TiO2 as SCR catalysts

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

653

Fe-vanadyl phosphates/TiO2 as SCR catalysts G. Bagnasco l, p. Galli 2, M. A. Larrubia3, M. A. Massucci 2, P. Patron04, G. Ramis 3, M. Turco ~ ~Dipartimento di Ingegneria Chimica, Universith "Federico II", P.le Tecchio 80, 80125 Napoli, Italy. 2Dipartimento di Chimica, Universith "La Sapienza", P.le Aldo Moro 5, Roxlm, Italy. 3Dipartimento di Ingegneria Chimica e di Processo "G.B. Bonino", Universifft di Genova, P.le J.F. Kennedy 1, Genova, Italy. 4IMA[ CNR, Via Salaria Km 29.600, 00016 Monterotondo Stazione, Roma, Italy. Fe-vanadyl phosphate (FeVOP) was precipitated in the presence of different amounts of TiO2. The samples also contained Ti(HPO4)2"H20 and probably amorphous FePO4. In the catalysts heat treated at 450~ besides TiO2 and anhydrous FeVOP, amorphous layered TiP207 was likely present. NH3 adsorption sites with medium and high strength were detected, that were related to Bronsted acidity of a pyrophosphate phase and Lewis acidity of FeVOP. The catalysts were noticeably more active and selective than pure FeVOP. NO conversion was increasing with FeVOP content, reaching 90% value at 400~ with unit selectivity to N2 and NH3/NO reaction ratio close to 1. 1. INTRODUCTION VOPO4 phases have been widely investigated, due to their role in VPO oxidation catalysts (1). Recently new materials have been obtained by isomorphous substitution of VO groups of VOPO4-2HzO with a trivalent metal such as A1, Cr, Fe, Ga, Mn (2). Such substitution modifies the adsorption properties of VOPO4 phase (3, 4). Moreover Fe-vanadyl phosphate gave high activity for NO reduction by NH3 (SCR process) if compared with conventional SCR catalysts (5). The activity of this compound can be related to the dehydrogenation properties of Fe, promoting the formation of species like amide, that reacts with gaseous NO (2,5). However the catalytic activity of Fe-vanadyl phosphate is limited by its low surface area (5 m2/g).Therefore these systems could be improved by dispersing the active phase on a suitable material. In this work, we have studied catalysts obtained by precipitating Fe-vanadyl phosphate in the presence of titanium dioxide. Such catalysts that were never reported before, are not simple supported systems, due to the presence of other phases formed during the preparation. These systems were studied for SCR catalytic properties and characterized for physical and chemical properties by means ofEDS, XRD, NH3 TPD and FT-IR techniques.

2. EXPERIMENTAL [Fe(H20)]o.2(VO)o.sPO4"2.25H20 (FeVOP) was prepared by refluxing for 16 h a suspension of V205, Fe(NO3)3"9H20 in H3PO4 3.3 M, with a yield of 60% (2). Materials A, B and C were

654 prepared by refluxing for 16 h the above suspensions in the presence of 12, 6 or 3 g of TiO2 (s. a.=125mEg ~) respectively. The catalysts A-450, B-450 and C-450 were obtained by treating the materials at 450~ for 12 h in He flow. A reference material TiP-TiO2 (s.a.-35mEg 1) was prepared by refluxing TiO2 with HaPO4 3.3 M in the same conditions. Elemental analysis was effected by EDS on a Philips XL30 apparatus. BET surface areas were measured on a Quantachrom Chembet 300. XRD measurements at room temperature (r. t.) and at 450~ were performed by Philips diffractometers PW 1100 and 1710 (HT-A.Paar diffraction camera) respectively. NI'-I3 temperature programmed desorption (TPD) was carried out in a flow apparatus at a rate of 10~ min -~. FT-IR spectra were recorded with a Nicolet Proteg6 460 instrument, using conventional IR cells with evacuation-gas manipulation apparatus. Catalytic activity tests were carried out in a flow apparatus with a fixed bed reactor at T=200-450 ~ contact time=8 9103 s. The feed mixture contained 700 ppm of NO and NH3, 27000 ppm of O2~He as balance. NO and NH3 were measured by continuous analyzers, N2 and N20 by gaschromatography. The nitrogen balance was verified within 5% error. 3. RESULTS AND DISCUSSION Table 1 Composition and surface areas of the materials Sample

FeVOP

P Ti mol% b) mol% b) A 10 34.0 61.7 B 20 30.7 64.4 C 40 27.0 56.2 a) nominal, assuming 60% yield in FeVOP; wtO~ a)

V mol% b) 2.12 2.10 11.5 b) EDS analysis,

Fe Surface area mol% b) m2g-~ 2.09 56 2.80 59 5.21 61 on oxygen free basis

Composition and surface areas of the samples are reported in Table 1. Surface areas are markedly higher than pure FeVOP and are unchanged after treatment at 450~ The vanadium content of the samples A and B would correspond to about 7wt% FeVOP, that of C to about 30wt% FeVOP. These percentages, quite lower than the nominal ones, suggest that the presence of TiO2 hinders in some way the formation of FeVOP, as more as higher is the TiOz amount in the preparation mixture. The phosphorous content is always largely exceeding the amount corresponding to FeVOP and the Fe/V ratio is higher than 1/4, that is the value usually obtained (2,5). These data indicate that the samples cannot be described by a simple FeVOP/TiO2 composition, and that other phases are produced when FeVOP is precipitated in the presence of TiO2. The XRD patterns are reported in Fig. 1. TiO2 shows the characteristic signals of the anatase and brookite phases. TiP-TiO2 shows the reflexions of layered cz-Ti(HPO4)z-H20 (TIP) (6), besides weak signals of TiO2. This indicates that refluxing TiO2 with H3PO4 leads to partial conversion of TiO2 into TIP. A similar behaviour was found in the treatment of TiO2 supported catalysts with H3PO4, leading to formation of titanium hydrogenphosphate (7). FeVOP shows its characteristic pattern (3). Different phases are detected in the XRD of A, B and C. The signals of TiO2 and TIP are always present, the intensity of TiP signals decreasing from A to C. The signal with d-7.11 A of FeVOP is not observable in the pattern of A, while it appears as a shoulder in B and is clearly evident in C that shows also other reflexions of FeVOP. Thus on going from A to C the amount of the FeVOP phase increases while that of

655 TiP decreases. This confirms that decreasing the amount of TiO2 in the preparation mixture

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favours the precipitation of FeVOP in respect to the formation of TiP. The presence of TiP in all samples explains why the phosphorous amounts exceed those corresponding to FeVOP (Table 1). Moreover the high Fe/V ratio suggests the presence of other Fe containing phases, such as FePO4, not detected by XRD because in too low amount or in an amorphous state. Fig. 2 reports the XRD patterns of the materials heated at 450 ~ After this treatment TiO2 shows an unchanged pattern, while FeVOP transforms into a well crystalline anhydrous phase (3). The pattern of TiP-TiO2 treated at 450~ (TiP-TiO2-450), shows the signals of TiOz and of layered titanium pyrophosphate (L-TIP207), formed by condensation of the TiP phase. In the XRD of the catalysts, besides the signals of TiO2, the strongest signal of anhydrous FeVOP with d=4.21 ,~ is well evident, while the reflexion with d=3.08 A becomes evident only in C-450. This suggests an increase of the amount of anhydrous-FeVOP from A-450 to C-450, that agrees with the increase of hydrated FeVOP in the corresponding precursors. It is worthnoting that the reflexions of L-TiP207 are not observable in the XRD of the catalysts, although they are present in the pattern of TiP-TiO2-450, suggesting that the condensation process of TiP in the catalysts is influenced by the presence of FeVOP (or other unidentified phases). It can be supposed that some reaction between TiP and FeVOP (or other phases) can occur during heat treatment, leading to formation of a disordered phase, not detected by XRD. NH3 TPD spectra of the catalysts and reference materials treated at 450~ are reported in Fig. 3. TPD spectrum ofTiO2-450 shows two bands due to Lewis acid sites of different strength,

656 due to coordinatively unsaturated Ti4+ ions (8). FeVOP-450 exhibits a broad band due to superficial Fe 3§ and VO 3+ ions acting as .~ ._ Lewis acid sites, with a large variety of acid strength (3). The TiP-TiO2-450 sample 't1 'r'= 1 ~ gives a spectrum very different from that of -~ TiO2. However the spectrum is very similar ~ to that reported for TiP phase treated at ~, • 450~ that is partially transformed into ~ L-TiP207 with signals due to adsorption g on Bronsted acid sites (9). This suggests ~, ~. that TiP-TiO2-450 has adsorption properties similar to L-TiP207. It can be g supposed that the TiO2 particles are ~0 o=~ z . , . z surrounded by TiP phase, formed by the reaction of T i O 2 with H3PO4, and 0 200 400 600 transformed into L-TiP207 during heat Temperature, ~ treatment. TPD spectra of the catalysts show broad desorption peaks due to NH3 Fig. 3. NH3 TPD of A-450, B-450, C-450 and adsorbing sites with strength from medium reference materials (right axis for FeVOPto very high. By taking into account surface 450) area values, the amoums ofdesorbed NH3 correspond to similar concemrations of surface sites (abt. 2-1014 cm2). The shape of the curve of A-450 is very similar to that TiPTIO2-450, suggesting the presence of a titanium pyrophosphate phase in the catalysts. XRD failed to detect this phase probably because it was amorphous. The shape of the curves gradually changes from A to C, as the 400~ component increases, while that at 550~ decreases. XRD shows that the content of FeVOP increases from A-450 to C-450. Therefore the increased intensity of the signal at 400~ can be related to an increase of the amount of FeVOP, since this phase shows a noticeable concentration of sites desorbing NH3 at 350400~ The decrease of the 550~ signal can be related to a decrease of the amount of the amorphous pyrophosphate phase. The nature of surface acid sites of the catalyst has been investigated by FT-IR technique (Fig. 4). According to (10,11) the band observed at 1605 cm1 is assigned to asymmetric deformation mode (Sas NH3) of ammonia coordinated to Lewis acid sites; the corresponding symmetric deformation (Ssym NH3) is not detectable because obscured by the cut-off of the transmittance of the sample due to the absorption of the bulk. Moreover, bands of NH4+, due to the adsorption of ammonia over Bronsted acid sites, can be observed at near 1680 and 1440 cml; these bands are respectively due to symmetric (Ssym NH4) and asymmetric (Sas NH4) deformation mode and the associated stretching. The relative intensities of the 8as NH4 with respect to the 8as NH3 seems higher in the spectrum of TiP-TiO2-450 and lower in the spectrum of pure FeVOP-450; in the case of C-450 and A-450 an intermediate trend is observed. These data indicate that ammonia adsorbs over all the catalysts in the form of molecularly coordinated species and of ammonium ions. The former are due to Fe 3§ and VO 3§ groups, the latter to HPO4 groups present on the surface of L-TiP207 (6). Moreover protonation of NH3 by water molecules coordinated to Fe 3+ ions cannot be excluded. The results of catalytic tests for the SCR reaction are reported in Fig. 5. The catalysts are very active, giving NO conversions up to 90%. TiO2 is inactive, FePO4 (5) and TiP-TiO2-450 have

657 very low activity, appreciable only at temperatures higher ), 13" than 300~ Thus the (/) 0 catalytic activity must o" I1} be related to the -.1 t") anhydrous FeVOP I11 phase. The catalysts r appear more selective than pure FeVOP. In fact the NH3/NO reaction ratio is close to 1 and no formation of N20 is observed in all conditions, except Wavenumbers (cm -1) for C-450 that gives Fig 4. FT-IR spectra of adsorbed species arising from contact of conversion to N20 of about 15% at 450~ NH3 over pure FeVOP-450 (a), C-.450 (b), A-450 (c) and TiPOn the other hand TIO1-450 (d). with pure FeVOP conversion to N 2 0 was observed starting fi'om 300~ (5). The catalytic activity towards NH3 oxidation has also been investigated, in the same conditions as SCR tests, but in the absence of NO. NH3 oxidation activity is negligible up to 300~ and markedly increases at higher temperatures (Fig. 6), giving N2 as the only product (traces of NO are produced at 450~ with 100

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C-450). However under SCR conditions, such NH3 oxidation activity is ineffective suggesting that NH3 reacts preferentially with NO rather than with 02. The activities of the three catalysts can be directly compared, since their surface areas are almost the same. The catalytic activity increases from A-450 to C-450, suggesting that it is related to the content of FeVOP phase.

658 However, since the catalysts are more selective than pure FeVOP, it can be supposed that either the FeVOP phase is someway modified by interaction with titanium phosphate, or other phases are involved in the catalysis. It can be supposed that a modified pyrophosphate phase, in which some V4+ ions replace T1.4+.ions, catalyzes NO reduction with higher selectivity, taking into account the catalytic properties observed for V4+ species in V2OJTiO2 catalysts (12). REFERENCES

1. G. Centi, Catal. Today, 16 (1993) 5. 2. K. Melanov~, J. Votinsky, L. Bene~ and V. Zima, Mat. Res. Bull., 30 (1995) 1115. 3. G. Bagnasco, L. Bene~, P. Galli, M. A. Massucci, P. Patrono, M. Turco and V. Zima, J. Therm. Anal., 52 (1998) 615. 4. G. Bagnasco, G. Busca, P. Galli, M. A. Larrubia, M. A. Massucci, P. Patrono, G. Ramis, M. Turco, J. Therm. Anal., in press. 5. G. Bagnasco, G. Busca, P. Galli, M. A. Massucci, K. Melanova, P. Patrono, G. Ramis, M. Turco, submitted to Appl. Catal. B: Envir. 6. G. Alberti, P. Cardini-Galli, U. Costantino and E. Torracca, J. Inorg. Nucl. Chem., 29 (1967) 571. 7. J. Blanco, P. Avila, C. Barthelemy, A. Bahamonde, J. A. Odriozola, J. F. Garcia de la Banda, H. Heinemann, Appl. Catal. 55 (1989), 151. 8. N. Y. Topsoe, J. Catal., 128 (1991) 499. 9. G. Bagnasco, P. Ciambelli, A. La Ginestra, M. Turco, Thermochim. Acta, 162 (1990) 91. 10. A. A. Tsyganenko, D. V. Pozdnyakov, V. N. J. Filimonov, Mol. Struct., 29 (1975) 299. 11. K. Nakamoto, in "Infrared and Raman Spectra of Inorganic and Coordination Compounds", 4 th ed., Wiley, New York (1986). 12. H. Bosch, F. Janssen, Catal. Today, 2 (1988) 369.