Catalytic phase material

Bulk Catalytic Phase Materials Xero- and Aerogels... 

Effective preparation methods of hexaaluminates for catalytic applications, such as the hydrolysis of alkoxides and the co-precipitation in aqueous medium, ensure high interspersion of the constituents in the precursor. This allows the formation of single phase materials with layered-alumina structure at reasonably low temperature (1100-1200 °C) and with high surface area. The hydrolysis of alkoxides was extensively studied and used for the industrial scale-up in the production of catalysts in the monolith shape. However, the co-precipitation in aqueous medium has much potential in view of the possible commercialization of these materials due to its simplicity and low cost. 

Thermally induced deactivation of catalysts is a particularly difficult problem in high-temperature catalytic reactions. Thermal deactivation may result from one or a combination of the following (i) loss of catalytic surface area due to crystallite growth of the catalytic phase, (ii) loss of support area due to support collapse, (iii) reactions/transformations of catalytic phases to noncatalytic phases, and/or (iv) loss of active material by vaporization or volatilization. The first two processes are typically referred to as "sintering." Sintering, solid-state reactions, and vaporization processes generally take place at high reaction temperatures (e.g. > 500°C), and their rates depend upon temperature, reaction atmosphere, and catalyst formulation. While one of these processes may dominate under specific conditions in specified catalyst systems, more often than not, they occur simultaneously and are coupled processes. 

This section shows, for four examples of increasing complexity, how precipitates are formed and how the properties of the precipitates are controlled to produce a material suitable for catalytic applications. The first two examples comprise silica, which is primarily used as support material and is usually formed as an amorphous solid, and alumina, which is also used as a catalytically active material, and which can be formed in various modifications with widely varying properties as pure precipitated compounds. The other examples are the results of coprecipitation processes, namely Ni/ AI2O3 which can be prepared by several pathways and for which the precipitation of a certain phase determines the reduction behavior and the later catalytic properties, and the precipitation of (VOjHPCU 0.5 H2O which is the precursor of the V/P/O catalyst for butane oxidation to maleic anhydride, where even the formation of a specific crystallographic face with high catalytic activity has to be controlled. 

In summary, both amorphous and crystalline material is found in these vanadium phosphate catalysts, and it cannot be stated with any certainty whether or not the amorphous phase is the active phase. However, experimental observations have added weight to the postulate that amorphous material is the catalytically active material. 

Levels of volatility that would lead to unacceptable rates of vapor transport-driven sintering, attrition of catalytically-active materials, or corrosion of catalytic materials or support oxides by transport from contaminants or substrate materials can be estimated given equilibrium vapor pressures and a few assumptions about evaporation rates and mass transport. In particular, the rate of condensation of a vapor species on its source solid phase at high temperatures is almost certainly non-activated and may show little configurational restriction. Therefore, using the principle of microscopic reversibility, we can take the rate constant for condensation to be approximately equal to the collision frequency. 

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. 

Besides boehmite and boehmite derived calcined alumina phases bayerite and eta alumina can also be produced via this technology. Also accessible is an almost unlimited variety of high purity mixed oxides such as silica aluminas and doped aluminas. Even other catalytic carrier materials for example, MgO can be obtained. 

It is clear that the majority of electrochemical hydrogenations - those carried out at a Pb or similar electrode - have very little in common with the stoicheiometrically similar gas-phase reaction. But in respect of the sizeable number of electrochemical reductions carried out on catalytic electrode materials (e.g., transition metals), it seems that further work is useful to establish how much the two types of process have in common. The only way to do such studies is design electrochemical experiments to match existing gas-phase results or vice versa. But it could well be that from such studies, a store of newly available quantitative information would become available to practitioners of either discipline, and if this were to be the case, such a study would be a most worthwhile and cost-effective exercise. 

These I Ls have been used as catalysts-solvents for the alkylation of benzene with 1-dodecene [33]. The reaction produces higher yields in 2-dodecylbenzene (46%/ other monoalkylbenzene isomers) than the conventional HF process [Eq. (12)]. In addition the IL does not present the drawback of sludge byproduct formation as is the case with AICI3. One of the points of interest of operating the reaction in I Ls is that alkylbenzenes are poorly miscible in ILs. The reaction proceeds in a biphasic mode, thus making catalyst recovery and recycling easier. In traditional processes, consecutive polyalkylation reactions may occur since the alkylated benzene hydrocarbons are more reactive than the nonalkylated starting material. In the biphasic IL mode, consecutive polyalkylation reactions are disfavored since the alkylated benzenes are less soluble in the catalytic phase than the nonalkylated benzenes. 

The electrodes must contain catalytically active materials to catalyze the electrochemical oxidation of hydrogen at the anode and the electrochemical reduction of oxygen at the cathode. These catalysts must be stable under potentials, humidity conditions and pH values encountered during all steady state and transient fuel cell operation phases. 

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