Sara Colussi

Nanofaceted PdO Sites in PdCe Surface Superstructures: Enhanced Activity in Catalytic Combustion of Methane†

An open superstructure: A Pd/CeO2 catalyst prepared by solution combustion synthesis is three to five times more active for CH4 combustion than the best conventional palladium-based systems. The catalyst contains an ordered, stable Pd-O-Ce surface superstructure (see picture; cyan arrow is a square-planar Pd site, red arrow is an undercoordinated O atom) and is an example of ultra-highly dispersed, stable PdO within an oxide carrier.

Opposite face sensitivity of CeO2 in hydrogenation and oxidation catalysis

The determination of structure-performance relationships of ceria in heterogeneous reactions is enabled by the control of the crystal shape and morphology. Whereas the (100) surface, predominantly exposed in nanocubes, is optimal for CO oxidation, the (111) surface, prevalent in conventional polyhedral CeO2 particles, dominates in C2H2 hydrogenation. This result is attributed to the different oxygen vacancy chemistry on these facets. In contrast to oxidations, hydrogenations on CeO2 are favored over low-vacancy surfaces owing to the key role of oxygen on the stabilization of reactive intermediates. The catalytic behavior after ageing at high temperature confirms the inverse face sensitivity of the two reaction families.

Surface Faceting and Reconstruction of Ceria Nanoparticles

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.

The effect of ceria on the dynamics of CuO-Cu2O redox transformation: CuO-Cu2O hysteresis on ceria

CuO supported on ceria and alumina is prepared by incipient wetness impregnation and its redox behavior is investigated by means of XRD, XPS and TPO experiments. CuO on ZrO2 and on ceria–zirconia are also synthesized for comparison. The support plays a major role on the dynamics of CuO–Cu2O transformation. On Al2O3 the formation of CuAl2O4 inhibits any oxygen exchange during TPO experiments, while on ceria, zirconia and ceria–zirconia supported catalysts CuO decomposition and re-oxidation take place with a hysteresis similar to that observed for Pd–PdO transformation. On CeO2 the hysteresis is significantly reduced, indicating a strong effect of ceria on CuO–Cu2O redox cycle.Graphical Abstract

Influence of Different Palladium Precursors on the Properties of Solution-Combustion-Synthesized Palladium/Ceria Catalysts for Methane Combustion

A series of Pd/CeO2 catalysts was prepared by solution combustion synthesis (SCS) using different Pd precursors. The powders were characterized by complementary techniques such as BET surface area measurements, X‐Ray diffraction analysis, X‐Ray photoelectron spectroscopy, temperature‐programmed reduction, temperature‐programmed oxidation, and high‐resolution TEM. The results obtained evidenced the formation of a Pd‐Ce solid solution on all SCS samples. This solid solution is in the form of an ordered supercell structure only in the SCS catalyst prepared from palladium nitrate. This was correlated to the heat of reaction between Pd(NO3)2 and the fuel. The samples were tested for methane catalytic combustion, and the reaction rates on all SCS samples were approximately twice that of the impregnated counterpart, irrespective of the precursor. This was attributed to the presence of the Pd‐O‐Ce solid solution, which gave rise to strong Pd–ceria interactions.

Outstanding Methane Oxidation Performance of Palladium-Embedded Ceria Catalysts Prepared by a One-Step Dry Ball-Milling Method

By carefully mixing Pd metal nanoparticles with CeO2 polycrystalline powder under dry conditions, an unpredicted arrangement of the Pd-O-Ce interface is obtained in which an amorphous shell containing palladium species dissolved in ceria is covering a core of CeO2 particles. The robust contact that is generated at the nanoscale, along with mechanical forces generated during mixing, promotes the redox exchange between Pd and CeO2 and creates highly reactive and stable sites constituted by PdOx embedded into CeO2 surface layers. This specific arrangement outperforms conventional Pd/CeO2 reference catalysts in methane oxidation by lowering light-off temperature by more than 50°C and boosting the reaction rate. The origin of the outstanding activity is traced to the structural properties of the interface, modified at the nanoscale by mechanochemical interaction.