10 research outputs found
Structures and Energetics of Pt Clusters on TiO<sub>2</sub>: Interplay between Metal–Metal Bonds and Metal–Oxygen Bonds
Depositing size-selected nanoclusters on a well-defined
support
surface provides a way to probe the metal–support interaction
and the size dependence of the catalytic activity; however, the detailed
structural information at such interface is often missing. Here we
examine from density functional theory the interfacial structure of
Pt<sub>4</sub> to Pt<sub>8</sub> clusters on rutile TiO<sub>2</sub>(110). We find that Pt<sub>4</sub> prefers a flat, nearly square
structure on TiO<sub>2</sub>(110), while larger clusters such as Pt<sub>5</sub>, Pt<sub>6</sub>, Pt<sub>7</sub>, and Pt<sub>8</sub> have
a two-layer structure with the top layer not interacting with the
support directly. The interaction strength generally increases with
the contact area between Pt<sub><i>n</i></sub> and TiO<sub>2</sub>(110). The interfacial structure is a result of optimizing
the Pt–Pt, Pt–O, and Pt–Ti interactions: Pt<sub>4</sub> prefers the square planar configuration on TiO<sub>2</sub>(110) with more Pt–Ti interaction over a two-layer, bi-triangle
configuration of more Pt–Pt bonds; Pt<sub>8</sub> prefers a
hut-like two-layer structure over an edge-sharing bi-pyramid structure
of greater internal strain. Our findings will be useful for understanding
the interface of size-selected clusters on a typical reducible support
such as TiO<sub>2</sub> and its catalytic activity for reactions such
as CO oxidation
Interaction of Gold Clusters with a Hydroxylated Surface
We explore the interaction between gold nanoclusters and a fully hydroxylated surface, Mg(OH)<sub>2</sub>’s basal plane, by using a density functional theory-enabled local basin-hopping technique for global-minimum search. We find strong interaction of gold nanoclusters with the surface hydroxyls via a short bond between edge Au atoms and O atoms of the −OH groups. We expect that this strong interaction is ubiquitous on hydroxylated support surfaces and helps the gold nanoclusters against sintering, thereby contributing to their CO-oxidation activity at low temperatures
Oxygen Vacancy-Assisted Coupling and Enolization of Acetaldehyde on CeO<sub>2</sub>(111)
The temperature-dependent adsorption and reaction of
acetaldehyde
(CH<sub>3</sub>CHO) on a fully oxidized and a highly reduced thin-film
CeO<sub>2</sub>(111) surface have been investigated using a combination
of reflection–absorption infrared spectroscopy (RAIRS) and
periodic density functional theory (DFT+U) calculations. On the fully
oxidized surface, acetaldehyde adsorbs weakly through its carbonyl
O interacting with a lattice Ce<sup>4+</sup> cation in the η<sup>1</sup>-O configuration. This state desorbs at 210 K without reaction.
On the highly reduced surface, new vibrational signatures appear below
220 K. They are identified by RAIRS and DFT as a dimer state formed
from the coupling of the carbonyl O and the acyl C of two acetaldehyde
molecules. This dimer state remains up to 400 K before decomposing
to produce another distinct set of vibrational signatures, which are
identified as the enolate form of acetaldehyde (CH<sub>2</sub>CHO¯).
Furthermore, the calculated activation barriers for the coupling of
acetaldehyde, the decomposition of the dimer state, and the recombinative
desorption of enolate and H as acetaldehyde are in good agreement
with previously reported TPD results for acetaldehyde adsorbed on
reduced CeO<sub>2</sub>(111) [Chen et al. <i>J. Phys. Chem. C</i> <b>2011</b>, <i>115</i>, 3385]. The present findings
demonstrate that surface oxygen vacancies alter the reactivity of
the CeO<sub>2</sub>(111) surface and play a crucial role in stabilizing
and activating acetaldehyde for coupling reactions
Adsorption and Reaction of Acetaldehyde on Shape-Controlled CeO<sub>2</sub> Nanocrystals: Elucidation of Structure–Function Relationships
CeO<sub>2</sub> cubes with {100} facets, octahedra with {111} facets,
and wires with highly defective structures were utilized to probe
the structure-dependent reactivity of acetaldehyde. Using temperature-programmed
desorption (TPD), temperature-programmed surface reactions (TPSR),
and <i>in situ</i> infrared spectroscopy, it was determined
that acetaldehyde desorbs unreacted or undergoes reduction, coupling,
or C–C bond scission reactions, depending on the surface structure
of CeO<sub>2</sub>. Room-temperature FTIR indicates that acetaldehyde
binds primarily as η<sup>1</sup>-acetaldehyde on the octahedra,
in a variety of conformations on the cubes, including coupling products
and acetate and enolate species, and primarily as coupling products
on the wires. The percent consumption of acetaldehyde ranks in the
following order: wires > cubes > octahedra. All the nanoshapes
produce
the coupling product crotonaldehyde; however, the selectivity to produce
ethanol ranks in the following order: wires ≈ cubes ≫
octahedra. The selectivity and other differences can be attributed
to the variation in the basicity of the surfaces, defects densities,
coordination numbers of surface atoms, and the reducibility of the
nanoshapes
Gold Nanoparticles Supported on Carbon Nitride: Influence of Surface Hydroxyls on Low Temperature Carbon Monoxide Oxidation
This paper reports the synthesis of 2.5 nm gold clusters
on the oxygen free and chemically labile support carbon nitride (C<sub>3</sub>N<sub>4</sub>). Despite having small particle sizes and high
enough water partial pressure these Au/C<sub>3</sub>N<sub>4</sub> catalysts
are inactive for the gas phase and liquid phase oxidation of carbon
monoxide. The reason for the lack of activity is attributed to the
lack of surface −OH groups on the C<sub>3</sub>N<sub>4</sub>. These OH groups are argued to be responsible for the activation
of CO in the oxidation of CO. The importance of basic −OH groups
explains the well documented dependence of support isoelectric point
versus catalytic activity
Acid–Base Reactivity of Perovskite Catalysts Probed via Conversion of 2‑Propanol over Titanates and Zirconates
Although perovskite
catalysts are well-known for their excellent
redox property, their acid–base reactivity remains largely
unknown. To explore the potential of perovskites in acid–base
catalysis, we made a comprehensive investigation in this work on the
acid–base properties and reactivity of a series of selected
perovskites, SrTiO<sub>3</sub>, BaTiO<sub>3</sub>, SrZrO<sub>3</sub>, and BaZrO<sub>3</sub>, via a combination of various approaches
including adsorption microcalorimetry, in situ FTIR spectroscopy,
steady state kinetic measurements, and density functional theory (DFT)
modeling. The perovskite surfaces are shown to be dominated with intermediate
and strong basic sites with the presence of some weak Lewis acid sites,
due to the preferred exposure of SrO/BaO on the perovskite surfaces
as evidenced by low energy ion scattering (LEIS) measurements. Using
the conversion of 2-propanol as a probe reaction, we found that the
reaction is more selective to dehydrogenation over dehydration due
to the dominant surface basicity of the perovskites. Furthermore,
the adsorption energy of 2-propanol (Δ<i>H</i><sub><i>ads,</i>2<i>–propanol</i></sub>) is
found to be related to both a bulk property (tolerance factor) and
the synergy between surface acid and base sites. The results from
in situ FTIR and DFT calculations suggest that both dehydration and
dehydrogenation reactions occur mainly through the E<sub>1cB</sub> pathway, which involves the deprotonation of the alcohol group to
form a common alkoxy intermediate on the perovskite surfaces. The
results obtained in this work pave a path for further exploration
and understanding of acid–base catalysis over perovskite catalysts
Rational Design of Bi Nanoparticles for Efficient Electrochemical CO<sub>2</sub> Reduction: The Elucidation of Size and Surface Condition Effects
We report an efficient electrochemical
conversion of CO<sub>2</sub> to CO on surface-activated bismuth nanoparticles
(NPs) in acetonitrile
(MeCN) under ambient conditions, with the assistance of 1-butyl-3-methylimidazolium
trifluoromethanesulfonate ([bmim]Â[OTf]). Through the comparison between
electrodeposited Bi films (Bi-ED) and different types of Bi NPs, we,
for the first time, demonstrate the effects of catalyst’s size
and surface condition on organic phase electrochemical CO<sub>2</sub> reduction. Our study reveals that the surface inhibiting layer (hydrophobic
surfactants and Bi<sup>3+</sup> species) formed during the synthesis
and purification process hinders the CO<sub>2</sub> reduction, leading
to a 20% drop in Faradaic efficiency for CO evolution (FE<sub>CO</sub>). Bi particle size showed a significant effect on FE<sub>CO</sub> when the surface of Bi was air-oxidized, but this effect of size
on FE<sub>CO</sub> became negligible on surface-activated Bi NPs.
After the surface activation (hydrazine treatment) that effectively
removed the native inhibiting layer, activated 36-nm Bi NPs exhibited
an almost-quantitative conversion of CO<sub>2</sub> to CO (96.1% FE<sub>CO</sub>), and a mass activity for CO evolution (MA<sub>CO</sub>)
of 15.6 mA mg<sup>–1</sup>, which is three-fold higher than
the conventional Bi-ED, at −2.0 V (vs Ag/AgCl). This work elucidates
the importance of the surface activation for an efficient electrochemical
CO<sub>2</sub> conversion on metal NPs and paves the way for understanding
the CO<sub>2</sub> electrochemical reduction mechanism in nonaqueous
media
Thiolate Ligands as a Double-Edged Sword for CO Oxidation on CeO<sub>2</sub> Supported Au<sub>25</sub>(SCH<sub>2</sub>CH<sub>2</sub>Ph)<sub>18</sub> Nanoclusters
The effect of thiolate ligands was
explored on the catalysis of
CeO<sub>2</sub> rod supported Au<sub>25</sub>(SR)<sub>18</sub> (SR
= −SCH<sub>2</sub>CH<sub>2</sub>Ph) by using CO oxidation as
a probe reaction. Reaction kinetic tests, in situ IR and X-ray absorption
spectroscopy, and density functional theory (DFT) were employed to
understand how the thiolate ligands affect the nature of active sites,
activation of CO and O<sub>2</sub>, and reaction mechanism and kinetics.
The intact Au<sub>25</sub>(SR)<sub>18</sub> on the CeO<sub>2</sub> rod is found not able to adsorb CO. Only when the thiolate ligands
are partially removed, starting from the interface between Au<sub>25</sub>(SR)<sub>18</sub> and CeO<sub>2</sub> at temperatures of
423 K and above, can the adsorption of CO be observed by IR. DFT calculations
suggest that CO adsorbs favorably on the exposed gold atoms. Accordingly,
the CO oxidation light-off temperature shifts to lower temperature.
Several types of Au sites are probed by IR of CO adsorption during
the ligand removal process. The cationic Au sites (charged between
0 and +1) are found to play the major role for low-temperature CO
oxidation. Similar activation energies and reaction rates are found
for CO oxidation on differently treated Au<sub>25</sub>(SR)<sub>18</sub>/CeO<sub>2</sub> rod catalysts, suggesting a simple site-blocking
effect of the thiolate ligands in Au nanocluster catalysis. Isotopic
labeling experiments clearly indicate that CO oxidation on the Au<sub>25</sub>(SR)<sub>18</sub>/CeO<sub>2</sub> rod catalyst proceeds predominantly
via the redox mechanism where CeO<sub>2</sub> activates O<sub>2</sub> while CO is activated on the dethiolated gold sites. These results
point to a double-edged sword role played by the thiolate ligands
on Au<sub>25</sub> nanoclusters for CO oxidation
Complexity of Intercalation in MXenes: Destabilization of Urea by Two-Dimensional Titanium Carbide
MXenes are a new class of two-dimensional
materials with properties that make them important for applications
that include batteries, capacitive energy storage, and electrocatalysis.
These materials can be exfoliated and delaminated to create high surface
areas with interlayers accessibility. Intercalation is known to be
possible, and it is critical for many applications including electrochemical
energy storage, water purification, and sensing. However, little is
known about the nature of the intercalant and bonding interactions
between the intercalant within the MXene. We have investigated urea
interaction within a titanium carbide based MXene using inelastic
neutron scattering (INS) to probe the state of intercalated species.
By comparison with reference materials, we find that under intercalation
conditions urea decomposes readily, leading to intercalation of ammonium
cations observable by INS and evolving carbon dioxide detected by
infrared spectroscopy. Reactive molecular dynamics calculations were
conducted to provide atomistic insights about reaction pathways and
their energetics. These results have implications for understanding
intercalation in active layered materials