Adsorption of Weakly Bonded Carbonaceous Species

However, as discussed in section 6 of this chapter, radiometric measurements alone do not allow to decide whether a dehydrogenation occurs during the adsorption process. Reversibility of this type does not necessarily imply an adsorption equilibrium in the thermodynamic sense but may represent [21] a balance between the rates of formation /p and hydrogenation /ny of C HpOq at potentials in the hydrogen region 

The rates /p, I y, and Iqx at constant pH are functions of the different coverages, the concentration of fuel adjacent to the electrode, and the potential. 

The steady-state coverage with carbonaceous species may be determined rather by the balance of the rates of different electrochemical reactions than by an adsorption equilibrium. The treatment [13] of the dependence of the coverage with C HpOq upon potential and methanol bulk concentration b M is an example based on Eq. 4. This treatment by Biegler and Koch predicts variations of coverage and current with bCv, which are similar to those obtained [11] by Bagotzky and Vasiliev under the assumption that the adsorption equilibrium is established and follows a Temkin isotherm. It is questionable for this reason whether the recent determination [22] of the heat of chemisorption of C HpOq, formed under anodic polarization from methanol on platinum in sulfuric acid solution, is meaningful. 

Adsorbed species that desorb when the pressure of the bulk species is greatly reduced by pumping, in gas phase studies, are designated weakly bonded. The analogous procedure in electrochemical studies consists in replacing the mixture of fuel and electrolyte by pure electrolyte in the case of liquid fuels or by reducing the pressure of gaseous fuels 

Experimental evidence for the presence of weakly bonded species was reported recently by Podlovchenko, Frumkin and Stenin [2 ]. The adsorption of carbonaceous species on a platinized platinum electrode with large real surface at open circuit was determined from the decrease of the methanol bulk concentration in O.5MH2SO4 at room temperature. Methanol (bCM = 4.5 10 M) was added at (/ = 0.5V. While the potential approached values between 0.156 V and 0.165 V, the bulk concentration dropped to about 2.15 10 M. Let us suppose [3] that only strongly bonded species are formed by dehydrogenation  

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