J. Phys. Chem. 1992,96, 1329-1334
1329
Electronic Metal-Support Interaction in Pt Catalysts under Deuterium-Ethene Reaction Conditions and the Microscopic Nature of the Active Sites Hideaki Yoshitaket and Yasuhiro Iwasawa* Department of Chemistry. Faculty of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan (Received: April 24, 1991) The electronic states and the nature of active sites of Pt/Y203,Pt/Zr02, Pt/V203, and Pt/Ti02 in the working state were investigated by X-ray absorption near-edge structure (XANES) spectroscopy, kinetics, and isotope-tracing techniques. The density of the unoccupied 5d state of Pt decreased as the reduction temperature of catalysts increased, but its degree strongly depended on the nature of support. High-temperature reduction of Pt/Zr02 and Pt/Y203 decreased the unoccupied d density of Pt more than Ti02 and Nb205,probably because the reduced region of support was restricted in the vicinity of the Pt particles. The unoccupied d density of Pt on V2O3 was independent of the reduction temperature due to the metallic nature of the support. For this catalyst, the decoration by the support oxide occurred by the reduction at 773 K. The d states of Pt were largely modified under the D2-ethenereaction conditions as proved by XANES. The electrons of Pt on Zr02were removed by adsorbed ethene, being negatively charged, while Pt on Y203behaved as an electron acceptor, leaving a positive charge on ethene. The nature of the active sites was also characterized by the D profile of ethane in the D2-ethenereaction. Pt/Ti02 reduced at 773 K had two kinds of sites with different surface H/D ratios under working conditions-it was suggested that one was the bare metal site of Pt mainly producing ethanad2and the other was the peripheral site of TiO, islands producing ethane-& The active site of Pt/V203 reduced at 773 K had oxide nature, suggeting that Pt particles may be completely covered with VOX.
Introduction Metal oxides have been used as supports for dispersed metal particles to stabilize and modify the small bare-metal clusters through metal-support interaction. One of the most manifesting discaveriesof mctal-support interaction is the strong metal-support interaction (SMSI) for noble metals on reducible oxides such as Ti02, Nb20S,Ta20s, and V20S. It has been originally reported as complete suppression of hydrogen adsorption without significant particle enlargement.'s2 There have been several other kinds of metal-support interactions in the literature whose origin, appearance, and effects on catalysis have extensively been studied." Bond classified metal-support interactions into three groups:3 strong (SMSI), medium (MMSI), and weak (WMSI) metalsupport interactions. He exemplified Ti02or Nb20Sfor the first class, zeolite for the second class, and Si02or A1203 for the third class. The alternation of the electronic states of metal particles upon contacting with oxide supports should be always considered more or less as a universal effect brought by the supports. The catalysis of metals is controlled by changing reduction temperature of catalysts, for example, SMSI catalysts and other-supported catalysts, V203,6*'Zr02,Srare-earth oxides? and so on. These supports are usually less reducible than Ti02 and the literature is relatively small so far. Mode of interaction between metal particles and reduced supports in supported metal catalysts could be the coverage of the metal surface with partially reduced support-oxide and/or the electronic effect through electron donor-acceptor interaction. For the small particle systems, X-ray absorption near-edge structure (XANES) is preferable to X-ray photoemission spectroscopy (XPS) in terms of uncoupling the change of the final state of d electrons from the initial state change. Further, the XANES spectra are measurable in situ in the presence of reaction gas. White limes appearing at the edge position of Pt have been used for determining the density of the unoccupied d state of Pt in the supportedt'F catalysts, though the intensity of the Pt white line may also be affected by the d-s rehybridization. It is sensitive enough to monitor the change in d state which arises from the nature of support,"-13 particle size,'* and a d s o r p t i ~ n . ~ ~ . ' ~ We have demonstrated that electrons are donated from N a 2 0 additives to the unoccupied d state of Pt in Na/Pt/Si02 catalysts *To whom all correspondence should be addressed. 'Present addrw: Department of Energy Engineering, Faculty of Engineering, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240, Japan.
and the electrons are extracted by di-a-ethene adsorbed at the bare metal sites of Pt particles under the ethene hydrogenation condition^.'^ The effects of the change in Pt d density on the elementary step of ethene hydrogenation on Na/Pt/Si02 catalysts have also been investigated.I6 In the present article we intend to generalize the electronic effect of Pt in relation to the metal-support interactions and to present a new criterion of support effect by means of XANES spectroscopy. The active site and the reaction environment of catalyst surface should be characterized electronically and structurally under reaction conditions. Reaction mechanism often involves the essential nature of active sites. The W t r i b u t i o n profiles in ethane and ethene in the D2-ethene reaction have been demonstrated to reflect the structural characteristics of the working ~urfaces.'~J' The microscopic environments of reaction sites for the D2-ethene reaction should be combined with the electronic state of Pt particles with influences on that of the active center.
Experimental Section Pt/Ti02, Pt/Nb205,Pt/Y203,Pt/V203,and Pt/Zr02 catalysts were prepared by an impregnation method using an aqueous (1) Tauster, S.J.; Fung,S.C.; Garten, R. L. J. Am. Chem. Sac.1978,100, 170. (2) Tauster, S.J.; Fung, S.C. J . Coral. 1978, 55, 29. (3) Bond, G. C. Metal-Support and Metal-Additive Eflects in Catalysis;
Imelik, B., Naccache, C., Coudurier, G., Praliaud, H., Meriaudcau, P., Gallezot, P., Martin, G. A., Vedrine, J. C., Eds.; Elsevier: Amsterdam, 1982; Chapter 1 . (4) (a) Solymosi, F.; Tombicz, I.; Koszta, J. J . Coral. 1985,95,578. (b) Solymosi, F.; ErdBhelyi, A.; Lann, M. J. Caral. 1985.95.567. (c) ErdBhelyi, A.; Phztor, M.; Solymosi, F. J. Catal. 1986, 98, 166. (5) Haller, G. L.; Resasco, D. E. Adv. Catal. 1989, 36, 173. (6) Bond, G. C.; Duarte, M. A. J . Caral. 1988, 1 1 I , 189. (7) Lm,Y. J.; Resasco, D. E.; Haller, G . L. J. Chem. Sac.,Faraday Trans. 1 1987,83, 209 1. (8) Dall'Agnol, C.; Gervasini, A.; Morazzoni. F.; Pinna, F.; Strukul, G.; Zanderighi, L. J. Caral. 1985, 96, 106. (9) Szymanski, R.; Charcosset, H.; Gallezot, P.; Massardier, J.; Toumayan, L.J . Catal. 1986, 97, 366. (10) Rieck, J. S.; Bell, A. T. J . Catal. 1986, 99. 278. (11) McHugh, B. J.; Larsen, G.; Haller, G. L. J . Phys. Chem., in press. (12) Short, D. R.; Mansour, A. N.; Cook,J. W., Jr.; Sayers, D. E.; Katzer, J. R. J . Catal. 1983.82, 299. (1 3) Gallezot, P.; Weber,R.; Dalla Betta, R. A,; Boudart, M. Z . Narurforsch., A: Phys., Phys. Chem., Kosmophys. 1979, A34, 40. (14) Fukushima, T.; Katzer, J. R.; Sayers, D. E.; Cook, J. W., Jr. Prac. Inr. Congr. Caral., 9rh 1988, 79. (15) Yoshitake, H.; Iwasawa, Y. J . Phys. Chem. 1991, 95, 7368. (16) Yoshitake, H.; Asakura, K.; Iwasawa, Y. J . Chem. Sac., Faraday Trans. 1 1988,84,4337. (17) Nazario, P.; Brenner, A. Prac. Inr. Conp. Caral., 9th 1988, 1020.
0022-3654/92/2096-1329$03.00/0Q 1992 American Chemical Society
1330 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
Yoshitake and Iwasawa
TABLE I: H/Pt for the Pt Catalysts Reduced at Various Temperahues (K)" 373 473 573 673 773 773-6730~-373 ~t /Y,o, 0.20 0.19 0.19 0.19 Pt/ZrO, 0.75 0.71 0.68 0.63 0.60 0.73 Pt/V,O3 0.24 0.13 0.17 0.19 0.14 0.24 0.001 0.70 Pt/TiO, 0.75 0.61 0.37 Pt/Nb,O, 0.38 0.30 0.04 u.d. u.d. '773-6730~-373 denotes the successive treatments of 773 K reduction, 673 K oxidation, and 373 K reduction. The reduction time of the catalyst was always 60 min. The hydrogen adsorption experiments were carried out at room temperature. F't on Y203reduced at 373 K was in an oxide state, which was evaluated from fd and it was reproduced after 773-6730~-373treatment. The other blanks of the table were not measured. u.d., under detection limit.
solution of H2PtC16-6H20 (Soekawa Chemical Co., Ltd., research grade). Ti02 (Nippon Aerosil P-25; surface area, 80 m2.g-'), Nb205(Soekawa, 99-58, 5 m2-g-'), Y203(Soekawa, 99.99%, 7 m2.g-'), V 2 0 3(Soekawa, 99%, 10 m2-g-'), and Z r 0 2 (Soekawa, 99% 12 m2*g-')were commercially available. Each impregnated sample was allowed to stand for 24 h. Water was removed at 320 K and dried at 393 K, followed by calcination at 773 K. The loading of platinum was 2.3% for all catalysts. The obtained samples were placed in a U-shaped tube combined in a closed circulating system with a base pressure below lo" Pa. Then they were oxidized with O2at 673 K and reduced at given temperatures in situ before catalytic reactions and hydrogen adsorption experiments. CH2=CH2(Takachiho Trading Co., Ltd., 99.9%) was purified by freeze-thaw cycles before use. Hydrogen and deuterium (Takachiho, research grade) were purified through a 5A molecular sieve trap at 77 K. The D2-CH2=CH2reaction was carried out in a closed circulating system connected to a gas chromatograph for the product analysis. The separation column used was VZ-10. XANES spectra were measured at BL-7C of the Photon Factory, National Laboratory for High Energy Physics (KEK-PF), in the transmission mode (proposal No. 89-007). The samples were prepared in a closed circulating system and transferred to X-ray absorption cells. The analysis of Pt L2- and L3-edge XANES spectra was performed in a similar way to that reported by Mansour et aL'* The total unoccupied d density of the Pt sample, hT, is given by comparing with that of the reference (Pt foil), h,, as follows hT = (1.0 +fd)hr (1) fd
(hA3
+ 1.11hAZ)/(A3r + 1.11'42r)
(2)
PA3 = A3 - A3r, PA2 = A2 - A2,
(3) where A3 is the white line intensity at the L3 edge for the sample, A2 is the white line intensity at the L2 edge for the sample, A3r is the white line intensity at the L3 edge for the reference, and AZris the white line intensity at the L2 edge for the reference. hr has been reported to be 0.30-0.40.'9920 The spectral area from the edge position to the next local minimum was integrated and normalized by the edge jump to give the intensities A2,A3, A2r, and A3,. Results The amounts of hydrogen adsorbed on the Pt catalysts reduced at various temperatures are summarized in Table I. The increase in the reduction temperature TR caused the decrease of H/Pt. The typical cases were Pt/Ti02 and Pt/NbZOS which are wellknown SMSI catalysts. The value markedly decreased from 0.75 at 373 K to 0.001 at 773 K for Ti02 and from 0.38 at 373 K to nearly zero at 773 K for Nb205. The H/Pt for Pt/Zr02 decreased (18) Mansour, A. N.; Cook,J. W.; Sayers, D. E. J . Phys. Chem. 1984,88, 2330. (19) Brown, M.; Peicrls, R. E.; Stern, E. A. Phys. Rev. B Solid Srare 1911, 15, 738. (20) Mattheiss, L. S.;Deitz, R. Phys. Reu. B: Condens. Matrer 1980, 22, 1663.
u'vO
5 tR"2/mirP2
10
Figure 1. Relative amount of hydrogen adsorption (H/R for the catalysts reduced at 373 K equals unity) as a function of the square root of reduction time (tR): hydrogen adsorption performed at PH2= 15.0 kPa and room temperature; 0,Pt/TiO, reduced at 623 K; 0 , Pt/V203reduced at 773 K, 0 , Pt/Nb,O, reduced at 500 K.
a small amount (0.75 at 373 K to 0.60 at 773 K), while that for Pt/Y203 showed almost no change (0.20 at 473 K and 0.19 at 773 K). The behavior of Pt/V203was complicated; H/Pt once decreased (0.24 to 0.13), then increased (0.13 to 0.19), and through a maximum finally decreased to 0.14. The reversibility of H/Pt upon high temperature reduction (HTR) was examined by the cycle treatment, 773 K reduction473 K oxidation-373 K reduction. H/Pt was completely reproduced for Pt/V203 and almost for Pt/Ti02, Pt/Zr02, and Pt/Nb,O,. Pt on Y203 was not reduced to a metallic state at 373 K but was still in an oxide state as elucidated by the XANES spectrum. This was also observed with Pt/Y203 sample treated by the 773 K reduction473 K oxidation-373 K reduction procedure. In order to examine the origin of the H/Pt suppression, kinetic experiments for H/Pt were conducted. Figure 1 shows the reduction time-dependent H/Pt profile for Pt/Ti02, Pt/V203, and Pt/Nb05, where a significant suppression in hydrogen adsorption was observed by the high temperature reduction. The H/Pt for these catalysts decreased in proportion to the square root of the reduction time (tR). The change in the L3 and L2 edges of XANES spectra for Pt/Zr02 with various reduction temperatures is shown in curves a-c and d-f of Figure 2, respectively. The intensities of white lines decreased with an increase of the reduction temperature. The calculated values of fd are plotted as a function of reduction temperature (TR)for Pt/Y2O3, Pt/Zr02, Pt/V203, Pt/Ti02, and Pt/Nb205in Figure 3. The density of the unoccupied d levels of Pt in the catalyst reduced at low temperatures was largest with Y203support and smallest with V203. fd of all these catalysts had a decreasing tendency against TR, but the degree or behavior was quite different from support to support as shown in Figure 3. The catalysts which showed the large difference infd between LTR and HTR were Pt/Yz03 and Pt/ZrO,, whereas little alternation could be observed for Pt/V2O3; Pt/Ti02 and Pt/NbZOS showed a moderate difference. Thefd for Pt/Y203reduced at 373 K was 2.39, which is the value comparable to Pt oxide. Arrhenius plots for the formation of deuterated ethane in the D2-ethene reaction on Pt/Y2O3 (TR = 573 K, 773 K), Pt/Zr02 ( T R= 373 K, 773 K), and Pt/Ti02 (TR= 373 K, 773 K) are shown in Figure 4. The calculated activation energies are listed in Table 11. The reaction rate for our experiment (0.1 < Pb/kPa < 5.0 and 0.1 < PCH-H /kPa < 5.0) over all the catalysts was well traced by eq 4 whick is based on the relatively strong adsorption of ethene and the rate-determining adsorption of deuterium.21 The rate constant k and the equilibrium constant for r = k P ~ , / ( 1 + W'CH,-CH,) (4)
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1331
XANES Study on Metal-Support Interaction
11.54
11.57
11.67
11.63
I
3.8
photon energy /keV
4.0
4.2
4.4
4.6
4.0
5.0
F i p e 4. Arrhenius plots for Dz-ethene reaction on the Pt catalysts: Pk = PE 1.3 kPa; 0, Pt/Zr02 (TR = 373 K); 0 , Pt/Zr02 (TR = 773 K); 0,Pt/Ti02 (TR = 373 K); m, Pt/TiOz (TR = 773 K);A, Pt/YZO, !TR = 573 K); A, Pt/Y203(TR= 773 K). The rates ( m i d ) are normalized with H/Pt of the catalyst reduced at 373 K (for Z r 0 2 and Ti02) or 573 K (for Y203). TABLE II: Kinetic Parameters for Ethene Deuteration' TR/K E/kJmol-l &/Pa-' k/Pa-l.min-' Pt/Y203 573 43 4.0 x 104 3.8 x 10-3 40 8.7 x 1 0 4 3.4 x 10-3 773 2.9 x 1 0 4 Pt/Zr02 373 119 2.2 x 104 773 119 2.0 x 104 8.7 x 104 Pt/Ti02 373 36 1.6 x 104 9.5 x 104 39 1.0 x 10-3 6.3 x 10-5 773
13.24
13.27
13.30
TR,reduction temperature of the catalysts; E, activation energy; KE and k, the coefficients in eq 4, determined at 245 K for Pt/Y203 and 215 K for P t / Z r 0 2 and Pt/Ti02. Base pressure: PCH+H~= PD2= 1.3 P a .
1334
photon energy/keV Figure 2. (1) Pt L3 edge XANES spectra for Pt/Zr02 reduced at (a) 373 K, (b) 573 K, and (c) 773 K. (2) Pt L2 edge XANES spectra for Pt/Zr02 reduced at (d) 373 K, (e) 573 K, and ( f ) 773 K. The photon energy is not adjusted.
I
373
473 573 TR/K
673
773
Figure 3. Variation of fd for the Pt catalysts with the reduction temperature, TR.The XANES spectra were measured in vacuum.
ethene adsorption KE are also listed in Table 11. The activation energim for Pt/Ti02 and Pt/Y203were 36-43 W.mol-', whereas
the activation energy for Pt/ZrOz was much larger (1 19 kJ-mol-'). The values were almost independent of the reduction temperature for each catalyst. The variation of KE and k with the reduction temperature was also different between Pt/Zr02 and Pt/Y203 or Pt/Ti02. For Pt/Zr02, KE showed almost no change and k increased when the reduction temperature increased, while KE for Pt/Yz03and Pt/TiOz increased and k decreased with an increase of reduction temperature. KE varied from 1 X lo4 to 1 X by changing the kind of support and the reduction temperature, but k was more sensitive to the support and the reduction tem(Yz03)to 6.3 X lo-* (Ti02) for 773 K perature, e.g., 3.4X reduction. Figure 5 shows thefd of the supported Pt particles under ambient ethene. Most parts of the catalyst surfaces are suggested to be covered with adsorbed ethene in a steady state of reaction since the reaction kinetics obeyed eq 4. Thusfd in Figure 5 reflects the unoccupied d-electron densities of the Pt particle in the working state of catalyst. Comparing Figure 5 with Figure 3, we can obtain the difference infd under vacuum and reaction conditions, Afd, for each catalyst. As shown in Figure 5 , thefd for both Pt/Zr02 increased in the presence of ethene and the difference, Ah, became larger as the reduction temperature increased,while& for Pt/Y203 decreased with ethene and Afd became larger with the reduction temperature. The fd for Pt/Vz03 under ambient ethene was the same as that in vacuum. The contents of deuterium incorporated in ethane in the D2ethene reaction are listed in Table 111. The order of percentage (21) Laidler, K. J. Chemical Kinetics; Harpcr & Row Publishers: New York, 1987; p 257. Boudart, M.; Mariadassou, G . D. Kinetics of HererogeWOKS Catalytic Reactions; Princeton University Press: Princeton, NJ, 1984; p 184. Schlatter, J. C.; Boudart, M. J. Card. 1972, 24, 482.
Yoshitake and Iwasawa
1332 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
TABLE I V Rate for Ethew Deuteration and Hydrogen Exchange = PIL= 1.3 kPa" Reaction at 223 I( and Pa-catalyst TR/K r,,
Pt/Y20B
\
0.0.
Pt/Zr02 Pt/V203 Pt/Ti02
I \
r-O
r-0- 0.1 0.1.
573 173 393 713 393 713b 393 113c
0.16 0.16 0.74 1.63 0.31 0,0070 0.63 0.025
0.026 0.096 0.083 0.092 0.0091 0.22 0.020
0.16 0.13 0.05 1 0.30 1.3 0.35 0.80
"The unit of the rate is m i d , which are normalized by the number of surface Pt atoms estimated by H/Pt. The rate for hydrogen exchange is defined as Ed,-ethene. bobserved at 333 K. 'Normalized by H/Pt for 393 K reduced Pt/Ti02.
- 0.2.
- 0.3
373
473
573
673
773
Tu/ K Figure 5. Variation of fd of the pt catalysts as a function of reduction temperature, TR.The XANES spectra were measured under 1.3 kPa of ethene.
0.4
I
0.2
'
ZrOl 0.22 0.0d
0.0
-0.2-
0
50
100
t R/min Figure 7. Rates for the formation of ethane-do, -dl, and -d2on Pt/Ti02 versus the catalyst reduction time at 623 K. The catalytic reactions were carried out at Pq = PE = 1.3 Wa and 223 K. The rates were normalized with H/Pt = 0.75.
- 0.4
'
1
J
,
373
473
573
673
773
TR/K Figure 6. Subtraction offd of F't in vacuum by that under ethene, Ah. These are related with the electron absorbed by ethene molecules adsorbed. TABLE III: Percentage Di~tributioaoin Ethane Formed in D,-CH,==CH, Reaction on Platinum Catalysts at 223 K" catalyst TR/K do dl d2 d, d4 d, d6 Zidt/6Zd, 23 Pt/Y20, 513 13 35 52 0 0 0 0 113
Pt/Zr02 pt/V203 Pt/Ti02
313 773 373 113 373 773
13 15 16 14 3 12 33
35 19 21 28
5 22 13
52 49 49 50 87 51 49
0 10 8 6 4 12 4
0 6 5 2 1 3 1
0 1 1 0 0 0 0
0 0 0 0 0 0 0
23 29 28 26 33 28 21
" P C H ~ = PQ H ~= 1.3 kPa. Catalysts were reduced for 1 h.
isotopes for deuterated ethane varied as dz> dl > do for Pt/TiOZ and Pt/Vz03 reduced at 373 K, and for Pt/Y203and Pt/ZrO, reduced at both low and high temperatures. On the contrary, the population was concentrated in dz for Pt/V203 reduced at 773 K. A different distribution, dz> do > dl, was also observed with Pt/TiO, reduced at 773 K.
The hydrogen exchange between ethene and deuterium occurred during the ethane formation. The rates for the hydrogen exchange reaction which are defined as Zidrethene are shown in Table IV. The rates for ethane formation are also listed. The deuterated ethene formed in the initial stage of reaction was mainly ethened, and -dz. The rates were normalized with the number of the surface Pt atoms which were estimated by H/R except R/TiOz reduced at 773 K. The exchange reaction was suppressed by the hightemperature reduction of catalyst, but the relative rate to the ethane formation on R / V 2 0 3 and Pt/Ti02 increased. Both the exchange and deuterogenation reaction were extremely slow on the 773 K reduced Pt/VZO3,compared to other catalysts. Figure 7 illustrates the reduction time dependence of the ethaned, formation rate on Pt/Ti02. The curves aie well expressed by least-square fitting as follows:
+ r = 0.14 - 0.0042rR1/2+ 0.001tR1/4 for ethane-dl r * 0.32 - 0.0096rR1/2+ 0.000tR1/4 for ethaned:,
r = 0.076 - 0.0632tR1/z 0.274tR1l4 for ethane-do
The formation of ethane-dl and ethane-dz showed nearly the square root dependence of the reduction time of catalyst (tR).On the contrary, the formation of ethane-do had a maximum at tR
XANES Study on Metal-Support Interaction
= 22 min and, consequently, there was the contribution of the tR1/4term in addition to tR1i2.
Discussion (i) Electronic Effects of Support and the Classification of Metal-Support Interactions. The suppression of hydrogen adsorption by the reduction of catalysts at high temperatures like 773 K has been studied especially in so-called SMSI catalysts involving the supports such as Ti02 and Nb205. It has been accepted that the suppression of hydrogen adsorption originated from the decoration of metal surface with reduced support oxides. One of the criteria for the migration of the partially reduced oxides to metal surfaces is the thermal diffusion of the support to the metal surface, where it is expected that H/Pt decreases as a function of square root of reduction time. Figure 1 shows good linear relations between H/Pt and tR1/2for Pt/V203, Pt/Ti02, and Pt/NhOS. Thus the decrease in H/Pt for Pt/V2O3, Pt/TiOz, and Pt/Nb205caused by the high temperature reduction in Table I is attributed to the migration of support oxides. The adsorption suppression on Pt/Nb205at 500 K reduction occurred faster than for Pt/Ti02 at 623 K and for Pt/V203at 773 K reduction. For supported Pt, the ease of SMSI is on the order Nb205> Ti02 > Y 2 0 3from Figure 1 and Table I. The hydrogen adsorption capacity was recovered after 773 K reduction-673 K oxidation373 K reduction, which is commonly observed in SMSI phenomenon. For Y203 and Zr02, the suppression of hydrogen adsorption is negligible or small as compared to SMSI supports. It is ambiguous whether the decrease of H/Pt of Pt/Zr02 in Table I resulted from the partial decoration of Pt by reduced ZrO, or from Pt-Zr bond formation because of the small difference in H/Pt between LTR and HTR. The complicated reduction profile of Pt/V203 agrees with the literature in which its origin has been attributed to the formation of vanadium bronze.’ fd depends on the particle size of P t . 1 2 9 1 5 For example,fd is 0.17 for 0.9% Pt/Si02 (H/Pt = 0.67), whilefd is 0.02 for 4.7% &/SO2 (H/Pt = O.O4).I5 Thus the comparison offd for different particle sizes, such as between Pt/Y203and Pt/Zr02, has certain limitation. We may focus on the effect of the reduction temperature on fd, The difference of fd between LTR and HTR vd(LTR) fd(HTR))can be explained in terms of the reducibility of supports. V203is metallic without reduction (resistibility: p / i l cm = 1 X at 300 K22) and remains in a metallic state through V,O, ( p = (1-3) X 10” except V,Oll when 2n > m > 1.5n).23 Thus the electrons may easily delocalize between d states of Pt particles and V z 0 3 and the degree would not change in the reduction temperature range. Z r 0 2 is an insulator ( p = 1 x monoclinic) and the reduction of Pt/Zr02 at high temperatures has been reported to partially make a Pt-Zr alloy? The amount of hydrogen consumed in the reduction of Pt/Zr02 at 773 K was below limitation of detection and hence the extent of the reduction is suggested to be within the vicinity of Pt particles. Thus the electron transfer from the support to Pt particles which generates the large difference offd(LTR) -fd(HTR) for Pt/Zr02 may be restricted in the small area of the interface (the bottom and periphery of Pt particles). The phenomenon observed with Y 2 0 3 is explained in the same manner. Ti02 is also an insulator ( p = 1.2 X 1O1Oat 800 K, for rutile22)but it is easily reduced to TiO, (n < 2) and the extent of reduction is measurable with the amount of H 2 0 evolved during the reduction. Most of TiO, compounds are metallic at room temperature. Thus Ti02 support has the intermediate character between V203and ZrO,. The reduced region spreads at the surface in Pt/Ti02 to some extent. Nb205 is likely similar to Ti02. Metalsupport interactions can be reasonably classified into three classes by the variation of H/Pt andfd with the reduction temperature: first, the decoration of the metal particles with support oxide without fd change (PtIV203); second, the large (22) Yoshitake, H.; Iwasawa, Y. J. Catal. 1990, 125, 227. (23) Weaat, R.C.Handbook of Chemistry and Physics; CRC Press: Boa Raton, FL, 1980.
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1333 change offd without the coverage of the metal particle (F’t/Zr02 and Pt/Y2O3); last, wering the metal particles by suboxides with somefd change (Pt/Ti02 and Pt/Nbz05). (ii) Nature of the Active Site for D2-Etaew Rerctioa. The adsorption of ethene on the noble metals is usually described in terms of the Dewar, Chatt, and Duncanson model (DCD model) which demonstrates that adsorption occurs through both the u-donating interaction of filled ethene ?r orbital with empty metal d orbital and the r-accepting interaction of empty ethene r* orbital with filled metal d orbital. Their theory has explained the difference of the strength of adsorption and the reaction rate of hydrogenation among metals, particularly in homogeneous systems. But the mechanism of adsorption on metal surfaces with additives is still disputable. An interesting phenomenon related to ethene adsorption on Pt is the reduction of the adsorption strength by the addition of either sodium (as an electron-donating additiv@ or oxygen (as an electron-withdrawinga d d i t i ~ e ) . Two ~ ~ kinds of adsorbates (di-u and Ir-ethene) on Pt/Si02 have been observed by IR, but the electron acceptor was found to be the di-u-ethene, not ~r-ethene.~~ In contrast to the case of oxygen which can be explained by the DCD model, the effect of alkali-metal addition on ethene adsorption is rather singular and the decrease in KE by the addition may be related with the suppression of udonating interaction%and the repulsion of the negatively charged etheneF4 The electronic modification of Pt/ZrOz by high-temperature reduction has a common feature to that for Na/Pt/Si0*4 infd and the reaction kinetics. It is suggested that electrons move from Zr02 support to platinum particles as a function of the reduction temperature as shown in Figure 3, whereas the significant amount of the electrons donated was removed to the adsorbed ethene as shown in Figures 5 and 6 (Afd= 0.0842). The effect of these electronic modifications on the reaction kinetics was similar to that observed with Na/Pt/Si02, i.e., weakening ethene adsorption and promoting hydrogen adsorption. The surface of platinum in Pt/Zr02 is mainly covered with ethene according to eq 4 and Table 11. The surface state of the Pt/Zr02 catalyst in the working state was similar to that for Na/Pt/Si02 in terms of the fa in the presence of ethene. Thus the electronic promotion of the Pt/Zr02 catalysis for ethene hydrogenation by the high temperature reduction is demonstrated to be similar to that by Na addition. Pt/Y203decreases its density of the unoccupied d density with an increase of the reduction temperature (Figure 3) without causing significant H/M suppression (Table I). This feature is the same as that for Pt/Zr02. It is to be noted thatfd at 473 K reduction (0.14) is larger than that of bulk platinum (0.00) or Pt/Si02 with similar dispersion (0.06 for H/M = 0.3). This indicates that in the low-temperature reduced Pt/Y203, Yz03 support had electron-withdrawingcharacter and this state may be compared with Pt decorated with adsorbed oxygen. Thefd decreased as the reduction temperature increased and the support becomes somewhat electron-donating at 773 K reduction (Figure 3). The change in kinetic parameter in Table I1 suggests that the strength of ethene adsorption increased with the reduction temperature. The reduction in the ethene adsorption was also observed with O/Pt( 111).25 In addition, the adsorption of ethene reduced thefd value in the catalyst r e d u d at low temperatures, which is likely that the adsorbed ethene worked as an electron donor to the Pt particles. This may be related with the weak adsorption of ethene as compared to the case of the 773 K reduced catalyst on which adsorbed ethene is nearly neutral as shown in Figure 6. These results suggest the presence of an optimum value of fd for ethene adsorption. The isotope distributions on these catalysts in Table I11 did not change significantly through the alternation of the reduction temperature. The comparison offd with or without ethene would not be adequate for Pt/Ti02, Pt/V2O3, and Pt/Nb205with H / R suppression induced by high temperature reduction because the (24) Yoshitake, H.; Iwasawa, Y. J. Catal. 1991, 131, 276. (25) Steininger, H.; Ibach, H.; Lewald, S. Surf. Sci. 1982. 117, 685. (26) Windham, R. G.; Bartram, M. E.; Koel, B. E. J. Phys. Chcm. 1988, 92, 2862.
1334
J. Phys. Chem. 1992,96, 1334-1340
surfaces at the SMSI state are heterogeneous according to migrated suboxides and ethene is probably adsorbed on at least two different ~ i t e s . ' ~ The J ~ , nature ~ ~ of active sites for these catalysts should be represented in the isotope distributions in reaction products. The profile of the isotope distribution on Pt/V203reduced at 773 K was much different from those on other catalysts, where almost 90% of the isotopes was ethane-d2 (Table 111); this is a typical distribution on oxide catalysts. Thus the active site of 773 K reduced Pt/V203is suggested to be VOXand the surface of Pt particles is fully covered with suboxides. The suppression of the activity is larger than that for the SMSI Pt/Ti02 (Table IV), which may also support this model. Independence of fd of the presence of ambient ethene in Figure 5 may be due not only to the delocalization of the charge but also to little adsorption of ethene. Measured H/Pt is likely to result from the formation of vanadium bronze. Isotope distribution in ethane formed on the 773 K reduced Pt/Ti02 was unique. The deuterium distribution is controlled by the reversible step of the associatively adsorbed ethene and half-hydrogenated state and also the surface H/D ratio during the reaction.I6 Appearance of two peaks of ethane-do and ethane-d2 in the ethane formation strongly suggests two kinds of reaction sites/reaction environments with different H/D ratios.I6 This can be confirmed by controlling the number of each active site. The bare metal area of Pt in Pt/Ti02 decreases as a function of square root of reduction time, as shown in Figure 1, while the peripheral sites of TiO, islands on Pt surface increase with tR1I4 dependence.I6 Ethane-dl and ethaned2showed the tR1/2dependence in Figure 7, suggesting no relation with peripheral site in their formation. Only ethane-do had the tR1/4term in the polynomial above described to fit the curve. Consequently, ethaned, is suggested to be produced at the peripheral sites of TiO, islands
on the Pt surface. This phenomenon is similar to our previous report on Rh/Nb205and Ir/Nb205.16
Conclusion The electronic modification of the density of the unocccupied 5d state of Pt by the reduction temperature strongly depends on the kind of support. Zr02and Y203 donate electrons to Pt without migration of the support oxides upon increasing reduction temperature. The change in the unoccupied d-electron density of Pt for Pt/Y203 and Pt/Zr02 is larger than the cases of TiOz and Nb2O5, which are known as SMSI supports. The density of the unoccupied 5d state of Pt on V203is independent of the reduction temperature but decoration by the support oxide occurs. These variations of the density of state may be due to the reduced region of the supports. Ethene is electron accepting on Pt/ZrO, at every reduction temperature, while it is electron donating on Pt/Y?03 at low-temperature reduction. The increase of the reduction temperature causes the increase of electron-accepting character for both catalysts. These are suggested to be the origin of the different behavior of the kinetic parameters. The active sites for D2-ethene reaction were characterized with kinetic parameters and isotope distributions. The active site of Pt/V203 reduced at 773 K has an oxide character, where the Pt surface may be fully covered with VOX. Pt/Ti02 reduced at 773 K has two kinds of active sites with different surface H/D ratios in D2-ethene reaction. One is the bare metal site of Pt on which ethane-d2 is mainly produced and the other is the peripheral site of the TiO, island producing ethane-do. Acknowledgment. We are grateful to the staff of the Photon Factory for their help with the XANES measurements. R e t r y NO.R,7440.06-4;Y203,1314-36-9;ZrO,, 1314-23-4; V203, 1314-34-7; Ti02, 13463-67-7; C2H4,74-85-1.
Actlve Specles of Molybdenum for Alcohol Synthesis from CO-H,+ Atsushi Muramatsu, Takashi Tatstmi,* and Hiro-o Tominaga Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, Hongo, Tokyo 113, Japan (Received: April 29, 1991)
Molybdenum supported on Si02catalyzed CO hydrogenation to produce higher alcohols as well as hydrocarbons. The alcohol formation was found to require two kinds of Mo species, metallic Mo and Moo2,after reduction with flowing H2. Marked increase in alcohol yield with time on stream suggested that active species for alcohol synthesis were formed during CO-H2 reaction. Because the carburization of Mo resulted in insignificant change in the number and nature of the sites for alcohol formation, Mo carbides were excluded from the active sites for alcohol synthesis. Treatment of Mo catalysts with flowing atmospheric CO or CO-H2 was remarkably effective for the formation of sites producing alcohols. The Mo 3d XPS spectra of the C0-H2 conditioned catalyst showed a shoulder on the low binding energy side of the Mo 3d5,, peak. These findings suggest the formation of CO-reduction-induced defects on MOO,, MOO^-^, during the CO hydrogenation reaction, resulting in the increase in the alcohol synthesis rate. On the other hand, the hydrocarbon synthesis appeared to be solely based on metallic Mo. A dual-site mechanism for the alcohol formation over Si02-supportedMo has been proposed, CO dissociates on metallic Mo to form surface carbide, followed by hydrogenation to carbene and/or methyl species. Addition of methylene unit to alkyl and the following hydrogenation and/or dehydrogenation of alkyl to give hydrocarbons are also catalyzed by species. The mechanism can account for the difference metallic Mo,whereas CO insertion leading to alcohols occurs on in selectivities to branched products between hydrocarbons and alcohols.
Introduction There is a
i n w t in the catalystsfor synthesis of mixed alcohols, since the alcohol mixture has proved effective as number enhancer for motorfuel.i.2 Problems caused by the use of methanol (phase separation, vapor lock, etc.) can be alleviated by incorporating C2+higher alcohols as a cosolvent into Author to whom correspondence should be addressed. Presented in part at the 7th Annual International Pittsburgh Coal Conference, Pittsburgh, PA, September 1990.
the methanol-gasoline mixture. Several investigators reported that an appropriate modification of the methanol synthesis catalyt resulted in the formation of higher alcohols together with methanoi.'4 Institut Francais de Petrole patented catalysts for the (1) Klier. K. In Catalysis oforganic Reaciions; MOM, W. R.,Ed.; Marcel Dekker: New York, 1981; p 195. (2) Xu,X.;Doesburg, E. B. M.;Scholten, J. J. F. Carol. Today 1987,2, 125. (3) Natta, G.; Colombo,U.; Pasquon, I. In Caralysis;Emmett, P. H., Ed.; Reinhold: New York, 1957; Vol. V, p 131.
0022-3654/92/2096- 1334$03.00/0 0 1992 American Chemical Society
