Thermal and Photon Stimulated Reactions on Metal Particles

<p>The interaction between molecules and solid surfaces influence our daily lives in many ways. One important example is heterogeneous catalysis where surface reactions, for example, are used for production of chemicals and exhaust after-treatment. As surface reactions are complex, various different approaches are needed to explore the fundamental processes.</p> <p>One route is to study reactions at well-defined single crystal surfaces under ultra-high vacuum (UHV) conditions. However, as the reaction pathways may depend on pressure, ideally we would like to track how and if the interactions change with pressure. Furthermore, technical catalysts generally consist of nanometre-sized particles supported by an oxide material. This introduces the complexity of particle size-dependence, as well as effects arising from interactions between the reactants and the support material.</p> <p>The studies in this thesis range from fundamental studies in UHV to experiments under ambient conditions. Structure-wise, the studies encompass single crystal substrates and model catalysts. Furthermore, surface processes are stimulated by both thermal and photon energy.</p> <p>The fundamental aspects of thermally driven interactions of NO and water with sodium supported on a single crystal C(0001) surface were studied in UHV. Moreover, the influence of silver particle size (and to some extent shape) on the efficiency of photon driven desorption of NO from the Ag/C(0001) system was explored.</p> <p>Model catalysts were studied under ambient conditions by use of indirect nanoplasmonic sensing (INPS). In particular, INPS was used to determine the apparent activation energy for hydrogen oxidation as a function of platinum nanoparticle size, and to track the sintering of platinum clusters.</p

Summaries of FY 1997 Research in the Chemical Sciences

The objective of this program is to expand, through support of basic research, knowledge of various areas of chemistry, physics and chemical engineering with a goal of contributing to new or improved processes for developing and using domestic energy resources in an efficient and environmentally sound manner. Each team of the Division of Chemical Sciences, Fundamental Interactions and Molecular Processes, is divided into programs that cover the various disciplines. Disciplinary areas where research is supported include atomic, molecular, and optical physics; physical, inorganic, and organic chemistry; chemical energy, chemical physics; photochemistry; radiation chemistry; analytical chemistry; separations science; heavy element chemistry; chemical engineering sciences; and advanced battery research. However, traditional disciplinary boundaries should not be considered barriers, and multi-disciplinary efforts are encouraged. In addition, the program supports several major scientific user facilities. The following summaries describe the programs

Computational and Spectroscopic Investigations of Intermolecular Interactions in Clusters

In this thesis, the study of intermolecular interactions within cluster systems is presented. Covalently and non-covalently bound clusters possess oftentimes unique and unexpected properties which can be tuned by adjustment of size, composition, and geometry to target desired properties for use in nanotechnologies. Additionally, clusters present a computationally tractable model of bulk systems such as reactive sites on bulk heterogeneous catalysts. Infrared spectra have been collected of various clusters and theoretical computations have been conducted to interpret spectra and provide predictions for other properties to guide future works. Investigations of the forces binding cluster species together are conducted to provide insight into the fundamental underpinning of molecular properties with applications in the field of nanomaterial design. A variety of clusters have been studied here. Computational studies of size-dependency in nitrous oxide reactions with rhodium sulphide clusters have been conducted. Barriers to competing N2O desorption and decomposition have been ascertained and compared with and without thermal corrections. Inclusion of the sulphur atom is found to alter which reaction pathway is preferred, as seen by comparison with analogous studies on pure rhodium clusters. Infrared multiple photon dissociation (IRMPD) spectroscopy is utilized to probe the additional clusters; a series of palladium coordination complexes and a series of clusters containing icosahedral [B12X12]2─ (X = H, halogen) cages complexed with a cationic transition metal atom, a cationic amine, or a neutral polar cyclohexane-based compound. This IRMPD technique successfully produced infrared spectra for these species in the gas phase and unique properties were observed for each cluster upon IR induced dissociation. Density functional theory calculations determined geometries, dissociation thresholds, and interpreted IR spectra. Additional theoretical tools quantified molecular orbital interactions and topographical parameters of the electron density

Preparación de nuevos materiales fotocatalizadores para la descontaminación de gases NOx

El desarrollo del presente trabajo de investigación ha permitido alcanzar las siguientes conclusiones generales. a) En lo que respecta al diseño de nuevos materiales fotocatalizadores avanzados: - Se demuestra la capacidad de la fase hematita del óxido de hierro, -Fe2O3, como fotocatalizador eficiente para la degradación de gases NOx. - Se ha confirmado que el mecanismo fotocatalítico De-NOx conlleva el siguiente proceso de foto-oxidación: NO NO2 NO3 - No obstante, y a pesar de su tamaño nanométrico, la eficiencia fotocatalítica de la hematita resulta ser pobre. - La eficiencia fotocatalítica de la fase hematita se mejora al incrementar su superficie específica. En este sentido, estructuras unidimensionales constituidas por nanohilos nanocristalinos muestran una mejorada actividad De-NOX. - La fase α-Fe2O3 se puede preparar por la técnica CVD con un control efectivo de su nano-organización estructural. De igual modo se pueden preparar depósitos de α-Fe2O3/TiO2 nanoestructurados. La fase α-Fe2O3 actúa en sinergia con la fase anatasa del TiO2 obteniendo unas láminas delgadas muy eficientes en el proceso De-NOX. - Se demuestra que la actividad De-NOX de la fase TiO2 anatasa puede ser mejorada. La preparación de una estructura mesoporosa nanoparticulada conlleva a la obtención de un óxido TiO2 no sólo activo a la luz UV sino también en el Visible, además de una sobresaliente mejora en la selectividad del proceso fotoquímico. b) Por otra parte, en relación al estudio de nuevos materiales fotocatalíticos obtenidos a partir de residuos sólidos procedentes de otras industrias: - Se ha demostrado que se pueden realizar transformaciones adecuadas de los residuos industriales que les aporten un nuevo valor añadido y propiedades para la mejora de la calidad medioambiental. - Se ha optimizado el proceso de transformado de un residuo industrial en agente fotocatalizador. - Los residuos ricos en hierro son utilizados eficazmente como aditivos fotocatalizadores en materiales de construcción para aplicaciones de auto-limpieza y descontaminación de gases NOx. - En los materiales de construcción que además de incorporar los residuos transformados también incorporan TiO2, se observa una acción sinérgica de ambos aditivos fotocatalizadores que conduce a una clara mejora de la eficiencia fotocatalítica última del material. - Se ha demostrado que las cenizas de procedentes de la combustión de la cascarilla de arroz sirven de soporte útil de material fotocatalizador. - El proceso térmico de preparación de composites Fe2O3/SiO2 condiciona la morfología de la fase hematita soportada, así como su superficie específica. - Los composites Fe2O3/SiO2 presentan actividad De-NOX mejorada respecto a la observada para la muestra de hematite nanocristalina en polvo. Todas las conclusiones anteriores evidencian que se pueden preparar nuevos materiales con propiedades fotocatalíticas mejoradas en lo que respecta a la eliminación de gases NOX en atmósferas contaminadas, objetivo principal planteado al iniciar esta Tesis Doctoral

Structure, Stability, Vibrational, Thermodynamic, And Catalytic Properties Of Metal Nanostructures: Size, Shape, Support, And Adsorbate Effects

Recent advances in nanoscience and technology have provided the scientific community with new exciting opportunities to rationally design and fabricate materials at the nanometer scale with drastically different properties as compared to their bulk counterparts. A variety of challenges related to nanoparticle (NP) synthesis and materials characterization have been tackled , allowing us to make more homogenous, well defined, size- and shape-selected NPs, and to probe deeper and more comprehensively into their distinct properties. In this dissertation, a variety of phenomena relevant to nanosized materials are investigated, including the thermal stability of NPs and coarsening phenomena in different environments, the experimental determination of NP shapes, gaining insight into NP-support interactions, epitaxial relationships, and unusual thermodynamic and electronic properties of NPs, including the effect of adsorbates on the electron density of states of small clusters, and the chemical, and structural evolution of NPs under reaction conditions. In chapter 2, a general description of different characterization tools that are used in this dissertation is provided. In chapter 3, the details of two different methods used for NP synthesis, namely inverse micelle encapsulation and physical vapor deposition (PVD) are described. Chapter 4 describes the thermal stability and coarsening behavior of Pt NPs supported on TiO2(110) and γ-Al2O3 as a function of the synthesis method, support pretreatment, and annealing environment. For the Pt/TiO2(110) system, micellesynthesized NPs showed remarkable stability against coarsening for annealing temperatures up to 1060°C in vacuum, in contrast to PVD-grown NPs. When comparing v different annealing environments (H2, O2, H2O), Pt NPs on γ-Al2O3 annealed in O2 were found to be the least affected by coarsening, followed by those heated in H2O vapor. The largest NP growth was observed for the sample annealed in H2. The role of the PtOx species formed under oxidizing conditions will be discussed. In chapter 5, the shape of Pt and Au NPs and their epitaxial relationship with the TiO2(110) support was extracted from scanning tunneling microscopy (STM) measurements. Three main categories of NP shapes were identified, and through shape modeling, the contribution of facets with different orientations was obtained as a function of the number of atoms in each NP. It was also shown that the micellesynthesized Pt and Au NPs have an epitaxial relationship with the support, which is evident from the fact that they always have one symmetry axis parallel to TiO2(110) atomic rows in [001] directions. Chapter 6 describes how the presence of NPs on TiO2(110) surface affects its reconstruction upon high temperature annealing in vacuum. In contrast to NP-free TiO2(110) substrates, long and narrow TiO2 stripes are observed for Pt NP-decorated surfaces. This phenomenon is explained based on the stabilization of TiO2, induced by Pt NPs, which hinders the desorption of oxygen atoms in TiO2 to vacuum. In chapter 7, a systematic investigation of the thermodynamic properties of γ- Al2O3-supported Pt NPs and their evolution with decreasing NP size is presented. A combination of in situ extended x-ray absorption fine structure spectroscopy (EXAFS), ex situ transmission electron microscopy (TEM) measurements, and NP shape modeling is used to obtain the NPs shape, thermal expansion coefficient, and Debye vi temperature. The unusual thermodynamic behavior of these NPs such as their negative thermal expansion and enhanced Debye temperature are discussed in detail. Chapter 8 presents an investigation of the electronic properties of size-controlled γ-Al2O3-supported Pt NPs and their evolution with decreasing NP size and adsorbate (H2) coverage. The hydrogen coverage of Pt NPs at different temperatures was estimated based on XANES data and was found to be influenced by the NP size, and shape. In addition, correlations between the shift in the center of the unoccupied d-band density of states (theory) and energy shifts of the XANES spectra (experiment) upon hydrogen chemisorption as well as upon modification of the NP structure were established. Chapter 9 is dedicated to an operando study, describing the evolution of the structure and oxidation state of ZrO2-supported Pd nanocatalysts during the in-situ selective reduction of NO in H2 via EXAFS and XANES measurements

37th Rocky Mountain Conference on Analytical Chemistry

Final program, abstracts, and information about the 37th annual meeting of the Rocky Mountain Conference on Analytical Chemistry, co-sponsored by the Colorado Section of the American Chemical Society and the Rocky Mountain Section of the Society for Applied Spectroscopy. Held in Denver, Colorado, July 23-27, 1995