In situ and theoretical studies for the dissociation of water on an active Ni/CeO2 catalyst: importance of strong metal-support interactions for the cleavage of O-H bonds

Promoting CO Electroreduction to C2+ Oxygenates by Distribution of Water Dissociation Sites

The electrocatalytic CO2 or CO reduction reaction is a complex proton-coupled electron transfer reaction, in which protons in the electrolyte have a critical effect on the surface adsorbed *H species and the multi-carbon oxygenate products such as ethanol. However, the coupling of *H and carbon-containing intermediates into C2+ oxygenates can be severely hampered by the inappropriate distributions of those species in the catalytic interfaces. In this work, the controlled distribution of highly dispersed CeOx nanoclusters is demonstrated on Cu nanosheets as an efficient CO electroreduction catalyst, with Faradaic efficiencies of ethanol and total oxygenates of 35% and 58%, respectively. The CeOx nanoclusters (2-5 nm) enabled efficient water dissociation and appropriate distribution of adsorbed *H species on the Cu surface with carbon-containing species, thus facilitating the generation of C2+ oxygenate products. In contrast, pristine Cu without CeOx tended to form ethylene, while the aggregated CeOx nanoparticles promoted the surface density of *H and subsequent H2 evolution.

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.

Structured Catalysts and Catalytic Processes: Transport and Reaction Perspectives

The structure of catalysts determines the performance of catalytic processes. Intrinsically, the electronic and geometric structures influence the interaction between active species and the surface of the catalyst, which subsequently regulates the adsorption, reaction, and desorption behaviors. In recent decades, the development of catalysts with complex structures, including bulk, interfacial, encapsulated, and atomically dispersed structures, can potentially affect the electronic and geometric structures of catalysts and lead to further control of the transport and reaction of molecules. This review describes comprehensive understandings on the influence of electronic and geometric properties and complex catalyst structures on the performance of relevant heterogeneous catalytic processes, especially for the transport and reaction over structured catalysts for the conversions of light alkanes and small molecules. The recent research progress of the electronic and geometric properties over the active sites, specifically for theoretical descriptors developed in the recent decades, is discussed at the atomic level. The designs and properties of catalysts with specific structures are summarized. The transport phenomena and reactions over structured catalysts for the conversions of light alkanes and small molecules are analyzed. At the end of this review, we present our perspectives on the challenges for the further development of structured catalysts and heterogeneous catalytic processes.

Comprehensive Density Functional and Kinetic Monte Carlo Study of CO2 Hydrogenation on a Well-Defined Ni/CeO2 Model Catalyst: Role of Eley-Rideal Reactions

A detailed multiscale study of the mechanism of CO2 hydrogenation on a well-defined Ni/CeO2 model catalyst is reported that couples periodic density functional theory (DFT) calculations with kinetic Monte Carlo (kMC) simulations. The study includes an analysis of the role of Eley-Rideal elementary steps for the water formation step, which are usually neglected on the overall picture of the mechanism, catalytic activity, and selectivity. The DFT calculations for the chosen model consisting of a Ni4 cluster supported on CeO2 (111) show large enough adsorption energies along with low energy barriers that suggest this catalyst to be a good option for high selective CO2 methanation. The kMC simulations results show a synergic effect between the two 3-fold hollow sites of the supported Ni4 cluster with some elementary reactions dominant in one site, while other reactions prefer the another, nearly equivalent site. This effect is even more evident for the simulations explicitly including Eley-Rideal steps. The kMC simulations reveal that CO is formed via the dissociative pathway of the reverse water-gas shift reaction, while methane is formed via a CO2 → CO → HCO → CH → CH2 → CH3 → CH4 mechanism. Overall, our results show the importance of including the Eley-Rideal reactions and point to small Ni clusters supported on the CeO2 (111) surface as potential good catalysts for high selective CO2 methanation under mild conditions, while very active and selective toward CO formation at higher temperatures.

Active site engineering of Zn-doped mesoporous ceria toward highly efficient organophosphorus hydrolase-mimicking nanozyme

Hydrolase-mimicking nanozymes have received increasing attention in recent years, but the effective rational design and development of these materials has not been realized, as they are not at present considered a critical research target. Herein, we report that Zn-doped mesoporous ceria (Zn-m-ceria) engineered to have an abundance of two different active sites with different functions-one that allows both co-adsorption binding of organophosphate (OP) and water and another that serves as a general base-has significant organophosphorus hydrolase (OPH)-like catalytic activity. Specifically, Zn-m-ceria exhibits a catalytic efficiency over 75- and 25-fold higher than those of m-ceria and natural OPH, respectively. First-principles calculations reveal the importance of Zn for the OPH-mimicking activity of the material, promoting substrate adsorption and proton-binding. The OPH-like Zn-m-ceria catalyst is successfully applied to detect a model OP, methyl paraoxon, in spiked tap water samples with excellent sensitivity, stability, and detection precision. We expect that these findings will promote research based on the rational engineering of the active site of nanozymes and efficient strategies for obtaining a diverse range of catalysts that mimic natural enzymes, and hence the utilization in real-world applications of enzyme-mimicking catalysts with properties superior to their natural analogs should follow.

Role of H2O in Catalytic Conversion of C1 Molecules

Due to their role in controlling global climate change, the selective conversion of C1 molecules such as CH4, CO, and CO2 has attracted widespread attention. Typically, H2O competes with the reactant molecules to adsorb on the active sites and therefore inhibits the reaction or causes catalyst deactivation. However, H2O can also participate in the catalytic conversion of C1 molecules as a reactant or a promoter. Herein, we provide a perspective on recent progress in the mechanistic studies of H2O-mediated conversion of C1 molecules. We aim to provide an in-depth and systematic understanding of H2O as a promoter, a proton-transfer agent, an oxidant, a direct source of hydrogen or oxygen, and its influence on the catalytic activity, selectivity, and stability. We also summarize strategies for modifying catalysts or catalytic microenvironments by chemical or physical means to optimize the positive effects and minimize the negative effects of H2O on the reactions of C1 molecules. Finally, we discuss challenges and opportunities in catalyst design, characterization techniques, and theoretical modeling of the H2O-mediated catalytic conversion of C1 molecules.

Renaissance of Strong Metal-Support Interactions

Strong metal-support interactions (SMSIs) have emerged as a significant and cutting-edge area of research in heterogeneous catalysis. They play crucial roles in modifying the chemisorption properties, interfacial structure, and electronic characteristics of supported metals, thereby exerting a profound influence on the catalytic properties. This Perspective aims to provide a comprehensive summary of the latest advancements and insights into SMSIs, with a focus on state-of-the-art in situ/operando characterization techniques. This overview also identifies innovative designs and applications of new types of SMSI systems in catalytic chemistry and highlights their pivotal role in enhancing catalytic performance, selectivity, and stability in specific cases. Particularly notable is the discovery of SMSI between active metals and metal carbides, which opens up a new era in the field of SMSI. Additionally, the strong interactions between atomically dispersed metals and supports are discussed, with an emphasis on the electronic effects of the support. The chemical nature of SMSI and its underlying catalytic mechanisms are also elaborated upon. It is evident that SMSI modification has become a powerful tool for enhancing catalytic performance in various catalytic applications.

CeO2/Cu2O/Cu Tandem Interfaces for Efficient Water-Gas Shift Reaction Catalysis

Metal-oxide interfaces on Cu-based catalysts play very important roles in the low-temperature water-gas shift reaction (LT-WGSR). However, developing catalysts with abundant, active, and robust Cu-metal oxide interfaces under LT-WGSR conditions remains challenging. Herein, we report the successful development of an inverse copper-ceria catalyst (Cu@CeO₂), which exhibited very high efficiency for the LT-WGSR. At a reaction temperature of 250 °C, the LT-WGSR activity of the Cu@CeO₂ catalyst was about three times higher than that of a pristine Cu catalyst without CeO₂. Comprehensive quasi-in situ structural characterizations indicated that the Cu@CeO₂ catalyst was rich in CeO₂/Cu₂O/Cu tandem interfaces. Reaction kinetics studies and density functional theory (DFT) calculations revealed that the Cu⁺/Cu⁰ interfaces were the active sites for the LT-WGSR, while adjacent CeO₂ nanoparticles play a key role in activating H₂O and stabilizing the Cu⁺/Cu⁰ interfaces. Our study highlights the role of the CeO₂/Cu₂O/Cu tandem interface in regulating catalyst activity and stability, thus contributing to the development of improved Cu-based catalysts for the LT-WGSR.

Atomic Scaled Depth Correlation to the Oxygen Reduction Reaction Performance of Single Atom Ni Alloy to the NiO2 Supported Pd Nanocrystal

This study demonstrates the intercalation of single-atom Ni (NiSA ) substantially reduces the reaction activity of Ni oxide supported Pd nanoparticle (NiO2 /Pd) in the oxygen reduction reaction (ORR). The results indicate the transition states kinetically consolidate the adsorption energy for the chemisorbed O and OH species on the ORR activity. Notably, the NiO2 /Ni1 /Pd performs the optimum ORR behavior with the lowest barrier of 0.49 eV and moderate second-step barrier of 0.30 eV consequently confirming its utmost ORR performance. Through the stepwise cross-level demonstrations, a structure-Eads -ΔE correspondence for the proposed NiO2 /Nin /Pd systems is established. Most importantly, such a correspondence reveals that the electronic structure of heterogeneous catalysts can be significantly differed by the segregation of atomic clusters in different dimensions and locations. Besides, the doping-depth effect exploration of the NiSA in the NiO2 /Pd structure intrinsically elucidates that the Ni atom doping in the subsurface induces the most fruitful NiSA /PdML synergy combining the electronic and strain effects to optimize the ORR, whereas this desired synergy diminishes at high Pd coverages. Overall, the results not only rationalize the variation in the redox properties but most importantly provides a precision evaluation of the process window for optimizing the configuration and composition of bimetallic catalysts in practical experiments.

Dual Function of Hypo-d-electronic Transition Metals in the Brewer Intermetallic Phase for the Highly Efficient Electrocatalytic Hydrogen Evolution Reaction in Alkaline Electrolytes

Reported are the synthesis, material characterization, and electrocatalytic hydrogen evolution reaction (HER) in acid and alkaline electrolytes for the Brewer intermetallic phase, Nb₆Co₇ and Mo₆Co₇. It was realized that the overpotential at a current density of 10 mA/cm² (η₁₀) for Nb₆Co₇ (η₁₀ = 62 mV) and Mo₆Co₇ (η₁₀ = 143 mV) are both much lower than that of using a single Co metal (η₁₀ = 253 mV) in alkaline electrolytes. The enhancement of electrocatalytic HER activity of Nb₆Co₇ and Mo₆Co₇ can be attributed to the hypo-hyper-d-electronic interaction between Nb/Mo and Co elements. Based on the result of density functional theory calculation, alloying between Nb/Mo and Co elements will increase the antibonding state population of the Co-Co bond near the Fermi level (E_F), which induces the synergistic effect to influence the adsorption energy of the H atom (ΔG_H) on the surface of Nb₆Co₇ and Mo₆Co₇. Moreover, the role of the Nb element is not only a simple electron donor but is also an anchor position for the OH molecule (i.e., dual function) due to the bonding character of the Nb-Co bond near E_F. It can reduce the OH position effect as well as the activation energy for water dissociation, which rationalizes the high and robust HER performance of Nb₆Co₇ to that of commercial Pt/C (η₁₀ = 67 mV) in alkaline electrolytes.

Bixbyite-type Ln2O3 as promoters of metallic Ni for alkaline electrocatalytic hydrogen evolution

The active-site density, intrinsic activity, and durability of Ni-based catalysts are critical to their application in industrial alkaline water electrolysis. This work develops a kind of promoters, the bixbyite-type lanthanide metal sesquioxides (Ln₂O₃), which can be implanted into metallic Ni by selective high-temperature reduction to achieve highly efficient Ni/Ln₂O₃ hybrid electrocatalysts toward hydrogen evolution reaction. The screened Ni/Yb₂O₃ catalyst shows the low overpotential (20.0 mV at 10 mA cm⁻²), low Tafel slope (44.6 mV dec⁻¹), and excellent long-term durability (360 h at 500 mA cm⁻²), significantly outperforming the metallic Ni and benchmark Pt/C catalysts. The remarkable hydrogen evolution activity and stability of Ni/Yb₂O₃ are attributed to that the Yb₂O₃ promoter with high oxophilicity and thermodynamic stability can greatly enlarge the active-site density, reduce the energy barrier of water dissociation, optimize the free energy of hydrogen adsorption, and avoid the oxidation corrosion of Ni.

While renewable H₂ evolution will require inexpensive, abundant catalysts, non-noble metals typically show relatively low activities. Here, authors examine lanthanide metal sesquioxide doped metallic Ni and show efficient, stable performances for alkaline H₂ evolution electrocatalysis.

Tuning Particle Sizes and Active Sites of Ni/CeO2 Catalysts and Their Influence on Maleic Anhydride Hydrogenation

Supported metal catalysts are widely used in industrial processes, and the particle size of the active metal plays a key role in determining the catalytic activity. Herein, CeO₂-supported Ni catalysts with different Ni loading and particle size were prepared by the impregnation method, and the hydrogenation performance of maleic anhydride (MA) over the Ni/CeO₂ catalysts was investigated deeply. It was found that changes in Ni loading causes changes in metal particle size and active sites, which significantly affected the conversion and selectivity of MAH reaction. The conversion of MA reached the maximum at about 17.5 Ni loading compared with other contents of Ni loading because of its proper particle size and active sites. In addition, the effects of Ni grain size, surface oxygen vacancy, and Ni–CeO₂ interaction on MAH were investigated in detail, and the possible mechanism for MAH over Ni/CeO₂ catalysts was deduced. This work greatly deepens the fundamental understanding of Ni loading and size regimes over Ni/CeO₂ catalysts for the hydrogenation of MA and provides a theoretical and experimental basis for the preparation of high-activity catalysts for MAH.

Tuning Selectivity in the Direct Conversion of Methane to Methanol: Bimetallic Synergistic Effects on the Cleavage of C-H and O-H Bonds over NiCu/CeO2 Catalysts

The efficient activation of methane and the simultaneous water dissociation are crucial in many catalytic reactions on oxide-supported transition metal catalysts. On very low-loaded Ni/CeO₂ surfaces, methane easily fully decomposes, CH₄ → C + 4H, and water dissociates, H₂O→ OH + H. However, in important reactions such as the direct oxidation of methane to methanol (MTM), where complex interplay exists between reactants (CH₄, O₂), it is desirable to avoid the complete dehydrogenation of methane to carbon. Remarkably, the barrier for the activation of C-H bonds in CH_x (x = 1-3) species on Ni/CeO₂ surfaces can be manipulated by adding Cu, forming bimetallic NiCu clusters, whereas the ease for cleavage of O-H bonds in water is not affected by ensemble effects, as obtained from density functional theory-based calculations. CH₄ activation occurs only on Ni sites, and H₂O activation occurs on both Ni and Cu sites. The MTM reaction pathway for the example of the Ni₃Cu₁/CeO₂ model catalyst predicts a higher selectivity and a lower activation barrier for methanol production, compared with that for Ni₄/CeO₂. These findings point toward a possible strategy to design active and stable catalysts which can be employed for methane activation and conversions.

Can oxygen vacancies in ceria surfaces be measured by O1s photoemission spectroscopy?

X-ray photoemission spectroscopy is a standard technique for materials characterization and the O 1s binding energy is commonly measured for oxides. Here we use density functional theory calculations to investigate how the O 1s binding energy in CeO₂(111) is influenced by the presence of oxygen vacancies. The case with point vacancies in CeO₂(111) is compared to complete reduction to Ce₂O₃. Reduction of CeO₂by oxygen vacancies is found to have a minor effect on the O 1s binding energy. The O 1s binding energy is instead clearly changed when the character of the chemical bond for the considered oxygen atom is modified by, for example, the formation of OH-groups or carbonates.

Toluene Steam Reforming over Ni/CeZrO2—The Influence of Steam to Carbon Ratio and Contact Time on the Catalyst Performance and Carbon Deposition

The formation of tars during coal or biomass gasification is a serious issue resulting in decreasing efficiency of the process and increased maintenance costs. The decomposition of tars can be conducted via catalytic steam reforming that enriches the produced gas in hydrogen. Nevertheless, the catalyst should be characterized by high activity, stability, and resistance towards carbon deposition. Ceria-zirconia supported nickel (Ni/CeZrO2) is a very good candidate to catalyze tar removal—Ni is an active phase for reforming reactions, while CeZrO2 provides the active sites that play important roles in protecting the catalyst from carbon deposition. Ni/CeZrO2 shows high activity in the steam reforming of model tar compounds. In this paper, its performance in the steam reforming of toluene and carbon deposition is discussed considering the changing parameters of the reaction: the temperature, steam to carbon ratio, and the contact time.

Bismuth single atom supported CeO2 nanosheets for oxidation resistant photothermal reverse water gas shift reaction

Bi single atoms supported on CeO2 nanosheets combined with a Ti2O3 based photothermal device showed oxidation resistance and outperforming weak solar driven RWGS with a CO production rate of 31.00 mmol g−1 h−1 under 3 sun units of irradiation.

Insights into the Interfacial Lewis Acid-Base Pairs in CeO2 -Loaded CoS2 Electrocatalysts for Alkaline Hydrogen Evolution

Despite the known efficacy of CeO₂ as a promoter in alkaline hydrogen evolution reaction (HER), the underlying mechanism of this effect remains unclear. CoS₂ , a pyrite-type alkaline HER electrocatalyst, suffers from sluggish HER kinetics and severe catalyst leaching due to its weak water dissociation kinetics and oxygen-related corrosion. Herein, it is demonstrated that the interfacial Lewis acid-base Ce∙∙∙S pairs in CeO₂ -loaded CoS₂ effectively improve the catalytic activity and durability. In CeO₂ -loaded CoS₂ nanowire array electrodes, these interfacial Lewis acid-base Ce∙∙∙S pairs with unique electronic and structural configurations efficiently activate water adsorptive dissociation and kinetically accelerate hydrogen evolution, delivering a low overpotential of 36 mV at 10 mA cm^-2 in alkaline media. Such Ce∙∙∙S pairs also weaken O₂ adsorption on CoS₂ , leading to undecayed activity over 1000 h. These findings are expected to provide guidance for the design of CeO₂ -based electrocatalysts as well as other hybrid electrocatalysts for water splitting.

Role of Fe Species of Ni-Based Catalysts for Efficient Low-Temperature Ethanol Steam Reforming

The suppression of methane and coke formation over Ni-based catalysts for low temperature ethanol steam reforming remains challenging. This paper describes the structural evolution of Fe-modified Ni/MgAl₂O₄ catalysts and the influence of iron species on methane and coke suppression for low temperature ethanol steam reforming. Ni-Fe alloy catalysts are gradually oxidized by water to generate Ni-rich alloy and γ-Fe₂O₃ species at steam-to-carbon ratio of 4. The electron transfer from iron to nickel within Ni-Fe alloy weakens the CO adsorption and effectively alleviates the CO/CO₂ methanation. The oxidation capacity of γ-Fe₂O₃ species promotes the transformation of ethoxy to acetate groups to avoid methane formation and the elimination of carbon deposits for anticoking. Ni10Fe10/MgAl₂O₄ shows a superior performance with a highest H₂ yield of 4.6 mol/mol ethanol at 400 °C for 15 h. This research could potentially provide instructions for the design of Ni-based catalysts for low-temperature ethanol steam reforming.

Nature of the Active Sites on Ni/CeO2 Catalysts for Methane Conversions

Effective catalysts for the direct conversion of methane to methanol and for methane’s dry reforming to syngas are Holy Grails of catalysis research toward clean energy technologies. It has recently been discovered that Ni at low loadings on CeO₂(111) is very active for both of these reactions. Revealing the nature of the active sites in such systems is paramount to a rational design of improved catalysts. Here, we correlate experimental measurements on the CeO₂(111) surface to show that the most active sites are cationic Ni atoms in clusters at step edges, with a small size wherein they have the highest Ni chemical potential. We clarify the reasons for this observation using density functional theory calculations. Focusing on the activation barrier for C–H bond cleavage during the dissociative adsorption of CH₄ as an example, we show that the size and morphology of the supported Ni nanoparticles together with strong Ni-support bonding and charge transfer at the step edge are key to the high catalytic activity. We anticipate that this knowledge will inspire the development of more efficient catalysts for these reactions.

Strong Metal-Support Interaction for 2D Materials: Application in Noble Metal/TiB2 Heterointerfaces and their Enhanced Catalytic Performance for Formic Acid Dehydrogenation

Strong metal-support interaction (SMSI) is a phenomenon commonly observed on heterogeneous catalysts. Here, direct evidence of SMSI between noble metal and 2D TiB₂ supports is reported. The temperature-induced TiB₂ overlayers encapsulate the metal nanoparticles, resulting in core-shell nanostructures that are sintering-resistant with metal loadings as high as 12.0 wt%. The TiO_x -terminated TiB₂ surfaces are the active sites catalyzing the dehydrogenation of formic acid at room temperature. In contrast to the trade-off between stability and activity in conventional SMSI, TiB₂ -based SMSI promotes catalytic activity and stability simultaneously. By optimizing the thickness and coverage of the overlayer, the Pt/TiB₂ catalyst displays an outstanding hydrogen productivity of 13.8 mmol g^-1 _cat h^-1 in 10.0 m aqueous solution without any additive or pH adjustment, with >99.9% selectivity toward CO₂ and H₂ . Theoretical studies suggest that the TiB₂ overlayers are stabilized on different transition metals through an interplay between covalent and electrostatic interactions. Furthermore, the computationally determined trends in metal-TiB₂ interactions are fully consistent with the experimental observations regarding the extent of SMSI on different transition metals. The present research introduces a new means to create thermally stable and catalytically active metal/support interfaces for scalable chemical and energy applications.

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.

DFT insights into structural effects of Ni-Cu/CeO2 catalysts for CO selective reaction towards water-gas shift

The water-gas shift (WGS) reaction is a key step in hydrogen production, particularly to meet the high-purity H2 requirement of PEM fuel cells. The catalysts currently employed in large-scale WGS plants require a two-step process to overcome thermodynamic and kinetic limitations. Ni-Cu/CeO2 solids are promising catalysts for the one-step process required for small-scale applications, as the addition of Cu hinders undesired methanation reactions occurring on Ni/CeO2. In this work, we performed calculations on Ni4-xCux/CeO2(111) systems to evaluate the influence of cluster conformation on the selectivity towards water-gas shift. The structure and miscibility of CeO2-supported Ni4-xCux clusters were investigated and compared with those of gas-phase clusters to understand the effect of metal-support interactions. The adsorption of CO onto apical Ni and Cu atoms of Ni4-xCux/CeO2(111) systems was studied, and changes in the C-O bond strength were confirmed at the electronic level by investigating shifts in the 3σ and 1π orbitals. The selectivity towards WGS was evaluated using Brønsted-Evans-Polanyi relations for the C-O activation energy. Overall, a strengthening of the C-O bond and an increase in CO dissociation energy were verified on Cu-containing clusters, explaining the improvement in selectivity of Ni4-xCux/CeO2(111) systems.

Modulating oxidation state of Ni/CeO2 catalyst for steam methane reforming: a theoretical prediction with experimental verification

Rational design of partially oxidized Ni/CeO₂ with both high activity and good stability by DFT for efficient methane steam reforming.

Improving water oxidation performance by implementing heterointerfaces between ceria and metal-oxide nanoparticles

The main technical challenge for the electrolytic production of hydrogen via water splitting lies in realizing a very stable material that effectively oxidizes water under low overpotential (η). Of all materials, metal oxides hold the greatest promise due to their inherited chemical stability in aqueous solutions; however, electrolytic effectiveness in water oxidation reactions (OERs) is limited to precious metals. In this study, we designed metal oxide/metal oxide (MO/MO) nanoparticle heterointerfaces to offer more active sites and enhance the overall performance of the OER. To demonstrate this improvement, we synthesized and characterized CeO₂/Co₃O₄, CeO₂/CuO, and CeO₂/NiO nanoparticles. In these structures, onset potential and photoactivity were significantly improved relative to a single MO. A cathodic shift of onset potential as high as ~0.4 or 0.3 V was recorded for CeO₂/Co₃O₄ relative to CeO₂ or Co₃O₄, respectively. This improvement was further investigated using density functional theory calculations, upon which adsorption preferability and reaction free energy at the CeO₂/Co₃O₄ heterointerface were found to play significant roles in OER enhancement.

The Effects of CeO2 and Co Doping on the Properties and the Performance of the Ni/Al2O3-MgO Catalyst for the Combined Steam and CO2 Reforming of Methane Using Ultra-Low Steam to Carbon Ratio

In this paper, the 10 wt% Ni/Al2O3-MgO (10Ni/MA), 5 wt% Ni-5 wt% Ce/Al2O3-MgO (5Ni5Ce/MA), and 5 wt% Ni-5 wt% Co/Al2O3-MgO (5Ni5Co/MA) catalysts were prepared by an impregnation method. The effects of CeO2 and Co doping on the physicochemical properties of the Ni/Al2O3-MgO catalyst were comprehensively studied by N2 adsorption-desorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), H2 temperature programmed reduction (H2-TPR), CO2 temperature programmed reduction (CO2-TPD), and thermogravimetric analysis (TGA). The effects on catalytic performance for the combined steam and CO2 reforming of methane with the low steam-to-carbon ratio (S/C ratio) were evaluated at 620 °C under atmospheric pressure. The appearance of CeO2 and Co enhanced the oxygen species at the surface that decreased the coke deposits from 17% for the Ni/MA catalyst to 11–12% for the 5Ni5Ce/MA and 5Ni5Co/MA catalysts. The oxygen vacancies in the 5Ni5Ce/MA catalyst promoted water activation and dissociation, producing surface oxygen with a relatively high H2/CO ratio (1.6). With the relatively low H2/CO ratio (1.3), the oxygen species at the surface was enhanced by CO2 activation-dissociation via the redox potential in the 5Ni5Co/MA catalyst. The improvement of H2O and CO2 dissociative adsorption allowed the 5Ni5Ce/MA and 5Ni5Co/MA catalysts to resist the carbon formation, requiring only a low amount of steam to be added.

Low Temperature Activation of Methane on Metal-Oxides and Complex Interfaces: Insights from Surface Science

ConspectusThe abundance of cheap, natural gas has transformed the energy landscape, whereby revealing new possibilities for sustainable chemical technologies or impacting those that have relied on traditional fossil fuels. The primary component, methane, is underutilized and wastefully exhausted, leading to anthropogenic global warming. Historically, the manipulation of methane remained "clavis aurea," an insurmountable yet rewarding challenge and thus the focus of intense research. This is primarily due to an inability to dissociate C-H bonds in methane selectively, which requires a high energy penalty and is an essential prerequisite for the direct conversion of methane into a large set of value-added products. The discovery of such processes would promise an energy gainful use of natural gas benefiting several essential chemical processes associated with C1 chemistry. This first C-H bond dissociation step of the methane molecule appears in numerous catalytic mechanisms as the rate-determining step or most essential barrier sequence for all subsequent steps that follow in the production of C-C, C-O, or C_x-H_y-O_z bonds found in value added products. A main goal is to catalytically reduce the energy barrier for the first C-H bond dissociation to be able to achieve the activation of methane at low or moderate temperatures. As such there is great value in understanding the fundamental nature of the active sites responsible for bond breaking or formation and thus be able to facilitate better control of this chemistry, leading to the development of new technologies for fuel production and chemical conversion. Surface science studies offer enhanced perspectives for a careful manipulation of bonds over the last layer atoms of catalyst surfaces, an essential factor for the design of atomically precise catalysts and unravelling of the reaction mechanism. With the advent of new surface imaging, spectroscopy, and in situ tools, it has been possible to decipher the surface chemistry of complex materials systems and further our understanding of atomic active sites on the surfaces of metals, oxides, and carbides or metal-oxide and metal-carbide interfaces. The once considered near impossible step of C-H bond activation is now observed at low temperatures with high propensity over a collection of oxide, metal-oxide, and metal-carbide systems in a conventional or inverse configuration (oxide or carbide on metal). The enabling of C-H activation at low temperature has opened interesting possibilities for the specific production of chemicals such as methanol directly from methane, a step toward facile synthesis of liquid fuels. We highlight the most recent of these results and present the key aspects of active site configurations engineered from surface science studies which enable such a simple reactive event through careful manipulation of the last surface layer of atoms found in the catalyst structure. New concepts which help in the activation and conversion of methane are discussed.

Co3O4-Impregnated NiO-YSZ: An Efficient Catalyst for Direct Methane Electrooxidation

Co₃O₄-impregnated NiO-YSZ (yttria-stabilized zirconia) is a possible electrocatalyst for direct methane electrooxidation with both high catalytic activity and the ability to mitigate coking. The physical and electrochemical properties of Co₃O₄-impregnated NiO-YSZ anodes are investigated and benchmarked against NiO-YSZ and CeO₂-impregnated NiO-YSZ anodes. The following methane electrooxidation activity trend: Co₃O₄-impregnated NiO-YSZ > CeO₂-impregnated NiO-YSZ > NiO-YSZ with i_o (exchange current density) values of 88, 83, and 2 mA cm^-2, respectively, is obtained in the high overpotential region. The high activity of Co₃O₄-impregnated NiO-YSZ is attributed to the changes in the electronic structure and microstructure with the incorporation of nickel into the lattice of Co₃O₄ as observed using X-ray photoelectron spectroscopy, temperature-programmed reduction, high-resolution transmission electron microscopy, and field emission scanning electron microscopy. Co₃O₄-impregnated NiO-YSZ also demonstrated the least coking during operation, confirming its utility as a methane electrooxidation catalyst.

Water-promoted interfacial pathways in methane oxidation to methanol on a CeO2-Cu2O catalyst

Highly selective oxidation of methane to methanol has long been challenging in catalysis. Here, we reveal key steps for the pro-motion of this reaction by water when tuning the selectivity of a well-defined CeO₂/Cu₂O/Cu(111) catalyst from carbon monoxide and carbon dioxide to methanol under a reaction environment with methane, oxygen, and water. Ambient-pressure x-ray photoelectron spectroscopy showed that water added to methane and oxygen led to surface methoxy groups and accelerated methanol production. These results were consistent with density functional theory calculations and kinetic Monte Carlo simulations, which showed that water preferentially dissociates over the active cerium ions at the CeO₂-Cu₂O/Cu(111) interface. The adsorbed hydroxyl species blocked O-O bond cleavage that would dehydrogenate methoxy groups to carbon monoxide and carbon dioxide, and it directly converted this species to methanol, while oxygen reoxidized the reduced surface. Water adsorption also displaced the produced methanol into the gas phase.

Ceria-Based Catalysts Studied by Near Ambient Pressure X-ray Photoelectron Spectroscopy: A Review

The development of better catalysts is a passionate topic at the forefront of modern science, where operando techniques are necessary to identify the nature of the active sites. The surface of a solid catalyst is dynamic and dependent on the reaction environment and, therefore, the catalytic active sites may only be formed under specific reaction conditions and may not be stable either in air or under high vacuum conditions. The identification of the active sites and the understanding of their behaviour are essential information towards a rational catalyst design. One of the most powerful operando techniques for the study of active sites is near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), which is particularly sensitive to the surface and sub-surface of solids. Here we review the use of NAP-XPS for the study of ceria-based catalysts, widely used in a large number of industrial processes due to their excellent oxygen storage capacity and well-established redox properties.

In situ growing of CoO nanoparticles on g-C3N4 composites with highly improved photocatalytic activity for hydrogen evolution

CoO/g-C₃N₄ hybrid catalyst is facilely prepared for application to photocatalytic H₂ evolution from water splitting by the vacuum rotation-evaporation and in situ thermal method. The physical and chemical properties of CoO/g-C₃N₄ are determined by a series of characterization methods. The g-C₃N₄ with 0.6 wt% Co loading exhibits superior photocatalytic hydrogen evolution activity with an H₂ evolution amount of 23.25 mmol g^-1 after 5 h. The obtained 0.6 wt% CoO/g-C₃N₄ can split water to generate 0.39 mmol g^-1 H₂ without sacrificial agent and noble metal, while the pure g-C₃N₄ is inactive under the same reaction conditions. The remarkable enhancement of photocatalytic H₂ evolution activity of CoO/g-C₃N₄ composites is mainly ascribed to the effective separation of electron-hole pairs and charge transfer. The work creates new opportunities for the design of low-cost g-C₃N₄-based photocatalysts with high photocatalytic H₂ evolution activity from overall water splitting.

Well-Dispersed Nickel- and Zinc-Tailored Electronic Structure of a Transition Metal Oxide for Highly Active Alkaline Hydrogen Evolution Reaction

The practical scale-up of renewable energy technologies will require catalysts that are more efficient and durable than present ones. This is, however, a formidable challenge that will demand a new capability to tailor the electronic structure. Here, an original electronic structure tailoring of CoO by Ni and Zn dual doping is reported. This changes it from an inert material into one that is highly active for the hydrogen evolution reaction (HER). Based on combined density functional theory calculations and cutting-edge characterizations, it is shown that dual Ni and Zn doping is responsible for a highly significant increase in HER activity of the host oxide. That is, the Ni dopants cluster around surface oxygen vacancy of the host oxide and provide an ideal electronic surface structure for hydrogen intermediate binding, while the Zn dopants distribute inside the host oxide and modulate the bulk electronic structure to boost electrical conduction. As a result, the dual-doped Ni, Zn CoO nanorods achieve current densities of 10 and 20 mA cm^-2 at overpotentials of, respectively, 53 and 79 mV. This outperforms reported state-of-the-art metal oxide, metal oxide/metal, metal sulfide, and metal phosphide catalysts.