Photoacoustic Spectroscopy of Catalyst Surfaces - ACS Publications

20 Photoacoustic Spectroscopy of Catalyst Surfaces Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 18, 2016 | http://pubs.acs.org Publication Date: April 5, 1984 | doi: 10.1021/bk-1984-0248.ch020

E. M. EYRING, S. M. RISEMAN, and F. E. MASSOTH Departments of Chemistry and Fuels Engineering, University of Utah, Salt Lake City, UT 84112

Microphonic Fourier transform infrared photoacoustic spectroscopy (FT-IR/PAS) has emerged as a useful tool for characterizing fractions of a monolayer of organic species adsorbed on opaque, high surface area samples. Such a study of calcined and sulfided hydrodesulfurization catalysts will be discussed. Specifics such as indications that Bronsted acidity may be associated with polymolybdate structure and the observation of a low frequency feature at 1310 reciprocal centimeters will be described along with generalizations regarding the present limitations of this technique. Although photoacoustic spectroscopy (PAS) was f i r s t conceived by Bell and h i s contemporaries over one hundred years ago ( 1 - 3 ) , the a p p l i c a t i o n s of PAS to the study of surfaces have a l l emerged within the l a s t ten y e a r s . The d e c i s i v e f a c t o r i n t h i s b e l a t e d renaissance of i n t e r e s t i n PAS was the p u b l i c a t i o n of the onedimensional Rosencwaig-Gersho (R-G) model (4) of PAS with microphonic d e t e c t i o n . In Figure 1 a c y l i n d r i c a l PAS sample c e l l i s depicted by a rectangle one end of which i s i l l u m i n a t e d by a beam of l i g h t chopped at an audio frequency. The l i g h t beam traverses a t i g h t l y s e a l e d , transparent window, passes through the transparent gas behind the window, and i s i n c i d e n t upon a s o l i d sample. Energy absorbed by the sample surface from the i n c i d e n t l i g h t beam may be converted by r a d i a t i o n l e s s t r a n s i t i o n s to a thermal wave that returns (by thermal d i f f u s i o n ) to the sample surface and warms the t h i n l a y e r of gas i n contact with the surface. This l a y e r of p e r i o d i c a l l y heated gas acts as a "thermal piston" on the r e s t of the gas in the c e l l and causes a sound wave of the same frequency as that at which the l i g h t beam was chopped but of delayed phase. These a c o u s t i c waves i n the gas impinge on a microphone l o c a t e d at the end of a duct (see Figure 2) t h a t prevents s c a t t e r e d l i g h t from s t r i k i n g the microphone and producing spurious s i g n a l s . Thermal p r o p e r t i e s of the material used as a backing to the sample can a l s o i n f l u e n c e the i n t e n s i t y and phase of the PA signal detected by the microphone. 0097-6156/84/0248-0399$06.00/0 © 1984 American Chemical Society In Catalytic Materials: Relationship Between Structure and Reactivity; Whyte, Thaddeus E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

CATALYTIC MATERIALS

MODULATED

LIGHT

TRANSPARENT WINDOW

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 18, 2016 | http://pubs.acs.org Publication Date: April 5, 1984 | doi: 10.1021/bk-1984-0248.ch020

TRANSDUCING GAS

THERMAL PISTON SAMPLE BACKING MATERIAL u

(L+L ) Figure 1. Schematic diagram of a photoacoustic c e l l used to develop the one-dimensional theory of microphonic PAS by Rosencwaig and Gersho.

MIRROR

WINDOW

STAINLESS STEEL CELL HOUSING FOR PREAMPLIFIER AND MICROPHONE

SAMPLE

ACOUSTIC

CHANNEL

Figure 2. Schematic diagram of a photoacoustic c e l l f o r s o l i d samples that d e p i c t s the a c o u s t i c channel (diameter exaggerated) to the microphone from the gas f i l l e d sample chamber.

In Catalytic Materials: Relationship Between Structure and Reactivity; Whyte, Thaddeus E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

20.

EYRING ET AL.

401

Photoacoustic Spectroscopy of Catalyst Surfaces


The s o l u t i o n to t h i s boundary value problem was approximated by Rosencwaig and Gersho f o r s i x d i f f e r e n t cases (4) one of which, a thermally t h i c k but o p t i c a l l y t h i n sample, often a p p l i e s to l a y e r s adsorbed on heterogeneous c a t a l y s t s . The photoacoustic signal a r i s e s from the chemisorbed species and the support. Optical p r o p e r t i e s of the chemisorbed monolayer are u s u a l l y paramount, and t h i s l a y e r i s much thinner than the substrate and support that experience h e a t i n g . The photoacoustic signal i n t e n s i t y Q i s given by the p r o p o r t i o n a l i t y

Qα 3yf

(1)

3 / 2

where 3 = o p t i c a l a b s o r p t i v i t y ( c m * ) , y Ξ thermal d i f f u s i o n length (cm), and f = beam chopping frequency ( s ' ) . An important experimental i n s i g h t follows from equation 1: The PA signal to noise r a t i o , S/N, r a p i d l y diminishes with i n c r e a s i n g chopping frequency. Thus microphonic PA measurements are often made at i n c i d e n t l i g h t chopping frequencies lower than 500Hz. High i n t e n s i t y of the i n c i d e n t l i g h t beam i s a l s o advantageous, and high wattage arc lamps are therefore f r e q u e n t l y used f o r PAS at u l t r a v i o l e t and v i s i b l e wavelengths. Some advantage i s found i n using helium to carry the sound wave from the sample surface to the microphone (because of the high thermal c o n d u c t i v i t y of He), but a more important c o n s i d e r a t i o n i n the choice of a gas i s i t s transparency: The PA e f f e c t i s much l a r g e r in gases absorbing electromagnetic r a d i a t i o n (where i t i s c a l l e d the o p t a c o u s t i c e f f e c t ) than in l i q u i d s or s o l i d s . Thus a t r a c e of C 0 ( g ) , f o r example, can overwhelm PA s i g n a l s from a surface i n the 2310 to 2380 cm" region of the i n f r a r e d spectrum (see Figure 3 ) . The depth i n the sample surface from which the PA signal comes depends on the beam chopping frequency. At low chopping frequencies s p e c t r a l information comes from greater depths i n the sample. In other words, i f one speeds up the motor of the d e v i c e , such as a fan b l a d e , that i s chopping the i n c i d e n t l i g h t beam, not only w i l l S/N d i m i n i s h , but the sample w i l l a l s o be probed at a shallower depth below i t s s u r f a c e . This a b i l i t y to y i e l d subsurface s p e c t r a l and thermal information i s a p e c u l i a r advantage of PAS over r e f l e c t a n c e and transmission spectroscopies that s t i l l remains to be widely e x p l o i t e d (5). In detector noise l i m i t e d spectroscopies such as PAS i t i s advantageous to enhance the throughput of energy (Jacquinot's advantage) by u t i l i z i n g a Michel son i n t e r f e r o m e t e r . One then F o u r i e r transforms (FTs) the r e s u l t i n g interferogram to y i e l d a PA spectrum that q u a l i t a t i v e l y resembles an absorption spectrum. Thus while one never sees commercial FT spectrometers f o r u l t r a v i o l e t - v i s i b l e (UV-VIS) absorption measurements (because p h o t o m u l t i p l i e r tubes are much q u i e t e r detectors than are microphones), FT-YIS/PA spectrometers have been b u i l t that permit speedier a c q u i s i t i o n of high S/N photoacoustic spectra ( 6 - 7 ) . -

1

2

1



402

CATALYTIC MATERIALS

NAVENUMBER5

Figure 3. Photoacoustic i n f r a r e d spectrum of gaseous C0£ obtained i n a microphonic PAS c e l l f o r s o l i d samples when the operator exhaled once i n t o the c e l l before c l o s i n g . 100 scans, 0.5 cm resolution. This i l l u s t r a t e s the l a r g e photoacoustic signal a r i s i n g from gas phase samples and the high r e s o l u t i o n a t t a i n a b l e .



403


20. EYRING ET AL.

Microphonic detection can be used in such FT/PA experiments i n which case the R-G t h e o r e t i c a l arguments (4) s t i l l apply. In s i t u a t i o n s where absorption of the i n c i d e n t r a d i a t i o n by the transducing gas i s troublesome a p i e z o e l e c t r i c transducer (made from barium t i t a n a t e , f o r example) can be attached to the sample (or sample cuvette i n the case of l i q u i d s ) to detect the thermal wave generated i n the sample by the modulated l i g h t (8,9). The low frequency, c r i t i c a l l y damped thermal wave bends the sample and transducer thus producing the p i e z o e l e c t r i c response. The p i e z o e l e c t r i c transducer w i l l a l s o respond to a sound wave in the s o l i d or l i q u i d but only e f f i c i e n t l y at a resonant frequency of the transducer t y p i c a l l y of the order of 10 to 100 KHz (see Figure 4 ) . Thus neither in the case of microphonic nor p i e z o e l e c t r i c detection i s the PA e f f e c t s t r i c t l y an a c o u s t i c phenomenon but rather a thermal d i f f u s i o n phenomenon, and the term "photoacoustic" i s a now well e s t a b l i s h e d misnomer. The chemist generally f i n d s i n f r a r e d s p e c t r a l data to be very much more informative than UV-VIS data f o r i d e n t i f y i n g species on surfaces. For t h i s reason the discovery by Rockley (10) and Y i d r i n e (11) that photoacoustic s p e c t r a l measurements can be performecTcbnveniently on commercial FT-IR spectrometers by s u b s t i t u t i n g a microhponic (or p i e z o l e c t r i c ) PA detector f o r the usual deuterated t r i g l y c i n e s u l f a t e (DTGS) i n f r a r e d detector was of c a p i t a l importance. A schematic representation of the adaptation at the U n i v e r s i t y of Utah of a Ni col e t 7199 FT-IR spectrometer f o r FT-IR/PAS i s shown i n Figure 5. Quality of the PA s p e c t r a l data can be improved by s e t t i n g the microphonic sample c e l l on v i b r a t i o n i s o l a t i o n mounts, foam rubber, or other damping m a t e r i a l s to i n t e r c e p t otherwise troublesome low frequency v i b r a t i o n s a r i s i n g from c r y o s t a t s or other mechanically noisy equipment i n the v i c i n i t y of the spectrometer. No beam chopping device i s shown i n Figure 5. Motion of the moving mirror in the Michel son interferometer i s e q u i v a l e n t to beam chopping and the frequency f i s given by f= 2 w c "

1

= 2 vv

(2)

where ν = mirror speed (cm s " ) and v= i n f r a r e d frequency (cm" ). If a s t e p - a n d - i n t e g r a t e mode i s s e l e c t e d f o r the m i r r o r motion, the photoacoustic measurements are a l l made at a s i n g l e audio frequency. This has the advantage that the "absorbances" measured at a l l wavelengths of the IR spectrum are f o r the same depth below the sample s u r f a c e . This also f a c i l i t a t e s l o c k - i n d e t e c t i o n thus improving S/N. Unfortunately, the t y p i c a l presently a v a i l a b l e commercial FT-IR spectrometer i s "rapid scan" and the mirror sweeps with a continuous motion that produces a higher chopping frequency at shorter wavelengths. Thus, f o r example, when the interferometer mirror i s moving at a speed of 0.112 cm s " the chopping frequency i s only 90 Hz at 400 cm" but has increased to 900 Hz at 4000 cm" . Thus the photoacoustic signal i s coming from d i s t i n c t l y d i f f e r e n t depths i n the sample 1

1

1

1

1


404

CATALYTIC MATERIALS

> ~ 300o c

" / 200-

3 Ο

100;

σ

-X30r


ο 10

1 1 -^H 100 IK Ι0Κ Chopping Frequency (Hz)

Ι00Κ

Figure 4. Photoacoustic signal measured in a sample l i q u i d with an attached p i e z o e l e c t r i c transducer having a resonant frequency of several tens of thousands of kilohertz. Note the change in scale of the amplitude and thus the much greater s e n s i t i v i t y of the detector at low l i g h t chopping f r e q u e n c i e s . Argon ion l a s e r l i g h t source, 400 mW, λ = 488 nm; sample 25 ug/mL BaS04 powder suspended i n aqueous g l y c e r i n e . Reproduced with permission from Ref. 21 copyright 1980, American Chemical Society. Fixed Mirror Moveable,, Mirror

JΊ X

Selectable Translation Velocity V

-J Polychromatic Light Source (e.g. Globar)

Beam/ Splitter

I Translator IR Window Duct

Preamp

Gas Filled Sample Chamber

ADC of Nicolet Computer

Fast Fourier Transform | Computer

X

Sample

Interface & Amplifier

Adjustable High and Low Pass Analog Filters

Microphone F T / P A Spectrum on Plotter

Figure 5. Schematic diagram of the adaptation of a N i c o l e t 7199 FT-IR spectrometer f o r photoacoustic measurements on s o l i d samples.



20.

EYRING ET AL.


405

depending upon i n c i d e n t wavelength and a complete mid-IR spectrum f o r a p a r t i c u l a r sample surface depth can only be made by changing the mirror motion to many d i f f e r e n t successive constant speeds and then combining information from d i f f e r e n t spectra to get a composite spectrum f o r a s i n g l e sample depth. A serious l i m i t a t i o n of the new, more a f f o r d a b l e , lower r e s o l u t i o n FT-IR spectrometers i s that they often do not o f f e r more than one r a p i d scan mirror speed. This poses no problem f o r ordinary FT-IR work but makes these u n i t s d i s t i n c t l y l e s s a t t r a c t i v e than the top of the l i n e FT-IR spectrometers f o r PAS work. One other operational d e t a i l merits b r i e f mention before a p p l i c a t i o n s to surface spectroscopy are considered. Infrared sources d e c l i n e markedly in i n t e n s i t y at longer wavelengths and therefore PA spectra must be source i n t e n s i t y normalized before peak heights can be a s c r i b e d any q u a n t i t a t i v e s i g n i f i c a n c e . It has sometimes been mistakenly supposed that the PA spectrum of graphite could be used to normalize i n f r a r e d PA s p e c t r a . Depending on the source of the g r a p h i t e , one obtains d i s t i n c t l y d i f f e r e n t IR/PA spectra (frequently caused by adsorbed species) and the response of the DTGS detector of an IR spectrometer turns out to be a more accurate measure of v a r i a b l e source i n t e n s i t y (12). A normalization technique (13) r e q u i r i n g measurement of the spectrum at two d i f f e r e n t mirror v e l o c i t i e s and c o r r e c t e d by black body spectra taken at the same two v e l o c i t i e s appears to be the best normalization method reported thus f a r . Light s c a t t e r i n g by the sample can cause c o r r e c t a b l e (14) e r r o r s in photoacoustic s p e c t r a , p a r t i c u l a r l y at v i s i b l e aTiH shorter wavelengths. However, at m i d - i n f r a r e d wavelengths t h i s i s no longer an important c o n s i d e r a t i o n . Methods of applying PAS to the study of l i q u i d s and h i g h l y transparent s o l i d s are now well e s t a b l i s h e d (9) but are inappropriate to the present d i s c u s s i o n . APPLICATIONS In seeking i n t e r e s t i n g a p p l i c a t i o n s of FT-IR/PAS one u s u a l l y looks f o r samples of maximum suface area and high o p a c i t y . Not s u r p r i s i n g l y many heterogenous c a t a l y t i c systems q u a l i f y . In the f i r s t stage of such an i n v e s t i g a t i o n one p r e f e r s to examine a sample system that has been p r e v i o u s l y c h a r a c t e r i z e d s u c c e s s f u l l y by conventional transmission-absorbance type s p e c t r a l measurements. Two such well studied systems are p y r i d i n e chemisorbed on alumina (15) and p y r i d i n e chemisorbed on s i l i c a - a l u m i n a (16). It had been p r e v i o u s l y shown that alumina contains only s i t e s which adsorb p y r i d i n e i n a Lewis acid-base fashion whereas s i l i c a alumina has both Lewis and Bronsted a c i d s i t e s . These two d i f f e r e n t kinds of s i t e s are d i s t i n g u i s h a b l e by the c h a r a c t e r i s t i c v i b r a t i o n a l bands of p y r i d i n e adducts at these s i t e s (see Table I). Photoacoustic and transmission r e s u l t s are compared i n Table II. Note that the PA signal strength depends on f a c t o r s such as sample p a r t i c l e s i z e and volumes of s o l i d sample and transducing


406

CATALYTIC MATERIALS


Table I. Assignments of Pyridine Chemisorbed on Silica-Alumina As Lewis Acid Sites (LPY) and Bronsted Acid Sites (BPY) vibrational assignment

LPY. cm"

8a "CC(N) 8b "CC(N) 19a "CC(N) 19b CC(N)

1620 1577 1490 1450

1

b

BPY. cm"

LPY. cm"

b

0

1

1

1621 1578 1493 1454

1638 1490 1545

BPY. cm"

C

1

1639 1493 1547

*Kline, C H . ; Turkevich, J . J . Chem. Phys. 1944, 12, 300. B a s i l a , M.R.; Kantner, T.R.; Rhee, K.H. J . Phys.Them. 1964, 68, 3197. Riseman, S.M.; Massoth, F.E.; Dhar, G.M.; Eyring, E.M. J . Phys. Chem. 1982, 86, 1760. b

c

Table II.

Vibrational Frequencies of Py-ridine Chemisorbed on γ-Alumina, cm"

transmission, , photoacoustic, 3

13

1453 1447

1495 1493

1578 1578

1614 1614

1622 1621

Mone, R. "Preparation of Catalysts", Delmon, B.; Jacobs, P . Α . ; Poncelet, G.; Eds.; Elsevier: Amsterdam, The Netherlands, 1976; pp. 381. Riseman, S.M.; Massoth, F.E.; Dhar, G.M.; Eyring, E.M. J . Phys. Chem. 1982, 86, 1760. a

b


20.

407

gas so that a simple c o r r e l a t i o n of a b s o r p t i v i t y and PA signal magnitude i s e l u s i v e . However, r e l a t i v e r a t i o s of a b s o r p t i v i t i e s can be deduced f o r the PA data when the thermal p r o p e r t i e s of the samples are maintained i n v a r i a n t (17). The p r i n c i p a l advantage of PA over transmission spectroscopy l i e s i n the determinaton of v i b r a t i o n a l species chemisorbed on opaque, l i g h t s c a t t e r i n g s u r f a c e s . This we have demonstrated by obtaining PA spectra of p y r i d i n e chemisorbed on reduced and s u l f i d e d M o / A l 0 and C o - M o / A l 0 c a t a l y s t s (18). The black s u l f i d e d samples are opaque at both v i s i b l e and i n f r a r e d wavelengths, but good q u a l i t y PA spectra of these surfaces are r e a d i l y obtained. Only Lewis a c i d s i t e s are detected on these surfaces (See Figure 6 ) . In a d d i t i o n , the high surface s e n s i t i v i t y of t h i s technique (a small f r a c t i o n of a monolayer) permits PA detection of a surface cobalt-aluminate type of domain which i s uninfluenced by the presence of molybdenum, i s r e s i s t a n t to s u l f i d i n g . and i s capable of adsorbing p y r i d i n e . This PA band (at 1310 cm" ) was not observed in transmission studies because such s p e c t r a l measurements of a t t e n t u a t i o n of a beam passing through the sample lack the r e q u i s i t i v e surface s e n s i t i v i t y . There are s i t u a t i o n s in which the s e n s i t i v i t y to gases of a FT-IR/PAS sample c e l l intended f o r s o l i d s i s advantageous. By p l o t t i n g PA i n t e n s i t y ( r a t i o e d to a s i l i c a PA i n t e r n a l standard i n the region 866 to 767 cm" ) versus the volume of C0(g) added to a s p e c i a l , microphonic PA c e l l one can develop a c a l i b r a t i o n curve. This curve can then be used to deduce the r e s i d u a l gas phase CO when carbon monoxide i s i n j e c t e d into a PA sample c e l l c o n t a i n i n g N i / S i Û 2 of predetermined surface area t h a t , u n l i k e pure SiOo, tends to adsorb CO. It was found (19) that 40% of the a c t i v e s i t e s on the N i / S i 0 c a t a l y s t had^BTorbed CO molecules (assuming a molecular cross s e c t i o n of 16 Â /CO molecule and s i n g l e occupancy of surface s i t e s . ) An inherent disadvantage of microphonic PA c e l l s i s t h e i r f r a g i l i t y f o r operation at the high temperatures and pressures t y p i c a l of commercial c a t a l y t i c processes. While Helmholtz resonance sample c e l l c o n f i g u r a t i o n s (20) can maintain a microphone at moderate temperatures while the PA sample i s at very low or at elevated temperatures, the high gas pressure problem i s not resolved in t h i s f a s h i o n . A most promising photothermal technique f o r i n f r a r e d s p e c t r a l measurements on high temperaturehigh pressure sample surfaces i s photothermal d e f l e c t i o n spectroscopy (PDS or sometimes a l s o "mirage e f f e c t " spectroscopy) (21). In a PDS experiment (see Figure 7) the i l l u m i n a t i o n of a surface by the focused output from a Michel son interferometer gives r i s e to thermal gradients that in turn produce a time dependent thermal lens i n the medium (gas or l i q u i d ) above the surface. A small-diameter probe l a s e r beam passing through t h i s thermal lens and almost grazing the surface i s then d e f l e c t e d through an angle whose magnitude and d i r e c t i o n i s measured with a p o s i t i o n sensing d e t e c t o r . In the special case of heterogeneous c a t a l y s t s at high temperatures and pressures a high pressure, 2



EYRING ET AL.

3

2

3

1

1

2



ι

1550

ι 1420

2

3

ι 1

I 1810

1

1550

1

1

1290

before (dashed line) and after exposure to pyridine.

1

2

3

1160

l.9%Co-6.9%Mo/AI 0

1420

Figure 6. Photoacoustic spectra of s u l f i d e d HDS catalysts. Frequencies (cm" ) of the most prominent absorbance bands of p y r i d i n e on the s u l f i d e d Μ ο / Α Ι ο Ο β and C0-M0/AI0O3 are i n d i c a t e d . Only bands r e p r e s e n t a t i v e of Lewis a c i d s i t e s are observed. 1

1

1680

F T - I R / P A spectra of sulfided

WAVENUMBERS

1160

before (dashed

1280

line) and after (solid line) exposure to pyridine.

F T - I R / P A spectra of sulfided M o / A I 0

ι

1680

I

1810


en

> r

ζ d Q ^ m

Q


LENS

NICOLET 7199

^ I

FT-IR INTERFACE

Sampler Reference

DIVIDER Ref. Sample

UDT43I Photodiode I Power Supply and Amplifier

LINEAR POSITION SENSING PHOTODIODES UDT L S C / 5 D

êflec^dâ^leêam^jz|

Reference Beom

SAPPHIRE WINDOWS

F T - I R OUTPUT

Figure 7. Schematic diagram of a photothermal d e f l e c t i o n spectroscopy (PDS) apparatus f o r i n f r a r e d s p e c t r a l measurements of surfaces at high temperatures and high pressures constructed at Utah by L.B. L l o y d .

PRISM

BEAMSPLITTING

He-Ne LASER

MIRROR

FOCUSING MIRROR


CATALYTIC MATERIALS

410

heated sample c e l l with three windows (and no microphone) as in Figure 7 permits i n f r a r e d spectral measurements under c o n d i t i o n s c l o s e l y approximating "the real t h i n g . " Acknowledgment


F i n a n c i a l support of t h i s work by a c o n t r a c t from the Department of Energy ( O f f i c e of Basic Energy Sciences) i s g r a t e f u l l y acknowledged.

Literature

Cited

1. 2. 3. 4. 5.

Bell, A.G. Am. J. Sci. 1880, 20, 305. Tyndall, J. Proc. Roy. Soc. London 1881, 31, 307. Rontgen, W.C. Philos. Mag. 1881, 11, 308. Rosencwaig, Α.; Gersho, A. J. Appl. Phys. 1976, 47, 64. Helander, P; Lundstrom, I.; McQueen, D.J. Appl. Phys. 1981, 52, 1146. 6. Farrow, M.M.; Burnham, R.K.; Eyring, E.M. Appl. Phys. Lett. 1978, 33, 735. 7. Lloyd, L . B . ; Burnham, R.K.; Chandler, W.L.; Eyring, E.M.; Farrow, M.M. Anal. Chem. 1980, 52, 1595. 8. Farrow, M.M.; Burnham, R.K. Auzanneau, M.; Olsen, S.L.; Purdie, N.; Eyring, E.M. Appl. Optics 1978, 17, 1093. 9. Patel, C.K.N.; Tam, A.C. Rev. Mod. P h y s . 1981, 53, 517. 10. Rockley, M.G. Chem. Phys. Lett. 1979, 68, 455. 11. Vidrine, D.W. Appl. Spectrosc. 1980, 34, 314. 12. Riseman, S.M.; Eyring, E.M. Spect. Lett. 1981, 14, 163. 13. Teng, Y.C.; Royce, B.S.H. Appl. Optics 1982, 14, 163. 14. Burggraf, L.W.; Leyden, D.E. Anal. Chem. 1981, 53, 759. 15. Mone ,R. in "Preparation of Catalysts," Delmon, B,; Jacobs, P.A.,; Poncelet, G., Eds.; Elsevier: Amsterdam, The Netherlands, 1976; p. 381. 16. Basila, M.R.; Kantner, T.R.; Rhee, K.H. J. Phys. Chem. 1964 68, 3197. 17. Riseman, D.M.; Massoth, F.E.; Dhar, G.M.; Eyring, E.M. J. Phys. Chem. 1982, 86, 1760. 18. Riseman, S.M.; Banyopadhyay,; Massoth, F.E.; E.M. Eyring, submitted for publication to J. Catalysis. 19. Gardella, J.A. Jr.; Jiang, D. -Z.; Eyring, E.M. Appl. Spectrosc. 1983, 37, 131. 20. Pelzl, J.; Klein, K; Nordhaus, O. Appl. Optics 1982, 21, 94. 21. For references see Aamodt, L.C.; Murphy, J.C. J. Appl. Phys. 1983, 54, 581. 22. Oda, S.; Sawada, T.; Moriguchi, T.; Kamada, H. Anal. Chem. 1980, 52, 650. R E C E I V E D November 1, 1983


Photoacoustic Spectroscopy of Catalyst Surfaces - ACS Publications

Recommend Documents