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Supported gold, rhodium and bimetallic rhodium-core-gold-shell catalysts were prepared. The supports were TiO 2 as well as titanate nanotube and nanowire formed in the hydrothermal conversion of titania. The catalytic properties were tested in the CO 2 hydrogenation at 493 K. The amount and the reactivity of the surface carbonaceous deposit were determined by temperatureprogrammed reduction. The surfaces of the materials were characterized by X-ray photoelectron and low-energy ion scattering spectroscopy (LEIS). The surface forms during the catalytic reaction were identified by DRIFT spectroscopy. On the XP spectra of bimetallic catalysts the existence of highly dispersed gold particles could be observed besides the metallic form on all supports. Small Rh particles could also be identified on the titanate supports. LEIS spectra demonstrated that Rh-core-Au-shell particles formed, since no scattering from Rh was detected. The main product of CO 2 hydrogenation was CH 4 on all catalysts. IR spectra revealed the existence of CO and formate species on the surface. In addition, a new band was observed around 1,770 cm-1 which was assigned as tilted CO. It is bonded to Rh and interacts with a nearby the oxygen vacancy of the support. Agglomeration of highly dispersed Rh was observed on bimetallic samples induced by reaction or reactant. Keywords CO 2 hydrogenation Á Titania nanowire Á Titania nanotube Á Core-shell structure Á Gold Á Rhodium

a b s t r a c tAu and Rh clusters, as well as Au-Rh bimetallic nanoparticles were prepared on titanate nanowires, nanotubes and on TiO 2 (1 1 0). They were characterized by X-ray photoelectron spectroscopy (XPS), low energy ion scattering spectroscopy (LEIS) and Fourier transform infrared spectroscopy (FTIR). By performing careful LEIS experiments, it was found that for appropriate Au and Rh coverage, a thin Au layer almost completely covers the Rh nanoparticles, a Rh core-Au shell structure was detected. The formation of this structure was not affected by alkali (K) adatoms. LEIS and FTIR measurements disclosed that adsorbed CO at 300 K causes the segregation of Rh atoms to the surface of metal clusters in order to bind to CO. Upon CO adsorption on Rh/titanate nanostructures the IR stretching frequencies characteristic of the twin form were dominant, whereas bimetallic nanosystems featured a pronounced linear stretching vibration as well. In spite of this structure adsorbed CO is detectable during the ethanol adsorption on gold-rhodium bimetallic cluster and the ethanol decomposition rate is twice higher than on Au/TiO 2 .

Keywords:Partial oxidation of ethanol Rh/Al2O3 catalyst Rh/SiO2 catalyst Rh/TiO2 catalyst Rh/CeO2 catalyst a b s t r a c t In this work, the effect of the nature of oxide support on the reaction mechanism of the partial oxidation of ethanol on Rh catalyst was studied by diffuse reflectance infrared (DRIFTS) and X-ray photoelectron spectroscopy (XPS). The conversion of ethanol and the product distribution were analysed by gas chromatography. The highest ethanol conversion was measured on Rh/CeO 2 catalyst. The results showed that the nature of the oxide support affected the reaction mechanism. On supported Rh catalysts ethanol adsorption gives rise to ethoxide species, which can be decomposed to CO 2 , CO and CH 4 and dehydrogenated, forming acetaldehyde. The latter species are oxidized to acetate or dehydrogenated to acetyl species on Rh/Al 2 O 3 and on Rh/CeO 2 . On Rh/TiO 2 the acetaldehyde can also be oxidized in a parallel process to HCOOH/HCOO (a) which forms CO 2 and water. Furthermore, the acetate species previously formed can be decomposed to CH 4 , CO and/or oxidized to CO 2 via carbonate species at higher temperature depending on the oxide support. On silica support acetaldehyde was the dominant intermediate and it desorbed or directly decomposed with or without oxygen to CO 2 and methane. The partial oxidation of ethanol proceeds on partially oxidized Rh sites on Rh/Al 2 O 3 , Rh/TiO 2 and on Rh/CeO 2 . In spite of the presence of O 2 , ceria is not fully oxidized during the partial oxidation of ethanol.