J. Phys. Chem. 1989, 93, 8323-8327 dipolar system, and the effect of the nonlinear polarization will make an important contribution to the elucidation of this problem. Finally, it will be instructive to compare our reaction coordinate x with those defined by the others. In Figure 4A, we draw the energy surface of f f and HP in the phase space (only two-dimensional coordinates q1 and q2 are shown due to the limitation of the drawing). We look for the hypersurface which holds the relation IF - ZP = x . As examples, we have chosen three values of c’, 2, and c3 for x . Those hypersurfaces are named SI,S2,and S3 and are represented by the curves in Figure 4A. In Figure 4, parts B and C, we draw these curves in the q1 and q2 coordinates. Marcus’ y is defined by the successive vertical line on each hypersurface.20 Warshel’s xi is defined as the minimum coordinate in each hypersurface Si. Therefore, this xi corresponds to a point in the phase space. Our c‘ c corresponds to his .‘C Warshel’s reaction coordinate is a locus of xi, retaining a molecular picture. Our x has no such molecular picture. However, the variation of x corresponds to travel along the different hypersurfaces such as SI S2 S3, which is rather similar to the propagation of y and to the transfer of x1 x2 x3. Therefore, our x will also deserve the name of reaction coordinate. The reaction coordinate y as defined by Calef and Wolynes22exactly corresponds to our x and accordingly it is usable in the nonlinear case. Now, the r defined by Marcus20 is equal to x c in our notation. The
+
--
--
+
8323
Marcus m is correlated to our x by the equation
where =
+m(ff-p)
(53)
We can see that only when the linear response holds is x proportional to m.
Acknowledgment. We express our sincere thanks to Dr. M. Tachiya and Prof. H. Sumi for a valuable discussion on the relation of the free energy curvatures between the initial and final states. We are very grateful to Prof. R. Marcus for giving us much valiable comments on our Monte Carlo simulation by freezing translational motions, on the controversy about the interpretation of Rehm and Weller’s data, and on improving the manuscript. T.K. was supported by Grants-in-Aid (63300014,01300009) from the Japanese Ministry of Education, Science and Culture. N.M. acknowledges the support by a Grant-in-Aid (62065006) from the Japanese Ministry of Education, Science and Culture. The computations of Monte Carlo simulation were done by using a FACOM VP200 in the Computation Center of Nagoya University and HITAC S-820/80 in the Institute for Molecular Science.
Temperature Dependence of Extended X-ray Absorption Fine Structure Spectra of Rh and Pd Catalysts in the Strong Metal-Support Interaction State Toshihiko Yokoyama,+Kiyotaka Asakura, Yasuhiro Iwasawa, and Haruo Kuroda* Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan (Received: January 19, 1989; In Final Form: June 20, 1989)
The temperature dependence of EXAFS spectrum was investigated on the normal and SMSI states of Rh/Ti02 and Pd/Ti02 catalysts. The mean square relative displacement (MSRD) of metal-metal bondings of supported metal was derived by the analysis of EXAFS data with the cumulant expansion technique, and the Debye temperature was estimated from the temperature dependence of MSRD. The Debye temperature of Rh-Rh bonds of Rh/Ti02 catalyst was found to be higher by 38 K in its SMSI state in comparison with the normal state. This difference was shown to be due to the significant increase of the surface Debye temperature of Rh metal particles. A similar result was obtained for Pd/Ti02 catalyst. These facts were concluded to support the “decoration model” for the SMSI state.
Introduction It is well-known that the H2 and C O adsorption capacities of noble metal catalysts supported on a reducible transition-metal oxide such as Ti02 and Nb205dramatically decrease when the catalysts are reduced by a high-temperature heat treatment in hydrogen atmosphere.’ Since this phenomenon was ascribed to the strong interaction between the metal and the support, it was called the “strong metal-support interaction” (SMSI). Horsley et al.2 first suggested that this phenomenon occurred through the charge transfer between the metal and the support, being accompanied by a transformation of metal particles into a raftlike structure. This model is known as the “charge-transfer pillbox model”.2 However, in a more recent work on Fe/Ti02 catalyst, Santos et aL3 showed that the SMSI behavior persisted even if the mean diameter of supported iron particles was larger than 200 A and pointed out that the observed SMSI phenomenon could not be understood by the charge-transfer model since the effect of the excess charge should be screened out in a metal particle of such a large diameter. They proposed that a diffusion of Ti02 over the metal particle surface took place during the high-tem‘Present address: Department of Materials Science, Faculty of Science, Hiroshima University, Naka-ku, Hiroshima 730, Japan.
0022-3654/89/2093-8323$01.50/0
perature reduction treatment. Later, Jiang et a1.4 also discussed the presence of titanium oxide species covering over metal particle surfaces in their study on the hydrogen uptake behaviors of several metal catalysts supported on Ti02. These models are known as the “decoration model”. Furthermore, Resasco et aLs found that the ethane hydrogenolysis activity of Rh/Ti02 catalyst followed a square-root dependence upon reduction time and pointed out that this behavior was in accord with the decoration model. There have been several attempts“’ to clarify the SMSI mechanism by studying a model system employing UHV surface (1) Tauster, S.J.; Fung, S.C.; Garten, R. L. J. Am. Chem. Soc. 1978,100, 170. Tauster, S.J.; Fung, S.C. J. C a r d 1978.55, 29. Smith, J. S.;Thrower, P.A.; Vannice, M. A. J. Carol. 1979, 56, 236. (2) Horsley, J. A. J. Am. Chem. Soc. 1979, 101, 2870. Tauster, S.J.; Fung, S.C.; Baker, R. T.; Horsley, J. A. Science 1981, 211, 1121. (3) Santos, J.; Phillips, J.; Dumesic, J. A. J. Caral. 1983, 81, 147. (4) Jiang, X. 2.; Hayden, T. F.; Dumesic, J. A. J. Carol. 1983, 83, 168. (5) Resasco, D. E.; Haller, G. L. J. Carol. 1983, 82, 168. (6) Kao, C. C.; Tsai, S.C.; Bahl, M.K.; Chung, Y. W.; Lo, W. J. Surf. Sci. 1980, 95, 1. Cairns, J. A,; Baglin, J. E. E.; Clark, G. J.; Ziegler, J. F.; J . Catal. 1983, 83, 301. KO, C. S.;Gorte, R. J. J. Catal. 1984, 90, 59. Sadeghi, H. R.; Henrich, V. E. J. Carol, 1984,87,279. Belton, D. N.; Sun, Y. H.; White, J. M. J. Phys. Chem. 1984.88, 5172. Raupp, G. B.; Dumesic, J. A. J. Catal. 1985, 95, 587. (7) Takatani, S.;Chung, Y. W. J. Carol. 1984, 90, 75.
0 1989 American Chemical Society
Yokoyama et al.
8324 The Journal of Physical Chemistry, Vol. 93, No. 26, 1989
science technique. Takatani et al.’ presented evidence for the migration of partially reduced titanium oxide TiO, over the surface of Ni particles deposited on Ti02, by use of Auger electron spectroscopy (AES) and high-resolution electron energy loss spectroscopy (HREELS). However, there remains some doubt in adopting such observations on a model system as a proof of the decoration model for the SMSI states of practical catalysts, since there is a significant difference in the CO adsorption behavior between the model system and the SMSI state of a practical catalyst. EXAFS spectroscopy is the most promising technique to study the SMSI state of a practical catalyst and has been actually used by several authors.*-I0 Sakellson et aL9 claimed, through the analysis of the EXAFS data of Rh/Ti02 catalyst in the SMSI state, that they could actually detect Rh-Ti bondings with a distance of 2.53 A, corresponding to the sum of the metallic radii of Rh and Ti, and attributed this to the bondings of surface Rh atoms with the decorating Ti02 species. Their proposal was immediately criticized by Koningsberger et a1.,I0 who investigated the SMSI state of a similarly prepared Rh/Ti02 catalyst and reported that they could detect Rh-0 bondings but no Rh-Ti metallic bond. They stated that, since no evidence could be found for the decoration of metal particles by TiO,, the SMSI phenomenon might be due to the electronic interaction between the metal and the support. Thus the conclusions given by those two works apparently contradict to each other. This contradiction may reflect the difficulty in getting direct information on the bondings on the surface of supported metal particles from the analysis of EXAFS data. All the previous works*-1° aimed to obtain direct information on the bondings of surface atoms employing catalysts with a very small particle size of supported metal. Even in such cases it may not be easy to separate without ambiguity the contribution of the bondings on the surface or interface of a metal particle from the strong contribution of the metal-metal bondings in the bulk of a metal particle. In the present study we took an alternative approach to understand the SMSI state. That is, we paid attention to the change in the mean square relative displacement of metal-metal bondings of supported metal, since the thermal vibration of surface atoms should be significantly affected if any decoration takes place over the particle surface. Experimental and Data Analysis The catalysts, Rh(2.0 wt %)/Ti02 and Pd(2.0 wt %)/Ti02, were prepared by the incipient wetness impregnation method; Ti02 powder (P-25, with the BET surface area of 70 m2/g) was impregnated with an aqueous solution of RhC13 (or Pd(NO,),), dried and reduced by heating to 773 K in hydrogen (1 50 Torr) for 2 h, and subsequently calcined at 673 K in oxygen atmosphere (1 50 Torr) for 2 h. The last treatment was done in order to avoid the sintering effect in the SMSI state. For further reduction treatments, the catalyst powder thus prepared was put into a Pylexmade reactor, to which a compact sample cell for EXAFS measurement had been connected. The reduction with hydrogen at 473 K (“low-temperature reduction”) gave the ”normal” state with an ordinary capacity of H2 (or CO) adsorption, while the reduction at 773 K (“high-temperature reduction”) yielded the ”SMSI” state. After these treatments, the catalyst was transferred into the EXAFS cell without breaking the vacuum, and the cell was sealed off from the reactor system. The EXAFS sample cell is made to be sufficiently airtight having Kapton windows for X-ray transmission, so that the sample catalyst can be kept in vacuo throughout the EXAFS experiment. A hydrogen chemisorption measurement was performed on each sample at the room temperature by the conventional volumetric (8) Short, D. R.; Mansour, A. N.; Cook, Jr., J. W.; Sayers, D. E.; Katzer,
J. R. J . Catal. 1983, 82, 299.
method. The resultant H / M value (total amount of irreversibly adsorbed hydrogen per total amount of supported metal) was 0.50 and 0.30 for the normal states of Rh/Ti02 and Pd/Ti02, respectively. The amount of chemisorbed (irreversibly adsorbed) hydrogen was confirmed to be negligible in the SMSI state. From the observed H/M values, the average diameter of supported metal particles was estimated to be about 30 and 50 A for the normal states of Rh/Ti02 and Pd/Ti02, respectively. The particle size in the SMSI state cannot be estimated by this method, but the fact that the coordination number of the first nearest neighbor Rh-Rh in the SMSI state of Rh/Ti02 catalyst was essentially the same as in the normal state indicates that particle size remained almost the same. Rh K-edge and Pd K-edge EXAFS spectra were measured by use of the EXAFS spectrometer” at BL-IOB of the Photon Factory in National Laboratory for High Energy Physics (KEK-PF). The operating condition of the storage ring during the present EXAFS measurements was 2.5 GeV in energy with the ring current of 130-300 mA. The sample cell containing catalyst was mounted in the cryostat equipped with a cryogenic refrigerator (CTI), which was placed at the sample stage of the EXAFS spectrometer. The Rh K-edge EXAFS spectrum was measured at 25, 125,240, and 355 K on the normal and SMSI states of Rh/Ti02 catalyst. In the case of Pd/Ti02 catalyst, the Pd K-edge spectrum was measured only at 200 and 295 K. EXAFS measurements were done also on the reference materials, Rh metal foil (25 pm) and Pd metal foil (25 pm). EXAFS data were analyzed by use of the cumulant expansion technique.I2 According to the well-established procedures such as the background subtraction and normalization, the EXAFS oscillation, ~ ( k )was , extracted from the observed spectrum. After Fourier transformation of k 3 x ( k ) ,inverse Fourier transformations into the real and imaginary spaces were carried out separately by setting a window in the region of a prominent peak in the Fourier transform. The amplitude A ( k ) and phase @ ( k )can be expressed by (1) and (2), respectivelyI2 A ( k ) = ( N / k R Z ) F ( k )exp[-2a2k2]
(1)
@ ( k ) = 2kR - 4 ( k ) - 4C3k3/3
(2)
where N is the coordination number of the neighboring atoms at the distance R from the absorbing atom, a2 is the mean square relative displacement (MSRD) between the atoms concerned, F(k) is the effective backscattering amplitude including damping factor due to many-electron effects, +(k) is the total phase shift function, and C3 is the mean cubic relative displacement associated with an asymmetric distribution. When the distribution function can be expressed with a Gaussian function with a narrow width, we can assume that C3 = 0. When we denote the amplitude and phase functions experimentally derived for the reference material as Ao(k) and 40(k), respectively, the following equations can be derived: In [ A ( k ) / A O ( k )=] In [NRo2/N&2] - 2(a2 - u?)k2 (3)
where C3have been assumed to be zero for the reference materials. Since the values appearing in the left sides of (3) and (4) can be derived from experimental data, we can obtain the structure parameters appearing in the right sides of the equations by means of the linear least-squares refinements. Results and Discussion Figure 1 shows the Rh K-edge EXAFS oscillation k 3 x ( k ) obtained at 25 K for the normal and SMSI states of Rh/TiOz catalyst. The oscillation associated with the first nearest Rh-Rh bondings dominantly appear in both states. Their Fourier
(9) Sakellson, S.;McMillan, M.; Haller, G . L. J . Phys. Chem. 1986, 90,
1777 .
(10) Koningsberger, D. C.; Martens, J. H. A.; Prins, R.; Short, D. R.; Sayers, D. E. J . Phys. Chem. 1986, 90, 3047. Martens, J. H. A,; Prins, R.; Zanbergen, H.; Koningsberger, D. C. J . Phys. Chem. 1988, 92, 1903.
(1 1) Oyanagi, H.; Matsushita, T.; Ito, M.; Kuroda, H. KEK-Rep. 1984, 83/30.
(12) Banker, G. Nucf. Instrum. Methods 1983, 207, 437.
EXAFS Spectra of Rh and Pd Catalysts in SMSI State U'
I
I
The Journal of Physical Chemistry, Vol. 93, No. 26, 1989 8325 TABLE I: Results of Rh K-Edge EXAFS Analysis for Rh/TiOj' sample
SMSI
normal
-36
1
(a) " " '
5
"
'
'
'
'
"
'
10
'
'
a*, A2
5.15 X 5.75 X 6.78 X 8.39 X 4.42 X 4.96 X 6.16 X 8.27 X
lo-'
lo-' lo-' lo-'
c3,A' (0) (0) 0.8 X 1.6 X (0) (0) 0.6 X 1.8 X
lo4 lo4
lo4 IO4
TABLE 11: Results of Pd K-Edge EXAFS Analysis for Pd/TiOZ0
'"I
-
1
sample SMSI normal
1
R, A 2.667 2.666 2.669 2.673 2.665 2.667 2.669 2.673
'Coordination numbers are assumed to be 9.7, which was determined by the analysis of 25 K data for both states.
" " '
WAVE NUMBER / 8 - l
-30
temp, K 25 125 240 355 25 125 240 355
temp, K 200 295 200 295
R, A 2.744 2.747 2.752 2.755
u2, A2
5.19 X lo-' 7.21 X lo-) 5.89 X IO-) 8.37 X
c3, A3 0.1 0.7 0.9 1.5
X X X X
lo4 lo4 lo4 lo4
'Coordination numbers are assumed to be 10.3 and 9.8 for the SMSI and the normal states, respectively. These values were determined by the analysis of 200 K data.
(b) ' '
'
' '
'
' ' '
'
" "
loo WAVE NUMBER / A - '
' ' '
" '
Figure 1. Rh K-edge EXAFS oscillation functions k 3 x ( k ) of Rh/TiO, catalyst, measured at 25 K on the normal state (a) and on the SMSI state (b).
of the metal-support bonding. Thus, we analyzed our EXAFS data by assuming that the observed oscillation is entirely due to Rh-Rh (or Pd-Pd) bondings. Although the Fourier transforms of the low-temperature EXAFS data shown in Figure 2 exhibit the peaks attributable to higher coordination shells up to at least the fourth nearest neighbors, we will concentrate our discussion exclusively on the peak due to the first nearest neighbors. One should note in Figure 2 that the intensity of this peak markedly increases on lowering the temperature from 355 to 25 K. This increase reflects the decrease of the mean square relative displacement (MSRD) of the Rh-Rh bond. The MSRD u2 generally consists of two independent terms and can be expressed as u2 =
DISTANCE / A 44 I
1
1 t2,, DISTANCE /
a
Figure 2. Fourier transforms of Rh K-edge EXAFS k3x(k)of Rh/TiOz catalyst measured at 25 and 355 K on the normal state (a) and on the SMSI state (b).
transforms over the k region of 3.5-13.5 A-' are shown in Figure 2, together with those derived from the EXAFS data at 355 K. Even in the Fourier transform of the EXAFS data at low temperature we can find neither Rh-Ti contribution nor Rh-0 contribution, in disagreement with the works by Sakellson et al? and by Koningsberger et al.IO This may be because of the larger particle size of supported metal. In our catalyst, the coordination number of the first nearest Rh atoms was estimated to be about 9, which is much larger than the value (about 5) in the catalysts studied by the previous workers. The metal particle size in our Pd/Ti02 catalyst is also too large to give appreciable contribution
UT2
4- us*
(5)
where uT2is the term due to thermal vibration and us2 is the one due to static structural disorder. Here, the former should vary with temperature, whereas the latter is expected to be temperature-independent. The temperature dependence of uTz can be correlated with Debye temperature by the correlated Debye model14 if the metal particle size is not too small. The static disorder may not be neglected for a small metal particle since a static disorder could be often introduced through the metalsupport interaction as suggested by Marques et aLl3 The contribution of thermal disorder uT2is also dependent of particle size. Previous investigations by X-ray diffractionI5 and EXAFS spectroscopy16 revealed that the Debye temperature of a small metal particle decreased on decreasing particle size. This can be easily understood when we consider the fact that surface atoms can vibrate more freely, thus their Debye temperature would be lower, in comparison with bulk atoms, and the ratio of surface atoms relative to bulk atoms increases on decreasing particle size. In the catalysts investigated here, about half of metal atoms are on the surface, so we can expect a significant lowering of average Debye temperature. Keeping these expectations in mind, we have investigated the temperature dependence of MSRD. In the case of Rh/Ti02 catalyst, the coordination number of the first nearest Rh-Rh was estimated to be 9.7 f 0.5 for both the SMSI and normal states, suggesting that there is no significant structural change between the two states. This is consistent with (13) Marques, E. C.; Sandstrom, D. R.; Lytle, F. W.; Greegor, R. B. J. Chem. Phys. 1982, 77, 1027. (14) Beni, G.;Platzman, P. M. Phys. Reu. 1976, 814, 1514. (15) Harada, J. Surf. Sci. 1981, 106, 51. (16) Balerna, A.; Mobilio, S. J . Phys. 1986, C8, 1009. Balerna, A.; Bernieri, E.; Picozzi, P.; Reale, A,; Santucci, S.;Burattini, E.; Mobilio, S.Phys. Reu. 1985, B 3 l . 5058.
8326 The Journal of Physical Chemistry, Vol. 93, No. 26, 1989 N
c
0.010
04:
2
Yokoyama et al. From the average particle size, we estimated that c, = 0.38 and = 0.62 in the present Rh/Ti02 catalyst. The value of (uTb)2 can be assumed to be equal to the value for the bulk metal. Putting these values into (6), we calculated the surface MSRD (uTS),,and then from its temperature dependence, we estimated the surface Debye temperature 8 D s , the results also being given in Table 111. The value of 8 D s for the SMSI state is 341 K while that for the normal state is 241 K, showing a markedly large difference between the two states. It should be noted also that ODs in the normal state is in good agreement with the surface Debye temperature, 260 K, of Rh single crystal, which has been reported from the study of the LEED data of Rh( 11 1) surface. Similar results are found also for the Pd/Ti02 catalyst. The coordination number for the first nearest neighbors was 10.3 and 9.8 in the SMSI and normal states, respectively, and the Pd-Pd distance was almost the same in the two states as shown in Table 11. The MSRD value is meaningfully different between the SMSI and normal states, giving 81, = 273 K for the SMSI state and 8 D = 248 K for the normal state. As in the case of Rh/Ti02, the Debye temperature in the SMSI state is higher in comparison with the normal state. Again we estimated ODS, separating the surface and bulk contributions by use of (6), assuming that c, = 0.22 and cb = 0.78. The value of ODs was estimated to be 241 K in the SMSI state and 121 K in the normal state. Here again 8 D s in the normal state well agrees with the surface Debye temperature reported from the analysis of LEED data of Pd metal, 8,‘ = 142 K. As we have described above, both in Rh/TiO, and in Pd/TiOz the value of ODs significantly increases on going from the normal state to the SMSI state up to the value almost equal to the bulk Debye temperature ODb. This change was not due to the aggregation of metal atoms since, as we have already mentioned, the coordination number remained almost the same between the two states. Instead, the above-mentioned change indicates that the surfaces of metal particles in the SMSI state are covered with some chemical species. This fact is consistent with the decoration model. On the other hand, according to the pillbox model, the fraction of the surface atoms should increase in the SMSI state; hence the average Debye temperature should decrease. This is in contradiction with what we have found in the present study. Therefore, we conclude that our experimental results support the decoration model. In the present study, we could not obtain any direct information about the bondings between the surface metal and the decorating chemical species. However, the fact that the surface Debye temperature became higher by as much as 60 K in the SMSI state suggests that the bondings formed on the particle surface are considerably strong. In this connection, it is worth noting that the static disorder, us, is markedly large in the SMSI state, being 3.43 X AZ in the SMSI state of Rh/TiO, catalyst and 2.49 X A2 in its normal state. This means that a considerable static disorder has been introduced into metal particles, possibly because of the strong interaction with the decorating chemical species. cb
0.005
W
cc
W
a
Rh metal
4
3 Q
fn
z W 4
I I
0.000 I
0
I
I
I
100
200
3 00
1
TEMPERATURE / K Figure 3. Mean square relative displacements of Rh atoms as a function
of temperature. TABLE III: Debye Temperature of Supported Rh and Pd Metal Particles and Bulk Metals Determined by EXAFS
state average SMSI average normal surface SMSI surface normal bulk metal single-crystal surface’
rhodium 354 K
316 K 341 K 241 K
362 K 260 K
palladium 213 K 248 K 241 K 121 K 282 K 142 K
‘Determined by LEED. the suggestion given by Short et aI.* and by Koningsberger et al.IO There should be a considerable change in coordination number if the dispersion state of the supported metal changes to a raftlike structure. Thus the pillbox model is not likely to be acceptable for the SMSI state of the present Rh/Ti02 catalyst. From the obtained coordination number, the average diameter of metal particles is estimated to be about 30 A, which is in good agreement with the value derived from the hydrogen chemisorption measurement. Furthermore, the Rh-Rh distance in the SMSI state was found to be the same as the value in the normal state as shown in Table I. This fact is again inconsistent with the charge-transfer pillbox model since a considerable change of Rh-Rh distance should sppear if a strong charge transfer has occurred in the SMSI state. The obtained values of MSRD are listed in Table I, and the temperature dependence is shown in Figure 3. A significant difference between the normal and SMSI states is found in MSRD as well as in its temperature dependence. The temperature gradient of MSRD is considerably smaller in the SMSI state than in the normal state. This implies that Debye temperature 8 D is higher in the SMSI state than in the normal state. According to the correlated Debye model,14 we evaluated Debye temperature from the observed temperature dependence of MSRD. The results are given in Table 111 together with the values that were obtained by the same method from the observed EXAFS data of the bulk metal foils. The Debye temperature in the SMSI state is higher by 40 K than the corresponding value in the normal state. Note that the Debye temperatures obtained here are the averaged values including the contributions of the bulk and surface. The average value of MSRD due to thermal vibration uT2can be decomposed into the surface and bulk contributions by use of the relation UT2
= c,( UTS)
+ cb( UTb)’
(6)
where c, and cb are the relative weights of the surface and bulk atoms and and (uTb)lare their MSRD values, respectively.
Conclusion
In the present study, we have studied the temperature dependence of EXAFS spectra of Rh/TiO, and Pd/TiO, catalysts, aiming to obtain information on the SMSI state by investigating the change in the mean square relative displacement of metalmetal bondings and Debye temperature, instead of getting direct information on the bondings between the metal and the support or those between the surface metal atoms and the decorating species. From the analysis of the experimental data, we could successfully determine the Debye temperature associated with the surface atoms of the supported metal particles. The results show that the surface Debye temperature significantly increases on changing from the normal state to the SMSI state both in the Rh/TiO, catalyst and in the Pd/TiO, catalysts. This fact indicates that, in the SMSI state, the metal particle surfaces are covered with strongly bound chemical species, possibly by TiO, which has
J. Phys. Chem. 1989, 93, 8327-8333 diffused out from the support. Thus our experimental results are consistent with the decoration model rather than the pillbox model. Finally, we believe that the present results demonstrate the potential use of the analysis of the temperature dependence of EXAFS even in the case of a relatively complicated system like
8327
supported metal catalysts.
Acknowledgment. We express our thanks to Dr. Nobuhiro for his advice and stimulating discussion. Registry No. Pt, 7440-06-4; Rh, 7440-16-6;Ti02, 13463-67-7.
Surface Chemistry of C-N Bonds on Rh(ll1). 1. C2N2and CH,NH2 S. Y. Hwang, A.
C.F. Kong, and L. D. Schmidt*
Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 (Received: January 23, 1989; In Final Form: June 9, 1989)
The adsorption and decomposition of C2N2and CH3NH2on Rh( 111) have been studied by using temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES). Both molecules adsorb readily on Rh( 11 1) at 300 K and totally decompose. At very low coverages, C2N2decomposes completely to N2 (desorbing at 820 K) and surface carbon residues, while at saturation coverage (6.7 X lOI4 molecules/cm2), -80% of C2N2desorbs intact and 20% of C2N2decomposes to N2 and surface carbon. Adsorption of CH3NH2at 300 K produces H2 and N2 as major products at low coverages. Almost equal amounts of N, (lo%), C2N2(9%), and HCN (7%) are produced with H2 (74%) at saturation coverage. CH3NH2 is totally dehydrogenated by 500 K to leave surface CN which then follows the same decomposition path as C2N2. The saturation density is -8.2 X lOI4 CH3NH2molecules/cm2at 300 K. The chemistry of the C-N bonds in C2N2and CH3NH2 is therefore similar in that 20% (in C2N2)and 39% (in CH3NHz)of the bond is cleaved to produce N2 and surface carbon residues while the rest desorbs intact as HCN and C2N2. Their reactivities and selectivities on Rh( 111) are much higher than those on Pt( 11 1) where no C-N bond cleavage occurs and lower than Ni where total C-N bond cleavage is observed.
Introduction It is becoming evident that group VI11 transition metals have large variations in their ability to break the C-N bond, with Pt yielding no dissociation and Ni producing complete dissociation. Rh is intermediate in adsorption properties, and its selectivity in C-N bond dissociation should therefore be instructive in interpreting bond-breaking processes. Interest in C N and CNO bond surface chemistry derives from many industrial reactions such as synthesis of HCN from CH,, NH,, and O2over Pt-Rh gauzes (the Andrussow process). These systems have been studied much less than C 4 bond systems, and, as is shown from this work, CNO bond chemistry is much more selective on noble metals than are CO bonds. Recently, we investigated the adsorption and decomposition of CH3NH21and CH3N02and C2N> on Pt( 111) using TPD, A B , and XPS. We found that C-N bonds in all three molecules are very stable on Pt( 1 11) in that no significant C-N bond scission (identified by N 2 formation) is observed in TPD up to 1250 K. Investigation of the decomposition and oxidation of CH3NH2on polycrystalline Pt wires3 at higher pressures (0.5-10 Torr) also revealed that the addition of oxygen over Pt wires at 1450 K did not significantly change the decomposition chemistry of C-N bonds in CH3NHz until oxygen was in excess. In this study, we use TPD and AES to examine the adsorption and decomposition of C2N2and CH3NH2on Rh(l1 I). Cyanogen should desorb intact or decompose by C-N bond scission CzN2 N2 (1) which leaves carbon residues on the metal surface. Methylamine decomposition should occur by dehydrogenation CH3NH2 HCN + 2H2 (2) 72C2N2 + 72H2 (3) or by C-N bond breaking
-
-
-
(1) Hwang, S.Y.; Seebauer, E. G.; Schmidt, L. D. Surf. Sci. 1987,188,
21. (2) Hwang, S.Y.; Kong, A. C. F.; Schmidt, L. D., submitted for publiin Surf. Sci. (3) Hwang, S. Y.; Schmidt, L. D. J . Catal. 1988, 114, 230.
cation
0022-3654/89/2093-8327$01.50/0
CH3NH2
-
-+
CH4 + 72N2
72N2 + 7ZH2
+ !/*HZ
(4)
(5)
which can also yield NH3 and surface carbon residues. The adsorption and decomposition of C2N2on clean metal surfaces has been investigated previously on Pt( 11 1),Z4q5Pt( 110): Ni( 11 l),]' CuPt( loo),' Rh( 11 1),* Pd( 11 1),9310Pd( (lll),lz,13Cu(1l0),l4Ag(llO),l5 and Ru(10O).l6 On Ag(ll0) C2Nzadsorbs only molecularly. It adsorbs dissociatively (as CNd) and desorbs as C2N2through recombination of C N radicals in TPD on Pt, Pd, and Cu surfaces. On Ru( 11 1) it adsorbs dissociatively and decomposes partially to N2 and surface C upon heating. On Ni( 11 1) it adsorbs dissociatively and decomposes completely to Nz and surface carbon species upon TPD. C2N2thermal desorption on Pt( 11 1) has been studied in detail in this laboratory.2 C2N2adsorbed on Pt( 111) at 300 K produces at least five desorption peaks at 380 (a),710 (y), 810 (pl), 910 (&), and 1100 K (&), in which the a state was attributed to molecularly adsorbed C2N2,the y state as the "polymeric state", and the /3 states as associative desorption of surface CN. No C-N
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(4) Kingsley, J. R.; Dahlgren, D.; Hemminger, J. C. Surf.Sci. 1984, 139, 417. (5) Hoffmann, W.; Bertel, E.;Netzer, F. P. J . Catal. 1979, 60, 316. (6) Bridge, M. E.; Lambert, R. M. Surf.Sci. 1977, 63, 315. (7) Conrad, H.; Kuppers, J.; Nitschke, F.; Netzer, F. P. Chem. Phys. Lert. 1977, 46, 571. (8) Solymosi, F.; Bugyi, L. Surf. Sci. 1984, 147, 685. (9) Kordesch, M. E.; Stenzel, W.; Conrad, H . J. Electron Spectrosc. Relat. Phenom. 1986, 39, 89. (10) Kordesch, M. E.; Stenzel, W.; Conrad, H. Surf. Sci. 1987,186,601. (1 1) Hemminger, J. C.; Muetterties, E. L.; Somorjai, G. A. J. Am. Chem. SOC.1979, 101, 62. (12) Solymosi, F.; Kiss, J. Surf.Sci. 1981, 108, 368. (13) Kordesch, M. E.; Stenzel, W.; Conrad, H.; Weaver, M . J. J . Am. Chem. SOC.1987, 109, 1878. (14) Outka, D. A.; Jorgensen, S.W.; Friend, C. M.; Madix, R. J. J. Mol. Catal. 1983, 21, 375. (15) Bridge, M. E.; Marbrow, R. A.; Lambert, R. M. Surf. Sci. 1976,57, 415. (16) Gudde, N. J.; Lambert, R. M. Surf. Sci. 1983, 124, 372.
0 1989 American Chemical Society
