Adsorption strength

Poisoning is operationally defined. Often catalysts beheved to be permanently poisoned can be regenerated (5) (see Catalysts, regeneration). A species may be a poison ia some reactions, but not ia others, depending on its adsorption strength relative to that of other species competing for catalytic sites (24), and the temperature of the system. Catalysis poisons have been classified according to chemical species, types of reactions poisoned, and selectivity for active catalyst sites (24). 

FIG. 4 Density n z) plotted as a function of distance z from the adsorbing wall and of adsorption strength e/k T for chain length = 16. Vertical dotted lines denote the density of the monomers sticking immediately at the wall for > 2.0 this density changes dramatically [13]. 

Figure 1.2 Volcano plot illustrating the Sabatier principle. Catalytic rate is maximum at optimum adsorption strength. On the left of the Sabatier maximum, rate has a positive order in reactant concentration, and on the right of Sabatier maximum the rate has a negative order.

An alternative approach to increase the oxidation rate is the use of alkaline solutions, because bases enhance the reactivity of L-sorbose and weaken the adsorption strength of 2-KLG. Unfortunately, the rate enhancement at higher pH is accompanied by a drop in selectivity due to the poor stability of 2-KLG in alkaline solutions. To circumvent this problem, we have modified the platinum catalysts by adsorbed tertiary amines and carried out the oxidation in neutral aqueous solution [57], This allowed to enhance the rate without increasing the pH of the bulk liquid, which leads to detrimental product decomposition. Small quantities of amines (molar ratio of amine sorbose = 1 1700, and amine Pts = 0.1) are sufficient for modification. Using amines of pKa a 10 for modification, resulted in a considerable rate enhancement (up to a factor of 4.6) with only a moderate loss of selectivity to 2-KLG. The rate enhancement caused by the adsorbed amines is mainly determined by their basicity (pKa). In contrast, the selectivity of the oxidation was found to depend strongly on the structure of the amine. 

The qualitative proportion between some model parameters should be as that known from existing knowledge e.g. the relative adsorption strength of various sjjecies on different metals is quite well known and a totally different order of strength found by regression can be considered as an indication that the model should be rejected. 

Matrix properties (nature, particle size, pore structure, water content, adsorptive strength)... 

Liquid-phase breakthrough experiments were also developed in order to characterize mesopores. The principle of the methodology relied on the analysis of the diffusion and adsorption of molecular probes with various molecular dimensions and adsorption strength. The relative proportion of occluded and accessible mesopores in the studied dealuminated Y zeolite could then be estimated. To allow this estimation, it is necessary to use molecular probes that can or cannot penetrate into the microporosity of the Y zeolite (see Figure 2). 

The adsorption strength of anions on Au electrodes follows the sequence of C104 < S042- < Cl < Br < I. The strong specific adsorption of halide ions leads to a partial charge transfer between the adsorbate and the metal electrode [234]. 

In the presence of specifically adsorbed ions, e.g., for halides at potentials E > Epzc, the trend changes and the position of the 1 G0 peaks shifts to higher values than the quantum conductance unit. The up-shifts follow the order of adsorption strength, Cl- < Br < I-. 

The toxicities of nine alkyl aromatics (4) are appreciably less than those of the polycyclic aromatics. No signihcant retardation is observed in benzene-deuterium oxide exchange reactions at 30° even when the poisons are present in benzene in concentrations of 3.6 mole %. The high temperature exchange rate (Table II) of sterieally unhindered ring positions in the alkylbenzenes indicates that adsorption strength... 

Trends in v Complex Adsorption Strength from Competitive Exchange... 

The results of the alkylbenzene series may also be readily explained in terms of ir complex adsorption. In this series, the molecular orbital symmetry of individual members remains constant while the ionization potential, electron affinity, and steric factors vary. Increased methyl substitution lowers the ionization potential and consequently favors IT complex adsorption. However, this is opposed by the accompanying increase in steric hindrance as a result of multiple methyl substitution, and decrease in electron affinity (36). From previous data (Tables II and III) it appears that steric hindrance and the decreased electron affinity supersede the advantageous effects of a decreased ionization potential. The results of Rader and Smith, when interpreted in terms of tt complex adsorption, show clearly the effects of steric hindrance, in that relative adsorption strength decreases with increasing size, number, and symmetry of substituents. 

Pr = It couples the role of adsorption strength together with the diffusion mass transport on the time scale of potential pulses. If the adsorption is very weak, Pr > 1.23, the response of reaction (2.144) is equivalent to the simple reaction of a dissolved redox couple (2.157). 

The physical meaning of the kinetic parameter m is identical as for surface electrode reaction (Chap. 2.5.1). The electrochemical reversibility is primarily controlled by 03 (Fig. 2.71). The reaction is totally irreversible for log(m) < —3 and electrochemically reversible for log(fo) > 1. Between these intervals, the reaction appears quasireversible, attributed with a quasireversible maximum. Though the absolute net peak current value depends on the adsorption parameter. Fig. 2.71 reveals that the quasireversible interval, together with the position of the maximum, is independent of the adsorption strength. Similar to the surface reactions, the position of the maximum varies with the electron transfer coefficient and the amphtude of the potential modrrlation [92]. 

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