GB2141634A

GB2141634A - Lead resistant catalyst

Info

Publication number: GB2141634A
Application number: GB08412303A
Authority: GB
Inventors: Dr Graham Butler
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1983-05-23
Filing date: 1984-05-14
Publication date: 1985-01-03
Also published as: GB8412303D0

Abstract

A catalyst suitable for the catalytic treatment of motor vehicle exhaust emissions comprises a refractory material substrate having a plurality of internal passageways for passage therethrough of reactant(s) and/or product(s), and catalytically active material for a chemical reaction carried by the substrate either directly or by refractory support material which is itself carried by the substrate. To overcome problems of degradation of the substrate in a lead-containing environment, the refractory material is wholly or partly fabricated of ceria.

Description

SPECIFICATION Lead resistant catalyst This invention relates to a lead resistant catalyst useful, for example, in the catalytic treatment of motor vehicle exhaust emissions.

The treatment of motor vehicle exhaust emissions to remove noxious components therein by e.g., catalytic oxidation of hydrocarbons or carbon monoxide, or catalytic reduction of oxides of nitrogen, is well known. The catalyst used may, for example, consist of a refractory monolithic substrate having a plurality of internal passageways for passage therethrough of reactant(s) and/or product(s) gases, and cataytically active material for a chemical reaction carried by the substrate either directly or by refractory support material which is itself carried by the substrate.

A problem with such catalysts is that the refractory material constituting the substrate, which may, for example, be cordierite (See, for example, I.R. Lachman et al, American Ceramic Society Bulletin, Vol 60 No 2, pages 202-205 (1981)), may be degraded in a lead containing enviroment such as a vehicle exhaust emission when a lead tetralkyl "anti-knock" additive has been used in the fuel for the vehicle.

The invention is intended to overcome the above problem by using a ceria-containing substrate. Thus, in one aspect, the invention provides a lead-resistant catalyst comprising a refractory material substrate having a plurality of internal passageways for passage therethrough of reactant(s) and/or product(s), and catalytically active material for a chemical reaction carried by the substrate either directly or by refractory support material which is itself carried by the substrate, characterised in that the substrate contains 50% or more by weight of ceria uniformly and homogeneously dispersed therethrough.

A substrate of the invention has been found in experimental tests described hereinafter to be considerably more resistant to a lead-containing enviroment than a known cordierite substrate.

The substrate of the invention preferably consists essentially of ceria. It may, however, consist of a mixture of ceria with other refractory materials such as cordierite.

The substrate may be in a range of configurations and sizes according to specific requirements. It may, for example, be in the form of a honeycomb having a plurality of parallel channels constituting the internal passageways. Such a honeycomb may, for example, be right cylindrical in shape wherein the curved surface is continuous and is substantially impermeable to the reactant(s) and products of the reaction(s) taking place in use of the substrate, and wherein the channels are parallel to the principle axis of the cylinder.

The substrate may be made from powdered constituent material by methods known in the art for fabricating ceramic bodies. For example, the powdered material may be mixed with binder(s) providing thermoplastic and setting properties (a plasticiser may also be included), shaping the mixture into sheets for defining the substrate, bonding the sheets together in the desired form of substrate and firing to give the final substrate. Alternatively, the substrate may be made by extrusion of a suitable mixture using an appropriate die, for example by vacuum extrusion.

The catalytically active material may be constituted by one or more platinum group metals, for example as known in the art for the catalytic treatment of motor vehicle emissions, either alone or in combination with one or more refractory oxides, such as ceria and alumina, as support material for the platinum group metal(s). The substrate may be provided with the catalytically active material by methods known in the art, for example by contacting the substrate with an aqueous solution of a salt(s) of the platinum group metal(s) convertible to the metal(s) themselves, followed by drying and conversion to the metal(s). A refractory oxide may be provided in dispersion in the solution (e.g. in the form of colloidal particles thereof) if it is desired to include a refractory oxide(s) in combination with the platinum group metal(s).

One way of carrying out the invention will now be described as follows as an example only.

Example Ceria powder (40 g) was dry mixed with polvinyl butyral (PVB) (15 g). A mixture of methyl ethyl ketone (MEK) (15 g) and dibutyl phthalate (DBP) (7.5 g) was then mixed into the dry mix using a Hobart mixer. The resultant "paste" was rolled in a two-roller mill (rollers steam heated to ca 90at) to yield a homogenous "rubberised" sheet ca 0.02-0.03 inch thick. The sheet was then passed through a four roll calendar with rollers as follows: rollers 1 to 3, oil heated to ca 40 C, roller 4 ambient temperature. The sheet thus obtained has a thickness of ca 0.006 inch.

Two samples of sheet were then fed through a corrugator to yield a sinusoidally corrugated (1 mm corrugations) sheet bound to a flat sheet. The corrugated composite sheet was then tightly hand rolled to yield corrugated monoliths (1 cm dia x 20 cm length). The "green" pieces were debonded by heating in air from room temperature to 400 C at 5 C min-'. The cooled pieces were then calcined at 1 1 50 C for 1 hour after being slowly elevated to this temperature over ca 4-5 hours. The pieces were cooled in the furnace and cut with a diamond saw to a length of 1.03 cm. The diameter was 1.01 cm.

Tests The 'life time' of the substrate produced in Example 1 when aged in hot lead oxide powder was measured and compared with the 'life time' of a known cordierite substrate of rectangular shape and dimensions 1 cm X 1 cm x ca 1 mm and having an internal honeycomb structure.

The procedure for determining 'life time' was as follows: AR lead oxide was placed in a 3 cm diameter a-alumina crucible to generate a bed of depth ca 3 mm. The test substrate was placed on the bed and more lead oxide added to partially immerse the substrate. The crucible with contents was placed in a muffle furnace pre-heated and maintained at a desired test temperature in order to subject the test substrate to the influence of PbO vapour. The test substrate was visually inspected at hourly intervals for possible failure which was deemed to have occurred when all mechanical integrity and shape had been lost from the honeycomb. The time at which such failure first occurred was noted; this is termed the 'life time'. The results are summarised in TABLE 1 below.

Substrate Temperature Lifetime (4C) (h) Example 1 800 > 16 (Sample D)** Example l 1000 > 24 (Sample E# Cordierite 800 3 (Sample A) Cordierite 800 4 (Sample B) Cordierite 800 3 (Sample C) *This sample was tested in a closed crucible, i.e in saturated PbO vapour. The other samples were tested in an open crucible.

4*These samples remained undergraded at the end of the test.

In the tests, the cordierite substrate samples formed a solid solution in the lead oxide during failure so that the honeycomb structure and powdered lead oxide became indistinguishable. The substrates of Example 1 were completely unaffected by the test procedure.

Claims

1. A lead-resistant catalyst comprising a refractory material substrate having a plurality of internal passageways for passage therethrough of reactant(s) and/or product(s), and catalytically active material for a chemical reaction carried by the substrate either directly or by refractory support material which is itself carried by the substrate, wherein the substrate contains 50% or more by weight of ceria uniformly and homogeneously dispersed therethrough.

2. A lead-resistant catalyst as claimed in claim 1 wherein the substate consists essentially of ceria.

3. A lead-resistant catalyst as claimed in either of the preceding claims wherein the substrate is in the form of a honeycomb having a plurality of parallel channels constituting the internal passageways.

4. A lead-resistant catalyst as claimed in claim 1 substantially as described herein with reference to the example.