Effect of cobalt incorporation on the stability of ionic Pd in the presence of carbon monoxide over Pd/BEA passive NOx adsorbers

Bimetallic PdCo/BEA zeolites were evaluated for the first time as potential passive NOx adsorbers (PNAs) in the presence of carbon monoxide (CO). The NOx:Pd ratio of Pd/BEA and PdCo/BEA catalysts decreased during 5 consecutive NOx + CO adsorption/desorption cycles due to loss of ionic Pd. The amount of NOx desorbed at T < 200 °C (weakly adsorbed NOx) was slightly increased over monometallic Pd/BEA, while the amount of NOx desorbed at T > 200 °C (strongly adsorbed NOx) was continuously decreased during 5 consecutive NOx + CO adsorption/desorption cycles. This behavior can be attributed to a decrease in the Pd²⁺ adsorption sites through formation of PdO. Specifically, part of Pd²⁺ can be reduced to Pd⁰ in the presence of CO and Pd⁰ can be further oxidized to PdO during oxidative pretreatments. Unlike Pd/BEA, the amount of weakly adsorbed NOx was decreased and the amount of strongly adsorbed NOx was amplified over bimetallic PdCo/BEA PNAs. Microscopic images, O₂-temperature programmed desorption, and H₂-temperature programmed reduction experiments over Pd/BEA and PdCo/BEA after cycling showed that Co incorporation in Pd/BEA zeolites suppressed PdO formation in the presence of CO. These results demonstrated that Co incorporation in Pd/BEA zeolites can stabilize the Pd²⁺ adsorption sites, thus provide higher amount of NOx desorption at temperatures > 200 °C. In situ diffuse reflectance infrared spectroscopy experiments showed that the amount of Pd²⁺ decreased upon Co incorporation, while the adsorbed NOx species remained unaltered. Density functional theory calculations found that Co experienced a direct bonding interaction with Pd, providing overall stability.