Catalyst active phase incorporation

The photocatalytic oxidation of alcohols constitutes a novel approach for the synthesis of aldehydes and acid from alcohols. Modification of Ti02 catalyst with Pt and Nafion could block the catalyst active sites for the oxidation of ethanol to CO2. Incorporation of Pt resulted in enhanced selectivity towards formate (HCOO ad)-Blocking of active sites by Nafion resulted in formation of significantly smaller amounts of intermediate species, CO2 and H2O, and accumulation of photogenerated electrons. The IR experimental teclmique has been extended to Attenuated Total Reflectance (ATR), enabling the study of liquid phase photocatalytic systems. 

The addition of zirconium to vanadium catalysts has been found to improve activity in a number of studies across compositions containing 1.5-15% Zr 98,146,165,169,171-180). Observations have shown that zirconium was not incorporated into the (VO)2P207 lattice, but instead was found in an amorphous phase, which is proposed to be the catalytically active phase 171). This interpretation provides a probable explanation for why zirconium was not found (by Ye et al. 168)) to increase the surface area of the catalyst. 

Various methods are possible to incorporate a catalytically active phase to the monolith [48-59,85-95]. Figure 3 shows the general scheme for preparing a monolithic catalyst structure from a washcoated monolith. In fact, no fundamental differences exist between incorporation of an active phase in a conventional support (beads, extrudates, spheres) and in monoliths. In practice, precautions are needed because, besides concentration profile on a particle scale, such profile over the length of the monolith also can easily arise. 

Impregnation is one of the most used techniques to incorporate an active phase in a support. It can also be used to deposit active phase to a monolith [85]. Usually, a high-surface-area monolith is dried, evacuated, and dipped in a solution containing a precursor of the active phase. After drying and calcination a monolithic catalyst is obtained. Often, an activation step is necessary to convert the precursor of the active phase into the active phase, e.g., the transformation of a metal oxide in the corresponding metal or metal sulfide. Monolithic catalysts with complex compositions of active phases can be prepared by sequential impregnations with suitable solutions or with a conunon solution containing various precursors of the components. 

Transition Metal Salts and Oxides on Alumina. Transition metal salts, particularly chlorides and nitrates, are frequently used as starting materials for the preparation of supported transition metal oxides or supported precursors for supported metal catalysts. Also, many catalytic materials, particularly supported molybdenum and tungsten oxide and sulfide catalysts, contain transition metal ions, namely Co, Ni , and Fe " as promoters. Thus, it is interesting to study the spreading and wetting behavior of salts of these transition metals and of their oxides. This is of particular importance for promoted catalyst materials, since in practice the incorporation of the active phase and the promoter should be possible in one step for economic reasons. 

Industrial catalysts are prepared exclusively by impregnation of the support material with aqueous solutions of the active phase materials, followed by subsequent drying and calcination. In the work described here, an alternative mode of catalyst preparation was chosen, based on a solventless, mechano-chemical method of incorporating the active catalytic components on the support carrier via ball-milling. 

DRS spectra indicated the change of promoter structure, with spinel formation at temperatures exceeding 700°C, i.e. incorporation of Ni(Co) in alumina support causing the destruction of previously formed active phases of the catalyst. 

The supported metal oxides wliich constitute the active phase of these catalysts were incorporated by impregnation on the support (SLCu, SLCuNi, STCuNi and SACuNi catalysts) or over a dispersion of alumina that was wash coated (5% by weight of the support) over the support (SA(RCuNi). 

The incorporation of the active phases on to the monolithic support after first applying an alumina washcoat greatly increased the selectivity of the catalyst where NO conversions of 75% could be achieved. This was probably due to a reduction in any diffiisional limitations. 

Both charge neutral and cation incorporating V/P/O phases exhibit as a recurring theme the isolation of high-temperature, dense-phase materials based on vanadium polyhedra and pyrophosphate P207 " subunits. Such materials have attracted considerable attention as components or models for industrial catalysts. While considerable controversy surrounds the nature of the active component of such catalysts (180-186), in the case of vanadyl pyrophosphate the active phase involved in the redox cycle for the organic oxidation appears to be structurally related to 8-VO(PO)4 (187). 

Zeolites are crystalline aluminosiHcates. Their unit cells are quite complex, as they have intricate microporous structures. Currently, around 200 frameworks are known for zeolites [10], and they all have one specific characteristic chaimels and pores in the size range 2 A to 1 nm, incorporated into the framework structure. This characteristic makes them appropriate for use as, for example, molecular sieves, cation-exchange materials, supports for catalytic active phases, and catalysts themselves [11, 12]. Controlled synthesis of zeoHte materials is still a challenge, and in this regard only a few selected zeolites have been studied in detail [13]. Silicalite-1 (MFI framework) has a pure-silica stmcture, but does not have active sites. The incorporation of, for example, heteroatoms such as aluminum (ZSM-5) makes it catalytically active [14, 15]. Nevertheless, silicalite-1 can be seen as an archetype system, of which its preparation has been characterized in great detail. 

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