13 research outputs found
Theoretical Study on the Mechanism of Photoreduction of CO<sub>2</sub> to CH<sub>4</sub> on the Anatase TiO<sub>2</sub>(101) Surface
Artificial
photosynthesis of CO<sub>2</sub> has recently attracted
intense attention as a potential solution for the energy crisis and
global warming. However, the molecular mechanism of the reaction is
quite complicated and is far from understood. We performed a first-principles
calculation on the thermodynamically feasible formaldehyde pathway:
CO<sub>2</sub> → HCOOH → H<sub>2</sub>CO → CH<sub>3</sub>OH → CH<sub>4</sub>. The interconversion of the C1
molecules has been systematically investigated. We find that a two-electron
process has a lower barrier than a one-electron process for the photoreduction
of all of the molecules under investigation except for methanol. On
the basis of the full potential energy surface for photoreduction
of CO<sub>2</sub> to methane, the rate-limiting step is found to be
the photoreduction of formic acid to formaldehyde, which contains
the elementary step that has the largest kinetic barrier. It will
be more efficient if CO instead of formic acid is the precursor of
formaldehyde. Then the rate-limiting step becomes the photoreduction
of CO<sub>2</sub> to CO. However, the barriers for the photoreduction
of the organic molecules are all higher than the barriers for their
photodecomposition reaction, which suggests that all of the C1 organic
molecules are more easily oxidized than reduced. Thus, charge separation
is crucial for improving the efficiency and selectivity of the reaction.
The intertwining of photoreduction and photooxidation reactions might
be one of the major reasons for the complexity and low efficiency
of the reaction. On the basis of the calculations, a new mechanism
for the reaction is proposed
New Mechanism for Photocatalytic Reduction of CO<sub>2</sub> on the Anatase TiO<sub>2</sub>(101) Surface: The Essential Role of Oxygen Vacancy
Photocatalytic reduction
of CO<sub>2</sub> into organic molecules
is a very complicated and important reaction. Two possible pathways,
the fast-hydrogenation (FH) path and the fast-deoxygenation (FdO)
path, have been proposed on the most popular photocatalyst TiO<sub>2</sub>. We have carried out first-principles calculations to investigate
both pathways on the perfect and defective anatase TiO<sub>2</sub>(101) surfaces to provide comprehensive understanding of the reaction
mechanism. For the FH path, it is found that oxygen vacancy on defective
surface can greatly lower the barrier of the deoxygenation processes,
which makes it a more active site than the surface Ti. For the FdO
path, our calculation suggests that it can not proceed on the perfect
surface, nor can it proceed on the defective surface due to their
unfavorable energetics. Based on the fact that the FH path can proceed
both at the surface Ti site and the oxygen vacancy site, we have proposed
a simple mechanism that is compatible with various experiments. It
can properly rationalize the selectivity of the reaction and greatly
simplify the picture of the reaction. The important role played by
oxygen vacancy in the new mechanism is highlighted and a strategy
for design of more efficient photocatalysts is proposed accordingly
Dehydrogenation of Propane to Propylene by a Pd/Cu Single-Atom Catalyst: Insight from First-Principles Calculations
The
catalytic properties of the single-Pd-doped Cu<sub>55</sub> nanoparticle
toward propane dehydrogenation have been systemically
investigated by first-principles calculations, and the possible reaction
mechanisms and effects of the single and multiple Pd doping on the
catalytic activity have been discussed. Calculations reveal that the
low-energy catalytic conversion of propane to propylene by the Pd/Cu
single-atom catalyst comprises the initial crucial C–H bond
breaking at either the methyl or methylene group, the facile diffusion
of detached H atoms on the Cu surface, and the subsequent C–H
bond dissociation activation of the adsorbed propyl species. The single-Pd-doped
Cu<sub>55</sub> nanoparticle shows remarkable activity toward C–H
bond activation, and the presence of relatively inactive Cu surface
is beneficial for the coupling and desorption of detached H atoms
and can reduce side reactions such as deep dehydrogenation and C–C
bond breaking. The single-Pd-doped Cu<sub>55</sub> cluster bears good
balance between the maximum use of the noble metal and the activity,
and it may serve as a promising single-atom catalyst toward selective
dehydrogenation of propane
Feasible Catalytic Strategy for Writing Conductive Nanoribbons on a Single-Layer Graphene Fluoride
An
accessible method for local reduction of graphene fluoride catalyzed
by the Pt-coated nanotip with the assistance of a mixture of hydrogen
and ethylene atmosphere is proposed and fully explored theoretically.
Detailed mechanisms and roles of hydrogen and ethylene molecules in
the cyclic reduction is discussed based on extensive first-principles
calculations. It is demonstrated that the proposed cyclic reduction
strategy is energetically favorable. This new strategy can be effectively
applied in scanning probe lithography to fabricate electronic circuits
at the nanoscale on graphene fluoride under mild conditions
Hydrophobicity and Hydrophilicity Balance Determines Shape Selectivity of Suzuki Coupling Reactions Inside Pd@meso-SiO<sub>2</sub> Nanoreactor
Molecular sorting
and catalysis directed by shape selectivity have
been extensively applied in porous extended frameworks for a low-carbon,
predictable, renewable component of modern industry. A comprehensive
understanding of the underlying recognition mechanism toward different
shapes is unfortunately still missing, owing to the lack of structural
and dynamic information under operating conditions. We demonstrate
here that such difficulties can be overcome by state-of-the-art molecular
dynamics simulations which provide atomistic details that are not
accessible experimentally, as exemplified by our interpretation for
the experimentally observed aggregation-induced shape selectivity
for Suzuki C–C coupling reaction catalyzed by Pd particles
in mesoporous silica. It is found that both aggregation ability and
aggregating pattern of the reactants play the decisive role in controlling
the shape selectivity, which are in turn determined by the balance
between the hydrophobicity and hydrophilicity of the reactants, or
in other words, by the balance between the noncovalent hydrogen bonding
interaction and van der Waals forces. A general rule that allows prediction
of the shape selectivity of a reactant has been proposed and verified
against experiments. We show that molecular modeling is a powerful
tool for rational design of new mesoporous systems and for the control
of catalytic reactions that are important for the petrochemical industry
Ruthenium/Graphene-like Layered Carbon Composite as an Efficient Hydrogen Evolution Reaction Electrocatalyst
Efficient water splitting through
electrocatalysis has been studied
extensively in modern energy devices, while the development of catalysts
with activity and stability comparable to those of Pt is still a great
challenge. In this work, we successfully developed a facile route
to synthesize graphene-like layered carbon (GLC) from a layered silicate
template. The obtained GLC has layered structure similar to that of
the template and can be used as support to load ultrasmall Ru nanoparticles
on it in supercritical water. The specific structure and surface properties
of GLC enable Ru nanoparticles to disperse highly uniformly on it
even at a large loading amount (62 wt %). When the novel Ru/GLC was
used as catalyst on a glass carbon electrode for hydrogen evolution
reaction (HER) in a 0.5 M H<sub>2</sub>SO<sub>4</sub> solution, it
exhibits an extremely low onset potential of only 3 mV and a small
Tafel slope of 46 mV/decade. The outstanding performance proved that
Ru/GLC is highly active catalyst for HER, comparable with transition-metal
dichalcogenides or selenides. As the price of ruthenium is much lower
than platinum, our study shows that Ru/GLC might be a promising candidate
as an HER catalyst in future energy applications
Theoretical Modeling of Plasmon-Enhanced Raman Images of a Single Molecule with Subnanometer Resolution
Under
local plasmonic excitation, Raman images of single molecules
can now surprisingly reach subnanometer resolution. However, its physical
origin has not been fully understood. Here we report a quantum-mechanical
description of the interaction between a molecule and a highly confined
plasmonic field. We show that when the spatial distribution of the
plasmonic field is comparable to the size of the molecule, the optical
transition matrix of the molecule becomes dependent on the position
and distribution of the plasmonic field, resulting in a spatially
resolved high-resolution Raman image of the molecule. The resonant
Raman image reflects the electronic transition density of the molecule.
In combination with first-principles calculations, the simulated Raman
image of a porphyrin derivative adsorbed on a silver surface nicely
reproduces its experimental counterpart. The present theory provides
the basic framework for describing linear and nonlinear responses
of molecules under highly confined plasmonic fields
Negative Differential Resistance in a Hybrid Silicon-Molecular System: Resonance between the Intrinsic Surface-States and the Molecular Orbital
It has been a long-term desire to fabricate hybrid silicon-molecular devices by taking advantages of organic molecules and the existing silicon-based technology. However, one of the challenging tasks is to design applicable functions on the basis of the intrinsic properties of the molecules, as well as the silicon substrates. Here we demonstrate a silicon-molecular system that produces negative differential resistance (NDR) by making use of the well-defined intrinsic surface-states of the Si (111)-√3 × √3-Ag (R3-Ag/Si) surface and the molecular orbital of cobalt(II)–phthalocyanine (CoPc) molecules. From our experimental results obtained using scanning tunneling microscopy/spectroscopy, we find that NDR robustly appears at the Co<sup>2+</sup> ion centers of the CoPc molecules, independent of the adsorption configuration of the CoPc molecules and irrespective of doping type and doping concentration of the silicon substrates. Joint with first principle calculations, we conclude that NDR is originated from the resonance between the intrinsic surface-state band S<sub>1</sub> of the R3-Ag/Si surface and the localized unoccupied Co<sup>2+</sup> <i>d</i><sub><i>z</i><sup>2</sup></sub> orbital of the adsorbed CoPc molecules. We expect that such a mechanism can be generally used in other silicon-molecular systems
Coagulation Behavior of Graphene Oxide on Nanocrystallined Mg/Al Layered Double Hydroxides: Batch Experimental and Theoretical Calculation Study
Graphene oxide (GO)
has attracted considerable attention because
of its remarkable enhanced adsorption and multifunctional properties.
However, the toxic properties of GO nanosheets released into the environment
could lead to the instability of biological system. In aqueous phase,
GO may interact with fine mineral particles, such as chloridion intercalated
nanocrystallined Mg/Al layered double hydroxides (LDH–Cl) and
nanocrystallined Mg/Al LDHs (LDH–CO<sub>3</sub>), which are
considered as coagulant molecules for the coagulation and removal
of GO from aqueous solutions. Herein the coagulation of GO on LDHs
were studied as a function of solution pH, ionic strength, contact
time, temperature and coagulant concentration. The presence of LDH–Cl
and LDH–CO<sub>3</sub> improved the coagulation of GO in solution
efficiently, which was mainly attributed to the surface oxygen-containing
functional groups of LDH–Cl and LDH–CO<sub>3</sub> occupying
the binding sites of GO. The coagulation of GO by LDH–Cl and
LDH–CO<sub>3</sub> was strongly dependent on pH and ionic strength.
Results of theoretical DFT calculations indicated that the coagulation
of GO on LDHs was energetically favored by electrostatic interactions
and hydrogen bonds, which was further evidenced by FTIR and XPS analysis.
By integrating the experimental results, it was clear that LDH–Cl
could be potentially used as a cost-effective coagulant for the elimination
of GO from aqueous solutions, which could efficiently decrease the
potential toxicity of GO in the natural environment
Visualizing Large Facet-Dependent Electronic Tuning in Monolayer WSe<sub>2</sub> on Au Surfaces
Two-dimensional transition metal dichalcogenides (TMDs)
have shown
great importance in the development of novel ultrathin optoelectronic
devices owing to their exceptional electronic and photonic properties.
Effectively tuning their electronic band structures is not only desired
in electronics applications but also can facilitate more novel properties.
In this work, we demonstrate that large electronic tuning on a WSe2 monolayer can be realized by different facets of a Au-foil
substrate, forming in-plane p–n junctions with remarkable built-in
electric fields. This facet-dependent tuning effect is directly visualized
by using scanning tunneling microscopy and differential conductance
(dI/dV) spectroscopy. First-principles
calculations reveal that the atomic arrangement of the Au facet effectively
changes the interfacial coupling and charge transfer, leading to different
magnitudes of charge doping in WSe2. Our study would be
beneficial for future customized fabrication of TMD-junction devices
via facet-specific construction on the substrate