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Improvement of DeNOx catalytic activity of nanosized mixed oxides of La, Sr and Co by Ag

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2011 Adv. Nat. Sci: Nanosci. Nanotechnol. 2 025005

(http://iopscience.iop.org/2043-6262/2/2/025005)

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IOP PUBLISHING ADVANCES IN NATURAL SCIENCES: NANOSCIENCE AND NANOTECHNOLOGY

Adv. Nat. Sci.: Nanosci. Nanotechnol. 2 (2011) 025005 (6pp) doi:10.1088/2043-6262/2/2/025005

Improvement of DeNOx catalytic activityof nanosized mixed oxides of La, Srand Co by Ag dopingThi Minh Nguyet Tran1, Van Le Tiep2, Que Chi Tran1,Thi Hoang Yen Quach1, Thi Toan Nguyen1 and Quoc Trung Nguyen1

1 Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc VietRoad, Cau Giay Dist, Hanoi, Vietnam2 Institute of Applied Materials Science, Vietnam Academy of Science and Technology, 1 Mac Dinh Chi,Ho Chi Minh City, Vietnam

E-mail: tranminhnguyet@hotmail.com

Received 11 November 2010Accepted for publication 1 April 2011Published 3 May 2011Online at stacks.iop.org/ANSN/2/025005

AbstractSingle phase perovskites, such as LaCoO3 and La1−x Srx CoO3, exhibit catalytic activity onlyin oxidation reactions. However, a mixture of La1−x Srx CoO3, SrCoOx and Co3O4 oxidespresents both oxidation and reduction properties in the DeNOx reaction. To improve DeNOx

activity, we have added 1–2 wt% of Ag to a mixture of nanosized oxides of La, Sr and Co.These mixtures (with and without Ag) were prepared by the sol–gel method using citric acid asthe solvent. Two kinds of mixtures, A = La1−x Srx CoO3 + Co3O4 + SrCoOx and B = A + Ag,were characterized by x-ray diffraction (XRD) analysis, field emission scanning electronmicroscope (FESEM) and energy dispersive x-ray spectroscopy (EDS). Their catalytic activitywas studied by temperature programmed surface reaction (TPSR). The results showed that(i) for catalyst A, the total C3H6-conversion temperature is in the range 210–500 ◦C, while forcatalyst B that is 200–400 ◦C; (ii) the mixed oxide A exhibits a weak DeNOx activity, whilethe silver doping leads to a remarkable increase in the NOx -conversion from 29.4% (in theabsence of Ag) to 82.4% at low temperature 220 ◦C in the lean burn condition.

Keywords: nanosized mixed oxides, DeNOx , silver doping, conversion temperature

Classification numbers: 4.04, 5.06

1. Introduction

Catalysis plays a major role in reducing pollution andimproving the atmosphere. Precious metals, such as Pt, Pd andAu, have high activity and stability for the oxidation of COand volatile organic compounds (VOCs). However, owing tothe scarcity and high cost of noble metals, worldwide effortsare being made to replace noble metal catalysts with otherones. To solve this problem, the catalytic properties of mixedmetal oxides, typically oxides of Mn, Co and Fe, were studied.Among them, several perovskite-type ABO3 (where positionA is for rare earth metals and position B for transition metals)exhibit high activity in the deep oxidation of hydrocarbonsand VOCs. A partial substitution of ions in position A and/orposition B by A′ and B′ produces crystal lattice defects or

creates oxygen vacancies in the lattice, and enhances thecatalytic properties of the materials [1–4].

The data in [2, 3] show that several perovskite oxidesexhibit activity in direct decomposition of NOx , but theiractivity is low and the effective temperature is high (above700 ◦C). In an effort to reduce the temperature of NOx

removal, many experiments have been carried out. Theauthors of [5–10] have indicated that the addition of silverto perovskite and/or oxides leads to an improvement incatalytic activity not only in oxidation (of CO, VOCs,aromatic hydrocarbons) and oxygen storage but also in thereactions of NOx removal at relatively low temperature.Miyadera [5] showed that several oxides (Co/Al2O3,Cu/Al2O3, Ag/Al2O3, V/Al2O3 and Cr/Al2O3) on an oxidealumina support exhibit different activities for the selective

2043-6262/11/025005+06$33.00 1 © 2011 Vietnam Academy of Science & Technology

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 2 (2011) 025005 T M N Tran et al

catalytic reduction (SCR) in humid conditions. The authorfound that 2% silver doped on alumina has the highest activity,with 80.5% NOx conversion at 400 ◦C in a reaction conditionwith 10% H2O.

The selective catalytic reduction of nitrogen oxides byhydrocarbons has attracted much attention because it hasthe potential ability to remove NOx from diesel exhaust andother oxygen-rich flue gases. In the preceding report [11],we found that the single phase perovskite La1−x Srx CoO3

presents high catalytic activity only in the oxidation reaction.Meanwhile the mixed oxides, including perovskite phase andmono-/intermediate oxides, such as Co3O4, La2O3, SrO andSrCoOx , due to their multifunction, are able to catalyze theDeNOx reaction (removal of NO and NO2).

In this work, in an effort to improve the catalyticproperties of catalysts, the mixed oxides La1−x Srx CoO3 andSrCoOx , La2O3, SrO and Co3O4 were doped with 2% weighmetal silver and were studied by a temperature programmedsurface reaction (TPSR) in the lean burn condition.

2. Experimental

2.1. Catalyst preparation

Reagents such as La2O3, Co(NO3)2, SrCO3, C6H8O7 · H2O,AgNO3, NH4OH, CH3COOH and other chemicals wereof analytical grade, the solutions being diluted with twicedistillated water. Two kinds (A and B) of mixed oxides wereprepared by the citrate sol–gel method from the followingstarting reagents, respectively: [La2O3 + Co(NO3)2 + SrCO3]and [La2O3 + Co(NO3)2 + SrCO3 + AgNO3].

The synthesis procedures were as follows. A weighedamount of La2O3 and SrCO3 were first dissolved in thepresence of nitric and acetic acids, respectively, then mixedwith a stoichiometric amount of Co(NO3)2 and acidic citrateaccording to the following molar ratio: metal : citrate =

3 : 2. The solution was adjusted to pH = 5.5–6.5 withaqueous ammonia or acetic acid while stirring and heating to atemperature of 60–80 ◦C for 4 h. A dark-rose gel product wasobtained. The gel was then heated to 100 ◦C for 24 h in air,calcined at 300 ◦C for 2 h, then the temperature was raised to800 ◦C for 4 h. We produced sample A.

Sample B was similarly prepared from the reagentmixture mentioned above and 2% of AgNO3.

2.2. Methods for catalyst characterization

X-ray diffraction analysis was carried out using a D5000diffractometer with a Cu-Kα cathode (λ = 1.5406 Å).

The FESEM images were obtained by Field EmissionScanning Electron Microscope using a Hitachi S4800 system.

The energy dispersive x-ray spectroscopy (EDS) analysiswas performed using an FESEM Hitachi S4800 anda JEOL-JEM 2010 high-resolution transmission electronmicroscope, operating at 200 kV, with a PGT IMIX-PC EDSdevice.

The catalytic activity of the materials were studied byTemperature Programmed Surface Reaction (TPSR) with aspecial Siemens’s instrument.

Figure 1. XRD patterns of sample A (a) and B (b).

3. Results and discussion

3.1. Identification of materials by x-ray diffraction

The XRD patterns of samples A and B are presented infigures 1(a) and 1(b), respectively. Figure 1(a) shows thatsample A contained mainly the perovskite La1−x Srx CoO3

phase mixed with a small amount of SrCoOx and SrO.In figure 1(b) for sample B, these are typical peaks ofLa1−x−yAgySrx CoO3, SrCoOx , SrO and Ag. We can see thatthe intensity of the SrCoOx peak in figure 1(b) is higher thanthat in figure 1(a). This difference indicates that the quantityof SrCoOx in sample A is larger than that in sample B. Thismeans that the presence of Ag leads to a change to the ratio ofoxides contained in the sample. Moreover, ion Ag in sampleB was converted into the metallic state. This phenomenonis similar to the result of Piotrowska and Landmesser [9],who considered the conversion of Ag2O into Ag whenthe calcining temperature was increased to more than600 ◦C.

3.2. Results of FESEM and EDS studies

Figure 2 presents the FESEM images of sample A (a) andB (b). It indicates that the particle size of both samplesvaried in the range 40–80 nm. Apart from the silver species,which are well dispersed into base metal oxides, we canstill see numerous small metallic silver nanoparticles on thesurface of mixed oxides. According to Shimizu et al [8], x-rayabsorption near-edge structure (XANES) spectra showed that

2

Adv. Nat. Sci.: Nanosci. Nanotechnol. 2 (2011) 025005 T M N Tran et al

Figure 2. FESEM images of samples A (a) and B (b).

both Ag metal as the dominant silver species and Ag+ ion asthe minor silver species are present on the sample.

Through EDS studies on different points over bothsamples, we can observe various phases: the major phase ofcomplex oxide with perovskite structure ABO3/or AA′BO3

and the transition mixed oxide SrCoOx and SrO. Figure 3exhibits the existence of La1−x Srx CoO3, SrCoOx and SrO insample A. These EDS patterns are in agreement with the XRDresults mentioned above.

Figure 4 shows that metallic silver is dispersed unevenlyon the oxides La1−x Srx CoO3, SrCoOx and SrO. Table 1contains the data for three different points of sample Bcorresponding to EDS-patterns, figures 4(a)–(c), respectively:in figure 4(a), the Ag concentration is 2.82%, in figure 4(b) itis 0.95% and in figure 4(c) it is 0.84%. We can see the EDSresult, which coincides with that of X-ray spectroscopy, asmentioned above: while the sample was calcined at 800 ◦C,the silver ion Ag+ was converted into metallic silver Ag. Thedata in table 1 indicate the non-uniform chemical compositionon the surface of the sample. The sample included not onlyperovskite La1−x−yAgySrx CoO3, intermediate oxide SrCoOx ,oxide SrO and metallic silver Ag but also oxides of La andCo, which had not been observed by x-ray spectroscopy.

3.3. Catalytic properties

Figures 5(a) and (b) present TPSR profiles of samples Aand B = A + Ag, respectively. The TPSR were taken inthe following conditions. The composition of the reactant

Figure 3. EDS patterns of sample A (a) La1−x Srx CoO3, (b) SrCoOx

and (c) SrO.

mixture in the deNOx runs was 340 ppm NOx , 680 ppm C3H6,8 vol% oxygen, nitrogen as carrier gas. The total flow rate was15 litres h−1, giving an hour space velocity of 75 000 h−1. In atypical run, 200 mg catalyst was introduced into the reactor.The catalyst was first pre-treated in air at 500 ◦C for 2 h. Thenit was cooled to room temperature. The reaction was carriedout with a heating rate of 10 K min−1 in a flowing reactionmixture.

Figure 5 shows that the behaviors of the C3H6 oxidationand NOx removal for the two catalysts A and B are different.This can be explained by analyzing the TPSR data in tables 2and 3, as follows.

3.3.1. C3 H6 oxidation process. Over sample A, the C3H6

oxidation begins from 200 ◦C and total oxidation at 500 ◦C,while over B, the C3H6 oxidation runs in the range200–400 ◦C (table 2). Moreover, both the starting andfinishing times of the reaction over B are earlier than thoseof A. Therefore, Ag doped in the catalyst does not change thestarting reaction temperature but decreases the total oxidationtemperature of propane on 100 ◦C and increases the reactionspeed over B.

According to Luo et al and Shimizu et al [7, 8], dopingsilver in the composite oxide catalysts have some combination

3

Adv. Nat. Sci.: Nanosci. Nanotechnol. 2 (2011) 025005 T M N Tran et al

Figure 4. EDS patterns of sample B (a) La1−x Srx CoO3, (b) SrCoOx

and (c) SrO.

Table 1. EDS data from the different points of sample B.

From figure 4(a) From figure 4(b) From figure 4(c)

Weight Atomic Weight Atomic Weight AtomicElement % % % % % %

OK 38.42 75.09 22.59 61.45 22.70 60.59CoK 25.54 13.55 14.14 10.44 23.94 17.35SrL 23.32 8.32 44.72 22.21 31.03 15.12AgL 2.82 0.82 0.95 0.38 0.84 0.33LaL 9.91 2.22 17.61 5.52 21.49 6.61Totals 100.00 100.00 100.00 100.00 100.00 100.00

with the base metal, such as (Ag–Co), which may createa cooperative action, breaking the activation energy barriersof the individual catalyst. This results in a decrease in theactivation energy of the rate-determining step and the latticeoxygen of the Ag-doped oxide catalysts is much more reactiveat the lower temperature than that of the rest catalyst. Owingto the higher reactivity of lattice oxygen, the Ag-doped

0 20 40 60 80 100

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NO2 CONO

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CO2

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)

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Figure 5. TPSR profiles over catalyst A (a) and B (b).

Cycle 3Cycle 1

NO2 + HC

Cycle 2

HCxOy + 2 NON2 + H2O +CO2NO + O2 NO2 + HC HCxOy + 2 NON2 + H2O +CO2

Global reaction: 2 NO + Reductant (HC) + Oxygen = N2 + carbon dioxide + water

Cycle 3Cycle 1

NO2 + HC

Cycle 2

HCxOy + 2 NON2 + H2O +CO2NO + O2 NO2 + HC HCxOy + 2 NON2 + H2O +CO2

Cycle 3Cycle 1

NO2 + HC

Cycle 2

HCxOy + 2 NON2 + H2O +CO2NO + O2 NO2 + HC HCxOy + 2 NON2 + H2O +CO2

Global reaction: 2 NO + Reductant (HC) + Oxygen = N2 + carbon dioxide + water

Scheme 1. The three-cycle model for DeNOx reactions in lean burnconditions [12].

oxides are catalytically more active in the propylene (C3H6)combustion at low temperature.

3.3.2. NO x removal process. Figure 5 shows that theadsorption of C3H6 and NOx over B was greater thanthat of A. There are three periods of NOx consumption inthe temperature range 120–600 ◦C from 120 ◦C, 220 ◦C and350 ◦C over A and 120 ◦C, 220 ◦C and 330 ◦C over B. Allthree periods started over B at a time earlier than over A. TheNOx conversion over B is also higher than that over A. At therelatively low temperatures of 120 ◦C and 220 ◦C, an increasein NOx conversion from 23.5% and 29.4% (over A) to 76.5%and 82.4% (over B), respectively, was observed (table 3). Themaximum NOx conversion was 82.4% at 220 ◦C.

The TPSR curve (figure 5) shows that at 120 ◦C thereis the NO consumption without both NO2 formation and adecrease in C3H6 concentration. In this period, the decrease

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 2 (2011) 025005 T M N Tran et al

Table 2. Temperature and time of C3H6 oxidation over catalysts A and B.

Catalyst Temperature of Starting time of Finishing time oftotal oxidation C3H6 oxidation C3H6 oxidation

C3H6 (◦C) (min) (min)

A = La1−x Srx CoO3 + SrCoOx + SrO 200–500 57 84B = A + Ag 200–400 40 62

Table 3. Data of process removing NOx over catalyst A and B.

Catalyst A B = A + Ag

Reaction starting time (min) 45 63 74 25 40 52Temperature of maximum decomposition (◦C) 120 220 350 120 220 330Conversion NOx (%) 23.5 29.4 41 76.5 82.4 50

in NO concentration could relate to the storage of NO orthe direct NO decomposition over catalytic centers as oxidesMOx−1 by the following reactions:

2NO + 2MOx−1 → N2 + 2MOx ,

2NO → N2 + O2.

When a small amount of Ag was added, the silvernanoparticles were well dispersed on the catalytic oxides andplayed the role of reinforcing the catalytic activity of thematerial, according to [5], and the metallic silver Ag couldparticipate in the following reaction:

2Ag + MOx → Ag2O + MOx−1.

Due to this reaction, the metallic oxide-type MOx−1 wasproduced incessantly on the surface of the catalyst, andMOx−1 continued to participate in the following reaction:

2MOx−1 + 2NO → N2 + 2MOx .

Combining these factors, we can see that the catalytic reactionof Ag-doped oxides is enhanced in comparison with that of theoxides without Ag doping.

In the two next periods from 220 ◦C and 330 ◦C (forB)/350 ◦C (for A), the NO consumption was observedalso without NO2 formation, but the C3H6 concentrationdecreased with CO2 emission at a ratio of near to 1C3H6 →

3CO2. Hence the samples exhibit both oxidation and DeNOx

properties at these temperatures by other mechanisms incomparison with the first period. According to the model ofthree-function DeNOx reaction [12], the mechanism of 2ndand 3rd periods could be as follows:

In particular, for catalyst B (with Ag-doped), the DeNOx

reaction at 220 ◦C was increased notably. It agreed with [5],whose authors proposed that Ag on a suitable support is ableto move NOx with high conversion in the presence of oxygenorganic compound Cx HyO (as ethanol, acetone . . . ), whichplays the role of reductant. In the present catalytic system,following the schema mentioned above, the 2nd cycle formingCx HyO (from C3H6) is an intermediate product but plays therole of reductant enhancing the 3rd cycle. When the reactiontemperature increased above 330 ◦C or 350 ◦C, the C3H6

concentration decreased (because of the oxidation reaction),leading to a decrease in Cx HyO concentration, hence NOx

conversion decreased, as we can see, by 41% and 50% over Aand B, respectively. At temperatures above 400 ◦C, C3H6 wastotally converted into CO2 by oxidation over both catalystsand the DeNOx reaction also stopped.

In summary, the modification of mixed oxides by asmall amount (about 1.5–2%) of silver nanoparticles leadsto an increase in catalytic activity of material, especiallyfor the DeNOx reaction. The present results are prospectivecompared with previous publications: the obtained materialcould catalyze to the DeNOx reaction at low temperature(220 ◦C) with relatively high NOx conversion as 82.4%.

4. Conclusions

Two catalytic kinds as mixed oxides, A = La1−x Srx CoO3 +SrCoOx + SrO and B = A + Ag, were synthesized by thecitrate sol–gel method. Most of the particles had sizes rangingfrom 40 nm to 80 nm.

The metal silver nanoparticles were well dispersed ontothe nano mixed oxides but not equally on the surface. The2% weight of Ag added to the mixed oxides reinforcedremarkably the catalytic properties; exceptionally it improvedthe DeNOx capability of material. The NOx conversionincreased from 29.4% (in the absence of Ag) to 82.4% (inthe presence of Ag) at the low temperature of 220 ◦C and thetemperature of the total oxidation of propane C3H6 decreasedto about 100 ◦C.

The obtained catalyst is not too expensive but has strongactivity at this low temperature. It could be applied inenvironmental technology for exhaust gas removal.

Acknowledgments

This work was supported in part by Vietnam’s NationalFoundation for Science and Technology Development,No. 103.99-2010.25. The authors are thankful to ProfessorAcad. Nguyen Van Hieu for his encouragement and interestin this research.

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