GB1581628A - Catalytic purification of automobile exhaust gases - Google Patents

Description

(54) IMPROVEMENTS IN AND RELATING TO CATALYTIC PURIFICATION OF AUTOMOBILE EXHAUST GASES (71) We, JOHNSON, MATTHEY & CO., LIMITED, a British Company, of 43 Hatton Garden, London, EC1N 8EE, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a catalytic process for use in the purification of automobile exhaust gases: The exhaust gases from an internal combustion engine include oxygen, carbon monoxide, unburnt hydrocarbons and nitrogen oxides. If the engine is fuelled by petrol, the oxygen is generally present in a stoichiometrically lean concentration, that is, the atmosphere is a "net reducing atmosphere". Under such conditions, the nitrogen oxides may be catalytically reduced in one catalyst bed and the carbon monoxide and hydrocarbons may be mixed with additional oxygen and catalytically oxidised in a second catalyst bed. If, on the other hand, the engine is fuelled by diesel, there is a stoichiometric excess of oxygen in the exhaust gases and the nitrogen oxides may only be catalytically reduced after adding an excess of a reducing fuel. The exhaust of a petrol-fuelled internal combustion engine will also be oxygen-rich under certain operating conditions.

According to the invention, a process of catalytically removing oxides of nitrogen present in an automobile exhaust gas and/or catalytically oxidising carbon monoxide and hydrocarbons present in the said gas comprises contacting the gas at an elevated temperature with a catalyst wherein the catalyst consists of rhodium or iridium supported on a refractory oxide support consisting of silica or a mixture or composition containing silica and alumina.

A process according to the present invention will remove oxides of nitrogen from a gas under either reducing or oxidising conditions. Under oxidising conditions, removal may occur either by decomposition or by selective reduction. Such a process represents a considerable advance in the technology of purification of petrol and diesel exhaust gases and nitric acid tail gases.

Under certain circumstances, the catalytic decomposition of nitrogen oxides may lead to the formation of nitrous oxide. A further option, therefore, is that the catalyst be associated with one or more catalytic metals or oxides to promote the reduction or decomposition of nitrous oxide. We have found that oxides of one or more of lanthanum, cerium and titanium are particularly useful for this purpose.

By "elevated temperature" we mean a temperature sufficient for catalytic decomposition of a significant quantity of the oxide of nitrogen etc. present or catalytic oxidation of a significant quantity of the organic compound or carbon monoxide present to occur as a result of contact with a catalyst such as described above. In general, the elevated temperature would not exceed about 500-600"C. and, depending upon the exact nature of the catalyst, the optimum temperature could be in the region of 300-450"C.

The refractory oxide on which the catalytic metal or mixture of metals is deposited or with which it is associated, may be in the form of pellets, granules or powders but is preferably in the form of fine grains, the individual grains being typically slightly coarser than particles of powder.

An example of a suitable refractory oxide is Davison 70 silica.

The catalyst is optionally ultimately supported on a substrate known in the art. Examples of preferred substrates which may be used are porous refractory ceramic honeycombs or modules formed from corrugated metallic sheets rolled up to form a cellular structure. Other supports may be used, however, for example particulate solids such as granules, pellets, shapes and extrudates which can be constructed of ceramic or metallic refractory materials.

Alternatively, the catalyst may be mixed with a catalytically inert refractory material, for example in the form of granules, as an extender or diluent.

The ultimate supporting structure may have a first coating, layer or deposit of a refractory oxide which may be the same as or different from the refractory oxide on which the catalytic metal is supported. This first refractory oxide is deposited on the ultimate support, preferably in the form of a film, and the film may have a thickness of 0.0004 to 0.001 inches.

Such oxides are calcined and preferably have a porous structure which has a relatively large internal pore volume and total surface area. Such oxides are referred to as "active", that is, catalytically active, refractory oxides.

Catalysts for use in the process of the invention may be deposited directly on the support without the intervention of an intermediate refractory oxide layer. Such an arrangement includes a catalyst comprising a rhodium or iridium supported on for example, silica and deposited on a ceramic or metallic ultimate support, so that, although no separate intermediate refractory oxide layer is included, the refractory oxide of the catalyst acts in lieu of such an intermediate layer.

In addition to Davison 70 silica a range of colloidal suspensions sold under the trade name SYTON (Registered Trade Mark) by Monsanto may be used. For example, "Syton X30" which has a silica concentration of approximately 30%, has been used for applying silica to both ceramic and metallic monoliths prior to applying the precious metal.

The general procedure of preparation of such catalysts is to dip the monolith directly into the "Syton" solution, blow off the excess from the channels and surface of the monolith, dry and fire, and then to apply the precious metal by dipping into an aqueous solution of precious metal salt followed by drying and reducing in a stream of H2/N2. Metal supports (e.g.

Fecralloy) should be pre-oxidised by heating for 1 hour at 1300C. This has the effect of improving corrosion resistance to the gases with which the catalyst will come into contact and also improving the adhesion of the silica layer. Also, since metal supports, unlike ceramic supports, are not porous, two immersions in the silica sol, each followed by drying etc, are necessary to build up the desired coating of silica.

In detail, one method of applying a catalyst according to the invention on a ceramic support will now be described.

A ceramic honeycomb monolith (Corning (Registered Trade Mark) M20; 300 cells per square inch) made from cordierite and being in the shape of a cylinder 2 inches in diameter and 3 inches in length and therefore having a volume of 9.42 cu. ins. was immersed in 500 ml of Syton X30, the silica therein having a surface area in excess of 200 m2g-1, for 2 minutes.

After withdrawal, the excess silica sol was first shaken off and then blown off at an air pressure of 15 p.s.i. The weight of the monolith had increased from 92 to 108 g. The coated monolith was then dried at about 100"C in a convection current of hot air flowing at > 300 ft.min.-' and was then roasted in a static kiln furnace at 500"C for a hour. The resulting coated monolith weighed 97.2 g, hence the weight of silica was 5.2 g which represented a loading of 0.55 g.ln- The film thickness represented by this loading falls within the range of the 0.0004 - 0.001 inch mentioned above.

To deposit the precious metal on the silica, the following method was used: The water absorption of the silica-coated monolith was measured and the amount of rhodium or iridium required to give a loading of 30 g.ft-3 of monolith was calculated. An aqueous solution of the metal salt, for example, RhCl3.xH20, at a concentration of 14 g.l- 1of Rh metal was made up and the silica-coated monolith immersed for 2 minutes in the solution, removed, allowed to drain and the excess salt solution blown off. The process was repeated, if and as necessary. until the predetermined weight, that is, of silica-coated monolith carrying the required quantity of solution corresponding to a metal loading of 30 g.ft- 3, was achieved.

The process was completed by hot air drying and reduction in a tube furnace for one hour under an atmosphere of H2/N2 containing 5% H2 at 4500C (for rhodium) and 220"C (for iridium).

Methods of deposition of oxide are described in our British Patent Specification No.

1,390,182. Descriptions of typical ceramic supports and useful metal supports which may be used are given in British Patent Specification No. 882,484 (Corning) and in our co-pending British Patent Specification No. 1492929. (published DOS No. 2,450,664).

The following descriptions. examples. tables and figures 1 to 13 accompanying the provi sional specification are illustrative of the invention. Figures 1 to 13 show graphically the % N2O formation, % NOX removal and % CO oxidation for each of the runs 1 to 13 given in Table 2 plotted against temperature.

Preparation of the Catalysts Catalysts comprising platinum, rhodium and iridium supported on Davison 70 silica were prepared by impregnation of the silica with a solution of the relevant chloride or acid chloride of the required strength and volume. The impregnated silica was then dried in vacuo (0.1 torr) at room temperature and reduced in a stream of hydrogen at 3500C. (for platinum) and 450"C. (for rhodium and iridium).

The following silica-supported catalysts were prepared using the above method.

TABLE 1 Catalyst A Rh/SiO2 6.14X 10 -2 At% (approx. 0.1 w/w%) Catalyst B Rh/SiO2 6.14X 10-3 At% (approx.0.1 w/w%) Catalyst C *Pt/SiO2 6.14 X 10-2 At% (0.2 wlw%) Catalyst D *Pt/SiO2 6.14 X 10 -3 At% (0.02 wlw%) Catalyst E Ir/SiO2 6.14X 10 -3 At% (approx. 0.02 w/w%) * For comparative purposes.

In the above table, atomic percentages are quoted as atoms of the metal calculated as a percentage of the number of moles of silica. In a similar way, weight percentages are expressed as weight of metal to weight of silica.

Experimental Test Rig The experimental runs, results of which are given below, were carried out in a silica reactor comprising two concentric tubes mounted vertically. The outer tube had an o.d. of about 1 -" and the inner tube had an o.d. of about w". The catalyst was carried on a bed of silica wool in the inner tube and the reaction gas stream was passed down the outer tube and up the inner.

The gases were passed through a pre-heating zone upstream of the catalyst. Reactor gas inlet temperature was measured by means of a chrome-aluminel thermocouple located in a well below the catalyst. The reactor was surrounded by a heater equipped to give variable heating rates. The inlet and outlet concentration of gases was monitored using a BOC Luminox Analyser and Perkin-Elmer F17 gas chromatograph.

Experimental Results and Discussion Two principal types of experiment were conducted on the catalysts. These were: (a) a profile of activity in removal of oxides of nitrogen versus inlet temperature at a fixed ratio of oxidising to reducing species in the inlet gas stream; and (b) an isothermal profile of activity against ratio at a temperature chosen from the result of experiment type (i).

The ratio (R) of oxidising to reducing species in the inlet gas stream is calculated according to the equation 2 x %02 + % NOx R = % CO Generally, the particular oxide of nitrogen (NOx) in the equation is nitric oxide, NO.

In the experiments described below, the weight of catalyst tested was generally 1 g. In some experiments, smaller weights were tested, typically 0.005 g., but in all experiments using less than 1 g. of catalyst, pure silica was added as an extender to the catalyst, to give a total weight of 1 g. in order to prevent bed fluidisation.

Space velocity was calculated on the basis of catalyst weight exclusive of the weight of added pure silica. if present. and is quoted in units of hr- Experimental runs were conducted as in the following table (Table 2) in which the "Catalyst" designation refers to Table 1. the "Experimental type" refers either to (a) or (b) as above. and the "R or temperature" refers to the fixed ratio R (for experimental type (a)) or to the isothermal temperature (for experimental type (b)).

TABLE 2 Run No. Wt. of Wt. of Space Experimental R or (also Fig. No.) Catalyst Catalyst added Velocity Type Temp.

SiO2 1 A 1 g - 2,200 a 1.9 2 A 0.005g 0.995g 440,000 a 1.9 3 A 0.005g 0.995g 440,000 b 395 C 4 B 1 g - 2,200 a 1.9 5 B 1 g - 2,200 b 325 C 6 C 1 g - 2,200 a 1.9 7 C 0.005g 0.995g 440,000 a 1.9 8 C 0.005g 0.995g 440,000 b 400 C 9 D 1 g - 2,200 a 1.9 10 D 1 g - 2,200 b 360 C 11 E 1 g - 2,200 a 1.9 12* E 1 g - 2,200 a 1.9 13 E 1 g - 2,200 B 329 C *Run 12 was repeat of Run 11 using the identical catalyst.

Run 1 shows the activity of Rh/SiO2 catalyst at an R value of 1.9 (i.e. - oxidising conditions). NOx decomposition showed a maximum of 95% at about 230"C. and the "half peak activity range" (that is, the active temperature range at half the maximum activity) was 220"C. Nitrous oxide formation (as a percentage of the total NOx input) reaches a maximum just above the NOx decomposition threshold temperature and decreases steadily to zero at about 450"C.

Run 2 was carried out under similar conditions to Run 1 using the same catalyst, except that the actual catalyst weight was 0.005g. instead of lg. The effect of this was to raise the temperature of maximum activity to 400"C. At this temperature, decomposition of NOx is still 75% despite the small amount of catalyst and the correspondingly high space velocity compared with Run 1. Formation of N2O did not rise above 18% and, at the temperature of maximum decomposition of NOx, was already dropping rapidly.

Run 3 was carried out using the same catalyst as Run 2 but under isothermal conditions, a temperature near the maximum activity of NOx decomposition from Run 2 being selected (395"C.). Maximum NOx decomposition (and minimum N2O formation) occurred at about the stoichiometric point (R = 1) and the value of R at 50% of the maximum NOx decomposition was 2.8.

Run 4 used ig. of a Rh/SiO2 catalyst of lower metal loading compared to Runs 1,2 and 3.

Diluting the metal loading results in a lower maximum of NOx decomposition at a higher temperature, and also a lower half-peak activity of 1600C.

Run 5 - the isothermal plot using the catalyst of Run 4 - showed that the catalyst was less resistant to oxygen poisoning between R values of about 1 and 5 and that production of N20 was higher than with the similar catalyst of higher metal loading. The value of R at 50% of maximum NOx decomposition was 2.5.

Run 6 used ig. of a Pt/SiO2 catalyst and is comparable to Run 1 using a similarly loaded Rh/SiO2 catalyst. The results of Run6 indicated that the maximum activity in NOx decomposition for Pt/SiO2 was only 50% compared with 95% for Rh/SiO2. However, this poor result does not necessarily reflect accurately the true activity of this catalyst since little is known as yet about the relative dispersions of the metals on silica. The half-peak activity for Pt/SiO2 was 200"C. compared to 220"C. for Rh/SiO2.

Run 7 used 0.005g. of the same catalyst and is therefore comparable to Run 2. As in Run 2, lowering the quantity of catalyst increased the temperature of maximum activity in NOx decomposition but the maximum decomposition was only 17%.

Run 8 was carried out under isothermal conditions at a temperature of 400"C. Compared to Run 3 (using Rh/SiO2 catalyst) the resistance to oxygen poisoning was poor. The value of R at 50% of maximum NOx decomposition was 1.25.

Run 9 was carried out using 1 g. of a Pt/SiO2 catalyst having a lower metal loading than that used in Runs 7 and 8. Run 9 therefore compares with Run 4 for Rh/SiO2 and the results show that Pt/SiO2 is inferior to Rh/SiO2 in respect of NOx decomposition and about the same for half peak activity.

Run 10 was carried out under isothermal conditions at a temperature of 360"C. Again, comparing the results with Run 5, Pt/SiO2 shows less resistance to oxygen poisoning than does Rh/SiO2. The value of R at 50% of maximum NOx decomposition was 1.7.

Run 11 used an Ir/ SiO2 catalyst and compres with Run 4 (Rh/ SiO2) and Run 9 (Pt/ SiO2).

Maximum Nox decomposition was 65%at 345 0C. and at this temperature production of N2O was virtually zero. Half peak activity was 1900C.

Run 12 was a re-run of Run 11, using the identical catalyst, and shows the remarkable effect of heating to 600"C. in Run 11. Maximum Nox decomposition was 90% at a temperature of 350"C. and production of N2O was still extremely low, peaking at 2%. The half peak range was comparatively sharp at 100"C.

Run 13, the isothermal experiment on Ir/SiO2 at 329"C., shows that this catalyst is considerably more resistant to oxygen poisoning than either Rh/SiO2 (Run 5) or Pt/SiO2 (Run 10). The value of R at 50% of maximum NOx decomposition was 5. N2O production is consistently either very low or zero throughout the entire range of R.

In conclusion. it may readily be seen that the catalysts of the invention show remarkable results for the decomposition of NOx under oxidising conditions. Furthermore, they showed high efficiency in the oxidation of carbon monoxide and it is anticipated that these catalysts will successfully oxidise the lower hydrocarbons typically present in the exhaust gas stream from an internal combustion engine.

Rh/SiO2 tended under certain conditions to cause the formation of nitrous oxide. We have found that certain additives may be incorporated or admixed with the catalysts to catalyse the reduction or decomposition of nitrous oxide. Examples of such additives are oxides of lanthanum, cerium and titanium.

Claims (6)

WHAT WE CLAIM IS:

1. A process of catalytically removing oxides of nitrogen present in an automobile exhaust gas and/or catalytically oxidising carbon monoxide and hydrocarbons present in the said gas comprising contacting the gas at an elevated temperature with a catalyst wherein the catalyst consists or rhodium or iridium supported on a refractory oxide support consisting of silica or a mixture or composition containing silica and alumina.

2. A process according to claim 1 wherein the catalyst includes at least one oxide of lanthanum or cerium or titanium.

3. A process according to claim 1 or claim 2 wherein the refractory oxide support is deposited upon a substrate.

4. A process according to claim 3 wherein the substrate is made from a refractory oxide in the form of a honeycomb.

5. A process according to claim 3 wherein the substrate is made from a metallic sheet material in the form of a cellular structure.

6. A process according to any preceding claim wherein the elevated temperature is within the range 300-600"C.