Kinetic, thermodynamic, and mechanistic studies of DeNOx catalysis over barium sodium-yttrium: The roles of nitrates and nitrites

BaNa-Y is a catalyst for reduction of nitrogen oxides (NO and NO2) with added reductants at temperatures of ∼200°C. Studies were performed over BaNa-Y and related catalysts to unravel the roles that nitrates and nitrites play in determining the processes and pathways involved in NOx reduction.;Ammonium nitrate and chemisorbed surface nitrates are potential catalyst deactivators for BaNa-Y. It is shown that NO reduces NH4NO 3 above its melting point, 170°C, while acidic solids catalyze this reaction even at temperatures below 100°C. A mechanism for the acid catalyzed reduction of NH4NO3 by NO is proposed, with HNO 3 as an intermediate. The use of acidic promoters in DeNO x systems could thus help mitigate catalyst deactivation at <150°C.;Nitric acid dissociates on BaNa-Y to form H+ and NO 3-. Temperature-programmed desorption (TPD) of these species was studied in the presence and absence of CO. The activation energy for desorption of the nitrates is found to be 160 to 190 kJ/mol. The kinetics and stoichiometry suggest that OH and NO2 are formed in the pores, and that when CO is present the OH produced reacts with CO to form CO 2 + H. The H atoms thus formed react with OH in preference to NO 2, consistent with zeolite pore walls causing a change in mechanism by acting as a third body. No reaction was observed between CO and NO 3 or NO3-.;TPD was performed on NO3- and NO2 - on Na-Y. The activation energies for desorption obtained span ∼100 to 200 kJ mol-1 for both species. The trends for the relative stabilities of nitrates and nitrites on Na-Y and BaNa-Y are presented, and the roles of these stabilities in low temperature DeNOx catalysis are discussed.;The kinetics of the reaction of NO + H+ + NO3 - was studied on BaNa-Y and Na-Y. The results display evidence of a lower dimensional diffusion limited A + B → 0 reaction, where H + and NO3- are A and B. To our knowledge, this is the first evidence for this type of kinetics on a surface.;Concurrently performed work on entropies of adsorption, geared towards weakly bound molecules, is also presented.