Non-Mean Field Treatments of Surface Chemistry: Incorporating Adsorbate-Adsorbate Interactions into Deterministic Kinetic Theories

Jeffrey Greeley of Purdue University is supported by an award from the Chemical Catalysis program in the Division of Chemistry to study heterogeneous catalysts with new theoretical methods. Heterogeneous catalysts accelerate the conversion of chemical species to valuable products, and they underpin a host of technologically important chemical processes, ranging from production of plastics to generation of electricity in fuel cells.

These catalysts are often composed of metal nanoparticles, the surfaces of which break and form chemical bonds in molecules to produce valuable products. Fundamental models of how these molecular transformations occur have been developed for many chemical reactions and have greatly enhanced both the understanding of these transformations and the design of new catalytic materials.

However, these models often make significant approximations in their descriptions of the relevant molecular processes, including the assumption that reacting molecules and products are randomly distributed across the catalyst surfaces and do not interact with one another. The development of improved theoretical descriptions that correct these approximations and explicitly describe how the reactions change when molecules interact strongly with one another, is, in turn, the central focus of this project.

The improved models will lead to more accurate descriptions of heterogeneous catalytic processes and may ultimately facilitate the design of improved catalysts. For broader impacts, the new capability and software will be made available to the catalysis community. Outreach and education efforts will focus on high school students at a very early stage in their academic path, to encourage the pursuit of STEM studies.

PI will make a focused effort to engage students from diverse and under-represented groups. This activity will be a continuation of a successful effort by the PI, whereby the students are paired with the PI and graduate students to pursue tailored research projects.

Mean field theories of surface kinetics permit crucial information about heterogeneous catalytic processes, including the overall reaction rates, effective activation barriers, and reaction orders, to be predicted from a combined knowledge of the elementary reaction steps occurring on a catalyst surface and the rate constants of these elementary steps. In spite of the power of these theories, however, the mean field approximation begins to break down under conditions of strong interactions between surface adsorbates, and there is a need to develop surface kinetic modeling techniques that directly incorporate non-mean field effects while, at the same time, preserving the powerful physical and mechanistic insights that emerge naturally from deterministic mean field descriptions.

In this work, analytical or semi-analytical corrections to mean field expressions for equilibrium constants and rate constants of elementary reactions will be developed on square planar and hexagonal catalyst surfaces. These expressions, which will be based on the Bethe-Peierls approximation to the Cluster Variation Method, will be used to identify mathematical and conceptual principles that describe when non-mean field effects will significantly impact surface catalysis.

The resulting formalisms, which will deterministically treat non-mean field effects while preserving the powerful mechanistic insights that can be obtained from mean field microkinetic analyses, will be benchmarked on two classic surface reaction chemistries, the direct and H2-assisted decomposition of NO and the synthesis or decomposition of ammonia from N2 and H2.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.