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1.
J Am Chem Soc ; 141(44): 17548-17557, 2019 Nov 06.
Artigo em Inglês | MEDLINE | ID: mdl-31607122

RESUMO

The construction of stable active site in nanocatalysts is of great importance but is a challenge in heterogeneous catalysis. Unexpectedly, coordination-unsaturated and atomically dispersed copper species were constructed and stabilized in a sintered copper-ceria catalyst through air-calcination at 800 °C. This sintered copper-ceria catalyst showed a very high activity for CO oxidation with a CO consumption rate of 6100 µmolCO·gCu-1·s-1 at 120 °C, which was at least 20 times that of other reported copper catalysts. Additionally, the excellent long-term stability was unbroken under the harsh cycled reaction conditions. Based on a comprehensive structural characterization and mechanistic study, the copper atoms with unsaturated coordination in the form of Cu1O3 were identified to be the sole active site, at which both CO and O2 molecules were activated, thus inducing remarkable CO oxidation activity with a very low copper loading (1 wt %).

2.
Nat Commun ; 10(1): 3470, 2019 Aug 02.
Artigo em Inglês | MEDLINE | ID: mdl-31375672

RESUMO

As the water-gas shift (WGS) reaction serves as a crucial industrial process, strategies for developing robust WGS catalysts are highly desiderated. Here we report the construction of stabilized bulk-nano interfaces to fabricate highly efficient copper-ceria catalyst for the WGS reaction. With an in-situ structural transformation, small CeO2 nanoparticles (2-3 nm) are stabilized on bulk Cu to form abundant CeO2-Cu interfaces, which maintain well-dispersed under reaction conditions. This inverse CeO2/Cu catalyst shows excellent WGS performances, of which the activity is 5 times higher than other reported Cu catalysts. Long-term stability is also very solid under harsh conditions. Mechanistic study illustrates that for the inverse CeO2/Cu catalyst, superb capability of H2O dissociation and CO oxidation facilitates WGS process via the combination of associative and redox mechanisms. This work paves a way to fabricate robust catalysts by combining the advantages of bulk and nano-sized catalysts. Catalysts with such inverse configurations show great potential in practical WGS applications.

3.
Langmuir ; 35(26): 8658-8666, 2019 Jul 02.
Artigo em Inglês | MEDLINE | ID: mdl-31244254

RESUMO

Supported CuO-CeO2 catalysts have been extensively studied for their outstanding catalytic activity in CO oxidation. Unfortunately, they are prone to sintering and deactivation when exposed to high-temperature automotive exhausts. Herein, taking advantage of the heat-resistant SiO2 microspheres, we fabricated a series of core-shell-structured yCuO- xCeO2@SiO2 ( x is the weight ratio of CeO2-SiO2 and y is the weight ratio of Cu-(CeO2@SiO2)) composite catalysts. All the small CeO2 particles were bound to the SiO2 spheres, forming an xCeO2@SiO2 structure, on the surface of which a certain amount of CuO was well-dispersed. The 5CuO-50CeO2@SiO2 catalyst exhibited good activity, with the full conversion of CO achieved at around 130 °C, and no obvious deactivation was observed in the stability test. Importantly, the interaction between CuO and CeO2@SiO2 enhanced its durability at high temperatures. Even at 800 °C and with a space velocity of 800 000 mL·gcat-1·h-1, CO conversion could be maintained at 90%, which is prospectively applied in a real CO elimination system. The result of the temperature-programmed reduction in hydrogen demonstrated that this special core-shell-structured 5CuO-50CeO2@SiO2 catalyst improved the reduction ability of the CuO species. In situ diffuse reflectance infrared Fourier transform spectroscopy measurements further confirmed that CO molecules preferred to be adsorbed on Cu(I) species to form reactive CO-Cu(I) that enhanced the reactivity of the 5CuO-50CeO2@SiO2 catalyst.

4.
J Am Chem Soc ; 141(11): 4613-4623, 2019 Mar 20.
Artigo em Inglês | MEDLINE | ID: mdl-30807152

RESUMO

The crucial role of the metal-oxide interface in the catalysts of the water-gas shift (WGS) reaction has been recognized, while the precise illustration of the intrinsic reaction at the interfacial site has scarcely been presented. Here, two kinds of gold-ceria catalysts with totally distinct gold species, <2 nm clusters and 3 to 4 nm particles, were synthesized as catalysts for the WGS reaction. We found that the gold cluster catalyst exhibited a superiority in reactivity compared to gold nanoparticles. With the aid of comprehensive in situ characterization techniques, the bridged -OH groups that formed on the surface oxygen vacancies of the ceria support are directly determined to be the sole active configuration among various surface hydroxyls in the gold-ceria catalysts. The isotopic tracing results further proved that the reaction between bridged surface -OH groups and CO molecules adsorbed on interfacial Au atoms contributes dominantly to the WGS reactivity. Thus, the abundant interfacial sites in gold clusters on the ceria surface induced superior reactivity compared to that of supported gold nanoparticles in catalyzing the WGS reaction. On the basis of direct and solid experimental evidence, we have obtained a very clear image of the surface reaction for the WGS reaction catalyzed by the gold-ceria catalyst.

5.
ACS Appl Mater Interfaces ; 10(40): 34172-34183, 2018 Oct 10.
Artigo em Inglês | MEDLINE | ID: mdl-30205674

RESUMO

Exploring the state-of-the-art heterogeneous catalysts has been a general concern for sustainable and clean energy. Here, Pt-embedded CuO x-CeO2 multicore-shell (Pt/CuO x-CeO2 MS) composites are fabricated at room temperature via a one-pot and template-free procedure for catalyzing CO oxidation, a classical probe reaction, showing a volcano-shaped relationship between the composition and catalytic activity. We experimentally unravel that the Pt/CuO x-CeO2 MS composites are derived from an interfacial autoredox process, where Pt nanoparticles (NPs) are in situ encapsulated by self-assembled ceria nanospheres with CuO x clusters adhered through deposition/precipitation-calcination process. Only Cu-O and Pt-Pt coordination structures are determined for CuO x clusters and Pt NPs in Pt/CuO x-CeO2 MS, respectively. Importantly, the close vicinity between Pt and CeO2 benefits to more oxygen vacancies in CeO2 counterparts and results in thin oxide layers on Pt NPs. Meanwhile, the introduction of CuO x clusters is crucial for triggering synergistic catalysis, which leads to high resistance to aggregation of Pt NPs and improvement of catalytic performance. In CO oxidation reaction, both Ptδ+-CO and Cu+-CO can act as active sites during CO adsorption and activation. Nonetheless, redundant content of Pt or Cu will induce a strongly bound Pt-O-Ce or Cu-[O x]-Ce structures in air-calcinated Pt/CuO x-CeO2 MS composites, respectively, which are both deleterious to catalytic reactivity. As a result, the composition-dependent catalytic activity and superior durability of Pt/CuO x-CeO2 MS composites toward CO oxidation reaction are achieved. This work should be instructive for fabricating desirable multicomponent catalysts composed of noble metal and bimetallic oxide composites for diverse heterogeneous catalysis.

6.
Phys Chem Chem Phys ; 19(22): 14533-14542, 2017 Jun 07.
Artigo em Inglês | MEDLINE | ID: mdl-28537308

RESUMO

A low-concentration cobalt (∼6 at%) and manganese (∼3 at%) bimetallic oxide catalyst supported on ceria nanorods (CoMnOx/CeO2), as well as its related single metal oxide counterparts (CoOx/CeO2 and MnOx/CeO2) was synthesized via a deposition-precipitation approach. The fresh samples after air-calcination at 400 °C were tested under the reaction conditions of CO oxidation, and showed the following order of reactivity: CoMnOx/CeO2 > CoOx/CeO2 > MnOx/CeO2. X-ray diffraction (XRD) and transmission electron microscopy (TEM) data identified that the structure of the CeO2 support was maintained during deposition of metal (Co, Mn) ions while the corresponding vis-Raman spectra verified that more oxygen vacancies were created after deposition-precipitation than those in pure ceria nanorods. Aberration-corrected, high-angle, annular dark-field scanning transmission electron microscopy (HAADF-STEM) images with the help of electron energy loss spectroscopy (EELS) analyses determined two types of cobalt species, i.e. ultra-fine clusters (<2 nm) and smaller nanocrystals (up to 5 nm) in CoOx/CeO2 while only bigger nanostructures (∼10 nm) of cobalt-manganese oxides in CoMnOx/CeO2. X-ray absorption fine structure (XAFS) measurements demonstrated the presence of a cubic Co3O4 phase in all the cobalt-based catalysts. The fitting results of the extended X-ray absorption fine structure (EXAFS) indicated that the introduction of the secondary metal (Mn) oxide significantly enhanced the two-dimensional growth of cobalt oxide nanostructures on the surface of CeO2. Therefore, the enhanced activity of CO oxidation reaction over the bimetallic cobalt-manganese oxide nanocatalyst can be attributed to the higher crystallinity of the Co3O4 phase in this work.

7.
Chemistry ; 23(33): 7990-7996, 2017 Jun 12.
Artigo em Inglês | MEDLINE | ID: mdl-28326629

RESUMO

An unprecedented reversible dynamic transformation is reported in a metal-organic framework involving bond formation, which is accompanied by two important structural changes; achiral to chiral and two- to three-dimensions. Using two bent organic ligands (diimpym=4,6-di(1H-imidazol-1-yl)pyrimidine; H2 npta=5-nitroisophthalic acid) and CoII (NO3 )2 ⋅6 H2 O the coordination polymer Co(diimpym)(npta)⋅CH3 OH, (1⋅CH3 OH), was obtained solvothermally. Its structure consists of knitted pairs of square layers (44 -sql net) of five-coordinated Co and disordered methanol, and it crystallized in the achiral Pbca space group at room temperature. It undergoes a single crystal to single crystal (SC-SC) transformation to a 3D interpenetrated framework (α-polonium-type net, pcu) of six-coordinated Co and ordered methanol in the chiral P21 21 21 space group below 220 K. Most unusual is the dynamic temperature-dependent shortening of a Co⋅⋅⋅O connection from a non-bonded 2.640 Š(298 K) to a bonded 2.347 Šdistance (100 K) transforming the square pyramidal cobalt polyhedron to a distorted octahedron. The desolvated crystals (1) obtained at 480 K retain the full crystallinity and crystallize in the achiral Pbca space group between 100 and 298 K but the dynamic shortening of the Co⋅⋅⋅O distance connecting the layers into the 3D pcu framework structure is observed. Following post-synthetic insertion of ethanol (1⋅CH3 CH2 OH) it does not exhibit the transformation and retains the knitted 2D achiral Pbca structure for all temperatures (100-298 K) and the ethanol is always disordered. The structural analyses thus conclude that the ordering of the methanol induces the chirality while the available space controls the dynamic motion of the knitted 2D networks into the 3D interpenetrated framework. Consequently, 1 selectively adsorbs CO2 to N2 and exhibits Type-III isotherms indicating dynamic motion of the 2D networks to accommodate the CO2 at 273 and 298 K in contrast to the rigidity of the 3D framework at 77 K preventing N2 from penetrating the solid. The magnetic properties are also reported.

8.
Nat Commun ; 7: 13481, 2016 11 16.
Artigo em Inglês | MEDLINE | ID: mdl-27848964

RESUMO

Small-size (<5 nm) gold nanostructures supported on reducible metal oxides have been widely investigated because of the unique catalytic properties they exhibit in diverse redox reactions. However, arguments about the nature of the gold active site have continued for two decades, due to the lack of comparable catalyst systems with specific gold species, as well as the scarcity of direct experimental evidence for the reaction mechanism under realistic working conditions. Here we report the determination of the contribution of single atoms, clusters and particles to the oxidation of carbon monoxide at room temperature, by the aid of in situ X-ray absorption fine structure analysis and in situ diffuse reflectance infrared Fourier transform spectroscopy. We find that the metallic gold component in clusters or particles plays a much more critical role as the active site than the cationic single-atom gold species for the room-temperature carbon monoxide oxidation reaction.

9.
Inorg Chem ; 55(8): 3992-9, 2016 Apr 18.
Artigo em Inglês | MEDLINE | ID: mdl-27032058

RESUMO

MoS2 ultrathin nanosheet-coated CeO2 hollow sphere (CeO2@MoS2) hybrid nanostructures with a 3D hierarchical configuration were successfully constructed from a facile two-step wet chemistry strategy: first, CeO2 formed on a silica core which served as a template and was subsequently removed by NaOH solution to attain hollow spheres, and then few-layered ultrathin MoS2 nanosheets were deposited on the CeO2 hollow spheres through a hydrothermal process. As a proof of concept application, the as-prepared CeO2@MoS2 hybrid nanostructures were used as catalytic material, which exhibited enhanced catalytic activity in ammonia decomposition for H2 production at high temperature. It was demonstrated that, even with a structural transformation from MoS2 to MoNx under harsh conditions of ammonia decomposition at high temperature (700 °C), the 3D hierarchical nanostructures of the CeO2@MoNx were well kept, indicating the important role of the CeO2 support.

10.
Nanoscale ; 7(45): 18971-83, 2015 Dec 07.
Artigo em Inglês | MEDLINE | ID: mdl-26510962

RESUMO

Effective separation and migration of photogenerated electron-hole pairs are two key factors to determine the performance of photocatalysts. It has been widely accepted that photocatalysts with heterojunctions usually exhibit excellent charge separation. However, the migration process of separated charges in the heterojunction structures has not been fully investigated. Herein, photocatalysts with heterojunctions are constructed by loading g-C3N4 nanoparticles onto BiOCl nanosheets with different exposed facets (BOC-001 and BOC-010). The g-C3N4 nanoparticles with decreasing size and increasing zeta potential could induce stronger coupling and scattering in the heterojunction. The relationship between the crystal facet orientation in the BiOCl nanosheets and charge separation/effective migration behaviours of the materials is investigated. The visible light photocatalytic activity of the composites is evaluated by methyl orange (MO) and phenol degradation experiments, and the results show that ng-CN/BOC-010 composites exhibit higher photocatalytic performance than that of ng-CN/BOC-001 composites. Both photoelectrochemical and fluorescence emission measurements indicate that the different exposed facets in ng-CN/BiOCl composites could induce the migration of the photogenerated electrons in different ways, but do not significantly alter the separation efficiencies. The separated electrons in ng-CN/BOC-010 undergo a shorter transport distance than that of ng-CN/BOC-001 to reach the surface reactive sites. The study may suggest that the crystal facet orientation in polar semiconductors is a critical factor for designing highly efficient heterojunction photocatalysts.

11.
J Am Chem Soc ; 137(35): 11407-18, 2015 Sep 09.
Artigo em Inglês | MEDLINE | ID: mdl-26301797

RESUMO

Co3O4 with a spinel structure is a very active oxide catalyst for the oxidation of CO. In such catalysts, octahedrally coordinated Co(3+) is considered to be the active site, while tetrahedrally coordinated Co(2+) is assumed to be basically inactive. In this study, a highly ordered mesoporous CoO has been prepared by H2 reduction of nanocast Co3O4 at low temperature (250 °C). The as-prepared CoO material, which has a rock-salt structure with a single Co(2+) octahedrally coordinated by lattice oxygen in Fm3̅m symmetry, exhibited unexpectedly high activity for CO oxidation. Careful investigation of the catalytic behavior of mesoporous CoO catalyst led to the conclusion that the oxidation of surface Co(2+) to Co(3+) causes the high activity. Other mesoporous spinels (CuCo2O4, CoCr2O4, and CoFe2O4) with different Co species substituted with non/low-active metal ions were also synthesized to investigate the catalytically active site of cobalt-based catalysts. The results show that not only is the octahedrally coordinated Co(3+) highly active but also the octahedrally coordinated Co(2+) species in CoFe2O4 with an inverse spinel structure shows some activity. These results suggest that the octahedrally coordinated Co(2+) species is easily oxidized and shows high catalytic activity for CO oxidation.

12.
Nanoscale ; 7(11): 4920-8, 2015 Mar 21.
Artigo em Inglês | MEDLINE | ID: mdl-25631762

RESUMO

Uniform Au nanoparticles (∼2 nm) with narrow size-distribution (standard deviation: 0.5-0.6 nm) supported on both hydroxylated (Fe_OH) and dehydrated iron oxide (Fe_O) have been prepared by either deposition-precipitation (DP) or colloidal-deposition (CD) methods. Different structural and textural characterizations were applied to the dried, calcined and used gold-iron oxide samples. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) showed high homogeneity in the supported Au nanoparticles. The ex situ and in situ X-ray absorption fine structure (XAFS) characterization monitored the electronic and short-range local structure of active gold species. The synchrotron-based in situ X-ray diffraction (XRD), together with the corresponding temperature-programmed reduction by hydrogen (H2-TPR), indicated a structural evolution of the iron-oxide supports, correlating to their reducibility. An inverse order of catalytic activity between DP (Au/Fe_OH < Au/Fe_O) and CD (Au/Fe_OH > Au/Fe_O) was observed. Effective gold-support interaction results in a high activity for gold nanoparticles, locally generated by the sintering of dispersed Au atoms on the oxide support in the DP synthesis, while a hydroxylated surface favors the reactivity of externally introduced Au nanoparticles on Fe_OH support for the CD approach. This work reveals why differences in the synthetic protocol translate to differences in the catalytic performance of Au/FeOx catalysts with very similar structural characteristics in CO oxidation.

13.
Nanoscale ; 6(5): 2649-59, 2014 Mar 07.
Artigo em Inglês | MEDLINE | ID: mdl-24442108

RESUMO

Fabricating heterojunction photocatalysts is an important strategy for speeding up the separation rate of photogenerated charge carriers, which is attracting greater interest. However, the choice of three factors, individual materials, band offsets, and effective interfaces, is still important for fabricating efficient heterojunction photocatalysts. Herein, efficient g-C3N4/Zn2GeO4 photocatalysts with effective interfaces were designed by controlling the surface charges of the two individual materials inside the same aqueous dispersion medium, making use of the electrostatic attraction between oppositely charged particles. The g-C3N4/Zn2GeO4 heterojunction with opposite surface charge (OSC) showed higher visible-light photocatalytic activity for degradation of methylene blue than those of pure g-C3N4, pure Zn2GeO4, and the g-C3N4/Zn2GeO4 with identical surface charge (ISC). The investigation of the light absorption spectrum, adsorption ability, and photocurrent responses revealed that the improved separation of photogenerated carriers was the main reason for the enhancement of the OSC g-C3N4/Zn2GeO4 sample's photocatalytic activity. By combining with theoretical calculations, we investigated the microscopic mechanisms of interface interaction and charge transfer between g-C3N4 and Zn2GeO4. The photogenerated electrons in the g-C3N4 N 2p states directly excited into the Zn 4s and Ge 4s hybrid states of Zn2GeO4. The strategy of designing and preparing a g-C3N4/Zn2GeO4 composite catalyst in this work is very useful for fabricating other efficient heterojunction photocatalysts.

14.
J Am Chem Soc ; 133(29): 11279-88, 2011 Jul 27.
Artigo em Inglês | MEDLINE | ID: mdl-21696181

RESUMO

A high surface area Co(3)O(4)-SiO(2) nanocomposite catalyst has been prepared by use of activated carbon as template. The Co(3)O(4)-SiO(2) composite, the surface of which is rich in silica and Co(II) species compared with normal Co(3)O(4), exhibited very high activity for CO oxidation even at a temperature as low as -76 °C. A rather unusual temperature-dependent activity curve, with the lowest conversion at about 80 °C, was observed with a normal feed gas (H(2)O content ~3 ppm). The U-shape of the activity curve indicates a negative apparent activation energy over a certain temperature range, which has rarely been observed for the heterogeneously catalyzed oxidation of CO. Careful investigation of the catalytic behavior of Co(3)O(4)-SiO(2) catalyst led to the conclusion that adsorption of H(2)O molecules on the surface of the catalyst caused the unusual behavior. This conclusion was supported by in situ diffuse reflectance Fourier transform infrared (DRIFT) spectroscopic experiments under both normal and dry conditions.

15.
Phys Chem Chem Phys ; 13(7): 2457-87, 2011 Feb 21.
Artigo em Inglês | MEDLINE | ID: mdl-21246127

RESUMO

Recent advances in the synthesis of colloidal metal nanoparticles of controlled sizes and shapes that are relevant for catalyst design are reviewed. Three main methods, based on colloid chemistry techniques in solution, i.e., chemical reduction of metal salt precursors, electrochemical synthesis, and controlled decomposition of organometallic compounds and metal-surfactant complexes, are used to synthesize metal nanoparticles. Their catalytic activity and selectivity depend on the shape, size and composition of the metal nanoparticles, and the support effect, as shown for many reactions in quasi-homogeneous and heterogeneous catalysis. A specially designed type of thermally stable catalysts--"embedded" metal catalysts, in which metal nanoparticles are isolated by porous support shells so that metal sintering is effectively avoided at high temperatures, are also introduced. The utilization of pre-prepared colloidal metal nanoparticles with tuned size, shape and composition as components of designed catalysts opens up new field in catalysis.


Assuntos
Nanopartículas Metálicas/química , Catálise , Coloides
17.
Chem Commun (Camb) ; 46(8): 1238-40, 2010 Feb 28.
Artigo em Inglês | MEDLINE | ID: mdl-20449262

RESUMO

Using iron oxide as catalyst, glycerol can be converted to allyl alcohol through a dehydration and consecutive hydrogen transfer.


Assuntos
Glicerol/química , Propanóis/química , Catálise , Compostos Férricos/química , Hidrogênio/química
18.
J Am Chem Soc ; 132(5): 1520-2, 2010 Feb 10.
Artigo em Inglês | MEDLINE | ID: mdl-20078052

RESUMO

The colloidal deposition method was used to prepare Au/Mg(OH)(2) (0.7 wt % gold) catalysts with gold particle sizes between 1.5 to 5 nm which exhibited very high activity for CO oxidation with specific rates higher than 3.7 mol(CO) x h(-1) x g(Au)(-1) even at temperatures as low as -89 degrees C.

19.
J Am Chem Soc ; 130(50): 16968-77, 2008 Dec 17.
Artigo em Inglês | MEDLINE | ID: mdl-19053430

RESUMO

We present an innovative approach to the production of single-crystal iron oxide nanorings employing a solution-based route. Single-crystal hematite (alpha-Fe2O3) nanorings were synthesized using a double anion-assisted hydrothermal method (involving phosphate and sulfate ions), which can be divided into two stages: (1) formation of capsule-shaped alpha-Fe2O3 nanoparticles and (2) preferential dissolution along the long dimension of the elongated nanoparticles (the c axis of alpha-Fe2O3) to form nanorings. The shape of the nanorings is mainly regulated by the adsorption of phosphate ions on faces parallel to c axis of alpha-Fe2O3 during the nanocrystal growth, and the hollow structure is given by the preferential dissolution of the alpha-Fe2O3 along the c axis due to the strong coordination of the sulfate ions. By varying the ratios of phosphate and sulfate ions to ferric ions, we were able to control the size, morphology, and surface architecture to produce a variety of three-dimensional hollow nanostructures. These can then be converted to magnetite (Fe3O4) and maghemite (gamma-Fe2O3) by a reduction or reduction-oxidation process while preserving the same morphology. The structures and magnetic properties of these single-crystal alpha-Fe2O3, Fe3O4, and gamma-Fe2O3 nanorings were characterized by various analytical techniques. Employing off-axis electron holography, we observed the classical single-vortex magnetic state in the thin magnetite nanorings, while the thicker rings displayed an intriguing three-dimensional magnetic configuration. This work provides an easily scaled-up method for preparing tailor-made iron oxide nanorings that could meet the demands of a variety of applications ranging from medicine to magnetoelectronics.

20.
J Am Chem Soc ; 127(39): 13492-3, 2005 Oct 05.
Artigo em Inglês | MEDLINE | ID: mdl-16190701

RESUMO

SnO2 nanorod arrays were hierarchically assembled onto the surface of alpha-Fe2O3 nanotubes via a facile solution method. Determined by the hexagonal geometrical nature of the alpha-Fe2O3 nanotubes, the heterostructures were of 6-fold symmetry. HRTEM characterizations demonstrated that the lattice mismatch at the interface was an important factor in determining the growth direction of the secondary nanorod arrays.

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