3. Combinational Effect of Particle Size and Support Morphology at the Platinum-Ceria
3.3. Results and Discussion
3.3.3. DFT Calculation for the Mechanism of CO Oxidation
particles are more active due to the increased total interfacial area. In addition, the sequence obtained from the T50 agrees well with the reduction temperature, indicating the MvK reaction order affects the total CO oxidation activity. Furthermore, the ΔT is greater in 1 nm sized Pt/CeO2 (80) than that of 3 nm Pt (63), suggesting that the effect caused by the support decreases when the Pt NPs size increases.
Figure 3.6. DFT-calculated energetics of the sequential CO adsorption over (a) Pt NP/CeO2(100) and (b) Pt NP/CeO2(111). The black squares 𝐸 denotes the binding energy of the 𝑛 CO molecule.
The red square presents the O2 binding energy competing with the first binding CO molecules.
saturated gas environments, theoretical calculation for the reaction mechanism study should be perform on the model surfaces with saturated coverages by reaction gas molecules.59,60 To realize the saturated coverage of the Pt surface on CeO2 by CO, we calculate a sequential CO binding energy, Ebind, over Pt9
NPs supported on CeO2(100) (Pt9-NP/CeO2(100)) and CeO2(111) (Pt9-NP/CeO2(111)) (Figure 3.6). To confirm the concentration of CO and O2 molecules on the Pt-NP/CeO2 surface, we perform the pre- DFT calculation. As a results of competitive adsorption of CO and O2 molecules by the DFT calculation, CO was presumably adsorbed at the initial adsorption site of Pt NP which was about 2.5 times stronger than that of oxygen in both Pt9-NP/CeO2(100) and Pt9-NP/CeO2(111) catalysts. It was found that O2
molecule could not be adsorbed even at the last CO adsorption site. These results suggest that the surface of Pt NPs with even 1 nm of size can be poisoned entirely by CO molecules due to the strong CO adsorption energy which was good agreement with the previous reports.24,33
The energetics of CO adsorption that change binding energy of Pt9-NP/CeO2(100) and Pt9- NP/CeO2(111) catalysts with increasing number of adsorbed CO molecules are provided in Figure 3.6a and Figure 3.6b, respectively. The sequential CO binding energy over Pt9-NP/CeO2(100) shows that the exposed Pt NP surface is completely saturated with 8 CO molecules. The Ebind value of last-binding CO molecule on Pt-NP/CeO2(100), which is -1.40 eV, allude that Pt NP have a strong CO-philic nature, thus suppress the binding of oxygen. On the other hand, the weak binding energy of last-binding CO molecule (-0.74 eV) on the Pt9-NP/CeO2(111) catalyst show that, considering the entropic contribution, only 6 CO molecules can be bound on the Pt NP surface under reaction condition. Since the binding process of reactants on the Pt surface is determined by the chemical state of Pt NP, this result indicates that the inherent oxidation state of Pt NP supported on each CeO2(100) and CeO2(111) is different.
Moreover, fundamentally, the difference of the electronic interaction between each oxide support and the Pt NP causes the change of the intrinsic electron configuration of Pt NP.
Interestingly, the DFT-estimated catalytic reaction pathways of CO oxidation of Pt9- NP/CeO2(100) and Pt9-NP/CeO2(111) catalysts are consistent with the previously mechanism of CO oxidation mechanism of Au/CeO2 catalysts. In our previous results, we reported that Ce4+ ions are reduced to Ce3+ ions by a stronger electron interaction at the Au and CeO2(100) interface than at the Au and CeO2(111) interface.24,33 In addition, we suggested that an O-C-O intermediate, formed by binding the electron-rich oxygen around the reduced Ce3+ ions and adsorbed CO molecule at the Au-CeO2(100) interface, can accelerate the Mvk-type CO oxidation reaction.
Through calculation of CO oxidation reaction over the carefully-calculated CO-saturated Pt/CeO2 models, we find the pathways of CO oxidation that is in good agreement with the experimentally suggested results (Figure 3.7). Since the strong electronic interaction between the Pt9- CeO2(100) interface than the Pt9-CeO2(111) interface, O-C-O type intermediates are spontaneously generated at the interface of CO-saturated Pt NP and CeO2(100) and promote the reaction activity, as shown in our previous report.24,33 The low energy required for desorption (0.20 eV) of the O-C-O type
intermediate of the CO oxidation reaction of the Pt9-NP/CeO2(100) catalyst, which is 7.9 times lower than the RDS (1.58 eV) of the Pt9-NP/CeO2(111) catalyst, clearly interpret the performance of the experimentally observed activity gap between 1 nm sized PCC and PCO catalysts.
Figure 3.7. DFT-estimated CO oxidation pathways on the (a) Pt cluster/CeO2(100) and (b) Pt cluster/CeO2(111). The Pt cluster-CeO2(100) interface, which spontaneously forms an O-C-O-type intermediate in the sequential adsorption step of CO molecules, produces CO2 without a high activation energy barrier. In contrast, the CO oxidation pathway catalyzed by the Pt cluster/CeO2(111) not only has an activation energy barrier of 0.66 eV, TS1, for O-C-O-type intermediate formation, but also requires a high energy of 1.58 eV, S4, for the 2nd CO2 desorption.
In generally, the theoretical-estimated activation energy barrier of the Mvk-type CO oxidation reaction of the catalyst composed of metals and oxides calculated at the CO2 formation stage (*CO +
*O → *CO2). However, we appropriately modeled the structure that is theoretically estimated the surface of the catalyst under the real reaction condition. Our results suggest that the precisely calculated reaction mechanism can reveal the nature of the catalytic activation of the hidden metal-oxide interface.
Figure 3.8. DFT-calculated vacancy formation energy, Evac, at the Pt9-NP/CeO2 and Pt56-rod/CeO2
interface. The vacancy formation energy is the average energy calculated at the red circle. The decrease of vacancy formation energy gap at each interface of Pt-CeO2(100) and Pt-CeO2 (111) indicates that the MSIs are becoming similar.