Few-Atomic Zero-Valent Palladium Ensembles for Efficient Reductive Dehydrogenation and Dehalogenation Catalysis.

Single-atom catalysts (SACs) offer immense potential in heterogeneous catalysis due to their maximized atomic utilization and high selectivity but suffer the problem of low reactivity in catalytic reductive reactions due to their high-valent state. Here, we demonstrate that supported palladium (Pd) ensembles consisting of a few zero-valent Pd atoms (Pd 1+c-red /CN) exhibit exceptional reactivity in formic acid (FA) dehydrogenation and 4-chlorophenol (4-CP) dechlorination. The initial FA dehydrogenation and 4-CP dechlorination rates of Pd 1+c-red /CN are 42-104 and 16-210 times higher than that of supported Pd SACs (Pd 1-ox /CN), respectively. Experimental results and density functional theory (DFT) calculations reveal that optimal adsorption sites of Pd 1+c-red /CN stimulate the formation of H*, which is indispensable for 4-CP dechlorination. Moreover, direct electron transfer from Pd atoms to FA with a high electron density on Pd 1+c-red /CN also contributes to the rapid 4-CP dechlorination. The superior dehalogenation capability of Pd 1+c-red /CN for organohalides of great environmental and health concerns suggested its immense application potential in environmental remediation. This work highlights the pivotal roles of the structure and valence state of Pd ensembles in catalytic reductive reactions and provides a strategy to broaden the application of Pd-based catalysts for dehydrogenation and dehalogenation.