Adsorption at low temperatures

Figure 5 - Correlation between the increase of v(CO) and the Av(OH) displacement during CO adsorption at low temperature...

It must be expected that the effect on the heat of absorption will be particularly pronounced at higher pressures and in the middle temperature range, let us say between —170° and 200°. The reason for this is that absorption, which is a slow activated process, will affect the fast adsorption at low temperatures very little. Again at the very high temperatures absorption will be less than adsorption because of its lower heats. On the other hand, at the very high temperatures another absorption process, the endothermic solution, can possibly enter the picture, although this effect is undoubtedly small at low pressures, let... 

At the cold start of the engine the catalyst is not able to oxidize carbon monoxide and hydrocarbons present in the exhaust. Therefore, zeolites are added into y-Al203-based catalytic washcoat for HC adsorption at low temperatures, resulting in an integrated adsorber-reactor system (Jirat et al., 2001 Kryl et al., 2005). For optimum operation of such a system, the consecutive HC desorption induced by increasing temperature should not occur earlier than the catalyst light-off. 

Fig. 10. Diagram of a trapdoor CAVERN allowing catalyst activation and shallow-bed adsorptions at low temperature. The inset shows the expanded view of the trapdoor assembly.

Table II is a collection of information about the dissociatively adsorbed species thought to be present at higher temperatures, usually 300 K or higher, on the various metal surfaces after ethyne adsorption at low temperatures.

To date, only propyne (methylacetylene) and but-2-yne (dimethylacety-lene) seem to have been studied as adsorbates. Nondissociative adsorption at low temperature is supported by the experimental results in all cases. We first discuss the results obtained from but-2-yne, as the adsorbed species are likely to be more symmetrical and hence, with the use of the MSSR, more effective for structure elucidation. 

VEEL spectroscopic evidence has been cited (without details) for the disproportionation of cyclopentadiene adsorbed on Pt(lll) at 95 K to give a mixture of 7j5-C5H5 and cyclopentene, followed by the further conversion of the cyclopentene fraction into t75-C5H5 at 315 K. On Ir(lll), following adsorption at low temperatures, cyclopentadiene has been reported to transform at 350 K into C5H5 as an unstable intermediate before the final formation of what is considered to be a cyclic C5H3 surface species at 450 K. 

Figure 2. Comparison of IR band of CO chemisorbed on a polycrystalline Cu film at room temperature with the bands at the completion of the first stage of adsorption at low temperatures on single crystal Cu surfaces of the indicated orientations (6)...

CO AND CO SEPARATION ON Na-LSX USING PRESSURE-SWING ADSORPTION AT LOW TEMPERATURES... 

CO and CO separation on Na-LSX using adsorption at low temperatures, Izumi, J., Tsutaya, H., Yasutake, A., Tomonaga, N., Saiki, H., Kinugasa, A., Abstract of I6fli Japan Adsorption Society Conference, p. 33(2002)... 

Isotope and Ortho-Para Separations of the Molecular Hydrogens by Adsorption at Low Temperatures... 

IR spectra observed after two-step adsorption using C 02 in the second step at 180 K show that isotopic composition of species-1 formed is nearly same as that of the original gas. This fact corroborates that, during the adsorption at low temperatures, first, no CO2 in the gas phase undergoes 0-isotope exchange with surface O or previously adsorbed speices-3 and -4, and second, no molecular substitution occurs between gaseous CO2 and preadsorbed species. 

In this section we describe the structures of the surface alloys formed by adsorption of the alkali metals on Al(lll) at room temperature. This is prefaced by an account of the ordered phases formed by adsorption at low temperature, since for several systems, these phases undergo order-preserving phase transitions to the room temperature phases. As discussed in Sec. 6, these phase transitions shed some light on the mechanisms of formation of the surface alloys. 

The surface geometries of the Al(ll 1)—( 3 x y3)/ 30°)—K, Rb, and Cs phases formed by adsorption at low temperature, which contain K, Rb and Cs atoms in on-top sites. The interlayer spacings between the i th and y th layers, measured from the outer surfaces in the case of rumpled layers, are denoted dij (A) and the vibrational amplitudes are denoted , (A), doi (A) is the vertical spacing from the alkali layer to the outer surface of the first, rumpled A1 layer, and mq (A) is the vibrational amplitude of an adsorbed alkali atom, r (A) is the effective hard-sphere radius of the adsorbed alkali atom. Azi (A) is the vertical spacing between the subplanes in the first rumpled layer. The final column gives the value of the R factor for the comparison of experimental and calculated LEED spectra. 

It is interesting to note that the effective hard-sphere radii of the adsorbed alkalis are only 5% less than the bcc metallic values. By contrast, as listed in Table 4, the corresponding hard-sphere radii of alkali atoms adsorbed in on-top sites in the ( 3 x y3)R30° phases formed by adsorption at low temperature are 20% less than the bcc metallic values. It can also be seen from Table 5 that the adsorption leads to a small, 2-3% contraction of the first interlayer spacing in the substrate. Somewhat surprisingly, the enhanced vibrations of first layer Al atoms in the clean Al(l 11) surface are not reduced by the adsorption of the alkalis, except for K. 

Adsorption of Li or Na at room temperature on Al(lOO) leads to the formation of c(2 X 2) phases. These phases grow as islands, which consolidate to well-ordered c(2 X 2) structures at 1/2 ML coverage. Early LEED studies [64, 65] of the c(2 X 2)—Na phase led to the conclusion that it contained Na atoms adsorbed in four-fold hollow sites on an unreconstructed substrate, as found more recently for the corresponding c(2 x 2)—Na phase formed by adsorption at low temperature, as described above in Sec. 4.1. However, SEXAFS [61] and CLS [66] measurements showed unequivocally that the structure of the room temperature phase differed from that of the low temperature phase. 

D. White EN. Lassettre (1960). J. Chem. Phys., 32, 72-84. Theory of ortho-para hydrogen separation by adsorption at low temperature, isotope separation. 

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