496
J. F. SCHULTZ, M. ABELSON,K. C. STEINAND R. B. ANDERSON
We are indebted to several others for their contributions: G. C. Castellion (adsorption studies), T. F. Comiskey (regeneration experiments), J. Byrne .(attrition studies), E. Thomas (light micro-
Vol. 63
scopical examination), W. Doughman (X-ray spectra) and F. Horvath (photography). We also express appreciation to the American Cyanamid Company for permission to publish this work.
STUDIES OB’ THE FISCHER-TROPSCH SYNTHESIS. XVIII. INFLUENCE OF CATALYST GEOMETRY ON SYNTHESIS ON IRON CATALYSTS BYJ. F. SHULTZ,’ M. ABELSON, K. C. STEIN’AND R. B. ANDERSON~ Bureau of Mines,Region V , Pittsburgh, Pa. Received October 7 , 1868
The influence of catalyst geometry on activit of reduced and reduced-and-nitrided fused iron catalysts in the FischerTropsch synthesis was studied by the Bureau of $lines by varying (a) the particle size, (b) the depth of reduced layer and (c) the average pore radius. For catalysts reduced at 400 and 450’ the activity curves may be approximated by assuming that only the outer layers of the particles to a depth of 0.01-0.02 cm. are active. When the average pore radius was varied, the activity of 6- to &mesh particles remained essentially constant, despite the fact that the surface area varied in an inverse manner to pore radius. Apparehtly the depth of effective catalyst increased sufficiently to compensate for the decreased surface area. Changes in the composition of the catalyst during synthesis and product distribution are considered with respect to a simplified picture of transport phenomena in pores.
Introduction Diffusion in the pores of catalysts is intimately related to the reactions at the surface, as the concentration gradients required for diffusion are produced by the catalytic reaction. Several investig a t o r ~ ~have - ~ solved differential equations for simple hypothetical catalytic reactions in which diffusion in pores was coupled with reaction at the surface. Wheeler6-6 has summarized and extended this work to include consideration of selectivity and poisoning in porous catalysts and an appraisal of well known catalytic reactions. Weiss and Prater? have treated dehydrogenation reactions on this basis, and Bokhoven and associatesshave considered diffusion and reactions in iron catalysts, Koelbel, Ackermann and Engelhardte considered mass transport in the Fischer-Tropsch synthesis with emphasis on liquid-cooled reactor synthesis. A previous paperlo has shown that the pore geometry of fused-iron catalysts can be “tailormade” within certain limits by varying the extent and temperature of reduction: (a) at all temperatures studied, 400 to 625”, the pore volume and porosity increase linearly with extent of reduction ; (b) the reduction, at least on a macroscopic scale, proceeds uniformly inward from the external surface; (c) the average pore diameter of the reduced portion of the catalyst increases with increasing temperature of reduction. (1) Physical Chemist, Region V, Bureau of Mines, U. 8. Department of the Interior, Pittsburgh, Pa. (2) E. W. Thiele, lad. Eng. Chem.. 81, 916 (1939). (3) C. Wagner, 2. physik. Chem., Al98, 1 (1943). (4) G. Damkohler. ibid., 8198, 10 (1943). (5) A. Wheeler, in “Advances in Catalysis,” VoI. 111, edited by W. G. Frankenburn, V. I . Komarewsky and E. K. Rideal. Academic Press, Inc., New York, N. Y., 1951, p. 250. (6) A. Wheeler in “Catalysis,” Vol. 11, edited by P. IT. Emmett, Reinhold Publ. Corp., New York, N. Y., 195.5, p. 105. (7) P. B. Weiss and C. D. Prater in ref. 5, Vol. VI, 1954, p. 143. (8) C. Bokhoven. C. van Heerden, R. Westrik and P. Zwietering in ref. fi, p. 265. (9) H. Konlbel, P. Ackerinann and F. Enpclliardt, Erdosl una’ Kohl?,
B, 153 ( I 9 3 i J . (10) W. K. Hall, W. H. Tarn and R. B . Anderson, J . A n . Chem. Soc., 79. 5-436 (1950); THISJOURNAL,66, 688 (1952).
The influence of the geometry of reduced and reduced-and-nitrided, fused-ammonia synthesis catalyst on rate of the Fischer-Tropsch reactions was studied by varying (a) the particle size, (b) the extent of reduction, and (c) the reduction temperature. Catalytic activity and composition changes were determined for all three series. In the present paper these data are interpreted by simple computations, and in a subsequent paper a special solution of the differential equations of Wheeler is applied. Experimental These tests employed our standard fused-iron catalyst, D-3001, containing 67.4% by weight of iron, 4.61 MgO, 0.57 K20, 0.71 Si02 and 0.65 Cre08. The original material of about 4- to 8-mesh was crushed in a roller mill and sieved into the desired size fractions. The catalyst was reduced in hydro8en at temperatures varying from 400 to 625”, as desired, in a metal block reactor,” and for nitrided catalyst: the sample was subsequently treated with ammonia a t 350 in this unit. The catalyst-testing units and mode of operation have been described in previous papers.11?’2 Tests were made with 1H2 I C 0 gas at 21.4 atmospheres. The space velocity was held a t about 300 hr.-’, and the temperature of the reactor was varied to maintain the apparent COa-free contraction at about 65%. The activity, in terms of volume of synthesis gas reacted per hour per gram of iron a t 240’. was computed by an empirical rate equation . I 3 Experiments a t a constant temperature with the flow rate varied to maintain a constant conversion would have been preferable; however, experiments at constant reactor temperature would require more than fivefold variations of total gas conversion, and localized overheating of the catalyst robably would have occurred at the higher space time yiells. Although the use of an empirical equation with a constant energy of activationla is incorrect, fortunately the errors introduced do not change the interpretation of the results, as will be shown in the next paper. Reproducibility of catalytic activity as computed by the empirical equation was fairly ood, as shown by duplicate tests of nitrided catalysts. 8ondensed products from the synthesis were divided into several fractions by simple one-plate distillation.11 The reproducibility of product characterizations
+
(11) R. B. Anderson, J. F. Shulte, B. Seligman. W. K . Hall and H. H. Storch, .I. A m . Ckem. Soc.. 7 3 , 3502 (1950). (12) R. B. Anderson, A. Krieg, B. Seligman and W. E. O’Neill. Ind. E n s . Chsm., 89, 1548 (1947). (13) R. B. Anderson, B. Seliaman, J. F. Shulte, R. E. Kelly and M. A. Elliott, ibid., 44, 391 (1951).
I
INFLUENCE OF CATALYST GEOMETRY ON SYNTHESIS ON IRON CATALYSTS
April, 1959 Mesh sire
Av. activity per g. Fe for weeks 2-6
6-8
75 77 42-60
1 1
207 222 was less satisfactory. and these data are used only to show major trends. In some experiments a sizable fraction of the catalyst disintegrated into finer particles during the pretreatment. On this basis the pretreated catalysts for the particle-size series were sieved to the desired mesh size under liquid heptane. Suitable precautions were taken to reclude oxldation of the catalyst in this manipulation. &en this proce.dure was adopted, the extent and temperature of reduction series was nearly completed; however, a few additional experiments with sieved catalysts were made to show that the trends observed in the earlier tests were essentially correct. I t was desired to select a value for activity at a suitable period after steady conditions were attained. The activity was somewhat erratic in the first two or three days of synthesis, but in the remaining period of synthesis the activity was relatively constant. The deviations (absolute) of the average activities of the second to sixth week from the activity in the second week averaged only 6% for all tests considered. On this basis the average value for the second to sixth week8 was used as an expression of activity. In subsequent considerations the particles will be assumed to be smooth spheres. A hypothetical spherical radius was computed from the avera e weight per particle of raw catalyst and the real density of this material. It is further assumed that the pore geometry of the pretreated catalyst may be used in activity correlations, despite the fact that these catalysts are oxidized to a large extent during synthesis .14J6 Subsequent sections considering reaction and mass transport in pores and oxidation of catalyst largely justify this postulate. The three grou s of experiments for both reduced and reduced-and-nitriid samples are reported: A. Particle Size Series.-The first group of catalysts was virtually completely reduced a t 450' and sieved after pretreatment. Catalysts of 6- to 8-mesh and 28- to 32-mesh were tested after reduction a t 600". B. Reduction Temperature Series.-Six- to 8-mesh samples were reduced essentially completely a t 450,500,550, 600 and 625', and tested in the synthesis. Particles of 28to 32-mesh were completely reduced at 450 and 600". Most of the tests were made without sieving catalyst after pretreatment. C. Extent of Reduction Series.-To ensure uniform reduction throughout the catalyst bed, 6- to 8-mesh particles were reduced a t 400'; a t this temperature the rate of reduction is low. Catalysts were not sieved after pretreatment. The geometry of the reduced catal st per gram of raw (unreduced) catalyst may be describedras: A. Hypothetical spherical radius: Mesh size
Particle radius, R , om.
4-6
0.151 .116 .0508 ,0225 .0135
6-8
12-16 28-32 42-60
I
13: Properties that are independent of reduction temperature: (1) external vol. of catalyst = 0.20 cc./g. (2) pore vol. = 0.089 f cc./g., where f i s the extent of reduction
(4) depth of reduced zone from external surface = R{1 - (1 - f)'/g} where R is the particle radius 114) R . B. Anderson, L. J. E. Hofer, E. M. Cohn and B. Seligman, J . Am. Chsm. Soc., 78, 944 (1951). (15) J. F. Shultr, B. Seligman. J. Lecky and R. B. Anderson, ibid., 74, 637 (1952).
I
497
I
e l >
t 2
2
so
0
Fig. 1.-Variation of activity with particle size: circles, reduced catalysts; squares, nitrided catalysts. C. Properties dependent on reduction temperature: 1. For completely reduced catalysts Reduction temp., O C .
Surface area, m.Vg.
Av. pore radius, I , A.
400 15.7 113 450 9.3 194 500 6.4 278 550 3.5 508 600 1.6 1112 625 1.6 1112 2. For partly reduced catalysts surface area = Sof, where So is the area of the compfetely reduced catalyst. For nitrided catalysts the same relationship holds, except that the external volume and pore volume are about 15% larger than for the reduced catalyst; however, to simplify calculations, the pore geometr data for reduced catalysts have been used for both redlced and nitrided samples. For nitrided catalysts of the extent of reduction series, the catalyst was reduced at 400' to the desired extent and then nitrided in ammonia at 350". At this temperature ammonia nitrides only the reduced part of the catalyst and does not reduce or nitride any of the remaining magnetite. Activity as a Function of Catalyst Geometry.-Tables I, I1 and I11 present activity data for the particle size, extent of reduction and temperature of reduction series, respectively. Figure 1 shows the variation of activity with particle size. According to Fig. 1, activity increases rapidly with reciprocal particle radius, which is proportional to external area per gram. With increasing values of 1/R, the slope decreases and eventually becomes zero. For catalysts reduced a t 600°, the slope apparently becomes zero $,a larger particle size than for samples reduced at 450 . These curves should become flat when the particle is sufficiently small so that the entire particle is effective. To illustrate the magnitude of the depth of the effective catalyst layer from the external surface, a simple calculation was made assuming (a) hypothetical spherical particles, and (b) that the catalytic activity was constant in the surface layers to a depth AR and was zero at depths greater than AR. On this basis the activity, A , is proportional to the weight of material in this active zone per unit weight of catalyst, and for values of R 2 AR A - 3 k xA R/ l - - AR
R
498
J. F. SCHULTZ, M. ABELSON, K. C. STEINAND R. B. ANDEMON
imposed on the log-log plot in Fig. 2, indicate that the equation above represents the experimental results satisfactorily. The values for the constants were
TABLE I VARIATIONOF ACTIVITYWITH PARTICLE SIZE (Catalysts sieved after metreatment)
--
-Reduced catalystNitrided catalystExtent of Extent of Atomic reduction. Activity, reduction, Activity, ratio I A' f A' N/Fi Reduction temp. = 450°, av. pore radius, ? = 1.94
Particle radius, R , cm.
A.
x
0.151 .116 .0508 .0225 .0135
0.915 .971 .977 .995 1.00
10-6 cm. 40 0.915 54 .971 105 .977 185 .995 210 ,979
75 77 156 228 222
Vol. 63
0.35 ,40
Reduction temp., OC.
Reduced catalyst Nitrided catalyst Reduced catalyst Nitrided cahlyst
A&, cm. 0.0110 ,0175 .05
450 450 600 600
--
k 211 221 76 N I 10
-
.03
The values for reduction at 600', based on only two points each, probably are not very accurate and are shown only t o illustrate the direction of change in these parameters with .43 increasing reduction temperature. It should be noted that .42 AR is an average depth of useful catalyst for a hypothetical B. Reduction temp. = 600', av. pore radius, T = 11.12 x system in which the activity is constant in the effective zone and is zero beyond this depth. Actually the effectiveom. ness decreases rapidly with distance from the surface, as 0.116 1.00 62 0.914b 70b 0.49 shown in Table I1 for the extent of reduction series; however, the quantity A R is useful in discussing the data in a 0.0225 1.00 76 1.00 111 0.47 5 Activity A defined as cc. (S.T.P.) of HZ .CO con- simple manner. With reduced catalysts the activity ceased verted per gram of iron per hour at 240'. This sample to increase a t an extent of reduction of about 28%, corresponding to a depth of reduced layer of 0.012 cm. in good not sieved. agreement with values above; however, for nitrided catalysts this depth must be taken as 0.026 om. These values should TABLE I1 be regarded as tentative, being based on catalysts that were VARIATION OF ACTIVITY WITH EXTENTOF REDUCTION not sieved after pretreatment. In the reduction temperature series (Table 111) for 6- to (Reduction temp. = 400', particle radius, R = 0.116 cm., 8-mesh reduced catalysts no definitive shange in activity av. poreradius, P = 1.13 X 10-6 cm.) was observed, although the average pore radius increased Reduced-Nitrided Depth Depth fivefold and the surface area decreased correspondingly. Extent of reExtent of reFor nitrided catalysts activity was constant for reduction of reduced of reduced Atomic duolayer duolayer ratio temperatures of 450-550', but decreased significantly for tion, X IO*, AAztivi%b ti?: XcA!g, Activit NIFeC 600 and 625'. As the pore geometry of catalysts reduced f cm. at 600 and 625p was essentially identical, the lower activity A. Catalyst not sieved after pretreatment at the higher temperature cannot be attributed to a smaller surface area. With 28- t,o 32-mesh catalysts a marked de0 0 2 .. 0 0 2 .. 0 craase in activity was observed for both reduced and ni0.082 0.336 39 476 0.090 0.360 45 500 0.48 trided catalysts when the reduction temperature was in,125 ,522 52 416 ,192 .80 62 323 .50 creased from 450 to 600'.
.40
+
--
.280 .546
-
1.205 75 268 2.67 63 115
.399 1.82 ,577 2.90
68 84
170 146
+
.41 .42
Reduction temp., O C .
B. Catalyst sieved after pretreatment 0.350 0.975
1.57 8.36
50 54
143 0.430 2.01 55 0.713 3.94
55 59
.+
128 0.46 83 .39
Reduced catalyst Nitrided catalyst
A g , cm.8 of Ht CO converted per g. of Fe per hr. at 2400 450 600
185 228
76 111
5 Activity defined as cc. (S.T.P.) of HZ CO converted Activity per gram of per gram of iron er hour a t 240'. active layer. Tiese values give atomic ratio N/Fe for the reduced layer.
Discussion In the medium-pressure synthesis at the temperatures used in the present study the pores of iron catalysts are filled with liquid hydrocarbons, and TABLE I11 mass transfer involves dissolution in and diffusion VARIATION OF ACTIVITYWITH TEMPERATURE OF REDUCTION through oil films and oil in pores. From the data (Particle radius, R = 0.116 cm.) of Tables I, I1 and 111 this summary relating to Reduced catalysts -Nitrided catalystscatalyst geometry is made Temp. Pore Extent Extent of reradius, duotion, t X 10-8, OC. om.
of reduction,f
Activity, A'
of reduction, f
Activity, A'
Atomic
A. Catalyst not sieved after pretreatment 450 500 550 600 625
1.94 2.78 5.08 11.12 11.12
0.964 .971 .977 ,995 1.00
73 79 69 76 66
0.964 .987 ,964 ,914 1.00
98 98 95 70 50
0.42 .49 .47 .49 .44
B. Catalysts sieved after pretreatment 450 600
1.94 11.12
0.977 1.00
51 62
0.975
70
Variation
gy%
0.38
C. Influence of temperature of reduction 011 smaller particles shown in Table I. a Activity defined as cc. (S.T.P.) of H z +GO converted per gram of iron per hour a t 240'. The constants k and AR were evaluated by a graphical method using R log-log plot of ( 3 A R / R ) ( 1 - A R / R A R 2 / 3 R z ) as a function of A R / R . Activity data, super-
+
Variable
How varied
of
activity
(1) External area Particle (2) Area of pore openings Size
Large
( 3 4 Surface area (3b) Surface area (4) Pore volume (5) Av. pore radius (6) Depth of pores
Slight Large Large Slight Large
Reduction temp. Extent of reduction Extent of reduction Reduction temperature Extent of reduction
The general aspects of items 1, 2, 3a and 5 could be explained by asscming that the slow step is the rate of dissolution and diffusion through oil films a t the external surface of the particle; however, the large variations in 3b, 4 and 6 req,uire a more complicated mechanism, probably involving diffusion (16) K. C. Stein, G. P. Thompson and R . B. Anderson, TIJIB JOUR61, 928 (1957).
NAL,
April, 1959
INFLUENCE OF CATALYST GEOMETRY ON SYNTHESIS ON IRON CATALYSTS
499
-
-
6-
y'
2
- --
Pore mouth
I &em.
Fig. 2.-Log-log plot of data for reduced 0 and reducedcatalysts in particle size series. Catalysts and-nitrided reduced a t 450".
in oil-filled pores coupled with reaction at the catalyst surf ace. Concentration The present results indicate that the average in oil phase depth of effective catalyst on the periphery of the particle is of the order of 0.01-0.02 cm. for samples I reduced at 400 and 450" and for samples reduced at DISTANCE FROM PORE MOUTH, 600" of the order of 0.04 cm. Thus, the average depth of effective catalyst increases with increas- Fig. 3.Schematic representation of a pore in a Fischering pore radius. For larger particles the increase Tropsch catalyst. of effective depth appears sufficient to compensate rather exactly for the decreased surface area, and 1.2I I I I I I I I the rate remains essentially constant as the temperature of reduction is increased from 450 to 550". For smaller particles a sufficientdepth of ineffective catalyst is apparently not available to compensate for the decrease in surface area as the reduction temperature was increased from 450 t o 600". A generalized picture of catalyst pores during synthesis is proposed, based on the data presented, and these hypotheses were examined with respect to changes of catalyst composition that occur during synthesis and selectivity. A typical simplified pore as shown in Fig. 3 is filled with liquid hydrocarbon, and the average and ultimate depths of active layer are shown by dotted lines. Hydrogen and carbon monoxide dissolve in the oil and diffuse into the I I I I pore; the concentration gradients that cause dif- w 254" 254' , 248' I , 2500 fusion are produced by the reaction on the walls of V3 0 the pores. Synthesis products-water, carbon di- a oxide and hydrocarbons-dissolve in the liquid hydrocarbons in the pore and diffuse to the pore mouth. As the solubilities of gaseous hydrocar- v) .8 bons, water and carbon dioxide in oil exceed those z of the reactants, it is not necessary to postulate Ua I the presence of a gas phase within the pores. High z .6 molecular weight oils produced in the pore cause a 0 Nitrided catalyst cv) 0 small net flow of oil out of the pore. The concentrations of hydrogen and carbon g .4 monoxide, as shown by a single curve in Fig. 3, r 0 decrease rapidly with increasing distance from the V . 2 pore mouth from values in equilibrium with the gas phase until the concentration is sufficiently low so 259" 12.45' I 2430 I I 242. that the rate is zero due to either kinetic or thermodynamic limitations. The concentrations of water aad carbon dioxide increase from values in equilibrium with the gas phase to high constant values at the point where the concentration of reactants becomes so low that synthesis does not occur. On this basis optimum conditions for oxidation of iron are found in the interior of the particle temperatures are shown on graphs.
500
HAROLD W. KOHNAND ELLISON H. TAYLOR
beyond the effectivedepth, and optimum conditions for formation of carbide and free carbon occur in the effective layer. I n the present experimenh with lHa+lCO gas carbon monoxide is used in greater amounts than hydrogen (the over-all usage ratio Ha:CO is about 0.7) ; thus the liquid hydrocarbon in the pores in the ineffective portion of the catalyst should contain sizable quantities of dissolved hydrogen, as well as dissolved water, carbon dioxide and light hydrocarbons. I n the ineffective zone, hydrocracking reactions leading to a decrease in molecular weight may occur. I n the experiments described in Tables I, I1 and I11 representative samples of the entire catalyst charge were taken after six weeks of synthesis. After extraction to remove adsorbed hydrocarbons, entire particles were analyzed for iron, carbon, nitrogen and oxygen, and the changes in concentrs tion of oxygen, carbon and nitrogen are expressed as differences of atom ratios with respect to iron. For all of the experiments, the changes in atom ratios are divided by the initial extent of reduction. In the next paper it will be shown that the depth of effective catalyst is approximately proportional to the average pore radius. On this basis the ratio of total pore length (particle radius or depth of reduced zone) to average pore radius was taken as a measure of the relative amounts of ineffective and effective catalyst, and the selectivity and composition change data were plotted against this quantity. Despite a number of complicating factom, the data show the following trends as the ratio of ineffective to effective catalyst was increased : (a) the oxidation of the catalyst increased, (b) the deposition of carbon and elimination of nitrogen (if present) decreased, and (c) the average molecular weight of the product decreased. Data for the temperature of reduction series in Fig. 4 are shown as an example of composition changes. The present postulates provide qualitative ex-
Vol. 63
planations for a number of observations on the Fischer-Tropsch synthesis at temperatures below about 290". 1. As synthesis temperature is increased, the depth of the effective layer should decrease, land this is consistent with available data showing that the rate of oxidation of the catalyst increases and the average molecular weight of the product decvases with increasing synthesis temperature. 2. The activity and selectivity of reduced or nitrided fused ~ a t a l y s t s ~ * - ~remains 6 ~ 1 ~ essentially constant for long periods, although the composition of the catalyst changes drastically. For example, the percentage of iron as oxide in reduced catalysts increases from zero to 80 in six weeks of synthesis at 21.4 atmospheres. These data can be explained by the postulate that the catalyst oxidizes predominantly at the interior of the particle. 3. Surface layers of activated steel lathe turning's and steel spheres,'g a thickness of active material of about 0.006 cm. on a core of massive iron, oxidized very much slower than fused catalyst, despite the fact that the massive iron catalysts were operated a t temperatures as high as 290" compared with a maximum of 270" for fused catalysts. These postulates certainly will not explain all aspects of the synthesis. The relative rates of reactions for chain growth, chain termination and hydrocracking probably do not increase at the same rate with increasing temperature, and the rates of reactions with the catalyst, such as oxidation and carburization, probably increase with increasing temperature. Finally, the occurrence of hydrocracking reactions to a significant extent on iron catalysts may be questioned in view of the observations of Koch and Titzenthaler.2'J (17) J. F. Shultz, M. Abelson, L. Shaw and R. B. Anderson, 2nd. Eng. Chem., 49, 2055 (1957). (18) H. E. Benson, J. F. Field, D. Bienstock and H. H. Storch, ibid., 46, 2278 (1954). (19) J. F. Shultr and M. Abelson, unpublished data of Bureau of Mines. (20) H. Koch and E. Titsenthaler, Brpnnsfof-Chcm., 8 1 , 212 (1950).
THE E F F E C T OF IONIZING RADIATION UPON ?-A1203 AS A CATALYST FOR H2-Dz EXCHANGE BY HAROLD W. KOHNAND ELLISON H. TAYLOR Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee (Operated by Union Carbide Corporation for the U.S. Atomic Energy Commission) &&Ed
October 7, I068
Relatively small doses of y-rays a t -78" have been found to enhance the Hz-DZ exchange activity of r-AlzOs. The enhancement by radiation depends upon the presence of a catalyst poison, H20, HZor CzH4 and in the case of H?O is a function of the extent of poisoning, being greatest for the most highly poisoned samples. dowever, A I 2 0 3 not activated by high temperature treatment is not made active by prolonged irradiation. The enhancement in activity produced by radiation decays fairly rapidly a t room temperature and above, and appreciable decay can be noted in some samples (highly poisoned ones, highly sensitive to radiation) even a t -78". Reactor radiation (fast neutrons and y-rays) and radon,produce similar enhancement, but in this case the effect does not decay markedly at room temperature. Although i t is not et possible to assign these effects surely to electron trapping (for gammas) and atom displacement (for heavy particles), tgese are the most attractive hypotheses a t present. I n any case, i t appears likely that further study of these phenomena will help to clarify the importance for catalysis of various crystal imperfections.
Introduction The modification by ionizing radiation of the activity of heterogeneous catalysts has been sought
for a number of years, originally, probably, as an empirical possibility, more recently with some plausible reasons for expecting such ZL phenomenon.
