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An Operando View of the Nanoscale
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serve as the foundation to each presentation in the session. Below, several papers that were presented at that symposium are summarized to highlight key areas where synchrotron-based methods have advanced our understanding of catalysis. In addition, as with any field, new discoveries often more clearly reveal important gaps and challenges that limit accurate measurement and prediction of catalyst activity under process-dependent conditions. These challenges will be briefly described as a way to point researchers toward addressing future challenges in this area. Perhaps Ashleigh Baber captured the central theme of the symposium best during her introductory remarks by stating that “The catalytic importance of metal−oxide interfaces has long been recognized, but the nanoscale determination of their properties and role is only now emerging”. Central to these studies is a growing arsenal of analytical capability, probes, reactor cells, and methodologies that have emerged in recent years, and when coupled with synchrotron sources, these assets provide exciting opportunities to perform in situ operando characterization of catalysts at the nanoscale (see Figure 1). For example, XAS is recognized as the primary workhorse for in situ operando characterization because the electron scattering technique is capable of providing detailed structural and electronic information at high temporal resolution. Variations of the method, such as X-ray near-edge emission spectroscopy (XANES) and extended X-ray absorption finite spectroscopy (EXAFS), provide fine structural resonance information on interactions of adsorbates within localized geometries. XAS methods, applied to deciphering the dynamics of metallic particles during catalysis, were the central theme of a paper presented by Anatoly Frenkel entitled, “Probing Cooperative Phenomena in Nanoscale Metal Catalysts by Operando Techniques”. In this insightful study, the complex interdependent nature of a γ-Al2O3-supported Pt catalyst was highlighted by demonstrating that the charged state of the nanometer-sized particles depends on competing interactions between the particle, support, and adsorbate. By coupling X-ray absorption and emission spectroscopies, IR spectroscopy, and in situ electron microscopy to first-principles modeling, the Frenkel group showed that localized strong particle/support interactions induce compressive strain in the Pt nanoparticles. These interactions manifest asymmetric variations in charge and metal−metal bond distances within the Pt nanoparticles and affect site-specific adsorption energies and surface reactivity. In an excellent complement to the Frenkel presentation, Ralph Nuzzo demonstrated the extraordinary dynamics of Pt particles during catalysis. In his presentation, “In Operando Characterization of the Structural Dynamics of Supported Heterogeneous Catalysts during Transformations of C−C and C−H Bonds”, Nuzzo showed how Pt clusters on silica supports underwent significant restructuring in reactiondriven transformations of the catalyst over length scales ranging
he August 2015 National Meeting of the American Chemical Society (ACS) was, as usual, host to many sessions in the field of catalysis and surface science. Many of these sessions focused on research aimed at constructing a fundamental understanding of catalyst behavior at the molecular as well as ensemble level, with the overall objective of developing strategies for guiding future catalysts tailored to achieve the desired outcome. Across the scientific community, whether the interest lies in energy and chemical conversion, electronics, medicine, or military defense, understanding the dynamics between a functional material’s activity, in particular, changes in surface−bulk structure and electronic properties, under relevant operating conditions, is critical to the successful design of highly efficient and robust materials. Of the multitude of methods now available to surface scientists for the exploration of catalyst behavior, few have the power and promise of the next-generation synchrotron-based techniques that are providing unprecedented insight into reaction dynamics under working conditions.
Of the multitude of methods now available to surface scientists for the exploration of catalyst behavior, few have the power and promise of the next-generation synchrotron-based techniques that are providing unprecedented insight into reaction dynamics under working conditions. While ultrahigh-vacuum (UHV) surface science methods have provided exquisite insight into catalysis and will continue to provide invaluable insight into surface chemistry,1 there always is the question of whether the pressure gap between UHV and ambient conditions alters the surface chemistry. Xray absorption spectroscopy (XAS) and scattering techniques enable characterization of structural properties and reaction dynamics under ambient conditions in ways that are inaccessible to many other methods. XAS-based experiments, especially when coupled with density functional theory (DFT), molecular dynamics (MD), and ab initio methodologies, can provide a comprehensive (continuum) understanding of the dynamics of real and model catalyst surfaces at the nanoscale.2 When effectively integrated, experimental and theoretical modeling techniques can yield instructive information to guide the design and preparation of highly efficient and stable catalysts that can function over a broad spectrum of process conditions. Such in situ and in operando methods for deciphering catalytic chemical reaction dynamics were the primary focus in one particular session at the ACS meeting entitled, “Operando Spectroscopic Approach to Quantifying Structure−Activity Relationships of Real Catalysts under Ambient Conditions”. References 3−24 list the citations that © 2015 American Chemical Society
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Figure 1. Schematic diagram of the gas-flow system coupled to a synchrotron X-ray source that conceptually shows how one may obtain information from XAS, XRD, mass spectrometry, and infrared spectroscopy under operational conditions. This figure was presented at the symposium by Anatoly Frenkel and used here with permission.
nanoclusters. The challenge is to perform simultaneous in situ operando experiments in a single reaction cell that provides the sensitivity of long-range order (XRD) and local structure (XAS) of mixed-phase (crystalline and amorphous) catalysts. In a presentation that brought together many of the primary themes that emerged from the overall session, Ashleigh Baber presented a paper from her research group on “Unraveling the Relationship between Structure and Activity using Model Catalysts under Near-Ambient Pressures”. In this study, Baber et al. investigated the reduction of Cu2O/Cu(111) by CO by a combination of in situ scanning tunneling microscopy (STM) and XPS from UHV to near-ambient pressure (NAP) conditions coupled with theory to provide insight into the highly reducing environment of the water−gas shift reaction on a model oxide surface. This study demonstrated that surface oxide species could be identified with atomic-scale detail under NAPs. Similarly, in an effort to understand the relationship between electronic states and interaction energies, a number of researchers discussed the use of multiple and simultaneous techniques, such as ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and XAS. For example, Aaron Geller et al. presented a paper on “In Operando Tracking of Surface Electrochemical Redox Activity in Solid Oxide Electrochemical Cells using Near Infrared Radiation Imaging”. Geller’s team used near-infrared (NIR) radiation imaging in conjunction with NAP-XPS to study chemical and electrochemical redox processes on a ceria-based solid oxide electrochemical cell (SOC). Geller demonstrated that AP-XPS ceria redox state analyses correlate strongly with NIR imaging data in which ceria redox states are tracked by monitoring changes in NIR radiation emission. In another presentation by Daniel Friebel titled “AP-XPS and HERFD XAS as Complementary Operando Probes in Electrocatalysis”, Friebel’s team employed XAS in the high-energy resolution fluorescence detection (HERFD) mode to obtain spectra that contained more detailed electronic structure information than conventional XANES. The technique, combined with computational modeling of the oxygen evolution reaction (OER), allowed them to identify the most active sites for OER in mixed Fe/Ni oxyhydroxides. The presentations summarized above, as well as others discussed at
from a single atom to large clusters during ethylene hydrogenation. By coupling multiple in situ operando spectroscopic and microscopic techniques, they were able to fully characterize the dynamic transformation of the nanoparticle catalyst. Both presentations by Frenkel and Nuzzo illustrate the complex nature of integrating experimental techniques with theory to provide a more accurate representation of dynamical changes of a nanocatalyst under operando conditions. A key challenge remains in developing higher spatial and temporal resolution techniques to reveal more clearly dynamical changes and kinetics across disordered states of nanoclusters. The move from ensemble-based averaging techniques to characterization of individual nanoparticles will greatly improve the ability to understand the heterogeneities of real catalysts at the nanoscale.
The catalytic importance of metal−oxide interfaces has long been recognized, but the nanoscale determination of their properties and role is only now emerging. Jeffery Miller moved the symposium beyond monometallic catalysts by presenting his group’s efforts to better quantify the effect of temperature on metal−metal bonding states during his presentation titled, “Structural Evolution of an Intermetallic Pd−Zn Catalyst Selective for Propane Dehydrogenation”. The Miller group effectively employed in situ synchrotron extended EXAFS, X-ray diffraction (XRD), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to follow structural changes on a Pd−Zn alloy supported on an Al2O3 catalyst. Miller showed that a bimetallic nanoparticle forms at about 230 °C and the number of Pd−Zn bonds increases as the temperature decreases. By combining local structural information obtained from EXAFS with information on varying degrees of disorder (high/low dimensionality) using XRD, an accurate picture can emerge describing the kinetics and growth of metal 4924
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Review: Effects of Complex Interactions on Structure and Dynamics of Supported Metal Catalysts. J. Vac. Sci. Technol., A 2014, 32, 020801. (4) Frenkel, A. I.; Rodriguez, J. A.; Chen, J. G. Synchrotron Techniques for In-Situ Catalytic Studies: Capabilities, Challenges, and Opportunities. ACS Catal. 2012, 2, 2269−2280. (5) Patlolla, E. V.; Carino, S. N.; Ehrlich, E.; Stavitski, E. E.; Frenkel, A. I. Application of Operando XAS, XRD, and Raman Spectroscopy for Phase Speciation in Water Gas Shift Reaction Catalysts. ACS Catal. 2012, 2, 2216−2223. (6) Alayoglu, S.; Krier, J. M.; Michalak, W. D.; Zhu, Z.; Gross, E.; Somorjai, G. A. In Situ Surface and Reaction Probe Studies with Model Nanoparticle Catalysts. ACS Catal. 2012, 2, 2250−2258. (7) Schuyten, S.; Guerrero, S.; Miller, J. T.; Shibata, T.; Wolf, E. E. Characterization and Oxidation States of Cu and Pd in Pd−CuO/ ZnO/ZrO2 Catalysts for Hydrogen Production by Methanol Partial Oxidation. Appl. Catal., A 2009, 352, 133−144. (8) Miller, J. B.; Gumuslu, G.; Yin, C.; Gellman, A. J. Catalytic Reactivity and Electronic Structure Across Continuous Cuxxpd1-X And Cuxauypd1-X-Y Composition Space. Am. Chem. Soc., Div. Energy Fuels 2014, 59, 352. (9) Mudiyanselage, K.; Senanayake, S.; Baber, A.; Kundu, S.; Stacchiola, D. Investigations of Reactions Over Inverse CeOx-CuyO/ Cu(111) and TiO2-CuyO/Cu(111) Model Catalysts. Am. Chem. Soc., Div. Energy Fuels 2013, 58, 486−488. (10) Mondloch, J. E.; Katz, M. J.; Isley, W. C., III; Ghosh, P.; Liao, P.; Bury, W.; Wagner, G. W.; Hall, M. H.; DeCoste, J. D.; Peterson, G. W.; Randall, Q.; Snurr, R. Q.; Cramer, C. J.; Hupp, J. T.; Farha, O. F. Destruction of Chemical Warfare Agents Using Metal−Organic Frameworks. Nat. Mater. 2015, 14, 512−516. (11) Li, J.; Cushing, S. K.; Meng, F.; Senty, T. R.; Bristow, A. D.; Wu, N. Plasmon-Induced Resonance Energy Transfer For Solar Energy Conversion. Nat. Photonics 2015, 9, 601−607. (12) Singh, J.; Im, J.; Watters, E. J.; Whitten, J. E.; Soares, J. W.; Steeves, D. M. Thiol Dosing of ZnO Single Crystals and Nanorods: Surface Chemistry and Photoluminescence. Surf. Sci. 2013, 609, 183− 189. (13) Friebel, D.; Louie, M. W.; Bajdich, M.; Sanwald, K. E.; Cai, Y.; Wise, A. M.; Cheng, M. J.; Sokaras, D.; Weng, T. C.; Alonso-Mori, R.; Davis, R. C.; Bargar, J. R.; Nørskov, J. K.; Nilsson, A.; Bell, A. T. Identification of Highly Active Fe Sites in (Ni,Fe)OOH for Electrocatalytic Water Splitting. J. Am. Chem. Soc. 2015, 137, 1305− 1313. (14) Yu, Y.; Mao, B.; Geller, A.; Chang, R.; Gaskell, K.; Liu, Z.; Eichhorn, B. W. CO2 Activation and Carbonate Intermediates: an Operando AP-XPS Study of CO2 Electrolysis Reactions on solid oxide electrochemical cells. Phys. Chem. Chem. Phys. 2014, 16, 11633− 11639. (15) Achtyl, J. L.; Vlassiouk, I. V.; Dai, S.; Geiger, F. Interactions of Organic Solvents at Graphene/A-Al2O3 and Graphene Oxide/AAl2O3 Interfaces Studied by Sum Frequency Generation. J. Phys. Chem. C 2014, 118, 17745−17755. (16) Swierk, J. R.; Klaus, S.; Trotochaud, L.; Bell, A. T.; Tilley, T. D. Role of Catalyst Preparation on the Electrocatalytic Activity of Ni1xFexOOH for the Oxygen Evolution Reaction. J. Phys. Chem. C 2015, 119, 19022−19029. (17) Jung, U.; Elsen, A.; Li, Y.; Smith, J. G.; Small, M. W.; Stach, E. A.; Frenkel, A. I.; Nuzzo, R. G. Comparative in Operando Studies in Heterogeneous Catalysis: Atomic and Electronic Structural Features in the Hydrogenation of Ethylene Over Supported Pd and Pt Catalysts. ACS Catal. 2015, 5, 1539−1551. (18) Lwin, S.; Keturakis, C.; Handzlik, J.; Sautet, P.; Li, Y.; Frenkel, A. I.; Wachs, I. E. Surface ReOx Sites on Al2O3 and Their Molecular Structure-Reactivity Relationships for Olefin Metathesis. ACS Catal. 2015, 5, 1432−1444. (19) Bridewell, V. L.; Alam, R.; Karwacki, C. J.; Kamat, P. V. CdSe/ CdS Nanorod Photocatalysts: Tuning the Interfacial Charge Transfer Process Through Shell Length. Chem. Mater. 2015, 27, 5064−5071. (20) Axnanda, S.; Zhu, Z.; Zhou, W.; Mao, B.; Chang, R.; Rani, S.; Crumlin, E.; Somorjai, G.; Liu, Z. In Situ Characterizations of
the symposium, clearly reflect a strong desire to pursue a variety of ambient-pressure-capable techniques, such as XPS, STM, and XAS, configured under operando conditions. The challenge in addition to the need for higher spatial-temporal resolution is to pursue alternative methods and capabilities for integrating multiple techniques in a space-constrained environment. For example, in recent years, greater attention has been devoted to improving the design of in situ reactor cells that function efficiently across multiple scattering techniques. In summary, for more than 3 decades, fundamental surface science in a UHV environment provided a detailed mechanistic picture of molecular processes on idealized catalytic surfaces. The challenge at the time was whether the low-pressure studies were indicative of the surface chemistry of an actual running catalyst, a problem often referred to as “the pressure gap”. Today, we are in a new age of surface chemistry studies, where new tools and methods allow us to measure the structure and function of site-specific reactivity under in situ operando conditions. Such measurements are revealing the dynamic complexity of the actual structure and chemical reactivity on few-atom clusters under realistic dynamic conditions but with the specificity and detail that we have come to expect from UHV surface science studies. It is a new dawn in operando nanoscale surface studies for understanding the chemical reactivity and dynamics of catalyst materials under ambient or operationally relevant conditions.
John R. Morris† John N. Russell, Jr.‡ Christopher J. Karwacki*,§ †
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Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, United States ‡ Chemistry Division, Naval Research Laboratory, Washington, District of Columbia 20375, United States § Protection and Decontamination Division, U.S. Army, Edgewood Chemical Biological Center, Aberdeen Proving Ground, Aberdeen, Maryland 21010, United States
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to acknowledge all of the participants who presented their research at the 250th ACS National Meeting & Exposition symposium on Operando Spectroscopic Approach to Quantif ying Structure−Activity Relationships of Real Catalysts under Ambient Conditions. The support of the Army Research Office (W911NF-15-1-0186) and the Defense Threat Reduction Agency (W911NF-15-2-0107) is gratefully acknowledged.
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REFERENCES
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Nanostructured SnOx/Pt(111) Surfaces Using Ambient-Pressure XPS (APXPS) and High-Pressure Scanning Tunneling Microscopy. J. Phys. Chem. C 2014, 118, 1935−1943. (21) Brozek, C. K.; Miller, J. T.; Stoian, S. A.; Dinca, M. NO Disproportionation at a Mononuclear Site-Isolated Fe2+ Center in Fe2+-MOF-5. J. Am. Chem. Soc. 2015, 137, 7495−7501. (22) Carenco, S.; Wu, C. H.; Shavorskiy, A.; Alayoglu, S.; Somorjai, G. A.; Bluhm, H.; Salmeron, M. Synthesis and Structural Evolution of Nickel-Cobalt Nanoparticles Under H2 and CO2. Small 2015, 11, 3045−3053. (23) Karim, A. M.; Wei, Z.; Tupy, S. A.; Vlachos, D. G.; Chen, J. G.; King, D. L.; Wang, Y. Bimetallic Pt−M Catalysts for Aqueous Phase Reforming of Glycerol. Am. Chem. Soc., Div. Energy Fuels 2015, 60, 258. (24) Anderson, J. S.; Gallagher, A. T.; Mason, J. A.; Harris, T. D. A Five-Coordinate Heme Dioxygen Adduct Isolated Within a MetalOrganic Framework. J. Am. Chem. Soc. 2014, 136, 16489−16492.
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