(40f) Confinement-Induced Reactivity

Julibeth M. Martinez de la Hoz and Perla B. Balbuena

Department of Chemical Engineering and Materials Science and Engineering Program

Texas A&M University, College Station, TX, 77843

Important to the field of heterogeneous catalysis is the analysis of the ability of a given metal to adsorb different reactants, as well as the estimation of activation energies for the formation of intermediates and final structures on the surface. Also, it has been found that solid metal surfaces separated by small distances (3 – 10 Å) can induce new interesting phenomena, related to changes in the chemical, electronic and magnetic properties of the metallic material. Therefore, understanding how the spatial confinement of adsorbates between metallic surfaces influences their adsorption energies and energy barriers for dissociation, offers the possibility of using the degree and/or nature of confinement to facilitate specific reactions.

In this work, we use DFT methods with the aim to elucidate whether a specific reaction is facilitated by spatial confinement of the adsorbate among metal surfaces separated by a few Angstroms. We analyze two systems: the well known O₂ dissociation on Pt (111) and the the dissociation of NO₂ on Fe (111). We found that molecular oxygen chemisorbed among Pt (111) surfaces needs a significantly smaller activation energy to be dissociated at surface – surface separations of 4.7 Å, compared to that needed on a single Pt(111) surface. The difference between these activation energies seems to be related to a larger charge transfer from the substrate to the confined O₂ molecule, which induces larger changes in the electronic structure of the molecule and weakens the O – O bond strength. Similar qualitative results are found for the dissociation of NO₂ on Fe (111); however, there are quantitative differences between the charges transferred to the adsorbate as a function of the surface-surface separation, and no significant variation of the activation energies in confinement with respect to the single surface. Consequently, it is shown that the degree of facilitation provided by the confinement, for a given reaction also depends strongly on the nature of the surface.