Monolithic catalysts abatement

The replacement of vanadia-based catalysts in the reduction of NOx with ammonia is of interest due to the toxicity of vanadium. Tentative investigations on the use of noble metals in the NO + NH3 reaction have been nicely reviewed by Bosch and Janssen [85], More recently, Seker et al. [86] did not completely succeed on Pt/Al203 with a significant formation of N20 according to the temperature and the water composition. Moreover, 25 ppm S02 has a detrimental effect on the selectivity with selectivity towards the oxidation of NH3 into NO enhanced above 300°C. Supported copper-based catalysts have shown to exhibit excellent activity for NOx abatement. Recently Suarez et al and Blanco et al. [87,88] reported high performances of Cu0/Ni0-Al203 monolithic catalysts with NO/NOz = 1 at low temperature. Different oxidic copper species have been previously identified in those catalytic systems with Cu2+, copper aluminate and CuO species [89], Subsequent additions of Ni2+ in octahedral sites of subsurface layers induce a redistribution of Cu2+ with a surface copper enrichment. Such redistribution... 

Monolithic catalysts have found a wide range of applications in the removal of pollutants from air, especially in the automotive industry. Specifically, the demand for large surface to small volume, high conversions achieved for low retention times, and low pressure drop led to the development of monolithic supports. More information on automotive catalytic converters has been given in Chapter 1. Usually, a thin layer of alumina is deposited onto a monolith for keeping the precious metal used for air pollutants abatement dispersed. The oxidations that take place are highly exothermic and the reaction rates achieved are in turn high. Hence, the reactants diffuse only a small distance... 

Monolithic catalysts are widely known as a consequence of their applications for automobile exhaust gas abatement. If one would ask a layman what a catalyst is, he would likely describe the exhaust cleaning device in his automobile, rather than give the much broader definition we use in the catalysis community. 

Alvarez, E., Blanco, J., Otero de Becerra, J., Olivares, J., Salvador, L. 2002. Platinum monolithic catalysts for SO2 abatement in Dust-Free Flue Gas from combustion units. Latin American Applied Research. 32,123-129. 

The transformation of straw and agrofood residues with high sulfur and ash content requires the development of materials for sulfur abatement at high temperature, tar cracking and as monolith for syngas production by exothermic or autothermal processes thanks to catalysts supported on materials with a high thermal conductivity. 

Monolith reactor This type of reactor is used extensively for the abatement of automobiles exhaust emissions. The gas flows continuously through the reactor, whereas the catalyst is a continuous phase consisting of a ceramic support and the active phase, which is dispersed onto the support. The support is structured in many channels and shapes that achieve large catalytic surface at small volume. A typical application of monolith reactors is the exhaust gas cleaning. 

Refractory high surface area oxides are deposited from slurries onto the walls of the channels that make up monoliths in order to provide an adequate surface area to support the active catalytic species. Washcoats such as AI2O3 and TiC>2 are commonly used for pollution abatement applications (auto exhaust, stationary NO abatement, etc.) where the monolith is usually a ceramic. Metal monoliths are finding increasing use however, they represent only a small percentage of the total monoliths used. Optical microscopy enables one to see that the catalyzed washcoat follows the contour of the ceramic surface. Figure 7 shows the AI2O3 washcoat-ceramic interface for a typical auto exhaust catalyst. In this case, no evidence of loss of adhesion between washcoat and ceramic can be seen. 

The catalysts tested in this study are the first monolithic systems developed for N2O abatement. These systems can be a way to reduce the N2O emission for industrial processes with relatively low installation cost and lower perturbation in the production process. 

As illustrated in fig. 2, the pollution abatement system that is being used in gasoline-fuelled engines is a complex one, consisting of several parts. From a chemical point of view, the catalytic converter is the place where the chemistry occurs. Monolith-type catalysts are now employed in the automotive industry (fig. 7). 

High-temperature catalytic combustion of gas and liquid fuels (Table 6) even in catelyst-supported regime (flame is located within La-Mn- monolith) sharply decreases emissions of NO and CO as compared with an open flame r ime. Earlier [11], such results were achieved only with Pd catalysts supported on metal monolith carrier. Flameless combustion on our catalysts appears to be even more efficient to abate NO j. The catalysts worked for two months without loss of activity and monolith integrity withstanding everyday start-ups and shut-offs. 

In some VOC applications, particulate catalysts are more suitable than monoliths. The basic hydrocarbon VOC abatement reaction is... 

The abatement system is usually designed for maximum heat recovery, as shown in the diagram in Fig. 7.2. The igniter is needed only to pre-heat the pollutant-laden inlet gas to initiate the oxidation reaction. If the inlet gas temperature becomes sufficiently higher than that necessary for light-ofF no external heat source is needed to sustain catalytic oxidation. The catalyst shown is a precious metal(s) deposited on a high surface area carrier such as y-Al203 washcoated onto the walls of the monolith structure. 

---