122 research outputs found
Identifying systematic DFT errors in catalytic reactions
Using CO2 reduction reactions as examples, we present a widely applicable method for identifying the main source of errors in density functional theory (DFT) calculations.</p
Decoupling Strain and Ligand Effects in Ternary Nanoparticles for Improved ORR Electrocatalysis
Ternary PtâAuâM (M = 3d transition metal) nanoparticles show reduced OH adsorption energies and improved activity for the oxygen reduction reaction (ORR) compared to pure Pt nanoparticles, as obtained by density functional theory.</p
Electrochemical Reduction of CO<sub>2</sub> on Ir<i><sub>x</sub></i>Ru<sub>(1â<i>x</i>)</sub>O<sub>2</sub>(110) SurfacesÂ
High
overpotentials and low faradic efficiencies plague metal catalysts
for direct conversion of CO<sub>2</sub> to methanol and other liquid
fuels. RuO<sub>2</sub>-based electrocatalysts have been observed to
evolve methanol at low overpotentials, which has been attributed to
an alternative reaction mechanism with oxygen-coordinated intermediates
that can circumvent the limitations imposed by the scaling relations
on metal catalysts. Here, we introduce an innovative concept of ligand
effects in oxide catalysts. Both IrO<sub>2</sub> and RuO<sub>2</sub> binds OH* and other intermediates from the electrochemical reduction
of CO<sub>2</sub> (CO2RR) strongly, but the stable and miscible system
Ir<sub><i>x</i></sub>Ru<sub>(1âx)</sub>O<sub>2</sub> exhibits anomalous weaker binding energy in the presence of CO*
spectators, because of RuâIr ligand effects. The weakened adsorbate
binding leads to a very low CO2RR onset potential (methanol evolution
at â0.2 V RHE). An Ir atom at the bridge site with Ru neighbors
binds intermediates such as OH* and OCHO* much weaker, because of
synergistic ligand effects and adsorbateâadsorbate interactions.
Consequently, a RuO<sub>2</sub> surface doped with Ir move close to
the top of the predicted CO2RR volcano for oxides, which offers a
significant improvement over state-of-the-art electrocatalysts for
conversion of CO<sub>2</sub> into methanol. Analysis of electronic
structure parameters with adsorbate binding energies indicates the
ligand effect depletes electrons from the Ir atom and shifts the t<sub>2g</sub> orbitals. The lack of electron donation from CO* spectators
to Ir at the active site cause favorable adsorbate binding
From 3D to 2D Co and Ni Oxyhydroxide Catalysts: Elucidation of the Active Site and Influence of Doping on the Oxygen Evolution Activity
Layered
oxyhydroxides (ox-hys) of Ni and Co are among the most
active catalysts for oxygen evolution in alkaline media. Their activities
can be further tuned by delamination into single-layer oxide sheets
or by means of doping. The active site for the reaction and how doping
and delamination promote the intrinsic activity, however, remain elusive.
To shed light on these open questions, we have undertaken a systematic
analysis of the stability, catalytic activity, and electronic conductivity
of Ni and Co ox-hys ranging from bulk (3D) to single-layer (2D) catalysts.
In both cases, we investigate the role of terrace and edge sites and
use stability, catalytic activity, and electronic conductivity as
evaluation criteria to pinpoint the best catalysts. We arrive at several
important conclusions: the ox-hy surface is fully oxidized under oxygen
evolution conditions, bulk terraces are ostensibly the most active
sites, and Ni ox-hy sheets are more electronically conductive in comparison
to their Co equivalents. Furthermore, we examine 25 different doped
Co and Ni ox-hy nanosheets (V, Cr, Mn, Fe, Co/Ni, Cu, Ru, Rh, Pd,
Ir, Pt, Ag, Al, Ga, In, Sn, Pb, Bi, Mg, Sc, Y, Ti, Nb, Zn, and Cd)
to further tailor the catalytic performance. We establish the dependence
of the electronic conductivity and activity on potential and find
that it is more energetically favorable to dope Ni in comparison to
Co ox-hys, with first-row transition and noble metals being the most
stable dopants. Finally, we extend the analysis to include bulk terminations
and reveal that most dopants, which are stable in the nanosheets,
have a large propensity to segregate to the surface of bulk materials,
and those that are less prone to segregation (Fe or Cr) are not electronically
conductive in the bulk. Overall, we identify Rh-doped Ni ox-hy to
be the best catalyst material
Functional Independent Scaling Relation for ORR/OER Catalysts
A widely
used adsorption energy scaling relation between OH* and
OOH* intermediates in the oxygen reduction reaction (ORR) and oxygen
evolution reaction (OER), has previously been determined using density
functional theory and shown to dictate a minimum thermodynamic overpotential
for both reactions. Here, we show that the oxygenâoxygen bond
in the OOH* intermediate is, however, not well described with the
previously used class of exchange-correlation functionals. By quantifying
and correcting the systematic error, an improved description of gaseous
peroxide species versus experimental data and a reduction in calculational
uncertainty is obtained. For adsorbates, we find that the systematic
error largely cancels the vdW interaction missing in the original
determination of the scaling relation. An improved scaling relation,
which is fully independent of the applied exchangeâcorrelation
functional, is obtained and found to differ by 0.1 eV from the original.
This largely confirms that, although obtained with a method suffering
from systematic errors, the previously obtained scaling relation is
applicable for predictions of catalytic activity
Giant onsite electronic entropy enhances the performance of ceria for water splitting
AbstractPrevious studies have shown that a large solid-state entropy of reduction increases the thermodynamic efficiency of metal oxides, such as ceria, for two-step thermochemical water splitting cycles. In this context, the configurational entropy arising from oxygen off-stoichiometry in the oxide, has been the focus of most previous work. Here we report a different source of entropy, the onsite electronic configurational entropy, arising from coupling between orbital and spin angular momenta in lanthanide f orbitals. We find that onsite electronic configurational entropy is sizable in all lanthanides, and reaches a maximum value of â4.7 kB per oxygen vacancy for Ce4+/Ce3+ reduction. This unique and large positive entropy source in ceria explains its excellent performance for high-temperature catalytic redox reactions such as water splitting. Our calculations also show that terbium dioxide has a high electronic entropy and thus could also be a potential candidate for solar thermochemical reactions.</jats:p
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