EP0711198A1

EP0711198A1 - Base metal catalyst and two-stage process for the purification of vehicle exhaust gases

Info

Publication number: EP0711198A1
Application number: EP94921750A
Authority: EP
Inventors: Ashok Kumar Bhattacharya
Original assignee: MG Rover Group Ltd
Current assignee: MG Rover Group Ltd
Priority date: 1993-07-29
Filing date: 1994-07-28
Publication date: 1996-05-15
Also published as: GB9600502D0; JPH09501348A; GB2295780A; GB9315679D0; WO1995003877A1

Abstract

A catalyst for treating motor vehicle exhaust gases comprising: (A) a copper component containing cuprous oxide and cupric oxide dispersed on, (B) a mixed support containing (i) one or more lanthanide oxides; (ii) one or more oxides of elements from Group II; (iii) one or more oxides of elements from Group IIIb; and (iv) one or more oxides of elements from Group IV of the Mendeleev Periodic Table.

Description

BASE METAL CATALYST, CATALYTIC SUPPORT AND TWO-STAGE PROCESS FOR THE PURIFICATION OF VEHICLE EXHAUST GASES

This invention relates to a catalyst for treating gases, and in particular to a catalyst for treating waste gases which are the products of combustion, such as motor vehicle exhaust gases.

Pollution of the atmosphere by exhaust gases from internal combustion engines is causing a great deal of public concern and many attempts have been made to eliminate pollutants such as unburnt hydrocarbons, nitrogen oxides, and carbon monoxide from such exhaust gases.

One common solution is to pass the exhaust gases through a catalyst bed intended to promote the conversion of pollutants to less harmful substances before the exhaust gases are discharged into the atmosphere. The catalytic component of such exhaust gas catalysts that are used at present consists of platinum in association with other platinum group metals. Platinum group metals are in short supply and their price is already high, and there are fears that the increasingly-widespread use of exhaust gas catalysts as a result of anti-pollution legislation will cause the cost of platinum group metals to rise still further until the cost of the exhaust gas catalyst becomes a significant proportion of the overall cost of the motor vehicle. One objective of this invention is therefore to provide an effective exhaust gas catalyst that does not rely on a scarce and expensive material such as platinum or other platinum-group metal.

Exhaust gas purification catalysts utilising certain base metals, in particular copper, have also been tried. However they suffer from the disadvantage that the catalytic activity degrades rapidly in use so that they are quite unsuitable for use in purifying vehicle exhaust gases where a long service life is essential. To further illustrate this point the readers attention is directed to Figures 1a and 1b which show that copper oxide, a typical base metal oxide, supported on 0 - I2O has a light off temperature of 250°C for the reduction of NO by CO (Fig. 1a) but its activity decreases dramatically after approximately 8 hours in the feed gas at 750°C (Fig. 1b) .

Furthermore, base metal catalytic systems have been found to exhibit varied resistance to sulphur poisoning and have specific activities lower than those of platinum-group metal systems.

The present invention overcomes the above problems.

According to the invention there is provided a catalyst comprising: A. a copper component as defined herein dispersed on B. a mixed support containing

(i) one or more lanthanide oxides; (ii) one or more oxides of elements from Group ii;

(iii) one or more oxides of elements from Group lllb, and (iv) one or more oxides of elements from Group IV of the Mendeleev Periodic Table. By "copper component" we mean a mixture containing A(i) cuprous oxide and cupric oxide either on their own or mixed with metallic copper or A(i) and A(ii) one or more oxides of transition metals, preferably selected from cobalt, manganese, nickel and iron.

It has been found that the presence of a transition metal oxide leads to an increase in the catalytic activity of the copper catalyst in the reduction of NO by carbon monoxide by a synergistic mechanism.

Alternatively, the transition metal may provide active sites for a two-site reaction involving decomposition of NO_∑ at a first site, migration of oxygen to the other site, and oxidation of CO to CO, at the other site with recombination of nitrogen (N+N=N^T,) taking place at the first site.

Advantageously the lanthanide oxide component B(i) is selected from oxides of cerium,

LE 26 lanthanum and yttrium. Preferably component B(i) is cerium dioxide either on its own or mixed with one or more other lanthanide oxides.

Preferably the Group II component B(ii) is selected from one or more oxides of magnesium, calcium, strontium and barium.

Preferably the Group IV component B(iv) is selected from oxides of one or more of zirconium, titanium and Hafnium.

The Group IIlb component B(iii) is preferably alumina ( I2O3).

The components of the mixed support B may each be present in an amount of from 0.005 to 99.0 per cent by weight based on the total weight (W) of the support, with the total amount of components B(i) to B(iv) always adding up to 100 wt %.

The constituents of the copper component to be disposed or loaded on the support may each be present in an amount of from 0.005 to 25 wt % based on the total weight (W) of the support.

In experiments using a synthetic exhaust gas mixture containing carbon monoxide (CO), nitric oxide (NO), propane (C_jHg) [representing all the hydrocarbons present in a real exhaust gas,] carbon dioxide {CO2), water (H2O), oxygen (O2) , hydrogen {H2₎ and nitrogen (N2)], the catalysts of the invention exhibited good stability and a catalytic activity comparable to or better than a commercial platinum group metal catalyst.

Catalysts of the invention can be prepared by a variety of different methods which are within the knowledge of a worker well versed in the art.

Suitable, but by no means the only methods for preparation of the support include: urea hydrolysis, hydrolysis of Propoxides, coprecipitation by ammonium hydroxide (NH^OH) or ammonium oxalate [ { NH^ fi.'Pq, E-p ] , coprecipitation as carbonate, mechanical mixing of acetates and decomposition, and decomposition of nitrates.

Suitable, but not the only methods for dispersion of the copper component on the support include deposition by copper acetate, copper formate, copper ammine complex, copper carbonate and copper nitrate.

In order to promote efficient NOx removal at high space velocities the catalyst should be prepared so that the following NOx removal reaction:-

2 CO + 2 NO s_* C0₂ + N₂ competes with the faster "shift reaction": CO ⁺ Ep ^- C^ ^{+ H}2' In a further aspect the invention provides a two-stage process for the purification of exhaust gases comprising:

(i) passing the gases over a first catalyst comprising the copper component A on the mixed support B as defined herein to remove carbon monoxide and hydrocarbons in a first stage; and

(ii) passing the gases over a second catalyst arranged in series with the first catalyst to remove N0_χ and residual hydrocarbons in a second stage; the second catalyst comprising the copper component A on a highly acidic support.

The highly acidic support may comprise:-

(a) one or more zeolites; or

(b) a solid solution comprising two or more of zirconium oxide (Zrθ2) preferably present in an amount of from 3 to 15% and more preferably 7%, tungsten oxide (WO3) molybdenum oxide (M0O3) , alumina (AI2O3); optionally carried on a high surface area support such as alumina ( Alp- ^- ) , preferably stabilized with magnesium oxide; or

(c) highly acidic compounds obtained by co-gelling alumina (AI2O3) with one or more of silicon oxide (Siθ2) , titanium oxide (Tiθ2) and zirconium oxide (Zr0₂).

Generally, as the air/fuel ratio ( .) in the exhaust gas changes through stoichiometry ( λ = 1 ) to oxidising conditions ( λ > 1) catalytic activity declines and NO_χ conversion decreases. However, using the above two-stage process NO conversion under oxidising conditions is enhanced at 500,000 hr GHΞV to over 20% of that achieved at stoichiometry.

Alternatively the first catalyst may comprise (ii) cuprous oxide and cupric oxide and optionally metallic copper on the mixed support B in combination with a second catalyst comprising a platinum group metal dispersed on the mixed support B.

Preferably the platinum group metal comprises palladium and a mixture of metallic palladium and palladium oxide is particularly preferred.

According to the above embodiment a palladium catalyst with less than 10% of the precious metal loading found on commercial catalysts achieved a NO_χ conversion of around 40% of that achieved at stoichiometry, that is, when the air to fuel ratio is equal to 14.67 (represented as λ = 1). This value corresponds to the amount of air required for complete oxidation.

In another aspect the invention provides the mixed oxide support B for a base metal or platinum- group metal catalyst used in the purification of exhaust gases. An example of a catalyst embodying the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figs. 1a and 1b show results obtained with a base metal catalyst outside the scope of the invention comprising copper oxide supported on ύ -

Fig. 2 shows the catalytic conversion of a synthetic exhaust gas mixture corresponding to A= 1

_1 at a space velocity of 5000 hr GHSV against temperature using a catalyst of the invention.

Preparation of Support B ^ 0.935 ^0.03 ^O.OS ^0.005 Al O^±δ Coprecipitation by NH^OH

Requisite quantities of nitrate were added to 400 ml of distilled water and heated to 60°C. To this solution, about 35 ml of (1:1) NH^OH was added quickly with stirring. Final pH was 9. The slurry was digested at 70°-80°C for 4 hours, filtered, washed 4 times, each with 200 ml of cold water, dried overnight at 110°C and powdered.

550°C,6 hr:191.O.m²/g; 1000°C,24 hr:57.8m²/g Support Preparation

Al (N0₃)₃9H i (246 mmol), Mg (N0₃)₂ ΘH^ (123 mmol), Ce (N0₃)₃.6H )(6.2 mmol) ZrO(N0₃)₂(6.2 mmol) and La( 0 ) .6H ( 1 mmol) were dissolved by stirring in 500 ml of distilled water at 50°C. 10% NH₄0H solution was then added slowly to adjust the pH to 9. The resulting precipitate was then digested at 80°C for 5 hours, filtered and washed with 3 X 300 ml water. After drying, the mixture was calcined at 550°C for 6 hours/1 °C min^", and sieved to 100μm. This resulted in a support (B) with nominal composition ^M9θ93^^Ce003^ZrO(B^L O005^^ ' ^wi^h ^a surface

2 area of 137 πr/g.

Metal Deposition

In the following deposition procedure, solution was added to both 550°C calcined support B and 1000°C c&lcined support B (heated to 1000°C for 2 hrs/10°C min ) . A total of 5% by weight of copper was loaded onto the support.

Deposition by Copper Ammine Complex Cu(N0₃)₂.5/2H2θ (11.6 mmol) was added to 19 ml of water at 50°C with stirring. 5 ml of 33% NH^H was then added with stirring to yield the deep violet cuprammonium nitrate solution. 1.2 ml of this solution was then added to 1.5g of Support B with mixing. The mixture was dried and the procedure repeated. Calcination was done at both 550°C/6 hours and 1000°C/2 hours as before. Fig. 2 shows the catalytic conversion of a synthetic gas composition comprising: CO(1.0%), NO (1500 ppm), 0₂ (0.85%), H₂(0.33%), CH₄(500ppm), C_jHg (167ppm), H^ (10%), C0₂ (15%), SO₂(20ppm), (balance) and corresponding 1, at a space velocity of 5000 hr GHSV against temperature using the above catalyst of the invention.

As shown, the catalyst of the invention is able to achieve over 90% conversion of NO at 525°C.

In addition, studies on the effect of sulphur poisoning were carried out on the above catalyst of the invention by introducing 0.05% S0₂ into the feed gas. Surprisingly, such treatment was found to have no effect on the activity of the catalyst.

Fig. 3 shows the catalytic conversion of a synthetic exhaust gas composition at a space velocity of 15000 hr GHSV in a two-stage process of the invention in which the catalyst in the first stage comprises the catalyst of Fig. 2 and the catalyst in the second stage comprises the copper component of the catalyst of Fig. 2 dispersed on a zeolite support. As shown, by using the two-stage process of the invention it is possible to obtain 100% NO conversion at a temperature of 225°C.

Fig. 4 shows the catalytic conversion of a synthetic exhaust gas composition at a space velocity -1 of 5000 hr GHSV of a synthetic exhaust gas composition in a two-stage process of the invention in which the catalyst in the first stage comprises the catalyst of Fig. 2 and the catalyst in the second stage comprises the mixed oxide support B of the catalyst of Fig. 2 having palladium dispersed thereon and an amount of 1% weight based on the total weight of th support. As shown, using only a 1% loading of palladium it is possible to obtain excellent NO conversion using the two-stage process of the invention.

Claims

CLAIMS :

1. A catalyst comprising:

A. a copper component containing

cuprous oxide and cupric oxide

dispersed on,

B. a mixed support containing

(i) one or more lanthanide oxides;

(ii) one or more oxides of elements from Group

II;

(iii) one or more oxides of elements from Group IIIb; and

(iv) one or more oxides of elements from Group IV of the Mendeleev Periodic Table.

2. A catalyst as claimed in claim 1 wherein the copper component A(i) further comprises metallic copper.

3. A catalyst as claimed in claim 1 or 2 wherein the copper component further comprises A(() one or more transition metal oxides.

4. A catalyst as derived in claim 3 wherein A(i) is selected from cobalt, manganese, nickel and iron.

5. A catalyst as claimed in any one of claims 1 to 4 wherein component B(i) is selected from oxides of cerium, lanthanum and yttrium.

6. A catalyst as claimed in claims 1 to 5 wherein B(i) is cerium dioxide on its own or mixed with one or more other lanthanide oxides.

7. A catalyst as claimed in any one of claims 1 to 6 wherein B(ii) is selected from oxides of one or more of magnesium, calcium, strontium and barium.

8. A catalyst as claimed in any one of claims 1 to 8 wherein B(iv) is selected from oxides of one or more oxides of zirconium, titanium and hafnium.

9. A catalyst as claimed in any one of claims 1 to 8 wherein B(iii) is alumina.

10. A catalyst as claimed in any one of claims 1 to 10 wherein the support B comprises oxides of magnesium (Mg), zirconium (Zr), cerium (Ce), lanthanum (La) and alumina (Al₂O₄).

11. A catalyst as claimed in any one of claims 1 to 10 wherein each of the components of the mixed support B is present in an amount of from 0.005 to 99.0 per cent by weight based on the total weight of the support.

12. A catalyst as claimed in any one of claims 1 to 11 wherein each of the constituents of the copper component is present in an amount of from 0.005 to 25 per cent by weight based on the total weight of the support B.

13. A catalyst as claimed in claim 10 wherein the components of the support B are present in the following proportions Mg_0.935, Zr_0.03, Ce_0.03, La_0.005, Al₂O₄±δ (MgO 25.42%, ZrO₂2.49%, CeO₂ 3.48%, La₂O₃0.55%).

14. A vehicle exhaust gas purification process comprising passing the gas over a catalyst as claimed in any one of claims 1 to 13.

15. A two-stage process for the

purification of exhaust gases comprising:

(i) passing the gases over a first catalyst comprising the copper component A on the mixed support B as claimed in any one of claims 1 to 13 to remove carbon monoxide and hydrocarbons in a first stage; and

(ii) passing the gases over a second catalyst arranged in series with the first catalyst to remove NO_x and residual hydrocarbons in a second stage; the second catalyst comprising the copper component A on a highly acidic support.

16. A second catalyst for use in the process as claimed in claim 15 wherein the highly acidic support comprises:-

(a) zeolites; or

(b) a solid solution comprising two or more of zirconium oxide (ZrO₂), tungsten oxide (WO₃),

molybdenum oxide (MoO₃), alumina (Al₂O₃); or (c) highly acidic compounds obtained by co-gelling alumina (Al₂O₃) with one or more of silicon oxide (SiO₂), titanium oxide (TiO₂) and zirconium oxide (ZrO₂).

17. A second catalyst as claimed in claim 16 wherein zirconium oxide is present in an amount of from 3 to 15%.

18. A second catalyst for use in the process as claimed in claim 14 comprising a platinum-group metal dispersed on the mixed support B.

19. A support for a base metal or platinum group metal catalyst used to purify exhaust gases comprising the mixed oxide support B as claimed in any one of claims 1, 5, 6 to 11 or 13.