Multi-metallic aerogels have emerged as a promising unsupported, high surface area-based metal material for different applications in heterogeneous catalysis and electrochemistry. The fabrication of these multi-metallic aerogels is based on a complex gelation process which is characterized by controlled aggregation of metallic nanoparticles to form a macroscopic network structure in aqueous solution. However, achieving structural homogeneity of the multi-metallic aerogels in terms of diameter of the nanochains and chemical composition at the nano- as well as at the macro-scale is still a great challenge. In this paper, we show the characterization of two Pd-Pt aerogels prepared by the two-step gelation method. The structural homogeneity and chemical distribution of both metals (Pd and Pt) inside the aerogels were analyzed using high-resolution (scanning) transmission microscopy (HR(S)TEM), energy-dispersive X-ray spectroscopy (EDX), extended X-ray absorption fine structure (EXAFS) spectroscopy, and cyclic voltammetry. Based on the microscopic and spectroscopic results, the Pd-Pt aerogels show the presence of Pd/Pt-rich domains inside the long-range framework. It is evident that the initial monometallic features dominate over alloying during the gelation process. Although the same synthetic approach for Pd-Pt aerogels with different atomic ratios was used, we observed that the sizes of these monometallic domains strongly varied between the Pd-rich and Pt-rich aerogels. For instance, the Pd-rich aerogels showed larger clusters with a size range from few nanometers up to several tens of nanometers, while the dimension of the clusters of the Pt-rich aerogels varies from the sub-nanometers to a few nanometers. The presence of the metal clusters strongly influenced the electrochemical robustness of these Pd-Pt aerogels. Electrochemical durability investigations revealed that the aerogels with a high content of Pd are less stable due to the gradual dissolution of the less noble metal in particular inside the Pd-rich domains. A better chemical and structural homogeneity might improve the life-time of the Pd-Pt aerogels under electrochemical conditions. In this work, we provide a better understanding about the structure and chemical distribution of the bimetallic aerogel framework prepared by the two step gelation process.

Multi-metallic aerogels have emerged as a promising unsupported, high surface area-based metal material for different applications in heterogeneous catalysis and electrochemistry. The fabrication of these multi-metallic aerogels is based on a complex gelation process which is characterized by controlled aggregation of metallic nanoparticles to form a macroscopic network structure in aqueous solution. However, achieving structural homogeneity of the multi-metallic aerogels in terms of diameter of the nanochains and chemical composition at the nano- as well as at the macro-scale is still a great challenge. In this paper, we show the characterization of two Pd-Pt aerogels prepared by the two-step gelation method. The structural homogeneity and chemical distribution of both metals (Pd and Pt) inside the aerogels were analyzed using high-resolution (scanning) transmission microscopy (HR(S)TEM), energy-dispersive X-ray spectroscopy (EDX), extended X-ray absorption fine structure (EXAFS) spectroscopy, and cyclic voltammetry. Based on the microscopic and spectroscopic results, the Pd-Pt aerogels show the presence of Pd/Pt-rich domains inside the long-range framework. It is evident that the initial monometallic features dominate over alloying during the gelation process. Although the same synthetic approach for Pd-Pt aerogels with different atomic ratios was used, we observed that the sizes of these monometallic domains strongly varied between the Pd-rich and Pt-rich aerogels. For instance, the Pd-rich aerogels showed larger clusters with a size range from few nanometers up to several tens of nanometers, while the dimension of the clusters of the Pt-rich aerogels varies from the sub-nanometers to a few nanometers. The presence of the metal clusters strongly influenced the electrochemical robustness of these Pd-Pt aerogels. Electrochemical durability investigations revealed that the aerogels with a high content of Pd are less stable due to the gradual dissolution of the less noble metal in particular inside the Pd-rich domains. A better chemical and structural homogeneity might improve the life-time of the Pd-Pt aerogels under electrochemical conditions. In this work, we provide a better understanding about the structure and chemical distribution of the bimetallic aerogel framework prepared by the two step gelation process.

Multi-metallic aerogels have recently emerged as a novel and promising class of unsupported electrocatalyst materials due to their high catalytic activity and improved durability for various electrochemical reactions. Aerogels can be prepared by a spontaneous one-step gelation process, where the chemical co-reduction of metal precursors and the prompt formation of the nanochain-consisting hydrogels, as a preliminary stage for the preparation of aerogels take place. However, detailed knowledge about the homogeneity and chemical distribution of these three-dimensional Pd-Pt aerogels at the nano-scale as well as at the macro-scale is still unclear to date. Therefore, we used a combination of spectroscopic and microscopic techniques to obtain a better insight into the structure and elemental distribution of the various Pd-rich Pd-Pt aerogels prepared by the spontaneous one-step gelation process. Synchrotron-based extended X-ray absorption fine structure (EXAFS) spectroscopy and high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) in combination with energy-dispersive X-ray spectroscopy (EDX) were employed in this work to uncover the structural architecture and chemical composition of the various Pd-rich Pd-Pt aerogels over a broad length range. The Pd80Pt20, Pd60Pt40 and Pd50Pt50 aerogels showed heterogeneity in the chemical distribution of the Pt and Pd atoms inside the macroscopic nanochain-network. Features of monometallic clusters were not detected by EXAFS or STEM-EDX, indicating alloyed nanoparticles. However, the local chemical composition of the Pd-Pt alloys strongly varied along the nanochains and thus within a single aerogel. To determine the electrochemically active surface area (ECSA) of the Pd-Pt aerogels for applications in electrocatalysis, we used the electrochemical CO stripping method. Due to high porosity and extended network structure, the resulting values of the ECSA for the Pd-Pt aerogels were higher than that for a commercially available unsupported Pt black catalyst. We show that the Pd-Pt aerogels possess a high utilization of catalytically active centers for electrocatalytic applications based on the nanostructured bimetallic framework. Knowledge about the homogeneity and chemical distribution of the bimetallic aerogels can help to further optimize their preparation by the spontaneous one-step gelation process and to tune their electrocatalytic reactivity.

Multi-metallic aerogels have recently emerged as a novel and promising class of unsupported electrocatalyst materials due to their high catalytic activity and improved durability for various electrochemical reactions. Aerogels can be prepared by a spontaneous one-step gelation process, where the chemical co-reduction of metal precursors and the prompt formation of the nanochain-consisting hydrogels, as a preliminary stage for the preparation of aerogels take place. However, detailed knowledge about the homogeneity and chemical distribution of these three-dimensional Pd-Pt aerogels at the nano-scale as well as at the macro-scale is still unclear to date. Therefore, we used a combination of spectroscopic and microscopic techniques to obtain a better insight into the structure and elemental distribution of the various Pd-rich Pd-Pt aerogels prepared by the spontaneous one-step gelation process. Synchrotron-based extended X-ray absorption fine structure (EXAFS) spectroscopy and high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) in combination with energy-dispersive X-ray spectroscopy (EDX) were employed in this work to uncover the structural architecture and chemical composition of the various Pd-rich Pd-Pt aerogels over a broad length range. The Pd80Pt20, Pd60Pt40 and Pd50Pt50 aerogels showed heterogeneity in the chemical distribution of the Pt and Pd atoms inside the macroscopic nanochain-network. Features of monometallic clusters were not detected by EXAFS or STEM-EDX, indicating alloyed nanoparticles. However, the local chemical composition of the Pd-Pt alloys strongly varied along the nanochains and thus within a single aerogel. To determine the electrochemically active surface area (ECSA) of the Pd-Pt aerogels for applications in electrocatalysis, we used the electrochemical CO stripping method. Due to high porosity and extended network structure, the resulting values of the ECSA for the Pd-Pt aerogels were higher than that for a commercially available unsupported Pt black catalyst. We show that the Pd-Pt aerogels possess a high utilization of catalytically active centers for electrocatalytic applications based on the nanostructured bimetallic framework. Knowledge about the homogeneity and chemical distribution of the bimetallic aerogels can help to further optimize their preparation by the spontaneous one-step gelation process and to tune their electrocatalytic reactivity.