Sept., 1960
O2 ADSORPTIONON X-Rsk- ABSORPTIONEDGEOF KIox ALUMINA
1103
THE EFFECTS OF OXYGEN ADSORPTIOS O S THE K X-RAY ABSORPTION EDGE OF ALUMINA SUPPORTED NICKEL BY P. H. LEWIS Texaco Research Center, Beacon, New York Received March 7 , 1960
.2 nen- aiial:;tical~F.chriiqueis applied to show differences in the nickel K X-ray absorption edge due to o~ygeiicheniisorption on small (30 A ) nickel crystals in an alumina support catalyst. These changes show that the nickel-oxygen bond formed on the nickel surface is similar to that in bulk nickel oxide. The experimental data are interpreted as showing that the chemisorbed oxygen reduces the p character of the nickel’s 4s band and increases that of the 4p band. I t can further be suggested that the oxygen creates a gap between the almost filled 3d band and the empty 4p band. The alumina support does not affect the small nickel crystals; the absorption edge for the “bare” nickel is the same as that for bulk nickel metal, within the experimental error. The X-ray absorption results are useful in measuring the extent of reduction to the metal of the supported nickel oxide and measuring the nickel content of the catalyst.
Introduction The infrared technique’ has been used to show in great detail the chemisorbed state of molecules of hydrocarbons, carbon nioiioxide and aliphatic acids on small metal catalyst particles. There is a converse problem of how gns rhemisorptioii affects the metal particles. This is a problem that has been attacked by making measurements of ferromagnetism,2electrical conductivity and work function3for the metal. Of these, onlv tbe magnetic measurements can be made 011 practical, supported metal catalysts. The metal muit be. however, ferromagnetic. The results for a11 three of these techniques are interpretable ewntially only 111 terms of whether electroil addition to or extraction from the metal ocrur< duriiig chemisorption. The ultimate in description of how chemisorption afbe 111 terms of the band strucfects a metal \~-~-oultl ture of the metal The methods of X-ray emission (for studyi,ig bands filled with electrons) and absorption spectronietrj- (for studying unfilled bands) are theoretically ne11 suited for this. The observed X-ray absorption and emission effects are due to all the atoms of the metal crystals forming the sample. To use the X-ray methods to study the 2ffects of gas chemisorption on surface atoms, the number of surface atoms must be a large fraction of the total number of metal atoms forming the sample. Otherwise, only the absorption characteristicc of unaffected, subsurface atoms would be observed. The fractioii of surface atoEs inrreases a j the cxrystal size diminishes For 30 A. nickel crystals chemisorption effects on the X-ray absorption ciirw ran be observed. For these crystals about 30yGof the atoms are on the surface. There is 110 artificiality about a study of suvh small rrystals bexiuse such m a l l metal crystals form the in:tin constituent CJf many catalysts. It is the object of this paper to describe the use of K X-ra!, ahsorption measuremeiits to study the rhanges in the mifilled energv bands of these nickel metal crystallites (supported on y-alumina) caused h v oxygen cxheinisorption. Besides elucidating the effect of t h e chemisorption of oxygen oil nickel, the X-ray absorption technique can be used tn study diether or not the smallness of size of nickel crystals (1) R I’ I i d l e n s a n d T% 4. Piiskin, Adintires i n (’ntnlya,s, 10, 1 ( l ‘ t i 8 ) l c a d e r i i i Press Inc heir York, 4 I( 7 ) P \I Stlmood ?bid S, 93 (1957) R r i i e n c d 11, R Siilirniinn % b i d ,7 , 303 (105;)
,
affects the band structure and whether or riot contact with the alumina affects the nickel crystals. The technique is useful in e s t i m a h g the extent of reduction of the metal and in determining the per cent. metal present in the catalyst. A somewhat iiovel method for analyzing the X-ray absorption result’sis described. There ha,ve been several X-ray absorption studies of bulk and supported catalysts t’o determine the valence state of the metal ion.*J This paper presents the application of the X-ray technique to the study of the effect of chemisorpt’ion on supported catalyst metal.
Experimental A . The X-Ray System.-Polychromatic radiation was obtained from a copper X-ray tube operated a t 20 kilovolts, about 4 kilovolts above the threshold potential for exciting radiation of half the wave length of t,he nickel edge. The tube output Tvas controlled by P h i l i p Electronics voltage and current stabilizers. To avoid the tungsten La2 emission line that, is superposed on the white radiation used for st,udying the nickel X-ray absorption edge, a fresh X-ray tube was employed. Some 200 hours of running time can be obtained before trhe tungsten contamination of the tube anode becomes severe. Monochromatic X-rays %-ere obtained from the X-ray tube’s continuous spectrum by means of a monochromator system based on the Sorelco diffractometer. The X-rays were collimated by passing through a 1/12’ divergence slit and then through a Soller slit system. These X-rays were monochromatized by a lithium fluoride crystal cleaved from a Harshaw crystal along t,he (100) plane. This crystal v a s chosen for the high intensity of its diffracted X-rays and for its dispersive power. The diffr:tct,edX-rays were collimated by a 0.003 inch receiving slit, a Soller slit system and a 1/12’ scatter slit,. This single crystal apparatus does not have the resolutiori of a double crystal unit,, the half width of the Cu Kal line being 5 volts instead of the 3 volts obtained by means of a double cryst,al For a snrrifice in resolut,ion, however, higher X-ray intensity (roughly 4 times as much) was obtairicd so that X-ray ahsorption curves can be ol!t,sined iii :L few- hours. A rapid accumiilation of data IS desirahle so :LS to minimize the time that, the sample is es osed to contamination effects. t h e success of an X-ray absorption spectrum st.udy using the Korelco diffractomet,er depends upoii accwrate main1,cnance of alignment of the monochromator crystal and the detector. If t,he former is a t an angle 8 with respert to the main X-ray beam, t,hc latter must always bc at an angle 28 over the angular range of the absorption edge. To do this the diflractomet>erwas adjusted by the nianufacturcr to remove excessive gear play and er juipped Kith ___ (4) H. P. Hanson and W. 0. LIilligan, THISJ o L , ~ ~ s6 0 ~, L1144 , 11Xti). ( 5 ) R. P. Keeling, Jr., d . Chem. Phys., 31, 279 [19;:1).
(6) A . H. Compton and 9. K. Allison, “X-Rays in Theory and Exiwrinient.” 11. Van h’ostranti C ,., X e v York, N. Y . . 1
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P. H. LEWIS
the new micro-adjuster for maintenance of the 2: I relationship. The rotation of the monochromator crystal was always made from low to high angles. The absolute angular settings of the monochromator (these determine the wave length of the radiation detected) rannot be trusted to better than several hundredths of a degree. The absorption measurements on the catalyst sample were therefore calibrated by also measuring the absorption of a nickel foil a t each angular setting.’ The incident and transmitted X-ray intensities, 10 and I , were measured with a Korelco pure xenon proportional counter, 62032, operated at 1675 volts, below the voltage that caused afterpulsing. The counter output was pream lified by means of a Hamner feedback stabilized unit S%-3 and then fed to an Atomic Model 204B linear zmplifier. The signal pulses were screened from noise and the residual half wave length radiation by means of an Atomic pulse height analyzer, model 510. The noise content of the measured signal was but 0.2 count/second or less. The measured signal was virtually free of half wave length radiation, the test being the identit of absorption coefficients measured for a nickel foil with $-ray tube voltages of 15 and 20 kilovolts. B. Vacuum System.-The sample was placed in a Vycor glass cell located on the X-ray tube side of the monochromator crystal. The cell was mounted with its long axis vertical. For this position the X-ray path is unchanged as the monochromator crystal is rotated. The X-rays pass through mylar-covered slits in the glass walls of this cell. The mylar was fastened to the glass with a thin coat of a polymer cement.* A conventional vacuum system (mechanical pump, oil diffusion pump and liquid nitrogen trap) was attached to the cell. High gas pressures in this cell were measured by means of a calibrated thermocouple gage, RCA 1946, and low pressures by means of a Miller cold cathode gage. Known volumes of oxygen were admitted to the sample cell b y means of a mercury-calibrated 0.1 cc. dosing stopcock. The pressure of this gas in the reservoir system was measured l)y means of a mercury manometer. With the exception of the mechanical pump (connected to the diffusion pump by means of a rubber hose) the entire system was mounted on a flat, wooden base (fitted with height adjusting screws) so as to facilitate the movement of the sample in and out of the X-ray beam. The board was cut so as to just 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 vacuum system was sufficiently good that pressures of the order of 10-6-10-6 mm. can be maintained for 5-6 hours without pumping. This was the time required to accumulate the X-ray absorption data. The sample was heated by means of focussed radiation from a 1000 watt projection lamp positioned at the focus of an ellipsoidal r e f l e ~ t o r . ~Its rays were directed perpendicular to the X-ray beam, in the horizontal plane. The sample plane was turned about its vertical axis so as to be 30” to the X-ray beam. In this position the sample can intercept both the X-rays and the heat rays. While the Vycor tube transmits a high percentage of the infrared radiation, enough is absorbed to overheat the Mylar windows. These were cooled, therefore, with air jets. The temperature of the sample was measured by means of an iron-constantan thermocouple mounted on the shade side of the sample. The temperature of the sample was not iiniform; the temperature measurement was good enough to ensure that sufficient heat was furnished to the sample to ieduce the nickel. C. Materials.-The nickel-alumina catalysts were pre1med by impregnating Alcoa F-10 alumina with a water solution of Raker’s reagent grade nickel nitrate. This powder after drying, was compressed into sheets about 0.02 cm. in thickness. The sample sheets weigh about 0.3 g. Additional sample was mounted in a cavity below the sample sheet, outsidc the X-ray beam, to bring the total weight of (7) This was suggested t o the author by H. Cole of the IBM Research Laboratories. Poughkeepsie, New York. ( 8 ) Bonding agent R-313, made by Carl H. Biggs Company of Los Angeles. (9) E. H. Nioollian, G. R. Cunther-Mohr and L. R. Weisberg, I B M J . Research and Development. 1 , 349 (1957). Dr. Gunther-Mohr was most helpful in grttiny us started in using these heaters.
VOl. 64
the sample exposed to gas to about a gram. The additional sample reduces somewhat the effect of contamination and increases the precision with which gas adsorptions can be measured. Electrolytic hydrogen, purified by passage over hot copper and through a liquid nitrogen trap, was used. Commercial oxygen was used without further purification. D. Experimental Procedure.-The “bare” nickel sample was made in the following manner. The nickel nitrate impregnated alumina sample was dehydrated by slowly raising its temperature to 350” while maintaining it under vacuum. The sample was then reduced under flowing hydrogen for 15 hour8 a t 350’. The reduction step was followed by evacuation a t 350” to a residual pressure of 10-4 mm. Following this procedure the pressure in the cell ranged from 10-6 to 10-8 mm. a t room temperature. The entire process was then repeated before obtaining X-ray absorption data. The sample was regarded as having “clean” nickel crystals. No change in the X-ray absorption data was observed upon repeating this procedure 6 more times. Known amounts of oxygen were added to this sample to study the effects of oxygen chemisorption. The X-ray intensity measurements at each angular setting (changed in steps of 0.01 degree, 28) were made in the following sequence: (1) incident beam, (2) beam transmitted through a nickel foil, (3) beam transmitted through catalyst sample and (4)another measurement of the incident beam. Each incident beam intensity measurement was made in terms of the time required to accumulate 12,800 counts. The time to accumulate 6400 counts was measured for the transmitted beams. The maximum difference between the two measurements of incident beam intensity tolerated was 3%. The error, estimated using 2u statistics, included both the statistical error and an error due to variation in the background counting rate of 0.1 count/second. The larger statistical error was selected to allow for X-ray tube output variations. Each logarithm (base 10) of the ratio of incident to transmitted beam intensity has an absolute error of &0.014. This logarithmic ratio is equal to the product of the absorption coefficient p and the sample thickness t divided by 2.303.
Method for Analysis of X-Ray Absorption Edge Data Rather poor experiences with the two conventional techniques (fitting X-ray absorption results to an arc tangent function*0and measurement of the wave length a t which the change in absorption is half the maximumll) for analyzing X-ray absorption results initiated a search for a better technique. Both of these conventional techniques utilize a rather small percentage of the total absorption data accumulated and indicate changes in but one part of the absorption edge. The analytical technique actually used avoids these difficulties. The method depends upon the additivity of absorption for the components of a multicomponent sample. For the two components of a nickel-alumina catalyst, s (fit).
= (PO1
+
(Po2
where component 1 is the nickel and component 2 is the alumina. This equation is rigorous for any measurement made a t any single wave length. The application of this equation t o the analysis of X-ray absorption data obtained over the range of wave lengths of the edge can be done in the following way. For the narrow wave length region of the nickel edge, (pLtjZis a constant, 2.303B. Only a statistical fluctuation in pure alumina absorption data was noted in measurements made by varying the double B r a g angle (28) from 43.15 to 43.45’ (the nickel edge is measured between these angles). (10) R. K. Ricltmyer, S. W. Barnes and E. Rsmberg, Phys. Rev., 46, 843 (1934). (11) €I. W. B. Skinner and J. E. Johnson, Proc. Rov. Soc. (London), 8161, 420 (1937).
02 ADSORPTION ON X-RAYABSOK~TION EDGEOF NICKELo x
Sept., 1960
.~LUMIXA
1103
\\-here p is density. The ( p t ) for the calibrating nickel foil, determined by making weight and area measurements, was found to be 8.99 X g./cm.l. The mms absorption coefficient for alumina, 28.5 cm.*/g., was obtained by interpolating between reportedg absorption data measured a t 1.39 and 1.54 A., using the cube dependence of absorption coefficient on X-rax wave length to get the coefficient at the wave length 1.49 A. The per cent. nickel was compared m-ith that determined by a gravimetric analysis involving the determination of both A1208 and Xi in the sample. The determination of’ both of the catalyst constituents prevents a systematic error due to water present in the alumina. The X-ray result was (4.52 f 0.03) .%; the gravimetric (4.42 & 0.03) yo. The reason for this discrepancy is not yet known.
Results
*t
oL
I I I I L 1.10 1.20 1.30 1.40 1.50 Log lo/l-sample. Fig. 1:-Absorption data; bare nickel catalyst
1.00
T 1*1°
.=‘ 0.90
f
w
,$
t
D A T A FOR N I C K E L
t
O N SURFACE
I e
0.10 0.950
k
/
%DATA
FOR CLEAN NICKEL
t I
1.15 1.25 1.35 1.45 Log Zo/l-sample. Fig. 2.-Absorption data-nickel catalyst with oxygen chemisorbed on it. 1.05
If the nickel dispersed on the alumina has bulk metal properties, the ratio ( ~ t ) ~ / ( p t )isr also a constant, A , ever the entire wave length range of the absorption edge. Here (pt)f is the absorption of the bulk nickel foil used for calibration. Hence (pt).
= A(pt)t
+ 2.303B
or log ( l o / I ) a = A log ( I o / i ) f
+B
(1)
In consequence, a logarithmic plot of the absorption of the sample us. the absorption of the nickel foil for all the X-ray edge data should be linear if no perturbation of the catalyst nickel has occurred. If A is constant, the weight percentage of nickel, XI, may be calculated from A and B using
The comparison of bare nickel-alumina catalyst absorption characteristics a t the nickel K edge with that of nickel foil is shown in Fig. 1. The plot is not made in terms of absolute absorption coefficients because of the difficulty of making an accurate estimate of the density and thickness of the sample wafer. The experimental data were taken every 1.83 volts (0.01 degree, 28). The plotted data adequately cover the energy region of X-ray edge associated with atomic characteristics in contrast to the high energy edge region, the so-called Kronig region. Absorption characteristics in this last energy region are to be associated with energy bands controlled by crystal structure. The straight line was derived from the data by the least squares method. Within the experimental error (indicated by the small box) the data are adequately represented by the straight line. The slope of this straight line can be reproduced with a 1% error, the intercept with but a 0.5% error. Data for the interaction of oxygen with the nickel were taken a t three oxygen/nickel ratios : 1/3.4, 1/6.5 and 1/6.2. The gas adsorption measurements showed that a monolayer coverage of oxygen occurred at the largest of these ratios. The evidence that the coverage of osygen was a monolayer a t the oxygen/nickel ratio l / 3 . 4 was based on the marked rise in pressure within the cell upon further addition of oxygen. A real change in the absorption curve could just be observed when one osygen was adsorbed per 6.2 nickel atoms. Figure 2 shows the comparison of a nickel-alumina catalyst with a nickel foil when the catalyst nickel has one oxygen adsorbed for every 5.5 nickel atoms in the sample. It is clear that a straight line relationship does not exist for these data. The straight line that is drawn (dashed) was determined using the bare nickel catalyst data shown in Fig. 1. For discussion purposes the data of Fig. 1 and Fig. 2 were converted into the more usual form for showing X-ray absorption edges. The smoothed curves are shown in Fig. 3. The curve for bare nickel is. within the exDezmental error. the same as for bulk nickel metaLi2 The effect of the oxygen is to cause changes in the A and B regions of the absorption edge. The absorption edge data for the other two oxygen-nickel samples were of the same type as shown in Figs. 2 and 3, the magnitudes of the distortions being appropriately bigger or smaller, depending on the oxygen/nickel ratio. In order t o understand the changes in X-ray (12) W. W. Beeman and H. Friedman, Phys. Rar., 66, 392 (1939).
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Vol. 64
-__ OXYGEN absorption edge characteristics caused by the chemiCHEMISORBED sorption of oxygen, the catalyst nickel was com- BARE pletely oxidized by heating the sample to 350" in oxygen a t a pressure of 10.9 cm. The absorption data for this sample were analyzed by the log-log plot for comparison with nickel foil. The more conventional representation of the oxidized catalyst absorption data along with the absorption curve for the sample clean of chemisorbed gas is shown in Ls, Fig. 4. It is clear that the effect of bulk oxidation on the nickel is similar in kind to that observed M 1.200 after oxygen chemisorption, but, of course, greater ? in magnitude. 1.100 Useful information may be derived from the maximum differences in X-ray absorption between bare and oxygen treated nickel catalysts. Plots of I .000 I I these differences in the A and B regions are shown 43.18 43.22 43.26 43.30 43.34 43.38 43.42 in Figs. 5 and 6 for the A and B regions, respectively. Crystal position '28. The oxygen/nickel ratios were calculated using the total weight of nickel in the X-ray absorption cell Fig. 3.-Absorption data-nickel catalyst oxygen chemisorbed. and the amounts of oxygen admitted to the cell by means of the calibrated doser. Figure 5 shows that ---COMPLETELY in the A region all the differences are linearly related OXIDIZED to the oxygenlnickel ratios. / , --BARE Figure 5 also shows that at an oxygen/nickel 1.600 ratio of about 0.1 the experimental error is the \ same size as the diff ereiice between oxidized and \ unoxidized nickel X-ray absorption. If the X-ray \ \ absorption data for the "bare" nickel catalyst in a d, a log-log plot (shown in Fig. 4) obeys a linear relaIO V O L T S tionship, one can conclude that the nickel must have been a t least 90% reduced. Using the crystal size derived from the oxygen adsorption data, 30 A., 2 J (assumption: each surfate atom adsorbs one oxygen atom) this corresponds to a surface a t least 67 yoclean of oxygen. These analyses do not preclude 1.200 the possibility that the extent of the reduction of the nickel is greater. The plot shown in Fig. 6 for the B region of the X-ray adsorption edge contrasts with that shown for the A\region. It is clear that the changes caused 1.000 J by small additions of oxygen (low 00%ratios) fall 43.18 43.22 43.26 43.30 43.34 43.38 43.42 011 a straight line. The experimental point for the Crystal position '28. sample containing nickel completely oxidized to data-nickel catalyst, completely X i 0 falls off the straight line by a good deal more Fig. 4.-Absorption oxidized. than the experimental error. For the B region the amount of the effect on the X-ray absorption of the nickel is different for the first oxygen atoms cussed subsequently are the 3d, 4s and 4p bands of adsorbed than for those that complete the conver- nickel. Each band is characterized by the number dension of the metal crystal to N O . sity of energy levels forming it. The number denDiscussion sity as a function of energy has been calculated by The interpretation of the effect of the chemisorp- Rudberg and Slater'3 for nickel and is shown in Fig. tion of oxygen on the nickel X-ray absorption 7. The figure shows that the 3d band extends over edge is to be made in terms of the band theory of an energy range from -13 to - 5 . 5 volts. The solids. d brief review of the fundamentals of density of 3d energy levels is high. However, band theory is in order. First, a band: in a crystal much of this band is filled with electrons. The the valence electrons of a constituent atom no Fermi level (F. L.) marks the boundary between longer occupy the discrete, sharp energy levels the energy levels that are filled and those that are that are associated with a free atom. Instead the empty (to be more rigorous it is the energy position electrons occupy broadened energy levels called a t which the probability of finding an electron has bands. Each band is formed by closely spaced fallen to 1/2). The 4s band occupies an energy (so closely that the levels merge into a continuum) range roughly coincident with the 3d band, extendatom level,., the number of levels forming each band ing from - 13to - 0.5 volts. The density of energy being equal to the number of atoms forming the levels is small. At point iLI the 4s and 4p bands crystal. Each band is named according to the almost meet. The failure to make perfect, contact atom levels used to form it. The bands to be dis(13) E. Rudberg a n d J. C. Stater, Phys Rev., SO, 160 (1936).
2t 3
t
I
I
1I I:.' I
!
a"
t
,
