Structure Sensitivity of Acetylene Semi-Hydrogenation on Pt Single Atoms and Subnanometer Clusters

Oxide-supported Pt-group single atoms and clusters in the subnanometer size regime maximize the metal utilization and have shown extraordinary catalytic properties for many reactions including selective hydrogenation. Establishing relations between the metal nuclearity and electronic and catalytic properties is crucial for catalyst design. Here, we varied the nuclearity of Pt supported on TiO2 from single atoms to subnanometer clusters to larger nanoparticles to develop such relations for acetylene hydrogenation. We show that, in contrast to the low selectivity on large Pt nanoparticles, in the subnanometer size regime, Pt exhibits remarkably high selectivity to ethylene. Through a combination of X-ray photoelectron spectroscopy and calorimetry, we demonstrate that the origin of high selectivity is the decreased electron density on Pt and destabilization of C2H4 as the Pt nuclearity decreases. However, as the Pt nuclearity decreased, the activity for H-2 activation and acetylene hydrogenation decreased, indicating a trade-off between activity and selectivity. The results show that, while different properties emerge in the subnanometer regime, Pt supported on TiO2 appears to be bound by similar scaling and Bronsted-Evans-Polanyi relationships as on metal surfaces.