J . Phys. Chem. 1989, 93, 2039-2043
2039
Support Effect on Surface Reaction Rates in CO Hydrogenation over Supported Rh
Yoshihiko Mori,**+Toshiaki Mori,t Akira Miyamoto,+.sNaoki Takahashi,+Tadashi Hattori,? and Yuichi Murakamit Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Furo-cho, Chikusa- ku, Nagoya 464-01, Japan, and Government Industrial Research Institute, Nagoya, Hirate-cho, Kita- ku, Nagoya 462, Japan (Received: June 20, 1988)
The effect of the support and dispersion on the rate of surface reaction of CO hydrogenation over Rh catalyst was investigated by pulse surface reaction rate analysis (PSRA). Ti02, AI2O3,and Si02 were used as the supports, and the dispersion of Rh was varied over a wide range by changing the Rh loading. The rate constants of C-0 bond dissociation and the hydrogenation of the surface carbon species varied with the support and dispersion. The wavenumber of the IR absorption band due to linearly adsorbed CO also varied with the support and dispersion. The results are discussed in the light of more essential factors, such as the structure sensitivity, the electron property of Rh, and the pair site of metal-support suboxide. It was concluded that the electronic state of Rh is the predominant factor leading to the changes in the surface reaction rate constants with the support and dispersion.
Introduction Among group VI11 metals, the most remarkable feature of Rh is a potential ability for the selective production of C2 oxygenates from C O hydrogenation, such as acetaldehyde, ethanol, and acetic acid. The use of appropriate supports and promoters is essential for the improvement of the activity and selectivity. For this reason, a large number of studies have been devoted to elucidating the effect of the support and promoter on the activity and selectivity of Rh catalysts for C O hydrogenation.14 C O hydrogenation or methanation is composed of several steps which include the adsorption of CO and H2, the dissociation of adsorbed co [(CO),,] to form surface carbon [(CH,),,] and oxygen [(OH,,),,] species, and hydrogenation of (CH,),, to hydrocarbons with various numbers of carbon atoms.+' In addition, the insertion of co into the metal-carbon bond in (CH,.),, is also a requisite step for the production of the C2 oxygenates. However, the conventional rate measurement can give only the rate of the overall reaction for these complicated processes. In order to determine the rate of the individual steps involved in an overall reaction, we have previously developed the pulse surface reaction rate analysis (PSRA) method and demonstrated its applicability to C O hydrogenation.' The purpose of the present study is, therefore, to clarify the effect of the support and the dispersion on the individual steps involved in the CO hydrogenation on supported Rh catalysts, especially on the C-O bond dissociation and on the hydrogenation of (CHJad. Experimental Section
Catalysts used were prepared by impregnating a support oxide with an aqueous solution of RhC13 or a basic solution of Rh(NH4)3C16,followed by drying overnight at 393 K and subsequent reduction at 673 K for 2 h. The support oxides were T i 0 2 (Japan Aerosil, P-25),A1203(reference catalyst of The Catalysis Society of Japan, JRC-ALO-4),* and Si02 (Fuji Davison 5D,Davison 57, and Nikki E96G3). Highly purified C O (>99.99%) was used as received. So that O2as a possible impurity was removed, H2was purified by passing it through the column of the prereduced Pd/A1203catalyst at room temperature. The amounts of CO and H2 adsorbed were determined respectively by conventional pulsesb and volumetric adsorption techniques. For some catalysts, the dispersion was determined by taking electron micrographs using a Hitachi H-800 analytical electron microscope. 'Nagoya University. . *Government Industrial Research Institute, Nagoya. Present address: Department of Hydrocarbon Chemistry, Faculty of Engineering, Kyoto University, Sakyo-ku, Kyoto 606, Japan.
0022-3654/89/2093-2039$01SO/O
TABLE I: Amount of CO Adsorbed, Dispersion, and Particle Size on the Catalysts
adsorbed, support, wt %
dispersion,
pmol g-'
CO
H,
51.8 156 33.1 316 514 678 665
22.3
%
CO
particle size, A
H,
CO
H,
TEM
92
10 14 65 17 21 32 80
12
a
18 22 34 80
17 18 27 77
'41203
0.5 2 2b 5 10 20 50
107
80 148 249 311 333
17 65 53 35 14
61 51 32 14
Ti02 0.5c 0.5 2 2b 10
26.2 20.6 72.2 9.3 136
9.5 48.3 117
54 43 37 4.8 14
39 50 24
20 26 28 230 79
28 22 46
51
47 50
61
Si02 5d 1O d 20d 2' 5e 1o c
51
123 229 337 143 218 29 1 143
113 213
25 24 17 74 45 31 29
23 22
43 47 64 15 25 36 37
'Not observed. bPrepared using a basic solution of Rh(NH,)& 'Reduction at 573 K. dSi02,Fuji Davison 5D. cSi02,Nikki, E96G3. /SO2, Davison D57.
The rate constant for the C-0 bond dissociation (kco) was determined by measuring the dynamics of (CO),, with an (1) (a) Levin, M. E.; Salmeron, M.; Bell, A. T.; Somorjai, G. A. J . Catal. 1987, 106, 401. (b) Ichikawa, M. Chemtech 1982, 674. (c) Kuznetsov, V. L.; Romanenko, A. V.; Mudrakovskii, I. L.; Matikhin, V. M.; Scmachkov, V. A.; Yermakov, Yu. I. Proc. 8th,Znt. Cong. Catal. 1984.5, 3. (d) Fukushima, T.; Arakawa, H.; Ichikawa, M. J . Phys. Chem. 1985,89,4440. (e) Watson, P. R.; Somorjai, G. A. J. Caral. 1981, 72,347. ( f ) Watson, P. R.; Somorjai, G. A. Zbid. 1982,84, 282. (g) Chuang, S. C.; Tian, Y. H.; Goodwin Jr., J. G.; Wender, I. Ibid. 1985, 96, 397. (h) Ichikawa, M. Bull. Chem. Soc. Jpn. 1978, 51, 2273. (i) Ichikawa, M.; Sekizawa, K.; Shikakura, K.; Kawai, M. J . Mol. Caral. 1981, 1 2 , 167. (j)Fukushima, T.; Ichikawa, M.; Matsushita, S.; Tanaka, K.; Saito, T. J . Chem. SOC.,Chem. Commun. 1985, 1209. (k) Orita, H.; Naito, S.; Tamaru, K. J. Caral. 1984,90, 183. (I) Ichikawa, M.; Fukushima, T. J . Chem. SOC.,Chem. Commun.1985,321. (m) Bhasin, M. M.; Bartley, W. J.; Ellgen, P. C.; Wilson, T. P. J . Caral. 1978, 54, 120. (n) Nakajo, T.; Sano, K.; Matauhira, S.; Arakawa, H. J. Chem. SOC.,Chem. Commun.1987, 647. ( 0 ) Ichikawa, M.; Fukushima, T.; Shikakura, K. Proc. 8rh Znt. Cong. Caral. 1984, 2, 69. (p) Chuang, S.C.; Goodwin Jr., J. G.; Wender, I. J . Catal. 1985,95,435. (9)van den Beg, F. G. A.; Glezer, J. H. E.; Sachtler, W. M. H. Ibid. 1985, 93, 340.
0 1989 American Chemical Society
2040
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 a
c t c
-ca: Y-
0 a: CT I
Mori et al. 1
I
1
i-
a 0 c
0
0 Q 0
PT Q,
2 LL
15 r
-------
0
" " " " ' J 100
200
300 400 Time ( s e c )
500
Figure 2. Relationship between relative reflectance of 2060-cm-I band ( r ) and time a t 445 K: 0.5 wt % Rh/AI2O3 (0)and IO wt 56 Rh/AI20, (0).
Time Figure 1. Dynamic behavior of adsorbed CO (top) and produced CH4 (bottom) after introduction of CO pulse to 2 wt % Rh/AI2O3at 448 K.
emissionless infrared diffuse reflectance spectrometer, EDR, (Japan Spectroscopic Co., EDR-31)9 and that for the hydrogenation of (CH,),d (kcH,)from the dynamics of CH4 produced with a flame ionization detector (FID) equipped downstream from the EDR in a manner identical with that described previously.' In situ IR absorption spectra of surface species were taken in a flow of the Hz/CO mixture ( H 2 / C 0 = 10) at 445 K by using the EDR. Results Dispersion of Rh. Table I summarizes the dispersions of various supported Rh catalysts, defined as the ratio of the amount of CO adsorbed to the number of total Rh atoms. The Rh particle size in Table I was calculated as d = 6 V Y / a M D ,where D is metal dispersion and aMand VMare respectively the effective average area occupied by a metal atom in the surface and the volume per metal atom in the bulk.1° For some catalysts, the Rh dispersion was also estimated from Hz adsorption and TEM measurements. The particle sizes determined by these methods agreed well with those determined from CO adsorption. The dispersion of Rh varied with both the metal loading and the kinds of support. The dispersion decreased with increasing Rh loading on all supports in agreement with published data.3*4~'1~1z (2) (a) Erdohelyi, A.; Solymosi, F. J . Caral. 1983,84,446. (b) Solymosi, F.; Tombacz, I.; Kocsis, M. Ibid. 1982, 75, 78. (c) Underwood, R. P.; Bell, A. T. Appl. Coral. 1986, 21, 157. (d) Levin, M. E.; Salmeron, M.; Bell, A. T.; Somorjai, G. A. J. Chem. SOC.Faraday Trans. 1 1987, 83, 2061. (e) Worley, S . D.; Mattson, G. A.; Caud 111, R. J . Phys. Chem. 1983, 87, 1671. (f) Iizuka, T.; Tanaka, Y.; Tanabe, K. J . Caral. 1982, 76, 1. (9) Vannice, M. A. Ibid. 1982, 74, 199. (h) Niwa, M.; Lunsfold, J. H. Ibid. 1982,75, 302. (i) Katzer, J. R.; Sleight, A. W.; Gajardo, P.; Michel, J. B.; Gleason, E. F.; McMillan, S . Faraday Discuss. Chem. SOC.1981, Z2, 121. (3) (a) Arakawa, H.; Takeuchi, K.; Matsuzaki, T.; Sugi, Y. Chem. L e r r . 1984, 1607. (b) Arakawa, H.; Fukushima, T.; Ichikawa, M.; Takeuchi, K.; Matsuzaki, T.; Sugi, Y. Ibid. 1985, 23. (4) Van't Blik, H. F. J.; Vis, J. C.; Huizinga, T.; Prins, R. Appl. Catal. 1985, 19, 405. (5) (a) Vannice, M. A. Caral. Rev. 1976, Z4, 153. (b) Ponec, V. Ibid. 1978, 18, 151. (c) Denny, P. J.; Whan, D. A. Caralysis (London) 1978, 2, 46. (d) Bell, A. T. Coral. Rev. 1981, 23, 203. (e) Biloen, P.; Sachtler, W. M. H. Adv. Catal. 1981, 30, 165. (6) Mori, T.; Masuda, H.; Imai, H.; Miyamoto, A.; Baba, S.;Murakami, Y. J. Phys. Chem. 1982,86, 2753. (7) Mori, T.; Miyamoto, A.; Niizuma, H.; Takahashi, N.; Hattori, T.; Murakami, Y. J . Phys. Chem. 1986, 90, 109. (8) (a) Murakami, Y. In Preparation of Caralysrs III; Poncelet, G., Grange, P., Jacobs, P. A., Eds.; Elsevier: Amsterdam, 1983; p 775. (b) Hattori, T.; Matsumoto, H.; Murakami, Y. Preparation of Catalysrs IV; Delmon, B., Grange, p . , Jacobs, P. A., Poncelet, G., Eds.; Elsevier: Amsterdam, 1987; p 815. (9) (a) Niwa, M.; Hattori, T.; Takahashi, M.; Shirai, K.; Watanabe, M.; Murakami, Y. Anal. Chem. 1979, 51, 46. (b) Hattori, T.; Shirai, K.; Niwa, M.; Murakami, Y. Ibid. 1981, 53. 46. (c) Hattori, T.; Shirai, K.; Niwa, M.; Murakami, Y. Bull. Chem. SOC.Jpn. 1981, 54, 1964. (10) Anderson, J. R. Srrucrure of Metallic Catalysrs; Academic Press: London/New York/San Francisco, 1975. (11) Underwood, R. P.; Bell, A. T. Appl. Caral. 1987, 34, 289.
n
s
13
20
Time (nee) Figure 3. Relationship between FID response and f a t 445 K. The solid line shows the values calculated from eq 3. Although u ( t ) is proportional to the FID response under the constant flow rate of the carrier gas, the proportionality constant is not known. The calculation was, therefore, carried out so that the first point of the experimental value agreed with the calculated one.
The Al2O3support gave higher dispersion than Ti02 and Si02. On A 1 2 0 3 , 100% dispersion was obtained at the lowest Rh loading, 0.5 wt %, and the lowest dispersion, 14%, was obtained at the 50 wt 5% Rh loading. Thus, the Rh dispersion ranging from 14 to 100% was obtained for A1203 support. In the case of the TiOz-supported Rh, the Rh dispersion ranging from 14 to 50% was obtained by varying Rh loading from 10 to 0.5%. Three Si02 supports were used to obtain a wide range of dispersion. Two of them (Fuji Davison 5D and Davison D57) gave rather low dispersion from 17 to 30%,while Nikki E96G3 gave relatively high dispersion from 30 to 70%. Thus, dispersion ranging from 17 to 70% was obtained in the case of SiOz-supported Rh. After all, the effect of the support materials on the rate constant for surface reactions can be investigated in the same range of the dispersion, Le., from 20 to 50%. Rate Constant for C-0 Bond Dissociation and for the Hydrogenation of Surface Carbon Species Determined by PSRA. On pulsing a small amount of CO onto a catalyst via the H2 carrier gas at elevated temperatures (>398 K), IR absorption bands appeared at 2060 and 1900 cm-', which are assignable to linear and bridge adsorbed CO species, respectively. The band intensity of linear CO gradually decreased with time. The FID response of CH4 produced from the hydrogenation of the pulsed CO immediately increased and then gradually decreased with time. Since the band intensity of bridge CO decreased more slowly than that of linear CO, the contribution of CH4 produced from bridge CO to the FID response can be disregarded. Figure 1 shows a typical (12) (a) Hamada, H.; Funaki, R.; Kuwahara, Y.; Kintaichi, Y.; Wakabayashi, K.; Ito, T. Appl. Caral. 1987,30,177. (b) McDonald, M. A.; Storm, D. A,; Boudart, M. J . Catal. 1986, 102, 386. (c) Jones, V. K.; Neubauer, L. R.; Bartholomew, C. H. J . Phys. Chem. 1986, 90, 4832. (d) Okuhara, T.; Kimura, T.; Kobayashi, K.; Misono, M.; Yoneda, Y. Bull. Chcm. SOC.Jpn. 1984, 57, 938. (e) Kellner, C. S.; Bell, A. T. J . Coral. 1982, 75, 251. (fl Reuel, R. C.; Bartholomew, C. H. Ibid. 1984, 85, 78.
C O Hydrogenation over Supported Rh Catalysts
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 2041
0. 1
0. 01
0
50
100
Dlrperalon ( X )
Figure 4. Effect of Rh dispersion on rate constants for kco (open symbols) and kCHx(closed symbols) over Rh-supported A1203(0),prepared from Rh(NH4)3C16(0);Ti02 (A),prepared from Rh(NH4),C16(V); and S O 2 (0)at 445 K.
example of the dynamics of linear CO and produced CH4. Since the decrease in the band intensity is due to the loss of the CO bond, Le., to the dissociation of C-O bond, but not to desorption of CO, the analysis of the dynamics of linear C O should lead to the determination of the rate constant for the C-O bond dissociation (kco). The amount of CO adsorbed on the catalyst (Nco) is given by eq 1 using the relative reflectance (r).13 (1 - r)2/2r = yNco (1) Assuming that the rate of the C-0 bond dissociation is proportional to Nc0, eq 2 can be obtained: In [ r / ( l - r)2] = kcot + In [ro/(l - ro)2] (2) as for the case on Ru/A1203.' Figure 2 shows the observed relationship between In [ r / ( l - r)2] and t. As shown, a good straight line is obtained, indicating that the applicability of PSRA is satisfactory. The rate constant for the hydrogenation of (CH,)ad (kCH,) can also be determined from the analysis of the dynamics of the produced CH4 by using the rate equation for the consecutive reaction of (CO)ad (CHJad CH4:
-
-
2200
2000
1800
Figure 5. Infrared spectra on Rh/A1203various weight loadings: 0.5 wt 7% (a); 2 wt % (b); 5 wt % (c); 10 wt % (d); 20 wt % (e). 2040
2080
50
100
Dtsperalon ( X I
Figure 6. Wavenumber of linear CO for Rh dispersion. Open symbols are Rh catalysts supported on A1203(0),TiOl (A),and Si02-5D ( O ) , -D57 (0),and -E96 (V). Closed symbols are Rh/Ti02 and Rh/AI2O3 prepared from Rh/(NH4)3C16. Conditions are 445 K and CO/H2 = 1/10,
where V(CH4) is the rate for methane formation and NOco is the initial concentration of (C0)ad. Figure 3 shows an example of the least-squares fit for the observed dynamics of the produced CHI. As shown, the observed dynamics is explained by eq 3 with the appropriate kco and kCH,, indicating again the satisfactory applicability. Figure 4 illustrates kco and kCH, for supported Rh catalysts against the dispersion of the Rh metal. Both rate constants are affected by the supports: Ti02 is the most efficient, while S i 0 2 is the least. The Rh dispersion also influences the rate constants: with increasing dispersion, kco decreases in a complicated way and kCHlalso decreases relatively moderately. The variation in kco with the kinds of support and the metal dispersion well agrees with the published results in which the turnover frequency of the steady-state reaction decreases according to the order of Rh/TiO, > Rh/A1203> Rh/Si02 and with increasing Rh dispersiom2 This agreement seems to be quite reasonable, because it is widely believed that the C-0 bond dissociation is the rate-determining step in the hydrogenation of C0.6*7*'4
The rate constants on Rh/Ti02 and Rh/A1203both prepared from Rh(NH4)3C16were virtually equal to the respective values on the catalysts prepared from RhC13. IR Spectra of Surface Species in CO Hydrogenation. In situ IR absorption spectra were taken during the steady-state C O hydrogenation at 445 K by using the EDR, and the results are shown in Figure 5. Sharp and broad I R absorptions appeared respectively at around 2060 and 1840-1880 cm-', the former being ascribable to linear C O and the latter to bridge C0.15 Although a Rh catalyst is known to yield a gem-dicarbonyl species as an additional adsorbed C O species, this was not observed under the present conditions. The absence of the twin C O is due to the difference in the catalyst temperat~re.~."Indeed, the IR absorption measurements of the Rh catalyst at an ambient temperature (namely, at 323 K) revealed that the twin C O clearly appeared at 2030 and 2090 cm-' in a flow of the H2/C0 mixture. The ratio of the intensity of bridge CO to that of linear C O increases with decreasing Rh dispersion (or increasing particle size). This intensity ratio also increased on Rh/Ti02 and Rh/Si02 with decreasing Rh dispersion. These results can be reasonably expected, because larger Rh particles have a higher concentration of Rh ensembles capable of chemisorbing CO in the bridge' mode. Similar results were observed by Arakawa et aL3 and Underwood and Bell."
(13) Kubelka, P.; Munk, F. Z . Tech. Phys. 1931, 22, 593. (14) (a) Palmer, R.L.; Vroom, D. A. J . Caral. 1977, 50, 244. (b) Zagli, A. E.; Falconer, J. L.; Keenan, C. A. Ibid. 1979, 56, 453. (c) Ho, S.V.; Harriott, P. Ibid. 1980, 64, 272.
(15) (a) Dubois, L. H.; Somorjai, G . A. Surf. Sci. 1979, 88, L13. (b) Yates, J. T.; Duncan, T. M.; Worley, S . D.; Vaughan, R. W. J . Chem. Phys. 1979, 70, 1219.
2042
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989
b
I
3100
I
I1
3000
2900
/
1
Figure 7. Infrared spectra on 0.5 wt % Rh/Al20, (a) and 10 wt % Rh/A1203 (b).
The band position in the IR spectrum varied with the kinds of support and the Rh dispersion. The wavenumbers of linear C O are plotted in Figure 6 against the Rh dispersion. As shown, the wavenumber decreases with increasing Rh dispersion on all the supports. The wavenumber is higher on S i 0 2 than on both T i 0 2 and Alz03. During the steady-state C O hydrogenation, some catalysts exhibited IR absorption bands at wavenumbers from 3000 to 2900 cm-I, which can be assigned to the C-H stretching vibration of either (CH,),d species or hydrocarbon fragments. However, the other catalysts gave no noticeable absorption band in the C-H stretching region. Typical results are shown in Figure 7. Since the rate constants, kco and kCH,,are for the formation and hydrogenation of (CHx)ad,respectively, a small ratio of kcHJkco should lead to an increase in the concentration of (CH,), on the Rh catalyst surface. It can be confirmed from IR measurements that the concentration of (CH,),d is higher on the catalyst with a smaller ratio of kcH,/kco As shown in Figure 7, an I R adsorption band, which can be assigned to the C-H stretching vibration of the (CH,),d species, was observed on 10 wt % Rh/A1203 catalyst with kcH,/kco = 0.43 but not on 0.5 wt % Rh/AlZO3catalyst with kcH,/kco = 2.1.
Discussion Applicability of PSRA to CO Hydrogenation on Rh Catalysts. In our previous studies, the rates were measured similarly by PSRA for the C - O bond dissociation and for the hydrogenation of (CHJad produced therefrom over Ru/A1203 catalyst^.^ Moreover, the activity and selectivity in the steady-state C O hydrogenation were favorably controlled on the basis of the rates measured.I6 For the C O hydrogenation on Rh catalyst, the rate constants for the C-0 bond dissociation and the hydrogenation of (CH,)ad can also be determined with B R A by measuring both dynamics of adsorbed C O and produced CH,. This is because CH4 is produced selectively from the CO pulse under the conditions of the PSRA experiments, although the CO insertion is usually involved in the C O hydrogenation on Rh catalyst to produce the C2 oxygenates under pressure. Since B R A gives the rate constants per active sites, the rate constant determined should reflect directly the nature of the active site on the catalyst. The most remarkable characteristic of Rh catalysts is that kco and kCH, are close to each other as shown in Figure 4. This is in contrast with the result that kcH, is much larger than kco on Ru, Pd, and Ni ~ata1ysts.l~The situation of _ _ _ _ _ _ _ ~ ~ ~~
(16) Takahashi, N.; Mori, T.; Miyamoto, A.; Hattori, T.; Murakami, Y. AppI. Catal. 1986, 22, 137; Ibid. 1988, 38, 61, 301. (17) (a) Mori, Y.;Mori, T.; Takahashi, N.; Miyamoto, A,; Hattori, T.; Murakami, Y . Chem. Lert. 1986, 205. (b) Mori, Y.;Mori, T.; Takahashi, N.; Miyamoto, A,; Hattori, T.; Murakami, Y . 2.Phys. Chem (Munich) 1987,
.-----
l J J , LLI.
Mori et al. kCH,close to kco, observed on Rh, may lead to a favorable condition for the production of the Cz oxygenates. This is because small values of kc, result in a high abundance of (CH,),, and because C2 oxygenates are considered to be produced through the insertion of C O into the bond between metal and carbon atoms in (CHJad, that is, through the combination of (CH,.),, with (C0)ad. Structure Sensitiuity. It has been reported that C O hydrogenation is a structure-sensitive reaction which requires large ensembles of surface atoms.I8 The turnover frequency of the steady-state methanation on the Rh supported on SiOz,3A1z03,4 TiOZ," and La2O3" increased with increasing particle size of the metal. The C-0 bond dissociation could also be structure sensitive, because it requires a vacant site adjacent to adsorbed CO molecule in order to accept the oxygen atom dissociated therefrom. Thus, the large metal ensemble results in the favorable situation for the C-O bond dissociation. As shown in Figure 4, kco increased with increasing particle size of Rh, i.e., with decreasing Rh dispersion on all the supports. This result may suggest that the ensemble size is one of the factors determining the support effect on the C-0 bond dissociation. However, this does not imply that the ensemble effect is a sole factor. This is because the effect of difference of the support materials is more significant than that of the dispersion, as shown in Figure 4. Another factor should be taken into consideration for the elucidation of the larger effect of support materials on the rate of surface reaction. Metal-Oxide Pair Site. Recently, it has been suggested that, on TiOz-supported Ni,I9 Pd,20and PtZ1catalysts, pair sites consisting of a metal atom and a cation in an oxide support are formed at the fringe of the metal particle or at the fringe of TiO, suboxide which has migrated on the metal surface. The formation of the pair site is also suggested on the addition of Ti0222or rare-earth oxidesz3 to Pd/Si02 catalysts and on the addition of oxophilic oxides such as Mn02 and TiOZto Rh/SiOz catalyst,z4resulting in the increase in the turnover frequency. Our previous study indicated that the addition of Vz05 to a Ru/AlZO3 catalyst remarkably increases the rate of C-0 bond d i s s ~ c i a t i o n . ~ ~ Therefore, it is possible that kco is influenced by the formation of the pair site, the so-called metal-support interaction, between the metal particle and the cation in the oxide support. If such metalsupport interaction is the main factor of the support effect, a higher dispersion should result in higher activity, because the catalyst with higher dispersion has a larger portion of the fringe. However, this is not the case for the result in the present work, because as shown in Figure 4, kc- is larger at lower dispersion. Another possibility leading to the formation of the pair site is the deposition of a suboxide on a metal surface during the preparation of supported catalysts. Recent studies on Rh/V203z6and Pt/ A1z03z7suggested that, when catalysts were prepared by the impregnation method in an acidic solution, the suboxide of a support is dissolved and deposited on metal particles. One might suspect that the highest activity of Rh/Ti02 could be attributed to the pair sites consisting of Rh and TiO, suboxide. For the examination of this possibility, Rh catalysts supported on Ti02 and A1203were prepared from the basic solution of Rh(NH4),C16, and the rate constants were measured. As shown in Figure 4, these catalysts gave essentially the same rate constants as those on the catalysts prepared in the acidic solution, indicating that the deBoudart, M.; McDonald, M. A. J. Phys. Chem. 1984, 88, 2185. Burch, R.; Flambard, A. R. J. Carol. 1982, 78, 389. Bracey, J. D.; Burch, R. Ibid. 1984, 86, 384. Vannice, M. A.; Sudhakar, C. J. Phys. Chem. 1984, 88, 2429. (22) Rieck, J. S.; Bell, A. T. J . Catal. 1986, 99, 262. (23) Rieck, J. S.; Bell, A. T. Ibid. 1986, 99, 278. (24) Ichikawa, M.; Fukushima, T. J . Phys. Chem. 1985, 89, 1564. (25) Mori, T.; Miyamoto, A.; Takahashi, N.; Fukagaya, M.; Hattori, T.; Murakami, Y. J . Phys. Chem. 1986, 90, 5197. (26) Bastein, A. G. T.M.; van der Boogert, W. J.; van der Lee, G.; Luo, H.; Schuller, B.; Ponec, V. Appl. Coral. 1987, 29, 243. (27) Castro, A. A,; Scelza, 0. A,; Benvenuto, E. R.; Baronetti, G. T.; DeMiguel, S. R.; Parera, J. M. Preparation of Catalysts I I t Poncelet, G., Grange, P., Jacobs, P. A,, Eds.; Elsevier: Amsterdam, 1983; p 47. (18) (19) (20) (21)
C O Hydrogenation over Supported Rh Catalysts
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 2043 electron density in the Rh metal. As shown, a volcano-type correlation is obtained between kco and the wavenumber: the C-O bond dissociation is suppressed on the Rh metals with high or low electron density and accelerated on the metal with the intermediate electron density. This volcano-type correlation may be explained as follows. As previously proposed, the CO hydrogenation proceeds via the sequence
2080
2070
2060 Linear
2050
co
2040 (cn-1)
Figure 8. Relationship between the rate constant for C-0 bond dissociation (kco) at 445 K and the wavenumber of linearly adsorbed CO by infrared at 445 K. Symbols are the same as those in Figure 4.
position of suboxide on metal surface, if any, does not promote the dissociation of the C-0 bond. This conclusion agrees with our recent study2*that Rh/Ti02 in the SMSI state did not exhibit the high intrinsic methanation activity. Thus, the formation of the pair site is not a factor of the support effect in the present case. Electronic Effect on the C - 0 Bond Dissociation. IR Band Shijt. It is accepted that the chemisorption of C O on a metal surface involves the 5a and 27r orbitals of a C O molecule. The enrichment of the electron density in the metal should result in an enhanced back-donation of electrons from the metal to antibonding 21r orbital, which leads to the weakening of the C-0 bond and thereby to the redshift of the IR absorption band of adsorbed CO. The electron density in a metal can, therefore, be estimated from the vibrational frequency of the adsorbed C O molecule: the higher the frequency, the lower the electron density (and vice versa). As shown in Figure 6,the band frequency of linear C O is higher on Rh/Si02 than on both Rh/Ti02 and Rh/A1,03, suggesting that the electron density in the Rh metal is higher on both T i 0 2 and A1203 than on SO2. This trend is in harmony with the electronegativity of cations in the support materials, Le., A13+ < Ti4+ C Si4+.29 It follows that the observed band shift is mainly caused by the electronic effect, because the electronegativity of cation can be a measure of the electron affinity of a support. It is very probable that the modification of the electronic state of the Rh metal by the support is altered depending on the Rh dispersion: the electron density is more enriched in the highly dispersed metal than in the low dispersed metal. This is because the former metal is influenced by the support more readily than the latter metal. Therefore, the variation in the band position with the support and the Rh dispersion is mainly ascribable to the variation in the electron density in the Rh metal. Rate Constants for C - 0 Bond Dissociation. As described above, the electron density in the Rh metal varied with the support and the dispersion. Our previous studies revealed that the C-0 bond dissociation is strongly influenced by the electronic state of the metal.30 Then, kco is plotted in Figure 8 against the wavenumber of linear CO. Here, the wavenumber refers to the (28) Mori, T.; Taniguchi, S.; Mori, Y.; Hattori, T.; Murakami, Y. J . Chem. SOC.,Chem. Commun. 1987, 1401. (29) Tanaka, K.; Ozaki, A. J . Caral. 1967, 8, 1 . (30) (a) Mori, T.; Miyamoto, A.; Takahashi, N.; Niizuma, H.; Hattori, T.; Murakami, Y. J. Caral. 1986,102, 199. (b) Mori, T.; Masuda, H.; Imai, H.; Miyamoto, A.; Niizuma, H.; Hattori, T.; Murakami, Y. J . Mol. Caral. 1984, 25, 263.
Here, (COH,)ad, being in equilibrium with (co)ad, is introduced from the finding that the H2-D2 inverse isotope effect is present in the C-0 bond Since the rate of the C-0 bond dissociation is proportional to the concentration of (COH,)ad, its high concentration results in the acceleration of the C-0 bond dissociation (and vice versa). A CO molecule is adsorbed on a metal through two kinds of interactions: the donation of electrons from the 50 orbital of the CO molecule to the unoccupied d orbital of the metal and the back-donation of electrons from the occupied d orbital of the metal to the antibonding 21r orbital of the C O molecule. The high electron density in the Rh metal should result in the enrichment of the back-donation, which leads to stabilize (C0)ad and thereby to decrease the concentration of (COH,)ad. The low electron density, on the other hand, should result in the enhanced donation, by which (C0)ad is stabilized and the concentration of (COH,)ad is also decreased. Consequently, k,, decreases in two extremes of the high and low electron density in the Rh metal, which may explain the observed volcano-type correlation between kco and the wavenumber. Rate Constant f o r the Hydrogenation of CH, Species. As shown in Figure 4, kCH, varied in a way similar to that for kco. Thus, kCH, was the largest on Rh/Ti02 and the smallest on Rh/Si02, and it decreased with increasing Rh dispersion. This may imply that the support effect on the hydrogenation of (CH,)& is mainly governed by the electronic property of Rh which is the main factor determining the support effect on the C-0 bond dissociation. However, the detailed inspection of Figure 4 indicates some difference between kCH,and km: km decreased significantly with increasing Rh dispersion, but kcHxdecreased to a lesser extent. This result may suggest that the hydrogenation of (CH,.), is less structure sensitive than the C-0 bond dissociation. This seems reasonable, because the C-0 bond dissociation requires a vacant site for drawing oxygen atom from adsorbed CO, but the hydrogenation of (CHx)addoes not. Conclusion The rate constant of the C-0 bond dissociation in the C O hydrogenation over supported Rh catalysts on Ti02, A1203,and Si02 was measured by pulse surface reaction rate analysis (PSRA). The rate constant varied remarkably with the support materials: it was the largest on Ti02-supported Rh and the smallest on Si02-supported Rh. It also decreased, but to a lesser extent, with the increasing dispersion of Rh on all the support materials. The volcano-type correlation was observed between the rate constant of the C-0 bond dissociation and the wavenumber of adsorbed CO species, the latter of which is a measure of the electronic property of Rh metal. It was concluded that the electronic property of Rh metal is the main factor determining the effect of the support and dispersion on the C-0 bond dissociation. Registry No. CO, 630-08-0; Rh, 7440-16-6
