Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Gold based environmental catalyst L. A. Petrov Institute of Catalysis, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria The performance of the highly active and stable 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, is described. The active complex of the catalyst consisting of gold and reducible oxide from the group of the transition metals is supported on carrier consisting of oxides of ceria and titanium. 1. INTRODUCTION Environmental protection is one of the major issues facing the world today. Internal combustion engines contribute significantly to increasing pollution levels in the atmosphere and this will become worse due to the strong tendency towards global urbanization. The working environment for a large number of people in today's industrial sites and administrative buildings has an air pollution of unacceptably high level of CO, ozone, and other noxious gases. One of the major ways used to achieve improving of the environmental conditions is the application of catalytic methods for reduction of pollutant concentration. A reduction of the emission of noxious gages from combustion engines may be achieved by the use of platinum group metals (PGM)-based three-way catalyst (TWC). However, these catalysts function satisfactory at temperatures higher than 300~ and presence of moisture and sulfur dioxide common for the exhaust gases, severely affect their performance at lower temperatures. It is a 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 catalyst are not effective. The PGM - based catalysts are even less effective for diesel vehicles where the temperature of the emitted gases is lower than the temperature of the exhaust gases from the gasoline passenger car. Further, the high working temperatures of PGM catalysts makes them unsuitable for air purification in buildings, aircraft, and industrial sites. Gold has long been regarded with far lower catalytic activity than the PGM. However, recent publications in the literature [1-4] have shown that gold, when highly dispersed on reducible oxides, can be active for low temperature oxidation of CO. Gold containing catalysts known in the literature show poor conversion at the high flow rate of reacting gases.
2346 2. E X P E R I M E N T A L
2.1. Catalyst composition The proposed catalyst consist of a porous mixed oxides support having captured there an active complex comprising gold and a reducible oxide of a transition metal selected from chromium, copper, cobalt, manganese, iron and promoters. The concentration of the gold is from 0.2 to 2.5 %, preferably less than 1.5 %, when the total concentration of the metals in the active component does not exceed 10 % from the total mass of the catalyst.
2.2. Catalyst's support Catalyst support comprises a mixture from oxides of cerium and titanium in specially selected ratio.
2.3. Catalyst preparation The catalyst was prepared, according to the method proposed in , by precipitation of gold and cobalt from water solutions of HAuC14.H20 and Co(NO3)2 6H20, heated to 60oC on powdered support consisting of CeO2 and TiO2. The temperature is maintained at 60oC and water solution of Na2CO3 is added slowly until the pH is elevated to 8.0 + 0.1. While stirring, the system is maintained at temperature 60oC for 60 min. Thereafter, the composition is left to precipitate and aged for another 60 min. The suspension is filtrated and the catalyst is washed with distilled water until complete removal of CI and NO3ions. The catalyst is dried for 4 hours at 120~ and calcined thereafter at 450oC for 6 hours. The calcination temperature is reached slowly. Before use, the catalyst was activated in the oxidizing atmosphere.
2.4. Catalytic activity test The catalytic activity of used catalysts is measured in a quartz laboratory flow type reactor containing 1 g catalyst mixed with 2 g quartz sand (mesh size 12 25). The reaction temperature was changed in the interval 25 - 600~ and measured by thermocouple located in the catalyst bed. The Gas Hourly Space Velocity (GHSV) of the reacting gas was varied between 12 000 to 120 000 h -1. The gas flow was controlled by the Matheson mass flow controllers. Gases used for the tests are analytically certified.
2.5. Analytical The inlet and outlet reactor concentrations' of the HC, oxygen, carbon monoxide and dioxide, sulfur dioxide and nitric oxide in the reaction mixture was performed by Rosemount gas analyzers connected on-line with the reactor's exit.
3. R E S U L T S The active component of the catalyst is a gold-reducible oxide complex, which is bonded chemically and physically to the mixed oxides support. The particles of the active component are finely dispersed throughout the support and are of a size between 20 - 40 nm. After formation of the gold metal configuration, the catalyst is heated to a temperature in the range of 100~ to 500oC, to form fine cluster particles immobilized on the surface of the support. The heating of the catalyst is maintained on oxidizing atmosphere or air.
3.1. I n f l u e n c e of the s u p p o r t c o m p o s i t i o n on the o x i d a t i o n activity of the catalysts The experimental results show that the support composition plays an important role for the catalyst activity. In Table 1 are compared the catalytic activity of catalysts supported on alumina and on mixed oxide carrier. The catalytic activity is measured at steady state regime, GHSV = 45 000 h -1, gas composition: 1% CO, 0.9% 02, 350 ppm C3H6, 15 ppm SO2, 95% humidity, balance N2 at temperatures from 25 to 400 oC. Table 1 Influence of the support composition on the oxidation activity of the catalysts
Temperature, ~ 25 100 200 300 400
Degree of conversion, % CO Hydrocarbons A1203 Mixed oxides A1203 Mixed oxides 7.0 86.0 0 9.0 35.0 96.0 0 60.0 46.0 100.0 13.0 65.0 100.0 100.0 61.0 79.0 100.0 100.0 91.0 98.0
3.2. M o i s t u r e effect on the catalyst activity for CO o x i d a t i o n The presence of water vapor in the gas composition enhances the activity in reaction of CO oxidation. The test simulates conditions of air purification in buildings or industrial sites: temperature 25~ gas composition 25 ppm CO, balance air and GHSV = 360 000 h 1. Results are presented on Table 2. Table 2 Moisture effect on the catalyst activity for CO oxidation Temperature, ~ 25
CO oxidation degree of conversion (%) Dry air Air with 95% Humidity 96.O 100.0
2348 3.3. I n f l u e n c e of the catalyst composition on catalytic activity at the cold start of an engine The presence of gold in the catalyst composition has beneficial effect on catalyst activity at cold start conditions. The catalytic activity test was performed at space velocity GHSV = 60 000 h -1, gas composition that simulate the exhaust emission from the internal combustion engine: 1% CO, 0.9% 02, 700 ppm HC (Calls : C3H6 = 1:1), 1000 ppm NOx, 15 ppm SO2, 95% humidity, balance N2 and temperature 25oC. It is obvious that the gold catalyst has much better performance than PGM catalyst. The results are presented on Tab. 3. Table 3 Catalytic activity of PGM and Gold catalysts at the cold start of an engine Catalyst PGM catalyst Gold catalyst
Degree of conversion at 25oC, % CO HC NOx 3.0 5.0 0 83.0 60.0 87.0
3.4. Catalytic performance in purification of exhaust g a s e s f r o m the internal c o m b u s t i o n engine. The gold containing catalysts has very good performance as a three-way catalyst. On Table 4 are presented results from catalytic activity tests at temperatures between 25 and 400oC, GHSV = 60 000 h -~, gas composition: 1% CO, 0.9% 02, 700 pprn HC (C3H8 : C3H6 = 1:1), 1000 ppm NOx, 15 ppm SO2, 95% humidity, balance N2. Table 4 Purification of exhaust gases from the internal combustion engine. Temperature, ~ 25 100 200 300 400
Degree of conversion, % CO HC NOx 89.0 9.0 0 99.0 58.0 78.0 100.0 68.0 91.0 100.0 79.0 95.0 100.0 98.0 98.0
3.5. Methane total oxidation The activity of gold catalyst in methane total oxidation was studied using two reaction mixtures methane-air containing 0.25% (Mixture A) and 2.5% (Mixture B) methane, at GHSV = 12 000 h 1 in the temperature interval 25 to 600oC. The results are presented in Table 5.
2349 Table 5 Methane total oxidation Temperature, oC
400 450 500 550 6OO
Reaction mixture composition, % Mixture A Mixture B CH4 degree of conversion, % 28.9 5.1 58.3 10.7 89.0 32.2 92.8 83.4 100 100
3.6. M e t h a n o l t o t a l o x i d a t i o n The activity of gold catalyst in methanol total oxidation was studied using reaction mixture methanol-air containing 10.5 % methanol at GHSV = 60 000 h -1 in the temperature interval 25 to 100~ The results are presented in Table 6. The catalyst can be effectively used in direct methanol fuel cells. Table 6 Methanol total oxidation Temperature, ~ 30 50 100
Degree of conversion, % Au catalyst Pt catalyst 99.0 50,0 100 80.0 100 99.0
3.6. D e g r a d a t i o n o f o z o n e . The catalyst obtained by the method described above is tested at temperature 25~ GHSV = 120 000 h ~ and gas composition: 0 . 0 1 % ozone and balance air. Complete 100 % degradation of ozone is recorded. 3.7. S i m u l t a n e o u s c o n v e r s i o n o f o z o n e a n d c a r b o n m o n o x i d e The catalytic activity of the catalyst is tested at temperature 25oC GHSV = 120 000 h -1 and gas composition: 0 . 0 1 % ozone, 0 , 1 % CO and balance air. Complete 100 % conversion of ozone and CO is recorded. 3.8. S e l e c t i v e o x i d a t i o n o f CO in t h e p r e s e n c e o f H2 The catalytic activity of the catalysts was measured in catalyst at different temperatures, space velocity 60 000 h -1 and gas composition which simulate the hydrogen fuel for hydrogen fuel cells technology: CO =0.5 %vol., H2 = 20.0 % vol., 02 =0.8 % vol. and balance N2. The results are shown in Table 7. The presence of water vapor suppresses the H2 oxidation.
Table 7 Selective oxidation of CO in the presence of H2 Temperature, oC 5O 8O 100 (dry gas) 100 (in presence of moisture)
CO conversion, % 5O.0 100 100 100
H2 conversion, % 1.0 3.0 5.0 3.0
4. D I S C U S S I O N
The gold catalyst has been found more effective than the PMG catalysts in the oxidation of carbon monoxide and hydrocarbons at low temperatures and in the presence of moisture and sulfur dioxide. The moisture enhances the catalyst's oxidation activity. The catalyst also has ability for simultaneous reduction of nitrous oxides at low and high temperature ranges. The catalyst can have application in systems for purification of exhaust gases from the combustion engine, and specifically for the colder gases emitted from diesel vehicles. 5. CONCLUSION The prepared gold containing catalysts could have application for: Cleaning of the exhaust gases from gasoline and diesel engines; Full oxidation of organic compounds and CO in the gas emissions from industry and thermal electric power stations; NOx reduction in the gas emissions from industry and thermal electric power stations; Ozone decomposition; Selective oxidation of CO in the presence of H2 which is of great importance for the purification process of hydrogen fuel for the hydrogen fuel cells technology; The catalyst can be effectively used in direct methanol fuel cells; The catalyst is effective in the degradation of ozone at ambient temperatures and in purification of air in offices, industrial sites, airplanes, spacecrafts and submarine cabins. References
1. M. Haruta, Catalysis Surveys of Japan, 1 (1997) 61 2. D.T. Thomson, Gold Bull., 31 (1998) 111 3. D.T. Thomson, Gold Bull., 32 (1999) 12 4. A. Ueda and M. Haruta, Gold Bull., 32 (1999) 3 5. L.A. Petrov, BulgarianPatent Reg.No 101490 (1997)