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BORELLO, ZECCHINA, MORTERRA, AND GHIOTTI
and 17 would seem to be equally justified by the experimental data. The question of the extent of the applicability and significance of the equations presented here will require further experimental and theoretical investigation. An important test would be to see if the equations apply to systems with more than two components. It would be interesting to determine whether these equations might, with suitable care, be applied to solutions which contain electrolytes. It would be interesting to investigate whether low values of ti for a component would consistently be cor-
related with the formation of associated species in solution and if so whether the values of t i might be used to measure the extent of complex formation. Acknowledgments. The author wishes to express his gratitude to Professor Louis J. Gosting for his support and encouragement. The author is indebted to Professor R. H. Stokes for his helpful suggestions for the development of this work. Special thanks go to Mrs. Sharon R. Albright for performing the numerical calculations and for her help in the preparation of the manuscript .
Infrared Study of Carbon Monoxide, Oxygen, and Carbon Dioxide Adsorption on Chromia-Silica.
I. Carbon Monoxide
by Enzo Borello, Adriano Zecchina, Claudio Morterra, and Giovanna Ghiotti Istituto d i Chimica-Fisica dell’ Universitd d i Torino, Tortno, Italy
(Received August 5 , 1 9 6 8 )
An investigation has been made of adsorption of carbon monoxide on a chromia-silica surface by infrared spectroscopy. Particular attention was paid to the bands in the 2210-2130-~m-~spectral region, which can be assigned to species adsorbed onto chromium ions. Two main types of sites are present which act ag electron acceptors via either a u bond or a u-T bond. Other finer effects, such as those arising from the influence of residual free holes on the electron affinity of the localized sites and the change of chromium coordination number, are revealed by shifts of the frequencies. Thus CO appears to be a good molecule for examining the surface of our catalyst.
Introduction Although chromium oxide is widely used as a catalyst in the chemical industry, very little is known about its surface properties towards CO. For this reason, a discussion of the first results obtained in this laboratory studying the system CO-CrzO8 by infrared spectroscopy seems to us to be of some interest. This paper will be followed hy others on the adsorption of COZ and CO-02 mixtures, on the interaction of COZ with preadsorbed CO, CO with preadsorbed COZ,COZwith preadsorbed 0 2 , O2 with preadsorbed CO, and 02 with preadsorbed COz, following a scheme similar to that devised by Courtois and Teichnerl for NiO. At present, infrared spectroscopy is a powerful method in studying molecules adsorbed on oxides. With regard to CO we recall the works of O’Neill and , ~ ThOz, Yatesz and Peri8 on NiO, Pichat, et ~ l . on Taylor and Ambergs on ZnO, and Gardner, et uZ.,~-* on several metals and oxides. The adsorption of CO and COz on chromium oxide has been studied by volumetric, calorimetric, and and gravimetric techniques by Dowden, et The Journal of Physical ChemiStTy
Van Reijen, et ul.,ll and by infrared spectroscopy by Little and Amberg.12
Experimental Section Our catalyst was obtained by impregnating Aerosil with a chromium nitrate solution, so as to get dried samples containing 6.5% chromium. (1) M . Courtois and 9. J. Teichner, J. Catal., 1 , 121 (1962). (2) C. E. O’Neill and D . J. 0. Yates, Spectrochim. A c t a , 17, 953 (1961). (3) J. B. Peri, Discussions Faraday Soc., 41, 121 (1966). (4) P. Pichat, J. VBron, B . Claudel, and M . V. Mathieu, J. C h l m . P h y s . , 63, 1026 (1966). (5) J. H. Taylor and 0. H . Amberg, Can. J . Chent., 39, 635 (1961). (6) R. A. Gardner and R. H. Petrucci, J . Amer. Chem. Soc., 82, 5051 (1960). (7) R. A. Gardner and R . H . Petrucci, J. P h y s . Chem., 67, 1376 (1963). (8) R . A. Gardner, J. Catal., 3, 22 (1964). (9) D. A. Dowden and W. E . Garner, J. Chem. Soc., 893 (1939). (10) R. A. Beebe and D . A. Dowden, J. Amer. Chem. Soc., 60, 2192 (1938). (11) L. L. Van Reijen, W. M . H. Sachtler, P. Cossee, and D . M. Brouwer. Proc. Intern. Congr. Catalysts, 3c, Paris, 1 9 6 4 , 829, 1965. (12) L. H.Little and C. H. Amberg, Can. J. Chem., 40, 1997 (1962).
1287
IR STUDYOF ADSORPTIONON CHROMIA-SILICA Scheme I
-------
Phase a
Starting sample (decomposed at 130" and activated a t 400" for 5 hr) black color
Admittance of 40 Torr of CO at room temperature
Ir spectrum (background) Ir spectrum
First cycle-
Phase b
Phase c
--
---Second
cycle----Phase a
Phase b
Reduction in CO at 200" Outgassing at 400" for Admittance of 40 Torr of CO a t room tem(first series of experi1 hr ( p = Torr) perature ments) or a t 400" (second series of experiments)
Ir spectrum (after cooling Ir spectrum (after cool- Ir spectrum ing down to room down to room temtemperature) perature)
The powdered catalyst was compressed under a pressure of 100 kg/cm2 into thin pellets, usually weighing about 75 mg, 0.20 mm thick and 25 mm in diameter, and decomposed at 130" in the infrared cell at a pressure of Torr. Then the samples were Torr). activated for 5 hr at 400" in vacuo ( p 5 In another series of experiments, the decomposition was obtained in air at 130". The results in both cases are similar from the qualitative point of view; in fact the catalyst obtained by decomposition in air shows merely a weaker intensity in some bands due to adsorbed CO. In the present paper the results obtained with the catalyst decomposed in vacuo are mainly discussed. The infrared analysis cell, described elsewherells allowed the decomposition and other thermal treatments to be performed in situ. The small path length of the cell (1 mm) enabled us to neglect the spectrum due to the gaseous phase. Spectra were run on a Beckman IR7 spectrophotometer with a slit width of about 2 cm-l. Owing to the presence of the support, the spectral region was limited to 1300 cm-l. The frequencies in the spectral region around 2200 cm-l are very accurate because the rotational contour of CO, contained in a separate cell, was simultaneously recorded. This was necessary because small shifts of the frequencies are observed in this region following the treatments of the samples (frequencies in this spectral region are given with an accuracy of f 1 cm-l) . Since the catalyst obtained in this way is very nonstoichiometric, as shown by its color and by titration methods, the oxygen excess was lessened by reduction with CO. The reduction process is very similar to that followed by D o ~ d e n this ; ~ treatment transforms the highly nonstoichiometric CrzOa into a normal p-type semiconductor with a small oxygen excess. The transformation of chromium oxide into an n-type semiconductor only occurs upon reduction with hydrogen or hydrocarb0ns.1~ Since the treatments of the samples are rather complicated, we shall illustrate them carefully. Two alternative reduction processes have been employed, using reactions with CO either at 200 or at 400". The sequence of the thermal treatments and of spectroscopic controls is illustrated in Scheme I.
Etc.
Etc.
This set of thermal treatments (which will be referred to as a "reduction cycle") was repeated five times when heating at 200" (phase b) and four times when heating at 400". Both series of cycles led to the formation of pellets of permanent green color, which we name "reduced" samples. The catalyst so obtained was shown by electron microscopy and X-rays o! contain very small CrzOa particles (diameter < 50 A ) . A typical spectrum of a starting sample is illustrated in Figure 1 (broken line). This base line is recorded after each phase c and is used as a background for the spectra in other phases. The catalyst which was only decomposed and activated does not exhibit any activity towards CO at room temperature. Even after 20 hr the spectra obtained in phase a of the first cycle are not different from the background of the starting sample. The spectra obtained in phase a of the following cycles reveal bands of adsorbed CO in the 2130-2210-cm-l range (see Figure 1). Since these bands are not different in frequency from those observed in phase b of each previous cycle, we only report the spectra of each
I
m'
1750 b50 1550 1450
2100
2300
,
I
49
4.5
I
47
I
5,5
I
6u ,
I
e5
1
7
1350
1
7,s
Figure 1. Infrared spectra of the catalyst (% transmission vs. wavelength in microns or frequency in reciprocal centimeters) : broken line, after outgassing for 5 hr a t 400" ("background"); smooth line, after phase b of the first "reduction cycle;" dotted line, after phase b of the last L'reductioncycle." (13) E. Borello, A. ZeccNna, and M . Castelli, Ann. Chirn., (Rome) 53, 690 (1963).
(14) P. B. Weisz, 0.D. Prater, and R . D. Rittenhouse, J. Chem. Phys. 21, 2236 (1953). Volume 79, Number 6 May 1969
1288
BORELLO, ZECCHINA,MORTERRA, AND GHIOTTI 2300
m-1
2300
2200
I
2200
em-1
1 93
92
42 Figure 2. Infrared spectra of the catalyst after each phase b of the “reduction cycles” a t 200”; weight of sample -75 mg (optical density us. wavelength in microns or frequency in reciprocal centimeters) : smooth line, first cycle; - 00-, second cycle; third cycle; -.-e-, fourth cycle; -A-n-, fifth cycle.
-iJ-o-,
phase b (Figures 2-5). The spectra of phases a and b only differ in the region 1800-1300 cm-l. In phase b the bands in this range decrease in intensity as the reduction proceeds. This is shown in Figure 1, which reports the spectrum after the first cycle and after the fourth cycle a t 400°.16 In phase a, bands only appear after several hours of contact with CO and always exhibit a very weak intensity. They vi11 be discussed in part 111 of this work. The 1800-1300-~m-~region
,,
,
I
2300
,
2200
, cm-1
,
4b r
Figure 4. Infrared spectra of the catalyst after each phase b of the “reduction cycles” a t 400”; weight of sample -75 mg (optical density us. wavelength in microns or frequency in reciprocal centimeters): 1, first cycle; 2, 3, 4, the other cycles.
of the b phases will not be discussed here, because we are mainly interested in the solid-gas interaction taking place at room temperature, Our present interest is in the CO species originating the absorptions in the 2210-2130-~m--~range; we shall not discuss in this work the surface species absorbing in the 1800-1300-~m-~ region. They most likely have carbonate, bicarbonate, or carboxylate structures and clearly originate from the interaction of CO with the oxygen of nonstoichiometric surfaces, for they are very weak after four or five reduction cycles (Figure 1). The spectra obtained for the b phases of the two series of experiments (reduction with CO at 200 and
4#7 /u
Figure 3. Infrared spectra of the catalyst after each phase b of the “reduction cycles” a t 200”; weight of sample -150 mg (optical density us. wavelength in microns or frequency in reciprocal centimeters): smooth line, first cycle; -o-O-, second cycle; third cycle; -.-e-, fourth cycle; fifth cycle.
-m-n-,
The Journal of Physical Chemistry
-A-n-,
Figure 5. Infrared spectra of the catalyst after each phase b of the “reduction cycles” at 400”; weight of sample -150 mg (optical density vs. wavelength in microns or frequency in reciprocal centimeters); smooth line, first cycle; -O-O-, second cycle; -D-O-, third cycle; -.-e-, fourth cycle. (15) In this and other flgures a band appears at 2349 cm-1 which is assigned to physically absorbed 0 0 %and will be discussed in following papers.
1289
IR STUDYOF ADSORPTIONON CHROMIA-SILICA 2300
2200
cm-1
Figure 6. Desorption experiments: the most intense spectrum is that obtained after the first reduction "cycle" at 200" (also reported in Figure 3, solid line). The others in order of decreasing intensity refer to the following desorption times: 10 sec, 40 sec, 4 min, 8 min, 25 min (optical density vs. wavelength in microns or frequency in reciprocel centimeters).
400") are illustrated in Figures 2 and 3 and in Figures 4 and 5, respectively. Figures 3 and 5 refer to heavier pellets ( m 150 mg), particularly suitable for desorption studies. The results of the desorptions in the two series of experiments are illustrated in Figures 6 and 7, the starting spectra being represented in Figures 3 and 5 by a solid line. Moreover, some experiments were done by admitting CO onto a surface which had previously chemisorbed oxygen in two different ways. In the first case, 0 2 (40 Torr) was in contact for 2 hr at 200" with a catalyst which had undergone a complete series of "reduction cycles," either at 200 or 400". After cooling down to
Figure 8. Infrared spectra of CO adsorbed on a pellet, oxidized a t 200'; the curves, in order of decreasing intensity, refer to different times of exposure to CO at 200": respectively, 135 min, 75 min,
45 min, 30 min, 20 min, 10 rnin (optical density vs. wavelength in microns or frequency in reciprocal centimeters).
room temperature and evacuating the gaseous phase, it was found that the spectrum was coincident with the base line (Figure 1, broken line). During these treatments, the pellet became black again. To such re-oxidized samples, CO (40 Torr) was admitted at room temperature and left for 20 hr; since no detectable spectrum was observed, the temperature was increased to 200". Figure 8 shows the spectra obtained in the 2400-2000-~m-~range for the same pellet at different times of reaction. In the second case samples which had undergone the complete series of "reduction cycles" either at 200 or 400" were in contact for 2 hr at room temperature with 40 Torr of 0 2 . After pumping down the gaseous phase, CO (40 Torr) was admitted and allowed to react for 14 hr at room temperature. Figure 9 shows the spectrum obtained in the 2400-2000-~m-~range for one of the examined samples. Bands between 1800 and 1300 cm-l were also present, but their representation and discussion will be considered in the third part of this study.
Discussion
Figure 7. Desorption experiments: the most intense spectrum is that obtained after the first reduction "cycle" a t 400" (also reported in Figure 5, solid line). The others, in order of decreasing intensity, refer to the following desorption times: 30 see, 1min, 60 min (optical density us. wavelength in microns or frequency in reciprocal centimeters).
The two groups of bands appearing between 22102187 and 2150-2130 cm-l are treated separately. Their final intensities seem to be competitive, indicating that different local surface environments are involved. Every time the series of cycles produces a rather strong absorption a t 2150-2130 cm-l, the higher absorbing band cannot reach a high intensity (see, for instance, Figure 5) and vice versa (Figure 3). It is interesting to observe that the relative intensities of these two bands a t the end of the first cycle in samples decomposed i n vacuo exhibit an unpredictable character, which is probably acquired during the decomposition. Volume 73, Number 6 M a y 1960
1290
BORELLO, ZECCHINA,MORTERRA, AND GHIOTTI
2201
082 -
i
A
46
4,7
Figure 9. Infrared spectra of CO adsorbed on a pellet oxidized at room temperature; the curves, in order of decreasing intensity, refer to different times of exposure t o CO a t room temperature: respectively, 14 hr, 60 min, 3 min (optical density os. wavelength in microns or frequency in reciprocal centimeters).
The adsorption of nitriles on samples which have previously adsorbed GO lowers the intensity of the band at 2187-2210 cm-l, so that at high coverage it completely disappears. Thus we feel, as Brown and Dahrensburg do, that the species absorbing at 21872210 em-' are linked to chromium ions via a weak u bond, with some release of electrons to the solid. For every incompletely reduced sample (Figures 2, 3, 5 , smooth lines) or for partially oxidized samples (Figures 8 and 9 ) , the absorptions in the 2187-2210em-l region are located at frequencies higher than the final position. As this fact is accompanied by an evident lack of symmetry, we think that under such conditions some new bands are present, as also revealed by desorption experiments (Figures G and 7). For example, a catalyst which has been reduced through a cycle at 200" shows the maximum at 2210 cm-l (Figure G ) , while one reduced through a cycle at 400" (Figure 7) absorbs a t 2206 ern-'. Also, a partially oxidized catalyst (Figures 8 and 9) exhibits a band at 2204 cm-l. We think that these small shifts have a real physical meaning because, in addition to the observed asymmetry of the bands, they always occur in all of our samples when comparable surface conditions are reached. These bands of higher frequency are more resistant to the desorption than the band at 2187 em-'. As the frequency of these bands clearly depends on the reduction degree of the samples, we think that some electronic factors are likely involved. It is very difficult to assign a weight to the various factors which seem t o govern these adsorptions, Le., the effect of local interactions with a single ion, the effect of the local environment which may change with the oxidation, and the effect of the general state of the solid as a whole (amount of residual nonstoichiometry) . We think that the various effects act simultaneously. If the concentration of positive holes (imagined as the main factor) is continuously increased (as passing backward from the last to the first "reduction cycle") we can expect that the electron acceptivity of the whole solid will continuously increase as well, and only one band will be observed in the spectrum, continuously shifted towards higher wave numbers. However, on passing from the last to the first cycle, new bands appear, indicating a discontinuous variation of the electron acceptivity. Local interactions must then be considered very important. As several bands are observed a t higher frequencies, it is not very difficult to explain the chemical nature of some of the absorbing sites. CO usually releases electrons on adsorption,
Since this property is retained throughout the course of treatments, we feel that it has a pronounced morphological character. This hypothesis is supported by the observation that in samples decomposed in air, the bands at 2150-2130 cm-l always exhibit a negligible intensity. Probably the low amount of residual water, always present during decomposition, has great importance in favoring the appearance of different structure environments on the chromia surface.ll The 2210-$187-Cm-1 Region. When the catalysts are completely reduced, only a band at 2187 cm-l is detectable, which is reversible at room temperature; it disappears by pumping down for several hours. Bands in the same frequency range have been observed for CO on Zn0,6 XO,3 oxidized Pt,la and Cr203A120a.12 The nature of the adsorbing sites and the type of involved linkage is at present a matter of discussion, and the recent work of Brown and Dahrensburg1' represents a valid attempt to clarify the situation. We think that in our case the adsorbing sites are chromium ions, because they also adsorb nitriles, acting as centers with electron-accepting character. The spectroscopic behavior of nitriles adsorbed on Crz03will be fully discussed in a later paper; neverthe(16) H. Heyne and F. C . Tompkins, Trans. Faraday SOC.,6 3 , 1274 less, some important features are anticipated for a (1967). better understanding of the following discussion. The (17) T. L. Brown and D. J. Dahrensburg, J. Inorg. Chem., 6 , 971 stretching bands of the adsorbed C=N groups occur (1967). (18) B. L. Ross, J. G. Grasselli, W.M~ Ritchey, and H . D. Kaess, at higher frequencies than in the free molecules, with i b i d . , 2 , 1023 (1963). enhanced intensity. A similar behavior is observed in (19) V. K. Filimonov and D. S . Bystrov, O p t . S p e c t r y . ( U S S R ) , adducts of nitriles with metal ions in ~ o l u t i o n . ~ * 12, ~ ~31 ~ (1962). The Journal of Physical Chemistry
1291
IR STUDYOF ADSORPTIONON CHROMIA-SILICA acting as a positive hole trap. On chromia, this fact could be identified with the interaction of a CO molecule with a Cr*+ion, involving a bonding energy larger than that expected for the interaction with Cra+ ions.'' As other ions of higher valence may be present,12 a wider variety of bands is expected. However, this piclure, suggesting an oversimplified correlation between observed localized interactions and electronic properties of the solid, is not completely convincing. In fact, if localized interactions with trapped holes are emphasized by the presence of different bands (and thus of different sites), other finer effects are completely lost. Among them, the effect of residual free holes on the electron affinity of the localized sites and the change of the chromium coordination number owing to the oxygen chemisorption seem to be particularly important. It is significant that when reduced Crz03is in equilibrium with CO, the only band at high frequency is located at 2187 cm-l; but on desorbing for a short time at room temperature, a shift of 4 cm-l towards higher frequencies is observed. This shift is always observed, and we feel that it requires some explanation. It is possible that at high coverages some lateral interactions among adsorbed molecules are present, producing a perturbation of the stretching frequencies. Another explanation, following the electronic theory of semiconductors, is that since CO releases electrons to the solid via surface chromium ions, the electron affinity should be expected to decrease with increasing coverage, leading to a lowering of the strength of t,he u bond, and hence of the CO stretching frequency.20 Further experiments are needed to resolve this question. The 2160-21S0-Cm-1 Region. This range is very near to the frequency of gaseous CO, and thus the CO species responsible for these bands might be expected to be very easily desorbed. On the contrary, they are the most resistant in the range 2400-2000 cm-1 (Figures 6 and 7). Another interesting feature of these bands, shown in Figures 2-5, is that the frequency of the maximum at the end of a complete series of cycles is different, depending on whether the reduction has been carried out at 200 or at 400". This fact differentiates these bands from those at 2187-2210 cm-1, for which the same final position is reached in every case. We observe that when reduction has been made at 200", the maximum is located at 2145 cm-l and is not appreciably changed during the five "reduction cycles ;" this band is usually broad and may have a complex structure. When reduction is carried out at 400°, after the first "reduction cycle" a broad band at 2147 cm-l is observed, but in the following cycles the maximum is constantly located at 2136 crn-l. As its shape is asymmetric, this band could also have a complex structure. It is a difficult problem to combine these results in a simple picture, because many uncertainties are still present; the primary one is that all the bands
show a nonsymmetrical shape, and hence are probably not simple in structure. Despite these difficulties, we think that some hypothesis for the whole group of bands can be postulated. Following Brown and Dahrensburg, this range of frequencies is typical for CO molecules linked through a bond in which a contribution of a interaction may be present. Due to this, the strength of the bond with the surface ions could be higher in this case than for CO bonded with a pure, weak (T contribution. This fact agrees with the higher stability of these species, as a mild heating in vucuo is required for a complete desorption. Since for these bands two contributions ( u character and a backdonation) act simultaneously, probably in a synergistic way, we expect electronic factors and coordinative situations which modify the crystal field to have a heavy influence on them. This viewpoint is supported by the observation that different final frequencies of the maximum are obtained if different reduction conditions are employed. In any case, the two situations are not independent, but probably only different from a quantitative point of view. In fact on a catalyst reduced at 200" the maximum is at 2145 cm-l, Le., a frequency near to that observed after just one reduction cycle at 400". Evidently a cycle at 400" is roughly equivalent, for the absorptions in this frequency range, to five reduction cycles at 200". In the following cycles at 400" (and in all the cycles at 200") the frequency of the maximum is no longer changed, but visible changes of the shape and symmetry are produced, showing that continuous modifications of the electronic make-up are likely taking place.
Conclusions The adsorption of CO on Crz03exhibits bands in the 2210-2130-cm-1 region, which can be assigned to species adsorbed onto chromium ions. Two types of surface sites were found: (a) sites which act as electron acceptors via a (i bond, and (b) sites which act as electron acceptors via a u-a bond. Thus CO is a good test molecule to reveal surface sites of different nature. But CO also reveals different electronic situations, as shown by the shifts of band frequency. At least from a qualitative point of view, these variations can be correlated with the adsorption of oxygen, which induces the formation of positive holes in the boundary layer and changes the coordination number of surface chromium ions, thus modifying the local crystal field. These facts suggest that a more accurate analysis, in which integrated intensities and different outgassing and reduction conditions are considered,12 should also be made. Acknowledgment. This research was supported by the Italian Consiglio Nazionale delle Ricerche. (20) G. Blyholder, J. P h y s . Chehem., 68, 2772 (1964).
Volume 79, Number 6 M a y 1969
