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1.
Science ; 366(6467): 850-856, 2019 11 15.
Artigo em Inglês | MEDLINE | ID: mdl-31727830

RESUMO

Development of efficient and robust electrocatalysts is critical for practical fuel cells. We report one-dimensional bunched platinum-nickel (Pt-Ni) alloy nanocages with a Pt-skin structure for the oxygen reduction reaction that display high mass activity (3.52 amperes per milligram platinum) and specific activity (5.16 milliamperes per square centimeter platinum), or nearly 17 and 14 times higher as compared with a commercial platinum on carbon (Pt/C) catalyst. The catalyst exhibits high stability with negligible activity decay after 50,000 cycles. Both the experimental results and theoretical calculations reveal the existence of fewer strongly bonded platinum-oxygen (Pt-O) sites induced by the strain and ligand effects. Moreover, the fuel cell assembled by this catalyst delivers a current density of 1.5 amperes per square centimeter at 0.6 volts and can operate steadily for at least 180 hours.

2.
J Phys Chem C Nanomater Interfaces ; 123(12): 7290-7298, 2019 Mar 28.
Artigo em Inglês | MEDLINE | ID: mdl-30949277

RESUMO

Understanding the intrinsic catalytic properties of perovskite materials can accelerate the development of highly active and abundant complex oxide catalysts. Here, we performed a first-principles density functional theory study combined with a microkinetics analysis to comprehensively investigate the influence of defects on catalytic CO oxidation of LaFeO3 catalysts containing single atoms of Rh, Pd, and Pt. La defects and subsurface O vacancies considerably affect the local electronic structure of these single atoms adsorbed at the surface or replacing Fe in the surface of the perovskite. As a consequence, not only the stability of the introduced single atoms is enhanced but also the CO and O2 adsorption energies are modified. This also affects the barriers for CO oxidation. Uniquely, we find that the presence of La defects results in a much higher CO oxidation rate for the doped perovskite surface. A linear correlation between the activation barrier for CO oxidation and the surface O vacancy formation energy for these models is identified. Additionally, the presence of subsurface O vacancies only slightly promotes CO oxidation on the LaFeO3 surface with an adsorbed Rh atom. Our findings suggest that the introduction of La defects in LaFeO3-based environmental catalysts could be a promising strategy toward improved oxidation performance. The insights revealed herein guide the design of the perovskite-based three-way catalyst through compositional variation.

3.
ACS Catal ; 8(7): 6552-6559, 2018 Jul 06.
Artigo em Inglês | MEDLINE | ID: mdl-30023135

RESUMO

Methane (CH4) combustion is an increasingly important reaction for environmental protection, for which Pd/CeO2 has emerged as the preferred catalyst. There is a lack of understanding of the nature of the active site in these catalysts. Here, we use density functional theory to understand the role of doping of Pd in the ceria surface for generating sites highly active toward the C-H bonds in CH4. Specifically, we demonstrate that two Pd2+ ions can substitute one Ce4+ ion, resulting in a very stable structure containing a highly coordinated unsaturated Pd cation that can strongly adsorb CH4 and dissociate the first C-H bond with a low energy barrier. An important aspect of the high activity of the stabilized isolated Pd cation is its ability to form a strong σ-complex with CH4, which leads to effective activation of CH4. We show that also other transition metals like Pt, Rh, and Ni can give rise to similar structures with high activity toward C-H bond dissociation. These insights provide us with a novel structural view of solid solutions of transition metals such as Pt, Pd, Ni, and Rh in CeO2, known to exhibit high activity in CH4 combustion.

4.
ACS Catal ; 8(1): 75-80, 2018 Jan 05.
Artigo em Inglês | MEDLINE | ID: mdl-29333329

RESUMO

Doping CeO2 with Pd atoms has been associated with catalytic CO oxidation, but current surface models do not allow CO adsorption. Here, we report a new structure of Pd-doped CeO2(111), in which Pd adopts a square planar configuration instead of the previously assumed octahedral configuration. Oxygen removal from this doped structure is favorable. The resulting defective Pd-doped CeO2 surface is active for CO oxidation and is also able to cleave the first C-H bond in methane. We show how the moderate CO adsorption energy and dynamic features of the Pd atom upon CO adsorption and CO oxidation contribute to a low-barrier catalytic cycle for CO oxidation. These structures, which are also observed for Ni and Pt, can lead to a more open coordination environment around the doped-transition-metal center. These thermally stable structures are relevant to the development of single-atom catalysts.

5.
Chem Mater ; 29(21): 9456-9462, 2017 Nov 14.
Artigo em Inglês | MEDLINE | ID: mdl-29170602

RESUMO

We carried out density functional theory calculations to investigate the ripening of Pd clusters on CeO2(111). Starting from stable Pd n clusters (n = 1-21), we compared how these clusters can grow through Ostwald ripening and coalescence. As Pd atoms have mobility higher than that of Pd n clusters on the CeO2(111) surface, Ostwald ripening is predicted to be the dominant sintering mechanism. Particle coalescence is possible only for clusters with less than 5 Pd atoms. These ripening mechanisms are facilitated by adsorbed CO through lowering barriers for the cluster diffusion, detachment of a Pd atom from clusters, and transformation of initial planar clusters.

6.
Nanoscale ; 7(21): 9563-9, 2015 Jun 07.
Artigo em Inglês | MEDLINE | ID: mdl-25947616

RESUMO

A novel, mild and effective method was designed for grafting of high-quality organic monolayers on a silicon surface to catalyze nanoparticles' growth. By using a white-light source, 3-mercaptopropionic acid (3-MPA) molecules were attached to hydrogen-terminated Si(111) surfaces at room temperature. The attached monolayers were characterized using X-ray photoelectron spectroscopy to provide detailed information. The in situ growth of Au nanoparticles (AuNPs) with dimensions below 20 nm was catalyzed on a silicon surface with highly uniform and compact structure morphology. The AuNPs can grow selectively in a certain region on a patterned Si-Si3N4 chip. p-Nitrothiophenol (p-NTP) was used as the probe to evaluate the SERS enhancement of the highly uniform and compact AuNP-Si substrate. In order to better understand the white light initiation of the addition reaction of 3-MPA on the Si(111)-H surface, the mechanism was elucidated by density functional theoretical (DFT) calculations, which indicated that the formation of the Si-O bond occurred at the PEC of the first singlet excited state (S1) with a very low activation barrier about 30% of the ground state (S0) value.

7.
Spectrochim Acta A Mol Biomol Spectrosc ; 139: 456-63, 2015 Mar 15.
Artigo em Inglês | MEDLINE | ID: mdl-25576943

RESUMO

As an important inter-unit of lignin, guaiacylglycerol-ß-guaiacyl (GG) ether has been synthesized, and characterized using terahertz time-domain spectroscopy in the frequency range of 5-85 cm(-1). Seven absorption peaks have been observed. Among these peaks, the 49.8 cm(-1) and 57.6 cm(-1) vibrations are propose to be characteristic absorption peaks of GG ether. Raman spectra were also measured in the range of 50-3500 cm(-1). The vibrations of the two lowest energy forms, i.e., erythro 1r4s and threo 1s4s, were calculated using density functional theory at the B3LYP/6-311G∗∗ level and assigned according to potential energy distribution. In addition, the contents of erythro and threo forms in GG sample could be estimated by comparing the waveform similarities between theoretical and observed curves in the 33.0-80.0 cm(-1) range. Results showed that the observed curve of GG sample is a combination of erythro 1s4r and threo 1s4s. The four absorption vibrations below 33.0 cm(-1) could be assigned to phonon, inter-molecular modes and/or hydrogen bond vibrations. Terahertz spectra and Raman spectra, together with theoretical calculations, could be powerful methods for predicting contents of different isomers in sample.


Assuntos
Guaifenesina/análogos & derivados , Análise Espectral Raman , Vibração , Guaifenesina/química , Isomerismo , Conformação Molecular , Refratometria , Espectroscopia Terahertz , Termodinâmica
8.
Chem Sci ; 6(11): 6091-6096, 2015 11 01.
Artigo em Inglês | MEDLINE | ID: mdl-28717449

RESUMO

Here we emphasise the importance of the dielectric environment on the electron transfer behavior in interfacial electrochemical systems. Through doping cobalt hexacyanide (Co(CN)63-) into single microcrystals of sodium chloride (NaCl), for the first time, we obtained the direct electrochemical behavior of Co(CN)63- which is hardly ever obtained in either aqueous or conventional nonaqueous solutions. DFT calculations elucidate that, as the Co(CN)63- anions occupy the lattice units of NaCl65- in the NaCl microcrystal, the redox energy barrier of Co(CN)63-/4- is decreased dramatically due to the low dielectric constant of NaCl. Meanwhile, the low-spin Co(CN)64- anions are stabilized in the lattices of the NaCl microcrystal. The results also show that the NaCl microcrystal is a potential solvent for solid-state electrochemistry at ambient temperature.

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