Theory and practice in catalyst design: Tailoring binding centers and their surroundings

This lecture develops, through a combination of theory and experiments, a methodology to address the rate and selectivity of chemical transformations in surface catalysis based of thermodynamic formalisms that underpin the concept of transition states as intermediates. The approach considers the properties of molecular species involved as reactive intermediates in catalytic sequences and of active centers that bind them and how they act in concert to select reaction channels, often against those favored by thermodynamics. When applied to acid-base and oxidation catalysis at oxide surfaces, this methodology has uncovered unprecedented details about the types of active centers involved and the elementary steps that they mediate. For instance, the energy required to deprotonate a solid acid and that gained by placing the proton on a reactant molecule determine reactivity and selectivity for solid acid catalysts, because transformations involve the transfer of protons and cationic moieties at transition states.

In contrast, oxidation catalysis on redox-active oxides occurs via H-abstraction from C-H bonds in reactants and the concomitant reduction of the metal centers in oxide catalysts. These steps are mediated by bound di-radical pairs with O-H and C-H bonds that are nearly formed and cleaved, respectively, thus making the energies of H-binding at surfaces and of C-H bond cleavage the relevant surface and molecular descriptors of reactivity. The environments that surround the binding centers complement their properties through solvation effects that are able to stabilize specific bound intermediates and transition states through concerted van der Waals or H-bonding interactions. Such stabilization becomes particularly evident (and consequential) when active centers reside within inorganic voids of molecular dimensions or are able to contact dense phases, such as liquids or bound adlayers. These emerging concepts and tools are bringing us closer to the purposeful design of surfaces and environments for specific chemical transformations.

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