6 research outputs found
Metal-support interaction and charge distribution in ceria-supported Au particles exposed to CO
Understanding how reaction conditions affect metal-support interactions in catalytic materials is one of the most challenging tasks in heterogeneous catalysis research. Metal nanoparticles and their supports often undergo changes in structure and oxidation state when exposed to reactants, hindering a straightforward understanding of the structure-activity relations using only ex situ or ultrahigh vacuum techniques. Overcoming these limitations, we explored the metal-support interaction between gold nanoparticles and ceria supports in ultrahigh vacuum and after exposure to CO. A combination of in situ methods (on powder and model Au/CeO2 samples) and theoretical calculations was applied to investigate the gold/ceria interface and its reactivity toward CO exposure. X-ray photoelectron spectroscopy measurements rationalized by first-principles calculations reveal a distinctly inhomogeneous charge distribution, with Au+ atoms in contact with the ceria substrate and neutral Au0 atoms at the surface of the Au nanoparticles. Exposure to CO partially reduces the ceria substrate, leading to electron transfer to the supported Au nanoparticles. Transferred electrons can delocalize among the neutral Au atoms of the particle or contribute to forming inert Auδ− atoms near oxygen vacancies at the ceria surface. This charge redistribution is consistent with the evolution of the vibrational frequencies of CO adsorbed on Au particles obtained using diffuse reflectance infrared Fourier transform spectroscopy
Pair Correlation Function of a 2D Molecular Gas Directly Visualized by Scanning Tunneling Microscopy
The
state of matter in fluid phases, determined by the interactions
between particles, can be characterized by a pair correlation function
(PCF). At the nanoscale, the PCF has been so far obtained experimentally
only by means of reciprocal-space techniques. We use scanning tunneling
microscopy (STM) at room temperature in combination with lattice-gas
kinetic Monte Carlo (KMC) simulations to study a two-dimensional gas
of highly mobile molecules of fluorinated copper phthalocyanine on
a Si(111)/Tl-(1×1) surface. A relatively slow mechanism of STM
image acquisition results in time-averaging of molecular occurrence
under the STM tip. We prove by the KMC simulations that in the proximity
of fixed molecules STM images represent the PCF. We demonstrate that
STM is capable of visualizing directly the pair correlation function
in real space
Chemisorption of Acetophenone on Si(111)‑7 × 7. Polar Aromatic Molecule on Electronically Complex Surface
Temperature-dependent
chemisorption of acetophenone molecules on
the Si(111)-7 × 7 reconstruction was studied by means of scanning
tunneling microscopy, scanning tunneling spectroscopy, and density
functional theory calculations. A configuration interpreted as a silyl
enolether has been repeatedly observed on the surface at room temperature
and after annealing to 75 °C. The most frequent structure on
the surface stable up to 150 °C is identified as a 1,6-adduct
to two adatoms of the neighboring half unit cells. The results suggest
that presence of the polar group in the molecule affects the chemisorption
in a way that leads to bonding with two adatoms