(Cr−V) Supported on Titania Catalysts for the Selective Catalytic

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Monolayer Binary Active Phase (Mo-V) and (Cr-V) Supported on Titania Catalysts for the Selective Catalytic Reduction (SCR) of NO by NH3 Kyriakos Bourikas, Christine Fountzoula, and Christos Kordulis* Department of Chemistry, University of Patras, GR-265 00 Patras, Greece and Institute of Chemical Engineering & High-Temperature Chemical Processes, FORTH/ICE-HT, P.O. Box 1414, GR-265 00 Patras, Greece Received April 14, 2004. In Final Form: August 24, 2004 Monolayer catalysts containing binary active phases (VOx-CrOx, VOx-MoOx) were prepared by simultaneous deposition of the corresponding transition metal-oxo species on the TiO2 (anatase) surface using the equilibrium deposition filtration technique. The prepared samples contained various amounts of each transition metal but almost the same total metal loading. They were characterized by atomic absorption spectroscopy, N2 adsorption, UV-vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, and tested for the selective catalytic reduction of NO by NH3 in the temperature range 250-450 °C. It was found that the transition-metal ionic species used for the preparation of these catalysts compete for the same surface sites of the TiO2 carrier upon co-deposition. Small amounts of the second phase (Mo- or Cr-oxo phase) are sufficient in order to promote the catalytic activity at relatively high temperatures, in contrast to what happens in the corresponding industrial catalysts prepared by conventional methods. An electronic interaction between V- and Cr-oxo species favored at a V/Cr atomic ratio around 3 is probably responsible for the relatively high catalytic performance of the corresponding TiCrV catalyst. The activity of the studied catalysts is well correlated with the intensity of a DRS absorption band that appeared at ca. 400 nm, which is considered as a measure of the magnitude of interactions exerted between the monolayer transition metal-oxo species and the TiO2 carrier. This correlation is independent of the transition metals combination used and follows the same linear relationship found previously for singleactive-phase catalysts.

Introduction Nitrogen oxides (NOx) emitted both from stationary and automotive sources contribute to a variety of environmental problems, such as the formation of acid rain, the photochemical destruction of ozone, and the harmful impact for the respiratory system of human beings.1 The selective catalytic reduction of NOx by ammonia (NH3SCR) is considered to be the best available technology for the control of the NOx emission from stationary sources.2 The NH3-SCR process is operated over commercial metal oxide catalysts usually consisted of homogeneous mixtures of titania, tungsta or molybdena, and vanadia.2-4 On the other hand, several studies have shown that chromia or chromia-vanadia supported on titania can display high activity for the NH3-SCR process.5-12 * Author to whom correspondence should be addressed. E-mail: [email protected]. Tel: +302610997125. Fax: +302610994796. (1) Busca, G.; Lietti, L.; Ramis, G.; Berti, F. Appl. Catal., B 1998, 18, 1. (2) Forzatti, P. Catal. Today 2000, 62, 51. (3) Casagrande, L.; Lietti, L.; Nova, I.; Forzatti, P.; Baiker, A. Appl. Catal., B 1999, 22, 63. (4) Lietti, L.; Nova, I.; Ramis, G.; Dall’ Acqua, L.; Busca, G.; Giamello, E.; Forzatti, P.; Bregani, F. J. Catal. 1999, 187, 419. (5) Curry-Hyde, E.; Baiker, A. Ind. Eng. Chem. Prod. Res. Dev. 1990, 29, 1985. (6) Ko¨hler, K.; Schla¨pfer, C. W.; von Zelewsky, A.; Nickl, J.; Engweiler, J.; Baiker, A. J. Catal. 1993, 143, 201. (7) Engweiler, J.; Nickl, J.; Baiker, A.; Ko¨hler, K.; Schla¨pfer, C. W.; von Zelewsky, A. J. Catal. 1994, 145, 141. (8) Scharf, U.; Schneider, H.; Baiker, A.; Wokaum, A. J. Catal. 1994, 145, 464. (9) Schneider, H.; Scharf, U.; Wokaum, A.; Baiker, A. J. Catal. 1994, 147, 545. (10) Ko¨hler, K.; Maciejewski, M.; Schneider, H.; Baiker, A. J. Catal. 1995, 157, 301.

Various methods have been reported for the preparation of NH3-SCR catalysts. Numerous investigations have been reported on single active phase V2O5/TiO2, WO3/TiO2, MoO3/TiO2, and Cr2O3/TiO2 catalysts prepared by conventional incipient wetness impregnation,13,14 wet impregnation,12,15,16 equilibrium adsorption,17-20 and grafting.2,15,21-25 On the other hand, relatively few studies have been devoted to binary-active-phase NH3-SCR catalysts prepared by various methods. Usually, binary active-phase V2O5-MxOy/TiO2 catalysts (where MxOy ) WO3, MoO3, Cr2O3) are prepared by incipient wetness,3,4,26-32 wet (11) Schneider, H.; Maciejewski, M.; Ko¨hler, K.; Wokaum, A.; Baiker, A. J. Catal. 1995, 157, 312. (12) Fountzoula, Ch.; Matralis, H. K.; Papadopoulou, Ch.; Voyiatzis, G. A.; Kordulis, Ch. J. Catal. 1997, 172, 391. (13) Dall’ Acqua, L.; Nova, I.; Lietti, L.; Ramis, G.; Busca, G.; Giamello, E. Phys. Chem. Chem. Phys. 2000, 2, 4991. (14) Nova, I.; Lietti, L.; Casagrande, L.; Dall’Acqua, L.; Giamello, E.; Forzatti, P. Appl. Catal., B 1998, 17, 245. (15) Degovaand, G. V. O.; Slavinskaya, E. M. Kinet. Catal. 2004, 4, 133. (16) Bond, G. C.; Flamerz Tahir, S. Appl. Catal. 1991, 71, 1. (17) Ciambelli, P.; Lisi, L.; Russo, G.; Volta, J. C. Appl. Catal., B 1995, 7, 1. (18) Fountzoula, Ch.; Spanos, N.; Matralis, H. K.; Kordulis, Ch. Appl. Catal., B 2002, 35, 295. (19) Georgiadou, I.; Papadopoulou, Ch.; Matralis, H. K.; Voyiatzis, G. A.; Kordulis, Ch.; Lycourghiotis, A. J. Phys. Chem. B 1998, 102, 8459. (20) Bourikas, K.; Fountzoula, Ch.; Kordulis, Ch. Appl. Catal., B 2004, 52, 145. (21) Reiche, M. A.; Hug, P.; Baiker, A. J. Catal. 2000, 192, 400. (22) Reiche, M. A.; Bu¨rgi, T.; Baiker, A.; Scholz, A.; Schnyder, B.; Wokaun, A. Appl. Catal., A 2000, 198, 155. (23) Reiche, M. A.; Ortelli, E.; Baiker, A. Appl. Catal., B 1999, 23, 187. (24) Engweiler, J.; Harf, J.; Baiker, A. J. Catal. 1996, 159, 259. (25) Marshneva, V. I.; Slavinskaya, E. M.; Kalinkina, O. V.; Odegova, G. V.; Moroz, E. M.; Lavrova, G. V.; Salanov, A. N. J. Catal. 1995, 155, 171.

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impregnation,33,34 and scarcely grafting.21,22,35 To the extent of our knowledge, binary-active-phase NH3-SCR catalysts prepared by equilibrium adsorption have not been studied so far. Research carried out in our group has shown that the application of an equilibrium adsorption methodology, called equilibrium deposition filtration (EDF), results to supported catalysts with very high active surface area, as it favors monolayer structures (isolated species and/or bidimensional islands) of the deposited phases.19,36-42 This is presumably the reason for which the transition metaloxo (tm-oxo) species supported on titania catalysts prepared by EDF were proved to be much more active for NH3-SCR than the corresponding ones prepared using conventional impregnation methods.18,19,37 Recently, we have studied a single active phase supported on TiO2 catalysts.20 Specifically, we investigated the behavior of transition metal ionic-oxo (tmi-oxo) species (tm ) V, Cr, Mo, and W) upon their adsorption from aqueous solution on the anatase surface. Moreover, we tried to correlate the activities of the corresponding singleactive-phase catalysts prepared by EDF with various physicochemical properties. A very good correlation was found between the activity expressed by the rate constant of the NO reduction by NH3 and the magnitude of the interactions exerted between the monolayer-supported phases and the anatase surface. The magnitude of these interactions has been estimated by the intensity of the absorbance appearing at ca. 400 nm in the corresponding diffuse reflectance spectra. The same correlation had been also established in a previous work concerning a series of TiMo catalysts with various Mo loadings.18 All these results, limited to single active phase supported on TiO2 catalysts, demonstrated that the above-mentioned correlation is independent of the kind of the tm-oxo species supported on the titania surface. The purpose of the present work is to extend the previous study to binary-active-phase catalysts supported on TiO2. Specifically, in the present study, we prepared two series of binary-tm monolayer/TiO2 catalysts (binary-tm monolayer, V-Mo, V-Cr) using the EDF methodology in order to investigate whether the previously found correlation can be also detected in more-complicated catalysts. Moreover, in this study, we investigated the possible (26) Alemany, J. L.; Lietti, L.; Ferlazzo, N.; Forzatti, P.; Busca, G.; Ramis, G.; Giamello, E.; Bregani, F. J. Catal. 1995, 155, 117. (27) Amiridis, M. D.; Solar, J. P. Ind. Eng. Chem. Res. 1996, 35, 978. (28) Ramis, G.; Busca, G.; Forzatti, P. Appl. Catal., B 1992, 1, L9. (29) Vuurman, M. A.; Wachs, I. E.; Hirt, A. M. J. Phys. Chem. 1991, 95, 9928. (30) Lietti, L.; Nova, I.; Forzatti, P. Topics Catal. 2000, 11/12, 111. (31) Lietti, L. Appl. Catal., B 1996, 10, 281. (32) Choo, S. T.; Yim, S. D.; Nam, I.-S.; Ham, S.-W.; Lee, J.-B. Appl. Catal., B 2003, 44, 237. (33) Fountzoula, Ch.; Matralis, H. K.; Papadopoulou, Ch.; Voyiatzis, G. A.; Kordulis, Ch. J. Catal. 1999, 184, 5. (34) Mastikhin, V. M.; Terskikh, V. V.; Lapina, O. B.; Filimonova, S. V.; Seidl, M.; Kno¨zinger, H. J. Catal. 1995, 156, 1. (35) Ko¨hler, K.; Engweiler, J.; Baiker, A. J. Mol. Catal. A: Chem. 2000, 162, 423. (36) Spanos, N.; Matralis, H. K.; Kordulis, Ch.; Lycourghiotis, A. J. Catal. 1992, 136, 432. (37) Papadopoulou, Ch.; Karakonstantis, L.; Matralis, H. K.; Kordulis, Ch.; Lycourghiotis, A. Bull. Soc. Chim. Belg. 1996, 105, 247. (38) Karakonstantis, L.; Matralis, H.; Kordulis, Ch.; Lycourghiotis, A. J. Catal. 1996, 162, 306. (39) Bourikas, K.; Georgiadou, I.; Kordulis, Ch.; Lycourghiotis, A. J. Phys. Chem. B 1997, 101, 8499. (40) Kordulis, Ch.; Lappas, A. A.; Fountzoula, Ch.; Drakaki, K.; Lycourghiotis, A.; Vasalos, I. A. Appl. Catal., A 2001, 209, 85. (41) Vakros, J.; Kordulis, Ch.; Lycourghiotis, A. Langmuir 2002, 18, 417. (42) Papadopoulou, Ch.; Vakros, J.; Matralis, H. K.; Kordulis, Ch.; Lycourghiotis, A. J. Colloid Interface Sci. 2003, 261, 146.

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development of interactions between the two active phases of the binary catalysts. Finally, on the basis of the deposition mechanism followed, we investigated whether the tmi-oxo species of the two tm compete for the same surface sites of the support. Experimental Section Co-Deposition Isotherms. Our previous works have shown that the deposition of tmi-oxo species on the anatase surface is favored at pH = 4.5.36,39,43,44 There are two reasons for this: first, because at this pH value, the number of the positively charged surface sites is relatively high and second, because at this pH value, surface dissolution of the support does not take place. Thus, in this work, we have selected pH 4.6 to study the codeposition of the V- and Mo-oxo species, as well as that of V- and Cr-oxo species on the anatase surface. The anatase used was prepared by hydrolysis of titanium isopropoxide, as described in detail previously.45 Equilibrium co-deposition experiments were done at 25 ( 0.1 °C. In each experiment, 0.03 g of the anatase was suspended in 0.014 dm3 of the suitable solution containing various concentrations of the two tmi-oxo species ranging between 1 × 10-4 and 4 × 10-3 mol of metal/dm3. The total metal concentration was selected to be 3.5 × 10-3 mol/dm3 for the Mo-V system and 4.0 × 10-3 mol/dm3 for the Cr-V system. The ionic strength was adjusted by ammonium nitrate at 0.1 mol/dm3. The suspension pH (4.6) was adjusted by the addition of small aliquots of HNO3 (1M). The surface concentration of each tm, Γ (mol/m2), was calculated from the concentration difference before and after deposition according to the following relationship:

C0 - Ceq WS

Γ)V

(1)

where C0, Ceq, V, W, and S denote, respectively, the solution concentration of each tm before and after deposition (mol/dm3), the suspension volume (dm3), the weight (g), and the specific surface area of the anatase (m2/g). Full details regarding the equilibrium adsorption experiments have been reported elsewhere.46 Catalyst Preparation. Following EDF, an amount of anatase equal to 5 g was added in the corresponding impregnation solution totaling 2.5 dm3. Each impregnation solution used in this work contained various ionic-oxo species of two tm with concentrations selected on the basis of the corresponding co-deposition isotherms in order for a desired final surface concentration of each tm to be achieved. The total metal concentration was constant in each impregnation solution and was equal to 3.5 × 10-3 mol/dm3 for the Mo-V system and 4.0 × 10-3 mol/dm3 for the Cr-V system. In all cases, the ionic strength of the solutions was adjusted to 0.1 N using NH4NO3. The pH of the suspension was regulated at 4.6 using HNO3. After being stirred for 20 h at 25 °C, the suspensions were filtered using membrane filters (Millipore, 0.22 µm). The resulting solids were dried at 120 °C for 2.5 h, and then they were calcined at 500 °C for 5 h in air. The metal content in the catalysts was determined using atomic absorption spectroscopy (AAS, Hewlett-Packard A Analyst 300). Catalytic Activity Measurements. Catalytic activity measurements were performed in a continuous-flow fixed-bed microreactor working under atmospheric pressure. The reaction mixture consisted of 800 ppm NO, 800 ppm NH3, and 4% O2 with nitrogen as balance (Air Liquide). The concentrations of NO, NO2, N2O, NH3, H2O, and O2 in the inlet and outlet of the reactor were determined using a computer-controlled on-line quadrupole mass spectrometer (VG-Sensorlab 200D). These concentrations were used for the determination of the NO conversion and the relative yield toward the desired product, N2. (43) Spanos, N.; Slavov, S.; Kordulis, Ch.; Lycourghiotis, A. Colloids Surf. 1995, 97, 109. (44) Matralis, H. K.; Bourikas, K.; Papadopoulou, Ch.; Kordulis, Ch.; Lycourghiotis, A. Pol. J. Appl. Chem. 1997, XLI, 275. (45) Georgiadou, I.; Spanos, N.; Papadopoulou, Ch.; Matralis, H.; Kordulis, Ch.; Lycourghiotis, A. Colloids Surf. 1995, 98, 155. (46) Spanos, N.; Vordonis, L.; Kordulis, Ch.; Lycourghiotis, A. J. Catal. 1990, 124, 301.

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Figure 1. Co-deposition isotherms of V and Mo obtained for the system V-Mo/TiO2 at pH 4.6. The deposition isotherms for the single-phase systems at the same pH value, taken from ref 20, are also presented. T ) 25 °C, I ) 0.1 M NH4NO3.

Figure 2. Co-deposition isotherms of V and Cr obtained for the system V-Cr/TiO2 at pH 4.6. The deposition isotherms for the single-phase systems at the same pH value, taken from ref 20, are also presented. T ) 25 °C, I ) 0.1 M NH4NO3.

Masses of 5 mg of catalyst diluted up to 100 mg with SiO2 (Silica gel 60, Merck) were used in these measurements. At the beginning, an O2-N2 mixture containing 10% O2 was passed through the reactor and the catalytic bed was heated in two successive steps: first from room temperature to 100 °C, remaining constant for 45 min, and then from 100 to 450 °C, remaining also constant for 45 min. The isothermal heating between the two steps was required in order for the signal of 18 amu (H2O), monitored by the mass spectrometer, to be stabilized. Then, the reaction mixture was fed in the reactor, with a flow rate of 150 mL/min (STP), and catalytic tests were carried out in the temperature range 250-450 °C. The reaction steady state was established after 30-45 min at each temperature. Specific Surface Area Measurements. Specific surface area (SSA) measurements of the prepared samples have been carried out in a laboratory-constructed apparatus by the three-point dynamic BET method. Pure nitrogen and helium (Air Liquide) were used as adsorption and carrier gas, respectively. A thermal conductivity detector of a gas chromatograph (SHIMADZU GC 8A) was used to detect the adsorbed amount of nitrogen at a given partial pressure. Diffuse Reflectance Spectroscopy (DRS). The diffuse reflectance spectra of the catalysts were recorded in the range 200-800 nm at room temperature (Slit Band Width ) 2 nm) using a UV-vis spectrophotometer (Varian Cary 3) equipped with an integration sphere. The support was used for the baseline correction and as a reference sample as well. The samples were mounted in a quartz cell, which provided a sample thickness greater than 3 mm. X-Ray Photoelectron Spectroscopy (XPS). The XPS analysis was performed at room temperature in a UHV chamber (base pressure ) 8 × 10-10 mbar) which consists of a fast specimen entry assembly, a preparation, and an analysis chamber. The residual pressure in the analyzer chamber was below 10-8 mbar. The latter was equipped with a hemispherical electron-energy analyzer (SPECS, LH 10) and a twin-anode X-ray gun for XPS. The unmonochromatized Al KR line at 1486.6 eV and a constant pass energy mode for the analyzer were used in the experiments. Pass energies of 36 and 97 eV resulted in a full width at halfmaximum (fwhm) of 0.9 and 1.6 eV, respectively, for the Ag 3d5/2 peak of a reference foil. The binding energies were calculated with respect to the C 1s peak (C-C, C-H) set at 284.6 eV.

cases a plateau was obtained, corresponding to the monolayer surface coverage. The experimental points are well fitted with Langmuir-type isotherms. This type of isotherm implies localized, Langmuir-type adsorption of the metal-oxo species in the inner Helmholtz plane developed in the region of the “impregnating solution/ anatase surface” interface with very weak, if any, lateral interactions.46,47 To explain the adsorption behavior of the Mo-V and Cr-V systems as it is observed in Figures 1 and 2, we have to take into account previous findings concerning the deposition of moybdenum, vanadium, and chromium on the anatase surface at pH 4.6.20 As may be seen from Figure 1, the maximum uptake of vanadium or molybdenum on the anatase surface, which can be achieved by adsorption, is about 10 µmol/m2. In contrast, as may be seen from Figure 2, the maximum uptake in the case of the chromium is quite lower and equal to about 2.4 µmol/ m2. It should be noticed that for the systems Mo/TiO2 and V/TiO2 a significant portion of the deposited metal is due to the adsorption of polymeric species on the surface of titania (mainly the Mo7O246- and V10O274- species, respectively).20 Ciambelli et al.48 studying vanadia-titania catalysts prepared by equilibrium adsorption have confirmed the adsorption of such species at low pH values using Raman spectroscopy. These species carry a large number of active metal atoms and thus increase considerably the loading of the catalyst. In the case of Cr/TiO2, it should be noticed the nonexistence of adsorbed polymers. Only monomers (CrO42-, HCrO4-) and oligomers (Cr2O72-) are deposited on the titania surface carrying a relatively small number of Cr atoms.20 Thus, the Cr loading achieved is quite lower than that of the other systems. The simultaneous deposition of Mo and V leads to a total uptake (Mo + V) almost equal to that achieved in each one of the single-phase systems (Mo/TiO2 or V/TiO2). This indicates competitive adsorption for the same surface sites of the molybdates and vanadates (mainly of the Mo7O246- and V10O274- species, respectively). Concerning the simultaneous deposition of Cr and V, the corresponding co-deposition isotherms indicate also

Results and Discussion Active-Phase Deposition. Figures 1 and 2 illustrate the co-deposition isotherms obtained for the two pairs of metal-oxo species studied in this work at pH 4.6. For comparison, the deposition isotherms for the single-phase systems20 are also presented. It may be seen that in all

(47) Giles, C. H.; Smith, D.; Huitson, A. J. Colloid Interface Sci. 1974, 47, 755. (48) Ciambelli, P.; Lisi, L.; Russo, G.; Volta, J. C. Appl. Catal., B 1995, 7, 1.

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Table 1. Notation of the Prepared Catalysts, the Initial Concentrations (Ctm) of the Impregnating Solutions (in Moles of the Transition Metal), the Metal Loading (L), the Specific Surface Area (SSA) of the Catalysts and the Atomic Surface Ratio of the Transition Metals (ASR) Ctm ×103 (M) catalyst TiVa TiMoV1 TiMoV2 TiMoV3 TiMoV4 TiMoa TiCrV1 TiCrV2 TiCrV3 TiCra a

Mo 1.0 1.5 2.0 2.5 1.75

V

Cr

2.5 2.5 2.0 1.5 1.0 2.0 2.0 1.5 2.5 1.0 3.0 1.2

L (% mol) Mo 1.27 1.47 1.54 1.71 2.87

V

Cr

3.13 1.87 1.51 1.36 0.94 2.20 0.64 1.88 0.83 1.71 1.0 1.01

SSA (m2/g) 25 42 41 44 41 42 30 34 31 34

ASR × 102 Mo 1.87 2.12 2.44 2.81 3.9

V

Cr

5.17 2.69 2.39 2.28 2.06 4.22 0.79 4.13 0.91 3.94 1.08 2.0

The corresponding values have been taken from ref 20.

a competitive adsorption between the chromates and the vanadates (see Figure 2) at relatively low equilibrium concentrations. In fact, the uptake of each species at a given equilibrium concentration is lower in the codeposition than in the single-phase deposition. However, at high equilibrium concentrations, where the co-deposition isotherms reach a plateau, the situation is different. The extent of deposition of vanadium is not influenced considerably by the presence of the chromium in the impregnating suspension. Thus, the maximum vanadium uptake is almost equal to that achieved at the single V/TiO2 system, whereas the total maximum uptake (Cr + V) is higher than the corresponding one obtained in the Mo-V system (compare Figures 1 and 2). This probably means that the deposition mechanism and the interaction between the chromates and the vanadates change significantly under these conditions and lead to a surface speciation which permits the achievement of a higher total maximum uptake on the anatase surface. The full elucidation of the co-deposition mechanism of the chromates and vanadates on titania surface could shed more light in this interesting point. Texture of the Catalysts. Inspection of Table 1 shows that all the catalysts prepared in this work have roughly the same total loading of tm (an average 2.85 mol%). In any case, the deposited amount for each tm does not exceed the saturation value determined from the plateau of the corresponding deposition isotherms. Thus, we may conclude that using EDF the loadings achieved in all binaryactive-phase catalysts have not exceeded those corresponding to the monolayer coverage of the anatase surface.12,38,48 Therefore, no drastic diminution in the specific surface area of the anatase was expected upon the active phase deposition. In fact, as it may be seen from Table 1, the specific surface areas of the prepared samples are similar or slightly lower than that of the anatase used as carrier (40 m2/g). By comparison of these values with that measured in a previous work20 for a singleactive-phase VOx/TiO2 sample (25 m2/g) having almost the same V loading and prepared also by EDF, it is obvious that the presence of Mo- and to a lesser extent of Cr-oxo species prevents the sintering of the TiO2 caused by the V-oxo species. This finding is in very good agreement with previous results1,3,4,30,33,48,49 and indicates that, indeed, both Mo- and Cr-oxo species act as “structural” promoters by preserving the morphological characteristics of the support. Structure of the Catalysts: Investigation for Electronic Interactions between the Supported (49) Chen, J. P.; Yang, R. T. Appl. Catal., A 1992, 80, 135.

Figure 3. Diffuse reflectance spectra of the various TiMoVsupported titania catalysts. The corresponding spectra of the single-phase catalysts, taken from ref 20, are also presented.

Figure 4. Diffuse reflectance spectra of the various TiCrVsupported titania catalysts. The corresponding spectra of the single-phase catalysts, taken from ref 20, are also presented.

Phases. The UV-vis/DR spectra of the samples illustrated in Figures 3 and 4 show a main absorption band centered at about 400 nm. Such a band is reported by several authors50,51 in the case of titania doped with transition elements. In previous works, we have demonstrated that the intensity of the aforementioned band is proportional to the amount of the Cr-, V-, and Mo-oxo species strongly interacted with the TiO2 surface.12,18,19 Studying single-active-phase tm/TiO2 catalysts prepared by EDF, we have measured the molecular absorption coefficient () for the above band of the corresponding monolayered active phases.20 These  values showed that both V- and Cr-oxo phases interact strongly with the anatase surface. Such a strong interaction has been reported several times in the literature.19,52,53 On the contrary, the interaction of the Mo-oxo phase with the anatase surface is less strong. (50) Matralis, H. K.; Papadopoulou, Ch.; Kordulis, Ch.; Aguilar Elguezabal, A.; Cortes Corberan, V. Appl. Catal., A 1995, 126, 365. (51) Del Arco, M.; Holgado, M.; Martin, C.; Rives, V. J. Catal. 1986, 99, 19. (52) Djerad, S.; Tifouti, L.; Crocoll, M.; Weisweiler, W. J. Mol. Catal. A: Chem. 2004, 208, 257. (53) Weckhuysen, B. M.; Keller, D. E. Catal. Today 2003, 78, 25.

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Table 2. Comparison of the Calculated Values of the Theoretical Intensity, Ical, for the DRS Peak at ca. 400 nm with the Corresponding Experimental Values, Iex, Extracted from Figures 2 and 3 for Each One of the Prepared Catalysts catalyst

Ical

TiVa TiMoV1 TiMoV2 TiMoV3 TiMoV4 a

2.186 1.782 1.613 1.139

Iex 3.581 1.941 1.491 1.463 1.043

catalyst

Ical

Iex

3.239 3.088 3.085

0.106 4.001 3.068 2.996 1.140

TiMoa TiCrV1 TiCrV2 TiCrV3 TiCra

The corresponding values have been taken from ref 20.

Using the above-mentioned  values and the tm loadings, an intensity (Ical) for the peak at ca. 400 nm may be calculated for each one of the prepared samples. Comparing these Ical values compiled in Table 2 with the experimental values (Iex) extracted from Figures 3 and 4, which are also compiled in the same Table, it is evident that in almost all cases Ical ≈ Iex. There is only one exception in the case of the TiCrV1 sample, where Ical < Iex. The above-mentioned equality indicates that in most of cases the V- and Cr-(Mo-)oxo species present in the binary active phase samples prepared by EDF do not perturb each other from the electronic point of view. Therefore, it may be inferred that these species are strongly anchored at the titania surface. However, it is noteworthy that evidences collected in previous works3,4,54 suggest the development of electronic interactions between V- and Mo-oxo species supported on the titania surface, presumably through the conduction band of the support. The relatively high Iex value observed in the case of TiCrV1 sample is probably related with an interaction between V- and Cr-oxo species favored at V/Cr ratio equal to 3.4. Such an interaction in a similar V/Cr ratio ()3) had been also observed by us using laser Raman spectroscopy.33,55 In contrast, Ko¨hler et al.35 studying grafted CrOx-VOx/TiO2 catalysts with V/Cr ratios in the range 1-2 have not observed any interaction between the oxo species of the two supported tm. In all samples, the main absorption DRS band was more or less asymmetric toward the high-wavelength side of the spectra. This asymmetry reveals contributions of badly resolved absorption bands attributed to the supported oxo species, where the corresponding tm exists at oxidation states other than its maximum.56-58 However, the observed asymmetry is less pronounced in the binary-active-phase samples. This probably indicates that Mo- and Cr-oxo phases interact with the V-oxo phase, and such an interaction stabilizes the tm’s at high oxidation states. The binding energy (BE) of V 2p3/2 photoelectrons for V5+ is reported to be equal to 517 ( 0.2 eV.59-62 In the XP spectra of all the samples, the corresponding peak (54) Forzatti, P. Appl. Catal., A 2001, 222, 221. (55) Giakoumelou, I.; Fountzoula, Ch.; Kordulis, Ch.; Boghosian, S. Catal. Today 2002, 73, 255. (56) Schoonheydt, R. A. In Characterization of Heterogeneous Catalysts; Delanay, F., Ed.; Dekker: New York/Basel, 1984; Chapter 4. (57) Cimino, A.; de Angelis, B.; Luchetti, A.; Minelli, G. J. Catal. 1976, 45, 316. (58) Ko¨hler, K.; Engweiler, J.; Viebrock, H.; Baiker, A. Langmuir 1995, 11, 3423. (59) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G. E. Handbook of X-Ray Photoelectron Spectroscopy: A Reference Book of Standard Data for Use in X-Ray Photoelectron Spectroscopy; Physical Electronics Division, Perkin-Elmer Corp.: Eden Prairie, MN, 1979. (60) Centi, G.; Pinelli, D.; Trifiro, F.; Ghaussoub, D.; Guelton, M.; Gengembre, L. J. Catal. 1991, 130, 238. (61) Centi, G.; Giamello, E.; Pinelli, D.; Trifiro, F. J. Catal. 1991, 130, 220. (62) Haber, J.; Machej, T.; Czeppe, T. Surf. Sci. 1985, 151, 301.

Figure 5. X-ray photoelectron spectra of the Cr 2p3/2 peak of the various TiCrV-supported titania catalysts. The corresponding spectrum of the TiCr catalyst, taken from ref 20, is also presented.

appeared at the above-mentioned BE, indicating the existence of V5+-oxo species on the catalyst surface. Figure 5 illustrates the X-ray photoelectron spectra of the Cr 2p3/2 peak of the TiCrV catalysts. The shape of the Cr 2p3/2 peak strongly suggests the existence of supported chromium in various valence states in accordance with our DRS results; namely Cr3+ (BE ) 577.2 eV), Cr6+ (BE ) 579.5 eV), and Cr5+ (BE(Cr 2p3/2) ) 577.5-578.5 eV).59 However, the increase of the vanadium content of the TiCrV samples provokes a shift of the maximum of the above-mentioned peak toward higher BEs. This shift could be considered as an indication that indeed Cr-oxo species, in which Cr exists at high oxidation states, are favored in the presence of the V-oxo phase. In the case of TiMoV samples, a doublet peak corresponding to Mo 3d photoelectrons appeared at 235.5 eV (Mo 3d3/2) and 232.5 eV (Mo 3d5/2), indicating the existence of Mo6+ oxo-species on the titania surface.59,63 The BE of the Ti 2p3/2 photoelectrons of the titania carrier was 458.5 ( 0.2 eV, whereas the BE difference, BE(Ti 2p1/2) - BE(Ti 2p3/2), was equal to 5.74 ( 0.02 eV for all samples. These values are typical for TiO2.59,64 Using the normalized intensities of the V 2p, Cr 2p, Mo 3d, O 2s, and Ti 2p XPS peaks, we have calculated the atomic surface ratio (ASR) of each supported tm, ASR ) Itm/(Itm + ITi + IO), over the catalysts studied. Actually, ASR expresses the number of atoms of the tm per total atoms existing in the surface layer, which is analyzed by XPS. The ASR values are compiled in Table 1. An inspection of this Table shows that the ASR of each supported tm increases with its loading. Activity and Selectivity. Figure 6 illustrates the NO conversion achieved over the TiMoV catalysts. Results concerning the single tm catalysts (TiV and TiMo) taken from ref 20 are also presented for comparison. It may be observed that the vanadium-containing catalysts are more active than the TiMo sample in the whole temperature range studied. The NO conversions measured over the vanadium-containing samples exhibit a bell shape with the temperature. This behavior, combined with a continuous increase of NH3 consumption observed, shows that besides the main reaction, eq 2, other side reactions involving NH3 but not NO, like eqs 3, 4, and 5, (63) Defresne, P.; Grimblot, J.; Bonnelle, J. P. Bull. Soc. Chim. France 1980, 3/4, I-89. (64) Andersson, S. L. T. Catal. Lett. 1990, 7, 351.

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Bourikas et al.

Figure 6. NO conversion achieved over the various TiMoVsupported titania catalysts in the temperature range 250-450 °C. The corresponding values achieved over the single-phase catalysts (TiV and TiMo), taken from ref 20, are also presented.

Figure 8. NO conversion achieved over the various TiCrVsupported titania catalysts in the temperature range 250-450 °C. The corresponding values achieved over the single-phase catalysts (TiV and TiCr), taken from ref 20, are also presented.

Figure 7. The yield to N2 achieved over the various TiMoVsupported titania catalysts in the temperature range 250-450 °C. The corresponding values achieved over the single-phase catalysts (TiV and TiMo), taken from ref 20, are also presented.

Figure 9. The yield to N2 achieved over the various TiCrVsupported titania catalysts in the temperature range 250-450 °C. The corresponding values achieved over the single-phase catalysts (TiV and TiCr), taken from ref 20, are also presented.

should take place at high reaction temperatures over those catalysts:

of the side reactions are negligible, the catalytic performance decreases as the vanadium loading decreases. Figure 8 illustrates the NO conversions measured over the TiCrV samples. Results concerning the single tm catalysts (TiV and TiCr) taken from ref 20 are also presented for comparison. An inspection of this figure shows that the previously observed bell-shape variation of the NO conversion with the temperature is also observed for the V-containing samples of this series of catalysts. At reaction temperatures lower than 350 °C, the activity decreases with the decrease of the V loading of the samples. However, a promoting effect of the Cr-phase is evident at reaction temperatures g350 °C. This conclusion is also supported by the results presented in Figure 9 concerning the yield to N2 achieved over the catalysts of this series. At this point, it should be stressed that according to the above results the best catalytic performance of the studied catalysts is achieved over the sample with the minimum promoter (Mo or Cr) loading of each series. This means that in the case of the monolayer binary active phase (MoV) and (Cr-V) supported on titania catalysts only small amounts of the promoters are sufficient in order for the maximum promoting effect to appear. This is in contrast to what happens in the corresponding industrial catalysts prepared by conventional methods. In the latter, the

4NO + 4NH3 + O2 f 4N2 + 6H2O

(2)

4NH3 + 3O2 f 2N2 + 6H2O

(3)

2NH3 + 2O2 f N2O + 3H2O

(4)

4NH3 + 5O2 f 4NO + 6H2O

(5)

However, the maximum conversion achieved over each catalyst shifts to higher temperatures (g400 °C) in the presence of the Mo phase. This indicates that the Mophase favors the main reaction, eq 2, instead of the side reactions. This is in accordance with the chemical promoting effect of the Mo-phase in such catalysts described several times in the literature.1,3,4,30,49 This effect renders the TiMoV catalysts more active than TiV at the highest reaction temperature studied in the present work. Figure 7 corroborates the above argument, showing that the yield toward N2 achieved over the binary-active-phase samples is indeed higher at high reaction temperatures (>350 °C) than that of the single-active-phase sample. On the other hand, at reaction temperatures e350 °C, where the rates

(Mo-V) and (Cr-V) Supported on Titania De-NOx Catalysts

Langmuir, Vol. 20, No. 24, 2004 10669

Table 3. Values of Reaction Rate Constant, k, for the NO Reduction by NH3 at Three Temperatures over All the Catalysts Prepared k (L g-1 s-1) catalyst

250 °C

300 °C

350 °C

TiVa

0.15 0.047 0.03 0.035 0.025 0 0.155 0.099 0.104 0.007

0.348 0.155 0.099 0.113 0.069 0.014 0.372 0.317 0.272 0.017

0.621 0.376 0.267 0.298 0.225 0.025 0.78 0.579 0.582 0.12

TiMoV1 TiMoV2 TiMoV3 TiMoV4 TiMoa TiCrV1 TiCrV2 TiCrV3 TiCra a

The corresponding values have been taken from ref 20.

concentration of the promoters is 6 (for Mo) or 10 (for W) times higher than the concentration of the vanadia phase.54 Concerning the TiCrV catalysts, the increased activity of the sample satisfying a V/Cr atomic ratio around 3 (TiCrV1) could be attributed to the electronic interaction between V- and Cr-oxo species, suggested by the abovepresented DRS results. Taking into account the above-discussed activity results, it is obvious that we may compare the activities with respect to eq 2 only at temperatures e350 °C. In agreement with numerous published results concerning steady-state kinetic studies of the NH3-SCR process, we have assumed a first-order dependence on NO and zero-order dependence on NH3.18,65-69 In such a case, treating the reactor as a plug flow integral one,70 we can write eq 6:

k)-

U ln(1 - x) w

(6)

Equation 6 permits the calculation of the reaction rate constant (k) from the volumetric feed (U), the mass of catalyst (w) used, and the conversion of NO (x) achieved. Table 3 compiles the values of k calculated for all samples studied in this work. An inspection of Table 3 reveals that the activity trends resulted from Figures 6 and 8 are the same as those drawn from the calculated k values. To explain the above-discussed catalytic behavior of the studied catalysts, we have tried to correlate their activity estimated by the reaction rate constant (k) with various physicochemical parameters, such as tm-loading, specific surface area, active surface area, and the extent of the active phase-support interactions measured by the intensity of the DRS peak at about 400 nm. A very good correlation is found between the rate constant (calculated at three reaction temperatures, 250, 300 and 350 °C) and the intensity of the above peak (see Figure 10). The other parameters do not correlate with the catalytic activity. The above-mentioned correlation indicates that the magnitude of the interactions exerted between the monolayered phases and the anatase surface is most probably a key factor for the activity of the corresponding catalysts for the reduction of NO by NH3. The same correlation has been also established in recent works concerning a series (65) Topsoe, N.-Y. Science 1994, 265, 1217. (66) Marangozis, J. Ind. Eng. Chem. Res. 1992, 31, 987. (67) Amiridis, M. D.; Wachs, I. E.; Deo, G.; Jehng, J.-M.; Kim, D. S. J. Catal. 1996, 161, 247. (68) Caraba, R. M.; Masters, S. G.; Eriksen, K. M.; Parvulescu, V. I.; Fehrmann, R. Appl. Catal., B 2001, 34, 191. (69) Efstathiou, A. M.; Fliatoura, K. Appl. Catal., B 1995, 6, 35. (70) Fogler, H. S. Elements of Chemical Reaction Engineering; Prentice Hall: Upper Saddle River, NJ, 1999; p 619.

Figure 10. Correlation of reaction rate constant (k) of SCR of NO by NH3 measured over binary-phase monolayer tm (V-Cr, V-Mo)-supported titania catalysts at three temperatures with the intensity of their DRS peak at ca. 400 nm. The corresponding data determined on the single-active-phase (V, Mo, Cr)supported titania catalysts, taken from ref 20, are also presented with open symbols.

of TiMo catalysts with various Mo loadings18 and singleactive-phase transition metal supported on TiO2 catalysts.20 However, the results of the present work demonstrate that this correlation is a more general phenomenon observed also in the case of more-complicated catalysts, such as the binary-active-phase transition metal supported on TiO2 catalysts. It is noteworthy that the experimental data [k vs F(R)] determined using the monolayer binary active phase (Mo-V) and (Cr-V) supported on titania catalysts follow the same linear expression with the corresponding data determined on the single-active-phase (V, Mo, Cr) catalysts. The latter are also presented in Figure 10. Conclusions The most important conclusions drawn from the present work may be summarized as follows. 1. Upon co-deposition of vanadates and molybdates or vanadates and chromates on titania surface, the transition metal ionic species compete for the same surface sites of the TiO2 carrier. 2. Concerning the binary-active-phase catalysts, a small amount of the second phase (Mo- or Cr-oxo phase) is sufficient to promote the catalytic activity at relatively high temperatures, in contrast to what happens in the corresponding industrial catalysts prepared by conventional methods. 3. An electronic interaction between V- and Cr-oxo species favored at V/Cr atomic ratio around 3 is probably responsible for the relatively high catalytic performance of the corresponding TiCrV catalyst. 4. The activity of the binary-active-phase (V-Mo, V-Cr) catalysts for the selective reduction of NO by NH3 is well correlated with the intensity of a DRS absorption band appearing at ca. 400 nm, which is considered to be a measure of the magnitude of the interactions exerted between the monolayer tm-oxo species and the TiO2 carrier. This correlation is independent of the tm combination used and follows the same linear relationship found previously for single-active-phase catalysts. LA049050Y