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ANALYTICAL CHEMISTRY, VOL. 51, NO. 1, JANUARY 1979
stearate remain unaffected in t h e presence of silver nitrate. In conclusion, t h e selectivity of a reversed phase chromatographic system can be enhanced by using mobile phases containing silver ions as complexation agents for double bonds.
LITERATURE CITED (1) H. S. Huang and D. S. Goodman, J . B i d . Chem., 7. 2839 (1965). (2) B. Ahluwalia and R . T. Holrnan, Lipids. 1. 197 (1966).
D. J. Weber, J . Pharm Sci.. 66, 744 (1975). R. J. Tscherne and G. Capitano, J . Chromatogr., 136, 337 (1977). G. Schombura and K . Zaaarski. J . Chrornafoor.. 114. 174 (1975). B Vonach acd G Schorn-burg J Chromatog;, 149, 417 (1977) (7) E G Blighand W J Dyer Can J B/ochem Phys!o/, 37, 9 1 1 (1959)
(3) (4) 15) (6)
RECEIVED for review August 21, 1978. Accepted October 13, 1978.
Design and Construction of Emissionless Infrared Diffuse Reflectance Spectrometer Miki Niwa, * Tadashi Hattori, Mamoru Takahashi, Kenji Shirai, Michio Watanabe, and Yuichi Murakami Department of Synthetic Chemistry, Faculty of Engineering. Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464, Japan
A new IR diffuse reflectance spectrophotometer was constructed for in situ IR spectrometric determination of reacting species on catalyst surface at elevated temperatures without interference from the emission of samples. The IR spectra of powdered CaCO, show the suitability of the instrument for the study of high temperature IR spectra. In situ measurement of a reacting species on a catalyst surface is illustrated with a reaction between adsorbed benzoate ion and ammonia on an alumina catalyst at 290 O c .
T h e application of infrared techniques t o the study of catalysis a n d surface chemistry has heen widespread. For elucidation of t h e catalysis and adsorption phenomena, it is important t o measure IR spectra of reacting species while a reaction is in progress. Some attempts have been made to measure IR spectra during a reaction at room temperature, b u t only a few a t elevated temperatures because of the interference from t h e radiation of furnace and sample. Eberly ( I ) , Force a n d Bell (21, and Dalla Betta and Shelef ( 3 ) measured IR spectra of adsorbed species on catalysts a t elevated temperatures with transmittance IR spectrophotometers which canceled t h e emission from the sample. In the case of the transmittance IR method. however. very careful a n d skillful techniques are necessary to prepare a disk of catalyst with good transmittance. PoFder diffuse reflectance spectrometry in the infrared region is recommended because of ease of sample preparation and the potential of measuring colored samples ( 4 , 5). However, not many studies have been published on IR diffuse reflectance spectrometry, a n d only a few studies have been reported on species adsorbed on catalysts. Furthermore. nothing has been reported on IR spectra a t high temperatures b y the diffuse reflectance methvd. I n t h e present study, a n IR diffuse reflectance spectrophotometer for high temperatures was constructed and its applicability was examined.
EXPERIMENTAL Optical Unit. The basic optical unit for emissionless IR diffuse reflectance spectrometry is shown in Figure 1. Since the infrared radiation is chopped before the sample and onlv the chopped radiation can he detected by the detecting system (chopping speed, 13 Hz), the continuous emission from sample and furnace is not recorded. In this system, however, only a quarter of the radiation 0003-2700/79/0351-0046$01 .OO/O
from the light source is utilized; a half is lost at the chopper and a half of that remaining is lost at the half mirror. Two ellipse mirrors are used to compensate for the energy loss. The IK beam converging at the half-mirror reflects or passes through it. The half-mirror is made of glass with holes treated with an evaporated film of aluminum, and its transmittance is designed to be just ,507~ regardless of the wavelength so that the beams from both sides should enter equally into the monochromator. The light source is made of a homogeneously-qualified nichrome rod-heater of 6-mm diameter. No problems were detected when energy from opposite sides of the source was taken. The detector is a high-sensitive vacuum thermocouple. The SIX ratio is doubly heightened as compared with the previous one, accompanied with the increase of its energy efficiency. The optical unit for an emissionless diffuse reflectance spectrometer was incorporated into a ,Japan Spectroscopic Co.. infrared spectrophotometer (IR.4-SS). The head of the infrared cell made of stainless steel for elevated temperature is depicted in detail in Figure 2. The K R r window with a diameter of 17 mm kvas fastened to a stainless steel holder with a silicone glue (Shinetsu Chemical Co.. KEI4RTY) and the holder was sealed against the body b y an aluminum gasket o r a \'iton O-ring. The fine powder of the sample was mixed with the fine powder of KBr and packed just under the KBr window. The sample )vas heated by the underlying heater. and the temperature !vas measured by a thermocouple just below the sample plate. The sample can be evacuated or exposed to gases through vacuum lines made of stainless steel. Materials. Calcium carbonate and alumina (Xlon G) as adsorbent \yere commercially supplied and used without further treatment. HY-type zeolite was prepared by ion-exchanging, in which Ka-Y zeolite !vas added to the NHICl solution at 80 "C. filtered, and dried. followed by calcination at 450 " C for 3 h in a nitrogen stream. Ammonia, henzaldehyde, and pyridine were purified h y vacuum distillation.
RESULTS A N D DISCUSSION C a l i b r a t i o n of S a m p l e T e m p e r a t u r e w i t h M e l t i n g P o i n t . Although the sample temperature was measured by a thermocouple lying just helow the sample plate, the measured temperature was not in agreement with the temperature of t h e sample under actual working condition. T h e sample temperature was calibrated with a unique phenomenum occurring near t h e melting point. T h e reflectance of t h e substance decreased abruptly with t h e melting of the solid because of' the distortion from solids to liquids. Such examples in the cases o f benzoic acid and ascorbic acid are shown in Figure 3. An infrared heam fixed at 4500 cm was used to monitor the variation o f the reflectance with time. T h e c 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 1, JANUARY 1979
47
1
Figure 1. Basic optical unit for emissionless I R diffuse reflectance spectrophotometer: 1, 2, focusing ellipse mirror ( F , = 80 mm, f , = 160 mm, 0 = 190 mm); 3, 5, 7, 12, sphere mirror: 4, 6. 8, 9. 11, 13, 15, 16, flat mirror; 10, half-mirror: 14, parabola mirror: 17, ellipse mirror; LS, light source: CH, chopper; S, sample holder: F, filter: G, grating; SL, slit: D. detector
v IO ?o
TIME
M. P.
Figure 3. Examples of calibration of sample temperature under vacuum with the use of I R reflectance: A. benzoic acid: B, ascorbic acid. Temperatures in the curve show values measured by the thermocouple Figure 2. Head of the sample holder: 1, KBr window: 2, sample: 3, heater; 4 , cooling water; 5, O-ring or aluminum gasket: 6, chromelalumel thermocouple; 7, vacuum line: 8, pipes for cooling water
temperature was gradually increased and read with t h e thermocouple. T h e abrupt decrease of reflectance shorn the melting of t h e substance. T h e sample temperature represented by the melting point was much lower than the thermocouple temperature. 4 plot of melting points vs. thermocouple temperature gave a straight line as shown in Figure 4. So much difference was not observed between vacuo and atmospheric conditions. T h e broken line in Figure 4 shows the relation without IR beam irradiation. which was measured by observing melting points by t h e eye through a convex lens. T h e lower value of the broken line indicates that the sample temperature is elevated by t h e irradiation of t h e IR beam during IR measurement. T h e calibration lines in Figure 4 were used to measure the sample temperature. a n d t h e sample temperature quoted hereafter is t h e calibrated temperature. Calcium Carbonate. T o examine the usefulness of the high temperature spectrum, IR diffuse reflectance spectra of CaCO, were measured a t various temperatures with t h e present newly-developed spectrometer and an old type diffuse reflectance IR spectrometer (.Japan Spectroscopic Co., DR-3). Spectrum E in Figure 5 is an emission spectrum of CaCO,{ heated a t 110 "C measured with t h e old-type spectrometer. Since this spectrometer has a rotating mirror after the sample b u t not a chopper before the sample, the emission spectrum
I 4%
2
!
~
2
_
I
-
-
-
-
-
100
T ti E R M 0 C OU PL E ('C 1
Figure 4. Calibration of sample temperature under vacuum (0)or atmospheric (0)condition with the IR beam, and under the atmospheric condition without the I R beam ( A ) : 1, benzoic acid; 2, ascorbic acid; 3, tin; 4 , bismuth, 5. lead
could be observed. Spectrum E was measured without t h e radiation of the IR beam. LVhen the IR beam was irradiated to CaCO,$a t 110 "C. the absorption bands were observed in the same positions as the emission spectrum with smaller intensities than the emission bands in Figure 5E. In the case of the new spectrometer. the emission spectrum !vas not observable. even when CaCO,,was heated a t 290 "C.
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 1, JANUARY 1979
I400
1500
1600
(CM-I)
1000
800
600
400
Figure 5. Infrared spectra of powdered CaCO, diluted with 85 wt YO KBr at room temperature (A), 226 OC (B), and 290 "C (C); and of nondiluted CaCO, at 290 "C (D); and emission spectrum of heated CaC0, at 110 OC (E)
T h e recorder response was zero when the IR beam was intercepted between the sample and the chopper. On the other hand, when IR was irradiated, sharp absorption bands were observed in the same position as above as shown in Figure SA, B, and C. T h e bands at 873 and 'ill cm-' were ascribable to characteristic absorptions of carbonate ion (6). As can be seen from the comparison among spectra A: B, and C in Figure 5, the intensity of absorption was almost independent of the temperature up to 290 "C. These results indicate the great ability to measure the high temperature IR spectrum. Spectrum D in Figure 5 was measured with nondiluted CaC03, while spectra A, B, and C were measured with CaCO:, diluted with KBr. As mentioned below, dilution sharpened the band. Pyridine Adsorbed on HY Zeolite. T h e IR spectrum of pyridine adsorbed on solid acid catalysts is one of the most extensively examined in catalytic chemistry. HY zeolite gives enough stably adsorbed species of pyridine for our purpose. T h e sample of pyridine adsorbed on HY zeolite was prepared with a conventional apparatus of pulse technique. HY zeolite was packed in a Pyrex glass tubing with an inside diameter of 5 mm and it was heated with an electrically heated furnace a t 449 "C for 2 h in a flow of dry nitrogen. HY zeolite was cooled to 82 "C, and three pulses of 6 ,uL of pyridine were injected into a flow of nitrogen through a rubber septum. Pyridine was carried with nitrogen to HY zeolite and was adsorbed on zeolite. T h e sample thus obtained was mixed with KBr and subjected to IR measurement. Figure 6 shows the spectra measured at room temperature at various sample concentrations. Absorption bands were observed in the region between 1400 and 1600 cm I , which agreed well with published results ( 7 ) . T h e absorption bands at 1542 and 1531 cm were ascribed to pyridinium ion adsorbed on Briinsted acid sites, and the 1450 and 1438 cm bands were due to the coordinated and hydrogen-bonded pyridines, respectively. The 1488 cm
'
Figure 6. Infrared spectra of adsorbed pyridine on HY zeolite with KBr diluent mixed at various concentrations. Concentration of zeolite by weignr: I , IUUYO; z, 3 1 . ~ 7 03, ; Z D . U Y O ; 4, I Z . / Y O ; 3, 0 . ~ 7 0 o, ; L.J-/o: 7, 0.87%
0
I
I
I
I
20
40
60
80
C O N C E N T R A T I O N OF Z E O L I T E
100
(90)
Figure 7. Dependence of reflectance (I/Io)of 1488- (O), 1534- ( A ) , and 1438- (0)cm-' bands on concentration of zeolite
band was due to all of these three types of adsorbed pyridine. As shown in Figure 6, intensities of these adsorption bands depend on the concentration of zeolite in the KBr diluent. T h e reflectance ( I / Z o ) was said to be approximately linear to
A N A L Y T I C A L CHEMISTRY, VOL. 51, N O . 1, J A N U A R Y 1979
C M-' I700
I
1
I
I600
1500
1400
1300
Figure 8. Infrared spectra taken after adsorption of benzaldehyde and reaction with ammonia: A, background; B, after adsorption of benzaldehyde: C, 15 min after introduction of ammonia, D. emission spectrum of heated benzoate ion on AI,O, at 150 O C
the logarithm of sample concentration in the low concentration region (8). As shown in Figure 7 , the reflectance decreased with zeolite concentration up t o about 15% by weight, but increased in a more concentrated region. In other words. the best spectrum was obtained with the sample concentration of 10-2070. Benzoate Ion Adsorbed in the Course of Reaction with Ammonia. As reported elsewhere ( 9 ) , ammoxidation of toluene (a synthetic reaction for producing benzonitrile from toluene, ammonia, and oxygen) on W20h/A1203proceeds through benzoate ion adsorbed on A1,0,. T h e adsorbed species was identified with IR transmittance spectrometry at room temperature. T h e present newly-developed diffuse
49
reflectance spectrometer made it possible t o identify the adsorbed species in the course of reaction a t elevated temperatures. Alumina catalyst packed in the infrared cell was evacuated at 315 "C for 1 h. exposed to 1 Torr of benzaldehyde a t 300 "C for 15 min, and subjected to the IR measurement a t 300 " C . T h e spectrum obtained, shown in Figure 8B, was ascribable to the benzoate ion, though the spectrum was not so clear because of the high concentration of the sample. T h e bands a t 1545 and 1424 cm-' were due to the as-ymmetric and symmetric stretching vibrations of carboxylate ion. and furthermore the absorption of C=c' in the benzene ring was found at 1595, 1493, and 1455 cm-'. An absorption a t 1400 cm-l was caused by the contamination of silicone glue sticking to the KBr window on the in situ cell. After a brief evacuation, ammonia was introduced to the cell to react with the adsorbed benzoate a t 280 "C. T h e intensity of the absorption band decreased gradually during the course of the reaction, as shown in Figure 8C. T h e spectrum has become distinct with a decrease in concentration of the adsorbed benzoate ion. This is consistent with the measurement of behavior of the benzoate ion adsorbed on A120, with the IR transmittance spectrophotometer a t room temperature. The adsorbed benzoate ion surely gives benzonitrile in the reaction with ammonia, as reported previously (9). These spectra show the real behavior of the adsorbed intermediate on the surface in the course of the reaction. Spectrum D is the emission of benzoate ion adsorbed on A1,03 a t 150 " C measured with the old type spectrometer (DR-3) as mentioned above. T h e emission bands were observed in the same position as the absorption bands shown by spectra B and C in Figure 8, and the intensity of the absorption band measured with the old type spectrometer was smaller than the emission bands, similar to the case of CaCO?. Pyridine adsorbed on zeolite also gave the same result; the emission bands with greater intensity than the latter appeared in the same position as the absorption bands. These results may suggest that the cancellation of emission from the sample is necessary to get the sharp absorption spectrum a t elevated temperatures with the IR diffuse reflectance spectrometer.
LITERATURE CITED (1) P. E. Eberly, Jr., J . Pbys. Cbem., 71, 1717 (1967). (2) E. D. Force and A. T. Bell, J . Catal., 38. 440 (1975). (3) R . A . Dalla Betta and M. Shelef. J . Catal.. 48. 11 1 11977) (4) W. W. Wendkind?, "Modern Aspect of Reflectance Spectroscopy". Plenum Press, New York, 1968. (5) W. W . Wendlandt and H. G. Hecht, "Reflectance Spectroscopy", Interscience Publishers, New Y o r k , London, Sydney, 1966. 16) F. A . Miller, G. L. Carlson. F. F. Bentlev. and W. H. Jones. Soectrochim. Acta, 16, 135 (1960) (7) E. G. Parry, J . Cafal.. 2, 371 (1963). (8) E. Ishii, M. Marniya, and T. Murakami. Nippon Kagaku Kaishi, 1972, 353. (9) M. Niwa, H. Ando, and Y . Murakami, J &fa/., 49, 92 (1977).
RECEILED for review June 19. 1978. Accepted September 12. 1978.
