Adsorption models and heat of adsorption of adsorbed ortho di-methyl benzene species on silica by using Temperature Programmed Adsorption Equilibrium methods

Abstract

The adsorption of ortho dimethyl benzene (o-DMB) at different adsorption temperatures Ta (Ta > 300 K) on a SiO2 solid pretreated at 723 K is studied by the Temperature Programmed Adsorption Equilibrium methods developed previously. These methods provide the evolutions of the adsorption equilibrium coverage of the adsorbed species θe (θe < 0.7) with the adsorption temperature Ta in quasi isobar conditions. These experimental curves θe = f(Ta) are compared to theoretical curves associated to adsorption models developed with the statistical thermodynamics formalism. These models assume either localized or mobile adsorbed species without and with interactions. It is shown that the Langmuir model (localized species without interaction) provides theoretical isobars overlapped with the experimental data for different adsorption pressures Pa considering a heat of adsorption of 61 kJ/mol consistent with the isosteric heat of adsorption. FTIR data show that the adsorption sites are mainly the free OH groups of SiO2 with a small contribution of superficial oxygen species. In line with the development of the experimental microkinetic approach of heterogeneous catalytic processes, and considering previous works dedicated to the adsorption of diatomic molecules such as CO, H2, NO on metal supported particles and metal oxides, it is concluded that adsorption models assuming localized adsorbed species without (Langmuir model) and with (Temkin model) interactions provide robust mathematical expressions, for (a) the adsorption coefficient and (b) the adsorption equilibrium coverage θe = f(Ta, Pa), consistent with the experimental data for Ta > 300 K.