Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session S21: Focus Session: Fundamental Issues in Catalysis III |
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Sponsoring Units: DCP Chair: Steve Overbury, Oak Ridge National Laboratory Room: Morial Convention Center 213 |
Wednesday, March 12, 2008 2:30PM - 3:06PM |
S21.00001: Transient FTIR spectroscopy for probing reaction pathways on Au catalysts Invited Speaker: Au is now well known to be an active catalyst if the Au particles are sufficiently small, less than about 5 nm. The causes for this structure sensitivity are now beginning to be better understood. Computational modeling and measurements of size dependence on a single catalyst are consistent with activity at sites with low coordination numbers, due in part to flexibility of adsorbate geometry in these sites. Although small size and low coordinate sites are important in catalyzing, e.g. the CO oxidation reaction, there appear to be other factors which control the observed activity as demonstrated by catalyst deactivation and unusual temperature dependence. We have performed studies of CO oxidation over Au/TiO$_{2}$, Au/SiO$_{2}$, Au/ZnO/TiO$_{2}$ and Au/FePO$_{4}$ catalysts to explore reaction pathways and the causes for activation and deactivation. Three different reactor systems, a fast gas transient FTIR spectrometer, a slower transient DRIFTS cell and a steady state plug flow reactor have been used to correlate activity with surface species. Using this \textit{operando} approach the elementary steps in the CO oxidation reaction have been explored. Striking differences between the supports are found. The effect of various pre-treatments, the evolution of the surface species during ``steady state'' reaction and the role of carbonate, oxygen storage, water, hydroxyl upon catalyst activation and deactivation have been explored and will be described. Reaction pathways and mechanisms will be proposed and compared for the different catalysts. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:42PM |
S21.00002: TBD Invited Speaker: |
Wednesday, March 12, 2008 3:42PM - 4:18PM |
S21.00003: Molecular Factors Determining Selectivity in Catalysis. Invited Speaker: Achieving high selectivities is arguably the main challenge in heterogeneous catalysis for the 21st century. In complex reaction with competing parallel pathways, small changes in the relative values of the different activation energies are sufficient to switch the selectivity of those processes from one product to another. We in our laboratory have been carrying out mechanistic studies on model metal surfaces to try to identify the key factors that control such selectivity. In this talk we will present several examples of increasing subtlety from that work, with focus on the conversion of hydrocarbons. Specifically, we will discuss issues of regioselectivity and stereoselectivity in early dehydrogenation steps, and how those affect selectivity in the conversion of olefins. Time permitting, we will also discuss issues related to the bestowing of enantioselectivity on solid surfaces. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S21.00004: The nucleation and growth of ordered Fe and FeO nanoparticles on reconstructed Au(111) surfaces Neetha Khan, Christopher Matranga Iron-based catalysts, including iron oxides, are an important class of materials with relevance to Fischer-Tropsch catalysis and gas-sensing applications. By growing nanostructured particles on single-crystal surfaces, we can create a model system to study size and shape effects on reactivity. We have studied the formation of monolayer thick iron oxide nanoparticles and thin films on the reconstructed Au(111) surface. STM, XPS, ISS, and LEED were used to evaluate the structure and composition of the iron oxide nanoparticles and films as a function of growth conditions. Iron oxide was grown by depositing iron on Au(111), followed by oxidation at room temperature and annealing to 700 K. XPS results indicate that the Fe is oxidized at room temperature, but the STM results indicate that the particles are not ordered until after annealing to 700 K. Atomically-resolved STM images show that at 0.3 ML Fe coverage, iron oxide nanoparticles, pseudo-hexagonal in shape are formed, with large defects occurring in the corners. STM images of FeO particles over 0.5 ML Fe also show evidence of a non-coincidence overlayer lattice with a short periodicity of 0.25-0.3 nm modulated by a larger periodicity of approximately 3.5 nm. The larger periodicity results from a moir\'{e} pattern formed between the iron oxide overlayer and the underlying Au(111) surface. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S21.00005: Dynamic structure in Pt nanoclusters on $\gamma$-alumina F. Vila, J.J. Rehr, J. Kas, R.G. Nuzzo, A.I. Frenkel Pt nanoclusters on $\gamma$-alumina exhibit a number of unusual phenomena including large structural disorder and bond-length contraction with increasing temperature. We investigate this behavior for a prototypical 10-atom Pt cluster using real-time, temperature-dependent simulations combining density functional theory/molecular dynamics and x-ray spectroscopy theory. We find that the cluster structure is dynamically varying in shape and topology on a time scale long compared with internal vibrations. Moreover, the clusters are not rigidly attached to the surface and occasionally pick up or discard a Pt-O bond. This real-time approach suggests that these nanoclusters are comprised of two distinct populations of Pt atoms depending on the charge transfer from the surface, and reproduces many of their unusual properties. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S21.00006: In Situ Characterization of Ethylene Hydrogenation on Pt Powder Using Mass Spectrometry-Sum Frequency Generation Technique Bryan Hsu, Shawn Dougal, Paul Stevens, Mohsen Yeganeh Bridging the pressure gap has been of paramount importance to the field of surface science. Unfortunately, the available techniques used to characterize catalytic surfaces have all been limited in some degree by a combination of factors (e.g. low pressure regimes, ex situ analysis, and low surface area catalysts), which do not fully replicate industrially relevant conditions. Here, we present in situ observation of ethylene hydrogenation of Pt powder in a high pressure regime. Using total internal reflection sum frequency generation (TIR-SFG) we are able to identify ethyl, ethylidyne, di-sigma-bonded ethylene, and pi-bonded ethylene surface intermediates and find that these are all present under reactive conditions as monitored with mass spectrometry (MS). [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S21.00007: Dissociation of water and Acetic acid on PbS from first principles Alessandra Satta, Paolo Ruggerone, Giovanni De Giudici The adsorption of complex molecules at mineral surfaces are crucial ingredients for understanding the mechanisms that rule the interaction between minerals and the biosphere and for predicting both the stability and the reactivity of minerals. The present work focuses mainly on the early stages of different adsorption reactions occurring at both the cleavage surface and a high-index vicinal surface of galena (PbS). We have studied the dissociation mechanism of water and acetic acid on the galena surfaces by means of ab initio calculations within the framework of the density functional theory in the generalized gradient approximation and of pseudopotential approach. The calculated adsorption energies of the molecules indicate the stepped surface as the most reactive, as expected. The free energy surface during the reaction process has been explored via metadynamics[1]. The optimized configurations of both reactants and products obtained, were then used to accurately calculate the dissociation energy via the Nudge Elastic Band method[2]. [1] A. Laio and M. Parrinello, PNAS 99, 12562 (2002). [2] G. Mills and H. Jonsson, Phys. Rev. Lett. 72, 1124 (1994). [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S21.00008: Characterization and Reactivity of Mo$_{6}$S$_{8}^{+}$ on Au (111) via Size-Selected Deposition Melissa J. Patterson, James M. Lightstone, Michael G. White Supported MoS$_{2}$ nanoparticles are known for their ability to catalyze a wide range of heterogenous reactions such as hydrodesulfurization (HDS)$^{1}$. However, understanding the role of size, structure, composition and support interactions of the MoS$_{2}$ particles in these heterogenous reactions has not yet been resolved due to the inhomogeneity of commercial catalysts. Work done in our laboratory is geared towards preparing homogenous samples in ultra high vacuum that can serve as model systems for these types of catalytic reactions. We are currently investigating the reactivity of size-selected transition metal clusters generated in the gas-phase and deposited on a Au(111) surface. Using a magnetron cluster source, we are able to produce a wide range of nanocluster stoichiometries including the Mo$_{6}$S$_{8}^{+}$ cluster, which has been observed as the metal core of the well-known Chevrel phase$^{2}$. The work presented focuses on characterization of the Mo$_{6}$S$_{8}^{+}$ cluster deposited on a Au(111) single crystal using techniques such as Auger, photoemission spectroscopy, and thermal desorption. In addition, preliminary reactivity studies will be presented of the supported Mo$_{6}$S$_{8}^{+}$ cluster with small sulfur containing molecules. 1. Topsoe, H.; et.al; \textit{Hydrotreating Catalysis}; Springer: New York, 1996. 2. Umarji, A. M.; et.al.; \textit{J. Phys. Chem. Solids} \textbf{1980}, $41$, 421. [Preview Abstract] |
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