Ruthenium(iii) polyethyleneimine complexes for bifunctional ammonia production and biomass upgrading

Abstract

As an effective strategic approach to produce ammonia (NH₃), electrocatalytic nitrogen reduction reactions (NRRs) under ambient conditions using renewable energy sources (e.g. solar) have attracted significant attention; however, the design of an efficient electrocatalyst for the NRR is a challenging task and has been of central research interest. Herein, we report the synthesis of ruthenium(III) polyethyleneimine (Ru(III)-PEI) catalysts supported on carboxyl-modified carbon nanotubes (Ru(III)-PEI@MWCNTs) by a self-assembly process driven by electrostatic forces at room temperature. Our newly designed Ru(III)-PEI@MWCNTs were employed as bifunctional catalysts for the NRR and 5-hydroxymethylfurfural (HMF) oxidation. At −0.10 V vs. the reversible hydrogen electrode (RHE), our Ru(III)-PEI@MWCNTs exhibited the high NH₃ yield rate of 188.90 μg_NH3 mg_cat.⁻¹ h⁻¹ and the faradaic efficiency (FE) of 30.93% at room temperature. Furthermore, owing to its favorable thermodynamics for HMF oxidation, the Ru(III)-PEI@MWCNT electrode demonstrated an impressive electrocatalytic HMF oxidation at 1.24 V, 220 mV lower than that for oxygen evolution. The two-electrode electrolyzer employing Ru(III)-PEI@MWCNTs as a bifunctional catalyst for both the cathode and the anode showed the current density of 0.50 mA cm⁻² with the cell voltage of only 1.34 V over 27 hours of stable electrolysis with a 94% FE for 2,5-furandicarboxylic acid (FDCA) production; this suggested an outstanding performance of this electrolyzer for the coupling of NRR with HMF oxidation. This study represents the first attempt at the ground demonstration of combining NH₃ production with biomass upgrading.