NO reduction with H2 or CO over La2O3 and Sr-promoted La2O3

NO reduction with either H-2 or CO was Studied between 773 and 973 K over La2O3 and Sr-promoted La2O3. In the absence of O-2 with H-2 as a reductant, the specific activity for NO disappearance (moles/s/m(2)) over 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 reductant 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 dramatically decreased activities because of direct combustion of the reductants. Specific activities were enhanced by the addition of 4% Sr to the La2O3 catalyst, particularly for N2O formation. Adsorption of NO, O-2 CO, 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 counted 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) at 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 well-behaved Arrhenius plot for N-2 formation and the significant bend-over in N2O formation as temperature increases. (C) 1998 Academic Press.