Preparation of NiCe Mixed Oxides for Catalytic Decomposition of N2O

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Preparation of NiCe Mixed Oxides for Catalytic Decomposition of N2O Haibo Zhou, Peilei Hu, Zhen Huang, Feng Qin, Wei Shen, and Hualong Xu* Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Laboratory of Advanced Materials, Fudan University, Shanghai 200433, China S Supporting Information *

ABSTRACT: The catalytic N2O decomposition was investigated over a series of NiCe mixed oxides. Characterizations of XRD, N2-adsorption, SEM, XPS, and H2-TPR were applied to correlate their properties with the corresponding catalytic performance. The catalyst Ni90Ce10 prepared by citrate acid method exhibited the highest catalytic activity among the mixed oxides, which could completely decompose N2O at 400 °C in the presence of oxygen. The introduction of CeO2 could prevent the agglomeration of NiO and preserve high surface area. A strong interaction between NiO and CeO2 was observed in the mixed oxides; the interaction enhanced oxygen mobility and resisted the inhibition of O2. CuO, NiO15 shows a much better catalytic performance in the N2O decomposition in the presence of O2. So, it is interesting to explore the catalytic behavior of Ni-based catalysts. In spite of being applied in partial oxidation of methane,25 steam reforming of methane,26 water gas shift reaction,27 and VOCs catalytic combustion,28 NiCe mixed oxides catalysts have seldom been researched in the N2O decomposition. In the present work, a series of NiCe mixed oxides with different Ni/Ce molar ratios are synthesized. All of these catalysts are tested for the catalytic N2O decomposition in the presence and absence of oxygen. Characterizations of XRD, N2adsorption, XPS, TEM, and H2-TPR are applied to correlate their properties with the corresponding catalytic performance.

1. INTRODUCTION Nitrous oxide (N2O) has been recognized as a potential contributor to the destruction of the ozone layer. Furthermore, it is a powerful greenhouse gas, due to its long lifetime (about 150 years) in the atmosphere, which is 310 times higher greenhouse potential with respect to CO2.1,2 The concentration of N2O in the atmosphere is increasing by 0.2−0.3% yearly, which is mainly caused by anthropogenic activities such as energy production and chemical processes (e.g., production of nitric acid and adipic acid).2,3 The continuous increase of its concentration entails the need of developing efficient catalysts for the N2O decomposition. Thus, many types of catalysts, such as pure oxides,4 mixed oxides,5−8 supported systems,9−11 and zeolites,12,13 have been studied for the N2O catalytic decomposition. In particular, noble metals, such as Rh and Ru,11,14 exhibited excellent catalytic performance. Nevertheless, the high cost of noble metals and their sensitivity to higher temperatures have limited their industrial application. Hence, metal oxides based on 3d transition metals, such as Co3O4,4,15 NiO,16 and CuO,17 are of great potential substitute catalysts for the N2O catalytic decomposition, due to lower cost and abundant resources. A small amount of O2 coexists in the real exhaust N2O gas, such as the tail gas of a nitric acid plant. O2 significantly inhibits the catalytic activity and stability.18 Therefore, the enhancement of the oxygen storage capacity and mobility is urgently required during the N2O decomposition. In doped ceria and mixed oxides, CeO2 exhibits strong oxygen mobility.19 CeO2 has been widely used as an additive in the three-way catalysts based on its high oxygen storage capacity.20 Besides, the introduction of CeO2 could preserve a high surface area and prevent the sintering of precious metals. So, Ce-containing mixed oxides (Fe−Ce,5 Co−Ce,21−23 and Cu−Ce24) have attracted much attention in the catalytic N2O decomposition. In our previous work,24 CuCe mixed oxides were prepared and tested in the N2O decomposition. The synergetic effect between CuO and CeO2 was observed, and the CuI species were identified as the active sites. Compared to the single oxide © 2013 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Catalysts Preparation. A series of NiCe mixed oxides catalysts with mole percentages of 100, 95, 90, 80, 70, 50, 20, and 0 Ni metal (Ce balance) were prepared by the citrate acid method, the coprecipitation method, and the ammoniaevaporation method. The NiCe catalyst prepared by the citrate acid method22 was synthesized as follows. Stoichiometric amounts of analytical purity grade of Ni(NO3)2·6H2O and Ce(NO3)3·6H2O were mixed in an aqueous solution. Citric acid was then added to the mixture (citric acid: (Ce + Ni) = 2:1 (molar ratio)) under continuous stirring until the solid was dissolved. The solution was heated at 80 °C to remove water until a viscous gel was formed. The gel was dried overnight in an oven at 120 °C, and an amorphous precursor was obtained. The precursor was calcined at 450 °C for 200 min to obtain the mixed oxides. These as-synthesized mixed oxides were denoted as Ni100 (pure NiO), Ni95Ce5, Ni90Ce10, Ni80Ce20, Ni70Ce30, Ni50Ce50, Ni20Ce80, and Ce100 (pure CeO2). Received: Revised: Accepted: Published: 4504

January 22, 2013 March 3, 2013 March 5, 2013 March 5, 2013 dx.doi.org/10.1021/ie400242p | Ind. Eng. Chem. Res. 2013, 52, 4504−4509

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°C. At each temperature the reactions were stabilized for 30 min before analysis. The products were automatically sampled and analyzed on line by Trace GC Ultra equipped with a thermal conductivity detector (TCD), with a 10 m Poraplot Q capillary column.

The NiCe catalyst prepared by the coprecipitation method21 was synthesized as follows. Ni(NO 3 ) 2 ·6H 2 O and Ce(NO3)3·6H2O were mixed in an aqueous solution (Ni/Ce molar ratio = 9/1), and then a solution of K2CO3 was added dropwise at room temperature until the pH of the solution reached 9−10. The mixture was stirred with a magnetic stirring bar for about 12 h. Then the resultant precipitate was washed with deionized water and dried at 80 °C overnight, followed by calcination at 450 °C for 200 min. This as-synthesized catalyst was denoted as Ni90Ce10-CP. The NiCe catalyst prepared by the ammonia-evaporation method29 was synthesized as follows. Ni(NO3)2·6H2O and Ce(NO3)3·6H2O were mixed in an aqueous solution (Ni/Ce molar ratio = 9/1), and then a solution of NH3 was added dropwise at room temperature until the pH of the solution reached 11−12. The mixture was heated at 80 °C, to allow for the evaporation of NH3 and the decrease of pH value. When the pH value of the mixture decreased to 6−7, the evaporation process was terminated. The resultant precipitate was washed with deionized water and dried at 80 °C overnight, followed by calcination at 450 °C for 200 min. This as-synthesized catalyst was denoted as Ni90Ce10-AE. 2.2. Catalysts Characterization. XRD patterns were recorded with a Bruker D4 powder X-ray diffractometer, which employed Ni-filtered Cu Kα radiation (λ = 1.5418 Å) and was operated at 40 kV and 40 mA. Diffraction patterns were recorded with scan step of 0.02° at the speed of 0.2 s/step for 2-theta between 10 to 80°. Nitrogen adsorption−desorption isotherms at 77 K were performed using a Micromeritics TRISTAR 3000 apparatus. The samples were degassed at 120 °C and high vacuum prior to the measurements. The BET model was used to estimate the surface area of the materials. Transmission Electron Microscopy (TEM) images were recorded on a JEOL 2011 electron microscope operating at 200 kV. Before being transferred into the TEM chamber, the samples dispersed with ethanol were deposited onto a carboncoated copper grid and then quickly moved into the vacuum evaporator. X-ray photoelectron spectroscopy (XPS) data were carried out with a Perkin-Elmer PHI 5000C system equipped with a hemispherical electron energy analyzer. The carbonaceous C 1s line (284.6 eV) was used as the reference to calibrate the binding energy (BE). H2-Temperature Programmed Reduction (TPR) was carried out in Micromeritics AutoChem II 2920, equipped with a quartz U-tube reactor coupled to a TCD detector for analyzing the H2 consumption. Before the test, the sample was pretreated in helium stream at 100 °C for 1 h, and then the temperature cooled down to 50 °C in helium. After that, a H2−Ar mixture (10% H2 by volume) with a flow rate of 50 mL·min−1 was switched on, and the temperature was increased within the range of 50−900 °C (heating rate = 5 °C·min−1). 2.3. Activity Tests. The activity tests for N2O catalytic decomposition were carried out in an automated eight flow reactor system. The catalysts with particle sizes of 40−60 mesh were packed in the isothermal part of the quartz tubular reactor. Prior to the reaction, all the samples were pretreated in a He flow at 400 °C for 1 h, and then the temperature was cooled down to 260 °C. The pressure was set to 0.3 MPa, and the feed to the reactor contained 2600 ppm N2O, (0 or 1%) O2, balanced by He with the total GHSV at 19,000 h−1. The reaction was started at 260 °C and raised at steps of 20 to 500

3. RESULTS AND DISCUSSION 3.1. Textural Properties of NiCe Catalyst. XRD patterns of NiCe mixed oxides with different Ni/Ce ratios prepared by the citrate acid method are shown in Figure 1. Pure NiO shows

Figure 1. XRD patterns of NiCe mixed oxides catalysts with different Ni/Ce ratios.

sharp diffraction peaks. The peaks ascribed to NiO weaken dramatically when 5 mol % CeO2 is introduced in the NiCe mixed oxides and weaken continuously with the increase of amount of CeO2. The NiO peaks are quite weak in Ni50Ce50 and totally disappear in Ni20Ce80. The results indicate that the NiO phase is either dispersing well on the surface of CeO2 or incorporating to the ceria to form a solid solution.25 The diffraction signal of CeO2 does not appear in the Ni95Ce5 catalyst, suggesting that such a low amount of CeO2 may be amorphous or form too small a crystal to be detected. The peaks of CeO2 start to appear in the Ni90Ce10 catalyst, and with the increase of CeO2 content, the intensity of the CeO2 signal increases as expected. New crystal phase diffraction peaks are not clearly detected except those for NiO and CeO2. Compared to the peaks of the pure oxides (NiO, CeO2), the mixed oxides (especially Ni80Ce20, Ni70Ce30, and Ni50Ce50) display weaker and wider peaks. This result declares that the introduction of another metal oxide prevents the crystal growth of both metal oxides. Figure 2 compares the diffraction signal of the Ni90Ce10 catalyst synthesized by different methods. Ni90Ce10 prepared by the citrate acid method exhibits weakest and widest peaks, indicating that this method could inhibit the crystallization of catalyst. N2 adsorption isotherms (shown in Figures S1 and S2) are used to calculate the specific surface areas of NiCe mixed oxides, and the results are summarized in Table 1. The surface area of pure NiO is only 23 m2/g, and the value rises to 67 m2/ g rapidly with the introduction of 5 mol % CeO2. Ni90Ce10 catalyst shows the maximum surface area of 77 m2/g. More than 10 mol % CeO2 introduction leads to a decrease of surface area. Ni20Ce80 exhibits a very low value of 31 m2/g. The above 4505

dx.doi.org/10.1021/ie400242p | Ind. Eng. Chem. Res. 2013, 52, 4504−4509

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Figure 2. XRD patterns of NiCe mixed oxides catalysts prepared by different methods.

Table 1. Specific Surface Area and the XPS Results of the NiCe Mixed Oxides Catalysts Ce/Ni atom ratio catalysts

surface area (m2 g−1)

NiO Ni95Ce5 Ni90Ce10 Ni80Ce20 Ni70Ce30 Ni50Ce50 Ni20Ce80 CeO2 Ni90Ce10-CP Ni90Ce10-AE

23 67 77 76 73 64 31 50 50 54

a

surfacea

bulk

Ratiosurface/ Ratiobulk

0 0.08 0.28 0.53 0.87 2.1 6.5 ∞

0 0.05 0.11 0.25 0.43 1.0 4.0 ∞

1.6 2.5 2.1 2.0 2.1 1.6 -

Figure 3. TEM images of NiCe mixed oxides catalysts: (A) Ni90Ce10 and (B) NiO.

Determined by the ratio of Ce 3d peak area and the Ni 2p3/2 area.

results indicate that an appropriate amount of CeO 2 introduction could decrease the crystal size of NiO and CeO2, effectively enlarging the surface area. The surface areas of NiCe mixed oxides prepared by different methods are also listed in Table 1. Compared with Ni90Ce10, Ni90Ce10-CP and Ni90Ce10-AE exhibit smaller surface area. The results are in good agreement with the results of XRD. TEM is used as a complementary technique to examine the morphology of catalysts. TEM images for Ni90Ce10 and NiO are shown in Figures 3A and 3B, respectively. The average particle size of NiO is about 30 nm, which is bigger than that of Ni90Ce10. It is reported that the introduction of CeO2 could preserve high surface area and prevent the agglomeration of metal oxides, such as Co3O421 and CuO.24 A similar interaction may exist between NiO and CeO2. So, smaller sizes of particles and larger surface areas are observed in the Ni90Ce10 catalyst. The XPS binding energies (BE) of Ni 2p3/2 electrons are illustrated in Figure 4. In the case of NiO, a characteristic peak of unsupported NiO is presented at 854.4 eV,30 together with a shakeup peak from 860.0 to 862.4 eV. The binding energy shifts to higher value with the introduction of CeO2. When CeO2 content reaches 50 mol %, the peak of binding energy is presented at 855.2 eV. This fact further confirms that the

Figure 4. X-ray photoelectron spectra of Ni2p3/2 obtained on NiCe mixed oxides catalysts.

introduction of another metal oxide prevents the crystal growth of both metal oxides. The small particles of NiO and CeO2 exhibit interaction with each other, which modifies the nature of Ni species and shifts the binding energy to higher value. The Ce/Ni atomic ratio on the catalysts’ surface is summarized in Table 1. As for the Ni-rich catalysts (such as Ni90Ce10), the Ce/Ni surface atomic ratio is much higher than the bulk value, indicating that Ce phase prefers to aggregate on the surface of mixed oxides. Initial CeO2 introduction leads to the enrichment of CeO2 on the surface, and with the increasing amount of CeO2, the Ratiosurface/Ratiobulk increases from 1.6 to 2.5. With further introduction of CeO2, the Ce/Ni surface 4506

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reduced at a low temperature; in contrast, the Ni species incorporated into the solid solution could not be reduced in the H2-TPR profiles. So, when the Ni content is less than 80%, the β peak shifts to lower temperature. Only part of CeO2 could be reduced in this temperature range,25 and there is one very weak peak located at 670 °C. 3.2. Catalytic Performance. Using Ni90Ce10 prepared by different methods as catalysts, the conversion of N2O as a function of temperature is shown in Figure 6. The catalytic

atomic ratio decreases. This result suggests that crystallization of CeO2 starts to replace the aggregating on the surface of NiO. Considering part of NiO incorporation to CeO2 lattice, the value of Ratiosurface is still higher than that of Ratiobulk.31 It was reported25,31 that the appropriated amount of CeO2 introduction led to a strong interaction between NiO and CeO2. The strong interaction could significantly promote the oxygen mobility in the mixed oxides. H2-TPR serves as a powerful tool to verify the interaction between transition metal oxides and ceria.24,32 The H2-TPR profiles of the NiCe samples are compared in Figure 5. The surface area of NiO is small (23

Figure 6. Conversion of N2O over NiCe mixed oxides catalysts prepared by different methods. Conditions: 2600 ppm N2O, balance He, P = 0.3 MPa, GHSV = 19,000 h−1. Figure 5. H2-TPR profiles of NiCe mixed oxides catalysts with different Ni/Ce ratios. Conditions: 10 vol. % H2−Ar 50 mL·min−1 and ramping rate of 5 °C·min−1.

performance correlates with the results of surface area. The Ni90Ce10 prepared by the citrate acid method, which reveals the highest surface area and smaller particles, exhibits the highest activity. So, the citrate acid method is chosen to synthesize NiCe mixed oxides with different molar ratios, and the catalytic performance is shown in Figure 7. It is found that CeO2 is inactive for N2O decomposition under 400 °C. In comparison, NiO is much more active than CeO2 for N2O decomposition, and 100% conversion is achieved at 420 °C. Only 5 mol % CeO2 introduction leads to a significant increase

m2 g−1), hindering the diffusion. So, the reduction of pure oxide NiO proceeds as follows: NiO → Niδ+ → Ni0.33 It gives two broad peaks. The peak appearing from 270 to 340 °C is named the β peak, attributed to the first reduction step, and the peak appearing from 480 to 770 °C is named the γ peak, attributed to the second reduction step. The intensity of the β peak is much higher than that of the γ peak, suggesting that NiO can be reduced to a great extent in the first reduction step. When 5 mol % CeO2 is added, the surface area of Ni95Ce5 comes to 67 m2 g−1. The hydrogen diffuses well in the catalyst, resulting in the disappearance of the γ peak. At the same time, the two main reduction peaks of NiO in the β range change to one main peak and a shouldered peak. With further introduction of CeO2, the shouldered peak disappears, and the reduction of NiO species in the mixed oxides presents only one main peak. Combined with the results of XRD and TEM, the introduced CeO2 prevents the crystal growth of NiO, which could facilitate the diffusion. Besides, due to the small size of mixed oxides, NiO and CeO2 contact well with each other, and the interaction is enhanced. The interaction significantly promotes the oxygen mobility between NiO and CeO2,25 so the reduction of NiO species proceeds in a uniform way, and only one main peak is observed. A new small peak, named the α peak, appears at 204 °C in the Ni80Ce20 catalyst, which could be ascribed to the reduction of adsorbed oxygen in CeO2,33 indicating the formation of NiCe solid solution. In the Ce-rich mixed oxides, the NiCe solid solution starts to form, and NiO disperses well on the surface of CeO2. The highly dispersed NiO could be

Figure 7. Conversion of N2O over NiCe mixed oxides catalysts with different Ni/Ce ratios. Conditions: 2600 ppm N2O, balance He, P = 0.3 MPa, GHSV = 19,000 h−1. 4507

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of surface area, and it could shift the T100 from 420 to 380 °C. Among the NiCe mixed oxides, Ni90Ce10, which shows the highest surface area, exhibits the best performance for N2O decomposition and reaches complete N2O decomposition at 360 °C. More than 10 mol % CeO2 introduction leads to a decrease of surface area. Besides, the decrease of NiO content may also explain the decrease of the catalytic activity. Tail gas from nitric acid plant usually contains a small amount of O2. O atoms desorbing form active sites is considered as the rate-determining step.34,35 In the presence of O2, O atoms hardly desorb and block the active sites. So, if the active sites cannot be regenerated promptly, then the activity would decrease. To investigate the influence of O2 on the activity of NiCe mixed oxides, we test the catalytic performance of NiCe mixed oxides in the presence of 1% O2 in the reaction feed. As shown in Figure 8, NiCe mixed oxides Figure 9. Conversion of N2O over NiCe mixed oxides catalysts with different Ni/Ce ratios. Conditions: 2600 ppm N2O, 1% O2, balance He, P = 0.3 MPa, GHSV = 19,000 h−1.

Table 2. T50 and T100 of NiCe Mixed Oxidesa T50 (°C) catalysts

O2

without O2

NiO Ni95Ce5 Ni90Ce10 Ni80Ce20 Ni70Ce30 Ni50Ce50 Ni20Ce80

382 344 332 337 342 356 440

328 316 304 327 328 342 412

T100 (°C) O2

without O2

ΔT50 (°C)

ΔT100 (°C)

460 420 400 400 420 420

420 380 360 380 400 400

54 28 28 10 14 14 28

40 40 40 20 20 20

ΔT50 = T50 (O2) − T50 (without O2). ΔT100 = T100 (O2) − T100 (without O2). a

Figure 8. Conversion of N2O over NiCe mixed oxides catalysts prepared by different methods. Conditions: 2600 ppm N2O, 1% O2, balance He, P = 0.3 MPa, GHSV = 19,000 h−1.

and Ni50Ce50 present strong interaction between NiO and CeO2. Hence, the ΔT50 and ΔT100 of these catalysts exhibit low values.

prepared by the citrate acid method exhibit the highest activity for N2O decomposition in the presence of O2, and N2O is completely decomposed at 400 °C. The activity results of NiCe mixed oxides with different molar ratios are shown in Figure 9. With 5−30 mol % CeO2 promotion, these mixed oxides exhibit a much better catalytic performance than that of NiO. To compare the catalytic activity in the presence and absence of oxygen, the values of T50 and T100 (at which temperature 50% and 100% N2O decomposition conversion is achieved) are summarized in Table 2. ΔT50 is defined as the value of T50 in the presence of oxygen minus T50 in the absence of oxygen: ΔT50 = T50 (O2) − T50 (without O2). T100 is defined as the value of T100 in the presence of oxygen minus T100 in the absence of oxygen: ΔT100 = T100 (O2) − T100 (without O2). When 1% O2 is added to the reaction feed, the activity NiO is significantly inhibited. T50 and T100 increase 55 and 40 °C, respectively. When 5−10 mol % CeO2 is introduced to the mixed oxides, the O2-resistant ability is improved. T50 only increases 28 °C. Combined with the results of H2-TPR and XPS, in the Ni-rich catalysts, CeO2 aggregates on the surface of NiO and strongly interacts with it. Based on its ability to store and release reversibly considerable oxygen, CeO2 could promote oxygen migration effectively.24,25 By oxygen migration, active sites could be regenerated, and the O2-resistant ability is facilitated. Among the mixed oxides, the Ni80Ce20, Ni70Ce30,

4. CONCLUSIONS In this work, a series of NiCe mixed oxides has been synthesized and tested for the catalytic decomposition of N2O in the presence and absence of oxygen. Compared to the Ni90Ce10 catalysts prepared by the coprecipitation method and the ammonia-evaporation method, the catalyst prepared by the citrate acid method shows the best activity for N2O decomposition in the presence of O2. In the Ni-rich catalysts, CeO2 aggregates on the surface of NiO, prevents the sintering of that, and preserves a high surface area. 5−10 mol % CeO2 introduction to NiO leads to a significant increase in the catalytic activity. Besides, an appropriated amount of CeO2 introduction to NiO leads to a strong interaction in the NiCe mixed oxides catalysts. The strong interaction significantly promotes the oxygen mobility and facilitates the catalytic activity of N2O decomposition in the presence of O2.

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ASSOCIATED CONTENT

S Supporting Information *

Two figures. This material is available free of charge via the Internet at http://pubs.acs.org. 4508

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Corresponding Author

*Phone: +86-21-65642401; Fax: +86-21-65641740; E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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dx.doi.org/10.1021/ie400242p | Ind. Eng. Chem. Res. 2013, 52, 4504−4509