Synergistic Effect of Electrocatalyst for Enhanced Oxygen Reduction Reaction: Low Pt-Loaded CuPt Alloy Nanoparticles Supported on N-Doped Hierarchical Porous Carbon

It is challenging to synthesize oxygen reduction reaction (ORR) electrocatalysts that are highly efficient, affordable, and stable for use in proton exchange membrane fuel cells. To address this challenge, we developed a low platinum-loading (only 6.68% wt) ORR catalyst (PtCu1-NC), comprising CuPt nanoparticles (average size: 1.51 nm) supported on the N-doped carbon substrates. PtCu1-NC possesses a high specific surface area of 662 m2 g-1 and a hierarchical porous structure, facilitating efficient mass transfer. The synergistic effect from introduced copper and the electron effect from nitrogen modify the electronic structure of platinum, effectively accelerating the ORR reaction and enhancing stability. Density functional theory calculations demonstrate the catalytic mechanism and further verify the synergistic effect. Electrochemical assessments indicate that PtCu1-NC exhibits specific activity and mass activity 5.3 and 5.6 times higher, respectively, than commercial Pt/C. The half-wave potential is 27 mV more positive than that of commercial Pt/C. The electrochemical active surface area value is 104.3 m2 g-1, surpassing that of Pt/C. Approximately 78% of current is retained after 10,000 s chronoamperometry measurement. These results highlight the effectiveness of alloying in improving the catalyst performance.

Activity and Stability of Pd Bimetallic Catalysts for Catalytic Nitrate Reduction

In this work, we study the effect of modifying the metal loading (0.5–1.5 wt.% Pd and 0.1–1 wt.% Sn or In), the impregnation order of noble or promoter metal (Pd–Sn or Sn–Pd), and the type of promoter metal (Sn or In) during the preparation process for a Pd bimetallic catalyst, supported on γ-alumina, used in the catalytic reduction of nitrate. The deposition of the noble metal over the promoter metal, especially with Pd:Sn ratios (wt.) of 1:10 and 1:2, favored the hydrogen spillover rate and increased the H concentration on the catalyst surface, enhancing NH4+ production. On the other hand, Pd–In catalysts showed higher activity than the Sn catalysts, as well as higher NH4+ selectivity. The stability of the Pd–Sn/Al2O3 (1.5–1 wt.%) catalyst was evaluated in long-term experiments for the treatment of synthetic water (100 mg L−1 NO3−) and three different commercial drinking waters. This Pd–Sn/Al2O3 catalyst achieved a stable nitrate conversion for a duration of 50 h in the synthetic water treatment. However, the catalyst showed a significant activity loss in the presence of other ions (different to NO3−) in the reaction medium, increasing slightly the selectivity to NH4+.

Modified Catalysts and Their Fractal Properties

Obtaining high-area catalysts is in demand in heterogeneous catalysis as it influences the ratio between the number of active surface sites and the number of total surface sites of the catalysts. From this point of view, fractal theory seems to be a suitable instrument to characterize catalysts’ surfaces. Moreover, catalysts with higher fractal dimensions will perform better in catalytic reactions. Modifying catalysts to increase their fractal dimension is a constant concern in heterogeneous catalysis. In this paper, scientific results related to oxide catalysts, such as lanthanum cobaltites and ferrites with perovskite structure, and nanoparticle catalysts (such as Pt, Rh, Pt-Cu, etc.) will be reviewed, emphasizing their fractal properties and the influence of their modification on both fractal and catalytic properties. Some of the methods used to compute the fractal dimension of the catalysts (micrograph fractal analysis and the adsorption isotherm method) and the computed fractal dimensions will be presented and discussed.

Highly N2-Selective Activated Carbon-Supported Pt-In Catalysts for the Reduction of Nitrites in Water

The catalytic reduction of nitrites over Pt-In catalysts supported on activated carbon has been studied in a semi-batch reactor, at room temperature and atmospheric pressure, and using hydrogen as the reducing agent. The influence of the indium content on the activity and selectivity was evaluated. Monometallic Pt catalysts are very active for nitrite reduction, but the addition of up to 1 wt% of indium significantly increases the nitrogen selectivity from 0 to 96%. The decrease in the accessible noble metal surface area reduces the amount of hydrogen available at the catalyst surface, this favoring the combination of nitrogen-containing intermediate molecules to promote the formation of N₂ instead of being deeply hydrogenated into NH⁴⁺. Several activated carbon-supported Pt-In catalysts, activated under different calcination and reduction temperatures, have been also evaluated in nitrite reduction. The catalyst calcined and reduced at 400°C showed the best performance considering both the activity and the selectivity to nitrogen. This enhanced selectivity is ascribed to the formation of Pt-In alloy. The electronic properties of Pt change upon alloy formation, as it is demonstrated by XPS.

Effect of Nanoparticle Size in Pt/SiO2 Catalyzed Nitrate Reduction in Liquid Phase

Effect of platinum nanoparticle size on catalytic reduction of nitrate in liquid phase was examined under ambient conditions by using hydrogen as a reducing agent. For the size effect study, Pt nanoparticles with sizes of 2, 4 and 8 nm were loaded silica support. TEM images of Pt nanoparticles showed that homogeneous morphologies as well as narrow size distributions were achieved during the preparation. All three catalysts showed high activity and were able to reduce nitrate below the recommended limit of 50 mg/L in drinking water. The highest catalytic activity was seen with 8 nm platinum; however, the product selectivity for N₂ was highest with 4 nm platinum. In addition, the possibility of PVP capping agent acting as a promoter in the reaction is highlighted.

Mesoporous carbon confined palladium–copper alloy composites for high performance nitrogen selective nitrate reduction electrocatalysis

A mesoporous carbon confined PdCu bimetallic electrocatalyst is fabricated, which delivers a superior nitrate conversion yield and nitrogen selectivity.