1073
August, 1960
SOTES
show no evideiice for the formation of appreciable quantities of the loner complex as is found with cysteine. While the two paths discussed above are much faster with cyteine it is interesting to note that no path involving a dihydroxy romplex is observed in the cysteine reactions. With mercaptoacetate,
wideiice is giveii for a path involving I;e(OH)&S-. Apparently, the aquo FeOHcy molecule is less acidic than is aquo I?eOHRS molecule. This again may be a consequence of the coordination to the iroii(II1) ion of the more basic amino group in cysteine compared to the roordination of the carboxylate in mercaptoacetate.
NOTES T H E EXTREME SESSITIT'ITT OF I'AllhlLI-IGNETIC SITES T O POISONJSG BY DESORBED GA4SES ET7.1LUATED BIT T H E LO\Y TEMPEILITURE ORTHOPAILIHYDROGES CONVERSIOS BY I ) ~ K G L A S. SCHAPIN, C. DICKPARK ANI) MYRON
pump was capable of evacuation of the svstem to 10-8 mm. or better. Pressure measurements with a type GIC-001 ionization gage attached to a, typical reaction vessel shoivrd a prcmnre of 7 X lO-'mm after a n outgassing of the reaction vessel for 24 hours above 450" followrd by 24 hourp a t 300". Samples of hydrogvn w r ( > analvzed for their orthoparahvdrogm content by thc Farkas microthcrmal ronductivity method.lj2
rJ. CORRIN
The Department o f Chemistry, University o f Arizona, Tucson, Arizoni Recezired September 8, 1989
Catalytic studies invariably involve questions of surface cleaiiliness and of the maintenance of t'he surface in a given state of cleanliness betweeii the time of the cleaning process and the time of the catalytic rate det'ermiiiation. In the case of ortho-parahydrogen conversion a t 77°K. on silica gel supported iieodymia the question arose as to whether an observed decrease in rate coiistaiit with time of standing at room temperature should he attribut'ed to further diffusion of ivater from the silica gel or to poisoning of the catalysts from gases desorbed from the Pyrex glass walls. Results show t'hat minute quantities of gas, probably C0, and H20, drastically poisoii the catalysis of the orthoparahydrogen conversion a t 77°K. Extreme precautions to exclude the poisoiiiiig gases from reaching a catalytic surface which is being studied are advisable. Gases desorbed from the Pyrex walls of the vacuum system or stopcock grease folloiving bake out of the catalyst are sufficieiit' to cause a fourfold reductioii in the rate constant. Liquid iiitrogen trapping \vus effective, I)ut8 solid CO, iiieffect ive io preventing the poisoiiiiig effect oil ;L >ilic:L gel supported neodymium ix oxide cat,aly:it.
-
200 300 400 500 Tirne (hours). I.'ig. I . -Ii;ffrc*tof trai) tc3inlxratiirr on the p(i' Experimental silica gc~l-nc~odymi:t c.:ttwlj.st, ton.:trd the ortho-pa 1)avisori SI). 02:j silivit \V:W \\-;tshtstl \\.it / I Iiot :aclu:l, rotiversioii me:isiiretl a t 7 7 ° K . :mtl a preasurc rtyi;t, hot c ~ o n t ~ ~ i i t r aiti ci i dI wicl, lioi. cy~i~vt,~~tr:ttc~d tiy(1ro- Fig. 1.2 shows t,hc j,rc.t,rr,:it,iiieiittiinc~-t~emperatiirehistory c.liIoric* w i t 1 ant1 ~ ~ x h a i i s t i v ~\v:tsh(d ~ly Ivitti tl~~iiiinc~rali~c~tl given nll rcactorr. Pig. 1I3 s h n w thv time-temperature \vatcsr. Thv gc.1 t1ic.n ivits tlrietl at 110' ~ n 8d g. u w mi%or>.for the t r a p s Fig. 1C gives the average rate conlmgiiated x i t h 25 w . of 0.26 .I[ S C ~ C in I , ~0.1 .\. HC1 l)y tnts foiintl after t h t ])retreatment indicated in Figs. 1.4 tumbling overnight in a. thcrmostated water-bath in a polyt l 113. T h c solid lincs refer to reactors 1-3 and the c~thyleno bott'tc. The catalyst' \vas sucked free of gross dotted lines refer to reactors 4-7 throughout. ~oliitionon a ,iintcrrd glass funnel, rinsed with one volumc~ of water and dried in an air oven at (30". Ten-mg. samplrs Results and Discussion wrre weighed into approximately 20-cc. bulbs and sealed on Rate data t'aken for the seven reactors a t four to 2 mm. i.d. capillary Pyrex tubes connected to a stopcock on the vacuuni sj,steni in the case of the first three reactors different times are given in Table I. Averages are and thfough L: IT-tube trap to a stoprock in t h e cape of the remitining remators. A two-stage mercurj- diffueion pump ( 1 ) A . Farkas, "Orthohydrogen. Parahydrogen and Heavy Hydrowas used t o evac:uat
