Water and Cu+ Synergy in Selective CO2 Hydrogenation to Methanol over Cu-MgO-Al2O3 Catalysts.

The CO2 hydrogenation reaction to produce methanol holds great significance as it contributes to achieving a CO2-neutral economy. Previous research identified isolated Cu+ species doping the oxide surface of a Cu-MgO-Al2O3-mixed oxide derived from a hydrotalcite precursor as the active site in CO2 hydrogenation, stabilizing monodentate formate species as a crucial intermediate in methanol synthesis. In this work, we present a molecular-level understanding of how surface water and hydroxyl groups play a crucial role in facilitating spontaneous CO2 activation at Cu+ sites and the formation of monodentate formate species. Computational evidence has been experimentally validated by comparing the catalytic performance of the Cu-MgO-Al2O3 catalyst with hydroxyl groups against that of its hydrophobic counterpart, where hydroxyl groups are blocked using an esterification method. Our work highlights the synergistic effect between doped Cu+ ions and adjacent hydroxyl groups, both of which serve as key parameters in regulating methanol production via CO2 hydrogenation. By elucidating the specific roles of these components, we contribute to advancing our understanding of the underlying mechanisms and provide valuable insights for optimizing methanol synthesis processes.

Vibrational frequencies of CO bound to all three low-index cerium oxide surfaces: A consistent theoretical description of vacancy-induced changes using density functional theory.

The facet-dependent adsorption of CO on oxidized and reduced CeO2 single crystal surfaces is reviewed, with emphasis on the effect of CO coverage and the ability of state-of-the-art quantum-mechanical methods to provide reliable energies and an accurate description of the IR vibrational frequency of CO. Comparison with detailed, high-resolution experimental infrared reflection absorption spectroscopy data obtained for single crystal samples allows the assignment of the different CO vibrational bands observed on all three low-index ceria surfaces. Good agreement is achieved with the hybrid density functional theory approach with the HSE06 functional and with saturation coverage. It is shown that CO is very sensitive to the structure of cerium oxide surfaces and to the presence of oxygen vacancies. The combined theoretical-experimental approach offers new opportunities for a better characterization of ceria nanoparticles and for unraveling changes occurring during reactions involving CO at higher pressures.

Highly Active and Stable Ni/La-Doped Ceria Material for Catalytic CO2 Reduction by Reverse Water-Gas Shift Reaction.

The design of an active, effective, and economically viable catalyst for CO2 conversion into value-added products is crucial in the fight against global warming and energy demand. We have developed very efficient catalysts for reverse water-gas shift (rWGS) reaction. Specific conditions of the synthesis by combustion allow the obtention of macroporous materials based on nanosized Ni particles supported on a mixed oxide of high purity and crystallinity. Here, we show that Ni/La-doped CeO2 catalysts─with the "right" Ni and La proportions─have an unprecedented catalytic performance per unit mass of catalyst for the rWGS reaction as the first step toward CO2 valorization. Correlations between physicochemical properties and catalytic activity, obtained using a combination of different techniques such as X-ray and neutron powder diffraction, Raman spectroscopy, in situ near ambient pressure X-ray photoelectron spectroscopy, electron microscopy, and catalytic testing, point out to optimum values for the Ni loading and the La proportion. Density functional theory calculations of elementary steps of the reaction on model Ni/ceria catalysts aid toward the microscopic understanding of the nature of the active sites. This finding offers a fundamental basis for developing economical catalysts that can be effectively used for CO2 reduction with hydrogen. A catalyst based on Ni0.07/(Ce0.9La0.1Ox)0.93 shows a CO production of 58 × 10-5 molCO·gcat-1·s-1 (700 °C, H2/CO2 = 2; selectivity to CO > 99.5), being stable for 100 h under continuous reaction.

Shape-Controlled Pathways in the Hydrogen Production from Ethanol Steam Reforming over Ceria Nanoparticles.

The ethanol surface reaction over CeO2 nanooctahedra (NO) and nanocubes (NC), which mainly expose (111) and (100) surfaces, respectively, was studied by means of infrared spectroscopy (TPSR-IR), mass spectrometry (TPSR-MS), and density functional theory (DFT) calculations. TPSR-MS results show that the production of H2 is 2.4 times higher on CeO2-NC than on CeO2-NO, which is rationalized starting from the different types of adsorbed ethoxy species controlled by the shape of the ceria particles. Over the CeO2(111) surface, monodentate type I and II ethoxy species with the alkyl chain perpendicular or parallel to the surface, respectively, were identified. Meanwhile, on the CeO2(100) surface, bidentate and monodentate type III ethoxy species on the checkerboard O-terminated surface and on a pyramid of the reconstructed (100) surface, respectively, are found. The more labile surface ethoxy species on each ceria nanoshape, which are the monodentate type I or III ethoxy on CeO2-NO and CeO2-NC, respectively, react on the surface to give acetate species that decompose to CO2 and CH4, while H2 is formed via the recombination of hydroxyl species. In addition, the more stable monodentate type II and bidentate ethoxy species on CeO2-NO and CeO2-NC, respectively, give an ethylenedioxy intermediate, the binding of which is facet-dependent. On the (111) facet, the less strongly bound ethylenedioxy desorbs as ethylene, whereas on the (100) facet, the more strongly bound intermediate also produces CO2 and H2 via formate species. Thus, on the (100) facet, an additional pathway toward H2 formation is found. ESR activity measurements show an enhanced H2 production on the nanocubes.

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 CHx (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.

Erratum: Vibrational Frequencies of Cerium-Oxide-Bound CO: A Challenge for Conventional DFT Methods [Phys. Rev. Lett. 125, 256101 (2020)].

This corrects the article DOI: 10.1103/PhysRevLett.125.256101.

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.

Facet-dependent stability of near-surface oxygen vacancies and excess charge localization at CeO2surfaces.

To study the dependence of the relative stability of surface (VA) and subsurface (VB) oxygen vacancies with the crystal facet of CeO2, the reduced (100), (110) and (111) surfaces, with two different concentrations of vacancies, were investigated by means of density functional theory (DFT + U) calculations. The results show that the trend in the near-surface vacancy formation energies for comparable vacancy spacings, i.e. (110) < (100) < (111), does not follow the one in the surface stability of the facets, i.e. (111) < (110) < (100). The results also reveal that the preference of vacancies for surface or subsurface sites, as well as the preferred location of the associated Ce3+polarons, are facet- and concentration-dependent. At the higher vacancy concentration, theVAis more stable than theVBat the (110) facet whereas at the (111), it is the other way around, and at the (100) facet, both theVAand theVBhave similar stability. The stability of theVAvacancies, compared to that of theVB, is accentuated as the concentration decreases. Nearest neighbor polarons to the vacant sites are only observed for the less densely packed (110) and (100) facets. These findings are rationalized in terms of the packing density of the facets, the lattice relaxation effects induced by vacancy formation and the localization of the excess charge, as well as the repulsive Ce3+-Ce3+interactions.

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 x O 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.

Metal-Support Interactions and C1 Chemistry: Transforming Pt-CeO2 into a Highly Active and Stable Catalyst for the Conversion of Carbon Dioxide and Methane.

There is an ongoing search for materials which can accomplish the activation of two dangerous greenhouse gases like carbon dioxide and methane. In the area of C1 chemistry, the reaction between CO2 and CH4 to produce syngas (CO/H2), known as methane dry reforming (MDR), is attracting a lot of interest due to its green nature. On Pt(111), high temperatures must be used to activate the reactants, leading to a substantial deposition of carbon which makes this metal surface useless for the MDR process. In this study, we show that strong metal-support interactions present in Pt/CeO2(111) and Pt/CeO2 powders lead to systems which can bind CO2 and CH4 well at room temperature and are excellent and stable catalysts for the MDR process at moderate temperature (500 °C). The behavior of these systems was studied using a combination of in situ/operando methods (AP-XPS, XRD, and XAFS) which pointed to an active Pt-CeO2-x interface. In this interface, the oxide is far from being a passive spectator. It modifies the chemical properties of Pt, facilitating improved methane dissociation, and is directly involved in the adsorption and dissociation of CO2 making the MDR catalytic cycle possible. A comparison of the benefits gained by the use of an effective metal-oxide interface and those obtained by plain bimetallic bonding indicates that the former is much more important when optimizing the C1 chemistry associated with CO2 and CH4 conversion. The presence of elements with a different chemical nature at the metal-oxide interface opens the possibility for truly cooperative interactions in the activation of C-O and C-H bonds.

Toward an Atomic-Level Understanding of Ceria-Based Catalysts: When Experiment and Theory Go Hand in Hand.

ConspectusBecause ceria (CeO2) is a key ingredient in the formulation of many catalysts, its catalytic roles have received a great amount of attention from experiment and theory. Its primary function is to enhance the oxidation activity of catalysts, which is largely governed by the low activation barrier for creating lattice O vacancies. Such an important characteristic of ceria has been exploited in CO oxidation, methane partial oxidation, volatile organic compound oxidation, and the water-gas shift (WGS) reaction and in the context of automotive applications. A great challenge of such heterogeneously catalyzed processes remains the unambiguous identification of active sites.In oxidation reactions, closing the catalytic cycle requires ceria reoxidation by gas-phase oxygen, which includes oxygen adsorption and activation. While the general mechanistic framework of such processes is accepted, only very recently has an atomic-level understanding of oxygen activation on ceria powders been achieved by combined experimental and theoretical studies using in situ multiwavelength Raman spectroscopy and DFT.Recent studies have revealed that the adsorption and activation of gas-phase oxygen on ceria is strongly facet-dependent and involves different superoxide/peroxide species, which can now be unambiguously assigned to ceria surface sites using the combined Raman and DFT approach. Our results demonstrate that, as a result of oxygen dissociation, vacant ceria lattice sites are healed, highlighting the close relationship of surface processes with lattice oxygen dynamics, which is also of technical relevance in the context of oxygen storage-release applications.A recent DFT interpretation of Raman spectra of polycrystalline ceria enables us to take account of all (sub)surface and bulk vibrational features observed in the experimental spectra and has revealed new findings of great relevance for a mechanistic understanding of ceria-based catalysts. These include the identification of surface oxygen (Ce-O) modes and the quantification of subsurface oxygen defects. Combining these theoretical insights with operando Raman experiments now allows the (sub)surface oxygen dynamics of ceria and noble metal/ceria catalysts to be monitored under the reaction conditions.Applying these findings to Au/ceria catalysts provides univocal evidence for ceria support participation in heterogeneous catalysis. For room-temperature CO oxidation, operando Raman monitoring the (sub)surface defect dynamics clearly demonstrates the dependence of catalytic activity on the ceria reduction state. Extending the combined experimental/DFT approach to operando IR spectroscopy allows the elucidation of the nature of the active gold as (pseudo)single Au+ sites and enables us to develop a detailed mechanistic picture of the catalytic cycle. Temperature-dependent studies highlight the importance of facet-dependent defect formation energies and adsorbate stabilities (e.g., carbonates). While the latter aspects are also evidenced to play a role in the WGS reaction, the facet-dependent catalytic performance shows a correlation with the extent of gold agglomeration. Our findings are fully consistent with a redox mechanism, thus adding a new perspective to the ongoing discussion of the WGS reaction.As outlined above for ceria-based catalysts, closely combining state-of-the-art in situ/operando spectroscopy and theory constitutes a powerful approach to rational catalyst design by providing essential mechanistic information based on an atomic-level understanding of reactions.

Ce=O Terminated CeO2.

Multiply bonded lanthanide oxo groups are rare in coordination compounds and have not previously been reported for a surface termination of a lanthanide oxide. Here we report the observation of a Ce=O terminated ceria surface in a CeO2 (111)-( 3 × 3 )R30° reconstruction of ≈3 nm thick ceria islands prepared on Pt(111). This is evidenced by scanning tunnelling microscopy (STM), low energy electron diffraction (LEED) and high-resolution electron energy loss spectroscopy (HREELS) measurements in conjunction with density functional theory (DFT) calculations. A Ce=O stretching frequency of 775 cm-1 is observed in HREELS, compared with 766 cm-1 calculated by DFT. The calculations also predict that the Ce=O bond is weak, with an oxygen vacancy formation energy of 0.85 eV. This could play an important role in the facile removal of lattice oxygen from CeO2 , accompanied by the reduction of CeIV to CeIII , which is a key attribute of ceria-based systems in connection with their unique catalytic properties.

Insight into the mechanism of the water-gas shift reaction over Au/CeO2 catalysts using combined operando spectroscopies.

The mechanism of the low-temperature water-gas shift (LT-WGS) reaction over Au/CeO2 catalysts with different ceria terminations, i.e., (111), (110), and (100) facets, was investigated. Using combined operando Raman and UV-Vis spectroscopy as well as isotope exchange experiments, we are able to draw conclusions about the reducibility behaviour and the exchange of surface oxygen. Additional density functional theory (DFT) calculations facilitate the vibrational bands assignments and enhance the interpretation of the results on a molecular level. A facet-dependent role of gold is observed with respect to the oxygen dynamics, since for the CeO2(111) facet the presence of gold is required to exchange surface oxygen, whereas the CeO2(110) facet requires no gold, as rationalized by the low defect formation energy of this facet. This behaviour suggests that surface properties (termination, stepped surface) may have a strong effect on the reactivity. While the reduction of the support accompanies the reaction, its extent does not directly correlate with activity, highlighting the importance of other properties, such as the dissociative adsorption of water and/or CO2/H2 desorption. The results of our facet-dependent study are consistent with a redox mechanism, as underlined by H218O isotopic exchange experiments demonstrating the ready exchange of surface oxygen.

Breaking Simple Scaling Relations through Metal-Oxide Interactions: Understanding Room-Temperature Activation of Methane on M/CeO2 (M = Pt, Ni, or Co) Interfaces.

The clean activation of methane at low temperatures remains an eminent challenge and a field of competitive research. In particular, on late transition metal surfaces such as Pt(111) or Ni(111), higher temperatures are necessary to activate the hydrocarbon molecule, but a massive deposition of carbon makes the metal surface useless for catalytic activity. However, on very low-loaded M/CeO2 (M = Pt, Ni, or Co) surfaces, the dissociation of methane occurs at room temperature, which is unexpected considering simple linear scaling relationships. This intriguing phenomenon has been studied using a combination of experimental techniques (ambient-pressure X-ray photoelectron spectroscopy, time-resolved X-ray diffraction, and X-ray absorption spectroscopy) and density functional theory-based calculations. The experimental and theoretical studies show that the size and morphology of the supported nanoparticles together with strong metal-support interactions are behind the deviations from the scaling relations. These findings point toward a possible strategy for circumventing scaling relations, producing active and stable catalysts that can be employed for methane activation and conversion.

Elucidating the Oxygen Storage-Release Dynamics in Ceria Nanorods by Combined Multi-Wavelength Raman Spectroscopy and DFT.

The oxygen storage-release dynamics in ceria nanorods is elucidated by using a combination of in situ multi-wavelength Raman spectroscopy and density functional theory calculations. Ceria nanorods exposing CeO2(100) and CeO2(110) facets are shown to be characterized by highly facet-dependent properties regarding molecular oxygen activation and decomposition as well as lattice oxygen dynamics. Temperature-dependent Raman results show that oxygen is stored in the form of peroxide species on the (100) facets, which are then released as gaseous oxygen, whereas lattice oxygen is involved with the (110) facets. On the latter, peroxide species first decompose into oxygen atoms that heal vacant lattice oxygen sites before being released as gaseous oxygen. The exposure of different facets makes ceria nanorods an interesting material for catalytic applications, because they allow the use of a mixture of oxygen storage-release functions, as well as their synergistic interactions, in a single system.

Vibrational Frequencies of Cerium-Oxide-Bound CO: A Challenge for Conventional DFT Methods.

In ceria-based catalysis, the shape of the catalyst particle, which determines the exposed crystal facets, profoundly affects its reactivity. The vibrational frequency of adsorbed carbon monoxide (CO) can be used as a sensitive probe to identify the exposed surface facets, provided reference data on well-defined single crystal surfaces together with a definitive theoretical assignment exist. We investigate the adsorption of CO on the CeO_{2}(110) and (111) surfaces and show that the commonly applied DFT(PBE)+U method does not provide reliable CO vibrational frequencies by comparing with state-of-the-art infrared spectroscopy experiments for monocrystalline CeO_{2} surfaces. Good agreement requires the hybrid DFT approach with the HSE06 functional. The failure of conventional density-functional theory (DFT) is explained in terms of its inability to accurately describe the facet- and configuration-specific donation and backdonation effects that control the changes in the C─O bond length upon CO adsorption and the CO force constant. Our findings thus provide a theoretical basis for the detailed interpretation of experiments and open up the path to characterize more complex scenarios, including oxygen vacancies and metal adatoms.

Oxygen-Vacancy Dynamics and Entanglement with Polaron Hopping at the Reduced CeO_{2}(111) Surface.

The migration of oxygen vacancies (V_{O}) in ceria-based systems is crucial to their functionality in applications. Yet, although the V_{O}'s structure and the distribution of the Ce^{3+} polarons at the CeO_{2}(111) surface has received extensive attention, the dynamic behaviors of V_{O}'s and polarons are not fully understood. By combining density functional theory calculations and ab initio molecular dynamics simulations, we show that the movements of V_{O}'s and polarons exhibit very distinct entanglement characteristics within a temperature range of 300-900 K, and that the positions of the Ce^{3+} polarons play a key role in the V_{O} migration. Long-distance vacancy migration occurs at moderate temperatures when the "suitable" Ce^{3+} distribution remains long enough to promote oxygen migration. This study provides microscopic insight into the interplay between the electronic and ionic charge transport in ceria that will be beneficial for the rational design of conductive ceria-based materials.

Experimental and Theoretical Study on the Nature of Adsorbed Oxygen Species on Shaped Ceria Nanoparticles.

Ceria is widely used in heterogeneous catalysis owing to its redox properties. Engineering the shape of ceria particles offers a powerful tool to develop materials with enhanced catalytic properties. In this study, we provide evidence for the shape-dependent dioxygen adsorption and activation of ceria nanoparticles with (111) and (100) facets by in situ Raman spectroscopy and relate these properties to unique adsorption sites employing density functional theory. Temperature- and gas-phase-dependent experiments demonstrate facilitated formation of peroxide, superoxide, and weakly bound dioxygen species on the (100) facets as rationalized by calculated vibrational frequencies of O22-, O2-, and O2 species on CeO2- x(100) surfaces. Our results show that localization of the excess charge, driving the Ce4+ → Ce3+ reduction, significantly affects the stretching vibrations. Our approach provides a powerful basis for future developments of ceria-based catalysts by bridging the materials gap between idealized and real catalytic systems.

Direct Conversion of Methane to Methanol on Ni-Ceria Surfaces: Metal-Support Interactions and Water-Enabled Catalytic Conversion by Site Blocking.

The transformation of methane into methanol or higher alcohols at moderate temperature and pressure conditions is of great environmental interest and remains a challenge despite many efforts. Extended surfaces of metallic nickel are inactive for a direct CH4 → CH3OH conversion. This experimental and computational study provides clear evidence that low Ni loadings on a CeO2(111) support can perform a direct catalytic cycle for the generation of methanol at low temperature using oxygen and water as reactants, with a higher selectivity than ever reported for ceria-based catalysts. On the basis of ambient pressure X-ray photoemission spectroscopy and density functional theory calculations, we demonstrate that water plays a crucial role in blocking catalyst sites where methyl species could fully decompose, an essential factor for diminishing the production of CO and CO2, and in generating sites on which methoxy species and ultimately methanol can form. In addition to water-site blocking, one needs the effects of metal-support interactions to bind and activate methane and water. These findings should be considered when designing metal/oxide catalysts for converting methane to value-added chemicals and fuels.