JP2000308829A - Manufacture of catalyst for cleaning exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing an NOx occluding reduction catalyst for cleaning exhaust gas, wherein a noble metal carried is not covered with the NOx occluding material carried and the noble metal can be carried in a uniform distribution state. SOLUTION: In this method for manufacturing an NOx occluding reduction catalyst comprising a carrier, a noble metal carried on the carrier and at least one kind of NOx occluding element selected from alkali metals, alkaline earth metals and rare-earth elements carried on the carrier, the NOx occluding element is carried on the carrier in an adsorptively carrying step using an NOx occluding element source which is a compound containing NOx occluding elements.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a process for the preparation of _the NO _x storage-reduction type exhaust gas purifying catalyst, in particular, to oxidize the carbon monoxide contained in the exhaust gas (CO) and hydrocarbons (HC) the NO _x in the exhaust gas contains excess oxygen than that required for the production method of efficiently purifying it the exhaust gas purifying catalyst.

[0002]

BACKGROUND ART In a lean-burn engine, normally is burned with oxygen excess fuel lean condition, the system reduces and purifies NO _x exhaust gas as a reducing atmosphere is developed by the intermittent fuel stoichiometric-rich condition, Has been put to practical use. And as the best catalysts for this system, occludes NO _x in lean atmosphere, stoichiometric ~ _the NO _x storage-reduction type exhaust purifying catalyst is developing with _the NO _x storage material that releases occluded NO _x in a rich atmosphere Have been.

[0003] For example, Japanese Patent Application Laid-Open No. Hei 5-317652 discloses a NO _x storage material using an alkaline earth metal such as Ba as a NO _x storage element and Pt as a noble metal acting to oxidize CO and HC. There has been proposed an exhaust gas purifying catalyst supported on a porous oxide carrier such as alumina. Japanese Patent Application Laid-Open No. Hei 5-317652 discloses a method in which a carrier is immersed in an aqueous solution of dinitrodiamine platinum to support Pt, and then the carrier is further immersed in an aqueous solution of barium acetate.
A method for carrying a is disclosed.

[0004] However, since in this case the way the noble metal supporting the bearing after _the NO _x storage material by, _the NO _x storage material so as to overlie the noble metal supported above is allowed to carry, the exhaust gas in a valid state However, there has been a problem that the amount of noble metal contacting with the metal decreases, that is, the utilization rate of the noble metal decreases. To avoid this problem, after carrying _the NO _x storage material, it is also conceivable to adsorb support a noble metal. However, in this case, the carrying of the NO _x occluding material may be uneven, and in the case where there is unevenness, the noble metal is supported unevenly in the portion where the NO _x occluding material is not carried, and the carrying distribution of the noble metal becomes uneven. The result was that this phenomenon was caused.

[0005]

SUMMARY OF THE INVENTION The present invention relates to a conventional NO.
has been made to solve the above _{problems x} storage reduction catalyst for purification of exhaust gas faced, without noble metal supported is covered with _the NO _x storage material to be supported, and uniform distribution of the noble metal It has an object to provide a manufacturing method of the NO _x storage-reduction type exhaust purifying catalyst can be in bearing.

[0006]

The method for producing an exhaust gas purifying catalyst according to the present invention comprises a carrier, a noble metal supported on the carrier, an alkali metal, an alkaline earth metal and a rare earth element supported on the carrier. At least one NO selected
A _the NO _x storage method for producing reduced form of the catalyst for purification of exhaust gas comprising a _x storage element, and a noble metal supporting step of supporting the noble metal on the support, after the noble metal supporting step, the _the NO _x storage element is a compound comprising using _the NO _x storage element source, is adsorbed bearing the _the NO _x storage element in said carrier N
Characterized in that it comprises a O _x storage element adsorption loading step.

Further, another method for producing an exhaust gas purifying catalyst according to the present invention comprises a carrier, a noble metal supported on the carrier,
Alkali metal supported on the carrier, and at least one NO _x storage element and the comprising at NO _x production method of storage-reduction type exhaust gas purifying catalyst selected from alkaline earth metals and rare earth elements, wherein with _the NO _x storage element source is a compound containing _the NO _x storage element, and _the NO _x storage element adsorbent loading step of adsorbing bearing the _the NO _x storage element in said carrier, after the _the NO _x storage material adsorption loading step, the And a step of adsorbing and supporting a noble metal on the carrier.

[0008]

Manufacturing method of the embodiment of the invention The exhaust gas purifying catalyst of the present invention, the loading of the carrier of the NO _x storage element is performed by adsorption carrier. The method of loading the carrier of the noble metal or _the NO _x storage element is roughly divided into impregnation and adsorption carrier. The impregnation loading is such that the noble metal or the NO _x storage element is physically attached to the carrier by immersing the carrier in a salt solution containing the cation as a cation, and then is dried, calcined, or the like to be supported on the carrier surface. . On the other hand, the adsorption and loading is carried out by utilizing a chemical adsorption reaction. As will be described in detail later, the carrier is made of a porous oxide such as alumina and silica, and these porous oxides have some hydroxyl groups due to the influence of moisture in the air and the like. In the adsorption and support, a noble metal or a NO _x storage element is attached to the surface of the carrier so that the hydroxyl groups serve as sites for adsorption and replace the hydroxyl groups, and then are dried and calcined to be supported.

FIG. 1 shows the surface of a catalyst produced by a conventional method of impregnating and supporting a NO _x storage element and the catalyst produced by the production method of the present invention which is a method of adsorbing and supporting a NO _x storage element (the present invention). ) Is conceptually shown. The one shown on the left side of the conventional method in FIG. 1 is one in which a noble metal is supported later. Generally NO
_{When the x-} occluding element is impregnated and supported, the supported NO _x occluding element tends to be in a bulk state (aggregated state), and the noble metal supported later is biased toward a portion where the NO _x occluding element is not supported. Will be carried. In FIG. 1, what is shown on the right side of the conventional method is one in which a noble metal is supported first. This case, the portion of _the NO _x storage element to be impregnated supported later noble metal supported above covers occurs.

In the method for producing an exhaust gas purifying catalyst according to the present invention, the NO _x storage element is supported on the carrier by adsorption. Accordingly, as shown in the present invention product 1, If the adsorbing supported noble metals, NO _x
Both the occlusion element and the noble metal are supported on the adsorption site of the carrier. From this, NO _x
Regardless of whether the adsorption / supporting step of the occlusion element or the adsorption / supporting step of the noble metal is performed first, the NO _x occlusion element and the noble metal are supported on the carrier in a state where both are uniformly distributed without covering each other. Will be done.

[0011] In the case of impregnating a noble metal, carried out a noble metal supporting step ahead, followed by performing the adsorption loading step of _the NO _x storage element, supported without _the NO _x storage element that covers the noble metal In addition, the object of the production method of the present invention of supporting a noble metal in a uniform distribution state is achieved. Next, main components of the exhaust gas purifying catalyst to which the present production method is applied will be described.

The exhaust gas purifying catalyst to which the production method of the present invention is applied includes a carrier, a noble metal supported on the carrier, and at least one selected from an alkali metal, an alkaline earth metal, and a rare earth element supported on the carrier. a NO _x storage-and-reduction type exhaust gas purifying catalyst comprising a one NO _x storage element. The support comprises a porous oxide. The monolith type catalyst and the pellet type catalyst have different configurations.
For example, in the case of forming a monolithic catalyst, the oxide powder may be formed into a slurry, coated on the surface of a honeycomb body made of ceramics such as cordierite, a heat-resistant alloy, etc. to form a carrier, and may be formed in a pellet type. In the case of the above catalyst, the oxide may be sintered into particles of a predetermined size to form a support.

Examples of the porous oxide that can be used include alumina (Al ₂ O ₃ ), zeolite, silica (SiO ₂ ), zirconia (ZrO ₂ ), titania (T
iO ₂₎ or the like and, TiO ₂ -Al ₂ O ₃ which these complexed,
SiO ₂ —Al ₂ O ₃ and ZrO ₂ —Al ₂ O ₃ can be exemplified. For the purpose of improving the purification performance, ceria (CeO ₂ ) or ceria stabilized with zirconia (ceria-zirconia composite oxide) may be added. Since γ-alumina contains a large amount of hydroxyl groups, it is desirable that the oxide constituting the carrier contains this γ-alumina.

The noble metals supported on the carrier are mainly CO,
At the same time as the HC is oxidized, NO, which is the main component of NO _{x in} the exhaust gas, is oxidized to NO ₂ and plays a role in facilitating occlusion in the NO _x storage material. Examples of the noble metal that can be used include Pt, Rh, Pd, Ir, and Ru. Depending on the characteristics of the catalyst, one or more of these may be selectively supported. The amount of the noble metal carried is, for example, Pt, Pd per liter of carrier volume if a monolithic catalyst is formed.
Is preferably 0.1 to 20 g, and 0.5 to 1 g.
More preferably, it is 0 g. In the case of Rh,
It is preferably 0.01 to 10 g, and 0.05 to 5 g
More preferably,

[0015] _the NO _x storage element to be supported on the carrier occludes NO _x under fuel-lean atmosphere, it plays a role of releasing the NO _x under fuel stoichiometric-rich atmosphere, alkali metals, alkaline earth metals and rare earth At least one selected from elements can be used. Li, Na, K, Rb and Cs are used as alkali metals, and Be, Mg, Ca, Sr and Ba are used as alkaline earth metals.
Sc, Y, La, Ce, Pr, N
d, Dy, Yb, etc. can be exemplified. Note that NO _x
Storage element, considering that alkalinity is excellent in high _the NO _x storage capacity, an alkali metal, more preferably selected from alkali earth metals.

For example, in the case of a monolithic catalyst, the amount of NO _x storage element supported is 0.05
Desirably, it is in the range of 1 to 1 mol. Loading amount is 0.05
If the amount is less than 1 mole, the NO _x storage capacity is small, and the NO _x purification performance is reduced. If the carried amount exceeds 1 mole, the effect may be saturated and a problem may occur due to a decrease in the amount of other components. Because.

Next, the production method of the present invention, in particular, the step of supporting the noble metal and the step of supporting the NO _x storage element will be described. First, the noble metal supporting step ahead, the case of performing later loading step of _the NO _x storage element. In this case, the method of loading the noble metal is not particularly limited, and any of impregnation loading and adsorption loading can be adopted. In the case of carrying out by impregnation loading, for example, using a compound containing the above metal as a cation as a noble metal source, immersing a carrier whose water content is measured in advance in an aqueous solution of a predetermined concentration of this compound, the noble metal is supported Then, the carrier to which the noble metal is attached may be dried, fired, or the like. Alternatively, a method may be employed in which the oxide powder constituting the carrier is immersed in an aqueous solution of a compound serving as a noble metal source, the noble metal is attached to the oxide powder itself, and the carrier is simultaneously formed and supported. . For example, zirconia powder is immersed in an aqueous solution of rhodium nitrate, dried and fired to obtain zirconia powder carrying Rh, and a mixed powder of the Rh-supported zirconia powder and alumina and titania is mixed to form a slurry. This is a case where a monolithic carrier is produced by coating a ceramic honeycomb carrier substrate.

When the noble metal is adsorbed and supported, generally,
A carrier of the above-mentioned noble metal is used as a noble metal source, and the carrier is immersed in a solution of this complex, followed by drying, baking and the like to support the carrier. Therefore, the present production method may also follow this already-adsorbed method. For example, when Pt is adsorbed and supported as a noble metal, a carrier is supported in the case of a monolith type or an oxide powder which is a precursor of the carrier in a case of a pellet type is immersed in a nitric acid solution of dinitrodiamine platinum in a predetermined concentration to be adsorbed. After that, drying, baking and the like may be performed.

NO carried out after the noble metal loading step_xOcclusion source
The element supporting step is performed by adsorption and supporting. Adsorption loading method
Is not particularly limited, as in the case of precious metals,
NO _xNO as the storage element source_xMetal complex containing occlusion elements
The method using body solution, NO_xAs a storage element source
NO above_xSeeds such as methods using alkoxides of occlusion elements
Various methods can be adopted. Above all, NO_xOcclusion
NO as element source_xThose who use alkoxides as storage elements
The method allows the carrier surface to be covered with an occluding material while avoiding precious metals.
Considering this point, the manufacturing method of the present invention
In the manufacturing method, NO_xThe storage element alkoxide is N
O_xIt is desirable to use as a storage element source for adsorption
New

When an alkoxide is used as a NO _x storage element source and a NO _x storage element is adsorbed and supported on a pellet type catalyst, for example, an oxide powder constituting a carrier is immersed in an alkoxide solution, and then this powder is used. The alkoxide can be supported by drying and then heating and refluxing to hydrolyze the alkoxide. The type of alkoxide is not particularly limited, either.
₃ OM: M is the NO _x storage element described above, ethoxide (C ₂ H ₅
OM), diisopropoxide (M (OiC)
₃ H ₇ ) ₂ ) or the like may be used. When an alkaline earth metal such as Ba is used as the NO _x storage element, it is desirable to use diisopropoxide because it is relatively stable even in air and does not require special equipment.

Next, prior to the step of loading the noble metal, NO
_The case where the step of supporting the _x- occluding element is performed will be described. In this case, the precious metal supporting step performed later must be performed by adsorption and supporting. This is because, when the step of supporting the noble metal is performed by impregnation and supporting, a portion where the noble metal is supported so as to cover the previously supported NO _x storage element is generated, which causes a reduction in the NO _x storage capacity of the catalyst. Because it becomes. The method of adsorbing and supporting the NO _x storage element may be in accordance with the above-described method, and the method of adsorbing and supporting the noble metal may be in accordance with the above-described method of supporting by adsorption.

The method for producing an exhaust gas purifying catalyst of the present invention comprises:
The above two manufacturing methods may be combined. That is, first, a step of supporting a noble metal is performed, and then N
This is a production method in which a step of adsorbing and supporting an O _x storage element is performed, and thereafter, a step of adsorbing and supporting another noble metal is performed. Two or more noble metals can be supported in different steps, and the application range of the method for producing an exhaust gas purifying catalyst of the present invention is expanded.

[0023]

EXAMPLES A pellet-type exhaust gas purifying catalyst was actually manufactured as an example by the manufacturing method of the present invention based on the above embodiment. Further, according to the conventional manufacturing method of impregnating carrying _the NO _x storage element, the two catalysts of the exhaust gas purifying catalyst obtained by post-carried ahead of the exhaust gas purifying catalyst and a noble metal that is supported noble metal, was prepared as a comparative example . Then, the area of the portion of the noble metal supported on the carrier surface that is exposed to the exhaust gas (the area of the portion supported on the outermost surface) is measured with respect to these exhaust gas purifying catalysts, and the pseudo exhaust gas is brought into contact therewith. An experiment was conducted to measure the NO _x storage amount. From these measurement results, the superiority of the exhaust gas purifying catalyst according to the production method of the present invention was confirmed. Hereinafter, this will be described as an example.

<Exhaust Gas Purification Catalyst of Example> A pellet-type exhaust gas purification catalyst produced by the production method of the present invention, in which the noble metal is supported and the NO _x storage element is also supported by adsorption. Using alumina porous oxide having a carrier for the alumina 120 g, the Ba and Pt as the precious metal 2g, as _the NO _x storage element 0.2mol
It is adjusted so as to be carried.

The exhaust gas purifying catalyst was produced as follows. First, 30 gamma-alumina powder was added to 300 mL of distilled water.
g and mixed. To this, 10.86 g (0.5 g) of a 4.604 wt% aqueous solution of a platinum dinitrodiamine complex was added.
After stirring for 20 minutes, the mixture was filtered by suction. After drying this at 120 ° C. for 3 hours, 45
Calcination at 0 ° C. for 2 hours. Next, 11.9 g of this powder and acetic acid were added to 680 g of cyclohexane as a solvent, and the mixture was stirred and sufficiently mixed. Then, 25 g of barium diisopropoxide (Ba (OiC ₃ H ₇ ) ₂ ) was added. And 8
The mixture was refluxed at 0 ° C. for 2 hours. Thereafter, the resultant was washed with distilled water while being subjected to suction filtration. After drying this at 120 ° C. for 3 hours,
The mixture was calcined at 550 ° C. for 2 hours and pelletized to obtain a pellet-type exhaust gas purifying catalyst.

<Exhaust Gas Purification Catalyst of Comparative Example 1> A conventional production method of adsorbing and supporting a noble metal and impregnating and supporting a NO _x storage element, in which a noble metal is supported and then a NO _x storage element is supported. It is an exhaust gas purification catalyst produced by the above. Carrier, construction of the noble metal and NO _x storage element was the same as the catalyst for purification of exhaust gas of the embodiment.

The exhaust gas purifying catalyst was produced as follows. First, 30 gamma-alumina powder was added to 300 mL of distilled water.
g and mixed. To this, 10.86 g (0.5 g) of a 4.604 wt% aqueous solution of a platinum dinitrodiamine complex was added.
After stirring for 20 minutes, the mixture was filtered by suction. After drying this at 120 ° C. for 3 hours, 45
Calcination at 0 ° C. for 2 hours. Up to this point, it is the same as the exhaust gas purifying catalyst of the above embodiment. Next, this powder was mixed with 300 mL of distilled water and stirred. The concentration is 12.77 wt.
100 g of an aqueous barium acetate solution (containing 0.05 mol of Ba) was added and stirred. The solution was concentrated to remove water, dried at 120 ° C. for 3 hours and calcined at 550 ° C. for 2 hours. This was pelletized to obtain a pellet-type exhaust gas purifying catalyst.

[0028] The noble metal <exhaust gas purifying catalyst of Comparative Example 2> adsorbed supported, a conventional manufacturing method of impregnating carrying _the NO _x storage element, manufacturing method of supporting a noble metal on After carrying _the NO _x storage element It is an exhaust gas purification catalyst produced by the above. Carrier, construction of the noble metal and NO _x storage element was the same as the catalyst for purification of exhaust gas of the embodiment.

The exhaust gas purifying catalyst was produced as follows. First, 30 gamma-alumina powder was added to 300 mL of distilled water.
g and mixed. 100 g (containing 0.05 mol of Ba) of an aqueous barium acetate solution having a concentration of 12.77 wt% was added thereto, followed by stirring. The solution is concentrated to remove water and 1
After drying at 20 ° C. for 3 hours, it was calcined at 550 ° C. for 2 hours. Next, this powder was mixed with 300 mL of distilled water and stirred. To this, 10.86 g (containing 0.5 g of platinum) of an aqueous solution of a platinum dinitrodiamine complex having a concentration of 4.604 wt% was added, and the mixture was stirred for 20 minutes and filtered by suction. After drying at 120 ° C. for 3 hours, it was calcined at 450 ° C. for 2 hours. This was pelletized to obtain a pellet-type exhaust gas purifying catalyst.

<Measurement of Noble Metal Surface Area> The CO adsorption method was applied to each of the exhaust gas purifying catalysts of the above Examples and Comparative Examples, and distributed on the outermost surface of the carrier of each catalyst (covered with another substance). No meaning) The surface area of the noble metal (Pt) was measured. The result is shown in FIG. By comparing the exhaust gas purifying catalyst of Comparative Example 2 and Comparative Example 1, as is clear from FIG. 2, is _the NO _x storage element impregnated supported catalyst of Comparative Example 1 was in after supporting the noble metal, most The surface area of the noble metal located on the surface is small.
It is considered that this is because the NO _x storage element is supported so as to cover the supported noble metal.

[0031] In contrast, the noble metal can be adsorbed supported, the exhaust gas purifying catalyst of Example was adsorbed _the NO _x storage element, the noble metal located on the outermost surface as compared to the exhaust gas purifying catalyst of Comparative Example 1 large surface area, the _the NO _x storage element carrier after noble metal has become comparable to that of the adsorption supported exhaust gas purifying catalyst of Comparative example 2 was. Therefore, the noble metal and NO _x storage element both the exhaust gas purifying catalyst produced by the production method of the present invention of adsorbing carrier of, in spite was supported _the NO _x storage element after the noble metal, a noble metal surface Can be confirmed to be carried without covering the NO _x storage element.

<Measurement of NO _x Storage Amount> The respective exhaust gas purifying catalysts of the above Examples and Comparative Examples were exposed to simulated exhaust gas to measure the NO _x storage amounts of the respective exhaust gas purifying catalysts. The NO _x occlusion amount is determined by flowing a pseudo exhaust gas which is a fuel lean atmosphere, and a NO _x occlusion amount (saturated NO _x occlusion amount) until saturation in the lean atmosphere and a pseudo exhaust gas which becomes a fuel rich atmosphere after the saturation. After flowing for 2 seconds, two measurements were taken, namely, the NO _x storage amount until reaching saturation again (rich spike (RS) NO _x storage amount).
The total exhaust gas flow rate was 6.6 L / min, the space velocity was 10,000 hour ^-1, and the compositions of the lean gas and the rich gas were as shown in Table 1 below.

[0033]

[Table 1] FIG. 3 shows the measured saturated NO _x storage amount and rich spike NO _x storage amount of the exhaust gas purifying catalysts of the example and the comparative example.

As is apparent from FIG. 3, the catalyst for purifying exhaust gas of the embodiment has a saturated N
Both the O _x storage amount and the rich spike NO _x storage amount are large and have high NO _x storage capacity. NO in the exhaust gas from being occluded become oxidized NO ₂ by the catalytic action of the noble metal, NO _x storage capacity, the surface area of the precious metal that is located on the outermost surface of the carrier, by its dispersibility, large It depends.

The exhaust gas purifying catalyst of the embodiment, NO _x storage amount be the surface area of the precious metal that is located on the outermost surface as compared with approximately equal in Comparative Example 2 a catalyst for purification of exhaust gas of the carrier is large, located on the outermost surface It can be seen that the surface area of the noble metal is large and the noble metal is uniformly dispersed. Accordingly, aspects of the exhaust gas purifying catalyst produced by the production method of the present invention to adsorb carrying both the noble metal and _the NO _x storage element, without the precious metal is covered with _the NO _x storage element, are uniformly dispersed catalyst Can be confirmed.

[0036]

The method for producing the exhaust gas purifying catalyst of the present invention.
Is different from the conventional manufacturing method. _xAbsorbs occlusion elements
It is configured to include a step of supporting on a carrier by wearing and supporting
Is what you do. With this configuration, the book
The method for producing an exhaust gas purifying catalyst of the present invention is carried on a carrier.
Noble metal is NO_xWithout being covered by occlusion elements
In other words, it is a method that can easily produce a uniformly dispersed catalyst.
For purifying exhaust gas produced by the production method of the present invention.
The catalyst is a catalyst having an excellent exhaust gas purification ability.

[Brief description of the drawings]

[1] and the conventional NO _x storage element the impregnation produced by the method of supporting the surface of the catalyst, NO _x
The surface of a catalyst (product of the present invention) produced by the production method of the present invention, which is a method of adsorbing and supporting an occlusion element, is conceptually shown.

FIG. 2 shows the surface area of the noble metal distributed on the outermost surface of the carrier, measured for the exhaust gas purifying catalysts of Examples and Comparative Examples.

FIG. 3 shows the saturated NO _x storage amount and the rich spike NO _x storage amount measured for the exhaust gas purifying catalysts of Examples and Comparative Examples.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4D048 AA06 AB02 BA03X BA14X BA30X BB01 4G069 AA03 AA08 BA01B BA27C BC01A BC08A BC13B BC38A BC69A BC75B CA02 CA03 CA09 CA13 DA05 FA02 FB19 FB61

Claims (3)

[Claims] 1. A carrier, a noble metal supported on the carrier,
Alkali metal supported on the carrier, and at least one NO _x storage element and the comprising at NO _x production method of storage-reduction type exhaust gas purifying catalyst selected from alkaline earth metals and rare earth elements, wherein a noble metal supporting step of supporting the noble metal on the support, after the noble metal supporting step, the _the NO _x storage element is a compound containing with _the NO _x storage element source, NO adsorbing bearing the _the NO _x storage element in said carrier _A method for producing an exhaust gas purifying catalyst, comprising: 2. A carrier, a noble metal supported on the carrier,
Alkali metal supported on the carrier, and at least one NO _x storage element and the comprising at NO _x production method of storage-reduction type exhaust gas purifying catalyst selected from alkaline earth metals and rare earth elements, wherein with _the NO _x storage element source is a compound containing _the NO _x storage element, and _the NO _x storage element adsorbent loading step of adsorbing bearing the _the NO _x storage element in said carrier, after the _the NO _x storage material adsorption loading step, the A process for adsorbing and supporting a noble metal on the carrier. 3. The method for producing an exhaust gas purifying catalyst according to claim 1, wherein the NO _x storage element source is an alkoxide of the NO _x storage element.