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FTIR spectroscopic characterization of the adsorption and desorption of ammonia on MgO surfaces
Surface Science 230 (1990) 237-244 North-Holland
231
FTIR SPECTROSCOPIC CHARACTERIZATION OF THE ADSORPTION AND DESORPTION OF AMMONIA ON MgO SURFACES
Ralf ECHTERHOFF
* and Erich KNGZINGER
**
Institut $2 Physikalische Chemie, Universitiil Siegen, Postfach 101240, D-5900 Siegen, Fed. Rep. of Germany
Received 5 September 1989; accepted for publication 8 December 1989
In the present paper an attempt is made to disentangle the complex IR spectra obtained in the course of the adsorption of NH, on high surface area MgG. For this purpose two simplifications were introduced in comparison to previous studies: (a) Only uniformly hydroxylated MgO surfaces (degassing temperature: 200 0 C) and completely dehydroxylated MgO surfaces (degassing temperature: 1000 o C) were used as adsorbent. (b) The equilibrium pressure of the adsorbate NH, was generally maintained below 200 Pa in all adsorption and desorption experiments. Under these conditions the position and the pressure dependence of the resulting bands may consistently be interpreted in terms of characteristic surface species. Only the completely dehydroxyiated MgG surface gives rise to a heterolytic dissociation into neighbouring NH, and OH groups. In addition, NH, is physically adsorbed on less reactive sites via diverse types of hydrogen bonds. Less well defined hydroxylation states give rise to considerably more complicated spectra. In a first approximation they may be constructed by linearly combining two spectral patterns, one related to a uniformly hydroxylated and the other to a dehydroxylated surface. NH, pressures around and beyond 1 kPa give rise to significant changes of band positions and band shapes compared to those observed below 200 Pa. The interpretation of these effects which have previously been ignored must necessarily rely on more complicated models.
1. Introduction
High surface area magnesium oxide has previously been studied with respect to the interaction of its surface sites with ammonia [l-6]. It has been shown by IR spectroscopy that a heterolytic dissociation occurs at the lowest coordinated (three-fold) surface ion pairs. In addition, molecular adsorption on diverse surface sites of hydroxylated and d~hydroxylat~ MgO surfaces appeared to play an important role. An intuitively intelligible model of possible surface complexes may easily be derived from the known properties of NH, and of the surface sites of MgO (Mg2+ and O*- ions as well as surface OH groups). In a recent paper f6] IR spectroscopic data is presented which was considered to * Present address: Bayer A.G. Dormagen, Polyurethane Division, Geb. Kl, D-4047 Dormagen, Fed. Rep. of Germany. * * To whom correspondence should be addressed. 0039-6028/90/$03.50
be in agreement with the occurrence of these complexes. All spectra were, however, recorded at relatively high NH, pressures which favour interactions between two or more nei~bou~ng adsorbate species as well as between adsorbate species and further NH3 molecules. These effects although undoubtedly present in the spectra were essentially ignored in the course of the interpretation. Aiming at a reliable interpretation we have, therefore, confined ourselves exclusively to the low NH, pressure domain and thus to the intrinsic elementary steps of adsorption of NH, on MgO surfaces. This necessarily implies a reduction of the signal-to-noise ratio. In light of these considerations the application of a research grade FTIR spectrometer equipped with a highly sensitive cryogenic detector is an absolutely necessary condition. On the basis of such experimental equipment minute spectral changes - a few thousands of an absorbance unit - induced in the
0 1990 - Elsevier Science Publishers B.V. (North-Holland)
238
R. Echterhoff,
E. Kniizinger
/ FTIR spectroscopy
course of an absorption or desorption process may then be evidenced by spectral subtraction.
of ammonia on MgO
3. Results and discussion 3.1. MgO surfaces
2. Experimental Self-supporting disks of MgO and CaO (Merck GmbH) were obtained by pressing about 50 mg of the oxide powder at pressures of 10 MPa. By degassing at less than 10-l Pa and 927” C for - 2 h all surface impurities were removed. Thereafter the oxide pellets did not exhibit any specific IR absorption above 1000 cm-‘. In order to create a certain hydroxylation state, the clean surface was then completely rehydrated at 2 kPa H,O vapour pressure and again outgassed at a properly selected temperature [7]. The procedure applied in the adsorption experiments is as follows. The pretreated MgO pellet located in the IR adsorption cell [8,9] is exposed to NH, gas via a needle valve. The prepressure has to be adequately selected in order to obtain an appropriate rate of increase for the NH, pressure in the cell. “Appropriate” means that the adsorption equilibrium is readily established in the time interval between the data collections for two subsequent IR spectra. This procedure permits to monitor the process of adsorption on a time scale which ranges from seconds to hours. The desorption experiments were carried out simply by connecting the IR cell volume containing the NH, charged MgO pellet with a large evacuated reservoir for a time interval which guarantees the establishment of thermodynamic equilibrium. The IR transmission spectra were recorded at about 30 o C with an evacuable FTIR spectrometer model IFS-113 v (Bruker Analytische Messtechnik GmbH, FRG) equipped with a LN,-cooled MCT detector. The resolving power applied was 2 cm ’ for most experiments. Higher resolving power did not create any changes in band shape. The specific surface area of the MgO samples was 60 m* gg’. It was determined by adsorption experiments using molecular N, as probe (“BET method”). The pressure applied on preparing the self-supporting MgO disks as well as the degassing temperatures used (200-1000” C) do not change the specific surface area of MgO to any significant extent.
Photoluminescence and UV reflectance spectroscopy clearly established the conclusion that 3-, 4-, and 5-coordinated oxygen and magnesium ions are present at the surface of high surface area MgO [lo]. The reactivity of these sites is undoubtedly related to the degree of coordination. Thus according to -Mg*+
-O;,;
-
+ NH,
H I I -Mg-(A,,-,
-
H,N
(1)
ammonia undergoes a heterolytic dissociation exclusively on 3-coordinated cation anion couples, i.e., on corners of the cubic microcrystals of the MgO powder [6]. H,O, on the other hand, reacts on 3- as well as on 4- and 5-coordinated sites giving rise to the formation of H bonded pairs of OH groups [lo] as visualized by -Mg*+
-O;~-+H20--_=
HO,....H I I -Mg-O,-.
(2)
The degree of coordination x = III, IV, V. They exhibit a sharp OH stretching band at 3750 cm-’ and a broad one around 3550 cm-’ for the OH groups acting as proton acceptor (free OH) and as proton donor (H bonded OH), respectively [7] (table 1). A MgO surface exposed to water vapour pressure of at least 2 kPa and subsequently to vacuum treatment at 250 “C is essentially uniformly covered with the OH pairs according to reaction (2) [7]. On heating the sample in vacua at 1000” C the OH groups are removed quantitatively from the surface. In the course of this study only the two well defined hydroxylation states described above will be considered (table 1). Much more complicated surface systems will arise when an intermediate degassing temperature is applied. A thermal treatment at 450” C, e.g., creates a non-uniform surface which incorporates different hydroxylated and dehydroxylated species
R. Echterhoff, E. Kniizinger / FTIR spectroscopy of ammonia on MgO Table 1 Characterization
of surface
species on hydroxylated Surface
Degassing temperature of MgO(“C)
1 Mg-6,-
/H-O,
MgO surfaces
Characteristic IR absorption pattern (cm-‘)
Assignment
a)
3750/3550
Y,~,,, of OH groups acting as proton acceptor and proton donor, respectively
a)
3750
species
H-?,..H
250
and dehydroxylated
239
-Mg-o,H
I -Mg-On,450
\
3737 aeon, of isolated OH groups with I, Ill, IV, and V coordinated oxygen, respectively
H
I
3722
-Mg-O,,H -Mg-b,\ 1000
-Mg
2+ _02_
‘) x = Ill, IV, or V.
(table 1). Each of them has to be considered for possible interactions with NH,. Under these conditions the complexity of the respective IR spectra appears to be beyond the scope of a reliable analysis. 3.2. Vibrational properties of ammonia NH, (point group C,,) exhibits 6 IR aktive fundamentals two of them being twofold degenerate. According to table 2 the IR spectrum of NH, should provide three spectral intervals with relevant structural information. In order to use NH, as a model compound for probing the surface these spectral intervals have to be carefully checked for the influence of the surface potential on both the intramolecular force field and on the symmetry of the probe molecule (physisorption). Thus the IR spectroscopic response to physical adsorption of NH, on a specific surface site consists in principle in 4 characteristic band shifts and possibly - in the appearance of 2 additional bands (one in the NH stretching and one in the NH bending region) owing to the lifting of degeneracy.
Intrinsic chemical reactions of NH, with surface sites (chemisorption) may easily be recognized on the basis of the IR spectra of the resulting products. For example, the formation of an amide group or an ammonium ion on the surface necessarily implies the appearance of characteristic bands located around 1560 [11,12] and 1400 cm-’ [13], respectively (table 2). On the other hand, the presence of a band around 1560 cm-’ is not a sufficient condition to conclude the occurrence of the heterolytic dissociation of NH,. Such a band
Table 2 IR spectroscopic products of NH, Species
NH,
-NH, NH;
characterization on MgO surfaces
Characteristic IR absorption pattern (cm-‘) 3500-3300 1700-1600 1300-!Joo -1560 -1400
of possible
chemisorption
Assignment
Stretching (a,. e) Antisymmetric bending (e) Symmetric bending (a,) Bending Bending
(“amide band”) (“ammonium ion band”)
R. Echterhoff, E. K&zinger
240
/ FTIR spectroscopy of ammonia on MgO
may also originate from ammonia molecules which have non-equivalent NH bonds (H bonding!). Desorption experiments may help to discriminate the two species in question. 3.3. NH, on hydroxylated
MgO surfaces
All spectra (figs. 1 and 2) were recorded in the difference mode (A.A = f (F)), i.e., the absorbance trace observed for the pure hydroxylated MgO substrate was subtracted from the spectrum obtained after NH, adsorption. An interaction of two or more NH, molecules with one surface site is - owing to the experimental conditions applied _ extremely unlikely. The IR spectra presented here refer to a NH, surface coverage of less than 1% with respect to the area of the bare MgO surface. The absence of any band around 1560 and/or 1400 cm-’ definitely precludes any chemical reaction of NH, with OH groups in the sense of a formation of surface NH: or surface -NH,, respectively (table 2). Thus the most likely interactions of NH, with the hydroxylated MgO surface should be hydrogen bonds. In fact, the negative band at 3750 cm-’ (fig. 1) clearly indicates the consumption of free surface OH groups [9], which are excellent proton donors and might be involved in the process presented by: H-O,... I -Mg-OX-
H I + NH,
====G
complex A
H,N...H-0,..**H I -Mg-00,-.
I (3)
The degree of coordination x = III, IV, V. This idea is confirmed by a broad and structured positive band which emerges between 3700 of and 3400 cm -’ in the course of the adsorption NH,. In this spectral interval stretching vibrations of H bonded OH groups (proton donor) are to be expected. The OH pairs of the hydroxylated MgO surface before NH, adsorption (left side of reaction (3)) exhibit, e.g., a band at 3550 cm-’ (see section 3.1, [7]). It is still present in the difference spectrum after NH, adsorption indicating that process (3) initiates a certain polarization of the H
Fig. 1. Difference IR spectra (OH and NH stretching region) of NH, adsorbed on uniformly hydroxylated MgO surfaces (degassing temperature 200 o C). AA = A, - A, where A, and A, are the spectra obtained before NH, admission and at a NH, equilibrium pressure p, respectively. The adopted values of p were 5.3 (lower trace), 24, 44, 64, 84, 103, 123, and 173 Pa (upper trace).
bond between neighbouring surface OH groups. Otherwise the intensity around 3550 cm-’ should be canceled by spectral subtraction. In addition, two positive bands at 3600 and 3489 cm-i are observed which have then to be attributed to surface OH species hydrogen bonded to NH, as proton acceptor. One such complex is shown in reaction (3) the other is most likely related to the process: H-0,.-.-H I -Mg-OX-
I + NH,
-
complex B
H,
N’
H-_O,....fi
I -Mg-OX-.
I (4)
The degree of coordination x = III, IV, V. Already at an early stage of adsorption a broad positive band centred around 3033 cm- ’ appears. Both position and width of this feature clearly
R. Echterhoff, E. Kniizinger / FTIR spectroscopy of ammonia on MgO
recommend an assignment to NH groups acting as proton donor. On the other hand, it is extremely unlikely that all H atoms of the adsorbed NH, molecules are involved in hydrogen bonds. Therefore, according to the model developed above NH stretching bands should appear in close proximity to 3444 and 3337 cm-‘, the values for isolated NH, molecules [14]. This is definitely not the case. It has to be assumed that the product of the respective absorbance coefficient and the surface coverage is too low for a detection. Coluccia et al. [6] found a band at 3365 cm-’ with a shoulder at 3390 cm- * for surface coverages of 1 to 2 molecules per 100 A*, i.e., 15-30% of the bare MgO surface. This is a by 1 to 2 orders of magnitude higher surface coverage than the one we used in our experiments. A lateral interaction of adsorbed NH, molecules may, therefore, not be excluded. The band positions observed by Coluccia et al. (61 are blue shifted as compared to the above mentioned gas phase value of the symmetric NH, stretching vibration. This is, in principle, not compatible with the presently accepted hydrogen bond theory. Thus, based on the relatively unspecific data on the OH and NH stretching region (fig. 1) essentially two different types of surface complexes of NH, on hydroxylated MgO surfaces have to be expected (reactions (3) and (4)). To corroborate this model additional information is required from the sy~et~c and ~tisy~et~c bending region (fig. 2). Two symmetric bending modes at 1127 and 1095 cm-’ are observed in fig. 2B. They are both blue shifted with respect to the gas phase data [14]. At low NH, ~uilib~~ pressures there is only the band at 1127 cm-’ which we assign to the complex presented in reaction (3). As the adsorption goes on (higher NH, pressures) the formation of a second species, most likely the complex described by reaction (4), becomes increasingly important and gives rise to a prominent shoulder at 1095 cm-‘. At even higher NH, pressures again the rate of growth of the band at 1127 cm-’ dominates the spectrum. This observation may tentatively be interpreted in terms of the formation of a so far unspecified third type of adsorption complex.
241
‘500
to
Fig. 2. Difference IR spectra (symmetric (B) and antisymmetric (A) NH bending regions) of NH, adsorbed on uniformly bydroxylated MgO surfaces. For further explanation see legend of fig, 1 (two traces for the NH, ~uilib~urn pressures 147 and 160 Pa were added).
The information provided by the antisymmetric NH, bending modes (1700-1500 cm-‘) is less specific than that obtained from the symmet~c bending modes. There is one broad band around 1640 cm-’ for the NH, pressures applied (fig. 2A). On increasing the pressure the position of the absorption maximum is shifted from about 1630 to 1654 cm-‘. This trend supports the model of at least two different types of NH, surface complexes on fully hydroxylated MgO. On reducing the equilibrium NH, pressure (desorption experiments) the observations made in the course of the adsorption experiments are essentially reversed. Thus the adsorption probability of the 2 different sites appears to be determined predominantly by the stability of the resulting surface complexes. Theoretical studies clearly show that NH, acceptor complexes with H,O are considerably more stable than the NH, donor complexes [15,16]. A similar situation should arise when NH, interacts with surface OH groups. In fact, our desorption experiments clearly evidence that NH, donor complexes, if they exist at all, are less stable than the complexes A and B (reactions (3) and (4), respectively). Additional support for the assign-
242
R. EchterhofJ E. Kniizinger / FTIR spectroscopy
ment presented here may be derived from experimentally determined hydrogen bond lengths d(OH . . . N) and d(0 .. . HN) of diverse complexes: they amount to 268-279 pm and 281-304 pm, respectively [17]. According to reactions (3) and (4) there should be less sterical hindrance for the formation of complex A (3) in comparison with B (4). Therefore, it appears reasonable to attribute the highest stability to complex A. NH, belongs to point group C,, in the two NH, proton acceptor complexes (reactions (3) and (4)) and to C, in the donor complexes (only 2 equivalent NH bonds). For NH, donor complexes one would, therefore, expect an additional NH bending mode similar to that observed for NH,D compared with NH, [18]. At an equilibrium NH, pressure of more than 2.7 kPa an extremely weak band appears at 1554 cm-‘. It could originate from the lifting of degeneracy for the antisymmetric bending vibration in an NH, proton donor complex. 3.4. NH, on dehydroxylated MgO surfaces The spectra in figs. 3-5 were recorded as described in section 3.3, i.e., in the difference mode. Since MgO does not exhibit any specific IR absorption after a vacuum treatment at 1000 o C, the respective difference spectra of NH, adsorbed on these samples comprise only positive bands (figs. 3-5). According to sections 3.1 and 3.2 the sharp feature at 3755 cm-’ (fig. 3) has to be attributed to free or isolated surface OH groups and thus provides conclusive evidence for the chemisorption process according to reaction (1). This model also allows us to assign the bands at 3436 and 3329 cm-’ (fig. 3) which should be related to the antisymmetric and symmetric stretching vibration of the resulting surface NH, groups, respectively. The unspecific continuous absorption around and above 3000 cm-’ recommends that some of the surface NH, groups interact with each other or with surface OH groups via hydrogen bonds. On the other hand, the model does not enable the assignment of the feature at 3565 cm-’ to be made with confidence. The additional assumption has to be adopted that a certain percentage of the resulting surface OH groups acts as proton donor
of ammonia on MgO
$00
3obo
300
i; /cm
-1
Fig. 3. Difference IR spectra (OH and NH stretching region) of NH, adsorbed on dehydroxylated MgO surfaces (degassing temperature: 1000 o C). For further explanations see legend of fig. 1 (the trace for the NH, equilibrium pressure of 103 Pa is not there).
in hydrogen bonds with neighbouring NH, groups or with NH, molecules. The stretching vibration of hydrogen bonded surface OH groups on hydroxylated MgO surfaces has, e.g., previously been
016l-
00 1700
1600
1500
-
00
1300
11M
l(lo
i7/cm
Fig. 4. Difference IR spectra (symmetric (B) and antisymmetric (A) NH bending regions) of NH, adsorbed on dehydroxylated MgO surfaces. For further explanations see legend of fig. 1 (two traces for the NH, equilibrium pressures 147 and 160 Pa were added).
R. Echterhoff, E. Kniizinger / FTIR spectroscopy of ammonia on MgO
C/cm-’
Fig. 5. As fig. 4B, but different NH, equilibrium pressures: 173 (lower trace), 347,427, 867, and 1293 Pa (upper trace).
determined around 3550 cm-’ (table 1). The OH and NH stretching region does not allow us to infer with certainty the presence of physisorbed NH, molecules on the bare dehydroxylated MgO surface. This is, however, clearly evidenced in the spectral intervals where the NH bending vibrations appear. Although NH, groups exhibit only one band of this type around 1550 cm-‘, we observed two features at 1110 and 1070 cm-’ (fig. 4B). They have undoubtedly to be attributed to the symmetric bending vibrations (see also section 3.3) of two differently physisorbed NH, species. The pressure dependence of the band intensities indicates that the adsorption probabilities are practically the same for both. If - as proposed by Coluccia et al. [6] - Mg2+ and O*- are adopted as the effective surface sites, this means that the heat of adsorption is essentially the same for the surface complexes in question. The physisorption according to Mg*++ O*-+
NH, NH,
+S Mg’+. + 0*-e..
. . . NH,, . H-NH
(5) 21
(6)
is also evidenced - at least at low NH, pressures - by the broad band at 1600 cm-’ (fig. 4A). It
243
originates from the asymmetric bending vibration of physisorbed NH,. At an early stage of adsorption there is no signal present around 1550 cm-‘, which would be indicative of the formation of surface NH, groups. Evidently the probability of chemisorption according to reaction (1) is much smaller than that of physisorption according to reactions (5) and (6). From previous studies of Coluccia et al. [6] we know that the heterolytic dissociation of NH, necessarily implies 3-coordinated ions on the corners of the cubic microcrystall&es of the MgO powder. In general, only a small fraction of such sites compared to the total of ion pairs present on the surface is available. Thus higher NH, pressures are required to manifest the chemisorption process in question (reaction (1)). In fact, at 44 Pa a band at 1555 cm-’ appears which may not be removed in the course of a subsequent desorption process. At even higher NH, pressures a shoulder grows out at 1544 cm-’ which rapidly disappears on evacuating the adsorption cell. Therefore, this latter feature is attributed to NH, molecules acting as proton donor in a hydrogen bond with O*- sites (reaction (6)). The inequivalence of the 3 NH bonds eliminates the twofold degeneracy of the antisymmetric bending mode and, consequently, a second band (1544 cm-‘), in addition to the one buried in the feature around 1600 cm-‘, must appear. There is conclusive evidence that the physisorption complexes (reactions (5) and (6)) are not precursor species involved in the process of heterolytic dissociation of NH,. Otherwise the pressure dependence of the intensity of the symmetric bending modes at 1110 and 1070 cm-‘, respectively, should be different, since it would be extremely unlikely if the energies of activation for the heterolytic dissociation were the same for the two NH, species in reactions (5) and (6). An influence of the degree of coordination of Mg*+ and O*- on the IR spectra of the two physisorption complexes in reactions (5) and (6) was not observed. As expected, the observations made in the course of the adsorption process are not simply reversed in the desorption experiments. The bands assigned to the chemisorption product NH, (amide group, reaction (1)) remain even after extensive degassing at room temperature. The NH,
244
R. Echterhoff,
E. Kniizinger / FTlR spectroscopy of ummonia on MgO
proton donor complex (reaction (6)) evidenced by the band at 1544 cm-’ may easily be removed from the surface whereas the NH, electron donor complex (reaction (5)) remains there even at comparatively low NH, pressures. As in the case of the hydroxylated MgO surfaces the complexes where NH, acts as proton donor appear to be the least stable ones. 4. Conclusion The IR spectra related to the adsorption of NH, on uniformly hydroxylated and on dehydroxylated MgO may be interpreted consistently in the NH, pressure range below 200 Pa. We found unambiguous evidence for the presence of two different NH, complexes on hydroxylated MgO surfaces (degassing temperature: 250 o C). Chemisorption does not occur under these conditions. The completely dehydroxylated MgO surface, in turn, allows for a heterolytic dissociation of NH, and, in addition, on less reactive sites NH, is physisorbed forming two complexes with Mg2+ and 02- surface ions. Deviations from this simple model describing the elementary steps of adsorption of NH, on well defined MgO surfaces are observed for samples predominantly covered with isolated OH groups. Different from the 3- and higher-coordinated isolated OH groups, the l-coordinated species do not lend themselves at room temperature as proton donors for H bonds with NH, [9,19]. Our adsorption and desorption experiments clearly show that both band positions and band shapes strongly depend on the NH, pressure. This is particularly true for pressures around and above 1 kPa. Therefore, we would abstain from attributing characteristic band positions to surface complexes resulting under these conditions. The comparison of fig. 4B and 5 demonstrates how simple the situation is on dehydroxylated MgO surfaces as long as we restrict ourselves to the pressure range of a few hundred Pa and how complicated it turns on approaching 1 kPa. Similar observations were made in all spectral intervals and for different degrees of hydroxylation of the MgO surfaces. MgO and CaO exhibit the same crystal structure. Therefore, the same types of surface centres
should be present on these materials. In fact, our IR data show that the same schemes of interaction of NH, with both MgO and CaO surfaces may be established - as long as the hydroxylation state is the same for both substrates. There is, however, an interesting phenomenon intimately related to the reduced electronegativities of Ca compared to Mg. IR desorption experiments clearly show that on hydroxylated and dehydroxylated MgO surfaces complexes with NH, as electron donor (reactions (3) (4) and (5)) are more stable than that with NH, as proton donor (electron acceptor, reaction (6)). Recent studies in our laboratory have provided unambiguous evidence that this sequence is _ as expected - reversed when CaO is used as substrate.
References 111A.J. Tenth and D. Giles, J. Chem. Sot. Faraday
Trans. 1, 68 (1972) 193. I21 A.J. Tenth, J. Chem. Sot. Faraday Trans. 1, 68 (1972) 197. D.V. Pozdnyakov and V.N. Filimonov, [31 A.A. Tsyganenko, J. Mol. Struct. 29 (1975) 299. E. Garrone and E. Borello, J. Chem. Sot. [41 S. Coluccia, Faraday Trans. 1, 79 (1983) 607. [51 A. Zecchina and G. Spoto, J. Catal. 96 (1985) 586. I61 S. Coluccia, S. Lavagnino and L. Marchese, J. Chem. Sot. Faraday Trans. 1, 23 (1987) 477. and E. Kniizinger, in preparation. [71 R. Echterhoff and E. Knozinger, Surf. Sci. 188 (1987) 181. [81 P. Hoffmann [91 E. KnBzinger, P. Hoffmann and R. Echterhoff, Mikrochim. Acta (Wien) 2 (1988) 27. HOI C. Coluccia, A.J. Tenth and R.L. Segall, J. Chem. Sot. Faraday Trans. 1, 75 (1979) 1769. [ill J.B. Peri, J. Phys. Chem. 69 (1965) 231. [121 J.B. Peri, J. Phys. Chem. 70 (1966) 2937. Infrared and Raman Spectra of Inorganic U31 K. Nakamoto, and Coordination Compounds, 3rd ed. (Wiley, New York, 1978). [141 G. Herzberg, Molecular Spectra and Molecular Structure, Vol. 2, Infrared and Raman Spectra of Polyatomic Molecules (Van Nostrand-Reinhold, New York, 1945). [151 P. Schuster, Z. Chem. 13 (1973) 41. and L.C. Allen, J. Am. Chem. Sot. 92 [161 P.A. Kollman (1970) 753. and A. Earnshaw, Chemistry of the [171 N.N. Greenwood Elements (Pergamon, Oxford, 1984). Spectra of Labelled [181 S. Pinchas and I. Laulicht, Infrared Compounds (Academic Press, New York, 1971). and E. KnBzinger, J. Mol. Struct. 174 [191 R. Echterhoff (1988) 343.
231
FTIR SPECTROSCOPIC CHARACTERIZATION OF THE ADSORPTION AND DESORPTION OF AMMONIA ON MgO SURFACES
Ralf ECHTERHOFF
* and Erich KNGZINGER
**
Institut $2 Physikalische Chemie, Universitiil Siegen, Postfach 101240, D-5900 Siegen, Fed. Rep. of Germany
Received 5 September 1989; accepted for publication 8 December 1989
In the present paper an attempt is made to disentangle the complex IR spectra obtained in the course of the adsorption of NH, on high surface area MgG. For this purpose two simplifications were introduced in comparison to previous studies: (a) Only uniformly hydroxylated MgO surfaces (degassing temperature: 200 0 C) and completely dehydroxylated MgO surfaces (degassing temperature: 1000 o C) were used as adsorbent. (b) The equilibrium pressure of the adsorbate NH, was generally maintained below 200 Pa in all adsorption and desorption experiments. Under these conditions the position and the pressure dependence of the resulting bands may consistently be interpreted in terms of characteristic surface species. Only the completely dehydroxyiated MgG surface gives rise to a heterolytic dissociation into neighbouring NH, and OH groups. In addition, NH, is physically adsorbed on less reactive sites via diverse types of hydrogen bonds. Less well defined hydroxylation states give rise to considerably more complicated spectra. In a first approximation they may be constructed by linearly combining two spectral patterns, one related to a uniformly hydroxylated and the other to a dehydroxylated surface. NH, pressures around and beyond 1 kPa give rise to significant changes of band positions and band shapes compared to those observed below 200 Pa. The interpretation of these effects which have previously been ignored must necessarily rely on more complicated models.
1. Introduction
High surface area magnesium oxide has previously been studied with respect to the interaction of its surface sites with ammonia [l-6]. It has been shown by IR spectroscopy that a heterolytic dissociation occurs at the lowest coordinated (three-fold) surface ion pairs. In addition, molecular adsorption on diverse surface sites of hydroxylated and d~hydroxylat~ MgO surfaces appeared to play an important role. An intuitively intelligible model of possible surface complexes may easily be derived from the known properties of NH, and of the surface sites of MgO (Mg2+ and O*- ions as well as surface OH groups). In a recent paper f6] IR spectroscopic data is presented which was considered to * Present address: Bayer A.G. Dormagen, Polyurethane Division, Geb. Kl, D-4047 Dormagen, Fed. Rep. of Germany. * * To whom correspondence should be addressed. 0039-6028/90/$03.50
be in agreement with the occurrence of these complexes. All spectra were, however, recorded at relatively high NH, pressures which favour interactions between two or more nei~bou~ng adsorbate species as well as between adsorbate species and further NH3 molecules. These effects although undoubtedly present in the spectra were essentially ignored in the course of the interpretation. Aiming at a reliable interpretation we have, therefore, confined ourselves exclusively to the low NH, pressure domain and thus to the intrinsic elementary steps of adsorption of NH, on MgO surfaces. This necessarily implies a reduction of the signal-to-noise ratio. In light of these considerations the application of a research grade FTIR spectrometer equipped with a highly sensitive cryogenic detector is an absolutely necessary condition. On the basis of such experimental equipment minute spectral changes - a few thousands of an absorbance unit - induced in the
0 1990 - Elsevier Science Publishers B.V. (North-Holland)
238
R. Echterhoff,
E. Kniizinger
/ FTIR spectroscopy
course of an absorption or desorption process may then be evidenced by spectral subtraction.
of ammonia on MgO
3. Results and discussion 3.1. MgO surfaces
2. Experimental Self-supporting disks of MgO and CaO (Merck GmbH) were obtained by pressing about 50 mg of the oxide powder at pressures of 10 MPa. By degassing at less than 10-l Pa and 927” C for - 2 h all surface impurities were removed. Thereafter the oxide pellets did not exhibit any specific IR absorption above 1000 cm-‘. In order to create a certain hydroxylation state, the clean surface was then completely rehydrated at 2 kPa H,O vapour pressure and again outgassed at a properly selected temperature [7]. The procedure applied in the adsorption experiments is as follows. The pretreated MgO pellet located in the IR adsorption cell [8,9] is exposed to NH, gas via a needle valve. The prepressure has to be adequately selected in order to obtain an appropriate rate of increase for the NH, pressure in the cell. “Appropriate” means that the adsorption equilibrium is readily established in the time interval between the data collections for two subsequent IR spectra. This procedure permits to monitor the process of adsorption on a time scale which ranges from seconds to hours. The desorption experiments were carried out simply by connecting the IR cell volume containing the NH, charged MgO pellet with a large evacuated reservoir for a time interval which guarantees the establishment of thermodynamic equilibrium. The IR transmission spectra were recorded at about 30 o C with an evacuable FTIR spectrometer model IFS-113 v (Bruker Analytische Messtechnik GmbH, FRG) equipped with a LN,-cooled MCT detector. The resolving power applied was 2 cm ’ for most experiments. Higher resolving power did not create any changes in band shape. The specific surface area of the MgO samples was 60 m* gg’. It was determined by adsorption experiments using molecular N, as probe (“BET method”). The pressure applied on preparing the self-supporting MgO disks as well as the degassing temperatures used (200-1000” C) do not change the specific surface area of MgO to any significant extent.
Photoluminescence and UV reflectance spectroscopy clearly established the conclusion that 3-, 4-, and 5-coordinated oxygen and magnesium ions are present at the surface of high surface area MgO [lo]. The reactivity of these sites is undoubtedly related to the degree of coordination. Thus according to -Mg*+
-O;,;
-
+ NH,
H I I -Mg-(A,,-,
-
H,N
(1)
ammonia undergoes a heterolytic dissociation exclusively on 3-coordinated cation anion couples, i.e., on corners of the cubic microcrystals of the MgO powder [6]. H,O, on the other hand, reacts on 3- as well as on 4- and 5-coordinated sites giving rise to the formation of H bonded pairs of OH groups [lo] as visualized by -Mg*+
-O;~-+H20--_=
HO,....H I I -Mg-O,-.
(2)
The degree of coordination x = III, IV, V. They exhibit a sharp OH stretching band at 3750 cm-’ and a broad one around 3550 cm-’ for the OH groups acting as proton acceptor (free OH) and as proton donor (H bonded OH), respectively [7] (table 1). A MgO surface exposed to water vapour pressure of at least 2 kPa and subsequently to vacuum treatment at 250 “C is essentially uniformly covered with the OH pairs according to reaction (2) [7]. On heating the sample in vacua at 1000” C the OH groups are removed quantitatively from the surface. In the course of this study only the two well defined hydroxylation states described above will be considered (table 1). Much more complicated surface systems will arise when an intermediate degassing temperature is applied. A thermal treatment at 450” C, e.g., creates a non-uniform surface which incorporates different hydroxylated and dehydroxylated species
R. Echterhoff, E. Kniizinger / FTIR spectroscopy of ammonia on MgO Table 1 Characterization
of surface
species on hydroxylated Surface
Degassing temperature of MgO(“C)
1 Mg-6,-
/H-O,
MgO surfaces
Characteristic IR absorption pattern (cm-‘)
Assignment
a)
3750/3550
Y,~,,, of OH groups acting as proton acceptor and proton donor, respectively
a)
3750
species
H-?,..H
250
and dehydroxylated
239
-Mg-o,H
I -Mg-On,450
\
3737 aeon, of isolated OH groups with I, Ill, IV, and V coordinated oxygen, respectively
H
I
3722
-Mg-O,,H -Mg-b,\ 1000
-Mg
2+ _02_
‘) x = Ill, IV, or V.
(table 1). Each of them has to be considered for possible interactions with NH,. Under these conditions the complexity of the respective IR spectra appears to be beyond the scope of a reliable analysis. 3.2. Vibrational properties of ammonia NH, (point group C,,) exhibits 6 IR aktive fundamentals two of them being twofold degenerate. According to table 2 the IR spectrum of NH, should provide three spectral intervals with relevant structural information. In order to use NH, as a model compound for probing the surface these spectral intervals have to be carefully checked for the influence of the surface potential on both the intramolecular force field and on the symmetry of the probe molecule (physisorption). Thus the IR spectroscopic response to physical adsorption of NH, on a specific surface site consists in principle in 4 characteristic band shifts and possibly - in the appearance of 2 additional bands (one in the NH stretching and one in the NH bending region) owing to the lifting of degeneracy.
Intrinsic chemical reactions of NH, with surface sites (chemisorption) may easily be recognized on the basis of the IR spectra of the resulting products. For example, the formation of an amide group or an ammonium ion on the surface necessarily implies the appearance of characteristic bands located around 1560 [11,12] and 1400 cm-’ [13], respectively (table 2). On the other hand, the presence of a band around 1560 cm-’ is not a sufficient condition to conclude the occurrence of the heterolytic dissociation of NH,. Such a band
Table 2 IR spectroscopic products of NH, Species
NH,
-NH, NH;
characterization on MgO surfaces
Characteristic IR absorption pattern (cm-‘) 3500-3300 1700-1600 1300-!Joo -1560 -1400
of possible
chemisorption
Assignment
Stretching (a,. e) Antisymmetric bending (e) Symmetric bending (a,) Bending Bending
(“amide band”) (“ammonium ion band”)
R. Echterhoff, E. K&zinger
240
/ FTIR spectroscopy of ammonia on MgO
may also originate from ammonia molecules which have non-equivalent NH bonds (H bonding!). Desorption experiments may help to discriminate the two species in question. 3.3. NH, on hydroxylated
MgO surfaces
All spectra (figs. 1 and 2) were recorded in the difference mode (A.A = f (F)), i.e., the absorbance trace observed for the pure hydroxylated MgO substrate was subtracted from the spectrum obtained after NH, adsorption. An interaction of two or more NH, molecules with one surface site is - owing to the experimental conditions applied _ extremely unlikely. The IR spectra presented here refer to a NH, surface coverage of less than 1% with respect to the area of the bare MgO surface. The absence of any band around 1560 and/or 1400 cm-’ definitely precludes any chemical reaction of NH, with OH groups in the sense of a formation of surface NH: or surface -NH,, respectively (table 2). Thus the most likely interactions of NH, with the hydroxylated MgO surface should be hydrogen bonds. In fact, the negative band at 3750 cm-’ (fig. 1) clearly indicates the consumption of free surface OH groups [9], which are excellent proton donors and might be involved in the process presented by: H-O,... I -Mg-OX-
H I + NH,
====G
complex A
H,N...H-0,..**H I -Mg-00,-.
I (3)
The degree of coordination x = III, IV, V. This idea is confirmed by a broad and structured positive band which emerges between 3700 of and 3400 cm -’ in the course of the adsorption NH,. In this spectral interval stretching vibrations of H bonded OH groups (proton donor) are to be expected. The OH pairs of the hydroxylated MgO surface before NH, adsorption (left side of reaction (3)) exhibit, e.g., a band at 3550 cm-’ (see section 3.1, [7]). It is still present in the difference spectrum after NH, adsorption indicating that process (3) initiates a certain polarization of the H
Fig. 1. Difference IR spectra (OH and NH stretching region) of NH, adsorbed on uniformly hydroxylated MgO surfaces (degassing temperature 200 o C). AA = A, - A, where A, and A, are the spectra obtained before NH, admission and at a NH, equilibrium pressure p, respectively. The adopted values of p were 5.3 (lower trace), 24, 44, 64, 84, 103, 123, and 173 Pa (upper trace).
bond between neighbouring surface OH groups. Otherwise the intensity around 3550 cm-’ should be canceled by spectral subtraction. In addition, two positive bands at 3600 and 3489 cm-i are observed which have then to be attributed to surface OH species hydrogen bonded to NH, as proton acceptor. One such complex is shown in reaction (3) the other is most likely related to the process: H-0,.-.-H I -Mg-OX-
I + NH,
-
complex B
H,
N’
H-_O,....fi
I -Mg-OX-.
I (4)
The degree of coordination x = III, IV, V. Already at an early stage of adsorption a broad positive band centred around 3033 cm- ’ appears. Both position and width of this feature clearly
R. Echterhoff, E. Kniizinger / FTIR spectroscopy of ammonia on MgO
recommend an assignment to NH groups acting as proton donor. On the other hand, it is extremely unlikely that all H atoms of the adsorbed NH, molecules are involved in hydrogen bonds. Therefore, according to the model developed above NH stretching bands should appear in close proximity to 3444 and 3337 cm-‘, the values for isolated NH, molecules [14]. This is definitely not the case. It has to be assumed that the product of the respective absorbance coefficient and the surface coverage is too low for a detection. Coluccia et al. [6] found a band at 3365 cm-’ with a shoulder at 3390 cm- * for surface coverages of 1 to 2 molecules per 100 A*, i.e., 15-30% of the bare MgO surface. This is a by 1 to 2 orders of magnitude higher surface coverage than the one we used in our experiments. A lateral interaction of adsorbed NH, molecules may, therefore, not be excluded. The band positions observed by Coluccia et al. (61 are blue shifted as compared to the above mentioned gas phase value of the symmetric NH, stretching vibration. This is, in principle, not compatible with the presently accepted hydrogen bond theory. Thus, based on the relatively unspecific data on the OH and NH stretching region (fig. 1) essentially two different types of surface complexes of NH, on hydroxylated MgO surfaces have to be expected (reactions (3) and (4)). To corroborate this model additional information is required from the sy~et~c and ~tisy~et~c bending region (fig. 2). Two symmetric bending modes at 1127 and 1095 cm-’ are observed in fig. 2B. They are both blue shifted with respect to the gas phase data [14]. At low NH, ~uilib~~ pressures there is only the band at 1127 cm-’ which we assign to the complex presented in reaction (3). As the adsorption goes on (higher NH, pressures) the formation of a second species, most likely the complex described by reaction (4), becomes increasingly important and gives rise to a prominent shoulder at 1095 cm-‘. At even higher NH, pressures again the rate of growth of the band at 1127 cm-’ dominates the spectrum. This observation may tentatively be interpreted in terms of the formation of a so far unspecified third type of adsorption complex.
241
‘500
to
Fig. 2. Difference IR spectra (symmetric (B) and antisymmetric (A) NH bending regions) of NH, adsorbed on uniformly bydroxylated MgO surfaces. For further explanation see legend of fig, 1 (two traces for the NH, ~uilib~urn pressures 147 and 160 Pa were added).
The information provided by the antisymmetric NH, bending modes (1700-1500 cm-‘) is less specific than that obtained from the symmet~c bending modes. There is one broad band around 1640 cm-’ for the NH, pressures applied (fig. 2A). On increasing the pressure the position of the absorption maximum is shifted from about 1630 to 1654 cm-‘. This trend supports the model of at least two different types of NH, surface complexes on fully hydroxylated MgO. On reducing the equilibrium NH, pressure (desorption experiments) the observations made in the course of the adsorption experiments are essentially reversed. Thus the adsorption probability of the 2 different sites appears to be determined predominantly by the stability of the resulting surface complexes. Theoretical studies clearly show that NH, acceptor complexes with H,O are considerably more stable than the NH, donor complexes [15,16]. A similar situation should arise when NH, interacts with surface OH groups. In fact, our desorption experiments clearly evidence that NH, donor complexes, if they exist at all, are less stable than the complexes A and B (reactions (3) and (4), respectively). Additional support for the assign-
242
R. EchterhofJ E. Kniizinger / FTIR spectroscopy
ment presented here may be derived from experimentally determined hydrogen bond lengths d(OH . . . N) and d(0 .. . HN) of diverse complexes: they amount to 268-279 pm and 281-304 pm, respectively [17]. According to reactions (3) and (4) there should be less sterical hindrance for the formation of complex A (3) in comparison with B (4). Therefore, it appears reasonable to attribute the highest stability to complex A. NH, belongs to point group C,, in the two NH, proton acceptor complexes (reactions (3) and (4)) and to C, in the donor complexes (only 2 equivalent NH bonds). For NH, donor complexes one would, therefore, expect an additional NH bending mode similar to that observed for NH,D compared with NH, [18]. At an equilibrium NH, pressure of more than 2.7 kPa an extremely weak band appears at 1554 cm-‘. It could originate from the lifting of degeneracy for the antisymmetric bending vibration in an NH, proton donor complex. 3.4. NH, on dehydroxylated MgO surfaces The spectra in figs. 3-5 were recorded as described in section 3.3, i.e., in the difference mode. Since MgO does not exhibit any specific IR absorption after a vacuum treatment at 1000 o C, the respective difference spectra of NH, adsorbed on these samples comprise only positive bands (figs. 3-5). According to sections 3.1 and 3.2 the sharp feature at 3755 cm-’ (fig. 3) has to be attributed to free or isolated surface OH groups and thus provides conclusive evidence for the chemisorption process according to reaction (1). This model also allows us to assign the bands at 3436 and 3329 cm-’ (fig. 3) which should be related to the antisymmetric and symmetric stretching vibration of the resulting surface NH, groups, respectively. The unspecific continuous absorption around and above 3000 cm-’ recommends that some of the surface NH, groups interact with each other or with surface OH groups via hydrogen bonds. On the other hand, the model does not enable the assignment of the feature at 3565 cm-’ to be made with confidence. The additional assumption has to be adopted that a certain percentage of the resulting surface OH groups acts as proton donor
of ammonia on MgO
$00
3obo
300
i; /cm
-1
Fig. 3. Difference IR spectra (OH and NH stretching region) of NH, adsorbed on dehydroxylated MgO surfaces (degassing temperature: 1000 o C). For further explanations see legend of fig. 1 (the trace for the NH, equilibrium pressure of 103 Pa is not there).
in hydrogen bonds with neighbouring NH, groups or with NH, molecules. The stretching vibration of hydrogen bonded surface OH groups on hydroxylated MgO surfaces has, e.g., previously been
016l-
00 1700
1600
1500
-
00
1300
11M
l(lo
i7/cm
Fig. 4. Difference IR spectra (symmetric (B) and antisymmetric (A) NH bending regions) of NH, adsorbed on dehydroxylated MgO surfaces. For further explanations see legend of fig. 1 (two traces for the NH, equilibrium pressures 147 and 160 Pa were added).
R. Echterhoff, E. Kniizinger / FTIR spectroscopy of ammonia on MgO
C/cm-’
Fig. 5. As fig. 4B, but different NH, equilibrium pressures: 173 (lower trace), 347,427, 867, and 1293 Pa (upper trace).
determined around 3550 cm-’ (table 1). The OH and NH stretching region does not allow us to infer with certainty the presence of physisorbed NH, molecules on the bare dehydroxylated MgO surface. This is, however, clearly evidenced in the spectral intervals where the NH bending vibrations appear. Although NH, groups exhibit only one band of this type around 1550 cm-‘, we observed two features at 1110 and 1070 cm-’ (fig. 4B). They have undoubtedly to be attributed to the symmetric bending vibrations (see also section 3.3) of two differently physisorbed NH, species. The pressure dependence of the band intensities indicates that the adsorption probabilities are practically the same for both. If - as proposed by Coluccia et al. [6] - Mg2+ and O*- are adopted as the effective surface sites, this means that the heat of adsorption is essentially the same for the surface complexes in question. The physisorption according to Mg*++ O*-+
NH, NH,
+S Mg’+. + 0*-e..
. . . NH,, . H-NH
(5) 21
(6)
is also evidenced - at least at low NH, pressures - by the broad band at 1600 cm-’ (fig. 4A). It
243
originates from the asymmetric bending vibration of physisorbed NH,. At an early stage of adsorption there is no signal present around 1550 cm-‘, which would be indicative of the formation of surface NH, groups. Evidently the probability of chemisorption according to reaction (1) is much smaller than that of physisorption according to reactions (5) and (6). From previous studies of Coluccia et al. [6] we know that the heterolytic dissociation of NH, necessarily implies 3-coordinated ions on the corners of the cubic microcrystall&es of the MgO powder. In general, only a small fraction of such sites compared to the total of ion pairs present on the surface is available. Thus higher NH, pressures are required to manifest the chemisorption process in question (reaction (1)). In fact, at 44 Pa a band at 1555 cm-’ appears which may not be removed in the course of a subsequent desorption process. At even higher NH, pressures a shoulder grows out at 1544 cm-’ which rapidly disappears on evacuating the adsorption cell. Therefore, this latter feature is attributed to NH, molecules acting as proton donor in a hydrogen bond with O*- sites (reaction (6)). The inequivalence of the 3 NH bonds eliminates the twofold degeneracy of the antisymmetric bending mode and, consequently, a second band (1544 cm-‘), in addition to the one buried in the feature around 1600 cm-‘, must appear. There is conclusive evidence that the physisorption complexes (reactions (5) and (6)) are not precursor species involved in the process of heterolytic dissociation of NH,. Otherwise the pressure dependence of the intensity of the symmetric bending modes at 1110 and 1070 cm-‘, respectively, should be different, since it would be extremely unlikely if the energies of activation for the heterolytic dissociation were the same for the two NH, species in reactions (5) and (6). An influence of the degree of coordination of Mg*+ and O*- on the IR spectra of the two physisorption complexes in reactions (5) and (6) was not observed. As expected, the observations made in the course of the adsorption process are not simply reversed in the desorption experiments. The bands assigned to the chemisorption product NH, (amide group, reaction (1)) remain even after extensive degassing at room temperature. The NH,
244
R. Echterhoff,
E. Kniizinger / FTlR spectroscopy of ummonia on MgO
proton donor complex (reaction (6)) evidenced by the band at 1544 cm-’ may easily be removed from the surface whereas the NH, electron donor complex (reaction (5)) remains there even at comparatively low NH, pressures. As in the case of the hydroxylated MgO surfaces the complexes where NH, acts as proton donor appear to be the least stable ones. 4. Conclusion The IR spectra related to the adsorption of NH, on uniformly hydroxylated and on dehydroxylated MgO may be interpreted consistently in the NH, pressure range below 200 Pa. We found unambiguous evidence for the presence of two different NH, complexes on hydroxylated MgO surfaces (degassing temperature: 250 o C). Chemisorption does not occur under these conditions. The completely dehydroxylated MgO surface, in turn, allows for a heterolytic dissociation of NH, and, in addition, on less reactive sites NH, is physisorbed forming two complexes with Mg2+ and 02- surface ions. Deviations from this simple model describing the elementary steps of adsorption of NH, on well defined MgO surfaces are observed for samples predominantly covered with isolated OH groups. Different from the 3- and higher-coordinated isolated OH groups, the l-coordinated species do not lend themselves at room temperature as proton donors for H bonds with NH, [9,19]. Our adsorption and desorption experiments clearly show that both band positions and band shapes strongly depend on the NH, pressure. This is particularly true for pressures around and above 1 kPa. Therefore, we would abstain from attributing characteristic band positions to surface complexes resulting under these conditions. The comparison of fig. 4B and 5 demonstrates how simple the situation is on dehydroxylated MgO surfaces as long as we restrict ourselves to the pressure range of a few hundred Pa and how complicated it turns on approaching 1 kPa. Similar observations were made in all spectral intervals and for different degrees of hydroxylation of the MgO surfaces. MgO and CaO exhibit the same crystal structure. Therefore, the same types of surface centres
should be present on these materials. In fact, our IR data show that the same schemes of interaction of NH, with both MgO and CaO surfaces may be established - as long as the hydroxylation state is the same for both substrates. There is, however, an interesting phenomenon intimately related to the reduced electronegativities of Ca compared to Mg. IR desorption experiments clearly show that on hydroxylated and dehydroxylated MgO surfaces complexes with NH, as electron donor (reactions (3) (4) and (5)) are more stable than that with NH, as proton donor (electron acceptor, reaction (6)). Recent studies in our laboratory have provided unambiguous evidence that this sequence is _ as expected - reversed when CaO is used as substrate.
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Trans. 1, 68 (1972) 193. I21 A.J. Tenth, J. Chem. Sot. Faraday Trans. 1, 68 (1972) 197. D.V. Pozdnyakov and V.N. Filimonov, [31 A.A. Tsyganenko, J. Mol. Struct. 29 (1975) 299. E. Garrone and E. Borello, J. Chem. Sot. [41 S. Coluccia, Faraday Trans. 1, 79 (1983) 607. [51 A. Zecchina and G. Spoto, J. Catal. 96 (1985) 586. I61 S. Coluccia, S. Lavagnino and L. Marchese, J. Chem. Sot. Faraday Trans. 1, 23 (1987) 477. and E. Kniizinger, in preparation. [71 R. Echterhoff and E. Knozinger, Surf. Sci. 188 (1987) 181. [81 P. Hoffmann [91 E. KnBzinger, P. Hoffmann and R. Echterhoff, Mikrochim. Acta (Wien) 2 (1988) 27. HOI C. Coluccia, A.J. Tenth and R.L. Segall, J. Chem. Sot. Faraday Trans. 1, 75 (1979) 1769. [ill J.B. Peri, J. Phys. Chem. 69 (1965) 231. [121 J.B. Peri, J. Phys. Chem. 70 (1966) 2937. Infrared and Raman Spectra of Inorganic U31 K. Nakamoto, and Coordination Compounds, 3rd ed. (Wiley, New York, 1978). [141 G. Herzberg, Molecular Spectra and Molecular Structure, Vol. 2, Infrared and Raman Spectra of Polyatomic Molecules (Van Nostrand-Reinhold, New York, 1945). [151 P. Schuster, Z. Chem. 13 (1973) 41. and L.C. Allen, J. Am. Chem. Sot. 92 [161 P.A. Kollman (1970) 753. and A. Earnshaw, Chemistry of the [171 N.N. Greenwood Elements (Pergamon, Oxford, 1984). Spectra of Labelled [181 S. Pinchas and I. Laulicht, Infrared Compounds (Academic Press, New York, 1971). and E. KnBzinger, J. Mol. Struct. 174 [191 R. Echterhoff (1988) 343.