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Oct 11, 2014 - Interaction of H2 (D2) with OH (OD) Groups in a ZSM‑5 Zeolite: FTIR ... shift of the OD modes, when corrected by the OH/OD isotopic s...
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Interaction of H2 (D2) with OH (OD) Groups in a ZSM‑5 Zeolite: FTIR Study of the Isotopic Effects Nikola Drenchev and Konstantin Hadjiivanov* Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria S Supporting Information *

ABSTRACT: Low-temperature adsorption of H2 and D2 on H−ZSM-5 and D−ZSM-5 zeolites was studied by FTIR spectroscopy. H2 and D2 are polarized when interacting with the surface OH/OD groups which results in a significant bathochromic (red) shift of the H−H, D−D, O−H, and O−D modes. When D2 interacts with the bridging OH groups, the D−D modes are observed at 2951 cm−1 (ΔνDD = 42 cm−1), while this value is 2952 cm−1 after interaction with the bridging OD groups (ΔνDD = 41 cm−1). The D2-induced shift of the OD modes, when corrected by the OH/OD isotopic shift factor, is by 3 cm−1 lower than the shift measured for the OH modes. These differences are attributed to the slightly lower acid strength of OD as compared to OH groups. Similar phenomena were observed when the bridging OH/OD groups interacted with H2. However, in this case the H−H modes appeared as a doublet (4109 and 4102 cm−1) due to the coexistence of orthoand para-H2. It was demonstrated that the interaction of H2 with the samples is significantly weaker than their interaction with D2: D2 affects more hydroxyls at the same equilibrium pressure, and the induced shift of ν(OH) is by 7 cm−1 larger as compared to H2. It was also confirmed that H2 is connected simultaneously to the proton of the OH group and to a basic oxygen site. Interaction between H2/D2 and SiOH/SiOD and AlOH/AlOD groups was also established. The different acidity of the OH/OD groups and the different basicity of H2 and D2 are discussed. possibilities are shown in Scheme 1. Structure “a” shows “end-on” bonding of hydrogen, analogous to the bonding

1. INTRODUCTION The increasing interest in the hydrogen adsorption forms is mainly provoked by the need of development of effective hydrogen storage materials.1 Hydrogen is also proposed as an IR probe molecule that is very sensitive to the characteristics of the adsorption site.2−5 As a homonuclear diatomic molecule, H2 has no IR active vibration, and the reference Raman value is at 4161 cm−1.6 However, upon adsorption, the symmetry is distorted and the H−H modes become IR active. They can be shifted down to ca. 3000 cm−1 when H2 interacts with Cu+ ions in zeolites.7 Although much smaller, the shift observed after interaction of H2 with surface hydroxyl groups is also important. This allows analyzing not only the OH stretching modes but also the position of ν(HH).2−5 The main disadvantages of hydrogen as a probe are (i) the relatively weak interaction with acidic sites which requires the experiments to be performed at low temperatures and relatively high H2 equilibrium pressures; (ii) the low intensity of the H−H stretching bands; (iii) the possibility to produce water when interacting with easily reducible surface species, and (iv) the coexistence of ortho- and para-hydrogen having different spectral performances (the bands due to para-hydrogen being at higher frequencies by 6−7 cm−1).8−10 At room and at higher temperatures ca. 75% of hydrogen molecules are in the orthoform, but lowering temperature favors the conversion to paraH2.11 This process has been nicely monitored at 20 K on ETS10.9 In addition, there is still no consensus on the adsorption geometry of dihydrogen bonded to OH groups. Some © XXXX American Chemical Society

Scheme 1. Possible Geometries of Dihydrogen Molecule Interacting with a Bridging Hydroxyl Group

accepted for CO and N2. However, in this case the shift of the ν(HH) caused by adsorption should be positive12 which is opposite to the experimental observations. The current prevailing opinion is for structure “c”, i.e., a side-on configuration with one H atom interacting with lattice oxygen.5,13−15 Deuterium is the stable isotope of protium and normally is expected to have the same chemical properties. However, the drastic difference between the masses of H and D is the reason for the measurable difference in the properties of their chemical compounds. The bonds with deuterium are stronger than Received: September 10, 2014 Revised: October 10, 2014

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3. EXPERIMENTAL RESULTS 3.1. Background Spectra of H−ZSM-5, D−ZSM-5, and H−D−ZSM-5. The spectra of the H−ZSM-5 material used in this work were already reported.19,20 Briefly, at 100 K the hydroxyl bands were detected at 3748 cm−1 (silanol groups), ca. 3730 cm−1 (shoulder, terminal silanols), 3670 cm−1 (Al− OH groups), 3619 cm−1 (bridging hydroxyls), and ca. 3490 (broad band, H-bonded hydroxyls). The respective bands for the D−ZSM-5 material were at 2763, ca. 2751, 2705, 2669, and 2578 cm−1 (See Figure S1 in the Supporting Information). 3.2. Adsorption of D2 on D−ZSM-5. We start the description of the results with the D2/D−ZSM-5 system because of the fact that the O−D and D−D stretching modes are observed in spectral regions where the noise is limited. The spectra of the D−ZSM-5 sample in the ν(OD) region recorded in the presence of different D2 equilibrium pressures are shown in Figure 1. Under 50 mbar equilibrium pressure the band at

bonds with hydrogen, and, consequently, the D-acids are weaker than the respective H-acids. 16,17 Consequently, deuterated compounds are stronger bases. These differences are small and have usually been neglected in the surface chemistry. Recently, we reported the measurable difference between the acidities of surface OH and OD groups by (i) comparing the adsorbate induced shift of the OH and OD modes,18 (ii) analyzing the isotope shift factors of perturbed and unperturbed OH/OD groups,19 as well as (iii) basing on the value of ν(CN) of CD3CN interacting with OH and OD groups.20 Adsorption of deuterium has been less studied than H2 adsorption.5,7,8,10,21−23 The reference Raman D−D band in the gas phase is observed at 2993.5 cm−1.6 The spectral difference between the ortho- and para-forms is very small, ca. 2 cm−1,6,8 and consequently the two forms are not resolved in the adsorbed state. Another property of D2 is that a maximum 1/3 of the molecules can be in the para-form at room temperature, and this value decreases with temperature lowering.11 Because the stretching modes with participation of deuterium are observed at significantly lower frequencies as compared to protium, the spectra of adsorbed deuterium are normally less noisy and of better quality. It was found that D2 perturbed more strongly the OH groups than H2 did.5 However, the effect was not discussed in detail and was attributed to the higher mass of deuterium. The theoretical isotopic shift factor, ν(H−H)/ν(D−D), is 1.4137. Due to the anharmonicity, the experimentally observed value by Raman measurements is slightly lower, i.e., 1.3900.6 A value of 1.389−1.393 can be calculated from data reported for H2/D2 interacting with hydroxyl groups19 and with alkali metal cation in zeolites.2,8,10,21,23 However, the isotopic shift factor of H2/D2 adsorbed on Cu+ or Ni+ ions in zeolites is significantly lower, i.e., 1.35−1.36.7,22 Although postulated that this phenomenon was due to anharmonicity changes,10 at this stage the reasons are far from clear. Here we report a careful comparison between the interaction of OH/OD groups with H2 and D2 and discuss the observed isotopic effects.

Figure 1. FTIR spectra (O−D stretching region) registered after adsorption of D2 at 100 K on D−ZSM-5 sample. Equilibrium D2 pressure of 50 (a), 40 (b), 25 (c), 20 (d), 15 (e), 10 (f), 5 (g), 2.5 (h), 1.5 (i), and 1 mbar (j) and after short evacuation (k).

2673 cm−1 (bridging OD groups) is substantially eroded and a new band, at 2635 cm−1, developed (Figure 1, spectrum a). The weak band at 2706 cm−1 (Al−OD groups) is also affected. The SiOD band remains practically unperturbed, and only slight changes in the region are seen. Decrease of the equilibrium pressure leads to a gradual decrease in intensity of the 2635 cm−1 band with simultaneous recovery of the band at 2673 cm−1 (Figure 1, spectra b−k), and after short evacuation the original spectrum is practically restored. An isosbestic point is clearly visible at 2659 cm−1 advocating the direct conversion of one species into another. Based on these results and taking into account earlier reports,5 we attribute the band at 2635 cm−1 to the O−D stretching modes of OD···D 2 adducts. For convenience, the band maxima and the band shifts observed when bridging OH/OD groups interact with H2 and D2 are summarized in Table 1. Figure 2, panels C and D, shows chosen difference spectra (allowing better analysis of the effects) in the O−D and D−D stretching regions, respectively. It is evident that the band at 2634 cm−1 changes in concert with the band at 2952 cm−1. Therefore, we assign the band at 2952 cm−1 to D−D stretching modes of D2 interacting with the bridging OD groups. Similar position was reported for D2 attached to bridging hydroxyls of an H−ZSM-5 sample.5 The D−D band at 2971 cm−1 is more difficult to assign. Sigl et al.5 discussed the possibility of the analogous band on H− ZSM-5 to be due to D2 interacting with silanol groups or to D2

2. EXPERIMENTAL SECTION The H−ZSM-5 sample investigated in this study was from Zeolyst and had a Si-to-Al ratio of 25. Deuteration was performed in situ as described below. Hydrogen (99.999% purity) and deuterium (99.7% purity) were supplied by Messer. D2O (purity 99.9%) originated from Cambridge Isotope Laboratories, Inc. Before use, H2 and D2 were additionally purified by passing through a liquid nitrogen trap. FTIR spectra were recorded at 100 K with a Nicolet 6700 FTIR spectrometer accumulating 64 scans at a spectral resolution of 2 cm−1. Self−supporting pellets (ca. 10 mg cm−2) were prepared from the sample powder and treated directly in a purpose-made IR cell. The cell was connected to a vacuum-adsorption apparatus with a residual pressure below 10−3 Pa. Prior to the adsorption experiments, the sample was activated by heating for 1 h at 673 K under oxygen and evacuation for 1 h at the same temperature. The D−ZSM-5 and H−D−ZSM-5 samples were obtained by treatments of activated H−ZSM-5 for 1 h at 373 K with D2O vapors (1 mbar), followed by evacuation at 673 K. The procedure was repeated until the exchange degree reached ca. 50 and >95% for the H−D−ZSM-5 and D−ZSM-5, respectively. B

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forms of zeolites) has been assigned to the higher-frequency satellite of the band at 2952 cm−1.7 We will show below that this assignment is incorrect. 3.3. Adsorption of D2 on H−ZSM-5. Similar phenomena were registered after adsorption of D2 on the H−ZSM-5 sample (Figure 2, panels A and B). However, there are several important differences that can be systematized as follows: •The D−D band of D2 interacting with the OH groups was observed at slightly lower wavenumbers as compared to the experiments with the D−ZSM-5 sample (see also Figure S3 from the Supporting Information). This indicates a slightly stronger interaction because the shift of the D−D modes relative to the gas phase is larger. In fact the position of the band is slightly coverage dependent. Comparing the spectra at the same equilibrium pressures (see Figure S3 from the Supporting Information) indicates that the difference in the band position was ca. 1 cm−1. These results are consistent with the reported weaker acidity of the surface OD groups as compared to the OH ones.18−20 •The intensities of the D−D bands at given equilibrium pressure of D2 interacting with the OH groups are higher than the intensities of the D−D when deuterium interacts with OD groups (note the different bars in panels B and D of Figure 2). Computer deconvolution indicates that the difference is almost two times (see Figure S3 from the Supporting Information). However, analysis of the intensities of the OH and OD bands indicates that the percentages of the affected OH and OD hydroxyls are very similar for a given D2 equilibrium pressure. Therefore, the differences in intensity are mainly due to differences in the molar absorptivity (extinction) coefficients. This observation indicates stronger perturbation of the D2 molecule by OH as compared to OD groups.27 •The D2-induced shift of the OD modes is smaller than expected on the basis of the isotopic shift. In fact the X-axis scale in panel C (as compared to panel A) was shifted by the experimental isotopic shift factor measured for the bridging hydroxyls. Thus, if the effect of D2 on the OH and OD groups was essentially the same, the shifted band in panel C should lie just below the shifted band in panel A. In fact, there is a difference of 3 cm−1 between the expected and observed values. The results are again consistent with the weaker acidity of the OD groups as compared to the OH ones. Comparison between the ν(DD) spectra when D2 is adsorbed on H−ZSM-5 and D−ZSM-5 allows drawing some more conclusions. At first, the band at 3003 cm−1 does not change in parallel with the 2951−52 cm−1 band and is rather connected with the band at 2971 cm−1. This implies that the current assignment7 of the 3003 cm−1 band to the higherfrequency satellite of the 2952 cm−1 band is not correct. The intensity of the band at 2971 cm−1 practically does not depend on the deuteration of the sample (see Figure S3 from the Supporting Information). This indicates that the assignment of the band mainly to D2 polarized by surface oxygen5 is reasonable. Probably a small fraction of this band is associated with D2 affected by SiOH/SiOD groups. Computer deconvolution (Figure S3 from the Supporting Information) reveals a band at 2962 cm−1 already assigned to D2 polarized by AlOH/AlOD. The intensity of this band is also sensitive to deuteration although to a smaller extent as compared to the band at 2952−51 cm−1. 3.4. Adsorption of H2 on H−ZSM-5 and D−ZSM-5. Figure 3 compares the changes in the spectra of H−ZSM-5 and D−ZSM-5 samples when put in contact with different

Table 1. Spectral Parameters of the Complexes of Bridging OH/OD Groups with H2/D2 system H2− OH H2− OD D2− OH D2− OD

ν(HH)/ ν(DD), cm−1

Δν(HH)/ Δν(DD), cm−1

ν(OH)/ ν(OD), cm−1

Δν(OH)/ Δν(OD), cm−1

4109, 4103

52, 58

3574

52

4109, 4103

52, 58

2640

35 (38)a

2951

42

3567

58

2952

41

2634

40 (43)

a

The shift values expected on the basis of the isotopic shift factor are presented in brackets.

Figure 2. Difference FTIR spectra of D2 adsorbed at 100 K on HZSM-5 (panels A, B) and D-ZSM-5 (panels C, D). OH/OD stretching regions (panels A/C, respectively). D−D stretching region (panels B and D). Equilibrium D2 pressure of 40 (a), 20 (b), 10 (c), and 5 mbar (d).

polarized by surface oxygen and favored the second possibility. Indeed, the band seems to be too intense as compared to the minor changes of the SiOH band. To obtain more information on the nature of the band, we have inspected the spectra in the 3000−2650 cm−1 region obtained by subtracting spectra registered at different D2 pressures. Selected spectra are presented in Figure S2 from the Supporting Information. The results suggest that at least part of the band at 2971 cm−1 is associated with D2 interacting with Si−OD groups. A closer inspection in the OD region indicates some heterogeneity of this interaction: a band at 2766 cm−1 is shifted to 2746 cm−1 and a band at 2760 cm−1, to 2728 cm−1. Indeed, it is wellknown that some silanols demonstrate enhanced acidity as measured by different probe molecules.24−26 We suggest that the band at 2971 cm−1 is due to interaction of D2 with surface oxygen atoms including those from silanol groups. This supposition accounts for both the high intensity of the band and the almost parallel changes of the Si−OD bands at 2766 and 2760 cm−1 (Figure S2 from the Supporting Information). Additional support of this hypothesis will be provided below. The band at 2710 cm−1 (Al−OD groups) is also shifted, although the position of the shifted band cannot be exactly determined due to superimposition with the intense negative band at 2674 cm−1. It seems that this band changes together with a component of the D−D band around 2962 cm−1 (see Figure S3 from the Supporting Information). Finally, a broad band centered at 3003 cm−1 was observed. The 3003 cm−1 band (observed after D2 adsorption on the HC

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−40 and −50 cm−1 have been reported for the bands of bridging hydroxyls in HBEA,28 HY,27,28 and HEMT27 zeolites. Larger shifts, up to −64 cm−1, have been measured with HMOR28,30 and HCHA.31 3.5. Experiments with the H−D−ZSM-5 Sample. The spectral differences in the adsorption of H2 and D2 are well seen from the presented results. However, no clear differences between the energetic of adsorption of H2 (or D2) on OH and OD groups are evident. Note that the effect could be very small and not observable as a result of even negligible differences in the experimental conditions when the measurements are performed with the H- and D-forms of the zeolite. To avoid this, we have studied H2 and D2 adsorption on an H−D−ZSM5 sample where about half of the OH groups are exchanged with deuterium. Thus, the interaction of the OH and OD groups with one adsorbate (H2 or D2) can be compared at strictly identical conditions. More details are shown in Figures S6−S9 from the Supporting Information. It is seen that a very efficient deconvolution of the hydroxyl bands can be made which allows precise quantitative conclusions. Figure 4A presents the adsorption isotherms H2 and D2 when interacting with OH and OD groups in H−D−ZSM-5.

Figure 3. Difference FTIR spectra of H2 adsorbed at 100 K on HZSM-5 (panels A, B) and D-ZSM-5 (panels C, D). OH/OD stretching region (panels A/C, respectively), H−H stretching region (panels B, D). Equilibrium H2 pressure of 40 (a), 20 (b), 10 (c), and 5 mbar (d).

equilibrium pressures of H2. Consider first the OH/OD regions. It is evident that the H2-induced shift of the OH modes is smaller, by 7 cm−1, than the D2-induced shift (see also Figure S4 from the Supporting Information). Similar results were already reported by Sigl et al.5 An analogous situation is observed with the bridging OD groups; here the difference between the H2- and D2-induced shift is 4 cm−1. Note also that hydrogen affects a smaller fraction of OH/OD groups than deuterium at the same equilibrium pressure. These phenomena show that D2 is a stronger base than H2. The spectra in the H−H region (Figure 3, panels B and D) are more complicated (and more noisy) as compared to the spectra in the D−D region registered after D2 adsorption. It is clear that the bands are split into two components. This phenomenon is explained by the existence of ortho- and parahydrogen which are roughly at equal concentrations at our experimental conditions. The ν(HH) band of H2 affected by OH groups is observed as a doublet with maxima at 4109 and 4103 cm−1, while the bands corresponding to H2 interacting with oxygen sites give rise to a doublet with maxima at 4131 and 4126 cm−1. Note that, due to the complexity of the spectra (and the fact that they are noisy), no clear difference between the H−H modes of H2 adsorbed on OH and OD groups was established. Therefore, deuterium seems to be advantageous as a probe molecule as compared to H2. Interestingly, contrary to the case of deuterium, the H−H bands of H2 interacting with bridging OD groups is slightly more intense when comparing to the band due to OH/H2 interaction (see Figure S5 from the Supporting Information). This phenomenon will be discussed later on. The interaction of the OH groups with ortho- and para hydrogen should lead to split of the shifted OH/OD bands (at 3574 and 2640 cm−1, respectively) into two components. However, the effect cannot be resolved in the spectra because the shifted bands are rather wide. Although the spectra registered with the H2/H−ZSM-5 system are the noisiest from the spectra of all systems studied in this work, there are numerous published data that should be used for comparison. Thus, the H2-induced shift of the ν(OH) modes observed in this study (−52 cm−1) is similar (although slightly larger) to the shift values reported with the same system: −45 cm−1 5 or −49 cm−1.27−29 Shifts values between

Figure 4. (A) Proportion of the bands due to bridging OH/OD groups on H−D−ZSM-5 affected at 100 K by adsorption of H2 or D2 (expressed as coverage) vs the gas equilibrium pressure. Open figures, interaction with OH groups; filled figures, interaction with OD groups; circles, adsorption of D2; triangles, adsorption of H2. The full lines are guides for the eye. (B) The same as for panel A, but the intensities are normalized toward the intensities of the bands registered under 40 mbar equilibrium pressure of H2 or D2.

They are obtained on the basis of the integral intensity of the perturbed (negative) bands of the bridging OH/OD groups and taking the intensities of the original bands (without any adsorbate) as 100%. It is seen that D2 is definitely more strongly adsorbed when interacting with both OH and OD groups. Also, whatever the pressure, the percentage of affected OH groups is slightly higher as compared to OD. This is valid for both adsorbates, H2 and D2. D

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bonding makes this molecule another probe able to distinguish between the properties of OH and OD groups. This, however, is not true for H2 because of the complexity of the spectra arising for the coexistence of ortho- and para-hydrogen and the high noise in the H−H stretching region. The proton affinity of H2 (422.3 kJ mol−1) is very similar to that of O2 (421 kJ mol−1).33 It was reported that the O2induced shift of the OH modes of the bridging hydroxyls in HZSM-5 is −32 cm−1. Therefore, the significantly larger shift observed after adsorption of H2 needs explanation. We infer that, due to interaction with basic surface oxygen (Scheme 1, c−e), the molecule of H2 (D2) is polarized which leads to increase of its proton affinity as compared to the free molecule. The above supposition is consistent with the very diverse values of the H2-induced shift of the OH modes in different zeolites and the lack of correlation with the CO-induced shifts. 4.3. Intensities of the ν(HH) and ν(DD) Bands. The H2 and D2 molecules are amphoteric and can be bound to both acid and basic sites. However, the D2 molecule is more basic, and this favors its interaction with acid sites. In contrast, the interaction with basic sites will be more essential for H2. These considerations could explain the intensities of the ν(HH) and ν(DD) bands when interacting with OH and OD groups. Consider first the D2 molecule with the assumption that the interaction is mainly with acid sites (OH/OD groups). In agreement with this, the more acidic OH groups polarize more strongly the D2 molecule than the OD groups do, and the ν(DD) band is more intense when D2 interacts with the more acidic OH groups. The interaction with basic sites will be more pronounced with dihydrogen. Therefore, the intensity of the ν(HH) band should depend to a less extent on the acidity of the OH/OD group. This is consistent with the observed similar intensities of H2 interacting with OH and OD groups. The fact that the band associated with OD/H2 interaction is even slightly more intense indicates a slightly higher basicity of the surface oxygen to which H2 is bound. The observed dual interaction of H2/D2 with the surface is in favor of structures “c”, “d”, and “e” from Scheme 1. However, the strong isotopic effects suggests that the basic site to which H2 is bound is the oxygen atom from the OH/OD group, and thus structures “d” and “e” seem more probable. In conclusion, we like to emphasize that the factors that strongly affect the intensity of the ν(HH) and ν(DD) bands hardly influence the enthalpy of adsorption.

The data obtained from the spectra of OH and OD groups interacting with adsorbate are reliable (see Figures S6 and S8 from the Supporting Information). However, we realize that the determination of the intensities of the unperturbed OH and OD bands could have been less exact. This could have affected the calculation of the coverage and could account for the different calculated values of the fractions of OH and OD groups affected at the same conditions. In order to eliminate this eventual error, we plotted the same data, but in this case the intensities of the bands registered under 40 mbar equilibrium pressure were taken as 100% (Figure 4, panel B). It is seen that the curves for H2 adsorption on OH and OD groups practically coincide. The results suggest a slightly stronger adsorption of D2 when interacting with OH groups as compared to OD groups (see the inset). Nevertheless, the differences are small and at the edge of the detection limit of the technique. Evidently, some spectral parameters, as band shifts and extinction coefficients, are more sensitive to the isotopic effects.

4. DISCUSSION 4.1. Adsorption of H2 vs Adsorption of D2. It is documented that D2 is more strongly adsorbed than H2, and this phenomenon is used for separation of the two isotopes.16 Here we shall discuss the spectral dimension of this effect. The shift of the OH stretching modes induced by different bases is widely used to estimate the acidity of the hydroxyls. Although there are some indications that various factors can affect the shift value,32 there are many reports on correlation between the shift value and the proton affinity (PA) of the probe molecule used. Using the proposed correlation, one can calculate that the PA of D2 is with ca. 8 kJ mol−1 higher than the PA reported for H2, i.e., about 430 kJ mol−1. Note that these values are rough, because of two main reasons: (i) the ortho- and para-forms should also possess different proton affinities, and (ii) as already stated, H2 is most probably adsorbed at side-on configuration simultaneously interacting with lattice oxygen.5,13−15 Therefore, the interaction is not solely with the OH groups. Based on the presented results, it can be stated that D2 is a better IR probe molecule than H2. This arises from several facts: (i) the D−D stretching modes are observed in a spectral region that is less noisy than the H−H region; (ii) D2 interacts stronger with electrostatic acid sites than H2 and produces more intense bands at the same experimental conditions, and (iii) the spectral difference between the ortho- and para-D2 is very small thus avoiding the split of the D−D bands into two components. Note also that at low temperature the expected concentration of the para-form is negligible. However, some of the disadvantages of H2 as a probe are also valid for D2: the still discussed geometry of adsorption and the possibility of interacting with easily reducible species thus producing D2O. 4.2. OH vs OD Groups. The slightly higher acid strength of the bridging OH groups in H−ZSM-5 as compared to the respective OD groups in D−ZSM-5 is confirmed by the results in this work. The phenomenon is indirectly monitored by the smaller value of the observed adsorbate-induced shift of the OD modes than the expected shift on the basis of the isotopic shift factor. The only reported suitable probe molecule for direct observation of the difference in acidity of OH and OD groups is CD3CN, where the difference between the CN modes of OH··· CD3CN and OD···CD3CN complexes is slightly larger than 1 cm−1.5 The fact that the D−D modes are highly sensitive to the

5. CONCLUSIONS •D2 is a stronger base than H2 which results in a stronger interaction with OH (OD) groups. The estimated proton affinity of D2 is ca. 8 kJ mol−1 higher than that of H2. •As a probe molecule D2 allows distinguishing between the properties of OH and OD groups through analysis of the D−D stretching bands. •Another advantage of D2 as a probe molecule, as compared to H2, is the lack of measurable split of the D−D modes due to existence of ortho- and para-forms. •Adsorbed H2 (D2) is simultaneously connected to a proton of the OH groups and to a basic surface site, most probably oxygen from the OH group. •The intensities of the ν(HH) and ν(DD) bands are highly sensitive to the acidity and basicity of the sites to which H2 (D2) are bound. E

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−9S with FTIR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +3592-979-3598. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Thanks are due to Dr. P. A. Georgiev for some helpful discussions. The authors are indebted to the Union Centre of Excellence (Contract No. DCVP 02-2/2009 with the National Science Fund).

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REFERENCES

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