Precise Identification of the Infrared Bands of the Polycarbonyl

Article pubs.acs.org/JPCC

Precise Identification of the Infrared Bands of the Polycarbonyl Complexes on Ni−MOR Zeolite by 12C16O−13C18O Coadsorption and Computational Modeling Hristiyan A. Aleksandrov,† Videlina R. Zdravkova,‡ Mihail Y. Mihaylov,‡ Petko St. Petkov,† Georgi N. Vayssilov,*,† and Konstantin I. Hadjiivanov*,‡ †

Faculty of Chemistry and Pharmacy, University of Sofia, Sofia 1126, Bulgaria Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria

‡

ABSTRACT: We report a combined computational and experimental IR study of polycarbonyl species on Ni−MOR zeolite formed after adsorption of 12C16O/13C18O isotopic mixtures, which allowed us to suggest a justified assignment of the vibrational bands in the complex IR spectra. For identification of all individual IR bands of the polycarbonyl complexes containing either the same or mixed isotopic probe molecules, we modeled the corresponding species with density functional method. In order to provide reliable computed values for comparison with experimentally measured vibrational frequencies of CO in the complexes, we combined periodic density functional calculations (with gradient corrected exchange-correlation functional) with isolated cluster calculations using hybrid functional. In this way, we confirm that the Ni+ cations in this zeolite material are able to form di- and tricarbonyl complexes and thus feature high coordinative unsaturation as suggested earlier.

1. INTRODUCTION One of the directions of the current catalysis research is the replacement of the heterogeneous catalysts based on platinum group metals currently employed in various industrial and environmental applications by more abounded and cheaper metals. Among the promising systems are materials based on coordinatively unsaturated cations of 3d transition metals such as Fe, Co, Ni, and Cu.1,2 Such coordinative unsaturation combined with the availability of the cations to the reactant molecules can be achieved in limited types of systems, for example, in cation-exchanged zeolites due to the rigidity of their framework compared to open surfaces of silica, alumina, or other oxide supports.1,3−5 The coordinative unsaturation of the metal cations in zeolites and on other supports can be determined by IR spectroscopy of small probe molecules, typically carbon monoxide.6 As the extensive literature in this field evidence, even with this simple diatomic molecule, one could have problems with the interpretation of the results by rationalization of the observed frequency shift of the CO stretching frequency via different oxidation states or different locations (local coordination) of the metal cation, formation of di- or polycarbonyls, and presence of other adsorption centers (as acidic hydroxyls, defects, etc.).6,7 These complications are partially clarified using isotopically labeled molecules and isotopic mixture 12C16O/13C16O. However, due to the small isotopic shift (ca. 50 cm−1), some individual bands of different carbonyl complexes overlap, and the proposed assignments are ambiguous. Here, we overcome this problem using a 12 16 C O/13C18O isotopic mixture, which features a higher isotopic shift, 100 cm−1, and thus allows better distinction of the bands corresponding to different types of polycarbonyl © 2012 American Chemical Society

complexes. However, although utilized for different purposes, C16O/13C18O isotopic mixtures have been scarcely used for determination of polycarbonyl structures. There are only few examples for matrix-isolated complexes8 and some of our recent works concerning surface species.9−12 The carbonyl chemistry of cationic nickel has been a subject of a steady interest. The achievements in the surface cationic nickel carbonyls up to 2002 have been summarized in ref 6. Briefly, it is considered that Ni3+ cations do not form carbonyls because they are strong oxidants. With supported Ni2+ species, CO forms carbonyls that are observed in the 2220−2180 cm−1 region. Due to the lack of back π donation, these carbonyls are unstable and decomposed at ambient temperature. Linear carbonyls of Ni+ are observed in the 2160−2130 cm−1 region. These species are much more stable as compared to Ni2+−CO because of the formation of a π bond and the synergism between the σ and π bonds. Often dicarbonyls are observed and characterized by νs at 2145−2131 cm−1 and νas at 2100−2081 cm−1. It was also reported13 that Ni+ ions in Ni−ZSM-5 can form at low temperature tricarbonyls (2156, 2124, and 2109 cm−1). Previously, we have reported that a significant fraction of Ni2+ sites in Ni−ZSM-5 can be converted to Ni+ after reduction with CO or hydrocarbons.13 This procedure has been used by different groups to produce Ni+ in Ni−ZSM-514−16 and Ni− BEA.17 In addition, Ni+ species have been detected with Ni− MOR,18 Ni−MCM-22,18 and Ni−MCM-41.19 12

Received: May 22, 2012 Revised: September 20, 2012 Published: September 21, 2012 22823

dx.doi.org/10.1021/jp304972u | J. Phys. Chem. C 2012, 116, 22823−22831

The Journal of Physical Chemistry C

Article

adsorption, CO and the CO isotopic mixtures were purified by passing through a liquid nitrogen trap. 2.3. Computational Method and Models. We carried out periodic DFT calculations with the PW91 exchangecorrelation functional22 using the Vienna ab initio simulation package (VASP).23,24 Ultrasoft pseudopotentials25,26 were used as implemented in the VASP package. Due to the large unit cell (see below) the Brillouin zone was sampled using only the Γ point.27 The valence wave functions were expanded in a planewave basis with a cutoff energy of 400 eV. All calculations were carried out in spin-polarized fashion. Ni+ and Ni2+ cations and complexes were located at a sixmembered ring, containing one and two Al centers, respectively, in mordenite structure (Figure 1a). The monoclinic unit cell of the zeolite framework was optimized for the pure silicate structure with dimensions a = b = 13.675 Å, c = 7.54 Å.28 This provides at least a 6.50 Å distance between planar complexes in two neighboring cells. For the larger

The goal of the present work is to clarify the complex spectra of the various di- and tricarbonyl species formed on nickel cations in Ni−MOR zeolites and to check the formation of tricarbonyl complexes of Ni+, suggested earlier.9 For identification of all individual IR bands of the polycarbonyl complexes containing either the same or mixed isotopic probe molecules, we modeled the corresponding species with the density functional method. Note that, although calculations according to the approximate force field model give good results for dicarbonyl species, they fail to predict unambiguously the spectral parameters of tri- and tetracarbonyls. In order to provide reliable values for comparison with experimentally measured vibrational frequencies for simulating the CO frequency shifts, we combined periodic density functional theory (DFT) calculations (with gradient corrected functional PW91) with isolated cluster calculations using a hybrid functional (B3LYP). The former calculation allows obtaining adequate location of the complex in the zeolite channels and properly accounts for both local and the long-range effects of the zeolite framework, while the latter calculations ensure the correct electronic density distribution in the complex.20,21 The reliability of this computational approach is confirmed (see below) by comparison with the experimental frequency shifts of mono- and homoligand dicarbonyl nickel complexes: the differences are below 10 cm−1. Thus, combining computational and experimental data, we report a justified assignment of the vibrational bands in the complex IR spectra of polycarbonyl species on Ni−MOR zeolite formed after adsorption of a 12 16 C O/13C18O isotopic mixture. In this way, we confirm that the Ni+ cations in this zeolite material are able to form di- and tricarbonyl complexes and thus feature high coordinative unsaturation.

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS 2.1. Materials. The starting Na−MOR material was supplied by Penta zeolite GmbH. The Ni−MOR sample was prepared by the conventional ion-exchange technique: 1 g of the parent zeolite was suspended in 50 mL of 0.1 M aqueous solution of Ni(NO3)2, and the mixture was stirred for 8 h at 363 K. The exchange procedure was repeated two times more after separation of the zeolite from the solution by centrifugation. The precipitate was filtered, washed thoroughly with deionized water, dried at 383 K, and calcined for 2 h at 723 K. According to chemical analysis, Ni−MOR contained 4.1 wt % Ni. 2.2. Techniques. Fourier transform IR (FTIR) spectra were recorded with a Nicolet Avatar 360 spectrometer accumulating 128 scans at a spectral resolution of 2 cm−1. Self-supporting pellets (ca. 10 mg cm−2) were prepared from the powdered sample and treated directly in a purpose-made IR cell allowing measurements at ambient and low temperatures. The cell was connected to a vacuum-adsorption apparatus allowing for the obtainment of a residual pressure below 10−3 Pa. Before CO adsorption, the sample was activated in situ in the IR cell by 1 h calcination at 673 K and 1 h evacuation at the same temperature (oxidized sample). To obtain a reduced sample, it was treated with CO (2 kPa) at 573 K for 10 min. Chemical analysis of nickel was performed with flame atomic absorption spectrometry. Carbon monoxide (>99.5% purity) used in IR experiments was supplied by Merck. The labeled carbon monoxide 13C18O (13C isotopic purity of 99% and 18O isotopic purity of 95%) was provided by Cambridge Isotope Laboratories, Inc. Before

Figure 1. Optimized structures in our periodic models: (a) general location of complexes in the cavity of the MOR structure, represented by Ni+(CO)2; local structures of (b) Ni+(CO), (c) Ni2+(CO), (d) Ni2+(CO)2, (e) Ni+(CO)3 complexes. Our isolated cluster models are also presented for CO complexes of (f) Ni+ cations, represented by Ni+(CO)3, and (g) Ni2+ cations, represented by Ni2+(CO)2. 22824



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Ni(CO)3+ and for the complexes of Ni2+ ion, we doubled the unit cell in the c direction (c = 15.08 Å) in order to reduce the interaction between the complexes in two neighboring unit cells and in the latter case to avoid location of Al centers at neighboring positions. Similarly to the real situation, one Si atom in the six-membered ring is replaced with an Al center, which results in a Si/Al ratio of the zeolite framework of 23 in the standard model and 47 in the double unit cell model. For the complexes of Ni2+ ions, two Al centers are present in the double unit cell model, which corresponds to a Si/Al ratio of the zeolite framework of 23. The negative charges of the zeolite framework around the Al centers are compensated by the Ni+ or Ni2+ cations or their complexes. During the geometry optimization, all the zeolite atoms and the adsorbate species were allowed to relax until the force on each atom was less than 2 × 10−4 eV/pm. The binding energy (BE) of the CO adsorbates per ligand is determined as

Since the calculated vibrational frequencies are harmonic and are affected both by the computational method and the model of the system,31−33 we employed a correction procedure for the calculated frequencies described below. All calculated values are initially shifted by the difference of the calculated harmonic frequency of the free CO obtained with the same computational approach and the experimentally measured anharmonic frequency of CO in gas phase, 2143 cm−1: νshifted = νcalculated − νcalculated(CO − gas) + 2143

In this way, the calculated values are corrected both for the anharmonicity (which is 38 cm−1 for gas phase CO) and the systematic error of the computational method. The final corrected vibrational frequencies νcorr for each type of complexes that are used for comparison with experimental values are calculated combining the shifted frequencies from the three model systems: νcorr = νshifted(PW91, periodic) + νshifted(B3LYP, fragment)

BE[M(CO)n x + /Zeo] = {−E[M(CO)n x + /Zeo]

− νshifted(PW91, fragment)

+ E[M x +/Zeo] + n × E[CO]}/n ,

This correction is in the spirit of the well-known ONIOM approach,34−36 which is originally applied for calculating the energy of the system.

(n = 1−3, x = 1 or 2)

where E[M(CO)nx+/Zeo] is the energy of the zeolite system together with the metal cation and adsorbed CO molecule(s) in the optimized geometry; and E[CO] and E[Mx+/Zeo] are the energies of the adsorbate molecule(s) in the gas phase and of the clean zeolite system together with the monovalent cation, respectively. With the above definition, positive values of BE imply a favorable interaction. The vibrational frequencies for periodic models were obtained from a normal-mode analysis where the elements of the Hessian were approximated as finite differences of analytical gradients, displacing each atomic center by 1.5 pm either way along each Cartesian direction. In order to check the sensitivity of the vibrational frequencies with respect to the position of the Ni2+ cation, we modeled also the coordination of the corresponding dicarbonyl complex to the oxygen centers of different rings, five-membered and eight-membered rings. Both the optimized structure of the complexes and the calculated vibrational frequencies at the three rings are very similar; the maximal difference is 6 cm−1. In order to correct the calculated frequencies as described below, we employed also isolated cluster models, using the PW91 exchange-correlation functional and the hybrid functional B3LYP29 with 6-311+G** type of basis sets for all atoms. These calculations were performed with the Gaussian09 program suit.30 Model clusters containing five and seven T atoms representing part of channel of the MOR structure are employed in modeling complexes of Ni+ and Ni2+, respectively (Figure 1f, g). As in the periodic calculations, the models contain one and two Al centers, respectively. In those isolated models, the dangling bonds were saturated by H atoms, and the direction of these bonds at the cluster boundary was fixed as obtained in the zeolite structure optimized from periodic calculations. The lengths of the corresponding O−H bonds were preoptimized, keeping all other atoms in the cluster fixed, followed by an optimization of the cluster with fixed positions of all saturating H atoms. In the subsequent geometry optimization steps, only the positions of the oxygen centers and the guest complex species were relaxed, while the positions of terminated H atoms and T atoms were fixed.

3. RESULTS AND DISCUSSION 3.1. Background Spectra. The background spectrum of the Ni−MOR sample recorded after activation (oxidized sample) contains in the OH region two bands with maxima at 3744 (with a low-frequency shoulder) and 3612 cm−1. According to data from the literature,37,38 these bands are assigned to the O−H vibrations of silanol groups and zeolitebridging hydroxyls, respectively. A very weak feature at around 3662 cm−1 can also be distinguished and is usually assigned to Al−OH groups.39 The sample background spectrum is hardly affected by reduction. Only the 3612 cm−1 band slightly increased in intensity, and the weak feature at 3662 cm−1 has slightly decreased. 3.2. Adsorption of CO on Activated Ni−MOR. Adsorption of CO (50 Pa equilibrium pressure) at 100 K on Ni−MOR (oxidized sample) results in the appearance of four main bands at 2213, 2174, 2160, and 2138 cm−1 (Figure 2). The 2213 cm−1 band has a pronounced shoulder at 2203 cm−1. The band at 2174 cm−1 is assigned to CO polarized by the zeolite-bridging hydroxyls, and the bands at 2160 and 2138 cm −1 are associated with carbonyls formed with the participation of residual Na+ ions.40,41 At higher coverages, CO polarized by Al−OH groups contributes to the absorbance around 2160 cm−1 and physically adsorbed CO to the peak at 2138 cm−1.6 The band at 2213 cm−1 is assigned to the Ni2+−CO species. The shoulder at 2203 cm−1 is very sensitive to evacuation, and when it declines, the band at 2213 cm−1 together with a component at 2220 cm−1 gains intensity (see the inset in Figure 2). This has been interpreted as conversion of di- (2203 cm−1) to monocarbonyls (2213, 2220 cm−1). After prolonged evacuation at ambient temperature, the Ni2+−CO band remained in the spectrum together with a very weak band at around 2164 cm−1. The latter can be assigned to a small amount of Ni+−CO species. 3.3. Adsorption of CO on Reduced Ni−MOR. Figure 3 compares the spectra of CO adsorbed at ambient temperature on oxidized and reduced (2 kPa, 573 K, 10 min) Ni−MOR. At 22825



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with CO-reduced sample, we reactivated the sample in order to minimize the amount of metal nickel. To establish the maximum number of CO molecules that can be adsorbed on one Ni+ site, we have studied the lowtemperature adsorption of CO on the reduced sample. Carbon monoxide (50 Pa equilibrium pressure) was introduced into the IR cell at 100 K, and then, the sample was progressively evacuated. At low coverages (Figure 4h, i), there are four

Figure 2. FTIR spectra of CO adsorbed at 100 K on oxidized Ni− MOR. (a) Equilibrium CO pressure of 50 Pa and (b−i) evolution of the spectra during evacuation at 100 K. The inset shows the difference (a − b) spectrum.

Figure 4. FTIR spectra of CO adsorbed at 100 K on reduced Ni− MOR. (a) Equilibrium CO pressure of 50 Pa and (b−f) evolution of the spectra during evacuation at 100 K and (g−i) increasing temperatures.

carbonyl bands in the spectrum: Ni2+−CO at 2213 cm−1, Ni+(CO)2 at 2139 and 2093 cm−1, and Ni+−CO at 2109 cm−1. The band at 2109 cm−1 develops during the coverage decrease at the expense of the bands at 2139 and 2093 cm−1 that indicates conversion of di- to monocarbonyls. However, analysis of the spectra shows that the dicarbonyl bands reach a maximal intensity at medium coverages (Figure 4g). Therefore, at high coverages, the dicarbonyl species are converted into polycarbonyls. According to literature data,13,17,41 tricarbonyls of Ni+ on Ni−ZSM-5 are characterized by three bands at 2156, 2124, and 2109 cm−1 that shows that their symmetry is lower than C3v. Although the analysis of the spectra at high coverages in our case is hindered because of the appearance of carbonyls formed with OH groups and Na+ cations (as in the oxidized sample), it is well seen that bands around these frequencies (2158, 2125−24, and 2112−10 cm−1) are formed in the spectra. There are no data suggesting intensity decrease of these bands at higher coverage, that is, no tetracarbonyls are formed. A peculiarity in our case, as compared to the reported spectra with Ni−ZSM-5,13,17 is that all dicarbonyls are converted, at high coverage, into tricarbonyls. 3.4. Adsorption of CO Isotopic Mixtures on Reduced Ni−MOR. In order to obtain spectral evidence of the di- and tricarbonyls structures, we have studied adsorption of CO isotopic mixtures. In this case, we utilized mixtures of 12C16O and 13C18O because, as already reported,9,11,44 these mixtures provide a better spectral resolution as compared to the widely used 12C16O/13C16O mixtures. Also, to obtain more information on the spectral performance of the mixed-ligand complexes, we have utilized two isotopic mixtures with 12 16 C O/13C18O molar ratios of 1:1 and 3:1, respectively.

Figure 3. FTIR spectra of CO (2 kPa equilibrium pressure) adsorbed at ambient temperature on Ni−MOR: (a) oxidized and (b) reduced sample.

these conditions, the principal carbonyl band in the spectrum registered with the oxidized sample was that of Ni2+−CO (2213 cm−1) (Figure 3a). The intensity of this band was lower with the reduced sample (Figure 3b). At the same time, two other intense bands are registered at 2138 and 2093 cm−1. These bands are assigned to the symmetric and antisymmetric CO modes, respectively, of Ni+(CO)2 species.6,14,17,23 Thus, the results clearly demonstrate that the reduction with CO has led to formation of a fraction of Ni+ cations at the expense of Ni2+ sites. A small amount of adsorbed Ni0(CO)4 is also observable (weak bands at 2030 and 1985 cm−1).42,43 Therefore, some of the cationic nickel has been reduced to metal. This accounts for the enhanced intensity of the band due to bridging hydroxyls in the reduced sample. There are two possibilities of the reduction mechanism, direct reduction of nickel ions or disproportionation of Ni+ to Ni2+ and Ni0.17 Further reduction with CO at the same conditions led to formation of more Ni(CO)4 evidenced by bands at 2130, 2070, 2058, 2030, 1998, and 1988 cm−1 (spectra not shown).9,14 That is why, before each experiment 22826



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The spectra of a 1:1 12C16O/13C18O isotopic mixture adsorbed on reduced sample are presented on Figure 5. Let

Ni+(12C16O)(13C18O), and Ni+(13C18O)2 species should be 1:2:1, that is, the bands due to mixed-ligand species should be most intense. Analysis of the spectra shows that indeed bands at 2124 and 2016 cm−1 are observed in the spectra. The observed frequencies are 3−4 cm−1 higher than the calculated ones with the AFF model. Similar small deviations are often observed using this CO isotopic mixture.9,11,43,44 A useful approach when analyzing the spectra of adsorbed isotopic mixtures is comparison with simulated “monocarbonyl” spectra.9 They represent the sum of spectra registered after 12 16 C O and 13C18O adsorption divided by two. In fact, the spectra of the isotopes were obtained by shifting the spectrum of CO along the x axes by the corresponding isotopic shift factor. Especially for the spectrum of 13C18O, we have used an experimentally obtained value for the isotopic shift (0.9535) that was slightly different from the calculated value (0.9531). We have also considered the extinction coefficients of 12C16O and 13C18O to be equal, although they slightly differ. If only monocarbonyls are formed, the simulated and registered spectra should practically coincide. Therefore, any deviations from the simulated spectra should be associated with bands characterizing polycarbonyl species. In more details, the bands due to complexes with different CO ligands are not present in the simulated spectra while the bands due to polycarbonyls with equal ligands should appear with reduced intensity in the real spectra. A comparison between the registered and simulated spectra for coverages at which dicarbonyls of Ni+ are mainly expected is shown in Figure 6. It is clearly seen that the bands due to

Figure 5. FTIR spectra of CO and 13C18O (molar ratio 1:1) coadsorbed at 100 K on reduced Ni−MOR. (a) Equilibrium pressure of 50 Pa and (b−g) evolution of the spectra during evacuation at 100K and (h−j) increasing temperatures.

us first consider low coverages (spectra h−j) where mono- and dicarbonyls are expected. Unfortunately, the Ni+−CO band cannot be clearly detected because it is masked by the strong band due to the Ni2+−13C18O species at 2110 cm−1 (see Table 1). Calculations show that the Ni+−13C18O band should be Table 1. Spectral Performance of Nickel Carbonyls in Ni− MOR species Ni −CO Ni2+(CO)2 Ni+−CO Ni+(CO)2 Ni+(CO)(13C18O) Ni+(13C18O)2 Ni+(CO)3 2+

Ni+(CO)2(13C18O) Ni+(CO)(13C18O)2 Ni+(13C18O)3 a b

ν(12C16O), cm−1 2213 2203 2109 2138, 2094 2124 (2121b) − 2158, 2124, and 2111 (2149, 2124) 2161, 2139 (2149, 2124) −

ν(13C18O), cm−1 calculated with AFFa 2109 2100 2010 − 2012 2038, 1996 −

ν(13C18O), cm−1 experimental 2110 2101 2011 2016 2040, 1998

− −

2026 2041, 1998

2057, 2024, and 2012

2058, 2025, and 2014

Figure 6. (a) FTIR spectrum of a 1:1 CO/13C18O isotopic mixture adsorbed at ambient temperature on reduced Ni−MOR. The spectrum is registered after introduction of the isotopic mixture (50 Pa equilibrium pressure) to the sample at 100 K, followed by prolonged evacuation at the same temperature. (b) Simulated spectrum (for details see text).

On the basis of the isotopic shift factor (0.9531) and AFF model. Calculated frequency (with AFF model).

observed at 2010 cm−1. Indeed, at very low coverages (Figure 5j), a band at 2011 cm−1 is dominating in the 2100−1900 cm−1 region. Approximate force field (AFF) model45 predicts that six bands should be observed for dicarbonyl structures: at 2138 and 2093 cm−1 for Ni+(CO)2 (these bands were already observed after adsorption of CO on the reduced sample); at 2038 and 1996 cm−1 for Ni+(13C18O)2; and at 2121 and 2012 cm−1 for the Ni+(12C16O)(13C18O) mixed-ligand species. In addition, the molar ratio between the Ni + ( 12 C 16 O) 2 ,

Ni+(12C16O)2 (2138 and 2094 cm−1) and Ni+(13C18O)2 species (2040 and 1998 cm−1) appear with reduced intensity as compared to the simulated spectra while two new bands for mixed-ligand species at 2124 and 2016 cm−1 are observed. This confirms the assignment of the bands at 2138 and 2094 cm−1 to dicarbonyl species. Calculation of the frequencies of tricarbonyls species on the basis of the AFF model is not easy and not unambiguous. Therefore, in this case, the analysis should be based on the 22827



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spectra. This is complicated because of the appearance of too many bands, some of them superimposing. One valuable approach is to compare the real and simulated spectra in order to distinguish the bands due to polycarbonyls. The registered and simulated spectra of a 1:1 CO/13C18O isotopic mixture adsorbed on reduced Ni−MOR at high coverages (where the

Figure 8. FTIR spectra of CO and 13C18O (molar ratio 3:1) coadsorbed at 100 K on reduced Ni−MOR. (a) Equilibrium pressure of 100 Pa and (b−i) evolution of the spectra during evacuation at 100 K and (j−l) increasing temperatures.

Figure 7. (a) FTIR spectrum of a 1:1 CO/13C18O isotopic mixture adsorbed on reduced Ni−MOR. The spectrum is registered after introduction of 100 Pa of the isotopic mixture to the sample at 100 K, followed by a brief evacuation at the same temperature. (b) Simulated spectrum (for details see text).

tricarbonyl species dominate) are shown in Figure 7. The main conclusions that can be made are as follows: • Bands at 2159 and 2124 cm−1 are indeed due to a Ni+(12C16O)3 species. The band at 2111 cm−1 of these tricarbonyls is masked by the strong band of Ni2+−13C18O. The respective bands for Ni+(13C18O)3 species are at 2058, 2025, and 2014 cm−1. • Bands at 2149, 2041, ∼2025, and 1998 cm−1 are due to mixed-ligand tricarbonyls. Another useful approach is to use isotopic mixtures with different molar ratios. In this case, the relative concentration between the different species will be different. In particular, the formation of Ni+(CO)3 and Ni+(CO)2(13C18O) species should be favored when the CO/13C18O molar ratio is higher than 1. The spectra of a 3:1 CO/13C18O mixture adsorbed on reduced NiMOR sample are presented in Figure 8, while Figure 9 presents a comparison between the spectra of two adsorbed isotopic mixtures (with 1:1 and 3:1 ratios). Analysis of the results leads to the following conclusions: • The bands around 2041 and 1998 cm−1 are more intense after adsorption of a 1:1 isotopic mixture, and thus, they can be attributed to Ni+(12C16O)(13C18O)2 species (Figure 9). • The band at 2139 cm−1 is roughly of the same intensity in the two spectra presented in Figure 9. Part of the band was already assigned to Ni+(CO)2 dicarbonyls. However, the concomitant band at 2094 cm−1 is definitely more intense in the spectra registered after adsorption of a 3:1 isotopic mixture. Therefore, a component of the band at 2139 cm−1 should also be assigned to the Ni+(12C16O)(13C18O)2 species.

Figure 9. FTIR spectra of CO/13C18O isotopic mixtures adsorbed on reduced Ni−MOR. The spectra are registered after introduction of the isotopic mixture (50 Pa equilibrium pressure) to the sample at 100 K, followed by a brief evacuation at the same temperature. CO to 13C18O molar ratio of (a) 1:1 and (b) 3:1.

• On the contrary, a band at 2026 cm−1 appears with almost the same intensity in the two spectra (Figure 9). In the spectrum of an adsorbed 1:1 mixture, Ni+(13C18O)3 species contribute to this band. However, after adsorption of a 3:1 mixture, these species are expected in negligible concentration, and therefore, a component of the band should be assigned to Ni+(12C16O)2(13C18O) species. It is seen that, even using different approaches, we were not able to distinguish unambiguously all bands characterizing mixed ligand species. For that reason, we compared the spectra with the values obtained on the basis of the DFT calculations. 3.5. Comparison with Quantum Chemical Calculations. While in hydrated or dehydrated FAU zeolites, Ni2+ is solvated by six oxygen atoms from the zeolite and/or water molecules;46 in the dehydrated MOR zeolite, the bare Ni2+ is coordinated to four zeolite oxygen centers. After addition of two CO ligands to this cation, during the geometry 22828



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Table 2. Comparison between Corrected Calculated and Experimental CO Frequencies (in cm−1) in the Carbonyl Complexes of Ni+ and Ni2+ complexes +

Ni (CO) Ni+(CO)2

Ni+(CO)3

Ni2+(CO) Ni2+(CO)2

a

ligands 12 16

C O 13 18 C O 212C16O 213C18O 12 16 C O/13C18O 12 16 3 C O 313C18O 212C16O/13C18O 12 16 C O/213C18O 12 16 C O 13 18 C O 212C16O 213C18O 12 16 C O/13C18O

νcorr(C−O)

intensitya

νexp(C−O)

Δ(νexp − νcorr)

2107 2007 2139, 2090 2036, 1990 2126/2001 2158, 2116, 2104 2055, 2016, 2004 2153, 2111/2014 2130/2032, 2012 2224 2117 2222, 2202 2117, 2099 2223/2097

581 531 318, 712 290, 652 459/531 211, 414, 667 192, 379, 611 330, 478/450 396/374, 456 267 244 320, 200 292, 183 283/216

2109 2013 2138, 2094 2040, 1998 2124/2016 2158, 2124, 2111 2061, 2025, 2015 2149, 2124/2026 2139/2041, 1998 2213 2110 -, 2203 -, 2101 -,-

2 6 −1, 4 4, 8 −2/15 0, 8, 7 6, 9, 11 −4, 13/12 9/9, −14 −11 −7 -, 1 -, 2 -,-

Calculated intensity of the band in km/mol, obtained from the isolated fragments at the B3LYP level.

experimental IR CO bands in Ni+(12C16O)2(13C18O) are 2149, 2124, and 2026 cm−1, and they fit to the calculated frequencies within 4−13 cm−1. The second frequency could be also ∼2109 cm−1, coinciding with the 12C16O frequency for Ni+(12C16O). By using the same analysis, the experimental CO bands corresponding to Ni+(12C16O)(13C18O)2 should be 2139, 2041, and 1998 cm−1, and they fit to the calculated frequencies within 9−14 cm−1. Alternatively, the last frequency can be also ∼2014 cm−1, and in this case, it would coincide with the 13C18O stretching mode in the Ni + ( 13 C 18 O) or Ni + ( 13 C 18 O) 3 complexes. 3.6. Stability of the Complexes. The calculations allowed us to estimate the stability of the formed complexes. As expected, due to the combined electrostatic, σ and π bonding, CO is bound stronger to Ni+, BE(CO) = 177 kJ/mol, than to Ni2+, where the CO coordination is dominated by electrostatic and σ bonding, BE(CO) = 125 kJ/mol. The BE of the second CO molecule to the monocarbonyl complexes is significantly lower than the first one, 81 and 39 kJ/mol at the Ni+ and Ni2+ ion, respectively. The BE of the third molecule to the Ni+(CO)2 complex is reduced further to 72 kJ/mol. According to these results, one would expect that, upon evacuation of a sample containing both Ni+ and Ni2+ cations and completely saturated by CO, first Ni2+(CO)2 complexes lose one ligand and form monocarbonyls Ni2+(CO); after that consequently, the Ni+(CO)3 complex is transformed into dicarbonyls and monocarbonyls, and finally, decomposition of Ni2+(CO) complex occurs followed by decomposition of Ni+(CO). The calculated binding energies of CO in di- and tricarbonyl complexes of Ni+ suggest that the formation of tricarbonyls from dicarbonyls is energetically favorable by 72 kJ/mol. This energy preference is sufficient to counteract to the entropy loss due to formation of the tricarbonyls via adsorption of CO on the Ni+(CO)2 dicarbonyl, ΔS = −0.15 and −0.20 kJ/mol·K for p(CO) of 104 and 10 Pa, respectively, which corresponds to the contribution of the entropy term in ΔG at 100 K of 15 and 20 kJ/mol, respectively. This conclusion is in agreement with the experimental observation reported above that, in the Ni−MOR sample, the dicarbonyls are completely transformed into tricarbonyls. However, this was not the case for the Ni+ carbonyls in Ni−ZSM-5, where according to earlier studies,13,17 part of the dicarbonyls were resistant to transformation in tricarbonyls. According to the estimated binding energy of CO

optimization, the whole dicarbonyl complex shifts in order to allow coordination of both CO ligands in the preferable shape of the complex, which leads to reduction of the coordinated oxygen atoms from the zeolite to only two (see Figure 1d, g). The calculated stretching vibrational frequencies of CO in the Ni+ and Ni2+ monocarbonyls of 12C16O and 13C18O (Table 2) differ by 2−11 cm−1 from the experimental values; the largest difference is for the Ni2+(12C16O) complex. The calculated lower CO frequency modes in the homoligand dicarbonyls Ni2+(12C16O)2 and Ni2+(13C18O)2, 2202 and 2099 cm−1, differ by only 1−2 cm−1 from the experimental values. The calculated vibrational frequencies of the higher frequency modes essentially coincide with the values for the monocarbonyl complex with the same ligand, and by this reason, the corresponding bands cannot be clearly distinguished in the spectra. In the mixed-ligand dicarbonyl complex Ni2+(12C16O)(13C18O), the CO frequencies are calculated to be 2223 and 2097 cm−1, which essentially coincide with the higher and lower frequency bands of the homoligand complexes Ni2+(12C16O)2 and Ni2+(13C18O)2, respectively, 2222 and 2099 cm−1. Due to this coincidence, the frequencies of the mixed-ligand dicarbonyl complex Ni2+(12C16O)(13C18O) cannot be proposed experimentally; the experimentally observed bands at 2213 and 2101 cm−1 are assigned to the monocarbonyl or homoligand dicarbonyl complexes.13 The frequencies for both homoligand Ni+(CO)2 dicarbonyl complexes are also close to the measured values, within 8 cm−1, while for the mixed Ni+(12C16O)(13C18O), the discrepancy with experimental data become larger, 2 and 15 cm−1, respectively for the calculated bands at 2126 and 2001 cm−1. Using this agreement, we used the calculated values to assign all individual frequencies corresponding to Ni+ tricarbonyl complexes with different ratio of the two isotopic CO molecules. The calculated values for the four complexes (Table 2) are also close to bands observed in the experimental spectra. Since in some cases experimental IR bands cannot be unequivocally determined, the calculated frequencies helped us to solve this problem. The calculated frequencies for the Ni+(C12O16)3 complex, 2158, 2116, and 2104 cm−1, and the Ni+(C13O18)3 complex, 2055, 2016, and 2004 cm−1, agree within 0−8 and 6−11 cm−1 with experimental results. From our calculations, one can conclude that the most probable 22829



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Table 3. Binding Energies per CO Ligand and Selected Structural Data for the Carbonyl Complexes of Ni+ and Ni2+ from the Periodic Models complex +

Ni (CO) Ni+(CO)2 Ni+(CO)3 Ni2+(CO) Ni2+(CO)2

NSa

BEb

1 1 1 2 0

177 129 110 125 82

R(Ni−O)c 210, 206, 212, 203, 193,

213 211 232 206, 208, 209 193

R(Ni−C)c

Δ(C−O)d

178 181, 182 185, 186, 186 187 178, 179

1.2 0.7, 0.8 0.4, 0.5, 0.5 −0.2 −0.1, −0.1

C−Ni−Ce 87 87, 102, 113 92

a

Number of unpaired electrons. bBE values per CO ligand in kJ/mol. cNi−C and Ni−O distances in pm. dDifference (in pm) between CO in the corresponding complex and in the gas phase CO. eAngle C−Ni−C.

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in the complexes, the reason for this difference is not connected with the stability of the tricarbonyls but likely with specific steric restrictions in some particular positions of the complexes in Ni−ZSM-5 channels. From the obtained energy and entropy values, one can also tentatively estimate that, at the lower pressure, the tricarbonyls are transformed into dicarbonyls around 350 K, while at the higher pressure, such a process occurs above 450 K. The variation of the C−O distance in the complexes completely follows the analysis of the type of bonding. Due to the combined electrostatic, σ and π bonding of CO to Ni+, the C−O bond extends by 1.2 pm in Ni+(CO) with respect to C−O distance in the gas molecule. This C−O elongation decreases to 0.7−0.8 pm and 0.4−0.5 pm when the second and third CO molecule is adsorbed, respectively. On the other hand, when a CO molecule is adsorbed to the Ni2+ cation, C− O bonds are slightly shortened by −0.2 pm and −0.1 pm in Ni2+(CO) and Ni2+(CO)2 complexes, due to electrostatic and σ bonding (see Table 3).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]fia.bg (G.N.V.); [email protected] (K.I.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support by Bulgarian National Science Fund (contracts DO02-184/08 and DCVP 02/2) is gratefully acknowledged. H.A.A., P.St.P., and G.N.V. are grateful also to FP7 project BeyondEverest.

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4. CONCLUSIONS In order to clarify the type of the carbonyl complexes formed on Ni−MOR zeolite after different treatments, we performed FTIR study of these samples using 12C16O/13C18O isotopic mixtures. Due to the complexity of the experimental IR spectra of various mono- and polycarbonyl species, these experimental studies were complemented by computational modeling that allowed us to assign the experimentally observed vibrational bands. The calculations were based on density functional method and combined periodic models with gradient corrected exchange-correlation functional and isolated fragments of the zeolite framework calculated with a hybrid functional. In this way, we identified mono-, di-, and tricarbonyls of Ni+ cations and mono- and dicarbonyls of Ni2+ cations in this zeolite. The successful assignment of the vibrational bands in the IR spectra of polycarbonyls containing the same or different isotopes confirms the crucial advantages of the 12C16O/13C18O isotopic mixture in identification of such species. The calculated binding energies of CO in di- and tricarbonyl complexes of Ni+ suggest that the formation of tricarbonyls from dicarbonyls is energetically favorable. According to the relative stability of different complexes obtained from the calculations, upon evacuation of a sample containing both Ni+ and Ni2+ cations and completely saturated by CO, first Ni2+(CO)2 complexes lose one ligand and form monocarbonyls Ni2+(CO); after that consequently, the Ni+(CO)3 complex is transformed into dicarbonyls and monocarbonyls, and finally, decomposition of the Ni2+(CO) complex occurs followed by decomposition of Ni+(CO). The same order is followed in the experimental FTIR study. 22830



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Precise Identification of the Infrared Bands of the Polycarbonyl

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