Catalyst and method for purification of diesel engine exhaust gas

Abstract

The invention provides a catalyst for catalytically reducing nitrogen oxides in diesel engine exhaust gas by use of unburnt carbon contained in the diesel engine exhaust gas as a reducing agent, the catalyst comprising:

(a) alumina, or

(b) alumina supporting an ion and/or an oxide of at least one transition metal selected from the elements of Period 4 of the Periodic Table, or

(c) alumina supporting an aluminate of at least one transition metal selected from the elements of Period 4 of the Periodic Table.

Description

TECHNICAL FIELD[0001]The present invention relates to a catalyst which is to be supported on a diesel particulate filter (DPF) and which is used not only for catalytically reducing nitrogen oxides, which are mainly comprised of NO and NO2 and are hereinafter referred to as NOx, contained in diesel engine exhaust gas, using as a reducing agent unburnt carbon contained in the diesel engine exhaust gas and captured by the DPF, in a wide temperature range even in the presence of sulfur oxides; at the same time for catalytically oxidizing the unburnt carbon to remove it. The invention also relates to a method for catalytically reducing NOx using unburnt carbon as a reducing agent and catalytically oxidizing the unburnt carbon to remove it in such a manner as mentioned above.BACKGROUND ART[0002]Exhaust gas emitted from diesel engines contains NOx, and particulate matter, so-called particulates (PM), such as oily particulate matter, carbonaceous particulate matter and sulfuric acid mist, a...

AI Technical Summary

Benefits of technology

[0012]The invention provides a catalyst for catalytically reducing nitrogen oxides in diesel engine exhaust gas by use of unburnt carbon contained in the diesel engine exhaust gas as a reducing agent, the catalyst comprising:

Problems solved by technology

However, when such a DPF is used, as unburnt carbon is accumulated in the DPF, the pressure loss of the filter increases to have adverse effect on combustion of fuel in an engine, and finally the function of the DPF will be lost.

However, there is a problem that fuel efficiency is decreased because fuel is consumed in order to increase the temperature of DPF when it is reproduced.

Moreover, in the oxidation reaction of unburnt carbon on a noble metal oxidation catalyst such as platinum, oxidation of unburnt carbon proceeds rapidly and unburnt carbon is exhausted promptly.

Therefore, even if NOx is purified, the purification reaction will stop instantly and, therefore, the amount of NOx purified by unburnt carbon is very small.

It, however, does not fundamentally improve the deterioration of fuel efficiency because it is still necessary to perform rich combustion of fuel.

It, however, seems impossible to purify NOx over a wide temperature range because combustion of carbon, carbon monoxide and hydrocarbons proceeds rapidly and, as a result, a reducing agent disappears rapidly.

It, however, seems impossible to purify NOx over a wide temperature range because this catalyst also has a high perfectly oxidizing ability and, therefore, combustion of carbon, carbon monoxide and hydrocarbons proceeds rapidly and, as a result, a reducing agent disappears rapidly.

In addition, this catalyst has a problem that the oxidizing ability and reducing ability thereof will be lost in the presence of sulfur oxides because the catalyst contains a metal having a low electronegativity.