(784g) Beneficial Influence of Surface Capping Agents on the Stability and Activity of Metal/Oxide Catalysts

Capping agents have long been used as chemical additives to control the size, morphology, and composition of nanoparticle catalysts during their synthesis. Strong interactions between surface atoms and capping agents can stabilize structures with greater abundance of particular facets and surface sites, which drastically improve reaction rates or selectivity. These catalysts often require some mild treatment or conditioning (extensive rinsing with solvent, UV-Ozone, or low temperature calcination) to free more active sites by removing surface capping agents from the surface without inducing dramatic surface reconstructions.

Alternatively, capping agents can also improve catalytic activity and selectivity by reducing surface coverage of reactive intermediates and preventing undesirable bimolecular reactions, which can lead to coking. Previously, our group has deposited different capping agents (thiols, silanes, phosphonates) to control the orientation of chemical species on active phase of different metal and metal oxide catalysts to drastically enhance the selectivity compared to uncoated catalysts. All these beneficial effects are contingent on the stability of capping agents, which can degrade over time.

In this work, we discuss recent results that have shown the deposition of capping agents on the inactive phase can also have a beneficial impact on the stability of metal nanoparticles deposited on oxide supports. Improved stability results from the modification of physical properties of the catalyst following the addition of different tail groups on capping agents. Coated Pd/Al₂O₃ catalysts were showed to be more stable in highly acidic solutions, under hydrothermal conditions, and at elevated temperatures under reaction conditions.