Session Q21: Focus Session: Fundamental Issues in Catalysis II

Tailoring Surface Reactivity of Metal Oxides

Titanium oxide is receiving continued attention because of its importance as catalyst support, as a material to harvest solar energy for chemical transformations, and as a model metal oxide. In this talk, I will focus on the structure and defects (extrinsic and intrinsic) of less-studied TiO$_{2}$ surfaces; i.e., rutile (011)-2x1 and anatase (101), and their influence on surface reactivity.   

Modulating the reactivity of Pt-based catalysts for PEMFC: A First Principles Study

Low-temperature Polymer electrolyte membrane fuel cells (PEMFCs) have been regarded as one of the most promising candidates to produce heat and electricity, especially for electric vehicles or residential co-generation systems. However, the CO poison at the anode and the slow kinetics of the ORR at the cathode for Pt based-catalysts limit its widespread application, which motivated extensive research for more effective catalysts with CO tolerant, highly active and lower Pt loading, and/or highly selective for CO PROX. Density functional theory calculations have been used to screen Pt-based catalysts for PEMFC. It is found that the direct contact with Pt catalysts (so-called Pt-skin) is essential. The reactivity of Pt-skin catalysts towards the oxygen reduction reaction (ORR) and the hydrogen evolution reaction (HER) can be modulated by stepwise increase of Ni contents, which are accomplished by the modification of the reactivity through ligand and geometrical effects. The overall reactivity is however balanced by effective adsorption and desorption of adsorbates. Our calculations show that among various Pt$_{x}$Ni$_{y}$ with Pt-skin, Pt$_{3}$Ni is the catalyst with the highest overall reactivity. The present work indicates that it may be a good candidate for CO preferential oxidation (PROX) in excess of the hydrogen.   

Surface Site Characterization of CO$_{ads}$ on Platinum

Nuclear magnetic resonance (NMR) spectroscopy is used in conjunction with cyclic voltammetry (CV) to explore the surface chemistry of CO on platinum electrocatalysts. Electrochemically prepared CO$_{ads}$ (from different sources and electrode potentials) are studied on platinum at various coverages in sulfuric acid electrolyte. A model is presented to parse the total oxidation current into its separate contributions and these are correlated with the type of surface site occupied by the adsorbate. Accounting of the CV oxidation currents suggest that the species left on the surface after partial oxidation of a saturated CO$_{ads}$ layer is a mixture of linear- and bridged-CO. $^{13}$C-NMR of the surface species resulting from electrochemically adsorbing labeled methanol provides direct insight into the surface electronic structure of the catalysts. We observe a shift in the $^{13}$C-NMR spectra associated with different surface preparations. These shifts correlate with the corresponding coverage of the adsorbate on different types of platinum sites. NMR is used to probe the dynamics of these species to elucidate the interaction of the adsorbate with the platinum surface.   

Does Pauli repulsion induce the dissociation energy barriers? A first principles study

We elucidate the origin of the formation analyzing the dissociation process of oxygen molecule on bridge-top-bridge site of Pt(111). The charge state is analyzed by the Bader method together with the spin states of the two oxygen atoms. The charge transfers to the dissociated oxygen molecule from the Au surface. The potential energy variation is in agreement with the energy variation of the separated in distance, charged, and spin polarized oxygen molecules that is calculated with real- space density functional method. Excluding the exchange term in the total energy calculation of the H$_2$/Au system leads to a monotonic increase of the potential energy surface in the dissociation process. The energy barriers in the H$_2$/Mg, H$_2$/Pt, and H$_2$/Au systems are in agreement with the energy variations of the charged, isolated, and separated hydrogen molecules. The barriers appear in late dissociations although no barrier for the nondissociated adsorptions. Their electronegativity differences determine the directions of the transfer in the cases investigated. So we have to reconsider the applicablity of the Pauli repulsion to the barrier formations.   

O$_{2}$ Dissociative Adsorption on Cu$_{2}$O(100) with O Vacancies

Cu$_{2}$O surfaces and nanoparticles have been shown to have high activity for CO oxidation [1]. As a result of consumption of the surface oxygen during the CO oxidation process on Cu$_{2}$O(100), the issue of restoration of the surface composition becomes critical. Through first principles electronic structure calculations of the geometry, activation energy barriers, reaction pathways, and the local densities of electronic states for O$_{2}$ dissociative adsorption on the Cu$_{2}$O(100) surface with O vacancies, we show that the healing of oxygen vacancies is accompanied by reconstruction of the surface. Our calculations are based on density functional theory in the generalized gradient approximation and usage of ultrasoft pseudopotential method in the plane wave representation. [1] B. White, M. Yin, A. Hall, D. Le, S. Stolbov, T. S. Rahman, N. Turro, and S. O'Brien, \textit{Nano Lett.,} \textbf{6}, 2095 (2006).   

Modeling the effects of the oxide substrate on O$_{2}$ dissociative adsorption on Au nanostructures

In this work we apply the density functional theory calculations to explore the mechanism of high reactivity of Au nanoparticles on oxide substrates. We test the idea that the substrate -- nanoparticle interaction makes the O$_{2}$ dissociative adsorption favorable on this system, in contrast to bulk Au, and then the O atoms, so adsorbed, are consumed by reactants for further oxidation. We exploit the observation that the 2-layer Au film on TiO$_{x}$ displays an exceptionally high reactivity as compared to a monolayer Au film, as well as those with 3 or more layers [1]. We calculate the energy $E_{da}$ of dissociative adsorption of O$_{2}$ on the surfaces 1, 2, 3, and 5 Au(111) layer structures in two environments: 1) free standing layers, 2) on TiO fragments (modeling a substrate). We find $E_{da}$ to be negative for the 2- and 3-layer Au films on the ``substrate'' while it is positive for all other systems under consideration. This result along with the experimental finding [1] point to the O$_{2}$ dissociative adsorption as being the main mechanism for the observed reactivity of Au nanostructures. Calculated local densities of electronic states and local charges in the system will be presented for further insights into the nature of the effect. [1] M. S. Chen, D. W. Goodman, Science \textbf{306}, 234 (2004).   

A Density Functional Theory study of Cobalt nanoparticle catalyst for Fischer-Tropsch Synthesis

In the Fischer-Tropsch synthesis Cobalt nanoparticles are widely used as catalysts in which the reaction of Carbon Monoxide and Hydrogen form hydrocarbons. Particle sizes in the range of 6-8 nm have shown to exhibit maximum catalytic activity which is attributed to their surface area and their ability to stabilize steps. Using ab-initio electronic structure calculations based on the density functional theory we study the energetics of adsorption and dissociation of Carbon Monoxide on various particle morphologies and coverages including flat and stepped surfaces and particles with a separation of a few angstroms. The local density of states will be calculated for the various configurations. This study will provide an in-depth understanding of the energetics of adsorption and dissociation of Carbon Monoxide on Cobalt particles and for the various coverages and the particle configurations that lower the dissociation barrier as well as the preferred adsorption sites of the atoms that give the lowest energy for the various particle geometries.