Low-coordinated Sites Research Articles

Eliminating dissolution of platinum-based electrocatalysts at the atomic scale.

A remaining challenge for the deployment of proton-exchange membrane fuel cells is the limited durability of platinum (Pt) nanoscale materials that operate at high voltages during the cathodic oxygen reduction reaction. In this work, atomic-scale insight into well-defined single-crystalline, thin-film and nanoscale surfaces exposed Pt dissolution trends that governed the design and synthesis of durable materials. A newly defined metric, intrinsic dissolution, is essential to understanding the correlation between the measured Pt loss, surface structure, size and ratio of Pt nanoparticles in a carbon (C) support. It was found that the utilization of a gold (Au) underlayer promotes ordering of Pt surface atoms towards a (111) structure, whereas Au on the surface selectively protects low-coordinated Pt sites. This mitigation strategy was applied towards 3 nm Pt3Au/C nanoparticles and resulted in the elimination of Pt dissolution in the liquid electrolyte, which included a 30-fold durability improvement versus 3 nm Pt/C over an extended potential range up to 1.2 V.

Crystal facet-dependent activity of α-Mn2O3 for oxygen reduction and oxygen evolution reactions

Electrocatalysts with different morphologies and specific exposed facets usually exhibit distinguished activities. In this work, two α-Mn2O3 with different morphologies and crystal facets were successfully prepared by solvothermal method and investigated as ORR/OER bifunctional catalysts for the first time. The results showed that the catalytic performance is affected by the crystal facet of α-Mn2O3, and the α-Mn2O3-cubic has better bifunctional activity than α-Mn2O3-octahedra. As revealed by catalyst characterization, the enhanced activity is originated with the nature of the exposed α-Mn2O3 (100) facet. The (100) facet with abundant low-coordinated surface oxygen sites makes the formation of more oxygen vacancies, which can improve the charge transfer and optimize the adsorption energies for intermediates, thereby enhancing the bifunctional activity for ORR/OER. This work highlights the correlation between OER/ORR activities and exposed crystal planes of α-Mn2O3 catalysts and it may deepen the understanding of facetdependent activity of α-Mn2O3 and points out a strategy to improve their catalytic activity by crystal facet engineering.

Kaolins of high iron-content as photocatalysts: Challenges of acidic surface modifications and mechanistic insights

Surface modifications of natural kaolins (Felsőpetény, Hungary) with iron-content up to 9% (m/m) were carried out by varying acid concentration (5, 8 and 11 M HCl) and treatment time (1, 3 and 6 h) in order to evaluate the influence of treatment conditions on photochemical activity, amount of low coordinate Al defect sites, porosity, and acid/base character of the surface. Nitrogen adsorption and temperature-programmed desorption measurements showed that the acid treatment slightly reduced the pore volume and the surface area, while the average pore diameter and the number of acidic centers were increased. Solid-state 27Al NMR spectroscopy demonstrated the presence of mechanistically relevant Al defect sites. The distribution, concentration, and particle size of the iron oxide co-minerals were found to be influential to photochemical activity. Greater than 90% photochemical degradation efficiency of oxalic acid can be achieved by acid-treated samples upon 3 h exposure to 8 M HCl solution with good reproducibility.

Controllable CO adsorption determines ethylene and methane productions from CO2 electroreduction

Among all CO2 electroreduction products, methane (CH4) and ethylene (C2H4) are two typical and valuable hydrocarbon products which are formed in two different pathways: hydrogenation and dimerization reactions of the same CO intermediate. Theoretical studies show that the adsorption configurations of CO intermediate determine the reaction pathways towards CH4/C2H4. However, it is challenging to experimentally control the CO adsorption configurations at the catalyst surface, and thus the hydrocarbon selectivity is still limited. Herein, we seek to synthesize two well-defined copper nanocatalysts with controllable surface structures. The two model catalysts exhibit a high hydrocarbon selectivity toward either CH4 (83%) or C2H4 (93%) under identical reduction conditions. Scanning transmission electron microscopy and X-ray absorption spectroscopy characterizations reveal the low-coordination Cu0 sites and local Cu0/Cu+ sites of the two catalysts, respectively. CO-temperature programed desorption, in-situ attenuated total reflection Fourier transform infrared spectroscopy and density functional theory studies unveil that the bridge-adsorbed CO (COB) on the low-coordination Cu0 sites is apt to be hydrogenated to CH4, whereas the bridge-adsorbed CO plus linear-adsorbed CO (COB + COL) on the local Cu0/Cu+ sites are apt to be coupled to C2H4. Our findings pave a new way to design catalysts with controllable CO adsorption configurations for high hydrocarbon product selectivity.

Improvement of photocatalytic hydrogen generation of leaves-like CdS microcrystals with a surface decorated by dealloyed Pt-Cox nanoparticles

Herein, we report the platinum-cobaltx (Pt-Cox) nanoparticles (NPs) as a cocatalyst were synthesized by solvothermal method, and then modified on the surface of cadmium sulfide (CdS) via means of photoinduced deposition. Under visible light irradiation, the hydrogen generation rate of Pt3Co/CdS reached to 45.09 mmol·h−1·g−1 (QE = 46.01%, λ = 420 nm), which was 2.7 times and 1.8 times higher than that of Co/CdS and Pt/CdS, respectively. The high photocatalytic performance could be attributed to the strong synergistic effect between Pt and Co. Astoundingly, the photocatalytic activity was enhanced after the Pt-Cox NPs by acid treatment. The hydrogen production activity of Pt3Co-HNO3-5h/CdS was high up to 48.66 mmol·h−1·g−1 (QE = 54.36%, λ = 420 nm), which was better than that of Pt3Co/CdS. It is proved that the enhanced photocatalytic activity was mainly due to the dealloyed Pt3Co NPs resulted in low coordination sites of Pt on the surface and provided more active sites, which enhanced the ability to capture photo-generated electrons, thus promoted the effective separation of carriers. In addition, XRD implied that Co atoms enter the Pt structure to form Pt-Co alloy phase. TEM showed that Pt-Co NPs has a polyhedral structure with size of about 13–15 nm. SEM/XPS revealed that Pt3Co NPs has been successfully decorated onto the surface of CdS. UV–Vis spectra of CdS nanoparticles interpreted its trait in visible light adsorption. PL spectra and I-T curves indicated that the Pt3Co-HNO3-5h/CdS and Pt3Co/CdS have higher electrons and holes separation efficiency.

Strain and Low-Coordination Effects on Monolayer Nanoislands of Pd and Pt on Au(111): A Comparative Analysis Based on Density Functional Results.

Recent experiments demonstrated that the catalytic centers for the hydrogen evolution reaction (HER) are different on Pd and Pt nanoislands on Au(111). Inspired by these experiments, we examined the geometric, energetic, electronic and hydrogen adsorption properties of monolayer model nanoislands of Pd and Pt supported on Au(111) with density functional theory calculations. Accordingly, Au-tensile strain effects can be nearly 50% larger on the geometric structure of nanoislands of Pd on Au(111) than their Pt analogs, resulting on different electronic properties for these nanoislands. Despite these differences between Pd and Pt nanoisland on Au(111), our computational modelling of the hydrogen adsorption suggests that the unique catalytic centers for the HER on Pd and Pt nanoislands supported on Au(111) derive from the existence of low-coordinated adsorption sites and the intrinsic properties of Pd and Pt, but not from Au-tensile strain effects.

Nanoporous Metals for Heterogeneous Catalysis: Following the Success of Raney Nickel.

Nanoporous metals (NPMs) with diversified shapes and compositions can be readily fabricated by dealloying monolithic alloys through chemical or electrochemical processes. Benefited from their high surface area, high density of low-coordinated sites on the ligament surface, and unsupported character, NPMs have attracted increasing attentions as a new class of heterogeneous catalysts with high activity, selectivity, and long-term stability, reminiscent of the great success of Raney nickel. In the present minireview, we summarize the recent advances in this exciting field and provide a critical discussion of the nature of their active sites and the structure-property correlation.

SnO2 Catalyzed Simultaneous Reinforcement of Carbon Dioxide Adsorption and Activation Towards Electrochemical Conversion of CO2 to HCOOH

The increasing CO2 content in the earth's atmosphere has provoked the concerns about global warming. Electrochemical CO2 reduction reaction (CO2RR) has been identified as an attractive route meeting both economic and environmental requirements. HCOOH with advantages of easy storage and transportation is an important chemical intermediate and renewable energy carrier. Therefore, transferring CO2 into HCOOH is a promising and desirable option to reduce the CO2 emissions and generate renewable energy simultaneously. Tin based materials are promising candidates for CO2RR to HCOOH. The unsaturated O=O bonds in metallic oxide can store CO2 by bonding the unshared pair of electrons of CO2. Simultaneously, the activation energy barrier of CO2RR is lower because those electrons can transfer back from defects to the adsorbates and alter the d-band center of catalysts interacting with the adsorbed CO2. Herein, a highly efficient wavy SnO2 (Denoted as NW-SnO2) electrocatalyst having abundant low coordinated sites, oxygen vacancies and gain boundaries was synthesized by a facile and cost-effective method, which is advantageous in terms of its large scale practical applications. Results Firstly, we highlight the synthesis of the wavy SnO2 with abundant oxygen vacancies, low-coordinated active sites and gain boundaries via one pot urea assisted hydrothermal method, as shown in Fig. 1a. In a typical synthesis of NW-SnO2, 0.08 g SnCl4·5H2O and 0.8 g urea were firstly dissolved in 32 mL deionized water. Subsequently, 1.6 mL of hydrochloric acid was added under ultrasonic treatment. The whole solution was then transferred into a 50 mL Teflon-lined stainless-steel autoclave to react at 90 °C for 15 h.The composition and structure crystallinity of the as-prepared wavy SnO2 and commercial nanoparticle SnO2 were initially characterized by performing X-ray diffraction (XRD) analysis and high-resolution transmission electron microscopy (HRTEM). As shown in Fig. 1b, those marked diffraction peaks can be indexed to the rutile tetragonal phase of SnO2 (JCPDS card No. 41-1445). Low-resolution TEM image (Fig. 1c) reveals that the NW-SnO2 mainly possesses a well-dispersed worm-like wavy morphology. From the HRTEM image shown in Fig. 1e and 1f, we can clearly see the wavy network of NW-SnO2. It should be noted that each nanowire is composed of a number of interconnected crystal building blocks, as shown in the model. The lattice spacing marked in Fig. 1g is 0.34 nm and 0.23 nm, corresponding to the (110) and (101) planes of SnO2, respectively, which indicates the crystal purity of NW-SnO2 and is highly consistent with the results of XRD. Electrocatalytic performance To evaluate the electrocatalytic CO2RR performance of the NW-SnO2 and NP-SnO2, the linear sweep voltammetry (LSV) polarization curves of the NW-SnO2 and NP-SnO2 electrocatalysts in CO2 saturated 0.5 M KHCO3 solution were obtained (Fig. 2a). Fig. 2b shows potential-dependent FE of all products from CO2RR recorded by NP-SnO2 and NW-SnO2 electrocatalyst, respectively. Specifically, the formation of HCOOH starts at -0.4 vs VRHE (Fig. 2b), and the FE of HCOOH is up to maximum (87.4%) at -1.0 vs VRHE. In contrast, the maximum selectivity of 61.7% at -1.1 vs VRHE for HCOOH on commercial NP-SnO2 is much lower than NW-SnO2. Notably, the onset potential of HCOOH on NW-SnO2 is only around 190 mV, 0.2 V lower than that of NP-SnO2. The maximum energy efficiency (EE) is 57.5% and 38.0% for NP-SnO2 and NW-SnO2, respectively (Fig. 2e). At -1.0 vs VRHE, the NW-SnO2 reaches the maximum partial current density of HCOOH (22 mA/cm2), which is 2.3 times higher than the value of the NP-SnO2 (9.7 mA/cm2). More importantly, the 18 hours long-term stability of NW-SnO2 was also measured using chronoamperometry at -0.8 vs VRHE. Therefore, our NW-SnO2 electrocatalyst not only shows good selectivity and lower onset potential under lower loading (0.576 mg/cm2), but also enjoys the advantage of its easier synthetic method, which is an important factor for commercial applications. Conclusion To summarize, we synthesized wavy SnO2 with higher ratio of low-coordinated active sites and abundant gain boundaries and oxygen vacancies by an economical and simple method. The NW-SnO2 shows an onset potential of 190 mV, 0.2 V lower than that of the commercial NP-SnO2. In addition, NW-SnO2 electrocatalyst could electroreduce CO2 into HCOOH with a partial current density up to 22 mA/cm2 and a maximum FEHCOOH of 87.4% at -1.0 vs VRHE. The abundant low-coordinated sites, gain boundaries and oxygen vacancies all play an essential role for the increased production of HCOOH and the suppressed yield of H2. Importantly, this study paves a new path to design other metal oxide based electrocatalysts for CO2RR via tuning their surface structures.

Dendritic Silver/Palladium Alloy Arrays for Electrochemical CO2 Reduction

Increased fossil fuel consumption has sparked concerns about developing new energy sources and reducing carbon dioxide emissions. The electrochemical reduction of CO2 using renewable energy sources, e.g. wind and solar is an appealing method for making a range of carbon-based products such as CO even more valuable hydrocarbons.1 There have been several significant advances in CO2 electrochemical reduction in recent years.2 However, currently there are no efficient catalysts for CO2 reduction as activation of CO2 requires a large input of energy; and the reaction usually results in the broad product distribution.3 Thus, the search for new catalysts, which can promote this reaction along a specific reaction pathway, is one of the most important tasks to make electrochemical CO2 reduction successful.Herein, we developed dendritic silver/palladium (Ag/Pd) alloys nanoarrays by a simple galvanic replacement reaction. Figure 1a demonstrates a typical SEM image of Ag/Pd dendrites with feather-like branches. The earlier study proposed the vacancies generated by the displacement reaction attract silver diffusing to the surface and form alloys with Pd.4 Compared with the alloys of Ag80Pd20 or Ag60Pd40, Ag70Pd30 shows the better performance that can selectively convert CO2 to CO with a maximum faradaic efficiency (FE) of 98.6% and a current density of ~ 9 mA/cm2 at the potential of −0.8 V vs. RHE (Figure 1b, c).The high performance of Ag/Pd alloy can be ascribed to the synergistic effects between Pd and Ag. Alloying can adjust electronic state to optimize the adsorption and coupling of intermediate. The calculated Gibbs free energy (ΔG) diagram is depicted in Figure 1d, representing CO2 reduction to CO through elementary reaction steps involving the intermediates of *COOH and *CO according to first principles calculations. The formation of *COOH with an energy barrier of 0.81 eV on Ag70Pd30 alloy surface, which is ~ 0.1 eV lower than on the pure Ag(111) surface. In addition, dendrite with multiple active sites and hydrophobic array might play an indispensable role in improving the catalytic performance. The dendritic vertical arrays structure has an increased active surface area, which will expose more low-coordinated surface sites. Contact angle measurements have confirmed the hydrophobic surface feature of this alloy (Figure 1e), which can enhance the capture of gas, in turn increase the concentration of CO2 on the electrode surface, therefore improve CO2 reduction selectivity.5 Figure 1. (a) SEM image of Ag70Pd30 sample. (b) Potential-dependent faradaic efficiency of different samples on electrocatalytic reduction of CO2 to CO. (c) Stability performance of Ag70Pd30 for CO2 reduction operated at the potential of −0.8 V vs. RHE for 8 h. (d) Free energy diagram for electrochemical CO2 reduction to CO. (e) Contact angel of silver piece and silver/palladium dendrites.

The effect of chlorine vacancy in CuCl2 (0 0 1) catalyst on the mechanism of SiCl4 dissociation into SiHCl3: A DFT study

In order to elucidate the role of chlorine vacancy on CuCl2 catalyst in species adsorption and the mechanism of SiCl4 dissociation into SiHCl3, density functional theory (DFT) calculations were performed over the CuCl2(0 0 1) surfaces with single and double Cl-vacancy, comparing with those over the perfect CuCl2(0 0 1). The calculation results show that Cl vacancies on the CuCl2(0 0 1) surface cause low-coordination site Cu atoms and the exposed Cu atoms work as the actually catalytic active sites, which can promote species adsorption and dissociation. The CuCl2 catalyst enriched with exposed Cu active sites exhibits higher catalytic activity and higher selectivity of SiHCl3. Bader charge analysis indicates that the smaller positive charge leads to the higher species adsorption energies and better catalytic selectivity and activity of SiCl4 dissociation into SiHCl3. The closer the d-band center is to the Fermi level, the higher the species adsorption energy and the better catalytic selectivity and activity of SiCl4 dissociation into SiHCl3. It suggests that the double Cl-vacancy CuCl2(0 0 1) catalyst has better activity and selectivity to produce SiHCl3 via SiCl4 decomposition, and the favorable pathway is SiCl4 → SiCl3 + Cl+(H) → SiHCl3, which can provide useful guidance on the development of efficient catalysts for SiCl4 dissociation into SiHCl3.

Highly disordered cobalt oxide nanostructure induced by sulfur incorporation for efficient overall water splitting

Exploitation of cost-efficient active electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) plays a significant role for scalable electricity-to-hydrogen energy conversion. Crystalline transition metal oxides as the promising non-noble catalysts, however, are often suffering from the large excess overpotential and unsatisfactory performance. To boost their intrinsic catalytic property, we report here an incorporation of electronegative sulfur into crystalline cobalt oxide (S-CoO x ) to create structural disorder via a facile room-temperature ion exchange strategy. Compared with its crystalline form, the disorder in S-CoO x catalyst enables the increased low oxygen coordination and rich defect sites, which endows S-CoO x a superior catalytic activity for both OER and HER in alkali. Intriguingly, a water electrolyser adopting S-CoO x as both OER and HER electrode catalysts requires mere 1.63 V to reach a current density of 10 mA cm −2 in 1 M KOH. This work highlights the effectiveness of designing high-performing electrocatalysts for water electrolysers based on disordered structural materials. • The structurally disordered S-CoO x catalyst was synthesized via a facile room-temperature sulfur ion exchange strategy. • The structurally disordered catalyst shows increased low oxygen coordination and more defect sites. • Experimental and DFT calculation results reveal that a preferable electronic state was generated for the disorderedS-CoO x catalyst. • This novel structurally disordered strategy enables the S-CoO x catalyst a superior catalytic activity for both OER and HER in alkaline electrolyte.

Single Atom Dynamics in Chemical Reactions.

ConspectusMany heterogeneous chemical reactions involve gases catalyzed over solid surfaces at elevated temperatures and play a critical role in the production of energy, healthcare, pollution control, industrial products, and food. These catalytic reactions take place at the atomic level, with active structures forming under reaction conditions. A fundamental understanding of catalysis at the single atom resolution is therefore a major advance in a rational framework upon which future catalytic processes can be built. Visualization and analysis of gas-catalyst chemical reactions at the atomic level under controlled reaction conditions are key to understanding the catalyst structural evolution and atomic scale reaction mechanisms crucial to the performance and the development of improved catalysts and chemical processes. Increasingly, dynamic single atoms and atom clusters are believed to lead to enhanced catalyst performance, but despite considerable efforts, reaction mechanisms at the single atom level under reaction conditions of gas and temperature are not well understood. The development of the atomic lattice resolution environmental transmission electron microscope (ETEM) by the authors is widely used to visualize gas–solid catalyst reactions at this atomic level. It has recently been advanced to the environmental scanning TEM (ESTEM) with single atom resolution and full analytical capabilities. The ESTEM employs high-angle annular dark-field imaging where intensity is approximately proportional to the square of the atomic number (Z). In this Account, we highlight the ESTEM development also introduced by the authors for real time in situ studies to reliably discern metal atoms on lighter supports in gas and high temperature environments, evolving oxide/metal interfaces, and atomic level reaction mechanisms in heterogeneous catalysts more generally and informatively, with utilizing the wider body of literature. The highlights include platinum/carbon systems of interest in fuel cells to meet energy demands and reduce environmental pollution, in reduction/oxidation (redox) mechanisms of copper and nickel nanoparticles extensively employed in catalysis, electronics, and sensors, and in the activation of supported cobalt catalysts in Fischer–Tropsch (FT) synthesis to produce fuels. By following the dynamic reduction process at operating temperature, we investigate Pt atom migrations from irregular nanoparticles in a carbon supported platinum catalyst and the resulting faceting. We outline the factors that govern the mechanism involved, with the discovery of single atom interactions which indicate that a primary role of the nanoparticles is to act as reservoirs of low coordination atoms and clusters. This has important implications in supported nanoparticle catalysis and nanoparticle science. In copper and nickel systems, we track the oxidation front at the atomic level as it proceeds across a nanoparticle, by directly monitoring Z-contrast changes with time and temperature. Regeneration of deactivated catalysts is key to prolong catalyst life. We discuss and review analyses of dynamic redox cycles for the redispersion of nickel nanoparticles with single atom resolution. In the FT process, pretreatment of practical cobalt/silica catalysts reveals higher low-coordination Co0 active sites for CO adsorption. Collectively, the ESTEM findings generate structural insights into catalyst dynamics important in the development of efficient catalysts and processes.

Direct Spectroscopic Observation of Cyanide-Induced Restructuring of Pt at the Solid–Liquid Interface

Obtaining insight into surface restructuring induced by adsorbates is essential for understanding surface (electro)chemical reactions and for better design of functional materials. Here, the microscopic restructuring of Pt(111) upon adsorption of 1,4-phenylene diisocyanide (PDI) at the solid–liquid interface is reported. With in situ vibrational sum-frequency generation spectroscopy supported by ex situ scanning tunneling microscopy studies, the surface-roughening process is followed in real-time by observing the vibrational signal of on-top-bound cyanide as a probe of the local coordination of the adsorption site. These studies show that PDI adsorbs initially on regular Pt(111) surface sites, which are transformed into low-coordinated sites with increasing exposure time. The low-coordinated sites are formed because of PDI-induced extraction of Pt atoms.

Categorization of atomic mixing patterns in bimetallic nanoparticles by the energy competition.

The superior properties of bimetallic nanoparticles are strongly related to their morphology. A better understanding of the morphological details would be the first step to design bimetallic nanoparticles for target applications. In this study, we discussed three possible categories of the atomic mixing patterns of bimetallic nanoparticles using the concept of competition between bond energy and surface energy. The categorization was confirmed with the thermodynamically stable structures of AgPt, AuPt, CuPt, PdPt, AgPd, AuPd, and CuPd obtained using Monte Carlo simulations. This work also proposed the phase diagrams of AuPt, CuPt, and PdPt nanoparticles, which displayed the details of atomic arrangements depending on the changes in size and atomic composition. The population in low-coordination sites and temperature effects were also intensively studied. The comprehensive understanding of these factors would facilitate the rational design and wide applications of bimetallic nanoparticles.

Structural evolution from exohedral to endohedral geometries, dynamical fluxionality, and structural forms of medium-sized anionic and neutral Au2Sin (n = 8-20) clusters.

The structural evolution of medium-sized anionic and neutral Au2Sin (n = 8-20) clusters is investigated by using density functional theory (DFT) calculations and CCSD(T) methods in combination with the particle swarm optimization (CALYPSO) global search algorithm. The geometries of anionic and neutral Au2Sin clusters change from exohedral to endohedral ones with the increasing cluster sizes, and the critical size of forming Au2-endohedral structures for both anionic and neutral clusters is confirmed to be n = 20, in which a C2h symmetric Au2-endohedral cage-like structure is observed. Anionic and neutral Au2Sin clusters are primarily dominated by prism-based geometries with most of them adopting different structural features. It is found that the two Au atoms prefer to occupy low coordination sites and interact with fewer Si atoms. The Au atoms carry negative charges because of the electron-transfer from Sin frameworks. Meanwhile, the two Au atoms have very weak interactions. The second-order energy differences and incremental binding energies of anionic and neutral Au2Sin clusters exhibit an odd-even alternation and Au2Si20-/0 clusters are proven to have the highest chemical stability among these Au-Si clusters. Interestingly, Au2Si8-, Au2Si9-, Au2Si13-, Au2Si15-, and Au2Si17- anions, along with Au2Si13, Au2Si14, and Au2Si17 neutrals, have multiplicity of structural forms and their low-lying isomers show dynamical fluxionality due to the low barrier energies.

Preference for low-coordination sites by adsorbed CO on small platinum nanoparticles.

FTIR spectra of 12CO adsorbed on poly(vinylpyrrolidone) (PVP)-stabilized colloidal platinum at room temperature were acquired and studied. Two new bands, at 2021 cm−1 and 1994 cm−1, were observed for the first time and were assigned to the stretching vibrations of CO linearly adsorbed on the Pt surface at edge and corner sites, respectively. The relative intensities of these two bands were found to vary with the coverage of CO, where the smallest particles showed the highest intensity, corresponding to the relative quantities of edge and corner sites per unit surface. The vibrational spectra signals reported for terrace sites on colloidal Pt red-shifted as the particle size was decreased, which showed the electronic interactions between the Pt surface and PVP, with PVP acting as an electron donor.

Identifying the composition and atomic distribution of Pt-Au bimetallic nanoparticle with machine learning and genetic algorithm

Bimetallic nanoparticles (AmBn) usually exhibit rich catalytic chemistry and have drawn tremendous attention in heterogeneous catalysis. However, challenged by the huge configuration space, the understanding toward their composition and distribution of A/B element is known little at the atomic level, which hinders the rational synthesis. Herein, we develop an on-the-fly training strategy combing the machine learning model (SchNet) with the genetic algorithm (GA) search technique, which achieve the fast and accurate energy prediction of complex bimetallic clusters at the DFT level. Taking the 38-atom PtmAu38-m nanoparticle as example, the element distribution identification problem and the stability trend as a function of Pt/Au composition is quantitatively resolved. Specifically, results show that on the Pt-rich cluster Au atoms prefer to occupy the low-coordinated surface corner sites and form patch-like surface segregation patterns, while for the Au-rich ones Pt atoms tend to site in the core region and form the core-shell (Pt@Au) configuration. The thermodynamically most stable PtmAu38-m cluster is Pt6Au32, with all the core-region sites occupied by Pt, rationalized by the stronger Pt-Pt bond in comparison with Pt-Au and Au-Au bonds. This work exemplifies the potent application of rapid global search enabled by machine learning in exploring the high-dimensional configuration space of bimetallic nanocatalysts.

Light emission induced by electric current at room temperature through the defect networks of MgO nanocubes

Magnesium oxide (MgO) is generally a wide band-gap oxide unable to conduct electric current in the bulk at room temperature. In this study, MgO nanocubes synthesized by self-burning micro-sized Mg metal powders in air showed electrical conductivity when they were sandwiched between two gold-mesh electrodes and steadily applied a voltage at room temperature (∼25 °C). In addition, a simultaneous light emission caused by the microdischarge of nitrogen molecules occurred adjacent to the cathode. The light emission was observed when traces of water vapor existed in the gas environment. In the case of a voltage pulse produced by switching off, transient emissions of Mg I and Mg II were detected on both sides of the electrodes. However, those steady and transient light emissions were not observed in the commercial MgO nanoparticles devoid of nanocubes. The light emissions shown in the cases of the steady-state might be caused by electron injection into the empty conductive states, which exist along the edges of MgO nanocubes, as a result of the spontaneous dissociation of water vapors at reactive sites of the nanocube surfaces as well as a result of the reduction of the energy barriers between the cathode and MgO nanocubes in contact. For transient emission, electrons trapped in the low coordinate sites were released with voltage pulse and neutralized the nearby Mg+ and Mg2+ ions, driving them into the excited neutral states, Mg I and Mg II.

Excellent long-term reactivity of inhomogeneous nanoscale Fe-based metallic glass in wastewater purification

Metallic glasses (MGs) have attracted great attention in wastewater treatment because of their high reactivity arising from amorphous structure, large residual stress and high density of low coordination sites. However, the reactivity of MGs would gradually slow down with time due to the passivation of active sites by corrosion products, resulting in limited long-term reactivity, which is also an unsolved key issue for established crystalline zero valent iron (ZVI) technology. Here, such problems are successfully overcome by introducing nanoscale chemical inhomogeneities in Fe-based MG (Fe-MGI), which apparently contributes to local galvanic cell effect and accelerates electron transfer during degradation process. More importantly, the selective depletion of Fe0 causes local volume shrinkage and crack formation, leading to self-peeling of precipitated corrosion products and reacted regions. Thereby fresh low coordination sites could be continuously provided, counteracting the mass transport and reactivity deteriorating problem. Consequently, Fe-MGI demonstrates excellent long-term reactivity and self-refreshing properties even in neutral solution. The present results provide not only a new candidate but also a new route of designing ZVI materials for wastewater treatment.

Some insight on the structure/activity relationship of metal nanoparticles in Cu/SiO2 catalysts

The activity of two Cu/SiO 2 catalysts prepared by the chemisorption hydrolysis technique has been tested in the hydrogenation reaction of 3-methyl-cyclohexanone. Both catalysts were found to be very active at 60 °C and 1 atm of H 2 . Characterization of the materials by FT-IR of adsorbed CO and TEM put in light the presence of well formed Cu cristallites. By assuming a cuboctahedral model we could show that the hydrogenation activity is linked to high coordination sites on the metal particle. A comparison is also reported with a sample prepared by ammonia evaporation that was found to be inactive in the hydrogenation reaction under the same experimental conditions. The activity of Cu/SiO 2 catalysts in the hydrogenation of 3-methyl-cyclohexanone was found to be linked to high coordination sites on the metal Cu particle, whereas acidic sites are represented by defective, low coordination sites.