https://nova.newcastle.edu.au/vital/access/ /manager/Index en-au 5 https://nova.newcastle.edu.au/vital/access/ /manager/Repository/uon:29404 x/CeO₂ and Au/MO_x/CeO₂, where M = Mn, Fe, Co and Ni, using a range of CeO₂ surface areas. The catalytic ability of CeO₂ depended on the specific surface area, and the addition of gold always increased activity. Similarly, addition of MO_x to CeO₂ (M:Ce = 1:10) increased catalytic activity, and there was a synergic interaction between the MO_x and CeO₂ phases. For Au/MO_x/CeO₂ catalysts the presence of gold nanoparticles did not affect the initial reaction temperature or that for 100% conversion, or the apparent activation energy compared to MO_x/CeO₂ catalysts. The rate determining step in these reactions is suggested to be C—H bond activation. This was supported by TG/DTA studies under 10% H₂ in N₂ that showed there was no correlation between the catalysis results and the temperature of initial mass loss of lattice oxygen. Gold nanoparticles (5 wt%) were introduced by adsorption and subsequent thermolysis of preformed n-hexanethiolate-stabilized gold nanoparticles. STEM and XRD studies showed that the average size of the gold nanoparticles depended on the surface area. Introduction of gold nanoparticles by this method introduces a small amount of sulfur as adsorbed sulfate, but this did not have any major poisoning effect on isobutane oxidation. Gold 4f_7/2 XPS studies on Au/MO_x/CeO₂ showed that the only common gold species was Au(0), suggesting that higher oxidation states were not important in the oxidation, while Ce 3d_5/2 studies established the presence of Ce(III) in addition to Ce(IV), indicating their involvement in the Mars-van Krevelen mechanism, including possible participation in reoxidation of reduced MO_x.]]> Sat 24 Mar 2018 07:36:18 AEDT ]]> https://nova.newcastle.edu.au/vital/access/ /manager/Repository/uon:26613 nSAu) nanoparticles (n = 6, 12, 16) have been studied as the source of gold nanoparticles supported on the metal oxides NiO, TiO₂ (anatase and rutile), CeO₂, and γ-Al₂O₃ following adsorption from n-hexane and thermolysis in air at 340°C. Adsorption times increase with an increase in length of the carbon chain and vary with the metal oxide, particularly with specific surface area. Evidence of oxidation of some of the thiolate sulfur to give sulfoxide- and sulfone-type species has been established. TG/DTA studies and FT-IR (attenuated total reflectance) spectroscopy show that the adsorbed n-C_nSAu nanoparticles thermally decompose at about 210-340°C depending on the metal oxide, with some decomposition products, particularly those containing sulfur, adsorbing onto the metal oxide surface. Following thermolysis at 340°C, XPS and the FT-IR studies, combined with laser-ablation mass spectrometry, show that all organic material decomposition products are generally lost, and that the residual sulfur exists as sulfate at about 0.2 wt% or lower. TEM/STEM studies have shown that the n-C_nSAu nanoparticles, originally about 2 nm in diameter, produce gold nanoparticles with a range of 2-4 nm in size on the oxide surface following thermolysis at 340°C. The final average size of the gold nanoparticles depends on the metal oxide. For NiO, HRSTEM images shows little evidence of preferred orientation following immediate adsorption of n-C₆SAu nanoparticles, indicating weak interaction with the oxide surface, while a preferred orientation occurs on thermolysis at 340°C, indicating a much stronger interaction. The total oxidation of a representative alkane, isobutane, over TiO₂ (both anatase and rutile) and NiO, together with the addition of 5 wt% Au nanoparticles has been studied. Anatase and rutile are initially inactive but addition of the gold nanoparticles generates active oxidation catalysts, with anatase slightly more active than rutile. For NiO and 5 wt% Au/NiO reaction begins at 205-215°C and complete oxidation occurs by 430-440°C. The presence of the gold nanoparticles reduces the apparent activation energy from 89 to 51 kJ/mol. All active catalysts show formation of CO as well as CO₂ at about 20% conversion of isobutane, but at 100% oxidation the main product is almost exclusively CO₂ (>99.0%). The presence of the sulfate from the decomposition of the n-C_nS⁻ ligands has minimal apparent poisoning effect on the oxidation of isobutane for anatase, rutile, or NiO.]]> Sat 24 Mar 2018 07:34:00 AEDT ]]> https://nova.newcastle.edu.au/vital/access/ /manager/Repository/uon:29715 β-MnO₂ > α-Fe₂O₃, with the position of NiO dependent on the Ni³⁺ content which changes with temperature. Preformed n-hexanethiolate-stabilized gold nanoparticles, following adsorption and thermolysis in air, introduce a small amount of sulfur as adsorbed sulfate. The sulfate appears to block the reoxidation step in the Mars-van Krevelen mechanism. This can have a significant effect on catalytic activity, as observed for β-MnO₂. TEM/STEM studies indicate that gold nanoparticles of 2–4 nm in diameter form, which depends on the identity of the metal oxide and its specific surface area. Gold nanoparticle size effects have been studied on NiO, and show that the apparent activation energy and temperature of initial reaction depend on nanoparticle size. Comparisons of the multicomponent Au/MO_x/γ-Al₂O₃ (M:Al = 1:10) catalysts, where M = Mn, Fe, Co, Ni, have also been studied, and all are more active catalysts than Au/γ-Al₂O₃, but less active than the unsupported catalysts. Gold 4f_7/2 XPS studies on Au/MO_x and Au/MO_x/γ-Al₂O₃ have shown that the only common species present is Au(0), suggesting that higher oxidation states of Au are not important in oxidation catalysis.]]> Sat 24 Mar 2018 07:33:24 AEDT ]]>