The electrochemical production of NH3 under ambient conditions represents an attractive prospect for sustainable agriculture, but electrocatalysts that selectively reduce N2 to NH3 remain elusive. In ...this work, we present insights from DFT calculations that describe limitations on the low‐temperature electrocatalytic production of NH3 from N2. In particular, we highlight the linear scaling relations of the adsorption energies of intermediates that can be used to model the overpotential requirements in this process. By using a two‐variable description of the theoretical overpotential, we identify fundamental limitations on N2 reduction analogous to those present in processes such as oxygen evolution. Using these trends, we propose new strategies for catalyst design that may help guide the search for an electrocatalyst that can achieve selective N2 reduction.
Towards renewable fertilizer: The synthesis of ammonia has played a key role in our society for its uses in fertilizer and other reactive nitrates. The traditional Haber–Bosch method of this synthesis requires extensive capital, making alternative technologies attractive. We outline an alternative route that might be achieved at ambient conditions. Using DFT, we demonstrate why an electrochemical process is severely limited by the energetics of metal catalysts.
The presence of layered cobalt oxides has been identified experimentally in Co-based anodes under oxygen-evolving conditions. In this work, we report the results of theoretical investigations of the ...relative stability of layered and spinel bulk phases of Co oxides, as well as the stability of selected surfaces as a function of applied potential and pH. We then study the oxygen evolution reaction (OER) on these surfaces and obtain activity trends at experimentally relevant electro-chemical conditions. Our calculated volume Pourbaix diagram shows that β-CoOOH is the active phase where the OER occurs in alkaline media. We calculate relative surface stabilities and adsorbate coverages of the most stable low-index surfaces of β-CoOOH: (0001), (011̅2), and (101̅4). We find that at low applied potentials, the (101̅4) surface is the most stable, while the (011̅2) surface is the more stable at higher potentials. Next, we compare the theoretical overpotentials for all three surfaces and find that the (101̅4) surface is the most active one as characterized by an overpotential of η = 0.48 V. The high activity of the (101̅4) surface can be attributed to the observation that the resting state of Co in the active site is Co3+ during the OER, whereas Co is in the Co4+ state in the less active surfaces. Lastly, we demonstrate that the overpotential of the (101̅4) surface can be lowered further by surface substitution of Co by Ni. This finding could explain the experimentally observed enhancement in the OER activity of Ni y Co1–y O x thin films with increasing Ni content. All energetics in this work were obtained from density functional theory using the Hubbard-U correction.
<正>Both scientiic discovery and technological development are at some point faced with the question of how to progress from a trial-and-error approach to a highly controlled design process.In ...heterogeneous catalysis,the search for the optimal active site of a catalyst for a given chemical reaction has been the central objective of research for almost a century.In
The electron occupation of orbitals in transition metal oxides guided the identification of an efficient oxygen evolution catalyst based on Earth-abundant elements.
Electrochemical splitting of water ...into molecular oxygen (O
2
), protons, and electrons could provide a way to store the electricity generated from sustainable but intermittent energy sources, such as wind and solar power, as fuels (
1
). Hydrogen would be the simplest fuel to make, but the protons and electrons could be used to produce hydrocarbons and alcohols from CO
2
or ammonia from N
2
. A major challenge is that efficient catalysts for water electrolysis are expensive and contain rare noble metals, so cost-effective approaches will require the discovery of efficient electrocatalysts that contain only Earth-abundant elements. On page 1383 of this issue, Suntivich
et al.
(
2
) describe a method for rational design of metal oxide catalysts for the oxygen evolution reaction. They discovered perovskite-structure catalysts based on non-noble metals that work with a higher efficiency than one of the state-of-the-art catalysts, iridium oxide.
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•Metal catalysis theory from scaling relations, activity maps, and d-band model.•Bond strength in Sabatier principle is given quantitatively from scaling relations.•Implications of ...quantitative metal catalysis theory for the search of new catalysts.•Computational approach illustrated by NH3 synthesis and CO hydrogenation reactions.
We discuss three concepts that have made it possible to develop a quantitative understanding of trends in transition-metal catalysis: scaling relations, activity maps, and the d-band model. Scaling relations are correlations between surface bond energies of different adsorbed species including transition states; they open the possibility of mapping the many parameters determining the rate of a full catalytic reaction onto a few descriptors. The resulting activity map can be viewed as a quantitative implementation of the classical Sabatier principle, which states that there is an optimum “bond strength” defining the best catalyst for a given reaction. In the modern version, the scaling relations determine the relevant “bond strengths” and the fact that these descriptors can be measured or calculated makes it a quantitative theory of catalysis that can be tested experimentally by making specific predictions of new catalysts. The quantitative aspect of the model therefore provides new possibilities in catalyst design. Finally, the d-band model provides an understanding of the scaling relations and variations in catalytic activity in terms of the electronic structure of the transition-metal surface.
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The Haber–Bosch process for ammonia synthesis has been suggested to be the most important invention of the 20th century, and called the ‘Bellwether reaction in heterogeneous ...catalysis’. We examine the catalyst requirements for a new low-pressure, low-temperature synthesis process. We show that the absence of such a process for conventional transition metal catalysts can be understood as a consequence of a scaling relation between the activation energy for N2 dissociation and N adsorption energy found at the surface of these materials. A better catalyst cannot obey this scaling relation. We define the ideal scaling relation characterizing the most active catalyst possible, and show that it is theoretically possible to have a low pressure, low-temperature Haber–Bosch process. The challenge is to find new classes of catalyst materials with properties approaching the ideal, and we discuss the possibility that transition metal compounds have such properties.
Two-dimensional transition metal carbides and nitrides, also known as MXenes, represent an attractive class of materials for a multitude of electrochemical and other applications. While single sheets ...of MXenes have been widely studied theoretically, there have been much fewer studies on layered bulk MXenes, which are more representative of multi- or few-layer MXenes used in actual applications. Herein, we investigate the structural and electronic effects of water intercalation, multiple functional groups and applied potential on layered bulk Ti2C and Mo2C MXenes using density functional theory. The out-of plane lattice parameter, c, was found to vary significantly with the functional group, and is greatly increased upon intercalation of water. Experimental results confirm the change in lattice constant due to addition or removal of intercalated water. Under zero applied potential, both Ti2C and Mo2C were found to be functionalized by one monolayer of O; bare MXenes were never found to be stable, regardless of the applied potential. Applying a potential changed the adsorbate coverage, changing the systems from O covered to H covered at negative potentials and, in some cases, giving rise to a metal–insulator transition. Understanding of the effects of surface functionalization and water intercalation of MXenes provides a better insight of their use for catalytic and electronic applications.
Earth-abundant first-row (3d) transition metal–based catalysts have been developed for the oxygen-evolution reaction (OER); however, they operate at overpotentials substantially above thermodynamic ...requirements. Density functional theory suggested that non-3d high-valency metals such as tungsten can modulate 3d metal oxides, providing near-optimal adsorption energies for OER intermediates. We developed a room-temperature synthesis to produce gelled oxyhydroxides materials with an atomically homogeneous metal distribution. These gelled FeCoW oxyhydroxides exhibit the lowest overpotential (191 millivolts) reported at 10 milliamperes per square centimeter in alkaline electrolyte. The catalyst shows no evidence of degradation after more than 500 hours of operation. X-ray absorption and computational studies reveal a synergistic interplay between tungsten, iron, and cobalt in producing a favorable local coordination environment and electronic structure that enhance the energetics for OER.
We introduce a general method for estimating the uncertainty in calculated materials properties based on density functional theory calculations. We illustrate the approach for a calculation of the ...catalytic rate of ammonia synthesis over a range of transition-metal catalysts. The correlation between errors in density functional theory calculations is shown to play an important role in reducing the predicted error on calculated rates. Uncertainties depend strongly on reaction conditions and catalyst material, and the relative rates between different catalysts are considerably better described than the absolute rates. We introduce an approach for incorporating uncertainty when searching for improved catalysts by evaluating the probability that a given catalyst is better than a known standard.
Co-based oxides are suitable electrode materials for the electrocatalytic oxygen evolution reaction (OER) with promising activity and stability, in addition to being widely available and relatively ...cheap. We investigate OER on Co3O4(001) and β-CoOOH (011̅2) surfaces using density functional theory calculations (DFT). We construct surface Pourbaix diagrams and investigate the theoretical overpotential for the elementary steps involved in OER on these surfaces. We show that inclusion of the Hubbard-U correction to DFT (DFT+U) is necessary to recover experimentally observed trends in the activity for the strongly correlated cobalt oxides. We find that the inclusion of the Hubbard-U correction lowers the activity of both Co3O4(001) and β-CoOOH(011̅2) when compared to results from pure DFT. In addition, the Hubbard-U correction shifts the location of Co3O4 and β-CoOOH from the strong binding leg to the weak binding leg of the OER volcano plot. The calculations also suggest that the theoretical overpotentials for Co3O4 and β-CoOOH are very nearly the same. We ascribe this to a similar local coordination environment of the active Co site in Co3O4 and CoOOH under OER conditions.
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