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Sep 22, 2010 - To find out whether isolated Co2+ and dimers [Co−O−Co]2+ were located inside or outside the zeolite channels we used as probe molec...
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J. Phys. Chem. C 2010, 114, 17812–17818

Location of Isolated Co2+ and [Co-O-Co]2+ in Co-MOR as Investigated by Means of FTIR with Acetonitrile and 2,4,5-Trimethylbenzonitrile as Probe Molecules Daniela Pietrogiacomi,*,† Maria Cristina Campa,‡ and Valerio Indovina† Dipartimento di Chimica, “Sapienza” UniVersita` di Roma, Piazzale Aldo Moro 5, 00185 Roma, Italy, and Gruppo “Materiali Inorganici e Catalisi Eterogenea” dell’Istituto ISMN (CNR), c/o Dipartimento di Chimica, “Sapienza” UniVersita` di Roma, Piazzale Aldo Moro 5, 00185 Roma, Italy ReceiVed: July 23, 2010; ReVised Manuscript ReceiVed: August 27, 2010

Co-Na-MOR and Co-H-MOR were prepared by ion-exchange of Na-MOR or H-MOR with Co(acetate)2 solutions at 350 K. In these Co-Na-MOR and Co-H-MOR samples exchanged to various extents with cobalt (Co/Al ) 0.1 to 0.5), isolated Co2+ is the most abundant species, the residual cobalt being present as [Co-O-Co]2+. To find out whether isolated Co2+ and dimers [Co-O-Co]2+ were located inside or outside the zeolite channels we used as probe molecules acetonitrile (AN) and 2,4,5-trimethylbenzonitrile (TMBN), hypothesizing that AN enters the MOR main channels, whereas TMBN, having a higher steric hindrance than AN, does not enter the MOR channels, and only detects external cobalt. The comparison discriminates between internal and external cobalt. This feature was further investigated by presaturating Co-MOR samples with AN or TMBN and subsequently adsorbing CO or NO. FTIR spectra were run at room temperature on samples heated in O2 at 793 K and evacuated at the same temperature for 1 h before exposure to CO, NO, AN, or TMBN. The FTIR results demonstrate that in all Co-MOR samples, irrespective of the cobalt content, cobalt is almost exclusively located inside the mordenite channels. 1. Introduction In Co-exchanged zeolites, cobalt species are the active sites for NO reduction with CH4 in the presence of excess O2 (Selective Catalytic Reduction, SCR). At variance with groups invoking isolated Co2+ as the active site for the SCR reaction,1-8 others have suggested a role for multinuclear cobalt oxo-alike species, whose chemical composition was not specified.9-15 In a recent study investigating the type of cobalt species, in CoMOR samples exchanged with cobalt to various extents,16 we concluded that the most abundant species was isolated Co2+, the residual cobalt being present as [Co-O-Co]2+. The way the catalytic activity depended on the cobalt content allowed us to suggest that all cobalt, isolated or in pairs, or a constant fraction of it was the active site for SCR.16 An open question was whether the active cobalt species in our Co-MOR samples were located inside or outside the zeolite channels. For the SCR reaction, Wichterlova´ et al.3 and Kaucky´ et al.4 have suggested a role for isolated Co2+ located inside the zeolite channels of MOR, MFI, and FER exchanged with cobalt to various extents. In Co-H-MOR, Co-H-MFI, and Co-H-FER, extensively exchanged with cobalt (Co/Al ) 0.6 to 0.7), to locate cobalt species, Montanari et al.17 used nitriles of various sizes and concluded that in these zeolites Co2+ ions were distributed between internal and external surfaces. In Co-H-MFI samples, they identified (i) Co2+ anchored to the external surface, (ii) exchanged Co2+, and (iii) small cobalt oxide clusters, both species located inside the zeolite channels, and they proposed that in Co-H-MFI samples small cobalt oxide clusters were key species for the SCR process.15 * To whom correspondence should be addressed. Phone: +39-0649913304. Fax: +39-06-490324. E-mail: [email protected]. † Dipartimento di Chimica. ‡ Gruppo “Materiali Inorganici e Catalisi Eterogenea” dell’Istituto ISMN.

To find out whether isolated Co2+ and dimers [Co-O-Co]2+ were located inside or outside the zeolite channels of our CoMOR samples exchanged with cobalt to various extent, we used the FTIR spectroscopy of adsorbed hindered-nitriles. Busca and co-workers devised this method to locate cations in zeolites.17-20 We used as probe molecules acetonitrile (AN) and 2,4,5trimethylbenzonitrile (TMBN), hypothesizing that AN enters the MOR main channels, whereas TMBN, having a higher steric hindrance than AN, does not enter the MOR channels. To discriminate between external and internal cobalt, we compared the results obtained with TMBN and AN. To investigate this feature further we presaturated Co-MOR samples with AN or TMBN and subsequently adsorbed CO or NO on these samples. The results from these experiments are compared with those obtained previously adsorbing CO or NO at room temperature on Co-MOR samples activated under vacuum at 773 K or in CO at 623 K.16,21 In the present investigation we used Co-MOR samples exchanged to various extents (Co/Al ) 0.1 to 0.5). These samples were portions of those we have previously (i) prepared by ion-exchange of Na-MOR or H-MOR with Co(acetate)2 solutions at 350 K, (ii) characterized by adsorption of CO6,16 and NO,21 and (iii) used as catalysts for the SCR reaction6 and for N2O decomposition.21 2. Experimental Section 2.1. Sample Preparation. Co-exchanged zeolites and the starting materials for their preparation are listed in Table 1. Na-MOR (Si/Al ) 9.2) and H-MOR (Si/Al ) 9.2), kindly supplied by Tosoh Corporation, were used for ion exchange. In the Na-MOR sample, the analytical Na content equaled the Al content calculated from the analytical Si/Al ratio given by the supplier (Si/Al ) 9.2). The transition metal impurity level of the Na-MOR and H-MOR was analyzed by means of Inductively Coupled Plasma-Optical Emission Spectroscopy

10.1021/jp106878s  2010 American Chemical Society Published on Web 09/22/2010

Location of Isolated Co2+ and [Co-O-Co]2+ in Co-MOR

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TABLE 1: Sample Features starting materials

sample label

Na-MOR (Si/Al ) 9.2) Na-MOR + Co(ac)2 Na-MOR + Co(ac)2 H-MOR (Si/Al ) 9.2) H-MOR + Co(ac)2 H-MOR + Co(ac)2 NH4-Na-Y (Si/Al ) 2.47) Y + Co(ac)2

Na-MOR Co-Na-MOR-23 Co-Na-MOR-89 H-MOR Co-H-MOR-24 Co-H-MOR-66 Y Co-Y-52

analytical analytical 2Co/Al Na/Al 0.23 0.89

1.00 0.79 0.26

0.24 0.66 0.52

0.18 0.06

(Varian Vista-MPX CCD Simultaneous ICP-OES), and yielded Fe ) 36.8 (Na-MOR) and 95.3 ppm (H-MOR), Ni ) 2.1 and 20.1 ppm, Cu ) 3.0 and 3.1 ppm, and Co ) 2.2 and 2.4 ppm. The Co-exchanged MOR (Co-Na-MOR and Co-H-MOR) were portions of those previously prepared, characterized and used as catalysts.6,16,21 Co-containing samples were ion-exchanged at 350 K by contacting a weighted amount of MOR with an aqueous solution of 0.05 M Co(acetate)2 for 6 h under stirring at 350 K. To obtain extensively exchanged samples, three exchange procedures were run in sequence. After the exchange procedure, samples were thoroughly washed with distilled water and dried overnight at 383 K. The FAU starting material, NH4-Na-Y (Si/Al ) 2.47, Na/Al ) 0.18) kindly supplied by Linde was exchanged with 2 M NH4NO3 for 12 h under stirring at 350 K (Na/Al ) 0.06 after exchange), and thereafter exchanged with an aqueous solution of 0.01 M Co(acetate)2 for 6 h under stirring at 350 K. The sodium and the cobalt content of wet samples (equilibrated at ca. 79% relative humidity over a saturated solution of NH4Cl) were determined by atomic absorption (Varian SpectrAA-30), and expressed as Na+/Al and 2Co2+/Al ratios. Coexchanged samples are labeled as Co-X-Z-a, where X specifies the zeolite used in the preparation (H, or Na), Z specifies the type of zeolite (MOR or Y), and a is the analytical Co-exchange percentage, expressed as % 2Co/Al (Table 1). 2.2. FTIR Characterization. The FTIR spectrometer (Perkin-Elmer 2000), equipped with a MCT detector, operated at a resolution of 4 cm-1. The powdered samples were pelletted (pressure, 2 × 104 kg cm-2) in self-supporting disks of ca. 10 mg cm-2, and put in an IR cell that allowed heating in vacuum or in a controlled atmosphere. IR spectra were run at room temperature on samples heated in O2 (SOL, 99.9%) at 793 K for 1 h and evacuated at the same temperature for 1 h (actiVated samples). Activated samples were exposed at room temperature to CO (SOL, 99.9%), NO (AIR LIQUIDE/SIO, 99.0%), acetonitrile (extra dry, Acros Organics, hereafter indicated as AN), or 2,4,5-trimethylbenzonitrile vapor in equilibrium with the solid (97%, Aldrich, hereafter indicated as TMBN). In some experiments, CO or NO was adsorbed on samples presaturated with AN or TMBN, and evacuated at room temperature. In one experiment, activated Co-Na-MOR-89 sample was heated in CO at 623 K, maintained at this temperature for 15 min, cooled in CO, and thereafter evacuated at room temperature, before exposing it to AN or TMBN at room temperature. 3. Results and Discussion 3.1. The Adsorption of AN on Co-MOR: The OH Stretching Region. The IR spectrum of Na-MOR consisted of a band at 3746 cm-1. In addition to this band, the spectra of H-MOR, Co-Na-MOR, and Co-H-MOR consisted of a band at 3610 cm-1 and a weak shoulder at 3655 cm-1 (Figure 1). In agreement

Figure 1. FTIR spectra of Na-MOR, H-MOR, and Co-MOR in the OH stretching region. Spectra recorded after activation (solid lines), and subsequent adsorption of AN (equilibrium pressure )75 Torr) at room temperature (dotted lines).

with early assignments,6,18 we assign the band at 3746 cm-1 to terminal silanols, located at the external surface of MOR, the band at 3610 cm-1 to Brønsted acid sites, located at the exchanging sites inside MOR channels, and the weak shoulder at 3655 cm-1 to OH on partially extra-framework Al3+ in the side pocket of MOR. The comparison of the FTIR spectra of Co-Na-MOR and CoH-MOR with those of the corresponding matrices shows that whereas the intensity of the silanol band (νOH at 3746 cm-1) is nearly the same, (i) in Co-H-MOR-66 the band intensity of Brønsted acid sites (band at νOH at 3610 cm-1) was far higher than that expected from the Co-exchange extent, and (ii) in CoNa-MOR-89 an unexpected Brønsted acid site band appeared at 3610 cm-1 (Figure 1). The unexpected formation of Brønsted acid sites in Co-Na-MOR and the fact that the band intensity of Brønsted acid sites in Co-H-MOR was far higher than that expected from the Co-exchange extent can be rationalized along the same lines we have previously proposed.6 Namely, the Co2+(H2O)n species exchanged with (-Al-ONa-Si-) sites of NaMOR, or with (-Al-OH-Si-) sites of H-MOR, yielding (-Si-O--Al-) and [-Al-O-Co2+(H2O)n-Si-]. On heating in O2 at 773 K, a fraction of the [-Al-O-Co2+(H2O)n-Si-] species underwent hydrolysis thereby causing the formation of Brønsted acid sites, (-Si-OH-Al-), and [-Al-O-Co(OH)+-Si-]. Adsorption of AN on Na-MOR, H-MOR, and Co-exchanged samples caused the silanol band at 3746 cm-1 to disappear, and yielded in parallel a broad and complex absorption at a lower wavenumber, with a maximum at about 3400 cm-1, due to the interaction of silanols with the nitrile group of AN. Superimposed on this broad and complex absorption, peaks at 2990 and 2935 cm-1 formed, arising from the CH stretching modes of AN (Figure 1). On H-MOR and Co-H-MOR samples, AN adsorption caused the bands at 3655 and 3610 cm-1 to disappear, and yielded an intense and complex profile with maxima at about 2800, 2450, and 1700 cm-1 (Figure 1, spectral region below 2400 cm-1 not shown). These three absorptions have been previously observed

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Figure 2. FTIR spectra of AN adsorbed at room temperature on activated Na-MOR, H-MOR, and Co-MOR. Spectra were recorded in the presence of AN at increasing pressure, from 0.1 to 75 Torr, at room temperature (solid lines), and after AN evacuation at room temperature for 5 min (dotted lines).

in H-MOR,22 and assigned to the so-called ABC trio of bands, arising from the hydrogen bond between Brønsted acid sites (-Si-OH-Al-) and the -CN group of AN. The relative intensities of the ABC components is typical of a neutral complex with a medium-strong hydrogen bond.22,23 When all samples were evacuated at room temperature for 5 min (i) the silanol bands were partially restored, (ii) the broad absorption arising from the interaction of Si-OH with -CN decreased, (iii) the bands of the methyl group slightly decreased, and (iv) the bands arising from the interaction of Brønsted acid sites with -CN were nearly unaffected (spectra not shown). On all MOR samples, in agreement with analogous findings on similar systems,24 we found that the AN molecule interacts (i) with Si-OH placed in the external surface, (ii) with Brønsted acid sites, located at the exchanging sites inside the MOR channels, and (iii) with OH in the side pocket of MOR. 3.2. The Adsorption of AN on Co-MOR: The CN Stretching Region. AN adsorption on Na-MOR caused an intense absorption to form consisting of two bands at 2300 and 2268 cm-1, with shoulders at about 2290 and 2260 cm-1, respectively (Figure 2, solid lines, spectra at increasing equilibrium pressure). Salla et al.19 have previously observed all these bands on NaMOR. Therein, they specified that the nitrile stretching mode of AN yields two bands due to the Fermi resonance between the νCN mode and the combination of the δCH3 + νC-C of -CN groups, and they assigned the two main components (2300 and

Pietrogiacomi et al. 2268 cm-1) to -CN interacting with Na+ and the two shoulders (2290 and 2260 cm-1) to -CN interacting with terminal silanols.19 Evacuation at room temperature caused the intensity of the two shoulders at 2290 and 2260 cm-1 to decrease markedly, and the intensity of the two main components to remain unchanged (Figure 2, dotted line). Collectively, the decrease in the intensity of the two shoulders and the aforementioned partial recovery of the silanol-band (section 3.1) reinforces the assignment of the two shoulders to -CN interacting with Si-OH. The adsorption of AN on H-MOR caused an intense and complex absorption band to form with maxima at about 2330, 2305, 2273, and 2254 cm-1 (Figure 2, solid lines, spectra at increasing equilibrium pressure). All these bands have been previously observed on H-zeolites (MOR, MFI, FER), and assigned to -CN interacting with (i) Al3+ Lewis acid sites (bands at 2330 and 2305 cm-1), (ii) Brønsted acid sites (bands at 2305 and 2273 cm-1), and (iii) Si-OH (weak shoulders at about 2290 and 2260 cm-1, hardly detectable in our spectra).25,26 The band at 2254 cm-1 has been previously observed, and assigned to -CN from liquid-like AN,26 the second component at 2293 cm-1 being undetectable owing to the intense and complex absorption mentioned in the foregoing experiment. In agreement with this assignment, the band at 2254 cm-1 disappeared upon evacuation at room temperature. After this evacuation treatment, all other bands, arising from species stable at room temperature, slightly decreased, and maintained their relative intensity ratio (Figure 2, dotted line). AN adsorption on Co-Na-MOR and Co-H-MOR samples exchanged to various extent caused, in addition to the bands observed in the corresponding matrices, Na-MOR and H-MOR, a new doublet to form at 2318 and 2290 cm-1 superimposed on matrix bands (Figure 2, solid lines, spectra at increasing equilibrium pressure). The higher the Co content in the sample, the higher the intensity of the doublet at 2318 and 2290 cm-1, and the lower the intensity of matrix bands, due to Na+ or H+ exchanged with Co2+. A doublet with components at 2318 and 2290 cm-1 has been previously observed on Co-H-MOR adsorbing AN at room temperature,27 and assigned to -CN interacting with Co2+. As on the pure matrices, evacuation at room temperature of Co-Na-MOR and Co-H-MOR samples caused the band at 2254 cm-1 from liquid-like AN to disappear. All other bands slightly decreased, maintaining their relative intensity ratio, showing that Co2+---NC- was a stable adduct (Figure 2, dotted line). 3.3. The Adsorption of TMBN on Co-MOR: The OH Stretching Region. As observed for AN adsorption, TMBN adsorption on Na-MOR, H-MOR, and on Co-exchanged samples caused the band at 3746 cm-1 to disappear. TMBN adsorption yielded in parallel a broad absorption with maxima at about 3650 and 3400 cm-1, due to the interaction of two different Si-OH families with the -CN group of TMBN. Superimposed on the tail of this broad absorption, weak bands in the region 3050-2850 cm-1 formed, arising from the vibrational modes of the -CH groups of TMBN. Unlike AN adsorption, TMBN adsorption had no effect on or hardly affected the band of Brønsted acid sites and that of OH on partially extra-framework Al3+ (Figure 3). When all samples were evacuated at room temperature for 5 min, all bands remained unchanged (spectra not shown), indicating that the TMBN silanol interaction was stronger than the corresponding AN interaction. On the whole, the TMBN adsorption-induced changes in the OH stretching region show that the TMBN molecule interacts

Location of Isolated Co2+ and [Co-O-Co]2+ in Co-MOR

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Figure 3. FTIR spectra of Na-MOR, H-MOR, and Co-MOR in the OH stretching region. Spectra were recorded after activation (solid lines), and subsequent exposure to TMBN vapor (equilibrium pressure ) 0.02 Torr) at room temperature for 45 min (dotted lines).

with Si-OH placed on the external surface of all MOR samples and with Brønsted acid sites located at the channel mouths. Specifically, TMBN does not enter the MOR channels, where the Brønsted acid sites and the OH of partially extra-framework Al3+ are located. 3.4. The Adsorption of TMBN on Co-MOR: The CN Stretching Region. TMBN adsorption yielded bands in the CN stretching region much weaker than those observed after AN adsorption. TMBN adsorption on Na-MOR caused a weak absorption band to form with components at about 2250 and 2230 cm-1, and a shoulder at 2222 cm-1. TMBN adsorption on H-MOR yielded a similar envelope with an additional maximum at 2274 cm-1. On Co-Na-MOR and Co-H-MOR samples, in addition to the aforementioned components, a new shoulder at about 2260 cm-1 formed. On all samples, evacuation at room temperature left the spectra unchanged (Figure 4, solid lines, spectra at increasing contact time; dotted lines, spectra after evacuation at room temperature). We tentatively assign the H-MOR band at 2274 cm-1 to TMBN interacting with Al3+ Lewis sites on the external surface. In agreement, by adsorbing 2,2-diphenylpropionitrile, which did not enter MOR channels, Montanari et al. identified strong Lewis sites on the external surface of H-MOR.20 The shoulder at 2260 cm-1 was present only on the Coexchanged MOR samples. On Co-H-MOR, by adsorbing benzonitrile, which entered MOR channels, Montanari et al.17 observed an intense band at 2250-60 cm-1, and assigned this band to -CN of benzonitrile interacting with Co2+. We therefore assign the shoulder at 2260 cm-1 to -CN of TMBN interacting with Co2+ ions located at the channel mouth of MOR samples. In all samples, we detected bands at about 2250 and 2230 cm-1, previously observed by Montanari et al.17 and Salla et al.19 upon benzonitrile or o-toluonitrile adsorption on H-zeolite (FER, MFI, and MOR) or alkali-MOR, and assigned to -CN interacting with Brønsted acid sites or Na+ (2250 cm-1) and to -CN interacting with silanols (2230 cm-1). Along these lines, we assign the very weak band at 2250 cm-1 to -CN interacting

Figure 4. FTIR spectra of TMBN adsorbed at room temperature on activated Na-MOR, H-MOR, and Co-MOR. Spectra recorded in the presence of TMBN at equilibrium pressure ) 0.02 Torr, after increasing contact time from 1 to 45 min, at room temperature (solid lines), and after evacuation at room temperature for 5 min (dotted lines).

with the Brønsted acid sites or the Na+ ions at the channel mouth of MOR, and the band at 2230 cm-1 to -CN interacting with Si-OH. Because liquid-like o-toluonitrile absorbed at 2225 cm-1,17 we assign the shoulder at 2222 cm-1 to liquid-like adsorbed TMBN. 3.5. The Adsorption of CO or NO on Co-MOR Samples Presaturated with AN or TMBN. On Na-MOR, CO adsorption at room temperature yielded weak carbonyl bands, the most intense one arising from CO adsorbed on Na+ located in the main channels (band at 2176 cm-1) and in the side pockets (shoulder at 2163 cm-1), as previously reported6 (Figure 5, section a, spectrum 1). On H-MOR, CO adsorption at room temperature yielded very weak bands, the most intense one arising from CO adsorbed on Brønsted acid sites (band at 2170 cm-1), as reported previously6 (spectrum not shown). We have previously reported in detail6 that CO adsorption at room temperature on Co-MOR, in addition to weak carbonyl matrix bands, yielded carbonyl bands arising from CO interacting with isolated Co2+ located in the main channels, and in the smaller channels. On Co-Na-MOR-89, CO adsorption at room temperature also yielded [(CO)nCo+-0-Co+(CO)n] species (n ) 2 or 3), arising from the [Co-O-Co]2+ dimer reduction, yielding CO216 (Figure 5, section b, spectrum 1). Preadsorption of AN on Na-MOR, H-MOR, Co-Na-MOR, and Co-H-MOR samples yielded the strong nitrile bands already described. The subsequent CO adsorption at room temperature

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Figure 5. The adsorption of CO on samples presaturated with AN. FTIR spectra of Na-MOR (section a) and Co-Na-MOR-89 (section b). Spectra were recorded (i) after exposure to CO at room temperature for 30 min (equilibrium pressure ) 75 Torr, spectra 1 in sections a and b), (ii) after exposure to AN at room temperature (equilibrium pressure ) 75 Torr, spectra 2), and (iii) after exposure to AN at room temperature, evacuation at room temperature, and subsequent exposure to CO at room temperature for 30 min (equilibrium pressure ) 75 Torr, spectra 3).

had no effect on the nitrile bands, and yielded neither carbonyl bands nor CO2 (Figure 5). All these findings show that AN preadsorption blocked the zeolite sites, hindering carbonyl formation and [Co-O-Co]2+ species reduction, when samples were subsequently exposed to CO. NO adsorption on Co-Na-MOR and Co-H-MOR yielded Con+-mononitrosyls (with n ) 2 or 3) and Co2+-dinitrosyls in the range 2000-1700 cm-1, as previously reported.21 When samples were presaturated with AN, during the subsequent NO adsorption no cobalt nitrosyl species formed, confirming that AN preadsorption blocked the cobalt sites (spectra not shown). TMBN and AN preadsorption yielded substantially different results. Specifically, TMBN preadsorption on Na-MOR, HMOR, Co-Na-MOR, and Co-H-MOR yielded the weak nitrile bands already described. On the subsequent CO adsorption at room temperature, nitrile bands remained unchanged, and carbonyl bands formed, identical with those observed by adsorbing CO on samples not previously exposed to TMBN (Figure 6). Because MOR channels are not accessible to TMBN, these results show that isolated Co2+, [Co-O-Co]2+ dimer, Na+ ions, and Brønsted acid sites are located inside the MOR channels. Analogous results were obtained upon NO adsorption on CoMOR samples presaturated with TMBN. Specifically, when CoNa-MOR-89 presaturated with TMBN was subsequently contacted with NO, cobalt nitrosyl species formed, identical with those observed when TMBN was not preadsorbed (Figure 7). This finding confirms that TMBN preadsorption does not block cobalt species located inside the MOR channels. In another experiment, we heated the Co-Na-MOR-89 sample at 623 K in CO, cooled it to room temperature, and evacuated it at the same temperature. After this treatment, bands from Co2+-CO and from [(CO)nCo+-0-Co+(CO)n] species formed, the latter species at their maximum intensity, as reported

Pietrogiacomi et al.

Figure 6. The adsorption of CO on samples presaturated with TMBN. FTIR spectra of Na-MOR (section a) and Co-Na-MOR-89 (section b). Spectra were recorded (i) after exposure to CO at room temperature for 30 min (equilibrium pressure ) 75 Torr, spectra 1 in sections a and b), (ii) after exposure to TMBN at room temperature for 45 min (equilibrium pressure ) 0.02 Torr, spectra 2), and (iii) after exposure to TMBN at room temperature, evacuation at room temperature, and subsequent exposure to CO at room temperature for 30 min (equilibrium pressure ) 75 Torr, spectra 3).

Figure 7. The adsorption of NO on samples presaturated with TMBN. FTIR spectra of Co-Na-MOR-89 recorded (i) after exposure to NO at room temperature (equilibrium pressure ) 75 Torr, spectrum 1), (ii) after exposure to TMBN at room temperature for 45 min (equilibrium pressure ) 0.02 Torr, spectrum 2), and (iii) after exposure to TMBN at room temperature, evacuation at room temperature, and subsequent exposure to NO at room temperature (equilibrium pressure ) 75 Torr, spectrum 3).

previously16 (Figure 8, spectrum 1). The subsequent exposure to TMBN left the carbonyl bands unchanged, and yielded nitrile bands (spectra 2 to 4 at increasing contact time). The nitrile bands in spectrum 4 are identical in intensity and position with those observed on Co-Na-MOR-89 evacuated at 773 K and exposed to TMBN (inset to Figure 8). The subsequent exposure to AN caused strong nitrile bands of AN to form and carbonyl bands to disappear (Figure 8, dotted line). Because TMBN, unlike AN, has no effect on cobalt carbonyl bands, the latter experiment confirms that both isolated Co2+ and [Co-O-Co]2+ species are located inside the MOR channels.

Location of Isolated Co2+ and [Co-O-Co]2+ in Co-MOR

Figure 8. FTIR spectra of TMBN adsorbed on Co-Na-MOR-89 previously exposed to CO. The sample was activated, prereduced in CO at 623 K, cooled in CO to room temperature (equilibrium pressure ) 75 Torr), and evacuated at room temperature (spectrum 1). The sample was thereafter exposed to TMBN (equilibrium pressure ) 0.02 Torr) at increasing contact time from 1 to 45 min (spectra 2 to 4). The sample was finally exposed to AN (equilibrium pressure ) 75 Torr, dotted line spectrum). In the inset, the spectrum difference in the CN region between spectrum 4 and spectrum 1 (4-1) is compared with the spectrum obtained after TMBN adsorption on the same activated sample not previously exposed to CO (spectrum 5).

Figure 9. FTIR spectra of AN or TMBN adsorbed on activated CoY-52 and Co-H-MOR-66. Spectra were recorded in the presence of AN at equilibrium pressure ) 75 Torr at room temperature (section a) or in the presence of TMBN at equilibrium pressure ) 0.02 Torr, after 45 min, at room temperature (section b).

3.6. The Adsorption of AN or TMBN on Co-Y. The finding that TMBN adsorption does not hinder the subsequent CO or NO adsorption most probably depends on the fact that cobalt is located inside the MOR channels, which TMBN does not enter. Another possibility is that Co2+ does not chemisorb TMBN because the nitrile group of TMBN is less basic than that of AN. To discriminate between these two possibilities, we adsorbed AN or TMBN on Co-Y-52, whose channel size cannot hinder the access of TMBN. AN adsorption on Co-Y-52 caused bands to form at an intensity and position similar to those on Co-H-MOR-66 (Figure 9, section a). We assign the shoulder at 2250 cm-1 to liquidlike AN, the band at 2260 cm-1 to AN interacting with silanols, and the bands at 2290 and 2318 cm-1 to the AN interacting with Co2+ Lewis sites. Conversely, TMBN adsorption on Co-

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Figure 10. FTIR spectra of TMBN adsorbed on Co-Y-52 previously exposed to NO. The sample was activated, exposed to NO at room temperature, and evacuated at room temperature (spectrum 1). The sample was thereafter exposed to TMBN (equilibrium pressure ) 0.02 Torr) at room temperature at increasing contact time from 1 to 45 min (spectra 2 to 6).

Y-52 yielded bands at 2240 and 2256 cm-1 much more intense than those observed on Co-H-MOR-66 in the same spectral region (Figure 9, section b). Both bands were stable upon evacuation at room temperature. Similar bands have been previously observed by Montanari et al.17 on H- and Co-silicaallumina and H- and Co-zeolites (MFI, FER, and MOR) by adsorption of benzonitrile or o-toluonitrile, and assigned to nitriles interacting with Brønsted acid sites (2240 cm-1) and with Co2+ (2260-50 cm-1). Along these lines, we tentatively assign the band we observed on Co-Y-52 at 2240 cm-1 to TMBN nitriles interacting with Brønsted acid sites, and that at 2256 cm-1 to nitriles interacting with Co2+. The assignment of the band at 2240 cm-1 to nitriles interacting with Brønsted acid sites of Co-Y-52 is consistent with the fact that the band of Brønsted acid sites decreases, and the ABC trio of bands at 2700, 2400, and 1700 cm-1, arising from the hydrogen bond of nitriles with Brønsted acid sites, increases (spectra not shown). CO adsorption at room temperature on Co-Y-52 yielded a band at 2206 cm-1 arising from cobalt-carbonyls. This band did not form when CO at room temperature was contacted with Co-Y-52 presaturated with TMBN, indicating that TMBN preadsorption blocked the cobalt sites (spectra not shown). NO adsorption at room temperature on Co-Y-52 yielded intense bands at about 1900 and 1820 cm-1 (Figure 10, spectrum 1). These bands have been previously observed on zeolites and assigned to Co2+-(NO)2.28 When Co-Y-52 was subsequently exposed to TMBN, the same nitrile bands described in the foregoing experiment formed: their intensity increased with the exposure time to TMBN, while the Co2+-(NO)2 band intensity decreased in parallel (Figure 10, spectra 2-6). 4. Conclusions The adsorption of AN or TMBN on Co-Y yielded stable cobalt-nitrile adducts. AN adsorption on Co-MOR samples exchanged with cobalt to various extents yielded analogous adducts. Owing to the size of the mordenite channels, TMBN adsorption on all Co-MOR samples yielded cobalt-nitrile adducts in an extremely low amount. This finding demonstrates that in all Co-MOR samples, irrespective of the cobalt content, cobalt is located almost exclusively within the mordenite channels. On all Co-MOR samples presaturated with AN, the subsequent CO or NO adsorption did not yield cobalt carbonyls or cobalt nitrosyls. Conversely, on the same samples presaturated with TMBN, the subsequent CO or NO adsorption yielded cobalt carbonyls or cobalt nitrosyls, identical in type and concentra-

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tion to those detected on the corresponding samples unexposed to TMBN. These findings confirm that all cobalt in Co-MOR samples is located inside the mordenite channels. On the whole, our results confirm that the method first proposed by Busca and co-workers, namely the FTIR spectroscopy of hindered nitriles, is a valuable approach to locate cobalt species in zeolites. The Co-MOR samples we used in the present investigation were portions of those we have previously prepared by ionexchange with an aqueous solution of cobalt acetate at 350 K. These samples were portions of those we used as catalysts for the SCR of NO with CH46 or N2O decomposition.21 We conclude that in Co-MOR samples prepared in this way, irrespective of the cobalt content, all cobalt, isolated Co2+ and [Co-O-Co]2+, is located inside the mordenite channels. References and Notes (1) Li, Y.; Armor, J. N. Appl. Catal., B 1993, 2, 239. (2) Campa, M. C.; De Rossi, S.; Ferraris, G.; Indovina, V. Appl. Catal., B 1996, 8, 315. (3) Wichterlova´, B. Top. Catal. 2004, 28, 131. (4) Kaucky´, D.; Vondrova´, A.; De˘decˇek, J.; Wichterlova´, B. J. Catal. 2000, 194, 318. (5) De˘deeˇek, J.; Kaucky´, D.; Wichterlova´, B. Top. Catal. 2002, 18, 283. (6) Campa, M. C.; Luisetto, I.; Pietrogiacomi, D.; Indovina, V. Appl. Catal., B 2003, 46, 511. (7) Aylor, A. W.; Lobree, L. J.; Reimer, J. A.; Bell, A. T. In Studies in Surface Science and Catalysis, Proceedings of the 11th International Congress on Catalysis-40th AnniVersary; Hightower, J. W., Delgass, W. N., Iglesia, E., Bell, A. T., Eds.; Elsevier Science Publ.: Amsterdam, The Netherlands, 1996; Vol. 101, p 661. (8) Wang, X.; Chen, H.; Sachtler, W. M. H. Appl. Catal., B 2001, 29, 47.

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