NOTES
1592
no vanadium was detected. However, the sensitivity of this method was 1 p.p m. while the catalytic perouidt peak n-as sensitive to even smaller concentrations of vanadium. The decrease in sample peak current after hours of standing was due to hydrogen peroxide decomposition catalvzed by the niobium or tantalum. In fact, if large concentrations of niobium (0.1 M ) were added t o hydrogen peroxide an evolution of huhblec was observed. -2ddi ,ion of a maximum suppressor, Triton X100. had no effect on the peak current, except that when i t was present in exceps of 0.01%, a small decrease was ohserved This would seem to indicate that the peak was not an adsorption phenomenon. However, elec~trncapillarycurves were obtained by measuring the drop time as a function of potential for potassium chloride solutions containing Triton X-100, 2nd theqe showed that although the Triton X-100 cihiftecl the maximum drop time to about -0 2 T' 11s S C.E., there was no change in drop time a t potentials more positive than 0 0 v. us. S C E. Thus, e,.en if this was an adsorption phenomenon it is not wrpric;ing that Triton X-100 did not decrease Ihe peak heirht. Electrocapillary curves could not be obtained with peroxide solutions containing 0 01 nil- niobium because bubbles of oxygen formed causing erratic dror, times. The qhift of potential with pH was close to 50 mv. ner pH imit t n m r d easier reduction in acid solution. The ncak Dotential n.as not a theoretically interpretable half-wave potential, but its shift with pH does shnn- that hydroyen ions were involved in the electrode reavtion. Since the peak current also varied with the pH, one may also sap that hydrogen ions were inrnlved in the kinetic reaction. Kolthoff and Parrv gave the following explanation for '.he cata!vtic vave due to vanadium, molybdenum or tungsten. The high current was caused by the chain reaction M 0 0 4 - - + T I 2 0 2 +Moor-MOO,-- i 2H' 2 ~ --+ ?\I004--
+
+ H20 + H20
(1)
(2)
The rate of reaction 1 was so fast that the current was not i function of diffusion of the metal peroxide complex to the elertrnde, but of the rate of formation of the complex. Thus. the current depended on the metal ion concentration, hydrogen peroxide concentration and pH. The sharp peak current indicated that the rate of reaction 1 was affected by the potential (of the electrode which may indicate that adsorption effects were causing thir dependence. Because of the shift in potential with pH, a type of reaction 2 seems plausible for the niobium and tantalum caqe3. However. niobium and tantalum exist as polvnuclear species in aqueous solution (as well as molvbdenum in weakly acid solutions) $Q that reaction 1 must be more complicated for these elements. Since the predominant species changes with pH this could explain why the current varied so markedly with pH. Adler and Hiskey showed the existence of four niobium peroxide complexes in different p H regions; in weakly acid solutiow w ~ as h Save the high peak currents, a 1:l complex was the most stable. In more acid solutions n 2 : l complex was found while in basic solution, the predominant species was a 4 : l complex.
Vol. 64
The peak current constitutes a sensitive test for niobium and tantalum, capable of detecting IO-' M concentrations of these two clenicnts. This method was used to determine the small amounts of niobium formed by beta decay in a Zr95-Nbg5solution. Zirconium mas found to have some catalytic effect on the hydrogen peroxide reduction, but gave no peak at +0.23 v. in a solution a t pH 5. Thus, niobium and tantalum have been found to belong to the group of metals whose peroxide complexes catalyze the reduction of hydrogen peroxide at the dropping mercurv electrode. Acknowledgment.-The author thanks Dr. E. A. Tomic of our laboratories for furnishing a sample of Zrgs oxalate and calculating the amount of Nbe5 present from the age of the sample. The author L. Altemus, also expresses his appreciation to IT. Jr., for performing much of the experimental work presented here, and to H. M. Hubbard for his helpful suggestions and reviewing of the manuscript. CALORIMETRIC HEATS O F ADSORPTION FOR HYDROGEK ON NICKEL, COPPER ASD SOME OF THEIR ALLOYS BY LOISS.SHIELDA N D W. WALKERRUSSELL Department of Chemiatry, Bromn h'nn~rrs~tv Received ApriE 87. 1960
I n a recent study' of the activity of nickel, copper and some of their alloys as catalysts for the ortho-parahydrogen interconversion a t - 196" and a t near room temperature, it was found that catalytic activity changed little in the alloy composition range from 5 to over 90% copper. However, outsideof this rangealloying a femper cent. of copper or nickel with nickel or copper sharply decreased and increased catalyst activity, respectively. It was thought that a better understanding of the foregoing phenomena might be obtained if differential heats of hydrogen adsorption mere made on the same type of catalysts. Experimental The all-glass gas handling system followed in principle that earlier described2 but incorporated a McLeod gauge, a mercury diffusion pump and calorimeter. Gas purification, R.E.T. measurements and catalyst preparation followed an earlier disclosure.' However, catalyst shrinkage during reduction made a preliminary reduction necessary, after which cooling and adsorption of carbon dioxide allowed the no longer pyrophoric catalyst to be packed into the calorimeter. After sealing in place final reduction was carried out also at 350'. Outgassing was carried out at 300°, a little helium added to hasten thermal equilibrium in the Calorimeter a t room temperature, a t which measurements were made, and the helium outgassed t o 10-6 mm. prior to a run. The calorimrters wwe in principle like that described by Beebe and Caniplin3; howcvor, the catalyst sample was placed in a perforated cylindrical copper bad& between narrow layers of small, thin copper discs. The single junction of a copper-constantan thermocouple was attached to the middle of the basket exterior. The basket was completely surrounded by a thin polished silver radiation shield. The basket and concentric shield were suspended from a narrow central spindle tube which also served as a gas exit tube dur(1) P. B. Shallcross and W. (1959).
W. Russell.
J. A m . Chem. Soc.. 81, 4132
(2) W. W.Russell and L. G . Ghering. {bid., 66, 4468 (1933). (3) R. A. Beebe and E. R. Camplin, THISJOURNAL, 63, 480 (1959).
Oct., 1960 ing catalyst reduction. A heavy copper shield into which was inserted the cold junction of the thermocouple fitted snugly around the outside of the vacuum jacket of the calorimeter. Around this and also submerged in the water-bath was a glass tube spiral t o equilibrate thermally gases entering the calorimeter. The thermocouple wires left the calorimeter either through pressed glass seals, or by short tungsten-glass seals. AS no calibration heater element was built into the calorimeter, the heat capacity was calculated from specific heat data a t 20" for Pyrex glass, pure copper and pure nickel, with linear interpolation in the case of the alloy catalysts. The thermocouple circuit and galvanometer, and also the method of determining circuit sensitivity followed the work of Gstrner and Veal.* The galvanometer had a sensitivity of 0.046 rv./mm., and a period of 5.8 sec. The circuitsensitivity was always at least 40 cm./deg. Galvanometer scale readings us. time plots were analyzed by the method of Cork.6 The fraction 8 of ratalyst surface covered by adsorbed hydrogen was calculated on the assumption that each hydrogen atom occupied one metal site and that 100, 110 and 111 surface planes were equally probable.
NOTES
1503
0
10 20
50 40 50 60 70 80 90 100 Atom % copper. Fig. 1.-Thc effect of catalyst composition upon: (a) the heats of hydrog:n adsorption, heavy curve; ( b ) catalytic activity, a t -196 , lower dotted curve; a t -2O", upper dotted curve.
Results and Discussion 'The differential heats of hydrogen adsorption were measured over the greatest range of monolayer coverage, for the pure nickel catalyst, and fell off various rates with increasing surface coverage, and nearly linearly from 24 (extrapolated) to 8 kca1.l it appears clear that any direct relation between mole as e increased from zero to 0.6. Heats of heats of hydrogen adsorption and hydrogenating hydrogen adsorption on the 5 and 62 atom % copper activity may depend strongly upon employing alloy catalysts also decreased approximately linearly heats obtained with the type and extent of surface from about 15 (extrapolated) to about 7 kcal./mole coverage which is appropriate for the chemical as 0 increased from zero to 0.3. As measurements reaction occurring upon the catalyst surface. In on the pure copper catalyst were limited to e values the aforementioned study1 of the ortho-parahydronear 0.15 no trend was ascertainable. The heavy gen interconversion on these catalysts the imporcurve in Fig. 1 shows the dependence of the heat of tance of fixed hydrogen was suspected and was hydrogen adsorption upon the atom % copper in stressed. Such hydrogen was assumed iiot to parthe catalyst at e = 0.15, a monolayer coverage in ticipate directly in the interconversion but rather the range actually measured for each catalyst. to exert an activating and/or a deactivating effect Alloying 5y0 or less of copper with nickel has upon the more loosely adsorbed, reactive hydrogen. caused the heat of hydrogen adsorption to drop over The present work appears to support this view. 40%, while further increasing the copper content to Such fixed hydrogen is believed to adsorb under reover 60% causes a drop of less than one kcal./mole, action conditions, or may be preadsorbed on the then to pure copper the additional drop is a little catalyst. Catalysts exhibiting high heats of hydroover one kcal./mole. This dependence of the heat of gen adsorption at low surface coverages would be hydrogen adsorption upon alloy catalyst composi- expected to be most effective in fixing hydrogen and tion is strikingly similar to the dependence found thus modifying the catalytic properties of their surfor the catalytic activity of these alloy catalysts in faces. When hydrogen was adsorbed nnd fixed on the ortho-parahydrogen interconversion both at the catalyst surfaces at an interconversion tempera-196" (lower dotted curve) and at -20" (upper ture of -20°, the catalytic activity dependence dotted curve). The detailed shapes of the curves shown in the upper dotted curve in Fig. 1 was obat high percentages of copper are less certain, and tained. However, when hydrogen was fixed by preespecially so close to 100% copper since the pres- adsorption a t 325", the pure nickel catalyst was ence of very small traces of nickel in "pure" copper partially poisoned while the activities of the nickelis known greatly to affect catalytic activity,6 and copper alloy catalysts were enhanced for reaction at -20°, the 62 atom % copper catalyst showing maxalso probably heats of adsorption. illthough the effect of alloying 5% copper with imum enhancement. In the present work similarly nickel was to drop sharply both the catalytic ac- preadsorbing hydrogen on this latter catalyst has tivity and the heats of hydrogen adsorption a t all decreased the initial heat of subsequent hydrogen surface coverages studied, it does not necessarily adsorption to 4 kcal./mole a t an equilibrium presfollow that high heats of hydrogen adsorption mean sure of 0.28 mm., and to 2.6 kcal.,/mole when the high catalytic hydrogenating activity. On the con- equilibrium pressure had risen to 7.2 mm. These trary for the hydrogenation of ethylene on the smaller heats of adsorption are believed to be much more characteristic of hydrogen adsorbed in a transition metals' Beeck found catalytic activity to reactive condition than zero coverage heats. decrease as the heat of hydrogen adsorption a t The lack sensitivity to alloy composition, over zero coverage increased. As in the present work, a wide alloyof composition range, shown by the inBeeck's heats of hydrogen adsorption7 decreased at terconversion reactions (no deliberately pread(4) W. E. Garner and P. J. Veal, J . Chem. Soe., 1436 (1935). sorbed hydrogen) and by the heats of adsorption at e ( 5 ) J. M. Cork, "Heat," John TViley and Sons, Inc., New York, = 0.15 represent a case in which fked hydrogen N . Y.,1942, p. 56. induced promoting and poisoning effects appear (6) B. B. Corson and V. N. Ipstieff, T H IJOURNAL, ~ 46, 431 (1041). ncnrly to balance out. An explanation appears (7) 0.Beeck, Disc. Faraday Soc., 8 , 118 (1950).
1594
T'ol. 64
possible in terms of the following model. Except near the endr; of the alloy composit,ion range, both nickel and copper atoms are present' in la'rge numbers and supposedly randomly dispersed in the catalyst surf,zccs which because of t'he mode of catalyst preparation must be far from ideal single f.c.c. crystals. Such surface heterogeneity should lend to the eo-existeiice of surface areas of Twious types capable of adsorbing hydrogen with a certain spectrum of heats of adsorption to yield fixed hydrogen and adjacent adsorbed hydrogen some of which may be highly activated and some of which may be less or lit'tle actimted. The integrated effect of such surface heterogeneity, as well as of induced het'erogeneity, appears to be that the over-all rates of t,he ortho-parahydrogen interconversion and, over' a limit'ed range of surface coverage, the heats of hydrogen adsorpt'ion appear nearly constant on the alloy surface (of highly imperfect crystals when the nickel-copper alloy coiit'ains more than a few per cent. of both components.
been determined by gas adsorptions and iodometric titrations. The correspondence between the two methods has also been established. The electronic properties of chromia catalysts are very dependent on the extent of oxidation of the surface. The electrical conductivity is much higher in the oxidized state than in the reduced state. In many respects, chromia exhibits some of the properties of a p-type semiconductor in the oxidized state and of an n-type semiconductor in the reduced state. Attempts have been made to measure the acidities of chromia catalysts by the vapor phase adsorptions of amines and aqueous titrations. Kone of these measurements have given any indication of the distributions of acid sites on these catalysts. The work reported in this paper shows the types of distributions of acid sites that are found on chromia catalysts and an attempt has been made to correlate these results with the extents of surface oxidation and catalytic properties.
HBMMETT ACIDITIES OF CHROMIA
Unsupported chromia was prepared from dilute solutions of chromic nitrate and ammonium hydroxide. The gel was dried a t 110" and then cycled several times in oxygen and hydrogen a t 500". Chromia-alumina (Series A, Grade 100) was obtained from the Houdry Process Corporation. A sample of this latter catalyst was impregnated with aqueous potassium hydroxide solution to give the equivalent of 1% K?O. The impregnated catalyst was dried a t 110' and calcined a t 500". Oxidized and reduced samples of these catalysts were prepared by treating ground portions of each catalyst with oxygen or hydrogen, respectively, a t 500' for four hours. F-10 alumina (Alcoa) and S-34 silica-alumina (Houdry Process Corp.) were used as indicator catalysts in the aridity determinations. These catalysts were dried a t 500". The extents of surface oxidation of the chromia catalysts nere determined by an iodometric method.7-9J2 For the acidity measurements, freshly activated catalysts were placed in weighed vials in a dry box under nitrogen. h weighed amount of 100-120 mesh indicator catalyst mas mixed with a weighed amount of -200 mesh chromia catalyst in each vial. Various amounts of a standardized solution of n-butylamine in benzene were added t o different vials. The vials were thoroughly shaken and equilibrated overnight. The proper Hammett indicator was added and the color of the alumina (or silica-alumina) particles of the mixture was determined under a microscope. In this way, the equivalence point of each mixture was measured and the acidity of the rhromia catalyst was then calculated readily. Butter Yellow, dicinnamalacetone and anthraquinone were used to measure the acidity a t pK, values of +3, -3 and -8, respectively. Ethylenediamine was purified by repeated distillations over sodium and standardized by titration iyith perchloric acid in glacial acetic acid.
Experimental
CATALYSTS BY 3. E. VOLTZ, A. E. HIRSCHLER AND A. SMITH Suu
Oi; C o m p a n y , .Ifarcus Hook,Pennsylvania Received A p r i l $7, 1960
During the past decade, significant progress has been made in the determination of the properties of solid acid catalysts. Acid dist'ributions have been determined by titration in non-aqueous medium with ainines in the presence of Hammett indicators. The result's have clearly shown the import'aiice of the acid strengths to the catalytic properties of both cracking and re-forming catalyst'S.1-4 The extents of surface oxidation of transition metal oxides, such as chromia, have been shown to be intimately related to their surface chemistry and catalytic p r ~ p e r t ~ i e s . ~ Chromia -'~ catalysts are more active for many hydrocarbon reactioiis in the reduced state than in the oxidized state. The extents of surface oxidation of these catalysts have (1) 0.Johnson, THISJOURNAL, 69, 827 (1955). ( 2 ) H. A. Benesi, J . Am. Chem. Soc., 78, 5490 (1956). Benesi, THIS JOURNAL, 61, 970 (1957). (4) A . E. Hirschlt!r and A. Schneider, Paper No. 5 6 . Division of Colloid Chemistry, Nai;ional A.C.S. Meeting, Atlantic City, N. J., Sept. 1959.
(5) S. E. Volts and S. Weller, J . $ m . Chem. Soc., 76, 5227 (1953). (6) S. E. Voltz arid 5. W. Weller, ibid., 75, 5231 (1953). ( 7 ) S. E. Volts and S. W. Weller, i b i d . , 76, 1586 (1954). (8) S. W. Weller , m d S. E. Volte, ibid., 76, 4695 (1954). (9) S. E. Voltz arid S. W.Weller, i b i d . , 76, 4701 (1954). ( 1 0 ) S. W. Welle- and S. E. Volts, 2. physik. Chem., N.F., 5, 100 (19.55). (11) s. E. Voltz and S. W. Weller, THISJOVRNAL, 59, 566 (1955). (12) S. E. Volt,z and S. W. Weller, ibid., 59, 569 (1955). (13) S.W. Weller and S. E. Voltz, "Advances in Catalysis,'. Vol. I X , .Academic Press, Inc., New York, N. Y . , 1957, p. 215. (14) $. Chaplin, :P. R. Chapman and R. H. Griffith. Proc. Roy. Soc. ( L o n d o n ) , Aa24, 412 (1954). ( 1 5 ) P. R .Chapn.an, R . H. Griffith and J. D. F. Marsh, ibid.,A224, 419 (1954). (16) R. €3. Griffith, J. D. F. Marsh and M. J. Martin, ibid., 8234, 426 (1954). (17) Y.Matsunap:a, Bull. Chem. Soe. ( J a p a n ) .8 0 , 868 (1957). (18) Y.Matsunaga, ibid., SO, 984 (1957). (19) C. D. H o l l a n d a n d P . G. Murdoch, A.I.Ch.E. J.,386 (1967).
Results The acidity of a dark chromia catalyst cannot be determined by titrating it directly with an organic base. The color changes of the Hammett indicator are too difficult to observe on such a dark solid. It is necessary, therefore, to mix the chromia catalyst with a white indicator catalyst and titrate the mixture. The acidity of the chromia catalyst then can be calculated readily. The Hammett acidities of the F-10 alumina were 0.29, 0.29 and 0.27 milliequivalent n-butylamine/g. catalyst a t pKa values of +3, -3, and -8, respectively. The corresponding values for s-34 silica-alumina were 0.16, 0.16 and 0.11 milliequivalent n-butylamine/g. catalyst. F-10 alumina contains practically all strong acid sites; 53-34
