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.