A density functional theory study on the adsorption and dissociation of N2O on Cu2O(1 1 1) surface

Density functional theory has been employed to investigate the adsorption and the dissociation of an N₂O at different sites on perfect and defective Cu₂O(1 1 1) surfaces. The calculations are performed on periodic systems using slab model. The Lewis acid site, Cu_CUS, and Lewis base site, O_SUF are considered for adsorption. Adsorption energies and the energies of the dissociation reaction N₂O → N₂ + O(s) at different sites are calculated. The calculations show that adsorption of N₂O is more favorable on Cu_CUS adsorption site energetically. Cu_CUS site exhibits a very high activity. The Cu_CUS-N₂O reaction is exothermic with a reaction energy of 77.45 kJ mol⁻¹ and an activation energy of 88.82 kJ mol⁻¹, whereas the O_SUF-N₂O reaction is endothermic with a reaction energy of 205.21 kJ mol⁻¹ and an activation energy of 256.19 kJ mol⁻¹. The calculations for defective surface indicate that O vacancy cannot obviously improve the catalytic activity of Cu₂O.     

The water/graphitic-carbon interaction energy

The water/graphitic-carbon interaction energy was obtained for a sample having a water surface site adsorption density of 13.3 μmol m⁻². The interaction energy was determined from the spreading pressure of water, its surface tension and the water contact angle and using a formula obtained by the combination of the Young equation with a general equation of pair interaction. The values obtained for contact angles 42° and 86° are 7.63 and 7.18 kJ mol⁻¹ of water are similar to the water binding energies obtained from molecular dynamic simulations of water droplets on a graphite surface: 6.7-8.33 kJ mol⁻¹.     

A theoretical and matrix-isolation FT-IR investigation of the conformational landscape of N-acetylcysteine

The conformational landscape of N-acetylcysteine (NAC) has been investigated by a combined experimental matrix-isolation FT-IR and theoretical methodology. This combination is a powerful tool to study the conformational behavior of relatively small molecules. Geometry optimizations at the HF/3-21 level resulted in 438 different geometries with an energy difference smaller than 22 kJ mol⁻¹. Among these, six conformations were detected with a relative energy difference smaller than 10 kJ mol⁻¹ at the DFT(B3LYP)/6-31++G∗∗ level of theory. These were finally subjected to MP2/6-31++G∗∗ optimizations which resulted in five minima. The vibrational and thermodynamical properties of these conformations were calculated at both the DFT and MP2 methodologies. Experimentally NAC was isolated in an argon matrix at 16 K after being sublimated at 323 K. The most stable MP2 form appeared to be dominant in the experimental spectra but the presence of three other conformations with ΔE_MP2 < 10 kJ mol⁻¹ was also demonstrated. The experimentally observed abundance of the H-bond containing conformations appeared to be in good accordance with the predicted MP2 value.     

Interaction of indigo carmine dye with silica modified with humic acids at solid/liquid interface

Two distinct humic acids, one extracted from Brazilian peat soil, HA_PS, and another one obtained from commercial source, HA_FL, were attachment onto silica gel modified with aminopropyltrimethoxysilane, producing two material named SiHA_PS and SiHA_FL, respectively. The ability of these materials in removing indigo carmine dye from aqueous solution was followed through series of adsorption isotherms adjusted to modified Langmuir equation. The maximum number of moles adsorbed gave 6.82 ± 0.12 × 10⁻⁴ and 2.15 ± 0.17 × 10⁻⁴ mol g⁻¹ for SiHA_PS and SiHA_FL, respectively. Same interactions were calorimetrically followed and the thermodynamic data showed endothermic enthalpic values: 12.31 ± 0.55 and 24.69 ± 1.05 kJ mol⁻¹ for SiHA_PS and SiHA_FL surfaces, respectively. Gibbs free energies for two adsorption processes of indigo carmine dye presented negative values, reflecting dye/surface interactions must be accompanied by an increased in entropy values, which are 65 ± 3 and 98 ± 5 J mol⁻¹ K⁻¹ for SiHA_PS and SiHA_FL materials, respectively. The adsorption processes for both materials were spontaneous in nature although they presented an endothermic enthalpy for the interaction, resulting in an entropically favored process.     

Intermittent temperature-programmed desorption study of perovskites used for catalytic purposes

A differential desorption technique, called intermittent temperature-programmed desorption (ITPD), was used to give new insights into the properties of La_1−xSr_xCo_0.8Fe_0.2O₃ perovskites as a contribution to improve their performances with respect to various important application fields such as catalysis, electrocatalysis and solid oxide fuel cells (SOFC). Both ITPD and interrupted TPD (carried out at different heating rates) evidenced two distinct oxygen adsorbed states, desorbing at temperatures lower than 400 °C, corresponding to less than 5% of a compact monolayer of oxide ions. The first one, for low desorption temperatures (lower than 290 °C) exhibits a heat of adsorption (ΔH) distribution from 101 to 121 kJ mol⁻¹. The second one, for higher desorption temperatures (between 290 and 400 °C) corresponds to ΔH = 146 ± 4 kJ mol⁻¹. Additionally, for temperatures higher than 400 °C, we observed a continuous desorption of oxygen species, probably originating from the sub-surface or semi-bulk, with an associated activation energy of desorption ≥175 kJ mol⁻¹.     

Carbon monoxide oxidation over well-defined Pt/ZrO2 model catalysts: Bridging the material gap

Four different Pt/ZrO₂/(C/)SiO₂ model catalysts were prepared by electron beam evaporation. The morphology of these samples was examined before and after the catalytic reaction by Rutherford back-scattering (RBS), transmission electron microscopy (TEM) and grazing-incidence small-angle scattering (GISAXS). The catalytic behavior of such model catalysts was compared to a conventional Pt/ZrO₂ catalyst in the CO oxidation reaction using different oxygen excess (λ = 1 and 2). The so-called material gap was observed: model catalysts were active at higher temperature (620-770 K) and resulted in higher activation energy values (E_a = 77-93 kJ mol⁻¹ at λ = 1 and 129-141 kJ mol⁻¹ at λ = 2) compared to the powdered Pt/ZrO₂ catalyst (370-470 K, E_a = 74-76 kJ mol⁻¹). This material gap is discussed in terms of diffusion limitations, reaction mechanism and apparent compensation effect. Diffusion processes seem to limit the reaction on planar samples in the reactor system that was shown to be appropriate for the evaluation of the catalytic activity of powder samples. Kinetic parameters obeyed the so-called apparent compensation effect, which is discussed in detail. Langmuir-Hinshelwood-type of reaction, between CO_ads and O_ads, was proposed as the rate-determining step in all cases. Pt particles deposited on planar structures can be used for modeling conventional powdered catalysts, even though some limitations must be taken into account.     

Kinetic modeling of the benzyl + HO2 reaction

Benzyl is a resonantly stabilized radical that commonly occurs as an intermediate in the combustion of aromatic compounds. The bimolecular reaction of benzyl with HO₂ is important in the oxidation of toluene, especially at low to moderate temperatures, where unimolecular decomposition of the benzyl radical is slow. We show that the addition of HO₂ to the methylene site in benzyl produces a vibrationally excited benzylhydroperoxide adduct, with over 60 kcal mol⁻¹ (251 kJ mol⁻¹) of excess energy above the ground state. RRKM simulations are performed on the benzyl + HO₂ reaction, using thermochemical and kinetic parameters obtained from ab initio calculations, with variational transition state theory (VTST) for treatment of barrierless radical + radical reaction kinetics. Our results reveal that the benzyl + HO₂ reaction proceeds predominantly to the benzoxyl radical + OH at temperatures of around 800 K and above, with the production of stabilized benzylhydroperoxide molecules dominating at lower temperatures. The heat of formation of the benzyl radical is calculated as 52.5 kcal mol⁻¹ (219.7 kJ mol⁻¹) at the G3B3 level of theory, in relative agreement with other recent determinations of this value.     

The water-silicas interfacial interaction energies

The water-silicas interfacial interaction energies were calculated for samples of quartz, silicas and silicas outgassed at high temperatures using own and published data of the spreading pressure of water, its surface tension, its contact angle and using formulas obtained by the combination of the Young equation with a general equation of pair interaction. The values obtained for 18 different samples were in the range 7.80-6.92 kJ mol⁻¹. Lower values of energies are for samples that contain relatively less amounts of water at P/P₀ = 0.25 and are characterized also by relatively low values of surface pressures.     

Binding of rare earth metal complexes with an ofloxacin derivative to bovine serum albumin and its effect on the conformation of protein

The water-soluble Pr (Ⅲ) and Nd (Ⅲ) complexes with an ofloxacin derivative have been prepared and characterized. The single-crystal X-ray diffraction showed that the Pr (III) and Nd (III) complexes have the similar molecular structure. Under physiological pH condition, the effects of [PrL(NO₃)₂(CH₃OH)](NO₃) and [NdL(NO₃)₂(CH₃OH)](NO₃) on bovine serum albumin (BSA) were examined using fluorescence spectroscopy in combination with UV-vis absorbance and circular dichroism (CD) spectra. The result reveals that the quenching mechanism of fluorescence of BSA by two complexes is a static quenching process and the number of binding sites is about 1 for both. The thermodynamic parameters (ΔH=−14.52 kJ mol⁻¹, ΔS=56.54 J mol⁻¹ K⁻¹ for [PrL(NO₃)₂(CH₃OH)](NO₃) and ΔH=−24.63 kJ mol⁻¹, ΔS=22.07 J mol⁻¹ K⁻¹ for [NdL(NO₃)₂(CH₃OH)](NO₃)) indicate that hydrophobic and electrostatic interactions are the main binding force in the complexes-BSA system. The binding average distance between complexes and BSA was obtained on the basis of Förster's theory. In addition, it was proved by the CD spectra that the BSA secondary structure was changed in the presence of complexes in an aqueous solution.     

Interaction of O2 with Pd single crystals in the range 1-150 Torr: Oxygen dissolution and reaction

The interaction of O₂ with Pd(1 1 1), Pd(1 1 0) and Pd(1 0 0) was studied in the pressure range 1-150 Torr by the techniques of temperature programmed decomposition (TPD), Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The oxidation of Pd was rate-determined by oxygen diffusion into Pd metal followed by the diffusion into PdO once the bulk oxide layer was formed. The dissolution of oxygen atoms into Pd metal followed the Mott-Cabrera model with diffusion coefficient 10⁻¹⁶ cm² s⁻¹ at 600 K and activation energy of 60-85 kJ mol⁻¹. The bulk oxide phase was formed when a critical oxygen concentration was reached in the near-surface region. The formation of PdO was characterized by a decrease in the oxygen uptake rate, the complete fading of the metallic Pd LEED pattern and an atomic ratio O/Pd of 0.15-0.7 as measured by AES. The diffusion of oxygen through the bulk oxide layer again conformed to the Mott-Cabrera parabolic diffusion law with diffusion coefficient 10⁻¹⁸ cm² s⁻¹ at 600 K and activation energy of 111-116 kJ mol⁻¹. The values for the diffusion coefficient and apparent activation energy increased as the surface atom density of the single crystals increased.     

Attenuated total reflectance spectroscopy of coumarin organosilane molecules adsorbed on a fused silica surface

Attenuated total reflectance (ATR) spectroscopy was used to investigate the adsorption of coumarin organosilane molecules onto a fused silica surface. The difference between the absorption spectra of the molecules on the surface and in solution was explained by the interaction of the adsorbed coumarin organosilane molecules with the hydroxyl groups on the fused silica surface. This interaction produces a perturbation of the π electron distribution and the electronic transitions of the coumarin chromophore of the organosilane molecules adsorbed on the surface. From the kinetics adsorption curves, the calculated enthalpy values of 74.8 ± 5.2 kJ mol⁻¹ and free energy of −38.22 ± 0.70 kJ mol⁻¹ at 23 °C indicates a chemisorption process. The high sensitivity of ATR spectroscopy allows the detection of a monolayer formed by a 10 nM concentration of coumarin organosilane molecules, which covers more than half of the maximum surface coverage at 60 °C.     

Study on the interaction of La with bovine serum albumin at molecular level

The interaction of La³⁺ to bovine serum albumin (BSA) has been investigated mainly by fluorescence spectra, UV-vis absorption spectra, and circular dichroism (CD) under simulative physiological conditions. Fluorescence data revealed that the quenching mechanism of BSA by La³⁺ was a static quenching process and the binding constant is 1.75×10⁴ L mol⁻¹ and the number of binding sites is 1 at 289 K. The thermodynamic parameters (ΔH=−20.055 kJ mol⁻¹, ΔG=−23.474 kJ mol⁻¹, and ΔS=11.831 J mol⁻¹ K⁻¹) indicate that electrostatic effect between the protein and the La³⁺ is the main binding force. In addition, UV-vis, CD, and synchronous fluorescence results showed that the addition of La³⁺ changed the conformation of BSA.     

Removal of nickel(II) ions from aqueous solutions using the natural clinoptilolite and preparation of nano-NiO on the exhausted clinoptilolite

The natural zeolite tuff (clinoptilolite) from a Serbian deposit has been studied as adsorbent for Ni(II) ions from aqueous solutions. Its sorption capacity at 298 K varies from 1.9 mg Ni g⁻¹ (for the initial solution concentration of 100 mg Ni dm⁻³) to 3.8 mg Ni g⁻¹ (for C₀ = 600 mg Ni dm⁻³) and it increases 3 times at 338 K. The sorption is best described by the Sips isotherm model. The sorption kinetics follows the pseudo-second-order model, the activation energies being 7.44, 5.86, 6.62 and 6.63 kJ mol⁻¹ for C₀ = 100, 200, 300 and 400 mg Ni dm⁻³, respectively. The sorption involves a film diffusion, an intra-particle diffusion, and a chemical cation-exchange between the Na⁺ ions of clinoptilolite and the Ni²⁺ ions. The sorption is endothermic (ΔH° being 37.9, 33.4, 30.0, 27.7 and 24.3 kJ mol⁻¹ for C₀ = 100, 200, 300, 400 and 600 mg Ni dm⁻³, respectively) and spontaneous in the 298-338 K temperature range. Thermal treatment of the Ni(II)-loaded clinoptilolite results in the formation of spherical nano-NiO particles of approx. 5 nm in diameter which are randomly dispersed in the clinoptilolite lattice.     

Influence of cetyltrimethylammonium bromide on the morphology of AWO4 (A = Ca, Sr) prepared by cyclic microwave irradiation

AWO₄ (A = Ca, Sr) was prepared from metal salts [Ca(NO₃)₂·4H₂O or Sr(NO₃)₂], Na₂WO₄·2H₂O and different moles of cetyltrimethylammonium bromide (CTAB) in water by cyclic microwave irradiation. The structure of AWO₄ was characterized by X-ray diffraction (XRD) and selected area electron diffraction (SAED). Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) revealed the presence of nanoparticles in clusters with different morphologies; spheres, peaches with notches, dumb-bells and bundles, influenced by CTAB. Six Raman vibrational peaks of scheelite structure were detected at 908, 835, 793, 399, 332 and 210 cm⁻¹ for CaWO₄ and 917, 833, 795, 372, 336 and 192 cm⁻¹ for SrWO₄, which are assigned as ν₁(A_g), ν₃(B_g), ν₃(E_g), ν₄(B_g), ν₂(A_g) and ν_f.r.(A_g), respectively. Fourier transform infrared (FTIR) spectra provided the evidence of W-O stretching vibration in [WO₄]²⁻ tetrahedrons at 793 cm⁻¹ for CaWO₄ and 807 cm⁻¹ for SrWO₄. The peaks of photoluminescence (PL) spectra were at 428-434 nm for CaWO₄, and 447-451 nm for SrWO₄.     

Photoinduced electron transfer between hen egg white lysozyme and anticancer drug menadione

The protein, hen egg white lysozyme, on photoexcitation undergoes electron transfer with menadione (2-methyl-1,4-naphthoquinone), a widely known anticancer drug. With the addition of menadione the fluorescence of lysozyme is quenched with the simultaneous formation of an excited state charge-transfer complex in the longer wavelength and a ground state complex. The former is further evident from laser flash photolysis studies, which indicate a tryptophan to menadione electron transfer. From fluorescence quenching studies the binding constant is found to be ∼1.7×10⁴ M⁻¹ with the corresponding changes in enthalpy (ΔH°) and entropy (ΔS°) as 12.24 kJ mol⁻¹ and 124.12 J mol⁻¹ K⁻¹, respectively, indicative of an entropy-driven process. The circular dichroism studies also show some structural changes with increase in α-helical content in the protein on interaction with menadione. Finally, docking studies give some insight into the role of Trp 108 of lysozyme in the interaction.     

Influence of ferroelectric polarization on the energetics of the reaction of 2-fluoroethanol on BaTiO3

The influence of ferroelectric polarization on the reaction of 2-fluoroethanol on ferroelectric BaTiO₃ thin films was characterized. 2-Fluoroethanol was found to absorb dissociatively on the BaTiO₃ surface to form an alkoxide that reacts upon heating to produce acetaldehyde, ethylene, and adsorbed fluorine atoms which remain on the surface. Temperature programmed desorption (TPD) studies showed that the activation energy for the production of the acetaldehyde product was a function the orientation of the ferroelectric dipoles with an ∼4 kJ mol⁻¹ higher value on the c+ termination relative to the c− termination.     

Variable-temperature FT-IR studies on the thermodynamics of carbon dioxide adsorption on a faujasite-type H-Y zeolite

Adsorption of carbon dioxide on a faujasite-type H-Y zeolite (Si:Al = 2.6:1) was studied by variable-temperature (200-290 K range) infrared spectroscopy. Adsorbed CO₂ molecules interact with the Brønsted acid Si(OH)Al groups located inside the zeolite supercage, bringing about a characteristic bathochromic shift of the O-H stretching mode from 3645 cm⁻¹ (free OH group) to 3540 cm⁻¹ (hydrogen-bonded CO₂ adsorption complex). Simultaneously, the asymmetric (ν₃) mode of adsorbed CO₂ is observed at 2353 cm⁻¹. From the observed variation of the integrated intensity of the 3645 and 2353 cm⁻¹ IR absorption bands upon changing temperature, corresponding values of standard adsorption enthalpy and entropy were found to be ΔH° = −28.5(±1) kJ mol⁻¹ and ΔS° = −129(±10) J mol⁻¹ K⁻¹. Comparison with the reported values of ΔH° for CO₂ adsorption on other zeolites is briefly discussed.     

Hydrogen adsorption on the faujasite-type zeolite Mg-X: An IR spectroscopic and thermodynamic study

Hydrogen adsorption (physisorption) on the faujasite-type zeolite Mg-X was studied by means of variable-temperature (80-140 K) FT-IR spectroscopy. Perturbation of the adsorbed H₂ molecules by the cationic adsorbing centres of the zeolite renders the H-H stretching mode IR active, at 4065 cm⁻¹. Simultaneous measurement of IR absorbance and hydrogen equilibrium pressure, for a series of spectra recorded at the increasing temperature, allowed standard adsorption enthalpy and entropy to be determined. They resulted to be ΔH⁰ = −13 kJ mol⁻¹ and ΔS⁰ = −114 J mol⁻¹ K⁻¹, respectively. Both, spectroscopic and thermodynamic results are discussed in the broader context of corresponding data for hydrogen adsorption on other alkali and alkaline-earth cation exchanged zeolites, showing that, while an approximate correlation exists between ΔH⁰ and H-H stretching frequency, deviations can be expected for the case of zeolites containing small metal cations.     

Thermodynamics of nitrogen adsorption on the zeolite H-FER

Adsorption (at a low temperature) of nitrogen on the protonic zeolite H-FER results in hydrogen bonding of the adsorbed N₂ molecules with the zeolite Si(OH)Al Brønsted acid groups. This hydrogen bonding interaction leads to activation, in the IR, of the fundamental NN stretching mode, which appears at 2331 cm⁻¹. From the infrared spectra taken over a temperature range, while simultaneously recording integrated IR absorbance, temperature and nitrogen equilibrium pressure, the thermodynamics of the adsorption process was studied. The standard adsorption enthalpy and entropy resulted to be ΔH° = −20(±1) kJ mol⁻¹ and ΔS° = −131(±10) J mol⁻¹ K⁻¹, respectively.     

Synthesis and utilization of Mg/Al hydrotalcite for removing dissolved humic acid

It has been synthesized Mg/Al layered double hydroxide anionic clay (Mg/Al hydrotalcite) through direct precipitation by adding 0.5 M NaOH solution into a mixed solution containing Mg(NO₃)₂ and Al(NO₃)₃ with molar ratio of 0.1:0.05 until the medium acidity reached pH 10.1. The synthesized Mg/Al hydrotalcite was then utilized to remove dissolved humic acid in aqueous medium. The humic acid was isolated from peat soil taken in Gambut District, South Kalimantan, Indonesia using the recommended procedure of IHSS (International Humic Substances Society). The removal of humic acid was mostly occurred through simple sorption process without accompanied by significant intercalation. The sorption was optimum at pH 9.0, with the first order rate constant, capacity and energy of sorption were 5.50 × 10⁻³ min⁻¹, 0.12 mmol g⁻¹ (69 mg g⁻¹), and 28.32 kJ mol⁻¹, respectively.