Octahedral gold-silver nanoframes with rich crystalline defects for efficient methanol oxidation manifesting a CO-promoting effect

Three-dimensional bimetallic nanoframes with high spatial diffusivity and surface heterogeneity possess remarkable catalytic activities owing to their highly exposed active surfaces and tunable electronic structure. Here we report a general one-pot strategy to prepare ultrathin octahedral Au3Ag nanoframes, with the formation mechanism explicitly elucidated through well-monitored temporal nanostructure evolution. Rich crystalline defects lead to lowered atomic coordination and varied electronic states of the metal atoms as evidenced by extensive structural characterizations. When used for electrocatalytic methanol oxidation, the Au3Ag nanoframes demonstrate superior performance with a high specific activity of 3.38 mA cm−2, 3.9 times that of the commercial Pt/C. More intriguingly, the kinetics of methanol oxidation on the Au3Ag nanoframes is counter-intuitively promoted by carbon monoxide. The enhancement is ascribed to the altered reaction pathway and enhanced OH− co-adsorption on the defect-rich surfaces, which can be well understood from the d-band model and comprehensive density functional theory simulations.

Comparing to those acquired in HClO4 (Supplementary Fig. 18), the voltammograms obtained here are more complicated as they involve a multi-step oxidation for both Au and Ag (eq. 1eq. 4), with the current peaks superimposed with each other. Taking the Au3Ag NFs as an example, the oxidation peaks emerge from ~0.80 V, followed by two prominent peaks at ~1.21 and 1.53 V, in accordance with the surface OHadsorption, the oxidation of Ag to Ag2O and AgO, and the Au oxidation 1 . On the cathodic sweep, three reduction peaks are prominent at 1.17, 0.90 and 0.82 V, corresponding to the reduction of AgO, Ag2O and AuO, coupled with the desorption of OH -.
By contrast, all oxidation peaks of Au2Ag NCs occur at significantly higher potentials than those of Au3Ag NFs, suggesting a lower surface activity. In addition, when comparing Au2Ag NCs with Au3Ag NFs, the convoluted AuO/Ag2O reduction peaks at 0.82 -0.9 V indicate a higher coverage of oxidized species on the Au3Ag NFs on account of their larger electroactive surface area. As for AuAg12 NPs, all major peaks on both anodic and cathodic sweeps are mainly relevant to the oxidation and reduction of AgO and Ag2O, except for the small reduction peak at 0.62 V, which is likely due to the reduction of AuO/Au-OH owing to the low Au content in AuAg12 NPs. These observations provide additional evidence for the predominant Au occupation at the surface of Au3Ag NFs with rich low coordination states and high chemical activities.

Supplementary Note 2
All calculations are carried out on the density functional theory (DFT), executed by the Vienna Ab-initio Simulation Package (VASP) code using the projector augmented wave (PAW) method to tackle electron-core interactions 2,3 . The exchangecorrelation interactions are handled by generalized gradient approximation with the Perdew-Burke-Ernzerhof (PBE) function 4 . The kinetic energy cutoff with plane wave basis set is set to 400 eV. The first Brillouin zone integrations are adopted with 2 × 3 × 1 k-points mesh by the Monkhorst-packing method 5 . We may also test higher accuracy simulations with the energy cutoff of 520 eV and 3 × 4 × 1 k-points mesh to calculate the adsorption of CH3OH molecule in Supplementary Table 3. The results of adsorption energy trend is similar to the 400 eV energy cutoff and 2 × 3 × 1 k-points mesh. For the geometry optimization, all the structures were relaxed fully until the total energy and total force per atom is less than 2 × 10 −5 eV and 0.02 eV/Å, respectively. The slabs exposed to (111) surface with 4 × 2√3 supercell and (410) surface with 3 × 1 supercell are cleaved from the bulk material and the thickness of the vacuum layer was about ~10 Å. The bottom two layers are fixed to the bulk position, and the other layers and adsorbates are fully relaxed. We use the Climbing Image-Nudged Elastic Band (CI-NEB) 6 method together with the Dimer method 7 to search for the transition state, which is just one imaginary frequency along the reaction pathway via the vibrational frequency calculations based on harmonic approximation.
The adsorption energy and bond distance of the CH3OH molecule is calculated on 16 the Au (111), Au3Ag (111), Au3Ag (410), Au3Ag-Auvac, and Pt (111) surfaces as shown in Supplementary Table 3 below. Eads = Etotal -Esurf -ECH3OH, where Etotal and Esurf are the total energy with and without CH3OH, respectively. ECH3OH is the molecule energy of CH3OH. Note that the more negative the adsorption energy, the stronger the bonding.
In the electrochemical oxidation of CH3OH, the elementary steps are referred to the pure Au and Pt surfaces as shown in Supplementary Figure 21 The value of U0 is calculated using the equation ΔG = G*A + (1/2GH2 -eU) -G*A -G*H, by determining the potential at which the reaction free energy given by Eq. (7) is zero. Then the barrier is extrapolated to other electrode potentials using Butler-Volmer formalism 10 . As such, the potential-dependent (U) activation barrier (Ea(U)) is calculated below: In Eq. (8), ′ is an effective symmetric factor, which is approximated as Where is a reaction symmetry factor between 0.3 and 0.7, with 0.5 indicating a symmetric reaction. We started with the approximation that is 0.5 for all the elementary steps and used an approximation to correct for an asymmetric reaction by considering variations in the interaction of surface dipole moment (μ) and interfacial electric field between the transition state and reactant state in Eq. (9). d is the the thickness of interfacial double layer, assumed as 3 Å.