Catalytically active filters metal oxides

The catalytic activity of metal oxide composites depends both on the acidity and basicity of the surface that can be characterized by the level of ammonia sorption and The catalyst should adsorb 100-200 pmol/g NH3 and its pAT, should be in the range 15-17.5 [31]. Catalysts having low basicity and acidity of the snrface have too low activities, but a lot of undesired by-products (dioxane and PEGs) are formed when their acidities are too high. The activity of Al-Mg composite oxide catalyst increases with an increase in the calcination temperature and reaches a maximum of approximately 700°C [22]. Further increase in the calcination temperature results in reduced activities, attributed to the sintering of the catalyst surface. The catalyst activity increases with inaeasing aluminum content, but less narrow distributed products are formed. Filtration of the catalyst is not always possible. Therefore, addition of water (180 g of water/1063 g of oxyethylation product) and the use of activated clay or diatomaceous earth as a filter aid are proposed [33]. 

A number of metal oxides was screened upon catalytic activity for soot oxidation by means of TGA/DSC. Several metal oxides appeared to be active soot oxidation catalysts. Contact between catalyst and soot was found to play a major role in this solid-solid-gas reaction varying this contact, activities for several catalysts ranged from active to hardly any activity. It is fUrther tentatively suggested that contact of soot, deposited on catalytic coated particulate filters, is poor, which has major implications for the development of soot oxidation catalysts under diesel operation conditions. 

MnOj thick film, deposited on the surface of metal oxides such as SnO and WO3 with the purpose of reducing the interference of O3 for NO sensors, acts as a catalytically active filter as well (Pijolat et al. 2003 Viricelle et al. 2006 Zhang et al. 2012). Fortunately, O3 is unstable and can be transformed to O3 by catalysts, e.g., manganese dioxide (MnO ) (Dhandapani and Oyama 1997). The results obtained have shown that the sensors with thin MnO filters are able to reduce the O3 interference while keeping a good response to NO. ... 

In order to obtain more fundamental catalytic activity data of the catalytic materials of interest a number of model catalysts consisting of alkali metal and precious metal were prepared and tested for their ability to promote the reactions of water and carbon dioxide with solid carbon. These tests provide basic information about the ability of the catalysts to catalyse soot combustion with CO2, H2O and O2. Results are summarized in Table 2. Both alkali metal and precious metal (PM) doped supports were used. Two supports were used which can be categorised as an inert and a reducible oxide support. Clearly the presence of the alkali metal has a significant effect on catalysing the soot combustion as anticipated. The effect of the reducible oxide support is not significant. In addition to the experiments summarised in Table 2 two further samples of alkali metal supported on an alumina foam and cordierite wall flow filter were prepared and coated with soot in a similar manner to that described above. Measurement of the soot combustion characteristics of these samples in O2, CO2 and H2O were very similar to the powder samples. 

Catalyst carriers are often manufactured in a two-step process. In the first step, the solid material, e.g., silica or alumina, is precipitated from a solution and filtered. Then the wet powder is brought into the desired shape, as grains or pellets of a certain size, e.g., by spray drying, pressing or extrusion. The resulting particles are then dried and sintered at elevated temperatures. The more active catalytic material may be formed by co-precipitation. At elevated temperatures oxides are usually formed, that may be reduced to obtain a pure metal. Sometimes oxides or sulfides are the active species. 

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