Stabilization of aqueous urea solutions containing organometallic catalyst precursors

Abstract

The present invention relates to the stabilization of aqueous urea solutions containing organometallic catalyst precursors. Stabilization can be achieved by monitoring and controlling the solution pH.

Description

[Technical field]

[0001] FIELD OF THEINVENTION The present invention relates to the stabilization of aqueous urea solutions containing organometallic catalyst precursors. Stabilization can be achieved by monitoring and controlling the solution pH. [Background technology]

[0002] Internal combustion engines, such as those found in cars and trucks, can produce combustion by-products and / or incomplete combustion products that are present in the engine exhaust and can be discharged into the environment. Due to emission regulations, the exhaust can be treated to reduce the concentration of such products, thus reducing pollution. Spark ignition (i.e., gasoline) engines may use three-way catalytic converters to meet emission regulations, but compression ignition (i.e., diesel) engines typically use two-way catalytic converters and cannot efficiently reduce nitrogen oxides (NOx). Thus, diesel engines may include selective catalytic reduction (SCR) systems to seek to reduce nitrogen oxide concentrations. Improving the performance of such systems remains an area of ​​ongoing research and development. Summary of the Invention [Means for solving the problem]

[0003] The present invention relates to a method for controlling the pH level of a diesel exhaust fluid reservoir, the reservoir containing an aqueous urea solution including a catalyst precursor including a water soluble organometallic compound, wherein the reservoir includes a pH probe for monitoring the pH of the solution, the pH being controlled to a value below 10.0.

[0004] In a related embodiment, the invention further relates to a method of controlling the pH level of a diesel exhaust comprising an aqueous urea solution including a catalyst precursor comprising a water-soluble organometallic compound, wherein a reservoir includes a pH probe for monitoring the pH of said solution, and the pH is controlled to a value of 10.0 or less by selecting and implementing at least one or more of the following pH adjustment procedures: (i) introducing exhaust gas including CO2 into said reservoir; (ii) introducing air into said reservoir; or (iii) introducing NOx into said reservoir.

[0005] In yet a further related embodiment, the invention relates to a diesel exhaust fluid system including a diesel exhaust fluid reservoir for containing an aqueous urea solution including a catalyst precursor including a water soluble organometallic compound, wherein the reservoir includes a pH probe for monitoring a pH of said solution. A controller is provided that is configured to monitor the pH and control the pH to a level of 10.0 or less by selecting and performing at least one of the following: (i) introducing exhaust gas including CO2 into the reservoir; (ii) introducing air into the reservoir; or (iii) introducing NOx into said reservoir.

[0006] Various aspects and advantages of the present disclosure may be better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings. [Brief description of the drawings]

[0007] [Figure 1] The following is an overview, in block diagram form, of a diesel engine discharging exhaust gases into an exhaust gas aftertreatment system. [Diagram 2] Explain the possible reactions of urea in the exhaust gas environment of an SCR system. [Figure 3A] The effect of adding 0.1 mol % ATO to an aqueous urea solution on deposit formation is described. [Figure 3B] The effect of adding 0.05 mol % ATO to an aqueous urea solution on deposit formation is described. [Figure 3C]The effect of aqueous urea solutions without the addition of ATO on deposit formation is described. [Figure 4] The effect of adding ATO to an aqueous urea solution, as assessed by thermogravimetric analysis (TGA), is described. [Diagram 5] 1 illustrates, in block diagram form, an overview of a diesel engine emitting exhaust gases and the use of pH control to regulate the pH of a diesel exhaust fluid reservoir. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Urea is used as a convenient source of ammonia (NH3) reductant in SCR systems to reduce NOx emissions from diesel engines. Thus, urea (H2NCONH2) is a component of what is called Diesel Exhaust Fluid (DEF) that is preferably introduced as a spray into the exhaust gas upstream of the SCR catalyst. The DEF evaporates and the urea pyrolyzes to NH3 and HNCO (isocyanic acid), followed by hydrolysis of HNCO to produce additional amounts of NH3 along with CO2: [ka]

[0009] The ammonia formed above can then be utilized as a reducing agent in selective catalytic reduction (SCR) to reduce NOx to nitrogen and water. Such reactions include: 4NH3+ 4NO + O2 → 4N2+ 6H2O 2NH3+ NO + NO2 → 2N2+ 3H2O 8NH3+ 6NO2 → 7N2+ 12H2O

[0010] However, urea has been observed to deposit on the walls of exhaust systems and / or SCR catalysts, either as solid urea or as a mixture of urea and isocyanic acid deposits. Over time, in the presence of relatively hot exhaust gases, secondary reactions can occur in which such deposits (urea and isocyanic acid) are converted into a variety of relatively high molecular weight compounds. Such high molecular weight compound deposits can include one or more of the following:

[0011] [Table 1]

[0012] [Table 2]

[0013] When deposits form, they act to reduce the efficiency of exhaust gas treatment by reducing exhaust gas flow and / or by reducing the efficiency of the SCR catalyst from reacting with ammonia, which is effective in reducing NOx gas concentrations. In addition, it can be appreciated that heating to high temperatures, for example in the range of 193°C to 700°C, can help break down the relatively high molecular weight and polymer-type deposits identified above, but this can add a degree of complexity to the system and other associated problems.

[0014] 1, there is shown in block diagram form an overview of a compression ignition engine 10, such as a diesel engine, that emits exhaust into an exhaust aftertreatment system 12, which may include a diesel oxidation catalyst (DOC) chamber 14, which connects to a diesel particulate filter (DPF) 16, which in turn feeds into a selective catalytic reduction (SCR) system 20. The DOC 14 may serve to convert NO to NO2 and remove hydrocarbons (HC) according to the following general reactions: 2NO + O2 → 2NO2 2CO + O2 → 2CO2 4HC + 5O2 → 4CO2 + 2H2O

[0015] A diesel exhaust fluid (DEF) injector and reservoir is provided at 18 which contains an aqueous urea solution which this time also contains a catalyst precursor. Preferably the aqueous urea based solution used will typically include one having between 30.0 wt.% and 40.0 wt.% urea, preferably 32.5 wt.% urea.

[0016] The catalyst precursors herein are preferably those that have the requisite amount of solubility in an aqueous urea solution and can be decomposed at temperatures similar to those required to convert urea to ammonia. Preferably, the temperature range for decomposition of the catalyst precursor is between 45°C and 180°C, more preferably between 75°C and 125°C. Such catalyst precursors, when decomposed at such temperatures in an exhaust gas aftertreatment system, act to provide a target catalyst that serves to accomplish one or more of the following: (1) hydrolyze isocyanic acid to form ammonia that aids in SCR; (2) reduce and / or eliminate the accumulation of relatively high molecular weight deposits that may otherwise form from isocyanic acid; and (3) decompose relatively high molecular weight deposit compounds (e.g., biuret, triuret, cyanuric acid, ammelide, ammeline, and / or melamine) found in exhaust gas aftertreatment systems such as the SCR system 20. It is contemplated that this decomposition occurs at temperatures lower than those typically required for their thermal decomposition (e.g., pyrolysis) and removal.

[0017] The catalyst precursors herein preferably include a water-soluble organometallic compound that upon decomposition can provide the above-mentioned catalytic activity. Preferably, such organometallic compounds include titanium(IV) acetate (CH), which is water-soluble and has a boiling point of 117° C., and thus suitable for use as part of diesel exhaust fluids. 12 Titanium(IV) acetates include titanium(IV) acetates such as O8Ti. Upon heating (exposure to high temperature exhaust gases), titanium(IV) acetate decomposes to provide catalytic titanium oxide, which may proceed according to the following general reaction: C8H 12 O8Ti + 8O2→ TiO2+ 8CO2+ 6H2O

[0018] Thus, titanium dioxide (TiO2) as a subject catalyst may now advantageously serve to improve exhaust gas treatment by hydrolyzing any isocyanic acid present to ammonia in order to reduce and / or eliminate the build-up of relatively high molecular weight deposits. In addition, as previously mentioned, TiO2 can decompose any relatively high molecular weight deposits that may be present, and can do so at relatively lower temperatures than are typically used for pyrolysis.

[0019] Another water-soluble organometallic that can be mixed into DEF is titanium(IV) bis(ammonium lactato)dihydroxide (CH 18 NOTi), which is available as a 50 wt.% aqueous solution (Sigma-Aldrich) with a flash point of 27°C. Thus, upon exposure to high temperatures in the exhaust gas, the formation of the desired titanium dioxide catalyst is explained by the following: [CH3CH(O-)CO2NH4]2Ti(OH)2+6O2→TiO2+6CO2+6H2O+2NH3

[0020] Another candidate catalyst precursor is contemplated to include titanium(III) oxalate decahydrate (Ti2(C2O4)3-10H2O). It may therefore be appreciated that, preferably, the catalyst precursor may thereby include one or more of titanium(IV) acetate, Ti(IV) bis(ammonium lactato) dihydroxide, or titanium(III) oxalate. Thus, mixtures of such water-soluble organometallic catalyst precursors may be used.

[0021] Other preferred catalyst precursors are contemplated to include ammonium titanyl oxalate (ATO), CONHT, having the following structure: [ka]

[0022] Even more preferred precursors include titanium oxyacetylacetonate, C 10 H 14 O5Ti included: [ka]

[0023] Even more preferred precursors include ZrO2, such as zirconia dioxide (ZrO2). 4+ Similar to TiO2 disclosed above, it is contemplated that ZrO2 may similarly function to improve exhaust gas treatment by hydrolyzing any isocyanic acid present to ammonia: [ka]

[0024] Thus, for a given aqueous urea solution containing a catalyst precursor comprising a water-soluble organometallic compound, such as those listed above, which decomposes to form TiO2, a ZrO2 4+ It is contemplated herein that compounds of the type may be included. As can be appreciated, if isocyanate deposits form on the walls of the exhaust system or on the SCR catalyst, ZrO2 can additionally serve to reduce such deposits and improve SCR performance.

[0025] Thus, in the broad context of the present disclosure, the organometallic compound is preferably selected from organotitanium compounds having solubility in water as described above, where the solubility is preferably at a level sufficient to decompose the organotitanium compound as a catalyst precursor in the exhaust gas and provide a sufficient amount of catalyst (TiO2) to hydrolyze the isocyanic acid present to ammonia. The water solubility of the organometallic compound catalyst precursor is contemplated to be at a level ranging from 0.1 wt.% to 25.0 wt.%, more preferably 0.1 wt.% to 10.0 wt.%, and 0.1 wt.% to 5.0 wt.%. The solubility is such that it is contemplated to be maintained at a temperature range between -11°C (the freezing point of a 32.5% urea solution in water) and 50°C.

[0026] It is further contemplated that the amount of catalyst derived from the water-soluble organometallic precursor is preferably formed in the exhaust gas aftertreatment system 12 at a level of 0.01 wt.% to 1.0 wt.% in the DEF with aqueous urea solution introduced therein. Thus, preferably, titanium(IV) acetate (CH) in aqueous urea solution is preferably formed in an amount such that the level of titanium oxide in the injected DEF upon delivery and decomposition to titanium oxide in the exhaust gas aftertreatment system is in the range of 0.01 wt.% to 1.0 wt.%. 12 O8Ti) precursors may be used. Additionally, as alluded to above, optionally, ZrO2 may be included in the urea solution to enhance its ability to reduce isocyanate deposits. Thus, it is contemplated that the level of ZrO2 in the urea solution will similarly range from 0.01 wt.% to 1.0 wt.%, either by itself or in combination with an organometallic precursor catalyst.

[0027] Turning now to Figure 2, another useful summary of the disclosure herein is now provided. As shown, DEF (aqueous urea solution) can decompose when exposed to heat from exhaust gas (approximately 100°C) to provide both isocyanic acid and ammonia, which is desirable for feeding to an SCR system. Isocyanic acid tends to be converted, by what are widely identified as undesirable reaction pathways, to deposits of relatively high molecular weight compounds, including one or more of biuret, cyanuric acid, ammelide, ammeline, melamine, and other high molecular and polymer-type compounds. Such compounds impair the efficiency of the SCR system to reduce NOx levels.

[0028] Thus, by placing the catalyst precursor herein in the DEF, as shown in FIG. 2, the catalyst precursor is such that when exposed to exhaust gas and a temperature preferably in the range of about 100° C. to 300° C., the catalyst precursor (a soluble organometallic compound) is converted to a catalyst (e.g., titanium oxide), which reduces or eliminates the conversion of isocyanic acid (HNCO) to relatively high molecular weight deposits. Instead, as shown, the isocyanic acid is converted to additional amounts of ammonia for the SCR process. It should be noted that the formation of deposits may be particularly pronounced at temperatures in the range of 185° C. to 250° C. Thus, it is particularly advantageous to use the catalyst precursor herein to reduce the formation of deposits that occur in the temperature range of 185° C. to 250° C.

[0029] In addition, the catalyst precursors herein can serve to promote the decomposition at relatively low temperatures of relatively high molecular weight deposit-forming compounds including biuret, cyanuric acid, ammelide, ammeline and melamine, where such decomposition can occur in the temperature range of 130°C to 300°C.

[0030] Referring now to FIG. 3, images identified as "A", "B" and "C" show how the addition of ATO to a urea aqueous solution can affect sediment formation. Image "A" in FIG. 3 has the highest concentration of ATO at 0.1 mol% (1.74 g ATO / 1 liter UWS). Image "B" in FIG. 3 has a concentration of ATO at 0.05 mol% (0.87 g ATO / 1 liter UWS). Image "C" in FIG. 3 is UWS with no ATO added. It is clear that increasing the concentration of ATO incorporated into the UWS results in a decrease in the amount of sediment formed.

[0031] Referring now to FIG. 4, the use of thermogravimetric analysis (TGA) in conjunction with a Fourier transform infrared (FTIR) spectrophotometer is identified. Samples were prepared by incorporating a specific concentration of ATO with urea by lyophilization (freeze drying). The samples were then subjected to TGA and analyzed for evolved gases using IR spectroscopy. FIG. 4 shows overlaid TGA data from three samples: pure urea, 0.5 mol% ATO in urea, and 2.0 mol% ATO in urea. The Gram-Schmidt value shown on the plot is an indication of the temperature at which the maximum amount of gas was evolved from the sample. As a general trend, as the concentration of ATO in the sample increased, the maximum off-gassing event occurred at a lower temperature. The decrease in temperature of the Gram-Schmidt maximum indicates that ATO catalyzes the total decomposition of urea, as shown in the following reaction: CO(NH2)2+ H2O → 2NH3+ CO2

[0032] To further confirm that the use of catalyst precursors herein reduces deposits, a spike test evaluation was performed. Two conditions were considered. Condition 1 consisted of a medium duty cycle injection at a relatively low temperature. More specifically, for 1.0 hour, the DEF injector duty cycle was 13.1%, the target gas temperature was 215°C, the engine speed was 1375 rpm, and the exhaust flow rate was 670 kg / hr. Condition 2 consisted of a high duty cycle injection at a high temperature. In both cases, after 1.0 hour, the exhaust pipe was disassembled and the deposits were collected and weighed.

[0033] In condition 1, utilizing DEF, i.e., urea solution, 0.4 g of sediment was recovered from the mixer region and 4.2 g from the elbow region after 1.0 hour of operation, for a total of 4.6 g of sediment. In contrast, when 0.2 mol.% ATO was used in the same urea solution, only 0.2 g of sediment was recovered from the mixer region and 0.7 g of sediment was recovered from the elbow region, for a total mass of sediment recovered of 0.9 g. Thus, as can be seen, in the absence of ATO catalyst precursor, the mass of sediment increased by 411% (0.9 g vs. 4.6 g). In condition 2, again utilizing urea solution, 6.4 g of sediment was recovered from the mixer region after 1.0 hour of operation. In contrast, when 0.2 mol.% ATO was used in the same urea solution, only 3.5 g of sediment was recovered from the mixer region. As can be seen, in the absence of ATO catalyst precursor, the mass of the deposit increased by 45% (6.4 g vs. 3.5 g).

[0034] Thus, some of the notable benefits of the above-mentioned use of the catalyst precursor herein include the ability to improve emissions thereby allowing for DEF injection at relatively low exhaust temperatures (e.g., 185°C to 250°C) and reduce the formation of deposits that impair the efficiency of SCR systems. In other words, the catalyst precursor herein will provide a reduction in cold start emissions in vehicles equipped with SCR catalysts. In addition, the reduction in deposit formation described herein is such that it is contemplated to reduce system back pressure. System back pressure, in turn, results in increased fuel consumption. Thus, reducing deposits will reduce the fuel consumption of the vehicle. Additionally, reducing deposits can reduce or avoid the need for high temperature regeneration periods to break down and remove deposits. Furthermore, the high molecular weight deposits identified herein are such that they tend to have a corrosive effect when in contact with metals utilized in exhaust pipes. Thus, the reduction or elimination of such deposits is contemplated to extend the life of metal exhaust systems and reduce or eliminate the loss of their structural integrity.

[0035] As will now be appreciated from all of the above, injection of an aqueous urea solution containing a catalyst precursor comprising a water-soluble organometallic compound is an effective method for reducing deposits. However, it has also been observed that the stability of the organometallic catalyst precursor can be problematic due to variations in pH of the aqueous urea solution containing the precursor. As the temperature of the aqueous urea solution increases, the alkalinity of the solution also increases due to an increase in the concentration of dissolved ammonium and hydroxide ions. This change in pH can cause the organometallic compound to precipitate out of solution, reducing its effectiveness. It has now been appreciated, therefore, that it is preferable to monitor the pH of the aqueous urea solution containing the organometallic catalyst precursor and maintain it within an appropriate pH window. More specifically, it is preferable to monitor the pH of the aqueous urea solution containing the organometallic catalyst precursor and maintain it at a pH less than or equal to 10.0 (i.e., 10.0 or less), and in a more preferred embodiment, at a pH in the range of 7.0 to 10.0.

[0036] Turning to Figure 5, tailpipe exhaust is shown to contain carbon dioxide (CO2) emissions that may be present at levels between 6.0% and 16.0% by volume. Thus, as shown in Figure 5, and as shown diagrammatically by path 22, it is contemplated that a portion of the tail gas containing CO2 can be recycled to the aqueous urea solution containing the organometallic catalyst precursor. The following can then occur to form carbonic acid: CO2+ H2O → H2CO3

[0037] As a result of the introduction of CO2 and the subsequent formation of carbonic acid (H2CO3), the alkalinity of the aqueous urea solution containing organometallic precursors contained in the reservoir 18 (which may otherwise tend to increase due to an increase in temperature) can be adjusted to maintain the pH at a value below 10.0, and in particular in the range of 7.0 to 10.0. It is therefore also preferred to place a pH indicator, such as an ion-selective electrode or pH probe with feedback, in the aqueous urea solution containing organometallic precursors, as shown diagrammatically at 24. Additionally, in a more preferred arrangement, the exhaust path 22 of the tailpipe may include a CO2 membrane filter 26 for filtering the tailpipe emissions and passing only CO2 therefrom. Such a membrane may preferably include a CO2 membrane as described in U.S. Pat. No. 8,454,732. It may therefore be appreciated that by monitoring the ion-selective electrode or pH indicator 24, the amount of CO2 can be selectively controlled to maintain the pH of the aqueous urea solution containing organometallic precursors at a desired level as referred to herein.

[0038] Additionally, as shown, reservoir 18 may be provided with an air intake 28. Thus, it is contemplated that if the aqueous urea solution containing the organometallic precursor becomes acidic (e.g., a pH below 7.0) due to a relatively high concentration of CO2 and subsequent carbonic acid formation, air may be introduced, which will reduce the CO2 concentration in reservoir 18, and thus an acidic pH may be minimized or avoided. Thus, pH probe 24 may be re-monitored thereby, and if the pH of the reservoir containing the aqueous urea solution and organometallic catalyst precursor becomes acidic or begins to decrease at an unacceptable rate, air may be introduced to attenuate or prevent such pH changes.

[0039] Additionally, within the DOC chamber 14, NO is being converted to NO2, and therefore a portion of the exhaust immediately upstream of the chamber 14 may be sent via path 30 to a reservoir 18 containing the urea solution and organometallic precursors. The introduction of NOx (NO and / or NO2) may be hydrolyzed in water to form nitric acid (HNO3) and / or nitrous acid (HNO2). The introduction of such acids into the chamber 18 will serve to control the pH of the urea solution and organometallic catalyst precursors, again thereby providing acidity and achieving a pH value greater than 10.0 due to the elevated temperature, as described herein.

[0040] From the above, it should now be appreciated that control of the pH level of a diesel exhaust fluid reservoir containing an aqueous urea solution containing a catalyst precursor comprising a water soluble organometallic compound may be achieved by performing one or more of the following pH adjustment procedures: (1) introducing exhaust gases including CO2 into the reservoir; (2) introducing air into the reservoir; and / or (3) introducing NOx (NO and / or NO2). Preferably, one, two or all three of these pH adjustment procedures may be selected and performed thereby. Such control may be performed by a computer processor programmed to monitor the pH of the reservoir 18, receive the pH readings, and select one or more of the pH adjustment procedures identified above to maintain a pH at a value of 10.0 or less, preferably within the range of 7.0 to 10.0 as mentioned.

Claims

1. 1. A method for controlling pH levels of a diesel exhaust fluid, comprising: (a) providing a diesel exhaust fluid reservoir containing an aqueous urea solution containing a catalyst precursor comprising a water soluble organometallic compound selected from organotitanium compounds having solubility in water, the reservoir including a pH probe for monitoring the pH of said solution; (b) controlling the pH to a value equal to or less than 10.0; The method includes:

2. The method of claim 1, wherein the pH is controlled at a value in the range of 7.0 to 10.

0.

3. The pH is ２ The method of claim 1 , wherein the pressure is controlled by introducing an exhaust gas containing

4. The method of claim 1 , wherein the pH is controlled by introducing air into the reservoir.

5. The method of claim 1 , wherein the pH is controlled by introducing NOx into the reservoir.

6. 2. The method of claim 1, wherein the water-soluble organometallic compound dissolves in the aqueous urea solution at a level of from 0.1 wt. % to 25.0 wt. %.

7. 10. The method of claim 1, wherein the water-soluble organometallic compound comprises an organotitanium compound that is converted to a titanium oxide catalyst.

8. 8. The method of claim 7, wherein the titanium oxide catalyst is formed in a concentration of 0.01 wt. % to 1.0 wt. % in the diesel exhaust fluid.

9. 8. The method of claim 7, wherein the organotitanium compound comprises titanium(IV) acetate.

10. 8. The method of claim 7, wherein the organotitanium compound comprises titanium(IV) bis(ammonium lactato) dihydroxide.

11. 8. The method of claim 7, wherein the organotitanium compound comprises titanium(III) oxalate.

12. The method of claim 7, wherein the organotitanium compound comprises ammonium titanyl oxalate.

13. The method of claim 7, wherein the organotitanium compound comprises titanium oxyacetylacetonate.

14. The method of claim 1 , wherein the catalyst precursor comprises zirconium dioxide.

15. 1. A method for controlling pH levels of a diesel exhaust fluid, comprising: (a) providing a diesel exhaust fluid reservoir containing an aqueous urea solution containing a catalyst precursor comprising a water soluble organometallic compound selected from organotitanium compounds having solubility in water, the reservoir including a pH probe for monitoring the pH of the solution; (b) pH adjustment procedure as follows: (i) CO ２ introducing an exhaust gas containing (ii) introducing air into said reservoir; (iii) introducing NOx into said reservoir; controlling said pH to a value equal to or less than 10.0 by selecting and implementing at least two of: The method includes:

16. The method of claim 15, wherein the pH is controlled to a value in the range of 7.0 to 10.

0.

17. A diesel exhaust fluid reservoir for containing an aqueous urea solution containing a catalyst precursor comprising a water soluble organometallic compound selected from organotitanium compounds having solubility in water, the reservoir including a pH probe for monitoring the pH of the solution; below: (i) CO ２ introducing an exhaust gas containing (ii) introducing air into the reservoir; or (iii) introducing NOx into said reservoir; a controller for monitoring the pH and configured to control the pH to a level equal to or less than 10.0 by selecting and executing at least one of:

23. A diesel exhaust fluid apparatus comprising:

Images