Redox Properties of Titanium Oxides on Pt3Ti

Abstract

The morphology and electronic structure of surface-segregated titanium oxides on Pt3Ti( 1 1 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. IntroductionThe 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-segregated oxides and their redox behavior on a Pt3Ti( 11 1) support. This is closely linked to the carrier's role as an active partner in titania-supported metal catalysts for CO hydrogenation.

Structural and compositional characterization of ultrathin titanium oxide films grown on Pt3Ti(111)

“…Yet, another interesting approach is the controlled oxidation of an alloy containing the respective metal as the constituent with the highest heat of oxide formation. This approach has proved to be very successful, for instance, in the formation of well defined aluminum oxide films on NiAl(110) [18] and Ni 3 Al(111) [19,20], and was also reported for the formation of titanium oxide films on a Pt 3 Ti(111) surface [7,[21][22][23][24][25][26][27][28][29][30][31][32][33] prior to the present study. The driving force for the formation of the TiO x overlayers originates from the respective heats of formation, which are much more negative than those of platinum oxides (TiO: −123.9 kcal mol −1 , TiO 2 : −224.9 kcal mol −1 , Ti 2 O 3 : −363.0 kcal mol −1 , Ti 3 O 5 : −586.7 kcal mol −1 , compared to, e.g., PtO: −17.0 kcal mol −1 , PtO 2 : −32.0 kcal mol −1 [34]) and also more negative than the heat of formation of the Pt 3 Ti alloy itself (−81.7 kcal mol −1 [35]).…”

Adsorbate-Induced Changes in the Surface Composition of Bimetallic Clusters: Pt−Au on TiO2(110)

“…The appearance of Pt at the surface of the bimetallic clusters is not well understood. We speculate that it might be related to the formation of Pt−Ti alloys, − which have been reported for TiO x films grown on Pt surfaces, but in any case this diffusion of Pt to the surface upon annealing is a small effect. For the pure 0.25 ML Au clusters (Figure b), the ∼40% loss in Au signal during heating to 1000 K is attributed to a combination of cluster sintering and Au desorption, which has an onset temperature of 1000 K .…”

“…The appearance of Pt at the surface of the bimetallic clusters is not well understood. We speculate that it might be related to the formation of Pt-Ti alloys, [50][51][52] which have been reported for TiO x films grown on Pt surfaces, 53 but in any case this diffusion of Pt to the surface upon annealing is a small effect. For the pure 0.25 ML Au clusters (Figure 5b), the ∼40% loss in Au signal during heating to 1000 K is attributed to a combination of cluster sintering and Au desorption, which has an onset temperature of 1000 K. 20 Similar changes in Au LEIS signal are observed for all of the Pt-Au bimetallic clusters although the signal drops off slightly more quickly for the Pt-rich clusters between 600 and 800 K. Note that the Au signal intensity is larger for the 75% and 50% Au clusters compared to the pure Au clusters.…”

Scanning Tunneling Microscopy Investigation of Ultrathin Titanium Oxide Films Grown on Pt3Ti(111)

“…Pt 3 Ti crystallizes, like Ni 3 Al, in the fcc Cu 3 Au structure. The bulk truncated (111) surface exhibits a p(2 × 2) superstructure of titanium atoms. − In a recent paper we have presented a detailed atomic emission spectroscopy (AES), low-energy electron diffraction (LEED), ultraviolet photoelectron spectroscopy (UPS), and high-resolution electron energy loss spectroscopy (HREELS) study of the surface oxidation, which disclosed great similarities to the long-range properties of the films obtained by reactive Ti evaporation on Pt(111) . In this paper we present STM measurements on the atomic structure of the obtained titania phases on Pt 3 Ti(111) and compare them to the work of Granozzi et al − on TiO x /Pt(111) as well as to different vanadium oxide phases. − Thereby, we show the similarities but, in particular, also emphasize the differences of the various titanium oxide films prepared by the two distinct approaches.…”