Helium adsorbed

When hydrogen is used as the adsorbate, helium will not adequately serve as a carrier gas because these two gases exhibit similar thermal conductivities. In this case nitrogen or any other nonreacting gas can serve as the carrier. 

The density of coal shows a notable variation with rank for carbon content (Figure 6.1) and, in addition, the methanol density is generally higher than the helium density because of the contraction of adsorbed helium in the coal pores as well as by virtue of interactions between the coal and the methanol, which results in a combined volume that is notably less than the sum of the separate volumes. Similar behavior has been observed for the water density of coals having 80 to 84% w/w carbon. 

The constant C may have either of two values, from which it is calculated that the adsorbed helium atoms can be held on lithium fluoride in one of two states, having energies of adsorption of 57 and 129 calories per mole. For helium and sodium fluoride the corresponding energies. were 80 and 193. 

In order to determine the amount adsorbed, it is necessary to know the volume of the dead space, and this is measured with a gas that is not adsorbed. Helium is commonly used for this purpose. The method is satisfactory when the adsorption is measured at low pressures, particularly when most of the gas introduced is adsorbed... 

Dogu and Smith (1972) used pellets made by compressing porous Boehmite particles. They provided the following data about the system of non-adsorbing helium in nitrogen at 24 and 1 atm. 

Lermard-Jones, J. E., and A. F. Devonshire. 1937. The interaction of atoms and molecules with solid surfaces VI. The behaviour of adsorbed helium at low temperatures. Proceedings of the Royal Society of London. Series A 158 242-252. 

In the narrowest of the microporosity, the dispersion forces are at their most intense, so intense, in fact, that the density of adsorbed helium exceeds that of solid helium. 

Starting with the pioneering studies of Greyson and Aston [28], experimental results for adsorbed helium at submonolayer coverages include adsorption isotherms, heat capacities, heats of adsorption, and neutron diffi action methods [54-57 and references therein]. Other references to studies of helium on different substrates, as well as an extensive review of the results obtained for helium on grafoil until 1973, can be found in Ref 55. For data about scattering experiments on solid adsorbed helium, see Refs. 58 and 59. Data focused on phase transition studies wUl be described in the following section. 

Adsorbates can physisorb onto a surface into a shallow potential well, typically 0.25 eV or less [25]. In physisorption, or physical adsorption, the electronic structure of the system is barely perturbed by the interaction, and the physisorbed species are held onto a surface by weak van der Waals forces. This attractive force is due to charge fiuctuations in the surface and adsorbed molecules, such as mutually induced dipole moments. Because of the weak nature of this interaction, the equilibrium distance at which physisorbed molecules reside above a surface is relatively large, of the order of 3 A or so. Physisorbed species can be induced to remain adsorbed for a long period of time if the sample temperature is held sufficiently low. Thus, most studies of physisorption are carried out with the sample cooled by liquid nitrogen or helium. 

Monolayers of alkanetliiols adsorbed on gold, prepared by immersing tire substrate into solution, have been characterized by a large number of different surface analytical teclmiques. The lateral order in such layers has been investigated using electron [1431, helium [144, 1451 and x-ray [146, 1471 diffraction, as well as witli scanning probe microscopies [122, 1481. Infonnation about tire orientation of tire alkyl chains has been obtained by ellipsometry [149], infrared (IR) spectroscopy [150, 151] and NEXAFS [152]. 

A study of Table 1.1 reveals interesting features as to the mobility of the adsorbed atoms. Thus, for an argon atom on the (100) face, the easiest path from one preferred site S to the next is over the saddle point P, so that the energy barrier which must be surmounted is (1251 — 855) or 396 X 10 J/molecule. Since the mean thermal energy kT at 78 K is only 108 J/molecule, the argon molecule will have severely limited mobility at this temperature and will spend nearly all of its time in the close vicinity of site S its adsorption will be localized. On the other hand, for helium on the... 

Fig. 12. Helium liquefaction process. A represents adsorber E, expander.

In the dynamic method the powder is flushed with an inert gas during degassing, nitrogen is then adsorbed on the powder in a carrier of helium gas at known relative pressure while the powder is in a container surrounded by hquid nitrogen. The changing concentration of nitrogen is measured by a cahbrated conductivity cell so that the amount adsorbed can be determined. 

Helium [7440-59-7] M 4.0. Dried by passage through a column of Linde 5A molecular sieves and CaS04, then passed through an activated-charcoal trap cooled in liquid N2, to adsorb N2, argon, xenon and krypton. Passed over CuO pellets at 300° to remove hydrogen and hydrocarbons, over Ca chips at 600° to remove oxygen, and then over titanium chips at 700° to remove N2 [Arnold and Smith 7 Chem Soc, Faraday Trans 2 77 861 1981]. 

In recent years there is a growing interest in the study of vibrational properties of both clean and adsorbate covered surfaces of metals. For several years two complementary experimental methods have been used to measure the dispersion relations of surface phonons on different crystal faces. These are the scattering of thermal helium beams" and the high-resolution electron-energy-loss-spectroscopy. ... 

This relationship of the metastable atom deactivation mechanisms is valid for atomically pure metal surfaces and is proved true in a series of works [60, 127, 128]. Direct demonstrations of resonance ionization of metastable atoms near a metal surface are given by Roussel [129]. The author observed rebound of metastable atoms of helium in the form of ions from a nickel surface in the presence of an adsorbed layer of potassium. In case of large coverages of the target surface with potassium atoms, when the work of yield becomes less than the ionization potential of metastable atoms of helium, the signal produced by rebounded ions disappears, i.e. the process of resonance ionization becomes impossible and the de-excitation of metastable atoms starts to follow the mechanism of Auger deactivation. 

The presence of adsorbed layers also affects the other parameters of the interaction between metastable atoms and a metal surface. Titley et al. [136] have shown that the presence of an adsorbed layer of oxygen on a W( 110) surface increases the reflection coefficient of helium metastable atoms. The reflection is of irregular nature and grows higher when the incidence angle of the initial beam increases. A series of publications [132, 136, 137] indicate that the presence of adsorbed layers causes an increase in the quantum yield of electron emission from a metal under the action of rare gas metastable atoms. 

Temperature programmed desorption (TPD) of NH3 adsorbed on the samples was carried out on an Altamira TPD apparatus. NH3 adsorption was performed at 50°C on the sample that had been heat-treated at 120°C in a helium flow. After flushing with helium, the sample was subjected to TPD from 50 to 600°C (AT = 10°C min 1). The evolved NH3, H20 and N2 were monitored by mass spectroscopy by recording the mass signals of m/e = 16, 18 and 28, respectively using a VG Trio-1 mass spectrometer. 

Although acetone was a major product, it was not observed by infrared spectroscopy. Flowing helium/acetone over the catalyst at room temperature gave a prominent carbonyl band at 1723 cm 1 (not show here). In this study, a DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) cell was placed in front of a fixed reactor DRIFTS only monitored the adsorbed and gaseous species in the front end of the catalyst bed. The absence of acetone s carbonyl IR band in Figure 3 and its presence in the reactor effluent suggest the following possibilities (i) acetone formation from partial oxidation is slower than epoxidation to form PO and/or (ii) acetone is produced from a secondary reaction of PO. 

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