Peter G. Hall

Physisorption of argon, nitrogen and oxygen by MCM-41, a model mesoporous adsorbent

Adsorption isotherms of argon, nitrogen and oxygen have been determined at 77 K on a sample of MCM-41, a novel form of aluminosilicate. All the isotherms are Type IV in the IUPAC classification. The argon and oxygen isotherms exhibit well defined hysteresis loops, whereas the nitrogen isotherm is completely reversible. This unusual character is attributed to capillary condensation taking place within a narrow range of tubular pores of effective width 3.3–4.3 nm. MCM-41 shows considerable promise as a model mesoporous adsorbent.

The adsorption of water vapour by lithium fluoride, sodium fluoride and sodium chloride

Abstract Water vapour adsorption isotherms have been determined volumetrically on samples of LiF, NaF and NaCl at 278.15, 288.15 and 298.15 K. The surface areas of the samples were determined by krypton adsorption at 77.5 K. Infrared measurements showed the presence of occluded solvents and complete removal of these species was effected under vacuum at the following temperatures; LiF, 823 K; NaF, 723 K; NaCl, 623 K. Except in the case of NaF, for which a BET Type II isotherm was obtained, the krypton isotherms exhibited single sharp steps at the following values of P P 0 ; LiF, 0.57; NaCl, 0.15. Similarly with water adsorption, the NaF and NaCl isotherms exhibited a single sharp step at the P P 0 values of 0.30 for NaF and 0.35 for NaCl. The isotherm for LiF outgassed at room temperature was of Type II, whilst for LiF outgassed at 673 K it was of Type III. Throughout the water runs no irreversible uptake was observed. The sharp steps in the krypton and water isotherms were attributed to phase transformations in the monolayer. Isosteric heats of adsorption for water vapour were generally invariant with coverage and close to the enthalpy of liquefaction of water. Theoretical adsorption potential energy calculations were carried out for adsorption of a water molecule above four sites on the (100) planes of the three adsorbents. The total adsorption potential was obtained as a sum of the dispersion, electrostatic, induction and repulsion interactions, and for each solid the most favourable adsorption site was above the cation. For adsorption directly above ions, the greatest contribution to the attractive potential was furnished by the electrostatic interaction. Except for the cation values of LiF and NaCl, the theoretical isosteric heats calculated from the potential curves were significantly lower than the experimental isosteric heats. All the samples possessed active sites depending on the adsorbate and outgassing temperature as follows; LiF (Kr, 298 K) 36%, LiF (Kr, 673 K) 15%, NaF (H 2 O, 298 K) 18%, NaCl (Kr, 623 K) 20%, NaCl (H 2 O, 623 K) 25%.

Physisorption of nitrogen and oxygen by MCM-41, a model mesoporous adsorbent

Physisorption measurements of nitrogen and oxygen reveal that MCM-41 has a well defined mesoporous structure and is a highly promising model adsorbent.

Physisorption of alcohols and water vapour by MCM-41, a model mesoporous adsorbent

Adsorption isotherms of methanol, ethanol, propan-1-ol, butan-1-ol and water vapour have been determined on MCM-41, a model mesoporous adsorbent. The isotherms of the alcohols are all of Type IV, whereas the water isotherm is of Type V in the IUPAC classification. Each adsorption isotherm exhibits a sharp step, indicative of capillary condensation within a narrow distribution of mesopores. The isotherms are reversible in the monolayer-multilayer region, but distinctive hysteresis loops are associated with the condensation-evaporation cycle. The area within the loop is dependent on the adsorptive, increasing in scale from methanol to butan-1-ol and water. It is evident that the large internal surface of MCM-41 is somewhat hydrophobic and that its mesopore structure is remarkably uniform and stable.

Adsorption of carbon dioxide, sulfur dioxide and water vapour by MCM-41, a model mesoporous adsorbent

Adsorption isotherms of carbon dioxide, sulfur dioxide and water vapour have been determined on a well characterized sample of MCM-41. The shape of the carbon dioxide isotherm (at 195 K) is similar to that given by mesoporous hydroxylated silica, whereas the water isotherm (at 303 K) is of Type V in the IUPAC classification and its shape is therefore indicative of a more hydrophobic behaviour. Isosteric enthalpies of adsorption, calculated from the sulfur dioxide isotherms (at 254 and 273 K), reveal strong energetic heterogeneity in the adsorbent-adsorbate interactions and it appears that the high-energy sites occupy ca. 30% of the total surface area.

Sorption of phosphine by activated carbon cloth and the effects of impregnation with silver and copper nitrates and the presence of water

Abstract The interaction of PH 3 and H 2 O vapour with an activated carbon cloth before and after impregnation with AgNO 3 and Cu(NO 3 ) 2 · 3H 2 O has been studied, as well as the systems PH 3 + AgNO 3 and PH 3 + Cu(NO 3 ) 2 · 3H 2 O. PH 3 sorption isotherms at 25°C, determined gravimetrically in the range 0–25 Torr before and after impregnation with the salts, showed that raw cloth had a strong affinity for PH 3 . Impregnation enhanced the sorption capacity of the cloth above a certain pressure limit, which depended on the metal content, but apparently reduced it below this. All isotherms exhibited sorption-desorption hysteresis, with the loops failing to close as the pressure approached zero, while a second sorption run on raw cloth was reversible. Impregnation also increased the amount of sorbate retained under vaccum. A Dubinin-Astakhov relationship was followed by some of the sorption data. With bulk salts on exposure to PH 3 at 25°C, linear plots of weight loss against equilibrium gas pressure indicated chemical reaction. H 2 O vapour adsorption isotherms at 25°C measured gravimetrically up to relative pressures of 0.8 on raw cloth were of BET Type V with a marked hysteresis loop. Impregnation with AgNO 3 had little effect on the isotherm shape, except at low pressure where a small second hysteresis loop appeared, which failed to close as the pressure approached zero. With a mixture of H 2 O and PH 3 , gravimetric studies showed that the sorption capacity of AgNO 3 impregnated cloth for PH 3 was markedly reduced by the presence of H 2 O. Gas infrared studies showed N 2 O present when AgNO 3 impregnated cloth interacted with PH 3 ; the presence of H 2 O markedly reduced the amount of N 2 O. No infraredactive species were detected when raw cloth was exposed to PH 3 , while N 2 O and NO were produced when bulk AgNO 3 and Cu(NO 3 ) 2 · 3H 2 O were exposed to PH 3 . SEM with ED X-ray analysis showed nodular growth of virtually pure Ag together with AgNO 3 on the surface of AgNO 3 impregnated cloth. After exposure to PH 3 , P was largely confined to areas away from the nodules. XPS showed a layer of AgNO 3 together with some metallic Ag on AgNO 3 impregnated cloth. After exposure to PH 3 , the formation of at least two P compounds, one airsensitive, was indicated, irrespective of the purity of the gas.

Physical adsorption of nitric oxide on graphite and silica and adsorption of gases on nitric oxide preadsorbed on carbon

Abstract Adsorption isotherms of nitric oxide on graphitized and ungraphitized samples of carbon black (Sterling FT) and on silica (Cab-O-Sil) have been measured at 77.5 and 90.2 K. Isosteric heats of adsorption on the graphitized carbon at low coverage (θ Adsorption isotherms at 77.5 K of nitrogen, argon, carbon monoxide, krypton and oxygen have been measured on the graphitized carbon modified with preadsorbed layers of NO. For all the adsorbates except Kr, the stepwise shape of the isotherm is eliminated by the preadsorption of NO. The isotherms on the modified surface are compared with those on the bare surface by means of comparison plots (1) of amounts adsorbed at equal pressures.

Inelastic neutron scattering by water adsorbed on silica

Abstract Inelastic neutron scattering measurements, using a beryllium filter spectrometer, have been used to investigate the librational motions of monolayer adsorbed water on the surface of silica. The inelastic spectrum at ca. 15°K, of samples of Spherisorb S20W (mesoporous, mean pore diameter, 89 A) equilibrated with water vapor to give ca. 0.3 to 1.0 adsorbed water molecule per surface -OH group, showed librational peaks which were shifted toward frequencies lower than those of the corresponding peaks (78 and 103 meV) in bulk ice at the same temperature. The frequency shift (ca. 10–25 meV) was independent of water coverage in the range investigated, and can be shown to be consistent with a simple model involving double hydrogen bonding of an adsorbed water molecule. No change was observed in the frequency of the libration at room temperature. With microporous silica (Gasil 200, calculated pore diameter, 21 A) similar measurements at a coverage of ca. 0.35 and 1.1 adsorbed water molecules per surface -OH group show a lower frequency shift (

Dielectric properties of water adsorbed by kaolinite clays

The dielectric loss observed for adsorbed water on kaolinite clays is interpreted in terms of a Debye dipolar mechanism in preference to a Maxwell–Wagner mechanism. The variation of relaxation times, τ, with coverage shows a decrease from low coverage values of ≈10–4 s to limiting values at monolayer coverage of ≈10–6 s. These times imply a very tightly bound adsorbate at low coverages. The more rigidly bound a water molecule is the longer it takes to orientate in the direction of the applied field. At monolayer coverages τ has values similar to the extrapolated value for ice at 298 K, indicating a monolayer that has a co-operative structure similar to that of ice.The effect of Na+ and Cs+ counter-ions on the structure of the adsorbate is discussed in terms of the enthalpy of hydration of the cations, and the strength to which they are bound to the surface; the weaker they are bound, the easier for the cations to form complete hydration shells.A low frequency loss, very marked for H2O + Cs kaolinite and H2O + Na kaolinite, is attributed to d.c. conductance in which mobile cations act as charge carriers with adsorbed water as the conducting path.

Sorption of nitrogen, water vapor, and benzene by charcoal cloth

Abstract Charcoal cloth has been characterized by small angle neutron scattering (SANS) and sorption techniques. A SANS gyration radius of 10Awas obtained for the charcoal cloth compared with a gyration radius in the range 6 to 9Afrom the Dubinin—Rudushkevich (DR) nitrogen and benzene “ B ” values. The charcoal cloth pores approximated a cylinder shape in the SANS analysis. Type I sorption isotherms of benzene (303 K) and nitrogen (77 K) on charcoal cloth confirm its predominantly microporous nature with a surface area in excess of 1000 m 2 g −1 , and obeyance of the Gurvitsch rule giving a pore volume of 0.65 cm 3 g −1 . A type V water (303 K) isotherm showing linear Dubinin behavior in the range 0.3 0 −1 is smaller than the N 2 BET monolayer capacity (14 mmole g −1 ).