The Cooperative Effects of the Rh-M Dual-Metal Atomic Pairs in Formic Acid Oxidation

The continuously increasing mass activity of precious metal in formic acid oxidation reaction (FAOR) is the key to achieving the practical application of direct formic acid fuel cells (DFAFCs). Herein, Rh-based dual-metal atomic pairs supported on nitrogen-doped carbon catalysts [DAP-(M, Rh)/CN] with adjacent interatomic Rh-M (M = V, Cr, Mn, Fe, Co, Ni, Cu) have been synthesized by a "host-guest" strategy. It is discovered that DAP-(Cr, Rh)/CN shows the highest mass activity of 64.1 A mg^-1, which is 3.8 times higher than that of the single atom Rh catalyst (17.0 A mg^-1) and two orders of magnitude higher than Pd/C (0.58 A mg^-1). Interestingly, the mass activity of DAP-(M, Rh)/CN first increases from 11.7 A mg^-1 (Rh-V) to 64.1 A mg^-1 (Rh-Cr) and then decreases to 21.8 A mg^-1 (Rh-Cu), forming a volcano curve of the reaction activity. Density functional theory calculations combined with in situ Fourier transform infrared spectrometer (FTIR) spectra reveal that formic acid oxidized on a series of DAP-(M, Rh)/CN catalysts through the formate route with the subsidiary M metal atoms binding the HCOO species and the Rh atom accepting the H atoms. The most suitable adsorption strength of HCOO on the Cr sites luckily contributes to two spontaneous elementary steps and thus accelerates the FAOR rates.