Palladium single atoms from melting nanoparticles on WO3− for boosted hydrogen evolution reaction

Surface-supported isolated atoms in single-atom catalysts (SACs) grant maximum utilization of metals in heterogeneous catalysis. Herein, we report a feasible pyrolysis strategy to synthesize Pd single atoms by thermally melting Pd nanoparticles on an oxygen-vacancy-rich tungsten-oxide matrix at reduction atmosphere. Near ambient pressure X-ray photoelectron spectroscopy was used to monitor the formation of zero-valence Pd single atoms and the increased metallic feature of WO3−x substrate. Accordingly, the as-obtained zero-valence Pd single-atom catalyst exhibits a markedly boosted HER activity with a low overpotential (η10 = 70 mV) at the current density of 10 mA/cm2 and a small Tafel slope (b = 68 mV/dec), nearly 150 mV and a 3.0-fold enhancement than those of Pd nanoparticles (η10 = 220 mV, b = 133 mV/dec) under the same conditions. In addition, quasi in situ XPS results suggest the hydrogen spillover effect is more likely to occur on Pd single atoms during the electrochemical process. Our work may pave an interesting route for the rational design of highly-efficient single-atom catalyst, and the elucidation of corresponding enhanced reaction mechanisms by the utilization of advanced characterization techniques.