3. Combinational Effect of Particle Size and Support Morphology at the Platinum-Ceria
3.3. Results and Discussion
3.3.1. Structure Identification of Pt/CeO 2 Catalysts …
respectively. We used a Gaussian smearing function with a finite temperature width of 0.05 eV to improve 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.44,45 A Bader charge analysis was performed to determine the local charge of atoms in the system.46,47
Figure 3.1. HAADF-STEM images, TEM images, and the Pt particle size distribution histograms (inset) of the Pt/CeO2 catalysts. (a) PCC-1, (b) PCC-2, (c) PCC-3, (d) PCO-1, (e) PCO-2, and (f) PCO-3.
uniform sized Pt NPs on two different CeO2 supports of cubes and octahedra, are shown in Figure 3.2.
Additional low magnification STEM and TEM images confirm the uniformity of the Pt NPs and the average particle sizes are determined by particle size histogram in Figure 3.1. Figure 3.2 demonstrates the finely controlled the size of Pt NPs and their homogeneous deposition on the CeO2 which was achieved by the microwave-assisted reduction environment. A schematic illustration of the Pt/CeO2
model shows the changes in the amount of interface-located and surface Pt atoms by increasing the particle size of Pt NPs. Regardless of the type of the CeO2 shape, the Pt size was tuned to be comparable for the effect of both Pt size and the CeO2 shape. The d-spacing determined by HRTEM is 2.7 and 3.1 Å for CeO2 cubes and octahedra, consistent with the (100) and (111) orientation, respectively (Figures 3.2c and 3.2f).
Figure 3.2. HAADF-STEM images of Pt NPs on CeO2 cubes. (a) PCC-1; (b) PCC-2, (c) PCC-3. Pt NPs on CeO2 octahedra. (d) PCO-1; (e) PCO-2; (f) PCO-3. The red circle denotes each Pt NPs. The scale bar represents 2 nm for all images.
The size control of Pt on CeO2 was also confirmed by in situ DRIFT spectra. As shown in Figures 3.3a and 3.3b, DRIFT spectra were collected after CO chemisorption on Pt/CeO2 catalysts. Obviously, as the size of the Pt NPs increases, the amount of adsorbed CO gradually increases. According to the
similar size of the controlled Pt NPs between the CeO2 cube and the octahedron, the NP size uniformity is confirmed and the CeO2 support effect can be directly compared. Also, regardless of the Pt particle size and support type, the shape of the peaks that appeared was always constant. The CO-Ptn+ peak was not observed because the Pt cations exist as a very stable square planar structure, making it difficult to be observed through DRIFTS.50 The multiple shoulders of the observed major peaks represent the linearly attached CO to various Pt metal sites, and specifically, the peak of 2063 cm−1 (in 3 nm Pt NPs) represents the linear CO attached to the terrace sites of the Pt metal.51 According to the CO-Pt0 peak (2063 cm−1), the peak is blue-shifted as the Pt particle size increases. This is because the proportion of CO adsorbed sites changes as the Pt particle size varies, and this trend was same for both CeO2 cubes and octahedra. The 2057 cm−1(3 nm Pt NPs) represents the CO adsorption on the step site of Pt metal.52 As the Pt particle size increases, terrace site (2063 cm−1) becomes dominant in Pt metal, resulting in a blue-shift due to the change in the dipole-dipole coupling between CO molecules.53,54
Figure 3.3. Structural information of the Pt/CeO2 catalysts. In situ DRIFT spectra after CO adsorption.
(a) Pt/CeO2-cube; (b) Pt/CeO2-octa. XPS results of Pt 4f region (c) Pt/CeO2-cube; (d) Pt/CeO2-octa.
The oxidation state of Pt NPs according to the size was investigated through XPS analysis (Figures 3.3c and 3.3d). When the Pt 4f spectra were corrected based on the Ce 4d spectra, a peak shift by Pt NPs size was observed. Based on Pt 4f , Pt metal has a value of 71.0 eV, which is constant in 3
nm Pt NPs (PCC-3 and PCO-3).However, as the Pt particle size decreases, the position of Pt 4f7/2 is moved forward (71.3 for PCC-2, 71.2 for PCO-2, 71.5 for PCC-1, and 71.6 eV for PCO-1). The spin- orbit component between Pt 4f5/2 and Pt 4f7/2 remains constant as 3.35 eV. Consequently, small Pt NPs (1 nm or 2 nm) have a slightly oxidized form which is conclusive evidence for the precise control of the Pt size. The oxide phase on the surface of metallic Pt NPs can exist, as the size of metallic Pt NPs become smaller to the cluster level.55,56 It was reported that the typical shift was dominated by the final state Coulomb charge in the photoemission process where particle size decreased.57,58 However, given that the change is rather constant in the case of cubes and octahedra, this is due to the difference in the Pt NPs size and can rule out the influence of the support on the synthesis behavior of the catalyst. Based on these results, it was proven that Pt NPs with a size of 1, 2, and 3 nm were uniformly controlled and deposited on two types of CeO2 NCs (cubes and octahedra) without a significant deviation caused by the morphology of the support. As the size of Pt NPs increases, the ratio of the atom located at the interface with CeO2 decreases. As a result, the MSI is dependent on the Pt particle size by changed ratio of Pt atoms.