Low-temperature desorption

Due to the presence of low-temperature desorption peak a new desorption site was included to phenomenological model of TPD experiments previously used for the description of the Cu-Na-FER samples [5], The fit of experimental TPD curves was performed in order to obtain adsorption energies and populations for individual site types sites denoted A (A1 pair), B (sites in P channel (A1 at T1 or T2)), C (sites in the M channel and intersection (A1 at T3 or T4)) [3] and D (newly introduced site). The new four-site model was able to reproduce experimental TPD curves (Figure 1). The desorption energy of site D is cu. 82 kJ.mol"1. This value is rather close to desorption energy of 84 kJ.mol"1 found for the site B , however, the desorption entropy obtained for sites B and D are rather different -70 J.K. mol 1 and -130 J.K. mol"1 for sites B and D , respectively. We propose that the desorption site D can be attributed to so-called heterogeneous dual-cation site, where the CO molecule is bonded between monovalent copper ion and potassium cation. The sum of the calculated populations of sites B and D (Figure 2) fits well previously published population of B site for the Cu-Na-FER zeolite [3], Because the population of C type sites was... 

Subsequently, the lattice parameters of Al(Mg) were calculated as shown in Table 3.1. It is clear that the lattice parameter of Al in the powders milled for 10 h is much larger after heating in DSC up 295, 350 and 425°C than that of the sample heated up to 180°C. The lattice expansion is consistent with the magnitude of the atomic radius of Al and Mg which is equal to 0.1432 and 0.1604 nm, respectively [121]. Fossdal et al. [114] claimed the formation of the Al(Mg) solid solution during low-temperature desorption of Mg(AlH4)2 below 180°C which is not confirmed here. Mamatha et al. [117, 118] never reported the formation of the Al(Mg) solid solution although Kim et al. [119] mentioned about increase in the lattice parameter of Al most probably due to formation of the Al(Mg) solid solution. 

Thus, it is clear that on the Ni/Y Al202 catalyst or the Ni/Si02 catalyst a large fraction of adsorbed CO is in the dissociated or nearly dissociated form. On the Ni/A.C. catalyst CO is adsorbed non-dissociatively and the bond between nickel and CO is weak, as suggested from the low temperature desorption spectrum. In contrast to Ni/y-Al202 and Ni/Si02, the C-0 bond dissociation (Equation 8) hardly proceeds on the Ni/A.C. and therefore the formation of methane (Equation 9) or CO2 (Equation 10) is small. 

The new low frequency shoulder appearing slightly above 400 cm- -corresponds to the metal-carbon stretch of the bridge bonded species. This weaker bond to the substrate for the new species can be correlated with a low temperature desorption peak appearing at high CO exposures in the TDS spectra of reference 23. 

Methanol desorbs from the (110) surface in low and high temperature states, as recently verified in separate studies by Henderson et al. [71,72] and Vohs et al. [73] The low temperature desorption state has been assigned to desorption of molecularly adsorbed methanol. The higher temperature states have been assigned as recombinative desorption of methanol with surface hydroxyl groups based on SSIMS and HREELS identifications of surface species [52,71-73]. 

Decomposition reactions of larger aliphatic alcohols have been examined in detail on the (Oil [-faceted TiO2(001) surface [80]. Ethanol adsorbed at 300 K exhibited a low temperature desorption peaks for ethanol and water at 365 K and a high temperature desorption state for decomposition products at 588 - 595 K. Half of the ethanol adsorbed on the surface desorbed as ethanol at 365 K. Half of the remaining surface ethoxide groups desorbed as ethanol at 588 K. The... 

H-covered surfaces. TPD studies [15,18,20,22] of CO and H2 co-adsorption on Ni(lOO) concluded that CO+H interactions in the co-adsorbed layer lead to formation of a new surface entity exhibiting new low-temperature desorption states of CO and H2. Relevant LEED investigations [22,25,26] brought about consistent results, by characterizing formation of CO+H interaction species. HREELS studies [18,25,26] of a similar system proved the existence of a strong link between the CO+H interaction species and a terminal-CO stretching vibration at 2100 cm , i.e. close to that observed here at 2095 cm for the terminal-CO species (spectrum (a). Fig. 3). These features were observed [18] only for CO and H2 coadsorption on low site-density surfaces ofNi°, namely, Ni(lOO). 

High-resolution ELS and LEED have also been used in a study of C2H2 adsorption on Ni(lll). At room temperature no LEED pattern was observed although the work function was lowered by 1.5 V. Only H2 is desorbed upon heating. From ELS after low exposures (<1.5 L) it was deduced that the C—C bonds of the surface species had an order of 1.15. Thermal desorption showed no low-temperature desorption of H2 and so it was thought that C2H2 was bonded associatively with likely tt and a interactions, with the two H atoms equivalent. 

The low-temperature desorption processes are best suited for removal of organics from sand, gravel, or rock fractions. The high sorption capacity of clay or humus decreases partitioning of organics to the vapor phase, making these materials difficult to process. The debris considered in this study will have little or no clay or humus. 

HS and combustion products of biomasses, and alkylbenzenes and thiophenes can be both evaporation/pyrolysis products from humic substances. At present, solvent extraction and low temperature desorption followed by TMAH Py-GC-MS have been proposed to distinguish combustion, evaporation and pyrolysis products in the pyrolysis. ... 

However, not all internal hydrogen is catalytically active for hydrogenation. Temperature program desorption measurements on Raney nickel revealed that surface hydrogen associated with the low-temperature desorption peak is involved primarily in hydrogenation [5] and neutron inelastic spectroscopy... 

Low-temperature thermal desorption It consists of heating the soils to drive off the volatile compounds and/or hydrocarbons. The heat from the burn unit volatihzes,but may not completely destroy, the contaminants. Low-temperature desorption usually can meet clean soil criteria and the impacted soils can be transported to either a permanent facihty or, if enough material is present, a mobile unit can be brought to the site. Special consideration must be given chlorinated compounds during low-temperature desorption due to their abihty to break down and form hydrochloric acid in the hot air stream, causing potential air emission problems. 

High temperature thermal treatment It is similar to low temperature thermal desorption, but is hot enough to thermally destroy volatile and hydrocarbon compounds. The cost of high temperature thermal treatment is, therefore, much more than low-temperature desorption, but the soil is typically completely depleted of volatile and heavy hydrocarbon compounds. PCBs are typically destroyed to 99.99% by this method. The use of the soil after high temperature thermal treatment depends on the original material s quality, as the soil can typically be used as a clean fill. In some cases, it is used as a feed stock for asphalt or concrete. 

Figure 21 TPD profiles of H2 after reduction at 623 K (—) and 723 K (—), (a) whole profile, (b) low-temperature desorption peaks/...

Catalyst N2 — TPD Low temperature desorption peak H2 — TPD Low temperature desorption peak H2 — TPD High temperature desorption peak ... 

Diamond oxidation occurs at 1,070 K preferentially at grain boundaries, local defects, and in the diamond-like carbon phase [86, 87], Molecular oxygen adsorption happens to the clean (111) and (110) surfaces of diamond at room temperature [88]. Thermal desorption produces CO from both surfaces. Apart firom a low-temperature desorption peak, TDS shows two CO desorption peaks at 1,060 and 1,300 K for the C(lll)—(2 x 1) surface, whereas only one desorption peak presents in the 1,030-1,160 K range for the C(llO) surface. 

From MS results the nature of acid sites can be determined. Upon heating in vacuum two stages of weight loss and two stages of product evolution can be observed. The molecules associated with low-temperature desorption can probably be assigned to a number of different types of species, including molecules associated with Lewis... 

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