9 research outputs found
Configurational Energies of Nanoparticles Based on Metal–Metal Coordination
Nanoparticle
sintering remains a fundamental problem in heterogeneous
catalysis, motivating mechanistic studies toward mitigating deactivation
of precious metal catalysts. We present a model based on the local
coordination environment of metal atoms that can be used to provide
total energy estimates for metal nanoparticles in a space of generic
configurations. All energies are based only on a small set of density
functional theory calculations of single metal atom adsorption on
metal slabs. A model that can provide accurate nanoparticle energies
is an important step toward the goal of understanding their sintering
behavior in practical catalytic contexts
Understanding the Thermal Stability of Palladium–Platinum Core–Shell Nanocrystals by <i>In Situ</i> Transmission Electron Microscopy and Density Functional Theory
Core–shell
nanocrystals offer many advantages for heterogeneous
catalysis, including precise control over both the surface structure
and composition, as well as reduction in loading for rare and costly
metals. Although many catalytic processes are operated at elevated
temperatures, the adverse impacts of heating on the shape and structure
of core–shell nanocrystals are yet to be understood. In this
work, we used <i>ex situ</i> heating experiments to demonstrate
that Pd@Pt<sub>4L</sub> core–shell nanoscale cubes and octahedra
are promising for catalytic applications at temperatures up to 400
°C. We also used <i>in situ</i> transmission electron
microscopy to monitor the thermal stability of the core–shell
nanocrystals in real time. Our results demonstrate a facet dependence
for the thermal stability in terms of shape and composition. Specifically,
the cubes enclosed by {100} facets readily deform shape at a temperature
300 °C lower than that of the octahedral counterparts enclosed
by {111} facets. A reversed trend is observed for composition, as
alloying between the Pd core and the Pt shell of an octahedron occurs
at a temperature 200 °C lower than that for the cubic counterpart.
Density functional theory calculations provide atomic-level explanations
for the experimentally observed behaviors, demonstrating that the
barriers for edge reconstruction determine the relative ease of shape
deformation for cubes compared to octahedra. The opposite trend for
alloying of the core–shell structure can be attributed to a
higher propensity for subsurface Pt vacancy formation in octahedra
than in cubes
Design of Chemoresponsive Liquid Crystals through Integration of Computational Chemistry and Experimental Studies
We
report the use of computational chemistry methods to design
a chemically responsive liquid crystal (LC). Specifically, we used
electronic structure calculations to model the binding of nitrile-containing
mesogens (4′-<i>n</i>-pentyl-4-biphenylcarbonitrile)
to metal perchlorate salts (with explicit description of the perchlorate
anion), which we call the coordinately saturated anion model (CSAM).
The model results were validated against experimental data. We then
used the CSAM to predict that selective fluorination can reduce the
strength of binding of nitrile-containing nematic LCs to metal-salt-decorated
surfaces and thus generate a faster reordering of the LC in response
to competitive binding of dimethylmethylphosphonate (DMMP). We tested
this prediction via synthesis of fluorinated compounds 3-fluoro-4′-pentyl[1,1′-biphenyl]-4-carbonitrile
and 4-fluoro-4′-pentyl-1,1′-biphenyl, and subsequent
experimental measurements of the orientational response of LCs containing
these compounds to DMMP. These experimental measurements confirmed
the theoretical predictions, thus providing the first demonstration
of a chemoresponsive LC system designed from computational chemistry
Synthesis and Characterization of Ru Cubic Nanocages with a Face-Centered Cubic Structure by Templating with Pd Nanocubes
Nanocages have received
considerable attention in recent years
for catalytic applications owing to their high utilization efficiency
of atoms and well-defined facets. Here we report, for the first time,
the synthesis of Ru cubic nanocages with ultrathin walls, in which
the atoms are crystallized in a face-centered cubic (fcc) rather than
hexagonal close-packed (hcp) structure. The key to the success of
this synthesis is to ensure layer-by-layer deposition of Ru atoms
on the surface of Pd cubic seeds by controlling the reaction temperature
and the injection rate of a Ru(III) precursor. By selectively etching
away the Pd from the Pd@Ru core–shell nanocubes, we obtain
Ru nanocages with an average wall thickness of 1.1 nm or about six
atomic layers. Most importantly, the Ru nanocages adopt an fcc crystal
structure rather than the hcp structure observed in bulk Ru. The synthesis
has been successfully applied to Pd cubic seeds with different edge
lengths in the range of 6–18 nm, with smaller seeds being more
favorable for the formation of Ru shells with a flat, smooth surface
due to shorter distance for the surface diffusion of the Ru adatoms.
Self-consistent density functional theory calculations indicate that
these unique fcc-structured Ru nanocages might possess promising catalytic
properties for ammonia synthesis compared to hcp Ru(0001), on the
basis of strengthened binding of atomic N and substantially reduced
activation energies for N<sub>2</sub> dissociation, which is the rate-determining
step for ammonia synthesis on hcp Ru catalysts
Synthesis and Characterization of Pt–Ag Alloy Nanocages with Enhanced Activity and Durability toward Oxygen Reduction
Engineering the elemental
composition of metal nanocrystals offers an effective strategy for
the development of catalysts or electrocatalysts with greatly enhanced
activity. Herein, we report the synthesis of Pt–Ag alloy nanocages
with an outer edge length of 18 nm and a wall thickness of about 3
nm. Such nanocages with a composition of Pt<sub>19</sub>Ag<sub>81</sub> could be readily prepared in one step through the galvanic replacement
reaction between Ag nanocubes and a Pt(II) precursor. After 10 000
cycles of potential cycling in the range of 0.60–1.0 V as in
an accelerated durability test, the composition of the nanocages changed
to Pt<sub>56</sub>Ag<sub>44</sub>, together with a specific activity
of 1.23 mA cm<sup>–2</sup> toward oxygen reduction, which was
3.3 times that of a state-of-the-art commercial Pt/C catalyst (0.37
mA cm<sup>–2</sup>) prior to durability testing. Density functional
theory calculations attributed the increased activity to the stabilization
of the transition state for breaking the O–O bond in molecular
oxygen. Even after 30 000 cycles of potential cycling, the
mass activity of the nanocages only dropped from 0.64 to 0.33 A mg<sup>–1</sup><sub>Pt</sub>, which was still about two times that
of the pristine Pt/C catalyst (0.19 A mg<sup>–1</sup><sub>Pt</sub>)
Atomic Layer-by-Layer Deposition of Platinum on Palladium Octahedra for Enhanced Catalysts toward the Oxygen Reduction Reaction
We systematically evaluated two different approaches to the syntheses of Pd@Pt<sub><i>n</i>L</sub> (<i>n</i> = 2–5) core–shell octahedra. We initially prepared the core–shell octahedra using a polyol-based route by titrating a Pt(IV) precursor into the growth solution containing Pd octahedral seeds at 200 °C through the use of a syringe pump. The number of Pt atomic layers could be precisely controlled from two to five by increasing the volume of the precursor solution while fixing the amount of seeds. We then demonstrated the synthesis of Pd@Pt<sub><i>n</i>L</sub> octahedra using a water-based route at 95 °C through the one-shot injection of a Pt(II) precursor. Due to the large difference in reaction temperature, the Pd@Pt<sub><i>n</i>L</sub> octahedra obtained <i>via</i> the water-based route showed sharper corners than their counterparts obtained through the polyol-based route. When compared to a commercial Pt/C catalyst based upon 3.2 nm Pt particles, the Pd@Pt<sub><i>n</i>L</sub> octahedra prepared using both methods showed similar remarkable enhancement in terms of activity (both specific and mass) and durability toward the oxygen reduction reaction. Calculations based upon periodic, self-consistent density functional theory suggested that the enhancement in specific activity for the Pd@Pt<sub><i>n</i>L</sub> octahedra could be attributed to the destabilization of OH on their Pt<sub><i>n</i>L</sub>*/Pd(111) surface relative to the {111} and {100} facets exposed on the surface of Pt/C. The destabilization of OH facilitates its hydrogenation, which was found to be the rate-limiting step of the oxygen reduction reaction on all these surfaces
Significant Quantum Effects in Hydrogen Activation
Dissociation of molecular hydrogen is an important step in a wide variety of chemical, biological, and physical processes. Due to the light mass of hydrogen, it is recognized that quantum effects are often important to its reactivity. However, understanding how quantum effects impact the reactivity of hydrogen is still in its infancy. Here, we examine this issue using a well-defined Pd/Cu(111) alloy that allows the activation of hydrogen and deuterium molecules to be examined at individual Pd atom surface sites over a wide range of temperatures. Experiments comparing the uptake of hydrogen and deuterium as a function of temperature reveal completely different behavior of the two species. The rate of hydrogen activation increases at lower sample temperature, whereas deuterium activation slows as the temperature is lowered. Density functional theory simulations in which quantum nuclear effects are accounted for reveal that tunneling through the dissociation barrier is prevalent for H<sub>2</sub> up to ∼190 K and for D<sub>2</sub> up to ∼140 K. Kinetic Monte Carlo simulations indicate that the effective barrier to H<sub>2</sub> dissociation is so low that hydrogen uptake on the surface is limited merely by thermodynamics, whereas the D<sub>2</sub> dissociation process is controlled by kinetics. These data illustrate the complexity and inherent quantum nature of this ubiquitous and seemingly simple chemical process. Examining these effects in other systems with a similar range of approaches may uncover temperature regimes where quantum effects can be harnessed, yielding greater control of bond-breaking processes at surfaces and uncovering useful chemistries such as selective bond activation or isotope separation
Nature of Lone-Pair–Surface Bonds and Their Scaling Relations
We investigate the (surface) bonding
of a class of industrially and biologically important molecules in
which the chemically active orbital is a 2<i>p</i> electron
lone pair located on an N or O atom bound via single bonds to H or
alkyl groups. This class includes water, ammonia, alcohols, ethers,
and amines. Using extensive density functional theory (DFT) calculations,
we discover scaling relations (correlations) among molecular binding
energies of different members of this class: the bonding energetics
of a single member can be used as a descriptor for other members.
We investigate the bonding mechanism for a representative (H<sub>2</sub>O) and find the most important physical surface properties that dictate
the strength and nature of the bonding through a combination of covalent
and noncovalent electrostatic effects. We describe the importance
of surface intrinsic electrostatic, geometric, and mechanical properties
in determining the extent of the lone-pair–surface interactions.
We study systems including ionic materials in which the surface positive
and negative centers create strong local surface electric fields,
which polarize the dangling lone pair and lead to a strong “electrostatically
driven bond”. We emphasize the importance of noncovalent electrostatic
effects and discuss why a fully covalent picture, common in the current
first-principles literature on surface bonding of these molecules,
is not adequate to correctly describe the bonding mechanism and energy
trends. By pointing out a completely different mechanism (charge transfer)
as the major factor for binding N- and O-containing unsaturated (radical)
adsorbates, we explain why their binding energies can be tuned independently
from those of the aforementioned species, having potential implications
in scaling-driven catalyst discovery
Atomic Layer-by-Layer Deposition of Pt on Pd Nanocubes for Catalysts with Enhanced Activity and Durability toward Oxygen Reduction
An effective strategy for reducing
the Pt content while retaining
the activity of a Pt-based catalyst is to deposit the Pt atoms as
ultrathin skins of only a few atomic layers thick on nanoscale substrates
made of another metal. During deposition, however, the Pt atoms often
take an island growth mode because of a strong bonding between Pt
atoms. Here we report a versatile route to the conformal deposition
of Pt as uniform, ultrathin shells on Pd nanocubes in a solution phase.
The introduction of the Pt precursor at a relatively slow rate and
high temperature allowed the deposited Pt atoms to spread across the
entire surface of a Pd nanocube to generate a uniform shell. The thickness
of the Pt shell could be controlled from one to six atomic layers
by varying the amount of Pt precursor added into the system. Compared
to a commercial Pt/C catalyst, the Pd@Pt<sub><i>n</i>L</sub> (<i>n</i> = 1–6) core–shell nanocubes showed
enhancements in specific activity and durability toward the oxygen
reduction reaction (ORR). Density functional theory (DFT) calculations
on model (100) surfaces suggest that the enhancement in specific activity
can be attributed to the weakening of OH binding through ligand and
strain effects, which, in turn, increases the rate of OH hydrogenation.
A volcano-type relationship between the ORR specific activity and
the number of Pt atomic layers was derived, in good agreement with
the experimental results. Both theoretical and experimental studies
indicate that the ORR specific activity was maximized for the catalysts
based on Pd@Pt<sub>2–3L</sub> nanocubes. Because of the reduction
in Pt content used and the enhancement in specific activity, the Pd@Pt<sub>1L</sub> nanocubes showed a Pt mass activity with almost three-fold
enhancement relative to the Pt/C catalyst