Metal/oxide interfacial effects on the selective oxidation of primary alcohols

A main obstacle in the rational development of heterogeneous catalysts is the difficulty in identifying active sites. Here we show metal/oxide interfacial sites are highly active for the oxidation of benzyl alcohol and other industrially important primary alcohols on a range of metals and oxides combinations. Scanning tunnelling microscopy together with density functional theory calculations on FeO/Pt(111) reveals that benzyl alcohol enriches preferentially at the oxygen-terminated FeO/Pt(111) interface and undergoes readily O–H and C–H dissociations with the aid of interfacial oxygen, which is also validated in the model study of Cu2O/Ag(111). We demonstrate that the interfacial effects are independent of metal or oxide sizes and the way by which the interfaces were constructed. It inspires us to inversely support nano-oxides on micro-metals to make the structure more stable against sintering while the number of active sites is not sacrificed. The catalyst lifetime, by taking the inverse design, is thereby significantly prolonged.

Calculation of benzyl alcohol density adsorbed at Pt(111) surface and FeO/Pt interface after exposure ~1 L benzyl alcohol at 300 K, followed by flash annealing to 350 K to remove trace diffusive adsorbates. Image sizes: 43 nm × 43 nm.

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Supplementary Discussion
The free energy profiles of benzaldehyde formation on Pt(111) Benzyl alcohol adsorbs mainly with the benzene ring sitting at the bridge(30) site ( Supplementary Fig. 13A) (1, 2), which is strongly exothermic by 2.57 eV including the van der Waals (vdW) interactions between the adsorbate and the Pt surface (3,4). The benzaldehyde formation can start with the O-H bond scission to remove the first hydrogen with an activation energy of 0.70 eV (pathway 1, Supplementary Fig. 13B). Subsequently, the second hydrogen can be removed by the C-H bond activation with a 0.42 eV barrier and 0.30 eV heat releasing. Alternatively, the C-H bond cleavage can firstly occur and then be followed by the O-H bond dissociation (pathway 2, Supplementary Fig. 40). In pathway 2, the activation energies of C-H and O-H cleavage (0.68 eV and 0.23 eV, respectively) are both lower than those in pathway 1, which implies that the pathway 2 would energetically be favorable for the benzaldehyde formation on Pt(111).

Ag-Cu2O nanocomposites
The XRD pattern of Ag-Cu2O nanocomposite displays characteristic diffractions of Ag (JCPDS No. 4-0783) and Cu2O (JCPDS No. 05-0667), demonstrating the sample comprises Ag and Cu2O phases. High angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image ( Supplementary Fig. 25) shows that the Ag phase (bright) and Cu2O (dark) phase are close to each other in space. The corresponding EDX mappings ( Supplementary Fig. 25) confirm the close contact of the two phase, indicating the formation of Ag-Cu2O composite.

Catalyst preparation
Pt and Pd nanoparticles (NPs), PdCu nano-alloy, Ag-Cu2O nanocomposites, Ag, Cu2O, NiO, CoO and Mn3O4 NPs were synthesized using procedures reported previously (5). In brief, 7.5 g of octadecylamine (ODA, Beijing Chemical Factory) was loaded in a 50 mL beaker and heated at 120 o C. Designed amounts of metal precursors were added in the ODA under magnetic stirring to form a clear solution.
The reaction mixture was further heated to desired temperature (T) and maintained at this temperature for 10 min before it was cooled down to 70 o C. The resultant solution was mixed with 20 mL of ethanol and kept stand at 70 o C for 30 min without magnetic stirring. The products were collected at the bottom of the beaker by decanting the supernatant and further washed with hexane and ethanol for several times. The amounts of metal precursors and the reaction temperatures are listed in the Supplementary Table 11.
Cu NPs were prepared as described elsewhere (6). Briefly, 10 mL of trioctylamine (TOA) was loaded in a three-neck flask and heated at 130 °C for 30 min under a N2 flow to remove dissolved water and oxygen. 1 mmol copper(I) acetate (CuOAc) and 0.5 mmol tetradecylphosphonic acid were added into TOA after cooling to room temperature. The solution was rapidly heated to 180 °C and maintained for 30 min under N2 atmosphere. The solution was further rapidly heated to 270 °C, and held there for an additional 30 min before it was cooled to room temperature. 5 mL ethanol was added into the colloidal solution and the products were collected by centrifugation.
As-synthesized NPs were treated at 300°C in air to remove the organic capping agents.and loaded on the corresponding supports before they were subjected to catalytic investigation.

Details of density functional theory (DFT) calculations
The spin-polarised calculations were performed with the Perdew-Burke-Ernzerhof (PBE) functional within the generalised gradient approximation as implemented in the VASP package (7,8). The project-augmented wave (PAW) method was used to represent the core-valence electron interaction (9). To model Pt(111), four-layer p(3×3) (Supplementary Fig. 41) and p(4×4) slabs ( Supplementary Fig. 13) with 4 × 4 × 1 and 3 × 3 × 1 k-point samplings, respectively, were used. For the Fe-terminated FeO/Pt(111) surface, a FeO ribbon covered p(2√3 × 7) Pt(111) slab was used for benzyl alcohol oxidation ( Supplementary Fig. 8B), which could further be oxidized to form the O-terminated FeO/Pt(111) surface. A 3 × 2 × 1 k-point sampling was applied and the on-site column repulsion was described by DFT + U with UFe -JFe = 3 eV, as used in previous studies (10). A vacuum layer of 15 Å was applied and the bottom two layers were fixed for all the slabs. The valence electronic states were expanded in plane wave basis sets with a cutoff energy of 450 eV. Atomic positions were optimized until the maximum force of each atom was less than 0.05 eV/Å. The van der Waals (vdW) interactions between the adsorbate and the substrate surface were considered using the DFT-D3 method (3,4). The benzene adsorption was tested on p(3×3) Pt(111) and the result shows that the adsorption is exothermic by 2.06 eV by the D3 method, which was closed to the calculation value in the work of Tkatchenko (1.96 eV) by the PBE + vdW surf (1) method and the experimental results in the same benzene coverage (1.57-1.91 eV) (11). The transition states were searched by using a constrained optimisation scheme (12)(13)(14) and were verified when (i) all forces on the atoms vanish and (ii) the total energy is a maximum along the reaction coordinate but a minimum with respect to the rest of the degrees of freedom. The vibration frequency calculations were performed for the transition state structures we obtained, which shows that the located TS structures are true saddle points. The adsorption energies Ead(adsorbate) were defined as Ead(adsorbate) = Eadsorbate/surface -Eadsorbate -Esurface.