Catalytic Activity of Titania-Supported Core–Shell Fe3O4@Au Nano-Catalysts for CO Oxidation

In continuation of our previous work (J. Phys. Chem. Lett. 2010, 1 (20), 3141 and J. Phys. Chem. C 2010, 114 (45), 19194), Fe3O4@Au core–shell types of nanoparticles were prepared by coating superparamagnetic iron oxide nanoparticles (SPIONS; ∼4.9 nm) with a thin layer of gold (∼0.5 nm) and supported on microporous TiO2. To remove the ligands attached to nanoparticles, the catalyst was treated at 200, 300, 400, and 500 °C in either (a) a reducing atmosphere with H2/Ar or (b) an oxidizing atmosphere with O2/He. The synthesized nanoparticles and freshly prepared catalysts were characterized by HRTEM, which revealed that the size of the Fe3O4@Au nanoparticles was 5.34 ± 0.71 nm and that of Fe3O4@Au/TiO2 was 5.96 ± 0.71 nm. Fresh and pretreated catalysts were further characterized by temperature programmed reduction, X-ray diffraction, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. To test the activity of Fe3O4@Au/TiO2 catalysts, CO oxidation was performed over catalysts from 30 to 500 °C. Results showed that the catalyst treated with H2/Ar at 500 °C had a lower light-off temperature and the highest CO conversion (∼68%) at 300 °C; however, such a treatment also resulted in catalyst sintering, leading to a net increase in particle size to 7.87 ± 1.59 nm. The higher catalytic activity of the catalyst treated with H2 at 500 °C can be attributed to the copresence of Fe0 and Au0 in the catalyst, the complete removal of organic ligands from the catalyst surface, and possibly the synergistic interaction between Au and Fe.