Alan W. Weimer

Efficient generation of H2 by splitting water with an isothermal redox cycle.

Isothermal Water Splitting Solar concentrators can create extremely high temperatures that can drive chemical reactions, including the thermal splitting of water to provide hydrogen. A metal oxide catalyst is needed that is usually cycled between hotter conditions where it is reduced and cooler conditions where it is reoxidized by water. This cycling can limit catalyst lifetime, which can be costly. Muhich et al. (p. 540; see the Perspective by Roeb and Sattler) developed an approach that allowed the redox cycle to be driven isothermally, using pressure swings. A thermal process for generating H2 from water uses pressure changes to recycle between catalyst redox states. [Also see Perspective by Roeb and Sattler] Solar thermal water-splitting (STWS) cycles have long been recognized as a desirable means of generating hydrogen gas (H2) from water and sunlight. Two-step, metal oxide–based STWS cycles generate H2 by sequential high-temperature reduction and water reoxidation of a metal oxide. The temperature swings between reduction and oxidation steps long thought necessary for STWS have stifled STWS’s overall efficiency because of thermal and time losses that occur during the frequent heating and cooling of the metal oxide. We show that these temperature swings are unnecessary and that isothermal water splitting (ITWS) at 1350°C using the “hercynite cycle” exhibits H2 production capacity >3 and >12 times that of hercynite and ceria, respectively, per mass of active material when reduced at 1350°C and reoxidized at 1000°C.

Atomic layer deposition of ultrathin and conformal Al2O3 films on BN particles

Abstract Ultrathin and conformal Al2O3 films were deposited on BN particles using alternating exposures of Al(CH3)3 and H2O. Transmission Fourier transform infrared spectroscopy performed in vacuum on high surface area BN particles was used to monitor the surface chemistry during the sequential exposures. The initial vibrational modes were consistent with BOH* and BNH2* surface species on the BN particles. These species were converted to AlCH3* species during Al(CH3)3 exposure. Subsequently, H2O exposure was used to convert the AlCH3* species into AlOH* species. Alternating Al(CH3)3 and H2O exposures yielded AlCH3* and AlOH* species, respectively, that sequentially deposited aluminum and oxygen with atomic layer control. The repetition of the Al(CH3)3 and H2O exposures in an ABAB… reaction sequence led to the appearance of bulk Al2O3 vibrational modes. The intensity of these bulk vibrational modes increased with the number of AB reaction cycles. Following Al2O3 deposition, the BN particles were also examined with transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The TEM studies revealed extremely uniform and conformal Al2O3 films on the BN particles with a thickness of ∼90 A after 50 AB reaction cycles. The absence of observable B and N photoelectron signals during XPS analysis was consistent with a continuous and conformal Al2O3 coating. These ultrathin Al2O3 films should help to increase BN particle loading in composite materials for thermal management applications without degrading the high thermal conductivity of the BN particles.

Solar-driven gasification of carbonaceous feedstock—a review

Given the future importance of solid carbonaceous feedstocks such as coal, coke, biomass, bitumen, and carbon-containing wastes for the power and chemical industries, gasification technologies for their thermochemical conversion into fluid fuels are developing rapidly. Solar-driven gasification, in which concentrated solar radiation is supplied as the energy source of high-temperature process heat to the endothermic reactions, offers an attractive alternative to conventional autothermal processes. It has the potential to produce high-quality synthesis gas with higher output per unit of feedstock and lower specific CO2 emissions, as the calorific value of the feedstock is upgraded through the solar energy input by an amount equal to the enthalpy change of the reaction. The elimination of an air separation unit further facilitates economic competitiveness. Ultimately, solar-driven gasification is an efficient means of storing intermittent solar energy in a transportable and dispatchable chemical form. This review article develops some of the underlying science, examines the thermodynamics and kinetics of the pertinent reactions, and describes the latest advances in solar thermochemical reactor technology.

ALD of SiO2 at Room Temperature Using TEOS and H 2 O with NH 3 as the Catalyst

Amine catalysts can reduce the high temperatures and long exposure times required for SiO 2 atomic layer deposition (ALD) using SiCl 4 and H 2 O reactants. One problem is that the reaction product, HCI, readily reacts with the amine catalysts to form a salt. Salt formation can be avoided by using organometallic silicon precursors. This study investigated catalyzed SiO 2 ALD on BaTiO 3 and ZrO 2 particles using alternating exposures of tetraethoxysilane (TEOS) and H 2 O at 300 K with NH3 as the catalyst. The sequential surface chemistry was monitored in a vacuum chamber using in situ transmission Fourier transform infrared (FTIR) spectroscopy. Alternating TEOS/NH 3 and H 2 O/NH 3 exposures yielded Si(OCH 2 CH 3 ) x * and SiOH* surface species, respectively, that sequentially deposited silicon and oxygen. Repetition of the TEOS and H 2 O exposures in an ABAB... reaction sequence led to the appearance of bulk SiO 2 vibrational modes. The infrared absorbance of these bulk SiO 2 vibrational modes increased with the number of AB reaction cycles. After SiO 2 deposition, the BaTiO 3 and ZrO 2 particles were examined using transmission electron microscopy (TEM). The TEM images revealed extremely uniform and conformal SiO 2 films. The measured SiO 2 film thicknesses were consistent with SiO 2 ALD growth rates of 0.7-0.8 A per AB reaction cycle. The NH 3 catalysis mechanism was also explored by monitoring the FTIR spectra of hydroxylated SiO 2 particles vs. NH 3 pressure at constant temperature and vs. temperature at constant NH 3 pressure. The spectra revealed strong hydrogen bonding between NH 3 and SiOH* surface species that activates the oxygen in SiOH* for nucleophilic attack. Catalyzed SiO 2 at room temperature should be useful for deposition of inorganic and insulating films on thermally fragile organic, polymeric, or biological substrates.

Rotary reactor for atomic layer deposition on large quantities of nanoparticles

Challenges are encountered during atomic layer deposition (ALD) on large quantities of nanoparticles. The particles must be agitated or fluidized to perform the ALD surface reactions in reasonable times and to prevent the particles from being agglomerated by the ALD film. The high surface area of nanoparticles also demands efficient reactant usage because large quantities of reactant are required for the surface reactions to reach completion. The residence time of the reactant in a fluidized particle bed reactor may be too short for high efficiency if the ALD surface reactions have low reactive sticking coefficients. To address these challenges, a novel rotary reactor was developed to achieve constant particle agitation during static ALD reactant exposures. In the design of this new reactor, a cylindrical drum with porous metal walls was positioned inside a vacuum chamber. The porous cylindrical drum was rotated by a magnetically coupled rotary feedthrough. By rotating the cylindrical drum to obtain a cen...

Rapid High Temperature Solar Thermal Biomass Gasification in a Prototype Cavity Reactor

High temperature biomass gasification has been performed in a prototype concentrated solar reactor. Gasification of biomass at high temperatures has many advantages compared with historical methods of producing fuels. Enhancements in overall conversion, product composition ratios, and tar reduction are achievable at temperatures greater than 1000°C. Furthermore, the utilization of concentrated solar energy to drive these reactions eliminates the need to consume a portion of the product stream for heating and some of the solar energy is stored as chemical energy in the product stream. Experiments to determine the effects of temperature, gas flow rate, and feed type were conducted at the high flux solar furnace at the National Renewable Energy Laboratory, Golden, CO. These experiments were conducted in a reflective cavity multitube prototype reactor. Biomass type was found to be the only significant factor within a 95% confidence interval. Biomass conversion as high as 68% was achieved on sun. Construction and design considerations of the prototype reactor are discussed as well as initial performance results.

Atomic layer deposition on gram quantities of multi-walled carbon nanotubes

Atomic layer deposition (ALD) was employed to grow coaxial thin films of Al(2)O(3) and Al(2)O(3) /W bilayers on multi-walled carbon nanotubes (MWCNTs). Although the MWCNTs have an extremely high surface area, a rotary ALD reactor was successfully employed to perform ALD on gram quantities of MWCNTs. The uncoated and ALD-coated MWCNTs were characterized with transmission electron microscopy and x-ray photoelectron spectroscopy. Al(2)O(3) ALD on untreated MWCNTs was characterized by nucleation difficulties that resulted in the growth of isolated Al(2)O(3) nanospheres on the MWCNT surface. The formation of a physisorbed NO(2) monolayer provided an adhesion layer for the nucleation and growth of Al(2)O(3) ALD films. The NO(2) monolayer facilitated the growth of extremely conformal coaxial Al(2)O(3) ALD coatings on the MWCNTs. Cracks were also observed in the coaxial Al(2)O(3) ALD films on the MWCNTs. After cracking, the coaxial Al(2)O(3) ALD films were observed to slide on the surface of the MWCNTs and expose regions of bare MWCNTs. The Al(2)O(3) ALD film also served as a seed layer for the growth of W ALD on the MWCNTs. The W ALD films can significantly reduce the resistance of the W/Al(2)O(3)/MWCNT wire. The results demonstrate the potential for ALD films to tune the properties of gram quantities of very high surface area MWCNTs.

Vibro-fluidization of fine boron nitride powder at low pressure

At low pressure (less than atmospheric), a vibro-fluidized bed was constructed for the fluidization of fine BN powder. This powder exhibits the characteristics of a Geldart C type powder, with significant cohesive behavior. Fine BN particles form lightly bonded agglomerates that remain in the bed during fluidization. The agglomerate size was measured in situ during fluidization using a pulsed laser coupled with a CCD camera. An increase in the vibrational force causes a corresponding decrease in the size of the agglomerates. Vacuum level does not affect agglomerate size. A balance of forces model is used to derive an equation that predicts the minimum fluidization velocity (umf), taking into account the forces due to vibration, cohesion, gravity and drag.

Rapid solar-thermal dissociation of natural gas in an aerosol flow reactor

A solar-thermal aerosol flow reactor process is being developed to dissociate natural gas (NG) to hy drogen (H2) and carbon black at high rates. Concentrated sunlight approaching 10 kW heats a 9.4 cm long×2.4 cm diameter graphite reaction tube to temperatures ~2000 K using a 74% theoretically efficient secondary concentrator. Pure methane feed has been dissociated to 70% for residence times less than 0.1 s. The resulting carbon black is 20–40 nm in size, amorphous, and pure. A 5 million (M) kg/yr carbon black/1.67 M kg/yr H2 plant is considered for process scale-up. The total permanent investment (TPI) of this plant is

Atomic layer deposition of boron nitride using sequential exposures of BCl3 and NH3

12.7 M. A 15% IRR after tax is achieved when the carbon black is sold for