TPD of nitric acid from BaNa-Y: evidence that a nanoscale environment can alter a reaction mechanism

The mechanism of temperature-programmed desorption (TPD) of nitric acid chemisorbed on BaNa-Y was studied over the temperature range from 200 to 400 degrees C, in the presence and absence of CO. Nitric acid dissociates to form H+ and NO3- when chemisorbed on BaNa-Y. The results of these experiments are consistent with H+ and NO3- either reacting directly to produce OH and NO2 or recombining to produce HNO3, which is desorbed and rapidly decomposes within the zeolite pores to OH and NO2. The kinetics and stoichiometry suggest that the hydroxyl radicals produced react with CO and NO2 to form CO2 + H and NO + HO2, respectively. The H atoms thus formed react with OH in preference to NO2, a change in mechanism consistent with literature rate constants and the expectation that the zeolite pore walls act as a third body for the reaction of H with OH. Finally, OH may react with NO2 to form HO2, which can undergo further reactions to form O-2, H2O, and/or H-2. No reaction between CO and NO3 or CO and surface-bound NO3- was observed.