Single Isolated Pd²⁺ Cations Supported on N-Doped Carbon as Active Sites for Hydrogen Production from Formic Acid Decomposition

Single-site heterogeneous catalysis with isolated Pd atoms was reported earlier, mainly for oxidation reactions and for Pd catalysts supported on oxide surfaces. In the present work, we show that single Pd atoms on nitrogen-functionalized mesoporous carbon, observed by aberration-corrected scanning transmission electron microscopy (ac STEM), contribute significantly to the catalytic activity for hydrogen production from vapor-phase formic acid decomposition, providing an increase by 2-3 times in comparison to Pd catalysts supported on nitrogen-free carbon or unsupported Pd powder. Some gain in selectivity was also achieved. According to X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) studies after ex situ reduction in hydrogen at 573 K, these species exist in a Pd²⁺ state coordinated by nitrogen species of the support. Extended density functional theory (DFT) calculations confirm that an isolated Pd atom can be the active site for the reaction, giving decomposition of the formic acid molecule into an adsorbed hydrogen atom and a carboxyl fragment, but only if it is coordinated by a pair of pyridinic-type nitrogen atoms located on the open edge of the graphene sheet. Hence, the role of the N-doping of the carbon support is the formation and stabilization of the new active Pd sites. A long-term experiment performed for more than 30 h on stream indicated an excellent stability of these Pd species in the reaction.