From Pd to Co: Performance Optimization and Mechanism Study of the Co-SSZ-13 Catalyst for Pulsed Nitrogen Oxide Adsorption Applications

In this article, the performance of Co-SSZ-13 catalysts as pulsed nitrogen oxide adsorption (PNA) catalysts was investigated, focusing on the effects of cobalt (Co) loading, silica/aluminum ratio (Si/Al), and hydrothermal aging temperature on their adsorption and desorption characteristics. It was found that an increase in Co loading, although capable of improving NO_x adsorption, would lead to a tighter binding of nitrate species to the molecular sieves, thus increasing the desorption temperature and not favoring the adsorption–desorption cycle of PNA. The effect of the silicon/aluminum ratio on the catalyst performance is more complicated: at a low silicon/aluminum ratio, Co ions are prone to form a Z₂Co structure, which reduces the NO_x adsorption capacity; at a high silicon/aluminum ratio, Co mainly exists in the form of oxides, which results in a weak adsorption capacity; and the catalysts with moderate silicon/aluminum ratios show strong oxidation capacity. In addition, the hydrothermal aging temperature had a significant effect on the performance of the Co-SSZ-13 catalysts; 750 °C aging of the catalysts still maintained good adsorption performance, but aging temperatures of 800 °C and above led to a significant decrease in the adsorption capacity. In contrast, the Pd-SSZ-13 catalyst exhibits better PNA properties after hydrothermal aging at 800 °C, but its performance decreases significantly at higher temperatures (e.g., 900 °C). This suggests that there are differences in hydrothermal stability between Co- and Pd-based catalysts, with Co-SSZ-13 showing better performance at lower aging temperatures, while Pd-SSZ-13 retains some activity at higher temperatures. The effects of Co loading and silicon/aluminum ratio on the pore structure and adsorbed species of the catalysts were further revealed by specific surface area analysis [Brunauer–Emmett–Teller (BET)] and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) analysis. It was demonstrated that the Co-SSZ-13 catalyst had good PNA performance at appropriate Co loading and silicon/aluminum ratio and was expected to be used as a non-precious metal catalyst to replace Pd-based catalysts for NO_x removal during cold start of internal combustion engines.