Stability issues in Pd-based catalysts: the role of surface Pt in improving the stability and oxygen reduction reaction (ORR) activity

Abstract

Carbon-supported Pd and Pd₃Co catalysts have been electrochemically characterized in 0.1 M HClO₄ solution and we found that both catalysts were unstable. On repeated potential cycling, the electrochemical surface area of the catalysts decreases and the oxygen reduction reaction (ORR) activity suffers. To stabilize surface Pd atoms of both Pd and Pd₃Co catalysts, we deposited Pt using adsorbed hydrogen on the catalytically active Pd sites. The Pt : Pd ratio of Pt-coated Pd and Pt-coated Pd₃Co catalysts suggests half-a-monolayer coverage of Pt (two hydrogen atoms required for reducing a Pt²⁺ ion). The Pt : Pd ratio of Pt-coated Pd₃Co catalyst obtained from the simple geometrical hard sphere model, energy-dispersive X-ray spectroscopy (EDS) line scan and bulk EDS agrees very well with that calculated from the hydrogen desorption (H_des) charge of Pd₃Co. At the same time, the Pt : Pd ratio of Pt-coated Pd calculated from the H_des charge of Pd catalyst is significantly lower than the ratio obtained from the other methods. Thus, the Pt : Pd ratio of the Pt-coated Pd catalyst estimated from the H_des region of Pd is an underestimation of the composition. This suggests that Pd forms an electrochemically inactive species from the H_upd region itself and Co in Pd₃Co seems to stabilize Pd against oxidation by delaying the formation of electrochemically inactive species to higher potentials above the H_upd region. The voltammograms along with the peroxide formation characteristics of the catalysts support the above observations. The deposited Pt on the surface of the Pd and Pd₃Co catalysts masks active Pd sites from the electrochemical environment and even partial coverage with Pt improves the stability and ORR activity of the catalysts when compared to that of the respective Pt-free counterparts.