6 research outputs found
Friction of Water on Graphene and Hexagonal Boron Nitride from <i>Ab Initio</i> Methods: Very Different Slippage Despite Very Similar Interface Structures
Friction is one of the main sources
of dissipation at liquid water/solid
interfaces. Despite recent progress, a detailed understanding of water/solid
friction in connection with the structure and energetics of the solid
surface is lacking. Here, we show for the first time that <i>ab initio</i> molecular dynamics can be used to unravel the
connection between the structure of nanoscale water and friction for
liquid water in contact with graphene and with hexagonal boron nitride.
We find that although the interface presents a very similar structure
between the two sheets, the friction coefficient on boron nitride
is â3 times larger than that on graphene. This comes about
because of the greater corrugation of the energy landscape on boron
nitride arising from specific electronic structure effects. We discuss
how a subtle dependence of the friction on the atomistic details of
a surface, which is not related to its wetting properties, may have
a significant impact on the transport of water at the nanoscale, with
implications for the development of membranes for desalination and
for osmotic power harvesting
Elucidating the Stability and Reactivity of Surface Intermediates on Single-Atom Alloy Catalysts
Doping
isolated single atoms of a platinum-group metal into the
surface of a noble-metal host is sufficient to dramatically improve
the activity of the unreactive host yet also facilitates the retention
of the hostâs high reaction selectivity in numerous catalytic
reactions. The atomically dispersed highly active sites in these single-atom
alloy (SAA) materials are capable of performing facile bond activations
allowing for the uptake of species onto the surface and the subsequent
spillover of adspecies onto the noble host material, where selective
catalysis can be performed. For example, SAAs have been shown to activate
CâH bonds at low temperatures without coke formation, as well
as selectively hydrogenate unsaturated hydrocarbons with excellent
activity. However, to date, only a small subset of SAAs has been synthesized
experimentally and it is unclear which metallic combinations may best
catalyze which chemical reactions. To shed light on this issue, we
have performed a widespread screening study using density functional
theory to elucidate the fundamental adsorptive and catalytic properties
of 12 SAAs (Ni-, Pd-, Pt-, and Rh-doped Cu(111), Ag(111), and Au(111)).
We considered the interaction of these SAAs with a variety of adsorbates
often found in catalysis and computed reaction mechanisms for the
activation of several catalytically relevant species (H<sub>2</sub>, CH<sub>4</sub>, NH<sub>3</sub>, CH<sub>3</sub>OH, and CO<sub>2</sub>) by SAAs. Finally, we discuss the applicability of thermochemical
linear scaling and the BrønstedâEvansâPolanyi relationship
to SAA systems, demonstrating that SAAs combine weak binding with
low activation energies to give enhanced catalytic behavior over their
monometallic counterparts. This work will ultimately facilitate the
discovery and development of SAAs, serving as a guide to experimentalists
and theoreticians alike
WaterâIce Analogues of Polycyclic Aromatic Hydrocarbons: Water Nanoclusters on Cu(111)
Water has an incredible ability to
form a rich variety of structures,
with 16 bulk ice phases identified, for example, as well as numerous
distinct structures for water at interfaces or under confinement.
Many of these structures are built from hexagonal motifs of water
molecules, and indeed, for water on metal surfaces, individual hexamers
of just six water molecules have been observed. Here, we report the
results of low-temperature scanning tunneling microscopy experiments
and density functional theory calculations which reveal a host of
new structures for waterâice nanoclusters when adsorbed on
an atomically flat Cu surface. The H-bonding networks within the nanoclusters
resemble the resonance structures of polycyclic aromatic hydrocarbons,
and waterâice analogues of inene, naphthalene, phenalene, anthracene,
phenanthrene, and triphenylene have been observed. The specific structures
identified and the H-bonding patterns within them reveal new insight
about water on metals that allows us to refine the so-called â2D
ice rulesâ, which have so far proved useful in understanding
waterâice structures at solid surfaces
Controlling Hydrogen Activation, Spillover, and Desorption with PdâAu Single-Atom Alloys
Key descriptors in hydrogenation
catalysis are the nature of the
active sites for H<sub>2</sub> activation and the adsorption strength
of H atoms to the surface. Using atomically resolved model systems
of dilute PdâAu surface alloys and density functional theory
calculations, we determine key aspects of H<sub>2</sub> activation,
diffusion, and desorption. Pd monomers in a Au(111) surface catalyze
the dissociative adsorption of H<sub>2</sub> at temperatures as low
as 85 K, a process previously expected to require contiguous Pd sites.
H atoms preside at the Pd sites and desorb at temperatures significantly
lower than those from pure Pd (175 versus 310 K). This facile H<sub>2</sub> activation and weak adsorption of H atom intermediates are
key requirements for active and selective hydrogenations. We also
demonstrate weak adsorption of CO, a common catalyst poison, which
is sufficient to force H atoms to spill over from Pd to Au sites,
as evidenced by low-temperature H<sub>2</sub> desorption
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 âź190 K and for D<sub>2</sub> up to âź140 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