NO REDUCTION WITH H-2 OR CO OVER LA2O3 AND SR-PROMOTED LA2O3

NO reduction with either H-2 or CO was Studied between 773 and 973 K o ver La2O3 and Sr-promoted La2O3. In the absence of O-2 with H-2 as a r eductant, the specific activity for NO disappearance (moles/s/m(2)) ov er La2O3 at 773 K was 10-fold greater than that with CH4, and at 973 K it was 19 times higher; however, the N-2/N2O product ratio was only 0 .27 at 773 K, although it increased to 1.4 at 973 K. With CO as the re ductant and no O-2, specific activities were 4.4 and 6.5 times higher than with CH4 at 773 and 973 K, respectively. In contrast to behavior with CH4, the inclusion of 1% O-2 in the feed with H-2 or CO dramatica lly decreased activities because of direct combustion of the reductant s. Specific activities were enhanced by the addition of 4% Sr to the L a2O3 catalyst, particularly for N2O formation. Adsorption of NO, O-2 C O, and H-2 was determined at 300 and 573 K, and substantial amounts of all but H-2 were adsorbed at the latter temperature. Based on sites c ounted by irreversible NO adsorption at 300 K, turnover frequencies on La2O3 for N-2 and N2O formation with H-2 were 0.074 and 0.059 s(-1) a t 973 K, whereas the respective values with CO were 0.026 and 0.019 s( -1). A model is proposed for each reaction in the absence of O-2 that provides a rate expression that correlates very well with both the wel l-behaved Arrhenius plot for N-2 formation and the significant bend-ov er in N2O formation as temperature increases. (C) 1998 Academic Press.