New Strategy toward a Dual Functional Nanocatalyst at Ambient Conditions: Influence of the Pd-Co Interface in the Catalytic Activity of Pd@Co Core-Shell Nanoparticles.

Abstract

Bimetallic nanostructures with a combination of noble and nonnoble metals hold promise for improving catalyst activity and selectivity. Here, we report the synthesis of Pd@Co (PC) core-shell morphology nanoparticles with three different ratios of palladium (Pd) and cobalt (Co), and a possibility to fine tune the ratio of core and shell thickness. PC exhibits superior and selective hydrogenation as well as oxidation catalytic activity at ambient or near-ambient conditions. Various characterization techniques have been employed to confirm the core-shell morphology. Without any pre-treatment or activation, fresh catalysts with different Pd to Co ratios, that is, 2:1, 1:1, and 1:2, were subjected to olefin (phenylacetylene) hydrogenation and oxidation (styrene to styrene oxide) reaction. The catalytic activity results demonstrate that the 1:1 ratio of Pd/Co is the most active composition for controlled and stepwise reduction of phenyl acetylene to styrene and then to ethyl benzene; 1:1 Pd/Co shows 100% styrene conversion in 30 min. with an order of magnitude higher turnover frequency than other catalysts. The 1:1 PC ratio is also the most active composition for selective oxidation of styrene to styrene oxide. NAPXPS (near-ambient pressure XPS) results show that the active sites for catalytic C═C hydrogenation and oxidation reaction are Co0 and Co3+, respectively. However, the superior catalytic performance can be attributed to Co0 (for reduction) or Co3+ (for oxidation), and the Pd-Co interface plays a critical role in stabilizing the required functional character. NAPXPS results confirm that the superior catalytic performance can be attributed not only to Co0 or Co3+, but also to the Pd-Co interface. The electronic effect and synergism between Co and Pd helps Co to stabilize in different oxidation states depending on the reaction conditions, and making it a dual functional catalyst.