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.

A Reusable Cobalt Catalyst for Reversible Acceptorless Dehydrogenation and Hydrogenation of N‐Heterocycles

“…Thus, we believe that all elementary reactions like, hydrogen adsorption, hydrogen dissociation, N-heterocycle adsorption, subsequent reaction, desorption of products, should occur in tandem for such complex reactions. [33] Thus, mixed interphase in Co-Phen@C catalyst helps in the present catalysis.…”

Electronic Structure Evolution of Pd@Co Nanocatalysts Under Oxidation and Reduction Conditions and Preferential CO Oxidation

“…However, as shown in Figure SI-1, there were no significant changes were observed in Pd core level spectra for all PC catalyst and it remains in metallic state. [16,21]…”

“…The PdÀ Co core-shell nanoparticles (PC NPs) with Pd/Co ratio were synthesised by a single step process, as described in ref 16. As prepared PC NPs were preserved in hexane after sonication for 5 min using an ultrasonic bath to prevent the surface oxidation of NPs.…”

“…Palladium chloride and Cobalt acetylacetonate used as precursor for Co and Pd. In the typical synthesis method, required molar ratios of cobalt and palladium precursors (0.2 mmol of Pd and Co precursors for PC11) [16] employed along with PVP were dispersed in ethylene glycol and stirred at room temperature for 15 min followed by heating at 393 K for 30 min. At 393 K the reduction of palladium precursor to Pd occurs which acts as seed and surrounded by cobalt ions.…”

“…Towards this direction, research on bimetallic nanoparticles reveals that the incorporation of noble metals in to the 3d-TM can enhance resistance towards oxidation, however these catalyst systems are less stable and shows catalyst restructuring under reaction conditions. [16][17][18] This fact restrict the application of such bimetallic systems only for hydrogenation reactions, since reduced state of 3d-TM NPs is not suitable for oxidation reactions. [13][14][15]20] Hence it is highly desirable to modify the bimetallic system in such a way that it can easily switch its oxidation state with the change in reaction condition; it should be easily reducible in the presence of reductive environment, and sufficiently oxidise in presence of oxidising environment, while retaining its less pyrophoric nature and catalyst structure.…”