NOx adsorption on Pt/K/Al2O3

This study explores NOx storage in a Pt/K/Al2O3 lean NOx trap (LNT) catalyst using in situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) in conjunction with chemisorption. The combination of these techniques allows the quantification of surface species for this catalyst system. A free nitrate ion, NO3−, is the primary form of stored NOx on the potassium phase at all temperatures, but there was significant nitrite formation observed below 200 °C during short adsorption times. The NO2 saturation storage on Pt/K/Al2O3 decreases from 6.7 μmol/m2 at 150 °C to 1.8 μmol/m2 at 400 °C, and DRIFTS indicates that the equilibrated surface species are identical over this entire temperature range. While the final state of Pt/K/Al2O3 is identical at all temperatures, the rates and observed surface species change significantly. After 1 min uptake, NOx adsorption varies from 0.35 μmol/m2 at 150 °C to a maximum of 3.2 μmol/m2 at 250 °C, and at 400 °C the adsorption decreases to 1.1 μmol/m2. This temperature sweep illustrates the limiting regimes associated with NOx adsorption: kinetic limitations at lower temperatures due to the strength of the Pt–O bond and insufficient storage sites at higher temperatures. This study propose three routes for the storage of NOx on Pt/K/Al2O3, two of them involving a potassium storage site adjacent to Pt sites, and a third route that involves a form of NO2 disproportionation on storage sites irrespective of their proximity to Pt.