Reverse chemisorption

Reversible chemisorption and hydrogen-deuterium exchange on zinc oxide have been observed (Taylor et al., 136,137 , Harrison and McDowell,... 

Because zinc oxide is a relatively well-understood oxide semiconductor, we shall first review its properties as a hydrogenation catalyst in the catalytic hydrogen-deuterium exchange reaction. Since the latter essentially measures the rate of reversible chemisorption of hydrogen at equilibrium, data on the hydrogen chemisorption will be included in this survey. Any theory of hydrogen chemisorption on zinc oxide must explain all the following well-established facts. 

Weakly adsorbed CO is characterized by a band around 2100 cm-1 and by bands below 2000 cm-1 (1935 and 1895 cm-1). All these bands are formed upon CO adsorption following hydrogen reduction of the Pd(II) ions. No intermediate species can be detected during this reduction, and the Pd+ ions observed by ESR are present in amounts too small to give strong bands. The reversible chemisorption must take place on the Pd°. 

Heats of adsorption are due to three fundamental processes, physical adsorption, reversible chemisorption, and irreversible chemisorption. Physical adsorption can be attributed to van der Waals forces (dispersion forces). Reversible and irreversible chemisorption are due to bond formation between the adsorbent and the adsorbate. Generally heats of adsorption less than 10-15 kcal/mole are considered due to physical adsorption alone, although some physical adsorption processes may exceed these values. Any chemisorption process generally involves all three processes so that the heat of adsorption value reflects the sum of the average contribution of each. The interaction of transition metals with unsaturated compounds is attributed by most authors, at least in part, to chemisorption. 

For the hydrogenation of propionaldehyde (CH3CH2CHO) to propanol (CH3CH2CH2OH) over a supported nickel catalyst, assume that the rate-limiting step is the reversible chemisorption of propionaldehyde and that dihydrogen adsorbs dissociatively on the nickel surface. 

Maclver and Tobin (56) have reported that there is a weak and nearly reversible chemisorption of hydrogen on a-Cr203 at —195° which is suppressed almost completely by chemisorption of carbon monoxide. The amount of this adsorption at 300 torr is about equal to that of carbon monoxide at —78°. Adsorption of hydrogen on a-Cr203 has also been studied by Weller and Voltz (8). 

An adsorbent is often tailor-made to suit a separation need or a process can be designed to best fit the properties of an adsorbent. Special adsorbents are also available for specific applications (e.g., removal of mercury vapor, drying of reactive fluids, resistance to acids, etc). More recently, adsorbents have been produced that use reversible chemisorption as the mechanism for gas separation. Creation of new adsorbents and modification of existing adsorbents continue to be an active area of research and development. 

The interaction of olefins with silver ions on the adsorbent surface can be classified as a rapidly reversible, weak chemisorption. Proton-substituted acetylene derivatives (R—C=C—H) adsorb on silver-impregnated adsorbents with strong, slowly reversible chemisorption (as the silver acetylide). This permits the clean-cut separation of such acetylene compounds (as a group) from other sample components (90). 

J) and of Garner and Dowden (5) on Cr203 certainly shows the existence of two mechanisms, and there may even be three, namely a) low temperature reversible chemisorption, responsible for H2/D2 exchange at 90°K,... 

A composite ceramic Zr02-Mg0 with porous structure and n-type semiconductivity is a promising material [30]. When the Zr02-MgO ceramic at high temperature between 400 °C and 700 "C is exposed to an ambient atmosphere containing water vapor, reversible chemisorption becomes dominant and the electrical conduction changes with gas chemisorption. 

Adsorption from solution is normally divided into two categories, physical adsorption and chemical adsorption. Physical adsorption which arises from weak interactions such as van der Waals bonding or hydrogen bonding is generally reversible. Chemisorption which, however, is a result of the formation of chemical bonds in many cases is not reversible. 

Their careful adsorption measurements proved that the chemisorpticn layer on W was far from complete at the low pressures used by Roberts and that the heat of reversible chemisorption fell from 15 kcal/ mole at 80% coverage to 3 kcal/mole at 100% coverage. ... 

In general terms, physical adsorption, or physisorption, refers to weak bonding of molecules to surfaces through the interactions of induced or permanent dipoles and/or quadrupoles, whereas chemisorption describes adsorption where transfer of chemical charge between adsorbate and surface takes place. Physisorption is characteristically observed at low temperatures, is not an activated process and is completely reversible. Chemisorption, by contrast, involves the formation of bonds, persists to elevated temperatures and can lead to chemical changes. For the adsorption of molecules on microporous solids, important physisorption interactions include the uptake of simple non-polar molecules such as dinitrogen and dioxygen on cationic forms of zeolites whereas the adsorption of molecules onto acid sites is the most important type of chemisorption, because of its importance in catalysis. 

Lee, K., Beaver, M., Caram, H. and Sircar, S. (2007) Reversible chemisorption of carbon dioxide- Simultaneous production of fuel-cell grade H2 and compressed CO2 from synthesis gas. Adsorption, 13,385-397. 

The metal areas of the unactivated metal-carbons (226A and 249A) measured by Hj chemisorption appeared to be zero ie the reversible and irreversible chemisorption isotherms were co-incident. Therefore, although a small amount of reversible chemisorption did take place, this was shown to be due to chemisorption on the carbon as chemisorption isotherms for the carbon containing no metal and sample 249A (Rh/C) were virtually superimposable. 

Torr) [32-36], Alternatively, higher H2 pressures at 300 K can be used to both form the hydride and saturate the Pd surface with H atoms, then an evacuation step is used to rapidly decompose the bulk hydride, and this is followed by obtaining a second isotherm. Similar to the situation in Figure 3.3, the difference, a, represents the irreversible H adsorption on the Pd surface. This approach may be preferred because it provides additional information about the Pd crystallites, i.e., once the surface Pds atoms are counted by the irreversible uptake, the remainder of the atoms can be attributed to bulk (Pdb) atoms, i.e., Pdb = Pdt Pds, and the apparent bulk hydride ratio can be determined [32]. Values near PdHo b are typically attained with large clean Pd crystalhtes, but on small Pd crystallites this apparent hydride ratio can become larger than 0.6 because reversible chemisorption on the Pds atoms can dominate the second isotherm as the Pd dispersion approaches unity. Consequently, valuable information can be obtained regarding surface cleanliness and metal-support interactions (MSI) [32,37]. An example of such an effort is provided by Illustration 3.1. 

A K-promoted HTlc was synthesized and tested for its reversible COj capacity between 250 and 500 °C. Non-equilibrium dynamic adsorption and desorption isotherms were measured between 65 and 980 torr using 20 or 50 torr steps and a 45 min duration between steps. The absolute CO2 capacity on K-promoted HTlc increased with decreasing temperature, with CO2 loadings of 2.25 and 1.02 mol/kg respectively at 250 and 500 °C and 980 torr. The reversible CO2 working capacity obtained between 65 and 980 torr exhibited a maximum at 450 "C, with a value of 0.55 mol/kg compared to 0.11 and 0.46 mol/kg at 250 and 500 "C, respectively. It was surmised that three temperature dependent, highly coupled, completely reversible, equilibrium driven but kinetically limited reactions were taking place, with the first one being a rapid and reversible chemisorption of CO2 that initiated the entire process. 

The film forming, oxygen-containing compounds examined to date all appear to undergo reversible chemisorption on steel in HCI, followed by polymerization reactions that form a barrier film over the entire surface. These general steps may be summarized with the following scheme ... 

Eleetroehemieal experiments and surface analyses show that the adsorbed oxygen speeies in solution on most transition metals at 25°C are likely to be hydroxyls. Thermodynamic data obtained from electrochemical experiments are presently available only for on copper [49]. Therefore is considered here on Fe, Ni, and Cr and on Cu. The standard Gibbs energies of formation (chemical potentials) for sulW and oxygen adsorbed on metal surfaces can be calculated [44—46] from literature thermodynamie data for reversible chemisorption at the metal-gas interface (see Chqj. 2). 

These simple models can be expanded in their forms to represent various situations such as fouling of polyfunctional catalysts, reversible chemisorptions, etc. 

Deactivated catalyst can be regenerated by simple heating if poisoning is due to reversible chemisorption. If the poisoning species is irreversibly chemisorbed, however, it has to be removed by oxidation or some other chemical means. It is not unusual, when deactivation is a severe problem, to have a guard reactor in which the impurities in the feed are captured before they enter the primary reactor. 

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