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

Abstract

The mechanism of temperature-programmed desorption (TPD) of nitric acid chemisorbed on BaNa–Y was studied over the temperature range from 200 to 400 °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 O₂, H₂O, and/or H₂. No reaction between CO and NO₃ or CO and surface-bound NO₃⁻ was observed.