Acidity constants adsorption-desorption

Table I. Rate Constants of the Adsorption/Desorption and Equilibrium Acidity Constants...

Li (1981) has proposed that the distribution coefficients reflect adsorption-desorption reactions at the surface of mineral grains. To emphasize this point, Li plotted a slightly different distribution coefficient (log Cop/Q , where Qp and Cs are the concentrations in oceanic pelagic clay sediments and seawater respectively vs the first hydrolysis constants of the metals or the dissociation constant of the oxyanion acids. The argument is that those elements that hydrolyze the strongest will adsorb the... 

The decomposition behavior of formic acid on the close-packed Ru(lOTO) surface parallels the reaction on nickel, except that the autocatalytic process was not observed (lOJ). Water was desorbed at 183 K by apparent second-order kinetics following adsorption of HCOOH at 100 K. Subsequent desorption of Hj, COj, and CO suggested the formation of the surface anhydride. The rate constant for decomposition was 2.6 x 10 sec exp —26.9 kcal/mol// r. ... 

Adsorption and desorption reactions of protons on iron oxides have been measured by the pressure jump relaxation method using conductimetric titration and found to be fast (Tab. 10.3). The desorption rate constant appears to be related to the acidity of the surface hydroxyl groups (Astumian et al., 1981). Proton adsorption on iron oxides is exothermic potentiometric calorimetric titration measurements indicated that the enthalpy of proton adsorption is -25 to -38 kj mol (Tab. 10.3). For hematite, the enthalpy of proton adsorption is -36.6 kJ mol and the free energy of adsorption, -48.8 kJ mol (Lyklema, 1987). 

The Langmuir equation has been used to describe adsorption of p carotene from solution onto activated bleaching clays (8) and free fatty acid from isooctane solution by acid-washed rice hull ash (9). Likewise, isotherm analysis of the commercial bleaching of rubber and melon seed oil by Fullers earth, activated carbon, and Fullers earth/activated carbon mixture (10) followed Langmuir behavior at 55°C and 80°C, suggesting the possibility of monolayer adsorption with little competition. This behavior was not observed at 30°C where the isotherm no longer applies, because of desorption. The amount of adsorbent was kept constant while varying... 

Acetal and ketal formation from aldehydes, resp. ketones and alcohols occurs over mordenite and other acidic zeolites [91] slightly above ambient temperatures in the liquid phase. The reaction is not confined to simple alcohols, diols can also be converted (e.g., cyclohexanone reacts with ethylglycol to 1,4, dioxaspiro(4,5)decane [2]). Note that it is likely that desorption controls the rate of such reactions as the product molecules are larger than the reactants and have, hence, a higher adsorption constant. 

This chapter describes the results of the acidity characterization of a selected silica surface with VT-DRIFT spectroscopy. Examples of the capabilities of the method are demonstrated by the qualitative determination of the adsorption and thermal desorption characteristics of pyridine on amorphous, porous silica gel. Procedures for the determination of isothermal desorption rate constants and activation energy of desorption are presented and discussed as a means of assessing acid site strength. 

The resultant materials were characterised by powder X-ray diffraction (XRD) and by nitrogen adsorption at 77K, using the adsorption isotherm to calculate BET surface area, and the desorption isotherm and BJH method to determine the pore size distribution. The concentrations of acid sites were measured by pH titration with standard NaOH solution, following exchange with excess NaCl solution. Water contents were measured by heating to constant weight at 150 C. 

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