59th Annual Report on Research 2014

In the second grant year of ACS PRF support, we have capitalized on initial results obtained in 2012-13, which has resulted in the publication of four new reports, each cantered around the use of microwave heating to prepare novel types of metallic nanoparticles. We have continued to elucidate important relationships between the action of microwave-assisted heating and the resulting size and shape selectivity that is observed. In a systematic manner, this has been compared directly with nanoparticles obtained under otherwise identical reaction conditions, using conventional heating.

In the past year, we have shown that microwave-assisted heating plays a clear and beneficial role in the synthesis of unusual hetero-structured core-shell metallic nanoparticles that consist of two different noble metals. In most cases, the same well-ordered structures cannot be obtained under conventional (convectively-heated) conditions. These types of structure are prime targets in our research because they provide a means to reduce the amount of the most precious and/or least abundant metal that is required per nanoparticle, by limiting its use to a thin shell above a sacrificial core comprised of a cheaper, and more earth-abundant metal. This is useful because metals such as Rh are catalytically very valuable in catalysis, but only surface-exposed metal can be active. Metal nanoparticles already have a favorable surface area-to-volume ratio compared with bulk metal; our current strategy allows us to further reduce the amount of Rh required to prepare stable catalysts. Specifically, we have perfected a preparative method for the synthesis of Ag-Rh and Au-Rh core-shell nanoparticles with sizes ranging between 2-8 nm and near monodispersity. It has been shown that the size of Rh overlayer grown on top of pre-formed Ag or Au nanoparticle ‘seeds’ can be varied from only 2-2.5 monolayers up to 6-8 monolayers. In the latter case, the resulting surface chemistry is akin to what is observed for pure Rh nanoparticles (albeit requiring less total Rh). However, for very thin shells of Rh on either Ag or Au, so-called stress or strain effects are found to accentuate the catalytic behavior at the surface of the core-shell nanoparticles. This is because there is a lattice mismatch between Rh and the more noble metal, which causes the thin Rh overlayers to become strained. We have shown that this further activates the Rh surface in vapor-phase hydrogenation studies: very high turnover numbers are observed for the first 2-4 hours on-stream, sometimes as much as 10x higher than is observed for pure Rh nanoparticles. Studies to elucidate the exact nature of the surface chemistry and possible reagent-induced restructuring mechanisms are ongoing at this time and will continue under alternative grant support.

Secondly, we have undertaken a new collaborative study with the Brutchey group at the University of Southern California, in which we have translated their methodology for the synthesis of nanoaprticles using ionic liquids into our microwave-assisted approach. We have published a recent paper that directly compares and contrasts the use of bulky organic ionic liquids (namely 1-butyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide) under convective and microwave-assisted heating. It was found that in general, size and shape control was better using microwave-assisted synthesis. One of the most interesting results from this study is the realization that a significantly different amount of the ionic liquid is incorporated into the resulting metallic nanoparticle, depending on the synthesis method. Surprisingly however, this did not have a large effect on the resulting hydrogenation activity of each type of nanoparticle catalyst (when anchored on amorphous silica substrate). This tends to indicate that the ionic liquid does not strongly cap to the nanoparticle surfaces and therefore does not strongly inhibit surface catalysis.

Thirdly, and in related directly related studies, ACS PRF grant support has been utilized to continue our studies into the synthesis of novel mesoporous materials that can subsequently be used for the in situ nucleation and growth of metallic nanoparticles, resulting in the formation of advanced composite catalyst materials. Our major target in the previous grant year was the identification of means to prepare well-ordered mesoporous cobalt oxide (Co₃O₄). This target was achieved and more recently we have utilized the new synthetic protocol to prepare a large quantity of mesoporous Co3O4 that has been extensively tested as an anode battery material. These recently published studies show that the meso-Co₃O₄ performs excellently under multiple (>200) cyclese of charge-discharge, without losing significant capacity. The tests made on this material relate to real-world applications and indicate that this material out-performs nanoparticles of Co₃O₄ under the same operating conditions.