2783
IRSTUDYOF 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 Cr203-02 interactions 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
2784
A. ZECCHINA, S. COLUCCIA, L. CERRUTI,AND E. BORELLO
100
100
80
80
60
60
T%
T%
40
40
20
20
1100
1000 cm-'
900
800
1100
1000 cm-1
900
800
Figure 1. Infrared spectra of a-Cr20a (% transmission vs. wavelength in crn-l): curve 1, background after phases 1-111; curves 2-4, a t increasing oxygen coverages at room temperature; curve 5, saturated with oxygen.
Figure 2. Infrared spectra of amorphous CrzOa(% transmission us. wavelength in cm-I): curve 1, background; curves 2-5, a t increasing oxygen coverages a t room temperature; curve 6, saturated with 02.
02 Adsorption at Room Temperature. Figure 1 shows ir spectra of various amounts of O2 adsorbed on a aCrz03sample that underwent the whole series of pretreatments described in part I. (These samples will be hereafter referred to as samples A.) Increasing oxygen coverages were obtained allowing small amounts of gas (approximately 2 X 10IRmolecules each) into the cell and assuming the chemisorption to be complete when a Torr mas read on the residual pressure of 5 X Pirani gauge. A few minutes was required for the first doses to be consumed, suggesting that a fast reaction was taking place. Higher oxygen pressures (5 X to 40 Torr) did not cause any rapid change of band intensity, but a slow chemisorption was revealed by a slight time-dependent increase of the bands in the 1040-75O-cm-' range. The reproducibility of 0 2 chemisorption at room temperature is evident if Figure 1 (curve 5 ) , Figure 5 (solid line), Figure 6 (broken line), and Figure 7 (solid line) are compared. They refer to different A samples and only minor differences are visible except for a peak at 1016 cm-', whose intensity depends on the dehydration of the surface (see Discussion). The spectrum of chemisorbed oxygen is thus expected to be sensitive to water contamination.
Oxygen chemisorption produces three groups of ir absorpt,ions at 1040-970, 900-880, and 850-750 cm-', the first one being definitely the most intense a t room temperature. It is complex and five components are visible at 1024, 1016, 995, 986, and 980 cm-'. Owing to band sharpness and good sample transparency, peak positions have been observed with high accuracy ( h1 cm-I). Also, i t has been possible to observe that most of the bands change their position from one experiment to another in a narrow range (*3 cm-l) probably due to minor causes that we cannot identify yet. Only crystalline samples exhibit the above band complexity. Figure 2 shows ir spectra relative to O2 chemisorbed a t room temperature onto amorphous CrpOa, coverages and all experimental conditions being as for Figure 1. A broad absorption is visible a t -1000 cm-l that does not exhibit any definite structure. Oxygen chemisorption has also been studied on crystalline samples of various surface hydration. An A sample was allowed to react with DzO for 1 hr a t 400" and pumped off at room temperature to a final pressurc of Torr. Heavy water was used for rehydration purposes to avoid absorption bands due to OH groups from 1100 to 700 cm-1 (part I). At this point the surface
The Journal of Phgsical ChemistTy, Vol. 7 6 , No. 18, 1971
2785
IRSTUDY OF SURFACE PROPERTIES OF ~-CHROMIA 100 I
I
I
I
I
900
800
i
80
60
1% 40
1100
1000
crn-1
Figure 3, Infrared spectra of a-CrzOI (% transmission 21s. wavelength in cm-1): oxygen (40 Torr) on samples a t various dehydration degrees. Dehydration times and temperatures are: curve 1, 1 hr a t room temperature; curve 2, 1 hr a t 200"; curve 3, 1 hr a t 300'; curve 4, 1 hr a t 400"; curve 5, 13 hr a t 400"; dotted curve, 1 hr a t 800".
was completely hydrated, as reported in part I. Oxygen was allowed into the cell, and the ir spectrum was recorded. This first rehydration-oxidation operation was followed by a reduction with CO a t 400" to remove chemisorbed oxygen, if any, and then degassing a t 400" to eliminate the reduction products. The whole series of operations (exhaustive rehydration, evacuation, oxygen exposure, reduction with CO, degassing a t 400") was then repeated several times just varying the evacuation temperature of the rehydrated sample. I n this way, different dehydration conditions were obtained in turn and oxygen chemisorption a t room temperature was checked with the usual spectroscopic technique. Notice that this set of operations did not produce appreciable variations of surface area and a drastic dehydration at 800" caused a loss of surface area of only 20%. Figure 3, showing the ir spectra of chemisorbed oxygen in various conditions, indicates (a) rehydrated samples that have been dehydrated a t temperatures up to 100" do not chemisorb oxygen; (b) 02-active sites start being produced by dehydration a t temperatures higher than 100"; (c) dehydration a t 100-300" originates 02-active sites leading to absorptions in the 1000-970-cm-' range (curves 2
Figure 4. Infrared spectra of a-CrzOs (% transmission us. wavelength in cm-1): --, background after phases 1-111; -O-O-, contacted with NzO (40 Torr) a t room temperature; -A-A-, after pumping off the gaseous phase a t room * after temperature (residual pressure -10-5 Torr); degaming a t 400", 1 hr (residual pressure -10-6 Torr).
.
-
s,
and 3) ; (d) a t 400" sites are formed that produce bands in the 1000-1040-~m-~range (curves 4 and 5 ) (the band at 995 em-' is intensified becoming the strongest of the whole spectrum); (e) an 800" dehydration mainly forms 02-active sites leading to the band a t 1016 cm-'. I t s intensity is still lower than on samples A or on samples whose activity towards oxygen has been completely restored by means of treatments of phases I1 and 111, which bring them back to the conditions of samples A. Infrared spectra of O2 chemisorbed on samples that have been reduced with Hz instead of CO are not reported, for they are all similar to the spectra of oxygen on samples rehydrated and dehydrated a t 400" (Figure 3, curve 5 ) . The latter are hereafter termed samples B; their surface hydration is higher than for samples A. Nitrous oxide has also been used as an oxidizing agent on samples A. Figure 4 clearly shows that NzO decomposes a t room temperature, giving ir absorption bands that are typical of chemisorbed oxygen, though coverages are lower than those obtainable with molecular oxygen. As previously observed by Winter6 KzO is also reversibly absorbed in a nondissociative fashion. Active sites for NzO molecules are Cr3+C.U.S. The Journal of Physical Chemistry, Vol. 76, No. l a 3 1071
A. ZECCHINA, S. COLUCCIA, L. CERRUTI,AND E. BORELLO
2786
1Ot
8C
6C 7 ,I,
4c
2c
00
1000
900
800
7(
cm-1
Figure 6. Infrared spectra of a-CrzOa (% transmission us. wavelength in cm-1): , background after phases 1-111; - - - -, full oxygen coverage a t room temperature ( p 40 Torr); -o-O-,after degassing a t 200", 1 hr; -A-A-, after degassing a t 400", 1 hr.
-
00
1000 -1
cm
Figure 5. Infrared spectra of (% transmission us. wavelength in cm-1): , full oxygen coverage a t room temperature ( p 40 Torr); -O-O-, after heating in 02 a t 200'; -A-A-, after heating in 0% a t 400'.
--
atoms (as defined in part I) that have not been saturated by oxygen atoms coming from the initial, fast decomposition. This matter will be discussed in more detail elsewhere. The spectrum of chemisorbed oxygen coming from decomposed N20 depends on whether undecomposed K20 is still present on the surface or whether it has been pumped off. When the NzO molecules are present, a band of medium intensity a t 1016 cm-' and a stronger and broader one at 975 b 3 cm-' are observed. Upon degassing at room temperature, the former absorption is strongly intensified, a new band appears a t 993 cm-l, and the broad one at 975 cm-l disappears. The phenomenon can be indefinitely reproduced through NzO exposure and evacuation cycles. In particular the final spectrum after evacuation is simpler than that coming from molecular oxygen. 0 2 Adsorption at Higher Temperatures. The continuous curve of Figure 5 refers to the maximum oxygen coverage obtainable a t room temperature on a sample A. When temperature is gradually raised, the following spectral modifications are observed (other curves of Figure 5): (a) the 995-cm-' band slightly grows; (b) The Journal of Physical Chemistry, Vol. 76,No. 18, 1971
the absorption at 1016-1024 cm-1 strongly increases; (c) the band between 900 and 880 cm-' also strongly increases; (d) a new absorption is formed between 820 and 850 cm-'. Stability of Surface Oxygen to Desorption. The various surface species originated by O2 chemisorption a t room temperature are very stable and are not eliminated by prolonged outgassing at 400" (Figure 6). Bands a t 980 and 986 cm-' only are severely weakened, when the outgassing temperature is increased, suggesting that the corresponding surface species could be reversible. On the other hand, an increased intensity a t 1016, a t 900-880, and a t 850-750 cm-' might mean that a surface rearrangement is involved, causing some species to be transformed into others of higher stability. I n other words, a t 400" surface oxygen would migrate on the surface rather than being desorbed. The spectrum after degassing at 400" (Figure 6) is very similar to that reported in part I, Figure 3, relative to oxygen chemisorption at 400' (equilibrium pressure 5X Torr). A Preliminary Investigation of Surjace Oxygen Mobility. The above observations made necessary an investigation of surface oxygen mobility. Two experiments have been performed, and the results are reported in Figures 7 and 4,respectively. The first one
IRSTUDYOF SURFACE PROPERTIES OF CY-CHROMIA 100
80
60 T% 40
20
noo
1000
cm-1
900
I
Figure 7. Infrared spectra of a-CrzOs (% transmission us. wavelength in cm-1): , background after phases 1-111 and full oxygen coverage; - - - -, after heating a t 200", 1 hr in a static vacuum; -.-.- , after heating a t 400", 1 hr in a static vacuum.
-
was run as follows. Oxygen was chemisorbed, and the excess was pumped off a t room temperature to a final pressure of Torr. The cell was then closed, the sample was warmed up at various temperatures up to 400') and the ir spectra were recorded after every thermal treatment. No pressure was developed in the cell a t any stage meaning that the amount of chemisorbed oxygen remained constant. The second experiment was as follows. N20 was allowed onto an A sample, and the excess was pumped off a t room temperature to complete the elimination of reversibly bonded NzO. The spectrum was recorded, and a series of thermal treatments were performed in a static vacuum as for the previous experiment. Also in this case no pressure was developed in the cell, and the amount of chemisorbed oxygen remained unchanged. The band a t 1016 cm-' was intensified to the detriment of bands a t 986 and 980 cm-' in the first case and a band a t 993 cm-' in the second. In both cases a new absorption was formed a t 1035 cm-l, and few bands a t lower frequencies were intensified. It is concluded that no oxygen is desorbed whatsoever and that there is a complex surface equilibrium which depends on temperature and coverage.
Discussion Surface Groups Originated by Oxygen Chemisorption. All of the reported spectra clearly indicate that oxygen
2787 is chemisorbed onto a-chromia to form surface species with characteristic ir absorptions. By comparison with several compounds and complexes of chromium and other transition metals7-14the conclusion might be drawn that such species involve oxygen atoms bonded to Cr ions with bonding indexes close to two. A first question arises on whether all of the observed bands are due to different Cr=O groupings or whether a few of them have to be attributed to surface complexes with more than one ir-active normal mode in the frequency range under consideration (e.g., Cr02, CrOs, Cr04, Cr207, etc.). We believe that a t least for absorptions a t 1040-970 cm-l the former hypothesis is true. In fact: (a) Bands at 1035, 1016, 995, 986, and 980 cm-' are all independent of one another and of absorptions a t lower frequencies, as suggested by the fact that their relative intensities are changeable with sample treatments (Figures 1 and 3) , thermal conditions (Figures 4-7), and oxidizing agent (Figure 4). (b) They exhibit different reaction rates with CO, H2, and D2. (The interaction of various gases on a-Crz03will be discussed in a following paper.) Absorptions located a t lower wave numbers are independent of the above but might be connected with one another and could be due to different normal modes of one surface complex only. In fact they grow together with increasing adsorpt'on temperature (Figure 5 ) and upon surface oxygen diffusion (Figures 4, 6, 7). Nevertheless the intensity ratio is rather changeable. The major difference between absorptions in the 1040-970- and those in the 900-750-cm-l range is that the latter are quite intense only when adsorption temperatures higher than room temperature are employed. The 1035-cm-' band is different from all of the others in that it only forms a t high temperatures in very special cases (Figures 4 and 7). Absorptions due to A40 groups a t such high frequencies have only been observed for VOCla15and VOBr316while no ir spectrum is reported for CrOCla although this compound has been isolated. From the above considerations we must conclude that oxygen is chemisorbed at room temperature with dissociation to form several Cr=O species. Our result only disagrees with some other author^^*^ conclusions in that we exclude that surface oxygen is in the form of 0- or 02groups, but it is linked to metal ions (7) C. G. Barraclough, J. Lewis, and R. S. Nyholm, J. Chem. Soc., 3552 (1959). ( 8 ) H . Stammreich, D. Sala, and K. Kawai, Spectrochim. Acta, 17, 226 (1961). (9) H. Kon, J . 17mrg. Nucl. Chem., 25, 933 (1963). (10) D. Brown, J . Chem. Soc., 4944 (1964). (11) W. P. Griffith, ibid., 245 (1954); ibid., A , 211 (1969); ibid., 2270 (1969). (12) I. R. Beattie and T. R. Gilson, ibid., 2322 (1969). (13) J. A . Campbell, Spectrochim. Acta, 21, 1333 (1965). (14) J. A . Campbell, ibid., 21, 851 (1965). (15) K. Ueno and A . E. Martell, J. Phys. Chem., 6 0 , 934 (1956). (16) F. A . Miller and W. K. Baer, Spectrochim. Acta, 17, 112 (1961).
The Journal of Physical Chemistry, Vol. 76, N o . 18, 1971
2788 through essentially covalent double bonds, as already suggested by Dowden and Garner.‘ No direct information can derive from the spectra on surface oxygen coverage. Nevertheless other authors’ results4 suggest that the adsorbed amount is much less than required for exhaustive saturation of all Cr3+ ions on a (001) face (9.8 ions for 100 dz). Spectroscopic evidence of Cr3+ ions still present on a-Crz03 containing preadsorbed oxygen has been obtained by adsorbing pyridine, H20, and COz (results submitted for publication). As far as oxygen chemisorbed at high temperature is concerned (bands at 900-750 cm-l), the assignment is rather involved. From a merely spectroscopic point of view, absorptions in that region could be due to complexes of the CrO4“- type, or to their polymers, where 2 is 2, 3, 4 . I 2 - l 4 It is quite likely that at least one of the observed bands is due to a normal mode of ill-0114 groups that are present in all [Cr04]-+ polymeric structures. In this regard, even chromium trioxide can be thought of as a polymer of the tetrahedral [CrOd] unit.’ Bands in the 900-750-cm-’ range are missing when room temperature chemisorption has taken place, and only form a t higher temperatures. This fact supports the above assignment in that monomeric and polymeric Crop- complexes would require a surface rearrangement to be formed that only could be brought about by temperature enhanced surface mobility. Other authors2previously suggested that oxygen a t high temperature produces surface islands of high valence oxides of poorly definable structure. The Nature of Active Sites. Oxygen chemisorption a t room temperature on samples A leads to five bands in the 1040-970-~m-~range, which is typical of Cr=O stretching modes. There is thus evidenced a heterogeneity of ar-CrzOasurface toward oxygen that ought to be clarified. On our a-chromia preparation, face (001) is by far the most abundant (see part I) and, as Davis” and IllcIver, et aLj3did, we shall limit our discussion to sites that can be generated on it. In other words we exclude that the observed heterogeneity is due to the presence of more than one crystallographic face. The experiment of Figure 3 clearly indicated that adsorptive capacity of face (001) towards O2 increased with dehydration. Therefore the nature of active centers that are created upon dehydration has to be investigated. Every surface chromium ion in the hydrated oxide is surrounded By six ligands (see part I), three of which are lattice oxygen ions and three are surface groupings (either OH- or coordinated water). Upon outgassing, HzO is eliminated that was coordinated or has been formed by condensation of two hydroxyls. I n the first steps of such a dehydration process Cr3+C.U.S. ions would be produced with one coordinative vacancy only, but later, ions with two vacancies could be formed. The latter sites are not favored by ligand field theory, but such possibility cannot be excluded in that surface strains force such a norifavored situation. The exisThe Journal of Physical Chemistry, Vol. 76,No. 18,1.971
A. ZECCHINA, S. COLUCCIA, L. CERRUTI,AND E. BORELLO Scheme I
OH’
02-
OH1
0” Cra+
‘V’
2
OH-
3
0“ Cra+
V
V
4
5
tence on the (001) face of Cr3+ C.U.S. ions with three coordinative unsaturations can be ruled out because their formation is unlikely also in severe dehydration conditions. Moreover, the mobility of surface oxygen, which is quite appreciable above 400” ,5, 18,19 certainly would further reduce their probability, for the surface oxygen is expected to move in order to maintain a local as well as an overall stoichiometry. Therefore let us conclude that a dehydrated (001) face contains Cr3+ C.U.S. ions with one or two coordinative unsaturations (see also Figure 8 of part I). This situation represents an important surface heterogeneity that could be indicated as a “coordinative heterogeneity.” Ions with one vacancy only (coordination 5, strained square pyramids) will be predominant for medium dehydration temperatures, and ions with two vacancies (coordination 4, strained tetrahedra) will be appreciably formed at higher temperatures. Obviously the determination of concentration ratios as a function of dehydration temperature is a t present almost impossible. A second type of heterogeneity must be considered that would be termed “ligand heterogeneity.” It comes from the fact that there are several ways to make Cr3+ c.u,s. ions, with either one or two vacancies, that are represented in the Scheme I. This type of heterogeneity is evidenced in Figures 8c and d of part I. In the above pictures dotted lines represent the layer of lattice 02-ions immediately below the surface. For sake of simplicity, it is supposed to be unstrained with respect to the bulk situation. On studying carbon monoxide chemisorption on variously hydrated a-Crz03samples (part I), we saw how CO can be thought as a test molecule for coordinative unsaturation. Chemisorbed oxygen probably gives information on both coordinative and ligand heterogeneity because in this case the stretching frequency of the metal-adsorbate bond can be directly observed. Oxygen chemisorption a t room temperature leads to the formation of Cr=O groups. Therefore five different chromium-oxygen stretching frequencies are (17) R. J. Davis, “Chemisorption,” W. E. Garner, Ed., Academio Press, New York, N. Y.,1957. (18) F. S. Stone, Advan. Catal., 13, 1 (1962). (19) K.Hauffe, ibid., 6, 213 (1955).
IRSTUDYOF SURFACE PROPERTIES OF CY-CHROMIA expected, corresponding to the five situations of Scheme I. Spectra of Figure 1 clearly show that five bands are present in the 1024-980-cm-' region, and, in particular, a doublet is at 1024-1016 and a triplet a t 995-980 cm-'. This correspondence does not seem to be fortuitous, because it is accompanied by several minor observations that are reported below. There is a clear separation of 20 wave numbers between the two groups of bands, while the doublet lies within 8 cm-l and the triplet within 15 cm-I. A great deal of spectroscopic literature leads to the conclusion that M=O stretching frequency increases with decreasing coordination number. This is particularly true in the case of VOC1315 (coordination number 4), V206 (coordination 5),12and VO complexes in which V has an octahedral coordination.z0 Therefore the doublet should be assigned to structures 4 and 5 of Scheme I and the triplet to structures 1-3. This tentative assignment seems to be necessary if other facts are taken into consideration: water and pyridine adsorption a t room temperature onto preadsorbed oxygen (part IV) quickly eliminates the doublet. In fact structures 4 and 5 of Scheme I after oxygen chemisorption still have the possibility of adsorbing water and pyridine because they are still C.U.S. This possibility is obviously missing in the case of structures 1-3, when saturated by chemisorbed 0 2 . Also spectra of Figure 3 are in agreement with the above assignment, for we observe that only one absorption at 980-986 cm-' is formed when oxygen is chemisorbed onto poorly dehydrated samples. When higher dehydration temperatures are employed and sites with two coordinative vacancies are probably formed, the higher frequency doublet is formed upon oxygen chemisorption. Finally our assignment is consistent with the results obtained in the experiments on surface oxygen mobility that are reported above. From all we have seen so far it might be concluded that what we termed "coordinative heterogeneity" plays a major role in the energetic differentiation of active sites and that the "ligand heterogeneity" causes a further minor differentiation. I n this discussion we only took into consideration the local situation of adsorbing chromium ions, every influence of neighboring groups being disregarded. This simplification is fair to more than a first approximation in that the spectroscopic behavior of our surface groupings is well defined and very similar to the behavior of mononuclear chromium complexes and compounds. On the other hand, it must be kept in mind that surface ligands (OH- and 02-C.U.S. ions) are also ligands of neighboring metals ions, and that newly introduced ligands, by gas adsorption, can partially saturate coordinative vacancies of nearby ions. These considerations recall the collective character of surface and would account for several slightly different situations t8hat might be produced around a given surface group on considering a t least the six surrounding positions.
2789 The collective character of surface also suggests that the adsorption of a ligand onto a metallic site can modify the electron situation of neighbors. I n particular, if adsorption takes place through electron transfer from adsorbate to adsorbent, the affinity of free sites is expected to decrease with increasing coverage. This fact is revealed in the ir spectra by a lowering of force constants that is particularly evident when pyridine is adsorbed onto chemisorbed oxygen. Besides the differentiations given by the first two types of heterogeneity, several minor effects can contribute to the overall complexity of the spectra and can also account for small frequency shifts that are observed in the oxygen bands owing to pretreatment conditions and coverage. Such minor effects can also explain the sensitivity of chemisorbed oxygen bands to the presence of K20 weakly bonded onto neighboring sites (see Figure 4). So far the discussion only dealt with oxygen bands in the 1024-980-cm-' range which are by far the most important in the case of room temperature chemisorption. Weak bands are also formed at 900-880 and 850-750 cm-l (Figure l),which become much stronger when oxygen is chemisorbed at higher temperature. Our assignment was to complex C r 0 P groupings, either monomeric or polymeric. They likely come from the rearrangement of species a t higher frequency. The required amount of energy for this rearrangement could be supplied a t room temperature by the chemisorption process itself, which is highly exothermic. Surface Layer Mobility and the Anomalous Band at 1036 Cm-l. The ir spectrum of chemisorbed oxygen remaining on the surface after a prolonged degassing at 400" (Figure 6) is practically the same (except for a band a t 1035 cm-') as the spectrum that is obtained when a sample that chemisorbed oxygen at room temperature is warmed up to 400" under a static vacuum (Figure 7). As already observed in the Experimental Section, this demonstrates that on raising the temperature all the observed modifications have to be attributed to enhanced surface mobility, oxygen desorption only playing a minor role.2 Experiments that are reported in Figures 4, 6, and 7 allow the following considerations: (a) Groups responsible for absorption a t 1000-970 cm-l decrease in concentration with temperature. In particular, the bands a t 980 and 986 cm-' are the most sensitive, and when they are completely eliminated the band a t 995 cm-l also starts declining. (b) The destruction of the above species gives rise to bands a t 1040-1000, 900-880, and 850-750 cm-l. When the operation is performed under a static vacuum (Figures 4 and 7) a weak absorption is also formed at 1035 cm-I, which has been previously assigned to a Cr=O stretching, where Cr is in a tetrahedral coordination. (20) J. Selbin, L. H. Holmes, Jr., and S. P. McGlynn, J. Inorg. Nucl. Chem., 25, 1359 (1963).
The Journal of Physical Chemistry, Vol. 76,N o . 18, 1971
A. ZECCHINA,S. COLUCCIA, E. GUGLIELMINOTTI, AND G. GHIOTTI
2790
What is observed above demonstrates that low absorbing species are produced by destruction of the higher absorbing ones. Surface rearrangement leading to those transformations is favored by surface mobility which becomes important between 200 and 400”. The destruction of CrO groups with Cr in an octahedral coordination also generates higher absorbing species, where Cr has a coordination number lower than 6. This suggests that at high temperatures ligands of octahedral species are removed from the central ion, leading to species of lower coordination. Removed ligands would migrate to form complex oxide islands of high valence. The formation of a band at 1035 cm-I is probably accounted for because a tetrahedrally coordinated Cr can be generated by migration of two ligands from an octahedral structure or of one ligand only from a square-pyramidal one. The absence of the 1 0 3 5 - ~ m -species, ~ when dynamic vacuum conditions are employed, might be ascribed to a lower stability.
are linked to the surface through covalent double bonds. (3) Active sites are generated by surface dehydration. (4) a-Chromia surface (001) is highIy heterogeneous, site differences being mainly brought about by initial coordination number of Cr ions and by the type of ligands. (5) At high temperature complex “chromate-like” groupings are formed. (6) At high temperature, surface mobility promotes an equilibrium among adsorbed ,species. The above conclusions still do not give a complete picture of the 02-Cr203 system, more experiments being required mainly concerning the “high temperature species” and the interaction a t lower temperature (from room temperature to 78°K). Different experimental techniques are at present under study, to approach the problem from different points of
Conclusion
(21) NOTE ADDEDIN PROOF. Since submission of this paper we have become acquainted with t w o papers by A. A. Davidov, et al. [Kinet. Katal., 10, 919, 1103 (1969)], in which they found some bands due to oxygen adsorbed on polycrystalline Crz03 in the spectral region 820-1020 cm-’. They assign them t o stretching modes of groups M-0 on different crystal faces having a band order between 1 and 2.
I r spectroscopy has been used to characterize chemisorbed oxygen onto a metallic oxide. The following information has been obtained. (1) O2 chemisorption a t room temperature is dissociative. ( 2 ) Oxygen atoms
Acknowledgment. This work has been supported by the Consiglio Xazionale delle Ricerche.
An Infrared Study of Surface Properties of a-Chromia.
111. Adsorption of Carbon Dioxide1 by A. Zecchina, S. Coluccia, E. Guglielminotti, and G. Ghiotti Istituto d i Chimica Fisica dell’ Universitb d i Torino, T u r i n , Italy
(Received October 28, 1970)
Publication costs assisted by the Consiglio Italiano delle Ricerche (C.N.R.)
An ir investigation has been done of COSadsorbed onto an a-CrzOa sample in which the (001) face is predominant. Two different types of interaction are observed: a strong chemisorption leading to bidentate carbonates and bicarbonates and a weak chemisorption whose products are “organic” carbonates. COZadsorption confirms a great heterogeneity even in a one-face system. Surface hydroxyls and Cr*+ c.u.s.-02C.U.S. couples are mainly revealed.
Introduction Parts 1 and 11 dealt .with H20,D20,co, and o2adsorption on a-Cr2Oa. In this paper the chemisorption Of dioxide Onto the Same is discussed and a few mechanisms are proposed. I r spectroscopy has been widely used to study C02 adsorption onto various o ~ i d e s ~and - ~ onto chromium oxide supported by and silica.“ No spectroThe Journal of Physical Chemistry, Vol. 76, N o . 18, 1071
scopic data are available relative to COz adsorption onto nonsupported chromia, but other techniques have (1). Correspondence should be sent t o E. Borello, Istituto di Chimica Fisica dell’Universith di Torino, Turin, Italy. (2) (a) J. H. Taylor and C. H. Amberg, Can. J. Chem., 39, 535 (1961) ; (b) M. Courtois and S. J. Teichner, J. Chim. Phys., 59, 272 (1962). (3) S.Matsushita and T.Nakata, J. Chem. Phys., 36, 665 (1962).
