Structural and Valence State Modification of Cobalt in CoPt Nanocatalysts in Redox Conditions

Abstract

Platinum is the primary catalyst for many chemical reactions in the field of heterogeneous catalysis. However, platinum is both expensive and rare. Therefore, it is advantageous to combine Pt with another metal to reduce cost while also enhancing stability. To that end, Pt is often combined with Co to form Co–Pt nanocrystals. However, dynamical restructuring effects that occur during reaction in Co–Pt ensembles can impact catalytic properties. In this study, model Co2Pt3 nanoparticles supported on carbon were characterized during a redox cycle with two in situ approaches, namely, X-ray absorption spectroscopy (XAS) and scanning transmission electron microscopy (STEM) using a multimodal microreactor. The sample was exposed to temperatures up to 500 °C under H2, and then to O2 at 300 °C. Irreversible segregation of Co in the Co2Pt3 particles was seen during redox cycling, and substantial changes of the oxidation state of Co were observed. After H2 treatment, a fraction of Co could not be fully reduced and incorporated into a mixed Co–Pt phase. Reoxidation of the sample increased Co segregation, and the segregated material had a different valence state than in the fresh, oxidized sample. This in situ study describes dynamical restructuring effects in CoPt nanocatalysts at the atomic scale that are crucial to understand in order to improve the design of catalysts used in major chemical processes.

Synthesis and Characterization of Core-Shell Cu-Ru, Cu-Rh, and Cu-Ir Nanoparticles

“…Some solutions developed in the past involve the synthesis of very small nanoparticles to the limit of single-atom catalysts. , Other approaches focus on the creation of alloyed structures, where PGMs are mixed with less expensive materials. − Both of these solutions have advantages and disadvantages. For single-atom catalysts, stability toward sintering into large clusters is a common problem that leads to reduced catalytic activity. , For alloyed structures, dynamical restructuring effects can cause migration of the inexpensive and inactive metal to the surface, leading to the coverage of the PGMs. − Consequently, the active material is no longer on the surface, leading to deactivation. Additional issues include poisoning effects, where gaseous molecules remain adsorbed on the surface, blocking the chemical reaction. , Thus, it is necessary to expand the range of available materials and synthesis techniques to improve the design of stable and active nanostructures.…”

Insights into Heterogeneous Catalysts under Reaction Conditions by In Situ/Operando Electron Microscopy

“…The bimetallic NPs could also form discrete Co and Pt regions across the NP, as demonstrated in a combined in‐situ STEM and X‐ray absorption spectroscopy (XAS) study of Co 2 Pt 3 /C upon redox cycling in O 2 and H 2 . [ 148 ] Irreversible segregation of Co, along with substantial changes in the oxidation state of Co in Co 2 Pt 3 during redox cycling is revealed. A fraction of Co could not be fully reduced and incorporated into a mixed Co–Pt phase.…”

Theoretical Perspective on Operando Spectroscopy of Fluxional Nanocatalysts

“…Similar to this, multimodal tracking via operando XAS and STEM-EELS of bimetallic NPs during redox cycling provided detailed insight into dynamic changes resulting from reduction and oxidation of the NPs. In both cases, the reducing conditions initially induced improved mixing of Co and Pt, or Ni and Cu, but upon reoxidation, the NPs underwent increased irreversible segregation, and specific components became more oxidized than the as-prepared NPs. In the case of NiCu NPs, these changes could be tracked separately via XAS of the Ni and Cu L edges, showing that Cu underwent the significant changes, while Ni did not .…”