A. ZECCHINA,S, COLUCCIA, E. GUGLIELMINOTTI, AND G. GHIOTTI
2774
An Infrared Study of Surface Properties of a-Chromia. I.
Preparation
and Adsorption of Water, Heavy Water, and Carbon Monoxide1 by A. Zecchina, S. Coluccia, E. Guglielminotti, and G. Ghiotti Istituto d i Chimica Fisica dell’ Universith. di Torino, T u r i n , Italy
(Received October 19, 1970)
Publication costs assisted by the Conaiglw Italiano delle Ricerche ( C . N . R . )
SampIes of a-chromia of high surface area and suitable for spectroscopic work have been prepared in a controlled manner. Crystals mainly expose the (001) face on which the adsorption of HzO, DzO, and CO has been studied. At low coverages, water adsorption at room temperature is dissociative and hydroxyl groups are formed. At higher coverages a nondissociativechemisorptionis observed and discussed. Carbon monoxide is adsorbed onto surface Cr*+ions to form u adducts, and the adsorbed amount solely depends on surface dehydration.
Introduction Ir studies have been reported of CO and COz adsorbed on chromium oxide supported on silica and alumina,2-5 but no spectroscopic data are available concerning unsupported chromium oxide, both in the amorphous and in the crystalline form. We chose to study the adsorptive properties of this oxide, because another study has been done in this laboratory dealing with chromium oxide supported on silica and also because this material should be very convenient for checking the application limits of Burwell’s “surface coordinative unsaturation” theory. I r technique is particularly useful in this field, for in the spectroscopic assignment of adsorbed species the enormous amount of work done with coordination compounds can be used as a reference. Unless otherwise stated, our experiments dealt with a microcrystalline form of a-Cr203,obtained from the thermal decomposition of ammonium dichromate. It is well known6 that in this way either an amorphous, nonstoichiometric modification or mixtures of that and a crystalline phase (CY)can be obtained. For this reason, special care has been spent in the preparation and characterization of the material, to get an oxide that is fully crystalline and reproducible. Both the sample preparation and the ir measurement technique will be carefully described.
Experimental Section Sample Preparation and Characterization. Ammonium dichromate vias decomposed in vacuo, as described , ~ a dark, bulky powder of by Harbard, et ~ l . yielding chromium oxide with a large excess of oxygen and exhibiting no X-ray diffraction lines due to crystalline phases. This amorphous oxide was compressed, under a pressure of 100-120 kg/cm2, into pellets of good mechanical resistance, that were about 0.2 mm thick and The Journal of Physical Chemistry, Vol. 76, No. 18, 1971
contained approximately 10 mg of chromium oxide per square centimeter. Since our aim was to check the surface properties of a-chromia, the amorphous samples were placed in an ir cell, described elsewhere,’ and underwent a thermal treatment in three steps whose final result was a reproducible crystalline modification. After each step, the state of the sample and its reproducibility were controlled by determining ir spectrum, BET surface area, X-ray diffraction spectrum, and electron micrographs. The three steps of the thermal treatment are as follows: phase 1, 16 hr of sample activation a t 400°, in vacuo (all vacuum treatments were carried out at a residual pressure Torr) ; phase 2, 6 hr of sample crystallization at 400” under a 5 X Torr pressure of oxygen, water vapor being condensed in a liquid nitrogen cold trap; phase 3, 4 hr of sample reduction at 400” with an excess of CO. Physical characteristics of the material in the various phases can be summarized. Phase I : The amorphous starting material exhibits a very poor transparency in the infrared (see Figures 1 and 2, curve 1). At the end of this phase I, the spectrum is as reported in Figures 1 and 2, curve 6. Curves 2 to 5 refer to intermediate activation temperatures. The sample is brown in color and still completely amorphous. The BET surface area now ranges between 120 and 130 m2/g. (1) All correspondence should be sent to E. Borello, Istituto di Chimica Fisica dell’Universith di Torino, Turin, Italy. (2) (a) E. Borello, A . Zecchina, C. Morterra, and G. Ghiotti, J . Phys. Chem., 73, 1286 (1969); (b) A. Zecchina, C. Morterra, G. Ghiotti, and E. Borello, ibid., 73, 1292 (1969). (3) A. Zecchina, G. Ghiotti, C. Morterra, and E. Borello, ibid., 73,
1295 (1969). (4) A. Zecchina, E. Guglieln~inotti, S. Coluccia, and E. Borello, J . Chem. Soc. A , 2196 (1969). ( 5 ) L. H. Little and C. H. Amberg, Can. J . Chem., 40, 1097 (1962). (6) E. H. Harbard and A. King, J . Chem. Soc., 955 (1938).
(7) E. Borello, A. Zecchina, and M. Castelli, Ann. C h i n . (Rome), 53, 690 (1963).
2775
IRSTUDYOF SURFACE PROPERTIES OF a-CHROMIA
70 T% 50
30
10 I
2000
I
I
1800
I
I
I
1600
I
I
1400
I
1200
I
I
1000
I
T-
800
cm -1 Figure 1. Infrared spectra of amorphous CrzO3 (% transmission us. wavelength iri cm-l): curve 1, before evacuation; curve 2, after evacuation at room temperature; curves 3-6, after degassing, times and temperatures being 1 hr a t looo, 1 hr at 200°, 1 hr at 300°, and 16 hr at 400".
Light scattering phenomena still predominate in the high frequency range, so that spectra of Figure 2, obtained with wide slits and heavy reference beam screenings, are much poorer than spectra of Figure 1. Phase I1: The infrared spectrum is now as reported in Figure 3, dotted line. Strong absorptions in the 1040-750cm-1 range are due to chemisorbed oxygen and will be discussed elsewhere (part 11). Oxygen contact at 400" has also caused the sample crystallization, as revealed by the presence of X-ray diffraction lines typical of achromia, just a little broader than usual, due to small crystal size. The color is now green; B E T surface area is 60-70 m2/g and the t-plot methods does not reveal the presence of micropores. Electron microscopy shows crystals whose linear dimensions are in the 300600-A range. The form is of very thin laminae of hexagonal or octagonal contour and (001) face is definitely predominant. Crystallization can be also promoted with higher oxygen pressures. The process is then faster and sometimes violent, but surface area of the final product falls in a wider range and in an unpredictable fashion. All these phenomena are quite well known10and will not be discussed further. Phase 111: Chemisorbed oxygen (bands in the 1040-750-cm-' range) cannot be removed by degassing alone at 400". Since we are interested in the adsorptive characteristics of pure chromia, the material is reduced a t 400" with
CO (the gaseous phase is renewed several times), and then degassed at 400" to get rid of CO and COzpossibly chemisorbed. The final spectrum is reported in Figure 3, solid line. BET surface area has not changed during phase 111. The spectrum of microcrystalline chromia (Figure 3, solid line) exhibits a few bands, weak but well defined, in the 1300-750-cm-' range, that are not present in the spectrum of amorphous chromia (Figure 1, curve 6). There is a general disagreement on the origin of these bands. Therefore, we have compared several samples that are quite different in surface area, crystallinity, nonstoichiometry, and degree of oxidation. Figure 4 shows the spectrum of an amorphous sample (BET surface area 130 mz/g), a microcrystalline sample free from chemisorbed oxygen (BET surface area 68 mz/g), a crystallized sample, obtained by sintering the microcrystalline one in air at about goo", and a microcrystalline sample reduced with hydrogen and degassed a t 800" (BET surface area 35 m2/g). This comparison led us
(8) G. A. Nioolaon, J . Chim. Phys., 66, 1783 (1969), and references therein. (9). P: Groth, "Chemische Krystallographie," Wilhelm Engelmann, Lelpzlg, 1906. (10) R. L. Burwell, Jr., G. L. Haller, K . C. Taylor, and J. F . Read, Advan. Catal., 19, 62 (1969).
The Journal of Physical Chemistry, VoE. 76, N o . 18,1971
A. ZECCHINA,S. COLUCCIA, E. GUGLIELMINOTTI, AND G. GHIOTTI
2776
90
I
7Q I-%
50
30
10
3800
3000
3400
2600
cm-1
Figure 2. Infrared spectra of amorphous Crz03 ("/c transmission us. vavelength i n cm-l): curve 1, before evacuation; curve 2, after evacuation at room temperature; ciirveb 3-6, aft,er degassing, times and temperatures being 1 hr at loo", 1 hr at 200", 1 hr at 300°, and 16 hr at 400'.
cm-'
Figure 4. Infrared spectra of various CraOa samples (% transmission us. wavelength in cm-1): --, sample sintered in air at 900"; -0-0-, crystalline sample reduced in hydrogen at 800" (surface area: 35 mz/g); crystalline sample after phases 1-111 (surface area 68 mz/g); -.-.-, amorphous sample after phase I (surface area 130 mz/g). 's..
2000
1800
IMK)
1400
cm-1
1200
1000
800
Figure 3. Infrared tpectra of a-Cr203 (70 transmission us. wavelength in cm-l): - - - - - -, after phase 11; --, after phase 111 (see text for definitions of phases I1 and 111).
to a complete assignment of the bands in the 1300750-cm-' region. Few adsorption experiments have been run at temperatures higher than 23". So we thought it was useful to test the stability of the material, prepared by means of the three phases described, towards various gases a t temperatures higher than room temperature. On repeating phase I1 and phase I11 four times, the surface area loss was lower than 5%. HzO, DzO, and COz contacted with the material at temperatures up to The Journal
of
Physical Chemistry, Vol. 76, N o . 18, 1971
e ,
400" had no effect on the surface area. A pressure of -40 Torr of oxygen does not alter the microcrystalline material, provided the temperature is equal to or lower than 400". Spectroscopic Technique in the Study of Adsorption Phenomeim All the ir spectra have been recorded on a Beckman IR7 double beam spectrophotometer. Arbitrary 100% transmission lines have been obtained, range by range, by screening the reference beam. Spectral slit widths were 3-6 cm-' in the 700-1800-~m-~ region, 6-10 cm-I in the 1800-2000-~m-~region, and 10-20 cm-1 in the 2000-3800-cm-' region. Water and CO Adsorption. Water vapor adsorption onto a microcrystalline sample (after phase I11 of the thermal treatments) gives rise to absorption in the following ranges: 3800-3000 cm-l, 1630-1580 cm-', 1100-750 cm-l. Spectral determinations in the first range are very poor due to heavy light scattering. Therefore parallel experiments have been run with water and heavy water: in the latter case the first range is shifted to 2700-2000 cm-', where better optical conditions allow reasonably good spectra to be recorded. Figure 5a shows the spectra of a-chromia, on which adsorption of D20 at room temperature (at different
2777
IRSTUDY OF SURFACE PROPERTIES OF a-CHROMIA
100
0.5
30
0.4
50
0.3
T 7.,
0.0.
10
0.2
20
0.1
Figure 5. (a) Infrared spectra of a-CrnOa(optical density us. wavelength in cm-l): curve I, background after phase 111; curves Torr for curves 2-5, 0.15 Torr for 2-7, at increasing DzO coverages a t room temperature (D20 equilibrium pressures are < curve 6, and 4 Torr for curve 7). (b) Infrared spectra of a-Cr203(optical density us. wavelength in cm-l): curve 6, 7, HzO adsorption (equilibrium pressures are 0.15 Torr and 4 Torr, respectively). (c) Infrared spectra of cu-Crz08 (% transmission us. wavelength in cm-I): curve I, background after phase 111; curves 2-7, increasing HzO coverages at room temperature (equilibrium pressures are as reported in Figure sa).
coverages) is taking place. Figures 5b and c refer to the adsorption of HzO. Spectra of Figures 5a and b have been run with a reference sample on the reference beam so as to balance scattering phenomena. Spectra of Figure 5c were obtained with the usual screening technique) owing to the presence of weak background absorptions in the 1300-700-~m-~range, that might cause ambiguous results, especially at very low water coverages. Numbers marked on spectra of Figure 5 refer to equal or almost equal coverages in the three adsorption series. Notice that in Figure 5c, spectra 2 and 3 are missing due to the very weak intensity, hardly distinguishable from the background. Figure 6a, dotted line, shows the spectrum of a microcrystalline sample, oxygen free, on which carbon monoxide" is chemisorbed at room temperature. Figure 6b shows the effect of carbon monoxide on an amorphous sample, desorption times and all conditions being the same as Figure 6a. Preadsorption of HzO (or D20) on
CrzOs completely inhibits the activity towards CO, activity that is recovered upon degassing at gradually higher temperatures. The experiment (Figure 7) has been run as follows. The sample that underwent the three phases of thermal treatment was contacted with 8 Torr of DzO, and the adsorption was assumed to be over when no further change was observed in the ir spectrum (this end point was attained in 30 min at 400" and in few hours a t room temperature, but the final spectrum was the same). No CO adsorption was exhibited by the sample that was degassed a t room temperature) nor at loo", but higher and higher adsorptive capacity toward CO mas developed by the hydrate oxide upon degassing at temperatures up to 400". Meanwhile, a decrease is observed (Figure 7a) in the intensity of absorptions in the 2700-2000-cm-' region, where stretching surface OD groups absorb. (11) CO was
5N grade, from SIO, Milano.
The Journal of Physical Chemistry, Vol. 76, No. 18, 1071
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A. ZECCHINA, S. COLUCCIA, E. GUGLIELMINOTTI, AND G. GHIOTTI
0
0 '0.0,
0
spectrum is quite similar to that of chromium oxide gels and is typical of amorphous Crz03 phase, no matter what the preparation method has been. Freshly prepared samples are very opaque in the whole ir range. Evacuation at room temperature eliminates a broad band at -1630 cm-', probably due to physically adsorbed water. At higher evacuation temperatures, another broad band gradually decreases at 1000-900 cm-l (surface hydroxyls bending) together with a complex band system at 3800-3000 cm-l, due to the stretching of both free and hydrogen-bonded hydroxyls. The activation process of phase I has likely caused a dehydration reaction that, according to that pr0posed~0-l~ for Cr203gel, could be
OH- OH0
02- H10 Cr3+ Cr3+ -+ Cr3+ Cr3+
Cr3+ions are thought to be coordinatively unsaturated (c.u.s.) and to represent adsorption centers for CO and
02. Ir Spectwn of a-Chromia. The major feature in the Figure 6. (a) Infrared spectra of a-CrzOa (optical density us. wavelength in cm-1): curve 1, CO adsorption a t room temperature (equilibrium pressure 40 Torr); curves 2-5, a t decreasing CO coverages (final pressure for curve 5 < 10-4 Torr). (b) Infrared spectra of amorphous CrlOa (optical density us. wavelength in cm-1): curve 1, CO adsorption at room temperature (equilibrium pressure 40 Torr); curves 2-5, decreasing CO coverages (conditions are as reported in Figure 6a).
A 13-hr degassing produced no further change of CO activity that was lower than the initial activity of the nonhydrated sample and that could only be reproduced by repeating phases I1 and I11 of the thermal treatment. Notice that on repeating the two phases the spectrum in the 2700-2000-~m-~region is also modified (Figure 7a, dotted line).
Discussion Ir Spectrum of Amorphous Cr203. Water adsorption on various oxides usually brings about absorptions in three spectral regions. The 3800-3000-cm-' region i s due to the stretching of surface hydroxyls originated by a dissociative water chemisorption and to the OH stretching of nondissociated water molecules. These absorptions are shifted to 2700-2000 cm-l in the caae of heavy water. The 1630-1680-cm-' region is due to bending modes of nondissociated water. Corresponding absorptions of heavy water are a t 1200-1180 cm-'. The 1000-700-~m-~region is due to bending modes of surface hydroxyls, originated by dissociative water chemisorption. No OD bending modes are observable on chromia, whose cut off is at 700 cm-1.12*13Amorphous chromium oxide originated by decomposition of ammonium dichromate (Figure 1, curve 6) has a good transparency around 1000 em -I, while at higher wavenumbers heavy light scattering predominates. The The JOUTMZ~ of Physical Chemistry, Vol. 76, No. 18, 1971
spectrum of a-chromia is a series of weak, characteristic bands in the 1300-750-~m--~ region (Figure 3, solid line). Roev, et u1.,15 assigned these absorptions to surface hydroxyls typical of high surface area oxides. Such an assignment contradicts what is observed in the case of amorphous chromia (samples after phase I) and of chromia gel,le where bands in the 1300-750-~m-~ range are entirely missing. Shopov, et ul.,16observed weak bands in the 1300-750-~m-~region only on samples that chemisorbed oxygen and were degassed at 340°, and they thought that the bands were due to nondesorbed oxygen. On the other hand, amorphous chromium oxide easily crystallizes in the presence of oxygen so that it is quite difficult to decide whether oxygen is still present on the surface or the observed bands are typical of crystalline chromia. To answer the question and to distinguish between background bands and oxygen bands in the 1300-750-cm-' range special care has been used in the assignment, and many experiments have been made to confirm or reject the following hypotheses. (a) If some of the bands were due to surface hydroxyls, they should not be observed on low area samples and should be shifted by isotopic exchange. (b) If they were due to chemisorbed oxygen, still present also after the reduction treatment of phase IIJ, they should be decreased in intensity by further reduction with hydrogen at 800" and should not be visible on low-area samples. (c) If they were due (12) J. E. Evans and T. L. Whateley, Trans. Faraday SOC.,63, 2769 (1967). (13) W. A. Benesi and A. C. Jones, J . Phys. Chem., 63, 179 (1959). (14) R.L.Burwell, Jr., J. F. Read, K. C.Taylor, and G. L. Haller, 2.Phys. Chem., 64, 18 (1969). (15) L. M . Roev and A. N . Terenin, Dokl. Akad. Nauk S S S R , 124, 373 (1969). (16) D.M ,Shopov and A. N.Palazov, Kinet. Katal., 6, 864 (1966).
IRSTUDY OF SURFACE PROPERTIES OF a-CHROMIA
2779
0.5
0.4
1.8
02
1.6
0.D.
0.D.
0.;
3,4
0.l
0.2
Figure 7 . (a) Infrared spectra of 4 3 2 0 8 (optical density vs. wavelength in cm-I): curve 1, after adsorption of 1hO (equilibrium pressure 4 Torr); curves 2-6, DzO degasuing, times and temperatures being 1 hr at loo", 1 hr at 2OOo, 1 hr at 300", 1 hr at 400", and 13 hr at 400'; dotted line, after phases 11-111. (b) Infrared spectra of cu-Ci-208 (optical density V S . wavelength in cm-l): curves 3-6, CO adsorption at room temperature (equilibrium pressure 40 Torr) on a sample at various hydration degrees. Ilegassing times are as reported in Figure 7a; dotted curve, CO adsorption at room temperature (equilibrium pressure 40 Torr) after phases 11-111.
to nonstoichiometric oxygen in the bulk, they should be particularly intense in the case of amorphous samples and decline by hydrogen reduction at 800°, when bulk mobility is certainly excited. Spectra of Figure 4 clearly demonstrate that none of the above hypotheses is true. I n particular a DzO exchange causes a gain of transparency in the 1000800-cm-' region, but none of the bands under discussion is eliminated. The only possible explanation is that they are characteristic of crystalline chromia and that any contribution of OH bending modes is unimportant. This seems to be confirmed by the weakness of OH stretching absorptions in the 3S00-300O-~m-~region, after the whole series of thermal treatments. Our assignment is also supported by the comparison of our on spectra with those obtained by Xarshall, et uZ.," chromia monocrystals (Table I). Few of our bands below 950 cm-' are not reported by Marshall, et al. We believe that they are still characteristic of the crystalline form and that their observation is particu-
larly difficult in the case of monocrystals, due to the very strong band intensity a t 600-700 cm-', brought about by the sample thickness. These bands can be easily explained according to Marhsall's hypothesis (Table I), and their assignment to crystal modes seems to be confirmed. The above comparison between amorphous and crystalline phases emphasizes the importance of ir spectroscopy in examining the crystalline status of transparent materials. Similar considerations have been reported by Burwell, et ul.,'O that used the low frequency range. Spectra of Figure 4 are also characterized by big differences in the 750-800-cm-' interval that cannot be explained with certainty. It is knovm that particle size strongly influences ir spectra of solids; therefore, we think that the observed differences might be mainly determined by the surface area of examined samples. (17) R. Marshall, S. S. Mitm, 1'. J. Gielisse, J. N. Plende, and L. C. Mansur, J . Chem. Phys., 43, 2893 (1965). The Journal of Physical Chemistry, Vol. 76,No. 18, 1071
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A. ZECCHINA, S. COLUCCIA, E. GUGLIELMINOTTI, AND G. GHIOTTI
Table I . Frequencies and Assignment of or-Chromia Marshall, et ~
1
.
880 975 1010 1040 1066 1097 1137 1167 1225 1247 1283 1332
~
This work
810 (2L’) 845 (El’ En’) 888 940 (El’ Ea’) 974 1013 1043 1075 1095 1142 1168 1225 1243 1285 1330
+ +
’ Reference 17.
Finally, the spectrum of a-chromia exhibits a couple of bands at approximately 1350 cm-’ of changeable intensity and peculiar behavior. Since we think that no crystal modes are involved, they will be discussed in more detail in part 111. The weakness of bands due to surface hydroxyl modes supports the hypothesis that, after phases I1 and 111, the (001) face is highly dehydrated. More arguments in favor of the above viewpoint are brought by the adsorbing capacity towards CO and will be discussed later. Figures 8a and b show two models for a fully dehydrated (ideal) (001) face. On it, half the amount of oxygen ions present in a close-packed structure is left as to maintain the electric neutrality. Models a and b are not the only models that can be drawn, but they are just two of the most likely; besides an overall neutrality they also maintain the local neutrality as much as possible. Chromium ions with coordination number 4 and 5 are present in equal concentration. We cannot exclude that ions with coordination 3 could be present, but they are quite unlikely. Our model of a (001) face is rather similar to that suggested by Stone1*for faces perpendicular to the (001). Real samples are not completely dehydrated (residual hydroxyls would prove it) and Cr ions of coordination 6 can thus be present. TWOsuch possibilities are presented in Figures 8c and d and have been obtained from a and b assuming the dissociative chemisorption of few water molecules. Water Adsorption at Room Temperature. The first doses of water (or heavy water) are quickly chemisorbed on a-CrzOs (Figures 5a and c, curves 2-4). The absence of any appreciable absorption a t -1600 cm-’ clearly indicates that in this phase the chemisorption is entirely dissociative. On the other hand, the spectral modifications in the 2700-2000- (in the case of D2O) The Journal of Physical Chemistry, VoE. 76, No. 18, 1871
and in the 1000-750-~m-~regions (in the case of H20) suggest that few different types of surface hydroxyls are formed, I n the former region (OD stretching) two bands are observed that are quite narrow, isolated, and centered at 2700 and 2675 cm-’, respectively, also a broad absorption with apparent maximum a t 2430 em-’, that is probably complex, for at higher coverages two shoulders are visible at -2600 and 2535 cm-’. Also, at higher coverages, this band grows much more than the couple at 2700 and 2675 cm-l. The beginning of a nondissociative adsorption (water pressure -0.5 Torr) is indicated by the appearance of a band at 1590 cm-l and a t water pressures of few Torr a second undissociated species is revealed by another peak a t 1630 cm-’ that becomes predominant for even higher amounts of HzO sorbed. This behavior and the frequency coincident with the bending mode of liquid water suggest that water molecules adsorbed at high coverages are hydrogen bonded to surface hydroxyls. Molecular water, whose bending mode falls at 1590 cm-’, is strongly held and cannot be removed at room temperature but only upon degassing at 150-200”. We think that these water molecules are adsorbed onto C.U.S. chromic sites through a coordinative bonding. I n fact the H20 bending mode in aquo complexes of transition metals normally falls at lower frequencies than in the vapor phase,lg and Costa, et observe the bending mode of the complex ( C H ~ C O O ) ~ C I - . ~ H ~ O in the 1610-1580-~m-~range. The adsorption of HzO at room temperature can be summarized as follows. (a) Dissociative chemisorption probably occurs through the mechanism
It presumably occurs on more unsaturated ions first. Hydroxyls so formed would be “bridged” between two metal ions and should have a spectroscopic behavior similar to “bridged” hydroxyls in polynuclear complexesaZ1 Figures 8a and b show that, in principle, there are no limitations to dissociative chemisorption of water, which should lead to a close-packed layer of OH- groups. In practice HnO chemisorption, both a t room temperature and at 400°, always leaves some Cr3+ C.U.S. ions able to coordinate undissociated molecules, proving that the dissociative mechanism is limited, possibly by collective electronic properties. (b) Coordinative chemisorption occurs through the mechanism (18) F, S. Stone, Chimia, 23, 490 (1969). (19) I. R.Beattie, T. It. Gilson, and G. A. Ozin, J . Chem. SOC.A , 4, 534 (1969). (20) G. Costa, E. Pauluzzi, and A. Puxeddu, Gazz. Chim. Ital., 87, 885 (1957). (21) K. Nakamoto, ”Infrared Spectra of Inorganic and Coordination Compounds,” Wiley, New York, N. Y., 1963.
2781
IRSTUDY OF SURFACE PROPERTIES OF WCHROMIA H
H
a
b
\ / 0
+H?O
Cr3+ (c.u.s.) -+ cr3+ active sites being chromium ions that have not been coordinatively saturated according to mechanism a. (c) Physical adsorption occurs through the formation of hydrogen bondings on a monolayer saturated by mechanisms a and b. CO Chemisoyption at Room Tempeyatwe. Several have studied the adsorption of CO onto a-chromia and chromia gel. It has been found that the differential heat of adsorption of CO on achromia strongly varies with coverage.22 All the authors observed that CO is reversibly sorbed by samples that are oxygen free. Spectra in Figures 6a and b relative to the chemisorption of CO at different,coverages on crystalline and amorphous chromia lead to the following observations. (a) The stronger of the two bands is at, higher frequency than gaseous CO and is quite similar to the band observed on chromia-silica.2a Such raised frequency can be ascribed to the formation of a u bond between CO and surface sites with none or very small ?r contribution, as discussed in a previous paper.2a Active sites would be Cr3+C.U.S. ions. In fact CO adsorption is completely inhibited by pyridine that is a hard base and forms a stable adduct with the hard Cr3+ ion (unpublished results). (b) The lower the coverage, the harder the desorption (Figure sa), in agreement with the differential heats situation.22 Also the frequency js influenced by coverage. I n particular, on passing from spectrum 1 to spectrum 2 there is a sudden rise (2170 to 2178 cm-'), and a t lower coverages any further frequency raise is gradual. This behavior and the fact, thatJ there is an intersection of the two curves seem t o suggest that the band a t 2178 cm-' is due to a new species, which is absent at higher coverages. When carefully inspected, the spectra relating to lower coverages (curves 2-5) are still complex, for they change in shape and half-bandwidth (this phenomenon is particularly evident on passing from curve 2 to 3). The residual absorption, that is centered a t 2184 cm-l, is clearly "tailed" on the low frequency side. This band cannot be completely eliminated on room temperature degassing, as observed by other authors.lO Frequency changes with changing coverage can be due either to lateral interactions or to site heterogeneity, and any distinction between the two causes is rather difficult. The former effect, particularly at high coverages, can only partially explain the abrupt peak shift from spectrum 1 to 2, for any coverage effect should be more gradual and would never originate distinct species. In a following paper the presence of Cr3+ C.U.S.ions
C
d
Figure 8. Two possible models (a and b) for a completely dehydrated (001) face: 0, 02- ions of the underlying layer; 0 , Cr3+ ions; a, 02-ions of the upper layer. Two possible models (c and d ) of a partially hydrated (001) face: a, surface hydroxyls.
with two coordinative vacancies will be considered. Therefore we might postulate that at high 8 values, two CO molecules could be coordinated, leading to spectroscopically different species. Due to the weakness of the metal-carbon bond, the asymmetric and symmetric CO stretching modes are not expected to be resolvable.
2170cm-'
2178-2184 cm-'
This hypothesis would require combined gravimetric and spectroscopic measurements to be proved. Frequency variations, as well as band-width and shape changes a t lower coverages (curves 2 4 , can be mainly attributed to surface heterogeneity, in that any perturbing effect due to dipole interactions should be negligible. We have already indicated that in our adsorbent the (001) face is definitely predominant, so that we think the observed heterogeneity not to be due to the presence of other crystallographic faces, but to the existence of different Cr3+ C.U.S. ions, yielding u bonds of different intensity. On the other hand, such a heterogeneity, though undoubted, when revealed by CO adsorption only originates a frequency shift range of few wave numbers and will not be discussed here any longer, for (22) D.A. Dowden and W. E. Garner, J . Chem. SOC.,893 (1939). (23) R.A. Beebe and D. A. Dowden, J . Amer. Chem. SOC.,60, 2912 (1938). (24) D. S. MacIver and H . H. Tobin, J . Phys. Chem., 64, 461 (1960). The Journal of Physical Chemistry, Vol. 76,No. 18, 1971
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A. ZECCHINA,S. COLUCCIA, E. GUGLIELMINOTTI, AND G. GHIOTTI
any tedious and lengthy reasoning would miss experiC.U.S. surface concentration. This hypothesis has also mental evidence. The argument will be discussed in been proposed by Burwell for the gel.lo The dehydramore detail in part 11, where the spectrum of chemition probably generates Cr3+C.U.S. ions with different sorbed oxygen can only be understood in terms of sevlocal configurations; they could differ either in the eral local configurations. number or in the nature of ligands, relative concentrations of the various types being a complex function of (c) Bands due to chemisorbed CO are broader in the case of amorphous nonstoichiometric Crz03than in dehydration degree. Heterogeneous sites can originate u bonds of different strength and a CO spectrum varythe case of a-Cr20~, as expected for a more heterogeneous, random material. Also this aspect will be further ing with surface dehydration is expected. Figure 7b confirms our viewpoint, in that the band maximum shown up by O2 chemisorption (part 11). The frequency of a-bonded CO is always higher in amorphous clearly varies with activation temperature. On the other hand, this frequency shift range is very narrow CrpOasamples and we think that this is accounted for by (2184-2170 cm-l), and no information on the real the presence of nonstoichiometric oxygen rather than nature of such surface sites heterogeneity can be deby a more pronounced heterogeneity. I n fact, this oxygen excess might produce surface chromium ions in duced. a higher oxidative status than three and thus capable As previously indicated, a 13-hr dehydration at 400" of stronger u bondings. of a rehydrated sample does not restore the whole CO activity as it was after phases I1 and 111. This fact (d) CO adsorbed on both amorphous and crystalline samples produces a broad, weak band at -2130 cm-l seems to suggest that samples after phases I1 and I11 that is very stable to desorption. An analogous abare in a more dehydrated surface condition. I n fact the whole CO activity is only restored if the degassing sorption has been observed on silica-supported chromi&,*& and the assignment is the same: few CO moleat 400" is followed by a treatment corresponding to phases I1 and I11 (Figure 7b, dotted line). Meanwhile, cules are adsorbed onto surface chromium ions, through a bond with some ?T contribution. These sites are the concentration of surface hydroxyls is further dethought not to be Cr2+ions produced during reduction creased (see OD stretching region in Figure 7a, dotted line). Of the two operations, only the oxidation of (phase 111))because they should not appear on merely activated, amorphous, and nonstoichiometric samples. phase I1 can bring about a hydroxyl decrease and a hydrated sample that underwent the treatment at phase Crystalline samples that chemisorbed O2 and were deI11 only did not exhibit any ability to adsorb CO. This gassed at 400" exhibit an even stronger intensity at idea has been already postulated by Burwell, et aL.,'O 2130 cm-l upon adsorption (unpublished results). when accounting for the difference between their rePart I1 will show how a 400" outgassing does not comsults and those of JIacIver, et al.,24suggesting that the pletely eliminate adsorbed oxygen thus excluding the latter authors, by using a several redox cycle techpresence of Cr2+ ions on the surface. In this way we nique, finally got a highly dehydrated surface situation. get another proof that no Cr2+ions are responsible for A guide in the assignment of surface hydroxyl bands the band at 2130 cm-'. can come from Figure 7a, where spectra of the OD CO Adsorbing Sites arid Surface Dehydration. After stretching region for different dehydration temperaa prolonged contact with H 2 0 at room temperature, no tures are reported. activity towards CO is exhibited by a-chromia. This The rehydration experiment (Figure 5a) seemed to fact is explainable in terms of the discussed rehydration suggest that only bands a t 2700 and 2675 em-l could reactions that eliminate Cr3+ C.U.S. ions. Adsorptive be assigned to free or nonstrongly interacting surface capacity of CO should be gradually restored upon deOD groups. A broader absorption at -2430 cm-', gassing at high temperatures, as already pointed out already present at the lowest coverages, might be due to with different techniques in the case of Cr203 gelsso hydrogen-bonded hydroxyls. Desorption spectra of Figures 7a and b, showing that CO adsorption begins Figure 7a partly confirm the above conclusions in that after a 200" degassing and increases thereafter for upon high temperature degassing the absorption a t higher activation temperatures, clearly demonstrate -2430 cm-I quickly declines. But another complex the close similarity between a-CrzOl and Crz03 gel. absorption, roughly centered at 2550 cm-' (with shoulWe have already indicated that coordinated water is desorbed a t -200" and therefore we think that Cr3+ ders a t 2600 and 2575 cm-') still remains and does not decline any faster than bands at 2700 and 2675 cm-l. C.U.S. sites, capable of chemisorbing CO, are first formed It must therefore be due to nonstrongly interacting at such temperature. Also a reaction opposite to the hydroxyls, though hydrogen-bonded species responsimechanism of the dissociative chemisorption would beble for a broad tail between 2500 and 2200 cm-' are still gin at 200". It becomes predominant at higher temon the surface (Figures 5 and 6 ) . A similar complex peratures, and new Cr3+ C.U.S. ions should be formed situation has also been observed by PeriZ5on y-alumina, causing a constant increase in the ability to adsorb CO. Our results tend to confirm that this is thecase and that adsorbed CO gives a direct estimate of Cr3+ (25) J. B . Peri, J . Phys. Chem., 69, 211 (1965). The Journal of Physical Chemistry, Vol. 76, N o . 18,I071
2783
IRSTUDY OF SURFACE PROPERTIES OF ~-CHROMIA where five bands due to "free" hydroxyls in different local configuration were present in the 3800-3700-cm-' range. On the (001) face of cr-CrtO3 the number of possible species, higher than in the case of r-Al203, would lead to an almost continuous variation of energy situation and could account for the complex spectrum, whose components are only partially resolved.
Conclusion From the above discussion, we conclude the following. (1) H20is chemisorbed on a-Crz03both through a dissociative mechanism and by forming a coordinative bonding. (2) There are several types of weakly
interacting surface hydroxyls, and they can be accounted for assuming that (001) face is predominant. (3) On tt dehydrated surface many Cr3+ ions are present with different coordinative situations. (4) CO is mainly chemisorbed through the formation of weak u bonds to Cr3+ C.U.S. ions, ( 5 ) Active sites can be generated on a-CrnOa by dehydration at high temperatures as well as on Cr203 gel. (6) Burwell's theory is quite useful in explaining the observed phenomena and can also be used for crystalline phases.
Acknowledgments. This research has been supported by the Consiglio Nazionale delle Ricerche.
An Infrared Study of Surface Properties of a-Chrornia. 11. Oxygen Chemisorption by A. Zecchina, S. Coluccia, L. Cerruti, and E. Borello* Istituto di Chimica Fiaica dell' Universith. di Torino, Turin, Italy
(Received October 19, 1970)
Publication costs assisted by the Consiglw Italian0 delle Ricerche (C.N.R.)
Oxygen interaction with a-CrzOahas been studied by ir spectroscopy. The chemisorption is dissociative, leading to different Cr=O surface groups. The Cr=O stretching frequency depends both on the coordination number of the adsorbing chromium ions (coordinative heterogeneity) and on the environment of the new formed groups (ligand heterogeneity), At high temperatures surface migration phenomena are observed and islands of high valence oxides are formed, as already suggested by other authors.
Introduction The O2-CrpO8 system has been studied by several authors, and few coherent conclusions have been reached. I n particular there is a good agreement on the room temperature chemisorption to be dissociative, as indicated by the high adsorption heat, On the other hand, no agreement exists on the nature of surface oxygen adsorbed a t various temperatures. Dowden and Garner' assume that O2 is chemisorbed at room temperature to form Cr=O groups. Weller and Voltz2 think that, a t room temperature, oxygen is in the form ions and that at higher temperature an oxide of 02layer is formed a t the surface with Cr valency up to 6. McIver and Tobin3 postulate the formation of 02or 0- ions in the dissociative chemisorption at low temperatures but do not justify the big difference that other authors1 observed in the adsorption heat between 0 and -196". Finally Burwell, et d.,* think that O2 chemisorption a t -78" leads to 022groups. For more details on oxygen chemisorption see also a review
by Winter.6 The only infrared study of surface oxygen mas carried out by Shopov and Palazov6 on CraOa gel but, due to the high temperatures employed, we think that their spectra refer to bulky layers of chromium oxides (like CrO3) more than to real surface complexes. Our purpose is to contribute to the understanding of the Cr203-02interactions in view of the powerful aid usually given by ir spectroscopy to band characterization problems and heterogeneity identifications.
Experimental Section a-Cr203 samples were prepared as described in part I. Also ir spectra have been run as described in the previous paper. (1) D. A. Dowden and W. E. Garner, J . Chem. Soc., 893 (1939). (2) W. Weller and S. E. Volts, J . Amer. Chem. SOC., 7 6 , 4695 (1954).
(3) D. S. McIver and H. H. Tobin, J . Phys. Chem., 64, 451 (1960). (4) R. L. Burwell, Jr., L. Ealler, X. C. Taylor, and J. 17. Read, Advan. Catal., 19, 62 (1969). ( 5 ) E. R. S. Winter, ibid., 10, 196 (1958). (6) D. M. Shopov and A. N. Palasov, Kinet. Katal., 6, 864 (1965).
The Journal of Physical Chemistry, Vol. 76, N o . 18, 1971
