Manuela Giustiniani

INFLUENCE OF GEOMETRIC AND ENERGETIC HETEROGENEITY ON ADSORPTION ISOTHERMS

The relationships between geometric roughness and energetic heterogeneity are discussed by considering a thermodynamically consistent model of adsorption isotherms (Keller model) which encompasses fractal scaling and the dependence of the adsorption energies on the coverage. Experimental results validate this model and indicate that it can be used not only to interpolate experimental data but also to predict adsorption equilibria of multicomponent rnixtures. The peculiar non-Henry behaviour of the Keller model at low pressure is discussed by considering a simple model of preferential adsorption on a rough energy landscape and including the effect of surface diffusion.

Thermodynamics and kinetics of adsorption in the presence of geometric roughness

Abstract We analyze in detail the thermodynamic properties and the kinetic features of adsorption on fractal structures, focusing on those topics that can be useful in practical adsorption-based processes and highlighting the theoretical results that can be applied in the design of suitable separation and sorption conditions. The development and the application of thermodynamically consistent models of adsorption isotherms, encompassing the fractality of the adsorbent and the volume-to-surface effects in microporous fractal structures, are analyzed.

Two-step adsorption models in molecular sieves

Starting from analysis of the behaviour of peak retention time vs. the adsorption constant and vs. the mass transfer coefficient, we propose a simple macroscopic model describing adsorption in molecular sieves at high Peclet numbers and at low temperatures. The model is based on the hypothesis that adsorption kinetics evolve through the superposition of an instantaneous adsorption step and a slower step controlled by mass transfer resistance. A detailed moment analysis is performed by comparing the results of the two-step model with those deriving from classical adsorption models (linear driving force model and macropore-micropore model) and with experimental chromatographic data of H2-D2 adsorption on NaCa-Mordenite. The experimental results at low temperatures validate the proposed model.

Projected Measures: A Simple Way to Characterize Fractal Structures and Interfaces

The properties of projected measures of fractal objects are investigated in detail. In general, projected measures display multifractal features which play a role in the evolution of dynamic phenomena on/through fractal structures. Closed-form results are obtained for the moment hierarchy of model fractal interfaces. The distinction between self-similar and self-affine interfaces is discussed by considering the properties of multifractal spectra, the orientational effects in the behavior of the moment hierarchies, and the scaling of the corresponding Fourier transforms. The implications of the properties of projected measures in the characterization of transfer phenomena across fractal interfaces are briefly analyzed.

Size Effects, Thermodynamic Consistency and Non-Ideal Energetic Behavior of Adsorption Isotherms in Microporous Materials

One of the fundamental problems in gas-solid adsorption on heterogeneous surfaces and microporous materials is the formulation of thermodynamically consistent models for single-component and multicomponent adsorption equilibria encompassing the influence of geometric and energetic heterogeneity.

NONLINEAR REACTION DIFFUSION SCHEMES IN CONTINUOUS KINETICS

We analyze reaction schemes in continuous mixtures in the presence of space gradients from the standpoint of the Banach theorem of contractive mappings. The particular case of reaction-diffusion kinetics in a catalyst pellet (slab-model) is considered by developing in detail the analysis for monotonically decaying kinetics and for generic (mass-conserving) reaction schemes. The latter case is addressed by making use of the Frechet decomposition theorem of continuous functionals. Kinetic schemes for which the conservation principle in continuous mixtures cannot be expressed in a straightforward manner are also discussed, focusing attention on the intrinsic dynamic complexity (instabilities, chaos, stochastic description) induced by continuous parametrization.