Catalytic processes for reducing air pollutants

Environmental catalysis finds application not only in refinery and chemical processes, but also in several applications for the treatment of emissions in other types of production (electronic, agro/food production, pulp and paper, leather and tanning, metal finishing companies, etc.), household or indoor applications (self-cleaning catalytic ovens, domestic burners, water purifiers, etc.), and in auto, ship and flight emissions control.

Environmental catalysts often operate in more extreme reaction conditions than catalysts for chemical production or refineries (very low or very high temperatures, in the presence of nonremovable poisons, with very high space velocities, with ultra-low concentrations, etc.) and sometimes also should operate efficiently with a range of different feeds or in the presence of fast changes in feed composition.

Catalysis has been traditionally associated with chemical and refinery production. Catalytic converters for treatment of car emissions constituted the first massive use of catalysis outside chemical and refinery production. In recent years, catalytic environmental technologies have been rapidly expanded to various new areas offering new opportunities: for a range of industrial sectors traditionally far from the use of catalysis and in user-friendly devices to improve the quality of life and the indoor environment.

The extension of the use of catalysis outside the traditional fields together with the basic problem that is often met in the environmental technologies, where it is difficult to choose the optimal reaction conditions which are determined by energy and feed constraints and/or conditions defined by upstream units, implies that a very innovative effort is necessary to develop new catalytic materials, devices and solutions.

Industrial chemical processes from the beginning have moved towards a more efficient use of resources and an improvement of selectivity, due to the fact that both aspects correspond to an improvement in process economics. Catalysis was a fundamental component of this innovation and therefore almost all new developments in catalytic industrial processes fall within the area of interest of environmental catalysis.

Catalytic technologies in emissions cleanup (NO_x or SO_x elimination from mobile or fixed sources, VOC control) is one of the traditional areas of use of environmental catalysts, and considerable research is still being carried out on this topic. The main area of interest in recent years includes the development and testing of catalysts for NO_x removal in lean burn or diesel engine emissions, catalytic combustion and VOC removal. The research attention was mainly focused on the study of the reaction mechanisms and the identification of the nature of the active sites. Also understanding the unsteady-state behaviour has become increasingly important, due to the key role of this aspect in understanding the behaviour of oxygen-storage components in three-way catalysts (TWCs) and the behaviour of NO_xstorage-reduction (NOxSR) catalysts, the most promising solution to solve the issue of NO_x removal in light-duty diesel engine emissions. Different from conventional «steady-state» catalysts, these catalysts work continuously under periodic changes in feed composition from lean conditions (where NOx is stored on the catalyst in the form of a nitrate) and rich conditions (where the stored NO_x species are reduced to N₂ by the H₂, CO and hydrocarbons present in the emissions).

The interesting concept on NO_xSR catalysts is the possibility of using a catalytic material with the double function of acting as a sorption material and a catalyst with periodic switching between the two functions. Another example of this concept is shown by the use of zeolitebased catalysts for VOC removal in rotating equipment. When the VOC concentration is low and the temperature of the emissions is also low (a typical example is the gas emissions from painting processes), the heating of large volumes of emissions to the temperature required for catalytic activity is expensive (if the VOC concentration is low, the heat of combustion cannot maintain the process autothermically). A solution is the use of a rotating monolith coated with zeolite-based catalysts which act as a sorbent when the monolith is in contact with the gas stream and as a catalyst in the other part of the monolith where the regeneration occurs. This is another example of the innovative and multifunctional possibilities of catalytic materials explored by environmental catalysis. Another example of this concept, but applied to the removal of pollutants in water is the abatement of chloroorganics by adsorption on a bed of Pd/active carbon and in situ periodic regeneration by a reducing treatment.

Source:

http://library.certh.gr/libfiles/PDF/MARNELLOS-EKETA-1638-CATALYTIC-PROCESSES-in-Seminar-2-PP-238–252-Y-2007. pdf%22