Immobilizing Pd nanoparticles on an energetic metal–organic farmwork MET-6-derived porous nitrogen-doped carbon nanocages for enhanced formic acid dehydrogenation

Abstract

It is essential yet challenging to assemble the heterogeneous catalysts with satisfactory activity and recyclability for formic acid (HCOOH, FA) dehydrogenation to employ FA as a valid hydrogen (H₂) carrier. In this research, through pyrolysis of an energetic metal–organic framework (MOF) MET-6 at different calcination temperature, various porous nitrogen-doped carbon nanocages (PNCNCs-x, x represents different temperatures of calcination with the unit of °C, and the value could be 700, 800, 900 or 1000 in this study) supports were obtained accordingly. Followed by a simple wet chemical reduction method, Pd nanoparticles (NPs) anchored on PNCNCs-x supports were prepared successfully, and the resultant catalysts Pd@PNCNCs-x could be applied in the H₂ generation by FA dehydrogenation. The optimal catalyst Pd@PNCNCs-900 demonstrated exceptional catalytic activity over FA dehydrogenation, the catalyst yields an initial turnover frequency (TOF) value TOF of 3253 h⁻¹ in the FA/SF system (1:2) at 60 °C. The catalyst achieves complete FA conversion and 100 % H₂ selectivity, outperforming other reference catalysts Pd@PNCNCs-x (x = 700, 800, and 1000) with different temperatures of the pyrolysis of MET and most MOF-derived carbon supported monometallic Pd catalysts. Moreover, optimized catalyst Pd@PNCNCs-900 maintained its excellent recyclability over five consecutive reaction runs, displaying a slight decline in catalytic activity. In addition, the activation energy (E_a) of the optimized catalyst Pd@PNCNCs-900 for the FA dehydrogenation could be calculated at 45.97 kJ/mol, lower than most reported MOF-derived carbon supported monometallic Pd systems. The outstanding catalytic performance of Pd@PNCNCs-900 could be ascribed to the highly dispersed Pd NPs with ultrasmall size (2.0 nm) as the catalytic active centers, the metal-support interaction (MSI) effect between the introduced NPs and PNCNCs-900 support, and the modulated N sites in the PNCNCs-900 support as the basic sites for the promotion of the cleavage of O–H bonds of FA molecules. This study provides a novel approach for constructing high-performance Pd-based catalysts for FA dehydrogenation using N-containing energetic MOF-derived carbon materials as the supports.