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The constituent elements in bimetallic nanoparticles

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The presence of electronic and strain effects usually leads to better catalytic performances in bimetallic nanocrystals than monometallic nanocrystals. In the bimetallic nanocrystals the electronic effect arises as the two constituent metals have different electronic structure which leads to surface d-band structure alteration such as lifting or lowering its center. This alteration eventually influences the surface reactivity to reactants. Similarly, difference in constituent metals causing lattice mismatch facilitates distortion over the metals interfaces and leads to strain effect. Moreover, a multimetallic nanoparticle provides multiple catalytic platforms for tandem or serial catalytic reactions. The constituent elements in bimetallic nanoparticles however can have different spatial distribution and it exhibits comprehensive properties such as,

  1. Atomic ordering in alloyed or intermetallic nanocomposites
  2. Twin defects or stacking faults in the crystal structures
  3. Different shapes and facets

Another example of multimetallic nanoparticles, although less explored till date, is the trimetallic nanoparticles. They possess similar electronic and strain effect and recently gaining much attention as this leads to new horizon for novel structure nanoparticles. These highly sought nanoparticles possess selective spatial distribution of its constituent atoms.

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Herein we have demonstrated the synthesis of bimetallic and trimetallic nanocrystals in a 3-steps Cu-templated method. Cu core was synthesized in a monodisperse cube morphology, which was then followed by epitaxial growth of Pt to result in Cu-CuPt core-frame rhombic dodecahedral morphology (RD).10 The uniqueness of this work lies in the preferential rapid kinetics of Pt epitaxial growth on Cu core, preventing the galvanic replacement of the Cu core that retains pure Cu core and alloy CuPt frame. The coreduction of Cu + and Pt2+ ions have been known as the cause of formation of CuPt alloy fame. Au precursor was subsequently added on the Pt surface. Interestingly the Au reduces selectively on the faces of the Cu-CuPt RD core-frame as CuAu alloy frame.

Olefin hydoformylation is one of the important reactions for energy conversion, which generates aldehyde from olefin, CO and H2. However, this work has been limited by the use of CO, being a poisonous gas as a raw material. This reaction was previously carried out with homogeneous Rh complexes, however the reaction used high pressure and tedious purification work.

Methanol is a good feedstock for methanol reforming to produce high purity H2 and CO, owing to its high H/C ratio. Moreover, it is easy to reform at lower temperature (200-300⁰C) as it has no C-C bond. Au has been one of the best metals for methanol reforming to generate H2 and CO with high activity.

Metal support has an important role in the catalysis. A keen selection of support leads to high selectivity of the desired reaction product. Pt loaded on Molybdenum oxide was reported as an ethylene hydrogenation catalyst with similar activity as Pt-SiO2 . However, the Pt-silica has a lower activation energy than Pt-Molybdenum oxide support for ethane hydrogenolysis.16 Therefore, in addition to the general adsorption sites, metal oxides also play an important role in the electronic modulation of the catalysis. A recent work3 demonstrated a novel strategy for tandem catalysis system on two neighboring metal-metal oxide, drop casted bilayer CeO2-Pt-SiO2 for the first time. The demonstrated catalysis reaction by this catalyst is based on two interfaces Pt-CeO2 which acts as a good catalyst for methanol oxidation leading to CO and H2. Which subsequently catalyzes the hydroformylation of ethylene with the in-situ generated CO and H2 on the Pt-SiO2. As a control reaction for comparison to the tandem catalytic platform same reaction was carried out on a physical mixture of Pt-CeO2 and Pt-SiO2 prepared by impregnation method with 1-5 wt% loading. However, all of the mixtures produced ethane as a major product with very small amount of propane as minor product which might be produced by successive hydrogenation of the propanal.

Another control reaction was carried out on a mixture of 3 wt% of each Pt-SiO2 and Pt-CeO2, that produced propane of 5.7*10-4 s-1 per Pt atom, whereas the Time of Flight (TOF) of propanal per Pt is 2.6*10-2s-1, much higher on the tandem catalysts. This proved how the involvement of multiple built-in metal oxide interfaces leads to efficiently high reaction activity and selectivity. This work reported a surprising propanal selectivity of more than 94%, whereas on the conventional Pt catalyst has long been known to prefer the competing ethylene hydrogenation over the hydroformylation. This study lifts up the concept of conventional heterogeneous catalysis.

However, the drop-casted CeO2-Pt-SiO2 catalyst exhibits poor thermal stability and less active site density which had been a limitation for mechanistic study of the tandem reaction. This work was then followed by [email protected], which eventually provides higher thermal stability than the drop casted bilayer CeO2-Pt-SiO2 and higher active site owing to the 3D coating of the mSiO2 than the drop casted bilayer CeO2-Pt-SiO2. The catalyst was employed on the tandem catalysis of methanol to propanal, by production of CO and H2 on the Pt-CeO2 and subsequent ethylene hydroformylation on the Pt-SiO2 interfacel, leading propanal at a high selectivity of 80%.

In the present work, we will demonstrate the formation of two metal-oxide interfaces as Pt/SiO2 and Au/SiO2, by embedding the CuPtAu cage nanoparticles in a mesoporous silica (mSiO2).


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