Method for preparation of a monolithic catalyst for the reduction of nitrogen oxides, VOC and carbon monoxide in an off-gas

Abstract

Method for the preparation of a monolithic catalyst for the reduction of nitrogen oxides VOC and carbon monoxide in an off-gas, the catalyst comprises at least one platinum group metal, vanadium, titania and optionally tungsten oxide.

Claims

1. Method for the preparation of a monolithic catalyst for the reduction of nitrogen oxides VOC and carbon monoxide in an off-gas, the catalyst comprises at least one platinum group metal, vanadium oxide, titania and optionally tungsten oxide, the method comprises the steps of: a) providing a monolithic substrate; b) providing an aqueous solution of a vanadium compound; c) providing an aqueous solution of one or more compounds of the at least one platinum group metal; d) adding particles of titania to the aqueous solution of the vanadium compound to obtain a suspension comprising the titania particles suspended in the aqueous vanadium compound solution; e) precipitating at least part of the vanadium compound in the suspension of step d) on surface of the titania particles to obtain a suspension of titania supported vanadium compound particles; f) adjusting the pH value of the suspension to a value of at least 6 and adding the aqueous solution of the one or more compounds of the at least one platinum group metal to the pH adjusted suspension of the titania supported vanadium compound particles to obtain a wash coat slurry containing a suspended solid phase with the one or more compounds of the at least one platinum group metal adsorbed on the titania supported vanadium compound particles; or adding the aqueous solution of the one or more compounds of the at least one platinum group metal to suspension of the titania supported vanadium compound particles and subsequently adjusting the pH value of the suspension to a value of at least 6 to obtain a wash coat slurry containing a suspended solid phase with the one or more compounds of the at least one platinum group metal adsorbed on the titania supported vanadium compound particles; g) coating the monolithic substrate with a layer of the wash coat slurry of step f); h) decreasing the pH value of the layer coated on the monolithic substrate below 6 and dissolving the one or more compounds of the at least one platinum group metal from the titania supported vanadium compound particles on and into walls of the monolithic substrate; and i) drying and calcining the wash coated monolithic substrate.

2. The method according to claim 1, wherein the pH value in step f) is adjusted to at least 8.

3. The method according to claim 1, wherein the vanadium compound is ammonium metavan date.

4. The method according to claim 1, wherein the at least one compound of the platinum group metal is one or more compounds of palladium.

5. The method according to claim 1, wherein the pH value of the washcoat layer in step h) is decreased by providing vanadium oxide and titania in the monolithic substrate provided in step a).

6. The method according to claim 5, wherein the vanadium oxide is vanadium pentoxide.

7. The method according to claim 1, wherein the one or more compounds of the at least one platinum group metal is palladium nitrate.

Description

(1) Pursuant to the above findings and observations, this invention provides a method for the preparation of a monolithic catalyst for the reduction of nitrogen oxides, VOC and carbon monoxide in an off-gas, the catalyst comprises of at least one platinum group metal, vanadium oxide, titania and optionally tungsten oxide, the method comprises the steps of:

(2) a) providing a monolithic substrate;

(3) b) providing an aqueous solution of a vanadium compound;

(4) c) providing an aqueous solution of one or more compounds of the at least one platinum group metal;

(5) d) adding particles of titania to the aqueous solution of the vanadium compound to obtain a suspension comprising the titania particles suspended in the aqueous vanadium compound solution;

(6) e) precipitating at least part of the vanadium compound in the suspension of step d) on surface of the titania particles to obtain a suspension of titania supported vanadium compound particles;

(7) f) adjusting the pH value of the suspension to a value of at least 6 and adding the aqueous solution of the one or more compounds of the at least one platinum group metal to the pH adjusted suspension of the titania supported vanadium compound particles to obtain a wash coat slurry containing a suspended solid phase with the one or more compounds of the at least one platinum group metal adsorbed on the titania supported vanadium compound particles; or

(8) adding the aqueous solution of the one or more compounds of the at least one platinum group metal to suspension of the titania supported vanadium compound particles and subsequently adjusting the pH value of the suspension to a value of at least 6 to obtain a wash coat slurry containing a suspended solid phase with the one or more compounds of the at least one platinum group metal adsorbed on the titania supported vanadium compound particles;

(9) g) coating the monolithic substrate with a layer of the wash coat slurry of step f);

(10) h) decreasing the pH value of the layer coated on the monolithic substrate below 6 and dissolving the one or more compounds of the at least one platinum group metal from the titania supported vanadium compound particles on and into walls of the monolithic substrate; and

(11) i) drying and calcining the wash coated monolithic substrate.

(12) The pH value of the suspension is one of the key parameters in the addition stage of solution of the palladium/platinum compounds and has to be carefully monitored to at least 6. The purpose of adjusting the pH value of the suspension is to prevent dissolution of the PGM compound(s) into the aqueous phase of the suspension in order to control a slower and even uptake of the compound(s) on the catalyst substrate.

(13) Experiments have shown that pH values decreasing below 6 results in dissolution of increasing amounts of the PGM compound(s) into the aqueous phase of the suspension and the washcoat prepared from the suspension.

(14) Based on these experiments, the suspension is preferably adjusted to pH 8, which also allows for a certain drop in pH value in the subsequent preparation steps.

(15) It has additionally shown that particularly ammonium metavanadate (AMV) enhances the adsorption of Pd compounds on the titania-vanadium compound particles in the slurry. Addition of AMV ensures that Pd does not dissolve in the aqueous phase down to a pH of 6.

(16) Thus, in a preferred embodiment of the invention, the vanadium compound used in the invention is ammonium metavanadate.

(17) It is known that metallic or oxidic palladium is the most useful catalyst in the combined catalytic removal of carbon monoxide, VOC and nitrogen oxides.

(18) Consequently, the at least one compound of the at least one platinum group metal is one or more compounds of palladium, which in the calcination step of the invention is converted to metallic and/or oxidic palladium.

(19) The slurry prepared as described above is used for washcoating monolithic substrates. Due to the PGM compound(s) are retained in the solid phase of the washcoat, the PGM compound(s) will be evenly distributed on the whole exterior of the walls in the monolithic substrate. At that point the pH value of the wet wash coat present on the wall is lowered below 6 and the PGM compound(s) gets dissolved from the titania supported vanadium compound particles and starts to migrate into the walls and finally ends up evenly distribute into and on the walls.

(20) It has further been observed that a monolithic substrate comprising vanadium oxide, titania and optionally tungsten before being washcoated with the washcoat prepared according to the invention, exhibits both Brnsted- and Lewis-acid sites, which decrease the pH value of the washcoat layer below 6 after the washcoat has been applied on surface of the monolithic substrate. At that point the acidity of vanadium oxide-titania contained in the monolithic substrate starts to decrease the pH value of the wet coat layer and the PGM compound(s) is dissolved from the titania supported vanadium compound particles in the washcoat layer and starts to migrate into the walls. Thereby, the PGM compound(s) are evenly distributed into the walls of the monolithic substrate.

(21) Thus, in a specific embodiment of the invention the monolithic substrate as provided in step a) of the method according to the invention is provided with vanadium oxide, titania and optionally tungsten oxide within the walls prior to be coated with the washcoat. The vanadium oxide comprised in the monolithic substrate is preferably vanadium pentoxide having the highest activity in the SCR reaction.

(22) A preferred PGM compound for use in the invention is palladium nitrate which is soluble in water and precipitates at a pH value of about 6 or higher, as desired in the invention.