11 research outputs found
One- or Two-Electron Water Oxidation, Hydroxyl Radical, or H<sub>2</sub>O<sub>2</sub> Evolution
Electrochemical or
photoelectrochemcial oxidation of water to form
hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) or hydroxyl radicals
(<sup>â˘</sup>OH) offers a very attractive route to water disinfection,
and the first process could be the basis for a clean way to produce
hydrogen peroxide. A major obstacle in the development of effective
catalysts for these reactions is that the electrocatalyst must suppress
the thermodynamically favored four-electron pathway leading to O<sub>2</sub> evolution. We develop a thermochemical picture of the catalyst
properties that determine selectivity toward the one, two, and four
electron processes leading to <sup>â˘</sup>OH, H<sub>2</sub>O<sub>2</sub>, and O<sub>2</sub>
Monocopper Active Site for Partial Methane Oxidation in Cu-Exchanged 8MR Zeolites
Direct conversion of methane to methanol
using oxygen is experiencing
renewed interest owing to the availability of new natural gas resources.
Copper-exchanged zeolites such as mordenite and ZSM-5 have shown encouraging
results, and di- and tri-copper species have been suggested as active
sites. Recently, small eight-membered ring (8MR) zeolites including
SSZ-13, -16, and -39 have been shown to be active for methane oxidation,
but the active sites and reaction mechanisms in these 8MR zeolites
are not known. In this work, we use density functional theory (DFT)
calculations to systematically evaluate monocopper species as active
sites for the partial methane oxidation reaction in Cu-exchanged SSZ-13.
On the basis of kinetic and thermodynamic arguments, we suggest that
[Cu<sup>II</sup>OH]<sup>+</sup> species in the 8MR are responsible
for the experimentally observed activity. Our results successfully
explain the available spectroscopic data and experimental observations
including (i) the necessity of water for methanol extraction and (ii)
the effect of Si/Al ratio on the catalyst activity. Monocopper species
have not yet been suggested as an active site for the partial methane
oxidation reaction, and our results suggest that [Cu<sup>II</sup>OH]<sup>+</sup> active site may provide complementary routes for methane
activation in zeolites in addition to the known [CuâOâCu]<sup>2+</sup> and Cu<sub>3</sub>O<sub>3</sub> motifs
Theoretical Investigations into Defected Graphene for Electrochemical Reduction of CO<sub>2</sub>
Despite
numerous experimental efforts that have been dedicated
to studying carbon-based materials for electrochemical reduction of
CO<sub>2</sub>, a rationalization of the associated trends in the
intrinsic activity of different active motifs has so far been elusive.
In the present work, we employ density functional theory calculations
to examine a variety of different active sites in N-doped graphene
to give a comprehensive outline of the trends in activity. We find
that adsorption energies of COOH* and CO* do not follow the linear
scaling relationships observed for the pure transition metals, and
this unique scaling is rationalized through differences in electronic
structure between transition metals and defected graphene. This finding
rationalizes most of the experimental observations on the carbon-based
materials which present promising catalysts for the two-electron reduction
of CO<sub>2</sub> to CO. With this simple thermodynamic analysis,
we identify several active sites that are expected to exhibit a comparable
or even better activity to the state-of-the-art gold catalyst, and
several configurations are suggested to be selective for CO<sub>2</sub>RR over HER
Ultrathin Cobalt Oxide Overlayer Promotes Catalytic Activity of Cobalt Nitride for the Oxygen Reduction Reaction
The oxygen reduction reaction (ORR) plays
a crucial role in various energy devices such as proton-exchange membrane
fuel cells (PEMFCs) and metalâair batteries. Owing to the scarcity
of the current state-of-the-art Pt-based catalysts, cost-effective
Pt-free materials such as transition metal nitrides and their derivatives
have gained overwhelming interest as alternatives. In particular,
cobalt nitride (CoN) has demonstrated a reasonably high ORR activity.
However, the nature of its active phase still remains elusive. Here,
we employ density functional theory calculations to study the surface
reactivity of rocksalt (RS) and zincblend (ZB) cobalt nitride. The
performances of the catalysts terminated by the facets of (100), (110),
and (111) are studied for the ORR. We demonstrate that the cobalt
nitride surface is highly susceptible to oxidation under ORR conditions.
The as-formed oxide overlayer on the facets of CoN<sub>RS</sub>(100)
and CoN<sub>ZB</sub>(110) presents a significant promotional effect
in reducing the ORR overpotential, thereby increasing the activity
in comparison with those of the pure CoNs. The results of this work
rationalize a number of experimental reports in the literature and
disclose the nature of the active phase of cobalt nitrides for the
ORR. Moreover, they offer guidelines for understanding the activity
of other transition metal nitrides and designing efficient catalysts
for future generation of PEMFCs
Theoretical Investigations of the Electrochemical Reduction of CO on Single Metal Atoms Embedded in Graphene
Single transition
metal atoms embedded at single vacancies of graphene
provide a unique paradigm for catalytic reactions. We present a density
functional theory study of such systems for the electrochemical reduction
of CO. Theoretical investigations of CO electrochemical reduction
are particularly challenging in that electrochemical activation energies
are a necessary descriptor of activity. We determined the electrochemical
barriers for key protonâelectron transfer steps using a state-of-the-art,
fully explicit solvent model of the electrochemical interface. The
accuracy of GGA-level functionals in describing these systems was
also benchmarked against hybrid methods. We find the first proton
transfer to form CHO from CO to be a critical step in C<sub>1</sub> product formation. On these single atom sites, the corresponding
barrier scales more favorably with the CO binding energy than for
211 and 111 transition metal surfaces, in the direction of improved
activity. Intermediates and transition states for the hydrogen evolution
reaction were found to be less stable than those on transition metals,
suggesting a higher selectivity for CO reduction. We present a rate
volcano for the production of methane from CO. We identify promising
candidates with high activity, stability, and selectivity for the
reduction of CO. This work highlights the potential of these systems
as improved electrocatalysts over pure transition metals for CO reduction
Trends in the Electrochemical Synthesis of H<sub>2</sub>O<sub>2</sub>: Enhancing Activity and Selectivity by Electrocatalytic Site Engineering
The
direct electrochemical synthesis of hydrogen peroxide is a
promising alternative to currently used batch synthesis methods. Its
industrial viability is dependent on the effective catalysis of the
reduction of oxygen at the cathode. Herein, we study the factors controlling
activity and selectivity for H<sub>2</sub>O<sub>2</sub> production
on metal surfaces. Using this approach, we discover two new catalysts
for the reaction, AgâHg and PdâHg, with unique electrocatalytic
properties both of which exhibit performance that far exceeds the
current state-of-the art
Nature of Lone-PairâSurface Bonds and Their Scaling Relations
We investigate the (surface) bonding
of a class of industrially and biologically important molecules in
which the chemically active orbital is a 2<i>p</i> electron
lone pair located on an N or O atom bound via single bonds to H or
alkyl groups. This class includes water, ammonia, alcohols, ethers,
and amines. Using extensive density functional theory (DFT) calculations,
we discover scaling relations (correlations) among molecular binding
energies of different members of this class: the bonding energetics
of a single member can be used as a descriptor for other members.
We investigate the bonding mechanism for a representative (H<sub>2</sub>O) and find the most important physical surface properties that dictate
the strength and nature of the bonding through a combination of covalent
and noncovalent electrostatic effects. We describe the importance
of surface intrinsic electrostatic, geometric, and mechanical properties
in determining the extent of the lone-pairâsurface interactions.
We study systems including ionic materials in which the surface positive
and negative centers create strong local surface electric fields,
which polarize the dangling lone pair and lead to a strong âelectrostatically
driven bondâ. We emphasize the importance of noncovalent electrostatic
effects and discuss why a fully covalent picture, common in the current
first-principles literature on surface bonding of these molecules,
is not adequate to correctly describe the bonding mechanism and energy
trends. By pointing out a completely different mechanism (charge transfer)
as the major factor for binding N- and O-containing unsaturated (radical)
adsorbates, we explain why their binding energies can be tuned independently
from those of the aforementioned species, having potential implications
in scaling-driven catalyst discovery
Copper Silver Thin Films with Metastable Miscibility for Oxygen Reduction Electrocatalysis in Alkaline Electrolytes
Increasing the activity
of Ag-based catalysts for the oxygen reduction reaction (ORR) is important
for improving the performance and economic outlook of alkaline-based
fuel cell and metalâair battery technologies. In this work,
we prepare CuAg thin films with controllable compositions using electron
beam physical vapor deposition. X-ray diffraction analysis indicates
that this fabrication route yields metastable miscibility between
these two thermodynamically immiscible metals, with the thin films
consisting of a Ag-rich and a Cu-rich phase. Electrochemical testing
in 0.1 M potassium hydroxide showed significant ORR activity improvements
for the CuAg films. On a geometric basis, the most active thin film
(Cu<sub>70</sub>Ag<sub>30</sub>) demonstrated a 4-fold activity improvement
vs pure Ag at 0.8 V vs the reversible hydrogen electrode. Furthermore,
enhanced ORR kinetics for Cu-rich (>50 at. % Cu) thin films was
demonstrated by a decrease in Tafel slope from 90 mV/dec, a commonly
observed value for Ag catalysts, to 45 mV/dec. Surface enrichment
of the Ag-rich phase after ORR testing was indicated by X-ray photoelectron
spectroscopy and grazing incidence synchrotron X-ray diffraction measurements.
By correlating density functional theory with experimental measurements,
we postulate that the activity enhancement of the Cu-rich CuAg thin
films arises due to the non-equilibrium miscibility of Cu atoms in
the Ag-rich phase, which favorably tunes the surface electronic structure
and binding energies of reaction species
Designing Boron Nitride Islands in Carbon Materials for Efficient Electrochemical Synthesis of Hydrogen Peroxide
Heteroatom-doped
carbons have drawn increasing research interest
as catalysts for various electrochemical reactions due to their unique
electronic and surface structures. In particular, co-doping of carbon
with boron and nitrogen has been shown to provide significant catalytic
activity for oxygen reduction reaction (ORR). However, limited experimental
work has been done to systematically study these materials, and much
remains to be understood about the nature of the active site(s), particularly
with regards to the factors underlying the activity enhancements of
these boronâcarbonânitrogen (BCN) materials. Herein,
we prepare several BCN materials experimentally with a facile and
controlled synthesis method, and systematically study their electrochemical
performance. We demonstrate the existence of <i>h</i>-BN
domains embedded in the graphitic structures of these materials using
X-ray spectroscopy. These synthesized structures yield higher activity
and selectivity toward the 2e<sup>â</sup> ORR to H<sub>2</sub>O<sub>2</sub> than structures with individual B or N doping. We further
employ density functional theory calculations to understand the role
of a variety of <i>h</i>-BN domains within the carbon lattice
for the ORR and find that the interface between <i>h</i>-BN domains and graphene exhibits unique catalytic behavior that
can preferentially drive the production of H<sub>2</sub>O<sub>2</sub>. To the best of our knowledge, this is the first example of <i>h</i>-BN domains in carbon identified as a novel system for
the electrochemical production of H<sub>2</sub>O<sub>2</sub>
Copper Silver Thin Films with Metastable Miscibility for Oxygen Reduction Electrocatalysis in Alkaline Electrolytes
Increasing the activity
of Ag-based catalysts for the oxygen reduction reaction (ORR) is important
for improving the performance and economic outlook of alkaline-based
fuel cell and metalâair battery technologies. In this work,
we prepare CuAg thin films with controllable compositions using electron
beam physical vapor deposition. X-ray diffraction analysis indicates
that this fabrication route yields metastable miscibility between
these two thermodynamically immiscible metals, with the thin films
consisting of a Ag-rich and a Cu-rich phase. Electrochemical testing
in 0.1 M potassium hydroxide showed significant ORR activity improvements
for the CuAg films. On a geometric basis, the most active thin film
(Cu<sub>70</sub>Ag<sub>30</sub>) demonstrated a 4-fold activity improvement
vs pure Ag at 0.8 V vs the reversible hydrogen electrode. Furthermore,
enhanced ORR kinetics for Cu-rich (>50 at. % Cu) thin films was
demonstrated by a decrease in Tafel slope from 90 mV/dec, a commonly
observed value for Ag catalysts, to 45 mV/dec. Surface enrichment
of the Ag-rich phase after ORR testing was indicated by X-ray photoelectron
spectroscopy and grazing incidence synchrotron X-ray diffraction measurements.
By correlating density functional theory with experimental measurements,
we postulate that the activity enhancement of the Cu-rich CuAg thin
films arises due to the non-equilibrium miscibility of Cu atoms in
the Ag-rich phase, which favorably tunes the surface electronic structure
and binding energies of reaction species