Synthesis of Pt/C Catalysts through Reductive Deposition: Ways of Tuning Catalytic Properties

Strong response of Pt clusters to the environment and conditions, formation of metastable states, and simple methods to trace the reversible changes

Subnanometric metal particles, the so-called "clusters", are known to be responsive to their surroundings, but the detection of occurring changes, understanding the causes, and predicting the consequences are still extremely difficult for such small particles. Our study was aimed at estimating the potential of adsorption-based methods for these purposes. Using carbon monoxide as a probing molecule, which readily adsorbs on both bare and H-covered Pt surface, we have probed the adsorption properties of highly dispersed Pt/γ-Al₂O₃ samples after treatments under different atmospheres and temperatures (H₂ or inert gas, 25-500 °C). The combined results of CO-chemisorption measurements, CO TPD, CO TPO, H₂-by-CO displacement, and H₂ TPD suggest that the system shuttles between two states: one with oxygen vacancies in the support and the other one with redox-active oxygen near the Pt clusters. These extreme states can be reversibly created and deleted, giving rise to innumerable intermediate structures that differ in the amount, binding strength, and/or reactivity of adsorbed species. Two adsorbates could act cooperatively, resulting in hydrogen spillover onto the support and making the adsorbate-metal-support interactions even more complex. Implications for better understanding the dynamic behavior of oxide-supported clusters and nanoparticles are discussed.

Design of Silica Nanoparticles-Supported Metal Catalyst by Wet Impregnation with Catalytic Performance for Tuning Carbon Nanotubes Growth

The catalytic activity of cobalt and iron nanoparticles for the growth of carbon nanotubes (CNTs) was studied by a specific reproducible and up-scalable fabrication method. Co and Fe catalysts were deposited over SiO2 nanoparticles by a wet-impregnation method and two different annealing steps were applied for the catalyst formation/activation. The samples were calcined at an optimal temperature of 450 °C resulting in the formation of metal oxide nano-islands without the detection of silicates. Further reduction treatment (700 °C) under H2 successfully converted oxide nanoparticles to Co and Fe metallic species. Furthermore, the catalytic efficiency of both supported-metal nanoparticles at 2 and 5% in weight of silica was evaluated through the growth of CNTs. The CNT structure, morphology and size dispersion were tailored according to the metal catalyst concentration.

Nitrogen Doped Carbon Nanotubes and Nanofibers for Green Hydrogen Production: Similarities in the Nature of Nitrogen Species, Metal–Nitrogen Interaction, and Catalytic Properties

The effect of nitrogen doped bamboo-like carbon nanotubes (N–CNTs) on the properties of supported platinum (0.2 and 1 wt %) catalysts in formic acid decomposition for hydrogen production was studied. It was shown that both impregnation and homogeneous precipitation routes led to the formation of electron-deficient platinum stabilized by pyridinic nitrogen sites of the N–CNTs. The electron-deficient platinum species strongly enhanced the activity and selectivity of the Pt/N–CNTs catalysts when compared to the catalysts containing mainly metallic platinum nanoparticles. A comparison of bamboo-like N–CNTs and herring-bone nitrogen doped carbon nanofibers (N–CNFs) as the catalyst support allowed us to conclude that the catalytic properties of supported platinum are determined by its locally one-type interaction with pyridinic nitrogen sites of the N–CNTs or N–CNFs irrespective of substantial structural differences between nanotubes and nanofibers.

Particle Size-Controlled Growth of Carbon-Supported Platinum Nanoparticles (Pt/C) through Water-Assisted Polyol Synthesis

A water-assisted control of Pt nanoparticle size during a surfactant-free, microwave-assisted polyol synthesis of the carbon-supported platinum nanoparticles (Pt/C) in a mixture of ethylene glycol and water using (NH₄)₂PtCl₆ as the Pt precursor is demonstrated. The particle size was tuned between ∼2 and ∼6 nm by varying either the H₂O volume percent or the Pt precursor concentration during synthesis. The electrochemical surface area (ECSA) and the oxygen-reduction reaction activity obtained for the Pt/C electrocatalyst show a catalytic performance competitive to that of the state-of-the-art commercial Pt/C electrocatalysts used for polymer electrolyte membrane fuel cell electrodes (ECSA: ∼70 m²/g; half-wave potential for oxygen reduction reaction: 0.83 V vs reversible hydrogen electrode). The synthesized Pt/C electrocatalysts show durability equivalent to or better than that of the commercial Pt/C. The durability was found to improve with increasing particle size, with the ECSA loss values being ∼70 and ∼55% for the particle sizes of 2.1 and 4.3 nm, respectively. The study may be used as a route to synthesize Pt/C electrocatalysts from a convenient and economic Pt precursor (NH₄)₂PtCl₆ and avoiding the use of alkaline media.

Factors Influencing the Performance of Pd/C Catalysts in the Green Production of Hydrogen from Formic Acid

Formic acid derived from biomass is known to be used for hydrogen production over Pd catalysts. The effects of preparation variables, structure of the carbon support, surface functional composition on the state of Pd, and catalytic properties of the samples in the vapor-phase decomposition of formic acid were studied. In all catalysts derived from Pd acetate, metal particles visible by conventional TEM had similar sizes, but the adsorption capacity towards CO responded strongly to N-doping of the carbon surface. Moreover, a decrease in the CO/Pd values was accompanied by a significant increase in the reaction rate. Taking account of X-ray photoelectron spectroscopy (XPS) and atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF/STEM) data, the trends observed were assigned to a larger fraction of single electron-deficient Pd atoms in the N-doped samples, which do not adsorb CO but interact with formic acid to produce hydrogen. This was confirmed by extended DFT studies. The obtained results are valuable for the development of Pd catalysts on carbon supports for different processes.

Metal-supported carbon-based materials: opportunities and challenges in the synthesis of valuable products

We summarize relevant applications and limitations of metal-supported carbons and nanocarbons in the synthesis of valuable products and the recent advances in this field.