A comparison study of the structural thermostability of SiO 2 supported Au@Pt and Au@Pd core-shell nanoparticles

A comparison study of the structural thermostability of SiO2 supported Au@Pt and Au@Pd core-shell nanoparticles Xiaohui Zhang, Qiaoqiao Guan, Mei Sun, Junling Lu School of Chemistry and Materials Science, Key Laboratory of Surface and Interface Chemistry and Energy Catalysis of Anhui Higher Education Institutes, University of Science and Technology of China, Hefei, Anhui, 230026, China Hefei National Research center for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China

Synthesis of SiO2 support. SiO2 spheres were prepared by the Stöber method 1 .
Typically, 17.8 mL tetraethyl orthosilicate (TEOS), 5.4 mL NH3· H2O, 358 mL C2H5OH and 30 mL H2O were added into a 500 mL beaker, then the solution mixtures were vigorously stirred at 25 °C for 24 h. After that, the colloid was centrifuged and washed by ethanol for several times and dried at 70 °C overnight. Finally, the obtained material was calcined at 700 °C under 10% O2/Ar at a flow rate of 40 mL· min -1 for 5 h in a tube furnace to get the sphere SiO2 support.
Synthesis of Au/SiO2 catalyst. An Au/SiO2 catalyst was prepared using the deposition-precipitation (DP) method 2 . Here 2 mL HAuCl4 aqueous solution (0.0485 M), 1.0 g spherical SiO2 and 150 mL deionized water were co-added into a three-necked bottle and mixed for 30 min under vigorous stirring at 65 °C, and ammonia was used to adjust the pH value between 9 and 10. Then, the system was continued vigorously stirred for another 12 h. The suspension was then centrifuged and washed with deionized water for several times and dried at 80 °C overnight. Finally, the resulting material was calcined at 300 °C in 10% O2/Ar at a flow rate of 40 mL· min -1 for 2 h to obtain the Au/SiO2 catalyst. continuous flow of Ar, a background spectrum was collected. Subsequently, the sample was exposed to 10% CO/Ar at a flow rate of 25 mL/min for about 0.5 h until saturation.
Next, the sample was purged with Ar at a flow rate of 25 mL/min for another 0.5 h to remove the gas-phase CO, and then the DRIFT spectrum was collected with 256 scans at a resolution of 4 cm −1 .

Metal dispersion measurements. The Pd or Pt dispersions of Au@Pd/SiO2 and
Au@Pt/SiO2 bimetallic catalysts were determined by CO pulse chemisorption, which were conducted on a Micromeritics AutoChem II chemisorption instrument. After loading a sample, the catalyst was first calcined in 10% O2/He at 150 °C and then reduced in 10% H2/He at different temperatures (150, 250, 350, 450 and 550 °C) for 1 h, respectively. Then, the catalysts were cooled to room temperature in He, and CO pulses were introduced to the catalyst surface using 10% CO/He until saturation. The 5 amount of chemisorbed CO was calibrated by using a thermal conductivity detector.
For Pd dispersion calculations, a stoichiometry of CO : Pd of 1 : 1 was assumed for Au@Pd/SiO2 catalyst reduced under 550 °C due to the high ratios of linear to bridgebonded CO in the DRIFTS CO spectra, while a stoichiometry of CO : Pd of 1 : 1.5 was applied to Au@Pd/SiO2 catalyst reduced at other temperatures, according to literature [3][4][5] . For Pt dispersion calculations, a stoichiometry of CO : Pt of 1 : 1 was assumed for Au@Pt/SiO2 catalyst with different reduction temperatures 6