(7a) Nanoparticle Size Effects on the Kinetics and Adsorbate-Induced Restructuring of Supported Pd1Aux Bimetallic Catalysts

Decreasing the size of bimetallic catalysts increases the fraction of atoms at surfaces, reduces site coordination, and may improve performance for catalytic reactions. Yet, the impact of size on the location of elements within bimetallic nanoparticles and the corresponding effects on kinetics remain poorly understood. Here, we examine how kinetics for the reduction of O₂ with H₂ over Pd₁Au_x nanoparticles respond to differences in nanoparticle size and Pd mole fraction (χ_Pd; 0 ≤ χ_Pd ≤ 1). Additionally, we probe the structural evolution of Pd₁Au_x on contact with relevant adsorbates (H*, O*, CO*), promoters (CO₂*), and during reactions using kinetics, in situ and operando spectroscopy, and DFT parameterized Monte Carlo predictions of 201 atom models.

Kinetic measurements reveal 1.8 nm Pd₁Au_x give greater H₂O₂ selectivities and formation rates across all χ_Pd compared to 11.8 nm Pd₁Au_x (Figure 1a). Infrared spectra of adsorbed CO and mixed ¹²CO-¹³CO adlayers confirm greater Pd isolation with decreasing size and χ_Pd, which agrees with the interpretation of measured kinetics and computational predictions. The smallest Pd₁Au_x nanoparticles (1.8 nm) restructure irreversibly during reactions, and CO confers the greatest extent of restructuring. H₂O₂ selectivities differ across combinations of H₂ and O₂ pressures in response to the accompanying changes in Pd-Pd coordination numbers (Figure 1b). Notably, the addition of CO₂ increases H₂O₂ formation rates and selectivities on 11.8 nm Pd nanoparticles due to a change in reaction mechanism and new low barrier pathways for proton-electron transfer. In contrast, the addition of CO₂ reduces rates and selectivities upon Pd₁Au_x nanoparticles of comparable size as a consequence of Pd ensemble formation (Figure 1c). This work expands the understanding of how nanoparticle sizes influence the structural evolution of PdAu nanoparticles and the corresponding impact on surface reactions.