All Publications

  • Cation Incorporation into Copper Oxide Lattice at Highly Oxidizing Potentials. ACS applied materials & interfaces Ostervold, L., Smerigan, A., Liu, M. J., Filardi, L. R., Vila, F. D., Perez-Aguilar, J. E., Hong, J., Tarpeh, W. A., Hoffman, A. S., Greenlee, L. F., Clark, E. L., Janik, M. J., Bare, S. R. 2023


    Electrolyte cations can have significant effects on the kinetics and selectivity of electrocatalytic reactions. We show an atypical mechanism through which electrolyte cations can impact electrocatalyst performance─direct incorporation of the cation into the oxide electrocatalyst lattice. We investigate the transformations of copper electrodes in alkaline electrochemistry through operando X-ray absorption spectroscopy in KOH and Ba(OH)2 electrolytes. In KOH electrolytes, both the near-edge structure and extended fine-structure agree with previous studies; however, the X-ray absorption spectra vary greatly in Ba(OH)2 electrolytes. Through a combination of electronic structure modeling, near-edge simulation, and postreaction characterization, we propose that Ba2+ cations are directly incorporated into the lattice and form an ordered BaCuO2 phase at potentials more oxidizing than 200 mV vs the normal hydrogen electrode (NHE). BaCuO2 formation is followed by further oxidation to a bulk Cu3+-like BaxCuyOz phase at 900 mV vs NHE. Additionally, during reduction in Ba(OH)2 electrolyte, we find both Cu-O bonds and Cu-Ba scattering persist at potentials as low as -400 mV vs NHE. To our knowledge, this is the first evidence for direct oxidative incorporation of an electrolyte cation into the bulk lattice to form a mixed oxide electrode. The oxidative incorporation of electrolyte cations to form mixed oxides could open a new route for the in situ formation of active and selective oxidation electrocatalysts.

    View details for DOI 10.1021/acsami.3c10296

    View details for PubMedID 37756387

  • Elucidating the role of Fe-Mo interactions in the metal oxide precursors for Fe promoted Mo/ZSM-5 catalysts in non-oxidative methane dehydroaromatization CHEMICAL ENGINEERING JOURNAL Hossain, M., Dhillon, G., Liu, L., Sridhar, A., Hiennadi, E. J., Hong, J., Bare, S. R., Xin, H., Ericson, T., Cozzolino, A., Khatib, S. J. 2023; 475
  • Probing of the Noninnocent Role of P in Transition-Metal Phosphide Hydrogen Evolution Reaction Electrocatalysts via Replacement with Electropositive Si CHEMISTRY OF MATERIALS Kong, S., Singh, P., Akopov, G., Jing, D., Davis, R., Perez-Aguilar, J., Hong, J., Lee, S. J., Viswanathan, G., Soto, E., Azhan, M., Fernandes, T., Harycki, S., Gundlach-Graham, A., Kolen'ko, Y. V., Johnson, D. D., Kovnir, K. 2023
  • Temporal Ni K-Edge X-ray Absorption Spectroscopy Study Reveals the Kinetics of the Ni Redox Behavior of the Iron-Nickel Oxide Bimetallic OER Catalyst JOURNAL OF PHYSICAL CHEMISTRY C Acharya, P., Hong, J., Manso, R., Hoffman, A. S., Kekedy-Nagy, L., Chen, J., Bare, S. R., Greenlee, L. F. 2023
  • CO Oxidation on Ir-1/TiO2: Resolving Ligand Dynamics and Elementary Reaction Steps ACS CATALYSIS Thompson, C. B., Liu, L., Leshchev, D. S., Hoffman, A. S., Hong, J., Bare, S. R., Unocic, R. R., Stavitski, E., Xin, H., Karim, A. M. 2023; 13 (12): 7802-7811
  • Observations of Ethylene-for-CO Ligand Exchanges on a Zeolite-Supported Single-Site Rh Catalyst by X-ray Absorption Spectroscopy. The journal of physical chemistry letters Hoffman, A. S., Muller, O., Hong, J., Canning, G. A., Fang, C., Perez-Aguilar, J. E., Gates, B. C., Bare, S. R. 2023: 4591-4599


    Quick-scanning X-ray absorption fine structure (QXAFS) measurements were used to characterize the exchanges of ethylene and CO ligands in a zeolite HY-supported single-site Rh complex at a sampling rate of 1.0 Hz. The two ligands were reversibly exchanged on the rhodium, with quantitative results determined for the C2H4-for-CO exchange that are consistent with a first-order process. The apparent rate constant for the exchange decreased with increasing temperature. Fourier-transform infrared spectra characterizing the C2H4 sorbed in the zeolite showed that the amount decreased with increasing temperature, consistent with the decrease in the exchange rate with increasing temperature. The results, illustrating the dynamics of ligand exchanges on a single-site supported metal catalyst, demonstrate the broad emerging applicability of the QXAFS technique for characterizing the dynamics of reactive intermediates on catalysts.

    View details for DOI 10.1021/acs.jpclett.3c00349

    View details for PubMedID 37166100

  • Memory-dictated dynamics of single-atom Pt on CeO2 for CO oxidation. Nature communications Zhang, Z., Tian, J., Lu, Y., Yang, S., Jiang, D., Huang, W., Li, Y., Hong, J., Hoffman, A. S., Bare, S. R., Engelhard, M. H., Datye, A. K., Wang, Y. 2023; 14 (1): 2664


    Single atoms of platinum group metals on CeO2 represent a potential approach to lower precious metal requirements for automobile exhaust treatment catalysts. Here we show the dynamic evolution of two types of single-atom Pt (Pt1) on CeO2, i.e., adsorbed Pt1 in Pt/CeO2 and square planar Pt1 in PtATCeO2, fabricated at 500°C and by atom-trapping method at 800°C, respectively. Adsorbed Pt1 in Pt/CeO2 is mobile with the in situ formation of few-atom Pt clusters during CO oxidation, contributing to high reactivity with near-zero reaction order in CO. In contrast, square planar Pt1 in PtATCeO2 is strongly anchored to the support during CO oxidation leading to relatively low reactivity with a positive reaction order in CO. Reduction of both Pt/CeO2 and PtATCeO2 in CO transforms Pt1 to Pt nanoparticles. However, both catalysts retain the memory of their initial Pt1 state after reoxidative treatments, which illustrates the importance of the initial single-atom structure in practical applications.

    View details for DOI 10.1038/s41467-023-37776-3

    View details for PubMedID 37160890

  • Dynamic Evolution of Palladium Single Atoms on Anatase Titania Support Determines the Reverse Water-Gas Shift Activity. Journal of the American Chemical Society Chen, L., Allec, S. I., Nguyen, M., Kovarik, L., Hoffman, A. S., Hong, J., Meira, D., Shi, H., Bare, S. R., Glezakou, V., Rousseau, R., Szanyi, J. 2023


    Research interest in single-atom catalysts (SACs) has been continuously increasing. However, the lack of understanding of the dynamic behaviors of SACs during applications hinders catalyst development and mechanistic understanding. Herein, we report on the evolution of active sites over Pd/TiO2-anatase SAC (Pd1/TiO2) in the reverse water-gas shift (rWGS) reaction. Combining kinetics, in situ characterization, and theory, we show that at T ≥ 350 °C, the reduction of TiO2 by H2 alters the coordination environment of Pd, creating Pd sites with partially cleaved Pd-O interfacial bonds and a unique electronic structure that exhibit high intrinsic rWGS activity through the carboxyl pathway. The activation by H2 is accompanied by the partial sintering of single Pd atoms (Pd1) into disordered, flat, 1 nm diameter clusters (Pdn). The highly active Pd sites in the new coordination environment under H2 are eliminated by oxidation, which, when performed at a high temperature, also redisperses Pdn and facilitates the reduction of TiO2. In contrast, Pd1 sinters into crystalline, 5 nm particles (PdNP) during CO treatment, deactivating Pd1/TiO2. During the rWGS reaction, the two Pd evolution pathways coexist. The activation by H2 dominates, leading to the increasing rate with time-on-stream, and steady-state Pd active sites similar to the ones formed under H2. This work demonstrates how the coordination environment and nuclearity of metal sites on a SAC evolve during catalysis and pretreatments and how their activity is modulated by these behaviors. These insights on SAC dynamics and the structure-function relationship are valuable to mechanistic understanding and catalyst design.

    View details for DOI 10.1021/jacs.3c02326

    View details for PubMedID 37145876

  • Dynamic Tracking of NiFe Smart Catalysts using In Situ X-Ray Absorption Spectroscopy for the Dry Methane Reforming Reaction ACS CATALYSIS Shah, S., Hong, J., Cruz, L., Wasantwisut, S., Bare, S. R., Gilliard-AbdulAziz, K. 2023
  • Structure and Site Evolution of Framework Ni Species in MIL-127 MOFs for Propylene Oligomerization Catalysis JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Yeh, B., Chheda, S., Prinslow, S. D., Hoffman, A. S., Hong, J., Perez-Aguilar, J. E., Bare, S. R., Lu, C. C., Gagliardi, L., Bhan, A. 2023; 145 (6): 3408-3418
  • Nano-sized Metallic Nickel Clusters Stabilized on Dealuminated beta-Zeolite: A Highly Active and Stable Ethylene Hydrogenation Catalyst JOURNAL OF PHYSICAL CHEMISTRY C Meloni, M., Hong, J., Hoffman, A. S., Holton, S., Kulkarni, A., Bare, S. R., Runnebaum, R. C. 2022; 126 (50): 21213-21222
  • Surface Fe clusters promote syngas reaction to oxygenates on Rh catalysts modified by atomic layer deposition JOURNAL OF CATALYSIS Nathan, S. S., Asundi, A. S., Hoffman, A. S., Hong, J., Zhou, C., Vila, F. D., Cargnello, M., Bare, S. R., Bent, S. F. 2022; 414: 125-136
  • Atomically Dispersed Platinum in Surface and Subsurface Sites on MgO Have Contrasting Catalytic Properties for CO Oxidation. The journal of physical chemistry letters Chen, Y., Rana, R., Huang, Z., Vila, F. D., Sours, T., Perez-Aguilar, J. E., Zhao, X., Hong, J., Hoffman, A. S., Li, X., Shang, C., Blum, T., Zeng, J., Chi, M., Salmeron, M., Kronawitter, C. X., Bare, S. R., Kulkarni, A. R., Gates, B. C. 2022: 3896-3903


    Atomically dispersed metals on metal oxide supports are a rapidly growing class of catalysts. Developing an understanding of where and how the metals are bonded to the supports is challenging because support surfaces are heterogeneous, and most reports lack a detailed consideration of these points. Herein, we report two atomically dispersed CO oxidation catalysts having markedly different metal-support interactions: platinum in the first layer of crystalline MgO powder and platinum in the second layer of this support. Structural models have been determined on the basis of data and computations, including those determined by extended X-ray absorption fine structure and X-ray absorption near edge structure spectroscopies, infrared spectroscopy of adsorbed CO, and scanning transmission electron microscopy. The data demonstrate the transformation of surface to subsurface platinum as the temperature of sample calcination increased. Catalyst performance data demonstrate the lower activity but greater stability of the subsurface platinum than of the surface platinum.

    View details for DOI 10.1021/acs.jpclett.2c00667

    View details for PubMedID 35471032

  • Steering CO2 hydrogenation toward C-C coupling to hydrocarbons using porous organic polymer/metal interfaces. Proceedings of the National Academy of Sciences of the United States of America Zhou, C., Asundi, A. S., Goodman, E. D., Hong, J., Werghi, B., Hoffman, A. S., Nathan, S. S., Bent, S. F., Bare, S. R., Cargnello, M. 2022; 119 (7)


    The conversion of CO2 into fuels and chemicals is an attractive option for mitigating CO2 emissions. Controlling the selectivity of this process is beneficial to produce desirable liquid fuels, but C-C coupling is a limiting step in the reaction that requires high pressures. Here, we propose a strategy to favor C-C coupling on a supported Ru/TiO2 catalyst by encapsulating it within the polymer layers of an imine-based porous organic polymer that controls its selectivity. Such polymer confinement modifies the CO2 hydrogenation behavior of the Ru surface, significantly enhancing the C2+ production turnover frequency by 10-fold. We demonstrate that the polymer layers affect the adsorption of reactants and intermediates while being stable under the demanding reaction conditions. Our findings highlight the promising opportunity of using polymer/metal interfaces for the rational engineering of active sites and as a general tool for controlling selective transformations in supported catalyst systems.

    View details for DOI 10.1073/pnas.2114768119

    View details for PubMedID 35135880

  • Reactivity of Pd-MO2 encapsulated catalytic systems for CO oxidation CATALYSIS SCIENCE & TECHNOLOGY Paz Herrera, L., Freitas, L., Hong, J., Hoffman, A. S., Bare, S. R., Nikolla, E., Medlin, J. 2022

    View details for DOI 10.1039/d1cy01916c

    View details for Web of Science ID 000749672000001

  • Dynamic Surface Reconstruction Unifies the Electrocatalytic Oxygen Evolution Performance of Nonstoichiometric Mixed Metal Oxides. JACS Au Samira, S., Hong, J., Camayang, J. C., Sun, K., Hoffman, A. S., Bare, S. R., Nikolla, E. 1800; 1 (12): 2224-2241


    Compositionally versatile, nonstoichiometric, mixed ionic-electronic conducting metal oxides of the form A n+1B n O3n+1 (n = 1 ; A = rare-earth-/alkaline-earth-metal cation; B = transition-metal (TM) cation) remain a highly attractive class of electrocatalysts for catalyzing the energy-intensive oxygen evolution reaction (OER). The current design strategies for describing their OER activities are largely derived assuming a static, unchanged view of their surfaces, despite reports of dynamic structural changes to 3d TM-based perovskites during OER. Herein, through variations in the A- and B-site compositions of A n+1B n O3n+1 oxides (n = 1 (A2BO4) or n = (ABO3); A = La, Sr, Ca; B = Mn, Fe, Co, Ni), we show that, in the absence of electrolyte impurities, surface restructuring is universally the source of high OER activity in these oxides and is dependent on the initial oxide composition. Oxide surface restructuring is induced by irreversible A-site cation dissolution, resulting in in situ formation of a TM oxyhydroxide shell on top of the parent oxide core that serves as the active surface for OER. The rate of surface restructuring is found to depend on (i) composition of A-site cations, with alkaline-earth-metal cations dominating lanthanide cation dissolution, (ii) oxide crystal phase, with n = 1 A2BO4 oxides exhibiting higher rates of A-site dissolution in comparison to n = ABO3 perovskites, (iii) lattice strain in the oxide induced by mixed rare-earth- and alkaline-earth-metal cations in the A-site, and (iv) oxide reducibility. Among the in situ generated 3d TM oxyhydroxide structures from A n+1B n O3n+1 oxides, Co-based structures are characterized by superior OER activity and stability, even in comparison to as-synthesized Co-oxyhydroxide, pointing to the generation of high active surface area structures through oxide restructuring. These insights are critical toward the development of revised design criteria to include surface dynamics for effectively describing the OER activity of nonstoichiometric mixed-metal oxides.

    View details for DOI 10.1021/jacsau.1c00359

    View details for PubMedID 34977894

  • Identifying higher oxygenate synthesis sites in Cu catalysts promoted and stabilized by atomic layer deposited Fe2O3 JOURNAL OF CATALYSIS Asundi, A. S., Nathan, S. S., Hong, J., Hoffman, A. S., Pennel, M., Bare, S. R., Bent, S. F. 2021; 404: 210-223
  • A Theory-Guided X-ray Absorption Spectroscopy Approach for Identifying Active Sites in Atomically Dispersed Transition-Metal Catalysts. Journal of the American Chemical Society Chen, Y., Rana, R., Sours, T., Vila, F. D., Cao, S., Blum, T., Hong, J., Hoffman, A. S., Fang, C., Huang, Z., Shang, C., Wang, C., Zeng, J., Chi, M., Kronawitter, C. X., Bare, S. R., Gates, B. C., Kulkarni, A. R. 2021


    Atomically dispersed supported metal catalysts offer new properties and the benefits of maximized metal accessibility and utilization. The characterization of these materials, however, remains challenging. Using atomically dispersed platinum supported on crystalline MgO (chosen for its well-defined bonding sites) as a prototypical example, we demonstrate how systematic density functional theory calculations for assessing all the potentially stable platinum sites, combined with automated analysis of extended X-ray absorption fine structure (EXAFS) spectra, leads to unbiased identification of isolated, surface-enveloped platinum cations as the catalytic species for CO oxidation. The catalyst has been characterized by atomic-resolution imaging and EXAFS and high-energy resolution fluorescence detection X-ray absorption near edge spectroscopy. The proposed platinum sites are in agreement with experiment. This theory-guided workflow leads to rigorously determined structural models and provides a more detailed picture of the structure of the catalytically active site than what is currently possible with conventional EXAFS analyses. As this approach is efficient and agnostic to the metal, support, and catalytic reaction, we posit that it will be of broad interest to the materials characterization and catalysis communities.

    View details for DOI 10.1021/jacs.1c07116

    View details for PubMedID 34806881

  • Unlocking the Catalytic Potential of TiO2-Supported Pt Single Atoms for the Reverse Water-Gas Shift Reaction by Altering Their Chemical Environment JACS AU Chen, L., Unocic, R. R., Hoffman, A. S., Hong, J., Braga, A. H., Bao, Z., Bare, S. R., Szanyi, J. 2021; 1 (7): 977-986
  • Reduction and Agglomeration of Supported Metal Clusters Induced by High-Flux X-ray Absorption Spectroscopy Measurements JOURNAL OF PHYSICAL CHEMISTRY C Albrahim, M., Thompson, C., Leshchev, D., Shrotri, A., Unocic, R. R., Hong, J., Hoffman, A. S., Meloni, M. J., Runnebaum, R. C., Bare, S. R., Stavitski, E., Karim, A. M. 2021; 125 (20): 11048-11057
  • Understanding Support Effects of ZnO-Promoted Co Catalysts for Syngas Conversion to Alcohols Using Atomic Layer Deposition CHEMCATCHEM Nathan, S. S., Asundi, A. S., Singh, J. A., Hoffman, A. S., Boubnov, A., Hong, J., Bare, S. R., Bent, S. F. 2020
  • Enhanced alcohol production over binary Mo/Co carbide catalysts in syngas conversion JOURNAL OF CATALYSIS Asundi, A. S., Hoffman, A. S., Chi, M., Nathan, S. S., Boubnov, A., Hong, J., Bare, S. R., Bent, S. F. 2020; 391: 446–58