K. Baán

Stability and Temperature-Induced Agglomeration of Rh Nanoparticles Supported by CeO2

The effects of reduction by H2 and by heat treatment in vacuum and in O2 flow on Rh particle size changes of Rh/CeO2 samples were studied by X-ray photoelectron spectroscopy (XPS), high-resolution electron microscopy (HRTEM), and CO adsorption followed by diffuse reflectance infrared spectroscopy (DRIFTS). Low-temperature (373-423 K) reduction of Rh without agglomeration is demonstrated. An average particle size of 2.3 ± 1.1 nm was measured by HRTEM regardless of the metal loading (1-5%). On Rh/CeO2, a significant particle size increase of the Rh particles was detected on heating (773 K). In this work, we suggest that the temperature-induced surface decrease resulting from the sintering of Rh is favored only for well-dispersed particles. XP spectra revealed that the mobile oxygens of CeO2 fundamentally determine the oxidation state of the supported metals. At elevated temperature, the oxidation of the reduced support surface as well as the metal component takes place because of the segregation of ceria oxygens. When the aggregated particles were reoxidized, the redispersion of Rh was observed probably because of the formation of Rh-O-Ce bonds.

Hydrogen evolution in the photocatalytic reaction between methane and water in the presence of CO2 on titanate and titania supported Rh and Au catalysts

The photocatalytic transformation of methane-water mixture over Rh and Au catalysts supported on protonated (H-form) titanate nanotube (TNT) was investigated. The role of the catalyst structure was analyzed using titania reference support. Furthermore the effect of carbon-dioxide addition was also investigated. The catalysts were characterized by high resolution transmission electron microscopy and X-ray photoelectron spectroscopy (XPS). Photocatalytic tests were performed with a mercury-arc UV source illuminating a continuous flow quartz reactor which was attached to a mass spectrometer. The surface of the catalysts was analyzed by diffuse reflectance infrared spectroscopy during the photoreactions. The changes of the catalysts due to photocatalytic usage were investigated by XPS and temperature programmed reduction methods as well. Most of the methane was generally transformed to hydrogen and ethane, and a small amount of methanol was also formed. The carbon dioxide addition enhanced the rate of the photocatalytic transformation of methane on Rh/TNT with increasing the lifetime of the electron–hole pairs. Bigger gold particles with mainly plasmonic character were more active in the reactions due to the photo induced activation of the adsorbed water. Surface carbon deposits were identified on the catalysts after the photoreactions. More oxidized carbon formed on the Au-containing catalysts than on the ones with Rh.Graphical Abstract

Gold Size Effect in the Thermal-Induced Reaction of CO2 and H2 on Titania- and Titanate Nanotube-Supported Gold Catalysts

In this paper, we study the thermal activation of CO₂ on the surface of small Au nanoparticles supported on TiO₂ and titanate nanotube. We characterize the catalysts with high resolution transmission electron microscopy (HR-TEM) and total gold content measurement. We performed catalytic test in flow reactors then we investigate the surface of the catalysts during the adsorption and reaction processes by diffuse reflectance infrared spectroscopy (DRIFTS). The size of gold nanoparticles on the surface has been found to have the most important effect on the final activity of the studied catalysts. Significantly higher TOF values were obtained when the size of Au were smaller on both TiO₂ and titanate nanotube supports. The size of the Au nanoparticles with the method of their preparation was controlled. The gold adatom promotes the adsorption and scission of CO₂, but the nature of the support has got important effect, too. The explored reaction schemes may pave the way towards novel catalytic materials that can solve challenges associated with the activation of CO₂ and thus contribute to a greener chemistry related to it.