Modified Layered Silicas as Catalysts for Conversion of Nitrogen Pollutants in Flue Gases—A Review

Abstract

This paper is focused on the recent achievements in the studies of modified layered zeolites and cationic layered clay minerals. These materials are very promising catalysts in green chemistry processes, such as selective catalytic reduction of NOx with ammonia (NH3-SCR) and selective catalytic oxidation of ammonia to dinitrogen (NH3-SCO). Special attention is paid to the roles of the micro- and mesoporous structures of the catalytic materials, the type and location of deposited transition metals, as well as surface acidity in the design of effective catalysts for the NH3-SCR and NH3-SCO processes. The majority of the presented analysis is based on the authors’ research.

Recent Progress in the Application of Transition‐Metal Containing MFI topologies for NH3‐SCR‐DeNOx and NH3 oxidation

“…Whilst there are many studies, which focus on the design and reaction mechanisms of microporous catalysts dedicated to NH 3 ‐SCR‐DeNO x or NH 3 ‐SCO, studies on micro‐/mesoporous catalysts are scarce. Chmielarz and Dziembaj reviewed layered zeolites (e. g., ITQ‐2, MCM‐36), their modifications and applications in NH 3 ‐SCR‐DeNO x and NH 3 ‐SCO [125] . Thus, this review, for the first time summarizes bottom‐up and top‐down approaches for the preparation of the ZSM‐5 zeolite‐based catalysts investigated for NH 3 ‐SCR‐DeNO x and NH 3 ‐SCO.…”

“…Chmielarz and Dziembaj reviewed layered zeolites (e. g., ITQ-2, MCM-36), their modifications and applications in NH 3 -SCR-DeNO x and NH 3 -SCO. [125] Thus, this review, for the first time summarizes bottom-up and topdown approaches for the preparation of the ZSM-5 zeolitebased catalysts investigated for NH 3 -SCR-DeNO x and NH 3 -SCO. The role of mesoporosity on performance is subsequently evaluated below.…”

Studies of NO Reduction with Ammonia Over Copper Species Deposited on MCM‐41 Nanospheres ‐ Experimental Evidence of Reaction Mechanism

“…The efficiency of the NO x conversion is not satisfactory below 300 °C, while above 400 °C the direct oxidation of ammonia by oxygen present in flue gases results in decreasing of NH 3 -SCR efficiency. [3] The operation with dusty flue gases is one of the main problems of currently used NH 3 -SCR units, which are located upstream of electrostatic precipitator (ESP), and therefore the channels in monolithic NH 3 -SCR converters can be blocked by dust particles. The changing of the ESP and NH 3 -SCR units order in the gas exhaust installation (NH 3 -SCR upstream of ESP) should result in protection of the monolithic catalyst from plugging by dust particles.…”

“…The commercially used NH 3 ‐SCR catalysts for conversion of nitrogen oxides in flue gases emitted by stationary sources, such as thermal power plants and industrial boilers, are based on V 2 O 5 ‐WO 3 ‐TiO 2 and V 2 O 5 ‐MoO 3 ‐TiO 2 metal oxide systems, which effectively operate in the temperature range of 300–400 °C, with high selectivity to nitrogen. The efficiency of the NO x conversion is not satisfactory below 300 °C, while above 400 °C the direct oxidation of ammonia by oxygen present in flue gases results in decreasing of NH 3 ‐SCR efficiency [3] . The operation with dusty flue gases is one of the main problems of currently used NH 3 ‐SCR units, which are located upstream of electrostatic precipitator (ESP), and therefore the channels in monolithic NH 3 ‐SCR converters can be blocked by dust particles.…”

Catalytic Performance of Bimetallic Systems (Cu-Fe, Cu-Mn, Fe-Mn) Based on Spherical MCM-41 Modified by Template Ion-Exchange in NH3-SCR Process

“…The problem is that the ESP units effectively operate only with relatively cold flue gases, at 250 • C or even lower. Thus, in such modified installations that operate with commercial vanadium-based catalysts of the NH 3 -SCR process, the de-dusted gas, downstream of the ESP unit, must be additionally heated to above 300 • C prior to being directed to the NH 3 -SCR converter [4,7]. Of course, such rearrangement of the exhaust gas purification system should result in protection of the monolithic catalysts against clogging by dust particles; however, it increases the operating costs of such an installation due to the additional heating of the exhaust gases downstream of the ESP unit.…”