Theoretical Studies on the Stability and Reactivity of the Metal-Doped CeO2(100) Surface: Toward H2 Dissociation and Oxygen Vacancy Formation

Does H2 Temperature-Programmed Reduction Always Probe Solid-State Redox Chemistry? The Case of Pt/CeO2

Redox reactions on the surface of transition metal oxides are of broad interest in thermo, photo, and electrocatalysis. H2 temperature-programmed reduction (H2-TPR) is commonly used to probe oxide reducibility by measuring the rate of H2 consumption during temperature ramps, assuming that this rate is controlled by oxide reduction. However, oxide reduction involves several elementary steps, such as H2 dissociation and H-spillover, before surface reduction and H2O formation occur. In this study, we evaluated the kinetics of H2 consumption over CeO2 and Pt/CeO2 with varying Pt loadings and structures to identify the elementary steps probed by H2-TPR. Literature often attributes changes in H2-TPR characteristics with Pt addition to increased CeO2 reducibility. However, our analysis revealed that the H2 consumption rate is measurement of the rate of H-spillover at Pt-CeO2 interfaces and is determined by the concentration of Pt species on Pt nanoclusters that dissociate H2. Therefore, lower temperature H2 consumption observed with Pt addition does not indicate higher CeO2 reducibility. Measurements on samples with mixtures of Pt single-atoms and nanoclusters demonstrated that H2-TPR can effectively quantify dilute Pt nanocluster concentrations, suggesting caution in directly linking H2-TPR characteristics to oxide reducibility while highlighting alternative material insights that can be gleaned.

Oxygen vacancy-engineered cerium oxide mediated by copper-platinum exhibit enhanced SOD/CAT-mimicking activities to regulate the microenvironment for osteoarthritis therapy

Cerium oxide (CeO2) nanospheres have limited enzymatic activity that hinders further application in catalytic therapy, but they have an "oxidation switch" to enhance their catalytic activity by increasing oxygen vacancies. In this study, according to the defect-engineering strategy, we developed PtCuOX/CeO2-X nanozymes as highly efficient SOD/CAT mimics by introducing bimetallic copper (Cu) and platinum (Pt) into CeO2 nanospheres to enhance the oxygen vacancies, in an attempt to combine near-infrared (NIR) irradiation to regulate microenvironment for osteoarthritis (OA) therapy. As expected, the Cu and Pt increased the Ce3+/Ce4+ ratio of CeO2 to significantly enhance the oxygen vacancies, and simultaneously CeO2 (111) facilitated the uniform dispersion of Cu and Pt. The strong metal-carrier interaction synergy endowed the PtCuOX/CeO2-X nanozymes with highly efficient SOD/CAT-like activity by the decreased formation energy of oxygen vacancy, promoted electron transfer, the increased adsorption energy of intermediates, and the decreased reaction activation energy. Besides, the nanozymes have excellent photothermal conversion efficiency (55.41%). Further, the PtCuOX/CeO2-X antioxidant system effectively scavenged intracellular ROS and RNS, protected mitochondrial function, and inhibited the inflammatory factors, thus reducing chondrocyte apoptosis. In vivo, experiments demonstrated the biosafety of PtCuOX/CeO2-X and its potent effect on OA suppression. In particular, NIR radiation further enhanced the effects. Mechanistically, PtCuOX/CeO2-X nanozymes reduced ras-related C3 botulinum toxin substrate 1 (Rac-1) and p-p65 protein expression, as well as ROS levels to remodel the inflammatory microenvironment by inhibiting the ROS/Rac-1/nuclear factor kappa-B (NF-κB) signaling pathway. This study introduces new clinical concepts and perspectives that can be applied to inflammatory diseases.

Spectroscopic and Theoretical Insights into H2 Activation on Uranium Monoxide: Homolytic H2 Cleavage Mediated by Intermediate OU(η2-H2)

Elucidating molecular-level interactions between dihydrogen (H2) and uranium oxides reveals fundamental insights into the intrinsic H2 activation mechanisms underlying processes such as heterogeneous catalysis over uranium oxides and corrosion of uranium induced by H2. Herein, the reactions of H2 with uranium monoxide (UO) molecules have been investigated via a combination of matrix-isolation infrared spectroscopy and quantum chemical calculations. A side-on bonded H2 complex, OU(η2-H2), is identified at 3733.7 and 800.3 cm-1. This species is regarded as a crucial intermediate along H2 activation pathways. Bonding analysis reveals cooperative U(π5f/6d) → H2(σ*) π// backdonation and U ← H2(σ) σ donation in OU(η2-H2) that facilitate the activation of the H2 moiety. Upon λ > 550 nm photoirradiation, OU(η2-H2) isomerizes into H2UO, indicating the homolytic H2 cleavage on UO. Mechanistic details of H2 adsorption and dissociation on UO molecules have been further elucidated.

How Pt Influences H2 Reactions on High Surface-Area Pt/CeO2 Powder Catalyst Surfaces

The addition of platinum-group metals (PGMs, e.g., Pt) to CeO2 is used in heterogeneous catalysis to promote the rate of redox surface reactions. Well-defined model system studies have shown that PGMs facilitate H2 dissociation, H-spillover onto CeO2 surfaces, and CeO2 surface reduction. However, it remains unclear how the heterogeneous structures and interfaces that exist on powder catalysts influence the mechanistic picture of PGM-promoted H2 reactions on CeO2 surfaces developed from model system studies. Here, controlled catalyst synthesis, temperature-programmed reduction (TPR), in situ infrared spectroscopy (IR), and in situ electron energy loss spectroscopy (EELS) were used to interrogate the mechanisms of how Pt nanoclusters and single atoms influence H2 reactions on high-surface area Pt/CeO2 powder catalysts. TPR showed that Pt promotes H2 consumption rates on Pt/CeO2 even when Pt exists on a small fraction of CeO2 particles, suggesting that H-spillover proceeds far from Pt-CeO2 interfaces and across CeO2-CeO2 particle interfaces. IR and EELS measurements provided evidence that Pt changes the mechanism of H2 activation and the rate limiting step for Ce3+, oxygen vacancy, and water formation as compared to pure CeO2. As a result, higher-saturation surface hydroxyl coverages can be achieved on Pt/CeO2 compared to pure CeO2. Further, Ce3+ formed by spillover-H from Pt is heterogeneously distributed and localized at and around interparticle CeO2-CeO2 boundaries, while activated H2 on pure CeO2 results in homogeneously distributed Ce3+. Ce3+ localization at and around CeO2-CeO2 boundaries for Pt/CeO2 is accompanied by surface reconstruction that enables faster rates of H2 consumption. This study reconciles the materials gap between model structures and powder catalysts for H2 reactions with Pt/CeO2 and highlights how the spatial heterogeneity of powder catalysts dictates the influence of Pt on H2 reactions at CeO2 surfaces.

Thermally Methanol Oxidation via the Mn1@Co3O4(111) Facet: Non-CO Reaction Pathway

Co3O4, as the support of single-atom catalysts, is effective in electron-structure modulation to get distinct methanol adsorption behaviors and adjustable reaction pathways for the methanol oxidation reaction. Herein, we considered the facets that constitute a Co vacancy of the Co3O4(111) facet and a foreign metal atom M (M = Fe, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, Au, Mn) leading to single-atom catalysts. The Mn1@Co3O4(111) facet is the facet considered the most favorable among all of the possible terminations. Oxygen adsorption, decomposition, and its co-adsorption with methanol are the vital steps of methanol oxidation at the exposed Mn1@Co3O4(111) facet, giving rise to the stable configuration: two O* and one CH3OH* adsorbates. Then, the Mn1@Co3O4(111) facet activates the O-H and C-H bonds within CH3OH*, advances CH3O* → H2CO* → HCOO* → COO*, and releases the products H2, H2O, and CO2 consecutively.

Atomic-scale insights into the interfacial charge transfer in a NiO/CeO2 heterostructure for electrocatalytic hydrogen evolution

To understand the underlying mechanism of the interfacial charge transfer and local chemical state variation in the nonprecious-based hydrogen evolution reaction (HER) electrocatalysts, a model system of the NiO/CeO₂ heterostructure was chosen for investigation using a combination of the advanced electron microscopic characterization and first-principles calculations. The results directly proved that interfacial charge transfer occurs from Ni to Ce, leading to reduction in the valence state of Ce and increased formation of V_O. This would optimize ΔG_H* and facilitate the hydrogen evolution process, resulting in outstanding HER performance in 1 M KOH with a low overpotential of 99 mV at the current density of 10 mA•cm^-2 and a modest Tafel slope of 78.4 mV•dec^-1 for the NiO/CeO₂ heterostructure sample. Therefore, the improved HER performance could be attributed to the synergistic coupling interactions and electron redistribution at the interface of NiO and CeO₂. These results concretely demonstrate the direct determination of the interfacial structure of the heterostructure and provide atomistic insights to unravel the underlying mechanism of interfacial charge transfer induced HER performance improvement.

Valence states of single Au atoms dictate the catalytic activity of Au1/CeO2(100)

We tune the valence state of single Au atoms anchored on CeO₂(100) by treating the catalyst in H₂ at different temperatures and obtain a series of Au₁/CeO₂(100). The transition from Au₁^+0.9 to Au₁^+0.3 leads to an enhancement of the CO oxidation activity of Au₁/CeO₂(100) by one order of magnitude. This work is of significance for an in-depth understanding of reaction mechanisms and rational design of high-performance catalysts.

Acetylene hydrogenation catalyzed by bare and Ni doped CeO2(110): the role of frustrated Lewis pairs

Ceria (CeO₂) has recently been found to catalyze the selective hydrogenation of alkynes, which has stimulated much discussion on the catalytic mechanism on various facets of reducible oxides. In this work, H₂ dissociation and acetylene hydrogenation on bare and Ni doped CeO₂(110) surfaces are investigated using density functional theory (DFT). Similar to that on the CeO₂(111) surface, our results suggest that catalysis is facilitated by frustrated Lewis pairs (FLPs) formed by oxygen vacancies (O_vs) on the oxide surfaces. On bare CeO₂(110) with a single O_v (CeO₂(110)-O_v), two surface Ce cations with one non-adjacent O anion are shown to form (Ce³⁺-Ce⁴⁺)/O quasi-FLPs, while for the Ni doped CeO₂(110) surface with one (Ni-CeO₂(110)-O_v) or two (Ni-CeO₂(110)-2O_v) O_vs, one Ce and a non-adjacent O counterions are found to form a mono-Ce/O FLP. DFT calculations indicate that Ce/O FLPs facilitate the H₂ dissociation via a heterolytic mechanism, while the resulting surface O-H and Ce-H species catalyze the subsequent acetylene hydrogenation. With CeO₂(110)-O_v and Ni-CeO₂(110)-2O_v, our DFT calculations suggest that the first hydrogenation step is the rate-determining step with a barrier of 0.43 and 0.40 eV, respectively. For Ni-CeO₂(110)-O_v, the reaction is shown to be controlled by the H₂ dissociation with a barrier of 0.41 eV. These barriers are significantly lower than that (about 0.7 eV) on CeO₂(111), explaining the experimentally observed higher catalytic efficiency of the (110) facet of ceria. The change of the rate-determining step is attributed to the different electronic properties of Ce in the Ce/O FLPs - the Ce f states closer to the Fermi level not only facilitate the heterolytic dissociation of H₂ but also lead to a higher barrier of acetylene hydrogenation.

Increasing Oxygen Vacancy of CeO2 Nanocrystals by Ni Doping and reduced Graphene Oxides Decoration towards the Electrocatalytic Hydrogen Evolution

The oxygen vacancy (VO) engineering is proved to be an effective approach for improving the hydrogen evolution reaction (HER) performance of low-cost metal oxides electrocatalysts. Cerium dioxide (CeO2), one of...

Hydrothermal Synthesis of a Ce-Zr-Ti Mixed Oxide Catalyst with Enhanced Catalytic Performance for a NH3-SCR Reaction

A series of mesoporous CeZrTiO_x catalysts were prepared by a facile hydrothermal method. Compared with CeTiO_x catalysts synthesized under the same conditions, the catalytic activity and anti-SO₂ performance of the Ce1Zr1TiO_x catalyst are greatly improved, and at the gas hourly space velocity (GHSV) of 60 000 h^-1, the NO_x removal efficiency is maintained at 90% in the temperature range of 290-500 °C. The catalytic effect of ZrO₂ on the Ce-Ti catalyst NH₃-SCR activity was elucidated through a series of characterizations. The results revealed that the doping of Zr could significantly improve and optimize the structure of Ce-Ti catalysts. At the same time, due to the doping of Zr, the synergistic effect between Ce and Zr in the CeZrTiO_x catalyst can effectively increase oxygen mobility, total acid content, and surface adsorbed oxygen species and lead to a larger pore volume. In addition, the introduction of ZrO₂ made the transformation of Ce⁴⁺ into Ce³⁺ more obvious, and the 2Ce⁴⁺ + Zr²⁺ ↔ 2Ce³⁺ + Zr⁴⁺ reaction greatly improved the reducibility of Ce1Zr1TiO_x. Among them, the improvement of SCR performance and H₂O/SO₂ tolerance is due to the electronic interaction between Zr and Ce.

Theoretical prediction by DFT and experimental observation of heterocation-doping effects on hydrogen adsorption and migration over the CeO2(111) surface

Hydrogen (H) atom adsorption and migration over the CeO₂-based materials surface are of great importance because of its wide applications to catalytic reactions and electrochemical devices. Therefore, comprehensive knowledge for controlling the H atom adsorption and migration over CeO₂-based materials is crucially important. For controlling H atom adsorption and migration, we investigated irreducible divalent, trivalent, and quadrivalent heterocation-doping effects on H atom adsorption and migration over the CeO₂(111) surface using density functional theory (DFT) calculations. Results revealed that the electron-deficient lattice oxygen (O_lat) and the flexible CeO₂ matrix played key roles in strong adsorption of H atoms. Heterocations with smaller valence and smaller ionic radius induced the electron-deficient O_lat. In addition, smaller cation doping enhanced the CeO₂ matrix flexibility. Moreover, we confirmed the influence of H atom adsorption controlled by doping on surface proton migration (i.e. surface protonics) and catalytic reaction involving surface protonics (NH₃ synthesis in an electric field). Results confirmed clear correlation between H atom adsorption energy and surface protonics.