Environment-mediated structure, surface redox activity and reactivity of ceria nanoparticles

Nanomaterials, with potential application as bio-medicinal agents, exploit the chemical properties of a solid, with the ability to be transported (like a molecule) to a variety of bodily compartments. However, the chemical environment can change significantly the structure and hence properties of a nanomaterial. Accordingly, its surface reactivity is critically dependent upon the nature of the (biological) environment in which it resides. Here, we use Molecular Dynamics (MD) simulation, Density Functional Theory (DFT) and aberration corrected TEM to predict and rationalise differences in structure and hence surface reactivity of ceria nanoparticles in different environments. In particular we calculate reactivity 'fingerprints' for unreduced and reduced ceria nanoparticles immersed in water and in vacuum. Our simulations predict higher activities of ceria nanoparticles, towards oxygen release, when immersed in water because the water quenches the coordinative unsaturation of surface ions. Conversely, in vacuum, surface ions relax into the body of the nanoparticle to relieve coordinative unsaturation, which increases the energy barriers associated with oxygen release. Our simulations also reveal that reduced ceria nanoparticles are more active towards surface oxygen release compared to unreduced nanoceria. In parallel, experiment is used to explore the activities of ceria nanoparticles that have suffered a change in environment. In particular, we compare the ability of ceria nanoparticles, in an aqueous environment, to scavenge superoxide radicals compared to the same batch of nanoparticles, which have first been dried and then rehydrated. The latter show a distinct reduction in activity, which we correlate to a change in the redox chemistry associated with moving between different environments. The reactivity of ceria nanoparticles is therefore not only environment dependent, but is also influenced by the transport pathway or history required to reach the particular environment in which its reactivity is to be exploited. © 2013 The Royal Society of Chemistry

Catalytic coatings on steel for low-temperature propane prereforming to solid oxide fuel cell (SOFC) application

Catalyst layers (4–20 lm) of rhodium (1 wt%) supported on alumina, titania, and ceria–zirconia (Ce0.5Zr0.5O2) were coated on stainless-steel corrugated sheets by dip-coating in very stable colloidal dispersions of nanoparticles in water. Catalytic performances were studied for low-temperature (6500 C) steam reforming of propane at a steam to carbon ratio equal to 3 and low contact time (0.01 s). The best catalytic activity for propane steam reforming was observed for titania and ceria–zirconia supports for which propane conversion started at 250 C and was more than three times better at 350 C than conversion measured on alumina catalyst. For all catalysts a first-order kinetics was found with respect to propane at 500 C. Addition of PEG 2000 in titania and ceria–zirconia sols eliminated the film cracking observed without additive with these supports. Besides, the PEG addition strongly expanded the porosity of the layers, so that full catalytic efficiency was maintained when the thickness of the ceria–zirconia and titania films was increased

Electrical properties and reactivity under air–CO flows of composite systems based on ceria coated carbon nanotubes

Nanocomposite systems of ceria nanoparticles and Double Walled Carbon Nanotubes (DWNTs) coated with nanoceria (nCeO2) were elaborated using a classical sol gel method. Three samples noted as [nCeO2 + xDWNTs] with variable weight fractions x = 0, 5 and 15 wt.% of DWNTs were obtained. The samples were characterized by Xray diffraction and electron microscopy. The electrical conductivity of [nCeO2 + xDWNTs] compacted pellets systems was determined from electrical impedance spectrometry, under air, between 120 and 400 °C. In this temperature range, all samples were semiconducting with a weak variation of activation energy. However, the conductivity strongly increased with the weight fraction of ceria coated carbon nanotubes. Finally, the solid–gas interactions between air–CO flows and these systems were studied as a function of time and temperature, by means of Fourier transform infrared (FTIR) spectroscopy. The oxidation kinetics of CO into CO2 was analyzed from the evolutions of FTIR absorption band intensities. As the carbon nanotube fraction x increased, the conversion efficiency was strongly improved

Enhanced hydrogen storage in Ni/Ce composite oxides

The properties of dried (but not calcined) coprecipitated nickel ceria systems have been investigated in terms of their hydrogen emission characteristics following activation in hydrogen. XRD and BET data obtained on the powders show similarities to calcined ceria but it is likely that the majority of the material produced by the coprecipitation process is largely of an amorphous nature. XPS data indicate very little nickel is present on the outermost surface of the particles. Nevertheless, the thermal analytical techniques (TGA, DSC and TPD-MS) indicate that the hydrogen has access to the catalyst present and the nickel is able to generate hydrogen species capable of interacting with the support. Both unactivated and activated materials show two hydrogen emission features, viz. low temperature and high temperature emissions (LTE and HTE, respectively) over the temperature range 50 and 500 °C. A clear effect of hydrogen interaction with the material is that the activated sample not only emits much more hydrogen than the corresponding unactivated one but also at lower temperatures. H2 dissociation occurs on the reduced catalyst surface and the spillover mechanism transfers this active hydrogen into the ceria, possibly via the formation and migration of OH− species. The amount of hydrogen obtained (0.24 wt%) is 10× higher than those observed for calcined materials and would suggest that the amorphous phase plays a critical role in this process. The affiliated emissions of CO and CO2 with that of the HTE hydrogen (and consumption of water) strongly suggests a proportion of the hydrogen emission at this point arises from the water gas shift type reaction. It has not been possible from the present data to delineate between the various hydrogen storage mechanisms reported for ceria