CO2 has a potentially bright future as a carbon resource
because it is very cheap and abundant. The conversion technology of
CO2 into useful chemicals therefore has gained growing
attention over recent years. Despite many attempts, there have not
yet been revolutionary successes for commercialization of such technology.
One of the main challenges in this field is to catalytically activate
the CO2 molecule on the surfaces of catalysts. Although
many researchers have studied the catalytic reactions involving CO2 on the surfaces, the activation process of CO2 is still controversial. Here, we performed density functional theory
calculations to understand the CO2 activation and dissociation
on a wide range of bimetallic alloy surfaces. To begin with, the adsorption
process of CO2 on pure metal surfaces was carefully examined
with the analyses of adsorption energetics, geometries, vibrational
frequencies, charge transfers, and density of states. From the activated
CO2 on the surfaces, we could precisely capture the transition
state of the dissociation reaction. On the basis of the information,
we found that Brønsted–Evans–Polanyi (BEP) relations
hold for CO2 dissociation reaction. It was also verified
that the sum of adsorption energies of CO and O is linearly scaled
with not only adsorption energy of CO2
δ− but also reaction energy for the CO2 dissociation. As
a result, the energy barriers of CO2 dissociation on pure
metal and bimetallic alloy surfaces could be rapidly screened by combining
the BEP relation, scaling relation, and surface mixing rule. Our results
will provide useful insight into designing transition metal catalysts
for the CO2-involved reactions.
We
investigated the influence of P incorporation into a Ni catalyst
on ethane dehydrogenation (EDH). Density functional theory calculations
on model Ni(111) and Ni2P(001) surfaces reveal that surface
P generally decreases adsorption energies of fragments relevant to
EDH at surface Ni sites but that P itself participates in binding
some of these intermediates. These nonlinear influences of P cause
CH3CH2–H activation to occur with similar
facility on metal and phosphide surfaces, while CH2CH–H
activation, an indicator of coking tendency, has much greater barriers
on the phosphide. We prepared Ni and Ni–P catalysts on an SBA-15
support to test these predictions. A Ni–P catalyst with a 2:1
ratio (Ni2P(2)/SBA-15), corresponding to the Ni2P phase, showed >80% ethylene selectivity during EDH at 873 K,
compared
to <1% ethylene selectivity on Ni/SBA-15, and maintained this selectivity
up to 4 h time-on-stream. Diffuse reflectance infrared Fourier transform
spectroscopy observations following ethylene exposure and heating
under an inert flow indicate the appearance of carbon deposits on
Ni/SBA-15 compared to ethylene desorption from Ni2P(2)/SBA-15,
consistent with predicted adsorption energy trends. Thermogravimetric
analysis of spent EDH catalysts indicates significantly less carbon
deposition on Ni2P(2)/SBA-15 relative to Ni/SBA-15. The
results highlight the potential of metal phosphides as selective and
robust alkane dehydrogenation catalysts.
The
high-temperature coelectrolysis system can be helpful to solve
environmental issues by reducing carbon dioxide emissions. The technology
is highly promising because of its high selectivity and conversion
efficiency toward the products. In addition, the produced syngas can
also be further converted into very useful synthetic fuels. In this
study, we investigated the series of reactions on a wide range of
transition metals to evaluate their ability to increase the activity
of the conventional Ni catalysts used in the fuel electrode of solid
oxide electrolyzer cells. We theoretically identified that the adsorption
energies of O and H are the common descriptors of coelectrolysis of
steam and carbon dioxide. We then combined microkinetic analysis with
density functional theory calculations to derive a volcano plot to
predict the activity of coelectrolysis on a variety of transition
metals. We could successfully suggest good candidates of Ni-based
bimetallic alloy catalysts with excellent activities in the coelectrolysis.
Our result will provide insight into improving the electrode catalysts
used in the high-temperature coelectrolysis system.
Understanding the adsorption phenomena of small adsorbates involved in surface reactions on transition metals is important because their adsorption strength can be a descriptor for predicting the catalytic activity. To explore adsorption energies on a wide range of binary transition metal alloys, however, tremendous computational efforts are required. Using density functional theory (DFT) calculations, here we suggest a "surface mixing rule" to predict the adsorption energies of H, O, S, CO and OH on bimetallic alloys, based on the linear interpolation of adsorption energies on each pure surface. As an application, the activity of CO oxidation on various bimetallic alloys is predicted from the adsorption energies of CO and O easily obtained by the surface mixing rule. Our results provide a useful tool for rapidly estimating adsorption energies, and furthermore, catalytic activities on multi-component metal alloy surfaces.
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