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React.Kinet.Catal.Lett. Vol. 71, No. 2, 253-262 (2000)

RKCL3689 PREPARATION AND CHARACTERIZATION OF SUPPORTED VANADIA CATALYST FOR THE SELECTIVE CATALYTIC REDUCTION OF NO WITH NH3 Claudia E.Quincoces, Ana K. de Figueiredoa, Araceli Lavata and María G. González Centro de Investigación y Desarrollo en Procesos Catalíticos, CONICET,UNLP, 47 N0 257, (1900) La Plata, Argentina. Fax: 54-21-425 42 77. e-mail:[email protected] a Facultad de Ingeniería, UNCPBA, Av. del Valle 5737, Olavarria, Argentina Received March 6, 2000 In revised form August 28, 2000 Accepted September 22, 2000

Abstract The structural characteristics and the catalytic behavior are analyzed on catalysts obtained by oxidative degradation of a planar VO complex on alumosilicate support (cordierite). By means of different techniques (FTIR, DRX, TGA), vanadium oxide formation was determined during the thermal decomposition of the organometallic precursor and the formation of NH4 species during the reaction. Besides, the catalytic activity was also determined for the selective reduction of NO with ammonia. Keywords: VO-catalyst, thermal oxidative degradation, NH3-SCR

INTRODUCTION One of the aspects to be considered for the design of active catalysts in the purification of exhaust and waste gas is the decrease of working temperature so that energy can be saved. According to reference [1], this can be achieved by means of catalysts based on loading the carriers by impregnation out of solutions or suspensions with planar or nearly planar complexes of transition metals, such as Cu, Mn, Fe, V, on aluminosilicate supports. The complexes are then decomposed and oxidized to products, mainly the respective metal oxides with different structural and catalytic properties. 0133-1736/2000/US$ 12.00. © Akadémiai Kiadó, Budapest. All rights reserved.

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Catalyst activation takes place as the result of formation of a disordered material with metal atoms very separated, anchored to the aluminosilicate surface in a stressed ligand field instead of an ordered oxide structure. This fact gives rise to a high activity and selectivity at low temperatures. In this paper, the transformation undergone by the complex is analyzed during its oxidative degradation and its catalytic performance for the selective reduction of NO with NH3 in excess of O2, used industrially for the decontamination of stationary emission sources [2,3]. The identification of species in the catalyst prior to and during the reaction is made by means of FTIR and XRD spectroscopy. EXPERIMENTAL Synthesis and preparation of samples The blue complex was prepared by the method of Rowe and Jones [4]. Purity was checked by IR spectra. Very pure cordierite powder was impregnated with chloroform solution of the planar complex in the following way. The concentrated solution, made out of 24% VO(acac)2 in 5 mL of chloroform, was added to 0.5 g of cordierite thoroughly mixed, stirring constantly during 30 min until all solvent evaporated. The resulting solid was light green. The catalyst precursor was transformed into the active catalyst by controlled heat treatment. In order to study the thermal behavior, the catalyst was heated in air from 25 to 5500C. Pure samples of the complex were treated as well. The thermolysis product and intermediates at several temperatures were analyzed by IR and DRX spectroscopy. Catalyst characterization Supported samples and pure complex were characterized by TGA, FTIR and XRD spectroscopy. The infrared spectra were recorded with a FTIR Nicolet Magna 550 using the KBr pellet technique. X-Ray diffraction patterns of powder samples were obtained with a Phillips PW 3710, Cu-Kα radiation. The thermogravimetric analysis was carried out in a Shimadzu TGA 50 analyzer over a 15 mg sample heated at a rate of 100C min-1 in a 20 mL min-1 air stream. The SCR of NO with NH3 was carried out in a microreactor using a mixture of NH3/ NO/O2 (0.085/0.075/2 (vol.%) and He as gas balance, with a programmed temperature increase between 200 and 4000C and a flow rate of


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50 cm3 min-1. The mass of the catalysts was 0.06 g. The inlet and outlet reactor gases were analyzed in a Shimadzu GC-8 gas cromatograph fitted with a thermal conductivity detector. Gas chromatographic analysis was done at 400C by using a CTR1 (Alltech) column packed with Porapack mixture and 5A molecular sieve. He gas ( 20 cm3 min-1) was used as carrier. RESULTS AND DISCUSSIONS Catalyst precursor degradation The thermal behavior of VO-catalyst precursor was investigated by controlled heat treatment in an oxidizing stream by TGA and by subsequent analysis of the IR spectra and XRD patterns of the pyrolysis residues at different temperatures. The degradation of this compound seems to occur practically in a single step. It begins at 1200C and extends up to 2000C with a great weight loss in this range. Another slight weight loss is observed at 3000C.

Fig. 1. FTIR spectra of VO catalyst. a) supported catalyst spectra, b) sample decomposes at 300oC, c) sample decomposes at 500oC

The supported catalyst FTIR spectra (Fig. 1a) show the most intense bands such as those belonging to absorptions of the support. The aluminosilicate cordierite is characterized by IR bands ranging from 1200 to 250 cm-1. The main absorptions are centered in 1200, 959, and ca. 800 cm-1, a few medium

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intensity over 600 cm-1 and a broad multiplet between 500-400 cm-1. The presence of such bands in the IR spectra imposes an unfortunate interference in the region of interest where the VO-complex and V-oxides exhibit their typical bands and therefore it is difficult to reach a clear assignment. In order to overcome this shortcoming and also to get a deeper insight into the species involved, after thermal decomposition we recorded the FTIR spectra of the pure compound. The structure of the VO-complex is known with certainty and the complete assignment of the IR bands was carried out based on known data for the free ligand as well as for different VO-complexes and salts [5-7]. The purity of the sample was checked by XRD. The study of the thermolysis product of the isolated complex, is compared with the supported catalyst to correlate these results with experiments in the reactor and discuss the probable role played during the catalytic reaction. X-Ray diffraction pattern of the residues at 2000C shows an amorphous solid but the material checked at 3000C is still not totally crystallized. Samples recrystallized at 5000C, according to XRD, give an ordered oxide structure and the product is almost pure V2O5. According to IR spectroscopic analysis, the sample decomposes at 2000C, the most important typical bands belonging to acac already disappear, giving rise to V-O vibrations only due to the fact that these bonds in V-oxides formed once the complex decomposes. Once the VO-complex decomposes a light brown yellowish colored oxide is formed. The presence of V2O5 and to a minor extent of V2O4 at 3000C (Fig. 1b) could be clearly confirmed by their characteristic IR spectra and XRD results. Upon heating at 5000C, almost pure V2O5 is obtained as the final product of oxidative degradation of the compound (Fig. 1c). Theoretical weight changes are in acceptable agreement with experimental changes for the overall decomposition in spite of the known nonstoichiometry of V2O5-x. As no significant weight gain is detected above 2000C, temperature at which V2O5 is already detected, the necessary oxygen is taken up during thermal decomposition of the ligand. The occurrence of a strong and sharp IR absorption near 990 cm-1 for V=O indicates monomeric V=O units in the complex [8]. On the other hand, it is also well known that the V-O multiple bond stretching frequency in VO2+depends upon the ligands attached to the oxocation entity. The most significant feature in the evolution of IR bands is that the intense doublet between 1025-980 cm-1 is always present during thermal degradation. Although some differences are worth mentioning, the relative intensity of the components is inverted and the highest one shifts to higher wavenumbers with the advancement of decomposition, while oxides are being formed. This fact is consistent with the reinforcement of V=O bond in the V(V)-oxide due to the higher oxidation state. Nevertheless, characteristic bands of V2O5 dominate the spectra of the pyrolysis product once the precursor has reacted.


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Another detail observed in FTIR spectra at 3000C is the presence of water bands evident from the absorption around 1600 cm-1 of the bending mode, as shown in Fig. 1-b. The corresponding stretching vibration ν(OH) located at 3440 cm-1 as a broad medium intensity band is also clear evidence of adsorbed water. A shoulder on high ν side of the latter band (3645 cm-1) is assigned to ν(OH) of surface V-OH [9,10]. All these bands disappear upon heating at 5000C. According to literature, the adsorption of water molecules takes place in certain planes with high excess + charge rendered from electron acceptors, resulting in the formation of Brönsted acid sites in the metal oxide. The lack of well resolved ν(OH) bands from the surface OH groups may result from the appearance of H-bonds [11]. The presence of water along with the partially amorphous nature of the V-oxides at 3000C in a disordered and stressed surrounding of O-atoms are responsible for the activated state of V in these conditions, in agreement with catalyst activation. Table 1 Assignment of bands in degraded precursor spectra Precursor at 300oC Frequency (cm-1) Assignment

Precursor at 500oC Frequency (cm-1)

Pure V2O5 Frequency (cm-1)

1012, s 994, s 886, s 839, s

ν (V=O) of V2O5 ν (V=O) of V2O4 V2O4 Coupled vib. ν(V=O) and νas V-O-V of V2O5

1020, vs 995, w

1023, s

836, vs

824, vs

534, vs 380, 361, m 295, m

νs of (VO) bridge deformations deformations

603, vs 385, m 300, m

526, vs 382, m 294, s

The spectroscopic evidence already mentioned suggests that when V is not still totally oxidized to +5 the ν(V=O) signal appears as a doublet and upon heating in oxidizing atmosphere it becomes a singlet displaced to higher wavenumbers typical of V5+ in oxygen environment as in V2O5. These facts are in agreement with other evidence previously pointed out in the literature [10,12,13] according to which, the stability of each species depends strongly on the experimental conditions, and the oxidation state of V must be kept pentavalent to ensure adequate catalytic activity [15]. The full assignment of bands in degraded precursor spectra at temperatures discussed above was carried out based on data available in the literature and presented in Table 1.

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Catalytic activity Prior to reaction, the supported VO(acac)2 was gradually activated in the reaction mixture at 3030C in agreement with the precursor degradation discussed above. After 3 h, the catalyst reached a steady state NO conversion of about 70%. The SCR activity as a function of reaction temperature is shown in Fig. 2, for two different NH3/NO ratios in excess of O2. It can be seen that at constant NO partial pressure the reaction conversion is affected significantly by changing NH3 partial pressure. Results show that a strong increase in SCR conversion is obtained when the reaction temperature is increased from 200 to 3000C. The principal feature of the catalysts obtained by degradation of the complex is the broad window of high activity. Experimental results showed that our catalyst, reached higher NO conversion between 300 and 4000C. Above 4000C, the NO conversion declines with increasing temperature, indicating that other reactions become important [14]. Figure 3 shows the effect of partial pressure of O2 on the reactivity at 3000C. The results show that the NO conversion on VO-catalyst is markedly accelerated by O2 increasing almost linearly with the concentration of oxygen up to 0.5%. Only small effects were found when the O2 concentration was increased from 0.5 to 2%. Effects of oxygen were previously reported [10,15] and significant differences were seen in absence of O2. These results suggest that O2 is involved in the reaction mechanism through the reoxidation of V-OH to V=O species. According to the literature [8,12,13,15], the surface of the catalyst keeps its activity due to a balanced content of V=O and V-OH groups by the following mechanism: NO + NH3 + V=O → N2 + H2O + V-OH

(3)

O2 or bulk V=O

2 V-OH → 2 V=O + H2O

(4)

where the vanadia component of the surface participates in an oxidation/reduction process which is necessary to provide sites for NH3 absorption and activation. Topsoe et al. [9,11] concluded from their IR studies on V/TiO2 catalyst that both V-OH and V=O species are involved in the SCR reaction. Activity measurements show that the SCR reaction stops when the


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Fig. 2. Effect of reaction temperature and NH3/NO ratio(R) on the catalytic activity. ( ▲) R= 1.18, (●) R= 0.66

oxygen is removed. These results indicate that V=O species cannot be regenerated in the absence of O2 to provide sites for ammonia activation.

Fig. 3. Effect of partial pressure on the catalytic activity at 300oC

In order to correlate the effect of SCR reaction on the species present in the supported catalyst, the samples extracted from the reactor were characterized by infrared spectroscopy. In the sample exposed to the reaction mixture at 2500C (Fig. 4b), two bands at 1262 and 1094 cm-1 are observed. That shows the simultaneous presence in the catalyst surface of V-OH and V=O groups in agreement with its catalytic behavior. On the other hand, for the catalyst treated up to 4000C in the reaction

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mixture (Fig. 4a), the previous bands disappear and new ones, though weak, appear at 3203 and 1416 cm-1, which would correspond to ammonia species. The band at 1416 cm-1 is attributed to NH4+ adsorbed on the surface, in agreement with Topsoe [10]. The IR spectra display a series of bands in the region between 1500 to 1900 cm-1 (Fig. 4a) and 1300 to 1900 cm-1 (Fig. 4b), typical of water absorption. The presence of such bands is reasonable

Fig. 4. FTIR spectra of catalyst samples extracted of the reactor, a) sample treated up to 400oC in the reaction mixture, b) sample exposed to reaction mixture at 250oC

as H2O is involved in the SCR reaction. On the other hand, some of these bands are also probably due to NO2 adsorbed groups, as has been observed in other studies with similar systems [10,12,13]. Due to overlapping, complete assignment is difficult. All bands subsequently decrease in intensity with increasing temperature. Therefore, as can be seen in spectra 4a, this region appears to be cleaner. Besides, depending on the pressure of NO, O2 and NH3 the possible adsorbates, NO2 and NH4+ may be observable concurrently at 1630 and 1416 cm-1, respectively [16]. However, under these circumstances we can not ascertain the presence of NO2 in the spectra. In agreement with our TPD experiments, no evidence for the presence of adsorbed NO is detected in the present study under SCR reaction conditions. Instead, it seems that probably NO reacts with adsorbed NH3 species in flowing O2 keeping the surface of the catalyst conveniently oxidized according to the enhancement of V=O typical


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bands or eventually V-OH, at a lower temperature (3000C) along with the absence of typical NH4+ bands located at 1216 and 3203 cm-1 [16]. Once the temperature rises up to 4000C, V=O and V-OH bands have disappeared indicating that these groups are being reduced by NH3 and/or that the amount of O2 is not enough to avoid this process. In this stage NH4+ species are clearly detected. The existence of NH4+ ions adsorbed on Brönsted acid sites of the catalyst surface is also confirmed by the presence of other bands at 3100, 3032, 2800 and as already mentioned, at 1216 cm-1 [10,16]. Although bands corresponding to stretching modes at 3350, 3253 and 3175 cm-1 are indicating adsorption of NH3, the lack of the corresponding deformational mode suggests that the amount of NH3 species adsorbed as such is smaller than that of NH4+. The preference of NH4+ adsorption by active sites instead of NH3 could indicate a Brönsted type behavior of the catalyst [12]. Although both adsorbed ammonia species should be reactive, according to the literature, NH4+ groups are more effective in the SCR reaction. CONCLUSIONS The following conclusions can be derived from this study. During the oxidative activation the presence of V2O5 and in a minor extent of V2O4 at 3000C was confirmed by IR spectra and XRD results. The NO conversion on VO-catalysts was markedly accelerated by oxygen in agreement with the presence of V-OH and V=O groups on the catalyst surface, which are responsible for catalytic activity. About 4000C, V=O and V-OH bands have disappeared, indicating that these groups are reduced by NH3, and NH4+ species are clearly detected. The experimental results suggest that the reaction occurs according to the mechanism given by eqs (3) and (4). REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

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