VEHICULAR EMISSION CONTROL : STUDIES ON NON-NOBLE METAL BASED CATALYSTS FOR CO AND PROPYLENE OXIDATION

Abstract

Indian cities are now beset with the problem of vehicular pollution. The automobiles emit significant quantities of carbon monoxide, hydrocarbons, nitrogen oxides and particulate matter. Most of the emitted components are pollutants and are harmful to environment in general and human life and vegetation in particular. In petrol driven vehicles, CO and hydrocarbons (HC) are the main pollutants in the emissions. CO concentration in vehicles exhaust using unleaded petrol varies between 2 to 5% while HC concentration varies between 500 to 3000 ppm. Along with engine modification, fuel modification and tuning, catalytic converters are found to be the most important devices for vehicular pollution control. The active ingredients currently being used in the catalytic converters are noble metals such as Pt, Pd and Rh . Extensive research is being conducted on the oxides of transition metals such as Cu, Co, Ni, Fe, Cr, V, Mn etc, in simple or complexed forms including perovskites, spinel, and promoted versions of these materials in the supported form as possible substitutes to noble metal-based catalysts. Unsupported catalysts include metallic alloys such as Monel metal, and ion-exchanged zeolites. Substitute catalysts must be cheap, should exhibit high initial activity and must retain their activity for a extended period of time under different conditions. Perovskites represented by the general formula, AB03j where A cation can be rare earth (e.g. La, Y) or alkaline earth (e.g. Ba, Ca), and B cation can be a transition metal, have been explored to be used as catalysts in auto exhaust pollution control. Perovskites can be prepared either from solid-solid reaction (ceramic route), liquid-solid reaction like dry evaporation, spray drying and freeze drying (physical route), coprecipitation, crystallization of single compound or mixture or complexation techniques (chemical route). Most of the studies with perovskites have been carried out in their unsupported form. Very little attention has been paid on the use of perovskites in their supported form. This study, therefore, has been undertaken to evaluate the performance of supported perovskites using various supports such as alumina, zirconia and their modified versions for the conversion (oxidation) ofCO and a model hydrocarbon (C3I \()). EXPERIMENTAL SET-UP The experimental set up comprised of the following : source of exhaust gas (synthetic gas mixture as well as petrol driven generator-set exhaust and air cylinder), arrangement for mixing of gases to get simulated exhaust composition, temperature indicator/controller ,tubular furnace for maintaining the desired test temperature, on-line flow meter and a device for fine control of flow rates. This set up was coupled with an on line gas chromatograph to analyze the feed and the reactor effluent gases. CO and O2 were analyzed using molecular sieve- 5 A column and C^Hf,, CO2, H2O were analysed using Porapak-Q column with TCD detector. A pyrex glass tube (2.46 mm i.d. x 315mm) housed in an electric furnace was used as the reactor. The catalyst particles were packed in the mid length portion of the reactor and 100-300 mg of the catalyst particles were packed in the reactor. The reactor temperature was measured by a ceramic sheathed chromel-alumel thermocouple. Gaseous mixtures ( CO+O2+C3H6+ H2O+N2 ) of varying compositions were fed into the reactor. The activity tests were performed at temperatures ranging from 100 to 550°C in 25°C/50°C interval. Screening of various catalysts were conducted at constant space velocity of 18950 h"1. Catalyst volume in all the tests was 0.046 cm3- Structural patterns ofperovskite catalysts were determined by X-ray diffraction. BET surface area, average pore dia., single point pore volume of a few representative samples of support, catalysts and supported catalysts (used and fresh) were determined using Micromeritics Flowsorb Instrument, U.S.A. Preparation of catalysts Majority of the catalysts were prepared by co-precipitation of stoichiometric mixtures of metal nitrate solutions with the addition of aqueous ammonium hydroxide. Substituted perovskite catalysts were prepared by evaporation method using rotary evaporator. Overnight drying was done at about 110°C. Different heating cycles alongwith repeated in- between grindings were used during calcination of simple perovskite and substituted perovskite catalysts to achieve the desired phase formation. The prepared catalysts were mixed with 10% stearic acid and pelletized. Particles in the size range of 250 to 600 urn were used in the reactor experiments. Supported catalysts were prepared by mixing the carrier oxide-alumina or zirconia (or their modified forms with ceria and lanthana) in the presence of 10% stearic acid. The catalysts were also prepared by impregnating carrier oxide with aqueous solutions of mixtures of desired metal nitrates. Perovskite catalysts based on (i) supported lanthanum cobaltate (ii) Ce and Sr substituted lantlianum cobaltate (iii) supported lanthanum and barium ferrites (iv) unsupported and zirconia supported CuO (v) mixture of supported perovskite and CuO were used as catalysts in the experiments. The supports included zirconia, alumina, modified zirconia and modified alumina. CO as well as C3H6 (model hydrocarbon) oxidations were studied. Temperature-conversion studies were conducted to vi determine the characterisitc temperatures. Effect of time-on-stream, catalyst loading, GHSV and reducing atmosphere on oxidation were also studied. Results & Discussion Lanthanum cobaltate catalysts Intermittent calcination time (24h) during the stepped calcination process in the preparation of LaCoOs catalyst facilitates in perovskite phase formation at a lower temperature of 600°C in comparison to 900°C as shown by thcrcmogravimctric analysis. 10% LaCo03/Zr02 catalyst showed the best performance giving 100% CO conversion at a temperature as low as 254°C and for propylene 100 % conversion was achieved at 400°C with 25% LaCo03/Zr02. CO conversion tests were also conducted at different GHSV (7000 h"1 to 19000 h"1) with 10 % LaCo03/Zr02 at 250°C. The results showed an overall increase from 54% to 100% conversion as the space velocity was decreased from 19000 to 10,000 h"1 at 250°C under the test conditions. Slight hysteresis effect was observed when the CO conversion runs were taken first in the increasing order and then in the decreasing order for 10% LaCo03/Zr02. Substituted cobaltate perovskites Partially A-site substituted perovskites synthesized at 600°C-24h , 850°C-10h and 960°C-14h with the substitution of strontium and cerium were all converted to perovskite form. Highest intensity of perovskite phase was achieved in 960°C-14h heating cycle. The catalysts tested for CO and C3H6 conversions were Lao.85Ceo.i5Co03/Zr02 and Lao.8Sro.2Co03 /Zr02. The catalyst La085Ce0.i5CoO3/ZrO2 was found to be superior than Lao.8Sr0.2Co03/Zr02 for CO conversion. With La0.85Ce0.i5CoO3/ZrO2, 100% CO conversion vii was achieved at a temperature as low as 225°C while with La0.8Sro.2Co03 /Zr02, 100% CO conversion was achieved at 250 C. Fe -based catalysts Fe (at B site) catalysts were prepared using La or Ba at A site.Three supports : alumina, Zr02 and brick dust were employed. Loading in each case was kept at 5.8 wt% for all the catalysts. Precursors were simultaneously precipitated with the carrier. Stearic acid was used in the pelletizing- fragmenting-heating process. Cement binding was also used for the catalysts.T50 (50% CO conversion temperature) for alumina supported La or Ba catalysts were found to be higher than 400°C. Among the catalysts only 5.8% BaFe03/Zr02 could achieve a T,oo (100% conversion temperature) of 400°C. Results with brick dust showed only 22.9% CO conversion at 450°C. Propylene conversion performance for La-Fe-O system was found to be marginally better than Ba-Fe-O system. Cement binding was found to be inferior as T)0o value for CO oxidation was higher by 100°C over that for stearic acid bound catalysts. CuO catalysts CuO catalysts were prepared to compare their performance with perovskite catalysts as well as to explore their performance with zirconia support. Calcination temperatureduration of 450°C-5h was used. At 250°C, the CO conversion for CuO and zirconia supported CuO (10 wt%) were 51.22% and 61.66 %, respectively. This indicated better dispersion and higher activity for catalysts with supports. viii Effect of support modification Zirconia and alumina supports were modified by the impregnation of (20%) ceria or lanthana. LaCo03 was loaded on these supports (10wt%). Support modification resulted in the decrease in the BET surface area in all cases with more pronounced effect on zirconia than on alumina. CO removal efficiency at 275°C for a feed stream of diluted Gen-set exhaust containing 2 % CO and 14-16 % 02 was in the order of Zr02 > (Al203+Ce02) > (Zr02+Ce02) > A1203 > (Al203+La203) > (Zr02+La203). The propylene conversion using synthetic feed stream showed marginal positive effect of Ce02 addition resulting in the following order of activities at 450°C : (Zr02+Ce02) > Zr02 > A1203 >(Al203+Ce02) > (Zr02+La203) > (Al203+La203). These results indicate insensitivity to the surface area and performance of the supports in the CO and HC removal and highlight the need to have active support rather than having simple mechanical support. Time-on - stream ( TOS ) studies CO oxidation performance was taken as reference for all the TOS studies. The catalysts used for these studies include 10%LaCoO3 supported on zirconia, alumina or their modified versions and 10% CuO/Zr02. The catalysts were tested for 6 hour continuous runs using diluted Gen-set exhaust. Temperature was kept constant at 300°C. Substantial deterioration in the performance was observed with alumina, modified alumina and lanthana modified zirconia supported catalysts. 10%LaCoO3/ZrO2, 10 %CuO/Zr02 and 10%LaCoO3/zirconia-ceria catalysts exhibited their resilience to activity deterioration. Effect of heat treatment All the three zirconia-based catalysts used in TOS study were subjected to extensive thermal treatment by heating them at 700°C for a period of 36 h in air. The catalysts were jx then again tested for their CO removal efficiency in TOS study at 300°C for 6hduration. The results showed CO removal efficiency remaining unchanged throughout the run. The performance of 10%CuO/ZrO2 under such severe conditions too remained unchanged. Furthermore, when these catalysts were once again tested for their CO conversion performance at varying temperatures, a substantial improvement was observed in their activities at all the temperatures. Comparison at 200°C showed an increase from 18.7% and 19.7% to 55.4% and 57.7% for 10%LaCoO3 and 10% CuO/ Zr02, respectively, while the performance of zirconia-ceria supported catalyst doubled at this temperature. This phenomenon may be explained by the even better perovskite phase formation and higher BET surface area obtained in used 10%LaCoO3/ZrO2 catalyst. Effect of absence of oxygen CO removal without any gas phase oxygen in the feed stream using 10%LaCoO3/ZrO2, 10%CuO/ZrO2 and 10%LaCoO3/zirconia-ceria catalysts were studied in order to find the role played by lattice-oxygen of the catalyst matrix. It was found that CO removal was as high as 56% with zirconia-ceria system at 475°C followed by zirconia supported La-cobaltate (23.5%). CuO/Zr02 catalyst did not perform well under reducing atmosphere. Activity after a duration of 60 minutes at 500°C also showed superior redox property of zirconia-ceria supported La-cobaltate. The performance of Zr02 supported catalysts is found to be better than those of Ab03 supported catalysts. Thismaybe due to many reasons, (a) homogeneous and higher dispersion of perovskite on Zr02 (b) Co atoms tend to diffuse into the bulk of the A1203 support and form spinel, so that most cobalt atoms can not contribute to the catalytic activity, (c) Zr02 acts as ionic oxygen carrier in contrast to Al203 which provides purely mechanical support and the inability of Zr4+ to enter into the perovskite form with anyof the added ions of La and Co, thereby ensuring phase formation of LaCo03 only. Conclusion Presence of perovskite phase in supported catalysts was found varying in its activity depending on the type of support and the calcination temperature used. Best formation of perovskite phase was achieved while using the calcination condition of 600°C- 24 hours and intermittent grinding. The higher catalytic activity of perovskites supported on Zr02 in comparison to Al203 support was due to the higher dispersion of perovskite on Zr02 surface. The catalyst Lao.85Ceo.i5Co03/Zr02 showed the best conversion efficiency of 100% at a temperature as low as 225°C for COoxidation and a conversion efficiency of 99.6% at a temperature of 350°C for C3H6 oxidation. 10% LaCo03/(Zr02+Ce02), 10% LaCo03/Zr02 and 10% CuO/Zr02 did not show any deterioration in activity during TOS studies. However, the above catalysts, when thermally aged at 700°C for 36 h duration, showed an improved performance.