Rutile phase

The heavier metal tantalum is distinctly less inclined than niobium to form oxides in lower oxidation states. The rutile phase TaOz is known but has not been studied, and a cubic rock-salt-type phase TaO with a narrow homogeneity range has also been reported but not yet fully characterized. TazOs has two well-established polymorphs which have a reversible transition temperature at 1355°C but the detailed structure of these phases is too complex to be discussed here. 

Taking titanium dioxide as an example, we may mention that PMC transients decay rapidly in the rutile phase (10"6 s) and much slower in the (catalytically more active) anatase phase (10"2-1 s).35 When a Ti02... 

However, no rutile phase is shown even though the molar ratio of TENOH/TTIP increased up to 1.0(Fig. 1(b)). In addition, the anatase particles with a high degree of crystallinity are formed compared to the HNOs-peptized particles. It is thought that the existence of OH- groups in the peptized sols can enhance the growth of anatase particles. 

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. 

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

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 present research showed a dependence of various ratios of rutile anatase in titania as a catalyst support for Co/Ti02 on characteristics, especially the reduction behaviors of this catalyst. The study revealed that the presence of 19% rutile phase in titania for CoATi02 (C0/RI9) exhibited the highest number of reduced Co metal surface atoms which is related the number of active sites present. It appeared that the increase in the number of active sites was due to two reasons i) the presence of ratile phase in titania can fadlitrate the reduction process of cobalt oxide species into reduced cobalt metal, and ii) the presence of rutile phase resulted in a larger number of reduced cobalt metal surface atoms. No phase transformation of the supports further occurred during calcination of catalyst samples. However, if the ratios of rutile anatase were over 19%, the number of active sites dramatically decreased. 

The weight fractions of the anatase and rutile phases in the samples were calculated from the relative intensities of the strongest peaks in XRD patterns corresponding to anatase and rutile as described by Spurr and Myers [4]. 

After gas-phase oxidation reaction finished, the reactor wall surfece was coated with a thick rough scale layer. The thickness of scale layer along axial direction was varied. The scale layer at front reactor was much thicker than that at rear. The SEM pictures were shown in Fig. 1 were scale layers stripped from the reactor wall surface. Fig. 1(a) was a cross sectional profile of scale layer collected from major scaling zone. Seen from right side of scale layer, particles-packed was loose and this side was attached to the wall surface. Its positive face was shown in Fig. 1(b). Seen from left side of scale layer, compact particles-sintered was tight and this side was faced to the reacting gases. Its local amplified top face was shown in Fig. 1(c). The XRD patterns were shown in Fig. 2(a) were the two sides of scale layer. Almost entire particles on sintered layer were characterized to be rutile phase. While, the particle packed layer was anatase phase. 

In general, the increase of preparation or calcination temperature helps to increase the crystallite size of anatase titania. However, anatase phase is thennally unstable and is easily converted to rutile phase. Moreover, reactive surface area decreases with increasing the... 

The properties of titania particles were investigated using X-ray diffraction (XRD, Model D/MAX-RB, Rigaku Ltd.), scanning electron microscopy (SEM, Model 535M, Philips Ltd.), transmission electron microscopy (TEM, Model 2000EX, JEOL Ltd.). The crystallite sizes were estimated by Scherrer s equation and the composition of rutile phase in titania were estimated from the respective integrated XRD peak intensities. 

Fig. 2. Average anatase crystallite sizes and fractions of rutile phase as a function of preparation temperature in conventional spray pyrolysis.

In 2006, Iida et a/.497,498 examined low temperature water-gas shift over Pt/Ti02 rutile phase catalysts. The authors tried a number of different Pt precursors, and concluded that the reaction over Pt/Ti02 was structurally insensitive relative to Pt, as a linear relationship was observed between dispersion and activity, as shown in Table 112. 

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