9 research outputs found
PySurf:A Framework for Database Accelerated Direct Dynamics
The greatest restriction to the theoretical study of the dynamics of photoinduced processes is computationally expensive electronic structure calculations. Machine learning algorithms have the potential to reduce the number of these computations significantly. Here, PySurf is introduced as an innovative code framework, which is specifically designed for rapid prototyping and development tasks for data science applications in computational chemistry. It comes with powerful Plugin and Workflow engines, which allows intuitive customization for individual tasks. Data is automatically stored through the database framework, which enables additional interpolation of properties in previously evaluated regions of the conformational space. To illustrate the potential of the framework, a code for nonadiabatic surface hopping simulations based on the Landau-Zener algorithm is presented here. Deriving gradients from the interpolated potential energy surfaces allows for full-dimensional nonadiabatic surface hopping simulations using only adiabatic energies (energy only). Simulations of a pyrazine model and ab initio-based calculations of the SO2 molecule show that energy-only calculations with PySurf are able to correctly predict the nonadiabatic dynamics of these prototype systems. The results reveal the degree of sophistication, which can be achieved by the database accelerated energy-only surface hopping simulations being competitive to commonly used semiclassical approaches
PySurf - A Framework for Database Accelerated Direct Dynamics
Here, PySurf is introduced as an innovative code framework, which is specifically designed for rapid prototyping and development tasks for data-science applications in computational chemistry. To illustrate the potential of the framework, a code for nonadiabatic surface-hopping simulations based on the Landau-Zener algorithm is presented here. The results reveal the degree of sophistication, which can be achieved by the database accelerated energy-only surface-hopping simulations being competitive to commonly used semi-classical approaches.</div
Solar Energy Harvesting with Carbon Nitrides: Do We Understand the Mechanism?
The photocatalytic splitting of water into molecular hydrogen and molecular oxygen with sunlight is the dream reaction for solar energy conversion. Since decades, transition-metal-oxide semiconductors and supramolecular organometallic structures have been extensively explored as photocatalysts for solar water splitting. More recently, polymeric carbon nitride materials consisting of triazine or heptazine building blocks have attracted considerable attention as hydrogen-evolution photocatalysts. The mechanism of hydrogen evolution with polymeric carbon nitrides is discussed throughout the current literature in terms of the familiar concepts developed for photoelectrochemical water splitting with semiconductors since the 1970s. We discuss in this perspective an alternative mechanistic paradigm for photoinduced water splitting with carbon nitrides, which focusses on the specific features of the photochemistry of aromatic N-heterocycles in aqueous environments. It is shown that a water molecule which is hydrogen-bonded to an N-heterocycle can be decomposed into hydrogen and hydroxyl radicals by two simple sequential photochemical reactions. This concept is illustrated by first-principles calculations of excited-state reaction paths and their energy profiles for hydrogen-bonded complexes of pyridine, triazine and heptazine with a water molecule. It is shown that the excited-state hydrogen-transfer and hydrogen-detachment reactions are essentially barrierless, in sharp contrast to water oxidation in the electronic ground state, where high barriers prevail. We also discuss in some detail the products of possible reactions of the highly reactive hydroxyl radicals with the chromophores. We hypothesize that the challenge of efficient solar hydrogen generation with carbon-nitride materials is less the decomposition of water as such, but rather the controlled recombination of the photogenerated radicals to the closed-shell products H2 and H2O2
Mechanism of Photocatalytic Water Oxidation by Graphitic Carbon Nitride
Carbon nitride materials
are of great interest for photocatalytic
water splitting. Herein, we report results from first-principles simulations
of the specific electron- and proton-transfer processes that are involved
in the photochemical oxidation of liquid water with heptazine-based
molecular photocatalysts. The heptazine chromophore and the solvent
molecules have been described strictly at the same level of electronic
structure theory. We demonstrate the critical role of solvent molecules
for the absorption properties of the chromophore and the overall photocatalytic
cycle. A simple model is developed to describe the photochemical water
oxidation mechanism. Our results reveal that heptazine possesses energy
levels that are suitable for the water oxidation reaction. We suggest
design principles for molecular photocatalysts which can be used as
descriptors in future experimental and computational screening studies
Solar Energy Harvesting with Carbon Nitrides: Do We Understand the Mechanism?
The photocatalytic splitting of water into molecular hydrogen and molecular oxygen with sunlight is the dream reaction for solar energy conversion. Since decades, transition-metal-oxide semiconductors and supramolecular organometallic structures have been extensively explored as photocatalysts for solar water splitting. More recently, polymeric carbon nitride materials consisting of triazine or heptazine building blocks have attracted considerable attention as hydrogen-evolution photocatalysts. The mechanism of hydrogen evolution with polymeric carbon nitrides is discussed throughout the current literature in terms of the familiar concepts developed for photoelectrochemical water splitting with semiconductors since the 1970s. We discuss in this perspective an alternative mechanistic paradigm for photoinduced water splitting with carbon nitrides, which focusses on the specific features of the photochemistry of aromatic N-heterocycles in aqueous environments. It is shown that a water molecule which is hydrogen-bonded to an N-heterocycle can be decomposed into hydrogen and hydroxyl radicals by two simple sequential photochemical reactions. This concept is illustrated by first-principles calculations of excited-state reaction paths and their energy profiles for hydrogen-bonded complexes of pyridine, triazine and heptazine with a water molecule. It is shown that the excited-state hydrogen-transfer and hydrogen-detachment reactions are essentially barrierless, in sharp contrast to water oxidation in the electronic ground state, where high barriers prevail. We also discuss in some detail the products of possible reactions of the highly reactive hydroxyl radicals with the chromophores. We hypothesize that the challenge of efficient solar hydrogen generation with carbon-nitride materials is less the decomposition of water as such, but rather the controlled recombination of the photogenerated radicals to the closed-shell products H2 and H2O2
Mechanism of Photocatalytic Water Splitting with Graphitic Carbon Nitride: Photochemistry of the Heptazine–Water Complex
Impressive
progress has recently been achieved in photocatalytic
hydrogen evolution with polymeric carbon nitride materials consisting
of heptazine building blocks. However, the fundamental mechanistic
principles of the catalytic cycle are as yet poorly understood. Here,
we provide first-principles computational evidence that water splitting
with heptazine-based materials can be understood as a molecular excited-state
reaction taking place in hydrogen-bonded heptazine–water complexes.
The oxidation of water occurs homolytically via an electron/proton
transfer from water to heptazine, resulting in ground-state heptazinyl
and OH radicals. It is shown that the excess hydrogen atom of the
heptazinyl radical can be photodetached by a second photon, which
regenerates the heptazine molecule. Alternatively to the photodetachment
reaction, two heptazinyl radicals can recombine in a dark reaction
to form H<sub>2</sub>, thereby regenerating two heptazine molecules.
The proposed molecular photochemical reaction scheme within hydrogen-bonded
chromophore–water complexes is complementary to the traditional
paradigm of photocatalytic water splitting, which assumes the separation
of electrons and holes over substantial time scales and distances
Photoinduced water oxidation in pyrimidine-water clusters: A combined experimental and theoretical study
International audienceThe photocatalytic oxidation of water with molecular or polymeric N-heterocyclic chromophores is a topic of high current interest in the context of artificial photosynthesis, that is, the conversion of solar energy to clean fuels. Hydrogen-bonded clusters of N-heterocycles with water molecules in a molecular beam are simple model systems for which the basic mechanisms of photochemical water oxidation can be studied under well-defined conditions. In this work, we explored the photoinduced H-atom transfer reaction in pyrimidine-water clusters yielding pyrimidinyl and hydroxyl radicals with laser spectroscopy, mass spectrometry and trajectory-based ab initio molecular dynamics simulations. The oxidation of water by photoexcited pyrimidine is unequivocally confirmed by the detection of the pyrimidinyl radical. The dynamics simulations provide information on the time scales and branching ratios of the reaction. While relaxation to local minima of the S1 potential-energy surface is the dominant reaction channel, the H-atom transfer reaction occurs on ultrafast time scales (faster than about 100 fs) with a branching ratio of a few percent. From the relaxed population in the S1 state, H-atom transfer yielding pyrimidinyl and hydroxyl radicals can occur on much longer time scales (picoseconds to nanoseconds) by H-atom tunneling
Dimerization of Linear Butenes on Zeolite-Supported
Nickel- and alkali-earth-modified LTA based zeolites catalyze the dimerization of 1-butene in the absence of Brønsted acid sites. The catalyst reaches over 95% selectivity to n-octenes and methylheptenes. The ratio of these two dimers is markedly influenced by the parallel isomerization of 1-butene to 2-butene, shifting the methylheptene/octene ratio from 0.7 to 1.4 as the conversion increases to 35%. At this conversion, the thermodynamic equilibrium of 90% cis- and trans-2-butenes is reached. Conversion of 2-butene results in methylheptene and dimethylhexene with rates that are 1 order of magnitude lower than those with 1-butene. The catalyst is deactivated rapidly by strongly adsorbed products in the presence of 2-butene. The presence of π-allyl-bound butene and Ni-alkyl intermediates was observed by IR spectroscopy, suggesting both to be reaction intermediates in isomerization and dimerization. Product distribution and apparent activation barriers suggest 1-butene dimerization to occur via a 1′-adsorption of the first butene molecule and a subsequent 1′- or 2′-insertion of the second butene to form octene and methylheptene, respectively. The reaction order of 2 for 1-butene and its high surface coverage suggest that the rate-determining step involves two weakly adsorbed butene molecules in addition to the more strongly held butene