58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

In the first year of PRF support we have successfully attained several synthetic targets, which have already been published. First, a generalized method for the preparation of highly defined noble metal nanoparticle catalysts via microwave-assisted heating. The preparation of small (2-10 nm) nanoparticles of pure Rh, Pd, Pt and Ag were all possible using this new technique, which involves controlled injection of metal halide precursors into organic solvents under microwave irradiation. Importantly, when directly compared to the nanoparticle products obtained using conventional (convective) heating under otherwise identical reaction conditions, the microwave-synthesized particles are more highly crystalline, and also have defined surface structures. As an example, it is possible to reproducibly prepare 10 nm Rh cubes using the method. The resulting heterogeneous catalytic properties of the nanoparticles have been extensively studied, specifically in the hydrogenation of alkenes with hydrogen gas at low temperature (20-50 degrees C). In comparison to conventionally-prepared nanoparticles, the microwave-obtained catalysts are also more highly active by a magnitude of 3-4 times. This is a direct result of the improved surface structure of the nanoparticles.

As a direct extension of this work, we have also been able to extend the microwave method to the synthesis of unusual hybrid core-shell nanoparticles, in which an Ag or Au core is surrounded with a thin (2-8 monolayer) Rh metal shell. This is a convenient means to limit the use of the most expensive and rarest metals in the preparation of nanoparticles: since only the surface is available to promote catalysis, catalytically inactive metals such as Ag may be employed as ‘sacrificial’ cores, above which the active metal is supported. In addition, the growth of very thin monolayers of Rh atop inert cores (2-3 monloayers) was shown to result in enhanced catalytic activity, due to so-called strain effects at the surface, imposed due to slight lattice mismatching between the core and shell constituents.

The aforementioned work laid the platform for a very exciting study that is presently under way, and for which preliminary results have recently been submitted for publication. We have been able to combine what has been learned from the synthesis of noble metal nanoparticles using microwave irradiation and apply it to the preparation of previously unknown alloyed phases. These include most notably, Rh_xM_y (M = Ag, Au) nanoparticles, in which x and y are broadly tunable. Such alloys have classically been considered to be immiscible, leading to phase segregation in the bulk. However, a combination of microwave-assisted synthesis and confinement effects unique to the nanoscale are allowing us to prepare truly stable alloys. Preliminary catalytic studies in a range of heterogeneous reactions (hydrogenation, NO_x reduction) are highly encouraging and show that it is possible to dilute expensive and rare metals such as Rh with less expensive and more available metals (e.g., Ag), whilst improving the overall activity of the former due to preferential electronic tuning in the alloy. Work is continuing on this project, and the results have been patent protected in conjunction with the University of Texas Office of Technology and Commercialization.

In a concurrent study, which forms an integral part of our ultimate target to generate noble metal nanoparticles inside micro- and mesoporous materials, we have investigated the synthesis of previously unknown transition metal oxide materials using easier and more reproducible methods. Specifically, we have determined a means to prepare oxides of Mn, Co and Ni using a one-step templating route that cuts out several intermediate steps presently required in so-called ‘nanocasting’ methods. We have shown in published work that porous Co₃O₄ with an ordered linear channel structure may be prepared in a one-pot approach using a surfactant template. The key to this method is that the entire reaction is performed under conditions of high pH, which is converse to most classically reported methods. The resulting materials have appreciable surface areas (100-300 m²/g) and are thermally stable. We have thus prepared catalyst materials by impregnation of these porous metal oxides with noble metal nanoparticles prepared using the microwave method, to prepare composite heterogeneous catalysts. Initial reactivity tests show that the chemically reactive nature of the transition metal oxide supports can be used to enhance the overall chemical reactivity of the nanoparticles through metal-support interactions. In the case of selective hydrogenation of α,β-unsaturated aldehydes, the use of porous Co₃O₄ greatly enhances the proportion of desirable unsaturated alcohol products, versus the use of innocent, inert support materials such as silica.

In the coming grant year, we plan to advance our understanding of the synthesis and catalytic properties of alloy noble metal nanoparticles, with additional collaborations with theoretical colleagues at U.T. Austin. In addition, we are embarking on a project to directly synthesize such nanostructures inside the pores of microporous ‘MOF’ type scaffolds, again using the now well-established microwave-assisted nucleation and growth method.