Modelling catalysis: Badly mixed reactions

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  • Published: Oct 8, 2015
  • Author: David Bradley
  • Channels: Chemometrics & Informatics
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Optimal model

Molecular modelling and theoretical simulations could be used to optimize catalytic reactions in which the reactants are known not to be well mixed, according to research from the US Department of Energy's Ames Laboratory.

Molecular modelling and theoretical simulations could be used to optimize catalytic reactions in which the reactants are known not to be well mixed, according to research from the US Department of Energy's Ames Laboratory.

It's not all in the mix. In fact, when it comes to catalytic chemistry, mixing is often a significant problem. Now, a team at Ames have taken a new approach to simulating catalytic reactions that are definitely not well mixed. Homogeneous catalysis in solution, for instance, usually assumes complete mixing of dissolved catalyst and reactants and standard theories of chemical kinetics describe how quickly and efficiently catalytic reactions occur under such circumstances. But, for heterogeneous reactions mixing is not possible. Catalytic reactions for fuel synthesis occur on the large surfaces of mesoporous particles or pollutants are removed in high-pressure systems via surface reactions. Conventional chemical kinetics in those situations needs to be refined.

Doesn't mix well

In two recent research papers, Jim Evans, Dajiang Liu and their research team - Andres Garcia, Jing Wang and Chi-Jen Wang, and David Ackerman - focused on modelling reactions in three different types of catalytic environment in which catalyst and reactants do not mix readily. For two of those classes, catalysis in the narrow pores of mesoporous particles and catalysis on metallic surfaces exposed to high pressure, it is the problem of reactant and product crowding that precludes mixing and so reduces efficiency. A third class of system is the low-pressure surface catalytic reaction in which interactions between the adsorbed reactants themselves cause these species to organize into ordered domains or "islands" rather to than mix randomly.

"For all of these scenarios, there are many 'extra' steps in the overall catalysis reaction process that must be built into chemical simulation models to reliably describe these systems that are not well stirred," explains Evans. "For example, for catalysis within crowded narrow pores, we simulate the entrance of the reactant into the pores, diffusion within the pore, conversion of the reactants to products within the pore, and diffusion out of the pore, rather than just assuming 'well mixed' conditions. All of these steps control the catalytic yield for these unique reaction processes. All of them must be built into the simulation models."

Model approach

The researchers modelled the transitions between strongly correlated and random steady states for the catalytic oxidation of carbon monoxide on surfaces at high-pressure. "We explore simple lattice-gas reaction models for CO-oxidation on 1D and 2D periodic arrays of surface adsorption sites with CO adsorption and desorption, dissociative O2 adsorption and recombinative desorption (at low rate), and CO + O reaction to form CO2," they explain. The outcome is a comprehensive simulation at the molecular level that is more realistic and more accurate than the conventional model used to describe what is occurring in a non-stirred catalytic systems. "The simulations can play the role of 'numerical experiments,' meaning if we do our work right, rather than having to do an experiment in a lab, our models can tell us what these types of catalytic reactions will do." The researchers have also now refined existing analytical theory on catalytic reaction rates so that they can be better applied to unmixed systems.

Related Links

Chem Rev 2015, 115, 5979-6050: "Kinetic Monte Carlo Simulation of Statistical Mechanical Models and Coarse-Grained Mesoscale Descriptions of Catalytic Reaction–Diffusion Processes: 1D Nanoporous and 2D Surface Systems"

J Chem Phys 2015, 142, 134703: "Transitions between strongly correlated and random steady-states for catalytic CO-oxidation on surfaces at high-pressure"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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