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1.
J Am Chem Soc ; 2020 Dec 31.
Artigo em Inglês | MEDLINE | ID: mdl-33382947

RESUMO

The electrochemical CO2 reduction reaction (CO2RR) using Cu-based catalysts holds great potential for producing valuable multi-carbon products from renewable energy. However, the chemical and structural state of Cu catalyst surfaces during the CO2RR remains a matter of debate. Here, we show the structural evolution of the near-surface region of polycrystalline Cu electrodes under in situ conditions through a combination of grazing incidence X-ray absorption spectroscopy (GIXAS) and X-ray diffraction (GIXRD). The in situ GIXAS reveals that the surface oxide layer is fully reduced to metallic Cu before the onset potential for CO2RR, and the catalyst maintains the metallic state across the potentials relevant to the CO2RR. We also find a preferential surface reconstruction of the polycrystalline Cu surface toward (100) facets in the presence of CO2. Quantitative analysis of the reconstruction profiles reveals that the degree of reconstruction increases with increasingly negative applied potentials, and it persists when the applied potential returns to more positive values. These findings show that the surface of Cu electrocatalysts is dynamic during the CO2RR, and emphasize the importance of in situ characterization to understand the surface structure and its role in electrocatalysis.

2.
ACS Appl Mater Interfaces ; 12(22): 25374-25382, 2020 Jun 03.
Artigo em Inglês | MEDLINE | ID: mdl-32383583

RESUMO

Bimetallic electrocatalysts provide a promising strategy for improving performance, especially in the enhancement of selectivity of CO2 reduction reactions. However, the first step of CO2 activation on bimetallic materials remains obscure. Considering bimetallic silver-copper (AgCu) as an example, we coupled ambient pressure X-ray photoelectron spectroscopy (APXPS) and quantum mechanics (QM) to examine CO2 adsorption and activation on AgCu exposed to CO2 with and without H2O at 298 K. The interplay between adsorbed species and the surface alloy composition of Cu and Ag is studied in atomic details. The APXPS experiment and density functional theory (DFT) calculations indicate that the clean sample has a Ag-rich surface layer. Upon adsorption of CO2 and surface O, we found that it is thermodynamically more favorable to induce subsurface Cu atoms substitution for some surface Ag atoms, modifying the stability and activation of CO2-related chemisorbed species. We further characterized this substitution effect by correlating the new adsorption species with the observed binding energy (BE) shift and intensity change in APXPS.

3.
ACS Appl Mater Interfaces ; 12(23): 26607-26613, 2020 Jun 10.
Artigo em Inglês | MEDLINE | ID: mdl-32423200

RESUMO

Because they deliver outstanding energy density, next-generation lithium metal batteries (LMBs) are essential to the advancement of both electric mobility and portable electronic devices. However, the high reactivity of metallic lithium surfaces leads to the low electrochemical performance of many secondary batteries. Besides, Li deposition is not uniform, which has been attributed to the low ionic conductivity of the anode surface. In particular, lithium exposure to CO2 gas is considered detrimental due to the formation of carbonate on the solid electrolyte interphase (SEI). In this work, we explored the interaction of Li metal with CO2 gas as a function of time using ambient pressure X-ray photoelectron spectroscopy to clarify the reaction pathway and main intermediates involved in the process during which oxalate formation has been detected. Furthermore, when O2 gas is part of the surrounding environment with CO2 gas, the reaction pathway is bypassed to directly promote carbonate as a single product.

4.
Nano Lett ; 20(2): 1403-1409, 2020 Feb 12.
Artigo em Inglês | MEDLINE | ID: mdl-31967840

RESUMO

Exploring the high-performance non-Pt electrocatalysts for oxygen reduction reaction (ORR), the bottleneck process in fuel cells, is desirable but challenging. Here, we report the Pd@PdFe core-shell icosahedra as an active and durable electrocatalyst toward ORR in alkaline conditions, which feature a three-atomic-layer tensile-strained PdFe overlayer on Pd icosahedra. Our optimized catalyst shows 2.8-fold enhancement in mass activity and 6.9-fold enhancement in specific activity than commercial Pt/C catalyst toward ORR, representing one of the best non-Pt electrocatalysts. Moreover, the boosted ORR catalysis is strongly supported by the assembled fuel cell performance using Pd@PdFe core-shell icosahedra as the cathode electrocatalyst. The density functional theory calculations reveal that the synergistic coupling of tensile strain and alloy effects enables the optimum binding strength for intermediates, thus causing the maximum activity. The present work suggests the coupling between multiple surface modulations endows larger room for the rational design of remarkable catalysts.

5.
Nanoscale ; 11(41): 19512-19519, 2019 Nov 07.
Artigo em Inglês | MEDLINE | ID: mdl-31573006

RESUMO

Titania nanocrystals have been investigated for fast color switching through photocatalytic reduction of dyes and hexacyanometalate pigments. Here we reveal that direct binding of sacrificial electron donors (SEDs) to the surface of titania nanocrystals can significantly promote the charge transfer rate by more efficiently scavenging photogenerated holes and releasing more photogenerated electrons for reduction reactions. Using diethylene glycol (DEG) as an example, we show that its binding to the nanoparticle surface, which can be achieved either during or after the nanoparticle formation, greatly enhances the photocatalytic reduction in comparison with the case where free DEG molecules are simply added as external SEDs.

6.
J Am Chem Soc ; 141(42): 16548-16552, 2019 Oct 23.
Artigo em Inglês | MEDLINE | ID: mdl-31535853

RESUMO

We report a generalized wet-chemical methodology for the synthesis of transition-metal (M)-doped brookite-phase TiO2 nanorods (NRs) with unprecedented wide-range tunability in dopant composition (M = V, Cr, Mn, Fe, Co, Ni, Cu, Mo, etc.). These quadrangular NRs can selectively expose {210} surface facets, which is induced by their strong affinity for oleylamine stabilizer. This structure is well preserved with variable dopant compositions and concentrations, leading to a diverse library of TiO2 NRs wherein the dopants in single-atom form are homogeneously distributed in a brookite-phase solid lattice. This synthetic method allows tuning of dopant-dependent properties of TiO2 nanomaterials for new opportunities in catalysis applications.

8.
J Am Chem Soc ; 141(17): 6946-6954, 2019 May 01.
Artigo em Inglês | MEDLINE | ID: mdl-30945541

RESUMO

The interaction of water with metal surfaces is at the heart of electrocatalysis. But there remain enormous uncertainties about the atomistic interactions at the electrode-electrolyte interface (EEI). As the first step toward an understanding of the EEI, we report here the details of the initial steps of H2O adsorption and complex formation on a Ag(111) surface, based on coupling quantum mechanics (QM) and ambient-pressure X-ray photoelectron spectroscopy (APXPS) experiments. We find a close and direct comparison between simulation and experiment, validated under various isotherm and isobar conditions. We identify five observable oxygen-containing species whose concentrations depend sensitively on temperature and pressure: chemisorbed O* and OH*, H2O* stabilized by hydrogen bond interactions with OH* or O*, and multilayer H2O*. We identify the species experimentally by their O 1s core-level shift that we calculate with QM along with the structures and free energies as a function of temperature and pressure. This leads to a chemical reaction network (CRN) that we use to predict the time evolution of their concentrations over a wide range of temperature (298-798 K) and pressure conditions (10-6-1 Torr), which agree well with the populations determined from APXPS. This multistep simulation CRN protocol should be useful for other heterogeneous catalytic systems.

9.
Nano Lett ; 19(5): 2928-2934, 2019 05 08.
Artigo em Inglês | MEDLINE | ID: mdl-30932498

RESUMO

The Mg/S battery is attractive because of its high theoretical energy density and the abundance of Mg and S on the earth. However, its development is hindered by the lack of understanding to the underlying electrochemical reaction mechanism of its charge-discharge processes. Here, using a unique in situ X-ray absorption spectroscopic tool, we systematically study the reaction pathways of the Mg/S cells in Mg(HMDS)2-AlCl3 electrolyte. We find that the capacity degradation is mainly due to the formation of irreversible discharge products, such as MgS and Mg3S8, through a direct electrochemical deposition or a chemical disproportionation of intermediate polysulfide. In light of the fundamental understanding, we propose to use TiS2 as a catalyst to activate the irreversible reaction of low-order MgS x and MgS, which results in an increased discharging capacity up to 900 mAh·g-1 and a longer cycling life.

10.
Nat Commun ; 10(1): 1875, 2019 04 23.
Artigo em Inglês | MEDLINE | ID: mdl-31015453

RESUMO

Converting carbon dioxide (CO2) into liquid fuels and synthesis gas is a world-wide priority. But there is no experimental information on the initial atomic level events for CO2 electroreduction on the metal catalysts to provide the basis for developing improved catalysts. Here we combine ambient pressure X-ray photoelectron spectroscopy with quantum mechanics to examine the processes as Ag is exposed to CO2 both alone and in the presence of H2O at 298 K. We find that CO2 reacts with surface O on Ag to form a chemisorbed species (O = CO2δ-). Adding H2O and CO2 then leads to up to four water attaching on O = CO2δ- and two water attaching on chemisorbed (b-)CO2. On Ag we find a much more favorable mechanism involving the O = CO2δ- compared to that involving b-CO2 on Cu. Each metal surface modifies the gas-catalyst interactions, providing a basis for tuning CO2 adsorption behavior to facilitate selective product formations.

11.
Phys Chem Chem Phys ; 21(10): 5402-5408, 2019 Mar 06.
Artigo em Inglês | MEDLINE | ID: mdl-30785434

RESUMO

The rational improvement of current and developing electrochemical technologies requires atomistic understanding of electrode-electrolyte interfaces. However, examining these interfaces under operando conditions, where performance is typically evaluated and benchmarked, remains challenging, as it necessitates incorporating an operando probe during full electrochemical operation. In this study, we describe a custom electrochemical flow cell that enables near-surface-sensitive operando investigation of planar thin-film catalysts at significant hydrogen evolution reaction (HER) rates (in excess of -100 mA cm-2) using grazing incidence X-ray methods. Grazing-incidence X-ray spectroscopy and diffraction were implemented on the same sample under identical HER conditions, demonstrating how the combined measurements track changing redox chemistry and structure of Cu thin-film catalyst surfaces as a function of electrochemical conditions. The coupling of these methods with improved mass transport and hydrodynamic control establishes a new paradigm for operando measurement design, enabling unique insights into the key fundamental processes occurring at the catalyst-electrolyte interface.

12.
J Am Chem Soc ; 140(45): 15157-15160, 2018 11 14.
Artigo em Inglês | MEDLINE | ID: mdl-30372055

RESUMO

Photo-catalytic fixation of nitrogen by titania catalysts at ambient conditions has been reported for decades, yet the active site capable of adsorbing an inert N2 molecule at ambient pressure and the mechanism of dissociating the strong dinitrogen triple bond at room temperature remain unknown. In this work in situ near-ambient-pressure X-ray photo-electron spectroscopy and density functional theory calculations are used to probe the active state of the rutile (110) surface. The experimental results indicate that photon-driven interaction of N2 and TiO2 is observed only if adventitious surface carbon is present, and computational results show a remarkably strong interaction between N2 and carbon substitution (C*) sites that act as surface-bound carbon radicals. A carbon-assisted nitrogen reduction mechanism is proposed and shown to be thermodynamically feasible. The findings provide a molecular-scale explanation for the long-standing mystery of photo-catalytic nitrogen fixation on titania. The results suggest that controlling and characterizing carbon-based active sites may provide a route to engineering more efficient photo(electro)-catalysts and improving experimental reproducibility.

13.
Proc Natl Acad Sci U S A ; 115(12): 2902-2907, 2018 03 20.
Artigo em Inglês | MEDLINE | ID: mdl-29507243

RESUMO

Atomically dispersed catalysts refer to substrate-supported heterogeneous catalysts featuring one or a few active metal atoms that are separated from one another. They represent an important class of materials ranging from single-atom catalysts (SACs) and nanoparticles (NPs). While SACs and NPs have been extensively reported, catalysts featuring a few atoms with well-defined structures are poorly studied. The difficulty in synthesizing such structures has been a critical challenge. Here we report a facile photochemical method that produces catalytic centers consisting of two Ir metal cations, bridged by O and stably bound to a support. Direct evidence unambiguously supporting the dinuclear nature of the catalysts anchored on α-Fe2O3 is obtained by aberration-corrected scanning transmission electron microscopy (AC-STEM). Experimental and computational results further reveal that the threefold hollow binding sites on the OH-terminated surface of α-Fe2O3 anchor the catalysts to provide outstanding stability against detachment or aggregation. The resulting catalysts exhibit high activities toward H2O photooxidation.

14.
J Phys Chem Lett ; 9(4): 785-790, 2018 Feb 15.
Artigo em Inglês | MEDLINE | ID: mdl-29376377

RESUMO

We investigated the X-ray absorption spectroscopy (XAS) fingerprint of EMImTFSI ionic liquid (IL) and its fragmentation products created by X-ray irradiation. To accomplish this, we used an open geometry where an IL droplet is directly exposed in the vacuum chamber and an enclosed geometry where the IL is confined in a cell covered by an X-ray transparent membrane. In the open geometry, the XAS signature was stable and consistent with experimental and theoretical spectra reported in the literature. In contrast, when the IL is enclosed, its XAS evolves continuously under X-ray illumination due to the accumulation of volatile fragmentation products inside the closed cell, while they evaporate in the open geometry. The changes in the XAS from the core levels of relevant elements (C, N, S, F) together with density functional theory calculations allowed us to identify the chemical nature of the fragment products and the chemical bonds most vulnerable to rupture under soft X-ray irradiation.

15.
J Phys Chem B ; 122(2): 927-932, 2018 01 18.
Artigo em Inglês | MEDLINE | ID: mdl-29090934

RESUMO

Photoelectrochemical (PEC) water splitting holds the potential as a direct route for solar energy conversation and storage. The performance of a PEC device is strongly influenced by the electronic properties of the photonanode surface. It has been shown that the synthesis methods can have a profound impact on the electronic properties and PEC performance of various photoelectrode materials such as hematite. Soft X-ray spectroscopic techniques, including O K-edge XAS and Fe L-edge XAS/XES, have been employed to investigate how the synthesis methods impact the electronic structure of resulting hematite materials. It is found that the hematite samples via solution regrowth methods show dramatically increased 3d-4sp band ratios in O K-edge XAS spectra and decreased relative elastic peak intensities in Fe L-edge RIXS spectra compared with samples synthesized via ALD or solution grown. The difference observed in O-K and Fe-L spectra indicated that solution regrowth strategy alters the O 2p-Fe 3d hybridization and hence the electronic structure of the hematite films, which proves to be beneficial for PEC performance of the hematite photoanode. Our findings provided new insights and potentially useful strategies for enhancing the PEC performance of photoanode materials.

16.
Rev Sci Instrum ; 88(7): 073904, 2017 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-28764488

RESUMO

X-ray emission spectroscopy is emerging as an important complement to x-ray absorption fine structure spectroscopy, providing a characterization of the occupied electronic density of states local to the species of interest. Here, we present details of the design and performance of a compact x-ray emission spectrometer that uses a dispersive refocusing Rowland (DRR) circle geometry to achieve excellent performance for the 2-2.5 keV range, i.e., especially for the K-edge emission from sulfur and phosphorous. The DRR approach allows high energy resolution even for unfocused x-ray sources. This property enables high count rates in laboratory studies, approaching those of insertion-device beamlines at third-generation synchrotrons, despite use of only a low-powered, conventional x-ray tube. The spectrometer, whose overall scale is set by use of a 10-cm diameter Rowland circle and a new small-pixel complementary metal-oxide-semiconductor x-ray camera, is easily portable to synchrotron or x-ray free electron laser beamlines. Photometrics from measurements at the Advanced Light Source show excellent overall instrumental efficiency. In addition, the compact size of this instrument lends itself to future multiplexing to gain large factors in net collection efficiency or its implementation in controlled gas gloveboxes either in the lab or in an endstation.

17.
Science ; 357(6349): 389-393, 2017 07 28.
Artigo em Inglês | MEDLINE | ID: mdl-28642235

RESUMO

The water-gas shift (WGS) reaction (where carbon monoxide plus water yields dihydrogen and carbon dioxide) is an essential process for hydrogen generation and carbon monoxide removal in various energy-related chemical operations. This equilibrium-limited reaction is favored at a low working temperature. Potential application in fuel cells also requires a WGS catalyst to be highly active, stable, and energy-efficient and to match the working temperature of on-site hydrogen generation and consumption units. We synthesized layered gold (Au) clusters on a molybdenum carbide (α-MoC) substrate to create an interfacial catalyst system for the ultralow-temperature WGS reaction. Water was activated over α-MoC at 303 kelvin, whereas carbon monoxide adsorbed on adjacent Au sites was apt to react with surface hydroxyl groups formed from water splitting, leading to a high WGS activity at low temperatures.

18.
Nano Lett ; 17(2): 755-761, 2017 02 08.
Artigo em Inglês | MEDLINE | ID: mdl-28094525

RESUMO

Developing efficient photoreversible color switching systems for constructing rewritable paper is of significant practical interest owing to the potential environmental benefits including forest conservation, pollution reduction, and resource sustainability. Here we report that the color change associated with the redox chemistry of nanoparticles of Prussian blue and its analogues could be integrated with the photocatalytic activity of TiO2 nanoparticles to construct a class of new photoreversible color switching systems, which can be conveniently utilized for fabricating ink-free, light printable rewritable paper with various working colors. The current system also addresses the phase separation issue of the previous organic dye-based color switching system so that it can be conveniently applied to the surface of conventional paper to produce an ink-free light printable rewritable paper that has the same feel and appearance as the conventional paper. With its additional advantages such as excellent scalability and outstanding rewriting performance (reversibility >80 times, legible time >5 days, and resolution >5 µm), this novel system can serve as an eco-friendly alternative to regular paper in meeting the increasing global needs for environment protection and resource sustainability.

19.
Angew Chem Int Ed Engl ; 55(50): 15656-15661, 2016 12 12.
Artigo em Inglês | MEDLINE | ID: mdl-27860131

RESUMO

Hydrogenation reactions are industrially important reactions that typically require unfavorably high H2 pressure and temperature for many functional groups. Herein we reveal surprisingly strong size-dependent activity of Pt nanoparticles (PtNPs) in catalyzing this reaction. Based on unambiguous spectral analyses, the size effect has been rationalized by the size-dependent d-band electron structure of the PtNPs. This understanding enables production of a catalyst with size of 1.2 nm, which shows a sixfold increase in turnover frequency and 28-fold increase in mass activity in the regioselective hydrogenation of quinoline, compared with PtNPs of 5.3 nm, allowing the reaction to proceed under ambient conditions with unprecedentedly high reaction rates. The size effect and the synthesis strategy developed herein may provide a general methodology in the design of metal-nanoparticle-based catalysts for a broad range of organic syntheses.

20.
Nano Lett ; 16(6): 3545-9, 2016 06 08.
Artigo em Inglês | MEDLINE | ID: mdl-27175936

RESUMO

Lithium-sulfur (Li-S) battery is a promising high energy storage candidate in electric vehicles. However, the commonly employed ether based electrolyte does not enable to realize safe high-temperature Li-S batteries due to the low boiling and flash temperatures. Traditional carbonate based electrolyte obtains safe physical properties at high temperature but does not complete reversible electrochemical reaction for most Li-S batteries. Here we realize safe high temperature Li-S batteries on universal carbon-sulfur electrodes by molecular layer deposited (MLD) alucone coating. Sulfur cathodes with MLD coating complete the reversible electrochemical process in carbonate electrolyte and exhibit a safe and ultrastable cycle life at high temperature, which promise practicable Li-S batteries for electric vehicles and other large-scale energy storage systems.

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