Designing Nanoparticles and Nanoalloys for Gas-Phase Catalysis with Controlled Surface Reactivity Using Colloidal Synthesis and Atomic Layer Deposition

Variable temperature in situ TEM mapping of the thermodynamically stable element distribution in bimetallic Pt-Rh nanoparticles

We report here the first variable temperature in situ transmission electron microscopy (TEM) study on smaller Pt-Rh nanoparticles (≤24 nm) under vacuum conditions. Well-defined 50 at% Pt/50 at% Rh Pt-Rh solid solution and Rh(core)-Pt(shell) nanoparticles, obtained via colloidal synthesis routes, were investigated between room temperature and 650 °C to elucidate the tendency of elemental mixing/segregation. Key findings are that Pt-Rh nanoparticles <13 nm are stable in a solid solution configuration over the entire studied temperature range, whereas nanoparticles >13 nm tend to segregate upon cooling. Such a cross-over in element distribution with nanoparticle size has not been reported for the Pt-Rh system previously. The results demonstrate the technique's ability to extract valuable information concerning the intricate dynamic processes that take place in the bimetallic Pt-Rh system at the nanoscale, which may be indispensable when optimizing, e.g., the metal composition in catalytically active materials.

Catalytic Design of Matrix-Isolated Ni-Polymer Composites for Methane Catalytic Decomposition

Targeted synthesis of C/composite Ni-based material was carried out by the method of matrix isolation. The composite was formed with regard to the features of the reaction of catalytic decomposition of methane. The morphology and physicochemical properties of these materials have been characterized using a number of methods: elemental analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, temperature programmed reduction (TPR-H₂), specific surface areas (SSA), thermogravimetric analysis, and differential scanning calorimetry (TGA/DSC). It was shown by FTIR spectroscopy that nickel ions are immobilized on the polymer molecule of polyvinyl alcohol, and during heat treatment, polycondensation sites are formed on the surface of the polymer molecule. By the method of Raman spectroscopy, it was shown that already at a temperature of 250 °C, a developed conjugation system with sp2-hybridized carbon atoms begins to form. The SSA method shows that the formation of the composite material resulted in a matrix with a developed specific surface area of 20 to 214 m²/g. The XRD method shows that nanoparticles are essentially characterized by Ni, NiO reflexes. The composite material was established by microscopy methods to be a layered structure with uniformly distributed nickel-containing particles 5-10 nm in size. The XPS method determined that metallic nickel was present on the surface of the material. A high specific activity was found in the process of catalytic decomposition of methane-from 0.9 to 1.4 g_H2/g_cat/h, X_CH4, from 33 to 45% at a reaction temperature of 750 °C without the stage of catalyst preliminary activation. During the reaction, the formation of multi-walled carbon nanotubes occurs.

Does CO2 Oxidize Ni Catalysts? A Quick X-ray Absorption Spectroscopy Answer

MgAl₂O₄-supported Ni materials are highly active and cost-effective CO₂ conversion catalysts, yet their oxidation by CO₂ remains dubious. Herein, NiO/MgAl₂O₄, prepared via colloidal synthesis (10 wt % Ni) to limit size distribution, or wet impregnation (5, 10, 20, and 40 wt % Ni), and bare, i.e., unsupported, NiO are examined in H₂ reduction and CO₂ oxidation, using thermal conductivity detector-based measurements and in situ quick X-ray absorption spectroscopy, analyzed via multivariate curve resolution-alternating least-squares. Ni reoxidation does not occur for bare Ni but is observed solely on supported materials. Only samples with the smallest particle sizes get fully reoxidized. The Ni-MgAl₂O₄ interface, exhibiting metal-support interactions, activates CO₂ and channels oxygen into the reduced lattice. Oxygen diffuses inward, away from the interface, oxidizing Ni entirely or partially, depending on the particle size in the applied oxidation time frame. This work provides evidence for Ni oxidation by CO₂ and explores the conditions of its occurrence and the importance of metal-support effects.

Innovative approach to controlled Pt-Rh bimetallic nanoparticle synthesis

Precise control of the elemental composition and distribution in bimetallic nanoparticles is of great interest for both fundamental studies and applications, e.g. in catalysis. We present a new innovative and facile synthesis strategy for the production of true solid solution Pt_1−xRh_x nanoparticles. This constitutes a development of the established heat-up method, where undesired shell formation is fully suppressed, despite utilizing metal precursors with different reaction rates. The concept is demonstrated through synthesis of selected Pt_1−xRh_x solid solution compositions via the polyalcohol reduction approach. In addition, we provide modified procedures, using the same surface stabilizing agent/metal precursors reaction matrix yielding controlled model Rh(core)–Pt(shell) and Pt(core)–Rh(shell) nanoparticles. Tunable bimetallic solid solution and core–shell nanoparticles with the same capping agent are of key importance in systematic fundamental studies, as functional materials properties may be altered by modifying the surface termination.

In this work, we establish an innovative protocol for the production of Pt–Rh solid solution/core–shell nanoparticles with excellent control of element distribution and composition, built upon the well-established heat-up method.

Catalysis by Hybrid Nanomaterials

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