Synthesis of MOFs catalyst containing Cu(I) sites for CO2 hydrogenation: Room-temperature controlled reduction via plasma reducing treatment strategy

Abstract

The design and synthesis of highly dispersible and accessible Cu(I)-containing catalysts as efficient catalysts for the hydrogenation of CO2 to alcohols has attracted considerable interest. Copper-based metal-organic framework (Cu-MOF) is considered one appropriate ideal precursor to prepare a porous Cu(I)-containing catalyst owing to their unique coordination assembly that can significantly prevent the aggregation of copper species during reduction. However, the thermal sensitivity of Cu-MOF makes it challenging to develop an efficient reduction strategy. Herein, we report a novel plasma reducing treatment (PRT) method that can reduce HKUST-1 in a DBD reactor at atmospheric pressure and near room temperatures (40°C). In addition, the PRT method requests short reduction time without the need for post-treatment. In this work, various reducing agents, including hydrogen gas, carbon monoxide, and methanol, have been investigated to manipulate the copper valence. The highly reactive reducing agents that are excited by the DBD plasma discharge at nearly ambient conditions, penetrate into the inner porous structure of Cu-MOFs to achieve efficient conversion of Cu(II) sites to Cu(I) without affecting the overall crystallinity. CO demonstrates the best reduction result. 79.9% Cu(II) was converted to Cu(I) within only 3 h, and the MOF crystal structure is well preserved. The obtained Cu(I)-HKUST catalyst exhibited good selectivity of alcohols in the plasma-assisted CO2 hydrogenation. The work confirms that the PRT method is a superior approach for reducing thermal-sensitive catalysts.