Proton transfer after adsorption, acidic

Proton transfer after adsorption of acidic molecules... 

As Smith (300) has shown by infrared spectroscopy, carboxylic acids are adsorbed either by hydrogen bonding of the carboxyl group or by proton transfer to the surface. Carboxylate absorptions were observed in the spectra. Very likely O " or OH ions acted as proton acceptors although no OH absorption bands could be detected after carboxylic acid adsorption. The isoelectric point of pure anatase is near pH 6.6 (305). 

The study observed fliat the O2 adsorption energy for one-fold end-on was 0.43 eV and for two-fold was 0.94 eV. Two-fold bonded oxygen was more stable than one-fold. The dissociation energy for two-fold bonded O2 was 0.74 eV, while the activation barrier for the first reduction step to OOH was less than 0.60 eV at 1.23 V electrode potential. In other words, the first electron transfer has a smaller barrier than that of O2 dissociation. Furthermore, the dissociation barrier for the first electron transfer product OOH was much smaller, 0.06 eV. So, the authors eoncluded that O2 did not dissociate before the first reduction step, and OOH easily dissociated once formed after the first electron transfer step. The paper also demonstrated that the electronic field of the proton increased the electron affinity of the reactant complex and therefore facilitated the reaction. Thus, they proposed that for oxygen reduction on Pt in acid, proton transfer would be involved in the rate determining step because of the ability of its electric field to enhance the electron attracting capability of flic surface-coordinated O2. The authors concluded... 

Examples of the lader include the adsorption or desorption of species participating in the reaction or the participation of chemical reactions before or after the electron transfer step itself One such process occurs in the evolution of hydrogen from a solution of a weak acid, HA in this case, the electron transfer from the electrode to die proton in solution must be preceded by the acid dissociation reaction taking place in solution. 

Unusual behaviour was observed for adsorption of ammonia on protonated ZSM zeolite at 416 K [50], Differential heat curve passed through a maximum at relatively low coverage. This behaviour was explained by conjunction of three independent phenomena immobile adsorption, mass-transfer limitations and preferential location of the most energetic acid sites in the internal pores of zeolite stmcture. The maximum was eliminated by heating of samples between doses to increase the surface mobility of the preadsorbed ammonia and allow it to migrate and adsorb on the most acidic sites. In this way most accessible sites are made available for new doses of ammonia at lower temperature. After this the heat curve showed a slightly higher value than the initial heat for the conventional method of adsorption, and it did not show any maximum. 

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