All Publications


  • Uniformity Is Key in Defining Structure-Function Relationships for Atomically Dispersed Metal Catalysts: The Case of Pt/CeO2. Journal of the American Chemical Society Resasco, J., DeRita, L., Dai, S., Chada, J. P., Xu, M., Yan, X., Finzel, J., Hanukovich, S., Hoffman, A. S., Graham, G. W., Bare, S. R., Pan, X., Christopher, P. 2019

    Abstract

    Catalysts consisting of atomically dispersed Pt (Ptiso) species on CeO2 supports have received recent interest due to their potential for efficient metal utilization in catalytic convertors. However, discrepancies exist between the behavior (reducibility, interaction strength with adsorbates) of high surface area Ptiso/CeO2 systems and of well-defined surface science and computational model systems, suggesting differences in Pt local coordination in the two classes of materials. Here, we reconcile these differences by demonstrating that high surface area Ptiso/CeO2 synthesized at low Pt loadings (<0.1% weight) exhibit resistance to reduction and sintering up to 500 °C in 0.05 bar H2 and minimal interactions with CO-properties previously seen only for model system studies. Alternatively, Pt loadings >0.1 weight % produce a distribution of sub-nanometer Pt structures, which are difficult to distinguish using common characterization techniques, and exhibit strong interactions with CO and weak resistance to sintering, even in 0.05 bar H2 at 50 °C-properties previously seen for high surface area materials. This work demonstrates that low metal loadings can be used to selectively populate the most thermodynamically stable adsorption sites on high surface area supports with atomically dispersed metals. Further, the site uniformity afforded by this synthetic approach is critical for the development of relationships between atomic scale local coordination and functional properties. Comparisons to recent studies of Ptiso/TiO2 suggest a general compromise between the stability of atomically dispersed metal catalysts and their ability to interact with and activate molecular species.

    View details for DOI 10.1021/jacs.9b09156

    View details for PubMedID 31815460

  • Palladium oxidation leads to methane combustion activity: Effects of particle size and alloying with platinum. The Journal of chemical physics Goodman, E. D., Ye, A. A., Aitbekova, A., Mueller, O., Riscoe, A. R., Nguyen Taylor, T., Hoffman, A. S., Boubnov, A., Bustillo, K. C., Nachtegaal, M., Bare, S. R., Cargnello, M. 2019; 151 (15): 154703

    Abstract

    Pd- and Pt-based catalysts are highly studied materials due to their widespread use in emissions control catalysis. However, claims continue to vary regarding the active phase and oxidation state of the metals. Different conclusions have likely been reached due to the heterogeneous nature of such materials containing various metal nanoparticle sizes and compositions, which may each possess unique redox features. In this work, using uniform nanocrystal catalysts, we study the effect of particle size and alloying on redox properties of Pd-based catalysts and show their contribution to methane combustion activity using operando quick extended x-ray absorption fine structure measurements. Results demonstrate that for all studied Pd sizes (3 nm-16 nm), Pd oxidation directly precedes CH4 combustion to CO2, suggesting Pd oxidation as a prerequisite step to methane combustion, and an oxidation pretreatment shows equal or better catalysis than a reduction pretreatment. Results are then extended to uniform alloyed PtxPd1-x nanoparticles, where oxidative pretreatments are shown to enhance low-temperature combustion. In these uniform alloys, we observe a composition-dependent effect with Pt-rich alloys showing the maximum difference between oxidative and reductive pretreatments. In Pt-rich alloys, we initially observe that the presence of Pt maintains Pd in a lower-activity reduced state. However, with time on stream, PdO eventually segregates under oxidizing combustion conditions, leading to a slowly increasing activity. Overall, across particle sizes and alloy compositions, we relate increased catalytic activity to Pd oxidation, thus shedding light on previous contrasting results related to the methane combustion activity of these catalysts.

    View details for DOI 10.1063/1.5126219

    View details for PubMedID 31640349

  • A versatile approach for quantification of surface site fractions using reaction kinetics: The case of CO oxidation on supported Ir single atoms and nanoparticles JOURNAL OF CATALYSIS Lu, Y., Kuo, C., Kovarik, L., Hoffman, A. S., Boubnov, A., Driscoll, D. M., Morris, J. R., Bare, S. R., Karim, A. M. 2019; 378: 121–30
  • Transition state and product diffusion control by polymer-nanocrystal hybrid catalysts NATURE CATALYSIS Riscoe, A. R., Wrasman, C. J., Herzing, A. A., Hoffman, A. S., Menon, A., Boubnov, A., Vargas, M., Bare, S. R., Cargnello, M. 2019; 2 (10): 852–63
  • Engineering of Ruthenium-Iron Oxide Colloidal Heterostructures Leads to Improved Yields in CO2 Hydrogenation to Hydrocarbons. Angewandte Chemie (International ed. in English) Cargnello, M., Aitbekova, A., Goodman, E., Wu, L., Boubnov, A., Hoffman, A., Genc, A., Cheng, H., Casalena, L., Bare, S. 2019

    Abstract

    Catalytic CO2 reduction to fuels and chemicals is one of the major pursuits in reducing greenhouse gas emissions. One such popular approach utilizes the reverse water-gas shift reaction, followed by Fischer-Tropsch synthesis, and iron is a well-known candidate for this process. Some attempts have been made to modify and improve its reactivity, but resuted in limited success. In this work, using ruthenium-iron oxide colloidal heterodimers we demonstrate that close contact between the two phases promotes the reduction of iron oxide via a proximal hydrogen spillover effect, leading to the formation of ruthenium-iron core-shell structures active for the reaction at significantly lower temperatures than in bare iron catalysts. Furthermore, by engineering the iron oxide shell thickness, we achieve a fourfold increase in hydrocarbon yield compared to the heterodimers. In general, our work shows how rational design of colloidal heterostructures can result in materials with significantly improved catalytic performance in CO2 conversion processes.

    View details for DOI 10.1002/anie.201910579

    View details for PubMedID 31545533

  • Catalyst deactivation via decomposition into single atoms and the role of metal loading NATURE CATALYSIS Goodman, E. D., Johnston-Peck, A. C., Dietze, E. M., Wrasman, C. J., Hoffman, A. S., Abild-Pedersen, F., Bare, S. R., Plessow, P. N., Cargnello, M. 2019; 2 (9): 748–55
  • 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
  • Understanding Structure-Property Relationships of MoO3-Promoted Rh Catalysts for Syngas Conversion to Alcohols. Journal of the American Chemical Society Asundi, A. S., Hoffman, A. S., Bothra, P., Boubnov, A., Vila, F. D., Yang, N., Singh, J. A., Zeng, L., Raiford, J. A., Abild-Pedersen, F., Bare, S. R., Bent, S. F. 2019

    Abstract

    Rh-based catalysts have shown promise for the direct conversion of syngas to higher oxygenates. Although improvements in higher oxygenate yield have been achieved by combining Rh with metal oxide promoters, details of the structure of the promoted catalyst and the role of the promoter in enhancing catalytic performance are not well understood. In this work, we show that MoO3-promoted Rh nanoparticles form a novel catalyst structure in which Mo substitutes into the Rh surface, leading to both a 66-fold increase in turnover frequency and an enhancement in oxygenate yield. By applying a combination of atomically controlled synthesis, in situ characterization, and theoretical calculations, we gain an understanding of the promoter-Rh interactions that govern catalytic performance for MoO3-promoted Rh. We use atomic layer deposition to modify Rh nanoparticles with monolayer-precise amounts of MoO3, with a high degree of control over the structure of the catalyst. Through in situ X-ray absorption spectroscopy, we find that the atomic structure of the catalytic surface under reaction conditions consists of Mo-OH species substituted into the surface of the Rh nanoparticles. Using density functional theory calculations, we identify two roles of MoO3: first, the presence of Mo-OH in the catalyst surface enhances CO dissociation and also stabilizes a methanol synthesis pathway not present in the unpromoted catalyst; and second, hydrogen spillover from Mo-OH sites to adsorbed species on the Rh surface enhances hydrogenation rates of reaction intermediates.

    View details for DOI 10.1021/jacs.9b07460

    View details for PubMedID 31724857

  • 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