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1.
Nano Lett ; 22(11): 4333-4339, 2022 06 08.
Artigo em Inglês | MEDLINE | ID: mdl-35584407

RESUMO

Achieving metal nanocrystals with metastable phase draws much attention due to their anticipated fascinating properties, wheras it is still challenging because their polymorphism nature and phase transition mechanism remain elusive. Here, phase stability of face-centered cubic (fcc) Pd nanocrystals was studied via in situ spherical aberration (Cs)-corrected transmission electron microscopy (TEM). By constructing a well-defined Pd/C composite structure, Pd nanocrystals encapsulated by graphite, the dispersion process of fcc Pd was observed through a nucleation and growth process. Interestingly, Cs-corrected scanning TEM analysis demonstrated that the newly formed Pd nanocrystals could adopt a metastable hexagonal phase, which was considered challenging to obtain. Accordingly, formation mechanism of the hexagonal Pd nanocrystals was proposed, which involved the combined effect of two factors: (1) templating of graphite and (2) size effect. This work is expected to offer new insight into the polymorphism of Pd nanocrystals and pave the way for the future design of metastable metal nanomaterials.


Assuntos
Grafite , Nanopartículas Metálicas , Nanoestruturas , Nanopartículas Metálicas/química , Microscopia Eletrônica de Transmissão , Nanoestruturas/química , Transição de Fase
2.
Adv Mater ; 34(30): e2202072, 2022 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-35580350

RESUMO

Surface oxygen vacancies have been widely discussed to be crucial for tailoring the activity of various chemical reactions from CO, NO, to water oxidation by using oxide-supported catalysts. However, the real role and potential function of surface oxygen vacancies in the reaction remains unclear because of their very short lifetime. Here, it is reported that surface oxygen vacancies can be well confined electrostatically for a polarization screening near the perimeter interface between Pt {111} nanocrystals and the negative polar surface (001) of ferroelectric PbTiO3. Strikingly, such a catalyst demonstrates a tunable catalytic CO oxidation kinetics from 200 °C to near room temperature by increasing the O2 gas pressure, accompanied by the conversion curve from a hysteresis-free loop to one with hysteresis. The combination of reaction kinetics, electronic energy loss spectroscopy (EELS) analysis, and density functional theory (DFT) calculations, indicates that the oxygen vacancies stabilized by the negative polar surface are the active sites for O2 adsorption as a rate-determining step, and then dissociated O moves to the surface of the Pt nanocrystals for oxidizing adsorbed CO. The results open a new pathway for tunable catalytic activity of CO oxidation.

3.
Nano Lett ; 21(17): 7309-7316, 2021 Sep 08.
Artigo em Inglês | MEDLINE | ID: mdl-34410724

RESUMO

Understanding surface reconstruction of nanocrystals is of great importance to their applications, however it is still challenging due to lack of atomic-level structural information under reconstruction conditions. Herein, through in situ spherical aberration corrected scanning transmission electron microscopy (STEM), the reconstruction of nanocrystalline SnO2 (110) surface was studied. By identifying the precise arrangements of surface/subsurface Sn and O columns through both in situ bright-field and high-angle annular dark-field STEM images, an unexpected added Sn2O model was determined for SnO2 (110)-(1 × 2) surface. The protruded Snδ+ of this surface could act as the active sites for activating O2 molecules according to our density functional theory (DFT) calculations. On the basis of in situ observation of atomic-level reconstruction behaviors and DFT calculations, an energy-driven reconstruction process was also revealed. We anticipate this work would help to clarify the long-standing debate regarding the reconstruction of SnO2 (110) surface and its intrinsic property.

4.
Angew Chem Int Ed Engl ; 60(41): 22339-22344, 2021 Oct 04.
Artigo em Inglês | MEDLINE | ID: mdl-34352928

RESUMO

The strong metal-support interaction (SMSI) is widely used in supported metal catalysts and extensive studies have been performed to understand it. Although considerable progress has been achieved, the surface structure of the support, as an important influencing factor, is usually ignored. We report a facet-dependent SMSI of Pd-TiO2 in oxygen by using in situ atmospheric pressure TEM. Pd NPs supported on TiO2 (101) and (100) surfaces showed encapsulation. In contrast, no such cover layer was observed in Pd-TiO2 (001) catalyst under the same conditions. This facet-dependent SMSI, which originates from the variable surface structure of the support, was demonstrated in a probe reaction of methane combustion catalyzed by Pd-TiO2 . Our discovery of the oxidative facet-dependent SMSI gives direct evidence of the important role of the support surface structure in SMSI and provides a new way to tune the interaction between metal NPs and the support as well as catalytic activity.

5.
Science ; 371(6528): 517-521, 2021 01 29.
Artigo em Inglês | MEDLINE | ID: mdl-33510028

RESUMO

The interface between metal catalyst and support plays a critical role in heterogeneous catalysis. An epitaxial interface is generally considered to be rigid, and tuning its intrinsic microstructure with atomic precision during catalytic reactions is challenging. Using aberration-corrected environmental transmission electron microscopy, we studied the interface between gold (Au) and a titanium dioxide (TiO2) support. Direct atomic-scale observations showed an unexpected dependence of the atomic structure of the Au-TiO2 interface with the epitaxial rotation of gold nanoparticles on a TiO2 surface during carbon monoxide (CO) oxidation. Taking advantage of the reversible and controllable rotation, we achieved in situ manipulation of the active Au-TiO2 interface by changing gas and temperature. This result suggests that real-time design of the catalytic interface in operating conditions may be possible.

6.
Chemosphere ; 263: 128109, 2021 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-33297102

RESUMO

Herein, the novel polyaniline@nano hollow carbon sphere (PANI@NHCS) adsorbents with different mass of NHCS were prepared by in-situ polymerization method. The microstructure of obtained PANI@NHCS-10, PANI@NHCS-20, PANI@NHCS-30 and PANI@NHCS-40 samples were observed through both scanning electron microscopy (SEM) and transmission electron microscopy (TEM), which showed that the PANI@NHCS-30 possessed hollow structure like lappaceum shell. Then, the performance of obtained PANI@NHCS-30 was studied for removing hexavalent chromium (Cr(VI)) from waste water. With the help of unique hollow structure and reduction ability of PANI@NHCS-30, the Cr(VI) was fleetly adsorbed and then reduced to less toxic Cr(III). The maximum adsorption capacity was 250.0 mg/g for PANI@NHCS-30 under the optimal condition. Moreover, the effects of initial Cr(VI) concentration, solution pH and different ions on the adsorption performance were investigated in detail. Importantly, the PANI@NHCS-30 still shows superb adsorption ability after five cycles, which suggests its satisfactory reusability ability. The accumulated data revealed the crucial role of PANI and hollow structure co-promoting effect on Cr(VI) reduction reactions over PANI@NHCS-30, which could be applied to the practical use.


Assuntos
Carbono , Poluentes Químicos da Água , Adsorção , Cromo/análise , Concentração de Íons de Hidrogênio , Poluentes Químicos da Água/análise
7.
ACS Appl Mater Interfaces ; 12(15): 17466-17473, 2020 Apr 15.
Artigo em Inglês | MEDLINE | ID: mdl-32212677

RESUMO

Layered germanium phosphide (GeP), a recently developed two-dimensional material, promises highly attractive theoretical capacity for use as a lithium-ion battery anode. Here, we comprehensively investigate its electrochemical performance and the modification mechanism. GeP flakes demonstrate large initial discharge/charge capacity and high initial Coulombic efficiency. However, the cycling performance is disappointing in the potential window of 0.001-3 V in which capacity retention is only ∼18% after 100 cycles. In situ transmission electron microscopy reveals that the poor cycling behavior results in the unexpected large volume change induced by complex reaction processes in cycles. Serious cracking and fracture appear clearly on the electrode surface after cycling. Narrowing the working voltage window to 0.001-0.85 V, cycling stability will be greatly enhanced, with 75% capacity retaining after 100 cycles and ∼50% left after 350 cycles due to the absence of the dealloying of Li3P in the narrowed working voltage window. Additionally, the electric contact among the electrode components has been enhanced by the alleviation of the electrode volume change in the narrowed working voltage window. Our work provides one effective method to give a deep understanding of the high-energy-density electrode failure and helps to narrow the huge gap between the microstructure and the performance of the electrode.

9.
Science ; 367(6476): 428-430, 2020 01 24.
Artigo em Inglês | MEDLINE | ID: mdl-31974251

RESUMO

Imaging a reaction taking place at the molecular level could provide direct information for understanding the catalytic reaction mechanism. We used in situ environmental transmission electron microscopy and a nanocrystalline anatase titanium dioxide (001) surface with (1 × 4) reconstruction as a catalyst, which provided highly ordered four-coordinated titanium "active rows" to realize real-time monitoring of water molecules dissociating and reacting on the catalyst surface. The twin-protrusion configuration of adsorbed water was observed. During the water-gas shift reaction, dynamic changes in these structures were visualized on these active rows at the molecular level.

10.
Angew Chem Int Ed Engl ; 59(6): 2171-2180, 2020 Feb 03.
Artigo em Inglês | MEDLINE | ID: mdl-31298462

RESUMO

The shape of metal nanoparticles (NPs) is one of the key factors determining their catalytic reactivity. Recent in situ TEM observations show that dynamic reshaping of metal NPs occurs under the reaction conditions, which becomes a major hurdle for fully understanding catalytic mechanisms at the molecular level. This Minireview provides a summary of the latest progress in characterizing and modeling the equilibrium shape of metal NPs in various reactive environments through the joint effort of state-of-the-art in situ environmental transmission electron microscopy experiments and a newly developed multiscale structure reconstruction model. The quantitative agreement between the experimental observations and theoretical modeling demonstrate that the fundamental mechanism of the reshaping phenomenon is driven by anisotropically changed surface energies under gas adsorption. The predictable reshaping of metal NPs paves the way for the rational design of truly efficient nanocatalysts in real reactions.

11.
Adv Mater ; 31(42): e1903719, 2019 Oct.
Artigo em Inglês | MEDLINE | ID: mdl-31475404

RESUMO

Nitrogen oxides are one of the major sources of air pollution. To remove these pollutants originating from combustion of fossil fuels remains challenging in steel, cement, and glass industries as the catalysts are severely deactivated by SO2 during the low-temperature selective catalytic reduction (SCR) process. Here, a MnOX /CeO2 nanorod catalyst with outstanding resistance to SO2 deactivation is reported, which is designed based on critical information obtained from in situ transmission electron microscopy (TEM) experiments under reaction conditions and theoretical calculations. The catalysts show almost no activity loss (apparent NOX reaction rate kept unchanged at 1800 µmol g-1 h-1 ) for 1000 h test at 523 K in the presence of 200 ppm SO2 . This unprecedented performance is achieved by establishing a dynamic equilibrium between sulfates formation and decomposition over the CeO2 surface during the reactions and preventing the MnOX cluster from the steric hindrance induced by SO2 , which minimized the deactivation of the active sites of MnOX /CeO2 . This work presents the ultralong lifetime of catalysts in the presence of SO2 , along with decent activity, marking a milestone in practical applications in low-temperature selective catalytic reduction (SCR) of NOX .

12.
Nano Lett ; 19(6): 4205-4210, 2019 06 12.
Artigo em Inglês | MEDLINE | ID: mdl-31145634

RESUMO

Layer-by-layer growth played a critical role in the fine design of novel materials and devices. Although it has been widely studied during materials synthesis, the atomic mechanism of the growth remains unclear due to the lack of direct observation at the atomic scale. Here, we report a new mode in layer-by-layer growth via surface reconstruction on MoO2 (011) by environmental transmission electron microscopy and density functional theory calculations. Our in situ environmental transmission electron microscopy results demonstrate that the layer-by-layer growth of MoO2 experiences two steps that occur in an oscillatory manner: (1) the formation of an atomic ledge by transforming a section of the reconstructed layer to the intrinsic surface layer and then (2) the spontaneous reconstruction of the newly formed intrinsic surface section. Thus, the surface reconstruction can be considered as an intermediated phase during the layer-by-layer growth of MoO2. A similar phenomenon was also observed in the MoO2 dissolution procedure.

13.
Chem Commun (Camb) ; 55(39): 5611-5614, 2019 May 09.
Artigo em Inglês | MEDLINE | ID: mdl-31025670

RESUMO

A hexagonal FeSe nanoparticle anode with a novel reaction mechanism and mechanical stability may fully facilitate the desirable rate capability and cycling performance in sodium-ion batteries. In situ TEM reveals that hexagonal FeSe nanoparticle transition to the Fe and Na2Se phase during sodiation, while the products transform to the tetragonal FeSe phase after desodiation.


Assuntos
Nanopartículas/química , Sódio/química , Espectroscopia Dielétrica , Condutividade Elétrica , Fontes de Energia Elétrica , Ferro/química , Selênio/química
14.
Phys Chem Chem Phys ; 21(6): 3134-3139, 2019 Feb 06.
Artigo em Inglês | MEDLINE | ID: mdl-30675619

RESUMO

Bimetallic core-shell nanoparticles have received considerable attention for their unique optical, magnetic and catalytic properties. However, these properties will be dramatically modified under ambient conditions by their structure and/or composition change. Thus, it is of primary importance to study the complex transformation pathway of core-shell nanoparticles at an elevated temperature. In this work, by using an aberration-corrected scanning transmission electron microscope equipped with an energy dispersive X-ray mapping system, the complete transformation process from a well-designed Pd@Au core-shell nanoparticle to a uniform alloy particle was visualized. It is revealed that this transformation process went through three steps, i.e., surface refacetting, particle resphering and complete alloying. Combining with a developed atomic kinetic Monte Carlo simulation, we found that surface energy is the driving force for shape variation, and the different atomic activation barriers of surface diffusion and bulk migration result in the multistep transformation pathway. Our results offered important information for understanding the structure evolution of bimetallic core-shell nanoparticles, which is beneficial for the rational design of nanoparticles with kinetic stability.

15.
Angew Chem Int Ed Engl ; 57(51): 16827-16831, 2018 Dec 17.
Artigo em Inglês | MEDLINE | ID: mdl-30397982

RESUMO

Preventing sintering of supported nanocatalysts is an important issue in nanocatalysis. A feasible way is to choose a suitable support. However, whether the metal-support interactions promote or prevent the sintering has not been fully identified. Now, completely different sintering behaviors of Au nanoparticles on distinct anatase TiO2 surfaces have been determined by in situ TEM. The full in situ sintering processes of Au nanoparticles were visualized on TiO2 (101) surface, which coupled the Ostwald ripening and particle migration coalescence. In contrast, no sintering of Au on TiO2 anatase (001) surface was observed under the same conditions. This facet-dependent sintering mechanism is fully explained by the density function theory calculations. This work not only offers direct evidence of the important role of supports in the sintering process, but also provides insightful information for the design of sintering-resistant nanocatalysts.

16.
Chem Commun (Camb) ; 54(62): 8587-8590, 2018 Aug 11.
Artigo em Inglês | MEDLINE | ID: mdl-29972158

RESUMO

We demonstrate an unexpected refacetting process of Pd nanoparticles, induced by N2 under atmospheric pressure at elevated temperatures, by in situ TEM observations. The morphology changes, with a notable increase in the fraction of Pd{110} facets, were visualized by atomic-scale TEM images and further explained by theoretical calculations. Firm evidence and rational understanding revealed that the "inert" gas N2 has the ability to modify the structure of metal nanoparticles. This surprising effect should be considered seriously in vast chemical applications that use N2 as a carrier gas or protective atmosphere.

17.
Angew Chem Int Ed Engl ; 57(35): 11344-11348, 2018 Aug 27.
Artigo em Inglês | MEDLINE | ID: mdl-29979826

RESUMO

Acquiring the kinetics of gas-nanoparticle fast reactions under ambient pressure is a challenge owing to the lack of appropriate in situ techniques. Now an approach has been developed that integrates time-resolved in situ electron diffraction and an atmospheric gas cell system in transmission electron microscopy, allowing quantitative structural information to be obtained under ambient pressure with millisecond time resolution. The ultrafast oxidation kinetics of Ni nanoparticles in oxygen was vividly obtained. In contrast to the well-accepted Wagner and Mott-Cabrera models (diffusion-dominated), the oxidation of Ni nanoparticles is linear at the initial stage (<0.5 s), and follows the Avrami-Erofeev model (n=1.12) at the following stage, which indicates the oxidation of Ni nanoparticles is a nucleation and growth dominated process. This study gives new insights into Ni oxidation and paves the way to study the fast reaction kinetics of nanoparticles using ultrafast in situ techniques.

18.
Environ Sci Technol ; 50(20): 11424-11432, 2016 Oct 18.
Artigo em Inglês | MEDLINE | ID: mdl-27668311

RESUMO

In this study, a reliable and steady PCDD/F generation system was utilized to investigate the performance of catalysts, in which 130 congeners of tetra- to octapolychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) vapors were studied under simulated flue gas with/without O3. TiO2 and carbon nanotubes (CNTs) supported vanadium oxides (VOX/TiO2-CNTs) modified with MnOX and CuOX, which were reported to be beneficial to the decomposition of model molecules, were found to have a negative effect on the removal of real PCDD/Fs in the simulated flue gas without O3. Moreover, the addition of MnOX presented different effects depending on whether CuOX existed in catalysts or not, which was also contrary to its effects on the degradation of model molecules. In an O3-containing atmosphere, low chlorination level PCDD/Fs congeners were removed well over VOX-MnOX/TiO2-CNTs, while high chlorination level PCDD/Fs congeners were removed well over VOX-CuOX/TiO2-CNTs. Fortunately, all PCDD/Fs congeners decomposed well over VOX-MnOX-CuOX/TiO2-CNTs. Finally, the effects of tetra- to octachlorination level for the adsorption and degradation behaviors of PCDD/Fs congeners were also investigated.

19.
Sci Rep ; 5: 17773, 2015 Dec 04.
Artigo em Inglês | MEDLINE | ID: mdl-26634815

RESUMO

High-ordered anatase TiO2 nanotube array films coated with exposed high-reactive {001} nanofacets were fabricated by a modified hydrothermal method using amorphous anodic TiO2 nanotube arrays (ATONAs) as starting materials. It was found that the reaction between gas phase HF and solid ATONAs played a key role in the transformation process from amorphous to anatase TiO2, and the TiO2 tubular structure kept unchanged during the surface modification with an exposed {001} facets up to 76.5%, which could be attributed to the low reaction temperature of 130 °C. Our study provided a novel route for the facile preparation of {001} facets exposed anatase TiO2.

20.
Nanoscale ; 4(24): 7832-41, 2012 Dec 21.
Artigo em Inglês | MEDLINE | ID: mdl-23151539

RESUMO

A controllable synthesis of various morphologies of CuO nanostructures with tuning by hetero-metal cations has been developed in aqueous solution at room temperature. The morphologies of CuO can be engineered from nanosheets to nanoparticles with different length ratios of the long axis to the short axis. The formation of many metal-ion complexes plays an important role in slowing the release rate of OH(-) and affects the reaction kinetics further. We found that the effect of hetero-metal cations on the final morphology of the CuO nanostructures was the same as that of the cooling temperature. A series of temperature-controlled experiments demonstrated this. Furthermore, among all the synthesized CuO nanostructures, the fascinating colloidal mesoporous CuO quasi-monocrystalline nanosheets prepared at 25 °C with a thickness of ca. 10 nm and large specific surface area of 80.32 m(2) g(-1) is investigated intensively. These CuO nanosheets demonstrate a superior catalytic activity for CO oxidation, with features of high CO conversion efficiency (47.77 mmol(CO) g(-1)(CuO) h(-1) at 200 °C), which is close to that reported for previously investigated supported-CuO catalysts, and a low apparent activation energy E(a) (53.3 kJ mol(-1)).

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