2. Shape Effect of Ceria in Gold-Ceria Catalyst for Carbon Monoxide Oxidation Reaction
2.3. Results and Discussion
2.3.3. Catalytic Performance of Au/CeO 2 in CO Oxidation
Catalytic CO oxidation was carried out over Au/CeO2 catalysts deposited on a silica substrate in a batch-type gas reactor. The reactant gases, CO and O2, were introduced with partial pressures of 0.05 and 0.13 bar. We found that the CO oxidation activity of unsupported Au NPs was marginally small.
The Au NPs were activated on CeO2 nanocrystals, qualitatively indicating the critical role of Au-CeO2
interface formation. Figure 2.6a shows the TOFs of the Au/CeO2 cubes and Au/CeO2 octahedra for CO oxidation. The overall TOF of the Au/CeO2 cubes was higher than that of the Au/CeO2 octahedra in the temperature range of 473‒513 K. The TOF of the Au/CeO2 cubes was 0.69 s-1 at 513 K, which was 4 times higher than the Au/CeO2 octahedra (0.17 s-1). Arrhenius plots of the Au/CeO2 catalysts are shown in Figure 2.6. The activation energies, Ea, of the Au/CeO2 cubes and the Au/CeO2 octahedra were 26.87 kcal‧mol-1 (1.17 eV) and 31.88 kcal‧mol-1 (1.38 eV), respectively. Although these values were higher than the DFT-estimated Ea values, the trend was well conserved. The catalytic activity of CO oxidation was much higher over the Au/CeO2 cubes than the Au/CeO2 octahedra, indicating that CeO2 (100) facets provided greater facilitation of Au-catalyzed CO oxidation, compared with CeO2 (111) facets.
Figure 2.6. Turnover frequencies (TOFs) and the Arrhenius plots for Au/CeO2 catalysts. (a) TOFs of CO oxidation over Au/CeO2 catalysts with different-shaped CeO2: Au/CeO2 cubes (red) and Au/CeO2
octahedra (blue). The TOFs were calculated by dividing the turnover number (TON) by the reaction time. (b) Arrhenius plots obtained by TOFs of Au/CeO2 catalysts at different temperatures.
Because the surface oxygen of CeO2 participates in CO oxidation, the change in the oxidation state of Ce4+ to Ce3+ and the corresponding reduction behavior of different CeO2 surfaces are important for understanding the reaction mechanism and promoting effect of CeO2. To measure the oxygen release capacity of CeO2 nanocrystals and Au/CeO2 catalysts, we conducted the H2-TPR experiment. In detail, 40 mg of CeO2 or Au/CeO2 powder was pretreated at 353 K under Ar flow (50 cm3/min) for 30 min.
The 4% H2/N2 gas mixture was introduced as a reference flow at a rate of 50 cm3/min from room temperature to 1173 K (5 K/min). In Figure 2.7, the characteristic peaks of the CeO2 and Au/CeO2
catalysts are shown in the H2-TPR plot in the temperature range of 293‒1173 K. Two main peaks were observed in the CeO2 cubes and octahedra, with the highest peaks (985 K for cubes and 1000 K for octahedra) originating from the lattice oxygen ions of CeO2 and the peaks in the middle (749 K for cubes and 763 K for octahedra) from the surface oxygen ions.67-68 The observation of peaks for the CeO2
cubes at a temperature 15 K lower than that at which the CeO2 octahedra peaks appeared suggested that the oxygen ions of the CeO2 cubes were released more easily and utilized for CO oxidation. In both Au/CeO2 catalysts, additional peaks were observed at low temperatures (at 434 K for Au/CeO2 cubes and 467 K for Au/CeO2 octahedra) due to splitting of the surface oxygen peak. Because splitting was not observed in the single CeO2 nanocrystals, it was attributed to the Au-CeO2 interaction. In particular, the surface oxygen ions at the interface between Au and CeO2, interfacial oxygen ions, the chemical nature of which was most strongly affected by Au NPs, were responsible for the low-temperature peaks.
Similar observations have also been reported for the temperature shift of the surface oxygen peak of oxide supports upon NP deposition.69-70 Because Au NPs donate electrons to the CeO2 support upon deposition,28, 43 the electron-rich interfacial oxygen ions can be easily released and utilized for CO
oxidation. The interfacial oxygen peak of the Au/CeO2 cubes appeared at a temperature 33 K lower than that at which the peak of the Au/CeO2 octahedra appeared, demonstrating that the reduction occurred more easily in Au/CeO2 cubes.
Figure 2.7. H2-TPR of CeO2 nanocrystals with different shapes and corresponding Au/CeO2 catalysts.
The changes in oxidation states in the CeO2 and Au/CeO2 catalysts were also confirmed by XPS (Figures 2.8). While binding energy shifts were clearly observed in the Au 4f spectra for both Au/CeO2
catalysts in XPS after the CO reaction, the peak shift was much greater in the Au/CeO2 cubes than in the Au/CeO2 octahedra. In addition, the calculated Ce3+ concentrations (Ce3+ / Ce3+ + Ce4+) by peak deconvolution of the Ce 3d spectra were 26.1 and 23.9% for the Au/CeO2 cubes and the Au-CeO2
octahedra, respectively. The relative Ce3+ concentration was higher in the Au/CeO2 cubes than in the Au/CeO2 octahedra, indicating that the concentration of oxygen vacancies in the Au/CeO2 catalysts depended on the morphology of CeO2. Because the empty 4f orbitals of the Ce ions attract and localize extra electrons from Au NPs, the higher concentration of localized hot electrons at the interface of Au- CeO2(100) than at Au-CeO2(111) could increase its activity toward CO oxidation. Previous studies showed that the flow of hot electrons across the metal–oxide interface is correlated with the catalytic activity.71-72 Kim et al. reported that the flow of hot electrons generated on the interface of Au-CeO2
during light irradiation was responsible for the change in the CO oxidation activity and was dependent on the size of the Au NPs.72
Our experimental findings: the appearance of new H2-TPR peaks at low temperature, the relatively low H2-TPR peak temperature observed in the Au/CeO2 cubes compared with the Au/CeO2
octahedra, and the higher Ce3+/Ce4+ ratio in Au/CeO2 cubes, clearly confirm our theoretical interpretation of the correlation between the electronic interactions between Au and CeO2 and the Evac. We are convinced that the interface-mediated M-vK mechanism of CO oxidation occurs at the Au-CeO2
interface.
Figure 2.8. XPS results of Au 4f and Ce 3d of Au/CeO2 catalysts. Au 4f spectra for (a) cubes and (b) octahedra. (c) Ce 3d spectra.