Use of Au@Void@TiO2 yolk-shell nanostructures to probe the influence of oxide crystallinity on catalytic activity for low-temperature oxidations

Abstract

A series of Au@Void@TiO2 yolk-shell structures were synthesized and characterized, and their catalytic activity was tested for the oxidation of carbon monoxide. The target of this work was to evaluate the effects of (1) the crystallinity of the TiO2 shell and (2) the presence of titanate phases. Transmission electron microscopy and X-ray diffraction data show that increasing calcination temperatures, from 850 to 1250 K, leads to the formation of larger titania crystallites. These crystallites are typically in the form of anatase, but some rutile is also made at high temperatures. The general yolk-shell nanostructures retain their basic characteristics upon calcination and upon treatment with HCl (used to remove the titanate phases), but a few structural and chemical changes do take place: (1) the void-space diameter is reduced by approximately 10% when going from TCalc = 850 K to TCalc = 1250 K, a change that is accompanied by a decrease in the surface area (estimated from N2 adsorption-desorption isotherms); (2) the shell thickness remains unaffected by either calcination or HCl treatment; (3) the pore volume also remains approximately constant with increasing calcination temperature in the catalysts free of titanates but diminishes significantly in the samples with titanate phases; (4) the mesoporosity is minimal in all yolk-shell nanostructures but more noticeable with the pure TiO2 shells; (5) the titanates have high Na content (measured by X-ray photoelectron spectroscopy), but that Na is fully removed upon treatment with HCl; (6) no Ti3+ was detectable in any of the samples, but silica, together with C and (minor amounts of) N, was present in all; (7) all Au is in metallic form; (8) two temperature regimes were observed for the catalytic oxidation of CO, as reported for other similar samples in the past, an Arrhenius regime around or above room temperature, and a cryogenic range going to temperatures as low as 120 K; (9) the latter regime is only seen with the samples containing titanate phases; (10) the titanates seem to also aid in the oxidation at higher temperatures; and (11) in general, increasing titania crystallinity leads to a decrease the catalytic activity.