2016-presentDistinguished Staff Scientist, SSRL, SLAC National Accelerator Lab
2010–2016Research Fellow, UOP LLC, Des Plaines, IL
2003–2010 Senior Research & Development Associate, UOP LLC, Des Plaines, IL
1996–2003Research & Development Associate, UOP LLC, Des Plaines, IL
1986–1996Staff Scientist, The Dow Chemical Company, Midland, MI
1984–1986Postdoctoral Research Associate, Materials & Molecular Research Division, Lawrence Berkeley National Laboratory, Berkeley
1982–1984Postdoctoral Research Associate, Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, NY
University of Liverpool, U.K.ChemistryB.Sc. (Honors) 1979
University of Liverpool, U.K.Physical ChemistryPh.D.1982
Conference Chair:“Operando-IV: Recent developments and future perspectives in spectroscopy of working catalysts,” April 2012, Brookhaven National Laboratory
Co-Chair:“Advanced x-ray techniques for catalyst characterization”, ACS National Meeting, April 2017.
Synthesis of Colloidal Pd/Au Dilute Alloy Nanocrystals and Their Potential for Selective Catalytic Oxidations.
Journal of the American Chemical Society
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
Direct observation of the kinetics of gas–solid reactions using in situ kinetic and spectroscopic techniques
Reaction Chemistry & Engineering
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
2018; 140 (42): 13736–45
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
2017; 23 (59): 14760–68
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
- Tuning the Selectivity of Single-Site Supported Metal Catalysts with Ionic Liquids ACS CATALYSIS 2017; 7 (10): 6969–72
- Uniform Pt/Pd Bimetallic Nanocrystals Demonstrate Platinum Effect on Palladium Methane Combustion Activity and Stability ACS CATALYSIS 2017; 7 (7): 4372–80
- Characterization of Coke on a Pt-Re/gamma-Al2O3 Re-Forming Catalyst: Experimental and Theoretical Study ACS CATALYSIS 2017; 7 (2): 1452–61
Coke Formation in a Zeolite Crystal During the Methanol-to-Hydrocarbons Reaction as Studied with Atom Probe Tomography
WILEY-V C H VERLAG GMBH. 2016: 11173–77
Understanding the formation of carbon deposits in zeolites is vital to developing new, superior materials for various applications, including oil and gas conversion processes. Herein, atom probe tomography (APT) has been used to spatially resolve the 3D compositional changes at the sub-nm length scale in a single zeolite ZSM-5 crystal, which has been partially deactivated by the methanol-to-hydrocarbons reaction using (13) C-labeled methanol. The results reveal the formation of coke in agglomerates that span length scales from tens of nanometers to atomic clusters with a median size of 30-60 (13) C atoms. These clusters correlate with local increases in Brønsted acid site density, demonstrating that the formation of the first deactivating coke precursor molecules occurs in nanoscopic regions enriched in aluminum. This nanoscale correlation underscores the importance of carefully engineering materials to suppress detrimental coke formation.
View details for DOI 10.1002/anie.201606099
View details for Web of Science ID 000383642300038
View details for PubMedID 27485276
Transmission and fluorescence X-ray absorption spectroscopy cell/flow reactor for powder samples under vacuum or in reactive atmospheres
REVIEW OF SCIENTIFIC INSTRUMENTS
2016; 87 (7): 073108
X-ray absorption spectroscopy is an element-specific technique for probing the local atomic-scale environment around an absorber atom. It is widely used to investigate the structures of liquids and solids, being especially valuable for characterization of solid-supported catalysts. Reported cell designs are limited in capabilities-to fluorescence or transmission and to static or flowing atmospheres, or to vacuum. Our goal was to design a robust and widely applicable cell for catalyst characterizations under all these conditions-to allow tracking of changes during genesis and during operation, both under vacuum and in reactive atmospheres. Herein, we report the design of such a cell and a demonstration of its operation both with a sample under dynamic vacuum and in the presence of gases flowing at temperatures up to 300 °C, showing data obtained with both fluorescence and transmission detection. The cell allows more flexibility in catalyst characterization than any reported.
View details for DOI 10.1063/1.4958824
View details for Web of Science ID 000382448600011
View details for PubMedID 27475549
- Surface analysis of zeolites: An XPS, variable kinetic energy XPS, and low energy ion scattering study SURFACE SCIENCE 2016; 648: 376–82
- Aberration-Corrected Transmission Electron Microscopy and InSitu XAFS Structural Characterization of Pt/-Al2O3 Nanoparticles CHEMCATCHEM 2015; 7 (22): 3779–87
- Expanding Beyond the Micropore: Active-Site Engineering in Hierarchical Architectures for Beckmann Rearrangement ACS Catalysis 2015; 5 (11): 6587–93
Spectroscopic and Computational Insights on Catalytic Synergy in Bimetallic Aluminophosphate Catalysts
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
2015; 137 (26): 8534-8540
A combined electronic structure computational and X-ray absorption spectroscopy study was used to investigate the nature of the active sites responsible for catalytic synergy in Co-Ti bimetallic nanoporous frameworks. Probing the nature of the molecular species at the atomic level has led to the identification of a unique Co-O-Ti bond, which serves as the loci for the superior performance of the bimetallic catalyst, when compared with its analogous monometallic counterpart. The structural and spectroscopic features associated with this active site have been characterized and contrasted, with a view to affording structure-property relationships, in the wider context of designing sustainable catalytic oxidations with porous solids.
View details for DOI 10.1021/jacs.5b03734
View details for Web of Science ID 000357964400039
View details for PubMedID 26076192
Determining the location and nearest neighbours of aluminium in zeolites with atom probe tomography
Zeolite catalysis is determined by a combination of pore architecture and Brønsted acidity. As Brønsted acid sites are formed by the substitution of AlO4 for SiO4 tetrahedra, it is of utmost importance to have information on the number as well as the location and neighbouring sites of framework aluminium. Unfortunately, such detailed information has not yet been obtained, mainly due to the lack of suitable characterization methods. Here we report, using the powerful atomic-scale analysis technique known as atom probe tomography, the quantitative spatial distribution of individual aluminium atoms, including their three-dimensional extent of segregation. Using a nearest-neighbour statistical analysis, we precisely determine the short-range distribution of aluminium over the different T-sites and determine the most probable Al-Al neighbouring distance within parent and steamed ZSM-5 crystals, as well as assess the long-range redistribution of aluminium upon zeolite steaming.
View details for DOI 10.1038/ncomms8589
View details for Web of Science ID 000358855900002
View details for PubMedID 26133270
- Characterization of a Fluidized Catalytic Cracking Catalyst on Ensemble and Individual Particle Level by X-ray Micro-and Nanotomography, Micro-X-ray Fluorescence, and Micro-X-ray Diffraction CHEMCATCHEM 2014; 6 (5): 1427–37
- Pressure-Dependent Effect of Hydrogen Adsorption on Structural and Electronic Properties of Pt/gamma-Al2O3 Nanoparticles CHEMCATCHEM 2014; 6 (1): 348–52
- Interplay between nanoscale reactivity and bulk performance of H-ZSM-5 catalysts during the methanol-to-hydrocarbons reaction JOURNAL OF CATALYSIS 2013; 307: 185–93
- Operando Effects on the Structure and Dynamics of PtnSnm/gamma-Al2O3 from Ab Initio Molecular Dynamics and X-ray Absorption Spectra JOURNAL OF PHYSICAL CHEMISTRY C 2013; 117 (24): 12446–57
- EXAFS Model of 2-Dimensional Platinum Clusters IOP PUBLISHING LTD. 2013
Electronic properties and charge transfer phenomena in Pt nanoparticles on gamma-Al2O3: size, shape, support, and adsorbate effects
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
2012; 14 (33): 11766–79
This study presents a systematic detailed experimental and theoretical investigation of the electronic properties of size-controlled free and γ-Al(2)O(3)-supported Pt nanoparticles (NPs) and their evolution with decreasing NP size and adsorbate (H(2)) coverage. A combination of in situ X-ray absorption near-edge structure (XANES) and density functional theory (DFT) calculations revealed changes in the electronic characteristics of the NPs due to size, shape, NP-adsorbate (H(2)) and NP-support interactions. A correlation between the NP size, number of surface atoms and coordination of such atoms, and the maximum hydrogen coverage stabilized at a given temperature is established, with H/Pt ratios exceeding the 1 : 1 ratio previously reported for bulk Pt surfaces.
View details for DOI 10.1039/c2cp41928a
View details for Web of Science ID 000307017800023
View details for PubMedID 22828479
- Controlled one-step synthesis of hierarchically structured macroscopic silica spheres MICROPOROUS AND MESOPOROUS MATERIALS 2011; 146 (1-3): 18–27
- Density functional theory study of the effect of subsurface H, C, and Ag on C2H2 hydrogenation on Pd(1 1 1) CATALYSIS TODAY 2011; 165 (1): 106–11
- Experimental (XAS, STEM, TPR, and XPS) and Theoretical (DFT) Characterization of Supported Rhenium Catalysts JOURNAL OF PHYSICAL CHEMISTRY C 2011; 115 (13): 5740–55
Simultaneous XAFS measurements of multiple samples
JOURNAL OF SYNCHROTRON RADIATION
2010; 17: 380–85
A four-channel ionization chamber has been designed, constructed and tested. This ionization chamber allows X-ray absorption spectra to be collected in transmission from up to four samples simultaneously. This results in spectra that are free of systematic uncertainty in relative energy alignment introduced by scan-to-scan stability of the monochromator or of numerical uncertainty associated with a post-processing alignment algorithm, allowing, in a single shot, an absolute measure of edge shift between four samples of different valence. As four samples can be measured in parallel, the time expended over the course of an experiment to cycle the measurement environment between its rest state and the measurement condition is substantially reduced. The ionization chamber is simple in design and could be implemented at virtually any XAFS beamline with a horizontal fan of radiation such as that provided by a bend magnet or wiggler.
View details for DOI 10.1107/S0909049510006230
View details for Web of Science ID 000276785600012
View details for PubMedID 20400837
Characterizing industrial catalysts using in situ XAFS under identical conditions
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
2010; 12 (27): 7702–11
In situ X-ray absorption spectroscopy (XAS) in catalysis research has traditionally been conducted by making one measurement at a time on a single sample. In an industrial research environment this is especially limiting as sample throughput (productivity) and turnaround time (direct project relevance) are critical issues in the use of XAS in a fast-moving technology delivery project. In order to address these issues we have developed and implemented a four-channel ionization chamber combined with two different in situ cells that allows XAS data to be collected simultaneously from four samples, or four regions, in transmission geometry without any sample or detector movement. In the development of this new capability it was realized that there are other benefits from this simultaneous detection in addition to increased productivity. Namely, (i) the use of EXAFS to determine the structure of a catalyst in situ axially at four different positions down a catalyst bed; (ii) the ability to collect up to four XAFS spectra simultaneously and thereby avoid any scan-to-scan uncertainties, and (iii) the added confidence in the ability to discriminate small differences in similarly prepared catalysts which is typical in the development of a commercial catalyst. Specific illustrations of each of these applications are shown. The methodology is simple to implement and could be used on any XAFS beamline with a horizontal fan of radiation, such as at a typical bending magnet or wiggler source beamline.
View details for DOI 10.1039/b926621f
View details for Web of Science ID 000279514500045
View details for PubMedID 20505854