Model TiO2 surface

The tltanla-based thin film catalyst models were constructed by first oxidizing the titanium surface In 5 x 10 torr of O2 for approximately 30 minutes at 775 K. This produced an AES llneshape consistent with fully oxidized TIO2. The metal was then vapor deposited onto the oxide support with the latter held at 130 K. The thickness of the metal overlayer and Its cleanliness were verified by AES. After various annealing and adsorption procedures, these thin films were further characterized using SSIMS, AES and TDS. For comparison, some work was done with Pt on Al20s. In this case a Mo foil covered with AI2O3 replaced the Tl(OOOl) substrate. 

Bowker M, Stone P, Morrall P, et al. Model catalyst studies of the strong metal-support interaction surface structure identified by STM on Pd nanoparticles on TiO2(110). J Catal. 2005 234 172-81. 

Hydration may modify the reactivity if the metallic sites are not available the acidity of the surface hydroxyl is less than that of the exposed metallic cations, H2O molecular adsorption on hydrated TiO2(H0) surface can result from these weak interactions (H-bonding with the surface hydroxyl groups)[22]Lindan, 1998 223], In this case, there is no direct interaction and the vibration frequencies should be recalculated on another model. Let us note that the basicity of the surface also varies in the case of Ti02(l 10) the lone pairs of a surface hydroxyl are less reactive than those of a naked surface oxygen for MgO it is the reverse the hydrated surface becomes more basic. 

Fig. 4. Model of the TiO2(110) surface. The relaxations of surface atoms, determined with SRXD are indicated [34]. The labels refer to the relaxations listed in Table 1.

Since the parallel components of the dynamic dipole are active in RAIRS, it is possible to use the azimuthal dependence to obtain the orientation of the adsorbate at the surface. A similar technique has been applied to adsorbates on metals in HREELS measurements made off specular in order to observe parallel modes through impact or resonant scattering processes. This was first demonstrated for the Rh(CO)2 molecule on anisotropic TiO2(110) surface [72]. The results of this study also allow a test of the three layer model theory (Fig.5,6) as applied to S-polarised radiation. Fig. 11 shows the FT-RAIRS spectrum for 1/3 monolayer of Rh(CO)2 on Ti02(l 10) measured with P and S polarised radiation. 

Photoreactions of organic compounds over model surfaces of wide band-gap oxide semiconductors have received considerable attention recently [43, 79-82]. The most-studied photocatalytic reactions on rutile TiO lllO) single-crystal surfaces include ethanol [43], acetic acid [78], trimethyl acetic acid [80, 81], and acetone [82]. In this section, we will focus on the photoreaction of ethanol over TiOj(llO). Ethanol is dissociatively adsorbed via its oxygen lone pair on fivefold coordinated Ti atoms to produce adsorbed ethoxide species (Fig. 7.6). STM studies of the adsorption of ethanol on TiO2(110) demonstrated the presence of both alkoxides and surface hydroxyls [83] confirming the adsorption is dissociative. Figure 7.11 is the XPS Cls spectra after the exposure of ethanol (9=0.5 with respect to Ti atoms). 

Figure 35 STM image (14 x 14nm ) of a stoichiometric 1x1 rutile TiO2(H0) surface. Dark rows on the terraces correspond to bridging oxygen rows, while the bright rows are due to titanium rows. The inset shows a ball-and-stick model of the rutile TiO2(H0)-(l x 1) surface. Large balls represent oxygen atoms, and small balls represent titanium atoms. (From Ref. 68.)...

Figure 18.1 Surface structure ofTi02(l 10) characterized by STM and non-contact atomic force microscope (NC-AFM). (a) An empty state STM image (7.3 x 7.3 nm2, Vs = 1.2 V, It = 0.15 nA) of a TiO2(110) surface at RT. (b) A structure model of a clean Ti02(l 10) surface. The fivefold coordinated Ti atoms are visualized, (c) A perspective view of a TiO2(110) surface with oxygen defect.

It has been emphasized by Bard et al. that there may be exceptions to the model derived above, insofar as Fermi level pinning by surface states may occur in a similar fashion to that at semiconductor-metal junctions [33]. Such an effect would lead to an unpinning of bands at the interface. There are some examples in the literature, such as FeS2 in aqueous solutions [34, 35] and Si in methanol [36] for which an unpinning of bands has been reported. In some cases, such as TiO2, experimental values of flatband potentials scatter considerably. This is mostly due to changes in surface chemistry and doping profiles. 

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