Excellent activity and selectivity of Pd/ZSM-5 catalyst in the selective catalytic reduction of NO_x by H₂

Superior de-NO_x activity and N₂ selectivity of the Pd/ZSM-5 catalyst was observed at low temperature (<200 °C) for the selective catalytic reduction of NO_x by H₂ (H₂-SCR). Various Pd/ZSM-5 catalysts were prepared by calcinating at different temperatures (e.g., 500 °C, 650 °C, 750 °C, and 850 °C) and treated at reductive conditions before the H₂-SCR reaction was performed. Among the prepared catalysts, the one prepared at the calcination temperature at 750 °C resulted in 96.7% NO_x conversion and 96.8% N₂ selectivity at 150 °C. Based on the H₂-O₂ reaction, the higher activity of the Pd/ZSM-5 catalyst calcined at 750 °C was attributed to its superior H₂ activation ability for the H₂-SCR reaction. The combined X-ray diffraction (XRD), temperature-programmed hydride decomposition (TPHD), and transmission electron microscopy (TEM) results revealed that highly dispersed Pd particles were generated on the catalyst calcined at 750 °C, while large Pd agglomerates were formed on the one calcined at 500 °C. It can be concluded that the catalytic activity of Pd/ZSM-5 improves by optimizing the calcination temperature, resulting in high Pd dispersion. Moreover, the Pd catalyst calcined at 750 °C showed high resistance to CO, maintaining >94% NO_x conversion at 175 °C under 1000 ppm CO in the feed gas. Therefore, the catalyst calcined at 750 °C can be potentially used for industrial applications because of its simple preparation method and high resistance to CO.