Synchrotron-Based In Situ Characterization of Carbon-Supported Platinum and Platinum Monolayer Electrocatalysts

A detailed understanding of oxidation/dissolution mechanisms of Pt is critical in designing durable catalysts for the oxygen reduction reaction (ORR), but exact mechanisms remain unclear. The present work explores the oxidation/dissolution of Pt and Pt monolayer (ML) electrocatalysts over a wide range of applied potentials using cells that facilitate in situ measurements by combining X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) measurements. The X-ray absorption near edge structure (XANES) measurement demonstrated that Pt nanoparticle surfaces were oxidized from metallic Pt to α-PtO₂-type oxide during the potential sweep from 0.41 to 1.5 V, and the transition state of O or OH adsorption on Pt and the onset of the place exchange process were revealed by the delta mu (Δμ) method. Only the top layers of Pt nanoparticles were oxidized, while the inner Pt atoms remained intact. At a higher potential over 1.9 V, α-PtO₂-type surface oxides dissolve due to local acidification caused by the oxygen evolution reaction and carbon corrosion. Pt oxidation of Pt_ML on the Pd nanoparticle electrocatalyst is considerably hampered compared with the Pt/C catalyst, presumably because preferential Pd oxidation proceeds at the defects in Pt MLs up to 0.91 V and through O penetrated through the Pt MLs by the place exchange process above 1.11 V

Manipulating the Microenvironment of Surfactant-Encapsulated Pt Nanoparticles to Promote Activity and Selectivity

Precise tuning of the local environment surrounding the active site is key to engineering catalytic materials. Here, we have explored a nanoconfined catalytic system that exhibits highly selective hydrogenation of C=O bonds relative to C=C bonds. Organic surfactants anchored on metal surfaces not only can modify the catalytic performance according to their steric properties but can also regulate the solvent distribution at the liquid–solid interface, creating nanopockets of adjustable hydrophobic/hydrophilic interactions. The cooperative action of organic ligands and water molecules leads to catalytic pathways distinct from those on catalysts without functionalization. By combining precise catalyst synthesis with in situ spectroscopic characterization, reaction kinetics analysis, and computational techniques, this study provides a quantitative description of a catalyst with a tunable local environment near the active site. This precise control is reminiscent of that of natural enzymes that can alter their scaffold structure to adjust the solvent composition in their inner cavities and control reaction selectivity

Manipulating the Microenvironment of Surfactant-Encapsulated Pt Nanoparticles to Promote Activity and Selectivity

Precise tuning of the local environment surrounding the active site is key to engineering catalytic materials. Here, we have explored a nanoconfined catalytic system that exhibits highly selective hydrogenation of C=O bonds relative to C=C bonds. Organic surfactants anchored on metal surfaces not only can modify the catalytic performance according to their steric properties but can also regulate the solvent distribution at the liquid–solid interface, creating nanopockets of adjustable hydrophobic/hydrophilic interactions. The cooperative action of organic ligands and water molecules leads to catalytic pathways distinct from those on catalysts without functionalization. By combining precise catalyst synthesis with in situ spectroscopic characterization, reaction kinetics analysis, and computational techniques, this study provides a quantitative description of a catalyst with a tunable local environment near the active site. This precise control is reminiscent of that of natural enzymes that can alter their scaffold structure to adjust the solvent composition in their inner cavities and control reaction selectivity

Aliovalent Doping of CeO₂ Improves the Stability of Atomically Dispersed Pt

Atomically dispersed supported catalysts hold considerable promise as catalytic materials. The ability to employ and stabilize them against aggregation in complex process environments remains a key challenge to the elusive goal of 100% atom utilization in catalysis. Herein, using a Gd-doped ceria support for atomically dispersed surface Pt atoms, we establish how the combined effects of aliovalent doping and oxygen vacancy generation provide dynamic mechanisms that serve to enhance the stability of supported single-atom configurations. Using correlated, in situ X-ray absorption, photoelectron, and vibrational spectroscopy methods for the analysis of samples on the two types of support (with and without Gd doping), we establish that the Pt atoms are located proximal to Gd dopants, forming a speciation that serves to enhance the thermal stability of Pt atoms against aggregation

Carbon-Supported IrNi Core–Shell Nanoparticles: Synthesis, Characterization, and Catalytic Activity

We synthesized carbon-supported IrNi core–shell nanoparticles by chemical reduction and subsequent thermal annealing in H2, and verified the formation of Ir shells on IrNi solid solution alloy cores by various experimental methods. The EXAFS analysis is consistent with the model wherein the IrNi nanoparticles are composed of two-layer Ir shells and IrNi alloy cores. In situ XAS revealed that the Ir shells completely protect Ni atoms in the cores from oxidation or dissolution in an acid electrolyte under elevated potentials. The formation of Ir shell during annealing due to thermal segregation is monitored by time-resolved synchrotron XRD measurements, coupled with Rietveld refinement analyses. The H2 oxidation activity of the IrNi nanoparticles was found to be higher than that of a commercial Pt/C catalyst. This is predominantly due to Ni-core-induced Ir shell contraction that makes the surface less reactive for IrOH formation, and the resulting more metallic Ir surface becomes more active for H2 oxidation. This new class of core–shell nanoparticles appears promising for application as hydrogen anode fuel cell electrocatalysts

Effect of Manganese Addition to the Co-MCM-41 Catalyst in the Selective Synthesis of Single Wall Carbon Nanotubes

The effect of manganese addition to the Co-MCM-41 catalyst on the synthesis of single wall carbon nanotubes (SWNT) by CO disproportionation was characterized. The ratio between the two metals in the MCM-41 framework was varied, and its effect on the resultant SWNT distribution was studied and compared with the results obtained for the monometallic Co-MCM-41 catalyst. Methods including temperature-programmed reduction, X-ray absorption fine structure, thermogravimetric analysis, TEM imaging, and Raman and fluorescence spectroscopy were employed to characterize the behavior of the catalysts under the SWNT synthesis conditions and the diameter and structure distribution of the resultant nanotubes. We found that addition of Mn to the Co-MCM-41 catalyst promotes the growth of SWNT, leading to synthesis of high yield, small diameter SWNT. Manganese does not act in the nucleation of SWNT but acts as an anchoring site for cobalt particles formed during the synthesis process as shown by X-ray absorption

Multiple Metal–Nitrogen Bonds Synergistically Boosting the Activity and Durability of High-Entropy Alloy Electrocatalysts

The development of Pt-based catalysts for use in fuel cells that meet performance targets of high activity, maximized stability, and low cost remains a huge challenge. Herein, we report a nitrogen (N)-doped high-entropy alloy (HEA) electrocatalyst that consists of a Pt-rich shell and a N-doped PtCoFeNiCu core on a carbon support (denoted as N–Pt/HEA/C). The N–Pt/HEA/C catalyst showed a high mass activity of 1.34 A mgPt–1 at 0.9 V for the oxygen reduction reaction (ORR) in rotating disk electrode (RDE) testing, which substantially outperformed commercial Pt/C and most of the other binary/ternary Pt-based catalysts. The N–Pt/HEA/C catalyst also demonstrated excellent stability in both RDE and membrane electrode assembly (MEA) testing. Using operando X-ray absorption spectroscopy (XAS) measurements and theoretical calculations, we revealed that the enhanced ORR activity of N–Pt/HEA/C originated from the optimized adsorption energy of intermediates, resulting in the tailored electronic structure formed upon N-doping. Furthermore, we showed that the multiple metal–nitrogen bonds formed synergistically improved the corrosion resistance of the 3d transition metals and enhanced the ORR durability

Structural and Chemical Evolution of an Inverse CeO_<i>x</i>/Cu Catalyst under CO₂ Hydrogenation: Tunning Oxide Morphology to Improve Activity and Selectivity

Small nanoparticles of ceria deposited on a powder of CuO display a very high selectivity for the production of methanol via CO2 hydrogenation. CeO2/CuO catalysts with ceria loadings of 5%, 20%, and 50% were investigated. Among these, the system with 5% CeOx showed the best catalytic performance at temperatures between 200 and 350 °C. The evolution of this system under reaction conditions was studied using a combination of environmental transmission electron microscopy (E-TEM), in situ X-ray absorption spectroscopy (XAS), and time-resolved X-ray diffraction (TR-XRD). For 5% CeOx/Cu, the in situ studies pointed to a full conversion of CuO into metallic copper, with a complete transformation of Ce4+ into Ce3+. Images from E-TEM showed drastic changes in the morphology of the catalyst when it was exposed to H2, CO2, and CO2/H2 mixtures. Under a CO2/H2 feed, there was a redispersion of the ceria particles that was detected by E-TEM and in situ TR-XRD. These morphological changes were made possible by the inverse oxide/metal configuration and facilitate the binding and selective conversion of CO2 to methanol

One-Step Facile Synthesis of High-Activity Nitrogen-Doped PtNiN Oxygen Reduction Catalyst

PtM alloy electrocatalysts (M = Fe, Co, Ni) have been the subject of many investigations aimed at increasing their attractive properties, in particular their oxygen reduction reaction (ORR) activity, while reducing total platinum-group-metal content and improving durability. Despite some success, these catalysts still have relatively high Pt content and lack the necessary durability, as M metals leach out from the alloys during potential cycling. Previously, we synthesized nitrogen (N)-doped PtMN/C catalysts consisting of thin Pt shells on M nitride cores by a two-step method, which showed higher ORR activity and stability than their PtM counterparts. In the present study, we developed a facile one-step synthesis method, which comprises a single thermal annealing process of the N-doped PtNiN/C alloy. The ORR performance of the one-step-synthesized PtNiN/C catalyst is much higher than that of the two-step-synthesized PtNiN/C, as revealed by rotating disk electrode measurements. Membrane electrode assembly fuel cell testing demonstrated superb durability and high activity. Formation of Pt monolayer shells on the nitrided (PtxNi1–x)4N cores was confirmed by in situ X-ray absorption spectroscopy. The origins of the enhanced activity and stability of the one-step-synthesized PtNiN/C catalyst are elucidated based on density functional theory calculations together with the experimental results

High-Temperature Pretreatment Effect on Co/SiO₂ Active Sites and Ethane Dehydrogenation

We report the synthesis, optimization, and characterization of Co/SiO2 for ethane nonoxidative dehydrogenation. Co/SiO2 is synthesized via strong electrostatic adsorption using the widely available Co(NO3)2 as the precursor. We demonstrate that high-temperature pretreatment (900 °C) in an inert atmosphere can significantly enhance the initial activity of the Co/SiO2 catalyst. X-ray absorption near-edge spectroscopy (XANES), temperature-programmed reduction (TPR), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) suggest that highly dispersed Co(II) clusters are more active than Co0 or CoOx nanoparticles. Fourier transform infrared (FTIR) and isopropanol (IPA) temperature-programmed desorption and density functional theory (DFT) calculations suggest that high-temperature treatment significantly increases the density of active Lewis acid sites, possibly via surface dehydroxylation of the catalyst