Rapid Synthesis of Rhodium–Palladium Alloy Nanocatalysts

Noble metals are widely used as catalysts in a variety of key industrial processes. In particular, rhodium is used to catalyze the reduction of NOx to N2 and O2 in exhaust gas abatement and is also used in the conversion of bulk hydrocarbon feedstocks and syngas into more highly oxygenated hydrocarbons (e.g. , hydroformylation). 2] Palladium also plays a myriad of critical roles, including CO-to-CO2 oxidation in three-way catalytic converters and carbon–carbon bond-forming reactions. Rh and Pd are also both widely employed in hydrogenation catalysis. However, because of their scarcity in the Earth’s crust, these metals are expensive to procure. Whereas fundamental research into the possible replacement of noble-metal catalysts with cheaper, “Earth-abundant” transition-metal catalysts proceeds in earnest, large-scale industry continues to rely primarily on Pd, Rh, and other scarce noble-metal catalysts because their chemistry is reliable and well understood. Therefore, a major challenge is to increase the activity and selectivity of catalysts containing these metals, which will lead to greater product yields with lower energy consumption and the formation of fewer waste byproducts. Heterogeneous noble-metal catalysts can be made more efficient by designing their structures to suit the application at hand. Nanoparticle (NP) catalysts offer a significant improvement in atom efficiency over their bulk-metal counterparts because a far greater proportion of the metal atoms are at the surface of the material. Compared to bulk grinding, so-called “bottom-up” synthesis methods using molecular precursors allow for greater control over the size and shape of the resulting NPs, which impacts both the number of accessible surface sites and their activity. In addition to NP morphology effects, the composition of the material can also be tailored on the atomic level to better suit the desired reaction. Alloying two or more metals has been shown to allow for fine and continuous tunability of binding energies between the NP surface and substrates. This type of approach can even lead to a substantial enhancement in overall catalytic activity in cases in which one metal is unable to effect the catalysis on its own (e.g. , RhAu or RhAg). This study focuses on determining fast (and therefore potentially scalable) and convenient means to prepare well-defined RhPd alloy NPs. The catalytic activity of the NPs was assessed by using cyclohexene hydrogenation as a model reaction, and the results were explored by using a state-of-the-art DFT approach. In the bulk phase, RhPd alloys can only be made by quenching mixtures of the two metals from co-melts heated above 1000 8C. Even upon formation under such harsh conditions, the kinetically trapped alloys are metastable and undergo segregation into the component pure metal phases upon adsorption of reactive species, that is, upon their use as heterogeneous catalysts. However, previous work by Schaak and Kitagawa and their respective co-workers demonstrated that RhPd NPs could be made at much lower temperatures (95 8C). In particular, Kitagawa and co-workers demonstrated that the NPs could resist segregation upon adsorption and desorption of H2, presumably as a result of size-confinement effects at the nanoscale. Their study showed that RhPd alloys reversibly stored and released hydride, which suggested to us that these materials should also be good candidates for hydrogenation catalysis. Our continued interest in the application of microwave irradiation as a means to generate unusual bimetallic noble-metal NPs is based on the premise that metal-ion precursors and polar reducing polyol solvents both strongly couple with dipolar irradiation. This enables a unique heating profile that is well matched to NP nucleation and growth, in which it is possible to mimic high-temperature chemistry in nontoxic solvents through so-called “hotspot” generation, while avoiding the 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 (111) 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<RhPd<Rh. Rh-to-Pd charge-transfer in the RhPd alloys was found to play an important role in modulating the H binding energy.