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Type-II g-GaN/Sc2CO2 van der Waals heterostructure with electronic properties has potential for nanoelectronics, optoelectronics and photovoltaic device applications.

The bandgap energy and band alignment of the CdS/SiH vdW heterostructure were modified by applying biaxial strain and an external electric field.

Polystyrene is an aromatic hydrocarbon plastic material with applications in the packaging, food, and construction industries. As a packaging material, it protects items against shocks and acts as an insulator in the construction of buildings and food preservation. The polystyrene bowl for food packaging has become a convention in the food and pastries industry in Ghana due to its clean, lightweight, and insulation properties. However, the corresponding waste litters its environment, chocks gutters, and does not decompose. For this reason, we explored the hot melt recycling technique to convert the PS wastes into valuable plastic plates that finds application in the artifact and souvenir industry. The incorporation of a 20% v/w lauric fatty acid (LFA) into the material matrix blocked micro holes in the finished product and induced a smooth surface. Characterizations included experiments such as Fourier Transform-Infra Red spectroscopy, melting point determination, tensile testing, and scanning electron microscopy (SEM). Generated data displayed the absence of a chemical bond between the PS molecules and the LFA additive. However, the methyl chains of the PS molecules were miscible with the saturated dodecyl chains of the LFA additive. Hence, the LFA additive was held in place by weak intermolecular forces between its chains and that of the PS molecules. The measured melting point of the recycled plastic with LFA content (rPS20) fell between 84-142 °C. Mechanically, the tensile strength dropped with the addition of LFA. The LFA induced a smooth surface morphology finishing in rPS20.

Understanding the mechanism of CO2 reduction on iron is crucial for the design of more efficient and cheaper iron electrocatalyst for CO2 conversion. In the present study, we have employed spin-polarized density functional theory calculations within the generalized gradient approximation (DFT-GGA) to elucidate the mechanism of CO2 reduction into carbon monoxide and formic acid on the Fe (100) facet. We also sort to understand the transformations of the other isomers of adsorbed CO2 on iron as earlier mechanistic studies are centred on the transformations of the C2v geometry alone and not the other possible conformations i.e., flip-C2v and Cs modes. Two alternative reduction routes were considered i.e., the direct CO2 dissociation against the hydrogen-assisted CO2 transformation through formate and carboxylate into CO and formic acid. Our results show that CO2 in the C2v mode is the precursor to the formation of both products i.e., CO and formic acid. Both the formation and transformation of CO2 in the Cs and flip-C2v is challenging kinetically and thermodynamically compared to the C2v mode. The formic acid formation is favoured over CO via the reverse water gas shift reaction mechanism on Fe (100). Both formic acid formation and CO formation will proceed via the carboxylate intermediate since formate is a stable intermediate whose transformation into formic acid is challenging both kinetically and thermodynamically. Graphic abstract

Insight into the detailed mechanism of the Sabatier reaction on iron is essential for the design of cheap, environmentally benign, efficient and selective catalytic surfaces for CO2 reduction. Earlier attempts to unravel the mechanism of CO2 reduction on pure metals including inexpensive metals focused on Ni and Cu; however, the detailed mechanism of CO2 reduction on iron is not yet known. We have, thus, explored with spin-polarized density functional theory calculations the relative stabilities of intermediates and kinetic barriers associated with methanation of CO2 via the CO and non-CO pathways on the Fe (111) surface. Through the non-CO (formate) pathway, a dihydride CO2 species (H2CO2), which decomposes to aldehyde (CHO), is further hydrogenated into methoxy, methanol and then methane. Through the CO pathway, it is observed that the CO species formed from dihydroxycarbene is not favorably decomposed into carbide (both thermodynamically and kinetically challenging) but CO undergoes associative hydrogenation to form CH2OH which decomposes into CH2, leading to methane formation. Our results show that the transformation of CO2 to methane proceeds via the CO pathway, since the barriers leading to alkoxy transformation into methane are high via the non-CO pathway. Methanol formation is more favored via the non-CO pathway. Iron (111) shows selectivity towards CO methanation over CO2 methanation due to differences in the rate-determining steps, i.e., 91.6 kJ mol−1 and 146.2 kJ mol−1, respectively.

The oxidation of ethylene catalyzed by manganese and technetium oxo complexes of the type MO

Iron and nickel are known active sites in the enzyme carbon monoxide dehydrogenases (CODH) which catalyzes CO2 to CO reversibly. The presence of nickel impurities in the earth abundant iron surface could provide a more efficient catalyst for CO2 degradation into CO, which is a feedstock for hydrocarbon fuel production. In the present study, we have employed spin-polarized dispersion-corrected density functional theory calculations within the generalized gradient approximation to elucidate the active sites on Fe (100)-Ni bimetals. We sort to ascertain the mechanism of CO2 dissociation to carbon monoxide on Ni deposited and alloyed surfaces at 0.25, 0.50 and 1 monolayer (ML) impurity concentrations. CO2 and (CO + O) bind exothermically i.e., -0.87 eV and − 1.51 eV respectively to the bare Fe (100) surface with a decomposition barrier of 0.53 eV. The presence of nickel generally lowers the amount of charge transferred to CO2 moiety. Generally, the binding strengths of CO2 were reduced on the modified surfaces and the extent of its activation was lowered. The barriers for CO2 dissociation increased mainly upon introduction of Ni impurities which is undesired. However, the 0.5 ML deposited (FeNi0.5(A)) surface is promising for CO2 decomposition, providing a lower energy barrier (of 0.32 eV) than the pristine Fe (100) surface. This active 1-dimensional defective FeNi0.5(A) surface provides a stepped surface and Ni-Ni bridge binding site for CO2 on Fe (100). Ni-Ni bridge site on Fe (100) is more effective for both CO2 binding or sequestration and dissociation compared to the stepped surface providing the Fe-Ni bridge binding site.

The oxygen adsorption and subsequent reduction on the {100} and {110} surfaces of 25% Ba-doped LaMnO3 (LBM25) have been studied at the density functional theory (DFT) with Hubbard correction and the results compared with adsorption on 25% Ca-doped LaMnO3 (LCM25) and Sr-doped LaMnO3 (LSM25). The trend in the reduction energies at the Mn cation sites are predicted to be in the order LSM25 2 precursors at the Mn cation sites of LCM25, LSM25 and LBM25 are thermodynamically stable, when compared directly with the adsorption energies (Eads = − 0.56 to − 1.67 eV) reported for the stable molecular O2 precursors on the Pt, Ni, Pd, Cu and Ir {111} surfaces. The predicted Gibbs energies as a function of temperature (T = 500–1100 °C) and pressures (p = 0.2 atm) for the adsorption and dissociation on the surfaces were negative, an indication of the feasibility of oxygen reduction reaction on the {100} and {110} surfaces at typical operating temperatures reported in this work.

Graphene despite its high surface area has very limited activity towards the oxygen reduction reaction (ORR), demonstrating selectivity towards the unfavorable two-electron mechanism. We have employed the spin polarized density functional theory (DFT) method to investigate the effect of the heteroatom p-type beryllium (Be) dopant on the oxygen reduction reaction activity of graphene. The preferred doping sites, active sites and reaction mechanism available on the doped graphene surfaces were investigated with increasing Be concentrations of 0.03 ML, 0.06 ML and 0.09 ML. Our results reveal that oxygen does not bind strongly to bare graphene, and Be at the lattice sites provides site for the oxygen adsorption and ORR. Oxygen binds dissociatively on the doped surfaces preferentially in the order 0.06 ML > 0.09 ML > 0.03 ML. From this studies introduction of Be impurities in a single honeycomb ring of graphene has significant impact on the binding and adsorbate-adsorbent interactions which leads to dissociative adsorption of oxygen, favouring the 4e- ORR pathway. The reaction is kinetically favoured most on the surface with a stronger O* binding and weaker OH* binding. Overall, the 0.03 ML and 0.06 ML doped surfaces are the most active facets for the ORR showing exergonic reaction energies at 0 V.

Despite series of experimental studies that reveal unique activities of late 3d transition metals and their role in microorganisms known for CO2 conversion, these surfaces are not industrially viable yet. An insight into the elementary steps of surface catalytic processes is crucial for effective surface modification and design. The mechanisms of CO2 transformation into CO, through the reverse water gas shift and methane reforming, are being studied. Mechanisms of CO2 methanation is also being explored by the Sabatier reaction into methane. This review covers both experimental and theoretical studies into the mechanisms of CO2 reduction into CO and methane, on single metals and bimetals of late 3d transition metals, i.e. Fe, Co, Ni, Cu and Zn. This paper highlights progress and gaps still existing in our knowledge of the reaction mechanisms. These mechanistic studies reveal CO2 activation and reduction mechanisms are specific to both composition and surface facet. Surfaces with least CO2 binding potential are seen to favour CO and O binding and provide higher barriers to dissociation. No direct correlation has been seen between binding strength of CO2 and its degree of activation. Hydrogen-assisted dissociation is seen to be generally favoured kinetically on Cu and Ni surfaces over direct dissociation except on the Ni (211) surface. Methane production on Cu and Ni surfaces is seen to occur via the non-formate pathway. Hydrogenation reactions have focused on Cu and Ni, and more needs to be done on other surfaces, i.e. Co, Fe and Zn.

Despite series of experimental studies that reveal unique activities of late 3d transition metals and their role in microorganisms known for CO

Transition-metal-mediated oxygen transfer reactions are of importance in both industry and academia; thus, a series of rhenium oxo complexes of the type ReO3L (L = O−, Cl−, F−, OH−, Br−, I−) and their effects as oxidation catalysts on ethylene have been studied. The activation and reaction energies for the addition pathways involving multiple spin states (singlet and triplet) have been computed. In all cases, structures on the singlet potential energy surfaces showed higher stability compared to their counterparts on the triplet potential energy surfaces (PESs). Frontier Molecular Orbital calculations show electrons flow from the HOMO of ethylene to the LUMO of rhenium for all complexes studied except ReO4− where the reverse case occurs. In the reaction between ReO3L (L = O−, Cl−, F−, OH−, Br−, and I−) and ethylene, the concerted [3 + 2] addition pathway on the singlet PES leading to the formation of dioxylate intermediate is favored over the [2 + 2] addition pathway leading to the formation of a metallaoxetane intermediate and subsequent rearrangement to the dioxylate. The activation and the reaction energies for the formation of the dioxylate on the singlet PES for the ligands studied followed the order O− > OH− > I− > F− > Br− > Cl− and O− > OH− > F− > I− > Br− > Cl−, respectively. Furthermore, the activation and the reaction energies for the formation of the metallaoxetane intermediate increase in the order O− > OH− > I− > Br− > Cl− > F− and O− > Br− > I− > Cl− > OH− > F−, respectively. The subsequent rearrangement of the metallaoxetane intermediate to the dioxylate is only feasible in the case of ReO4−. Of all the complexes studied, the best dioxylating catalyst is ReO3Cl (singlet surface) and the best epoxidation catalyst is ReO3F (singlet surface).

The mechanisms of LMnO3 (L = O−, Cl, NPH3, CH3, and Cp)-catalyzed oxidation of ethyne has been studied on the singlet and triplet hypersurfaces at the M06/6-311G(d) level of theory. For the first step, the [3 + 2] pathways to the formation of the metalla-2,5-dioxol-3-ene intermediate are kinetically and thermodynamically the most favored pathways in all the complexes studied; it is favored over the [2 + 2] addition pathways to the metallaoxetene intermediate. The formation of the oxirene precursor that could give the oxirene the reported key intermediates in the ozonolysis of alkynes would most likely result from the oxidation of ethyne by MnO3Cl on the triplet potential energy surface (PES).

Lake Bosomtwe in the Ashanti Region of Ghana is one of the largest naturally occurring lake in West Africa. The lake is surrounded by about 36 farming and fishing communities that use synthetic pesticides and herbicide in the cultivation of crops. These chemicals are likely to leak into the tributaries, contaminate the lake, and threaten the life of the aquatic animals. For this reason, a study of the levels of the active components of the pesticides in the lake were measured to monitor the water quality. A Gas Chromatographic equipped with Electron Capture Detector (GC-ECD) was employed in the measurements and levels of organochlorine, organophosphorus and pyrethoids. The GC-ECD analysis revealed that, the mean concentrations of organochlorines in the water sample were, Beta – HCH (0.03µg/L), Gamma – HCH (0.015 µg/L), Delta – HCH (0.012 µg/L), Endrin (0.014 µg/L), Beta-endosulfan (0.01 µg/L), Endosulfan (0.028 µg/L), and p, p’- DDT (0.012 µg/L). Heptachlor, Alpha-endosulfan, p,p’- DDE and Lambda-cyhal had concentrations less than 0.01 µg/L. Chloropyrifos was the only organophosphate detected with a mean concentration of 0.013 µg/L. The mean concentrations of the pyrethoids present were, permethrin 0.024 µg/L, cyfluthrin 0.016 µg/L and deltamethrin 0.017µg/L. A comparison of the measured concentrations with the WHO pesticides limit in water displayed lower levels and compliance to the standards. Detection of these pesticides residues indicates that, regulating farming activities along the boundaries of the lake will reduce the leakages with corresponding positive effect on the aquatic life.

The mechanisms of the oxidation of tetramethylethylene (TME) by permanganyl chloride (MnO3Cl) have been explored on the singlet and triplet potential energy surfaces at the B3LYP LANL2DZ/6-31G (d) level of theory. The results show that the pathway leading to the formation of the five-membered dioxylate through concerted [3 + 2] addition is favored kinetically and thermodynamically over the three other possible pathways, namely the [2 + 2] addition via the transient metallaoxetane intermediate, epoxidation, and hydrogen transfer pathways. The epoxide precursor that on hydrolysis would yield the epoxide product will most likely arise from a stepwise path through the intermediacy of an organometallic intermediate. This pathway affords the product that is more stable (thermodynamically favorable). However, kinetically, both the stepwise and the concerted [2 + 1] addition pathways leading to the epoxide precursors are very competitive (activation barrier difference of

We present a Hubbard-corrected density functional theory (DFT+U) study of the adsorption and reduction reactions of oxygen on the pure and 25% Ca-doped LaMnO3 (LCM25) {100} and {110} surfaces. The effect of oxygen vacancies on the adsorption characteristics and energetics has also been investigated. Our results show that the O2 adsorption/reduction process occurs through the formation of superoxide and peroxide intermediates, with the Mn sites found to be generally more active than the La sites. The LCM25{110} surface is found to be more efficient for O2 reduction than the LCM25{100} surface due to its stronger adsorption of O2, with the superoxide and peroxide intermediates shown to be energetically more favorable at the Mn sites than at the Ca sites. Moreover, oxygen vacancy defect sites on both the {100} and {110} surfaces are shown to be more efficient for O2 reduction, as reflected in the higher adsorption energies calculated on the defective surfaces compared to the perfect surfaces. We show from Löwdin population analysis that the O2 adsorption on the pure and 25% Ca-doped LaMnO3 surfaces is characterized by charge transfer from the interacting surface species into the adsorbed oxygen πg orbital, which results in weakening of the O-O bonds and its subsequent reduction. The elongated O-O bonds were confirmed via vibrational frequency analysis.

We present a Hubbard-corrected density functional theory (DFT+U) study of the adsorption and reduction reactions of oxygen on the pure and 25% Ca-doped LaMnO3 (LCM25) {100} and {110} surfaces. The effect of oxygen vacancies on the adsorption characteristics and energetics has also been investigated. Our results show that the O2 adsorption/reduction process occurs through the formation of superoxide and peroxide intermediates, with the Mn sites found to be generally more active than the La sites. The LCM25{110} surface is found to be more efficient for O2 reduction than the LCM25{100} surface due to its stronger adsorption of O2, with the superoxide and peroxide intermediates shown to be energetically more favorable at the Mn sites than at the Ca sites. Moreover, oxygen vacancy defect sites on both the {100} and {110} surfaces are shown to be more efficient for O2 reduction, as reflected in the higher adsorption energies calculated on the defective surfaces compared to the perfect surfaces. We show from Lowdin population analysis that the O2 adsorption on the pure and 25% Ca-doped LaMnO3 surfaces is characterized by charge transfer from the interacting surface species into the adsorbed oxygen πg orbital, which results in weakening of the O–O bonds and its subsequent reduction. The elongated O–O bonds were confirmed via vibrational frequency analysis.

The mechanisms of oxidation of ethylene by transition metal-oxo complexes of the type LTcO 3 (L = O − , Cl, CH 3 , OCH 3 , Cp, NPH 3 ) have been explored by computing the activation barriers and reaction energies for the concerted and stepwise addition pathways at the density functional theory B3LYP/LACVP * level of theory. The results indicate that in the reaction of LTcO 3 (L = O − , Cl, CH 3 , OCH 3 , Cp, NPH 3 ) with ethylene, the formation of the dioxylate intermediate through the concerted [3 + 2] addition pathway on the singlet potential energy surface is favored kinetically and thermodynamically over its formation through the two-step process via the metallaoxetane intermediate. The activation barrier for the formation of the dioxylate on the singlet PES for the ligands studied is found to follow the order: O − > CH 3 > NPH 3 > CH 3 O − > Cl − > Cp while the reaction energies follow the order: Cl − > O − > CH 3 > NPH 3 > CH 3 O − > Cp. On the doublet PES, the [2 + 2] addition leading to the formation of the four-membered metallacycle intermediate is favored kinetically and thermodynamically for the ligands when L = NPH 3 . The direct [2 + 1] addition of ethylene across the oxo- ligand of doublet TcO 3 (CH 3 ) to form the epoxide precursor is favored when L = CH 3 . The activation barriers for the formation of the dioxylate intermediate are found to follow the order: Cl − 3 O − 3 whiles the reaction energies follow the order Cl − 3 O − 3 . The re-arrangement of the metallaoxetane intermediate to the dioxylate is not a feasible pathway for the formation of the dioxylate. The formation of the epoxide precursor will not result from the reaction of LTcO 3 (L = O − , Cp) with ethylene on all the surfaces explored. There does not appear to be a spin-crossover in any of the pathways studied.

The periselectivity and chemoselectivity of the addition of transition metal oxides of the type ReO3L (L = Cl, CH3, OCH3 and Cp) to ethenone have been explored at the MO6 and B3LYP/LACVP* levels of theory. The activation barriers and reaction energies for the stepwise and concerted addition pathways involving multiple spin states have been computed. In the reaction of ReO3L (L = Cl(-), OCH3, CH3 and Cp) with ethenone, the concerted [2 + 2] addition of the metal oxide across the C=C and C=O double bond to form either metalla-2-oxetane-3-one or metalla-2,4-dioxolane is the most kinetically favored over the formation of metalla-2,5-dioxolane-3-one from the direct [3 + 2] addition pathway. The trends in activation and reaction energies for the formation of metalla-2-oxetane-3-one and metalla-2,4-dioxolane are Cp Cl(-) OCH3 CH3 and Cp OCH3 CH3 Cl(-) and for the reaction energies are Cp OCH3 Cl(-) CH3 and Cp CH3 OCH3 Cl CH3. The concerted [3 + 2] addition of the metal oxide across the C=C double of the ethenone to form species metalla-2,5-dioxolane-3-one is thermodynamically the most favored for the ligand L = Cp. The direct [2 + 2] addition pathways leading to the formations of metalla-2-oxetane-3-one and metalla-2,4-dioxolane is thermodynamically the most favored for the ligands L = OCH3 and Cl(-). The difference between the calculated [2 + 2] activation barriers for the addition of the metal oxide LReO3 across the C=C and C=O functionalities of ethenone are small except for the case of L = Cl(-) and OCH3. The rearrangement of the metalla-2-oxetane-3-one-metalla-2,5-dioxolane-3-one even though feasible, are unfavorable due to high activation energies of their rate-determining steps. For the rearrangement of the metalla-2-oxetane-3-one to metalla-2,5-dioxolane-3-one, the trends in activation barriers is found to follow the order OCH3 Cl(-) CH3 Cp. The trends in the activation energies for the most favorable [2 + 2] addition pathways for the LReO3-ethenone system is CH3 CH3O(-) Cl(-) Cp. For the analogous ethylene-LReO3 system, the trends in activation and reaction energies for the most favorable [3 + 2] addition pathway is CH3 CH3O(-) Cl(-) Cp [10]. Even though the most favored pathway in the ethylene-LReO3 system is the [3 + 2] addition pathway and that on the LReO3-ethenone is the [2 + 2] addition pathway, the trends in the activation energies for both pathways are the same, i.e. CH3 CH3O(-) Cl(-) Cp. However, the trends in reaction energies are quite different due to different product stabilities. The formation of the acetic acid precursor through the direct addition pathways was unsuccessful for all the ligands studied. The formation of the acetic acid precursor through the cyclization of the metalla-2-oxetane-3-one is only possible for the ligands L = Cl(-), CH3 whiles for the cyclization of metalla-2-oxetane-4-one to the acetic acid precursor is only possible for the ligand L = CH3. Although there are spin-crossover reaction observed for the ligands L = Cl(-), CH3 and CH3O(-), the reactions occurring on the single surfaces have been found to occur with lower energies than their spin-crossover counterparts.

The oxo complexes of group VII B are of great interest for their potential toward epoxidation and dihydroxylation. In this work, the mechanisms of oxidation of ethylene by rhenium-oxo complexes of the type LReO3 (L = O−, Cl, CH3, OCH3, Cp, NPH3) have been explored at the B3LYP/LACVP* level of theory. The activation barriers and reaction energies for the stepwise and concerted addition pathways involving multiple spin states have been computed. In the reaction of LReO3 (L = O−, Cl, CH3, OCH3, Cp, NPH3) with ethylene, it was found that the concerted [3 + 2] addition pathway on the singlet potential energy surfaces leading to the formation of a dioxylate intermediate is favored over the [2 + 2] addition pathway leading to the formation of a metallaoxetane intermediate and its re-arrangement to form the dioxylate. The activation barrier for the formation of the dioxylate on the singlet PES for the ligands studied is found to follow the order O− > CH3 > NPH3 > CH3O− > Cl− > Cp and the reaction energies follow the order CH3 > O− > NPH3 > CH3O− > Cl− > Cp. On the doublet PES, the [2 + 2] addition leading to the formation the metallaoxetane intermediate is favored over dioxylate formation for the ligands L = CH3, CH3O−, Cl−. The activation barriers for the formation of the metallaoxetane intermediate are found to increase for the ligands in the order CH3 < Cl− < CH3O− while the reaction energies follow the order Cl− < CH3O− < CH3. The subsequent re-arrangement of the metallaoxetane intermediate to the dioxylate is only feasible in the case of ReO3(OCH3). Of all the complexes studied, the best dioxylating catalyst is ReO3Cp (singlet surface); the best epoxidation catalyst is ReO3Cl (singlet surface); and the best metallaoxetane formation catalyst is ReO3(NPH3) (triplet surface).