Control of Molecular Bonding Strength on Metal Catalysts with Organic Monolayers for CO Reduction.

The development of separate levers for controlling the bonding strength of different reactive species on catalyst surfaces is challenging but essential for the design of highly active and selective catalysts. For example, during CO reduction, production of CO often requires balancing a trade-off between the adsorption strength of the reactant and product states: weak binding of CO is desirable from a selectivity perspective, but weak binding of CO leads to low activity. Here, we demonstrate a new method of controlling both CO adsorption and CO desorption over supported metal catalysts by employing a single self-assembly step where organic monolayer films were deposited on the catalyst support. Binding of phosphonic acid monolayers on supported Pt and Pd catalysts weakened CO binding via a through-support effect. The weakened CO adsorption was generally accompanied by decreased adsorption and reactivity of CO. However, by the incorporation of basic amine functions at controlled positions in the modifying film, strong CO adsorption and hydrogenation reactivity could be restored. Thus, both through-surface and through-space interactions could be manipulated by design of the organic modifiers. After surface modification, the catalysts exhibited significantly improved selectivity (up to ∼99% at conversions near 50%) and activity toward CO production. Moreover, the rate of deactivation was notably reduced due to prevention of CO poisoning.