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.

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.

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.

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.

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.

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.

Interaction of Hydrogen with Ceria: Hydroxylation, Reduction, and Hydride Formation on the Surface and in the Bulk.

The study reports the first attempt to address the interplay between surface and bulk in hydride formation in ceria (CeO2 ) by combining experiment, using surface sensitive and bulk sensitive spectroscopic techniques on the two sample systems, i.e., CeO2 (111) thin films and CeO2 powders, and theoretical calculations of CeO2 (111) surfaces with oxygen vacancies (Ov ) at the surface and in the bulk. We show that, on a stoichiometric CeO2 (111) surface, H2 dissociates and forms surface hydroxyls (OH). On the pre-reduced CeO2-x samples, both films and powders, hydroxyls and hydrides (Ce-H) are formed on the surface as well as in the bulk, accompanied by the Ce3+ ↔ Ce4+ redox reaction. As the Ov concentration increases, hydroxyl is destabilized and hydride becomes more stable. Surface hydroxyl is more stable than bulk hydroxyl, whereas bulk hydride is more stable than surface hydride. The surface hydride formation is the kinetically favorable process at relatively low temperatures, and the resulting surface hydride may diffuse into the bulk region and be stabilized therein. At higher temperatures, surface hydroxyls can react to produce water and create additional oxygen vacancies, increasing its concentration, which controls the H2 /CeO2 interaction. The results demonstrate a large diversity of reaction pathways, which have to be taken into account for better understanding of reactivity of ceria-based catalysts in a hydrogen-rich atmosphere.

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.

Site Sensitivity of Interfacial Charge Transfer and Photocatalytic Efficiency in Photocatalysis: Methanol Oxidation on Anatase TiO2 Nanocrystals.

Photocatalytic oxidation of methanol on various anatase TiO2 nanocrystals was studied by in situ and time-resolved characterizations and DFT calculations. Surface site and resulting surface adsorbates affect the surface band bending/bulk-to-surface charge migration processes and interfacial electronic structure/interfacial charge transfer processes. TiO2 nanocrystals predominantly enclosed by the {001} facets expose a high density of reactive fourfold-coordinated Ti sites (Ti4c ) at which CH3 OH molecules dissociate to form the CH3 O adsorbate (CH3 O(a)Ti4c ). CH3 O(a)Ti4c localized density of states are almost at the valence band maximum of TiO2 surface, facilitating the interfacial hole transfer process; CH3 O(a)Ti4c with a high coverage promotes upward surface band bending, facilitating bulk-to-surface hole migration. CH3 O(a)Ti4c exhibits the highest photocatalytic oxidation rate constant. TiO2 nanocrystals enclosed by the {001} facets are most active in photocatalytic methanol oxidation.

Boosting Interfacial Charge-Transfer Kinetics for Efficient Overall CO2 Photoreduction via Rational Design of Coordination Spheres on Metal-Organic Frameworks.

The recombination of electron-hole pairs severely detracts from the efficiency of photocatalysts. This issue could be addressed in metal-organic frameworks (MOFs) through optimization of the charge-transfer kinetics via rational design of structures at atomic level. Herein, a pyrazolyl porphyrinic Ni-MOF (PCN-601), integrating light harvesters, active catalytic sites, and high surface areas, has been demonstrated as a superior and durable photocatalyst for visible-light-driven overall CO2 reduction with H2O vapor at room temperature. Kinetic studies reveal that the robust coordination spheres of pyrazolyl groups and Ni-oxo clusters endow PCN-601 with proper energy band alignment and ultrafast ligand-to-node electron transfer. Consequently, the CO2-to-CH4 production rate of PCN-601 far exceeds those of the analogous MOFs based on carboxylate porphyrin and the classic Pt/CdS photocatalyst by more than 3- and 20-fold, respectively. The reaction avoids the use of hole scavengers and proceeds in a gaseous phase which can take full advantage of the high gas uptake of MOFs. This work demonstrates that the rational design of coordination spheres in MOF structures not only reconciles the contradiction between reactivity and stability but also greatly promotes the interfacial charge transfer to achieve optimized kinetics, providing guidance for the design of highly efficient MOF photocatalysts.

Interactions of Oxide Surfaces with Water Revealed with Solid-State NMR Spectroscopy.

Hydrous materials are ubiquitous in the natural environment and efforts have previously been made to investigate the structures and dynamics of hydrated surfaces for their key roles in various chemical and physical applications, with the help of theoretical modeling and microscopy techniques. However, an overall atomic-scale understanding of the water-solid interface, including the effect of water on surface ions, is still lacking. Herein, we employ ceria nanorods with different amounts of water as an example and demonstrate a new approach to explore the water-surface interactions by using solid-state NMR in combination with density functional theory. NMR shifts and relaxation time analysis provide detailed information on the local structure of oxygen ions and the nature of water motion on the surface: the amount of molecularly adsorbed water decreases rapidly with increasing temperature (from room temperature to 150 °C), whereas hydroxyl groups are stable up to 150 °C, and dynamic water molecules are found to instantaneously coordinate to the surface oxygen ions. The applicability of dynamic nuclear polarization for selective detection of surface oxygen species is also compared to conventional NMR with surface selective isotopic-labeling: the optimal method depends on the feasibility of enrichment and the concentration of protons in the sample. These results provide new insight into the interfacial structure of hydrated oxide nanostructures, which is important to improve performance for various applications.

Clarifying the impacts of surface hydroxyls on CO oxidation on CeO2(100) surfaces: a DFT+U study.

In heterogeneous catalysis, surface hydroxylation is well recognized as a common phenomenon under realistic reaction conditions. However, even for the versatile ceria-based materials that have attracted extensive studies, the results and causes of the hydroxyl effect on the catalytic reactivity remain largely elusive. In this work, density functional theory calculations corrected by on-site Coulomb interaction were conducted to clarify the CO oxidation pathways and also the impacts of surface hydroxyls on the catalytic performance at the two most stable reconstructions of CeO2(100). It is found that the presence of hydroxyl groups can boost the CO oxidation activity on the O-terminal surface but shows an opposite effect on the CeO4-terminal one. Further analyses regarding the structural distortions, electronic structures and orbital interactions reveal that the stretched Ce-O distance via in-plane hydrogen bonds and the electron redistributions induced by additional hydroxyl coordination are the main reasons for the different hydroxyl effects on the O- and CeO4-terminal surfaces. Our results not only uncover the dual-character of surface hydroxyls in heterogeneous catalysis, but they also underline the significance of moderate moisture in the reaction system that may endow ceria catalysts with both good thermostability and high catalytic activity.

Methanol Synthesis at a Wide Range of H2 /CO2 Ratios over a Rh-In Bimetallic Catalyst.

There is increasing interest in capturing H2 generated from renewables with CO2 to produce methanol. However, renewable hydrogen production is expensive and in limited quantity compared to CO2 . Excess CO2 and limited H2 in the feedstock gas is not favorable for CO2 hydrogenation to methanol, causing low activity and poor methanol selectivity. Now, a class of Rh-In catalysts with optimal adsorption properties to the intermediates of methanol production is presented. The Rh-In catalyst can effectively catalyze methanol synthesis but inhibit the reverse water-gas shift reaction under H2 -deficient gas flow and shows the best competitive methanol productivity under industrially applicable conditions in comparison with reported values. This work demonstrates a strong potential of Rh-In bimetallic composition, from which a convenient methanol synthesis based on flexible feedstock compositions (such as H2 /CO2 from biomass derivatives) with lower energy cost can be established.

A DFT+U revisit of reconstructed CeO2(100) surfaces: structures, thermostabilities and reactivities.

Cerium dioxide (CeO2) shows wide catalytic applications by virtue of its excellent oxygen storage capacity. The CeO2(100) surface has aroused particular interest because of its intrinsic polarity; however, it suffers from structural reconstruction, which consequently hinders experimental and theoretical studies. In this work, we performed density functional theory calculations with on-site Coulomb interaction correction to investigate and further correlate the geometric and catalytic properties of reconstructed CeO2(100) surfaces. By introducing CeO2 units on a previous O-terminal model, the surface exposed CeO4 pyramids with gradual increase in coverage and eventually transformed into a Ce-terminal structure. The corresponding thermostabilities were evaluated by calculating the surface energy and oxygen vacancy formation energy. We also showed that the CO oxidation on the reconstructed CeO2(100) surfaces favored the Mars-van-Krevelen mechanism. The most stable CeO4-terminal type of reconstruction, covered with a half overlayer of CeO4 pyramids on the surface, was capable of directly producing CO2 without forming bent CO2 intermediates and carbonate byproducts. Moreover, coordinatively unsaturated Ce ions at the pyramid apex provided extra accommodation to the reacting CO, thus lowering the reaction barrier of the key CO coupling step relative to that of the O-terminal surface. We finally generalized a unified picture of the dynamic changes in the thermostability and catalytic activity along with the structural reconstruction of the CeO2(100) surface. The CeO4-terminal type of reconstruction was theoretically predicted to be highly efficient for catalyzing CO oxidation.

A DFT+U study of the catalytic degradation of 1,2-dichloroethane over CeO2.

The catalytic degradation of 1,2-dichloroethane (DCE) at CeO2(111) was investigated using periodic density functional theory calculations corrected by on-site Coulomb interactions. From thorough calculations of possible elementary steps, we are able to identify the lowest energy reaction pathway for the catalytic oxidation of DCE at CeO2(111). It proceeds via two successive C-Cl bond breaking processes to form adsorbed CH2CH2 species, and after further dehydrogenation and C-C bond scission, the surface species are finally oxidized to CO2 and H2O. The surface oxygen vacancies were found to be important for the catalytic decomposition of DCE, by providing the adsorption sites, as well as for charge transfer to favor C-Cl bond breaking. We are also able to illustrate the effect of H2O on the catalytic activity of CeO2(111) for DCE oxidation.

Catalytic Activity Control via Crossover between Two Different Microstructures.

Metal nanocatalysts hold great promise for a wide range of heterogeneous catalytic reactions, while the optimization strategy of catalytic activity is largely restricted by particle size or shape control. Here, we demonstrate that a reversible microstructural control through the crossover between multiply twinned nanoparticle (MTP) and single crystal (SC) can be readily achieved by solvent post-treatment on gold nanoparticles (AuNPs). Polar solvents (e.g., water, methanol) direct the transformation from MTP to SC accompanied by the disappearance of twinning and stacking faults. A reverse transformation from SC to MTP is achieved in nonpolar solvent (e.g., toluene) mixed with thiol ligands. The transformation between two different microstructures is directly observed by in situ TEM and leads to a drastic modulation of catalytic activity toward the gas-phase selective oxidation of alcohols. On the basis of the combined experimental and theoretical investigations of alcohol chemisorption on these nanocatalysts, we propose that the exposure of {211}-like microfacets associated with twin boundaries and stack faults accounts for the strong chemisorption of alcohol molecules on MTP AuNPs and thus the exceptionally high catalytic activity.

Black Tungsten Nitride as a Metallic Photocatalyst for Overall Water Splitting Operable at up to 765†nm.

Semiconductor photocatalysts are hardly employed for overall water splitting beyond 700 nm, which is due to both thermodynamic aspects and activation barriers. Metallic materials as photocatalysts are known to overcome this limitation through interband transitions for creating electron-hole pairs; however, the application of metallic photocatalysts for overall water splitting has never been fulfilled. Black tungsten nitride is now employed as a metallic photocatalyst for overall water splitting at wavelengths of up to 765 nm. Experimental and theoretical results together confirm that metallic properties play a substantial role in exhibiting photocatalytic activity under red-light irradiation for tungsten nitride. This work represents the first red-light responsive photocatalyst for overall water splitting, and may open a promising venue in searching of metallic materials as efficient photocatalysts for solar energy utilization.

Clustering of Oxygen Vacancies at CeO2(111): Critical Role of Hydroxyls.

By performing density functional theory calculations corrected by an on site Coulomb interaction, we find that the defects at the CeO_{2}(111) surface observed by the scanning tunneling microscopy (STM) measurements of Esch et al. [Science 309, 752 (2005)] are not mere oxygen vacancies or fluorine impurities as suggested by Kullgren et al. [Phys. Rev. Lett. 112, 156102 (2014)], but actually the hydroxyl-vacancy combined species. Specifically, we show that hydroxyls play a critical role in the formation and propagation of oxygen vacancy clusters (VCs). In the presence of neighboring hydroxyls, the thermodynamically unstable VCs can be significantly stabilized, and the behaviors of oxygen vacancies become largely consistent with the STM observations. In addition to the clarification of the long term controversy on the surface defect structures of CeO_{2}(111), the "hydroxyl-vacancy model" proposed in this work emphasizes the coexistence of hydroxyls and oxygen vacancies, especially VCs, which is important for understanding the catalytic and other physicochemical properties of reducible metal oxides.

Methanol Conversion into Dimethyl Ether on the Anatase TiO2(001) Surface.

Exploring reactions of methanol on TiO2 surfaces is of great importance in both C1 chemistry and photocatalysis. Reported herein is a combined experimental and theoretical calculation study of methanol adsorption and reaction on a mineral anatase TiO2(001)-(1×4) surface. The methanol-to-dimethyl ether (DME) reaction was unambiguously identified to occur by the dehydration coupling of methoxy species at the fourfold-coordinated Ti(4+) sites (Ti(4c)), and for the first time confirms the predicted higher reactivity of this facet compared to other reported TiO2 facets. Surface chemistry of methanol on the anatase TiO2(001)-(1×4) surface is seldom affected by co-chemisorbed water. These results not only greatly deepen the fundamental understanding of elementary surface reactions of methanol on TiO2 surfaces but also show that TiO2 with a high density of Ti(4c) sites is a potentially active and selective catalyst for the important methanol-to-DME reaction.