Chemical desorption

The second stage of the reaction to produce molecular hydrogen may occur through either of two mechanisms. In the first of these, known as chemical desorption or chemical recombination, two adsorbed hydrogen atoms combine to produce a hydrogen molecule ... 

In the chemical desorption step the adsorbed H atoms diffuse about on the metal surface, either by threading their way through adsorbed water molecules or by pushing them aside, until two collide to form an Hj molecule which escapes into the solution. This chemical step will be independent of overpotential, since charge transfer is not involved, and the rate will be proportional to the concentration or coverage of adsorbed H,, (see equation 20.39) and may occur at coverages that range from very small to almost complete. 

The ability to use the Tafel slope as a diagnostic criterion can be exemplified by considering a discharge-chemical desorption mechanism for the h.e.r. in which either discharge or chemical desorption may be rate determining. ... 

Thus a Tafel slope of -0-118 V/decade could be diagnostic of a discharge-chemical desorption mechanism in which proton discharge is the r.d.s. 

If chemical desorption is rate determining, the rate will be independent of overpotential since no charge transfer occurs in this step, and... 

Now although the chemical desorption step is independent of rj, the surface coverage will increase as rj becomes more negative and this will affect tic.D - Since two electrons will be required for the overall reaction, equation 20.98 can be expressed in terms of i... 

Step 2a Chemical desorption step (CD step) or atom-atom combination step ... 

Thus, there are two kinetic paths for the hydrogen evolution. The first path consists of charge transfer (CT) followed by chemical desorption (CD) path CT-CD. The second path consists of charge transfer (CT) followed by electrochemical desorption (ED) path CT-ED. Within each path, either of the consecutive steps can be slow and thus can be the rate-determining step (RDS). Each of these paths has two pKJSsible mechanisms. 

The reaction sequence of the Volmer reaction (6a) and the Heyrovsky reaction (6b) is not the only one possible desorption of the adsorbed hydrogen may also proceed according to the so-called Tal el reaction (6c) by chemical desorptive dimerization of two adsorbed hydrogen atoms for which, however, the same fundamental considerations prevail... 

This trend for a rate-determining proton discharge followed by a chemical desorption step at low q s and an electrochemical one at higher rj s seems to survive the change to alkaline solution shown here of course, the proton discharge occurs from water ... 

It is known that the imperfections in a metal include voids that are larger than atomic dimensions, say about 100 A across. On reaching these regions, the absorbed hydrogen atoms feel they have reached an exposed surface. They become adsorbed hydrogen atoms and combine to form hydrogen molecules a chemical desorption... 

After transport to a laboratory, gases are introduced into an analytical instrument for quantitative determination of the constituents of interest. Soil air in a container is introduced directly to the instmment, whilst adsorbed gas is released by thermal of chemical desorption. The instrumental methods most widely used for gas analyses include gas chromatography, mass spectrometry and atomic absorption spectrophotometry. For quantifying the radiation scars on film, image analysis methods are employed. 

This reaction can be followed by either chemical desorption and recombination of two adsorbed hydrogen atoms ... 

Either the discharge or one of the desorption reactions can be the RDS. The other reactions in the process are then considered to be fast and in equilibrium. An example is slow discharge followed by fast chemical desorption ... 

For this mechanism, v = 2 since the RDS must happen twice for the full reaction to proceed once. Fast discharge followed by slow chemical desorption is given by ... 

The subsequent desorption step depends on the metal and the cathodic current density. It can occur by either chemical desorption, which is also referred to as recombination,... 

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