35 Search Results

Here the catalytic C—H activation and borylation of arenes by trispyrazolylborate complexes is reported. Trispyrazolylborate rhodium and iridium complexes have been previously shown to activate a variety of C—H bonds. Here, we show the catalytic borylation of arenes by the trispyrazolylborate ethylene complexes Tp'Rh(C₂H₄)₂, and Tp'Ir(C₂H₄)₂ .

Water electrolysis is the key to a decarbonized energy system, as it enables the conversion and storage of renewably generated intermittent electricity in the form of hydrogen. However, reliability challenges arising from titanium-based porous transport layers (PTLs) have hitherto restricted the deployment of next-generation water-splitting devices. Here, it is shown for the first time how PTLs can be adapted so that their interface remains well protected and resistant to corrosion across ˜4000 h under real electrolysis conditions. It is also demonstrated that the malfunctioning of unprotected PTLs is a result triggered by additional fatal degradation mechanisms over the anodic catalystmore » layer beyond the impacts expected from iridium oxide stability. Now, superior durability and efficiency in water electrolyzers can be achieved over extended periods of operation with less-expensive PTLs with proper protection, which can be explained by the detailed reconstruction of the interface between the different elements, materials, layers, and components presented in this work.« less

We present ultralow Ir-loaded (ULL) proton exchange membrane water electrolyzer (PEMWE) cells that can produce enough hydrogen to largely decarbonize the global natural gas, transportation, and electrical storage sectors by 2050, using only half of the annual global Ir production for PEMWE deployment. This represents a significant improvement in PEMWE's global potential, enabled by careful control of the anode catalyst layer (CL), including its mesostructure and catalyst dispersion. Using commercially relevant membranes (Nafion 117), cell materials, electrocatalysts, and fabrication techniques, we achieve at peak a 250× improvement in Ir mass activity over commercial PEMWEs. An optimal Ir loading of 0.011more » mg_Ir cm^-2 operated at an Ir-specific power of ~100 MW kg_Ir^-1 at a cell potential of ~1.66 V versus RHE (85% higher heating value efficiency). Here, we further evaluate the performance limitations within the ULL regime and offer new insights and guidance in CL design relevant to the broader energy conversion field.« less

The production of olefins via on-purpose dehydrogenation of alkanes allows for a more efficient, selective and lower cost alternative to processes such as steam cracking. Silica-supported pincer-iridium complexes of the form [(≡SiO–^R4POCOP)Ir(CO)] (^R4POCOP=κ³-C₆H₃-2,6-(OPR₂)₂) are effective for acceptorless alkane dehydrogenation, and have been shown stable up to 300 °C. However, while solution-phase analogues of such species have demonstrated high regioselectivity for terminal olefin production under transfer dehydrogenation conditions at or below 240 °C, in open systems at 300 °C, regioselectivity under acceptorless dehydrogenation conditions is consistently low. In this work, complexes [(≡SiO–^tBu4POCOP)Ir(CO)] (1) and [(≡SiO–^iPr4PCP)Ir(CO)] (2) were synthesized via immobilization ofmore » molecular precursors. These complexes were used for gas-phase butane transfer dehydrogenation using increasingly sterically demanding olefins, resulting in observed selectivities of up to 77 %. Finally, the results indicate that the active site is conserved upon immobilization.« less

Pincer-ligated iridium complexes of the form [(^R₄PCP)IrL] (^R₄PCP = κ³-C₆H₃-2,6-(XPR₂)₂; X = CH₂, O; R = tBu, iPr) have previously been shown competent for acceptorless alkane dehydrogenation when supported on silica. It was observed by postcatalysis solid-state NMR that silica-tethered [(≡SiO-^tBu₄POCOP)Ir(C₂H₄)] (3-C₂H₄) was converted fully to [(≡SiO-^tBu₄POCOP)Ir(CO)] (3-CO) at 300 °C. In this work, the characterization of species under dehydrogenation reaction conditions far from equilibrium between butane and butenes (approach to equilibrium Q/K_eq = 0.3 at 300 °C) is performed with operando Diffuse Reflectance Infrared Fourier–Transform Spectroscopy (DRIFTS) to show the kinetics of species conversion from 3-C₂H₄ to 3-CO. Itmore » is further found that [(≡SiO-^tBu₄POCOP)IrHCl] (3-HCl), a species considered to be a precatalyst for alkane dehydrogenation, is also fully converted to 3-CO. Here, a mechanism of decomposition is proposed that implicates surface silanol groups, while carbon monoxide acts as a “stabilizer” for the catalyst by promoting their reductive elimination and maintaining the complex in the I oxidation state.« less

A trade-off between catalytic activity and structural stability generally exists in oxygen evolution electrocatalysis, especially in acidic environment. This dilemma limits the development of higher-performance electrocatalysts that are required by next-generation electrochemical technologies. In this work it is demonstrated that the inverse catalytic activity–structural stability relation can be broken by alloying catalytically inert strontium zirconate with the other catalytically active perovskite, strontium iridate. This strategy results in an alloyed perovskite electrocatalyst with simultaneously improved iridium mass activity and structural stability, by about five times, for the oxygen evolution reaction under acidic conditions. The experimental and theoretical results suggest that themore » alloying strategy generates multiple positive effects, mainly including the reduction of catalyst size, the decrease of catalyst covalency, and the weakening of surface oxygen-binding ability. The synergistic optimization of bulk and surface properties, as a result, enhances the intrinsic activity and availability of surface iridium sites, whilst significantly inhibiting the surface cation corrosion during electrocatalysis.« less

Using a newly designed and developed parallelized photoreactor and colorimetric detection method, a large sampling of bimetallic co-catalysts (Pd/Sn, Pd/Mo, Pd/Ru, Pd/Pb, Pd/Ni, Ni/Sn, Mo/Sn and Pt/Sn) for photocatalytic water reduction have been tested in this work. Of these co-catalysts, the combination of palladium and tin showed the highest synergistic behavior and peak hydrogen gas production at a low relative fraction of palladium. The resulting Pd/Sn bimetallic co-catalysts were characterized using high-angle annular dark-field (HAADF) and scanning transmission electron microscopy energy dispersive X-ray spectroscopy (STEM-EDS), and specifically, investigation of the palladium/tin mixture indicated that palladium and tin elements reside withinmore » the same particle. The experimental catalytic activity for the palladium/tin mixture was compared to density functional theory-derived energy values associated with the adsorption of hydrogen onto a surface. This comparison demonstrated that the typical peak found in electrochemical Sabatier volcano plots at ΔG_H* = ~0 eV was replicated in the experimental photocatalytic system with a peak activity observed at ΔG_H* = -0.036 eV. Computational confirmation of the results expressed here demonstrates the efficacy of colorimetric detection of hydrogen in parallel and presents a model for increasingly rapid catalyst screening.« less

We used low energy electron diffraction (LEED) and temperature programmed desorption (TPD) to investigate the structure and reactivity of iridium oxide layers prepared by oxidizing Ir(100) at 765 K and O₂ pressures ranging from 0.05 to 5 Torr. Our LEED results provide evidence that Ir(100) oxidation at O₂ pressures up to 0.20 Torr produces a mixture of Ir oxide structures present as small domains, including a commensurate IrO₂(101) structure that coexists with other structures. Oxidizing from 0.50 to 1 Torr causes formation of a commensurate IrO₂(110)R27° structure and a sharp rise in the oxygen uptake from ~8 to 20 MLmore » (monolayer) as the films exhibit signs of roughening. Further increasing the O₂ pressure from 1 to 5 Torr causes the IrO₂(110)R27° structure to be replaced with a so-called IrO₂(110)-aligned structure, for which the IrO₂(110) lattice vectors align with those of the Ir(100) substrate. We find that the oxidized Ir(100) surfaces become increasingly reactive toward the dissociation and oxidation of CH₄ as IrO₂(110) develops on the surface, and observe that the IrO₂(110)-aligned structure is more reactive than the IrO₂(110)R27° phase. Furthermore, our findings demonstrate that the oxide phase evolution on Ir(100) is sensitive to the O₂ pressure in the range from 0.05 to 5 Torr, and that the development of reactive IrO₂(110) structures requires elevated O₂ pressures (> 0.5 Torr) and temperature.« less

Atomically dispersed iridium complexes were anchored on a reduced graphene aerogel (rGA) by the reaction of Ir(CO)₂(acac) [acac = acetonylacetonato] with oxygen-containing groups on the rGA. Characterization by X-ray absorption, infrared, and X-ray photoelectron spectroscopies and atomic resolution aberration-corrected scanning transmission electron microscopy demonstrates atomically dispersed iridium, at the remarkably high loading of 14.8 wt %. The rGA support offers sites for metal bonding comparable to those of metal oxides, but with the advantages of high density and a relatively high degree of uniformity, as indicated by the same turnover frequencies for catalytic hydrogenation of ethylene at low and highmore » iridium loadings. We note the atomic dispersion at a high metal loading—and the high density of catalytic sites per unit of reactor volume, a key criterion for practical catalysts—set this catalyst apart from those reported.« less

When graphene is placed on a crystalline surface, the periodic structures within the layers superimpose and moiré superlattices form. Small lattice rotations between the two materials in contact strongly modify the moiré lattice parameter, upon which many electronic, vibrational, and chemical properties depend. While precise adjustment of the relative orientation in the degree- and sub-degree-range can be achieved via careful deterministic transfer of graphene, we report on the spontaneous reorientation of graphene on a metallic substrate, Ir(111). Here, we find that selecting a substrate temperature between 1530 and 1000 K during the growth of graphene leads to distinct relative rotationalmore » angles of 0°, ± 0.6°, ±1.1°, and ±1.7°. When modeling the moiré superlattices as two-dimensional coincidence networks, we can ascribe the observed rotations to favorable low-strain graphene structures. The dissimilar thermal expansion of the substrate and graphene is regarded as an effective compressive biaxial pressure that is more easily accommodated in graphene by small rotations rather than by compression.« less