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Dependence of Gas-Phase Crotonaldehyde Hydrogenation Selectivity and Activity on the Size of Pt Nanoparticles (1.7-7.1 nm) Supported on SBA-15

Description: The selectivity and activity for the hydrogenation of crotonaldehyde to crotyl alcohol and butyraldehyde was studied over a series of Pt nanoparticles (diameter of 1.7, 2.9, 3.6, and 7.1 nm). The nanoparticles were synthesized by the reduction of chloroplatinic acid by alcohol in the presence of poly(vinylpyrrolidone) (PVP), followed by encapsulation into mesoporous SBA-15 silica. The rate of crotonaldehyde hydrogenation and selectivity towards crotyl alcohol both increase with increasing particle size. The selectivity towards crotyl alcohol increased from 13.7 % to 33.9 % (8 Torr crotonaldehyde, 160 Torr H{sub 2} and 353 K), while the turnover frequency increases from 2.1 x 10{sup -2} s{sup -1} to 4.8 x 10{sup -2} s{sup -1} with an increase in the particle size from 1.7 nm to 7.1 nm. The decarbonylation pathway to form propene and CO is enhanced over the higher proportion of coordinatively unsaturated sites on the smaller nanoparticles. The apparent activation energy remains constant ({approx} 16 kcal mol{sup -1} for the formation of butyraldehyde and {approx} 8 kcal mol{sup -1} for the formation of crotyl alcohol) as a function of particle size. In the presence of 130-260 mTorr CO, the reaction rate decreases for all products with a CO reaction order of -0.9 for crotyl alcohol and butyraldehyde over 7.1 nm Pt particles; over 1.7 nm Pt particles, the order in CO is -1.4 and -0.9, respectively. Hydrogen reduction at 673 K after calcination in oxygen results in increased activity and selectivity relative to reduction at either higher or lower temperature; this is discussed with regards to the incomplete removal and/or change in morphology of the polymeric surface stabilizing agent, poly(vinylpyrrolidone) used for the synthesis of the Pt nanoparticles.
Date: August 3, 2008
Creator: Grass, Michael; Rioux, Robert & Somorjai, Gabor A.
Partner: UNT Libraries Government Documents Department

The 13th International Symposium on Relations between Homogeneous and Heterogeneous Catalysis -- AnIntroduction

Description: Over forty years, there have been major efforts to aim at understanding the properties of surfaces, structure, composition, dynamics on the molecular level and at developing the surface science of heterogeneous and homogeneous catalysis. Since most catalysts (heterogeneous, enzyme and homogeneous) are nanoparticles, colloid synthesis methods were developed to produce monodispersed metal nanoparticles in the 1-10 nm range and controlled shapes to use them as new model catalyst systems in two-dimensional thin film form or deposited in mezoporous three-dimensional oxides. Studies of reaction selectivity in multipath reactions (hydrogenation of benzene, cyclohexene and crotonaldehyde) showed that reaction selectivity depends on both nanoparticle size and shape. The oxide-metal nanoparticle interface was found to be an important catalytic site because of the hot electron flow induced by exothermic reactions like carbon monoxide oxidation.
Date: February 6, 2008
Creator: Somorjai, Gabor A.
Partner: UNT Libraries Government Documents Department

Major Successes of Theory-and-Experiment-Combined Studies in Surface Chemistry and Heterogeneous Catalysis.

Description: Experimental discoveries followed by theoretical interpretations that pave the way of further advances by experimentalists is a developing pattern in modern surface chemistry and catalysis. The revolution of modern surface science started with the development of surface-sensitive techniques such as LEED, XPS, AES, ISS and SIMS, in which the close collaboration between experimentalists and theorists led to the quantitative determination of surface structure and composition. The experimental discovery of the chemical activity of surface defects and the trends in the reactivity of transitional metals followed by the explanations from the theoretical studies led to the molecular level understanding of active sites in catalysis. The molecular level knowledge, in turn, provided a guide for experiments to search for new generation of catalysts. These and many other examples of successes in experiment-and-theory-combined studies demonstrate the importance of the collaboration between experimentalists and theorists in the development of modern surface science.
Date: November 21, 2009
Creator: Somorjai, Gabor A. & Li, Yimin
Partner: UNT Libraries Government Documents Department

Nanoscale Advances in Catalysis and Energy Applications

Description: In this perspective, we present an overview of nanoscience applications in catalysis, energy conversion, and energy conservation technologies. We discuss how novel physical and chemical properties of nanomaterials can be applied and engineered to meet the advanced material requirements in the new generation of chemical and energy conversion devices. We highlight some of the latest advances in these nanotechnologies and provide an outlook at the major challenges for further developments.
Date: May 12, 2010
Creator: Li, Yimin & Somorjai, Gabor A.
Partner: UNT Libraries Government Documents Department

Colloid Science of Metal Nanoparticle Catalysts in 2D and 3D Structures. Challenges of Nucleation, Growth, Composition, Particle Shape, Size Control and their Influence on Activity and Selectivity

Description: Recent breakthroughs in synthesis in nanosciences have achieved control of size and shapes of nanoparticles that are relevant for catalyst design. In this article, we review the advance of synthesis of nanoparticles, fabrication of two and three dimensional model catalyst system, characterization, and studies of activity and selectivity. The ability to synthesize monodispersed platinum and rhodium nanoparticles in the 1-10 nm range permitted us to study the influence of composition, structure, and dynamic properties of monodispersed metal nanoparticle on chemical reactivity and selectivity. We review the importance of size and shape of nanoparticles to determine the reaction selectivity in multi-path reactions. The influence of metal-support interaction has been studied by probing the hot electron flows through the metal-oxide interface in catalytic nanodiodes. Novel designs of nanoparticle catalytic systems are discussed.
Date: February 13, 2008
Creator: Somorjai, Gabor A. & Park, Jeong Y.
Partner: UNT Libraries Government Documents Department

The Effects of Oxygen Plasma on the Chemical Composition and Morphology of the Ru Capping Layer of the Extreme Ultraviolet (EUV) Mask Blanks

Description: Contamination removal from extreme ultraviolet (EUV) mask surfaces is one of the most important aspects to improve reliability for the next generation of EUV lithography. We report chemical and morphological changes of the ruthenium (Ru) mask surface after oxygen plasma treatment using surface sensitive analytical methods: X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and transmission electron microscopy (TEM). Chemical analysis of the EUV masks shows an increase in the subsurface oxygen concentration, Ru oxidation and surface roughness. XPS spectra at various photoelectron takeoff angles suggest that the EUV mask surface was covered with chemisorbed oxygen after oxygen plasma treatment. It is proposed that the Kirkendall effect is the most plausible mechanism that explains the Ru surface oxidation. The etching rate of the Ru capping layer by oxygen plasma was estimated to be 1.5 {+-} 0.2 {angstrom}/min, based on TEM cross sectional analysis.
Date: June 7, 2008
Creator: Belau, Leonid; Park, Jeong Y.; Liang, Ted & Somorjai, Gabor A.
Partner: UNT Libraries Government Documents Department

Structure Sensitivity of Carbon-Nitrogen Ring Opening: Impact of Platinum Particle Size from below 1 to 5 nm upon Pyrrole Hydrogenation Product Selectivity over Monodisperse Platinum Nanoparticles Loaded onto Mesoporous Silica

Description: The ability to control fundamental properties (e.g., particle size, surface structure, and metal-oxide interface) in order to design highly selective heterogeneous catalysts would greatly reduce energy intensive separations. Particle size dependence (i.e., structure sensitivity) upon selectivity can now be examined with well defined nanoparticles (NPs) because of advances in synthetic chemistry. Colloidal chemistry has provided means for synthesizing monodisperse Pt NPs as small as {approx}2 nm. Using a dendrimer templated approach, Pt NPs smaller than 1 nm--a new size regime for studying size induced effects in heterogeneous catalysis--can be synthesized (Scheme 1). In this contribution, we report that ring opening for pyrrole hydrogenation is distinctly different for Pt NPs smaller than 2 nm. This insight has not been demonstrated for hydrogenation of cyclic heteroatom bonds to the best of our knowledge. This finding adds fundamental insight into hydrodenitrogenation (HDN) chemistry, which is important for fuel processing and involves removal of N-containing organics. Advances in HDN catalysis are needed to meet new fuel quality regulations because N-containing organics inhibit hydrodesulfurization (HDS) through competitive adsorption and poison acid catalysts, which are used for downstream processing and as supports for HDS catalysts. Pyrrole was selected as the reactant because organics with 5-member N-containing rings are the most common components in fuel.
Date: July 1, 2008
Creator: Kuhn, John N.; Huang, Wenyu; Tsung, Chia-Kuang; Zhang, Yawen & Somorjai, Gabor A.
Partner: UNT Libraries Government Documents Department

Colloidally Synthesized Monodisperse Rh Nanoparticles Supported on SBA-15 for Size- and Pretreatment-Dependent Studies of CO Oxidation

Description: A particle size dependence for CO oxidation over rhodium nanoparticles of 1.9-11.3 nm has been investigated and determined to be modified by the existence of the capping agent poly(vinylpyrrolidone) (PVP). The particles were prepared using a polyol reduction procedure with PVP as the capping agent. The Rh nanoparticles were subsequently supported on SBA-15 during hydrothermal synthesis to produce Rh/SBA-15 supported catalysts for size-dependent catalytic studies. CO oxidation by O{sub 2} at 40 Torr CO and 100 Torr O{sub 2} was investigated over two series of Rh/SBA-15 catalysts: as-synthesized Rh/SBA-15 covering the full range of Rh sizes and the same set of catalysts after high temperature calcination and reduction. The turnover frequency at 443 K increases from 0.4 to 1.7 s{sup -1} as the particle size decreases from 11.3 to 1.9 nm for the as-synthesized catalysts. After calcination and reduction, the turnover frequency is between 0.1 and 0.4 s{sup -1} with no particle size dependence. The apparent activation energy for all catalysts is {approx}30 kcal mol{sup -1} and is independent of particle size and thermal treatment. Infrared spectroscopy of CO on the Rh nanoparticles indicates that the heat treatments used influence the mode of CO adsorption. As a result, the particle size dependence for CO oxidation is altered after calcination and reduction of the catalysts. CO adsorbs at two distinct bridge sites on as-synthesized Rh/SBA-15, attributable to metallic Rh(0) and oxidized Rh(I) bridge sites. After calcination and reduction, however, CO adsorbs only at Rh(0) atop sites. The change in adsorption geometry and oxidation activity may be attributable to the interaction between PVP and the Rh surface. This capping agent affect may open new possibilities for the tailoring of metal catalysts using solution nanoparticle synthesis methods.
Date: February 12, 2009
Creator: Grass, Michael E.; Joo, Sang Hoon & Somorjai, Gabor A.
Partner: UNT Libraries Government Documents Department

Molecular Components of Catalytic Selectivity

Description: Selectivity, that is, to produce one molecule out of many other thermodynamically feasible product molecules, is the key concept to develop 'clean manufacturing' processes that do not produce byproducts (green chemistry). Small differences in potential energy barriers for elementary reaction steps control which reaction channel is more likely to yield the desired product molecule (selectivity), instead of the overall activation energy for the reaction that controls turnover rates (activity). Recent studies have demonstrated the atomic- or molecular-level tailoring of parameters such as the surface structures of active sites that give rise to nanoparticle size and shape dependence of turnover rates and reaction selectivities. Here, we highlight seven molecular components that influence reaction selectivities. These include: surface structure, adsorbate-induced restructuring, adsorbate mobility, reaction intermediates, surface composition, charge transport, and oxidation states for model metal single crystal and colloid nanoparticle catalysts. We show examples of their functioning and describe in-situ instruments that permit us to investigate their roles in surface reactions.
Date: July 2, 2008
Creator: Somorjai, Gabor A. & Park, Jeong Y.
Partner: UNT Libraries Government Documents Department

Molecular Surface Chemistry by Metal Single Crystals and Nanoparticles from Vacuum to High Pressure.

Description: Model systems for studying molecular surface chemistry have evolved from single crystal surfaces at low pressure to colloidal nanoparticles at high pressure. Low pressure surface structure studies of platinum single crystals using molecular beam surface scattering and low energy electron diffraction techniques probe the unique activity of defects, steps and kinks at the surface for dissociation reactions (H-H, C-H, C-C, O{double_bond}O bonds). High-pressure investigations of platinum single crystals using sum frequency generation vibrational spectroscopy have revealed the presence and the nature of reaction intermediates. High pressure scanning tunneling microscopy of platinum single crystal surfaces showed adsorbate mobility during a catalytic reaction. Nanoparticle systems are used to determine the role of metal-oxide interfaces, site blocking and the role of surface structures in reactive surface chemistry. The size, shape and composition of nanoparticles play important roles in determining reaction activity and selectivity.
Date: April 5, 2008
Creator: Somorjai, Gabor A. & Park, Jeong Y.
Partner: UNT Libraries Government Documents Department

Evolution of the Surface Science of Catalysis from Single Crystals to Metal Nanoparticles under Pressure

Description: Vacuum studies of metal single crystal surfaces using electron and molecular beam scattering revealed that the surface atoms relocate when the surface is clean (reconstruction) and when it is covered by adsorbates (adsorbate induced restructuring). It was also discovered that atomic steps and other low coordination surface sites are active for breaking chemical bonds (H-H, O=O, C-H, C=O and C-C) with high reaction probability. Investigations at high reactant pressures using sum frequency generation (SFG)--vibrational spectroscopy and high pressure scanning tunneling microscopy (HPSTM) revealed bond breaking at low reaction probability sites on the adsorbate-covered metal surface, and the need for adsorbate mobility for continued turnover. Since most catalysts (heterogeneous, enzyme and homogeneous) are nanoparticles, colloid synthesis methods were developed to produce monodispersed metal nanoparticles in the 1-10 nm range and controlled shapes to use them as new model catalyst systems in two-dimensional thin film form or deposited in mesoporous three-dimensional oxides. Studies of reaction selectivity in multipath reactions (hydrogenation of benzene, cyclohexene and crotonaldehyde) showed that reaction selectivity depends on both nanoparticle size and shape. The oxide-metal nanoparticle interface was found to be an important catalytic site because of the hot electron flow induced by exothermic reactions like carbon monoxide oxidation.
Date: March 6, 2008
Creator: Somorjai, Gabor A. & Park, Jeong Y.
Partner: UNT Libraries Government Documents Department

Sum Frequency Generation Vibrational Spectroscopy of Pyridine Hydrogenation on Platinum Nanoparticles

Description: Pyridine hydrogenation in the presence of a surface monolayer consisting of cubic Pt nanoparticles stabilized by tetradecyltrimethylammonium bromide (TTAB) was investigated by sum frequency generation (SFG) vibrational spectroscopy using total internal reflection (TIR) geometry. TIR-SFG spectra analysis revealed that a pyridinium cation (C{sub 5}H{sub 5}NH{sup +}) forms during pyridine hydrogenation on the Pt nanoparticle surface, and the NH group in the C{sub 5}H{sub 5}NH{sup +} cation becomes more hydrogen bound with the increase of the temperature. In addition, the surface coverage of the cation decreases with the increase of the temperature. An important contribution of this study is the in situ identification of reaction intermediates adsorbed on the Pt nanoparticle monolayer during pyridine hydrogenation.
Date: February 22, 2008
Creator: Bratlie, Kaitlin M.; Komvopoulos, Kyriakos & Somorjai, Gabor A.
Partner: UNT Libraries Government Documents Department

Reaction Selectivity in Heterogeneous Catalysis

Description: The understanding of selectivity in heterogeneous catalysis is of paramount importance to our society today. In this review we outline the current state of the art in research on selectivity in heterogeneous catalysis. Current in-situ surface science techniques have revealed several important features of catalytic selectivity. Sum frequency generation vibrational spectroscopy has shown us the importance of understanding the reaction intermediates and mechanism of a heterogeneous reaction, and can readily yield information as to the effect of temperature, pressure, catalyst geometry, surface promoters, and catalyst composition on the reaction mechanism. DFT calculations are quickly approaching the ability to assist in the interpretation of observed surface spectra, thereby making surface spectroscopy an even more powerful tool. HP-STM has revealed three vitally important parameters in heterogeneous selectivity: adsorbate mobility, catalyst mobility, and selective site-blocking. The development of size controlled nanoparticles from 0.8 to 10 nm, of controlled shape, and of controlled bimetallic composition has revealed several important variables for catalytic selectivity. Lastly, DFT calculations may be paving the way to guiding the composition choice for multi-metallic heterogeneous catalysis for the intelligent design of catalysts incorporating the many factors of selectivity we have learned.
Date: February 2, 2009
Creator: Somorjai, Gabor A. & Kliewer, Christopher J.
Partner: UNT Libraries Government Documents Department

Pyrrole Hydrogenation over Rh(111) and Pt(111) Single-Crystal Surfaces and Hydrogenation Promotion Mediated by 1-Methylpyrrole: A Kinetic and Sum-Frequency Generation Vibrational Spectroscopy Study

Description: Sum-frequency generation (SFG) surface vibrational spectroscopy and kinetic measurements using gas chromatography have been used to study the adsorption and hydrogenation of pyrrole over both Pt(111) and Rh(111) single-crystal surfaces at Torr pressures (3 Torr pyrrole, 30 Torr H{sub 2}) to form pyrrolidine and the minor product butylamine. Over Pt(111) at 298 K it was found that pyrrole adsorbs in an upright geometry cleaving the N-H bond to bind through the nitrogen evidenced by SFG data. Over Rh(111) at 298 K pyrrole adsorbs in a tilted geometry relative to the surface through the p-aromatic system. A pyrroline surface reaction intermediate, which was not detected in the gas phase, was seen by SFG during the hydrogenation over both surfaces. Significant enhancement of the reaction rate was achieved over both metal surfaces by adsorbing 1-methylpyrrole before reaction. SFG vibrational spectroscopic results indicate that reaction promotion is achieved by weakening the bonding between the N-containing products and the metal surface because of lateral interactions on the surface between 1-methylpyrrole and the reaction species, reducing the desorption energy of the products. It was found that the ring-opening product butylamine was a reaction poison over both surfaces, but this effect can be minimized by treating the catalyst surfaces with 1-methylpyrrole before reaction. The reaction rate was not enhanced with elevated temperatures, and SFG suggests desorption of pyrrole at elevated temperatures.
Date: March 4, 2008
Creator: Kliewer, Christopher J.; Bieri, Marco & Somorjai, Gabor A.
Partner: UNT Libraries Government Documents Department

Preparation of Size-tunable, Highly Monodisperse PVP-Protected Pt-nanoparticles by Seed-mediated Growth

Description: We demonstrate a preparative method which produces highly-monodisperse Pt-nanoparticles of tunable size without the external addition of seed particles. Hexachloroplatinic acid is dosed slowly to an ethylene glycol solution at 120 C and reduced in the presence of a stabilizing polymer poly-N-vinylpyrollidone (PVP). Slow addition of the Pt-salt first will first lead to the formation of nuclei (seeds) which then grow further to produce larger particles of any desired size between 3 and 8nm. The amount of added hexachloroplatinic acid precursor controls the size of the final nanoparticle product. TEM was used to determine size and morphology and to confirm the crystalline nature of the nanoparticles. Good reproducibility of the technique was demonstrated. Above 7nm, the particle shape and morphology changes suddenly indicating a change in the deposition selectivity of the Pt-precursor from (100) towards (111) crystal faces and breaking up of larger particles into smaller entities.
Date: April 2, 2008
Creator: Koebel, Matthias Michael; Jones, Louis C. & Somorjai, Gabor A.
Partner: UNT Libraries Government Documents Department

John Pendry: His Contributions to the Development of LEED Surface Crystallography

Description: In this paper we discuss the pivotal role played by Sir John Pendry in the development of Low Energy Electron Diffraction (LEED) during the past three decades; the earliest understanding on the physics of LEED to the development of sophisticated methods for the structural solution of complex surfaces.
Date: October 15, 2007
Creator: Somorjai, Gabor A. & Rous, P. J.
Partner: UNT Libraries Government Documents Department

Hydrogen Oxidation-Driven Hot Electron Flow Detected by Catalytic Nanodiodes

Description: Hydrogen oxidation on platinum is shown to be a surface catalytic chemical reaction that generates a steady state flux of hot (>1 eV) conduction electrons. These hot electrons are detected as a steady-state chemicurrent across Pt/TiO{sub 2} Schottky diodes whose Pt surface is exposed to hydrogen and oxygen. Kinetic studies establish that the chemicurrent is proportional to turnover frequency for temperatures ranging from 298 to 373 K for P{sub H2} between 1 and 8 Torr and P{sub O2} at 760 Torr. Both chemicurrent and turnover frequency exhibit a first order dependence on P{sub H2}.
Date: July 20, 2009
Creator: Hervier, Antoine; Renzas, J. Russell; Park, Jeong Y. & Somorjai, Gabor A.
Partner: UNT Libraries Government Documents Department

Size Characterization of Colloidal Platinum Nanoparticles by MALDI-TOF Mass Spectrometry

Description: In this work, matrix assisted laser desorption ionization-time-of-flight (MALDI-TOF) mass spectrometry (MS) has been utilized to characterize colloidal platinum nanoparticles synthesized in the 1-4 nm size range. The nanoparticles were prepared via a solution-based method in which the size could be controlled by varying reaction conditions, such as the alcohol used as the reductant. Poly(vinylpyrrolidone), or PVP, (MW = 29,000 g/mol) was employed as a capping agent to stabilize the synthesized nanoparticles in solution. A model for determining the size of the metallic nanoparticle core from MALDI-TOF mass spectra has been developed and verified through correlation with particle sizes from transmission electron microscopy (TEM) and X-ray diffraction (XRD) measurements. In this model it was assumed that 1.85 nm nanoparticles are capped by one PVP chain, which was verified through experiments performed with capped and uncapped nanoparticles. Larger nanoparticles are capped by either two (2.60 and 2.94 nm) or three (3.69 nm) PVP chains. These findings clearly indicate the usefulness of MALDI-TOF MS as a technique for fully characterizing nanoscale materials in order to elucidate structure-property relationships.
Date: August 15, 2009
Creator: Navin, Jason K.; Grass, Michael E.; Somorjai, Gabor A. & Marsh, Anderson L.
Partner: UNT Libraries Government Documents Department

Seedless Polyol Synthesis and CO Oxidation Activity of Monodisperse (111) and (100)-Oriented Rhodium Nanocrystals in Sub-10 nm Sizes

Description: Monodisperse sub-10 nm (6.5 nm) sized Rh nanocrystals with (111) and (100) surface structures were synthesized by a seedless polyol reduction in ethylene glycol, with poly(vinylpyrrolidone) as a capping ligand. When using [Rh(Ac){sub 2}]{sub 2} as the metal precursor, (111)-oriented Rh nanopolyhedra containing 76% (111)-twined hexagons (in 2D projection) were obtained; whereas, when employing RhCl{sub 3} as the metal precursor in the presence of alkylammonium bromide, such as tetramethylammonium bromide and trimethyl(tetradecyl)ammonium bromide, (100)-oriented Rh nanocubes were obtained with 85% selectivity. The {l_brace}100{r_brace} faces of the Rh nanocrystals are stabilized by chemically adsorbed Br{sup -} ions from alkylammonium bromides, which led to (100)-oriented nanocubes. Monolayer films of the (111)-oriented Rh nanopolyhedra and (100)-oriented Rh nanocubes were deposited on silicon wafers in a Langmuir-Blodgett trough to make model 2D nanoarray catalysts. These nanocatalysts were active for CO oxidation by O{sub 2}, and the turnover frequency was independent of nanoparticle shape, consistent with that previously observed for Rh(111) and Rh(100) single crystals.
Date: March 15, 2010
Creator: Zhang, Yawen; Grass, Michael E.; Huang, Wenyu & Somorjai, Gabor A.
Partner: UNT Libraries Government Documents Department

Shaping metal nanocrystals through epitaxial seeded growth

Description: Morphological control of nanocrystals has becomeincreasingly important, as many of their physical and chemical propertiesare highly shape-dependent. Nanocrystal shape control for both single andmultiple material systems, however, remains fairly empirical andchallenging. New methods need to be explored for the rational syntheticdesign of heterostructures with controlled morphology. Overgrowth of adifferent material on well-faceted seeds, for example, allows for the useof the defined seed morphology to control nucleation and growth of thesecondary structure. Here, we have used highly faceted cubic Pt seeds todirect the epitaxial overgrowth of a secondary metal. We demonstrate thisconcept with lattice matched Pd to produce conformal shape-controlledcore-shell particles, and then extend it to lattice mismatched Au to giveanisotropic growth. Seeding with faceted nanocrystals may havesignificant potential towards the development of shape-controlledheterostructures with defined interfaces.
Date: February 17, 2008
Creator: Habas, Susan E.; Lee, Hyunjoo; Radmilovic, Velimir; Somorjai,Gabor A. & Yang, Peidong
Partner: UNT Libraries Government Documents Department

The Role of Organic Capping Layers of Platinum Nanoparticles in Catalytic Activity of CO Oxidation

Description: We report the catalytic activity of colloid platinum nanoparticles synthesized with different organic capping layers. On the molecular scale, the porous organic layers have open spaces that permit the reactant and product molecules to reach the metal surface. We carried out CO oxidation on several platinum nanoparticle systems capped with various organic molecules to investigate the role of the capping agent on catalytic activity. Platinum colloid nanoparticles with four types of capping layer have been used: TTAB (Tetradecyltrimethylammonium Bromide), HDA (hexadecylamine), HDT (hexadecylthiol), and PVP (poly(vinylpyrrolidone)). The reactivity of the Pt nanoparticles varied by 30%, with higher activity on TTAB coated nanoparticles and lower activity on HDT, while the activation energy remained between 27-28 kcal/mol. In separate experiments, the organic capping layers were partially removed using ultraviolet light-ozone generation techniques, which resulted in increased catalytic activity due to the removal of some of the organic layers. These results indicate that the nature of chemical bonding between organic capping layers and nanoparticle surfaces plays a role in determining the catalytic activity of platinum colloid nanoparticles for carbon monoxide oxidation.
Date: December 17, 2008
Creator: Park, Jeong Y.; Aliaga, Cesar; Renzas, J. Russell; Lee, Hyunjoo & Somorjai, Gabor A.
Partner: UNT Libraries Government Documents Department

Influence of Particle Size on Reaction Selectivity in Cyclohexene Hydrogenation and Dehydrogenation over Silica-Supported Monodisperse Pt Particles

Description: The role of particle size during the hydrogenation/dehydrogenation of cyclohexene (10 Torr C{sub 6}H{sub 10}, 200-600 Torr H{sub 2}, and 273-650 K) was studied over a series of monodisperse Pt/SBA-15 catalysts. The conversion of cyclohexene in the presence of excess H{sub 2} (H{sub 2}:C{sub 6}H{sub 10} ratio = 20-60) is characterized by three regimes: hydrogenation of cyclohexene to cyclohexane at low temperature (< 423 K), an intermediate temperature range in which both hydrogenation and dehydrogenation occur; and a high temperature regime in which the dehydrogenation of cyclohexene dominates (> 573 K). The rate of both reactions demonstrated maxima with temperature, regardless of Pt particle size. For the hydrogenation of cyclohexene, a non-Arrhenius temperature dependence (apparent negative activation energy) was observed. Hydrogenation is structure insensitive at low temperatures, and apparently structure sensitive in the non-Arrhenius regime; the origin of the particle-size dependent reactivity with temperature is attributed to a change in the coverage of reactive hydrogen. Small particles were more active for dehydrogenation and had lower apparent activation energies than large particles. The selectivity can be controlled by changing the particle size, which is attributed to the structure sensitivity of both reactions in the temperature regime where hydrogenation and dehydrogenation are catalyzed simultaneously.
Date: July 11, 2008
Creator: Rioux, R. M.; Hsu, B. B.; Grass, M. E.; Song, H. & Somorjai, Gabor A.
Partner: UNT Libraries Government Documents Department

Chemical Effect of Dry and Wet Cleaning of the Ru Protective Layer of the Extreme ultraviolet (EUV) Lithography Reflector

Description: The authors report the chemical influence of cleaning of the Ru capping layer on the extreme ultraviolet (EUV) reflector surface. The cleaning of EUV reflector to remove the contamination particles has two requirements: to prevent corrosion and etching of the reflector surface and to maintain the reflectivity functionality of the reflector after the corrosive cleaning processes. Two main approaches for EUV reflector cleaning, wet chemical treatments [sulfuric acid and hydrogen peroxide mixture (SPM), ozonated water, and ozonated hydrogen peroxide] and dry cleaning (oxygen plasma and UV/ozone treatment), were tested. The changes in surface morphology and roughness were characterized using scanning electron microscopy and atomic force microscopy, while the surface etching and change of oxidation states were probed with x-ray photoelectron spectroscopy. Significant surface oxidation of the Ru capping layer was observed after oxygen plasma and UV/ozone treatment, while the oxidation is unnoticeable after SPM treatment. Based on these surface studies, the authors found that SPM treatment exhibits the minimal corrosive interactions with Ru capping layer. They address the molecular mechanism of corrosive gas and liquid-phase chemical interaction with the surface of Ru capping layer on the EUV reflector.
Date: April 10, 2009
Creator: Belau, Leonid; Park, Jeong Y.; Liang, Ted; Seo, Hyungtak & Somorjai, Gabor A.
Partner: UNT Libraries Government Documents Department

A New Scanning Tunneling Microscope Reactor Used for High Pressure and High Temperature Catalysis Studies

Description: We present the design and performance of a home-built high-pressure and high-temperature reactor equipped with a high-resolution scanning tunneling microscope (STM) for catalytic studies. In this design, the STM body, sample, and tip are placed in a small high pressure reactor ({approx}19 cm{sup 3}) located within an ultrahigh vacuum (UHV) chamber. A sealable port on the wall of the reactor separates the high pressure environment in the reactor from the vacuum environment of the STM chamber and permits sample transfer and tip change in UHV. A combination of a sample transfer arm, wobble stick, and sample load-lock system allows fast transfer of samples and tips between the preparation chamber, high pressure reactor, and ambient environment. This STM reactor can work as a batch or flowing reactor at a pressure range of 10{sup -13} to several bars and a temperature range of 300-700 K. Experiments performed on two samples both in vacuum and in high pressure conditions demonstrate the capability of in situ investigations of heterogeneous catalysis and surface chemistry at atomic resolution at a wide pressure range from UHV to a pressure higher than 1 atm.
Date: May 12, 2008
Creator: Tao, Feng; Tang, David C.; Salmeron, Miquel & Somorjai, Gabor A.
Partner: UNT Libraries Government Documents Department