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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 present a comparison of the quantitative information obtained about the energetic heterogeneity of silica-supported nickel catalysts by an analysis of an experimental TPD peak using the absolute rate theory, in the form of the Wigner−Polanyi equation, and the statistical rate theory of interfacial transport. A new method of evaluating the desorption energy distribution, based on an improved condensation approximation approach and applicable to thermodesorption kinetics in both the above approaches, has been developed for this purpose. To make the comparison, a literature report of a study of adsorption equilibria in the hydrogen/SiO2−Ni system is used together with a TPD study of our own sample of the catalyst. The use of the Wigner−Polanyi approach resulted in the recovery of energy distribution functions showing the solid surface to be more energetically heterogeneous than it is when one uses the SRTIT approach on the same data. The broader surface energetic heterogeneity calculated using the Wigner−Polanyi approach is especially dramatic in the systems where significant readsorption occurs. This is because readsorption affects TPD peaks in a similar way to that due to surface heterogeneity. Despite this, the Wigner−Polanyi approach is commonly applied, neglecting the readsorption term, because of fundamental problems connected with using the full ART formulation, with the readsorption term included. We find that in general, for the same set of physical parameters characterizing a given gas/solid adsorption system, the Wigner−Polanyi approach will generate theoretical TPD peaks which are narrower, and shifted toward lower temperatures, compared with matching theoretical TPD peaks generated by the SRTIT approach. Clearly, one of these approaches is misleading, and on the basis of various considerations presented below, arguments are put forward favoring the use of the SRTIT approach for the interpretation of TPD peaks.

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.

A group of Ni and Ni, Li/Al2O3catalysts were prepared using impregnation and controlled surface reaction methods. The various steps in the preparation were characterised by IR spectroscopy. The catalysts were reduced under the same conditions, while fixed conditions were also employed for the subsequent reactions in order to effect a better comparison. The total and metal surface areas, the degree of nickel reduction and the activity of the catalysts towards CO2methanation were all measured in order to compare the effect of preparation method on the structural properties as well as on the catalyst activity and selectivity. Nickel catalysts modified with lithium, either by controlled surface reaction or double impregnation utilising H2Li2EDTA, exhibited a higher activity in CO2methanation than any other catalyst samples investigated.

Small TiO2 additions decrease both the resistance to carbon formation and (at amounts>1wt.%) the specific activity of Ni catalysts. Particularly harmful are falls of temperature<700 K, causing a rapid and irreversible deactivation of catalysts.

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.

Dispersed Ni/γ−Al2O3 catalysts have been prepared by the method of double impregnation. The application of ethylenediaminetetraacetic acid (H4Y) and modified alumina support with gaseous ammonia permit to prepare well dispersed nickel catalysts with relatively high metal loading.