Site Communication in Direct Formation of H2O2 over Single-Atom Pd@Au Nanoparticles

Single atom alloy catalysts offer possibilities to obtain turnover frequencies and selectivities unattainable by their monometallic counterparts. One example is direct formation of H2O2 from O2 and H2 over Pd embedded in Au hosts. Here, a first-principles-based kinetic Monte Carlo approach is developed to investigate the catalytic performance of Pd embedded in Au nanoparticles in an aqueous solution. The simulations reveal an efficient site separation where Pd monomers act as active centers for H2 dissociation, whereas H2O2 is formed over undercoordinated Au sites. After dissociation, atomic H may undergo an exothermic redox reaction, forming a hydronium ion in the solution and a negative charge on the surface. H2O2 is preferably formed from reactions between dissolved H+ and oxygen species on the Au surface. The simulations show that tuning the nanoparticle composition and reaction conditions can enhance the selectivity toward H2O2. The outlined approach is general and applicable for a range of different hydrogenation reactions over single atom alloy nanoparticles.

: Constrained molecular dynamics simulations for proton transfer from the metal to the water solution over Pd@Au(111), Au(111) and Pd(111). To reduce the noise from O−H vibrations, each step in the trajectory is a running average over 600, 800 and 600 steps, for Pd@Au(111), Au(111) and Pd(111), respectively.
A simple decomposition of the energy contributions is made to elucidate the factors governing the exothermicity of the H + transfer to the solution. A schematic is shown in Figure S2 considering the case with complete charge separation to H + and e -. Table S1: H 2 adsorption energies (E ads ), H 2 dissociation barrier (E a,f ) and association barrier (E a,b ) over Pd@Au(100) and Pd@Au(211).

Surface
Reaction Equation E ads (eV) E a,f (eV) E a,b (eV) Pd@Au(100) The potential energy surfaces presented in the main text show the H 2 O 2 formation route where protons react with O * 2 and OOH * via the water solution. The corresponding reactions, where surface-bound H * reacts with O * 2 and OOH * (Langmuir-Hinshelwood mechanism) are shown in Table S2.

Scaling Relations
To describe the potential energy surface over the range of under-coordinated Au sites on the NP, scaling relations are used. The adsorption energy is determined using generalized coordination numbers as a descriptor. S2,S3 The data used to obtain the scaling relations are shown in Table S3. O 2 , OH and OOH are preferably adsorbed at bridge sites, whereas O is adsorbed either at bridge or hollow sites. The adsorption energies are given with respect to the bare surface and gas phase H 2 and O 2 .  (111) facets or in edges in the NP, is increased from one to eight (0.37 % to 2.9 %). The TOF and selectivity as a function of the number of Pd monomers are shown in Figure S3. When Pd i located in the (111) surface or in the (111) facets of the NP, the TOF and selectivity decreases only slightly with an increased number of Pd monomers. However, when the concentration of Pd monomers embedded in edges increases, the selectivity is significantly reduced, from around 66 % to 14 %. This is owing to the strong adsorption energy of O 2 and facile OOH * dissociation over Pd@NP(edge). In conclusion, the number of Pd monomers in the NP can be increased, as long as the monomers are located in Au (111) facets.

TOF as a Function of Partial Pressures
The number of Pd monomers per NP is experimentally not restricted to one. Therefore, to facilitate comparison between our results and experimental results, a truncated Au octahedron with eight Pd monomers located at different (111)-facets are used. The turn-over frequency, as a function of H 2 pressure (left) and O 2 pressure (right) are shown in Figure   S4.   The adsorption energy of O 2 is also relatively unchanged over a 116 atom truncated Au octahedron, regardless whether the surface is charged or not. DFT calculations are performed for the structure shown in Figure S5.  Table S5. The O-O bond length and O 2 Bader charge is significantly increased when water is included in the calculations. However, an excess electron has only a slight effect on the adsorption configuration. In water, O 2 is negatively charged when adsorbed on the Pd monomer, i.e., the surface is positively charged. Upon the proton-transfer to the water solution, 0.65e is donated to the surface/adsorbate structure. Only a small part of this charge is located on the adsorbed O 2 molecule; most of the electron is delocalized over the surface. S-10