Thermodesorption Studies of Energetic Properties of Ni/MgO−Al₂O₃ Catalysts. Determination of Adsorption Energy Distribution Functions

Abstract

The thermodesorption spectra of hydrogen from coprecipitated catalysts (70-x)NiO-xMgO-30Al(2)O(3) (x = 0-50%(wt)) are reported. The catalysts were calcined at 400 degrees C and reduced with H(2) at 20-800 degrees C and for 3 h at 800 degrees C. NiO reduction degree was between 49.3 and 92.1%. The active surface areas changed from 8.4 to 32.4 m(2)/g whereas mean size of nickel crystallites was between 3.7 and 9.7 nm. The TPD spectra were next analyzed in order to determine the adsorption energy distributions functions. To obtain these functions a theoretical model of adsorption/desorption kinetics based on the statistical rate theory (SRT) was applied. This approach allows for determination of the adsorption energy at nonequilibrium conditions as well as at quasiequilibrium conditions. The resulting distribution functions reveal the presence of two main bands of adsorption energy. Some correlation is found between the determined distributions of adsorption energy and the size of nickel crystallites determined using the XRD method. The presence of MgO favors creation of high energy adsorption sites on Ni crystallites.

Sticking coefficient and pressure dependence of desorption rate in the statistical rate theory approach to the kinetics of gas adsorption. Carbon monoxide adsorption/desorption rates on the polycrystalline rhodium surface

“…[7][8][9][10] The above considerations lead to the conclusion that in most of the experimental situations it is sufficient to take for analysis only simplified versions of the SRT kinetic equation, i.e. The gas phase in such an experiment is always pumped off and the number of molecules in the gas phase is kept low.…”

“…Set (24) describes the case of isothermal desorption, however, more typical is the thermodesorption process, i.e. 10 Eqn (24) can be expressed in terms of dimensionless quantities using the set of substitutions (14)(15)(16)(17). In such a case set (24) is still valid, however, one has to remember that the temperature is then a certain function of time, usually dT/dt = b where b is the heating rate.…”

“…Originally the SRT master equation was developed for an isolated system consisting of a constant number of molecules. [6][7][8][9][10] However, despite many successful applications of the SRT kinetic equation in the analysis of experimental data, 3-10 one can notice some peculiar features of this equation. [6][7][8][9][10] However, despite many successful applications of the SRT kinetic equation in the analysis of experimental data, 3-10 one can notice some peculiar features of this equation.…”

Methane steam reforming at low temperature over Ni-MgO-Al2O3 catalysts: activity and resistance to coking

“…The TPD-H 2 profiles of catalysts with the variable MgO/NiO ratios were subjected to a quantitative analysis in ref. [42]. The determined adsorption energy distribution functions showed that the catalysts containing ≤ 27.3 wt.% of MgO revealed qualitatively similar bands of the adsorption energy.…”

“…However, the increasing amount of MgO alters the population of various types of adsorption centers found on catalysts active phases. Thus, the amount of MgO modifies significantly the morphology of nickel crystallites formed during the reduction process [42].…”