Atomistic insights from DFT calculations into the catalytic properties on ceria-lanthanum clusters for methane activation

Improving the catalytic performance of materials based on cerium oxide (CeO2) for the activation of methane (CH4) can be achieved through the following strategies: mixture of CeO2 with different oxides (e.g., CeO2-La2O3) and the use of particles with different sizes. In this study, we present a theoretical investigation of the initial CH4 dehydrogenation on (La2Ce2O7)n clusters, where n = 2, 4, and 6. Our framework relies on density functional theory calculations combined with the unity bond index-quadratic exponential potential approximation. Our results indicate that chemical species arising from the first dehydrogenation of CH4, that is, CH3 and H, bind through the formation of C-O and H-O bonds with the clusters, respectively. The coordination of the adsorption site and the chemical environment plays a crucial role in the magnitude of the adsorption energy; for example, species adsorb more strongly in the low-coordinated topO sites located close to the La atoms. Thus, it affects the activation energy barrier, which tends to be lower in configurations where the adsorption of the chemical species is stronger. During CH4 dehydrogenation, the CH3 radical can be present in a planar or tetrahedral configuration. Its conformation changes as a function of the charge transference between the molecule and the cluster, which depends on the CH3-cluster distance. Finally, we analyze the effects of the Hubbard effective parameter (Ueff) on adsorption properties, as the magnitude of localization of Ce f-states affects the hybridization of the interaction between the molecule and the clusters and hence the magnitude of the adsorption energies. We obtained a linear decrease in the adsorption energies by increasing the Ueff parameter; however, the activation energy is only slightly affected.

Carbon Monoxide Oxidation on Ceria-Supported Nanoclusters

Periodic density functional theory is used to evaluate the minimum energy pathways of CO oxidation on cerium oxide-supported platinum and palladium nanoclusters (Pt/CeO2 and Pd/CeO2). For Pt/CeO2, the oxidation process involves the participation of lattice oxygen from CeO2 at the boundary sites of the cluster-ceria interface, which exhibits an exceptionally low energy barrier. Conversely, on Pd/CeO2, oxidation predominantly occurs through oxygen species bound to the Pd cluster. Experimental analysis using the temperature-programmed reduction of the oxidized Pd/CeO2 catalyst reveals a lower CO oxidation temperature compared to Pt/CeO2. This observation aligns with the anticipated decrease in the energy barrier for CO oxidation due to the oxygen coverage of the Pd cluster.

Supported Ce/Zr pyrochlore monolayers as a route to single cerium atom catalysts with low temperature reducibility

The combination of structural characterization at atomic resolution, chemical data, and theoretical insights has revealed the unique nanostructures which develop in ceria supported on yttria-stabilized zirconia (YSZ) after being submitted to high-temperature reducing treatments. The results show that just a small ceria loading is needed for creating a supported Zr-rich pyrochlore (111) nanostructure, resembling the structure of single cerium atom catalysts. The specific atomic arrangement of this nanostructure allows to explain the improvement of the reducibility at low temperature. The reduction mechanism can be extrapolated to ceria-zirconia mixed oxides with pyrochlore-like cationic ordering, exposing Zr-rich (111) surfaces. The results gathered here provide key information to understand the redox behavior of these types of systems, which may contribute to improving the design of new ceria-zirconia based materials, with lower content of the lanthanide element, nearly 100% cerium atom utilization, and applications in environmental catalysis.

Modulate the metal support interactions to optimize the surface-interface features of Pt/CeO2 catalysts for enhancing the toluene oxidation

Metal support interactions modulation is one of the effective strategies to enhance the catalytic performance. Herein, we reported that modulating metal support interactions by switching the strength (CO, H₂, NH₃) and temperature (200, 300, 400 °C) of reducing gases is a facile way to improve the catalytic performance of Pt/CeO₂ for toluene oxidation. The distinct reduction treatments will stepwise enhance the reducibility, ratio of Pt⁰ and oxygen vacancy concentration, which dominated the activity. The metal support interactions modulation can significantly affect toluene deep oxidation (from benzoate to formate or monodentate carbonate) via enhancing the mobility of surface/lattice oxygen and activation ability towards O₂ molecules, since the main activation sites for O₂ molecules expand from Pt⁰ sites to oxygen vacancies and Pt⁰ sites with temperature increasing.

Effect of Ag doping on Pd/Ag-CeO2 catalysts for CO and C3H6 oxidation

To achieve high fuel efficiency and low emission in automobiles, it is necessary to develop highly active diesel oxidation catalysts (DOCs). Pd/CeO₂ catalysts have been widely used as active catalysts for CO and C₃H₆ oxidation reactions. Additionally, Ag has been reported to enhance the oxygen storage capacity (OSC) of CeO₂, which contributes to the oxidation ability of Pd/CeO₂. In this study, Pd/Ag-CeO₂ catalysts were used for CO and C₃H₆ oxidation reactions. When CeO₂ was doped with appropriate amounts of Ag, reducibility and CO desorption rate were increased, which confirmed the high OSCs of Ag-doped catalysts. However, Ag particles were formed and the Ce³⁺/Ce⁴⁺ ratio decreased when CeO₂ was doped with excess amounts of Ag. In addition, reduced Pd (Pd⁰), which is an active species for C₃H₆ oxidation, was formed and maintained even under oxidative reaction conditions. Since the removal of C₃H₆ is important for the oxidation of CO and C₃H₆, the catalyst with the highest Pd⁰ fraction (Pd/0.1Ag-CeO₂ and Pd/0.3Ag-CeO₂) presented improved catalytic activity. Consequently, the optimal amount of Ag enhanced the OSC of Pd/Ag-CeO₂ catalysts and formed active Pd⁰ species under oxidative conditions, which resulted in the excellent catalytic activity of Pd/Ag-CeO₂ for the CO and C₃H₆ oxidation reaction.

The role of platinum on the NOx storage and desorption behavior of ceria: an online FT-IR study combined with in situ Raman and UV-vis spectroscopy

The role of platinum on the room temperature NOx storage mechanism and the NOx desorption behavior of ceria was investigated by combining online FT-IR gas-phase analysis with in situ Raman and UV-vis spectroscopy. The type of pretreatment, leading to the presence of different platinum states (Pt0, and mixed Pt0/Pt2+), is shown to have a major effect on the NOx storage and desorption properties. Upon loading of ceria with platinum (1 wt%), NOx storage capacities decrease except for reductively pretreated Pt/CeO2, enabling new reaction pathways via activation of gas-phase oxygen. In the absence of oxygen, NO is reduced by metallic platinum leading to N2O and N2 formation. In situ Raman spectra provide mechanistic information, by monitoring changes in ceria surface and subsurface oxygen, as well as PtOx during NOx storage. In the presence of gas-phase oxygen, NOx storage is related to the consumption of (sub)surface oxygen and PtOx, and proposed to involve NO2 or [NO + O2] intermediates reacting with surface oxygen. The NOx desorption behavior is shown to be strongly related to the stored NOx species. Oxidative pretreatment of ceria resulted in the largest amount of stored nitrates, consistent with NOx being mostly desorbed at elevated temperatures, i.e., within 300-500 °C. Reductive pretreatment and/or addition of platinum significantly increased the fraction of stored nitrite, thereby shifting the main NOx desorption temperature to values <300 °C. Storage and subsequent desorption of NOx in PtOx/CeO2 was associated with PtOx reduction and reoxidation, as monitored by in situ UV-vis and Raman spectra. Through detailed analysis we were able to elucidate the influence of platinum on NOx storage/desorption and demonstrate the participation of different platinum states in room temperature NOx storage, with each platinum state opening a distinct new reaction pathway.

Monodisperse-porous cerium oxide microspheres as a new support with appreciable catalytic activity for a composite catalyst in benzyl alcohol oxidation

Monodisperse porous ceria microspheres as a support with individual catalytic activity, facile post-functionalization and high surface area for heterogeneous catalysis.

A hybrid-DFT investigation of the Ce oxidation state upon adsorption of F, Na, Ni, Pd and Pt on the (CeO2)6 cluster

The formation of small polarons in CeO_2−x compounds has been investigated mainly on solids, compact surfaces, and large nanoparticles.

Ultrasmall Au clusters supported on pristine and defected CeO2: Structure and stability

The atomistic simulation of supported metal catalysts has long been challenging due to the increased complexity of dual components. In order to determine the metal/support interface, efficient theoretical tools to map out the potential energy surface (PES) are generally required. This work represents the first attempt to apply the recently developed SSW-NN method, stochastic surface walking (SSW) global optimization based on global neural network potential (G-NN), to explore the PES of a highly controversial supported metal catalyst, Au/CeO₂, system. By establishing the ternary Au-Ce-O G-NN potential based on first principles global dataset, we have searched for the global minima for a series of Au/CeO₂ systems. The segregation and diffusion pathway for Au clusters on CeO₂(111) are then explored by using enhanced molecular dynamics. Our results show that the ultrasmall cationic Au clusters, e.g., Au₄O₂, attaching to surface structural defects are the only stable structural pattern and the other clusters on different CeO₂ surfaces all have a strong energy preference to grow into a bulky Au metal. Despite the thermodynamics tendency of sintering, Au clusters on CeO₂ have a high kinetics barrier (>1.4 eV) in segregation and diffusion. The high thermodynamics stability of ultrasmall cationic Au clusters and the high kinetics stability for Au clusters on CeO₂ are thus the origin for the high activity of Au/CeO₂ catalysts in a range of low temperature catalytic reactions. We demonstrate that the global PES exploration is critical for understanding the morphology and kinetics of metal clusters on oxide support, which now can be realized via the SSW-NN method.

Total Oxidation of Propane over a Ru/CeO2 Catalyst at Low Temperature

Ruthenium (Ru) nanoparticles (∼3 nm) with mass loading ranging from 1.5 to 3.2 wt % are supported on a reducible substrate, cerium dioxide (CeO₂, the resultant sample is called Ru/CeO₂), for application in the catalytic combustion of propane. Because of the unique electronic configuration of CeO₂, a strong metal-support interaction is generated between the Ru nanoparticles and CeO₂ to stabilize Ru nanoparticles for oxidation reactions well. In addition, the CeO₂ host with high oxygen storage capacity can provide an abundance of active oxygen for redox reactions and thus greatly increases the rates of oxidation reactions or even modifies the redox steps. As a result of such advantages, a remarkably high performance in the total oxidation of propane at low temperature is achieved on Ru/CeO₂. This work exemplifies a promising strategy for developing robust supported catalysts for short-chain volatile organic compound removal.

Ag/CeO2 Composites for Catalytic Abatement of CO, Soot and VOCs

Nowadays catalytic technologies are widely used to purify indoor and outdoor air from harmful compounds. Recently, Ag–CeO2 composites have found various applications in catalysis due to distinctive physical-chemical properties and relatively low costs as compared to those based on other noble metals. Currently, metal–support interaction is considered the key factor that determines high catalytic performance of silver–ceria composites. Despite thorough investigations, several questions remain debating. Among such issues, there are (1) morphology and size effects of both Ag and CeO2 particles, including their defective structure, (2) chemical and charge state of silver, (3) charge transfer between silver and ceria, (4) role of oxygen vacancies, (5) reducibility of support and the catalyst on the basis thereof. In this review, we consider recent advances and trends on the role of silver–ceria interactions in catalytic performance of Ag/CeO2 composites in low-temperature CO oxidation, soot oxidation, and volatile organic compounds (VOCs) abatement. Promising photo- and electrocatalytic applications of Ag/CeO2 composites are also discussed.

Morphology of size-selected Ptn clusters on CeO2(111)

Supported Pt catalysts and ceria are well known for their application in automotive exhaust catalysts. Size-selected Pt clusters supported on a CeO₂(111) surface exhibit distinct physical and chemical properties. We investigated the morphology of the size-selected Pt_n (n = 5-13) clusters on a CeO₂(111) surface using scanning tunneling microscopy at room temperature. Pt_n clusters prefer a two-dimensional morphology for n = 5 and a three-dimensional (3D) morphology for n ≥ 6. We further observed the preference for a 3D tri-layer structure when n ≥ 10. For each cluster size, we quantitatively estimated the relative fraction of the clusters for each type of morphology. Size-dependent morphology of the Pt_n clusters on the CeO₂(111) surface was attributed to the Pt-Pt interaction in the cluster and the Pt-O interaction between the cluster and CeO₂(111) surface. The results obtained herein provide a clear understanding of the size-dependent morphology of the Pt_n clusters on a CeO₂(111) surface.

Tuning the Pd-catalyzed electroreduction of CO2to CO with reduced overpotential

The electrochemical CO₂reduction to CO can be greatly enhanced by controlling the Pd–ceria interface and doping with tellurium atoms.

2D–3D structural transition in sub-nanometer PtN clusters supported on CeO2(111)

Transition metal particles dispersed on oxide supports are used as heterogeneous catalysts in numerous applications.

CO adsorption on small Aun (n = 1-4) structures supported on hematite. I. Adsorption on iron terminated α-Fe2O3 (0001) surface

This is the first of two papers dealing with the adsorption of Au and formation of Aun nanostructures (n = 1-4) on hematite (0001) surface and adsorption of CO thereon. The stoichiometric Fe-terminated (0001) surface of hematite was investigated using density functional theory in the generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE) form with Hubbard correction U, accounting for strong electron correlations (PBE+U). The structural, energetic, and electronic properties of the systems studied were examined for vertical and flattened configurations of Aun nanostructures adsorbed on the hematite surfaces. The flattened ones, which can be viewed as bilayer-like structures, were found energetically more favored than vertical ones. For both classes of structures the adsorption binding energy increases with the number of Au atoms in a structure. The adsorption of Aun induces charge rearrangement at the Aun/oxide contact which is reflected in work function changes. In most considered cases Aun adsorption increases the work function. A detailed analysis of the bonding electron charge is presented and the corresponding electron charge rearrangements at the contacts were quantified by a Bader charge analyses. The interaction of a CO molecule with the Aun nanostructures supported on α-Fe2O3 (0001) and the oxide support was studied. It is found that the CO adsorption binding to the hematite supported Aun structures is more than twice as strong as to the bare hematite surface. Analysis of the Bader charges on the atoms showed that in each case CO binds to the most positively charged (cationic) atom of the Aun structure. Changes in the electronic structure of the Aun species and of the oxide support, and their consequences for the interactions with CO, are discussed.

Adsorption of water and ethanol on noble and transition-metal substrates: a density functional investigation within van der Waals corrections

We report an extensive density functional theory investigation of water and ethanol adsorption on several Cu-, Pt-, and Au-based substrates including substrates with low-coordinated sites due to intermixing of Pt–Cu and Pt–Au in the topmost surface.

Real-space observation of strong metal-support interaction: state-of-the-art and what's the next

The real-space resolving of the encapsulated overlayer in the well-known model and industry catalysts, ascribed to the advent of dedicated transmission electron microscopy, enables us to probe novel nano/micro architecture chemistry for better application, revisiting our understanding of this key issue in heterogeneous catalysis. In this review, we summarize the latest progress of real-space observation of SMSI in several well-known systems mainly covered from the metal catalysts (mostly Pt) supported by the TiO2 , CeO2 and Fe3 O4 . As a comparison with the model catalyst Pt/Fe3 O4 , the industrial catalyst Cu/ZnO is also listed, followed with the suggested ongoing directions in the field.