STRONGLY ADSORBING SPECIES

None of the selectively adsorbed components is removed from the adsorption vessel until the countercurrent depressurization (blowdown) step. During this step, the strongly adsorbed species are desorbed and recovered at the adsorption inlet of the bed. The reduction in pressure also reduces the amount of gas in the bed. By extending the blowdown with a vacuum (ie, VSA), the productivity of the cycle can be greatiy increased. 

Interest in fuel cells has stimulated many investigations into the detailed mechanisms of the electrocatalytic oxidation of small organic molecules such as methanol, formaldehyde, formic acid, etc. The major problem using platinum group metals is the rapid build up of a strongly adsorbed species which efficiently poisons the electrodes. 

It is only since 1980 that in situ spectroscopic techniques have been developed to obtain identification of the adsorbed intermediates and hence of reliable reaction mechanisms. These new infrared spectroscopic in situ techniques, such as electrochemically modulated infrared reflectance spectroscopy (EMIRS), which uses a dispersive spectrometer, Fourier transform infrared reflectance spectroscopy, or a subtractively normalized interfacial Fourier transform infrared reflectance spectroscopy (SNIFTIRS), have provided definitive proof for the presence of strongly adsorbed species (mainly adsorbed carbon monoxide) acting as catalytic poisons. " " Even though this chapter is not devoted to the description of in situ infrared techniques, it is useful to briefly note the advantages and limitations of such spectroscopic methods. 

The different species formed by steps (18) to (20) or (18 ) to (20 ) have been detected by in situ infrared reflectance spectroscopy, and such dissociative steps are now widely accepted even if the exact nature of the species formed during (20) or (20 ) is still a subject of discussion. Several groups proposed the species (COH)3js as the main, strongly adsorbed species on the platinum surface, even though no absorption infrared band can be definitely attributed to (COH),, . However, the formyl-like species ( CHO), , . has been formally identified, since it gives an IR absorption band ataroimd 1690cm . ... 

Platinum is the only acceptable electrocatalyst for most of the primary intermediate steps in the electrooxidation of methanol. It allows the dissociation of the methanol molecule hy breaking the C-H bonds during the adsorption steps. However, as seen earlier, this dissociation leads spontaneously to the formation of CO, which is due to its strong adsorption on Pt this species is a catalyst poison for the subsequent steps in the overall reaction of electrooxidation of CHjOH. The adsorption properties of the platinum surface must be modified to improve the kinetics of the overall reaction and hence to remove the poisoning species. Two different consequences can be envisaged from this modification prevention of the formation of the strongly adsorbed species, or increasing the kinetics of its oxidation. Such a modification will have an effect on the kinetics of steps (23) and (24) instead of step (21) in the first case and of step (26) in the second case. 

The identity of the strongly adsorbed species responsible for poisoning the platinum anode was eventually narrowed down to two possibilities ... 

Assume there are two main paths for the oxidation of methanol one is direct oxidation to C02 the other is oxidation to COad or strongly adsorbed species and its oxidation to CO2 os shown in the following scheme ... 

In all above mentioned applications, the surface properties of group IIIA elements based solids are of primary importance in governing the thermodynamics of the adsorption, reaction, and desorption steps, which represent the core of a catalytic process. The method often used to clarify the mechanism of catalytic action is to search for correlations between the catalyst activity and selectivity and some other properties of its surface as, for instance, surface composition and surface acidity and basicity [58-60]. Also, since contact catalysis involves the adsorption of at least one of the reactants as a step of the reaction mechanism, the correlation of quantities related to the reactant chemisorption with the catalytic activity is necessary. The magnitude of the bonds between reactants and catalysts is obviously a relevant parameter. It has been quantitatively confirmed that only a fraction of the surface sites is active during catalysis, the more reactive sites being inhibited by strongly adsorbed species and the less reactive sites not allowing the formation of active species [61]. 

In any case, the treatment of the catalyst results in a more selective (regarding the formation of primary amine) hydrogenation of nitriles. This effect could be explained on a molecular basis by electronic (work function) and/or steric effects by strongly adsorbed species that function to partition the wide-open active surface of Ni and Co catalysts into smaller active sites composed of a controlled number of... 

A significant advantage of the methodology of NMR spectroscopy is that it allows application of pulse sequences for the discrimination of nuclei in specific local structures, if these nuclei are characterized by a coupling with other nuclei. Examples are dipolar-dephasing techniques such as those used in TRAPDOR experiments (Section II.C). CP experiments can be applied for the discrimination of nuclei in various structures, for example, for the identification and investigation of strongly adsorbed species with low mobility. 

The absence of any direct, i.e. molecular, means of identifying the adsorbed species in situ rendered the controversy unresolvable and it remained undecided over the ensuing fifteen years. However, in 1981 Beden et til. published EMIRS spectra that were destined to have a major impact on this dispute, as discussed in section 2.1.6. This early paper concluded that C=Oads is the dominant strongly adsorbed species (poison) and it is present at high coverage. Some Pt CO is also present but there is no evidence of COHads under the experimental conditions employed (non steady-state, potential perturbation at 8.5 Hz and with the dissociative chemisorption of methanol slow). The principal assignments of the paper were very quickly verified by Russell et al. (1982) using the IRRAS technique. 

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