57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

The principal research objective of this project is to elucidate the role of the high-surface area oxide catalyst support in modulating the selectivity and activity rhodium (Rh) catalysts in syngas (CO + H₂) conversion to ethanol. This is a variant of the conventional Fischer-Tropsch process, generating long-chain liquid hydrocarbon oxygenates from gasified biomass / coal. Since thermodynamics favors the formation of methane and water, generation of alternative (more useful) products (like ethanol) must necessarily exploit subtle modifications of reaction kinetics enabled by catalyst modification. Experimentally, it has been found that changes in the oxide support from silica (SiO₂) to titania (TiO₂) yields significant increases in the production of C₂₊ oxygenates. Thus in this reaction, as in many others, the support modulates activity either directly (by catalyzing reactions on the oxide or at the nanoparticle/oxide interface) or indirectly (by modifying the electronic structure of the metal nanoparticle itself, or nanoparticle geometry.) The ultimate goal of our research is to understand the detailed role of the oxide support in this specific reaction, and also develop a more comprehensive understanding on support effects in general.

We are using state-of-the-art computational chemistry methods to explore the role of the oxide support in various elementary steps involved in syngas conversion. In particular, based on experimental STM and EXAFS data, we modeled the catalyst as a hemispherical Rh₃₇ cluster, either: in the gas phase (as a model of an unsupported nanoparticle); or supported on an explicit SiO₂ surface; or on TiO₂. We also compare with a Rh(111) single crystal surface. The direct comparison of these four related systems allows us to separate geometric effects (changes in nanoparticle geometry induced by the support), from indirect effects (modification of the nanoparticle electronic structure), from direct effects (for example, reaction at interfacial sites at the nanoparticle/support interface). A picture of the model system with an adsorbed CO is shown below. In all cases, we evaluate our computational model using plane wave density functional theory (PWDFT) calculations using the VASP software package.

Nearly all previous computational heterogeneous catalysis calculations neglect the support, treating the reaction as if it took place on a simple single-crystal metal surface. As such, these calculations are intrinsically unable to address support effects. The present calculations are, to the best of our knowledge, the first to attempt to address the role of the support in Rh-catalyzed syngas conversion. Nonetheless, the calculations are extremely computationally expensive. As such, we previously obtained a preliminary 50,000 CPU-hour allocation on the NSF Teragrid (a collection of supercomputers). Just last week, our request for an additional 860,000 CPU-hour allocation was approved, which should dramatically enhance our ability to efficiently conduct these calculations.

We have divided our study into two phases. In phase one (nearly complete), we examined the role of the support in modifying the thermodynamics of the syngas conversion process; in phase two (partially underway), we will complete our study by examining the modifications to reaction kinetics. In our study of syngas conversion thermodynamics, we first developed optimized models for the various supported and unsupported nanoparticles, as described above. We subsequently identified all kinetically relevant reactants, products and intermediates leading from CO/H₂ to alkane/oxygenate products, including their most stable binding sites and binding energies on the both the supported and unsupported nanoparticles. Here, our preliminary data shows that binding of nearly all species is very similar between the unsupported Rh₃₇ nanoparticle and the single-crystal Rh(111) surface. As such, geometric changes in the nanoparticle are unlikely to contribute to changes in the reaction thermodynamics. Nonetheless, we see significant binding energy differences between adsorbates on the unsupported vs. supported nanoparticles, and also significant differences between silica and titania supports. These changes persist even when the adsorbate is bound to the "top" of the nanoparticle, rather far from the nanoparticle/support interface. As such, it appears that the support is modulating the thermodynamics indirectly by modifying the electronic structure of the nanoparticle itself. We are currently using a variety of analysis approaches, including projected densities of states, Bader analysis, and Natural Bond Order analysis to understand the mechanism through which these support effects are mediated. Corresponding studies of kinetics are also in preliminary stages. We have identified several transition states for kinetically-relevant reaction steps, and we will contrast the activation energies for the various supported and unsupported nanoparticles.

The bulk of this research has been conducted by Glen Jenness, a postdoctoral associate in my group who is funded via this grant. Glen came with extensive (but rather narrow) experience in computational chemistry but no specific experience in PWDFT or materials science / catalysis experience. The complementary skill set acquired during his time in my group will be a big asset for Glen and will contribute to his marketability when he applies for positions at a national lab (his ultimate career goal). I have also worked extensively with Glen to help him continue to grow a scientist. In particular, I have spent a great deal of time help Glen to think deeply in terms of our computational analysis and allowing him to play an integral role in guiding the evolution of our work. Both of these are essential skills for an independent scientist.

Although not directly funded by this grant, Glen works in close coordinate with 3^rd year graduate student Benjamin Dunnington. Benjamin is working to develop new PWDFT analysis approaches that allow us to probe the bonding and reactivity of solid-state catalytic systems in novel manners. Benjamin has developed an extension of the Natural Bond Orbital (NBO) analysis technique to PWDFT calculations on catalytically-relevant materials. NBO allows us to interpret bonding in complex bulk or interfacial catalyst systems in simple general chemistry "Lewis-like" terms. We recently applied these ideas to several simple catalyst model systems; Benjamin and Glen are coordinating to apply these ideas to obtain a more "chemical" understanding of the catalysis that is taking place over our supported Rh nanoparticles.