WO1996031275A1

WO1996031275A1 - Water soluble noble metal catalyst precursors

Info

Publication number: WO1996031275A1
Application number: PCT/US1995/004332
Authority: WO
Inventors: Kevin Allison King
Original assignee: Pgp Industries, Inc.
Priority date: 1994-03-31
Filing date: 1995-04-03
Publication date: 1996-10-10
Also published as: AU2243595A; EP0819025A1; ZA952679B

Abstract

This invention pertains to the technology of making noble metal catalysts in a manner that avoids the production of toxic emissions. More specifically, the invention is directed to the production of water soluble complexes of noble metals with organic acids, which complexes can be used to deposit the metals on an appropriate solid matrix in a catalytically active form.

Description

WATERSOLUBLENOBLEMETALCATALYSTPRECURSORS

This invention pertains to the technology of making noble metal catalysts in a manner that avoids the production of toxic emissions. More specifically, the invention is directed to the production of water soluble complexes of noble metals with low molecular weight carboxylic acids, in particular hydroxy substituted carboxylic acids. These complexes can be used to deposit noble metals on an appropriate solid matrix in a catalytically active form. Suitable solid matrices include, without limitation, alumina, silica, and molecular sieves (zeolites).

Catalysts precursors according to the invention, and the resulting catalysts, are suitable for a variety of end uses, including automotive catalyst applications, stationary pollution-abatement catalysts, hydrocracking catalysts, fuel cells, etc.

BACKGROUND OF THE INVENTION

Pollution control catalysts for internal combustion engines utilize platinum group metals to break down harmful exhaust gasses into less noxious components. Typically, platinum, palladium and rhodium dispersed onto alumina washcoated ceramic or metal supports are used to destroy exhaust gasses. Common forms of the platinum group metals currently used in the industry for catalyst precursors include chloroplatinic acid, palladium chloride, palladium nitrate, rhodium chloride, and rhodium nitrate. During catalyst preparation, the washcoated support is impregnated with aqueous solutions of the platinum group metals. After soaking, the

"wetted" catalyst tiles are heated in ovens to the point where the ligands (Cl^" or NO₃ ^") are released from the metal ion to leave only the metal bound to the alumina washcoat.

The ligands decompose into HC1 in the case of the chlorides, and into NO_x in the case of the nitrates. Both HC1 and NO_x are corrosive and toxic which places special requirements on the catalyst manufacturer. Equipment must be of suitable construction to prevent corrosion, and an oven scrubber system is typically required to prevent release of these gasses to the environment.

This invention provides a new type of catalyst precursor which is water soluble, yet when subjected to thermolysis it is substantially free of NO_x, chloride, and sulfur oxides.

Most commercial catalyst preparations utilize "catalyst precursors" in the form of aqueous solutions containing precious metal salts such as chloroplatinic acid, palladium nitrate, and rhodium nitrate, for deposition of platinum, palladium, and rhodium, respectively. Each of these salts is "wet impregnated" onto a high surface area support (typically gamma alumina) which is itself anchored onto a honeycomb monolith (either ceramic or metal). Substrates impregnated with these metal salt solutions are then subjected to high temperature thermolysis to convert the noble metal salts into the catalytically active forms of the metals. During the thermolysis reaction, toxic emissions may evolve via decomposition of the metal salts. Typically, noble metal salt solutions containing halide, nitrogen, or sulfur emit toxic and corrosive gases upon thermolysis (HX, NO_x, and SO., respectively.) For example, US Patent 5,179,060 discloses a method of catalyst production in which the noble metal (in the form of salt) is deposited on the matrix in a one-step multi-component reaction, using platinum group metal salts, a water soluble acid of the form RCOOH, and an alumina substrate. This method would appear to have the disadvantage of forming toxic substances upon subsequent heating of the impregnated matrix.

Typical known solvent systems include aqueous acid solutions of chlorides, nitrates or sulfates.

Chlorides, such as aqueous HC1, produce toxic fumes and can corrode plant machinery. Such solvents can also corrode the metallic monolith honeycomb used as a catalyst substrate. The industry is tending to favor metallic monoliths over ceramic (eg Cordierite) substrates, which makes the chloride solvents even less desirable. Nitrate solvents, such as aqueous HNO₃ are similarly corrosive, and also present strong oxidizer hazards. Scrubbers may be needed to control nitric acid and nitrogen oxides. The same is true of sulfates and sulfites, such as aqueous H₂SO₄ or H₂SO₃.

Other known methods rely on organic (non-aqueous) solvents which tend to be flammable or toxic. During drying and firing of a ceramic or metal honeycomb impregnated with catalyst precursor, scrubbers are required to prevent the release of incomplete combustions products and potentially harmful volatile organic compounds (VOCs). Industry is currently tooled for aqueous systems, and re-tooling would be required to handle organic solvent systems. SUMMARY OF THE INVENTION

The present invention teaches a method of impregnating a catalytic carrier matrix with catalytically active forms of noble metals without producing toxic emissions. For this purpose, the metal or metal oxide is combined with lactic acid (or derivatives thereof). This reaction results in the formation of a water-soluble complex of the metal with lactic acid or its derivatives, which complex can be separated from the unreacted metal. The soluble complex can then be deposited on the carrier matrix in a subsequent reaction.

The present invention is substantially free of elements or compounds that result in toxic emissions during the thermolysis process (e.g. , halides, nitrogen, and sulfur). By design, only water and CO₂ will be produced during activation of the matrix. Furthermore, the water soluble feature of the invention makes it particularly attractive for manufacturing, since current machinery can be used and highly flammable organic solvents are avoided. The present invention thus provides environmental advantages to the catalyst manufacturer without compromising the performance of the catalyst.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 shows a calibration curve generated from known amounts of NaNO^ dissolved in a trapping solution.

Figure 2 is a bar graph comparing the amounts nitrogen oxides generated during thermolysis of various palladium catalyst species. DETAILED DESCRIPTION OF THE INVENTION

In general terms, a catalyst precursor is prepared by mixing a solution of a noble metal (or its oxide) with lactic acid or a related compound in the presence of water. The concentration of the metal or metal oxide may vary from about 5 to about 30%, and is preferably about 12% for palladium-based reactions (PdO ^• xH₂O), 29% for platinum-based reactions (PtO₂ ^• xH₂O), and 13% for rhodium-based reactions

(Rh₂O₃ - xH₂O). In the reaction mixture, (PdO ^■ xH₂O) may vary from about 30- 80%, (PtO₂ ^• xH₂O) may vary from about 20-70%, and (Rh₂O₃ - xH₂O) may vary from about 25-65 % . The concentration of lactic acid may vary from about 2 to 20 moles lactic acid per mole of Palladium (Pd) or Platinum (Pt). If rhodium is used, the lactic acid may vary from about 4 to 20 moles of lactic acid per mole of Rhodium (Rh).

The reaction mixture is brought to a temperature ranging from room temperature to about 40°to 80^CC for palladium systems, about 85-90°C for platinum systems, and about 75-85 °C for rhodium systems. The reaction is allowed to proceed until all of the metal or metal oxide is fully dissolved. Any unreacted metal or metal oxide is then separated from the soluble metal-lactic acid complexes by filtration. Finally, the crude (concentrated) noble metal solutions are assayed for metal content and then adjusted to the appropriate dilution by the addition of water. A final assay to verify the desired metal content is then performed.

For example, a gravimetric assay can be used to determine the resulting concentration of precious metal in the precursor solution. In this conventional procedure, a sample is dissolved into aqua regia (3:1 HCl:HNO₃), nitrate is driven off with HC1, and the HC1 is driven off with water. The precious metal remaining is reduced with dropwise addition of a 10% NaBH, solution, and is then collected and washed in ashless filter paper. The metal and paper are burned at 1200°F, and subsequently for Pd or Rh (not Pt) is reduced with a hydrogen flame. The soluble complex concentration of precious metal is determined by weighing the precious metal and calculating the noble metal concentration, which preferably ranges from about 5 to 15% .

Water-soluble noble metal catalyst precursors can be prepared from carboxylic acids of the general form R-COOH where R is an alkyl, substituted alkyl, or aryl group. Acids having six carbon atoms or less are generally preferred. It is recognized that the entire class of water soluble carboxylic acids of the general form

R-COOH may perform equally well as the specific lactic acid examples described below. For example, alpha-hydroxy carboxylic acids are particularly well suited to the invention, and beta-hydroxy and gamma-hydroxy carboxylic acids are also contemplated. While not wishing to be bound by theory, the applicants believe that the OH group promotes hydrogen bonding with water, thereby enhancing the solubility of the metal carboxylates in water.

The present invention is described below in specific working examples using lactic acid (i.e. R= CH₃-CH(OH)-). It will be understood that these examples illustrate the invention without limiting its scope.

Example 1 : Platinum Catalyst Precursor

Platinum oxide (PtO^ hydrate is combined with aqueous lactic acid and heated followed by filtration to give a concentrated noble metal solution. As an example, 103.7g PtO₂ (30% Pt) is combined with 136g lactic acid, 24g water. This mixture is heated to 85 °C. The reaction is allowed to proceed until the formation of the R-lactic acid complex is indicated by a resultant dark solution, usually after one hour. The solution is then filtered to obtain the platinum precursor solution in a 50-

80% yield. In some cases the yield can reach 90% or better, depending on stirring efficiencies, temperature control, and other factors. For quality control, this precursor solution can be assayed for noble metal content prior to storage, shipment, or use.

The chemical reaction believed to occur during this process is as follows.

RO₂ 4- 3CH₃CH(OH)CO₂H → R(CH₃CH(OH)CO₂)₂ + CH₃COCO₂H + 2H₂O

The precursor solution is used to make a noble metal catalyst composition by known methods. For example, a catalyst suitable for automotive use (/^'_.. in a catalytic converter) can be made by diluting the precursor solution (generally to a concentration of about 1-2%), applying the diluted solution to the washcoat on a supporting ceramic or metallic honeycomb substrate. Usually the matrix substrate is simply immersed in the catalyst solution. Excess solution is removed by gravity

(dripping), which can be aided by pressurized air. The substrate is the dried and fired to produce the final catalyst product.

Example 2: Palladium Catalyst Precursor

Sample A (Made From Palladium Sponge)

Palladium sponge (i.e. Pd metal) and lactic acid are stirred with a slow dropwise addition of concentrated nitric acid, after which the reaction mixture is filtered to yield a concentrated noble metal solution. As an example, 31.1 grams of Pd sponge are combined with 86 g of 85 % lactic acid and 2 drops of concentrated hydrochloric acid and 6 ml of water. The mixture is stirred, followed by the dropwise addition of 15 ml of concentrated nitric acid over several days. Once the nitric acid has been added, the mixture is warmed to 30-35 °C for several more days. The resulting purple solution is diluted with 17 ml of water and is then filtered to remove unreacted Pd metal. This yields the desired Pd-lactic acid complex in solution.

The reaction which is believed to occur during this process is as follows.

3Pd° + 6CH₃CH(OH)CO₂H + 2HNO₃ → 3Pd(CH₃CH(OH)CO₂)₂ + 2NO + 4H₂O Unlike conventional procedures, the majority of the nitrogen (nitrate) used in the present invention is lost during the reaction, without forming a harmful or corrosive complex. While not wishing to be bound by any particular theory, it is believed that NO is released and lost during oxidation of the palladium metal.

Furthermore, there is much less nitrate present here than in conventional methods. In commercial processes currently in use, the molar ratio of Pd to nitrate during the reaction is significantly greater than 1, at about 1 to 3.5. According to the invention, the molar ratio of Pd to nitrate during reaction is less than 1, at about 1 to 0.8. The final product contains even less nitrate, and for commercial purposes is substantially nitrate-free. The invention also differs from conventional methods by employing only a trace of hydrochloric acid, in a molar ratio of Pd to Cl of about 1400: 1. In conventional methods, PdCl₂ is formed, with a molar ratio of Pd to Cl of 1:2. Here, the trace amount of HC1 is used in a preferred embodiment to assist in the dissolution of the palladium by generating a small amount of aqua regia (HC1 : HNC^).

Sample B (Made From Palladium Nitrate Intermediate.

The above procedure can be modified to complex a palladium nitrate intermediate with lactic acid. Palladium metal is dissolved in nitric acid, to form a palladium nitrate intermediate. This intermediate is then treated with caustic soda (NaOH) to form a palladium oxide by hydrolysis. The oxide is washed with deionized water to remove salts. The resulting oxide is heated with lactic acid to form a palladium lactate complex according to the invention. For example, a mixture of 194.4g hydrated palladium (II) oxide (16% Pd) and 248g of 85 % lactic acid is heated to 75 °C and stirred for several hours. The reaction mixture is filtered to remove any unconverted palladium oxide.

The reaction believed to occur is as follows: PdO + 2CH₃CH(OH)COOH → Pd(CH₃CH(OH)COO)₂ + H₂O

The concentrated precursor solution is diluted for use in making a catalyst as described above.

Example 3: Rhodium Catalyst Precursor

Sample A: (Made From Lactic Acid)

Rhodium sesquioxide (Rh₂O₃) hydrate is combined with lactic acid with stirring followed by filtration to yield the concentrated noble metal solution. As an example, 239.3g Rh₂O₃ (13% Rh) hydrate is combined with 256g lactic acid, and the mixture is gently heated overnight with stirring. The resulting emerald green solution is filtered to remove unreacted Rh₂O₃, yielding the Rh-lactic acid complex in solution.

The reaction believed to occur is as follows. Rh₂O₃ + 5CH₃CH(OH)CO₂H → Rh₂(CH₃CH(OH)CO₂)₄ + CH₃COCO₂H + 3H₂O The concentrated precursor solution is diluted for use in making a catalyst matrix as described above.

Sample B: (Made From Acetic Acid) A mixture of 239.3g Rh₂O₃ hydrate (13 % Rh content) and 146g acetic acid is stirred for five hours at 75-80°C. The mixture is filtered to remove unconverted oxide to give the catalyst precursor solution. In this example the Rh is in the +3 oxidation state in contrast to example A above where the Rh is in the +2 oxidation state. This difference in oxidation state effects the solubility of the material in water. An analogous Rh +2 complex with acetic acid is known, however the solubility in water is low. The preparation of Rh(H) acetate typically requires the intentional addition of a reducing agent. The lactic acid probably reduces the Rh+3 to Rh+2 in example A.

In the case of Rh complexed to acetate the key to water solubility is the oxidation state. In lactate complexes, the presence of the OH group out weighs the effects of oxidation state and renders the Rh +2 species water soluble. Also note the Pd and R acetates have low solubility in water, while the lactates are water soluble and it is our belief the OH group on the lactate is key. The reaction believed to occur is: Rh₂O₃ + 6 CH₃COOH → 2 Rh(CH₃COO)₃ + 3 H₂O Example 4: Performance of Palladium Catalyst Precursor The performance of an exemplary organopalladium catalyst precursor made according to Example 2 was evaluated. To make this evaluation, ceramic substrates having standard cell configurations of 400 cpsi and a volume of 93 inches³ were washcoated, and half of each substrate was overcoated with a standard palladium chloride solution (HjPdCl . The other half of each substrate was overcoated with a 1-2 % diluted organopalladium solution of Example 2. The target palladium loading was 100 grams of palladium per cubic foot. Part of each resulting catalyst material was aged on a stand engine dynamometer for 100 hours, and was thereafter evaluated with an air/fuel-sweep test and a light-off test.

"Light-Off" is the temperature at which 50% of the hydrocarbon (taken as propane) in the exhaust is combined into CO₂ and water. An air/fuel sweep is used to evaluate catalyst activity by simulating engine performance. The air/fuel ratio is varied around a chemically balanced or stoichiometric ratio, usually about 14.6/1 for test fuel. At stoichiometry, and within a narrow fraction on each side (usually about 0.3), a three-way catalyst can efficiently convert NO_x, HC and CO. Thus, an air/fiiel sweep is a measure of catalyst efficiency as the air/fuel mixture changes during engine use.

The light-off and air/fuel sweep results are summarized below.

These studies showed that a catalyst made using this precursor, in comparison with a conventional palladium chloride precursor, was equivalent in HC, CO, and NO_x performance. The catalyst of the invention had improved light-off, in comparison with standard palladium chloride. Example 5: Nitrogen Oxides Generated During Thermolysis

This example compares NO_x evolution during thermolysis of two exemplary palladium catalyst precursor solutions and a standard palladium nitrate solution. NO, evolution was evaluated by a trapping technique based on ASTM number D 3608-77T, Nitrogen oxides (combined) content in the atmosphere by the Griess-Saltzman reaction. A sample was weighed in a combustion chamber attached to a separatory funnel and the system was evacuated. A Bunsen burner was used to heat the sample with resultant decomposition. Trapping solution was introduced into the separatory funnel utilizing a vacuum and was shaken on occasion. After 15 minutes the trapping solution was transferred to a 500 ml volumetric flask, the combustion chamber and separatory funnel were rinsed with several portions of trapping solution which was also transferred to the volumetric flask. The solution was then diluted with trapping solution to the 500 ml mark. The absorbance was measured at 550 nm. The amount of NO. was determined from a calibration curve generated from known amounts of NaNO₂ dissolved in trapping solution (Figure 1). NO_x values are indicated as weights NaNO₂; however, these values can be converted to weights of NO₂ using a conversion described in the ASTM method.

Palladium Lactate A was made according to Example 2, using palladium sponge suspended in lactate acid with vigorous stirring and a slow addition of concentrated nitric acid. After filtration the material was ready for use.

Palladium Lactate B was prepared from hydrated palladium oxide generated by caustic soda hydrolysis of palladium nitrate, or palladium chloride. Freshly prepared hydrated oxide was washed extensively with deionized water to remove salts below 10,000 ppm Na. The hydrated oxide was then combined with lactic acid followed by gentle heating and resultant filtering.

Palladium Nitrate Pd(NO₃)₂ was obtained from PGP Chemical Products.

Palladium Nitrate/Acetic Acid Pd(NO₃)₂/ Acetic Acid was made according to U.S. Patent No. 5, 179,060 in which a 0.0835 g Pd/cc solution of

Pd(NO₃)₂ is diluted to 0.0167g Pd/cc with 50:50 (v/v) glacial acetic acid:deionized water.

The total nitrogen content was used as a measure of the NO_x generating potential for each material upon thermolysis. Each sample had approximately the same concentration of palladium. As anticipated, the palladium nitrate sample had more nitrogen than the Palladium Lactate moieties (Table 2). Sample A had more nitrogen than Sample B as expected by the fact that nitric acid is used in its manufacture. The total nitrogen content for Sample B fell below the detection limits of the method. On

a per weight of palladium basis, Pd(NO₃)₂ had over five times the amount of nitrogen. In brief, the elemental analysis confirms the designed-in low nitrogen content of

Palladium Lactate, and in particular the embodiment of Sample B.

TABLE 2 Elemental Analysis Of Catalyst Precursors

Below detection limits of method Table 3 shows the data from NO_x trapping tests. These tests are based on the Griess-

Saltzman reaction in which NO₂ forms an adduct with sulfanilic acid which then combines with N-1-Napthylethylenediamine dihydrochloride to form a pink colored azo-dye. The azo-dye has an absorbance maximum in the visible range of 550 nm. Any NO generated during thermolysis is assumed to combine with oxygen in the air to convert to NO₂. NO_x generated from palladium nitrate was 2.5 times greater than Palladium Lactate A, and 5 times greater than Palladium lactate B.

Figure 2 is a bar graph comparing the amounts of NO, generated in these experiments for each palladium species tested. The nitrate present in Sample B

probably originates from nitrate which was not fully washed out of the hydrated palladium oxide (formed by hydrolysis of palladium nitrate, used in the manufacture of the Sample B). With more careful washing of the hydrated oxide this amount of NO_x should be able to be reduced even further. An alternative route to the hydrated oxide includes using palladium chloride which would further reduce the chance of trace nitrate remaining in the product.

TABLE 3 NO_λ Generated During Thermolysis (In units of grams NaNO,/ grams PdNO

" Prepared from hydrolysis of Pd(NO₃)₂. ^2> Prepared from hydrolysis of PdCl_:.

A key advancement for the manufacture of exhaust catalysts would be a precious metal precursor which is water soluble and yet when thermally decomposed produces only non-toxic by-products — CO₂ and water. Current catalysts makers use water based nitrate and chloride based systems. The present technology provides a significant environmental advantage by eliminating most harmful nitrogen oxides, without compromising performance or requiring new processing equipment.

Claims

WHAT IS CLAIMED IS: 1. A water soluble noble metal catalyst precursor comprising a carboxylic acid in combination with a noble metal composition selected from the group consisting of palladium, platinum, rhodium and oxides thereof.

2. A noble metal catalyst preparation comprising a dilute aqueous solution of an alpha-hydroxy carboxylic acid and a noble metal composition selected from the group consisting of palladium, platinum, rhodium, and oxides thereof.

3. A method for preparing a water soluble noble metal catalyst precursor comprising the step of reacting a short-chain carboxylic acid with a noble metal composition selected from the group consisting of palladium, platinum, rhodium and oxides thereof.

4. A water soluble noble metal catalyst precursor comprising lactic acid in combination with a noble metal composition selected from the group consisting of palladium, platinum, and rhodium and oxides thereof.

5. A catalyst precursor according to claim 1, wherein the concentration of noble metal or oxide is from 5 to 30% and the concentration of carboxylic acid is from 2 to 20 moles of acid per mole of noble metal.

6. A catalyst precursor according to claim 4, wherein the concentration of noble metal or oxide is from 5 to 30% and the concentration of lactic acid is from 2 to 20 moles of acid per mole of noble metal.

7. A method according to claim 3 , wherein the concentration of acid is from 2 to 20 moles of acid per mole of noble metal, the concentration of noble metal or oxide is from 5 to 30 % , and the noble metal or oxide is initially combined with water in a complex comprising from 20 to 80% of the reaction mixture.

8. A method according to claim 3 wherein the acid is lactic acid.

9. A method according to claim 7 wherein the acid is lactic acid.

10. A method according to claim 3 wherein the reaction occurs at a temperature ranging from room temperature to approximately 90 °C.

11. A catalyst preparation according to claim 2 wherein the acid is lactic acid.

12. A catalyst precursor according to claim 1 wherein the acid is lactic acid and the noble metal is palladium or its oxide in an initial concentration of from 5 to 30%.

13. A catalyst precursor according to claim 1 wherein the acid is lactic acid and the noble metal is platinum or its oxide in an initial concentration of from 5 to 30% .

14. A catalyst precursor according to claim 1 wherein the acid is lactic acid and the noble metal is rhodium or its oxide in an imtial concentration of from 5 to 30% .

15. A water soluble noble metal catalyst precursor comprising the reaction product of an aqueous alpha-hydroxy carboxylic acid and an aqueous noble metal or its oxide selected from the group consisting of palladium, platinum and rhodium.