Structure, Mobility, and Composition of Transition Metal Catalyst Surfaces. High-Pressure Scanning Tunneling Microscopy and Ambient-Pressure X-ray Photoelectron Spectroscopy Studies

Surface structure, mobility, and composition of transition metal catalysts were studied by high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) at high gas pressures. HP-STM makes it possible to determine the atomic or molecular rearrangement at catalyst surfaces, particularly at the low-coordinated active surface sites. AP-XPS monitors changes in elemental composition and chemical states of catalysts in response to variations in gas environments. Stepped Pt and Cu single crystals, the hexagonally reconstructed Pt(100) single crystal, and Pt-based bimetallic nanoparticles with controlled size, shape and composition, were employed as the model catalysts for experiments in this thesis. Surface reconstruction at low-coordinated step sites at high gas pressures was first explored on a stepped Pt(557) single crystal surface under O2. At 298 K, 1 Torr of O2 is able to create nanometer-sized clusters that are identified as surface Pt oxide by AP-XPS, which covers the entire Pt(557) surface. On the flat Pt(111) surface under 1 Torr of O2, Pt oxide clusters can form but are mostly accumulated within 2 nm from the steps. The hexagonal oxygen chemisorption pattern is observed on the terraces. At lower pressures such as 10-7 Torr, O2 only adsorbs at the step edges on Pt(557). The majority of the Pt oxide clusters disappear on both Pt(557) and Pt(111) surfaces after O2 is evacuated to the 10-8 Torr range. Quantitative XPS analysis with depth profiles indicates that the Pt oxide formed on Pt(557) is less than 0.6 nm thick and that the Pt oxide concentration at surface together with oxygen coverage varies reversibly with the O2 pressure. The disappearance of Pt oxide clusters upon O2 evacuation is ascribed to reactions of Pt oxide towards H2 and CO in the vacuum background gases. The structure and surface chemistry of the Pt(557) surface was therefore studied under H2-O2 and CO-O2 mixtures. After exposing Pt(557) to approximately 1 Torr of O2 to induce the formation of Pt oxide clusters, H2 was slowly added into the system. Both HP-STM and AP-XPS results show that the Pt oxide coverage decreases with the H2 partial pressure and that all the Pt oxide disappears at H2 partial pressures above 43 mTorr. Pt steps are restored with the removal of Pt oxide clusters. Water is produced in the gas-phase, which co-adsorbs with hydroxyl species on Pt(557). Detailed analysis shows that the consumption of surface Pt oxide is exclusively responsible for the decrease of oxygen coverage on Pt(557). In the coexistence of 1 Torr of CO and 1 Torr of O2, Pt oxide clusters are not observed like under the H2-O2 mixture. Instead, triangular Pt clusters and double-sized terraces induced by CO are observed. Influences of step configuration on the surface restructuring processes were studied on Pt(557) and Pt(332) that differ only in the step orientation. 500 mTorr of CO creates Pt clusters shaped as triangles and parallelograms on Pt(557) and Pt(332), respectively. When 500 mTorr of C2H4 was introduced afterwards, Pt clusters are removed on Pt(332) but preserved on Pt(557). The three-fold hollow sites at the (111) steps enable the Pt(332) surface to accommodate ethylidyne even covered by CO. As a result, kink Pt atoms at the cluster edges are driven to diffuse to form straight steps, so as to admit more ethylidyne at steps. In contrast, Pt(557) has (100) steps on which ethylidyne does not adsorb, therefore keeping the island structure after the introduction of C2H4. When 500 mTorr of C2H4 was added first into the high-pressure cell, a periodic pattern is resolved at step edges on Pt(332). In contrast, some bright species separated by more than 1 nm are observed on Pt(557). Further introducing 500 mTorr of CO does not facilitate the formation of Pt clusters. The structure and mobility under C2H4, H2, and CO were also studied on the Pt(100) surface, whose topmost layer is rearranged into a hexagonal overlayer in vacuum. Under 1 Torr of C2H4, the hexagonal reconstruction is preserved on Pt(100), which is covered by highly mobile adsorbates. Pt atoms on the hexagonal layer can also move as a result of the weakened interaction between the surface layer and the bulk. The mobility is enhanced under 1 Torr of 1:1 C2H4-H2 mixture because the Pt(100)-hex surface is active in ethylene hydrogenation. The surface mobility along with the catalytic reaction is quenched after introducing 3 mTorr of CO. Meanwhile, the hexagonal reconstruction is lifted by the adsorption of CO. At 5 × 10-6 Torr of C2H4, CO from background gases can also adsorb on Pt(100), creating Pt islands that do not revert to the hexagonal surface when the C2H4 pressure was further increased to 1 Torr. In order to understand the effect of substrates on surface reconstruction, the structure of the stepped Cu(557) surface was monitored in equilibrium with high pressures of gases. Cu generally binds to the reducing gases such as CO, H2, and C2H4 weaker than Pt, leading to a lower coverage on Cu than on Pt at the same gas pressure. Accordingly, 12 Torr of CO is required to induce clusters on Cu(557), because higher CO pressures are needed to keep a sufficient amount of CO that can stabilize clusters. At 1 Torr, large terraces with an average width of 23 nm are observed on Cu(557), because of the low diffusion barrier for Cu atoms both on terraces and along the steps. 500 mTorr of H2 results in step coalescence on Cu(557), giving rise to 6 nm wide terraces. C2H4 adsorption at 500 mTorr results in 5 nm large clusters. CO does not change the Cu(557) surface structure while adding into C2H4, but causes the appearance of large terraces while co-adsorbing with H2. Under oxidizing gases, for example 1 Torr of O2, the Cu(557) surface is significantly oxidized, forming thick layers of Cu oxide. Pt-based bimetallic nanoparticle catalysts were also investigated with AP-XPS under reaction conditions to study their surface chemistry. PtFe nanoparticles do not undergo any surface segregation at 298 K when the gas environment changes, but surface Fe atoms are partially reduced under the C2H4-H2 mixture and partially oxidized under O2. Neither does the surface composition of Pt9Co-Co core-shell nanoparticles change while heating under H2 even to 673 K nor do oxidation states. In Pt-Ni systems, at 393 K, Ni is oxidized under O2 and migrates to the surface because Ni is more susceptible to oxidation than Pt. In contrast, when the surface is reduced by H2, Pt segregates to the surface since the surface free energy of Pt is lower. Such segregation does not occur at 353 K owing to the low atomic mobility in lattice

Effects of Coronal Density and Magnetic Field Distributions on a Global Solar EUV Wave

We investigate a global extreme-ultraviolet (EUV) wave associated with a coronal mass ejection (CME)-driven shock on 2017 September 10. The EUV wave is transmitted by north- and south-polar coronal holes (CHs), which is observed by the Solar Dynamics Observatory (SDO) and Solar Terrestrial Relations Observatory A (STEREO-A) from opposite sides of the Sun. We obtain key findings on how the EUV wave interacts with multiple coronal structures, and on its connection with the CME-driven shock: (1) the transmitted EUV wave is still connected with the shock that is incurvated to the Sun, after the shock has reached the opposite side of the eruption; (2) the south CH transmitted EUV wave is accelerated inside an on-disk, low-density region with closed magnetic fields, which implies that an EUV wave can be accelerated in both open and closed magnetic field regions; (3) part of the primary EUV wavefront turns around a bright point (BP) with a bipolar magnetic structure when it approaches a dim, low-density filament channel near the BP; (4) the primary EUV wave is diffused and apparently halted near the boundaries of remote active regions (ARs) that are far from the eruption, and no obvious AR related secondary waves are detected; (5) the EUV wave extends to an unprecedented scale of ~360{\deg} in latitudes, which is attributed to the polar CH transmission. These results provide insights into the effects of coronal density and magnetic field distributions on the evolution of an EUV wave, and into the connection between the EUV wave and the associated CME-driven shock.Comment: 16 pages, 8 figures, and 3 animations available at http://doi.org/10.13140/RG.2.2.12408.29442 , http://doi.org/10.13140/RG.2.2.25830.06723 , and http://doi.org/10.13140/RG.2.2.19119.18088 ; published in Ap

Chemo-Mechanical Modeling of Stress Evolution in All-Solid-State Lithium- Ion Batteries Using Synchrotron Transmission X-ray Microscopy Tomography

In this study, a chemo-mechanical modeling framework was developed by adopting a reconstructed three-dimensional morphology of all-solid-state lithium-ion battery (ASSB) composite electrodes, using a synchrotron transmission X-ray microscopy tomography system. The developed model aimed to elucidate the effects of the electrode microstructure, specifically solid electrolyte/active material (SE/AM) interface and void space, toward the lithiation-induced stress evolution. The results show that the peak stress points happen at the SE/AM interface, while void space can partially accommodate the AM swelling and alleviate the stress formation. Although applying higher pressing pressure during the electrode fabrication can improve the ion pathways, it adversely affects the stress formation and may cause crack propagation. The results reveal that SE stiffness has a key impact on stress formation and AM displacement. Although employing SE with a lower stiffness can attenuate the stress within the microstructure, it can exacerbate the anisotropic displacement of AM particles. In contrast, applying external pressing pressure can prevent anisotropic displacement of AM particles. The developed framework highlights the significance of microstructural design of ASSBs and provides invaluable insights

PLASMA AND MAGNETIC FIELD CHARACTERISTICS OF SOLAR CORONAL MASS EJECTIONS IN RELATION TO GEOMAGNETIC STORM INTENSITY AND VARIABILITY

The largest geomagnetic storms of solar cycle 24 so far occurred on 2015 March 17 and June 22 with D[subscript st] minima of -223 and -195 nT, respectively. Both of the geomagnetic storms show a multi-step development. We examine the plasma and magnetic field characteristics of the driving coronal mass ejections (CMEs) in connection with the development of the geomagnetic storms. A particular effort is to reconstruct the in situ structure using a Grad–Shafranov technique and compare the reconstruction results with solar observations, which gives a larger spatial perspective of the source conditions than one-dimensional in situ measurements. Key results are obtained concerning how the plasma and magnetic field characteristics of CMEs control the geomagnetic storm intensity and variability: (1) a sheath-ejecta-ejecta mechanism and a sheath-sheath-ejecta scenario are proposed for the multi-step development of the 2015 March 17 and June 22 geomagnetic storms, respectively; (2) two contrasting cases of how the CME flux-rope characteristics generate intense geomagnetic storms are found, which indicates that a southward flux-rope orientation is not a necessity for a strong geomagnetic storm; and (3) the unexpected 2015 March 17 intense geomagnetic storm resulted from the interaction between two successive CMEs plus the compression by a high-speed stream from behind, which is essentially the "perfect storm" scenario proposed by Liu et al. (i.e., a combination of circumstances results in an event of unusual magnitude), so the "perfect storm" scenario may not be as rare as the phrase implies.China. Recruitment Program of Global ExpertsNational Natural Science Foundation (China) (Grant 41374173)State Key Laboratories of China (Specialized Research Fund

Chromo-fluorogenic detection of aldehydes with a rhodamine based sensor featuring an intramolecular deoxylactam

A chromogenic and fluorogenic detection of aldehydes was achieved via analyte triggered opening of the deoxylactam of N-(rhodamine B)-deoxylactam-ethylenediamine (dRB-EDA). The utility of the sensor was demonstrated by fluorescent labeling of aldehyde-displaying sialoproteins on cell surfaces.NSF China[20802060, 21072162, 30830092, 30921005, 91029304, 81061160512]; Natural Science Foundation of Fujian Province of China[2011J06004]; Fundamental Research Funds for the Central Universities[2011121020]; 973 program[2009CB522200

FOXD1 Promotes Cell Growth and Metastasis by Activation of Vimentin in NSCLC

Background/Aims: Forkhead box D1 (FOXD1) has a well-established role in early embryonic development and organogenesis and functions as an oncogene in several cancers. However, the clinical significance and biological roles of FOXD1 in non-small cell lung cancer (NSCLC) remain largely unknown. Methods: A total of 264 primary NSCLC tissue samples were collected. The expression levels of FOXD1 in these samples were examined by immunohistochemical staining. The expression of FOXD1 was knocked down by lentiviral shRNA. The relative expression of FOXD1 was determined by qRT-PCR, Western blotting and immunofluorescence image. The functional roles of FOXD1 in NSCLC were demonstrated cell viability CCK-8 assay, colony formation, cell invasion and migration assays, and cell apoptosis assay in vitro. In vivo mouse xenograft and metastasis models were used to assess tumorigenicity and metastatic ability. The Chi-square test was used to assess the correlation between FOXD1 expression and the clinicopathological characteristics. Survival curves were estimated by Kaplan-Meier method and compared using the log-rank test. The Cox proportional hazards model was used for univariate and multivariate analyses. Results: We determined that higher levels of FOXD1 were present in NSCLC tissues, especially in metastatic NSCLC tissues. FOXD1 was also higher in all NSCLC cells compared with normal human bronchial epithelial cells. A higher expression level of FOXD1 was associated with malignant behavior and poor prognosis in NSCLC patients. Knockdown of FOXD1 significantly inhibited proliferation, migration, and invasion in vitro and tumor growth and metastasis in vivo, and it increased the apoptosis rates of NSCLC cells. Mechanistic analyses revealed that FOXD1 expressed its oncogenic characteristics through activating Vimentin in NSCLC. Multivariate Cox regression analysis indicated that FOXD1 was an independent prognostic factor both for overall survival (OS) and disease-free survival (DFS) in NSCLC patients. Conclusion: Our results indicated that FOXD1 might be involved in the development and progression of NSCLC as an oncogene, and thereby might be a potential therapeutic target for NSCLC patients

Ion Doping Effects on the Lattice Distortion and Interlayer Mismatch of Aurivillius-Type Bismuth Titanate Compounds

Taking Bismuth Titanate (Bi4Ti3O12) as a Aurivillius-type compound with m = 3 for example, the ion (W6+/Cr3+) doping effect on the lattice distortion and interlayer mismatch of Bi4Ti3O12 structure were investigated by stress analysis, based on an elastic model. Since oxygen-octahedron rotates in the ab-plane, and inclines away from the c-axis, a lattice model for describing the status change of oxygen-octahedron was built according to the substituting mechanism of W6+/Cr3+ for Ti4+, which was used to investigate the variation of orthorhombic distortion degree (a/b) of Bi4Ti3O12 with the doping content. The analysis shows that the incorporation of W6+/Cr3+ into Bi4Ti3O12 tends to relieve the distortion of pseudo-perovskite layer, which also helps it to become more stiff. Since the bismuth-oxide layer expands while the pseudo-perovskite layer tightens, an analytic model for the plane stress distribution in the crystal lattice of Bi4Ti3O12 was developed from the constitutive relationship of alternating layer structure. The calculations reveal that the structural mismatch of Bi4Ti3O12 is constrained in the ab-plane of a unit cell, since both the interlayer mismatch degree and the total strain energy vary with the doping content in a similar trend to the lattice parameters of ab-plane