Composition–Structure–Activity Relationships for Palladium-Alloyed Nanocatalysts in Oxygen Reduction Reaction: An Ex-Situ/In-Situ High Energy X‑ray Diffraction Study

Understanding how the composition and atomic-scale structure of a nanocatalyst changes when it is operated under realistic oxygen reduction reaction (ORR) conditions is essential for enabling the design and preparation of active and robust catalysts in proton exchange membrane fuel cells (PEMFCs). This report describes a study of palladium-alloyed electrocatalysts (PdNi) with different bimetallic compositions, aiming at establishing the relationship between catalyst’s composition, atomic structure, and activity for ORR taking place at the cathode of an operating PEMFC. Ex-situ and in-situ synchrotron high-energy X-ray diffraction (HE-XRD) coupled to atomic pair distribution function (PDF) analysis are employed to probe the structural evolution of the catalysts under PEMFC operation conditions. The study reveals an intriguing composition–activity synergy manifested by its strong dependence on the fuel cell operation induced leaching process of base metals from the catalysts. In particular, the synergy sustains during electrochemical potential cycling in the ORR operation potential window. The alloy with Pd:Ni ratio of 50:50 atomic ratio is shown to exhibit the highest possible surface Pd–Pd and Pd–Ni coordination numbers, near which an activity is observed. The analysis of the Ni-leaching process in terms of atomic-scale structure evolution sheds further light on the activity–composition–structure correlation. The results not only show a sustainable alloy characteristic upon leaching of Ni consistent with catalytic synergy but also reveal a persistent fluctuation pattern of interatomic distances along with an atomic-level reconstruction under the ORR and fuel cell operation conditions. The understanding of this type of interatomic distance fluctuation in the catalysts in correlation with the base metal leaching and realloying mechanisms under the electrocatalytic operation conditions may have important implications in the design and preparation of catalysts with controlled activity and stability

Composition–Structure–Activity Correlation of Platinum–Ruthenium Nanoalloy Catalysts for Ethanol Oxidation Reaction

Understanding the evolution of the composition and atomic structure of nanoalloy catalysts in the ethanol oxidation reaction (EOR) is essential for the design of active and robust catalysts for direct ethanol fuel cells. This article describes a study of carbon-supported platinum–ruthenium electrocatalysts (PtRu/C) with different bimetallic compositions and their activities in the EOR, an important anode reaction in direct ethanol fuel cells (DEFCs). The study focused on establishing the relationship between the catalyst’s composition, atomic structure, and catalytic activity for the EOR. Ex situ and in situ synchrotron high-energy X-ray diffraction (HE-XRD) experiments coupled with atomic pair distribution function (PDF) analysis and in situ energy-dispersive X-ray (EDX) analysis were employed to probe the composition and structural evolution of the catalysts during the in situ EOR inside a membrane electrode assembly (MEA) in the fuel cell. The results revealed an intriguing composition–structure–activity relationship for the PtRu electrocatalysts under EOR experimental conditions. In particular, the alloy with a Pt/Ru ratio of ∼50:50 was found to exhibit a maximum EOR activity as a function of the bimetallic composition. This composition–activity relationship coincides with the relationship between the Pt interatomic distances and coordination numbers and the bimetallic composition. Notably, the catalytic activities of the PtRu electrocatalysts showed a significant improvement during the EOR, which can be related to atomic-level structural changes in the nanoalloys occurring during the EOR, as indicated by in situ HE-XRD/PDF/EDX data. The findings shed some new light on the mechanism of the ethanol oxidation reaction over bimetallic alloy nanocatalysts, which is important for the rational design and synthesis of active nanoalloy catalysts for DEFCs

Understanding Composition-Dependent Synergy of PtPd Alloy Nanoparticles in Electrocatalytic Oxygen Reduction Reaction

Gaining an insight into the relationship between the bimetallic composition and catalytic activity is essential for the design of nanoalloy catalysts for oxygen reduction reaction. This report describes findings of a study of the composition–activity relationship for PtPd nanoalloy catalysts in oxygen reduction reaction (ORR). Pt_<i>n</i>Pd_{100‑<i>n</i>} nanoalloys with different bimetallic compositions are synthesized by wet chemical method. While the size of the Pt₅₀Pd₅₀ nanoparticles is the largest among the nanoparticles with different compositions, the characterization of the nanoalloys using synchrotron high-energy X-ray diffraction (HE-XRD) coupled to atomic pair distribution function (PDF) analysis reveals that the nanoalloy with an atomic Pt:Pd ratio of 50:50 exhibits an intermediate lattice parameter. Electrochemical characterization of the nanoalloys shows a minimum ORR activity at Pt:Pd ratio close to 50:50, whereas a maximum activity is achieved at Pt:Pd ratio close to 10:90. The composition–activity correlation is assessed by theoretical modeling based on DFT calculation of nanoalloy clusters. In addition to showing an electron transfer from PtPd alloy to oxygen upon its adsorption on the nanoalloy, a relatively large energy difference between HOMO for nanoalloy and LUMO for oxygen is revealed for the nanoalloy with an atomic Pt:Pd ratio of 50:50. By analysis of the adsorption of OH species on PtPd (111) surfaces of different compositions, the strongest adsorption energy is observed for Pt₉₆Pd₁₀₅ (Pt:Pd ≈ 50:50) cluster, which is believed to be likely responsible for the reduced activity. Interestingly, the adsorption energy on Pt₂₄Pd₁₇₇ (Pt:Pd ≈ 10:90) cluster falls in between Pt₉₆Pd₁₀₅ and Pd₂₀₁ clusters, which is believed to be linked to the observation of the highest catalytic activity for the nanoalloy with an atomic Pt:Pd ratio of 10:90. These findings have implications for the design of composition-tunable nanoalloy catalysts for ORR