June, 1963
COUNTERION DISTRIBUTION IN POLYELECTROLYTE SOLUTIONS
1293
CHARACTERIZATION OF COUNTERIOX DIS'TRIBUTIOK I N POLYELECTROLYTE SOLUTIONS. 11. THE EFFECT OF THE DISTRIBUTION OF ELECTROSTATIC POTENTIAL ION THE SOLVOLYSIS OF CATIONIC ESTERS I N POLYMERIC ACID SOLUTION1i2 BY H. MORAWETZ AND J. A. SHAFER Department of Chemistry, Polytechnic Institute of Brooklyn, Brooklyn, iY. Y Received December 28, 1961 A polyion is expected to inhibit reactions of two low molecular weight species if one is positively and the other negatively charged, while reactions of two species carrying charges of the same sign should be accelerated. The theory of the effect as a means to characterize the distribution of electrostatic potential in polyelectrolyte solutions is developed. The effect is then demonstrated on the inhibition of the hydroxyl ion-catalyzed hydrolysis of cationic esters carrying one and two positive charges by partially ionized polycarboxylic acids. The data are interpreted on the basis of a simple model for the polyelectrolyte solution, in which a region containing a uniform density of fixed charges is in Donnan equilibrium with a region from which the fixed charges are excluded.
In previous communications from this Laboratory*s3 it was pointed out that the rates of reactions involving two charged lorn molecular weight species should be affected by the uneven distribution of electrostatic potential iii solutions containing polymeric ions. A comparison of the rates of such reactions in conventional buffers and in polyelectrolyte solutions should, therefore, yield a parameter related to the distribution of the potential. Using then reactions involving different combinations of charged species it should, in principle, be possible to obtain a number of parameters from which the counterion distribution could be approxima1,ed to a high degree of precision. In the present work these ideas were applied to the hydroxyl ion-catalyzed hydrolysis of singly and doubly charged cationic esters in buffers and solutions of polymeric acids.
Theory Consider a second-order reaction involving species A and B which carry m and n unit charges, respectively. Assume that these species are added to a polyelectrolyte solution a t such lorn concentration that the distribution is not altered signifiof the electrostatic potential cantly. The local concentration a t a point characterized by a potential $ is then
+
where e is the magnitude of the electronic charge, Fi is the Boltzmann constant, and T the temperature. In any small volume element the reaction rate ill depend on the local reagent concentration. Since no work is carried out against the field of the polyion in bringing the two reagents together to form the transition-state complex, the reaction rate constant in any volume element will be equal to the intrinsic rate constant ICo observed in simple electrolyte solutions a t low ionic strength. The observed rate A will then be (1) This research was supported b y grants of the Officeof Naval Research and the National Institutes of Health. It is part of a Ph.D. thesis to'be submitted by J. A. Shafer t o the Graduate School of the Polytechnio Institute of Brooklyn in June, 1963. (2) P a r t I of this series: J . Polymer Sci., 42, 126 (1960). (3) H. Morawetz and E. W. Westhead, Jr., ibid.,16, 273 (1955).
where the square brackets indicate an averaging process over the volume of the system. But the observed ral;e constant k is conventionally expressed in terms of the volume arerage concentration of the reagents
R = k (CA) (c,)
=
~ C A ' C B(Xn) ~
(2")
(3)
Comparing (2) and ( 3 ) me obtain for the relationship of the intrinsic and the observed rate constant
k @ / k = ( P )( X " ) / ( X " + " )
(4)
Qualitatively, thm result predicts IC > ka if m and n have the same sign, while IC < k @if the signs of the charges carried by the reagents are opposite. This may be understood by considering that reagents of the same sign are concentrated into the same region of space, being either attracted or repelled by the polyion. If the reagents ca,rry charges of opposite sign, the polyion will tend to separate them, since it will attract one and repel the other. If we have data for only one combination of m and ??, we have to assume the general form of the distribution of x and eq. 4 will then yield one parameter characterizing that distribution. Obtaining then data for reagents of other charge types, we obtain other ratios of volume average values of various powers of x which will allow us to refine gradually our model, so that all the parameters can be fitted simultaneously. I n interpreting the data from the present investigation, we used a model in which the vicinity of each polyion contains a sufficient excess of counterions over by-ions to render the region electroneutral. The mobile ions in the polymer regions may then be assumed to be in Donnan equilibrium with the electrolyte in that part of the systeni from which the polyions are excluded. The applicability of such a model to polyelectrolyte solutions has been discussed by many ~ o r k e r s . ~ -We ~ have used as a first approximation a model in which the fixed charge densities have a coiistant value p in the region containing the polyions, E$O that the electrostatic potential has only two values defined as $0 = 0 outside and $ = inside the regions containing the polyions corresponding to x,, = 1 and (4) G. E. Xlmball, M. Cutler, and H. Samelson, J . Phys. Chem., 66, 57 (1952). (5) P. J. Flory, J . C h e n Phys., 21, 162 (1953). (6) F. Oosawa, N. Imai, and I. K a g a a a , J . Polymer Scz , l a , 93 ( l b 5 4 ) . (7) F. T. Wall and J. Berkomtz, J . C h e n . Phys., 26, 111 (1957). (8) S. Lifson, tbzd., 27, 700 (1957).
H. MORAJTETZ AND J. A. SHAFER
1294
x,, respectively. If the fraction of the total volume assigned to the regions containing polyions is cp and if the system contains only polyioiis with univalent counterions and a uni-univalent electrolyte of a volume average concentration s, then the electrolyte concentration so outside the polymer region is obtained from
Vol. 67
z =
s = so [(I -
cp)
[(I -
cp)
s = so
+ cpxi] + qz,-l]
(cationic polymers) (anionic polymers)
I1
(5)
From the requirement of equal chemical potential of the mobile ions inside and outside the polymer regions 802
=
so2 =
+
soxi(s0zi p) s0xi-l ( ~ ~ z i - 1 p )
+
(cationic polymer) (anionic polymers)
(6)
Solving (6) for x,,we obtain then
+ ( p * + 4s02)1'2]/2s0 [ - p + ( p 2 + 4s02)"']/2so
x, = [ - p =
(cationic polymers)
(anionic polymers)
(7)
0
(CH3)sNCHzCO +
a
I
/
CH+O--(=J.NO~ 0
I11
In the case of reagent I carrying two ester fuiictions the two steps of the hydrolysis are characterized by different rate constants designated as IC, and k2. It was found that kl was reduced substantially below the value klo observed in buffer solution, when a polymeric acid was added to the system, while kz mas reduced below k2O by a much smaller factor. Qualitatively, this is the expected pattern of behavior, since the polyaiiions will interact much more powerfully with the unreacted reagent with its doubly positive charge than with the half-reacted species IT' which has a net charge of only
We may then use the xi values from (7) in relation 4, so that the ratio of the intrinsic and the observed rate
constant will be interpreted in terms of tlie model for the polyelectrolyte solution described above by
+ 1 - ( P / p ) ] [(P/p)z.in + 1 -
k0/k = [(P/p)~i"
+
(P/P> I/ C(P/p>zimSn 1 - (P/P>I (8) since p = P / p where P is the bulk average conceiitration of fixed charges. We may note that for any experimental value of s and P the value of xi depends only on the choice of p, so that the rate constant ratio k o / k is also a function of this single adjustable parameter. Results and Discussion To test the theory outlined in the previous section and to use it in the characterization of the distribution of electrostatic potential in polyelectrolyte solutions, we have studied the effect of four polymeric acids, PMA, PAA, PAA/,1IIA, and PVME/hlA, on the hydroxide ion catalyzed hydrolysis of esters I, 11, and I1i. CH3 CH,
I I
I
(-C--CH~-C-CHZ-)
I
I
COOH
n
I COOH
(-CH---CH--CH--CH~-)
I
I
COOH COOH COOH PAA/ M A (-C--C---CH-CH2-)
I
I
n
I
COOH COOH OCH3 PVMElhIA
TABLEI THE EFFECTOF COUNTERION COXCESTRATIOS o s THE EBFICIESCY OF 0.01 Ar PMA AS AS INHIBITOR IN THE HYDROLYSIS OF (I) AND ( I V ~ T = 25.8"; pH 7.87 f.0.04; a 2 0.75 klQ/kl
k%Q/kZ
0,0099 5.20 1.98 ,0125 4.37 1.61 .0325 2.14 1.35 ,0575 1.43 1.20 .lo93 1.17 0.98 1.01 .3156 1.09 a /2l0 and kzO correspond to the rate constants for kt and lcz in the absence of polymer and a t the same pH a t which 1c1 and kp were measured.
PAA
I
$1. Table I lists inhibition factors klo/kl and l i ~ ~ / k ? for the reaction of ester I in P V A solutions a t a pH of 7.87 and at varying concentrations of added simple electrolyte. The trend of klo/lil is also illustrated in Fig. 1. The inhibition effect decreases sharply with increasing counterioii concentration reflecting the decreasing electrostatic potential in the neighborhood of the polyion. It may be noted that this happens in spite of a slight increase in the degree of ionization 01 of the polyioii as salt is being added to the system while the pH is kept constant. It is estimated that a! rises from a value of 0.75 at the lowest electrolyte concentration employed, to about 0.97 in solutions containing 0.3 N sodium ion.
Counterion oonon. (moles/l.)
n
COOH COOH PMA (-CH-CHZ-CH-CH~-)
IV
Table I1 illustrates tlie effect of polyioii charge density and charge distribution on the inhibition factor in the absence of simple electrolyte. As would be expected, PAX/MA which carries three ionizable carboxyls for four carbon atoms in the chain backbone as against one carboxyl on every second backbone atom of PlIA, is a more powerful inhibitor if the two polymeric acids are compared at equal degrees of ionization. The effect of tlie charge density along the chain backbone on the inhibition factor is shown in Fig. 2 . I t
COUNTERIOX DISTRIBUTION IN POLYELECTROLYTE SOLUTIONS
June, 1963
1295
TABLE I1 THE EFFECTOF SOME POLYANIOSS O N THE HYDROLYSIS OF (I)I N THE ABSEXCEOF ADDEDSIMPLEELECTROLYTE Conon. Polymer
MA PMA PMA PAA PMA
(base moles/l.)
a
Charges per backbone atom
pH
Inhibition factora
0 0100 ,0100
0.333 0.167 6.62 1.65 ,667 .333 7.54 3.43 .loo .667 ,333 6.83 1.03 .0100 .667 .333 7.42 4.23 .0100 .750 .375 8.02 6.68 PVME/MA ,0100 .750 ,375 9.05 21.6 PAA/RIIA .0112 .591 .443 7.88 12.3 PAA/M A ,0112 .668 .501 8.67 26.3 a Calculated from klO/kl, where kl0 is calculated from the equa(1.28 X 10-Io)/(H+) and kt is the obtion k10 = 4.8 X 10-4 served rate constant for the hydrolysis of ( 1 ) a t 25.8".
+
may be seen that on this basis the effect of PRIA, PAA, and PAA/MA appear to be quite similar, while PVRIE/MA falls sharply out of line. This can be easily understood since neighboring carboxyl groups are not expected to ionize in PA4A/MAif the degree of ionization is below two-thirds. In this range, the charge distribution mill, therefore, be similar at equal charge densities for PAA/MA, PilIA, and PAA. By contrast, oarboxyls attached to neighboring atoms of the chaiii backbone must ionize in PVblE/AIA for degrees of ionization above 0.5 ( i e . , a t charge densities higher than 0.25 per atom of the chain backbone), resulting in higher values of the local electrostatic potential. The larger effect on reaction kinetics observed with polyions deviating further from a uniform distribution of ionizable groups along the chain is a consequence of the same situation which leads for such polyanions to a steeper titration curve. Before considering in greater detail the quantitative interpretation of the inhibition by the polyanions of the hydroxide-catalyzed hydrolyses of the cationic esters, we have to assess the possible contribution of non-electrostatic forces to the formation of molecular association complexes between the polyanion and the ester. This was done by comparing the rate of basic hydrolysis of p-nitropheiiyl acetate (111) in buffer solution and in 2/3 ionized PAA/i\IA. These two rates were found to be identical within the experimental error, indicating that the fluctuations in the local concentration of the uncharged ester are negligible. Applying now eq. 7 and 8 to the hydroxide ion catalyzed hydrolysis of the doubly charged cationic ester I (ie.,7% = 2, n = - 1) we obtain for the inhibition factors Iclo/lcl a t a constant counterion concentration as a function of the bulk average concentration of fixed charges P a family of curves, each corresponding to a given value of p , the assumed concentration of fixed charges within the region assigned to the polyelectrolyte. The relation of these theoretical curves to the observed inhibition factors in solutions containing 2/3 ionized PAA/MA and counterion concentrations of 0.0367 and 0.0184 N , respectively, is shown in Fig. 3. For both counterion concentrations, the experimental data fall into the range between the curves calculated for p = 0.2 and p = 0.3 N . Our model can now be tested by cornparing the inhibition factor which would be predicted for the hydrolysis of the singly charged cationic ester I1 with that observed under conditions where the values given above for the param-
+
tLJ I
I
0.05 0.1 0.15 Caunterion concn., moles/l.
Fig. 1.-The effect of counterion concentration on the inhibition of the hydrolysis of cationic esters by PMA. Ester I, 0; ester IV, e. PMA concn. 0.01 N , temp. 25.8", p H 7.87 =t0.04, a 2 0.75.
0.1
0.2 0.8 0.4 Charsea per backbone atom.
0.5
Fig. 2.-The effert of polyanion charge density on the hydrolysis rate of ester I. PMA, 0 ; PAA, Ei; PAA/LIA, e;PVME/ M A , Q . Polyanion concn. 0.01 S , temp. 25.S0, no simple eleotrolyte added.
eter p are applicable. It can be seen in Table PIP that the values of p which fit the results with the doubly charged ester lead to a calculated inhibition factor which agrees closely with that observed with the singly charged ester 11. It has been pointed out by Strauss and Andergthat the radius of curvature of a polyion chain is generally large compared to the distance over which the forces due to the fixed charges affect the distribution of the counterions in the presence of a simple electrolyte. They have therefore proposed that when treating the polyelectrolyte solution as a Donnan system, the region assigned (9)
U.P. Strauss and P. Ander, J. Am. Chem. Soc., 8 0 , 6494 (1958).
H. MORAWETZ AND J. A. SHAFER
1296
Vol. 67
titration behavior of the polymeric acid used as a n inhibitor. The titration of polymeric acids may be represented by"
Kapp= KOexp(-e$/LT)
7.5
=
KOxi
(10)
where Kap, is the apparent ionization constant given by Kapp
(H+)&/(l - a )
(11)
y'
2 5.0
2.5
0.005
0.0025
0.007Z
P.
Fig. 3.-Dependence of inhibition factor on the concentration of 2/3 neutralized PAA/MA. [Na+] = 0.0367, 0 ; [Na+! = 0.0184, 0 . Temp. 1.8'. Full lines calculated from equations 5-8.
TABLE I11 THEEFFECTOF 0.0112 X PAA/RIA ON THE HYDROLYSIS OF SOME ESTERS" 0.02 a! 2/3, pH = 8.96
-
Eater
Temp.. OC.
Obsd. inhibition Calcd. inhibition factor factor from eq. 7 and 8 kQ/k p = 0.2 M p = 0.3 M .
p-Nitrophenyl acetate 25.8 1.00 1.00 1.00 N,N,N-Trimethyl2-acetoxyphenylammonium cation 1.8 1.18 1.17 1.19 Ester IV 1.8 1.16 1.17 1.19 1.8 2.13 Ester I 2.12 2.83 a Cation concentration = 0.0367 'VI N a f , k O was obtained within 0.03 unit of the pH a t which k was measured. The value of k0 was corrected to the p H a t which k was measured by assuming that kO is proportional to hydroxide ion concentration in this range.
to the fixed charges should be represented by a cylinder whose radius r is given by r =d
- S/K
(9)
while KO is the intrinsic ionization constant to be expected in the absence of the electrostatic interaction of the charges of a polyion. The z i values corresponding to the inhibition factors observed in this study lead to predicted pKapp - pKo values of 1.4 or less, while potentiometric titration gave for 2/3 neutralized PAA/?tlA pKapp - pKo = 3.5. This discrepancy indicates that the Donnan model is not adequate for predicting the titration behavior in this case. Qualitatively this is not surprising, since the small hydrogen ion can approach the polyion much more closely (and thus can penetrate to regions of much higher electrostatic potential) than the bulky ester reagent. It should also be noted that the treatment outlined above for the interpretation of the effect of a polyion on the reaction rate of a multiply charged reagent does not take into account the spatial separation of the charges carried by the reagent. The limitations of such a treatment have become apparent in a recent investigation of the dialysis equilibrium involving polyanions and bis-quaternary ammonium ions, which indicates that the interaction of a polyion with a bolaform counterion is quite sensitive to the spacing of the two cationic charges.1z
Experimental Materials.-The preparation of phenyl 2-bromoethanoate, ethylene bis-[N,N-dimethyl-N-(phenyl)-carboxymethylammonium] bromide, and the copolymerization of acrylic acid and maleic anhydride are described elsewhere.la Poly-(methacrylic acid)(PMA) was prepared by heating (1.5 hours under nitrogen a t 60") 120 ml. of methacrylic acid with 350mg. of aeobisisobutyronitrile in 550 ml. of butanone. The resulting polymer was collected by suction filtration, washed repeatedly with ether, and freeze-dried from a water solution. Poly-(acrylic acid) (PAA) was prepared by heating (2 hours under nitrogen a t 60") 100 ml. of acrylic acid in 700 ml. of benzene containing 500 mg. of azobisisobutyronitrile. The same procedure employed to collect and purify poly-(methacrylic acid) was also used here. The copolymer of vinyl methyl ether and maleic anhydride (PVME/MA) was from General Aniline and Film Gorp. (Batch 30.31).
where d is the radius of the rod representing the polyN,N,N-Trimethyl-N-(phenyl)-carboxymethylammonium Broion, K is the Debye-Huckel parameter, and Xis a constant mide.-Anhydrous trimethylamine (4.0 g., 0.068 mole) was close to unity. Basing the calculations of K on the added t o phenyl 2-bromoethanoate (4.0 g., 0.019 mole) in 50 concentration of mobile ioizslowe obtain for the counterml. of dry acetone (dried over type 4-4 molecular sieve). After ion concentrations of 0.0184 and 0.0367 N with the 1 hour the precipitated product was filtered and washed with polyelectrolyte conceiitra$ions used, value: of 1 / ~ acetone (dec. 197-198"). Anal. Calcd. for C11H1BBrS02:C, 48.19; H, 5.88; Br, ranging from 23.9 to 24.8 A. and 16.2 to 17.0 A., respes29.15; pu', 5.11. Found: C, 48.20; H, 5.96; Br, 29.05; tively. These values may be compared to r = 32 A. N , 5.20. corresponding to a cylindrical region within which the Polyacid Solutions.-The letter iV when used to indicate concentrations, refers to the normality of carboxyl groups in the fixed charge concentration p is 0.2 N and a radius of solution. I n all cases only sodium and ester cations were presr = 26 A. corresponding to p = 0.3 AT. Jn these ent. When the counterion concentration exceeded the concencalculations the charge density of the polyanion is tration of the formally ionized carboxyls on the polyanion, based on an all-trans conformation of the chain backbone. the excess counterions were introduced from stock solutions of partially neutralized sodium hydrogen phosphate and partially Considering the uncertainties of the proper values of neutralized boric acid. The degree of neutralization of these d and S the agreement of r with 1 / may ~ be considered acids was such that these stock solutions had a pH equal to the satisfactory. final p H of the polyacid solution. Yet another check of our analysis is available in a (11) J. T. G. Overbeek, Bull. soc. chzm. Belges, 57,252 (1848). comparison of the observed inhibition factors with the (12) H. Morawetz and A. Kandanian, in preparation. ( I O ) A. Katchalsky and
S.Lifson, J . Polymer
Sce., 11, 409 (1956).
(13) H. Morawetr and J. Shafer, Beopolymers, 1,71 (1963).
June, 1963
PHYSICAL P R O P E R T I E S OF TRa4XSITIOK M E T A L
The p H measurements were made with a Cambridge Research Model p H meter. The symbol OL refers to the ratio of equivalents NaOH added to equivalents of carboxyl originally present. Poly-( acrylic acid-maleic acid) 1-1 copolymer stock solution (0.1128 N ) was prepared by dissolving poly-( acrylic acid-maleic anhydride) 1-1 copolymer (6.400 g., 0.1128 carboxyl equivalent) in a 1liter volumetric flask and making up to volume with de-ionized water. The solution was then kept in a 85" bath overnight to hydrolyze the anhydride groups. 314-Neutralized Poly-(vinylmethyl ether-maleic acid) Copolymer Stock Solution (0.100 N).-The anhydride copolymer was dried to constant weight in a vacuum oven and 3.90 g. (0.05 carboxyl equivalent) was placed in a 500-ml. volumetric flask containing 17.53 ml. of 2.139 M (0.0375 mole) sodium hydroxide. De-ionized water (approximately 350 ml.) was added to the flask. The flask was then kept in a 60" bath overnight in order to hydrolyze the anhydride groups. The solution was cooled t o room temperature and filled to volume with de-ionized water. Rate Measurements.-In the case of the cationic esters, release of phenol was followed by the rise in the optical density a t 273 mp on a Beckman DU spectrophotometer. A special waterjacketed cylindrical cell equipped with a mechanical stirrerdesigned to prevent bubble formation was used. The cell was 10 cm. long and had a capacity of 50 ml. An aliquot of the ester stock solution (in the neighborhood of 0.1 ml.) was introduced by means of a blowout pipet to start a kinetic run. Prior to each run stock solutions of esters I and I1 were prepared in 0.01 N HC1 and methanol, respectively. In the 10 cm. cell, esters I and I1 were present in concentrations of 0.6 mg./50 ml. and 0.7 rag./50 ml., respectively. The rate of disappearance of p-nitrophenyl acetate was followed a t 273 mp. The test solution containing 0.64 mg. of the ester in 50 ml. was thermostated and at various time intervals aliquots were removed from the flask. The optical densities were measured in a 1 cm. cell. In all cases the temperature was controlled to a t least 0.05'. Kinetics.-The hydrolysis of I in basic solution may be depicted by
CATALYSTS
1297
For the two consecutive reactions shown above, the fraction of ester groups ( E )a t an,y time ( t ) is given byI4
where CE represents the molar concentration of ester bonds, COthe molar concentration of ( I ) a t t = 0 and y = k1/2 ( k l kz). The fraction of ester bonds a t any time was calculated from the optical density D of the solution
D
=
CEeE
+ (2C0 - CE)e, + I:
where and ep are the extinction coefficientsassociated with the ester bonds, and with phenol, respectively, and U is the optical density due to background which is unaffected by the reaction. The initial slope of a plot of -In (D, - D) against time (where Dm is the optical density a t the conclusion of the reaction) equals k1/2, and the final slope is equal to kz if kl > kz and to kl if kz > kl. Although the initial slopes could be used to give good estimates of k l , the final slopes could not be used for an accurate estimate of kz, because kl/kz was not large enough to observe the disappearance rate of ester bonds in I1 without some contribution from I. Thus, the method of initial slopes was used when only values of kl were desired. In order to calculnte easily both kl and kz a graphical method was employed in whieh the observed dependence of In E on t was compared with curves calculated for various values of y. As expected, the pseudo-first-order rate constants of ester hydrolysis in conventional buffer solutions varied with hydrogen ion concentration according to
In order t o represent the data, the following values were taken for ~ ( H ~ and o ) k ( o ~ ) .At 25.8'
Ester I, ~
( H ~= o )
4.5 X
sec.-l,
1. mole-' sec.-l
k ( ~ ~ ~ =) l1.28 < ~X Ester IV, J Z ~ H ~ O )= l.0 X l o w 4sec.-l,
k(oHjKw = 0.375 X 10-lO1. mole-lsec.-l
IA -t @OH
-
The kinetics of the hydrolysis of esters I1 and I11 in buffer solution were first order. Plots of - ln(Dm D ) were linear in time, and were used to obtain the pseudo-first-order rate constants.
-
IA
kz
(14) A. A. Frost and 1%. G. Pearson, "Kinetics and Mechanism," John Wiley and Sons, Xew York, S . Y . , 1953,p. 153.
ADSORPTION STATISTICS ,4ND THE: PHYSICAL PROP'ERTIES OF TRAKSITION METAL CATALYSTS BY CHARLES P. POOLE, JR. Gulf Research & Development Company, Pittsburgh, Penntrylvania Receive8 December $6, 196.3 The number of small clusters of transition metal ions formed on an alumina surface by adsorption from solution was calculated from a statistical model, and the resulting variation of clusters with metal content was correlated with electron spin resonance, nuclear magnetic resonance, magnetic susceptibility, and gas adsorption data for alumina impregnated with chromium, cobalt, and nickel salts.
I. Introduction Over the paat fern years several members of this Laboratory ha,ve been studying catalysts prepared by impregnating and with transition metal oxides. For example, chromia on alumiiia was studied by electron spin resonance, (1) D. E. O'Reilly, Adtan. Catalysts, 12, 31 (1960). (2) D. E. O'Reilly and D. 8. MacIver, J. Phys. Chem., 66, 276 (1962).
magnetic susceptibility,3 nuclear magnetic resonance ( n . r n ~ . )and , ~ catalytic techniques5; cobalt on alumina X-ray a bwas studied by magnetic (3) J. R. Tomllnson and G. T. Rymer, preprints, Division of Petroleum Chemistry, American Chemical society National Lfeeting, April 5-10, 1959, Mass. (4) D. E. O'Reilly and IC. P. Poole, Jr., t o be published. ( 5 ) J. 11. Bridges, D. 8 , MacIver, and H. H.Tobin, Paper No. 110, Second International Congress on Catalysis, Paris, France (July, 1960).
