Unravelling Charge-transfer in Pd to pyrrolic-N bond for superior electrocatalytic performance

Fuel cells require large quantities of Pt for oxygen reduction reaction (ORR) to subvert the activity-loss during prolonged use. Pd can complement Pt in the near future by exhibiting a similar activity and stability in alkaline fuel cells. Herein we show that by depositing Pd atom-by-atom on an N-doped reduced graphene oxide (NRGO), it is possible to create a strong bond between Pd and pyrrolic-fraction of the N-moieties. This bond further strengthens in the presence of an by a from the Pd 3d-orbitals to the 2p-orbitals of C, N and O, thereby lowering the Pd- 3d binding-energy and the resulting Pd/NRGO exhibits a very high ORR activity (E1/2 = 0.93 V vs. RHE) and stability (∆E1/2 = 0.013 V after 15000. Usually pyridinic-N is considered for imparting high-performance while N-doping creates nearly as many pyrrolic-N in graphene-substrates, the role of which is evidenced in this study

Prospects in Engineering Congested Molecular Diffusion at the Stabilizer Layer of Metal Nanocrystals for Ultrahigh Catalytic Activity

Electron transfer processes between a catalyst and a reactant molecule are inefficient beyond a couple of angstroms distance. However, the stabilizers of metal nanocrystals or ligands often create an outer shell that may extend beyond a few nanometers, which is considerably larger than the efficient electron-transfer length scales and suggests that the reactants must therefore diffuse through the shell toward the catalytic surface with a restrained diffusion rate to potentially slow the reaction. However, the effect of such diffusion behavior has so far been neglected as a contributing factor toward achieving high catalytic activities by noble metal nanocrystals. Herein, we examine this hypothesis using Pd nanocrystals having identical surface electronic structures but stabilized by shells of vinylpyrrolidone molecules in different fashions to show that (i) molecular diffusion near the catalyst surface can vary significantly and (ii) the diffusion barrier can improve severalfold, resulting in Pd nanocrystals exhibiting the highest turnover frequencies (TOF) reported to date for a variety of hydrogenation reactions, Suzuki–Miyaura cross-coupling reactions, and nitroarene reduction reactions. The work demonstrates the tailoring of the reactant diffusion barrier near the surface of a heterogeneous catalyst may offer new possibilities for improving the catalytic activity of noble metal nanocrystals

3D Porous Polymeric-Foam-Supported Pd Nanocrystal as a Highly Efficient and Recyclable Catalyst for Organic Transformations

The efficient recovery of noble metal nanocrystals used in heterogeneous organic transformations has remained a significant challenge, hindering their use in industry. Herein, highlycatalytic Pd nanoparticles (NPs) were first prepared having a yield of >98% by a novel hydrothermal method using PVP as the reducing cum stabilizing agent that exhibited excellent turnover frequencies of ∼38,000 h$^{−1}$ for Suzuki−Miyaura cross-coupling and ∼1200 h$^{−1}$ for catalytic reduction of nitroarene compounds in a benign aqueous reaction medium. The Pd NPs were more efficient for cross-coupling of aryl compounds with electrondonating substituents than with electron-donating ones. Further, to improve their recyclability, a strategy was developed to embed these Pd NPs on mechanically robust polyurethane foam (PUF) for the first time and a “dip-catalyst” (Pd-PUF) containing 3D interconnected 100−500 μm pores was constructed. The PUF was chosen as the support with an expectation to reduce the fabrication cost of the “dip-catalyst” as the production of PUF is already commercialized. Pd-PUF could be easily separated from the reaction aliquot and reused without any loss of activity because the leaching of Pd NPs was found to be negligible in the various reaction mixtures. We show that the Pd-PUF could be reused for over 50 catalytic cycles maintaining a similar activity. We further demonstrate a scale-up reaction with a single-reaction 1.5 g yield for the Suzuki−Miyaura cross-coupling reaction

Self-immobilized Pd nanowires as an excellent platform for a continuous flow reactor: efficiency, stability and regeneration

Despite extensive use of Pd nanocrystals as catalysts, the realization of a Pd-based continuous flow reactor remains a challenge. Difficulties arise due to ill-defined anchoring of the nanocrystals on a substrate and reactivity of the substrate under different reaction conditions. We demonstrate the first metal (Pd) nanowire-based catalytic flow reactor that can be used across different filtration platforms, wherein, reactants flow through a porous network of nanowires (10–1000 nm pore sizes) and the product can be collected as filtrate. Controlling the growth parameters and obtaining high aspect ratio of the nanowires (diameter = ∼13 nm and length > 8000 nm) is necessary for successful fabrication of this flow reactor. The reactor performance is similar to a conventional reactor, but without requiring energy-expensive mechanical stirring. Synchrotron-based EXAFS studies were used to examine the catalyst microstructure and Operando FT-IR spectroscopic studies were used to devise a regenerative strategy. We show that after prolonged use, the catalyst performance can be regenerated up to 99% by a simple wash-off process without disturbing the catalyst bed. Thus, collection, regeneration and redispersion processes of the catalyst in conventional industrial reactors can be avoided. Another important advantage is avoiding specific catalyst-anchoring substrates, which are not only expensive, but also non-universal in nature