Nanoalloying and phase transformations during thermal treatment of physical mixtures of Pd and Cu nanoparticles

Nanoscale alloying and phase transformations in physical mixtures of Pd and Cu ultrafine nanoparticles are investigated in real time with in situ synchrotron-based x-ray diffraction complemented by ex situ high-resolution transmission electron microscopy. The combination of metal-support interaction and reactive/non-reactive environment was found to determine the thermal evolution and ultimate structure of this binary system. At 300 degrees C, the nanoparticles supported on silica and carbon black intermix to form a chemically ordered CsCl-type (B2) alloy phase. The B2 phase transforms into a disordered fcc alloy at higher temperature (\u3e 450 degrees C). The alloy nanoparticles supported on silica and carbon black are homogeneous in volume, but evidence was found of Pd surface enrichment. In sharp contrast, when supported on alumina, the two metals segregated at 300 degrees C to produce almost pure fcc Cu and Pd phases. Upon further annealing of the mixture on alumina above 600 degrees C, the two metals interdiffused, forming two distinct disordered alloys of compositions 30% and 90% Pd. The annealing atmosphere also plays a major role in the structural evolution of these bimetallic nanoparticles. The nanoparticles annealed in forming gas are larger than the nanoparticles annealing in helium due to reduction of the surface oxides that promotes coalescence and sintering

Nanoscale Alloying in Electrocatalysts

In electrochemical energy conversion and storage, existing catalysts often contain a high percentage of noble metals such as Pt and Pd. In order to develop low-cost electrocatalysts, one of the effective strategies involves alloying noble metals with other transition metals. This strategy promises not only significant reduction of noble metals but also the tunability for enhanced catalytic activity and stability in comparison with conventional catalysts. In this report, some of the recent approaches to developing alloy catalysts for electrocatalytic oxygen reduction reaction in fuel cells will be highlighted. Selected examples will be also discussed to highlight insights into the structural and electrocatalytic properties of nanoalloy catalysts, which have implications for the design of low-cost, active, and durable catalysts for electrochemical energy production and conversion reactions

PdCu Nanoalloy Electrocatalysts in Oxygen Reduction Reaction: Role of Composition and Phase State in Catalytic Synergy

The catalytic synergy of nanoalloy catalysts depends on the nanoscale size, composition, phase state, and surface properties. This report describes findings of an investigation of their roles in the enhancement of electrocatalytic activity of PdCu alloy nanoparticle catalysts for oxygen reduction reaction (ORR). Pd_<i>n</i>Cu_{100–<i>n</i>} nanoalloys with controlled composition and subtle differences in size and phase state were synthesized by two different wet chemical methods. Detailed electrochemical characterization was performed to determine the surface properties and the catalytic activities. The atomic-scale structures of these catalysts were also characterized by high-energy synchrotron X-ray diffraction coupled with atomic pair distribution function analysis. The electrocatalytic activity and stability were shown to depend on the size, composition, and phase structure. With Pd_<i>n</i>Cu_{100–<i>n</i>} catalysts from both methods, a maximum ORR activity was revealed at Pd/Cu ratio close to 50:50. Structurally, Pd₅₀Cu₅₀ nanoalloys feature a mixed phase consisting of chemically ordered (body-centered cubic type) and disordered (face-centered cubic type) domains. The phase-segregated structure is shown to change to a single phase upon electrochemical potential cycling in ORR condition. While the surface Cu dissolution occurred in PdCu catalysts from the two different synthesis methods, the PdCu with a single-phase character is found to exhibit a tendency of a much greater dissolution than that with the phase segregation. Analysis of the results, along theoretical modeling based on density functional theory calculation, has provided new insights for the correlation between the electrocatalytic activity and the catalyst structures

Nanoalloying and phase transformations during thermal treatment of physical mixtures of Pd and Cu nanoparticles

Nanoscale alloying and phase transformations in physical mixtures of Pd and Cu ultrafine nanoparticles are investigated in real time with in situ synchrotron-based x-ray diffraction complemented by ex situ high-resolution transmission electron microscopy. The combination of metal–support interaction and reactive/non-reactive environment was found to determine the thermal evolution and ultimate structure of this binary system. At 300 °C, the nanoparticles supported on silica and carbon black intermix to form a chemically ordered CsCl-type (B2) alloy phase. The B2 phase transforms into a disordered fcc alloy at higher temperature (> 450 °C). The alloy nanoparticles supported on silica and carbon black are homogeneous in volume, but evidence was found of Pd surface enrichment. In sharp contrast, when supported on alumina, the two metals segregated at 300 °C to produce almost pure fcc Cu and Pd phases. Upon further annealing of the mixture on alumina above 600 °C, the two metals interdiffused, forming two distinct disordered alloys of compositions 30% and 90% Pd. The annealing atmosphere also plays a major role in the structural evolution of these bimetallic nanoparticles. The nanoparticles annealed in forming gas are larger than the nanoparticles annealing in helium due to reduction of the surface oxides that promotes coalescence and sintering

Atomic-Structural Synergy for Catalytic CO Oxidation over Palladium-Nickel Nanoalloys

DOE-BES [DE-SC0006877]; DOE [AC02-06CH11357]; DOE's Office of Biological and Environmental ResearchAlloying palladium (Pd) with other transition metals at the nanoscale has become an important pathway for preparation of low-cost, highly active and stable catalysts. However, the lack of understanding of how the alloying phase state, chemical composition and atomic-scale structure of the alloys at the nanoscale influence their catalytic activity impedes the rational design of Pd-nanoalloy catalysts. This work addresses this challenge by a novel approach to investigating the catalytic oxidation of carbon monoxide (CO) over palladium nickel (PdNi) nanoalloys with well-defined bimetallic composition, which reveals a remarkable maximal catalytic activity at Pd:Ni ratio of similar to 50:50. Key to understanding the structural-catalytic synergy is the use of high-energy synchrotron X-ray diffraction coupled to atomic pair distribution function (HE-XRD/PDF) analysis to probe the atomic structure of PdNi nanoalloys under controlled thermochemical treatments and CO reaction conditions. Three-dimensional (3D) models of the atomic structure of the nanoalloy particles were generated by reverse Monte Carlo simulations (RMC) guided by the experimental HE-XRD/PDF data. Structural details of the PdNi nanoalloys were extracted from the respective 3D models and compared with the measured catalytic properties. The comparison revealed a strong correlation between the phase state, chemical composition and atomic-scale structure of PdNi nanoalloys and their catalytic activity for CO oxidation. This correlation is further substantiated by analyzing the first atomic neighbor distances and coordination numbers inside the nanoalloy particles and at their surfaces. These findings have provided new insights into the structural synergy of nanoalloy catalysts by controlling the phase state, composition and atomic structure, complementing findings of traditional density functional theory studies

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

Nanoalloy Printed and Pulse-Laser Sintered Flexible Sensor Devices with Enhanced Stability and Materials Compatibility

While conformal and wearable devices have become one of the most desired formats for printable electronics, it is challenging to establish a scalable process that produces stable conductive patterns but also uses substrates compatible with widely available wearable materials. Here, we describe findings of an investigation of a nanoalloy ink printed and pulsed-laser sintered conductive patterns as flexible functional devices with enhanced stability and materials compatibility. While nanoparticle inks are desired for printable electronics, almost all existing nanoparticle inks are based on single-metal component, which, as an electronic element, is limited by its inherent stabilities of the metal such as propensity of metal oxidation and mobility of metal ions, especially in sintering processes. The work here has demonstrated the first example in exploiting plasmonic coupling of nanoalloys and pulsed-laser energy with controllable thermal penetration. The experimental and theoretical results have revealed clear correlation between the pulsed laser parameters and the nanoalloy structural characteristics. The superior performance of the resulting flexible sensor device, upon imparting nanostructured sensing materials, for detecting volatile organic compounds has significant implications to developing stable and wearable sensors for monitoring environmental pollutants and breath biomarkers. This simple “nanoalloy printing–laser sintering–nanostructure printing” process is entirely general to many different sensor devices and nanostructured sensing materials, enabling the ability to easily construct sophisticated sensor array

Resolving Atomic Ordering Differences in Group 11 Nanosized Metals and Binary Alloy Catalysts by Resonant High-Energy X‑ray Diffraction and Computer Simulations

Resonant high-energy X-ray diffraction coupled to atomic pair distribution function analysis and computer simulations is used to study the atomic-scale structure of group 11 nanosized metals and binary alloy catalysts. We find that nanosized Cu is quite disordered structurally whereas nanosized Ag and especially Au exhibit a very good degree of crystallinity. We resolve Cu–Cu and Ag–Ag atomic correlations from Au-involving ones in Au–Cu and Au–Ag nanoalloys and show that depending on the synthetic route group 11 binary alloys may adopt structural states that obey or markedly violate Vegard’s law. In the latter case, Cu and Ag atoms undergo substantial size expansion and contraction by as much as 0.3 and 0.03 Å, respectively, while heavier Au atoms remain practically intact. The size change of Cu and Ag atoms does not follow Pauling’s rule of electronegativity predicting charge flow toward the more electronegative Au but occurs in a way such that Cu/Au and Ag/Au atomic size ratios in the nanoalloys become closer to one. Atomic size adjusting and the concurrent charge redistribution result in a synergistic effect of oxygen inactive Au and oxygen very active Cu and Ag leading to nanoalloys with very good activity for low-temperature oxidation of CO

Resolving Atomic Ordering Differences in Group 11 Nanosized Metals and Binary Alloy Catalysts by Resonant High-Energy X‑ray Diffraction and Computer Simulations

Resonant high-energy X-ray diffraction coupled to atomic pair distribution function analysis and computer simulations is used to study the atomic-scale structure of group 11 nanosized metals and binary alloy catalysts. We find that nanosized Cu is quite disordered structurally whereas nanosized Ag and especially Au exhibit a very good degree of crystallinity. We resolve Cu–Cu and Ag–Ag atomic correlations from Au-involving ones in Au–Cu and Au–Ag nanoalloys and show that depending on the synthetic route group 11 binary alloys may adopt structural states that obey or markedly violate Vegard’s law. In the latter case, Cu and Ag atoms undergo substantial size expansion and contraction by as much as 0.3 and 0.03 Å, respectively, while heavier Au atoms remain practically intact. The size change of Cu and Ag atoms does not follow Pauling’s rule of electronegativity predicting charge flow toward the more electronegative Au but occurs in a way such that Cu/Au and Ag/Au atomic size ratios in the nanoalloys become closer to one. Atomic size adjusting and the concurrent charge redistribution result in a synergistic effect of oxygen inactive Au and oxygen very active Cu and Ag leading to nanoalloys with very good activity for low-temperature oxidation of CO