SCR CATALYST

Abstract

A highly practical SCR catalyst excellent in NO.sub.x purification performance is provided. The SCR catalyst includes a blend of an aluminosilicate molecular sieve that has supported thereon copper as an extra-framework metal and that has a CHA framework, and a silicoaluminophosphate molecular sieve that has a CHA framework, and is adapted to perform selective catalytic reduction of NO.sub.x. In the SCR catalyst, the silicoaluminophosphate molecular sieve and the aluminosilicate molecular sieve contain silicoaluminophosphate and aluminosilicate, respectively, in a molar ratio of silicoaluminophosphate:aluminosilicate of 0.1:1.0 to 0.4:1.0.

Claims

1. An SCR catalyst adapted to perform selective catalytic reduction of NO.sub.x, comprising a blend of an aluminosilicate molecular sieve having supported thereon copper as an extra-framework metal and having a CHA framework, and a silicoaluminophosphate molecular sieve having a CHA framework, wherein the silicoaluminophosphate molecular sieve and the aluminosilicate molecular sieve contain silicoaluminophosphate and aluminosilicate, respectively, in a molar ratio of silicoaluminophosphate:aluminosilicate of 0.1:1.0 to 0.4:1.0.

2. The SCR catalyst according to claim 1, wherein a mass ratio of the extra-framework metal of the aluminosilicate molecular sieve to that of the silicoaluminophosphate molecular sieve is 1.0:0.0 to 1.0:1.0.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1 is a graph that shows results of an experiment for verifying the relation between the molar proportion of silicoaluminophosphat/aluminosilicate and the NO.sub.x purification rate at an evaluation temperature of 450 C.;

[0024] FIG. 2 is a graph that shows results of the experiment for verifying the relation between the molar proportion of silicoaluminophosphat/aluminosilicate and the NO.sub.x purification rate at an evaluation temperature of 410 C.;

[0025] FIG. 3 is a graph that shows results of the experiment for verifying the relation between the molar proportion of silicoaluminophosphat/aluminosilicate and the NO.sub.x purification rate at an evaluation temperature of 330 C.;

[0026] FIG. 4 is a graph that shows results of an experiment for verifying the relation between a catalyst coating amount and catalyst performance; and

[0027] FIG. 5 is a graph that shows results of an experiment for verifying the relation between a catalyst coating amount and a pressure loss.

DETAILED DESCRIPTION

(Embodiment of the SCR Catalyst)

[0028] The SCR catalyst of the present disclosure includes a catalyst layer that contains a blend of an aluminosilicate molecular sieve that supports thereon copper as an extra-framework metal and has a CHA framework and a silicoaluminophosphate molecular sieve that has a CHA framework, and a substrate. The catalyst layer is formed on a cell wall surface of the substrate so as to form the overall structure of the SCR catalyst.

[0029] The SCR catalyst is provided in an exhaust gas purification system (not shown). The exhaust gas purification system includes, for example, an internal combustion engine that discharges exhaust gas, a diesel oxygen catalyst (DOC), a diesel particulate filter (DPF), an urea tank that supplies an exhaust path with urea water, the SCR catalyst, and an ammonia slip catalyst (ASC).

[0030] The substrate of the SCR catalyst is a carrier with a honeycomb structure capable of supporting the catalyst layer, and is made of ceramics, SiC, metal, and the like.

[0031] Further, the aluminosilicate molecular sieve of the catalyst layer that has supported thereon copper as an extra-framework metal and has a CHA framework is a copper ion-exchanged zeolite, such as Cu-SSZ13 and Cu-SSZ62, and the silicoaluminophosphate molecular sieve that has a CHA framework is a proton-type zeolite, such as H-SAPO34, H-SAPO44, and H-SAPO47.

[0032] Herein, the silicoaluminophosphate molecular sieve and the aluminosilicate molecular sieve contain silicoaluminophosphate and aluminosilicate, respectively, in a molar ratio of silicoaluminophosphate:aluminosilicate of 0.1:1.0 to 0.4:1.0 (which is represented as a molar proportion of silicoaluminophosphate/aluminosilicate of 0.1 to 0.4).

[0033] Furthermore, the aluminosilicate molecular sieve has supported thereon Cu as the extra-framework metal, while the silicoaluminophosphate molecular sieve does not have an extra-framework metal supported thereon.

[0034] When an aluminosilicate molecular sieve that has copper supported thereon is exposed to high temperature, copper oxide particles are formed so that oxidation of ammonia is increased, and as a result, NO.sub.x purification performance is lowered. In contrast, in the SCR catalyst of the present disclosure, copper oxide particles are captured by the silicoaluminophosphate molecular sieve to suppress the formation of copper oxide particles, so that oxidation of ammonia can be suppressed, and as a result, the NO.sub.x purification performance can be improved.

[0035] In the SCR catalyst of the present disclosure, the number of moles of silicoaluminophosphate is reduced as much as possible so as to set the molar ratio of silicoaluminophosphate:aluminosilicate to 0.1:1.0 to 0.4:1.0, such that the silicoaluminophosphate serves as an auxiliary material for trapping copper oxide particles. This effectively suppresses deterioration of the SCR catalyst due to water adsorption and desorption, thereby making it a highly practical SCR catalyst.

(Results of an Experiment for Verifying the Relation Between the Molar Proportion of Silicoaluminophosphat/Aluminosilicate and the NO.SUB.x .Purification Rate)

[0036] The present inventors produced, through the following process, an SCR catalyst test sample with variations of the molar proportion of silicoaluminophosphate/aluminosilicate shown in Table 1 below, and conducted an experiment for verifying the NO.sub.x purification rate of the sample at evaluation temperatures of 450 C., 410 C., and 330 C.

[0037] Herein, the SCR catalyst test sample was produced through the following process: Cu-SSZ13 was prepared so as to contain 3.0 mass % Cu and have a molar ratio of Si:Al of 13:2, and H-SAPO34 was prepared so as to have a molar ratio of Si:Al:P of 17:50:33. The catalyst was prepared through the following process: the Cu-SSZ13 and H-SAPO34, SiO.sub.2 sol, and H.sub.2O were mixed and agitated so as to form slurry, which was then applied to a cordierite honeycomb substrate, dried at 150 C., and baked at 550 C. for two hours in the air, and the SCR catalyst test sample was thus produced.

[0038] In this experiment, a catalyst with a volume of 15 cc was cut out to be used as the test sample, and a transient evaluation of simulated SCR reaction of the test sample was conducted using a model gas evaluation apparatus. Herein, the composition of gas in each of rich and lean states is shown in Table 2 below. The state of gas was switched between rich and lean states with 10 seconds for the rich state and 60 seconds for the lean state, and the space velocity (SV) was 85700 (1/h).

TABLE-US-00001 TABLE 1 SAPO/SSZ SAPO/SSZ proportion proportion Cu-SSZ13 H-SAPO34 (mass ratio) (molar ratio) (g/L) (g/L) () () 150 0 0.00 0.00 144 6 0.04 0.04 136 14 0.10 0.10 125 25 0.20 0.20 107 43 0.40 0.41 100 50 0.50 0.51 90 60 0.67 0.68 Note: g/L represents each of the masses of Cu-SSZ13 and H-SAPO34 per catalyst with a volume of one litter.

TABLE-US-00002 TABLE 2 O.sub.2 NO NH.sub.3 H.sub.2O (%) (ppm) (ppm) (%) Rich 0 150 550 5 Lean 10 50 0 5

[0039] The results of the experiment are shown in FIGS. 1 to 3. Herein, FIGS. 1 to 3 show the results of the experiment for verifying the relation between the molar proportion of silicoaluminophosphat/aluminosilicate and the NO.sub.x purification rate at evaluation temperatures of 450 C., 410 C., and 330 C., respectively. Each of FIGS. 1 to 3 shows an approximate curve representing plots of the results of the experiment.

[0040] It can be understood from FIG. 1 showing the results of the experiment at an evaluation temperature of 450 C. that with SAPO/SSZ molar proportions of 0.1 and 0.4, inflection points appear; with a SAPO/SSZ molar proportion in the range of 0.1 to 0.4, high NO.sub.x purification performance with a NO.sub.x purification rate of 60% or higher is exhibited; and that with a SAPO/SSZ molar proportion of less than 0.1 or greater than 0.4, the NO.sub.x purification rate significantly decreases.

[0041] Further, FIG. 2 showing the results of the experiment at an evaluation temperature of 410 C. shows a tendency similar to that of FIG. 1. It can be understood from FIG. 2 that with SAPO/SSZ molar proportions of 0.1 and 0.4, inflection points appear; with a SAPO/SSZ molar proportion in the range of 0.1 to 0.4, quite high NO.sub.x purification performance with an NO.sub.x purification rate of 90% or higher is exhibited; and that with a SAPO/SSZ molar proportion of less than 0.1 or greater than 0.4, the NO.sub.x purification rate decreases.

[0042] Furthermore, it can be understood from FIG. 3 showing the results of the experiment at an evaluation temperature of 330 C. that with a SAPO/SSZ molar proportion of greater than 0.4, the NO.sub.x purification rate gradually decreases.

[0043] It can be understood from each of FIGS. 1 to 3 that at an evaluation temperature of 450 C., with a SAPO/SSZ molar proportion in the range of 0.1 to 0.4, a higher NO.sub.x purification rate was exhibited as compared to that with a SAPO/SSZ molar proportion in other ranges. It is considered that this is because with a SAPO/SSZ molar proportion in the range of 0.1 to 0.4, copper oxide particles are captured by SAPO so as to become innoxious when catalyst performance is exhibited.

[0044] Based on the results of the experiment, the SCR catalyst of the present disclosure was defined such that the silicoaluminophosphate molecular sieve and the aluminosilicate molecular sieve contain silicoaluminophosphate and aluminosilicate, respectively, in a molar ratio of silicoaluminophosphate:aluminosilicate of 0.1:1.0 to 0.4:1.0 (which is represented as a molar proportion of silicoaluminophosphate/aluminosilicate of 0.1 to 0.4).

(Results of Experiments for Verifying the Relations Between a Catalyst Coating Amount and Catalyst Performance and Between a Catalyst Coating Amount and a Pressure Loss)

[0045] The present inventors further conducted experiments for verifying the relations between a catalyst coating amount and catalyst performance and between a catalyst coating amount and a pressure loss with variations of the masses of the aluminosilicate molecular sieve and the silicoaluminophosphate molecular sieve and the catalyst coating amount, as shown in Table 3 below.

[0046] In the experiments, the gas shown in Table 4 below was circulated for five hours at a temperature of 800 C. The state of the gas was switched between rich and lean states with 10 seconds for the rich state and 60 seconds for the lean state, and the space velocity SV was 114000 (1/h).

TABLE-US-00003 TABLE 3 Cu-SSZ13 H-SAPO34 Total coating amount (g/L) (g/L) (g/L) 120 0 120 95 25 60 60 150 0 150 125 25 90 60 180 0 180 155 25 120 60

TABLE-US-00004 TABLE 4 O.sub.2 CO H.sub.2O (%) (%) (%) Rich 0 2 10 Lean 10 0 10

[0047] The results of the experiments are shown in FIGS. 4 and 5. Herein, FIG. 4 shows the results of the experiment for verifying the relation between a catalyst coating amount and catalyst performance, and FIG. 5 shows the results of the experiment for verifying the relation between a catalyst coating amount and a pressure loss.

[0048] It can be understood from FIG. 4 that the catalysts containing H-SAP034 with total coating amounts of 150 g/L and 180 g/L exhibit high catalyst performance.

[0049] In addition, it can be understood from FIG. 5 that with a greater total coating amount, the pressure loss of the catalyst increases.

[0050] In view of the results shown in FIGS. 4 and 5, it is considered that with a total coating amount of 150 g/L or greater and replacement of a portion of Cu-SSZ with H-SAP034, the increase in the pressure loss is suppressed so that the catalyst performance is improved.

[0051] Although the embodiments of the present disclosure have been described in detail with reference to the drawings, specific structures are not limited thereto, and any design changes that may occur within the spirit and scope of the present disclosure are all included in the present disclosure.