Session P36: Focus Session: Environment III: Nanoparticles, Surfaces and Catalysis

Nanominerals, Mineral Nanoparticles, and Earth Processes: Details on How Nanoparticles Work in the Environment

Naturally occurring inorganic nanoparticles have been one of the principal catalytic components of Earth throughout its history. Yet these ubiquitous materials have largely escaped our close scrutiny until very recently. They are illusive and difficult to study. They have properties that change significantly with their exact size, shape, aggregation state, and surrounding environment. In the past, it has not even been clear how they accumulate, disperse, and move around the planet, nor even for sure what their major sources and sinks are. We have now compiled and derived a global budget for naturally occurring inorganic nanoparticles, including an assessment of their sources and sinks, as well as their fluxes between various Earth compartments (atmosphere, continents, continental shelves, and open oceans). In addition, these kinds of budgets provide a basis for fundamental understanding such as residence and transfer times between compartments. Specific findings include the following: 1) The primary producer of Earth's inorganic nanoparticles is soil through terrestrial weathering processes; 2) rivers, and to a lesser extent glaciers, bring 0.1\% to 0.01\% of the Earth's continental nanomaterial reservoir to the continental edge/ocean margins each year; 3) only about 1.5\% of this material makes it to the deep oceans due to aggregation and settling in saline ocean margins; and 4) the airborne and waterborne inputs of nanominerals and mineral nanoparticles to the open oceans are very similar. These kinds of results, along with a much better understanding of the characteristics of naturally occurring inorganic nanoparticles via direct observation in the field coupled with laboratory studies, provide a useful foundation upon which to predict the behavior and fate of manufactured nanoparticles, many of which are very similar to naturally occurring varieties. Even when specific correlations between naturally occurring and manufactured nanoparticles cannot be made, important clues in manufactured nanoparticle behavior in complex environments can be obtain by observing natural systems.   

Surface Reactivity of Core Shell Iron-Iron Oxide Nanoclusters towards Breakdown of Carbon Tetrachloride

Zero-valent iron (ZVI) is one of the technologies for groundwater remediation to reduce contaminants by removal of mobile chlorinated hydrocarbons. Iron-Iron oxide (Fe/Fe$_{3}$O$_{4})$ nanoclusters (NCs) made in our laboratory using cluster deposition technique have enhanced reactivity towards targeted contaminants due to the presence of ZVI protected by a passivated oxide shell. Here, we investigate the effectiveness of the Fe/Fe$_{3}$O$_{4 }$NCs in reducing carbon tetrachloride (CT) under laboratory conditions. The reactivity of the NCs was investigated by conducting unbuffered aqueous batch experiments to reduce CT at room temperature. Initial results show that 80{\%} of the degradation of CT resulted in the formation of dichloromethane (DCM) and chloroform (CF); the remainder likely followed a competing pathway to yield nonhazardous products such as CO. The production of undesirable hydrogenated products such as DCM and CF suggests that the dominant reaction pathway occurs through hydrogen (H) atom transfer via H atoms generated by corrosion of the iron. Comparative experiments with ZVI NCs prepared by other methods are underway and the results will be reported. Future work is to analyze and understand factors that control the reaction pathways between desirable and undesirable products.   

Environmental Catalysis at the Boundary between Metals and Metal Oxides

Growing theoretical and computational evidence points to the participation of partially to completely oxidized catalyst surfaces during catalytic oxidations under realistic conditions. These catalytic oxidations are of both fundamental scientific interest and of practical importance in a variety of contexts, including in particular environmental NOx remediation, yet fundamental understanding of the coupling between surface structure and composition, reactive environment, particle size, and catalytic reactivity is still in its infancy. In this work we use density functional theory (DFT) models to consider the coupling between catalytic oxidation activity and the transformation of metal to oxide surface, taking as our model CO and NO oxidation on oxygen-covered to oxidized Pt surfaces. We describe DFT-parameterized cluster expansions (CEs) of O on Pt that capture the transition from metal to oxide, spectroscopic signatures of these transformations, and the incorporation of the metal-to-oxide transition into kinetic models of surface reactivity.   

Effect of substrate-catalyst interaction on Spin-dependent chemical reactions: CO oxidation

First-principles calculations have been performed to investigate and compare the catalytic reactivity of Ni(Pd)$_{1}$/TiO$_{2}$(110) and Ni(Pd)$_{2}$/TiO$_{2}$, for CO oxidation. A recent experiment showed that the catalysis of small Ni(Pd)$_{2}$ clusters deposited on rutile TiO$_{2}$(110) surface exhibit very different performance for CO oxidation, compared with the single atom cases, Ni(Pd)/TiO$_{2}$(110). However, the underlying mechanism of this interesting phenomenon is still unclear. Our calculations show that the catalyst-substrate interaction plays a key role in both the thermodynamic and kinetic process of the catalytic reactions. Particularly, the spin degree of freedom of the complex oxide is found to dominate the reaction rate. Essentially, the oxidation of CO on the single atom cases is a spin-forbidden reaction, while it is spin-permitting for the dimer cases. This work provides valuable guidance for high efficient catalyst design at the atomic scale.   

Assessing Actinide-Oxygen Covalency by K-edge X-ray Absorption Spectroscopy.

The development of many essential nuclear technologies requires a comprehensive grasp of the electronic ground and valence states of molecular actinide bonding interactions. Identifying itinerant or delocalized electrons -- in molecular nomenclature, ionic or covalent bonds -- is a longstanding problem in actinide science. Recent advances have shown that the transition intensities measured by ligand K-edge X-ray absorption spectroscopy (XAS) directly relate to coefficients of covalent orbital mixing. Ligand K-edge XAS has been employed successfully to describe the valence states of materials containing predominantly ionic metal--Cl and metal--S bonds, however, it remains experimentally challenging to obtain quantitative intensity information at the K-edge for light atoms such as C, O, N, and F. Insights regarding the nature and extent of orbital mixing in actinide--O bonds are now within reach through a combination of XAS with a scanning transmission X-ray microscope (STXM) and hybrid density functional theory calculations (DFT). A new effort to employ these techniques for non-conducting molecular systems containing interactions between actinide and oxygen-based ligands will be discussed. Oxygen K-edge XAS measurements and DFT for a series of six structurally related transition metal oxides suggest that metal nd and O 2p orbital mixing increases with increasing Z. The actinyl ions were chosen for the first O K-edge XAS measurements with actinides because they represent the most important high-valent actinide species in the environment. Features in the polarized XAS of the actinyls follow anticipated trends based on the 5f and 6d orbital energies and occupancies. Results from an ongoing collaboration with theorists ties these experimental trends in actinide--O orbital mixing to changes in 3d, 4d, 5d, and 6d/5f valence orbital occupancies.   

Submonolayers of Au/Pd on the hematite (0001) and magnetite (111) surfaces

Ultra-thin films and nanostructures formed by noble metals on oxide surfaces exhibit enhanced catalytic activity for CO oxidation. We used the spin-polarized density functional theory (DFT) and the DFT+U method, accounting for the strong on-site Coulomb correlations, to study the submonolayer adsorption of Au/Pd atoms on two stable iron-oxide surfaces: hematite (0001) and a magnetite (111). For each surface, adsorption on two terminations has been studied: one terminated with iron and the other with oxygen. Both Au and Pd bind strongly to hematite and magnetite surfaces and induce large changes in their geometry. DFT and DFT+U provide qualitatively similar surface geometries but they differ much in the prediction of the surface energetics and the electronic and magnetic properties of the oxides. Pd binds stronger than Au both to hematite and magnetite surfaces and the Au/Pd bonding to the O-terminated surface is distinctly stronger than that to the Fe-terminated one. For hematite, the DFT+U bonding is by 0.3-0.6 eV weaker than DFT on the Fe-terminated surface and about 2 eV stronger on the O-terminated one. For magnetite, in each case, DFT+U gives stronger bonding than DFT. The differences between DFT and DFT+U results are discussed based on the calculated electronic structure.