Langmuir 1991, 7, 1413-1418
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Stepwise Thermolysis of Tris(ally1)rhodium to Rhodium Metal: Elucidation of Thermal Processes on Bulk Titania through Ultrahigh Vacuum Studies on Ti02(001) Tuwon Chang, Steven L. Bernasek,' and Jeffrey Schwartz' Department of Chemistry, Princeton University, Princeton, New Jersey 08544-1009 Received October 12,1990. I n Final Form: January 25, 1991 Deposition of tris(ally1)rhodium onto hydroxylated bulk Ti02 was carried out at room temperature. Thermolysis of the supported organometallic occurred in stages and was analyzed by stoichiometricmeans and by IR spectroscopy: between 60 and 100 OC, TiOz-Rh(allyl)H was obtained; between 120 and 150 "C, TiOz-RhHt was produced; above 400 "C under Hz, Rh metal was formed. Deposition of tris(ally1)rhodium onto hydroxylated single crystal Ti02(001) was also carried out at room temperature under ultrahigh vacuum. With long exposure (2000 langmuirs) ca. 5% Rh atomic composition of the surface region was determined by Auger electron spectroscopy. Thermolysis of the single crystal oxide supported organometallic was followed by ultraviolet photoelectron spectroscopy (for 0 2p) and X-ray photoelectron spectroscopy (XPS) (for Rh 3d6p). Correlationswere drawn with results of reactions performed on "bulk" TiO2. Results suggest covalency between surface Rh complexes and the support oxygens. Aggregation of metallic Rh on Ti02 was noted (by XPS analysis) after long reaction times.
Introduction Chemical vapor deposition of a reactive organometallic complex onto a support surface followed by mild thermal degradation is an important technique for the preparation of surface-attached metals.lI2 In preliminary investigations, we noted that [titania]-O*-Rh(allyl)2 (0*= surface oxygen), prepared3*' from hydroxylated titania and Rhallyl)^, underwent thermolysis in stages involving rhodium hydride complex intermediates and, finally, rhodium metal,s as revealed by IR and stoichiometric analysis. Single-crystal supports provide an excellent medium for understanding details of surface organometallic chemical processes, and we have previously noted6 the deposition of (al1yl)sRh onto hydroxylated TiOz(OOl)' to give [TiOz(OOl)]-O*-Rh(allyl)~under ultrahigh vacuum (UHV) and ita subsequent hydrogenolysis.6g8 Data obtained by ultraviolet photoelectron (UP) and X-ray photoelectron (XP) spectrocopies elucidated the covalent nature of the Rh complex-oxide support interaction.6 We now report details of the thermolysis of [Ti0~(OO1)]-O*-Rh(allyl)~ in UHV. (1) (a) Yermakov, Y. I.; Kuznetaov, B. V.; Zakharov, V. A. Catalysis by Supported Complexes. In Studies in Surface Science and Catalysis 8; Elsevier: Amsterdam, Oxford, New York, 1981; p 345. (b) Guo, X.; Yang, Y.; Deng, M.; Li, H.; Lin, Z. J. Catal. 1986,99,218. (c) Hucul, D. A.; Brenner, A. J. Phys. Chem. 1981,85,496. (d) Iwasawa, Y.; Chiba, T.; Ito, N. J. Catal. 1986,99,95. (2) For example, see: (a) Prakash, H. h o g . Cryst. Growth Charact. 1986,12,243. (b) Steigerwald,M. L.;Rice, C. E. J. Am. Chem. SOC.1988, 110,4228. (3) Ward, M. D.; Schwartz, J. J. Mol. Catal. 1981,11, 397. (4) A report of the stoichiometry of deposition of tris(ally1)rhodium onto titania, based on thermolytic degradation, has appeared (Iwasawa, Y.; Sato, H. Chem. Lett. 1986, 507). This study concludes that only 1 equivof allylgroupremains,following deposition. We note that (titania)Rh(al1yl)l hydrolyzes to give 2 equiv of propene, but pyrolyzes to give slightly lees than 1 equiv of volatile materials. Stoichiometric analysis of a supported complex by thermal degradation can, therefore, be misleading. (5) Chang, T.; Bernasek, S.L.; Schwartz, J. J. Am. Chem. SOC.1989, 111, 768. (6) (a) Smith, P. B.; Bernaeek, 5.L.; Schwartz, J.; McNulty, G . S.J. Am. Chem. SOC.1986, 108, 5654. (b) Smith, P. B.; Bernasek, S. L.; Schwartz, J. Surf. Sci. 1988, 204, 374. (7) Smith, P. B.; Bemasek, S. L. Surf. Sci. 1987, 188, 241. (8) Smith, P. B. Ph.D. Thesis, Princeton University, 1986.
0743-7463/91/2407-1413$02.50/0
Thermal Decomposition of Bulk [Titania]-0*-Rh(ally1)z Formation of hydrides by allyl ligand decomposition of [silica]-O*-Rh(allyl)~~and ita iridium analoguelohas been noted. Both bis(ally1)- and (ally1)iridium hydride complexes yield [silicaI-IrHz a t 100 "C (Scheme I). Infrared analysis showed one terminal hydride (Q-H 2020 cm-l) for [silica]-0*-Ir(ally1)H and two terminal hydrides for [silica]-O*-IrH2 ( Y ~ 2081, H 2018 cm-l). Rapid thermolysis of [alumina]-O*-Rh(allyl)z or [titania]-O*-Rh(all y l ) ~(120 "C) produced a mixture of oxide-bound (allyl)Rh-H and -RhH2 s p e ~ i e s . ~ Hydrogenolysis J~ of either alumina- and titania-bound bis(ally1)rhodium yielded this same mixture, each component of which has been characterized stoichiometrically.3J~Jz Staged thermolysis was carefully done to bulk [titania]0*-Rh(ally1)z (prepared on P-25). Gas analysis of deposition reaction products formed according to Scheme I showed 0.02 mmol of Rh(allyl13 per gram of titania (0.16 wt % Rh) deposition as [titania]-0*-Rh(al1yl)z. The IR spectrum showed characteristic stretching (centered at 3057 cm-l, with minor contribution near 2920 cm-l) and bending (1458 cm-l) modes of allylic C-H groups. The sample was contaminated with residual deposition solvent (octane or toluene), even following evacuation for 6-12 h (at lo4 Torr). The solvent peak is prominent a t 2960 cm-l with a shoulder near 2900 cm-l. After the sample was heated a t 60 "C for 2 h in vacuo, a new absorption appeared (UN-H 2016 cm-l) (Figure 1A) assigned to [titania]-O*-Rh(al1yl)H by spectral comparison with known material. No absorption was observed attributable to bridging hydride ligands,"11J2 consistent with IR data for [titania]-O*-Rh(C0)2, which suggest a monomeric structure for the c o m p l e ~ .Under ~ these thermal conditions no significant amount of [titania]-O*-RhHz was formed. When the temperature was increased to 120 OC,a spectrum (9) McNulty, G . S.;Cannon, K.; Schwartz, J. Znorg. Chem. 1986,25, 2919. (10) Kitajima, N.; Schwartz, J. Unpublished resulta. (11) Cannon, K. Ph.D. Thesis, Princeton University, 1987. (12) Ward, M. D. Ph.D. Thesis, Princeton University, 1981.
0 1991 American Chemical Society
Chang et al.
1414 Langmuir, Vol. 7, No. 7, 1991
'!
J
2022
15
10
5
0 ('El)
B i n d i n g Energy, e V
Figure 2. UP spectra for [Ti0~(001)]-O*-Rh(ally!)z: (A) TiOz(001) after 200 langmuirs of water exposure; (B)T102(001) after subsequent 2000 langmuirs exposure to Rh(al1yl)s.
!I 3000
Scheme 11. Deposition of Tris(ally1)rhodium onto a Hydroxylated Oxide Surface
2500 wavcnumkrs
2000
Figure 1. IR spectra for thermolysisof [titania]-O*-Rh(allyl)z: (A) 60 "C,2 h; (B)120 OC,2 h; (C)400 O C , 2 h, Hz.Asterisk denotes residual solvent peak. Scheme I. Thermolysis of Bulk Metal Oxide Supported Metal Allyls
(oxide)-O*-M(allyl)H
loo e
(oxide)-O*44Hz
oxide = Si02or Top;M = Rh or I r
was obtained after 2 h (Figure 1B) showing no absorptions due to allylic C-H in either the 3000- or 1450-cm-l regions and two peaks of comparable intensity ( Y ~ - H2088,2022 cm-l; antisymmetric and symmetric stretching modes), indicating complete conversion to [titania]-0*-RhHz. Under 1atm Hz, heat treatment a t 400 "C for 2 h resulted in disappearance of all absorption peaks (Figure IC), including those due to residual solvent. The material had turned deep blue, attributed to the creation of Ti3+.
Reaction of Rh(ally1)s with TiOz(001) Procedures used for cleaning and hydroxylation of the TiOz(001) surface have been described.7 The substrate surface was hydroxylated by exposure to 200 langmuirs of water a t room temperature. Following this treatment, three peaks were observed in the UPS for hydroxylated TiOz(001);assignments have been discussed.7*8 Two major peaks for the oxygen 2p region appear a t approximately 5.2 and 7.1 eV (Figure 2A).7 The third peak was attributed to Ti3+ (arising from 0-vacancy defects) and appears a t 0.8 eV below the Fermi level. Surface hydroxyl groups react with Rh(ally1)gwith propylene evolution to give [TiO2(001)]-O*-Rh(allyl)~ according to Scheme 11. Auger analysis,l3-l6as described in the Experimental Section, shows that after 2000 lang-
muirs of exposure to Rh(ally1)a a t room temperatrue,I7 5 % Rh atomic concentration of the complex on the hydroxylated surface was achieved.'3J4 No significant further increase in Rh AES signal was observed up to 5000 langmuirs of total exposure (Figure 2).8 The UP spectrum for the oxygen 2p region originally exhibited peaks a t 7.1 and 5.2 eV. The UP spectrum recorded for [Ti0~(001)]-0*Rh(allyl)z, after 2000 langmuirs of exposure to Rh(allyl)3, showed a new binding energy maximum a t 8.1 eV, with only a negligible change in the peak a t 5.2 eV (Figure 2B); replacing the proton of [Ti02(001)]-0*-H by -Rh(allyl)z has a net electron-withdrawing effect on oxygen? In other words, these particular oxygen valence electrons of [TiOz(001)]-0*-Rh(allyl)z are more strongly bound than are those in [TiO2(OOl)]-O*-H; apparently -Rh(allyl)z is a stronger electron acceptor than is a proton. The interaction between the surface oxygens, "O*",and the Rhallyl)^ moiety is assumed to be covalent since only one of the two observed O* states (at 7.1 eV) shifted to higher binding energy; were the Rh-O* interaction to have been entirely ionic, both peaks would have been expected to shift. A more rigorous explanation, based on extended Huckel band calculations, has recently appeared.18 These calculations, based on two-dimensional slab models of the oxide-organorhodium complex interaction, as well as on discrete molecular analogues of the surface-adsorbate complex, suggest that the major bonding interaction ~~
(13) Davis, L.E.; Palmberg, P. W.; Riach, G. E.; Weber, R. E.; MacDonald, N. C. Handbook of Auger Electron Spectroscopy; Physical Electronics Industries, Inc.: Eden Prairie, MN, 1972. (14) Bishop, H. E.; Riviere, J. C.; Coak, J. P. Surf. Sci. 1971, 24, 1. (16) Grant, J. T.Appl. Surf. Sci. 1982, 13, 4. (16) Minni, E. Appl. Surf. Sci. 1983,15, 270. (17)Wenotethatdepoeitionoftris(allyl)rhodiumontoTiO2(001)OCCUIB below200Kwithhighcoverage (byAugeradysi.9). Thereaultingmatarisl may be structurally different from that which is obtained by room temperature deposition; pyrolysis chemistries of the low- and room-temperature-deposited materials are not comparable. Dai, A.; Miller, J. B.; Bernasek, S. L.; Schwartz, J. Unpublished results. (18) Halet, J.-F.; Hoffmann, R. J. Am. Chem. SOC. 1989, 111, 3648.
Langmuir, Vol. 7, No. 7, 1991 1415
Thermolysis of Tris(ally1)rhodium.to Rhodium Metal
4 309. o .*
1
458.5
A 318
314 310 Binding Energy
306 / ev
302
Figure 3. XP spectra for [TiO?(00l)]-o*-Rh(ayl)2: (A) clean surface; (B)after 2000 langmmra of exposure to Rh(allyl)3. involves the u lone pair of the surface oxygens and the 3al orbital of the bis(a1lyl)rhodium adsorbate. Two new features also appeared in the UP spectrum of the [Ti02(001)]-0*-Rh(allyl)2 surface, at 10.8 and 3.0 eV (Figure 2B). The 10.8-eV peak may be due to allyl ligand molecular orbitals, perhaps C-H bonds;lg the 3.0-eV peak is assigned to a Rh (4d)-7r2 allyl ligand interaction.8 Ranges in core-level binding energies observed by XPS for a metal in a family of complexes can be attributed to the formal oxidation state, the molecular or ligation environinent, or the type of metal atom lattice site.20To probe the oxidation state of Rh in [Ti02(001)]-0*-Rhallyl)^, XPS analysis was done by using Mg K a radiation (1253.6 eV) and each spectrum was separately referenced to the binding energy of T i 2pap electrons (458.5 eV) (see the inset spectrum of Figure 3). After 2000 langmuirs exposure, an X P spectrum for [Ti02(001)]-0*-Rh(allyl)2, was obtained (Figure 3B) showing both characteristic Rh peaks (binding energy for Rh 3d6p = 309.0 eV). This value compares well with other formally Rh3+complexes;21 however, it is the real charge (or electron) density, not the formal oxidation state, which affects binding energies.22
Thermal Decomposition of
[TiOa(001)]-O*-Rh(allyl)2 Thermolysis of [TiO2(00l)]-O*-Rh(allyl)2was performed in stages up to 150 "C in order to correlate data for bulk oxide-supported complexes with that for species formed in UHV. The sample was heated to the desired temperature and annealed for 2 h, with both UPS and XPS determinations made following each increase in annealing temperature. UPS results are shown in Figure 4 C-G and show significant changes in the 0*(2p) region. Peak maxima shifted to lower values as the temperature is increased and the metal undergoes partial reduction. At 60 "C, the 8.1 eV feature shifted to 7.8 eV (C vs B), and (19) Green, J. C. Struct. Bonding 1981,43, 37. (20) Brigp, D.; Seah, M. P.Practical Surface Analysis by Auger and X-ray PhotoelectronSpectroscopy;John Wiley&Sons: New York, 1983. (21) (a) Leigh, G.H.; Bremser, W. J. Chem. SOC.,Dalton Tram. 1972, 1217. (b) Nefedov, V. I. J. Electron Spectrosc. 1977,12,459. (22) For example, see Chatt, J.; Leigh, G. J. Angew. Chem., Int. Ed. Engl. 1978,17,400.
5
10 Binding
0i'E~)
Energy. e V
Figure 4. UP spectra for [TiOz(ool)]-o*-Rh(ayl)*and its thermal derivatives: (A) hydroxylated surface; (B) -O*-Rh(al1yl)n; ( C ) 60 "C; (D)80 "C;(E) 100 O C ; (F)120 "C;(G)150 "C; (H)400 "C under Hz;(I) 5 days of aging at room temperature.
it remained close to this value up to 100 "C. In the region 120-150 "C, a further small shift to 7.6 eV was observed (F-G). However,the 5.2-eV peak (attributed to nonbonding electrons of the surface oxygens) remained fixed throughout the entire thermolysis. These results suggest that covalency between the O*and Rh was retained, that (at least) two relatively stable intermediates exist in this thermolysisprocedure,one predominating between 60 and 100 "C and the other between 120 and 150 "C, and that these two intermediates are formed sequentially. Futhermore, these data imply that the electron-withdrawing effect of the Rh moiety decreases with thermolysis. When the sample was heated to 400 "C under hydrogen, conditions under which metallic Rh is prepared from various inorganic p r e ~ u r s o r s the , ~ ~O(2p) ~ ~ 7.6-eV peak shifted back to its original, predeposition value, 7.1 eV, indicating that covalency no longer existed between the surface and the overlayer. The UP spectrum (Figure 4H) also showed increased emission near the Fermi level, suggesting metallic rhodium had been formed. To elucidate UPS shifts observed for 0*(2p) bonding electrons, concomitant XPS studies were carried out on Rh (Figure 5). For [Ti02(001)]-O*-Rh(allyl)2, the binding energy for Rh3dsp was measured a t 309.0 eV vs Ti(2ps 2). Annealing the sample in the 60-100 "C range results(iin a Rh3dsp binding energy near 308.5 eV. Annealing a t 120-150 "C resulted in a further shift to 308.0 eV. For Rh metal prepared in situ (400 "C, Hz), this binding energy was measured at 307.8 eV. A plot of the combined UPS and XPS data is shown in Figure6. Photoelectron binding energies plotted in Figure 6 were obtained by averaging peak positions for several spectra recorded during repeated deposition and thermolysis runs. Error bars are estimates based on approximate peak widths. Two temperature regions of roughly constant binding energy were observed, at 60-100 "C and at 120-150 "C, for this thermolysis ~
~~
(23) Lin, Y. J.; Fenoglio,R. J.; Reeaeco, D. E.; Haller, G. L. Preparation
ofCatalpisIV. InStudiesonSurfaceScienceand Catalysis31;Elsevier: Amsterdam-Oxford-New York-Tokyo, 1987; p 125. (24) Huizinga, T.; van't Blik, H. F.J.; Vis, J. C.; Prins, R. Surf. Sci. 1983,135,580.
1416 Langmuir, Vol. 7, No.
Chang et al.
7,1991
aos
a io
ais
Binding Enrrgy'. oV
Figure 5. XP spectra for (TiOz(OOl)]~*-Rh(allyl)zand its thermal derivatives: (A) hydroxylated surface; (B)-O*-Rh(al(C) 60 "C; (D)80 "C;(E) 100 "C; (F) 120 "C; (G) 150 "C; 1~1)~; (H)400 "C under Hz;(I) 5 days of aging at room temperature.
..
? I
n
n
8.0
309
if'
0 U
z W
0
O
>1
r
G
I
C
Y
308
7.5
.m
50
100
Temperature
,
150
C
Figure 6. Binding energy changes for surface 0 2p bonding electrons and Rh 3dsp electrons: filled circles for O*; open circles for Rh.
process. The correlation between the UPS and XPS data suggests that binding energies for the O* 2p electrons change as a function of the electronegativity of the Rh in the surface complex which, in turn, is a function of ligation at Rh. Data correlations for UPS, XPS,and IR studies are given in Table 11.
Scheme 111. Thermolysis of [Ti0,(001)]-0*-Rh(a11y1),
-
60-100 o c
Ti02(001)-0*-Rh(allyl),
-
120-150 e c
TiO,(OOl)-O*-Rh( ally1)H
-
Hz,(00 'C
Ti02(001)-0*-Rh(H)2
Ti02(001)/Rh
Further XPS studies were performed to investigate the unusually high binding energy (307.8eV) found for freshly
Table I. Binding Energies of Rh 3ds/a Electrons for Various Rh Systems compound BE, eV ref a 310.2 RhCls.3H20 b 309.7 C 310.3 d 308.8 a 309.5 C 309.1 e Rhfoil 307.0 d Rh/ y-alumina 307.5 20 307.5-307.1 Rh/silica 307.25 f 20 307.4-307.1 Rh/titania silica-Rh(allyl)2 308.9 g silica-Rh(ally1)H 308.0 f d alumina-Rh(CO)z 308.7 32 titania-Rh(ally1)z 308.6 h TiO&Ol)-Rh(allyl)z 309.0 h 308.5 Ti02(001)-Rh(allyl)H h 308.0 TiOZ(001)-Rh(H)z h 307.8-307.1 Ti02(001)/Rh a Anderson, S. T.; Scurell,M. S. J. Catal. 1979,59,340.* Imanaka, T.; Kaneda, K.; Teranishi, T.; Teraeawa, M. Proceedings Sixth International Congress on Catalysis, London, 1976; Bond, G. C., Wells, P. B., Tompkins, F. C., Eds.; The Chemical Society: London, 1977; Vol. 1, p 509. Contour, J. P.; Mouvier, G.; Hoogewijs, M.; Leclere, C. J. Catal. 1977,48,217. d van't Blik, H. F. J.; van Zon, J. B. A. D.; Huizinga, T.; Vis, J. C.; Koningsberger, D. C.; Prins, R. J. Am. Chem. SOC.1985,107,3139.e Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976,8, 129. f Schwartz, J. Acc. Chem. Res. 1985,18,302.8Foley, H. C.; DeCanio, S.J.; Tau, K. D.; Chao, K. J.; Onuferko,J. H.; Dybowski, C.; Gates, B. C. J. Am. Chem. SOC.1983, 105, 3074. h This work.
prepared Rh metal (400 O C under H2). This value is higher by 0.8 eV than that found for rhodium foil (see Table I). When this "freshly prepared metal" was aged in UHV at room temperature for 5 days, a slow shift in the Rh 3d5/2 binding energy was observed, which eventually moved to 307.1 eV. The binding energy for surface O(2p) did not change during this time (spectrum I in Figure 41, consistent with the notion of weak (or no) covalent interaction between metallic rhodium and the oxide surface. This shift in rhodium 3ds 2 binding energies can be interpreted in terms of a partic e size effect9 As the metal particle grows by aggregation,binding energies fall. This has been explained by considering the extraatomic relaxation of metal particles of different sizes: In small particles, there is less effective screening of core holes created during photoemission than exists in larger aggregate^.^^ Values observed herein for this effect (307.8-307.1 eV) compare well with those reportedu for Rh metal of varying particle size on titania and suggestthat rhodium metal on Ti02(001) produced initially by controlled thermolysis of the vapor phase deposited organometallic complex may approach the "monodispersive", single atom limit. Aggregation of metals on oxides has been investigated from several perspectives, including adatom displacement,% migration of small crystallites (not necessarily single atoms):' and "ripeningn28*W in which single atoms are mobile and small crystallites decrease in size by losing atoms while larger ones increase in size by gaining atoms.
f
(25) (a) Oberli,L.; Monot, R.; Matthieu, H. J.; Landolt, D.; Buffet, J. Surf. Sci. 1981,106,301. (b) Mason, M. G.; Baetzold, R. C. J. Chem. Phys. 1974,64, 271. (26) (a) Ehrlich,G. Discws. Faraday SOC.1966,41,7. (b) Ehrlich, G.; Kirk, C. F. J. Chem. Phys. 1968,48,1456. (c) Ehrlich, G.; Hudda, F. G. J. Chem. Phys. 1966,44, 1039. (27) (a) Ruckenstein,E.; Pulvermacher, B. J. Catal. 1979,29,224. (b) Ruckenstein,E.; Dadyburjor, D. J. Catal. 1977,48,73. (c) Ruckenstein, E.; Chu, Y.F. J. Catal. 1978,55, 281. (28) Wynblatt,P.;Gjostein,N.A. Prog. Solid State Chem. 1976,9,21. (29) (a) Flynn, P. C.; Wanke, S. E. J. Catal. 1974,34,390. (b) Flynn, P. C.; Wanke, S. E. J. Catal. 1974, 34,400.
Thermolysis of Tris(ally1)rhodium to Rhodium Metal Table 11. Correlation of UPS and XPS Data for Ti0,(001)and IR Data for Titania-Susmrted Com~lexes single crystal Ti0&01) bulk titania species 0*(2p);Rh(3ds/r),eV VCH; WH, cm-l -OH 7.1, 5.2 -O*-Rh(allyl)z 8.1, 5.2; 309.0 3057 (m) -O*-Rh(ally1)H 7.8, 5.2; 308.5 3055 (m); 2016 ( 8 ) -O*-Rh(H)2 7.6, 5.2; 308.0 2088, 2022 (s) -OH/Rh 7.1, 5.2; 307.8-307.1 Surface migration also causes surface redispersion of aggregates under some reaction condition^.^^^^^ These surface migration processes all require heat treatment under various reaction conditions. Therefore, these processes might not completely account for the aggregation of Rh metal observed in this study, which occurred over 5 days at room temperature. Since “freshly prepared Rh metal” on TiOz(001) approaches the “monodispersive limit” (at least by this XPS measure), the aggregation observed here may be a simple model for “ripening”. Small differences exist between complexes prepared thermally on “bulk” titania and analogues obtained on TiOZ(001) in UHV regarding temperature ranges in which each intermediate is stable. The allyl(Rh) hydride complex prepared on bulk titania starts to decompose to the dihydride at 80 “C, while the analogue prepared on the single crystal substrate appears stable to 100 “C. The dihydride prepared on the single crystal at 120 “C is stable to 150 “C,but at this temperature the one prepared on “bulk” titania begins to decompose, presumably to the metallic state. These relative stabilities may be due to differences in hydroxyl group availabilities on the two surfaces, which might be mechanistically necessary for decomposition.
Conclusions
A correlation in thermolysis behavior was observed for organometallic complexes produced in UHV and those prepared under “normal” conditions, at atmospheric pressure on powdered oxides. IR data for two hydridecontaining intermediates thermally prepared from [titania]-O*-(allyl)~ provided structural assignments for analogs observed in UHV. UPS data for [Ti02(001)]-0*Rh(ally1)z were in good agreement with literature values, and XPS studies indicated that the oxidation state of the rhodium center was comparable to that of other Rh(II1) compounds. These UPS and XPS data support the notion of covalency between the surface oxygens and the bis(al1yl)rhodium unit. Stepwise thermolysis of [TiOz(OOl)]0*-Rh(ally1)z yielded two intermediates prior to formation of rhodium metal. The first intermediate (dominant in the region 60-100 “C)was assigned as [Ti02(001)]-0*Rh(allyl)H, and the second one (dominant from 120 to 150 “C)as [Ti02(001)]-0*-RhHz. UPS data for these two intermediates suggested covalent interactions between the surface oxygens and the rhodium complex overlayer. XPS results (for Rh3dap electrons) indicated that the real electron density on the metal center for these two intermediates was higher than that found for simple inorganic Rh(111) complexes, consistent with the donor properties of the hydride ligand.22 XPS data for rhodium metal prepared by thermal decomposition (307.8-307.1 eV) showed what may be, initially, the monodispersion (30) (a) Ruckenstein, E.; Malhotra, M. L. J. Catal. 1976,41,303. (b) Ruckenstein, E.; Chu, Y.F. J. Catal. 1979,59, 109. (31) Baker, R. T. K. J. Catal. 1980,63,523. (32) This small difference in reactivitymight be due to actualoxidation state differences for the Rh center on the two supports, attributable to varying ligation geometries in the coordination sphere of the Rh center on rutile Ti02(001)vs polycrystalline anatase (Table I). The authors acknowledge the help of Mr. H. Eric Fischer and Dr. Dennis Andereon in obtaining these data.
Langmuir, Vol. 7, No. 7, 1991 1417 limit for this metal on TiOZ(001). Thus, complementary studies performed on “normal” and UHV analogues enable a better understanding of structure and bonding than could be obtained by analyzing each system individually.
Experimental Section The Ultrahigh Vacuum Apparatus. A stainless steel bell jar of 12-in. diameter (Varian) was used as the main UHV chamber. Five 30 L/s ion pumps were installed at the bottom of the chamber and a 60 L/s ion pump was added at the top of the chamber opposite to the cylindrical mirror analyzer (CMA). This arrangement gives a total pumping speed of 210 L/s, and, typically, a base pressure of 5 X 10-’0Torr was obtained. A threefilament titanium sublimation pump equipped with a cryopanel located at the bottom of the chamber was frequently used to attain the desired base pressure for an experiment. Major surface probes included low energy electron diffraction (LEED),Auger electron spectroscopy (AES), ultraviolet photoelectron spectroscopy (UPS), and X-ray photoelectron spectroscopy (XPS). A double pass CMA which has a coaxial electron gun was used as an electron energy analyzer for AES, UPS, and XPS. He I light (21.2 eV) produced from a discharge lamp was used for UPS, and spectra were recorded with a pass energy of 15 eV. A Mg anode (1253.6 eV) was used for XPS and a CMA pass energy of 50 eV. The pulse counting mode was used for data collection in UPS and XPS. All spectra were taken at room temperature with an interfaced Apple 11+ personalcomputer. Two leak valves were used for gas exposure. One of these valves was used for argon, hydrogen, oxygen, and carbon monoxide. The other was used for tris(allyl)rhodiums3and water. All lines were pumped to Torr through mechanicaland glass diffusion pumps prior to any gas exposure. Tris(ally1)rhodiumwas handled in inert atmosphere and was transferred in a glass tube to the UHV chamber. Auger electron spectra were obtained with a 3-keV primary electron beam energy, and typically 10pA current to the sample. A 2-eV peak to peak modulation voltage was used. Quantitative estimates for rhodium and carbon percent composition were obtained by the use of relative sensitivity factors for the most prominent Rh, Ti, 0, and C peaks, as outlined in ref 13. The atomic percent compositionsdetermined in thie way refer to atomic compositions of the sampled volume. Signals from Ti and 0 derive from the top three or four layers, while the Rh and C signals are from the top layer only. Thus, the actual surface coverages are likely to be somewhat higher than the 5% atomic concentration quoted for the Rh composition. XPS binding energies for Rh were referenced to the Ti(2ps/g) peak (458.2 eV) for each spectra. Digitally recorded spectra were smoothed by using a Blackman window low pass filter with a cutoff frequency of 0.4Hz/point prior to plotting (see Figure 3). UHV Procedures. A singlecrystalTi02boule was purchased from Atomergic Chemetals Corp. and was oriented to the desired crystallographic Miller plane within 0.5’ using the Laue back reflection technique. The crystal was then cut and mechanically polished to the final size of ca. 0.6-1 cm2area X 1mm thickness. In the mechanical polishing procedure, abrasive papers (240600 grit), diamond paste (15,6,1 pm), and alumina powder (0.05 pm) were used successively. The polished crystal was cleaned with water and ethanol and was ultrasonicated in ethanol for 10 min. Before the crystal was brought into the UHV chamber,the crystal was heated at 500 “C for 2 h under a hydrogen stream (1 atm). After this treatment, the color of the crystal changed from clear, light yellow to deep blue, which indicates that Tia+was produced in the bulk. This small amount of Ti9+in the bulk converts the Ti02 crystal to a semiconductor so that problems associated with charging could be avoided. The crystal was mounted onto Mo foil (2 cm X 4 cm). Small flaps were cut and folded at the top and the bottom part of the Mo foil to hold the crystalin place. Thie foil was then mounted onto the manipulator with Mo rods. Solvent cleaned, high-purity copper braids were used to connect the Mo rods to high current feed throughs. The sample crystal was heated resistively, and temperatures were measured with a chromel-alumel thermocouple spot-welded to the Mo foil close to the crystal. All heating elements were isolated from electrical ground by Macor blocks or plates and glass wool (33) Powell, J.; Shaw, B. L. Chem. Commun. 1966, 323.
1418 Langmuir, Vol. 7, No. 7, 1991 m 10
diffusion pump
Figure 7. A schematic of the high vacuum line apparatus. tube. After the sample was mounted in the UHV system and the base pressure was achieved, the crystal surface was cleaned by cycles of argon ion bombardment and annealing under oxygen. Typical sputtering conditions were 3 x 106 Torr argon, 2-keV beam energy, 20-mA emission current, and 15-min sputtering time. Under these conditions, a 4-PA current to the crystal was measured during sputtering. Annealing at 600 O C for 1h under 0 2 following sputtering usually gave a Ti/O surface ratio in the AES spectrum of 1.5-2.2 without disturbing the semiconducting properties of the bulk. The absence of contaminants (mostly sulfur, coming from the bulk during annealing) was verified by Auger electron spectroscopy. Details for the deposition of tris(ally1)rhodium onto Ti02(001)have reported.8 High Vacuum Line Techniques. An illustration of the glass high vacuum manifold employed is shown in Figure 7. Pressures of 1O-g mmHg were attainable by use of a silicone oil diffusion pump. The following procedure was employed to isolate light hydrocarbons (usually up to C,) from high boiling solvents (e.g., toluene or HzO). The right-side trap was calibrated for PV readings on the attached manometer. In a typical experiment, reaction flask 1contained solvent and the light hydrocarbon (e.g., propene) product. The system was evacuated to 1O-g mmHg up to the stopcock of the reaction flask with the manometer stopcock and stopcocks 5,7, and 8 open. The trap on the left was immersed in a dry ice/acetone bath (195 K) and the calibrated trap was placed in a liquid nitrogen bath (77 K). Stopcock 5 was then closed and the reaction flask was opened slowly to the traps. After 15 min, valve 5 was slowly opened. Care was taken to prevent 'bumping", which can send oxide through the apparatus. Distillation was performed until all solvent was removed from flask 1. In this way the toluene was trapped in the left-hand trap and the propene in the calibrated trap. Light volatiles were then transferred to flask 2. A standard, such as butane, was then measured into the calibrated trap and transferred to flask 2. After flask 2 was allowed to thaw, the equilibrated gas mixture was analyzed by GC and the product was quantitatively compared with a standard. General Procedures for Bulk TiO2. All operations were carried out under purified nitrogen with standard Schlenk glassware,either in a drybox or on a bench line. A high vacuum line manifold equipped with a glass diffusion pump capable of obtaining pressures of 10-6 Torr and a Tbpler pump for noncondensable gas transfer were used for gas handling and for trapping volatiles. Solvents were distilled from sodium benzophenone ketyl under nitrogen. Allyl chloride was passed through a column of basic alumina (Baker) prior to use, and methyllithium (Aldrich, 1.4 M in THF) was used after standardization. A solution of allylmagnesium chloride in THF was prepared by standard proceduresMand was titrated%before use. Rhodium chloride trihydrate (Johnson Matthey) was used as (34) Grummitt, 0.; Budewitz, E.; Chudd, C. Organic Synthe8e8, Collectiue Volumes; Wiley: New York, 1963; Vol. IV,p 751. (35) Vlismas, T.;Parker R. J. Orgonomet. Chem. 1967,10, 193.
Chang et al. received. Chloro(dicarbony1)rhodium dimer was prepared in an apparatus detailed by Wardu and was stored under an atmosphere of carbon monoxide until use. Titanium dioxide powder (Degussa Corp., Titanium Dioxide P-25,50 mz/g) was loaded into a quartz tube which was then placed in a tube furnace. The sample was evacuated and heated to 200 OC for 12 h. Dry powder samples prepared in this way were stored in the drybox for subsequent use. The concentration of hydroxyl groups on the titanium dioxide was measured by titration with methyllithium.12 A solution of methyllithium (20 mL, 1.4M) was transferred by syringe intoa 60-mL tube equipped with a 4-mm Teflon vacuum valve and a 20-mm greaseless pan joint. After it was degassed by several freeze/thaw cycles on a high vacuum line fitted with a glass diffusion pump, the methyllithium solution was brought into the drybox. This tube was then attached to a flask containing 1 g of oxide and a stir bar. This flask was also equipped with a 4-mm Teflon valve and a pan joint. The apparatus was brought out from the drybox and was attached to a high vacuum line and the bottom part containing the oxide sample was evacuated for 1h while the upper part was kept closed. The methyllithium solution was then added to the oxide and was stirred for 1h. All volatile products were pumped with a Tbpler pump into a calibrated manometer through a liquidnitrogen-cooled trap, which condensed everything except methane. The gas collected was verified to be methane by GC/MS. Methyllithium titration cannot be used to distinguish between hydrogen-bonded and non-hydrogen-bonded hydroxylgroups on the oxide surface. Infrared Techniques. Pelleta (1 cm diameter) of optical quality for use in IR studies were obtained by pressing approximately 30 mg of a mixture of KBr and the titania-supported sample with a Beckman P-16 hydraulic press. A typical mixture was composed of 70 mg of KBr and 30 mg of titania sample and was dispersed with a Crescent mixer. Typically, the pellets were loaded into airtight cells fitted with demountable windows. A mixture of KBr and the unreacted oxide (in the same ratio) was used aa a reference. All manipulations were carried out in an inert atmosphere drybox. Infrared spectra were taken at room temperature with a Perkin-Elmer 1710 IR spectrometer. Preparation of Oxide-Bound Rhodium Complexes. Approximately 2.0 g of dried oxide was placed in a 100-mLSchlenk flask containing a magnetic stirring bar. Tris(ally1)rhodium (in 40 mL. of toluene) was added to the flask, which was closed immediately with a high vacuum adapter attachable to the high vacuum line; the mixture was then stirred at ambient temperature for 24 h. The evolved propene was separated and quantified by GC and GC/MS as described above. Typically, > O M equiv of propene/equiv of added Rh(ally1)l was obtained. The sample was then washed with an excess of pentane and was dried at 1o-B Torr. On titania, maximum loadings are typically 0.1 mmol of Rh(allyl)s/g of oxide powder. For this study, 0.02 mmol of Rhallyl)^ was loaded per gram of titania. In the inert atmosphere drybox, [titania]-O*-Rh(allyl)z (150 mg) was put in a 50-mL Schlenk flask which was then attached to the high vacuum line. The flask was slowly evacuated to 10-6 Torr for 1 h at room temperature. Desired temperatures were then set by using an oil bath, and the reaction mixture was heated in vacuo for 2 h for each chosen temperature. Unreacted titania was also treated in the same way to obtain reference pellets for IR analyses. To convert surface-bound rhodium complexes to Rh metal, [titania]-O*-Rh(allyl)z was heated to 400 O C under a hydrogen stream for 2 h in a tube furnace followed by evacuation on the high vacuum line. All heat-treated samples were pressed into pellets in the drybox for IR studies.
Acknowledgment. We acknowledge support for this work provided by the National Science Foundation. We also thank Mr. H. Eric Fischer (Princeton) and Dr. Dennis Anderson (Englehard Industries) for helping obtain XP spectra for bulk titania-supported samples. Registry No. Rh(allyl)s, 12082-48-3;TiOz, 13463-67-7.
