3. Combinational Effect of Particle Size and Support Morphology at the Platinum-Ceria
3.2. Experimental Methods
3.2.1 Synthesis of CeO2 Nanocrystals and Pt/CeO2 Catalysts
CeO2 cubes and octahedra were synthesized using the hydrothermal method. For CeO2 cubes, sodium hydroxide (9.6 g) was dissolved in deionized water (40 mL) and cerium nitrate hexahydrate (0.868 g) was added. The resulting solution was placed into a Teflon-lined reactor and kept in an oven at 180 °C for 24 h. For CeO2 octahedra, sodium phosphate tribasic dodecahydrate (0.004 g) was dissolved in deionized water and cerium nitrate hexahydrate (0.434 g) was added. The resulting solution was placed into the Teflon-lined reactor and kept in the oven at 200 °C for 20 h. The precipitate was collected by centrifugation with deionized water and ethyl alcohol. The obtained CeO2 NCs were dried at 80 °C for 8 h and calcined at 400 °C for 4 h.
Pt/CeO2 catalysts with different Pt size were prepared by the microwave-assisted reduction method. For detail, CeO2 NCs (40 mg) were dissolved in ethylene glycol (40 mL), and the resulting solution was sonicated for 30 min. By varying the amounts of chloroplatinic acid hydrate solution in the mixture, 1, 2, and 3 nm size of Pt NPs loaded on CeO2 NCs were obtained (0.05, 0.2, and 0.5%, respectively). Finally, the mixture was transferred to the Teflon-vessel and placed in the microwave reactor (MARS, CEM Corporation). The reaction was monitored during the microwave-assisted reaction at 800 W of the power (2.45 GHz) for 30 s. The resulting catalysts were collected by centrifugation and washed with ethanol. The obtained Pt/CeO2 catalysts were dried at 80 °C for 8 h.
3.2.2 Characterization Methods
Transmission electron microscopy (TEM) and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) analyses were conducted using a JEM-2100F (JEOL) instrument with acceleration voltages of 200 kV. Scanning electron microscope (SEM) images were obtained by using S-4800 model (Hitachi High Technologies) at an acceleration voltage of 5 kV.
Inductively coupled plasma-optical emission spectrometry (ICP-OES) using a 700-ES model instrument (Varian) was used to determine the Pt concentration of the catalysts. The concentration of Pt was calibrated using a standard Pt solution (Sigma-Aldrich). X-ray diffraction (XRD) patterns were obtained by a powder X-ray diffractometer PANalytical X’Pert Pro (Philips) operating at 40 kV and 30 mA. N2 adsorption measurement was performed using a BELSORP-max model to estimate the surface area. CO chemisorption was measured by pulse injection using a BELCAT Ⅱ instrument. CO (5%) with He (50 mL min−1) flowed with the pulse size (0.4 mL) and the time interval (4 min). Pt dispersion was determined by assuming the stoichiometry of CO adsorption on the Pt atom to be 1:1. X-ray photoelectron spectroscopy (XPS) measurement for Pt 4f was carried out on beamline 8A2 of the Pohang Accelerator Laboratory (PAL). The system was manufactured by SPECS GmbH, consisting of a PHOIBOS NAP 150 hemispherical analyzer. The spectrum was obtained under UHV with photon energy of 265 eV (Cr X-ray source). The binding energies of obtained Pt 4f were calibrated based on the position of Ce4+ component in Ce 4d spectra (~ 122 eV). All spectra were presented without further manipulations. The diffuse reflectance infrared spectroscopy (DRIFTS) experiments performed on a Nicolet iS10 FTIR spectrometer equipped with a mercury cadmium-telluirde (MCT) detector. 20 mg of the Pt/CeO2 catalyst was loaded into the DRIFTS cell and pretreated at 100 °C under He flow for 30 min (50 mL min−1). The FT-IR spectra were obtained using a KBr background. CO adsorption was carried out under a 1% CO/He flow (50 mL min−1) at 25 °C. The spectrum was collected after He purging to remove the gaseous CO. All spectra were obtained by averaging 128 scans at a resolution of 4 cm−1.
3.2.3 Catalytic Tests
CO oxidation reaction was carried out in a quartz fixed-bed reactor. Typically, 20 mg of catalyst mixed with 0.5 g of quartz sand was loaded into reactor. Prior to the measurements, the catalyst was pretreated at 100 °C in a Ar flow for 1 h (50 mL min−1). Catalytic CO oxidation was performed with a feed stream of 2% CO and 2% O2, balanced with Ar. The total flow rate was 50 mLmin−1 and the heating rate was 5 °Cmin−1. The temperature was stabilized by holding it at the target temperature for 10 min, and it was measured three times at each point. The products were detected by a gas chromatography (GC) equipped with a Carboxen 1000 column (supelco) and a thermal conductivity detector (TCD).
CO-temperature programmed reaction (CO-TPR) tests to verify the operation of the Mars-van Krevelen
mechanism the Pt/CeO2 catalysts were performed in a fixed-bed quartz flow microreactor. The CO2
concentration was monitored in real time with a quadrupole mass spectrometer (QMS, PFEIFFER Vacuum GSD320) connected to the reactor outlet. Catalysts were pretreated at 100 °C for 1 h under Ar.
The 100 mg of catalyst mixed with 200 mg of quartz sand and loaded on the quartz wool. The reaction gas consisted of 1% CO and 99% Ar and was fed at 100 mL∙min−1, corresponding to a weight hourly space velocity (WHSV) of 60,000 mL∙g−1∙h−1. A ramping rate of 1 °C∙min−1 applied from 50 °C to 300 °C and the catalyst was maintained at 300 °C for 4 h.
3.2.4 Density Functional Theory Calculations
The interfaces between 1 and 3 nm size of Pt NPs and CeO2 NCs are constructed by a two-layered FCC-type Pt9 NPs and a one-dimensional Pt56 rod horizontally supported on CeO2(100) and CeO2(111) slab models respectively. In our previous study, we modeled a reliable crystalline Au9 NPs supported on CeO2(100) and CeO2(111) for complete understanding of CO oxidation catalyzed by Au-CeO2
interface.24,33 We also reported that the CO oxidation mechanism of an appropriately constructed Au9/CeO2 catalyst via theoretical design were in close agreement with the experimental catalytic reaction under various reaction environments by controlling the reaction temperature and partial pressure of CO and O2. In this study, because the FCC-like nanoparticles supported on CeO2 can adequately describe the metal-oxide interface, the models of the thermodynamically stable structural isomers of Pt9 NPs34 supported on defect-free CeO2(100) 3 3 2 and CeO2(111) 5 5 2 slab were used to reproduce the 1nm sized Pt NPs supported on a shape-controlled CeO2 cube and octahedral.
To interpret mechanism of the active catalyst at metal-support interface on a practical metal NP scale, the interface of the metal-support have accurately modeled in several previous reports.35-37 Kumar et al. and Duan et al. reported the morphology of the Au rod and TiO2 interfacial boundary in order to identify the specific catalytic activity at the interface.35,36 Mehta et al. presented a systematic analysis of metal rod and MgO boundary to understand the adsorption behavior of catalytic intermediates.37 We constructed a morphology of the interface consisting of 3 nm sized Pt NPs and CeO2 cube and octahedral using three layers Pt56 rod and CeO2(100) 5 6 2 and CeO2(111) 6 6 2 supercell. We did not consider oxygen defects of CeO2 surface for a consistent comparison of catalytic activity by the interface of each Pt rod supported on CeO2(100) and CeO2(111) catalysts. The bottom layer of CeO2
were fixed during all geometric optimization.
We performed spin-polarized DFT calculations with the Vienna ab-initio simulation package (VASP)38 and the PW91functional.39 To treat the highly localized Ce 4f-orbital, DFT+U40 with Ueff = 4.5 eV was applied.24,41,42 The interaction between the ionic cores and the valence electrons was described by the projector-augmented wave method.43 Valence electron functions were extended with the plane-wave basis to an energy cutoff of 400 eV. The Brillouin zone was sampled at the -point. The convergence criteria for the electronic structure and the geometry were set to 10−4 eV and 0.01 eV/A,
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.