Thermal annealing processes for supported Pt-based nanoparticles are usually developed based on trial and error iterations and findings resulting from ex-situ characterization of pre- and post-annealed samples. Such an approach, however, offers limited insight into processes occurring during the heating step. In this work, we first exemplify typical findings that are accessible by ex-situ investigation using typical conventional techniques such as transmission electron microscopy (TEM), X-ray diffraction (XRD), and thin film – rotating disc electrode (TF-RDE). As a model system we select a well-researched Pt-Cu alloy which, as demonstrated, offers exciting new insights into the dynamics occurring during heat treatment on the nano-to-atomic scale. This dynamics can be viewed by upgrading the ex-situ findings with a high resolution TEM imaging in combination with carefully designed in-situ heating protocol. This way one can directly observe the particle growth mechanisms during heat treatment. Such direct observations, in turn, provide new understanding of morphologyperformance correlations in alloys. For example, it is shown that the enhanced activity of the present PtCu3/C electrocatalyst is due to Cu enrichment during heat treatment. This enrichment, however, is only possible due to the presence of relatively large excess CuO needlelike particles left over from the previous double passivation galvanic displacement step. Very importantly, we further show that the mechanism of Cu enrichment at elevated temperatures involves migration of Cu single atoms via the carbon support. At moderate temperatures (up to 500 °C), other effects have also been observed such as reshaping into a sphere-like shape as well as ordering of the crystal lattice which could not occur without enrichment of the initial Pt-Cu nanoparticles with Cu. In that region, Cu enrichment is also responsible for the initial growth of PtCu nanoparticles. By contrast, upon heating till 800 °C, the growth is mainly due to coalescence. Ostwald ripening, on the other hand, does not seem to play a significant role in the increase in the nanoparticle size. The new general insights can be readily extended to various other similar alloy systems.
COBISS.SI-ID: 6670618
Catalytic properties of advanced functional materials are determined by their surface and near-surface atomic structure, composition, morphology, defects, compressive and tensile stresses, etc; also known as a structure–activity relationship. The catalysts structural properties are dynamically changing as they perform via complex phenomenon dependent on the reaction conditions. In turn, not just the structural features but even more importantly, catalytic characteristics of nanoparticles get altered. Definitive conclusions about these phenomena are not possible with imaging of random nanoparticles with unknown atomic structure history. Using a contemporary PtCu-alloy electrocatalyst as a model system, a unique approach allowing unprecedented insight into the morphological dynamics on the atomic-scale caused by the process of dealloying is presented. Observing the detailed structure and morphology of the same nanoparticle at different stages of electrochemical treatment reveals new insights into atomic-scale processes such as size, faceting, strain and porosity development. Furthermore, based on precise atomically resolved microscopy data, Kinetic Monte Carlo (KMC) simulations provide further feedback into the physical parameters governing electrochemically induced structural dynamics. This work introduces a unique approach toward observation and understanding of nanoparticles dynamic changes on the atomic level and paves the way for an understanding of the structure–stability relationship.
COBISS.SI-ID: 6623002
To date, copper is the only monometallic catalyst that can electrochemically reduce CO2 into high value and energy-dense products, such as hydrocarbons and alcohols. In recent years, great efforts have been directed towards understanding how its nanoscale structure affects activity and selectivity for the electrochemical CO2 reduction reaction (CO2RR). Furthermore, many attempts have been made to improve these two properties. Nevertheless, to advance towards applied systems, the stability of the catalysts during electrolysis is of great significance. This aspect, however, remains less investigated and discussed across the CO2RR literature. In this Minireview, the recent progress on understanding the stability of copper-based catalysts is summarized, along with the very few proposed degradation mechanisms. Finally, our perspective on the topic is given.
COBISS.SI-ID: 6800666