A DFT+U study of structure and reducibility of CenO2n−x (n⩽4, 0⩽x⩽n) nanoclusters

Low Temperature Infrared Study of Carbon Monoxide Adsorption on Rh/CeO2

Fundamental studies of the interaction of adsorbates with metal oxides alone and on which a noble metal is deposited provide information needed for catalytic reactions. Rh/CeO2 is one of the textbook catalysts for many reactions including syngas conversion to ethanol, water gas shift reaction (WGSR), and ethanol steam reforming. In this work, the adsorption of CO is studied by infrared (IR) spectroscopy, over CeO2 and 0.6 at. % Rh/CeO2 at a temperature range of 90 to 300 K. CeO2 is in the form of nanoparticles with sizes between 5 and 10 nm and exposing predominantly {111} surface termination in addition to non-negligible fraction of the {100} termination, determined from high resolution transmission electron microscopy (HRTEM). The as prepared Rh/CeO2 contained metallic Rh as well Rh cations in higher oxidation states. At 90 K two IR bands were observed at 2183–2186 and 2161–2163 cm−1, with the former saturating first. The 2163 cm−1 peak was more sensitive to CO pressure than the 2186 cm−1. Heating resulted in the depopulation of the 2163 cm−1 before the 2186 cm−1 peak. The desorption energy computed, assuming a first-order desorption kinetic, was found to be 0.35 eV for the 2186 cm−1 and 0.30 for the 2163 cm−1 IR peak (+/−0.05 eV). The equilibrium constant at 90 K was computed equal to 1.83 and 1.33 Torr−1 for the 2183 and 2161 cm−1, respectively. CO adsorption at 90 K on Rh/CeO2 resulted (in addition to the bands on CeO2) in the appearance of a broad band in the 2110–2130 cm-1 region that contained two components at 2116 and 2126 cm−1. The high frequency of this species is most likely due to adsorption on Rh clusters with very small sizes. The desorption energy of this species was found to be equal to 0.55 eV (+/−0.05 eV). Heating the CO covered Rh/CeO2 surface accelerated the disappearance of CO species over CeO2 and resulted in the appearance of CO2 bands (at about 150 K) followed by carbonate species. At 300 K, the surface was mainly composed of carbonates.

The stability and unexpected chemistry of oxide clusters

The stability of Fe_mO_n clusters is determined by second energy differences.

Simulations of ceria nanoparticles

Atomistic computer simulations, using classical potential models, have been used to model ceria nanoparticles (NPs) with diameters of approximately 1 and 2 nm. Lattice expansion is observed in the stoichiometric 1 nm NP, consistent with experiment, indicating that reduction is not the primary driver for such expansion. Furthermore, on reduction, the 1 nm NP is found to distort significantly, offering a possible explanation for its reduced oxygen storage capacity compared to the 2 nm NP. Point defect calculations on the 2 nm NP indicate that while doping with La is energetically favourable, Fe incorporation is not.

A DFT + U computational study on stoichiometric and oxygen deficient M–CeO2 systems (M = Pd1, Rh1, Rh10, Pd10 and Rh4Pd6)

Molecular and dissociative adsorption processes of ethanol on stoichiometric and O-defected CeO₂(111) surfaces alone as well as in the presence of one metal atom (Pd or Rh) are studied using spin-polarized density functional theory (DFT) with the GGA + U method (U_eff = 5.0 eV).

Oxygen vacancy formation in CeO2 and Ce(1-x)Zr(x)O2 solid solutions: electron localization, electrostatic potential and structural relaxation

Ceria (CeO(2)) and ceria-based composite materials, especially Ce(1-x)Zr(x)O(2) solid solutions, possess a wide range of applications in many important catalytic processes, such as three-way catalysts, owing to their excellent oxygen storage capacity (OSC) through the oxygen vacancy formation and refilling. Much of this activity has focused on the understanding of the electronic and structural properties of defective CeO(2) with and without doping, and comprehending the determining factor for oxygen vacancy formation and the rule to tune the formation energy by doping has constituted a central issue in material chemistry related to ceria. However, the calculation on electronic structures and the corresponding relaxation patterns in defective CeO(2-x) oxides remains at present a challenge in the DFT framework. A pragmatic approach based on density functional theory with the inclusion of on-site Coulomb correction, i.e. the so-called DFT + U technique, has been extensively applied in the majority of recent theoretical investigations. Firstly, we review briefly the latest electronic structure calculations of defective CeO(2)(111), focusing on the phenomenon of multiple configurations of the localized 4f electrons, as well as the discussions of its formation mechanism and the catalytic role in activating the O(2) molecule. Secondly, aiming at shedding light on the doping effect on tuning the oxygen vacancy formation in ceria-based solid solutions, we summarize the recent theoretical results of Ce(1-x)Zr(x)O(2) solid solutions in terms of the effect of dopant concentrations and crystal phases. A general model on O vacancy formation is also discussed; it consists of electrostatic and structural relaxation terms, and the vital role of the later is emphasized. Particularly, we discuss the crucial role of the localized structural relaxation patterns in determining the superb oxygen storage capacity in kappa-phase Ce(1-x)Zr(1-x)O(2). Thirdly, we briefly discuss some interesting findings for the oxygen vacancy formation in pure ceria nanoparticles (NPs) uncovered by DFT calculations and compare those with the bulk or extended surfaces of ceria as well as different particle sizes, emphasizing the role of the electrostatic field in determining the O vacancy formation.