Bio


The focus of my research is implementation of advanced methods to gain structural information from X-ray absorption spectra of heterogeneous catalysts under operating conditions. Structural dynamics can be very subtle and chemical structures yet unknown. By applying high-sensitivity detection methods (e.g. HERFD-XAS*) and theoretically calculating spectra of hypothetical structures, I enhance the sensitivity of conventional techniques and create shortcuts to new discoveries.
Particular systems of interest are single-site catalysts, serving as excellent models for examining reaction mechanisms and intermediate species on well-defined sites. Furthermore, I study activation and de-activation of catalysts for globally important energy-related processes.

Expertise
•Experimental X-ray absorption spectroscopy: sample optimization, data acquisition and analysis
•In situ cell design
•Molecular models for EXAFS refinement
•XANES and L-DOS calculations using FEFF9

*High-energy-resolution fluorescence-detected X-ray absorption spectroscopy, a method with monochromatic fluorescence detection, unlike energy-dispersive fluorescence detection with a much lower energy resolution.

Professional Education


  • Ph. D., Karlsruhe Institute of Technology, Chemical Technology (2014)
  • Master of Science, University of Copenhagen, Nanoscience (2009)
  • Bachelor of Science, University of Copenhagen, Nanotechnology (2007)

All Publications


  • Structural evolution of atomically dispersed Pt catalysts dictates reactivity. Nature materials DeRita, L., Resasco, J., Dai, S., Boubnov, A., Thang, H. V., Hoffman, A. S., Ro, I., Graham, G. W., Bare, S. R., Pacchioni, G., Pan, X., Christopher, P. 2019

    Abstract

    The use of oxide-supported isolated Pt-group metal atoms as catalytic active sites is of interest due to their unique reactivity and efficient metal utilization. However, relationships between the structure of these active sites, their dynamic response to environments and catalytic functionality have proved difficult to experimentally establish. Here, sinter-resistant catalysts where Pt was deposited uniformly as isolated atoms in well-defined locations on anatase TiO2 nanoparticle supports were used to develop such relationships. Through a combination of in situ atomic-resolution microscopy- and spectroscopy-based characterization supported by first-principles calculations it was demonstrated that isolated Pt species can adopt a range of local coordination environments and oxidation states, which evolve in response to varied environmental conditions. The variation in local coordination showed a strong influence on the chemical reactivity and could be exploited to control the catalytic performance.

    View details for DOI 10.1038/s41563-019-0349-9

    View details for PubMedID 31011216

  • Role of Co2C in ZnO-promoted Co Catalysts for Alcohol Synthesis from Syngas CHEMCATCHEM Singh, J. A., Hoffman, A. S., Schumann, J., Boubnov, A., Asundi, A. S., Nathan, S. S., Norskov, J., Bare, S. R., Bent, S. F. 2019; 11 (2): 799–809
  • Synthesis of Colloidal Pd/Au Dilute Alloy Nanocrystals and Their Potential for Selective Catalytic Oxidations. Journal of the American Chemical Society Wrasman, C. J., Boubnov, A., Riscoe, A. R., Hoffman, A. S., Bare, S. R., Cargnello, M. 2018

    Abstract

    Selective oxidations are crucial for the creation of valuable chemical building blocks but often require expensive and unstable stoichiometric oxidants such as hydroperoxides and peracids. To date, many catalysts that contain a single type of active site have not been able to attain the desired level of selectivity for partially oxidized products over total combustion. However, catalysts containing multiple types of active sites have proven to be successful for selective reactions. One category of such catalysts is bimetallic alloys, in which catalytic activity and selectivity can be tuned by modifying the surface composition. Traditional catalyst synthesis methods using impregnation struggle to create catalysts with sufficient control over surface chemistry to accurately tune the ensemble size of the desired active sites. Here we describe the synthesis of colloidal nanocrystals of dilute alloys of palladium and gold. We show that when supported on titania (TiO2), tuning the composition of the Pd/Au nanocrystal surface provides a synergistic effect in the selective oxidation of 2-propanol to acetone in the presence of H2 and O2. In particular, we show that certain Pd/Au surface ratios exhibit activity and selectivity far superior to Pd or Au individually. Through precise structural characterization we demonstrate that isolated atoms of Pd exist in the most active catalysts. The synergy between isolated Pd atoms and Au allows for the formation of reactive oxidizing species, likely hydroperoxide groups, responsible for selective oxidation while limiting oxygen dissociation and, thus, complete combustion. This work opens the way to more efficient utilization of scarce noble metals and new options for catalyzed selective oxidations.

    View details for PubMedID 30220200

  • Low-Temperature Restructuring of CeO2-Supported Ru Nanoparticles Determines Selectivity in CO2 Catalytic Reduction. Journal of the American Chemical Society Aitbekova, A., Wu, L., Wrasman, C. J., Boubnov, A., Hoffman, A. S., Goodman, E. D., Bare, S. R., Cargnello, M. 2018; 140 (42): 13736–45

    Abstract

    CO2 reduction to higher value products is a promising way to produce fuels and key chemical building blocks while reducing CO2 emissions. The reaction at atmospheric pressure mainly yields CH4 via methanation and CO via the reverse water-gas shift (RWGS) reaction. Describing catalyst features that control the selectivity of these two pathways is important to determine the formation of specific products. At the same time, identification of morphological changes occurring to catalysts under reaction conditions can be crucial to tune their catalytic performance. In this contribution we investigate the dependency of selectivity for CO2 reduction on the size of Ru nanoparticles (NPs) and on support. We find that even at rather low temperatures (210 °C), oxidative pretreatment induces redispersion of Ru NPs supported on CeO2 and leads to a complete switch in the performance of this material from a well-known selective methanation catalyst to an active and selective RWGS catalyst. By utilizing in situ X-ray absorption spectroscopy, we demonstrate that the low-temperature redispersion process occurs via decomposition of the metal oxide phase with size-dependent kinetics, producing stable single-site RuO x/CeO2 species strongly bound to the CeO2 support that are remarkably selective for CO production. These results show that reaction selectivity can be heavily dependent on catalyst structure and that structural changes of the catalyst can occur even at low temperatures and can go unseen in materials with less defined structures.

    View details for PubMedID 30252458