Thermal heterogeneous catalytic processes

Due to the low temperatures, the thermal heterogeneous catalytic processes which prevail in the atmosphere are simple hydrolytic reactions (such as the hydrolysis of N2O5 in acidic water droplets to form nitric acid). However, photocatalysis provides more complicated redox reactions (such as ammonia synthesis from N2 and H2O over aerosols containing Ti02). 

Thermal Heterogeneous Catalytic Processes over Ice Particles 217... 

Mkrostructured reactors (MSR) for heterogeneous catalytic processes mostly consist of a large number of parallel flow channels. At least one dimension of these channels is smaller than 1 mm, but rarely <100 pm. This leads to an increased heat transfer in the direction of the smallest dimension. The volumetric heat transfer performance in microstructured devices is several magnitudes higher than in conventional reactors. Therefore, even highly exothermic or endothermic reactions can be operated under near isothermal conditions and thermal runaway can be avoided (see Chapter 5). In addition, mass transfer between the bulk phase... 

Catalyst support due to their ability to be tailored to specific needs, carbon nanotubes are candidate supports in heterogeneous catalytic processes. Carbon nanotubes are also employed as catalytic support due to their high surface area, chemical and thermal stability (in a non oxidative enviromnents). Carbon nanofibres, soot and graphite are used in these applications. 

In the domain of atmospheric de-pollution, still the most important is to remove several main classes of pollutants (CO2, NOx, SOx and VOCs) from the gas-phase effluents. The scientific efforfs have been, and still are, directed towards several classes of procedures that can be generalized in a following way the usage of various solid materials as adsorbents, chemical treatment of effluent gases either in heterogeneous catalytic processes or with ozone, the treatment of pollutant gases with non-thermal plasmas, and plasma-assisted adsorption and/or catalytic processes. 

The activation barrier for the oligomerization of alkynes may be overcome thermally or catalytically. Because the classical thermal transformation of ethyne into benzene in a hot tube is rather ineffective (47), more emphasis has been placed on working out catalytic routes for the synthesis of linear oligomers, cyclooligomers, and polymers. Transition metal compounds have proved to act as effective catalysts in homogeneous as well as in heterogeneous processes (48). 

Heterogeneous thermal and photocatalysis are bound to play a key role in those solutions. However, while thermal heterogeneous catalysis is widespread in chemical industries, and photocatalysis not yet, there is still a strong demand in both areas for research in order to understand the process and its elementary steps as well as to rationally design the catalytic material. 

It has been pointed out that catalytic processes are the only realistic way of increasing the reactor yield. Indeed, under plasma conditions it is experimentally found tlmt the catalyst in a heterogeneous reaction shifts the steady-state concentrations of the product. There is no analogue for this in the field of thermal catalysis where the catalyst merely diminishes the relaxation time, without affecting the equilibrium concentration of the product. 

A large amount of work has been done on the thermal (uncatalyzed) oxidation of acetylene. Since it did not seem practical to use this method for the present purpose because of the relatively high temperatures involved, and since its mechanism is apparently quite different from that of the heterogeneous reaction, it will not be discussed here. It is worth noting, however, that the catalytic process seems to be the simpler of the two in regard to the products of reaction though peroxide formation with the eventual production of rather complex molecules—... 

Novel and innovative approaches for the selective catalytic homogeneous oxidation of alkanes and alkenes to produce industrially important alcohols, aldehydes, ketones, and epoxides aie still urgently needed. Homogeneous catalytic oxidation is a process that is more selective to products and, as well, the reactions aie carried out at lower temperatures in comparison to heterogeneous catalytic oxidation reactions. However, the separation of the homogeneous catalyst from the oxidation products is energy intensive for example, distillation, as well as possible thermal decomposition of the catalyst during distillation. We will discuss... 

In contrast to heterogeneous catalysis, classical homogeneous catalysis takes place in the bulk of a solvent. However, due to the lack of a phase boundary it is much more complicated in this case to isolate the fully dissolved transition metal complex from the desired reaction product. Often, distillation fails due to the thermal instability of the dissolved catalyst or the fact that unselective reactions occur in the bottom of the distillation column. Catalyst recovery and recycling strategies for homogeneous catalytic processes can therefore sometimes be rather complicated... 

The release of VOCs into the environment has widespread environmental imph-cations. Pollution by VOCs has been linked to the increase in photochemical smog and ozone depletion. In addition, many VOCs are themselves toxic and/or carcinogenic. The US Clean Air Act of 1990 was one of the first measures to call for a 90% reduction in the emissions of 189 toxic chemicals, with 70% of these classed as VOCs, by 1998. Hence, in recent years, the development of effective technologies for the removal of VOCs from the atmosphere has increased in importance with the introduction of legislation to control their release. Various methods have been proposed, and one of the best is heterogeneous catalytic oxidation. This has the advantage over the more common original thermal oxidation process, since it requires less supplementary fuel and is therefore a less expensive process. However, the characteristics of the catalyst selected for this process are of vital importance for successful operation, and potential problems such as lifetime and deactivation must be solved if catalytic oxidation is to be employed universally. Catalysts currently in use include noble metals, notably platinum and palladium, and those based on metal oxides, however, irrespective of the type of catalyst, the most important characteristics are activity and selectivity for total oxidation. 

---