Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/34—Chemical or biological purification of waste gases
  • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
  • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
  • B01D53/944—Simultaneously removing carbon monoxide, hydrocarbons or carbon making use of oxidation catalysts
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
  • B01J23/70—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper
  • B01J23/76—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36
  • B01J23/84—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36 with arsenic, antimony, bismuth, vanadium, niobium, tantalum, polonium, chromium, molybdenum, tungsten, manganese, technetium or rhenium
  • B01J23/847—Vanadium, niobium or tantalum or polonium
  • B01J23/8472—Vanadium
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
  • B01J23/70—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper
  • B01J23/89—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper combined with noble metals
  • B01J23/8933—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper combined with noble metals also combined with metals, or metal oxides or hydroxides provided for in groups B01J23/02 - B01J23/36
  • B01J23/898—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper combined with noble metals also combined with metals, or metal oxides or hydroxides provided for in groups B01J23/02 - B01J23/36 with vanadium, tantalum, niobium or polonium
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
  • F02B—INTERNAL-COMBUSTION PISTON ENGINES; COMBUSTION ENGINES IN GENERAL
  • F02B3/00—Engines characterised by air compression and subsequent fuel addition
  • F02B3/06—Engines characterised by air compression and subsequent fuel addition with compression ignition

Definitions

• the present invention relates to a method for the combustion of soot from diesel engines and to a catalyst for use in this method.
• the invention makes it possible to efficiently purify the exhaust gases from diesel engines both with regard to CO and unburnt hydrocarbons and with regard to soot.
• Diesel engines have two major advantages over spark ignition engines. The fuel economy is 25-45% better, and the emissions of carbon monoxide (CO) and hydrocarbons are low. Because of the excellent fuel economy, the number of diesel-powered vehicles is expected to increase in the future, especially in a situation with high fuel prices. Although the emissions of carbon monoxide and unburnt hydrocarbons from diesel engines are low, something must be done in order to decrease the emissions of nitrogen oxides and soot particles.
• diesel exhaust gases seldom reach temperatures above 573°K. This is a dilemma and will result in an extensive accumulation of diesel soot in the particle trap, and this again causes a further problem since the combustion process is highly exothermic, causing the temperature to rise dramatically when the soot eventually ignites. At such temperatures, the trap will crack or even melt down.
• TGA thermogravimetric analysis
• Watabe et al pri ⁇ marily aimed at evaluating carrier materials for soot par ⁇ ticle traps, especially porous ceramical bodies, so-called ceramic foam.
• Watabe et al indicate that the catalysts of Cu-K-Mo and Cu-K-V in the ratios 4:4:2 should
• soot has an extremely complicated composition.
• the soot contains sul ⁇ phur compounds and transition metals as well as hydrocar ⁇ bons. All of these components will affect the combustion process.
• An important problem is the fact that large amounts of S0 2 will desorb during the heating of the die ⁇ sel soot. This results in an immediate poisoning of oxides susceptible to sulphur, such as oxides of Mn and Pb.
• Soot particles are built up of small spherical pri- mary particles composed mainly of carbon and having a diameter in the range of 20-60 nm, which is typical of soot formation in the gaseous phase. These primary par ⁇ ticles agglomerate to form secondary aggregates. The agglomeration already starts in the combustion chamber and continues all the way through the exhaust system. The secondary particles can become rather large, but more than 90% of these particles have a diameter below 300 nm. Large hydrocarbon molecules are condensed on the soot particles at the temperatures prevailing in diesel exhausts. The chemical composition of soot particles has been reported in literature [e.g. M. Mashaly, F. Nevejans, P. Sandra and M. Verzek, Proc. Int.
• a representative diesel soot contains approxi ⁇ mately 70 wt% C, 20 wt% 0, 3 wt% S, 1.5 wt% H, ⁇ 1 wt% N and ⁇ 1 wt% trace elements.
• the composition of elementary components in the soot may vary considerably depending on the fuel quality, the engine efficiency and the running conditions. According to Otto et al [K. Otto, M.H. Sieg, M. Zinbo and L. Bartsiewicz, "SAE Congress and Exposi- tion", SAE 800336, 277 (1980)] the amount of volatile hydrocarbons can vary between 10 and 90 wt%. Fresh soot particles contain a higher level of volatile hydrocabons than soot particles aged in the exhaust manifold.
• diesel fuel contains sulphur
• relatively large amounts of sulphur are also found in the soot particles.
• the nitrogen content of diesel soot has -its origin in nitrogen oxides formed during the combustion process. Through reactions between nitrogen oxides and aromatic hydrocarbons, aroma ⁇ tic nitro and nitroso compounds can be produced. Some of these compounds are highly mutagenic.
• the trace amounts of other elements, mainly metals, have their origin in the fuel or in the lubricating oil. These trace elements are incorporated into the soot particles during the combustion process. As mentioned before only few studies exist on the combustion of diesel soot. These studies show that com ⁇ bustion rates are much higher with diesel soot than with graphite or activated carbon.
• the hydrocarbons in the soot can be classified into different groups, depending on their chemical characteris ⁇ tics [cf. G. Hunter, J. Scholl, F. Hibbler, S. Bagley, D. Leddy, D. Abata and J. Johnson (SAE Spec. Publ.), SAE, 810263 (1981) 111].
• SAE Spec. Publ.
• SAE 810263 (1981) 111.
• Several of these are mutagenic sub ⁇ stances. Since the soot particles are very small, they can easily reach far down into lung tissue when inhaled. For this reason, these emissions have a decided impact on human and animal health. This indicates the importance of an efficient purification of diesel engine exhausts, espe ⁇ cially with regard to soot.
• the European Document EP-A-0,092,023 mentioned above relates to a catalyst which may contain vanadium and copper in mixture ratios which, according to the Examples, are given as 50:50 and 20:80, respectively.
• Watabe et al emphasise catalysts of Cu-K-Mo and Cu-K-V in the ratios 4:4:2 which are said to be the most effective catalysts.
• EP-A-0,225,595 relates to a highly active catalyst for burning of hydrocarbons. This publication does not mention soot burning which differs substantially from the mechanisms of hydrocarbon burning. Thus, this document is not relevant to the burning of soot. No catalysts with vanadium and copper ratios accord ⁇ ing to the present invention are mentioned.
• German Publication DE-A-3,623,600 relates to a catalyst which contains alkali metal and copper vanadate Cu 3 V 2° 8 and is used for lowering the ignition temperature of diesel soot. Also this document mentions no catalysts containing vanadium and copper in ratios according to the present invention.
• Known catalytic and non-catalytic methods for burning soot have different drawbacks which have limited their use. Therefore, there is a great need for highly active catalysts which can both enhance the burning of diesel soot and be regenerated continuously during their use.
• One object of the present invention is, therefore, to develop a highly active catalyst for burning diesel soot. Other objects will appear from the description below.
• the present invention is based on the insight that burning of diesel soot can be performed catalytically under the action of metal oxides and metals.
• the charac ⁇ teristic features of the invention are set forth in claims 1 and 5, especially preferred embodiments of the invention beiijag stated in the sub-claims.
• the present invention relates to a method of purify ⁇ ing exhaust gases from diesel enginges by catalytic burn ⁇ ing of soot and hydrocarbons, in which method the exhausts are conveyed at a temperature of at least 300°C, prefer- ably at least 350°C, over catalyst-containing vanadium and copper oxides carried by a carrier, this method being cha ⁇ racterised by using a catalyst in which the molar ratio of V:Cu is from 85:15 to 95:5.
• the invention also relates to a catalyst for purify- ing exhaust gases from diesel engines, this catalyst com ⁇ prising a carrier and a catalytic material comprising oxides of vanadium and copper, the catalyst being charac- terised in that it contains vanadium and copper in a molar ratio of V:Cu of from 85:15 to 95:5.
• V 2 0 5 has a superior activity at tempera ⁇ tures above 623°K, while CuO is the most active catalyst below this temperature. Since the soot phase is burnt mainly at temperatures above 623°K, V 2 0 5 is the most active catalyst for burning the non-volatile components of the diesel soot. Oxides of Mn and Cr as well as elementary Ag and Pt have high activities in the combustion of hydro ⁇ carbons that desorb from the diesel soot. On the other hand, oxides of Co, Fe, Mo and Pb have low activities in the whole temperature range 573-723°K studied.
• the composite catalyst which, ac ⁇ cording to the invention, contains V and Cu in a molar ratio of 85:15 to 95:5, is not either affected to a higher degree by sulphur, which imparts to this catalyst excel- lent stability and long useful life. This is an advantage in a catalyst indended for use in the purification of ex ⁇ hausts from diesel engines.
• V 2 0 g The reason for the high activity of V 2 0 g could pos ⁇ sibly be explained by its low melting point (963°K).
• the atoms in the structure obtain an appreciable mobility. This temperature is about half the temperature of the melting point.
• Baker and Chludzin- ski [R.T.K. Baker and J.J. Chludzinski Jr., Carbon, 19 (1981) 75] state in explanation of the observed mobility of the particles of CuO in a graphite matrix that the tem ⁇ perature was above the corresponding Tamman temperature (799°K).
• these authors state that CuO is transformed, at this temperature, from the non-wetting state into the wetting state.
• V 2 0 5 Since 2 0 5 becomes wetting at the Tamman temperature (482°K), the contact efficiency between -the soot and the catalyst will be promoted.
• Another explanation of the high activity of V 2 0 5 could be that vanadium can exist in several oxidation states at small energy differences. Because of this, vanadium can easily change ' its oxidation state during the catalytic prod ss. According to McKee [D.W. McKee, Carbon, 8 (1970) 623], the catalytic effect of _0 5 in the oxidation of graphite resulted from the interaction of graphite and V «0 ⁇ to give g 0 13 . Vg0 13 was then re-oxidised back to V ⁇ O by ambient oxygen.
• oxides of vanadium are highly active in the catalytic combustion of diesel soot at temperatures above 573°K, especially above 623°K. Further, the experiments have confirmed other authors' statements that Ag, Pt and oxides of Cu, Cr and Mn have high activities at low tempe- ratures, when the combustion of desorbing hydrocarbons is important. Further, since copper oxides seem to have a somewhat higher resistance to sulphur than the above-men ⁇ tioned substances, this oxide has been used in the present invention. Another and much more important reason for com ⁇ bining oxides of vanadium and copper is the discovery of a synergistic effect between these oxides within a specific range of mixing ratios.
• the oxides of vanadium and copper may advantageously be used in combination with Ag, Pt, Rh or oxides of Mn.
• the catalytically active material can be dispersed on a coating or a carrier of the type 2C-alumina for simulating the conditions in a particle trap.
• a wire netting filter the wires of which have been coated with a layer which, in its turn, is impregnated with the catalyst mate ⁇ rial.
• the carrier preferred at present is a body of honeycomb structure. Such bodies are used to a great extent for exhaust purification devices for gasoline- powered vehicles and therefore are easily obtainable.
• these carriers may be coated with a layer which is impreg ⁇ nated with a catalytic material.
• the cata ⁇ lytic material may be impregnated directly into the car ⁇ rier.
• tube filters can be used, the tubes of which have been coated with the catalytically active material.
• the choice of carrier system depends on the construction of the exhaust system and can be left to the user of the method and the catalyst according to the present inven ⁇ tion. However, it is important that the carrier can with- stand the present high temperatures so that it is not necessary to impose temperature limitations during use of the exhaust purification device.
• the exhaust gases are brought into contact with the catalyst at a temperature of at least 300°C, prefer- ably at least 350°C.
• the upper temperature limit is set mainly by the ability of the carrier material and the en ⁇ closing receptacle to withstand high temperatures.
• a prac ⁇ tical upper limit can be 450°C or 500°C.
• the amount of active material i.e. oxides of vana- dium and copper
• the amount of active material is depending on the carrier used and the method used for coating the carrier with the active material.
• the carrier used consists of a wash coat of aluminum oxide or another com- parable high-temperature resistant material
• a loading of 5-25 wt% active material, based on the mixture of carrier and active material may be suitable, if the active mate ⁇ rial is distributed homogeneously in the coating.
• this coating can be applied on another substrate, such as a monolith of honeycomb type.
• the lower limit may preferably be increased to 7.5 wt%.
• the upper limit has been given as 25 wt%, but higher loadings than 20 wt% do not signifi- cantly increase the activity. For economical reasons, the upper limit may therefore preferably be 20 wt%. Corre ⁇ spondingly, loadings of 0.01-25 wt%, based on the mixture of carriers (wash coat material), oxide of vanadium and copper and precious metal, can be used as starting points when dealing with loadings of platinum, palladium and rhodium.
• Fig. 1 is a diagram showing the logarithmic combustion rate of diesel soot as a function of the inverted value of the temperature in a preliminary experi ⁇ ment.
• Fig. 2 is a diagram showing the selectivity for the pro ⁇ duction of CO as a function of the temperature in another preliminary experiment.
• Fig. 3 is a diagram showing the combustion rate as a function of the molar content of vanadium in a composite oxide catalyst according to the present invention.
• Fig. 1 is a diagram showing the logarithmic combustion rate of diesel soot as a function of the inverted value of the temperature in a preliminary experi ⁇ ment.
• Fig. 2 is a diagram showing the selectivity for the pro ⁇ duction of CO as a function of the temperature in another preliminary experiment.
• Fig. 3 is a diagram showing the combustion rate as a function of the molar content of vanadium in a composite oxide catalyst according to the present invention.
• Fig. 1 is a diagram showing the logarithmic combustion rate
• Fig. 4 is a diagram showing the relative EDAX signal from Al as a function of the content of active phase in a catalyst according to the present invention.
• Fig. 5 is a diagram showing the combustion rate as a func ⁇ tion of the content of active phase (90 mole% V, 10 mole% Cu) in a catalyst according to the presen invention.
• Fig. 6 is a diagram showing the selectivity for CO pro- duction as a function of the temperature for one example of a catalyst according to the present invention.
• Fig. 7 is a diagram showing the combustion rate as a func tion of the Pt content of a composite catalyst of vanadium and copper oxides and platinum according to the present invention.
• Fig. 8 is a diagram showing the combustion rate as a func tion of the calcination temperature in one example of a catalyst according to the present invention.
• Fig. 9 is a phase diagram for the system V 2 0 5 and CuO.
• Fig. 10 shows schematically an apparatus used in the expe ⁇ riments with the present invention.
• the invention is based on the in ⁇ sight that vanadium pentoxide in combination with copper oxide is an effective catalyst for the combustion of diesel soot.
• results are presented from an investigation of a number of catalytically active mate ⁇ rials, mainly metal oxides, in the burning of diesel soot under conditions which resemble the conditions in particle 5 traps.
• soot particles used in the experiments presented below were collected from the exhaust streams from diesel engines at Volvo Truck Corporation in Gothenburg, Sweden. After collection, the soot was kept in a closed vessel in 10 order to avoid desorption of volatile hydrocarbons.
• the chemical composition of the soot used in the experiments is given in Table 16, below.
• Fig. 10 shows schematically the apparatus used for the,experiments with the invention.
• This apparatus com- 15 prised three units, namely a gas mixing unit, a reactor unit and ⁇ ap. analysis unit.
• the gas mixing unit comprised needle valves 1, rotameters 2, shut-off valves 3, reducing valves 4, and gas tubes 3 for oxygen and nitrogen.
• the gas mixing unit further comprised another needle valve 1A for l ⁇ O regulating the mixed flow of oxygen and nitrogen, and an outlet valve IB.
• a rotameter 2A was connected in the line from the needle valve A, ahead of a manometer 6.
• N 2' °2 an / 0 P ionall y- S0 2 t *• N 2•• were supplied from separate gas tubes to the gas mixing unit.
• the different 35 gas flows were measured by the rotameters 2, the measured values of which were corrected with respect to the abso ⁇ lute pressure in the equipment.
• the partial gas flows were united to a total flow which was measured by a rotameter 2A.
• a flow divider consisting of the needle valve IB was installed to make it possible to vary the total flow through the reactor within wide limits, without having to change the partial flows from the various gas tubes. The inaccuracy of the flow setting was at most 5%.
• the preheater 7 for preheating the gas mixture before the inlet end of the reactor unit consisted of a tube hav ⁇ ing a length of one meter and being coiled with a resis- tance wire for electrical heating. After preheating, the gas mixture had a temperature of about 200°C. Before the introduction in the furnace, steam was added using the injection pump 8. The water injection was carried out with a syringe directly into the preheated gas mixture. After the water injection, the gas mixture was conveyed into the furnace where the final preheating to the prevailing tem ⁇ perature took place in a helically coiled tube having a length of 3 m. The furnace was designed for a maximum operation temperature of 700°C.
• the furnace was heated by resistant heating. A fan had been installed to increase the convection in the furnace.
• the temperature stability of the furnace was very good, and the temperature fluctua ⁇ tions were less than 0.2°C at 600°C. At maximum heating efficiency, the temperature could be increased by more than 30°C/min. in the furnace. The temperature in the furnace could be increased successively.
• thermocouples were con ⁇ nected in series so that the temperature difference between the soot bed and the reactor could be recorded. This made it possible to use the reactor temperature as a reference temperature, and the heating effect in the reac- tor zone could be followed with high accuracy; temperature differences of 1°C could be recorded.
• the gas mixture After leaving the reactor, the gas mixture was con ⁇ ducted through a cooler 10 which was a refrigerator in which the water was removed by condensation. When leaving the refrigerator 10, the gas mixture had a dew point of +4°C.
• the gas mixture was conveyed to an IR instrument 11 (Siemens Ultramat 22P) for 3 sec. after leaving the reac ⁇ tor, this indicating that the useless volumes were small.
• the IR instrument used could detect CO in the interval
• Both the IR instrument and the gas chromatograph were calibrated regularly, using a gas mixture with known con ⁇ tents of CO, C0 2 and S0 2 .
• the con- centratiors of CO and C0 2 were measured and the combustion rate (r. . , expressed as nmole burnt carbon per second) was calculated.
• the gas mix- ture also contained S0 2 .
• the experiments were conducted to detect a possible catalyst deactivation due to sulphur poisoning.
• the running of the reactor was varied, but the maximal relative consumption of oxygen through the reactor never exceeded 5%.
• the standard deviation in the all the calculated combustion rates was less than 10%.
• the catalytically active material mainly consisted of metal oxides.
• the catalytical active material was distributed on a coating (often y-Al 2 0 3 ) formed as a wash coat.
• a coating often y-Al 2 0 3
• J.-A1 2 0 3 particles having a diameter of 0.25 mm as carrier material.
• the carrier mate ⁇ rial for the experiments was obtained from Alcoa Chemi- 2 c'als, USA.
• the specific surface area was 290 m /g before impregnation.
• the catalysts were prepared by impregnating the car ⁇ rier material with aqueous solutions of different metal salts. All of the chemicals used were of analytical grade, and the water used was distilled twice. For platinum, a solution of H 2 PtCL ⁇ was used, and for forming V 2 0_ and Mo0 3 use was made of solutions of NH.V0 3 (oxalic acid) and (NH.)gMo 7 0 24 -4H 2 0, respectively. Otherwise, corresponding nitrates were used. After impregnation, the catalysts were dried at 383°K in an oven for 24 h. Calcination was per ⁇ formed in a reactor at 773°C in air for 1 h.
• the Pt-cata ⁇ lyst was reduced in similar manner for one hour in an atmosphere containing 5% H 2 , the balance being N ? . All catalysts, except the Pt-catalyst, contained 10 wt% active material (calculated on the corresponding metal). The Pt- content of the Pt-catalyst was 1 wt%. Higher levels are not realistic due to the high price of precious metals. Since the outer surface of the particles should be important in the burning of soot, small fractions should not be prepared by milling after impregnating the carrier material with the active phase. Instead, the carrier mate ⁇ rial should be milled to the suitable grain size before impregnation. The experimental procedure was as follows:
• soot is in excess during the whole experiment. If the experiment takes too long (i. ⁇ _ too low a temperature rise rate), or if the initial soot:catalyst ratio is too low, the catalyst will be in excess at the end of experiment. Conversely, if the tempe ⁇ rature-rise rate is too high, the conditions will be far from stationary, with ensuing local overheating in the bed and a decided risk of ignition. If the soot: catalyst ratio is too high, it is difficult to detect the catalytic activity as soot is combusted also in the absence of cata ⁇ lysts. Practical tests showed that a temperature rise rate of 5°K/min. and an initial soot:catalyst ratio of 0.1 (on weight basis) satisfy the above requirements.
• soot was burnt catalytically in two different ways, on the one hand in the form of mixtures of soot and carrier material and, on the other hand, as mixtures of catalysts, carrier material and soot.
• EXAMPLE 1 This Example is a preliminary experiment for compari ⁇ son, showing that diesel soot is combusted also in the absence of catalyst material.
• a mixture of 0.05 g soot and 0.5 g #-alumina was placed in the reactor.
• the experi- ment was considered important to verify that alumina nor ⁇ mally is inactive at combustion reactions.
• the total gas flow rate was 901/h.
• the temperature increase rate was 5 c K/min., and the gas mixture consisted of 6% 0 « and 7% H 2 0, the balance being N 2>
• Fig. 1 shows the combustion rate as a function of the inverted value of the tempera ⁇ ture in an Arrhenius graph. The measured values are given in Table 1, below.
• the combustion rate increased from 200 nmole/s. to 280 nmole/s. when the temperature was raised from 673°K to 773°K.
• the product gas was analysed for both CO and CO- to make it possible to calculate the selectivities. These selectivities have no fundamental significance, since they are the result of secondary reactions at which CO is oxidised to C0 2 . However, in this case it is important to know how much CO has been produced during combustion. If too much CO is reduced, this would lead to an increased emission of CO from the diesel vehicle.
• the combustion rate obtained must be related to the com ⁇ bustion rates valid for mixtures of soot and alumina with- out any catalytically active material.
• the ratio between these combustion rates at different temperatures are reported in Table 3.
• Deactivation was performed in a specially designed reactor.
• the catalysts were analysed in respect of their performance during combustion.
• Table 4 gives the ratio between the combustion rates for deactivated and fresh catalyst using 0.05 g soot and 0.5 g catalyst (containing 10 wt% active phase and 90 wt% car ⁇ rier material), a total flow rate of 90 1/h, and a gas composition of 6% 0 2 , 7% H 2 0 and 100 ppm S0 2 , the balance being N 2 .
• V 2 0 5 as a catalyst for the combustional soot from diesel engines
• further experiments were carried out in order to investigate the value of mixtures of oxides or vanadium and copper in the catalytic combustion of diesel soot.
• the catalysts used in the following were prepared by repeated impregnation of y-alumina with aqueous solutions of NH.VO (oxalic acid) and Cu(N0 3 ) 2 . After impregnation, the cata ⁇ lysts were dried at 383°K for 24 h and then calcined at 773°K in *ir for 1 h.
• the catalysts nominally contained 10 wt% active phase and had different V:Cu ratios.
• the combustion experiments were carried out using a mixture of soot and catalyst (0.05 g soot + 0.5 g cata ⁇ lyst) in a flow reactor.
• the catalyst contained 10 wt% active phase.
• the gas mixture contained 10% 0 2 and 7% H ⁇ , the balance being N 2 , and was conducted through the reac- tor at a total flow rate of 901/h. The temperature was increased at a temperature rise rate of 5°K/min. to 698°K.
• the specific surface areas of the catalysts used in the following were measured using an area meter (FlowSorb II from MicroMeritics) through adsorption of N 2 according to the one-point BET method.
• the catalysts were flushed with N 2 at 623°K for 1 h prior to these measurements.
• the combustion rates are shown as a function of the molar fraction of vanadium in the catalyst surface in Fig. 3.
• the corresponding numerical values are given in Table 6.
• the activity of pure 2 0 5 is relatively low at temperatures below 623°K. Conversely, this low-temperature activity is rather high in the case of pure CuO at tempe ⁇ ratures above 623°K.
• the isotherms have a maximum as a function of the molar fraction of vanadium (maximum at V - 0.9, at temperatures above 598°K. This synergistic effect is pronounced at temperatures between 648°K and 698°K. In this temperature range, the combustion rates are 50% higher than those obtained for pure V 2 O j -.
• the low temperature activity is maintained up to a molar fraction of 0.95 in respect of vanadium.
• V 2°5 ⁇ Cu0 is shown in Fi _f- 9, from which it appears that a melting point minimum (903°K) occurs at a molar amount of 0.82 V, i.e. very close to the activity maximum observed at the molar amount of 0.9 according to what is stated above.
• the molar fractions which fall within the invention have been marked with a hatched area. If the molar amount * of Cu is increased, the melting point quickly rises. This may be an explanation of the lower activity of catalysts rich in Cu. As appears from Fig. 3, the activity at low temperatures ( ⁇ 623°K) is high for catalysts rich in Cu. At such low temperatures, the hydrocarbon combustion is considerable.
• CuO is much more active than V ._.0o, in com- bustion of hydrocarbons.
• the high low-temperature activity is extended to such high molar fractions of vanadium as 0.95. This means that the catalyst having the best high- temperature activity (molar fraction 0.9 V) also has a high low-temperature activity.
• the metal in M V.,0,- is single- ionised, whereas the corresponding numb x er of V5+ is re- prised to V 4+.
• the net oxidation state of the vanadium phase will be somewhat less than +5.
• the choice of carrier is also important to the development of lattice defects. If the interaction between the carrier and the oxide phase is high, as in the case of alumina, the lattice defects will be stabilised and could survive a high mobility of the structure (i.e. high temperatures).
• EXAMPLE 6 Experiments were also performed to determine the com- bustion rate as a function of the amount of active phase (V 2 0 5 /Cu0: 90 mole% V, 10 mol% Cu) at different tempera ⁇ tures. The results are given in Fig. 5 and Table 9. The combustion rates increase up to a loading of 10 wt% of active phase. A further increase in the loading does not increase the activity of the catalyst. EXAMPLE 7
• This experiment aims at determining how an addition of platinum will influence a catalyst which is intended for combustion of diesel soot and is based on V tract __0,_ and CuO and contains V and Cu in the nominal mole ratio 90:10.
• the same test conditions as in Example 4 were used.
• catalysts Nos. 1 and 2 the car ⁇ rier material was first impregnated with a solution of H 2 PtClg. This Pt precursor was then decomposed in an oxidising (catalyst No. 1) or in a reducing (catalyst No. 2) atmosphere. After this step, the catalysts were impregnated with solutions of NH.V0 3 and Cu(N0 3 ) 2 , dried and calcined in the usual manner. Catalysts Nos.
• the active phase contained 90 mole% V and 10 mole% Cu.
• the Pt loading was 0.10 wt%.
• the test conditions were the same as in Example 4. Fig. 6 and Table
• the temperature of diesel exhaust gases is normally below 573°K. At high speed and heavy load operation, how ⁇ ever, the temperature may get as high as 873°K. If the exhaust temperature is low ( ⁇ 573°K) for a long period of time, this will lead to an extensive accumulation of par ⁇ ticles in the particle trap. If the exhaust gas tempera ⁇ ture now rises sufficiently, the soot will ignite. The heat generated during the combustion will lead to a dra- mati ⁇ increase of the temperature in the trap (tempera- tures as high as 1273°K are possible). It is therefore of importance to investigate the temperature stability of the catalyst.
• Table 13 the catalyst compositions, the EDAX analysis and the specific surface areas are given. The specific surface area has decreased dramatically through the heat treatment, from 196 m 2 /g at 773°K to about 2 m 2 /g at 1073°K. This reduc ⁇ tion is caused by sintering of the carrier material and transformation of y-alumina into alumina.
• the car- rier material mainly is in the form of y alumina in the catalyst which was calcined at 1073°K. As the calcination temperature is increased, 8 alumina and ⁇ alumina are formed. After calcination at 1073°K, also a alumina could be detected. It is apparent that an extensive sintering and phase transformation has occurred during the high temperature calcination. In addition, several reflexes from the active phase could be observed for the catalyst calcined at temperatures above 873°K. As the specific surface area has decreased to such an extent, the dis ⁇ persion of the active phase becomes poor, i.e. bulk com ⁇ pounds are produced.
• the combustion rate is shown as a function of the calcination temperature in Fig. 8.
• the corresponding values are given in Table 14.
• the activities at low tem ⁇ peratures decrease dramatically as the calcination tem ⁇ perature is increased, which is due to the structural break-down of the catalyst leading to a dramatic reduction in the specific surface area.
• Mainly hydrocarbons are com ⁇ busted at temperatures below 623°K. This combustion pro ⁇ cess takes place also on the inner surfaces of the cata ⁇ lyst.
• the combustion of the solid soot phase however, mainly occurs on the outer surfaces of the catalyst. As the outer surface area is relatively unaffected by the calcination, the high temperature activity should be main ⁇ tained.
• Fig. 8 shows that the activities at temperatures above 623°K even increase as the calcination temperature is increased.
• the combustion of the solid soot phase takes place mainly on the outer surfaces of the catalyst grains, whereas CO and hydrocarbons are combusted also on the inner surfaces of the catalyst.
• the activity increases with the loading of active phase up to a value of 10 wt%.
• Very small amounts ( ⁇ 0.05 wt%) of platinum, palladium or rhodium dispersed on the active phase by decomposition of H 2 PtCl g in a reducing atmosphere greatly enchance the low-temperature activity.
• the carrier y alumina
• sinters and is transformed into other forms of alumina when subjected to high tempe ⁇ ratures. If the catalyst is subjected to high temperatures (1073°K), the specific surface area therefore is reduced dramatically due to sintering.
• the activity of the cata- lyst regarding the combustion of hydrocarbon decreases, whereas its activity regarding the combustion of the solid soot phase is maintained.

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