Heterogeneous Catalysis - ACS Publications - American Chemical

32 Leonard C. Drake-His Contributions to the Development of Mercury Porosimetry for Catalyst Characterization

Downloaded by FUDAN UNIV on December 20, 2016 | http://pubs.acs.org Publication Date: June 3, 1983 | doi: 10.1021/bk-1983-0222.ch032

BURTRON H. DAVIS University of Kentucky, Institute for Mining and Minerals Research, Lexington, ΚY 40512 Preparation of controlled porosity materials becomes an increasingly important function in catalyst research and preparation. Mercury porosimetry is a unique characterization technique since it permits measurement of the full range of pore diameters. The contributions that Dr. Leonard C. Drake made to the design and construction of the first instrument and in showing the utility of the technique are reviewed. Diffusion plays an important role in heterogeneous catalysis. All too frequently the impact of diffusion on experimental results is not recognized, even by experts in catalysis. In 1939 Thiele gave a clear account of the role that heterogeneous catalyst particle size may play on the rate of catalyzed reactions (1). While this appears to be the first widely circulated publication, as well as the first one to make an important impact on the role of diffusion in heterogeneous catalysis, it was not the first one. Likewise, even though it was published prior to the development of mercury porosimetry, it did not play a direct role in initiating the development of the porosimeter. The following, from the introduction to the first Ritter and Drake publication (2), provides an excellent introduction to mercury porosimetry (reference numbers changed to conform to the present manuscript): "Determination of total pore volume is a routine measurement in most laboratories dealing with porous materials. The value usually is calculated as the difference of two specific volumes (reciprocal density). Thus the internal pore volume is the difference between the reciprocals of real density and particle density; the intergranular (void) volume is the difference between the reciprocals of bulk density and particle density; and the sum of pore and void volumes is the difference between the reciprocals of bulk and real densities. [The nomenclature of McBrain (3) is followed in identifying the several densities, 0097-6156/83/0222-0451$06.00/0 © 1983 American Chemical Society Davis and Hettinger; Heterogeneous Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


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assuming (with some e r r o r ) that real and true d e n s i t i e s are equal.] The t o t a l i n t e r n a l pore volume i s then c a l c u l a t e d from observations of the r e a l and p a r t i c l e d e n s i t i e s , determined, f o r example, by the usual pycnometric method using water and mercury, r e s p e c t i v e l y , as the displacement l i q u i d s . In processes i n v o l v i n g d i f f u s i o n rates and the a v a i l a b i l i t y of i n t e r n a l surface to large molecules, a knowledge of t o t a l pore volume i s less important than a knowledge of the f r a c t i o n of t o t a l pore volume contributed by pores i n a given s i z e r a n g e — i . e . , of the d i s t r i b u t i o n of pore s i z e s . It i s convenient to classify the internal pores of porous m a t e r i a l s roughly i n two ranges. Present usage (4) applies the name "micropores to those having r a d i i smaller than 100 A.; "macropores" to those larger than 100 A. The d i v i s i o n of the pore volume of a given porous material into micro- and macropores implies the existence of a d i s t r i b u t i o n i n s i z e , yet l i t t l e work has been done i n the determination of such d i s t r i b u t i o n functions. Rabinowitsh and Fortunatow (5) have determined the r e s p e c t i v e f r a c t i o n s of micro- and macropores i n a number of porous s o l i d s by means of the K e l v i n equation...The adsorption equation of Brunauer, Deming, Deming and T e l l e r (6) may be of some use i n t h i s connection, but i s open to the c r i t i c i s m that i t does not satisfactorily combine the simultaneous effects of adsorption and capillary condensation. J e l l i n e k and Fankuchen (7) have used the s c a t t e r i n g of x-rays at very small angles to evaluate pore s i z e (or p a r t i c l e s i z e ) but assumed a constant average s i z e . The unpublished work of S h u l l (8) on low-angle x-ray scattering takes into consideration a pore-size distribution; but, this method not only cannot conveniently be used f o r pores l a r g e r than perhaps 500 A. i n radius, but the r e s u l t s i n terms of pore s i z e may also be open to question. This paper presents a method for determining the macropore-size d i s t r i b u t i o n i n a porous s o l i d as w e l l as the derived d i s t r i b u t i o n s for some t y p i c a l porous m a t e r i a l s . Washburn (9) has pointed out the fact that surface tension opposes the entrance i n t o a small pore of any l i q u i d having an angle of contact greater than 90° (the common phenomenon of c a p i l l a r y depression); that this opposition may be overcome by the application of external pressure; and that the pressure required to f i l l a given pore i s a measure of the s i z e of the pore. Henderson, Ridgway, and Ross (10) have used t h i s p r i n c i p l e in a very l i m i t e d way... The r e l a t i o n (quoted by Washburn) g i v i n g the pressure 11

Davis and Hettinger; Heterogeneous Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


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required to force l i q u i d into a pore of given s i z e i s pr = 20 cos © (1) where ρ i s the pressure, r the pore r a d i u s , 6 the surface tension, and θ the contact angle. I t may be derived as f o l l o w s : In a pore of c i r c u l a r cross s e c t i o n , the surface tension acts along the c i r c l e o f contact over a length equal to the perimeter of the circle. The force i s 2 77>ur. -Normal to the plane o f the c i r c l e of contact, the force tending to squeeze the l i q u i d out of the pore i s -2îf/ltf cos©. (The negative sign a r i s e s from the fact that the angle between the d i r e c t i o n of a c t i o n of the surface tension and the p o s i t i v e normal to the plane of contact is7f-0 Since θ >90°, the term -21t/v(f cos 9 i s i n t r i n s i c a l l y positive.) Opposing this force i s the applied pressure a c t i n g over the area of the c i r c l e of contact with a force equal to 7fr?9. At e q u i l i b r i u m these opposing forces are equal I-27f7itf" cosQ = 77/1,0, whence Equation 1 follows immediately. I From t h i s r e l a t i o n i t appears that a porous m a t e r i a l under zero pressure w i l l "absorb none of any nonwetting l i q u i d i n which i t i s immersed. When the pressure i s r a i s e d to some f i n i t e value, the l i q u i d w i l l penetrate and f i l l a l l pores having r a d i i greater than that c a l c u l a t e d from Equation l...As the pressure i s increased the amount of l i q u i d "adsorbed" increases monotonically at a rate proportional to the differential pore volume due to pores of s i z e corresponding to the instantaneous pressure. Thus, a given pore-size d i s t r i b u t i o n gives r i s e to a unique pressuring curve; and, conversely, a given pressuring curve a f f o r d s a unique determination of the pore-size distribution...Differentiation of Equation 1 and e l i m i n a t i o n of ρ give 11

=-Aetane ( 2 ) as the f r a c t i o n a l e r r o r incurred i n c a l c u l a t e d pore radius by an e r r o r ofA0in contact angle. For θ i n the neighborhood of 1 4 0 ° , for a 1° e r r o r i n contact angle i s only about 1.5%." Reference 2 and a companion p u b l i c a t i o n (11) defined the method and c a p a b i l i t i e s for mercury porosimetry. By 1949, Drake had increased the c a p a c i t y of h i s equipment to 60,000 p s i so that he was able to penetrate mercury i n t o pores down t o 18A i n radius

(12). Graduate Studies Dr. A. F. Benton received h i s Ph.D. at Princeton working under the d i r e c t i o n o f Prof. H. S. T a y l o r . Dr. P. H. Emmett recognizes Dr. Benton (Emmett's t h e s i s d i r e c t o r at C a l i f . I n s t . Tech.) for h i s outstanding experimental techniques and h i s sound


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t r a i n i n g i n adsorption as w e l l as h i s a b i l i t y to teach these subjects (13). Benton was at the C a l i f o r n i a I n s t i t u t e of Technology only briefly prior to joining the chemistry department at the U n i v e r s i t y of V i r g i n i a . At V i r g i n a , he quickly established a very productive research program emphasizing adsorption s t u d i e s . Drake worked under Benton and received h i s Ph.D during t h i s p e r i o d . His t h e s i s research included a d e t a i l e d study of chemisorption of oxygen by s i l v e r and the s t a b i l i t y of s i l v e r oxide. Thus, Drake l e f t the U n i v e r s i t y of V i r g i n i a at the height of the 1930 s United States' depression with a Ph.D., a sound t r a i n i n g i n adsorption and experimental techniques, and no job. Drake r e t a i n e d h i s y o u t h f u l c u r i o s i t y thought h i s graduate studies and employment. This i s r e f l e c t e d i n h i s drive to improve and to develop new experimental approaches. His research also reflected his independent and self-reliant character. Drake valued work, modesty in promoting accomplishments and, above a l l , honesty. These values s t r o n g l y influenced h i s i n t e r a c t i o n s with colleagues. For Drake, l i k e many others, the challenge of the next problem was more important than the writing about completed work.


1

E a r l y Years at Socony Vacuum A f t e r a period of unemployment and s e v e r a l months with the U. S. G e o l o g i c a l Survey i n Washington, where he a s s i s t e d i n a survey of n a t u r a l bleaching c l a y s , Drake joined Socony Vacuum (now Mobil) at t h e i r Paulsboro, N. J . Research Laboratory where he was the lone Ph.D. chemist in a group of engineers. He worked for s e v e r a l years on solvent r e f i n i n g , dewaxing and wax processing problems. His f i r s t contact with the Houdry cracking process was i n the application of molten salts as high temperature heat t r a n s f e r agents for e a r l y commercial cracking units. In the late t h i r t i e s he constructed low temperature p h y s i c a l adsorption equipment to measure the surface area of porous c a t a l y s t s by the Point Β method. Routine small g l a s s cracking u n i t s were also i n s t a l l e d to simulate Houdry commercial cracking u n i t s and to evaluate cracking c a t a l y s t s . Socony Vacuum wanted to use t h e i r r e c e n t l y developed moving bed (TCC) cracking process. This process permitted continuous movement of the c a t a l y s t from the cracking zone, purging and regeneration of the spent c a t a l y s t with a i r and r e i n t r o d u c t i o n into the r e a c t o r . The i n i t i a l development of t h i s process began with 30/60 mesh c a t a l y s t but a need for l a r g e r p a r t i c l e s was evident and more a t t r i t i o n r e s i s t a n t p e l l e t s were r e q u i r e d . Fortunately a concurrent Socony Vacuum development (by Dr. M a r i s i c ) of s y n t h e t i c s i l i c a alumina gel p a r t i c l e s (beads) solved some of these problems. A large plant to manufacture these beads was b u i l t i n Paulsboro and they replaced extruded and p e l l e t e d types. However, M a r i s i c l e f t Socony Vacuum before the bead c a t a l y s t was commercially developed. Drake was concerned with the laboratory evaluation and commercial



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development of t h i s new catalyst form, using coke burning, surface area, gaseous d i f f u s i o n and various a t t r i t i o n methods to measure hardness and abrasion r e s i s t a n c e as w e l l as c a t a l y t i c activity. During the first years of Drake's employment, the management of the Paulsboro Lab were quite r e s t r i c t i v e i n permitting discussions of laboratory research with people outside the lab. In t h i s respect Socony Vacuum was no d i f f e r e n t than other labs. R e s t r i c t i o n s and enforcement p o l i c i e s on exchange of research r e s u l t s and ideas appear to c y c l e between periods of utmost secrecy and periods where exchange and p u b l i c a t i o n were encouraged. Brunauer and Emmett c a r r i e d out and published much pioneering work on the a p p l i c a t i o n of p h y s i c a l and chemical adsorption to c a t a l y s t c h a r a c t e r i z a t i o n during the 1930's culminating i n 1938 with the p u b l i c a t i o n of the BET equation for c a l c u l a t i n g surface areas. However, Drake honored the r e s t r i c t i o n s so that when he attended the ACS meeting i n Boston i n 1939, he avoided d i s c u s s i o n of adsorption and c a t a l y s t c h a r a c t e r i z a t i o n with both Brunauer and Emmett. S h o r t l y a f t e r the Boston meeting a t t i t u d e at the l a b o r a t o r y changed. Roland Hansford had joined another group at the Paulsboro labs and had co-patented a method for thiophene production by r e a c t i n g hydrocarbons and s u l f u r over c a t a l y s t s . The development of t h i s process was l e f t to others and Hansford was made the head of M a r i s i c ' s group. He introduced a program of more fundamental c a t a l y s t s t u d i e s . Drake j o i n e d t h i s group at i t s i n c e p t i o n and a s s i s t e d i n s e t t i n g up a laboratory f o r p h y s i c a l and chemical s t u d i e s of porous c a t a l y s t s . Hansford, as a former student of Professor Paul Emmett, was of course f a m i l i a r with adsorption research. However, more important to Drake was Hansford's support and encouragement to pursue new ideas, both w i t h i n and outside the laboratory, and Drake considers t h i s research a t t i t u d e as a major f a c t o r i n developing the mercury porosimeter. Hansford and Drake, i n the e a r l y f o r t i e s adapted the McBain-Bakr quartz f i b e r spring technique to low temperature n i t r o g e n adsorption surface area measurements. This required at that time the c o n s t r u c t i o n of a s p r i n g winding device for making springs from quartz f i b e r s and a t r a v e l i n g microscope reading device for m u l t i p l e unit r o u t i n e measurements. Improved models of t h i s equipment are s t i l l i n use at Mobil f o r surface area measurements. It must be r e a l i z e d that adsorption was an a c t i v e research area in the t h i r t i e s so that the s i x McBain-Bakr balances would be considered a "modern c h a r a c t e r i z a t i o n instrument." Mercury

Porosimeter Washburn had used l i q u i d penetration to estimate the p o r o s i t y of a range of ceramics; however, he employed l i q u i d s such as water or hydrocarbons and only used low pressures. Even so, he c l e a r l y recognized the advantage of mercury and published



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the theory f o r i t s use i n 1921 (9). This paper, encountered by Drake i n a search f o r new methods for measuring p o r o s i t y , provided a b a s i s f o r undertaking the experimental program. Drake, together with a recent employee (H. L. R i t t e r ) and the encouragement of Hansford, started work on t h i s i n the e a r l y 1940's. R i t t e r remained at Socony Vacuum f o r only a b r i e f period and had moved to the U n i v e r s i t y of Wisconsin p r i o r to p u b l i c a t i o n of the two papers he authored with Drake (2, 11). While at Socony Vacuum R i t t e r also used small-angle x-ray s c a t t e r i n g to measure pore s i z e d i s t r i b u t i o n i n porous m a t e r i a l s and published a paper with L. C. Rich on t h i s i n 1948 (14). He went to Miami U n i v e r s i t y i n Ohio a f t e r a few years at Wisconsin. A f t e r leaving Scocony-Vacuum he continued to use x-rays i n h i s research but he did not again work on mercury porosimetry. High pressure techniques were not commonly used at t h i s time so that many problems were encountered. Precision-bore g l a s s tubing was just becoming commercially a v a i l a b l e . Without t h i s , a d i f f e r e n t method for measuring penetration would have been r e q u i r e d . Even so, the p r e c i s i o n - b o r e tubing was the component that l i m i t e d the accurracy of the measurement. The f i r s t bomb used by R i t t e r and Drake was l i m i t e d to 2000 p s i and used a commercial i n s u l a t e d e l e c t r i c a l wire lead to the dilatometer they had developed. After publishing their f i r s t two papers, the authors learned that the Rusha Instrument Corp., Houston, Texas has marketed f o r a number of years a "high pressure porometer" which forced mercury by means of a t i g h t p i s t o n into o i l - w e l l (12). However, the Rusha instrument only operated to 1000 p s i (2100 A pores). In the high pressure 60,000 p s i u n i t , a P. W. Bridgeman (awarded a Nobel P r i z e f o r h i s high pressure work) designed i n s u l a t e d e l e c t r i c a l lead, made i n the Socony Vacuum shops, was at f i r s t l i m i t e d t o , at most, a few runs because of e l e c t r i c a l shorting due to wire i n s u l a t i o n f a i l u r e (Figure 1). This leakage problem caused months of f r u s t r a t i o n . The s u b s t i t u t i o n of T e f l o n coated copper wire allowed many runs to be made before eventual f a i l u r e . This wire was made a v a i l a b l e by Dr. Wahl, a f r i e n d at the Wilmington duPont Research Laboratory, as a r e s u l t of a d i s c u s s i o n . This d i s c u s s i o n , and the assistance that i t provided, c o n t r a s t s markedly with the s i t u a t i o n a few years earlier. A gas was used i n the p r e s s u r i z a t i o n procedure and t h i s , together with the l i m i t e d experience of manufactures of high pressure v e s s e l c o n s t r u c t i o n , provided an a d d i t i o n a l problem. "The f i r s t bomb used i n t h i s work s p l i t from end to end while at 50,000 pounds per square inch, throwing water and thermostatic equipment over the surrounding area. The sudden expansion of the 400cc of n i t r o g e n (compressed from 2000 to 50,000 pounds per square inch) also l i f t e d the r e l a t i v e l y heavy bombshed roof several f e e t . " (12) This i s a rare mention of the experimental problems encountered i n the development of t h i s different



F i g u r e 1. Schematic of the f i r s t mercury porosimeter. (Reproduced from Ref. 2. Copyright 1945, American Chemical Society.)



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experimental procedure and even i t was presented i n the form of a s a f e t y precaution. Never one to be s a t i s i f i e d with a method, Drake t r i e d many ways to improve the accuracy and ease of operation of the method. In the o r i g i n a l dilatometer the penetration volume was obtained by measuring the r e s i s t a n c e of a platinum wire i n the c a l i b r a t e d bore (Figure 2 ) . As the mercury l e v e l i n the bore d e c l i n e d as pressure forced mercury i n t o the c a t a l y s t pores the length of Pt wire i n the mercury decreases; hence, the increase i n r e s i s t a n c e provided a measure of the penetration volume. In the o r i g i n a l design, a gas was used to p r e s s u r i z e to 2,000 p s i . In the high pressure u n i t , t h i s gas was further compressed by an o i l ram. O i l could not be t o l e r a t e d i n the c a l i b r a t e d bore area since i t fouled the c a l i b r a t e d c a p i l l a r y bore. A sight was developed i n order to v i s u a l l y measure the mercury l e v e l s but t h i s was not considered to be an improvement over the r e s i s t a n c e method. When t h i s approach was not s u c c e s s f u l , Drake's e f f o r t to make v i s u a l measurements was terminated. However, t h i s method was incorporated into a commercial unit so that Rootare, in a 1968 review (15), stated that i t was the most common way to measure volume changes. There i s some u n c e r t a i n t y about the contact angle that mercury makes with c a t a l y s t m a t e r i a l s . Drake used Woods Metal and sodium, heated above the melting point, to replace mercury i n an attempt to learn the i n f l u e n c e of contact angle. However, l i t t l e d i f f e r e n c e was observed f o r the few r e s u l t s obtained with the two metals and the experimental d i f f i c u l t i e s were many. Other C a t a l y s t C h a r a c t e r i z a t i o n Techniques While the mercury porosimeter was Drake's most noteable success during this period, he worked on many catalyst c h a r a c t e r i z a t i o n methods that, while s u c c e s s f u l , were never published except i n i n t e r n a l r e p o r t s . One of the l a s t c a t a l y s t c h a r a c t e r i z a t i o n problems Drake worked on was with Ρ ί - Α ^ Ο β reforming c a t a l y s t s . About 1950, Ρ ί - Α ^ Ο β was accepted as the p r e f e r r e d reforming c a t a l y s t . The need to measure the c r y s t a l l i t e s i z e of the low loading o f Pt supported on alumina was apparent. X-ray l i n e broadening was not p o s s i b l e f o r the dual component c a t a l y s t since the most intense Pt peak was a shoulder on an intense alumina peak; with small p a r t i c l e Pt and low concentrations there was i n s u f f i c i e n t Pt i n t e n s i t y to even make an estimate of the p a r t i c l e s i z e . However, i t was found that acetylacetone e x t r a c t i o n s e l e c t i v e l y d i s s o l v e d the alumina. The Pt could be recovered and analyzed f o r c r y s t a l l i t e s i z e by x-ray l i n e broadening. I t was also confirmed that they were not j u s t recovering the large p a r t i c l e s by the e x t r a c t i o n . The Pt line f o r a supported large p a r t i c l e Pt formed by high temperature s i n t e r i n g was so sharp that i t could be separated from the alumina peak. Thus, by determining that the x-ray peak area per gram of Pt was the same f o r the s i n t e r e d , supported c a t a l y s t as f o r the extracted Pt, they concluded that the


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BAKELITE SCREW

GROUNDED PLATINUM CONDUCTOR INSULATED PLATINUM CONDUCTOR LUCITE CUSHIONING WASHER

LOCK NUT


STRETCHING NUT • THRUST COLLAR-LUCITE HEAVY WALL PYREX -4mm I.D. 32 B&S GAUGE PLATINUM WIRE HOLES CALIBRATED BORE

Il II Il II Il II

2 OR 4mm I.D. HEAVY WALL GLASS TUBING

INTERNAL GLASS BRIDGE

C A T A L Y S T BULB 12 mm O.D. PYREX

F i g u r e 2. Schematic o f the d i l a t o m e t e r mercury porosimetry measurements.

used f o r the i n i t i a l



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e x t r a c t i o n method recovered a l l of the reduced Pt i n i t i a l l y on the c a t a l y s t . While the method was used at Mobil for years, i t was never published i n the open l i t e r a t u r e , but was d i s c l o s e d to other outside c a t a l y s t groups by other Mobil researchers. It is interesting to follow the development of the s c i e n t i f i c thought from the d i s c u s s i o n sections of the three Drake manuscripts (2,11 and 12). In the f i r s t paper (2) the concern was with e s t a b l i s h i n g the v a l i d i t y of the experimental r e s u l t s and with showing that the r e s u l t s were c o n s i s t e n t with geometric models for the pores. Deviations were i n the d i r e c t i o n expected f o r inkwell pores. In the companion paper (11) r e s u l t s for a wide range of porous m a t e r i a l s are given as the authors e s t a b l i s h e d the scope of the method. H y s t e r e s i s was noted and a phenomenological d i s c u s s i o n of i t was provided. Only i n the t h i r d paper (12) had the d i s c u s s i o n a t t a i n e d the s c i e n t i f i c maturity that one frequently observes a f t e r a few years intense work and thought by a pioneer. Among the conclusions were: a) the volume of a coke deposid on aged silica-alumina c a t a l y s t s was greater than that of amorphous carbon and the carbon deposited r a t h e r uniformly throughout the pore s i z e range during c a t a l y s t ageing, b) p r e s s u r i z a t i o n , and the mercury r e t a i n e d by h y s t e r e s i s , only s l i g h t l y a l t e r e d the o r i g i n a l surface area showing that the p o r o s i t y survived the p r e s s u r i z a t i o n of the measurement, c) observed h y s t e r e s i s , and the slow, nonequilibrium nature of the d e p r e s s u r i z a t i o n curve and noted that, at 25 p s i a on the d e p r e s s u r i z a t i o n curve, very l i t t l e more mercury was r e t a i n e d a f t e r a 60,000 p s i than f o r a 10,000 p s i p e n e t r a t i o n . d) v e r i f i e d mercury r e t e n t i o n by weight of the c a t a l y s t a f t e r penetration as w e l l as by r e s i s t a n c e measurements, and e) showed that the 60,000 p s i data i n d i c a t e d that there was no reason to suspect the breakdown of the concept of surface tension even i n pores with a diameter of 5 mercury atoms. To date, i t does not appear that the l a s t observation has been adequately recognized. The experimental design of R i t t e r and Drake was the b a s i s for the American Instrument Company developing and marketing a commercial instrument. Experimental d i f f i c u l t i e s and the time consuming operation required for a measurement delayed the wide usage of the technique. However, w i t h i n the past years two U. S. companies have introduced two types of automated instruments that should make t h i s method as popular as the widely used nitrogen adsorption method. Hopefully t h i s account w i l l help to introduce Leonard Drake to a new generation of s c i e n t i s t s who, with improved commercial instruments, will participate in g r e a t l y expanding the use of the technique. The author was fortunate to be associated with Leonard Drake during four years while employed at Mobil. His f r i e n d s h i p was a valued acquisition during t h i s period. Technical


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information on the development of the mercury porosimeter was provided by Leonard during a v i s i t with the author during May, 1982 as w e l l as by a review of the d r a f t of t h i s manuscript; however, the author i s responsible for the f i n a l version as w e l l as the conclusions. The photo f o r Figure 3, showing Leonard i n the author's lab viewing a commercial porosimeter, was taken during the May, 1982 v i s i t .

F i g u r e 3. P i c t u r e (1981) of L. C. Drake with a mercury porosimeter.

commercial


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Literature Cited 1. 2. 3. 4. 5.


6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

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RECEIVED November

29, 1982


Heterogeneous Catalysis - ACS Publications - American Chemical

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