Designing and Probing Photovoltaic and Photocatalytic Materials

Meeting Program – November 2012

 
Jason B. Baxter
Department of Chemical and Biological Engineering
Drexel University
Philadelphia, PA

 
Abstract – The sunlight incident on the earth provides 10,000 times more power than is needed to meet global demand. However, converting this energy into electricity or fuels efficiently and cost effectively remains a great challenge. Nanostructured solar cells present opportunities to inexpensively convert sunlight to electricity through the use of architectures tailored on the nanometer to micrometer length scale. Planar solar cells are subject to opposing constraints where thick films are required for light absorption while thinner films are desirable for efficient charge separation. Extremely thin absorber (ETA) solar cells can decouple these constraints by using a thin absorber at the interface between highly structured p- and n-type layers. In this talk, I will describe our work on ETA solar cells that use a thin CdSe coating on a ZnO nanowire array to absorb light and inject electrons into the oxide. Rational design of these architectures requires control over morphology and microstructure of the materials, as well as knowledge of material properties such as photoexcited carrier lifetimes and mobilities. Our approach utilizes a combination of solar cell measurements and ultrafast transient absorption spectroscopy to understand the effects of CdSe thickness, annealing conditions, and interfacial treatments on the dynamics and efficiency of charge carrier separation, and ultimately on the solar-to-electric energy conversion efficiency. These studies provide guidelines for architecture design and materials selection for ETA solar cells.
 

Jason B. Baxter

Jason B. Baxter

Biography – Dr. Jason B. Baxter is an Assistant Professor in the Department of Chemical and Biological Engineering at Drexel University in Philadelphia, PA, where he began in 2007. He received his B.Ch.E. from the University of Delaware in 2000, where he did undergraduate research on dye sensitized solar cells at the Institute of Energy Conversion under the guidance of Prof. T.W. Fraser Russell. He earned his Ph.D. in chemical engineering from the University of California Santa Barbara in 2005. Advised by Prof. Eray S. Aydil and funded by an NSF Graduate Research Fellowship, he investigated growth and characterization of ZnO nanowires and their application in dye sensitized solar cells. From 2005-2007, Dr. Baxter was an ACS Petroleum Research Fund Alternative Energy Postdoctoral Fellow at Yale University. There he worked with Prof. Charles A. Schmuttenmaer in the Chemistry Department on the application of time-resolved terahertz spectroscopy to probe transient photoconductivity in oxide thin films, nanoparticles, nanowires, and bulk crystals.

Dr. Baxter’s current research interests are in designing, fabricating, and probing semiconductor nanomaterials and thin films for solar energy conversion. Most current efforts focus on solar-to-electric energy conversion, but the group has growing interest in photocatalytic water splitting for clean and renewable hydrogen production. Various projects in the group include extremely thin absorber solar cells, organic solar cells, microreactor deposition of graded thin films for high-throughput characterization, and ultrafast pump-probe spectroscopy to measure charge carrier dynamics. The general focus of the group is on striving to understand how materials and interfaces affect device performance, and how these materials and interfaces can be controlled during the fabrication process. Low-temperature solution processing methods are used whenever possible to provide a pathway to low-cost, scalable manufacturing.
Dr. Baxter advises a group of 4 PhD students, 2 MS students, and 8 BS students. He has published nearly 25 papers, which have collectively garnered well over 1000 citations. He has been awarded over $1 million in funding as lead investigator and another $3 million as co-investigator. He received the NSF CAREER Award in 2009.

Pervasiveness of Surface Metal Oxide Phases In Mixed Oxide Catalysts

Meeting Program – October 2012

 
Israel E. Wachs
Operando Molecular Spectroscopy & Catalysis Laboratory
Department of Chemical Engineering
Lehigh University
Bethlehem, PA 18015 USA

 
Abstract – Mixed oxide catalytic materials possess two or more metal oxide components as found in bulk mixed metal oxides (stoichiometric oxides as well as solid solutions), polyoxo metalates (POMs), molecular sieves, zeolites, clays, hydrotalcites and supported metal oxides. Although it is now well established that two-dimensional surface metal oxide phases are present for supported metal oxides on traditional supports (e.g., Al2O3, TiO2, ZrO2, SiO2, etc.), it is not currently appreciated that such surface metal oxide species or phases are also present for other types of mixed oxides. For example, recent surface analyses have demonstrated that stoichiometric bulk mixed metal oxides also possess surface metal oxide phases that control their catalytic activity. For example, the catalytic active sites for methanol oxidation to formaldehyde over the bulk Fe2(MoO4)3 mixed oxide catalyst are surface MoOx species and not the bulk Fe2(MoO4)3 phase as previously thought in the catalysis literature. The nanometer sized clusters in POMs also possess surface species when a second metal oxide component is introduced (e.g., H3+xPW12-xMxO40). Deposition of metal oxides into molecular sieves, zeolites, clays and hydrotalcites also results in the metal oxide additive usually being present as surface metal oxide species that are the catalytic active sites for many redox and acid reactions. The formation of these surface metal oxide phases is driven by their low surface free energy and low Tammann temperature for many metal oxides of interest in catalysis (e.g., VOx, MoOx, CrOx, ReOx, WOx, etc.).
 
Biography – Israel E. Wachs received his undergraduate education at The City College of The City University of New York where he graduated with a B.E. (ChE) in June, 1973. He received several recognitions upon graduation (AIChE Award for Outstanding Senior, Heller Memorial Award for Outstanding Scholastic Achievement, and White ChE Alumni Award). He continued his graduate ChE education at Stanford University under the mentorship of Professor Robert J. Madix in the area of surface science, and graduated with a PhD (ChE) in 1978. His research findings are considered the first application of surface science to catalysis, and his thesis publications are extensively cited in the surface science and catalysis literature.
 
INDUSTRIAL YEARS (1977-1986) – Israel joined Exxon Research & Engineering Company in their Corporate Research Labs towards the end of 1977. At Exxon, he was involved with many different catalytic technologies over the years (selective oxidation, acid catalysis, synthesis of synthetic fuels, hydrodesulfurization (HDS) and hydrocarbon conversion). He obtained 100 USA and international patents during his industrial career. One of his inventions on the selective oxidation of o-xylene to phthalic anhydride became the leading international industrial catalyst for this technology and is still used around the world. At Exxon, he received the Research Incentive Award for one of his inventions on the synthesis of synthetic fuels and was also selected to be an Exxon Fellow for the spring semester of 1986 at California Institute of Technology (CalTech). He departed for academia at the end of 1986.
 
ACADEMIC YEARS (1987-present) – He joined the Chemical Engineering Department of Lehigh University in January 1987. At Lehigh, he taught many different courses over the years: Heterogeneous Catalysis, Reactor Engineering, Fluid Mechanics, Professional Development, Unit Operations, Environmental Catalysis, and Air Pollution Control. He set up a world-class catalysis research laboratory focusing on mixed metal oxide catalytic materials and their characterization under reaction conditions (in situ and operando spectroscopy). These studies have established the foundation for the molecular/electronic structure – activity/selectivity relationships and the molecular engineering of mixed metal oxide catalysts. The research performed by Wachs and his students is well known around the world. This is reflected in the many national and international honors he has received over the years as well as ~17,000 citations to his publications with an H-index of 70 (one of the highest among heterogeneous catalysis researchers).

The current focus of Wachs’ catalysis laboratory is to develop catalyst characterization techniques under reaction conditions, referred to as operando spectroscopy in the recent literature. The term operando spectroscopy implies that the catalyst characterization information is being conducted simultaneously with online product analysis. Along these lines, Professor Wachs has developed instrumentation that can simultaneously obtain Raman, IR and UV-vis spectroscopic information and product analysis with an online mass spectrometer/GC system. This cutting-edge instrument is allowing Professor Wachs’ catalysis research group to rapidly develop molecular/electronic structure – catalytic activity/selectivity relationships for many different catalytic materials and reactions (selective hydrocarbon oxidation, hydrocarbon conversion with solid acid catalysts, gas-to-liquids, photocatalytic splitting of water, enzyme catalysis, CO2 capture, WGS, nanocatalysis, rational catalyst design, etc.).

Water Gas Shift over Industrial Cu Catalysts: A Mechanistic and Microkinetic Investigation

Meetimg Program – September 2012

 
Rostam J. Madon
BASF Corporation
25 Middlesex/Essex Turnpike
Iselin, NJ, USA 08830
rostam.madon@basf.com

 
Abstract – Low temperature water gas shift (LTS) is a commercially important reaction that takes place over a Cu-ZnO-Al2O3 catalyst. A large number of fundamental studies have been carried out for this reaction including investigations of the reaction mechanism as typified by Refs. [1-4]. In short, discussions have centered around (a) the redox mechanism in which adsorbed H2O is dissociated to O* and OH* and the O* is removed via CO* to form CO2 – where * is an active site, and (b) formate as a reactive intermediate. Recently, Gokhale et al. [5] using a DFT investigation of the LTS reaction on Cu(111) proposed a new mechanism that involves a reactive surface carboxyl. Our study is aimed at resolving which elementary steps best describe the catalytic cycle for the LTS reaction. To achieve this, we used the microkinetic modeling methodology pioneered by Dumesic [6], and analyzed our reactivity data using all elementary steps, including those that described the redox mechanism, the formate mechanism, and the carboxyl mechanism. Thus, we ensured that there was no bias towards any particular reactions to fit our data. We found the closed catalytic cycle for LTS on Cu consists of eight elementary steps that include the formation of COOH*, and its reaction with OH* to form CO2* and H2O*. The cycle does not include the reaction of CO2* and H* to form surface formate. However, this is an important side reaction, which ensures significant coverage of bidentate formate species on the Cu surface. Bidentate formate is a spectator species whose coverage increases with increasing pressure and decreases with increasing temperature. In summary, our investigation demonstrates that the redox and formate mechanisms are not relevant, and that the LTS catalytic cycle involves the formation and reaction of surface carboxyl. Several related aspects of the LTS reaction on Cu will also be discussed.
 

References

  1. Ovesen, C. V., et al. J. Catal. 158, (1996), 170.
  2. Koryabkina, N. A. et al. J. Catal. 217, (2003), 233.
  3. Rhodes, C., Hutchings G.J., and Ward A.M. Catal. Today 23, (1995), 43.
  4. Herwijnen, T.V., and de Jong, W. A. J. Catal. 63, (1980), 83 and 94.
  5. Gokhale, A. A., Dumesic, J. A., and Mavrikakis, M. J. Am. Chem. Soc. 130, (2008), 1402.
  6. Dumesic, J. A., et al. “The Microkinetics of Heterogeneous Catalysis”, American Chemical Society, Washington, D. C., 1993.

 
Speaker Bio – Ross completed his undergraduate studies in chemical engineering at the University Department of Chemical Technology, Mumbai, India. He did his graduate work at Stanford University, obtaining his Ph.D. under the guidance of Professor Michel Boudart. After completing his post-doctoral work with Professor W. Keith Hall at the University of Wisconsin – Milwaukee, Ross joined Exxon Research and Engineering Company. After 12 years with Exxon’s Corporate Research Laboratories, Ross joined Engelhard Corporation. Ross recently completed 25 years at Engelhard/BASF Corporation where he is currently a Senior Research Associate.

Ross has made pioneering contributions to the chemistry and engineering of catalytic processes. Early in his career with his advisor Michel Boudart, he developed an experimental method to address artifacts in kinetic data; a test accepted today as being definitive for kinetic control in catalysis. At Exxon, Ross’ studies in Fischer-Tropsch synthesis demonstrated the crucial role intraparticle diffusion played in enhancing hydrocarbon chain length and in changing selectivity. At Engelhard, he developed important concepts in fluid catalytic cracking to help design commercial catalysts. He elucidated the mechanism by which vanadium causes structural degradation of Y zeolite in FCC catalysts, and used this understanding to minimize its deleterious effect. His studies provided a definite assessment of the role of ZSM-5 additives in FCC to form light olefins and high octane gasoline. And, he defined the critical role rare earth cations play in Y-based FCC catalysts, demonstrating how the presence of rare earth influences hydride transfer reactions and product selectivity in FCC. Most recently, at BASF, Ross, together with colleagues in academia, elucidated the mechanism of the water gas shift reaction on copper, evincing parameters that could significantly improve catalytic activity. Importantly, though, Ross has used his conceptual and mechanistic approach to catalyst research to design commercial catalysts. He is the coinventor and developer of the Reduxion – Maxol® family of FCC catalysts and of the IsoPlus® and Ultrium® families; all of which have been used commercially worldwide. He coinvented the Flex-Tec® resid cracking catalyst which has been widely and successfully deployed in demanding resid cat-cracking processes. And most recently he has developed several copper based catalysts for the petrochemical industry.

Ross chaired the 1996 Gordon Research Conference on Catalysis, and in 2009 was awarded the AIChE Catalysis and Reaction Engineering Division Practice Award.