J. Phys. Chem. 1995,99, 12892-12895
12892
Redox Properties of Titanium Oxides on Pt3Ti W. Chen,? S. Cameron,' M. Gdthelid,? M. Hammar,? and J. Paul**+ KTH/Royal Institute of Technology, Physics III, 100 44 Stockholm, Sweden, and EXXON Research and Development, CR, Route 22 E, Annandale, New Jersey 08801 Received: January 20, 1995; In Final Form: May 2, 1 9 9 9
The morphology and electronic structure of surface-segregated titanium oxides on Pt3Ti( 11 1) are presented. Core level photoemission spectra at grazing emission reveal two states of oxidation: a dominant and reducible four-valent oxide together with a small amount of a three-valent oxide is produced by oxidation in 0 2 at and below 400 "C; an irreducible three-valent oxide by oxidation in 0 2 at and above 450 "C. The ratio between the active four-valent and the inactive three-valent oxides decreases with increasing oxidation temperature. The probability for reduction by CO is almost unity for the Ti4+ oxide, and the conclusion must be that the four-valent oxide plays an active role for catalytic reactions. Scanning tunneling measurements relate these observations to changes in the dispersion and nucleation of the oxide overlayer. The four-valent oxide grows as islands with remaining areas open for CO adsorption while the three-valent oxide spreads on and blocks the crystal surface. Photoemission spectra relate these dispersion effects to an electronic interaction between the Ti3+ oxide and adjacent Pt atoms. The above observations are in accordance with the common picture of dispersion effects in titania-supported SMSI catalysts and prove that interfacial energies play a crucial role whether the dominant phase is metallic or an oxide.
1. Introduction The ordered Pt3Ti bimetallic alloy is a valid model system for the SMSI effect (strong metal-support interaction). The SMSI state is obtained when titania-supported platinum catalysts, exposed to reducing gas mixtures (CO/H2) at high temperatures, undergo dramatic morphological changes resulting in altered catalytic properties. Surface science measurements have documented that platinum is engulfed by the Ti02 carrier under these conditions and that the behavior is reversible during redox cycles. The free energy of Pt3Ti is unusually large for a bimetallic system but smaller than that of the partially reduced system (Ti3/pt0). Nevertheless, nucleation of the ordered intermetallic alloy has been observed for the bulk catalyst2. The above morphological changes, including the formation of thin coatings and new phases thermodynamically not stable in the bulk, point at the importance of interfacial energies. The present work addresses morphological changes of a surface oxide during sequential oxidation and reduction cycles. We characterize different surface-segregatedoxides and their redox behavior on a Pt3Ti(111) support. This is closely linked to the carrier's role as an active partner in titania-supported metal catalysts for CO hydrogenation.
emission and planar samples are essential to reveal reduction/ oxidation cycles in the topmost layer. We have previously measured similar changes at 30" emission angle with a nonmonochromatic X-ray source, and we could hardly see any changes in the Ti 2p region despite a large difference in highresolution electron energy loss ~ p e c t r a . ~ One simply has to calculate the effective escape depth to find that grazing emission is essential and that this is the relevant procedure for standard photon energies even at the price of low count rates. The situation is different for variable photon energies where one can minimize the mean free path by calculating the kinetic energy. Such measurements are, however, complicated by strong variations in the cross section for d-electron emission around 80-150 eV. The preparation chamber with a LEED system constitutes a separate part of this ultrahigh vacuum system. Scanning tunneling measurements (STM) were done in a different ultrahigh vacuum system, likewise equipped with an intergrated preparation chamber with LEED optics.6 The main chamber of this system is equipped with an Omicron microscope and data-handling routines.
2. Experimental Section The Pt3Ti(l11) crystal was gold brazed to a tantalum foil and cleaned in situ by repeated cycles of Ar+-bombardment (300 K, 1 kV, 15 min) and annealing in vacuo (1100 K, 5 Cleanliness was verified by photoemission spectroscopy or by scanning tunneling microscopy. The clean surface gave a shape ~ ( 2 x 2 LEED ) pattern. X-ray photoemission spectra were measured at low emission angle to maximize the surface sensitivity. Spectra were obtained with monochromatic A1 K a X-ray radiation (1486.7 eV) and a hemispherical analyzer equipped for multidetection: Low angle
3.1. Photoemission. Ti 2p photoelectron spectra at grazing emission from clean Pt3Ti(lll) and Pt3Ti(lll) oxidized at different temperatures are displayed in Figure 1. Titanium in clean Pt3Ti(lll) is in a metallic state (Ti*) with a Ti 2~312 binding energy of 455.4 eV, which is higher than the 454.0 eV binding energy of pure t i t a n i ~ m . ~ - ~When - ' ~ the crystal is exposed to 0 2 at and below 400 "C, three states of titanium were found, denoted as Ti*, Ti3+, and Ti4+. The binding energies of Ti3+ 2~312and Ti4+ 2~312are 456.3 and 458.0 eV for oxidation at 375 "C and shift 0.4 and 0.3 eV toward higher binding energy when the temperature during exposure is increased to 400 "C. The XPS ratio Ti4+/Ti3+decreases with the increase of the oxidation temperature. When the Pt3Ti(111) surface was dosed with 0 2 at and above 450 "C, only two states of titanium were found, denoted as Ti* and Ti3+. The binding
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* To whom correspondence should be addressed.
' KTHRoyal Institute of Technology, Physics 111
* EXXON Research and Development. @
Abstract published in Advance ACS Absfracts, July 15, 1995.
0022-365419512099-12892$09.0010
3. Results
0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 34, 1995 12893
Redox Properties of Titanium Oxides on Pt3Ti
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Binding Energy (eV)
Binding Energy (eV) Figure 1. PES spectra at grazing emission for clean PtjTi(ll1) and Pt3Ti(lll) oxidized in 0 2 (50 L) at different temperatures: (a) clean
Figure 3. PES spectra at grazing emission from Pt3Ti( 11 1): (a) clean surface; (b) oxidized in 0 2 (50 L) at 800 "C; (c) reduced by CO (100
surface; (b) oxidation at 375 "C; (c) oxidation at 400 "C; (d) oxidation at 450 "C; (e) oxidation at 650 "C; (f) oxidation at 800 "C. * corresponds to the metallic Ti* peak, 3 to Ti3+, and 4 to Ti4+. 1 L = 1 Langmuir = 1 x Torr s.
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of ~ ( 2 x 2 images ) in the nonoxidized areas from clean Pt3Ti(1 1l), as can be seen from high resolution image in Figure 4 (bottom). In contrast, the oxide overlayer covers the whole surface with no p(2 x 2) images when the crystal was exposed to 0 2 above 450 OC.
4. Discussion Evidence has been found for oxygen-induced segregation of titanium to form titanium oxides on the (1 11) surface of Pt3Ti. Such segregation of the most reactive component of an alloy to form an oxide overlayer is expected from thermodynamics and has been observed for this and other binary metal
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Binding Energy (eV) Figure 2. PES spectra at grazing emission from Pt3Ti( 11 1): (a) clean surface; (b) oxidized in 0 2 (50 L) at 375 OC; (c) reduced by CO (100
L). energy of Ti3+ 2 ~ 3 is, ~456.6 eV, and the intensity of Ti* 2p significantly weakens with the increase of the temperature. Figures 2 and 3 present Ti 2p photoelectron spectra at grazing emission from clean Pt3Ti( 111) and PtsTi( 111) oxidized in 0 2 at 375 and 800 "C, followed by reduction with CO. The reduction cycle was done by exposure at cryogenic temperature and flashing in vacuo to above the desorption temperature for excess C0.53738Apparently, the oxide in the Ti4+ state produced at low temperature can be reduced to Ti3+ by CO while the oxide formed at higher temperature is irreducible under these mild conditions. 3.2. Scanning Tunneling Microscopy. The STM images of PtsTi(111) oxidized in 0 2 below 400 "C and above 450 "C, corresponding to the above two titanium oxides, are demonstrated in Figures 4 and 5. These images clearly show that the oxide formed at 'low temperature exhibits a distinctly different morphology compared to the oxide produced at high temperature. Below 400 "C, the oxide grows in the form of islands on the surface with no site-blocking effect on the remaining "open" areas of the crystal. This is evidenced by the existence
Photoemission and STM measurements clearly reveal two distinctly different oxidation processes. Below 400 "C a dominant four-valent oxide, assigned as Ti02, is formed while above 450 "C a trivalent oxide, assigned as Ti2O3, is formed. A small portion of Ti203 is also formed below 400 "C, but this portion shrinks with increasing oxidation temperature. The Ti02 oxide can be readily reduced by the adsorption of a monolayer of CO. S T M shows that this oxide nucleates preferentially at defects in contrast to the oxide formed by oxidation above 450 "C. The latter trivalent oxide grows from step edges and is irreducible under the above mild conditions. This is attributed to a different nucleation process, in tum an effect of the interfacial energies. Obviously, it also requires more energy to reduce the three-valent oxide than the four-valent oxide. The interfacial energies also govem the growth modes, islands with poor wetting to the metal substrate for Ti02 and well-dispersed overlayers for Ti2O3. The reduced intensity of the Ti* 2~312 signal comes from inelastic scattering of photoemitted electrons traversing the well-dispersed Ti203 overlayer. CO titration of bare metallic patches agrees with the STh4 images and indicates that remaining open areas during low temperature oxidation are neither blocked nor electronically effected by the oxide island^.^*^,* These areas can readily adsorb CO at low temperature, and this accounts for the high probability for reduction of the four-valent oxide. Our conclusion is also supported by vibrational spectra of adsorbed CO and of the reducible oxide during reduction/oxidation cycle^.^ The growth of the three-valent oxide blocks CO adsorption sites. It should be noted that after oxidation of Pt3Ti( 111) the Ti4+ and Ti3+binding energies of Ti 2p core levels for the two oxides are found at lower values compared to those of bulk Ti02 and
12894 J. Plzys. Chem., Vol. 99, No. 34, 1995
Figure 4. (Top) STM image of Pt3Ti( 1 I 1) oxidized in O2 (50 L) at 350 "C. The surface is partly covered by oxide islands with open patches of the metal surface in between. The clean surface displays a p(2 x 2) pattern. (Bottom) High-resolution image of Pt3Ti( 1 11) oxidized in 0 2 (50 L) at 350 "C.
Chen et al. (spectra not shown). These changes clearly indicate that interaction between titanium oxides and platinum atoms occurs upon the oxidation of Pt3Ti(ll l), probably due to titanium oxides in contact with platinum atoms. Such a Pt-Ti bond at the interface between the Pt3Ti( 111) substrate and titanium oxides should be partially ionic with Pt negatively charged.I6ll7 Our observations support previous work on TiOzPt, which revealed bonding between titanium oxide and metal surfaces.'* The oxide-metal interaction influences platinum atoms which indicates that the titanium oxide rides atop a film of pure platinum, segregated from the bulk Pt3Ti alloy, similar to the situation demonstrated for the clean Pt3Ti( 111) s u r f a ~ e . ~ ~ ' ~ J ~ Titanium atoms in Pt3Ti do not display a surface core level shift for clean Pt3Ti( 111) nor does the metallic Ti* 2p3/2 level exhibit an interfacial shift for Pt3Ti covered by an oxide overlayer. Hence, we can conclude that the surface of Pt3Ti concists of pure platinum both toward a reducing gas atmosphere and toward a surface-segregated oxide. The formation of different titanium oxides under different oxidation conditions is understandable from the kinetics of diffusion. The above mentioned oxygen-induced segregation of titanium should be governed by the diffusion of titanium in the Pt3Ti alloy such that the amount of segregation of titanium will increase with the oxidation temperature. It is known that the main structural difference between Ti203 and Ti02 is the coordination of oxygen atoms by titanium atoms.20 The oxygen atoms are coordinated by four titanium atoms in Ti203 and by three titanium atoms in TiO2. We believe that the preference of Ti203 formation at high temperature is attributed to the increased coordination number of oxygen atoms by diffusion of more titanium atoms from the bulk. At low temperature the titanium atoms mostly diffuse from the region near the surface, i.e., from the outermost layers of the alloy. The formation of two different oxides is of relevance for the preparation of titania-supported catalysts or bimetallic catalysts containing titanium. The four-valent oxide must be considered an active part in any catalytic reaction under reducing or alternating reducing and oxidizing atmospheres. The trivalent oxide can, in comparison, be characterized as an inactive carrier for an active component. The two oxides have different wetting behavior toward a metallic phase, and this will affect the dispersion and the electron structure of a titania-supported catalyst. 5. Conclusions
Figure 5. STM image of Pt3Ti( 1 1 1) oxidized in 0 2 (50 L) at 625 "C. The oxide wets the metal surface with no open patches.
XPS and STM have been used to characterize the surface of Pt3Ti( 111) following oxidation and reduction cycles. Oxidation of Pt3Ti( 111) in 0 2 segregates titanium to the surface. The oxide formed at low temperature (
