Construction of Ultrafine PtIr Clusters Supported on Co3O4 Nanoflowers for Enhanced Overall Water Splitting.

Green hydrogen production through electrochemical overall water splitting has suffered from sluggish oxygen evolution reaction (OER) kinetics, inferior conversion efficiency, and high cost. Herein, ultrafine PtIr clusters are synthesized via an electrodeposition method and decorated on the Co3O4 nanoflowers assembled by nanowires (PtIr-Co3O4). The encouraging performances in electrochemical OER and hydrogen evolution reaction (HER) are achieved over the PtIr-Co3O4 catalyst, with the overpotentials as low as 410 and 237 mV at 100 mA cm-2, respectively, outperforming the commercial IrO2 and Pt/C catalysts. Due to the ultralow loading of PtIr clusters, the PtIr-Co3O4 catalyst exhibits 1270 A gIr -1 for OER at the overpotential of 400 mV. Our detailed analyses also show that the strong interactions between the ultrafine PtIr clusters and the Co3O4 nanoflowers enable the PtIr-Co3O4 catalyst to afford 10 mA cm-2 for the overall water splitting at the potential of 1.57 V, accompanied by high durability for 100 h.

Traces of Potassium Induce Restructuring of the Anatase TiO2(001)-(1×4) Surface from a Reactive to an Inert Structure.

Reconstruction of solid surfaces is generally accompanied by changes in surface activities. Here, via a combined experimental and theoretical study, we successfully identified that a trace amount of potassium dopant restructures the mineral anatase TiO2(001) single-crystal surface from an added molecule (ADM) termination to an added oxygen (AOM) one without changing the (1×4) periodicity. The anatase TiO2(001)-(1×4)-ADM surface terminated with 4-fold coordinated Ti4c and 2-fold coordinated O2c sites is (photo)catalytically active, whereas the anatase TiO2(001)-(1×4)-AOM surface terminated with O2c and inaccessible 5-fold coordinated Ti5c sites is inert. These results unveiled a mechanism of dopant-induced transformation from a reactive to an inert TiO2(001)-(1×4) surface, which unifies the existing arguments about the surface structures and (photo)catalytic activity of anatase TiO2(001)-(1×4).

Dual-functional single-atomic Mo/Fe clusters-decorated C3N5 via three electron-pathway in oxygen reduction reaction for tandemly removing contaminants from water.

Inspired by the development of single-atom catalysts (SACs), the fabrication of multimetallic SACs can be a promising technical approach for the in situ electro-Fenton (EF) process. Herein, dual-functional atomically dispersed Mo-Fe sites embedded in carbon nitride (C3N5) (i.e., MoFe/C3N5) were synthesized via a facile SiO2 template method. The atomically isolated bimetallic configuration in MoFe/C3N5 was identified by combining the microscopic and spectroscopic techniques. The MoFe/C3N5 catalyst on the cathode exhibited a remarkable catalytic activity toward the three electron-dominated oxygen reduction reaction in sodium sulfate, leading to a highly effective EF reaction with a low overpotential for the removal of organic contaminants from wastewater. The new catalyst showed a superior performance over its conventional counterparts, owing to the dual functions of the dual-metal active sites. Density functional theory (DFT) analysis revealed that the dual-functional 50-MoFe/C3N5 catalyst enabled a synergistic action of the Mo-Fe dual single atomic centers, which can alter the adsorption/dissociation behavior and decrease the overall reaction barriers for effective organic oxidation during the EF process. This study not only sheds light on the controlled synthesis of atomically isolated catalyst materials but also provides deeper understanding of the structure-performance relationship of the nanocatalysts with dual active sites for the catalytic EF process. Additionally, the findings will promote the advanced catalysis for the treatment of emerging organic contaminants in water and wastewater.

Contracted Fe-N5-C11 Sites in Single-Atom Catalysts Boosting Catalytic Performance for Oxygen Reduction Reaction.

Promoting the catalyst performance for oxygen reduction reaction (ORR) in energy conversion devices through controlled manipulation of the structure of catalytic active sites has been a major challenge. In this work, we prepared Fe-N-C single-atom catalysts (SACs) with Fe-N5 active sites and found that the catalytic activity of the catalyst with shrinkable Fe-N5-C11 sites for ORR was significantly improved compared with the catalyst bearing normal Fe-N5-C12 sites. The catalyst C@PVI-(TPC)Fe-800, prepared by pyrolyzing an axial-imidazole-coordinated iron corrole precursor, exhibited positive shifted half-wave potential (E1/2 = 0.89 V vs RHE) and higher peak power density (Pmax = 129 mW/cm2) than the iron porphyrin-derived counterpart C@PVI-(TPP)Fe-800 (E1/2 = 0.81 V, Pmax = 110 mW/cm2) in 0.1 M KOH electrolyte and Zn-air batteries, respectively. X-ray absorption spectroscopy (XAS) analysis of C@PVI-(TPC)Fe-800 revealed a contracted Fe-N5-C11 structure with iron in a higher oxidation state than the porphyrin-derived Fe-N5-C12 counterpart. Density functional theory (DFT) calculations demonstrated that C@PVI-(TPC)Fe-800 possesses a higher HOMO energy level than C@PVI-(TPP)Fe-800, which can increase its electron-donating ability and thus help achieve enhanced O2 adsorption as well as O-O bond activation. This work provides a new approach to tune the active site structure of SACs with unique contracted Fe-N5-C11 sites that remarkably promote the catalyst performance, suggesting significant implications for catalyst design in energy conversion devices.

Directing Highly a-Axis-Oriented ZSM-5 Nanosheets with Pre-estimated Bifunctional Imidazole Cations.

An MFI-topology nanosheet zeolite with a highly a-axis-oriented structure has rarely been reported but with a great potential for industrial applications. Theoretical calculations on the interaction energies between the MFI skeleton and ionic liquid molecules indicated the possibility of preferential crystal growth along a specific direction, according to which highly a-oriented ZSM-5 nanosheets were synthesized from commercially available 1-(2-hydroxyethyl)-3-methylimidazolium and layered silicate sources. The imidazolium molecules directed the structure formation and meanwhile acted as zeolite growth modifiers to restrict the crystal growth perpendicular to the MFI bc plane, which induced unique a-axis-orientated thin sheets with ∼12 nm thickness. The a-oriented ZSM-5 exhibited more competitive propylene selectivity and longer lifetime than bulky crystals in methanol-to-propylene (MTP) reaction. This research would provide a versatile protocol for the rational design and synthesis of shape-selective zeolite catalysts with promising applications.

Synergistic promotions between CO2 capture and in-situ conversion on Ni-CaO composite catalyst.

The integrated CO2 capture and conversion (iCCC) technology has been booming as a promising cost-effective approach for Carbon Neutrality. However, the lack of the long-sought molecular consensus about the synergistic effect between the adsorption and in-situ catalytic reaction hinders its development. Herein, we illustrate the synergistic promotions between CO2 capture and in-situ conversion through constructing the consecutive high-temperature Calcium-looping and dry reforming of methane processes. With systematic experimental measurements and density functional theory calculations, we reveal that the pathways of the reduction of carbonate and the dehydrogenation of CH4 can be interactively facilitated by the participation of the intermediates produced in each process on the supported Ni-CaO composite catalyst. Specifically, the adsorptive/catalytic interface, which is controlled by balancing the loading density and size of Ni nanoparticles on porous CaO, plays an essential role in the ultra-high CO2 and CH4 conversions of 96.5% and 96.0% at 650 °C, respectively.

Strategies to Improve the Oxygen Reduction Reaction Activity on Pt-Bi Bimetallic Catalysts: A Density Functional Theory Study.

Decreasing the level of use of Pt in proton exchange membrane fuel cells is of great research interest both academically and industrially. In this work, we systematically studied the oxygen reduction reaction (ORR) following the four-electron association mechanism at various Pt-Bi surfaces with density functional theory calculations. The results showed that the introduction of Bi changes the potential-determining step of ORR. Moreover, the hydroxy adsorption free energy (GOH*) can be used as a descriptor of ORR activity, and 0.74 eV is the ideal GOH* for it to reach its maximum. Notably, we also found that the tensile strain introduced by Bi and electron transfer between Pt and Bi synergize to modulate the d-band of Pt to contract, shift downward, and break the 5d96s1 valence electron configuration of Pt, and accordingly, PtBi(100), with the lowest d-band center, gives the best ORR activity, which is even slightly higher than that of Pt(111).

Acid-catalyzed Disulfide-mediated Reversible Polymerization for Recyclable Dynamic Covalent Materials.

Poly(1,2-dithiolane)s are a family of intrinsically recyclable polymers due to their dynamic covalent disulfide linkages. Despite the common use of thiolate-initiated anionic ring-opening polymerization (ROP) under basic condition, cationic ROP is still not exploited. Here we report that disulfide bond can act as a proton acceptor, being protonated by acids to form sulfonium cations, which can efficiently initiate the ROP of 1,2-dithiolanes and result in high-molecular-weight (over 1000 kDa) poly(disulfide)s. The reaction can be triggered by adding catalytic amounts of acids and non-coordinating anion salts, and completed in few minutes at room temperature. The acidic conditions allow the applicability for acidic monomers. Importantly, the reaction condition can be under open air without inert protection, enabling the nearly quantitative chemical recycling from bulk materials to original monomers.

Uncovering Structure-Activity Relationships in Pt/CeO2 Catalysts for Hydrogen-Borrowing Amination.

The hydrogen-borrowing amination of alcohols is a promising route to produce amines. In this study, experimental parameters involved in the preparation of Pt/CeO2 catalysts were varied to assess how physicochemical properties influence their performance in such reactions. An amination reaction between cyclopentanol and cyclopentylamine was used as the model reaction for this study. The Pt precursor used in the catalyst synthesis and the properties of the CeO2 support were both found to strongly influence catalytic performance. Aberration corrected scanning transmission electron microscopy revealed that the most active catalyst comprised linearly structured Pt species. The formation of these features, a function result of epitaxial Pt deposition along the CeO2 [100] plane, appeared to be dependent on the properties of the CeO2 support and the Pt precursor used. Density functional theory calculations subsequently confirmed that these sites were more effective for cyclopentanol dehydrogenation-considered to be the rate-determining step of the process-than Pt clusters and nanoparticles. This study provides insights into the desirable catalytic properties required for hydrogen-borrowing amination but has relevance to other related fields. We consider that this study will provide a foundation for further study in this atom-efficient area of chemistry.

Unveiling the Surface Structure of ZnO Nanorods and H2 Activation Mechanisms with 17O NMR Spectroscopy.

ZnO plays a very important role in many catalytic processes involving H2, yet the details on their interactions and H2 activation mechanism are still missing, owing to the lack of a characterization method that provides resolution at the atomic scale and follows the fate of oxide surface species. Here, we apply 17O solid-state NMR spectroscopy in combination with DFT calculations to unravel the surface structure of ZnO nanorods and explore the H2 activation process. We show that six different types of oxygen ions in the surface and subsurface of ZnO can be distinguished. H2 undergoes heterolytic dissociation on three-coordinated surface zinc and oxygen ions, while the formed hydride species migrate to nearby oxygen species, generating a second hydroxyl site. When oxygen vacancies are present, homolytic dissociation of H2 occurs and zinc hydride species form from the vacancies. Reaction mechanisms on oxide surfaces can be explored in a similar manner.

Surface differences of oxide nanocrystals determined by geometry and exogenously coordinated water molecules.

Determining the different surfaces of oxide nanocrystals is key in developing structure-property relations. In many cases, only surface geometry is considered while ignoring the influence of surroundings, such as ubiquitous water on the surface. Here we apply 17O solid-state NMR spectroscopy to explore the facet differences of morphology-controlled ceria nanocrystals considering both geometry and water adsorption. Tri-coordinated oxygen ions at the 1st layer of ceria (111), (110), and (100) facets exhibit distinct 17O NMR shifts at dry surfaces while these 17O NMR parameters vary in the presence of water, indicating its non-negligible effects on the oxide surface. Thus, the interaction between water and oxide surfaces and its impact on the chemical environment should be considered in future studies, and solid-state NMR spectroscopy is a sensitive approach for obtaining such information. The work provides new insights into elucidating the surface chemistry of oxide nanomaterials.

Photoinduced inverse vulcanization.

The inverse vulcanization (IV) of elemental sulfur to generate sulfur-rich functional polymers has attracted much recent attention. However, the harsh reaction conditions required, even with metal catalysts, constrains the range of feasible crosslinkers. We report here a photoinduced IV that enables reaction at ambient temperatures, greatly broadening the scope for both substrates and products. These conditions enable volatile and gaseous alkenes and alkynes to be used in IV, leading to sustainable alternatives for environmentally harmful plastics that were hitherto inaccessible. Density functional theory calculations reveal different energy barriers for thermal, catalytic and photoinduced IV processes. This protocol circumvents the long curing times that are common in IV, generates no H2S by-products, and produces high-molecular-weight polymers (up to 460,000 g mol-1) with almost 100% atom economy. This photoinduced IV strategy advances both the fundamental chemistry of IV and its potential industrial application to generate materials from waste feedstocks.

Isolating Single Sn Atoms in CuO Mesocrystal to Form Ordered Atomic Interfaces: An Effective Strategy for Designing Highly Efficient Mesocrystal Catalysts.

Tuning the electronic structures of mesocrystals at the atomic level is an effective approach to obtaining unprecedented properties. Here, a lattice-confined strategy to obtain isolated single-site Sn atoms in CuO mesocrystals to improve catalytic performance is reported. The Sn/CuO mesocrystal composite (Sn/CuO MC) has ordered Sn-O-Cu atomic interfaces originated from the long-range ordering of the CuO mesocrystal itself. X-ray absorption fine structure measurements confirm that the positively charged Sn atoms can tune the electronic structure of the Cu atoms to some extent in Sn/CuO MC, quite different from that in the conventional single-atom Sn-modified CuO nanoparticles and nanoparticulate SnO2 -modified CuO mesocrystal catalysts. When tested for the Si hydrochlorination reaction to produce trichlorosilane, Sn/CuO MC exhibits significantly better performances than the above two catalysts. Theoretical calculations further reveal the electronic modification to the active Cu component and the induced improvement in HCl adsorption, and thus enhance the catalytic performance. This work demonstrates how to design efficient metal oxide mesocrystal catalysts through an electronic structure modification approach.

Achieving Active and Stable Amorphous IrVOxOHy for Water Splitting.

Evaluating the structural and electronic-state characteristics of long-range disordered amorphous iridium (Ir)-based oxides is still unsatisfying. Compared with the benchmark IrO2, the higher oxygen evolution reaction (OER) performance brought by IrOxOHy was normally considered to be associated with the pristine IrIII-containing species. However, such a conclusion conflicts with the opinion that high-valence metals can create excellent OER activity. To resolve such contradictions, we synthesized a pure amorphous Lu1.25IrOxOHy (Lu = lutetium) catalyst in this work. In combination with the comprehensive electrochemical evaluation in alkaline and acidic media, ex situ Ir L3-edge and O K-edge X-ray absorption spectroscopy and theoretical calculations revealed that the ultrahigh OER performance of reconstructed IrOx/Lu1.25IrOxOHy in acidic media was identified to be driven by the more d-hole-containing electronic state of IrV created by cationic vacancies. The pristine properties of IrIII-containing Lu1.25IrOxOHy conversely inhibit the OER activity in alkaline media. Additionally, the high edge-shared [IrOx]-[IrOx] motif proportion structure in amorphous Lu1.25IrOxOHy achieves a stable OER process, which exhibits a high S-number stability index similar to IrO2. We demonstrate that the key factor of the edge-shared [IrOx]-[IrOx] motif with cationic vacancies in IrVOxOHy could rationally reveal the source for most of the high-performance Ir-based materials.

A unique Co@CoO catalyst for hydrogenolysis of biomass-derived 5-hydroxymethylfurfural to 2,5-dimethylfuran.

The development of precious-metal-free catalysts to promote the sustainable production of fuels and chemicals from biomass remains an important and challenging target. Here, we report the efficient hydrogenolysis of biomass-derived 5-hydroxymethylfurfural to 2,5-dimethylfuran over a unique core-shell structured catalyst, Co@CoO that affords the highest productivity among all catalysts, including noble-metal-based catalysts, reported to date. Surprisingly, we find that the catalytically active sites reside on the shell of CoO with oxygen vacancies rather than the metallic Co. The combination of various spectroscopic experiments and computational modelling reveals that the CoO shell incorporating oxygen vacancies not only drives the heterolytic cleavage, but also the homolytic cleavage of H2 to yield more active Hδ- species, resulting in the exceptional catalytic activity. Co@CoO also exhibits excellent activity toward the direct hydrodeoxygenation of lignin model compounds. This study unlocks, for the first time, the potential of simple metal-oxide-based catalysts for the hydrodeoxygenation of renewable biomass to chemical feedstocks.

Unique catalytic mechanisms of methanol dehydrogenation at Pd-doped ceria: A DFT+U study.

Pd-doped ceria is highly active in promoting oxidative dehydrogenation (ODH) reactions and also a model single atom catalyst (SAC). By performing density functional theory calculations corrected by on-site Coulomb interactions, we systematically studied the physicochemical properties of the Pd-doped CeO2(111) surface and the catalytic methanol to formaldehyde reaction on the surface. Two different configurations were located for the Pd dopant, and the calculated results showed that doping of Pd will make the surface more active with lower oxygen vacancy formation energies than the pristine CeO2(111). Moreover, two different pathways for the dehydrogenation of CH3OH to HCHO on the Pd-doped CeO2(111) were determined, one of which is the conventional two-step process (stepwise pathway) with the O-H bond of CH3OH being broken first followed by the C-H bond cleavage, while the other is a novel one-step process (concerted pathway) involving the two H being dissociated from CH3OH simultaneously even with a lower energy barrier than the stepwise one. With electronic and structural analyses, we showed that the direct reduction of Pd4+ to Pd2+ through the transfer of two electrons can outperform the separated Ce4+ to Ce3+ processes with the help of configurational evolution at the Pd site, which is responsible for the existence of such one-step dehydrogenation process. This novel mechanism may provide an inspiration for constructing ceria-based SAC with unique ODH activities.

Identification of CO2 adsorption sites on MgO nanosheets by solid-state nuclear magnetic resonance spectroscopy.

The detailed information on the surface structure and binding sites of oxide nanomaterials is crucial to understand the adsorption and catalytic processes and thus the key to develop better materials for related applications. However, experimental methods to reveal this information remain scarce. Here we show that 17O solid-state nuclear magnetic resonance (NMR) spectroscopy can be used to identify specific surface sites active for CO2 adsorption on MgO nanosheets. Two 3-coordinated bare surface oxygen sites, resonating at 39 and 42 ppm, are observed, but only the latter is involved in CO2 adsorption. Double resonance NMR and density functional theory (DFT) calculations results prove that the difference between the two species is the close proximity to H, and CO2 does not bind to the oxygen ions with a shorter O···H distance of approx. 3.0 Å. Extensions of this approach to explore adsorption processes on other oxide materials can be readily envisaged.

Subtle structure matters: boosting surface-directed photoelectron transfer via the introduction of specific monovalent oxygen vacancies in TiO2.

Oxygen vacancies (Ov) are widely considered to play crucial roles in photocatalysis, but how and why they contribute to improved performances remains controversial. In this work, we studied the promotional effect of Ov on photoelectron transfer in TiO2, using first-principles density functional theory calculations with correction for on-site Coulomb interactions. We explicitly identified three types of Ov with different charge states (i.e., charge-neutral , monovalent , divalent Ov2+) via electronic structure analysis. Electron transfer energy calculations revealed that the ionized Ov in anatase TiO2 are able to collect excess electrons whereas those in the rutile phase are not. The presence of ionized Ov further endows anatase TiO2 with directional electron transfer along the [100] orientation, in favor of anatase TiO2(101) for photocatalytic reduction surpassing the (001) termination. After examining various combination modes of ionized Ov involving different charge states and spatial distributions, we demonstrated that the vertical chain in anatase TiO2(101) is the most catalytically effective Ov pattern in TiO2. These results signify the importance of subtle defects in photocatalysis and may assist future photocatalyst design toward higher photocatalytic efficiency.

Photo-induced hydrophilicity at the ZnO(112̄0) surface: an evolutionary algorithm-aided density functional theory study.

Evolutionary algorithm-aided density functional theory calculations were utilized to determine the stable adsorption structures of H2O at ZnO(112̄0) extensively under different coverages. By decomposing the adsorption energetics, we illustrate that H2O dissociation to a large extent is actually hampered by the barrier for induced distortion of the ZnO surface, and once the surface becomes less difficult to be distorted it will exhibit higher hydrophilicity or even superhydrophilicity. Specifically, photo-stimulation modelling suggests that the surface Zn-O bonds can be weakened by photo-excitation, and the layer of fully dissociated H2O can be then facilitated to form. Accordingly, a novel mechanism for photo-induced superhydrophilicity is proposed.

Surface Reconstruction for Forming the [IrO6]-[IrO6] Framework: Key Structure for Stable and Activated OER Performance in Acidic Media.

The surface reconstruction of iridium-based derivatives (AxIryOz) was extensively demonstrated to have an excellent oxygen evolution reaction (OER) performance in an acidic medium. It is urgent to use various spectroscopy and computational methods to explore the electronic state changes in the surface reconstruction process. Herein, the underestimated Lu2Ir2O7 was synthesized and investigated. Four typical forms of electrochemistry impedance spectra involved in the reconstruction process revealed three dominating forms of reconstructed pyrochlore in the OER stage, including the inner intact pyrochlore, mid metastable [IrO6]-[IrO6] framework, and the outer collapse amorphous layer. The enhancing electron transport efficiency of the corner-shared [IrO6]-[IrO6] framework was revealed as a critical role in acidic systems. The density of state (DOS) for the constructed [IrO6]-[IrO6] framework corroborated the enhancement of Ir-O hybridization and the downshift of the d-band center. Additionally, we contrast the pristine and reconstruction properties of the Pr2Ir2O7, Eu2Ir2O7, and Lu2Ir2O7 in alkaline and acidic media. The DOS and the XANES results reveal the scale relationship between the O 2p band center and the intrinsic activity for bulk pyrochlore in alkaline media. The highest O 2p center and the highest Ir-O hybridization of Lu2Ir2O7 exhibited the best OER performance among the Ir-based pyrochlore, up to a ninefold improvement in Ir-mass activity compared to IrO2. Our findings emphasize the electrochemical behavior of the reconstruction process for activated water-splitting performance.