Escaping the trap of complication and complexity in multiscale microkinetic modelling of heterogeneous catalytic processes

In this feature article, the development of methods to enable a hierarchical multiscale approach to the microkinetic analysis of heterogeneous catalytic processes is reviewed. This methodology is an effective route to escape the trap of complication and complexity in multiscale microkinetic modelling. On the one hand, the complication of the problem is related to the fact that the observed catalyst functionality is inherently a multiscale property of the reacting system and its analysis requires bridging the phenomena at different time and length scales. On the other hand, the complexity of the problem derives from the system dimension of the chemical systems, which typically results in a number of elementary steps and species, that are beyond the limit of accessibility of present-day computational power even for the most efficient implementation of atomistic first-principles simulations. The main idea behind the hierarchical approach is to tackle the problem with methods of increasing accuracy in a dual feed-back loop between theory and experiments. The potential of the methodology is shown in the context of unravelling the WGS and r-WGS catalytic mechanisms on Rh catalysts. As a perspective, the extension to structure-dependent microkinetic modelling is discussed

Molecular-level understanding of the WGS and reverse WGS reactions on Rh through hierarchical multiscale approach

Hierarchically combining semi-empirical methods and first-principles calculations we gain a novel and noteworthy picture of the molecular-level mechanisms that govern the water-gas-shift (WGS) and reverse water-gas-shift (r-WGS) reactions on Rh catalysts. Central to this picture is that the WGS and r-WGS follow two different dominant reaction mechanisms: WGS proceeds according to a carboxyl (COOH) mechanism, whereas r-WGS proceeds according to a redox (CO2 {\to} CO + O) mechanism. The obtained results furthermore underscore the danger of common first-principles analyses that focus on a priori selected dominant paths. Not restricted to such bias, our herein proposed hierarchical approach thus constitutes a promising avenue to properly transport and incorporate the ab initio predictive-quality to a new level of system complexity.Comment: 11 page

Prediction of morphological changes of catalyst materials under reaction conditions by combined: Ab initio thermodynamics and microkinetic modelling

In this article, we couple microkinetic modelling, ab initio thermodynamics and Wulff-Kaishew construction to describe the structural variation of catalyst materials as a function of the chemical potential in the reactor. We focus specifically on experiments of catalytic partial oxidation (CPO) of methane on Rh/α-Al2O3. We employ a detailed structureless microkinetic model to calculate the profiles of the gaseous species molar fractions along the reactor coordinate and to select the most abundant reaction intermediates (MARIs) populating the catalyst surfaces in different zones of the reactor. Then, we calculate the most stable bulk and surface structures of the catalyst under different conditions of the reaction environment with density functional theory (DFT) calculations and ab initio thermodynamics, considering the presence of the MARIs on the catalyst surface in thermodynamic equilibrium with the partial pressures of their reservoirs in the gas phase surrounding the catalyst. Finally, we exploit the Wulff-Kaishew construction method to estimate the three-dimensional shape of the catalyst nanoparticles and the distribution of the active sites along the reactor coordinate. We find that the catalyst drastically modifies its morphology during CPO reaction by undergoing phase transition, in agreement with spectroscopy studies reported in the literature. The framework is also successfully applied for the analysis and interpretation of chemisorption experiments for catalyst characterization. These results demonstrate the crucial importance of rigorously accounting for the structural effect in microkinetic modeling simulations and pave the way towards the development of structure-dependent microkinetic analysis of catalytic processes

Potenzialita di una linea ferroviaria: confronto tra blocco fisso e blocco mobile

Nella prima parte dell'elaborato vengono introdotti dei concetti basilari del sistema ferroviario necessari al proseguo della trattazione: i sistemi di circolazione utilizzati in Italia, le sezioni di blocco, i sistemi di sicurezza e di controllo della marcia, il sistema ERTMS a livello europeo. Nella seconda parte, dopo aver introdotto il concetto di capacità, vengono analizzati nel dettaglio e confrontati, in termini di capacità teorica, i sistemi basati sul blocco fisso e sul blocco mobile. Vengono quindi proposti i metodi utilizzati per il calcolo della capacità reale, prestando particolare attenzione al metodo dei coefficienti di ritardo specifico D e di stabilità X. Da quest' ultimo e con l'introduzione dei livelli di servizio, viene analizzato il rapporto che lega la capacità con la qualità della circolazione. Infine viene proposto un confronto tra le linee convenzionali e le linee AV/AC in Italia, evidenziando il rapporto tra le caratteristiche di velocità e di capacità

A fundamental investigation of gas/solid mass transfer in open-cell foams using a combined experimental and CFD approach

In this work, we combine numerical (CFD) simulations and experimental measurements in a fundamental investigation of the fluid-solid mass transfer properties of open-cell foams, which are promising support for catalytic applications limited by external heat and mass transfer. CFD simulations are exploited to gain insight into the complex transport mechanisms and to enable a parametric analysis of the geometrical features by means of virtually-generated structures. Catalytic activity experiments under diffusion control are used to validate the CFD results and to extend the range of conditions and foam morphologies investigated. Analysis of the flow field by CFD simulations provides a rational basis for the choice of the average strut size as a physically sound characteristic length for mass transfer correlations. Results from both numerical simulations and experimental tests are interpreted according to a fully-theoretically based geometrical model for the prediction of the specific surface area, which accounts for the detailed node-strut geometry. The effects of cell size and strut shape are properly included in the functional dependence of the Sherwood number on the Reynolds number. The effect of porosity requires one additional dependence, wherein the Sherwood number is inversely proportional to the square of the void fraction. The resulting Sherwood–Reynolds correlation is in excellent agreement with experimental data and CFD simulations. It enables accurate (±15%) estimation of the external mass transfer coefficients for open-cell foams when coupled with the proposed geometrical model from two readily accessible pieces of geometrical information, i.e. the void fraction and either the cell size or the pore diameter of the foam. The derived correlation can be applied to the design of novel enhanced open-cell foam catalyst substrates and structured reactors

A systematic procedure for the virtual reconstruction of open-cell foams

Open-cell foams are considered a potential candidate as an innovative catalyst support in many processes of the chemical industry. In this respect, a deeper understanding of the transport phenomena in such structures can promote their extensive application. In this contribution, we propose a general procedure to recover a representative open-cell structure starting from some easily obtained information. In particular, we adopt a realistic description of the foam geometry by considering clusters of solid material at nodes and different strut-cross sectional shapes depending on the void fraction. The methodology avoids time-consuming and expensive measuring techniques, such as micro-computed tomography (μCT) or magnetic resonance imaging (MRI). Computational Fluid Dynamics (CFD) could be a powerful instrument to enable accurate analyses of the complex flow field and of the gas-to-solid heat and mass transport. The reconstructed geometry can be easily exploited to generate a suitable computational domain allowing for the detailed investigation of the transport properties on a realistic foam structure by means of CFD simulations. Moreover, the proposed methodology easily allows for parametric sensitivity analysis of the foam performances, thus being an instrument for the advanced design of these structures. The geometrical properties of the reconstructed foams are in good agreement with experimental measurements. The flow field established in complex tridimensional geometries reproduces the real foam behavior as proved by the comparison between numerical simulations and experiments

First‐principles Assessment of the Role of Water in the Reduction Half Cycle of Low‐Temperature NH3‐SCR over Cu‐CHA

Dispersion corrected density functional theory calculations show that the presence of H2O in the Reduction Half-Cycle (RHC) of NH3-SCR affects the free energy of the kinetically-relevant transition state (TS) leading to a reduction in the rate and activation energy with respect to dry conditions. In particular, H2O enthalpically stabilizes the kinetically-relevant TS by 20 kJ mol(-1) with respect to the dry counterpart. Such enthalpic stabilization vanishes when van der Waals (vdW) interactions are excluded from the calculations, thus showing the preeminent role of non-specific dispersion forces in the reduction of the activation enthalpy. At the same time, the enthalpic stabilization is more than compensated by the additional entropy losses of the TS brought forth by the presence of H2O in the CHA cage. Calculated enthalpy and entropy changes with respect to the dry case agree quantitatively with the experimental measurements and reflect the modified reacting environment in the presence of H2O. As a result, this study provides theoretical underpinnings on the mechanistic role of H2O in the RHC and, on a more general basis, highlights the importance of the molecular scale description of the reaction environment in voids of molecular dimensions

First-principles theoretical assessment of catalysis by confinement: NO-O2 reactions within voids of molecular dimensions in siliceous crystalline frameworks

Density functional theory methods that include dispersive forces are used to show how voids of molecular dimensions enhance reaction rates by the mere confinement of transition states analogous to those involved in homogeneous routes and without requiring specific binding sites or structural defects within confining voids. These van der Waals interactions account for the observed large rate enhancements for NO oxidation in the presence of purely siliceous crystalline frameworks. The minimum free energy paths for NO oxidation within chabazite (CHA) and silicalite (SIL) frameworks involve intermediates similar in stoichiometry, geometry, and kinetic relevance to those involved in the homogeneous route. The termolecular transition state for the kinetically-relevant cis-NOO2NO isomerization to trans-NOO2NO is strongly stabilized by confinement within CHA (by 36.3 kJ mol-1in enthalpy) and SIL (by 39.2 kJ mol-1); such enthalpic stabilization is compensated, in part, by concomitant entropy losses brought forth by confinement (CHA: 44.9; SIL: 45.3, J mol-1K-1at 298 K). These enthalpy and entropy changes upon confinement agree well with those measured and combine to significantly decrease activation free energies and are consistent with the rate enhancements that become larger as temperature decreases because of the more negative apparent activation energies in confined systems compared with homogeneous routes. Calculated free energies of confinement are in quantitative agreement with measured rate enhancements and with their temperature sensitivity. Such quantitative agreements reflect preeminent effects of geometry in determining the van der Waals contributions from contacts between the transition states (TS) and the confining walls and the weak effects of the level of theory on TS geometries. NO oxidation reactions are chosen here to illustrate these remarkable effects of confinement because detailed kinetic analysis of rate data are available, but also because of their critical role in the treatment of combustion effluents and in the synthesis of nitric acid and nitrates. Similar effects are evident from rate enhancements by confinement observed for Diels-Alder and alkyne oligomerization reactions. These reactions also occur in gaseous media at near ambient temperatures, for which enthalpic stabilization upon confinement of their homogeneous transition states becomes the preeminent component of activation free energies

Biophysical parameters control signal transfer in spiking network

IntroductionInformation transmission and representation in both natural and artificial networks is dependent on connectivity between units. Biological neurons, in addition, modulate synaptic dynamics and post-synaptic membrane properties, but how these relate to information transmission in a population of neurons is still poorly understood. A recent study investigated local learning rules and showed how a spiking neural network can learn to represent continuous signals. Our study builds on their model to explore how basic membrane properties and synaptic delays affect information transfer.MethodsThe system consisted of three input and output units and a hidden layer of 300 excitatory and 75 inhibitory leaky integrate-and-fire (LIF) or adaptive integrate-and-fire (AdEx) units. After optimizing the connectivity to accurately replicate the input patterns in the output units, we transformed the model to more biologically accurate units and included synaptic delay and concurrent action potential generation in distinct neurons. We examined three different parameter regimes which comprised either identical physiological values for both excitatory and inhibitory units (Comrade), more biologically accurate values (Bacon), or the Comrade regime whose output units were optimized for low reconstruction error (HiFi). We evaluated information transmission and classification accuracy of the network with four distinct metrics: coherence, Granger causality, transfer entropy, and reconstruction error.ResultsBiophysical parameters showed a major impact on information transfer metrics. The classification was surprisingly robust, surviving very low firing and information rates, whereas information transmission overall and particularly low reconstruction error were more dependent on higher firing rates in LIF units. In AdEx units, the firing rates were lower and less information was transferred, but interestingly the highest information transmission rates were no longer overlapping with the highest firing rates.DiscussionOur findings can be reflected on the predictive coding theory of the cerebral cortex and may suggest information transfer qualities as a phenomenological quality of biological cells