The activation of O2 on metal surfaces is a critical process for heterogeneous catalysis and materials oxidation. Fundamental studies of well-defined metal surfaces using a variety of techniques have given crucial insight into the mechanisms, energetics, and dynamics of O2 adsorption and dissociation. Here, trends in the activation of O2 on transition metal surfaces are discussed, and various O2 adsorption states are described in terms of both electronic structure and geometry. The mechanism and dynamics of O2 dissociation are also reviewed, including the importance of the spin transition. The reactivity of O2 and O toward reactant molecules is also briefly discussed in the context of catalysis. The reactivity of a surface toward O2 generally correlates with the adsorption strength of O, the tendency to oxidize, and the heat of formation of the oxide. Periodic trends can be rationalized in terms of attractive and repulsive interactions with the d-band, such that inert metals tend to feature a full d band ...

O2 Activation by Metal Surfaces: Implications for Bonding and Reactivity on Heterogeneous Catalysts

We propose and validate with quantum mechanics methods a unique catalyst for electrochemical reduction of CO2 (CO2RR) in which selectivity and activity of CO and C2 products are both enhanced at the borders of oxidized and metallic surface regions. This Cu metal embedded in oxidized matrix (MEOM) catalyst is consistent with observations that Cu2O-based electrodes improve performance. However, we show that a fully oxidized matrix (FOM) model would not explain the experimentally observed performance boost, and we show that the FOM is not stable under CO2 reduction conditions. This electrostatic tension between the Cu+ and Cu0 surface sites responsible for the MEOM mechanism suggests a unique strategy for designing more efficient and selective electrocatalysts for CO2RR to valuable chemicals (HCOx), a critical need for practical environmental and energy applications.

https://www.pnas.org/content/pnas/114/26/6685.full.pdf

Cu metal embedded in oxidized matrix catalyst to promote CO2 activation and CO dimerization for electrochemical reduction of CO2.

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Projector-Augmented Wave Method: An introduction

First-principles-based multiscale models are ever more successful in addressing the wide range of length and time scales over which material–function relationships evolve in heterogeneous catalysis. They provide invaluable mechanistic insight and allow screening of vast materials spaces for promising new catalysts — in silico and at predictive quality. Here, we briefly review methodological cornerstones of existing approaches and highlight successes and ongoing developments. The biggest challenge is to overcome presently largely static couplings between the descriptions at the various scales to adequately treat the dynamic and adaptive nature of working catalysts. On the road towards a higher structural, mechanistic and environmental complexity, it is, in particular, the fusion with machine learning methodology that promises rapid advances in the years to come. First-principles-based multiscale models provide mechanistic insight and allow screening of large materials spaces to find promising new catalysts. In this Review, Reuter and co-workers discuss methodological cornerstones of existing approaches and highlight successes and ongoing developments in the field.

First-principles-based multiscale modelling of heterogeneous catalysis

We present a first-principles-based formalism to provide a quantitative measure of the thermodynamic instability and propensity for electrochemical stabilization, passivation, or corrosion of metastable materials in aqueous media. We demonstrate that this formalism can assess the relative Gibbs free energy of candidate materials in aqueous media as well as their decomposition products, combining solid and aqueous phases, as a function of pH and potential. On the basis of benchmarking against 20 stable as well as metastable materials reported in the literature and also our experimental characterization of metastable triclinic-FeVO4, we present quantitative estimates for the relative Gibbs free energy and corresponding aqueous regimes where these materials are most likely to be stable, form inert passivating films, or steadily corrode to aqueous species. Furthermore, we show that the structure and composition of the passivating films formed on triclinic-FeVO4 are also in excellent agreement with the Point D...

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Electrochemical Stability of Metastable Materials

Density-functional theory is used to evaluate the mechanism of copper surface oxidation. Reaction pathways of O2 dissociation on the surface and oxidation of the sub-surface are found on the Cu(100), Cu(110), and Cu(111) facets. At low oxygen coverage, all three surfaces dissociate O2 spontaneously. As oxygen accumulates on the surfaces, O2 dissociation becomes more difficult. A bottleneck to further oxidation occurs when the surfaces are saturated with oxygen. The barriers for O2 dissociation on the O-saturated Cu(100)-c(2×2)-0.5 monolayer (ML) and Cu(100) missing-row structures are 0.97 eV and 0.75 eV, respectively; significantly lower than those have been reported previously. Oxidation of Cu(110)-c(6×2), the most stable (110) surface oxide, has a barrier of 0.72 eV. As the reconstructions grow from step edges, clean Cu(110) surfaces can dissociatively adsorb oxygen until the surface Cu atoms are saturated. After slight rearrangements, these surface areas form a “1 ML” oxide structure which has not been reported in the literature. The barrier for further oxidation of this “1 ML” phase is only 0.31 eV. Finally the oxidized Cu(111) surface has a relatively low reaction energy barrier for O2 dissociation, even at high oxygen coverage, and allows for facile oxidation of the subsurface by fast O diffusion through the surface oxide. The kinetic mechanisms found provide a qualitative explanation of the observed oxidation of the low-index Cu surfaces.

Calculations of oxide formation on low-index Cu surfaces

The energetics and kinetics of the missing-row reconstruction (MRR) and missing-row island formation on the Cu(100) surface are investigated using density functional theory calculations. We find that copper ejection from the c(2 × 2) surface is made energetically possible by the presence of surface-adsorbed O2 molecules. The barrier for MRR formation via this ejection mechanism is calculated to be 0.96 eV, consistent with the experimentally observed formation temperature of 400 K. The reaction pathways between Cu and O2 result in the formation of Cu–O chains on the c(2 × 2) surface, which can grow from the −O–Cu–O– trimer at least up to the −Cu–O–Cu–O–Cu– pentamer. Remarkably, these chains can both diffuse rapidly and change their orientation on the surface, allowing them to assemble into longer Cu–O chains. Facile diffusion of the Cu–O chains occurs via a collective mechanism which limits the number of broken Cu–O bonds. Perpendicular to the Cu–O rows, the chains hop first at one end and then the other. ...

Calculations of Oxygen Adsorption-Induced Surface Reconstruction and Oxide Formation on Cu(100)

Density functional theory calculations are used to study the elementary processes of the formation of the (2 × 1)-O reconstruction on the Cu(110) surface. The (2 × 1)-O reconstruction requires additional Cu atoms to form Cu-O rows on top of the surface. Both terrace and step sites are considered as the source of Cu adatoms. On terraces, adsorbed oxygen induces the ejection of Cu atoms to form -O-Cu-O- units, leaving Cu vacancies behind. The barrier for subsequent unit growth, however, is prohibitively high. Cu(110) step sites are also considered as a source of Cu atoms. Dissociated oxygen triggers the formation of stable Cu-O chains along the [001] step edges. This process, however, blocks the diffusion of Cu atoms so that it is not a viable mechanism for the (2 × 1)-O reconstruction. Oxygen adsorption on the [11¯0] edges also allows the nucleation of [001] oriented Cu-O rows. The short Cu-O rows act as diffusion channels for Cu atoms that detach from the step, which append to the end of the Cu-O chains. Our calculations of the formation of the (2 × 1)-O phase on Cu(110) provide a mechanistic description of the experimentally observed reconstruction.

Renlong Liu

Papers

Calculations of oxide formation on low-index Cu surfaces

Calculations of Oxygen Adsorption-Induced Surface Reconstruction and Oxide Formation on Cu(100)

Communication: Calculations of the (2 × 1)-O reconstruction kinetics on Cu(110).