Structural Evolution of Pt Nanoclusters Driven by CO Reactant Pressure and Catalyst Temperature

The atomic-scale structure of a metal catalyst surface controls its catalytic performance. Through a combination of high-pressure scanning tunneling microscopy (HP-STM), ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), and machine learning-accelerated computational studies, we uncovered that Pt nanoclusters, formed by restructuring of hex-Pt(100) under reaction conditions involving CO, experience major structural evolution when exposed to increasing CO pressure in the range of 2 × 10 –8 –750 Torr. Atom-resolved images and simulations demonstrate that CO binds strongly on these nanoclusters, leading to a CO coverage of one molecule per Pt atom, while the lateral CO–CO repulsion is released by tilting CO molecules outward at the nanocluster edge. Metal nanoclusters break down along as CO pressure is increased from 2 × 10 –8 to 1 Torr with the average size decreasing from 3.2 ± 1.5 to 2.3 ± 1.0 nm, consistent with Pt 4f 7/2 photoemission feature evolution observed with AP-XPS. In contrast, in the pressure range of 1–750 Torr at 25 °C, HP-STM observed a decrease of nanocluster density by 4–5 times, consistent with the growth of the average nanocluster size from 2.3 ± 1.0 nm in 1 Torr CO to 4.6 ± 1.8 nm in 750 Torr. This uncovers a reactant pressure-driven coalescence of nanoclusters even at room temperature. Nanoclusters formed at 25 °C in 750 Torr CO require annealing to 100–130 °C to reach equilibrium size, indicating akinetic control of nanocluster growth at low preparation temperatures, such as room temperature. Our neural network potential (NNP) coupled with basin-hopping (BH) simulations determined the optimal CO coverage and configuration at various CO pressures and showed, in agreement with experiments, an optimum nanocluster size resulting from a competition between the size-dependent energy cost of nanocluster formation and the energy gain through CO adsorption. The formation of Pt nanoclusters, followed by their breakdown with increasing CO pressure from 2 × 10 –8 to 1 Torr and coalescence in CO pressure from 1 to 750 Torr highlights the significance of imaging catalyst nanoparticle surfaces in gas phase at a specific reactant pressure toward establishing a direct structure–catalytic performance correlation.