Langmuir 1986,2, 773-776 The lead hydroxide film produced under these conditions is highly likely to be hydrated and will undoubtedly have a much lower density than the underlying mineral. Thus, relatively small percentages of surface reaction can lead to high coverages of hydrophilic lead hydroxide on the galena surface. The fact that collectorless flotation of galena, which is attributed to the hydrophobicity of the sulfur-rich oxidation product, is inhibited at potentials4 where only 10-20% of the surface reaction leads to the (13) Hamilton, I. C.; Woods, R. In Proceedings of the International Symposium on Electrochemistry in Mineral and Metal Processing; Richardson, P. E., Srinivasan,S., Woods, R., Eds.; The Electrochemical Society: Pennington, NJ, 1984; p 259.
773
production of the hydrophilic products, Pb(OH)2 and S2032-,12 gives some support to this view. Thus, the inhibition of the flotation of galena a t high potentials with xanthate collectors could result from oxidation of the mineral to form sufficient lead hydroxide to counteract the influence of dixanthogen. Furthermore, dixanthogen will be attached only weakly to the increasingly hydrophilic surface. In these circumstances, the turbulent hydrodynamic conditions in a flotation cell could result in stripping of dixanthogen and hence a diminution of the amount of this hydrophobic entity at the mineral surface. Registry No. Pb(C2H50CS2),,23810-93-7; KCzH,0CS2, 140-89-6;Pb(N03)2, 10099-74-8;Pb(OH)Z,19783-14-3;Pb, 743992-1; Au, 7440-57-5;dixanthogen, 502-55-6;galena, 12179-39-4.
EXAFS Studies of Rh/NaY and RhCr/NaY Zeolite Catalysts: Evidence for Direct Bonding between Metal Particles and Anchoring Ions M. S. TZOU, B. K. Teo,t and W. M. H. Sachtlerh Ipatieff Laboratory, Chemistry Department, Northwestern University, Euanston, Illinois 60201 Received M a y 13, 1986. I n Final Form: August 15, 1986 The Rh K-edge EXAFS of Rh/NaY and RhCr/NaY zeolite catalysts, prepared by ion exchange of NaY with Cr(N03)3and Rh(NH3)5C13solutions, was measured. It was found that the Rh-Rh distance in these catalysts is 0.06 A shorter than in rhodium metal. Cr ions are found to interact directly with Rh particles; the Rh-Cr interatomic distance in Rh/CrNaY is 2.50 A. Particle size and coordination number of Rh are decreased by the chromium ions which act as chemical anchors.
Introduction Recently we found' that for Pt in NaY supports higher metal dispersion could be obtained by using transitionmetal ions such as Fe2+ or Cr3+ as "chemical anchors". When heated in hydrogen at 500 "C, the anchored samples also displayed a superior maintenance of the metal dispersion. In the present paper this work is extended to Rh in NaY supports using Cr3+ions as the chemical anchor. T o ascertain whether indeed a short-range interaction exists between Rh and Cr, we have used EXAFS which has been proven by several authors to be able to provide significant information for such i n t e r a ~ t i o n . ~We - ~ refer to published work on supported bimetallic clusters including Rh-Cu, Pt-Ir, Os-Cu, Ir-Rh,2 Rh-Co? and Rh-Fe.4 Also for highly dispersed monometallic catalysts, e.g., Rh/ y-A1203, the interfacial distance between rhodium and oxygen of the support was detected by EXAFS.5 Experimental Section Catalyst Preparation. For this study, two Rh-containingNaY zeolite catalysts were prepared by ion exchange of Linde NaY(LZY-52)and CrNaY, dispersed in water (1g/200 mL), with an aqueous solution containing 30 ppm Rh(NH3)&13(Strem Chemical) at 70 "C for 24 h. The CrNaY was obtained at room temperature by exchange of a 0.01 M Cr(N03)3.9H20solution with NaY (1 g/200 mL of water) which was previously adjusted by * T o whom correspondence should be addressed. +Current address: Department of Chemistry, University of Illinois-Chicago, Chicago, IL 60680.
0743-7463/S6/2402-0773$01.50/0
diluted HN03to near pH 4.0. As analyzed by atomic absorption and Galbraith Laboratory, Inc., the rhodium content is 4.0 wt YO in both catalysts and the chromium content is 1.43 wt % in Rh/CrNaY. The X-ray diffraction suggests that CrNaY retains a high degree of crystallinity. The Rh-containing zeolite powder for EXAFS measurements was placed in a specially designed aluminum-glass tubing cell containing two Kapton-film (500-wm thickness) windows. This allowed treatments at different temperatures under various atmospheres and v a c u ~ m Both . ~ catalysts were heated from room temperature to 360 or 550 "C in flowing 02,with a heating rate of 0.5 "C/min. This calcination was followed by purging with He for 1 h, reducing with H2for 2 h, and purging again with He for 1 h at the same temperature. The zeolite powder was then cooled to room temperature and transferred to the aluminum part of the cell. Finally, the glass arm was sealed off to keep the reduced rhodium in a pure He atmosphere. The X-ray adsorption measurements were performed at room temperature at the Cornel1 High Energy Synchrotron Source (CHESS) on the C-2 EXAFS beam line at Wilson Laboratory at Cornel1 University. Data Analysis. The raw EXAFS data in energy space (ln(lo/4 vs. E ) were reduced to photoelectron wavevector ( k ) space as described in the literature6with E, = 23.205 eV. The resulting (1)Tzou, M. S.; Jiang, H. J.; Sachtler, W. M. H. Appl. Catal. 1986,20, 231. (2) Sinfelt, J. H.; Via, G. H.; Lytle, F. W. Catal. Reu. Scc. Eng. 1984, 26, 81. (3) Van't Blik, H. F. J.; Koningsberger, D. C.; Prins, R. J. Catal. 1986, 97, 210.
(4) Ichikawa, M.; Fukushima, T.; Yokoyama, T.; Kosugi, N.; Kuroda, H. J . Phys. Chem. 1986,90, 1222. (5) Koningsberger, D. C.; van Zon, J. B. A. D.; van't Blik, H. F. J.; Vissez, G. J.;Prins, R.; Mansour, A. N.; Sayers, D. E.; Short, D. R.; Katzer, J. R. J . Phys. Chem. 1985,89, 4075.
0 1986 American Chemical Society
Tzou e t al.
774 Langmuir, Vol. 2, No. 6, 1986 10
0
5
IO
k
(
15
R
Figure 1. Rh K-edge EXAFS spectra k 3 x ( k ) vs. k for (a) LTRhNaY, (b) HT-RhNaY, (c) LT-Rh/CrNaY, and (d) HT-Rh/ CrNaY.
k 3 x ( k ) function vs. k data are shown in Figure la-d for four different samples. The corresponding Fourier transforms (FT) are shown in Figure 2a-d. For the purpose of curve fitting (CF), the high-frequency noise was removed by a Fourier filtering technique and the resulting filtered data (dashed curves) are subsequently truncated a t 4.5 and 14.9 A-' and fitted with theoretical phase and amplitude functions& (solid curves) as depicted in Figure 3a-d.
Results and Discussions (I) Rh/NaY. The metal in the Rh/NaY catalyst treated at 360 OC in O2 and H2,designated as LT-RhNaY (sample a), has a high dispersion; we measured H/Rh and CO/Rh ratios of 1.00 and 2.00, respectively. These results are in fair agreement with those reported by Primet7 and Shannon et a1.8 They show that highly dispersed Rh particles in Y zeolite can be obtained from Rh(NH,),Cl2+/NaY. A t this dispersion every Rh atom ad(6)(a) Teo, B. K.; Lee, P. A. J. Am. Chem. SOC.1979, 101, 2185. (b) Teo, B.K.; Shulman, R. G.; Brown, G. S.; Meixner, A. E.Ibid. 1979,101, 5624. ( c ) Teo, B. K. Acc. Chem. Res. 1980, 13, 412. (d) Teo, B. K. In EXAFS Spectroscopy: Techniques and Applications; Teo, B. K., Joy, P. C., Eds.; Plenum Press: New York, 1981;pp 15-38. (e) Teo, B. K.; Antonion, M. R.; Averill, B. A. J. A m . Chem. SOC.1983, 105, 3751. (7) Primet, M. J . Chem. Soc., Trans. Faraday 1 1978, 74, 2570. (8)Shannon, R.D.;Vedrine, J. C.; Naccache, C.; Lefebvre, F. J. Catal. 1984, 88, 431.
7
6
4
6
1
0
X ) Figure 2. Fourier transforms of the k3-weighted EXAFS spectra for (a) LT-RhNaY, (b) HT-RhNaY, (c) LT-Rh/CrNaY, and (d) HT-Rh/CrNaY. r (
Table I. Chemisorption and Structural Parameters of RhNaY and RhCrNaY Catalysts Rh-Rh Rh-Cr H/Rh CO/Rh r,A" N b N' r , A a Nc Rh foil 2.67 12 (a) LT-RhNaY 1.00 2.00 2.61 1.5 6.5 (b) HT-RhNaY 0.55 0.85 2.63 2.6 11.3 (c) LT-RhCrNaY 0.75 2.12 2.58 0.9 3.8 2.60 0.7 3.1 2.49 3.4 (d) HT-RhCrNaY 0.20 0.08 2.61 1.2 4.8 2.50 1.7 "The uncertainty in r is h0.02 A. bBest fit based on theory (BFBT) with a = 0.057. CFine adjustment based on model (FABM) with CY = 0.069.
sorbs two CO molecules to give Rh1(C0)2. When treating our catalysts at 550 "C in O2 followed by H2 reduction at the same temperature (designated as HT-RhNaY or sample b), the H/Rh and CO/Rh ratios decrease to 0.55 and 0.85, respectively, indicative of a significant decrease in the dispersion (cf. Table I). The low dispersion indicates the formation of large Rh aggregates upon high-temperature treatment. The only predominant feature in the FT of LT-RhNaY (sample a) is a single peak at near r = 2.4 A. After curve fitting and phase correction, the Rh-Rh interatomic coordination distance is 2.61 A (cf. Table I), which is 2.4% shorter than in the Rh foil ( r = 2.67 A).
Langmuir, Vol. 2, No. 6, 1986 775
EXAFS of RhlNaY and RhCrlNaY Zeolite Catalysts For the HT-Rh/NaY (sample b), the EXAFS data and
FT functions shown in Figures 1 and 2 are qualitatively similar to those of LT-Rh/NaY with the following exceptions. First, the rhodium interatomic distance is 2.63 A, which is still 1%shorter than that in bulk Rh metal but slightly longer than that in LT-RhNaY. Second, the amplitude function of this sample is larger than that of LTRh/NaY. Third, the peak intensity of the higher coordination shell a t larger r is increased. In other words, in going from H T (sample b) to L T (sample a), (1) Rh to Rh distance decreased, (2) the amplitude function decreased, and (3) peaks of higher coordination shells decreased. This is consistent with the chemisorption results mentioned above in that the LT (sample a) represents a higher degree of dispersion than the H T (sample b)). This EXAFS and the chemisorption results presented here are also fully consistent with those reported in the literature for Pt and Rh particles supported on alumina or silica. Sinfelt et al.9J0 and Van't Blik et al.ll found the following trends when metal dispersion increases for Pt and Rh on A1,0, or Si02: (1)the metal-metal distance decreases by 0.03-0.05 A, (2) the EXAFS amplitude of the first shell decreases significantly, and (3) the signal from outer shells decreases much faster than that of the first shell. Moraweck et al.', observed a contraction of 0.12 A in the Pt-Pt coordination distance of 12-A Pt particles in a PtNaY zeolite when preventing chemisorption by analyzing the catalyst in vacuum. (11) Rh/CrNaY. For Rh/CrNaY thermally treated at 360 "C and designated as LT-Rh/CrNaY (sample c), the EXAFS data in k space and also the corresponding FT data in r space bear a certain degree of analogy to those of LT-RhNaY (sample a), except that the amplitude is even smaller for the Cr-containingsample. One-term curve fitting was employed in the data analysis. It was found that the rhodium interatomic distance is 2.58 A. To detect a contribution of the chromium ions, we also performed two-term fits (Rh-Rh and Rh-Cr) and found that there is indeed a nonnegligible contribution of Rh-Cr to the EXAFS. The interatomic distance of first-shell coordination for Rh-Rh and Rh-Cr are 2.60 and 2.49 A, respectively. On the other hand, when the Rh/CrNaY sample was heated a t 550 "C, designated as HT-Rh/CrNaY (sample d), its EXAFS and FT functions are very distinctive, as illustrated in (d) of Figures 1and 2. Part of the EXAFS function a t low k is diminished and this might be due to the interference of Rh-Rh and Rh-Cr EXAFS oscillations. The F T peak in Figure 2d is broader and appears at r = 2.2 8, which is shorter than any of the previous three corresponding spectra a-c in Figure 2. It c a n n o t be fitted with a one-term (Rh-Rh) model. A subsequent two-term curve fitting analysis of the filtered data confirms that chromium is indeed present in the first coordination shell with r(Rh-Rh) and r(Rh-Cr) of 2.64 and 2.50 A, respectively, as illustrated in Figure 3d. The contribution of nearest-neighborrhodium and chromium as backscattering atoms to the EXAFS function is shown in Figure 4. The interatomic distances and coordination numbers of Rh in the above two catalysts treated at two different
5 .O 2.5
0 -7.5
-5.0 10 5
n -5 -in L
7
0 -2 -4
10
k
12
(x
14
Figure 3. Nonlinear least-square curve fitting (solid curve) o f the Fourier-filtered k 3 x ( k )vs. lz EXAFS spectra (dashed curve) for (a) LT-RhNaY, (bjHT-RhNaY, (c) LT-Rh/CrNaY, and (d) HT-Rh/CrNaY. 1
1
L
h
.
G
(9)Sinfelt, J. H.; Via, G. H.; Lytle, F. W. J. Chem. Phys. 1978, 68, 2009. (10) Via, G. H.; Meitzer, G.; Lytle, F. W.; Sinfelt, J. H. J. Chem. Phys. 1983, 79,1527. (11)Van't Blik, H.F. J.; van Zon, J. B. A. D.; Koningsberger, D. C.; Prins, R. J.Mol. Catal. 1984, 25, 379. (12)Moraweck, B.; Renouprez, A. J. Surf. Sci. 1981, 106, 35. (13)Van Zon, J. B. A. D.; Koningsberger, D. C.; van't Blik, H. F. J.; Sayers, D. E. J . Chem. Phys. 1985, 82, 5742.
8
6
.
..
I . .
..
8
I
. .
10
.
. .,,, 1
. .
12
J 14
8 -'I Figure 4. Contribution of nearest-neighbor rhodium (solid curve of (a))and chromium (solid curve of (b)) back-scattering atoms to the Fourier-filtered k 3 x ( k ) vs. k EXAFS spectrum of HTRh/CrNaY (dashed curves in (a) and (b)). k
(
temperatures are summarized in Table I. The small coordination numbers of Rh, evaluated according to the best
776 Langmuir, Vol. 2, No. 6, 1986
fit based on theory (BFBT)6eusing rhodium foil as the model, are very small, ranging from 0.7 to 2.6 (Table I). These values are even smaller than those reported by Van Zon et al.13 for highly dispersed Rh particles on A1203but comparable to those reported by Via et a1.,I0viz., 1.5 for 0.5 wt % Rh on yA1203. It is known that absolute values of coordination numbers derived from EXAFS must be interpreted with caution. What we consider of much greater importance is the significant influence of chromium ions and of the reduction temperature on the coordination number and particle size of rhodium which is evident from the present results. Since the particle size of rhodium in the zeolite is small, as evidenced by chemisorption and the EXAFS amplitude function, their structure may differ from that in the rhodium foil and it might be debatable whether this foil should be used as a model. A small particle with icosahedral structure has been claimed to be energetically more favorable than a fcc symmetry.14a Sattler et al.14breport for some elements that clusters formed in atomic beams with 13 atoms (magic number) are abundant compared to clusters with 12 and 14 atoms. It is, therefore, not impossible that the Rh particles in sample (a) may have an icosahedral structure wih an average coordination number 6.5. Such a Rh particle has diameter of 8.1 A, which is larger than the aperture of supercage (7.5 A) and should be trapped in the supercages to give a high dispersion H/Rh = 1.00. If we use sample (a) as a model instead of Rh foil, the coordination numbers for the other samples, (b)-(d), can be estimated (fine adjustment based on model, FABM)6eand these numbers are listed in Table I. We realize it is inappropriate to use the S* value for one-term and two-term fits, where S* is the amplitude reduction factor. However, in the absence of a well-defined model compound containing both Rh and Cr, we take the S*Rh from the one-term fit as a crude estimate of the S*Rh for the two-term fits. For a bimetallic catalyst (Pt + Mo)/Si02 prepared by the successive decomposition of deposited allylic complexes, Yermakov15suggests a chemical bonding between Pt atoms and Mo ions resulting in an enhancement of the (14) (a) Gordon, M. B.; Cyrot-Lackmann, F.; Desjonqdres, M. C. Surf. Sei. 1979, 80, 150. (b) Sattler, K. Surf. Sci. 1985, 156, 292. (15) Yermakov, Yu. I. Catal. Rec.--Sci. Eng. 1976, 13, 77.
Tzou et al. thermal stability of the catalyst. It was also reported that the adsorption of Cr(CO)6 on NiY followed by oxidation to form Cr203could inhibit the agglomeration of nickel into large crystallines.16 From our present study, it appears that chromium ions interact with rhodium atoms. As it is certain that these Cr ions are electrostatically bound to the zeolite oxygen anions, we may conclude that they act as anchoring sites. This anchoring phenomenon is likely to inhibit migration of Rh atoms or small Rh particles through the zeolite channels and, therefore, results in a lower average particle size of the Rh particles. By assuming that after calcination all Cr ions are hexavalent and still in extralattice positions, it is possible to calculate the oxidation state of these ions after reduction from the quantity of hydrogen consumed. In this manner an oxidation state of 1or 2 results for the reduced Cr ions." However, the interatomic distance r(Rh-Cr) = 2.50 A is larger than the sum r(Rho) r(Crl+) = 2.15 A or r(Rho) + r(Cr2+)= 2.23 A, but it is shorter than r(Rho) r(Cro) = 2.59 A.ls While we are aware that conclusions based on adding radii of rigid atoms and ions are of limited significance, it appears not unlikely that the d electrons of Rh spill over to adsorbed Cr"' ions which results in an increase in the effective radius of the latter ions; Le., the effective positive charge on each metal is probably very small and smeared out over the Cr ion and its Rh neighbors. It would be interesting to carry out EXAFS measurement on the Cr K-edge. However, the present results strongly indicate that an interaction of rhodium atoms and chromium anchoring ions takes place and is responsible for the superior dispersion and its maintenance upon heating in Hz.
+
+
Acknowledgment. We gratefully acknowledge support from the U S . Department of Energy under contract DEAC02-84ER13157. Registry No. Rh, 7440-16-6; Cr, 7440-47-3; H2,1333-74-0;CO, 630-08-0. (16) Lawson, J. D.; Rase, H. F. Ind. Eng. Chem. Prod. Res. Deo. 1970, 9, 317. (17) Tzou. M. S.: Sachtler. W. M. H.. unuublished data. (18) CRC Handbook o f Chemistry and Physics, Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1984; pp F170, F172.
