FT-IR and TPD Investigation of the NOx Storage Properties of BaO

12732

J. Phys. Chem. B 2001, 105, 12732-12745

FT-IR and TPD Investigation of the NOx Storage Properties of BaO/Al2O3 and Pt-BaO/ Al2O3 Catalysts F. Prinetto,*,† G. Ghiotti,† I. Nova,*,‡ L. Lietti,‡ E. Tronconi,‡ and P. Forzatti‡ Dipartimento di Chimica IFM, UniVersita` di Torino, Via P. Giuria 7, 10125 Torino, Italy, and Dipartimento di Chimica Industriale e Ingegneria Chimica “Giulio Natta”, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy ReceiVed: July 13, 2001; In Final Form: August 26, 2001

The interaction of NO, NO2, or NO/O2 mixtures with Pt/Al2O3, Ba/Al2O3, and Pt-Ba/Al2O3 catalysts has been investigated by IR spectroscopy and temperature-programmed desorption. Upon NO interaction, small amounts of nitrites, nitrates, and hyponitrite species were formed on the Ba-containing samples. The NOx storage capacity of the catalysts was highly enhanced upon adsorption of NO/O2 mixtures and further upon NO2 admission. Upon adsorption of NO/O2 on Pt/Al2O3 sample nitrites, nitrates and NO2δ+ species were mainly formed, showing a moderate thermal stability. Barium markedly increased the amount and stability of the stored NOx species, which were bidentate and monodentate nitrites and, in minor amounts, nitrates. Nitrites were removed below 750 K and/or transformed into ionic Ba nitrates, stable up to 800-900 K. Upon NO2 adsorption, huge amounts of nitrates, but no nitrites, were formed on all the samples. Also in this case, Ba increased the amount and stability of the stored NOx species. The nature and the amounts of the stored NOx species formed upon adsorption of NO, NO/O2, or NO2 were similar on Ba/Al2O3 and Pt-Ba/Al2O3 catalysts, whereas Pt slightly decreased their thermal stability. Bulky Ba nitrate was formed during the thermal desorption of NO2 (and to a less extent of NO/O2), inducing an extensive decomposition of the Ba carbonate or oxycarbonate phase which was detected on the calcined samples.

Introduction The growing need to reduce the fuel consumption of vehicles and the related CO2 emissions has led the car manufacturers to develop engines working under “lean” conditions, i.e., in the presence of excess oxygen. As a matter of fact, it has been proven that a lean burn engine can decrease fuel consumption up to 30% compared with a stoichiometric engine.1 However, the presence of excess oxygen in the exhaust dramatically reduces the efficiency of the catalytic converters in the reduction of NOx. Accordingly, the development of catalytic systems able to reduce NOx in the presence of excess of oxygen has become a topical problem nowadays.2 Among the different solutions presented so far, the so-called “NOx storage-reduction” (NSR) catalysts present interesting features.3 These catalysts are used in engines which operate alternatively under lean and rich conditions: during lean operation, NOx in the exhaust gases is adsorbed onto the catalyst, while during rich operation, the stored NOx is converted to nitrogen by unburned hydrocarbons. The efficiency of the process is guaranteed if one can alternate sequences of long NOx adsorption periods with short regeneration stages. NSR catalysts are constituted by a NOx storage component (typically an alkaline or earth-alkaline metal oxide) and by a noble metal which reduces the stored NOx. Early formulations developed by Toyota included Ba as NOx storage compound and Pt as noble metal,4 but different formulations have also been patented5 in order to optimize both the NOx storage capacity and the NOx reduction properties. * Corresponding authors. F.P.: fax, +39 0116707855; e-mail, prinetto@ ch.unito.it. I.N.: fax, +39 02 70638173; e-mail, [email protected]. † Universita ` di Torino. ‡ Politecnico di Milano.

Recently, the NSR concept has been considered for stationary applications as well (small-medium electricity generating industry) under the trade name SCONOX.6 Like the NSR concept, an alumina-supported Pt-alkaline (or alkaline earth carbonate) catalyst has been proposed for the removal of NOx. In this case, the NOx removal is accomplished by adsorbing the NOx contained in the flue gases on a major fraction of the catalyst bed, while the other part of the catalyst is regenerated by an appropriate stream of reducing gas (hydrogen or hydrocarbons) in the absence of oxygen. Accordingly, the two catalyst fractions are isolated from each other by using sets of louvres and valves. Several studies have been recently devoted to the investigation of both the NOx adsorption and reduction steps over NSR catalysts.3,4,7-10 Early investigations by Takahashi et al.4 provided indications that NOx are stored on NSR catalysts in the form of nitrates after NO oxidation on precious metals and reaction with a neighboring NOx storage compound. They also concluded that the reduction of NOx to N2 during rich conditions takes place at noble metal sites. Similar conclusions have also been reported by Bogner et al.,8 who investigated the performances of NSR catalysts using both synthetic and real engine exhausts. Along similar lines, Fridell et al.9 investigated the influence of key process parameters on the performances of barium oxide-based NSR catalysts. They reported maximum NOx storage at about 650 K, slow increase in stored NOx with increasing oxygen concentration, little difference with respect to the reducing agent (C3H6, C3H8, CO, or H2), limited decrease in the presence of CO2, and IR evidence for the formation of nitrate species upon NOx storage. Along similar lines, Mahzoul et al.10 suggested that two kinds of sites operate, namely, Pt sites close to Ba crystallites, which are responsible for nitrate formation, and Pt sites far from Ba crystallites, which behave

10.1021/jp012702w CCC: $20.00 © 2001 American Chemical Society Published on Web 11/30/2001

Investigation of NOx storage properties as oxidation centers. However, further investigation is required in order to better elucidate the NOx storage mechanisms as well as the role of the noble metal in NOx storage. For this purpose, in the present paper, FT-IR spectroscopy and temperatureprogrammed desorption (TPD) measurements have been used as complementary techniques to gain information on the NOx storage capability of typical NSR catalysts and the characteristics of the stored NOx species. In particular, we have investigated the nature, relative amounts, and thermal stability of the species formed upon interaction of NO, NO2, and NO/O2 mixtures with Pt/Al2O3, BaO/Al2O3, and Pt-BaO/Al2O3 reference catalysts. Experimental Section Sample Preparation. The γ-alumina support was obtained by calcination at 973 K of a commercial alumina material (Versal 250 from La Roche Chemicals). Pt/Al2O3 (1/100 w/w) and Ba/Al2O3 (20/100 w/w) catalysts were prepared by incipient wetness impregnation of the alumina support with aqueous solutions of dinitrodiammine platinum (Stream Chemicals, 5% Pt in ammonium hydroxide) or barium acetate (Stream Chemicals, 98.5%). The powders were then dried overnight in air at 353 K and finally calcined at 773 K for 5 h. The Pt-Ba/Al2O3 (1/20/100 w/w) sample was prepared by impregnation of calcined Pt/Al2O3 with a solution of barium acetate. The powders were dried overnight at 353 K in air, followed by calcination in air at 773 K for 5 h. Characterization Techniques and Procedures. XRD diffraction analyses on the powder samples calcined at 773 K in atmospheric air were collected with a Philips PW 1050/70 vertical goniometer diffractometer using a Ni-filtered Cu KR1 radiation. XRD spectra were analyzed to estimate the mean crystal size of the detected phases by means of the Scherrer equation. Surface areas of the powder samples calcined at 773 K were determined by N2 adsorption at 77 K with the BET method using a Carlo Erba Sorptomatic 1900 series instrument. Pore size distribution measurements were obtained by N2 adsorptiondesorption at 77 K with the same apparatus used for surface area measurements and by the mercury penetration method using a Carlo Erba Porosimeter 2000 series instrument. The Pt dispersion over the Pt/Al2O3 and Pt-Ba/Al2O3 samples calcined at 773 K and subsequently reduced in H2 at 573 K was estimated from hydrogen chemisorption at 273 K using a TPD/R/O ThermoQuest Instrument. Absorption/transmission IR spectra were run at RT on a Perkin-Elmer FT-IR 1760-X spectrophotometer equipped with a Hg-Cd-Te cryodetector, working in the range of wavenumbers 7200-580 cm-1 at a resolution of 2 cm-1 (number of scans, ∼60). For IR analysis, powder samples were pelletized in selfsupporting disks (10-15 mg cm-2) and placed in a quartz IR cell allowing thermal treatments in vacuo or in controlled atmosphere. Pellets were activated by heating in vacuo at 773 K (1023 K in some cases) subsequently in dry oxygen at the same temperature and cooled in oxygen. IR spectra were recorded at room temperature (RT) before and after interaction with CO, NO, NO2, or freshly prepared NO/O2 1:4 mixtures and after subsequent evacuation at increasing temperature from RT to 773 K. CO and O2 gases (Ucar) were used without further purification. NO (Ucar) was freshly distilled before use; NO2 was prepared by contacting for 2-3 weeks freshly distilled NO with excess O2, followed by purification from O2 by freezing. In some experiments, 15NO and 18O2 (CIL) were used. A new catalyst sample was generally used in each experiment, except

J. Phys. Chem. B, Vol. 105, No. 51, 2001 12733 for devoted experiments where the adsorption and subsequent desorption of NO, NO/O2, and NO2 were performed repeatedly on the same sample. For the purpose of evaluating the relative amounts of surface species on the different catalysts, spectra were normalized to an average sample weight of 12 mg cm-2. Spectra of BaCO3, Ba(NO2)2, and Ba(NO3)2 reference compounds (Aldrich) in KBr were also collected. Temperature-programmed desorption experiments (TPD) were performed in a quartz tubular fixed-bed microreactor by using 120 mg of catalysts (100-150 µm). In a typical run, the sample was activated in-situ by heating in synthetic air to 773 K and cooled in synthetic air and then saturated at 313 K with NO (1000 ppm), NO (1000 ppm) + O2 (2% v/v), or NO2 (1000 ppm) in He. NO/He, NO2/He, and O2/He mixtures (Sapio) were used without further purification. After flushing He at 313 K for 30 min in order to remove physisorbed species, the catalyst was heated to 1173 K at 15 K min-1 under flowing He (60 cm3/min STP) while continuously monitoring the reaction products by on-line mass spectrometry and gas chromatography. A new sample was generally used in each TPD run, except for devoted TPD experiments where the adsorption/desorption of NO2 was performed repeatedly on the same portion of sample. Accordingly, in this case, the temperature ramp was limited to 873 K. Temperature-programmed decomposition experiments (TPDC) were also performed. A procedure similar to that employed for TPD experiments was used, but the saturation step was omitted in this case. Results and Discussion Characterization of the Samples. Structural and Morphological Characterization. The specific surface areas of the pure γ-Al2O3 support and of the Pt/Al2O3 sample were near 200 m2 g-1, while lower values were determined for the Ba-containing samples (133 and 160 m2 g-1 for Ba/Al2O3 and Pt-Ba/Al2O3, respectively). The contraction of surface area was accompanied by a slight reduction of the pore volume from 1.1 cm3/g for γ-Al2O3 and Pt/Al2O3 down to 0.82 and 0.63 cm3/g for Ba/ Al2O3 and Pt-Ba/Al2O3, respectively. The pore radii were in the range 90-110 Å. The XRD patterns of the pure γ-Al2O3 support and of the Pt/Al2O3 sample exhibited the reflections typical of microcrystalline γ-Al2O3 (JCPDS file n. 10-425), with a mean crystallite dimension (estimated by the Scherrer equation) of 70 Å. No Pt crystalline phases were detected in the Pt/Al2O3 and Pt-Ba/ Al2O3 samples. In the Ba/Al2O3 sample, the presence of a BaCO3 whiterite phase (JCPDS file n. 5-378) was observed in addition to γ-Al2O3, whereas in the fresh Pt-Ba/Al2O3 catalyst, both the monoclinic (JCPDS 78-2057) and orthorhombic (whiterite, JCPDS 5-378) polymorphic forms of BaCO3 were detected. The formation of the metastable monoclinic phase was likely induced by the fast and highly exothermic decomposition of Ba acetate catalyzed by Pt, as evidenced by DTA-TG experiments reported elsewhere.11 However, the Pt-Ba/Al2O3 sample aged in air for several months showed only the presence of orthorhombic BaCO3. A quantitative XRD analysis performed over the aged Ba/Al2O3 and Pt-Ba/Al2O3 samples indicated that in both cases nearly 30% of Ba is present as crystalline BaCO3, the other fraction (possibly Ba carbonate or Ba oxide) being present as amorphous or highly dispersed, XRD-undetectable phases. Hydrogen chemisorption measurements performed on the prereduced Pt/Al2O3 and Pt-Ba/Al2O3 samples evidenced a high Pt dispersion (>95 and 60%, respectively). The lower value

12734 J. Phys. Chem. B, Vol. 105, No. 51, 2001

Prinetto et al.

Figure 1. FT-IR absorbance spectra of Pt/Al2O3 (curve 1), Ba/Al2O3 (curve 2), and Pt-Ba/Al2O3 (curve 3) samples activated at 773 K. Inset: spectra in the OH stretching region.

found for the Ba-containing sample is likely related to a lower accessibility of Pt sites due to the coverage by the dispersed Ba phase. In fact, due to the catalyst preparation procedure herein adopted (impregnation of Pt followed by calcination and Ba deposition), it is expected that the spreading of Ba carbonate leads to coverage of the support and of the Pt component as well. FT-IR Characterization. FT-IR measurements showed that the activation at 773 K of the samples previously calcined in air caused extensive dehydration, dehydroxylation, and surface decarbonation of the catalysts. In particular, the only surface species present on the Pt/Al2O3 sample (Figure 1 curve 1) and pure alumina (not reported in the figure) activated at 773 K were small amounts of free hydroxyl species of various type (bands at 3776, 3755, 3730, and 3682 cm-1)12 and carbonates (bands at 1600-1300 cm-1). In the case of the Ba-containing samples, the activation treatment under vacuum induced a marked decrease in intensity (around 40%) of the bands assigned to Ba carbonate (1436 cm-1 with shoulders at 1550, 1400, and 1374 cm-1),13 which, however, still dominate the spectra of Ba/ Al2O3 and Pt-Ba/Al2O3 catalysts activated at 773 K (Figure 1 curves 2 and 3). This datum evidenced an extensive decomposition of BaCO3 during the catalyst activation, which occurs at temperatures well below those required for the decomposition of bulk BaCO3 (>1200 K).14,15 These findings suggest the presence of an alumina-supported Ba oxide or oxycarbonate phase on the activated samples. Note that from IR spectra it is not possible to distinguish the contributions of the various crystalline and highly dispersed carbonate phases evidenced by XRD. In the OH stretching region, only the components at 3730 and 3682 cm-1 were detected on the Ba-containing samples; furthermore, the overall integrated intensity of the hydroxyl bands was markedly decreased (around 60-70%) in comparison with that of the Pt/Al2O3 sample (see Inset of Figure 1). These findings are likely related to the coverage of the alumina support by the dispersed Ba phase.

Figure 2. CO interaction. FT-IR spectra after admission of CO (10 mbar) at RT (solid lines) and subsequent evacuation at RT (dotted lines) on samples activated at 773 K: Al2O3 (curve 1), Pt/Al2O3 (curve 2), and Pt-Ba/Al2O3 (curve 3). Curve 4: admission of CO on Pt-Ba/ Al2O3 sample previously submitted to NO2 adsorption and desorption at 773 K. Each spectrum is reported as difference from the spectrum before CO admission and translated along the y axis for the sake of clarity.

To analyze the nature and accessibility of the Pt sites after the addition of barium, the catalysts activated at 773 K were further characterized by adsorption of CO at RT, followed by IR spectroscopy (Figure 2). On pure alumina (curve 1), CO chemisorbed on Al3+ Lewis acid sites exhibiting a weak band at 2212 cm-1, reversible upon evacuation at RT.12 Upon CO admission on the Pt/Al2O3 sample (curve 2), in addition to the Al3+(CO) species, an intense band at 2115 cm-1, stable under vacuum up to 523 K, was observed and assigned to Ptδ+ linear carbonyls formed upon partial reduction of PtO by CO at RT. Accordingly, the formed CO2 was coordinated onto the alumina surface, giving “organic-like” carbonates (bands at 1850-1750

Investigation of NOx storage properties

J. Phys. Chem. B, Vol. 105, No. 51, 2001 12735

TABLE 1: NOx Species and Related IR Bands Formed upon Adsorption of NO, NO/O2 or NO2 at RT on Al2O3, Pt/Al2O3, Ba/Al2O3 and Pt-Ba/Al2O3 Samplesa band positions (cm-1) sample Al2O3 and Pt/Al2O3

Pt/Al2O3 Ba/Al2O3 and Pt-Ba/Al2O3

NOx species

NO

bidentate nitrites

traces

bridging bidentate nitrates

“

chelating bidentate nitrates

“

monodentate nitrates

n.f.

NO2δ+ Pt mononitrosyls hyponitrites

n.f. 1800 1375, 1310

ionic bidentate nitrites monodentate nitrites

1050-950 n.d. 1220-1180 n.f.

bidentate nitrates

1600 n.d.

ionic nitrates

1470 n.d.

a

NO/O2 1320 1230 1660-1590 1250-1240 1060-1000 1570 1290 1060-1000 1500-1420 1400-1300 1970 1860 n.d. or n.f. 1330-1320 1220-1180 1500-1400 1200-970 1620-1500 n.d. 1040-970 n.d.

NO2 n.f. 1660-1590 1250-1240 1060-1000 1570 1290 1060-1000 1500-1420 1400-1300 ∼ 2000 n.f. n.f. n.f. n.f. 1650-1540 1320-1200 (*) 1050-970 1460-1400, 1360-1300 (*) 1038

assignments νsym (NO2) νasym (NO2) ν(NdO) νasym (NO2) νsym (NO2) ν(NdO) νasym (NO2) νsym (NO2) νasym (NO2) νsym (NO2) νasym (NO2) ν(NO) ν(NN) νasym, νsym (NO2) νsym (NO2) νasym (NO2) ν(NdO) ν(N-O) ν(NdO) νasym (NO2) νsym (NO2) νasym (NO3) νsym (NO3)

n.d. ) band not detectable because superimposed on other bands. n.f. ) not formed. (*) ) partially overlapped.

and 1300-1100 cm-1) and hydrogen carbonates (bands at 1650-1600, 1500-1400, and 1235 cm-1) similar to those observed upon CO2 interaction on Al2O3.16,17 A further component at 2168 cm-1 was assigned to Pt2+ carbonyls.18 The spectrum of CO adsorbed on Pt-Ba/Al2O3 catalyst (Figure 2 curve 3), if compared to previous spectra, showed several interesting features. The band at 2212 cm-1, associated with Al3+(CO) species, was absent. This datum, associated with the strong diminution of the band assigned to Ptδ+ carbonyls, pointed out an extensive coverage of the alumina support and of the Pt sites by the Ba phase. This is in qualitative agreement with the decrease of the Pt accessibility evidenced by H2 chemisorption measurements on the reduced samples changing from Pt/Al2O3 to Pt-Ba/Al2O3. Notably, on Pt-Ba/Al2O3 catalyst, the band associated with Pt carbonyls exhibited a lower frequency (2080 cm-1) which is typical of Pt0(CO),19 thus indicating a higher reducibility of Pt oxide by CO. Furthermore, additional components at 2020 and 1970 cm-1 could be noted. These components are ascribed to CO linearly chemisorbed on negatively charged Pt sites and account for a strong interaction between Pt and the strongly basic oxygen anions of the Ba phase.20,21 This may suggest that the exposed Pt sites and the Ba component are in close contact, a key-requisite for obtaining active NSR catalysts.5 Finally, a comparison of spectra 2 and 3 of Figure 2 shows the different nature of carbonates formed upon CO adsorption on Pt/Al2O3 and Pt-Ba/Al2O3 catalysts, mainly of bidentate type on the latter (bands at 1650-1550, 1350-1250 cm-1),17 in accordance with the increased surface basicity induced by Ba. Interaction with NO. FT-IR Spectroscopy. IR spectra of NO adsorbed at RT on Al2O3, Pt/Al2O3, Ba/Al2O3, and Pt-Ba/Al2O3 samples activated at 773 K are displayed in Figure 3. Band positions and assignments are summarized in Table 1. Upon NO adsorption on pure alumina (Figure 3 curve 1) and on Pt/ Al2O3 sample (Figure 3 curve 2), negligible amounts of surface nitrites and nitrates (very weak bands in the range 1700-1200

Figure 3. NO interaction. FT-IR spectra after admission of NO (5 mbar) at RT on samples activated at 773 K: Al2O3 (curve 1), Pt/Al2O3 (curve 2), Pt-Ba/Al2O3 (curve 3), and Ba/Al2O3 (curve 4). Subsequent evacuation at 623 K (curve 5) on Ba/Al2O3 sample, taken as an example. Each spectrum is reported as difference from the spectrum before NO admission and translated along the y axis for the sake of clarity.

cm-1) species were formed. On the latter catalyst, small amounts of linear Ptδ+ mononitrosyls (1800 cm-1) were also detected, formed upon partial reduction of Pt2+ ions by NO at RT.22 All of these species were removed upon evacuation below 623 K. Different features were observed for the Ba-containing samples (Pt-Ba/Al2O3 and Ba/Al2O3 samples, Figure 3 curves 3 and 4, respectively), exhibiting a couple of sharp bands at 1375 and 1310 cm-1, a complex absorption at 1050-950 cm-1 with maximum at 1025 cm-1 and bands of minor intensity at 1600, 1470, and 1220-1180 cm-1. Moreover, the band around 1800 cm-1 associated with Ptδ+ mononitrosyls, observed in the case of Pt/Al2O3 sample, was not detected on Pt-Ba/Al2O3. Coming


Prinetto et al.

to the band assignment, made on the basis of the band correlation and of the literature,13,23,24 the weak components at 1220-1180 cm-1 can be ascribed to surface nitrite species and those at 1600 and 1470 cm-1 to nitrates, possibly of bidentate and ionic type, respectively. These species are expected to have further vibrational modes in the range 1300-950 cm-1, which, however, could not be distinguished due to the high number of overlapped bands in this region. The couple of bands at 1375 and 1310 cm-1 showed to be correlated with the bands around 1050-950 cm-1 (however, nitrates are expected to contribute in part to this absorption). The assignment of these bands is not straightforward and the literature is controversial. On one hand, Raman bands at 1323, 1235, and 807 cm-1 have been assigned to nitro groups N-bonded to Ba2+ ions; on the other hand, sets of bands at 1335, 1235, and 820 cm-1 and 1350, 1049, and 822 cm-1 have been ascribed to barium nitrites.25-27 However none of these proposals completely accounts for the spectral features here observed. In our opinion, alternative assignments are more likely, e.g., to hyponitrite species which have been proposed to form upon NO adsorption on basic oxides such as MgO.28,29 Along similar lines, we suggest that hyponitrite, nitrite, and nitrate species formed on the Ba-containing samples arise from NO interaction with strongly basic oxygen ions, according to reactions 1 and 2, followed by the dimerization of NO- (3):

2 NO(g) + O2-(s) f NO-(s) + NO2-(s)

(1)

4 NO(g) + 2 O2-(s) f 3 NO -(s) + NO3-(s)

(2)

2 NO -(s) f N2O22-(s)

(3)

The N2O22- anion can exist in cis and in trans configurations, exhibiting N-N stretching modes at 1314 and 1419 cm-1, respectively, and symmetric and antisymmetric N-O stretching modes at 1057 and 857 cm-1 (cis form) and 1120 and 1030 cm-1 (trans form).24 Strong evidence in favor of this assignment comes from the experiments carried out with 15NO (spectra not reported). Indeed, for a nitrito or nitro group, the red shifts of the bands at 1375 and 1310 cm-1 due to the isotopic substitution should be comparable to those calculated for the NO oscillator (∆ν ) -25 and -24 cm-1, respectively) or even smaller, considering ONO group vibrations. In contrast, ∆ν values of -38 and -33 cm-1, respectively, were observed, likely accounting for N-N bonds, such as those in hyponitrites. The presence of two ν(NN) modes can account for two hyponitrite species with different structure and/or coordination, exhibiting νasym and νsym (NO) modes strongly overlapped at 1050-950 cm-1. In agreement, the isotopic shift observed for the components at 1050-950 cm-1 (∆ν ) -19 cm-1) is that expected for N-O modes. Further evidence comes from the use of 1:1 14NO/15NO mixtures, although results are not completely satisfactory. Indeed, the curve-fitting of the spectra thus obtained gave indication of the presence of two components around 1360 and 1298 cm-1, i.e., with wavenumbers near those calculated for 14N-15N oscillators, in addition to the bands due to 14N-14N and 15N-15N oscillators. However, the intensity ratios of the six peaks were not those expected on the basis of the statistics. The thermal stability of the NOx species present on Ba/Al2O3 and Pt-Ba/Al2O3 sample was next examined. Hyponitrites were removed by evacuation at 423 K. Concurrently, a band around 1220 cm-1 developed, reaching the maximum intensity upon heating under vacuum at 623 K (Figure 3 curve 5 for Ba/Al2O3,

taken as an example). By comparison with the spectrum of pure Ba(NO2)2 compound, exhibiting bands at 1235, 1335 cm-1, this bands was ascribed to the νasym (ONO) mode of barium nitrites (νsym around 1330 cm-1, overlapped with nitrate modes) formed upon evolution of the adsorbed hyponitrites. The assignment of Ba nitrites is further supported by the high thermal stability of these species (up to 700 K) in comparison to nitrites coordinated on alumina (vide infra). The small amounts of nitrates present were removed at 700-850 K. TPD Experiments. The results obtained upon performing NO-TPD experiments over Pt/Al2O3, Ba/Al2O3, and Pt-Ba/ Al2O3 samples are reported in Figure 4, left side. In the case of the Pt/Al2O3 sample, negligible amounts of NO were desorbed near 600 K. This indicates the weak adsorption capability of Pt/Al2O3 toward NO, in agreement with the IR results previously reported. Small amounts of O2, not accompanied by NO evolution, were also observed above 850 K, i.e., at temperatures higher than that of the sample oxidative pretreatment (see Experimental). Such an oxygen evolution is possibly related to the desorption of O2 adsorbed during the oxidative pretreatment. The Ba-containing samples exhibited different features with respect to Pt/Al2O3, with respect on one hand to the higher overall amounts of NO released and on the other hand to the presence of NO desorption peaks at high temperature (up to 1000 K). In particular, three NO desorption peaks were evident over Ba/Al2O3, namely, a low-temperature peak centered around 400 K and two major peaks near 750 and 900 K, the latter accompanied by oxygen evolution. The high-temperature peaks showed maxima slightly shifted toward lower temperatures (∼50 K) in the case of Pt-Ba/Al2O3 as compared to the maxima of the Ba/Al2O3 catalyst. On the basis of the comparison with the Pt/Al2O3 sample and in agreement with the IR data previously reported, the NO evolution at low temperature can be ascribed to the removal of Ba hyponitrites, while the peaks in the range 670-900 K likely arise from the decomposition of Ba nitrites and nitrates. In particular, it is suggested that the evolution of NO around 750 K is associated with nitrite decomposition, whereas nitrate decomposition is responsible for the desorption of NO + O2 at T > 800 K. The decomposition of Ba nitrites and nitrates may occur according to the following stoichiometry:

Ba(NO2)2 f 2NO + 0.5O2 + BaO

(4)

3Ba(NO2)2 f 4NO + Ba(NO3)2 + 2BaO

(5)

Ba(NO3)2 f 2NO + 1.5O2 + BaO

(6)

Ba(NO2)2 f 2NO + BaO2

(7)

Ba(NO3)2 f 2NO + O2 + BaO2

(8)

Ba(NO3)2 f 2NO2 + 0.5O2 + BaO

(9)

Ba(NO3)2 f 2NO2 + BaO2

(10)

BaO2 f BaO + 0.5O2

(11)

Since oxygen is not evolved in correspondence of the peaks in the 550-750 K region (which have been associated with nitrite decomposition), it is speculated that nitrites are transformed into nitrates with release of NO, according to reaction 5, as described in the literature.10,15 Nitrates decompose thereafter at higher temperatures originating the NO + O2 peak at 800-900 K. As a matter of fact, the measured O2/NO ratio slightly exceeds the



Figure 4. TPD profiles (concentration versus temperature) following saturation at 313 K of Pt/Al2O3, Ba/Al2O3, and Pt-Ba/Al2O3 with He + NO (2000 ppm) (left side) or with He + NO (2000 ppm) + O2 (2% v/v) (right side).

stoichiometric ratio expected from nitrate decomposition according to reaction 6 (1 vs 0.75). Other routes for the decomposition of Ba nitrites and nitrates can occur, leading to the formation of BaO2 according to reactions 7 and 8. In particular, reaction 7 could justify the absence of oxygen evolution in the 550-750 K region, while BaO2 decomposition according to reaction 11 could account for the O2 excess released at high temperature. Indeed, peroxide ions in or on defective BaO have been observed to form by Raman during the decomposition of Ba(NO3)2. Moreover, crystalline BaO2 was detected if the decomposition was carried out in the presence of oxygen.26-27 The presence of BaO2 or of peroxide ions at the surface and subsurface layers of BaO could not be detected by IR. Finally, the absence of NO2 in the TPD profiles indicates that routes 9-10 are negligible and/or that NO2 decomposes to NO + O2 upon desorption. As seen, the presence of platinum did not influence significantly both the nature and the amount of the stored NOx species but slightly decreased their thermal stability. This effect could be ascribed to the formation of reduced Pt at the temperatures

required for the removal of NOx species, which would favor the decomposition of nitrites and nitrates being thereafter reoxidized. Interaction with NO + O2. FT-IR Spectroscopy. The nature and thermal stability of the NOx surface species formed after admission of freshly preformed NO/O2 mixtures were next examined. Analogous results were obtained by admitting in sequence NO and O2. The band assignments, not straightforward due to the great number of NOx species formed and the superposition of their vibrational modes, have been made on the basis of correlation of the band intensities, of the use 15NO/ O2 and NO/18O2 isotopic mixtures (spectra herein not reported) and of the literature.13,23,24 For the sake of clarity, the band positions and assignments are collected in Table 1. Upon admission of the NO/O2 mixture on pure alumina (Figure 5a) and on the Pt/Al2O3 sample (Figure 5b), surface bidentate nitrites (1230, 1320 cm-1) and nitrates of various type (several bands in the range 1700-1000 cm-1) were formed in increasing amounts on increasing the contact time. Nitrites reached the maximum intensity after 10 min of contact, while the amounts of nitrates continued to increase up to 30 min. The


Prinetto et al.

Figure 5. Adsorption of NO/O2 mixtures on Al2O3 (section a) and Pt/Al2O3 (section b) activated at 773 K. FT-IR spectra after admission at RT of 20 mbar of a freshly preformed NO/O2 1:4 mixture; spectra recorded after 5 min, (curves 1) and 30 min of contact (curves 2) and after the subsequent evacuation at 623 K (curve 3, only in section b). Each spectrum is reported as the difference from the spectrum before admission of NO/O2 and translated along the y axis for the sake of clarity.

main surface species observed after 30 min of contact with the NO/O2 mixture were bridging and chelating bidentate nitrates (Figure 5 curves 2). Monodentate nitrates, NO2δ+ species formed on Al3+ sites with a high acidic strength (broad band around 1970 cm-1), and Pt2+ mononitrosyls (1860 cm-1, only on Pt/ Al2O3) were detected in smaller amounts. The surface species described above showed a moderate resistance upon heating under vacuum; nitrates, and particularly the chelating ones, are the most stable species. After evacuation at 623 K (Figure 5 curve 3 on Pt/Al2O3 sample) the overall amount of NOx surface species was reduced to around 20% of those initially present, and they were completely removed at temperatures below 700 K. The possible routes leading to the formation of nitrites and nitrates on a metal oxide surface are reported below:

2NO + 0.5O2 + O2-(s) f 2NO2-(s)

(12)

2NO + 1.5O2 + O2-(s) f 2NO3-(s)

(13)

NO2-(s) + 0.5O2 f NO3-(s)

(14)

NO + 0.5O2 f NO2

(15)

2NO2 + 0.5O2 + O2-(s) f 2NO3-(s)

(16)

2NO2 + O2-(s) f NO3-(s) + NO2-(s)

(17)

These reactions involve the oxidation of NO by O2 (gaseous or activated on the catalyst surface) and the interaction with partially uncoordinated surface oxygen anions (mainly O2-). However, the formation of NO2 by gaseous oxygen according to reaction 15, although favored by thermodynamics, is slow so that reactions 12-14 are suggested to occur rather than reactions 16 and 17. Moreover, since nitrites are observed to form first, while nitrate formation occurs with lower rate, NO2 dismutation according to route 17 seems to be negligible. The nitrites/nitrates transformation has also been pointed out by Westerberg et al. over similar catalyst.30 Finally, we notice that experiments do not emphasize a significant influence of platinum on the nature and amounts of the surface NOx species. Ba-containing catalysts exhibit interesting differences with respect to the Al2O3 and Pt/Al2O3 samples upon adsorption of NO/O2 mixtures (Figure 6). A first point concerns the absence of NO2δ+ species and Pt nitrosyls (on Pt-Ba/Al2O3 catalyst), in line with the previously evidenced high coverage of the alumina support and of the Pt sites by the Ba phase. A second point concerns the different nature and thermal stability of NOx species formed. On Ba/Al2O3 (Figure 6a) and Pt-Ba/Al2O3 (Figure 6b) catalysts, a variety of bands in the 1700-950 cm-1 region, all of intensity increasing with the contact time, were formed and assigned to different types of NOx surface species. The presence of platinum did not influence significantly the relative and total amounts of the NOx species formed. The most abundant species after 30 min of contact with the NO/O2 mixture were bidentate nitrites (1330, 1220 cm-1) and monodentate nitrites (several overlapped components at 1500-1400 cm-1 and 1200-1000 cm-1). The complexity of the absorptions assigned to monodentate nitrites could reflect the heterogeneity of surface sites, but it could be also related to the presence of nitrite ions coordinated in different ways. Indeed at least five possible structures for surface NO2- ion are reported in the literature.23 Chelating and/or bridging bidentate nitrates were formed in minor amounts (Figure 6 a,b curves 2). The presence of monodentate and/or ionic nitrates, if any, could not be ascertained due to the strong superposition of their vibrational modes with those of nitrites in the 1500-1300 cm-1 region. The nitrite and nitrate formation upon NO/O2 adsorption can occur through the mechanisms previously proposed. However, whereas on Al2O3 and Pt/Al2O3 nitrates were mainly formed, on the Ba-containing catalysts nitrites are the prevailing species, accounting for the higher basicity of the oxygen sites involved in the Ba2+O2- pairs. Moreover, nitrites formed on Bacontaining catalysts show high thermal stability (up to 673 K, Figure 6c,d) as compared to those observed on Al2O3 and Pt/ Al2O3, suggesting that they are coordinated to Ba ions. In the temperature region corresponding to nitrite decomposition (473-673 K), the bands due to bidentate nitrates decreased in intensity, while a couple of bands at 1460-1400 and 13601300 cm-1 progressively developed, reaching the maximum intensity upon evacuation at 623 K (Figure 6c,d curves 4). The spectrum thus obtained was very similar to that of bulk Ba(NO3)2 (not reported), exhibiting bands at 1415 and 1350 cm-1 which correspond to the splitting of the degenerate νasym(ONO) mode due to the lowering of the simmetry upon coordination. Therefore, the bands developing upon NO/O2 desorption were



Figure 6. Adsorption of NO/O2 mixtures on Ba/Al2O3 (sections a and c) and Pt-Ba/Al2O3 (sections b and d) catalysts activated at 773 K. FT-IR spectra after admission at RT of 20 mbar of a freshly preformed NO/O2 1:4 mixture; spectra recorded after 5 min (curves 1) and 30 min of contact (curves 2) and after the subsequent evacuation at 473 K (curves 3), 623 K (curves 4), and 773 K (curves 5). For comparison, adsorption of pure NO (5 mbar) on Pt-Ba/Al2O3 catalyst is reported, (curve 0 in section b). Each spectrum is reported as difference from the spectrum before admission of NO/O2 or NO.

ascribed to ionic Ba nitrates formed at the expense of surface bidentate nitrates and possibly of a fraction of nitrites. Bidentate nitrates were totally removed by evacuation at 773 K (Figure 6c,d curves 5) and ionic Ba nitrates at T > 800 K. Notably, the temperature required for the complete elimination of nitrites and nitrates was slightly lower (∼50 K) for the Pt-Ba/Al2O3 sample in comparison to that for Ba/Al2O3. As mentioned above, a large fraction of ionic nitrates still remained after evacuation at 773 K, as evidenced by the persistence of the component at 13601300 cm-1 (Figure 6c,d curves 5). The component at 14601400 cm-1 could not be observed because masked by the appearance of a deep negative band centered at 1440 cm-1 in the difference spectra. This negative band arose from the strong superposition of the vibrational modes of ionic nitrates with those of carbonates which decrease during the thermal desorption treatments. This point will be addressed in the following. Temperature-Programmed Desorption Experiments. The results obtained after adsorption of the NO/O2 mixtures over Pt/ Al2O3, Ba/Al2O3, and Pt-Ba/Al2O3 samples are reported in Figure 4, right side. In line with IR data, the TPD profiles showed integrated intensities markedly higher than those obtained in the case of pure NO for all the examined catalysts. In the case of the Pt/Al2O3 sample, two major NO desorptions were observed, the first one in the range 400-600 K with maximum around 470 K and the second (of lower intensity) with maximum near 700 K. The high-temperature NO desorption peak was accompanied by oxygen evolution. On one hand, the low-temperature peak could be ascribed to the removal of nitrites, nitrates, and NO2δ+ surface species detected by IR spectroscopy. Unfortunately, the individual contributions of these different species could not be distinguished. On the other

hand, the high-temperature NO + O2 desorption peak was likely associated primarily with the elimination of the small fraction of bidentate nitrates (and particularly of the chelating ones), which have been shown by FT-IR to possess the highest thermal stability. In the case of the Ba/Al2O3 sample, a complex desorption spectrum was apparent, with two major NO desorption peaks centered at 550 K (with a shoulder near 650 K) and 870 K, this latter accompanied by oxygen evolution. A similar picture was apparent for the Pt-Ba/Al2O3 sample, with NO and NO + O2 desorption peaks observed near 570 and 800 K, respectively. A comparison with the TPD spectra recorded in the case of the Pt/Al2O3 sample pointed out a marked increase of the NOx storage properties at high temperature, induced by the presence of barium. In fact, a NO desorption peak (accompanied by O2 evolution) was apparent above 700 K over the Ba-containing samples. On the basis of the IR results previously discussed, the peaks in the 550-650 K region can be associated with the decomposition of monodentate and bidentate Ba nitrites, while the high-temperature peaks correspond to the decomposition of ionic Ba nitrates, showing high thermal stability. Again, the presence of platinum slightly decreased the stability of the stored NOx species, in line with results reported by Weterberg et al.30 The decomposition of nitrites and nitrates may occur in this case via the same mechanisms previously proposed. In the case of Ba/Al2O3 and Pt-Ba/Al2O3, on assuming that NO desorption is associated with the decomposition of Ba nitrite and nitrate species, it was estimated that roughly 15-18% of barium present on the catalyst samples was involved in the storage of NOx. Interaction with NO2. FT-IR Spectroscopy. Upon NO2 admission on Al2O3 and Pt/Al2O3 samples (spectra not reported),

12740 J. Phys. Chem. B, Vol. 105, No. 51, 2001 chelating and bridging bidentate nitrates and NO2δ+ species were formed in amounts depending on the NO2 pressure (see Table 1). At high NO2 pressure (>10 mbar), weakly adsorbed N2O4 was also detected (spectra not reported). Note that nitrites were not observed while nitrates were formed immediately and in higher amounts than those obtained after prolonged contact with the NO/O2 mixture. NO2δ+ species were desorbed at 500 K, while nitrates were removed at temperatures below 700 K. In the case of the Ba-containing catalysts, the main species formed upon NO2 adsorption were ionic and bidentate (chelating and/or bridging) nitrates, whereas nitrites and NO2δ+ species were not detected (Figure 7a,b curves 1). Also in this case, weakly adsorbed N2O4 was detected at high NO2 coverage. The overall amount of NOx species formed (evaluated from the integrated intensity of the bands in the 1700-970 cm-1 region) was greater than that in the case of the Al2O3 and Pt/Al2O3 samples. It was also greater than that obtained on the Bacontaining catalysts upon prolonged contact with the NO/O2 mixture. Again, very similar features were observed on Ba/Al2O3 and Pt-Ba/Al2O3 catalysts, concerning both the nature and the amounts of the adsorbed NOx species (compare curves 1 in Figure 7 a,b). Bidentate nitrates were progressively removed by evacuation at increasing temperature in the range 550-700 K, while the desorption of ionic nitrates started at T > 700 K. As in the case of the NO/O2 mixture, a positive band at 13601300 cm-1 and negative band around 1440 cm-1 were noted in the difference spectra after NO2 desorption at 773 K (Figure 7a,b curves 3). Again, these findings were ascribed to the presence of a fraction of ionic nitrates not yet decomposed, exhibiting νasym(ONO) modes partially overlapped to those of carbonates. This suggestion is nicely confirmed by the NO2 adsorption experiments carried out on a Ba/Al2O3 catalyst sample previously activated at 1023 K, i.e., where the major fraction of BaCO3 has been decomposed to BaO (see inset of Figure 7c). Indeed, upon NO2 adsorption and subsequent evacuation at 773 K, the negative band was no longer observed, and both the modes at 1460-1400 and 1360-1300 cm-1 of residual ionic nitrates could be detected (Figure 7c curve 3). Notice that the NOx species (bidentate and ionic nitrates) formed upon admission of NO2 at RT on the catalysts activated at 773 or 1023 K exhibit very similar features (compare curves 1 in Figure 7a,c). This happens despite the different nature of the Ba phases present on the catalyst activated at 773 K, which were both BaO and BaCO3 (or oxycarbonate phase), while mainly BaO was present on that activated at 1023 K. Such findings suggest that in both cases the Ba2+O2- pairs of the supported Ba oxide phase are mainly involved in the NOx storage process, at least at low temperature. We underline that on all the investigated catalysts the nitrate formation upon NO2 admission is fast and does not require the presence of O2 in the gas phase. Moreover, the absence of nitrites apparently suggests that reaction 17 does not occur to an appreciable extent. These results account for a one-step mechanism involving the coordination of NO2 through its nitrogen atom to a surface oxygen anion and through its oxygen atoms (one or both) to the surface cations, accompanied by a charge redistribution. Temperature-Programmed Desorption Experiments. TPD spectra obtained upon NO2 adsorption over Pt/Al2O3, Ba/Al2O3, and Pt-Ba/Al2O3 are shown in Figure 8. On the Pt/Al2O3 sample, NO2, NO, and O2 were observed to desorb in the range 500-700 K, accounting for the decomposition of nitrate species. In addition, weak NO2 and NO desorption peaks centered around

Prinetto et al. 400 K were observed and ascribed to the removal of weakly adsorbed N2O4 and of NO2δ+ species. In the case of the Ba-containing catalysts, significant amounts of NO2 desorbed in a wide temperature range (300-850 K), exhibiting a weak peak around 400 K and a major peak around 700 K. In addition, a broad NO desorption peak, accompanied by oxygen evolution, was observed in the temperature range 600-900 K. Notably, the overall amount of desorbed NOx (particularly at high temperature) was greater than that in the case of Pt/Al2O3. The desorption profiles showed very similar features over both the Ba/Al2O3 and the Pt-Ba/Al2O3 samples, but the peak maxima were shifted some 50-60 K toward lower temperatures in the case of the Pt-containing system. In line with FT-IR data, the weak NO2 peak around 400 K can be related to the removal of weakly adsorbed N2O4, while the desorption of NO2, NO and O2 in the range 600-900 K is ascribed to the decomposition of nitrate species. In particular, the desorption of NO2 can be associated to the decomposition of bidentate nitrates, which showed a lower thermal stability on the basis of FT-IR data. Since NO2 desorption is not accompanied by evolution of oxygen, we suggest that decomposition of nitrates occurs through the reverse mechanism of that proposed for their formation, rather than following stoichiometry 9. However, reaction 10 may also occur. The hightemperature NO + O2 peaks corresponded well to those observed in the case of the NO/O2 TPD experiments and, accordingly, can be associated with the decomposition of ionic Ba nitrate. Noticeably, TPD data showed that the amounts of the adsorbed NOx species on the Ba/Al2O3 and Pt-Ba/Al2O3 samples were higher by a factor of 3 if compared to those obtained upon saturation with NO + O2, and they correspond to the participation of roughly 50-60% of the barium present on the Ba/Al2O3 and Pt-Ba/Al2O3 samples. Changes of the Ba Phases Induced by NOx Adsorption. As already implied by the IR difference spectra previously reported, the high-temperature desorption of the NO/O2 mixtures and of NO2 caused significant changes in the nature of the Ba phases. This aspect is better investigated by examining the IR absorbance spectra of the Ba/Al2O3 and Pt-Ba/Al2O3 catalysts in the carbonate region before and after adsorption of the various gases and mixtures and their subsequent desorption at 773 K. The spectra before and after NO interaction were coincident (Figure 9a,b curves 1 and 2). On the contrary, after adsorption of NO/O2 mixtures and subsequent desorption at 773 K, the bands due to Ba carbonates (and particularly the component at 1440 cm-1) decreased in intensity, while the components at 1360-1300 cm-1, ascribed to Ba nitrates, appeared (Figure 9a,b curves 3). The decrease of the bands of carbonates was more pronounced in the case of adsorption/desorption of NO2 (Figure 9a,b curves 4). Actually, owing to the partial superposition of the IR modes of Ba carbonates and Ba nitrates, it is not possible to evaluate by FT-IR spectra the fraction of carbonates which have been removed during the thermal desorption of NO/O2 or NO2. It is worthy of note that the amount of nitrates present after adsorption of NO2 (or NO/O2) and subsequent evacuation at 773 K, evaluated from the integrated intensity of the mode at 1360-1300 cm-1, was lower on Pt-Ba/Al2O3 than on the Ba/Al2O3 catalyst. These data indicate that Pt decreases the thermal stability of Ba nitrates, as already pointed out by IR and TPD results previously reported. The effect of the NOx adsorption and thermal desorption on the BaCO3 phase could be also nicely investigated by monitoring the evolution of CO2 during the TPD experiments following



Figure 7. NO2 adsorption on Ba/Al2O3 and Pt-Ba/Al2O3 catalysts activated at 773 K (sections a and b, respectively) and on Ba/Al2O3 catalyst activated at 1023 K (section c). FT-IR spectra after adsorption of 5 mbar of NO2 at RT (curves 1) after the subsequent evacuation at 623 K (curves 2) and 773 K (curves 3). Each spectrum is reported as difference from the spectrum before NO2 admission. Inset of section c: IR spectra of Ba/Al2O3 catalyst activated at 773 K (curve 1) and 1023 K (curve 2) before NO2 admission.


Prinetto et al.

Figure 8. TPD profiles (concentration versus temperature) following saturation at 313 K of Pt/Al2O3, Ba/Al2O3, and Pt-Ba/Al2O3 with He + NO2 (2000 ppm).

Figure 9. FT-IR absorbance spectra of Ba/Al2O3 (section a) and Pt-Ba/Al2O3 (section b) catalysts activated at 773 K, (curves 1) after adsorption and subsequent evacuation at 773 K of NO (curves 2), NO/O2 (curves 3), or NO2 (curves 4). Each experiment is performed on a fresh catalyst sample.

NO, NO + O2, and NO2 adsorption. Figure 10 compares the desorption traces of CO2 monitored in the case of the Ba/Al2O3 and Pt-Ba/Al2O3 catalysts during the NO (traces 2), NO + O2 (traces 3), and NO2 (traces 4) TPD experiments. The CO2 desorption trace recorded upon heating an oxidized catalyst sample (TPDC experiment) in He has been also reported for

comparison (traces 1). During the TPDC experiment, CO2 evolution was observed starting from 773 K, corresponding to the temperature of catalyst pretreatment. The CO2 desorption peak presented a broad shape extending in the 800-1150 K temperature range, with a maximum around 950 K. The evolution of CO2 is related to the decomposition of BaCO3



Figure 10. CO2 evolution recorded during the various TPD experiments: without preadsorption (TPDC experiment, curves 1) and after adsorption of NO (curves 2), NO+O2 (curves 3), and NO2 (curves 4) on BaO/Al2O3 and Pt-BaO/Al2O3 catalysts pretreated in-situ at 773 K. Each experiment was performed with a new catalyst sample.

species (crystalline and/or amorphous); a quantitative evaluation of the CO2 peak indicates that the amount of evolved CO2 roughly corresponds to 50% of the Ba nominal content of the catalyst. The desorption of CO2 observed during the NO-TPD experiments (traces 2) closely resembled that recorded in the case of the TPDC runs, thus suggesting that the adsorption of NO did not significantly affect the decomposition of BaCO3. Conversely, during the NO2-TPD (traces 4), CO2 evolution was observed at low temperature (400-750 K), showing a complex feature with a maximum near 570 K. The CO2 evolution following NO + O2 interaction (traces 3) showed somehow intermediate features between those observed in the cases of NO and NO2 TPD experiments. On the basis of IR and TPD results, the evolution of CO2 at temperatures below 750 K is related to the formation of bulky Ba nitrate species at the expense of Ba carbonates. This suggestion is confirmed by the XRD analysis performed on Pt-Ba/Al2O3 samples following NO2 adsorption at high temperatures (not reported). Indeed, the

lines of Ba(NO3)2 nitrobarite (JCPDS 24-53) appeared, while those of the BaCO3 phase markedly decreased in intensity. As in the case of IR measurements, replicated NO2 TPD experiments were performed over the same Pt-Ba/Al2O3 catalyst sample. The data, not reported herein for the sake of brevity, clearly pointed out that CO2 evolution from the catalyst sample only occurs during the first TPD run. Interestingly, the TPD profiles of the desorbing species (NO, NO2 and O2) during the replicated runs were practically superimposed. This occurs despite the different degree of carboxylation of the catalyst sample at the beginning of the first and of the subsequent TPD runs. All of these results indicate that in the temperature range 400-750 K surface and lattice cations and anions (NO3-, CO32-, O2-) have mobility enough to migrate at the surface and from the surface to the bulk of the Ba phases and vice versa. Accordingly, it is suggested that the formation of bulky Ba nitrate during the TPD of NO2 (and to a minor extent of NO/


Prinetto et al.

O2) proceeds via surface nitrate species, bound over the alumina supported Ba oxide phase, through reactions 18 and/or 19:

BaCO3 + 2NO3-(s) f Ba(NO3)2 + CO2 + O2- (18) BaO + 2NO3-(s) f Ba(NO3)2 + O2-

(19)

Finally, it was noted that the thermal desorption of NO/O2 or NO2 not only induced structural changes in the Ba-containing phase but, in the case of the Pt-Ba/Al2O3 catalyst, also modified the accessibility of Pt sites. This could be pointed out by using CO as a probe molecule: indeed, by comparing the spectra of CO adsorbed on the Pt-Ba/Al2O3 catalyst before (Figure 2 curve 3) and after admission of NO2 and subsequent evacuation at 773 K (Figure 2 curve 4), it was noted that the overall integrated intensity of the bands due to carbonyl species was reduced by about 40%, thus accounting for a lower accessibility of Pt sites. A similar behavior was also observed, but to a less extent, in the case of the interaction with the NO/O2 mixture. These findings suggest a spreading of the Ba-containing phase over the alumina support induced by the formation of Ba(NO3)2 at the expense of BaCO3, which occurs during the adsorption/ desorption of NO2 or NO/O2. Conclusions The present study aimed at an investigation of the NOx storage properties of Pt-Ba/Al2O3 “NOx storage-reduction” (NSR) catalytic systems, recently proposed for the NOx abatement in the presence of excess oxygen in both mobile and stationary applications. For this purpose, the interaction of NO, NO2, or NO/O2 mixtures with Pt-Ba/Al2O3, Ba/Al2O3, and Pt/Al2O3 samples was examined by means of FT-IR spectroscopy and temperature-programmed desorption measurements. The combination of these complementary techniques proved to be very useful in obtaining information on the nature, relative amounts, and thermal stability of the stored NOx species and on the transformations induced on the catalyst during the storage process. Both IR and TPD experiments pointed out that, on all the investigated catalysts, the amounts of the adsorbed NOx species follow the order NO , NO + O2 < NO2. The presence of barium increases the amount and thermal stability of the stored NOx species and influences their nature. In particular, NO is adsorbed in negligible amounts on the Al2O3 and Pt/Al2O3 samples, while hyponitrite species are formed on the Bacontaining catalysts, along with nitrites and nitrates which are removed upon heating at temperatures above 700 K. The NOx storage capacity of all the catalysts is enhanced by 1 order of magnitude if NO adsorption is carried out in the presence of oxygen. Notably, high amounts of NOx species are observed to form also in the absence of Ba, involving in this case the alumina support. However the thermal stability of such species is lower if compared to those formed over the Ba-containing catalysts. In particular, nitrites, chelating and bridging bidentate nitrates, and NO2δ+ species were the major species formed over the alumina and Pt/Al2O3 samples. These species were almost completely removed by heating at temperatures below 700 K. In the case of the Ba-containing samples, the main NOx species stored upon adsorption of NO/O2 mixtures were ionic bidentate nitrites and monodentate nitrites, likely coordinated to Ba2+ sites and, in minor amounts, chelating and/or bridging bidentate nitrates. It was estimated that roughly 15-18% of Ba was involved in the NOx storage. Nitrites showed lower thermal stability if compared to nitrates, being removed below 700 K

and/or transformed into thermally more stable ionic Ba nitrates, which decomposed only upon heating at 750-950 K. On all the investigated catalysts, nitrate species were mainly detected upon adsorption of NO2, while nitrites were not formed. In particular, on the Al2O3 and Pt/Al2O3 catalysts, chelating and bridging bidentate nitrates and NO2δ+ species were formed, while on the Ba-containing catalysts, the main species were ionic nitrates, along with bidentate nitrates. Again, the presence of barium increases the amounts and thermal stability of the stored NOx species. 50-60% of Ba was involved in this case in the storage of NOx. The high thermal stability of the NOx species stored on the Ba-containing catalysts is related to the presence of bulky Ba nitrate, whose formation proceeds during the thermal desorption of NO2 (and to a lower extent of NO/O2) via surface bound nitrate species. The formation of bulky Ba(NO3)2 compounds also induces an extensive decomposition in the 400-700 K temperature range of the Ba carbonate or oxycarbonate phase which was detected on the calcined samples. However, experiments performed over the catalyst samples activated at high temperature or submitted to repeated NOx adsorption/desorption cycles evidence that the alumina supported Ba oxide phase is mainly involved in the NOx storage at RT. Results here reported do not support an influence of platinum addition to Ba/Al2O3 in the NOx storage at RT, and particularly on the nature, relative, and total amounts of the stored NOx species. Conversely, Pt slightly decreases the thermal stability of the stored NOx species. This possibly accounts for the formation of reduced Pt during the thermal desorption of NOx species, favoring their decomposition. However, one cannot exclude that at the working temperatures of the Pt catalyst could have a role in the NO oxidation processes. Obviously, the noble metal is required for the reduction of the stored NOx species during the rich phase of the catalytic cycle and consequently for the regeneration of the storage capacity of the catalysts. In this respect, the close contact between the exposed Pt sites and the Ba component evidenced by IR results upon CO adsorption plays a key role in determining the NSR activity. Acknowledgment. This work was supported by MURST (Rome), Cofin 2000 - Project “Catalysis for the reduction of the environmental impact of mobile sources emissions”. References and Notes (1) Heck, R. M.; Farrauto, R. J. Catalytic Air Pollution Control; Van Nostrand Reinhold: New York, 1995. (2) Parvulescu, V. I.; Grange, P.; Delmon, B. Catal. Today 1998, 46, 233. (3) Shinjoh, H.; Takahashi, N.; Yokota, K.; Sugiura, M. Appl. Catal., B: EnViron. 1998, 15, 189. (4) Takahashi, N.; Shinjoh, H.; Iijima, T.; Szuki, T.; Yamazaki, K.; Yokota, K.; Suzuki, H.; Miyoshi, N.; Matsumoto, S.; Tanizawa, T.; Tanaka, T.; Tateishi, S.; Kasahara, K. Catal. Today 1996, 27, 63. (5) Toyota Patent. European Patent Application no. 0573672A1, 1992. (6) Staff Report; Modern Power Systems: 2000; p 23. (7) Matsumoto, S.; Ikeda, Y.; Suzuki, H.; Ogai, M.; Miyoshi, N. Appl. Catal., B: EnViron. 2000, 25, 115. (8) Bogner, W.; Kramer, M.; Krutzsch, B.; Pishinger, S.; Voigtlander, D.; Wenninger, G.; Wirbeleit, F.; Brogan, M. S.; Brisley, R. J.; Webster, D. E. Appl. Catal., B: EnViron. 1995, 7, 153. (9) Fridell, E.; Skoglundh, M.; Westerberg, B.; Johanson, S.; Smedler, G. J. Catal. 1999, 183, 196. (10) Mahzoul, H.; Brilhac, J. F.; Gilot, P. Appl. Catal., B: EnViron. 1999, 20, 47. (11) Lietti, L.; Forzatti, P.; Nova, I.; Tronconi, E. J. Catal. 2001, 204, 175. (12) Morterra, C.; Magnacca, G. Catal. Today 1996, 27, 497. (13) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed.; Wiley: New York, 1986.

Investigation of NOx storage properties (14) . Handbook of Chemistry and Physics, 66th ed.; CRC Press: Florida, 1985/1986. (15) Pascal, P. NouVeau Traite´ de Chimie Mine´ rale; Ed. Masson: Paris, 1958. (16) Peri, B. J. Phys. Chem. 1966, 70, 3168. (17) Busca, G.; Lorenzelli, V. Mater. Chem. Phys. 1982, 7, 89. (18) Sheppard, N.; Nguyen, T. T. In AdVances in Infrared Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Heyden, London, 1978; p 67. (19) Lokhov, Yu. A.; Davydov, A. A. Kinet. Katal. 1980, 21, 1523. (20) Kazanski, V. B.; Borovkov, V. Yu.; Derouane, E. G. Catal. Lett. 1993, 19, 327. (21) Gandao, Z.; Coq, B.; de Menorval, L. C.; Tichit, D. Appl. Catal., A: Gen. 1996, 147, 395. (22) Boccuzzi, F.; Guglielminotti, E. Surf. Sci. 1992, 271, 149.

J. Phys. Chem. B, Vol. 105, No. 51, 2001 12745 (23) Hadjiivanov, K. I. Catal. ReV. Sci. Eng. 2000, 42 (1-2), 71. (24) Laane, J.; Olsen, J. R. Prog. Inorg. Chem. 1980, 27, 465. (25) Nakagawa, I.; Shimanouchi, T.; Yamasaki, K. Inorg. Chem. 1964, 3, 772. (26) Mestl, G.; Rosynek, M. P.; Lunsford, J. H. J. Phys. Chem. B 1997, 101, 9321. (27) Mestl, G.; Rosynek, M. P.; Lunsford, J. H. J. Phys. Chem. B 1997, 101, 9329. (28) Kapteijn, F.; Singoredjo, L.; van Drill, M.; Andreini, A.; Moulijn, J. A.; Ramis, G.; Busca, G. J. Catal. 1994, 150, 105. (29) Cerruti, L.; Modone, E.; Guglielminotti, E.; Borello, E. J. Chem. Soc., Faraday Trans. 1 1974, 70, 729. (30) Westerberg, B.; Fridell, E. J. Mol. Catal., A: Chem. 2001, 165, 249.

FT-IR and TPD Investigation of the NOx Storage Properties of BaO

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