19 research outputs found
Scaling Relations and Kinetic Monte Carlo Simulations To Bridge the Materials Gap in Heterogeneous Catalysis
Scaling relations combined with kinetic
Monte Carlo simulations
are used to study catalytic reactions on extended metal surfaces and
nanoparticles. The reaction energies are obtained by density functional
theory calculations, where the site-specific values are derived using
generalized coordination numbers. This approach provides a way to
handle the materials gap in heterogeneous catalysis. CO oxidation
on platinum is investigated as an archetypical reaction. The kinetic
simulations reveal clear differences between extended surfaces and
nanoparticles in the size range of 1–5 nm. The presence of
different types of sites on nanoparticles results in a turnover frequency
that is orders of magnitude larger than on extended surfaces. For
nanoparticles, the reaction conditions determine which sites dominate
the overall activity. At low pressures and high temperatures, edge
and corner sites determine the catalytic activity, whereas facet sites
dominate the activity at high pressures and low temperatures. Furthermore,
the reaction conditions are found to determine the particle-size dependence
of the turnover frequency
Adsorbate Entropies with Complete Potential Energy Sampling in Microkinetic Modeling
The
influence of different approximations on adsorbate entropies
is investigated for density functional theory based mean-field kinetic
modeling. Using CO oxidation over Pt(111) as a prototypical reaction,
we compare four approximations: the harmonic approximation, the hindered
translator, the free translator, and complete potential energy sampling
(CPES). The CPES method results in particularly good agreement with
previously measured experimental data. Given its general applicability
and moderate computational cost, the CPES method stands out as a preferable
option to describe adsorbate entropies
Ensemble Effects in Adsorbate–Adsorbate Interactions in Microkinetic Modeling
Adsorbates on a surface
experience lateral interactions
that result
in a distribution of adsorption energies. The adsorbate–adsorbate
interactions are known to affect the kinetics of surface reactions,
which motivates efforts to develop models that accurately account
for the interactions. Here, we use density functional theory (DFT)
calculations combined with Monte Carlo simulations to investigate
how the distribution of adsorbates affects adsorption and desorption
of CO from Pt(111). We find that the mean of the average adsorption
energy determines the adsorption process, whereas the desorption process
can be described by the low energy part of the adsorbate stability
distribution. The simulated results are in very good agreement with
calorimetry and temperature-programmed desorption experiments and
provide a guideline of how to include adsorbate–adsorbate interactions
in DFT-based mean-field kinetic models
Methane Oxidation over PdO(101) Revealed by First-Principles Kinetic Modeling
The
catalytic oxidation of methane to carbon dioxide and water
over PdO(101) is investigated with first-principles based microkinetic
modeling. Extensive exploration of the reaction landscape allows for
determination of preferred pathways at different reaction conditions.
The predicted kinetic behavior is in good agreement with a range of
experimental findings including reaction orders in methane, water,
and oxygen as well as apparent activation energies. The results consolidate
the role of the PdO(101) surface in the activity of PdO catalysts
and offer starting points for computational design of materials with
improved catalytic activity. Moreover, the study demonstrates the
predictive power of first-principles based kinetic modeling for oxide
surfaces when hybrid functionals are applied in conjugation with kinetic
models that go beyond the mean-field approximation
Ensemble Effects in Adsorbate–Adsorbate Interactions in Microkinetic Modeling
Adsorbates on a surface
experience lateral interactions
that result
in a distribution of adsorption energies. The adsorbate–adsorbate
interactions are known to affect the kinetics of surface reactions,
which motivates efforts to develop models that accurately account
for the interactions. Here, we use density functional theory (DFT)
calculations combined with Monte Carlo simulations to investigate
how the distribution of adsorbates affects adsorption and desorption
of CO from Pt(111). We find that the mean of the average adsorption
energy determines the adsorption process, whereas the desorption process
can be described by the low energy part of the adsorbate stability
distribution. The simulated results are in very good agreement with
calorimetry and temperature-programmed desorption experiments and
provide a guideline of how to include adsorbate–adsorbate interactions
in DFT-based mean-field kinetic models
Analysis of Porphyrines as Catalysts for Electrochemical Reduction of O<sub>2</sub> and Oxidation of H<sub>2</sub>O
Bioinspired
structures are promising as improved catalysts for
various redox reactions. One example is metal hangman-porphyrines
(MHP), which recently have been suggested for oxygen reduction/evolution
reaction (ORR/OER). The unique properties of the MHPs are attributed
to both the hangman scaffold and the C<sub>6</sub>F<sub>5</sub> side
groups. Herein, the OER/ORR over various transition metal MHPs is
investigated by density functional theory calculations within an electrochemical
framework. A comparison of the reaction landscape for MHP, metal porphyrine
(MP) and metaltetrafluorophenyloporphyrines (MTFPP), allow for a disentanglement
of the different roles of the hangman motif and the side groups. In
agreement with experimental studies, it is found that Fe and Co are
the best MHP metal centers to catalyze these reactions. We find that
the addition of the three-dimensional moiety in the form of hangman
scaffold does not break the apparently universal energy relation between
*OH and *OOH intermediates. However, the hangman motif is found to
stabilize the oxygen intermediate, whereas addition of C<sub>6</sub>F<sub>5</sub> groups reduces the binding energy of all reaction intermediates.
Our results indicate that the combination of these two effects allow
new design possibilities for macromolecular systems with enhanced
catalytic OER/ORR activity
Catalysis at the Rim: A Mechanism for Low Temperature CO Oxidation over Pt<sub>3</sub>Sn
Metal
alloying is commonly used as a design strategy for catalyst
optimization. The mechanistic understanding of this class of systems
is, however, obscured by reaction induced segregation phenomena. Herein,
the case of CO oxidation over Pt<sub>3</sub>Sn is investigated using
density functional theory calculations combined with ab initio thermodynamics
and first-principles based microkinetic modeling. It is found that
Pt<sub>3</sub>Sn segregates under typical operating conditions into
SnO<sub><i>X</i></sub> and an Sn deficient metal phase.
The segregation is driven both by the stability of the metal oxide
and the strong bonding of CO to Pt. The catalytic consequences of
a metal supported SnO<sub>2</sub> phase are explored by comparing
CO oxidation at an SnO<sub><i>X</i></sub>/Pt interface with
oxidation over Pt and Pt/Pt<sub>3</sub>Sn skin models. The reaction
is found to proceed with lower barriers at the interface as compared
to the metal-only systems and the cocatalytic role of the SnO<sub><i>X</i></sub> rim is manifested by low temperature activity.
The present work highlights the effects of reaction-induced metaloxide/metal
interfaces and elucidates the role of Sn in PtSn alloys for CO oxidation
reactions
Simulated Photoemission Spectra of Hydroxylated MgO(100) at Elevated Temperatures
Density functional theory has been used to investigate
photoemission
O1s core-level shifts (CLS) of hydroxylated MgO(100). Rapid proton
exchange at elevated temperatures (300 K) yields broad features in
the simulated photoemission signal, in good agreement with experimental
observations. The results provide further evidence that the stable
structure of hydroxylated MgO(100) consists of a partly dissociated
water monolayer. Analysis of the CLS for adsorbed hydroxyl groups
at different coverage reveals a pronounced effect on hydrogen bonding
to neighboring H<sub>2</sub>O molecules. The inclusion of exact exchange
by use of the hybrid PBE0 functional leads to quantitatively similar
results as the gradient corrected PBE functional
Cutting intervals in marandu grass (brachiaria brizantha (hochst. ex a. rich.) stapf cv. marandu): dry matter yield, nutritive value and silage fermentation losses.
Os objetivos do estudo foram avaliar os efeitos do intervalo entre cortes e da estação do ano, sobre a produção de matéria seca (MS), composição morfológica e químico-bromatológica do capim-Marandu. Os tratamentos consistiram em seis intervalos entre cortes (T15, T30, T45, T60 T75 e T90), com forragens colhidas, respectivamente, aos 15, 30, 45, 60, 75 e 90 dias de rebrotação, conduzidos ao longo de 12 meses. Parte da forragem colhida foi ensilada em janeiro (verão) e julho (inverno), exceto T75. Foram realizados dois ensaios, no primeiro houve a caracterização da produção de MS, composição morfológica e químico-bromatológica da forragem. Num segundo ensaio foi caracterizada a composição químico-bromatológica das silagens de capim-Marandu e foram avaliados alguns parâmetros físicos das mesmas, como densidade da massa verde (DMV), da massa seca (DMS), tamanho médio das partículas, condutividade elétrica (CE) e atividade de água (Aw). O processo fermentativo foi avaliado indiretamente através da quantificação das perdas por gases, produção de efluente e taxa de recuperação de MS das silagens. O delineamento experimental constituiu-se de blocos completos casualizados avaliando seis tratamentos para o primeiro ensaio e cinco no segundo. Para o ensaio de produção e qualidade da forragem os dados foram agrupados em ciclos de cortes de 90 dias para efeito de comparação entre os tratamentos. As produções de MS nos tratamentos T30 e T45 (21 t ha -1 ano -1 ) foram inferiores (P<0,01) aos demais (27 t ha -1 ano -1 ) e a produção de MS digestível (16,2 t ha -1 ano -1 ) seguiu a mesma tendência. As alterações encontradas nas variáveis estudadas foram, sobretudo, decorrentes das variações ambientais, que levaram a alterações na composição morfológica e químico-bromatológica. Em geral, maiores intervalos entre cortes proporcionaram redução na porcentagem de folhas e aumento na de hastes que, como conseqüência, levaram ao declínio nos teores de proteína bruta (PB) e na digestibilidade verdadeira in vitro da matéria seca (DVIVMS), além de aumentos nos teores de fibra em detergente neutro (FDN). A silagem de inverno promoveu discreta alteração na composição químico-bromatológica entre os tratamentos, restringindo-se a variações nos teores de PB, FDN e celulose que levaram a alterações na DVIVMS. Na silagem de verão observou-se consistência de redução do valor nutritivo com a maior extensão do intervalo entre cortes da forragem ensilada, com aumento das frações da parede celular, exceto hemicelulose, e diminuição nos componentes do conteúdo celular. Os valores de pH (4,9), apesar de elevados, estiveram dentro da amplitude de variação esperada para silagens de capins tropicais. O menor tamanho de partículas (31 mm) proporcionou maiores (P<0,01) DMV (633 kg m -3 ). Os maiores teores de MS nas silagens confeccionadas no inverno levaram a maiores perdas por gases e menores produções de efluente, com maiores taxas de recuperação de MS. A otimização do intervalo entre cortes, visando conciliar elevada produção de MS e valor nutritivo satisfatório, ocorreu ao redor dos 60 dias de crescimento vegetativo, quando a baixa digestibilidade (63,1%) foi compensada pela maior produção de MS (27 t ha -1 ano -1 ) e menores perdas (3,45% MS) no processo fermentativo.The objectives of this study were to evaluate cutting intervals and season of the year on dry matter (DM) yield, morphological and chemical composition of Marandu grass. Treatments consisted of Marandu grass harvested at 15, 30, 45, 60, 75 and 90-d intervals and was carried out during 12 months. Part of the harvested forage was ensiled, both in the summer (January) and in the winter (July) cuts, except that of 75-d interval. In the first trial was evaluated the dry matter (DM) yield, morphological composition and nutritive value of Marandu grass. In the second trial, silage samples were submitted to chemical and physical analysis (electrical conductivity - EC, water activity - Aw, particle size - PS, fresh matter density - FMD, dry matter density - DMD, effluent yield, gases losses and dry matter recovery rate-DMRR). Randomized blocks with six treatments in the first trial and five treatments in the second one was the experimental design chosen. In the first trial data were pooled in 90-d interval to compare treatments. DM yield was lower (P<0.01) at 30 and 45-d interval (21 t ha -1 year -1 ) as compared to 15, 60, 75 and 90-d interval (27 t ha -1 year -1 ). Digestible DM yield (16.2 t ha -1 year -1 ) followed the same pattern. Values observed in the measured parameters were probably due to environmental conditions, leading to morphological and chemical composition changes. In general, samples harvested at longer cutting intervals showed a decrease in leaves, crude protein (CP) content and in vitro true DM digestibility (IVTDMD) and an increase in stems and neutral detergent fiber (NDF) content. Among treatments, the winter cut silages showed minor chemical composition changes as compared to the summer cut silages. The summer cut silages showed a trend for increasing cell wall fractions, except hemicellulose and decreasing cell soluble contents as the cutting intervals were increased. Average silage pH values were 4.9. Small particle size (31 mm) silages had higher (P<0.01) FMD (633 kg m -3 ). The higher DM content observed in the winter cut silages led to increased gases losses and DMRR and lower effluent yield. The 60-d cutting interval is recommended based on productivity (27 t ha -1 year -1 ), digestibility (63.1%) and fermentation losses (3.4% DM) values