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Langmuir 1997, 13, 4628-4637
Ammonia Adsorption and Oxidation on Cu/Mg/Al Mixed Oxide Catalysts Prepared via Hydrotalcite-Type Precursors Marcella Trombetta,† Gianguido Ramis,† Guido Busca,*,† Beatrice Montanari,‡ and Angelo Vaccari‡ Istituto di Chimica, Facolta` di Ingegneria, Universita` , P.le Kennedy, 16129 Genova, Italy, and Dip. di Chimica Industriale e dei Materiali, Viale del Risorgimento 4, 40136 Bologna, Italy Received July 9, 1996. In Final Form: May 9, 1997X Cu/Mg/Al catalysts obtained by controlled calcination of hydrotalcite-type (HT) anionic clays may be new interesting and cheap catalysts for the selective catalytic reduction (SCR) of NO by NH3. In this paper the ammonia adsorption and oxidation on CuxMg0.710-xAl0.290 catalysts (x ) 0.022, 0.046, and 0.072, as atomic ratio), obtained by calcination for 14 h at 923 K of HT precipitates, have been investigated and compared with those of the corresponding Mg0.710Al0.290 sample. The presence of copper strongly increases the SCR activity and the selectivity to nitrogen in ammonia oxidation, while the Mg/Al catalyst did not show SCR activity in these conditions and formed significant amounts of nitrogen oxides by ammonia oxidation. All samples adsorbed coordinatively ammonia on medium-week Lewis acid sites, while no Brønsted acidity was found, showing that protonic acidity is not necessary for both SCR and ammonia oxidation. With an increase in the copper content, the ammonia gave rise by oxidation to adsorbed hydrazine (likely via amide intermediates) and other adsorbed species, tentatively identified as imido or nitroxyl fragments and nitrogen anions. These surface species were probably involved in either selective or unselective ammonia oxidation, this last occurring via a Mars-van Krevelen-type mechanism. In order to have more information on the SCR activity of the Cu/Mg/Al catalysts, the NO adsorption also was investigated, showing that on the Mg/Al-mixed oxide free surface, NO disproportionates to nitrogen dioxide and to a species identified as hyponitrite anions. On the other hand, over the Cu-containing centers NO gave rise mainly to surface nitrosyl, being also oxidized to nitrates. On the basis of these data, it was hypothesized that on the Cu/Mg/Al catalysts the SCR took place between NO or nitrosyls and amide species, which were likely common intermediates in either SCR and ammonia oxidation.
1. Introduction Copper oxide-based materials have been investigated deeply in the field of environmental catalysis. Cu-ZSM-5 catalysts, usually overexchanged with copper, appear to be most promising for the purification of the diesel exhaust gases through the reduction of NOX by hydrocarbons.1-4 It is established that Cu-ZSM-5 shows a high catalytic activity and selectivity to N2 in the presence of excess oxygen, whereas in the absence of oxygen it is either not active or leads to NO reduction to ammonia. Cu-ZSM-5 shows an interesting catalytic activity also in the decomposition of NO to nitrogen, although still far below that needed for industrial application. CuO-based catalysts, like CuO-TiO25-7 and CuO-ZrO25,6 and CuO-Al2O36 have also been reported to be active in the selective catalytic reduction (SCR) of NOx by ammonia, through the main reaction:
4NO + 4NH3 + O2 a 4N2 + 6H2O
(1)
This process is now well established and is widely used for the abatement of nitrogen oxides from waste gases of stationary sources.8,9 The industrial catalysts are based on V2O5-TiO2-anatase with addition of either WO3 or †
Istituto di Chimia. Dip. di Chimica Industriale e dei Materiali. X Abstract published in Advance ACS Abstracts, July 1, 1997. ‡
(1) Iwamoto, M.; Hamada, H. Catal. Today 1991, 10, 57. (2) Li, Y.; Armor, J. N. Appl. Catal. 1992, B1, L31. (3) Petunchi, J. O.; Hall, W. Keith Appl. Catal. 1993, B2, L17. (4) Centi, G.; Perathoner, S. Appl. Catal. A 1995, 132, 179. (5) Iizuka, T.; Ikeda H.; Okazaki, S. J. Chem. Soc., Faraday Trans. 1 1986, 82, 61. (6) Centi, G.; Nigro, C.; Perathoner, S.; Stella, G. Catal. Today 1993, 17, 159. (7) Ramis, G.; Yi, Li; Busca, G.; Turco, M.; Kotur, E.; Willey, R. J. J. Catal. 1995, 157, 523.
S0743-7463(96)00673-7 CCC: $14.00
MoO3. CuO-based catalysts are slightly less selective than the industrial ones, because of their excessively high activity in oxidizing ammonia to N2, N2O, and NO.7 CuO-Al2O3 powders have also been proposed as regenerable adsorbant catalysts for the DeSOx-DeNOx processes where SO2 and SO3 are adsorbed in the form of sulfates and NOx are simultaneously reduced by ammonia.10,11 Finally, catalysts like CuO-TiO2 and MnOx-CuOTiO212 have been proposed for the selective catalytic oxidation (SCO) of NH3 to N2, following the reaction
2NH3 + 3/2O2 a N2 + 3H2O
(2)
This reaction has been proposed recently as an industrial process for the abatement of slipped ammonia after the SCR reactors.12,13 In previous papers, the use of CuO-based catalysts obtained by controlled calcination of Cu/Mg/Al hydrotalcite-type (HT) precursors in the SCR of NO by ammonia were investigated, providing evidence of a specific behavior of these catalysts, also due to their base properties.14 Furthermore, it was also reported that HT anionic clays (8) Bosch, H.; Janssen, F. Catal. Today 1988, 2, 369. (9) Wood, S. C. Chem. Eng. Prog. 1994, 90 (1), 32. (10) Pollack, S. S.; Chisholm, W. P.; Obermyer, R. T.; Hedges, S. W.; Ramanathan. M.; Montano, P. A. Ind. Eng. Chem. Res. 1988, 27, 2276. (11) Centi, G.; Passarini, N.; Perathoner, S.; Riva, A. In Environmental Catalysis; Armor, J. N., Ed.; American Chemistr Society: Washington, DC, 1994; p 233. (12) Wollner,A.; Lange, F.; Schmelz, H.; Kno¨zinger, H. Appl. Catal. 1993, A94, 181. (13) deBoer, M.; van Dillen,; Koningsberger, D. C.; Janssen, F. J. J. G.; Koerts, T.; Geus, J. W. In New Developments in Selective Oxidation by Heterogeneous Catalysis; Centi, G., Trifiro`, F., Eds.; Elsevier: Amsterdam, 1992; p 133.
© 1997 American Chemical Society
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Table 1. Composition and Physical Characterization of the Hydrotalcite-Type Precipitates after Overnight Drying at 363 K and Calcination for 14 h at 923 K S.a (m2g-1)a
XRDb
crystallographic parameters (Å)
sample
Cu:Mg:Al (atom ratio %)
CuO (wt %)
363 K
923 K
363 K
923 K
ac
cc
ad
CAT 1 CAT 2 CAT 3 CAT 4
0.0:71.0:29.0 2.2:68.8:29.0 4.6:66.4:29.0 7.2:63.8:29.0
0.0 4.0 8.0 12.5
96 74 69 75
191 139 106 112
HT HT HT HT
oxide oxide oxide oxide
0.3035(2) 0.3064(6) 0.3062(6) 0.3069(8)
2.272(2) 2.310(7) 2.298(6) 2.313(9)
0.4179(8) 0.4178(8) 0.422(4) 0.423(4)
a S.a. ) surface area; b XRD ) phases identified by XRD analysis. HT ) hydrotalcite-type phase; oxide ) mixed oxide phase. c Referred to the HT phase. d Referred to the oxide phase.
may be useful precursors of high surface area supports to prepare transition metal supported catalysts for the same reaction.15 Moreover these catalytic materials appear to be promising in relation to their low cost and environmental friendless. On the other hand, the mechanism of the reduction of NO by ammonia over CuO-TiO27,16 and by propane over Cu-ZSM-517 have been investigated by Fourier transform infrared (FT-IR) spectroscopy. We report here a study of the adsorption and oxidation of ammonia on catalysts produced by calcination of Cu/Mg/ Al HT precursors, with the aim to shed light on the reaction mechanism over these catalysts.
used to calibrate the mass spectrometer and check periodically the response of the instrument. Fluctuations in the operating pressure of the mass spectrometer were compensated for by using helium as an internal standard. The mass data were transferred to a computer which elaborates the data before correcting the mass intensities to take into account the possible presence of multiple fragmentations. Successively, the mass intensities were converted to concentration and conversion. Catalytic tests refer to steady-state activity of the samples: after attainment of this condition, the activity behavior was monitored checking that the catalytic data did not change by more than the experimental error (ca. 5%). The gas feedstock was as follows: NH3 ) 5000 ppm, O2 ) 17 500 ppm, remainder He (total flow 6 dm3/h, gas hourly space velocity ) 10000-12000 h-1).
2. Experimental Section The Mg71.0Al29.0 and CuxMg(71.0-x)Al29.0 (atomic ratio) HT precursors (Table 1) were prepared by coprecipitation at pH 10.0 upon pooring a solution containing the nitrates of the elements in to a Na2CO3 solution under energetic stirring. The precipitates were kept in suspension at 333 K for 30 min under stirring and then filtered and washed with distilled water until a Na2O content lower than 0.02 wt % was obtained. The composition of the dried precipitates was confirmed by chemical analysis. The precipitates were dried overnight at 363 K and calcined for 14 h at 923 K. X-ray diffraction (XRD) powder analysis was carried out using a Philips PW1050/81 diffractometer and Ni-filtered Cu KR radiation (λ ) 0.15418 nm) (40 kV, 40 mA). A 2θ range from 10° to 80° was investigated at a scanning speed of 60° h-1. The data were processed on an Olivetti M240 computer. The lattice constants were determined by least-squares refinements, from the well-defined position of the five most intense peaks. The crystallographic parameters of the HT phases were calculated for a hexagonal cell on the basis of a rhombohedral R3m space group. The surface areas of the samples were determined using a Carlo Erba Sorpty Model 1750, by means of N2 adsorption. FT-IR spectra were performed with a Nicolet 5ZDX Fourier transform spectrometer (4 cm-1 resolution) using self-supporting pressed disks of the pure catalyst powders, previously pretreated by calcination in the IR cell at 673 K for 2 h and outgassing at 673 K for 30 min. Ammonia and NO were taken from commercial cylinders from SIAD (Milano, Italy). The catalytic tests were carried out using 0.40 g of catalyst (600-850 µm particle size) in a quartz flow reactor operating at atmospheric pressure in the 373-773 K range. A preliminary test verified the absence of inter- and intraparticle diffusional limitations on the reaction rate under the experimental conditions. Due to low concentrations of reactants, temperature gradients within the pellets may be considered to be negligible. The absence of an axial temperature profile was verified using a thermocouple sliding inside the catalytic bed. The inlet and outlet gases were continuously monitored by an on-line VG-SX200 quadrupole mass spectrometer. All the lines from the reactor to the mass detector were heated at 423 K in order to avoid adsorption or condensation phenomena. The feed was prepared starting from calibrated mixtures of the single reagents diluted in helium (SIAD Milano, Italy). The same mixtures were also (14) Montanari,B.; Vaccari, A.; Kassner, P.; Papp, H.; Pasel, J.; Dziembaj, R.; Makowski, W. Appl. Catal., B. in press. (15) Montanari,B.; Gazzano, M.; Vaccari, A. In Actas XV Simp. Iberoamericano de Catalisis; Herrero, E., Ammunziata, O., Peret, C., Eds.; Univ. Nacional de Cordoba: Argentina, 1996; Vol. 3, p 1967. (16) Ramis, G.; Yi, Li; Busca, G. Catal. Today 1996, 28, 373. (17) Hadjiivanov, K.; Klissurski, D.; Ramis, G.; Busca, G. Appl. Catal., B 1996, 7, 251.
3. Results 3.1. Characterization of the Samples. XRD powder patterns of the precipitates dried overnight at 363 K show for all samples the presence of only well-crystallized HT phases,18 according to the nature and amounts of the ions present.19-21 On the basis of the crystallographic parameters a and c of the dried precipitates (Table 1), the Mg/Al sample (CAT 1) exhibits the best packing of the ions in the HT structure, while a very low substitution of Mg2+ ions with Cu2+ ions causes a significant increase of the cell parameters. This can be taken as evidence of an increase of disorder in the structure.20-21 Furthermore, the Cu/Mg/Al HT precipitates (CAT 2-4) show surface area lower than that of the Mg/Al sample. The XRD powder patterns of the samples calcined at 923 K show only the presence of a poorly crystallized oxide phase, in agreement with formation of a rock-salt-type mixed oxides, without any evidence of segregation of CuO.18 As reported in a previous paper,14 the UV-visiblenear-IR diffuse reflectance (DR) spectra show for CAT 2 and 3 the presence of Cu2+ ions mainly in octahedral coordination, with negligible amounts of Cu2+ ions in tetrahedral coordination. On the other hand, the DR spectrum of the sample with higher copper content (CAT 4) shows the additional formation of a small amount of copper oxides clusters, the amount of which increases with increasing the calcination temperature and time, with a corresponding decrease in the absorption bands associated with the presence of octahedrally coordinated Cu2+ ions. An increase of the surface area of about 70% may be observed for all samples calcined at 923 K; the extent of this increase suggests that evolving steam and CO2 escape through holes in the crystal surface without any extensive change in crystal morphology,22 with higher values for the magnesium rich samples. (18) Powder Diffraction File - Inorganic Phases, International Centre for Diffraction Data, Swartmore PA, 1991. (19) Reichle, W. T. CHEMTECH 1986, 16, 58. (20) Cavani, F.; Trifiro`, F.; Vaccari, A. Catal. Today 1992, 11, 174. (21) Trifiro`, F.; Vaccari, A. In Comprehensive Supramolecular Chemistry; Atwood, J. L., MacNicol, D. D., Davies, J. E. D., Vo¨gtle, F., Eds.; Vol. 7, Pergamon: Oxford, 1996; Vol. 7, p 254. (22) Reichle, W. T.; Kang, S. Y.; Everhardt, D. S. J. Catal. 1971, 101, 617.
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Figure 1. FT-IR spectra of the surface species arising from NH3 adsorption on the Mg/Al mixed oxides CAT 1, CAT 2, and CAT 3 at r.t. (a) and successive outgassing at 300 K (b), 373 K (c), 473 K (d), and 573 K (e).
3.2. Adsorption of Ammonia. The FT-IR spectra resulting from ammonia adsorption on the Mg/Al mixed oxide (CAT 1) are reported in Figure 1. The bands observed at 1614 and 1162 cm-1, after outgassing at 300
K, are assigned to the asymmetric and symmetric deformation modes (δas(NH3) and δsym(NH3), respectively) of ammonia coordinated on Lewis acid sites. No traces of ammonium ions arising from ammonia protonation by
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Langmuir, Vol. 13, No. 17, 1997 4631
Figure 2. FT-IR spectra of the surface species arising from NH3 adsorption on the Cu/Mg/Al mixed oxide CAT 4 (CuO ) 12.5 wt %) at room temperature (a) and successive outgassing at 300 K (b), 323 K (c), 373 K (d), 473 K (e), and 573 K (f).
Brønsted acid sites are found, as indeed expected. As is well-known,23,24 the position of the δsym(NH3) mode of coordinated ammonia is very sensitive to the strength of the adsorbing Lewis acid site, shifting from near 1050 cm-1 (value for liquid ammonia) toward higher frequencies. The value measured here is almost coincident with that observed on MgO25,26 and MgFe2O47 while it is definitely shifted downward with respect the value observed for ammonia coordinated on the Lewis sites of alumina (12951220 cm-1 27,28). It must be pointed out that the Mg/Al mixed oxide has a higher Mg content in comparison to the stoichiometric MgAl2O4 spinel. The δsym(NH3) value thus indicates that the Lewis sites are very weak and can be identified as coordinatively unsaturated Mg2+ cations. Outgassing at 373 K already causes the complete desorption of ammonia, without the formation of any new adsorbed species. The negative band appearing in the spectra near 1000 cm-1 is due to the perturbation of the surface metal-oxygen modes by adsorbed ammonia, like previously found for ammonia on TiO2.29 The adsorption of ammonia on the Cu-containing samples (CAT 2 and CAT 3) gives rise to similar spectra (Figure 1), with the bands of the species coordinated on Lewis sites. The trend of the band intensities suggests that the amount of coordinated ammonia resisting to (23) Tsyganenko,A. A.; Pozdnyakov, D. V.; Filimonov, V. N. J. Mol. Struct. 1975, 29, 299. (24) Nakamoto,K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed.; Wiley: New York, 1986. (25) Kagami, S.; Onishi, T.; Tamaru, K. J. Chem. Soc., Faraday Trans. 1 1983, 79, 29. (26) Martra, G.; Borello, E.; Giamello, E.; Coluccia, S. In Acid-Base Catalysis II; Hattori, H., Misono, M., Ono, Y., Eds.; Elsevier: Amsterdam, 1994; p 169. (27) Kno¨zinger, H. Advan. Catal. 1976, 25, 184. (28) Kapteijn, F., Singoredjo, L.; van Driel, M.; Andreini, A.; Moulijn, J. A.; Ramis, G.; Busca, G. J. Catal. 1994, 150, 105. (29) Hadjiivanov, K.; Busca, G. J. Chem. Soc., Faraday Trans. 1 1991, 87, 175.
outgassing at room temperature (r.t.) and 373 K increases by increasing the Cu content. Moreover, the position and shape of the δsym(NH3) modes are also modified: a component increases in intensity at the higher frequency side of the main band centered, in all cases, near 11601165 cm-1. This new component, observed near 1225 cm-1, could be assigned to species coordinated either on Al3+ or, more likely, to Cun+ (n ) 2 or 1) sites. The δas(NH3) modes are also observed to shift slightly up to 1615-1620 cm-1. The picture is definitely modified for the sample with the highest copper content (CAT 4) (Figure 2); in this case the features of coordinated ammonia at r.t. are superimposed to those of other species, responsible for sharp bands at 1605, 1575 (very weak), 1488 (weak), 1446, 1221, 1150, 1070, and 1041 cm-1. These bands are formed after contact at r.t., become predominant at slightly higher temperatures and are still present at 373 K. In the region 4000-2000 cm-1 we observed weak bands that can be assigned to N-H stretchings and a pronounced band near 2220 cm-1 (Figure 3). The spectrum observed here is comparable to that we observed on CuO-TiO2 catalysts7 and on other transition metal oxide supports on TiO2.30 In those cases it was evident that the bands in the range 2300-2000 cm-1 and near 1490 and 1445 cm-1 behave differently with each other and with respect to all others in the spectra. Moreover, in those cases the band near 1490 cm-1 has a stronger rise in relative intensity than that in the present case. Moreover, bands in the region 2300-2000 cm-1 also frequently appear in these conditions. On the basis of the considerations reported previously7,30 as well as on the spectra of liquid, coordinated and adsorbed hydrazine (Table 2) and of hydrazine intercalated in oxide matrices,31 (30) Sanchez Escribano, V.; Gallardo Amores, J. M.; Ramis, G.; Busca, G. Appl. Catal. B: Environmental, in press.
4632 Langmuir, Vol. 13, No. 17, 1997
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Figure 3. FT-IR spectra (4000-2000 cm-1 region) of the surface species arising from NH3 adsorption on the Cu/Mg/Al mixed oxide CAT 4 (CuO ) 12.5 wt %) at room temperature (a) and successive outgassing at 300 K (b), 373 K (c), 473 K (d), and 573 K (e); (f) sample activate at 873 K for 1 h. Table 2. Wavenumbers (cm-1) of the IR Absorption Band Spectra of Hydrazine Species CAT 4 NH3
CuTiO2 NH3 or N2H4
3350
1221 1150 1070 1041 this work
γ-Fe2O3 NH3 or N2H4
3375 3272 3157 3078 2971
3100 2950 1605 1570
TiO2 N2H4
Fe-MgO N2H4
ZnCl2 × 2N2H4 complex
N2H4 liq
adsorbant adsorbate
3270 3200
1611 1560 1350 1280 1180
1606
1612
1225 1186
1221 1185
ref 7
ref 30
ref 7
1605 1580 1205 1165
1610 1570 1345 1310 1170 1150
1608 1608 1324 1283 1098 1042
NH2 scissoring NH2 scissoring NH2 wagging NH2 wagging N-N stretching NH2 rocking
ref 33
ref 34
refs 34, 35
refs 34, 35
1360
the bands at 1605, 1575, 1221, 1150, 1070, and 1041 cm-1 can be assigned to adsorbed hydrazine. Hydrazine is a product of ammonia oxidation. The industrial production of hydrazine implies the oxidation of ammonia with reagents such as NaClO through the intermediary of either NH2Cl (Rasching process) or dimethyloxazirane with the assistance of acetone (Bayer process).32 In the present case the formation of hydrazine is thought to occur via the previous formation and (31) Cantero, M.; Martinez-Lara, M.; Bruque, S.; Moreno-Real, L. Solid State Ionics 1993, 63-65, 500. (32) Bu¨chner, W.; Schliebs, R.; Winter, G.; Bu¨che, K. H. Industrial Inorganic Chemistry; VCH: Berlin, 1989. (33) Brill, R.; Jiru, P.; Schulz, G. Z. Phys. Chem. (Munich) 1969, 64, 216. (34) Sathyanarayana,D. N.; Nicholls, D. Spectrochim. Acta 1978, A34, 263. (35) Durig, J. R.; Bush, S. F.; Mercer, E. E. J. Chem. Phys. 1966, 44, 4238.
dimerization of amide NH2 species, likely bonded to copper centers:
NH3 f NH2 + H+ + e-
(3)
2NH2 f N2H4
(4)
The bands found at 1488 and 1445 cm-1, frequently present upon ammonia oxidation over transition metal oxide surfaces7,30 have been assigned tentatively to ν(NO) stretching of HNO (nitroxyl) and to the deformation of NH (imido) fragments, respectively. A reasonable assignment for the band at 2220 cm-1 is, according to Martra et al.26 and Garrone et al.,36 to dinitrogen anion N2- surface complexes. In any case, the formation of these bands (36) Garrone, E.; Coluccia, S.; Giamello, E. Spectrochim. Acta 1987, A43, 1567.
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Figure 4. FT-IR spectra of the surface species arising from NO adsorption at room temperature and successive outgassing at 300 K on Mg/Al or Cu/Mg/Al mixed oxides: (a) CAT 1 (CuO ) 0.0 wt %); (b) CAT 2 (CuO ) 4.0 wt %); (c) CAT 3 (CuO ) 8 wt %).
(whose assignments are more or less tentative) is a certain indication of an oxidation process for ammonia occurring at the surface when copper oxide species are present in a sufficient concentration. The data reported here for the Cu/Mg/Al catalyst (CAT 4) are in good agreement with those reported previously for Cu-TiO2 and show that copper centers are very active in oxidizing ammonia, in agreement with the excellent catalytic activity in ammonia oxidation by oxygen and by NO. On the other hand, it is well-known that hydrazine, found here as a product of ammonia oxidation, can be easily further oxidized to nitrogen by high valency metal cations.37 3.3. Adsorption of NO. The adsorption of NO on the Mg/Al mixed oxide CAT 1 at r.t. gives rise to the complex spectrum reported in Figure 4. NO is a diatomic molecule giving rise in the IR spectrum to a single band at 1875 cm-1 in the gas. It is evident that the complex spectrum obtained is due to different transformation products of NO. No molecularly adsorbed NO can be found in these conditions, like on MgO at r.t.38 The spectrum observed in the region 1700-1000 cm-1 is very similar to that previously obtained by NO adsorption on γ-Al2O3;28 however NO adsorption on MgO gives rise to bands in the same regions.38 These bands are due to NxOy species formed by mutual reaction between NO molecules. A reasonable assignment for the band observed at 1637 cm-1, disappearing after outgassing at 300-373 K, is to the asymmetric ONO stretching of adsorbed NO2 (1617 cm-1 for the gas39). It is in fact known that the electron withdrawal from the NO2 molecule, due to coordination on Lewis sites, causes an increase of the νas(NO2) frequency.40 This assignment is supported by the thermal (37) Stedman, G. Adv. Inorg. Chem. Radiochem. 1979, 22, 113. (38) Cerruti, L.; Modone, E.; Guglielminotti, E.; Borello, E. J. Chem. Soc., Faraday Trans. 1 1974, 70, 729. (39) Laane, J.; Ohlsen, J. R. Prog. Inorg. Chem. 1980, 27, 465. (40) Pozdnyakov, D. V.; Filimonov, V. N. Kinet. Katal. 1973, 14, 760.
lability of this adsorbed species, which desorbs completely by outgassing at 373 K. The oxidation of NO to NO2 on a nonoxidizing surface implies that other NO molecules are reduced, resulting in a disproportionation. Accordingly, the strongest band at 1220 cm-1 and its shoulder near 1050 cm-1 could be assigned to the N-N stretching and to the symmetric N-O stretchings, respectively, of a cis-hyponitrite anion, cis-N2O22- (1304 and 1057 cm-1 for K2N2O2, respectively39). Similar species have been supposed to be formed also on MgO.38,41 Thus, the occurrence of the following disproportionation, as a main reaction at the surface of the Mg/Al mixed oxide CAT 1, can be hypothesized:
3NO + O2- f NO2 + cis-N2O22-
(5)
The species identified as hyponitrites disappears only partially upon outgassing at 473 K, probably giving rise to the following reaction:
cis-N2O22- f N2O + O2-
(6)
The sum of the last two reactions gives rise to the overall reaction
3NO f NO2 + N2O
(7)
which is thermodynamically favored and could be catalyzed by the basic sites at the surface of the Mg/Al mixed oxide catalyst (CAT 1). The broader bands centered near 1420 and 1330 cm-1 could be associated to the split components of the asymmetric NO stretching of bidentate nitrate ions24,39 (41) Escalona Platero, E.; Spoto, G.; Zecchina, A. J. Chem. Soc., Faraday Trans. 1 1985, 81, 1283.
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Figure 5. FT-IR spectra of the surface species arising from NO adsorption over the Cu/Mg/Al mixed oxide CAT 4 (CuO ) 12.5 wt %) at room temperature (a) and successive outgassing at 300 K (b), 373 K (c), 473 K (d), and 573 K (e).
arising from NO and/or NO2 through a similar disproportionation reaction. The spectrum of these species is nearly unchanged by the addition of increasing amounts of copper oxide up to 8% (Figure 4). On this sample, however, a band at 1885 cm-1 appears, weak, and disappears by outgassing at r.t. This band can be assigned to NO species linearly coordinated on weakly electron withdrawing centers, likely Cu2+. This assignment is in agreement with those given previously for NO adsorbed on CuO-TiO2 catalysts,7 on Cu-ZSM-5 catalysts,42 on overexchanged Cu-ZSM-5,30 and on bulk CuO.43 According to these papers, the spectra of Cu2+-NO species are reported to give a single band in the 1895-1865 cm-1 region in all cases and seems to shift up by increasing the acidity of the matrix. For the sample CAT 4 (Figure 5) the band now assigned to nitrosyl species is much stronger at 1887 cm-1 and disappears again by outgassing at r.t., while broad bands at 1480 and 1310 cm-1 dominate this spectrum. These bands are stable (or even increased in intensity) upon outgassing at 373 K, although shifting at 1518 and 1285 cm-1, and only partially disappear at 473 K. These couples of bands, together with a third weaker component found at 1030-1050 cm-1, are typical of two different forms of bidentate and/or bridging nitrate ions. They are due to the NO asymmetric stretching (with a splitting for asymmetrical nitrates, due to loss of degeneracy, of the strong single band near 1400 cm-1 of symmetrical NO3ions) and to the symmetric NO stretching (weak near 1050 cm-1, IR inactive for symmetric nitrate ions, but weakly activated and poorly shifted for asymmetric nitrates).24,39 The main band of hyponitrites and that of NO2 are also evident near 1220 and 1633 cm-1. These data indicate (42) Spoto, G.; Bordiga, S.; Scarano, D.; Zecchina, A. Catal. Lett. 1992, 13, 39. (43) Davydov, A. A.; Budneva, A. A. React. Kinet. Catal. Lett. 1984, 25, 121.
that the above disproportionation reactivity is still present on the sample with the highest copper content (CAT 4), suggesting that part of the Mg/Al oxide is still uncovered by copper. Additionally, the coordination of NO and its oxidation to nitrates seem to represent specific reactivities of supported copper oxide on this catalyst:
NO + 2O2- f NO3- + 3e-
(8)
3.4. Interaction of NO with Adsorbed NH3. The comparison of the above spectra shows that the strongest bands of the adsorbed species arising from NH3 and NO over our catalysts are superimposed. This makes it difficult the study of the cointeraction of these adsorbates. When NO gas is put into contact with ammonia-covered surfaces of the samples with copper oxide content up to 8% (CAT 2 and 3), the bands assigned above to NO2 and cis-N2O22- are evident. However, they can mask the bands of adsorbed ammonia, so that it is not possible to determine whether the reaction occurs between the adsorbates. From these results, however, we can suppose that either the site on which the SCR reaction occurred with ammonia oxidation to water and nitrogen or the adsorption sites allowing the formation of the disproportionation of NO are different from those where NH3 adsorbs. If the last alternative is true, a difference is found with respect to the picture found on vanadia-titania-based catalysts, where NO is adsorbed essentially in the form of nitrosyls and these sites are poisoned by ammonia.44,45 On the other hand, reaction 5 certainly occurs at basic oxide anions, which are not sites for strong ammonia adsorption. (44) Ramis, G.; Busca, G.; Bregani, F.; Forzatti, P. Appl. Catal. 1990, 64, 259. (45) Ramis, G.; Busca, G.; Bregani, F.; Forzatti, P. In Catalytic Science and Technology; Yoshida, S., Takezawa, N., Ono, T. Eds.; Kondasha: Tokyo, 1991; Vol. 1, p 189.
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Figure 6. FT-IR spectra of the surface species arising from interaction of NO with absorbed NH3 on the Cu/Mg/Al mixed oxide CAT 4 (CuO ) 12.5 wt %) at different temperatures: 300 K (a), 373 K (b), 473 K (c), and 573 K (d).
A quite different picture is observed for CAT 4 (CuO 12.5 wt %) (Figure 6): in this case the presence of adsorbed ammonia modifies significantly the spectrum of the species arising from NO adsorption. In particular, the sharp band at 1887 cm-1, assigned to Cu2+ nitrosyls, is no longer formed. After contact at r.t. the spectrum observed does not show the bands assigned to nitrate species, strong when NO is put into contact with the ammonia-free surface. The spectrum of NO in contact with NH3-covered catalyst can be interpreted as due to the superimposition of the absorptions of coordinated ammonia and (NO2 + N2O22-). This suggests that ammonia poisons both the sites for NO coordination, like on vanadia-titania based catalysts44,45 and CuO-TiO2,7 and the sites for nitrates formation. So, it seems reasonable to conclude that NO coordination on Cu2+ is a previous step for NO oxidation to nitrates on this catalyst. However, if contact is carried out at 373 K, a sharp weak band at 1890 cm-1 and strong bands at 1480, 1305, and 1030 cm-1 appear, showing that part of ammonia is either desorbed or destroyed and some of the sites for nitrosyl and nitrate formation are made free. Moreover, the band at 1221 cm-1 disappeared. This could indicate that hyponitrites and coordinated ammonia react together. However, this is unlikely, because both species are reducing agents. A weaker band is still present near 1630 cm-1 and it can be due, in this case, to the superimposition of the bands of adsorbed NO2 and water. According to the above data, it seems reasonable to conclude that the disproportionation giving rise to species identified as hyponitrites occurs on the Mg/Al oxide surface while the coordination of NO to nitrosyls and the oxidation to nitrates occurs on CuO. Moreover it seems certain that ammonia interacts with the latter phenomenon, not with the former. This strongly suggests that a reaction occurred between adsorbed ammonia and NO on the CuO particles near 373 K, giving rise to the disappearance of ammonia
and restoring the reactivity of ammonia-free CuO surface. So, the disappearance of hyponitrites on the free Mg/Al mixed oxide surface should be due to their successive reaction with water or with nitrates formed over copper oxide particles. 3.5. Catalytic Oxidation of Ammonia. The catalytic activity of the Cu/Mg/Al in the SCR of NO by NH3 has been reported in a previous paper,14 providing evidence of very different behavior as a function of the copper content and, mainly, of the presence of O2 excess in the feedstock. In the catalytic tests performed feeding a O2free gas mixture (NO ) 10 500 ppm, NH3 ) 10 000 ppm in He) CAT 1 was almost inactive while the samples CAT 2-4 showed very similar behavior (Figure 7, for CAT 3), regardless of the amount of copper present, with a significant increase in NO conversion and N2 production in a narrow range of temperature (573-323 K), with the formation of small amounts of N2O as a byproduct in the 543-643 K range. On the contrary, the presence of O2 excess in the reaction gas mixture discriminates dramatically between the behavior of the different coppercontaining catalysts,14 with the appearance of a high NO conversion in the low-temperature range and its decrease at higher temperature due to the NH3 combustion reaction (Figure 7 for CAT 3). The decrease of the copper content and/or the increase the calcination time and temperature led to considerably poorer performances, and catalytic behavior similar to that of a CuO-supported catalyst was observed.14,15 In the presence of oxygen CAT 1 is active only above 673 K. Therefore, in order to shed light on the catalytic performances of the catalysts obtained by calcining Cu/ Mg/Al HT precursors and in particular on the role of base surface sites in the NH3 combustion reaction, the behavior of the Mg/Al mixed oxide CAT 1 and those of two Cu/ Mg/Al samples with different copper content (CAT 3 and CAT 4) was investigated feeding a mixture of ammonia
4636 Langmuir, Vol. 13, No. 17, 1997
Trombetta et al.
Figure 7. Catalytic behavior in the SCR of NO by NH3 of the Cu/Mg/Al mixed oxide CAT 3 (CuO ) 8.0 wt %): (9, b) NO conversion; (2, 0) amount of N2O formed. Reaction gas mixture: (A) NO ) 15 000 ppm, NH3 ) 10 000 ppm, He remainder (9,2); (B) NO ) 6000 ppm, NH3 ) 7500 ppm, O2 ) 30 000 ppm, He remainder (b,0).
and oxygen, but using the same amount of catalyst and GHSV of the above mentioned SCR tests. All samples show negligible activity up to 450 K (Figure 8) which rises to almost complete conversion near 723 K for Cucontaining catalysts. On the other hand NO and N2O production is definitively higher for Cu-free catalyst than for Cu-containing catalysts. The similar catalytic behavior for the Cu/Mg/Al catalysts in the NH3 oxidation, regardless of the copper content, may be justified by hypothesizing that on account of the high surface area values of the mixed oxides obtained by calcination at 923 K (Table 1), also for CAT 4, the amount of copper is not sufficient to cover the surface with a monolayer of CuO. Therefore, the presence of a Mg/Al nonstoichiometric spinel-type phase46,47 at the surface of these catalysts may be considered responsible for the NH3 oxidation, in agreement with that recently reported in the literature for other magnesium-rich spinel-type catalysts.48 On the other hand, the presence on the surface of Cu-containing sites favor the selective SCO of NH3. The worsening of the catalytic performances in SCR of NO observed for CAT 4 by increasing the calcination time or temperature14 supports this hypothesis, considering that segregation and sintering of CuO increased the amount of Mg/Al free surface. On the other hand, NO and N2O production is definitively higher on Mg/Al sample CAT 1 (Figure 8a) than on the Cu-containing samples CAT 3 and CAT 4 (Figure 8b,c). This last sample showed conversely higher selectively toward N2 (for example at 693 K NO conversion of ca. 96% and selectivity N2 ca.80%). The comparison of the IR and catalytic data reported above strongly suggests that the oxidation of ammonia (46) Vaccari, A.; Gazzano, M. In Preparation of Catalysts VI; Poncelet, G., Martens, J., Delmon, B., Jacobs, P. A., Grange, P., Eds.; Elsevier: Amsterdam, 1995; p 893. (47) Rebours,B.; d’Espinose de la Caillere, J. B.; Clause, O. J. Am. Chem. Soc. 1994, 116, 1707. (48) Busca,G.; Daturi, M.; Kotur, E.; Olivieri, G.; Wiley, R. J. In Preparation of Catalysts VI; Poncelet, G., Martens, J., Delmon, B., Jacobs, P. A., Grange, P., Eds.; Elsevier: Amsterdam, 1995; p 667.
Figure 8. Catalytic behavior in the SCO of NH3 of some Cu/ Mg/Al mixed oxide catalysts (a) CAT 1 (CuO ) 0.0 wt %), (b) CAT 3 (CuO ) 8.0 wt %), and (c) CAT 4 (CuO ) 12.5 wt %): (9) NH3 conversion; (b) O2 coversion; (2) amount of N2O formed; (0) amount of NO formed. Reaction gas mixture: NH3 ) 5000 ppm, O2 ) 17 500 ppm, He remainder.
Ammonia Adsorption and Oxidation
over Cu-containing HT catalysts occurs with a redox type Mars-van Krevelen mechanism, at the expense of oxidized copper centers, that are later reoxidized by dioxygen. In fact we observe the ammonia oxidation to hydrazine and other compounds (likely including N2-) in the absence of oxygen over the preoxidized surface. The SCO reaction is likely competitive with the SCR reduction over these catalysts and previously discussed for the CuO-TiO2 catalysts.7 The oxidation of ammonia mainly to NOx over the copper-free Mg/Al catalyst cannot be explained with a similar redox mechanism, because the catalyst is virtually irreducible. In this case a Langmuir-Hinshelwood type mechanism is likely, like that determined over other MgObased systems by Escalona Platero et al.49 involving superoxide ions and giving rise to nitrates. Accordingly the Mg/Al catalyst is quite inactive in the SCR reaction (conversion of NO raised 45% at 773 K versus 96% at 550 K for Cu-containing catalysts). 4. Conclusions From our IR spectroscopic study we can draw the following conclusions: 1. Cu-containing phases and/or well-dispersed CuO clusters provide the strongest adsorption sites for ammonia on Cu/Mg/Al mixed oxide catalysts. 2. The SCR activity over these materials does not involve Brønsted acidity, which is in fact absent from our surfaces. This confirms that Brønsted acidity is not a necessary requirement for SCR activity. 3. In the absence of NO, ammonia is oxidized to several different products. On the copper-free catalyst unselective oxidation of ammonia occurs, probably via a LangmuirHinshelwwod type mechanism. On Cu/Mg/Al mixed oxide (49) Escalona-Platero, E.; Coluccia, S.; Zecchina, A. J. Catal. 1987, 103, 270.
Langmuir, Vol. 13, No. 17, 1997 4637
catalysts selective oxidation to N2 predominates, likely via hydrazine and other NtN triple bond containing species, as surface intermediates. 4. IR experiments show that over Cu/Mg/Al mixed oxide catalysts the reactivity of ammonia to give hydrazine and other compounds is not found in the presence of NO gas. This is explained assuming that the earlier ammonia oxidation intermediate (likely amide NH2) reacts faster with NO giving rise to nitrogen. 5. The SCR reaction occurs between coordinated ammonia and gas-phase NO, so that the formation of a common intermediate in SCR and SCO reactions is very likely and can be identified as an adsorbed amide species. This occurs only over Cu/Mg/Al mixed oxide catalysts, providing evidence for the role of Cu-containing sites in both reactions. 6. The amount of copper oxide needed to have good SCR performances is higher in the case of Cu/Mg/Al mixed oxide catalysts than that required in Cu-containing zeolites. This is likely required due to the necessity of covering the entire Mg/Al mixed oxide surface that gives rise to nonselective ammonia oxidation, in relation to the its unusual acid-base properties.50-54 Acknowledgment. The financial support from the Ministero per l’Universita` e la Ricerca Scientifica e Tecnologica (MURST, Roma) is gratefully acknowledged. LA960673O (50) Rossi,P. F.; Busca, G.; Lorenzelli, V.; Waqif, M.; Saur, O.; Lavallay, J. C. Langmuir 1991, 7, 267. (51) McKenzie, A. L.; Fishel, C. T.; Davis, R. J. J. Catal. 1992, 138, 547. (52) Shen, J.; Kobe, J. M.; Chen, Y.; Dumesic, J. A. Langmuir 1994, 10, 3902. (53) Tichit, D.; Hassane Lhouty, M.; Guida, A.; Chice, B. H.; Figueras, F.; Auroux, A.; Bartalini, D.; Garrone, E. J. Catal. 1995, 151, 50. (54) Corma,A.; Forne´s, V.; Rey, F. J. Catal. 1994, 148, 205.
