August, 1961
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(cf. Table I) on a solid which is a catalyst for the thermal dealkylation suggests the possibility that a solid may to some extent direct absorbed radiation energy into that reaction for which it is a thermal catalyst. It is intended to test this speculation by the use of solids of varying catalytic activity. YIELDS(GTY There are several other points of interest in this Hydrogen 0 174 0 182 0 43 2 23 work. The fact that GT(C~&) is approximately Methane constant from F = 0.01 down to F = 0.0017, which 0 0068 0 089 1 92 3 50 Acetylene 0 0 004 0 15 0 05 corresponds to about 1% surface coverage, sugEthene 0 0 001 0 11 0 24 gests that transfer of energy from solid to adsorbate Ethane 0 0002 0 003 0 01 0 07 is rapid relative to the decay time of the responsible Propene 0 0009 0.013 0 09 0 58 transfer entity. The sharp decline of G T ( C ~ H ~ ) Propane 0 0047 0 013 0 06 0 52 above F = 0.01 suggests saturation at this compo0 0017 0 0003 Isobutane sition of those surface sites which are effective in Isopropylbenzene formation so that above F = 0.01 the benzene 3 3 1 8 5 16 additional isopropylbenzene occupies sites which 1 0 0 05 0.3 2 8 Bmzene compete for the transferred energy but are relatively Ethylbenzene 0 02 0 04 0.6 1 5 inefficient in benzene formation, The value of F Isopropyl= 0.01 corresponds to 0.01 g. of isopropylbenzene cycloheuanp 0 07 per g. ofosolid. Using the surface area of 400 m.*/g. Electron fraction of isopropylbenzene = 0.01069. and 50 As2as the cross-section of isopropylbeiizene, * Pressure of 1 atm. c Based on total energy absorbed. one obtains 1.3 x 10'' sites/m.2 which corresponds to 6% surface coverage. h question of considerrnation are somewhat more striking, and the behavior of the benzene yields is unique. The "liquid able interest arises as to whether this value correline" of Sutherland and Allen expresses the behavior sponds to a steady-state concentration of sites expected if the absorbed radiation is initially parti- created by the irradiation dose-rate used or to the tioned betmeen the two phases in proportion to their number of thermally active sites. Oblad2 has electron fractions (an explicit assumption in all presented data for a similar catalyst (12.5% that follows) and if adsorbed isopropylbenzene alumina) from which a value is obtained of 1.3 behaves exactly like the liquid without energy X l O I 7 sites/m.2 for quinoline chemisorption. exchange or interaction with the solid. GT(HZ) SOCONY MOBILOIL COMPANY, Isc. ROBERT R. HEhTZ DEPARTMEKT immediately rises above the "liquid line" from the RESEARCH N. J. value of zero a t F = 0 to a value of 0.18 (about PRINCETON, RECEIVED JCKE14, 1961 equal to the pure liquid value) a t F = 0.0068 and then remains approximately constant to F = 1.0. The variation in benzene yield, however, is (2) G , A . Mills, E. R. B o d e k e r a n d A . 0 . Oblad. J . S m . Chem. even more striking; GT(C&&) rises abruptly from S o c , 72, 1554 (1950). zero to a d u e of 0.96 a t F = 0.0017 (lowest F studied a t present time) and then remains essenTHE 11'IAGIUETIC SUSCEPTIBILITY OF tially constant to F = 0.01 above which a sharp SMALL PALLADIUM CRYST4LS decline occurs followed by a gradual decrease to the pure liquid value of GT(CCHB) = 0.05 at F = 1.0. Sir: GT = 0.96 obtained a t F = 0.0017 requires that In preparing for a study of the pwsible changrs for loopG energy transfer from solid t o adsorbate, in thc paramagnetism of palladium during its G(C6H6) = /&(C6H6) = 0.96. If no energy trans- adsorption of gaseous nitric oxide, NO, it was fer is assumed, then a yield of benzene based on necessary to determine the susceptibility of the only that radiation energy directly deposited in the metal adsorbents prior to the adsorption of the gas. adsorbate may be calculated as G = G T / F = 5.6 One of the palladium adsorbents was a finely x 102 which gives a value of 0.18 e.v. (lOOFIGT) divided crystallin$ sample having an average crysof radiation energy directly deposited iii adsorbate tallite size of 134 A. and a surface area of 3 2 . i square for earh molecule of benzene formed. A compari- meters per gram. One hundred five milligrams of *on may be made with the energy requirements of this sample were weighed into a small quartz bucket the gas phase dealkylation of isopropylbenzerie to which was suspendcd from a calibrated quartz henzene and propylenc for which AFo = 0.57 C.V. spiral spring enclosed in an all glass v a c u ~ ~ynsi t e m . and AH" = 1.0 C.V. at 2.7'. Such results as the After thorough outgassing, t hc mo-c-ahlc rlwtroforegoing strongly suggest that radiation energy magnet, having a maximum field of !NO0 gauss, wits deposited in the solid i. ronverted to a form which is positioned so that the sample was in the rcgioti efficiently transferred to the adsorbed isopropyl- of maximum field gradient. The magnetic force benzene where it 1s ubed for benzene formation on the sample was determined by measuring the n ith an efficiency considerably greater than in extension of the quartz spring in the presence of the liquid phase radiolysis. Further, the unique be- magnetic field. Using the value of this magnetic havior of the hcnzene yields and the higher per- force at 301OK. the mass of the sample and the cent. conversion of iwpropylbeiizene to benzene value of the field strength, determined by previous calibration with samples of known susceptibility, (1) J. W. Sutherland a n d A 0 Allen. J . .4m Chem Soc., 83, 1040 the magnetic susceptibility of the finely divided (1901). TABLEI YIELDSIN ISOPROPYLBENZENE RADIOLYSIS Gasb Gasb Phase Adsorbed" Liquid Temp, "C. 178 385 36 36 5 4 2 11 yo Dec. 18
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palladium turned out to be 12.0 X c.g.s. units or 2.34 times the value of 5.14 X c.g.s. units reported by Hoare and his co-workers' for bulk palladium. Spectrochemical analysis showed that trace impurities were present to the extent of hundredths of a per cent. or less except for platinum and calcium, each of which was present in amounts less than 0.2'%. This seems to rule out magnetic impurities as the cause of this increased susceptibility. It was then decided to prepare a number of silica gel samples having increasing amounts of palladium, reduced from adsorbed Pd(NHJ42+, upon the gel surfaces. X-Ray examinations of the gels containing the larger amounts of Pd showed the deposits to be definitely crystalline. The magnetic susceptibilities of eight different samples containing varying amounts of Pd reduced on each of two silica gels are shown in Fig. 1. The relative susceptibilities of the samples, xS/xo,are plotted as ordinates while the abscissas are the amounts of Pd, expressed as percentage weights of the gels. There can be little doubt but that, in the finer states of subdivision, a paramagnetic metal, such as Pd, exhibits remarkable variations in susceptibility. The samples containing the least Pd had susceptibilities, h,as high as four times that of the bulk metal, XO. The values fell as the amounts of Pd increased and then rose again to a maximum of about three times that of the bulk metal after which the measured values fell, approaching that reported for the metal. Since the samples had a very large surface to
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pendently for each ratio of surface to bulk. If differing surface states do exist and if they are Tamm2 states, then one may extend Hulbert'sa discussion to this problem. Surface atoms are assumed to extract unfilled d-orbitals from the bulk atoms returning electrons to the interior. Finally, a t a high surface to volume ratio, all of the vacant dorbitals will be found in the surface leaving the sample with no magnetic properties. As the surface increases further, d-electrons will be pulled into the surface and will be unpaired, according to Hund's rules. From this point the susceptibility mill increase rapidly as the surface increases. Calculations show that at a volume to surface ratio of 12.4 (i.e., one surface atom per 12.4 volume atoms) the magnetic susceptibility of the palladium should be 2.34 times that of massive palladium, which is what was found in this study. The ratio calculated from the specific surface, in the case of the small unsupported crystals used, turned out to be 12 which is in good agreement with the calculations based on surface states. In qualitntive agreement with this work Kobozev and coworkers4in Russia report large increases in the magnetic susceptibility of cobalt and nickel salts when these salts are highly dispersed on a carrier such as silica gel. These investigators also claim to have found extremely high susceptibility values for platinum when prepared in thin atomic layers on charcoal. The present authors are fully aware of the work of Trzebiatowski and his co-workers". which found that the susceptibility of Pd dispersed on alumina gel was lower than that of the bulk metal. As has been shown in this laboratory6 alumina gel pairs electrons with the free electroil of adsorbed nitric oxide while silica gel does not. The reasonable explanation for the discrepancy between the results here given and those of Trzebiatowski lies in the fact that the alumina is able to pair electrons with the atoms of Pd reduced on the surface thus reducing the susceptibility of thc Pd on the alumina gel. The implications of these higher suceptibility values for catalytic activity should be self evident. This study was supported in part I)y a gr:ult froin the Xatioiial Science Foundation.
L-
15 % Pd
Fig. l.-ktelative magnetic susceptibility of P d us. thc amounts dispersed on silica gel.
volume ratio as compared to the material used by Hoare,' it is suggested that the surface states of palladium differ markedly in magnetic character from the bulk states and that this difference is dependent on the surface to volume ratio. If one assumes that the number of surface states corresponds exactly to the number of surface atoms, then one may treat the surface and bulk states inde(1) F. E. Hoare a n d J. C. Walling, Proc. Phys. Soc. (London), B I B , 337 (1951), a n d Pioc. Roy. Soc. (London), A212, 137 (1952).
SCHOOL OF CHEMISTRY
UNIVERSITY O F MINNESOTA hTINNEAPOLIS, hfINKESOTA
LLOYDH. R E Y E R ~ O N .YAGE SOLBAKKEV Rxcli.\nn IT.ZUEHLKE
RECEIVED JUNE S, 1961 (2) I. Tamni, J . Phys. Z. Sowjel, 1, 733 (1942). (3) 13. M. Hulburt, "The Nature of Catalytic Surfaces," Catalysis edited by P. H. E m m e t t Reinhold Publishing Corporation, New
York. N. Y., Vol. 1, 1954, 1G7-232. (4) Kobozev, Evdomikov, Zuhovich a n d hlalttsev, Zhur. 1''~s. Khim., 26, 1349 (1952). (5) TV. Trzehiatowski, 11. Kubioka and 8 . Silva, IZoczniki Chem., 31, 497 (1957). ( 6 ) Aaee Solbakken a n d Lloyd €1. Reyoraon, J. P h y s . Chem., 64, 1903 (1960).
