Reversible adsorption/desorption

Figure 13.9 Reaction scheme for Ci molecule oxidation on a Pt/C catalyst electrode, including reversible diffusion from the bulk electrolyte into the catalyst layer, (reversible) adsorption/ desorption of the reactants/products, and the actual surface reactions. The different original reactants (educts) and products are circled. For removal/addition of H, we do not distinguish between species adsorbed on the Pt surface and species transferred directly to neighboring water molecule (H d, H ) therefore, no charges are included (H, e ). For a description of the individual reaction steps, see the text.

Filippone GA, Fish SS, Lacouture PG, Scavone JM, Lovejoy FH. Reversible adsorption (desorption) of aspirin from activated charcoal. Arch Intern Med 1987 147 1390-2. 

For Ag(lOO) crystals, a similar electrochemical behavior was observed with quasi-reversible adsorption/desorption of Cd and surface alloying, faster than for Ag(lll) [286]. Electrochemical and AFM experiments have shown that the alloying process consisted of two steps a very fast reaction occurring within a few atomic layers, and a much slower one, represented by a solid-state diffusion process [244]. 

In Scheme 1, steps I, V, and VII are the processes of reversible adsorption-desorption of the alcohol reactant and butene and ether products onto or from the Al—0(H)—Si site. Steps II and III are reversible interconversions of R H R R... 

Sedlak and Andren (1991b) modeled hydroxyl radical reaction kinetics in the presence of particulate. They found that the reaction kinetics for PCB oxidation in the presence of particulate resulted from the complex interplay between solution-phase OH reactions and reversible adsorption-desorption reactions. A model predicting the reaction kinetics can be described by the following equation ... 

From these considerations we can see an outline of the kinetics and mechanism of catalyst decay. While the catalyst remains in the presence of the reactant-product stream, on each active site the processes which dominate are the "fiuitful" processes of the attached carbenium ions, involving protolysis, P-cracking, disproportionation, and the reversible adsorption-desorption of product olefins. These events, in combination, constitute the dKiin mechanism of cracking 4) and yieldthe major products of the "cracking" reactioa None of these processes results in an irreversible reduction of catalyst activity, although the various carbenium ions present will undergo various mainline reactions at different rates. 

Figure 4.1 shows that with n solution species and m surface species involving the adsorbate of interest (the species are distinguished in terms of their stoichiometry surface heterogeneity is not taken into account) there are nxm possible reversible adsorption-desorption reactions. Moreover, there are hx( -1)/2 possible reversible reactions between the solution species, and mx(m-l)/2 possible reversible reactions between the surface species. This makes together (m + n) x (m + n-l)/2 reversible reactions. 

FIG. 2 Principles of SECMID using H+ as a model adsorbate. Schematic of the transport processes in the tip/substrate domain for a reversible adsorption/desorption process at the substrate following the application of a potential step to the tip UME where the reduction of H+ is diffusion-controlled. The coordinate system and notation for the axisymmetric cylindrical geometry is also shown. Note that the diagram is not to scale as the tip/substrate separation is typically <0.01 rs. 

One of the main disadvantages of the Damjanovic s scheme is that it does not consider possible weak adsorptions and the reversible adsorption/desorption of hydrogen peroxide at the interface. The mechanism proposed by Wroblowa et al. [104] considers the adsorption/desorption equilibrium either for the oxygen reactant or the hydrogen peroxide intermediate. They also proposed the chemical decomposition of the intermediate (1(4), besides the electrochemical reduction to water through k3 (Scheme 2.5). 

Experiments were again fully reproducible suggesting reversible adsorption / desorption processes for all probe molecules. 

The latest developments in the issue are indicating that the view based on the H adsorption model is subject of some revision. In References 23 and 24 the voltammetric contribution of some specifically adsorbed anions (acetate, oxalate, chloride and bromide) was studied in the case of Pt(lll) electrodes by means of experiments involving the displacement of the adsorbed species by CO in acidic medium. The conclusion of this study was that the usual states correspond to the reversible adsorption/desorption of hydrogen, whereas the so-called unusual states would correspond to the adsorption/ desorption of anions. 

As soon as solvent is injected, 5h drops sharply due to desorption of the polymer. The rate of this decrease is initially rapid, as can be seen, but soon takes a much lower pace. Upon reinjection of polymer, the initially measured layer thickness was rapidly restored.The first point to note is that in contrast to F, 5h is indeed remarkably sensitive to exposure to pure solvent very small desorbed amounts give rise to considerable decreases in layer thickness. Another interesting aspect is that the experiment is entirely reversible adsorption/desorption cycles can be repeated many times without observable differences. 

Figure 16.4 (a) Reversible adsorption-desorption of oxygen in thermogravimetric O2/TPD experiments in solids La i SrJ (Fe +/Fe +)03 . The steps are as follows In step (1) the sample is heated from RT to 700 °C under Fie flow and releases oxygen. In step (2) the temperature is kept to 700 °C under Fie and the TG curve is stabilized. In step (3) the temperature Is lowered to 550°C, the Fie flow Is switched to O2, and a sharp adsorption occurs. In step (4) the temperature is lowered to 70 °C and further O2 adsorption takes place. Finally, in step (5), calcination... 

Note that step 6 is just a reversible adsorption/desorption step that allows N2O to leave the surface as a product as well as to react further, and this step is not part of the catalytic cycle to form N2. Derive rate laws for the areal rates of a) NO disappearance, b) N2 formation, and 3) N2O formation. You may assume that NO is the MARI on the sites and that H coverage of the S sites is very low. 

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