Multiscale modelling surface reactions

There have been many hybrid multiscale simulations published recently in other diverse areas. It appears that the first onion-type hybrid multiscale simulation that dynamically coupled a spatially distributed 2D KMC for a surface reaction with a deterministic, continuum ODE CSTR model for the fluid phase was presented in Vlachos et al. (1990). Extension to 2D KMC coupled with ID PDE flow model was described in Vlachos (1997) and for complex reaction networks studied using 2D KMC coupled with a CSTR ODEs model in Raimondeau and Vlachos (2002a, b, 2003). Other examples from catalytic applications include Tammaro et al. (1995), Kissel-Osterrieder et al. (1998), Qin et al. (1998), and Monine et al. (2004). For reviews, see Raimondeau and Vlachos (2002a) on surface-fluid interactions and chemical reactions, and Li et al. (2004) for chemical reactors. 

The mathematical descriptions in a multiscale model can be part of a single simulation paradigm (e.g., only continuum) or of a combination of different simulation paradigms (e.g., stochastic model describing a surface reaction coupled with a continuum description of reactant transport phenomena). In the latter, one speaks about multiparadigm models. Multiparadigm models can be classified in two classes direct or indirect. 

Very recently, Viswanathan et al. " developed a multiscale model for simulating linear sweep voltammetry of electrochemical solid-liquid interfaces of H2O on Pt(l 11) and on Pt3Ni(l 11) facets. In the model, DFT was used to parameterize the reaction kinetics KMC was used to capture the kinetic steps of the electrochemical oxidation, and conventional MC was used to equilibrate the surface between kinetic steps. The calculated cyclic voltammograms are in good agreement with experimental CV - CS d the experimental XPS results (Figure 8). 

In molecular models, a surface site is modeled using an analogous molecular reaction. This is the simplest approach, which requires the least amount of computational resources. The selection of a molecule that can more or less adequately reproduce the properties of the surface site under study determines the success or failure of the approach. This approach was used in Ref. [20] in the multiscale simulation of zirconium and hafnium oxide film growth. 

Existing quantum mechanics and molecular dynamics methods have not yet advanced to the stage where water can be reliably modeled by itself [148-151], much less when involved in electrochemical reactions at a surface [152]. An important requirement ofany general multiscale systems framework is that it must be able to enable the resolution of the unknowns in complex heterogeneous mechanisms. 

One of the ultimate goals in modeling heterogeneous catalytic reaction systems would be the development of a multiscale approach that could simulate the myriad of atomic scale transformations that occur on the catalyst surface as they unfold as a function of time, processing conditions and catalyst structure and composition. The simulation would establish all of the elementary physicochemical paths available at a specific instant... 

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