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Structure-dependent microkinetic modelling of heterogeneous catalytic processes

Periodic Reporting for period 3 - SHAPE (Structure-dependent microkinetic modelling of heterogeneous catalytic processes)

Reporting period: 2019-05-01 to 2020-10-31

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.
"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 Brønsted–Evans–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 – 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 “catalysis by confinement”. 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, the confinement effects are clearly shown to reflect vdW forces.

We also developed and tested methods for the prediction of shape and size of the nanoparticles in reaction conditions and their coupling with reactor simulations. The methodology has been tested and validated in the context of catalytic partial oxidation of methane on Rh and allowed for a comprehensive interpretation of the CO and H2 chemisorption experiments for the calculation of the catalyst dispersion. On the basis of these results, we also extended our investigation to a detailed study of the reactivity of the active site, thus paving the way towards the full understanding of the identity and of the nature of the active site in reaction conditions.

We have also developed a methodology for the coupling of microkinetic models and CFD-DEM (Euler-Lagrange approach) simulations. Our methodology allows for gaining insights into the interplay between chemistry and fluid dynamics with a direct impact also on non-conventional applications such as chemical vapor deposition and nanoparticle dynamics. We are currently extended the methodology to Euler-Euler approaches and to the implementation of machine learning techniques to reduce the computational cost of the simulations.

For what concerns the experimental activities, we are extending a previously developed operando-Raman-annular reactor to the inclusion of UV-Vis spectroscopy in order to be able to monitor in detail the structural changes of catalyst material in reaction conditions, which is a very important step for the integration and comparison with the theoretical analysis developed in parallel. We also planned in detail the design of the experiments for the kinetic investigation and the relation with the structural properties of the catalyst material. Preliminary experiments (CO oxidation, CH4 CO2 reforming, CPO partial oxidation on Rh) were performed to test and assist the optimization of the rig and of the experimental campaign. These tests were performed on in-house prepared catalysts (Rh based) in relation to the on-going design of the experimental rig and of the experimental campaign.

As a whole, the project so far has resulted in 5 (plus one in press and three currently in preparation) scientific papers in international journals (including 2 papers classified as ""hot article"" by the Editor and 1 inside front cover of the journal issue), about 20 contributed talks in international conferences, invited talks at international conferences, workshops, and research centers."
"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."
Aim of the ERC-SHAPE project