Stanford Advisors


All Publications


  • Chemically Controllable Porous Polymer-Nanocrystal Composites with Hierarchical Arrangement Show Substrate Transport Selectivity CHEMISTRY OF MATERIALS Riscoe, A. R., Wrasman, C. J., Menon, A., Dinakar, B., Goodman, E. D., Kunz, L. Y., Yacob, S., Cargnello, M. 2020; 32 (13): 5904–15
  • Determining number of sites on ceria stabilizing single atoms via metal nanoparticle redispersion CHINESE JOURNAL OF CATALYSIS Aitbekova, A., Wrasman, C. J., Riscoe, A. R., Kunz, L. Y., Cargnello, M. 2020; 41 (6): 998–1005
  • Dilute Pd/Au Alloys Replace Au/TiO2 Interface for Selective Oxidation Reactions ACS CATALYSIS Wrasman, C. J., Riscoe, A. R., Lee, H., Cargnello, M. 2020; 10 (3): 1716–20
  • 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

  • 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
  • Highly tunable platform for biomimetic catalysis from nanocrystal-polymer composites Cargnello, M., Riscoe, A., Wrasman, C., Herzing, A., Bare, S. AMER CHEMICAL SOC. 2019
  • Artificial inflation of apparent photocatalytic activity induced by catalyst-mass-normalization and a method to fairly compare heterojunction systems ENERGY & ENVIRONMENTAL SCIENCE Kunz, L. Y., Diroll, B. T., Wrasman, C. J., Riscoe, A. R., Majumdar, A., Cargnello, M. 2019; 12 (5): 1657–67

    View details for DOI 10.1039/c9ee00452a

    View details for Web of Science ID 000473083100015

  • General Self-Assembly Method for Deposition of Graphene Oxide into Uniform Close-Packed Monolayer Films Holm, A., Kunz, L., Riscoe, A. R., Kao, K., Cargnello, M., Frank, C. W. AMER CHEMICAL SOC. 2019: 4460–70

    Abstract

    Depositing a morphologically uniform monolayer film of graphene oxide (GO) single-layer sheets is an important step in the processing of many composites and devices. Conventional Langmuir-Blodgett (LB) deposition is often considered to give the highest degree of morphology control, but film microstructures still vary widely between GO samples. The main challenge is in the sensitive self-assembly of GO samples with different sheet sizes and degrees of oxidation. To overcome this drawback, here, we identify a general method that relies on robust assembly between GO and a cationic surfactant (cationic surfactant-assisted LB). We systematically compared conventional LB and cationic surfactant-assisted LB for three common GO samples of widely different sheet sizes and degrees of oxidation. Although conventional LB may occasionally provide satisfactory film morphology, cationic surfactant-assisted LB is general and allows deposition of films with tunable and uniform morphologies-ranging from close-packed to overlapping single layers-from all three types of GO samples investigated. Because cationic surfactant-assisted LB is robust and general, we expect this method to broaden and facilitate the use of GO in many applications where precise control over film morphology is crucial.

    View details for DOI 10.1021/acs.langmuir.8b03994

    View details for Web of Science ID 000463679300007

    View details for PubMedID 30836748

  • Photocatalytic CO2 reduction using hybrid nanoporous catalysts Kao, K., Riscoe, A., Yang, A., Huang, W., Holm, A., Frank, C., Cargnello, M. AMER CHEMICAL SOC. 2019
  • Colloidal nanocrystals for heterogeneous catalysis NANO TODAY Losch, P., Huang, W., Goodman, E. D., Wrasman, C. J., Holm, A., Riscoe, A. R., Schwalbe, J. A., Cargnello, M. 2019; 24: 15–47
  • 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

  • Langmuir-Blodgett deposition of graphene oxide - identifying Marangoni flow as a process that fundamentally limits deposition control Holm, A., Wrasman, C., Riscoe, A., Cargnello, M., Frank, C. AMER CHEMICAL SOC. 2018
  • 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
  • Well-defined nanocrystals catalysts as active phases and premier materials for spectroscopic studies of catalyst restructuring Cargnello, M., Goodman, E., Aitbekova, A., Wrasman, C., Riscoe, A., Yang, A., Abild-Pedersen, F., Bare, S. AMER CHEMICAL SOC. 2018
  • Highly tunable platform for biomimetic catalysts from nanocrystal-amorphous polymer composites Cargnello, M., Riscoe, A., Wrasman, C., Aitbekova, A., Goodman, E., Herzing, A., Bare, S. AMER CHEMICAL SOC. 2018
  • Langmuir-Blodgett Deposition of Graphene Oxide-Identifying Marangoni Flow as a Process that Fundamentally Limits Deposition Control. Langmuir : the ACS journal of surfaces and colloids Holm, A., Wrasman, C. J., Kao, K., Riscoe, A. R., Cargnello, M., Frank, C. W. 2018

    Abstract

    Langmuir-Blodgett deposition is a popular route to produce thin films of graphene oxide for applications such as transparent conductors and biosensors. Unfortunately, film morphologies vary from sample to sample, often with undesirable characteristics such as folded sheets and patchwise depositions. In conventional Langmuir-Blodgett deposition of graphene oxide, alcohol (typically methanol) is used to spread the graphene oxide sheets onto an air-water interface before deposition onto substrates. Here we show that methanol gives rise to Marangoni flow, which fundamentally limits control over Langmuir-Blodgett depositions of graphene oxide. We directly identified the presence of Marangoni flow by using photography, and we evaluated depositions with atomic force microscopy and scanning electron microscopy. The disruptive effect of Marangoni flow was demonstrated by comparing conventional Langmuir-Blodgett depositions to depositions where Marangoni flow was suppressed by a surfactant. Because methanol is the standard spreading solvent for conventional Langmuir-Blodgett deposition of graphene oxide, Marangoni flow is a general problem and may partly explain the wide variety of undesirable film morphologies reported in the literature.

    View details for PubMedID 30025460

  • Understanding the Preferential Oxidation of Carbon Monoxide (PrOx) Using Size-Controlled Au Nanocrystal Catalyst AICHE JOURNAL Beck, A., Yang, A., Leland, A. R., Riscoe, A. R., Lopez, F. A., Goodman, E. D., Cargnello, M. 2018; 64 (8): 3159–67

    View details for DOI 10.1002/aic.16206

    View details for Web of Science ID 000441215000022

  • Understanding activity loss in precious-metal combustion catalysts using well-defined nanocrystals Goodman, E., Riscoe, A., Tahsini, N., Abild-Pedersen, F., Johnston-Peck, A., Cargnello, M. AMER CHEMICAL SOC. 2018
  • 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
  • Highly tunable platform for biomimetic catalysts from nanocrystal-polymer composites Riscoe, A., Wrasman, C., Menon, A., Hilbert, M., Dai, A., Vargas, M., Goodman, E., Yang, A., Beck, A., Wu, L., Cargnello, M. AMER CHEMICAL SOC. 2018
  • Systematic Identification of Promoters for Methane Oxidation Catalysts Using Size- and Composition-Controlled Pd-Based Bimetallic Nanocrystals. Journal of the American Chemical Society Willis, J. J., Goodman, E. D., Wu, L. n., Riscoe, A. R., Martins, P. n., Tassone, C. J., Cargnello, M. n. 2017; 139 (34): 11989–97

    Abstract

    Promoters enhance the performance of catalytic active phases by increasing rates, stability, and/or selectivity. The process of identifying promoters is in most cases empirical and relies on testing a broad range of catalysts prepared with the random deposition of active and promoter phases, typically with no fine control over their localization. This issue is particularly relevant in supported bimetallic systems, where two metals are codeposited onto high-surface area materials. We here report the use of colloidal bimetallic nanocrystals to produce catalysts where the active and promoter phases are colocalized to a fine extent. This strategy enables a systematic approach to study the promotional effects of several transition metals on palladium catalysts for methane oxidation. In order to achieve these goals, we demonstrate a single synthetic protocol to obtain uniform palladium-based bimetallic nanocrystals (PdM, M = V, Mn, Fe, Co, Ni, Zn, Sn, and potentially extendable to other metal combinations) with a wide variety of compositions and sizes based on high-temperature thermal decomposition of readily available precursors. Once the nanocrystals are supported onto oxide materials, thermal treatments in air cause segregation of the base metal oxide phase in close proximity to the Pd phase. We demonstrate that some metals (Fe, Co, and Sn) inhibit the sintering of the active Pd metal phase, while others (Ni and Zn) increase its intrinsic activity compared to a monometallic Pd catalyst. This procedure can be generalized to systematically investigate the promotional effects of metal and metal oxide phases for a variety of active metal-promoter combinations and catalytic reactions.

    View details for PubMedID 28800226