Low Heat Of Adsorption Research Articles

CO2Adsorption-Based Separation by Metal Organic Framework (Cu-BTC) versus Zeolite (13X)

The potential for the metal organic framework (MOF) Cu-BTC to selectively adsorb and separate CO2 is considered. Isotherms for CO2, CH4, and N2 were measured from 0 to 15 bar and at temperatures between 25 and 105 °C. The isotherms suggest a much higher working capacity (×4) for CO2 adsorption on Cu-BTC relative to the benchmark zeolite 13X over the same pressure range. Higher CO2/N2 and CO2/CH4 selectivities in the higher pressure range (1−15 bar) and with lower heats of adsorption were also demonstrated. Cu-BTC was observed to be stable in O2 at 25 °C, but its crystallinity was reduced in humid environments. The CO2 adsorption capacity was progressively reduced upon cyclic exposure to water vapor at low relative humidity (<30%), but leveled out at 75% of its original value after several water adsorption/desorption cycles.

Schiff bases of ethylenediamine/triethylenetetramine with benzaldehyde/cinnamic aldehyde/salicylaldehyde as corrosion inhibitors of zinc in sulphuric acid

PurposeA survey of technical literature indicates that there is a shortage of excellent corrosion inhibitors for zinc in non‐oxidizing acids, particularly sulphuric acid. This paper aims to describe the behaviour of ethylenediamine‐N‐N′‐dibenzylidene, ethylenediamine‐N‐N′‐disalicylidene, ethylenediamine‐N‐N′‐dicinnamylidene, triethylenetetramine tribenzylidene and triethylenetetramine trisalicylidene as corrosion inhibitors for zinc in sulphuric acid solutions. The objective of this research work also is to have an insight into the action mechanism of these inhibitors.Design/methodology/approachThe effect of the various parameters affecting the action of the above‐mentioned corrosion inhibitors has been studied using weight‐loss data and polarisation measurements. Adsorption data also were utilized.FindingsThe inhibitors showed excellent corrosion inhibition (&gt;99 per cent) at effective inhibitor concentrations. The two salicylidenes were better corrosion inhibitors than were corresponding benzylidenes. It appeared from this study that an efficient inhibitor is characterised by a relatively greater decrease in free energy of adsorption, lower entropy of adsorption and relatively lower heat of adsorption. Basically, these inhibitors were cathodic, as was revealed by polarisation data, and the inhibitors followed Langmuir adsorption isotherm behaviour. In general, the conjoint action of the inhibitor and the cathodic current was synergistic.Research limitations/implicationsPowerful Schiff bases still need to be synthesized so as to be effective at extremely low concentrations. The behaviour with other metals and alloys in diverse media also requires to be investigated.Originality/valueVery few inhibitors demonstrate such excellent corrosion inhibition of zinc in aggressive corrosive media. Such detailed investigations on corrosion inhibitors are unusual.

Hyperpolarized 129Xe Nuclear Magnetic Resonance Studies of Isoreticular Metal-Organic Frameworks

The pore environments of a series of isoreticular metal-organic frameworks (IRMOF) have been studied using hyperpolarized (HP) 129Xe nuclear magnetic resonance (NMR) spectroscopy. Xenon gas behaved as an efficient probe molecule for interrogating the variability of adsorption sites in functionalized IRMOF materials through variations in the NMR chemical shift of the adsorbed xenon. The xenon adsorption enthalpies extracted from variable-temperature HP 129Xe NMR were found to be lower than published values for the physisorption of xenon. The low heats of adsorption were corroborated by xenon adsorption measurements that revealed two atoms per pore under one atmosphere of pressure at 19 °C. Average pore diameters estimated from the empirical chemical shift and pore size correlations based on a geometrical model were compared with X-ray crystallography data. The exchange processes of xenon in IRMOFs also were explored using 2D exchange spectroscopy (EXSY) 129Xe NMR. It was found the exchange of xenon from adsorption sites within the IRMOF to the free gas space is much slower than that between the adsorption sites within the lattice. Cross-polarization experiments showed that the preferred adsorption sites were spatially removed from the phenylene rings of the network. This agrees with previous spectroscopic, structural, and computational studies of gas adsorption (H2, N2, Ar) in IRMOFs that indicate the preferred binding sites reside near the carboxylate groups of the inorganic clusters.

Interaction of Alcohols and Ethers with a-CFx Films

The surfaces of the magnetic data storage hard disks used in computers are coated with a thin film of amorphous carbon and a layer of perfluoropolyalkyl ether (PFPE) lubricant. Both protect the surface of the magnetic layer from contact with the read-write head flying over the disk surface. Although the most commonly used carbon films are amorphous hydrogenated carbon, a-CH(x), it has been suggested that the thermal properties of amorphous fluorinated carbon films, a-CF(x), might be superior. This work has probed the interaction of small fluorinated ethers and alcohols with the surfaces of a-CF(x) films to understand the effects of carbon film fluorination on the interaction of the lubricant with its surface. Temperature-programmed desorption was used to measure the desorption energies of small fluorocarbons from the a-CF(x) surface and to compare their desorption energies with those from the surfaces of a-CH(x) films. These measurements reveal that, similarly to a-CH(x) films, a-CF(x) films expose a heterogeneous surface on which fluorocarbons adsorb at sites with a range of binding energies. The fluorocarbon ethers all have lower heats of adsorption than their hydrocarbon counterparts, suggesting that the ethers adsorb by donation of electron density from the oxygen lone-pair electrons to sites on the surface. Fluorinated alcohols have roughly the same heats of adsorption as their hydrocarbon counterparts. There is little significant difference between the interactions of fluorinated ethers (or alcohols) with the surfaces of either a-CF(x) or a-CH(x) films.

Calorimetric and FTIR Study of the Acid Properties of Sulfated Titanias

Titanium oxides of different surface areas were sulfated then calcined to convert the solid to a strong acid. The amount of sulfur retained by the solid and the thermal stability of the resulting sulfate are controlled by the dispersion of the initial oxide. The acid properties were determined by gravimetry at 383 K, calorimetry using ammonia adsorption at 353 K, and by quantitative analysis of the infrared spectra of pyridine retained after evacuation at 423 K. A good agreement was observed between the different determinations. At low coverage of ammonia, sulfated titanias show a much lower heat of adsorption, and the IR study of NH3 adsorption shows that the first doses of NH3 dissociate at the surface with the formation of OH species. The lower heat of adsorption is then attributed to the contribution of NH3 dissociation to the differential heat of adsorption. IR spectroscopy indicates that NH3 reacts with sulfates and may lead to the transformation of disulfate species into monosulfate species on sulfated titania dioxide. A band at ca. 3574 cm-1 has been assigned to nu(OH) of monosulfate species. This particular behavior makes it difficult to appreciate the initial acidity of these sulfated oxides.

Effect of Surface Resistances on the Diffusion of Binary Mixtures in the Silicalite Single Crystal Membrane

Diffusion of methane and argon mixtures through the silicalite single-crystal membrane is studied using the dual-control volume-grand canonical molecular dynamics method to understand how surface resistances alter selectivity and permeance. Comparison of results from intracrystalline transport and entrance simulations for binary mixtures of CH4 and Ar shows that the selectivity of silicalite membranes toward Ar is enhanced in the presence of the surface resistances. In both cases, however, diffusion of faster Ar molecules was inhibited by slower diffusing CH4 molecules, whereas diffusion of the latter remained unaffected. This behavior was explained by the difference between the magnitudes of surface resistances for two molecules, which is much smaller for Ar because of its smaller permeant-crystal interaction size. We find that selectivity of the membrane at the surface depends strongly on total feed pressure and temperature, whereas this dependence is weak for intracrystalline diffusion. Furthermore, we show that the selectivity at the surface diminishes with crystal thickness until a certain thickness is reached, whereas the intracrystalline selectivity remains constant with increasing thickness. Finally, a study of diffusion of C2H6 and CF4 mixtures shows that the diatomic ethane molecules diffuse faster inside the zeolite channels, but their desorption is hindered to a larger extent than that of a spherical molecule with larger diameter and lower heat of adsorption. This observation indicates that the difference in molecular geometry is also a significant factor to explain the exit effect.

Adsorption of sodium dodecyl benzenesulfonate onto coal

The adsorption behavior of sodium dodecyl benzenesulfonate (NaDDBS) on a raw (as received) coal sample and its demineralized variety with 11.3% and 1.2% ash contents respectively has been studied. The samples have been characterized by their proximate analysis, particle size distribution, surface area, porosity, density, points of zero charge, etc. Adsorption of NaDDBS on these two samples has been studied as a function of concentration of NaDDBS, temperature, pH, and presence of indifferent electrolyte in the medium. It has been observed that the isotherm exhibits two adsorption plateaus below and above the critical micelle concentration (CMC) of NaDDBS. Low heats of adsorption suggest weak hydrophobic bonding between adsorbent and adsorbate. The present work aims to correlate the adsorption of surfactant onto coal particles with the rheological behavior of coal–water slurry (CWS). The results reveal that addition of a very small amount of NaDDBS (0.3 wt% of coal) to 60% (w/w) CWS results in a marked reduction of the apparent viscosity of the CWS at a shear rate of 100 s −1. The effect of pH on the apparent viscosity of CWS with and without the presence of the surfactant is also investigated.

Schiff bases of ethylenediamine as corrosion inhibitors of zinc in sulphuric acid

The work described here deals with the performance of ethylenediamine N, N ′-dibenzylidene, ethylenediamine N, N ′-di( p-methoxybenzylidene), ethylenediamine N, N ′-disalicylidene as corrosion inhibitors for zinc in sulphuric acid. The effect of various parameters on the efficiency of these inhibitors has been studied. Ethylenediamine N, N ′-di( p-methoxybenzylidene) and ethylenediamine N, N ′-disalicylidene give 99% protection under a variety of conditions. Activation energies in the presence and absence of inhibitors have been calculated. It appears that an efficient inhibitor is characterized by a relatively greater decrease in free energy of adsorption, relatively lower entropy of adsorption and relatively lower heat of adsorption. Galvanostatic polarisation studies indicate that these are basically cathodic inhibitors. Cathodic protection in the presence of these inhibitors has been studied. With an efficient inhibitor, cathodic protection is achieved at potentials much less negative than that required for plain acid. The difference between protective potential and corrosion potential appears to be less for an effective inhibitor. Mechanism of the action of inhibitors has been provided.

Comparative adsorption of simple molecules on carbon nanotubes

Adsorption isotherms of krypton between 77.5 and 83.7 K and ethylene between 90 and 100.5 K on carbon multi-walled nanotubes have been measured. The comparison of the results with those obtained under the same conditions on graphite are discussed in the light of the nanotube morphology. The curvature of the graphene sheets constituting the nanotube walls mainly results in higher condensation pressures and a lower heat of adsorption with respect to those on a planar surface. Near the completion of the krypton first condensed layer, a transition to a phase of higher density occurs, the nature of which is discussed. The wetting transition temperature above which a second ethylene monomolecular layer can condense on the surface is higher on nanotubes than on graphite.

Studies on methanol fuel cell

A fuel cell using methanol dissolved in 6 N KOH solution with (Fe(III) and Al(III)) and without treated graphite cathode at ambient temperature (25°C) was studied for the following parameters: (a) current–voltage behaviour at ambient temperature; (b) voltage drop with time; (c) methanol concentration as a function of time; (d) carbon dioxide generation as a function of time; (e) effect of catalyst on the performance of the cell. The OCV of the cell was 0.273, 0.850 and 1.4 V, while current density was 0.5, 4.52 and 17.2 Am −2 with untreated and treated (Fe(III) and Al(III)) graphite electrode, respectively. The highest current density was 180 Am −2 at 0.3 V and 540 Am −2 at 1.04 V for Fe(III) and Al(III), respectively. Polarisation was found to be higher in Al(III) than Fe(III)-treated graphite electrode due to the low heat of adsorption of Al(III) [406 kJ mol −1 vs. 518 kJ mol −1 for Fe(III)]. The catalytic oxidation of methanol in terms of electrical energy output shows efficiency of about 78% for Al(III) and 80% for Fe(III)-treated graphite electrode.

Microcalorimetric and infrared spectroscopic studies of C 2H 4 adsorption on Ni/SiO 2 and NiBi/SiO 2 catalysts

Microcalorimetric adsorption and FTIR were used to study the surface species and energy of surface bonding for ethylene adsorption on Ni/SiO 2 and NiBi/SiO 2 catalysts. Microcalorimetric adsorption of H 2 was employed to titrate the surface sites of Ni in the catalysts. It was found that addition of a small amount of Bi (Ni/Bi = 64 and 16) significantly decreased the number of Ni surface sites for H 2 adsorption. The adsorption of ethylene at room temperature on the Ni/SiO 2 and Ni 64Bi/SiO 2 produced mainly ethylidyne species on the surface plus adsorbed hydrogen atoms, generating the initial adsorption heats of 170 and 160 kJ/mol, respectively. In contrast, the adsorption of ethylene on the Ni 16Bi/SiO 2 at room temperature produced mainly molecularly adsorbed surface species (di-σ or π-bonded species) with much lower adsorption heat. The enrichment of Bi on the surface may change the ensemble size of Ni for the adsorption of ethylene, thereby changing the adsorption state and energy of ethylene on the surface of Ni.

Surface properties of catalytic aluminas modified by alkaline-earth metal cations: A microcalorimetric and IR-spectroscopic study

Lewis acidic properties of transition aluminas whose surfaces have been doped with alkaline-earth metal cations (Ca2+ and Ba2+) were studied by means of the room temperature adsorption of carbon monoxide. The vibrational features of CO adsorbed at the surface of doped aluminas were investigated by IR spectroscopy in comparison with pure parent aluminas, while the quantitative and energetic features were studied by adsorption microcalorimetry. Various CO adspecies were found to form at the surface of both pure and doped-alumina, owing to the structural heterogeneity of the Al2O3 surface and to the presence of alkaline-earth metal cations. The surface heterogeneity was revealed by different vco stretching frequencies, namely vco≈2230, 2218 and 2205 cm−1 for coordinatively unsaturated tetrahedral Al3+ cations in different crystallographic configurations, and vco≈2186 and 2172 cm−1 for coordinatively unsaturated Ca2+ and Ba2+ cations, respectively. Heats of adsorption of ≈80, 70 and 55 kJ/mol were assigned to the formation of Al3+/CO complexes, ≈45 kJ/mol for Ca2+/CO and ≈30 kJ/mol for Ba2+/CO complexes. The latter value was estimated through a correlation curve existing between vco stretching frequencies and adsorption enthalpies. This correlation, already proposed in the past for CO adsorbed on non-d/d0/d10 metal cations, has been revisited and confirmed here, by including Al2O3 data for which an apparent lack of correlation between the two parameters was first observed. With respect to pure alumina, the population of Lewis acidic sites was found to be significantly depressed by the presence of alkaline-earth cus metal cations. These acidic sites are intrinsically weaker than tetrahedral cus Al3+ cations, as witnessed by smaller upward shifts of the vco stretching frequencies with respect to CO gas and lower heats of adsorption, in accordance with expectations from the charge/ionic radius ratios. Ca2+ cations were found to compete in adsorbing CO with Al3+ cations more efficiently than the larger Ba2+ cations. In the case of CaO/Al2O3 systems outgassed at 1023 K, a thin surface layer of calcium aluminate, not detected by XRD or HRTEM, was suggested to form.

Structure Sensitive Hydrogen Adsorption: Effect of Ag on Ru/SiO2Catalysts

The dynamics and energetics of hydrogen chemisorption on silica-supported ruthenium and ruthenium–silver bimetallic catalysts were studied by1H NMR spectroscopy and microcalorimetry. It was observed that the amount of hydrogen adsorbed on Ru particles having intermediate and low heats of adsorption was significantly reduced with increasing amounts of silver. The desorption and adsorption rate constants, determined from selective excitation NMR experiments, were lower on Ru–Ag catalysts than those on Ru catalysts at the same temperature and hydrogen coverage. The apparent sticking coefficients of hydrogen on a Ru–Ag catalyst with 10 at% Ag were more than 10 times lower than those on a Ru catalyst and were comparable to sticking coefficients reported in the literature for hydrogen adsorbing on Ru single crystal. Thus silver was found to greatly affect both the dynamics and energetics of hydrogen chemisorption on Ru/SiO2. However, ensemble and electronic effects did not play any role in causing these effects. It is postulated here that the influence of silver was due to its tendency to selectively segregate to edge, corner, and other low coordination structures on the Ru particle surface. Hence, hydrogen adsorption on these surfaces was found to be structure sensitive.

Coverage and structure of deuterium on Cu(111)

The absolute coverages of atomic deuterium adsorbed on the Cu(111) surface at a temperature of 100 K have been determined using nuclear reaction analysis (NRA), and correlated with thermal desorption spectra (TDS) and deuterium-induced low energy electron diffraction (LEED) patterns for Cu(111). It was found that the adsorption of deuterium saturates at a coverage of 0.67 ± 0.03 ML, significantly lower than that observed for fcc(111) transition metal surfaces. The D coverage for a (2 × 2) LEED pattern was measured to be 0.5 ML, in common with those of H Ni(111) and H(D) Ag(111) . No bulk absorption was observed. The low saturation coverage of D on Cu(111) is attributed to the combination of a low heat of adsorption and D-D repulsion on the surface.

Mechanisms of surface processes in silicon etching

A unified view on the mechanism allowing one to explain the experimental features governing spontaneous silicon etching by atomic fluorine is presented. Analysis of the phenomenological equation of adsorption shows a significant difference between etching mechanisms at high and low heat of adsorption on the surface being etched. As follows from the parameter estimates, one or another case can be realized under different experimental conditions. At steady-state the etching is argued to be determined only by the processes taking place on the SiF. film surface. To describe the process, it is necessary to understand the mechanism of overcoming the surface barrier for fluorine penetration into the film. At low heat of fluorine adsorption the barrier is overcome by thermal activation. In the opposite case the etching mechanism includes electron tunneling from silicon to adatoms and creation of a surface electric field. The field lowers the high energetic barrier for fluorine penetration. Based on the kinetic equations describing the electronic and atomic processes on the surface, the equation of the field strength is obtained. This equation is analyzed in different limit cases. The observed features are shown to be reproduced at some conditions on the parameters. Definite predictions on the temperature dependence of the etch rate are made.

Calorimetric heats of adsorption at the silica/isooctane interface

The heats of immersion of a well-characterized mesoporous silica gel (Davisil 62) in solutions of six organic compounds (2-methylnaphthalene, benzothiophene, 2-methylbenzothiophene, pyridine, isoquinoline, and quinaldine) in isooctane (2,2,4-trimethylpentane) have been measured as a function of surface coverage by batch solution microcalorimetry. Adsorption isotherms at the silica/isooctane interface were obtained by conventional surface excess methods. The heat of adsorption of gaseous pyridine on silica was derived by correcting the heat of adsorption from the liquid for the bulk heats of mixing and phase change and the measured heats of wetting of silica covered with varioius preadsorbed amounts of pyridine in pyridine/isooctane solution. Connection was then made to the gas-phase adsorption data available for pyridine by the use of a rigorous thermodynamioc cycle. All of the compounds studied appeared to have site-limited adsorption (∼ 1 adsorbate per 2 surface silanol groups). 2-Methylnaphthalene and its sulfur aromatic analogs had very similar isotherms and low heats of adsorption from isooctane, in agreement with chromatographic experience. The one- and two-ring N-bases had high adsorption heats from isooctane which varied strongly with coverage (pyridine) or with adsorbate size (pyridine vs two-ring). It is argued, based upon the heats and entropies of adsorption, that the optimum hydrogen bond is the plane of the silica surface and that the observed large adsorbate steric effect reflected the geometry sensitivity of the hydrogen bond.

Adsorption of propylene and allene on TiCl3·1/3 AlCl3 and polymerization of allene with the system TiCl3·1/3 AlCl3/AlEt2Cl

AbstractAdsorption of propylene and allene from the gas phase on TiCl3·1/3 AlCl3 and TiCl3·1/3 AlCl3/AlEt2Cl catalyst was measured volumetrically. In the ranges of 296–334 K and 1–100 kPa, the adsorbed amounts of both gases was found to be approximately proportional to the gas pressure. The adsorbed amount of allene is about twice as high as that of propylene under the same conditions. Low heats of adsorption of both propylene and allene support the assumption of the physical nature of the adsorption. Addition of AlEt2Cl to the TiCl3·1/3 AlCl3 catalyst did not change substantially the sorption properties of the catalyst. Polymerization of allene with the system TiCl3·1/3 AlCl3/AlEt2Cl in heptane was found to be first order in monomer and the rate decreases with increasing yield. A mechanism of retardation by the polymer formed is probably operative. Both heptane‐soluble and heptane‐insoluble polyallenes are produced. The soluble polymer has M̄n ≈ 103 and a very narrow molecular weight distribution (M̄w/M̄n = 1,2 ± 0,2). Polyallene comprises mainly vinylidene units.

A gas-solid chromatographic study of the adsorption of hydrocarbons and alcohols on graphite

The Henry's law region of the adsorption isotherms for several hydrocarbons and alcohols on a graphite was studied by gas-solid chromatography. The thermodynamic functions at zero surface coverage were determined. The low heats of adsorption indicate a highly homogeneous surface. A constant increment of the standard free energy with the number of carbon atoms in the lineal adsorbates was found. Attemps were made to estimate the London contribution to the surface free energy of the adsorbents. A non-specific interaction between the graphite and the adsorbates was shown.

Comparison between the adsorption properties of Pd(111) and PdCu(111) surfaces for carbon monoxide and hydrogen

Thermal desorption spectroscopy and LEED have been used to investigate the interaction of CO and hydrogen with a Pd 0.75Cu 0.25(111) single crystal surface with surface composition of about Pd 0.7Cu 0.3. The main objective was to make a comparison with the previously studied Pd 0.67Ag 0.33(111) (surface composition Pd 0.1Ag 0.9) and Pd(111) surfaces. In addition, the effect of preadsorbed H on subsequent CO dosage and the effect of adsorbed CO on postdosed hydrogen are described. Marked differences were found in the adsorption behaviour of the three surfaces towards CO and hydrogen. The maximum amount of H and CO that can be adsorbed at 250 K and pressures below 10 −9 mbar is much lower on the PdCu surface than expected on the basis of the surface composition. This effect appears to be caused by a low heat of adsorption of hydrogen and CO and Pd singlet sites. Arguments are presented that singlet Pd sites or isolated Pd atoms in a Cu or Ag matrix are able to trap and dissociate the hydrogen molecule at 250 K. The CO desorption spectra are not influenced by pre- or postexposed hydrogen. Adsorbed CO hampers the uptake of hydrogen upon subsequent exposure to hydrogen. Postdosed CO causes adsorbed H adatoms to move to the bulk (adsorbed H). CO exposure at 250 K results in a very broad desorption plateau between 310 and 425 K with hardly discernable maxima. The results can be explained in terms of the size and relative concentration of the various Pd sites present on the surface (triplet, doublet and singlet sites). It can be concluded that for Pd (111) the heat of adsorption of both CO and H differ appreciably for the triplet, doublet and singlet sites. The effect of site has a larger contribution to the decrease of the heat of adsorption with coverage than the effect of lateral interaction in the adlayer. For Pd(111), PdCu(111) and PdAg(111) the effect of the available Pd sites is the major effect that determines the heat of adsorption, followed by the effect of lateral interaction and for the alloy surfaces the electronic or ligand effect.

Adsorption of hydrocarbons on graphites and graphitized carbon black at zero surface coverage

The Henry's law region of the adsorption isotherms for a homologous series of n-alkanes on graphites and a graphitized carbon black was studied by gas chromatography. The three thermodynamic functions at zero surface coverage were determined. A linear relationship between the standard free energy, the enthalpy and the entropy and the number of carbon atoms was found. The low heats of adsorption at zero coverage indicated a virtual absence of high-energy sites. Attempts were made to estimate the London contribution to the surface free energy of the adsorbents; it was estimated to be 100 mN m −1 for the graphitized carbon black and 80 mN m −1 for the graphites.