IR Study of CO on Aerosil at Low Temperature
The Journal of Physical Chemistty, Vol. 83,No. 22, 1979 2863
Infrared Study of Low Temperature Adsorption. 1. CO on Aerosil. An Interpretation of the Hydrated Silica Spectrum Giovanna Ghiotti," Edoardo Garrone, Claudio Morterra, and Flora Boccurzi Istituto di Chimica Fisica, Universit.4 di Torino. 10125 Torino, Ita& (Received November 8, 1978; Revised Manuscript Received June 29, 1979) Publication costs assisted by the Univefsity of Turin
IR spectra of CO physically adsorbed on Aerosil were recorded at 123 K. Spectral modifications have been examined as a function of coverage and surface hydroxylation in the region 4000-700 cm-l. Computer simulation of IR spectra in the CO stretching region was performed. The free silanols act as interaction centers via a weak hydrogen bond, as clearly shown by the perturbation of OH stretching and bending modes. We interpret that on highly dehydroxylated samples the interacting silanols are isolated, whereas on highly hydroxylated samples the majority are present as free partners in couples of silanols, H bonded with different strengths. On the dehydrated part of the surface interaction arises via dispersion forces; two different sites for such interaction are present on highly dehydrated samples. Reactivity of the acidic Lewis sites is also observed. In all cases the degrees of freedom of the adsorbed molecules seem to be restricted to libration only.
Experimental Section Introduction Aerosil (Degussa 2491, specific surface area -330 m2g-l) Infrared spectra of physisorbed species are, in principle, was used, since its surface properties have been largely vital in the characterization of the state of the adsorbate. studied in our The powder is tableted at The shape and width of the bands can yield information 100 MN m-2 together with a gold gauze which helps in about the rotational degrees of freedom of physisorbed the attainment of a uniform sample temperature during molecules1 which can be used, together with their vibraboth preliminary outgassing and cooling. Samples of about tional frequencies, for the computation of thermodynamic 10 mg cm-2 were used unless otherwise stated. properties of the adsorbate.2 The pellet is inserted into a suitable IR cell which allows In practice, such information is difficult to obtain. both heating and cooling in situ. Such a cell, somewhat Adsorbate modes are often not very sensitive to surface related to that used by P r i t ~ h a r dis, ~sketched in Figure fields, and the majorfeature of IR spectra of physisorbed 1. Like any low temperature (LT) IR cell, it recalls species turns out to be the activation of some Raman basically a Dewar flask. The outer part, made of Pyrex, modes because of surface field a~ymrnetry.~ For these carries the NaCl windows (W); the reentrant (mainly in reasons, after some flourishing some 20 years ago, visilica; GS is the graded seal) ends in two parallel U-tubes brational studies of physisorbed species have been, to our (U) between which the sample is held. The optical path knowledge, abandoned. is kept as small as possible (5 cm only) so as to minimize A possible alternative use of such vibrational spectra could be the characterization of the adsorbent itself. In the gas phase signal. The sample holder (not shown in the fact, only a minor fraction of a surface is usually involved figure) is a small rectangular block of gold with both a slit in any room temperature adsorption, whereas the features in which the pellet is inserted and a hole allowing the IR of the whole surface can be revealed by a weak interaction. beam to pass through. The sample can be heated by For such a study, small polarizable molecules like CO and means of two Nichrome coils placed in the U-tubes (not NO are best suitable, provided that the temperature is low shown). To avoid heat loss by radiation during heating, enough. the lower part of the reentrant is surrounded by a gold box In a previous paper,* some of us studied CO physi(not shown) with two holes in the proper position. Cooling sorption on Aerosil at 77 and 163 K. Two kinds of inis done by filling the inner part of the Dewar flask with teraction (revealed by two CO bands) were found: a weak liquid nitrogen. hydrogen bonding on free surface silanols and a nonspecific A good adherence of the sample holder to the inner walls interaction with the dehydrated part of the surface. That of the U-tubes is vital for efficient cooling of the sample, paper mainly dealt with the nature of the two CO bands, as well as the adherence of the sample to its holder. A thermocouple (T) in contact with the sample holder shows which other authors5 had assigned in a different way, and no inference was drawn for the adsorbent structure. that a temperature around 150 K is reached under vacuum. In the present paper we reexamine this phenomenon In all cases considerably lower temperatures are reached over a much wider range (4000-750 cm-l), as recent ~ o r k ~ , when ~ gases are admitted, as thermal conduction between has led to the understanding of new features for the surface the sample and the coolant is increased. A t high gas modes of silica. pressures, thermal conduction also occurs between the Both hydrated and dehydrated samples are examined, reentrant and the outer walls of the cell, causing the sample and it is shown that some information is gained on the temperature to rise. Moreover, the outer walls tend to structure of the adsorbent, in particular at high hydration, condense moisture from the ambient air and the optical through both the mere decrease in temperature and the windows have to be kept clear by blowing warm air over physisorption of the gas. them. As a consequence, the temperature of the sample is observed, as a function of the gas pressure, first to In a future paper, NO physisorption on the same support will be described. decrease markedly and then to increase somewhat. With
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0022-3654/79/2083-2863$01 .OO/O
0 1979 American Chemical Society
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The Journal of Physical Chemistty, Vol. 83, No. 22, 1979
Figure 1.
IR cell described in the text.
CO a minimum temperature of 108 K has been measured at 670 N m-2, whereas it was 133 K at 8 kN m-2; with He the lowest temperature was 83 K at 260 N m-2. During IR measurements, a further increase in the sample temperature of some 15 K occurs due to the heating effect of the beam. Samples outgassed 8 h at room temperature (RT) and a t 350,423,573,673,923, and 1073 K have been studied. For the sake of brevity, spectra are given only for two extreme degrees of hydroxylation, and the results concerning other samples are quoted only when necessary. The highest attainable dehydration, without provoking any sintering of the samples, is represented by sample B, outgassed at 1073 K (noH nm-2 = 1.5, ref 8). As far as highly hydrated samples are concerned, it is noted that Aerosil, unlike other kinds of silica, hold molecular water very loosely, and the mere room temperature outgassing for 8 h completely eliminates ahy water molecules from the surface. This is in agreement with the literature,7-10-12 and it is confirmed by the absence in all cases of any signal in the region around 1630 cm-’. Instead of the sample outgassed at room temperature, we have, however, chosen to represent the highly hydrated surface, sample A OH nm-2 > 6, ref 8), outgassed at 423 K, because it yields the clearest low temperature spectra. In order to achieve complete dehydroxylation, sample C has been prepared by methoxylation with CH30H at 623 K13 OH nm-2 = 0; n0cHsnm-2 = 7-8). Measurements have been run as follows. After thermal treatment, the spectrum of the sample was taken at room temperature and then at 123 K (liquid nitrogen in the flask, 53 N m-2 of He in the cell) to obtain the background for subsequent CO physisorption. After He evacuation, CO was admitted at increasing pressures. All spectra were recorded on a Beckman IR 7 double beam spectrophotometer. Those in the 2000-2300-~m-~ range were recorded most accurately, Le., with a 10 cm-’ in-’ expansion, a 10 cm-’ min-’ recording speed, and a slit width of only 3 cm-l, to resolve all components in the spectra and to observe any band shift caused by sample pretreatment. Slit widths were about 8 and 1.8 cm-l over the 3000-4000- and 600-1000-cm-’ ranges, respectively. Frequencies have been calibrated in the CO stretching region by means of rotovibrational transitions of gaseous
Ghiotti et al.
Flgue 2. CO interaction on sample B at 123 K (a) OH stretching region (absorbance vs. wavenumber); (b) the silica “window”(transmission vs. wavenumber). Bold soild curves, background at room temperature; dotted curves, background at 123 K; bold broken curves, 10.4 kN m-2
of
co.
Flgure 3. CO interaction on sample A at 123 K: (a) OH stretching region (absorbance vs. wavenumber); (b) the silica “wlndow” (transmission vs. wavenumber). Bold soild curves, background at room temperature; dotted cwves, background at 123 K; solid thin curve, 1.3 kN m-2 of C O bold broken curves, 10.4 kN mA2of CO.
CO, in the 600-1000-cm~’region by means of gaseous NH3, and in the OH stretching region by means of water vapor. For computational purposes spectra in the CO stretching region were digitized at 2-cm-’ intervals between 2090 and 2270 cm-’. Band-fitting computations have been carried out on the IBM 370/ 158 machine at Turin University. The main features of the computer program have been described briefly else~here.’~ Moreover, it has been modified to take into account the gas phase signal which, despite the short optical path of the cell, is always not neglibible. Results Spectral modifications due to a decrease in temperature or CO physisorption occur only in the silanol stretching region (3150-3800 cm-’), in the so-called silica “window” (700-1000 cm-’), and in the CO stretching region (2100-2200 cm-’). For the sake of brevity, we postpone the discussion in little detail of the spectra in the last range. The spectra of samples B and A are shown in Figures 2 and 3, respectively. The figures are both divided into two sections (a and b), i.e., the high- and low-frequency ranges. The ordinate scale is absorbance in sections a, whereas it is transmission in sections b.
IR Study of CO on Aerosii at Low Temperature
Bold solid curves refer to the room temperature spectra of the samples alone; bold dotted curves are the spectra at 123 K before physisorption; bold broken curves are the spectra run when in contact with 10.4 kN m-2 of CO. For the sake of clarity one spectrum at intermediate CO coverage is shown in Figure 3a. The spectra in sections b refer to lighter (and thus more transparent) samples. Room Temperature Spectra of Samples A and B. A great deal of IR data can be found in the literature1s2J5 concerning the stretching mode of free silanols, hereafter labeled v(OH), in highly dehydroxylated samples (Figure 2a, band at 3748 cm-’) and its change upon adsorption. Much less attention has been paid both to the other modes of surface silanols and to the spectra of highly hydroxylated samples. As far as we are aware, room temperature spectra of such samples have been reported by few authors.2*6J2J6-U The band at about 3750 cm-l has been assigned to isolated free hydroxyls,16already present on the highly hydroxylated surface. It is noteworthy that this band falls in our case at 3743 cm-l, that it does not coincide in frequency with the one for B, and that its definitely larger width suggests a composite nature. The broad absorption below 3740 cm-l has been assigned to stretching modes of hydrogen bonded silanols, hereafter called v(OH-). The broadness of the band and the presence of ill-defined maxima (as in Figure 3a) have been interpreted as due to the occurrence of H bonds of different strengths.18 As far as the low-frequency region (Figures 2b and 3b) is concerned, recent work6v7has led to the following assignments of the other silanols modes: 6(OH) = 760 cm-l; 6(OH-.) = 760-1000 cm-l (very broad); v(Si-OH) = 980 cm-’; v(Si-OH-.) = 935 cm-’ (broad). In the present case, v(Si-OH) and v(Si-OH-) will not be seen since they are overlapped by the strong absorption due to bulk modes at about 1100 and 800 cm-l, so that the only mode that will be clearly visible is the bending of silanols 6(OH) and 6(OH--). The two weak bands at 908 and 888 cm-l in Figure 2b are due to localized Si-0 modes associated with highly distorted surface structures, which arise by outgassing at temperatures higher than 850 Ka6 Low Temperature Spectra of Samples A and B. The decrease in temperature causes some interesting changes. As far as sample B is concerned, the 3748-cm-l band (Figure 2a) undergoes an upward shift of about 3 cm-l, as does the 888- and 908-cm-’ pair, which is now found at 892 and 912 cm-’ (Figure 2b). All bands are observed to sharpen and intensify somewhat: the 754-cm-l band is in particular magnified, while sharpening of the two bulk modes causes a marked increase of transparency in the “window”. On the contrary, the transparency of the sample decreases below 780 cm-l; a possible explanation for this behavior is the quenching upon cooling of the IR emission by the sample at room temperature. The effect of cooling in the high-frequency region of sample A (Figure 3a) deserves some attention. The free silanols stretching mode moves to 3748 cm-l and intensifies, as expected by similarity with sample B; moreover, this band reveals, upon cooling, its composite nature, guessed above, as at least two components become visible. The transparency of the sample increases in the 37003730-cm-I range, and a new band at about 3720 cm-l is observed. The transparency decreases below 3700 cm-l and more definite maxima are observed at 3680, 3640, 3540, and 3450 cm-’. As far as the low-frequency range (Figure 3b) is concerned, the lowering of the temperature brings about a
The Journal of Physical Chemistry, Vol. 83, No. 22, 1979 2865
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zim
zim
2140
2100
2120 vcm.’
mea
Flgure 4. Comparison among the spectra of CO physisorbedat 123 K: CO stretching region (transmission vs. wavenumber). Pressures of CO are 0.53, 1.06, and 0.66 kN m-2 for cases A, B, and C, respectively.
decrease in transparency below 780 cm-l, as in the previous case, and the appearance of a shoulder at about 755 cm-l. Effect of CO Adsorption o n the Spectrum of Samples A and B. Contact with CO brings about spectacular changes in the spectrum of both A and B samples. As far as sample B is concerned, the 3751-cm-’ band decreases (Figure 2a, bold broken curve), giving rise to a new band at 3673 cm-l, -50 cm-l wide and much more intense. In Figure 2b a marked erosion of the band at 754 cm-l is observed, which does not correspond to the appearance of any new band. Below 750 cm-’ the transparency increases, most likely because of some effect on the lower frequency modes of silica7 not studied here. The behavior of the couple of bands in the “window” (Figure 2b) is interesting; the higher frequency one undergoes a small but definite decrement, whereas the lower frequency one seems to move downward and intensify. As far as sample A is concerned, Figure 3a shows that CO physisorption goes far beyond the expected erosion of the 3748-cm-l composite band. In fact, the transparency of the sample increases down to 3700 cm-l; a band with apparent maximum at 3640 cm-l is formed, which appears broader and more intense than in the previous case. Also, a very broad band is observed starting from 3530 cm-’, which is absent in Figure 2a. Taking into account the spectrum at intermediate CO coverage, we inferred that (i) the crossing point at 3695 cm-’ acts as a definite isobestic point and (ii) all low temperature curves have about the same absorbance at -3530 cm-’. Figure 3a also shows that the room temperature spectrum passes through the isobestic point. This is likely a mere matter of chance, as it does not happen if the sample has been pretreated at slightly higher temperatures or upon physisorption of other molecules, e.g., NO. In Figure 3b the following features are noted: (i) the bending mode of silanols is largely eroded and (ii) a broad absorption at 855-840 cm-’ is formed, with a definite shoulder at about 870 cm-’. Spectra of Adsorbed CO. Figure 4 compares the spectra of A, B, and C in the CO stretching region taken at nearly the same pressure and hence at approximately the same temperature. The presence of two absorptions at about 2137 and 2157 cm-’ is evident (except for C), as well as some additional features. The absorption at about 2137 cm-l seems to be single in cases A and C, whereas it is surely double in case B; the absorption at about 2157 cm-’ has different positions and half-widths in cases A and B.
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The Journal of Physical Chemistty, VoL 83, No. 22, 1979
These differences escaped detection in our previous investigation, where the spectra were interpreted as made up of two bands with constant position and half-width. All spectra in this region (not reported here) have been computer simulated. The following results were obtained: (i) all peaks have a strict Lorentzian nature; (ii) within a set of spectra concerning the same sample, changes only occur in the intensities of the peaks, all other features (shape, position, and half-width) remaining constant; (iii) all spectral peaks for the B and C samples have a halfwidth which is constant within few percent (around 10 cm-l). Computer simulation of sample A spectra in this region is briefly discussed later.
Discussion Room and Low Temperature Spectra of A and B Samples. The changes in the spectrum of sample B due to the marked decrease in temperature are not unexpected. The slight upward shift of the bands and their intensification is accounted for by the decrease in thermal agitation of the solid; the increase in the u(OH) frequency is in agreement with what is already known.lg Much more stimulating is the case of sample A. No low temperature spectrum of this or similar samples has, to our knowledge, even been reported. Many features are observed in this spectrum which are nearly smeared out at room temperature. The decrease in thermal agitation seems to cause a surface situation in which, despite the amorphous nature of Aerosil, “a few distinct types of surface silanol groups predominate”, as suggested by MacDonald.lS We point out that groupings of H-bonded silanols should end (if not clustered together to form a ring) with free hydroxyls, whose stretching mode (hereafter referred to as v’(0H)) is expected to fall at frequencies lower than that of isolated silanols. The occurrence of such terminal hydroxyls could be neglected only if very large groupings occurred. As no band in the literature has even been assigned to u’(0H) modes, as far as hydrated surfaces are concerned, such an assumption seems to have been made more or less implicitly up to now. We think, on the contrary, that the whole IR spectrum can be interpreted as made up of, besides isolated free silanols, u’(0H) and v(OH-) modes of a limited number of isolated couples of silanols, one obviously free and the other H bonded to the former with different strengths. Reasons for such an interpretation are as follows. (1) Morrow and Cody6found, when studying the first stages of rehydration of a highly hydroxylated silica, a couple of associated bands at 3720 and 3520 cm-l, which were assigned to the u’(0H) and u(OH-.) modes of dimer I. o/H -------0/H
I
I
Si
Si
I
Taking into account some shift in position caused by the temperature difference, we assume that the 3720- and 3540-cm-l bands in Figure 3a have a strictly similar nature. (2) Pairs of silanols (one free and the other H bonded) can be singled out also for the case of the commonly accepted ideal model yielding the highest concentration of hydroxyls, the (100) face of fl-cristobalite,20 which shows rows of geminal hydrogen-bonded hydroxyls. In fact, taking into account the involved geometry,10,21 we feel that a realistic representation of a row would be as shown in 11. Such pairs can be regarded as isolated, from a vibrational point of view, if the coupling between the vibration of adjacent
Ghiotti et al.
- ---o
OH----
0
\ /O-H---o \ /o-H --- \ /o-H---o \, /Si
si-
A\
A\
5a-
’si
Si
A\
A\
I1 pairs due to the interposed Si atom is low enough. This seems quite likely, as it is generally accepted that no coupling occurs between the vibration of the two hydroxyls of isolated geminal pairs. The actual surface of sample A cannot be thoroughly described by I1 because of the following: (i) other crystal faces have been invoked as ideal models, e.g., the (0001) face of P-tridimite,16to account for the presence of free isolated hydroxyls at any stage of dehydration; (ii) some dehydration of the surface has already occurred because of the mild pretreatment, likely transforming couples of adjacent geminal pairs into vicinal pairs, which should be close to the structure I proposed by Morrow and Cody. The interaction between hydroxyls in structure I1 is likely the strongest one which can occur on this surface. The corresponding modes are thus expected to fall at lower frequencies than in the previous case. Thus, tentatively we assign the 3680- and 3450-cm-l bands respectively to the u’(0H) and u(OH.-) modes of such a dimer. As a check, we compared the low temperature spectra of a sample outgassed first at room temperature only, and then at about 350 K; a preferential erosion of the above couple was in fact observed. As far as the 3640-cm-l band is concerned, some papers in the literature, e.g., ref 12, show that internal hydroxyls (silanols not readily reacting with proper molecules, like BC13, for steric hindrance) absorb at that frequency. It is clear, however, that such internal hyroxyls are responsible for only a fraction of the 3640-cm-l band in samples pretreated in a way similar to ours.12 Thus, we feel that the assignment of the major fraction of the band under discussion is still an unresolved problem. If it were related to the u(OH-) mode of a dimer, the question would arise where the corresponding v’(0H) mode would fall. A possible location for it could be the shoulder at about 3745 cm-l, up to now considered as due to a free isolated hydroxyl. In such a totally tentative hypothesis the two bands would correspond to a dimer of weakly interacting OH’S. The proposed overall assignment of surface silanols will be checked by means of CO physisorption in the following. CO Interaction. The results in the present paper are not in disagreement with the ones in our previous inve~tigation.~The presence is confirmed of two main mechanisms of interaction of CO with the surface, Le., hydrogen bonding onto surface silanols and interaction with other parts of the surface via dispersion forces. This is revealed by the presence of two band envelopes in the CO stretching region, one close to the frequency of CO in condensed phasesz2or in mat rice^,^^-^^ and the other at definitely higher frequencies, which is absent in case C. Before moving to a detailed assignment, some preliminary points have to be discussed. At about 123 K (average measurement temperature) the vapor pressure of CO is around 240 kN m-2. Relative whereas pressures adopted ( p / p o )are in the range of a p / p o of around 4 X corresponds to a monolayer coverage, as evaluated by the BET method from physisorption isotherms at 77 K.4 The CO coverages i n the present experiments are thus fairly low. The question arises to what extent CO molecules adsorbed at low coverages interact with one another, and in
I R Study of CO on Aerosil at Low Temperature
particular whether the surface can induce a clustering of CO, even at low concentration, similar to that observed in matrix isolation rnea~urernents.~~ Such an idea could account in particular for the presence of two components in the 2137-cm-’ envelope in case B. We point out that, should any clustering occur, the intensity of some bands in the CO stretching region ought to increase at the expense of some other bands when increasing the surface coverage. This is never so; computer simulation shows that the intensity of all bands in all three cases grows smoothly with increasing pressures. CO molecules at the surface are thus best thought of as single noninteracting entities, physisorbed onto definite sites. This is in agreement with the marked Lorentzian nature of CO bands. The small and fairly constant values of the half-width point out that all CO species have similar degrees of freedom, which are likely restricted to librational modes only.22 Should any rotational mode be retained, half-widths would be definitely larger, as it is easily checked by a computation similar to that given by Sheppard and YatesS3Half-widths around 10 cm-’ have been reported for CO bands in condensed phases.22 It has to be noticed, however, that, due to the poor transparency of our samples, optical conditions are definitely worse in the present case than those occurring in such studies. If a Lorentzian shape is assumed for our instrument slit function, a rough estimate of the real half-widths of CO bands yields only 4-5 cm-l, i.e., of the same order of magnitude for bands due to CO in matrices. In our opinion, the data here presented allow the indentification of the following sites for physisorption. ( A )Isolated and Terminal Silanols. Results in Figure 2 show quite clearly that CO interacts with the isolated hydroxyls of a dehydroxylated surface. The stretching mode (band at 3751 cm-’) is decreased, and a new band appears at 3673 cm-l, due to the same mode of silanols hydrogen bonded onto CO (hereafter labeled v(OH-40)). In fact the downward shift of the stretching mode and its intensification and broadening are typical features of such a process. The bending mode (initially at 754 cm-l) is expected to shift upward; as no new band is observed, it is likely hidden in the intense bulk mode at -800 cm-’. This is not surprising; on the basis of the spectroscopical literature on H bonding,26a shift of some 20 cm-’ would be expected in this case. A strict proportionality exists, as required, between the decrease of the 3751-cm-‘ band and the increase of the 3673-cm-l one. All free silanols can act as sites for CO interaction: full depletion of 3751-cm-l band was not reached in the present experiments but it was in the previous ones4at higher p / p o values. The stretching mode of CO involved in such interaction is easily identified with the peak at 2157 cm-l. In fact a strict proportionality exists between the intensities of such peak yielded by computer fitting, and those of the band at 3673 cm-l. In our previous paper we also showed that the intensity of the 3751-cm-l band before interaction in samples outgassed at least at 650 K (Le., where only free silanols are present) was proportional to the intensity of the 2157-cm-l band at full CO overage.^ It is interesting to note that, in a study of CO-HC1 matrix mixtures,23a band has been found at 2157 cm-l which was assigned to a CO-HC1 complex, in close agreement with the present assignment. In that paper, CO was assumed to interact via the oxygen lone pair; in our previous paper we favored instead the interaction through the carbon atom. It is not easy to decide between the two interactions, one reason being the extremely small dipole
The Journal of Physical Chemistry, Vol. 83, No. 22, 1979 2887
moment of CO, whose direction still is unknown. The work by Hush and Williams would, however, indicate that our choice is the valid one,n so that, in conclusion, the complex formed can be depicted as shown by 111.
i
Si
111
As already pointed out, the most relevant feature of CO interaction with sample A is the marked erosion of the spectrum in the region above 3700 cm-l, i.e., the reactivity toward CO of both free hydroxyls (3751-cm-l band) and other silanols absorbing at lower frequencies. This fact is in full agreement with the proposed assignment of the bands at about 3745,3720, and 3680 cm-l to the v’(0H) mode of terminal silanols. The terminal hydroxyls in I and I1 are in fact protruding out of the surface and well available for CO binding, just as the isolated silanols are. An example of the complexes formed is (by reference to structures 11) shown in IV.
I
Si
I
Si IV
The polarizing power of the various kinds of hydroxyls is expected to be of the same order of that of free silanols, and in fact CO does not discriminate between free and terminal silanols, and hence an isobestic point is observed in Figure 3a. Because of the heterogeneity in the involved hydroxyls, the bands due to CO-OH interaction in case A fall at slightly different positions than in case B, and are somewhat broader. The v(OH.-CO) modes cause the intense band at 3640 cm-l. Its width can be semiquantitatively accounted for by the charge-transfer-no-bond t h e ~ r yaccording , ~ ~ ~ ~to~which in a set of XH-aM bonds, the ratio (vl - v2)/v1 (vl and u2 being unperturbed and perturbed XH stretching frequencies, respectively) is a function of the ionization potential of H-bonded M molecule. In our case, in the simplifying assumption that all kinds of hydroxyls have equal polarizing power, such a ratio is a constant, whose numerical value is easily computed from case B; for case A silanols a nearly constant shift of some 75 cm-l is expected. This indicates a lower limit of about 3605 cm-l for the position of v ’ ( O H 4 0 ) modes, in nice agreement with the position of the lower frequency tail of the 3640-cm-’ band. This result also means that the presence of the broad band below 3530 cm-l cannot be accounted for by any v or v’(OH..CO) mode. We thus assign it to the perturbation of v(OH-) modes due to interaction with the terminal hydroxyls. By doing so, we assume that the coordination of a CO molecule onto the terminal silanol of dimer causes a shift of the stretching mode of the H-bonded partner to lower frequencies. No data are available in the literature for H-bonded molecules to directly support this point. A perusal of the literature shows however that v’(0H) and v(OH-.) frequencies are indeed correlated in that an increase in the grouping size always decreases both frequencies. The perturbation of v(OH-.) modes should lead to an isobestic point, just as in the case of v’(0H) modes. This is not observed, however, in Figure 3a because of the
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The Journal of Physical Chemistry, Vol. 83,No. 22, 1979
growing of the 3640-cm-I band, which overcomes the decrease of the v(OH-) unperturbed modes; as a consequence, instead of an isobestic point, all spectra have a common point at about 3530 cm-l. As a final comment on the broad band starting from 3530 cm-l, we point out that the perturbation of v(OH-) modes in a silanol grouping is acceptable only if the size is fairly small, and that it becomes likely if OH dimers are involved. We thus assume the presence of this broad band as further evidence in favor of the proposed model of the hydrated surface. If all kinds of interacting silanols had exactly the same polarizing power, actually coincident with the one of free silanols, a single CO stretching band would be expected, 10 cm-l wide. The spectrum of sample A in Figure 4 shows that it is not strictly so. The spectral simulation of this band with only one peak is rather unsatisfactory, and a larger width is, however, obtained. A simulation by two peaks at least is needed; the peak at higher frequency nicely coincides with the peak in case B and it is assigned to CO interacting with isolated silanols and, likely, those hydroxyls causing the shoulder at about 3745 cm-l. The latter peak is probably due to the merging of the stretching modes of CO interacting with the other terminal silanols (bands at 3720 and 3680 cm-l). The slightly lower frequency indicates that the polarizing power of the terminal hydroxyls likely decreases upon increasing H-bonding strength in the dimers, Such decrease seems, however, small, and it does not contradict the inferences previously made by neglecting any difference among the reactive silanols. In conclusion, the simple model of a 1:l CO linear interaction with all kinds of protuding silanols can account satisfactorily for all observed sdectral features. No other mechanism of interaction seems to be operative, particularly not bridging of CO molecules onto adjacent hydroxyls. Should any bridging occur, it would induce a partial sp2 character in the carbon atom and this would lead to a definite lowering in the CO stretching frequency. We observe on the contrary a limited shift within the 2157-cm-l envelope, which is revealed by the larger half-width of this band. The mechanism of CO adsorption seems thus to be definitely different from the one observed for the C02 intera~tion.~~ As far as bending modes are concerned, both 6(OH) and 6’(OH) modes are expected; the former cause the shoulder at about 755 cm-l, whereas the latter (obviously at higher frequency) are not clearly seen, although the comparison before and after CO contact shows some erosion of the 775-cm-l edge. By similarity with what was found for sample B, the 6(OH) mode moves upward, upon CO contact, and thus is hidden in the bulk mode at 800 cm-l. The 6’(OH) modes are expected to undergo an upward shift of about the same magnitude so that they are made partially visible by the shift of the 850-cm-l edge (Figure 3b). Finally, we assign the shoulder at -870 cm-l to the perturbation of the 6(OH.-) mode corresponding to the very broad band in Figure 3a below 3530 cm-‘. (B)Acidic Sites. Upon outgassing at temperatures higher than 850 K, highly distorted surface structures are formed which cause the couple of weak bands at 892 and 912 cm-l. A detailed assignment is not yet available; it is, however, known that such structures (probably involving an exposed Si atom) show some Lewis acidity. For instance, upon pyridine adsorption: the high-frequency band
Ghiotti et al.
is decreased, whereas the low-frequency one seems to be unaffected, or at most to shift somewhat to lower frequencies. The spectra shown in Figure 2b indicate that CO interacts with these Lewis centers very weakly, in agreement with Morrow and Cody’s observation that no interaction occurs at room temperatures6 Since the surface concentration is fairly low, no corresponding CO band can be detected. ( C ) Other Sites in the Covalent Parts of the Surface. As already stated, the CO bands at about 2137 cm-l have been assigned in our previous paper to merely physisorbed molecules interacting with the adsorbent via nonspecific dispersion forces. Figure 4 shows, however, that small but definite differences exist in this spectral region between samples A, B, and C. This means that even in the case of CO interaction with the covalent parts of the surface, the CO molecule reveals slight differences in the surface fields of differently pretreated samples. In the case of sample C, whose surface can be regarded to as homogeneous, a single band is observed. As far as sample A is concerned, the presence of a band in this region shows, in our opinion, that, even in the case of a slightly dehydroxylated surface, covalent patches are available for mere physisorption. Due to the weakness of the band and the lack of a detailed model of the surface, no feasible hypothesis can be made on the ultimate nature of this site. In the case of sample B, the presence of two sites is observed. The splitting of the band around 2137 cm-’ is seen clearly first after outgassing at about 650 K, at which temperature the formation of strained siloxane bridges beginsa8 For higher pretreatment temperatures, the two CO bands are observed to grow, with roughly constant relative intensity. The couple of bands seems ascribable to the presence of strained bridges. In our opinion, physisorption will not necessarily occur onto the siloxane bridges themselves, but more likely onto two different sites strictly related to those, e.g., sort of cavities on the surface brought about by the formation of strained bridges. Conclusions The CO molecules adsorbed at low temperature (physisorbed) onto Aerosil are a sensitive probe of the surface structure. On a completely methoxylated surface, plain physisorption takes place; only one band is observed in the stretching region, very close to the liquid or solid value. On highly dehydroxylated samples, free isolated silanols act as interaction centers: both the OH stretching and bending modes are perturbed, in agreement with the well-known features of H bonding. On the dehydrated parts of the surface, two different sites for physisorption are observed. Reactivity of the Lewis centers toward CO is also observed. On highly hydrated samples, CO mainly interacts with free silanols. It is shown that such silanols exist on the surface with stretching frequencies down to 3700 cm-l at least, which up to now were regarded to as H bonded. These are interpreted as the free partners in couples of silanols H bonded with different strengths, and a tentative assignment is proposed of the many bands observed in the low temperature spectrum of the bare sample (unresolved at room temperature). Although the CO stretching frequency is sensitive to different environments of the molecule, both H bonding and interaction via dispersion forces have comparable energies in that both processes always occur simultaneously. Similarly, in all cases the other degrees of freedom of the adsorbed molecule seem to be restricted to librations
The Journal of Physical Chemistry, Voi. 83, No. 22, 1979 2869
Spectral Study of Irradiated Naphthalene Crystal
only, as the computer simulation of the spectra show that all peaks are Lorentzian in nature with a very limited real half-width.
(1 1) A. V. Kiselev, V. A. Loknzsievskii, and V. I.Lygin, Zh. Fis. Chim., 49, 1796 (1975). (12) A. J. Tyler, F. M. Hambleton, and J. A. Hockey, J . Catal., 13, 35 (1969). (13) C. Morterra and M. J. D. Low, J. Phys. Chem., 73, 321 (1969). (14) C. Morterra, G. Ghiotti, E. Garrone, and F. Boccuzzi, J . Chem. Soc., Faraday Trans. 7 , 72, 2722 (1976). (15) M. L. Hair, "Infrared Spectroscopy in Surface Chemistry", Marcel Dekker, New York, 1967. (16) C. G. Armistead, A. J. Tyler, F. M. Hambleton, S.A. Mitchell, and J. A. Hockey, J . Phys. Chem., 73, 3347 (1969). (17) P. G. Rouxhet and R. E. Sempels, Trans. FaraAy SOC.,70, 70 (1974). (18) R. S. MacDonald, J . fhys. Chem., 62, 1168 (1958). (19) P. R. Ryason and B. G. Russel, J . Phys. Chem., 79, 1276 (1975). (20) J. B. Peri and A. L. Hensley, Jr., J . Phys. Chem., 70, 3168 (1966). (21) R. L. Mozzi and B. E. Warren, J. Appl. Crystallogr., 2, 164 (1969). (22) G. E. Ewing, J. fhys. Chem., 37, 2250 (1962); G. E. Ewing and G. C. Pimentel, ibid., 35, 925 (1961). (23) J. B. Davies and H. E. Hallam, Trans. Faraday Soc., 87, 3176 (1971); J. Chem. Soc., Faraday Trans. 2 , 68, 509 (1972). (24) G. B. Leroi, G. E. Ewing, and G. C. Pimentel, J . fhys. Chem., 40, 2298 (1964). (25) S.W. Charles and K. 0. Lee, Trans. Faraday Soc., 61, 614 (1965). (26) . . G. C. Pimentel and A. L. McLellan, "The Hydrogen . - Bond", W. H. Freeman, San Francisco, 1960. (27) N. S.Hush and M. L. Williams, J . Mol. Spectrosc., 50, 349 (1974). (28) J. A. Cusumano and M. J. D. Low, J . Catal., 23, 214 (1971). (29) H. Knozinger, Surf. Sci., 41, 339 (1974). (30) A. Ueno and C. 0. Bennett, J . Catal., 54, 31 (1978).
Acknowledgment. This work has been carried out with the partial financial support of the C.N.R. We gratefully thank Professor A. Zecchina for helpful discussions. References and Notes (1) L. H. Little, "Infrared Spectra of Adsorbed Species", Academic Press, New York, 1966, p 306. (2) A. V. Kiselev and V. I.Lygin, "Infrared Spectra of Surface Compounds", Wiley, Toronto, 1975, p 363. (3) N. Sheppard and D. J. C. Yates, f r o c . R . SOC.London, Ser. A , 238, 69 (1956). (4) A. Zecchina, G. Ghiotti, L. Cerruti, and C. Morterra, J. Chim. fhys., 68, 1479 (1971) (5) P. J. Fenelon and H. E. Rubalcava, J. fhys. Chem., 51, 961 (1968). (6) B. A. Morrow and I.A. Cody, J. fhys. Chem., 79, 761 (1975); 80, 1995, 1998 (1976); L. S. M. Lee, ibid., 80, 2761 (1976). (7) F. Boccuzzi, S.Coluccia, G. Ghlotti, C. Morterra, and A. Zecchina, J. fhys. Chem., 82, 1298 (1978). (8) E. Borello, A. Zecchina, C. Morterra, and G. Ghiotti, J. fhys. Chem., 71, 2945 (1967), and references therein. (9) A. M. Bradshaw and J. Pritchard, Surf. Sci., 17, 372 (1969). (10) V. M. Bermudez, J. fhys. Chem., 75, 3243 (1971).
High-Resolution Absorption and Fluorescence of the I-Hydrobinaphthyl Radical in the Irradiated Naphthalene Crystal T. Nakayama and S. J. Sheng" Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received January 15, 1979) fubllcation costs assisted by the US. Depatfment of Energy
The strong absorption band at 710 nm in an irradiated naphthalene crystal was studied at 4.2 K. Evidence from the deuterium shift, the absorption spectrum in a 5050 CloH8-Cl$, crystal, and the fluorescence spectrum indicates that the radical responsible for 710-nm absorption is the 1-hydrobinaphthyl radical. This radical is formed through the bimolecular reaction between a naphthyl radical and a naphthalene molecule and is stable at room temperature in the crystalline lattice.
Introduction When aromatic crystals are subjected to ionizing irradiation, some hydrogen atoms will be produced through the breakage of carbon-hydrogen bonds. The hydrogen atoms can add to aromatic rings, taking away one of the T electrons in forming cyclohexadienyl type radicals' which are rather stable. The phenyl type radical, on the other hand, will react with another aromatic ring to form phenylcyclohexadienyl type radicals. The absorption of phenylcyclohexadienyl radicals has not yet been observed. In the case of naphthalene, Chong and Itoh2p3and Piccini and Whitten4 have observed a strong absorption around 710 nm and attributed it to the dimer radical (I). The evidence for their assignment came from the observation that the absorption band at 710 nm gradually increases for a naphthalene crystal irradiated at 77 K and annealed at room temperature. Recently we have used a 50:50 isotopic mixed crystal to demonstrate the nature of the hydrogen atom addition in naphthalene crystals by studying the absorption bands at 'The research described herein was supported b y the office of Basic Energy Sciences o f the Department of Energy. This i s Document No. NDRL-1958 from the N o t r e Dame Radiation Laboratory.
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0022-3654/79/2083-2869$0 1.OO/O
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539 and 635 nm. These two bands correspond to the 1-hydronaphthyl radical5 and 2-hydronaphthyl radical,6 respectively. The same type of experiment should provide additional information on the connection between the 710-nm absorption and the postulated reaction between naphthyl radicals and naphthalene molecules. From an analysis of the splitting in the 710-nm band of the spectrum of the 5050 mixed crystal and from the magnitude of the deuterium shift of the electronic origin, it will be shown that the 710-nm transition is indeed due to the dimer radical (I). 0 1979 American Chemical Society
