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67 Infrared Study of the Catalyzed Oxidation of CO
67 Infrared Study of the Catalyzed Oxidation of CO R,. P. EISCHENS
AND
W. A. PLISKIN
The Texas Company, Beacon, New York Infrared techniques, which make i t possible t o obtain spectra of adsorbed molecules while reactions are in progress have been used t o study the oxidation of CO over a nickel-nickel oxide catalyst system. During the reaction a band is observed a t 4.56 p which behaves as though i t is related t o an intermediate in the oxidation reaction. This band cannot be accounted for on the basis of simple adsorption of any of the components of the reaction system. The band position and the method by which it is obtained suggest that i t is due t o the structure Ni - - ()=C=O.
I. INTRODUCTION
A major advantage of the infrared method of studying molecules adsorbed on surfaces is that these molecules can be observed while reactions are in progress. Knowledge of the spectra produced by chemisorption of reactants and products is essential in interpreting spectra obtained during the course of a catalyzed reaction. For this reason, an infrared investigation of CO and COz on nickel and on nickel oxide was carried out in conjunction with the study of the oxidation of CO. Moreover, it was necessary to determine the spectrum of physically adsorbed COz because the high specific absorptivity of this molecule makes it possible to detect bands due to physically adsorbed COzunder conditions where physical adsorption would not ordinarily be expected to occur.
11. EXPERIMENTAL PROCEDURE The method of preparing samples suitable for observation of the spectra of molecules adsorbed on metals has been described previously (1). In this case samples containing 9.2 wt. % nickel supported on Cab-o-sil were prepared by impregnating Cab-o-sil with nickel nitrate and then reducing with hydrogen. The nickel oxide samples were prepared by exposing the Cab-o-sil-supported nickel to oxygen. An excess of oxygen was introduced at 25" and the temperature raised to 300". After one half-hour at 300", the excess oxygen was pumped out and the sample cooled to 25" for subsequent adsorption measurements. 662
67.
CATALYZED OXIDATION OF
CO
663
Unless otherwise specified, the work reported here was carried out in a n
in situ cell which had been developed for the infrared study of chemisorbed gases (1) .
111. INFRaRED
ADSORBED MOLECULES
STUDY OF
1. CO on Nickel
The spectrum of CO chemisorbed on reduced nickel has been discussed in another paper ( 1 ) . The essential features of this spectrum are bands in the 4.84.9-p region, which are attributed to CO adsorbed with a linear structure, Ni-CEO, and bands in the 5.1-5.4-p region which are attributed to the bridged structure 0
II
C
Ni
/ \
Ni
2. C02 on Nickel Spectrum a of Fig. 1 is due t o C02 chemisorbed on Cab-o-sil-supported nickel a t 25" and 1.2-mm pressure. The nickel sample had been reduced with Hz at 300" for 16 hrs. prior to admission of the CO, . This spectrum shows a strong band a t 6.4 and a weaker band a t 7.1 p. Bands in these positions a.re characteristic of the carboxylate ion (2). This indicates that in the present system they are due t o the structure 0
-0
'\ / C
I
Ni
The weak bands a t 6.1 and 7.2 p are attributed t o a [CO& ion which is formed by react>ionof the C02 with a small amount of residual oxygen on the nickel surface. The latter ion will be discussed in the next section. If the nickel is exposed to C02 a t 100" instead of a t 25", bands attributable t o CO chemisorbed on nickel are observed. These are probably due t o reduction of the COZ together with diffusion of the extra oxygen into the interior of the metal particles. 3. COZ o n Nickel Oxide
Spectrum b of Fig. 1 is due to COZ chemisorbed on nickel oxide a t 25" and 1.6-mm pressure. The bands a t 6.1 and 7.2 p are similar t o those of a bicarbonate ion ( 3 ) .This indicates that the adsorbed species is
664
R. P. EISCHENS AND W. A. PLISKIN
0
-0
'\ / C
I
0
I
Ni
This structure might be described as a carbonate ion bonded to the surface through one of the oxygens. It will be referred to as a bicarbonate ion, however, because the oxygens are not all equivalent and the spectrum more closely resembles that of the bicarbonate ion in this region. Spectra of carbonate ions are characterized by a single strong band in the 6.9-7.0-p region (3). Observation of the spectrum attributable to the bicarbonate ion obviously has a direct bearing on the [CO,] complex theory which has been postulated to explain results of adsorption experiments with CO and COz on nickel oxide (4).At present it does not appear that the infrared results can be used to support the [Coal complex theory. Although a [C03]- ion is observed when COZ is adsorbed on nickel oxide, this fact alone is not confirmation of the theory, because formation of such an ion would be expected on the basis of conventional chemical principles. The significant point involved in the [CO,] complex theory is that this complex is stable and can be formed from all suitable combinations of CO, COZ, and oxygen. Attempts to obtain a [C03]- ion by methods suggested by this theory have not been successful. For example, it has not been possible to form a [C03]ion by treating the carboxylate ion with oxygen. Moreover, the carboxylate
67.
CATALYZED OXIDATION OF
CO
665
ion is more stable than the bicarbonate ion. The latter can be almost entirely removed from the surface by pumping at 25" for one half-hour, while the carboxylate ion is stable up to 150".
4. CO on Nickel Oxide Attempts to observe the spectrum of CO adsorbed on nickel oxide have not been successful. This result was unexpected on the basis of the adsorption of CO on nickel oxide (4, 5 ) . It is difficult to predict the minimum amount of adsorbed material which would be detectable when the specific absorptivity of that species is not known. On the basis of experience with CO and C 0 2 , it had been assumed that a surface coverage of 10% of a monolayer of CO on nickel oxide would be sufficient to produce a detectable band. Since the purpose of these preliminary experiments w & ~ to get information needed to interpret bands which are observed during the oxidation of CO, failure to observe a band in this case was not a serious obstacle in the interpretation of the oxidation experiments. 5 . Physically Adsorbed COZ
The area of the Cab-o-sil support is about ten times as large as that of the nickel in the sample. Previous references to monolayers apply only to the nickel area. When physical adsorption is studied, the area of the Cab-o-sil must also be considered. The extreme sensitivity of COZ makes it possible to detect bands due to 0.01 % of a monolyaer on the total surface. Hence, it is necessary to consider the possibility of physically adsorbed COz under conditions where physical adsorption would not ordinarily be expected to be an important factor. The infrared spectrum of gaseous COz has a vibration-rotation doublet at 4.23 and 4.28 1.1. Physically adsorbed C02 was expected to produce a single band between 4.23 and 4.28 1.1 because the COz molecules would not be rotating freely. Physically adsorbed COZ was studied to check this prediction and to insure that a band at 4.56, which is observed during the oxidation of CO, was not due to some unforseen factor which would shift the band position of the asymmetric carbon-oxygen stretching frequency in the physically adsorbed state. The cell used in this physical adsorption was a simple glass cylinder with CaFz windows sealed on the ends and with a side tube for admission of gases. The path length in the cell was reduced to 4 mm. by inserting salt plates, and about half of this space was filled with Cab-o-sil. In Fig. 2, spectrum a is that of gaseous COz in the blank cell. Spectrum b is due to a combination of gaseous COz plus COz physically adsorbed on Cab-o-sil at room temperature and 200-mm pressure. Spectrum c, which has a strong band at 4.26, is attributed to the physically adsorbed C O z .
666
R. P. EISCHENS AND W. A. PLISKIN
i
100-
z 0
90
-
80
-
VI
I?
170-
z fn
gI-
c60-
z 0 w
!SO-
40
(c)
-
-
4.2
4.3
FIG.2. Spectra of (a) gaseous Cog, (b) gaseous plus physically adsorbed COZ, and physically adsorbed COZ.
Spectrum c was obtained from spectrum b by subtracting the adsorption equivalent to the amount of gaseous COz indicated by the 4.23-p band in b. Thus, the small band a t 4.22 in c is an artifact caused by a slight displacement of the bands and has no significance regarding the question of whether the physically adsorbed COZ is rotating freely. The intensity of the band due to physically adsorbed COz indicates that the surface coverage is 1%. IV. CATALYZED OXIDATION OF CO I . Spectra during Course of Oxidation
A sequence of spectra obtained during the oxidation of CO at 25" is shown in Fig. 3. In these experiments O2was admitted to the system, which contained CO chemisorbed on nickel plus a gaseous CO atmosphere at 2-mm pressure. Spectrum a is due to the chemisorbed CO. After 2 mm. of 0 2 was admitted (total pressure 4 mm.), the most significant change was the appearance of a band a t 4.56 p. This band is evident in spectra b and c. Spectra a , c, and d were taken from a single run, and b was taken from a similar run. The initial reaction rate is fast compared with the rate a t which the spectrum is scanned, so that it is difficult to get a single spectrum which shows the
67. CATALYZED OXIDATION OF CO
A 4.5
I
'
l
t
t
5 .O WAVELENGTH I N MICRONS
t
667
t 5.5 "
FIG.3. Infrared study of the oxidation of CO: (a) chemisorbed CO, (b) and ( c ) intermediate stages, (d) termination of reaction.
band a t 4.56 p a t its maximum intensity and which also shows moderately strong bands due to chemisorbed CO. It requires about 30 sec. to scan from 4.56 to 4.83 p. Spectrum d was obtained one hour after c. During this period, the 4.56-p band gradually diminished until it was no longer evident. Strong bands, which are found a t 6.4 and 7.1 p in the higher wavelength regions of b, c, and d , show that carboxylate ions are formed on the surface. These ions are probably due t o the adsorption of gaseous COz , and it is likely that it is this adsorption which causes the reaction t o slow down or stop. The band a t 4.27 p in c and d is attributed to gaseous or physically adsorbed COz , or both. Integrated intensity measurements indicate that 0.5 % of a physically adsorbed monolayer would be sufficient t o produce this band. The band a t 4.56 p is of interest because it appears t o he related to the intermediate in the oxidation. This band has also been observed when nickel oxide was reduced with CO a t 200" and a t 25" over nickel when oxygen is admitted prior to CO and vhen O2 and CO are admitted simultaneously. When 0 2 is admitted prior to CO, the adsorbed product has the bicarbonate structure. Thus far, all cases except one, which will be discussed later, in
668
R. P. EISCHENS AND W. A. PLISKIN
which a band has been observed at 4.56 p are consistent with the view that it is due to the oxidation intermediate. It has not been observed when nickel was treated only with either CO or 0 2 . The specific absorptivity of the species producing the 4.56-p band is not known. If it is assumed that it is of the same order as that of chemisorbed CO, then the maximum concentration is about 1 % of a monolayer on the nickel surface, 9. Suggested Structure of the Oxidation Intermediate
A band due to carbon-oxygen vibrations which occurs in the 4.56-p region could be due to a carbon between a triple and a single bond or a carbon between two double bonds. Since the 4.56-p band is produced by adding oxygen to CO, it is likely that the species producing the band has a t least two oxygens. On the basis of the band position and the method by which ,it is obtained, it appears that the structure of the observed intermediate is Ni- - - O-C=O. Broken lines are used to represent some of the bonds because the exact bond order is not known. 3. Decomposition of Nickel Nitrate
A broad band with an absorption maximum a t 4.54-4.56 p is observed during the decomposition of nickel nitrate when the sample is being prepared. In order to be sure that the 4.56-p band observed during the oxidation of CO was due to a species containing carbon, C13O was used in an oxidation reaction. This shifted the 4.56-p band to 4.70 p and is proof that carbon is present in the species producing this band. It now appears that the broad band near 4.56 p which is observed during the nitrate decomposition is due to a structure similar to that postulated for the oxidation intermediate, with the exception that it contains nitrogen instead of carbon, Ni- - - O-N=O. ACKNOWLEDGMENTS We are grateful to Dr. L. C. Roess and Dr. S.A . Francis for interesting discussions
relating to this work and to E. J. Bane, M. Lahey, and J . Webber for help with the experimental work.
Received: February 23, 1956 REFERENCES 1 . Eischens, R . P., Francis, S. A., and Pliskin, W. A., J . Phys. Chem. 60, 194 (1956). 2. Bellamy, L. J., “The Infrared Spectra of Complex Molecules,” p. 149. Wiley, New York, 1954. 3. Miller, F. A., and Wilkins, C. H . , Anal. Chem. 24, 1253 (1952). 4 . Dell, R . M., and Stone, F. S., Trans. Faraday SOC.60,501 (1954). 5. Teichner, S.J., and Morrison, J. A., Trans. Famday SOC.61, 961 (1955).
AND
W. A. PLISKIN
The Texas Company, Beacon, New York Infrared techniques, which make i t possible t o obtain spectra of adsorbed molecules while reactions are in progress have been used t o study the oxidation of CO over a nickel-nickel oxide catalyst system. During the reaction a band is observed a t 4.56 p which behaves as though i t is related t o an intermediate in the oxidation reaction. This band cannot be accounted for on the basis of simple adsorption of any of the components of the reaction system. The band position and the method by which it is obtained suggest that i t is due t o the structure Ni - - ()=C=O.
I. INTRODUCTION
A major advantage of the infrared method of studying molecules adsorbed on surfaces is that these molecules can be observed while reactions are in progress. Knowledge of the spectra produced by chemisorption of reactants and products is essential in interpreting spectra obtained during the course of a catalyzed reaction. For this reason, an infrared investigation of CO and COz on nickel and on nickel oxide was carried out in conjunction with the study of the oxidation of CO. Moreover, it was necessary to determine the spectrum of physically adsorbed COz because the high specific absorptivity of this molecule makes it possible to detect bands due to physically adsorbed COzunder conditions where physical adsorption would not ordinarily be expected to occur.
11. EXPERIMENTAL PROCEDURE The method of preparing samples suitable for observation of the spectra of molecules adsorbed on metals has been described previously (1). In this case samples containing 9.2 wt. % nickel supported on Cab-o-sil were prepared by impregnating Cab-o-sil with nickel nitrate and then reducing with hydrogen. The nickel oxide samples were prepared by exposing the Cab-o-sil-supported nickel to oxygen. An excess of oxygen was introduced at 25" and the temperature raised to 300". After one half-hour at 300", the excess oxygen was pumped out and the sample cooled to 25" for subsequent adsorption measurements. 662
67.
CATALYZED OXIDATION OF
CO
663
Unless otherwise specified, the work reported here was carried out in a n
in situ cell which had been developed for the infrared study of chemisorbed gases (1) .
111. INFRaRED
ADSORBED MOLECULES
STUDY OF
1. CO on Nickel
The spectrum of CO chemisorbed on reduced nickel has been discussed in another paper ( 1 ) . The essential features of this spectrum are bands in the 4.84.9-p region, which are attributed to CO adsorbed with a linear structure, Ni-CEO, and bands in the 5.1-5.4-p region which are attributed to the bridged structure 0
II
C
Ni
/ \
Ni
2. C02 on Nickel Spectrum a of Fig. 1 is due t o C02 chemisorbed on Cab-o-sil-supported nickel a t 25" and 1.2-mm pressure. The nickel sample had been reduced with Hz at 300" for 16 hrs. prior to admission of the CO, . This spectrum shows a strong band a t 6.4 and a weaker band a t 7.1 p. Bands in these positions a.re characteristic of the carboxylate ion (2). This indicates that in the present system they are due t o the structure 0
-0
'\ / C
I
Ni
The weak bands a t 6.1 and 7.2 p are attributed t o a [CO& ion which is formed by react>ionof the C02 with a small amount of residual oxygen on the nickel surface. The latter ion will be discussed in the next section. If the nickel is exposed to C02 a t 100" instead of a t 25", bands attributable t o CO chemisorbed on nickel are observed. These are probably due t o reduction of the COZ together with diffusion of the extra oxygen into the interior of the metal particles. 3. COZ o n Nickel Oxide
Spectrum b of Fig. 1 is due to COZ chemisorbed on nickel oxide a t 25" and 1.6-mm pressure. The bands a t 6.1 and 7.2 p are similar t o those of a bicarbonate ion ( 3 ) .This indicates that the adsorbed species is
664
R. P. EISCHENS AND W. A. PLISKIN
0
-0
'\ / C
I
0
I
Ni
This structure might be described as a carbonate ion bonded to the surface through one of the oxygens. It will be referred to as a bicarbonate ion, however, because the oxygens are not all equivalent and the spectrum more closely resembles that of the bicarbonate ion in this region. Spectra of carbonate ions are characterized by a single strong band in the 6.9-7.0-p region (3). Observation of the spectrum attributable to the bicarbonate ion obviously has a direct bearing on the [CO,] complex theory which has been postulated to explain results of adsorption experiments with CO and COz on nickel oxide (4).At present it does not appear that the infrared results can be used to support the [Coal complex theory. Although a [C03]- ion is observed when COZ is adsorbed on nickel oxide, this fact alone is not confirmation of the theory, because formation of such an ion would be expected on the basis of conventional chemical principles. The significant point involved in the [CO,] complex theory is that this complex is stable and can be formed from all suitable combinations of CO, COZ, and oxygen. Attempts to obtain a [C03]- ion by methods suggested by this theory have not been successful. For example, it has not been possible to form a [C03]ion by treating the carboxylate ion with oxygen. Moreover, the carboxylate
67.
CATALYZED OXIDATION OF
CO
665
ion is more stable than the bicarbonate ion. The latter can be almost entirely removed from the surface by pumping at 25" for one half-hour, while the carboxylate ion is stable up to 150".
4. CO on Nickel Oxide Attempts to observe the spectrum of CO adsorbed on nickel oxide have not been successful. This result was unexpected on the basis of the adsorption of CO on nickel oxide (4, 5 ) . It is difficult to predict the minimum amount of adsorbed material which would be detectable when the specific absorptivity of that species is not known. On the basis of experience with CO and C 0 2 , it had been assumed that a surface coverage of 10% of a monolayer of CO on nickel oxide would be sufficient to produce a detectable band. Since the purpose of these preliminary experiments w & ~ to get information needed to interpret bands which are observed during the oxidation of CO, failure to observe a band in this case was not a serious obstacle in the interpretation of the oxidation experiments. 5 . Physically Adsorbed COZ
The area of the Cab-o-sil support is about ten times as large as that of the nickel in the sample. Previous references to monolayers apply only to the nickel area. When physical adsorption is studied, the area of the Cab-o-sil must also be considered. The extreme sensitivity of COZ makes it possible to detect bands due to 0.01 % of a monolyaer on the total surface. Hence, it is necessary to consider the possibility of physically adsorbed COz under conditions where physical adsorption would not ordinarily be expected to be an important factor. The infrared spectrum of gaseous COz has a vibration-rotation doublet at 4.23 and 4.28 1.1. Physically adsorbed C02 was expected to produce a single band between 4.23 and 4.28 1.1 because the COz molecules would not be rotating freely. Physically adsorbed COZ was studied to check this prediction and to insure that a band at 4.56, which is observed during the oxidation of CO, was not due to some unforseen factor which would shift the band position of the asymmetric carbon-oxygen stretching frequency in the physically adsorbed state. The cell used in this physical adsorption was a simple glass cylinder with CaFz windows sealed on the ends and with a side tube for admission of gases. The path length in the cell was reduced to 4 mm. by inserting salt plates, and about half of this space was filled with Cab-o-sil. In Fig. 2, spectrum a is that of gaseous COz in the blank cell. Spectrum b is due to a combination of gaseous COz plus COz physically adsorbed on Cab-o-sil at room temperature and 200-mm pressure. Spectrum c, which has a strong band at 4.26, is attributed to the physically adsorbed C O z .
666
R. P. EISCHENS AND W. A. PLISKIN
i
100-
z 0
90
-
80
-
VI
I?
170-
z fn
gI-
c60-
z 0 w
!SO-
40
(c)
-
-
4.2
4.3
FIG.2. Spectra of (a) gaseous Cog, (b) gaseous plus physically adsorbed COZ, and physically adsorbed COZ.
Spectrum c was obtained from spectrum b by subtracting the adsorption equivalent to the amount of gaseous COz indicated by the 4.23-p band in b. Thus, the small band a t 4.22 in c is an artifact caused by a slight displacement of the bands and has no significance regarding the question of whether the physically adsorbed COZ is rotating freely. The intensity of the band due to physically adsorbed COz indicates that the surface coverage is 1%. IV. CATALYZED OXIDATION OF CO I . Spectra during Course of Oxidation
A sequence of spectra obtained during the oxidation of CO at 25" is shown in Fig. 3. In these experiments O2was admitted to the system, which contained CO chemisorbed on nickel plus a gaseous CO atmosphere at 2-mm pressure. Spectrum a is due to the chemisorbed CO. After 2 mm. of 0 2 was admitted (total pressure 4 mm.), the most significant change was the appearance of a band a t 4.56 p. This band is evident in spectra b and c. Spectra a , c, and d were taken from a single run, and b was taken from a similar run. The initial reaction rate is fast compared with the rate a t which the spectrum is scanned, so that it is difficult to get a single spectrum which shows the
67. CATALYZED OXIDATION OF CO
A 4.5
I
'
l
t
t
5 .O WAVELENGTH I N MICRONS
t
667
t 5.5 "
FIG.3. Infrared study of the oxidation of CO: (a) chemisorbed CO, (b) and ( c ) intermediate stages, (d) termination of reaction.
band a t 4.56 p a t its maximum intensity and which also shows moderately strong bands due to chemisorbed CO. It requires about 30 sec. to scan from 4.56 to 4.83 p. Spectrum d was obtained one hour after c. During this period, the 4.56-p band gradually diminished until it was no longer evident. Strong bands, which are found a t 6.4 and 7.1 p in the higher wavelength regions of b, c, and d , show that carboxylate ions are formed on the surface. These ions are probably due t o the adsorption of gaseous COz , and it is likely that it is this adsorption which causes the reaction t o slow down or stop. The band a t 4.27 p in c and d is attributed to gaseous or physically adsorbed COz , or both. Integrated intensity measurements indicate that 0.5 % of a physically adsorbed monolayer would be sufficient t o produce this band. The band a t 4.56 p is of interest because it appears t o he related to the intermediate in the oxidation. This band has also been observed when nickel oxide was reduced with CO a t 200" and a t 25" over nickel when oxygen is admitted prior to CO and vhen O2 and CO are admitted simultaneously. When 0 2 is admitted prior to CO, the adsorbed product has the bicarbonate structure. Thus far, all cases except one, which will be discussed later, in
668
R. P. EISCHENS AND W. A. PLISKIN
which a band has been observed at 4.56 p are consistent with the view that it is due to the oxidation intermediate. It has not been observed when nickel was treated only with either CO or 0 2 . The specific absorptivity of the species producing the 4.56-p band is not known. If it is assumed that it is of the same order as that of chemisorbed CO, then the maximum concentration is about 1 % of a monolayer on the nickel surface, 9. Suggested Structure of the Oxidation Intermediate
A band due to carbon-oxygen vibrations which occurs in the 4.56-p region could be due to a carbon between a triple and a single bond or a carbon between two double bonds. Since the 4.56-p band is produced by adding oxygen to CO, it is likely that the species producing the band has a t least two oxygens. On the basis of the band position and the method by which ,it is obtained, it appears that the structure of the observed intermediate is Ni- - - O-C=O. Broken lines are used to represent some of the bonds because the exact bond order is not known. 3. Decomposition of Nickel Nitrate
A broad band with an absorption maximum a t 4.54-4.56 p is observed during the decomposition of nickel nitrate when the sample is being prepared. In order to be sure that the 4.56-p band observed during the oxidation of CO was due to a species containing carbon, C13O was used in an oxidation reaction. This shifted the 4.56-p band to 4.70 p and is proof that carbon is present in the species producing this band. It now appears that the broad band near 4.56 p which is observed during the nitrate decomposition is due to a structure similar to that postulated for the oxidation intermediate, with the exception that it contains nitrogen instead of carbon, Ni- - - O-N=O. ACKNOWLEDGMENTS We are grateful to Dr. L. C. Roess and Dr. S.A . Francis for interesting discussions
relating to this work and to E. J. Bane, M. Lahey, and J . Webber for help with the experimental work.
Received: February 23, 1956 REFERENCES 1 . Eischens, R . P., Francis, S. A., and Pliskin, W. A., J . Phys. Chem. 60, 194 (1956). 2. Bellamy, L. J., “The Infrared Spectra of Complex Molecules,” p. 149. Wiley, New York, 1954. 3. Miller, F. A., and Wilkins, C. H . , Anal. Chem. 24, 1253 (1952). 4 . Dell, R . M., and Stone, F. S., Trans. Faraday SOC.60,501 (1954). 5. Teichner, S.J., and Morrison, J. A., Trans. Famday SOC.61, 961 (1955).