Au L3-Edge XANES Analyses for spr-Au/Al2O3 Catalyst
Nobuyuki Ichikuni, Syougo Shimazu, Takashi Fujikawa, Takayoshi Uematsu,
Au L3-edge XANES analyses for spr-Au/Al2O3 catalyst
S. Nagamatsu(1), N. Ichikuni(2), S. Shimazu(2), T. Fujikawa(3) and T. Uematsu(2)
Center for Frontier Electronics and Photonics, Chiba University(1)
Faculty of Engineering, Chiba University(2)
Graduate School for Science, Chiba University(3)
Catalytic activity of gold has become widely recognized for various reactions. [1-3] Catalytic properties depend on the particle size of gold and interactions between the gold and the metal oxide support. Therefore, various methods for the preparation of supported gold catalysts have been developed (impregnation, co-precipitation, sol-gel, deposition-precipitation, etc.).
For long years, gold was known as inert material, although the gold nano-particle dispersed on metal oxide supports exhibit remarkable catalytic activities. Generally, catalytic active gold particle is found over about 2-3 nm of diameter. The gold catalysts prepared from suspended spray reaction method (spr) provides gold particle with about 20 nm of diameter. However, the spr catalyst (spr-Au/Al2O3) shows catalytic activity for NO-CO reaction.
Taking the advantage of spray technique, we have developed a new type of preparation method for multi-component composite system and investigated various applications. We have already applied spray reaction technique to supported metal catalysts preparation (Ni/ZrO2, Ru/Al2O3 and Ru/TiO2) [4-6]. In this study, we have applied this technique to prepare the supported gold catalyst, and found catalytic activity in the multi-component composite with gold. In previous work, two kinds of supported gold catalysts, Au/TiO2 and Au/Al2O3 were prepared and investigated CO oxidation over Au/TiO2  and for NO-CO reaction over spr-Au/Al2O3 , respectively.
In this paper, we discuss the local structures of spr-Au/Al2O3 catalysts on the study of Au L3-edge X-ray absorption near edge structure (XANES) spectra. In order to investigate the local structures of Au-sites, multiple scattering approaches to XANES spectra are used. Theoretical calculations allowed us to obtain XANES spectra for each sites.
First, we prepared spr-Au/Al2O3 and imp-Au/Al2O3 and these catalysts treated four different pretreatments (H2-reduction, O2-oxidition, H2-reduction + O2-oxidation, and O2-oxidation + H2-reduction). NO-CO reaction results are strongly depended on these pretreatment methods. O2 pretreated and O2+H2 pretreated samples showed remarkable catalytic activity. These results suggest that Au oxide or hydroxide play an important role in this reaction. Gold particle size is determined by XRD analyses, and revealed that about 20 nm of diameter in spr-Au/Al2O3 and about 30 nm in imp-Au/Al2O3.
XANES spectra measured at KEK PF BL-10B in transmission mode at room temperature (Proposal No. 2001G321). The gold hydroxide (Au(OH)3) and the gold oxide (Au2O3) gave similar XANES spectra. Both of them have large whiteline structures. Whiteline structure indicate resonance like 2p3/2-5d transition and existence of Au 5d-hole state. For Au L3-edge XANES spectrum, there is no whiteline peak and observed only shoulder-like structure in the energy range of whiteline. In more high energy region, the shoulder-like structure (11935 eV) and one peak (11945 eV) were observed. Latter structures were not observed in oxide and hydroxide. XANES spectra for spr-Au/Al2O3 and imp-Au/Al2O3 had whiteline and peak structure at 11945 eV. Peak structure at 11945 eV was smaller than that for the bulk state (gold foil).
In order to discuss XANES spectra, we classified the supported gold particle into 4 sites (bulk, surface, interface, and peripheral) and calculate XANES spectra by using multiple scattering theory . Theoretical calculations allowed us to separate these contributions. However, the XANES spectra affected not only these 4 sites, but also gold hydroxide and oxide.
The gold particles in spr-Au/Al2O3 were about 20 nm. The gold particles are fcc structure . From the preparation condition, gamma-alumina structure is expected. Therefore, the structure model was constructed fcc-gold and gamma-alumina. For simplicity, landing model was used. Electronic structure was obtained by real space multiple scattering technique  and DV-Xalpha method . For bulk state, our results are consist tight-binding results . First, we calculate Au L3-edge XANES spectrum for fcc-gold and obtained good agreement with observed spectrum. The peak height at 11945 eV is calculated in the order bulk > interface > surface and peripheral. Of course, also the surface effect causes this change.
Experimental result can explain linear combination of these 4 atomic site + Au oxide and hydroxide. In this case, gold particle was about 20 nm, and almost all gold site (>90%) was bulk site. Therefore, it is appropriate decision to approximate bulk + Au oxide and hydroxide. We can expect that Au oxide distributed in gold-alumina interface area. Interface site is less than 10% of gold particle. However, spr-Au/Al2O3 experimental result requires about 30% of Au oxide or hydroxide. Therefore, we can expect that many gold nano-particles are present in the bulk of alumina and few atomic layers of interface are changed gold oxide phase. Electronic structure calculations show that 2-3 layers of Au particle from surface or interface are different from bulk state.
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Au/Al2O3 catalyst, Au L3-edge XANES, Gold-support interaction, multiple scattering theory