IR STUDYOF ADSORPTION ON CHROMIA-SILICA CBr3COO-, which are represented by Spinner as CC13-COO and CBr3-COO. If we suppose that on the surface of CrzOa, COS can be bonded onto sites of different electron-donor activity, we might expect surface groups like COO-, or groups COO*without a complete charge transfer. The former species should give rise to the spectrum of a typical acetate ion (1600-1580 and 1420-1400 cm-I) and should be strongly bonded. In the latter structure, the COO modes I and I1 should be more split and the corresponding surface species weakly bonded. Hence we believe that the bands a t 1630 and 1350 cm-l can be assigned to surface species of this structure and the bands a t 1600 and 1415 cm-I to species with a complete charge transfer. In fact these absorptions are hardly eliminated upon evacuation. Finally, we assign the band at -1700 cm-l to the mode I of an even more weakly bonded species carrying a very small charge. This surface compound should
1295 be very similar to the “quasi-neutral COPads”suggested by Hauffe as an intermediate in the oxidation of CO on semiconductor oxides.’ In this case the mode I1 is expected to lie off our spectral region. These structural assignments lead us to the following observation. If C02 is adsorbed onto the surface by subtracting electrons, it must induce surface modifications similar to those obtained by the adsorption of 0 2 . We observed (part I) that CO chemisorbed on a partially oxidized sample gives rise to bands at a frequency greater than or equal to 2200 cm-’ instead of at 2187 om-’. A similar behavior can be observed (Figure 6) when CO is adsorbed onto a sample with preadsorbed C02 or when COZ interacts with the surface which preadsorbed CO.
Acknowledgment. This research was supported by the Italian Consiglio Nazionale delle Ricerche. (7) K. Hauffe, Adean. Catal., VII, 213 (1955).
Infrared Study of Carbon Monoxide, Oxygen, and Carbon Dioxide Adsorption on Chromia-Silica, 111. Carbon Monoxide-Oxygen Interaction by Adriano Zecchina, Giovanna Ghiotti, Claudio Morterra, and Enzo Borello Istftuto di Chimica-Fisica dell ‘Uniwersitb d i Torino, Torino, Italy (Received August 5 , 1 9 6 8 )
The interaction between carbon monoxide and oxygen at room temperature on a chromia-silica catalyst has been studied by infrared spectroscopy. Only the adsorbed CO which causes the band at 2187 cm-’ actively participates in the oxidation reaction, thus acting as an intermediate of great activity. Oxygenated species, mainly of the carboxylate type, have also been detected in the 1800-1300-~m--~frequency range.
Introduction In this paper the interaction between CO and oxygen adsorbed on a CrzOs surface is investigated, following the steps: (1) interaction of gaseous CO with surface oxygen of noncompletely “reduced” Cr203;l (2) interaction of gaseous CO with oxygen chemisorbed on previously “reduced” Cr203; (3) interaction of gaseous O2with CO reversibly adsorbed on previously “reduced” Cr203; (4) interaction of CO-02 mixtures on a “reduced” Cr203surface.
Results and Discussion Step 1. The first point deals with the formation on a noncompletely reduced surface of oxygenated species of “carbonate,” %arboxylate,” or “bicarbonate” type. The contact of gaseous CO with samples in phase a of each “reduction cycle’’ a t 200” gives rise to the slow
formation of weak bands in the 1800-1300-~m-~range. They are shown in Figure la. Since these absorptions are very weak, thick samples have been used (av wt 150 mg) . The behavior of these bands is particularly interesting, and we observe (Figure l a ) : (a) no detectable absorption in the first cycle; (b) absorptions with intensity increasing from the second to the fourth cycle (spectra 1-3) ; and (c) absorptions with intensity decreasing from the fifth to the sixth cycle (spectra 4 and 5 ) . The intensity of these bands is extremely weak if more than six cycles are performed. Since the absorptions in this spectral region are certainly assignable t o oxygenated species,2 we conclude (1) The meaning of “reduced” CrzOr is reported in part I, J . Phys. Chem., 7 3 , 1288 (1969). See also part 11, i b f d . , 73, 1292 (1969). (2) L. H.Little, ”Infrared Spectra of Adsorbed Species,” Academic Press, Inc., New York, N.Y., 1966.
Volume 79, Number 6 May 1060
ZECCHINA,GHIOTTI,MORTERRA, AND BORELLO
1296 1750
1650
1550
1450
1350
1750
1650
1550
1450
1350
cm.1
014 083 042 011
Figure 1. Infrared spectra of the catalyst (optical density us. wavelength in microns or frequency in reciprocal centimeters): spectra 1, 2,3, 4,5a, CO adsorbed in phase a of the 2,3,4, 5, 6th reduction oycle at 200’; spectrum lb, CO adsorbed in phase a of all reduction cycles at 400”; spectrum 2b was obtained after heating at NO”.
that reactive oxygen ions are present on the surface in phase a of all “reduction cycles” up to the sixth. The reactive oxygen layer is most likely formed through a diffusion process from the bulk to the surface, occurring during the activation a t 400” which always precedes every phase a. It is possible to try to assign our bands, even if the direct comparison of spectra of bulk compounds with the spectra of twodimensional structures is always doubtful. The main feature of Figure l a is the presence of quite strong bands in the range 1600-1530 cm-’, which are not coupled with other bands of comparable intensity in the range 1440-1350 cm-’. Therefore, we cannot consider as prevalent on the surface any monodentate species (carbonate, bicarbonate, or carboxylate), as they constantly exhibit several bands of comparable intensity within the above regions.2 Bidentate carbonates show a larger separation of the two bands, so that the absorption between 1600 and 1630 om-l might be assigned to the mode of higher frequency, the second mode being shifted towards lower frequencies, off the transparent region.2 It is quite surprising that no detectable bands are produced when CO is exposed, at room temperature, to the completely oxygenated surface in phase a of the first cycle. We think that this difficulty can be overcome if we look at the mechanism by which the “carbonate” species are most likely formed.* cogas
COada
+ 2oad8-
coada
COSbid. Sd2-
Coabid. ad?- can be formed only if c o a d a is present. ‘Our spectra of CO at room temperature on samples just activated clearly showed (part I) that no bands were detectable in the 2210-2130-~m-~ region. Thus the formation of “carbonate” groups is greatly delayed, as a consequence of the very low surface concentration of the “precursors,” which we identify with CO molecules linked to chromium ions (2210-2130 Om-’). We wish to point out that probably bidentate C O P groups are not the only oxidized species, since The Jowrnal of Phyaicai Chemistry
the spectra look very complicated. Other oxygenated species could be present in a lower but not negligible concentration. As Figure l b shows, the first reduction with CO at 400” is so effective in lessening the excess oxygen concentration that during the following activation not enough active oxygen is diffused to the sample surface to give a detectable spectrum of carbonate species (spectrum 1). Only on heating at 200” do weak bands appear and are located at several different frequencies (spectrum 2). As their position is very similar to that of C02 adsorbed ontjo reduced catalysts (part 11),we suppose that different mechanisms are involved in the reductions at higher temperatures. Step 9. Weak bands are produced in the 18001300-cm-’ range when gaseous CO interacts with oxygen adsorbed at room temperature on a previously reduced catalyst. They are located at different frequencies than those discussed in step 1, and their relative intensities have changed (see Figure 2a). We observe two peaks of comparable intensity, which are located at 1600-1610 and 1440-1410 cm-’, respectively. Both the position and intensity of these bands are changeable and depend on the sample, the band a t lower frequency varying within a wider range than the band at higher frequency. We suppose that this fact is mainly due to the poor optical conditions obtainable around 1400 cm-1, especially for the case of thick samples. These bands grow very slowly and are very similar, both in position and shape, to those observed (part 11) for C02 irreversibly adsorbed onto “reduced” samples. We conclude that “carboxylate” ions are the prevalent product and that oxygen is probably adsorbed at room temperature in the form of 0- ions. This conclusion has also been obtained by other a ~ t h o r s . ~ , ~ Besides absorptions in the 1800-1300-~m-~range, bands in the 2210-2130-~m-~region are also formed, meaning that the contact of the “reduced” sample (3) F. 9. Stone, Advan. Calal., 13, 1 (1962). (4) K. Hauffe, t b t d . , 6, 213 (1955).
IR STUDYOF ADSORPTIOSON CHROMIA-SILICA
1297 2200
2200
em-1
em-1
a2
D 0.1
45
Figure 2. Infrared spectra of the catalyst (optical density us. wavelength in microns or frequency in reciprocal centimeters) : spectra 1, 2, 3a, CO adsorbed on a sample oxidized a t room temperature (exposure times 10 min, 1 hr and 14 hr, respectively); spectra 1, 2, 3b, 0 2 gas (4 Torr) interaction with reversibly adsorbed CO (exposure times 10 min, 1hr 30 min, and 4 hr, respectively); spectra 1, 2, 3, 4c, stoichiometric mixture (40 Torr) immediately after admission and after 15 min, 1hr 30 min, and 4 hr, respectively; spectra 1, 2, 3d), nonstoichiometric mixture (40 Torr) immediately after admission and after 15 min and 23 hr, respectively.
with oxygen a t room temperature did not produce a full monolayer of adsorbed oxygen, some cationic sites being still available on the surface. On bringing CO into contact with a surface which has adsorbed oxygen a t 200°, no bands are detected in the 1800-1300-~m-~ range, even if the amount of chemisorbed oxygen is larger than in the previous case,6 so that the formation of oxygenated species should be favored. The explanation comes from the fact that, under such conditions, the bands at 2210-2130 cm‘’ are also missing, meaning that, like the case of the freshly prepared surface, the lack of a sufficient concentration of “precursors” greatly delays the formation of oxidized species. On the other hand, if the temperature is increased to 200°, the bands of “precursors” appear (see part I) and absorptions in the 1800-1300-~m-~ range also form quickly. This fact is probably connected with the increased mobility of the adsorbed oxygen, as some authors have already pointed o ~ t . ~ nA~t even * ~ higher temperatures (-400°), the bulk mobility is also ex~ited.~m~~6 Step 3. The interaction of gaseous 0 2 with reversibly adsorbed CO was studied by bringing oxygen into contact with “reduced” samples which had previously adsorbed CO, originating the bands a t 2210-2130 cm-l. The pressure of CO was then lowered to less than lo-’ Torr. In Figures 3a and b, the behavior of two samples, decomposed in vacuo and in air, respectively, is illustrated. The presence of gaseous 0 2 immediately decreases the intensity of the band a t 2191 cm-l, while that of the band at 2136 cm-’ (Figure 3a) is slightly increased. In both cases the band at 2191 cm-’ is substituted for by a weak absorption a t 2204-2206 cm-l, which is
4;s &
47
45
40
#.u
4,7
Figure 3. Infrared spectra of the catalyst (optical density us. wavelength in microns or frequency in reciprocal centimeters) : spectrum la, CO (0.1 Torr) on vacuum-decomposed “reduced” sample; spectra 2, 3, 4a, after exposure to oxygen (4 Torr) for 10 min, 1 hr, and 4 hr, respectively; spectrum lb, CO (0.1 Torr) on air-decomposed “reduced” sample; spectra 2, 3,4b, after exposure to oxygen (4 Torr) for 10 min, 1 hr, and 4 hr, respectively.
characteristic of CO adsorbed onto a surface reoxidized a t room temperature (part I ) . The observed phenomena indicate that the species absorbing at 22102190 cm-1 are very reactive towards 02, while the species absorbing at 2150-2130 cm-I are unaffected by 02, a t room temperature. Only on increasing the temperature are the bands at 2150-2130 cm-’ lowered and, above 200°, eliminated. The increase in intensity of the bands a t 2150-2130 cm-1 when gaseous 02 interacts with adsorbed CO is strictly analogous to the effect of gaseous COZ on adsorbed CO. We think that the explanation of this fact may be quite similar in the two cases (see part 11). The oxidation of the species absorbing at 2191 cm-I forms bands a t 1600-1610 and 1420-1440 cm-I, which are shown in Figure 2b. They are coincident with those observed on a reduced catalyst upon adsorption of Cot, followed by a long evacuation or reaction with 0 2 (see part 11). We conclude that also in this case, mainly COO- groups are formed on the surface. A band increasing with the time of exposure to 02 also appears at 2349 cm-l, showing the formation of gaseous c02. Step 4. The effect of CO-O2 mixtures, both stoichiometric (2: 1) and nonstoichiometric (7: 1), was studied by exposure of reduced samples to 40 Torr of the gaseous phase. When stoichiometric mixtures are employed, the quick formation of the band at 2349 cm-l (weakly adsorbed Con) and its increase with time clearly reveal that oxidation of CO is taking place. At the same time, the bands a t 1600-1610 and 14401420 cm-l appear (Figure 2c). They are not different in intensity, position, or shape from those observed in steps 2 and 3; thus, the assignment is likely the same. Figure 4 shows the interesting phenomena observed in the 2210-2130-cm-’ region, both for a vacuum-de(5) W. Weller and 9. E. Voltz, (6) E. R. 9.
J. Amer. Chem. Sac., 7 6 , 4695 (1954). Winter, Advun. Cutal., 10, 196 (1958). Volume 75, Number 6 May 1969
1298
ZECCHINA, GHIOTTI,MORTERRA, AND BORELLO 2200
Figure 4. Infrared spectra of the catalyst (optical density 8s. wavelength in microns or frequency in reciprocal centimeters) : spectra 1, 2, 3a, stoichiometric mixture (40 Torr) on vacuumdecomposed “reduced” sample immediately after admission and after 30 min and 14 hr, respectively; spectra 1, 2, 3b, stoichiometric mixture (40 Torr) on air-decomposed “reduced” sample immediately after admission and after 30 min and 14 hr, respectively.
composed sample (Figure 4a) and for an air-decomposed one (Figure 4b). It appears that (a) the band in the 2150-2130-cm-l range does not change with exposure time, showing that these CO species do not participate in the reaction. (b) The band at 2187 cm-’ is missing and the remaining maximum is located a t higher frequency. It follows that this type of adsorbed CO actively participates in the reaction. Figure 4 also shows the band at 2348 cm-1 for one of the two samples. Usually the air-decomposed samples exhibit a better capacity for forming COz, but no explanation can be supplied and quantitative measurements are probably needed. The frequency of the residual band 2210-2187-cm-1 range is located at 2200-2204 cm-l, as in the case of CO adsorbed onto a partially oxidized surface. Thus we conclude that the rate at which adsorbed oxygen is removed by CO does not balance the quantity of oxygen which is adsorbed, and that a state is reached in which the oxygen coverage is quite high. -4quite different behavior is observed when the catalyst is exposed to a nonstoichiometric mixture (CO/Oz = 7: 1). Figures 5a and b show (a) the presence of two bands in the 2187-2210-~m-~range. The first one is located a t 2188 cm-I and is typical of reduced samples (part I). The second one (ijmax > 2200 cm-l) looks like a shoulder on the high-frequency side of the first band. The first band slowly decreases in intensity with exposure time, while the second one is unaffected and thus progressively more easily observed. It follows that when an excess of CO is used, the state of the catalyst is characterized by a considerably lower oxygen coverage. (b) The band at 2150-2130 cm-l does not participate in the reaction, as already observed in the case of stoichiometric mixtures. In Figure 2d the spectral behavior in the range 1800-1300 cm-l for a The Journal of Physical Chemistrv
1
43
cm-7
2t,50
I
4.6
1
y
4,7
I
4,5
2260
2150
I
4,6 a ,
I
4.7
Figure 5. Infrared spectra of the catalyst (optical density vs. wavelength in microns or frequency in reciprocal centimeters) : spectra 1, 2a, nonstoichiometric CO/OZ (7 :I) mixture (40 Torr) on air-decomposed “reduced” sample 15min and 24 hr after admission; spectra 1, 2, 3b, nonatoichiometric mixture (40 Torr) on a vacuum-decomposed “reduced” sample immediately after admission and after 15 min and 23 hr, respectively.
mixture 7 : l is illustrated. The spectrum is intermediate between that of COZ adsorbed onto reduced Crz03before desorption or action of gaseous 02 (part 11, Figure 4, solid line), and that obtained after admission of O2a t room temperature (part 11, Figure 4, broken line). This fact confirms the hypothesis that the oxygen coverage is not very high when CO is in large excess with respect with 02. From the above results, we can conclude that (1) the surface state reached through the interaction of gaseous CO with adsorbed oxygen, of gaseous 0 2 with reversibly adsorbed CO, or for stoichiometric mixtures on “reduced” samples is very much the same as far as the 1800-1300 -cm-l range is concerned. As previously discussed, in such cases carboxylate groups COO- are mainly formed on the surface. When the contacting mixture contains a great excess of CO, a different surface situation is reached, likely characterized by the presence of surface groups like COO*- or COO* (quasineutral groups, see part 11) for which the charge transfer is not complete. (2) The bands observed in the 2210-2187-cm-’ range are located at frequencies which are strongly affected by the oxygen coverage. In every case these bands are due to “precursors,” which seem to be of fundamental importance in the process of oxidation of CO. (3) The formation of oxygenated species absorbing in the 1800-1300-~m-~ range is only possible when the precursors are present on the surface. (4) The CO species which absorb in the 2150-2130-cm-’ range are not active as intermediates in the oxidation process at room temperature. ( 5 ) The oxygen diffused from the bulk to the surface during the activation at 400” forms a surface complex with CO which is different from that observed
GEOMETRY , REACTIVITY, AND SPECTRUM OF CYCLOPROPANE in the case of adsorbed oxygen. Thus we conclude that different types of oxygen are involved even if, as in the first case, we cannot postulate any reasonable model. Our conclusions introduce into the chemistry of catalytic oxidation of GO on semic0,nductoroxides (we recall the analysis contained in the review of Stone3) the concept of “precursors.11 They are most likely GO molecules reversibly linked to surface metal ions, and play a determinant role in the formation of oxygenated complexes. The process GO,,, -+ CO,d, which is considered, in the usual mechanism] as the preliminary
1299
step is in our opinion suitably characterized by means of ir spectroscopy. The generic definition of CO,d, can thus be identified with the GO reversibly linked to metal ions through a weak u bond. Finally, we think that the oxygenated complexes might be identified with carboxylate structures, even if the complexity of our spectra do not exclude the possibility of the presence of other species on the surface. Acknowledgment. This research was supported by the Italian Consiglio Nazionale delle Ricerche.
Theoretical Study of the Geometry, Reactivity, and Spectrum of Cyclopropane by Robert J. Buenker Department of Chemistry, University of Nebraska, Lincoln, Nebraska
68508
and Sigrid D. Peyerimhoff Institut f u r Theoretische Physik, Justus Liebig-Universitut, 63 Giessen, Lahn, West Germany
Ab initio SCF-MO and CI calculations have been carried out for the cyclopropane molecule CsHs for a series of CCC internuclear angles in order to study the geometry of this molecule in its ground and excited states; from this treatment an equilibrium CCC angle of 65’ is indicated. A basic similarity between cyclopropane and the series of symmetric AB2 molecules (and others) is pointed out; all possess CzVsymmetry throughout their respective bending processes and their angular correlation diagrams show a general agreement between shapes of corresponding orbital energy curves. The great disparity in the geometries of cyclopropane and its isoelectronic AB2 and HABz counterparts ozone and formate ion is thereupon related to the fact that these systems possess different ground-state electronic configurations; in turn it is shown that this distinction is caused by the ability of hydrogen AO’s to selectively alter the stability of the orbitals of a parent AB2 molecule, Development of this concept also allows a consistent explanation for the unusual reactivity of cyclopropane. C I calculations are employed to study the electronic spectrum of C& and they indicate that the majority of the known ultraviolet absorptions of this system cannot reasonably be assigned to vertical transitions; instead 0-0 excitations involving upper states with wide-angle equilibrium geometries are suggested.
I. Introduction The cyclopropane molecule C3H6 is well known to exhibit anomalous behavior among organic systems with regard to binding and reactivity. According to the simplest valence bond description the molecule possesses pure CC single bonds (experimental bond 1ength’J 1.51 A) and yet its CCC angles are all approximately 60°, much smaller than the tetrahedral angle characteristic of other saturated organic molecules. Furthermore, cyclopropane undergoes certain addition reactions such as hydrogenation in the presence of a catalyst and bromination in carbon tetrachloride, also in definite contrast to the behavior generally observed for singly bonded hydrocarbons. Several explanations have previously been offered
for the unusual properties and reactivity of cyclopropanels perhaps the most celebrated of which are due to Walsh4and Coulson and Moffitt.6 It is the purpose of the present paper to discuss these matters in considerably more quantitative detail, by means of ab initio LCAO-MO SCF calculations, than has been possible previously ; a simple qualitative theory is sought to explain the anomalous characteristics of cyclopropane in a manner which is still clearly coil(1) 0. Bastiansen, B. N. Fritsch, and K. Hedberg, Acta CTyst., 17, 538 (1963). (2) L. C. Snyder and 8. Meiboom, J. Chem. Phys., 47, 1480 (1967). (3) W. A. Benett, J . Chem. Educ., 44, 17 (1967), and references therein. (4) A. D.Walsh, Trans. Faraday SOC.,4 5 , 179 (1949). (5) 0.A. Coulson and W. E. MofBtt, Phil. Mag., 40, 1 (1949). Volume 73, Number 6 May 186#
