Engineering the morphology of palladium nanostructures to tune their electrocatalytic activity in formic acid oxidation reactions

Boosting the Electrocatalytic Formic Acid Oxidation Activity via P-PdAuAg Quaternary Alloying

Direct formic acid fuel cells (DFAFCs) are considered promising sustainable power sources due to their high energy density, nonflammability, and low fuel crossover. However, serious CO poisoning and activity attenuation of the anodic formic acid oxidation reaction (FAOR) greatly restrict the output and durability of DFAFCs. Inspired by the specific relationship between the composition, type, and property of alloys, in this work, we synthesize a series of hybrid substitutional/interstitial quaternary alloys P-PdAuAg by means of a novel polyphosphide route to address these issues. Due to the simultaneous interstitial P-doping and metal (Au, Ag, Pd) co-reduction, the P-PdAuAg quaternary alloy obtained is only 3 nm in diameter with abundant defects. It not only achieves a new high mass activity of 8.08 A mg_Pd^-1 (6.78 A mg_catalyst^-1) but also maintains high stability in the high potential range and harsh reaction conditions. Both the activity and anti-poisoning ability are far exceeding those of the currently reported FAOR catalysts. Detailed density functional theory (DFT) calculations reveal that the superb electrochemical performances originate from the shift of the d-band center of Pd as a result of the synergistic electronic/ligand effects between Pd, Au, Ag, and P. The introduction of interstitial P inhibits the occurrence of an indirect reaction pathway on Pd, while Au and Ag suppress the adsorption of CO and optimize the sequential dehydrogenation steps, leading to boosted reaction kinetics and CO tolerance. This work pioneered a facile way for the synthesis of Pd-based substitutional/interstitial hybrid alloys, providing a promising means of further improving the performance of alloying catalysts.

PdAu Nanosheets for Visible-Light-Driven Suzuki Cross-Coupling Reactions

Combining a two-dimensional (2D) morphology and plasmonic photocatalysis represents an efficient design for light-driven organic transformations. We report a one-pot synthesis of surfactant templated PdAu nanosheets (NSs). Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analyses show the formation of 2D PdAu structures was initiated through nanoparticle seeds dispersed in the alkyl ammonium salt surfactant which acted as a template for the growth into NSs. The PdAu NSs were used for visible-light-enhanced Suzuki cross coupling. The PdAu bimetallic NSs outperformed monometallic Pd NSs and commercial Pd/C in room-temperature Suzuki cross-coupling reactions. The high catalytic activity is attributed to a combination of the 2D morphology giving rise to plasmon-enhanced catalysis and a high density of surface atoms, the electron-rich Pd surface due to alloying, and the presence of weakly bound amines. A comparative study of surfactant-assisted NSs and CO-assisted NSs was also carried out to assess the influence of surface ligands on the catalytic and photocatalytic enhancement of NSs with similar morphology. The surfactant-assisted NSs showed substantially superior performance compared to the CO-assisted for room-temperature Suzuki coupling reactions.

Boosting the anti-poisoning ability of palladium towards electrocatalytic formic acid oxidation via polyphosphide chemistry

In this work, we reported a novel polyphosphide strategy for the synthesis of phosphorus doped Pd (P-Pd) using red phosphorus as the starting material at quasi-ambient conditions. Polyphophide anions, as the key reaction intermediates, served as the reducing agent and phosphorus source to modulate the surface electronic structure of Pd. The P-Pd obtained exhibited topmost CO tolerance and electrocatalytic activity to formic acid oxidation among the state-of-arts reports. The mass activity and turnover frequency of P-Pd reached 4413 mA mg^-1_Pd and 16.04 s^-1 at 0.8 V, which were 23.7 and 6.4 times that of commercial Pd/C respectively. After 1000 repeated cycles, 82% initial activity was reserved. Combined with the electrochemical analysis and the density functional theory calculation, the boosted electrochemical performances can be attributed to the size and electronic effects induced by the P doping, which increase the surface actives sites, inhibit the adsorption of CO and change the reaction pathway to favorable CO₂ route. A full cell was also assembled to demonstrate the practical potential of the P-Pd, which showed a maximum power density of 21.56 mW cm^-2. This polyphophide-based reaction route provides a new strategy for the preparation of efficient and durable phosphorus doped alloys for electrocatalysis.

Mechanistic effects of blending formic acid with ethanol on Pd activity towards formic acid oxidation in acidic media

The direct formic acid fuel cell (DFAFC) is one of the most promising direct liquid fuel cells. Pd is the most active catalyst towards formic oxidation, however, it suffers from CO-like poisoning and instability in acidic media. Blending formic acid with ethanol is known to synergistically enhance the Pt catalytic activity of Pt. However, it has not been studied in the case of Pd. In this study, ethanol/formic acid blends were tested, aiming at understanding the effect of ethanol on the formic acid oxidation mechanism at Pd and how the direct and indirect pathways could be affected. The blends consisted of different formic acid (up to 4 M) and ethanol (up to 0.5 M) concentrations. The catalytic activity of a 40% Pd/C catalyst was tested in 0.1 M H₂SO₄ + XFA + YEtOH using cyclic voltammetry, while the catalyst resistance to poisoning in the presence and absence of ethanol was tested using chronopotentiometry. The use of these blends is found to not only eliminate the indirect pathway but also slowly decrease the direct pathway activity too. That is believed to be due to the different ethanol adsorption orientations at different potentials. This study should open the door for further studying the oxidation of FA/ethanol blends using different pHs and different Pd-based catalysts.

Ethanol changes the Pd selectivity towards the different pathways of formic acid oxidation by eliminating the indirect pathway and slowly decreasing the direct pathway activity, owing to ethanol potential depdant adsorption orientations.