A stable low-temperature H2-production catalyst by crowding Pt on α-MoC.

The water-gas shift (WGS) reaction is an industrially important source of pure hydrogen (H2) at the expense of carbon monoxide and water1,2. This reaction is of interest for fuel-cell applications, but requires WGS catalysts that are durable and highly active at low temperatures3. Here we demonstrate that the structure (Pt1-Ptn)/α-MoC, where isolated platinum atoms (Pt1) and subnanometre platinum clusters (Ptn) are stabilized on α-molybdenum carbide (α-MoC), catalyses the WGS reaction even at 313 kelvin, with a hydrogen-production pathway involving direct carbon monoxide dissociation identified. We find that it is critical to crowd the α-MoC surface with Pt1 and Ptn species, which prevents oxidation of the support that would cause catalyst deactivation, as seen with gold/α-MoC (ref. 4), and gives our system high stability and a high metal-normalized turnover number of 4,300,000 moles of hydrogen per mole of platinum. We anticipate that the strategy demonstrated here will be pivotal for the design of highly active and stable catalysts for effective activation of important molecules such as water and carbon monoxide for energy production.

Can the Rate of a Catalytic Turnover Be Altered by Ligands in the Absence of Direct Binding Interactions?

Second sphere coordination effects ubiquitous in enzymatic catalysis occur through direct interactions, either covalent or non-covalent, with reaction intermediates and transition states. We present herein evidence of indirect second sphere coordination effects in which ligation of water/alkanols far removed from the primary coordination sphere of the active site nevertheless alter energetic landscapes within catalytic redox cycles in the absence of direct physicochemical interactions with surface species mediating catalytic turnovers. Density functional theory, in situ X-ray absorption and infrared spectroscopy, and a wide array of steady-state and transient CO oxidation rate data suggest that the presence of peripheral water renders oxidation half-cycles within two-electron redox cycles over µ3-oxo-bridged trimers in MIL-100(M) more kinetically demanding. Communication between ligated water and the active site appears to occur through the Fe-O-Fe backbone, as inferred from spin density variations on the central µ3-oxygen 'junction'. Evidence is provided for the generality of these second sphere effects in that they influence different types of redox half-cycles or metals, and can be amplified or attenuated through choice of coordinating ligand. Specifically in the case of MIL-100(M) materials, the Cr isostructure can be made to kinetically mimic the Fe variant by disproportionately hindering oxidation half-cycles relative to the reduction half-cycles. Kinetic and spectroscopic inferences presented here significantly expand both the conceptual definition of second sphere effects as well as the palette of synthetic levers available for tuning catalytic redox performance through chemical ligation.

Silicon Nitride Surface Enabled Propane Dehydrogenation Catalyzed by Supported Organozirconium.

Mesoporous silicon nitride (Si3N4) is a nontraditional support for the chemisorption of organometallic complexes with the potential for enhancing catalytic activity through features such as the increased Lewis basicity of nitrogen for heterolytic bond activation, increased ligand donor strength, and metal-ligand orbital overlap. Here, tetrabenzyl zirconium (ZrBn4) was chemisorbed on Si3N4, and the resulting supported organometallic species was characterized by Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), Dynamic Nuclear Polarization-enhanced Solid State Nuclear Magnetic Resonance (DNP-SSNMR), and X-ray Absorption Spectroscopy (XAS). Based on the hypothesis that the nitride might enable facile heterolytic C-H bond activation along the Zr-N bond, this material was found to be a highly active (1.53 molpropene molZr-1 h-1 at 450 °C) and selective (99% to propylene) catalyst for propane dehydrogenation. In contrast, the homologous silica supported complex exhibited negligible activity under these conditions.

Dynamic Copper Site Redispersion through Atom Trapping in Zeolite Defects.

Single-site copper-based catalysts have shown remarkable activity and selectivity for a variety of reactions. However, deactivation by sintering in high-temperature reducing environments remains a challenge and often limits their use due to irreversible structural changes to the catalyst. Here, we report zeolite-based copper catalysts in which copper oxide agglomerates formed after reaction can be repeatedly redispersed back to single sites using an oxidative treatment in air at 550 °C. Under different environments, single-site copper in Cu-Zn-Y/deAlBeta undergoes dynamic changes in structure and oxidation state that can be tuned to promote the formation of key active sites while minimizing deactivation through Cu sintering. For example, single-site Cu2+ reduces to Cu1+ after catalyst pretreatment (270 °C, 101 kPa H2) and further to Cu0 nanoparticles under reaction conditions (270-350 °C, 7 kPa EtOH, 94 kPa H2) or accelerated aging (400-450 °C, 101 kPa H2). After regeneration at 550 °C in air, agglomerated CuO was dispersed back to single sites in the presence and absence of Zn and Y, which was verified by imaging, in situ spectroscopy, and catalytic rate measurements. Ab initio molecular dynamics simulations show that solvation of CuO monomers by water facilitates their transport through the zeolite pore, and condensation of the CuO monomer with a fully protonated silanol nest entraps copper and reforms the single-site structure. The capability of silanol nests to trap and stabilize copper single sites under oxidizing conditions could extend the use of single-site copper catalysts to a wider variety of reactions and allows for a simple regeneration strategy for copper single-site catalysts.

Silica Supported Organometallic IrI Complexes Enable Efficient Catalytic Methane Borylation.

Catalytic C-H borylation is an attractive method for the conversion of the most abundant hydrocarbon, methane (CH4), to a mild nucleophilic building block. However, existing CH4 borylation catalysts often suffer from low turnover numbers and conversions, which is hypothesized to result from inactive metal hydride agglomerates. Herein we report that the heterogenization of a bisphosphine molecular precatalyst, [(dmpe)Ir(cod)CH3], onto amorphous silica dramatically enhances its performance, yielding a catalyst that is 12-times more efficient than the current standard for CH4 borylation. The catalyst affords over 2000 turnovers at 150 °C in 16 h with a selectivity of 91.5% for mono- vs diborylation. Higher catalyst loadings improve yield and selectivity for the monoborylated product (H3CBpin) with 82.8% yield and >99% selectivity being achieved with 1255 turnovers. X-ray absorption and dynamic nuclear polarization-enhanced solid-state NMR spectroscopic studies identify the supported precatalyst as an IrI species, and indicate that upon completion of catalysis, multinuclear Ir polyhydrides are not formed. This is consistent with the hypothesis that immobilization of the organometallic Ir species on a surface prevents bimolecular decomposition pathways. Immobilization of the homogeneous IrI fragment onto amorphous silica represents a unique and simple strategy to improve the TON and longevity of a CH4 borylation catalyst.

Intermetallic alloy structure-activity descriptors derived from inelastic X-ray scattering.

Synchrotron spectroscopy and Density Functional Theory (DFT) are combined to develop a new descriptor for the stability of adsorbed chemical intermediates on metal alloy surfaces. This descriptor probes the separation of occupied and unoccupied d electron density in platinum and is related to shifts in Resonant Inelastic X-ray Scattering (RIXS) signals. Simulated and experimental spectroscopy are directly compared to show that the promoter metal identity controls the orbital shifts in platinum electronic structure. The associated RIXS features are correlated with the differences in the band centers of the occupied and unoccupied d bands, providing chemical intuition for the alloy ligand effect and providing a connection to traditional descriptions of chemisorption. The ready accessibility of this descriptor to both DFT calculations and experimental spectroscopy, and its connection to chemisorption, allow for deeper connections between theory and characterization in the discovery of new catalysts.

Dynamically Unveiling Metal-Nitrogen Coordination during Thermal Activation to Design High-Efficient Atomically Dispersed CoN4 Active Sites.

We elucidate the structural evolution of CoN4 sites during thermal activation by developing a zeolitic imidazolate framework (ZIF)-8-derived carbon host as an ideal model for Co2+ ion adsorption. Subsequent in situ X-ray absorption spectroscopy analysis can dynamically track the conversion from inactive Co-OH and Co-O species into active CoN4 sites. The critical transition occurs at 700 °C and becomes optimal at 900 °C, generating the highest intrinsic activity and four-electron selectivity for the oxygen reduction reaction (ORR). DFT calculations elucidate that the ORR is kinetically favored by the thermal-induced compressive strain of Co-N bonds in CoN4 active sites formed at 900 °C. Further, we developed a two-step (i.e., Co ion doping and adsorption) Co-N-C catalyst with increased CoN4 site density and optimized porosity for mass transport, and demonstrated its outstanding fuel cell performance and durability.

Evolution Pathway from Iron Compounds to Fe1(II)-N4 Sites through Gas-Phase Iron during Pyrolysis.

Pyrolysis is indispensable for synthesizing highly active Fe-N-C catalysts for the oxygen reduction reaction (ORR) in acid, but how Fe, N, and C precursors transform to ORR-active sites during pyrolysis remains unclear. This knowledge gap obscures the connections between the input precursors and the output products, clouding the pathway toward Fe-N-C catalyst improvement. Herein, we unravel the evolution pathway of precursors to ORR-active catalyst comprised exclusively of single-atom Fe1(II)-N4 sites via in-temperature X-ray absorption spectroscopy. The Fe precursor transforms to Fe oxides below 300 °C and then to tetrahedral Fe1(II)-O4 via a crystal-to-melt-like transformation below 600 °C. The Fe1(II)-O4 releases a single Fe atom that diffuses into the N-doped carbon defect forming Fe1(II)-N4 above 600 °C. This vapor-phase single Fe atom transport mechanism is verified by synthesizing Fe1(II)-N4 sites via "noncontact pyrolysis" wherein the Fe precursor is not in physical contact with the N and C precursors during pyrolysis.

Synergetic effect on catalytic activity and charge transfer in Pt-Pd bimetallic model catalysts prepared by atomic layer deposition.

Pt-Pd bimetallic nanoparticles were synthesized on TiO2 support on the planar substrate as well as on high surface area SiO2 gel by atomic layer deposition to identify the catalytic performance improvement after the formation of Pt-Pd bimetallic nanoparticles by surface analysis techniques. From X-ray absorption near edge spectra of Pt-Pd bimetallic nanoparticles, d-orbital hybridization between Pt 5d and Pd 4d was observed, which is responsible for charge transfer from Pt to Pd. Moreover, it was found from the in situ grazing incidence X-ray absorption spectroscopy study that Pt-Pd nanoparticles have a Pd shell/Pt core structure with CO adsorption. Resonant photoemission spectroscopy on Pt-Pd bimetallic nanoparticles showed that Pd resonant intensity is enhanced compared to that of Pd monometallic nanoparticles because of d-orbital hybridization and electronic states broadening of Pt and Pd compared monometallic catalysts, which results in catalytic performance improvement.

Evidence for Redox Mechanisms in Organometallic Chemisorption and Reactivity on Sulfated Metal Oxides.

The chemical and electronic interactions of organometallic species with metal oxide support materials are of fundamental importance for the development of new classes of catalytic materials. Chemisorption of Cp*(PMe3)IrMe2 on sulfated alumina (SA) and sulfated zirconia (SZ) led to an unexpected redox mechanism for deuteration of the ancillary Cp* ligand. Evidence for this oxidative mechanism was provided by studying the analogous homogeneous reactivity of the organometallic precursors toward trityl cation ([Ph3C]+), a Lewis acid known to effect formal hydride abstraction by one-electron oxidation followed by hydrogen abstraction. Organometallic deuterium incorporation was found to be correlated with surface sulfate concentration as well as the extent of dehydration under thermal activation conditions of SA and SZ supports. Surface sulfate concentration dependence, in conjunction with a computational study of surface electron affinity, indicates an electron-deficient pyrosulfate species as the redox-active moiety. These results provide further evidence for the ability of sulfated metal oxides to participate in redox chemistry not only toward organometallic complexes but also in the larger context of their application as catalysts for the transformation of light alkanes.

Insight into the Catalytic Mechanism of Bimetallic Platinum-Copper Core-Shell Nanostructures for Nonaqueous Oxygen Evolution Reactions.

The oxygen evolution reaction (OER) plays a critical role in multiple energy conversion and storage applications. However, its sluggish kinetics usually results in large voltage polarization and unnecessary energy loss. Therefore, designing efficient catalysts that could facilitate this process has become an emerging topic. Here, we present a unique Pt-Cu core-shell nanostructure for catalyzing the nonaqueous OER. The catalysts were systematically investigated with comprehensive spectroscopic techniques, and applied in nonaqueous Li-O2 electrochemical cells, which exhibited dramatically reduced charging overpotential (<0.2 V). The superior performance is explained by the robust Cu(I) surface sites stabilized by the Pt core in the nanostructure. The insights into the catalytic mechanism of the unique Pt-Cu core-shell nanostructure gained in this work are expected to serve as a guide for future design of other nanostructured bimetallic OER catalysts.

Concerted Growth and Ordering of Cobalt Nanorod Arrays as Revealed by Tandem in Situ SAXS-XAS Studies.

The molecular and ensemble dynamics for the growth of hierarchical supercrystals of cobalt nanorods have been studied by in situ tandem X-ray absorption spectroscopy-small-angle X-ray scattering (XAS-SAXS). The supercrystals were obtained by reducing a Co(II) precursor under H2 in the presence of a long-chain amine and a long-chain carboxylic acid. Complementary time-dependent ex situ TEM studies were also performed. The experimental data provide critical insights into the nanorod growth mechanism and unequivocal evidence for a concerted growth-organization process. Nanorod formation involves cobalt nucleation, a fast atom-by-atom anisotropic growth, and a slower oriented attachment process that continues well after cobalt reduction is complete. Smectic-like ordering of the nanorods appears very early in the process, as soon as nanoparticle elongation appears, and nanorod growth takes place inside organized superlattices, which can be regarded as mesocrystals.

Cu(II)-Cu(I) synergistic cooperation to lead the alkyne C-H activation.

An efficient alkyne C-H activation and homocoupling procedure has been studied which indicates that a Cu(II)/Cu(I) synergistic cooperation might be involved. In situ Raman spectroscopy was employed to study kinetic behavior, drawing the conclusion that Cu(I) rather than Cu(II) participates in the rate-determining step. IR, EPR, and X-ray absorption spectroscopy evidence were provided for structural information, indicating that Cu(I) has a stronger interaction with alkyne than Cu(II) in the C-H activation step. Kinetics study showed Cu(II) plays a role as oxidant in C-C bond construction step, which was a fast step in the reaction. X-band EPR spectroscopy showed that the coordination environment of CuCl2(TMEDA) was affected by Cu(I). A putative mechanism with Cu(I)-Cu(II) synergistic cooperation procedure is proposed for the reaction.

Atomic layer deposition overcoating: tuning catalyst selectivity for biomass conversion.

The terraces, edges, and facets of nanoparticles are all active sites for heterogeneous catalysis. These different active sites may cause the formation of various products during the catalytic reaction. Here we report that the step sites of Pd nanoparticles (NPs) can be covered precisely by the atomic layer deposition (ALD) method, whereas the terrace sites remain as active component for the hydrogenation of furfural. Increasing the thickness of the ALD-generated overcoats restricts the adsorption of furfural onto the step sites of Pd NPs and increases the selectivity to furan. Furan selectivities and furfural conversions are linearly correlated for samples with or without an overcoating, though the slopes differ. The ALD technique can tune the selectivity of furfural hydrogenation over Pd NPs and has improved our understanding of the reaction mechanism. The above conclusions are further supported by density functional theory (DFT) calculations.

Structural and reactive evolution of oxidatively grafted Pd catalysts on MnO2 for the low-temperature oxidation of CO.

Isolated Pd atoms supported on high surface area MnO2, prepared by the oxidative grafting of (bis(tricyclohexylphosphine-palladium(0)), catalyze (>50 turnovers, 17 h) the low temperature (≤325 K) oxidation of CO (7.7 kPa O2, 2.6 kPa CO) with results of in situ/operando and ex situ spectroscopic characterization signifying a synergistic role of Pd and MnO2 in facilitating redox turnovers.

CO2 Hydrogenation: Na Doping Promotes CO and Hydrocarbon Formation over Ru/m-ZrO2 at Elevated Pressures in Gas Phase Media.

Sodium-promoted monoclinic zirconia supported ruthenium catalysts were tested for CO2 hydrogenation at 20 bar and a H2:CO2 ratio of 3:1. Although increasing sodium promotion, from 2.5% to 5% by weight, slightly decreased CO2 conversion (14% to 10%), it doubled the selectivity to both CO (~36% to ~71%) and chain growth products (~4% to ~8%) remarkably and reduced the methane selectivity by two-thirds (~60% to ~21%). For CO2 hydrogenation during in situ DRIFTS under atmospheric pressure, it was revealed that Na increases the catalyst basicity and suppresses the reactivity of Ru sites. Higher basicity facilitates CO2 adsorption, weakens the C-H bond of the formate intermediate promoting CO formation, and inhibits methanation occurring on ruthenium nanoparticle surfaces. The suppression of excessive hydrogenation increases the chain growth probability. Decelerated reduction during H2-TPR/TPR-MS and H2-TPR-EXAFS/XANES at the K-edge of ruthenium indicates that sodium is in contact with ruthenium. A comparison of the XANES spectra of unpromoted and Na-promoted catalysts after H2 reduction showed no evidence of a promoting effect involving electron charge transfer.

La- and Mn-doped cobalt spinel oxygen evolution catalyst for proton exchange membrane electrolysis.

Discovery of earth-abundant electrocatalysts to replace iridium for the oxygen evolution reaction (OER) in a proton exchange membrane water electrolyzer (PEMWE) represents a critical step in reducing the cost for green hydrogen production. We report a nanofibrous cobalt spinel catalyst codoped with lanthanum (La) and manganese (Mn) prepared from a zeolitic imidazolate framework embedded in electrospun polymer fiber. The catalyst demonstrated a low overpotential of 353 millivolts at 10 milliamperes per square centimeter and a low degradation for OER over 360 hours in acidic electrolyte. A PEMWE containing this catalyst at the anode demonstrated a current density of 2000 milliamperes per square centimeter at 2.47 volts (Nafion 115 membrane) or 4000 milliamperes per square centimeter at 3.00 volts (Nafion 212 membrane) and low degradation in an accelerated stress test.

Oxidative Grafting for Catalyst Synthesis in Surface Organometallic Chemistry.

The development of new methods of catalyst synthesis with the potential to generate active site structures orthogonal to those accessible by traditional protocols is of great importance for discovering new materials for addressing challenges in the evolving energy and chemical economy. In this work, the generality of oxidative grafting of organometallic and well-defined molecular metal precursors onto redox-active surfaces such as manganese dioxide (MnO2) and lithium manganese oxide (LiMn2O4) is investigated. Nine molecular metal precursors are explored, spanning groups 4-11 and each of the three periods of the transition metal series. The byproducts of the oxidative grafting reaction, a mixture of protodemetalation and ligand homocoupling for several organometallic precursors, was found to provide insights into the mechanism of the grafting reaction, suggesting oxidation of both the metal d-orbitals, as well as the metal-carbon σ-bonds, resulting in ejection of the ligand radical fragment. Analysis of the supported structures and oxidation state by X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) suggests that several of the chemisorbed metal ions are intercalated into interstitial vacancies of the surface structure while other complexes form intact molecular fragments on the surface. Proof of concept for the use of this metalation protocol to generate diverse, metal-dependent catalytic performance is demonstrated by the application of these materials in the conversion of cyclohexane to K/A oil (cyclohexanol and cyclohexanone) with tert-butyl hydroperoxide, as well as in the low-temperature (T ≤ 50 °C) oxidation of carbon monoxide to carbon dioxide.

Bypassing Formation of Oxide Intermediate via Chemical Vapor Deposition for the Synthesis of an Mn-N-C Catalyst with Improved ORR Activity.

A significant barrier to the commercialization of proton exchange membrane fuel cells (PEMFCs) is the high cost of the platinum-based oxygen reduction reaction (ORR) cathode electrocatalysts. One viable solution is to replace platinum with a platinum-group metal (PGM) free catalyst with comparable activity and durability. However, PGM-free catalyst development is burdened by a lack of understanding of the active site formation mechanism during the requisite high-temperature synthesis step, thus making rational catalyst design challenging. Herein we demonstrate in-temperature X-ray absorption spectroscopy (XAS) to unravel the mechanism of site evolution during pyrolysis for a manganese-based catalyst. We show the transformation from an initial state of manganese oxides (MnOx) at room temperature, to the emergence of manganese-nitrogen (MnN4) site beginning at 750 °C, with its continued evolution up to the maximum temperature of 1000 °C. The competition between the MnOx and MnN4 is identified as the primary factor governing the formation of MnN4 sites during pyrolysis. This knowledge led us to use a chemical vapor deposition (CVD) method to produce MnN4 sites to bypass the evolution route involving the MnOx intermediates. The Mn-N-C catalyst synthesized via CVD shows improved ORR activity over the Mn-N-C synthesized via traditional synthesis by the pyrolysis of a mixture of Mn, N, and C precursors.

In situ anomalous small-angle X-ray scattering studies of platinum nanoparticle fuel cell electrocatalyst degradation.

Polymer electrolyte fuel cells (PEFCs) are a promising high-efficiency energy conversion technology, but their cost-effective implementation, especially for automotive power, has been hindered by degradation of the electrochemically active surface area (ECA) of the Pt nanoparticle electrocatalysts. While numerous studies using ex situ post-mortem techniques have provided insight into the effect of operating conditions on ECA loss, the governing mechanisms and underlying processes are not fully understood. Toward the goal of elucidating the electrocatalyst degradation mechanisms, we have followed Pt nanoparticle growth during potential cycling of the electrocatalyst in an aqueous acidic environment using in situ anomalous small-angle X-ray scattering (ASAXS). ASAXS patterns were analyzed to obtain particle size distributions (PSDs) of the Pt nanoparticle electrocatalysts at periodic intervals during the potential cycling. Oxide coverages reached under the applied potential cycling protocols were both calculated and determined experimentally. Changes in the PSD, mean diameter, and geometric surface area identify the mechanism behind Pt nanoparticle coarsening in an aqueous environment. Over the first 80 potential cycles, the dominant Pt surface area loss mechanism when cycling to 1.0-1.1 V was found to be preferential dissolution or loss of the smallest particles with varying extents of reprecipitation of the dissolved species onto existing particles, resulting in particle growth, depending on potential profile. Correlation of ASAXS-determined particle growth with both calculated and voltammetrically determined oxide coverages demonstrates that the oxide coverage is playing a key role in the dissolution process and in the corresponding growth of the mean Pt nanoparticle size and loss of ECA. This understanding potentially reduces the complex changes in PSD and ECA resulting from various voltage profiles to a response dependent on oxide coverage.