The effect of proximity between Pt and BaO on uptake, release, and reduction of NO x on storage catalysts

Abstract

In this study, we compared the NO x storage reduction characteristics of four systems: BaO/Al 2O 3 alone, a sequential system with Pt/SiO 2 ahead of the BaO/Al 2O 3, a combined system with the Pt/SiO 2 and BaO/Al 2O 3 physically mixed, and BaO/Al 2O 3 with Pt deposited on it. We also investigated the isotopic exchange between 15NO and stored NO x under storage conditions for the latter two systems. The rate of exchange was more than five times as fast for Pt/BaO/Al 2O 3 than for the combined Pt/SiO 2 + BaO/Al 2O 3 system, demonstrating spillover of NO x species between Pt and BaO in close proximity. Movement of NO x in this way can also explain why storage from NO/O 2 reached completion much faster for Pt/BaO/Al 2O 3 than for the sequential or combined systems, where the influence of Pt was confined to the oxidation of NO to NO 2 with storage by disproportionation to nitrate and NO alone. The initial product of the decomposition of stored NO x in He was NO 2 with the sequential system and NO with the combined system, where Pt in the same bed decomposed NO 2. However, NO 2 in excess of the NO 2/NO equilibrium ratio was seen during release of stored NO x from Pt/BaO/Al 2O 3, reflecting transfer back to the metal, which was deactivated for NO 2 decomposition by high oxygen coverage. A large fraction of the NO x stored on Pt/BaO/Al 2O 3 was immediately converted to N 2 alone when exposed to H 2, with formation of NH 3 evident only after H 2 breakthrough. This is consistent with the reduction of stored NO x species as they transfer back to Pt particles. Reduction in the combined system requires considerably higher temperature and forms NH 3 as the major product. This is consistent with the decomposition of stored NO x to gaseous NO x and subsequent reduction by a catalytic reaction on Pt particles located remotely from BaO. In the sequential system, reduction was largely confined to the conversion of NO 2 to NO. Measurement of the O:N ratio in the products formed by temperature-programmed decomposition showed that NO was stored on BaO/Al 2O 3 in the presence of O 2 as a species with the formula NO 2. Similar measurements with NO 2, which is stored in much larger quantities, were consistent with conversion to nitrate, in agreement with existing knowledge.