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At least 1,171 records · Page 65
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Effect of the Molecular Structure of Surface Vanadia on Activity and Regenerability of VO $x$ /In 2 O 3 Catalysts for CO 2 -Assisted Oxidative Dehydrogenation of Propane

Our recent work has reported that higher propylene selectivity and improved stability can be achieved by combining redox-active VO $x$ and basic In 2 O 3 for CO 2 -assisted oxidative dehydrogenation of propane (CO 2 -ODHP). In the present work, we continued to explore the stability and regenerability of V/In catalysts. In particular, our interest lies in identifying the effect of mono- and polyvanadate on catalytic performance and regenerability. A V/In catalyst with an increased proportion of monovanadate was prepared using the Schlenk line under moisture-free conditions (V/In–S), while the fully polymerized vanadate catalyst was prepared through a regular impregnation (V/In) for comparison. The Schlenk-line-prepared catalyst, namely, V/In–S, not only exhibits a 17–30% enhanced propylene yield at high temperatures (500–540 °C) over V/In but also presents improved stability and regenerability with nearly 88% activity recovered after regeneration in O 2 . Detailed characterizations have been performed to reveal the catalyst structure–performance relationship, including chemisorption (NH 3 /CO 2 -temperature-programmed desorption, NH 3 /CO 2 -TPD), H 2 -temperature-programmed reduction (H 2 -TPR), and spectroscopic studies [Raman spectroscopy, UV–vis diffuse reflectance spectroscopy (UV–vis DRS), near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS), and high-sensitivity low-energy ion scattering (HS-LEIS)]. Characterization results demonstrate that compared with polyvanadates, monovanadates lead to strengthened interaction with In 2 O 3 and a more stabilized V/In surface and subsurface, as well as improved redox properties of VO $x$ . These advantages give rise to the observed enhancement in activity, stability, and regenerability. In conclusion, these findings advance the understanding of the relationship between the activity/stability and the molecular structure of surface oxide species (vanadia) and the interplay between acid–base interactions and redox properties of mixed metal-oxide catalysts for efficient CO 2 -ODHP.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Role of Catalyst Preparation on the Electrocatalytic Activity of Ni 1– x Fe x OOH for the Oxygen Evolution Reaction

Ni 1– x Fe x OOH thin films prepared via cathodic electrodeposition have been demonstrated to be highly active catalysts for the oxygen evolution reaction (OER) in basic media. Integration of these catalysts with light-absorbing semiconductors is required for photoelectrochemical fuel generation. However, the application of cathodic potentials required for typical electrochemical catalyst deposition limits the library of compatible photoanode materials. Sputter deposition of catalysts circumvents this limitation by enabling facile catalyst layering without cathodic potentials. In this work, we compare the structure and OER activity of sputter-deposited and electrodeposited Ni 1– x Fe x OOH thin films. Electrochemical cycling converts sputtered Ni 1-x Fe x metallic films to the desired oxides/(oxy)hydroxides. Both film preparation methods give catalysts with similar electrochemical behavior across all compositions. Additionally, OER activity is comparable between the deposition methods, with maximum activity for films with ~20% Fe content (320 mV overpotential at j = 10 mA cm -2 geometric). Electrochemical cycling to convert sputtered metallic Ni 1-x Fe x films to metal oxides/(oxy)hydroxides is found to lower the Fe/Ni ratio, while the electrodeposited films exhibit comparable Fe/Ni ratios before and after electrochemical cycling and characterization. Structurally, Fe is found to incorporate within the Ni(OH) 2 /NiOOH lattice for films formed through both sputter-deposition and electrodeposition. Layered films were also compared to codeposited 1:1 Fe/Ni films. It is found that, for layered films, an Fe top layer inhibits the electrochemical conversion of metallic Ni to Ni(OH) 2 /NiOOH, thus reducing the amount of Ni 1– x Fe x OOH OER-active phase formed. In contrast, migration of metals within Ni-on-top films occurs readily during electrochemical cycling, resulting in films that are structurally and electrochemically indistinguishable from codeposited Ni 1– x Fe x OOH. These findings enable direct application of Ni 1– x Fe x OOH sputtered films to a wider library of photoanodes for light-driven water-splitting applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Deep Understanding of Strong Metal Interface Confinement: A Journey of Pd/FeO x Catalysts

Tuning the atomic interface configuration of noble metals (NMs) and transition-metal oxides is an effective straightforward yet challenging strategy to modulate the activity and stability of heterogeneous catalysts. Herein, Pd supported on mesoporous Fe 2 O 3 with a high specific surface area was rationally designed and chosen to construct the Pd/iron oxide interface. As a versatile model, the physicochemical environments of Pd nanoparticles (NPs) could be precisely controlled by taming the reduction temperature. The experimental and density functional theory calculation results unveiled that the catalyst in the support–metal interface confinement (SMIC) state showed significantly enhanced catalytic activity and sintering resistance for CO oxidation. The constructed Fe sites at the interfaces between FeO x overlayers and Pd NPs not only provided additional coordinative unsaturated ferrous sites for the adsorption and activation of O 2 , thereby facilitating the activation efficiency of O 2 , but also impressively changed the reaction pathway of CO oxidation. As a result, the catalyst followed the Pd/Fe dual-site mechanism instead of the classical Mars–van Krevelen mechanism. For the catalyst in the strong metal–support interaction (SMSI) state, its catalytic activity was seriously suppressed because of the excessive encapsulation of the active Pd sites by FeO x overlayers. Thus, the present study therefore provides detailed insights into the SMIC and SMSI in ferric oxide-supported Pd catalysts, which could guide the preparation of highly efficient supported catalysts for practical applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Hydrogenation of CO 2 to Methanol on a Au δ+ –In 2 O 3–x Catalyst

CO 2 hydrogenation to methanol has attracted increasing attention with the development of renewable hydrogen. A big challenge is to identify catalysts able to achieve high conversion and selectivity. In this work, we report an In 2 O 3 -supported Au catalyst that exhibits excellent performance for hydrogenation of CO 2 selectively to methanol. In situ characterizations using time-resolved X-ray diffraction, ambient-pressure X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy confirm that a strong metal–support interaction leads to a reactive Au δ+ –In 2 O 3– x interface for activation and hydrogenation of CO 2 to methanol. An effective gold-indium oxide bonding favors the dispersion of the noble metal and prevents its sintering under reaction conditions. The methanol selectivity reaches 100% at temperatures below 225 °C and is more than 70% at 275 °C over the Au δ+ –In 2 O 3– x catalyst. It is even 67.8% with a space time yield of methanol of 0.47 g MeOH /(h·g cat ) at 300 °C, 5 MPa, and 21,000 cm 3 h –1 g cat –1 . The results obtained here represent the highest selectivity and activity ever reported for CO 2 hydrogenation over supported gold catalysts. Our study shows that the strong Au/In 2 O 3 interaction and the intrinsic chemical activity of In 2 O 3 can be used to significantly improve the catalytic performance of Au catalysts, providing promising routes for the rational design and application of Au catalysts beyond CO 2 hydrogenation.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Dynamics of CrO 3 –Fe 2 O 3 Catalysts during the High-Temperature Water-Gas Shift Reaction: Molecular Structures and Reactivity

A series of supported CrO 3 /Fe 2 O 3 catalysts were investigated for the high-temperature water-gas shift (WGS) and reverse-WGS reactions and extensively characterized using in situ and operando IR, Raman, and XAS spectroscopy during the high-temperature WGS/RWGS reactions. The in situ spectroscopy examinations reveal that the initial oxidized catalysts contain surface dioxo (O=) 2 Cr 6+ O 2 species and a bulk Fe 2 O 3 phase containing some Cr 3+ substituted into the iron oxide bulk lattice. Operando spectroscopy studies during the high-temperature WGS/RWGS reactions show that the catalyst transforms during the reaction. The crystalline Fe 2 O 3 bulk phase becomes Fe 3 O 4 ,and surface dioxo (O=) 2 Cr 6+ O 2 species are reduced and mostly dissolve into the iron oxide bulk lattice. Consequently, the chromium–iron oxide catalyst surface is dominated by FeO x sites, but some minor reduced surface chromia sites are also retained. The Fe 3–-x Cr x O 4 solid solution stabilizes the iron oxide phase from reducing to metallic Fe0 and imparts an enhanced surface area to the catalyst. Isotopic exchange studies with C 16 O 2 /H 2 → C 18 O 2 /H 2 isotopic switch directly show that the RWGS reaction proceeds via the redox mechanism and only O* sites from the surface region of the chromium–iron oxide catalysts are involved in the RWGS reaction. The number of redox O* sites was quantitatively determined with the isotope exchange measurements under appropriate WGS conditions and demonstrated that previous methods have undercounted the number of sites by nearly 1 order of magnitude. The TOF values suggest that only the redox O* sites affiliated with iron oxide are catalytic active sites for WGS/RWGS, though a carbonate oxygen exchange mechanism was demonstrated to exist, and that chromia is only a textural promoter that increases the number of catalytic active sites without any chemical promotion effect.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Ultra-Low Amounts of Transition Metals in Carbon-Based Catalysts Improve Their Alkaline OER Performance: A Systematic Study

The pursuit of green hydrogen production highlights a persistent gap in electrocatalyst research: while academic efforts prioritize cost-efficiency via activity enhancement, industrial viability demands greater emphasis on electrochemical stability. Carbon-based electrocatalysts, particularly those incorporating transition metals, have shown promise in alkaline oxygen evolution (OER) due to their high activity, cost-efficiency, and resource-efficiency. However, these catalysts suffer from insufficient stability under oxidizing conditions compared to pure transition metal catalysts due to erosion of the carbon support resulting from carbon corrosion, among other degradation mechanisms. In this systematic study, the influence of ultra-low amounts (<1 wt %) of iron, cobalt, nickel, and their most common combinations on the stability of a hydrothermally derived, N-doped carbon support and the overall catalyst performance during alkaline OER is systematically explored. By identifying critical stability descriptors and correlating them with synthesis conditions and catalyst properties, primary and secondary corrosion pathways are unraveled. Subsequently, through careful adjustment of carbonization temperature and composition of incorporated transition metals, overall catalyst corrosion can be suppressed immensely. Especially, the inclusion of Ni and Fe is paramount for the formation of stable catalyst materials under laboratory conditions (10 mA/cm 2 in 0.1 M KOH), which is surprisingly unconstrained by the degree of graphitization of the carbon support. Mixing this electrochemically stable material with a graphitic carbon powder results in an excellent stability of over 400 h at 100 mA/cm 2 in 1 M KOH, implying great potential for the future improvement of carbon-based electrodes under oxidizing conditions toward industrial application.

N-doped carbon↗
DOE OSTI2018

In Situ Elucidation of the Active State of Co–CeO x Catalysts in the Dry Reforming of Methane: The Important Role of the Reducible Oxide Support and Interactions with Cobalt

The activation of methane and its dry reforming with CO 2 was systematically studied over a series (2–30 wt %) of Co (~5 nm in size) loaded CeO 2 catalysts, with an effort to elucidate the interplay between Co and CeO 2 during the catalytic process using in situ methods. The results of in situ time-resolved X-ray diffraction (TR-XRD) show a strong interaction of methane with the CoOx–CeO 2 systems at temperatures between 200 and 350 °C. The hydrogen produced by the dissociation of C–H bonds in methane leads to a full reduction of Co oxide, Co 3 O 4 → CoO → Co, and a partial reduction of ceria with the formation of some Ce 3+ . Upon the addition of CO 2 , a catalytic cycle for dry reforming of methane (DRM) was achieved on the CoOx–CeO 2 powder catalysts at temperatures below 500 °C. A 10 wt % Co–CeO 2 catalyst was found to possess the best catalytic activity among various cobalt loading catalysts, and it exhibits a desirable stability for the DRM with a minimal effect of carbon accumulation. The phase transitions and the nature of active components in the catalyst were investigated under reaction conditions by in situ time-resolved XRD and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). These studies showed dynamic evolutions in the chemical composition of the catalysts under reaction conditions. CO 2 attenuated the reducing effects of methane. Under optimum CO- and H 2 -producing conditions, both XRD and AP-XPS indicated that the active phase involved a majority of metallic Co with a small amount of CoO, both supported on a partially reduced ceria (Ce 3+ /Ce 4+ ). Finally, we identified the importance of dispersing Co, anchoring it onto the ceria surface sites, and then utilizing the redox properties of CeO 2 for activating and then oxidatively converting methane while inhibiting coke formation. Furthermore, a synergistic effect between cobalt and ceria and likely the interfacial sitee are essential to successfully close the catalytic cycle.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Revealing the Synergy between Oxide and Alloy Phases on the Performance of Bimetallic In–Pd Catalysts for CO 2 Hydrogenation to Methanol

In 2 O 3 has recently emerged as a promising catalyst for methanol synthesis from CO2. In this work, we present the promotional effect of Pd on this catalyst and investigate structure–performance relationships using in situ X-ray spectroscopy, ex situ characterization, and microkinetic modeling. Catalysts were synthesized with varying In:Pd ratios (1:0, 2:1, 1:1, 1:2, 0:1) and tested for methanol synthesis from CO 2 /H 2 at 40 bar and 300 °C. In:Pd(2:1)/SiO 2 shows the highest activity (5.1 μmol MeOH/g InPd s) and selectivity toward methanol (61%). While all bimetallic catalysts had enhanced catalytic performance, characterization reveals methanol synthesis was maximized when the catalyst contained both In–Pd intermetallic compounds and an indium oxide phase. Experimental results and density functional theory suggest the active phase arises from a synergy between the indium oxide phase and a bimetallic In–Pd particle with a surface enrichment of indium. We show that the promotion observed in the In–Pd system is extendable to non precious metal containing binary systems, in particular In–Ni, which displayed similar composition–activity trends to the In–Pd system. As a result, both palladium and nickel were found to form bimetallic catalysts with enhanced methanol activity and selectivity relative to that of indium oxide.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

End-On Bound Iridium Dinuclear Heterogeneous Catalysts on WO 3 for Solar Water Oxidation

Heterogeneous catalysts with atomically defined active centers hold great promise for high-performance applications. Among them, catalysts featuring active moieties with more than one metal atom are important for chemical reactions that require synergistic effects but are rarer than single atom catalysts (SACs). The difficulty in synthesizing such catalysts has been a key challenge. Recent progress in preparing dinuclear heterogeneous catalysts (DHCs) from homogeneous molecular precursors has provided an effective route to address this challenge. Nevertheless, only side-on bound DHCs, where both metal atoms are affixed to the supporting substrate, have been reported. The competing end-on binding mode, where only one metal atom is attached to the substrate and the other metal atom is dangling, has been missing. Here, we report the first observation that end-on binding is indeed possible for Ir DHCs supported on WO 3 . Unambiguous evidence supporting the binding mode was obtained by in situ diffuse reflectance infrared Fourier transform spectroscopy and high-angle annular dark-field scanning transmission electron microscopy. Density functional theory calculations provide additional support for the binding mode, as well as insights into how end-on bound DHCs may be beneficial for solar water oxidation reactions. The results have important implications for future studies of highly effective heterogeneous catalysts for complex chemical reactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Late-Transition-Metal-Modified β-Mo 2 C Catalysts for Enhanced Hydrogenation during Guaiacol Deoxygenation

Molybdenum carbide has been identified as a promising bifunctional catalyst in the deoxygenation of a variety of pyrolysis vapor model compounds. Although high deoxygenation activity has been demonstrated, complementary hydrogenation activity has been limited, especially for lignin-derived, aromatic model compounds. The ability to control the relative site densities of acidic and hydrogenation functionalities represents a catalyst design challenge for these materials with the goal to improve hydrogenation activity under ex situ catalytic fast pyrolysis (CFP) conditions. Here in this paper, we demonstrate that the addition of Pt and Ni to Mo 2 C resulted in an increase in the H*-site density with only a minor decrease in the acid-site density. In contrast, the addition of Pd did not significantly alter the H* or acid site densities. High conversions (>94%) and high selectivities to 0-oxygen products (>80%) were observed in guaiacol deoxygenation under ex situ CFP conditions (350 °C and 0.44 MPa H 2 ) for all catalysts. Pt addition resulted in the greatest deoxygenation, and site-time yields to hydrogenated products over the Pt/Mo 2 C catalyst were increased to 0.048 s -1 compared to 0.015-0.019 s -1 for all other catalysts. The Pt/Mo 2 C catalyst demonstrated the highest hydrogenation performance, but modification with Ni also significantly enhanced hydrogenation performance, representing a promising lower-cost alternative.

09 BIOMASS FUELS↗
DOE OSTI2017

Inverse Catalysts for CO Oxidation: Enhanced Oxide–Metal Interactions in MgO/Au(111), CeO 2 /Au(111), and TiO 2 /Au(111)

Au(111) does not bind CO and O 2 well. The deposition of small nanoparticles of MgO, CeO 2 , and TiO 2 on Au(111) produces excellent catalysts for CO oxidation at room temperature. In an inverse oxide/metal configuration there is a strong enhancement of the oxide–metal interactions, and the inverse catalysts are more active than conventional Au/MgO(001), Au/CeO 2 (111), and Au/TiO 2 (110) catalysts. An identical trend was seen after comparing the CO oxidation activity of TiO2/Au and Au/TiO 2 powder catalysts. In the model systems, the activity increased following the sequence: MgO/Au(111) < CeO 2 /Au(111) < TiO 2 /Au(111). Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) was used to elucidate the role of the titania–gold interface in inverse TiO 2 /Au(111) model catalysts during CO oxidation. Stable surface intermediates such as CO(ads), CO 3 2– (ads), and OH(ads) were identified under reaction conditions. CO 3 2– (ads) and OH(ads) behaved as spectators. The concentration of CO(ad) initially increased and then decreased with increasing TiO 2 coverage, demonstrating a clear role of the Ti–Au interface and the size of the TiO 2 nanostructures in the catalytic process. Overall, our results show an enhancement in the strength of the oxide–metal interactions when working with inverse oxide/metal configurations, a phenomenon that can be utilized for the design of efficient catalysts useful for green and sustainable chemistry.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Degradation mechanisms of Platinum Nanoparticle Catalysts in Proton Exchange Membrane Fuel Cells: The Role of Particle Size

Five membrane-electrode assemblies (MEAs) with different average sizes of platinum (Pt) nanoparticles (2.2, 3.5, 5.0, 6.7, and 11.3 nm) in the cathode were analyzed before and after potential cycling (0.6 to 1.0 V, 50 mV/s) by transmission electron microscopy. Cathodes loaded with 2.2 nm and 3.5 nm catalyst nanoparticles exhibit the following changes during electrochemical cycling: (i) substantial broadening of the size distribution relative to the initial size distribution, (ii) presence of coalesced particles within the electrode, and (iii) precipitation of sub-micron-sized particles with complex shapes within the membrane. In contrast, cathodes loaded with 5.0 nm, 6.7 nm and 11.3 nm size catalyst nanoparticles are significantly less prone to the aforementioned effects. As a result, the electrochemically-active surface area (ECA) of MEA cathodes loaded with 2.2 nm and 3.5 nm nanoparticle catalysts degrades dramatically within 1,000 cycles of operation, while the electrochemically-active surface area of MEA cathodes loaded with 5.0 nm, 6.7 nm and 11.3 nm nanoparticle catalysts appears to be stable even after 10,000 cycles. The loss in MEA performance for cathodes loaded with 2.2 nm and 3.5 nm nanoparticle catalysts appears to be due to the loss in electrochemically-active surface area concomitant with the observed morphological changes in these nanoparticle catalysts

DISSOLUTION↗
DOE OSTI2015

Effective Strategy for Improving Electrocatalyst Durability by Adhesive Immobilization of Catalyst Nanoparticles on Graphitic Carbon Supports

We have found that Ta-based additive films in our catalyst system act as adhesives, which improves electrocatalyst durability by immobilizing the catalyst Pt NPs on the graphitic Vulcan carbon support. Furthermore, we suggest that this can be a general design principle in producing higher-durability electrocatalysts on graphitic supports. By electrochemically probing the contributing roles of the tantalum oxide (Ta 2 O 5 ) and the polyphosphate (PPA) components in separate samples, we show that these combine to produce the observed improvement in activity and durability of our best catalyst, the tantalum polyphosphate (TaOPO 4 )-treated sample. To control variables for a valid electrochemical comparison, such as dissimilar catalyst particle size distributions and variations in surface coverage, four new catalyst samples closely matched in every way were prepared: (1) Pt/VC, (2) Pt/[PPA/VC], (3) Pt/[Ta 2 O 5 /VC], and (4) Pt[TaOPO 4 /VC]. We present HR-TEM/HAADF-STEM, EDS elemental mapping, PXRD, XPS, and electrochemical activity and durability evidence, showing that the TaOPO4 and Ta2O5 additives act as adhesives, effectively tethering the NPs to the VC graphitic support surface. Pt/[Ta 2 O 5 /VC] exhibited 3× better durability as compared with the Pt/VC control because of better catalyst nanoparticle immobilization by the Ta 2 O 5 adhesive. Pt[TaOPO 4 /VC] is the overall best performer, exhibiting both a high MA of 0.82 A mg Pt –1, the highest ORR MA after heat treatment, as well as 1.75× greater durability over the Pt/VC control.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Intrinsically Bifunctional and Tunable Tungsten Carbide Catalysts Enable Efficient PVC-Compatible Polyolefin Hydrocracking

Hydrocracking is a promising route for the chemical recycling of polyolefins (PO), converting them into short hydrocarbons over bifunctional catalysts with metal sites for hydrogenation and dehydrogenation, and Brønsted acid sites (BAS) for isomerization and C–C bond cleavage. However, PO feedstocks containing polyvinyl chloride (PVC) can release chlorine (Cl) under reaction conditions, deactivating conventional noble metal/zeolite catalysts. Moreover, the lack of site intimacy and the presence of micropores within conventional catalysts create challenges around the transport of high-molecular-weight, sterically encumbered polymer intermediates. Here, in this study, we report tungsten carbides (W x C) as a novel type of bifunctional catalysts that address these challenges. W/W 2 C phases on W x C offer “metal” sites, and −OH on WO x species introduces BAS in close proximity. The “metal”:BAS ratio can be tuned through carburization temperature, leading to a volcano-shaped activity trend reflecting the requirement for metal–BAS balance. Kinetic data demonstrate that each PO chain undergoes sequential cleavage, while trends in cracking ideality and selectivity follow those in short-alkane hydrocracking. On the per-BAS basis, W x C is more efficient than conventional bifunctional catalysts by more than an order of magnitude, due to enhanced polymer transport. They maintain or show increased activity with 10 wt % PVC in the substrate. This work establishes transition-metal carbides as earth-abundant bifunctional catalysts with unique site proximity and heteroatom compatibility. These features, along with the broad structure space for rational tuning, make them promising options to tackle specific challenges that polymer feedstocks present in hydrocracking.

Nwachukwu, Uchenna C. [Univ. of Akron, OH (United ↗
DOE OSTI2016

Metal–Organic Frameworks Stabilize Solution-Inaccessible Cobalt Catalysts for Highly Efficient Broad-Scope Organic Transformations

New and active earth-abundant metal catalysts are critically needed to replace precious metal-based catalysts for sustainable production of commodity and fine chemicals. We report here the design of highly robust, active, and reusable cobalt-bipyridine- and cobalt-phenanthroline-based metal–organic framework (MOF) catalysts for alkene hydrogenation and hydroboration, aldehyde/ketone hydroboration, and arene C–H borylation. In alkene hydrogenation, the MOF catalysts tolerated a variety of functional groups and displayed unprecedentedly high turnover numbers of ~2.5 × 10 6 and turnover frequencies of ~1.1 × 10 5 h –1 . Structural, computational, and spectroscopic studies show that site isolation of the highly reactive (bpy)Co(THF) 2 species in the MOFs prevents intermolecular deactivation and stabilizes solution-inaccessible catalysts for broad-scope organic transformations. Computational, spectroscopic, and kinetic evidence further support a hitherto unknown (bpy•–)CoI(THF) 2 ground state that coordinates to alkene and dihydrogen and then undergoing σ-complex-assisted metathesis to form (bpy)Co(alkyl)(H). Reductive elimination of alkane followed by alkene binding completes the catalytic cycle. MOFs thus provide a novel platform for discovering new base-metal molecular catalysts and exhibit enormous potential in sustainable chemical catalysis.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Increasing Stability and Activity of Core-Shell Catalysts by Preferential Segregation of Oxide on Edges and Vertexes: Oxygen Reduction on Ti-Au@Pt/C

We describe a new class of core-shell nanoparticle catalysts having edges and vertexes covered by refractory metal oxide that preferentially segregates onto these catalyst sites. The monolayer shell is deposited on the oxidefree core atoms. The oxide on edges and vertexes induces high catalyst’s stability and activity. The catalyst and synthesis are exemplified by fabrication of Au nanoparticles doped by Ti atoms that segregate as oxide onto low–coordination sites of edges and vertexes. Pt monolayer shell deposited on Au sites has the mass and specific activities for the oxygen reduction reaction about 13 and 5 times higher than those of commercial Pt/C catalysts. The durability tests show no activity loss after 10000 potential cycles from 0.6 to 1.0V. The superior activity and durability of the Ti-Au@Pt catalyst originate from protective Ti oxide located at the most dissolution-prone edge and vertex sites, and Au-supported active and stable Pt shell.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Direct Observation by Rapid-Scan FT-IR Spectroscopy of Two-Electron-Reduced Intermediate of Tetraaza Catalyst [Co II N 4 H(MeCN)] 2+ Converting CO 2 to CO

In the search for the two-electron-reduced intermediate of the tetraaza catalyst [Co II N 4 H(MeCN)] 2+ (N 4 H = 2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),2,11,13,15-pentaene) for CO 2 reduction and elementary steps that result in the formation of CO product, rapid-scan FT-IR spectroscopy of the visible-light-sensitized catalysis, using Ir(ppy) 3 in wet acetonitrile (CD 3 CN) solution, led to the observation of two sequential intermediates. The initially formed one-electron-reduced [Co I N 4 H] + --CO 2 adduct was converted by the second electron to a transient [Co I N 4 H] + --CO 2 - complex that spontaneously converted CO 2 to CO in a rate-limiting step on the second time scale in the dark under regeneration of the catalyst (room temperature). The macrocycle IR spectra of the [Co I N 4 H] + --CO 2 - complex and the preceding one-electron [Co I N 4 H] + --CO 2 intermediate show close similarity but distinct differences in the carboxylate modes, indicating that the second electron resides mainly on the CO 2 ligand. Vibrational assignments are corroborated by 13 C isotopic labeling. The structure and stability of the two-electron-reduced intermediate derived from the time-resolved IR study are in good agreement with recent predictions by DFT electronic structure calculations. This is the first observation of an intermediate of a molecular catalyst for CO 2 reduction during the bond-breaking step producing CO. The reaction pathway for the Co tetraaza catalyst uncovered here suggests that the competition between CO 2 reduction and proton reduction of a macrocyclic multi-electron catalyst is steered toward CO 2 activation if the second electron is directly captured by an adduct of CO 2 and the one-electron-reduced catalyst intermediate.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗