Supports titania

Chromium Oxide-Based Catalysts. Chromium oxide-based catalysts were originally developed by Phillips Petroleum Company for the manufacture of HDPE resins subsequendy, they have been modified for ethylene—a-olefin copolymerisation reactions (10). These catalysts use a mixed sihca—titania support containing from 2 to 20 wt % of Ti. After the deposition of chromium species onto the support, the catalyst is first oxidised by an oxygen—air mixture and then reduced at increased temperatures with carbon monoxide. The catalyst systems used for ethylene copolymerisation consist of sohd catalysts and co-catalysts, ie, triaLkylboron or trialkyl aluminum compounds. Ethylene—a-olefin copolymers produced with these catalysts have very broad molecular weight distributions, characterised by M.Jin the 12—35 and MER in the 80—200 range. 

The preparation method of titania support was described in the previous paper [6]. Titanium tetraisopropoxide (TTIP 97%, Aldrich) was used as a precursor of titania. Supported V0x/Ti02 catalysts were prepared by two different methods. The precipitation-deposition catalysts (P-V0x/Ti02) were prepared following the method described by Van Dillen et al. [7], in which the thermal decomposition of urea was used to raise homogeneously the pH of a... 

The present study revealed effects of various rutile/anatase ratios in titania on the reduction behaviors of titania-supported cobalt catalysts. It was found that the presence of rutile phase in titania could facilitate the reduction process of the orbalt catalyst. As a matter of fact, the number of reduced cobalt metal surface atoms, which is related to the overall activity during CO hydrogenation increased. 

The various ratios of rutileranatase in titania support were obtained by calcination of pure anatase titania (obtained fi om Ishihara Sangyo, Japan) in air at temperatures between 800-1000°C for 4 h. The high space velocity of air flow (16,000 h" ) insured the gradual phase transformation to avoid rapid sintering of samples. The ratios of rutile anatase were determined by XRD according to the method described by Jung et al. [5] as follows ... 

Co/Rn titania support containing n% of rutile phase (R)-supported cobalt... 

Transmission electron microscopy (TEM) The dispersion of cobalt oxide species on the titania supports were determined using a JEOL-TEM 200CX transmission electron spectroscopy operated at 100 kV with 100k magnification. 

In this present study, we basically showed dependence of the number of reduced cobalt metal surface atoms on dispersion of cobalt oxides along with the presence of rutile phase in titania. Both XRD and SEM/EDX results (not shown) revealed good distribution of cobalt oxides over the titania support. However, it can not differentiate all samples containing various ratios of rutile/anatase phase. Thus, in order to determine the dispersion of cobalt oxide species on titania, a more powerful technique such as TEM was applied with all samples. The TEM micrographs for all samples are shown in Figure 1. The dark spots represented cobalt oxides species present after calcination of samples dispersing on titania consisting various... 

The commonly used catalyst today is a vanadia on a titania support, which is resistant to the high SO2 content. Usually the titania is in the anatase form since it is easier to produce with large surface areas than the rutile form. Several poisons for the catalyst exist, e.g. arsenic and potassium. The latter is a major problem with biomass fuel. In particular, straw, a byproduct from grain production, seems to be an attractive biomass but contains potassium, which is very mobile at reaction tern-... 

Figure 1. TEM image of a titania supported gold catalyst (1.7wt.% Au) prepared by deposition-precipitation (gold particle size = 5.3+ 0.3 nm, dispersion = 36%). (Reprinted from Reference [84], 2000, with permission from American Chemical Society).

Hayden BE, Pletcher D, Suchsland J-P. 2007. Enhanced activity for electrocatal)4ic oxidation of carbon monoxide on titania-supported gold nanoparticles. Angew Chem Int Ed 46 3530-3532. 

The graduation of material across a wafer was achieved using a wedge shutter controlling the deposition profile of each source independently the principle is discussed in detail elsewhere [Guerin and Hayden, 2006]. For uniform depositions such as carbon and titania support materials, the sample holder was equipped with a motor drive that allowed rotation of the substrate during deposition. 

Figure 16.6 TEM micrographs of titania-supported Au particles. The nominal thickness of An was (a) 0.13 nm (h) 0.78nm (c) 1.56nm (d) 2.33 nm. The Au deposition rate was 2.6 X 10 nms. Particle size distributions of Au for various deposition times are shown in the plot, with the distrihutions fitted to a normal Gaussian function.

Figure 16.7 Specific activity for oxygen reduction at 0.3 V vs. RHE on carbon- and titania-supported Au nanoparticles [Guerin et al., 2006b] (a) C/Au (b) TiO /Au. In the case of TiO c/Au (b), results are shown for data obtained on arrays of electrodes (O) and on rotating disk electrodes (RDE) (A).

Oxygen reduction on both carbon- and titania-supported Pt particles is dependent on particle size. A deactivation of the catalytic activity is observed for decreasing particle size on both supports. In addition, there is no evidence of any activation of the Pt above that of bulk Pt on either support. 

Oxygen reduction on both carbon- and titania-supported Au exhibits a similar dependence on particle size to that observed for Pt, namely, a decrease in activity with decreasing particle size. This decrease occurs at particle sizes below about 3 nm. In addition to the decrease in activity, a small increase in activity is also observed for titania-supported Au nanoparticles. 

CO electro-oxidation exhibits a strong particle size dependence on both carbon-and titania-supported Au catalysts a strong deactivation of the reaction is observed for particle sizes below about 3 nm. In the case of the titania supports, however, a distinct activation of the reaction is also evident. This manifests itself in a strong decrease in the overpotential for the reaction, and an increase in activity as the particle size decreases in the range 8-3 nm. The result is a maximum in the catalytic activity with particle size. 

The activation observed in titania-supported Au electrocatalysts is unlikely to arise from electronic effects in monolayer or bilayer Au [Valden et al., 1998 Chen and Goodman, 2004], since the electrocatalytic activity was correlated with the size of three-dimensional titania-supported Au particles [Guerin et al., 2006b Hayden et al., 2007a, c]. The possibility that titania-induced electronic modification of three-dimensional particles below 6.5 nm is responsible for the induced activity, however, could not be excluded. It was pointed out, though, that such electronic effects should dominate for the smaller particle regime (<3 nm), where deactivation of the Au is observed on all supports. 

Hayden BE, Fletcher D, Suchsland J-F, Williams LJ. 2009. The influence of Ft particle size on the surface oxidation of platinum—Fart 1 Reduced titania support. Fhys Chem Chem Fhys. DOI 10.1039/b817553e. 

Kantcheva, M. (2001) Identification, Stability, and Reactivity of NO, Species Adsorbed on Titania-Supported Manganese Catalysts, J. Catal., 204, 479. 

Corbella, B.M. et al., Performance in a fixed-bed reactor of titania-supported nickel oxide as oxygen carriers for the chemical-looping combustion of methane in multicycle tests, I EC Res., 45(1), 157, 2006. 

Storsaeter S., Totdal B., Walmsley J.C., Tanem B.S., and Holmen A. 2005. Characterisation of alumina-, silica- and titania-supported cobalt Fischer-Tropsch catalysts. 7. Catal. 236 139-52. 

Morales F., de Smit E., de Groot F.M.F., Visser T., and Weckhuysen B.M. 2007. Effects of manganese oxide promoter on the CO and H2 adsorption properties of titania-supported cobalt Fischer-Tropsch catalysts. J. Catal. 246 91-99. 

Madikizela-Mnqanqeni N.N. and Coville N.J. 2004. Surface and reactor study of zinc on titania-supported Fischer-Tropsch cobalt catalysts. Appl. Catal. A Gen. 272 339 46. 

Zennaro, R., Tagliabue, M., and Bartholomew, C. 2000. Kinetics of Fischer-Tropsch synthesis on titania-supported cobalt. Catal. Today 58 309-19. 

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