Promotion of H2O2 decomposition activity over β-MnO2 nanorod catalysts

Colloids and Surfaces A: Physicochem. Eng. Aspects 304 (2007) 60–66

Promotion of H2O2 decomposition activity over ␤-MnO2 nanorod catalysts Weixin Zhang ∗ , Hua Wang, Zeheng Yang, Fang Wang School of Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, People’s Republic of China Received 24 November 2006; received in revised form 28 March 2007; accepted 12 April 2007 Available online 18 April 2007

Abstract The relationship among the ␤-MnO2 nanorod preparation conditions, morphology and catalytic performance on H2 O2 decomposition has been investigated in the work. ␤-MnO2 nanorod catalysts were prepared by heating different ␥-MnOOH nanorod precursors at 250–450 ◦ C. They were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and nitrogen adsorption. The experimental results indicate that ␥-MnOOH nanorod precursors prepared at higher hydrothermal temperature make final ␤-MnO2 nanorods uniform. The catalytic performance on H2 O2 decomposition shows that ␤-MnO2 nanorod catalysts have excellent catalytic activity on the decomposition of H2 O2 , which has not been reported to our knowledge. ␤-MnO2 nanorods which were prepared by calcining ␥-MnOOH precursors at 250 ◦ C have higher catalytic activity than those at higher calcination temperatures. The reaction of H2 O2 decomposition over ␤-MnO2 nanorod catalysts was found to follow first-order kinetics. © 2007 Elsevier B.V. All rights reserved. Keywords: H2 O2 decomposition; ␤-MnO2 ; Nanorod; Catalyst; Reaction kinetics

1. Introduction Decomposition of H2 O2 has been a subject of long-standing interest. It has been used as a source of power for rocket [1,2], robot [3,4] and a source of the hydroxyl radical (• OH) as oxidant for the removal of toxic pollutants from water [5–10]. The catalytic decomposition of H2 O2 has been performed both in homogeneous [11–13] and heterogeneous [13–19] media. Homogeneous catalysts are very efficient, but their recovery from the treated effluents is difficult and brings about additional costs. This problem can be overcome by using heterogeneous catalysts. Transition metal oxides have been used as catalysts for decomposition of H2 O2 [14–20] because of their significant activities. It was reported that pure and mixed manganese oxides have good activity for decomposition of H2 O2 [16–22]. Nanomaterials have received great attention because of their unique properties caused by very small physical dimensions. The high volume fraction of atoms located both on the surface and

∗

Corresponding author. E-mail address: [email protected] (W. Zhang).

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interfaces results in an increasing surface energy, which may make the catalytic activity improved greatly. The electroanalytical methods for detecting H2 O2 using MnO2 nanoparticles [23] have been investigated and it was found that the activity of the MnO2 nanoparticles was much higher than that of the MnO2 powders. The carbon paste electrode modified with the nanostructured cryptomelane-type manganese oxides exhibits significant electrocatalytic activity [24]. Recently, nanoscaled one-dimensional structures attract much attention due to their low dimensionalities. Such 1D nanostructures have potential applications in wide-ranging sectors including catalysis, sensing, electronics, and photonics, with performances that are anticipated to be superior to those of their bulk counterparts [25]. Kawi et al. [26] reported a high catalytic activity of nanorods of Zn–Al oxides for the reduction of nitrogen oxides NOx in the presence of excess oxygen. Li et al. [27] reported that the CeO2 nanorods had higher catalytic activity for CO oxidation than CeO2 nanoparticles. In this manuscript, we report a different manganese oxide catalyst—␤-MnO2 nanorods, which shows excellent catalytic activity on the decomposition of H2 O2 . To our knowledge, decomposition of H2 O2 over the ␤-MnO2 nanorods has not been reported. The ␤-MnO2 nanorods were prepared by calcining

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␥-MnOOH nanorods, which were prepared through hydrothermal method. The effects of hydrothermal and calcination temperatures on catalyst activity for decomposition of H2 O2 have been investigated. The kinetics of H2 O2 decomposition over different ␤-MnO2 nanorod catalysts was studied as well. The relationship among the ␤-MnO2 nanorod preparation conditions, morphology and catalytic performance on H2 O2 decomposition has been investigated in the work. 2. Experimental 2.1. Reagents All reagents were analytical grade and used without further purification. Distilled water was used in the preparation. The initial concentration of H2 O2 (30%, w/w) was determined by titration with a 0.1 M solution of KMnO4 . The commercial ␤-MnO2 was obtained from the Shanghai Chemicals Company. 2.2. Catalysts ␥-MnOOH nanorods were prepared through a hydrothermal method, which was reported in our previous paper [28,29]. Four grams of KMnO4 , 4 mL of CH3 CH2 OH and 400 mL of distilled water were put in a stainless steel autoclave of 1 L capacity with a magnetic stirrer. The autoclave was sealed and maintained at 100–200 ◦ C for 24 h under autogenous pressure and continuous stirring. The product was filtered, washed with distilled water for several times, and dried in vacuum at 60 ◦ C for 24 h. The obtained samples were denoted by MnOOH{A}, in which A stands for the hydrothermal temperature for preparing ␥-MnOOH. The ␤-MnO2 nanorods were prepared by calcining ␥-MnOOH at 250–450 ◦ C in air for 4 h. The samples were denoted by MnO2 {A}[B, C, D], which were prepared by calcining MnOOH{A} precursor at different temperatures of B, C or D. 2.3. Catalyst characterization X-ray diffraction (XRD) patterns were carried out on a Japan Rigaku D/max-␥B X-ray diffractometer equipped with graphite monochromatized high-intensity Cu K␣ radia˚ Transmission electron microscopy (TEM) tion (λ = 1.54178 A). images were taken on a Philips CM200 transmission electron microscope at an accelerating voltage of 200 kV. The Brunauer–Emmett–Teller (BET) surface area (SBET ) and pore parameters of the samples were determined from nitrogen adsorption isotherm obtained at −196 ◦ C measurements on a Beckman Coulter SA3100 nitrogen adsorption apparatus. Before carrying out the measurements, each sample was degassed under a reduced pressure of 10−4 Torr at 150 ◦ C for 12 h. The BET surface area was determined by the multipoint BET method with the adsorption data in the relative pressure (P/P0 ) range of 0.05–0.2. The pore size distribution was calculated by the Barrett–Joyner–Halenda (BJH) method using adsorption isotherms.

Fig. 1. Experimental apparatus for catalytic decomposition of H2 O2 . 1, magnetic stirrer with a temperature controller; 2, water-bath; 3, thermometer; 4, magnetic bar; 5, injector; 6, pressure gauge; 7, glass container; 8, U-type glass jar; 9, water; 10, tube; 11, beaker; 12, electronic balance.

2.4. Apparatus and procedures The apparatus used for catalytic studies is shown in Fig. 1. A catalyst was loaded into a flask. The inlet of the flask was covered by a septum with a hole, and the hole was connected with a glass tube to a big glass container. One end of a U-type glass jar was connected with the container. The other end of the U-type glass jar was connected to a beaker. The container and U-type glass jar were filled with water. When the reaction took place, the evolved gaseous oxygen forced water out of the U-type glass jar into the beaker. The mass of the water was weighed on an electronic balance, then it was conveniently translated into the volume. The different catalysts were studied through the decomposition of H2 O2 (H2 O2 → H2 O + 1/2O2 ) at temperatures within 5–25 ◦ C. The kinetics of the reaction was monitored by measuring the volume of O2 (at 273 K, 1 atm) gas liberated at different time intervals until no further liberation of O2 took place. In each experiment 20 mg of catalyst powder was added to the flask containing 18 mL of distilled water. Then the flask was mounted on a magnetic stirrer to make the catalyst uniformly disperse in the water. Two milliliters of H2 O2 of known concentration was injected into the flask and time responses were observed. Plots of volume of O2 versus time were obtained. The hydrostatic pressure generated in the U-type glass apparatus was taken into account when the volume of evolved oxygen was determined. Self-decomposition of H2 O2 during reaction can be ignored. 3. Results and discussion 3.1. Structures of catalysts Fig. 2A–C shows XRD patterns of the precursors for preparing the ␤-MnO2 catalysts including MnOOH{100}, MnOOH{150} and MnOOH{190}, which were prepared at 100, 150 and 190 ◦ C for 24 h by the hydrothermal method. All the samples can be indexed to monoclinic ␥-MnOOH (JCPDS 411379). When the hydrothermal temperature was increased to 200 ◦ C, the XRD pattern (Fig. 2D) of the sample MnOOH{200} has Mn3 O4 phase as impurity besides ␥-MnOOH. Fig. 3A–C shows XRD patterns of the catalysts including MnO2 {100}[350], MnO2 {150}[350] and MnO2 {190}[350]

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3.3. Catalytic activities of various catalyst samples

Fig. 2. XRD patterns of the precursors for preparing the ␤-MnO2 catalysts: (A) MnOOH{100}, (B) MnOOH{150}, (C) MnOOH{190}, and (D) MnOOH{200}.

samples, which were prepared by calcining the corresponding MnOOH samples at 350 ◦ C for 4 h. The products can be indexed to tetragonal ␤-MnO2 (JCPDS 24-0735). The XRD patterns of the samples calcined at 250–450 ◦ C for 4 h can also be indexed to ␤-MnO2 phase. 3.2. Morphologies of catalysts TEM images (Fig. 4a–c) respectively show the morphologies of the samples including MnO2 {100}[250], MnO2 {150}[250] and MnO2 {190}[250], which are composed of ␤-MnO2 nanorods. Their diameters are in the range of 50–150 nm, and the lengths are 5–10 ␮m. The morphology of MnO2 {190}[250] is more uniform than those of MnO2 {100}[250] and MnO2 {150}[250]. It indicates that more uniform distribution of the lengths and diameters exists in ␤-MnO2 nanorods based on ␥-MnOOH nanorod precursors prepared at higher hydrothermal temperature. Compared the image of MnOOH{190} (Fig. 4d) with that of MnO2 {190}[250], the morphology of the precursor was retained in the calcination product.

Fig. 3. XRD patterns of the catalysts: (A) MnO2 {100}[350], (B) MnO2 {150}[350], and (C) MnO2 {190}[350].

H2 O2 decomposition activity of test catalysts was measured in solution (H2 O2 → H2 O + 1/2O2 ). The reaction kinetics was followed isothermally at 25 ◦ C by determining the volume of oxygen released (Vt ) as a function of time. Figs. 5–7 show comparison of catalytic decomposition of H2 O2 over different ␤-MnO2 nanorod catalysts at 25 ◦ C. The ␤-MnO2 nanorod samples were prepared by calcining MnOOH{100}, MnOOH{150} and MnOOH{190} precursors at 250, 350 and 450 ◦ C, respectively. Fig. 8 displays catalytic activities of commercial ␤-MnO2 catalysts. The results reveal that all ␤-MnO2 nanorod catalysts have higher activity for H2 O2 decomposition. The percentage of decomposition of H2 O2 over various ␤-MnO2 nanorods reached 95% (the theoretical volume of O2 decomposed from all H2 O2 is 239 mL) within 100 s. However, it almost took 70 min to decompose the same amount of H2 O2 over commercial ␤-MnO2 . The rate of H2 O2 decomposition over ␤-MnO2 nanorods prepared in our experiment is much faster than that over commercial ␤-MnO2 catalysts. The XRD patterns of the ␤-MnO2 nanorods before and after decomposition of H2 O2 showed no obvious differences (see supporting information) as expected for a catalyst. According to our experiment, the ␤-MnO2 nanorods were reusable and could work several times without obvious decrease of the catalytic activity. If the reaction of H2 O2 decomposition follows first-order reaction, the first-order kinetic equation can be determined as kt = ln V∞ − ln(V∞ − Vt ), where V∞ is the volume of O2 released when all of the H2 O2 is decomposed and Vt is the volume of O2 released at a given time. Then linear plots of ln(V∞ − Vt ) versus t can be made. The slopes of the plots allow ready determination of the reaction rate constant, k, measured at a given temperature over a given catalyst sample. Here in the present work, insets of Figs. 5–7 show linear plots of ln(V∞ − Vt ) versus t based on H2 O2 decomposition over all ␤MnO2 nanorods. Thus it can be concluded that the reactions over ␤-MnO2 nanorod catalysts follow first-order kinetics in all cases, while it is not observed in the case of commercial ␤-MnO2 . Fig. 9 shows the plots of the calculated reaction rate constant k versus calcination temperature for different catalysts. For ␤-MnO2 nanorods which were prepared by calcining MnOOH{100} respectively at 250, 350 and 450 ◦ C, the reaction rate constants k are 0.0321, 0.0326 and 0.0325 s−1 correspondingly. With increasing calcination temperature from 250 to 450 ◦ C, the value of k fluctuates within 2%. For ␤-MnO2 nanorods which were prepared by calcining MnOOH{150} respectively at 250, 350 and 450 ◦ C, the reaction rate constants k are 0.0431, 0.0307 and 0.0252 s−1 correspondingly. The activity increases by 134% compared with MnO2 {100}[250] at the same calcinations temperature of 250 ◦ C. But the activity decreases by 29% and 42% respectively with increasing calcination temperature to 350 and 450 ◦ C. For ␤-MnO2 nanorods which were prepared by calcining MnOOH{190} respectively at 250, 350 and 450 ◦ C, the reaction rate constants k are 0.0942, 0.0567 and 0.0213 s−1 correspondingly. The activity increases by 293% compared with MnO2 {100}[250] at the same calcinations tem-

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Fig. 4. TEM images of samples: (a) MnO2 {100}[250], (b) MnO2 {150}[250], (c) MnO2 {190}[250], and (d) MnOOH{190}.

perature of 250 ◦ C. The activity decreases by 40% and 77% with increasing calcination temperature to 350 and 450 ◦ C, respectively. It can be seen that the catalytic activities of the ␤-MnO2 samples calcined at 250 ◦ C significantly increase with increasing the hydrothermal temperatures for MnOOH precursors from 100 to 190 ◦ C. On the other hand, the catalytic activities of ␤-MnO2 nanorods decease with increasing the calcination temperature of MnOOH precursors from 250 to 450 ◦ C and show more sensitivity to calcination temperature based on MnOOH nanorod precursors prepared at higher hydrothermal temperature. Among the three MnOOH precursors, MnOOH{190} has the greatest impaction on catalytic activity of ␤-MnO2 nanorods under different calcination temperature. The experiments show that all ␤-MnO2 nanorod catalysts have much higher activities than commercial ␤-MnO2 for H2 O2 decomposition. Inset of Fig. 8 shows that commercial ␤-MnO2 powder is made up of micro-sized particles. The surface area of the commercial ␤-MnO2 powder was 0.11 m2 g−1 .

In comparison with the surface areas of ␤-MnO2 nanorods in Table 1, too low surface area may bring about few amounts of active sites, which leads to much lower activity for H2 O2 decomposition. The activities of ␤-MnO2 nanorod catalysts for H2 O2 decomposition vary from one to another. Fig. 9 indicates that different hydrothermal temperatures greatly influence the activities of ␤MnO2 nanorods calcined at different temperatures. Based on MnOOH prepared at 100 ◦ C, the activity of ␤-MnO2 prepared at different calcinations temperatures changes little. Based on Table 1 SBET and pore volume of samples Catalyst

SBET (m2 g−1 )

Pore volume (ml g−1 )

MnOOH{190} MnO2 {190}[250] MnO2 {190}[450]

13.90 16.20 16.17

0.027 0.053 0.030

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Fig. 5. Catalytic decomposition of H2 O2 over ␤-MnO2 nanorod samples at 25 ◦ C, based on MnOOH{100} precursors: (A) MnO2 {100}[250], (B) MnO2 {100}[350], and (C) MnO2 {100}[450].

Fig. 8. Catalytic decomposition of H2 O2 over commercial ␤-MnO2 at 25 ◦ C. Inset: SEM image of commercial ␤-MnO2 .

Fig. 6. Catalytic decomposition of H2 O2 over ␤-MnO2 nanorod samples at 25 ◦ C, based on MnOOH{150} precursors: (A) MnO2 {150}[250], (B) MnO2 {150}[350], and (C) MnO2 {150}[450].

MnOOH prepared at 150 ◦ C, the activity of ␤-MnO2 decreases a little with increasing calcinations temperatures. Based on MnOOH prepared at 190 ◦ C, the activity of ␤-MnO2 prepared at different calcinations temperatures changes greatly. ␤-MnO2 nanorods which were prepared by calcining MnOOH at 250 ◦ C have the highest activity for H2 O2 decomposition. Then with increasing calcinations temperatures, the activity of ␤-MnO2 decreases sharply. In order to investigate the influences of the hydrothermal and calcination temperatures on the activities of ␤-MnO2 nanorod catalysts for H2 O2 decomposition, the surface areas and pore diameters of the catalysts were measured. Table 1 shows that the surface areas of MnOOH{190}, MnO2 {190}[250] and MnO2 {190}[450] are 13.90, 16.17 and 16.12 m2 g−1 , respectively. Fig. 10 shows the adsorption isotherms and pore diameter distributions of the samples. All the samples have many mesopores with the diameters of 3–10 nm. Compared MnO2 {190}[250] with MnOOH{190}, large pores ranging from 20 to 110 nm were produced with increasing SBET and pore volume due to calcination at 250 ◦ C. This may be caused by

Fig. 7. Catalytic decomposition of H2 O2 over ␤-MnO2 nanorod samples at 25 ◦ C, based on MnOOH{190} precursors: (A) MnO2 {190}[250], (B) MnO2 {190}[350], and (C) MnO2 {190}[450].

Fig. 9. Plots of the calculated reaction rate constant k vs. calcination temperature for different ␤-MnO2 nanorod catalysts prepared from different precursors: (A) MnOOH{100}, (B) MnOOH{150}, and (C) MnOOH{190}.

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4. Conclusions

Fig. 10. Nitrogen adsorption isotherms and pore size distributions derived from adsorption for the samples: (A) MnOOH{190}, (B) MnO2 {190}[250], and (C) MnO2 {190}[450].

losing molecular water during calcination. Thus lots of active sites were produced and the catalytic activity for decomposing H2 O2 was increased greatly. The increasing mesopores makes H2 O2 easily reach the active sites. Although MnO2 {190}[250] and MnO2 {190}[450] have similar surface areas (Table 1), their catalytic activities for H2 O2 decomposition decrease with increasing calcination temperature according to Fig. 9. The reason may be that the quantity of pores ranging from 20 to 110 nm was decreased due to some merging pores at higher calcination temperature. Thus the pore volume was decreased and fewer amounts of active sites left. So ␤-MnO2 nanorods, which were prepared by calcining MnOOH obtained at higher hydrothermal temperature, show more sensitive catalytic activities to calcination temperature. In our work, the hydrothermal temperature on the one hand adjust the morphology of catalysts, on the other hand changes the catalysts activity. ␤-MnO2 is constructed from chains of {MnO6 } octahedral. It has a rutile structure, with the oxygen atoms forming a slightly distorted hexagonal closely packed (hcp) array. The basic motif of this tetragonal structure is an infinite chain of octahedra-sharing opposite edges, with each chain corner-linked with four similar chains. The unit cell parameters can be calculated from XRD data, and then the volume of the unit cell can be obtained. The catalysts of MnO2 {100}[250,350,450], MnO2 {150}[250,350,450], and MnO2 {190}[250,350,450] have unit cell volume range of ˚ 3 , respec55.353–54.898, 55.494–54.933, and 55.579–55.566 A tively. The increase of unit cell volume results in a decrease of stability of unit cell [30]. About manganese oxides it has been proved that a favorable couple of Mn3+ to Mn4+ is essential for catalytic decomposition of H2 O2 through electron exchange [19,22]. In this case, the unit cell volume of ␤-MnO2 nanorod sample which were prepared by calcining MnOOH at higher hydrothermal temperature is bigger and thus the Mn–O bond strength becomes weaker. This may make the sample have higher concentration of ion pairs (Mn3+ to Mn4+ ) on the surface of catalysts and accordingly have higher activity.

The work has revealed the relationships among the ␤-MnO2 nanorod preparation conditions, morphology and catalytic performance on H2 O2 decomposition. ␤-MnO2 nanorods with diameters of 50–150 nm and lengths of 5–10 ␮m were prepared by a hydrothermal method. The ␤-MnO2 nanorod catalysts converted from the MnOOH prepared at higher hydrothermal temperature have more uniform morphology. All ␤-MnO2 nanorod catalysts have higher activity on H2 O2 decomposition than commercial ␤-MnO2 and known manganese oxides. However, the ␤-MnO2 nanorod catalysts obtained by calcining MnOOH precursors which were prepared at higher hydrothermal temperature have higher activity for H2 O2 decomposition. ␤-MnO2 nanorods which were prepared by calcining MnOOH at 250 ◦ C have the highest activity and then with the increasing calcination temperature, the activity of ␤-MnO2 decreases. The reactions of H2 O2 decomposition over all ␤-MnO2 nanorod catalysts follow first-order kinetics. Acknowledgements The project was financially supported by the National Natural Science Foundation of China (NSFC 20576024), Anhui Provincial Natural Science Foundation (070414165) and the Excellent Young Teachers Program of the Ministry of Education of China. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.colsurfa.2007.04.022. References [1] H. Tian, T. Zhang, X.Y. Sun, D.B. Liang, L.W. Lin, Performance and deactivation of Ir/g-Al2 O3 catalyst in the hydrogen peroxide monopropellant thruster, Appl. Catal. A 210 (2001) 55–62. [2] J. Guerrero, B. Hamilton, R. Burton, D. Crockett, Z. Taylor, Upper stage flight experiment (USFE) integral structure development effort, Compos. Struct. 66 (2004) 327–337. [3] Y.H. Choi, S.U. Son, S.S. Lee, A micropump operating with chemically produced oxygen gas, Sensor. Actuators A 111 (2004) 8–13. [4] J.W. Raade, H. Kazerooni, Analysis and design of a novel hydraulic power source for mobile robots, Autom. Sci. Eng. 2 (2005) 226–232. [5] C. Catrinescu, C. Teodosiu, M. Macoveanu, J. Miehe-Brendl´e, R.L. Dred, Catalytic wet peroxide oxidation of phenol over Fe-exchanged pillared beidellite, Water Res. 37 (2003) 1154–1160. [6] T. Nakajima, T. Kawabata, H. Kawabata, H. Takanashi, A. Ohki, S. Maeda, Degradation of phenylarsonic acid and its derivatives into arsenate by hydrothermal treatment and photocatalytic reaction, Appl. Organoment. Chem. 19 (2005) 254–259. [7] I. Arslan, I.A. Balcioglu, Advanced oxidation of raw and biotreated textile industry wastewater with O3 , H2 O2 /UV-C and their sequential application, J. Chem. Technol. Biotechnol. 76 (2001) 53–60. [8] L.F. Pedersen, P.B. Pedersen, O. Sortkjær, Dose-dependent decomposition rate constants of hydrogen peroxide in small-scale bio filters, Aquaculture Eng. 34 (2006) 8–15. [9] P. Verma, P. Baldrian, J. Gabriel, T. Trnka, F. Nerud, Copper–ligand complex for the decolorization of synthetic dyes, Chemosphere 57 (2004) 1207–1211.

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