NO REDUCTION BY CH4 IN THE PRESENCE OF O-2 OVER LA2O3 SUPPORTED ON AL2O3

Dispersing La2O3 on delta- or gamma-Al2O3 significantly enhances the r ate of NO reduction by CH4 in 1% O-2, compared to unsupported La2O3. T ypically, no bend-over in activity occurs between 500 degrees and 700 degrees C, and the rate at 700 degrees C is 60% higher than that with a Co/ZSM-5 catalyst. The final activity was dependent upon the La2O3 p recursor used, the pretreatment, and the La2O3 loading. The most activ e family of catalysts consisted of La2O3 on gamma-Al2O3 prepared with lanthanum acetate and calcined at 750 degrees C for 10 h. A maximum in rate (mol/s/g) and specific activity (mol/s/m(2)) occurred between th e addition of one and two theoretical monolayers of La2O3 on the gamma -Al2O3 surface. The best catalyst, 40% La2O3/gamma-Al2O3, had a turnov er frequency at 700 degrees C of 0.05 s(-1), based on NO chemisorption at 25 degrees C, which was 15 times higher than that for Co/ZSM-5. Th ese La2O3/Al2O3 catalysts exhibited stable activity under high convers ion conditions as well as high CH4 selectivity (CH4 + NO vs. CH4 + O-2 ) The addition of Sr to a 20% La2O3/gamma-Al2O3 Sample increased activ ity, and a maximum rate enhancement of 45% was obtained at a SrO loadi ng of 5%. In contrast, addition of SO4= to the latter Sr-promoted La2O 3/Al2O3 catalyst decreased activity although sulfate increased the act ivity of Sr-promoted La2O3. Dispersing La2O3 on SiO2 produced catalyst s with extremely low specific activities, and rates were even lower th an with pure La2O3. This is presumably due to water sensitivity and si licate formation. The La2O3/Al2O3 catalysts are anticipated to show su fficient hydrothermal stability to allow their use in certain high-tem perature applications. (C) 1997 Elsevier Science B.V.