Universal relation between the boundary temperatures of a basic cycle of sorption heat machines

For analyzing a basic cycle of an adsorption heat machine (AHM) an empiric rule was suggested, which manifests that adsorption isosters and equilibrium line for the pure sorbate intersect at T approaching infinity. This rule prompts how to plot the cycle, gives a link between the boundary temperatures of the cycle and allows estimation of a minimal temperature of an external heat source that is necessary to drive the cycle. In this paper the validity of the estimation was justified for working pairs which are most commonly used for adsorption units: water–silica gel, water–zeolite 13X, water–zeolite 4A, water–selective water sorbents (SWSs), CO₂–carbon, methanol–carbons (AC-35, TA90), methanol–hydrophobic zeolite CBV 901 Y and ammonia–carbon PX31. Four main working pairs for absorption heat machines—ammonia–water, water–LiBr, methanol–LiBr and R22–isobutylacetate—are also analyzed. This allowed the formulation of requirements to an optimal adsorbent to be used in a single-effect non-regenerative cycle of an AHM. The accuracy of the estimation was examined for each pair. Moreover, it was shown that Trouton's rule is always valid if sorption equilibrium obeys the Polanyi potential theory, i.e., the equilibrium sorption is a unique function of the sorption potential ΔF=-RTln(P/P₀). For chemical reactions between various salts and sorbates this rule is violated because of a large difference between the standard changes of the entropy and enthalpy in the course of reaction and evaporation. In this case can be calculated from the Clausius–Clapeyron and Vant-Hoff equations.     

Permeation behavior during the catalytic oxidation of CO in a Pt-loaded Y-type zeolite membrane

A Pt-loaded Y-type zeolite (Pt/NaY) membrane was prepared on the surface of a porous α-Al₂O₃ support tube by hydrothermal synthesis and then ion-exchanged with platinum. The thickness of the zeolite layer and the amount of Pt loaded were ca. and , respectively. The membrane was employed in the form of a cylindrical thin catalyst for the selective oxidation of CO in an H₂-rich mixture. A mixture of CO, O₂ and H₂ was fed to the outer surface of the membrane, and CO was selectively oxidized during its permeation through the thin layer. The permeation fluxes for H₂ and CO were determined at 423-. Permeation fluxes also calculated by means of a mathematical model using effective diffusion coefficients and reaction kinetics. The effective diffusion coefficients through the zeolite membrane were estimated from gas permeation test data, obtained at 423-, and the oxidation rates of CO were determined over a particulate catalyst that had the same composition as the Pt/NaY membrane. As a result, the diffusion coefficients of O₂, N₂ and CO were determined to be (0.7-1.0) at 423-, and the activation energies for the rate constants for CO oxidation were 61-. The predicted permeation fluxes of H₂ and CO using the mathematical model were in good agreement with the experimental data, when the oxidation selectivity of CO to H₂ was assumed to be 80% in the model calculation.     

Kinetic analysis on LiFePO4 thin films by CV, GITT, and EIS

LiFePO₄ thin films were deposited on Ti substrates by pulsed laser deposition (PLD). The apparent chemical diffusion coefficients of lithium in the films, , were measured by cyclic voltammetry (CV), galvanostatic intermittent titration technique (GITT), and electrochemical impedance spectroscopy (EIS). The average values calculated from CV results were in the order of 10⁻¹⁴ cm² s^–1. The values obtained by GITT, and EIS techniques were in the range of 10^–14–10^–18 cm² s^–1, 10^–14–10^–18 cm² s^–1, respectively. The values obtained by the two methods show a minimum point at x ∼ 0.5 for Li_1−xFePO₄. However, the overpotential values of the LiFePO₄ thin film electrodes obtained from the GITT results and the diffusion impedance deduced from the impedance spectra also show the minimum values at x ∼ 0.5 for Li_1–xFePO₄. This contradict could be caused by the improper use of GITT and EIS techniques for measuring the chemical diffusion coefficient of Li in Li_1–xFePO₄ which constitutes two phase, i.e., LiFePO₄ and FePO₄ in this region.     

Deposition of ultrafine aerosol particles on wire screens by simultaneous diffusion and image force

This paper presents the results of an experimental investigation on the deposition of multiply charged particles on wire screens by the combined mechanisms of diffusion and image force. Experiments were performed with particles having diameters between 25 and 65 nm (transition regime), carrying 0, +1, +2 or +3 elementary charges, and using three different flow rates, two types of wire screen, and two types of test aerosol. The single fiber efficiencies for the mechanisms of image force, η_IM, and diffusion, η_D, are of the same order of magnitude and, furthermore, they are both much smaller than one. Under these conditions, the total capture efficiency can be approximated as the sum of the efficiencies by diffusion and image force deposition. Theoretically, η_IM is proportional to the square root of a dimensionless number, K_IM, which includes all the relevant parameters cited above (i.e., particle size and charge, aerosol flow rate and screen geometry). The available correlations for η_IM, obtained from experiments with particles carrying a large number of elementary charges (K_IM>10^-5), predict that image force should not have any effect in the case of the small particles with very few number of charges that we have tested in our experiments (in our experimentation, K_IM ranged between 10^-7 and 10^-5). Our results, the only ones available to date for this particle size range, show that there is indeed a clear, measurable effect. Although our experimental results are best fitted by the correlation , it is shown that the expression , which is in agreement with the theoretical 1/2 exponent for K_IM, also reproduces reasonably well the measured values.     

Preparation of bipolar zeolite exchangers by gas-phase modification using NOx

Zeolites with varying Si/Al ratio, were synthesized using silica and alumina powder and by matching the XRD patterns with those given in the JCPDS files their chemical formula and crystal structure were determined. In order to make these into bipolar zeolite exchangers, they were modified (called Z₂ zeolite) using a gas-phase reaction with NO_x at 225°C and the formation of the oxynitride groups was confirmed with FTIR, ESCA and the elemental analysis. The XRD patterns of the modified zeolites were found to have changed and did not match with any of the patterns given in the JCPDS files. For these modified zeolites, we have determined the crystal structures and these were found to change from cubic for the unmodified to orthorhombic for the modified zeolites with Si/Al ?1.2 and to the tetragonal for Si/Al >1.2.The oxynitride covalent bond in Z₂ zeolites could be reduced by reacting with hydrazine hydrate to give bipolar zeolite exchangers (denoted as Z₃ zeolite) containing imine groups, thus making the zeolite bipolar in nature. The cation exchange capacities were determined and found to have the same order of magnitude as reported in the literature. The elemental analysis of the modified zeolites was carried out and the experimentally determined nitrogen value of gives 0.9-1.2 oxynitride group per formula unit of the material. The experimentally determined anion exchange capacity of the Z₃ zeolites ( for sample A1) suggests that for this value of exchange capacity, about two out of three formula units have an imine group. The difference between these two independent experiments suggests that not all imine groups participate in the anion exchange phenomena due to their non-availability. We found that Z₃ zeolite responds to organic reactions, and can be reacted with dichloroethane and subsequently quarternized with trimethylamine. Their anion exchange capacity is found to increase significantly.     

Direct oxidation of sodium borohydride on Pt, Ag and alloyed Pt–Ag electrodes in basic media: Part II. Carbon-supported nanoparticles

We studied the borohydride oxidation reaction (BOR) by voltammetry in 0.1 M NaOH/10⁻³ M BH₄⁻ on carbon-supported Pt, Ag and alloyed PtAg nanoparticles (here-after denoted as Pt/C, Ag/C and Pt–Ag/C). In order to compare the different electrocatalysts, we measured the BOR kinetic parameters and the number of electrons exchanged per BH₄⁻ anion (faradaic efficiency). The BOR kinetics is much faster for Pt/C than for Ag/C (i_Pt=0.15, i_Ag=3.1×10⁻⁴ A cm⁻² at E=−0.65 V vs. NHE at 25 °C), but both materials present similar Tafel slope values. The n value involved in the BOR depends on the thickness of the active layer of electrocatalysts. For a “thick layer” (approximately 3 m), n is nearly 8 on Pt/C and 4 on Ag/C, whereas n decreases for thinner Pt/C active layers (n2 for thickness <1 m). These results are in favour of the sequential BH₄⁻ hydrolysis (yielding H₂) followed by hydrogen oxidation reaction (HOR), or direct sequential BOR on Pt/C, whereas Ag/C promotes direct but incomplete BOR (Ag has no activity regarding hydrogen evolution reaction, HER). The n value close to 8 for the thick Pt/C layer displays the sufficient residence time of the molecules formed (H₂ by heterogeneous hydrolysis or BOR intermediates) within the active layer, which favours the complete HOR and/or BOR. Two PtAg/C nanoparticles alloys have been tested (noted APVES-4C and APVES-E1). They show different behavior; the borohydride oxidation reaction kinetics is faster on APVES-E1 than on APVES-4C (b=0.15, and b=0.31 V dec⁻¹,  A cm⁻², respectively, at 25 °C), but the n values are higher on APVES-4C than APVES-E1 (nearly 8 vs. 3, respectively, at 25 °C). These discrepancies probably originate from the heterogeneity of such bimetallic materials, as observed from physicochemical characterizations.     

Systematics of salt precipitation in complexes of polyethylene oxide and alkali metal iodides

Conductivity measurements in PEO₃₀MI polymer electrolytes with M=Li, Na, K, Rb, or Cs over the temperature range from about 65 to 200 °C show an increasing tendency for salt precipitation with increasing cation size. The salt precipitation in these complexes upon heating is revealed by the decrease of the dc conductivity starting at a critical temperature T_c. Whereas LiI and NaI complexes do not show precipitation effects, T_c monotonically decreases from about 140 to 65 °C when changing the salt component from KI via RbI to CsI. For the PEO-RbI system, precipitation is further investigated by nuclear magnetic resonance (NMR) and tracer diffusion experiments. NMR analysis unambiguously demonstrates the onset of RbI salt precipitation and the increase of the precipitate fraction with increasing temperature. In diffusion experiments on PEO₃₀RbI with the radiotracers and , the precipitation effect is manifested by anomalous features in the penetration profiles, however, without noticeable changes in their depth range. Combining the resulting tracer diffusion coefficients with the dc conductivity data enables us to assess crucial parameters characterizing ionic transport in PEO₃₀RbI.     

Ion and solvent diffusion and ion conduction of PC-DEC and PC-DME binary solvent electrolytes of LiN(SO2CF3)2

Two binary mixed solvent systems typically used for lithium batteries were studied by measuring the self-diffusion coefficients of the solvent, lithium ion and anion, independently by using the multi-nuclear pulsed field-gradient spin-echo (PGSE) , and NMR method. One system was propylene carbonate (PC) and diethyl carbonate (DEC) system and the other binary system was PC and 1,2-dimethoxyethane (DME), and the lithium salt used was LiN(SO₂CF₃)₂ (LiTFSI). The relative ratio of the PC was changed from zero (pure DME and DEC) to 100% (pure PC) in the DME-PC and the DEC-PC systems, respectively. The self-diffusion coefficients of the solvents were measured with and without the lithium salt, and the two solvents had almost the same diffusion coefficient in the DEC-PC system, while DME diffused faster than PC in the DME-PC system. In the electrolytes the solvents diffused the fastest, followed by the anion with the lithium ion diffusing the slowest. The degree of ion dissociation was estimated for each electrolyte by comparing the ionic conductivities estimated from the ion diffusion and those measured directly by the electrochemical method.     

Unusual iron-mediated C–N bond formation and synthesis of the Fe(III) complex of a polypyridine ligand with one carboxamide group

A high-spin iron(III) chloro complex [(PaPPy₃)Fe(Cl)](ClO₄) (1, where =N,N-bis(2-pyridylmethyl)amine-N-propyl-2-pyridine-2-carboxamide) has been synthesized via the “template effect” and structurally characterized. The template reaction leads to the formation of a new carbon–nitrogen bond from coupling of a primary alkyl chloride with a secondary amine.     

Os(dppe)(dppe monoxide)(CO)Cl2 as an active intermediate in the synthesis of strongly luminescent divalent osmium complexes

Reaction of K₂OsCl₆ with 1,2-bis(diphenylphosphino)ethane (dppe) in 1:2 ratio afforded the osmium(II) Os(dppe)(dppeO)(CO)Cl₂ complex (where dppeO = 1,2-bis(diphenylphosphino)ethane monoxide) with the molecular structure confirmed by X-ray investigations. The Os(dppe)(dppeO)(CO)Cl₂ intermediate can be simply converted to species in reaction with N ∩ N chelating diimines like 1,10-phenanthroline, 4,7-diphenyl-1,10-phenanthroline or 3,4,7,8-tetramethyl-1,10-phenanthroline. The obtained mixed complexes feature strong (with _em up to 0.57) long lived (with τ_em up to 25.2 μs) MLCT emission.     

Anion exchanged polymerized ionic liquids: High free volume single ion conductors

In this study, we investigate the isolated effect of anion type on the chemical, thermal, and conductive properties of imidazolium-based polymerized ionic liquids (PILs). PILs with various anions at constant average chain length were prepared by ion exchange with a water-soluble PIL precursor, (poly(1-[(2-methacryloyloxy)ethyl]-3-butylimidazolium bromide) (poly(MEBIm-Br)). NMR, IR, and elemental analysis confirm that anion exchange of ploy(MEBIm-Br) with bis(trifluoromethanesulfonyl) imide (TFSI), tetrafluoroborate (BF₄), trifluoromethanesulfonate (Tf), and hexafluorophosphate (PF₆) in water resulted in nearly fully exchanged PILs. As a function of anion type, the glass transition temperature plays a dominant role, but not the sole role in determining ion conductivity. Other factors affecting ionic conductivity include the size and symmetry of the anion and dissociation energy of the ion pair. Both the Vogel-Fulcher-Tammann (VFT) and Williams-Landel-Ferry (WLF) equations were employed to investigate the temperature dependent ionic conductivities. The (9.03) and (168 K) values obtained from the WLF regression of these PILs greatly deviate from the classical WLF values originally obtained from the mechanical relaxation of uncharged polymers ( = 17.44,  = 51.6 K) and the WLF values obtained from the conductive properties of other polymer electrolytes. This suggests that the fractional free volume (f (T_g) = B/(2.303)) and Vogel temperature (T₀ = T_g − ) are strong functions of ion concentration, where high free volume allows for ion mobility at temperatures farther below the glass transition temperature of the polymer.     

Hierarchical structures in agar hydrogels

Small angle neutron scattering experiments were performed on agar solutions and gels to explore their differential microscopic structures. In solution state, the wave vector, q, dependence of static structure factor, I(q), could be described by . Statistical analysis gave: R_g = 18 nm and α = 0.85 ± 0.07 indicating the existence of rod-like rigid structures of length, R_g ≈ 63 nm. In gels, which had discernible Gaussian, power-law and Kratky–Porod regimes in the low, intermediate and high-q regions. Regression analysis yielded a characteristic length, Ξ = 3.3 − 4 nm for gels with agar concentration, c = 0.1 − 0.3% (w/v). The exponent β = 1.2 ± 0.2 and the cross-sectional radius of cylindrical fibres, R_c = 1.5 ± 0.3 nm remained invariant of agar concentration. This assigned a value 5 nm to the persistence length of the fibres in the solution phase that reduced to 3 nm in the gel phase indicating differential hydration of the fibres.     

Dynamics of diffusion with reversible binding in microscopically heterogeneous membranes: General theory and applications to dermal penetration

Essentially all biological membranes and tissues exhibit microscopic heterogeneity in the form of cellular, lamellar or other organization, and molecular diffusion in these materials is frequently slowed by binding to elements of the microstructure (“trapping”). This paper addresses situations where binding is describable as a linear reversible process at the microscale, with forward (“on”) and reverse (“off”) rate constants k_f(x) and k_r(x) that vary with position. Very commonly it is tacitly assumed that the macroscopically observable binding behavior should follow the same rate law with the substitution of appropriate effective (tissue-average) rate constants and . This assumption is probed theoretically for spatially periodic microstructures using a judicious application of numerical calculations and asymptotic analysis to prototypical one-dimensional transport problems. We find that smooth microscopic variations produce an anomalous macroscopic exchange between free and bound solute populations that is not well described by a single pair of forward and reverse rate constants, i.e., violates the usual paradigm. In contrast, discontinuous variations (as in two-phase composite media) are evidently well described by the usual paradigm. For the latter case we derive simple and general algebraic equations giving and , and generalize them to any three-dimensional unit cell representing the tissue microstructure. Validity of the formulas is demonstrated with reference to a concrete example describing molecular diffusion through the stratum corneum (barrier) layer of skin, comprising lipid (intercellular) and corneocyte (cellular) phases. Our analysis extends coarse-graining (homogenization, effective transport) theory for irreversible trapping systems to the reversible case.     

NMR evidence of different conformations of structure-directing cyclohexylamine in high-doped AlPO4-44 materials

A series of high-crystalline ZnAPSO-44 samples have been successfully synthesised using cyclohexylamine as structure-directing agent, keeping a constant (Si + Zn)/(Al + P) ratio of 0.25 and systematically varying the Si/Zn ratio. Chemical analyses by ICP shows a good correlation between the heteroatom (Si and/or Zn) content of the samples and that of the initial gels. A combination of single crystal and powder X-ray diffraction techniques evidences a high crystallinity and a total purity for all samples. In addition, single crystal X-ray diffraction studies of the samples from which large crystals could be isolated, allow us to certify that either the whole or the majority of the cyclohexylamine molecules adopt a NH₂- or -equatorial conformation inside of the pores. Furthermore, for the first time, ¹H MAS NMR spectroscopy was used as a proof of the protonation of SDA molecules within the microporous materials, unequivocally indicating that cyclohexylamine molecules were protonated. ¹³C MAS NMR studies indeed corroborated such protonation state of the SDA molecules and, more importantly, allowed to detect the presence of a mixture of -equatorial and -axial conformers of cyclohexylamine in the Si-richest samples, the former being always the dominant. Based on both ¹³C and ²⁹Si cross-polarization MAS NMR measurements as well as on the calculated interaction energies of the two conformers as a function of the framework composition, it is proposed that this non-described -axial conformation of cyclohexylammonium within the microporous materials was associated to the presence of SiO₂ islands into the AlPO₄ framework, whereas the -equatorial conformer was the only one found in the case of isolated incorporation of Si⁴⁺ (and/or Zn²⁺) ions.     

Selective mesoporous adsorbents for and Cu separation

NH₂–MCM-41 and COONa–MCM-41 were prepared by grafting aminopropyls and propionates on the pore wall of the mesoporous silica MCM-41 for the separation of and Cu²⁺. Single and binary component adsorptions show that has a 100% selectivity for adsorption at low pH (<3.5 i.e., point of zero charge, p.z.c.), while COO⁻Na⁺–MCM-41 adsorbs only Cu²⁺ over the entire pH range of the study (i.e., pH 1.5–5.5). Adsorption uptakes were rapid on both adsorbents reaching equilibrium in less than 3 h and in most cases less than 10 min. The adsorption capacity of for is high, approaching the theoretical values of two aminopropyls for one , the Cu²⁺ adsorption on COO⁻Na⁺–MCM-41 was less efficient requiring five propionates to adsorb one Cu²⁺. High purity hydrogen dichromate and copper chloride solutions were respectively recovered by a simple acid wash and the regenerated adsorbents were reused without a loss of performance.     

The effect of tert-butanol adsorption on the two-step electroreduction of Zn and its dependence on the NaClO4 concentration

The influence of water molecules on the adsorption of organic substances and kinetics of electroreduction of inorganic depolarizers is very often overlooked. The electroreduction of Zn²⁺ is a typical example of a reaction controlled by both diffusion and charge transfer. This process in 1.0, 0.5 and 0.1 M NaClO₄ solutions at a mercury electrode in the presence of tert-butanol is expected to involve two consecutive one-electron transfer steps in the overall reaction. Solutions of tert-butanol were prepared to cover the concentration range from 0.01 to 0.3 M. Measurements were performed using an impedance method for a wide range of both the potential and frequency. The difference between the potential of the anodic and cathodic peaks, which was obtained from the cyclic voltammetry method, increases with an increase in the concentration of both tert-butanol and NaClO₄. From the dependences of , the true standard rate constants and the constants and characterizing the stage of the first electron transfer and the stage of the second electron exchange, respectively, were determined. The obtained results indicate that the inhibiting properties of tert-butanol are the weakest in 0.1 M NaClO₄. In addition the stage of the second electron transfer is less sensitive to the inhibiting effect of tert-butanol than the stage of the first electron exchange.     

Determination of adsorption and diffusion parameters in zeolites through a structured approach

Temporal analysis of products (TAP) is a transient pulse-response technique that allows to extract kinetic information from reacting and adsorbing systems. In a previous work (Chem Eng Sci 57(2002) 1835), a detailed-transport model for the Multitrack set-up, a TAP-like system, was developed, which allows studying systems with a low bed resistance. The use of structured beds, having both a low bed resistance and small sorbent particles, is required to determine adsorption and diffusion parameters when strong adsorption and slow diffusion occurs. A method is presented to extend the range of measurable adsorption and diffusion parameters in zeolitic sorbents in the TAP technique by a structured approach. Small zeolite crystals are coated on larger non-porous glass beads. Adsorption and diffusion parameters for n-butane, SF₆ and 3-methylpentane have been determined in MFI-type zeolites. Absolute diffusivity values in the zeolite coating are estimated by using a well-defined silicalite sample as a reference to determine the effective diffusion length in the coating.Two criteria have been derived, one for the characteristic time for transport through the bed, , and one for the ratio of the latter and the characteristic diffusion time in the zeolite crystal, 0.01<α<200t_bed/(1+t_bed), which should be satisfied both to be able to determine values of the zeolite diffusivity.