distances characteristic of the two pure salts. Thc niimcrical factor (140 kcal./molc) is of the ordrr of magnitudc of tlie lnt,ticc energies of the salts. The simplicity of this equation has aroused consider:il)le interest. I t bcgs the question whether equally simple relations may apply for the oiher cxccss thcrmodpnamic functions, and notably for thc cxccss volumes considered in thc present work. In ordcr to obtain an a n s m r to this question TW h a w plotted our excess volumc data against diffcrent powcrs of the parametcr (d, - d2),/(dl+d2). It‘igurc 1 shows that wc obtain a reasonably good straight line that passcs through the origin in a phi, against the fourth power of this parameter. This s i q e s t s for the excess volumc an empirical rc.Iation ol thc type AITi =
For
t h c k
+V’x(l - A’)[(dr -
&)/(dl
-k
&)I4
considcred alkali nitrate mixtures, the
value of the numerical constant V’ is about 22,000 cc./mole. It, was shown by Longuet-Higgins,’ in his thcory of conformal solutions, that any first-order solution thcory will predict the same sign for all the excess thermodynamic functions (APE, AHE, ASE and A P ) . Thus, our volume and enthalpy data for the considcred alkali nitrate systems demonstrate that a satisfactory theory for them mixtures must bc sccond ordcr or higher. An attcmpt has been made recently to account for the observed enthalpies of mixing by mcans of sccond-order conformal solution tlieory.* However, no similar attack has becn madc as yet on the problem of the excess volumes. (7) 11. C. I,onguet-€Tingins, P r o r . Rov. Soc. (London), A205, 247 (1951). (8) 13. Reiss, J. L. Katz and 0. J. Kleppa, J. Chem. I’hus. (in press).
PERTUItBATIONS OF THE NICKEL METAL I< X-RAY ABSORPTION EDGE DUE TO SNALL CRYSTAL SIZE ,4XD IlYDROGES CHEAfISOllPTION BY P. 11. LEWIS Tcxaco Research Center, Beacon, New I’arlc Receztrd Julv SI,1961
‘The X-ray absorption edge of small nickrl crystals (ca 30 A.) has been found t o have small pcrturbutions from that of bulk nic-ltel. Thc pertiirbation has tlilTerctit chractciristics from those obscrwd whcn gases arc adsort)cd ’Yhe small crvsr,al pwtnrbationc, have h n tentatively associated with the posr-ession of atom-like energy statcu. ‘ l h effert of chemisorbed hydrogen on the electron-empty energy levels of the small cr) st:d nicbel IS q u d i t ativcly the same as that of c~liemisorbrd osygen, but quantitatively about half ns small. It is suggestcd that thc changes in cnutalyst absorption edge spectrum due to gas chemisorption are better correlutcd u ith the gas molcculcs spicading surface ut o m apart rrtthrr thaii n ith an increase in potential ficld about each stom.
Introduction since they have no clectrorls in them. It is with Although catalytic reactions on the sixfaces of these last bands that this paper will tic concerned. small metal crystals have bcen studicd extensively, The X-rav spectral methods measure the density the knowlcdgc of many fundamental aspects is of cncrgy levels, the extent of electron filling, and still incompletc. Little is known about horn the their angular momentum quantum niimber. The first object of this paper is to show the diffcrenergy lei& of these crystals tire affected by their crystal sizc, their contact with support niai crials or ences bctween the I< X-ray absorption edge of with adsorbed gases. X-Itav spectroscopy can be very small nickel crystals and that of bulk metal. used to study those energy levels (closely grouped The second object is to compare the catalyst nickel to form bands) that arc affected when the metal X-ray absorption edge after the chemisorption of crystals are small enough (ca. 50 A. or smaller). hydrogen with that after the chemisorption of Then the atoms on the surface form a sufficiently oxygen. The similarities found in this comparison large fraction of the total so that perturbation ef- tend to lead to a unified picture of t,he effect of fects arc iiot diluted bcyond observation by h r g o chemisorbed gas on a metal. Since the mathematical methods used to study numbers of unaffected interior atoms. The bands that arc affected are in an encrgy rcgion roughly 40 the X-ray absorption edge were riot conventional, C.V.wide and, for nickel, 8 k.e.v. above the Bohr a description of thcse preccdcs thc experimental IC-level. I h r g y levels in the lower 10 e.v. part resid t s. Use has been made of X-ray absorption edge contain valence electrons For nickel metal thcse valcnc(J electrons occupy t hc 3d and the lower part spectra to mcnsure solid state characteristics of of the I s band. Sincc thcse bands contain elcc- catalysts. The valence stsatc of siipported transifrons, they should be studied using the profiles of tion metal oxides2 has been detcrmincd. A study X-my emission 1int.s. The energy bands in thc has been made of the absorption spcctnim 10-100 The problems uppcr 30 e.v. section: the higher enerqy part of the e.v. above the cdge dis~ontinuity.~ nickel 4s hand and the entire 4p band, can be connccf cd with the obscrvation of small changes studied using met,al absorption edge’ measurements in the X-ray absorption spectrum of metals due to chemisorbed gas have been shown to be surmount(1) An absorption edge occitrs wlirn the X-rays have just solfirient enrrgy t o raise a n inner electron to the empty levels. .4 discontinuous change in abaorntion cw(tioient as a function of X-ray a a v c length characterizes tlie edge.
(2) (a) 11. P. Hanson and W.0. hlilligan, J. Phv8. Cham., 60,1144 (1958): (b) R. 0. Keelinq, Jr., J . Chem. Phua , 91,279(1959). (3) R. A. Van Nordntrand, Aduanceszn Catalums, 12, 1 4 0 (1960).
106
P. H. LEWIS SUPPORT T U B E
ROUND G L A S S
I N T E R N A L STAN DAR D T A P E R JOINT KOVAR-GLASS SEALS F O R THERMOCOUPLE WIRES P STOPCOCK
Fig. 1.-Sample
TO VACUUM PUMPS, GAS DOSING
support.
/A r
-I
PYREX FLAT
SUPPORT TUBE
Fig. 2.-Sample
cell.
able in a study of the effect of chemisorbed oxygen on n i ~ k e l . ~ The chemisorption of hydrogen on nickel has been studied with conductivity,6 magnetic susceptibility6 and surface dipole measuremeiits.6 These investigations have shown that the bond formed is probably covalent and the nickel-hydrogen dipole is small, the hydrogen being negative. Experimental A. The X-Ray System and Procedures.-The X-ray apparatus, method of collection of data and analysis of error have been d e ~ c r i b e d . ~Some check runs were made using a Von der Hyde double-crystal attachment to the Philips spectrometer and germanium crvstalb . B. Vacuum System.-FTith the exception of the mechanical pump, the entire vacuum system was mounted on a flat, wooden board (equipped with height adjusting screws) so that the sample could be removed from the X-ray beam. This board mas cut so as to fit the angle formed by the diffractometer base and the X-ray tube housing. This permits reproducible positioning of the sample to the X-ray beam. The sample was supported on a glass tube that fits into the internal standard taper joint of a glass base made as shown in Fig. 1. (4) P. H. Lewis, J . Ph2s. Chem., 64, 1103 (1960). ( 5 ) (a) G. C. A. Schuit and L. L, van Reijen, Advances in Catalyazs, 10,242-5 (1958); (b) P.W.Selwood, ibid , 9,93 (1957). (0) M. €I. Bachtler and G. J. H. Dorgelo, J . chim. phys., 64, 27 (1957).
Vol. 66
This glass base was cemented to the sample cell, shown in Fig. 2, with DeKhotinsky cement. The cell's 6.35 cm. diameter glass cylinder has slits cut into it for the passage of X-rays. These were covered with 1 mil mylar that is cemented to the glass with Bonding Agent' R-313. These windows were cooled with air jets. Rays from a 1000 watt projection lamp in an ellipsoidal reflector8 mere used to heat the sample. The sample wafer was mounted in slots cut in the end of an 18 mm. diameter glass tube a t an angle of 45" with respect to the tube axis. This position permits the sample to intercept both X-rays and heat rays. The attachment of the cell to vacuum pumps and gas dosing system was made demountable. The connection was sealed with Apiezon IT. The pumping system consisted of a mechanical pump, an oil diffusion pump and a liquid nitrogen trap. Gas, stored in a bulb, was admitted to the sample cell by means of a calibrated, 0 , l cc. dosing stopcock. The gas pressure in the bulb was measured with a mercury manometer. The temperature of the sample was measured with an iron-constantan thermocouple. Higher gas pressures in the sample cell were measured with a thermocouple gage, RCA 1946; lower pressures with a cold cathode Miller gage. C. Materials.-The catalyst was prepared by impregnating Davison silica gel 12 with a water solution of Baker's reagent grade nickel nitrate. This silica support was chosen because very small nickel crystals with high hydrogen adsorptive capacity had been obtained.6s The powder, after drying, was compressed into sheets 0.02 cm. in thickness. A 2 X 2 em. square was cut from the sheet and placed in the slots of the 18 mm. glass tube shown in Fig. 2. This sheet attenuates the intensity of the X-rays by a factor of 20 on the high energy side of the absorption edge. Additional catalyst powder was placed just beneath this sheet in the sample support tube. This additional catalyst material helps to improve gas adsorption measurements and reduce contamination. The total sample in the cell weighed about 0.3 g. X-Ray absorption measurements were used to calculate4 the nickel content of the catalyst to be 6.9%. The maximum number of hydrogen atoms adsorbed per nickel atom, measured a t 25' and 0.1 mm. pressure, was 0.12 (24 cc. (STP)/g. Ni). This capacity compares favorably with that previously reported6 for Davison silica, supported nickel, 40 cc. (STP)/g. Ni, measured a t -78" and a t 100 mm. KO measurement of the crystallite size of the catalyst nickel was attempted. The apparatus used for the X-ray absorption work is not suited for determination of the catalyst sample nickel crystal size by X-ray diffraction, high resolution electron microscopy, magnetic measurements, or gas adsorption a t low temperatures. The work of Schuit and van Reijensa with the same nickel on Davison silica gel catalyst that was use6 s h o w that the nickel crystak have a diameter of about 30 A . Matheson electrolytic hydrogen, cleaned of residual oxygen by passage through a tube filled with hot (350') copper and thence through a liquid nitrogen-cooled trap, was used. The results subsequently were checked using palladiumfiltered hydrogen. Linde oxygen was used without further purification. D. Experimental Procedure.-All the work was based on a "bare" nickel catalyst in which the nickel is free of oxygen. The preparation was begun by raising the sample temperature to 350' under a vacu'im of mm. The degassed sample was reduced with hydrogen for 15 hr. a t 350". The cell subsequently was pumped down to a rfsidual pressure of 1 x IO-* mm. with the sample a t 350 When the sample was cooled to room temperature, the mm. The entire propressure in the cell fell to 1 X cedure was repeated before obtaining X-ray data. The effect of hydrogen on this bare nickel was obtained by passing hydrogen over the sample a t room temperature, maintaining the pressure a t 1atm.
.
Analysis of the X-Ray Data.-The raw experimeiital data of an X-ray absorption edge study are the ratios of the intensity of the monochromatic X-ray beam before passing through the (7) Made by Carl H. Biggs Company of Los Angeles. (8) E. H Niooliean, G. R. Gunther-Mohr and L. R. Weisberg, I.B.N. J . Ressarch and Deselopment, 1, 349 (1957).
Jan., 1962
PERTURBATIOK OF KICKEL METALX-RAYABSORPTION EDGE
107
sample, Io,to those after passage, I . These ratios are related to the mass absorption coefficient of the sample, ,urn, the density of the sample, p , and the thickness of the sample, t, by log ( I o / I ) =
P
pLmpt/2303
It is the measurements of the mass absorption coefficient of !,he nickel that mill show how the energy levels of the metal are affected. One cannot make direct use of catalyst absorption measurements alont; to study the state of catalyst nickel. First, the absorption by the support must be subtracted. Second, the change in metal absorption is a very small fraction of the catalyst absorption. Third, because angular settings of the monochromator crystal cannot be determined exactly, there is a need for a continuous calibration as the wave length of the monochromatic X-rays is changed. The basic technique4 for overcoming these difficulties is baqed on taking absorption data for the catalyst sample, s, and for a thin, bulk metal foil, f, a t each angular position of the monochromator crystal. If the catalyst nickel has the same energy band characteristics as that of the bulk nickel metal, the logarithmic plot of these X-ray absorption edge data will obey the straight line relationship log ( I d I h = A log (IO/l)f B Here, the intercept B is equal to ,urn t/2.303 for the silica support. It is a constant for the small range of X-ray wave lengths used in studying an absorption edge. The slope A is the ratio of the product, pmt, for the catalyst metal to that for the bulk nickel foil. When the t x o nickel metals are in the same state, the mass absorption coefficients for the two will be equal for X-rays of every wave length, A mill be constant, and the linear plot of X-ray absorption data for catalyst vs. bulk nickel will be observed. When the catalyst nickel is perturbed, its mass absorption coefficient no longer need equal that for bulk metal for all the wave lengths of the X-rays used in studying the absorption edge, Then A will vary with wave length and a non-linear plot will bo observed. In the previous non-linear behavior was noted when individual data points fell off the straight line by more than the experimental error. This method becomes useless when each deviation from the siraight line is less than the experimental error calculated for a single point. Small non-linear deviations still can be detected by examining the data for trends. This can be understood by looking at Fig. 3. While the sums of the squared deviations from each line are the same, the data in one scatter randomly and in the other shorn a sinusoidal trend. The detection of a non-random data trend can be made in two ways. The first is the method of mean square successive difference^.^ This technique uses two data. The first, Z,, is the perpendicular distance of each point to the mean square straight liw. The second, & + I - Zi,is the difference between the perpendicular distances for two adjacent data points. Each of these is squared and then summed using the total number of data points,
+
(9) C A Bennett and N. L Franklin, “Stat1stlcel Snalysls in Chernistry and the Chemical Industry,” John Wiley and Sons, Ino., New York N Y., 1954, pp. 677-684.
1.50 6 1.40
e
8 1.30
-
0.90
-1
/
6
,
,+/
------i-----ti-i
0
0.40 0.60 0.80 1.00 Log Io/l - Ni foil. Fig. .l.---X-Ray absorption bare nickel.
0.20
1.20
n. The first sum, called S 2and equal t o (l/n)2Zi2, will be larger than the second sum, called a2 and equal to (l/(n - l))z’(Z: -+ZJz, ~ if a non-random trend occurs. The actual ratio, Sz/S2,called IC, has been analyzed statisticallygto determine the probability that a trend is tion-random. For a given n, the smaller k is, the less random the data. The second technique used to analyze for trends is based on the trite axiom that the slope and intercept of any part of a straight line must be equal to the slope and intercept of the entire line. The slope and intercept determined using a part of the X-ray data must be equal to those determined using all the X-ray data, if the metal in the sample has the same state as the bulk metal. If the catalyst metal is perturbed, the inequality of the appropri-
P. 11. LEnm
108
Vol. 66
1.40
1.30 .
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.-CATALYST &--BULK
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1.10
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43.14
43.18 43.22 Fig. 5.-X-Ray
43.26 43.30 “20. absorption bare nicltel.
ate straight line parameters describcs the kind of perturbation and its amount. The method will be callcd the incqi1:Llity technique. A number of considcrations must be hecdcd in this analysis. It is necessary that the numbcr of data points in any given subdivision of the data be large so that thc calculated slope and intercept have sufficient precirion. Second, the subdivision must be made in the same manner for c:ich run and should havc physical significance. Third, the method cannot be used in studies involviiig thc greater pcrturbations of X-ray absorption dnt:i that occur in plots of data for nickcl mctal us. completely oxidized catalyst nickcl. Then the mid-range data can no longer be approsimated by a straight linc. Segrcgatiori of thc data is done as follows. I’rcevamination of Figs. 4 and 6 shows that thc loglog plots consist of two clusters of data conncctcd by a string of data points. Least squares analysis using only the data in thc two cliistcrs leads to “end-point” results; the string of data points, “mid-range r c s i i l t s ” ; and all the data, “all data.” The clusters of data occur where the mctal Xray absorption is relatively invariant with wave length. The lower cluster is connccted with the excitation of nickel 1,-shcll clectrons to very high energv bands. The magnitude of X-ray absorption diic to this excit,ation is constant ovcr the absorption edge. Only statistical scatter of the data points is to be expected. ‘I7lic upper cluster is associatcd with the excitation of nickcl Iadvant,age of both methods is the great improvement8 in sensitivity over that of the “ o m point” type used previously.* One oxygen atom pcr 30 nickcl atoms can bc dctccted instcad of but one per 10. Experimental Results X-Ray Absorption Characteristics of Bare, Small Crystal Nickel.-A log-log plot of observed X-ray absorption data for the bare, small crystal nickel on lhvison silica gel-I2 us. that for h l k nickel foil is
1
43.34
43.38
shown in Fig. 3. Thc straight line was obtained by n least sqwres treatment of the cnd-point d a h
Thc expcrimcntal points hnving abscissa 0.7-1.0 tend to have high ordinates. A more conventional plot of the X-ray absorption data for both catalyst :ind hulk nickel foil us. angular setting of the monochromator is in Icig. 5 . I n Fig. 5 the catalyst data arc plotted :is observed. The bulk nickcl foil data wcrc scaled from observed data by multiplying by the slope, A , and adding the intercept, B, to each product using cnd-point values for R and 8. The plot shows that thc general shape of the cataIrst nickcl X-ray absorption curve is the same as that of huIk mctal evccpt for a small increase in absorption coefficient in the cent rd part of the data, 43.18-43.32O (28). The accumulated analytical results for nine runs are listed in ‘l’ablc I. ‘rA6tE 1 X-RAY ABSOILPTION RESULTSFOR BARE,Sci.aL CRYSTAL NICKEL
The ineqiislity technique Av. slope
Av. intercept
All dnta 0.492 f 0.008 0 805 f 0.005 ,802 f .008 llid-range .SO3 L .007 End-point q 4!)0 f ,008 ,803 i 005 T-value (Mid-range, end-point dope datrt) 3 25 C h i c e of coinridenve 1 in 400 The nicthod of mean sqnare successive dil’frrrnres ’I’ypi(x1 k VILIIIPS 1 04, 0 BO, 1.22 1’rol):iI)ility of noli-random run 0.999
The tnblc shows the averages of mid-range and cwd-point doprs t o differ by about twice the root me:in square twor. A Stiident’s Tes1,10811 was appllcd to this difference to show that coincidcncc of thew two slopes tias a probability of less than 1 in (IO) The fartor T 1s calculated by
where
21,k2 are the averages of
data in each group, and Sn =
each group. A’[. h’z are the
[---t-+ ---I”*. Nlulr n-1
.\‘ruz’
.\12
2
nuliller
oi
Tho vali~oofII’
calculated is correlated with thc grohrrbility t h a t XI a n d ?.Y be dinwent in reference 11. (11) W. .J, IXxon a n d F. J. hlnssey, “Introduction to Strrtistical An:rlysis.” ~frGraw-llill nook Co.. New York, N. Y., 1051, p. 101105.
400. Typical IC values correspond to a high probability of a non-random deviation from liiicarity. These measurements of noli-linear behavior show the small crystal nickel is not iii the same electronic state as that of bulk metal. The perturbation due to small crystal size is characterized by differences between slopes calculated using the three methods of collecting data that are not observed in the intercept data. Some concern was given to the possibility that the small crystal effect might be due to a systematic instnimcntal factor. To test this, the X-ray abtion data for nickel catalysts having a smaller hydrogen adsorptive capacity as comparcd to the nickel/ DJvison silica gel 12 catalyst were run. The smaller hydrogen adsorption w'as indicative of larger nickel crystals.12 For thcsc the differences in slope were less than the root mean error. Moreover, a large k. value, 1.9, indicated a very low probability of a nonrandom trend. It was concluded that what had been observed for the small iiickel crystals was not due to instrumental factors. 'Lhere is still the possibility that the small crystal effect might really be due to contamination. This might8occur either through incomplete reductioil of the nickel or through iricomplete removal of residual g:is from the cell. Incomplete reduction does not seem likely because the X-ray absorption characteristics of the small crystal sample do not change after a total of 180 hr. of reduction. Usually 36 hr. is more than adcquate to obtain catalyst metal having the X-ray absorption characteristics of hulk metal when the metal crystals are 1:irgcr. The maximum amount of surface oxygen contamination possible was calculated to be less than t2y0 of a m~nolayer.'~The small crystal effect moreo\-er, will be, shown to be different in Xray characteristics from that observed when the small metal crystals have adsorbed gas on their surface. Schuit and van R ~ i j e nhave ~ ~ shown that a thin skin of silica can be formed on nickel crystals. The conditions for forming this skin include forming the catalyst by coprccipitation and heating it to temperatures :ibove 400'. Since the sample used in the present X-ray work was made by impregnation :tiid never heated above 3 j O o , it was concluded that the thin silica skin never was formed on the nickel crystals used. The small crystal X-ray effect observed therefore is not to be correlated with the formation of this silica coating. The Hydrogen Effect on Nickel X-Ray Absorption Characteristics.-The easy observation of the effect of hydrogen on the nickel X-ray edge is a nasty problem in that an optinium crystallite size is required. If the nickel crystals are too big. the .m:d amount c f hydrogen adwrbed will produce no rneasumble X-ray effect. If the crystallites are too smd1. one is faccd with thc problem (not inairmountable) that the weak hydrogen effect on 1he bulk metal state of the catalyst nickel occurs simultniicously with a relatively large small crystal per(12) S. F. Adler and J. .J. Keavney, J . Phys. Chem., 64, 208 (1960); L. Ppenitdel and M Boudart, tbtd., 64, 201 (1960).
(13) Oue oxygen atom per 30 nickel atoms can be detcctod by the X-ray niethod. For 30 A. crystals. 26% of the metal atoms are on the surfacc.
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must be, therefore, hydrogen. The work was further checked by using palladium-filtered hydrogen.
Discussion A picture for interpreting the X-ray results obtained after the chemisorption of oxygen by nickel has been described4 To review the fundamentals: (1) The X-ray absorption edge can be divided into two energy sections. The first section, associated with the 4p band, lies between 43.18 and 43.29' (2e) in Fig. 7. The second section, associated with the 4s band, lies between 43.29 and 43.36' (28). (2) The magnitude of an absorption coefficient for X-rays of energy E is governed by the product of the density of empty levels at energy E above the K level (or level of origin of the excited electron) times the probability that the X-rays can excite an electron. (3) This probability is governed by the selection rule that an electron can be excited from one level to another if the resulting change of angular momentum is one. The absorption coefficient measured with K electron excitation is, therefore, a mirror of the amount of p character of the bands. (4) A metal, due to hybridization, has a 4p band with decreased p character and a 4s band with some p character. (5) The effect of gas chemisorption is to reduce the metal's hybridization: the p content of the 4s band is reduced and that of the 4p band increased. (6) The change iii the angular momentum makeup of the bands is due to the increase in the depth of the potential well in which the metal atom exists upon transfer of charge. Since hydrogen affects the absorption edge in much the same way as oxygen does, this same interpretation should be attempted. It generally is successful except for the last statement, (6). Comparing the differences in slope and intercept that are observed when hydrogen is adsorbed with those due to an equal amouiit of oxygen, the oxygen has 2 times the effect that hydrogen does. The error of this ratio is extremely large, i.1. This ratio should be compared with that obtained by magnetic measurements. The decrease in magnetization was found to be equal for oxygen and hpdrogens& The large error in the X-ray absorption result makes unprofitable any speculation about the discrepancy between the X-ray and magnetic
I
I
methods, interesting as it may be. It is surprising that hydrogen should exert as much influence on the nickel as it does. The nickel-oxygen bond has a 50% ionic character; the nickel-hydrogen bond but 370.14 The X-ray results, at least, do not respond in a way proportional to the charge displacement that the ionic character calculations indicate. To attribute the X-ray effects obserred as being due to the deepening of the potential well about the metal atom seems difficult. An alternate explanation might be mentioned. Goodenough15 has shown that the conductivity of 3d transition metal oxides decreases both with atomic number and distance between atoms. He suggests that this intermetal distance is quite critical. Thus for small distances between metal atoms, electrons are delocalized; for larger distances, localized to each atom site. To put it slightly differently, when the metal atoms are closely spaced the valence electron eiiergy levels are common to the crystal as a whole and are metallic in character (have mixed angular momentum character) and become atomic in character (have pure angular momentum characteristics) when the atoms are further apart. To apply this theory to chemisorption it must be assumed that chemisorbed atoms spread the surface metal atoms apart by situating themselves in the interstices between the atoms of the surface. Some estimate of this deformation of the metal surface may be obtained by examining the lattice parameters, ao, of nickel compounds. TABLE I11 LATTICE CONSTANTS OF NICKELCOMPOUNDS
Nickell" Sickel hydrideI7 Nickel oxideLG
ao, A. 3.53 3.73,3.83 4.18
Deformation,
52
0 5.7,8 5 18.4
114) Calculated from electronegativity data from L. Pauling, "Nature of the Chemical Bond," Cornell Unlverslty Press, Ithaca, N.Y. 1939 and from A. L. Sllred and E. G . Rochow, J. Inorp. & Nudear Chem., 6, 264 (1058). (15) J. B. Goodenough, Phys. Ben., 117, 1442 (1960). (16) H F. Swanson a n d E. Tatge, "Standard X-ray DIffraction Powder Patterna," Vol. 1, Natl. Bur. Standards Circular 539, P. 13, 47. (17) A. Janko and Pierre Miohel, Compt. r e n d , 261, 1001 (1960) Two nickel hydride phases have been found. Both are face-centered cubic b u t have different lattice constants.
Jan., 1962
DETECTION OF ZINC IONHYDROLYSIS BY COAGULATIOX
It is notable that the ratio of deformations, 3.2 to 1, or 2.2 to 1, depending on the lattice constant for nickel hydride, is closer to the X-ray effects observed than that calculated using per cent. ionic character. The placing of the hydrogen (and oxygen) atoms in the interstices of the metal surface layer fits in with the lack of observation of a Si-H absorption band in the infrared.I8 This negative result was interpreted by these authors as being due to bonding of the hydrogen to two or more mer,al atoms. The interstitial position of the adsorbed gas atom also has been suggested by Mignoletlg and Farnsworth.20 Sufficient theoretical background has not been developed to enable one to determine from the Xray absorption data whether a given bond is covalent or ionic. Indeed, if it is the latter, it is not clear from the X-ray data which way the dipole is oriented. It should be noted, however, that the effect of hydrogen on nickel is not confined to an effect on the 3d electrons but further encompasses perturbations of the 4s and 4p bands of the metal. While a mode has been developed for interpreting the X-ray absorption edge changes that occur after gas adsorption, an interpretation of the small crystal effects is by no means clear. It is not surprising that a perturbation of the X-ray absorption edge should be observed for a sample of metal crystals approaching atoms in size. ?Ilott*l has, (18) R. P. Eischens and W. A. Pbskin, Advances 2% Caatalyszs, 10, 1 (1958). (19) J. C. P. Mignolet, Bull w c chim. Belges, 61, 358 (1958). (20) T. H George, H. E. Farnsworth and R. E. Sohlier J Chem. Phys., 31, 89 (1959).
111
in fact, suggested that the main effect of atomization of a large metal crystal would be that the bands would lose their hybrid symmetry characteristics. While this is a plausible explanation for the enhanced X-ray absorption in the small crystal’s 4p band, a decrease in absorption’in the 4s band was not observed. It is conceivable that the enhanced X-ray absorption is due to the additional surface energy levels proposed by Tamm.22 No firm correlation can be made, however, of the present X-ray observations with the Tamm levels. One importance of the observation of a small crystal size effect on the X-ray absorption data is the possibility .of extending X-ray size measurenients below 50 A. For smaller crystals than this12the X-ray diffraction lines become so broad as t o become indistinguishable from background. The perturbation of the X-ray absorption edge should be greater (and hence more easily observed) the smaller the crystal size. It is evident that empirical and theoretical work is required before the X-ray absorption edge measurements can be used to make quantitative measurements of crystallite size. Acknowledgment.-The author would like t o thank Mr. McNelly for obtaining thc X-ray data and making the mathematical computations. Discussions with members of the Physical Research Section of the Texaca Research Center have been most helpful. (21) N. P. X o t t , Froc. Phys. Soc., (London), 62A,416 (1949). (22) See, for example, F. Seitz, “Modern Theory of Solids,” MoGraw-Hi11 Book Co., New York, N. Y., 1940, pp. 320-326.
DETECTION O F METAL ION HYDROLYSIS BY COAGULATIOX. IVel ZINC2 BY E. MATIJEVI~, J. P. COVCH AXD 31. KERKER Department of Chemistry, Clarkson College of Technology, Potsdanz, New York Received August
4, 1961
The coagulation method has been used to determine the charge of zinc ions in aqueous solution a t various pH’s. Coagulation concentrations of zinc nitrate solutions for negative silver bromide sol in statu nascendi were obtained over a pH range from 2.0 to 8.7. At pH
