José A. Rodriguez

Activity of CeOx and TiOx Nanoparticles Grown on Au(111) in the Water-Gas Shift Reaction

The high performance of Au-CeO2 and Au-TiO2 catalysts in the water-gas shift (WGS) reaction (H2O + CO→H2 + CO2) relies heavily on the direct participation of the oxide in the catalytic process. Although clean Au(111) is not catalytically active for the WGS, gold surfaces that are 20 to 30% covered by ceria or titania nanoparticles have activities comparable to those of good WGS catalysts such as Cu(111) or Cu(100). In TiO2-x/Au(111) and CeO2-x/Au(111), water dissociates on O vacancies of the oxide nanoparticles, CO adsorbs on Au sites located nearby, and subsequent reaction steps take place at the metal-oxide interface. In these inverse catalysts, the moderate chemical activity of bulk gold is coupled to that of a more reactive oxide.

Active sites for CO2 hydrogenation to methanol on Cu/ZnO catalysts

Synergy between copper and zinc oxide on a catalyst surface facilitates methanol synthesis via CO2 hydrogenation. Metal-oxide synergy The hydrogenation of carbon dioxide is a key step in the industrial production of methanol. Catalysts made from copper (Cu) and zinc oxide (ZnO) on alumina supports are often used. However, the actual active sites for this reaction—Zn-Cu bimetallic sites or ZnO-Cu interfacial sites—are debated. Kattel et al. studied model catalysts and found that ZnCu became as active as ZnO/Cu only after surface oxidation formed ZnO. Theoretical studies favor a formate intermediate pathway at a ZnO-Cu interface active site. Science, this issue p. 1296 The active sites over commercial copper/zinc oxide/aluminum oxide (Cu/ZnO/Al2O3) catalysts for carbon dioxide (CO2) hydrogenation to methanol, the Zn-Cu bimetallic sites or ZnO-Cu interfacial sites, have recently been the subject of intense debate. We report a direct comparison between the activity of ZnCu and ZnO/Cu model catalysts for methanol synthesis. By combining x-ray photoemission spectroscopy, density functional theory, and kinetic Monte Carlo simulations, we can identify and characterize the reactivity of each catalyst. Both experimental and theoretical results agree that ZnCu undergoes surface oxidation under the reaction conditions so that surface Zn transforms into ZnO and allows ZnCu to reach the activity of ZnO/Cu with the same Zn coverage. Our results highlight a synergy of Cu and ZnO at the interface that facilitates methanol synthesis via formate intermediates.

In Situ and Theoretical Studies for the Dissociation of Water on an Active Ni/CeO2 Catalyst: Importance of Strong Metal–Support Interactions for the Cleavage of O–H Bonds†

Water dissociation is crucial in many catalytic reactions on oxide-supported transition-metal catalysts. Supported by experimental and density-functional theory results, the effect of the support on OH bond cleavage activity is elucidated for nickel/ceria systems. Ambient-pressure O 1s photoemission spectra at low Ni loadings on CeO2 (111) reveal a substantially larger amount of OH groups as compared to the bare support. Computed activation energy barriers for water dissociation show an enhanced reactivity of Ni adatoms on CeO2 (111) compared with pyramidal Ni4 particles with one Ni atom not in contact with the support, and extended Ni(111) surfaces. At the origin of this support effect is the ability of ceria to stabilize oxidized Ni(2+) species by accommodating electrons in localized f-states. The fast dissociation of water on Ni/CeO2 has a dramatic effect on the activity and stability of this system as a catalyst for the water-gas shift and ethanol steam reforming reactions.

Dry Reforming of Methane on a Highly-Active Ni-CeO2 Catalyst: Effects of Metal-Support Interactions on C-H Bond Breaking.

Ni-CeO2 is a highly efficient, stable and non-expensive catalyst for methane dry reforming at relative low temperatures (700 K). The active phase of the catalyst consists of small nanoparticles of nickel dispersed on partially reduced ceria. Experiments of ambient pressure XPS indicate that methane dissociates on Ni/CeO2 at temperatures as low as 300 K, generating CHx and COx species on the surface of the catalyst. Strong metal-support interactions activate Ni for the dissociation of methane. The results of density-functional calculations show a drop in the effective barrier for methane activation from 0.9 eV on Ni(111) to only 0.15 eV on Ni/CeO2-x (111). At 700 K, under methane dry reforming conditions, no signals for adsorbed CHx or C species are detected in the C 1s XPS region. The reforming of methane proceeds in a clean and efficient way.

In-situ characterization of heterogeneous catalysts

CONTRIBUTORS vii Introduction: Goals and Challenges for the In-situ Characterization of Heterogeneous Catalysts 1 Jose A. Rodriguez, Jonathan C. Hanson, and Peter J. Chupas 1 QEXAFS in Catalysis Research: Principles, Data Analysis, and Applications 23 Anatoly I. Frenkel, Syed Khalid, Jonathan C. Hanson, and Maarten Nachtegaal 2 Spatially Resolved X-ray Absorption Spectroscopy 49 Christian G. Schroer and Jan-Dierk Grunwaldt 3 Energy-Dispersive EXAFS: Principles and Application in Heterogeneous Catalysis 75 Mark A. Newton and Andrew J. Dent 4 In-situ Powder X-ray Diffraction in Heterogeneous Catalysis 121 Jonathan Hanson and Poul Norby 5 Pair Distribution Function Analysis of High-Energy X-ray Scattering Data 147 Karena W. Chapman and Peter J. Chupas 6 Neutron Scattering for In-situ Characterization of Heterogeneous Catalysis 169 Ashfi a Huq and Wei-Ren Chen 7 Visualization of Surface Structures of Heterogeneous Catalysts under Reaction Conditions or during Catalysis with High-Pressure Scanning Tunneling Microscopy 191 Luan Nguyen, Shiran Zhang, Yingchun Ye, Yuan Zhu, Lei Wang, and Franklin (Feng) Tao 8 In-situ Infrared Spectroscopy on Model Catalysts 209 Kumudu Mudiyanselage and Dario J. Stacchiola 9 Infrared Spectroscopy on Powder Catalysts 241 Eli Stavitski 10 Structural Characterization of Catalysts by Operando Raman Spectroscopy 267 Gerhard Mestl and Miguel A. Banares 11 In-situ Electron Paramagnetic Resonance of Powder Catalysts 293 Angelika Bruckner 12 Application of Ambient-Pressure X-ray Photoelectron Spectroscopy for the In-situ Investigation of Heterogeneous Catalytic Reactions 315 David E. Starr, Hendrik Bluhm, Zhi Liu, Axel Knop-Gericke, and Michael Havecker 13 Combined X-ray Diffraction and Absorption Spectroscopy in Catalysis Research: Techniques and Applications 345 Anatoly I. Frenkel and Jonathan C. Hanson 14 Combining Infrared Spectroscopy with X-ray Techniques for Interrogating Heterogeneous Catalysts 369 Mark A. Newton and Marcos Fernandez-Garcia 15 XRD Raman and Modulation Excitation Spectroscopy 411 Wouter van Beek, Atsushi Urakawa, and Marco Milanesio 16 Catalyst Imaging Using Synchrotron-Based Multitechnique Approaches 441 Andrew M. Beale, Javier Ruiz-Martinez, and Bert M. Weckhuysen INDEX 475

Inverse Oxide/Metal Catalysts in Fundamental Studies and Practical Applications: A Perspective of Recent Developments

Inverse oxide/metal catalysts have shown to be excellent systems for studying the role of the oxide and oxide-metal interface in catalytic reactions. These systems can have special structural and catalytic properties due to strong oxide-metal interactions difficult to attain when depositing a metal on a regular oxide support. Oxide phases that are not seen or are metastable in a bulk oxide can become stable in an oxide/metal system opening the possibility for new chemical properties. Using these systems, it has been possible to explore fundamental properties of the metal-oxide interface (composition, structure, electronic state), which determine catalytic performance in the oxidation of CO, the water-gas shift and the hydrogenation of CO2 to methanol. Recently, there has been a significant advance in the preparation of oxide/metal catalysts for technical or industrial applications. One goal is to identify methods able to control in a precise way the size of the deposited oxide particles and their structure on the metal substrate.

Ambient pressure XPS and IRRAS investigation of ethanol steam reforming on Ni–CeO2(111) catalysts: an in situ study of C–C and O–H bond scission

Ambient-Pressure X-ray Photoelectron Spectroscopy (AP-XPS) and Infrared Reflection Absorption Spectroscopy (AP-IRRAS) have been used to elucidate the active sites and mechanistic steps associated with the ethanol steam reforming reaction (ESR) over Ni-CeO2(111) model catalysts. Our results reveal that surface layers of the ceria substrate are both highly reduced and hydroxylated under reaction conditions while the small supported Ni nanoparticles are present as Ni(0)/NixC. A multifunctional, synergistic role is highlighted in which Ni, CeOx and the interface provide an ensemble effect in the active chemistry that leads to H2. Ni(0) is the active phase leading to both C-C and C-H bond cleavage in ethanol and it is also responsible for carbon accumulation. On the other hand, CeOx is important for the deprotonation of ethanol/water to ethoxy and OH intermediates. The active state of CeOx is a Ce(3+)(OH)x compound that results from extensive reduction by ethanol and the efficient dissociation of water. Additionally, we gain an important insight into the stability and selectivity of the catalyst by its effective water dissociation, where the accumulation of surface carbon can be mitigated by the increased presence of surface OH groups. The co-existence and cooperative interplay of Ni(0) and Ce(3+)(OH)x through a metal-support interaction facilitate oxygen transfer, activation of ethanol/water as well as the removal of coke.

Demonstration of a threshold response in a proteolytic feedback system: control of the autoactivation of factor XII.

Mathematical analysis of positive feedback loops in proteolytic systems has previously suggested that when the active enzymes are subject to inhibitory control these systems will exhibit threshold behavior. This is demonstrated in the present study, for the autolytic activation of factor XII in the presence of a contact activator and an irreversible inhibitor of factor XIIa. The threshold between the two system states – complete factor XII activation, or complete stability – is dependent on the kinetic balance between the catalytic rate of autoactivation and rate of enzyme (factor XIIa) inhibition. Activation of the system can be brought about by either increasing the catalytic rate (in this study, by using more potent contact-activation conditions), or by lowering the enzyme inhibition rate. Previous mathematical work predicted complete stability in a positive-feedback system that is below threshold, and this has been experimentally confirmed.

Three-dimensional ruthenium-doped TiO2 sea urchins for enhanced visible-light-responsive H2 production

Three-dimensional (3D) monodispersed sea urchin-like Ru-doped rutile TiO2 hierarchical architectures composed of radially aligned, densely-packed TiO2 nanorods have been successfully synthesized via an acid-hydrothermal method at low temperature without the assistance of any structure-directing agent and post annealing treatment. The addition of a minuscule concentration of ruthenium dopants remarkably catalyzes the formation of the 3D urchin structure and drives the enhanced photocatalytic H2 production under visible light irradiation, not possible on undoped and bulk rutile TiO2. Increasing ruthenium doping dosage not only increases the surface area up to 166 m(2) g(-1) but also induces enhanced photoresponse in the regime of visible and near infrared light. The doping introduces defect impurity levels, i.e. oxygen vacancy and under-coordinated Ti(3+), significantly below the conduction band of TiO2, and ruthenium species act as electron donors/acceptors that accelerate the photogenerated hole and electron transfer and efficiently suppress the rapid charge recombination, therefore improving the visible-light-driven activity.

Response to Comment on “Active sites for CO2 hydrogenation to methanol on Cu/ZnO catalysts”

In their Comment on the our recent Report, Nakamura et al. argue that our x-ray photoelectron spectroscopy (XPS) analysis was affected by the presence of formate species on the catalyst surface. This argument is not valid because the reactant gases were evacuated at temperatures from 525 to 575 kelvin, conditions under which formate is not stable on the catalyst surface. An analysis of the XPS results obtained after exposing zinc oxide/copper (111) [ZnO/Cu(111)] surfaces to hydrogen (H2) and mixtures of carbon dioxide (CO2)/H2 show an absence of carbon (C) 1s signal, no asymmetries in the oxygen (O) 1s peak, and a Zn:O intensity close to 1:1. Thus, the most active phase of these catalysts contained a ZnO-Cu interface.