J. Phys. Chem. 1981, 85, 1232-1235
1232
Investigation of Supported Cobalt and Nickel Catalysts by X-ray Absorption Spectroscopy Robert B. Greegor, Farrel W. Lytle, The Boeing Company, Seattle, Washington 98 124
Roland L. Chin, and David M. Hercules' Depattment of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: November 24, 1980; In Final Form: January 27, 198 1)
X-ray absorption spectroscopicinvestigations of Ni and Co catalysts amplify the results of the surface-sensitive techniques of ion-scattering spectroscopy (ISS) and X-ray photoelectron spectroscopy (ESCA). The measured characteristics of the catalysts were affected by the metal used, the percent loading, and the calcination temperature. In particular the coordination number increased with increasing loading and/or decreasing calcination temperature. At higher loadings Ni or Co is thought to be on the surface in the form of NiO or Co304. A t lower loadings Co is tetrahedrally coordinated by oxygen in the tetrahedral lattice sites of y-alumina. Additionally at 2-3 % loading the oxygen coordination of Ni is higher than the Co. This is consistent with the postulate that Ni2+ions have greater octahedral site preference than Co ions. N
Introduction X-ray photoelectron spectroscopy (ESCA) and ionscattering spectroscopy (ISS) have been used to study both Col and Ni2 catalysts as a function of loading on alumina supports. In the study reported here measurements on the same catalyst samples have been broadened to include the extended X-ray absorption fine structure (EXAFS), and near edge structure of the Co and Ni K edges. The results indicated by these data further support and amplify the previous studies. Of primary interest is the effect of the percent loading of the metal and the support interaction on the resultant structure of the catalysts. ESCA and ISS probe the surface of the catalyst while EXAFS is predominantly a bulk technique. However, because supported catalysts have metals in the surface layer, comparisons between the techniques are valid. Experimental Section The Co catalysts were prepared' by impregnation of r-A1203(90 m2/g) with aqueous solutions of cobalt nitrate, drying at 110 "C, and calcination at 400 or 600 "C in air for 5 h. The Ni catalysts were prepared2 by impregnation with Ni(N03)2with the same support, drying temperature, and calcination period as for the Co. The resulting catalyst metal loading varied from -2 to 20 w t 9o for the catalysts studied here. EXAFS data were taken at the Stanford Synchrotron Radiation Laboratory3 (SSRL). The critical beam energy was 2 GeV with a storage ring current of 10 mA. Beam line 4 (wiggler) was used which added -1 GeV to the effective energy. The samples were maintained at 77 f 5 K for the duration of the EXAFS scans.
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Results Examples of the k weighted EXAFS spectra for Co and Ni at high metal loading are shown in Figure 1. The (1) R. Chin and D. Hercules, J . Phys. Chem., submitted for publication.
(2) (a) M. Wu, R. Chin, and D. Hercules, Spectrosc. Lett., 11, 615 (1978); (b) M. Wu and D. M. Hercules, J . Phys. Chem., 83,2003 (1979). (3) H. Winick and A. Bienenstock, Ann. Reu. Nucl. Part. Sei., 28,33 (1978).
EXAFS oscillations can be described by single scattering theories4 as
In this representation, Nj is the number of atoms in the j t h coordination shell, at distance Rj from the absorbing atom. F j ( k ) is the back-scattering amplitude for an element in the j t h shell, and u,2 is the mean square relative displacement between the aisorbing and scattering atoms along Rp 4j(k)is the total phase shift due to the interaction of the ejected photoelectron with the potential of the absorbing and scattering atom. Both Fj(k)and 4 j ( k ) depend upon the kind of absorbing and back-scattering atom.6 For oxygen neighbors the scattering amplitude function increases with decreasing k. Thus, information concerning the oxygen is contained at low k values between approximately 4 and 10 A-l. Evidence of oxygen coordination is clearly evident in the Fourier transforms of EXAFS shown in Figure 1 (righthand side). The peaks in the radial distribution function correspond to the different coordination shells. The peaks indicated by the arrows in Figure 1 are due to nearestneighbor oxygen atoms. The position of this peak can be used to estimate the bond length by using a standard sample having a known Rj so that a ARj correction can be made due to the effect of 4j(k).6 The height and width of the peak are related to N j and uj but precise determination of N j or ujis difficult due to convolution effects of the Fourier transforms of F,(k)and the other x ( k ) terms in R space.' A more reliable method to determine Nj and uj is by isolating a given peak in the radial distribution function and performing an inverse Fourier transform. This procedure decouples the contribution of adjacent coordination (4) (a) D. Sayers, E. Stern, and F. Lytle, Phys. Rev. Lett., 27, 1204 (1971); (b) E. Stern, D. Sayers, and F. Lytle, Phys. Rev. B, 11, 4836 (1975);. (c) P. Lee and J. Pendry, ibid., 11, 2795 (1975); (d) P. Lee and G. Beni, ibid., 15, 2862 (1977). (5) B. Teo and P. Lee, J. Am. Chem. SOC.,101, 2816 (1979). (6) E. Stern, D. Sayers, and F. Lytle, Phys. Rev. B, 11, 4836 (1975). (7) R. Greegor and F. Lytle, Phys. Reu. B, 20,4902 (1979).
0022-3654/81/2085-1232$01.25/0 0 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85, No. 9, 198 1
X-ray Study of Supported Ni and Co Catalysts
1233
1.5 20% co
20% co
h
1 .o
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0
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shells so that the effect of only the coordination shell of interest is preserved. The resultant x for one shell can be used in a least-squares fitting routine to determine N a n d Q. However, for simple systems having only one kind of atom in the coordination sphere of interest a ratioing technique6 can be implemented. The natural logarithm of the ratio of the inverse Fourier transforms of standard and unknown plotted as a function of k2 has a slope which is proportional to Au2 and an intercept proportional to NJN,. Analytically this gives
TABLE I: Summary of EXAFS Results on Cobalt and Nickel Catalysts cata1yst
N( f 20%)
R( 2 0.05), A
20% coa 12%coa 12% co 6% Co 2% co
4.9 5.1 5.5 4.1 3.5
1.84 1.78 1.84 1.84 1.78
15%Ni 12.5 Ni 10%Ni 2.5%Ni
5.3 5.2 5.3 4.8
2.08 2.08 2.03 2.03
a Calcined at 600 "C, others calcined at 400 ' C. 7Alumina support for all catalysts.
which is of the form y = mk2
+b
where m = 2Aa2 and b = In (NJNJ. Knowing N, for the standard allows calculation of Nu for the unknown. The standard should be as chemically similar to the unknown as possible. Care must also be exercised in keeping the standards and unknowns thin or of the same effective thickness so that more accurate values of Nu can be obtained8 The In of the ratio technique was used in this study (see Figure 2) and the standard used was NiA1204. After values for Nuwere determined, as shown in Table I, a ratio of the Nuat a given loading to the Nuof a fixed loading was calculated. For the Co system it was shown by ESCA and ISS1that the 12% Co catalyst calcined at 400 "C had the highest relative percentage of Co304of all catalysts analyzed. Hence, the Nuof this catalyst was used in the ratio for Co. The Nu of the 15% sample was used for the Ni ratio. The plot of these data in Figure 3 shows (8) E. Stern, B. Bunker, and S. Heald, Phys. Rev. B , 21,5521 (1980).
the dependence of Nuon percent loading for Co or Ni with calcination temperatures of 400 or 600 "C. The dashed lines are used only to indicate the trend of increasing coordination number with increasing metal loading. The near-edge structure (edge f 50 eV) was also useful in analyzing the catalysts studied here. The near-edge structures for the 2.0% Co and 2.5% Ni are shown in Figure 4. These spectra exhibit a weak absorption peak near threshold followed by a strong absorption peak at 17-20 eV above the edge. The first peak has been identified as a 1s 3d transition while the second peak has ' ~ 1s 3d been identified as a 1s 4p t r a n s i t i ~ n . ~ ,The feature has been observed to be more intense in tetrahedral than in octahedral coordination of the absorbing atom.g Thus, this feature can be used as a measure of the coordination of transition metal complexes exhibiting the
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(9) R. Shulman, Y. Yafet, P. Eisenberger, and W. Blumberg, Proc. Natl. Acad. Sci. U.S.A., 73, 1384 (1976). (10) R. Bair and W. Goddard, 111, Phys. Rev. B , 22, 2767 (1980).
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The Journal of Physical Chemistty, Vol. 85, No. 9, 1981
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Flgure 4. The near-edge spectra for representative Co and Ni samples. The expanded inset shows the Increase In lntenslty of the 1s 3d transition as the metal atoms tend toward fourfold coordination: (a) - 20% Co, ---- 2 % Co, COO;(b)- 15% Ni, 2.5% Ni, NiO.
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Figure 3. The ratio of coordination number for a given loading to high loading as a function of metal content. The calculated value was estimated by assuming that all the Co was in the form of Co,04 (fourand sixfold) at 12% loading and all CoAI,04 (fourfold) at 2% loading: (0)CO calcined at 400 OC; (A)Co calcined at 600 OC; (0)Ni calcined at 400 OC; (H) calculated.
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transition (i.e., with d vacancies). We examined the 1s 3d feature in the catalysts studied and found support for
Discussion In a previous study ESCA and ISS were used to investigate metal-support interactions in Co/A1203catalysts. Interactions arise from diffusion of Co ions during calcination into lattice sties of the r-A1203where they occupy tetrahedral (Co-t) or octahedral ((20-0)sites." The relative distribution of the interaction species was determined to be influenced by such factors as metal loading and/or calcination temperature. For catalysts with low cobalt content the percentage of cobalt existing as the interaction species (Co-t) is high. Increasing the metal loading results in a shift of the distribution of cobalt species from Co-t (11) (a) J. T. Richardson and L. W. Vernon, J. Phys. Chem., 62,1153 (1958); (b) M. LoJacono, J. L. Verbeek, and G. C. A. Schwitt, J. Catal., 29, 463 (1973).
X-ray Study of Supported Ni and Co Catalysts
The Journal of Physical Chemistry, Vol. 85, No. 9, 198 1
to Co-0. Eventually the segregation of bulklike Co304is observed on the surface of the catalyst with further increases in metal loading. Formation of the interaction species was also found to be enhanced by increasing the calcination temperature at a given metal loading. Evaluation of ESCA and ISS Co/A1 intensity ratios vs. metal loading indicated that major compositional changes were occurring on the surface of the catalysts at certain metal concentrations. These compositional changes were correlated with changes in the distribution of Co species on the catalysts. As a result of the alteration in the distribution of c o species changes in the average c o coordination number should result. The trend of changing coordination number was found in the EXAFS results of Figure 3. Co-t is characterized by fourfold coordination to oxygen whereas Co-o has 6 oxygen nearest neighbors. Co304being a normal spinel has two Co3+ ions in sixfold coordination and one Co2+in fourfold coordination. The net coordination number (N) for Co304will be 2(6) + 1(4)/3 or 5.33. The two extreme situations which may characterize the surfaces of the catalysts are when Co-t (low metal loading, high calcination temperature) or Co304(high metal loading, low calcination temperature) predominate as the surface species. Thus an estimate of the ratio of oxygen coordination for Co-t and Co304is 4/5.33 or 0.75. From the previous ESCA and ISS results' it was determined that Co-t was the major Co species on the surface of the 2% Co catalyst calcined at 600 "C whereas the percentage of Co304was highest for the 12% Co catalyst calcined at 400 OC. The ratio of N as determined for the 2 and 12% Co catalysts yields a value of 0.64 (from Table I) which is in reasonable agreement with the calculated value of 0.75. The difference between the experimental and calculated values is attributable to uncertainties in the determination of N (f20 % ). As estimate of the percentage of Co atoms having sixfold or fourfold coordination can be made by assuming that (1) the EXAFS derived coordination number represents the product of the fraction of Co in four and six coordination, respectively, times the nominal coordination of four and six, and (2) the sum of the fraction in four and six coordination is unity.12 For the 20% (600 "C) Co catalyst this gives approximately 45% sixfold and 55% fourfold or about 70% of Co appears as Co304with the remainder as Co-t. Such calculations are intended only as an illustrative approximation due to the uncertainties in N. In addition the formation of Co-o has been neglected since its concentration is low and therefore contributes little to N. Because N for the 2% Co (600 "C) catalyst is 3.5 (Table I) we assumed that essentially all of the Co at this loading is fourfold coordinated as Co-t. Similar estimates can be made by using the 1s 3d near-edge feature shown (expanded inset) in Figure 4a. Here we assumed that the peak height of COOis representative of sixfold coordination, and the peak height of the 2% Co (600 " C ) catalyst represents fourfold coordination. Since the 20% Co catalyst has a mixture of four and six coordination its peak height falls between the two. Using these three peak heights we have estimated the percent of Co in four and six coordination. The results, -45% octahedral and -55% tetrahedral, agree with and substantiate the EXAFS results discussed above. The Co-0 bond lengths as determined from the Fourier transform peaks are summarized in Table I. In general, we see shorter bond lengths (indicative of tetrahedral co-
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(12) D. R. Sandstrom, F. w. Lytle, P. Wei, R. Greegor, J. Wong, and P. Schultz, J. Noncryst. Solids, 41,201 (1980).
1235
ordination) for the lower loadings and higher calcination temperatures. ESCA and ISS were previously used to study Ni on y-A1203catalysts.2 As in the Co system, metal-support interactions result in the diffusion of Ni2+into tetrahedral (Ni-t) and octahedral (Ni-o) sites of the support. The factors which influence the extent of this interaction are metal loading and calcination temperature. However, the relative distributions of tetrahedral and octahedral sites at a given metal loading and calcination temperature are much different for the Ni and Co systems due to differences in site preference. Ni has a higher octahedral site preference than C0,13 and thus, Ni-t and Ni-o are both formed even at low nickel loadings whereas Co-t is the dominant species at low cobalt loadings. As the Ni concentration is increased bulklike NiO (N = 6) is formed at the surface of the catalyst. Changes in the average Ni coordination are observed in the EXAFS data of Figure 3. At low nickel loadings (Le., 2.5% Ni) the dominant Ni species are those resulting from metal-support interactions: Ni-t and Ni-o. In this case, iV for Ni would be the weighted average due to four- and six-fold coordination. As the Ni content is inqeased, bulklike NiO formation becomes significant and N shifts toward a higher value. The percentage of the Ni atoms in four- and sixfold coordination sites may be calculated in the same manner as the Co catalysts. At 15% Ni loading the distribution was evaluated as -65% octahedral and -35% tetrahedral. At 2.5% Ni loading this distribution was reversed, that is -40% octahedral and -60% tetrahedral. The 1s 3d edge features for representative Ni samples are shown in Figure 4b. Since a pure fourfold Ni system was not measured no estimates of percent of Ni in a given coordination geometry could be made. However, we qualitatively note that the lower 2.5% Ni loading has a slightly higher 1s 3d absorption peak which is consistent with greater tendency toward fourfold geometry. The bond lengths for Ni are shown in Table I. As with the Co, the Ni also exhibits shorter bond lengths for those systems tending toward tetrahedral coordination. Comparison of the Co and Ni data in Figure 3 shows that at -2-3% loading the oxygen coordination of Ni is higher than for Co. This is consistent with the postulate that Ni2+has greater octahedral site preference than Co ions.13 This observation must be qualified somewhat by the fact that the Co was calcined at a higher temperature than Ni (600 OC vs. 400 "C) at 2-3% loading. In general, we expect higher calcination temperature to cause greater diffusion of Co into the support which leads to a lower average coordination number since more tetrahedral sites become available for a given percent loading. This postulate is substantiated by the Co samples at -12-13% loading. At this loading the 400 "C calcined sample has higher oxygen coordination than the 600 "C. The results indicated by the X-ray absorption spectroscopy are viewed as consistent in trend with the results shown earlier by ESCA and ISS. The combined studies of ESCA, ISS, and X-ray absorption spectroscopy demonstrate the suitability and complementary nature of these techniques.
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Acknowledgment. The authors thank the Stanford Synchrotron Radiation Laboratory with the financial support of the National Science Foundation for beam time. This research was supported by NSF Grants CHE7611255, CHE79-18084, and CHE78-00876. (13) M. Shelef, J. Wheeler, and H. Yao, Surf. Sci., 47, 697 (1975).
