www.acsprf.org

This research project—a new direction for the PI into the areas of nanomaterials and hydrogen storage—seeks to elucidate the fundamental dynamical interactions between hydrogen and carbon-based nanomaterials, which have not been previously well understood.  Certainly, traditional chemical intuition, in terms of local covalent bonding models and the like, is found to be lacking in this context, vis-à-vis providing a quantitative description of the electronic structure. Moreover, classical trajectory simulations (CTS), commonly used to model such systems, fail to capture quantum dynamical effects associated with the nuclear motion, which are sure to be important, given that hydrogen is the lightest and “most quantum” atom.  Perhaps even more important though, a new paradigm is needed for describing and understanding the essential physics of such systems, which requires new ways of framing the problem.  This research project addresses all of the above limitations,  and continues to shed important insights and mechanistic understanding that could well be of substantial benefit to material scientists and engineers working on hydrogen storage and related applications. 

Specifically, we have performed accurate first-principles theoretical investigation of hydrogen atom adsorbates, bound exohedrally to a single-walled carbon nanotubes (SWNT).  The adsorbates are presumed to have been loaded via a catalytic “spillover” mechanism, with the subsequent migration of bound adsorbates down the SWNT backbone serving as the focus of this research project.  The adsorbate migration step is the bottleneck for the entire loading process, and therefore it is important to understand the mechanism accurately and comprehensively.  Prior to the work of this project, the  migration mechanism was very poorly understand, both quantitatively and qualitatively. Our efforts have led to an explanation for several “mysterious” aspects observed in experimental adsorbate migration, which were previously not understood.

 In the second grant year, the emphasis was on obtaining a better understanding of the multiple adsorbate electronic structure and dynamics. In particular, we focused on the dominant full coverage migration pathway occurring via hole defects, which only allow a single adsorbate to migrate at a time.  Also, in order to better understand which features of our proposed mechanism are “universal,” vs. those which are unique to the (5,5) SWNT, we performed multi-adsorbate electronic structure calculations for a variety of SWNTs, ranging over different chiralities and radii.  We discovered that the pronounced multiple binding enhancement effect, discovered in the first grant year, persists across all armchair SWNTs, with binding energy magnitudes decreasing with SWNT radius, but relative multiple binding enhancement increasing. For a more detailed discussion of the terminology used here, and of the original goals of this research project and corresponding findings, please consult the 2010 Annual Report, and the original proposal.

This work has led, in the second grant year, to a new lengthy publication, i.e. J. L. McAfee and B. Poirier, J. Chem. Phys. 134, 074308 (2011), directly addressing the multiple adsorbate case.  Like its single adsorbate predecessor, i.e. J. L. McAfee and B. Poirier, J. Chem. Phys. 130, 064701, both articles have been selected for joint publication in the Virtual Journal of Nanoscale Science and Technology [in 19 (2009) and  23 (2011), respectively].  The work above was primarily performed by Jason McAfee, a recently graduated Ph.D. student, who gave oral presentations at two regional meetings, one of which led directly to a postdoctoral researcher position at U North Texas.  I am pleased to report that Jason McAfee is now a Professor at Howard College in Big Spring, TX.  Jason was assisted by undergraduate Karl Gillenwater, who persisted in the group after Jason had departed.

The PI himself continues to give many talks and seminars on this work, more by far than for any other funded area of his research program.   In particular, six seminars were given since the start of the second grant year period, three of which were at international conferences (France, Germany, Israel) and two of which were at non-TTU universities (Marquette and New Mexico State).  Also, two additional publications currently in preparation, will be submitted within the next couple of months. Finally, as anticipated in last year’s report, this work has indeed lead to several important collaborations.  In particular, the PI is working with Juergen Eckert of Los Alamos National Labs and the University of Southern Florida, to coorganize the 2012 XVIth International Workshop
on
Quantum Atomic and Molecular Tunneling in Solids and other Condensed
 Phases, to be held in Santa Fe next summer.   The two of us, working together with Ivana Matanovic, also plan to submit a larger scale proposal to the DoE, building directly on the work achieved in the scope of this research project.

The latter development—though completely in keeping with the goals of the ACS PRF ND program—nevertheless will require a certain rethinking of goals. Simply put, the future of hydrogen fuel research in the U.S. has become very uncertain since the time that this ACS PRF ND proposal was originally submitted, to the extent that any proposal submitted with a pure SWNT-hydrogen storage focus will not likely be competitive.   Our DoE grant will therefore focus on pincer ligand and related organometallic catalysis—of broad interest, not only for hydrogen storage, but for many other application areas as well.  Accordingly, ACS PRF ND support (for student Corey Petty and postdoc Benhui Yang) is now being used to retool the technologies developed here for broader use.