Wang Y.-G.,Tsinghua University |
Wang Y.-G.,Institute for Integrated Catalysis |
Mei D.,Institute for Integrated Catalysis |
Li J.,Tsinghua University |
And 2 more authors.
Journal of Physical Chemistry C | Year: 2013
We present the results of an extensive density functional theory based electronic structure study of the role of 4f-state localized electron states in the surface chemistry of a partially reduced CeO2(111) surface. These electrons exist in polaronic states, residing at Ce3+ sites, which can be created by either the formation of oxygen vacancies, OV, or other surface defects. Via ab initio molecular dynamics, these localized electrons are found to be able to move freely within the upper surface layer, but penetration into the bulk is inhibited as a result of the higher elastic strain induced by creating a subsurface Ce3+. We found that the water molecule can be easily dissociated into two surface bound hydroxyls at the Ce4+ site associated with OV sites. This dissociation process does not significantly affect the electronic structure of the excess electrons at reduced surface, but does lead to a favorable localization on Ce3+ sites in the vicinity of the resulting OH groups. In the presence of water, a proton-mediated Mars-van Krevelen mechanism for CO oxidation via the formation of bicarbonate species is identified. The localized 4f electrons on the surface facilitate the protonation process of adsorbed O2 species and thus decelerate the further oxidation of CO species. Overall, we find that surface hydroxyls formed via water dissociation at the CeO2 surface lead to inhabitation of the CO oxidation reaction. This is consistent with the experimental observation of requisite elevated temperatures, on the order of 600 K, for this reaction to occur. © 2013 American Chemical Society.
News Article | February 20, 2017
In a pioneering study, researchers definitively measured the stability of water molecules as they land on an oxide. There is a slight preference to keep water molecules intact. When the water’s do break, it is because surface forces align the molecules long before they hit the surface. Credit: Pacific Northwest National Laboratory ater is behind creating certain biofuels, sequestering carbon, and forming corrosive rust. If and how water (H2O) breaks when it hits a metal oxide surface, such as a catalyst or a pipe, matters. In a pioneering study, scientists at Pacific Northwest National Laboratory (PNNL), led by Dr. Zdenek Dohnálek and Dr. Roger Rousseau, definitively measured the stability of adsorbed water compared to the hydroxyl (-OH) fragments. They showed that there is a slight preference to keep water molecules intact. The research team showed that when they do break, it is because surface forces align water molecules in a specific way, long before they hit the surface and dissociate. For decades, scientists studied the reactions between water and oxides. They obtained substantial amounts of data; however, it did not definitively resolve water's stability, or how much energy is needed to make the water molecules fall apart. The PNNL team ended the controversy by precisely measuring stability of water and its fragments; they found that molecular water is only 0.035 eV more stable than its dissociated counterparts (hydroxyl groups). Supplying the definitive answer for the scientific community will benefit those working to efficiently and effectively improve the catalysts involved in producing fuels and chemicals. "The approach lets us look at how bonds break in catalysis," said Dohnálek, who led the experimental research. "However, it applies much more broadly. Any dissociative process that has an energy barrier will benefit from this study." Directly measuring the amount of energy needed to split bonds inside a water molecule when it hits an oxide surface was a major challenge. The team at PNNL spent several years getting the exact answer. "The problem wasn't that there wasn't enough data," said Rousseau. "Everyone had tons of answers. They just weren't what we needed." To determine water's stability on titanium dioxide, a prototypical surface, experts in surface science and chemical theory came together. Because conventional instruments couldn't measure water's stability, the team first designed an instrument able to measure the energy and see the collisions. They combined a supersonic molecular beam with scanning tunneling microscopy and froze the molecular fragments by working at low substrate temperatures. It allowed the scientists to distinguish the molecules from the fragments. By iterating between experiments and simulations, the team made numerous discoveries about the way water interacts with the oxide. For example, they found that the oxide surface influences the water molecules from relatively far away, which means a few molecular lengths in this case. While such "steering" of the molecules is seen on metal surfaces, it occurs at much shorter distances. "When I saw the data, it hit me that this can only be explained by looking at how the water molecules approach the surface," said Dr. Vanda Glezakou, who suggested this analysis. In the end, they determined that when the water fragmented to form two hydroxyl groups, the amount of energy needed to activate the bond that splits off the proton is relatively small, just 0.36 eV and the amount of energy to put the water back together is only 0.035 eV smaller, showing that water is only tiny bit more stable. The team's technique can provide answers on other systems where bond activation is crucial, including in biomass production and carbon storage. The PNNL team is applying the approach to catalytic problems as part of the Institute for Integrated Catalysis. Specifically, they are studying how molecules other than water break on surfaces. They are examining the different intermediate products formed. Further, they are seeing how heat influences dissociation in different reactions. Explore further: Some catalysts contribute their own oxygen for reactions More information: Zhi-Tao Wang et al. Probing equilibrium of molecular and deprotonated water on TiO(110), Proceedings of the National Academy of Sciences (2017). DOI: 10.1073/pnas.1613756114
News Article | February 15, 2017
RICHLAND, Wash. - Two researchers at the Department of Energy's Pacific Northwest National Laboratory have been elected to membership in the prestigious National Academy of Engineering. Ruby Leung and Johannes Lercher are among the 106 new members elected worldwide to the 2017 class. The NAE is a private, independent, nonprofit institution that is part of The National Academies of Sciences, Engineering, and Medicine. NAE focuses on maintaining a strong engineering community and bringing together experts to provide independent advice to the federal government on engineering and technology challenges. Lercher and Leung join emeritus staff member Subhash Singhal, who is a National Academy of Sciences member, as PNNL researchers in the National Academies. "I am thrilled that the exceptional contributions of two of our researchers have been recognized by the National Academy of Engineering," said PNNL director Steven Ashby. "Membership in the NAE is among the highest honors that a researcher can achieve, and Ruby and Johannes are most deserving. Congratulations to both of them!" Ruby Leung is an atmospheric scientist at PNNL and also an affiliate scientist at the National Center for Atmospheric Research. She was elected based on her leadership in regional and global computer modeling of the Earth's climate and water cycles. Leung's research has advanced understanding and modeling of the regional and global water cycles, with implications for managing water, agriculture and energy. She has organized key workshops sponsored by environmental agencies, served on panels that define future priorities in climate modeling, and has developed computer climate models that are used globally. She has published more than 200 peer-reviewed journal articles and is a fellow of the American Association for the Advancement of Science, the American Geophysical Union and the American Meteorological Society. She earned a bachelor's degree in physics and statistics from the Chinese University of Hong Kong, and a master's degree and a doctorate in atmospheric science from Texas A&M University. Johannes Lercher is a chemist and holds a joint appointment at PNNL and the Technische Universität München in Germany. At PNNL, he serves as the director of the Institute for Integrated Catalysis, and at TUM he is a professor in the Department of Chemistry and holds the chair of the Institute for Technische Chemie. He was elected based on his catalysis research, which focuses on the details of how catalysts work at the elementary level and using that insight to design and build better catalysts for industrial applications, including cleaner fossil fuels and renewable, biology-based fuels. He has published more than 500 peer-reviewed journal articles, is editor-in-chief of the Journal of Catalysis and was previously elected to the Austrian Academy of Sciences, the Academia Europaea and the European Academy of Sciences. He has won numerous awards, including the David Trim and Noel Cant Lectureship given by the Catalysis Society of Australia, the Eni Award for energy research, and the Francois Gault lectureship of the European Association of Catalysis Societies. He earned undergraduate and graduate degrees, as well as a doctorate in chemistry from the Technische Universität Wien, Austria. The newly elected class brings the NAE's total U.S. membership to 2,281 and the number of foreign members to 249. Lercher and Leung will be inducted at a ceremony in Washington, DC in October. Interdisciplinary teams at Pacific Northwest National Laboratory address many of America's most pressing issues in energy, the environment and national security through advances in basic and applied science. Founded in 1965, PNNL employs 4,400 staff and has an annual budget of nearly $1 billion. It is managed by Battelle for the U.S. Department of Energy's Office of Science. As the single largest supporter of basic research in the physical sciences in the United States, the Office of Science is working to address some of the most pressing challenges of our time. For more information on PNNL, visit the PNNL News Center, or follow PNNL on Facebook, Google+, Instagram, LinkedIn and Twitter.
Abellan P.,Daresbury Laboratory |
Parent L.R.,Fundamental and Computational science Dir. |
Parent L.R.,University of California at San Diego |
Al Hasan N.,Institute for Integrated Catalysis |
And 5 more authors.
Langmuir | Year: 2016
Synthesizing nanomaterials of uniform shape and size is of critical importance to access and manipulate the novel structure-property relationships arising at the nanoscale, such as catalytic activity. In this work, we synthesize Pd nanoparticles with well-controlled size in the sub-3 nm range using scanning transmission electron microscopy (STEM) in combination with an in situ liquid stage. We use an aromatic hydrocarbon (toluene) as a solvent that is very resistant to high-energy electron irradiation, which creates a net reducing environment without the need for additives to scavenge oxidizing radicals. The primary reducing species is molecular hydrogen, which is a widely used reductant in the synthesis of supported metal catalysts. We propose a mechanism of particle formation based on the effect of tri-n-octylphosphine (TOP) on size stabilization, relatively low production of radicals, and autocatalytic reduction of Pd(II) compounds. We combine in situ STEM results with insights from in situ small-angle X-ray scattering (SAXS) from alcohol-based synthesis, having similar reduction potential, in a customized microfluidic device as well as ex situ bulk experiments. This has allowed us to develop a fundamental growth model for the synthesis of size-stabilized Pd nanoparticles and demonstrate the utility of correlating different in situ and ex situ characterization techniques to understand, and ultimately control, metal nanostructure synthesis. © 2016 American Chemical Society.