49 research outputs found
Activity Descriptors for CO<sub>2</sub> Electroreduction to Methane on Transition-Metal Catalysts
The electrochemical reduction of CO<sub>2</sub> into
hydrocarbons and alcohols would allow renewable energy sources
to be converted into fuels and chemicals. However, no electrode catalysts
have been developed that can perform this transformation with a low
overpotential at reasonable current densities. In this work, we compare
trends in binding energies for the intermediates in CO<sub>2</sub> electrochemical reduction and present an activity âvolcanoâ
based on this analysis. This analysis describes the experimentally
observed variations in transition-metal catalysts, including why copper
is the best-known metal electrocatalyst. The protonation of adsorbed
CO is singled out as the most important step dictating the overpotential.
New strategies are presented for the discovery of catalysts that can
operate with a reduced overpotential
Methanol to Dimethyl Ether over ZSM-22: A Periodic Density Functional Theory Study
Methanol-to-DME
conversion over ZSM-22 Brønsted acid sites
is investigated on the basis of periodic density functional theory
calculations. DME formation has been speculated to take place via
the dissociative or associative pathway. It is shown that the dissociative
pathway is the predominant pathway. We find that water lowers the
activation energies of key reactions but that the lowering of the
activation energies is insufficient to increase the rate because of
the entropy loss associated with water adsorption. The consequence
of acid strength on the methanol-to-DME conversion pathways is investigated
on the basis of Al-, Ga-, or In-induced Brønsted acid sites.
We show that linear correlations between activation energies and acid
strength exist. It is found that weaker acidity leads to higher activation
energies. We find that changes in acidity will not change the conclusion
that the dissociative pathway is the predominant pathway
Theoretical Insights into Methane CâH Bond Activation on Alkaline Metal Oxides
In
this work, we investigate the role of alkaline metal oxides
(AMO) (MgO, CaO, and SrO) in activating the CâH bond in methane.
We use Density Functional Theory (DFT) and microkinetic modeling to
study the catalytic elementary steps in breaking the CâH bond
in methane and creating the methyl radical, a precursor prior to creating
C<sub>2</sub> products. We study the effects of surface geometry on
the catalytic activity of AMO by examining terrace and step sites.
We observe that the process of activating methane depends strongly
on the structure of the AMO. When the AMO surface is doped with an
alkali metal, the transition state (TS) structure has a methyl radical-like
behavior, where the methyl radical interacts weakly with the AMO surface.
In this case, the TS energy scales with the hydrogen binding energy.
On pure AMO, the TS interacts with AMO surface oxygen as well as the
metal atom on the surface, and consequently the TS energy scales with
the binding energy of hydrogen and methyl. We study the activity of
AMO using a mean-field microkinetic model. The results indicate that
terrace sites have similar catalytic activity, with the exception
of MgO(100). Step sites bind hydrogen more strongly, making them more
active, and this confirms previously reported experimental results.
We map the catalytic activity of AMO using a volcano plot with two
descriptors: the methyl and the hydrogen binding energies, with the
latter being a more significant descriptor. The microkinetic model
results suggest that CâH bond dissociation is not always the
rate-limiting step. At weak hydrogen binding, the reaction is limited
by CâH bond activation. At strong hydrogen binding, the reaction
is limited due to poisoning of the active site. We found an increase
in activity of AMO as the basicity increased. Finally, the developed
microkinetic model allows screening for improved catalysts using simple
calculations of the hydrogen binding energy
A Theoretical Study of Methanol Oxidation on RuO<sub>2</sub>(110): Bridging the Pressure Gap
Partial
oxidation catalysis is often fraught with selectivity problems,
largely because there is a tendency of oxidation products to be more
reactive than the starting material. One industrial process that has
successfully overcome this problem is partial oxidation of methanol
to formaldehyde. This process has become a global success, with an
annual production of 30 million tons. Although ruthenium catalysts
have not shown activity as high as the current molybdena or silver-based
industrial standards, the study of ruthenium systems has the potential
to elucidate which catalyst properties facilitate the desired partial
oxidation reaction as opposed to deep combustion due to a pressure-dependent
selectivity âswitchâ that has been observed in ruthenium-based
catalysts. In this work, we find that we are able to successfully
rationalize this âpressure gapâ using near-ab initio
steady-state microkinetic modeling on RuO<sub>2</sub>(110). We obtain
molecular desorption prefactors from experiment and determine all
other energetics using density functional theory. We show that, under
ambient pressure conditions, formaldehyde production is favored on
RuO<sub>2</sub>(110), whereas under ultrahigh vacuum pressure conditions,
full combustion to CO<sub>2</sub> takes place. We glean from our model
several insights regarding how coverage effects, oxygen activity,
and rate-determining steps influence selectivity and activity. We
believe the understanding gained in this work might advise and inspire
the greater partial oxidation community and be applied to other catalytic
processes which have not yet found industrial success
Screened Hybrid Exact Exchange Correction Scheme for Adsorption Energies on Perovskite Oxides
The bond formation between an oxide
surface and oxygen, which is
of importance for numerous surface reactions including catalytic reactions,
is investigated within the framework of hybrid density functional
theory that includes nonlocal Fock exchange. We show that there exists
a linear correlation between the adsorption energies of oxygen on
LaMO<sub>3</sub> (M = ScâCu) surfaces obtained using a hybrid
functional (e.g., HeydâScuseriaâErnzerhof) and those
obtained using a semilocal density functional (e.g., PerdewâBurkeâErnzerhof)
through the magnetic properties of the bulk phase as determined with
a hybrid functional. The energetics of the spin-polarized surfaces
follows the same trend as corresponding bulk systems, which can be
treated at a much lower computational cost. The difference in adsorption
energy due to magnetism is linearly correlated to the magnetization
energy of bulk, that is, the energy difference between the spin-polarized
and the non-spin-polarized solutions. Hence, one can estimate the
correction to the adsorption energy as obtained from a semilocal functional
directly from the bulk magnetization energy from a hybrid functional
AlâAir Batteries: Fundamental Thermodynamic Limitations from First-Principles Theory
The Alâair battery possesses
high theoretical specific energy
(4140 W h/kg) and is therefore an attractive candidate for vehicle
propulsion. However, the experimentally observed open-circuit potential
is much lower than what bulk thermodynamics predicts, and this potential
loss is typically attributed to corrosion. Similarly, large Tafel
slopes associated with the battery are assumed to be due to film formation.
We present a detailed thermodynamic study of the Alâair battery
using density functional theory. The results suggest that the maximum
open-circuit potential of the Al anode is only â1.87 V versus
the standard hydrogen electrode at pH 14.6 instead of the traditionally
assumed â2.34 V and that large Tafel slopes are inherent in
the electrochemistry. These deviations from the bulk thermodynamics
are intrinsic to the electrochemical surface processes that define
Al anodic dissolution. This has contributions from both asymmetry
in multielectron transfers and, more importantly, a large chemical
stabilization inherent to the formation of bulk AlÂ(OH)<sub>3</sub> from surface intermediates. These are fundamental limitations that
cannot be improved even if corrosion and film effects are completely
suppressed
Tuning the MoS<sub>2</sub> Edge-Site Activity for Hydrogen Evolution via Support Interactions
The hydrogen evolution reaction (HER)
on supported MoS<sub>2</sub> catalysts is investigated using periodic
density functional theory,
employing the new BEEF-vdW functional that explicitly takes long-range
van der Waals (vdW) forces into account. We find that the support
interactions involving vdW forces leads to significant changes in
the hydrogen binding energy, resulting in several orders of magnitude
difference in HER activity. It is generally seen for the Mo-edge that
strong adhesion of the catalyst onto the support leads to weakening
in the hydrogen binding. This presents a way to optimally tune the
hydrogen binding on MoS<sub>2</sub> and explains the lower than expected
exchange current densities of supported MoS<sub>2</sub> in electrochemical
H<sub>2</sub> evolution studies
Theoretical Limits to the Anode Potential in Aqueous MgâAir Batteries
The aqueous Mgâair battery
is an attractive candidate for
some electric vehicle applications due to its high theoretical specific
energy, environmentally and physiologically benign properties, and
implied low cost from using earth-abundant materials. However, the
experimentally observed potentials (1.6â1.2 V) are far from
the thermodynamically predicted value of 3.09 V, based on the free
energy of formation for the reaction Mg (s) + 1/2 O<sub>2</sub> (g)
+ H<sub>2</sub>O (l) â MgÂ(OH)<sub>2</sub> (s). It is generally
believed that this large difference is principally due to the presence
of Mg corrosion giving rise to a net corrosion potential, and that
it would be possible to nearly obtain the full potential of 3.09 V
if corrosion were completely suppressed. In this contribution, we
present a density functional theory study of the hydroxide-assisted
Mg anodic dissolution mechanism in the aqueous Mgâair battery.
We show that the Mg surface is expected to be highly OH*-covered in
the anodic dissolution process, and that the calculated intrinsic
limiting potentials are in fact in reasonable agreement with experimentally
observed potentials. These limiting potentials are dictated by sequential
electrochemical adsorption of hydroxide to the Mg surface, and therefore,
the bulk free energy of MgÂ(OH)<sub>2</sub> (s) formation cannot be
used to predict the intrinsic anode potential in the aqueous Mgâair
battery. These intrinsic limits imply that completely suppressing
Mg corrosion will not significantly increase the potential available
for the Mgâair battery
Role of Subsurface Oxygen on Cu Surfaces for CO<sub>2</sub> Electrochemical Reduction
Under
ambient conditions, copper with oxygen near the surface displays
strengthened CO<sub>2</sub> and CO adsorption energies. This finding
is often used to rationalize differences seen in product distributions
between Cu-oxide and pure Cu electrodes during electrochemical CO<sub>2</sub> reduction. However, little evidence exists to confirm the
presence of oxygen within first few layers of the Cu matrix under
relevant experimental reducing conditions. Using density functional
theory calculations, we discuss the stability of subsurface oxygen
from thermodynamic and kinetic perspectives and show that under reducing
potentials subsurface oxygen alone should have negligible effects
on the activity of crystalline Cu
Naravne oblike gibanja kot sredstvo razvoja moÄi v mali odbojki (10-12 let)
RuO<sub>2</sub> has been reported
to reduce CO<sub>2</sub> electrochemically
to methanol at low overpotential. Herein, we have used density functional
theory (DFT) to gain insight into the mechanism for CO<sub>2</sub> reduction on RuO<sub>2</sub>(110). We have investigated the thermodynamic
stability of various surface terminations in the electrochemical environment
and found CO covered surfaces to be particularly stable, although
their formation might be kinetically limited under mildly reducing
conditions. We have identified the lowest free energy pathways for
CO<sub>2</sub> reduction to formic acid (HCOOH), methanol (CH<sub>3</sub>OH), and methane (CH<sub>4</sub>) on partially reduced RuO<sub>2</sub>(110) covered with 0.25 and 0.5 ML of CO*. We have found that
CO<sub>2</sub> is reduced to formic acid, which is further reduced
to methanol and methane. At 0.25 ML of CO*, the reduction of formate
(OCHO*) to formic acid is the thermodynamically most difficult step
and becomes exergonic at potentials below â0.43 V vs the reversible
hydrogen electrode (RHE). On the other hand, at 0.5 ML of CO*, the
reduction of formic acid to H<sub>2</sub>COOH* is the thermodynamically
most difficult step and becomes exergonic at potentials below â0.25
V vs RHE. We have found that CO<sub>2</sub> reduction activity on
RuO<sub>2</sub> changes with CO coverage, which suggests that CO coverage
can be used as a tool to tune the CO<sub>2</sub> reduction activity.
We have shown the mechanism for CO<sub>2</sub> reduction on RuO<sub>2</sub> to be different from that on Cu. On Cu, hydrocarbons are
formed at high Faradaic efficiency through reduction of CO* at âź1
V overpotential, while on RuO<sub>2</sub>, methanol and formate are
formed through reduction of formic acid at lower overpotentials. Using
our understanding of the CO<sub>2</sub> reduction mechanism on RuO<sub>2</sub>, we suggest reduction of formic acid on RuO<sub>2</sub>,
which should lead to methanol and methane production at relatively
low overpotentials