Langmuir 1991, 7, 1498-1505
1498
Adsorption and 2HNMR of Ammonia, Pyridine, Dimethylamine, and Benzene on Rutile and Anatase B. Boddenberg' and K. Eltzner Lehrstuhl fiir Physikalische Chemie II, Universitat Dortmund, Otto-Hahn-Strasse 6, 0-4600 Dortmund 50, West Germany Received October 9, 1990.I n Final Form: January 21, 1991 Adsorption isotherms and deuteron (2H) NMR spectra of ammonia, pyridine, dimethylamine, benzene, and water on microcrystalline rutile were measured. For comparison purposes, 2H NMR spectra of the first three adsorptives on anatase were obtained. In all cases except benzene it was found that after 12 h of pumping at ambient temperature, 'irreversibly" adsorbed amounts of these molecules are retained on the surface. The results obtained can be rationalized in terms of a model that ascribes about 85% of the surface exposed to (110)planes, which carry the most active Lewis acidic Ti'+ centers. At the level of irreversible adsorption, the nitrogen base molecules occupy practically every other site, whereas water occupies each of these centers. Under adsorption equilibrium conditions, further ammonia and pyridine molecules are Lewis coordinated to the centers left unoccupied, whereas this is not feasible in the case of dimethylamine.
Introduction The surface of hydrated and dehydrated titania in both modifications, rutile and anatase, exhibits predominantly Lewis acidic properties. This is explained as being due to the exposure of coordinately unsaturated (cus) Ti4+ions which may occur in different stereochemical environments. On the basis of idealizing models for microcrystalline rutile, such different environments were attributed to the exposure of a limited number of crystallographically welldefined cleavage planes of which (110)is the most frequent and, at the same time, the most active of these planes.' The present contribution pursues the aim to use nitrogen bases as electron pair donor molecules in order to determine the number of the Lewis acid sites on well-defined titania and to learn about the orientation and the reorientational motions of the molecules on these sites. It was also intended to confirm previous conclusions concerning the contribution of the most active (110) planes to the total surface area of rutile that were obtained with water and 2-propanol as the adsorbate molecules.2 Experimental Section Materiale. Crystallographically pure anatase was obtained by 4 h of calcination at 540 O C of a precipitate prepared from tetraisopropyltitanate according to a procedure described elsewhere.2 The crystallographically pure rutile (British Tioxide, Code No. CLDD/887 A) was a gift of the late Professor Parfitt. The particles of both materials were of spheroidal shapes with sizes in the range 50-180 nm. Prior to all loadings with the adsorptives under study, the adsorbents were subjectedto a standard pretreatment consisting of cycles of evacuation and oxygen treatments as described elsewhere.2 The gaseous adsorptives (NH3and (CH&NH from Messer-Griesheim,Frankfurt, Germany;ND3and (CD&NHfrom MSD, Montreal, Canada) were used as obtained; the liquid adsorptives (C& and C5HsN from Merck, Darmstadt, Germany; DzO, CeDe, and CsD5N from Roth, Karlsruhe, Germany) were degassed by repeated freeze-pump-thaw cycles. Adsorption and NMR Sample Preparation. Low-temperature (77.7 K) nitrogen ad- and desorption isotherms were measured with the aid of a conventional volumetric apparatus. The ad- and desorption isotherms of all other adsorbates were measured gravimetricallywith a Cahn electrobalance(HzO,C6H6) (1) Jones, P.; Hockey, J. A. Trans. Faraday SOC.1971,67, 2679. (2) Boddenberg, B.; Horstmann, W. Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 519.
0743-7463/91/2401-1498$02.50/0
or with a McBain quartz-spring balance (NH3, (CH&NH, pyridine-hs)at temperaturesthat will be given at appropriate places in the text. After completionof the desorptionruns, the samples were warmed to ambient temperature and evacuated (p lo6 mbar) overnight, and the adsorbate remaining on the samples (nh,"irreversibly" adsorbed amount) was determined. NMR samples of standard pretreated anatase and rutile carrying these irreversibly or some other well-defined amounts of the deuteriatedadsorbateswere prepared in thin-walledglasstubes (10mm diameter)and sealed off for use in the spectrometer. The deuteron (ZH)NMR measurementswere performed with the aid of a FT-NMR spectrometer (CXP 100,Bruker-Physik, Karlsruhe) in conjunction with a cryogenic magnetic system at the resonance frequency, 4 2 s = 52.7 MHz. The 2H spectra were obtained by Fourier transformation of the free induction decays (FID)or of the quadrupoleechoes generated by the quadrupole echo (QE)pulse sequence3and detected in quadrature in cases where motionally averaged singlet or powder pattern type spectracame out, respectively. The pulse widthsand pulse delays of the QE sequence were 4 and 40-50 ps, respectively. Due to the low surface area of the adsorbents and the comparativelylow surface coverages of the deuteriated molecules, a great number of scans ranging from several hundred to about 10 OOO had to be employed.
-
Rssults Adsorption Isotherms. Figure 1 shows the nitrogen ad- and desorption isotherms at 77.7 K of standard pretreated rutile and anatase. From the BET plots being linear in the relative pressure range 0.05 I p / p o I0.35, the monolayer capacities n,, the C values, and the specific surface areas S N ~obtained , by using the conventional nitrogen molecule cross section CTN~= 0.162 nm2, were determined. These data are collected in Table I. The coincidence of the ad- and desorption isotherms over the whole pressure range studied proves that both adsorbents are nonporous. The solid line represents the nitrogen adsorption isotherm of the self-prepared rutile (SN~ = 13.3 m2/g) used in a previous investigation2 but which is drawn here referenced to the specific surface area of the presently used rutile ( S N=~ 17.4 m2/g). Besides slight deviations at relative pressures 20.9, the isotherms are perfect agreement, showing that their surface properties should be nearly identical. (3) Davis, J. H.; Jeffrey, K. R.; Bloom, M.;Valic, M.I.; Higge, T.P. Chem. Phys. Lett. 1976,42,390.
0 1991 American Chemical Society
Langmuir, Vol. 7,No. 7,1991 1499
Adsorption on Rutile and Anatase ns/mmol g-1 8 0
.
2.0-
I
1.0
0.5
0.5- c
0
15-
I 0
I
01 0.2 0.3 0.
I I
1.0-
0
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i
..
I
.*
a
P/Po I
0
1
!
0.2
OL
0.6
I
0.8
I
1
1.0
Figure 2. Adsorption (0) and desorption).( isotherms of pyridine (18O C , a),ammonia (-32 O C , b), and dimethylamine(0 O C , c) on rutile. The insert shows the BET plots of the isotherms
for ammonia ( 0 )and dimethylamine (0).
Figure 1. Ad- and desorption isotherms (77.7K)of nitrogen on
Table 11. Irreversible ( n h )and Monolayer (nm) Adsomtion on Rutile and AnataW
rutile (a,.) and anatase(0, 0 ) :opensymbols, adsorption;closed symbols, desorption. Insert: BET plot. The solid line represents the ad- and desorption isotherm on rutile from ref 2 referenced to the specific surface area of the rutile used here. Table I. Monolayer Capacities, nm,C Values, and Specific Surface Areas, S, from the BET Analysis of Adsorption Isotherms on Rutile (R)and Anatase (A)
,, pmolg-1
;I
S/m2 g-l 0
NzoR N:!A NHsR 178 334 150 230 70 150 17.4 32.6
P y R DMAR 140 163 400 15
CgHgR 52 160
n d
pmol gl
66 (2.3)
64 (2.2)
pmol gl
BET point B
150 (5.2) 160 (6.5)
163 (5.6) 145 (5.0)
186 (3.4)
164 (3.0)
52
(13) 175 (6.1)
78 (2.1)
Anatase nhl
149 (2.8) a The figures'in parenthesesdenote the adsorbed quantities in the unit molecules/nmz. Data from ref 2 but referenced to the specific surface area of the rutile used in the present investigation. pmol g-l
(4) Gfegg, S.J.; Sing, K. 5.W .Adsorption, Surface Area and Porosity; Academic: London, 1982.
Rutile 54 (1.9)
nml
Adsorbate.
Figure 2 shows the ad- and desorption isotherms of pyridine, ammonia, and dimethylamine on standard pretreated rutile a t the temperatures denoted in the figures. As in the case of nitrogen adsorption on rutile (Figure l), the ad- and desorption isotherms coincide and are of type I1 in Brunauer's classification. The pyridine adsorption isotherm shows a welldeveloped monolayer saturation behavior before multilayer formation sets in. Most remarkably, the amount adsorbed does not exceed the equivalence of about three monolayers close to the saturation pressure. The inserts of the figures show that the adsorption isotherms of ammonia and dimethylamine can be well represented by the BET model in the usual range of relative pressure between about 0.05 and 0.3. The monolayer capacities n, of NH3, (CH&NH, and CSHSNfrom the BET analysis and/or from "point B"' of the isotherms as well as the
.
*
irreversibly adsorbed amounts n h after prolonged evacuation (ca. 12 h) under high vacuum are collected in Table 11. Figure 3 shows the ad- and desorption isotherms of water and benzene on rutile at the temperatures 18 and 16 O C , respectively. As with the previous adsorbates the ad- and desorption runs yield practically the same curves. The monolayer capacities n,from "point B" (water) and from the BET analysis (benzene) as well as the irreversibly adsorbed amount n h of water are collected in Table 11. Benzene could be completely desorbed under vacuum at the measuring temperature ( n h = 0). Inspection of Table I1 reveals a very good agreement of the irreversibly adsorbed amounts n h of ammonia and pyridine on rutile between each other and with 2-propan01from a previous study.2 For DMA, a somewhat lower value is obtained, whereas the amount of irreversibly bound
Boddenberg and Eltzner
1500 Langmuir, Vol. 7,No. 7, 1991 Rutile
y,
I
01
GZ
A
*
PIPo
0.3 -
Anatase
03
A d
I
A A
A
I I CI
293 K
I
293 K
C
.
I I
Li I
293 K PlP,
0
0.2
0.L
06
0.8
,
1.0
Figure 3. Adsorption (open symbols) and desorption (closed symbols) of water (18 O C , A, A) and benzene (15 "c,0, m) on rutile. Insert showsBET plot of the benzene adsorption isotherm.
water is practically twice these figures. The 1:2 ratio between irreversibly held 2-propanol and water was already found previously.2 On anatase the values of n k of ammonia, pyridine, and water are of comparable magnitude. The monolayer capacities n, of the nitrogen bases on rutile may be considered to be practically the same taking into account the uncertainties inherent in the BET and "point B" procedures of analysis. The n, value of water is somewhat larger, whereas for 2-propanol and benzene considerably lower figures are observed. 2H NMR Spectra. Parts a and b of Figure 4 show the deuteron NMR spectra of pyridine-d5 adsorbed on rutile and anatase in amounts corresponding to the irreversible (nim) and the monolayer (n,) adsorption. In spite of the poor signal to noise (SIN)ratio in Figure 4a, the spectra obtained may be considered as Pake type quadrupole patterns exhibiting the full static line width (edge splitting c 140 kHz). When the coverage is 1.5 monolayers (Figure 4C), the spectrum consists of the static powder pattern as before and, in addition, a central singlet of comparatively narrow line width. In Figures 5 and 6 are shown the 2H NMR spectra of ammonia and dimethylamine, respectively, adsorbed on both anatase and rutile at the coverages corresponding to nirr. In each case temperature independent Pake type quadrupole patterns are obtained. The edge splittings are AY = 60 and 40 kHz for adsorbed ammonia and dimethylamine, respectively. Figure 7 shows the 2H NMR spectra of benzene at one monolayer coverage on rutile at several selected temperatures between 290 and 75 K. In contrast to the adsorbed nitrogen bases (Figures 4-6), the spectra are strongly temperature dependent, changing shape from narrow Lorentzian type singlet into Pake type quadrupole pattern of edge splitting Au = 70 kHz. From the prominent edge splittings, Av, of the exper-
Figure 4. 2HNMR spectra of pyridine ( C a y ) on rutile and anatase at coverages corresponding to irreversible adsorption (a), one mono-layer (b), and 1.5 monolayers (c).
r
-Anatase
I
Rutiie
1
1 I
298 K
298 K
73 K
7L K
DQCC = (4/3)Au (1) By use of the well-known formula for the shape of quadrupole patterns with axial symmetry of the electric field gradient: more accurate couplingconstants were evaluated with the aid of computer simulations taking into account Lorentzian type individual line broadening5 of 4 and 2 kHz for the nitrogen bases and benzene, respectively. The (5) Barnes, R. G. Adu. Nucl. Quadrupole Reson. 1974,1,336.
(6)Spiess, H. W. In NMR Basic Principles and Progrese; Diehl, P., Fluck,E., Kosfeld, R., Eds.; Springer: Berlin, 1978; Vol. 16, p 66.
Adsorption on Rutile and Anatase
1
I
Anatase
Langmuir, Vol. 7,No. 7,1991 1501
I
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293 K
298 K
90 K
100 K
290 K
263K
1
182 K
132 K
1
A
,E”.
113 K
Figure 7. *HNMR spectraof benzene (C&) adsorbed at monolayer coverage on rutile.
data obtained by this procedure are collected in Table I11 together with couplingconstants taken from the literature.
Discussion 2H NMR Spectra. It is general experience that the electric field gradient (EFG) tensor q at the sites of deuterons (spin I = 1,electric quadrupole moment Q)in carbon- and nitrogen-hydrogen bonds of molecules is almost axially symmetric, and its distinct principal axis component has values such that the corresponding quadrupole coupling constant e2qQ/his in the range 150-230 k H ~ . ~ An , 8 experimentally determined value of DQCC in (7)Mantach, H.H.;Saito, H.; Smith, I. C. P. h o g . NMR Spectrosc. 1977, 11, 211. (8!Brevard, C.; Kintzinger, J. P. In NMR and the Periodic Table; Harris,R.K., Mann, B. E., Eds.; Pergamon: New York, 1978; p 107.
this range indicates that any molecular rotation proceeds = DQCC-1 = slowly on the NMR time scale, T= (e2qQ/h)-l (slow orientational exchange). With the estimates given before, TNMR amounts to several microseconds. Values of DQCC determined from experiment that are considerably smaller than e2qQ/h are an indication of a rapid uniaxial rotation (rate >> e2qQ/h)of the deuterium carrying molecules (rapid orientational exchange). Under these circumstances the quadrupole coupling constant is given by the relation6@ DQCC = (1/2)13 cos2 A - lle2qQ/h (2) where A is the angle between the carbon/nitrogenhydrogen bond axis and the axis of rotation. Eventually, if the axis of preferred rotation is isotropically moved in space at a rate >>DQCC-l, then the spectrum consists of a singlet of, in general, Lorentzian shape which narrows with increasing rate of the isotropic rotation. It is worthwhile to state that the type and mechanism of the anisotropic and isotropic rotations considered so far, e.g. Brownian rotation and symmetric large angle jump type motion, cannot be determined from the spectra under the conditions of slow or rapid orientational exchange. The foregoing considerations are sufficient to analyze the 2H NMR spectra shown in Figures 4-7 as follows. Benzene on Rutile. The value DQCC = 93 kHz evaluated from the low-temperature quadrupole patterns (Figure 7) is typical for benzene molecules rotating rapidly about their 6-fold symmetry axis. By use of eq 2 with A = 90°, the rigid coupling constant comes out as e2qQ/h = 186 kHz, which is in agreement with literature data for benzene in the bulk solid stategJOand in adsorption layers on various solid substrates.11-17 This result suggests that the molecules are adsorbed flat on the surface since any other orientation of the rapidly spinning molecules is hardly imaginable. This suggestion is in accordance with the notion that the benzene molecules are ?r bonded to the surface cations.18 Most probably, the “lying flat” orientation still maintains at temperatures higher than about 180K, where the quadrupole patterns have collapsed into singlets. The underlying isotropic averaging is considered to be due to the rapid translational diffusion of the molecules across the surface of the rutile microcrystallites (mean diameter ca. 1pm), which corresponds to a reorientation of the EFG tensor in a spatially fixed frame of reference.17 Pyridine on Rutile and Anatase. With DQCC = 180 kHz, the quadrupole patterns of pyridine on both titania modifications exhibit the static spectrum width since DQCC is in agreement with e2qQ/h = 178 kHz obtained for pyridine in the bulk solid state.lg It follows that in (9)Millett, F. S.;Daily, B. P. J. Chem. Phys. 1972,56, 3249. (10)Andrew, E.R.;Eades, R. G.Proc. R. SOC.London, A 1953,218, 539. (11)Boddenberg,B.;Grosse,R.Z.Naturforsch.,A: Phys.,Phys. Chem., Kosmophys. 1986,41A,1361. (12)Boddenberg, B. In Lectures on Surface Science; Castro, G . R., Cardona, M.; Eds.; Springer: Berlin, 1987;p 226. (13)Hasha,D.L.;Miner,V.W.;Garces,J.M.;Rocke,S.C.InCatalyst Characterization Science; Deviney, M. L., Gland, J. L., Eds.; ACS Symposium Series 288, American Chemical Society: Washington, DC, 1985;p 485. (14)Eckman, R.R.;Vega, A. J. J.Phys. Chem. 1986,90,4679;Zeolites 1988,8,19. (15)Zibrowiua, B.;Caro, J.; Pfeifer, H. J. Chem. SOC.,Faraday Tram. 1 1988,a. (16)Boddenberg, B.;Burmeister, R. Zeolites 1988,8,488. (17)Boddenberg, B.; Beerwerth, B. J. Phys. Chem. 1989,93,1435. (18)Nagao, M.; Suda, Y. Langmuir 1989,5,42. (19)Barnes, R. G.;Bloom, J. W. J. Chem. Phys. 1972,57,3082.
1502 Langmuir, Vol. 7, No. 7, 1991
Boddenberg and Eltzner
Table 111. Deuterium Quadrupole Coupling Constants (DQCC) of Ammonia, Pyridine, and Dimethylamine (DMA) molecule ammonia-d6 pyridine-& dimethylamine (DMA)-de
DQCC, kHz 78 i 4 (this work); 72.5 f 0.9 (solid, 159-186 K);B 73.0 f 0.4 (in TaS2);B 74.7 f 0.4 (in TiS2);% 71.6 f 0.5 (Ag(ND&+ 300 K);S164 (Co(ND3)eS+,190 K);32 156 i 7 (solid, 75 K);% 245 f 25 (lis., 202 K);39200 f 20 (gas);" 282 f 12 (NHzD, gas)" 180 f 6 (this work); 178 f 1.2 (solid, 77 K);19 -90 (in CdzPzSe, 300-210 K);42 164 f 1 (in 2H-TaS2,280-333 K)" 48 f 2 (this work); 47.8 (MMA in zeolite Rho; 290 K)4
contrast to benzene the pyridine molecules at coverages up to one monolayer are in the slow exchange regime with respect to both reorientation in place and surface diffusion even at ambient temperature. From a practical point of view, the molecules can be considered to be held rigidly in place. This result suggests that pyridine is coordinatively bonded via the N atom to the surface Ti4+ions since K bonding allows rapid rotation around the axis normal to the molecular plane20 as in the case of benzene. This conclusion is consistent with the results of numerous infrared studies concerning the bonding of pyridine to the surface of The narrow line superimposed on the quadrupole pattern at the coverage of 1.5 monolayers (Figure 4c) has to be ascribed to a fraction of very mobile molecules for which strongly bonding sites are not available. Since for reasons of time consumption the relaxation behavior was not studied in detail, the fraction of the mobile molecules cannot be quantitatively determined from the corresponding signal portion. Ammonia on Rutile and Anatase. The value DQCC = 80 kHz evaluated from the quadrupole patterns (Figure 5c) is of similar magnitude as for ammonia in the bulk solid25and intercalated in chalcogenides26but is considerably lower than the rigid coupling constant for which values in the range 230-290 kHz have been reported (see Table 111). This result indicates that the adsorbed ammonia molecules are rapidly spinning around their CS symmetry axis in the range of temperature covered by experiment. The most reasonable conclusion is that the molecules are coordinated via the electron lone pair to the Lewis acidic surface Ti4+sites, thus confirming infrared re~ults.22~23.27v28 Generally, it is expected that the coordinative bonding of ammonia entails changes of the geometry as well as the charge distribution in the N-D bond of the molecule in comparison to the gaseous or the bulk solid state. Such changes should effect the quadrupole coupling constant. In the gas and in the solid the D-N-D angles are cp 106.78' 29 and 110.4O,3O respectively. The corresponding apex angles, A, are 112.04' and 108.5O, respectively. By use of eq 2 with DQCC from experiment and cp in the range given by the data before, e2qQ/h is calculated to be in the range 277-229 kHz. Values of the rigid coupling constant in this range are found for gaseous and liquid ammonia (see Table 111)suggesting that on adsorption on titania a t least (20) Boddenberg, B.; Grundke, V. Unpublished. (21) Parfitt, G. D.; Ramsbotham,J.; Rochester, C. H. Trans. Faraday SOC.1971,67,1500.
(22) Morterra, C.; Ghiotti, G.;Garrone, E.; Fisicaro, E. J. Chem. SOC., Faraday Trans. 1 1980, 76, 2102. (23) Primet, M.; Pichat, P.; Mathieu, M.-V. J . Phys. Chem. 1971, 75, 1221; J . Phys. Chem. 1971, 75,1216. (24) Busca, G.;Saussey, H.; Saur, 0.;Lavalley, J. C.; Lorenzelli, V. Appl. Catal. 1985, 14, 245. (25) Rabideau, S. W.; Waldstein, P. J. Chem. Phys. 1966, 45, 4600. (26) Silbernagel,B. G.; Gamble, F. R. J. Chem. Phys. 1976,65, 1914. (27) Parfitt, G.D.; Ramsbotham,J.; Rochester, C. H. Trans. Faraday SOC.1971,67, 841. (28) Tanaka, K.; White, J. M. J. Phys. Chem. 1982,86,4708. (29) Weiss, M. T.; Strandberg, M. W. P. Phys. Rev. 1961,83, 567. (30) Reed, J. W.; Harris, P. M. J. Chem. Phys. 1961,35, 1730. (31) Maurer, H. M.; Weise, A. J. Chem. Phys. 1978, 69, 4046.
drastic changes of the molecular geometry and the intramolecular charge distribution are not probable. Similar conclusions have been drawn for ammonia in complexes with silver9 and cobalt ions.32*33 Dimethylamine (DMA) on Rutile. The value DQCC = 48 kHz evaluated from the experimental quadrupole patterns (Figure 6) is typical for rapidly rotating methyl groups which are attached to an otherwise rigidly fixed molecule. Assuming the angle N-C-D to be tetrahedral, i.e. A = 109.47', eq 2 gives e2qQ/h= 144 f 12 kHz, which is in the range of coupling constants for deuterons attached to carbon in sp3 hybridized bond^.^^^^* These results indicate that DMA is strongly bound to the Lewis acidic Ti4+centers on rutile with the molecular framework rigidly held in place even at ambient temperature. Adsorption on Rutile. The discussion of the 2HNMR spectra has given strong indication that the sites responsible for the irreversible adsorption of the nitrogen bases and for the reversible adsorption of pyridine up to monolayer completion have to be identified with Lewis acidic Ti4+centers of the surface. In this section it is undertaken to rationalize the data gained from the analysis of the adsorption isotherms (ni,and n,, Table 11)in terms of a model of the rutile surface concerning the number of Ti4+ sites available and their distribution over the various crystallographic planes exposed.2 This model, which was developed on the basis of adsorption and NMR data for 2-propanol and water on rutile, states that the surface of microcrystalline rutile is made up of the low index crystallographic planes (110), (101),and (100) (Figure 8) where the former and the latter two represent about 85 and 15% of the surface area exposed, respectively. These figures come out by assuming that at the level of irreversible adsorption, water and 2-propan01 are bonded to every and every other of the coordinatively unsaturated (cus) Ti4+ ions on (110), respectively, and the reversibly adsorbed 2-propanol molecules up to monolayer completion occupy every other of such sites on the remaining planes (101) and (100). The preference of the cus Ti4+ sites on (110) for the irreverisble water a n d 2 - p r o p a n o l a d s o r p t i o n was attributed-following earlier suggestionsof other authors94 -to the easy access of the adsorptive molecules to these sites in contrast to the other planes where steric hindrance is imposed. It was further assumed that the crystallography of the exposed planes is identical with the corresponding cleavage planes of the bulk crystaLS5 According to this model the numbers of cus Ti4+sites exposed for the presently used rutile of surface area 17.4 m2/g are twice the figures for 2-propanol given in Table 11,Le. 126pmollg on (110) and 30 pmol/g on (101) plus (loo), giving a total of 156 pmol/g. The close agreement of the irreversibly held amounts nkr of both ammonia and pyridine with 2-propanol (Table 11)suggests that these nitrogen bases likewise occupy every (32) Ito, T.; Chiba, T. Bull Chem. SOC.Jpn. 1969, 42, 108. (33) Ito, T. Bull. Chem. SOC.Jpn. 1972,45,3507. (34) Munuera, G.;Stone, F. S. Discuss. Faraday SOC.1971,62, 206. (35) v. Hippel, A.; Kalnajs, J.; Westphal, W. B. J. Phys. Chem. Solids 1962, 23, 779.
Langmuir, Vol. 7, No. 7, 1991 1503
Adsorption on Rutile and Anatase
(100)
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A
B
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a0.5464
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-1
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@
0 i n p l a n e of T i
0
0 above p l a n e of T i
Figure 8. Crystallographic cleavage planes of rutile. Table IV. Van der Waals Diameters (a). and Potential Energy Curve Parameters (a, c/k)of Nitrogen Bases molecule d/nm a/nm (c/k)/K ref ammonia
0.38 ( I C 3 )
pyridine
0.63 (in plane, IC2) 0.34 (Iplane) 0.67;0.47 (Ilone pair-N axis)
dimethylamine
(a) 0.29 (b) 0.26 (c) 0.315 (d) 0.344 0.305
692 320 358 146.8 1070
36 36 36 36 38
I, Calculated according to atomic (H)and group (CHa) van der Waals radii given in ref 45. second Ti4+ site on (110) at the stage of irreversible adsorption. The less than 1:2 correspondence found for dimethylamine (DMA) may be due to the bulky methyl groups, which prevent a close approach of the DMA nitrogen atom to the Ti4+sites leading to a weaker coordinative bond strength than for ammonia and pyridine and so to a partial removement of the molecules by the 12-h pumping procedure applied. The prerequisite for the correctness of this proposal is, of course, that the sizes of the adsorbed species allow such arrangements on (110) where the next nearest neighbor Ti4+ distance is d(Ti-Ti) = 0.297 nm. Taking as the sizes of the nitrogen bases the appropriate van der Waals diameters according to Pauling (Table IV), the requirement of fitting can easily be achieved with molecular orientations as in Figure 9. From this geometrical point of view the spinning of the ammonia molecules around the C3 axis and, on the other hand, the absence of anisotropic reorientational motions of pyridine and DMA as deduced from the 2HNMR spectra are readily understood. The proposed orientation of the pyridine molecules with their plane normal to the row A direction (Figure 9) is likely for symmetry reasons. At the monolayer coverage the adsorbed amount of each of the nitrogen bases is approximately twice the amount
of 2-propanol and, therefore, corresponds to the total number of Ti4+sites available according to the proposed model of the rutile surface. These findings suggest that the molecules adsorbed reversibly up to monolayer completion either are Lewis coordinated to the Ti4+sites left unoccupied by the irreversibly adsorbed species or are adsorbed on other types of sites whose number is 1:lrelated to the number of the unoccupied sites. The first of these alternatives, resulting in a more dense packing of the molecules than can be accomplishedwith the van der Waals sizes, requires investigating the intermolecular interaction energy of the adsorbed molecules. Of most interest is the situation encountered on the most frequent (110) planes where the distance of the next nearest neighbor adsorption sites is d(Ti-Ti) = 0.297 nm. For ammonia, several intermolecular potential functions based on gas-phase second virial coefficient and viscosity data have been set up, the parameters of which are collected in Table 111. Here u and c are the mass center distance at zero potential energy and the depth of the potential well, respectively. For an estimation of the interaction energy for a pair of NH3 molecules separated by the site distance 0.297 nm, the potential of the "approximate angleindependent" potential3s (d) of Table IV) will be used. It consists of the Lennard-Jones (6-12) potential and a dipolar term, which latter will be neglected here because the electron lone pair of the nitrogen atom can be assumed to be compensated effectively by the dative bond. With these assumptions the center of mass distance a t the potential minimum is calculated to be 0.386 nm, which is in nice agreement with the Pauling diameter. The potential energy at distance d(Ti-Ti) comes out as +16.7 kJ mol-' per interacting pair, the corresponding value at 2d(Ti-Ti) is -0.17 kJ mol-'. According to these data the differential heat of adsorption in the regimes of coverage where the half and full occupation of the cus Ti4+ sites takes place should show up a difference of the order of about 2 X (16.7 + 0.2) = 34 kJ mol-l. Experimental data of
(36)Hirschfelder, J. 0.;Curtiss, C. F.; Bird, R. B. Molecular Theory Gases and Liquids; Wiley: New York, 1964.
1504 Langmuir, Vol. 7, No. 7, 1991 E
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-r
-r
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0 . 2 9 5 9 nm
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!0.6496 nm
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-I
0.6496 nm
Figure 9. Arrangement of ammonia, pyridine, and dimethylamine molecules on the (110) surface planes at the stage of irreversible adsorption. The adsorbed molecules are depicted with their van der Waals sizes. for ammonia on rutile, which could serve for comparison purposes, are not available. In a recent paper37 the differential heats of adsorption of ammonia of an otherwise not defined titania have been reproted to be about 60 and 25-35 kJ mol-l at coverages corresponding to the less and more than half occupation, respectively. Although the comparison with these data must be considered with care, the rather good agreement, nevertheless, signifies that the estimates given above seem to have a realistic basis. The conclusion is that in the interplay of attractive Lewis coordination and repulsive intermolecular interaction, the latter is sufficiently low to allow the full occupancy of the Lewis sites under adsorption equilibrium conditions and the destabilization of this configuration under the pumping conditions applied. For the reversible adsorption of pyridine, a similar estimation procedure is applied as before. The only realistic arrangement is with the molecules standing upright and their planes parallel to each other. For this stacked configuration the intermolecular potential of Evans and Watts38 for benzene molecules is used as an approximation. The parameter data Q and t (Table IV) as well as the center of mass distance at the potential minimum, ?,in = 0.347 nm, were taken from the drawing in this paper. The value of rminagrees satisfactorily with the corresponding Pauling thickness given in Table 111. At the distance d(Ti-Ti) = 0.297 nm, the potential is repulsive at an estimated value of about +5 kJ mol-', at 2d(Ti-Ti) the corresponding value is about -1.3 kJ mol-'. These data indicate that the configuration with every cus Ti4+on (110) occupied is stable under adsorption equilibrium and is destabilized under vacuum conditions, leaving half occupancy corresponding to the condition of irreversible adsorption. Considering the situation on the (101) and (100) planes, the occupancy of every second site by both ammonia and pyridine molecules can easily be accepted. The less favorable steric arrangement of the Ti4+sites in comparison to the (110) planes is considered to be responsible for the ease of removal of the molecules from these sites. Whether beyond such a configuration every site becomes occupied
1
Rakhmatkariev,G. U.; Zhalalov, Kh. R. Uzb.Khim. Zh.1988,32. Evans, D. J.; Watts, R. 0. Mol. Phys. 1976, 31, 83. Powles, J. G.;Rhodes, M. Mol. Phys. 1967, 12, 399. Herrmann, G.J. Chem. Phys. 19S8,29, 875. Thaddeus.. P.:. Krishner.. L. C.;. Cahill. P. J. Chem. Phvs. 1964.41,
1542. (42) Lifshitz, E.; Vega, S.; Luz, Z.; Francis, A. H.; Zimmermann, H. J. Phys. Chem. Solids 1986,47, 1045. (43) McDaniel, P. L.; Barbara, T. M.; Jones, J. J . Phys. Chem. 1988, 92, 626. (44) Vega, A. J.; Luz, Z. Zeolites 1988, 8, 19. (45) Pauling, L. The Nature of the Chemical Bond; Cornel1University Press: New York, 1960.
at monolayer completion will not be considered here because this is a matter of the subtle interplay between the attractive coordinative and the repulsive intermolecular interactions. The difference of the adsorbed amounts between the alternatives of half and full occupation amounts to 15 pmol/g, which is in the range of uncertainty of the determination of n,. The dimethylamine molecules have van der Waals sizes too large to interpret the near coincidence of n, with the number of Ti4 sites available in the same way as for ammonia and pyridine. Furthermore, the very low BET c value evaluated in this case is indication that the molecules adsorbed beyond the half-filling of the sites experience a similar type of bonding as in the bulk liquid state. Therefore it is suggested that on (1lo), the monolayer completion is achieved by the formation of a hydrogen bond between the irreversibly and reversibly adsorbed species leading to the 1:l correspondence between the numbers of adsorbed molecules and the cus Ti4+sites as is required to explain the experimental data. Similar arguments as advanced before for ammonia and pyridine may be applied concerning the adsorption on the less frequent (101) and (100) planes. If on these planes every other site is occupied as in the case of 2-propanol which is of similar size and on (110) the 1:l correspondence proposed above is realized, n, is predicted to be 141 pmol/ g. The difference of about 20 pmol/g against the value evaluated with the aid of the BET procedure is considered to be of no relevance, bearing in mind that the application of the BET formalism in the case of low c values is problematic.
Summary and Conclusions In this paper it was shown that the results obtained from the adsorption isotherms and deuteron NMR spectra of ammonia, pyridine, and dimethylamine adsorbed on microcrystalline rutile can be rationalized in terms of a model of the rutile surface2stating that about 85 and 80% of the surface area and of the coordinatively unsaturated (cus)Ti4+sites exposed, respectively,come from faces being identical with the (110) cleavage planes of the bulk rutile crystal. Including the previously studied 2-propanol and water molecules, some evidence is obtained that on these most frequent (110) planes where the cus Ti4+ sites are arranged in rows, each of the molecules investigated are Lewis coordinated, yielding ordered structures in the regimes of both irreversible adsorption and monolayer completion. A t the stage of irreversible adsorption 2-propanol as well as the three nitrogen bases studied occupy every other cus Ti4+site on (110), whereas the water molecules occupy each of these sites leaving open here the question of
Adsorption on Rutile and Anatase
dissociative or molecular adsorpti0n.a The NMR spectra give evidencethat on the time scale of NMR (microsecond range) 2-propanol," pyridine, and dimethylamine are rigidly held in space, whereas the ammonia molecules are in rapid reorientational motion around their symmetry axis. Under adsorption equilibrium conditions, the molecules investigated behave differently with respect to their ability to coordinate to the cus Ti4+sites on (110) left unoccupied by the irreversibly bound species. Due to the bulky methyl groups, 2-propanol and dimethylamine exhibit van der Waals sizes too large to allow bonding to the sites left free. On the other hand, both ammonia and pyridine are Lewis coordinated to all sites available, although their van der Waals dimensions are considerably larger than the next nearest neighbor distance of the sites (0.297 nm). Estimates of the intermolecular potential energy using known potential functions, however, indicate that in spite of the repulsive character of the intermolecular interaction at the site distance, the free energy change for adsorption is (46)Boddenberg, B.; Horstmann,W. Ber. Buneen-Gee.Phys. Chem. 1988,92, 525. (47) Boddenberg, B.; Horetmann, W. Ber. Bumen-Ges. Phys. Chem. 1988, 92, 531.
Langmuir, Vol. 7, No. 7, 1991 1505
still negative due to the strong Lewis coordination. In the case of pyridine where the molecules must be assumed to have their planes oriented parallel to each other, this notion is substantiated by the 2H NMR spectra. Here, with adsorbed amounts corresponding to the full occupation of the sites, all molecules are found to be rotationally immobile even at ambient temperature, whereas additionally adsorbed pyridine molecules exhibit large translational mobility. The circumstance that the adsorption properties of a variety of adsorptives can be treated on the basis of a unifying approach, proves microcrystalline rutile to be a well-suited solid to study molecule/surface interactions. Anatase, on the other hand, for which conflicting results have been obtained,a seems to be much less suited for studies of this kind although the NMR results presented in this work indicate great similarity with rutile. Acknowledgment. Financial support of this work by Deutsche Forschungsgemeinschaft and Fonds-der Chemischen Industrie is gratefully acknowledged. Registry No. TiOl,13463-67-7;NHs, 7664-41-7;C s H a , 11086-1; (CH*)zNH, 124-40-3;C&, 71-43-2; HzO, 7732-18-5. (48)Eltzner, K.; Boddenberg, B. Unpublished results.
