The Carbonate-catalyzed Transesterification of Sunflower Oil for Biodiesel Production: in situ Monitoring and Density Functional Theory Calculations

Biodiesel has emerged as a promising alternative fuel to replace dwindling fossil-based resources, particularly in view of its added environmental merit of  reducing additional air pollution. Its specific attraction stems from the similarity of its physical properties to fossil fuel-derived diesel. Although the  production of biodiesel is a relatively straightforward process, reaction progress monitoring and product analysis require costly specialist equipment,  such as gas chromatography and mass spectrometry. In this study, we investigate the use of pH in monitoring the progress of carbonate-catalyzed  transesterification reactions. Specifically, we focus on potassium and sodium carbonates and sunflower oil. Our results are consistent with the results  obtained by other studies using different methods of monitoring. To test the generality of the method, pH measurements were also used to monitor the  progress of the potassium carbonate transesterification reaction in the presence of added water, glycerol and gamma-valerolactone (GVL). The obtained  results are as expected, with a limited amount of water increasing the transesterification rate; glycerol slowing the reaction slightly in accord with Le  Chatellier’s principles; and GVL increasing the rate due to co-solvent effects. Atomic-level insights into the adsorption mechanism of methanol and water  on the (001) surfaces of Na2CO3 and K2CO3 catalysts are provided by first-principles DFT calculations, which explain the increase in transesterification    reaction rate upon the addition of water

Exploring the Redox Properties of the Low-Miller Index Surfaces of Copper Tungstate (CuWO₄): Evaluating the Impact of the Environmental Conditions on the Water Splitting and Carbon Dioxide Reduction Processes

Photocatalysis has gained significant attention and interest as an environmentally friendly and sustainable approach to the production of hydrogen through water splitting and the reduction and conversion of CO2. Copper tungstate (CuWO4) is a highly promising candidate for these applications owing to its appropriate bandgap and superior stability under different conditions. However, the redox behavior of the CuWO4 surfaces under different environments and their impact on the morphology of the material nanoparticles, as well as the electronic properties, remain poorly understood. In this study, we have employed density functional theory calculations to investigate the properties of the bulk and pristine surfaces of CuWO4 and how the latter are impacted by oxygen chemisorption under the conditions required for photocatalytic water splitting and carbon dioxide reduction processes. We have calculated the lattice parameters and electronic properties of the bulk phase, as well as the surface energies of all possible nonpolar, stoichiometric, and symmetric terminations of the seven low-Miller index surfaces and found that the (010) and (110) facets are the thermodynamically most stable. The surface-phase diagrams were used to derive the equilibrium crystal morphologies, which show that the pristine (010) surface is prominent under synthesis and room conditions. Our crystal morphologies suggest that the partially oxidized (110) surface and the partially reduced (011) surface may play an important role in the photocatalytic splitting of water and CO2 conversion, respectively. Our results provide a comprehensive understanding of the CuWO4 surfaces under the conditions of important photocatalytic applications

Role of Interfacial Morphology in Cu₂O/TiO₂ and Band Bending: Insights from Density Functional Theory

Photocatalysis, a promising solution to environmental challenges, relies on the generation and utilization of photogenerated charge carriers within photocatalysts. However, the recombination of these carriers often limits efficiency. Heterostructures, especially Cu₂O/TiO₂, have emerged as effective solutions to enhance charge separation. This study systematically explores the effect of interfacial morphologies on the band bending within Cu₂O/TiO₂ anatase heterostructures by employing density functional theory. Through this study, eight distinct interfaces are identified and analyzed, revealing a consistent staggered-type band alignment. Despite variations in band edge positions, systematic charge transfer from Cu₂O to TiO₂ is observed across all interfaces. The proposed band bending configurations would suggest enhanced charge separation and photocatalytic activity under ultraviolet illumination due to a Z-scheme configuration. This theoretical investigation provides valuable insights into the interplay between interfacial morphology, band bending, and charge transfer for advancing the understanding of fundamental electronic mechanisms in heterostructures

The carbonate-catalysed transesterification of sunflower oil for biodiesel production: in situ monitoring and density functional theory calculations

Biodiesel has emerged as a promising alternative fuel to replace dwindling fossil-based resources, particularly in view of its added environmental merit of reducing additional air pollution. Its specific attraction stems from the similarity of its physical properties to fossil fuel-derived diesel. Although the production of biodiesel is a relatively straightforward process, reaction progress monitoring and product analysis require costly specialist equipment, such as gas chromatography and mass spectrometry. In this study, we investigate the use of pH in monitoring the progress of carbonate-catalyzed transesterification reactions. Specifically, we focus on potassium and sodium carbonates and sunflower oil. Our results are consistent with the results obtained by other studies using different methods of monitoring. To test the generality of the method, pH measurements were also used to monitor the progress of the potassium carbonate transesterification reaction in the presence of added water, glycerol and gamma-valerolactone (GVL). The obtained results are as expected, with a limited amount of water increasing the transesterification rate; glycerol slowing the reaction slightly in accord with Le Chatellier's principles; and GVL increasing the rate due to co-solvent effects. Atomic-level insights into the adsorption mechanism of methanol and water on the (001) surfaces of Na2CO3 and K2CO3 catalysts are provided by first-principles DFT calculations, which explain the increase in transesterification reaction rate upon the addition of water

Molecular-level understanding of interfacial carbonates in stabilizing CuO-ZnO(Al2O3) catalysts

A descriptor of active CuO-ZnO(Al2O3) methanol-synthesis and water–gas-shift catalysts is the presence of high-temperature carbonates (HT-CO3) in the oxidic catalyst precursor. Previous reports have shown that such HT-CO3 lead to an appropriate interaction between the oxides; as a result, a high Cu surface area (or Cu-Zn or Cu/ZnO interphase areas) can be achieved. Yet, their nature is not well understood. In this study, the nature of these carbonates was investigated by experimental and theoretical methods in the oxidic precatalyst. A calcined Cu-Zn-Al hydrotalcite model compound revealed to have well-dispersed ZnO and CuO phases, together with highly stable HT-CO3. It was hypothesized that these HT-CO3 groups may be placed at critical locations at nano-scale as a glue, thus avoiding the growth of the oxide crystallites during calcination. This is an excellent pre-condition to achieve a high Cu surface area (or Cu-Zn or Cu/ZnO interphase areas) upon reduction, and therefore a high activity. To prove that, first-principles calculations were carried out based on the density functional theory (DFT); alumina was not considered in the model as the experimental data showed it to be amorphous but it may still have an effect. Comprehensive calculations provided evidence that such carbonate groups favourably bind the CuO and ZnO together at the interface, rather than being isolated on the individual oxide surfaces. The results strongly suggest that the HT-CO3 groups are part of the structure, in the calcined precatalyst, where they would prevent thermal sintering through a bonding mechanism between CuO and ZnO particles, which is a novel interpretation of this important catalyst descriptor

Experimental and theoretical studies of the mechanism of oxidation of arsenopyrite in the presence of hydrogen peroxide

Arsenopyrite, which is a typical gold-bearing mineral, is widely distributed in gold tailings. In this study, we have examined the surface morphology of arsenopyrite before and after oxidation, the types of oxidation products, and the oxidation mechanism. We have carried out froth flotation and contact angle measurements and found that the wettability, which is a descriptor of the hydrophilicity of the arsenopyrite surfaces, increases with the levels of surface oxidation. X-ray photoelectron spectroscopy (XPS) was used to analyze the elemental composition before and after oxidation, which has demonstrated that both Fe(II) and As(III) oxidize to Fe(III) and As(V), respectively. The microscopic changes of the surface at the atomic level were also examined by atomic force microscopy (AFM), which indicated that the oxidation products gradually and partially cover the surface of the arsenopyrite upon reaction with hydrogen peroxide. These observations explain why the arsenopyrite XPS signal and the uniform change in wettability do not vanish completely during oxidation. Finally, density functional theory (DFT) calculations were used to describe the oxidation of the surface As atoms by the O of the chemisorbed hydrogen peroxide molecules. The computational results also show that the hydrogen peroxide dissociates into two hydroxyl groups that coordinate the Fe and As atoms on the surface of the arsenopyrite. The results of this study have important implications for the reuse of gold tailings containing arsenopyrite

Activation and dissociation of CO2 on the (001), (011), and (111) surfaces of mackinawite (FeS): A dispersion-corrected DFT study

Iron sulfide minerals, including mackinawite (FeS), are relevant in origin of life theories, due to their potential catalytic activity towards the reduction and conversion of carbon dioxide (CO2) to organic molecules, which may be applicable to the production of liquid fuels and commodity chemicals. However, the fundamental understanding of CO2 adsorption, activation, and dissociation on FeS surfaces remains incomplete. Here, we have used density functional theory calculations, corrected for long-range dispersion interactions (DFT-D2), to explore various adsorption sites and configurations for CO2 on the low-index mackinawite (001), (110), and (111) surfaces. We found that the CO2 molecule physisorbs weakly on the energetically most stable (001) surface but adsorbs relatively strongly on the (011) and (111) FeS surfaces, preferentially at Fe sites. The adsorption of the CO2 on the (011) and (111) surfaces is shown to be characterized by significant charge transfer from surface Fe species to the CO2 molecule, which causes a large structural transformation in the molecule (i.e., forming a negatively charged bent CO2 −δ species, with weaker C—O confirmed via vibrational frequency analyses). We have also analyzed the pathways for CO2 reduction to CO and O on the mackinawite (011) and (111) surfaces. CO2 dissociation is calculated to be slightly endothermic relative to the associatively adsorbed states, with relatively large activation energy barriers of 1.25 eV and 0.72 eV on the (011) and (111) surfaces, respectively

Cation doping and oxygen vacancies in the orthorhombic FeNbO4 material for solid oxide fuel cell applications: A density functional theory study

The orthorhombic phase of FeNbO4, a promising anode material for solid oxide fuel cells (SOFCs), exhibits good catalytic activity toward hydrogen oxidation. However, the low electronic conductivity of the material specifically in the pure structure without defects or dopants limits its practical applications as an SOFC anode. In this study, we have employed density functional theory (DFT + U) calculations to explore the bulk and electronic properties of two types of doped structures, Fe0.9375A0.0625NbO4 and FeNb0.9375B0.0625O4 (A, B = Ti, V, Cr, Mn, Co, Ni) and the oxygen-deficient structures Fe0.9375A0.0625NbO3.9375 and FeNb0.9375B0.0625O3.9375, where the dopant is positioned in the first nearest neighbor site to the oxygen vacancy. Our DFT simulations have revealed that doping in the Fe sites is energetically favorable compared to doping in the Nb site, resulting in significant volume expansion. The doping process generally requires less energy when the O-vacancy is surrounded by one Fe and two Nb ions. The simulated projected density of states of the oxygen-deficient structures indicates that doping in the Fe site, particularly with Ti and V, considerably narrows the bandgap to ∼0.5 eV, whereas doping with Co at the Nb sites generates acceptor levels close to 0 eV. Both doping schemes, therefore, enhance electron conduction during SOFC operation