4164
J . Phys. Chem. 1993,97, 4164-4166
Studies of the Segregation Dependence of Proton Knight Shifts in Ru-Cu Bimetallics B. C. Khanra and T . S. King’ Ames Laboratory, U.S.Department of Energy and Department of Chemical Engineering, 231 Sweeney Hall, Iowa State University, Ames, Iowa 5001 1 Received: October 2, 1992; In Final Form: February 12, 1993
Theconcentration dependence of proton Knight shifts associated with chemisorbed hydrogen on Ru-Cu bimetallics has been studied in terms of conduction electron polarization and surface segregation. Comparison of the calculated results with the experimental results suggests that the proton Knight shift for Ru-Cu particles results mainly from the hydrogen atoms experiencing a hyperfine interaction with three Ru first nearest neighbors.
1. Introduction Recently, a significant amount of N M R work has been done using solid-state N M R to characterize SiOz-supported bimetallic catalysts such as Ru-Cu, Pt-Cu, Pt-Ag, and Pt-Au, etc.I4 In these studies the influence of group 1b metals on the Knight shift of the reversibly adsorbed hydrogen was investigated, and the results were used to obtain a rough estimate of the surface composition of the bimetallic particles. Thus, a relationship was established between the surface compositions and the proton Knight shifts of bimetallic particles. In the present work, we have attempted to quantify the phenomenon of proton Knight shifts in Ru-Cu bimetallic particles from our understanding of (a) particle morphology, (b) segregation behavior of small particles, and (c) conduction electron polarization in transition metals and their alloys. In section 2 we develop expression for the Knight shift in terms of the surface composition and the number of first nearest neighbors of one of the bimetallic components. In section 3 we calculate the surface composition of the bimetallic particles of varying dispersion. In section 4 we briefly describe the experimental conditions for proton Knight shift measurements. Insection 5 we present thecalculated Knight shift results and compare them with the experimental results for various overall compositions of Ru-Cu particles. Conclusions are drawn in section 6.
2. Conduction Electron Polarization and Knight Shift For the H/Ru-Cu system, there are hydrogen atoms with differing numbers of Ru and Cu nearest neighbors. The magnitude of the H Knight shift as observed experimentally is proportional to the conduction electron polarization and may be expressed as the sum of contributions due to protons with n Ru neighbors and protons with other than n Ru nearest neighbors. If P,(x,) be the probability that any H atom has n Ru nearest neighbors, then one may write for a system with N adsorbed hydrogen atoms on Ru-Cu particles the total proton Knight shift K as where NP,(xs) is the number of H atoms with n Ru neighbors and [ 1 - NP,(x,)] is the number of H atoms with other than n Ru nearest neighbors. xsis the surface concentration of Ru. a& and b a a r e the constants that may be determined from the Knight shifts of hydrogen adsorbed on limiting surface concentrations. Such analysis has previously given a good amount of Knight shifts for bulk and surface ~ y s t e m s . ~In. ~view of the fact that for particles of size 4 nm the number of 3-fold hollow sites would constitute a major fraction of the surface sites and given that the hydrogen atoms preferentially adsorb on these sites,’ eq 1 is
* To whom
all correspondence should be addressed.
0022-3654/93/2091-4164%04.00/0
expected to describe reasonably well the proton Knight shift on this bimetallic system with the maximum value of n being 3. Now, the probability that a proton will have n number of Ru neighbors on 3-fold sites is given by the usual binomial expression
However, the probability function P,,(x,) expressed by relation 2 considers a random distribution of Ru and Cu atoms on the particle surface. In reality, the Ru-Cu particle surface has copper atoms more in the form of copper islands than as random distribution (as may be observed from Figure l b obtained from a Monte Carlo simulation4). Therefore, we must modify eq 2 to take into consideration the effect of the clustering tendency of Cu atoms on Ru particles. We have done this modification by a quasi-chemical treatmentas In this treatment, the number of Cu-Ru atom pairs is less than its value for random distribution of Ru and Cu atoms on the surface. Therefore, the probability of a hydrogen atom having three Ru nearest neighbors would be higher compared to its value for random distribution of Ru and Cu atoms. P3(xs) would then be modified as8 (3) where @ is a parameter determined from the heat of mixing and is given by the relation
+
p = { 1 4x,( 1 - x,)(T’ - l))’/’ (4) x, being the Ru surface concentration and 7 = ewbkT.Here, 2w/z is the energy required to change an AA pair and a BB pair into two AB pairs where z is the coordination number. Since for Ru-Cu the heat of mixing reaction is positive (endothermic), w is positive and 7 > 1 and j3 > 1. Thus, the overall probability function P3(x,) is greater than xs3,since (j3 + 1)/2 > 1. We can write for Ru-Cu system
+
P ~ ( ~= , ) i/2X,3[{1 + 4xs(1 - x , ) ( ~- ~I ) ) I / ~ 11
(5)
3. Segregation in Bimetallic Particles
In order to calculate the Knight shift as a function of the composition of the bimetallic particles, we must know xs. For the present work we use the x, values as obtained from a Monte Carlo ~imulation.~ The Monte Carlo simulations assumed a cubooctahedron as an initial particle morphology. In all cases the shape did not change from this energetically stable configuration. Since Ru and Cu do not mix in the bulk due to a large endothermic heat of mixing, Cu totally segregates to the surface. One can easily plot the x,(Ru) vs ~ ( C U )where , ~ ( C U is) the average Cu concentration of the particle. (In the case of small particles, the 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 16, 1993 4165
Proton Knight Shifts in Ru-Cu Bimetallics
A
1.o
1
I
h
0.8 3
E.
1
1
non-random Cu distribution (Theory)
-60 ".. h
I
I
- Expt.
-65
.......
0.6
random Cu distribution -
(I)
X
0.4 -
0.2 0.0
0.0
0.2
0.4
x
0.6 (CU)
0.2
0.4
-
0.6
0.8
1.0
x (CU)
1.0
0.8
Figure 2. Comparison of calculated and experimental proton Knight shifts of Ru-Cu particles as a function of average Cu concentration in the particle (0,experimental points).
(N03)3and C U ( N O ~ )Thevolumetric ~. hydrogen chemisorption was carried out in a high-vacuum apparatus' at room temperature after the catalyst sample was reduced in flowing hydrogen (50 cm3/min) at an elevated temperature (450 "C for Ru and 400 OC for Pt) for 2 h and evacuated at the reduction temperature. Previous studies indicated that this reduction procedure was sufficient to ensure that the metal particles had reached equilibrium.2.3 A speciallydesigned needle bellows device made from stainless steel was used for direct reduction of a catalyst sample in flowing hydrogen inside a 5-mm NMR tube. After reduction and evacuation purified hydrogen was dosed through the needle to the samples, and the system was allowed to equilibrate for 4 h. The NMR tube containing the sample was then immersed in a water bath and sealed off with a microtorch. The home-built NMR spectrometer used was operated at 220 MHz for proton resonance.
5. Calculation of Proton Knight Shifts and Comparison witb Experimental Results
- 1
r
(9 Figure 1. (A) Surfaceconcentrationof Ru atoms as a functionof average Cu concentrationin Ru-Cu particles with three different total number of atoms (T = 550 K). (B) Monte Carlo simulation results for Ru-Cu particles with total metal dispersion of 31% and Cu atom % of (a) 296, (b) 596, (c) 1095, (d) 15'36, (e) 2096, and (f) 30% (T = 550 K).
bulk concentration is not well-defined and hence we use the average concentration.) In Figure 1A is plotted x,(Ru) vs Z(Cu) for three particle sizes consisting of 4033, 2406, and 586 atoms, respectively. A Monte Carlo calculation by Wu et al.4gives the occupation of surface sites by Cu atoms as shown in Figure 1b. The results correspond to a temperature 550 K. It may be noted from Figure 1B that irrespectiveof the overall Cu concentration in the particle, ~ ( C U )the , Cu atoms do not mix with the Ru atoms but form a linear and/or two-dimensional cluster on the particle surface. This is a result of endothermic heat of mixing.
4. Summary of Experimental The Ru-Cu/SiOz bimetallic catalysts were prepared by a coimpregnation method using an aqueous solution of Ru(N0)-
From the segregation results plotted in Figure 1A we have calculated the K for various values of particle-average Cu concentration, ~ ( C U ) We . compare the results with available experimental res~lts.2.~ To find the constants a a in eq 1, we adjust eq 1 with K for ~ ( C U=) 0, x,(Ru) = 1, and K = -63 ppm, the value of proton Knight shift on pure ruthenium. For bdV we consider the experimental value ~ ( C U=) 0.3, x,(Ru) = 0.04, and K = -48.5 ~ p m Thus, . ~ we have for n = 3
K = 63x:
+ 4 8 3 1 - x:)
(6)
The interpolated results are shown in Figure 2, and the results are extrapolated to ~ ( C U = ) 0.8. We have compared the calculated results for particles with dispersion D = 0.31 (2406 atoms) with experimental proton Knight shifts. The theoretical NMR shifts are very close to the experimental shifts. We have also calculated the Knight shift with random probability distribution function P3(xs)= x.: Please note, the quasi-chemical approach to take into account the clustering of Cu atoms on the surfacegives better agreementwith experimentalresults. In view of the statistical nature of the analytical model, the qualitative as well as semiquantitative agreement with the experimental results is highly significant. The analysis presented above is based on approximating the Ru-Cu particles with stable fcc cubooctahedron geometry. Although Ru has a hcp structure, an fcc structure assumed in the model is expected to give a good approximation of the metal particles. For such particles with dispersion D = 0.31 (total number of atoms in the particles being 2406), only 13%of the
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The Journal of Physical Chemistry, Vol. 97, No. 16, 1993
TABLE I: Comparison of Theoretical and Experimental Proton Knight Shifts ( K ) on Ru-Cu Bimetallics (for Explanation See Text) X(Cu) 0.1 0.2
K (ppm) n=2
expt
n=0
n=1
-56.3 -50.5
-59.7 -57.8
-30.0 +large
+large +large
n=3 -54.3 -49.3
surface sites allow hydrogen atoms to have four surface nearest neighbors. (These are the hydrogen atoms adsorbed on fcc (100)type faces.) On rest of the surface sites hydrogen atoms can have fewer than four surface nearest neighbors. The present model analyzes, therefore, the experimental Knight shifts as a sum of the Knight shifts coming from hydrogen atoms with different number of Ru nearest neighbors-the maximum being three. In Table I we compare the calculated Knight shifts at two concentrations for different values of n where n is a number such that the Knight shift K is assumed to come from hydrogen atoms with n Ru nearest neighbors and from hydrogen atoms with other than n Ru nearest neighbors. It may be noticed that three Ru nearest neighbors gives the best fit with the experimental results.
Khanra and King For estimating the surface composition of bimetallic particles, however, there exists a simpler phenomenological model9 based on a priori knowledge of the heats of segregation on different sites of a particle. There is no known way how to calculate these site-dependent heats of segregation. The segregation results obtained from Monte Carlo calculations4 and used in this calculation avoid evaluation of the heats of segregation and, therefore, give a better estimate of the surface composition. Within the limitations of these assumptions, the present study shows the relationship between surface segregation and proton Knight shifts in supported bimetallic systems. This study also throws light on the possibleoccupation of surfacesites by different species. The main result of the present investigation is that for Ru-Cu bimetallics theshift may very well beinterpreted as arising from the adsorbed hydrogen atoms with three Ru first nearest neighbors.
Acknowledgment. This work was supported by the Biological and Chemical Technologies Research Program (Advanced Industrial Concepts Division) of the U S . Department of Energy through the Ames Laboratory which is operated for the US. DOE by Iowa State University under Contract W-7405-Eng-82.
6. Conclusions
In the present work we have endeavored to understand experimental proton N M R results of hydrogen chemisorbed on Ru-Cu bimetallics in terms of conduction electron polarization in transition-metal alloys and segregation in small particles. We have made several assumptions in this calculation. For example, we have assumed that the Knight shifts arise mostly from hydrogen atoms adsorbed on 3-fold sites in fcc (1 1 1)-like basal planes, though for particles with dispersion D = 0.3 1 the number of such sites is 4 5 % . Secondly, our calculation are for particles with a fixed number of atoms, whereas the experimental systems have particle size distributions. To the best knowledge of the authors, there has been no previous analysis of the proton knight shift data for supported bimetallic particles in terms of surface segregation. The present work is the first of its kind.
References a d Notes (1) Wu, X.; Gerstein, B. C.; King, T. S. J . Cutal. 1989,118, 238. (2) Wu, X.; Gerstein, B. C.; King, T. S. J . Cutal. 1990,121, 271. (3) Wu, X.; Gerstein, B. C.; King, T. S. J . Cutal. 1990,123, 43. (4) Wu, X.; Bhatia, S.; King, T. S. J . Vac. Sci. Technol. A 1992,10, 2729. (5) Craig, P. C.; Mozer, B.; Segnan, R. Phys. Rev. Lett. 1965,14,895. (6) Modak, S.; Khanra, B. C. Phys. Rev. B 1983,28,2279; J . Phys. C 1985,is,~ 8 9 7 . (7) Khanra, B. C.; Saha, S. K. Chem. Phys. 1983,76, 255. Chou, M. Y . ;Chelikowski, J. R. Phys. Rev. Lett. 1987,59, 1737; Phys. Rev. B 1989, 39,5623. Lauth, G.;Schwartz, E.; Christman, K. J . Chem. Phys. 1989.91, 3729. (8) Guggenheim, E. A. Mixtures; Clarendon Press: Oxford, 1952; p 38. (9) Helm, C. R. In Interfacial Segregation; Johnson, W. C., Blakely, J. M., Eds.; American Society for Metals: Metal Parks, OH, 1979; p 175.
