Studies on chelating adsorption properties of novel composite material polyethyleneimine/silica gel for heavy-metal ions

Firstly, the coordination processes of line-type polyethyleneimine with Cu²⁺, Cd²⁺ and Zn²⁺ were studied by using visible light absorption spectroscopy and chelation conductivity titration method, and the structures of the chelates were determined. Afterwards, polyethyleneimine (PEI) was grafted onto the surface of silica gel particles via the coupling effect of γ-chloropropyl trimethoxysilane (CP), and the novel composite adsorption material PEI/SiO₂ with strong adsorption ability towards heavy-metal ions was prepared. The chelating adsorption properties of PEI/SiO₂ for Cu²⁺, Cd²⁺ and Zn²⁺ were researched by both static (batch) and dynamic (flow) methods. The experiment results show that water-soluble polyamine PEI with line-type structure reacts with Cu²⁺, Cd²⁺ and Zn²⁺ easily and quantitatively, and water-soluble chelates with four ligands are formed. The composite material PEI/SiO₂ possesses very strong chelating adsorption ability for heavy-metal ions, and the saturated adsorption amount can reach 25.94 mg g⁻¹ and 50.01 mg g⁻¹ for Cu²⁺ under static and dynamic conditions, respectively. The isothermal adsorption data fit to Langmuir equation, and the adsorption is typical chemical adsorption with monomolecular layer. The adsorbing ability of PEI/SiO₂ towards the three kinds of the ions follows the order of Cu²⁺ > Cd²⁺ > Zn²⁺. The pH value has great influence on the sorption, and at pH 6-7, the adsorption capacity is the greatest. The fact that adsorption capacity increases with temperature rising indicates the adsorbing process of PEI/SiO₂ for metal ions is endothermic. As diluted hydrochloric acid is used as eluent, the adsorbed heavy-metal ions are eluted easily from PEI/SiO₂, and the regeneration and reuse without decreasing sorption for PEI/SiO₂ are demonstrated.     

Preparation of activated carbon from a renewable bio-plant of Euphorbia rigida by H2SO4 activation and its adsorption behavior in aqueous solutions

The use of activated carbon obtained from Euphorbia rigida for the removal of a basic textile dye, which is methylene blue, from aqueous solutions at various contact times, pHs and temperatures was investigated. The plant material was chemically modified with H₂SO₄. The surface area of chemically modified activated carbon was 741.2 m² g⁻¹. The surface characterization of both plant- and activated carbon was undertaken using FTIR spectroscopic technique. The adsorption process attains equilibrium within 60 min. The experimental data indicated that the adsorption isotherms are well described by the Langmuir equilibrium isotherm equation and the calculated adsorption capacity of activated carbon was 114.45 mg g⁻¹ at 40° C. The adsorption kinetics of methylene blue obeys the pseudo-second-order kinetic model and also followed by the intraparticle diffusion model up to 60 min. The thermodynamic parameters such as ΔG°, ΔH° and ΔS° were calculated to estimate the nature of adsorption. The activation energy of the system was calculated as 55.51 kJ mol⁻¹. According to these results, prepared activated carbon could be used as a low-cost adsorbent to compare with the commercial activated carbon for the removal textile dyes from textile wastewater processes.     

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.     

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.     

Adsorption and solid phase extraction of 8-hydroxyquinoline from aqueous solutions by using natural bentonite

The nitrogen-heterocyclic compound 8-hydroxyquinoline (8HQ) is one of the components of coal tar and has a wide variety of uses in industry. Because of its toxicity for aquatic organisms and harmful effects for human health, the removal of 8HQ from aqueous solutions by adsorption onto natural bentonite was investigated in the present work. The experimental results show that the optimum pH value of 2.5 is favourable for the 8HQ adsorption. The experimental data were fitted well with the pseudo-second-order kinetic and Langmuir adsorption isotherm models at all studied temperatures. The maximum adsorption capacity obtained from the Langmuir isotherm model at 20 °C was 120.6 mg g⁻¹. The calculated thermodynamic results such as ΔG° (−24.3 kJ mol⁻¹) and ΔH° (−9.56 kJ mol⁻¹) indicate that the adsorption process is spontaneous and exothermic in nature. Solid phase extraction of 8HQ was also performed. The X-ray diffractometry (XRD), Fourier Transform Infrared (FTIR) and thermogravimetric (TG) analyses were carried out in order to confirm the 8HQ adsorption onto bentonite. According to the obtained results, natural bentonite can be a reusable and effective adsorbent for the removal of 8HQ.     

Preparation and characterization of hexadecyl functionalized magnetic silica nanoparticles and its application in Rhodamine 6G removal

In this paper, a new adsorbent, hexadecyl functionalized magnetic silica nanoparticles (C₁₆/SiO₂-Fe₃O₄ NPs), was prepared by a facile method. The final product was characterized by X-ray diffractometer, transmission electron microscope, Fourier transform infrared spectrometer and vibration sample magnetometer. The preparation and adsorption conditions of the adsorbent were optimized. The adsorbent prepared maintaining volume ratio of tetraethylorthosilicate to hexadecyltrimethoxysilane at 1:0.5 and their total volume at 1100 μL exhibited high adsorption capacity. The optimum pH value for the adsorption experiments was 11.00. The adsorption behavior of Rhodamine 6G onto C₁₆/SiO₂-Fe₃O₄ NPs obeyed pseudo-second-order kinetic model and Langmuir isotherm. Thermodynamic data indicated that the adsorption process was spontaneous and exothermic. The adsorption capacity of the adsorbent could reach to 35.6 mg g⁻¹, owing to the hydrophobic attraction and the enhanced electrostatic attraction. The saturation magnetization of the magnetic adsorbent was 35 emu g⁻¹, which ensured the magnetic separation after adsorption.     

Kinetics and thermodynamics of Pb(II) adsorption onto modified spent grain from aqueous solutions

Spent grain, a main by-product of the brewing industry, is available in large quantities, but its main application has been limited to animal feeding. Nevertheless, in this study, spent grain modified with 1 M NaCl solution as a novel adsorbent has been used for the adsorption of Pb(II) in aqueous solutions. Isotherms, kinetics and thermodynamics of Pb(II) adsorption onto modified spent grain were studied. The equilibrium data were well fitted with Langmuir, Freundlich and Dubinin-Radushkevick (D-R) isotherm models. The kinetics of Pb(II) adsorption followed pseudo-second-order model, using the rate constants of pseudo-second-order model, the activation energy (E_a) of Pb(II) adsorption was determined as 12.33 kJ mol⁻¹ according to the Arrhenius equation. Various thermodynamic parameters such as ΔG^ads, ΔH^ads and ΔS^ads were also calculated. Thermodynamic results indicate that Pb(II) adsorption onto modified spent grain is a spontaneous and endothermic process. Therefore, it can be concluded that modified spent grain as a new effective adsorbent has potential for Pb(II) removal from aqueous solutions.     

Simultaneous observation of oxygen uptake curves and electronic states during room-temperature oxidation on Si(0 0 1) surfaces by real-time ultraviolet photoelectron spectroscopy

Ultraviolet photoelectron spectroscopy was used to measure the oxygen uptake, changes in work function due to the surface dipole layer of adsorbed-oxygen atoms, Δ?_SDL, and changes in band bending due to the defect-related midgap state, ΔBB, simultaneously during oxidation on Si(0 0 1) surface at room-temperature, RT, under an O₂ pressure of 1.3 × 10⁻⁵ Pa. The oxygen dosage dependence of Δ?_SDL revealed that dissociatively adsorbed-oxygen atoms occupy preferentially dimer backbond sites at the initial stage of Langmuir-type adsorption, which is associated with a rapid increase of ΔBB. When raising temperature to ∼600 °C, such preferential occupation of the dimer backbond sites by oxygen atoms is less significant and ΔBB becomes smaller in magnitude. The observed relation between Δ?_SDL and ΔBB indicates that point defects (emitted Si atoms + vacancies) are more frequently generated by oxygen atoms diffusing to the dimer backbond sites at lower temperature in RT −600 °C.     

Adsorption performance and mechanism of 2,4,6-trinitrotoluene on a novel adsorption material polyvinylbenzyl acid/SiO2

Polyvinylbenzene (PVB, namely polystyrene, PSt) was grafted on the surface of silica gel particles by “grafting from” in solution polymerization system, and grafting particles PVB/SiO₂ were obtained. The chloromethylation reaction of the grafted polyvinylbenzene was performed using a novel chloromethylation reagent, 1,4-bis (chloromethyoxy) butane that is un-carcinogenic, and grafting particles CMPVB/SiO₂ were obtained. Subsequently, chloromethyl groups on grafting particles CMPVB/SiO₂ were hydrolyzed and oxidized, and finally adsorption material polyvinylbenzyl acid/SiO₂ (PVBA/SiO₂) was prepared. The adsorption performances and mechanism of 2,4,6-trinitrotoluene (TNT) on PVBA/SiO₂ were investigated through static methods. The experimental results showed that PVBA/SiO₂ possessed strong adsorption ability for TNT with adsorption amount of 26.84 mg g⁻¹. The empirical Freundlich isotherm was also found to agree well with the equilibrium adsorption data. In addition, pH was found to have great influence on the adsorption amount. Finally, PVBA/SiO₂ was observed to possess excellent reusability as well.     

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.     

Enhanced photoluminescence properties of Zn2SiO4:Mn co-activated with Y/Li under VUV excitation

A series of green phosphors Zn_1.92−2xY_xLi_xSiO₄:0.08Mn²⁺ (0≤x≤0.03) were prepared by solid-state synthesis method. Phase and lattice parameters of the synthesized phosphors were characterized by powder X-ray diffractometer (XRD) and the co-doped effects of Y³⁺/Li⁺ upon emission intensity and decay time were investigated under 147 nm excitation. The results indicate that the co-doping of Y³⁺/Li⁺ has favorable influence on the photoluminescence properties of Zn₂SiO₄:Mn²⁺, and the optimal photoluminescence intensity of Zn_1.90Y_0.01Li_0.01SiO₄:0.08Mn²⁺ is 103% of that of commercial phosphor when the doping concentration of Y³⁺/Li⁺ is 0.01 mol. Additionally, the decay time of phosphor is much shortened and the decay time of Zn_1.90Y_0.01Li_0.01SiO₄:0.08Mn²⁺ is 3.39 ms, shorter by 1.83 ms than that of commercial product after Y³⁺/Li⁺ co-doping.     

Fourier transform emission spectroscopy of the EΠ-XΣ transition of CaH and CaD

The emission spectra of CaH and CaD have been recorded at high resolution using a Fourier transform spectrometer and bands belonging to the E²Π-X²Σ⁺ transition have been measured in the 20 100-20 700 cm⁻¹ region. A rotational analysis of 0-0 and 1-1 bands of both the isotopologues has been carried out. The present measurements have been combined with the previously available pure rotation and vibration-rotation data to provide improved spectroscopic constants for the E²Π state. The constants ΔG(½) = 1199.8867(34) cm⁻¹, B_e = 4.345032(49) cm⁻¹, α_e = 0.122115(92) cm⁻¹, r_e = 1.986633(11) Å for CaH, and ΔG(½)=868.7438(46) cm⁻¹, B_e = 2.212496(51) cm⁻¹, α_e = 0.036509(97) cm⁻¹, r_e = 1.993396(23) Å for CaD have been determined.     

Heat capacities of the electron acceptor 7,7,8,8-tetracyano- quinodimethane (TCNQ) and its radical-ion salt NH4(TCNQ) showing spin-Peierls transition

Heat capacities of the electron acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its radical-ion salt NH₄-TCNQ have been measured at temperatures in the 12-350 K range by adiabatic calorimetry. A λ-type heat capacity anomaly arising from a spin-Peierls (SP) transition was found at 301.3 K in NH₄-TCNQ. The enthalpy and entropy of transition are Δ_trsH=(667±7) J mol⁻¹ and Δ_trsS=(2.19±0.02) J K⁻¹ mol⁻¹, respectively. The SP transition is characterized by a cooperative coupling between the spin and the phonon systems. By assuming a uniform one-dimensional antiferromagnetic (AF) Heisenberg chains consisting of quantum spin (S=1/2) in the high-temperature phase and an alternating AF nonuniform chains in the low-temperature phase, we estimated the magnetic contribution to the entropy as Δ_trsS_mag=0.61 J K⁻¹ mol⁻¹ and the lattice contribution as Δ_trsS_lat=1.58 J K⁻¹ mol⁻¹. Although the total magnetic entropy expected for the present compound is R ln 2 (=5.76 J K⁻¹ mol⁻¹), a majority of the magnetic entropy (∼4.6 J K⁻¹ mol⁻¹) persists in the high-temperature phase as a short-range-order effect. The present thermodynamic investigation quantitatively revealed the roles played by the spin and the phonon at the SP transition. Standard thermodynamic functions of both compounds have also been determined.     

Electrical conductivity and dynamics of quasi-solidified lithium-ion conducting ionic liquid at oxide particle surfaces

Lithium ion conducting solid-state composites consisting of lithium ion conducting ionic liquid, lithium bis(trifluoromethanesulfonyl)amide (Li-TFSA) dissolved 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)amide (EMI-TFSA), denoted by [yMLi⁺][EMI⁺][TFSA⁻] in this study, and various oxide particles such as SiO₂, Al₂O₃, TiO₂ (anatase and rutile) and 3YSZ are synthesized via a liquid route for the molar concentration of lithium, y, to be 1. The composite consists of SiO₂ and the ionic liquid with y = 0.2 was also prepared. The ionic liquid are quasi-solidified at the above oxide particle surfaces when x is below 40 for y = 1 and x is below 30 for y = 0.2, corresponding to the confinable thickness of the ionic liquid at the oxides' surfaces to be approximately 5-10 nm regardless of the oxide compositions. The electrical conductivities of x vol.%[yMLi⁺][EMI⁺][TFSA^-]-SiO₂, Al₂O₃, TiO₂s or 3YSZ composites are evaluated by ac impedance measurements. The quasi-solid-state composites exhibited liquid-like high apparent conductivity, e.g. 10^− 3.3-10^− 2.0 S cm^− 1 in the temperature range of 323-538 K for SiO₂-ionic liquid composites with y = 1. The self-diffusion coefficients of the constituent species of x vol.% [yMLi⁺][EMI⁺][TFSA⁻] (x is below 40, y = 0.2 and 1) − SiO₂ are evaluated by the Pulse Gradient Spin Echo (PGSE)-NMR technique in the temperature range of 298-348 K. By the quasi-solidification of the ionic liquid at SiO₂ particle surfaces, the absolute values of the diffusion coefficients of all constituent species decreased. The SiO₂ surfaces work to promote ionization of ion pair, [EMI⁺][TFSA⁻], while significant influence on the solvation coordination, [Li(TFSA)_n + 1]ⁿ⁻, was not observed. The apparent transport numbers of Li-containing species both in the bulk and the quasi-solidified ionic liquid showed similar values with each other, which was evaluated to be in the range of 0.010-0.017 for y = 0.2 and 0.051-0.093 for y = 1 in the abovementioned temperature range.     

Interatomic potentials for the ground state X 0 and the two excited states 1, 0 of the intercombination cadmium line 326.1 nm broadened by Kr pressure

The temperature dependence of the Cd line absorption profile at 326.1 nm perturbed by Kr has been carefully studied over a spectral range extending from 800 cm⁻¹ in the blue wing to 1200 cm⁻¹ in the red wing using a high-resolution double-beam spectrometer. The atomic densities of krypton (N_Kr) and cadmium (N_Cd) were (2.015±0.07)×10¹⁹ and (3.62±0.05)×10¹⁸ cm⁻³, respectively. The temperature dependence of the studied line profile was analyzed in the framework of the quasi-static theory. The van der Waals coefficient differences between the ground ¹0⁺ state and the two excited states ³0⁺ and ³1 (ΔC₆⁰ and ΔC₆¹) were obtained from the near red wing profile using Kuhn's law. The values of ΔC₆⁰ and ΔC₆¹ are found to be equal to 37.8±2 and 58.5±3 eV Å⁶, respectively. The ground (X ¹0⁺), and the excited (³1, ³0⁺) state potentials at the internuclear separations from 3.2 to 6.3 Å were determined. The well depths with their positions for these states are respectively equal to 134±7 cm⁻¹, 3.95±0.2 Å; 72.3±4 cm⁻¹, 4.95±0.3 Å; and 471±12 cm⁻¹, 3.6 Å. The obtained well depths with their allowable errors are in good agreement with the values obtained before for the Cd-Kr system from some theoretical results and molecular beams experiments.     

Ultrasensitive and selective spectrofluorimetric determination of Hg(II) using a dimercaptothiadiazole fluorophore

The present work describes the ultrasensitive and selective spectrofluorimetric determination of Hg(II) using 2,5-dimercaptothiadiazole (DMT) as a fluorophore. DMT shows an emission maximum at 435 nm while exciting at 330 nm. The colorless solution of DMT changes into a highly emittive yellow color immediately after the addition of 0.5 μM Hg(II) and nearly 245-fold increase in emission intensity at 435 nm was observed. These changes were ascribed to the complex formation between Hg(II) and DMT. Based on the fluorescence enhancement, the concentration of Hg(II) was determined. The binding constant value (K_A=1.8620×10⁴ mol⁻¹ L) suggests that there is a strong binding force between Hg(II) and DMT. The fluorescence quantum yield of DMT-Hg(II) complex was found to be 4-fold higher than that of DMT, indicating that the DMT-Hg(II) complex was highly emittive than the DMT. Interestingly, the emission intensity was increased even in the presence of 0.1 pM Hg(II). The fluorophore showed an extreme selectivity towards 100 nM Hg(II) in the presence of 50,000-fold higher concentrations of Na⁺, K⁺, Ca²⁺, Mg²⁺, Fe²⁺, Fe³⁺, Cd²⁺, Cr³⁺, Mn²⁺, Zn²⁺, Co²⁺, Ni²⁺, Cl⁻, SO₄²⁻, NO₃⁻ ions and 1000-, 500- and 200-fold higher concentrations of Cu²⁺, Pb²⁺ and Ag⁺ ions, respectively, as interferences. The lowest detection of 18 pg L⁻¹ Hg(II) (LOD=3S/m) was achieved for the first time using DMT by fluorimetry. The proposed method was successfully utilized for the determination of Hg(II) in tap water, river water and industrial waste water samples.     

Studies of the electrochemical reduction processes of Bi, HTeO2 and their mixtures

The reduction process of Bi³⁺, HTeO₂⁺ and their mixtures on Au electrode surface was studied by cyclic voltammetry, linear sweep voltammetry, electrochemical impedance spectroscopy and chronoamperometry. XRD and EDS methods were also used to measure the reductive products prepared under different potentials and provide the evidences of the reactions. The results indicate that the reduction of HTeO₂⁺ occurs at more positive potential than that of Bi³⁺, but its reduction rate is slower and adsorption phenomenon exists during its reduction process. Bi₂Te₃ compound can be obtained potentiostatically at a proper potential in all the mixed solutions with concentration ratio CHTe+O₂/CBi3+ in our research range (0.1-10). But pure Bi₂Te₃ compound can only be obtained at 42 mV in the solution with concentration ratio CHTe+O₂/CBi3+ equaling to 1. And the formation of Bi₂Te₃ compound is an inductive co-depositing process: (1) HTeO₂⁺ + 4e⁻ + 3H⁺ → Te⁰ + 2H₂O, (2) 3Te⁰ + 2Bi³⁺ + 6e⁻ → Bi₂Te₃.     

The bΣ(b0) → XΣ(X10,X21) and aΔ(a2) → X21 transitions of AsBr

Emission spectra of the b¹Σ⁺(b0⁺) → X³Σ⁻(X₁0⁺,X₂1) and a¹Δ(a2) → X₂1 transitions of AsBr have been measured in the near-infrared spectral region with a Fourier-transform spectrometer. The arsenic bromide radicals were generated in fast-flow systems by reaction of arsenic vapor (As_x) with bromine and were excited by microwave-discharged oxygen. The most prominent features in the spectrum are the Δv = +1,0,−1, and −2 band sequences of the b¹Σ⁺(b0⁺) → X³Σ⁻(X₁0⁺) transition in the range 11 700-12 700 cm⁻¹. With lower intensities, the Δv = 0 and −1 sequences of the b¹Σ⁺(b0⁺) → X³Σ⁻(X₂1) sub-system show up in the same range. Further to the red, between 6000 and 6700 cm⁻¹, the Δv = 0, +1, and −1 sequences of the hitherto unknown a¹Δ(a2) → X₂1 transition are observed. Analyses of medium- and high-resolution spectra have yielded improved molecular constants for the X₁0⁺, X₂1, and b0⁺ states and first values of the electronic energy and the vibrational constants of the a2 state.     

Nanofibrous membrane of polyacrylonitrile with efficient adsorption capacity for cadmium ions from aqueous solution: Isotherm and kinetic studies

The nanofibrous membrane of polyacrylonitrile (NMP) was successfully synthesized after NaOH and NaHCO₃ treatment aiming its functionalization using electrospinning for cadmium ion (Cd²⁺) adsorption. Field emission scanning electron microscopy (FE-SEM) revealed that small particles attached to the surface of functionalized PAN nanofibers. Equilibrium was attained after 60 min following a rapid uptake of Cd²⁺ with maximum adsorption capacity and percentage removal at an optimum solution pH of 7.0. The adsorbent dose of 0.3 g and 90 mg L⁻¹ of initial Cd²⁺ concentration yielded the maximum adsorption capacity. The pseudo-second-order kinetic model was the best fitted to the adsorption data, indicating that the chemisorption is the controlling mechanism of adsorption. The physisorption was proposed based on the calculated values of the mean free energy of adsorption from the D–R isotherm (E < 8 kJ mol⁻¹). Furthermore, three-parameter isotherm models indicated the homogeneous and heterogeneous Cd²⁺ adsorption onto NMP adsorbent.     

Ethylene adsorption on the Pt-Cu bimetallic catalysts. Density functional theory cluster study

The adsorption of ethylene on Cu₁₂Pt₂ clusters has been studied within the density functional theory (DFT) approach to understand the high ethylene selectivity of Cu-rich Pt-Cu catalyst particles in the reaction of hydrogen-assisted 1,2-dichloroethane dechlorination. The structural parameters for Cu₁₂Pt₂ clusters with D_4h, D_2d, and C_3v symmetry have been calculated. The relative stability of the isomeric Cu₁₂Pt₂ clusters follows the order: C_3v > D_2d > D_4h. Each isomer has an active site for ethylene adsorption that consists of a single Pt atom surrounded by Cu atoms. The interaction of ethylene with the active site yields a π-C₂H₄ adsorption complex. The strongest π-C₂H₄ complex forms with the cluster of C_3v symmetry; the bonding energy, ΔE_π(C₂H₄), is −15.6 kcal mol⁻¹. The bonding energies for the π-C₂H₄ complex with Cu₁₄ and Pt₁₄ clusters are −6.5 and −18.8 kcal mol⁻¹, respectively.The addition of Pt to Cu modifies the valence spd-band of the cluster as compared to a Cu₁₄ cluster. The DOS near the Fermi level increases when C₂H₄ adsorbs on the Cu₁₂Pt₂ cluster. As well, the center of the d-band shifts toward lower binding energies. Ethylene adsorption also induces a number of states below the d-band. These states correspond to those of gas-phase C₂H₄.The vibrational frequencies of C₂H₄ adsorbed on the clusters of D_4h and C_3v symmetry have been calculated. The phonon vibrations occur below 250 cm⁻¹. The intense bands around 200 cm⁻¹ are attributed to stretching vibrations of the Pt-Cu bonds normal to the cluster surface. The stretching vibrations of the Pt-C bonds depend on the local structure of the active site: ν_s(Pt-C) = 268 cm⁻¹ and ν_as(Pt-C) = 357 cm⁻¹ for the cluster of the D_4h symmetry; ν_s(Pt-C) = 335 cm⁻¹ and ν_as(Pt-C) = 397 cm⁻¹ for the cluster of the C_3v symmetry. Bands in the range of 800-3100 cm⁻¹ are attributed to vibrations of the adsorbed C₂H₄ molecule. The signature frequencies of the π-C₂H₄ adsorption complex are the δ_s(CH₂) deformation vibration at ∼1200 cm⁻¹ and the ν(C-C) stretching vibration at ∼1500 cm⁻¹. These vibration are absent for di-σ-C₂H₄ adsorption complexes.