Time Resolved Operando X-ray Techniques in Catalysis, a Case Study: CO Oxidation by O2 over Pt Surfaces and Alumina Supported Pt Catalysts

Hydrogen Spillover Induced PtCo/CoOx Interfaces with Enhanced Catalytic Activity for CO Oxidation at Low Temperatures in Humid Conditions

The development of catalysts with abundant active interfaces for superior low-temperature catalytic CO oxidation is critical to meet increasingly rigorous emission requirements, yet still challenging. Herein, this work reports a PtCo/CoOx/Al2O3 catalyst with PtCo clusters and enriched Pt─O─Co interfaces induced by hydrogen spillover from the Pt sites and self-oxidation process in air, exhibiting excellent performance for CO oxidation at low temperatures and humid conditions. The combination of structural characterizations and in situ Fourier transform infrared spectroscopy reveals that the PtCo cluster effectively prevents CO saturation/poisoning on the Pt surface. Additionally, the presence of Pt─O─Co interfaces in the PtCo/CoOx/Al2O3 catalyst provides a significant number of active sites for oxygen activation and ─OH formation. This facilitates efficient generation of CO2 at ambient temperature by coupling with nearby adsorbed CO molecules, resulting in superior low-temperature activity and long-term stability for CO oxidation under humid conditions. This work provides a facile route toward rationalizing the design of catalysts with more active interfaces for superior low-temperature CO oxidation under humid conditions for practical applications.

Catalytic behavior of gold nanoparticles supported on a TiO2-Al2O3 mixed oxide for CO oxidation at low temperature

The present work highlights the versatility of a TiO₂-Al₂O₃ mixed oxide bearing highly dispersed gold nanoparticles that was applied in the CO oxidation reaction at room temperature. The TiO₂, Al₂O₃, and TiO₂-Al₂O₃ supports were synthesized by the sol-gel method, while gold nanoparticles were added by the deposition-precipitation with urea method using a theoretical Au loading of 2 wt.%. A promotional effect of the TiO₂-Al₂O₃ support on the activity of gold catalysts with respect to TiO₂ and Al₂O₃ was observed; Au/TiO₂-Al₂O₃ showed outstanding CO oxidation, being active from 0 °C and stable throughout a 24-h test. As for the alumina content (5, 10, and 15 wt.%) in TiO₂, it improved the textural properties by retarding the crystal growth and anatase-rutile phase transformation of TiO₂, suppressing the deposition of carbon on the catalyst surface and stabilizing the Au nanoparticles even at high temperatures. Gold was highly dispersed with nanoparticle sizes ranging from 1 to 2 nm when H₂ was used to treat thermally the Au/TiO₂-Al₂O₃, Au/TiO_2, and Au/Al₂O₃ materials. In addition, the XPS technique helped elicit that Au⁰ and Au¹⁺ boosted their interaction with the TiO₂, Al₂O₃, and TiO₂-Al₂O₃ supports by means of charge transfer, which resulted in outstanding CO oxidation activity from 0 °C. Likewise, the key factors that control the peculiar catalytic performance in the CO oxidation reaction are discussed, which represents a step forward in the versatility behavior of gold catalysts supported on mixed oxide catalysts.

Near-ambient pressure velocity map imaging

We present a new velocity map imaging instrument for studying molecular beam surface scattering in a near-ambient pressure (NAP-VMI) environment. The instrument offers the possibility to study chemical reaction dynamics and kinetics where higher pressures are either desired or unavoidable, adding a new tool to help close the “pressure gap” between surface science and applied catalysis. NAP-VMI conditions are created by two sets of ion optics that guide ions through an aperture and map their velocities. The aperture separates the high pressure ionization region and maintains the necessary vacuum in the detector region. The performance of the NAP-VMI is demonstrated with results from N2O photodissociation and N2 scattering from a Pd(110) surface, which are compared under vacuum and at near-ambient pressure (1 × 10−3 mbar). NAP-VMI has the potential to be applied to, and useful for, a broader range of experiments, including photoelectron spectroscopy and scattering with liquid microjets.

A near-ambient pressure flow reactor coupled with polarization-modulation infrared reflection absorption spectroscopy for operando studies of heterogeneous catalytic reactions over model catalysts

The model catalyst approach is often used for fundamental investigations of complex heterogeneous catalysis, in which operando characterizations are critical. A flow reactor is usually adopted for gas-solid heterogeneous catalytic reactions. Herein, we report a home-designed near-ambient pressure (NAP) flow reactor coupled with polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS) and an online quadrupole mass spectrometer for operando studies of heterogeneous catalytic reactions over model catalysts. A unique gas supply system is designed and manufactured to enable a stable gas inlet to the NAP flow reactor at pressures up to ∼100 mbar. An ultrahigh vacuum chamber equipped with the facilities for x-ray photoelectron spectroscopy, low-energy electron diffraction, thermal desorption spectroscopy, E-beam evaporation source, and ion sputtering gun is connected to the NAP flow reactor via a gate valve for preparations and routine characterizations of model catalysts. The functions of the system are demonstrated by in situ PM-IRAS characterization of CO adsorption on Pt(111) and operando characterizations of CO oxidation on Pt(111) under NAP conditions.

Strong response of Pt clusters to the environment and conditions, formation of metastable states, and simple methods to trace the reversible changes

Subnanometric metal particles, the so-called "clusters", are known to be responsive to their surroundings, but the detection of occurring changes, understanding the causes, and predicting the consequences are still extremely difficult for such small particles. Our study was aimed at estimating the potential of adsorption-based methods for these purposes. Using carbon monoxide as a probing molecule, which readily adsorbs on both bare and H-covered Pt surface, we have probed the adsorption properties of highly dispersed Pt/γ-Al₂O₃ samples after treatments under different atmospheres and temperatures (H₂ or inert gas, 25-500 °C). The combined results of CO-chemisorption measurements, CO TPD, CO TPO, H₂-by-CO displacement, and H₂ TPD suggest that the system shuttles between two states: one with oxygen vacancies in the support and the other one with redox-active oxygen near the Pt clusters. These extreme states can be reversibly created and deleted, giving rise to innumerable intermediate structures that differ in the amount, binding strength, and/or reactivity of adsorbed species. Two adsorbates could act cooperatively, resulting in hydrogen spillover onto the support and making the adsorbate-metal-support interactions even more complex. Implications for better understanding the dynamic behavior of oxide-supported clusters and nanoparticles are discussed.

Advanced Materials for Energy-Water Systems: The Central Role of Water/Solid Interfaces in Adsorption, Reactivity, and Transport

The structure, chemistry, and charge of interfaces between materials and aqueous fluids play a central role in determining properties and performance of numerous water systems. Sensors, membranes, sorbents, and heterogeneous catalysts almost uniformly rely on specific interactions between their surfaces and components dissolved or suspended in the water-and often the water molecules themselves-to detect and mitigate contaminants. Deleterious processes in these systems such as fouling, scaling (inorganic deposits), and corrosion are also governed by interfacial phenomena. Despite the importance of these interfaces, much remains to be learned about their multiscale interactions. Developing a deeper understanding of the molecular- and mesoscale phenomena at water/solid interfaces will be essential to driving innovation to address grand challenges in supplying sufficient fit-for-purpose water in the future. In this Review, we examine the current state of knowledge surrounding adsorption, reactivity, and transport in several key classes of water/solid interfaces, drawing on a synergistic combination of theory, simulation, and experiments, and provide an outlook for prioritizing strategic research directions.

Synchronizing gas injections and time-resolved data acquisition for perturbation-enhanced APXPS experiments

An experimental approach is described in which well-defined perturbations of the gas feed into an Ambient Pressure X-ray Photoelectron Spectroscopy (APXPS) cell are fully synchronized with the time-resolved x-ray photoelectron spectroscopy data acquisition. These experiments unlock new possibilities for investigating the properties of materials and chemical reactions mediated by their surfaces, such as those in heterogeneous catalysis, surface science, and coating/deposition applications. Implementation of this approach, which is termed perturbation-enhanced APXPS, at the SPECIES beamline of MAX IV Laboratory is discussed along with several experimental examples including individual pulses of N₂ gas over a Au foil, a multi-pulse titration of oxygen vacancies in a pre-reduced TiO₂ single crystal with O₂ gas, and a sequence of alternating precursor pulses for atomic layer deposition of TiO₂ on a silicon wafer substrate.

Shedding Light on CO Oxidation Surface Chemistry on Single Pt Catalyst Nanoparticles Inside a Nanofluidic Model Pore

Investigating a catalyst under relevant application conditions is experimentally challenging and parameters like reaction conditions in terms of temperature, pressure, and reactant mixing ratios, as well as catalyst design, may significantly impact the obtained experimental results. For Pt catalysts widely used for the oxidation of carbon monoxide, there is keen debate on the oxidation state of the surface at high temperatures and at/above atmospheric pressure, as well as on the most active surface state under these conditions. Here, we employ a nanoreactor in combination with single-particle plasmonic nanospectroscopy to investigate individual Pt catalyst nanoparticles localized inside a nanofluidic model pore during carbon monoxide oxidation at 2 bar in the 450-550 K temperature range. As a main finding, we demonstrate that our single-particle measurements effectively resolve a kinetic phase transition during the reaction and that each individual particle has a unique response. Based on spatially resolved measurements, we furthermore observe how reactant concentration gradients formed due to conversion inside the model pore give rise to position-dependent kinetic phase transitions of the individual particles. Finally, employing extensive electrodynamics simulations, we unravel the surface chemistry of the individual Pt nanoparticles as a function of reactant composition and find strongly temperature-dependent Pt-oxide formation and oxygen spillover to the SiO2 support as the main processes. These results therefore support the existence of a Pt surface oxide in the regime of high catalyst activity and demonstrate the possibility to use plasmonic nanospectroscopy in combination with nanofluidics as a tool for in situ studies of individual catalyst particles.

In Situ/Operando Electrocatalyst Characterization by X-ray Absorption Spectroscopy

During the last decades, X-ray absorption spectroscopy (XAS) has become an indispensable method for probing the structure and composition of heterogeneous catalysts, revealing the nature of the active sites and establishing links between structural motifs in a catalyst, local electronic structure, and catalytic properties. Here we discuss the fundamental principles of the XAS method and describe the progress in the instrumentation and data analysis approaches undertaken for deciphering X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra. Recent usages of XAS in the field of heterogeneous catalysis, with emphasis on examples concerning electrocatalysis, will be presented. The latter is a rapidly developing field with immense industrial applications but also unique challenges in terms of the experimental characterization restrictions and advanced modeling approaches required. This review will highlight the new insight that can be gained with XAS on complex real-world electrocatalysts including their working mechanisms and the dynamic processes taking place in the course of a chemical reaction. More specifically, we will discuss applications of in situ and operando XAS to probe the catalyst's interactions with the environment (support, electrolyte, ligands, adsorbates, reaction products, and intermediates) and its structural, chemical, and electronic transformations as it adapts to the reaction conditions.

Characterizing electronic and atomic structures for amorphous and molecular metal oxide catalysts at functional interfaces by combining soft X-ray spectroscopy and high-energy X-ray scattering

Amorphous thin film materials and heterogenized molecular catalysts supported on electrode and other functional interfaces are widely investigated as promising catalyst formats for applications in solar and electrochemical fuels catalysis. However the amorphous character of these catalysts and the complexity of the interfacial architectures that merge charge transport properties of electrode and semiconductor supports with discrete sites for multi-step catalysis poses challenges for probing mechanisms that activate and tune sites for catalysis. This minireview discusses advances in soft X-ray spectroscopy and high-energy X-ray scattering that provide opportunities to resolve interfacial electronic and atomic structures, respectively, that are linked to catalysis. This review discusses how these techniques can be partnered with advances in nanostructured interface synthesis for combined soft X-ray spectroscopy and high-energy X-ray scattering analyses of thin film and heterogenized molecular catalysts. These combined approaches enable opportunities for the characterization of both electronic and atomic structures underlying fundamental catalytic function, and that can be applied under conditions relevant to device applications.

Propene oxidation catalysis and electronic structure of M55 particles (M = Pd or Rh): differences and similarities between Pd55 and Rh55

Propene oxidation is one of the important reactions that occurs in the presence of a three-way catalyst but its reaction mechanism is unclear. The reaction mechanisms and differences in catalysis between Pd and Rh particles were investigated by DFT calculations employing Pd₅₅ and Rh₅₅ as the model catalysts. The O-attack mechanism, in which the O atom adsorbed on the Pd₅₅ and Rh₅₅ surfaces attacks the C[double bond, length as m-dash]C double bond of propene, needs to overcome a large activation barrier (E_a). On the other hand, C-H bond cleavage of the methyl group of propene easily occurs with moderate E_a; the mechanism initiated by this C-H activation is named H-transfer mechanism. In this mechanism, the next step is allyl alcohol formation, followed by the second C-H bond activation of the CH₂OH species of allyl alcohol, and the final step is proton transfer from OH-substituted π-allyl species to the OH group on the metal surface to yield acrolein and water molecules with the regeneration of M₅₅. The rate-determining step is the second C-H bond activation. Its E_a is 17.4 kcal mol^-1 for the reaction on Pd₅₅ and 34.4 kcal mol^-1 for the reaction on Rh₅₅. These results indicate that Pd particles are more active than Rh particles in propene oxidation, which agrees with the experimental findings. The larger E_a for Rh₅₅ than that for Pd₅₅ arises from the stronger Rh-OH bond than the Pd-OH bond. The higher energy d-valence band-top of Rh₅₅ than that of Pd₅₅ is the origin of the stronger Rh-OH bond than the Pd-OH bond. Thus, the d-valence band-top energy is an important property for understanding and designing catalysts for alkene oxidation.

Ensembles of Metastable States Govern Heterogeneous Catalysis on Dynamic Interfaces

Heterogeneous catalysis is at the heart of the chemical industry. Being able to tune and design efficient catalysts for processes of interest is of the utmost importance, and for this, a molecular-level understanding of heterogeneous catalysts is the first step and indeed a prime focus of modern catalysis research. For a long time, the single most thermodynamically stable structure of the catalytic interface attained under the reaction conditions had been envisioned as the reactive phase. However, some catalytic interfaces continue to undergo structural dynamics in the steady state, triggered by high temperatures and pressures and binding and changing reagents. Among particularly dynamic interfaces are such widely used catalysts as crystalline and amorphous surfaced supporting (sub)nanometallic clusters. Recently, it became clear that this dynamic fluxionality causes the supported clusters to populate many distinct structural and stoichiometric states under catalytic conditions. Hence, the catalytic interface should be viewed as an evolving statistical ensemble of many structures (rather than one structure). Every member in the ensemble contributes to the properties of the catalyst differently, in proportion to its probability of being populated. This new notion flips the established paradigm and calls for a new theory, new modeling approaches, operando measurements, and updated design strategies. The statistical ensemble nature of surface-supported subnanocluster catalysts can be exemplified by oxide-supported and adsorbate-covered Pt, Pd, Cu, and CuPd clusters, which are catalytic toward oxidative and nonoxidative dehydrogenation. They have access to a variety of 3D and quasi-2D shapes. The compositions of their thermal ensembles are dependent on the cluster size, leading to size-specific catalytic activities and the famous "every atom counts" phenomenon. The support and adsorbates affect catalyst structures, and the state of the reacting species causes the ensemble to change in every reaction intermediate. The most stable member of the ensemble dominates the thermodynamic properties of the corresponding intermediate, whereas the kinetics can be determined by more active but less populated metastable catalyst states, and that suggests that many earlier studies might have overlooked the actual active sites. Both effects depend on the relative time scales of catalyst restructuring and reaction dynamics. The catalyst may routinely operate off-equilibrium. Ensemble phenomena lead to surprising exceptions from established rules of catalysis, such as scaling relations and Arrhenius behavior. Catalyst deactivation is also an ensemble property, and its extent of mitigation can be predicted through the new paradigm. These findings were enabled by advances in theory, such as global optimization and subsequent utilization of multiple local minima and pathways sampling as well as operando catalyst characterization. The fact that the per-site and per-species resolution is needed for the description and prediction of catalyst properties gives theory the central role in catalysis research, as most experiments provide ensemble-average information and cannot detect the crucial minority species that may be responsible for the catalytic activity.

Shining light on the solid–liquid interface: in situ/operando monitoring of surface catalysis

Many industrially important chemical transformations occur at the interface between a solid catalyst and liquid reactants. In situ and operando spectroscopies offer unique insight into the reactivity of such catalytically active solid–liquid interfaces.

Unravelling the Different Reaction Pathways for Low Temperature CO Oxidation on Pt/CeO2 and Pt/Al2O3 by Spatially Resolved Structure-Activity Correlations

Spatially resolved operando HERFD-XANES (high energy resolution fluorescence detected X-ray absorption near edge structure) complemented by CO concentration gradient profiles along the catalyst bed (SpaciPro) was used to identify the dominant reaction paths for the low and high temperature CO oxidation on Pt/CeO₂ and Pt/Al₂O₃. At low temperatures, features associated with CO adsorption on Pt were found for both catalysts. During the oxidation reaction light-off, the evolution of the spectral and catalytic profile diverged along the catalyst bed. The CO oxidation rate was high on Pt/CeO₂ from the beginning of the catalyst bed with CO being adsorbed on Pt, whereas low CO conversion due to strong CO poisoning was found on Pt/Al₂O₃. This correlation of the CO concentration gradient with unique insight by HERFD-XANES gave direct proof of the crucial contribution of the Pt-CeO₂ perimeter sites overcoming the CO self-inhibition effect at low temperatures.

Characterization of Nanoparticles: Advances

Over the past two decades, the unique properties of engineered nanoparticles (NPs) have placed them at the centre of revolutionary advancements in many sectors of science, technology and commerce. Multi-technique and multi-disciplinary analytical approaches are required to identify, quantify, and characterize the chemical composition, size and size distribution, surface properties and the number and concentration of NPs. In this chapter, an overview of the recent advances in the characterization of NPs will be presented.

Critical Review of Low-Temperature CO Oxidation and Hysteresis Phenomenon on Heterogeneous Catalysts

There is a growing demand for new heterogeneous catalysts for cost-effective catalysis. Currently, the hysteresis phenomenon during low-temperature CO oxidation is an important topic in heterogeneous catalysis. Hysteresis provides important information about fluctuating reaction conditions that affect the regeneration of active sites and indicate the restoration of catalyst activity. Understanding its dynamic behavior, such as hysteresis and self-sustained kinetic oscillations, during CO oxidation, is crucial for the development of cost-effective, stable and long-lasting catalysts. Hysteresis during CO oxidation has a direct influence on many industrial processes and its understanding can be beneficial to a broad range of applications, including long-life CO2 lasers, gas masks, catalytic converters, sensors, indoor air quality, etc. This review considers the most recent reported advancements in the field of hysteresis behavior during CO oxidation which shed light on the origin of this phenomenon and the parameters that influence the type, shape, and width of the conversion of the hysteresis curves.

Investigation of periodically driven systems by x-ray absorption spectroscopy using asynchronous data collection mode

We report the development, testing, and demonstration of a setup for modulation excitation spectroscopy experiments at the Inner Shell Spectroscopy beamline of National Synchrotron Light Source - II. A computer algorithm and dedicated software were developed for asynchronous data processing and analysis. We demonstrate the reconstruction of X-ray absorption spectra for different time points within the modulation pulse using a model system. This setup and the software are intended for a broad range of functional materials which exhibit structural and/or electronic responses to the external stimulation, such as catalysts, energy and battery materials, and electromechanical devices.

Identification of dopant site and its effect on electrochemical activity in Mn-doped lithium titanate

Doped metal oxide materials are commonly used for applications in energy storage and conversion, such as batteries and solid oxide fuel cells. The knowledge of the electronic properties of dopants and their local environment is essential for understanding the effects of doping on the electrochemical properties. Using a combination of X-ray absorption near-edge structure spectroscopy (XANES) experiment and theoretical modeling we demonstrate that in the dilute (1 at. %) Mn-doped lithium titanate (Li4/3Ti5/3O4, or LTO) the dopant Mn2+ ions reside on tetrahedral (8a) sites. First-principles Mn K-edge XANES calculations revealed the spectral signature of the tetrahedrally coordinated Mn as a sharp peak in the middle of the absorption edge rise, caused by the 1s → 4p transition, and it is important to include the effective electron-core hole Coulomb interaction in order to calculate the intenisty of this peak accurately. This dopant location explains the impedance of Li migration through the LTO lattice during the charge-discharge process, and, as a result - the observed remarkable 20% decrease in electrochemical rate performance of the 1% Mn-doped LTO compared to the pristine LTO.