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

  • Density-dependent deactivation mechanism in supported catalysts by high-temperature decomposition of particles into single atoms Goodman, E., Johnston-Peck, A., Dietze, E., Wrasman, C., Hoffman, A., Abild-Pedersen, F., Bare, S., Plessow, P., Cargnello, M. AMER CHEMICAL SOC. 2019
  • 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
  • In situ observation of phase changes of a silica-supported cobalt catalyst for the Fischer-Tropsch process by the development of a synchrotron-compatible insitu/operando powder X-ray diffraction cell. Journal of synchrotron radiation Hoffman, A. S., Singh, J. A., Bent, S. F., Bare, S. R. 2018; 25 (Pt 6): 1673–82

    Abstract

    In situ characterization of catalysts gives direct insight into the working state of the material. Here, the design and performance characteristics of a universal insitu synchrotron-compatible X-ray diffraction cell capable of operation at high temperature and high pressure, 1373 K, and 35 bar, respectively, are reported. Its performance is demonstrated by characterizing a cobalt-based catalyst used in a prototypical high-pressure catalytic reaction, the Fischer-Tropsch synthesis, using X-ray diffraction. Cobalt nanoparticles supported on silica were studied insitu during Fischer-Tropsch catalysis using syngas, H2 and CO, at 723 K and 20 bar. Post reaction, the Co nanoparticles were carburized at elevated pressure, demonstrating an increased rate of carburization compared with atmospheric studies.

    View details for PubMedID 30407177

  • 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

  • Synergistic effect in colloidal Pd/Au single atom alloy nanocrystals for selective oxidations Wrasman, C., Riscoe, A., Hoffman, A., Boubnov, A., Bare, S., Cargnello, M. AMER CHEMICAL SOC. 2018
  • Beating Heterogeneity of Single-Site Catalysts: MgO-Supported Iridium Complexes ACS CATALYSIS Hoffman, A. S., Debefve, L. M., Zhang, S., Perez-Aguilar, J. E., Conley, E. T., Justl, K. R., Arslan, I., Dixon, D. A., Gatesr, B. C. 2018; 8 (4): 3489–98
  • Biomimetic oxidation catalyst from polymer-nanocrystal composite material Riscoe, A., Wrasman, C., Hoffman, A., Menon, A., Boubnov, A., Goodman, E., Bare, S., Cargnello, M. AMER CHEMICAL SOC. 2018
  • Uniform Pt/Pd bimetallic nanocrystals demonstrate platinum effect on palladium methane combustion activity and stability Goodman, E., Dai, S., Yang, A., Wrasman, C., Gallo, A., Bare, S., Hoffman, A., Jaramillo, T., Graham, G., Pan, X., Cargnello, M. AMER CHEMICAL SOC. 2018
  • Direct observation of the kinetics of gas–solid reactions using in situ kinetic and spectroscopic techniques Reaction Chemistry & Engineering Hoffman, A. S., Azzam, S., Zhang, K., Xu, Y., Liu, Y., Bare, S. R., Simonetti, D. A. 2018; 3: 668-675

    View details for DOI 10.1039/C8RE00020D

  • 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

  • High-Energy-Resolution X-ray Absorption Spectroscopy for Identification of Reactive Surface Species on Supported Single-Site Iridium Catalysts CHEMISTRY-A EUROPEAN JOURNAL Hoffman, A. S., Sokaras, D., Zhang, S., Debefve, L. M., Fang, C., Gallo, A., Kroll, T., Dixon, D. A., Bare, S. R., Gates, B. C. 2017; 23 (59): 14760–68

    Abstract

    We report high-energy-resolution X-ray absorption spectroscopy detection of ethylene and CO ligands adsorbed on catalytically active iridium centers isolated on zeolite HY and on MgO supports. The data are supported by density functional theory and FEFF X-ray absorption near-edge modelling, together with infrared (IR) spectra. The results demonstrate that high-energy-resolution X-ray absorption spectra near the iridium LIII (2p3/2 ) edge provide clearly ascribable, distinctive signatures of the ethylene and CO ligands and illustrate effects of supports and other ligands. This X-ray absorption technique is markedly more sensitive than conventional IR spectroscopy for characterizing surface intermediates, and it is applicable to samples having low metal loadings and in reactive atmospheres and is expected to have an increasing role in catalysis research by facilitating the determination of mechanisms of solid-catalyzed reactions through identification of reaction intermediates in working catalysts.

    View details for PubMedID 28749554

  • Understanding and controlling the activity and stability of Pd/Pt oxide catalysts for methane activation Cargnello, M., Goodman, E., Yang, A., Dai, S., Wrasman, C., Bare, S., Hoffman, A., Graham, G., Pan, X. AMER CHEMICAL SOC. 2017
  • Uniform Pt/Pd Bimetallic Nanocrystals Demonstrate Platinum Effect on Palladium Methane Combustion Activity and Stability ACS CATALYSIS Goodman, E. D., Dai, S., Yang, A., Wrasman, C. J., Gallo, A., Bare, S. R., Hoffman, A. S., Jaramillo, T. F., Graham, G. W., Pan, X., Cargnello, M. 2017; 7 (7): 4372–80