Transition-metal exchanged zeolites
are known to convert methane
to methanol with high selectivity, in a stepwise process, involving
exposure to oxidants, followed by exposure to methane, and finally
by exposure to water vapor. However, a comprehensive theoretical study
on the nature of the possible active sites and their respective changes
during this stepwise process is still lacking. Here, we use a combination
of density functional theory calculations in its generalized-gradient
approximation (DFT-GGA) and post-DFT methods to identify the thermodynamically
preferred sites in Cu-exchanged zeolite SSZ-13 during the stepwise
conversion of methane to methanol. We develop a thermodynamic model
for an extensive set of possible active sites, that is, Cu monomers,
dimers, and trimers, which are anchored in different ring structures
and supported by a series of different local Al distributions. Subsequently,
phase diagrams are constructed and used to identify thermodynamically
favored sites at each step during the stepwise conversion of methane
to methanol. We find that during exposure to O2, hydroxylated
dimersCu2O2H2 and, depending
on the local Al configuration, Cu2OHare preferred.
Upon exposure to methane, site-bound methanol molecules are formed.
With the subsequent increase in water vapor pressure, a thermodynamic
preference for monoatomic Cu and the release of methanol are observed.
Furthermore, we compare our predicted results to experimental measurements
published in the literature and find close agreement in terms of Cu
coordination number and bond distances for some of the sites considered.
We expect that the insights obtained here can be used to improve our
understanding of the reaction mechanism and to optimize the stepwise
conversion of methane to methanol.
Graphene-based single-atom catalysts are promising alternatives to platinum-based catalysts for fuel cell applications. Different transition metals have been screened using electronic structure methods by estimating onset potentials from the most endergonic elementary reaction step. We calculate onset potentials for the oxygen reduction reaction on metal atoms embedded in Nsubstituted graphene di-vacancies by virtue of first-principlesinformed microkinetic analysis. We find that for more oxophilic metals (Cr, Fe, Mn, and Ru), purely thermodynamic models systematically underestimate onset potentials. Furthermore, the oxophilic metals (Cr, Fe, Mn, and Ru) are oxidized under reaction conditions, leading to an increase in activity compared to their reduced state. Importantly, coadsorbed O m H n species actively participate in the reaction, which requires a dynamic treatment of spectator species. These findings highlight the limitations of thermodynamic analyses for electrocatalytic processes, which commonly assume the same oxidation state for each metal, and show that deviations between computational and experimental onset potentials cannot be solely attributed to the shortcomings of the electronic structure methods.
A combination of
periodic density functional theory (DFT, PW91-GGA)
calculations, reaction kinetics experiments, and mean-field microkinetic
modeling is used to derive insights on the reaction mechanism and
determine the nature of the active site under reaction conditions
for the vapor-phase decomposition of formic acid (FA, HCOOH) over
Pt/C catalysts. Microkinetic models formulated using DFT energetics
derived on the clean Pt(100) and Pt(111) required large parameter
adjustments to reproduce the experimentally measured apparent activation
energies and reaction orders. Further, these models predicted high
surface coverage of adsorbed carbon monoxide (CO*), inconsistent with
the environment of the active site in the DFT calculations on the
clean surfaces. Consequently, we reperformed DFT calculations for
the entire reaction network on partially CO*-covered (4/9 monolayer,
ML) Pt(111) and Pt(100). The resultant microkinetic models, with thermochemistry
and kinetics explicitly dependent on CO* coverage, were able to reproduce
the experimentally determined activation energies and reaction orders,
in addition to being self-consistent in CO* coverage. Our results
suggest that Pt(100) is likely poisoned by CO* under typical reaction
conditions and does not contribute significantly to the experimentally
observed reactivity. Instead, we find that Pt(111) better represents
the active site for FA decomposition reaction on Pt/C catalysts. The
optimized model on 4/9 ML CO*-covered Pt(111) suggests that the reaction
occurs via the carboxyl (COOH*) intermediate and that the spectator
CO*-assisted pathways play a significant role under reaction conditions.
This study underscores the importance of spectator species on the
energetics and the mechanism of a catalytic reaction and their key
role in developing a model that better addresses the nature of the
active site under realistic catalytic reaction conditions.
Cu-exchanged zeolites are promising materials for the selective conversion of methane to methanol. Their activity is attributed to the presence of small Cu-oxo and Cu-hydroxy clusters, but the nature of...
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