DE2712009A1 - METHOD OF CATALYTIC CLEANING OF GAS MEDIA - Google Patents

Description

The invention relates to a method for the catalytic cleaning of air and other gas media, in particular for the removal of nitrogen oxides, preferably in the presence of an excess of oxygen, and for the removal of carbon monoxide and organic compounds such as lower hydrocarbons.

The exhaust gases from internal combustion engines contain oxygen, carbon monoxide, unburned carbons and nitrogen oxides. In the exhaust gases of a gasoline engine, the oxygen is generally contained in a stoichiometrically weak concentration, i.e. it is a "purely reducing atmosphere". Under this condition, as is known, the nitrogen oxides must be catalytically reduced in one catalyst support and the hydrocarbons must be oxidized with the addition of additional oxygen in a second catalyst support. However, when diesel fuel is used as fuel, there is an excess of oxygen in the exhaust gases and the nitrogen oxides can be catalytically reduced if an excess of a reducing fuel is added. Under certain operating conditions, gasoline exhaust gases can also be rich in oxygen.

The gases produced during the production of nitric acid contain a considerable amount of NO and other nitrogen oxides, which should be extracted before the gas is released into the atmosphere.

The invention is based on the object of creating a method of the type mentioned at the beginning with which both reducing and oxidizing gases can be cleaned.

The object is achieved according to the invention in that the gas is brought into contact at a higher temperature with a catalyst which contains at least one metal from the platinum group, gold and silver and is applied to a refractory oxide or bonded to such an oxide.

By "platinum group" is meant platinum, rhodium, iridium, osmium, palladium and ruthenium, of which platinum, rhodium and iridium are preferably used. The metals can be alloyed with one another or as mixtures or alloys ments with one or more base metals or precious metals that do not belong to the platinum group.

Refractory oxide can be an oxide or a mixture of oxides from elements of groups IIA, IIB or IVB of the periodic table or the transition metals. Such elements can, for example
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Magnesium, calcium, scandium, yttrium, titanium, zirconium, aluminum, silicon or germanium. In addition, compounds such as alum earth / silicate can be used. Either silica or alum earth or mixtures and compounds with silica and alum earth or alum-silica are preferably used.

With the method according to the invention, nitrogen oxides can be removed from both reducing and oxidizing gases. In the case of the latter, it occurs either through decomposition or through selective reduction. Such a process brings a great advance in the technology of cleaning exhaust gases from gasoline and diesel fuel, as well as from nitric acid gases.

If desired, the catalyst can be associated with one or more other catalytic metals or metal oxides to further promote oxidation.

Under certain conditions, the catalytic decomposition of nitrogen oxides forms nitrogen oxide. In this case, it is also proposed according to the invention to combine the catalyst with one or more catalytic metals which promote the reduction or decomposition of the nitrogen oxide. One or more oxides of lanthanum, cerium and titanium can advantageously be used for this purpose.

By "higher temperature" is meant a temperature sufficient to catalytically decompose a major amount of nitrogen oxides, etc., or to oxidize a significant amount of organic compounds or carbon monoxide when the gas is contacted with a catalyst as described above will. As a rule, the temperature does not exceed 500-600 ° C. Depending on the catalyst, the optimum temperature is between 300 and 450 ° C.

The catalytic metal or metal mixture is deposited on the refractory oxide, which can be in the form of spheres, granules or powder, with a Fine-grained oxide is preferable because the individual grains are somewhat coarser than powder particles.

A suitable refractory oxide is, for example, "Davison 70 silica".

Finally, the catalyst can be applied to a substrate surface of a known type. The substrate can advantageously be a porous, refractory ceramic honeycomb or an element with a network-like structure which is produced by rolling a corrugated metal sheet. Solid particles such as granules, spheres, briquettes and compacts made from either ceramic or metallic refractory materials are other examples of substrates. It is optionally provided that the catalyst is mixed with a catalytically inactive refractory material serving as an extender or diluent.

Finally, the carrier element can be provided with a first layer, a coating or a coating made of a refractory oxide, which is the same as the carrier material or can be a different material. The first refractory oxide is preferably applied to the final support to form a 0.010 to 0.025 mm thick film.

Such oxides are fired and they preferably have a porous structure with a relatively large pore space and total surface area. These oxides belong to the "active", namely catalytically active refractory oxides.

Suitable refractory oxides are silica, alum earth, silica / alum earth, titanium, zircon, magnesium, quartz and zeolite sieved through a molecular sieve.

It is also possible to deposit the catalyst for the process according to the invention directly on the support without using an intermediate layer of a refractory oxide. In this case, a catalyst is provided which consists of a platinum group metal carried by, for example, silica or bonded to it, and deposited on a final ceramic or metallic support. Here, the refractory oxide of the catalyst takes on the function of the oxide layer.

In addition to "Davison 70" silica, a number of colloidal suspensions known under the trade name SYTOM from Monsanto can be used. For example, the "Sytom X30", which contains approx. 30% silica used to apply silica to either ceramic or metallic substrates prior to the noble metal.

A general method of making such catalysts involves immersing the support directly in the "Syton" solution, blowing the excess from the furrows and from the surface of the support, drying and firing the support, and finally in an aqueous solution immersing a precious metal salt, drying it and reducing it in a stream of H [deep] 2 / N [deep] 2. Metal supports (such as Fe-Cr alloy) should be pre-oxidized by heating at 180 ° C for one hour. The purpose of this treatment is to improve the corrosion resistance to the gases with which the catalyst will come into contact and the adhesion of the silica layer. In addition, since the metal carrier is not porous in contrast to the ceramic carrier, it must be dipped twice into the silica solution and dried after each bath and treated accordingly in order to obtain the desired silica coating.

A method according to the invention for applying the catalyst to a ceramic support is described below.

A ceramic honeycomb carrier ("Corning M20, 47 cells per cm [high] 2) made of dichroic was used, which had the shape of a 5 cm thick and 7.5 cm long cylinder with a volume of 154 cm [high] 3. This carrier was immersed for 2 minutes in 500 ml of Sytom X30, in which the silica had a surface area of over 200 m [high] 2g [high] 1. After removal, the excess silica was first shaken off and then with an air stream of 15 psi, the weight of the backing had increased from 92 g to 108 g. The coated backing was then dried at 100 ° C. in a stream of hot air having a flow rate of over 300 ft.min [high] -1 and then annealed for half an hour in a stationary kiln at 500 ° C. According to this, the carrier had a weight of 97.2 g, which means that the silica applied was 5.2 g or 0.04 g cm [high] -3.

The film thickness is in the above-mentioned range between 0.010 and 0.025 mm.

The following procedure was used to apply the noble metal to the silica.

The water absorption of the coated silica support was measured and the amount of precious metal required to apply 1 g.dm [high] -3 precious metal to the support was calculated from this. An aqueous solution of a noble metal salt, for example RhCl [deep] 3.xH [deep] 2O, with a concentration of 14 g.1 [high] -1 Rh. The carrier was then immersed in the solution for 2 minutes, removed and drained left and the excess solution vented. This process was repeated until the desired amount of 1 g.dm [high] -3 of noble metal was charged. The process was ended by completely drying the carrier with hot air and finally placing it in a tube furnace under an atmosphere of H [deep] 2 / N [deep] 2 with 5%
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was reduced at 450 ° C (for rhodium) or 220 ° C (for iridium) for 1 hour.

Methods of applying oxides are described in British Patent 1,390,182. A description of typical ceramic supports and useful metal supports is contained in British patent specification 882,484 and in DT-OS 2,450,664 (corresponding to British applications 49501/73 and 32703/74).

Embodiments according to the invention are explained in more detail in the following descriptions, examples, tables and drawings.

Manufacture of the catalyst.

A catalyst containing platinum, rhodium and iridium coated on Davison 70 silica was prepared by impregnating silica with a required amount of a solution of the appropriate chloride or chloric acid at the required concentration. The impregnated silica was then dried in vacuo (0.1 torr) at room temperature and then reduced in a stream of hydrogen at 350 ° C. (for platinum) and 450 ° C. (for rhodium and iridium). Using this method, the following silica-supported catalysts were prepared.

Table 1

Catalyst A Rh / SiO [low] 2 6.14 x 10 [high] -2At% (approx. 0.1 wt.%)

Catalyst B Rh / SiO [low] 2 6.14 x 10 [high] -3At% (approx. 0.1% by weight)

Catalyst C Pt / SiO [low] 2 6.14 x 10 [high] -2At% (0.2 wt.%)

Catalyst D Pt / SiO [low] 2 6.14 x 10 [high] -3At% (0.02 wt.%)

Catalyst E Ir / SiO [low] 2 6.14 x 10 [high] -3At% (approx. 0.02% by weight).

The atomic percentages given in the table are the ratios of the metal atoms to the number of moles of the silica. Similarly,% by weight means the ratio between the weight of metal and silica.

Experimental test.

The experimental procedure described below was carried out in a silica reaction vessel consisting of two tubes arranged vertically and concentrically. The clear diameter of the outer tube was about 3 cm and that of the inner tube 0.9 cm. The catalyst was carried on silica wool in the inner tube and the reaction gas stream was guided downward in the outer tube and upward in the inner tube. The gas flows through a preheated zone downstream of the catalytic converter. The inlet temperature of the reaction gas was measured with a thermocouple placed on the wall below the catalyst. Furthermore, the reaction vessel was surrounded by a heating apparatus in order to be able to generate different levels of heating. Finally, a "BOC Luminox" analyzer and a "Perkin-Elmer F 17" chromatograph were provided to measure the inlet and outlet gas concentration of gases.

Experimental results and discussion

Two experimental procedures were carried out with the catalysts, namely:

a) To record the activity profile in the removal of nitrogen oxides as a function of the inlet temperature at a certain oxidizing to reducing inlet gas ratio and

b) to determine an isothermal activity profile as a function of the gas ratio at a temperature selected from the method according to a).

The gas ratio
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between oxidizing and reducing gas types in the inlet stream was determined using the formula calculated.

The nitrogen oxide (NOx) in the formula is generally NO.

In the experiments described below, generally 1 g of catalyst was tested and only sometimes less, here mostly 0.005 g. In the last cases, however, the catalyst was extended to 1 g by adding pure silica in order to avoid dilution of the support.

The space velocity was calculated on the basis of the weight of the catalyst without any added pure silica and given in units of St [high] -1.

The experiments were carried out according to Table 2. In this table, the "catalyst" designation relates to Table 1, "Process type" relates to the above-mentioned processes a) or b) and "R or temperature" relates to the corresponding ratio R (in process type a) or the isothermal temperature (for process type b)).

Table 2

Experiment 1 shows the activity of a Rh / SiO [deep] 2 catalyst with a
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of 1.9 (i.e. an oxidizing state); the maximum decomposition of NOx at 230 ° C with 95%. The "half peak activity range" (ie the activity temperature range at which the activity is half the maximum activity) was measured at 220 ° C. The formation of nitrogen oxide (as% of the total NOx input quantity) reaches a maximum just above the NOx decomposition threshold temperature and then decreases continuously until it reaches zero at around 450 ° C.

Experiment 2 was carried out under similar conditions and the same catalyst as in experiment 1, only that 0.005 g was used instead of 1 g. As a result, the temperature rose to 400 ° C for maximum activity. Despite the small amount of catalyst and the high space velocity in relation to experiment 1, the decomposition of the NOx at 400 ° C. is still 75%. The formation of N [deep] 2O did not exceed 18% and already decreased rapidly at the temperature of maximum activity.

Experiment 3 was carried out with the same catalyst from Experiment 2 but under isothermal conditions. guided. A temperature close to the temperature of maximum activity in the decomposition of NOx in test 2 was selected (395 ° C.). The maximum decomposition of NOx (and the lowest N [deep] 2O formation) takes place approximately at the stoichiometric point (R = 1), and the value of R was 2.3 at 50% of the maximum NOx decomposition.

For experiment 4, 1 g of a Rh / SiO [deep] 2 catalyst with a lower metal charge than for experiments 1-3 was used. If the metal charge is diluted, the maximum NOx decomposition decreases at a higher temperature. Half of the peak activity is also reduced to 160 ° C.

Experiment 5. The catalyst from Experiment 4 was used for the isothermal diagram. Experiment 5 showed that the catalyst is less resistant to oxygen "poisoning" at R values between 1 and 5 and that the N [deep] 2O formation was higher than with the similar catalyst with a higher metal charge. At 50% of the maximum NOx decomposition, the R value was 2.5.

Experiment 6, in which 1 g of a Pt / SiO [deep] 2 catalyst was used, is identical to experiment 1 in which a similarly charged Rh / SiO [deep] 2 catalyst was used, comparable. The result of experiment 6 showed, compared to the 95% maximum activity for Rh / SiO [deep] 2 in the NOx decomposition, only a maximum activity for Pt / SiO [deep] 2 of 50%. Even so, this inferior result does not necessarily reflect the correct activity of this catalyst, especially since very little is known about the relative dispersion of metals on silica. Half the peak activity for Pt / SiO [deep] 2 was 200 ° C in contrast to 220 ° C for Rh / SiO [deep] 2.

In experiment 7, 0.005 g of the same catalyst as for experiment 2 was used. The two experiments can therefore be compared with one another. In this case, just as in Experiment 2, the temperature for the maximum activity of NOx decomposition also increased when the amount of catalyst was decreased, but the maximum decomposition was only 17%.

Experiment 8 was carried out under isothermal conditions at 400 ° C. In contrast to experiment 3, the resistance to oxygen poisoning for Rh / SiO [deep] 2 was very low in this case. The value of R at 50% maximum NOx decomposition was 1.25.

Experiment 9 was carried out with 1 g of Pt / SiO [deep] 2 catalyst, which has a lower metal charge than the catalysts used in experiments 7 and 8. Experiment 9 can be compared with experiment 4 for Rh / SiO [deep] 2, and the result showed that Pt / SiO [deep] 2 is inferior to Rh / SiO [deep] 2 in terms of NOx decomposition, and that they have roughly the same half peak activity.

Experiment 10 was carried out isothermally at 360 ° C. In comparison with experiment 5, Pt / SiO [deep] 2 shows a lower resistance to oxygen poisoning than Rh / SiO [deep] 2. The R value at 50% maximum NOx decomposition is equal to 1.7.

Experiment 11 was carried out with Ir / SiO [deep] 2 and is comparable to experiments 4 (Rh / SiO [deep] 2) and 9 (Pt / SiO [deep] 2). The maximum NOx decomposition was 65% at 345 ° C. At this temperature the N [deep] 2O formation was essentially zero. Half the peak activity was 190 ° C.

Run 12 was a repeat run of 11 with an identical catalyst. It shows the remarkable effect that was caused by the heating to 600 ° C. in experiment 11. The maximum NOx decomposition was 90% and at 350 ° C.

Only a maximum of 2% N [deep] 2O was formed. Half of the peak activity was relatively accurate at 100 ° C.

Experiment 13, which was carried out isothermally at 329 ° C. on Ir / SiO [deep] 2, shows that this catalyst has a significantly higher resistance to oxygen poisoning than Rh / SiO [deep] 2 (experiment 5) or Pt / SiO [deep] 2 (experiment 10). R was equal to 5 at 50% of the maximum NOx decomposition.

In summary, these results show that the catalysts mentioned give a remarkable result with regard to the decomposition of NOx under oxidizing conditions. Furthermore, all have shown high efficiency in the oxidation of carbon monoxide, and it is predicted that these catalysts, either alone or in conjunction with known oxidation catalysts, for example noble metal alloys such as Pt / Rh, the lower hydrocarbons that are characteristic in the exhaust gases of internal combustion engines , will oxidize successfully.

Rh / SiO [deep] 2 and Pt / SiO [deep] 2 lead to the formation of nitrogen oxide under certain conditions. It has been found that additives can be added to the catalyst to catalyze the reduction or decomposition of the nitrogen oxide. As examples of the additives, lanthanum, cermetall and titanium can be mentioned.

Claims (12)

1. A method for the catalytic elimination of nitrogen oxides contained in a gas and / or for the catalytic oxidation of carbon monoxide and hydrocarbons, characterized in that the gas is brought into contact at an elevated temperature with a catalyst which contains at least one metal from the platinum group, gold and silver and is connected to or deposited on a refractory oxide carrier, the oxide carrier being made from silica, alum earth or mixtures or compounds with silica and / or alum earth. 2. The method according to claim 1, characterized in that the metals are contained as alloys or mixtures. 3. The method according to claim 1 or 2, characterized in that a catalyst is used which additionally contains one or more base metals or noble metals that are not listed in claim 1. 4. The method according to claim 1, characterized in that the catalyst contains at least one additional metal or oxide to promote the reduction or decomposition of nitrogen oxide. 5. The method according to claim 4, characterized in that the oxide is at least one oxide of lanthanum, cermetall and titanium. 6. The method according to claim 1, characterized in that the catalyst metal is rhodium, platinum or iridium, and that the refractory oxide carrier is SiO [deep] 2. 7. The method according to any one of the preceding claims, characterized in that the gas is the exhaust gases from motor vehicles. 8. The method according to any one of the preceding claims, characterized in that the refractory oxide carrier is deposited on a substrate. 9. The method according to claim 8, characterized in that the substrate is made of a refractory oxide in the form of a honeycomb. 10. The method according to claim 8, characterized in that the substrate is made from a metal sheet and has a cell-like structure. 11. The method according to claim 1, characterized in that the elevated temperature is in the range of 300-600 ° C. 12. The method according to claim 11, characterized in that the elevated temperature is in the range between 300 and 450 ° C.