Reducibility of the Support

Reducibility of the Support. - The creation of anion vacancies in the surface of Ti02 is believed to be an essential first step in the development of TiOj is known to be reducible, and will lose O2 even during heating in vacuum. Reduction in H2 is fairly slow at low temperatures and 

Gajardo, Proceedings 7th International Congress on Catalysis, Tokyo, 1980, p.1462. 

Rh/TiOa catalyst at 623 K. They attribute this to an electron transfer from Ti to a Rh particle. If this is the correct explanation why Apple etal. find no e.s.r. signal after R673, it is significant to note that their Rh/Ti02 catalyst still adsorbed H2 normally. 

---

In the geometrical, covering theory the reducibility of the support, as well as the formation of surface defects by dehydration at high temperatures, are essential to explain the migration of reduced support species onto the metal particles (5., 7., 14). 

Ceria-modified mesoporous zirconia catalysts show much higher activity than pure mesoporous zirconia catalysts especially at lower reaction temperatures. The characterization results reveal that ceria decreases the degree of zirconia crystallinity and particle size. Also ceria interacts with the mesoporous zirconia and causes the strong effect on the reducibility of the support. The high and stable WGS activity could be related to the high stability of the gold dispersion and to larger number of active sites located at the Au/ceria-modified mesoporous zirconia interface. 

The ease of reduction and subsequent performance of the catalyst will be strongly influenced by the extent and nature of the interaction between the metal and the oxide support. In ammonia synthesis studies the most commonly used oxides have been alumina and magnesia. " Some work has also been carried out with basic materials such as calcia, etc., although they have little commercial potential due to their low surface areas. In the case of magnesia and alumina, high and reasonably stable dispersions can be produced but complete reduction is sometimes very difficult to achieve. In the case of coprecipitated iron/alumina the spinels formed can only be reduced at temperatures above 1073 The reducibility of the supported systems also tends to be retarded to a greater extent by low vapor pressures of water than less highly dispersed catalysts. ... 

The quality control of galvanic anodes is reduced mainly to the analytical control of the chemical composition of the alloy, to the quality and coating of the support, to an adequate joint between support and anode material, as well as to restricting the weight and size of the anode. The standards in Refs. 6, 7, 22, 27, 31 refer to magnesium and zinc anodes. Corresponding specifications for aluminum anodes do not exist. In addition, the lowest values of the rest potentials are also given [16]. The analytical data represent the minimum requirements, which are usually exceeded. 

The angle of repose is sensitive to the conditions of the supporting surface the smoother the surface, the smaller the angle. The angle may also be reduced by vibrating the supporting surface. 

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. 

Similarly, Pd, Ag, and Pd-Ag nanoclusters on alumina have been prepared by the polyol method [230]. Dend-rimer encapsulated metal nanoclusters can be obtained by the thermal degradation of the organic dendrimers [368]. If salts of different metals are reduced one after the other in the presence of a support, core-shell type metallic particles are produced. In this case the presence of the support is vital for the success of the preparation. For example, the stepwise reduction of Cu and Pt salts in the presence of a conductive carbon support (Vulcan XC 72) generates copper nanoparticles (6-8 nm) that are coated with smaller particles of Pt (1-2 nm). This system has been found to be a powerful electrocatalyst which exhibits improved CO tolerance combined with high electrocatalytic efficiency. For details see Section 3.7 [53,369]. 

A striking difference in selectivity was observed. According to the non-acidic character of the support Cu/Si02 showed to be an excellent hydrogenation catalyst. The conjugated double bond was selectively reduced giving citronellal with fairly good selectivity and then citronellol. 

A significant increase of activity can be observed on Rh catalysts supported on reducible oxides. Activity exaltation is severely annihilated when the catalysts are treated in the reaction mixture. Nevertheless, the presence of chlorine largely upset the results. Cl probably slows down the reduction of the support, particularly in the CO+NO mixture. 

The promotion by reduced ceria could be due to a spillover phenomenon of O species from metal to support. In fact, this is not sufficient to explain all the results of Mullins and Overbury. An exposure of the Rh/CeOx surface to water leads to a re-oxidation accompanied by a hydroxylation of the support while the metal surface is left unchanged. In fact, it seems that preferential orientation of Rh surface on reduced ceria may also explain the specific role of CeOx surface. This is consistent with the fact that NO dissociation occurs at lower temperatures on Rh (110) and on Rh (100) than on Rh (111) [83,84],... 

---