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

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% H-2 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.