The present invention is aimed at selective catalytic reduction (SCR) articles and systems suitable for use in treating exhaust of an internal combustion engine.

BACKGROUND

Molecular sieves such as zeolites are employed in the catalysis of certain chemical reactions for example the selective catalytic reduction (SCR) of nitrogen oxides with a reductant such as ammonia, urea or hydrocarbons. Zeolites are crystalline materials having rather uniform pore sizes which, depending upon the type of zeolite and the type and amount of cations included in the zeolite lattice, range from about 3 to about 25 angstroms in diameter. Zeolites having 8-ring pore openings and double-six ring secondary building units, particularly those having cage-like structures are of interest as SCR catalysts. Included in this category are zeolites having a chabazite (CHA) crystal structure, which are small pore zeolites with 8 member-ring pore openings (ca. 3.8 angstroms) accessible through its 3-dimensional porosity. A cage like structure results from the connection of double six-ring building units by 4 rings.

Catalysts employed in the SCR process ideally should be able to retain good catalytic activity over the wide range of temperature conditions of use, for example from about 150° C. to about 600° C. or higher, under hydrothermal condition s. Hydrothermal conditions are encountered in practice because water is a byproduct of fuel combustion and high temperature hydrothermal conditions occur in diesel exhaust applications, such as during the regeneration of a soot filter, a component of exhaust gas treatment systems used for the removal of carbonaceous particles.

The SCR process converts nitrogen oxides (NOx) to nitrogen (N2) and water (H2O). An undesired SCR byproduct is nitrous oxide (N2O). Desired are improved articles, systems and processes to selectively convert NOx within internal combustion engine exhaust streams to N2while minimizing the formation of undesired N2O. Undesired N2O formation may be observed as molar percent conversion of (NO+NO2) to N2O.

Nitrogen oxides (NOx) may include N2O, NO, N2O3, NO2, N2O4, N2O5or NO3.

SUMMARY

In one or more embodiments, the present disclosure provides catalytic articles, systems and methods for treating exhaust gas streams containing nitrogen oxides. The articles, systems and methods will exhibit high NOx conversion while at the same time minimize N2O formation. In certain embodiments, the articles, systems and methods are suitable for treating lean exhaust gas streams of diesel internal combustion engines.

In one or more embodiments, the present disclosure relates to selective catalytic reduction articles, systems incorporating such articles, and methods utilizing such articles and systems. The selective catalytic reduction articles advantageously provide for high NOx conversion with simultaneously low formation of N2O because of the utilization of at least two different catalytically active molecular sieves. More specifically, at least a first steam-activated iron-containing molecular sieve and a second copper-containing molecular sieve are utilized. The steam-activated iron-containing molecular sieve and the copper-containing molecular sieve are coated on at least one substrate. For example, a first coating layer of the first steam-activated iron-containing molecular sieve and a second coating layer of the second copper-containing molecular sieve are coated on the at least one substrate. The first and second coating layers can be coated on the same substrate or on different substrates. For example, a first substrate can have the first coating layer with the first steam-activated iron-containing molecular sieve provided thereon (e.g., in the form of a washcoat), and a second substrate can have the second coating layer with the second copper-containing molecular sieve provided thereon (e.g., in the form of a washcoat). Preferably, the first substrate with the first coating layer is provided upstream of the second substrate with the second coating layer relative to the flow path of an exhaust stream (the second substrate with the second coating layer thus being downstream of the first substrate with the first coating layer). As another example, a substrate can have a first coating layer with a first steam-activated iron-containing molecular sieve provided thereon (e.g., in the form of a washcoat), and the same substrate can have a second coating layer with the second copper-containing molecular sieve provided thereon (e.g., in the form of a washcoat). In such configuration, the second coating layer is preferably provided downstream of the first coating layer relative to the flow path of an exhaust stream (the first coating layer thus being upstream from the second coating layer). The substrate is thus zoned so that a first zone includes the first coating layer and the second zone includes the second coating layer. The zones (and thus the coating layers) may overlap if desired or may be non-overlapping. It has been found that providing a first steam-activated iron-containing molecular sieve and a second copper-containing molecular sieve beneficially achieves the results described herein in relation to NOx conversion and low N2O formation, particularly when the first steam-activated iron-containing molecular sieve is positioned upstream from the second copper-containing molecular sieve. The present disclosure particularly provides, in some embodiments, a selective catalytic reduction article comprising a substrate having a front upstream end and a rear downstream end defining an axial length and having a catalytic coating thereon, where the catalytic coating comprises a first coating layer comprising an iron-containing molecular sieve and a second coating layer comprising a copper-containing molecular sieve.

Advantageously, the catalytic coating is zoned and comprises a first zone comprising the first coating layer comprising the iron-containing molecular sieve and a second zone comprising the second coating layer comprising the copper-containing molecular sieve. The iron-containing molecular sieve for example comprises steam-activated FeCHA powder.

Also disclosed is a selective catalytic reduction system comprising a first selective catalytic reduction article comprising a first substrate comprising a first catalytic coating layer comprising an iron-containing molecular sieve and a second selective catalytic reduction article comprising a second substrate comprising a second catalytic coating layer comprising a copper-containing molecular sieve, where the first and second articles are in fluid communication.

Also disclosed is an exhaust gas treatment system comprising the reduction article or reduction system and a reductant injector in fluid communication with and upstream of the article or system.

Also disclosed are methods for treating an exhaust stream containing NOx, comprising passing the exhaust stream through the reduction articles, reduction systems or treatment systems.

Also disclosed are methods for activating an iron-containing molecular sieve, the methods comprising adding iron to a molecular sieve followed by steam-calcination of the resulting iron-containing zeolite powder from about 500° C. to about 800° C. for a period of from about 20 minutes to about 12 hours in the presence of water vapor or preferably from about 650° C. to about 750° C. for a period of from about 20 minutes to about 2 hours in the presence of water vapor. Pre-activated iron-containing zeolite applied to a substrate provides outstanding SCR performance without further hydrothermal treatment of the coated substrate.

Also disclosed are selective catalytic reduction articles or systems capable of providing NOx conversion of ≥90% (preferably ≥99%), and N2O formation of ≤50% (preferably ≤40%) of that of an article or system, respectively, containing a uniform concentration of CuCHA as the only SCR catalyst under transient engine testing conditions. More particularly, selective catalytic reduction articles, systems, or methods of the present disclosure can be configured for providing >90% NOx conversion while also providing ≤1.5% N2O formation, particularly the N2O formation is ≤40% of that compared to an article, system, or method containing or employing a uniform concentration of high Cu-containing CuCHA as the only SCR catalyst under transient engine testing conditions as otherwise described herein. Embodiments of the present disclosure also relate to improved performance by utilizing zoning. For example, a selective catalytic reduction article, system, or method according to the present disclosure can include a substrate with a front upstream zone including a catalytic coating and a second downstream zone including a catalytic coating, and can be adapted for providing >90% total NOx conversion, particularly wherein the front upstream zone provides from about 30% to about 80% of the total NOx conversion.

Also disclosed are SCR articles comprising a substrate having a front upstream end and a rear downstream end defining an axial length and having a catalytic coating thereon, where the catalytic coating comprises steam-activated FeCHA powder and CuCHA.

The invention includes, without limitation, the following embodiments:

Embodiment 1

A method for activating an iron-containing molecular sieve powder, the method comprising: adding iron into a molecular sieve to form an iron-containing molecular sieve; and carrying out steam-calcination of the iron-containing molecular sieve powder at a temperature of about 650° C. to about 750° C. for a period of about 20 minutes to about 2 hours in the presence of water vapor.

Embodiment 2

The method of any preceding or subsequent embodiment, wherein adding the iron into the molecular sieve comprises a process selected from the group consisting of ion-exchange, impregnation of an iron salt, and mixing a molecular sieve with iron oxide.

Embodiment 3

A method of preparing a catalytic article, the method comprising applying to a substrate a catalytic coating comprising a steam-activated iron-containing molecular sieve powder prepared according to any preceding embodiment.

Embodiment 4

The method any preceding or subsequent embodiment, wherein the coated substrate is adapted to achieve greater than 90% NOx conversion at 250° C. under fast SCR conditions without further steam-treatment.

Embodiment 5

A selective catalytic reduction article comprising a substrate having a front upstream end and a rear downstream end defining an axial length and having a catalytic coating thereon, where the catalytic coating comprises: a first coating layer comprising a steam-activated iron-containing molecular sieve; and a second coating layer comprising a copper-containing molecular sieve.

Embodiment 6

The selective catalytic reduction article of any preceding or subsequent embodiment, wherein the catalytic coating is zoned and comprises: a first, upstream zone comprising the first coating layer comprising the steam-activated iron-containing molecular sieve; and a second, downstream zone comprising the second coating layer comprising the copper-containing molecular sieve.

Embodiment 7

The selective catalytic reduction article of any preceding or subsequent embodiment, wherein the substrate is a porous wall-flow filter or a flow-through monolith.

Embodiment 8

The selective catalytic reduction article of any preceding or subsequent embodiment, wherein one or both of the following conditions apply: a weight ratio of the steam-activated iron-containing molecular sieve to the copper-containing molecular sieve is about 1:10 to about 10:1; a weight ratio of iron oxide in the steam-activated iron-containing molecular sieve to copper oxide in the copper-containing molecular sieve is about 1:15 to about 15:1.

Embodiment 9

The selective catalytic reduction article of any preceding or subsequent embodiment, wherein the steam-activated iron-containing molecular sieve includes iron oxide in an amount of about 1 wt % to about 15 wt %, based on the total weight of the iron-containing molecular sieve, and the copper-containing molecular sieve includes copper oxide in an amount of about 1 wt % to about 10 wt %, based on the total weight of the copper-containing molecular sieve.

Embodiment 10

The selective catalytic reduction article of any preceding or subsequent embodiment, wherein one or both of the following conditions apply: the copper-containing molecular sieve has a Cu/AI atomic ratio of about 0.05 to about 0.55; the steam-activated iron-containing molecular sieve has a Fe/AI atomic ratio of about 0.05 to about 0.5.

Embodiment 11

The selective catalytic reduction article of any preceding or subsequent embodiment, wherein the first coating layer extends a distance from the front, upstream end of the substrate towards the rear, downstream end of the substrate and overlays a portion of the second coating layer, which extends from the rear, downstream end of the substrate a distance towards the front, upstream end of the substrate.

Embodiment 12

The selective catalytic reduction article of any preceding or subsequent embodiment, wherein the first coating layer extends a from the front, upstream end of the substrate to the rear, downstream end of the substrate and overlays an entirety of the second coating layer, which extends from the rear, downstream end of the substrate to the front, upstream end of the substrate.

Embodiment 13

The selective catalytic reduction article of any preceding or subsequent embodiment, wherein the second coating layer extends a distance from the front, upstream end of the substrate towards the rear, downstream end of the substrate and overlays a portion of the first coating layer, which extends a distance from the rear, downstream end of the substrate towards the front, upstream end of the substrate.

Embodiment 14

The selective catalytic reduction article of any preceding or subsequent embodiment, wherein the second coating layer extends from the front, upstream end of the substrate to the rear, downstream end of the substrate and overlays an entirety of the first coating layer, which extends from the rear, downstream end of the substrate to the front, upstream end of the substrate.

Embodiment 15

The selective catalytic reduction article of any preceding or subsequent embodiment, wherein the first coating layer and the second coating layer are adjacent and do not overlay each other.

Embodiment 16

The selective catalytic reduction article of any preceding or subsequent embodiment, wherein the first coating layer and the second coating layer are in direct contact with each other.

Embodiment 17

The selective catalytic reduction article of any preceding or subsequent embodiment, wherein the steam-activated iron-containing molecular sieve and the copper-containing molecular sieve are each 8-ring small pore molecular sieves.

Embodiment 18

The selective catalytic reduction article of any preceding or subsequent embodiment, wherein the steam-activated iron-containing molecular sieve and the copper-containing molecular sieve are both independently zeolites having a structure selected from the group consisting of AEI, AFT, AFX, CHA, EAB, ERI, KFI, LEV, SAS, SAT and SAV.

Embodiment 19

The selective catalytic reduction article of any preceding or subsequent embodiment, wherein each of the steam-activated iron-containing molecular sieve and the copper-containing molecular sieve have a CHA crystal structure.

Embodiment 20

The selective catalytic reduction article of any preceding or subsequent embodiment, wherein each of the steam-activated iron-containing molecular sieve and the copper-containing molecular sieve are aluminosilicate zeolites having a CHA crystal structure and a silica to alumina ratio (SAR) of about 5 to about 40.

Embodiment 21

The selective catalytic reduction article of any preceding or subsequent embodiment, wherein the substrate includes an undercoat comprising an AMOx catalyst in the downstream zone.

Embodiment 22

A selective catalytic reduction system comprising: a first selective catalytic reduction article formed of a first substrate including a first catalytic coating layer comprising a steam-activated iron-containing molecular sieve; and a second selective catalytic reduction article formed of a second substrate including a second catalytic coating layer comprising a copper-containing molecular sieve: wherein the first selective catalytic reduction article and the second selective catalytic reduction article are in fluid communication.

Embodiment 23

The selective catalytic reduction system of any preceding or subsequent embodiment, wherein the first substrate of the first selective catalytic reduction article is zoned so as to have a first zone comprising the first catalytic coating layer and a second zone comprising a co-catalyst layer comprising a high copper-containing molecular sieve.

Embodiment 24

The selective catalytic reduction system of any preceding or subsequent embodiment, wherein the first substrate and the second substrate are each independently selected from the group consisting of a porous wall-flow filter and a flow-through monolith.

Embodiment 25

The selective catalytic reduction system of any preceding or subsequent embodiment, wherein the second substrate includes an undercoat comprising an AMOx catalyst.

Embodiment 26

An exhaust gas treatment system comprising: a selective catalytic reduction article or a selective catalytic reduction system according to any preceding or subsequent embodiment; and a reductant injector in fluid communication with and upstream of the selective catalytic reduction article or the selective catalytic reduction system.

Embodiment 27

The exhaust gas treatment system of any preceding or subsequent embodiment, further comprising one or more of a diesel oxidation catalyst, a soot filter, and an ammonia oxidation catalyst.

Embodiment 28

The exhaust gas treatment system of any preceding or subsequent embodiment, further comprising an internal combustion engine in fluid communication with and upstream of the selective catalytic reduction article or the selective catalytic reduction system.

Embodiment 29

A method for treating an exhaust stream containing NOx, comprising passing the exhaust stream through a selective catalytic reduction article, a selective catalytic reduction system, or an exhaust gas treatment system of any preceding embodiment.

These and other features, aspects, and advantages of the disclosure will be apparent from a reading of the following detailed description together with the accompanying drawings, which are briefly described below. The invention includes any combination of two, three, four, or more of the above-noted embodiments as well as combinations of any two, three, four, or more features or elements set forth in this disclosure, regardless of whether such features or elements are expressly combined in a specific embodiment description herein. This disclosure is intended to be read holistically such that any separable features or elements of the disclosed invention, in any of its various aspects and embodiments, should be viewed as intended to be combinable unless the context clearly dictates otherwise. Other aspects and advantages of the present invention will become apparent from the following.

