DFT-studied CO Oxidation Pathway

2. Shape Effect of Ceria in Gold-Ceria Catalyst for Carbon Monoxide Oxidation Reaction

2.3. Results and Discussion

2.3.1. DFT-studied CO Oxidation Pathway

convergence criteria for the electronic structure and geometry were set to 10⁻³ eV and 0.01 eV/A, respectively. We used a Gaussian smearing function with a finite temperature width of 0.05 eV to improve the convergence of states near the Fermi level. The location and energy of transition states (TSs) were calculated with the climbing-image nudged elastic band method.^61-62

Figure 2.1. DFT-calculated ∆G of multiple CO adsorption on (a) Au/CeO2(100) and (b) Au/CeO2(111).

nCO denotes the Au/CeO2 model with n adsorbed CO molecules. Under practical CO oxidation conditions (p(O2) = 0.21 bar and 0.02 bar ≤ p(CO) ≤ 0.20 bar), 8 CO molecules can be stabilized on the Au NP of Au/CeO2(100), whereas 4 CO molecules can be stabilized on the Au NPs of Au/CeO2(111).

Figure 2.2. The schematic CO oxidation pathways. (a) Au/CeO2(100) and (b) Au/CeO2(111) with 8 or 4 adsorbed CO molecules show that CO oxidation occurs at the Au-CeO2 interface, although an additional O-C-O formation step (step 2, b) is required for Au/CeO2(111).

Because the Au NPs were covered with CO molecules, CO oxidation occurred at the Au-CeO2

interface via the M-vK mechanism (Figure 2.2a and 2.2b). We attempted to determine the available binding sites for O2 molecules near the Au-CeO2 interface. However, Ebind of O2 was consistently positive. Several theoretical studies, including our previous study, have reported the energetically favorable adsorption of O2 at the interface of Au and TiO2 or CeO2.^{28, 63-64} Under-coordinated Ti ions or reduced Ce³⁺ ions formed upon oxygen vacancy formation might anchor O2 at Au-oxide interfacial sites and utilize for oxidation of Au-bound CO, Au-CO^*. However, we found no available case of O2

chemical adsorption at the Au-CeO2 interfaces of our catalyst models. Very recently, Schlexer et al.

reported that TiO2-supported Au NPs are positively charged, thus stabilizing an oxygen vacancy by acting as an electron reservoir.⁶⁵ We also found that our Au9 NPs on CeO2(100) and CeO2(111) were positively charged and that the released electrons were localized on adjacent Ce ions (Figure 2.3).

However, although the Au9 NPs of Au9/CeO2(100) had a greater positive charge than that of Au9/CeO2(111) (Figure 3), we also found that the newly formed Au-Ce³⁺ interface in Au9/CeO2(100) cannot strongly bind O2 (as mentioned above, Ebind was positive). Presumably, the strong electronic

interaction between the Au NPs and CeO2 is not sufficient for facile adsorption and activation of O2 at the Au-CeO2 interface. In their early study of O2 activation by the unsupported Au12 cluster, Nørskov and coworkers found that the low-coordinated Au corner site binds and activates an O2 molecule.⁶⁶ In addition, we recognized that the O2 binding site (Au-Ce³⁺) found on the Au13/CeO2(111) system reported in our previous study also involves a low-coordinated Au site.²⁸ Presumably, the O2 adsorption and activation at the Au-CeO2 interface is limited to the specific sites that provide both the reduced Ce³⁺ ion and the low-coordinated Au. Another interesting theoretical and experimental finding of Li and coworkers showed that the surface of CeO2-supported Au NPs is covered with CO molecules under CO oxidation conditions.⁴² Based on DFT calculations and environmental TEM analyses, they also confirmed that CO molecules rather than O2 molecules occupy the Au sites at the Au-CeO2 interface.⁴² Their interpretations and experimental findings are consistent with our previous theoretical findings.

We also experimentally observed a positive correlation between the p(CO) and TOF of Au/CeO2

catalysts, presenting a dominant effect of CO on the activation of CO oxidation (discussed below).

Figure 2.3. Electronic interaction between Au9 NPs and CeO2 surfaces. Bader charge analysis of (a) Au9/CeO2(100) and (c) Au9/CeO2(111). ∆ρ represents the excess charge (e) originating from the Au- CeO2 interaction.

Although we excluded the initial O2 binding and activation at the Au-CeO2 interface in our current study, such O2 activation occurs during CO oxidation by the M-vK mechanism. The second half of the M-vK mechanism of CO oxidation (Figure 2.2a and 2.2b) involves O2 activation at a Au-CeO2

interfacial site adjacent to an oxygen vacancy and the Au-CeO2 interface strongly binding an O2

molecule. This step is generally regarded as vacancy healing, although the oxygen atom protruding

from the original oxygen vacancy occasionally sits near the Au-Ce³⁺ interface. The protruding oxygen atom of the O2 that heals the vacancy can oxidize another Au-CO^*. This step is usually faster than the first step of the M-vK mechanism of CO oxidation. A 2~3 nm Au NP supported on CeO2 may have closely packed pairs of reaction sites at the Au-CeO2 interface; therefore, adjacent pairs of oxygen vacancies could be formed. Under high p(O2) oxidation conditions, an O2 molecule may heal two adjacent oxygen vacancies at once, as proposed by Schlexer et al.⁶⁵

The DFT-estimated energetics and pathways of CO oxidation at the Au-CeO2(100) and Au- CeO2(111) interfaces suggested that Au/CeO2(100) is a better catalyst for CO oxidation (Figure 2.2a and 2.2b). The original data published in our previous report⁴¹ were adopted and reorganized as schematic diagrams, as shown in Figure 2a and 2b. Because the surface of Au NPs of Au/CeO2(100) was covered with 8 CO molecules, the O-C-O type intermediate formed spontaneously at the Au- CeO2(100) interface upon adsorption of the last CO molecule, and only 0.4 eV was required for desorption of the first CO2 molecule. In contrast, an activation energy barrier, Eb, of 0.86 eV was required to convert the last and fourth bound CO molecule on the Au NP of Au/CeO2(111) into the O- C-O type intermediate. Based on the DFT-estimated energetics of CO oxidation by Au/CeO2(100) and Au/CeO2(111), we performed microkinetic modeling and presented the rate maps for both catalysts as a function of p(CO) and temperature (discussed below).

The oxygen vacancy formation energy, Evac, of an oxide support is often used as an energetic descriptor of the activity of M-vK type CO oxidation. Indeed, the low Evac of the step-edge oxygen ions of CeO2(111) or the oxygen ions of doped CeO2(111) facilitates the M-vK type CO oxidation mechanism at the Au-CeO2 interface. Figure 2.3 presents the electronic accumulation or depletion accompanied by the Au-CeO2 interaction. As discussed above, the relatively stronger electronic interactions between Au and CeO2(100) leads to the reduction of three Ce⁴⁺ ions into Ce³⁺ ions, whereas Ce⁴⁺ ions in CeO2(111) were relatively weakly reduced upon Au deposition. As previously discussed by Behm and coworkers,⁶⁵ positively charged Au NPs on oxide supports can act as electron reservoirs that take up the released electrons upon oxygen vacancy formation.⁶⁵ The positively charged Au NPs therefore facilitate the formation of oxygen vacancies from the lattice of CeO2. We also found a positive correlation between the presence of reduced Ce ions (positively charged Au NPs) and the Evac values.

The Evac values of CeO2(111) (2.63 eV) and CeO2(100) (1.92 eV) decreased to 2.52 eV and 1.77 eV upon deposition of Au NPs on CeO2. Based on our theoretical findings and the previous discussion by Behm and coworkers,⁶⁵ the electronic interaction between the supported Au NPs and the supporting CeO2 lowers the Evac of CeO2, activating the oxygen ions at the Au-CeO2 interface for CO oxidation.

The corresponding experimental findings of the modified Evac upon Au deposition on CeO2 will be discussed below.

Figure 2.4. XRD patterns and N2 adsorption-desorption isotherms of CeO2 cubes and octahedra.