Exploring the evolution of magnesium oxidation mechanisms by density functional theory

Mechanism and activity of CO oxidation on (001) and (110) surfaces of spinel Co3O4, NiCo2O4 and NiFe2O4: A DFT + U study

A series of zeolite catalysts, M(Cu, Fe, Co)-ZSM-5, was prepared by an impregnation method and evaluated for the selective catalytic combustion of acrylonitrile (AN-SCC). Cu-ZSM-5, exhibiting the highest AN conversion activity and best N2 yield, was further selected for an AN-SCC mechanism investigation, wherein both experimental [in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)] and theoretical [density functional theory (DFT)] approaches were employed. The in situ DRIFTS revealed that AN-SCC followed a hydrolysis mechanism at T < 300 °C via intermediates of acylamino species (-CONH2) and NH3, while it followed an oxidation mechanism at T > 300 °C via an intermediate of NCO. The DFT simulations gave much deeper insights suggesting that: (i) the NCO could be generated by oxidation of AN over [Cu]+ active sites, with an assistance of dissociated atomic O from gaseous O2; (ii) three types of reaction routes could be proposed for the further reaction of NCO to produce N2, namely NCO direct dissociation, NCO coupling, and NO + NCO reaction; and (iii) the last route (NO + NCO), possessing the lowest energy barrier, was the most probable reaction pathway, wherein the NO could be produced by oxidation of NCO. The DFT energy calculation results and microkinetic analyses revealed that the NCO generation step, possessing an energy barrier of 17.0 kcal mol-1 and a forward reaction rate constant of 2.20 × 107 s-1, was the rate-determining step of the whole catalytic cycle.

Butadiyne (diacetylene, HC(4)H) is produced during combustions, and has been quantified in different flames as well as a biomass burning emission. Its reaction with the hydroxyl radical, HO((2)Π(3/2)), under combustion conditions, was investigated in a thorough RRKM study by J. P. Senosiain, S. J. Klippenstein, and J. A.Miller (Proc. Combust. Inst. 2007, 31, 185−192). The present densityfunctional theory (DFT) study focuses on the mechanism of further oxidation by O(2)(3Σ(g)(−)). The DFT(M06-2X)/cc-pVTZ reaction energy hypersurface for the system C(4)H(2)/HO•/O(2) is studied to define a variety of reaction pathways, and the relevant thermochemistry for temperatures ranging from 200 to 2500 K is assessed, thus encompassing combustive, postcombustive, and tropospheric conditions.Energies are then recomputed at the coupled cluster level[CCSD(T)/cc-pVTZ], and combined with the DFT thermochemistry.Finally, the role of the different reaction channels is assessed by RRKM-ME simulations in the same temperature range for P = 1 atm,to comprise the situation of emission in the troposphere and those pertaining to different flames. This shows that, when considering HO addition to the triple bond, dioxygen takes part in C(4)H(2) oxidation with higher efficiency at lower temperatures, whereas, as T rises, the O(2) adducts are inclined to redissociate: for instance, a 50% redissociation is estimated at T = 1800 K. For 200 < T < 1100 K, two polycarbonyl products (CHO.CO.CCH and CHO.CO.CHCO) and two fragmentation products (HCOOH plus OC(•)−CCH) are the main species predicted as products from the addition channel (fragmentation is entropy-favored by higher T values). However, at higher temperatures, aninitial H abstraction by HO can give the but adiynyl radical (HC(4)(•)) as the starting point for subsequent dioxygen intervention.Then, new pathways opened by O(2) addition become accessible and bring about fragmentations mainly to HC(3)(•) + CO(2) and also to HC(3)(•)O + CO.

Density functional theory study of the selective oxidation of 5-Hydroxymethylfurfural (HMF) to 5-Hydroxymethyl-2-furancarboxylic acid (HMFCA) on the Silver oxide surface (001)

In order to investigate and analyze the effect of MgO gangue on the surface adsorption and oxidation behavior of magnetite, the physicochemical properties of substances in the magnetite oxidation process are investigated in this study by using thermogravimetric experiments, density functional theory, and ab initio molecular dynamics methods (AIMD) methods. The oxidation mechanism of magnetite and the inhibition mechanism of magnesium oxide on the oxidation properties of magnetite are elucidated. The results of the thermogravimetric experimental study show that the initial oxidation temperature of magnetite tends to increase with the increase of MgO content. At the same time, the presence of MgO leads to the migration of oxidation reactions to the high‐temperature region. AIMD study shows that the presence of the gangue element Mg prolongs the adsorption and dissociation time of O2 molecules on the surface and reduces the interfacial chemical reaction rate of Fe3O4. Moreover, the chemical bonds formed in the system after doping Mg atom are more stable, which is not conducive to the migration of oxygen atoms between Fe3O4 crystal structures, thus affecting the comprehensive oxidation performance of minerals.

Density functional theory (DFT) calculations have been performed to investigate the catalytic mechanism for the oxidation of veratryl alcohol to veratraldehyde by Cu–phen catalyst. In this work, the most favorable mechanism has been predicted and we hope the obtained results could provide useful insights for the reaction process.

In order to investigate the molecular mechanisms of the heat-induced cis/trans isomerization and oxidative cleavage of trilinolein, a highly purified sample was heated at a range of temperatures (120, 140, 160, 180, 200, 220 °C) for 5 h. The reaction process of cis/trans isomerization of C18:2 was studied by a combination of the gas chromatogram (GC) method with density functional theory (DFT). When trilinolein was heated to 180 °C, a small amount of trans/trans C18:2 was obtained (0.074 mg g−1). As the temperature increased to 220 °C, the amount of trans C18:2 reached 0.198 mg g−1. This study shows that C18:2-9c12t and C18:2-9t12c were the main trans fatty acids in heated trilinolein. The molecular mechanisms of isomerization and oxidative cleavage were verified by Gaussian 09 W software. All the geometry was optimized using DFT at the B3LYP/6-31*G level. The energy difference between cis and trans linoleic acid was equal to 6.2 kJ mol−1. Therefore, vegetable oil with a higher linoleic acid content should be maintained at 140 °C or less to avoid the formation of trans linoleic acid.

The antiradical properties and possible mechanisms of action of the tautomers of curcumin, caffeic acid phenethyl ester (CAPE), and chicoric acid (CA) have been studied within density functional theory (DFT). We calculated global chemical reactivity descriptors from conceptual DFT, pKa, bioavailability, and toxicity to evaluate the antiradical properties and characterize these species. Our final level of theory is the M06-2X functional with the 6-31 + G* basis set; we selected this level after performing a benchmark calibration and validation among different levels. Solvent effects were modeled via the continuum solvation model based on density (SMD). We used water and pentyl ethanoate as solvents to simulate the physiological conditions. The free radical scavenger capacity was analyzed for three possible oxidative stress mechanisms: single electron transfer (SET), hydrogen atom transfer (HAT), and xanthine oxidase (XO) inhibition. The results indicate that neutral curcumin, CA, and CAPE behave as antireductants. The most favorable mechanism turns out to be HAT, where CA and CAPE stand out. In conclusion, our DFT study strongly indicates that neutral curcumin, CAPE, and CA would very likely perform well as antiradical drugs with recommended therapeutic use, supported by their non-toxic nature.

Since the introduction of Lithium-ion batteries (LIBs) into commercial use, great improvements have been achieved in their performance such as energy density, durability, and safety. However, there still remain many technical issues to meet the increasing demands for a longer driving range of electric vehicles, a larger storage for renewable energy. Research and development of electrode active materials are extensively progressing to respond those market needs, and R&D for electrolyte solutions are also active in order to utilize the fullest extent of the newly developed electrode materials.Various kinds of characteristics are required for electrolyte solutions in LIBs. Ideally, the electrochemical stability in a wide potential range is needed, but almost all the electrolyte solutions reductively decompose on negative electrodes, and the decomposition products precipitate on the electrode to form a protective surface film, which is referred to as SEI (solid electrolyte interphase). Because the nature of SEI is influenced by chemical structures of electrolyte solutions, the solvents and lithium salts need to be combined properly to realize LIBs with better performance. Although the conventional electrolyte solutions show satisfactory performance to some extent, there still remain some disadvantages; instability against moisture and temperature, and volatility and flammability of carbonate ester solvents. Great efforts have been made to overcome those disadvantages in the conventional electrolyte solutions, for example, utilization of ionic liquid, polymer electrolytes and inorganic solid state electrolytes, and recently, highly concentrated electrolyte solution gathers a lot of attention.Based on these background, solid electrolyte interphase (SEI) formed on graphite in conventional and highly concentrated electrolyte solutions was thoroughly characterized by a combined experimental and computational study. The comprehensive understanding revealed that a chemical composition of SEI, as well as the solvation structures unstable to reductive potential, can be predicted by a profound understanding of density functional theory (DFT) calculation results of the solvates containing a counter anion. The solvation structures were determined by Raman spectroscopy to evaluate electron affinity (EA) and LUMO of the solvates containing an anion by DFT calculation. The chemical composition of SEI was quantitatively analyzed by X-ray photoelectron spectroscopy (XPS), and the results were compared with a prediction based on the calculation. The formation mechanism of SEI during charge and discharge process can be partly estimated by the combination of experiment and theoretical calculation. These results indicate that electrolyte solutions can be efficiently designed by predicting the physicochemical properties of SEI through the more effective utilization of DFT calculation. Thus, a combination of DFT prediction and experimental analysis is proved to be an effective approach to reveal SEI formation mechanisms, and can be utilized as a versatile approach to develop new solvents, electrolyte salts, and additives, and to design electrolyte solutions with appropriate concentration.It is expected that the oxidative stability and reaction mechanism of electrolyte solutions at the positive electrode can be estimated using similar approaches. Here, it is necessary to predict oxidative stability using ionization potentials (IP), rather than EA. In the same way, other solvents than carbonate, such as ethers and sulfones, which have been difficult to put into practical use in conventional electrolytes, can also be the target to be studied. By analyzing the solvation structure and estimating its chemical stability, we believe it will be possible to get many insights on the reactions that proceed at the interface between the electrode and the electrolyte. Figure 1

Global energy demands are predicted to increase through 2040. In the spirit of meeting these demands, work focusing on increasing the efficiency of existing energy technologies, as well as improving energy storage is necessary. This work takes a catalytic approach to these challenges, focusing on Co, Rh, and Ir catalysts with pincer and bipyridine ligands. Density functional theory (DFT) can be used in order to gain a deeper understanding of how these catalysts behave. In the realm of improving existing technologies, the mechanism for oxidation of methane to methanol by Phebox Ir (Phebox = bis(oxazolinyl)phenyl) is investigated with a focus on understanding how subtle substitutions to the ligand can help or hinder this reaction. It is shown that in this catalyst, two unwanted intermediates on the potential energy surface (an IrIV state leading to catalyst deactivation and an IrV state leading to over-oxidation) can potentially be avoided by adding trifluoromethyl groups to the ligand. For production of fuels from solar energy, two reactions are studied. Experimentally, CO2 reduction to formate by (POCOP)Ir (POCOP = C6H3-2,6-[OP(tBu)2]2) has been shown to selectively occur at moderate potentials. The mechanism by which this catalyst reduces CO2 is elucidated. In particular, the impressive product selectivity afforded this catalyst for formate over hydrogen production is rooted in kinetics: high barriers for protonation inhibit the creation of H2 adducts. In addition to this, substitutions to the ligand and metal center are investigated to further illuminate the relationship between kinetics and thermodynamics. Hydrogen evolution in Cp*Rh(bpy) (bpy = 2,2'-bipyridine, Cp* = pentamethylcyclopentadienyl) is investigated, centering on unexpected protonation at the Cp* ligand rather than the metal center. This state is on the path for hydrogen evolution in the case of using weak acids, but in the presence of strong acids, the path through the traditional hydride is most likely. Finally, the attachment of these catalysts to electrode surfaces is discussed with the aim of making molecular catalysts a more viable option in industry It is shown that chlorine present in the attachment process enables easy catalyst dissociation from the surface. Several non-halogen options are discussed as replacements. Throughout the thesis two themes emerge: the constant interaction between thermodynamics and kinetics to control mechanistic paths and products, and the ability of small modifications to have huge impacts on catalytic cycles.

Anticancer drugs (ADs) have been detected in the environment and represent a risk to aquatic organisms, necessitating AD removal in drinking water and wastewater treatment. In this study, ozonation of the most commonly used antimetabolite ADs, namely 5-fluorouracil (5-FU) and its prodrug capecitabine (CAP), was investigated to determine reaction kinetics, oxidation mechanisms, and residual toxicity. The specific second-order rate constants between aqueous ozone and 5-FU, 5-FU−, 5-FU2−, CAP, and CAP− were determined to be 7.07(±0.11)×104 M−1·s−1, 1.36(±0.06)×106 M−1·s−1, 2.62(±0.17)×107 M−1·s−1, 9.69(±0.08)×103 M−1·s−1, and 4.28(±0.07)×105 M−1·s−1, respectively; furthermore, the second-order rate constants for •OH reaction with 5-FU and CAP at pH 7 were determined to be 1.85(±0.20)×109 M−1·s−1 and 9.95(±0.26)×109 M−1·s−1, respectively. Density functional theory was used to predict the main ozone reaction sites of 5-FU (olefin) and CAP (olefin and deprotonated secondary amine), and these mechanisms were supported by the identified transformation products. Carboxylic acids constituted a majority of the residual organic matter for 5-FU ozonation; however, carboxylic acids and aldehydes were important components of the residual organic matter generated by CAP. Ozone removed the toxicity of 5-FU to Vibrio fischeri, but the residual toxicity of ozonated CAP solutions exhibited an initial increase before subsequent removal. Ultimately, these results suggest that ozone is a suitable technology for treatment of 5-FU and CAP, although the residual toxicity of transformation products must be carefully considered.

Insight into the overpotential and thermodynamic mechanism of hydroxyl radical formation on diamond anode

High temperature intergranular oxidation and corrosion of metal alloys is one of the primary causes of materials degradation in nuclear systems. In order to gain insights into grain boundary oxidation processes, a mesoscale metal alloy oxidation model is established by combining quantum Density Functional Theory (DFT) and mesoscopic Poisson-Nernst-Planck/classical DFT with predictions focused on Ni alloyed with either Cr or Al. Analysis of species and fluxes at steady-state conditions indicates that the oxidation process involves vacancy-mediated transport of Ni and the minor alloying element to the oxidation front and the formation of stable metal oxides. The simulations further demonstrate that the mechanism of oxidation for Ni-5Cr and Ni-4Al is qualitatively different. Intergranular oxidation of Ni-5Cr involves the selective oxidation of the minor element and not matrix Ni, due to slower diffusion of Ni relative to Cr in the alloy and due to the significantly smaller energy gain upon the formation of nickel oxide compared to that of Cr2O3. This essentially one-component oxidation process results in continuous oxide formation and a monotonic Cr vacancy distribution ahead of the oxidation front, peaking at alloy/oxide interface. In contrast, Ni and Al are both oxidized in Ni-4Al forming a mixed spinel NiAl2O4. Different diffusivities of Ni and Al give rise to a complex elemental distribution in the vicinity of the oxidation front. Slower diffusing Ni accumulates in the oxide and metal within 3 nm of the interface, while Al penetrates deeper into the oxide phase. Ni and Al are both depleted from the region 3-10 nm ahead of the oxidation front creating voids. The oxide microstructure is also different. Cr2O3 has a plate-like structure with 1.2-1.7 nm wide pores running along the grain boundary, while NiAl2O4 has 1.5 nm wide pores in the direction parallel to the grain boundary and 0.6 nm pores in the perpendicular direction providing an additional pathway for oxygen diffusion through the oxide. The proposed theoretical methodology provides a framework for modeling metal alloy oxidation processes from first principles and on the experimentally relevant length scales.

Mechanism of formaldehyde oxidation catalyzed by doped graphene single atom catalysts: Density functional theory study

Complete reaction mechanisms of mercury oxidation on halogenated activated carbon