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The development of catalysts for propylene oxide production from direct epoxidation using propylene and oxygen remains a challenge. Compared to ethylene epoxidation, where selectivity on silver catalysts is high, the low selectivity to produce propylene oxide over silver is partially attributed to the lack of electrophilic oxygen under propylene epoxidation reaction conditions. Here, we investigate how to mediate the chemical reactivity of oxygen by theory-inspired experiments for propylene epoxidation. We show how adding electrophilic-O via SO4 oxyanions to the surface of silver increases epoxide selectivity. Moreover, we show how the addition of Cl to the SO4-modified catalyst activates the oxyanion, giving a more than 4-fold increase in selectivity to propylene oxide. Finally, we explore different systems using DFT and draw a picture on how the next catalyst/co-catalyst systems should be tuned to design a catalyst with high selectivity for direct propylene oxidation.

Platinum group metal (PGM)-based catalysts are known to be highly active in the total combustion of lower hydrocarbons. However, through an alternative catalyst design reported in this paper by isolating PGM-based active sites in a tungsten phosphate matrix, we present a class of catalysts for selective oxidation of n-butane, propane, and propylene that do not contain Mo or V as redox-active elements. Two different catalyst concepts have been pursued. Concept A: isolating Ru-based active sites in a tungsten phosphate matrix coming upon as ReO3-type structure. Concept B: dilution of PGM-based active sites through the synthesis of X-ray amorphous Ru tungsten phosphates supported on SiO2. Using a high-throughput screening approach, model catalysts over a wide compositional range were evaluated for C3 and C4 partial oxidation. Bulk crystalline and supported XRD amorphous phases with similar Ru/W/P compositions showed comparable performance. Hence, for these materials, composition is more crucial than the degree of crystallinity. Further studies for optimization on second-generation supported systems revealed even better results. High selectivity for n-butane oxidation to maleic anhydride and propane oxidation to an acrolein/acrylic acid has been achieved.

Renewable energy storage via water electrolysis strongly depends on the design of electrified electrode–electrolyte interfaces at which electricity is converted into chemical energy. At the core of the hydrogen evolution reaction (HER) and the oxygen evolution reaction conversion efficiency are interfacial processes with complex dynamic mechanisms, whose further acceleration is practically impossible without a thorough fundamental understanding of electrocatalysis. Here, we communicate new experimental insights into HER, which will potentially further deepen our general understanding of electrocatalysis. Of special note is the very surprising observation that the most active metals (i.e., noble metals) for HER, which exhibit the lowest overpotentials at a defined current density, exhibit the highest activation energies in comparison to the other metals from the d-block. This suggests a major, if not dominant, impact of the frequency factor on activity trends and the need for deeper reconsideration of the origins of electrocatalytic activity.

Advanced in situ X-ray absorption spectroscopy characterization ofelectrochemically co-electrodeposited bi-element copper alloy electrodesshows that zinc yields the formation of a stable cationic Cu species during theelectroreduction of CO2 at high cathodic polarization. In contrast, theformation/stabilization of cationic Cu species in copper oxides, or doping Cuwith another element, like Ni, is not possible. It is found that the pure andmixed Cu:Zn electrodes behave similarly in term of electrocatalytic selectivityto multi-carbon products. At higher Zn concentrations the electrode behaveslike the pure Zn catalyst, which indicates that the Cu cationic species do nothave a significant influence on the selectivity to multi-carbon products. It isfound that in the non-monotonically distribution of products is dominated interm of surface energy in which copper prefers the surface. Otherwise, thiswork highlights the importance of in situ characterization to uncover themechanisms mediating the catalytic reactions in contrast to ex situ or postmortem analysis, which can be a source of misinterpretation.

Spinel-type catalysts are promising anode materials for the alkaline oxygen evolution reaction (OER), exhibiting low overpotentials and providing long-term stability. In this study, we compared two structurally equal Co2FeO4 spinels with nominally identical stoichiometry and substantially different OER activities. In particular, one of the samples, characterized by a metastable precatalyst state, was found to quickly achieve its steady-state optimum operation, while the other, which was initially closer to the ideal crystallographic spinel structure, never reached such a state and required 168 mV higher potential to achieve 1 mA/cm2. In addition, the enhanced OER activity was accompanied by a larger resistance to corrosion. More specifically, using various ex situ, quasi in situ, and operando methods, we could identify a correlation between the catalytic activity and compositional inhomogeneities resulting in an X-ray amorphous Co2+-rich minority phase linking the crystalline spinel domains in the as-prepared state. Operando X-ray absorption spectroscopy revealed that these Co-rich domains transform during OER to structurally different Co3+-rich domains. These domains appear to be crucial for enhancing OER kinetics while exhibiting distinctly different redox properties. Our work emphasizes the necessity of the operando methodology to gain fundamental insight into the activity-determining properties of OER catalysts and presents a promising catalyst concept in which a stable, crystalline structure hosts the disordered and active catalyst phase.

The oxygen evolution reaction (OER) is crucial to future energy systems based on water electrolysis. Iridium oxides are promising catalysts due to their resistance to corrosion under acidic and oxidizing conditions. Highly active iridium (oxy)hydroxides prepared using alkali metal bases transform into low activity rutile IrO2 at elevated temperatures (>350 °C) during catalyst/electrode preparation. Depending on the residual amount of alkali metals, we now show that this transformation can result in either rutile IrO2 or nano-crystalline Li-intercalated IrOx. While the transition to rutile results in poor activity, the Li-intercalated IrOx has comparative activity and improved stability when compared to the highly active amorphous material despite being treated at 500 °C. This highly active nanocrystalline form of lithium iridate could be more resistant to industrial procedures to produce PEM membranes and provide a route to stabilize the high populations of redox active sites of amorphous iridium (oxy)hydroxides.

The cleavage of the N2 triple bond on the Fe(111) surface is believed to be the rate limiting step of the famed Haber-Bosch ammonia catalysis. Using a combination of machine learning potentials and advanced simulation techniques, we study this important catalytic step as a function of temperature. We find that at low temperatures our results agree with the well-established picture. However, if we increase the temperature to reach operando conditions the surface undergoes a global dynamical change and the step structure of the Fe(111) surface is destroyed. The catalytic sites, traditionally associated with the Fe(111) surface appear and disappear continuously. Our simulations illuminate the danger of extrapolating low-temperature results to operando conditions and indicate that the catalytic activity can only be inferred from calculations that take dynamics fully into account. More than that, they show that it is the transition to this highly fluctuating interfacial environment that drives the catalytic process.

Artificial intelligence (AI) can accelerate materials design by identifying the key parameters correlated with the performance. However, widely used AI methods require big data, and only the smallest part of the available data in heterogeneous catalysis meets the quality requirement for data-efficient AI. Here, we use rigorous experimental procedures, designed to consistently take into account the kinetics of the catalyst active states formation, in order to measure 55 physicochemical parameters as well as the reactivity of 12 catalysts towards ethane, propane, and n-butane oxidation. These catalyst materials are based on vanadium or manganese redox-active elements (RAEs) and present diverse phase compositions, crystallinities, and catalytic behaviors. By applying the sure-independence-screening-and-sparsifying-operator (SISSO) approach to the consistent data set, we identify nonlinear property-function relationships depending on several key parameters, reflecting the intricate interplay of underlying processes governing selective oxidation. This approach indicates the most relevant characterization techniques and shows how the catalyst properties may be tuned in order to achieve the desired performance. For example, to achieve high olefin yields, the catalyst must have a high specific surface area, a low concentration of surface RAE, and the ability to change the surface RAE oxidation states under reaction conditions with respect to vacuum. These parameters are measured by N2 adsorption, x-ray photoelectron spectroscopy (XPS), and near-ambient-pressure in situ XPS. They reflect the relevance of local transport, site isolation, surface redox activity, and the materials dynamical restructuring under reaction conditions. Although the relationship describing the even more challenging oxygenate yields shares similarities with that for olefin yields, a parameter reflecting the importance of specific surface sites, derived from the analysis of the carbon 1s XPS spectra, is additionally identified as key for high selectivity to oxygenates.

Cobalt oxide (assigned as CoOx) is an efficient oxygen evolution reaction (OER) nanocatalyst, which has been extensively studied as a replacement to noble metal-based catalysts. The recent observations and understandings for the interfacial state, adsorbed intermediate products, and rate-determining steps (RDS) on CoOx, however, have remained elusive because of the dynamic transformation of different Co ions and the transient nature of the intermediates formed during the OER process. In this work, we propose that under the chosen experimental conditions, the redox process between Co(III) and Co(IV) species does not follow a proton-coupled electron transfer mechanism that is thought to be common prior to the OER, but it involves a proton-decoupled electron transfer, clarified by isotope labeling experiments and in situ electrostatic modulation. The interfacial state of CoOx is negatively charged prior to the formation of Co(IV)═O species. The theoretical concentration of the resulting Co(IV)═O species is approximately 0.1229 × 1019 cm–2. The Co(IV)═O species are demonstrated to directly regulate the OER performance. Moreover, we experimentally monitor the dynamic evolution behaviors of Co(IV)═O, Co(O)O–, OOH*, and O2–* intermediates during the OER with in situ time-resolved infrared spectroscopy, and the following elementary step OOH* + OH– → OO–* + H2O is likely to be the unexpected RDS in the OER process.

Methane oxidation using O2 over transition metal oxides often requires severe conditions ( >500 °C) to achieve detectable conversion. In this study, NO was used to transfer oxygen atoms from O2, through the facile gas-phase formation of NO2 at moderate conditions (0.1 MPa and 300–400 °C), to oxidize methane over silica-supported transition metal oxides (VOx, CrOx, MnOx, NbOx, MoOx, and WOx). In situ infrared spectroscopy measurements indicated that the reaction likely proceeded by the formation of surface monodentate nitrate intermediates. These nitrate species were formed by the interaction between adsorbed NO2 and the supported metal oxides. During the reaction, the oxides of vanadium, molybdenum, and tungsten formed formaldehyde and CO2, whereas the oxides of chromium, manganese, and niobium produced only CO2. These results are consistent with the known hydrocarbon oxidation chemistry of the metal oxides. Contact time measurements on VOx/SiO2 indicated that formaldehyde was a primary product and CO2 was the final product; conversely, analogous measurements on MnOx/SiO2 showed that CO2 was the sole product. The formaldehyde production rate on VOx/SiO2, MoOx/SiO2, and WOx/SiO2, based on surface sites measured by high temperature oxygen chemisorption, compared favorably to oxygenate production rates for stronger oxidants (N2O and H2O2) reported in the literature.

Ammonia synthesis remains one of the most important catalytic processes since it enables efficient hydrogen storage and provides the basis for the production of fertilizers. Herein, complementary bulk and local analytical techniques were combined to investigate the effect of selected promoters (Al, K, Ca) on the reduction of wuestite into α-iron and their catalytic performance for ammonia synthesis. The use of promoters appears to have a positive effect on the wuestite-derived catalyst in ammonia synthesis. The promoters seemingly act as a binder for wuestite grains and impede the reduction and disproportionation events of wuestite precursors resulting in an increased catalytic performance. This effect is associated with an increase of surface area and mesoporosity. The study delivers new insights into the interplay of structure and promoters in wuestite-based catalysts.

Automation of experiments is a key component on the path of digitalization in catalysis and related sciences.

Perovskites are interesting oxidation catalysts due to their chemical flexibility enabling the tuning of several properties. In this work, we synthesized LaFe1−xCoxO3 catalysts by co-precipitation and thermal decomposition, characterized them thoroughly and studied their 2-propanol oxidation activity under dry and wet conditions to bridge the knowledge gap between gas and liquid phase reactions. Transient tests showed a highly active, unstable low-temperature (LT) reaction channel in conversion profiles and a stable, less-active high-temperature (HT) channel. Cobalt incorporation had a positive effect on the activity. The effect of water was negative on the LT channel, whereas the HT channel activity was boosted for x > 0.15. The boost may originate from a slower deactivation rate of the Co3+ sites under wet conditions and a higher amount of hydroxide species on the surface comparing wet to dry feeds. Water addition resulted in a slower deactivation for Co-rich catalysts and higher activity in the HT channel state.

Several in situ studies have revealed spatiotemporal dynamics on heterogeneous catalysts surfaces under chemical stimuli1-4, which presumably control the activity, selectivity, and productivity5-11. However, operando validations of sufficient spacetime resolution12 are often missing, and hence, the effect of these dynamics on catalytic performance may not be entirely clear. Here, using dry reforming of methane over Ni as an example, we demonstrate the relevance of catalytic redox dynamics for reaction performance and determine their genesis from adaptive chemistry and continual catalytic cycling. By combining operando scanning electron microscopy and near-ambient-pressure X-ray photoelectron spectroscopy, we found that activation sites for methane and carbon dioxide differed but continually transformed into each other during the reaction. This behavior enabled a self-sustained oscillating regime evincing the sequential formation of active sites. We also found that not all spatiotemporal dynamics accounted for the catalytic function. We highlight the importance of oscillating reactions for mechanistic studies and propose that the generation of mechanical strain at the catalyst during redox cycling acted as a feedback element for the oscillations. These observations lead to deeper understanding of fundamental catalysis and open new opportunities for tuning catalytic performances.

NiFe-based oxides are one of the best-known active oxygen evolution electrocatalysts. Unfortunately, they rapidly lost performance in Fe-purified KOH during the reaction. Herein, tetraphenylporphyrin (TPP) was loaded on a catalyst/electrolyte interface to alleviate the destabilization of NiFe (oxy)hydroxide. We propose that the degradation occurs primarily due to the release of thermodynamically unstable Fe. TPP acts as a protective layer and suppresses the dissolution of hydrated metal at the catalyst/electrolyte interface. In the electric double layer, the nonpolar TPP layer on the NiFe surface also invigorates the redeposition of the active site, Fe, which leads to prolonging the lifetime of NiFe. The TPP-coated NiFe was demonstrated in anion exchange membrane water electrolysis, where hydrogen was generated at a rate of 126 L h

The oxygen evolution reaction (OER) plays a crucial role in (photo)electrochemical devices that use renewable energy to produce synthetic fuels. Recent measurements on semiconducting oxides have found a power law dependence of the OER rate on surface hole density, suggesting a multihole mechanism. In this study, using transient photocurrent measurements, density functional theory simulations and microkinetic modelling, we have uncovered the origin of this behaviour in haematite. We show here that the OER rate has a third-order dependence on the surface hole density. We propose a mechanism wherein the reaction proceeds by accumulating oxidizing equivalents through a sequence of one-electron oxidations of surface hydroxy groups. The key O–O bond formation step occurs by the dissociative chemisorption of a hydroxide ion involving three oxyl sites. At variance with the case of metallic oxides, the activation energy of this step is weakly dependent on the surface hole coverage, leading to the observed power law.

Glassy carbon is frequently used in electrochemical research due to its presumed robust electrochemical performance. Although it is widely utilized as a rotating disc electrode material, the modification of glassy carbon during electro-catalytic process is rarely emphasized or characterized. In this report, we investigated the structural modification of glassy carbon imparted by electrochemical oxidation in acidic media and compared the behavior with graphite. The functional groups generated from electrochemical oxidation in both electrodes possess similar electrochemical properties. However, above an oxidation potential of 1.8 V (vs. reversibly hydrogen electrode), glassy carbon exhibits a lower electrochemical capacitance compared to graphite. We propose that the existence of electrochemically inactive species, originating from the non-graphitic portion of glassy carbon is attributed to such deterioration. Additionally, high resolution scanning electron microscopy (HR-SEM) and high-resolution transmission electron microscopy (HR-TEM) images corroborate how electrochemical oxidation prevails for glassy carbon electrodes at oxidative potentials. The overall analysis leads us to propose a corrosion mechanism for glassy carbon in acidic solution.

This review article deals with the challenge to identify catalyst materials from literature studies for the ammonia decomposition reaction with potential for application in large-scale industrial processes. On the one hand, the requirements on the catalyst are quite demanding. Of central importance are the conditions for the primary reaction that have to be met by the catalyst. Likewise, the catalytic performance, i.e., an ideally quantitative conversion, and a high lifetime are critical as well as the consideration of requirements on the product properties in terms of pressure or by-products for potential follow-up processes, in this case synthesis gas applications. On the other hand, the evaluation of the multitude of literature studies poses difficulties due to significant varieties in catalytic testing protocols.

It is of general interest to combine the faradaic processes based high energy density of a battery with the non-faradaic processes based high power density of a capacitor in one cell. Surface area and functional groups of electrode materials strongly affect these properties, but a mechanistic explanation for the interplay between such process and how to balance them is largely missing. For the anode material Li4Ti5O12 (LTO), we suggest a polaron based mechanism that influences Li ion uptake and mobility. Electrolytes containing a lithium salt induce an observable change in the bulk NMR relaxation properties of LTO nano particles. The longitudinal 7Li NMR relaxation time of bulk LTO can change by almost an order of magnitude and, therefore, reacts very sensitively to the cation and its concentration in the surrounding electrolyte. The reversible effect is largely independent of the used anions and of potential anion decomposition products. It is concluded that lithium salt containing electrolytes increase the mobility of surface polarons. These polarons and additional lithium cations from the electrolyte can now diffuse through the bulk, induce the observed enhanced relaxation rate and enable the non-faradaic process. This picture of a Li+ ion equilibrium between electrolyte and solid may help with improving the charging properties of electrode materials.

ChemCatChem : the European Society Journal for Catalysis 14(19), e202200300 (2022). doi:10.1002/cctc.202200300 special issue: "Catalysts and Reactors under Dynamic Conditions for Energy Storage and Conversion", Published by Wiley-VCH, Weinheim

Detrimental to the performance of solid oxide cells (SOCs) is the interface between electrolyte and each electrode: for purely electron-conducting electrodes, it is part of the triple phase boundary where the reactions take place, and for mixed ionic and electronic conductor (MIEC) electrodes, the oxygen ions coming from or moving into the electrolyte are transported through this phase boundary. As such, the performance of SOFCs and SOECs is strongly dependent on the chemical nature of this interface. By combining state-of-the-art electron microscopy, synchrotron-based X-ray spectroscopy and theoretical calculations, we were able to obtain an atomically resolved picture of the interface between yttria-stabilized zirconia (YSZ, with 8 mol% Y2O3) and lanthanum strontium manganite (LSM, (La0.8Sr0.2)0.95MnO3-δ).[1] Energy-dispersive X-ray (EDX) spectroscopy measurements in the transmission electron microscope (TEM) reveal the presence of a significant amount of inter-diffusion between YSZ and LSM. Only strontium does not show any diffusion into YSZ, resulting in a shift of its intensity distribution profile of approximately 0.8 nm with respect to all other elements. All this is in agreement with Monte-Carlo-based simulations that were used to theoretically model the YSZ/LSM boundary including the ion- as well as site-specific swapping probabilities for the diffusion process.[1] According to these simulations, this diffusion region is slightly amorphous. By means of atomically resolved scanning TEM (STEM), this reduction of long-range order was observed experimentally for an approximately 1.5 nm wide slab on the YSZ side of the boundary, clearly distinguishing it from the bulk fluorite structure. In materials science, distinct ‘2D-like’ layers at grain boundaries and interfaces are often referred to as ’complexions’. While these can in general occur in different types, depending on thickness and retained order,[2] they are all characterized by a self-limited (finite) width and thermodynamic stability obtained only by confinement in between two bulk phases (i.e. they cannot exist on their own without neighboring phases).[2] These complexions have recently also been discovered in other energy-related systems such as battery materials.[3] The presence of such a stable complexion might be the reason why the YSZ/LSM interface is not as prone to the formation of lanthanum or strontium zirconates as other perovskites such as lanthanum strontium cobalt ferrite (LSCF).[4] Consequently, it might have significant influence on the chemical stability of such SOCs and a thorough understanding of the complexions between electrolyte and electrode may in the future allow fine tuning of SOC performance and stability. This requires the determination of electrochemical and thermodynamic properties of these interface complexions for which we will, in a next step, employ theoretical simulations and experimental techniques in order to find out how different transport phenomena, widths and stabilities of the layers behave with temperature, time and chemical environment. [1] H. Tuerk, F.-P. Schmidt, T. Götsch et al. in preparation [2] S. J. Dillon et al. Acta Mater. 2007, 55, 6208–6218 [3] J. Timmermann et al. Phys. Rev. Lett. 2020, 125, 206101 [4] W. Wang et al. J. Electrochem. Soc. 2006, 153, A2066

ACS applied materials & interfaces 13(25), 30187-30197 (2021). doi:10.1021/acsami.1c06771, Published by American Chemical Society, Washington, DC

X-ray photoemission spectroscopy (XPS) measurements in near-ambient pressure (NAP) conditions result in a signal loss of the primary spectrum as a result of inelastic scattering of photoelectrons in the gas phase. The inelastic scattering of the primary electrons gives rise to a secondary signal that can result in additional and often unwanted features in the measured spectrum. In the present work, we derive equations that can be used to model the resulting signal and provide equations that can be used to simulate or remove the inelastic scattering signal from measured spectra. We demonstrate this process for photoemission spectra of a wide range of kinetic energies, measured from Au, Ag, and Cu, in a variety of gases (N2, He, H2, and O2). The work is supplemented with an open-source software in which the algorithms described here have been implemented and can be used to remove the gas phase inelastic scattering signal.

Within the Carbon2Chem® network, basic research is mandatory for a successful implementation and realization of sustainable technologies for CO2 emission reduction. For this purpose, the exchange of knowledge between the project partners in the individual subareas is as essential as obtaining precise data on the fundamental parameters on a laboratory scale in order to transfer them later to large-scale plants. Therefore, the Carbon2Chem® laboratory offers a platform to gain detailed insights into the individual sub-processes and to then apply these findings at the technical center in Duisburg.

Copper-exchanged zeolites are promising catalysts for the direct methane-to-methanol reaction, but the design of improved catalysts has been hampered by limited understanding of the active site str...

As-prepared materials tested for a catalytic reaction are usually only precatalysts that become active and/or selective under specific conditions. During this initial formation phase, catalysts can...