Attain Adsorption Equilibrium Research Articles

Experimental investigation of cow dung based organic desiccant for dehumidification in evaporative cooling

ABSTRACT Aim of this study is to explore the alternate of costly desiccants such as silica gel, zeolite, etc., in the dehumidification process of evaporative cooling and food storage. Therefore, the new cow dung-based novel desiccant (CDND) material has been developed which is very cost effective and readily available organic material. This CDND has been tested experimentally to explore the suitability to replace commercially available desiccants. The experiments were carried out in two stages. First, 100 g of CDND was tested, and the adsorption and desorption characteristics were evaluated. In second stage, 2500 g of CDND in Special Design Pipe (CDNDSDP) is tested to study adsorption and regeneration and how long it takes to attain adsorption equilibrium without regeneration under different inlet air conditions. In the present study, it is found that this novel desiccant is regenerated in only 28 mins and ready for dehumidification processes for next 120 mins, which demonstrates continuous regeneration of CDND is not required. In addition, the regeneration of CDND is found to be 54ºC that is quite significant and can be attained easily using low-grade energy. Further, CDND is organic material and reutilization will help in terms of solid waste management.

Enhanced adsorption and desorption of Cr(VI) from aqueous solution using hydrous Ce1– x Zr x O2: Isotherm, kinetics and thermodynamic evaluation

Hydrous Ce1– x Zr x O2 (x = 0.25; 0.5; 0.75) mixed oxides were used as potential Cr(VI) adsorbents, expecting to take an advantage of benefits of their corresponding single oxides (e.g. high adsorption capacity) and to eliminate their weaknesses (e.g. limited reusability, long adsorption time). Chemical incorporation of ZrO2 into CeO2 lattice significantly changed the texture and the morphology of hydrous single oxides, enhancing their adsorptive and desorptive properties. Cr(VI) removal efficiency depended on Ce/Zr molar ratio, increasing in the following order: CeO2<Ce0.75Zr0.25O2<Ce0.5Zr0.5O2< ZrO2<Ce0.25Zr0.75O2. Hydrous Ce0.25Zr0.75O2 showed the highest adsorption capacity (ca. 35.5 mg g−1), desorption efficiency (ca. 34.5 mg g−1), short time needed to attain adsorption equilibrium (30 min) and chemical stability (90% of original Cr(VI) adsorption capacity in the third cycle). Cr(VI) adsorption on hydrous Ce1– x Zr x O2 oxides obeyed the Langmuir isotherm model and followed a pseudo-second-order kinetic model. The values of thermodynamic parameters indicated exothermic and spontaneous adsorption. For hydrous Ce–Zr oxides, adsorption was governed mainly by physisorption and ion exchange, making possible effective and fast regeneration. For hydrous CeO2 and ZrO2, Cr(VI) adsorption was not completely reversible. Reduction of Cr(VI) to Cr(III), on CeO2 surface, and formation of ≡Zr–OH2 +–HCrO4 − complex on ZrO2 surface, were also considered as suppliers of Cr(VI) removal.

Boronic acid-decorated metal-organic frameworks modified via a mixed-ligand strategy for the selective enrichment of cis-diol containing nucleosides

Functional metal–organic frameworks (MOFs) constructed via a pre-installation strategy of introducing mixed organic ligands have attracted considerable interest in various fields. In the present study, boronic acid decorated magnetic Zr-MOFs were successfully synthesized by introducing 3-carboxyphenylboronic acid ligands as fragments. The prepared material was used as an adsorbent for the enrichment of cis-diol-containing nucleosides. The adsorbent has excellent performance with regard to the enrichment and separation of the nucleosides. This may be attributed to its abundant boronic acid functional groups, and the convenience of magnetic separation it provides. The obtained material was chemically stable over a large pH range. The degree of linearity of the nucleosides was excellent (0.02–10 μg mL−1), and the detection and quantification limits were low (0.006–0.016 μg mL−1 and 0.02–0.05 μg mL−1, respectively). Furthermore, it was possible to attain adsorption equilibrium within 10 min. The high efficiency of this method makes it suitable for the successful extraction of nucleosides from human urine samples, with satisfactory recoveries of 88–146%, and 1.7–9.4% precision. We believe that the fabricated functional magnetic MOFs have great potential for the analysis of other cis-diol-containing target, and the pre-installation strategy could be adapted for the wider application of MOFs.

Water-insoluble cyclodextrin polymer crosslinked with citric acid for paraquat removal from water

The anionic water-insoluble cyclodextrin polymer (polyCTR-β-CD) was crosslinked between β-cyclodextrin (β-CD) and citric acid (CTR) at 180 °C during 30 minutes to eliminate paraquat (PQ) from water. The reaction yield was equal to 70.2%, the ionic exchange capacity corresponded to 3.29 mmol·g−1 and the β-CD content was 0.29 mmol·g−1. Then, samples were characterized by SEM, ATR-FTIR, TGA, BET and stereoscopic microscope. Adsorption experiments were investigated with different factors such as pH of the solution, contact time, initial concentration of paraquat and adsorption temperature. The relevant pH was equal to 6.5 and the optimal contact time was 120 minutes to attain adsorption equilibrium. At 30 °C, the adsorption capacity was increased (9.4, 17.4 and 20.8 mg·g−1) when the initial concentration of paraquat was raised (25, 50 and 200 mg·L−1 respectively). Adsorption kinetics was appropriated to the pseudo-second-order model and adsorption isotherm was fitted to the Langmuir model. For thermochemistry parameters at different temperatures, the negative ΔG° showed a spontaneous adsorption process, the negative ΔH° indicated an exothermic process and the positive ΔS° exhibited an increase disorder. Finally, the reusability of the insoluble polymer was reached 78.3% after four regeneration cycles in methanol.

Creating magnetic ionic liquid-molecularly imprinted polymers for selective extraction of lysozyme.

A novel magnetic (Fe3O4) surface molecularly imprinted polymer (MIP) based on ionic liquid (IL) (Fe3O4@VTEO@IL-MIPs) was prepared for the selective extraction of lysozyme (Lys). As the functional monomer of the MIPs, an imidazolium-based IL with vinyl groups was prepared. It can provide multiple interactions with template molecules. The amount of IL was optimized (200 mg). Fourier transform infrared spectrometry (FT-IR), transmission electron microscopy (TEM), dynamic light scattering (DLS), thermogravimetric analysis (TGA) and a vibrating sample magnetometer (VSM) were used to characterize the MIP. The results indicate the successful formation of an imprinting polymer layer. The concentration of Lys in the supernatant was determined by UV-vis spectrophotometry at a wavelength of 280 nm. The maximum adsorption capability of the MIP is 213.7 mg g−1 and the imprinting factor (IF) is 2.02. It took 2.5 h for the MIP to attain adsorption equilibrium. The structure of the protein was evaluated using circular dichroism (CD) spectra and UV-visible spectra. The adsorption performance was further investigated in detail by selective adsorption experiments, competitive rebinding tests, and reusability and stability experiments. Furthermore, it was utilized to separate the template protein from a mixture of proteins and real samples successfully because of the high adsorption capacity for Lys.

Synthesis, Characterization, and Adsorption Properties of a Graphene Composite Sand (GCS) and Its Application in Remediation of Hg(II) Ions

In this study, graphene-based adsorbent was successfully prepared following a thermal treatment method. The prepared material, named as graphene-coated sand (GCS), was used as an adsorbent for the removal of Hg(II) ions from aqueous solutions. Structure, composition, and morphology of the GCS were characterized by Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), X-ray diffractometer (XRD), Raman spectroscopy, electron diffraction (ED) measurements, energy dispersive X-ray spectroscopy (EDX), surface area measurements, particle size, and zeta potential measurements, respectively. A batch adsorption method was used to assess the ability of GCS towards removal of Hg(II) ions from aqueous solutions. The results of batch studies revealed that the GCS required a pH value 6.0, contact time 120 min, and adsorbent dose of 200 mg to attain adsorption equilibrium. Langmuir, Freundlich, Temkin, and D-R adsorption isotherm models were employed to evaluate the isotherm constants and other parameters related to the adsorption process. The Hg(II) ions uptake by the GCS was found to follow Freundlich isotherm model with R 2 value of 0.97695, under optimized conditions and at 40 °C with a maximum adsorption capacity of 299.40 mg/g. The adsorption process followed the second-order kinetic path. The thermodynamic parameters such as ΔH°, ΔS°, and ΔG° were also calculated which suggested that the adsorption processes of Hg(II) ions onto the GCS was endothermic and entropy favored. The values of ΔG° at 283, 303, and 313 K were − 1.10, − 0.025, and − 4.55 kJ, respectively, and ΔH°, ΔS° were calculated to be 26.60 kJ mol−1 and ΔS° 1.35 J mol−1 K−1, respectively. The obtained results revealed that the prepared materials could be effectively and economically beneficial.

Investigation of Mixture Effects on the Adsorption Rates of Aromatics from Aqueous Solution on Clay and Sandy Soil

The adsorption behavior of a mixture of two polycyclic aromatic hydrocarbons (PAHs); naphthalene and pyrene and one cyclic aromatic hydrocarbon using clay and sandy soil as adsorbents was examined under ambient conditions in this study. The adsorption from the aqueous solution was observed to be time dependent and the time taken to attain adsorption equilibrium for benzene, naphthalene and pyrene in the mixture were 22, 16 h, 18 and 14 h, and 26 and 18 h in clay and sandy soil, respectively. Among the tested kinetics model in this study, the pseudo-second order equation successfully predicted the adsorption. The rate of adsorption using the pseudo-second order rate expression for benzene, naphthalene and pyrene were 0.00096, 0.00072, 0.00092 min-1 and 0.00091, 0.00080, 0.00090 min-1 for clay and sandy soils respectively. These results suggest that benzene was more selectively adsorbed than naphthalene and pyrene in both clay and sandy soil.

Adsorption of Naphthalene on Clay and Sandy Soil from Aqueous Solution

The adsorption behavior of naphthalene using clay and sandy soil as adsorbents is examined under ambient conditions. The adsorption equilibrium of naphthalene on clay and sandy soil was evaluated by the Langmuir, Freundlich and Tempkin isotherms. The results showed that the equilibrium data for naphthalene fitted the Freundlich model best within the concentration range studied for both clay and sandy soil. Experimental results showed that the time taken to attain adsorption equilibrium for naphthalene was 26 hrs and 20 hrs for clay and sandy soil, respectively. Among the tested kinetic models in this study, the pseudo-second order successfully predicted the adsorption process.

Manganese dioxide/iron oxide/acid oxidized multi-walled carbon nanotube magnetic nanocomposite for enhanced hexavalent chromium removal

This study presents a simple route for the fabrication of manganese dioxide/iron oxide/acid oxidized multi-walled carbon nanotube magnetic nanocomposites (MnO2/Fe3O4/o-MWCNTs). The materials were characterized by transmission electron microscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, vibrating sample magnetometry, and Brunauer, Emmett, and Teller surface area measurement. MnO2/Fe3O4/o-MWCNTs are more effective for hexavalent chromium ion adsorption compared with other materials. Hexavalent chromium adsorption by MnO2/Fe3O4/o-MWCNTs is strongly pH dependent. The Langmuir isotherm model is consistent with the experimental data at different temperatures. The maximum adsorption capacity was determined to be qmax=186.9mgg−1. A contact time of different initial concentrations was about 150min to attain adsorption equilibrium. The kinetic adsorption of different initial concentrations can be described by the pseudo-second-order rate equation. The overall rate process was apparently influenced by external mass transfer and intraparticle diffusion. Moreover, the thermodynamic parameters indicated that the adsorption process was spontaneous and endothermic, and that the adsorption mechanism included both the physical and the chemical adsorption mechanisms. After adsorption, MnO2/Fe3O4/o-MWCNTs can be conveniently and quickly separated from the media by an external magnetic field, and adsorption capacity can remain up to 85% after five times of usage. Thus, MnO2/Fe3O4/o-MWCNTs are good candidate for efficient hexavalent chromium removal from wastewater and for the deep-purification of polluted water.

Decolourisation of Metanil Yellow by visible-light photocatalysis with N-doped TiO2 nanoparticles: influence of system parameters and kinetic study

A nitrogen-doped photocatalyst (N-TiO2) is prepared from 15% TiCl3 and 25% aqueous NH3 solution as precursor. The synthesised material is characterised by XRD, BET, TEM, DRS and XPS and is used for photocatalytic decolourisation of Metanil Yellow under visible-light illumination. The experimental parameters, viz, photocatalyst dose, initial dye concentration and solution pH influence the decolourisation process. At pH 3.5, N-TiO2 can decolourise almost 89% of the dye at equilibrium within 240 min. COD study reveals ∼92% mineralisation of the dye in 360 min of irradiation. The photocatalytic reaction is allowed to proceed only after the attainment of adsorption equilibrium between N-TiO2-Metanil Yellow. The adsorption is carried out by batch process under different experimental conditions, namely, N-TiO2 dose, initial dye concentration and solution pH. The adsorption process attains equilibrium within 60 min and follows Lagergren first-order kinetics. On the other hand, the photocatalytic decolourisation of Metanil Yellow is suitably fitted with the modified Langmuir–Hinselwood model. Modification of TiO2 improved the decolourisation of the dye by ∼8 times compared to TiO2 P25.

Adsorption behavior of MnO2 functionalized multi-walled carbon nanotubes for the removal of cadmium from aqueous solutions

Manganese dioxide (MnO2) was coated on acid oxidized multi-walled carbon nanotubes (MnO2/o-MWCNTs) by immersing o-MWCNTs into a KMnO4 aqueous solution. The materials were characterized by different techniques, such as Fourier-transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, and BET surface area measurement. MnO2/o-MWCNTs were the most effective materials for cadmium ion adsorption among other adsorption materials synthesized at different conditions. Cadmium ion adsorption by MnO2/o-MWCNTs was strongly pH dependent. The Langmuir isotherm model is consistent with the experimental data at different temperatures; the maximum adsorption capacity was determined (qmax=41.6mgg−1). A contact time of at least 150min was required to attain adsorption equilibrium, and kinetic adsorption can be described by the pseudo-second-order rate equation. The overall rate process was apparently influenced by external mass transfer, intraparticle diffusion, and chemical adsorption. Moreover, the thermodynamic parameters indicated that the adsorption process was spontaneous and endothermic, and that the adsorption mechanism included both physical and chemical adsorption mechanisms.

Comparative Study of the Adsorption and Desorption Behavior of Single and Multi-Ring Aromatics in Sediment Fractions

The sorption behaviour of benzene, toluene, ethyl benzene, xylene and naphthalene using clay and sand sediments under ambient conditions is examined in this study. Experimental results showed that, the time taken to attain adsorption equilibrium for naphthalene, and BTEX were 28, 30, 30, 32, 28 hrs and 20, 22, 22, 24, 22 hrs while the desorption equilibrium time were 10, 13, 12, 15, 12 hrs and 9, 9, 9, 11, 10 hrs in clay and sand respectively. All of the naphthalene, and BTEX were adsorbed at the different equilibrium times, using clay while the amount of naphthalene and BTEX adsorbed by sand, at different equilibrium times were 117, 121, 127, 123 and 134 mg. Following the results of the adsorption/desorption experiments, quantitative measurements showed that sand exhibited higher affinity for the solute as retained more chemicals (as high as between 58% - 66%) within it pores while nearly all the chemicals adsorbed by the clay were released at the attainment of equilibrium. The implication of this is that occlusion within the sand particles may likely be the resultant effect of continued sand-chemicals contact. The amount of contaminant solute adsorbed and desorbed affirmed that clay has a better capacity to retain naphthalene and BTEX than sand and this may not be unrelated to its large surface area, high porosity and higher hydraulic conductivity for the solutes arising from its good binding sites (small pore sizes) that tend to hold the adsorbates to its particles.

Mesoporous organic–inorganic hybrid aerogels through ultrasonic assisted sol–gel intercalation of silica–PEG in bentonite for effective removal of dyes, volatile organic pollutants and petroleum products from aqueous solution

Hybrid mesoporous materials with high surface area (1144m2/g) and enhanced thermal stability were achieved through ultrasonication assisted sol–gel-intercalation technique using bentonite clay and PEG grafted sol gel silica. Solvent extraction with mixed solvents showed efficient aerogel formation of the hybrids. The products were characterised by small angle X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, scanning electron microscopy, transmission electron microscopy, zeta potential measurement and BET surface area analysis. The XRD analysis showed interlayer spacing of ∼43.28Å. The increasing spacing is interesting in a way that the available interior pore volume can be tailored for the enhanced adsorption of different pollutants. Clay-hybrid porous materials showed hydrophobic nature with high adsorption capacities for organic dyes (methylene blue and malachite green), volatile organic pollutants (phenol and toluene) and petrochemical derivatives (kerosene, engine oil and diesel). The contact time necessary to attain adsorption equilibrium and the optimum pH were found to be 2h and 5.0–6.0, respectively. Kinetics of adsorption was rapid film diffusion with a pseudo-second-order rate constant. The best interpretation for the equilibrium data was given by the Langmuir isotherm indicative of homogenous surface and maximum adsorption capacity of clay-hybrid towards methylene blue, malachite green, phenol and toluene from aqueous solution was found to be 101.55, 98.42, 116.75 and 114.10mg/g, respectively, and was found to be greater than commercial adsorbents like activated carbon, zeolite and activated alumina. Thermal regeneration studies were carried out by calcinating the exhausted adsorbents. Clay-hybrid aerogels were then demonstrated using different petroleum products to exhibit excellent oil absorption properties.

Adsorption of two coffee aromas from synthetic aqueous solution onto granular activated carbon derived from coconut husks

Soluble coffee production requires several processes that cause the loss and degradation of aromas, which reduces the quality of the final product. However, such aromas can be recovered into aqueous solution during the production process. Aromas must be separated from water, and pleasant compounds, such as benzaldehyde, must be separated from unpleasant compounds, such as acetic acid. In this study, the kinetic and thermodynamic parameters of benzaldehyde and acetic acid adsorption from synthetic single-solute aqueous systems onto commercial grade granular activated carbon derived from coconut husks were investigated. The contact time required to attain adsorption equilibrium of benzaldehyde was 6 h, while that of acetic acid was 10 h. Adsorption isotherm data revealed that the adsorption of benzaldehyde and acetic acid was exothermic and spontaneous in nature. Moreover, the isosteric heat of adsorption indicated that the adsorbent surface was energetically heterogeneous. The adsorption equilibrium was fitted according to Langmuir, Freundlich and Temkin isotherms.

Removal of copper by calcium alginate encapsulated magnetic sorbent

In this study, the adsorption performance of copper on calcium alginate encapsulated magnetic sorbent is investigated via equilibrium and kinetics study. Results showed the sorption performance is greatly affected by the initial solution pH, the background ionic strength, the mechanical stirring speed, and the presence of humic acid. The optimum copper sorption was achieved at initial solution pH > 5. The copper uptake is poor in the presence of higher background ionic strength (sodium perchlorate). Langmuir equation can be well used to describe the adsorption isotherm data. The maximum sorption capacity ( q max) and Langmuir constant ( b) decrease from 60 to 49 mg g −1 and 1.43 to 0.35 L mg −1 as the ionic strength is increased from 0 to 0.05 M. Kinetics study shows the sorption equilibrium can be obtained within 3 h, and the adsorption kinetics data are well described by the intraparticle pore diffusion model. The mechanical stirring speed greatly enhances the mass transfer rate of copper ions onto the sorbent, and the external mass transfer coefficient ( k f) increases from 2.5 × 10 −5 to 2.5 × 10 −4 m s −1 when stirring speed is increased from 120 to 220 rpm. The presence of humic acid decreases the k f from 2.10 × 10 −4 to 5 × 10 −5 m s −1, and increases the time for copper to attain adsorption equilibrium due to clogging of surface pore which apparently reduce the surface reaction site.

Removal of lead(II) from aqueous solution by adsorption onto manganese oxide-coated carbon nanotubes

Mn oxide-coated carbon nanotubes (MnO 2/CNTs) were prepared and used to remove Pb(II) from aqueous solution. The XRD revealed that MnO 2 covered MnO 2/CNTs was an amorphous phase. The Pb(II) removal capacity of MnO 2/CNTs decreased with the decrease of pH. From the Langmuir isotherms, maximum adsorption capacity ( Q 0) of MnO 2/CNTs towards Pb(II) was determined ( Q 0 = 78.74 mg/g). Comparing with CNTs, significant improvement of Pb(II) adsorption shows MnO 2/CNTs can be good Pb(II) absorbers. Although Pb(II) adsorption by MnO 2/CNTs occurred rapidly within the first 15 min of contact time, contact time needed at least 2 h in order to attain adsorption equilibrium and the kinetic adsorption can be described by the pseudo-second-order rate equation. Thermodynamics of Pb(II) was studied at various temperatures of 283, 303 and 323 K and the thermodynamic parameters were also determined.

An Unsaturated Transient Flow Method for Determining Solute Adsorption by Variable‐Charge Soils

The amount of reactive ions adsorbed during transport in variable‐charge soils is often smaller than those predicted from batch adsorption experiments. This overestimation of adsorption for invading ions by the batch method is likely to be caused by excessive desorption of indigenous, strongly adsorbed ions. We propose an unsaturated transient flow method by which equilibrium adsorption of weakly reactive ions can be determined without causing appreciable desorption of indigenous ions. In our method, water is imbibed into horizontal columns packed with soil that has been mixed with a salt solution at different concentrations to attain adsorption equilibrium. We make use of the observation that during imbibition of water, the antecedent solution is pushed ahead by the invading solution and accumulates in the region beyond the plane of separation, with the solution concentration unchanged. From linear plots of the solute content vs. soil water content in this region, the amount of solutes adsorbed by soil and the equilibrium solution concentration prior to the water imbibition are obtained. The advantages of the method are that adsorption isotherms can be determined under conditions similar to those expected during transport processes in soil, and that it is exempt from uncertainty about attainment of adsorption equilibrium as in the miscible displacement methods. The proposed method is best suited for variable‐charge soils where adsorption of weakly reactive ions is largely due to an increase in the exchange capacity.

A rapid equilibrium approach to the interfacial kinetics of lipid hydrolysis by a candidal lipase

The relations having previously been established between the initial reaction rates and the amount of enzyme adsorbed at the oil/ water interface in th In the adsorption of Candidal lipase at an n-heptane/water interface, induction times as long as 2.5 h are required to attain adsorption equilibrium. U

Selective Adsorption of Molybdenum(VI) and/or Vanadium(V) Using Cross-linked Polystyrene Resins with Bis(2-hydroxyethyl)amino, Bis(carboxymethyl)amino, and Bis(phosphonomethyl)amino Groups

Abstract Styrene–divinylbenzene copolymers containing –N(CH2CH2OH)2, –N(CH2CO2H)2, and –N(CH2PO(OH)2)2 moieties have been synthesized and applied to the selective adsorption of molybdenum and/or vanadium from aqueous solution. The equilibrium adsorption capacity vs. pH correlation and exchange kinetics of the resins for MoVI and VV have been studied. The resin containing bis(2-hydroxyethyl)amino group is particularly effective for the selective recovery of MoVI and/or VV by column operation due to its high adsorption capacity and faster exchange rate, whereas other resins required much longer time to attain adsorption equilibrium. The bis(2-hydroxyethyl)amino resin was used for the selective recovery of molybdenum from an acid-leaching solution of waste desulfurization catalyst.

Ammonia synthesis and related reactions over iron-cobalt and iron-nickel alloy catalysts. Part I. Catalysts reduced at 853 K

Studies have been undertaken of ammonia synthesis, nitrogen chemisorption and nitrogen isotope exchange over a series of iron-cobalt and iron-nickel alloy catalysts containing 3% of alumina as a structural stabiliser. Catalysts were reduced for 18 h at 853 K before use and metal surface areas were measured by adsorption of carbon monoxide. While nitrogen chemisorption could be observed below 673 K the rate decreased with coverage so markedly that temperatures of 773 K or more were required to attain adsorption equilibrium within a reasonable period. Ammonia synthesis was observed from 550 K but at 700 K or over the rate of synthesis was affected by the back reaction becoming significant. Higher temperatures in the range from 773 K to 853 K were required to give measurable rates of nitrogen isotope exchange. Thus the temperatures for ammonia synthesis correspond to the start of nitrogen chemisorption and those for exchange to the higher temperatures needed for reversible adsorption and the establishment of the adsorption equilibrium. The pattern of activity with composition for the cobalt alloys showed a small maximum at a composition of iron (95%) cobalt (5%) for all three processes (adsorption, synthesis and exchange). A similar but less well-defined maximum was found with catalysts containing from 5% to 15% of nickel. Possible reasons for the enhanced activity of these alloys are discussed.