Oxidation of zeolite acid sites in NO/O2 mixtures and the catalytic properties of the new site in NO oxidation

• Acid and sodium-exchanged zeolites (BEA, MFI, CHA) catalyze NO oxidation from ambient temperature up to 623 K.
• Two reaction regimes for NO oxidation identified in all samples.
• NO reaction order changes from 2 (T < 423 K) to 1 (T > 423 K).
• NO+ forms at zeolite acid sites and is a reaction intermediate in the NO oxidation above 423 K.
• Adsorbed NO+ is in equilibrium with gas-phase NO and desorbs (as NO) to yield an oxidized acid site (SiOAl).

The NO oxidation reaction kinetics over a number of zeolite structures were investigated at temperatures above 423 K using plug-flow microreactors and in situ FTIR spectroscopy. Fast catalytic NO oxidation rates (up to 1000×) are observed over H- and Na-exchanged zeolite frameworks (BEA, MFI, and CHA) at temperatures above 423 K with respect to the gas-phase reaction. NO oxidation rates increase with temperature over H- and Na-zeolites, while siliceous zeolites display little activity. Activation energies scale in the order of CHA < MFI < BEA (40.8–55.5 kJ mol−1), and application of transition state theory shows that the formation of the activated complex is an enthalpically controlled process that benefits from smaller zeolite pore size. In all samples, rates are proportional to NO and O2 concentrations, in contrast to low-temperature catalytic rates Loiland et al. [33], which are second order in NO and first order in O2. This difference indicates that a new reaction mechanism is operative above 423 K. In-situ FTIR studies reveal that NO+ coordinated at framework sites in the zeolite pores (SiO−(NO+)Al) plays a direct role in the catalysis, and they also reveal that the NO oxidation rate is proportional to the amount of NO+ present in the sample. Furthermore, it is found that NO+ is in equilibrium with gas-phase NO and that desorption of NO+ (as NO) yields an oxidized acid site (SiOAl). No evidence of NO+ formation is observed over the siliceous zeolite samples. A working model of the reaction mechanism for the high-temperature NO oxidation reaction is proposed for acidic zeolites that is consistent with the observed form of the rate equation and the observed NO+ reaction intermediate.

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