Modelling of the hydroxyl group population using an energetic analysis of the temperature-programmed desorption of pyridine from silica gel

Abstract

The temperature-programmed desorption (TPD) of pyridine from silica gel has been analysed with the variable heating, constant coverage procedure proposed by Richards and Rees (R. E. Richards and L. V. C. Rees, Zeolites, 1986, 6, 17). This results in curves of activation energies of desorption as a function of coverage. A weak heterogeneity of the silica gel surface is observed in contrast with other pyridine sorption studies. Two adsorption sites, the free and bridged hydroxyl groups, contribute to the surface heterogeneity. The relative amounts of the two sites on the surface are deduced from the calculated activation energy vs. coverage profiles. An attempt is made to extrapolate the absolute concentrations of free and bridged hydroxyls from the relative site coverages

On the Characterization of Chemical Surface Groups of Carbon Materials

“…Similar IA at different temperatures and under dynamic conditions has been reported not only for ammonia adsorption but also for other polar molecules on zeolites (9,(33)(34)(35). The data in literature usually relate IA to acidic chemical surface groups (33)(34)(35). This is supported by the data of samples GAeox1 and GAe-ox2 for which an increase in IA with respect to GAe is observed (Table 5).…”

“…Similar IA at different temperatures and under dynamic conditions has been reported not only for ammonia adsorption but also for other polar molecules on zeolites (9,(33)(34)(35). The data in literature usually relate IA to acidic chemical surface groups (33)(34)(35).…”

Thermal desorption of C6H6 from surfaces of astrophysical relevance

“…A value of 10 13 s −1 was used, the value typically assumed for chemisorbed adsorbates, in order to account for the likely relatively strong hydrogen bonding interaction between C 6 H 6 and surface hydroxyl groups. These silanol groups are expected to exist on the surface, which was not annealed to temperatures above 500 K. 49 The resulting curves were then fitted with third order exponential decay functions in order to facilitate incorporation in the desorption model. This functional form was chosen arbitrarily to obtain good agreement with the experimentally derived curve, and should be regarded as purely empirical.…”

“…The desorption energy in the first layer is most likely to be influenced by the distribution of silanol groups on the surface. SiO 2 surfaces are generally hydroxylated as a result of exposure to atmosphere unless heated to temperatures well in excess of 500 K. 49 The silanol groups are generally considered to be important binding sites for adsorbate molecules on SiO 2 surfaces. 50 Binding is likely to be strongest on those sites where an -OH group to which the C 6 H 6 molecule can hydrogen bond is present.…”

The surface chemistry of amorphous silica. Zhuravlev model

“…15) has been made by Vansant, Van Der Voort and Vrancken in their monograph [263], where they considered seven different models: (1) the model of Zhuravlev [302,303] as a reference one with an error region of 90.5 OH nm − 2 (the shaded band, Fig. 15); (2) the model of Gillis-D'Hamers et al [238], which is based on pyridine desorption from the silica surface; (3) the model of Haukka et al [253,254], which is based on 1 H NMR data for isolated and vicinal silanols; (4) the model of Gillis-D'Hamers et al [263], which is based on the water desorption from the silica surface; (5) the model of Gillis-D'Hamers et al [238], which is based on the IR band shift of free isolated OH groups as a function of temperature; (6) the model of Van Der Voort et al [237,263], which is based on the integration of IR bands due to different types of OH groups; and (7) the model of Fink et al [190], which is based on the deconvolution of the summed-up IR band into constituent components belonging to different types of silanol groups. It can be seen from Fig.…”