H2 Activation and Spillover on Catalytically Relevant Pt-Cu Single Atom Alloys

Platinum is a key component in many heterogeneous hydrogenation catalysts. Because of its high price, fairly strong interaction with intermediates, and susceptibility to CO poisoning, it is often mixed with other elements. These bimetallic alloys have complex surface structures, and the atomic structure of their active sites is not well understood. In this study, we examine the effect of the geometric arrangement of dilute Pt-Cu alloys on H-2 activation, spillover, and release. Using scanning tunneling microscopy, we directly visualize the atomic arrangement of Pt-Cu alloys and show that small amounts of Pt (similar to 1%) exists as isolated atoms in the Cu surface. These Pt monomers are capable of facile H-2 dissociation and spillover to Cu at temperatures as low as 85 K. Additionally, the low-temperature desorption of H-2 (230 K) suggests a reduced desorption barrier compared to monometallic Pt or Cu. We find these single atom alloy surfaces are robust to multiple adsorption/desorption and heating cycles to 450 K. Larger Pt ensembles in Cu exhibit higher temperature desorption profiles due to the stronger binding of H to extended Pt ensembles, demonstrating how the geometric arrangement of Pt atoms in Cu impacts the binding of H to catalytic surface sites. Overall, dilute Pt-Cu alloys containing only isolated Pt atoms are most favorable for H-2 activation, spillover, and release and hence should be capable of catalyzing hydrogenation reactions with a greatly reduced concentration of the precious metal.