Linkage Isomerism of CO Adsorbed on Alkali Halides - The Journal of

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Linkage Isomerism of CO Adsorbed on Alkali Halides Roman Belykh, Maria Maevskaya, Irina Krauklis, and Alexey Tsyganenko* Department of Physics, Saint-Petersburg State University, 1 Ulianovskaya Str., Peterhof, Saint-Petersburg 199034, Russia ABSTRACT: Variable-temperature FTIR spectra of CO adsorbed on NaCl and KBr films are shown to reveal linkage isomerism, that is, formation of C- and O-bonded adsorption complexes with the same cationic sites. Not all of the sites can be involved in that, only the three- or four-coordinated cations located on kinks, edges, or steps of microcrystals. For NaCl, these adsorption complexes account for the high-frequency (HF) C−O stretching bands at 2175 and 2160 cm−1. Coresponding LF bands of O-bonded CO were found at 2115 and 2124 cm−1. In the spectrum of KBr, the HF band is poorly resolved, and only one LF band can be clearly seen, near 2124 cm−1. The value of the isomerization enthalpy for complexes that account for the bands at 2160 and 2124 cm−1 of CO adsorbed on NaCl, estimated from the experiment, is 4.0 ± 0.2 kJ/mol. Quantum mechanical calculations by DFT methods applied to NaCl model clusters interacting with the CO molecule confirm the existence of two potential wells with adsorption and isomerization energies close to the experimentally measured values, and enable us to estimate the height of the potential barrier between the two adsorption states.

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the lone pairs of C and O atoms.5,7 Therefore, it forms two kinds of linear complexes with the cations, while with the anions side-on (T-shaped) complexes are energetically more favorable. In zeolites with high Si/Al ratio, weak negative charge is delocalized over the framework, the electric field of the cations is only partially neutralized by the nearest environment, and interaction of the CO molecule with those cations is almost the same as that with a bare cation. For X zeolites, where the negative charge of the framework is higher, as well as for metal oxides, interaction of CO with oxygen anions is stronger. This leads to the lowering or even disappearance of the potential barrier between the two isomeric states, and linkage isomerism in such systems is not possible any more. Hence, linkage isomerism can be anticipated for systems with a weaker field of anions. Besides the compounds with polyanions, these can be metal halides where anions have lower charge and greater radii, as compared with corresponding oxides. Thus, the aim of our work was to study the spectra of CO adsorbed on alkali-metal halides at variable temperatures in order to find out the manifestations of isomeric states. IR spectra of molecules adsorbed on sublimated alkali and alkali-earth halide films were first studied by Kozirovski and Folman.9−12 Making use of relatively low sublimation temperatures of alkali and alkali-earth halides, they prepared porous films by vacuum deposition of the salt onto transparent substrate of the same material. Unlike oxide adsorbents, the

INTRODUCTION Linkage isomerism of adsorbed molecules was first observed for CO on Y and ZSM-5 zeolites with alkali-metal cations.1−5 The CO molecule was shown to form on the same adsorption site two kinds of complexes, via carbon or via an oxygen atom. The two isomers coexist in thermodynamic equilibrium and account for two bands of CO stretching vibrations, shifted, respectively, to higher (HF band) and lower (LF band) wavenumbers, as compared with the frequency of the free CO molecule in a gas phase. The adsorption energy of the isomers is not the same; the difference is the isomerization energy, which varies from about 1.5 kJ/mol for CO molecules hydrogen-bonded to silanol groups of silica6 up to 10 kJ/mol or more for complexes with doubly charged cations, such as Ca2+ or Sr2+, according to refs 7 and 8, respectively. The phenomenon of isomerism of adsorbed species could have a great importance for catalysis because the energetically less favorable isomer has chemical properties different from those of the usual one and possesses an excess of energy, which could be used to pass the activation barriers. It can be thus considered as an activated intermediate state in catalytic reactions. This inspired us to conduct an extensive search of systems with linkage isomerism of adsorbed CO and other molecules using variable-temperature FTIR spectroscopy. However, no isomerism was found for CO adsorbed either on X zeolites having the same alkali-metal cations or on metal oxides. To find out the factors that account for the existence of two potential wells, a simple electrostatic model was used. According to it, the CO molecule is a linear quadrupole with a positive charge in the center and unequal negative charges of © XXXX American Chemical Society

Special Issue: Markku Räsänen Festschrift Received: July 23, 2014 Revised: September 15, 2014

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described device for film preparation was used,21 attached to the low-temperature vacuum cell described elsewhere.22 Some experiments were carried out with the support kept at ambient temperature. The specific surface area of NaCl films prepared at such conditions is about S = 50 m2/g.10 Much more disperse films, with S ≈ 300 m2/g according to ref 10, can be prepared by depositing on the cold support. To this purpose, a modified construction was used, where the sample holder for deposition was placed in a copper appendix cooled by liquid nitrogen. After deposition, the cold support was moved quickly to the bottom of the cell preliminarily also cooled down to 77 K. Band intensities of CO adsorbed in similar conditions confirm that in the latter case, the surface area of films was about twice higher than that of films prepared at ambient temperature. The above procedure enabled us to perform adsorption and to register spectra without contact of the newly prepared samples with air. The films were very transparent (A ≤ 0.5), and their thickness was not limited by absorption or scattering. The attempts to prepare a thicker film so as to see better weak bands of M−OC isomers led to its detaching from the support. Thicker samples could be prepared by the traditional method of pellets pressed in the air from a fine powder of the salt. The powder was prepared by the same way of deposition in vacuum on the cold part of Dewar vessel that was then opened; the deposit was scratched off, gathered, and used for pressing the pellet. In this way, the contact with the air moisture could not be avoided, and the pellets had to be cleaned by heating in vacuum at 65 °C to remove most of the adsorbed water. In this case, thick samples could be obtained, but the bands of adsorbed CO were not much more intense, apparently because of the decreased surface area. The spectra of cooled samples were recorded in the presence of about 0.2 Torr of helium to provide thermal contact of the support with the cold part of the cell. The spectra were collected before adsorption and upon admitting CO by small portions at 77 K. Then, liquid nitrogen was removed, and a series of spectra was obtained upon raising the temperature in the cell with simultaneous pressure monitoring. After desorption of most of the CO, part of the gas was removed, the cell was cooled again, and another series of spectra was registered. Three such series were usually done for the analysis of thermodynamics. Spectra were registered by a Nicolet 710 FTIR spectrometer with 2 or 4 cm−1 resolution. For the analysis of band intensities, spectral contours were fitted with Lorentzian curves by the MagicPlot software. Several theoretical methods can be used to describe interactions between molecules and surfaces. The most efficient DFT method can be applied to the slab models with periodic boundary conditions or to finite cluster models of the surface. The cluster approach is known as best suited for describing local phenomena such as interactions on catalytically active sites, while the slabs are good for regular faces of single crystals.23 Because in our work we deal with highly dispersed materials with special attention to defect sites, we have chosen the former, making use of the advantage of its comparatively low computational cost and the vast choice of method variations and bases. DFT cluster calculations were performed using Gaussian 0924 provided by the Resource Center “Computer Center of SPbU”. Crystallographic data for NaCl were acquired from http://nanocrystallography.org/.

films are transparent in a wide spectral region and have specific surface area of hundreds of m2/g, (even higher when sputtered upon the support cooled with liquid nitrogen). Among other gases, the authors have registered spectra of CO adsorbed at low temperatures on NaCl, NaBr, NaI,10,11 CsCl, CsBr, and CsI12 at different surface coverages. According to Gevirzman and Kozirovski,12 bands of CO appear as a doublet, with its components at lower and higher wavenumbers with respect to the frequency of a free CO molecule in the gas phase. For CO/CsCl, the frequency shifts and adsorption heats were calculated for different adsorption centers and orientations of the CO molecule. A good agreement with experimental data was found if the two bands were attributed to molecules adsorbed perpendicular or parallel to the surface. Another interpretation was suggested by B. Rao and M. J. Dignam,13 where the authors have shown the same data to fit quantitatively if the CO band is supposed to split due to the influence of the electrostatic field on the adsorbed molecules with lowered local symmetry in the case of the planar orientation. Ewing et al.14,15 have studied IR spectra of CO on NaCl films before and after annealing and observed a sharp strong band at 2154 cm−1 and a weaker one at 2160 cm−1, assigned to molecules adsorbed on regular cationic sites of the (100) face and on the edges or steps, respectively. This assignment was supported by comparison with the spectra of CO adsorbed on single crystals, where an extremely narrow band at 2155 cm−1 was detected16 with the half-width of 0.08 cm−1 at 6 K.17 Measurements in polarized light have shown that the CO molecules in the first adsorption layer are perpendicular to the surface, while those in the next layers, which account for the band at 2138 cm−1, are rather parallel or randomly oriented.18 From the isotherms of adsorption, the isosteric heat of adsorption was calculated. At half-monolayer coverage, it was found to be 17.1 kJ/mol.18 For the film, however, the value of 13 ± 3 kJ/mol at 83−99 K was obtained.14 The band position of adsorbed CO depends on the surface coverage. For the single crystal of NaCl, its maximum shifts from about 2158 up to 2155 cm−1 with the increasing amount of adsorbate; however, if a small portion of 12C16O is diluted by the 13C16O isotope at monolayer conditions, it moves further, up to 2151−2150 cm−1.19 This means that lateral interactions, both static and dynamic coupling, account for the observed spectra changes, shifting the bands by −8 and +5 cm−1, respectively. Lateral interactions of CO molecules adsorbed on NaCl films were thoroughly studied by Zecchina et al.20 Bands positions were close to those observed for single crystals (2157.5−2156 instead of 2155 cm−1), while the values of static and dynamic shifts were a little bit smaller, −6 and +3 cm−1. Thus, the studies of CO adsorption on alkali halide films reveal the presence of several complexes associated with different active sites. Admitting the possibility of various orientations of adsorbed CO molecules, the authors considered only the cases of molecules directed parallel or perpendicular to the surface. The case of alternate linkage via C or O atoms was not discussed at that time. Temperature dependence, which could give evidence for linkage isomerism of adsorbed molecules, was not yet investigated.

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EXPERIMENTAL SECTION The films of alkali halides were prepared by sputtering a batch of corresponding salt from a quartz pot heated by tungsten wire upon the flat support of the same salt. For that, the earlierB

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band at 2102 cm−1 due to its 13CO isotope analogue, a shoulder is also clearly seen at 2160 cm−1. The temperature dependence of the spectrum is shown in Figure 3a. Upon raising the sample temperature, while the

RESULTS KBr. Figure 1 shows the spectrum of CO adsorbed on KBr film as a function of temperature. Addition of about 9 Torr of

Figure 1. IR spectrum of CO adsorbed on KBr film deposited at 300 K and registered at 77 (1), 90 (2), 95 (3), 100 (4), and 110 K (5).

CO at 77 K to the film (deposited at 300 K) leads to a band with a maximum at 2151 cm−1. After the removal of liquid nitrogen as the temperature grows, this band starts to decrease in intensity while its maximum moves to higher frequencies, finally up to 2153 cm−1. Spectra of films deposited at low temperature registered with better resolution (2 cm−1) enabled us to see that this band has a poorly resolved shoulder at 2160 cm−1. Another, much weaker, band at 2101 cm−1, apparently due to 13CO molecules at the natural concentration of the 13C isotope (1.1%), decreases simultaneously at the same rate. Besides, one can see a weak band at 2123 cm−1 whose intensity changes in quite a different way. First, it starts to grow when the temperature increases while other bands of CO decrease in intensity. Then, after reaching its maximum intensity, it starts to diminish and finally disappears, together with the other bands of adsorbed CO. Such a behavior is typical of the LF band in the spectra of O-bonded CO molecules in zeolites. NaCl. The spectrum of CO adsorbed on NaCl film is more complicated. Figure 2 shows the evolution of the spectrum upon gradual coverage increase at 77 K. Besides the most intense band at 2155 cm−1, which moves to 2153 cm−1 with the growing amount of adsorbate and a weak

Figure 3. (a) Evolution of the IR spectrum of CO adsorbed on the NaCl film deposed at 300 K with a temperature increase from 89 (top curve) to 133 K (bottom curve). (b) The same spectra with the expanded absorbance scale.

absorption at 2155−2153 cm−1 weakens, another shoulder becomes well resolved at 2170 cm−1. As the temperature exceeds 100 K, a weak band appears at 2124 cm−1, and there is even a weaker one at 2115−2111 cm−1 (more visible in Figure 3b). Both of these bands grow in intensity at first and then decrease and disappear, together with the other CO bands. Almost the same spectra were obtained for a pressed NaCl pellet. The intensity of the band at 2155−2153 cm−1 for such a sample was approximately the same, but the high-frequency shoulders were weaker, and the low-frequency bands were hardly visible. The bands at 2170 and 2160 cm−1 are more visible in the spectra of CO adsorbed on films deposited on the support cooled with liquid nitrogen. These spectra are shown in Figure 4, together with an expanded scale inset. Thermodynamic Measurements. Simultaneous measurement of band intensities, pressure, and temperature for several series of spectra registered upon heating the cell with different initial amounts of gas enabled us to estimate the isosteric heat (enthalpy) of adsorption ΔHa°. For that, the graph of ln P against 1/T was plotted for the points from different series where the intensities of the corresponding bands of adsorbed molecules were equal. The value obtained from the slope of such a plot for the 2153−2151 cm−1 band of CO on KBr turned out to be ΔHa° = 11 ± 1 kJ/mol.

Figure 2. Evolution of the IR spectrum of NaCl film deposed at 300 K, with the increasing amount of CO adsorbed at 77 K (1−7). C

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ΔE°i values. On the other hand, the εLF/εHF ratio for zeolites was shown to be always more than 1;25 therefore, the real ΔE°i could be higher. For KBr, neither the intensity of the 2160 cm−1 shoulder nor the εLF/εHF ratio can be estimated with sufficient accuracy. Thus, the obtained figure of 1.5 kJ/mol can be considered as a rough estimation. In the spectrum of the NaCl film, the different bands of adsobed CO are better resolved, and we can confront the two LF bands at 2124 and 2115 cm−1 with the shoulder at 2160 cm−1 and the band 2170 cm−1. The correct tracing of the van’t Hoff plot is complicated because of a strong overlapping of poorly resolved bands with coverage-dependent positions of maxima. However, after fitting the band contour for each spectrum by Lorentz curves, it was possible to estimate the intensity ratios of the 2160 and 2124 cm−1 bands with reasonable accuracy (see Table 1). To take into account the

Figure 4. Evolution of the IR spectrum of CO adsorbed on the NaCl film deposited at 77 K, with a temperature increase from 83 to 129 K (1−8).

Table 1. Observed Intensity Ratios and Obtained Isomerization Energies ΔE°i for the 2160 and 2124 cm−1 Bands of CO Adsorbed on NaCl

The enthalpy of isomerization ΔH°i can be found from the van’t Hoff plot, which describes the concentration ratio of two isomeric forms as a function of reciprocal temperature. In our case, we can plot the ratio of the intensities of HF and LF bands, AHF and ALF, against the reciprocal absolute temperature ⎛A ⎞ ⎛ε ⎞ ΔHi° ΔSi° ln⎜ HF ⎟ = − + + ln⎜ LF ⎟ RT R ⎝ εHF ⎠ ⎝ ALF ⎠

(1)

Then, the slope of the corresponding straight line gives us the ΔHi° value. We can assume that the HF band of CO on KBr that corresponds to the LF one at 2124 cm−1 is the whole band at 2153−2151 cm−1. In this case, the resulting van’t Hoff plot is not linear and leads to ΔHi° values between 5 and 9 kJ/mol. Another method to estimate the isomerization enthalpy, especially in the cases when the intensity ratio could not be followed accurately over a sufficiently large temperature range is based on Boltzmann’s law. According to it, the population ratio for the two states with equal statistical weights at any temperature should follow the exponential dependence on the energy gap between them ⎛ ΔE ° ⎞ NM−CO A ε = HF LF = exp⎜ i ⎟ ⎝ RT ⎠ NM−OC ALFεHF

T, K

ln(AHF/ALF)

ΔE°i , kJ/mol

100 103 106 109 113 116 118 121 125 129

4.17 3.83 3.61 3.27 3.39 3.23 3.18 3.35 2.98 2.83

4.14 3.97 3.89 3.70 3.95 3.90 3.91 4.18 3.94 3.90

inequality of absorption coefficients for the two isomeric forms, we can take for the εLF/εHF ratio the value of 4.5/2.0 = 2.25, that is, ln(εLF/εHF) = 0.81 obtained earlier for NaY zeolite.24 Calculated from Boltzmann’s formula, the values of isomerization energy ΔE°i for the two sites are close to each other, scattered between 3.7 and 4.2 kJ/mol. The van’t Hoff plot for these data, shown in Figure 5, is close to linear and gives the value of ΔHi° = 4.0 kJ/mol, in a fair agreement with the results obtained from Boltzmann’s formula. DFT Calculations of Cluster Models. CO interaction with clusters modeling different sites of the NaCl surface was studied

(2)

Here, N is the number of C- or O-bonded complexes, and ΔE°i is the difference of energies between the two isomeric states. At the conditions of the experiment, ΔEi° should be close to the isomerization enthalpy ΔHi°, as found earlier for zeolites with alkali cations.5,7 However, estimates made for the spectra where the LF band is more intense and less distorted by superimposed absorption of weakly bound species lead to ΔEi° values near 1.5 kJ/mol. Such a dissimilarity of isomerization energy values can be explained if we assume that not all of the sites of CO adsorption are capable of isomerism and the LF band at 2124 cm−1 corresponds to the high-frequency shoulder of the CO band detectable near 2160 cm−1. Then, the most reliable values of ΔE°i can be obtained from the Boltzman formula applied to the spectra measured at the highest temperature when the intensity of the LF bands is high enough while the band at 2153−2151 cm−1 is not so intense as compared with that at 2160 cm−1. As far as the contribution of absorption at 2153− 2151 cm−1 cannot be neglected, the above application of the Bolzmann formula leads to overestimated AHF/ALF and, hence,

Figure 5. Van’t Hoff plot for the 2160 and 2124 cm−1 bands of CO adsorbed on the NaCl film. D

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To find out whether some of the surface cationic sites of the NaCl surface are capable of isomerism with adsorbed CO molecules, the dependence of the cluster energy on the Na− O−C angle was calculated. The angle was changed from 0 to 180° in 20 steps, and at each step, the full optimization procedure was performed with a variable position of the CO molecule and the intramolecular C−O distance. The data on the calculated adsorption and isomerization energies, as well as the barrier heights and CO stretching frequencies, are given in Table 2. With a single Na+ ion, the CO molecule was shown to form two isomeric linear complexes, bonded via carbon or oxygen, with the energy difference of 15.4 kJ/mol and a more stable Cbonded structure. The two states are separated by a high potential barrier of about 40 kJ/mol (with respect to the more stable state). For the Na4Cl4 cubic cluster, from the modeling of the CO interaction with a 3c Na+ ion, a kind of isomerism was also established. The most stable C-bonded complex with the 3c Na+ ion is separated from another state by a barrier that is much lower than that in the previous case. Another isomeric structure is not a linear Na−OC complex with a sharp potential well but a smooth plateau on the angular dependence when the CO molecule is located near a 3c Cl− anion forming something like a side-on complex with it. The whole picture is changed, however, if these chlorine anions have one more neighboring sodium in the Na7Cl7 cluster. The barrier becomes higher, and the more narrow second potential well corresponds now to a linear structure with a molecule O-bonded to the same Na+ cation. For the Na6Cl6 cluster (Figure 7), the CO molecule, initially positioned in the middle of the edge attached to the 4c Na+ cation, with a changing of the angle, migrates to a 3c Na on the kink. However, if we let the 4c Na move under the influence of the adsorbed molecule, the result is different. The molecule then remains at the same site, and the energetic diagram of the system has two distinct minima, corresponding to Na−OC and Na−CO conformations. These are the data for such a “flexible” cluster presented in Table 2. Modeling of CO adsorption on a 5c cationic site of both Na5Cl5 clusters, where the Na ion has four chlorine ions around it, leads to potential wells with two minima, although the isomerization energy and the barrier height are lower as compared with those of CO on the 3c sites. It is noteworthy that the energy minimum achieved for these clusters is not for the perpendicular orientation with respect to the surface but for molecules tilted by 10−15° from the normal to the NaCl4 plane. Moreover, C−O stretching frequencies for the two isomeric states, unlike other structures, are very close to each other and almost coincide with that of a free molecule, calculated in the same way (2136 cm−1). The Na6Cl6 cluster with a movable central sodium atom reveals even lower adsorption and isomerization energies and the barrier height. Moreover, for this cluster, the frequency of the O-bonded structure turns out to be higher than that of the C-bonded one.

by gradient-corrected correlation functional PBEPBE26 with the 6-311G(d)27 basis set. This method was chosen because it reproduces well the vibrational frequencies of free CO molecules (calculated 2136 and 2089 cm−1 versus experimental 2143 and 2096 cm−1 for 12C16O and 13C16O, respectively. Although most GGA functionals cannot describe the longrange interactions,28 they are good for equilibrium structures of adsorbed CO complexes, including those on ionic surfaces. In this case, the electrostatic interactions dominate, and the energy of CO adsorption on the cations calculated as that of electrostatic interaction between the molecular dipole and quadrupole with the fields of surface ions is in a perfect accordance with the experimentally measured values.29 The electrostatic model predicts also the formation of side-on CO complexes with the surface anions, as was shown for basic zeolites with the highly negative charge on the framework oxygen ions.5,30 The computations were carried out for the CO molecule interacting with single Na+ and K+ ions, the three-coordinated (3c) kink site of a cubic Na4Cl4 or pyramidal Na7Cl7 cluster (Figure 6a), and with a 4c cationic site at the edge of a Na6Cl6

Figure 6. Na7Cl7 (a) and Na5Cl5 (b) clusters.

Figure 7. Dependence of the adsorption energy on the NaOC angle α. (Inset) The scheme of the Na6Cl6 cluster with the CO molecule bound to a 4c Na+ cation.

cluster, as shown in Figure 7. To model a 5c Na+ site, the Na5Cl5 cluster was considered, where the sodium atom was surrounded by four 2c Cl− anions of the surface layer, and under them, there was a layer of five ions of opposite sign (Figure 6b). The geometry of all of the clusters was usually fixed with the Na−Cl distance of 2.80 A, taken from the literature.31−33 In the Na6Cl6 cluster and one series of calculations for the Na5Cl5 cluster, the position of the central Na atom was optimized at every step in order to see the influence of relaxation.

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DISCUSSION The obtained results are mostly in agreement with earlier reported data for CO adsorption on alkali halides.9−20 Band positions of adsorbed molecules and their shifts with coverage changes are close to those from the literature. The HF band at 2153−2155 cm−1 and a shoulder at 2160 cm−1 in the spectra of NaCl films were reported earlier by E

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Table 2. Calculated Values of Vibrational Frequencies ν (cm−1), Distances between the Metal Cation and the Nearest Atom of the Adsorbed Molecule D (Å), Adsorption Eads and Isomerization Energies Ei (kJ/mol), as Well as the Potential Barrier Height Ebar (kJ/mol) between Isomeric States for the Studied Model Clusters

a

cluster

ν(M−C−O)

ν(M−O−C)

D (M−CO)

D (M−OC)

Eads

Ei

Ebar

Na+ Na4Cl4 Na7Cl7 Na6Cl6a Na5Cl5 Na5Cl5a

2220 2170 2173 2149 2136 2134

2072 2111 2107 2125 2135 2139

2.58 2.69 2.68 2.69 2.69 2.69

2.38 2.52 2.51 2.62 2.64 2.59

51.3 23.0 27.6 20.2 27.9 18.8

15.4 6.3 7.2 5.7 5.2 4.2

40.2 14.6 20.9 11.8 16.0 14.2

Flexible clusters with a movable central sodium atom.

Ewing et al.14,15 and assigned to molecules on regular 5c Na+ sites of the (100) face and 4c Na+ ions on the edges or steps. The third band observed by us at 2170 cm−1 could be associated with even less abundant 3c Na+ sites at the kinks of microcrystals. In the spectra of CO adsorbed on a KBr film, where the electric field of cations is weaker and frequency shifts are smaller, adsorption on kink sites could be associated with the shoulder near 2160 cm−1. Our measurements at variable temperatures reveal for both KBr and NaCl films the LF bands, whose intensity dependences on temperature are similar to those of the bands due to Obonded CO species in the spectra of ZSM-5 or Y zeolites. However, we cannot associate these bands with adsorption on the sites that account for the most intense bands of CO in the spectra of corresponding halides. Indeed, in that case, unlike zeolites, the shifts of the HF and LF bands from that of CO gas are different (+8−10 and −20 cm−1, respectively, for KBr), the van’t Hoff plot is not linear, and the isomerization enthalpy values obtained from it are much higher than the energies calculated from Boltzmann’s law. These contradictions disappear if we match the LF band at 2123 cm−1 in the spectrum of KBr with the shoulder near 2160 cm−1 and the HF bands of NaCl at 2170 and 2160 cm−1 with the LF bands at 2124 and about 2115 cm−1. Then, the values of frequency shifts of corresponding HF and LF bands with respect to the gas are now almost equal. For KBr, these are about +17 and −20 cm−1, while for NaCl, they are +27 and −28 cm−1 for one pair of bands and +17 and −19 cm−1 for another. The isomerization enthalpy value obtained from the van’t Hoff plot and ΔEi° from Boltzmann’s formula for the pair of bands at 2160 and 2124 cm−1 in the spectrum of NaCl, whose relative intensity can be measured with sufficient accuracy, now coincide within the experimental error. Moreover, extrapolation of the van’t Hoff plot to 1/T = 0 gives the intersection point at ∼−0.8. Then, using the εLF/εHF value adopted above, from eq 1, we obtain a quite reasonable magnitude for the isomerization entropy ΔS°i = −13 J/mol·K. The results of quantum mechanical calculations within the DFT/PBEPBE method are in agreement with the suggested assignment. The existence of two potential wells was established for all of the clusters, and generally, the adsorption energy and the barrier height diminish with the increase of the coordination number of the cation. Indeed, CO interaction with a single Na+ ion leads to two isomeric states with a high barrier between them. The barrier height (40.2 kJ/mol) amounts to 78% of the adsorption energy Ea (51 kJ/mol). The latter value, which is the energy of NaCO+ complex formation from the Na+ ion and CO molecule, is much above the experimental value of Ea for CO on NaCl (16 ± 4 kJ/

mol11) or calculated before for the (100) face of the NaCl crystal (17−20 kJ/mol29). However, with the increasing size of clusters, Ea has a tendency to decrease to almost 20 kJ/mol, while the barrier height lowers and becomes less than 60% of Ea and the calculated isomerization energy decreases from 10 to almost the experimentally measured value of 4.0 kJ/mol. For the Na4Cl4 cluster, the presence of the second state cannot be considered as a real isomerism because in this case, the CO molecule is rather located at another, adjacent anionic 3c Cl− site. The O-bonded state of the CO molecule adsorbed at the same Na+ site appears when the coordination number of adjacent Cl− anions is increased up to 4 in the Na7Cl7 cluster. This result is in agreement with the consequence of the electrostatic model that linkage isomerism is not possible for the system with a strong field of adjacent anions. These could be O2− anions of oxides or framework oxygen atoms of X zeolites. We can add now that besides the charge and the size of anions, their coordination is also important. As we anticipated, the existence of isomeric states was found also for 4c sodium sites of the Na6Cl6 cluster. It is interesting that isomeric transition on the rotation of the CO molecule in this case occurs only for a flexible site, when the Na+ ion is allowed to move at every step during optimization. This result points to an important role of relaxation induced by adsorption, which is known to influence the adsorption energy and lateral interactions in the adsorbed layer.34 As we can see now, it also affects the phenomenon of isomerism. Modeling of the CO interaction with a 5c Na+ sites reveals some new peculiarities of adsorption at the (100) face. On one hand, for both clusters, there are two isomeric states with reasonable isomerization energy, separated by a rather high potential barrier. On the other hand, for both clusters, the molecule is tilted with respect to the NaCl4 plane, apparently due to interaction with the adjacent Cl− ions, and this interaction can make an important contribution to the adsorption energy. This result is in coherence with an earlier observation of CO adsorbed at the (100) face of the NaCl crystal at low temperatures (T < 38 K) The calculated frequencies of stretching C−O vibrations for the clusters modeling isomeric states on 3c and 4c Na+ sites are in a good agreement with the observed spectra. The frequency increase of CO adsorbed on 4c and 3c reflects the increasing charge on the cations; the calculated Mulliken charges on 5c, 4c, and 3c chlorine ions increase steadily from ∼0.69 up to 0.78. In parallel, the distances between the site and the molecule shorten (see Table 2), demonstrating the strengthening of the molecule−surface interaction. The deviations of calculated frequencies by several wavenumbers can well be caused by the limited size of clusters, relaxation induced by adsorption, lateral interactions between the adsorbed moleF

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calculated isomerization energy decreases from 10 to almost the experimentaly measured value of 4.0 kJ/mol. The existence of a second potential well, which corresponds to CO molecules O-bonded to the cations, depends on the anionic environment. While 3c Na+ sites of Na7Cl7 clusters exhibit two isomeric states, in the simpler Na4Cl4 cluster with the same 3c Na+ centers, the CO molecule after passing over the barrier prefers to remain in the state that can be considered as a side-on complex with the 3c Cl− sites. The example of the Na6Cl6 complex shows that surface relaxation, which is the mobility of the cation in the cluster, is also important for the isomerism. Modeling of the 5c Na+ site interaction with CO molecules infers that even if the two isomeric states are possible, CO stretching frequencies for these complexes can be so close to each other that they may be indistinguishable in the experimental spectra. This can happen if the molecule is tilted, and a comparatively strong interaction with the adjacent Cl− ions accounts for the frequency lowering. The presented results support our notion about the conditions necessary for isomerism of adsorbed species. This information should be taken into account when considering the mechanism of CO reactions catalyzed by oxides and zeolites or in the search for new systems with two stable states for possible use in the optoelectronic devices.

cules, anharmonicity, zero-point vibration energy, or some other factors not taken into account in the model or calculation procedure. This is not the case of clusters modeling 5c Na+ sites. Frequencies of two isomeric states differ here by 2−5 cm−1 only. Such splitting could hardly be resolved in the spectra of polycrystalline films, where the bandwidth was higher, typically about 7−10 cm−1. Thus, the presence of isomeric states predicted by quantum mechanical calculation cannot be confirmed experimentally, and CO adsorbed at the predominating (100) face of NaCl should account for the only one absorption band. Besides, the tilted form of CO adsorbed on NaCl monocrystals was observed only at the lowest temperatures, while in our temperature-dependent spectra, manifestations of isomerism could be seen at T > 77 K. At temperatures below 38 K, when the tilted form was detected, the concentration of the O-bonded isomers had to be negligible. What was not reproduced in the calculation is the band position of the most abundant form. Instead of the observed high-frequency shift by ∼10 cm−1, the model for the C-bonded isomeric form predicts a frequency equal to or 2 cm−1 below that calculated for a free CO molecule. The red shift of the CO band is typical of side-on interaction with anionic sites,23 and because the tilted molecules interact with the adjacent Cl− ions, the final band position is a result of two counteracting factors, a positive shift caused by the influence of the cation and a negative shift brought about by Cl− anions. It is very possible that due to lower coordination of the anions, that is, 2c or 3c instead of 5c for the regular (100) face, the influence of anions upon the frequency is greater. Further calculations should clarify this question and explain the abnormal frequency of the O-bonded isomer in the relaxed Na5Cl5 cluster.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +7 (812) 428-45-71. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to Carlos Otero Arean for his comments and helpful advice. The work was supported by a Grant of the Government of Russian Federation, No. 14.Z50.31.0016

CONCLUSION

Detailed analyses of the temperature dependence of band positions and intensities of CO adsorbed on evaporated films of NaCl as well as KBr persuade us that the weak low-frequency bands observed in the 2125−2110 cm−1 region of the spectra of CO adsorbed on NaCl or KBr films are due to the O-bonded CO molecules. Not all of the surface cations are capable of isomerism. Two low-frequency maxima at ∼2115 and 2124 cm−1 in the spectrum of NaCl correspond to the high-frequency CO bands at 2172 and 2160 cm−1, respectively, which, in accordance with the earlier studies,14 are due to the strongest cationic sites at the kinks or edges and steps of the rocksalt microcrystals. For the pair of bands at 2160 and 2124 cm−1, the value of the isomerization enthalpy ΔH°i obtained from the van’t Hoff plot was found to be the same as that estimated from the Boltzman’s law and equal to 4.0 kJ/mol. Almost the same spectra were observed for KBr films, where the frequency shifts upon adsorption were smaller and the only HF band near 2160 cm−1 was scarcely resolved. The results of quantum mechanical calculations of cluster models support and complete the experimental data. Two isomeric states were established for CO complex with a lone Na+ ion. The energy of formation Ea of such complexes is rather high, above 40 kJ/mol. With the increasing size of clusters, Ea decreases to almost 20 kJ/mol, while the barrier height lowers and becomes less that 60% of Ea and the

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ABBREVIATIONS FTIR, Fourier Transform Infrared; PBE, Perdew−Burke− Ernzerhof functional

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REFERENCES

(1) Otero Arean, C.; Tsyganenko, A. A.; Escalona Platero, E.; Garrone, E.; Zecchina, A. Two Coordination Modes of CO in Zeolites: A Temperature-Dependent Equilibrium. Angew. Chem., Int. Ed. 1998, 37, 3161−3163. (2) Tsyganenko, A. A.; Escalona Platero, E.; Otero Arean, C.; Garrone, E.; Zecchina, A. Variable Temperature IR Spectroscopic Studies of CO Adsorbed on Na-ZSM-5 and Na-Y Zeolites. Catal. Lett. 1999, 61, 187−192. (3) Tsyganenko, A. A., Otero Arean, C., Escalona Platero, E. Infrared Spectroscopic Evidence for Two Ways of Adsorbed CO Coordination. In Studies in Surface Science and Catalysis; Corma, D. A., Melo, F. V., Mendioros, S., Fierro, J. L. G., Eds.; Elsevier: New York, 2000; Vol. 130, pp 3143−3148. (4) Otero Arean, C.; Turnes Palomino, G.; Tsyganenko, A. A.; Garrone, E. Quantum Chemical and FTIR Spectroscopic Studies on the Linkage Isomerism of Carbon Monoxide in Alkali-MetalExchanged Zeolites: A Review of Current Research. Int. J. Mol. Sci. 2002, 3, 764−776. (5) Storozhev, P. Yu.; Yanko, V. S.; Tsyganenko, A. A.; Turnes Palomino, G.; Rodriguez Delgado, M.; Otero Areán, C. Isomeric States

G

dx.doi.org/10.1021/jp507394s | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

Article

of Polar Molecules on Ionic Surfaces: Electrostatic Model and FTIR Studies. Appl. Surf. Sci. 2004, 238, 390−394. (6) Storozhev, P.Yu.; Otero Areán, C.; Garrone, E.; Ugliengo, P.; Ermoshin, V. A. Tsyganenko A.A. FTIR Spectroscopic and Ab Initio Evidence for the Amphipathic Character of CO Bonding with Silanol Groups. Chem. Phys. Lett. 2003, 374, 439−445. (7) Tsyganenko, A. A.; Storozhev, P.Yu.; Otero Areán, C. IRSpectroscopic Study of the Binding Isomerism of Adsorbed Molecules. Kinet. Catal. 2004, 45, 530−540. (8) Garrone, E.; Bonelli, B.; Tsyganenko, A. A.; Rodríguez Delgado, M.; Turnes Palomino, G.; Manoilova, O. V.; Otero Areán, C. Spectroscopic and Thermodynamic Characterization of Strontium Carbonyls Formed Upon Carbon Monoxide Adsorption on the Zeolite. J. Phys. Chem. B 2003, 107, 2537−2542. (9) Kozirovsky, Y.; Folman, M. Infrared Studies of Molecules Adsorbed on Alkali-Halide Films. J. Chem. Phys. 1964, 41, 1509−1510. (10) Kozirovski, Y.; Folman, M. The Influence of Crystalline Structure of the Adsorbent on the Infrared Spectrum of the Adsorbate. Israel J. Chem. 1966, 4, 56. (11) Gevirzman, R.; Kozirovski, Y.; Folman, M. Infrared Spectra and Spectral Shifts of CO Adsorbed on Evaporated Alkali-Halides. Part 1. Films of Sodium Salts. Trans. Faraday Soc. 1969, 65, 2206−2214. (12) Gevirzman, R.; Kozirovski, Y. Infrared Spectra and Spectral Shifts of CO Adsorbed on Evaporated Alkali-Halides. Part 2. Films of caesium salts. Trans. Faraday Soc. 1971, 67, 2686−2698. (13) Rao, B.; Dignam, M. J. Interpretation of the Infra-Red Spectra of CO Adsorbed on Evaporated Alkali Metal Halides. J. Chem. Soc., Faraday Trans. 1974, 70, 492−496. (14) Richardson, H. H.; Baumann, C.; Ewing, G. E. Infrared Spectroscopy and Thermodynamic Measurements of CO on NaCl Films. Surf. Sci. 1987, 185, 15−36. (15) Richardson, H. H.; Chang, H.-C.; Noda, C.; Ewing, G. E. Infrared Spectroscopy of CO on NaCl (l00). I. Photometry. Surf. Sci. 1989, 216, 93−104. (16) Richardson, H. H.; Ewing, G. E. Infrared Spectrum of CO on NaCl(100). J. Phys. Chem. 1987, 91, 5833−5835. (17) Heidberg, J.; Kampshoff, E.; Kuhnemuth, R.; Schonekas, O.; Suhren, M. High Resolusion IR Spectroscopy of Adsorbates on Ionic Crystal Surfaces. J. Electron Spectrosc. Relat. Phenom. 1990, 54/55, 945−950. (18) Heidberg, J.; Suhrer, M.; Weiss, H. Two-Dimensional Phase Transitions and Correlation Field Splitting in Unimolecular Layers of CO Physisorbed on NaCl(100) as Detected by Polarizaton FourierTransform IR Spectroscopy. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1680−1682. (19) Noda, C.; Ewing, G. E. Infrared Spectroscopy of CO on NaCl (100). III. Submonolayers and Isotopic Mixtures. Surf. Sci. 1990, 240, 181−192. (20) Zecchina, A.; Scarano, D.; Garrone, E. Dynamic and Static Interactions in CO Overlayers on Sintered NaCl Films. Surf. Sci. 1985, 160, 492−508. (21) Sekushin, N. A.; Tsyganenko, A. A. Infrared Investigation of the Surface Properties of Amorphous Films of Tungsten and Molybdenum Trioxides. Russ. J. Phys. Chem. 1987, 61, 82−85. (22) Otero Areán, C.; Manoilova, O. V.; Tsyganenko, A. A.; Turnes Palomino, G.; Peñarroya Mentruit, M.; Geobaldo, F.; Garrone, E. Thermodynamics of H-Bonding between CO and the Supercage Brønsted Acid Sites of the H-Y Zeolite: Studies from Variable Temperature IR Spectrometry. Eur. J. Inorg. Chem. 2001, 7, 1739− 1743. (23) Lousada, C. M.; Johansson, A. J.; Brinck, T.; Jonsson, M. Reactivity of Metal Oxide Clusters With Hydrogen Peroxide and Water  A DFT Study Evaluating the Performance of Different Exchange−Correlation Functionals. Phys. Chem. Chem. Phys. 2013, 15, 5539−5552. (24) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, revision C.1, Gaussian, Inc.: Wallingford, CT, 2009.

(25) Kondratieva, E. V.; Manoilova, O. V.; Tsyganenko, A. A. Integrated Extinction Coefficient of Adsorbed CO. Kinet. Catal. 2008, 49, 451−456. (26) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (27) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. SelfConsistent Molecular Orbital Methods. XX. A Basis Set for Correlated Wavefunctions. J. Chem. Phys. 1980, 72, 650−654. (28) Zhao, Y.; Truhlar, D.G. Benchmark Databases for Nonbonded Interactions and Their Use to Test Density Functional Theory. J. Chem. Theory Comput. 2005, 1, 415−432. (29) Ewing, G. E. A Model System for the Study of Structure and Dynamics of Molecules on Surfaces: CO on NaCl(100). Int. Rev. Phys. Chem. 1991, 10, 391−425. (30) Tsyganenko, A. A.; Kondratieva, E. V.; Yanko, V. S.; Storozhev, P. Yu. FTIR Study of CO Adsorption on Basic Zeolites. J. Mater. Chem. 2006, 16, 2358−2363. (31) Gražulis, S.; Daškevič, A.; Merkys, A.; Chateigner, D.; Lutterotti, L.; Quirós, M.; Serebryanaya, N. R.; Moeck, P.; Downs, R. T.; LeBail, A. Crystallography Open Database (COD): An Open-Access Collection of Crystal Structures and Platform for World-Wide Collaboration. Nucleic Acids Res. 2012, 40, D420−D427. (32) Grazulis, S.; Chateigner, D.; Downs, R. T.; Yokochi, A. T.; Quiros, M.; Lutterotti, L.; Manakova, E.; Butkus, J.; Moeck, P.; Le Bail, A. Crystallography Open Database  An Open-Access Collection of Crystal Structures. J. Appl. Crystallogr. 2009, 42, 726−729. (33) Downs, R. T.; Hall-Wallace, M. The American Mineralogist Crystal Structure Database. Am. Mineral. 2003, 88, 247−250. (34) Tsyganenko, A. A.; Zverev, S. M. Mechanism of Lateral Interactions between Molecules Adsorbed on Oxide Surfaces. React. Kinet. Catal. Lett. 1988, 36, 269−274.

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dx.doi.org/10.1021/jp507394s | J. Phys. Chem. A XXXX, XXX, XXX−XXX