The influence of water vapor on methane catalytic combustion was studied over a Pd@CeO₂/Si-Al₂O₃ catalyst, carefully designed to maximize Pd-CeO₂ interaction and prevent metal sintering and compared to a conventional impregnated catalyst with identical chemical composition. Although the nanostructured Pd@CeO₂/Si-Al₂O₃ catalyst is thermally stable, the addition of water to the reaction feed leads to a transient deactivation at low temperatures, consistent with the well documented competitive adsorption. In addition to this, the hierarchically structured catalyst exhibits an additional severe deactivation after methane oxidation in the presence of water vapor at 600 °C that can be reversed only by heating the catalyst above 700 °C. The presence of water in the reaction feed deactivates the conventional impregnated catalyst less severely and the activity largely returns upon water removal. Catalytic FTIR and CO-chemisorption data indicate that this severe deactivation process in the hierarchical catalyst is due to the formation of stable OH groups on the surface of the ceria nanoparticles. These hydroxyl groups are suggested to significantly inhibit the oxygen spillover from the CeO₂ nanoparticles to Pd, preventing its efficient re-oxidation, as observed by operando X-ray absorption near edge spectroscopy (XANES) experiments. At the same time, their presence can contribute to limit the gas phase accessibility of Pd, as indicated by the decrease of CO chemisorption capability. The presence of hydroxyls plays a minor role on the deactivation of the conventional catalyst at 600 °C.

The front cover artwork for Issue 14/2015 is provided by the groups of Prof. Raymond J. Gorte and Prof. Dr. Paolo Fornasiero. The image shows the deactivation of hierarchical Pd@CeO₂ catalysts for methane combustion in the presence of water. See the Full Paper itself at http://dx.doi.org/10.1002/cctc.201402717.

A method, based on self assembly, for preparing core–shell nanostructures that are dispersible in organic solvents is demonstrated for Pd and Pt cores with CeO₂, TiO₂, and ZrO₂ shells. Transmission electron microscopy (TEM) of these nanostructures confirmed the formation of distinct metal cores, approximately 2 nm in diameter, surrounded by amorphous oxide shells. Functional catalysts were prepared by dispersing the nanostructures onto an Al₂O₃ support; and vibrational spectra of adsorbed CO, together with adsorption uptakes, were used to demonstrate the accessibility of the metal core to CO and the porous nature of the oxide shell. Measurements of water-gas-shift (WGS) rates demonstrated that these catalysts exhibit activities similar to that of conventional supported catalysts despite having lower metal dispersions. Pd-based CeO₂ and TiO₂ core–shell catalysts exhibit significant transient deactivation, which is probably caused by a decrease in the exposed metal surface area due to the ease of reduction of the shells. Alternatively, Pt-based analogous core–shell catalysts do not exhibit such a transient decrease. Both Pd- and Pt-based ZrO₂ core–shell catalysts deactivate at a significantly lower rate due to the less reducible nature of the ZrO₂ shell.

Improved cathodes are required for low-temperature operation of solid-oxide fuel cells (SOFCs). Recent work has shown that electrode fabrication and modification by infiltration of active components into a porous scaffold can result in outstanding electrochemical performance. In this paper we review the literature on this new approach for cathode preparation and discuss the insights that this work has provided for understanding the relationships between the materials properties, electrochemical performance, and electrode stability.