25 research outputs found
Electrochemical oxidation of CO on Cu single crystals under alkaline conditions
We perform a joint experimental-theoretical study of the electrochemical
oxidation of CO on copper (Cu) under alkaline conditions. Using cyclic
voltammetry on Cu single crystal surfaces, we demonstrate that both Cu terraces
and steps show CO oxidation activity at potentials just slightly positive
(0.03-0.14 V) of the thermodynamic equilibrium potential. The overpotentials
are 0.23-0.12 V lower than that of gold (approx. 0.26 V), which up until now
has been considered to be the most active catalyst for this process. Our
theoretical calculations suggest that Cu's activity arises from the
advantageous combination of simultaneous *OH adsorption under CO oxidation
potentials and surmountable *CO-*OH coupling barriers. Experimentally observed
onset potentials are in agreement with the computed onsets of *OH adsorption.
We furthermore show that the onsets of *OH adsorption on steps are more
affected by *CO-*OH interactions than on terraces due to a stronger competitive
adsorption. Overall, Cu(100) shows the lowest overpotential (0.03 V) of the
facets considered.Comment: 16 pages, 3 figures plus supplementary informatio
Solvation of furfural at metalâwater interfaces: Implications for aqueous phase hydrogenation reactions
Metal-water interfaces are central to understanding aqueous-phase heterogeneous catalytic processes. However, the explicit modeling of the interface is still challenging as it necessitates extensive sampling of the interfaces' degrees of freedom. Herein, we use ab initio molecular dynamics (AIMD) simulations to study the adsorption of furfural, a platform biomass chemical on several catalytically relevant metal-water interfaces (Pt, Rh, Pd, Cu, and Au) at low coverages. We find that furfural adsorption is destabilized on all the metal-water interfaces compared to the metal-gas interfaces considered in this work. This destabilization is a result of the energetic penalty associated with the displacement of water molecules near the surface upon adsorption of furfural, further evidenced by a linear correlation between solvation energy and the change in surface water coverage. To predict solvation energies without the need for computationally expensive AIMD simulations, we demonstrate OH binding energy as a good descriptor to estimate the solvation energies of furfural. Using microkinetic modeling, we further explain the origin of the activity for furfural hydrogenation on intrinsically strong-binding metals under aqueous conditions, i.e., the endothermic solvation energies for furfural adsorption prevent surface poisoning. Our work sheds light on the development of active aqueous-phase catalytic systems via rationally tuning the solvation energies of reaction intermediates
Kinetics of Graphene Growth on Liquid Copper by Chemical Vapor Deposition
We report a combined experimental and computational study of the kinetics of
graphene growth during chemical vapor deposition on a liquid copper catalyst.
The use of liquid metal catalysts offers bright perspectives for controllable
large-scale, high-quality synthesis technologies of two-dimensional materials.
We carried out a series of growth experiments varying CH4-to-H2 pressure ratios
and deposition temperature. By monitoring the graphene flake morphology in real
time during growth using in situ optical microscopy in radiation mode, we
explored the morphology and kinetics of the growth within a wide range of
experimental conditions. Following an analysis of the flakes' growth rates, we
conclude that the growth mode was attachment-limited. The attachment and
detachment activation energies of carbon species are derived as 1.9 +- 0.3 eV
and 2.0 +- 0.1 eV, respectively. We also conducted free-energy calculations by
a moment tensor potential trained to density functional theory data. Our
simulations propose that carbon dimers are most likely the active carbon
species during growth, with attachment and detachment barriers of 1.71 +- 0.15
eV and 2.09 +- 0.02 eV, respectively, being in good agreement with the
experimental results
A foundation model for atomistic materials chemistry
Machine-learned force fields have transformed the atomistic modelling of
materials by enabling simulations of ab initio quality on unprecedented time
and length scales. However, they are currently limited by: (i) the significant
computational and human effort that must go into development and validation of
potentials for each particular system of interest; and (ii) a general lack of
transferability from one chemical system to the next. Here, using the
state-of-the-art MACE architecture we introduce a single general-purpose ML
model, trained on a public database of 150k inorganic crystals, that is capable
of running stable molecular dynamics on molecules and materials. We demonstrate
the power of the MACE-MP-0 model - and its qualitative and at times
quantitative accuracy - on a diverse set problems in the physical sciences,
including the properties of solids, liquids, gases, chemical reactions,
interfaces and even the dynamics of a small protein. The model can be applied
out of the box and as a starting or "foundation model" for any atomistic system
of interest and is thus a step towards democratising the revolution of ML force
fields by lowering the barriers to entry.Comment: 119 pages, 63 figures, 37MB PD
Unified Approach to Implicit and Explicit Solvent Simulations of Electrochemical Reaction Energetics
One of the major open challenges in ab initio simulations of the electrochemical
interface is the determination of electrochemical barriers under a constant driving force.
Existing methods to do so include extrapolation techniques based on fully explicit
treatments of the electrolyte, as well as implicit solvent models which allow for a
continuous variation in electrolyte charge. Emerging hybrid continuum models have
the potential to revolutionize the field, since they account for the electrolyte with
little computational cost while retaining some explicit electrolyte, representing a âbest
of both worldsâ method. In this work, we present a unified approach to determine
reaction energetics from both fully explicit, implicit, and hybrid treatments of the
electrolyte based on a new multi-capacitor model of the electrochemical interface. A given electrode potential can be achieved by a variety of interfacial structures; a crucial
insight from this work is that the effective surface charge gives the true driving force
of electrochemical processes. In contrast, we show that the traditionally considered
work function gives rise to multi-valued functions depending on the simulation cell
size. Furthermore, we show that the reaction energetics are largely insensitive to the
countercharge distribution chosen in hybrid implicit/explicit models, which means that
any of the myriad implicit electrolyte models can be equivalently applied. This work
thus paves the way for the accurate treatment of ab initio reaction energetics of general
surface electrochemical processes using both implicit and explicit electrolyte.
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Implications of Occupational Disorder on Ion Mobility in Li<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub> Battery Materials
Lithiumâtitanium-oxide
(Li<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub>, LTO) is unique among
battery materials due to its exceptional
cyclability and high rate capability. This performance is believed
to derive at least partly from the occupational disorder introduced
via mixed Li/Ti occupancy in the LTO spinel-like structure. We explore
the vast configuration space accessible during high-temperature LTO
synthesis by Monte Carlo sampling and indeed find lowest-energy structures
to be characterized by a high degree of microscopic inhomogeneity.
Dynamical simulations in corresponding configurations reveal the dominant
fraction of Li ions to be immobile on nanosecond time scales. However,
Ti antisite-like defects stabilized by the configurational disorder
give rise to a novel correlated ion diffusion mechanism. The resulting
fast but localized diffusion could be a key element in the sudden
rise in conductivity found in LTO in the early stages of charging
and questions the validity of ion mobility measurements for this and
other configurationally disordered materials
Number of sites-based solver for determining coverages from steady-state mean-field micro-kinetic models
Kinetic models parameterized by ab-initio calculations have led to significant improvements in understanding chemical reactions in heterogeneous catalysis. These studies have been facilitated by implementations which determine steady-state coverages and rates of mean-field micro-kinetic models. As implemented in the open-source kinetic modeling program, CatMAP, the conventional solution strategy is to use a root-finding algorithm to determine the coverage of all intermediates through the steady-state expressions, constraining all coverages to be non-negative and to properly sum to unity. Though intuitive, this root-finding strategy causes issues with convergence to solution due to these imposed constraints. In this work, we avoid explicitly imposing these constraints, solving the mean-field steady-state micro-kinetic model in the space of number of sites instead of solving it in the space of coverages. We transform the constrained root-finding problem to an unconstrained least-squares minimization problem, leading to significantly improved convergence in solving micro-kinetic models and thus enabling the efficient study of more complex catalytic reactions
First step of the oxygen reduction reaction on Au(111): A computational study of O2 adsorption at the electrified metal/water interface
Local field effects at the electrical double layer change the energies of reaction intermediates in heterogeneous electrocatalysis. The resulting dependence on (absolute) electrode potential can be pivotal to a catalyst\u27s performance in acid or alkaline media. And yet, such local field effects are very difficult to describe theoretically and are often ignored. In this work, we focus on O2 adsorption as the first step of the oxygen reduction reaction (ORR) on Au(111). Different physical effects of the local field are elucidated and compared by systematically improving the model of the double layer: from an applied saw-tooth potential in vacuum, to an implicit solvent model, and explicitly modeled water via ab initio molecular dynamics (AIMD). We find all models predict a dominant dipole-field type interaction that significantly strengthens O2 binding at increasingly reducing conditions. However, only an atomically-resolved solvent model such as provided by AIMD can properly capture the additional stabilization due to explicit H-bonding from the water network. This contribution comes with the formation of a peroxo-like surface species and a more dramatic field response around the ORR onset. Our results overall demonstrate the importance of including local electric field effects in models of the electrochemical interface, while assessing the level of detail that is required to be accounted for