18 research outputs found
Insights into the mechanism of electrochemical ozone production via water splitting on the Ni and Sb Doped SnO2 catalyst
The H2O splitting mechanism is a very attractive alternative used in electrochemistry for the formation of O3. The most efficient catalysts employed for this reaction at room temperature are SnO2-based, in particular the Ni/Sb-SnO2 catalyst. In order to investigate the H2O splitting mechanism Density Functional Theory (DFT) was performed on a Ni/Sb-SnO2 surface with oxygen vacancies. By calculating different SnO2 facets, the (110) facet was deemed most stable, and further doped with Sb and Ni. On this surface, the H2O splitting mechanism was modelled paying particular attention to the final two steps, the formation of O2 and O3. Previous studies on β-PbO2 have shown that the final step in the reaction (the formation of O3) occurs via an Eley-Rideal style interaction where surface O2 desorbs before attacking surface O to form O3. It is revealed that for Ni/Sb-SnO2, although the overall reaction is the same the surface mechanism is different. The formation of O3 is found to occur through a Langmuir-Hinshelwood mechanism as opposed to Eley-Rideal. In addition to this the relevant adsorption energies (Eads), Gibbâs free energy (ÎGrxn) and activation barriers (Eact) for the final two steps modelled in the gas phase have been shown; providing the basis for a tool to develop new materials with higher current efficiencies
Effect of mass transport on the electrochemical oxidation of alcohols over electrodeposited film and carbon-supported pt electrodes
Š 2018 The Author(s) Electrochemical oxidation of four different alcohol molecules (methanol, ethanol, n-butanol and 2-butanol) at electrodeposited Pt film and carbon-supported Pt catalyst film electrodes, as well as the effect of mass transport on the oxidation reaction, has been studied systematically using the rotating disk electrode (RDE) technique. It was shown that oxidation current decreased with an increase in the rotation rate (Ď) for all alcohols studied over electrodeposited Pt film electrodes. In contrast, the oxidation current was found to increase with an increase in the Ď for Pt/C in ethanol and n-butanol-containing solutions. The decrease was found to be nearly reversible for ethanol and n-butanol at the electrodeposited Pt film electrode ruling out the possibility of intermediate CO ads poisoning being the sole cause of the decrease and was attributed to the formation of soluble intermediate species which diffuse away from the electrode at higher Ď. In contrast, an increase in the current with an increase in Ď for the carbon supported catalyst may suggest that the increase in residence time of the soluble species within the catalyst layer, results in further oxidation of these species. Furthermore, the reversibility of the peak current on decreasing the Ď could indicate that the surface state has not significantly changed due to the sluggish reaction kinetics of ethanol and n-butanol
Role of Water and Adsorbed Hydroxyls on Ethanol Electrochemistry on Pd: New Mechanism, Active Centers, and Energetics for Direct Ethanol Fuel Cell Running in Alkaline Medium
First principles calculations with
molecular dynamics are utilized
to simulate a simplified electrical double layer formed in the active
electric potential region during the electrocatalytic oxidation of
ethanol on Pd electrodes running in an alkaline electrolyte. Our simulations
provide an atomic level insight into how ethanol oxidation occurs
in fuel cells: New mechanisms in the presence of the simplified electrical
double layer are found to be different from the traditional ones;
through concerted-like dehydrogenation paths, both acetaldehyde and
acetate are produced in such a way as to avoid a variety of intermediates,
which is consistent with the experimental data obtained from <i>in situ</i> FTIR spectroscopy. Our work shows that adsorbed
OH on the Pd electrode rather than Pd atoms is the active center for
the reactions; the dissociation of the CâH bond is facilitated
by the adsorption of an OH<sup>â</sup> anion on the surface,
resulting in the formation of water. Our calculations demonstrate
that water dissociation rather than H desorption is the main channel
through which electrical current is generated on the Pd electrode.
The effects of the inner Helmholtz layer and the outer Helmholtz layer
are decoupled, with only the inner Helmholtz layer being found to
have a significant impact on the mechanistics of the reaction. Our
results provide atomic level insight into the significance of the
simplified electrical double layer in electrocatalysis, which may
be of general importance
Effect of the Presence of MEA on the CO<sub>2</sub> Capture Ability of Superbase Ionic Liquids
The miscibility of monoethanolamine
(MEA) in five superbase ionic
liquids (ILs), namely the trihexyl-tetradecylphosphonium benzotriazolide
([P<sub>66614</sub>]Â[Bentriz]), trihexyl-tetradecylphosphonium benzimidazolide
([P<sub>66614</sub>]Â[Benzim]), trihexyl-tetradecylphosphonium 1,2,3-triazolide
([P<sub>66614</sub>]Â[123Triz]), trihexyl-tetradecylphosphonium 1,2,4-triazolide
([P<sub>66614</sub>]Â[124Triz]), and trihexyl-tetradecylphosphonium
imidazolide ([P66614]Â[Im]) was determined at 295.15 K using <sup>1</sup>H NMR spectroscopy. The solubility of carbon dioxide (CO<sub>2</sub>) in equimolar (IL + MEA) mixtures was then studied experimentally
using a gravimetric technique at 295.15 K and 0.1 MPa. The effect
of MEA on the CO<sub>2</sub> capture ability of these ILs was investigated
together with the viscosity of these systems in the presence or absence
of CO<sub>2</sub> to evaluate their practical application in CO<sub>2</sub> capture processes. The effect of the presence of MEA on the
rate of CO<sub>2</sub> uptake was also studied. The study showed that
the MEA can enhance CO<sub>2</sub> absorption over the ideal values
in the case of [P<sub>66614</sub>]Â[123Triz] and [P<sub>66614</sub>]Â[Bentriz] while in the other systems the mixtures behave ideally.
A comparison of the effect of MEA addition with the addition of water
to these superbase ILs showed that similar trends were observed in
each case for the individual ILs studied
Understanding the Optimal Adsorption Energies for Catalyst Screening in Heterogeneous Catalysis
The fundamental understanding of
the activity in heterogeneous
catalysis has long been the major subject in chemistry. This paper
shows the development of a two-step model to understand this activity.
Using the theory of chemical potential kinetics with BrønstedâEvansâPolanyi
relations, the general adsorption energy window is determined from
volcano curves, using which the best catalysts can be searched. Significant
insights into the reasons for catalytic activity are obtained
Mechanistic Study of 1,3-Butadiene Formation in Acetylene Hydrogenation over the Pd-Based Catalysts Using Density Functional Calculations
Green
oil, which leads to the deactivation of the catalysts used
for the selective hydrogenation of acetylene, has long been observed
but its formation mechanism is not fully understood. In this work,
the formation of 1,3-butadiene, known to be the precursor of green
oil, on both Pd(111) and Pd(211) surfaces is examined using density
functional theory calculations. The pathways containing C<sub>2</sub> + C<sub>2</sub> coupling reactions as well as the corresponding
hydrogenation reactions are studied in detail. Three pathways for
1,3-butadiene production, namely coupling plus hydrogenation and further
hydrogenation, hydrogenation plus coupling plus hydrogenation, and
a two step hydrogenation followed by coupling, are determined. By
comparing the effective barriers, we identify the favored pathway
on both surfaces. A general understanding toward the deactivation
process of the industrial catalysts is also provided. In addition,
the effects of the formation of subsurface carbon atoms as well as
the Ag alloying on the 1,3-butadiene formation on Pd-based catalysts
are also investigated and compared with experimental results
A Comparative Study on the Thermophysical Properties for Two Bis[(trifluoromethyl)sulfonyl]imide-Based Ionic Liquids Containing the Trimethyl-Sulfonium or the Trimethyl-Ammonium Cation in Molecular Solvents
Herein, we present a comparative study of the thermophysical
properties
of two homologous ionic liquids, namely, trimethyl-sulfonium bisÂ[(trifluoromethyl)Âsulfonyl]Âimide,
[S<sub>111</sub>]Â[TFSI], and trimethyl-ammonium bisÂ[(trifluoromethyl)Âsulfonyl]Âimide,
[HN<sub>111</sub>]Â[TFSI], and their mixtures with propylene carbonate,
acetonitrile, or gamma butyrolactone as a function of temperature
and composition. The influence of solvent addition on the viscosity,
conductivity, and thermal properties of IL solutions was studied as
a function of the solvent mole fraction from the maximum solubility
of IL, <i>x</i><sub>s</sub>, in each solvent to the pure
solvent. In this case, <i>x</i><sub>s</sub> is the composition
corresponding to the maximum salt solubility in each liquid solvent
at a given temperature from 258.15 to 353.15 K. The effect of temperature
on the transport properties of each binary mixture was then investigated
by fitting the experimental data using Arrheniusâ law and the
VogelâTammanâFulcher (VTF) equation. The experimental
data shows that the residual conductivity at low temperature, e.g.,
263.15 K, of each binary mixture is exceptionally high. For example,
conductivity values up to 35 and 42 mS¡cm<sup>â1</sup> were observed in the case of the [S<sub>111</sub>]Â[TFSI] + ACN and
[HN<sub>111</sub>]Â[TFSI] + ACN binary mixtures, respectively. Subsequently,
a theoretical approach based on the conductivity and on the viscosity
of electrolytes was formulated by treating the migration of ions as
a dynamical process governed by ionâion and solventâion
interactions. Within this model, viscosity data sets were first analyzed
using the JonesâDole equation. Using this theoretical approach,
excellent agreement was obtained between the experimental and calculated
conductivities for the binary mixtures investigated at 298.15 K as
a function of the composition up to the maximum solubility of the
IL. Finally, the thermal characterization of the IL solutions, using
DSC measurements, showed a number of features corresponding to different
solidâsolid phase transitions, <i>T</i><sub>SâS</sub>, with extremely low melting entropies, indicating a strong organizational
structure by easy rotation of methyl group. These ILs can be classified
as plastic crystal materials and are promising as ambient-temperature
solid electrolytes
Origin of the Increase of Activity and Selectivity of Nickel Doped by Au, Ag, and Cu for Acetylene Hydrogenation
Activity and selectivity are both important issues in
heterogeneous
catalysis and recent experimental results have shown that Ni catalysts
doped by gold exhibit high activity for the hydrogenation of acetylene
with good selectivity of ethylene formation. To unravel the underlying
mechanism for this observation, the general trend of activity and
selectivity of Ni surfaces doped by Au, Ag, and Cu has been investigated
using density functional theory calculations. Complete energy profiles
from C<sub>2</sub>H<sub>2</sub> to C<sub>2</sub>H<sub>4</sub> on Ni(111),
Au/Ni(111), Ag/Ni(111) and Cu/Ni(111) are obtained and their turnover
frequencies (TOFs) are computed. The results show that acetylene adsorption
on Ni catalyst is strong which leads to the low activity while the
doping of Au, Ag, and Cu on the Ni catalyst weakens the acetylene
adsorption, giving rise to the increase of activity. The selectivity
of ethylene formation is also quantified by using the energy difference
between the hydrogenation barriers and the absolute value of the adsorption
energies of ethylene. It is found that the selectivity of ethylene
formation increases by doping Au and Ag, while those of Cu/Ni and
Ni are similar
Are Alkyl Sulfate-Based Protic and Aprotic Ionic Liquids Stable with Water and Alcohols? A Thermodynamic Approach
The knowledge of the chemical stability as a function
of the temperature
of ionic liquids (ILs) in the presence of other molecules such as
water is crucial prior to developing any industrial application and
process involving these novel materials. Fluid phase equilibria and
density over a large range of temperature and composition can give
basic information on IL purity and chemical stability. The IL scientific
community requires accurate measurements accessed from reference data.
In this work, the stability of different alkyl sulfate-based ILs in
the presence of water and various alcohols (methanol, ethanol, 1-butanol,
and 1-octanol) was investigated to understand their stability as a
function of temperature up to 423.15 K over the hydrolysis and transesterification
reactions, respectively. From this investigation, it was clear that
methyl sulfate- and ethyl sulfate-based ILs are not stable in the
presence of water, since hydrolysis of the methyl sulfate or ethyl
sulfate anions to methanol or ethanol and hydrogenate anion is undoubtedly
observed. Such observations could help to explain the differences
observed for the physical properties published in the literature by
various groups. Furthermore, it appears that a thermodynamic equilibrium
process drives these hydrolysis reactions. In other words, these hydrolysis
reactions are in fact reversible, providing the possibility to re-form
the desired alkyl sulfate anions by a simple transesterification reaction
between hydrogen sulfate-based ILs and the corresponding alcohol (methanol
or ethanol). Additionally, butyl sulfate- and octyl sulfate-based
ILs appear to follow this pattern but under more drastic conditions.
In these systems, hydrolysis is observed in both cases after several
months for temperatures up to 423 K in the presence of water. Therein,
the partial miscibility of hydrogen sulfate-based ILs with long chain
alcohols (1-butanol and 1-octanol) can help to explain the enhanced
hydrolytic stability of the butyl sulfate- and octyl sulfate-based
ILs compared with the methyl or ethyl sulfate systems. Additionally,
rapid transesterification reactions are observed during liquidâliquid
equilibrium studies as a function of temperature for binary systems
of (hydrogen sulfate-based ionic liquids + 1-butanol) and of (hydrogen
sulfate-based ionic liquids + 1-octanol). Finally, this atom-efficient
catalyst-free transesterification reaction between hydrogen sulfate-based
ILs and alcohol was then tested to provide a novel way to synthesize
new ILs with various anion structures containing the alkyl sulfate
group
Origin of low CO2 selectivity on platinum in the direct ethanol fuel cell
Calculated answer: First-principles calculations have been applied to calculate the energy barrier for the key step in CO formation on a Pt surface (see picture; Pt blue, Pt atoms on step edge yellow) to understand the low CO 2 selectivity in the direct ethanol fuel cell. The presence of surface oxidant species such as O (brown bar) and OH (red bar) led to an increase of the energy barrier and thus an inhibition of the key step