Reaction of NO and O 2 to NO 2 on Pt: Kinetics and catalyst deactivation

Abstract

The kinetics of NO oxidation was determined on two Pt catalysts with average particle sizes of 2.4 nm (fresh) and 7.0 nm (sintered). The degree of surface oxidation after reaction, as measured by CO titration and XPS, determined the surface reactivity. The turnover rate (TOR) for NO oxidation at 300 °C with 300 ppm of NO, 170 ppm of NO 2, 10% of O 2, and the balance N 2 and at atmospheric pressure on a Pt/Al 2O 3 catalyst with a Pt particle size of 2.4 nm was 3.5 × 10 −3 mol NO / ( mol Pt s ) . On the sintered catalyst, the TOR was 14.7 × 10 −3 s −1 , a four-fold increase with respect to the fresh one. Measuring the rate of reaction on the fresh catalyst under a set of experimental conditions obtained using the central composite design statistical method, in which the interaction among the variables temperature and species concentration can be tested, confirmed the assumption in our previously published results that there is no interaction among the variables. The reaction was nearly first order with respect to NO and O 2 and nearly negative first order with respect to NO 2, and the apparent activation energy ( E a ) was 81.8 ± 5 kJ mol −1 . With respect to the fresh catalyst, the sintered catalyst showed a similar E a ( 80.9 ± 5 kJ mol −1 ) and apparent reaction orders for NO and NO 2, with a lower O 2 order ( 0.7 ± 0.04 ). After the NO oxidation reaction attained steady state, both fresh and sintered catalysts showed an average oxygen uptake of about 1.5 times the number of Pt surface atoms. When the oxygen uptake was increased to the equivalent of two oxygen atoms per surface Pt by a different pretreatment, the NO oxidation TOR decreased by 85% with respect to the original steady-state level. XPS measurements suggested that over-oxidation of Pt was concomitant with this TOR decrease. Titration studies of surface oxygen using CO suggested that the rate of CO 2 formation was also higher on larger Pt particles.