Dynamic Reaction Conditions Research Articles

Unraveling the Potential Dependence of Active Structures and Reaction Mechanism of Ni-based MOFs Electrocatalysts for Alkaline OER.

Nickel-based metal-organic frameworks (MOFs) with flexible structure units provide a broad platform for designing highly efficient electrocatalysts, especially for alkaline oxygen evolution reaction (OER). However, the stability of MOFs under harsh and dynamic reaction conditions poses significant challenges, resulting in ambiguous structure-activity relationships in MOFs-based OER research. Herein, Ni-benzenedicarboxylic acid-based MOF (NiBDC) is selected as prototypical catalyst to elucidate its real active sites for OER and reaction pathway under different reaction states. Electrochemical measurements combined with X-ray absorption spectroscopy (XAS) and Raman spectroscopy reveal that the complete reconstruction of NiBDC to β-NiOOH in the chronoamperometry activation process is responsible for significantly increased OER performance. In situ XAS and Raman results further demonstrate the electro-oxidation of β-NiOOH into γ-NiOOH at high-potential state (above 1.6V vs RHE). Furthermore, the collective evidences from key reaction intermediates and isotope-labeled products definitely unravel the potential dependence of OER mechanism: OER process at low-potential state proceeds mainly through the lattice oxygen-mediated mechanism, while adsorbate evolution mechanism emerges as the predominant pathway at high-potential state. Interestingly, the dynamically changing OER mechanism can not only reduce the required overpotential at the low-potential state but also improve the electrochemical stability of catalysts at high-potential state.

Structural Changes of Ni and Ni-Pt Methane Steam Reforming Catalysts During Activation, Reaction, and Deactivation Under Dynamic Reaction Conditions.

Ni-based catalysts are the most widely used materials to produce H2 in large-scale methane steam reformers under stationary conditions. For domestic applications such as fuel cells, H2 production involves the exposure of the catalysts to more dynamic conditions due to the daily startup and shutdown operation mode, making Ni-based catalysts susceptible to oxidation and deactivation. In this context, we report a systematic investigation of the structural changes occurring for monometallic Ni/MgAlOx and bimetallic NiPt/MgAlOx catalysts during methane steam reforming under transient conditions, comprising catalyst activation, operation, and deactivation processes. Besides extensive catalytic tests, the samples prepared by incipient wetness impregnation were characterized by complementary methods, including N2-physisorption, X-ray diffraction, H2-temperature-programmed reduction, and electron microscopy. Next, the structure of the Ni and Pt species was monitored under reaction conditions using time and spatially resolved in situ/operando X-ray absorption spectroscopy. The results obtained show that before catalyst activation by H2-reduction, nickel diffuses into the support lattice and forms mixed oxides with magnesium. In the activated catalysts, Ni is present in the metallic state or alloyed with Pt. A clear beneficial effect of the noble metal addition was identified on both the activity and stability of the bimetallic NiPt/MgAlOx catalyst. In contrast, the pronounced oxidation and reincorporation of Ni into the support lattice were observed for the monometallic sample, and these catalyst deactivation effects are hindered in the bimetallic Ni-Pt catalyst. Overall, the outcome of our study not only helps in understanding the catalyst activation/deactivation processes at an atomic level but also provides the basis for the rational development of improved methane steam reforming catalysts.

Dynamic Surface Evolution of Metal Oxides for Autonomous Adaptation to Catalytic Reaction Environments.

Metal oxides possessing distinctive physical/chemical properties due to different crystal structures and stoichiometries play a pivotal role in numerous current technologies, especially heterogeneous catalysis for production/conversion of high-valued chemicals and energy. To date, many researchers have investigated the effect of the structure and composition of these materials on their reactivity to various chemical and electrochemical reactions. However, metal oxide surfaces evolve from their initial form under dynamic reaction conditions due to the autonomous behaviors of the constituent atoms to adapt to the surrounding environment. Such nanoscale surface phenomena complicate reaction mechanisms and material properties, interrupting the clarification of the origin of functionality variations in reaction environments. In this review, the current findings on the spontaneous surface reorganization of metal oxides during reactions are categorized into three types: 1) the appearance of nano-sized second phase from oxides, 2) the (partial) encapsulation of oxide atoms toward supported metal surfaces, and 3) the oxide surface reconstruction with selective cation leaching in aqueous solution. Then their effects on each reaction are summarized in terms of activity and stability, providing novel insight for those who design metal-oxide-based catalytic materials.

Operando QEXAFS Study of Pt–Fe Ammonia Slip Catalysts During Realistic Driving Cycles

Bifunctional Fe–Pt ammonia slip catalysts were studied by operando quick-scanning extended X-ray absorption fine structure spectroscopy (QEXAFS) under conditions mimicking rapid temperature variations that occur in an automotive exhaust gas aftertreatment system during real driving. Two catalysts, Pt/Al2O3 and Fe-ZSM-5, were tested individually, as mixtures and in dual bed arrangements. Applying QEXAFS allowed to track changes of active metal state with high time resolution. It uncovered a strong dependence of the active metal state on reaction conditions and catalyst bed layout. For example, proximity to platinum stabilized iron species in their more active oxidized state and led to higher Fe-ZSM-5 activity. On the contrary, isolated iron species were more susceptible to overreduction by ammonia which led to deactivation and low selectivity. The use of transient conditions uncovered the influence of non-equilibrium phenomena on catalytic performance under industrially relevant conditions. Specifically, the effect of ammonia storage on the increase of activity was shown. This was also accompanied by elevated N2O production not observed during tests with gradual heating. Additionally, unusually high NOx selectivity was detected for Fe-ZSM-5 under these conditions. Lastly, tracking catalyst state under dynamic reaction conditions disclosed that Fe-ZSM-5 activity did not grow directly with temperature increase but rather depended on the oxidation state of Fe and surface concentration of ammonia.

Steering the structure and selectivity of CO2 electroreduction catalysts by potential pulses

Convoluted selectivity trends and a missing link between reaction product distribution and catalyst properties hinder practical applications of the electrochemical CO2 reduction reaction (CO2RR) for multicarbon product generation. Here we employ operando X-ray absorption and X-ray diffraction methods with subsecond time resolution to unveil the surprising complexity of catalysts exposed to dynamic reaction conditions. We show that by using a pulsed reaction protocol consisting of alternating working and oxidizing potential periods that dynamically perturb catalysts derived from Cu2O nanocubes, one can decouple the effect of the ensemble of coexisting copper species on the product distribution. In particular, an optimized dynamic balance between oxidized and reduced copper surface species achieved within a narrow range of cathodic and anodic pulse durations resulted in a twofold increase in ethanol production compared with static CO2RR conditions. This work thus prepares the ground for steering catalyst selectivity through dynamically controlled structural and chemical transformations.

Versatile in situ/operando Setup for Studying Catalysts by X‐Ray Absorption Spectroscopy under Demanding and Dynamic Reaction Conditions for Energy Storage and Conversion

AbstractX‐ray absorption spectroscopy (XAS) is one of the powerful operando tools to track structural variations in heterogeneous catalysts. The nature of active sites in catalyst research is of great relevance, especially given the growing importance of energy storage using CO2 as feedstock and the need for dynamic availability of electric power. Due to the pressure/temperature prerequisite of catalyst performance, the characterization of catalyst structure during catalysis under such high‐pressure reaction conditions is important to further improve catalyst design at a molecular level. Investigating catalysts in a controlled reaction atmosphere, while probing with X‐rays, offers an excellent opportunity for developing infrastructure at the synchrotron. Herein, a mobile setup with a robust spectroscopic cell for in situ and operando XAS applications, including a high‐pressure gas dosing equipment for such catalytic systems, is presented. The in situ/operando cell is operational for both the transmission and the fluorescence XAS mode at up to 50 bar and 450 °C. The setup comes with a protective box with Kapton windows, which holds the cell and serves as a miniature fume hood, and on‐line product analysis. Furthermore, the gas dosing equipment is compact, light‐weighted and can be easily transported to different synchrotrons and allows an optimum pre‐mix of gas flows and pressure build‐up. Methanol and Fischer‐Tropsch syntheses are used as examples for the highly flexible instrumentation.

Bidentate and Monodentate Binding of Chromium (VI) on the Acid Scavenger Exchange Resin, MP-Carbonate, as a Function of pH

Hexavalent chromium or Cr (VI) is a serious health risk that has been identified in drinking water and generally originates as industrial by-products and hazardous waste sites, although natural sources have been reported. Common methods to remove Cr (VI) employ adsorption, filtration, ion exchange technologies, and redox reactions. We report on a macro porous polystyrene anion exchange resin, a solid-supported equivalent of tetra-alkyl ammonium carbonate (Biotage® MP-Carbonate), to effectively remove Cr (VI) from synthetic wastewater with bidentate or monodentate binding modes of chromate, depending on the pH environment. Sorption capacity is pH dependent with 332, 161, and 163 mg CrO42− per gram of MP-Carbonate adsorbed at pH 2, 6.8, and 10, respectively. Experimental data indicate an exergonic and endothermic adsorption process. Static vs. dynamic reaction conditions are discussed. Density functional theory calculations parallel the Gibbs free energy results of the bidentate binding of the large chromate ion compared to carbonate with the ammonium ions. These studies identify the potential uses of MP-Carbonate in the remediation of Cr (VI) wastewater unrelated to the pH of the water source.

Regulating Cu(II)-benzimidazole coordination structure in rigid-rod aramid fiber and its composites enhancement effects

Coordination crosslinking, as the kind of strongest supramolecular interaction, has been widely introduced into flexible polymer chains, including rubbers and aerogels, while that in rigid-rod polymer is much rare, due to the difficulty in rigid-rod macromolecules' configuration change. Moreover, the exact coordination structure inside those polymers is not clear. Herein, this coordination interaction was incorporated into a traditional rigid-rod polymer, aramid fiber, by Cu2+-benzimidazole coordination reaction, through the careful control of the dynamic reaction conditions, and the structural/performance relationships of fiber and its composites are detailed studied. It is seen that two different coordination structures, S-coordination style (Cu2+ coordination with single benzimidazole unit) and M-coordination style (Cu2+ coordination with multiple benzimidazole units from different macromolecular chains), could be both obtained inside fiber, using different solvents as the reaction medium. Moreover, using reaction kinetics equilibrium, we presented a mathematical method to investigate those two coordination structures in detail, including their average coordination numbers of Cu2+ and reaction equilibrium constant (K). Further, the experimental results show that the fiber with average coordination number of 2 exhibits a comprehensive improved performance in heat/solvent resistance, fluorescence emission as well as fiber's transverse properties, with the corresponding increase of 54.2% and 47.1% for its composites interfacial properties and compressive strength, compared with untreated fiber.

Role of Iron on the Structure and Stability of Ni3.2Fe/Al2O3 during Dynamic CO2 Methanation for P2X Applications

AbstractAn energy scenario, mainly based on renewables, requires efficient and flexible Power‐to‐X (P2X) storage technologies, including the methanation of CO2. As active Ni0 surface sites of monometallic nickel‐based catalysts are prone to surface oxidation under hydrogen‐deficient conditions, we investigated iron as “protective” dopant. A combined operando X‐ray absorption spectroscopy and X‐ray diffraction setup with quantitative on‐line product analysis was used to unravel the structure of Ni and Fe in an alloyed Ni−Fe/Al2O3 catalyst during dynamically driven methanation of CO2. We observed that Fe protects Ni from oxidation and is itself more dynamic in the oxidation and reduction process. Hence, such “sacrificial” or “protective” dopants added in order to preserve the catalytic activity under dynamic reaction conditions may not only be of high relevance with respect to fine‐tuning of catalysts for future industrial P2X applications but certainly also of general interest.

Integrated multiscale modeling of fixed bed reactors: Studying the reactor under dynamic reaction conditions

Fixed-bed reactors are widely used for large-scale, heterogeneously-catalyzed gas-phase reactions, such as steam reforming, selective hydrogenations and partial oxidations. These reactions have strong heat effects; exothermic reactions can exhibit “hot-spots”, while endothermic reactions require a large supply of heat. Steady-state reactor analysis does not give the whole picture; when dynamics are considered then hot-spots, extinction waves and “wrong-way” behavior (increased downstream temperature following inlet feed temperature decrease) can result. We investigate here the interactions between the local pellet-scale dynamic responses, and the bed-scale global fields, using computational fluid dynamics (CFD). This method allows detailed simulation of the fluid around the catalyst, coupled to transport and reaction inside it. To date, reactor dynamics have been based on simplified reactor models. We have developed a methodology to use 3D detailed models to investigate local dynamic hot-spot formation and wrong-way responses to inputs and their connections to bed-scale flow and transport.

Contactless temperature measurements under static and dynamic reaction conditions in a single-pass fixed bed reactor for CO2 methanation

Resolving the temperature profile in catalytic fixed bed reactors is essential for the investigation of reactions. However, this is not readily possible at laboratory scale without affecting the flow regime. Thermography tracks temperature profiles contactless with a high spatial resolution and without influencing the flow regime in the reactor. This study applies thermography for a powder fixed bed reactor at laboratory scale. A pa​rameter investigation was conducted to identify the dependency of hot spot temperatures during the highly exothermic CO2 methanation reaction on major process parameters like reactant gas dilution, volume flow and cooling temperature. Further, the impact of an upstream cold bed on the temperature distribution in the catalytic bed was investigated. It was found that the cold bed was able to remove the heat of reaction efficiently to prolong an overheating of the catalyst bed. Thermography was also used to study a dynamic phenomenon, namely the in situ sulfur poisoning of a nickel-alumina catalyst. The temperature of the fixed bed was thereby spatially resolved at any point in time during the reaction. It was found that poisoning causes the reactive zone to move through the catalytic bed at a constant velocity. A simple poisoning model was applied to correlate the observed moving velocity to the catalyst's specific active surface area as determined by static H2 chemisorption at room temperature.

Future Challenges in Heterogeneous Catalysis: Understanding Catalysts under Dynamic Reaction Conditions.

In the future, (electro‐)chemical catalysts will have to be more tolerant towards a varying supply of energy and raw materials. This is mainly due to the fluctuating nature of renewable energies. For example, power‐to‐chemical processes require a shift from steady‐state operation towards operation under dynamic reaction conditions. This brings along a number of demands for the design of both catalysts and reactors, because it is well‐known that the structure of catalysts is very dynamic. However, in‐depth studies of catalysts and catalytic reactors under such transient conditions have only started recently. This requires studies and advances in the fields of 1) operando spectroscopy including time‐resolved methods, 2) theory with predictive quality, 3) kinetic modelling, 4) design of catalysts by appropriate preparation concepts, and 5) novel/modular reactor designs. An intensive exchange between these scientific disciplines will enable a substantial gain of fundamental knowledge which is urgently required. This concept article highlights recent developments, challenges, and future directions for understanding catalysts under dynamic reaction conditions.

Dynamic transformation of small Ni particles during methanation of CO2 under fluctuating reaction conditions monitored by operando X-ray absorption spectroscopy

A 10 wt.-% Ni/Al2O3 catalyst with Ni particles of about 4 nm was prepared and applied in the methanation of CO2 under dynamic reaction conditions. Fast phase transformations between metallic Ni, NiO and NiCO3 were observed under changing reaction atmospheres using operando X-ray absorption spectroscopy (XAS). Removing H2 from the feed gas and, thus, simulating a H2 dropout during the methanation reaction led to oxidation of the active sites. The initial reduced state of the Ni particles could not be recovered under methanation atmosphere (H2/CO2 = 4); this was only possible with an effective reactivation step applying H2 at increased temperatures. Furthermore, the cycling of the gas atmospheres resulted in a steady deactivation of the catalyst. Operando XAS is a powerful tool to monitor these changes and the behavior of the catalyst under working conditions to improve the understanding of the catalytic processes and deactivation phenomena.

Selective catalytic reduction of nitric oxide with hydrogen on supported Pd: Enhancement by hydrogen spillover

Two high-activity supported-Pd catalysts (Pd/V2O5/TiO2/SBA-15 and Pd/V2O5/TiO2/MCM-41) for selective catalytic reduction of NO with H2 were studied. They differ only by the mesoporous-silica support: SBA-15 vs. MCM-41. Although the SBA-15 supported catalyst had a lower surface area, it showed a higher NO conversion (95% vs. 84%, at a high space velocity of 4.6×104 1/h). Two pieces of direct and related evidence are shown for the role of hydrogen spillover in enhancing the catalytic activities of these two catalysts. A comparison of hydrogen adsorption isotherms (via the Benson-Boudart method) show more spillover on the SBA-15 supported catalyst. The results from the transient kinetics experiments also showed that the amount of spiltover hydrogen stored on the Pd/V2O5/TiO2/SBA-15 under dynamic reaction conditions was significantly higher than that on Pd/V2O5/TiO2/MCM-41 (7.3μmol/g vs. 2.1μmol/g). Catalytic performance of the two catalysts for practical applications is reported. The favorable effect of hydrogen spillover on H2-SCR could be used as a new strategy for improving the performance of noble-metal H2-SCR catalysts.

Catalysts at work: From integral to spatially resolved X-ray absorption spectroscopy

Spectroscopic studies on heterogeneous catalysts have mostly been done in an integral mode. However, in many cases spatial variations in catalyst structure can occur, e.g. during impregnation of pre-shaped particles, during reaction in a catalytic reactor, or in microstructured reactors as the present overview shows. Therefore, spatially resolved molecular information on a microscale is required for a comprehensive understanding of theses systems, partly in ex situ studies, partly under stationary reaction conditions and in some cases even under dynamic reaction conditions. Among the different available techniques, X-ray absorption spectroscopy (XAS) is a well-suited tool for this purpose as the different selected examples highlight. Two different techniques, scanning and full-field X-ray microscopy/tomography, are described and compared. At first, the tomographic structure of impregnated alumina pellets is presented using full-field transmission microtomography and compared to the results obtained with a scanning X-ray microbeam technique to analyse the catalyst bed inside a catalytic quartz glass reactor. On the other hand, by using XAS in scanning microtomography, the structure and the distribution of Cu(0), Cu(I), Cu(II) species in a Cu/ZnO catalyst loaded in a quartz capillary microreactor could be reconstructed quantitatively on a virtual section through the reactor. An illustrating example for spatially resolved XAS under reaction conditions is the partial oxidation of methane over noble metal-based catalysts. In order to obtain spectroscopic information on the spatial variation of the oxidation state of the catalyst inside the reactor XAS spectra were recorded by scanning with a micro-focussed beam along the catalyst bed. Alternatively, full-field transmission imaging was used to efficiently determine the distribution of the oxidation state of a catalyst inside a reactor under reaction conditions. The new technical approaches together with quantitative data analysis and an appropriate in situ catalytic experiment allowed drawing important conclusions on the reaction mechanism, and the analytical strategy might be similarly applied in other case studies. The corresponding temperature profiles and the catalytic performance were measured by means of an IR-camera and mass spectrometric analysis. In a more advanced experiment the ignition process of the partial oxidation of methane was followed in a spatiotemporal manner which demonstrates that spatially resolved spectroscopic information can even be obtained in the subsecond scale.

Evolution of catalyst structure under reaction conditions from time-resolved in situ XAS investigations

Time-resolved x-ray absorption spectroscopy (TR-XAS) possesses excellent capabilities to reveal quantitative phase composition and average valence together with the evolution of the local structure of a system under dynamic reaction conditions. The work discussed here focused on time-resolved in situ XAS investigations aiming, first, at understanding structural evolution under dynamic conditions and, second, at revealing properties of the system studied not available from investigations under stationary conditions. In order to demonstrate the potential of TR-XAS in heterogeneous catalysis, TR-XAS investigations combined with mass spectrometry of the structural evolution of Keggin type heteropolyoxomolybdates (HPOM) during thermal treatment in propene and in propene and oxygen in the temperature range from 300 K to 773 K are presented. It is concluded, that the undistorted Keggin ion in the as-prepared HPOM has to be regarded as a precursor of the active catalyst, while the formation of lacunary Keggin ions is a prerequisite for catalytic activity.

Application of time-resolved in-situ X-ray absorption spectroscopy in solid-state chemistry.

Time-resolved X-ray absorption spectroscopy (TR-XAS) possesses excellent capabilities to reveal quantitative phase composition and average valence together with the evolution of the local structure of a system under dynamic reaction conditions. The work discussed here focused on time-resolved in-situ XAS investigations aiming, first, at understanding structural evolution under dynamic conditions and, second, at revealing properties of the system studied not available from investigations under stationary conditions. Hence, not only was the local structure of a material studied under reaction conditions, but characteristic properties of the reaction, such as reaction intermediates or the kinetics of the reaction, were also elucidated. The solid-gas reactions presented here clearly demonstrate the potential of TR-XAS investigations to extend the suitability of XAS for in-situ studies in solid-state chemistry to investigations under dynamic conditions.