/Al2O3 catalyst in PROX due to increased oxygen mobility. Lu et al.20 also reported that partially substituting Ce for Mn in Mn-Cu oxide 3



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

catalysts resulted in higher activity for catalytic toluene oxidation due to enhanced oxygen transport. The main goal of this study is to combine the advantages of Mn-Cu and dopant systems by developing a ternary Mn-Cu-M (M = Ce, Co, Cr, and Fe) oxide catalyst with high activity in catalytic CO oxidation. Therefore, we prepared different Mn-Cu binary oxide catalysts with different ratios of Mn to Cu and confirmed the most appropriate one for catalytic oxidation of CO. After that, a series of M-added ternary Mn-Cu-M oxide catalysts was prepared. The catalytic performance for CO oxidation was measured and compared to that of prepared oxide catalysts. Additionally, prepared catalysts were characterized to investigate the relationship between physicochemical properties and reaction characteristics. The prepared catalysts were characterized using XRD, SEM, nitrogen adsorption desorption isotherms, XPS, and H2-TPR analysis.

2. EXPERIMENTAL SECTION

2.1. Preparation of the catalysts Potassium permanganate (KMnO4, Samchun, 99.3%), manganese(II) acetate tetrahydrate (Mn acetate, Sigma-Aldrich, >99%), Cu(II) nitrate trihydrate (Cu nitrate, Sigma-Aldrich, >99%), cobalt(II) nitrate hexahydrate (Co nitrate, Samchun, 97%), chromium(III) nitrate enneahydrate (Cr nitrate, Samchun, 98%), iron(III) nitrate enneahydrate (Fe nitrate, Samchun, 99%), and cerium(III) nitrate hexahydrate (Ce nitrate, Samchun, 99%) were used as starting materials without further purification. The MnO2, Mn-Cu and Mn-Cu-M (M = Ce, Co, Cr, and Fe) catalysts were synthesized using the redox precipitation method as follows: 5 g of KMnO4 was dissolved in 50 mL of deionized water (DI water). At the same time, 20.25 g of Mn acetate and a calculated amount of a dopant metal precursor were dissolved together in DI water for 1 h. After they had dissolved completely, a KMnO4 solution was slowly added to mixed metal precursor solution under stirring. The solution was stirred for 24 h at room temperature and brownish precipitant resulted. The precipitated product was filtered and washed several times using DI water and ethanol. The filtered resultant was dried under vacuum at 60 °C. The dried product was calcined in air at 300 °C for 2 h. The samples were denoted as Mn-Cu(x)

4


Page 4 of 23

Page 5 of 23

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


for binary catalyst, where x represents the nominal molar content of Cu in precursor solution (Mn:Cu = 1:x). The composition of M in Mn-Cu-M ternary catalysts was fixed to 0.05 (Mn:Cu:M = 1.0:0.1:0.05). Table 1 provides the nominal and bulk compositions of few selected catalysts. Bulk composition of the catalysts was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES, Optima 7300 DV, PerkinElmer) after being dissolved it in HCl/H2O2 solution.

Table 1. Atomic composition of MnO2, Mn-Cu(0.1) and M (M=Co, Ce, Cr, and Fe) doped catalysts Composition of [Mn]:[Cu]:[M]

Nomenclature

Nominal

Bulk*

MnO2

100:0:0

100:0:0

Mn-Cu(0.1)

90.9:9.1

90.8:9.2

Mn-Cu-Co

87.0:8.7:4.3

86.7:8.6:4.7

Mn-Cu-Ce

87.0:8.7:4.3

87.5:8.1:4.5

Mn-Cu-Cr

87.0:8.7:4.3

87.4:8.8:3.7

Mn-Cu-Fe

87.0:8.7:4.3

86.7:8.8:4.5

*Chemical analysis were obtained from ICP-AES.

2.2. Catalytic performance test A schematic diagram of the CO oxidation apparatus is shown in Figure 1. CO oxidation was performed in a Pyrex tubular reactor (outside diameter of 1.1 cm, total length of 36 cm). First, 100 mg of prepared catalyst with a size of 40 - 60 mesh was loaded in the center of the reactor. Then, it was heated under an Ar gas flow (30 mL min-1) at 300 °C for 1 h as a pretreatment. Once it had cooled to room temperature, it was re-heated to the desired reaction temperature, and then, 1% CO (20% air in Ar) was fed into the reactor for the CO oxidation reaction. The effluent gases of the reactor were simultaneously analyzed using a gas chromatograph (GC) equipped with a thermal conductivity detector (TCD) and a carbosphere column. No carbon-containing compounds other than the CO2 and unreacted CO in the effluent gases were detected for any of the oxide catalysts tested. Therefore, we calculated the CO conversion as follows:

5



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

CO conversion (%) =

x 100

where [CO]in is the CO concentration of the feed gas, and [CO]out is the CO concentration of the effluent gas.

Figure 1. A schematic diagram of the fixed bed reactor for CO oxidation.

2.3. Characterization of the catalysts The crystal structures of the prepared catalysts were analyzed using X-ray diffraction (XRD, D/max III-B, Rigaku) with CuKα radiation with a wavelength of 1.5418 Å. XRD analysis (10 < 2θ < 80o) was performed with a voltage of 40 kV and a current of 150 mA. Scanning electron microscopy (SEM, S4800, Hitachi) was used to analyze morphological properties. The specific surface areas of the catalysts were determined using a Micromeritics ASAP-2400 analyzer. Before the analysis, 50 mg of each sample was pretreated under vacuum at 300 °C for 12 h. Each sample’s nitrogen adsorption/desorption was analyzed at -196 °C using liquid nitrogen. Its specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method. X-ray photoelectron spectroscopy (XPS)

6


Page 6 of 23

Page 7 of 23

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


was used to investigate the chemical binding state and composition of the surface. XPS analysis was performed using a MultiLab 2000 (Thermo Electron Corp. Thermo Fisher Scientific) instrument. In addition, Al Kα was used as an X-ray source. The reduction properties of the prepared catalysts were studied using the temperature-programmed hydrogen reduction (H2-TPR) technique. First, 100 mg of catalyst was loaded into a quartz reactor (outside diameter = 1.1 cm, length = 36 cm); then, they were pretreated at 300 °C for 1 h under an Ar flow and cooled to room temperature. An H2-TPR analysis was performed under a stream of 5 vol% H2 gas in Aras the temperature increased from room temperature to 500 °C at a rate of 5 °C min-1. The effluent gases were analyzed in real time using a GC (DS-6200, Donam) equipped with a thermal conductivity detector (TCD).

3. RESULTS AND DISCUSSION

3.1. Structural and textural properties of the catalysts XRD analysis was performed to determine the crystalline structure of prepared catalysts after calcination at 300 °C in static air for 2 h (Figure 2). The main crystal phase of MnO2 with weak and broad peaks are visible at 2θ = 37.3

o

and 65.6 o; these imply that the MnO2 was of an amorphous

birnessite type (JCPDS 42-1317) and that nano-sized particles formed.22 Only the MnO2 phase was detected with low Cu contents for Mn-Cu(0.05) and Mn-Cu(0.1) catalysts, but the peaks of MnO2 were slightly shifted to lower angles indicates that the lattice volume increased due to the substitution of Cu for Mn.14 With the increase of the Cu contents, new CuO (JCPDS 80-1268) and Mn-Cu species with low crystallinity emerged, which is attributed to the Cu1.5Mn1.5O4 (JCPDS 35-1172) phase. Njagi et al.15 reported that incorporation of Cu into Mn oxide framework observed linear relationship up to 20 mol% but drastically decreased beyond that by redox precipitation method. This suggests that access amounts of Cu in Mn-Cu precursor may lead to the formation of Mn-Cu solid solution and CuO.15,29 Similar weak and broad diffraction peaks for Mn-Cu(0.1) and Mn-Cu-Co catalysts were observed. It suggests that Mn-Cu-Co is amorphous phase and Co dopant did not effectively affect the

7



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

structural changes of the MnO2 framework. The absence of Co related visible diffraction lines in the Mn-Cu-Co due to doped transition metal or any derivative hardly detectable at low doping contents (< 4.3%) or well dispersed on the surface of catalysts.31

Figure 2. XRD patterns of MnO2 and the Mn-Cu and Mn-Cu-Co oxide catalysts.

Figure 3 shows SEM images of the catalysts. All catalysts showed nano-sized irregular shape with porous structure morphology. It was observed that the size of catalysts were increased by increasing Cu contents in Mn-Cu precursor solutions, especially for the Mn-Cu(0.25) and Mn-Cu(0.5) catalysts. The particle may be due to the formation of Mn1.5Cu1.5O4 or CuO as observed in XRD results (Figure 2). However, the particle size and morphology of metal doped Mn-Cu-M catalysts were similar with that of the Mn-Cu(0.1).

8


Page 8 of 23

Page 9 of 23

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 3. SEM images of MnO2, Mn-Cu and Mn-Cu-M (M = Ce, Co, Cr, and Fe) oxide catalysts.

Figure 4(a) and (b) show the N2 adsorption/desorption isotherms of MnO2, Mn-Cu binary catalysts, and Mn-Cu-M ternary catalysts, and their textural properties were summarized in Table 2. The adsorption/desorption isotherms of the prepared catalysts for Mn-Cu catalysts were Type IV and Type II with, according to the IUPAC classification. Hysteresis loop of MnO2 was observed at a relative pressure of 0.6 to 1.0. The specific surface area and total pore volume for MnO2 were 189 m2·g-1 and 0.42 cm3·g-1. However, both specific surface area and total pore volume were decreased significantly to 32 m2·g-1 and 0.09 cm3·g-1 for Mn-Cu(0.5) with increasing Cu content in Mn-Cu catalysts. It may be due to the pore blocking or formation of different structure (Mn1.5Cu1.5O4) which were observed in Table 2 and Figure 2. These result were also associated with SEM results in Figure 3 showing that the particle size increased with the Mn-Cu ratio. Cho et al.21 also reported the similar results that the specific surface area of Cu-Mn mixed oxide decreased significantly from 205 m2·g-1 to 17 m2·g-1 when the molar ratio of Cu in Cu/(Cu+Mn) increased to 1 due to the formation of CuO and Cu-Mn phases. The effect of adding dopants to the Mn-Cu-M catalysts on the N2 adsorption/desorption isotherms is shown in Figure 4(b). Similar isotherm patterns corresponding to type IV were observed for all prepared ternary catalysts. The differences in the specific surface area and the pore characteristics 9



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

were confirmed in accordance with the type of dopant. In particular, specific surface areas and total pore volume (Table 2) of the Mn-Cu-Co (176 m2·g-1 and 0.75 cm3·g-1) and Mn-Cu-Fe (166 m2·g-1 and 0.71 cm3·g-1) catalysts are remarkably increased than that of the Mn-Cu(0.1) catalyst. These characterizations demonstrated that the textural properties of the Mn-Cu-M catalysts were influenced by dopants.

Figure 4. Nitrogen adsorption/desorption isotherms for MnO2, Mn-Cu, and Mn-Cu-M (M = Ce, Co, Cr, and Fe) oxide catalysts.

10


Page 10 of 23

Page 11 of 23

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 2. Specific surface area and total pore volume of prepared catalysts Catalyst

Textural properties 2

-1

SBET (m ·g )*

Vtotal (cm3·g-1)

MnO2

189

0.42

Mn-Cu(0.05)

111

0.28

Mn-Cu(0.1)

133

0.39

Mn-Cu(0.25)

39

0.15

Mn-Cu(0.5)

32

0.09

Mn-Cu-Ce

117

0.47

Mn-Cu-Co

176

0.75

Mn-Cu-Cr

140

0.36

Mn-Cu-Fe

166

0.71

*Specific surface area calculated by BET method.

XPS spectra of O 1s, Mn 2p, Cu 2p, and Co 2p are used to obtain detailed information the oxidation states of elements on the surfaces of the MnO2, Mn-Cu(0.1) and Mn-Cu-Co catalysts. As shown in Figure 5(a), two peaks displayed at the binding energy of 642.3±0.1 eV and 654.1±0.1 eV for the Mn 2p3/2 and Mn 2p1/2 in MnO2, Mn-Cu(0.1), respectively. The value is in the range of that for Mn4+ in literature15,32 and indicated that the prepared catalysts contained Mn in the Mn4+ state.14,21-23 The Mn 2p binding energy decreased when Co was added. The Mn 2p3/2 spectra can be deconvoluted into two peaks at 641.5 eV and 642.5 eV which is assigned to Mn3+ and Mn4+, respectively. The surface atomic ratio calculated by XPS result for Mn3+/Mn4+ was about 0.25 indicates that oxidation state of Mn decreased to 3.75. It was reported that the catalytic activity on oxidation reaction using manganese oxide is preferable for higher valence states of manganese29 and also affected by the transforming ability between Mn3+ and Mn4+, reflecting to oxygen mobility in the oxide lattice.8-10 The corresponding XPS spectra of O 1s shown in Figure 5(b) could be used to explain the effect of Co in Mn-Cu-Co catalyst for oxygen mobility. The binding energy of O 1s observed at 529.8 eV for MnO2 and Mn-Cu(0.1) catalysts is shifted to lower at 529.2 eV for Mn-Cu-Co catalyst. This indicates that the mobility of the lattice oxygen species was enhanced and it might be provide more surface active

11



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

oxygen species for oxidation reaction.15,34 Interestingly, Mn-Cu-Co catalyst was observed the best catalytic activity for low temperature CO oxidation among tested catalysts in this study, even though the oxidation state of Mn in Mn-Cu-Co catalyst was decreased slightly compared to that of MnO2 and Mn-Cu(0.1) catalysts. It suggests that the improvement of oxygen mobility and textural properties for Mn-Cu-Co catalyst are more favorably influenced on the enhanced catalytic activity. The Co 2p XPS analysis shown in Figure 5(c) confirmed the presence and the oxidation state of Co. The characteristic peaks of Co were not observed in the XPS spectra of the MnO2 and Mn-Cu(0.1) catalysts. In case of the Mn-Cu-Co catalyst, the Co 2p3/2 and Co 2p1/2 peaks were observed at 780.2 and 795.1 eV, respectively.26-28 Regarding the oxidation state of Co, it is difficult to know to use binding energy only because of binding energy for main Co 2p peak is almost similar among cobalt oxides and hydroxides such as CoO, Co2O3, Co3O4 and CoOOH.35 The Cu 2p XPS spectra observed in Figure 5(d) are characterized by two main lines of Cu 2p3/2 and Cu 2p1/2 peaked at 933.8 and 953.9 eV, respectively which are consistent with those reported by others.24,25 In the result of XPS of the MnO2 catalysts, the characteristic peaks of Cu were not observed. At present, however, the role of Cu in XPS study is unclear because the binding energies of Mn 2p, O 1s, and Cu 2p for MnO2 and Mn-Cu(0.1) are almost similar.

12


Page 12 of 23

Page 13 of 23

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 5. XPS spectra for Mn 2p, O 1s, Co 2p, and Cu 2p of MnO2, Mn-Cu(0.1), and Mn-Cu-Co catalysts.

Figure 6 shows the H2-TPR profiles of the MnO2 and series of Mn-Cu catalysts. According to previous reports,29 the reduction of manganese oxide is described by the following steps; MnO2 → Mn2O3 → Mn3O4 → MnO. The MnO2 catalyst prepared for this study showed a broad reduction profile at temperatures between 175 and 330 °C. The peaks maximum at 264 °C and 286 °C can be assigned of MnO2/Mn2O3 to Mn3O4 and Mn3O4 to MnO, respectively. The results are in good accordance with the TPR profiles for bulk MnOx catalysts reported previously.29,30 Compared with the TPR profiles of Mn-Cu catalysts and MnO2 catalyst, the reduction process began at lower temperatures and seemed to be more complicated. Many published studies for Mn-Cu mixed catalytic system reported that the reduction of Mn-Cu mixed oxide is completed at lower temperature compared with that of pure MnOx due to the existence of structural defects associated to oxygen vacancies or highly dispersed MnOx in mixed metal oxide.21,31,33 From the results, the shift of peak maxima for Mn-Cu(0.05) and Mn-Cu(0.1) from 264 °C to 250 °C and 233 °C may be due to Cu ions

13



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

distributed in the manganese lattice created a promoting effect with the oxygen species in the manganese lattice which caused enhanced mobility of the lattice oxygen species and improved reduction properties of Mn-Cu(0.05) and Mn-Cu(0.1) catalysts. The reduction profiles of the MnCu(0.25) and Mn-Cu(0.5) oxide catalysts were observed at lower temperatures. The observed broad shoulder peaks maxima at 211 °C and 203 °C in the low temperature side could be related to the reduction of copper species.23,31,36 These results were associated with the XRD results in Figure 2 that showed the characteristic peaks of CuO and Cu1.5Mn1.5O4 for the Mn-Cu(0.25) and Mn-Cu(0.5) catalysts. By considering the reduction results, the presence of Cu species with improved reducibility might be contributed to the enhancement of CO oxidation at low temperature but access amounts of Cu addition is hampered by decreasing contents of active Mn-Cu component in catalysts.

Figure 6. H2-TPR profiles for MnO2 and the Mn-Cu oxide catalysts.

Figure 7. shows the H2-TPR profiles of the Mn-Cu-M oxide catalysts. Similar with the Mn-Cu catalysts, broad and complicated reduction profiles were observed. All the Mn-Cu-M oxide catalysts exhibited low-temperature reduction peaks shifted approximately 8–23 °C less than where the corresponding peaks for Mn-Cu(0.1) appeared. This indicates that doping of small amounts of metal makes Mn-Cu catalysts facilitate reduction. 14


Page 14 of 23

Page 15 of 23

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 7. H2-TPR profiles for the Mn-Cu-M (M = Co, Fe, Cr, Ce) oxide catalysts.

3.2. Catalytic performance for CO oxidation The experiments on CO oxidation were conducted to clarify the effect of Cu contents on catalytic performance of MnO2 and series of Mn-Cu catalyst. As shown in Figure 8, CO conversions of 15% and 83% were observed for MnO2 catalyst at 100 °C and 150 °C, respectively. The results shows the increasing catalytic activity by 100% CO conversion at a reaction temperature of 150 °C Mn-Cu catalysts. The Mn-Cu oxide catalysts performed significantly better than that of MnO2 catalyst. This result indicates that adding Cu to manganese oxide improved its catalytic activity for CO oxidation. However, the catalytic activities at 100 °C are significantly influenced depending on the contents of Cu. It was surprising to find that increasing the amounts of Cu more than 20% in Mn-Cu catalysts did not affect the catalytic performance, but lead to a sharp decrease in catalytic activity to 89, 84 and 53% for Mn-Cu (0.25), Mn-Cu(0.5), and Mn-Cu (1), respectively. These results imply that Mn-Cu mixed oxide plays a more important role in CO oxidation rather than CuO. In particular, Mn-Cu(0.1) converted 100% of the CO at a reaction temperature of 100 °C. From the results, the optimal Cu/Mn molar ratio of 0.1 (Mn-Cu(0.1)) for CO catalytic oxidation was confirmed under these experimental conditions.

15



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

Page 16 of 23

Figure 8. Effect of Cu content on the catalytic performance of MnO2 and the Mn-Cu catalysts in the CO oxidation reaction after 50 min on-stream. Catalyst: 100 mg, feed gas: 1% CO, 20% air in Ar, 18,000 cm3·g-1·h-1 GHSV. Each empty and full symbol represents Mn-Cu catalysts; Mn-Cu (0.05),

Mn-Cu (0.1),

Mn-Cu(0.25),

Mn-Cu (0.5), and

MnO2,

Mn-Cu (1).

The catalytic performance of Mn-Cu-M (M = Ce, Co, Cr, and Fe) catalysts for CO oxidation were performed to investigate the effect of third components. As shown in Figure 9, the catalytic activities of Mn-Cu-M catalysts were enhanced at low temperatures. The Mn-Cu (0.1), Mn-Cu-Co and Mn-CuFe catalysts converted the CO completely at a reaction temperature of 100 °C, whereas the MnO2 catalyst converted only 15% under the same reaction conditions. The values of T50, T70 and T90, which are the reaction temperatures at which the amount of CO converted reached 50%, 70% and 90%, respectively, are shown in Table 3. The values of T70 are in the following order: Mn-Cu-Co < Mn-CuFe < Mn-Cu (0.1) < Mn-Cu-Cr < Mn-Cu-Ce

Oxide Catalysts for Low-Temperature - American Chemical Society

Recommend Documents