Redox Activity and NO Storage Capacity of MnOx−ZrO2 with

Ind. Eng. Chem. Res. 2010, 49, 1725–1731

1725

Redox Activity and NO Storage Capacity of MnOx-ZrO2 with Enhanced Thermal Stability at Elevated Temperatures Qiang Zhao,*,† Wan Y. Shih,‡ Hsiao-Lan Chang,§ and Wei-Heng Shih† Department of Materials Science and Engineering and School of Biomedical Engineering, Science, and Health Systems, Drexel UniVersity, Philadelphia, PennsylVania 19104, and Johnson Matthey, Emission Control Technology, Wayne, PennsylVania 19087

MnOx-ZrO2 mixed oxides are very active catalysts for oxidation or combustion applications due to their excellent redox activity. Recently, they have also shown promising results for NO storage in emission control. Despite the versatile applications, a systematic study on the redox activity and NO storage capacity of MnOx-ZrO2 with their thermal stability at high temperatures is still missing. In this paper the evolution of these properties in the temperature range 500-900 °C is reported. It was found that there is a dramatic deterioration in redox activity and NO storage capacity when the calcination temperature increases to 900 °C, which is attributed to the loss of thermal stability at this high temperature. On the other hand, doping with La can hinder the sintering of MnOx-ZrO2 particles and therefore increase the surface area of the mixed oxides calcined at 900 °C. Correspondingly, La-doped MnOx-ZrO2 mixed oxides show much improved redox activity and NO storage capacity even after 900 °C heat treatment. 1. Introduction

2. Experimental Section

Manganese based oxides are very active catalysts for the oxidation of methanol,1 ethanol,2 benzene,3 CO,4 and propane,5 and for the combustion of volatile organic compounds (VOC).6,7 The catalytic activity is related to the redox property of Mn ions in response to the oxygen lean/rich environments. On the other hand, zirconia has drawn a lot of attention as a catalyst support material due to its unique surface bifunctional (acidity and basicity) property.8 Additionally zirconia also has a synergic effect on the oxygen transport in the catalyst. For example, it can promote the oxygen mobility in the ceria lattice.9

2.1. Sample Preparation. Manganese nitrate powder (Mn(NO3)2 · xH2O, 98%, Aldrich) and zirconyl nitrate solution (Zr, 199 g/L; Hf, 3.98 g/L, specific gravity, 1.435 g/cm3; Johnson Matthey) were used as received without further purification. The ZrO2-MnOx mixed oxides were made by use of the coprecipitation method as follows. Different mole ratios (varying from 10/1 to 1/1) of ZrO(NO3)2 and Mn(NO3)2 were dissolved in distilled water to make binary salt solutions. The concentration of ZrO(NO3)2 in these solutions was kept at 0.33 mol/L. After stirring for 20 min, 50 mL of the binary salt solutions was added dropwise to 200 mL of 5.02 N ammonium hydroxide solution (Alfa Aesar) with rigorous stirring. Upon the completion of the addition to ammonium hydroxide, the precipitated suspensions were stirred for another 1/2 h before they were centrifuged and washed with distilled water four times and dried in air at 70 °C overnight. The single oxide, ZrO2 or MnOx, was made from a similar procedure with the single salt precursor solution, respectively. Lanthanum-manganese-zirconium oxides were made by the coprecipitation method from 50 mL of La(NO3)3-Mn(NO3)2-ZrO(NO3)2 aqueous solution dropping into 200 mL of 5.02 N ammonium hydroxide solution. The concentration of Zr in the precursor solution was 0.33 mol/L and the La to Zr molar ratio was fixed at 3/100, but the Mn to Zr ratio was varied from 1/5 to 1/1. All oxides were calcined at designated temperatures for 4 h in air before characterization tests. 2.2. X-ray Diffraction. The powder X-ray diffraction (XRD) patterns of the samples were obtained by a Siemens D-500 X-ray diffractometer using Cu KR radiation (1.5416 Å). The crystallite size of the tetragonal zirconia was calculated from the broadening of the (111) peak, according to the Scherrer formula

MnOx-ZrO2 based systems have been studied extensively, aiming for better oxidation and combustion catalysts.10-16 Besides applications as oxidation catalysts, reports on the reversible NOx trapping by MnOx-ZrO2 mixed oxides have brought a new interest in the MnOx-ZrO2 system as NOx storage materials.17-21 One important issue for practical applications is the thermal stability of MnOx-ZrO2 at elevated temperatures. However, to our knowledge, there are only limited studies on the thermal stability of MnOx-ZrO2 around or above 800 °C.22-24 It was reported that MnOx could stabilize the tetragonal phase of zirconia by forming a solid solution with a limited solubility ( MnOx g MnOx-ZrO2 at 900 °C. 3.2. Effect of La Dopant on the Thermal Stability of MnOx-ZrO2. In order to improve the thermal stability of MnOx-ZrO2 at high temperatures, a possible solution is to introduce a dopant into the system since doping is an effective method to increase the surface area of pure zirconia.26,27 Lanthanum was chosen as the dopant candidate because it has been shown to be helpful for the thermal stability of zirconia.27 In addition, lanthanum and manganese oxide compounds are potential oxidation catalysts, too.28,29 Figure 5 shows the effect of La on the surface area of MnOx-ZrO2. The surface area values of ZrO2, La2O3-ZrO2, and MnOx-ZrO2 are also shown for comparison. After 500 °C calcination both Mn and La have positive effects on the surface area of ZrO2. Also, these positive effects can be added on each

1728

Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010

Figure 4. SEM images of (a) ZrO2, (b) MnOx-ZrO2 (Mn/Zr ) 0.2), (c) MnOx-ZrO2 (Mn/Zr ) 1), and (d) MnOx after 4-h calcination at 900 °C.

Figure 5. Specific surface area of ZrO2 based oxide powders with different kinds of dopants at elevated temperatures. The calcination was conducted at various temperatures for 4 h. The numbers in the legend are molar ratios between the dopant and Zr.

other, which is shown by even higher surface area values of La-Mn-ZrO2 ternary oxide systems compared with those of binary systems. Calcination at higher temperatures decreases the specific surface area. The surface areas of the samples containing Mn decrease more dramatically compared with those without Mn. At 900 °C the surface area of MnOx-ZrO2 is very low (0.8 ( 0.2 m2/g), but the La-Mn-ZrO2 system has a specific surface area of 21 ( 2 m2/g, which is comparable to that of pure zirconia. The XRD results are shown in Figure 6. Lanthanum dopant helps stabilize the tetragonal zirconia phase even after 900 °C heat treatment. Also, there are no crystalline phases related to Mn or La when the Mn content is small (Mn/Zr ) 0.2). Figure 7 shows the ESEM images of the oxide systems after 4-h heat treatment at 900 °C. Compared with MnOx-ZrO2 powders, the La-Mn-ZrO2 system has a much smaller gain size and is more

Figure 6. XRD results of ZrO2 based oxide powders with different kinds of dopants at elevated temperatures. The calcination was conducted at 900 °C for 4 h. The numbers in the legend are molar ratios between the dopant and Zr.

porous, so it has a much higher surface area. Figure 8 indicates that a small amount of La dopant can significantly improve the thermal stability of MnOx-ZrO2 powders over a wide range of Mn contents. Accordingly, this improvement in thermal stability will increase the potential of MnOx-ZrO2 for catalytic applications that may involve exposure to high temperatures. 3.3. Redox Activity of MnOx-ZrO2 System. The TPR results of coprecipitated MnOx-ZrO2 binary oxides with different Mn contents heat treated at 700 °C for 4 h are shown in Figure 9. The pure manganese oxide powder (dashed line in Figure 9) shows a light off temperature at ∼240 °C with two reduction peaks: one at 300 °C and the other at 425 °C. For MnOx-ZrO2 the light off temperature decreases to ∼100 °C and the first reduction peak is at ∼140 °C. Reduction can be initiated at much lower temperatures likely due to the facts that MnOx-ZrO2 mixed oxides have higher surface areas and MnOx

Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010

1729

Figure 7. SEM images of (a) La-Mn-ZrO2 (La/Mn/Zr ) 0.03/0.2/1) and (b) Mn-ZrO2 (Mn/Zr ) 0.2) after 4-h heat treatment at 900 °C.

Figure 8. Effect of La doping on the surface area of MnOx-ZrO2 powders at 900 °C. The numbers in the legend are molar ratios between the dopant and Zr.

Figure 9. TPR results of MnOx-ZrO2 samples with different Mn/Zr molar ratios. All samples were heat treated at 700 °C for 4 h.

is well dispersed in the ZrO2 matrix. For the sample with an Mn/Zr ratio of 1 (thick solid line in Figure 9), there are two other peaks at ∼350 and ∼425 °C, respectively. These two peaks are very close to the TPR peaks of the pure manganese oxide. The XRD result of this sample shows a certain amount of R-Mn2O3 phase, which is also shown in the pure manganese oxide (Table 1). Thus it is reasonable to say that these two peaks at ∼350 and ∼425 °C are the reduction of crystalline R-Mn2O3 in the MnOx-ZrO2 (Mn/Zr ) 1) sample. For the other two, Mn/Zr ) 0.5 (thinner solid line in Figure 9) or 0.2 (the thinnest solid line in Figure 9), there are also two similar peaks but appearing at lower temperatures, 270-320 °C and 250-280 °C, respectively. According to Table 1, there are no crystalline manganese oxide phases in the XRD results of these two samples. The above information suggests that manganese oxide

Figure 10. TPR results of MnOx-ZrO2 samples with and without La dopant calcined at (a) 700 and (b) 900 °C for 4 h. Mn/Zr ) 0.2 for all the samples and La/Zr ) 0.03 for La-containing samples.

is fully dispersed in the composite powders with lower Mn contents and therefore can be more easily reduced. Lanthanum doping slightly increases the reduction temperatures of the ZrO2-MnOx sample calcined at 700 °C (Figure 10a). However, the peak profiles (solid line vs dashed line in Figure 10a) are very similar between samples with and without La dopant. However, for samples calcined at 900 °C, the reduction of La-ZrO2-MnOx starts at much lower temperatures than that of ZrO2-MnOx (Figure 10b). This significant improvement in reduction activity is due to the enhanced thermal stability (SSA ∼20 m2/g) of the La-doped sample. 3.4. NO Storage Capacity. The results of NO storage tests at 100 °C for the 500 °C calcined samples are shown in Figure 11. There is no apparent difference in the NO adsorption behavior between MnOx-ZrO2 (Mn/Zr ) 1/1) and La-

1730

Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010

Figure 11. NO adsorption behavior on ZrO2-MnOx (Zr/Mn ) 1/1) based samples with and without La dopant (La/Zr ) 0.03) at 100 °C.

Figure 12. Gas content analysis at the outlet of the flow reactor during NO storage tests shown in Figure 11. Table 2. NO Storage Capacities of ZrO2-MnOx Based Powders samples, calcination conditions ZrO2-MnOx, 500 °C/4 h La-ZrO2-MnOx, 500 °C/4 h ZrO2-MnOx, 700 °C/4 h La-ZrO2-MnOx, 900 °C/4 h

test temp (°C)

amt NO adsorption (×10-4 mol/g)

100 300 100 300 300 300

5.02 1.12 4.99 1.18 0.58 0.35

ZrO2-MnOx (La/Zr/Mn ) 0.03/1/1) samples. Both powders can completely remove NO from the gas stream within the initial ∼400 s. After that time, NO cannot be completely adsorbed and NO breakthrough occurs after 4000-s adsorption. The gas contents at the outlet of the flow reactor were analyzed by mass spectroscopy. Both NO and NO2 were detected at the outlet (Figure 12). The input gases contained only NO, so the NO2 was from the oxidation of NO. According to Matsukuma et al., converting NO to NO2 by MnOx was a necessary step before NO2 could be adsorbed as NO3- on the surface.20 From Figure 12 it seems that oxidizing NO to NO2 was not the bottleneck for NO storage under our testing conditions because the NO2 signal was seen all the time, even when the sample had been saturated by the adsorption. The limiting factor for NO storage is the number of available adsorption sites on the powder surface. The NO storage capacities of ZrO2-MnOx and LaZrO2-MnOx at 100 °C are listed in Table 2. These two samples have very similar NO storage capacity, which can be attributed to the similar surface areas of these two samples (201 vs 181 m2/g). When the testing temperature was increased to 300 °C,

Figure 13. NO adsorption behavior on ZrO2-MnOx (Zr/Mn ) 1/1) based samples with and without La dopant (La/Zr ) 0.03) at 300 °C.

the NO storage capacities of ZrO2-MnOx and La-ZrO2-MnOx (calcined at 500 °C for 4 h) samples became lower compared with those tested at 100 °C. The durations of 100% NO removal at 300 °C are ∼300 and ∼150 s, respectively (Figure 13). They are shorter than those tested at 100 °C, ∼400 s (Figure 11). The total time before saturation at 300 °C is shorter than that at 100 °C, too: 1500 s vs 4000 s (refer to Figures 13 and 11). The data in Figure 13 also show the influence of powder calcination temperatures on the NO storage capacity. ZrO2-MnOx calcined at 700 °C and La-ZrO2-MnOx calcined at 900 °C have smaller NO storage capacities compared with the same powders calcined at 500 °C due to the surface area reduction. However, those powders calcined at high temperatures still completely remove NO from the gas stream for ∼3070 s. Figure 14 provides the information about the gas contents at the outlet of the flow reactor during the test at 300 °C. The oxidation of NO to NO2 happened all the time during the storage test, and actually more NO2 was generated at 300 °C than at 100 °C (refer to the relative intensity of NO2 compared with that of NO in Figures 14 and 12, respectively). This observation is consistent with the results of Eguchi et al.19 According to the DTA results on hydrous zirconia26 and the decomposition temperature of Mn(NO3)2 in the literature,30 the nitrate group binding with Zr or Mn starts to decompose around 300 °C, which may be a possible reason why the powders have higher NO storage capacities at 100 °C than at 300 °C; i.e., the adsorbed NOx species are not stable at g300 °C. 4. Conclusions Coprecipitated ZrO2-MnOx mixed oxides have higher surface areas at 500 and 700 °C than the single component oxide and the surface area value increases with the Mn content up to 30-50 mol %. However, at 900 °C MnOx phases segregate from ZrO2 and the sintering and grain growth of binary oxide particles are enhanced, so their specific surface areas decrease dramatically. Lanthanum dopant can hinder the sintering of ZrO2-MnOx and increase the surface area by 1 order of magnitude at 900 °C. It also stabilizes the tetragonal zirconia in the ZrO2-MnOx system. Redox activity and the NO storage capacity of ZrO2-MnOx are highly correlated with the surface area. The light off temperature of MnOx in H2-TPR tests can be decreased by as much as 150 °C when coprecipitating MnOx with ZrO2 to achieve high surface area and good dispersion of MnOx in ZrO2 matrix. Lanthanum dopant further improves the redox activity of ZrO2-MnOx powders calcined at 900 °C due to the enhanced

Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010

Figure 14. Gas content analysis at the outlet of the flow reactor during NO storage tests shown in Figure 13.

thermal stability. The NO storage capacity of mixed oxides decreases as the testing temperature increases from 100 to 300 °C due to the instability of adsorbed nitrate species at 300 °C. ZrO2-MnOx or La-ZrO2-MnOx powders calcined at higher temperatures (700 or 900 °C) have smaller NO storage capacities compared with those powders calcined at 500 °C because of the surface area loss. Literature Cited (1) Baltanas, M. A.; Stiles, A. B.; Katzer, J. R. Development of supported manganese oxides catalysts for partial oxidation: Preparation and hydrogenation properties. Appl. Catal. 1986, 28, 13. (2) Pettersson, L. J.; Wahlberg, A. M.; Jaras, S. G. Catalysis and automotiVe pollution control IV; Elsevier: Amsterdam, The Netherlands, 1998; pp 465-475. (3) Naydenov, A.; Mehandjiev, D. Complete oxidation of benzene on manganese dioxide by ozone. Appl. Catal., A: Gen. 1993, 97, 17. (4) Craciun, R.; Nentwick, B.; Hadjiivanov, K.; Kno¨zinger, H. Structure and redox properties of MnOx/Yttrium-stabilized zirconia (YSZ) catalyst and its used in CO and CH4 oxidation. Appl. Catal., A: Gen. 2003, 243, 67. (5) Baldi, M.; Finocchio, E.; Pistarino, C.; Busca, G. Evaluation of the mechanism of the oxy-dehydrogenation of propane over manganese oxide. Appl. Catal., A: Gen. 1998, 173, 61. (6) Baldi, M.; Escribano, V. S.; Gallardo-Amores, J. M.; Milella, F.; Busca, G. Characterization of manganese and iron oxides as combustion catalysts for propane and propene. Appl. Catal., B: EnViron. 1998, 17, L175. (7) Baldi, M.; Finocchio, E.; Milella, F.; Busca, G. Catalytic combustion of C3 hydrocarbons and oxygenates over Mn3O4. Appl. Catal., B: EnViron. 1998, 16, 43.

1731

(8) Tanabe, K.; Yamaguchi, T. Acid-base bifunctional catalysis by ZrO2 and its mixed oxides. Catal. Today 1994, 20, 185. (9) Kasˇpar, J.; Fornasiero, P.; Graziani, M. Use of CeO2-based oxides in the three-way catalysis. Catal. Today 1999, 50, 285. (10) Arena, F.; Torre, T.; Raimondo, C.; Parmaliana, A. Structure and redox properties of bulk and supported manganese oxide catalysts. Phys. Chem. Chem. Phys. 2001, 3, 1911. (11) Zaki, M. I.; Hasan, M. A.; Pasupulety, L.; Fouad, N. E.; Kno¨zinger, H. CO and CH4 total oxidation over meanganese oxide supported on ZrO2, TiO2, TiO2-Al2O3 and SiO2-Al2O3 catalysts. New J. Chem. 1999, 23, 1197. (12) Kantzer, E.; Do¨bber, D.; Kiessling, D.; Wendt, G. Scientific Bases for the Preparation of Heterogeneous Catalysts; Elsevier: Amsterdam, The Netherlands, 2002; pp 489-497. (13) Li, W. B.; Chu, W. B.; Zhang, M.; Hua, J. Catalytic oxidation of toluene on Mn-containing mixed oxides prepared in reverse microemulsions. Catal. Today 2004, 93-95, 205e. (14) Do¨bber, D.; Kiessling, D.; Schmitz, W.; Wendt, G. MnOx/ZrO2 catalysts for the total oxidation of methane and chloromethane. Appl. Catal., B: EnViron. 2004, 52, 135. (15) Choudhary, V. R.; Deshmukh, G. M.; Pataskar, S. G. Low temperature complete combustion of dilute toluene and methyl ethyl ketone over Mn-doped ZrO2 (cubic) catalysts. J. Chem. Technol. Biotechnol. 2005, 80, 934. (16) Kantserova, M. R.; Orlik, S. N.; Kazimirov, V. P. Effect of structural, redox and acid characteristics of MxOy/ZrO2(Al2O3) (M-Mn, Co, Cr) oxide nanocomposites on their catalytic properties in the deep oxidation of methane. Theor. Exp. Chem. 2007, 43, 399. (17) Eguchi, K.; Watabe, M.; Ogata, S.; Arai, H. Removal of Dilute Nitrogen Oxide by the Absorption in Mn-Zr Oxide. Bull. Chem. Soc. Jpn. 1995, 68, 1739. (18) Eguchi, K.; Kondo, T.; Hayashi, T.; Arai, H. Sorption of nitrogen oxides on MnOy-ZrO2 and Pt-ZrO2-Al2O3. Appl. Catal., B: EnViron. 1998, 16, 69. (19) Eguchi, K.; Watabe, M.; Ogata, S.; Arai, H. Reversible Sorption of Nitrogen Oxides in Mn-Zr Oxide. J. Catal. 1996, 158, 420. (20) Matsukuma, I.; Kikuyama, S.; Kikuchi, R.; Sasaki, K.; Eguchi, K. Development of zirconia-based oxide sorbents for removal of NO and NO2. Appl. Catal., B: EnViron. 2002, 37, 107. (21) Li, W. B.; Yang, X. F.; Chen, L. F.; Wang, J. A. Adsorption/ desorption of NOx on MnO2/ZrO2 oxides prepared in reverse microemulsions. Catal. Today 2009, 148 (1-2), 75–80. (22) Lo´pez, E. F.; Escribano, V. S.; Gallardo-Amores, J. M.; Resini, C.; Busca, G. Structural and morphological characterization of Mn-Zr mixed oxides prepared by a sol-gel method. Solid State Sci. 2002, 4, 951. (23) Lo´pez, E. F.; Escribano, V. S.; Resini, C.; Gallardo-Amores, J. M.; Busca, G. A study of coprecipitated Mn-Zr oxides and their behaviour as oxidation catalysts. Appl. Catal., B: EnViron. 2001, 29, 251. (24) Valigi, M.; Gazzoli, D.; Dragone, R.; Marucci, A.; Mattei, G. Manganese oxide-zirconium oxide solid solutions. An X-ray diffraction, Raman spectroscopy, thermogravimetry and magnetic study. J. Mater. Chem. 1996, 6, 403. (25) Keshavaraja, A.; Jacob, N. E.; Ramaswamy, A. V. Thermal decomposition of co-precipitated oxide hydrates of zirconium and manganese. Thermochim. Acta 1995, 254, 267. (26) Zhao, Q.; Shih, W.-H.; Chang, H.-L.; Andersen, P. The effect of curing on the thermal stability of Si-doped ZrO2 powders. Appl. Catal., A: Gen. 2004, 262, 215. (27) Mercera, P. D. L.; Ommen, J. G. V.; Doesburg, E. B. M.; Burggraaf, A. J.; Ross, J. R. H. Stabilized tetragonal zirconium oxide as a support for catalysts: Evolution of the texture and structure on calcination in static air. Appl. Catal. 1991, 78, 79. (28) Liu, Y.; Zheng, H.; Liu, J.; Zhang, T. Preparation of high surface area La1-xAxMnO3 (A)Ba, Sr or Ca) ultra-fine particles used for CH4 oxidation. Chem. Eng. J. 2002, 89, 213. (29) Cimino, S.; Colonna, S.; Rossi, S. D.; Faticanti, M.; Lisi, L.; Pettiti, I.; Porta, P. Methane Combustion and CO Oxidation on Zirconia-Supported La, Mn Oxides and LaMnO3 Perovskite. J. Catal. 2002, 205, 309. (30) Kappenstein, C.; Wahdan, T.; Duprez, D.; Zaki, M. I.; Brands, D.; Poels, E.; Bliek, A. Preparation of Catalysts VI; Elsevier: Amsterdam, The Netherlands, 1995; pp 699-706.

ReceiVed for reView August 4, 2009 ReVised manuscript receiVed December 8, 2009 Accepted December 10, 2009 IE901234X