In Situ Phase Separation of NiAu Alloy Nanoparticles for Preparing Highly Active Au/NiO CO Oxidation Catalysts

H eterogeneous catalysts, often consisting of metal nanoparticles supported on high-surface-area oxide solids, are common in industrial chemical reactions. Researchers have increasingly recognized the importance of oxides in heterogeneous catalysts: that they are not just a support to help the dispersion of supported metal nanoparticles, but rather interact with supported metal nanoparticles and affect the catalysis. The critical role of oxides in catalytic reactions can become very prominent when oxides cover metal surfaces forming the inverse catalysts.The source of the catalytic activity in homogeneous catalysts and metalloenzymes is often coordinatively unsaturated (CUS) transition metal (TM) cations, which can undergo facile electron transfer and promote catalytic reactions. Organic ligands and proteins confine these CUS cations, making them highly active and stable. In heterogeneous catalysis, however, confining these highly active CUS centers on an inorganic solid so that they are robust enough to endure the reaction environment while staying flexible enough to perform their catalysis remains a challenge.In this Account, we describe a strategy to confine the active CUS centers on the solid surface at the interface between a TM oxide (TMO) and a noble metal (NM). Among metals, NMs have high electron negativity and low oxygen affinity. This means that TM cations of the oxide bind strongly to NM atoms at the interface, forming oxygen-terminated-bilayer TMO nanostructures. The resulting CUS sites at the edges of the TMO nanostructure are highly active for catalytic oxidation reactions. Meanwhile, the strong interactions between TMOs and NMs prevent further oxidation of the bilayer TMO phases, which would otherwise result in the saturation of oxygen coordination and the deactivation of the CUS cations. We report that we can also tune the oxideÀmetal interactions to modulate the bonding of reactants with CUS centers, optimizing their catalytic performance.We review our recent progress on oxide-on-metal inverse catalysts, mainly the TMO-on-Pt (TM = Fe, Co, and Ni) systems and discuss the interface-confinement effect, an important factor in the behavior of these catalytic systems. We have studied both model catalyst systems and real supported nanocatalysts. Surface science studies and density functional theory calculations in model systems illustrate the importance of the oxideÀmetal interfaces in the creation and stabilization of surface active centers, and reveal the reaction mechanism at these active sites. In real catalysts, we describe facile preparation processes for fabricating the oxide-on-metal nanostructures. We have demonstrated excellent performance of the inverse catalysts in oxidation reactions such as CO oxidation. We believe that the interface confinement effect can be employed to design highly efficient novel catalysts and that the inverse oxide-on-metal catalysts may find wide applications in heterogeneous catalysis.

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“…26 Moreover, the active surface oxide phases can be confined on Au surface, producing highly active TMO-on-Au catalytic systems. 16 …”

Section: Discussionmentioning

confidence: 99%

Interface-Confined Oxide Nanostructures for Catalytic Oxidation Reactions

2013

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“…Our group synthesized NiAu nanoparticles via a co-reduction method employing butyllithium as the reducing agent and trioctylphosphine as the protecting agent [88,90]. These nanoparticles were loaded onto an amorphous SiO 2 support to obtain NiAu/SiO 2 .…”

Section: In Situ Transformation Of Niau/sio2 Into Au-niomentioning

confidence: 99%

“…This sample was transformed into Au-NiO/SiO 2 after low-temperature reduction and subsequent hightemperature oxidation pretreatment (Fig. 14) [88]. Interestingly, the size of the gold nanoparticles was quite small due to the protection afforded by the surrounding NiO, whereas Au/SiO 2 underwent significant sintering on calcination.…”

Section: In Situ Transformation Of Niau/sio2 Into Au-niomentioning

confidence: 99%

“…However, only limited attention has been paid to the use of such core-shell structures [252,[254][255][256][257][258][259][260][261] or their supported versions [262][263][264] in the fabrication of new catalysts [265,266]. Some researchers have developed new Figure 14 Schematic representation of the phase transformation from a NiAu alloy to a Au/NiO hetero-aggregate [88]. The SiO 2 support is omitted for simplicity.…”

Section: Dispersion Of Au@oxide Core-shell Structures On a Supportmentioning

confidence: 99%

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Development of novel supported gold catalysts: A materials perspective

2010

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Since Haruta et al. discovered that small gold nanoparticles finely dispersed on certain metal oxide supports can exhibit surprisingly high activity in CO oxidation below room temperature, heterogeneous catalysis by supported gold nanoparticles has attracted tremendous attention. The majority of publications deal with the preparation and characterization of conventional gold catalysts (e.g., Au/TiO 2 ), the use of gold catalysts in various catalytic reactions, as well as elucidation of the nature of the active sites and reaction mechanisms. In this overview, we highlight the development of novel supported gold catalysts from a materials perspective. Examples, mostly from those reported by our group, are given concerning the development of simple gold catalysts with single metal-support interfaces and heterostructured gold catalysts with complicated interfacial structures. Catalysts in the first category include active Au/SiO 2 and Au/metal phosphate catalysts, and those in the second category include catalysts prepared by pre-modification of supports before loading gold, by post-modification of supported gold catalysts, or by simultaneous dispersion of gold and an inorganic component onto a support. CO oxidation has generally been employed as a probe reaction to screen the activities of these catalysts. These novel gold catalysts not only provide possibilities for applied catalysis, but also furnish grounds for fundamental research.

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“…Early work by Koeppel et al (18), for example, showed that the oxidation of amorphous Au 25 Zr 75 produces Au nanoparticles on ZrO 2 , which are active in CO 2 hydrogenation. Similarly, the oxidation of an Au-Ni alloy leads to the formation of coupled Au-NiO aggregates in the form of Au particles connected to NiO particles; on SiO 2 support these heterostructures showed enhanced activity in low-temperature CO oxidation (19,20). Both Au-Cu (21-23) and Au-Sn (24) alloy nanoparticles are transformed to core-shell structures upon oxidation, consisting of Au cores and CuO x or SnO 2 shells, respectively.…”

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confidence: 98%

Oxidation of nanoscale Au–In alloy particles as a possible route toward stable Au-based catalysts

Sutter

Tong

Jungjohann

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

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The oxidation of bimetallic alloy nanoparticles comprising a noble and a nonnoble metal is expected to cause the formation of a single-component surface oxide of the nonnoble metal, surrounding a core enriched with the noble metal. Studying the room temperature oxidation of Au-In nanoparticles, we show that this simple picture does not apply to an important class of bimetallic alloys, in which the oxidation proceeds via predominant oxygen diffusion. Instead of a crystalline In 2 O 3 shell, such oxidation leads to an amorphous shell of mixed Au-In oxide that remains stable to high temperatures and whose surface layer is enriched with Au. The Au-rich mixed oxide is capable of adsorbing both CO and O 2 and converting them to CO 2 , which desorbs near room temperature. The oxidation of Au-In alloys to a mixed Au-In oxide shows significant promise as a viable approach toward Au-based oxidation catalysts, which do not require any complex synthesis processes and resist deactivation up to at least 300°C.transmission electron microscopy | temperature programmed desorption | X-ray photoelectron spectroscopy T he importance of metal nanoparticles for a wide variety of applications, e.g., in catalysis, sensing, etc., has sparked interest in understanding and controlling their oxidation. The formation of an oxide layer negatively affects performance in applications that require pure metal surfaces. However, oxidation can be advantageous as it opens a way for engineering complex structures. Our understanding of room temperature oxidation has been established primarily in studies on the oxidation of bulk materials or planar films (1). Most of this insight should apply directly to the oxidation of nanoparticles, but important aspects arise in the geometry of small particles. For example, the high curvature in a nanoparticle can drive oxidation to larger thicknesses than in the planar case (2-4). Nanoscale junctions and interfaces to other materials can promote enhanced oxidation, as well as the formation of well-ordered epitaxial oxide segments (5). The oxidation of nanoparticles is also a powerful mechanism to produce nanoscale heterostructures. Metals that oxidize via predominant anion diffusion [In (2, 5), Pb (6), Sn, among others] form metal-oxide core-shell nanoparticles. For metals that oxidize via fast cation diffusion [Co (7), Al (6), Fe (8), Ni (9), among others], nanoscale porosity buildup creates hollow oxide nanocrystals that can be used as cages, e.g., to be filled with different materials (7) for storage or protection.Bimetallic (alloy, core-shell, etc.) nanoparticles promise widely tunable properties, and much progress has recently been made in their controlled synthesis (10). The oxidation of bimetallic nanoparticles has been studied much less than that of elemental metals, but it could provide even more interesting opportunities for the fabrication of functional nanomaterials, for example in catalysis.Alloys that comprise a noble and a less noble metal component are of interest because their oxidation can give ...

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In Situ Phase Separation of NiAu Alloy Nanoparticles for Preparing Highly Active Au/NiO CO Oxidation Catalysts

Cited by 91 publications

References 29 publications

Interface-Confined Oxide Nanostructures for Catalytic Oxidation Reactions

Interface-Confined Oxide Nanostructures for Catalytic Oxidation Reactions

Development of novel supported gold catalysts: A materials perspective

Oxidation of nanoscale Au–In alloy particles as a possible route toward stable Au-based catalysts

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