Periodic trends of oxygen vacancy formation and C–H bond activation over transition metal-doped CeO2 (111) surfaces

Revisiting Heterolytic Cleavage Mechanism of Methane C-H Bond Activation over Metal Oxide Surfaces

The understanding of C-H bond activation could facilitate the design of improved catalysts for the conversion of methane to valuable products. However, its mechanism remains controversial, particularly with regard to metal oxides. This study aims to shed light on this issue by systematically investigating methane C-H bond activation across various pristine metal oxide surfaces and revisiting the prevailing heterolytic cleavage mechanism. It is found that the so-called "heterolytic cleavage mechanism" for methane activation could be classified into two distinct mechanisms on bare metal oxide surfaces: the real heterolytic cleavage mechanism over flat nonreducible alkali and alkaline-earth metal oxide surfaces (N-MOSs) and a ligand-to-metal charge transfer (LMCT)-enabled hydrogen atom transfer (HAT) mechanism over reducible metal oxide surfaces (R-MOSs). The dominant mechanism is determined by the Coulomb interaction between methane and the surface at the transition state and the energy of LMCT (ELMCT). Strong Coulomb interactions favor the heterolytic cleavage mechanism on bare N-MOSs, while the opposite favors the LMCT-enabled HAT mechanism on R-MOSs. Nevertheless, the heterolytic cleavage mechanism might have difficulty occurring under the reaction conditions of methane oxidation since the strong chemisorption of dioxygen over alkali and alkaline metal oxide surfaces would render the methane far from the surface, significantly weakening the Coulomb interaction. Doping can manipulate the electronic structure of lattice oxygen, potentially reducing ELMCT and even bypassing LMCT to directly generate reactive oxygen radicals, thus accelerating C-H activation. Additionally, these distinct mechanisms can influence subsequent steps, such as C-O coupling. C-H bond activation through the LMCT-enabled mechanism would be a prerequisite to trigger C-O coupling. This study provides valuable insights into the design of targeted catalysts with desired activity and selectivity for efficient and controlled methane conversion.

Efficient photothermal catalytic methane dry reforming over rich oxygen vacancy catalysts

The catalysts of Ni nanoparticles supported on ZrO2, La2O3 and La2Zr2O7 were prepared and employed in photothermal catalytic DRM. High yield of H2 and CO (76.2 and 99.1 mmol g-1 min-1) and excellent durability (50 h) were achieved merely under focused light irradiation. 10Ni/La2O3 exhibited better resistance to carbon deposition compared to 10Ni/ZrO2 and 10Ni/La2Zr2O7. Structural characterization and experimental data indicated that La2O3 has abundant oxygen vacancies, which can adsorb and activate CO2 to produce reactive oxygen species. Oxygen species transfer to nickel nanoparticles through the strong Ni-La2O3 interface to accelerate carbon oxidation. The experimental results indicated that light illumination can not only drive DRM reactions through photothermal conversion, but also improve catalytic activity by reducing the activation energy of reaction molecules and stability by increasing the oxygen vacancies.

Effect of the surface morphology of alkaline-earth metal oxides on the oxidative coupling of methane

Alkaline-earth metal oxides with the rocksalt structure, which are simple ionic solids, have attracted attention in attempts to gain fundamental insights into the properties of metal oxides. The surfaces of alkaline-earth metal oxides are considered promising catalysts for the oxidative coupling of methane (OCM); however, the development of such catalysts remains a central research topic. In this paper, we performed first-principles calculations to investigate the ability of four alkaline-earth metal oxides (MgO, CaO, SrO, and BaO) to catalyze the OCM. We adopted five types of surfaces of rocksalt phases as research targets: the (100), (110), stepped (100), oxygen-terminated octopolar (111), and metal-terminated octopolar (111) surfaces. We found that the formation energy of surface O vacancies is a good descriptor for the adsorption energy of a H atom and a methyl radical. The energies related to the OCM mechanism show that, compared with the most stable surface, the minor surfaces better promote the C - H bond cleavage of methane. However, as the trade-off for this advantage, the minor surfaces exhibit increased affinity for the methyl radical. On the basis of this trade-off relationship between properties, we identified several surfaces that we expect to be promising OCM catalysts. Our investigation of the temperature dependence of the Gibbs free energy indicated that, at higher temperatures, the step (100) surface exhibits properties that might benefit the OCM mechanism.

The Structure, Oxidation States, and Energetics of Co Nanoparticles on CeO2(111): An STM and DFT Study

Co nanoparticles (NPs) dispersed on ceria have been widely studied as active catalytic materials for many industrially relevant reactions. The detailed nature of such particles and the factors affecting their interaction with ceria remain to be better understood. In this study, a very low coverage (∼0.02 ML) of Co is deposited on a model CeO2(111) thin-film surface and is examined using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). The Co NPs that nucleate on terrace sites grow with coverage in this range to a maximum size of ca. 40 Co atoms, with an average diameter and height of 16.1 and 1.1 Å, respectively. Global minimization of the structures of Co NPs consisting of up to 23 Co atoms on CeO2(111) is performed based on the minima hopping algorithm and density functional theory (DFT) calculations, and the energetic and chemical properties of the resulting NPs are analyzed. While the theoretical findings are consistent with the STM observations on the strong Co-ceria interactions and the prevalence of oxidic Co species, some notable discrepancies are identified, including inconsistent aspect ratios and the existence of a low oxidation state Coδ+ species. The combined experimental and theoretical findings provide new insights into Co NPs formed on ceria and identify areas requiring further investigation.

Electronic and Structural Properties of Thin Iron Oxide Films on CeO2

Modification of CeO2 (ceria) with 3d transition metals, particularly iron, has been proven to significantly enhance its catalytic efficiency in oxidation or combustion reactions. Although this phenomenon is widely reported, the nature of the iron-ceria interaction responsible for this improvement remains debated. To address this issue, we prepared well-defined model FeOx/CeO2(111) catalytic systems and studied their structure and interfacial electronic properties using photoelectron spectroscopy, scanning tunneling microscopy, and low-energy electron diffraction, coupled with density functional theory (DFT) calculations. Our results show that under ultrahigh vacuum conditions, Fe deposition leads to the formation of small FeOx clusters on the ceria surface. Subsequent annealing results in the growth of large amorphous FeOx particles and a 2D FeOx layer. Annealing in an oxygen-rich atmosphere further oxidizes iron up to the Fe3+ state and improves the crystallinity of both the 2D layer and the 3D particles. Our DFT calculations indicate that the 2D FeOx layer interacts strongly with the ceria surface, exhibiting structural corrugations and transferred electrons between Fe2+/Fe3+ and Ce4+/Ce3+ redox pairs. The novel 2D FeOx/CeO2(111) phase may explain the enhancement of the catalytic properties of CeO2 by iron. Moreover, the corrugated 2D FeOx layer can serve as a template for the ordered nucleation of other catalytically active metals, in which the redox properties of the 2D FeOx/CeO2(111) system are exploited to modulate the charge of the supported metals.

Two decades of ceria nanoparticle research: structure, properties and emerging applications

Cerium oxide nanoparticles (CeNPs) are versatile materials with unique and unusual properties that vary depending on their surface chemistry, size, shape, coating, oxidation states, crystallinity, dopant, and structural and surface defects. This review encompasses advances made over the past twenty years in the development of CeNPs and ceria-based nanostructures, the structural determinants affecting their activity, and translation of these distinct features into applications. The two oxidation states of nanosized CeNPs (Ce3+/Ce4+) coexisting at the nanoscale level facilitate the formation of oxygen vacancies and defect states, which confer extremely high reactivity and oxygen buffering capacity and the ability to act as catalysts for oxidation and reduction reactions. However, the method of synthesis, surface functionalization, surface coating and defects are important factors in determining their properties. This review highlights key properties of CeNPs, their synthesis, interactions, and reaction pathways and provides examples of emerging applications. Due to their unique properties, CeNPs have become quintessential candidates for catalysis, chemical mechanical planarization (CMP), sensing, biomedical applications, and environmental remediation, with tremendous potential to create novel products and translational innovations in a wide range of industries. This review highlights the timely relevance and the transformative potential of these materials in addressing societal challenges and driving technological advancements across these fields.

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.

Unraveling the formation of oxygen vacancies on the surface of transition metal-doped ceria utilizing artificial intelligence

Ceria has been extensively utilized in different fields, with surface oxygen vacancies playing a central role. However, versatile oxygen vacancy regulation is still in its infancy. In this work, we propose an effective strategy to manipulate the oxygen vacancy formation energy via transition metal doping by combining first-principles calculations and analytical learning. We elucidate the underlying mechanism driving the formation of oxygen vacancies using combined symbolic regression and data analytics techniques. The results show that the Fermi level of the system and the electronegativity of the dopants are the paramount parameters (features) influencing the formation of oxygen vacancies. These insights not only enhance our understanding of the oxygen vacancy formation mechanism in ceria-based materials to improve their functionality but also potentially lay the groundwork for future strategies in the rational design of other transition metal oxide-based catalysts.

Tailored Platinum Group Metal/Spinel Oxide Catalysts for Dynamically Enhanced Methane Oxidation

A combined experimental and molecular modeling study identifies a family of spinel oxides that in combination with PGM (platinum group metals) provide enhanced methane oxidation activity. With a reduction in greenhouse gas (GHG) emissions urgently needed, there is renewed interest in the use of natural gas vehicles (NGVs) and engines (NGEs) for transportation, commerce, and industrial applications. NGVs and NGEs emit less CO2 than their petroleum-derived counterparts but may emit uncombusted methane, an even more potent GHG. For stoichiometric engines, methane oxidation catalysts containing PGM and spinel oxide in layered architectures offer increased methane oxidation activity and lower light-off temperatures (T50). The reducible spinel oxide has direct and indirect roles that are effectively described by the bulk oxygen vacancy formation energy (Evac). We apply density functional theory (DFT) to identify several earth-abundant, cobalt-rich spinel oxides with favorable Evac, shown to correlate with dynamic oxygen storage capacity (DOSC) and CO and H2 oxidation activity. We experimentally rank-order the DFT-identified spinel oxides in combination with Pt+Pd for their methane oxidation activity measurements, under both time-invariant and modulated feed conditions. We show good agreement between the activity and the DFT-computed reducibility of the spinel oxide. The findings suggest spinel reducibility is a key factor in achieving enhanced low-temperature methane conversion, enabled through a balance of methane activation on the PGM sites and subsequent oxidation of the intermediates and byproducts on spinel oxides. In agreement with its computationally predicted Evac, NiCo2O4 was confirmed to have the highest DOSC and lowest T50 among the tested spinel samples.

Effect of CeO2 support structure on the catalytic performance of ammonia synthesis in an electric field at low temperatures

Ammonia is an extremely important storage and transport medium for renewable energy, and technology is expected to produce it on demand and onsite using renewable energy. Applying a DC (direct current) to a solid catalyst layer with semiconducting properties makes ammonia synthesis highly efficient, even at low temperatures (approximately 400 K). In this process, oxide supports with semiconducting properties play important roles as metal supports and conduction fields for electrons and protons. The influence of the degree of particle aggregation on the support properties and ammonia synthesis using an electric field was evaluated for CeO2, which is the best material for this purpose because of its semiconducting properties. The results showed that controlling the aggregation structure of the crystalline particles could significantly influence the surface conductivity of protons and electrons; thus, the activity could be largely controlled. The Ru-CeO2 interaction could also be controlled by changing the crystallinity, which suppressed the aggregation of the supported Ru and significantly improved the ammonia synthesis activity using an electric field at low temperatures.

Doping SnO2 with metal ions of varying valence states: discerning the importance of active surface oxygen species vs. acid sites for C3H8 and CO oxidation

To elucidate the valence state effect of doping cations, Li+, Mg2+, Cr3+, Zr4+ and Nb5+ with radii similar to Sn4+ (CN = 6) were chosen to dope tetragonal SnO2. Cr3+, Zr4+ and Nb5+ can enter the SnO2 lattice to produce solid solutions, thus creating more surface defects. However, Li+ and Mg2+ can only stay on the SnO2 surface as nitrates, thus suppressing the surface defects. The rich surface defects facilitate the generation of active O2-/Oδ- and acid sites on the solid solution catalysts, hence improving the reactivity. On the solid solution catalysts active for propane combustion, several reactive intermediates can be formed, but are negligible on those with low activity. It is confirmed that for propane combustion, surface acid sites play a more vital role than active oxygen sites. Nevertheless, for CO oxidation, the active oxygen sites play a more vital role than the acid sites.

Pyro-Catalytic Degradation of Pyrene by Bentonite-Supported Transition Metals: Mechanistic Insights and Trade-Offs with Low Pyrolysis Temperature

Transition metal catalysts can significantly enhance the pyrolytic remediation of soils contaminated with polycyclic aromatic hydrocarbons (PAHs). Significantly higher pyrene removal efficiency was observed after the pyrolytic treatment of Fe-enriched bentonite (1.8% wt ion-exchanged content) relative to natural bentonite or soil (i.e., 93% vs 48% and 4%) at the unprecedentedly low temperature of 150 °C with only 15 min treatment time. DFT calculations showed that bentonite surfaces with Fe3+ or Cu2+ adsorb pyrene stronger than surfaces with Zn2+ or Na+. Enhanced pyrene adsorption results from increased charge transfer from its aromatic π-bonds to the cation site, which destabilizes pyrene allowing for faster degradation at lower temperatures. UV-Vis and GC-MS analyses revealed pyrene decomposition products in extracts of samples treated at 150 °C, including small aromatic compounds. As the pyrolysis temperature increased above 200 °C, product distribution shifted from extractable compounds to char coating the residue particles. No extractable byproducts were detected after treatment at 400 °C, indicating that char was the final product of pyrene decomposition. Tests with human lung cells showed that extracts of samples pyrolyzed at 150 °C were toxic; thus, high removal efficiency by pyrolytic treatment does not guarantee detoxification. No cytotoxicity was observed for extracts from Fe-bentonite samples treated at 300 °C, inferring that char is an appropriate treatment end point. Overall, we demonstrate that transition metals in clay can catalyze pyrolytic reactions at relatively low temperatures to decrease the energy and contact times required to meet cleanup standards. However, mitigating residual toxicity may require higher pyrolysis temperatures.

Thermocatalytic Decomposition of Methane: A Review on Carbon-Based Catalysts

The global initiatives on sustainable and green energy resources as well as large methane reserves have encouraged more research to convert methane to hydrogen. Catalytic decomposition of methane (CDM) is one optimistic route to generate clean hydrogen and value-added carbon without the emission of harmful greenhouse gases, typically known as blue hydrogen. This Review begins with an attempt to understand fundamentals of a CDM process in terms of thermodynamics and the prerequisite characteristics of the catalyst materials. In-depth understanding of rate-determining steps of the heterogeneous catalytic reaction taking place over the catalyst surfaces is crucial for the development of novel catalysts and process conditions for a successful CDM process. The design of state-of-the-art catalysts through both computational and experimental optimizations is the need of hour, as it largely governs the economy of the process. Recent mono- and bimetallic supported and unsupported materials used in CDM process have been highlighted and classified based on their performances under specific reaction conditions, with an understanding of their advantages and limitations. Metal oxides and zeolites have shown interesting performance as support materials for Fe- and Ni-based catalysts, especially in the presence of promoters, by developing strong metal-support interactions or by enhancing the carbon diffusion rates. Carbonaceous catalysts exhibit lower conversions without metal active species and largely result in the formation of amorphous carbon. However, the stability of carbon catalysts is better than that of metal oxides at higher temperatures, and the overall performance depends on the operating conditions, catalyst properties, and reactor configurations. Although efforts to summarize the state-of-art have been reported in literature, they lack systematic analysis on the development of stable and commercially appealing CDM technology. In this work, carbon catalysts are seen as promising futuristic pathways for sustained H2 production and high yields of value-added carbon nanomaterials. The influence of the carbon source, particle size, surface area, and active sites on the activity of carbon materials as catalysts and support templates has been demonstrated. Additionally, the catalyst deactivation process has been discussed, and different regeneration techniques have been evaluated. Recent studies on theoretical models towards better performance have been summarized, and future prospects for novel CDM catalyst development have been recommended.

Unlocking High-Efficiency Methane Oxidation with Bimetallic Pd-Ce Catalysts under Zeolite Confinement

Catalytic complete oxidation is an efficient approach to reducing methane emissions, a significant contributor to global warming. This approach requires active catalysts that are highly resistant to sintering and water vapor. In this work, we demonstrate that Pd nanoparticles confined within silicalite-1 zeolites (Pd@S-1), fabricated using a facile in situ encapsulation strategy, are highly active and stable in catalyzing methane oxidation and are superior to those supported on the S-1 surface due to a confinement effect. The activity of the confined Pd catalysts was further improved by co-confining a suitable amount of Ce within the S-1 zeolite (PdCe_0.4@S-1), which is attributed to confinement-reinforced Pd-Ce interactions that promote the formation of oxygen vacancies and highly reactive oxygen species. Furthermore, the introduction of Ce improves the hydrophobicity of the S-1 zeolite and, by forming Pd-Ce mixed oxides, inhibits the transformation of the active PdO phase to inactive Pd(OH)₂ species. Overall, the bimetallic PdCe_0.4@S-1 catalyst delivers exceptional outstanding activity and durability in complete methane oxidation, even in the presence of water vapor. This study may provide new prospects for the rational design of high-performance and durable Pd catalysts for complete methane oxidation.

Promotional Effect of Pt-Doping on the Catalytic Performance of Pt−CeO2 Catalyst for CO Oxidation

Growing interest in the development of a hydrogen economy means that CO oxidation is increasingly important for upgrading H2-rich fuel gas streams for fuel cells. CeO2-supported catalysts are the most promising candidates for the catalytic oxidation of CO because of their high activity. In the present work, DFT+U calculations were performed to investigate the stability and CO oxidation reactivity of Ptn (n = 1−4) clusters supported on CeO2(111) (Pt/CeO2) and Pt-doped CeO2(111) (Pt/(Pt−Ce)O2) surfaces. The Pt clusters showed similar nucleation behavior on both CeO2 and (Pt−Ce)O2 surfaces. Further, the formation of oxygen vacancies (Ov) was facilitated because of surface charge depletion caused by the dopant Pt. Our DFT results suggest that the interfacial OV plays an important role in the CO oxidation reaction cycle, and the calculated energy barrier for the CO oxidation reaction on the Pt/(Pt−Ce)O2 surface is approximately 0.43 eV lower than that on the surface of the undoped catalyst, suggesting enhanced CO oxidation reactivity. Therefore, the chemical modification of the CeO2 support via doping is an effective strategy for improving the catalytic performance of Pt/CeO2.

Elucidation of the reaction mechanism on dry reforming of methane in an electric field by in situ DRIFTs

With increasing expectations for carbon neutrality, dry reforming is anticipated for direct conversion of methane and carbon dioxide: the main components of biogas. We have found that dry reforming of methane in an electric field using a Pt/CeO2 catalyst proceeds with sufficient rapidity even at a low temperature of about 473 K. The effect of the electric field (EF) on dry reforming was investigated using kinetic analysis, in situ DRIFTs, XPS, and DFT calculation. In situ DRIFTs and XPS measurements indicated that the amount of carbonate, which is an adsorbed species of CO2, increased with the application of EF. XPS measurements also confirmed the reduction of CeO2 by the reaction of surface oxygen and CH4. The reaction between CH4 molecules and surface oxygen was promoted at the interface between Pt and CeO2.

Enhanced C–H bond activation by tuning the local environment of surface lattice oxygen of MoO3

The lattice oxygen on transition metal oxides serves as a critical active site in the dehydrogenation of alkanes, whose activity is determined by electronic properties and environmental structures. Hydrogen affinity has been used as a universal descriptor to predict C–H bond activation, while the understanding of the environmental structure is ambiguous due to its complexity. This paper describes a combined theoretical and experimental study to reveal the activity of lattice oxygen species with different local structures, taking Mo-based oxides and C–H bond activation of low-carbon alkanes as model catalytic systems. Our theoretical work suggests that oxygen species with convex curvature are more active than those with concave curvature. Theoretically, we propose an interpretative descriptor, the activation deformation energy, to quantify the surface reconstruction induced by adsorbates with various environmental structures. Experimentally, a Mo-based polyoxometalate with the convex curvature structure shows nearly five times the initial activity than single-crystal molybdenum oxide with the concave one. This work provides theoretical guidance for designing metal oxide catalysts with high activity.

Tuning of the environmental structure near the lattice oxygen of molybdenum oxides can form a favorable spatial structure to enhance the intrinsic activity for C–H bond activation.

Factors Governing Oxygen Vacancy Formation in Oxide Perovskites

The control of oxygen vacancy (V_O) formation is critical to advancing multiple metal-oxide-perovskite-based technologies. We report the construction of a compact linear model for the neutral V_O formation energy in ABO₃ perovskites that reproduces, with reasonable fidelity, Hubbard-U-corrected density functional theory calculations based on the state-of-the-art, strongly constrained and appropriately normed exchange-correlation functional. We obtain a mean absolute error of 0.45 eV for perovskites stable at 298 K, an accuracy that holds across a large, electronically diverse set of ABO₃ perovskites. Our model considers perovskites containing alkaline-earth metals (Ca, Sr, and Ba) and lanthanides (La and Ce) on the A-site and 3d transition metals (Ti, V, Cr, Mn, Fe, Co, and Ni) on the B-site in six different crystal systems (cubic, tetragonal, orthorhombic, hexagonal, rhombohedral, and monoclinic) common to perovskites. Physically intuitive metrics easily extracted from existing experimental thermochemical data or via inexpensive quantum mechanical calculations, including crystal bond dissociation energies and (solid phase) reduction potentials, are key components of the model. Beyond validation of the model against known experimental trends in materials used in solid oxide fuel cells, the model yields new candidate perovskites not contained in our training data set, such as (Bi,Y)(Fe,Co)O₃, which we predict may have favorable thermochemical water-splitting properties. The confluence of sufficient accuracy, efficiency, and interpretability afforded by our model not only facilitates high-throughput computational screening for any application that requires the precise control of V_O concentrations but also provides a clear picture of the dominant physics governing V_O formation in metal-oxide perovskites.

Reaction Pathway for Coke-Free Methane Steam Reforming on a Ni/CeO2 Catalyst: Active Sites and the Role of Metal-Support Interactions

Methane steam reforming (MSR) plays a key role in the production of syngas and hydrogen from natural gas. The increasing interest in the use of hydrogen for fuel cell applications demands development of catalysts with high activity at reduced operating temperatures. Ni-based catalysts are promising systems because of their high activity and low cost, but coke formation generally poses a severe problem. Studies of ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) indicate that CH₄/H₂O gas mixtures react with Ni/CeO₂(111) surfaces to form OH, CH_x, and CH_xO at 300 K. All of these species are easy to form and desorb at temperatures below 700 K when the rate of the MSR process is accelerated. Density functional theory (DFT) modeling of the reaction over ceria-supported small Ni nanoparticles predicts relatively low activation barriers between 0.3 and 0.7 eV for complete dehydrogenation of methane to carbon and the barrierless activation of water at interfacial Ni sites. Hydroxyls resulting from water activation allow for CO formation via a COH intermediate with a barrier of about 0.9 eV, which is much lower than that through a pathway involving lattice oxygen from ceria. Neither methane nor water activation is a rate-determining step, and the OH-assisted CO formation through the COH intermediate constitutes a low-barrier pathway that prevents carbon accumulation. The interactions between Ni and the ceria support and the low metal loading are crucial for the reaction to proceed in a coke-free and efficient way. These results pave the way for further advances in the design of stable and highly active Ni-based catalysts for hydrogen production.

Design Aspects of Doped CeO2 for Low-Temperature Catalytic CO Oxidation: Transient Kinetics and DFT Approach

CO elimination through oxidation over highly active and cost-effective catalysts is a way forward for many processes of industrial and environmental importance. In this study, doped CeO₂ with transition metals (TM = Cu, Co, Mn, Fe, Ni, Zr, and Zn) at a level of 20 at. % was tested for CO oxidation. The oxides were prepared using microwave-assisted sol-gel synthesis to improve catalyst's performance for the reaction of interest. The effect of heteroatoms on the physicochemical properties (structure, morphology, porosity, and reducibility) of the binary oxides M-Ce-O was meticulously investigated and correlated to their CO oxidation activity. It was found that the catalytic activity (per gram basis or TOF, s^-1) follows the order Cu-Ce-O > Ce-Co-O > Ni-Ce-O > Mn-Ce-O > Fe-Ce-O > Ce-Zn-O > CeO₂. Participation of mobile lattice oxygen species in the CO/O₂ reaction does occur, the extent of which is heteroatom-dependent. For that, state-of-the-art transient isotopic ¹⁸O-labeled experiments involving ¹⁶O/¹⁸O exchange followed by step-gas CO/Ar or CO/O₂/Ar switches were used to quantify the contribution of lattice oxygen to the reaction. SSITKA-DRIFTS studies probed the formation of carbonates while validating the Mars-van Krevelen (MvK) mechanism. Scanning transmission electron microscopy-high-angle annular dark field imaging coupled with energy-dispersive spectroscopy proved that the elemental composition of dopants in the individual nanoparticle of ceria is less than their composition at a larger scale, allowing the assessment of the doping efficacy. Despite the similar structural features of the catalysts, a clear difference in the O_lattice mobility was also found as well as its participation (as expressed with the α descriptor) in the reaction, following the order α_Cu > α_Co> α_Mn > α_Zn. Kinetic studies showed that it is rather the pre-exponential (entropic) factor and not the lowering of activation energy that justifies the order of activity of the solids. DFT calculations showed that the adsorption of CO on the Cu-doped CeO₂ surface is more favorable (-16.63 eV), followed by Co, Mn, Zn (-14.46, -4.90, and -4.24 eV, respectively), and pure CeO₂ (-0.63 eV). Also, copper compensates almost three times more charge (0.37e^-) compared to Co and Mn, ca. 0.13e^- and 0.10e^-, respectively, corroborating for its tendency to be reduced. Surface analysis (X-ray photoelectron spectroscopy), apart from the oxidation state of the elements, revealed a heteroatom-ceria surface interaction (O_a species) of different extents and of different populations of O_a species.

Manipulation of CO adsorption over Me1/CeO2 by heterocation doping: Key roles of single-atom adsorption energy

The performance of metal atoms chemically bonded to oxide supports cannot be explained solely by the intrinsic properties of the metals such as the d-band center. Herein, we present an in-depth study of the correlation between metal-oxide interactions and the properties of the supported metal using CO adsorption on Me₁ (Fe₁, Co₁, and Ni₁) loaded over CeO₂ (111) doped with divalent (Ca, Sr, and Ba), trivalent (Al, Ga, Sc, Y, and La), and quadrivalent (Hf and Zr) heterocations. CO adsorption over Me₁ is strongly dependent on the binding energies of Me₁. Two factors led to this trend. First, the extent of the Me₁-surface oxygen (Me₁-O) bond relaxation during CO adsorption played a key role. Second, the d-band center shifted drastically because of charge transfer to the oxides. The shift is related to the oxophilicity of metals. Adsorption energies of Me₁ over oxides include the contributions of the factors described above. Therefore, we can predict the activities of Me₁ using the strength of anchoring by oxide supports. Results show that smaller ionic radii of the doped heterocations were associated with more tightly bound Me₁. This finding sheds light on the possibility of heterocation-doping manipulating the reactivity of the Me₁ catalyst based on theoretical predictions.

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.

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.

Preferential location of zirconium dopants in cerium dioxide nanoparticles and the effects of doping on their reducibility: a DFT study

Structural properties and reducibility of zirconium-doped cerium dioxide systems were studied using periodic plane-wave calculations based on density functional theory. A systematic analysis of the results for nanoparticles of two sizes, Ce40-nZrnO80 ∼ 1.5 nm large and Ce140-nZrnO280 ∼ 2.4 nm large, in comparison with slab model data for Ce1-xZrxO2(111) surface has been performed focusing on specific nanoscale effects. Several loadings of Zr dopants ranging from 0.7 to 50 atomic metal percent have been considered. Subsurface cationic sites of ceria are calculated to be energetically most favourable for doping Zr4+ ions in all models. The system stability with several zirconium ions is defined by the relative stability of the occupied individual Zr4+ positions when only one zirconium ion is present. Data for the Ce70Zr70O280 nanoparticle with an equal number of Ce4+ and Zr4+ cations reveal that atomic orderings of neither separated oxide (Janus-type) particles nor randomly intermixed ones are more stable than the distribution of Zr atoms occupying all cationic positions inside the nanoparticle to minimize the presence of surface zirconium. The basicity of surface oxygen centers in nanoparticles is predicted to be decreased when Zr dopants are located in surface positions. The presence of Zr4+ dopants in CeO2 systems can notably lower the oxygen vacancy formation energy and shows interesting peculiarities at higher Zr loadings. Among them is the higher stability of inner oxygen vacancies in Zr-containing nanoparticles and enhanced oxygen mobility beneficial for application in catalysis and as a solid electrolyte with oxygen ions as charge carriers. Similar to pure ceria, Zr-doped ceria nanoparticles exhibit notably higher reducibility than the corresponding extended systems.

Modified Single Iteration Synchronous-Transit Approach to Bound Diffusion Barriers for Solid-State Reactions

Herein, we detail an approach to accelerate the computational screening of materials for properties dictated by the kinetics of solid-state diffusion through reliably and rapidly identifying upper and lower bounds to the transition state (TS) energy through our proposed modified single iteration synchronous-transit (MSIST) approach. While this sacrifices providing detailed information of the explicit TS structure, it requires only 30% of the force evaluations of a full nudged elastic band (NEB) TS search and reduces the computational demand to compute estimated diffusion barriers by ∼70% on average. In all 53 cases in which we explicitly compared our results to those of an NEB calculation, the upper and lower bounds identified using this approach bracketed the TS energy calculated with explicit NEB calculations. We use the applications of diffusion of Na⁺ in potential sodium-ion battery electrodes and oxygen vacancy diffusion in solid-oxide fuel cell electrodes and redox mediators for solar thermochemical hydrogen production to demonstrate the power of MSIST for analyzing the kinetics of bulk diffusion. For Na⁺ diffusion through 13 proposed electrode materials in which the average diffusion barrier was 0.28 eV, the average difference between the upper and lower bounds was 0.08 eV. An iterative application of this approach to the three materials with the largest difference between their upper and lower bounds further narrowed the average range of the bounded TS energies to 0.04 eV while still requiring fewer force evaluations than an NEB TS calculation. When applied in a high-throughput manner to study 514 diffusion pathways in 97 different materials, the average difference between the upper and lower bounds was 0.33 eV and the average barrier, as calculated by the average of all upper and lower bounds, was ∼1.7 eV. Because the MSIST approach produces explicit errors, i.e., the difference between the upper and lower bounds energies, even predicted barrier ranges with large errors can be reliably modeled with weighted regression techniques. MSIST enables the analysis of the kinetics of solid-state diffusion across larger sets of materials and can thus efficiently provide data to train statistically learned models of diffusion and to develop physical insights into the diffusion process.

Hydrogen in Nanocatalysis

Hydrogen is ubiquitous in catalysis. It is involved in many important reactions such as water splitting, N₂ reduction, CO₂ reduction, and alkane activation. In this Perspective, we focus on the hydrogen atom and follow its electron as it interacts with a catalyst or behaves as part of a catalyst from a computational point of view. We present recent examples in both nanocluster and solid catalysts to elucidate the parameters governing the strength of the hydrogen-surface interactions based on site geometry and electronic structure. We further show the interesting behavior of hydride in nanometal and oxides for catalysis. The key take-home messages are: (1) the in-the-middle electronegativity and small size of hydrogen give it great versatility in interacting with active sites on nanoparticles and solid surfaces; (2) the strength of hydrogen binding to an active site on a surface is an important descriptor of the chemical and catalytic properties of the surface; (3) the energetics of the hydrogen binding is closely related to the electronic structure of the catalyst; (4) hydrides in nanoclusters and oxides and on surfaces offer unique reactivity for reduction reactions.

Heteroatom doping effects on interaction of H2O and CeO2 (111) surfaces studied using density functional theory: Key roles of ionic radius and dispersion

Understanding heteroatom doping effects on the interaction between H₂O and cerium oxide (ceria, CeO₂) surfaces is crucially important for elucidating heterogeneous catalytic reactions of CeO₂-based oxides. Surfaces of CeO₂ (111) doped with quadrivalent (Ti, Zr), trivalent (Al, Ga, Sc, Y, La), or divalent (Ca, Sr, Ba) cations are investigated using density functional theory (DFT) calculations modified for onsite Coulomb interactions (DFT + U). Trivalent (except for Al) and divalent cation doping induces the formation of intrinsic oxygen vacancy (O_vac), which is backfilled easily by H₂O. Partially OH-terminated surfaces are formed. Furthermore, dissociative adsorption of H₂O is simulated on the OH terminated surfaces (for trivalent or divalent cation doped models) and pure surfaces (for Al and quadrivalent cation doped surfaces). The ionic radius is crucially important. In fact, H₂O dissociates spontaneously on the small cations. Although a slight change is induced by doping as for the H₂O adsorption energy at Ce sites, the H₂O dissociative adsorption at Ce sites is well-assisted by dopants with a smaller ionic radius. In terms of the amount of promoted Ce sites, the arrangement of dopant sites is also fundamentally important.

Promotion effect of cerium doping on iron–titanium composite oxide catalysts for selective catalytic reduction of NOx with NH3

Doping with a suitable amount of Ce enhances the SCR performance of FeTi catalysts.

Computational Study of Methane Activation on γ-Al2O3

The C-H activation of methane remains a longstanding challenge in the chemical industry. Metal oxides are attractive catalysts for the C-H activation of methane due to their surface Lewis acid-base properties. In this work, we applied density functional theory calculations to investigate the C-H activation mechanism of methane on various sites of low-index facets of γ-Al₂O₃. The feasibility of C-H activation on different metal-oxygen (acid-base) site pairs was assessed through two potential mechanisms, namely, the radical and polar. The effect of surface hydroxylation on C-H activation was also investigated to examine the activity of γ-Al₂O₃ under realistic catalytic surface conditions (hydration). On the basis of our calculations, it was demonstrated that the C-H activation barriers for polar pathways are significantly lower than those of the radical pathways on γ-Al₂O₃. We showed that the electronic structure (s- and p-band center) for unoccupied and occupied bands can be used to probe site-dependent Lewis acidity and basicity and the associated catalytic behavior. We identified the dissociated H₂ binding and final state energy as C-H activation energy descriptors for the preferred polar pathway. Finally, we developed structure-activity relationships for the C-H activation of methane on γ-Al₂O₃ that account for surface Lewis acid-base properties and can be utilized to accelerate the discovery of catalysts for methane (and shale gas) upgrade.