Fundamentals and Applications of Diffuse Reflectance Infrared Fourier

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13 Fundamentals and Applications of Diffuse Reflectance Infrared Fourier Transform (DRIFT) Spectroscopy Mark B. Mitchell Department of Chemistry, Clark Atlanta University, Atlanta, G A 30314

Diffuse reflectance spectroscopy is a powerful tool for the study of materials such as polymers and catalysts. The bulk or surface morphology of these materials is many times an important experimental parameter that can be altered by sample preparation methods used in the more common spectroscopic techniques. Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy has been shown to be more sensitive to surface species than transmission measurements and to be an excellent in situ technique. The applications of DRIFT spectroscopy to the investigation of polymer surfaces and surface structures of both fibers and films are particularly germane to this symposium and are covered in this review. The foundations of the diffuse reflectance technique in the mid-infrared range are presented, and methods that have been used to obtain spectra for polymer powders, films, and fibers are outlined, as is the use of DRIFT spectroscopy for depth-profiling studies. The use of DRIFT spectroscopy for the investigation of catalytic processes is also reviewed.

DIFFUSE

REFLECTANCE

INFRARED

FOURIER

TRANSFORM

(DRIFT)

spec

troscopy is an infrared sampling m e t h o d that involves m i n i m a l sample p r e p a ration i n terms o f time a n d sample manipulation. D R I F T spectroscopy is particularly useful f o r p o w d e r samples, b u t c a n also b e used to investigate samples such as p o l y m e r fibers a n d films. T h e manipulations i n v o l v e d i n preparation o f a sample f o r transmission measurements,

as a n example,

involve m i x i n g the sample w i t h an appropriate diluent a n d compressing the 0065-2393/93/0236-0351$07.25/0 © 1993 American Chemical Society

In Structure-Property Relations in Polymers; Urban, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1993.


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sample into a transparent pellet. T h i s preparation can easily result i n irre versible changes i n sample m o r p h o l o g y a n d surface properties. T h u s , m i n i m i z i n g sample manipulation becomes an important consideration w h e n sample m o r p h o l o g y a n d surface properties are investigated, or are important sample variables. I n addition, because the sample is not compressed for D R I F T spectroscopy, good gas contact w i t h the entire sample is possible, a distance advantage for i n situ measurements. Gas flow t h r o u g h the sample is a capability that makes D R I F T spectroscopy useful as one segment o f a hyphenated analytical system or as a technique for the study o f gas-surface interactions. D i f f u s e reflectance has b e e n used extensively i n the visible a n d ultravio let regions o f the electromagnetic spectrum for the characterization o f solid materials such as inorganic powders. M a n y o f the effects a n d constraints that govern diffuse reflectance spectroscopy were d e t e r m i n e d as a result o f experiments w i t h ultraviolet a n d visible radiation. I n those investigations, the K u b e l k a - M u n k expression was particularly useful for relating the observed reflectance spectrum to the optical constants o f the material u n d e r study. References b y W e n d l a n d t a n d H e c h t ( I ) a n d b y K o r t u m ( 2 ) provide an excellent b a c k g r o u n d a n d i n c l u d e reviews o f these earlier investigations. I n 1976, W i l l e y ( 3 ) d e s c r i b e d the optical design o f an instrument developed to extend the technique o f diffuse reflectance into the infrared. H i s F o u r i e r transform I R ( F T I R ) spectrometer was specifically designed to carry out i n f r a r e d diffuse reflectance measurements, although it c o u l d also be used for transmission measurements. Since the w o r k o f F u l l e r a n d Griffiths i n 1978 a n d 1980 (4, 5), the D R I F T spectroscopy technique has b e e n u s e d for the analysis o f a w i d e variety o f sample types, i n c l u d i n g p o l y m e r powders, films, a n d fibers; hetero geneous catalysts; a n d high-temperature superconductors, as w e l l as " n o r m a l " inorganic a n d organic p o w d e r materials. F u l l e r a n d Griffiths l a i d m u c h o f the groundwork necessary for the use o f the technique as an analytical tool. T h i s review outlines the foundation l a i d b y F u l l e r a n d G r i f f i t h s and by other investigators a n d presents results that demonstrate the utility of the D R I F T spectroscopy technique for a variety of sample types. A variety o f i n f r a r e d spectroscopic methods are available for use to investigate polymers, and many o f the applications o f these techniques to polymers have b e e n discussed (6). Transmission spectroscopy is one o f the most c o m m o n methods, but, as m e n t i o n e d earlier, the pressure n e e d e d to f o r m pellets o f many polymers can affect surface functionality. These effects were d o c u m e n t e d b y B l i t z et al. ( 7 ) i n their investigation o f trimethoxymethylsilane o n a silica surface. T h e authors showed that the pressures i n v o l v e d i n m a k i n g a pellet i n d u c e d c h e m i c a l changes i n the surface functional groups. A l s o , many samples o f interest simply are not optically t h i n enough to be studied b y transmission. F i b e r - r e i n f o r c e d composites are an example o f such a case.


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Attenuated total reflectance ( A T R ) is one o f the best ways to obtain i n f r a r e d spectra o f p o l y m e r films. O n e o f the requirements o f A T R is that the sample make optical contact w i t h a n i n f r a r e d crystal. Some samples are n o t soft enough to f o r m g o o d contact a n d are difficult to study w i t h this m e t h o d . F i b e r s are very difficult to measure w e l l using A T R . C h i l d e r s a n d P a l m e r (8) a n d U r b a n et a l . ( 9 ) c o m p a r e d photoacoustic spectroscopy ( P A S ) a n d D R I F T spectroscopy. T h e general conclusion is that the methods are comparable: D R I F T spectroscopy usually gives better signal to noise ( S / N ) ratios f o r most samples, b u t P A S is better suited for strongly absorbing or highly reflective samples. A n o t h e r general conclusion is that D R I F T spectroscopy is very sensitive to sample morphology (with p o w d e r particle size b e i n g a critical parameter), whereas P A S is relatively insensitive to sample morphology a n d is perhaps better suited to the study o f carbon-filled rubbers, for example, because it is difficult to control particle size f o r a D R I F T sample. P A S c a n b e used f o r depth-profiling ( 9 , 10), w h i c h is not as easily accomplished using D R I F T spectroscopy, although some concentration versus d e p t h i n f o r m a t i o n c a n b e generated using D R I F T w i t h K B r overlayers as is discussed later.

Theory T h e r e are t w o general types o f reflected light: specular a n d diffuse. A c o m m o n example o f specular reflection (also called front-surface, regular, o r F r e s n e l reflection) is light reflected f r o m mirrors o r other p o l i s h e d surfaces. Specular reflectance occurs at any interface between t w o materials w i t h different refractive indices. Specularly reflected light is characterized b y the rule that the angle o f reflection is equal to the angle o f incidence. C o m m o n examples o f diffusely reflecting surfaces are the matte surfaces characteristic o f certain types o f paper a n d powders. D i f f u s e l y reflected radiation is the fight reflected f r o m a diffusely reflecting sample f o r w h i c h the angle o f reflection does not equal the angle o f incidence.

Optical Constants. T h e intensive property that characterizes t h e reflectance a n d transmittance o f a material is the complex refractive index (n ), w h i c h is the s u m o f t h e index o f refraction (n) (called t h e real component) a n d the absorption index (k ) (called the imaginary component): f

f

ri = η + ik'

(1)

F o r nonabsorbing materials, k' = 0. Specularly reflected light is w e l l d e scribed b y FresneFs equations (see reference 1), w h i c h express the reflection o f electromagnetic radiation at an interface as a function o f the refractive indices o f the t w o m e d i a that f o r m t h e interface. T h e difference i n the refractive indices leads to reflection. T h e differences i n refractive index that


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l e a d to reflection can be due to differences i n the real o r the imaginary components. I n the B e e r - L a m b e r t equation, the loss o f transmitted intensity, I, due to absorption processes is given b y I = I e- * 0

k

(2)


w h e r e k is the absorption coefficient (Φ k') a n d χ is the path length t h r o u g h the sample. T h e absorption index is related to the absorption coefficient b y Jfc'=Jfc/4irv

(3)

w h e r e ν is the frequency o f the radiation (per centimeter) a n d the absorption coefficient is related to the absorptivity, a, a n d the concentration, c, o f a n absorber b y k = 2.303ac

(4)

Kubelka-Munk Expression. I n any spectroscopic technique it is important to be able to characterize the response o f the technique to the concentration o f the analyte. A n u m b e r o f models have b e e n d e v e l o p e d that y i e l d expressions relating observed diffuse reflectance to concentration ( J , 2, 9). T h e n u m b e r o f i n d e p e n d e n t experimental variables that must b e evalu ated for each expression is related to the complexity o f the m o d e l . T h e most c o m m o n l y u s e d expression is the two-parameter K u b e l k a - M u n k ( K M ) ex pression ( I , 2, 11-13): f(R ) œ

= (l-R f/2R^-k/s œ

(5)

w h e r e R is the absolute reflectance o f an " i n f i n i t e l y t h i c k " sample, f(R ) represents the value o f the K u b e l k a - M u n k f u n c t i o n , k is the absorption coefficient d e f i n e d previously, a n d s is a scattering coefficient d e f i n e d for p u r e l y scattering samples b y x

œ

1 = I e~" 0

(6)

It is generally assumed that s is a constant or at most a slowly varying function o f frequency a n d is not a f u n c t i o n o f analyte concentration. A n infinitely thick sample is one t h r o u g h w h i c h no light is transmitted. A l l light is either reflected or absorbed. T h e infinitely thick criterion is generally satis fied for samples that are 3 - 5 m m thick. T h i s thickness does not satisfy the infinitely thick c r i t e r i o n for a l l samples or for many samples over the w h o l e m i d - i n f r a r e d range; these considerations are discussed later. Nonetheless, D R I F T spectra o f most c o m p o u n d s can be at least semiquantitative w i t h samples o f this thickness. A s an aside, Fraser a n d G r i f f i t h s (14) argue that


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because the effective path length increases b y a factor o f 2 f o r diffuse reflectance, eqs 2 a n d 6 s h o u l d b e m o d i f i e d such that the exponents are — 2 fee a n d — 2sx, respectively. I n p r i n c i p l e , this change s h o u l d have no effect o n the remission f u n c t i o n ( e q 5) calculated f o r the infinitely thick sample, because the factor o f 2 w i l l cancel. H o w e v e r , this possible discrep ancy s h o u l d b e kept i n m i n d w h e n c o m p a r i n g k data f r o m different groups. Typically, the absolute reflectance o f e q 5 is not evaluated. Instead, the reflectance relative to some nonabsorbing standard is measured. R is substituted f o r R i n e q 5, w h e r e f

œ


œ

R' = R (sample)/Β» œ

œ

(standard)

(7)

R (sample) is the reflectance o f the sample a n d R (standard) is the reflectance o f a nonabsorbing standard, such as K B r o r K C l . F o r samples that obey the K u b e l k a - M u n k expression, plots o f f(R'«) versus concentration (c) y i e l d a straight line w i t h a slope o f 2 . 3 0 3 « / s . I n their original paper, F u l l e r a n d Griffiths (4) presented results that showed the dependence o f the D R I F T spectra o n particle size a n d concen tration o f the absorber. Plots o f the K M intensity o f a particular b a n d showed deviations f r o m linearity at h i g h absorber concentrations. T h e K M equation is d e r i v e d w i t h the assumption that the sample is optically dilute: the concentra tion o f the absorber is l o w o r the absorption coefficient is small. T h i s condition is generally m e t b y d i l u t i n g the sample o f interest i n a nonabsorb ing matrix such as K B r o r K C l , although for weak i n f r a r e d absorptions, this is not necessary. F o r samples that satisfy the optically dilute criterion, small changes i n concentration result i n p r o p o r t i o n a l changes i n the observed K M intensities. œ

œ

Fresnel Reflection.

T o a i d a discussion o f several o f the causes a n d possible solutions o f nonlinearity associated w i t h D R I F T spectroscopy, it is useful to consider the sample material as a composition o f small grains o f diluent a n d analyte (see F i g u r e 1). W h e n radiation interacts w i t h the sample, three different events, w h i c h y i e l d three classes o f reflected radiation, c a n occur. T h e first class results f r o m the simple reflection o f incident radiation at a sample grain interface that is parallel to the macroscopic surface o f the sample; this is c a l l e d specular F r e s n e l reflectance, after B r i m m e r a n d G r i f fiths (15). T h i s reflection f r o m the t o p surface o f the p o w d e r is n o t isotropically scattered; the angle o f reflection is e q u a l to the angle o f incidence. T h e reflection f r o m the t o p surface also retains the polarization o f the incident beam. T h e second class o f reflected radiation is similar to the first i n that it is not transmitted t h r o u g h any sample grains. M a n y o f the grain interfaces that the radiation encounters w i l l not b e parallel to the surface o f the macroscopic sample. Incident radiation that encounters such an interface w i l l b e reflected


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Incident Radiation

Figure 1. Schematic diagram of the DRIFT interaction with three classes of interaction shown. The top ray is used to indicate the Fresnel specular component of the reflected radiation, which is reflected from the first interface it encounters at the top of the sample. The center ray indicates the Fresnel diffuse component. After two reflective interactions and no transmission through any sample grain, the sample is reflected out of the sample. The lowest ray corresponds to the Kubelka-Munk diffuse reflectance, which is radiation that has been reflected from some grains and transmitted through others.

at an angle that is equal to the angle o f incidence at t h e grain interface, yet not equal to the angle o f incidence to the macroscopic sample. T h i s radiation w i l l carry n o i n f o r m a t i o n about the analyte, b u t it is diffusely reflected i n the sense that it is isotropically scattered a n d its polarization is scrambled relative to that o f the incident radiation. T h i s radiation is called diffuse

Fresnel

reflectance. M u l t i p l e reflection w i t h i n the sample w i l l also generate radiation e m e r g i n g f r o m the sample that has not b e e n transmitted through any sample grain, a n d this radiation falls into the class o f diffuse F r e s n e l reflectance. T h e t h i r d a n d final class o f radiation reflected f r o m the sample is called the diffuse K u b e l k a - M u n k reflectance. T h i s radiation has b e e n transmitted t h r o u g h at least one sample grain, contains a l l the i n f o r m a t i o n regarding the analyte that is available i n the reflected radiation, is isotropically scattered, and its polarization is s c r a m b l e d relative to the incident field. T h e intensity o f this radiation obeys the K u b e l k a - M u n k relation f o r the intensity o f analyte absorption bands relative to concentration o f the absorber, u n d e r the l i m i t a tions o f that relation. The

presence o f F r e s n e l

the detector

reflectance i n the radiation collected b y

leads to m u c h o f the nonlinearity associated w i t h


DRIFT


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spectroscopy. Specular F r e s n e l reflectance is relatively easy to eliminate f r o m the collected radiation. O n e scheme is to collect radiation i n directions that exclude the plane o f incidence o f the incident radiation (16, 17). A second scheme is to polarize the incident radiation a n d collect radiation t h r o u g h an analyzing polarizer oriented at 90° to the polarization o f the incident radiation (17, 18). These two optical configurations are illustrated i n F i g u r e 2. N e i t h e r o f the two methods w i l l eliminate the diffuse F r e s n e l reflectance. O n e m e t h o d that apparently decreases diffuse F r e s n e l reflectance is to g r i n d the sample a n d diluent to very small particle sizes. T h i s m e t h o d decreases the fraction o f incident radiation that is reflected out o f the sample without b e i n g transmitted t h r o u g h a sample grain. F r e s n e l reflectance does not obey the K u b e l k a - M u n k relation w h e t h e r specularly or diffusely reflected, a n d it is important to k n o w w h e n an observed spectrum can be characterized using the K u b e l k a - M u n k relation. Intensity variations i n the F r e s n e l reflectance components o f the collected radiation are directly dependent o n variations i n b o t h the index o f refraction (n) a n d the absorption index (k ) o f the sample. I n the n e i g h b o r h o o d o f an absorption b a n d , the index o f refraction fluctuates rapidly as a f u n c t i o n o f frequency a n d takes o n the first derivative shape characteristic o f a d a m p e d oscillator. T h i s fluctuation is k n o w n as anomalous dispersion. F o r moderate to weak absorption bands, the F r e s n e l reflectance is d o m i n a t e d b y the fluctua tions o f the index o f refraction, a n d spectra that contain a large F r e s n e l reflectance c o m p o n e n t w i l l show characteristic derivative-shaped bands. F o r absorption bands w i t h very h i g h absorption indices, the F r e s n e l reflectance is d o m i n a t e d b y the absorption index, a n d the sample is actually m o r e reflective i n the n e i g h b o r h o o d o f these absorption bands than i n spectral regions i n w h i c h the sample is transparent. These " i n v e r t e d " reflectance bands are f

called reststrahlen bands. A n o m a l o u s dispersion a n d reststrahlen bands a n d their effects have b e e n discussed b y several groups (16, 18-20). W i t h respect to the absorption index scale, strong i n f r a r e d chromophores such as the 6 7 4 - c m C - H out-of-plane absorption o f benzene have absorp tion indices o n the order o f 1.5 (21). T h e weakly absorbing C - H stretch vibrations o f benzene have absorption indices o f 0 . 0 1 - 0 . 0 5 . F o r inorganic materials, the absorption indices associated w i t h m e t a l - o x y g e n a n d m e t a l - h a l i d e vibrations can be significantly greater than 3.0. S u c h absorp tions almost always lead to distorted peak shapes w h e n measured u s i n g D R I F T spectroscopy. - 1

Optical Accessories. Several companies manufacture optical acces sories that can be u s e d to obtain D R I F T spectra. T h e majority o f these accessories correspond to one o f the configurations shown i n F i g u r e 2 A a n d B . H o w e v e r , the on-line configuration is used most c o m m o n l y without the polarizers. U n d e r such conditions, the o n - a n d off-line configurations p e r f o r m m u c h differently w i t h regard to their ability to reject F r e s n e l reflectance.


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Analyzer

Radiation from

To detector

interferometer Polarizer []

Sample


(A)

To detector

Radiation from interferometer

U

Sample

(B)

Radiation from interferometer

Detector

Analyzer (C) Figure 2. Schematic diagram of three commonly used optical configurations that eliminate to a great degree the contribution of Fresnel specular reflectance to the collected radiation. The top diagram (A) represents the in-line configuration in which polarizers are used to eliminate the specular component. The middle diagram (B) shows the off-line configuration in which exclusion of the plane of incidence from the collection optics is used to eliminate the specularly reflected component. The bottom diagram (C) is a configuration built by Fuller and Griffiths (3). (Reproduced with permission from reference 17. Copyright 1948.)



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Comparisons o f these two configurations have b e e n carried out b y at least three groups (16-19), a n d it was observed that the optical off-hne configura tion leads to better rejection o f the F r e s n e l components o f the reflected radiation than the on-line configuration w h e n the polarizers are not used f o r the optical i n - l i n e configuration. H e m b r e e a n d S m y r l ( 1 9 ) observed that even for relatively weak c a l c i u m carbonate absorptions, the on-fine configuration gave nonlinear results as a f u n c t i o n o f concentration even at l o w concentra tions. T h e use o f a device to physically b l o c k specularly reflected radiation i n the on-line configuration significantly r e d u c e d the throughput o f the configuration.

Sample Preparation. O n e o f the limitations o f diffuse reflectance spectroscopy w i t h regard to its use as a truly quantitative technique involves the reproducibility o f the technique. Because the sample materials are typically powders, the physical characteristics o f the diluent a n d the analyte play a n important role i n the reproducibility o f the results. I n their first paper, F u l l e r a n d Griffiths (4) investigated the effects o f particle size o n the observed spectra. T h e spectra were clearly shown to b e dependent o n particle size, a n d samples w i t h smaller average particle sizes (average particle size < 10 μ ι η ) gave better spectra w i t h r e d u c e d peak widths c o m p a r e d to samples w i t h large average particle sizes (average particle size > 90 μ ι η ) . A n average particle size o f 10 μ m is a realistic goal f o r quantitative measurements o f powders. Average particle sizes o f 5 0 - 1 0 0 μ m have b e e n shown to y i e l d useful qualitative results (22). F u l l e r a n d Griffiths also illustrated the requirement for optically dilute samples; they c o u l d not effectively carry out spectral subtraction over large frequency ranges unless the sample was d i l u t e d i n a nonabsorbing matrix such as K C l . L a t e r studies b y G r i f f i t h s ' group a n d b y other groups showed that it is not always a requirement that the sample b e diluted; p o l y m e r films a n d fibers are a n important example o f systems that c a n b e examined without dilution, i n many cases w i t h excellent results (see later discussion). W i t h regard to the use o f pressure as a way to overcome the n o n r e p r o ducibility i n the observed spectra associated directly w i t h the sample prepara tion step, Y e b o a h et a l . (23) f o u n d that the K M intensities o f t w o o f the i n f r a r e d absorption bands o f caffeine increased b y almost an order o f magnitude between a sample p r e p a r e d w i t h n o pressure treatment a n d one subjected to a pressure o f 1 2 , 0 0 0 p s i for 5 m i n . T h i s increase i n / ( l O was p r e s u m e d to b e completely due to a corresponding decrease i n the scattering coefficient, an effect that (as they point out) is c o m m o n l y observed i n pressing pellets for i n f r a r e d analysis. T h i s decrease occurs because the scattering at a K C l - a n a l y t e interface is less than the scattering at a c o m b i n a tion K C l - a i r - a n a l y t e interface because the refractive index f o r K C l is similar to that f o r many organic analytes. F o r diffuse reflectance measurements though, once the sample becomes transparent, the K u b e l k a - M u n k f o r m u l a tion is n o longer valid because diffuse reflectance is n o longer the dominant


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process. I n general, pressure increases the spectral contrast o f the observed spectra a n d increases the r e p r o d u c i b i l i t y between measurements; the stan d a r d deviation approaches ± 3 % for samples p r e p a r e d w i t h the same pres sure a p p l i e d for the same length o f t i m e . H e m b r e e a n d S m y r l ( 1 9 ) also investigated the use o f pressure as a way o f i m p r o v i n g reproducibility a n d f o u n d that b y careful sample preparation without pressure, the standard deviation approached + 3 % . T h e use o f pressure was f o u n d to decrease the magnitude o f the diffusely reflected radiation and increase the specular component. T h e effect o f using infrared-absorbing materials as diluents was investi gated b y B r i m m e r a n d Griffiths (24). A n important example o f such a situation is a surface layer o n a p o l y m e r fiber or film. A l t h o u g h the p o l y m e r fiber or film substrate is not technically a diluent i n such a sample, many o f the considerations that B r i m m e r a n d Griffiths discuss are applicable. I n situ catalytic studies also provide an example o f a case w h e n the use o f diluents is not desirable. A s a diluent, K C l is a source o f many potential uncertainties i n such an investigation: Potassium is a w e l l k n o w n p r o m o t e r i n many catalytic systems a n d c h l o r i d e ions can act as a catalyst poison, so the use o f these materials for i n situ investigations can clearly distort results. I n such cases, it is many times preferable to dilute the sample w i t h a chemically inert support material such as a l u m i n a o r silica, b o t h o f w h i c h , however, have strong absorptions i n the infrared. B r i m m e r a n d Griffiths r e p o r t e d that the effect o f an absorbing matrix is very complex. I n general, the authors observed an apparent decrease i n the absorptivity o f analyte bands that o c c u r r e d i n regions o f i n f r a r e d absorption b y the diluent. T h e authors postulated two possible causes. T h e increased average absorption o f the sample c a n result i n a decrease i n the effective penetration d e p t h o f the radiation into the powder. A l s o , because the absorption index (&') for the diluent is nonzero i n frequency regions w h e r e it absorbs, m o r e specular reflectance f r o m the front surface o f the sample p o w d e r may be observed i n those regions than i n the case o f a nonabsorbing diluent. T h e c o m b i n e d result is that the use o f infrared-absorbing diluents may significantly distort analyte absorption bands, especially the intensity o f one analyte b a n d relative to another, c o m p a r e d to absorption bands observed i n a transmission spectrum. Recently, Frasier a n d Griffiths (14) investigated the effect o f changes i n the scattering coefficient s (eq 6) o n the observed D R I F T spectra; specifi cally, the effect o f the scattering coefficient o n the infinite-depth criterion o f the K u b e l k a - M u n k expression. T h e scattering coefficient o f K C l p o w d e r that h a d b e e n pressed slightly, for samples o f thicknesses greater than about 500 μπι, was f o u n d equal to 25 c m , a n d the scattering coefficient v a r i e d as a f u n c t i o n o f frequency (although b y n o m o r e than about a factor o f 2 over the m i d - i n f r a r e d range) a n d as a f u n c t i o n o f sample thickness. T h e calculations o f F r a s i e r a n d Griffiths indicate that the infinite d e p t h approximation w i l l not be v a l i d for nonabsorbing samples u n t i l the sample thickness approached 1 c m . - 1


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H o w e v e r , i f a sample does have i n f r a r e d absorption bands, k' w i l l decrease the effective d e p t h o f penetration a n d , over the frequency region o f the absorption, the infinite depth approximation m a y be valid f o r samples m u c h less than 1 c m thick. M a n d e l i s et a l . ( 2 5 ) d e r i v e d a treatment, based o n M e l a m e d ' s statistical approach ( 2 6 ) , for calculating the diffuse reflectance a n d diffuse transmittance o f powders w i t h a particle size that is very large w i t h respect to the wavelength o f the incident radiation. G o o d agreement was obtained f o r samples made u p o f particles w i t h (d) ~ 7k, w h e r e (d) is the average particle diameter a n d λ is the wavelength o f the radiation; however, relatively p o o r agreement was f o u n d f o r smaller particles, w h e r e the K M expression becomes useful.

Polymer Studies Using DRIFT Spectroscopy T h e r e are t w o different types o f p o l y m e r investigations that have b e e n carried out w i t h D R I F T spectroscopy. T h e first type results f r o m the study o f a p o l y m e r p o w d e r . I n this sort o f investigation, the considerations regarding the sample itself are no different than those that w o u l d b e i n v o l v e d f o r a typical organic analyte: particle size, sample homogeneity, a n d sample pack ing. L e e et a l . ( 2 7 ) carried out just such a study o n complexes o f poly (vinyl phenol) ( P V P h ) a n d p o l y ( 2 - v i n y l pyridine) ( P 2 V P ) . T h e complex precipitated f r o m solution as a p o w d e r a n d gave very p o o r K B r pellet transmission spectra due to the large amount o f scattering caused b y the analyte p o w d e r . U s i n g D R I F T spectroscopy, these investigators were able to study the interactions between the t w o species that comprise the complex; the spectra indicate the formation o f a 1:1 complex. F i g u r e 3 shows the 2 2 0 0 - 3 8 0 0 - c m " region o f the D R I F T spectrum o f a complex o f P V P h - P 2 V P , p u r e P V P h , a n d p u r e P 2 V P . T h e two b r o a d , intense absorptions at 3525 a n d 3330 c m " f o r p u r e P V P h are associated w i t h free a n d h y d r o g e n - b o n d e d O - H groups. T h e s e bands are not present i n the complex; rather, a n e w b r o a d feature centered at about 3000 c m " is apparent. T h i s n e w absorption is assigned to a strongly p e r t u r b e d O - H vibration; the perturbation is caused b y interaction w i t h the nitrogen atom o f the P 2 V P . 1

1

1

Siesler (28) used silica p o w d e r to m i m i c glass fibers a n d treated the p o w d e r w i t h 7-propyltrimethyloxysilane, a c o u p l i n g agent u s e d to increase reinforcement effectiveness i n fiber reinforced plastics. T h i s investigator was able to m o n i t o r the decrease i n free O H (3740 c m " ) o n the glass surface a n d the growth o f C H (2900 c m ) a n d C = 0 ( 1 7 0 0 - 1 7 2 5 c m " ) d u e to the c o u p l i n g agent, as a f u n c t i o n o f the amount o f coating. 1

2

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Savolahti ( 2 9 ) used D R I F T to study the t h e r m a l degradation o f barley protein. T h e sample was l y o p h i l i z e d o r spray-dried. Samples w e r e m i x e d w i t h K B r a n d heated i n a c o n t r o l l e d environment c e l l (from Spectra T e c h ) . Loss



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3800


3000 Cm"

1

Figure 3. Scale-expanded DRIFT spectra in the range 3800 to 2200 cm ~ of solid solutions in KCl (1:50 weight ratio): PVPh~P2VP polymer complex formed with 4:1 initial composition (A); pure PVPh (B); and pure P2VP (C). (Reproduced with permission from reference 27. Copyright 1984.) 1

o f the olefinic C - H stretch absorption was assigned t o oxidation o f the double b o n d . T h e growth o f a strong C = 0 absorption b a n d was observed as was the loss o f intensity associated w i t h O - H , N - H , C - H , a n d C - O groups. T h e amide I and I I bands decreased at 1650 a n d 1540 c m , respectively. - 1

Polymer Fibers. T h e second type o f p o l y m e r investigation using D R I F T spectroscopy is carried out w h e r e g r i n d i n g p o l y m e r fibers o r films w o u l d change the chemistry a n d m o r p h o l o g y o f the system too drastically to allow spectroscopic correlation o f the results w i t h the u n g r o u n d sample. I n 1980, M a u l h a r d t a n d K u n a t h ( 3 0 ) used D R I F T spectroscopy w i t h their o w n optical device a n d a c o m m e r c i a l F T I R to study a silane c o u p l i n g agent o n glass fibers. C o n t r i b u t i o n s f r o m the C - H , C = 0 , a n d C = C v i b r a tions o f the c o u p l i n g agent w e r e clearly seen, b u t useful i n f o r m a t i o n f o r frequencies less than about 1500 c m was not obtained because o f the strong i n f r a r e d absorptions o f the silica substrate. -

1

Investigation o f c o u p l i n g agents o n glass fibers was o f interest to K o e n i g a n d co-workers. A l t h o u g h m u c h o f their early w o r k was c o n d u c t e d using transmission spectroscopy (31), K o e n i g a n d co-workers carried out a n u m b e r o f investigations using D R I F T spectroscopy to study p o l y m e r fibers a n d films.



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C u l l e r et a l . ( 6 ) characterized several o f the i n f r a r e d methods that m a y b e used to obtain i n f r a r e d spectra o f polymers, i n c l u d i n g D R I F T spectroscopy, and p o i n t e d out several o f the advantages a n d disadvantages o f D R I F T spectroscopy. These advantages i n c l u d e g o o d sensitivity a n d little sample preparation; the disadvantages i n c l u d e d the n e e d for a consistent particle size for quantitative results, l i m i t e d d e p t h penetration, a n d that strongly absorbing modes i n a material l i m i t the sensitivity o f the technique i n spectral regions that those modes overlap. T h e authors r e p o r t e d initial spectra o f a c o u p l i n g agent o n silica fibers that closely resembled those o f M a u l h a r d t a n d K u n a t h (30). I n 1984, the technique o f using K B r overlayers to obtain D R I F T spectra o f p o l y m e r fibers was i n t r o d u c e d b y M c K e n z i e et a l . ( 3 2 ) , w h o investigated the c o u p l i n g agent 7-aminopropyltriethoxysilane (7-APS) o n Ε-glass fibers. Ε-glass fiber mats were cut into circles to fit the D R I F T spectroscopy cell. H a d the samples obeyed the K u b e l k a - M u n k relation, it w o u l d have b e e n possible to obtain the spectrum o f the c o u p l i n g agent o n the fibers b y subtracting a spectrum o f the fibers alone f r o m that o f the fibers treated w i t h 7 - A P S . Unfortunately, this m e t h o d d i d not w o r k w e l l . Subtraction o f the spectrum d u e to the Ε-glass substrate f r o m the spectrum o f Ε-glass treated w i t h 7 - A P S y i e l d e d a spectrum i n w h i c h some o f the substrate bands h a d not b e e n subtracted completely a n d some h a d b e e n oversubtracted. T h i s result was an obvious indication o f n o n - K u b e l k a - M u n k behavior, presumably d u e to anomalous dispersion. A n additional p r o b l e m that M c K e n z i e et al. e n c o u n tered was that rotation o f the sample generated p r o f o u n d changes i n the observed spectrum. A s i n their previous study (31) a n d the study b y M a u l hardt a n d K u n a t h ( 3 0 ) , the useful range o f the D R I F T spectra was 4 0 0 0 - 1 6 0 0 c m . T h e lower l i m i t is a significant l i m i t a t i o n because it eliminates virtually all o f what is typically referred to as the fingerprint region o f the i n f r a r e d spectrum f r o m study. U s e o f a relatively t h i n (approximately 5 0 - m g ) layer o f K B r o n top o f the sample scrambled the incident radiation a n d r e m o v e d the observed rotational dependence o f the spectra, as w e l l as the apparent contribution o f specular reflectance. Subtractions c o u l d b e carried out over the range f r o m 4000 to 900 c m w i t h good e l i m i n a t i o n o f the Ε-glass bands a n d w i t h clear spectral contributions f r o m the c o u p l i n g agent. T h e a d d e d K B r r e d u c e d the spectral intensities o f all bands, b u t the r e d u c t i o n factor was greater for the substrate than for the c o u p l i n g agent o n the surface o f the fiber. M c K e n z i e et a l . postulate that the K B r overlayer m e t h o d makes the incident a n d reflected radiation isotropic, w h i c h removes the orientational dependence, a n d increases the average angle o f incidence o f the radiation to the fibers, w h i c h causes greater reflection at the fiber surface a n d less penetration into the fiber itself. M c K e n z i e a n d K o e n i g ( 3 3 ) c o m p a r e d the use o f g r o u n d K B r overlayers o n oriented fibers to the technique o f averaging several different sample orientations ( 3 3 , 34) for obtaining D R I F T spectra o f glass fibers treated w i t h - 1

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a c o u p l i n g agent. E a c h orientation y i e l d e d a different spectrum, but averag i n g several different sample orientations y i e l d e d linear K M plots as a f u n c t i o n o f surface treatment. Unfortunately, the standard deviation o f the average c o m p u t e d i n this way was as h i g h as 3 5 % , a n d the concentration breakpoint corresponding to association o f the c o u p l i n g agent c o u l d not be d e t e r m i n e d . M c K e n z i e a n d K o e n i g then u s e d coarse-ground ( 1 2 0 - 1 8 0 - μ π ι particle size) K B r overlayers to remove the orientation dependence o f the observed spectra. T h e overlayers r e d u c e d the intensity o f the observed bands b y about a factor o f 2, b u t e l i m i n a t e d the orientation dependence o f the spectra a n d enabled the use o f subtraction techniques to study the onset o f association o f the c o u p l i n g agent. B o t h D r i f t spectroscopy a n d photoacoustic spectroscopy ( P A S ) were u s e d b y U r b a n et al. ( 9 ) to investigate stretching-induced phase transitions o f p o l y ( b u t y l terephthalate) fibers a n d to characterize annealed and d r a w n fibers. T h e spectra obtained using the two methods were similar; the P A S studies h a d the a d d e d ability to characterize the orientation of surface species. U s i n g D R I F T spectroscopy, the authors were able to characterize the a l p h a - b e t a phase transition observed u p o n d r a w i n g the fibers a n d observe the relaxation o f the fibers u p o n annealing. C h a t z i et al. ( 3 5 ) used D R I F T spectroscopy w i t h the K B r overlayer technique to study water absorbed i n polyamide fibers ( K e v l a r 49). U s e o f gradually increasing amounts o f K B r as the overlayer demonstrated the loss o f absorbed water due to a nitrogen purge as a f u n c t i o n o f d e p t h i n the p o l y m e r fibers. T h e H 0 i n f r a r e d absorptions were characterized w i t h respect to the nature o f the intermolecular interactions i n the polyamide fibers. 2

Polymer Films. C u l l e r et al. ( 3 6 ) u s e d the K B r overlayer technique to study p o l y m e r films. T h e y investigated a m o d e l system consisting o f a 1.5-μ m poly (vinyl fluoride) ( P V F ) layer over a 2 9 ^ m - t h i c k substrate o f poly(ethylene terephthalate) ( P E T ) . A d d i t i o n o f a surface layer o f K B r increased the relative c o n t r i b u t i o n o f the P V F layer i n the spectra a n d decreased the c o n t r i b u t i o n o f the P E T substrate. F i n e K B r p o w d e r (average particle size less than 75 μ ι η ) h a d a m o r e dramatic effect than course p o w d e r (average particle size 1 0 5 - 2 5 0 μ η ι ) . T h e authors suggest that the presence o f the K B r overlayer increases the average angle o f incidence to the films a n d increases the amount o f light reflected at the film interface, w h i c h makes the technique m o r e sensitive to surface groups as K B r is added. T h e sensitivity o f the m e t h o d to a very t h i n film o f a 7 - A P S c o u p l i n g agent o n P E T was also demonstrated. T h e K B r overlayer technique brought out spectral c o n t r i b u tions f r o m the c o u p l i n g agent, whereas the film o n P E T was transparent i n transmission measurements. C u l l e r et al. postulate that the i n f r a r e d radiation may make several reflections inside the thin-surface film i n this m e t h o d that enhances the sensitivity o f the technique.


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D R I F T spectroscopy was c o m p a r e d to attenuated total reflectance a n d transmittance measurements b y C o l e et al. ( 3 7 ) for the study o f a c a r b o n epoxy composite consisting o f a w o v e n carbon fiber material impregnated w i t h an epoxy resin. T h e sample was p r e p a r e d b y cutting the p r e p r e g (fiber-reinforcement impregnated w i t h partially c u r e d resin) into 1 - c m X 1c m squares that were p l a c e d i n the sample c u p . A sulfone index a n d an epoxide index were calculated f r o m the observed spectra, parameters that are indicative o f t h e hardener to epoxy ratio a n d the degree o f polymerization, respectively. T h e m e t h o d that gave the poorest results i n terms o f quantita tive performance was D R I F T spectroscopy. Some o f the spectra clearly showed the effects o f a large contribution f r o m F r e s n e l reflectance; that is, the derivative-shaped bands d u e to the anomalous dispersion. Several differ ent methods to reduce the F r e s n e l reflectance w i t h K B r overlayers were attempted. A n opaque layer r e d u c e d the spectral contrast o f the spectra to the extent that the m e t h o d c o u l d not b e used. T w o other methods were used to generate very t h i n K B r overlayers, b o t h o f w h i c h increased the spectral contrast a n d r e d u c e d the apparent contribution o f F r e s n e l reflectance, b u t neither o f w h i c h i m p r o v e d the precision o f the measurements to any great extent. T h e variation i n the thickness o f the surface resin layer, w h i c h contributes to variations i n the F r e s n e l reflectance o f the samples, was p r o p o s e d to b e the m a i n reason for the lack o f reproducibility. C o l e et a l . only used one orientation to acquire the spectra: a n d orientation w i t h approximately 8 0 % o f the fibers aligned w i t h the b e a m direction o f the instrument. C o l e et al. also used D R I F T spectroscopy to measure the degree o f crystallinity o n the surface o f a composite material (38). A p r e p r e g (different f r o m the one i n the foregoing study) that consisted o f a unidirectional arrangement o f carbon fibers impregnated w i t h h i g h molecular weight polyphenylene sulfide ( P P S ) was investigated. N o K B r overlayer a n d no rotation o f the sample were used, a n d the orientation was w i t h carbon fibers parallel to the b e a m direction o f the instrument. E x c e l l e n t spectra that resembled spectra obtained w i t h transmission measurements were obtained. T h e spectrum o f a sample w i t h l o w crystallinity was subtracted f r o m the spectrum o f a sample w i t h h i g h crystallinity, a n d a pair o f peaks i n the resulting difference spectrum, whose ratio changed significantly as a f u n c t i o n of annealing, was d e t e r m i n e d . C o l e et a l obtained an excellent correlation between the intensity ratio a n d the enthalpy o f crystallization f o r the p r e p r e g that indicates that the intensity ratio is a good measure o f the crystalfinity o f the composite.

Foams. C h a l m e r s a n d M a c k e n z i e ( 3 9 ) carried out a variety o f investi gations a i m e d at illustrating the utility o f D R I F T spectroscopy for industrial applications. O n e o f the m o r e interesting applications i n their study was the use o f D R I F T spectroscopy f o r the study o f foams. F o a m s are sometimes


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difficult to study by A T R because the pressure p l a c e d o n a f o a m sample is so critical i n the A T R measurement. H o w e v e r , the D R I F T measurement was easily carried out b y s i m p l y c u t t i n g the f o a m to the appropriate size, a n d the identification o f a variety o f foams was demonstrated.


Catalytic Studies Using DRIFT Spectroscopy H a m a d e h et al. (40, 41) w e r e the first to develop a heatable-evacuable c e l l that c o u l d interface w i t h D R I F T optics. T h i s c e l l was u s e d to study the i n f r a r e d absorption spectroscopy o f alumina-supported r h o d i u m clusters u n der varying amounts o f C O , w h i c h represented different coverages. T h e observed peaks i n the D R I F T spectrum w e r e assigned b y comparison o f the D R I F T spectra w i t h previously assigned i n f r a r e d spectra o f r h o d i u m carbonyl species. I n a recent extension o f these studies, V a n E v e r y a n d Griffiths (42) obtained detection limits approaching 1 0 " monolayers o f C O adsorbed o n alumina-supported r h o d i u m a n d observed that equilibration of the spectra w i t h t i m e is a c o m p l i c a t i n g factor that depends o n the pressure o f C O to w h i c h the catalyst is exposed. S m a l l amounts o f C O equilibrate relatively rapidly o n the surface o f the catalyst whereas large amounts o f C O equilibrate over l o n g times, presumably because this equilibration involves diffusion into the pores o f the support. Baseline changes i n the spectra as a function o f the pressure o f a d m i t t e d C O w e r e also observed a n d assigned to pressure-induced changes i n the sample scattering coefficient. T h e results o f V a n E v e r y a n d Griffiths i n d i c a t e d that m e r c u r y c a d m i u m telluride detectors are susceptible to c o n d u c t i o n b a n d saturation effects w h e n hot samples are b e i n g observed, an effect that w i l l lead to signal loss for high-temperature samples. T h e y showed that deuterated triglycine sulfate or triglycine sulfate detectors do not exhibit this susceptibility. 6

W i t h regard to using D R I F T spectroscopy as an i n situ technique, F i g u r e 4 is a schematic diagram o f an environmental c h a m b e r that has b e e n used i n our laboratory to obtain i n situ D R I F T spectra. T h e c h a m b e r is similar i n design to c o m m e r c i a l l y available cells, although most currently available cells are m u c h more sophisticated i n design than the c e l l shown. T h e sample sits o n top o f a post that can be heated v i a the cartridge heaters, a n d the temperature o f the c e l l is m o n i t o r e d w i t h the t h e r m o c o u p l e . M o r e sophisti cated designs call for a t h e r m o c o u p l e to b e p l a c e d directly i n contact w i t h the sample, so that an accurate reading o f the sample temperature can be obtained (43, 44). T h e cell allows for control o f the internal atmosphere, either v a c u u m or a gas mixture, t h r o u g h connections to an external gas m a n i f o l d a n d a v a c u u m p u m p . C o m m e r c i a l l y available cells allow for c o o l i n g the section o f the c e l l containing the w i n d o w s , because the w i n d o w seals are usually the l i m i t i n g consideration w i t h regard to the ultimate temperature attainable. T h e flow p a t h o f any gases f r o m an external gas m a n i f o l d takes the



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Figure 4. Schematic diagram of a DRIFT spectroscopy environmental chamber.

gases directly t h r o u g h the sample material. T h i s flow-through o f gases compresses the p o w d e r to a certain extent, w h i c h gives rise to enhanced b a n d intensities c o m p a r e d w i t h intensities observed without the flow. T h e samples should not, however, be mechanically pressed to any great extent, because compression prohibits gas flow through the sample (40). I n addition, after flowing through the sample material, the gas stream can b e s a m p l e d b y gas chromatography ( G C ) or gas chromatography-mass spectroscopy ( G C - M S ) , an application suggested b y H a m a d e h et al. (40) a n d i m p l e m e n t e d b y M a r o n i et al. (43). M a r t i n a n d Zabransky (45) studied various forms o f the zeolite Z S M - 5 ( H - Z S M - 5 , F e - Z S M - 5 , a n d N a - Z S M - 5 ) a n d sihcalite for the conversion o f


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m e t h a n o l to d i m e t h y l ether. T o study the adsorption process a n d products, neat samples o f the zeolites i n a c o m m e r c i a l environmental chamber w e r e used. Changes i n the O - H stretching region ( 3 5 0 0 - 3 7 5 0 c m ) that oc c u r r e d as methanol adsorbed o n the zeolites w e r e studied to determine w h i c h sites w e r e b e i n g o c c u p i e d b y the m e t h a n o l as it adsorbed as a f u n c t i o n o f temperature. A t r o o m temperature, adsorption o f methanol onto the zeolites o c c u r r e d at only one O H site; at 150° C , two sites were i n v o l v e d for the active zeolites H - Z S M - 5 a n d F e - Z S M - 5 , a n d o n l y one site is i n v o l v e d for Na-ZSM-5. Downloaded by NORTH CAROLINA STATE UNIV on January 22, 2013 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/ba-1993-0236.ch013

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M e t h a n o l fragmentation products w e r e studied b y observation o f the infrared absorption pattern i n the C - H stretching region ( 2 7 0 0 - 3 1 0 0 c m ) . O n H - Z S M - 5 , adsorption o f m e t h a n o l gave rise to i n f r a r e d bands that c o u l d be assigned to the methoxide group at r o o m temperature. A t higher tempera ture (150 °C), the absorption pattern changed a n d the result c o u l d no longer be i d e n t i f i e d as a methoxide. F e - Z S M - 5 gives the same sequence o f p r o d ucts. M e t h a n o l adsorption o n N a - Z S M - 5 a n d silicalite resulted i n the f o r m a tion o f the surface methoxide o n l y at r o o m temperature a n d at 200 °C. - 1

T h o m p s o n a n d P a l m e r ( 2 2 ) u s e d i n situ D R I F T spectroscopy to study the reaction o f S 0 w i t h C a C 0 . C a C O is u s e d as an adsorbent for S 0 i n dry-injection flue gas cleanup systems. T h e evolution o f adsorbed S 0 as a function o f t i m e a n d temperature f r o m sulfite to sulfate was investigated. B a s e d o n their assignment o f the observed vibrational bands a n d o n their previous i n f r a r e d photoacoustic measurements o n p u r e C a C 0 , T h o m p s o n a n d P a l m e r p r o p o s e d a m e c h a n i s m for the adsorption o f S 0 a n d the conversion of the physisorbed species to S 0 or S O f depending on temperature. 2

3

s

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3

2

3

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-

O n e o f the limitations o f the D R I F T technique is the n e e d to dilute strongly absorbing samples i n a nonabsorbing matrix to avoid reststrahlen effects. I n their D R I F T studies, T h o m p s o n a n d P a l m e r u s e d N a C l as the diluent (1:10 C a C 0 : N a C l ) , a n d i n their photoacoustic studies using p u r e C a C O , bands that w e r e assigned to C a S 0 w e r e observed. These bands w e r e not observed i n the D R I F T studies. Because N a C l , as w e l l as other alkali additives, is k n o w n to accelerate the conversion o f C a S 0 to C a S 0 , T h o m p s o n and P a l m e r point out that the n e e d to use the diluent i n the D R I F T studies is potentially a very serious drawback, especially at elevated temperatures. 3

s

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M a r o n i a n d co-workers (43, 46, 47) studied a variety o f molecular sieve catalysts b y using D R I F T spectroscopy. T o avoid problems associated w i t h alkali halide diluents, p u r e zeolite materials w e r e used a n d regions o f the spectrum that can show contributions f r o m reststrahlen effects ( w h i c h i n cludes m u c h o f the region b e l o w 2000 c m for these types o f samples) w e r e not investigated. Investigations w e r e l i m i t e d to the O - H a n d C - H (or O - D and C - D ) stretching regions ( f r o m approximately 2000 to 4000 c m ) . Because m u c h o f the catalytic activity o f these zeolite catalysts is d e t e r m i n e d - 1

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b y the nature o f the acid sites, a significant amount o f i n f o r m a t i o n c o u l d be obtained even w i t h this l i m i t a t i o n . T h e differences i n the n u m b e r a n d type o f O - H sites were studied b y M a r o n i a n d collaborators o n three different zeolites as a f u n c t i o n o f metal atom substitution ( H - Z S M - 5 c o m p a r e d to A F S - H - Z S M - 5 , a ferrisilicate zeolite) a n d as a f u n c t i o n o f aluminosilicate framework ( H - Z S M - 5 c o m p a r e d to H - o f f r e t i t e ) . D R I F T spectroscopy w i t h subsequent gas c h r o m a t o g r a p h y -


mass spectrometry ( G C - M S ) was then used f o r determination o f products to compare the reactivity o f the ferrisilicate zeolite A F S - H - Z S M - 5 a n d its sodium exchanged f o r m A F S - N a - Z S M - 5 t o w a r d the conversion o f m e t h a n o l to higher hydrocarbons as a function o f temperature a n d composition. T h e experimental setup i n c l u d e d a provision f o r sampling the exhaust gases o f the D R I F T environmental c h a m b e r w i t h a n on-line gas chromatograph a n d w i t h a sample b u l b for G C - M S analysis. T h e sodium-exchanged forms were f o u n d to b e 2 orders o f magnitude less reactive. T h e authors p o i n t out that probably the most effective use o f the D R I F T technique i n i n situ measurements is not as an absolute technique, b u t rather as a m o n i t o r o f changes that o c c u r as a f u n c t i o n o f experimental variables. A D R I F T spectrum is obtained o f some initial sample; t h e n this spectrum is subtracted f r o m subsequent spectra. T h e set o f difference spectra shows changes i n the i n f r a r e d spectrum as a f u n c t i o n o f t i m e , temperature, a n d reaction conditions i n a very clear way. A stacked plot o f these spectra is a dramatic m e t h o d f o r showing spectroscopic changes as a function o f the various experimental parameters. M a r o n i a n d co-workers (46) used the same experimental arrangement to study the formation o f Z S M - 5 f r o m the template t e t r a p r o p y l a m m o n i u m cation ( T P A - Z S M - 5 ) a n d to study the reaction o f methanol, ethanol, a n d ethylene o n H - Z S M - 5 f o r t h e formation o f higher hydrocarbons. U s i n g the D R I F T technique i n conjunction w i t h on-line G C analysis o f reaction p r o d ucts, the investigators were able to show that O H groups finking a l u m i n u m a n d silicon atoms ( S i - O H - A l sites) are the sites at o r near w h i c h the alcohol decomposition products adsorb, a n d that t e r m i n a l O H groups b o u n d to silicon atoms ( S i O H sites) are labile b u t d o not appear to b e active i n the catalysis. T h e shape o f the C - H stretch absorption d u e to the organic fragment is indicative o f the activity o f the catalyst for a particular reactant. A t temperatures Τ > 673 Κ, the reaction continues to generate products as shown b y the G C results, b u t the surface-adsorbed species are not h e l d tightly enough to r e m a i n o n the surface after a nitrogen purge. T h e onset o f a b r o a d i n f r a r e d absorption i n the range 3 0 0 0 - 3 2 0 0 c m

-

1

is attributed to

polycyclic aromatic coke precursors. Iton et a l . (47) used D R I F T spectroscopy to examination the stabiliza tion a n d redox chemistry o f C o (III) i n aluminophosphate zeolites. T h e y were able to study the oxidation o f H

2

b y the stabilized C o (III) at elevated

temperatures t h r o u g h an increased O H absorption, a n d the 50 a n d 100 °C


370

STRUCTURE-PROPERTY

oxidation of N O to N O


POLYMERS

b y C o (III) through the infrared absorption o f the

+

nitrosyl cation at 2162 c m

- 1

a n d coordinated nitrosyl at 1674 a n d 1562 c m

- 1

.

T h e species adsorbed o n a silver catalyst d u r i n g the oxidation of acetaldehyde was analyzed b y K a n n o a n d Kobayashi (48)

using D R I F T spectroscopy.

T h e D R I F T spectra obtained f r o m the reaction were c o m p a r e d w i t h similar spectra for silver salts, such as silver acetate, silver lactate, silver ketenide, a n d silver carbonate. T h e s e comparisons, facilitated assignment of observed


peaks i n the i n f r a r e d spectrum o f the catalyst as b e i n g due to silver acetate intermediates, a strongly adsorbed acetaldehyde, a n d an adsorbed alcohol. T h e spectra obtained w i t h the D R I F T technique were c o m p a r e d w i t h spectra for similar samples obtained b y transmission methods; the D R I F T m e t h o d y i e l d e d m u c h better spectra i n the lower frequency region (1100 c m

- 1

> v).

A l t h o u g h K a n n o a n d Kobayashi d i d dilute their catalyst samples i n K B r d u r i n g the actual oxidation reactions, w h i c h w e r e c a r r i e d out at

elevated

temperature (160 °C), they d i d not address the possibility of perturbation o f the catalysis results due to the h i g h concentration o f alkali halide. D R I F T spectroscopy was u s e d b y F r i e s a n d M i r a b e l l a (49) commercial T i C l - A l C l 3

3

to study

catalysts used i n the polymerization of propylene.

T h e y were able to m o n i t o r the decay o f the 1 6 4 0 - c m " absorption due to the 1

C=C

group o f gas-phase propylene a n d the simultaneous growth of an

absorption at 1370 c m

- 1

due to a m e t h y l deformation m o d e of polypropylene

o n the catalyst surface. T h e 1 8 2 0 - e m

- 1

b a n d o f propylene, w h i c h corre

sponds to the first overtone o f the t e r m i n a l C H

2

wag, p r o v i d e d a m o r e

reliable measure o f the loss o f propylene d u e to the lack o f overlap b y other spectral bands. S m y r l a n d F u l l e r ( 5 0 ) u s e d i n situ diffuse reflectance spectroscopy to examine the reactions o f neat coal samples. T h e s e authors point out that coal samples n e e d no diluent to be studied b y D R I F T spectroscopy, because coal effectively forms its o w n dispersion m e d i u m . B r i m m e r a n d Griffiths

(24)

suggested a reason for this effect, based o n the idea that the coal particles absorb so strongly that the effective penetration d e p t h is very small (on the order o f 10 μ η ι ) so that a sample n o r m a l l y considered too strongly absorbing to study b y D R I F T spectroscopy actually yields quite good spectra. V e r y careful D R I F T investigations b y S m y r l a n d F u l l e r y i e l d e d well-resolved spectra for the entire m i d - i n f r a r e d region. U s e o f the acetylation reaction as a probe of the O H sites o n the coal a n d use o f the i n f r a r e d results to determine exactly what types o f esters w e r e f o r m e d b y the acetylation allowed S m y r l a n d F u l l e r to propose that the particular type o f coal u n d e r study contained only phenolic-type O H groups a n d n o alkyl-type O H groups. T h e m e t h o d holds m u c h promise for the study o f coal O H - s i t e distribution a n d coal chemistry i n general. T h e use of D R I F T spectroscopy for the study o f carbon-supported metal catalysts, i n particular metal carbonyls, was explored b y V e n t e r et al. (44,


13.

MITCHELL

51-53). T h e i r initial investigations (44, 51, 52) focused o n the adsorption o f F e ( C O ) , R u ( C O ) , and O s ( C O ) o n amorphous carbon black. T h e i r studies investigated, i n a very careful manner, the effects o f a variety o f experimental parameters o n the observed spectra, b o t h theoretically a n d experimentally. T h e effects o f m e t a l loading, choice o f b a c k g r o u n d spectrum, various forms o f baseline correction, and the diluent, b o t h the d i l u t i o n ratio a n d the type o f diluent, w e r e observed and the linearity o f the K u b e l k a - M u n k transform of the data for a range o f metal loadings o n these carbon-supported samples was established. T h i s range d e p e n d e d o n the d i l u t i o n ratio, a n d the choice o f d i l u t i o n ratio i n v o l v e d a c o m p r o m i s e between detection limitations and linearity o f the transformed data. T h e final choice for d i l u t i o n ratio was 1:200 b y weight for carbon-supported catalyst to diluent. T h e choice for the diluent itself was a c o m p r o m i s e between the effect o f the diluent o n the observed chemistry a n d the effect o n the observed spectroscopy. A l O a n d S i 0 h a d the least effect o n the chemistry but t e n d e d to adsorb water, w h i c h caused baseline drifts i n the spectra a n d made the spectra h a r d to interpret. Ultimately, C a F was chosen as the diluent. 3


371


1 2

3

1 2

3

1 2

2

s

2

2

T h e choice o f material to generate the b a c k g r o u n d spectrum also af fected the final K u b e l k a - M u n k spectra. Rather than use a nonabsorbing sample (i.e., p u r e diluent) as the b a c k g r o u n d " s a m p l e " , V e n t e r et al. used a sample consisting o f decarbonylated carbon black i n the appropriate ratio, 1:200, i n the diluent. T h u s , the K u b e l k a - M u n k spectra w e r e calculated relative to a b a c k g r o u n d that absorbed radiation at the same frequencies as the catalyst support. T h i s b a c k g r o u n d was not expected to have any effect o n the linearity o f the K M intensities for the bands due to the adsorbed C O - c o n t a i n i n g species, a n d the use o f this material to generate the back g r o u n d spectrum y i e l d e d m u c h better signal-to-noise ratios a n d better base line correction. I n later investigations, V e n t e r et al. (53) used the technique d e m onstrated i n earlier studies to examine the t h e r m a l decomposition o f five different metal carbonyls o n carbon: F e ( C O ) , N E t [ F e M n ( C o ) ] , M n ( C O ) , K [ H F e ( C O ) ] , a n d K [ F e M n ( C O ) ] . D R I F T spectroscopy alone was used to measure the rate constant for decarbonylation as a f u n c t i o n o f temperature a n d to calculate the activation energy o f decarbonylation for these five carbon-supported species u n d e r H e a n d H . T h e decomposition kinetics o f all o f the species f o l l o w e d first-order kinetics, a n d the rates as a function o f temperature w e r e fitted w e l l w i t h the A r r h e n i u s expression. T h e Fe (CO) clusters w e r e the least stable, w i t h rate constants for decomposi tion o f about 1 m i n a n d E values o f 7 5 - 8 8 k j / m o l . M n ( C O ) p r o v e d to be the most stable c o m p o u n d w i t h decomposition rate constants o f approxi mately 0.001 m i n at 350 Κ a n d activation energy E values for the decomposition o f 1 3 4 - 1 5 5 k j / m o l . T h e decomposition routes o f the M n containing clusters w e r e f o u n d to d e p e n d o n the atmosphere u n d e r w h i c h the decomposition took place. 3

2

1 0

3

n

2

1 2

4

2

1 2

1 2

2

3

1 2

- 1

- 1

A

2

1 0

A



372


NAVENUMBER

Figure 5. Evolution of the DRIFT spectrum of supported Cu(acac), as a function of metal loading. The lowest spectrum is that of a 0.92% wt/wt Cu sample on Si0 (Cab-O-Sil). The spectra of six different metal loadings are shown in sequence from bottom to top representing metal loadings of 0.92, 1.7, 2.5, 3.8, 4.5, and 5.6% Cu on Si0 , respectively. 2

2

A l l o f the foregoing data was generated b y studying the i n f r a r e d absorp tion o f the C O species, n o m i n a l l y 1 9 0 0 - 2 1 0 0 c m . T h e region o f C H stretch absorption y i e l d e d some i n f o r m a t i o n c o n c e r n i n g the types o f products that might b e expected. T h e K - c o n t a i n i n g catalysts showed significantly greater i n f r a r e d absorption d u e to C H (as opposed to C H ) groups, p r e s u m ably o n the support surface. T h i s i n f o r m a t i o n is consistent w i t h earlier observations that the presence o f potassium promotes chain growth i n the products o f the syngas reaction. - 1

2

3

K e n v i n a n d co-workers (54-56) showed that D R I F T spectroscopy is very useful for the study o f supported catalyst precursors o n oxide surfaces. T h e i r study o f C u ( a c a c ) o n S i 0 demonstrated that the D R I F T technique c o u l d be used to show the onset o f multilayer formation o f the supported complex a n d that subtraction o f K M plots o f the first-layer species f r o m spectra o f samples that h a d m u l t i p l e layers o f C u ( a c a c ) o n the silica surface y i e l d e d a spectrum o f the overlayers; that is, the C u ( a c a c ) species not i n direct contact w i t h the silica surface. F i g u r e 5 shows a spectrum o f the impregnated silica p o w d e r as a f u n c t i o n o f metal loading. T h e onset o f the absorption at 1552 c m " is thought to indicate the presence o f m u l t i p l e layers o f C u ( a c a c ) o n the silica surface. F i g u r e 6 is a spectrum that results f r o m a subtraction o f a less-than-a-monolayer spectrum f r o m a spectrum o f a sample containing 2

2

2

2

1

2



13.

MITCHELL


373

m o r e than o n e monolayer o f complex. T h e result is i n remarkably g o o d agreement w i t h that o f p u r e C u ( a c a c )

2

as shown i n F i g u r e 7. K e n v i n

(55)

a p p l i e d the D R I F T technique to the study o f a variety o f similar complexes, including Pd(acac) , Pt(acac) , and Cr(acac) . T h e palladium and platinum 2

2

3

complexes showed behavior similar to that shown b y the copper system, although less adsorption for these complexes was observed. T h e c h r o m i u m complex showed very little adsorption a n d n o spectral changes f o r the material that

d i d adsorb, w h i c h might b e expected

f o r the octahedral

complex.

Summary D i f f u s e reflectance spectroscopy i n the m i d - i n f r a r e d range is a p o w e r f u l technique for the study o f certain types o f samples, i n c l u d i n g polymers, heterogeneous catalysts, a n d n o v e l materials. D R I F T has great potential f o r the study o f a w i d e variety o f processing p r o b l e m s a n d for i n situ investiga tions, b o t h as a stand-alone technique a n d i n conjunction w i t h G C a n d G C - M S . W h a t must b e kept i n m i n d i f useful results are expected are the approximations inherent i n the technique such as optically dilute samples, the problems a n d the solutions associated w i t h specular a n d diffuse reflectance, a n d the n e e d f o r reproducible sample preparation.


Fresnel


374


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RECEIVED for review July 15, 1991. ACCEPTED revised manuscript September 25,

1992.


Fundamentals and Applications of Diffuse Reflectance Infrared Fourier

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