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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - SHAPE (Structure-dependent microkinetic modelling of heterogeneous catalytic processes)

Teaser

Summary

SHAPE is a project focusing on the development of an experimental and theoretical methodology for structure dependent microkinetic modeling in heterogeneous catalysis.

Heterogeneous catalysis plays a crucial role in modern societies both in the production of important chemicals (for instance, ammonia, the basis for production of fertilizers), energy applications (for example, fuel production) and environmental protection (for example, pollution abatement from vehicles). SHAPE aims at incorporating the effect of the catalyst structure in the microkinetic description of heterogeneous catalytic processes by hierarchically combining novel methods at different levels of accuracy in a dual feedback loop between theory and experiments. This approach requires the development and assessment of novel methods belonging to different disciplines, from physical chemistry to material science and chemical engineering.

The successful accomplishment of the objectives of the project will allow the prediction of structural changes of the catalyst during the reaction by achieving an atomistic-level description of the structure-activity relation. The potential contribution of SHAPE on catalysis science and technology is very high by making possible the engineering of the chemical transformation at the atomic level. The possibility to predict the catalyst structure under reacting conditions has a direct impact on the fundamental analysis and design of the structure-activity relation, thus paving the way towards the nano-engineering of the catalyst structure and composition to tailoring activity and selectivity for advanced process intensification in applications of technological relevance.

Work performed

In the first 36 months, we have developed and tested a hierarchical methodology for structure-dependent microkinetic modeling of catalytic processes. This has included the thorough assessment of methodologies for the parametrization of chemical kinetics on different active sites of a nanoparticle, the inclusion of the structural dynamics of the catalyst material in multiscale simulations, the analysis of the kinetic consequences of confinement in kinetics, the development of multiscale methodologies for the coupling between detailed microkinetic models and CFD simulations.

In particular, for the first time in the literature, we were able to give a systematic analysis of the validity of Bronsted-Evans-Polanyi (BEP) and UBI-QEP relationships in the context of structure-sensitive reactions. In particular, we clarified that these general relationships can be applied to the same surface and different metals, but not at a similar extent to the same metal and different surfaces. This is mainly related to the fact that different surfaces may induce changes in the reaction path, thus influencing the character of the transition state. Thanks to our analysis, we proposed and assessed a criterion for the application of BrÃ¸nstedâ€“Evansâ€“Polanyi (BEP) relations for dissociation reactions at surfaces. We clarified that both the activation energy and the reaction energy can be decomposed into two contributions that reflect the influence of reactant and products in determining either the activation energy or the reaction energy. We showed that the applicability of the BEP relation implies that the reaction energy and activation energy correlate to these two contributions in the range of conditions to be described by the BEP relation. A lack of correlation between these components for the activation energy is related to a change in the character of the transition state (TS) and this turns out to be incompatible with a BEP relation because it results in a change of the slope of the BEP relation. Our analysis reveals that these two contributions follow the same trends for the activation energy and for the reaction energy when the path is not characterized either by the formation of stable intermediates or by the change of the binding mechanism of the reactant. As such, one can assess whether a BEP relation can be applied or not for a set of conditions only by means of thermochemical calculations and without requiring the identification of the TS along the reaction pathway.

Given the very recent importance of confinement effects for the interpretation of the structure-activity relation in catalysis (that is one the main aims of the SHAPE project), we have also extended our analysis to the assessment of confinement effect in catalysis. This has been done in collaboration with the University of California â€“ Berkeley, USA (Group of Prof. Enrique Iglesia), that recently reported in the literature experimental pieces of evidence on the importance of confinement effects for NO oxidation in purely siliceous zeolites. Our analysis provided the first theoretical underpinning of the so-called â€œcatalysis by confinementâ€. We found that confinement does not substantially affect the relevant energies of each individual elementary step because the reactants, products and transition states share the same molecularity and size and are thus stabilized by confinement to the same extent. Measured rates reflect, however, the energy of the relevant termolecular transition states relative to those of their unconfined gaseous reactants, making reactivity strongly dependent on the enthalpic stabilization of transition states provided by vdW dispersion forces upon confinement. This enthalpic stabilization more than compensates the loss of entropy upon confinements at low temperatures, thus giving a lower free energy of activation than for the homogeneous reaction. This enthalpic stabilization vanishes when vdW interactions are excluded in the calculations. Thus,

Final results

\"All the work so far performed consists of important progress with respect to the state-of-the-art in microkinetic modeling analysis, which is at the moment \"\"structure-less\"\". In particular:

1) The hierarchical methodology, published in Catalysis Science and Technology (8, 2018, 3493), represents the first step in the literature towards the structure-dependent microkinetic analysis of a catalytic process of industrial relevance. Bulk transitions and dynamics of the nanoparticles as a function of the operating conditions are correctly predicted in agreement with the experimental literature on the topic.

2) The first-principles analysis of the catalysis by confinement, published in Physical Chemistry Chemical Physics (20, 2018, 15725), is the first theoretical underpinning of the effect of confinement on reactivity, even in the absence of specific binding sites. These findings are very general and they can have a great impact on the investigation of several classes of reactions.

3) The extension of the multiscale modeling methodology to fluidized reacting systems, published in Reaction Chemistry and Engineering (3, 2018, 527), allows for gaining insights into the interplay between chemistry and fluid dynamics, thus making it possible to reach an unprecedented understanding of the fluidized systems with a direct impact also on nonconventional applications such as chemical vapor deposition and nanoparticle dynamics.

The successful accomplishment of all the objectives of SHAPE will make it possible to use the structure of the catalyst as an \"\"engineering\"\" parameter to tune activity and selectivity. In particular, it is foreseen that integration of the theoretical part with the experimental investigation (currently ongoing) will make it possible to reach a detailed understanding of the relation between structure and activity in heterogeneous catalysis in terms of identity and nature of the active site at given operating conditions in the reactor. This will play a crucial role in the development of novel and sustainable processes for energy and chemicals production.\"