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The thermodesorption spectra of hydrogen from coprecipitated catalysts (70−x)NiO-xMgO-30Al2O3 (x = 0−50%wt) are reported. The catalysts were calcined at 400 °C and reduced with H2 at 20−800 °C and for 3 h at 800 °C. NiO reduction degree was between 49.3 and 92.1%. The active surface areas changed from 8.4 to 32.4 m²/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.

Theoretical analysis of thermodesorption from energetically heterogeneous solid surfaces is commonly based on the absolute rate theory in the form of the Wigner−Polanyi (W−P) equation, which neglects readsorption. As a consequence, the results of such analyses contain errors of unknown type and magnitude. Here we show that the application of the statistical rate theory of interfacial transport, instead of the W−P equations, leads to adsorption/desorption rate equations which simplify taking readsorption effects into account. An extensive model investigation is presented to show how neglecting readsorption can affect the theoretical analysis of experimental TPD peaks. Our investigation shows that readsorption can mimic effects which are actually due to surface energetic heterogeneity and/or to interactions between the adsorbed molecules. In addition to illustrative model calculations, the role of readsorption is also demonstrated by a quantitative analysis of spectra of hydrogen thermodesorption from a nickel catalyst.

We have recently proposed a new method of estimating the surface energetic heterogeneity of solids from temperature-programmed desorption (TPD) spectra analyzed in terms of the statistical rate theory of interfacial transport (SRTIT) (Chem. Anal. 1996, 41, 1057; In Equilibria and Dynamics of Gas Adsorption on Heterogeneous Solid Surfaces; Elsevier:  New York, 1996.). SRTIT is a new theoretical approach linking the rate of desorption kinetics to the chemical potential of the adsorbed molecules and the molecules in the gas phase. While applying SRTIT to analyze TPD spectra, one arrives at the condensation approximation (CA) for the adsorption energy distribution. Having obtained the CA function, one can calculate the exact adsorption energy distribution using well−known methods (Adsorption of Gases on Heterogeneous Surfaces; Academic Press:  London, 1992). The analysis of TPD spectra in terms of SRTIT is improved here by taking into account the readsorption kinetics. As an example of its application, an analysis of a TPD spectrum of hydrogen desorption from an alumina-supported catalyst is presented.

Recently a new theoretical method of a quantitative analysis of surface energetic heterogeneity from the experimental TPD spectra, based on the statistical rate theory of interfacial transport has been proposed by Rudzinski and co-workers. The purpose of the present publication is to show the predictive features of that new method. Also prospects of a further generalization of that method toward taking into account readsorption effects are discussed.