SCR of NO by propene over nanoscale LaMn1−xCuxO3 perovskites

Applied Catalysis A: General 307 (2006) 85–97 www.elsevier.com/locate/apcata

SCR of NO by propene over nanoscale LaMn1
xCuxO3 perovskites Runduo Zhang a, Adrian Villanueva a, Houshang Alamdari b, Serge Kaliaguine a,* a

Department of Chemical Engineering, Laval University, Ste Foy, Que., Canada G1K 7P4 b Nanox Inc., 4975 rue Rideau, Local 100, Que., Canada G2E 5H5 Available online 27 April 2006

Abstract Nanoscale LaMn1
xCuxO3 perovskites with high specific surface areas were prepared by reactive grinding and characterized by N2 adsorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), H2-temperature programmed reduction (TPR), O2-, NO + O2- and C3H6temperature programmed desorption (TPD) and NO + O2-temperature programmed surface reduction (TPSR) under C3H6/He flow. The samples were then submitted to activity tests in the selective catalytic reduction (SCR) of NO by C3H6 with or without O2. The catalytic performances over unsubstituted LaMnO3 is observed with maximum N2 yield of 62% and a C3H6 conversion of 80% at 550 8C at a space velocity of 50,000 h
1 (3000 ppm NO, 3000 ppm C3H6, 1% O2 in helium). The N2 yield is however significantly improved by Cu incorporation into the lattice, achieving a remarkable N2 yield of 86% at 500 8C at 20% Mn substitution by Cu. The content of a-oxygen over lanthanum manganite is enhanced by Cu substitution, but the opposite occurs for excess oxygen. The better performance of Cu-substituted samples is likely to correspond to the facility in the formation of adsorbed nitrate species via the oxidation of NO by a-oxygen in addition to the intrinsic effect of Cu in NO transformation. However, the excessive a-oxygen content observed over LaCo0.8Cu0.2O3 accelerated the unselective hydrocarbon oxidation and suppressed the formation of organo nitrogen compounds, which led to a poor N2 yield with respect to Mn-based perovskites. A mechanism involving the formation of an organic nitrogen intermediate, which further converts into N2, CO2 and H2O via isocyanate, was proposed. The gas phase oxygen acts as a promoter when its concentration is lower than 1000 ppm because of the promotion of nitrate formation and organo nitrogen compounds transformation. O2 acts however as an inhibitor when its concentration is higher than 5000 ppm due to the heavily unselective combustion of C3H6 by O2, in the reaction of NO and C3H6 over LaMn0.8Cu0.2O3 at 400 8C. # 2006 Published by Elsevier B.V. Keywords: SCR of NO; Propene; Reactive grinding; High surface area; Mn-based; Perovskite; TPR; TPD

1. Introduction The purification of auto exhaust gases from gaseous impurities, including NO, hydrocarbons and CO is regarded as one of the main objectives of environmental catalysis. Great success has been achieved by applying three-way catalysts (TWC) in NO reduction using CO as a reducing agent [1]. These catalysts mainly consist of noble metals (platinum, palladium and rhodium) deposited on a CeO2-g-Al2O3 washcoat. Taking into account the limited supply and high cost of noble metals and the need to eliminate hydrocarbons from the exhaust gases, the study of NO reduction using hydrocarbons as reducing agents over low cost oxide-based catalysts appears as highly necessary. In these studies, C3H6 is often used as a model reducing hydrocarbon gas. Recently, special attention has been

* Corresponding author. Tel.: +1 418 656 2708; fax: +1 418 656 3810. E-mail address: [email protected] (S. Kaliaguine). 0926-860X/$ – see front matter # 2006 Published by Elsevier B.V. doi:10.1016/j.apcata.2006.03.019

paid to the perovskite-type mixed oxides due to their excellent redox properties, early pointed out by Libby [2]. Among the perovskites, lanthanum manganite has been investigated intensively because it is one of the few perovskite systems that display a wide range of deviations from stoichiometry (oxygen excess). This deviation can be modified by partial substitution of A and B cations, with significant changes in their physicochemical properties [3–5]. Moreover, Mn2O3 was also reported as a valuable additive to some selective catalytic reduction (SCR) catalysts such as Ce-ZSM-5 [6] and Au/Al2O3 [7]. Hence, LaMnO3 and its derivatives can be considered as promising candidates for NO reduction. Although the effect of the partial substitution of cation A, by elements having a valence state different from 3+, on catalytic performance in the NO reduction by C3H6 has been widely studied [8], the effect of the B site substitution on NO elimination has been much less investigated. Copper containing catalysts are of special interest because they are active in various reactions for the transformation of

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nitrogen oxides, as reviewed by Centi and Perathoner [9]. Low coordination isolated Cu ions are regarded as the active sites for SCR of NO over Cu-zeolites [10]. Our recent work showing that Cu/MCM-41 was also an active catalyst [11] indicates that the surface environment of the Cu ion is of crucial importance for the catalyst activity. Introducing Cu cations into the B sites of a perovskite structure is thus likely to yield good catalytic performances. Due to their preparation which usually involves a temperature as high as 800 8C to ensure the formation of the crystalline phase, perovskites are synthesized with the low specific surface areas of several m2/g by the usual ceramic method which suppresses their activity and limits their application [12]. A new preparation method designated as reactive grinding was developed in our group to synthesize perovskites at room temperature via high-energy-ball milling, achieving a relatively high surface area, on the order of 100 m2/ g when grinding additives are used [13–15]. A series of LaMn1
xCuxO3 perovskites with various atomic ratios x = 0, 0.1 and 0.2 was prepared by reactive grinding. The aim of this work is therefore to study the influence of Cu substitution in the B site of ABO3 solids on their physicochemical properties and their catalytic performances in NO reduction by propene. It is furthermore attempted to clarify the role of oxygen in NO reduction and propene oxidation, to determine the correlation between physicochemical properties and catalytic behavior, finally, to propose a reaction mechanism in NO-SCR over these perovskites. This work is therefore a continuation of our recently reported study of LaCo1
xCuxO3 catalysts [16]. 2. Experimental 2.1. Preparation of perovskites LaMn1
xCuxO3 perovskites were prepared by reactive grinding, by fully mixing powders of La2O3 (Alfa, 99.99%), Mn3O4 (Baker & Adamson, 98.22%) and CuO (Aldrich, 99.98%) in a high-energy-ball mill as described in ref. [14]. La2O3 was calcined at 600 8C for 24 h. Grinding was conducted in two steps of 8 h for synthesis and 10 h for refining with ZnO as the grinding additive. 2.2. Characterization of perovskites BET surface areas of the materials calcined at 500 8C for 5 h were measured by nitrogen adsorption at
196 8C using an automated gas sorption system (NOVA 2000; Quantachrome) operating in continuous mode. The specific surface area was determined from the linear part of the BET curve (P/P0 = 0.01– 0.10). Pore volume and average diameter were obtained from the pore size distribution curve, which was calculated from the desorption branch of N2 isotherms using the Barrett–Joyner– Halenda (BJH) formula. The chemical composition (Mn, Cu) of the prepared samples and the impurities (Fe, Zn) were analyzed by atomic absorption spectroscopy (AAS) using a Perkin-Elmer 1100B spectrometer. The La content of the perovskites was established using an

inductively coupled plasma spectrometer (ICP; Optima 4300DV; Perkin-Elmer). Powder X-ray diffraction (XRD) patterns were recorded using a Siemens D5000 diffractometer and Cu Ka radiation ˚ ) with a 0.058 step scan from 208 to 708 in 2u (l = 1.5406 A angle. Crystal domain sizes (D) were evaluated by means of the Scherrer equation after Warren’s correction for instrumental broadening. The identification of the crystal phases took place using the JCPDS data bank. The solids obtained after calcination at 500 8C were observed with magnification of 100,000 by scanning electron microscopy (SEM) using a JEOL JSM 840A instrument operated at 110 kV. Transient studies were carried out with a multifunctional catalyst testing and characterization system (RXM-100; ASDI), equipped with a quadrupole mass spectrometer (UTI 100) and a thermal conductivity detector (TCD). Prior to H2-TPR, the samples (50 mg) were pretreated under 10% O2/He flow at 20 cm3/min total flow rate (STP) for 1 h at 500 8C, cooled down to room temperature under the same atmosphere, purged with 20 cm3/min of helium for 40 min to remove the physically adsorbed O2, and then heated under a 20 cm3/min total flow rate of 5% H2/Ar stream with temperature rising up to 900 8C at a constant heating rate of 5 8C/min. The water in the effluent gas of the temperature programmed reduction (TPR) process was condensed via a cold trap with a mixture of dry-ice and ethanol. H2 consumption was monitored continuously by TCD using a 20 cm3/min flow of 5% H2/Ar as reference gas. Prior to the temperature programmed desorption (TPD) of O2, NO + O2 and C3H6, 50 mg samples were treated under an atmosphere of 10% O2, 3000 ppm NO + 1% O2, 3000 ppm C3H6, respectively, with a total flow rate of 20 cm3/min at 500 8C for 1 h and then cooled down to room temperature under the same flow, subsequently, flushed with 20 cm3/min He for 40 min to remove the physically adsorbed molecules. The temperature was then raised up to 500 8C (800 8C for TPD of O2) at a rate of 10 8C/ min. O2, NO, N2O and N2 desorbed during O2- and NO + O2TPD experiments were simultaneously detected and recorded on-line by mass spectrometry (MS) with the mass numbers of 32, 30, 44 and 14, respectively. Moreover, C3H6, CO and CO2 desorbing during C3H6-TPD experiments were monitored with the mass numbers of 41, 28 and 44, respectively. Temperature programmed surface reduction (TPSR) of NO + O2 under C3H6/He flow was performed with the same sample pre-treatment as that during TPD of NO + O2 experiment. The thermodesorption was however performed under 1000 ppm C3H6/He instead of He. The desorbed C3H6, NO, O2 together with generated CO2, N2, H2O were monitored by MS with mass number of 41, 30, 32, 44, 14 and 18, respectively. The gas responses obtained by MS were calibrated using standard mixtures. 2.3. Activity measurements The catalytic tests were performed in a fixed-bed quartz reactor under an atmosphere of 3000 ppm NO, 3000 ppm C3H6, with 1% or without O2, balanced by He at a space velocity of

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Table 1 Properties of prepared LaMn1
xCuxO3 solid solutions after calcination at 500 8C for 5 h Sample

Chemical composition

Specific surface area (m2/g)

Crystallite size (nm)

Pore volume (cm3/g)

Pore diameter (nm)

LaMnO3 LaMn0.9Cu0.1O3 LaMn0.8Cu0.2O3

La0.98Mn1.0O3d La0.97Mn0.91Cu0.09O3d La1.1Mn0.78Cu0.22O3d

40.6 40.7 42.6

11.3 13.0 14.4

0.15 0.20 0.20

10.8 12.4 10.8

Iron and zinc can be detected as contaminant with a weight percent less than 2 and 3%, respectively, of the total weight of prepared LaMn1
xCuxO3 samples.

50,000 h
1. The reactor was regulated using a temperature controller (CN3240; Omega) achieving temperatures from 200 to 700 8C by steps of 50 8C. The effluent gases (NO, N2O, NO2 and C3H6) were analyzed using a FT-IR gas analyzer (FTLA 2000; ABB). N2 and O2 were monitored using a gas chromatograph (GC; 5890; Hewlett Packard) equipped with TCD and separated by columns of molecular sieve 13 combined with a Silicone OV-101 column. Nitrogen oxides were also analyzed using a chemiluminescence NO/NO2/NOx analyzer (Model 200AH; Advanced Pollution Instrumentation). Organo nitrogen compounds, mentioned in the literature [17–19] as intermediates of NO reduction by hydrocarbons, can be detected by comparing the different NO2 values from the NOx analyzer and from the FT-IR gas analyzer. The higher NO2 value from the NOx analyzer was ascribed to the organo nitrogen compounds which are observed as an NO2 signal after the NO2/ NO converter of this analyzer. These organo nitrogen compounds were further identified by GC–MS (CP 3800-Saturn 2200; Varian) and confirmed to be mainly composed of C3H7NO2. 3. Characterization 3.1. Characterization results 3.1.1. Catalyst physicochemical characterization Table 1 lists the chemical compositions, BET surface areas, pore volumes and average diameters, and crystallite sizes of Mnbased samples synthesized by reactive grinding after calcination at 500 8C for 5 h. The tested catalysts had acceptable specific surface areas with values of approximately 40 m2/g even after calcination. The X-ray diffraction patterns of LaMn1
xCuxO3, where x is equal to 0, 0.1 and 0.2, solid solutions generated by reactive grinding are illustrated in Fig. 1. A comparison of these spectrum with JCPDS charts indicated that all Mn-based samples presented the rhombohedral perovskite structure and with a formula of LaMnO3.15 in the case of the parent perovskite (JCPDS card 50-0298). Minor phases such as CuO (JCPDS card 80-1268) in the case of LaMn0.8Cu0.2O3 and La2O2CO3 (JCPDS card 37-0804) in the case of LaMnO3 and LaMn0.8Cu0.2O3, were detected in addition to the major ABO3 perovskite phase. It was reported that stoichiometric LaMnO3, with a distorted orthorhombic perovskite structure as a consequence of the Jahn–Teller distortion of the oxide octahedron around the d4+ Mn3+ ion, could merely be prepared under a reducing or inert atmosphere [20]. In air, overstoichiometric LaMnO3+d with an oxygen excess would be synthesized accompanied by the formation of Mn4+ ions in order to reduce the static Jahn–Teller

Fig. 1. XRD patterns of LaMn1
xCuxO3 samples.

distortion of Mn3+. This results in a smaller deviation from the ideal cubic symmetry so that a symmetry changes from orthorhombic found in stoichiometric LaMnO3 to primitive rhombohedral found in overstoichiometric LaMnO3+d [21,22] was also observed in the present study. The crystallite sizes of prepared perovskites calculated from X-ray lines broadening are also reported in Table 1 with values in excess of 10 nm in all cases. Nanoscale LaMn1
xCuxO3 perovskites prepared by reactive grinding show higher surface areas than those prepared by the ceramic method [23,24]. Considering that both average pore diameters and crystallite sizes of those materials are around 10 nm, it was therefore deduced that the prepared perovskites had a porous structure formed via clustering of primary

Fig. 2. H2-TPR profiles of LaMn1
xCuxO3 perovskites.

LaMn0.314+Mn0.693+O3.15 LaMn0.284+Mn0.623+Cu0.1O3.10 LaMn0.304+Mn0.503+Cu0.2O3.05 – 0.96 1.15

c

d

Calculated by deconvolution of the O2 desorption curves. Calculated with 4 mmol/m2 of oxygen per monolayer (ref. [14]). Complete Cu2+ ! Cu+ would consume 206 mmol/g for LaMn0.9Cu0.1O3 and 411 mmol/g for LaMn0.8Cu0.2O3. The excess oxygen calculated according to the second O2 desorption peak at 600–650 8C. a

b

0.10 0.14 0.23 – (–) 155.7c (760 8C) 196.1c (763 8C)

b-O2 (mmol/g)

1.95 1.16 0.62 316.7 (609 8C) 189.3 (646 8C) 106.8 (599 8C) 16.8 (324 8C) 22.6 (323 8C) 40.1 (301 8C)

b-O2 a-O2 Excess O2 (mmol/g) a-O2 (mmol/g)

Excess O2

Number of monolayers desorbedb Amount of oxygen desorbeda

LaMnO3 LaMn0.9Cu0.1O3 LaMn0.8Cu0.2O3

Fig. 3. TPD of O2 profiles over LaMn1
xCuxO3 perovskites.

Table 2 Amount of O2 desorbed from LaMn1
xCuxO3 perovskites during O2-TPD experiments

3.1.2. Temperature programmed reduction and desorption 3.1.2.1. Temperature programmed reduction by hydrogen (H2-TPR). For investigating the reducibility of LaMn1
xCuxO3 samples, H2-TPR tests were performed and the results are plotted in Fig. 2. In the TPR profile of LaMnO3 a first minor peak was centered at 212 8C, whereas a broad feature appeared with one maximum close to 350 8C and another approximately at T > 700 8C. Additional peaks were also obtained with a maximum at 225–250 8C in the TPR profiles of LaMn1
xCuxO3 with x = 0.1 or 0.2, respectively. The minor peak at 212 8C was only slightly shifted to lower temperature in LaMn0.8Cu0.2O3 whereas the broad feature at 350 8C has essentially vanished at both x = 0.1 and 0.2. The existence of overstoichiometric oxygen (namely excess oxygen) in lanthanum manganite generated by reactive grinding was already confirmed by XRD in the present study. This kind of overstoichiometric oxygen was believed to be associated with Mn4+ and could be easily reduced due to its low coordination number with cation in perovskite lattice, so the former peak centered at 212 8C in the profile of LaMnO3 was attributed to the reaction of overstoichiometric oxygen. The subsequent peak at 350 8C and the shoulder at its rising part could be ascribed to the reduction of Mn3+ ! Mn2+ in bulk and over surface. The third peak observed above 700 8C was associated with the partial reduction of the Mn2+ ! Mn0. After Cu substitution, the sharp reduction peak related to Cu2+ ! Cu+ was observed at 250 and 225 8C for x = 0.1 and 0.2, respectively. The reduction of Cu+ ! Cu0 was believed to be related to the broad feature roughly centered at 580 8C (according to the literature [11,16,26]), overlapping with another broad feature at 420–430 8C. The latter was also thought to correspond to the complete reduction of extralattice copper oxide CuO [11], which was indeed observed in the

Experimental formula of perovskites d

perovskite particles in good agreement with ref. [25]. Furthermore, scanning electron micrographs of Mn-based perovskites (not shown) reveal sponge-like morphologies with spherical cluster of 50–100 nm, which was believed to be composed by individual nanoscale primary particles, forming a somewhat porous microstructure.

0.44 0.46 0.61

Mn4+/Mn3+

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Sample

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Fig. 4. TPD of NO + O2 profiles over LaMn1
xCuxO3 perovskites MS signal of: (a) NO, (b) N2O, (c) N2 and (d) O2.

XRD pattern of LaMn0.8Cu0.2O3. Thus, the downward shift of the Mn2+ ! Mn0 peak, centered at 900 8C in LaMnO3, upon Cu substitution was possibly associated with atomic hydrogen formed on metallic copper. It is quite remarkable that the Mn3+ ! Mn2+ reduction peak observed for LaMnO3 diminishes in the profiles of the Cusubstituted materials. This may be related to the dilution of Mn3+ concentration after Cu substitution as well as to the presence of oxygen vacancies in the LaMn1
xCuxO3 materials which suppresses the intercalated oxygen associated with the Mn3+ ions in LaMnO3. 3.1.2.2. Temperature programmed desorption of oxygen (O2-TPD). The mobility of O2 for Mn-based samples was

studied via TPD of O2 experiments as depicted in Fig. 3. The amount of O2 released from perovskites was calculated after deconvolution of O2 desorption curve and shown in Table 2. Two kinds of desorbing oxygen designated as aO2 and b-O2, respectively, were well illustrated in the previous O2-TPD studies for perovskites [14]. The a-O2 is ascribed to molecular O2 adsorbed in oxygen (anion) vacancies, while b-O2 is the oxygen liberated from lattice with reduction of the cation and the generation of oxygen vacancies. The excess oxygen, different from the a-O2 and bO2, can also be desorbed from lanthanum manganite during O2-TPD process due to the essential oxygen overstoichiometry of LaMnO3, however, without the generation of oxygen vacancies.

Table 3 Amounts of NO and O2 desorbed from LaMn1
xCuxO3 perovskites during NO + O2-TPD experiments Sample

LaMnO3 LaMn0.9Cu0.1O3 LaMn0.8Cu0.2O3 a b

NOa (mmol/g) 1

2

3

Total

5.8 (167 8C) 6.6 (184 8C) 2.3 (177 8C)

18.7 (283 8C) 12.3 (268 8C) 21.1 (271 8C)

11.4 (394 8C) 26.1 (386 8C) 36.4 (422 8C)

35.9 45.0 58.8

O2 (mmol/g)

Third NO/O2 b

N2 (mmol/g)

N2O (mmol/g)

11.8 24.1 38.2

0.95 1.08 0.97

3.3 3.7 4.6

5.8 6.7 8.3

Calculated by deconvolution of NO desorption curves. The molar ratio between amount in the third NO desorption peak (at T > 350 8C) and amount of O2 desorbed.

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The O2-TPD spectra show two series of peaks for LaMnO3: the former minor one occurred over the temperature below 400 8C with a small value compared to that calculated for the monolayer capacity. This desorption showed a slight increase as the Cu substitution increases to x = 0.1 and 0.2. In light of Rojas et al.’s conclusion that the charge compensation in LaMn1
xCuxO3 oxides is achieved by an oxygen loss with the eventual appearance of oxygen vacancies [27], this peak was ascribed to the desorption of a-O2 adsorbed on surface oxygen vacancies. Subsequently, an intense peak with a maximum at 608 8C was observed over LaMnO3, whose intensity declined with the increase of Cu content. Lisi et al. [28] pointed out that the introduction of Cu ions in lattice of Mn-based perovskites suppressed their excess (overstoichiometric) oxygen, so this peak was likely ascribed to the desorption of the excess oxygen. This oxygen yields the 212 8C peak in H2-TPR patterns of LaMn1
xCuxO3 (Fig. 2) which indeed decreases in intensity as the Cu content is raised. The amount of excess oxygen desorbed was quantified using the measured amounts of desorbed oxygen and found to be in a good agreement with the formula of LaMnO3.15 determined by XRD in the case of unsubstituted LaMnO3 perovskite (see Table 2). Moreover, the molar percent of Mnn+ ion in perovskites was calculated and is also shown in Table 2 according to the charge neutralization. Although the ratio of Mn4+/Mn3+ was gradually enhanced after Cu substitution, Mn4+ concentrations in lattice of LaMn1
xCuxO3 (x = 0.1 or 0.2) slightly decreased due to the dilution by Cu2+ ion incorporation compared to that of LaMnO3. O2 desorption at approximately 760 8C was observed over Cu-substituted samples. This is designated as b-O2 peaks and related to the desorption of O2 from the lattice. Considering that these peaks only occurred in Cu-substituted samples, they likely corresponded to the reduction of lattice Cu2+ to Cu+ in accordance with the increase in their intensities with Cu content. Moreover, no obvious b-O2 desorption peak was found over unsubstituted LaMnO3 at 700–800 8C possibly corresponding to the difficult reduction of Mn cation in lattice, which likely occurs above 800 8C during O2-TPD experiments. The trace amounts of a-O2 desorbing from LaMn0.8Cu0.2O3 suggest that Cu substitution can indeed produce oxygen vacancies which may combine with the overstoichiometric oxygen in the lattice. 3.1.2.3. Temperature programmed desorption of NO + O2 (NO + O2-TPD). The MS signals of NO (m/e = 30), N2O (m/ e = 44), N2 (m/e = 14) and O2 (m/e = 32) as functions of temperature were recorded during NO + O2-TPD analysis for LaMn1
xCuxO3 as indicated in Fig. 4a–d. The deconvolution and integration of these desorption peaks were carried out as shown in Table 3. Three severely overlapping NO desorption peaks were observed for all Mn-based perovskites with maxima in the region of 160–200, 250–300 and 380–410 8C depending on the chemical composition (Fig. 4a). Interestingly, the peak at 380–410 8C increased drastically in intensity with Cu substitution. The N2O and N2 desorptions (Fig. 4b and c) were detectable over the range of 100–300 8C followed by

Fig. 5. TPD of C3H6 profiles over LaMn1
xCuxO3 perovskites MS signal of: (a) C3H6, (b) CO and (c) CO2.

O2 desorption at T > 300 8C over Mn-based perovskites (Fig. 4d). 3.1.2.4. Temperature programmed desorption of C3H6 (C3H6TPD). Fig. 5a–c shows the desorption signals of C3H6 (m/ e = 41), CO (m/e = 28) and CO2 (m/e = 44) during the TPD of C3H6 experiments as functions of chemical composition of Mn-based perovskites. A quantitative analysis of the various gases desorbed from perovskites is presented in

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3.2. Characterization discussion

Fig. 6. The MS signals during TPSR of NO + O2 under C3H6/He flow over LaMn0.8Cu0.2O3 perovskite.

Table 4. In the case of LaMnO3, one sharp C3H6 desorption peak was observed at 70 8C. This C3H6 desorption was severely suppressed after Cu substitution in lattice, accompanied with higher CO and CO2 desorptions starting at T > 220 and 250 8C, respectively. The result showed that a transformation from C3H6 into CO2 could take place over Mnbased perovskites during the C3H6-TPD process, resulting in a CO2 desorption at high temperature. This C3H6 transformation was promoted by Cu substitution, leading to the suppression of the C3H6 desorption peak and enhancement of CO and CO2 desorption peaks at elevated temperature. Taking into account the result that a-oxygen of Mn-based catalysts can be further developed by Cu substitution, it was thus thought that a-O2 bonded to oxygen vacancies plays an important role in the C3H6 transformation, in concordance with the early report that the total oxidation of propene occurs through a suprafacial catalysis mechanism in which a-O2 is the dominant species participating in the reaction [29]. 3.1.2.5. TPSR of NO + O2 under C3H6/He flow. Fig. 6 presents the TPSR of NO + O2 desorption in C3H6/He flow over LaMn0.8Cu0.2O3. A large amount of oxygen desorbs from the perovskite surface under the atmosphere of 1000 ppm C3H6/He. This signal rapidly disappeared over the range of 160–240 8C simultaneously with a consumption of C3H6. At T < 300 8C, the NO desorption features were similar to those shown in Fig. 4a for the thermodesorption of NO. However, the third NO desorption peak at T > 300 8C in Fig. 4a was not observed in Fig. 6. A N2 desorption peak was observed simultaneously with NO desorption centered at 240 8C. The appearance of CO2 coincided with the disappearance of C3H6. Table 4 Amounts of C3H6, CO and CO2 desorbed from LaMn1
xCuxO3 during C3H6TPD experiments Sample

C3H6 (mmol/g)

CO (mmol/g)

CO2 (mmol/g)

LaMnO3 LaMn0.9Cu0.1O3 LaMn0.8Cu0.2O3

16.5 5.3 3.1

10.2 21.3 22.1

52.3 226.9 283.4

3.2.1. O2-TPD study The overstoichiometric LaMnO3 perovskite prepared by reactive grinding in air exhibits a predominant excess oxygen desorbed from lattice compared to the a-oxygen desorbed from surface during O2-TPD experiments. Furthermore, the substitution of Mn by Cu2+ leads to a positive charge deficiency which is compensated by oxygen vacancies resulting in the slight enhancement of a-oxygen content. Simultaneously, an obvious diminution in excess oxygen desorption takes place after Cu incorporation into the lattice. A similar result was found in the case of La1
xSrxMnO3 [30]. At x = 0.2 these authors report a decrease in excess oxygen desorption whereas at x = 0.4 the substitution brings about the appearance of a small a-oxygen desorption accompanied by a further decrease in excess oxygen desorption. Based on our previous description in ref. [14], the following process was assumed to occur during calcination of LaMnO3 perovskite: (1) Transient generation of anion vacancy during calcination of fresh sample:

(1) (2) Instantaneous formation of a-O2 by adsorbing O2 at anion vacancy: (2) The two reactions (3) and (4) are proposed for the a-O2 desorption process: (3)

(4)

Copper substitution would yield additional adsorption sites. (5) The oxygen desorption peaks observed at 600–650 8C in the TPD profiles of O2 over Mn-based perovskites was ascribed to the desorption of excess oxygen. 4Mn4þ þ 2O2
ð&Þ ! 4Mn3þ þ O2 excess oxygen

(6)

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In Eq. (6), the excess oxygen charge is compensated by cation vacancies (noted here as ‘&’) following the suggestions of van Roosmalen et al. [21]. Subsequently, the b-oxygens desorbing above 700 8C, in accordance with the temperatures previously reported for similar compounds [31,32], are defined as the oxygens liberated from the lattice leaving oxygen vacancies and reduced cations. The initial temperatures of b-oxygen desorption is thus generally considered as a measure of lattice oxygen mobility. For Cu containing samples, the b-oxygen peak centered at around 760 8C was corresponding to the Cu2+ ! Cu+ reduction, which seems easier than Mn3+ ! Mn2+ according to our H2-TPR experiments. (7)

Fig. 7. The correlation between O2 and the corresponding third NO desorption at T > 300 8C during NO + O2-TPD experiments.

The Mn3+ ! Mn2+ reduction and anion vacancy generation process after the desorption of lattice oxygen from the surface, observed in H2-TPR profiles (Fig. 2), was believed to occur at relatively high temperature (above 800 8C) during O2-TPD tests involving the following steps:

temperature NO peak in amount that essentially increases with Cu substitution (see Table 3). It is remarkable that the high temperature NO peaks and O2 peaks are essentially located at the same temperature and even the shapes of the peaks look alike. This is likely related to the report of Coq et al. [34] that the NO3
species would appear as a NO desorption peak at high temperature (T > 300 8C) with a parallel O2 desorption. Furthermore, the numbers of moles of NO desorbed as the high temperature peak are plotted against those of O2 (Table 3) in Fig. 7. Given the imprecisions associated with curve fitting of NO traces, the data are reasonably fitted with a line of slope of 1, indicating that the ratio NO/O2 is essentially 1 over these catalysts. This strongly suggests that the high temperature NO desorption is associated with the desorption of an oxidized nitrogen oxide species having the general formula NO3
. The oxidation of NO was likely realized by a-oxygen as O2
ion radicals form (according to Eq. (2)) during O2 adsorption:

(8) Thereafter, b-oxygen desorption proceeds via the diffusion of oxygen from the bulk to the surface:

(9)

3.2.2. NO + O2-TPD study Infrared spectroscopic studies of NO adsorbed over LaMnO3 [33] have been reported, showing bands related to mononitrosyl, dinitrosyl species interacting with B site cations as well as nitrates bonding to oxygen vacancies. The formation of nitrosyl, nitrite or nitrate species during NO-TPD over Cu/ MCM-41 was also observed in our earlier work [11]. In the present study, the adsorbed species formed during coadsorption of NO and O2 over Mn-based perovskites were investigated by recording the MS signals of NO, N2O, N2 and O2 in the effluent of NO + O2-TPD tests.

(10)

Compared to Fig. 3, no a-oxygen desorbing below 300 8C is observed in Fig. 4d, indicating that reaction (10) involving aoxygen occurs during NO + O2-TPD process. The desorption of nitrate species leads to the formation of NO and O2 with a molar ratio NO/O2 equal to 1.

(11)

Although the NO desorption traces (m/e = 30) for Mnbased perovskites, reported in Fig. 4a, show essentially three overlapping peaks, the oxygen traces (Fig. 4d, m/e = 32) show that oxygen desorbs simultaneously with only the high

Besides the NO desorption related to nitrate species, the other two peaks in the NO trace of NO + O2-TPD (Fig. 4a) show similar features among the three Mn-based perovskites, as does also the desorption of N2O at 100–300 8C (Fig. 4b). The

R. Zhang et al. / Applied Catalysis A: General 307 (2006) 85–97

thermal stability of nitrogen containing adspecies over Cu/ ZSM-5 was reported to enhance with the increase in oxidation state of nitrogen [9]. Valyon and Hall [35] also found that both mononitrosyl and dinitrosyl species are weakly adsorbed on the same catalyst compared to nitrate species. Therefore, the former two NO desorptions likely correspond to mononitrosyl and dinitrosyl species, respectively. The adsorption of NO to form mononitrosyl and dinitrosyl species

93

over Mn-based perovskites, likely realized via suprafacial oxidation by a-O2 bonded to anion vacancies. From our C3H6TPD study, the following scheme may be proposed for the adsorption of C3H6 over perovskites. Using the site described by the left-hand side of Eq. (3), the olefine should adsorb on the electron deficient part of the site as described in Eq. (18). The – OC3H6 radical (allylic adspecies) formed can now react with the surrounding a-O2 to form CO2 and water as described in ref. [36].

(18)

(12)

(13)

From our TPD-O2 results we know that the surface concentration of a-O2 is higher on Cu-substituted samples. The higher content of adsorbed carbonaceous species in the latter samples (see Table 4) indicates that the total primary adsorption of propene is also enhanced. This might suggest that the surface concentration of the electron deficient site similar to is also increased upon Cu substitution. An other explanation could be that the enhanced a-O2 content allows a more complete rapid oxidation of the adsorbed olefine thereby

is described by Eqs. (12) and (13), respectively. The desorption of both mononitrosyl and dinitrosyl species generates NO: (14)

(15)

The NO dissociation yielding N2O and nitrogen in NO + O2TPD experiments is believed to involve the following steps:

(16)

(17)

3.2.3. C3H6-TPD study A significant C3H6 conversion, which was further accelerated by Cu substitution, was observed during C3H6 desorption

regenerating the

site for further adsorption.

3.2.4. TPSR of NO + O2 under C3H6/He flow It is clearly observed from Fig. 6 that the NO desorption features observed at T < 300 8C are similar to the two peaks ascribed to mononitrosyl and dinitrosyl species in the TPD of NO + O2 and reported in Fig. 4a. The NO desorption at high temperature (T > 300 8C) in Fig. 4a, which was ascribed to nitrate species, is not present any more in the traces of Fig. 6 suggesting that among the adsorbed NO, only the nitrate species is highly active toward C3H6, and completely consumed by this reducing agent. A significant NO conversion was indeed found at temperatures higher than 350 8C in the activity tests (Fig. 8a). At such high temperature, only the nitrate species is present on the perovskite surface giving another clue for the high reactivity of nitrate species in the C3H6-SCR process. As seen in Fig. 6 molecular oxygen desorbs even in the presence of C3H6 in the gas phase for temperatures below 240 8C, indicating that the surface oxidation which consumes a-oxygen is not completed below this temperature. The CO2 formed by this reaction desorbs from the surface essentially above 240 8C which implies that either CO2 or an oxygenated carbonaceous species leading to its formation is adsorbed on the perovskite surface above 240 8C. N2 was also detected during this experiment again suggesting that the reduction of the nitrate surface species by propene is associated with the SCR process.

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than 5%) was found in the region of 200–300 8C, while the significant conversion to N2 begins at 300 8C over the three perovskites. The NO conversion to N2 passed through a maximum of 62% at 550 8C and then declined down to 39% at 650 8C and finally recovered up to 56% at 700 8C over the unsubstituted LaMnO3. An enhancement in conversion to N2 was observed at low temperature (T < 500 8C) over lanthanum manganites after Cu substitution, resulting in N2 yields of approximately 80 and 86% at 450 8C over LaMn0.9Cu0.1O3 and LaMn0.8Cu0.2O3, respectively. Results obtained in a previous work for a LaCo0.8Cu0.2O3 catalyst in the same conditions [16] are also shown for comparison. Fig. 8b shows that the generation of organo nitrogen compounds occurs simultaneously with the decrease in NO conversion to N2 observed at 550–650 8C over LaMn1
xCuxO3 perovskites (Fig. 8a). These organo nitrogen compounds are widely mentioned in the literature as intermediates during SCR of NO by hydrocarbons [37]. Yields as high as 40% were measured at temperatures in the 600–650 8C range (Fig. 8b). The catalytic conversion of C3H6 over the Mn-based perovskites is described in Fig. 8c, with a rapid increase up to about 75% at 200–400 8C and then a more progressive enhancement at higher temperature. This conversion level at T < 400 8C was moderately improved after Cu substitution. NO2 and N2O productions (less than 5%) over Mn-based perovskites are neither shown nor further discussed in the present study. The comparison between catalytic properties of LaMn0.8Cu0.2O3 and LaCo0.8Cu0.2O3 indicates that the former shows a higher conversion to nitrogen at 350–500 8C and a lower C3H6 conversion at T < 400 8C whereas the cobalt containing perovskite yields much less organo nitrogen compounds. The catalytic performance of LaMn0.8Cu0.2O3 in NO reduction by C3H6 in absence of oxygen is presented in Fig. 9, showing a maximum N2 yield (78%) at 400 8C followed by a decline (58%) at 500 8C and a final development (95%) at 700 8C. C3H6 oxidation conversion increased monotonically with increasing temperature, reaching about 40% at 700 8C which is quite lower than the value obtained with 1% O2. Organo nitrogen compounds were generated during this reaction with a maximum of 33% at 500 8C over LaMn0.8Cu0.2O3.

Fig. 8. (a) NO conversion to N2 in C3H6 + NO + O2 reaction over LaMn1
xCuxO3 and LaCo0.8Cu0.2O3 perovskites. (b) Yield of organo nitrogen compounds in C3H6 + NO + O2 reaction over LaMn1
xCuxO3 and LaCo0.8Cu0.2O3 perovskites. (c) C3H6 conversion in C3H6 + NO + O2 reaction over LaMn1
xCuxO3 and LaCo0.8Cu0.2O3 perovskites; conditions: GHSV = 50,000 h
1, 3000 ppm C3H6, 3000 ppm NO and 1% O2.

4. Activity tests and reaction mechanism 4.1. Activity test results The temperature dependence of NO conversion to N2 over LaMn1
xCuxO3 is shown in Fig. 8a. A slight N2 formation (less

Fig. 9. Reaction of C3H6 and NO in the absence of O2 over LaMn0.8Cu0.2O3; conditions: GHSV = 50,000 h
1, 3000 ppm C3H6 and 3000 ppm NO.

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perovskites (Figs. 8b and 9), which strongly implies a mechanism with organo nitrogen compounds as intermediates. It was reported that the formation of organo nitrogen compounds via reaction of adsorbed hydrocarbons with the surface nitrate would be the rate-determining step of the whole SCR of NO process [40–42]. A kinetic study by FT-IR showed that the rate of nitrate consumption was close to that of N2 formation over Ce/ZSM-5 [42] and Al2O3 [43]. Hence, the formation of 1-nitropropane must involve a reaction between the adsorbed species formed in reaction (18) and the nitrate species:

Fig. 10. O2 effect on catalytic performance in NO reduction by C3H6 over LaMn0.8Cu0.2O3; conditions: T = 400 8C, GHSV = 50,000 h
1, 3000 ppm C3H6, 3000 ppm NO and 0–20,000 ppm O2.

(19) The catalytic performance, including yields of N2 and organo nitrogen compounds, conversion of C3H6, as functions of O2 concentration over LaMn0.8Cu0.2O3 at 400 8C are depicted in Fig. 10 to clarify the role of gaseous O2 during SCR of NO. With an increase in O2 concentration, N2 yield initially increased, and then passed through a maximum of about 82% at 1000 ppm O2, finally decreased down to 2% with further raise in O2 concentration up to 2%. A similar effect has been mentioned by other authors for La0.8Sr0.2Mn0.5Cu0.5O3/Al2O3 at 375 8C [38] or Cu/ZSM-5 at 300 8C [39]. Gaseous O2 (1000 ppm) also improves the yield of organo nitrogen compounds achieving a maximum of 6%, however, further increase of O2 feed concentration leads to a decline and complete suppression when O2 concentration exceeded 1%. More O2 introduction resulted in a monotonic increase in C3H6 conversion. 4.2. Activity test discussion and reaction mechanism The slight N2 formation observed over LaMn1
xCuxO3 perovskites below 300 8C (Fig. 8a) seems related to the NO dissociation, found over the region of 200–300 8C during NO + O2-TPD tests (Fig. 4b and c). Simultaneously, the role of C3H6 during NO dissociation is to generate oxygen vacancies and maintain a reduced surface, which seems necessary for NO decomposition (see Eqs. (12) and (13)). C3H6 is oxidized in this process. Note that the NO reduction over Mn-based perovskites (Figs. 8a and 9) are only significant at temperature higher than 350 8C, where nitrate is the only remaining species over perovskites according to the NO + O2-TPD study. The high activity of nitrate species to react with propene was established via TPSR of NO + O2 desorption in C3H6/He tests which indicated that the formation of nitrate species is the first important step for NO reduction. This is in agreement with our previous finding in C3H6-SCR of NO over Cu-MCM-41 [11]. Furthermore, organo nitrogen compounds, mainly composed of 1nitropropane (C3H7–NO2), were detected in the region of 450–700 8C during the NO reduction over Mn-based

Among the three Mn-based perovskite samples, the worst NO conversion to N2 was achieved over unsubstituted LaMnO3 with a maximum of 62% at 550 8C. A significant improvement in N2 yield at T < 500 8C after Cu substitution into B site was observed. Besides the essential effect of copper ions in the transformation of nitrogen oxides [9], Cu substitution promoted the formation of nitrate species due to the enhancement of aoxygen thereby further accelerating the generation of organo nitrogen compounds from nitrate and adsorbed propene, which is believed to be the rate-determining step in SCR of NO by C3H6. The reactivity of 1-nitropropane was investigated by Haj et al. [44]. It was found more rapidly decomposed in O2 or NO + O2 compared to C3H6, the final products being N2, CO2 and H2O. This result stresses the fact that such species are highly reactive in the presence of O2. In our case the organo nitrogen compounds were only detectable in the gas phase at T > 400 8C whereas N2 formation occurred as low as 300 8C. This seems to indicate too low desorption rate to allow detection under these conditions. Based on the suggestion of Blower and Smith [45], a further transformation of R-NO2 into isocyanate (R-NCO) by a cyclic intermediate may be formulated as:

(20) Finally, the C2H5NCO was proposed to react with NO via the coupling of nitrogen atoms to yield the product of N2 and CO2 according to the assumption of Witzel et al. [46]. At the same time, the participation of O2 (especially a-oxygen) can promote the oxidation of ethyl group into CO2 and H2O and accelerate the formation of N2 via the coupling of nitrogen atoms.

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(21)

At the temperature range of 450–600 8C, a faster generation and accumulation of organo nitrogen compounds over Mn-based perovskites leads to an inhibition of N2 yield (Fig. 8a and b). However, this organo nitrogen compounds can be well decomposed at elevated temperature with a recovery of NO conversion to N2. O2-TPD experiments in the present study exhibited a small a-oxygen and a predominant excess oxygen desorption from Mn-based perovskites. Larger amounts a-oxygen were observed in our previous O2-TPD study of LaCo0.8Cu0.2O3 [16]. A comparison of catalytic behaviors between LaCo0.8Cu0.2O3 and LaMn1
xCuxO3 was also shown in Fig. 8a–c in order to clarify the correlation between catalytic properties of perovskites with different non-stoichiometric oxygen [2,29]. It is clear from Fig. 8c that a higher C3H6 oxidation was obtained over LaCo0.8Cu0.2O3 compared to Mn-based samples, which is believed to be related to the abundance of a-oxygen over LaCo0.8Cu0.2O3. Compared to Mn-based solids, a poor N2 yield at low temperature (T < 500 8C) associated with a lower yield of organo nitrogen compounds at 500–700 8C was found over LaCo0.8Cu0.2O3 strongly suggesting that organo nitrogen compounds are more difficult to form over LaCo0.8Cu0.2O3. It was proven that a suitable amount of a-oxygen is necessary for the formation of nitrate species and organo nitrogen compounds resulting in an enhancement in N2 yield over Mnbased perovskites after Cu substitution. Nevertheless, in Cobased perovskites too much a-oxygen would diminish the number of reductant molecules available for the SCR reaction leading to suppression of organo nitrogen compounds and N2 formation due to the competitive hydrocarbon oxidation as illustrated in Eq. (18). On the other hand, abundant a-oxygen over LaCo0.8Cu0.2O3 would accelerate the decomposition of organo nitrogen compounds giving another reason for the scarce organo nitrogen compounds formation. Several points can be raised about the crucial role of O2 based on the proposed mechanism. As a promoter, O2 can oxidize NO into strongly adsorbed nitrate species and accelerate the transformation of organo nitrogen compounds and isocyanate to get the desired products; as an inhibitor, O2 can lead to the consumption of the reducing agent by the complete oxidation of C3H6. In the absence of O2, the investigation of catalytic behavior in C3H6 + NO reaction over LaMn0.8Cu0.2O3 was carried out, as depicted in Fig. 9, achieving a higher N2 yield at T < 400 8C and a quite lower C3H6 conversion compared with those in the presence of 1% O2. The maximum in organo nitrogen compounds yield of 33% shifts down to 500 8C. Furthermore, a detailed study of the O2 effect on catalytic performance of LaMn0.8Cu0.2O3 at 400 8C is reported in Fig. 10, showing that NO conversion to N2 and organo nitrogen compounds yield would pass through a maximum at 1000 ppm O2, and then tend to decline at higher O2 concentration. The slight increase in N2 yield by a little

amount of O2 (less than 1000 ppm) can be ascribed to the facility in formation of nitrate species. On the contrary, N2 yield will be inhibited at O2 concentration higher than 5000 ppm because the unselective combustion of the reducing agent with O2 becomes faster than the NO-SCR over Mn-based perovskites and diminishes the number of reductant molecules remaining for the SCR reaction. It can indeed be seen from Fig. 10 that C3H6 was consumed significantly with the increase in O2 concentration. 5. Conclusion LaMnO3 and corresponding Cu-substituted perovskites were synthesized by reactive grinding with rhombohedral structure, crystal domain sizes around 10 nm and high specific surface areas of about 40 m2/g. The NO reduction by propene was mainly controlled by a mechanism involving organo nitrogen compounds, possibly generated from the interaction between nitrate species and adsorbed C3H6. Subsequently, an isocyanate intermediate forms from organo nitrogen compounds and reacts with NO and/or O2 to get the final products. Meanwhile, the slight contribution of NO dissociation for N2 yield over Mn-based perovskites was also observed at T = 200–300 8C. Although a predominant excess oxygen was found desorbing in O2-TPD experiments over Mnbased perovskites, a-oxygen likely makes the dominant contribution in catalytic activity of SCR process. a-Oxygen surface concentration over lanthanum manganite was enhanced likely due to new anion vacancies generated upon Cu substitution in lattice. This increase simultaneously accelerated the formation of nitrate species as established in TPD experiments. The good performance achieved over Cu-substituted perovskites can be ascribed to the easy formation of nitrate species, which was considered as a crucial precursor with high reactivity in reaction with C3H6 according to TPSR tests. A study of the effect of gaseous O2 on catalytic performance of LaMn0.8Cu0.2O3 was performed. A small amount of O2 is necessary to facilitate the formation of nitrate species resulting in an improvement in the NO reduction and C3H6 oxidation, whereas higher O2 partial pressure reduces the N2 yield by depleting the reducing agent through complete C3H6 oxidation. Acknowledgements The financial support of NSERC through its industrial chair program is gratefully acknowledged. We thank Nanox Inc. for the preparation of the perovskite samples. References [1] H.S. Gandi, G.W. Graham, R.W. McCabe, J. Catal. 216 (2003) 433. [2] W.F. Libby, Science 171 (1971) 499.

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