ISTRODUCTIOS
In 'I wries of p i c ~ i o i commirnic*ations i~ (7, $1. 10. 1 I ) home new light \\:is thio\\ n upon the catalytic intci action of the components in catalytic alloys. For the giq-pliase dehydrogenntion of formic acid. catalyzed by I-lume-Rothm y :tlloys, the following rules have been found to Iiold: ( / ) IVitIiin the range of existence of n phase (the cubic face-centered CY- o r the Iieuagonal €-phase),the activation energy invreases ith increasing concenti ation of the niultivitlent component, or, at variation of the component, I\ ith increasing valence. ( 2 ) On comparing diffcrent phases, the saturated C Y - , e-, and v-phabes roughly - h o ~the same activation energy, while the (deformed space-centered) y-phase shov s a sharp maximum. b t h findings can lie elpicssed in common h y the statement that the activation energy depends on the> electron concentration being higher according as the 1;space of the first I3iillouin zone in the reciprocal lattice is filled up by the sphere of inipiilbe of the F'ermi distilbution. For example, the I3rillouin zone of the y-plinbe, because of i t h nearly spherical shape. 1- nearly entirely filled by the Fermi qphere at the saturation concentration uf electronb, x-hereai in the cy-. E-. and 7 phases considerable regions oi the X-space remain empty. The bearing of these le-ults on catalytic activation is that t1ierm:d activation is needed to force valent P clcc*troiii of the srrl),trate into free energy level5 of the metal electron glib. In mother papci ((;Ii t 11:~sbeen -lio\i n that the acativation meigics c4iihit ;I ytijking paiallelism n(Jt onls-, a;, i- to he eyiec-ted, v i t h the electricd rc~istanrr of the respective alloyi, hiit &o ivith their mechanical strength, e x p r e s d for practical purposes as 13rinell hnrdn -ineuplanation has been attempted on thc assumption thdt ;L iieall- full Iloiiin zone elerts a strong re-istmcc t o lattice distortion. or zone c'ompiewoii'I'hc resiilts desvi ihed hcre hare heen obtained on different sill er and copper alloy system5 111the pre5ent inrestig2Ltioii they are extended t o a 5ystem containing gold as a iiniralent component \\-? have meninred the activation energy of the reaction
-
EICOOH
-+132
+ COZ
on goltl-c.atlmnim nllo; ~i diffeient cornpodion-. 'l'hr haidneb5 of all but the been meazuiecl 1)v ttic umal Ihinell method of sphere s-cry brittle qpecimens s for the first time an oppor tunity indenting. The sv-tem qold-radniiirni ~ i r - c113 \ t < ~ t ( i iRi c w r i e
I i i i v e i b i t \ , Clc\cIlaritl, Ohio
of exaniining the catalytic behavioi of a space-centered citbic $-phase, \vliich in the alloy systems hitherto examined i. not stable in the temperature inten-a1 of the catalytic reaction. A%ccordingto N o t t and Jones (3), in the p-plinie the Brillouin zone is far from being electronically saturated, and thus \re must (J YI,cct the activation energy and h a i d n ~ s sto have the order of magnitude of the other phase:, except the y-phase. -\PI’ i R \TUS
The experimental procedure. differing bomen-hat from that iibed in the esperiments cited above, has been described recently in this Journal (12). The method, based on reflus circulation of formic acid vapor over the catalyst and measurement of the rate of product formation with a floivmeter (see figure 1 in reference 12). has been improved in two respects: (1) the evaporation tube I< has been filled with fine longitudinal glass capillaries, giving the same effect of qniooth boiling; ( 2 ) the thermocouple tube A I , instead of being sealed in, has been inserted n i t h a ground-glass joint a t the top of the dephlegmator chamlwi F. By this means the catalyst can be rcmoved after use by holding the emptiml and dried apparatus head down, and n new catalyst can he placed in E without cutting the apparatus. T’essel A contained liquid formic acid (98 per cent). In all other respects the 11 ork \vas carried out in the manner prwioiisly clescrit)o(i. c iT.1LlSTS
The gold used was prepared from commercial gold by the modified method of Iiruss (2). i.e., dissolution in aqua regia, evaporation. dilution, filtration, piecipitation with osalic acid, boiling n-ith concentrated sulfiiric and nitric acid.. repeated fusion u-ith potassium bisulfate, and then repetition of all of these opt’iations. Finally, the precipitate TI as melted on charcoal in an alcohol-ldo\\ pipe flame. and rolled into foils of about 0.03 mm. The cadmium used was Pchering-l~alilbaum’smctallicirv7 p w i ~ s . The alloys were prepared by melting weighed portions of both metals in piircelain crucibles under borax in a blon-ing furnace. Because of the considerable heat of formation, great care i p needed lest the temperature exceed the lo\\. h i l ing point of the alloys and so cause losses. -4s in the case of silver alloys, thc final concentration of the alloys u-st? calculated from their might on the assumption that only cadmium is oxidized and i r a s controlled by s-rny analysis (qee below). On a flat side of the reguli two or three spherical indentations were made with a Brinell sclerometer, the reguli tempered for 21 hr. a t about 400°C., and the hardness measurement repeated. Then the reguli Tf-erebroken into small lumps of at most 1 mm. ,size. The surface of the catalyst samples v-as calculated approximately on the ba*i. very qimilar diagram concerning the apstems silver-antimony and copper-tin has been published by Schn-ab and I \\e \\auld have expected. V-ithin the a-phase the hardening effect of added cadmnim is cnlearly seen, although the maximum is not found at the saturation limit. Some anomaly seems to manifest itself in that a t 8 per cent cadmium tempering does not, as usual, loner the hardness. Probably here an anomalous ha1 dening, comparable to that of duralumin, takeplace, due t o the formation oi t h e hrht tr:tcrs oi the (ordered) a'-phase. I n any case for the hardness, as for the activation energy, the @-phaseshows it value lover than the highest of the cu-pli:ibe, and the y-phase iq the hardest.
+
5UMJZIItY
The t,\w constants (if the -1rrhenius equation have been measured for the dehydrogenation of gaseous formic acid \\-ith gold-cadmium alloys as catalysts. functional relationship between the t\vo has been observed and discussed. The true energy of activation increases n-it,hin the a-phase with increa,sing electron concentration up to a limiting value; in the @- and e-phases it' is lower than this, while the y-phase shows a sharp maximum. -4s for the cy-! y-, and €-phases the present results confirm our former results on Hume-Rothery catalysts and their explanation; as for t'he &phase, they show for the first time that t'his case t'oo fit's in with the general rules. I n accordance with a former statement, the Brinell hardness of t'he alloys seems t o run parallel t o the activation energies. IIE;FF:R ESCES 11.:D e r .-lrcjbnf( der %~~~eislo~f/cfitliic?i unge,i. J . Spritiger, Berlin (1936). FL-XK I f ., : Ilarstellittzg der J l e t a l l e i n Labomtorirtnz, C ; . : -%nn.238, 30 (1887); 11. 113, F. E n k e , Stuttgart (19 I, J O K E S , H . : T h e Y'heory UJ the Properties of Metals and Alloys. University Press, Oxford (1936). EX,
s.
CATALYTIC ACTIOS O F SALT PAIRS
1053
(4) OELASDER,A%. : %. I i r i s t . 83, 11.5 (1932). ( 5 ) OELANDER -1.: J. Am. Chem. Soc. 64, 3819 (1932). (6) sCHW.4B, G , - l I , : Esperientia 2, 103 (1916). (7) S c ~ u - a nG.-hI.: , T r a n s . F a r a d a y Soc. 42, 689 (1946). (8) ScHwAn, G.->I.,. i s 1 1 C R E M E R , E . : %. physik. Chem. A144, 243 (19291; B6,406 (1929). G , - M , ,. i s n HOLZ. G . : %. anorg. Chem. 262, 205 (1944); S a t u r x i s s e n s c h a f t e n (9) SCHWAB, 31, 345 (1913). (10) SCHJTAEI. G.-lI.. ASD I ~ A R A T Z .I.: AS %., Elektrochem. 60, 242 (1914); Chem. Abstracts 40,4942 (1946). (11) S c ~ r n . 4 G ~ ;. - l I . , ASD S C H ~ ~ - . I B - . ~ G ~ ~EL L L IYDBer. : I S , 76, 1228 (1913) ; S a t u r n i s s e n schaften 31, 322 (1943). S . :J . I'hys. Chcm. 50, 127 11046). (12) ScHn-.in, G , - l I . , A S D THCOPHILIDES,
ISTRODUCTIOS
Those catalytic reactions which, on account of their technical importance, have been examined with respect to relation between catalytic action and phase composition are mainly hydrogenations, dehydrogenations, oxidations, ammonia synthesis, and petroleum synthesis. I n most of them homopolar or interstitial (2) catalyst compounds are to be considered as intermediate states. The catalyst systems are seldom suitable for thermal analysis; thus usually the proper problem concerns the nature of the catalytically active phase. -in exception is provided by dehydrogenation reactions on alloy hyatems, where recently distinct relations betn-een the phase-determining electron concentration and the catalytic action were established ( i ) .I n order to gain more fundamental knowledge as to the relation betiveen catalysis and the phase diagram, n-e have investigated another type of reaction, in which again the phases are n-ell characterized by thermal analysis. This is the salt catalysis of the decomposition of ethyl chloride : CZH,C'l
+
('?Hi
+ HC'1
Here, probably, a more polar adsorption of the type CH2-CHp
-+ (-)C1+- +H(+) - -+ +- + - + forms the intermediate state. The reaction has been studied kinetically b y
