Pranaw Kunal

Rapid Synthesis of Rhodium–Palladium Alloy Nanocatalysts

The chemistry of metastable RhPd alloys is not well understood, and well‐characterized nanoparticle (NP) examples remain rare. Well‐defined and near‐monodisperse RhPd NPs were prepared in a simple one‐pot approach by using microwave‐assisted or conventional heating in reaction times as short as 30 s. The catalytic hydrogenation activity of supported RhPd NP catalysts revealed that short synthesis times resulted in the most‐active and most‐stable hydrogenation catalysts, whereas longer synthesis times promoted partial Rh‐Pd core–shell segregation. Relative to Rh NPs, RhPd NPs resisted deactivation over longer reaction times. Density functional theory (DFT) was employed to estimate the binding energies of H and alkenes on (1 1 1) Rh, Pd, and Rh0.5Pd0.5 surfaces. The DFT results concurred with experiment and concluded that the alkene hydrogenation activity trend was of the order Pd

Computationally Assisted STEM and EXAFS Characterization of Tunable Rh/Au and Rh/Ag Bimetallic Nanoparticle Catalysts

The acceleration of rational catalyst design by computational simulations is only practical if the theoretical structures identified can be synthesized and experimentally verified. Of particular interest are bi-functional/bimetallic catalysts, which can have the potential to exceed the selectivity and efficiency of a single-component system [1]. However, adding a second metal greatly increases the complexity of the system; variation in the elements’ mixing patterns and reconfiguration can affect the reaction mechanisms and thus catalytic performance [2].

Organoarsine Metal–Organic Framework with cis-Diarsine Pockets for the Installation of Uniquely Confined Metal Complexes

ACM-1 is the first example of an organoarsine metal-organic framework (MOF), prepared using a new pyridyl-functionalized triarylarsine ligand coordinated to Ni(II) nodes. ACM-1 has micropores that are decorated with cis-diarsine coordination pockets. Postsynthetic metalation of ACM-1 with AuCl under facile conditions studied by single-crystal X-ray diffraction reveals the installation of dimeric Au2Cl2 complexes via the formation of As-Au bonds. The Au(I) dimers display exceptionally short aurophilic bonds (2.76 Å) induced by the rigidity of the MOF, which acts as a unique solid-state ligand.

Optothermophoretic Manipulation of Colloidal Particles in Nonionic Liquids

The response of colloidal particles to a light-controlled external temperature field can be harnessed for opto-thermophoretic manipulation of the particles. The thermoelectric effect is regarded as the driving force for thermophoretic trapping of particles at the light-irradiated hot region, which is thus limited to ionic liquids. Herein, we achieve opto-thermophoretic manipulation of colloidal particles in various non-ionic liquids, including water, ethanol, isopropyl alcohol and 1-butanol, and establish the physical mechanism of the manipulation at the molecular level. We reveal that the non-ionic driving force originates from a layered structure of solvent molecules at the particle-solvent interface, which is supported by molecular dynamics simulations. Furthermore, the effects of hydrophilicity, solvent type, and ionic strength on the layered interfacial structures and thus the trapping stability of particles are investigated, providing molecular-level insight into thermophoresis and guidance on interfacial engineering for optothermal manipulation.

Thermal Stability Study of Classically Immiscible Rh-Ag Alloy Nanoparticles by in situ TEM

Surface structure, composition and segregation properties of bimetallic nanoalloys are crucial in determining chemical reactivity and activity [1,2]. RhAg nanoalloy is of interest in catalysis since Rh, although expensive and scarce, is used in wide range of catalytic process. Alloying Rh with Ag seems reasonable due to Ag’s abundance in nature and low cost. Unfortunately, there are no stable phase for RhAg below 2177 or 2139 K and 1 atm [3] resulting to immiscibility and it’s chemical properties not well-understood. Here, stable RhAg nanoalloys were prepared using a novel microwave-assisted technique. To determine the viability of the RhAg nanoalloys for catalysis, the RhAg nanoalloy’s thermal stability was investigated using electron microscopy to understand the driving forces for the nanoalloy’s instabilities. Rh-Ag mixed nanoalloys with diameter of 8 nm were synthesized. These nanoparticles were dispersed in hexane and drop-casted on an Aduro MEMS heating device. Using a Protochips Aduro heating holder inserted in a FEI Titan transmission electron microscope with a ChemiSTEM system, thermal annealing was performed with energy-dispersive X-ray spectra (EDS) acquired after each annealing step. The nanoparticles were annealed from 50C until agglomeration of the nanoparticles was observed by Z contrast imaging. Subsequently, the elemental and quantitative distributions of Rh and Ag within the nanoparticle were determined from the acquired EDS maps. Figure 1 shows the powder X-ray diffraction (PXRD) of the RhAg alloy with 1:1 molar ratio heated from 25°C to 350°C indicating a phase separation staring at 300°C[4]. With in situ TEM technique using similar conditions used from the PXRD, the phase segregation of Ag (green on the EDS maps of Figure 2) was identified to occur at the nanoalloy’s surface at 350°C. The bulk phase diagram of RhAg indicates immiscibility for all compositions below 1400 K however due to size-effects at the nanoscale a metastable mixture can form. In this case, the Ag segregation was driven by the difference in the surface energies of Rh and Ag at 350C where Ag diffusion was induced. Furthermore, catalysis experiments were conducted resulting to increased catalytic activity of Rh-rich RhAg nanoalloys[4]. Additional in situ TEM studies using Rh-rich RhAg nanoalloys to higher temperatures similar to operating temperatures in catalytic converters (~800C) are underway to further investigate the thermal and structural stabilities of the RhAg nanoalloys as viable catalysts for such applications [5].