Titania surface

Thermal desorption spectra of CO2 from a titania surface are shown in figure 2. It revealed two desorption peaks at temperature ca. 175 and 200 K. As reported, surface of titania have two structures which is similar to the results fomd by Tracy et al. [7]. Based on their study, it was confirmed that one peak at ca. 170 K was attributed to CO2 molecules bound to regular five-coordinate Ti site considered as the perfected titania structure. The second peak at ca. 200 K considered as the CO2 molecules bound to Ti referred to the... 

In our experiment, photocatalytic decomposition of ethylene was utilized to probe the surface defect. Photocatalytic properties of all titania samples are shown in table 2. From these results, conversions of ethylene at 5 min and 3 hr were apparently constant (not different in order) due to the equilibrium between the adsorption of gaseous (i.e. ethylene and/or O2) on the titania surface and the consumption of surface species. Moreover it can be concluded that photoactivity of titania increased with increasing of Ti site present in titania surface. It was found that surface area of titania did not control photoactivity of TiOa, but it was the surface defect in titania surface. Although, the lattice oxygen ions are active site of this photocatalytic reaction since it is the site for trapping holes [4], this work showed that the presence of oxygen vacancy site (Ti site) on surface titania can enhance activity of photocatdyst, too. It revealed that oxygen vacancy can increase the life time of separated electron-hole pairs. 

Lanthanides in combination with transition metals have been shown to have a positive effect in promoting heterogeneous catalytic reactions. The bimetallic Yb—Pd catalyst obtained from the precursor (pMF)i0Yb2 Pd(CN)4]3 K on a titania surface offers improved performance over a palladium-only catalyst for the reduction of NO by CH4 in the presence of 02.99 100 The structure, shown in Figure 6, consists of two inverted parallel zigzag chains that are connected through the lanthanide atoms by trans-bridging [Pd(CN)4]2- anions.101... 

Figure 6 Bimetallic Yb—Pd catalyst obtained from the precursor (DMF)1oYb2[Pd(CN)4]3

To see that this is true, qualitative models of the titania surface produced following each of the three pretreatments used in this study (i.e., evacuation at 600 K, evacuation at 720 K, and hydrogen reduction at 720 K) are developed below on the basis of earlier studies of Ti02 surfaces (5-6,10-22). Surface Type I Following outgassing at about 600 K the Ti02 surface should be almost entirely free of molecular water (except on the rutile fraction), but about one half of the surface should be covered... 

An appreciable amount of Ti + may also exist in such samples. Indeed, materials treated at this temperature are gray, suggesting the presence of reduced forms of titania. Surface Type III Hydrogen reduction at 720 K probably produces a surface in which there is no molecular water and only a small number of hydroxyl groups. Furthermore, the surface following this treatment may have a high concentration of Ti + species. Materials treated in this manner were found to be pastel blue in color. The surface "type" of each sample is given in Table I and II. 

Figure 10.15 CCT STM image (/ = 2.0 nA, V = +2.0 V) and the corresponding STS data acquired for Au clusters of varying sizes on the Ti02(110)-(1 x 1) surface. The STS curve of the bare titania surface is also shown. (Reprinted from Meier, D.C. et al., in Surface Chemistry and Catalysis, A.F. Carley et al., Eds., Kluwer, New York, 2002, pp. 147-189. With permission from Springer Science and Business Media.)...

Homogeneous, nanosized, copper-loaded anatase titania was synthesized by an improved sol-gel method [197], These titania composite photocatalysts were applied to the photoreduction of carbon dioxide to evaluate their photocatalytic performance. Methanol was found to be the primary hydrocarbon product [198], Under calcination conditions, small copper particles are well dispersed on the surface of anatase titania. According to XAS and XPS analysis, the oxidation state of Cu(I) was suggested to be the active species for C02 photoreduction [199], Higher copper dispersion and smaller copper particles on the titania surface are responsible for a great improvement in the performance of C02 photoreduction. 

Kinetics of the photooxidation of organic water impurities on illuminated titania surfaces has been generally regarded to be based on the Langmuir-Hinshelwood equation with first-order reaction kinetics vs. initial substrate concentration was established univocally by many authors... 

The solid-state Si SPE NMR spectra of SBA-15 and the titania surface-coated SBA-15 (Ti-SBA-15) are in accord with this expectation. The spectrum of SBA-15 displays a broad as)mimetric peak at 109 ppm (Q" sites) with shoulders at —101 ppm (Q sites) and 90 ppm(Q sites) in the area ratio 79 19 2. The NMR spectrum of Ti-SBA-15 (one layer) shows a reduction of the band intensity relative to the intensity. The normalized Q Q Q site populations become 85 13 2. No asymmetry is observed in the Q site band. Repetition of the monolayer deposition to form a double layer of titania on silica yields a material whose Si NMR spectrum is indistinguishable from that of the Ti-SBA-15 with a monolayer coverage. As expected, the titania-insulated silica resonances are unperturbed by the second titania layer. 

Scheme 1.5 Silica, alumina and titania surface oxygens behaving as ligands in the M.L.H. Green formalism [9] after reaction of r -tris(allyl)rhodium with a partially dehydroxylated surface [39].

In conclusion, the EPR results demonstrate the presence of tetravalent vanadium atoms in some sites of the sample. The Vlv atoms could be included in a new compound, as in the NaV5vV,vC>15 vanadate identified in the sample by X-ray spectroscopy. They could also interact with the zeolite surface as reported for V205 supported on silica or n alumina (22) and titania surfaces (23). 

Unfortunately, the redox potential of the Pt4 + /3+ couple is not known in literature. Although some stable Ptm compounds have been isolated and characterized (37), the oxidation state III is reached usually only in unstable intermediates of photoaquation reactions (38-40) and on titania surfaces as detected by time resolved diffuse reflectance spectroscopy (41). To estimate the potential of the reductive surface center one has to recall that the injection of an electron into the conduction band of titania (TH) occurs at pH = 7, as confirmed by photocurrent measurements. Therefore, the redox potential of the surface Pt4 + /3+ couple should be equal or more negative than —0.28 V, i.e., the flatband potential of 4.0% H2[PtClal/ TH at pH = 7. From these results a potential energy diagram can be constructed as summarized in Scheme 2 for 4.0% H2[PtCl6]/TH at pH = 7. It includes the experimentally obtained positions of valence and conduction band edges, estimated redox potentials of the excited state of the surface platinum complex and other relevant potentials taken from literature. An important remark which should be made here is concerned with the error of the estimated potentials. Usually they are measured in simplified systems - for instance in the absence of titania - while adsorption at the surface, presence of various redox couples and other parameters can influence their values. Therefore the presented data may be connected with a rather large error. 

Gan S., Liang Y., Baer D.R. et al. (2001) Effect of Platinum Nanocluster Size and Titania Surface Structure upon CO Surface Chemistry on Platinum-Supported Ti02 (110), J. Phys. Chem. B. 105(12), 2412-2416. 

Volumetric measurements at room temperature showed that by far the greater part of the adsorption of carbon monoxide on Au/TiC>2 occurred on the support it followed the Langmuir equation and most of it was removable by pumping.23,83 About one-third of the titania surface was able to retain it, but there was little uptake on Au/SiC>2. Use of the double-isotherm method with Au/MgO showed that adsorption onto the metal was complete at about 1 atm, but on various samples the coverage never rose above 18%. On model Au/MgO(100) the maximum coverage attained using a pulsed molecular beam at room temperature was < 10%.54... 

Bulk Au is a noble metal. Goodman and co-workers,1301 however, found that Au nanocrystals supported on a titania surface show a marked size-effect in their catalytic ability for CO oxidation reaction, with Au nanoparticles in the range of... 

Pelizetti et al. detailed the photocatalytic mineralisation of ortho-, para-and mefa-cresols [116]. As with chlorophenol, methyl catechol and hydro-quinone were detected as major by-products. The o- and p-crcsols were observed to degrade with first order kinetics while the m-cresol degradation followed zero order kinetics at pH 3. Under 3 h photocatalysis time in alkaline conditions m-cresol kinetics become first order which was proposed to be due to lower surface coverage of the cresol at this pH due to electrostatic repulsion with the titania surface. In air saturated solutions the time for mineralisation was around 8 h. The mineralisation time was, however, reduced to 2.5 h in suspensions sparged with oxygen. 

Figure 10.6. Surfactant demand curves of 70% (weight) dispersions of Ti02 pigments with various levels of alumina treatment. Dispersions were prepared in DIDP plasticizer on a highspeed disk mill. Survactant used was Disperbyk I. Pigment A titania surface, no alumia surface treatment. Pigment B 1.5% alumina surface treatment, minimum viscosity achieved at 2.36% surfactant based on pigment weight. Pigment C 3.0% alumina surface treatment, minimum viscosity at 3.9% surfactant.

---