Catalytic hydrogen production from renewables is a promising method for providing energy carriers in the near future. Photocatalysts capable of promoting this reaction are often composed of noble metal nanoparticles deposited on a semiconductor. The most promising semiconductor at present is TiO₂. The successful design of these catalysts relies on a thorough understanding of the role of the noble metal particle size and the TiO₂ polymorph. Here we demonstrate that Au particles in the size range 3-30 nm on TiO₂ are very active in hydrogen production from ethanol. It was found that Au particles of similar size on anatase nanoparticles delivered a rate two orders of magnitude higher than that recorded for Au on rutile nanoparticles. Surprisingly, it was also found that Au particle size does not affect the photoreaction rate over the 3-12 nm range. The high hydrogen yield observed makes these catalysts promising materials for solar conversion.

Engineering the shape and size of catalyst particles and the interface between different components of heterogeneous catalysts at nanometer level can radically alter their performances. This is particularly true with CeO2-based catalysts, where the precise control of surface atomic arrangements can modify the reactivity of Ce 4+ /Ce 3+ ions, changing the oxygen release/uptake characteristics of ceria, which, in turn, strongly affects catalytic performance in several reactions like CO, soot and VOC oxidation, WGS, hydrogenation, acid-base reactions and so on. Despite many of these catalysts are polycrystalline with rather ill-defined morphologies, experimental and theoretical studies on well-defined nanocrystals have clearly established that the exposure of specific facets can increase/decrease surface oxygen reactivity and metal-support interaction (for supported metal nanoparticles), consequently affecting catalytic reactions. Here, we want to address the most recent developments in this area, showing that shape (and size) modification, surface/face reconstruction and faceting of ceria at the nanoscale level can offer an important tool to govern activity and stability in several reactions and imagine how this could contribute to future developments.

In this study, a series of conventional polycrystalline ceria and single-crystalline ceria nanorods and nanocubes were prepared by hydrothermal methods, and their structural, redox, and morphological properties were investigated using XRD, SEM, HRTEM, BET, temperature-programmed reduction, and oxygen storage capacity measurements. According to HRTEM, they are characterized by exposure of different surfaces: {100} surface for nanocubes; {100}, {110}, and in part {111} for nanorods; and mainly {111} for conventional polycrystalline ceria, with a morphology dominated by {111}-enclosed octahedral particles. The presence of more-reactive exposed surfaces affects the reaction of soot oxidation positively, with an increase in activity in nanoshaped materials compared with conventional ceria. Thermal aging, although detrimental for surface area, is shown to affect morphology by promoting irregular truncation of edges and corners and development of more reactive surface combinations in all crystal shapes. It is likely that thermal treatment, starting from either cubes or octahedral particles, induces the formation of a similar particle geometry whose activity is dependent on the type of plane exposed and by the number an extension of edge and corners, thus linking reactivity of octahedral particles in conventional ceria powders with that of cubes in nanoshaped materials. The results indicate that soot oxidation is also a surface-dependent reaction, and catalyst design for this purpose should allow for surface structure morphology and its evolution against temperature.

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Challenges in energy and the environment call for the development of highly active catalysts, allowing for a more efficient and cleaner use of energy supplies.[1] Catalytic combustion of methane is a leading technology in emission prevention and cleanup.[2] Its main advantage over traditional flame combustion is to stabilize complete oxidation of fuel at low temperature while simultaneously controlling NO x emissions. Catalysts yielding the highest activity at low temperatures consist of noble metals dispersed on high-surface-area oxide supports. PdO particles dispersed on oxide carriers are the most active methane combustion catalysts, but they still suffer from inadequate activity at low temperature (below 673 K) and deactivation at high temperature (above 973 K) owing to formation of metallic Pd from PdO particles.[3] This transformation is regulated by a complex dynamic of formation and decomposition of PdO to Pd under reaction conditions, which is affected by the temperature and the reaction mixture.[4] One possibility for avoiding this transformation is to disperse Pd already in the ionic form over an oxide support. Stabilization of precious metals as ionic moieties over reducible supports such as ceria (CeO 2 ) has been shown to be effective for several reactions, such as the water-gas shift reaction and total oxidation, [5] and the ability of ceria to stabilize Pd in a highly dispersed state is wellrecognized.[6] Insertion of the precious metal into the metal oxide lattice would lead to the highest degree of dispersion for a given metal loading, with important consequences in several catalytic applications. Isolated encapsulated Pd metal in ceria as a result of a strong metal-support interaction was reported in early studies of noble-metal / ceria systems. [6,7] Solid solutions based on PdO/CeO 2 of composition Ce 0.99 Pd 0.01 O 2Àd or Ce 0.76 Zr 0.19 Pd 0.05 O 2Àd were reported more recently and found to be active in CO/NO reaction and methane combustion; [8] this finding is also corroborated by recent density functional theory (DFT) calculations suggesting that insertion of Pd into CeO 2 surfaces provides a lower energy barrier for dissociative adsorption of methane.[9] However, stabilization of Pdsubstituted ceria is difficult, and Pd segregation out of the oxide to form PdO or metallic Pd crystallites is commonly observed at high temperatures.[8]Herein we report an ordered and stable Pd-O-Ce surface superstructure as revealed by DFT calculations on the basis of high-resolution (HR) TEM data. It results from a complex reconstruction of the (110) CeO 2 surface and leads to the opening of wide surface channels exposing highly undercoordinated oxygen atoms.We have prepared two Pd/CeO 2 catalysts by one-step solution combustion synthesis (SCS). The new catalysts contain between 1 and 1.71 wt % Pd and are denoted SCS1 and SCS2 (Table 1). We also prepared samples of conventional Pd/CeO 2 catalysts by incipient wetness impregnation (IWI). These catalysts were prepared from two different samples of commerc...

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The surface atomic arrangement of metal oxides determines their physical and chemical properties, and the ability to control and optimize structural parameters is of crucial importance for many applications, in particular in heterogeneous catalysis and photocatalysis. Whereas the structures of macroscopic single crystals can be determined with established methods, for nanoparticles (NPs), this is a challenging task. Herein, we describe the use of CO as a probe molecule to determine the structure of the surfaces exposed by rod-shaped ceria NPs. After calibrating the CO stretching frequencies using results obtained for different ceria single-crystal surfaces, we found that the rod-shaped NPs actually restructure and expose {111} nanofacets. This finding has important consequences for understanding the controversial surface chemistry of these catalytically highly active ceria NPs and paves the way for the predictive, rational design of catalytic materials at the nanoscale.