Realistic Reaction Conditions Research Articles

Temperature-driven evolution of ceria-zirconia-supported AuPd and AuRu bimetallic catalysts under different atmospheres: insights from IL-STEM studies.

The evolution of the structure and composition of the system of particles in two Ce0.62Zr0.38O2-supported bimetallic catalysts based on Au and a 4d metal (Ru or Pd) under high temperature conditions and different reducing and oxidizing environments has been followed by means of Identical Location Scanning Transmission Electron Microscopy (IL-STEM). As an alternative to in situ microscopy, this technique offers valuable insights into the structural modifications occurring in chemical environments with the characteristics of a macro-scale reactor. By tracking exactly the same areas on a large number of metallic entities, it has been possible to reveal the influence of particle size and the nature of the redox environment on the temperature-driven mobilization of the different metals involved. Thus, oxidizing environments evidenced a much higher capacity to mobilize the three metals, preferentially Au. Moreover, the typical storage conditions (under air) of catalysts during the prolonged exposure time has been proved to induce significant modifications in these bimetallic systems, even at room temperature. Regardless of the type of redox environment, bimetallic systems showed better thermal resistance, which demonstrates a beneficial effect of the second metal. In summary, IL-STEM is an invaluable and complementary methodology for characterizing heterogeneous catalysts under realistic reaction conditions and is within the reach of most laboratories.

A Polycrystalline Pd Surface Studied by Two-Dimensional Surface Optical Reflectance during CO Oxidation: Bridging the Materials Gap.

Industrial catalysts are complex materials systems operating in harsh environments. The active parts of the catalysts are nanoparticles that expose different facets with different surface orientations at which the catalytic reactions occur. However, these facets are close to impossible to study in detail under industrially relevant operating conditions. Instead, simpler model systems, such as single crystals with a well-defined surface orientation, have been successfully used to study gas-surface interactions such as adsorption and desorption, surface oxidation, and oxidation/reduction reactions. To more closely mimic the many facets exhibited by nanoparticles and thereby close the so-called materials gap, there has also been a recent move toward using polycrystalline surfaces and curved crystals. However, these studies are limited either by the pressure or spatial resolution at realistic pressures or by the number of surfaces studied simultaneously. In this work, we demonstrate the use of reflectance microscopy to study a vast number of catalytically active surfaces simultaneously under realistic and identical reaction conditions. As a proof of concept, we have conducted an operando experiment to study CO oxidation over a Pd polycrystal, where the polycrystalline surface acts as a collection of many single-crystal surfaces. Finally, we visualized the resulting data by plotting the reflectivity as a function of surface orientation. We think the techniques and visualization methods introduced in this work will be key toward bridging the materials gap in catalysis.

Modelling of metal nanoparticles’ structures and dynamics under reaction conditions

Metal nanoparticles (NPs) are widely used in heterogeneous catalysis. Their performance in catalytic reactions is closely related to the shape, surface structure, and composition. In the past two decades, a number of in-situ experiments have observed that under real reaction conditions, the structures of metal NPs are not static, but undergo remarkable dynamic evolution due to the presence of gas molecules. Therefore, it is imminent to develop reliable theoretical models to provide accurate predictions and comprehensive understandings of the structure reconstructions and dynamic behaviors of catalysts in response to the different reactive environments. In this review, we summarize a series of progress and achievements made by first-principle-based theoretical models in analyzing the shape evolution of metal NPs and surface segregation of alloys under different gas conditions in recent years. We also discuss the understanding of the catalytic performance of NPs by considering the reaction-condition-dependent structures. In addition, the real-time dynamic simulation methods of catalysts under reaction conditions are introduced. The perspective of simulating the kinetic process of in situ structural change is provided at last.

Scanning tunneling microscopy under chemical reaction at solid–liquid and solid–gas interfaces

The task of bridging the pressure gap between ideal ultrahigh vacuum conditions and more realistic reaction conditions involving gas and liquid phases is crucial in surface and interfacial chemistry. Scanning tunneling microscopy (STM) has played a key role in addressing this challenge by enabling atomic-scale probing of the interface. STM enabled us to study surface structure, electronic structure, atomic manipulation, dynamics of molecules and atoms, and chemical properties of the surface at the atomic scale. Over the past four decades, the field of STM has undergone explosive growth. This review article focuses on recent advances in operando STM, specifically in the study of solid–liquid and solid–gas interfaces. It highlights the latest works in ambient-pressure STM, which has enabled the observation of atomic features under various gas and reaction conditions. This information sheds light on the surface mobility of adsorbates and atomic structures of reaction intermediates. The review also addresses research on electrochemical STM, which investigates the evolution of surface morphology under electrochemical processes and provides insights into atomic-scale reaction mechanisms. Finally, the article outlines future challenges and perspectives for operando STM techniques.

Grand canonical DFT based constant charge method for electrochemical deuterium/hydrogen evolution reaction micro-kinetics on Pt (1 1 1)

Applied external potential is considered to have significant impact on the reaction mechanisms and kinetic behaviors for electrochemical reactions. The electrochemical isotope deuterium/hydrogen (D/H) evolution shows significant different kinetic behavior under applied potential conditions, but the intrinsic mechanisms from the electrochemical isotope effect remains unclear. In this work, the grand canonical density functional theory (GC-DFT) based constant charge method was used to systematically investigate the electrochemical D/H evolution reactions (DER/HER) on the Pt (111) surface. By using this method, the linear and quadratic relationship between charge distribution (q), electronic energy (Eelect) and applied potential (U) were observed respectively, which are resulted from solving of linear Poisson-Boltzmann equation and Kohn-Sham-Mermin equations in combination of solvent modification. Equilibrium Pourbaix diagrams of D/H adsorbing on Pt (111) surface was subsequently investigated to revealed the electrode structure in real reaction condition, with θD/H coverages on the cathode surface calculated to be 0.0 monolayer (ML) and 1/6 ML for alkaline and acidic solvent at −1.23 V to 0.0 V vs. SHE. Mechanistic studies of HER/DER on the Pt (111) surface revealed that Volmer-Tafel reaction pathway is more favorable than the Volmer-Heyrovsky pathway. Analysis of the applied potential effect on the reaction energy barriers show that the acidic Volmer step is very sensitive toward the applied potential condition, with the structure of transition state likely to shift. Comparisons of the calculated current–potential polarization curves between HER and DER revealed that in the acidic condition, the calculated current density of HER is close to DER, revealing that the H*/D* migration and coupling on the electrode is comparable to each other when the solution is saturated with H3O+/D3O+. In the alkaline reaction condition, discrepancies of j value between DER and HER becomes slightly larger, revealing that jointly control effect from Volmer and Tafel step was enhanced. This work provides the new insight for investigating the microkinetic properties of electrochemical isotope reaction.

Reconstructed Ir‒O‒Mo species with strong Brønsted acidity for acidic water oxidation

Surface reconstruction generates real active species in electrochemical conditions; rational regulating reconstruction in a targeted manner is the key for constructing highly active catalyst. Herein, we use the high-valence Mo modulated orthorhombic Pr3Ir1−xMoxO7 as model to activate lattice oxygen and cations, achieving directional and accelerated surface reconstruction to produce self-terminated Ir‒Obri‒Mo (Obri represents the bridge oxygen) active species that is highly active for acidic water oxidation. The doped Mo not only contributes to accelerated surface reconstruction due to optimized Ir‒O covalency and more prone dissolution of Pr, but also affords the improved durability resulted from Mo-buffered charge compensation, thereby preventing fierce Ir dissolution and excessive lattice oxygen loss. As such, Ir‒Obri‒Mo species could be directionally generated, in which the strong Brønsted acidity of Obri induced by remaining Mo assists with the facilitated deprotonation of oxo intermediates, following bridging-oxygen-assisted deprotonation pathway. Consequently, the optimal catalyst exhibits the best activity with an overpotential of 259 mV to reach 10 mA cmgeo−2, 50 mV lower than undoped counterpart, and shows improved stability for over 200 h. This work provides a strategy of directional surface reconstruction to constructing strong Brønsted acid sites in IrOx species, demonstrating the perspective of targeted electrocatalyst fabrication under in situ realistic reaction conditions.

New insights into the relationship between Cu–Ce interaction and reactive Cu species in CuOx-CeO2 catalysts for preferential oxidation of carbon monoxide in excess hydrogen

A group of CuOx-CeO2 catalysts for preferential oxidation of carbon monoxide (CO-PROX) is designed by constructing different distribution status of Cu species on similar CeO2 support to adjust the Cu–Ce interaction strength. Cycled temperature programmed reduction by hydrogen (H2-TPR) indicates the strength of Cu–Ce interaction can impact the amount and redox properties of surface highly dispersed CuOx and strongly bonded Cu-[Ox]-Ce species, which respectively determines the catalytic performance at low and high temperature. Temperature programed desorption of H2 (H2-TPD) reveals the presence of large amounts of Cu-[Ox]-Ce species can inhibit H2 adsorption and dissociation, which increases the O2 selectivity toward CO oxidation. Moreover, temperature programmed desorption of CO2 (CO2-TPD) and in situ diffuse reflectance infrared Fourier transform spectra (DRIFTs) illustrates that H2O and CO2 influences the low-temperature catalytic performance mainly by competitive adsorption with CO on surface highly dispersed reactive Cu+ sites. This work aims to shed light on the underlying Cu–Ce interaction regime, and guide the design of high-performance CuO–CeO2 catalysts in realistic reaction conditions.

In situ catalytic cells for x-ray absorption spectroscopy measurement.

In catalysis, determining the relationship between the dynamic electronic and atomic structure of the catalysts and the catalytic performance under actual reaction conditions is essential to gain a deeper understanding of the reaction mechanism since the structure evolution induced by the absorption of reactants and intermediates affects the reaction activity. Hard x-ray spectroscopy methods are considered powerful and indispensable tools for the accurate identification of local structural changes, for which the development of suitable in situ reaction cells is required. However, the rational design and development of spectroscopic cells is challenging because a balance between real rigorous reaction conditions and a good signal-to-noise ratio must be reached. Here, we summarize the in situ cells currently used in the monitoring of thermocatalysis, photocatalysis, and electrocatalysis processes, focusing especially on the cells utilized in the BL14W1-x-ray absorption fine structure beamline at the Shanghai Synchrotron Radiation Facility, and highlight recent endeavors on the acquisition of improved spectra under real reaction conditions. This review provides a full overview of the design of in situ cells, aiming to guide the further development of portable and promising cells. Finally, perspectives and crucial factors regarding in situ cells under industrial operating conditions are proposed.

Mechanistic Insights Gained by High Spatial Resolution Reactivity Mapping of Homogeneous and Heterogeneous (Electro)Catalysts.

The recent development of high spatial resolution microscopy and spectroscopy tools enabled reactivity analysis of homogeneous and heterogeneous (electro)catalysts at previously unattainable resolution and sensitivity. These techniques revealed that catalytic entities are more heterogeneous than expected and local variations in reaction mechanism due to divergences in the nature of active sites, such as their atomic properties, distribution, and accessibility, occur both in homogeneous and heterogeneous (electro)catalysts. In this review, we highlight recent insights in catalysis research that were attained by conducting high spatial resolution studies. The discussed case studies range from reactivity detection of single particles or single molecular catalysts, inter- and intraparticle communication analysis, and probing the influence of catalysts distribution and accessibility on the resulting reactivity. It is demonstrated that multiparticle and multisite reactivity analyses provide unique knowledge about reaction mechanism that could not have been attained by conducting ensemble-based, averaging, spectroscopy measurements. It is highlighted that the integration of spectroscopy and microscopy measurements under realistic reaction conditions will be essential to bridge the gap between model-system studies and real-world high spatial resolution reactivity analysis.

The high activity of Co-Mn-based solid solution catalysts for lean methane combustion

Developing non-noble metal-based catalysts for methane combustion with high catalytic activity and stability is important in the new context of a changing global energy landscape. In this study, we reported an Co5Mn1-O catalyst achieved 90% conversion of methane to CO2 at 305 °C, which was about 60 ℃ lower than the catalysts reported. The synthetic strategy via lattice distortion that inhibits the deep oxidation of Mn2+ with prepared a highly catalytic reactive Co-Mn-based solid solution material. The results show that introduction of Mn atoms into the Co3O4 spinel structure caused lattice distortion, which modulated the oxygen coordination environment and oxygen vacancies of the catalyst, and the Co5Mn1-O (Optimum) catalyst prepared under optimized conditions obtained excellent methane activation capacity and lattice oxygen mobility. The higher lattice oxygen mobility of the Co5Mn1-O catalyst facilitated the adsorption, activation, and oxidation of methane, especially the rate-determining step of methane oxidation, the conversion of CH4 to CH3-. The conversion rate decreases slightly in the presence of 5 vol% or 10 vol% H2O, but it also still maintained good catalytic activity and stability under high temperatures and long-term reactions. In addition, in situ infrared specra was used to explore the reaction mechanism of methane combustion under real reaction conditions.

Formation of active sites on transition metals through reaction-driven migration of surface atoms.

Adopting low-index single-crystal surfaces as models for metal nanoparticle catalysts has been questioned by the experimental findings of adsorbate-induced formation of subnanometer clusters on several single-crystal surfaces. We used density functional theory calculations to elucidate the conditions that lead to cluster formation and show how adatom formation energies enable efficient screening of the conditions required for adsorbate-induced cluster formation. We studied a combination of eight face-centered cubic transition metals and 18 common surface intermediates and identified systems relevant to catalytic reactions, such as carbon monoxide (CO) oxidation and ammonia (NH3) oxidation. We used kinetic Monte Carlo simulations to elucidate the CO-induced cluster formation process on a copper surface. Scanning tunneling microscopy of CO on a nickel (111) surface that contains steps and dislocations points to the structure sensitivity of this phenomenon. Metal-metal bond breaking that leads to the evolution of catalyst structures under realistic reaction conditions occurs much more broadly than previously thought.

Elucidation of the Active Site for the Oxygen Evolution Reaction on a Single Pt Atom Supported on Indium Tin Oxide.

Single-atom catalysts (SACs) have attracted attention for their high catalytic activity and selectivity, but the nature of their active sites under realistic reaction conditions, involving various ligands, is not well-understood. In this study, we use density functional theory calculations and grand canonical basin hopping to theoretically investigate the active site for the oxygen evolution reaction (OER) on a single Pt atom supported on indium tin oxide, including the influence of the electrochemical potential. We show that the ligands on the Pt atom change from Pt-OH in the absence of electrochemical potential to PtO(OH)4 in electrochemical conditions. This change of the chemical state of Pt is associated with a decrease of 0.3 V for the OER overpotential. This highlights the importance of accurately identifying the nature of the active site under reaction conditions and the impact of adsorbates on the electrocatalytic activity. This theoretical investigation enhances our understanding of SACs for the OER.

Promoted coke resistance of Ni by surface carbon for the dry reforming of methane.

Dry reforming of methane (DRM) is an efficient process to transform methane and carbon dioxide to syngas. Nickel could show good catalytic activity for DRM, whereas the deactivation of nickel surfaces by the formation of inert carbon structures is inevitable. In this study, we carry out a detailed investigation of the evolution and catalytic performance of the carbon-covered surface structure on Ni(100) with a combined density functional theory and microkinetic modeling approach. The results suggest that the pristine Ni(100) surface is prone to carbon deposition and accumulation under reaction conditions. Further studies show that over this carbon-covered reconstructed Ni(100) surface, a carbon-based Mars-van-Krevelen mechanism would be favored, and the activity and coke resistance is promoted. This surface state and reaction mechanism were rarely reported before and would provide more insights into the DRM process under real reaction conditions and would help design more stable Ni catalysts.

Shape-Dependent CO2 Hydrogenation to Methanol over Cu2O Nanocubes Supported on ZnO.

The hydrogenation of CO2 to methanol over Cu/ZnO-based catalysts is highly sensitive to the surface composition and catalyst structure. Thus, its optimization requires a deep understanding of the influence of the pre-catalyst structure on its evolution under realistic reaction conditions, including the formation and stabilization of the most active sites. Here, the role of the pre-catalyst shape (cubic vs spherical) in the activity and selectivity of ZnO-supported Cu nanoparticles was investigated during methanol synthesis. A combination of ex situ, in situ, and operando microscopy, spectroscopy, and diffraction methods revealed drastic changes in the morphology and composition of the shaped pre-catalysts under reaction conditions. In particular, the rounding of the cubes and partial loss of the (100) facets were observed, although such motifs remained in smaller domains. Nonetheless, the initial pre-catalyst structure was found to strongly affect its subsequent transformation in the course of the CO2 hydrogenation reaction and activity/selectivity trends. In particular, the cubic Cu particles displayed an increased activity for methanol production, although at the cost of a slightly reduced selectivity when compared to similarly sized spherical particles. These findings were rationalized with the help of density functional theory calculations.

Methanol Adsorption and Reaction on TiO2(110) at Near Ambient Pressure

Although methanol on rutile TiO2(110) has been widely investigated under ultrahigh vacuum (UHV) conditions as a prototypical model, the adsorption structure and reaction of this system under or near real reaction conditions are still unclear. Surface-sensitive sum frequency generation vibrational spectroscopy (SFG-VS) has been utilized to in situ study the adsorption and reaction of methanol on TiO2(110) at near ambient pressure and 400 K. The SFG results indicate that the bridge-bonded methoxy (CH3Ob) is the dominant adsorbed species on TiO2(110) at an elevated pressure of methanol, and its density rises with increasing the methanol pressure. By evacuating the methanol gas, only CH3Ob species leave behind on the TiO2(110) surface, which is also confirmed by ultraviolet (UV) irradiation experiments. Furthermore, by a comparative SFG study of methanol adsorption on TiO2(110) as a function of methanol pressure at 250 K, we concluded that the recombinative desorption of bridging hydroxyl (OHb) groups in the form of water can facilitate the continuous dissociative adsorption of methanol on bridging oxygen vacancy (Ov) sites and the continuous generation of CH3Ob. Our results not only shed light on the adsorption and reaction of methanol on TiO2(110) at near ambient pressure but also demonstrate the studies of catalytic reaction under or near real reaction conditions at the molecular level.

Redox mediators promote electrochemical oxidation of nitric oxide toward ambient nitrate synthesis

Under ambient conditions, using low concentration NO (1%), ultrahigh nitrate formation rates in a two-electrode system can be achievedviathe eCOR-NOOR. The structure changes of electrode materials under real reaction conditions are revealed.

The role of lanthanum hydride species in La2O3 supported Ru cluster catalyst for ammonia synthesis

To exploit the ammonia as a carrier for sustainable energy, development of efficient catalysts is highly desired. Oxide supported Ru catalysts have been widely studied in ammonia synthesis for their superior catalytic performances under mild conditions. Here, sub-nanometer Ru clusters on La2O3 (Ru clusters/La2O3) catalyst has been synthesized and exhibits superior catalytic performance in ammonia synthesis under mild conditions. Multiple characterizations results reveal that surface La-H species, in situ generated on the surface of catalyst, can significantly boost the activity of Ru clusters/La2O3 catalyst in ammonia synthesis. Detailed mechanistic studies demonstrate that the active La-H species on the La2O3 can not only promote activation and dissociation of nitrogen molecules but also directly involve in ammonia synthesis reaction process via a La-H species-mediated reaction pathway. This work promotes molecular level understanding the active sites and reaction mechanism under real reaction conditions, and will also offer opportunity for guiding the design of efficient Ru-based catalysts for ammonia synthesis.

Operando Identification of Dynamic Photoexcited Oxygen Vacancies as True Catalytic Active Sites

The complex structural evolution of catalysts under realistic reaction conditions causes great difficulty in identifying true active sites and further understanding the correlations between the dynamic behavior and the catalytic property. Herein, we developed a set of operando electron paramagnetic resonance (EPR) measurements to investigate the dynamic behavior of true active sites, particularly the atomic surface structures and interfacial processes under reaction conditions. By constructing reduced TiO2 as the catalyst model, supported by the developed operando EPR and multiple in situ characterization results, we confirm the generation of previously unrevealed active sites of photoexcited oxygen vacancies (Vo) and further dissect the formation, rebirth, and evolvement of dynamic Vo at the electronic level. Subsequently, the consistent theoretical and experimental results comprehensively justify the dynamic photoexcited Vo as the true active site directly determining the catalytic performance, subverting the classical perception of predesigned Vo as the active site. The relationship between dynamic active site evolution and catalytic performance during operation enables us to understand the interfacial mechanisms at work and design better catalysis systems.

Evolution of a GDE setup: Beyond ambient conditions

Gas diffusion electrode (GDE) setups recently and increasingly received attention as testing platforms for fuel cell catalysts. GDE setups provide realistic reaction conditions while remaining simple and efficient to use. Therefore, GDE setups bridge the gap between rotating disk electrode (RDE) measurements and membrane electrode assembly (MEA) testing. Here, we describe our initial GDE design development to its latest improvements that allow application of high temperatures. We point out experimental challenges we overcame, yet also discuss properly applying our new technique to avoid wrongful use. In particular, we advocate for implementing GDE setups in catalyst stability investigations where the technique so far has been used infrequently.

Sm doped NiO catalysts with excellent H2O resistance for N2O decomposition under simulated nitric acid plant exhaust condition

Sm0.06Ni with excellent H2O resistance for eliminating N2O was prepared by co-precipitation method. Research indicated that the introduction of Sm into Sm0.06Ni has the following effects: (1) Sm species (Sm2O3) highly dispersed on the surface of Sm0.06Ni catalyst, which not only decreased the crystallite size of NiO, but also produced a certain number of edge dislocations and defects. (2) Sm2O3 weakened the Ni-O band and increased the number of active sites. (3) The redox cycle of Ni2+ + Sm3+ ↔ Ni3+ + Sm2+ was formed on the surface of Sm0.06Ni during the reaction. Based on the above changes, Sm0.06Ni exhibited superior catalytic performance and much lower Ea for the reaction compared with pure NiO. Over Sm0.06Ni, the TOF at 350 ℃ was 4.1 × 10−3 s−1, 37 times of that over pure NiO. More importantly, Sm0.06Ni exhibited excellent H2O resistance performance under submitted real reaction conditions, about 76 % N2O conversion can be obtained at 400 ℃.