Characterization and reactivity of TiO2-supported MoO3 De-Nox SCR catalysts

Applied Catalysis B: Environmental 17 (1998) 245±258

Characterization and reactivity of TiO2-supported MoO3 De-Nox SCR catalysts I. Novaa, L. Liettia, L. Casagrandea, L. Dall'Acquab, E. Giamellob, P. Forzattia,* a

Dipartimento di Chimica Industriale e Ingegneria Chimica ``Giulio Natta'', Politecnico di Milano, P. zza L. da Vinci 32, 20133 Milano, Italy b Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, UniversitaÁ di Torino, Viale P. Giuria 9 , Torino, Italy Received 27 November 1997; received in revised form 10 February 1998; accepted 10 February 1998

Abstract TiO2-supported MoO3 catalysts, with MoO3 loading similar to that of commercial DeNOx-SCR catalysts (in the range 0± 11% w/w) have been prepared, characterized and tested in the NO reduction by NH3. The structural and morphological characteristics of the anatase-TiO2 support are not signi®cantly changed upon MoO3 addition. For MoO3 loading below or near 9% (w/w) (corresponding to the formation of the theoretical monolayer), evidence for the presence of isolated molybdenyl surface species have been collected by FT-IR, Laser Raman and EPR spectroscopies. Above this loading, the presence of crystalline MoO3 is observed. The MoO3/TiO2 catalysts are active in the reduction of NO by NH3: the reactivity of the catalysts increases on increasing the MoO3 loading, whereas the N2 selectivity decreases due to the formation of undesired N2O. The formation of N2O is primarily ascribed to a reaction between NH3 and NO, and not to the ammonia oxidation reaction. A comparison with a WO3/TiO2 catalyst having similar molar composition indicates that the WO3- and MoO3-containing samples exhibit similar structural and morphological characteristics, but different reactivity: the WO3/TiO2 sample is less active but more selective in the SCR reaction. On the basis of EPR data, the different catalytic behavior has been tentatively ascribed to the different redox characteristics of the samples. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Selective catalytic reduction; MoO3/TiO2 catalysts; NO reduction

1. Introduction The selective catalytic reduction (SCR) of NOx by NH3 is the best developed and widespread method for the NOx removal from stationary sources due to its ef®ciency and selectivity [1,2]. The SCR process is based on the reaction between NO and NH3 to produce *Corresponding author. Tel.: +39 (2) 2399-3238; fax: +39 (2) 70638173; e-mail: [email protected] 0926-3373/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-3373(98)00015-0

water and nitrogen according to the reaction: 4 NO ‡ 4 NH3 ‡ O2 ! 4 N2 ‡ 6 H2 O

(1)

Commercial SCR catalysts are constituted by a highsurface area TiO2 carrier (in the anatase form) that supports the active components tungsten (or molybdenum) trioxide and vanadium pentoxide. Vanadia is responsible for the activity of the catalyst in the reduction of NOx but also for the undesired oxidation of SO2 to SO3; accordingly the V2O5 content is

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generally low (0.3±1.5% w/w). WO3 is employed in larger amounts (10% w/w): it acts as a promoter by enlarging the temperature window of the SCR reaction [3,4] and imparts superior thermal stability and better mechanical properties to the catalysts. Commercial catalysts containing MoO3 instead of WO3 are also used: these catalysts have been reported to be less active than the analogous V2O5±WO3/TiO2 samples, but more tolerant to As [5,6]. In spite of the fact that MoO3-containing catalysts have been developed to a commercial scale, the physico-chemical characteristics and the catalytic properties of V2O5±MoO3/TiO2 in the SCR reaction have not been investigated in details so far [7,8]. Accordingly, an investigation has been undertaken in our laboratories aiming at a better understanding of the chemico-physical properties and of the reactivity in the SCR reaction of TiO2-supported MoO3based catalysts. As a ®rst part of this study, binary MoO3/TiO2 model catalysts have been investigated. The molybdenum content of these samples was varied in the range 0±11% (w/w), i.e. with amounts comparable to that of commercial catalysts and accordingly of interest for SCR applications. The catalyst samples have been characterized by means of different techniques including surface area (BET) and pore size distribution, XRD, FT-IR, FT-Laser Raman, EPR and TPD of NH3. The reactivity in the SCR reaction of the samples has also been studied, and compared with that of WO3/TiO2 reference catalysts. In a forthcoming paper, the physico-chemical properties and the reactivity of ternary V2O5±MoO3/TiO2 model catalysts will be addressed. 2. Experimental 2.1. Catalyst preparation TiO2-supported MoO3 catalysts, with nominal MoO3 content of 3.1, 6.0, 8.7 and 11.3% (w/w) were prepared by the incipient wetness impregnation method. Support material was titania in the anatase form prepared by neutralization at constant pH of Titetrachloride (Fluka), followed by washing of the precursor, drying at 383 K and calcination for 4 h at 823 K. The TiO2 support was then impregnated with aqueous solution of ammonium heptamolybdate

(Carlo Erba RPE) complexated with citric acid (J.T. Baker). The resulting precursors were dried at 383 K overnight and then calcinated at 823 K for 3 h. A reference WO3/TiO2 sample with WO3ˆ9.5% (w/w) (corresponding on a molar basis to the sample with 6% (w/w) MoO3) was also prepared by impregnating the same titania support with a solution of ammonium paratungstate (Fluka) and citric acid. In the following, catalysts are denoted as MO3(x)/ TiO2, where MˆMo or W and x represents the % (w/w) metal oxide loading. 2.2. Catalyst characterization Surface area has been measured by N2 adsorption at 77 K with the BET method using a Carlo Erba Sorptomatic 1900 Series instrument. Pore size distribution measurements have been obtained by N2 adsorption± desorption at 77 K with the same apparatus used for surface area measurements and by the mercury penetration method using a Carlo Erba Porosimeter 2000 Series instrument. Powder X-ray diffraction analyses have been performed with a Philips vertical goniometer PW 1050/ 70 and Ni-®ltered CuK radiation. The FTIR spectra of all samples (KBr pressed disk method) were recorded by a Perkin±Elmer 147 FTIR spectrometer. FT-Laser-Raman spectra (pure powder pressed disk method) were collected by using a Nicolet 910 LaserRaman spectrometer (1064 nm). The EPR spectra have been obtained by a Varian E 109 Spectrometer operating between 9.2 and 9.5 GHz microwave frequency and with a ®eld of modulation of 100 kHz; the spectrometer was equipped with a dual cavity and connected to a personal computer for spectra recording and elaboration. NH3-TPD experiments were performed with the same apparatus used for catalytic activity runs (see below). In a typical experiment, the catalyst is oxidized in He‡20% O2 at 773 K for 1 h, subsequently cooled down at 313 K and saturated for 20 min with a stream of NH3 (2000 ppm) in He (total ¯ow rateˆ120 cm3/min (STP)). Then pure He is ¯owed over the catalyst sample at the same temperature for 30 min to remove weakly adsorbed ammonia. Finally the catalyst is heated in He (120 cm3/min) up to 773 K at 15 K/min.

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2.3. Reactivity measurements Catalytic activity measurements have been performed in a quartz tubular ®xed-bed microreactor (i.d.ˆ7 mm), operating at atmospheric pressure and inserted into an electric furnace driven by a proportional-integral-derivative temperature controller/programmer (Eurotherm 812). The temperature of the reactor was measured and controlled by means of a Ktype thermocouple (o.d.ˆ0.5 mm) directly immersed in the catalyst bed. In a typical run 160 mg of catalyst (100±150 mesh) were used and a stream of NH3 (800 ppm)‡NO (800 ppm)‡O2 (9000 ppm) in He (total ¯ow rateˆ120 cm3/min (STP)) was fed to the reactor. For the analysis of the exiting gases, the reactor outlet was connected in a parallel arrangement to both a quadrupole mass detector (Balzers QMS 200) and a gas chromatography equipped with a Poraplot Q capillary column (Chromopack). The following mass-to-charge ratios were used to monitor the concentration of products and reactants: 17 (NH3), 18 (H2O), 28 (N2), 30 (NO), 32 (O2), 44 (N2O) and 46 (NO2); the data were quantitatively analyzed by taking into account the response factors and the fragmentation patterns of the various species experimentally determined. N-balances, calculated on the gas exiting the reactor, always closed within 5%, being typically lower than 2%. 3. Results 3.1. XRD and morphology The effect of the MoO3 loading on the catalysts structural and morphological properties was at ®rst investigated. Table 1 reports the phase composition identi®ed by XRD, the mean crystal size (dcryst), the

247

surface area (Sa), the pore volume (Vp), the mean pore radius (rp) and the MoO3 surface coverage (Mo) calculated from the nominal MoO3 loading and the speci®c surface area of the sample by assuming a monolayer capacity of 0.12% (w/w) for MoO3/m2 [9,10]. As reported in Table 1, the theoretical MoO3 monolayer coverage is approached in the sample with MoO3 content of 8.7% (w/w) and is exceeded for higher Mo loading. The anatase polymorphic form of TiO2 is the only detected phase by XRD, and no signi®cant differences are observed in the crystallite dimensions of anatase on increasing the MoO3 content. No diffraction lines assignable to crystalline MoO3 are detected: the absence of the bulk phase MoO3 in the XRD patterns implies that for these samples the molybdenum oxide is present in either a non-crystalline state or as small crystallites (less than 4 nm in diameter). The morphological characteristics of the samples are slightly modi®ed upon MoO3 addition: the BET surface area and the pore volume of the catalysts decrease progressively, from 80 to 67.5 m2/g and from 0.31 to 0.11 cm3/g, respectively, as the molybdena loading increases from 0 to 11.3% (w/w). For the purpose of comparison, the morphological characteristics of the WO3(9.5)/TiO2 sample have also been investigated. This sample presents a BET surface area of 69 m2/g and a pore volume of 0.28 cm3/g. Only the anatase polymorphic form of TiO2 has been detected by XRD, with a mean crystal dimension of Ê . The WO3 surface the TiO2 crystallites of 153±161 A coverage, calculated assuming a monolayer capacity of 7 mmol WO3/m2 [11], is near 0.85. 3.2. Vibrational studies Information concerning the structural and vibrational characteristics of molybdenum oxide on the

Table 1 Structural and morphological data of MoO3(x)/TiO2 samples (xˆ0±11.3% w/w), calcinated at 823 K MoO3 (% w/w)

0

3.1

6

8.7

11.3

Phase Ê) dcryst (A Sa (m2/g) Vp (cm3/g) Ê) rp (A Mo

Anatase 150±160 80 0.31 70 0

Anatase 158±174 60.6 0.26 60 0.42

Anatase 152±162 79.4 0.28 60±70 0.63

Anatase 157±162 75 0.22 40±50 0.98

Anatase 159±163 67.5 0.11 50 1.4

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TiO2 surface are provided by the spectroscopic analyses (FT-IR, Laser Raman and EPR). The IR spectra of all the MoO3/TiO2 sample (not reported for brevity) are characterized by a strong absorption below 800 cmÿ1, corresponding to the absorption of the TiO2 anatase support, and by a weak shoulder near 960 cmÿ1, that can be assigned to the stretching mode of molybdenyl species in an hydrated form [8,9,12,13]. In the case of the samples with highest MoO3 loading (i.e. MoO3ˆ8.7 and 11.3% w/w) a shoulder also appears at 890±900 cmÿ1, that has been associated to the presence of bulk microcrystalline MoO3. This phase has not been observed in the XRD spectra due to the lower sensibility of this technique. The Raman spectra obtained in air using the pure powders are reported in Fig. 1. All the spectra show bands at 396, 514, 637 cmÿ1 due to the Raman active fundamentals of anatase [14]. A weak band near 950 cmÿ1, characteristic of the M=O stretching mode of molybdenyl species [15], is also well evident in all the samples. The catalysts with MoO3 loading of 8.7 and 11.3% (w/w) show additional broad bands near 820 and 980 cmÿ1, associated with the presence of micro-crystalline MoO3. Accordingly, the spectroscopic data reported above indicate for all the investigated model catalysts the presence of dispersed molybdenyl species. For the samples with highest MoO3 loading (8.7 and 11.3% w/w) the presence of micro-crystalline MoO3 is also observed: this is in line with the estimates of the molybdena coverage (Table 1) showing that for these samples the theoretical monolayer coverage is reached and exceeded. No direct evidences have been obtained for the existence of polymeric MoxOy species, whose presence, however, cannot be excluded on the basis of the present data. It is noteworthy that on increasing the molybdenum content a signi®cant decrease in the intensity of the anatase peaks is evident. As discussed elsewhere [14] in the case of WO3-containing catalysts, this phenomenon may be associated to the production of quasi-free electrons in the samples, that accordingly is more pronounced at high MoO3 loading. The vibrational studies performed in the case of the WO3(9.5)/TiO2 catalyst showed the presence of dispersed monomeric wolframyl species, in line with previous studies [11,14]: it is then con®rmed that

accordingly the Mo and W species in TiO2-supported MoO3 and WO3 catalyst samples exhibit similar structural characteristics. 3.3. EPR spectra The MoO3/TiO2 samples containing 3.1, 6 and 8.7% (w/w) of MoO3, along with the WO3(9)/TiO2 samples were also investigated by electron paramagnetic resonance (EPR) spectroscopy. This experimental technique is able to detect paramagnetic species only: possible candidates for EPR detection are, in the present case, Ti3‡, Mo3‡, Mo5‡ and W5‡ ions. The Mo5‡ is a d1 ion and its spectrum arises from a hyper®ne interaction with the two isotopes 95 Mo and 97 Mo of natural abundance of 15.78% and 9.60%, respectively, both with the nuclear spin Iˆ5/2. An extensive EPR analysis of TiO2 supported MoO3 systems is not available in the literature [7]: in fact most of the published work concerns either systems in which Mon‡ ions are stabilized in the bulk of TiO2 [16,17] or MoO3/SiO2 [18,19]. Upon calcination at 823 K, the MoO3/TiO2 samples are white for MoO3 loading up to 3.1% (w/w), and light gray for higher MoO3 percentage (6±8.7% w/w). A weighted amount of each catalyst was introduced in a quartz EPR cell that was then evacuated at room temperature up to 1.3310ÿ3 Pa. In these conditions the EPR spectrum (recorded at RT) of the MoO3(6)/ TiO2 sample, taken as example, shows a very weak signal due to Mo5‡ and a second isotropic signal at gˆ2.0034 ascribable to a paramagnetic electron defect of TiO2. Upon evacuation of the same sample for one hour at 423 K, an intense signal of Mo5‡ is observed (Fig. 2(A)), in addition to the signal of the lattice defect previously reported. The features of the spectrum in Fig. 2 (A) have been reproduced by computer simulation. Comparison with the literature data [7] indicates that the spectrum in Fig. 2 (A) corresponds to a surface molybdenylic ion (MoO)3‡ in axial symmetry. In particular two distinct species slightly different in the coordinative environment (labeled a and b) are present. Species a exhibits gkˆ1.9041, g?ˆ1.9504 and A?ˆ40.38, whereas species b is characterized by gkˆ1.8817, g?ˆ1.9450, and A?ˆ 45.40. No signi®cant differences are observed in the EPR spectra upon further evacuation at 623 K for 1 h.

Fig. 1. FT-Laser Raman spectra of MoO3(3.1)/TiO2 (a), MoO3(6)/TiO2 (b), MoO3(8.7)/TiO2 (c) and MoO3(11)/TiO2 (d) catalyst.

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Fig. 2. EPR spectra of MoO3(6)/TiO2 catalyst: (A) sample calcined at 823 K and evacuated for 1 h at 423 K, spectrum registered at R.T.; (B) sample evacuated at 423 K for 1 h, contacted with NH3 (666.612 Pa) at R.T. and finally evacuated at the same temperature (spectrum registered at 77 K)

The same thermal treatments as above were performed for all the investigated MoO3/TiO2 samples, and similar results have been obtained in terms of the spectral shape of the Mo5‡ species. The various spectra, however, differ in intensity indicating that different amounts of Mo5‡ are present on the various systems. In particular, a quantitative study based on the comparison of the intensities with those of known standard samples showed that in the samples evacuated at 423 and 623 K the abundance of Mo5‡ is in the range 5±12% of the total Mo (Motot) present on the catalyst. The higher Mo5‡/Motot ratio corresponds to the samples with lower MoO3 loading. To test the coordinative capability of the molybdenyl species present in the MoO3(6)/TiO2 sample an additional experiment was carried out. A new portion

of the catalyst was treated in the conditions above described (evacuation at 423 K for 1 h) and then the EPR spectrum was recorded a 77 K, obtaining again a spectrum similar to that reported in Fig. 2(A). A weak amount of NH3 (666.612 Pa) was then adsorbed on the sample at 77 K, and the temperature was slowly raised to RT to allow a gradual adsorption of ammonia. The sample was ®nally evacuated to remove excess ammonia and ®nally the EPR spectrum was again recorded at 77 K (Fig. 2(B)). Inspection of Fig. 2(B) shows that after NH3 adsorption species a disappears, species b is reduced in intensity, while a new species appears (species c), characterized by a higher coordination of the Mo5‡ ions. As derived from computer simulation the new species c exhibits gkˆ1.8755, g?ˆ 1.9237, A?ˆ46.66. The remarkable modi®cation of the g? value in comparison to those observed for species a and b indicates that ammonia probably coordinates molybdenylic sites in equatorial position with respect to the axis of the molybdenylic surface species. The three observed Mo5‡ species show a progressive increase of coordination in the order cb>a, and accordingly the structures depicted in Scheme 1 can be tentatively proposed for these species. It is noted that the EPR spectra did not evidence the presence of polymeric molybdenum (V) clustered species, characterized by very broad line width. This analysis is in line with literature data on variously supported MoO3 [13,19,20] and supports the vibrational characterization data indicating the presence, on the catalyst surface, of isolated molybdenyl species, having a geometric structure similar to that proposed for the surface W oxide species in TiO2-supported WO3-based catalysts [14,19,21]. EPR analyses were also performed over the WO3/ TiO2 sample, following the same experimental procedure reported above in the case of the MoO3/TiO2 samples. However, in this case the formation of W5‡

Scheme 1. Proposed structure for the Mo surface species

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paramagnetic ion has never been observed, even if the catalyst sample is progressively heated up to 873 K in vacuum and/or in the presence of ammonia. These treatments certainly lead to the reduction of the catalyst, as pointed out by the color of the catalyst sample (changing from light yellowish to brown), and by the decrease of the quality factor of the EPR cavity that causes the practical impossibility of tuning the system and therefore of recording any spectrum. This situation is a kind of ®ngerprint of the reduction of the whole sample with a consequent increase of the electron population in the conduction band of the solid. 3.4. Temperature programmed desorption (TPD) of ammonia Temperature programmed desorption (TPD) experiments of NH3 have been performed with the aim of investigating the acid characteristics of the catalysts. Fig. 3 shows the results of the ammonia TPD runs performed over the MoO3(x)/TiO2 catalysts (xˆ0± 8.7% w/w). In all the cases the spectra show similar features, with ammonia and water as the main desorption products. Minor amounts of N2 have also been observed to desorb at high temperature in the case of the MoO3-containing samples. In the case of the TiO2 support, the evolution of ammonia occurs with a broad shape in a wide temperature range (350±770 K), showing maxima at 400 and 550 K. This clearly points out the presence of several NH3 adsorbed species with different thermal stability. The very strong acid character of pure titania, able to hold ammonia up to 770 K, is also evident. Along with ammonia, the desorption of water is also observed in the whole temperature range. The nonnegligible amount of water desorbed in the TPD experiments in the range 400±700 is likely to be related to two distinct effects: (i) presence of water impurities in the NH3 feed, and (ii) surface dehydroxylation. The ®rst effect is possibly of minor importance if compared to the surface dehydroxylation due to both the expected low amounts of impurity of water and to the lower basicity of water if compared to ammonia, that is known to displace water from the catalyst acid sites [22]. Under the experimental conditions used for catalyst activation prior to NH3 adsorption (heating at 773 K) it is expected that the

Fig. 3. TPD profiles (concentration vs. temperature) following saturation of the MoO3/TiO2 samples in He ‡ NH3 (2000 ppm) at 313 K for 20 min: (a) TiO2, (b) MoO3(3.1)/TiO2, (c) MoO3(6)/ TiO2, and (d) MoO3(8.7)/TiO2

surface of the catalyst still contains isolated hydroxyl groups whose desorption in the form of molecular water requires proton migration from one hydroxyl to the other to form the neutral molecule. This phenomenon has been proposed for other oxide surfaces [23] and takes place at temperature higher than 773 K. In the present case water desorption during the TPD experiments at temperatures lower than that attained in the preliminary activation treatment could be associated with the presence of ammonia which is a vehicle for proton migration, for instance, via NH4‡ groups which are known to form [22] upon adsorption of ammonia at Brùnsted acidic sites. Upon addition of MoO3 to the TiO2 support, the features of the NH3-TPD are only slightly changed:

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the maximum at 550 K in the NH3 desorption peak is progressively reduced upon increasing the MoO3 loading, so that in the catalyst with MoO3ˆ8.7% (w/w) this maximum is no more evident. These results clearly indicate that addition of MoO3 modi®es the acid properties of titania. Molybdenyl species may act either as Lewis or Brùnsted acid sites, thus originating molecularly coordinated NH3 species or NH‡ 4 ammonium ions, respectively [24]. Molecularly coordinated ammonia species are thermally more stable than ammonium ions [24]: these species cannot be distinguished by TPD, but the features of the NH3-TPD spectra eventually suggest that during impregnation the strongest TiO2 Lewis acid sites have been covered by Mo surface species, acting either as Lewis or Brùnsted acid sites, characterized by lower acid strength than TiO2. It is noteworthy that upon addition of molybdena to titania, the formation of small amounts of N2 are also observed above 600 K. This clearly indicates that, at high temperature, a small fraction of the adsorbed ammonia is oxidized to N2. The NH3 oxidation is promoted by the MoO3 component, since no oxidation products have been observed in the case of the pure titania sample. The ammonia oxidation is also accompanied by water evolution, as evident in the shoulders clearly visible in the H2O-TPD spectra.

Fig. 4. Results of catalytic activity runs performed over the MoO3/ TiO2 samples: (A) NO conversion (filled symbols) and NH3 conversion (open symbols) vs. temperature; (B) N2 selectivity vs. temperature ± (a) TiO2, (b) MoO3(3.1)/TiO2, (c) MoO3(6)/TiO2, and (d) MoO3(8.7)/TiO2.

3.5. Catalytic activity measurements The results of the catalytic activity tests in the reduction of NO by NH3 performed over samples with different molybdenum content (MoO3ˆ0, 3.1, 6, 8.7% w/w) are shown in Fig. 4 (A) in terms of NO and NH3 conversion (®lled and empty symbols, respectively) and in Fig. 4 (B) as N2 selectivity. The N2 selectivity is de®ned as SN2ˆ[N2]/([N2]‡[N2O]). Fig. 4(A) clearly shows that the reactivity of the catalysts in the SCR reaction is signi®cantly increased on increasing the MoO3 loading: the temperature required to achieve 50% NO conversion is progressively lowered from  710 to 530 K by passing from the MoO3-free sample to the catalyst with MoO3ˆ8.7% (w/w). The increase of the MoO3 loading also affects the nitrogen selectivity, that is signi®cantly reduced by MoO3 addition: the SN2 value estimated at 733 K for the MoO3(8.7)/TiO2 is near only 30%.

For all the samples at high temperatures the measured NH3 conversion is slightly higher than that of NO, that in fact over the samples with the highest Mo loading shows a maximum in the high temperature region. This possibly indicates that ammonia is involved in oxidation reactions other than the SCR reaction (1), from which a 1/1 ratio between the NH3 and NO conversion is expected. To investigate these aspects more in details, the reactivity of NH3 in the presence of oxygen has been investigated, and a catalytic activity run has been performed over the MoO3(6)/TiO2 sample in the absence of NO in the feed (i.e. NH3 (800 ppm)‡O2 (1% v/v)‡He balance). The productivity of N2, N2O and NO measured in this case is reported in Fig. 5, where they are compared with the corresponding data estimated during the NH3‡NO reaction performed over the same catalyst sample. Fig. 5 clearly shows that the reactivity of

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Fig. 5. N2, N2O and NO productivity in the NH3‡NO‡O2 and NH3‡O2 reaction over MoO3(6)/TiO2.

ammonia in the absence of NO is signi®cantly lowered, since the appearance of the reaction products (N2 and minor amounts of N2O, with NO negligible) occurs roughly about 150 K above the temperature at which the formation of the same species is observed in the NH3‡NO reaction. Also, the major reaction product is N2, the formation of other species being negligible. Accordingly these data clearly show that: (i) ammonia is involved in oxidation reactions over the MoO3(6)/TiO2 sample, but only at high temperatures; (ii) the presence of NO does accelerate the rate of NH3 consumption; (iii) the NH3 oxidation reaction is apparently not responsible for the decrease in the N2 selectivity observed during the SCR experiments. Finally, the reactivity of the MoO3(6)/TiO2 sample has been compared with that of a WO3/TiO2 catalyst having the same metal oxide loading (on a molar basis), i.e. WO3ˆ9.5% (w/w). Fig. 6 shows a comparison of the NO conversion and N2 selectivity measured over the MoO3- and WO3-containing catalysts. The two samples exhibit a very different catalytic behavior: the WO3(9.5)/TiO2 catalyst shows a lower reactivity but a signi®cantly higher selectivity if compared to the corresponding MoO3/TiO2 sample. As a matter of fact, the WO3-based catalyst does exhibit a N2 selectivity higher than 90% in the whole investigated temperature range, whereas for the MoO3(6)/TiO2 sample the N2 selectivity monotonically decreases with temperature, to 30% at 773 K.

Fig. 6. Results of catalytic activity runs performed over the MoO3/ TiO2 and the WO3/TiO2 samples: (A) NO conversion (filled symbols) and NH3 conversion (open symbols) vs. temperature; (B) N2 selectivity vs. temperature ± (a) MoO3(6)/TiO2 and (b) WO3(9.5)/TiO2.

4. Discussion 4.1. Characteristics of the TiO2-supported MoO3 oxide species Different TiO2-supported MoO3 catalysts, with MoO3 loading similar to that of commercial catalysts, have been prepared and characterized in this study. It has been found that the structural and morphological characteristics of the starting TiO2 support (in the form of anatase) are not changed to a signi®cant extent upon increasing the MoO3 loading for amounts slightly above those corresponding to the formation of the theoretical monolayer coverage. This occurs when the MoO3 loading exceeds  9% (w/w), in line with literature indications [9,10,25]. Above this level, the formation of MoO3 as a segregate phase has been

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observed by FT-IR and FT-laser Raman but not by XRD, due to the lower sensibility of this technique. Accordingly our data suggest that, for sub-monolayer samples, molybdenum oxide is mainly dispersed as isolated molybdenyl species over the TiO2 surface. Information concerning the nature of the dispersed Mo oxide surface species have been collected by EPR, FT-IR and FT-laser Raman. For sub-monolayer samples, FT-IR and Laser Raman indicates that the Mo species are likely associated with surface isolated molybdenyl species, stabilized on the TiO2 surface. These data are further con®rmed by the EPR analysis, clearly indicating the presence of surface molybdenylic ions in axial symmetry. Three species have been detected by EPR, showing slight differences in the coordinative environment. Furthermore, the EPR analysis did not evidence the presence of polymeric molybdenum (V) clustered species, in line with the indications provided by vibrational spectroscopies. However, their presence cannot be excluded at high Mo loadings, as reported by other authors [9]. The catalyst surface of all the investigated samples is characterized by strong acidity, probed by NH3TPD. The spectra always exhibit a broad desorption peak, indicative of the presence of several NH3 adsorbed species with different thermal stability. These species are likely associated with both Lewis (Ti4‡ ions and molybdenyl surface species) and Brùnsted (hydrated molybdenyl and Ti surface species) acid sites: on increasing the MoO3 loading the amount of ammonia retained over the catalyst surface at high temperature decreases, possibly due to the formation NH4‡ ions on Brùnsted acid sites (associated with the Mo component) that have been reported to be thermally less stable than NH3 coordinated on Lewis acid sites [6,22]. Accordingly, it can be concluded that the MoO3/TiO2 catalysts present a good capacity to adsorb ammonia and that the strongest TiO2 Lewis acid sites have been covered by Mo surface species, acting either as Lewis or Brùnsted acid sites, but characterized by lower acid strength than TiO2. Also, upon addition of MoO3, the formation of small amounts of NH3 oxidation products (mainly N2) is evident, thus indicating the increased redox properties of the catalyst surface.

4.2. Reactivity in the selective catalytic reduction of NO by NH3 All the MoO3/TiO2 catalysts investigated in the present study are active in the reduction of NO by NH3. Their activity and selectivity is strongly dependent on the MoO3 content: indeed the reactivity of the catalysts in the reaction is signi®cantly increased on increasing the MoO3 loading, but an opposite effect has been observed on the N2 selectivity. Accordingly, the temperature window for the SCR reaction, i.e. the temperature range where high NO conversions are achieved with almost complete N2 selectivity, is not very wide over the investigated catalyst samples. In the high temperature region, the decrease in the formation of N2 can be primarily ascribed to two different factors: (i) decrease in the N2 selectivity; and (ii) decrease in the NO conversion. The decrease in the N2 selectivity is related to the formation of N2O. The mechanisms leading to the formation of N2O have not been fully elucidated in the literature, but it has been suggested that this species may be originated via the direct ammonia oxidation, i.e. reaction (2): 2NH3 ‡ 2O2 ! N2 O ‡ 3H2 O

(2)

or via the so-called non-selective catalytic reduction (NSCR) process: 4NH3 ‡ 4NO ‡ 3O2 ! 4N2 O ‡ 6H2 O

(3)

Isotopic labeling experiments performed over vanadia/titania catalysts [26±31] showed that, under SCR conditions, one of the two N-atoms of N2O comes from ammonia and the other from NO. This eventually suggests that over these catalysts nitrous oxide is primarily formed from NO‡NH3 and not from NH3 alone. To check whether these conclusions do apply also to the MoO3/TiO2 system, catalytic activity runs have been performed with NH3‡O2 mixtures in the absence of NO over the MoO3(6)/TiO2 sample, and results have been compared with those obtained in the NH3‡NO reaction. As shown in Fig. 5, the investigated catalyst is active in the oxidation of NH3, but at temperatures signi®cantly higher than those corresponding to the onset of the SCR reaction. Also, the formation of N2O and NO is limited if compared to that of N2. Accordingly, these data suggest that the direct NH3 oxidation reaction (2) is not the main route

I. Nova et al. / Applied Catalysis B: Environmental 17 (1998) 245±258

responsible for the decrease in the N2 selectivity observed during the SCR experiments, in line with data reported for V2O5-based catalysts [26±31]. In this respect, it cannot be excluded that also in the case of the NH3 oxidation experiments the formation of the low amounts of N2O involves the preliminary oxidation of ammonia to NO, that however, has not been detected in signi®cant amounts in the reaction products because it is rapidly consumed in reactions (1) and (3). The other factor responsible for the decrease in the SCR temperature window in the high temperature region is the decrease in the NO conversion. The decrease in the NO conversion cannot be ascribed to the formation of N2O via the NSCR reaction (3), which implies equimolar consumption of NO and NH3 (i.e. the same NH3/NO ratio than the SCR reaction (1)). Accordingly, different routes must be invoked, e.g. the direct ammonia oxidation to N2 and/or NO (reactions (4) and (5), respectively): 4NH3 ‡ 3O2 ! 2N2 ‡ 6H2 O

(4)

4NH3 ‡ 5O2 ! 4NO ‡ 6H2 O

(5)

It is noteworthy that among reactions (2)±(5), reactions (4) and (5) only affect the NO conversion, reaction (2) affects both the N2 selectivity and the NO conversion whereas reaction (3) affects the N2 selectivity only. In view of the low NO and N2O formation observed during the NH3 oxidation experiments, associated with the high nitrogen formation, it is likely that the decrease in the NO conversion observed at high temperature during the NO‡NH3 experiments in the presence of oxygen can be primarily ascribed to the occurrence of reaction (4). To investigate more into details the catalytic behavior of the samples and to get additional information on the reactivity of the surface Mo oxide species, the

255

molybdenum turnover frequencies (TOFMo, mols of NO converted sÿ1 molMoÿ1) over the various catalysts have been estimated. Accordingly the NO conversion vs. temperature activity data have been analyzed assuming an isothermal plug ¯ow reactor model and a ®rst order power law rate equation (Rˆk8 exp(ÿEact/RT) CNO) [14], after verifying the absence of interphase and intraparticle diffusion limitations [32]. This led to the estimates of the speci®c rate of reaction rNO (moles of NO converted sÿ1 gÿ1) at various temperatures where the N2O formation is negligible. Then, by assuming that all the Mo atoms are accessible to the reactants and by considering the contribution of the exposed Ti atoms to the NO consumption, the TOFMo can be estimated at various temperatures according to the following equation: TOFMo ˆ

rNO ÿ TOFTi


molTi gcat

molMo =gcat

;

(6)

where molMo and molTi are the moles of Mo and Ti present at the catalyst surface, respectively, and TOFTi is the TOF of the TiO2 support. TOFTi has been estimated from the activity data obtained over the pure TiO2 sample (TOFTiˆrNO/molTi/gcat), by assumÊ 2 [13]). ing that the area occupied by a Ti atom is 19A The values of the TOFMo estimated at 500, 550 and 600 K are reported in Table 2 where they are compared with the TOFW estimated for the reference WO3/ TiO2 catalyst sample. From Table 2 it is evident that there is a more than linear increase in the TOFs upon increasing the molybdena loading: this clearly indicates that the reactivity in the SCR reaction of the surface Mo oxide species increases on increasing the MoO3 content. Similar effects have also been reported on several SCR catalysts, including V2O5/TiO2 [33,34] and V2O5/ZrO2 [35]. In the case of TiO2-supported

Table 2 Calculated Turnover frequency (TOF, molNO converted sÿ1 molMoÿ1 or molWÿ1) for the Molybdena/titania and the Tungsta/titania catalysts Catalyst

Mo or W coverage

MoO3(3.1)/TiO2 MoO3(6)/TiO2 MoO3(8.7)/TiO2 WO3(9.5)/TiO2

0.42 0.63 0.98 0.85

Mo±W turnover frequency (TOF) 500 K

550 K

600 K

9.46310ÿ5 1.29010ÿ4 2.06010ÿ4 7.4610ÿ6

2.83010ÿ4 5.3110ÿ4 9.04010ÿ4 8.3010ÿ5

6.89010ÿ4 1.70010ÿ3 3.08010ÿ3 6.21010ÿ4

256

I. Nova et al. / Applied Catalysis B: Environmental 17 (1998) 245±258

V2O5-based catalysts, several explanations have been proposed to account for the increase in the TOF observed upon increasing the vanadia loading, e.g. increase in the catalyst redox properties, a dual-site mechanism for the SCR reaction, or, alternatively, the role of Brùnsted acidity. Accordingly, Bell and coworkers [34] attributed the higher reactivity of the V2O5-rich samples to the presence of very active polyvanadate species, whose existence has been proved by Raman spectroscopy. These species are more active (but also less selective) than isolated vanadyls due to the greater lability of their oxygen atoms. Along similar lines, on the basis of TPSR and TPR data, Lietti and Forzatti [36] pointed out an increase of the catalyst redox properties upon increasing the catalyst V loading. A different explanation has been proposed by Wachs et al.[33,37]. These authors observed similar effects, but reported that the redox properties of the catalysts, probed by the methanol oxidation reaction, do not depend on the vanadia loading. Accordingly a dual site mechanism for the SCR reaction has been proposed, involving a surface redox site and an adjacent surface non-reducible metal oxide site. Along similar lines, the involvement of a catalyst redox site and a distinct acid function has also been suggested in the mechanism proposed by Topùse [38],Topsùe et al. [39,40]. Finally, other authors attributed a key-role in the SCR reaction to the presence of Brùnsted acidity, and accordingly the increase in the catalyst activity has been associated to the increased number of Brùnsted acid sites [3,33]. In the case of the TiO2-supported MoO3 catalysts used in this study, no clear and direct evidence has been derived in favor of one or the other of the hypotheses mentioned above. However, it is noted that signi®cant amounts of N2O are produced upon Mo addition, indicating that the increase in the catalyst activity is also accompanied by a decrease in the catalyst selectivity. This observation, already reported for V2O5/TiO2 catalysts [34], clearly outlines a role of the redox properties of the system in the DeNOx reaction. Previous work from our laboratories on a series of binary and ternary systems based on V2O5 and/or WO3 supported on TiO2 allowed some of us to propose [21] a satisfactory relationship between the redox properties of the various solids (including, according to the sample composition, formation of

reduced isolated centers or of free electrons in the solid) and the band structure of TiO2 as modi®ed by the presence of the different supported oxide phases. In this light it may be speculated that crystalline or aggregated MoO3 species, formed at high Mo loadings, are characterized by different redox properties with respect to isolated molybdenyl species. This is in line with both TPD data (pointing out that the formation of the NH3 oxidation products, i.e. N2 and H2O, occurs at lower temperatures on increasing the MoO3 loading), and also with the observed decrease of the intensity of the Raman anatase peaks that is evident on the high-loading samples. 4.3. Reactivity of MoO3/TiO2 vs. WO3/TiO2 The results reported above indicate that in spite of the similar structural and morphological characteristics, TiO2-supported MoO3- and WO3-catalyst samples having the same metal oxide loading (on a molar basis) exhibit a very different catalytic behavior. Indeed, as shown in Fig. 5, the WO3(9.5)/TiO2 catalyst investigated in the present study shows a slightly lower reactivity but a signi®cantly higher selectivity if compared to the corresponding MoO3/TiO2 sample. This is also con®rmed by the values of the estimated TOFMo that are roughly one order of magnitude higher than the corresponding TOFW in the range 500±600 K (e.g. 2.010ÿ4 vs. 7.510ÿ6 at 500 K and 3.1 10ÿ3 vs. 6.210ÿ4 at 600 K). Several factors may be responsible for the differences observed in the catalytic behavior of the TiO2supported MoO3- and WO3- catalysts. Among these, the EPR data clearly point out to the different redox properties of the molybdenyl vs. the wolframyl surface species. As a matter of fact, the presence of Mo5‡ ions is usually observed over the MoO3/TiO2 samples, both after calcination at 773 K and/or after evacuation treatments at different temperatures. The amounts of the Mo5‡ ions signi®cantly increase upon evacuating the samples in the temperature range RT ÿ623 K, and represent a signi®cant fraction of the total Mo loading. Accordingly Mo5‡ ions are easily localized at the Mo atoms in the MoO3/TiO2 samples. A different behavior is apparent for the WO3/TiO2 catalysts investigated in the present study. Indeed in these cases no evidences for the presence of W5‡ paramagnetic ions have been collected neither in the

I. Nova et al. / Applied Catalysis B: Environmental 17 (1998) 245±258

calcined samples nor upon evacuation at increasing temperatures, up to 873 K. These treatments certainly lead to the reduction of the catalyst, as visually indicated by the progressive darkening of the catalyst sample. However, the absence of EPR signals clearly indicates that the electrons are not localized at the W atoms. Similar results have also been obtained in the case of ternary V2O5±WO3/TiO2 and V2O5±MoO3/ TiO2 catalyst samples: indeed the presence of Mo5‡ is well evident in the case of the MoO3-containing samples, but no evidences have been collected for the formation of W5‡ in V2O5±WO3/TiO2 catalysts [21]. Accordingly, these data clearly point out the different electronic characteristics of MoO3/TiO2 catalysts if compared to WO3/TiO2 samples. The different redox properties of the TiO2-supported MoO3- and WO3- catalysts samples have also been pointed out by a dedicated TPSR study showing that both the temperature threshold for the SCR reaction and for the catalyst re-oxidation by gas-phase oxygen (representative of the catalyst redox properties [4,41]) are lower over the MoO3/TiO2 sample with respect to the corresponding WO3/TiO2 catalyst. The role of these properties in determining the activity and selectivity in the reduction of NO is still unclear, but considering that the SCR reaction involves a redox cycle it is likely that the different catalyst redox properties play a role in determining the performances of the different samples. 5. Conclusions TiO2-supported MoO3 catalysts, with MoO3 loading similar to those of commercial catalysts, have been prepared, characterized and tested in the NO reduction by NH3. The following conclusions were derived: 1. The structural and morphological characteristics of the TiO2 support (in the form of anatase) are not signi®cantly modi®ed upon MoO3 addition. Only a slight reduction of the surface area and of the pore volume has been observed. 2. For MoO3 loading below or near 9% (w/w) (corresponding to the formation of the theoretical monolayer), Mo oxide is dispersed over the TiO2 surface in the form of isolated molybdenyl surface species. Above this loading, the presence of crystalline MoO3 has been observed, along with molyb-

3.

4.

5. 6.

257

denyl surface species. The presence of polymeric MoxOy groups, although not detected in the present study, cannot be excluded. All the investigated samples are characterized by strong surface acidity, probed by NH3 TPD. The presence of several NH3 adsorbed species with different thermal stability can be deducted from the shape of the NH3 TPD desorption peaks. These species are likely to be associated with both Lewis (Ti4‡ ions and molybdenyl surface species) and Brùnsted (Ti and Mo) acid sites, in line with literature indications. It has also shown that the surface Mo oxide species cover the strongest TiO2 acid sites, thus leading to a decrease in the amounts of ammonia desorbed in the high temperature region. The MoO3/TiO2 catalysts are active in the reduction of NO by NH3. Their activity and selectivity strongly depend on the MoO3 content: the reactivity of the catalysts (and of the Mo surface species) increases on increasing the MoO3 loading, whereas the N2 selectivity decreases due to the formation of N2O. Accordingly, the temperature window for the SCR reaction, i.e. the temperature range where high NO conversions are achieved with almost complete N2 selectivity, is narrow over the investigated catalyst samples. The N2O formation over MoO3/TiO2 catalysts is principally ascribed to the reaction between NH3 and NO, and not to the direct ammonia oxidation. The higher reactivity and lower selectivity of MoO3/TiO2 as compared to WO3/TiO2 can be associated with the different redox properties of the samples.

Acknowledgements The authors wish to thank Professors Guido Busca (University of Genova) for useful discussions. References [1] [2] [3] [4]

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