WO1998051401A1

WO1998051401A1 - Gold based catalyst for exhaust gas purification

Info

Publication number: WO1998051401A1
Application number: PCT/BG1998/000010
Authority: WO
Inventors: Lachezar Angelov Petrov
Original assignee: Laman Consultancy Limited
Priority date: 1997-05-15
Filing date: 1998-05-15
Publication date: 1998-11-19
Also published as: BG62687B1; AU7201398A; EP0981402A1; BG101490A; JP2001524030A

Abstract

The invention relates to a gold catalyst for oxidation of carbon monoxide and hydrocarbons, reduction of nitrous oxides and degradation of ozone, for use in the protection of the environment. The active component of the catalyst consists of complex of gold and reducible oxide from the group of the transition metals. The concentration of gold in the catalyst is from 0.1 % to 2.5 % and the total concentration of the metals in the active component of the catalyst is from 0.1 % to 10 % of the total mass. The support consists of oxides of ceria and titanium. The surface of the catalyst is between 80 m2/g and 400 m2/g and the catalyst working temperature is from 0 °C to 400 °C.

Description

GOLD BASED CATALYST FOR EXHAUST GAS PURIFICATION

Background of the invention

The invention relates to a gold catalyst for the oxidation of carbon monoxide and hydrocarbons, reduction of nitrous oxides and the degradation of ozone.

Internal combustion engines contribute significantly to increasing pollution levels in the atmosphere. This will become worse in the future with the strong tendency towards global urbanisation. The working environment for a large number of people in today's industrial sites and administrative office buildings has an air pollution of unacceptably high level of carbon monoxide and other noxious gases. Computers and laser printers that are vastly used in our daily routines have increased ozone concentrations to dangerous levels and, thus have further deteriorated the working environment.

Environmental protection and reduction of pollution caused by industry, transport and way of life are major issue facing the world today. Various ways are used to achieve that: improvement of existing technologies, introduction of new, more effective methods in industry, more stringent legislation for reduction of pollutants, etc.

Catalysts of Prior art

A reduction of the emission of noxious gases from the combustion engines may be achieved by the use of PGM-based catalysts. However this catalysts function satisfactorily at temperatures higher than 300°C. Moisture and sulphur dioxide common for the exhaust gases, severely affect their performance at lower temperatures. It is known fact that 80% of the noxious gases from the combustion engine are emitted during the cold start of the engine, the first 3 to 5 minutes, where the conventional catalysts are not affective. The PGM-based catalysts are even less effective for diesel vehicles where the temperature of the emitted gases in lower than the temperature of the exhaust gases from the gasoline passenger car. Conversion of the TPM, CO, HC and NOx (4-way catalyst) is considered a major challenge for diesel vehicle application in the future. Further, the high working temperatures of the PGM catalysts make them unsuitable for air purification in buildings, aircrafts and industrial sites.

Gold has been long regarded as far less catalytically active than the platinum group metals (PGM's). Recent publications in the literature have shown that gold, when highly dispersed on reducible oxides, can be active for low temperature oxidation of CO. However, gold containing catalysts as shown in the literature, ere either to costly, with gold concentration up to 12%, or show poor conversion at the higher flow rate of gases common for the working conditions. Thus, these catalysts are not suitable for industrial applications and do not have real commercial value.

For example German patent No. 3914294 describes a catalyst in which the gold is captured on an iron oxide-containing support which includes also alumina or aluminosilicate. However this catalyst has poor conversion of carbon monoxide at higher space velocity and is severely affected by moisture and sulphur dioxide. Examples of gold catalysts supported on cobalt oxide, titanium oxide and iron oxide described in the literature are:

M. Haruta et al, "Mechanistic studies of CO oxidation on highly dispersed gold catalysts for use in room temperature air purification". Proceedings of the 10 International Congress of Catalysts 19-24 July 1992, Budapest, Hungary (2657-2660), and H. Kageyama et al, "XAFS studies of ultra-fine gold catalyst supported on hematite prepared from co-precipitated precursors", Physica B 158 (1989) 183-184.

Summary of the Invention

According to the invention, a catalyst for simultaneous oxidation and reduction reactions comprises a porous mixed oxides support having captured thereon a complex comprising gold and reducible oxide of a transition metal selected from chromium, copper, cobalt, manganese, iron or a combination of this metals. The concentration of the gold is from 0.1%-2.5%, preferably less than 1.5%, when the total concentration of the metals in the active component should not exceed 10% from the total mass of the catalyst.

The gold-reducible oxide complex contains chemical and physical bonding and is bonded to the mixed oxides support.

The support of the catalytic composition comprises individual or mixed oxides with a large surface area typically 80 m²/g to 400 m²/g. Oxides composition is selected from ceria and titania oxides. The concentration of ceria oxide is from 30-95% and of the titania oxide from 5-25%. The catalyst may be in the form of powder, pellets, molded or deposited on a suitable carrier as foams, honeycombs (ceramics or metals).

The gold-transition metal oxide particles are deposited on the mixed oxides support by the methods of the known art: impregnation, precipitation, co- precipitation, wet incipient dryness, or a combination of these techniques. The particles of the active component are finely dispersed throughout the support and should be of a size less than 40 nm, preferably less than 20 nm. The pH of the process of preparation of gold-metal oxide catalyst is of significance and should be in the range of 7.0 to 12.0, preferably 8.0 to 10.5. The adjustment of the pH value in the prescribed range is achieved by the use of alkaline compound, for example, sodium or potassium carbonates, hydroxides or ammonia. After formation of the gold-metal oxide configuration, the catalyst is heated to a temperature in the range of 100°C to 500°C, to form fine cluster particles, immobilized on the surface of the support. The heating of the catalyst is maintained on oxidizing atmosphere or air.

The catalyst working temperature is from 0°C to 500°C.

The catalyst will have application also in the fuel cells technology.

The advantages of the Invention consist of:

1. The catalyst of the invention is more effective than similar catalyst of the prior art in the oxidation of carbon monoxide and hydrocarbons at law temperatures and in presence of moisture and sulphur dioxide;

2. The presence of moisture even enhances the catalyst's oxidation activity;

3. The catalyst tolerates presence of sulphur dioxide;

4. The catalyst has the ability for simultaneous reduction of nitrous oxides and oxidation of carbon monoxide and hydrocarbons at low and high temperature;

5. The catalyst is highly effective in the degradation of ozone at ambient temperature;

6. The catalyst has high catalytic activities in the simultaneous oxidation of carbon monoxide, hydrocarbons and degradation of ozone at ambient temperature and presence of moisture; 7. The catalyst could be used concurrently with the PGM catalysts to deal with their deficiency during the cold start of the combustion engine.

Description of Examples

Example 1

Preparation of the catalyst

1.82 g of HAuCl₄.H₂0 and 24.7 g of Co(N0₃)₂.6H₂0 are dissolved in 500 cm³ of distilled water heated to 60°C. Support mixture consisting of Ce0₂ and Ti0₂ is added to the solution. The temperature is maintained at 60°C and solution of 50 g Na₂C0₃ in 500 cm³ distilled water is added slowly until the pH is elevated to 8.0± 0.1. While stirring, the system in maintained at 60°C for 60 minutes. Thereafter, the composition is left to precipitated and aged for another 60 minutes. The suspension is filtrated and the catalyst is washed with distilled water until complete removal of Cl^" and N0₃ ^" ions. The catalyst is dried for 4 hours at 120°C and calcined thereafter at 450°C for 6 hours. The calcination temperature is reached slowly.

Figure 1 shows the bonding between gold and the reducible oxide of the transition metal, Co₂0₃ in the active cluster, while Figure 2 shows the binding between the cluster and the oxides of the support. Example 2

The effect of the support on the catalyst's activity

The catalyst obtained in Example 1 and a catalyst obtained in the same way, but with A1₂0₃ support, are tested at various temperatures in a reactor containing 1 g of the catalyst, gas flow rate 45 000 h ¹ and gas composition 1% CO, 0.9% 0₂, 350 ppm C₃H₆, 350 ppm C₃H₈, 15 ppm S0₂, humidity 95% and balance N₂. The results in Table 1 show the effect of the support on the activity of the gold catalyst in the oxidation of carbon monoxide and hydrocarbons.

Table 1

Example 3

The positive effect of moisture on the oxidation of carbon monoxide by the gold catalyst

The catalyst obtained by the method described in Example 1 is tested in a reactor containing 1 g of the catalyst at a temperature of 25°C, gas flow rate 360 000 h^"1 and composition of the gas 25 ppm CO, the balance dry air and air with humidity 95%.

The results are shown in Table 2. Table 2

The results show the positive effect of moisture on the activity of the gold catalyst for oxidation of carbon monoxide.

Example 4

Comparison between the activities of the gold catalyst of the invention and a catalyst of the PGM group, during the cold start of the engine (the first 180 seconds)

A. The catalyst obtained by the method described in Example 1 on the support of the invention and PGM catalyst on A1₂0₃ support are tested in a reactor containing 1 g of the catalyst at temperature of 25°C, gas flow rate 60 000 h ¹ and composition of the gas 1% CO, 0.9% 0₂, 350 ppm C₃H₆, 350 ppm C₃H₈, 1000 ppm NOx, 15 ppm S0₂, humidity 95% and the balance N₂.

The results are shown in Table 3 and demonstrate the advantages of the invention in conversion of gases emitted from the internal combustion engines at ambient temperature. Table 3

B. Design for the inclusion of the gold catalyst into the configuration of the existing auto catalytic system which could resolve the cold start deficiency of the commercial PGM catalysts is shown in Fig.3. The heat generated from the exothermal oxidation reaction is emitted to the adjoining PGM and assists in the heating of the platinum catalyst. When the PGM catalyst working temperature is obtained, 300°C, the thermostatic valve switch of the flow of the exhaust gases to the gold catalyst and the purification continues on the PGM catalytic surface. Modified schematic configuration with one tale pipe is given in Fid. 4.

Example 5

Conversion of exhaust gases on the surface of the catalyst of the invention at various temperatures of the combustion cycle

The catalyst obtained by the method described in Example 1 on the support of the invention, is tested on a reactor containing 1 g of the catalyst at various temperatures, gas flow rate 60 000 h ¹ and composition of the gas l%CO, 0.7- 0.9% 0₂, 350 ppm C₃H₆, 350 ppm C₃H₈, 1000 ppm NOx, 15 ppm S0₂, humidity 95% and the balance N₂. The results shown in Table 4 are demonstrating the high activity of the catalyst of the invention, specifically at the cold start of the combustion engine.

Table 4

Example 6

Degradation of ozone on the gold catalyst's surface

The catalyst obtained by the method described in Example 1 is tested in a reactor containing 1 g of the catalyst at temperature 25°C, , gas flow rate 120 000 h^"1 and gas composition 0.01% ozone and the balance air.

Complete 100% degradation of ozone is recorded. Example 7

Simultaneous conversion of ozone and carbon monoxide on the gold catalyst's surface

The catalyst obtained by the method described in Example 1 is tested in a reactor containing 1 g of the catalyst at temperature 25°C, , gas flow rate 120 000 h^"1 and gas composition 0.01% ozone, 0.1% CO and the balance air.

Complete 100% conversion of ozone and carbon monoxide is recorded.

Claims

P A T E N T C L A I M S

1. A gold catalyst for simultaneous oxidation of carbon monoxide and hydrocarbons, reduction of nitrous oxides and degradation of ozone, comprising an active complex and a porous support, where the active complex contains gold-reducible oxide clusters and is applied on a support consisting of oxides of ceria and titanium and the concentration of the metals in the active complex is from 0.1% to 10% of the total mass and gold concentration is between 0.1% and 2.5%.

2. A gold catalyst according to Claim 1, where the reducible oxide can be one or more oxides of CU, Cr, Co, Mn and Fe.

3. A gold catalyst according to Claim 1, where the concentration of gold in the active component is preferably between from 0.1% to 1.5% from the total weight of the catalyst.

4. A gold catalyst according to Claim 1, where the clusters of gold and transition metal oxides have chemical bonding as well physical bonds.

5. A gold catalyst according to Claim 1, where the support of the active composition consists of oxides selected from ceria and titanium or a mixture of these.

6. A gold catalyst according to claims 1 to 5, where the support consists preferably from mixed oxides of ceria and titanium.

7. A gold catalyst according to claims 1 to 6, where the concentration of the ceria oxide could be from 30% to 95% and titania oxide in the range between 5% to 25%.

8. A gold catalyst according to Claim 1, where the surface of the catalyst is from 80 m²/g to 400 m²/g and the catalyst working temperature is from 0┬░C to 400┬░C.