Intermediates In Catalytic Processes Research Articles

S heteroatom doping in highly porous carbonaceous spheres for boosted oxygen reduction reaction of atomically dispersed Fe–N4 active sites

Tuning of the coordination environment of atomically dispersed single atoms (SAs) by heteroatom doping is a promising strategy to boost the electrocatalytic activity for oxygen reduction reaction (ORR). The synergistic effect of dual heteroatom doping provides additional degrees of freedom to achieve higher ORR efficiency through decreased energy barriers of the intermediates in catalytic processes. Herein, Fe–SAs anchored within the N, S co-doped porous nanosized carbonaceous spheres (Fe–SAs@N/S-PCSs) was fabricated through the pyrolysis of anchored heteroatom sources and ferric precursors. Under the low catalyst loading capacity of 0.255 mg cm−2, the obtained Fe–SAs@N/S-PCSs exhibits exceptionally high performance with ORR onset potential (Eonset) of 1.02 V, and half wave potential (E1/2) of 0.90 V (vs. RHE) in 0.1 M KOH solution, outperforming benchmark Pt/C (Eonset = 1.01 V, E1/2 = 0.86 V). Superior oxygen evolution reaction (OER) activity (450 mV at 10 mA cm−2) is also demonstrated, thus displaying outstanding performance for Zn-air battery. The porous structure, rich accessible Fe SAs, and the optimal binding of ORR-related species induced by S doping in carbon matrix generate the excellent ORR activity and durability. This work provides insights into the rational design of cost-effective single atom catalysts (SACs) with impressive electrocatalytic activities for efficient energy conversion applications.

Electron Paramagnetic Resonance Signature of Tetragonal Low Spin Iron(V)-Nitrido and -Oxo Complexes Derived from the Electronic Structure Analysis of Heme and Non-Heme Archetypes.

Iron(V)-nitrido and -oxo complexes have been proposed as key intermediates in a diverse array of chemical transformations. Herein we present a detailed electronic-structure analysis of [FeV(N)(TPP)] (1, TPP2– = tetraphenylporphyrinato), and [FeV(N)(cyclam-ac)]+ (2, cyclam-ac = 1,4,8,11-tetraazacyclotetradecane-1-acetato) using electron paramagnetic resonance (EPR) and 57Fe Mössbauer spectroscopy coupled with wave function based complete active-space self-consistent field (CASSCF) calculations. The findings were compared with all other well-characterized genuine iron(V)-nitrido and -oxo complexes, [FeV(N)(MePy2tacn)](PF6)2 (3, MePy2tacn = methyl-N′,N″-bis(2-picolyl)-1,4,7-triazacyclononane), [FeV(N){PhB(t-BuIm)3}]+ (4, PhB(tBuIm)3– = phenyltris(3-tert-butylimidazol-2-ylidene)borate), and [FeV(O)(TAML)]− (5, TAML4– = tetraamido macrocyclic ligand). Our results revealed that complex 1 is an authenticated iron(V)-nitrido species and contrasts with its oxo congener, compound I, which contains a ferryl unit interacting with a porphyrin radical. More importantly, tetragonal iron(V)-nitrido and -oxo complexes 1–3 and 5 all possess an orbitally nearly doubly degenerate S = 1/2 ground state. Consequently, analogous near-axial EPR spectra with g|| < g⊥ ≤ 2 were measured for them, and their g|| and g⊥ values were found to obey a simple relation of g⊥2 + (2 – g∥)2 = 4. However, the bonding situation for trigonal iron(V)-nitrido complex 4 is completely different as evidenced by its distinct EPR spectrum with g|| < 2 < g⊥. Further in-depth analyses suggested that tetragonal low spin iron(V)-nitrido and -oxo complexes feature electronic structures akin to those found for complexes 1–3 and 5. Therefore, the characteristic EPR signals determined for 1–3 and 5 can be used as a spectroscopic marker to identify such highly reactive intermediates in catalytic processes.

The effect of oxygen content of carbon nanotubes on the catalytic activity of carbon-based iron phthalocyanine for oxygen reduction reaction

The development of cost-effective non-precious metal catalysts has attracted much research attention because of their high oxygen reduction reaction (ORR) performance. Herein, based on three kinds of carbon nanotubes (CNTs) with different oxygen content, we prepare three CNTs-based iron phthalocyanine (FePc) composites via only a simple physical method. In surprise, the FePc-CNTs composites with the most oxygen content exhibit the highest ORR catalytic activity in comparison with commercial Pt/C in alkaline media, especially its half-wave potential (E1/2) and durability as well as methanol tolerance. A large amount of experimental measurements show that these superb catalytic activities can be mainly attributed to the possible formation of novel interactions between FePc and oxidized CNTs. It not only provides more active sites, but also is in favor of stabilizing active centers and removing intermediates in catalytic process. This work displays the effects of oxygen content and roughness of CNTs on the interactions between FePc and CNTs and their important role in ORR catalysis, providing a new insight into simple application of carbons materials.

Structure and Reactivity of a Model Oxide Supported Silver Nanocluster Catalyst Studied by Near Ambient Pressure X-ray Photoelectron Spectroscopy

The photocatalytic activity of anatase TiO2 decorated with metal clusters has been widely documented, but the nature of the metal–metal oxide interface and reaction intermediates in catalytic processes are still not well understood. This in part is due to the fact that use of photoelectron spectroscopy to deduce the surface chemistry of catalytic systems has long been hampered by the huge pressure difference between real-world operation and the requirement of high vacuum for electron detection. Here, the in situ growth of silver nanoparticles on a model metal-oxide catalyst support and their reactivity with a CO/H2O gas mixture has been investigated in detail. Using synchrotron X-ray photoelectron spectroscopy, near-ambient pressure X-ray photoelectron spectroscopy, and scanning tunneling microscopy, the interaction of Ag with the anatase TiO2 surface leads to metal-surface charge transfer and low mobility of Ag on the surface. Upon exposure to a 1.5 mbar CO/1.5 mbar H2O gas mixture, partial oxidation of ...

Operando X-Ray Absorption Spectroscopy Measurement Studies on Oxygen Reduction Reaction of Ionic-Liquid-Encapsulated Pd/Pt Core Shell Catalysts

For the widespread use of Polymer Electrolyte Fuel Cell (PEFC), it is needed to reduce the amount of platinum used for electrode catalyst materials from the viewpoint of improving oxygen reduction reaction (ORR) activity of platinum. Recently, it is reported by Erlebacher1), 2) that the ORR activity of nanoporous NiPt alloy catalysts encapsulated by ionic liquid (IL) is higher than that of this material not encapsulated by. Based on the report3) that promoting the oxygen solubility at the interface between catalysts and electrolyte improves the ORR activity, this result implies that the high oxygen solubility of IL mainly contributes to high ORR activity. However, the reason of improvement of the ORR activity has not been clarified yet. In this study, we prepared Pd/Pt core shell catalyst, which is one of the most notable nanomaterials showing substantially high ORR activity, and encapsulated the surface of this catalyst with IL. Then we studied on the ORR of IL-encapsulated Pd/Pt core shell nanoparticles (NPs) by operando X-ray absorption spectroscopy (XAS) method. Pd/C (particle diameter of Pd: 3.6 nm) was applied in this study. Cu monolayer has been deposited on Pd NPs using under potential deposition (UPD) of Cu in 10 mM CuSO4 solution. A single UPD Cu monolayer replacement with Pt in K2PtCl4 solution formed Pt monolayer film on Pd NPs (Pt/Pd/C). The surface of Pt/Pd/C was coated with [MTBD][NTf2] 2), 3) (IL). Convection voltammetry was performed at 25 ˚C in O2-saturated 0.1 M HClO4 aq and H2SO4 aq using rotating disc electrode (RDE). RDE was rotated at 300, 500, 700, 900, 1200, 1600, and 2500 rpm. Electrochemical activity value at 0.9 V vs. Reversible Hydrogen Electrode (RHE) was calculated from Koutecky-Levich plot. Operando XAS measurements for Pt L Ⅲ -edge and L Ⅱ -edge of Pt/Pd/C catalysts were carried out by using the beamline BL14B2 in SPring-8 (Japan). These measurements were performed by fluorescence method in O2-saturated 0.1 M HClO4 aq and H2SO4 aq. Potentials were maintained at 0.5, 0.85, and 1.15 V vs. RHE. Specific activity value of Pt/Pd/C in 0.1 M H2SO4 aq was much lower than that of Pt/Pd/C in 0.1 M HClO4 aq. Pt-Pt bond distances were calculated by analyzing EXAFS spectra of Pt L Ⅲ -edge. At each potentials, there are no significant differences between Pt-Pt bond distances of Pt/Pd/C in 0.1 M HClO4 aq and H2SO4 aq. It implied that the irreversible specific adsorption of SO4 2- on the surface of Pd/Pt NPs decreased the number of active site of catalysts. The specific activity value of IL-encapsulated Pt/Pd/C was 1.31 times higher than that of non-encapsulated Pt/Pd/C in 0.1 M HClO4 aq. Pt-Pt bond distance estimated by EXAFS spectra of Pt L Ⅲ -edge of IL-encapsulated Pt/Pd/C was definitely longer than that of non-encapsulated Pt/Pd/C. These results suggested that coating with IL on the surface of Pt/Pd/C have lattice strain. The quantitative lattice strain establishes a direct correlation to monolayer Pt shell ORR activity and the strain effect induced d-band shift regulates the adsorption properties of rate-limiting intermediates in catalytic processes.4)

PtZnH5(-), A σ-Aromatic Cluster.

We report a joint photoelectron spectroscopic and theoretical study of the PtZnH5(-) cluster anion. This cluster exhibited an unprecedented planar pentagonal coordination for Pt and an unusual stability and high intensity in the mass spectrum. Both are due to the σ-aromaticity found in the H5-cycle supported by the 5d orbitals on the Pt atom. σ-Aromaticity in all-H systems has been predicted in the past but never found in experimentally observed species. Besides fundamental importance, mixed transition-metal hydrides can be found as intermediates in catalytic processes, and thus, the unexpected stability facilitated by σ-aromaticity can be appreciated also in practical applications.