NO adsorption/desorption pathways over Pd/SSZ-13 low temperature NOx adsorbers investigated by operando FT-IR spectroscopy and microreactor study

In this study a Pd-doped SSZ-13 zeolite sample is synthesized, characterized and evaluated for potential use in the low-temperature NOx adsorption. TEM/HR-TEM and in-situ CO/NO adsorptions followed by FT-IR spectroscopy are used to gain information on the nature and accessibility of the Pd sites. It is found that over the calcined sample Pd is present as isolated Pd+ and Pd2+ cations formed by ion exchange with the Brønsted acid sites of the zeolite, and as Pd+ and Pd2+ species belonging to PdOx particles on the external surface of the zeolite. Isolated Pd2+ ions prevail on the calcined fresh sample. The NOx adsorption/desorption performances, investigated by microreactor studies and operando FT-IR under relevant operating conditions, indicate that significant amounts of NO are stored on the catalyst (i.e. ca. 80–95 μmol/gcat of NOx in the temperature range 50–150 °C) which suggests the participation in the NOx uptake of both isolated Pdn+ sites and PdOx particles. NO is adsorbed as nitrosyls (anhydrous and hydrated) over both Pd2+ and Pd+; at low temperatures the formation of nitrates is also observed. The NO uptake as nitrosyls occurs with evolution of NO2 according with a NO storage mechanism which implies (i) the initial adsorption of NO on Pd2+, followed by (ii) the reduction of Pd2+ to Pd+ by NO accompanied by NO2 release and eventually (iii) the adsorption of NO on Pd+. The suggested interconversion of Pd2+ into Pd+ during NO adsorption is herein discussed based on FT-IR evidences. The stored NOx species are decomposed upon heating; nitrates are responsible for low temperature NOx desorption (below 200 °C), whereas nitrosyls show much higher stability and lead to a high temperature NO desorption (above 200 °C). The presence of NO2 in the feed stream increases the NOx adsorption capacity and results in the formation of mainly surface/bulk nitrates and nitro-compounds at the expense of nitrosyls.