Elementary reaction processes involving atomic and molecular oxygen on ZrB2 (0001)surface

The interaction of atomic and molecular oxygen along with the atomic recombination on thin ZrB2(0001) Zr- and B-terminated surfaces were studied using density functional theory (GGA/PBE) calculations. The adsorption of atomic oxygen is predominantly produced on threefold hollow sites for the Zr-finished surface and on B-B bridge sites for the B-finished surface. The experimental specular HREELS loss peaks and their shifts at high O exposures can be satisfactory explained by the present calculations. The interaction of O2 over both terminated surfaces produces mainly its dissociation through non-activated processes. This fact is in agreement with the observed open dissociation at room temperature. The atomic oxygen recombination over both ZrB2 surfaces shows that the Eley-Rideal reaction will be much more important than the Langmuir-Hinshelwood reaction at all temperatures and even more accessible in the case of the B-finished surface

DFT and kinetics study of O/O2 mixtures reacting over a graphite (0001) basal surface

Spin-polarised density functional calculations were used to investigate the interaction of atomic and molecular oxygen on the basal graphite surface at several atomic coverages. Two carbon layers were enough to represent the graphite surface. Oxygen atoms bind mainly over C-C bridge sites forming an epoxide-like structure with a two carbon puckering and with adsorption energies in the 0.95-1.28 eV range, depending on the atomic coverage. Molecular oxygen only shows a very weak physisorption. Atomic adsorption and diffusion along with atomic recombination via Eley-Rideal and Langmuir-Hinshelwood mechanisms were studied. All surfaces processes were activated with energy barriers that decreased for lower atomic coverages. Relaxation effects were non-negligible. A microkinetic model with six elementary surface processes was proposed to see the overall behavior of several initial O/O2 mixtures flowing over a graphite surface at 300-1000 K. Thermal rate constants were derived from Density Functional Theory (DFT) data and standard Transition State Theory. A very low steady-state atomic coverage ( < 0.5 %) was predicted and also very low atomic recombination coefficients were observed (gamma_O < 5x10-4). The Eley-Rideal together with the adsorption and desorption processes were much more important than the Langmuir-Hinshelwood reaction

Adsorption of atomic oxygen and nitrogen at β-cristobalite (100): a density functionaly theory study

The adsorption of atomic oxygen and nitrogen on the -cristobalite (100) surface is investigated from first principles density functional calculations within the generalized gradient approximation. A periodic SiO2 slab model (6 layers relaxing 4 or 6) ended with a layer of Si or O atoms is employed throughout the study. Several adsorption minima and diffusion transition states have been characterized for the two lowest spin states of both systems. A strong chemisorption is found for either O or N in several sites with both slab endings (e.g., it is found an average adsorption energy of 5.89 eV for O (singlet state) and 4.12 eV for N (doublet state) over the Si face). The approach of O or N on top O gives place to the O2 and NO abstraction reactions without energy barriers. Atomic sticking coefficients and desorption rate constants have been estimated (300 - 1900 K) by using the standard transition state theory. The high adsorption energies found for O and N over silica point out that the atomic recombination processes (i.e., Eley-Rideal and Langmuir-Hinshelwood mechanisms) will play a more important role in the atomic detachment processes than the thermal desorption processes. Furthermore, the different behavior observed for the O and N thermal desorption processes suggests that the published kinetic models for atomic O and N recombination reactions on SiO2 surfaces, based on low adsorption energies (e.g., 3.5 eV for both O and N ), should probably be revised

ReaxFF molecular dynamics simulations of CO collisions on an O-preadsorbed silica surface

A quasiclassical trajectory dynamics study was performed for carbon monoxide collisions over an oxygen preadsorbed beta-cristobalite (001) surface. A reactive molecular force field (ReaxFF) was used to model the potential energy surface. The collisions were performed fixing several initial conditions: CO rovibrational states (v = 0-5 and j = 0, 20, 35), collision energies (0.05 ≤ Ecol ≤ 2.5 eV), incident angles (0°, 45°) and surface temperatures (Tsurf = 300 K, 900 K). The principal elementary processes were the molecular reflection and the non-dissociative molecular adsorption. CO2 molecules were also formed in minor extension via an Eley-Rideal reaction although some of them were finally retained on the surface. The scattered CO molecules tend to be translationally colder and internally hotter (rotationally and vibrationally) than the initial ones. The present study supports that CO + Oad reaction should be less important than O + Oad reaction over silica for similar initial conditions of reactants, in agreement with experimental data

Ab initio study of the two lowest triplet potential energy surfaces involved in the N(4S) + NO(X2Π) reaction

This work presents ab initio electronic structure calculations of the two possible N(4S) + NO(X2) abstraction reaction channels on the lowest 3A' and 3A' potential energy surfaces (PESs). Complete active space self-consistent-field (CASSCF) calculations, second-order perturbation calculations (CASPT2) and multireference configuration interaction calculations (MR-CI) based on CASSCF wave functions, along with some coupled cluster (CC) calculations were carried out by using the standard correlation-consistent (cc-pVnZ and aug-cc-pVnZ, n=D,T,Q,5) Dunning's basis sets. It was shown that there was no energy barrier along the minimum energy path in the 3A' PES for the N-abstraction reaction channel. However, an energy barrier (6.74 kcal/mol) was located in the 3A' PES. This energy barrier was considerably smaller than the previously reported MR-CCI value (14.4 kcal/mol). It was established that the N and O 2s electron correlation, neglected in previous studies of these authors, was the main source of this energy decrease. As a result, the present ab initio data will produce larger values of the thermal rate constants at high temperatures. High-energy barriers were found for the O-abstraction reaction channel in both PESs (41.13 and 30.77 kcal/mol for 3A' and 3A', respectively), which agree with the accepted idea that this channel will be only important at high collision energies. Nonetheless, current ab initio results show that this channel will be open at not very high collision energies (e.g., over 30 kcal/mol could take place). Experimental studies on the O-abstraction reaction channel are missing and would be useful to confirm its ab initio expected importance

Assessing salt-surfactant synergistic effects on interfacial tension from molecular dynamics simulations

In the recent years, many efforts have been carried out trying to comprehend how surfactants and salts interact among each other at the oil/brine interface to reduce the interfacial tension (IFT). To that end, the interfacial properties of several combinations of surfactants, salts and oils have been measured experimentally confirming the existence of a synergistic effect. Unfortunately, many of the proposed mechanisms for that effect arise from experimental observations, so this work, based on molecular dynamics simulations, intends to reproduce and explain this kind of phenomenon from a molecular point of view. The correct understanding of these phenomena can have application in many fields, especially in Enhanced Oil Recovery, where reducing IFT can potentially increase oil production. In this article we evaluate the effect of adding three different salts (i.e., NaCl, CaCl2 and MgCl2) on the IFT of a water/oil system with different non-ionic surfactants. We have evaluated the effect that the ions of salt produce to surfactants, as well as the perturbation that surfactants produce on the ions. From our results, we can assess that salts (especially NaCl) and surfactants are able to interact with each other, being both active species in reducing the IFT of the system

Non-adiabatic quantum dynamics of the electronic quenching OH(A(2)sigma(+)) + Kr

We present the dynamics of the electronic quenching OH(A2S+) + Kr(1S)-OH(X2P) + Kr(1S), withOH(A2S+) in the ground ro-vibrational state. This study relies on a new non-adiabatic quantum theorythat uses three diabatic electronic statesS+,P0, andP00, coupled by one conical-intersection and nineRenner-Teller matrix elements, all of which are explicitly considered in the equation of the motion. Thetime-dependent mechanism and initial-state-resolved quenching probabilities, integral cross sections,thermal rate constants, and thermally-averaged cross sections are calculatedviathe real wavepacketmethod. The results point out a competition among three non-adiabatic pathways:S+2P0,S+2P00,andP02P00. In particular, the conical-intersection effectsS+-P0are more important than theRenner-Teller couplingsS+-P0,S+-P00, andP0-P00. Therefore,P0is the preferred product channel.The quenching occursviaan indirect insertion mechanism, opening many collision complexes, and theprobabilities thus present many oscillations. Some resonances are still observable in the cross sections,which are in good agreement with previous experimental and quasi-classical findings. We also discussthe validity of more approximate quantum methods

Combining soft-SAFT and COSMO-RS modeling tools to assess the CO2-SO2 separation using phosphonium-based ionic liquids

The development of efficient CO2 separation techniques from post-combustion flue gases is a key area of research to green-house gas control. However, CO2 capture is typically affected by the presence of other acid impurities, such as traces of SO2. In that sense, this work assesses CO2 separation from a CO2/SO2 mixture with a set of phosphonium-based ILs. Two different modeling tools, soft-SAFT and COSMO-RS, have been used cooperatively to study the CO2 gas separation on ILs. From one side, the soft-SAFT equation of state, which has been employed for the first time in this family of ILs, has been used to effectively reproduce the absorption properties of these promising CO2 absorbents in a wide range of pressures/temperatures. Additionally, COSMO-RS, employed to evaluate the charge distribution so as to develop representative models for soft-SAFT, has been capable of reproducing the low-pressure absorption region in a purely predictive way. In both cases, the enthalpy and entropy of dissolution and the selectivity of the mixtures are predicted. Also, several ternary diagrams have been built to analyze different acid gas compositions

SABER 2.0 in STEM: rewarded correction and subject content-active learning practical matching strategies

The present paper addresses a 2.0 implementation of a practical classroom strategy to increase university students' performance, with emphasis given to STEM subjects (Science, Technology, Engineering and Mathematics). METHOD. The starting point is a scheme based on the flipped classroom (FC) concept. The work, however, starts in the classroom, using a synchronous FC, and modifications are introduced to increase students' ability to work autonomously. The practical methodology is known as SABER (after the Spanish Supervisión del Aprendizaje Básico con Ejercicios y autoReflexión). RESULTS. The paper describes a 2.0 version that incorporates (a) rewarded mistake correction as a key part in students' consolidation of concepts; and (b) substantial changes in how subject content is introduced to students. In the latter case, comparison experiments and compared macroscopic physical properties have been used to introduce difficult concepts. DISCUSSION. This approach presents content from an experimental perspective that is much closer to students' existing knowledge. The paper also provides some specific examples and practical tips to demonstrate how easily the methodology can be implemented

Quasiclassical dynamics and kinetics of the N + NO → N+O, NO+N atmospheric reactions

The kinetics and dynamics of the title reactions was studied, using the quasiclassical trajectory (QCT) method and two ab initio analytical potential energy surfaces (PESs) developed by our group. In addition to the rate constant (T: 10-5000 K), we also considered a broad set of dynamic properties as a function of collision energy (up to 1.0 eV) and the rovibrational state of NO(v=0-2,j=1,8,12). The production of N2 + O, reaction (1), dominates the reactivity of the N + NO system over the conditions studied, as expected from the large energy barriers associated to the NO + N exchange reaction, reaction (2). Moreover, the ground PES, which is barrierless for reaction (1), plays a dominant role. Most of the results were interpreted according to the properties of the PESs involved and the kinematics of the system. The QCT rate constants of reaction (1) are in agreement with the experimental data (T: 47-3500 K), including very recent low temperature measurements, and also with variational transition state kinetics and most of quantum dynamics calculations. In addition, the QCT average vibrational energy content of the N2 product also agrees with the experimental and quantum data. The PESs used here could also be useful to determine equilibrium and non-equilibrium reaction rates at very high temperatures (e.g., 5000-15000 K)