Titania hydrogen form

As catalysis proceeds at the surface, a catalyst should preferably consist of small particles with a high fraction of surface atoms. This is often achieved by dispersing particles on porous supports such as silica, alumina, titania or carbon (see Fig. 1.2). Unsupported catalysts are also in use. The iron catalysts for ammonia synthesis and CO hydrogenation (the Fischer-Tropsch synthesis) or the mixed metal oxide catalysts for production of acrylonitrile from propylene and ammonia form examples. 

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. 

In the case of anatase allotropic form of titania, the redox potential for the photogenerated holes vs. the standard hydrogen electrode is of +3.1 V, and that for the conduction band electrons of +0.5 V (Figure 12.2). These values show that holes created by light excitation have a strong oxidizing potential. 

The reaction of propylene on ZrC>2 exhibits the same characteristics as on other oxides. Propane-d2, for example, is selectively formed in the deuteration process, with no hydrogen exchange in propylene215. New features appear, however, when zirconia is dispersed on other oxides (alumina, silica, titania)215,216. A considerable rate increase is observed and exchange in propylene proceeds simultaneously with addition via the associative mechanism through the common intermediate n-propyl and s-propyl species. 

A large number of heterogeneous catalysts have been tested under screening conditions (reaction parameters 60 °C, linoleic acid ethyl ester at an LHSV of 30 L/h, and a fixed carbon dioxide and hydrogen flow) to identify a suitable fixed-bed catalyst. We investigated a number of catalyst parameters such as palladium and platinum as precious metal (both in the form of supported metal and as immobilized metal complex catalysts), precious-metal content, precious-metal distribution (egg shell vs. uniform distribution), catalyst particle size, and different supports (activated carbon, alumina, Deloxan , silica, and titania). We found that Deloxan-supported precious-metal catalysts are at least two times more active than traditional supported precious-metal fixed-bed catalysts at a comparable particle size and precious-metal content. Experimental results are shown in Table 14.1 for supported palladium catalysts. The Deloxan-supported catalysts also led to superior linoleate selectivity and a lower cis/trans isomerization rate was found. The explanation for the superior behavior of Deloxan-supported precious-metal catalysts can be found in their unique chemical and physical properties—for example, high pore volume and specific surface area in combination with a meso- and macro-pore-size distribution, which is especially attractive for catalytic reactions (Wieland and Panster, 1995). The majority of our work has therefore focused on Deloxan-supported precious-metal catalysts. 

Conclusions. Submonolayer deposits of titania grow on the surface of Rh in the form of two-dimensional islands until a coverage of nearly a monolayer is achieved, at which point some three-dimensional growth of the islands is observed. The titania islands exclude CO chemisorption on Rh sites covered by the titania. The Ti + ions in the overlayer are readily reduced to TP+. This process begins at the perimeter of the islands and extends inwards as reduction proceeds. Titania promotion of Rh enhances the rate of CO hydrogenation by up to a factor of three and increases the selectivity to C2+ hydrocarbons. By contrast, the activity of Rh for the hydrogenolysis of ethane decreases monotonically with increasing titania promotion. 

The influence of the support is undoubted and spillover was further confirmed by the excess of hydrogen chemisorbed by a mechanical mixture of unsupported alloy and TJ-A1203 above that calculated from the known values for the separate components. It was also observed that the chemisorption was slower on the supported than on the unsupported metal and that the greater part of the adsorbate was held reversibly no comment could be made on the possible mediation by traces of water. On the other hand, spillover from platinum-rhenium onto alumina appears to be inhibited for ratios Re/(Pt Re) > 0.6. In an infrared investigation of isocyanate complexes formed between nitric oxide and carbon monoxide, on the surface of rhodium-titania and rhodium-silica catalysts, it seems that the number of complexes exceeded the number of rhodium surface atoms.The supports have a pronounced effect on the location of the isocyanate bond and on the stability of the complexes, with some suggestion of spillover. 

Finally, one may ask why the attack of titania by nickel (e.g., formation of pits) has not been observed in previous studies where nickel/titanium oxide samples had been treated in hydrogen and examined by electron microscopy (6,12,13). In these previous cases, the nickel particles were considerably smaller (5 to 10 nm) than the ones formed in the present experiments ( 100 nm) and therefore the extent of reaction may have been only sufficient to cause removal of a few monolayers of titania, which would be difficult to detect. 

Also, Marsh and co-workers [145] showed that gold on cobalt oxide particles, supported on a mechanical mixture of zirconia-stabilised ceria, zirconia and titania remains active in a gas stream containing 15 ppm SO2. Haruta and co-workers [207] found that although the low-temperature CO oxidation activity of Ti02-supported Au can be inhibited by exposure to SO2, the effect on the activity for the oxidation of H2 or propane is quite small. Venezia and co-workers [208] reported that bimetallic Pd-Au catalysts supported on silica/alumina are resistant to sulphur poisoning (up to 113 ppm S in the form of dibenzothiophene) in the simultaneous hydrogenation of toluene and naphthalene at 523 K. 

A few reports in the last two decades have described exciton confinement in small TiOi particles. Anpo and co-workers (1987) prepared particles of titania with sizes (diameters) ranging from 55 A to 2000 A for rutile, and from 38 A to 530 A for the anatase form of TiOi, and noted that these crystallites display size quantisation in optical properties and in the photocatalytic hydrogenation of methylacetylene, even for such large sizes. For a 120 A rutile particle the bandgap increased by 67 meV relative to the bulk rutile bandgap Eg 3.03 eV) for anatase Eg increased by 156 meV (from the bulk Eg of -3.18 eV). Moreover, quantum yields of photocatalytic activity of TiOi appeared to increase with the magnitude of the blue shift of the effective bandgap. Kormann et at. (1988) indicated that small particles of TiOi, prepared by the arrested hydrolysis of either TiCU or Ti(i-PrO)4, showed size... 

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