Molecular Dynamics Simulations of Model Perhydrogenated and Perfluorinated Alkyl Chains, Droplets, and Micelles

Molecular dynamics simulations have been performed to study chain conformation and internal structure for three models of n-alkanes (PHA) and n-perfluoroalkanes (PFA). These systems include single chains of 7−29 identical segments in a solvent of segments, “oil” droplets (aggregates of 24 chains), and micelles (experimental mean-sized aggregates of 24 octyl PHA or 40 perfluoroheptyl PFA chains with “nonionic sulfate head” groups attached to one end). Three different united-atom potential models of PHA chains and five models of PFA chains have been used to study the effect of chain potential. A computationally efficient solvent force field represented segment and headgroup interactions with an aqueous solvent and maintained aggregate sizes and shapes. For single chains, the overall trans bond fraction and distribution were independent of chain length while the scaling exponent for end-to-end distances for longer chains agrees with theoretical scaling laws. Several different statistical analyses showed that the conformational and structural properties for chains in droplets and micelles are similar except for slightly higher trans bond fractions for the PFA chains. The bond orientation parameters of micelles had preferential near-surface orientations but were random in the core. Aggregate shapes were changed in ellipticity from 0.5 (oblate) to 2.0 (prolate) but the only effect was a small change of bond orientation parameter in the PHA micelles

Dealumination of the H‑BEA Zeolite via the <i>S</i>_<b>N2</b> Mechanism: A Theoretical Investigation

To understand the stability of aluminosilicate zeolites in the presence of steam, a computational study on the dealumination process of the H-BEA zeolite is carried out. High-dimensional free energy profiles characterizing stepwise hydrolysis steps of four Al–O bonds have been mapped out using ab initio molecular dynamics simulations combined with the enhanced sampling methodology. It has been found that the dealumination process of the H-BEA zeolite can be elucidated as the SN2 mechanism. For each hydrolysis step, the protonation of the Al–OAl bond is initialized by proton transfer. Then, one water molecule at the antiposition of the protonated OAl atom attacks the Al atom, resulting in one Al–OAl bond breaking and one Al–OW bond formation simultaneously. Free energy barriers for four hydrolysis steps are calculated to be 16.6, 20.8, 69.3, and 50.0 kJ/mol, respectively. The breakage of the first two Al–OAl bonds within a small five-member ring is facile while the last two Al–OAl bonds within a large 12-member ring are hard to break. The leaving Al atom from the zeolitic framework is in the form of Al­(OH)3(H2O)

Origin of Support Effects on the Reactivity of a Ceria Cluster

The interaction between an active oxide and an oxide support plays a critical role in controlling the reactivity of oxide-on-oxide catalysts. In the present study, the reactivity of a small ceria cluster (Ce2O4) supported on the reducible monoclinic zirconia and the irreducible γ-alumina was investigated using the first-principles density functional theory method. Our results showed that the binding energies of the Ce2O4 cluster on the supporting ZrO2(111) and γ-Al2O3(100) substrates are −5.32 and −4.06 eV, respectively, indicating a very strong interaction. On the basis of these oxide-on-oxide model catalysts, the effects of supports on the reactivity of Ce2O4 cluster were probed by the adsorption of CO2 and CO. The acidic CO2 molecule chemisorbs at the O sites of the cluster, forming a carbonate-like (CO32−) species through an acid−base interaction. Neither ZrO2(111) nor γ-Al2O3(100) exhibits a significant effect on CO2 adsorption over the supported Ce2O4 cluster. In contrast, the reactive adsorption of CO on the supported Ce2O4 cluster shows a strong dependence on the supporting oxides: The reactive adsorption energy for CO on the γ-Al2O3(100)-supported Ce2O4 is −4.33 eV, whereas that on the ZrO2(111)-supported cluster is only −0.55 eV. This reactive adsorption was accompanied by the reduction of Ce4+ to Ce3+ in the Ce2O4 clusters, leading to the formation of (Ce2O2)2+CO32−, which can be considered as an intermediate for CO oxidation to CO2. The very different stabilities of the (Ce2O2)2+CO32− intermediate on the two oxide supports were analyzed in the context of CO oxidation catalyzed by ceria. The ZrO2(111)-supported Ce2O4 cluster is expected to be highly active for CO oxidation, whereas the turnover from CO to CO2 on the γ-Al2O3(100)-supported and the unsupported Ce2O4 clusters is hindered by the desorption of CO2 from the (Ce2O2)2+CO32− intermediates

Artificial Neural Network Potential for Encapsulated Platinum Clusters in MOF-808

Metal-organic frameworks (MOFs) have been recognized as one of the ideal supporting sintering-resistant catalyst materials because of their high specific surface area and unique nano-porous structure capable of manipulating the sizes and shapes of encapsulated metal clusters. To explore the binding sites, the stability, and migration mechanisms of encapsulated metal clusters in MOF materials, a robust potential model that accurately describes the interaction between metal clusters and MOF materials is highly desired for large-scale atomic simulations. Herein, as a demonstration case, an artificial neural network potential for encapsulated platinum (Pt) clusters in MOF-808 was developed using the machine learning-based global neural network (G-NN) technique. The artificial G-NN potential was tested and validated against a series of density functional theory calculation data, including structure optimization, adsorption energies, and the migration energy barrier of Ptn (n = 1–13) clusters in MOF-808. The newly developed Pt-MOF G-NN potential was further used to predict the adsorption and migration behaviors of Ptn clusters in MOF-808. It is found that the most stable adsorption site varies with the Ptn cluster size. The migration possibility of the Ptn cluster is strongly correlated with the adsorption energies of the Ptn clusters. Finally, the CO adsorption on the single Pt atom would effectively promote the aggregation of Ptn clusters via the Ostwald ripening mechanism

Density Functional Theory Study of Acetaldehyde Hydrodeoxygenation on MoO₃

Periodic spin-polarized density functional theory calculations were performed to investigate acetaldehyde (CH3CHO) hydrodeoxygenation on the reduced molybdenum trioxide (MoO3) surface. The perfect O-terminated α-MoO3(010) surface is reduced to generate an oxygen defect site in the presence of H2. H2 dissociatively adsorbs at the surface oxygen sites forming two surface hydroxyls, which can recombine into a water molecule weakly bound at the Mo site. A terminal oxygen (Ot) defect site thus forms after water desorption. CH3CHO adsorbs at the O-deficient Mo site via either the sole O−Mo bond or the O−Mo and the C−O double bonds. The possible reaction pathways of the adsorbed CH3CHO with these two configurations were thoroughly examined using the dimer searching method. Our results show that the ideal deoxygenation of CH3CHO leading to ethylene (C2H4) on the reduced MoO3(010) surface is feasible. The adsorbed CH3CHO first dehydrogenate into CH2CHO by reacting with a neighboring terminal Ot. The hydroxyl (OtH) then hydrogenates CH2CHO into CH2CH2O to complete the hydrogen transfer cycle with an activation barrier of 1.39 eV. The direct hydrogen transfer from CH3CHO to CH2CH2O is unlikely due to the high barrier of 2.00 eV. The produced CH2CH2O readily decomposes into C2H4 that directly releases to the gas phase and regenerates the Ot atom on the Mo site. As a result, the reduced MoO3(010) surface is reoxidized to the perfect MoO3(010) surface after CH3CHO deoxygenation

Theoretical Study on the Catalytic CO₂ Hydrogenation over the MOF-808-Encapsulated Single-Atom Metal Catalysts

The search for new catalytic agents for reducing excess CO2 in the atmosphere is a challenging but essential task. Due to the well-defined porous structures and unique physicochemical properties, metal–organic frameworks (MOFs) have been regarded as one of the promising materials in the catalytic conversion of CO2 into valuable platform chemicals. In particular, introducing the second metal (M) atom to form the MII–O–Zr4+ single-atom metal sites on the Zr nodes of MOF-808 would further greatly improve the catalytic performance. Herein, CO2 hydrogenation reaction mechanisms and kinetics over a series of MOF-808-encapsulated single-atom metal catalysts, i.e., MII–MOF-808 (MII = CuII, FeII, PtII, NiII, and PdII), were systematically studied using density functional theory calculations. First, it has been found that the stability for the encapsulation of a divalent metal ion follows the trend of PtII > NiII > PdII > CuII > FeII, while they all possess moderate anchoring stability on the MOF-808 with the Gibbs replacement energies ranging from −233.7 to −310.3 kcal/mol. Two plausible CO2 hydrogenation pathways on CuII–MOF-808 catalysts, i.e., formate and carboxyl routes, were studied. The formate route is more favorable, in which the H2COOH*-to-H2CO* step is kinetically the most relevant step over CuII–MOF-808. Using the energetic span model, the relative turnover frequencies of CO2 hydrogenation to various C1 products over MII–MOF-808 were calculated. The CuII–MOF-808 catalyst is found to be the most active catalyst among five MII–MOF-808 catalysts

Active Oxygen Vacancy Site for Methanol Synthesis from CO₂ Hydrogenation on In₂O₃(110): A DFT Study

Methanol synthesis from CO₂ hydrogenation on the defective In₂O₃(110) surface with surface oxygen vacancies has been investigated using periodic density functional theory calculations. The relative stabilities of six possible surface oxygen vacancies numbered from O_v1 to O_v6 on the perfect In₂O₃(110) surface were examined. The calculated oxygen vacancy formation energies show that the D1 surface with the O_v1 defective site is the most thermodynamically favorable while the D4 surface with the O_v4 defective site is the least stable. Two different methanol synthesis routes from CO₂ hydrogenation over both D1 and D4 surfaces were studied, and the D4 surface was found to be more favorable for CO₂ activation and hydrogenation. On the D4 surface, one of the O atoms of the CO₂ molecule fills in the O_v4 site upon adsorption. Hydrogenation of CO₂ to HCOO on the D4 surface is both thermodynamically and kinetically favorable. Further hydrogenation of HCOO involves both forming the C–H bond and breaking the C–O bond, resulting in H₂CO and hydroxyl. The HCOO hydrogenation is slightly endothermic with an activation barrier of 0.57 eV. A high barrier of 1.14 eV for the hydrogenation of H₂CO to H₃CO indicates that this step is the rate-limiting step in the methanol synthesis on the defective In₂O₃(110) surface

First-Principles Thermodynamics Study of Spinel MgAl₂O₄ Surface Stability

The surface stability of all possible terminations for three low-index (100, 110, 111) structures of spinel MgAl₂O₄ was studied using a first-principles-based thermodynamic approach. The surface Gibbs free energy results indicate that the 100_AlO₂ termination is the most stable surface structure under ultrahigh vacuum at <i>T</i> = 1100 K regardless of an Al-poor or Al-rich condition. With increasing oxygen pressure, the 111_O₂(Al) termination becomes the most stable surface in the Al-rich condition. The oxygen vacancy formation is thermodynamically favorable over the 100_AlO₂, 111_O₂(Al), and (111) structures with Mg/O connected terminations. On the basis of the surface Gibbs free energies for both perfect and defective surface terminations, 100_AlO₂ and 111_O₂(Al) are the most dominant surfaces in Al-rich conditions under atmospheric conditions. This is also consistent with our previously reported experimental observation

DFT+U Study on the Localized Electronic States and Their Potential Role During H₂O Dissociation and CO Oxidation Processes on CeO₂(111) Surface

We present the results of an extensive density functional theory based electronic structure study of the role of 4f-state localized electron states in the surface chemistry of a partially reduced CeO2(111) surface. These electrons exist in polaronic states, residing at Ce3+ sites, which can be created by either the formation of oxygen vacancies, OV, or other surface defects. Via ab initio molecular dynamics, these localized electrons are found to be able to move freely within the upper surface layer, but penetration into the bulk is inhibited as a result of the higher elastic strain induced by creating a subsurface Ce3+. We found that the water molecule can be easily dissociated into two surface bound hydroxyls at the Ce4+ site associated with OV sites. This dissociation process does not significantly affect the electronic structure of the excess electrons at reduced surface, but does lead to a favorable localization on Ce3+ sites in the vicinity of the resulting OH groups. In the presence of water, a proton-mediated Mars-van Krevelen mechanism for CO oxidation via the formation of bicarbonate species is identified. The localized 4f electrons on the surface facilitate the protonation process of adsorbed O2 species and thus decelerate the further oxidation of CO species. Overall, we find that surface hydroxyls formed via water dissociation at the CeO2 surface lead to inhabitation of the CO oxidation reaction. This is consistent with the experimental observation of requisite elevated temperatures, on the order of 600 K, for this reaction to occur