Non-activated chemisorption

Two Colorado oil shale samples one from the Parachute Creek Member and the other from the C-a tract, were retorted, de-charred and then subjected to temperatures between 800 K and 1100 K in order to study the mineral reactions which take place. Comparisions between these two samples include the reversible nature of ankeritic dolomite and free calcite as well as the temperatures at which significant silication takes place. Results for the C-a tract samples indicated silication appears to take place in stages and that ankeritic dolomite decomposition can be prevented by relatively low CO2 concentrations. Ankeritic dolomite and calcite decomposition rates were similar for the two samples and there was strong evidence that calcite recarbonation takes place via non-activated chemisorption of C(>2 ... 

This suggested mechanism is consistent with a number of observations made here and with previous work reported in the literature. For example it was found that recarbonation rates were relatively insensitive to temperature. This would indicate non-activated chemisorption and, as Fischbeck and Snaidt report (10), mineral recarbonation is often independent of temperature when the temperature dependencies of the decarbonation rate conr-stant and the equilibrium constant are similar. This is exactly what is observed in western oil shales (Ref (9), Equations (6) and (7)). Previous work has also pointed to the role of chemisorption phenomena in mineral decomposition reactions. Spencer and Topley (11) have suggested that finely grained oxides can chemisorb HoO as well as H2 and Soni and Thomson (12) observed higher recarbonation rates when CO2 was produced on the surface during oil shale char combustion. 

The above conclusion applies as well to non-activated chemisorption, fn chemisorption, however, the chemical specificity of the reaction is such that factors like the crossing of electronic curves, the existence of steric constraints, and the coupling with the heat reservoir are expected to play a fundamental role, so that no general rule can be specified. 

Fig. 4.11 Schematic potential energy curves for activated and non-activated chemisorption of hydrogen on a clean metal surface and exothermic or endothermic solution in the bulk. A more pronounced minimum just below the surface allows for subsurface hydrogen (onedimensional Lennard-Jones potential, Somorjai (1987) Ref [33]).

There are two major chemisorption patterns. One is non-activated chemisorption, which can happen at low temperatures. The adsorption rate is very fast, and does not require any activation energy. For example, H2 can be quickly adsorbed in Ni, Pt and Pd at 77 K. Another pattern is activated chemisorption, which is referred as activated adsorption, which requires a higher temperature. It is characterized by the activation energy need, just as the real chemical reaction. 

Regardless of the exact extent (shorter or longer range) of the interaction of each alkali adatom on a metal surface, there is one important feature of Fig 2.6 which has not attracted attention in the past. This feature is depicted in Fig. 2.6c, obtained by crossploting the data in ref. 26 which shows that the activation energy of desorption, Ed, of the alkali atoms decreases linearly with decreasing work function . For non-activated adsorption this implies a linear decrease in the heat of chemisorption of the alkali atoms AHad (=Ed) with decreasing > ... 

In view of the potential-work function equivalence of solid state electrochemistry (Eq. 4.30 or 5.18) and of the fact that for non-activated adsorption, AEd>Pt=0=A AHo,pt, where AHo.pt is the enthalpy of chemisorption of O on Pt, these equations can also be written as ... 

To improve accuracy, usually data are collected at various pressures, followed by the extrapolation of the adsorbed amount of gas to zero pressure. In commercial equipment this is often done in the so-called increasing pressure mode by the stepwise injection of small amounts of gas. Note that these methods can only be used easily for non-activated adsorption (Reuel and Bartholomew, 1984), e.g. for CO chemisorption. 

Aharoni, C, and Ungarish, M. (1976). Kinetics of activated chemisorption. I. The non-Elovichian part of the isotherm. J. Chem. Soc., Faraday Trans. 72, 400-408. 

We recently reported that a heterogeneous copper catalyst prepared with a non-conventional chemisorption-hydrolysis technique is able to promote a hydrogen transfer reduction using a donor alcohol. In this case, the role of copper is cricial, both for activity and selectivity [20]. 

In Fig. 14 we show HREEL spectra of ethylene adsorbed at Ag(4 1 0) and at Ag(2 1 0) at T = 105 K and compare them with the spectra recorded for Ag(l 00). On stepped surfaces (upper two spectra) C2H4 was dosed with a pure beam. Non-activated adsorption is witnessed by the loss in the 121-125 meY region. No adsorption takes place, on the other hand, on the extended (100) terraces of Ag(l 00) [90] up to much higher energies (see spectrum recorded at Ex = 0.31 eV in Fig. 14). Chemisorption on Ag(l 0 0) is observed when the ethylene exposure is performed with Ex — 0.35 eV. Adsorption on the flat surface is therefore translationally activated for extended (100) terraces and non activated for stepped surfaces. Physisorbed molecules do not contribute to the HREEL spectra since desorption takes place within a few seconds after the end of the dose at 105 K (as evident from Fig. 2) and recording a spectrum requires many minutes. 

The adsorption process is usually fast on evaporated films. However, on bulk solids, e.g. porous catalyst carriers, adsorption rates are usually slow and activated with activation energies typically in the range 10-40 kcal/g mole (Hayward and Trapnell, 1964). Some activation energies for typical activated chemisorption process are given in Table 5.9. In their investigation of the catalytic dehydration of methylcyclohexane, Sinfelt et al. (1960) found that to obtain a suitable kinetic expression finite rates of the adsorption-desorption process must be taken into consideration. In this section allowance is made for finite rates of adsorption and both activated and non-activated adsorption are considered ... 

It is clear that the material given in this chapter is quite classical and has been known in the literature since the 1930s and 1940s in the field of surface chemistry and catalysis. In fact this is the extent of knowledge used to date in the derivation of rate equations for gas solid catalytic reactions. To be more specific most of the studies on the development of gas-solid catalytic reactions do not even use the information and knowledge related to the rates of chemisorption (activated or non-activated) and desorption. Even the most detailed kinetic studies, usually rely on the assumption of equilibrium adsorption-desorption and use one of the well known equilibrium isotherms (usually the Langmuir isotherm) in order to relate the surface concentration to the concentration of the gas just above the surface of the catalyst. 

Two schematic combined potential energy wells for the interaction of a gaseous species with a surface are shown in Fig. 2, illustrating the importance of the crossover point of the chemisorption and physisorption wells for adsorption and desorption kinetics. In the first case, adsorption is activated in the second, it is non-activated. (There are, in fact, only a few well-documented cases of activated chemisorption.) Recently, Lundqvist et al. [43] have made detailed calculations of the potential interaction between H2 and a magnesium surface which substantiate the presence of two minima. Their work is reviewed elsewhere [44]. It must be borne in mind that diagrams such as Fig. 2 grossly oversimplify the... 

Fig. 2. Crossed potential energy curves for physisorption and chemisorption, (a) Non-activated adsorption (b) activated adsorption.

Figure 26.11 displays two examples of dissociative chemisorption, which depend on the shapes of the PESs. In one case one has a non-activated dissociative... 

The most general case requires solving Eqs.,(l) and (2) simultaneously, with the given rate expressions Rg and Rg. In the present study, however, the chemisorption process on the Pt surface is assumed to rapidly equilibrate according to the Langmuir isotherm. This assumption seems reasonable because CO adsorption on Pt is a fast, non-activated process (18, 19), and the desorption rate also appears to be reasonably fast at our reaction temperatures [around 2Q0°C or higher (18, 20, 21)]. 

In addition, chemisorption is often an activated process, which means that the formation of a chemisorptive bond requires overcoming the activation barrier (Fig. 4. lb) it may be slow and irreversible. By contrast, physisorption is rapid, non-activated and reversible process. 

Dissociative chemisorption of a diatomic molecule can also happen through the dissociation in a gas phase and a creation of two gas phase atoms these two atomic species can be then adsorbed on the surface (this way is almost always non-activated). If the curves describing molecular and atomic adsorption intersect at or below the zero potential energy line, then the precursor physisorbed molecule can experience non-activated dissociation, followed by chemisorption (Fig. 4. la). In contrast, if the energetic for these two pathways are such that the intersection occurs above the zero eneigy plane, then chemisorption wiU be activated with activation energy, Ead, as indicated in Fig. 4. lb. 

Fig. 4.1 Potential energy curves for (7) physical and (2) chemical adsorption (a) non-activated (b) activated. Epot - potential energy, Qc - heats of chemisorption, Qp - heats of physisorption, Ead -energy of activation for desorption, Ediss - dissociation energy for the diatomic molecule. The sum AEdes = Ead + Qc is the the heat of hemisorption, in the activated processes [8]...

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