Gold nanoparticles on metal oxide surfaces derived from n-alkanethiolate-stabilized gold nanoparticles; investigations of the adsorption mechanism and sulfate formation during subsequent thermolysis

Abstract

n-Alkanethiolate-stabilized gold (n-CnSAu) 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-CnSAu 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 0.2wt% or lower. TEM/STEM studies have shown that the n-CnSAu nanoparticles, originally about 2nm in diameter, produce gold nanoparticles with a range of 2–4nm 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 5wt% 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 5wt% 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 51kJ/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-CnS− ligands has minimal apparent poisoning effect on the oxidation of isobutane for anatase, rutile, or NiO.