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Abstract

Single-atoms on carbon–nitrogen supports are considered catalysts for a multitude of reactions. However, doubts remain whether really these species or subnanometer clusters formed under reaction conditions are the active species. In this work, we investigate the dynamics of palladium single-atoms on graphitic carbon nitride during ethylene hydrogenation and H₂-D₂ exchange. By employing aberration-corrected scanning transmission electron microscopy, x-ray photoelectron spectroscopy and x-ray absorption spectroscopy, we will show that palladium, originally present as single-atoms, agglomerates to clusters at 100 °C in a gas atmosphere that contains both ethylene and hydrogen. This agglomeration goes in hand with the emergence of catalytic activity in both ethylene hydrogenation and H₂-D₂ exchange, suggesting that clusters, rather than single-atoms, are the active species. The results presented herein highlight the potential of analytics over the course of reaction to identify the active species and provide new insights into the influence of gas atmosphere on metal speciation.