Jackie Y. Ying
Description of Research Program
Research activities in my laboratory are focused on the processing of nanostructured materials, which have unique microstructure and exceptional size-dependent properties. The technological potential of this class of materials is to be realized through our ability to tailor the materials for novel catalytic, sensors, membranes, structural, electric, magnetic, and biomaterials applications. By tying together chemical engineering principles, surface chemistry and materials science, my group seeks to understand the process chemistry and structural physics of nanocrystalline, nanoporous and microemulsion systems.
1) Synthesis and Surface Chemistry of Nanocrystalline Oxide
(P. Pitukmanorom, M. D. Fokema, A. Tschöpe, and J. Y. Ying)
Nanocrystalline oxides are prepared through inert gas condensation to derive high surface reactivity, ultrahighly dispersed catalytic materials. We have been investigating CeO2-x-based nanostructured materials for catalytic SO2 reduction by CO. In this case, a highly non-stoichiometric fluorite-structured oxide is used to tailor high oxygen vacancy concentration and mobility in a catalyst to facilitate the redox mechanism in the catalytic removal of SO2. This unique material is generated in the nanocrystalline form by magnetron sputtering from a metallic target, followed by controlled oxidation. Surface segregation is utilized to further promote the catalytic reactivity of the CeO2-x by providing the nanocrystals with a highly dispersed 'surface coating' of Cu. The Cu/CeO2-x catalyst is designed to lower the enthalpy of formation of oxygen vacancy through the creation of Schottky junction between Cu and CeO2-x, and to enhance the adsorptivity of the catalysts with a 'spillover' effect from Cu to CeO2-x. Starting out with a nanocrystalline Cu-Ce alloy, the system can be carefully post-oxidized such that the two components gradually lose their miscibility in their respective oxide forms. The surface-segregated Cu is found to greatly improve the catalytic activity in this model catalyst design. Complete conversion of SO2 to elemental sulfur is achieved at temperatures as low as 420°C, and CO oxidation can be accomplished with this supported base metal catalyst at a remarkably low temperature of 80°C. The surface and interfacial chemistry, chemisorption characteristics, and oxygen conductivity of this novel material are being examined for a fundamental understanding of its exceptional catalytic behavior.
Other nanostructured rare earth oxides are being investigated as catalysts for NOx reduction. The catalytic removal of NOx from a conventional exhaust stream is a challenging problem because it involves the reduction of NO to N2 in a strongly oxidizing atmosphere. Commonly, a reducing agent such as ammonia is added to the exhaust stream to aid in the reduction of NO. Methane is a much more attractive reducing agent due to its low cost, wide availability, and non-corrosiveness, but the current metal--exchanged zeolitic catalysts for this application suffer from poor hydrothermal stability. We have synthesized nanocrystalline Group IIIB and lanthanide oxide catalysts via controlled chemical precipitation for the selective reduction of NOx with methane. Yttrium oxide catalysts with crystallite sizes under 10 nm and surface areas above 110 m2/g have been prepared. These materials are active for the reduction of NOx with CH4 above 400°C, with a maximum in activity near 600°C. These nanocrystalline catalysts are the most active non-zeolitic catalysts for NOx reduction with CH4 in excess oxygen, and exhibit activities nearly comparable to the conventional zeolitic systems, with no irreversible hydrothermal deactivation. The reaction mechanism has been found to involve a series of heterogeneous and homogeneous reaction steps. The selectivity of these oxide catalysts has been directly related to their surface chemistry through catalytic, infrared, and thermogravimetric studies.
2) Catalytic Combustion with Nanostructured Barium Hexaaluminate--based
(Y. S. Su, J. T. McCue, N. Sangar, M. Wolf, A. J. Zarur, H. H. Hwu, and J. Y. Ying)
Catalytic combustion of methane has been widely studied as an alternative to gas-phase homogeneous combustion. It allows combustion to occur at high levels of excess air, leading to more complete reaction and reduced hydrocarbon emissions. Furthermore, it enables combustion to proceed at lower temperatures, significantly reducing the NOx production. Traditionally, noble metal systems, such as platinum and palladium, have been used as combustion catalysts. However, noble metal clusters tend to sinter or vaporize at the high combustion temperatures of > 1000°C. Our research objective is to develop complex metal oxide systems that offer superior thermal resistance and high catalytic activity simultaneously.
To achieve this, we have successfully synthesized nanocrystalline barium hexaaluminate (BHA) materials using a novel reverse microemulsion-mediated sol-gel technique. In this novel approach, a reverse microemulsion is used to effectively confine the hydrolysis and polycondensation reactions to nanometer-sized aqueous domains. The resulting nanostructured morphology and improved chemical uniformity enable nanocrystalline BHA to maintain surface areas in excess of 100 m2/g even upon extended exposure at 1300°C.
We have also significantly improved the low-temperature catalytic activity of BHA through the introduction of transition metal and rare earth oxide dopants and surface deposits. Of the systems studied, CeO2 surface deposits on BHA showed particularly exciting results. Utilizing our reverse microemulsion synthesis approach, we have been able to obtain CeO2 nanocrystals on the surface of BHA nanoparticles. These supported CeO2 nanocrystals retained grain sizes finer than 20 nm even after calcination to 1300°C, whereas unsupported CeO2 nanocrystals prepared by chemical precipitation exhibited grain growth to > 100 nm by 700°C. The CeO2/BHA nanocomposite demonstrated very high activity for methane combustion, achieving 10% conversion of a stream of 1% CH4 in air by 400°C, while attaining full conversion by ~ 550°C. Our system is competitive even with the expensive noble metal systems, which possess inferior high-temperature stability and poisoning resistance.
3) Photocatalytic Activity and Electronic Structure of Nanocrystalline
(C.-C. Wang, Z. Zhang, and J. Y. Ying)
Photocatalysis represents an attractive low-temperature, non-energy-intensive approach for treating chemical wastes. Through appropriate choice of semiconductor catalysts, charge carriers can be generated by ultraviolet or visible radiation to initiate reduction and oxidation reactions with adsorbed reactants, leading to the destruction of the pollutants. The current bottleneck in photocatalysis lies in the low quantum efficiency, due to the uncontrolled electron/hole recombination process and slow interfacial charge transfer. This project seeks to develop superior semiconductor photocatalytic materials with increased surface reactivity and reduced charge carrier recombination to substantially improve the photoefficiency.
Nanocrystalline TiO2 catalysts have been successfully developed to enable significant increase in the catalytic activity of photo-assisted decomposition of halogenated organics. Research efforts have been directed towards the design, synthesis, characterization and catalytic testing of a novel TiO2-based nanocrystalline system with tailored size, crystal phase, doping and metal deposition. We have identified the optimal grain size to be 10 nm for anatase crystals for both liquid-phase and gas-phase photo-oxidation of chlorinated compounds under ultraviolet radiation. Further improvement in photocatalytic activity has been achieved by introducing appropriate dopants and/or noble metals. The effectiveness of selective dopants such as Fe3+ and Nb5+ and/or Pt deposition towards enhancing photoactivity was noted to have a strong dependency on the TiO2 crystallite size and crystalline phase. To expand the application of photocatalysis to include not only ultraviolet but also visible light radiation, the TiO2 system has been modified with (i) metal doping and (ii) incorporation of a second semiconductor oxide. Excellent photocatalytic activity has been achieved over the resulting nanocomposite systems under visible light.
4) Gerneration of Hydrogen by Steam Reforming of Hydrocarbons
(S. E. Weiss, J. Cui, J. Xu, E. J. N. Smith, D. B. Myers, P. W. Cheung, and J. Y. Ying)
Steam reforming of hydrocarbons to produce synthesis gas (i.e. H2 and CO) has been used in industry for more than fifty years. Conventional steam reforming catalysts include nickel supported on alumina or magnesia, which are active at temperatures of 800-1000°C and pressures of 20-30 atm. Our goal is to design a catalyst that is active for steam reforming at significantly lower temperatures and pressures.
We are working with a variety of supported catalysts for steam reforming of methane and higher alkanes. New support materials that are stable in the presence of high-pressure steam, and resistant against coking and sulfur poisoning are being designed. Novel processing methods developed in our laboratory has enabled the production of nanocrystalline complex oxides that can maintain high surface areas in the presence of water vapor at temperatures in excess of 1000°C. The active transition metal or noble metal species are deposited onto these nanocrystalline oxide supports with ultrahigh dispersions via vapor grafting of volatile organometallic complexes. The resulting catalysts present attractive steam reforming characteristics at relatively low temperatures (500-600°C) and at atmospheric pressure, with excellent long-term stability.
We are also studying reforming of alternative fuels such as ethanol and methanol for use in on-board production of hydrogen for fuel-cell vehicles. On-board reforming presents several unique challenges. Besides being compact, lightweight and efficient, such fuel-processing reactors would also need to be stable to rapid startups and shutdowns. Traditional low-temperature water-gas shift catalysts used in methanol reforming are pyrophoric, and large exotherms may result upon re-oxidation. Alternative catalysts are being developed that show superior catalytic activity and stability. Lessons learned from these catalysts may also be applicable to other reforming systems.
5) Synthesis and Processing of Nanocrystalline Nitride Ceramics
(D. T. Castro and J. Y. Ying)
With their high strength and resistance to heat, wear and corrosion,
the nitride family of ceramics demonstrates great promise in a
number of engineering applications. However, there are many difficulties
in sintering and forming pure nitrides due to their covalent bonding
and high melting points, as well as the low quality of the commercially
available powders. The goal of this research lies in synthesizing
ultrafine non-oxides to address current challenges to structural
ceramic technology through nanocrystalline processing and grain
boundary engineering of nitride materials. Nanocrystalline processing
has been successfully employed to attain low-temperature sinterability
and unusual ductility in oxide ceramics. However, due primarily
to the synthesis and handling difficulties, there has been much
less investigation of nanocrystalline nitride systems. Therefore,
our objective is to synthesize and develop nitride-based materials
with enhanced sinterability, superplasticity and high-temperature
strength by exploiting the ultrafine microstructure, and high
diffusivity and reactivity of nanocrystalline materials.
We have designed and constructed a novel forced flow reactor for the large-scale synthesis of nanocrystalline materials. The reactor operates using a replenishable thermal evaporation source in a forced gas flow. The gas stream can quickly remove particles generated from the hot growth zone over the crucible to preserve the ultrafine particle size with minimal agglomeration. A microwave-generated nitrogen plasma allows in situ nitridation of evaporated metals. A variety of nanocrystalline metals (Si and Al) and nitrides (silicon nitride, titanium nitride and aluminum nitride) have been produced with particle size of ~ 10 nm. The nitride nanoparticles produced in our reactor are among the very highest, if not the highest quality powders cited in the literature. Careful handling/processing procedures that address the oxidation tendencies of the nanocrystalline nitrides have enabled us to produce 99% dense TiN with a pressureless sintering process at 1400°C. The resulting ultrafine-grained TiN (~ 250 nm) demonstrates superb hardness and fracture toughness.
6) Synthesis and Sintering of Nanocrystalline Alumina, Aluminum
Nitride, and Alumina-Zirconia
(M. L. Panchula, J. T. McCue, R. E. Rogers, Jr., and J. Y. Ying)
It has been well illustrated over the past few decades that microstructural control of ceramics is necessary to optimize the mechanical and thermal properties. For instance, the presence of pores, secondary phases, and other defects can greatly reduce the thermal conductivity of ceramics. The preparation, therefore, of high thermal conductivity Al2O3 and AlN typically involves high-temperature sintering to obtain dense and defect-free materials. We are interested in generating these materials in the nanocrystalline form for improved densification and possible net--shape forming. Our starting point for nanocrystalline a-Al2O3 is a high surface area g-Al2O3 produced by chemical precipitation and low-temperature calcination. The sintering of the transitional aluminas to full density is difficult to achieve due to the grain coarsening associated with the transformation of the reactive transitional phases (g, d, q) to the thermodynamically stable a-phase. To solve this problem, we have made use of mechanical energy to successfully nucleate the a-Al2O3 phase in the g-Al2O3 powder. The resulting material exhibits a lower phase transformation temperature and an ultrafine grain size. The a-Al2O3 seeds facilitate densification and may allow superplastic forming.
AlN possesses a higher thermal conductivity than Al2O3,
and has the added advantage that its thermal expansion coefficient
is close to that of silicon. Conventionally, AlN requires a high
sintering temperature even with the use of oxide sintering additives.
Synthesis of AlN in the nanocrystalline form may enable densification
without the use of oxide additives, thus increasing the thermal
conductivity. The reactive nature of the nanocrystalline ceramic
also offers the possibility of low-temperature metallization,
allowing cheaper metal interconnects to be utilized. We have successfully
synthesized nanocrystalline AlN powders from gas condensation
and in situ nitridation in a forced flow reactor. These unique
materials have been pressurelessly sintered successfully without
additives. Nanocrystalline Y2O3
additives can be introduced to further enhance the low-temperature
densification of nanocrsytalline AlN. Excellent thermal conductivities
have been obtained by the resulting nitride compacts. The particle
size and morphology of nanocrystalline nitride can be controlled
by the synthesis atmosphere and conditions. Besides spherical
nanoparticles, needle-shaped AlN particles have been obtained.
When hot-pressed, the latter produces a highly textured material
that may be of interest for high-temperature piezoelectric applications.
Increased reliability and effectiveness of thermal barrier coatings (TBC) depend primarily on improvements in the material systems from which they are constructed. TBC, which are generally made up of yttria-stabilized zirconia and an underlying bond coat, are subjected to high-temperature, corrosive environments that can chemically attack the TBC or the underlying layers. The environmental degradation may be minimized by creating composite TBC that provides both thermal and environmental protection. The Al2O3-ZrO2 system is one such composite system that deserves investigation. Al2O3 and ZrO2 are nearly insoluble in each other and, although Al2O3 has a higher thermal conductivity than ZrO2, it has much lower oxygen conductivity and environmental sensitivity so that a composite made up of these materials can result in improved stability. We are currently examining the processing of alumina-zirconia nanocomposites that would lead to enhanced thermal and chemical stabilization for TBC applications.
7) Processing and Properties of Nanostructured Biomaterials
for Orthopedic Applications
(E. S. Ahn, S. Moudgil, T. Oei, A. H. Choi, N. J. Gleason, A. Nakahira, and J. Y. Ying)
In this research, nanostructure processing is applied to hydroxyapatite-based materials to achieve the desired mechanical characteristics and enhanced the surface reactivities for multifunctional implant systems tailored towards different hard tissue replacements. Hydroxyapatite (HAP) is a bioactive ceramic with a crystal structure similar to native bone and teeth minerals. It has generated great interest in the search for advanced orthopedic and dental implant materials as it elicits a favorable biological response and forms a bond with the surrounding tissues. However, applications of HAP are currently limited to powders, coatings, porous bodies and non-load-bearing implants due to processing difficulties and the poor mechanical properties of conventional HAP. This material is sensitive to non-stoichiometry and impurities in its synthesis and processing due to its complex composition and crystal structure (Ca10(PO4)6(OH)2, P63/m). As a result, conventionally processed HAP materials lack phase purity and homogeneity. They are very challenging to sinter; densification has typically required high temperatures, which result in grain growth and decomposition into undesired phases with poor mechanical and chemical stability. To circumvent densification at high temperatures, glassy additives can be introduced to promote liquid-phase sintering at a lower temperature. However, the presence of a secondary glassy phase gives rise to poor mechanical characteristics.
Nanostructure processing of HAP-based composites allows for
the design of structural and surface features inspired by the
architecture of bone. It has allowed chemical homogeneity and
microstructural uniformity to be achieved for HAP so that fully
dense bioceramics can be generated at low sintering temperatures
with a significant reduction in flaw size. Our HAP powders can
be densified easily to > 98% theoretical density by pressureless
sintering at 1000°C or by pressure-assisted sintering at 900°C.
Translucent, ultrafine-grained (125 nm) compacts thus obtained
exhibits superior compressive (900 MPa) and bending (200 MPa)
strengths and fracture toughness (1.3 MPa.m1/2).
The nanometer-sized grains and high volume fraction of grain boundaries
of nanostructured HAP have also been found to increase osteoblast
adhesion, proliferation and mineralization. Zirconia nanocrystals
have been introduced to further toughen the HAP matrix. The resulting
nanocomposite achieves > 98% theoretical density and grain
sizes of < 125 nm by pressure-assisted sintering at 1000°C.
The incorporation of highly dispersed zirconia significantly enhances
the fracture toughness (2.0 MPa.m1/2)
and bending strength (280 MPa) of our HAP-based bioceramics for
load-bearing implant applications.
8) Nanostructured Drug Delivery Systems
(T. C. Zion, N. T. Zaman, N. Z. Mehenti, H. H. Tsang, A. J. Zarur, and J. Y. Ying)
The objective of this project is to synthesize intelligent nanostructured carriers for oral delivery of insulin and other active substrates. These carriers should have tailored sizes in the 5-100 nm range to ensure reasonable uptake through the gastrointestinal tract and avoid digestion by macrophages. They should also protect the active substrate from the action of acidic or enzymatic environments. The surface chemistry of the carriers is designed so as to allow for precise control of the drug release as a response to external stimuli or pathological conditions. To accomplish these goals simultaneously, nanostructure processing is employed to tailor drug delivery systems with unique surface chemistries.
Reverse microemulsions are employed as nanoreactors for the synthesis of polymeric nanocomposite carriers for various drugs and proteins. They are prepared using FDA-approved, biocompatible oils and surfactants to prevent drug carriers from exposure to toxic compounds. Biocompatible polymeric nanospheres are synthesized in the nanometer-sized aqueous domains of the reverse microemulsion. This allows us to achieve controlled particle size and morphology so as to maximize their uptake through the gastrointestinal tract, and to target specific organs or tissues in the body. The composition of the carriers is used to manipulate their circulation time, and the release pattern of the active substrates in the body. These carriers also serve to protect the active substrates from acidic and enzymatic degradation in the gastrointestinal tract. We further functionalize the surface chemistry of the polymeric carriers to allow them to respond to specific environmental conditions. For example, we are creating a glucose-sensitive insulin delivery system that will release the protein when the blood sugar levels reach a specified level. The novel nanostructured drug delivery systems would provide significantly more convenient insulin administration than the conventional subcutaneous procedures, and have the potential of improving patients' quality of life and compliance. Additionally, the intelligent drug delivery system would mimic the physiological secretion of insulin in the body, minimizing the risk of hypoglycemia and other diseases associated with high insulin doses.
9) Surfactant-Templated Mesoporous Materials for Solid Acid
(M. S. Wong, J. Xu, Y. S. Su, E. S. Jeng, H. C. Huang, T. Sun, D. M. Antonelli, and J. Y. Ying)
An exciting new class of materials called MCM-41 was reported by Mobil researchers in 1992 using amphiphilic molecules as a supramolecular templating agent. The surfactants self-assemble into micelles and micellar rods in the presence of aluminum and silicon salts. The inorganics condense around the micellar rods, forming a mesostructured precipitate. The removal of the organics by calcination reveals an open aluminosilicate framework of hexagonally packed one-dimensional pores with enormous surface areas (> 1000 m2/g). The uniform pore diameters of MCM-41 can be systematically tailored between 20-100 Å by changing surfactant chain length and adding swelling agents. The resulting class of mesoporous materials overcomes the conventional limitations of microporous zeolites and should lead to exciting possibilities in a wide variety of catalytic applications, including solid acid catalysis. Solid acid catalysts are being sought as substitutes for liquid acids, which are corrosive, difficult to handle and recover, and harmful to the environment. Surface acidity of MCM-41 is generated when a second metal cation is doped into the framework. There are a myriad of acidic materials that are not silicate-based, however, and would benefit from engineering mesoporosity into their microstructure. We are interested in developing mesoporous materials with novel compositions for acid catalysis and the supramolecular chemistry necessary to achieve this.
Our laboratory has created a series of mesoporous and microporous transition metal oxide molecular sieves (termed TMS) based on a ligand-assisted templating route. This approach makes use of covalent-bonding interaction between the surfactant head group and metal alkoxide precursor, as opposed to electrostatic or hydrogen-bonding interactions. We have recently derived thermally stable mesoporous phosphated zirconia called Zr-TMS, with controlled mesopores (15-26 Å) and surface areas (230-360 m2/g) greater than those of conventional zirconia materials. Zr-TMS has been found more catalytically active than MCM-41 in gas-phase (butene isomerization) and liquid-phase (acetalization of aldehydes) reactions. We have also generated high surface area mesoporous and microporous silicates containing up to an unprecedented 20 wt% zirconium. The high dispersion of Zr throughout the porous silicate framework makes this series of materials highly active for hydrocarbon conversion and partial oxidation reactions.
Yet another approach undertaken by our laboratory to achieve mesoporous acid catalysts is to construct the inorganic framework using nanoparticles as building blocks. A deficiency in non-silicate mesoporous materials has been the relatively low thermal stability, which severely limits their applicability as catalysts. The root of the problem lies with the amorphousness of mesoporous materals, e.g. amorphous transitions metal oxides tend to crystallize and sinter below 450°C. The introduction of nanocrystallinity into the framework was found to solve this problem. WM-TMS14 materials (where M is the metal oxide, such as Zr, Ti and Al) represent the successful incorporation of nanoparticles into a well-defined mesoporous framework. Such materials have acidic functionality arising from the WO3 species on the surface. They demonstrate superb catalytic activity and coking resistance in acid catalyzed reactions, such as isomerization and cracking of hydrocarbons.
10) Palladium-Grafted Mesoporous Materials for Heterogeneous
(C. P. Mehnert, J. S. Lettow, D. W. Weaver, and J.Y. Ying)
This research focuses on the synthesis and application of modified mesoporous MCM-41 materials that have active species attached to the framework via host-guest interactions, creating discrete and uniform catalyst sites on the inner walls of the porous systems. We have synthesized palladium-grafted mesoporous materials, designated Pd-TMS11, and investigated their application as heterogeneous catalysts in Heck reactions. The palladium catalyzed carbon-carbon bond-forming Heck reaction represents one of the most versatile tools in modern synthetic chemistry and has a great potential for industrial applications. Although recent advances in homogeneous and heterogeneous Heck catalysis have attracted considerable attention, the properties and activities of the existing catalysts have, as yet, limited industrial applications.
Pd-TMS11 is synthesized by vapor deposition of a volatile palladium complex onto the inner walls of the porous framework, followed by reduction. The success of grafting a highly dispersed metal film within the cylindrical pores is strongly dependent on the properties of the volatile organometallic complex and the reaction conditions. To maintain the uniform structure, high surface area and accessibility of the mesoporous materials, it is important to minimize the cluster growth inside and outside the porous material. The concept of vapor grafting enables a uniform distribution of volatile complex and ensures the discrete deposition of metal fragments throughout the porous structure of the material without cluster formation. The Pd-grafted molecular sieves thus obtained have been found to be an excellent catalyst for the Heck carbon-carbon coupling reaction of activated or non-activated aryl halides with butyl acrylate or styrene as the vinylic substrate. With its remarkable activity, ease of recovery and exceptional stability, Pd-TMS11 represents a new generation of heterogeneous Heck catalysts that rivals or even surpasses some of the best homogeneous Heck catalysts.
11) Organometallic Catalysts Immobilized on Novel Silica
Supports for the Heterogeneous Catalysis of Sterically Demanding
(J. S. Lettow, D. Huang, T. M. Lancaster, B. J. Schultz, C. P. Mehnert, and J. Y. Ying)
Two significant areas of research in recent years have been the development of mesoporous materials, such as MCM-41, and the design of organometallic complexes for use as homogeneous catalysts in a wide variety of fine chemical syntheses. We have combined these two areas of research to create composite heterogeneous catalysts that show excellent activity for a number of industrially important reactions.
The supports for our catalysts are mesoporous silica structures. While mesoporous materials like MCM-41, which consists of an hexagonal array of cylindrical pores with pore diameters of 2-4 nm, have been studied in depth over the past eight years, the small pore dimensions of such materials limit their utility for immobilizing bulky metal complexes that react with large substrates. Hence, we have used recently discovered polymer-templated mesoporous silicas that have pore diameters of 5-35 nm as our catalyst supports. Furthermore, through the use of non-polar additives, we have developed several new high surface area materials with open-pore, foam-like structures that are well suited for catalyzing sterically demanding reactions.
The first step toward converting the mesoporous silicas into a highly specific and active catalyst is to introduce an organic anchor group to the silica surface. To this first organic unit, a number of metal-chelating groups can be attached. Transition metal complexes are then fixated to the surface immobilized ligands, resulting in novel heterogenized catalysts. The nanocomposite catalysts retain the excellent catalytic activity and selectivity associated with the tailored organometallic complexes, without the diffusion limitations imposed by many traditional porous supports. The heterogeneous catalysts further provide superior stability and ease of separation compared to homogeneous catalysts, making them highly attractive for applications in fine chemicals and pharmaceuticals synthesis.
12) Matrix-Mediated Synthesis of Superparamagnetic Iron
Oxide Clusters and Supported Iron Porphyrin Oxidation Catalysts
(L. Zhang, J. Y. Ying, and G. C. Papaefthymiou)
Matrix-mediated approach is developed as a means to derive unique magnetic nanoclusters and active supported metalloporphyrin catalysts in this project. Nanoporous silicate-based matrices generated by modified sol-gel processing are used to stably host clusters and complexes through designed guest--host interactions. The matrix microstructure and surface chemistry are further tailored to control the size, morphology, dispersion and interfacial characteristics of the guest species. Iron oxide nanoclusters have been successfully synthesized and stabilized within three different matrices: (i) a sol-gel derived sulfonated silica matrix, (ii) an inert silica coating, and (iii) a hexagonally -packed mesoporous MCM-41 aluminosilicate matrix. Magnetization studies and Mössbauer spectroscopy have indicated that the three types of iron oxide/silica nanocomposites exhibit interesting superparamagnetic behavior. The surface magnetic structure of the g-Fe2O3 clusters and the strain effects imposed by the silica matrix play a critical role in determining the overall magnetic and optical characteristics of these nanocomposites. The coercivities and superparamagnetic barrier energies can be manipulated through modification of the host microstructure and the cluster/matrix interface. Tailoring of the magnetic properties of these novel systems is possible through the control of matrix structure and cluster synthesis conditions.
Matrix-mediated synthesis is also used to derive novel supported catalysts of organometallic complexes, such as metalloporphyrins, which offer unique chemoselectivity and excellent oxidation activity under mild reaction conditions. The development of an effective heterogeneous system that supports these complexes would prevent self-oxidation of the active centers and allow for facile recovery of the catalyst. A design strategy has been developed in this project to encapsulate the large metalloporphyrin complexes within the pore structure of mesoporous MCM-41 silicates doped with Al or Nb. In the former, Al doping provides more surface hydroxyl groups for interaction between the matrix and the Fe center of phthalocyanine complexes. In the latter, covalent bonding is possible between the Nb dopants on the support pore walls and the amine terminal groups of Fe(III)meso-(tetra-aminophenyl)porphyrin bromide. This second approach is particularly effective at immobilizing metalloporphyrins within the porous matrix, preventing catalyst leaching during the oxidation reactions. The supported metalloporphyrin catalysts have demonstrated excellent catalytic activity and selectivity for cyclohexene and cyclooctene epoxidation and cyclohexane hydroxylation at ambient conditions. By controlling the structural characteristics of the mesoporous matrix materials, such as the nature and concentration of dopant, pore size and surface area, the local catalytic environment for hosting metalloporphyrins and the diffusion characteristics of reactants and products are successfully tailored to optimize the catalyst performance.
13) Fabrication, Characterization and Transport Properties
of Bismuth Nanowire Systems
(Y.-M. Lin, Z. Zhang, J. Y. Ying, and M. S. Dresselhaus)
Bismuth (Bi) is an attractive material for the study of low-dimensional systems and a promising candidate for thermoelectric applications (e.g. refrigeration and power generation) because it has a very small electron effective mass and a highly anisotropic Fermi surface. However, bulk bismuth is a semimetal with equal numbers of electrons and holes. Therefore, the contributions to the Seebeck coefficient (thermoelectric power) of both carriers cancel each other, resulting in a low thermoelectric efficiency, although each carrier (electrons or holes) can have a large Seebeck coefficient. We have investigated the improvement of the thermoelectric performance of bismuth by confining the carriers in quasi-one-dimensional quantum wires. A significant enhancement of the thermoelectric figure of merit is predicted for Bi nanowires of diameters smaller than 10 nm at 77 K.
Bi nanowires have been produced in a dielectric matrix using a template-assisted fabrication process. Anodic alumina, which has a hexagonally packed array of parallel nanometer-sized channels, is used as the template. By controlling the processing conditions (e.g. nature and concentration of electrolyte, temperature, voltage) for aluminum substrate anodization, we can systematically vary the channel diameters (7-120 nm), channel packing densities (as high as 7x1010/cm2), and channel lengths (2-100 mm) in the resulting anodic alumina template. Bi nanowire arrays are then fabricated by pressure injection of the bismuth liquid melt into this porous template. We have developed a wetting process to reduce the contact angle between liquid bismuth and the anodic alumina to facilitate the filling process. With this novel technique, pure and Te-doped Bi nanowire arrays with wire diameters as small as 10 nm have been achieved. The large bandgap of anodic alumina confines the carriers within the embedded nanowires. We have also developed an etching process that dissolves the anodic alumina template selectively, yielding freestanding Bi nanowires. The nanowires produced by this process are found to be highly crystalline, with similar crystal structure and lattice parameters as bulk bismuth. They are of uniform diameter along the length of the wires, and share a common crystalline orientation along their wire axes.
A theoretical model has been developed to describe the electronic structure and the transport properties of these unique one-dimensional systems, taking into consideration the circular boundary condition and the anisotropic nature of Bi. Various transport measurements, such as resistance, magneto-resistance, Seebeck measurements, have been designed and performed over a wide range of temperatures (2-300 K) for Bi nanowire arrays with different diameters. These results, along with optical transmission measurements, provide evidence for a semimetal-to-semiconductor transition that has been predicted in our theoretical calculations for Bi nanowires with diameters smaller than 55 nm, 40 nm and 49 nm in the trigonal, binary and bisectrix directions, respectively. To obtain the absolute resistivity value, a four-probe measurement on a single nanowire has been developed by using electron beam lithographic patterning technique. We have also investigated the effect of Te doping by resistance and Seebeck measurements, and the results agree well with the theoretical model.
14) Nanostructured Gas Sensor Materials
(J. T. McCue, R. E. Rogers, Jr., and J. Y. Ying)
The use of gas sensors for detecting chemical species is important for numerous industrial and consumer processes. It has led to dramatic improvements in process control; the market size for gas sensors is expected to further increase with increasing use of automation. Current gas sensors suffer from poor long-term stability, response reproducibility and response time, and do not distinguish among different gases. The objective of this project is to develop nanostructured gas sensor systems that will address these issues.
Specifically, two classes of highly stable and selective gas sensors are being developed. The first involves anodic alumina thin films as a platform for microcalorimetric sensing. Active components, such as Pd, are vapor-grafted with ultrahigh dispersion within the nanoporous channels of anodic alumina. The resulting materials provide for the detection of combustible gases via a catalytic oxidation mechanism. We have varied the film microstructure and Pd loading of these devices to achieve superb response and high selectivity towards CO and CH4.
We have also developed novel semiconducting oxide nanocomposites as gas sensor systems. These materials exhibit superb thermal stability, resistance to humidity, and sensitivity to ppm levels of CO and NOx. By controlling the nanocomposite composition and dopant introduction, semiconducting sensors with outstanding selectivity for CO and NOx have been successfully attained.
15) Nanocrystalline Metal Oxides for the Remediation of
(J. T. Sweeney, R. I. Rubí, and J. Y. Ying)
Sulfur-containing compounds represent an environmental hazard and a poison for many industrial catalysts. The major contributors to these problems are SO2 and H2S, respectively. Current technologies for H2S removal are both energy- and equipment-intensive, leading to high operating costs and significant space requirements. We are designing novel nanocrystalline absorbents that address these issues. Aided by thermodynamic data and experimental analysis, simple oxides have been examined in the preliminary H2S absorption analyses. Current studies involve nanostructured mixed oxides, which offer superior thermal stability, absorption capacity and regenerability.
With environmental regulations becoming more stringent, the impetus towards developing a lean-burn engine for automotive applications has increased. However, the absorbent system necessary to ensure NOx requirements are met is readily poisoned by the SO2 present in the vehicle exhaust. We are developing an effective SO2 absorbent that protects the NOx storage system. Focus is placed on nanocrystalline mixed oxides with a high degree of non-stoichiometry. These materials presents efficient SO2 absorption at low temperatures, and provides for rapid regeneration under the appropriate atmospheres.
16) Nanostructured Palladium-based Membrane Materials
(K. J. Bryden and J. Y. Ying)
Palladium-based membranes are of technological interest as they possess a very high permselectivity for hydrogen. Diffusion through the palladium is often the rate-limiting step in hydrogen separation. The flux through these membranes can be increased by tailoring a microstructure that allows for higher hydrogen diffusivity. Nanostructured palladium has a much higher hydrogen diffusivity than conventional palladium due to its large volume fraction of grain boundaries, thus providing greater hydrogen fluxes and better performance. By doping palladium with another element, enhanced stability against grain growth, suppression of the a-to-b phase transition that causes membrane cracking, and improved poisoning resistance can be achieved.
We have produced nanostructured Pd-based alloy membranes by pulsed electrodeposition. This unique method of making nanostructured materials allows for the flexible tailoring of the elemental composition and microstructure of metal thin films. By controlling precursor concentrations, additives, pH, temperature, current density and pulse times, nucleation of new crystals can be favored over growth of existing crystals, resulting in films with ultrafine grain size. We have synthesized palladium--iron films that are stable against grain growth to 400°C and do not experience the a-to-b phase transition. The palladium-iron nanoalloys are also found to exhibit significantly improved surface properties compared to conventional materials. They do not require activation, and they recover readily from hydrogen sulfide poisoning. Besides hydrogen separation, these nanoalloy films are successfully applied in membrane-assisted hydrogenation and dehydrogenation reactions.
17) Nanocrystalline Oxide Membranes for Oxygen Separation
and Partial Oxidation Reactions
(N. Sangar, J. Cui, R. Chakravorty, and J. Y. Ying)
Separation of oxygen from air using oxide membranes is a promising alternative to cryogenic distillation. These ceramic membranes are non-porous, solid electrolytes that transport oxygen as oxide ions. The key challenge is to develop membrane materials with high oxide ion conductivity at relatively low temperatures (600-800°C). In addition, these membranes should have high electronic conductivity to preclude the necessity of external electrodes. Nanocrystalline oxides have the potential for very high oxide ion conductivities due to an increased concentration of high-diffusivity paths (grain boundaries) and a higher concentration of oxygen vacancies.
We have successfully synthesized various nanocrystalline multicomponent oxides by chemical techniques, such as chemical precipitation, reverse micelle-mediated processing, combustion synthesis, and freeze drying. The synthesis conditions have been carefully controlled to achieve nanometer-sized grains, high surface areas, compositional homogeneity, and phase purity. We are currently examining the effects of composition and grain size on the mixed conductivity of these oxides.
Besides oxygen separation, these membranes are being investigated for membrane-assisted oxidation of methane. An oxide conducting membrane is used to feed oxygen and methane separately, providing a controlled flux of oxygen ions to a catalyst surface where the ions may react with the activated methane molecules. The advantage of such membrane-assisted catalysis is that non-selective gas-phase and surface reactions can be minimized by appropriately matching the catalyst activity with the membrane conductivity. The potential of these membranes for selective partial oxidation of methane is being investigated.
18) The Reactivity of Metastable Mixed Oxides Synthesized
from Layered Double Hydroxides
(D. Levin and J. Y. Ying)
Layered double hydroxides (LDH) containing a volatile interlayer anion can be thermally decomposed by dehydration, dehydroxylation, and loss of the interlayer anion to produce metastable mixed oxide phases at temperatures below 600°C. These metastable phases undergo reaction in aqueous media to result in: (i) the reconstitution of the original LDH structure, or (ii) the intercalation of large ions (pillaring), or (iii) a chimie douce reaction generating new crystalline phases. The existence of these three different reaction pathways has been shown by us to be dependent on the composition of the mixed oxide phase, as well as the solution components. The LDH-precursor processing approach flexibly allows us to generate materials with (i) unique layered structures, (ii) designed interlayer surface adsorption sites, (iii) homogeneous doping of a large selection of elements, and (iv) specified pore sizes. An understanding of the chemistry of this synthesis is applied by us to generate new catalytic materials from the metastable oxide phases derived. Control of the processing parameters enables us to tailor the structural outcome of the material, thereby creating unique catalytic reactivity and selectivity for a wide selection of mixed divalent-trivalent metal oxides. For example, we have created a class of layered ammonium transition-metal molybdates for use as precursors to non-stoichiometric transition-metal molybdates. These materials are catalytically active for the oxidative dehydrogenation of lower alkanes to alkenes. Compositional tuning of these precursors has enabled the maximization of catalytic activity by the synthesis of a precursor having the lowest possible transition-metal to molybdenum ratio. Knowledge of the structure-property relationship of these materials enhances our ability to design new catalysts by molecular-level engineering.
19) Synthesis and Combustion Chemistry of Water-Fuel Microemulsions
(A. J. Zarur, N. Z. Mehenti, and J. Y. Ying)
Emulsified mixtures of water and organic fuels can be combusted in regular internal combustion engines to reduce pollutant emissions. These microemulsions are prepared by mixing a package of surfactants and stabilizers with water and an organic phase. The water aggregates into discrete nanometer-sized micelles within the continuous organic phase. By varying the nature and concentration of surfactants, as well as the water/fuel ratios, micelles of 10-1000 nm in size have been obtained. We are interested in the microstructure and chemistry of these reverse microemulsions and their effects on the phase stability and combustion behavior of the aqueous fuels. Typically, combustion occurs at a lower temperature for water-fuel mixtures compared to pure gasoline and diesel fuels. This substantially decreases the amount of NOx and particulate (soot) emissions. The thermodynamic, transport and chemical kinetics implications of the presence of water in fuel combustion are being examined. These studies would provide us with a better understanding of the aqueous fuel systems, and help us in optimizing their fuel efficiency and engine operation.