Heterogeneous Catalysis is a critical technology for producing fuels and chemicals. Chemical production accounts for over 25% of the energy usage today. In the Friend group, we are testing new, nanoporous catalyst materials for selective oxidation catalysis. Our objective is to develop highly selective processes using sustainable resources. Experimental capabilities used in this work include two vapor phase flow reactors that use mass spectrometry and gas chromatography to measure reaction rates; a temporal analysis of products (TAP) reactor in our laboratory. Nanoporous catalyst materials are synthesized both in our laboratory and by collaborators at Lawrence Livermore National Laboratories (https://www-pls.llnl.gov/?url=about_pls-scientific_staff-biener_m). We also make use of excellent electron microscopy facilities in the Harvard Center for Nanoscale Systems (www.cns.harvard.edu).
Nanoporous Gold is an exciting new nanostructured alloy material that has been shown to be highly active for selective alcohol oxidation reactions, an important class of reactions for the chemicals industry. Under reaction conditions this material is highly dynamic, resulting in a high degree of atomic diffusion at the surface. In order to gain a better understanding of the atomic-scale origins of the catalytic activity, we are using reaction studies at various pressure regimes (UHV to atmospheric) as well as a number of in-situ characterization techniques, such as environmental transmission electron microscopy (E-TEM) and atmospheric pressure X-ray photoelectron spectroscopy (AP-XPS) to follow these atomic-level changes as they happen.
Surface Chemistry provides molecular-level understanding of reactivity and materials properties of both well defined single-crystal surfaces and complex porous catalyst materials. The insight obtained from our work provides us with a framework for designing materials and process conditions for sustainable catalytic processes. The arsenal of tools available in our laboratory include several ultrahigh vacuum systems with the capability for temperature programmed reaction measurements, low energy electron diffraction and Auger electron spectroscopy. We also have specialized tools, including scanning tunneling microscopy, infrared absorption reflection spectroscopy, X-ray photoelectron spectroscopy (XPS) and high resolution electron energy loss spectroscopy.
Surface Photochemistry is a potential means of harnessing solar energy for chemical synthesis, energy storage, and for environmental remediation. We seek to understand how plasmonic structures influence the surface chemistry that is critical to the overall photochemical processes, be it through facilitating exciton generation, production of hot charge carriers, and cooperative site binding of reactant species. Titania and monolayer molybdenum disulfide serve as model photocatalysts, driving hole- and electron-mediated reactions at their surfaces respectively. By combining these materials with plasmonic nanoparticles, or incorporating them into plasmonic nanoporous metal supports, we seek to understand how the photochemistry of these materials is influenced. Using Temperature Programmed Desorption and Reaction Spectroscopy and, as well as and Scanning Tunneling Microscopy (STM), we seek to elucidate the details of these effects to drive rational design in the development of plasmonically enhanced photocatalytic materials.
Theoretical Simulation of Photochemical Processes - The computational simulations are aimed at attaining a fundamental understanding of the chemical reactions driven by photo-chemical excitation of semiconductor catalyst. The main method is Ehrenfest dynamics based on the Density Functional Theory. Such approach allows to investigate the evolution of catalytic systems in real time, and directly monitor the transition from reactants to products, including the intermediates. Current process of interest is alcohol oxidation on rutile titania. For methanol, the process of photo-oxidation to formaldehyde was successfully simulated. The interpretation of the first-principle simulations has already resulted in proposing new concepts regarding the mechanisms of the photo-chemical reactions, and current work focuses on extending those concepts to a broader range of chemical reactions.
Theory Collaborations with the Kaxiras group at Harvard (http://scholar.harvard.edu/efthimios_kaxiras) and the Tkatchenko group at the Fritz Haber Institute in Germany (http://www.fhi-berlin.mpg.de/~tkatchen/) provide detailed knowledge about bonding and reactions on surfaces.
Transition Metal Carbide Catalysts are integral for methane oxidation in low-temperature solid oxide fuel cells. We are developing theses catalyst and screening is performed using a flow reactor to study the electrocatalytic behavior of metal carbide films under various conditions of temperature, pressure, and gas composition. These films are also characterized before and after reaction using XPS and SEM. All of this information about the reaction mechanism is then used to develop more efficient catalyst materials.