Catalyst or membrane precursor systems, catalyst or membrane sys

Details Catalyst or membrane precursor systems, catalyst or membrane sys Catalyst or membrane precursor systems, catalyst or membrane sys

1993-09-30

1995-12-05

McFarland, Anthony

Catalyst, solid sorbent, or support therefor: product or process

Miscellaneous

502238, 502263, 502304, 502349, 502350, B01J 2108

Patent

active

054729270

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

This invention relates to a catalyst or membrane precursor system and a catalyst or membrane system as well as a method for preparing such systems.


DESCRIPTION OF THE PRIOR ART

In an increasing number of catalytic processes, catalysts are used which are composed of monoliths having catalytic material applied on their surface. Monoliths as referred to herein are ceramic or metallic bodies having such--relatively low--porosity that they offer a relatively low flow resistance to gases flowing through. The best known applications of monoliths are found in the catalytic purification of the exhaust gases of motor vehicles and in the catalytic purification of flue gases. In those applications, particularly monoliths prepared from ceramic materials are used, the channels in this ceramic material generally having a honeycomb structure. In particular in the selective catalytic reduction of nitrogen oxides in flue gas with ammonia, ceramic monoliths are generally used.
A drawback of such ceramic structures, however, is the minor mechanical strength and the low thermal conductivity thereof. Therefore, monoliths prepared from metal are sometimes used. These are typically prepared by rolling up a metal plate provided with ridges or the like and welding together the contact surfaces thus obtained. Although metal monoliths are relatively expensive, the use thereof is attractive, considering the high mechanical strength and the high heat conductivity in the purification of exhaust gases of motor vehicles, in particular more expensive ones.
In addition to finding application in the purification of flue gas and exhaust gas, monoliths can advantageously be used in other catalytic processes, in particular where it is significant to maintain a low pressure drop.
It is difficult to apply catalytic material on the wall of the internal channels in the monoliths in the desired manner and to the desired extent. Because the catalytic activity is proportional to the active surface per unit volume of the catalyst, small to extremely small particles of the active, catalyzing material are desirable. This applies in particular to the use of precious metals because when they are used, it is desirable to have as many metal atoms on the surface of the internal channels as is possible, also in view of the high cost price.
At the temperatures at which catalysts are thermally pretreated and/or used, however, small particles of most active materials sinter fast. This involves a strong reduction of the active surface and leads to a considerable decrease of the catalytic activity. To avoid this problem, the catalytically active component is generally applied on a so-called carrier, i.e., a thermostable, high-porous solid material which is usually not catalytically active itself. Known carriers include aluminium oxide, silicon oxide and activated carbon. The catalytically active particles are applied on the carrier in such a manner that they are not in mutual contact. By thus applying the catalytically active particles on a carrier, sintering of those particles is generally prevented.
When applying catalytically active components on the surface of the channels in monoliths, one generally starts from small particles, for instance of an average particle size of 100 .mu.m, of a suitable carrier material, on which the active component is applied. According to a suitable technique, a suspension is made of the loaded carrier particles in a suitable liquid. Then the monolith is dipped in this suspension, whereafter the monolith is allowed to drip clean. This method is known as the so-called "dip-coating method". In the technical production of monolith-catalysts, this dip-coating is usually carried out by robots. Studies of catalysts thus prepared demonstrate that the distribution of the loaded carrier particles over the surface of the internal channels of the monoliths is far from being homogeneous. It has also been found that the density of the loaded carrier particles present on the surface is often low. This is a great disadv

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