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The use of heterogeneous photocatalysts for the abatement of environmental problems has received increased attention during the last three decades. The photocatalyst used in most of these studies was titanium dioxide. This fact relates to the unique physical and chemical properties of titanium dioxide and similar semiconductor materials, such as ZnO, MgO,WO3, Fe2Oa, CdS, etc. that may be utilized for a wide range of potential applications. Specifically, upon ultraviolet irradiation these materials exhibit photocatalytic activity that enables the oxidative destruction of a wide range of organic compounds and biological species on their surface. In addition, these materials may also exhibit photocatalytically induced superhydrophilicity that converts the hydrophobic character of the surface to hydrophilic when exposed to UV light. This causes the formation of uniform water films on the surface of these materials, which prevents the adhesion of inorganic or organic components and thus retains a clean surface on the photocatalyst. Photocatalytic materials may be deployed on surfaces of various substrates, such as glass, ceramics or metals to provide layers that exhibit photocatalytic activity when they are exposed to light. Excitation of a photocatalyst leads to the creation of electrons and holes in the semiconductor material. Furthermore, these electrons and holes interact with molecules adsorbed on the semiconductor and can induce charge transfer process that results in the degradation of the adsorbate. The commercial potentials of heterogeneous photocatalysis are huge, including medical applications, application in the field of architecture (particularly for the cultural heritage purposes, facade paints, etc.), automotive and food industries (cleaner technologies, non-fogging glass and mirrors, product safety), textile and glass industry, as well as in environmental protection (water and air purification and disinfection). After the discovery that the photocatalytic degradation of organic compounds is generally more efficient in the gas phase than in the liquid phase, and the fact that the treatment cost may be significantly lower than that of the water phase photocatalytic treatment, the scientific interest has shifted towards the application of photocatalysis for air treatment. This paper describes the basics of heterogeneous photocatalysis, mainly on TiO2 and the application of photocatalytic processes for air treatment purposes. Review of more interesting practical application of heterogeneous photocatalysis for the treatment of polluted air is presented. Special efforts are made to describe the technical aspects of the photocatalytic processes and to characterize different photocatalytic reactors in use for air treatment.

Monolithic (honeycomb) catalysts are continuous unitary structures containing many narrow, parallel and usually straight channels (or passages). Catalytically active components are dispersed uniformly over the whole porous ceramic monolith structure (so-called incorporated monolithic catalysts) or are in a layer of porous material that is deposited on the walls of channels in the monolith's structure (washcoated monolithic catalysts). The material of the main monolithic construction is not limited to ceramics but includes metals, as well. Monolithic catalysts are commonly used in gas phase catalytic processes, such as treatment of automotive exhaust gases, selective catalytic reduction of nitrogen oxides, catalytic removal of volatile organic compounds from industrial processes, etc. Monoliths continue to be the preferred support for environmental applications due to their high geometric surface area, different design options, low pressure drop, high temperature durability, mechanical strength, ease of orientation in a reactor and effectiveness as a support for a catalytic washcoat. As known, monolithic catalysts belong to the class of the structured catalysts and/or reactors (in some cases the distinction between "catalyst" and "reactor" has vanished). Structured catalysts can greatly intensify chemical processes, resulting in smaller, safer, cleaner and more energy efficient technologies. Monolith reactors can be considered as multifunctional reactors, in which chemical conversion is advantageously integrated with another unit operation, such as separation, heat exchange, a secondary reaction, etc. Finally, structured catalysts and/or reactors appear to be one of the most significant and promising developments in the field of heterogeneous catalysis and chemical engineering of the recent years. This paper gives a description of the background and perspectives for application and development of monolithic materials. Different methods and techniques used in the preparation of monolithic catalysts are described. Several commercial applications of monolithic catalysts are presented. New applications and the associated technical challenges for the monolithic catalyst and reactors are discussed as well.