Nanoparticle effects on partial and complete oxidation of isobutane over metal oxide catalysts

Abstract

Research Doctorate - Doctor of Philosophy (PhD)Gold nanoparticles, derived from preformed n-alkanethiolate-stabilised gold nanoparticles, have been supported on metal oxide catalysts and pyrovanadates for the complete oxidation of isobutane and the oxidative dehydrogenation of isobutane to isobutene, respectively. The initial adsorption of gold nanoparticles on metal oxides depends on the length of the alkyl carbon chain and the surface area of the metal oxide, and also forms alkyl-sulfoxide and -sulfone species. On thermolysis (340°C) to decompose the organic alkylthiolate ligands, residual adsorbed sulfate (0.2-0.3 wt%) remains on the catalyst surface. TEM/STEM studies have shown that the sizes of gold nanoparticles, originally 1.8-2.0 nm, increase in diameter following adsorption and thermolysis. This depends on the surface area of the support. <i>n</i>-Hexanethiolate-stabilised gold nanoparticles were used to obtain 5 wt% Au nanoparticles supported on β-MnO₂, α-Fe₂O₃, Co₃O₄ and NiO. The oxides, with and without gold nanoparticles, were investigated for the complete oxidation of isobutane. Some effect from residual sulfate was detected, although this depended on the metal oxide. TEM/STEM and XRD studies were used to monitor the sizes of the gold nanoparticles, which were 2-4 nm in size. Analogous studies, with and without gold nanoparticles, on supported metal oxides of the type MO_x/γ-Al₂O₃ and MO_x/CeO₂ (M = Mn, Fe, Co, Ni), were studied for the complete oxidation of isobutane. Gold 4f XPS studies showed that it was likely that gold nanoparticles were the key to the oxidation catalysis and not any higher oxidation states of gold. The transition metal pyrovanadates, M₂V₂O₇ (M^II = Mn, Co, Ni, Cu and Zn), again with and without gold nanoparticles (5 wt%) have been investigated for the oxidative dehydrogenation of isobutane to isobutene from 300-450°C under reducing conditions (isobutane:O₂ = 20:1). Pyrovanadate reduction occurred to produce V(IV) and/or V(III) products that depended on the identity of M(II) and the presence of gold nanoparticles. The conversions reached as high as 11% and 16% in the absence and presence of gold nanoparticles, and the selectivities to isobutene were as high as 40-50%. Gold 4f XPS studies showed that the highest yields of isobutene correlated with a lower Au(I) content. Co₂VO₄ and ZnV₂O₄ were also found to be active catalysts for this reaction