Exceptional size-dependent catalytic activity enhancement in the room-temperature hydrogen generation from formic acid over bimetallic nanoparticles supported by porous carbon

Abstract

In this work, we report a prominent size-dependent activity in the catalytic dehydrogenation of formic acid (HCOOH, FA) over carbon-supported AuPd alloy nanoparticles (NPs) directed by amino acids (lysine, serine and glutamic acid) with different isoelectric points. Through decreasing the average size of AuPd NPs from 12.8 ± 0.5 to 3.8 ± 0.5 nm using different amino acids as structure-directing agents, drastic activity enhancement in the generation of hydrogen without CO impurity from FA was observed, and the initial turnover frequency (TOF) value was enhanced from 14 to 718 h⁻¹ at 298 K, which was among the highest values for the reported heterogeneous catalysts for FA dehydrogenation under the same conditions. Through optimizing the molar ratio of Au/Pd and the metal loading in the catalysts, the initial TOF value for FA dehydrogenation was enhanced to 1153 h⁻¹ at 298 K. In addition, the catalyst with the smallest size of AuPd NPs gave an initial TOF value of 2972 h⁻¹ at 323 K with 100% H₂ selectivity, comparable to the values acquired from the most active homogeneous catalysts. The investigation of UV-vis spectroscopy showed that the reaction between mixed metal ions (Pd²⁺ and AuCl₄⁻) and the three amino acids before the reduction by NaBH₄ led to the formation of three different coordination complex intermediates, which induced the formation of different-sized AuPd alloy NPs featuring remarkably different catalytic performances for FA dehydrogenation.