3 research outputs found
<i>In Situ</i> Studies of Carbon Monoxide Oxidation on Platinum and PlatinumāRhenium Alloy Surfaces
CO
oxidation has been investigated by near ambient pressure X-ray photoelectron
spectroscopy (NAP-XPS) on Pt(111), Re films on Pt(111), and a PtāRe
alloy surface. The PtāRe alloy surface was prepared by annealing
Re films on Pt(111) to 1000 K; scanning tunneling microscopy, low
energy ion scattering, and X-ray photoelectron spectroscopy studies
indicate that this treatment resulted in the diffusion of Re into
the Pt(111) surface. Under CO oxidation conditions of 500 mTorr O<sub>2</sub>/50 mTorr CO, CO remains on the Pt(111) surface at 450 K,
whereas CO desorbs from the PtāRe alloy surface at lower temperatures.
Furthermore, the PtāRe alloy dissociates oxygen more readily
than Pt(111) despite the fact that all of the Re atoms are initially
in the subsurface region. Mass spectrometer studies show that the
PtāRe alloy, Re film on Pt, and Pt(111) all have similar activities
for CO oxidation, with the PtāRe alloy producing ā¼10%
more CO<sub>2</sub> than Pt(111). The Re film is not stable under
CO oxidation conditions at temperatures ā„450 K due to the formation
and subsequent sublimation of volatile Re<sub>2</sub>O<sub>7</sub>. However, the PtāRe alloy surface is more resistant to oxidation
and therefore also more stable against Re sublimation
Nucleation, Growth, and Adsorbate-Induced Changes in Composition for CoāAu Bimetallic Clusters on TiO<sub>2</sub>
The nucleation, growth, and CO-induced changes in composition
for
CoāAu bimetallic clusters deposited on TiO<sub>2</sub>(110)
have been studied by scanning tunneling microscopy (STM), low energy
ion scattering (LEIS), X-ray photoelectron spectroscopy (XPS), temperature-programmed
desorption (TPD), and density functional theory (DFT) calculations.
STM experiments show that the mobility of Co atoms on TiO<sub>2</sub>(110) is significantly lower than of Au atoms; for equivalent or
lower coverages of Co, the number of clusters is higher and the average
cluster height is smaller than for Au deposition. Consequently, bimetallic
clusters are formed by first depositing the less mobile Co atoms,
followed by the addition of the more mobile Au atoms. Furthermore,
the reverse deposition of Au followed by Co results in clusters of
pure Co coexisting with clusters that are Au-rich. For clusters with
a total coverage of 0.25 ML, the cluster density increases and average
cluster height decreases as the fraction of Co is increased. Annealing
to 800 K results in cluster sintering and an increase of ā¼3ā5
Ć
in average height for all compositions. LEIS experiments indicate
that the surfaces of the bimetallic clusters are 80ā100% Au
for bulk Au fractions greater than 50%, but Co and Au coexist at the
surfaces when there are not enough Au atoms available to completely
cover the surfaces of the clusters. After heating to 800 K, pure Co
clusters become partially encapsulated by titania, and for bimetallic
clusters, the Co is selectively encapsulated at the cluster surface.
The desorption of CO from the bimetallic clusters demonstrates that
the presence of the CO adsorbate induces diffusion of Co to the cluster
surface, but the extent of this diffusion is less than what is observed
in the NiāAu and PtāAu systems. Density functional theory
calculations confirm that for a 50% Co/50% Au bimetallic structure:
the surface is predominantly Au in the absence of CO; CO induces diffusion
of Co to the cluster surface; and this CO-induced diffusion is less
extensive on CoāAu than on the NiāAu and PtāAu
surfaces
Electronic Properties of Bimetallic MetalāOrganic Frameworks (MOFs): Tailoring the Density of Electronic States through MOF Modularity
The development of
porous well-defined hybrid materials (e.g.,
metalāorganic frameworks or MOFs) will add a new dimension
to a wide number of applications ranging from supercapacitors and
electrodes to āsmartā membranes and thermoelectrics.
From this perspective, the understanding and tailoring of the electronic
properties of MOFs are key fundamental challenges that could unlock
the full potential of these materials. In this work, we focused on
the fundamental insights responsible for the electronic properties
of three distinct classes of bimetallic systems, M<sub><i>x</i>ā<i>y</i></sub>Mā²<sub><i>y</i></sub>-MOFs, M<sub><i>x</i></sub>Mā²<sub><i>y</i></sub>-MOFs, and M<sub><i>x</i></sub>(ligand-Mā²<sub><i>y</i></sub>)-MOFs, in which the second metal (Mā²)
incorporation occurs through (i) metal (M) replacement in the framework
nodes (type I), (ii) metal node extension (type II), and (iii) metal
coordination to the organic ligand (type III), respectively. We employed
microwave conductivity, X-ray photoelectron spectroscopy, diffuse
reflectance spectroscopy, powder X-ray diffraction, inductively coupled
plasma atomic emission spectroscopy, pressed-pellet conductivity,
and theoretical modeling to shed light on the key factors responsible
for the tunability of MOF electronic structures. Experimental prescreening
of MOFs was performed based on changes in the density of electronic
states near the Fermi edge, which was used as a starting point for
further selection of suitable MOFs. As a result, we demonstrated that
the tailoring of MOF electronic properties could be performed as a
function of metal node engineering, framework topology, and/or the
presence of unsaturated metal sites while preserving framework porosity
and structural integrity. These studies unveil the possible pathways
for transforming the electronic properties of MOFs from insulating
to semiconducting, as well as provide a blueprint for the development
of hybrid porous materials with desirable electronic structures