Oct,., 1961
CATALYTIC ACTIVITY OF
LANTHANIDE OXIl)IG3
1%"
('ATALY'I'TC ACTIVITY OF THE LANTHANIDE OXIDES FOR THI3 DEHYDROGENATION OF CYCLOHEXANE BYC. B. MCGOUGH AND G. HOUGHTOK ChenLzcul Enyaneerzng L)epnrtnLal, Unzverszty of Pzllshuigh, Pattsburgh, Penna Rereaved June 5 1961
A rising temperature flow reactor has been iisctl to determine the activation energies, frcqiiency factors and kinetics for thc. heterogeneous dehydrogenation of cyclohexane over Laz03,Cc:Op,PrzOz,Ndz03and RmpOr. The effect of crystal structure was evaluated by preparing the oxides in their different, crystal forms (A, U or C stritctiires) and the effect of a support w:is investigated by depositing the oxides on ahimina. Within the error of experiment, no effect of the support, crystal structuro. paramagnetism or valence state could be detected, indicating perhaps that the catalytic activity of the lanthanide oxides is associated with outer electronic states. instead of a thermal conductivity gauge. This method of Introduction analysis was possible by virtue of the fact In a previous communication Bridges and Hough- continuous that the benzene-hydrogen-nitrogen mixture formed by ton' have evaluated the activities, for the dehydro- complete dehydrogenation had an ionization considerabl!. genat,ion of cyclohexane, of certain transition metal different from that of the cyclohexane-nitrogen feed The strength of the ionizing source was 10 oxides and corripounds using a rising temperature ture. giving a current changing with gas composition iri flow reactor. :In general the transition elements curies, the range 5 X 10+0 t o 5 X l o 4 amp. a t a potential of 300 exhibit several valence states and their oxides are volts applied between the central electrode and the wall of semiconductors that have a pronounced para- the ionization chamber, which had 'a free volume of 5.9 The ionization current passed through a 2 X IO8 magnetism caused by holes in the d-band. In cc. ohm resistor, the voltage developed being converted to t,hese respects the rare earth oxides provide a C-10 mv. at an impedance of about 100 ohms for recordin4 contrast since valence states other than trivalent by means of a k e d s and Northrup chopper stabilizcd p H are relatively u:ncommon and their paramagnetism meter. Potentiometers were provided to adjust the 0 and 1 0 0 ~ oconversion points to be exactly 100 divisions apar! arises from inco~nplet~e 4f orbitds which are screened for calibration purposes. Mass spectrometric analyses of th(, from external .influence by 5s arid 5p elect,rons. product gases showed that the recorder scale was linear Furthermore, three different, crystal forms have within =tl% conversion in the range C-lOO% and thai been found in the rare-earth oxides, the most com- thermal decomposition of cyclohexane by rare rart,h oxide. not appreciable below 610". mon being the X aid C structures in which the was In the present equipment the carrier stream of dry oxygencoordinatjon number of t,he metal ion is 7 and 6, free nitrogen was saturated with cyclohexane (Spectroscopic. respectively, and the R structure about which grade) in two bubblers maintained at 20.90 +O.0lo and :I lit.tle is known. To evuluatt: the effects of para- constant pressure of 1090 mm. giving a feed composition o mole yo cyclohexane for nitrogen flow rates in the rang(' magnetism, crystal structure and crystal field on 7.3 75-150 cc./min. The feed passed into a Vycor reactor of catalytic activitty, thr present communication is the design used previously' containing about 13 cc. of concerned with the activity, for the dehydrogena- rrttalyst and then through capillary connecting t.ubing to tion of cyclohexane, of the lanthanide series of the Sr-90 conversion detector. *4s in the previous apparatus thc flow through the detector was maintained at a constant rare-earth oxides in their various crystal forms. value, in this case 60 cc./min., by using a bubble type presIn this connection E;omarewsky2 has demonstrated sure regulator containing glycerol. Except for the mod(, t>hat Nd203 and Smz03show activity for the de- of generation of the steadily changing reference voltagc. hydrogenat,ion of alcohols and cyclohexane as (cf. Redfield7), the temperature programming circuit was similar to that described earlier' in the present case thv well as for the cyclization of n-heptane and 1- linearly varying reference voltage was obtained by applyiiir: octene, while Taylor and Diamond3 have shown a constant voltage to the input of an analog computer that the par:magnetjic oxides Gd203 and Nd20a integrator. The catalyst temperature was measured by :t are more active in c.atalyzing t,he ort>ho-para hy- Chromel-Aliimel thermocouple in a stainless steel thrrnmodrogen conwrsion than the non-paramagnct,ic well placed in thc catalyst hed and the teniperature was recorded to within + l o by a C-700" strip chart. rccordt.r IJa203. Hopkins and Taebe14 have sumniarixnd running at the same chart, spctd (0.667 in./min.) as th(L the earlier work on t,he catalyt,ic activity of the rare (:onversionrecorder. Thc cqiiipmt:nt was oprrat,rtl as drscribed by Bridge8 aut1 eart,hs for oxidatlion, hydrogenation, decomposi1 foiighton,l the final data boing t,aken at const.ant Irrt.ivity tion and syir t he!+ of organic compounds. with i i t,c:mperaturt: ramp of 2 O per minilt,e and it flow r:tt(of I00 c*ct./min., whcri no significant difference WRS otiwrvExperimental
Equipment. the apparatus it~eclto inrasure t,he cainlytic activity of rare earth oxides for the dehydrogenatioii of cyclohex:tne RBE a modification of the rising temperaturcn flow r t w t o r previously described by Bridges and Houghton.' T h e major innovation in the present apparatus was that the gas leaving t,he reactor was analyzed continuously hy a Sr-W ionization detector of the hvelockS type (cf. Walter6) (1) J . M. Bridges and (3. Hougbton, J . A m . Chem. S O ~ .81, , 1334 (1959). (2) V. I. Koniarewsky, Ind. Eng. Chem.. 49, 264 (1957). (3) H. S. Taylor and H. Diamond, J . Am. Chem. SOC.,67, 1251 (1935). (4) B. S. Hopkins a n d DI'. A . Taehel, Tram. Electrochem. Soc., 71, 397 11934) (5) J. E. Lovelock, Nature. 181, 1460 (1958). (6)
J . F . W-alter. M.S. Thesis. University of Pittsburgh, 1959.
:~th betwecw t.hc rising and falling tcmperat,ure cyc:l(bs.
In order to test the accuracy of t,hr calibrations and to ?how that t l i c b t,iinc. constants of the thc:rmocoupl(., t.r:triirpurt linw and detector w ( w nc!gligiMe, tho convrrsion-1 vmper:iture curves for a highly active fluorine-promot,td Pt /.4l?O catalyst werc determined a t decreasing flow rator until t.hr curves hrcamct identical and independent of flow ratc, thus indicating that rquilibrium had been r e a c h d . Th(Jresultin# curves agreed, within experimental error ( f14.'; conversion > , with curves calculated from the free energy data of Rossini .* The standard free energy for cyclohexane dehydrogenatioti interpolated a t 227" from the present work is 5.4 kcal./g._.___
-
( 7 ) .J. A . Redfield, M.S. Thesis. Lnitersity- of I'ittshurgh. 1959. (8) F. n. Rossini. et al.. "Selected Values of Physical and Thernio-
dynaniir Proljerties of Hydrocarbon and Relatcd C:oiiiporinds." Csrnegie Press, Pittsburgh. Pennsylvania. IY.53.
C.B. MCGEOUGH AND G.HOUGHTON
1888
Vol. 65
TABLE I SUMMARY OF EXPERIMENTAL DATAA N D RESULTS Symbol for figures
'a i
L
)
,
P
Crystal structure
Paramagnetic susceptibility, x , a t 670°, cni.l/o. X 10'
Rutile
0
11.0
A+C
I O . 5-5.9"
C
Surface area, m.'/g. Used
K 5 t i c data Temp. range, "C.
kcal./g.-mole
9.1
530-610
33.0 f 2
10.3
10.6
530410
36.7 f 2
11.4
9.2
9.2
530-610
36.8 f 2
C
5.5
13.1
13.1
530-610
35.1 f 2
A
0
15.9
9.8
530-610
34.3 f 2
Fresh
e
A S C
10.5
8.5
9.7
530-610
37.9 f 2
c,
A
11.4
2.7
3.1
560-625
39
f 4'
u
I3
5.5
1.0
1.2
570-625
36
f 4b
A
..
0
181
167
490-610
37.0 f 1
a
..
0
..
157
490-610
39.9 f 1
e
..
I O . 5-5.9"
1 68
155
490-610
34.9 f 1
..
11.4
168
159
49O-Gl0
32.1 i 1
..
5.5
196
168
4YO-610
32.9 f 1
..
0
195
193
550-625
43
+
H X
-
w,
f 5Ib
a I'nramagnetic susceptibility of 10.5 X 10-6 for PrlOl and 5.9 x 10-8 om.a/g. for Pr6011. The amount of PrsOli is unknown. * Appreciable correction for thermal decomposition was necessary. Calculations arc based on zero-order kinetics.
mole, compared with the value of 5.17 calculated from the Ptandard heats of forniat.ion of benzene and ryclohexane given by Rossini.8 The rare earth oxides used in t.he invest,igation were La& CrO,, Pr203,Nd20s and Smz03,with a minimum purit,y of $19.9%, olbtained from Research Chemicals Incorporated, l h r b a n k , California. Supported (on alumina) and unsiipported oxides were used in the investigation. Unsupported Rare Earth Oxides.-The unsupported oxide catalysts, witaht,he exception of Ce02, were prepared in thc,ir various ci:,ystal modifications by heat treatment as described by Iandelli.9 As received from Research Chemicals Incorporated, the oxides Nd20a and SmzOoshowed the type C structure, which is a partial face-centered cubic structure of the TlZO8 type (rf. Iandelli9 and Wycltoff'o). The type C structure occurs when the oxides are formed a t low temperatures and is essentially a distorted fluorite structure in which trivalent rare earth ions replace calcium and three fourths of the fluorine positions are occupied by oxygen ions, the remaining positions being vacant. The praseodymium oxide, even aftrr use in the reactor, was found to be a mixture of types A and C PrpOs together with some Pr6011. In the case of LazOait waa not possible to investigate the activity of the C structure, since the temperat.ure of reaction was si$ficient to convert this oxide completely into type A wit,hin the few hours necessary to reach a state of constant activity. This agrees with the observations of Iandelli,s who found that temperatures in the range 500-600" were sufficient to convert type C: La201 into type A, whereas temperatures above 800" were required for the other lanthanide oxides. By heating the oxides a t 1200' for 3 hours in an oxygenfree nit,rogen atmosphere La2OI and NdZO3 wcre converted to t h e ty e A st,rueture, Smz03 to the type B structure, while Prz& provided a mixture of types A and C with no (9) A. Iandelli. Cwa. cham. i t 4 17, 312 (1947). (IO) R. W. G . Wyckoff. "Crystal Structures," VoL 11. Part V. Intwscience l'ublishers, Inc., New York, N. Y., 1951.
PrlOll. In order to ensure that no PrsOll was present before heat treating the oxide, it was first reduced a t 600" for 3 hours. The type A structure is hexagonal and charact,erized by a complex 7-coordination of t,he metal ion [rf. Iandelli? Wyekoff'o and I'auling") while type R , althorigti clearly distinct from types A and C, has not ))em positively classified, although Doug1:lss and Staritjzky'z alternately describe it M pscudotrigonal, ortJiorlioml)ic or monoclinic. The Ce02 ovide was not heat treated and the sample rised was found to have the normal rutile st,ructure in which the Ce4 ion is 6-coordinated. In order to convert the finely powdered oxides into n form that could ,he used in the flow reactor without incurriiig an ~ S C C S S I V C pressure drop, thev were first pulletd a t 12,000 lh./in.z, using deionized water as :t binder. The pellets then were sintered in nitrogen a t 400" for 2-3 horirs to remove water vapor. The sintering teinpcrat,ure was low enough to prevent any further changes in the crystal structure. The sintered pellets then were crushed and sized hetwern 30-100 mesh screens. The resulting innteri:tl had a surface aren of about 10 m.Z/g., the exact valiiw for each oxide before and after use being reported in Table I as det,ermined by the BET method using low temperature nitrogen adsorption. The crystal structures of the unsupported oxides were identified after their use as catalysts by powder X-ray methods, using a General Electric XRD3 spectrometer with molybdenum radiation (K, = 0.07926 A.) and a zirconium filter. Supported Rare Earth Oxides.--The 1:tnthanide rare earth oxides were sl1DDOrted on 28-48 mesh Alcoa F1 alumina by first dissolving (him in nitric acid, evaporating to dryness, dissolving the nitrate in enough water to fill the pore volume of the alumina (0.25 C C . / ~ . ) and then impregnating t,he support under vacuum. The impregnated alumina +
(11) L. I'auling, 2. Krisf., 69, 415 (1929). (12) R. M. Douglasa and E. Staritsky, Anal. Cirem.. 28, 652 (1956).
CATALYTIC ACTIVITYOF LANTHANIDE OXIDES
Oct., 1961
Temp., “C 600 550
was dried at 150” for 3 hours and then sintered at 525” for 4 hours to decompose the nitrates. The final catalyst contained 15 wt. % of rare earth oxide in each case and had a BET surface arca of about .I50 m.’/g., the exact values before and after use bemg given m Table I.
Results The activation energy E and frequency factor B in the Arrhenius equation for the velocity constant k, = B exp(-E/RT) were determined from the experimental temperature-conversion curves by plotting log & versus 1/T as described previously by Bridges and Houghton.’ For a certain amount of catalyst in the reactor the function qh, depended onIy upon the conversion and the order of the reaction n, since the feed rate, feed composition and total pressure were the same for all the experiments. If y = 0.072 is the mole fraction of cyclohexane in the feed, z is the fractional conversion of cyclohexane to benzene and hydrogen and 6 = 3 is the increase in molts of the reacting system per mole of cyclohexanc mverted, then & and 61 for zero and fmt-ordc kinetics, respectively, are defined by the relation! +o = x (1) 9 1 = -- 1 ~ 6 2+ ( 1 ~ 6 In ) (1 - x)l (2) In all cases the best line through the data was deterniiiicd by lea ;t squares methods which also gave the indirect precision of E and B reported in Table I. In order to determine whether thermal decomposition N ~ significant S in the temperature range 4WG1O0 cver which the lanthanide oxides were active, the reactor was packed with Vycor wool and the thermal decomposition measured over a Ride ten-perature range. Except in the cases of M 2 0 3and hcat treated ?;dz03 and Smz03, any corrections for thermal decomposition below 610’ \*:ere less than 4% conversion and did not appreciably affect the numerical values of E and B within the experimentai error-furthermore, mass spectrometr,c analyses showed that the small amount of thermal cracking that did occur in the rare carths could l)e corrected for by using the data obtained with \‘ycor wool. However, in the cases of the heat treated NdzOp and SmzOsactive in the range 56W25’, i,he surface areas of these oxides were low, so that a significant correction mas requircd for thermal decomposition and the error of nicasurement of E and B has been adjusted accordiiigly in Table I. In addition, Table I and Figs. 2 and 3 shorn that although Alz03 is considerably less nctive than either the supported or unsupported rare earths, it does possess some activity after correction for thermal decomposition that InnesI3has ascribed to the presence of trace quantities of iron or other impurities. In order to compare the present results for the rare earth oxides with those obtained for the transition metal oxides, the temperature-conversion curves shown in Fig. 3 were computed on the basis of the same feed rate per unit surface area of 0.0184 cc./min., m.2used by Bridges and Houghton.’ Order of Reaction.-Figure 1 shows typical plots of log & versus 1/T for various supported and unsupported oxides in different crystal forms. The experimental data of Fig. 1 were obtained
+
(13) W. B. Innes. "Catalysis," Vol. 2, Reinhold Publ. Co., New York. N . Y.. 19%.
1889
r-
1
e
2.0
-
I
500 I
I
1
1.1
1.2 1.3 (I/T) x 103, OK.-’. Fig. 1.-Plots of log +,, verersu.s 1/T (cf. Table I for a list of symbols): -, zero-order kinetics; - - - -, first-order
kinetics.
with roughly equal volumes of catalyst (12-14 cc.) and no attempt has been made to correct the data to either a fixed area or volume of catalyst, since the slopes and shapes of these curves are unaffected by the amount of catalyst, which is only important in computing the frequency factor, B. Plots similar to Fig. 1 were obtained for the other catalysts in Table I and an analysis of the slopes showed curvature in the first-order case but not in the zero-order case. That the reaction appears to be zero order indicates that the active surface is always covered by adsorbed reactant. However, in the cases of M203arid heat treated r\id& and Sm20R,the conversions after correction for thermal decomposition are low so that it is impossible to distinguish betu een zero and first-order kinetics, so that the data in Table I are reported for zeroorder kinetics, the values for first-ordcr kinetics being virtually the same for these oxides. Activation Energies and Frequency Factors.Table I summarizes the nctivnt ion eiieigies, E, and frequency factors, B, together with their accuracy. Since the rcactioii appears to be zero order, the values girrn in Table I are “true” activation energies arid do not require correction for the heats of adsorption and desorption (cf. Hinshelwood’4). All the catalysts, supported and unsupported, regardless of crystal structure, have activation energies in the narrow band 3 2 4 0 kcal./g.-mole, indicating that the surface and bulk characteristics of the solid have little influence on the energy barrier for dehydrogenation. It is interesting to note that when log B was plotted against E/T, a straight line of slope 0.22 was obtained, Cremer16has interpreted this linear relationship as due to quantum leakage of electrons on the catalyst surface by a “tunnel effect.’’ However, Barrerl6 has pointed out that the variation in velocity constant k, = B exp(-E/RT) is often considerably less than the variation in (14) C. N. Hinshelwood, “The Kinetics of Chemical Change,” Oxford Universitv Prese. 1940. (15) E. Oreme;, J . ohim Qhys.. 47, 439 (1960). (16) R. M. Barrer, ibib, 47,445 (1960).
1890
C. R. MCGOUGH AND G. HOUGHTON
Vol. 65
relation between paramagnetism and catalytic activity in the lanthanide oxides, suggesting that t'he unpaired 4f elect,rons do not participat,e in the mechanism. This might be expected, since the 4f electrons are effectively screened by the outer shells to sucah an extent that the paramagnetism is not influenced by crystal struct'ure or the presence of a support (cf. Selwood"). That Taylor arid Iliamond3 have observed an effect of rare earth paramagnetism on t,he ortho-para, hydrogen conversion is not contrary to the present measurements, since it is readily explained on the basis t,hat. spin isomerism does not necessitate electron exchange (cf. Kalckar and Tellerlg and SelwoodI7), (f L L - . L l - u L J . L - d the only requirement being the existence of a 0 2 4 6 8 10 12 rnagtietic field. l':~r:iiiiagiictic*susceptibility, x, c n ~ . ~ j X g . 108. Inspection of Table I indicates that in the case Fig. 2 . ---h;ffc:ct of paraniagrietic susceptibility and crystal .!ipport,ed oxides, t,hc activation energy structure o i l rcaction rate, T (at 5 i 0 ° ) , activatiorr energy, E , of the factlor tend to reach a maximum at, >tiid frequcricy factor, H . (cf. Tablr I for :i list of symbols unci values of ;( :it 570".) the point ilcre the paramagnetic susceptibility is the largc.&\ '11 each series, the re\-erse being true for the supported oxides. However, the varjatsions are on the borderline of the experimental error, so that. if pnrarnagnet~ismhas any effect at all, it is small. E'urt,hermore, if the specific reaction rate, r , is computed from G and B i n Table I, t'heii no apparent, t,rend in this variable wit,h paramagiietism can be detect'ed (cf. l i g . 2 ) . Effect of Crystal Structure.---I:igure 2 and Table I summarize the data for the effect, of crystd struct'ure on the activit,y of the unsupport,ed lanthanide oxides. It is evident from these data that thc. cffect of crystal st#ructnre is not. significant. -1iiO 500 450 500 500 550 On t.he basis of a geometric model of mtalysis this Temp., "C. Fig. 3.-~.lonversion-temper~ture curves calculated at a might' be expected, sincc in t'he type C: sesquioxides f w d rate per u n i t area of 0.0784cc./min., (cf. Table I the metal-oxygen interionic dist:tnc:e lies between for a. list of symbols). -~ , caalcd. from experimental tem2.34 and 2.44 h.,nhilc in the t,ype h oxides char~terature-conversion (l:Lt,a. hclow 610"; - - - -, extr:ipolated acterized by 7-coordinat,ion each metal ion has iisirrg constant,s E ariti /3 irr Table I. four oxygeris ai. about 2.4 A. and three at 2.7 A., i:xp( - E/ Zi2'1, i n whicali c ~ s clog li,, m:ty 1)c taken in no case a very large variation. However, if :LS approximately constant, n.hcvi the slope of log t'he hypothesis is introduced that crystal field K versus E ! T is simply 1/2.303R = 0.22. Hence splitting of bonding levels can ha\-e an effect on in the prcseirt cas(', no physicd significance can the activation energy, it might lw expected that. he attichod to thc linearity of the plots of log B the differenve hetween 7 and 6 coiirdination in wrsus F , / l structures A aiid C might have a significant effect on the activit'y, and furt,hrrmore t h a t the €3 strucDiscussion t j i m of SmpOamight show difiewtiws from the A It is now possible t o discuss the iiiflueilc~of para- and C structures. That; no such differences were magnetism and crystal structure 011 the avtivity of observed tends l m oindic*atjethat the cbryst,al field is t,he lanthanide oxides. lint' a significwit)cont,rihut,ioii to thr :wtivity of t,he Effect of Puamagnetisrn.--ManJ. iiivestigathw, rare earth oxides. particularly those of S e l ~ o o d , 'have ~ inc1ic:atetf hnot her iriterest,ing case is that. of praseodymium tjhat c;~t,algtinactivity and paramag~iat~isniarc related in t,htl transition elements. Figure 2 shmvs oxide, Ivhich is a mixt>uraof t3hcA and C structures plot of rc:wt)ion rate, T , activation energy, h', t oget,her wit.h some Pr6OI1i n t,he sample that was a.nd frequcwcy fact>or, K , URTSUS paramagnetic. iiot. heat t.rcated at, 1200'. It>might, be expect>ed susccpt ibili ty for thp rare eart-h oxides c.alcnlal,ed t'hat sinw t.he Iat ter s:imple contained a mixturc at, t.he temperature of 570', using the suscept,it)ility of valewe stoaksthat, impurit,y levels in the solid data of Selwc~od~' and Rabideau.ls Iii the cas(: of st,atc sense might afft*rttt,hc surface hnetics; no Z'r,jOll the value of 5.9 x ~ m . ~ / gwas . est,i- wch effect,s were observed. lTurthcrmore, Ce02 mated by assuming t,hat>the Curie-Weiss law was shows the same activation energy and kinetics as obeyed above 2 5 O , as indicated by the data of the sesquioxides, again indicating that the valence Rabidcau i n the range -70 to 25'. It is evident state of the metal ion is riot' an important criterion. It would be erroneous to conclude from the abow from Fig. 2 t'hat t>hereappears to be little corobservations that crystal and defect st,ructure had (17) P. FI'. Gelwood, "Magnetochemistry." Interscience Publishers, Inc., NePv Y o r k . N. Y.,1948. (18) S, W. Rabideau, J . Chem. Phys.. 19, 874 (1951).
(19) F. Kalckar and E. Teller, P w c . Roy. Soc. (London). AMO, 520 (1935).
Oct., 1961
CATALYTIC ACTIVITYOF LANTHANIDE OXIDES
1891
no effect upon ca,talytic activity, since the surface structure of the [solid may be quite different from that of the bulk as measured by X-ray spectroscopy. However, Schwa,b and Martinz0 have similarly found no effect of crystal structure in that the P and y forms of EZI have the same activity for the decomposition of ethanol, and in no case did they observe any significant change in catalytic activity for the decomposition of NH, and various organic compounds cata1,yzed by the different crystal forms of CUI,Gaz03,&I, T1I and Ka2SOi. Comparison of Activities.-Although the supported oxides were more active on an equal volume basis than the unsupported oxides, E'ig. 3 shows that on an equal area basis the unsupported oxides are more active. At the same feed rate per unit area of 0.0184 cr:./min., m.2 t,he supported oxides would be act'ive in the temperat,ure range 470570', while the unsupported oxides would be active in the range 430-530'. This result is not unexpect'ed since the total surface area of the catalysts was used in the calculations and the B E T method cannot idistinguish between total surface area of the catalyst, and the surface area of the deposited oxide. The present observations tend to lead t,o the conclusion that the surface area of the deposited oxide is lower than that of the unsupported oxide on the basis of an equal total area of catalyst and that the support, has lit,tle effect on t,he activity. The solid region of the curves in Fig. 3 terminst'es a t the temperature (610') in the actual measuirements, on an equal volume basis, above which thermal decomposit'ioii might affect the results. The broken line in l'ig. 3 represent's the extrapolatmionof the dat,a using values of E and B obtained in the temperature region below 610'. In the case of the heat treated forms of NdzOaand SmzOa the solid curves in 1:ig. 3 are quite short,, because t'he heat' treatmentj of these oxides at, 1200' mused pronounced sintering wit'h consequent reduction of the surface area to 1-3 m.?'g., as shown by Table I. It is evident from Table I that, with the exception of YdzO, and Smz03,the lanthanide oxides are thermrtlly quite st'able. comparison now can be made of the lanthanide rare earth oxides and some transit>ionmetal oxides using the data {of Bridges and Houghton.' For instance, 011 :in equal area basis CmOa is act'ive i i i the temperature range 380-440' while the
lanthanide oxides are active in the raiige 430540', representing a lower specific activity i i i t,he latter case. I n the case of 14.7% Crz03on rU203 the temperature range is 340470' for a waterwashed sample and 400-540' for an unwashed sample, while the 15T0 lanthanide oxides on alumina are active in the range 450-590' on an equal total area basis, again indicating a lower activity. The activation energies for Cr203 and CrzOaAlz03 cat'alysts are in the range 20-30 kcal./g.mole, while the supported and unsupported lanthanide oxides give activation energies in the range 32-40 kcal./g.-molz, representing a larger energy barrier for surface reaction in the rare earths. Finally, it may perhaps be concluded from these experiments that the lanthanide rare earth oxides are uniformly active for the dehydrogenation of cyclohexane and that the catalytic activity is iiot,ably independent of crystal structure, paramagnetism and, in the case of praseodymium, independent of the valence state within experimental error. The lanthanide oxides are also less active than certain transition metal oxides and compounds. A possible explanation for these observations is that the activity of the rare earths is associated with bonding due to the outer 5s and 5p electrons, and that this bonding is localized to specific ions in the sense that it' is relatively unaffected by defect struct8ure and cryst,al field splitting arising from neighboring ions. In the case of the transition metal oxides, the bonding electrons also might be the electrons responsible for paramagnetism, so that catalyt,ic activity and paramagnetism would be closely allied. The present observations tend to support those of Halpernzlmho has shown that singlc ions in aqueous solution, such as Cu2+,Ag+ and Hg2+,are act'ive in catalyzing the homogeneous hydrogenation of other dissolved subst'ances, suggesting that the activity is associated with the metal ion and is relatively independent of environment,. Acknowledgments.-The authors are grateful t,v the Gulf Research and Development Company for the mass spectrometric analyses, to Mr. R. L. Zanowick and Dr. W. E. Wallace of t~heUniversity of Pittsburgh for obtaining the X-ray powdei diffraction patterns, and one of 11s (C.B.M.) is indebted to the West'inghouse Corporatioil for a graduate fellowship.
'201 G . Sclrwab :and H. 11. hlartiri, Z. " l k i r o c h e m . , 43, ti10 ( 1 9 3 7 ) : 44, 724 (1938).
(21) J. Halpern. "Advances i n Catalgsis," V d . I X , .\rarlciriic Press, Inc., New York, N. Y . , 1957.
