Isolated Co2+ and [Co−O−Co]2+ Species in Na-MOR Exchanged with

Mar 11, 2008 - On evacuation at increasing temperature, [(CO)2Co+−Co+(CO)2] progressively disappeared and transformed into a stable bridged species,...
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J. Phys. Chem. C 2008, 112, 5093-5101

5093

Isolated Co2+ and [Co-O-Co]2+ Species in Na-MOR Exchanged with Cobalt to Various Extents: An FTIR Characterization by CO Adsorption of Oxidized and Prereduced Samples Valerio Indovina,*,† Maria Cristina Campa,‡ and Daniela Pietrogiacomi† Dipartimento di Chimica, UniVersita` degli Studi di Roma “La Sapienza”, Piazzale Aldo Moro 5, 00185 Roma, Italy, and Sezione “Materiali Inorganici e Catalisi Eterogenea” dell’Istituto ISC (CNR) c/o Dipartimento di Chimica, UniVersita` degli Studi di Roma “La Sapienza”, Piazzale Aldo Moro 5, 00185 Roma, Italy ReceiVed: NoVember 20, 2007; In Final Form: January 9, 2008

A commercial Na-MOR was exchanged to various extents with cobalt. The sodium and cobalt contents were determined by atomic absorption. Samples were characterized by UV-vis and by FTIR. The FTIR results with CO show that Co-MOR heated in O2 at 793 K and evacuated at the same temperature contained isolated Co2+ (86 to 100% of cobalt detected by CO adsorption at RT) and [Co-O-Co]2+. After exposure of samples to CO at RT, FTIR showed that only a minute fraction of [Co-O-Co]2+ underwent reduction yielding [(CO)nCo+-Co+(CO)n], with n ) 2 or 3, and CO2. After exposure to CO at increasing temperature up to 623 K, the subsequent adsorption of CO at RT yielded increasing amounts of [(CO)nCo+-Co+(CO)n] and CO2. Whereas isolated Co2+ did not undergo reduction, Co2+ in [Co-O-Co]2+ reduced to Co+. On evacuation at RT, [(CO)3Co+-Co+(CO)3] completely and reversibly transformed into [(CO)2Co+-Co+(CO)2]. On evacuation at increasing temperature, [(CO)2Co+-Co+(CO)2] progressively disappeared and transformed into a stable bridged species, [Co+(CO)Co+], and [(CO)Co+-Co+(CO)]. As the total cobalt content in Co-MOR samples increases, the [Co-O-Co]2+ amount increases exponentially. Hence, we infer that the [Co-O-Co]2+ species is not the active site for NO abatement with CH4 in the presence of O2.

1. Introduction Co-exchanged zeolites are active for NO reduction with CH4 in the presence of excess O2 (Selective Catalytic Reduction, SCR). Li and Armor1 found that the catalytic activity of CoMFI increased with the cobalt content. In agreement with Li and Armor, Campa et al.2 reported that the activity of Co-MFI increased linearly with the cobalt content and suggested that Co2+ ions exchanged in the framework of the MFI matrix were active in the SCR reaction, whereas the cobalt of the dispersed Co3O4 phase (i) contributed to CH4 oxidation with O2 and (ii) did not contribute to the SCR reaction. Wichterlova´3 and Kaucky´ et al.4 reported that Co-MFI was more active than Co-FER, and both were far more active than Co-MOR. Hence, they suggested that, within each zeolite matrix, isolated Co2+ in specific sites (R-type Co in MOR and FER and β-type Co in MFI) possessed the highest catalytic activity.4,5 Campa et al.6 found that the activity of Co-MOR samples increased linearly with the Co content, as in Co-MFI.2 They also found that the activity of Co-MOR approached that of Co-MFI and suggested that isolated Co2+ in the main channels of MOR were the active sites for the SCR reaction.6 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 * To whom correspondence should be addressed. Fax: +39-06-490324. E-mail address: [email protected]. † Dipartimento di Chimica, Universita ` degli Studi di Roma. ‡ Sezione “Materiali Inorganici e Catalisi Eterogenea” dell’Istituto ISC.

In an FTIR study, Hadjiivanov et al.16 investigated a prereduced Co-MFI sample, containing 2 wt % cobalt. They found that CO adsorption caused the formation of Co+(CO)n species, with n ) 2 to 4 and speculated that Co+ might have formed from [Co-O-Co]2+ during prereduction at 673 K with CO. On extensively exchanged Co-MOR samples, heated in O2 at 773 K and evacuated at the same temperature, Campa et al.6 in addition to strong bands of Co2+(CO) detected extremely weak bands at 2114 and 2038 cm-1, which they tentatively assigned to Co carbonyls with cobalt in an oxidation state lower than two. The same authors suggested that the reduced cobalt species formed on the surface of Co3O4 traces, possibly present in the extensively exchanged Co-MOR samples.6 A comparison of these results6 with those obtained by Hadjiivanov et al.16 suggests that the weak bands at 2114 and 2038 cm-1 arose not from Co carbonyls on the surface of Co3O4 but from Co+(CO)2 (Table 1 of ref 16). These findings prompted us to investigate the formation of Co carbonyls on either oxidized or prereduced Co-MOR samples. At variance with Co-MFI, on which Hadjiivanov limited the investigation to one Co concentration only,16 in CoMOR we extended the investigation to several samples, covering the whole range of Co-exchange extent. In some experiments, the Co-MOR samples we used for the FTIR investigation were portions of those we had previously used as catalysts for the SCR reaction.6 These samples were prepared by ion exchange of Na-MOR with Co(acetate)2 at 350 K. For the present investigation, we also prepared new Co-MOR samples either by using the identical preparation method as that used in our previous catalysis study6 or by ion exchange of Na-MOR with

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TABLE 1: Cobalt-exchanged Mordenite: Starting Materials for Sample Preparation, Analytical Cobalt, and Sodium Amounts starting materials

samples

H-MOR-9.2 Na-MOR-9.2 Na-MOR + NH4NO3 Na-MOR + Co(CH3COO)2 “ ” + Co(CH3COO)2 “ ” + Co(CH3COO)2 “ ” + Co(CH3COO)2 “ ” + Co(CH3COO)2 “ ” + Co(CH3COO)2 “ ” + Co(CH3COO)2 “ ” + Co(CH3COO)2 “ ” + Co(NO3)2 Co-MOR-92 + Co(CH3COO)2

H-MOR Na-MOR NH4-MOR Co-MOR-11 Co-MOR-23 Co-MOR-41 Co-MOR-61 Co-MOR-89 Co-MOR-92 Co-MOR-104b Co-MOR-73 Co-MOR-74 Co3O4/Co-MOR-92c

T/Ka

350 350 350 350 350 350 350 298 298

2 Co/Al

Na/Al

(2 Co + Na)/Al

0.11 0.23 0.41 0.61 0.89 0.92 1.04 0.73 0.74 1.26

1.00 0.02 0.91 0.79 0.57 0.40 0.26 0.36 0.28 0.32 0.32 0.36

1.02 1.02 0.98 1.01 1.15 1.28 1.32 1.05 1.06 1.62

a Temperature at which the ion exchange was performed. b The aqueous solution used for the preparation of this sample was 10 times less concentrated than that used for the preparation of Co-MOR-89 and Co-MOR-92, which contained a similar Co amount. c Sample prepared by impregnating Co-MOR-92 with a solution of Co(CH3COO)2. The sample was thereafter dried overnight at 383 K and calcined at 823 K for 5 h.

Co(acetate)2 or Co(nitrate)2 at RT rather than at 350 K. Our investigation had several aims: (i) to assess the presence of [Co-O-Co]2+ in Co-MOR, (ii) to specify the conditions which favor its formation, and (iii) to evaluate the possible role of this species as an active site in the SCR reaction. 2. Experimental Section 2.1. Sample Preparation. Co-exchanged mordenite samples (Co-MOR) and the starting materials for their preparation are listed in Table 1. Na-MOR (Si/Al ) 9.2, Tosoh Corporation) was used for ion exchange, and H-MOR (Si/Al ) 9.2, Tosoh Corporation) was used as a reference compound. In the NaMOR 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 NaMOR sample was analyzed by means of Inductively Coupled Plasma-Optical Emission Spectroscopy (Varian Vista-MPX CCD Simultaneous ICP-OES) and yielded Fe ) 36.8 ppm, Ni ) 2.1 ppm, Cu ) 3.0 ppm, and Co ) 2.2 ppm. A portion of Na-MOR was converted into NH4-MOR by contacting it with an aqueous solution of NH4NO3 at 350 K for 6 h. After this treatment, nearly all the Na+ ions were exchanged with NH4+ (Na/Al e 0.02, Table 1). Co-MOR samples are labeled as Co-MOR-x, where x specifies the analytical Co-exchange extent percent, calculated assuming that one Co balanced two Al atoms. The Na+ and the Co2+ content of 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 2 Co/Al ratios. The various Co-MOR samples were prepared by contacting a weighted amount (1.5 g) of Na-MOR with an aqueous solution of Co(CH3COO)2 (0.06 to 0.002 M) or Co(NO3)2 (0.008 M) for 6 h under stirring at 350 K or at 298 K, as specified in Table 1. To obtain extensively exchanged samples (x ) 61 or higher), we used a solution containing a Co2+ amount higher than that corresponding to the maximum exchange capacity of the MOR (2 Co/Al ) 1). In particular, for the Co-MOR-61 sample, the Co2+ amount in solution corresponded to 2 Co/Al ) 1.02, and for Co-MOR samples with x in the range of 73 to 104, the Co2+ amount in solution corresponded to 2 Co/Al ) 2.16 to 2.70. Ion exchange was repeated three times, each time by replacing the solution with a fresh portion having the same concentration. After the exchange procedure, all samples were thoroughly washed with distilled water and dried overnight at 383 K (asprepared samples).

The Co3O4/Co-MOR-92 sample was prepared by impregnating a portion of the Co-MOR-92 sample with a solution of Co(CH3COO)2. The sample was thereafter dried overnight at 383 K and calcined at 823 K for 5 h. 2.2. Characterization Techniques. As-prepared samples and samples heated in O2 (SOL, 99.9%) at 793 K for 1 h were characterized by means of UV-vis DRS spectroscopy. The UV-vis DRS spectra were recorded in the wavelength range of 200-2500 nm using a Varian Cary 5E spectrometer equipped with a computer for data acquisition and analysis (software Cary Win UV). Infrared spectra were run at RT on samples either (i) heated with O2 (100 Torr) at 793 K for 1 h and evacuated at the same temperature for 1 h (standard activated samples) or (ii) standard activated and heated with CO (75 Torr, SOL, 99.9%) at increasing temperature up to 623 K, for 5 min at each temperature, and in some experiments, as specified, up to 673 K (prereduced samples). Before recording spectra, prereduced samples were cooled in CO at RT. In one experiment, the sample was prereduced with 13CO (Stohler, 90% 13Cenriched CO) or with a mixture 12CO + 13CO (12CO:13CO = 1:1). 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 which allowed heating in vacuum or in a controlled atmosphere. For band integration and curve fitting, we used the software program “Curvefit in Spectra Calc.” (Galactic Industries). 3. Results and Discussion 3.1. Cobalt-exchange Process. The largest amount of sodium of Na-MOR we exchanged with cobalt was the same at 350 K and at 298 K (about 70% of total sodium), whereas the corresponding amount of cobalt we introduced was larger at 350 K than at 298 K (2 Co/Al ) 1.04 at 350 K and 2 Co/Al ) 0.74 at 298 K, Table 1). Due to the fact that under our conditions all sodium in Na-MOR was exchangeable with ammonium (see Experimental Section), the amount of sodium which could be exchanged with cobalt (70% of total sodium) must be limited by a structural constraint. Several authors17-20 suggested that the structural variable allowing the exchange of two monovalent ions with a divalent one was the Al-Al distance. The reason why we introduced a larger amount of cobalt at 350 K than at 298 K is that the cobalt species present in solution differ at the two temperatures. At 298 K, the main species in solution are

Isolated Co2+ and [Co-O-Co]2+ Species

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Figure 2. FTIR spectra of standard activated Na-MOR and Co-MOR in the OH stretching region. Samples: Na-MOR (spectrum 1), CoMOR-11 (spectrum 2), Co-MOR-23 (spectrum 3), Co-MOR-41 (spectrum 4), Co-MOR-61 (spectrum 5), Co-MOR-73 (spectrum 6), CoMOR-74 (spectrum 7), Co-MOR-89 (spectrum 8), Co-MOR-92 (spectrum 9), and Co-MOR-104 (spectrum 10).

Figure 1. UV-vis DRS spectra of Na-MOR and Co-MOR samples. Samples as-prepared (section a): Na-MOR (spectrum 1), Co-MOR-73 (spectrum 2), Co-MOR-74 (spectrum 3), Co-MOR-41 (spectrum 4), Co-MOR-61 (spectrum 5), Co-MOR-89 (spectrum 6), and Co-MOR104 (spectrum 7). Samples heated in O2 at 793 K (section b): NaMOR (spectrum 1), Co-MOR-41 (spectrum 2), Co-MOR-61 (spectrum 3), Co-MOR-73 (spectrum 4), Co-MOR-89 (spectrum 5), Co-MOR92 (spectrum 6), Co-MOR-104 (spectrum 7), and Co3O4/Co-MOR-92 (spectrum 8).

monomeric Co-aquo complexes, Co2+(H2O)n. At 350 K, in addition to Co2+(H2O)n, Co-hydroxo complexes, such as Co(OH)+ and Co2(OH)3+, form.21 Co-hydroxo complexes form in solution by hydrolysis of Co-aquo complexes.22 The UV-vis DRS spectra of the as-prepared Co-MOR ion exchanged at 298 K showed bands at 20000-22000 cm-1 typical of monomeric Co2+ octahedral aquo complexes (Figure 1a, spectra 2 and 3), similar to those previously observed by Deˇdecˇek et al. in CoH-MOR23 and Co-Na-MFI,24 ion exchanged at RT. Accordingly, in the Co-MOR samples ion exchanged at 298 K, the chemical analysis suggests that one Co2+ aquo complex replaces two Na+ ions, therefore yielding (2 Co + Na)/Al values = 1 (Table 1). The UV-vis DRS spectra of the as-prepared Co-MOR ion exchanged at 350 K, in addition to the bands of Co2+(H2O)6, showed a broad absorption above 25000 cm-1, whose intensity increased with the Co2+ content (Figure 1a, spectra 4-7). We assign this broad absorption to monomeric or oligomeric hydrated cobalt-hydroxo species or both. Accordingly, in the samples ion exchanged at 350 K, the chemical analysis suggests that Co(OH)+ or Co2(OH)3+, or both, exchanged a number of Na+ lower than that corresponding to 1 Co2+ per 2 Na+, therefore yielding (2 Co + Na)/Al > 1 (1.15 to 1.32, Table 1). Whereas the UV-vis DRS spectra of as-prepared Co-MOR samples ion exchanged at 298 K differed from those of as-prepared Co-MOR ion exchanged at 350 K, the UV-vis DRS spectra of the same samples heated in O2 at 793 K were nearly identical (Figure 1b, spectra 2-7). Hence, the type of cobalt species formed after heating in O2 did not depend on the cobalt species present in solution. After heating in O2 at 793 K, spectra

of all Co-MOR samples consisted of poorly defined bands at about 8000, 15000, and 22000 cm-1, typical of Co2+ in octahedral sites.25 The band intensity increased with the Co content (Figure 1b, spectra 2-7). In the Co3O4/Co-MOR-92 sample, the bands of Co2+ in octahedral sites were obscured by intense bands at 7000, 14000, and 22500 cm-1 (Figure 1b, spectrum 8), with all of these bands being typical of Co3O4.26,27 The FTIR analysis of the OH stretching region of the standard activated Co-MOR helps to clarify the Co-exchange process. In Co-MOR, whereas the intensity of the silanol band (νOH at 3746 cm-1)28 remained nearly unchanged, an unexpected Brønsted acid site band (νOH at 3610 cm-1)28 appeared, whose intensity increased with the cobalt content, reached a maximum on samples in which the Co-exchange extent was about 73%, and slightly decreased thereafter (Figure 2). Because Brønsted acid sites (-Si-OH-Al-) were absent in Na-MOR, their presence in Co-MOR was apparently due to the introduction of Co2+. This unexpected formation of Brønsted acid sites can be rationalized along the same lines we have previously proposed.6 Namely, the Co2+(H2O)n species exchanged with (-Al-ONaSi-) sites of Na-MOR, yielding (-Si-O--Al-) and [-AlO-Co2+(H2O)n-Si-]. Upon 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, (-SiOH-Al-) and [-Al-O-Co(OH)+ -Si-]. The percent amount of Co2+(H2O)n species which underwent hydrolysis can be calculated as [100 x (ICo-MOR/IH-MOR)], where ICo-MOR is the integrated intensity of the 3610 cm-1 band in Co-MOR and IH-MOR is that of the same band in the H-MOR sample. The percent amount of Co2+(H2O)n species which underwent hydrolysis was maximum on Co-MOR-73, on which [100 x (ICo-MOR/IH-MOR)] = 20. In agreement with our previous results,6 Ulla et al.29 observed by DRIFT spectroscopy the formation of (-Si-OH-Al-) and Co(OH)+ in one sample CoMOR, with Co/Al ) 0.24, prepared from Na-MOR. 3.2. CO Adsorption at RT on Samples Standard Activated. On Na-MOR, the adsorption of CO at RT yielded an intense band consisting of two components at 2176 and 2163 cm-1 and two weak bands at 2135 and 2112 cm-1 (Figure 3a, spectrum 1). After evacuation at RT, all these bands disappeared. These

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Figure 3. FTIR spectra of CO adsorbed at RT on standard activated Na-MOR and Co-MOR, after exposure to CO for 5 min (section a) and for 60 min (section b), with PCO ) 75 Torr. Samples: Na-MOR (spectra 1), Co-MOR-11 (spectra 2), Co-MOR-23 (spectra 3), Co-MOR41 (spectra 4), Co-MOR-61 (spectra 5),Co-MOR-73 (spectra 6), CoMOR-74 (spectra 7), Co-MOR-89 (spectra 8), Co-MOR-92 (spectra 9), and Co-MOR-104 (spectra 10).

bands have been previously observed on Na-MOR.6,28,30,31 In agreement with early assignments, we assign the bands at 2176 and 2163 cm-1 to CO adsorbed on Na+ located in the main channels (2176 cm-1) and in the side pockets (2163 cm-1)28 and the band at 2112 cm-1 to CO adsorbed Via the O atom on Na+ located in the main channels.30 In agreement with the early suggestion by Salla et al.,31 we ascribe the band at 2135 cm-1 to a multiple interaction of CO with two adjacent Na+ ions or Na+ and oxygen species. On all Co-MOR samples, the adsorption of CO at RT for a short time (5 min) yielded a band at 2205 cm-1, with a shoulder at 2191 cm-1, both bands arising from cobalt carbonyls. The intensity of these two bands increased with increasing cobalt content, and their position was independent of CO pressure and cobalt content, indicating that CO adsorbed on isolated Co2+ sites (Figure 3a, spectra 2-10). These Co2+ carbonyls were almost completely removed on evacuation at RT for 10 min. We previously assigned the band at 2205 cm-1 to the C-O stretching vibration of CO interacting with Co2+ located in the main channels and the band at 2191 cm-1 to CO interacting with Co2+ in the smaller channels.6 As already mentioned in the Introduction, on extensively exchanged Co-MOR samples (x g 61), Campa et al.6 detected extremely weak bands at 2114 and 2038 cm-1, in addition to bands of Co2+(CO). These authors6 tentatively assigned the weak bands at 2114 and 2038 cm-1 to Co carbonyls with cobalt in an oxidation state lower than two, which formed on the surface of Co3O4 traces, possibly present in the extensively exchanged Co-MOR samples.6 We now show that also on not extensively exchanged Co-MOR samples (x g 41), in addition to bands of Co2+(CO), CO exposure at RT for a much longer time (60 min) leads to the formation of bands at 2359, 2138, and 2089 cm-1 with a shoulder at 2079, 2114, and 2036 cm-1. All these bands were absent on Co-MOR-11 and Co-MOR-23 (Figure 3b, spectra 2 and 3), and their intensity markedly increased with the cobalt content (Figure 3b, spectra 4-10). We assign the weak band at 2359 cm-1 (not shown in Figure 3) to linear chemisorbed CO2 and those at 2138, 2089, 2079, 2114, and 2036 cm-1 to Com+(CO)n with m < 2. The type and the amount of all carbonyls formed upon CO adsorption at RT, after 5 min or 60 min, did not depend on (i) the cobalt-exchange temperature (compare spectrum 6 with spectrum 8 in Figure 3), (ii) the concentration

Indovina et al.

Figure 4. FTIR spectra of CO adsorbed at RT on Co-MOR-104, with PCO ) 75 Torr, as a function of the contact time with CO: 1 min (spectrum 1), 3 min (spectrum 2), 5 min (spectrum 3), 20 min (spectrum 4), 40 min (spectrum 5), and 60 min (spectrum 6).

Figure 5. FTIR spectra of CO adsorbed at RT on Co-MOR-89 at various equilibrium pressures. PCO ) 20 Torr (spectrum 1), 9 Torr (spectrum 2), 4 Torr (spectrum 3), 1.5 Torr (spectrum 4), 0.6 Torr (spectrum 5), 0.2 Torr (spectrum 6), 0.1 Torr (spectrum 7), and 0.01 Torr (spectrum 8), after CO evacuation at RT (spectrum 9, dotted line).

of the solution used in the exchange (compare spectrum 8 with spectrum 10 in Figure 3), and (iii) the salt used in the exchange, nitrate or acetate (compare spectrum 6 with spectrum 7 in Figure 3). The assignment of bands at 2138, 2089, 2079, 2114, and 2036 cm-1 to carbonyls of reduced cobalt species is consistent with their wavenumbers which are substantially lower than those of Co2+(CO) and with the fact that their intensity and that of CO2 increased in parallel as a function of the contact time with CO (Figure 4). During evacuation at RT, as the equilibrium CO pressure diminished, the intensity of bands at 2138, 2089, and 2079 cm-1 decreased, and the intensity of those at 2114 and 2036 cm-1 increased, with an isosbestic point at 2052 cm-1, indicating the presence of two interconvertible Com+(CO)n species (Figure 5). Bands with wavenumbers close to those observed here and having an analogous intensity dependence on the CO pressure have been previously detected by Hadjiivanov at al.16 on a CoH-MFI sample prereduced with CO at 673 K. By means of adsorption experiments with labeled-CO mixtures, Hadjiivanov at al.16 assigned these bands to Co+(CO)3 and Co+(CO)2. On the basis of these assignments,16 we assign the bands at 2138, 2089, and 2079 cm-1 to Co+(CO)3 species with a symmetry lower than C3V and those at 2114 and 2036 cm-1 to Co+(CO)2 (νsym ) 2114 cm-1 and νasym ) 2036 cm-1). The assignment of the 2114 and 2036 cm-1 bands to a Co+(CO)2 species in Co-MOR was further substantiated by performing adsorption experiments with labeled-CO (vide infra), analogous to those

Isolated Co2+ and [Co-O-Co]2+ Species

Figure 6. Experiments on prereduced samples. FTIR spectra of CO adsorbed on Co-MOR-41 (section a) and Co-MOR-89 (section b) with PCO ) 75 Torr, at various prereduction temperatures. Co-MOR samples were heated in CO from RT up to 623 K, kept for 5 min at each temperature, and cooled to RT, before recording spectra in the presence of CO. Section a: sample Co-MOR-41; CO at RT (spectrum 1), 573 K (spectrum 2), 596 K (spectrum 3), 613 K (spectrum 4), and evacuation at RT for 10 min (dotted-line spectrum). Section b: sample Co-MOR89; CO at RT (spectrum 1), 573 K (spectrum 2), 608 K (spectrum 3), 618 K (spectrum 4), 623 K (spectrum 5), and evacuation at RT for 10 min (dotted-line spectrum).

of Hadjiivanov et al. on Co-H-MFI.16 Evacuation for 10 min at RT completely and reversibly transformed Co+(CO)3 into Co+(CO)2, which remained stable under vacuum (Figure 5, spectrum 9). The stability of Co+(CO)2 in Co-MOR was analogous to that observed by Hadjiivanov et al.16 for Co+(CO)2 in Co-HMFI. These authors16 explained the high stability of Co+(CO)2 species in MFI in the same way as others32-34 explained the stability of Rh+(CO)2 in DAY32,35 or MFI,36 namely, the formation of stable 16- or 18-electron complexes. As mentioned previously, in Co-MOR-x samples with x g 41, exposed to CO at RT for a long time and evacuated at RT, Co+(CO)3 completely transformed into Co+(CO)2. The intensity ratio Iasym/Isym of Co+(CO)2 bands (integrated area νasym at 2036 cm-1/integrated area νsym at 2114 cm-1) did not depend on the Co content, being that Iasym/Isym ) 3.5 ( 0.1 on all samples. The θ angle between the two CO molecules, calculated as θ ) 2arctg xIasym/Isym,36 was 124° on all samples, somewhat higher than the 106° reported for Rh+(CO)2 in Rh+-DAY32 and the 101° reported for in Rh+-MFI,36 in agreement with the fact that the ionic radius of Co+ is smaller than that of Rh+. In Co-H-MFI, Hadjiivanov et al.16 speculated that the Co+ formed during the reduction with CO at 673 K originated from [Co-O-Co]2+. In Co-MOR, we suggest that upon adsorption of CO at RT, depending on the adsorption time and on the Co content, (i) a small fraction of [Co-O-Co]2+ reduces with CO at RT already, yielding the simultaneous formation of CO2 and [(CO)nCo+-Co+(CO)n], with n ) 2 or 3 and (ii) a large fraction of [Co-O-Co]2+ does not reduce, yielding [(CO)Co2+-OCo2+(CO)]. The Co2+-carbonyl band arising from [(CO)Co2+-

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Figure 7. FTIR spectra of the Co+(CO)2 species in the various CoMOR samples (section a), and the correspondent total integrated intensity of the νasym bands (2036 and 2000 cm-1) of both type I and type II Co+(CO)2 species (section b). The figure in parentheses specifies the spectrum in section a and the corresponding integrated intensity in section b for the various samples: Na-MOR (1), Co-MOR-11 (2), CoMOR-23 (3), Co-MOR-41 (4), Co-MOR-73 (5), Co-MOR-74 (6), CoMOR-89 (7), and Co-MOR-104 (8).

Figure 8. FTIR spectra of Co2+(CO) species and Co+(CO)2 species formed upon exposure of Co-MOR-104 to three labeled CO mixtures: pure 12CO (spectrum 1), pure 13CO (spectrum 2), and 12CO:13CO = 1:1 (spectrum 3). The sample was prereduced at 623 K with the three mixtures, cooled in the mixtures, and evacuated at RT. Asterisks indicate the two bands arising from Co+(12CO)(13CO), at 2088 and 2008 cm-1.

O-Co2+(CO)] occurs at nearly the same wavenumber as that of isolated Co2+ (vide infra), by far the most abundant species. 3.3. CO Adsorption at RT on Samples Prereduced with CO. Because [(CO)nCo+-Co+(CO)n] arose from [Co-OCo]2+, we could evaluate how the amount of [Co-O-Co]2+ depended on the Co amount by inspecting the band intensity of Co+(CO)n as a function of the prereduction temperature of the various Co-MOR samples. To this aim, we prereduced the

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Figure 9. Experiments on samples evacuated at increasing temperature. FTIR spectra of Co2+(CO) species and Co+(CO)2 species on Co-MOR samples prereduced in CO at 623 K (PCO ) 75 Torr), cooled in CO to RT, and evacuated at increasing temperature from RT up to various temperatures in the range 383-533 K, as specified. Section a: CoMOR-23 evacuated at RT (spectrum 1), 378 K (spectrum 2), and 388 K (spectrum 3). Section b: Co-MOR-41 evacuated at RT (spectrum 1), 368 K (spectrum 2), 373 K (spectrum 3), and 383 K (spectrum 4). Section c: Co-MOR-104 evacuated at RT (spectrum 1), 363 K (spectrum 2), 388 K (spectrum 3), 403 K (spectrum 4), 413 K (spectrum 5), and 428 K (spectrum 6), and in the inset, 428 K (spectrum 6), 453 K (spectrum 7), 513 K (spectrum 8), and 533 K (spectrum 9).

samples in CO at increasing temperature, up to 673 K. After maintaining the samples for 5 min at each temperature, we cooled them to RT, before recording spectra in the presence of CO. Heating Co-MOR-11 in CO up to 673 K yielded carbonyl bands whose wavenumber and intensity were identical to those of the standard activated sample, showing that cobalt did not reduce. Conversely, on all other Co-MOR samples, the same treatment yielded CO2, whose amount increased with Co content and with temperature, showing that all these samples underwent reduction. Heating also caused the carbonyl bands to change. In particular, as the prereduction temperature increased, (i) the band intensity of Co2+(CO) slightly decreased, reaching the minimum intensity at 623 K and remaining unchanged at higher temperature, (ii) the band intensity of Co+(CO)n markedly increased, reaching the maximum intensity at 623 K and decreasing at higher temperature, and (iii) a new band at 2000 cm-1 appeared, whose intensity paralleled that of Co+(CO)n. As an example, spectra of Co-MOR-41 and Co-MOR-89 prereduced at various temperatures up to 623 K are reported in Figure 6 (spectra after prereduction at higher temperature are not shown). To explain the decreased Co2+(CO) band intensity, we recall that this band consists of various components: an intense

Indovina et al.

Figure 10. Reversible interconversion of [Co+(CO)Co+] into Co+(CO)2. Section a: Co-MOR-23 prereduced in CO at 623 K and evacuated at 388 K (spectrum 1), after addition of small doses of CO at RT (0.05 µmol), in sequence, spectra 2-6 (equilibrium pressure,