Catalyst Architecture Research Articles

(Invited) Towards a Molecular Understanding of Dynamic Fe-Based Oxygen Evolution Catalysts

Heterogeneous electrocatalysts for the oxygen evolution reaction (OER) are complicated materials with dynamic structures. They exhibit potential-induced phase transitions, potential-dependent electronic properties, variable oxidation and protonation states, and disordered local/surface phases. These properties make understanding the OER, and ultimately designing higher-efficiency catalysts, challenging. Measurements of intrinsic activity show that, by far, the most-active phases for OER under alkaline conditions are Fe-containing mixed-metal oxyhydroxides, but exactly how they function remains controversial. I will discuss our work to understand the key properties of these catalysts, including morphology, composition, and molecular/electronic structure, and how they evolve and are dynamic under active catalytic conditions. These concepts inform design strategies for higher-performance catalyst architectures and for their incorporation into practical electrolyzer devices to make clean hydrogen fuel from inexpensive renewable electricity. Figure 1

Functional Electrospun Nanocomposites for Efficient Oxygen Reduction Reaction

Electrospinning with a simple and controllable process has extremely received considerable concerns by virtue of the fabrication and development of nanofibers. Moreover, nanofibers are playing an increasing impact on energy conversion and storage devices, especially for fuel cells based on oxygen reduction reaction(ORR), in view of the rich porosity, large surface area, excellent mass transportation and simply tunable composition, as well as good mechanical strength. In this review, we mainly introduce the primary principle of electrospinning technique, electrochemical reaction mechanism of ORR and synthetic strategies, and summarize the recent advances of unique non-noble-metal nanofibers on the basis of metal-organic framework(MOF) derivatives, single-atom catalysts(SACs) and transition metal oxides. More importantly, we emphasize on the influences of the components, morphology and architecture of advanced electrospun catalysts on their corresponding electrochemical performances towards ORR. Finally, the remaining puzzles and perspectives for further development of the electrospinning nanofibers involving electrocatalysis are presented. It is envisioned that this review would offer an important direction in designing novel electrocatalysts based on electrospinning nanofibrous structures and developing their potential.

Bridging NiCo layered double hydroxides and Ni3S2 for bifunctional electrocatalysts: The role of vertical graphene

Abstract In this work, we report a bifunctional electrocatalyst with nickel sulphide (Ni3S2) as the template, vertical graphene (VG) as the bridging material, and nickel–cobalt layered double hydroxides (NiCo LDHs) nanosheets as the active catalyst. The hybrid Ni3S2/VG@NiCo LDHs catalyst exhibits excellent activity in alkaline solution for both OER (overpotential ~ 320 mV at a current density of 100 mA cm−2) and HER (overpotential ~ 120 mV at a current density of 10 mA cm−2). In addition, the hybrid catalyst possesses superior stability with 99% retention of voltage upon a continued current density of 20 mV cm−2 for over 24 h. It is found that the transitions of Ni2+/Ni3+ and Co2+/Co3+ ions enable excellent HER and OER performances, and VG bridging between NiCo LDHs and Ni3S2, enable fast charge-transfer and a high density of active sites, resulting in the improved electrical conductivity, intrinsic activity, and electrochemical stability. This work provides a guideline to design the architecture of bifunctional catalysts for highly efficient water splitting applications.

Hierarchical fibrous bimetallic electrocatalyst based on ZnO-MoS2 composite nanostructures as high performance for hydrogen evolution reaction

To understand new fundamental questions related to electrocatalyst design to achieve very efficient and stable nonprecious catalysts for water splitting and to strengthen renewable energy reservoirs and nonprecious nanocomposites for efficient water splitting will strengthen the renewable energy reservoirs, thereby it will bring more sustainable and environment friendly society with minimum global warming effect. The synthesized nanocomposite material demonstrates efficient catalytic kinetics for HER, due to the exposure of best active edge sites of MoS 2. The as prepared the nanocomposite was characterized successfully by XRD, SEM, TEM, XPS & EDS were executed that elucidates the crystalline nature of the synthesized nanocomposite material. In an acidic medium, ZnO 30% providing the best ZnO-MoS 2 content achieved at 0.26 V. The ZnO-MoS 2 Tafel slope is 56 mV/decades the ever reported for hydrogen production in an acidic media for ZnO-MoS 2 -based electrocatalysts. The electrocatalyst exhibit excellent stability and durability. Chronopotentiometry at 10 mA/cm 2 current density suffers not a significant loss in the potential for 10 h in an acidic media, respectively. The sample of less ZnO concentration results in increased catalytic properties for MoS 2 , by providing active basal planes for hydrogen evolution reaction along with Mo and S-edges, as predicted by theoretical studies. Interestingly, the catalyst material, architecture and use of nanostructured catalysts have been found useful for improving the efficiency by showing high active reaction sites and the electrical connection of these sites for swift charge kinetics. Therefore, the morphology and structure of fabricated electrocatalysts can have advantageous features for catalysis performance. These findings provide a new stable and efficient electrocatalysts for the development of electro catalyzer devices for the hydrogen gas production based on cost-effective methodology and earth-abundant materials. This work will open a new gateway for the development of functional catalysts in renewable energy reservoirs, which will be scientifically significant and will also provide a roadmap for the development of industrial catalysts.

Nickel‐Catalyzed Migratory Hydrocyanation of Internal Alkenes: Unexpected Diastereomeric‐Ligand‐Controlled Regiodivergence

AbstractA regiodivergent nickel‐catalyzed hydrocyanation of a broad range of internal alkenes involving a chain‐walking process is reported. When appropriate diastereomeric biaryl diphosphite ligands are applied, the same starting materials can be converted to either linear or branched nitriles with good yields and high regioselectivities. DFT calculations suggested that the catalyst architecture determines the regioselectivity by modulating electronic and steric interactions. In addition, moderate enantioselectivities were observed when branched nitriles were produced.

Nickel‐Catalyzed Migratory Hydrocyanation of Internal Alkenes: Unexpected Diastereomeric‐Ligand‐Controlled Regiodivergence

A regiodivergent nickel-catalyzed hydrocyanation of a broad range of internal alkenes involving a chain-walking process is reported. When appropriate diastereomeric biaryl diphosphite ligands are applied, the same starting materials can be converted to either linear or branched nitriles with good yields and high regioselectivities. DFT calculations suggested that the catalyst architecture determines the regioselectivity by modulating electronic and steric interactions. In addition, moderate enantioselectivities were observed when branched nitriles were produced.

Solid Acid Electrochemical Cell for the Production of Hydrogen from Ammonia

Summary Production of high-purity hydrogen by thermal-electrochemical decomposition of ammonia at an intermediate temperature of 250°C is demonstrated. The process is enabled by use of a solid-acid-based electrochemical cell (SAEC) in combination with a bilayered anode, comprising a thermal-cracking catalyst layer and a hydrogen electrooxidation catalyst layer. Cs-promoted Ru on carbon nanotubes (Ru/CNT) serves as the thermal decomposition catalyst, and Pt on carbon black mixed with CsH2PO4 is used to catalyze hydrogen electrooxidation. Cells were operated at 250°C with humidified dilute ammonia supplied to the anode and humidified hydrogen supplied to the counter electrode. A current density of 435 mA/cm2 was achieved at a potential of 0.4 V and ammonia flow rate of 30 sccm. With a demonstrated faradic efficiency for hydrogen production of 100%, the process yields hydrogen at a rate of 1.48 m o l H 2 / g c a t h .

Single Cobalt Sites Dispersed in Hierarchically Porous Nanofiber Networks for Durable and High-Power PGM-Free Cathodes in Fuel Cells.

Increasing catalytic activity and durability of atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts for the oxygen reduction reaction (ORR) cathode in proton-exchange-membrane fuel cells remains a grand challenge. Here, a high-power and durable Co-N-C nanofiber catalyst synthesized through electrospinning cobalt-doped zeolitic imidazolate frameworks into selected polyacrylonitrile and poly(vinylpyrrolidone) polymers is reported. The distinct porous fibrous morphology and hierarchical structures play a vital role in boosting electrode performance by exposing more accessible active sites, providing facile electron conductivity, and facilitating the mass transport of reactant. The enhanced intrinsic activity is attributed to the extra graphitic N dopants surrounding the CoN4 moieties. The highly graphitized carbon matrix in the catalyst is beneficial for enhancing the carbon corrosion resistance, thereby promoting catalyst stability. The unique nanoscale X-ray computed tomography verifies the well-distributed ionomer coverage throughout the fibrous carbon network in the catalyst. The membrane electrode assembly achieves a power density of 0.40 W cm-2 in a practical H2 /air cell (1.0bar) and demonstrates significantly enhanced durability under accelerated stability tests. The combination of the intrinsic activity and stability of single Co sites, along with unique catalyst architecture, provide new insight into designing efficient PGM-free electrodes with improved performance and durability.

Catalytic upcycling of high-density polyethylene via a processive mechanism

The overconsumption of single-use plastics is creating a global waste catastrophe, with widespread environmental, economic and health-related consequences. Here we show that the benefits of processive enzyme-catalysed conversions of biomacromolecules can be leveraged to affect the selective hydrogenolysis of high-density polyethylene into a narrow distribution of diesel and lubricant-range alkanes using an ordered, mesoporous shell/active site/core catalyst architecture that incorporates catalytic platinum sites at the base of the mesopores. Solid-state nuclear magnetic resonance revealed that long hydrocarbon macromolecules readily move within the pores of this catalyst, with a subsequent escape being inhibited by polymer–surface interactions, a behaviour that resembles the binding and translocation of macromolecules in the catalytic cleft of processive enzymes. Accordingly, the hydrogenolysis of polyethylene with this catalyst proceeds processively to yield a reliable, narrow and tunable stream of alkane products.

Chinese ink-promoted co-assembly synthesis of 3D hierarchically structured and porous MoCx/C nanocomposites for highly efficient hydrogen evolution reaction

To explore high-performance carbide catalysts for hydrogen evolution reaction (HER), the rational design of hierarchically porous catalyst benefitting active-site exposure and fast mass transport is extremely demanded. Herein, Chinese ink (CI), used for writing and drawing for thousands of years, is adopted to promote the formation of co-assembly of carbon nanoparticles and Mo-based lamellar organic-inorganic hybrids, which, after pyrolysis in Ar atmosphere, is further converted to 3D hierarchically structured and porous MoCx/C nanocomposites (HSP-MoCx/C) for high-efficient HER. Their skeleton structure, porosity, carbon amounts, phase composition and active species dispersity are well modulated by simply tuning the CI dosage in starting mixture, and its mechanism is monitored via a series of physicochemical characterization techniques. The optimal composite exhibits remarkable HER activity with low overpotentials of 112 and 86 mV at current density of 10 mA cm−2, and the small Tafel slopes of 52 and 50 mV dec−1 in 0.5 M H2SO4 and 1 M KOH solution, respectively. Such performance stems from the combined effects of enhanced ion/mass transport, abundant exposed active sites and high conductivity due to the 3D hierarchically skeleton architecture of catalysts. This work paves a new strategy to design hierarchically porous materials for chemical conversion and energy storage.

Chain Mail for Catalysts.

Encapsulating transition-metal nanoparticles inside carbon nanotubes (CNTs) or spheres has emerged as a novel strategy for designing highly durable nonprecious-metal catalysts. The stable carbon layer protects the inner metal core from the destructive reaction environment and thus is described as chain mail for catalysts. Electron transfer from the active metal core to the carbon layer stimulates unique catalytic activity on the carbon surface, which has been utilized extensively in a variety of catalytic reaction systems. Here, we elaborate the underlying working principle of chain mail for catalysts as well as the key factors that determine their catalytic properties, and provide insights into the physicochemical nature of such catalyst architectures for further application of the strategy in rational catalyst design.

Pore Size Engineering Enabled Selectivity Control in Tandem Catalytic Upgrading of Cyclopentanone on Zeolite-Encapsulated Pt Nanoparticles

Selectivity control is the central challenge in the development of tandem catalytic processes. Zeolite encapsulated nanoparticle has been shown to be an effective catalyst architecture to regulate ...

Mechanism by which three-dimensional ordered mesoporous CeO2 promotes the activity of VOx-MnOx/CeO2 catalysts for NOx abatement: Atomic-scale insight into the catalytic cycle

Submonolayer (below one monolayer coverage) vanadium oxide (VOx)-supported catalysts exhibit high activity, which can be attributed to the nature of the active surface VOx species and electronic effects. Here, a catalyst architecture of submonolayer dimeric VOx species and MnOx supported on three-dimensional (3-D) ordered mesoporous CeO2 was successfully synthesized (VOx-MnOx/kit-CeO2) to enhance the selective catalytic reduction (SCR) of NOx with ammonia activity at low temperatures (<300 °C). The VOx-MnOx/kit-CeO2 catalyst performed excellent NH3-SCR activity at 250 °C under a high gas hourly space velocity of 160000 h−1. The larger surface area and 3-D mesoporous channels of the VOx-MnOx/kit-CeO2 catalyst endowed the material with a significantly strong NH3 adsorption capacity, surface reducibility and surface oxygen activity, which were crucial to determine the low-temperature NH3-SCR performance. The highly dispersed submonolayer dimeric VOx sites over the large surface area (>100 m2/g) and 3-D mesoporous channels of VOx-MnOx/kit-CeO2 produced abundant Brønsted and Lewis acid sites, subsequently enhancing the acid cycle of NH3-SCR. The high ratio of Ce3+/(Ce3++Ce4+) and Mn4+/(Mn3++Mn2+) and easy electron transfer sped up the NH3-SCR redox cycle. Ammonia species on both Brønsted and Lewis acid sites rapidly reacted with bridging nitrite species to yield N2 and H2O.

Isotactic Poly(propylene oxide): A Photodegradable Polymer with Strain Hardening Properties.

Leakage and accumulation of highly stable commercial plastics has led to substantial contamination of the environment. Highly isotactic poly(propylene oxide) (iPPO) was investigated as a potential high-strength thermoplastic with greater susceptibility toward degradation under ambient conditions. Various stereoregular forms of iPPO including enantiopure, enantioenriched, racemic, and stereoblock were synthesized with a single catalyst architecture in the presence of chain transfer agents. These materials were found to possess the same approximate ultimate tensile strength (UTS) via uniaxial tensile elongation analysis (∼75 MPa). A serrated tensile response corresponding to stress oscillations was observed in all forms of iPPO. An investigation on strain rate dependence showed that an increase in strain rate results in the decay and disappearance of the serrated response. Further evaluation of iPPO revealed its dramatic strain hardening afforded an UTS comparable to that of nylon-6,6. Exposing iPPO to UVA light (365 nm) resulted in photolytic degradation. Following 30 days of continuous exposure at 250 μW cm-2, the Mn decreased from 93 kDa to 21 kDa, while samples not exposed to UVA light remained unchanged. Through selective stabilization with antioxidant additives, we believe iPPO could be a suitable replacement for nylon-6,6 in environmentally susceptible applications.

Cu/ZnO Catalysts Derived from Bimetallic Metal-Organic Framework for Dimethyl Ether Synthesis from Syngas with Enhanced Selectivity and Stability.

Direct conversion of syngas to dimethyl ether (DME) through the intermediate of methanol allows more efficient DME production in a simpler reactor design relative to the conventional indirect route. Although Cu/ZnO-based multicomponent catalysts are highly active for methanol synthesis in this process, the sintering issue of Cu during the prolonged reaction generally deteriorates their performance. In this work, Cu/ZnO catalysts in a novel octahedron structure are prepared by a two-step pyrolysis of Zn-doped Cu-BTC metal-organic framework (MOF) in N2 and air. The catalyst CZ-350/A, hybrid of MOF-derived Cu/ZnO sample CZ-350 and γ-Al2 O3 for methanol dehydration, displays the best activity for DME formation (7.74% CO conversion and 70.05% DME selectivity) with the lowest deterioration rate over 40 h continuous reaction. Such performance is superior to its counterpart CZ-CP/A made via the conventional coprecipitation method. This is mainly due to the confinement of Cu nanoparticles within the octahedron matrix hindering their migration and aggregation. Besides, partial reduction of ZnO in the activated CZ-350 prompts the formation of Cu+ -O-Zn, further facilitating the DME production with the highest selectivity compared to literature results. The results clearly indicate that Cu and ZnO distribution in the catalyst architecture plays an important role in DME formation.

Nanoscale architecture of ceria-based model catalysts: Pt–Co nanostructures on well-ordered CeO2(111) thin films

We have prepared and characterized atomically well-defined model systems for ceria-supported Pt–Co core–shell catalysts. Pt@Co and Co@Pt core–shell nanostructures were grown on well-ordered CeO2(111) films on Cu(111) by physical vapour deposition of Pt and Co metals in ultrahigh vacuum and investigated by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy. The deposition of Co onto CeO2(111) yields Co–CeO2(111) solid solution at low Co coverage (0.5 ML), followed by the growth of metallic Co nanoparticles at higher Co coverages. Both Pt@Co and Co@Pt model structures are stable against sintering in the temperature range between 300 and 500 K. After annealing at 500 K, the Pt@Co nanostructure contains nearly pure Co-shell while the Pt-shell in the Co@Pt is partially covered by metallic Co. Above 550 K, the re-ordering in the near surface regions yields a subsurface Pt–Co alloy and Pt-rich shells in both Pt@Co and Co@Pt nanostructures. In the case of Co@Pt nanoparticles, the chemical ordering in the near surface region depends on the initial thickness of the deposited Pt-shell. Annealing of the Co@Pt nanostructures in the presence of O2 triggers the decomposition of Pt–Co alloy along with the oxidation of Co, regardless of the thickness of the initial Pt-shell. Progressive oxidation of Co coupled with adsorbate-induced Co segregation leads to the formation of thick CoO layers on the surfaces of the supported Co@Pt nanostructures. This process is accompanied by the disintegration of the CeO2(111) film and encapsulation of oxidized Co@Pt nanostructures by CeO2 upon annealing in O2 above 550 K. Notably, during oxidation and reduction cycles with O2 and H2 at different temperatures, the changes in the structure and chemical composition of supported Co@Pt nanostructures were driven mainly by oxidation while reduction treatments had little effect regardless of the initial thickness of the Pt-shell.

Unveiling the Delicate Balance of Steric and Dispersion Interactions in Organocatalysis Using High-Level Computational Methods.

High-level quantum electronic structure calculations are used to provide a deep insight into the mechanism and stereocontrolling factors of two recently developed catalytic asymmetric Diels–Alder (DA) reactions of cinnamate esters with cyclopentadiene. The reactions employ two structurally and electronically very different in situ silylated enantiopure Lewis acid organocatalysts: i.e., binaphthyl-allyl-tetrasulfone (BALT) and imidodiphosphorimidate (IDPi). Each of these catalysts activates only specific substrates in an enantioselective fashion. Emphasis is placed on identifying and quantifying the key noncovalent interactions responsible for the selectivity of these transformations, with the final aim of aiding in the development of designing principles for catalysts with a broader scope. Our results shed light into the mechanism through which the catalyst architecture determines the selectivity of these transformations via a delicate balance of dispersion and steric interactions.

Enhanced NO, CO and C3H6 conversion on Pt/Pd catalysts: Impact of oxygen storage material and catalyst architecture

Simultaneous conversion of NO, CO and C3H6 under stoichiometric conditions was carried out on washcoated monoliths containing oxygen storage material (OSM)-promoted mixed Platinum group metals (PGM: Pt(95 %)/Pd(5 %)), to evaluate OSM promotion and to identify the best architecture. The studied OSMs include ceria-zirconia (Ce0.3Zr0.7O2; CZO) and mixed metal oxide spinel (Mn0.5Fe2.5O4; MFO). PGM supported directly on the OSM was found to be the best design for CZO, leading to a ∼70 °C simultaneous decrease in the light-off temperature (T80; feed temperature giving 80 % conversion) for NO, CO and C3H6 conversion compared to the alumina-supported PGM. Close coupling between the PGM and CZO is responsible for the activity promotion. In contrast, a dual-layer architecture (top PGM layer, bottom MFO layer) outperformed the MFO-supported PGM catalyst. The dual-layer PGM-MFO catalyst decreased the T80 by ∼50 °C for NO, CO and C3H6 conversion compared to the PGM-only catalyst. Enhancement obtained with the dual-layer PGM-MFO architecture relies on a direct catalytic contribution from the spinel. During lean/rich modulation the addition of CZO or spinel helps to mitigate the detrimental impact of modulation on NO, CO and C3H6 conversion above 400 °C. The data suggest that both CZO and MFO can buffer lean/rich modulation but exhibit preference to different reductants; CZO-supported PGM and dual-layer PGM-MFO show no conversion decrease during modulation for CO or C3H6 modulation, respectively. Dynamic oxygen storage capacity (DOSC) measurements show that the spinel exhibits ∼10 times higher DOSC than CZO for both CO and C3H6 reductants. Both CO and C3H6 are equally effective reductants for the MFO but CO is better for CZO and PGM-deposited CZO. The study provides useful insight into benefits of OSM and provides guidance for optimization of the multi-component PGM + OSM catalyst composition and architecture.

Hierarchical NiS decorated CuO@ZnFe2O4 nanoarrays as advanced photocathodes for hydrogen evolution reaction

Rationally designed architecture and smart components of catalysts can greatly accelerate the hydrogen evolution reaction in photoelectrochemical water splitting. Herein, hierarchical NiS quantum dots decorated CuO nanowires@ZnFe2O4 nanosheets core/shell nanoarrays were prepared by a viable multi-step synthesis approach. First, CuO nanowire arrays were prepared through the thermal treatment of copper mesh. Then, CuO@ZnO core/shell nanowire arrays were prepared via an impregnation-calcination process. Next, the CuO nanowire arrays with different ZnFe2O4 nanosheet contents were prepared through wet chemical reaction and subsequent thermal treatment. The further NiS quantum dots decoration was realized through a chemical bath deposition. The CuO nanowire arrays covered with ZnFe2O4 porous nanosheets not only offer abundant active sites to react with the electrolyte but also improve visible light utilization. Moreover, the hierarchical nanoarray structure provides a direct electron transport pathway with a graded interface for better charge flow. As a result, remarkably enhanced photoelectrochemical performance and excellent cycling stability were obtained for the CuO@ZnFe2O4 nanoarray photocathodes due to the synergistic effects of ideal components and hierarchical nanoarray structure. Additionally, the further NiS decoration makes CuO@ZnFe2O4 exhibit a significantly enhanced photocathodic current density.

Enantiodivergence by minimal modification of an acyclic chiral secondary aminocatalyst

The development of enantiodivergent catalysis for the preparation of both enantiomers of a chiral compound is of importance in pharmaceutical and bioorganic chemistry. With the design of a class of reactive and stereoselective organocatalysts, acyclic chiral secondary amines, a method for achieving the enantiodivergence is developed simply by changing the secondary N-i-Bu- to N-Me-group within the catalyst architecture while maintaining the same absolute configuration of the catalysts, which modulates the catalyst conformation. This catalyst-controlled enantiodivergent method not only enables challenging asymmetric transformations to occur in an enantiodivergent manner but also features a high level of stereocontrol and broad scope that is demonstrated in eight different reactions (90 examples), all delivering both enantiomers of a range of structurally diverse products including hitherto less accessible, yet important, compounds in good yields with high stereoselectivities.