Simulating Structural Dynamics of Metal Catalysts under Operative Conditions

Structural reconstructions of metal catalysts have been recognized as common phenomena during catalytic reactions, which play a key role in their activities in heterogeneous catalysis. Precisely identifying the structures under the operative conditions becomes a prerequisite to establish a reliable structure-activity relationship and further rationalize the design of metal catalysts. However, real-time capture of the structural variations of catalysts at the atomic level with high-temporal resolution is a grand challenge for present in situ characterizations. During the past decade, significant progress has been made in theory to couple the structures with the reaction conditions to reproduce the experimental observations and predict the adsorbate-induced changes of catalysts in composition, morphology, size, etc. Modeling the dynamic correlation between the structure and activity of the metal catalysts brings us advanced knowledge of heterogeneous catalysis and becomes indispensable for accurate evaluation of the performance of metal catalysts.

Unveiling the Structure of Oxygen Vacancies in Bulk Ceria and the Physical Mechanisms behind Their Formation

Understanding the structures of oxygen vacancies in bulk ceria is crucial as they significantly impact the material's catalytic and electronic properties. The complex interaction between oxygen vacancies and Ce3+ ions presents challenges in characterizing ceria's defect chemistry. We introduced a machine learning-assisted cluster-expansion model to predict the energetics of defective configurations accurately within bulk ceria. This model effectively samples configurational spaces, detailing oxygen vacancy structures across different temperatures and concentrations. At lower temperatures, vacancies tend to cluster, mediated by Ce3+ ions and electrostatic repulsion, while at higher temperatures, they distribute uniformly due to configurational entropy. Our analysis also reveals a correlation between thermodynamic stability and the band gap between occupied O 2p and unoccupied Ce 4f orbitals, with wider band gaps indicating higher stability. This work enhances our understanding of defect chemistry in oxide materials and lays the groundwork for further research into how these structural properties affect ceria's performance.

The dynamic interplay between water and oxygen vacancy at the near-surface of ceria

Water, even at trace concentrations, strongly increases the CO oxidation activities of the reducible metal oxide supported noble-metal catalysts, where the transfer of proton plays a key role. In this paper, we performed a thorough investigation of the interplay between water molecules and the reduced CeO₂(111) surface. It was found that water molecules can induce the migration of oxygen vacancies which in turn results in the formation of surface protons. The proton then entangles with the near-surface polaron to form polaron-proton pair due to their mutual attractive interactions. The hopping of the polaron can easily trigger the long-range or short-range diffusion of protons mediated by water molecules at the CeO₂(111) surface. These findings provide new insights into the key roles of oxygen vacancies and polarons in reducible oxide based heterogeneous catalysis, which is beneficial for the understanding of the increased activity of reducible oxide supported metal nanoparticles in the presence of water.

Synergistic Effect of Simultaneous Doping of Ceria Nanorods with Cu and Cr on CO Oxidation and NO Reduction

Ceria particles play a key role in catalytic applications such as automotive three-way catalytic systems in which toxic CO and NO are oxidized and reduced to safe CO2 and N2 , respectively. In this work, we explore the incorporation of Cu and Cr metals as dopants in the crystal structure of ceria nanorods prepared by a single-step hydrothermal synthesis. XRD, Raman and XPS confirm the incorporation of Cu and Cr in the ceria crystal lattices, offering ceria nanorods with a higher concentration of oxygen vacancies. XPS also confirms the presence of Cr and Cu surface species. H2 -TPR and XPS analysis show that the simultaneous Cu and Cr co-doping results in a catalyst with a higher surface Cu concentration and a much-enhanced surface reducibility, in comparison with either undoped or singly doped (Cu or Cr) ceria nanorods. While single Cu doping enhances catalytic CO oxidation and Cr doping improves catalytic NO reduction, co-doping with both Cu and Cr enhances the benefits of both dopants in a synergistic manner employing roughly a quarter of dopant weight.

Synergistic Effects of Ternary PdO-CeO2-OMS-2 Catalyst Afford High Catalytic Performance and Stability in the Reduction of NO with CO

We developed a robust ternary PdO-CeO₂-OMS-2 catalyst with excellent catalytic performance in the selective reduction of NO with CO using a strategy based on combining components that synergistically interact leading to an effective abatement of these toxic gases. The catalyst affords 100% selectivity to N₂ at the nearly full conversion of NO and CO at 250 °C, high stability in the presence of H₂O, and a remarkable SO₂ tolerance. To unravel the origin of the excellent catalytic performance, the structural and chemical properties of the PdO-CeO₂-OMS-2 nanocomposite were analyzed in the as-prepared and used state of the catalyst, employing a series of pertinent characterization methods and specific catalytic tests. The experimental as well as theoretical results, based on density-functional theory calculations suggest that CO and NO follow different reaction pathways, CO is preferentially adsorbed and oxidized at Pd sites (Pd^II and Pd⁰), while NO decomposes on the ceria surface. Lattice oxygen vacancies at the interfacial perimeter of PdO-CeO₂ and PdO-OMS-2, and the diffusion of oxygen and oxygen vacancies are proposed to play a critical role in this multicenter reaction system.

NO reduction with CO over CuOx/CeO2 nanocomposites: influence of oxygen vacancies and lattice strain

The morphology-dependent population of oxygen vacancies in CuOx/CeO2 nanocomposites used for NO reduction with CO and its pivotal role in the reaction mechanism are examined in this combined experimental and first-principles study.

Temperature reversible synergistic formation of cerium oxyhydride and Au hydride: a combined XAS and XPDF study

In situ studies on the physical and chemical properties of Au in inverse ceria alumina supported catalysts have been conducted between 295 and 623 K using high energy resolved fluorescence detection X-ray absorption near edge spectroscopy and X-ray total scattering. Precise structural information is extracted on the metallic Au phase present in a 0.85 wt% Au containing inverse ceria alumina catalyst (ceria/Au/alumina). Herein evidence for the formation of an Au hydride species at elevated temperature is presented. Through modelling of total scattering data to extract the thermal properties of Au using Grüneisen theory of volumetric thermal expansion it proposed that the Au Hydride formation occurs synergistally with the formation of a cerium oxyhydride. The temperature reversible nature, whilst remaining in a reducing atmosphere, demonstrates the activation of hydrogen without consumption of oxygen from the supporting ceria lattice.

Simultaneous enzyme mimicking and chemical reduction mechanisms for nanoceria as a bio-antioxidant: a catalytic model bridging computations and experiments for nanozymes

The bio-antioxidant ability of nanoceria has been mainly ascribed to its ability to mimic superoxide dismutase (SOD) and catalase (CAT), and its mechanisms are thought to be analogous to those of the natural enzymes. Accordingly, nanoceria has been called a nanozyme, a nanomaterial mimicking enzymes. Because they overlook the real structural features of nanoceria, these hypothetical mechanisms cannot explain the important antioxidant experiments of nanoceria and have little predictive power. We hereby study the O₂˙^- and H₂O₂ scavenging mechanisms of nanoceria using first principles calculations, taking into account the role of oxygen vacancies that are practically abundant in nanoceria. The results reveal atomistic-level mechanisms responsible for the SOD and CAT mimetic activities of nanoceria. The newly created surface defect states in the electronic band structures of the shortly-lived intermediate species, called transient surface defect states (TSDSs), play critical roles in the enzyme mimetic catalysis and can serve as the bridge between computations and experiments at the atomistic level. The energy levels of TSDSs, which depend on the concentration and distribution of oxygen vacancies, determine whether the nanoceria is eligible for the catalysis. Besides the known enzyme mimicking mechanisms, the non-catalytic chemical reduction mechanisms are also responsible for the scavenging of O₂˙^- and H₂O₂, in which nanoceria serves as a reducing agent rather than a catalyst. The chemical reduction pathways poison the active sites of nanoceria which serve to mimic SOD and thus deteriorate its SOD mimetic activity. The results provide guidance for the engineering of nanoceria for bio-antioxidant applications. In particular, the proposed catalytic model can be generalized for the screening and design of high-performance nanozymes based on semiconductor nanomaterials.