CO oxidation over MnO2 catalysts prepared by a

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JIEC-2282; No. of Pages 8 Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx

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CO oxidation over MnO2 catalysts prepared by a simple redox method: Influence of the Mn (II) precursors Jung-Hyun Park a, Dong-Chang Kang a, Sang-Jun Park b, Chae-Ho Shin a,* a b

Department of Chemical Engineering, Chungbuk National University, Chungbuk, 361-763, South Korea Development team for Environmental Solution Division, ECOPRO, Ochang-eup, Cheongwon-gun, 363-886, South Korea

A R T I C L E I N F O

Article history: Received 26 August 2014 Received in revised form 14 October 2014 Accepted 3 November 2014 Available online xxx Keywords: MnO2 Redox method Mn precursors CO-pulse experiment Labile oxygen species

A B S T R A C T

MnO2 was prepared by a simple redox method using potassium permanganate and various Mn2+ precursors (Mn-chloride, Mn-nitrate, Mn-acetate, and Mn-sulfate) and the influence of Mn-precursors in the CO oxidation was investigated. The light-off temperatures (T50%) in the dry conditions increased as following order: Mn-chloride < Mn-nitrate < Mn acetate < Mn-sulfate. In the presence of CO2 and H2O vapor, the T50% value significantly shifted to higher temperatures compared to its value in the absence of them. MnO2 catalyst prepared from Mn-chloride exhibited a high quantity of labile oxygen and better reducibility compared with the other catalysts. ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

Introduction The catalytic oxidation of carbon monoxide (CO) at low temperature is an important reaction due to its various applications in industry and in environmental fields [1–4]. The supported noble metal catalysts, such as Pt, Pd, and Au, have been widely used as a catalyst for the removal of CO for several decades due to their high catalytic performance [5–9]. However, the high cost and low thermal stability of the noble metals inhibit their use in practical applications for the catalytic oxidation of CO. Compared to the noble metal catalysts, unsupported or supported transition metal oxides are low in cost with high thermal stability and are considered to be potential candidates for some practical applications [10,11]. Therefore, it is of great scientific and practical interest to investigate the physicochemical properties of transition metal oxides and to improve their catalytic performance. Among the transition metal catalysts, manganese oxides (MnO2) have been extensively studied as catalysts and materials in batteries due to their high electron storage capacity, low cost, environmental friendliness, and natural abundance [12–14]. In addition, MnO2 exhibits good oxygen storage/release ability due to its easy reduction–oxidation cycles through its interaction with

* Corresponding author. Tel.: +82 43 261 2376; fax: +82 43 269 2370. E-mail address: [email protected] (C.-H. Shin).

reducing and oxidizing agents [15]. MnO2 can form different structural phases, such as a–, b–, g–, d- and e-MnO2, when its octahedral basic unit [MnO6] is linked together in different ways. b-MnO2 has a 1 1 (2.3 2.3 A˚) tunnel, while g-MnO2 and aMnO2 possess 1 2 (2.3 4.6 A˚) and 2 2 (4.6 4.6 A˚) tunnels, respectively [16,17]. Of these different structural phases, b-MnO2 is the most stable phase of MnO2 [18]. The different polymorphic forms are believed to be responsible for their physicochemical properties and catalytic performance. For example, Xu et al. [12] reported that a-MnO2 and b-MnO2 nanowires/nanorods were effective catalysts for CO oxidation, and that a-MnO2 exhibited a higher catalytic performance than b-MnO2 due to the better reducibility of a-MnO2 nanowires. Jin et al. [19] reported that a gMnO2 octahedral molecular sieve showed excellent catalytic performance and good selectivity for toluene oxidation. Furthermore, Li et al. [20] reported that a-MnO2 catalyst was superior to b-MnO2 and g-MnO2 for acetaldehyde removal. The structural, morphological, and catalytic properties of MnO2 catalysts are significantly influenced by the preparation methods used. Extensive efforts have been made to control the synthesis method to obtain MnO2 with desirable morphologies and crystal structures to improve the catalytic performance and characteristics, and various synthesis methods have been proposed, e.g. sol– gel [21,22], reflux [23,24], hydrothermal [25,26], and redox method [27–29], etc. Recently, Frey et al. have synthesized the non-stoichiometric Mn oxides (MnOx and MnOy) having high

http://dx.doi.org/10.1016/j.jiec.2014.11.001 1226-086X/ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

Please cite this article in press as: J.-H. Park, et al., J. Ind. Eng. Chem. (2014), http://dx.doi.org/10.1016/j.jiec.2014.11.001

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specific surface areas via temperature-programmed oxidation of Mn-oxalates [30]. They reported that both catalysts showed high catalytic performance for CO oxidation at room temperature and had identical CO oxidation reaction rates. In this study, MnO2 catalysts were prepared by a redox reaction between potassium permanganate (KMnO4) and various Mn2+ precursors, and their catalytic performance for CO oxidation under different reaction conditions was studied. The redox method allowed for room temperature synthesis of MnO2 catalysts with high surface areas, and high catalytic performances for CO oxidation. The effects of the different Mn2+ precursors on the physicochemical characteristics and catalytic performance for CO oxidation of the MnO2 catalysts were investigated. Experimental Preparation of the catalysts All the chemicals used in this study were of analytical grade and were used without any further purification. The MnO2 catalysts were prepared by the redox method using potassium permanganate (KMnO4, Samchun, 99.3%), manganese chloride (MnCl24H2O, Junsei, 98%), manganese nitrate (Mn(NO3)26H2O, Junsei, 97%), manganese acetate (Mn(CH3COO)4H2O, Junsei, 97%), and manganese sulfate (MnSO4H2O, Samchun, 98%). The typical procedure to synthesize the catalysts was as follows: KMnO4 (0.03 mol) and various Mn precursors (0.045 mol) were individually dissolved in deionized water (75 mL). The Mn precursor solution was then added to the KMnO4 solution under vigorous stirring. At that time, we did not control the pH value using any precipitants. The pH value of the mixture solution was ca. 1. It is speculated that these phenomena are caused by the acidic materials formed during the redox reaction. The resulting suspensions were aged for 18 h under vigorous stirring at room temperature. The final products were filtered and washed two times with 3 L of deionized water. The filtered-product was then dried at 100 8C for 12 h and calcined at 300 8C for 2 h in flowing air. The catalysts are denoted by MnO2-C (Mn-chloride as a precursor), MnO2-N (Mn-nitrate as a precursor), MnO2-A (Mn-acetate as a precursor), and MnO2-S (Mn-sulfate as a precursor). Catalyst characterization The bulk and crystalline structure of the catalysts was recorded by powder X-ray diffraction (XRD) using a Rigaku Ultima IV diffractometer with Ni-filtered CuKa radiation (1.5418 A˚) operating at 40 kV and 40 mA at a scan rate of 1.28 min1. Thermogravimetric analyses (TGA) were performed on a TA Instruments SDT 2960 analyzer. The sample was heated from 30 8C to 800 8C at a heating rate of 10 8C min1 in flowing air. The scanning electron microscope images and energy dispersive spectroscopy (SEM– EDS) of the catalysts were acquired on a Hitachi S-2500C scanning electron microscope at an acceleration voltage of 5 kV. The surface areas and total pore volume of the samples were determined from the adsorption and desorption isotherms at liquid nitrogen temperature, 196 8C with a volumetric method using Micromeritics ASAP 2020 instruments. Prior to the measurements, the 0.2 g catalysts were degassed under a flow of N2 at 150 8C for 4 h. The Brunauer–Emmett–Teller (BET) method was used to determine the surface area that was measured at P/Po values in the range of 0.05–0.20. The total pore volumes were measured at P/Po = 0.995. The average pore diameters were calculated using the equation pore diameter = 4 total pore volume/BET surface area assuming that the pore shape is cylindrical. The method of Barrett, Joyner, and Halenda (BJH) was used to determine the pore size distribution of the catalysts by using the adsorption isotherms.

X-ray photoelectron spectroscopy (XPS) measurements were carried out on a PHI Quantera II spectrometer with an Mg Ka X-ray source (1253.6 eV). Each sample was degassed under 1 106 torr for 4 h to remove the contaminants and the pressure inside the analysis chamber was maintained under 5 1010 torr. The binding energies were calculated with reference to the energy of the C 1s peak at 284.6 eV from adventitious carbon. The reduction properties of the MnO2 catalysts were studied by temperature-programmed reduction of H2 or CO (H2- or CO-TPR) through mass spectroscopy (MS, Pfeiffer vacuum QMS 200). The reactor was made up of a quartz U-shape tube (I.D. 12 mm) on densely packed quartz wool. Each sample (50 mg) was pretreated at 300 8C for 1 h under Ar flow (30 cm3 min1) and then cooled down to RT. It was then reduced by increasing the temperature to 600 8C at a ramp rate of 10 8C min1 in stream of 5% H2/Ar or 5% CO/ Ar (30 cm3 min1). The mass signals of m/z = 2 (H2) and 44 (CO2) were detected by MS. Temperature-programmed desorption of CO (CO-TPD) experiments were carried out in order to elucidate the adsorption– desorption behavior of CO and H2O. Prior to analysis, a 50 mg catalyst was loaded in a quartz reactor and pretreated at 300 8C in an Ar flow (30 cm3 min1) for 1 h, and then cooled down to RT under the same atmosphere. After cooling to RT, the adsorbents, (i) 5% CO, and (ii) 3% H2O adsorption, followed by 5% CO adsorption, were passed through on the catalyst bed at a flow rate of 30 cm3 min1 for 0.5 h. The samples were heated up to 600 8C at a ramp rate of 10 8C min1. The mass signal of m/z = 44 (CO2) was detected by MS. The CO-pulse response experiments were performed at 190 8C, at atmospheric pressure using the fixed bed reactor. Each sample (0.1 g) was placed in the U-shape quartz reactor with densely packed quartz wool. All catalysts were first pretreated at 300 8C under Ar flow (30 cm3 min1) for 0.5 h and subsequently cooled down to the reaction temperature. After that, pulse experiments were carried out by contacting fixed amounts of CO with the catalyst using Ar as a carrier gas. The pulse experiments were repeated five times and each pulse contained 3.6 mmol of CO. The outlet gases were detected with the signal of m/z = 44 (CO2). Catalytic performance tests Catalytic performance tests were carried out in a fixed bed reactor (I.D. 12 mm) at atmospheric pressure. The 0.1 g catalyst sample (40–60 mesh) was loaded in the quartz reactor and pretreated at 300 8C for 1 h in N2 flow, and then cooled down to 30 8C. The reactant compositions consisted of 1 vol% CO, 4 vol% O2, 0 vol% (dry conditions) or 3 vol% H2O (wet conditions) and balanced N2 with a total flow rate of 200 cm3 min1; H2O vapor was introduced continuously through the saturator. The conversion of CO in the reaction was calculated in terms of the percentage consumed. The concentration of CO in the product stream was measured every second by using an on-line CO analyzer (TELEDYNE 7600 Instrument Analyzer of IR-ways). Results and discussion Characterization of the catalysts Fig. 1 shows the XRD patterns of the MnO2 catalysts. The prepared catalysts present similar peaks at 2u = 37.5 and 66.78, which was identified as amorphous birnessite-type MnO2 (JCPDS 42-1317) [31]. In addition, all the catalysts did not show any strong crystalline peaks. This means that the prepared catalysts were not fully crystallized, with a large segment present as an amorphous phase. The TGA profiles of the catalysts heated in the air are shown in Fig. 2. The thermal behavior of all the catalysts, except for the


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Fig. 1. XRD patterns of (a) MnO2-C, (b) MnO2-N, (c) MnO2-A, and (d) MnO2-S catalysts calcined at 300 8C.

MnO2-A catalyst, is very similar, the only difference being the degree of weight loss between 30 8C and 800 8C. Four major weight losses were observed for the MnO2-C, MnO2-N, and MnO2-S catalysts. The first major weight loss at 200 8C resulted from the removal of physically adsorbed H2O. The second weight loss at 200–490 8C was due to the desorption of chemically absorbed H2O. The rapid weight loss at ca 500 8C was attributed to oxygen release upon transformation to Mn2O3. The last weight loss above 520 8C could be assigned to the evolution of lattice oxygen. The TGA profile for the MnO2-A catalyst showed the three weight loss stages and one weight increase stage in these temperature ranges. The three weight loss stages were very similar to those of other catalysts, which consisted of the loss of physically adsorbed H2O, followed by chemically adsorbed H2O and the evolution of lattice

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Fig. 2. TGA profiles of (a) MnO2-C, (b) MnO2-N, (c) MnO2-A, and (d) MnO2-S catalysts heated in a stream of air.

oxygen. A slight weight increase was observed between 450 and 560 8C. It might be attributed to the buoyance force of acetate anion during the analysis. Some authors reported that the weight increase was attributed to significant oxidation of Mn3O4 [32]; however, the cause of this phenomenon is currently unclear. Fig. 3 shows SEM images of the MnO2 catalysts. The MnO2-C and MnO2-N catalysts displayed a similar morphology with an irregular shape and porous structure (Fig. 3(a) and (b)), and the diameter of the particles was mostly ca. 30 nm. In the MnO2-A catalyst, most of the particles showed closely packed sphericaltype morphology (Fig. 3(c)), and the microspheres were roughly 2– 4 nm in diameter. The MnO2-S catalyst was flower-like in its morphology (Fig. 2(d)), and the diameter of the flower-like sphere was typically ca. 1.5 mm. The SEM images indicate that the Mn precursors obviously play an important role in determining the

Fig. 3. SEM images of (a) MnO2-C, (b) MnO2-N, (c) MnO2-A, and (d) MnO2-S catalysts.


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Table 1 Physicochemical properties and catalytic performance of MnO2 catalysts calcined at 300 8C. Catalyst

MnO2-C MnO2-N MnO2-A MnO2-S a b c d

SBET (m2 g1)

174 151 260 56

Pore volume (cm3 g1)

0.320 0.267 0.205 0.113

Elemental analysis (mol%)a

Temperature at a fixed conversion of CO (8C) Dry conditionsb

Wet conditionsc

K

S or Cl

T50%

T90%d

T50%

T90%

5.52 4.01 5.01 4.65

0.55 – – 0.84

84 104 127 197

134 169 267 263

182 187 186 214

212 226 – 248

Determined from EDS analysis. 0.1 g catalyst, CO/O2/N2 (vol%) = 1/4/95, total flow rate = 200 cm3 min1. Continuous addition of 3 vol% H2O vapor. Temperature reached to 50% and 90% conversions of CO, respectively.

morphology of the resulting MnO2 catalyst. The EDS analysis observed K cations of 2–6 mol% and S or Cl anions below 1 mol% (Table 1), which indicates that the cations or anions were not completely washed during the preparation of the MnO2 catalysts. The surface area and total pore volume of the MnO2 catalysts are summarized in Table 1. The N2 sorption isotherms of all the catalysts can be classified as being type IV with hysteresis loops at P/P0 = 0.4–0.8, which is typical for mesoporous materials. However, the hysteresis loop of MnO2-S is much smaller than those of other catalysts due to the formation of large mesopores or macropores in the MnO2-S catalyst (not shown). The surface areas of the MnO2 catalysts were observed to be in the range of 56– 260 m2 g1, for the MnO2-A catalyst exhibiting the largest BET surface area of 260 m2 g1. The pore volume of the MnO2-C catalyst is 0.320 cm3 g1, which is higher than the values of 0.267, 0.205, and 0.113 cm3 g1 of the MnO2-N, MnO2-A, and MnO2-S catalysts, respectively. The pore size distributions of all the catalysts, with the exception of the MnO2-S catalyst, show multimodal pores. Both the MnO2-C and MnO2-A catalysts exhibit a bimodal pore size distribution, one mode consisting of broad pores in the range of 1– 10 nm and the other centered at 14.2 nm. The pore size distribution shows that the MnO2-C catalyst is mesoporous and relatively uniform with pores centered at 1.8 nm and 3.8 nm. The MnO2-S has no distinct pore size distribution in the ranges of the analysis. It is known that the surface area of a catalyst is one of the important parameters which determine the catalytic performance. In this work, the MnO2-C catalyst showed the most superior catalytic performance. However, the surface area of the MnO2-C catalyst appeared to have a medium-range value (174 m2 g1) compared to the others. This result indicates that there was no direct correlation between the BET surface area of the catalyst and the catalytic properties. That is to say, other factors appear to be responsible for the better performance of the MnO2-C catalyst. XPS analyses were carried out to identify the oxidation state of Mn (Fig. S1 in the supplementary data). The Mn 2p band showed that the binding energies of 2p3/2 and 2p1/2 are 642.2 0.1 eV and 653.9 0.1 eV, respectively, with spin–orbit separation of 11.6 0.1 eV which indicates that the Mn of the samples possess the oxidation state of Mn(IV) (Fig. S1(A)). In Fig. S1(B), two kinds of oxygen species are observed on the O 1s XPS spectrum. The one peak observed around 529.7 0.1 eV region was assigned to oxygen involved in Mn–O–Mn bonds for the tetravalent oxide and the other (531.5 0.1 eV) might be attributed to the interaction between surface hydroxides and Mn (Mn–OH) [33,34]. CO-TPR measurements were carried out to investigate the reduction properties of the catalysts (Fig. 4(A)). All the catalysts showed three reduction peaks which can be classified as being at 190–220 8C (peak I), 240–270 8C (peak II), and 384–402 8C (peak III). The maximum CO2 formation (CO consumption) occurred at 190–220 8C (peak I). Peak I is ascribed to the reaction between the CO and the labile oxygen species of the catalysts [35]. Peak I in

all samples excluding the MnO2-S catalyst is observed to occur at the same temperature and exhibited the highest intensity in the MnO2-C catalyst. The high intensity of peak I could suggest a higher quantity of labile oxygen species, and a higher quantity of labile oxygen species leads to a higher catalytic performance for CO oxidation. The intensity of the labile oxygen decreased in the order of MnO2-C > MnO2-N > MnO2-A > MnO2-S, in basic agreement with the sequence of the catalytic performance of these catalysts. Peaks II and III can be assigned to the reduction of Mn2O3 to Mn3O4 and the transformation of Mn3O4 to MnO, respectively [36]. The temperature of peak II is shifted to a higher temperature, indicating

Fig. 4. (A) CO-TPR and (B) H2-TPR profiles of (a) MnO2-C, (b) MnO2-N, (c) MnO2-A, and (d) MnO2-S catalysts.


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that the reducibility of Mn species in MnO2 catalysts is lowered in the following order: MnO2-C > MnO2-N > MnO2-A > MnO2-S. In the case of peak III, the MnO2-A catalyst exhibited the highest temperature, suggesting a lower reducibility in comparison with the other catalysts. Similar results are observed in the H2-TPR measurements (Fig. 4(B)). All the catalysts have two reduction peaks and can be classified into two categories, Mn2O3 to Mn3O4 (a peak), and Mn3O4 to MnO (b peak) [37,38]. Taking into consideration the ending reduction temperature, the reducibility of the MnO2 catalysts decreased with the same tendency as the COTPR results. It is generally known that a higher reducibility means a higher mobility of oxygen species [39]. Thus, it can be deduced that the MnO2-C catalyst has the highest oxygen mobility. Catalytic performance The CO conversion profiles in the absence (dry condition) or presence of H2O vapor (wet condition) and the reaction temperatures T50% and T90% (corresponding to CO conversions of 50% and 90%, respectively) over MnO2 catalysts are shown in Fig. 5 and Table 1. It is clear that the nature of the Mn precursors would have influence on the performance for CO oxidation. The MnO2-C catalyst exhibited the highest catalytic performance compared to the other catalysts; the T50% result of the catalyst was 84 8C, and increased in the following order: MnO2-C < MnO2-N < MnO2A < MnO2-S. For the verification of the effect of H2O on CO oxidation, 3% H2O was introduced into the reactant stream (Table 1). The catalytic performances were significantly decreased in the presence of H2O vapor and showed somewhat different behavior compared to the results for dry conditions at lower temperatures. It is observed that the MnO2-A catalyst was better at lower temperatures (80–180 8C) than the other catalysts, but inferior above 180 8C compared to the other catalysts in terms of the catalytic performance. The reproducibility tests over the MnO2-C catalyst, which has seen the best catalytic performance in the CO oxidation among the catalysts, were carried out using one sample and the results are shown in Fig. S2. The reactions were repeated three times under dry reaction conditions. Fig. 6 shows that the complete CO conversion for each run was achieved below 250 8C and deactivation was not observed during the repetitive reactions. For CO oxidation, the catalyst was reduced by the supplied CO gas, and the reduced catalyst could be replenished by the gas phase oxygen [32]. Thus, the re-oxidation ability of the catalyst is an important

Fig. 5. CO conversion profiles as a function of the reaction temperature of (a) MnO2C, (b) MnO2-N, (c) MnO2-A, and (d) MnO2-S catalysts. Reaction conditions: 0.1 g catalyst, total flow rate = 200 cm3 min1, CO/O2/N2 (vol%) = 1/4/95.

Fig. 6. (A) CO conversion profiles and (B) XRD patterns of MnO2-C catalyst calcined at different temperatures; (a) 300 8C, (b) 400 8C, (c) 500 8C, and (d) 600 8C.

factor in determining the catalytic activity and stability. From the result of the tests, it can be seen that the MnO2-C catalyst possesses the property of facile re-oxidation and could be a promising candidate for CO oxidation. The effect of calcination temperature on the catalytic performance of MnO2-C catalyst is shown in Fig. 6(A). The test results revealed that the catalytic performance of MnO2-C catalyst decreased with increasing the calcination temperature from 300 8C to 600 8C and the T50% value increased in the order of 300 8C < 400 8C 500 8C < 600 8C. In addition, Fig. 6(B) shows the XRD patterns and surface areas of MnO2-C catalyst with different calcination temperatures. With increasing the calcination temperatures, the crystalline structures of MnO2-C were transformed to pure a-MnO2 phase or the mixture phase of a-MnO2 and Mn2O3 from birnessite-type and the crystallinity of a-MnO2 and Mn2O3 increased. The sample calcined at 400 8C showed a pure a-MnO2 phase and the samples at above this temperature, the Mn2O3 phase was observed as a predominant phase than a-MnO2 phase. Increase in crystallinity was caused a decrease in the surface areas of MnO2-C catalysts. It was reported that large surface area of the samples favors in the catalytic performance for oxidation reaction [40], and the MnO2-C catalyst calcined at 300 8C possess the highest surface area; thus we believe that the surface area was partially influenced the catalytic performance. Although the sample calcined at 400 8C has higher surface area compared to that of 500 8C, the T50% reveals similar value; this result is


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speculated by the difference of crystalline structure and of interaction between CO and Mn3+ or Mn4+ [39]. In addition, the oxygen mobility and reduction property were important factors to influence the catalytic performance in the CO oxidation. The results of CO-TPR analyses depict in Fig. S3. The reduction temperatures were shifted to higher temperature with increasing the calcination temperatures. This result indicates that the oxygen mobility of the catalysts was gotten worse with respect to the calcination temperature, and was good agreement with the tendency of catalytic performance. Therefore, the decrease in oxygen mobility and the difference of crystalline structures were caused a decrease in the catalytic performance of MnO2-C catalysts. In order to investigate the influence of the feed composition on the catalytic performance over the MnO2-C catalyst, the oxidation of CO under CO2 and H2O feeding or CO2 and H2O cofeeding was performed as a function of the reaction temperature, as shown in Fig. 7. The dry reaction conditions were CO/O2/N2 = 1/4/95 with total flow rate of 200 cm3 min1. In addition, 3% CO2 or H2O was continuously introduced in the catalyst bed. As aforementioned, the T50% of the MnO2 catalyst under dry conditions was 84 8C and complete CO oxidation was achieved below 250 8C. Upon addition of CO2 to the feed, T50% was shifted to ca. 145 8C in comparison to that of dry conditions; however, complete CO oxidation showed a similar pattern to that of the dry conditions. The CO conversion was significantly shifted to a higher temperature in the presence of H2O or CO2 and H2O. Temperature-programmed CO oxidation was reconducted after stopping the addition of CO2 and H2O; the catalytic performance in this reaction condition showed similar result with under dry conditions (not shown). The results show that the addition of external H2O vapor and CO2 plays an inhibiting role in the CO oxidation rather than a deactivating role, and that H2O vapor has a stronger inhibiting effect than that of CO2 alone. The higher-temperature shifts of the CO oxidation temperature were attributed to the competitive adsorption of CO2, H2O, and CO on the active sites of the catalyst [41–43]. In order to probe the poor catalytic performance in the presence of CO2 or H2O vapor, CO-TPD was carried out. The CO-TPD profiles of the MnO2-C catalyst after only CO adsorption and CO2 or H2O adsorption followed by CO adsorption are shown in Fig. 8. The CO2 produced from the reaction of adsorbed CO and oxygen species of the catalyst was detected by MS. The quantity of CO2 desorption after only CO adsorption was larger than that after CO2 or H2O and

Fig. 7. CO conversion profiles with various feed composition over MnO2-C catalyst. Reaction conditions: 0.1 g catalyst, total flow rate = 200 cm3 min1, CO/O2/N2 (vol%) = 1/4/95 (Dry), and the addition of 3% CO2 or/and 3% H2O.

Fig. 8. (A) CO-TPD profiles of the MnO2-C catalyst after the adsorption of only CO, and CO and CO2 or H2O co-adsorption and (B) H2O-TPD profile after the adsorption of only H2O.

CO adsorption. The order of CO2 evolution was as follows; only CO > CO2 + CO > H2O + CO. This result indicates that the supplied CO2 or H2O and CO molecules competitively adsorb on the active sites of the catalyst and are in good agreement with the CO conversion for various feed conditions. Therefore, the large difference in the T50% results could be attributed to the competitive adsorption of CO and CO2 or H2O molecules on the same active sites as those for CO oxidation [43]. To further verify the desorption behavior of H2O vapor, H2O-TPD analysis was performed and the result exhibits in Fig. 8(B). The H2O-TPD was followed the same procedure as CO-TPD analysis. The mass signal of H2O (m/z = 18) started to desorb immediately after the onset of the temperature increase, and showed maxima at around 140 8C. In addition, the H2O signal was observed until the temperature reached at 500 8C. It means that the adsorbed H2O vapor was not completely desorbed at reaction temperature. Thus, decrease in catalytic performance can be deduced resulting from the strong adsorption of H2O vapor on the active sites. The oxygen vacancies and mobility of the catalyst are crucial factors affecting the catalytic performance in CO oxidation. A number of studies have reported the occurrence of a Mars van Krevelen (MvK) mechanism during CO oxidation [32,44]. To explore the quantity of oxygen vacancies in the catalysts, CO-pulse experiments were carried out at 190 8C. Fig. 9(A) shows the profile of the CO2 formation after the CO injection with respect to time.


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Mn2+ precursors. The catalysts have large specific surface areas in the range of 56–260 m2 g1 and show structural features in birnessite-like amorphous MnO2. The catalytic performance for CO oxidation in the absence or presence of H2O vapor in the reactant stream was in the following order: Mn-chloride > Mn-nitrate > Mn-acetate > Mn-sulfate. This was in fairly good agreement with the tendency towards lower reduction temperatures and high oxygen mobility in the H2-TPR experiments and of the quantity of labile oxygen species in the CO-pulse experiments. These results led us to the conclusion that better reducibility and a higher quantity of labile oxygen species were responsible for the higher catalytic performance of the MnO2 catalysts. Acknowledgment This work was supported by the Energy Efficiency and Resources of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry and Energy, Republic of Korea (No. 20132020500100). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jiec.2014.11.001. References

Fig. 9. (A) CO2 formation profiles during the CO-pulse reaction without oxygen, and (B) correlation curve between the T50% value in dry conditions and the amount of formed CO2. Reaction conditions: 0.1 g catalyst, T = 190 8C, total flow rate = 30 cm3 min1, and each pulse contained 3.6 mmol of CO.

CO2 was formed from the reaction between the supplied CO and the oxygen species of the catalysts as following equation, CO + &oxygen = CO2, where &oxygen refer to the surface or lattice oxygen species of the catalyst. The peak intensity and area of the CO2 formed during the reaction decreased in the following order: MnO2-C > MnO2-N > MnO2-A > MnO2-S. To investigate the correlation between the catalytic performance and the quantity of CO2 formed during the CO-pulse reaction, the correlation curve between the T50% value in dry conditions and CO2 formation was plotted and is shown in Fig. 9(B). The quantity of CO2 refers to the oxygen vacancies of the catalysts, which can be calculated by integrating the observed CO2 peak. The MnO2-C catalyst shows the largest quantity of CO2, and this value linearly decreases with increasing T50%. This result indicates that the MnO2-C catalyst has the largest quantity of oxygen vacancies, and that this is well correlated with the catalytic performance. Conclusions In this study, MnO2 catalysts were synthesized by employing a facile redox method using KMnO4 and various Mn2+ precursors. A simple redox method allowed for room temperature synthesis of the MnO2 catalysts. The physicochemical properties and catalytic properties for CO oxidation highly depended on the nature of the

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