Relative Oxidative Stability Research Articles

Valorization of fish waste for sustainable biofuel production: A case study in Ghana

Fish processing plants generate significant waste annually, which harms the environment. To tackle this, the waste can be converted into biofuel, pharmaceuticals, fertilizer, and animal feed. Biofuels, specifically biogas and biodiesel, are emphasized for their eco-friendly properties that help combat global warming. The study focuses on producing biodiesel through transesterification and biogas via anaerobic digestion of fish waste. Analyses of the rate constant (K), activation energy (Ea), entropy (ΔS), enthalpy (ΔH), and Gibbs free energy (ΔG) at varied temperatures show highest Ea, K, ΔH, and ΔS values of 60.74 kJ/mol, 31.43 s−1, 66.01 kJ/mol, and 30.99 kJ/mol K, respectively. The biodiesel was also characterized by Fourier transform infrared (FTIR) spectroscopy and kinematic viscosity. Additionally, the properties; relative density, viscosity, oxidation stability, pour point, cloud point, flash point, and cetane number were found to be 0.669, 5.090 mm2s−1, >7, −7 °C, 0.999 °C, 151 °C, and 51, respectively. Biogas production using varying ratios of fish waste and cow dung (1:1,1:2, 1:3, and 1:4) indicates that a 1:4 ratio yields optimal results with 69% methane (CH4) and 21.5% carbon dioxide (CO2), suitable for scalable biogas production. The process is efficient at a neutral pH (7.1–7.3) and 27 °C, offering a viable solution to reduce fish waste pollution while generating renewable energy. Finally, a proposed model of a biodigester for optimum biogas production is discussed.

Efficiency of δ-Tocopherol in Inhibiting Lipid Oxidation in Emulsions: Effects of Surfactant Charge and of Surfactant Concentration.

Charged interfaces may play an important role in the fate of chemical reactions. Alterations in, for instance, the interfacial acidity of emulsions induced by the charge of the surfactant head group and associated counterions may change the ionization status of antioxidants, modifying their effective concentrations. The chemical reactivity between interfacial reactants and charged species of opposite charge (protons, metallic ions, etc.) is usually interpreted in terms of pseudophase ion-exchange models, treating the distribution of charged species in terms of partitioning and ion exchange. Here, we focus on analyzing the effects of charged interfaces on the oxidative stability of soybean oil-in-water (o/w) emulsions prepared with anionic (sodium dodecyl sulfate, SDS), cationic (cetyltrimethylammonium bromide, CTAB) and neutral (Tween 20) surfactants, and some of their mixtures, in the presence and absence of δ-tocopherol (δ-TOC). We have also determined the effective concentrations of δ-TOC in the oil, interfacial and aqueous regions of the intact emulsions. In the absence of δ-TOC, the relative oxidative stability order was CTAB < TW20 ~ TW20/CTAB < SDS. Surprisingly, upon the addition of δ-TOC, the relative order was SDS ≈ TW20 << TW20/CTAB < CTAB. These apparently surprising results can be rationalized in terms of the nice correlation that exists between the relative oxidative stability and the effective interfacial concentrations of δ-TOC in the various emulsions. The results emphasize the importance of considering the effective interfacial concentrations of antioxidants in interpreting their relative efficiency in emulsions.

Improvement of physical and thermal properties of polyvinyl butyral coating with nanosilica

In this paper, polyvinyl butyral (PVB) coatings with different nanosilica contents (0- 1.5 weight percent (wt%) were prepared for improving properties such as mechanical properties, corrosion protection, thermal oxidation stability of coating. Presence of nanosilica in coating was characterized by FT-IR. Corrosion resistance of PVB coatings containing nanosilica was investigated by salt mist testing. Effects of nanosilica on adhesion, flexural strength and relative hardness and thermal oxidation stability of PVB coatings were also examined. Nanosilica with content of 1.2 wt% significantly improved mechanical properties, corrosion resistance and thermal oxidation stability of PVB coatings.

Correct and Accurate Polymorphic Energy Ordering of Transition-Metal Monoxides Obtained from Semilocal and Onsite-Hybrid Exchange-Correlation Approximations

The relative energetic stability of the structural phases of common antiferromagnetic transition-metal oxides (MnO, FeO, CoO, and NiO) within the semilocal and hybrid density functionals are fraught with difficulties. In particular, MnO is known to be the most difficult case for almost all common semilocal and hybrid density approximations. Here, we show that the meta-generalized gradient approximation (meta-GGA) constructed from the cuspless hydrogen model and Pauli kinetic energy density (MGGAC) can lead to the correct phase as the ground-state of MnO. The relative energy differences of zinc blende (zb) and rock salt (rs) structures as computed using MGGAC are found to be in nice agreement with those obtained from high-level correlation methods like the random phase approximation or quantum Monte Carlo techniques. Besides, we have also applied the onsite hybrid functionals (closely related to DFT+U) based on GGA and meta-GGA functionals, and it is shown that a relatively high amount of Hartree–Fock exchange is necessary to obtain rs as the ground-state phase. Our present investigation suggests the semilocal MGGAC and onsite hybrids, both being computationally cheap, as methods of choice for the calculation of the relative stability of antiferromagnetic transition-metal oxides having potential applications in solid-state physics and structural chemistry.

Metals extraction on Mars through carbothermic reduction

In situ resource utilization (ISRU), and specifically the extraction and production of structural metals from Martian soil, is important for supporting further exploration, habitat and industrial developments on Mars. Considering Mars's atmospheric conditions and soil compositions, systematic analyses (supported by thermodynamic calculations) on possible metal extraction processes have been carried out. The analyses suggest that carbothermic reduction of Martian regolith would be the preferable starting point considering the abundance of carbon in the form of CO2 in the Martian atmosphere. It was proposed that carbon would be sourced from the cooling of carbon monoxide produced from CO2 electrolysis (MOXIE) from the Martian atmosphere. An Ellingham diagram showing the relative stability of oxides at Mars's atmospheric pressure of 7 mbar has been constructed; detailed equilibrium calculations to evaluate the carbothermic reduction of Martian regolith have also been carried out. The thermodynamic calculations predicted the formation of liquid Fe alloy with 99.9% conversion when the regolith was reacted with carbon (at approximately 10 wt% addition) at 1120 °C and 7 mbar. The impurities in the liquid Fe alloy were predicted to be mainly Si, C, Cr, and P. The hot gas produced from the process was predicted to be rich in CO (91%) and could be used for the preheating of the regolith. The CO could also be recycled and condensed to produce C (that could be re-used for the carbothermic reduction). The minimum energy requirement for the carbothermic reactor was calculated to be 3.37 MWh/tonne of liquid iron alloy (while the total energy requirement including MOXIE and agglomeration was calculated to be 15.51 MWh/tonne of liquid iron alloy). A generic process flowsheet has been developed considering these results.

Unexpected Antioxidant Efficiency of Chlorogenic Acid Phenolipids in Fish Oil-in-Water Nanoemulsions: An Example of How Relatively Low Interfacial Concentrations Can Make Antioxidants to Be Inefficient.

Selecting effective antioxidants is challenging since their efficiency in inhibiting lipid oxidation depends on the rate constants of the chemical reactions involved and their concentration at the reaction site, i.e., at the interfacial region. Accumulation of antioxidants at the interface of emulsions is key to modulate their efficiency in inhibiting lipid oxidation but its control was not well understood, especially in emulsions. It can be optimized by modifying the physicochemical properties of antioxidants or the environmental conditions. In this work, we analyze the effects of surfactant concentration, droplet size, and oil to water ratio on the effective interfacial concentration of a set of chlorogenic acid (CGA) esters in fish oil-in-water (O/W) emulsions and nanoemulsions and on their antioxidant efficiency. A well-established pseudophase kinetic model is used to determine in the intact emulsified systems the effective concentrations of the antioxidants (AOs). The relative oxidative stability of the emulsions is assessed by monitoring the formation of primary oxidation products with time. Results show that the concentration of all AOs at the interfacial region is much higher (20–90 fold) than the stoichiometric one but is much lower than those of other phenolipid series such as caffeic or hydroxytyrosol derivatives. The main parameter controlling the interfacial concentration of antioxidants is the surfactant volume fraction, ΦI, followed by the O/W ratio. Changes in the droplet sizes (emulsions and nanoemulsions) have no influence on the interfacial concentrations. Despite the high radical scavenging capacity of CGA derivatives and their being concentrated at the interfacial region, the investigated AOs do not show a significant effect in inhibiting lipid oxidation in contrast with what is observed using other series of homologous antioxidants with similar reactivity. Results are tentatively interpreted in terms of the relatively low interfacial concentrations of the antioxidants, which may not be high enough to make the rate of the inhibition reaction faster than the rate of radical propagation.

Fabrication of 3D printed complex concentrated alloys using oxide precursors

In this article we report on the first reduction of a compositionally complex concentrated CoCrFeMnNi alloy processed entirely from oxide precursors and printed using materials extrusion additive manufacturing (AM) techniques. The chemical and structural properties of this material are explored, while transmission electron microscopy (TEM) is used to investigate potential chemical segregation within the well-annealed grains. The relative thermodynamic stability of Mn and Cr oxides would suggest that MnO and Cr2O3 will not reduce in a hydrogen atmosphere. However, we find that the unique local thermodynamic environment present in the samples allows for full reduction of Mn and Cr while diffusion into the grain allows for the development of a single-phase FCC structure.

Immobilized Lipase Based on Hollow Mesoporous Silicon Spheres for Efficient Enzymatic Synthesis of Resveratrol Ester Derivatives.

Enzymatic esterification of resveratrol is crucial for its potential application in lipophilic foods and drugs. However, the poor activity of the free enzyme hinders the reaction. In this work, the highly efficient enzymatic synthesis of resveratrol ester derivatives was achieved by immobilized lipase on hydrophobic modified hollow mesoporous silicon spheres (HMSS-C8). We preliminarily explored the use of Candida sp. 99-125 lipase (CSL) for the acylation of resveratrol, with a regioselectivity toward 3-OH- over 4'-OH-acylation. HMSS-C8 provided ideal accommodation for CSL with a loading capacity of up to 652 mg/g. The catalytic efficiency of CSL@HMSS-C8 was 15 times higher than that of free CSL, and the conversion of resveratrol reached 98.7% within only 2 h, which is the fastest value recorded in the current literature. After 10 cycles, the conversion remained up to 86.3%. Benefiting from better lipid solubility, the relative oxidation stability index values of oil containing monoester derivatives were 43.1%-68.8% and 23.9%-33.2% higher than that of refined oil and oil containing resveratrol, respectively. This research provides a new pathway for efficient enzymatic synthesis of resveratrol ester derivatives and demonstrates the potential application of resveratrol monoester derivatives as a group of excellent lipid-soluble antioxidants.

A fluorinated polycarbonate based all solid state polymer electrolyte for lithium metal batteries

Poly(ethylene oxide) (PEO) is a promising matrix for solid polymer electrolyte, but its inferior mechanical strength and relative low oxidation stability especially at elevated temperatures hamper its further applications. A novel fluorinated polycarbonate polymer of poly(2,2,3,3-tetrafluoro butyl carbonate) with cyano ends (cPTFBC) was synthesized for the first time and was composited with PEO based electrolyte by physical blending to resolve the above issues. The addition of cPTFBC can effectively enhance the mechanical strength at elevated temperature. In addition, the as-prepared PEO-cPTFBC based electrolyte shows an enlarged electrochemical window up to 4.7 V at 60 °C and a high Li ion transference number (0.33). More importantly, the PEO-cPTFBC based all solid state polymer electrolyte presents excellent compatibility with lithium metal and improved LiCoO2/Li battery performance compared with PEO based electrolyte. These outstanding performance of the cPTFBC based electrolyte make it promising as solid polymer electrolyte for lithium metal batteries.

Alternative Mechanisms of Betacyanin Oxidation by Complexation and Radical Generation.

The use of natural pigments such as betalains in food and health-related products is often limited by said pigments' relative oxidative stabilities in the products or physiological matrices. Determination of the mechanism of oxidation may inform future development and delivery of better stabilized molecules for improved outcomes. In order to best determine the oxidation mechanism of betanin, a natural food colorant, our efforts were directed toward structural elucidation (LCMS-IT-TOF and NMR) of previously tentatively identified key dehydrogenation products that had been generated as a result of betanin, decarboxylated betanin, and neobetanin oxidation by 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) cation radicals. The resultant oxidation products, the neo-derivatives, were the most stable and survived the preparative isolation and purification process. Structural analyses subsequently confirmed that these compounds, as well as neobetanin, were also the key products of alternative pathways of betanin and 2-decarboxy-betanin oxidation when catalyzed by Cu2+ cations in aqueous solutions at pH close to neutral. Therefore, the structures of the following five neo- or xanneo-derivatives (14,15- or 2,3,14,15-dehydrogenated derivatives, respectively) were confirmed: neobetanin, 2-decarboxy-neobetanin, 2-decarboxy-xanneobetanin, 2,17-bidecarboxy-xanneobetanin, and 2,15,17-tridecarboxy-xanneobetanin. This research confirmed that Cu2+-catalyzed oxidation of betanin and 2-decarboxy-betanin results in generation of neo-derivatives of betanin. In contrast, Cu2+-catalyzed oxidation of 17-decarboxy-betanin and 2,17-bidecarboxy-betanin resulted mostly in formation of betanin xan-derivatives. A relevant mechanism of Cu2+-catalyzed oxidation of the pigments is proposed herein that suggests that the oxidation of betanin can possibly occur in the region of the dihydropyridinic ring and can omit the stage of methide quinone formation in the dihydroindolic system.

Evaluation of oxidative stability, fatty acid profile, and antioxidant properties of black cumin seed oil and extract

Black cumin is a strong aromatic seed which can be used as a nutraceutical or medicinal food. Our aim was to evaluate the fatty acid profile and oxidative stability of black cumin seed (BCS) oil as well as the antioxidant activity of water–methanol extract of BCS compared with BHT. In fact, it was going to understand what kind of fatty acids exist in BCS oil and whether is it possible to apply BCS extract as a natural antioxidant or not. The physicochemical properties of BCS oil included an iodine value of 105.17 g/100 g oil, a PV of 11.88 meq/kg, an oxidative stability index of 16.48 h, viscosity of 21.3 mPas, and a refractive index of 1.45. BCS oil contained more than 79% unsaturated fatty acids. Its saturated fatty acids were composed mainly of palmitic acid (8.38%) and stearic acid (2.26%). The BCS extract contained 955.77 mg/kg total phenolics. In a DPPH assay, the IC50 of the BCS extract was measured as 104.76 mg/mL, while for BHT it was 8.06 mg/mL. In the incubation assay, the BCS extract inhibited the formation of oxidation primary products in raw soybean oil at a concentration of 100 mg/mL. To conclude, BCS oil had a high content of polyunsaturated fatty acids and in spite of its high degree of unsaturation, the presence of phenolic compounds in BCS oil led to an increase in its relative oxidative stability. Also, at higher concentrations, BCS extract can compete with BHT in terms of antioxidant effects, and thus can be added into edible oils.

Physicochemical properties, phenolic acids and volatile compounds of oil extracted from dry alhydwan (&lt;em&gt;Boerhavia elegana Choisy&lt;/em&gt;) seeds

In this study, the chemical composition, physicochemical properties, phenolic acids and volatile compounds of alhydwan (Boerhavia elegana Choisy) seed oil were evaluated. The crude oil content was 11.49%, ash 6.88%, moisture 6.12%, protein content 14.60%, total carbohydrate 24.77% and fiber 36.13%. The oil contain a high quantity of unsaturated fatty acids (74.63 mg·100 g−1) with oleic (C18:1) (57.77%), palmitic (C16:0) (18.65%) and linoleic (C18:2) (12.88%) acids as the most abundant. The relative density was 0.88 and the iodine value 105.59. The color analysis showed a value of 28.33 Y+1.43 R. The oil also had a high relative oxidative stability. The tocol composition showed that α-tocotrienol, γ-tocopherol and γ-tocotrienol were in a higher concentration than the rest. Seven phenolic acids (caffeic, vanillic, galic, p-coumaric, ascorbic, cinnamic and ferulic) were detected, with ascorbic acid as the predominant one (5.44 mg·100 g−1). In relation to the volatile composition, 48 compounds were found with Z-10-Pentadecen-1-ol (56.73%); Hexadecenoic acid, Z-11- (18.52%); 9,12-Octadecadienoic acid (Z,Z)- (3.93%) and 9,12-Octadecadienoic acid (Z,Z)-, 2-hydroxy-1-(hydroxymethyl) ethyl ester (3.04%) as the most abundant. These findings demonstrated the potential of alhydwan seeds to be used as a good source of quality edible oil.

Oxidation stabilization of peanut-linen blend lipids

Food quality is crucial in terms of impact on human health and life expectancy. The task of preserving the quality of fat-and-oil products lies in protecting lipids from oxidation, during which substances that not only degrade the quality characteristics of the product, but also can be harmful to human health are formed. To prevent oxidation of products, antioxidant-containing raw materials are widely used.Analysis of the content of antioxidants of oil extracts of sage leaves, black currant leaves, garlic, rose hips was conducted by the method, which consists in calculating the dependence of the amount of absorbed oxygen on time during initiated oil oxidation at increased temperature.It was found that the investigated oil extracts have an inhibitory effect and their introduction in the amount of 5% increases the oxidative stability of peanut-linen blend by 1,2-1,7 times. According to the content of antioxidants and relative oxidation stability, the studied oil extracts can be arranged in the following series: sage leaves> rose hips> black currant leaves> garlic.The studies show that the oil extracts contain substances that have antioxidant properties and allow to recommend them as an additive that inhibits oil oxidation processes. This blended oil has a balanced composition of polyunsaturated fatty acids, is stable to oxidative spoilage and enriched with biologically active substances. The developed product is recommended for people with weak health and for preventive purposes.

Thermodynamic Control of Phase Composition and Crystallization of Metal‐Modified Silicon Oxycarbides

Silicon oxycarbides modified with main group or transition metals (SiMOC) are usually synthesized via pyrolysis of sol‐gel precursors from suitable metal‐modified orthosilicates or polysiloxanes. In this study, the phase composition of different SiMOC systems (M = Sn, Fe, Mn, V, and Lu) was investigated. Depending on the metal, different ceramic phases formed. For M = Mn and Lu, MOx/SiOC ceramic nanocomposites were formed, whereas other compositions revealed the formation of M/SiOC (M = Sn), MSix/SiOC (M = Fe) or MCx/SiOC (M = V) upon pyrolysis. The different phase compositions of the SiMOC materials are rationalized by a simple thermodynamic approach which generally correctly predicts which type of ceramic nanocomposite is expected upon ceramization of the metal‐modified precursors. Calculations show that the thermodynamic stability of the MOx phase with respect to that of the C–O system is the most important factor to predict phase formation in polymer‐derived SiMOC ceramic systems. A secondary factor is the relative stability of metal oxides, silicates, carbides, and silicides.

Comparative study of the oxidative and physical stability of liposomal and nanoliposomal polyunsaturated fatty acids prepared with conventional and Mozafari methods

The relative oxidative stability of freshly prepared and stored liposomal and nanoliposomal systems of docosahexaenoic acid (DHA, 22:6 n−3) and eicosapentaenoic acid (EPA, 20:5 n−3) were investigated. The effects of organic solvents on the oxidative stability of liposomal polyunsaturated fatty acids (PUFAs) produced by two methods, the Bangham thin-film hydration (conventional rotary evaporation method and using organic solvents) and Mozafari (direct hydration and without using organic solvents) methods, were compared. The highest physicochemical stability was observed in PUFA liposomes prepared by the Mozafari method, followed by conventional liposomes and bulk PUFAs. There was no significant change in physicochemical stability during 10months of cold storage (4°C) in the dark. Moreover, the comparison between liposomes (>200nm) and nanoliposomes (50–200nm) revealed that the surface charge, physical stability and oxidative stability of liposomal PUFAs increased as the size of the liposomes decreased. The differences in the oxidative stability of PUFAs may be due to the protective effects of aqueous systems, which indicate the advantage of using non-organic solvent (water and CO2) techniques in liposome manufacturing.

Effect of Antioxidants on Oxidative Stability and Quenching Performance of Soybean Oil and Palm Oil Quenchants

Abstract Although the quenching performance of various vegetable oils has been reported in the past, reports on the effect of antioxidants on the oxidative stability of these seed oils and on the initial quenching performance are less common. The objective of this work was to assess the effect of the relative oxidative stability of two readily available seed oils with significantly different degrees of saturation, soybean oil and palm oil, on quenching performance and to determine the ability of readily available antioxidants to stabilize these oils. In addition to the determination of relative oxidative stability, a further objective of this work was to assess the effect of antioxidants on the initial quenching performance of soybean oil and palm oil using a cooling curve analysis. The results of these studies are reported here.

ChemInform Abstract: The Effect of pH on the Corrosion of Nuclear Fuel (UO2) in Oxygenated Solutions,

Abstract The oxidative dissolution of UO2 has been studied in NaClO4 and Na2SO4 solutions as a function of pH over a range of 0.8 ⩽ pH ⩽ 12 using a combination of electrochemical and X-ray photoelectron spectroscopic techniques. The relative stability and solubility of solid uranium oxides and uranium speciation in aqueous solutions were examined using thermodynamic calculations. In neutral to alkaline solutions (pH ≥5), dissolution is preceded by the growth of a thin film of UO2 33 on the UO2 surface. This film achieves a steady-state thickness (∼6 nm) in 5 to 10 h, and the thickness increases with an increase in pH. Over the next 10 to 100 h, further oxidation to a hydrated form of UO3 occurs, after which steady-state dissolution conditions are achieved. Formation of UO3 · xH2O is a precursor to dissolution and its presence appears to be confined mainly to the grain boundaries. In acidic solutions UO2.33 formation does not occur. Oxidation proceeds directly to the UVI state (possibly to UO3 · xH2O) and the dissolution rate, measured by extrapolation of steady-state currents to the corrosion potential, is approximately 50 times greater at pH = 2.5 than it is at pH = 9.7. The electrochemically measured corrosion rate value of ∼45 × 10−8 g cm−2 d−1 at pH = 2.5 compares well with the value of ∼ 25 × 10−8 g cm−2d−1 measured chemically by Thomas and Till. The dependence of the steady-state corrosion potentials on pH suggests possible rate control by the anodic dissolution reaction for pH > 10. For pH values between 2 and 10, the corrosion potential varies only slightly with pH, and it is unclear whether the anodic or cathodic reaction is rate-controlling in this region.

A Novel Heme Peroxidase from Raphanus sativus Intrinsically Resistant to Hydrogen Peroxide

AbstractAll known heme peroxidases are susceptible to self‐inflicted oxidative damage. Substrate oxidation in peroxidases is a naturally imperfect process and damage emerges from the utilization of protein elements as electron sources through the catalytic intermediate Compound III. Given their wide application potential, the search for more stable heme peroxidases has been actively pursued in the past, albeit with limited success. Here, the identification and biochemical characterization of a novel H2O2‐resistant isoenzyme is reported which was isolated from daikon radish (Raphanus sativus L. cv. daikon) and named Zo peroxidase (ZoPrx). Partial protein sequencing demonstrated that ZoPrx is more related to anionic isoenzyme 53 from Arabidopsis thaliana and to anionic isoenzyme 2 from Armoracia rusticana (HRPA2) than to any other protein known to date. The stability of ZoPrx was demonstrated by rigorous evaluation of resistance to H2O2 in the absence or presence of exogenous reductants which was consistent with porphyrin integrity. ZoPrx systematically outperformed the reference HRPA2 isoenzyme, a heme peroxidase known for its relative oxidative stability. The catalytic cycle of ZoPrx is conventional in the sense that the usual intermediate Compounds I and II were identified although reaction rates are significantly different compared to HRPA2. The distinctive characteristic of ZoPrx is that when Compound III is formed in the presence of excess H2O2, it is particularly more stable and, instead of engaging into the regular deactivation decay pathways, it commits to the ground state restoration decay pathway. ZoPrx is the only known case of a naturally occurring stable heme peroxidase based on intrinsic properties. Deeper study of the structural determinants underlying this property may eventually allow us to graft it into other peroxidases with established commercial potential.

Effect of Temperature on the Oil Stability Index (OSI) of Biodiesel

Biodiesel, an alternative fuel derived from lipid feedstocks, such as vegetable oil or animal fat, is mainly composed of saturated and unsaturated fatty acid alkyl esters. Fuel suppliers, terminal operators, and users are becoming more concerned with monitoring and maintaining good biodiesel fuel quality with respect to oxidative degradation during storage. The oil stability index (OSI), a parameter that measures the relative oxidative stability of fatty materials, is typically measured isothermally at elevated temperatures to accelerate oxidation. The present work investigates the effects of block temperature (T) on the OSI of biodiesel from soybean oil fatty acid methyl esters (SME) and used cooking oil fatty acid methyl esters (UCOME). Results were compared to those for pure methyl oleate (MO). An increasing temperature accelerated the oxidation reaction causing a decrease in OSI. Response factors (RF) determined with MO as the reference methyl ester showed little variation with respect to the temperat...

Surface oxides of the oxygen–copper system: Precursors to the bulk oxide phase?

To gain an initial understanding of the copper-based catalysts in commercially important chemical reactions such as the oxygen-assisted water–gas shift reaction, we performed density-functional theory calculations, investigating the interaction of oxygen and copper, focusing on the relative stability of surface oxides and oxide surfaces of the O/Cu system. By employing the technique of “ ab initio atomistic thermodynamics”, we show that surface oxides are only metastable at relevant pressures and temperatures of technical catalysis, with no stable chemisorption phase observed even at very low coverage. Although exhibiting only metastability, these surface oxides resemble the bulk oxide material both geometrically and electronically, and may serve as a precursor phase before onset of the bulk oxide phase. Having identified the bulk oxide as the most stable phase under realistic catalytic conditions, we show that a Cu 2O(1 1 1) surface with Cu vacancies has a lower free energy than the stoichiometric surface for the considered range of oxygen chemical potential and could be catalytically relevant.