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Single-atom catalysts (SACs) have recently attracted broad scientific interests due to their unique structural feature, the single-atom dispersion. Optimized electronic structure as well as high stability are required for single-atom catalysts to enable efficient electrochemical production of H2O2. Herein, we report a facile synthesis method that stabilizes atomic Pd species on the reduced graphene oxide/N-doped carbon hollow carbon nanospheres (Pd1/N-C). Pd1/N-C exhibited remarkable electrochemical H2O2 production rate with high faradaic efficiency, reaching 80%. The single-atom structure and its high H2O2 production rate were maintained even after 10,000 cycle stability test. The existence of single-atom Pd as well as its coordination with N species is responsible for its high activity, selectivity, and stability. The N coordination number and substrate doping around Pd atoms are found to be critical for an optimized adsorption energy of intermediate *OOH, resulting in efficient electrochemical H2O2 production.

Summary High-entropy alloys (HEAs) provide a near-continuous distribution of adsorption energies. With a minority of sites having optimal properties that dominate the catalysis, the overall catalytic activity can increase. In this article, we focus on the oxygen reduction reaction (ORR). We present density functional theory (DFT) calculated *OH and *O adsorption energies on a random subset of available binding sites on the surface of the HEA IrPdPtRhRu. Employing a simple machine learning algorithm, we predict remaining adsorption energies, finding good agreement between calculated and predicted values. With a full catalog of available adsorption energies, an appropriate expression for predicting catalytic activity was used to optimize the HEA composition. The HEA then becomes a design platform for the unbiased discovery of new alloys by promoting sites with exceptional catalytic activity. Setting different optimization constraints led to a new HEA composition and binary alloy IrPt, showing significant enhancements over pure Pt(111).

Summary Anodic oxygen evolution has gained significant prominence in electrochemical research in the last decade in connection with renewable electricity storage. With water being the only available fossil free source of hydrogen, which is deemed the primary storage medium, the water electrolysis optimization is one of the biggest challenges of today's electrochemistry. A development of novel oxygen evolution reaction (OER) catalysts is motivated to increase the feasibility of the current OER catalysts in terms of activity, stability and price. This review summarizes the most recent synthetic approaches in OER catalyst synthesis for acid and alkaline electrolyzers stressing the structural aspects of the proposed catalysts. The reported novel catalyst approaches are aligned with existing theoretical and experiment-based descriptors for rational of oxygen evolution activity improvement.

Achieving ultra-deep hydrodesulfurization means enabling removal of the last fractions of sulfur, contained in refractory molecules, from oil. Improving the state-of-the-art Co-promoted MoS2 (CoMoS) catalyst or the development of novel catalysts is crucial for this. Improving CoMoS requires more insight in the way sulfur containing molecules interact with it. Herein, we model the adsorption of sulfur containing molecules on the S-edge, M-edge, corner and basal plane of CoMoS using density functional theory. The obtained adsorption configurations and energies point to a preference towards physisorption at the S-edge and chemisorption in vacancies at the M-edge and corner. Smaller molecules, such as thiophene and methylthiol, were found to prefer vacancies when adsorbing while larger, sterically hindered molecules as 4,6-dimethyldibenzothiophene prefer physisorption on the brim of the edges or the basal plane through van der Waals interactions. Hydrogenation generally leads to a preference towards adsorption at vacancies for thiophene and dibenzothiophene while for 4,6-dimethyldibenzothiophene hydrogenation leads to preferential adsorption on the S-edge brim, possibly explaining why 4,6-dimethyldibenzothiophene does not get desulfurized directly but follows a hydrogenation route. Thiolate formation energies were also calculated for the different molecules and used to predict which sites are most likely to be involved in breaking carbon-sulfur bonds. The thiolate formation energies show the inert nature of the basal plane towards breaking carbon-sulfur and sulfur-hydrogen bonds. Additionally, activation energies for thiophene and dibenzothiophene carbon-sulfur bond scission indicate that both molecules follow the direct desulfurization route on under-coordinated sites or vacancies.

Currently, no catalysts are completely selective for the electrochemical CO2 Reduction Reaction (CO2RR). Based on trends in density functional theory calculations of reaction intermediates we find that the single metal site in a porphyrine-like structure has a simple advantage of limiting the competing Hydrogen Evolution Reaction (HER). The single metal site in a porphyrine-like structure requires an ontop site binding of hydrogen, compared to the hollow site binding of hydrogen on a metal catalyst surface. The difference in binding site structure gives a fundamental energy-shift in the scaling relation of ∼0.3 eV between the COOH* vs. H* intermediate (CO2RR vs. HER). As a result, porphyrine-like catalysts have the advantage over metal catalyst of suppressing HER and enhancing CO2RR selectivity.

The low catalytic hydrodesulfurization (HDS) activity toward sterically hindered sulfur-containing molecules is a main industrial challenge in order to obtain ultra-low sulfur diesel. In this study we report a combined Scanning Tunneling Microscopy (STM) and Density Functional Theory (DFT) investigation of the adsorption of the sterically hindered sulfur-containing molecule 4,6-dimethyldibenzothiophene (4,6-DMDBT) onto a hydrotreating model catalyst for the Co promoted MoS2 (CoMoS) phase. The molecular adsorption occurs exclusively on the Co-promoted S-edge, most predominantly in a precursor-like diffusive physisorption referred to as delocalized ππ-mode. 4,6-DMDBT adsorption directly in a S-edge sulfur vacancy is observed exclusively in S-edge corner vacancies in an adsorption configuration reflecting a σσ-coordination. STM movies reveal dynamic conversion between the σσ-mode and an on-top ππ-adsorption providing a link between different adsorption sites and hence between the hydrogenation and direct desulfurization pathways in HDS. The low overall direct desulfurization activity of 4,6-DMDBT and related molecules is consistent with the low occurrence of S-vacancies on CoMoS S-edges predicted under HDS conditions in this study.

Electrocatalysis on high-entropy alloys (HEAs) is a very young field. There are only a few articles on the theoretical understanding of catalysis on these surfaces. Activity descriptors and a ‘theory of catalysis’ exist for uniform surfaces. In this article, we address scaling relations and Bronsted–Evans–Polanyi relations on HEAs which are fundamental for finding activity descriptors. We find that some relations hold, whereas others disappear on HEAs. This reveals some of the future challenges in modeling electrocatalysis on HEAs.

Using a combination of a sophisticated modeling protocol and well-established electrochemical techniques, we unravel the chemical composition of the low-index surfaces of copper in groundwater environments at different ion concentrations, pHs, and redox potentials. By carefully linking density functional theory (DFT) and cyclic voltammetry (CV), we are able to extract fundamental information on interfaces of broad significance. Herein, we focus on the case of groundwater found in deep geological environments of importance to the planned constructions of disposal repositories for spent nuclear fuel around the world. Within the error margins of DFT, we can assign adsorption structures and compositions to the current peaks of the CVs. It is found that among the groundwater ions of main interest (i.e. sulfide, bisulfide, sulfate, chloride and bicarbonate), sulfides (HS−, S2−) bind strongest to the surface, and are likely to dominate at the interfaces under the deep geological conditions relevant for repositories of spent nuclear fuel.

We study the oxygen reduction (ORR) and the oxygen evolution reaction (OER) and based on previous obtained mechanistic insight we provide a unified general analysis of the two reactions simultaneously. The analysis shows that control over at least two independent binding energies is required to obtain a reversible perfect catalyst for both ORR and OER. Often only the reactivity of the surface is changed by changing from one material to another and all binding energies scale with the reactivity. We investigate the limitation in efficiency imposed by these linear scaling relations. This analysis gives rise to a double volcano for ORR and OER, with a region in between, forbidden by the scaling relations. The reversible perfect catalyst for both ORR and OER would fall into this “forbidden region”. Previously, we have found that hydrogen acceptor functionality on oxide surfaces can improve the catalytic performance for OER beyond the limitations originating from the scaling relations. We use this concept to search for promising combinations of binding sites and hydrogen donor/acceptor sites available in transition metal doped graphene, which can act as a catalyst for ORR and OER. We find that MnN4-site embedded in graphene by itself or combined with a COOH is a promising combination for a great combined ORR/OER catalyst.

PtxY and PtxGd exhibit exceptionally high activity for oxygen reduction, both in the polycrystalline form and the nanoparticulate form. In order to understand the origin of the enhanced activity of these alloys, we have investigated thin films of these alloys on bulk Pt(111) crystals, i.e. Y/Pt(111) and Gd/Pt(111). These surfaces exhibit a 4-fold improvement over Pt(111). We observe the formation of a thick Pt overlayer after the electrochemical measurements, both on Y/Pt(111) and Gd/Pt(111). Using surface sensitive X-ray diffraction we revealed that crystalline closely packed Pt overlayers were formed. The diffraction experiments showed that the strain and crystallinity of the overlayers are strongly dependent on the electrochemical treatment, and in general show lateral compression.

The pH dependence of rate of the hydrogen evolution/oxidation reaction HER/HOR is investigated. Based on thermodynamic considerations, a possible explanation to the low exchange current for hydrogen reactions in alkaline is put forward. We propose this effect to be a consequence of the change in configurational entropy of the proton as it approaches the surface. As a proton crosses the outer Helmholtz plane, it will lose a fraction of its entropy before it can interact with the electrode surface, which gives rise to an entropic barrier. The size of this barrier will depend on the electrostatic environment in the double layer region. The entropic barrier can be rate determining only when the surface catalysis is fast. Therefore the effect of pH is most pronounced on good catalysts and for fast reactions. This entropic barrier is also in a good agreement with the unusually low prefactor measured in experiments of good catalysts such as Pt. In such catalysts, the enthalpy barrier of 0.1–0.2 eV of the rate-determining step does not come from any of the surface reactions (Volmer, Tafel or Heyrovsky) but instead from the proton transfer into the outer Helmholtz layer.

The paper focuses on theoretical and experimental aspects of an oxide surface optimization for oxygen reduction reaction (ORR). Various doped α-MnO2 based electrocatalysts were prepared by microwave-assisted hydrothermal synthesis and electrochemically characterized to validate density functional theory (DFT) based predictions of the oxidation state and local structure effects on the catalytic activity of α-MnO2 catalysts in ORR. Both theory and experiments conclude that the highest activity in ORR is to be expected in the case of clustered Mn3+ sites which yield activity comparable with that of the polycrystalline Pt. These active sites have to be formed under in-operando conditions and their formation is hindered in doped α-MnO2 catalysts. The activity of the other conceivable active sites based on non-clustered Mn3+ or Mn4+ is inferior to that of clustered Mn3+. The activation of Mn3+ or Mn4+ based active sites leads to a shift in selectivity of the ORR process towards 2 electron formation of hydrogen peroxide.

The oxygen evolution reaction in acidic electrolyzers requires the presence of stable catalysts and current collectors at the anode. IrO2 catalysts and Ti current collectors are among the best in this regard. We show evidence of iridium and titanium corrosion and subsequent membrane crossover in long-term experiments of proton-exchange membrane electrolyzers for H2O2 production. The accumulation of trace iridium at the cathode was linked to degraded performance and increased cathodic current from hydrogen evolution. Detection of trace metal content at the cathode electrodes was enabled by prompt-gamma ray activation analysis and neutron activation analysis. These findings are not just relevant for H2O2 electrolyzers but to any system using iridium-based anode catalysts, including CO2 electroreduction.

In this work, we examine the thermochemistry of methanol synthesis intermediates using density functional theory (DFT) and analyze the methanol synthesis reaction network using a steady-state micro-kinetic model. The energetics for methanol synthesis over Zn-terminated ZnO (0 0 0 1) are obtained from DFT calculations using the RPBE and BEEF-vdW functionals. The energies obtained from the two functionals are compared and it is determined that the BEEF-vdW functional is more appropriate for the reaction. The BEEF-vdW energetics are used to construct surface phase diagrams as a function of CO, H 2 O, and H 2 chemical potentials. The computed binding energies along with activation barriers from literature are used as inputs for a mean-field micro-kinetic model for methanol synthesis including the CO and CO 2 hydrogenation routes and the water–gas shift reaction. The kinetic model is used to investigate the methanol synthesis rate as a function of temperature and pressure. The results show qualitative agreement with experiment and yield information on the optimal working conditions of ZnO catalysts.

A scheme for evaluating symmetry factors of elementary electrode reactions using a density functional theory (DFT) based model of the electrochemical double layer is presented. As an illustration, the symmetry factor is determined for hydrogen adsorption via the electrochemical Volmer reaction. The DFT results are compared with predictions of an analytical single electron transfer model with quadratic reactant and product free energy surfaces. The analytical model, fed with input parameters calculated by DFT, is shown to give symmetry factors in reasonable agreement with those of the full-scale DFT model. This suggests that qualitative estimates of symmetry factors can be obtained at a moderate cost by combining comparatively cheap DFT calculations with reasonably sophisticated analytical models.

Alignment of metal and molecular electronic energy levels at electrode–electrolyte interfaces is investigated using density functional theory. Three different regimes exhibiting qualitatively different energy level alignments are observed. The regimes are roughly defined by the size of the metal work function relative to the ionization potential and/or electron affinity of the electrolyte. It is demonstrated that proper matching of these quantities is essential for successful ab initio modeling of electrochemical interfaces and it is further discussed how such matching can be obtained by careful tailoring of the interfacial atomic structure.

We study electronic hole localization in rutile and anatase titanium dioxide by means of Δ-Self-Consistent Field Density Functional Theory. In order to compare stabilities of the localized and the delocalized hole states we introduce a simple correction to the wrong description of the localization processes within DFT. The correction removes the non-linearity of energy for fractional excitations. We show that the self-trapped and the delocalized hole states have comparable stability in rutile TiO 2 whereas in anatase the former is favoured. The theoretical prediction of the adiabatic Potential Energy Surfaces for the hole localization compares well with published photoluminescence measurements.

We study the oxygen reduction reaction (ORR) mechanism on a Pt(1 1 1) surface using density functional theory calculations. We find that at low overpotentials the surface is covered with a half dissociated water layer. We estimate the barrier for proton transfer to this surface and the barrier for proton transport parallel to the surface within the half dissociated water network. We find both barriers to be small. The only potentially dependent step is the proton transfer from water to the half dissociated water layer. We find that ORR proceeds via four direct e− reductions without significant peroxide formation. We show that the oxygen–oxygen bond breaking is dependent on the local surface environment. The minimum energy pathway is constructed and we confirm that OH removal from the surface determines the overpotential.

A detailed atomistic model based on density functional theory calculations is presented of the charged solid–electrolyte interface. Having protons solvated in a water bilayer outside a Pt(1 1 1) slab with excess electrons, we show how the interface capacitance is well described and how the work function can be related directly to the potential scale of the normal hydrogen electrode. We also show how finite-size effects in common periodic slab-type calculations can be avoided in calculations of activation energies and reaction energies for charge transfer reactions, where we use the Heyrovsky reaction for hydrogen oxidation over a Pt(1 1 1) electrode as an example. 2008 Elsevier B.V. All rights reserved. An understanding of the structure and dynamics of the interface between a solid and an electrolyte is a prerequisite to a molecularlevel understanding of many adsorption phenomena and of electrochemical processes. Modelling the solid–electrolyte interface is extremely demanding. One needs to describe the solid surface, the liquid, the ions solvated in the liquid and the charge-transfer taking place at the interface during electrochemical reaction. In addition one needs to be able to describe the effect of varying the electrical potential of the electrode. While models of the interface have been formulated for decades [1–5] the methods of density functional theory (DFT), which have been very important for the present understanding of the gas–solid interface, have only been applied recently [6–20] and still in a quite approximate way. In the present Letter we propose a method for calculating the energetics of interface reactions on the basis of standard DFT slab calculations and show that the method gives values for the interface capacitance in agreement with experiments and provide a measure of the vacuum potential relative to the normal hydrogen electrode (NHE). Consider a metal electrode in contact with water with ions dissolved. Since the electrolyte is conducting the interface region must be charge neutral and the charge on the solid surface will be counteracted by an opposite charge built up by ions just outside the surface. This interface region, the Helmholtz layer, is often approximated by a ca. 3 A thick electrical double layer [21]. The double layer has a strong electrical field, which is central to the properties of the interface. The strength of the field is given by the charge, which in turn is determined by the chemical potential of the electrons. The potential is measured relative to a counter electrode which is also immersed in the electrolyte. Because there is no electrical field in the electrolyte, the electrode and the counter electrode can be considered independent of each other and can be studied separately. However, with an electrochemical reaction running, the system is out of equilibrium. Locally at the interface charge is consumed or produced, which gives rise to a small field driving the charge to or from the interface. Fig. 1 shows the main components of an atomistic model of a metal–electrolyte interface. It is possible to carry out DFT calculations for such a system using slab calculations to model the surface [19], but the calculations are hampered by the fact that for realistic electrode potentials the charge on the surface is small, often considerably less than one electron charge per 10 surface atoms. Unit cells with a large surface area are therefore needed. An even more severe problem is that if a charge transfer reaction takes place – say a proton is transferred to the surface – and the unit cell area is small, the potential changes significantly during the reaction. This finite-size effect can severely affect the results. Recent developments have made important steps towards a method that can treat the electrified solid–liquid interface. Filhol and Neurock have performed calculations where they charge the surface by adding or subtracting electrons from the slab modelling the metal electrode [17]. This means that a homogenous background charge of the opposite sign is introduced implicitly to make the system neutral. The advantage of this method is that a fraction of an electron can be introduced, and the charge can be varied continuously. Large area unit cells can therefore be avoided. The drawback is that the counter-ions are now not in the water layer just outside the electrode surface but spread out everywhere in space. The field just outside the surface is, therefore, too small [22]. Otani and Sugino [18] and Sugino et al. [20] have introduced the possibility of charging the surface and placing the counter charge in a layer beyond the water, ca. 15–20 A from the electrode. Again the charge can be varied continuously and due to the polarization of the water between the counter charge and the surface, most of the potential drop happens correctly just outside the surface. The drawback is that again the counter charge is placed far from the electrode and the polarization of the water may therefore not be entirely

We have investigated the anode reaction in direct methanol fuel cells using a database of adsorption free energies for 16 intermediates on 12 close-packed transition metal surfaces calculated with periodic, selfconsistent, density functional theory (DFT–GGA). This database, combined with a simple electrokinetic model of the methanol electrooxidation reaction, yields mechanistic insights that are consistent with previous experimental and theoretical studies on Pt, and extends these insights to a broad spectrum of other transition metals. In addition, by using linear scaling relations between the adsorption free energies of various intermediates in the reaction network, we find that the results determined with the full database of adsorption energies can be estimated by knowing only two key descriptors for each metal surface: the free energies of OH and CO on the surface. Two mechanisms for methanol oxidation to CO2 are investigated: an indirect mechanism that goes through a CO intermediate and a direct mechanism where methanol is oxidized to CO2 without the formation of a CO intermediate. For the direct mechanism, we find that, because of CO poisoning, only a small current will result on all non-group 11 transition metals; of these metals, Pt is predicted to be the most active. For methanol decomposition via the indirect mechanism, we find that the onset potential is limited either by the ability to activate methanol, by the ability to activate water, or by surface poisoning by CO * or OH

This paper presents a detailed analysis of the steam reforming process from first-principles calculations, supported by insight from experimental investigations. In the present work we employ recently recognised scaling relationships for adsorption energies of simple molecules adsorbed at pure metal surfaces to develop an overview of the steam reforming process catalyzed by a range of transition metal surfaces. By combining scaling relationships with thermodynamic and kinetic analysis, we show that it is possible to determine the reactivity trends of the pure metals for methane steam reforming. The reaction is found to be kinetically controlled by a methane dissociation step and a CO formation step, where the latter step is found to be dominant at lower temperatures. The particle size of the metal catalysts particles have been determined by transmission electron microscopy (TEM) and the turn over frequency observed to be linearly dependent on the dispersion, supporting the theoretical notion that the active sites are most likely present as one dimensional edges. It has been found that determination of the correct particle size distribution of small (2–4 nm) Ru particles requires in situ TEM measurements under a hydrogen atmosphere. The overall agreement between theory and experiment (at 773 K, 1 bar pressure and 10% conversion) is found to be excellent with Ru and Rh being the most active pure transition metals for methane steam reforming, while Ni, Ir, Pt, and Pd are significantly less active at similar dispersion.

Density functional theory calculations are used as the basis for an analysis of the electrochemical process, where by water is split to form molecular oxygen and hydrogen. We develop a method for obtaining the thermochemistry of the electrochemical water splitting process as a function of the bias directly from the electronic structure calculations. We consider electrodes of Pt(1 1 1) and Au(1 1 1) in detail and then discuss trends for a series of different metals. We show that the difficult step in the water splitting process is the formation of superoxy-type (OOH) species on the surface by the splitting of a water molecule on top an adsorbed oxygen atom. One conclusion is that this is only possible on metal surfaces that are (partly) oxidized. We show that the binding energies of the different intermediates are linearly correlated for a number of metals. In a simple analysis, where the linear relations are assumed to be obeyed exactly, this leads to a universal relationship between the catalytic rate and the oxygen binding energy. Finally, we conclude that for systems obeying these relations, there is a limit to how good a water splitting catalyst an oxidized metal surface can become.

2008 Published by Elsevier B.V. In recent years computational methods based on density functional theory (DFT) have been extremely successful in describing chemical processes taking place at metal surfaces. It has become possible to account quantitatively for the dynamics of gas–surface interactions, and the kinetics of surface catalyzed reactions have been described in some detail [1–4]. A similar treatment of chemical transformations at the surface of electrodes has been hampered by the complexity of the electrified solid–liquid interface. A complete theoretical description would have to include the solid surface, the liquid, the solvated ions and the effect of changes in the chemical potential of the electrons in the solid. Several very interesting approaches have been proposed recently [5–12] but they are still quite approximate and in addition they are often also quite cumbersome. In an exciting paper in this issue Nilekar andMavrikakis take a much simpler approach to understand the electrochemical oxygen reduction reaction (ORR) [13]. The ORR is the cathode reaction in e.g. PEM fuel cells, and since most of the energy loss is associated with this reaction even over the best catalyst, Pt, there is an enormous impetus to find better (and cheaper) electrode catalysts [14]. The ORR involves the sequential addition of protons and electrons to adsorbed O2. Rather than calculating the free energy of the protons in solution and the electrons (H + e (U)) at a potential, U, Nilekar and Mavrikakis exploit the fact that if U is measured relative to the so-called Standard Hydrogen electrode, the free energy of (H + e (U)) is the same as the free energy of 1/2 H2 at standard conditions corrected for the effect of the difference in potential, –eU, relative to the potential U = 0 where the reaction (H + e )M 1/2 H2 is at equilibrium [15]. In this way they can get all reaction energies as a function of potential for the elementary electrochemical reactions from standard adsorption energy calculations of the intermediates, O, O2, OH, OOH, HOOH, and H2O. While solvation of H in the liquid is included implicitly, solvation effects at the surface are not. They can be included, but generally amount to a few tenths of an eV for the H-containing species and considerably less for adsorbed O [16]. Nilekar and Mavrikakis provide an overview of the reaction energetics for several possible reaction paths. Even more importantly, they use the calculations to investigate the effect of having an overlayer of one metal on top of another. Recent experiments have shown that Pt deposited on a number of metals (Au, Pd, Rh, Ir, and Ru) [17] has large changes in ORR activity compared to pure Pt. The calculations explain this in a detailed way. In fact, the calculations allow a systematic approach to understand trends in ORR activity with potential. They also identify an important descriptor of the electrocatalytic activity, the oxygen adsorption energy [15]. Fig. 1 shows that when Elsevier B.V. 45 4593 2399. ).

We use density functional theory calculations to study the oxygen reduction reaction and methanol activation on selenium and sulfur-containing transition metal surfaces. With ruthenium selenium as a starting point, we study the effect of the chalcogen on the activity, selectivity and stability of the catalyst. Ruthenium surfaces with moderate content of selenium are calculated active for the oxygen reduction reaction, and insensitive to methanol. A significant upper limit for the activity of transition metal chalcogenides is estimated.