Local structure and NO adsorption/desorption property of Pd2+ cations at different paired Al sites in CHA zeolite

Low-Temperature Methane Combustion Using Ozone over Coβ Catalyst

Catalytic methane (CH4) combustion is a promising approach to reducing the release of unburned methane in exhaust gas. Here, we report Co-exchanged β zeolite (Coβ) as an efficient catalyst for CH4 combustion using O3. A series of ion-exchanged β zeolites (Co, Ni, Mn, Fe, and Pd) are subjected to the catalytic test, and Coβ exhibits a superior performance in a low-temperature region (<100 °C). The results of X-ray absorption spectroscopy (XAS) and catalytic tests for Coβ with different Co loadings indicate the isolated Co species is the plausible active site. The reaction mechanism of CH4 combustion over the isolated Co2+ cation is theoretically investigated by the single-component artificial force-induced reaction (SC-AFIR) method to thoroughly search for possible reaction routes. The resulting path toward CO2 formation shows an activation energy of 73 kJ/mol for the rate-determining step and an exothermicity of 1025 kJ/mol, which supports the experimental results. During a long-term catalytic test for 160 h without external heating, the CH4 conversion gradually decreases from 80 to 40%, but the conversion fully recovers after dehydration at 500 °C (0.5 h). The copresence of H2O and CO exhibits a negative impact on the catalytic activity, while NO and SO2 do not markedly change the catalytic activity.

Na Cocations and Hydrothermal Aging Cooperatively Boost the Regeneration of Phosphorus-Poisoned Pd/SSZ-13 for Passive NOx Adsorption

Pd/SSZ-13 has been proposed as a passive NOx adsorber (PNA) for low-temperature NOx adsorption. However, it remains challenging for Pd/SSZ-13 to work efficiently when suffering from phosphorus poisoning. Herein, we report a simple and efficient strategy to regenerate the phosphorus-poisoned Pd/SSZ-13 based on the cooperation between hydrothermal aging treatment and Na cocations. It was found that hydrothermal aging treatment enabled the redispersion of Pd and P-containing species in phosphorus-poisoned Pd/SSZ-13. Meanwhile, the presence of Na cocations significantly reduced the formation of AlPO4 and retained more paired Al sites for highly dispersed Pd2+ ions, which was of great importance for the recovery of adsorption performance. To our satisfaction, the restoration ratio of the adsorption capacity of poisoned Pd/SSZ-13 was >90% after regeneration. Strikingly, the NOx adsorption activities of phosphorus-poisoned Pd/SSZ-13 with phosphorus loadings of 0.2 and 0.4 mmol g-1 almost completely recovered upon regeneration. This study demonstrates the promoting effect of Na cocations on the regeneration of phosphorus-poisoned Pd/SSZ-13 by hydrothermal aging treatment, which provides useful guidance for the design of PNA materials with excellent durability for cold-start application.

An automated reaction route mapping for the reaction of NO and active species on Ag4 clusters in zeolites

A computational investigation of the catalytic reaction on multinuclear sites is very challenging. Here, using an automated reaction route mapping method, the single-component artificial force induced reaction (SC-AFIR) algorithm, the catalytic reaction of NO and OH/OOH species over the Ag₄²⁺ cluster in a zeolite is investigated. The results of the reaction route mapping for H₂ + O₂ reveal that OH and OOH species are formed over the Ag₄²⁺ cluster via an activation barrier lower than that of OH formation from H₂O dissociation. Then, reaction route mapping is performed to examine the reactivity of the OH and OOH species with NO molecules over the Ag₄²⁺ cluster, resulting in the facile reaction path of HONO formation. With the aid of the automated reaction route mapping, the promotion effect of H₂ addition on the SCR reaction was computationally proposed (boosting the formation of OH and OOH species). In addition, the present study emphasizes that automated reaction route mapping is a powerful tool to elucidate the complicated reaction pathway on multi-nuclear clusters.