Key role of chemistry versus bias in electrocatalytic oxygen evolution.

The oxygen evolution reaction has an important role in many alternative-energy schemes because it supplies the protons and electrons required for converting renewable electricity into chemical fuels1-3. Electrocatalysts accelerate the reaction by facilitating the required electron transfer4, as well as the formation and rupture of chemical bonds5. This involvement in fundamentally different processes results in complex electrochemical kinetics that can be challenging to understand and control, and that typically depends exponentially on overpotential1,2,6,7. Such behaviour emerges when the applied bias drives the reaction in line with the phenomenological Butler-Volmer theory, which focuses on electron transfer8, enabling the use of Tafel analysis to gain mechanistic insight under quasi-equilibrium9-11 or steady-state assumptions12. However, the charging of catalyst surfaces under bias also affects bond formation and rupture13-15, the effect of which on the electrocatalytic rate is not accounted for by the phenomenological Tafel analysis8 and is often unknown. Here we report pulse voltammetry and operando X-ray absorption spectroscopy measurements on iridium oxide to show that the applied bias does not act directly on the reaction coordinate, but affects the electrocatalytically generated current through charge accumulation in the catalyst. We find that the activation free energy decreases linearly with the amount of oxidative charge stored, and show that this relationship underlies electrocatalytic performance and can be evaluated using measurement and computation. We anticipate that these findings and our methodology will help to better understand other electrocatalytic materials and design systems with improved performance.

Low-Temperature Methane Activation Reaction Pathways over Mechanochemically-Generated Ce4+/Cu+ Interfacial Sites.

Methane is a valuable resource and its valorization is an important challenge in heterogeneous catalysis. Here it is shown that CeO2/CuO composite prepared by ball milling activates methane at a temperature as low as 250 °C. In contrast to conventionally prepared catalysts, the formation of partial oxidation products such as methanol and formaldehyde is also observed. Through an in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) and operando Near Edge X-Ray Absorption Fine Structure Spectroscopy (NEXAFS) approach, it can be established that this unusual reactivity can be attributed to the presence of Ce4+/Cu+ interfaces generated through a redox exchange between Ce3+ and Cu2+ atoms facilitated by the mechanical energy supplied during milling. DFT modeling of the electronic properties confirms the existence of a charge transfer mechanism. These results demonstrate the effectiveness and distinctiveness of the mechanical approach in creating unique and resilient interfaces thereby enabling the optimization and refining of CeO2/CuO catalysts in methane activation reactions.

Direct and indirect role of Fe doping in NiOOH monolayer for water oxidation catalysis.

Water oxidation activity of pristine NiOOH is greatly enhanced by doping it with Fe. However, the precise role of Fe is still being debated. Using a first-principles DFT+U approach, we investigate the direct and indirect roles of Fe in enhancing the oxygen evolution reaction (OER) activity of NiOOH monolayers. Considering two Mars-Van-Krevelen mechanisms of OER based on the source of O-O bond formation, we show that a mechanism involving the coupling of lattice oxygen is generally more favorable than water nucleophilic attack on lattice oxygen. On doping with Fe, the overpotential of NiOOH is reduced by 0.33 V, in excellent agreement with experimental findings. Introducing Fe at active sites results in different potential determining steps (PDS) in the two mechanisms. The Ni sites in pristine and Fe-doped NiOOH have the same PDS regardless of the mechanism. The Fe sites not only have the lowest overpotential but also decrease the overpotential for Ni sites.

The band structure and optical absorption of hematite (α-Fe2O3): a first-principles GW-BSE study.

Hematite (α-Fe2O3) is a widely investigated photocatalyst material for the oxygen evolution reaction, a key step in photoelectrochemical water splitting. Having a suitable band gap for light absorption, being chemically stable and based on earth abundant elements, hematite is a promising candidate for the fabrication of devices able to split water using sunlight. What limits its performance is the high rate of bulk recombination following the optical excitation, so that few charge carriers are able to reach the surface to perform the redox reactions. In an effort to better understand the light absorption properties of hematite, in this work we perform a theoretical first-principles investigation of its band structure and optical absorption properties. We use state-of-the-art many-body perturbation theory, including the effects of electron-hole interaction by solving the Bethe-Salpeter equation. Our approach provides good agreement with the available photoemission and absorption measurements and shows that the onset of light absorption is mostly due to ligand-to-metal charge transfer excitations, giving rise to fairly localized excitons. This is at variance with the previously accepted view that these excitations are due to Fe d-d transitions.

Understanding the electrochemical double layer at the hematite/water interface: A first principles molecular dynamics study.

Using first principles molecular dynamics simulations, we probe the electrochemical double layer formed at the interface between the hematite surface and water. We consider two terminations of the (001) surface, viz., the fully hydroxylated (OH) and the stoichiometric (FeO3Fe) termination. We explicitly incorporate the counterions (Na+ and F-) in the solution, and model both specific and nonspecific adsorption of F- ions. We find that F- ions prefer to bind directly to the Fe ions (specific adsorption), with a substantial energy gain (0.75 eV/ion). We investigate the effect of the interface and the counterions on the dipole of individual water molecules. We find significant deviations of +0.2/-0.15 D for dipoles of the first solvation shell water molecules of F-/Na+ ions, respectively. Additionally, the hydration layers at the interface show an enhancement in the dipole moment resulting from stronger hydrogen bonding interactions between the water molecules and surface charged species. Furthermore, we analyze the electrostatic potential profile at the solid/liquid interface as a function of the kind of counterion present in the double layer and compute the capacitance of the compact (Helmholtz) layer. We find that our results (40.3 ± 3.5 µF/cm2 for the OH termination and 51 ± 5 µF/cm2 for the FeO3Fe termination) compare favorably with values reported by potentiometric titration based experimental studies (10-100 µF/cm2).

Water oxidation surface mechanisms replicated by a totally inorganic tetraruthenium-oxo molecular complex.

Solar-to-fuel energy conversion relies on the invention of efficient catalysts enabling water oxidation through low-energy pathways. Our aerobic life is based on this strategy, mastered by the natural Photosystem II enzyme, using a tetranuclear Mn-oxo complex as oxygen evolving center. Within artificial devices, water can be oxidized efficiently on tailored metal-oxide surfaces such as RuO2. The quest for catalyst optimization in vitro is plagued by the elusive description of the active sites on bulk oxides. Although molecular mimics of the natural catalyst have been proposed, they generally suffer from oxidative degradation under multiturnover regime. Here we investigate a nano-sized Ru4-polyoxometalate standing as an efficient artificial catalyst featuring a totally inorganic molecular structure with enhanced stability. Experimental and computational evidence reported herein indicates that this is a unique molecular species mimicking oxygenic RuO2 surfaces. Ru4-polyoxometalate bridges the gap between homogeneous and heterogeneous water oxidation catalysis, leading to a breakthrough system. Density functional theory calculations show that the catalytic efficiency stems from the optimal distribution of the free energy cost to form reaction intermediates, in analogy with metal-oxide catalysts, thus providing a unifying picture for the two realms of water oxidation catalysis. These correlations among the mechanism of reaction, thermodynamic efficiency, and local structure of the active sites provide the key guidelines for the rational design of superior molecular catalysts and composite materials designed with a bottom-up approach and atomic control.

Thermodynamic and spectroscopic properties of oxygen on silver under an oxygen atmosphere.

We report on a combined density functional theory and the experimental study of the O1s binding energies and X-ray Absorption Near Edge Structure (XANES) of a variety of oxygen species on Ag(111) and Ag(110) surfaces. Our theoretical spectra agree with our measured results for known structures, including the p(N× 1) reconstruction of the Ag(110) surface and the p(4 × 4) reconstruction of the Ag(111) surface. Combining the O1s binding energy and XANES spectra yields unique spectroscopic fingerprints, allowing us to show that unreconstructed atomic oxygen is likely not present on either surface under equilibrium conditions at oxygen chemical potentials typical for ethylene epoxidation. Furthermore, we find no adsorbed or dissolved atomic species whose calculated spectroscopic features agree with those measured for the oxygen species believed to catalyze the partial oxidation of ethylene.

Interface structure and reactivity of water-oxidation Ru-polyoxometalate catalysts on functionalized graphene electrodes.

We combine classical empirical potentials and density functional theory (DFT) calculations to characterize the catalyst/electrode interface of a promising device for artificial photosynthesis. This system consists of inorganic Ru-polyoxometalate (Ru-POM) molecules that are supported by a graphitic substrate functionalized with organic dendrimers. The experimental atomic-scale characterization of the active interface under working conditions is hampered by the complexity of its structure, composition, as well as by the presence of the electrolyte or solvent. We provide a detailed atomistic model of the electrode/catalyst interface and show that the catalyst anchoring is remarkably dependent on water solvation. A tight host-guest binding geometry between the surface dendrimers and the Ru-POM catalyst is predicted under vacuum conditions. The solvent destabilizes this geometry, leads to unfolding of the dendrimers and to their flattening on the graphitic surface. The Ru-POM catalyst binds to this organic interlayer through a stable electrostatic link between one POM termination and the charged terminations of the dendrimers. The calculated dynamics and mobility of the Ru-POM catalyst at the electrode surface are in fair agreement with the available high-resolution transmission electron microscopy data. In addition, we demonstrate that the high thermodynamic water-oxidation efficiency of the Ru-POM catalyst is not affected by the binding to the electrode, thus rationalizing the similar electrochemical performances measured for homogeneous and heterogeneous Ru-POM catalysts.

Adsorbate induced vacancy formation on silver surfaces.

The energy required to form and remove vacancies on metal surfaces mediates the rate of mass transport during a wide range of processes. These energies are known to be sensitive to environmental conditions. Here, we use electronic structure density functional theory calculations to show that the surface vacancy formation energy of silver changes markedly in the presence of adsorbed and dissolved oxygen. We found that adsorbed atomic oxygen can reduce the surface vacancy formation energy of the Ag(111) surface by more than 30%, whereas surface vacancy formation becomes exothermic in the presence of pure subsurface oxygen. We went on to show that the total directionality of the topologically defined bond paths can be used to understand these changes. The resulting structure-property relationship was used to predict the behavior of silver in different atmospheres. We show that the surface vacancy formation energy decreases when electronegative elements are adsorbed on the surface, but that it can increase when electropositive elements are adsorbed.

Photo-driven oxidation of water on α-Fe2O3 surfaces: an ab initio study.

Adopting the theoretical scheme developed by the Nørskov group [see, for example, Nørskov et al., J. Phys. Chem. B 108, 17886 (2004)], we conducted a density functional theory study of photo-driven oxidation processes of water on various terminations of the clean hematite (α-Fe2O3) (0001) surface, explicitly taking into account the strong correlation among the 3d states of iron through the Hubbard U parameter. Six best-known terminations, namely, Fe−Fe−O3− (we call S1), O−Fe−Fe−(S2), O2−Fe−Fe−(S3), O3−Fe−Fe− (S4), Fe−O3−Fe− (S5), and O−Fe−O3−(S6), are first exposed to water, the stability of resulting surfaces is investigated under photoelectrochemical conditions by considering different chemical reactions (and their reaction free energies) that lead to surfaces covered by O atoms or/and OH groups. Assuming that the water splitting reaction is driven by the redox potential for photogenerated holes with respect to the normal hydrogen electrode, UVB, at voltage larger than UVB, most 3-oxygen terminated substrates are stable. These results thus suggest that the surface, hydroxylated in the dark, should release protons under illumination. Considering the surface free energy of all the possible terminations shows that O3­S5 and O3­S1 are the most thermodynamically stable. While water oxidation process on the former requires an overpotential of 1.22 V, only 0.84 V is needed on the latter.

Rigid- and polarizable-ion potentials for modeling Ru-polyoxometalate catalysts for water oxidation.

This work assesses the predictive power and capabilities of classical interatomic potentials for describing the atomistic structure of a fully inorganic water-oxidation catalyst in the gas phase and in solution. We address a Ru-polyoxometalate molecule (Ru-POM) that is presently one of the most promising catalysts for water oxidation due to its efficiency and stability under reaction conditions. The Ru-POM molecule is modeled with two interatomic potentials, the rigid ion model and the shell model potentials, which are used to perform molecular dynamics simulations. The predictions of these two approaches are discussed and compared to the available ab-initio data. These results allow us to establish the suitable level of theory to model complex heterogeneous interfaces between the Ru-POM and electrodes in solution.

Correction to "Understanding Anomalous Gas-Phase Peak Shifts in Dip-and-Pull Ambient Pressure XPS Experiments".

[This corrects the article DOI: 10.1021/acs.jpcc.4c00113.].

Ag-Cu catalysts for ethylene epoxidation: selectivity and activity descriptors.

Ag-Cu alloy catalysts for ethylene epoxidation have been shown to yield higher selectivity towards ethylene oxide compared to pure Ag, the unique catalyst employed in the industrial process. Previous studies showed that under oxidizing conditions Cu forms oxide layers on top of Ag. Using first-principles atomistic simulations based on density functional theory, we investigate the reaction mechanism on the thin oxide layer structures and establish the reasons for the improved selectivity. We extend the range of applicability of the selectivity descriptor proposed by Kokalj et al. [J. Catal. 254, 304 (2008)], based on binding energies of reactants, intermediates, and products, by refitting its parameters so as to include thin oxide layer catalysts. We show that the selectivity is mainly controlled by the relative strength of the metal-carbon vs. metal-oxygen bonds, while the height of the reaction barriers mostly depend on the binding energy of the common oxametallacycle intermediate.

A first principles study of water oxidation catalyzed by a tetraruthenium-oxo core embedded in polyoxometalate ligands.

We present a computational study addressing the catalytic cycle of a recently-synthesized all-inorganic homogeneous catalyst capable to promote water oxidation with low overpotential and high turnover frequency [Sartorel et al., J. Am. Chem. Soc., 2008, 130, 5006; Geletii et al., Angew. Chem., Int. Ed., 2008, 47, 3896]. This catalyst consists of a tetraruthenium-oxo core [Ru(4)O(4)(OH)(2)·(H(2)O)(4)](6+)capped by two polyoxometalate [SiW(10)O(36)](8-) units. The reaction mechanism underpinning its efficiency is currently under debate. We study a reaction cycle involving four consecutive proton-coupled electron transfer (PCET) processes that successively oxidize the four Ru(IV)-H(2)O units of the initial state (S(0)) to the four Ru(V)-OH centers of the activated intermediate (S(4)). The energetics of these electrochemical processes as well as the structural and electronic properties of the reaction intermediates are studied with ab initio Density Functional Theory (DFT) calculations. After characterizing these reaction intermediates in the gas phase, we show that the solvated tetraruthenate core undergoes a solvent-induced structural distortion that brings the predicted molecular geometry to excellent agreement with the experimental X-ray diffraction data. The calculated electronic properties of the catalyst are instead weakly dependent on the presence of the solvent. The frontier orbitals of the initial state as well as the electronic states involved in the PCET steps are shown to be localized on the tetraruthenium-oxo core. The reaction thermodynamics predicted for the intermediate reaction steps is in good agreement with the available cyclic voltammetry measurements up to S(3), but the calculated free energy difference between the initial and the activated state (S(0)/S(4)) turns out to be significantly lower than the thermodynamic limit for water oxidation. Since the oxidizing power of the S(0)/S(4) couple is not sufficient to split water, we suggest that promoting this reaction would require cycling between higher oxidation states.

Structure and Energetics of Dye-Sensitized NiO Interfaces in Water from Ab Initio MD and Large-Scale GW Calculations.

The energy-level alignment across solvated molecule/semiconductor interfaces is a crucial property for the correct functioning of dye-sensitized photoelectrodes, where, following the absorption of solar light, a cascade of interfacial hole/electron transfer processes has to efficiently take place. In light of the difficulty of performing X-ray photoelectron spectroscopy measurements at the molecule/solvent/metal-oxide interface, being able to accurately predict the level alignment by first-principles calculations on realistic structural models would represent an important step toward the optimization of the device. In this respect, dye/NiO surfaces, employed in p-type dye-sensitized solar cells, are undoubtedly challenging for ab initio methods and, also for this reason, much less investigated than the n-type dye/TiO2 counterpart. Here, we consider the C343-sensitized NiO surface in water and combine ab initio molecular dynamics (AIMD) simulations with GW (G0W0) calculations, performed along the MD trajectory to reliably describe the structure and energetics of the interface when explicit solvation and finite temperature effects are accounted for. We show that the differential perturbative correction on the NiO and molecule states obtained at the GW level is mandatory to recover the correct (physical) interfacial energetics, allowing hole transfer from the semiconductor valence band to the highest occupied molecular orbital (HOMO) of the dye. Moreover, the calculated average driving force quantitatively agrees with the experimental estimate.

Alloy catalyst in a reactive environment: the example of ag-cu particles for ethylene epoxidation.

Combining first-principles calculations and in situ photoelectron spectroscopy, we show how the composition and structure of the surface of an alloy catalyst is affected by the temperature and pressure of the reagents. The Ag-Cu alloy, recently proposed as an improved catalyst for ethylene epoxidation, forms a thin Cu-O surface oxide, while a Ag-Cu surface alloy is found not to be stable. Several possible surface structures are identified, among which the catalyst surface is likely to dynamically evolve under reaction conditions.

Formation of a 2D Meta-stable Oxide by Differential Oxidation of AgCu Alloys.

Metal alloy catalysts can develop complex surface structures when exposed to reactive atmospheres. The structures of the resulting surfaces have intricate relationships with a myriad of factors, such as the affinity of the individual alloying elements to the components of the gas atmosphere and the bond strengths of the multitude of low-energy surface compounds that can be formed. Identifying the atomic structure of such surfaces is a prerequisite for establishing structure-property relationships, as well as for modeling such catalysts in ab initio calculations. Here, we show that an alloy, consisting of an oxophilic metal (Cu) diluted into a noble metal (Ag), forms a meta-stable two-dimensional oxide monolayer, when the alloy is subjected to oxidative reaction conditions. The presence of this oxide is correlated with selectivity in the corresponding test reaction of ethylene epoxidation. In the present study, using a combination of in situ, ex situ, and theoretical methods (NAP-XPS, XPEEM, LEED, and DFT), we determine the structure to be a two-dimensional analogue of Cu2O, resembling a single lattice plane of Cu2O. The overlayer holds a pseudo-epitaxial relationship with the underlying noble metal. Spectroscopic evidence shows that the oxide's electronic structure is qualitatively distinct from its three-dimensional counterpart, and because of weak electronic coupling with the underlying noble metal, it exhibits metallic properties. These findings provide precise details of this peculiar structure and valuable insights into how alloying can enhance catalytic properties.

Water adsorption on the stoichiometric and reduced CeO2(111) surface: a first-principles investigation.

We present a density functional theory investigation of the interaction between water and cerium oxide surfaces, considering both the stoichiometric and the reduced surfaces. We study the atomic structure and energetics of various configurations of water adsorption (for a water coverage of 0.25 ML) and account for the effect of temperature and pressure of the environment, containing both oxygen and water vapor, employing the ab initio atomistic thermodynamics approach. Through our investigation, we obtain the phase diagram of the water-ceria system, which enables us to discuss the stability of various surface structures as a function of the ambient conditions. For the stoichiometric surface, we find that the most stable configuration for water is when it is bonded at the cerium site, involving two O-H bonds of hydrogen and oxygen atoms at the surface. If oxygen vacancies are introduced at the surface, which is predicted under more reducing conditions, the binding energy of water is stronger, indicating an effective attractive interaction between water molecules and oxygen vacancies. Water dissociation, and the associated activation energies, are studied, and the role of oxygen vacancies is found to be crucial to stabilize the dissociated fragments. We present a detailed analysis of the stability of the water-ceria system as a function of the ambient conditions, and focus on two important surface processes: water adsorption/desorption on the stoichiometric surface and oxygen vacancy formation in the presence of water vapor. A study of the vibrational contribution to the free energy allows us to estimate the effect of this term on the stability range of adsorbed water.

Quantitative Analysis of the Oxidation State of Cobalt Oxides by Resonant Photoemission Spectroscopy.

Quantitative assessment of the charge transfer phenomena in cobalt oxides and cobalt complexes is essential for the design of advanced catalytic materials. We propose a method for the evaluation of the oxidation state of cobalt oxides with mixed valence states using resonant photoemission spectroscopy. The method is based on the calculation of the resonant enhancement ratio (RER) from the heights of the resonant features associated with the Co3+ and Co2+ states. The nature of the corresponding states was corroborated by means of density functional calculations. We employed a well-ordered Co3O4(111) film to calibrate the RER with respect to the atomic Co3+/Co2+ ratio. The method was applied to monitor the reduction of a well-ordered Co3O4(111) film to CoO(111) upon annealing under exposure to isopropanol. We demonstrate that this method yields the stoichiometry of cobalt oxides at a level of accuracy that cannot be achieved when fitting the Co 2p core level spectra.