Effects of different types of γ-Al2O3 on the activity of gold nanoparticles for CO oxidation at low-temperatures

Catalytic properties of Au nanoparticles supported on a one-dimensional gamma-Al2O3(1-D) nanofiber and commercial gamma-Al2O3 have been investigated during CO oxidation at low temperatures (298 similar to 373 K). The kinetic data showed that the activity of Au catalysts could be remarkably improved by using gamma-Al2O3 nanofibers as support compared to the commercial gamma-Al2O3. The measured turnover of frequency (TOF) and apparent activation energy (Ea) at room temperature were 0.25 s(-1) and 2.7 kJ/mol, respectively, for the Au/y-Al2O3-nanofibers catalyst (Au-AF), while those for the Au/gamma-Al2O3-commercial catalyst (Au-AC) were 0.07 s(-1) and 23.7 kJ/mol, respectively. To our knowledge, the Au-AF catalyst developed in this study is the most active alumina supporting Au catalysts ever reported. X-ray diffraction (XRD) results verified that the alumina nanofibers consist of solely gamma-type; meanwhile, a transition from gamma- to delta/theta-type was identified for the commercial alumina. The results of transmission electron spectroscopy (TEM) and XRD obtained from the fresh and spent catalysts revealed that the size distribution and morphology of Au particles were independent of the nature of alumina. Also, the X-ray photoelectron spectroscopy (XPS) spectra showed that the catalyst surface was predominant with metallic An species, which were slightly negatively charged because of interacting with gamma-Al2O3 matrix. However, in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) in the region of 3800-3000 cm(-1) showed that the OH groups bonded to both gamma-Al2O3 surfaces were quite different. In particular, the surface Al atoms of the nanofibers are supposed to be highly isolated; they probably favor the creation of small Au clusters, which have more step/edge sites at interface compared to that for the Au-AC considering the different kinetic behaviors for both catalysts. DRIFTS recorded from the oxidation and adsorption of CO further suggests that the OH groups linking to Au and alumina on the surface of Au-AF may involve the CO oxidation. It is confirmed that a tiny modification of the structure of gamma-Al2O3 matrix may have strong influence on the structure and activity of the Au nanoparticles.