Tuning the Electronic Structures of Anchor Sites to Achieve Zero-Valence Single-Atom Catalysts for Advanced Hydrogenation.

Single-atom catalysts (SACs) have recently become highly attractive for selective hydrogenation reactions owing to their remarkably high selectivity. However, compared to their nanoparticle counterparts, atomically dispersed metal atoms in SACs often show inferior activity and are prone to aggregate under reaction conditions. Here, by theoretical calculations, we show that tuning the local electronic structures of metal anchor sites on g-C3N4 by doping B atoms (BCN) with relatively lower electronegativity allows achieving zero-valence Pd SACs with reinforced metal-support orbital hybridizations for high stability and upshifted Pd 4d orbitals for high activity in H2 activation. The precise synthesis of Pd SACs on BCN supports with varied B contents substantiated the theoretical prediction. A zero-valence Pd1/BCN SAC was achieved on a BCN support with a relatively low B content. It exhibited much higher stability in a H2 reducing environment, and more strikingly, a hydrogenation activity, approximately 10 and 34 times greater than those high-valence Pd1/g-C3N4 and Pd1/BCN with a high B content, respectively.

An Efficient Way to Model Complex Iron Carbides: A Benchmark Study of DFTB2 against DFT.

Iron carbides have attracted increasing attention in recent years due to their enormous potential in catalytic fields, such as Fischer-Tropsch synthesis and the growth of carbon nanotubes. Theoretical calculations can provide a more thorough understanding of these reactions at the atomic scale. However, due to the extreme complexity of the active phases and surface structures of iron carbides at the operando conditions, calculations based on density functional theory (DFT) are too costly for realistically large models of iron carbide particles. Therefore, a cheap and efficient quantum mechanical simulation method with accuracy comparable to DFT is desired. In this work, we adopt the spin-polarized self-consistent charge density functional tight-binding (DFTB2) method for iron carbides by reparametrization of the repulsive part of the Fe-C interactions. To assess the performance of the improved parameters, the structural and electronic properties of iron carbide bulks and clusters obtained with DFTB2 method are compared with the previous experimental values and the results obtained with DFT approach. Calculated lattice parameters and density of states are close to DFT predictions. The benchmark results show that the proposed parametrization of Fe-C interactions provides transferable and balanced description of iron carbide systems. Therefore, spin-polarized DFTB2 is valued as an efficient and reliable method for the description of iron carbide systems.

Tuning the CO2 Hydrogenation Selectivity of Rhodium Single-Atom Catalysts on Zirconium Dioxide with Alkali Ions.

Tuning the coordination environments of metal single atoms (M1 ) in single-atom catalysts has shown large impacts on catalytic activity and stability but often barely on selectivity in thermocatalysis. Here, we report that simultaneously regulating both Rh1 atoms and ZrO2 support with alkali ions (e.g., Na) enables efficient switching of the reaction products from nearly 100 % CH4 to above 99 % CO in CO2 hydrogenation in a wide temperature range (240-440 °C) along with a record high activity of 9.4 molCO gRh -1 h-1 at 300 °C and long-term stability. In situ spectroscopic characterization and theoretical calculations unveil that alkali ions on ZrO2 change the surface intermediate from formate to carboxy species during CO2 activation, thus leading to exclusive CO formation. Meanwhile, alkali ions also reinforce the electronic Rh1 -support interactions, endowing the Rh1 atoms more electron deficient, which improves the stability against sintering and inhibits deep hydrogenation of CO to CH4 .

Probing Interfacial Electronic Effects on Single-Molecule Adsorption Geometry and Electron Transport at Atomically Flat Surfaces.

Clarifying interfacial electronic effects on molecular adsorption is significant in many chemical and biochemical processes. Here, we used STM breaking junction and shell-isolated nanoparticle-enhanced Raman spectroscopy to probe electron transport and adsorption geometries of 4,4'-bipyridine (4,4'-BPY) at Au(111). Modifying the surface with 1-butyl-3-methylimidazolium cation-containing ionic liquids (ILs) decreases surface electron density and stabilizes a vertical orientation of pyridine through nitrogen atom σ-bond interactions, resulting in uniform adsorption configurations for forming molecular junctions. Modulation from vertical, tilted, to flat, is achieved on adding water to ILs, leading to a new peak ascribed to CC stretching of adsorbed pyridyl ring and 316 % modulation of single-molecule conductance. The dihedral angle between adsorbed pyridyl ring and surface decreases with increasing surface electronic density, enhancing electron donation from surface to pyridyl ring.

Pinpointing the active sites and reaction mechanism of CO oxidation on NiO.

Exploring the active sites and reaction mechanisms of CO oxidation on metal oxides is of great significance in the field of heterogeneous catalysis. NiO has been attracting increasing attention in this field due to its high performance and low cost. Nevertheless, the active sites and reaction mechanism of NiO still remain controversial due to the complexity of the experiments involved, the limitations of characterization techniques, and the difficulty in searching for the global reaction pathway in theory. In this work, terraced and stepped NiO(100) surfaces, with and without oxygen vacancies, were established based on the Wulff construction, and the active sites and reaction mechanism were revealed at the atomic level using a novel global pathway searching method. Theoretical results indicate that the coordination-unsaturated Ni ions are the active sites for CO oxidation; O2 interacts with the low-coordinated Ni ions to form reactive oxygen species; then they react with CO to form CO2; oxygen species on stepped NiO(100) have a low barrier and sustain a catalytic cycle. The present work reveals that the best direction for the design and development of NiO-based catalysts with high performance is to prepare NiO catalysts with more defects and low-coordinated Ni ions. We anticipate that the approach adopted in this work can be applied to a wide range of heterogeneous catalysts for exploring the active sites and mechanisms.

Application of Ag/AgBr/GdVO4 composite photocatalyst in wastewater treatment.

Ag/AgBr/GdVO4 composite photocatalysts were designed and synthesized in this paper. The physical and chemical structures, as well as optical properties of the synthesized composite were investigated via XRD, XPS, TEM, and UV-vis. It is found that the composite showed a ternary heterojunction structure of Ag, AgBr and GdVO4. Meanwhile, it has a high intensity of light current, indicating its high separation efficiency of electron and hole. Photocatalytic oxidation of rhodamine B (RhB) under visible light irradiation was performed to investigate the activity of the Ag/AgBr/GdVO4 composite. Result indicates that it shows excellent photocatalytic activity. Under visible light irradiation for 12min, about 80% of RhB (30µmol/L) was degraded. The degradation rate is estimated to be 0.253 min-1, which is three times higher than that of pure AgBr. The high photoactivity can be ascribed to the synergetic effect of AgBr, GdVO4, and Ag nanoparticle in separation of electron-hole pairs.

Effects of strong interactions between Ti and ceria on the structures of Ti/CeO2.

The effects of strong interactions between Ti and ceria on the structures of Ti/CeO2(111) are systematically investigated by density functional theory calculation. To our best knowledge, the adsorption energy of a Ti atom at the hollow site of CeO2 is the highest value (-7.99 eV) reported in the literature compared with those of Au (-0.88--1.26 eV), Ag (-1.42 eV), Cu (-2.69 eV), Pd (-1.75 eV), Pt (-2.62 eV) and Sn (-3.68 eV). It is very interesting to find that Ti adatoms disperse at the hollow site of CeO2(111) to form surface TiOx species, instead of aggregating to form Ti metal clusters for the Ti-CeO2 interactions that are much stronger than those of Ti-Ti ones. Ti adatoms are completely oxidized to Ti4+ ions if they are monatomically dispersed on the next near hollow sites of CeO2(111) (xTi-NN-hollow); while Ti3+ ions are observed when they locate at the near hollow sites (xTi-N-hollow). Due to the electronic repulsive effects among Ti3+ ions, the adsorption energies of xTi-N-hollow are slightly weaker than those of xTi-NN-hollow. Simultaneously, the existence of unstable Ti3+ ions on xTi-N-hollow also leads to the restructuring of xTi-N-hollow by surface O atoms of ceria transferring to the top of Ti3+ ions, or oxidation by O2 adsorption and dissociation. Both processes improve the stability of the xTi/CeO2 system by Ti3+ oxidation. Correspondingly, surface TiO2-like species form. This work sheds light into the structures of metal/CeO2 catalysts with strong interactions between the metal and the ceria support.

New application of Z-scheme Ag3PO4/g-C3N4 composite in converting CO2 to fuel.

This research was designed for the first time to investigate the activities of photocatalytic composite, Ag3PO4/g-C3N4, in converting CO2 to fuels under simulated sunlight irradiation. The composite was synthesized using a simple in situ deposition method and characterized by various techniques including Brunauer-Emmett-Teller method (BET), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (DRS), photoluminescence spectroscopy (PL), and an electrochemical method. Thorough investigation indicated that the composite consisted of Ag3PO4, Ag, and g-C3N4. The introduction of Ag3PO4 on g-C3N4 promoted its light absorption performance. However, more significant was the formation of heterojunction structure between Ag3PO4 and g-C3N4, which efficiently promoted the separation of electron-hole pairs by a Z-scheme mechanism and ultimately enhanced the photocatalytic CO2 reduction performance of the Ag3PO4/g-C3N4. The optimal Ag3PO4/g-C3N4 photocatalyst showed a CO2 conversion rate of 57.5 µmol · h(-1) · gcat(-1), which was 6.1 and 10.4 times higher than those of g-C3N4 and P25, respectively, under simulated sunlight irradiation. The work found a new application of the photocatalyst, Ag3PO4/g-C3N4, in simultaneous environmental protection and energy production.

Choosing a proper exchange-correlation functional for the computational catalysis on surface.

To choose a proper functional among the diverse density functional approximations of the electronic exchange-correlation energy for a given system is the basis for obtaining accurate results of theoretical calculations. In this work, we first propose an approach by comparing the calculated ΔE0 with the theoretical reference data based on the corresponding experimental results in a gas phase reaction. With ΔE0 being a criterion, the three most typical and popular exchange-correlation functionals (PW91, PBE and RPBE) were systematically compared in terms of the typical Fischer-Tropsch synthesis reactions in the gas phase. In addition, verifications of the geometrical and electronic properties of modeling catalysts, as well as the adsorption behavior of a typical probe molecule on modeling catalysts are also suggested for further screening of proper functionals. After a systematic comparison of CO adsorption behavior on Co(0001) calculated by PW91, PBE, and RPBE, the RPBE functional was found to be better than the other two in view of FTS reactions in gas phase and CO adsorption behaviors on a cobalt surface. The present work shows the general implications for choosing a reliable exchange-correlation functional in the computational catalysis of a surface.

Theoretical study of the effects of surface Cu coordination environment on CO2 hydrogenation to CH3OH.

The coordination environment of Cu (the coordination number and arrangement of surface atoms) plays an important role in CO2 hydrogenation to CH3OH. Compared with the extensive studies of the effects of coordination number, the comprehensive effects of coordination number and arrangement of surface atoms were seldom explored in literature. To unravel the effects of surface Cu coordination environment on CO2 hydrogenation to CH3OH, the adsorption and reaction behaviors of H2 and CO2 on Cu(111), (100), (110) and (211) with different coordination numbers and arrangement of surface Cu were systematically calculated by density functional theory (DFT) and kinetic Monte Carlo (kMC) simulation. It was found that the adsorption energies of intermediates in CO2 hydrogenation on Cu surfaces increase with the decrease of coordination number. When the Cu coordination numbers are similar, the charge density on the open surface derived from the different atom arrangement becomes larger and leads to stronger interaction with intermediates than that on the compact one. DFT calculation and kMC simulation indicate that methanol formation pathway follows CO2*→HCOO*→HCOOH*→H2COOH*→H2CO*→CH3O*→CH3OH* on four Cu facets; CO formation is via CO2 direct dissociation on Cu(111), (100) and (110) but COOH* dissociation on (211). The low-coordinated surface Cu with more openness on Cu(211) is the highly active site for CO2 hydrogenation to CH3OH with high turnover of frequency (3.71 × 10-4 s-1) and high selectivity (87.17 %) at 600 K, PCO2 = 7.5 atm and PH2 = 22.5 atm, which is much higher than those on Cu(111), (100) and (110). This work unravels the effects of coordination environment on CO2 hydrogenation at the molecular level and provides an important insight into the design and development of catalysts with high performance in CO2 hydrogenation to CH3OH.

Exploring microstructures of metal-doped oxides via simulated Raman spectrum.

Solid solution of metal-doped oxide has been widely used in material industry and catalysis process. Its performance is highly correlated with the distribution of doped ions. Due to the complex distribution of doped ions in solid solution and its variation with temperatures, to obtain the microstructures of metal-doped ions in solid solution remains a substantial challenge. Taken Ce1-xZrxO2 as a model, the global structure searching, structures proportion with temperature determined by Boltzmann distribution, and the weighted simulation Raman spectra were integrated to explore the microstructures of metal-doped solid solution oxides. It was further verified by application into rutile and anatase TiO2 mixture, indicating that the present method is feasible to deduce the microstructure of metal composite oxides. We anticipate that it provides a powerful solution to explore microstructures of solid solution and complex metal oxides.

A DFT study of the structures of Au(x) clusters on a CeO2(111) surface.

Studying the structures of metal clusters on oxide supports is challenging due to their various structural possibilities. In the present work, a simple rule in which the number of Au atoms in different layers of Au(x) clusters is changed successively is used to systematically investigate the structures of Au(x) (x=1-10) clusters on stoichiometric and partially reduced CeO(2)(111) surface by DFT calculations. The calculations indicate that the adsorption energy of a single Au atom on the surface, the surface structure, as well as the Au-Au bond strength and arrangement play the key roles in determining Au(x) structures on CeO(2)(111). The most stable Au(2) and Au(3) clusters on CeO(2)(111) are 2D vertical structures, while the most stable structures of Au(x) clusters (x>3) are generally 3D structures, except for Au(7). The 3D structures of large Au(x) clusters in which the Au number in the bottom layer does not exceed that in the top layer are not stable. The differences between Au(x) on CeO(2)(111) and Mg(100) were also studied. The stabilizing effect of surface oxygen vacancies on Au(x) cluster structures depends on the size of Au(x) cluster and the relative positions of Au(x) cluster and oxygen vacancy. The present work will be helpful in improving the understanding of metal cluster structures on oxide supports.

Synthesis of functionalized 2,3-dihydroisoxazoles by domino reactions in water and unexpected ring-opening reactions of 2,3-dihydroisoxazoles.

α,α-Dicyanoolefins react with hydroxylamine to afford 2,3-dihydroisoxazoles (2,3-dihydroisoxazoles can be easily isolated by filtration) in excellent yields under mild and environmentally benign conditions. A one-pot reaction in tandem with an unexpected ring-opening of 2,3-dihydroisoxazoles has been developed as well.

Hydrogen fluoride adsorption and reaction on the α-Al2O3(0001) surface: a density functional theory study.

The adsorption and reaction behaviors of HF on the α-Al(2)O(3)(0001) surface are systematically investigated using density functional theory method. By increasing the number of HF molecules in a p(2 × 1) α-Al(2)O(3)(0001) slab, we find that HF is chemically dissociated at low coverage; while both physical and dissociative adsorption occurs at a 3/2 monolayer (ML) coverage. At the same coverage (1.0 ML), diverse configurations of the dissociated HF are obtained in the p(2 × 1) model; while only one is observed in the p(1 × 1) slab due to its smaller surface area compared with the former one. Preliminary fluorination reaction study suggests that the total energy of two dissociated HF in the p(2 × 1) slab increases by 1.00 and 0.72 eV for the formation and desorption of water intermediate, respectively. The coadsorption behaviors of HF and H(2)O indicate that the pre-adsorbed water is unfavorable for the fluorination of Al(2)O(3), which is well consistent with the experimental results. The calculated density of states show that the peak of σ(H-F) disappears, while the peaks of σ(H-O) and σ(Al-F) are observed at -8.4 and -5 to -3 eV for the dissociated HF. Charge density difference analysis indicates that the dissociated F atom attracts electrons, while no obvious changes on electrons are observed for the surface Al atoms.

Step Site-Specific Semihydrogenation of Acetylene on the Au Surface.

Supported Au catalysts are highly selective and size-sensitive in catalytic hydrogenation of alkynes under mild conditions. Using thermal-programmed desorption and density functional theory calculations, we study the hydrogenation reactions of C2 hydrocarbons with atomic H and clarify the site-specific selective hydrogenation of C2H2 on Au(997) at low temperatures. On atomic H(a) covered Au(997), hydrogenation of C2H2 goes with 100% selectivity to C2H4 at steps, yet no hydrogenation occurs at terraces; adsorbed C2H4 on neither steps nor terraces reacts with H(a). DFT calculations suggest that the increased adsorption free energies and appropriate reaction barriers of C2 species at steps lead to the step-site specific semihydrogenation of C2H2. These results elucidate the elementary surface reactions between C2 hydrocarbons and atomic H on Au surfaces at the molecular level and significantly deepen the fundamental understanding of the unique selectivity of Au catalysts.

A combined DFTB nanoreactor and reaction network generator approach for the mechanism of hydrocarbon combustion.

We explored the mechanism of ethylene combustion by combining a density functional tight-binding based nanoreactor molecular dynamic method (DFTB-NMD) and a hidden Markov model (HMM) based reaction network generator approach. The results demonstrate that the DFTB-NMD is a promising method to predict the mechanism of complicated combustion reactions.

Oxidative Coupling of Methanol with Molecularly Adsorbed Oxygen on Au Surface to Methyl Formate.

Supported Au catalysts efficiently catalyze the oxidative coupling of methanol with O2 to methyl formate, in which the atomic O species (O(a)) formed via O2 dissociation on the Au surface has been considered as the active oxygen species. Herein we report for the first time that the oxidative coupling of methanol can occur facilely with molecularly adsorbed O2 species (O2(a)) on a Au(997) surface at temperatures as low as around 125 K, while that with O(a) occurs at around 175 K. Both experimental and theoretical calculation results demonstrate a novel reaction mechanism of oxidative coupling of CH3OH with O2(a) via a dioxymethylene (H2COO) intermediate, resulting in the production of both HCOOCH3 and HCOOCH3. These results reveal the unique reactivity of molecularly adsorbed O2 species on Au surfaces for low-temperature oxidation reactions.

The active sites of Cu-ZnO catalysts for water gas shift and CO hydrogenation reactions.

Cu-ZnO-Al2O3 catalysts are used as the industrial catalysts for water gas shift (WGS) and CO hydrogenation to methanol reactions. Herein, via a comprehensive experimental and theoretical calculation study of a series of ZnO/Cu nanocrystals inverse catalysts with well-defined Cu structures, we report that the ZnO-Cu catalysts undergo Cu structure-dependent and reaction-sensitive in situ restructuring during WGS and CO hydrogenation reactions under typical reaction conditions, forming the active sites of CuCu(100)-hydroxylated ZnO ensemble and CuCu(611)Zn alloy, respectively. These results provide insights into the active sites of Cu-ZnO catalysts for the WGS and CO hydrogenation reactions and reveal the Cu structural effects, and offer the feasible guideline for optimizing the structures of Cu-ZnO-Al2O3 catalysts.

Conductive One-Dimensional Coordination Polymers with Tunable Selectivity for the Oxygen Reduction Reaction.

Conductive materials involving nonprecious metal coordination complexes as electrocatalysts for the oxygen reduction reaction (ORR) have received increasing attention in recent years. Herein, we reported efficient ORR electrocatalysts containing M-S2N2 sites with tunable selectivity based on simple one-dimensional (1D) coordination polymers (CPs). The 1D CPs were synthesized from M(OAc)2 and 2,5-diamino-1,4-benzenedithiol (DABDT) by a solvent thermal method. Due to their good electrical conductivities (10-6-10-2 S cm-1), the 1D CPs could be used as ORR catalysts in low catalytic amounts without the addition of carbon materials. Cobalt-based CPs showed a well-organized structure of nanosheets with Co-S2N2 sites exposed and exhibited remarkable electrocatalytic ORR activity (Eonset = 0.93 V vs reversible hydrogen electrode (RHE), E1/2 = 0.82 V, n = 3.85, JL = 5.22 mA cm-2, Tafel slope of 63 mV dec-1) in alkaline media. However, nickel-based CPs favored a 2e- ORR process with ∼87% H2O2 selectivity and an Eonset of 0.78 V. This work provides new opportunities for the construction of ORR catalysts based on conductive nonprecious metal CPs.

A density functional theory study of the CH(2)I(2) reaction on Ag(111): Thermodynamics, kinetics, and electronic structures.

Density functional theory calculation was performed to study the adsorption and reaction of CH(2)I(2) on Ag(111). Thermodynamically favorable reactions of CH(2)I(2) on Ag(111) are C-I bond ruptures and CH(2) coupling to form ethylene. The energy barriers for the C-I bond ruptures of chemisorbed CH(2)I(2) on Ag(111) are 0.43-0.48 eV, whereas the activation energy for the C-H bond rupture of chemisorbed CH(2) on Ag(111) is 1.76 eV. The coupling reaction barrier of neighboring chemisorbed CH(2) to form C(2)H(4) on Ag(111) was much less than those of the C-I bond ruptures of CH(2)I(2)(a) and the migration of chemisorbed CH(2) on Ag(111). The adsorption behaviors of different surface species on Ag(111) were well explained in terms of the charge density difference.