Stabilization of Catalytic Surfaces Using Bimetallic Core-Shell Structures with Different Surface Free Energies (SFE)

Catalyst stability is often as important as catalyst selectivity. The loss of active sites and catalyst deactivation often make a catalytic system inadequate for chemical reactions. Therefore, techniques to enhance the stability of catalytic surfaces are important in catalyst development. A possible way to stabilize these nanoparticles is through differences in surface free energies (SFE) of core and shell metals. By placing a lower surface free energy shell on top of a higher surface free energy core, a system is favored where the core metal anchors the shell metal to lower the overall surface free energy of the bimetallic system.

The Ir metallic cores synthesized by Strong Electrostatic Adsorption (SEA) on γ-alumina were highly dispersed (< 2 nm diameter) as determined by both XRD and chemisorption. After thermal treatments to 800°C, monometallic Ir catalysts sintered to an average XRD particle size of 19 nm. A monometallic Ag catalyst also treated at the same temperatures sintered to an average XRD particle size of 33 nm.

The addition of a Ag shell to the Ir cores causes a suppression in amount of sintering observed for both Ag and Ir components compared to the monometallic catalysts. Moreover, H2 chemisorption results for the bimetallic catalysts treated at 800°C indicated a higher Ir dispersion, thus confirming the enhanced stability of the catalytic surface. This enhanced stability of the Ag-Ir surface and cause(s) for it were investigated using high resolution XPS measurements.

Bimetallic Ag shell-Ir core catalysts maintained higher dispersions compared to monometallic Ir and Ag catalysts after thermal treatments up to 800°C, indicating the capability to stabilize catalytic surfaces through core-shell structures where the SFE of the shell component is lower than the SFE of the core metal.