CATALYST FOR HETEROGENEOUS CATALYSIS CONSISTING OF AN ALLOY OF TRANSITION METALS

Abstract: A catalyst for heterogenic catalyzation through reactions with a gaseous substance with a reduction or oxidation agent, also in a gaseous state. The catalyst comprises an alloy of a number of transition metals, whereby both the specific properties of the different alloying components and their mutual coaction provide a catalyst which has a low activation temperature, high activity and a long effective life.

Description: Catalyst for eterogenous catalysis consisting of an alloy of transition metals. The present invention relates to a catalyst of the kind used to effect chemical conversion by so-called hetero- genous catalyzation, and more specifically causing a gaseous substance to react with a similar gaseous oxi¬ dizing agent or reduction agent on the surface of a solid catalyst which is, itself, a substance which actively takes part in the reaction by facilitating said reaction, although without being consumed itself. The presence of a catalyst may instigate reactions which would not otherwise take place or may cause reac¬ tions to be incomplete, or may result in end products which are faulty in some other way. An illustrative example of a catalyzed oxidation reac¬ tion is the reaction of sulfur dioxide with oxygen. This reaction is employed on a large scale within the chemical industry in the manufacture of sulfuric acid, which constitutes one of the most important base chemi¬ cals in the chemical industry and for reasons of environmental care and protection, to prevent the emission of sulfur dioxide to atmosphere and after catalytic oxidation to sulfur trioxide with subsequent conversion to sulfuric acid to achieve an end product which has a market value. The most common methods employed for the removal of sulfur dioxide from flue gases, is to react the sulfur dioxide with lime, with results in the production of large quantities of gypsum. The gypsum thus produced presents dumping problems, above all in densely-built areas in which the desulfurization of flue gases is employed. An example of catalyzed reduction reactions is found in the reaction of oxides of nitrogen with ammonia. The ammonia is admixed with the gas at a temperature of between 300 and 400°C, and the hot mixture is then brought into contact with a solid catalyst which may be in the form of plates or have honeycomb construction, where the reduction process takes place. This reaction is employed to remove the oxides of nitrogen formed to a greater or lesser extent in combustion reactions where nitrogen is present. The reaction between the nitrogen oxides and the ammonia results in the forma¬ tion of nitrogen and water, i.e. substances which can be safely released to atmosphere, as opposed to impure gases which when released to atmosphere cause damage to the surroundings in the form of acidification and excessive fertilization of ground and water. Oxides of nitrogen, however, can also be caused to react directly with ammonia or other reduction agents at temperatures of about 1,000°C. However, since the reaction begins immediately the reduction agent is mixed with the gas, zones occur which contain either surplus or insufficient reduction agent, and the degree of purification achieved is not the same as that achieved with catalytic reduction. It is essential in organic chemical industries where specific substances are produced by reactions across a catalyst that the catalyst will provide a high yield of the specific substance and that no secondary products are formed, or only small quantities of such products. This problem is of a minor nature in the aforegiven examples, although progressively more notice is being given to the problem concerned with the production of nitrous oxides in the reduction of nitrogen oxides. Instead, endeavours have been directed ih the aforesaid examples primarily to attempts to obtain a highly ac¬ tive catalyst, in other words, a catalyst with which the largest possible amount of material is converted per unit of time by the amount of catalyst used. This will allow the catalyst charge and the size of the reactor to be reduced, which affords significant econo¬ mic advantages. Endeavors are also made to obtain catalysts which are highly reactive at low temperatures, which in the case of the example concerning oxidation of sulfur dioxide can have the advantage of enabling a smaller heat- exchanger to be used for the purpose of bringing the gas to reaction temperature and to obtain a higher yield from reactions at lower temperature, as the result of a more favourable equilibrium. Another factor to be observed in this context is the useful life of the catalyst, which is contingent on a number of factors. Among other things, the influence of temperature, dust, chemical substances and mechanical strain can poison, block, disintegrate or sinter the catalyst or its carrier substance. These catalyst im¬ pairments will change the properties of the catalyst, such as its activity and its activation temperature, and in the case of granular catalysts will result in an increased pressure drop through the bed, with the sub- sequent negative result of increasing operational costs and/or reduction in plant capacity. It will be evident from the aforegoing that a number of different requirements are demanded on a good catalyst, namely: Low activation temperature. - High activity per unit of area. Large active surface. - Selectivity, i.e. the minimum formation of undesir¬ able secondary products. - Will not disintegrate or sinter at high tempera¬ tures. - Is resistant to wear and pressure. - Will not be influenced by dust or chemical impuri¬ ties in the gas. Has a low pressure drop. Whereas the latter property is partly contingent on the configuration of the catalyst used, e.g. a cylindrical, circular, plate-like or honeycomb configuration, etc., the remaining properties are contingent on the con¬ struction and composition of the catalyst with regard to both the active substance and, when found, carrier material, and also on the relative size and specific properties of the active surface. A catalyst for heterogenic catalyzation may include one or more active substances, and transition metals in particular have been found to have good catalytic pro¬ perties in the most widely separated contexts in res¬ pect of both organic and inorganic reactions. For in¬ stance, pure metals or alloyed metals from this group, optionally placed on a carrier, are used for the oxida- tion of ammonia to nitrogen oxides, for cleansing ex¬ haust gases from petrol engines, for oxidizing sulfur dioxide to sulful trioxide, and for hydration and dehy¬ dration processes within the petrochemical industry. For instance, oxides of one or more transition metals, optionally placed on a carrier, are used in the desul- furization (hydration) of petroleum products, ammonia synthesis and in the production of formaldehyde. Even when the catalyst is essentially in metal form, the active surface can still nevertheless consist of a thin oxide layer which is responsible for the catalytic activity. Those transition metals found to have particularly useful catalytic properties are the platinum-group metals, although endeavors have been made to substitute these metals with other substances, because of the high prices of such metals. In this regard, transition me- tals in the fourth group of the periodic table have primarily obtained an important significance, and not solely in metal form but also in the form of alloys and oxides. The object of the present invention is to provide a catalyst which includes an alloy of different tran- sission material in the absence of appreciable amounts of platinum-group metals and which will nevertheless fulfill the aforesaid requirements asked of a good catalyst and which is equally as good as a known cata¬ lyst found to have catalytic properties and comprising alloys of different transition material. As examples of such known alloys can be mentioned: - Stainless steel rings doped in diluted sulfuric acid. These rings have been found to function as catalysts in the reduction of nitrogen monoxide with ammonia. Preactivated stainless steel rings have been used as catalysts in the reduction of sulfur dioxide to elementary sulfur. When sampling combustion gas through a stainless steel tube, it has been observed that the oxides of nitrogen are reduced by carbon monoxide so as to form nitrogen and carbon dioxide. A catalyst comprising one or more of the substances Cr, Mo, Co, Ni and Cu has been found somewhat use¬ ful for cleansing vehicle exhaust gases from carbon monoxide, nitrogen oxides and hydrocarbons. - A catalyst which comprises in percent by weight Cr 23, Ni 18, Mo2, C 0.5 and Fe 56.5 has been used for the oxidation of ammonia to nitrogen monoxide. The object of the invention is realized by the inven- tive catalyst having the characteristic features de- t ιed in the following Claims, and more specifically c. -.racterized in that it includes complex alloys of at least some of the following alloying components, namely V, Cr, Mn, Fe, Co, Ni, Mo and Si at low carbon con- tents, preferably the smallest possible carbon con¬ tents. It has surprisingly been found that at least with regard to the reduction of nitrogen oxides with ammonia, the inventive catalyst exhibits a greater activity that the aforesaid alloys which have been foxind to possess certain catalytic properties, and even greater than known, commercially available flue-gas catalysts which consist of vanadium pentoxide disposed on a carrier, and this even with a substantially lower content of vanadium in the inventive catalyst than in the known flue-gas catalysts. The reason for this unexpected high activity is assumed to lie in the coaction between the various alloying components, each of which is able to adopt, individu- ally, different oxidation states between which said components change during the reaction processes. Because the transitions between the oxidation states of these different metals have different redox potentials, reactions promoted by the transitions which have the highest redox potentials can be made easier. Because the inventive catalyst has a composition simi¬ lar to that of stainless steel, the inventive catalyst is highly stable against degradation, sintering, chemi- cal influence and the influence of temperature, in addition to possessing a low activation temperature and high activity. The inventive catalyst includes an alloy of a plurality of transition metals and comprises at least vanadium, nickel and chromium and preferably, but not neces¬ sarily, also one or more of the following metals: moly¬ bdenum, manganese, silicon, iron and cobalt, with the least possible carbon content. Expressed in percent by weight, the proportions in which said metals are pre¬ sent may be: vanadium 5-20, preferably between 5 and 15; chromium 5-30, preferably between 15 and 25; nickel 10-40, preferably between 15 and 30; manganese 0-10, preferably between 3 and 8; iron 0-40, preferably be- tween 20 and 30; cobalt 0-20, preferably between 5 and 15; molybdenum 0-10, preferably between 3 and 8; sili¬ con 0-10, preferably between 0 and 5. The carbon con¬ tent of the inventive catalyst will be at most 0.1 percent by weight, and preferably 0.06 percent by weight. A number of examples are described below. Example 1 An inventive catalyst of the following composition, expressed in percent by weight, namely V 8.9; Cr 21.5; Mn 4.68; Fe balance (approximately 25); Co 10.6; Ni 22.6; Mo 4.9; si 2.1; C 0.055; and a specific surface of 0.1 m 2/g was used to reduce chemically 560 ppm i•n a gas containing 730 ppm of NH-. The catalyst had a pro¬ nounced activity at a temperature as low as 160°C, and the activity per unit of surface area within the inves¬ tigated temperature range was more than three times that of a catalyst consisting of 20 percent V 2 °5 on sio 2 - io 2 . Example 2 100 g of alloy according to Example 1 was alloyed with 50 g Al and the new alloy wasw leached in a warm NaOH- solution until the generation of hydrogen-gas ceased. The resultant sample had a specific surface area of 63 2 m /g and was tested for the chemical reduction of 600 ppm NO with 700 ppm of NH . Activities per unit of surface area corresponding to those in Example 1 were achieved subsequent to having activated the surface at a temperature of about 350°C. Example 3 A study was carried out a catalyst according to Example 2 which had been coated with soot and the soot com¬ busted to carbon monoxide and then to carbon dioxide. The activity per unit of surface area in both of these part reactions and in the total reaction was found to be comparable with the results achieved with catalysts that comprised 2 0 5 on A1_0 3 and including CuO on A1 2 0 3 . The surface of the catalyst can be enlarged, by alloy- ing with a lightmetal, e.g. alximinium, with subsequent electrolytic dissolution of the lightmetal. The present invention is not restricted to the afore- described, since modifications and changes can be made within the scope of the inventive concept as defined in the following Claims.

Claims: CLAIMS 1. A catalyst intended for preferably for hetero- genous gas-phase reactions of the kind in which a sxib- stance is reduced or oxidized and comprising an alloy of a plxirality of transition metals, c h a r a c ¬ t e r i z e d in that the alloy includes at least vanadium, nickel and chromium. 2. A catalyst according to Claim 1, c h a r a c ¬ t e r i z e d in that the alloy also includes one or more of the following metals: molybdenum, manganese, silicon, iron and cobalt, with the least possible carbon content. 3. A catalyst according to Claim 1 or 2, c h a r ¬ a c t e r i z e d by the following composition in percent by weight: V 5-20; Cr 5-30; Mn 0-10; Fe 0-40; Co 0-20; Ni 10-40; Mo 0-10; Si 0-10; C max 0.1. 4. A catalyst according to Claim 3, c h a r a c ¬ t e r i z e d in that vanadium is present in a propor¬ tion of between 5 and 15 percent by weight. 5. A catalyst according to Claim 3 or 4, c h a r ¬ a c t e r i z e d in that chromium is present in a proportion of between 15 and 25 percent by weight. 6. A catalyst according to any one of Claims 3-5, c h a r a c t e r i z e d in that manganese is present in a proportion of between 3 and 8 percent by weight. 7. A catalyst according to any one of Claims 3-6, c h a r a c t e r i z e d in that iron is present in a proportion of between 20 and 30 percent by weight. 8. A catalyst according to any one of Claims 3-7, c h a r a c t e r i z e d in that cobalt is present in a proportion of between 5 and 15 percent by weight. 9. A catalyst according to any one of Claims 3-8, c h a r a c t e r i z e d in that nickel is present in a proportion of between 15 and 30 percent by weight. 10. A catalyst according to any one of Claims 3-9, c h a r a c t e r i z e d in that molybdenum is pre¬ sent in a proportion of between 3 and 8 percent by weight. 11. A catalyst according to any one of Claims 3-10, c h a r a c t e r i z e d in that silicon is present in a proportion of between 0 and 5 percent by weight. 12. A catalyst according to any one of Claims 2-11, c h a r a c t e r i z e d in that the carbon content of the catalyst is at most 0.6 percent by weight. 13. A catalyst according to any one of the preceding Claims 1-10, c h a r a c t e r i z e d in that the catalyst has a specific surface area which has been enlarged by disintegration. 14. A catalyst according to any one of the preceding Claims 1-12, c h a r a c t e r i z e d in that the catalyst has a specific surface area which has been enlarged by admixing a component therewith and then removing said component. 15. A catalyst according to any one of the preceding Claims 1-12 and Claim 14, c h a r a c t e r i z e d in that the specific surface area of the catalyst has been enlarged by alloying the catalyst with a light¬ metal and sxibjecting the catalyst to a sxibsequent leaching process. 16. A catalyst according to Claim 15, c h a r a c ¬ t e r i z e d in that the specific surface area of the catalyst has been enlarged by alloying the catalyst with aluminixim and subsequently leaching with caustic liquor. 17. A catalyst according to any one of Claims 1-12 and Claim 14, c h a r a c t e r i z e d in that the spe¬ cific sxirface area of the catalyst has been enlarged by alloying with a lightmetal and sxibsequent electrolytic dissolution of the lightmetal. 18. A catalyst according to Claim 17, c h a r a c ¬ t e r i z e d in that the specific surface area of the catalyst has been enlarged by alloying with aluminium with sxibsequent electrolytic dissolution of the aluminium.

Inventors: SOLBU ERIK, SOLBU, ERIK
Applicants: SOLBU ERIK AB, ERIK SOLBU AB
Application year: 1990
Main classification: B01D53
IPC codes: B01D53, B01J23, B01J25, F02B1