Modulation of active metal species in MOF-derived catalysts for efficient NO reduction by CO

Abstract

Transition metal oxides are frequently used as catalysts for NO selective catalytic reduction by CO (CO-SCR), but their catalytic activity is limited by the content of active metal species, the lower surface area, and the underdeveloped pore structure. Herein, we prepared three MOF-derived materials (Cu/C, Cu-CuO_x/C, and CuO/C), in which active copper species (Cu⁰, Cu⁺, and Cu²⁺), specific surface areas, and porosity were modulated through different heat treatment conditions. Moreover, these composite materials can be used as efficient catalysts for CO-SCR. In particular, it is demonstrated that the Cu/C catalysts containing only Cu⁰ species could exhibit 100% NO conversion from 350 °C to 500 °C, nearly 100% N₂ selectivity, and devoid of any by-product N₂O generation. Cu/C is one of the best MOF-derived catalysts reported to date. Thermal catalytic experiments revealed that the occupancies of reductive copper species (Cu⁰) showed a positive correlation with the materials' catalytic activity. In situ FTIR and DFT analysis elucidated that Cu/C catalysts with Cu⁰ as the active site have lower activation energy and higher energy gain. The high catalytic activity of Cu/C is attributed to the fact that its Cu⁰ species has low activation energy and a low reaction energy barrier in the ONNO dissociation step, which is always the decisive step in CO-SCR, and thus this makes the Cu/C catalysts more susceptible to CO-SCR reactions in the range of the conversion temperature. This work provides a new strategy to modulate the ratio of active metal species, specific surface area, and pore structure of CO-SCR catalysts, which can exhibit high catalytic activity.