Adsorption and Reaction Mechanisms of Direct Palladium Synthesis by ALD Using Pd(hfac)2 and Ozone on Si (100) Surface

Palladium nanoparticles made by atomic layer deposition (ALD) normally involve formaldehyde or H2 as a reducing agent. Since formaldehyde is toxic and H2 is explosive, it is advantageous to remove this reducing step during the fabrication of palladium metal by ALD. In this work we have successfully used Pd(hfac)2 and ozone directly to prepare palladium nanoparticles, without the use of reducing or annealing agents. Density functional theory (DFT) was employed to explore the reaction mechanisms of palladium metal formation in this process. DFT results show that Pd(hfac)2 dissociatively chemisorbed to form Pd(hfac)* and hfac* on the Si (100) surface. Subsequently, an O atom of the ozone could cleave the C–C bond of Pd(hfac)* to form Pd* with a low activation barrier of 0.46 eV. An O atom of the ozone could also be inserted into the hfac* to form Pd(hfac-O)* with a lower activation barrier of 0.29 eV. With more ozone, the C–C bond of Pd(hfac-O)* could be broken to produce Pd* with an activation barrier of 0.42 eV. The ozone could also chemisorb on the Pd atom of Pd(hfac-O)* to form O3-Pd(hfac-O)*, which could separate into O-Pd(hfac-O)* with a high activation barrier of 0.83 eV. Besides, the activation barrier was 0.64 eV for Pd* that was directly oxidized to PdOx by ozone. Based on activation barriers from DFT calculations, it was possible to prepare palladium without reducing steps when ALD conditions were carefully controlled, especially the ozone parameters, as shown by our experimental results. The mechanisms of this approach could be used to prepare other noble metals by ALD without reducing/annealing agents.

Low potential electron generating enzymes serve different functions during aerobic nitrogen fixation in Azotobacter vinelandii

Biological nitrogen fixation requires large amounts of energy in the form of ATP and low potential electrons to overcome the high activation barrier for cleavage of the dinitrogen triple bond. The model aerobic nitrogen-fixing bacteria, Azotobacter vinelandii, generates low potential electrons in the form of reduced ferredoxin (Fd) and flavodoxin (Fld) using two distinct mechanisms via the enzyme complexes Rnf and Fix. Both Rnf and Fix are expressed during nitrogen fixation, and deleting either rnf1 or fix genes has little effect on diazotrophic growth. However, deleting both rnf1 and fix eliminates the ability to grow diazotrophically. Rnf and Fix both use NADH as a source of electrons, but overcoming the energetics of NADH's endergonic reduction of Fd/Fld is accomplished through different mechanisms. Rnf harnesses free energy from the proton motive force, whereas Fix uses electron bifurcation to effectively couple the endergonic reduction of Fd/Fld to the exergonic reduction of quinone. Different stoichiometries and gene expression analyses indicate specific roles for the two reactions under different conditions. In this work, complementary physiological studies and thermodynamic modeling reveal how Rnf and Fix simultaneously balance redox homeostasis in various conditions. Specifically, the Fix complex is required for efficient growth under low oxygen concentrations, while Rnf sustains homeostasis and delivers sufficient reduced Fd to nitrogenase under standard conditions. This work provides a framework for understanding how the production of low potential electrons sustains robust nitrogen fixation in various conditions.

Current trends and perspectives on emerging Fe-derived noble metal-free oxygen electrocatalysts

Oxygen electrochemistry that comprises oxygen evolution and reduction reactions (OER/ORR) is vital for sustainable, clean and efficient energy generation. However, the high activation barrier due to the multi-electron transfer process...

Bypassing the Inertness of Aziridine/CO2 Systems to Access 5-Aryl-2-Oxazolidinones: Catalyst-Free Synthesis Under Ambient Conditions

The development of sustainable synthetic routes to access valuable oxazolidinones via CO₂ fixation is an active research area, and the aziridine/carbon dioxide coupling has aroused a considerable interest. This reaction is featured by a high activation barrier, so to require a catalytic system, and may present some other critical issues. Here, we describe the straightforward gram-scale synthesis of a series of 5-​aryl-​2-oxazolidinones at ambient temperature and atmospheric CO₂ pressure, in the absence of any catalyst/co-catalyst. The key to this innovative procedure consists in the direct transfer of the pre-formed amine/CO₂ adduct (carbamate) to common aziridine precursors (dimethylsulfonium salts), replacing the classical sequential addition of amine (intermediate isolation of aziridine) and then CO₂. The reaction mechanism has been investigated by NMR studies and DFT calculations applied to model cases.<br>

Bypassing the Inertness of Aziridine/CO2 Systems to Access 5-Aryl-2-Oxazolidinones: Catalyst-Free Synthesis Under Ambient Conditions

The development of sustainable synthetic routes to access valuable oxazolidinones via CO₂ fixation is an active research area, and the aziridine/carbon dioxide coupling has aroused a considerable interest. This reaction is featured by a high activation barrier, so to require a catalytic system, and may present some other critical issues. Here, we describe the straightforward gram-scale synthesis of a series of 5-​aryl-​2-oxazolidinones at ambient temperature and atmospheric CO₂ pressure, in the absence of any catalyst/co-catalyst. The key to this innovative procedure consists in the direct transfer of the pre-formed amine/CO₂ adduct (carbamate) to common aziridine precursors (dimethylsulfonium salts), replacing the classical sequential addition of amine (intermediate isolation of aziridine) and then CO₂. The reaction mechanism has been investigated by NMR studies and DFT calculations applied to model cases.<br>

Balancing the Activity and Selectivity of Propane Oxidative Dehydrogenation on NiOOH (001) and (010)

Propane oxidative dehydrogenation (ODH) is an energy-efficient approach to produce propylene. However, ODH suffers from low propylene selectivity due to a relatively higher activation barrier for propylene formation compared with that for further oxidation. In this work, calculations based on density functional theory were performed to map out the reaction pathways of propane ODH on the surfaces (001) and (010) of nickel oxide hydroxide (NiOOH). Results show that propane is physisorbed on both surfaces and produces propylene through a two-step radical dehydrogenation process. The relatively low activation barriers of propane dehydrogenation on the NiOOH surfaces make the NiOOH-based catalysts promising for propane ODH. By contrast, the weak interaction between the allylic radical and the surface leads to a high activation barrier for further propylene oxidation. These results suggest that the catalysts based on NiOOH can be active and selective for the ODH of propane toward propylene.

Bypassing the Inertness of Aziridine/CO2 Systems to Access 5-Aryl-2 Oxazolidinones: Catalyst-Free Synthesis in Water Under Ambient Conditions

<div>The development of sustainable synthetic routes to access valuable oxazolidinones via CO2 fixation is </div><div>currently a hot topic of research, and the aziridine/carbon dioxide coupling has aroused a particular </div><div>interest. This reaction is featured by a high activation barrier, thus it requires a catalytic system and </div><div>often presents some other critical issues. Here, we describe the first gram-scale synthesis of a large </div><div>number of 5-aryl-2-oxazolidinones at ambient temperature and atmospheric CO2 pressure, in the </div><div>absence of any catalyst/co-catalyst and using water as solvent. The key to this innovative procedure </div><div>consists in the direct transfer of the CO2/amine adduct (carbamate) to common aziridine precursors </div><div>(dimethylsulfonium salts), replacing the classical sequential addition of amine (intermediate isolation </div><div>of aziridine) and then CO2. The reaction mechanism has been elucidated by NMR studies and DFT </div><div>calculations applied to model cases. </div>

The surprisingly high activation barrier for oxygen-vacancy migration in oxygen-excess manganite perovskites

Cation vacancies diminish the oxygen-vacancy diffusivity, raise the activation enthalpy, and cause the diffusivity to depend on oxygen activity.

Effects of Chiral Ligands on the Asymmetric Carbonyl-Ene Reaction

The carbonyl-ene reaction is one of the most well-known reactions for C–C bond formation. Based on frontier molecular orbitals (FMO), carbonyl-ene reactions occur between the highest occupied molecular orbital (HOMO) of the ene compound bearing an active hydrogen atom at the allylic center and the lowest unoccupied molecular orbital (LUMO) of the electron-deficient enophile, which is a carbonyl compound. A high activation barrier enforces the concerted ene reaction rather than a Diels–Alder reaction at high temperature. Employing a catalytic system can eliminate defects in the ene reaction, and chiral catalysts promote the reaction under mild conditions to produce optically active compounds. In this account, we highlight investigations on the effects of various classes of chiral ligands on intermolecular and intramolecular carbonyl-ene reactions.1 Introduction2 Biaryl-Type Chiral Ligands3 C 1- and C 2-Symmetric Bis(oxazoline) Ligands4 Schiff Base Ligands5 N,N′-Dioxide Ligands6 Conclusions

Inside-protonated 1-azaadamantane: computational studies on the structure, stability, and generation

Post Hartree–Fock and density functional theory methods have been employed to study inside-protonated 1-azaadamantane 7 and its complexes with the fluoride counterion (contact ion pairs) 10 and 11. The study also involved 1-azaadamantane 4, its outside-protonated form 8, and 1-azaadamantane radical cation 17. Inside-protonated 1-azaadamantane 7 is more than 82 kcal mol−1less stable than out-isomer 8. The repulsive interaction between the internal N+–H group and the azaadamantane cage and a substantial deformation of this cage greatly weaken the C–N and C–C bonds and, consequently, lead to a low kinetic stability of in-ion 7 in the studied unimolecular and bimolecular reactions involving the removal of the encapsulated proton from the cage. Among these reactions, a 7 → 8 rearrangement through a reversible cage opening at the C–N bond was found to be the main transformation channel ([Formula: see text] < 16 kcal mol−1) for in-ion 7. This rearrangement can be catalyzed by an external base, e.g., the fluoride anion. A 1,4-hydrogen migration in 1-azaadamantane radical cation 17 as a possible pathway to the inside-protonated 1-azaadamantane 7 was explored. It was found that this process has a prohibitively high activation barrier, [Formula: see text] > 104 kcal mol−1, and is not able to compete with the α-C–C cleavage of the azaadamantane cage ([Formula: see text] < 26 kcal mol−1).