16 research outputs found
First-Principles Analysis of Potential-Dependent Proton Coupled Electron Transfer between Polypyridyl–Ruthenium Complexes and Oxygen-Modified Graphene Electrodes
Proton
coupled electron transfer reactions which are pervasive
throughout electrochemistry control a number of energy conversion
strategies. First-principles density functional theoretical calculations
are used herein to examine proton coupled electron transfer between
a homogeneous mononuclear polypyridyl–ruthenium catalyst used
in the catalytic oxidation of water and surface ketone groups on oxygen-modified
graphene electrode surfaces. The potential-dependent interface energies
were calculated for two proton transfer states: Ru<sup>III</sup>OH···OC–graphene
and Ru<sup>IV</sup>O···HO–C–graphene.
The reactivity for interfacial proton coupled electron transfer was
found to be controlled by functional groups that terminate surface
defect sites as well as graphene edge sites. The energy gap between
the two proton transfer states becomes smaller as the number of surface
ketone groups increases. Ab initio molecular dynamics simulations
clearly show that increases in the number of surface ketone groups
increase the hydrophilicity of the graphene basal plane. This significantly
decreases the energy for proton transfer, thus providing a low-energy
path that extends over a wide range of potentials. The surface ketone
groups at the armchair edges of the graphene plane appear to be the
most reactive oxygens on the graphene surface as they lead to direct
reversible proton coupled electron transfer between the polypyridal–Ru
complexes and the CO groups at the edges where the two proton
transfer potential energy surfaces crossing at 0.2 V vs SHE. The graphene
armchair edge helps to stabilize the Ru<sup>IV</sup>O···HO–C–graphene-edge
proton transfer state which is an important step in water oxidation
catalysis
On the Yield of Levoglucosan from Cellulose Pyrolysis
Fast pyrolysis is a thermochemical
process to fragment large biopolymers
such as cellulose to chemical intermediates which can be refined to
renewable fuels and chemicals. Levoglucosan (LGA), a six-carbon oxygenate,
is the most abundant primary product from cellulose pyrolysis with
LGA yields reported over a wide range of 5–80 percent carbon
(%C). In this study, the variation of the observed yield of LGA from
cellulose pyrolysis was experimentally investigated. Cellulose pyrolysis
experiments were conducted in two different reactors: the Frontier
micropyrolyzer (2020-iS), and the pulse heated analysis of solid reactions
(PHASR) system. The reactor configuration and experimental conditions
including cellulose sample size were found to have a significant effect
on the yield of LGA. Four different hypotheses were proposed and tested
to evaluate the relationship of cellulose sample size and the observed
LGA yield including (a) thermal promotion of LGA formation, (b) the
crystallinity of cellulose samples, (c) secondary and vapor-phase
reactions of LGA, and (d) the catalytic effect of melt-phase hydroxyl
groups. Co-pyrolysis experiments of cellulose and fructose in the
PHASR reactor presented indirect experimental evidence of previously
postulated catalytic effects of hydroxyl groups in glycosidic bond
cleavage for LGA formation in transport-limited reactor systems
Generalized Temporal Acceleration Scheme for Kinetic Monte Carlo Simulations of Surface Catalytic Processes by Scaling the Rates of Fast Reactions
A novel
algorithm is presented that achieves temporal acceleration
during kinetic Monte Carlo (KMC) simulations of surface catalytic
processes. This algorithm allows for the direct simulation of reaction
networks containing kinetic processes occurring on vastly disparate
time scales which computationally overburden standard KMC methods.
Previously developed methods for temporal acceleration in KMC were
designed for specific systems and often require a priori information
from the user such as identifying the fast and slow processes. In
the approach presented herein, quasi-equilibrated processes are identified
automatically based on previous executions of the forward and reverse
reactions. Temporal acceleration is achieved by automatically scaling
the intrinsic rate constants of the quasi-equilibrated processes,
bringing their rates closer to the time scales of the slow kinetically
relevant nonequilibrated processes. All reactions are still simulated
directly, although with modified rate constants. Abrupt changes in
the underlying dynamics of the reaction network are identified during
the simulation, and the reaction rate constants are rescaled accordingly.
The algorithm was utilized here to model the Fischer–Tropsch
synthesis reaction over ruthenium nanoparticles. This reaction network
has multiple time-scale-disparate processes which would be intractable
to simulate without the aid of temporal acceleration. The accelerated
simulations are found to give reaction rates and selectivities indistinguishable
from those calculated by an equivalent mean-field kinetic model. The
computational savings of the algorithm can span many orders of magnitude
in realistic systems, and the computational cost is not limited by
the magnitude of the time scale disparity in the system processes.
Furthermore, the algorithm has been designed in a generic fashion
and can easily be applied to other surface catalytic processes of
interest
Mechanistic Insights on the Hydrogenation of α,β-Unsaturated Ketones and Aldehydes to Unsaturated Alcohols over Metal Catalysts
The selective hydrogenation of unsaturated ketones (methyl
vinyl
ketone and benzalacetone) and unsaturated aldehydes (crotonaldehyde
and cinnamaldehyde) was carried out with H<sub>2</sub> at 2 bar absolute
over Pd/C, Pt/C, Ru/C, Au/C, Au/TiO<sub>2</sub>, or Au/Fe<sub>2</sub>O<sub>3</sub> catalysts in ethanol or water solvent at 333 K. Comparison
of the turnover frequencies revealed Pd/C to be the most active hydrogenation
catalyst, but the catalyst failed to produce unsaturated alcohols,
indicating hydrogenation of the CC bond was highly preferred
over the CO bond on Pd. The Pt and Ru catalysts were able
to produce unsaturated alcohols from unsaturated aldehydes, but not
from unsaturated ketones. Although Au/Fe<sub>2</sub>O<sub>3</sub> was
able to partially hydrogenate unsaturated ketones to unsaturated alcohols,
the overall hydrogenation rate over gold was the lowest of all of
the metals examined. First-principles density functional theory calculations
were therefore used to explore the reactivity trends of methyl vinyl
ketone (MVK) and benzalacetone (BA) hydrogenation over model Pt(111)
and Ru(0001) surfaces. The observed selectivity over these metals
is likely controlled by the significantly higher activation barriers
to hydrogenate the CO bond compared with those required to
hydrogenate the CC bond. Both the unsaturated alcohol and
the saturated ketone, which are the primary reaction products, are
strongly bound to Ru and can react further to the saturated alcohol.
The lower calculated barriers for the hydrogenation steps over Pt
compared with Ru account for the higher observed turnover frequencies
for the hydrogenation of MVK and BA over Pt. The presence of a phenyl
substituent α to the CC bond in BA increased the barrier
for CC hydrogenation over those associated with the CC
bond in MVK; however, the increase in barriers with phenyl substitution
was not adequate to reverse the selectivity trend
Selective Catalytic Oxidative-Dehydrogenation of Carboxylic AcidsAcrylate and Crotonate Formation at the Au/TiO<sub>2</sub> Interface
The
oxidative-dehydrogenation of carboxylic acids to selectively
produce unsaturated acids at the second and third carbons regardless
of alkyl chain length was found to occur on a Au/TiO<sub>2</sub> catalyst.
Using transmission infrared spectroscopy (IR) and density functional
theory (DFT), unsaturated acrylate (H<sub>2</sub>CCHCOO) and
crotonate (CH<sub>3</sub>CHCHCOO) were observed to form from
propionic acid (H<sub>3</sub>CCH<sub>2</sub>COOH) and butyric acid
(H<sub>3</sub>CCH<sub>2</sub>CH<sub>2</sub>COOH), respectively, on
a catalyst with ∼3 nm diameter Au particles on TiO<sub>2</sub> at 400 K. Desorption experiments also show gas phase acrylic acid
is produced. Isotopically labeled <sup>13</sup>C and <sup>12</sup>C propionic acid experiments along with DFT calculated frequency
shifts confirm the formation of acrylate and crotonate. Experiments
on pure TiO<sub>2</sub> confirmed that the unsaturated acids were
not produced on the TiO<sub>2</sub> support alone, providing evidence
that the sites for catalytic activity are at the dual Au–Ti<sup>4+</sup> sites at the nanometer Au particles’ perimeter. The
DFT calculated energy barriers between 0.3 and 0.5 eV for the reaction
pathway are consistent with the reaction occurring at 400 K on Au/TiO<sub>2</sub>
Mechanistic Insights into the Catalytic Oxidation of Carboxylic Acids on Au/TiO<sub>2</sub>: Partial Oxidation of Propionic and Butyric Acid to Gold Ketenylidene through Unsaturated Acids
The
partial oxidation of model C<sub>2</sub>–C<sub>4</sub> (acetic,
propionic, and butyric) carboxylic acids on Au/TiO<sub>2</sub> catalysts
consisting of Au particles ∼3 nm in size
was investigated using transmission infrared spectroscopy and density
functional theory. All three acids readily undergo oxidative dehydrogenation
on Au/TiO<sub>2</sub>. Propionic and butyric acid dehydrogenate at
the C2–C3 positions, whereas acetic acid dehydrogenates at
the C1–C2 position. The resulting acrylate and crotonate intermediates
are subsequently oxidized to form β-keto acids that decarboxylate.
All three acids form a gold ketenylidene intermediate, Au<sub>2</sub>CCO, along the way to their full oxidation to form
CO<sub>2</sub>. Infrared measurements of Au<sub>2</sub>CCO
formation as a function of time provides a surface spectroscopic probe
of the kinetics for the activation and oxidative dehydrogenation of
the alkyl groups in the carboxylate intermediates that form. The reaction
proceeds via the dissociative adsorption of the acid onto TiO<sub>2</sub>, the adsorption and activation of O<sub>2</sub> at the dual
perimeter sites on the Au particles (Au–O–O-Ti), and
the subsequent activation of the C2–H and C3–H bonds
of the bound propionate and butyrate intermediates by the weakly bound
and basic oxygen species on Au to form acrylate and crotonate intermediates,
respectively. The CC bond of the unsaturated acrylate and
crotonate intermediates is readily oxidized to form an acid at the
beta (C3) position, which subsequently decarboxylates. This occurs
with an overall activation energy of 1.5–1.7 ± 0.2 eV,
ultimately producing the Au<sub>2</sub>CCO species
for all three carboxylates. The results suggest that the decrease
in the rate in moving from acetic to propionic to butyric acid is
due to an increase in the free energy of activation for the formation
of the Au<sub>2</sub>CCO species on Au/TiO<sub>2</sub> with an increasing size of the alkyl substituent. The formation
of Au<sub>2</sub>CCO proceeds for carboxylic acids
that are longer than C<sub>2</sub> without a deuterium kinetic isotope
effect, demonstrating that C–H bond scission is not involved
in the rate-determining step; the rate instead appears to be controlled
by C–O bond scission. The adsorbed Au<sub>2</sub>CCO
intermediate species can be hydrogenated to produce ketene, H<sub>2</sub>CCO(g), with an activation energy of 0.21
± 0.05 eV. These studies show that selective oxidative dehydrogenation
of the alkyl side chains of fatty acids can be catalyzed by nanoparticle
Au/TiO<sub>2</sub> at temperatures near 400 K
Localized Partial Oxidation of Acetic Acid at the Dual Perimeter Sites of the Au/TiO<sub>2</sub> CatalystFormation of Gold Ketenylidene
Chemisorbed acetate species derived from the adsorption
of acetic
acid have been oxidized on a nano-Au/TiO<sub>2</sub> (∼3 nm
diameter Au) catalyst at 400 K in the presence of O<sub>2</sub>(g).
It was found that partial oxidation occurs to produce gold ketenylidene
species, Au<sub>2</sub>CCO. The reactive acetate
intermediates are bound at the TiO<sub>2</sub> perimeter sites of
the supported Au/TiO<sub>2</sub> catalyst. The ketenylidene species
is identified by its measured characteristic stretching frequency
ν(CO) = 2040 cm<sup>–1</sup> and by <sup>13</sup>C and <sup>18</sup>O isotopic substitution comparing to calculated frequencies
found from density functional theory. The involvement of dual catalytic
Ti<sup>4+</sup> and Au perimeter sites is postulated on the basis
of the absence of reaction on a similar nano-Au/SiO<sub>2</sub> catalyst.
This observation excludes low coordination number Au sites as being
active alone in the reaction. Upon raising the temperature to 473
K, the production of CO<sub>2</sub> and H<sub>2</sub>O is observed
as both acetate and ketenylidene species are further oxidized by O<sub>2</sub>(g). The results show that partial oxidation of adsorbed acetate
to adsorbed ketenylidyne can be cleanly carried out over Au/TiO<sub>2</sub> catalysts by control of temperature
Consequences of Metal–Oxide Interconversion for C–H Bond Activation during CH<sub>4</sub> Reactions on Pd Catalysts
Mechanistic assessments based on
kinetic and isotopic methods combined
with density functional theory are used to probe the diverse pathways
by which C–H bonds in CH<sub>4</sub> react on bare Pd clusters,
Pd cluster surfaces saturated with chemisorbed oxygen (O*), and PdO
clusters. C–H activation routes change from oxidative addition
to H-abstraction and then to σ-bond metathesis with increasing
O-content, as active sites evolve from metal atom pairs (*–*)
to oxygen atom (O*–O*) pairs and ultimately to Pd cation-lattice
oxygen pairs (Pd<sup>2+</sup>–O<sup>2–</sup>) in PdO.
The charges in the CH<sub>3</sub> and H moieties along the reaction
coordinate depend on the accessibility and chemical state of the Pd
and O centers involved. Homolytic C–H dissociation prevails
on bare (*–*) and O*-covered surfaces (O*–O*), while
C–H bonds cleave heterolytically on Pd<sup>2+</sup>–O<sup>2–</sup> pairs at PdO surfaces. On bare surfaces, C–H
bonds cleave via oxidative addition, involving Pd atom insertion into
the C–H bond with electron backdonation from Pd to C–H
antibonding states and the formation of tight three-center (H<sub>3</sub>C···Pd···H)<sup>⧧</sup> transition states. On O*-saturated Pd surfaces, C–H bonds
cleave homolytically on O*–O* pairs to form radical-like CH<sub>3</sub> species and nearly formed O–H bonds at a transition
state (O*···CH<sub>3</sub><sup>•</sup>···*OH)<sup>⧧</sup> that is looser and higher in enthalpy than on bare
Pd surfaces. On PdO surfaces, site pairs consisting of exposed Pd<sup>2+</sup> and vicinal O<sup>2–</sup>, Pd<sub>ox</sub>–O<sub>ox</sub> , cleave C–H bonds heterolytically via σ-bond
metathesis, with Pd<sup>2+</sup> adding to the C–H bond, while
O<sup>2–</sup> abstracts the H-atom to form a four-center (H<sub>3</sub>C<sup>δ−</sup>···Pd<sub>ox</sub>···H<sup>δ+</sup>···O<sub>ox</sub>)<sup>⧧</sup> transition state without detectable Pd<sub>ox</sub> reduction. The latter is much more stable than transition states
on *–* and O*–O* pairs and give rise to a large increase
in CH<sub>4</sub> oxidation turnover rates at oxygen chemical potentials
leading to Pd to PdO transitions. These distinct mechanistic pathways
for C–H bond activation, inferred from theory and experiment,
resemble those prevalent on organometallic complexes. Metal centers
present on surfaces as well as in homogeneous complexes act as both
nucleophile and electrophile in oxidative additions, ligands (e.g.,
O* on surfaces) abstract H-atoms via reductive deprotonation of C–H
bonds, and metal–ligand pairs, with the pair as electrophile
and the metal as nucleophile, mediate σ-bond metathesis pathways
Theoretical Insights into the Effects of KOH Concentration and the Role of OH<sup>–</sup> in the Electrocatalytic Reduction of CO<sub>2</sub> on Au
The active and selective electrochemical reduction of
CO2 to value-added chemical intermediates can offer a sustainable
route
for the conversion of CO2 to chemicals and fuels, thus
helping to mitigate greenhouse gas emissions and enabling intermittent
energy from renewable sources. Alkaline solutions are often the preferred
media for the electrocatalytic CO2 reduction reaction (CO2RR) as they provide high current densities and low overpotentials
while suppressing the hydrogen evolution side reaction. Recent experiments
carried out on Au and Ag in KOH, as well as other electrolytes, including
KHCO3, K2CO3, and KCl, showed that
increasing electrolyte concentration lowered onset potentials, increased
Faradaic efficiencies to CO, and improved current densities. Herein,
we carry out potential-dependent ab initio molecular dynamic (AIMD)
simulations along with density functional theory (DFT) calculations
using explicit KOH electrolyte and H2O solution molecules
to examine the influence of OH– anions and the KOH
electrolyte on the elementary steps and their corresponding energetics
in the mechanism for CO2 reduction. The simulations indicate
that the first electron transfer step to CO2 to form the
adsorbed *CO2(•−) radical anion
is rate-limiting, while the subsequent proton and electron transfer
steps are facile and downhill in energy at reducing potentials. The
OH– anions present in the solution can adsorb on
the Au cathode down to potentials as low as ∼ −3 V (SCE).
This enables the OH– anions to transfer electrons
to the Au cathode and into antibonding 2π* orbitals of CO2, thus facilitating the rate-determining adsorption and electron
transfer to CO2 to form the adsorbed *CO2(•−) radical anion. Increasing the concentration
of the K+OH– electrolyte reduces the
barrier for the electrocatalytic reduction of CO2 and thus
improves the current density, consistent with the reported experimental
results. The *CO2(•−) radical
anion that forms subsequently undergoes facile proton transfer from
a vicinal water molecule in solution to form the hydroxy carbonyl
(*HOCO) intermediate that readily undergoes subsequent proton and
electron transfer from a second water molecule to form CO and OH– at a potential of ∼ −1.2 V SCE. While
the formation of formate (HCOO–) is thermodynamically
favorable, the direct hydrogenation of *CO2(•−) as well as the intramolecular proton transfer via *HOCO to form HCOO– are kinetically unfavored.
The presence of OH– anions near the surface also
facilitates the formation of bicarbonate (HCO3–) at lower potentials. The bicarbonate that forms can be converted
to the reactive *HOCO intermediate at more negative potentials that
subsequently reacts to form CO and regenerate OH–. The results discussed herein help provide a more detailed understanding
of the interplay between the OH–, K+,
H2O, and reaction intermediates on the Au surface in the
electric double layer and their influence on the onset potential,
electrocatalytic activity, and selectivity for CO2RR
Inhibition at Perimeter Sites of Au/TiO<sub>2</sub> Oxidation Catalyst by Reactant Oxygen
TiO<sub>2</sub>-supported gold nanoparticles exhibit
surprising
catalytic activity for oxidation reactions compared to noble bulk
gold which is inactive. The catalytic activity is localized at the
perimeter of the Au nanoparticles where Au atoms are atomically adjacent
to the TiO<sub>2</sub> support. At these dual-catalytic sites an oxygen
molecule is efficiently activated through chemical bonding to both
Au and Ti<sup>4+</sup> sites. A significant inhibition by a factor
of 22 in the CO oxidation reaction rate is observed at 120 K when
the Au is preoxidized, caused by the oxygen-induced positive charge
produced on the perimeter Au atoms. Theoretical calculations indicate
that induced positive charge occurs in the Au atoms which are adjacent
to chemisorbed oxygen atoms, almost doubling the activation energy
for CO oxidation at the dual-catalytic sites in agreement with experiments.
This is an example of self-inhibition in catalysis by a reactant species