DRIFTS studies on CO and NO adsorption and NO + CO reaction over Pd2+-substituted CeO2 and Ce0.75Sn0.25O2 catalysts

Abstract

Sequential adsorption of CO and NO as well as equimolar NO+CO reaction with variation of temperature over Pd2+ ion–substituted CeO2 and Ce0.75Sn0.25O2 supports has been studied by DRIFTS technique. The results are compared with 2at.% Pd/Al2O3 containing Pd0. Both linear and bridging Pd0–CO bands are observed over 2at.% Pd/Al2O3. But, band positions are shifted to higher frequencies in Ce0.98Pd0.02O2−δ and Ce0.73Sn0.25Pd0.02O2−δ catalysts that could be associated with Pdδ+–CO species. In contrast, a Pd2+–CO band at 2160cm−1 is observed upon CO adsorption over Ce0.98Pd0.02O2−δ and Ce0.73Sn0.25Pd0.02O2−δ catalysts pre-adsorbed with NO and a Pd+–CO band at 2120cm−1 is slowly developed on Ce0.73Sn0.25Pd0.02O2−δ over time. An intense linear Pd0–NO band at 1750cm−1 found upon NO exposure to CO pre-adsorbed 2at.% Pd/Al2O3 indicates molecular adsorption of NO. On the other hand, a weak Pd2+–NO band at 1850cm−1 is noticed after NO exposure to Ce0.98Pd0.02O2−δ catalyst pre-adsorbed with CO indicating dissociative adsorption of NO which is crucial for NO reduction. Pd0–NO band is initially formed over CO pre-adsorbed Ce0.73Sn0.25Pd0.02O2−δ which is red-shifted over time along with formation of Pd2+–NO band. Several intense bands related to nitrates and nitrites are observed after exposure of NO to fresh as well as CO pre-adsorbed Ce0.98Pd0.02O2−δ and Ce0.73Sn0.25Pd0.02O2−δ catalysts. Ramping the temperature in a DRIFTS cell upon NO and CO adsorption shows the formation of N2O and NCO surface species, and N2O-formation temperature is comparable with the reaction done in a reactor.