Following the Structural Changes of Iron Oxides during Reduction under Transient Conditions.

Iron is considered as attractive energy carrier in a carbon-free circular energy economy. The reduction of iron oxide is crucial for its applica-tion as a metal fuel as it determines the efficiency of the cycle. Temperature programmed reduction of α-Fe2O3 was monitored by complementary X-ray absorption spectroscopy (XAS) and diffraction (XRD) to obtain the phase composition with high time resolution. Synchrotron Mössbauer spectroscopy (SMS) was additionally employed due to its high sensitivity to the different iron species. Theoretical calculations of surface and bulk adsorption processes were performed to establish the potential reaction pathways and the corresponding energy barriers. A kinetic particle model was then developed to bridge the experimental data and theoretical calculations, which reproduced the reduction onset and behavior. The reduction process was found to be strongly dependent on the heating rate in terms of the reduction window and the observed intermediate species. We propose that a core-shell mechanism determines the reaction by forming an iron layer which subsequently hinders diffusion of water out of the porous particles leading to some unreduced FeO at high temperature. This study demonstrates the need for complementary methods for describing complex heterogeneous systems and overcoming the chemical sensitivity limitations of any single method.

Highly Active Oxidation Catalysts through Confining Pd Clusters on CeO2 Nano-Islands.

CeO2-supported noble metal clusters are attractive catalytic materials for several applications. However, their atomic dispersion under oxidizing reaction conditions often leads to catalyst deactivation. In this study, the noble metal cluster formation threshold is rationally adjusted by using a mixed CeO2-Al2O3 support. The preferential location of Pd on CeO2 islands leads to a high local surface noble metal concentration and promotes the in situ formation of small Pd clusters at a rather low noble metal loading (0.5 wt %), which are shown to be the active species for CO conversion at low temperatures. As elucidated by complementary in situ/operando techniques, the spatial separation of CeO2 islands on Al2O3 confines the mobility of Pd, preventing the full redispersion or the formation of larger noble metal particles and maintaining a high CO oxidation activity at low temperatures. In a broader perspective, this approach to more efficiently use the noble metal can be transferred to further systems and reactions in heterogeneous catalysis.

L3-edge X-ray spectroscopy of rhodium and palladium compounds.

L3-edge high-energy-resolution fluorescence-detection X-ray absorption near-edge structure (XANES) spectra for palladium and rhodium compounds are presented, with focus on their electronic structures. The data are compared with transmission XANES spectra recorded at the K-edge. A correlation between the absorption edge energy and the metal ion oxidation state is not observed. Despite the different filling of the 4d orbitals and different local coordination, the Rh and Pd compounds show remarkably similar spectral shapes. Calculation of the density of states and of the L3-XANES data reproduce the experimental results.

Operando Spectroscopy to Understand Dynamic Structural Changes of Solid Catalysts.

Solid materials like heterogeneous catalysts are highly dynamic and continuously tend to change when exposed to the reaction environment. To understand the catalyst system under true reaction conditions,operando spectroscopy is the key to unravel small changes, which can ultimately lead to a significant difference in catalytic activity and selectivity. This was also the topic of the 7th International Congress on Operando Spectroscopy in Switzerland in 2023. In this article, we discuss various examples to introduce and demonstrate the importance of this area, including examples from emission control for clean air (e.g. CO oxidation), oxidation catalysis in the chemical industry (e.g. oxidation of isobutene), future power-to-X processes (electrocatalysis, CO2 hydrogenation to methanol), and non-oxidative conversion of methane. All of these processes are equally relevant to the chemical industry. Complementary operando techniques such as X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and Raman spectroscopy were utilized to derive the ultimate structure of the catalyst. The variety of conditions requires distinctly different operando cells that can reach a temperature range of 400-1000 °C and pressures up to 40 bar. The best compromise for both the spectroscopy and the catalytic reaction is needed. As an outlook, we highlight emerging methods such as modulation-excitation spectroscopy (MES) or quick-extended X-ray absorption fine structure (QEXAFS) and X-ray photon in/out techniques, which can provide better sensitivity or extend X-ray based operando studies.

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.

Fundamental Structural and Electronic Understanding of Palladium Catalysts on Nitride and Oxide Supports.

The nature of the support can fundamentally affect the function of a heterogeneous catalyst. For the novel type of isolated metal atom catalysts, sometimes referred to as single-atom catalysts, systematic correlations are still rare. Here, we report a general finding that Pd on nitride supports (non-metal and metal nitride) features a higher oxidation state compared to that on oxide supports (non-metal and metal oxide). Through thorough oxidation state investigations by X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), CO-DRIFTS, and density functional theory (DFT) coupled with Bader charge analysis, it is found that Pd atoms prefer to interact with surface hydroxyl group to form a Pd(OH)x species on oxide supports, while on nitride supports, Pd atoms incorporate into the surface structure in the form of Pd-N bonds. Moreover, a correlation was built between the formal oxidation state and computational Bader charge, based on the periodic trend in electronegativity.

Aqueous Phase Reforming over Platinum Catalysts on Doped Carbon Supports: Exploring Platinum-Heteroatom Interactions.

A series of platinum catalysts supported on carbon nanofibers with various heteroatom dopings were synthesized to investigate the effect of the local platinum environment on the catalytic activity and selectivity in aqueous phase reforming (APR) of ethylene glycol (EG). Typical carbon dopants such as oxygen, nitrogen, sulfur, phosphorus, and boron were chosen based on their ability to bring acidic or basic functional groups to the carbon surface. In situ X-ray absorption spectroscopy (XAS) was used to identify the platinum oxidation state and platinum species formed during APR of EG through multivariate curve resolution alternating least-squares analysis, observing differences in activity, selectivity, and platinum local environment among the catalysts. The platinum-based catalyst on the nitrogen-doped carbon support demonstrated the most favorable properties for H2 production due to high Pt dispersion and basicity (H2 site time yield 22.7 h-1). Direct Pt-N-O coordination was identified by XAS in this catalyst. The sulfur-doped catalyst presented Pt-S contributions with the lowest EG conversion rate and minimal production of the gas phase components. Boron and phosphorus-doped catalysts showed moderate activity, which was affected by low platinum dispersion on the carbon support. The phosphorus-doped catalyst showed preferential selectivity to alcohols in the liquid phase, associated with the presence of acid sites and Pt-P contributions observed under APR conditions.

Reversible and Irreversible Structural Changes in Cu/ZnO/ZrO2 Catalysts during Methanol Synthesis.

The structure and chemical state of heterogeneous catalysts are closely related to their operational stability. Knowing these relationships as precisely as possible is thus essential for further catalyst development. This work focuses on the deactivation of a Cu/ZnO/ZrO2-type catalyst for methanol synthesis. Experiments were performed in a parallel setup, with which time-dependent changes in the catalyst material can be observed. Elucidation of potential deactivation pathways is described for catalyst aging at different times on stream (0, 50, 935 h). Data from X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, N2 physisorption, and transmission electron microscopy measurements reveal that sintering of Cu0 domains and restructuring within ZnO domains mainly contribute to deactivation. Subsequent reactivation by reduction (in H2/N2) reverts the observed structural changes only to a limited extent. Moreover, this work highlights the participation of ZrO2 as a promoter and reveals redispersion of zirconia after initial reduction.

Highly loaded bimetallic iron-cobalt catalysts for hydrogen release from ammonia.

Ammonia is a storage molecule for hydrogen, which can be released by catalytic decomposition. Inexpensive iron catalysts suffer from a low activity due to a too strong iron-nitrogen binding energy compared to more active metals such as ruthenium. Here, we show that this limitation can be overcome by combining iron with cobalt resulting in a Fe-Co bimetallic catalyst. Theoretical calculations confirm a lower metal-nitrogen binding energy for the bimetallic catalyst resulting in higher activity. Operando spectroscopy reveals that the role of cobalt in the bimetallic catalyst is to suppress the bulk-nitridation of iron and to stabilize this active state. Such catalysts are obtained from Mg(Fe,Co)2O4 spinel pre-catalysts with variable Fe:Co ratios by facile co-precipitation, calcination and reduction. The resulting Fe-Co/MgO catalysts, characterized by an extraordinary high metal loading reaching 74 wt.%, combine the advantages of a ruthenium-like electronic structure with a bulk catalyst-like microstructure typical for base metal catalysts.

Unlocking a Dual-Channel Pathway in CO2 Hydrogenation to Methanol over Single-Site Zirconium on Amorphous Silica.

Converting CO2 into methanol on a large scale is of great significance in the sustainable methanol economy. Zirconia species are considered to be an essential support in Cu-based catalysts due to their excellent properties for CO2 adsorption and activation. However, the evolution of Zr species during the reaction and the effect of their structure on the reaction pathways remain unclear. Herein, single-site Zr species in an amorphous SiO2 matrix are created by enhancing the Zr-Si interaction in Cu/ZrO2 -SiO2 catalysts. In situ X-ray absorption spectroscopy (XAS) reveals that the coordination environment of single-site Zr is sensitive to the atmosphere and reaction conditions. We demonstrate that the CO2 adsorption occurs preferably on the interface of Cu and single-site Zr rather than on ZrO2 nanoparticles. Methanol synthesis in reverse water-gas-shift (RWGS)+CO-hydro pathway is verified only over single-dispersed Zr sites, whereas the ordinary formate pathway occurs on ZrO2 nanoparticles. Thus, it expands a non-competitive parallel pathway as a supplement to the dominant formate pathway, resulting in the enhancement of Cu activity sixfold and twofold based on Cu/SiO2 and Cu/ZrO2 catalysts, respectively. The establishment of this dual-channel pathway by single-site Zr species in this work opens new horizons for understanding the role of atomically dispersed oxides in catalysis science.

Methyl radical chemistry in non-oxidative methane activation over metal single sites.

Molybdenum supported on zeolites has been extensively studied as a catalyst for methane dehydroaromatization. Despite significant progress, the actual intermediates and particularly the first C-C bond formation have not yet been elucidated. Herein we report evolution of methyl radicals during non-oxidative methane activation over molybdenum single sites, which leads selectively to value-added chemicals. Operando X-ray absorption spectroscopy and online synchrotron vacuum ultraviolet photoionization mass spectroscopy in combination with electron microscopy and density functional theory calculations reveal the essential role of molybdenum single sites in the generation of methyl radicals and that the formation rate of methyl radicals is linearly correlated with the number of molybdenum single sites. Methyl radicals transform to ethane in the gas phase, which readily dehydrogenates to ethylene in the absence of zeolites. This is essentially similar to the reaction pathway over the previously reported SiO2 lattice-confined single site iron catalyst. However, the availability of a zeolite, either in a physical mixture or as a support, directs the subsequent reaction pathway towards aromatization within the zeolite confined pores, resulting in benzene as the dominant hydrocarbon product. The findings reveal that methyl radical chemistry could be a general feature for metal single site catalysis regardless of the support (either zeolites MCM-22 and ZSM-5 or SiO2) whereas the reaction over aggregated molybdenum carbide nanoparticles likely facilitates carbon deposition through surface C-C coupling. These findings allow furthering the fundamental insights into non-oxidative methane conversion to value-added chemicals.

Identifying the Structure of Supported Metal Catalysts Using Vibrational Fingerprints from Ab Initio Nanoscale Models.

Identifying active sites of supported noble metal nanocatalysts remains challenging, since their size and shape undergo changes depending on the support, temperature, and gas mixture composition. Herein, the anharmonic infrared spectrum of adsorbed CO is simulated using density functional theory (DFT) to gain insight into the nature of Pd nanoparticles (NPs) supported on ceria. The authors systematically determine how the simulated infrared spectra are affected by CO coverage, NP size (0.5-1.5 nm), NP morphology (octahedral, icosahedral), and metal-support contact angle, by exploring a diversity of realistic models inspired by ab initio molecular dynamics. The simulated spectra are then used as a spectroscopic fingerprint to characterize nanoparticles in a real catalyst, by comparison with in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments. Truncated octahedral NPs with an acute Pd-ceria angle reproduce most of the measurements. In particular, the authors isolate features characteristic of CO adsorbed at the metal-support interface appearing at low frequencies, both seen in simulation and experiment. This work illustrates the strong need for realistic models to provide a robust description of the active sites, especially at the interface of supported metal nanocatalysts, which can be highly dynamic and evolve considerably during reaction.

Soot Formation in Methane Pyrolysis Reactor: Modeling Soot Growth and Particle Characterization.

Methane pyrolysis is a very attractive and climate-friendly process for hydrogen production and the sequestration of carbon as solid material. The formation of soot particles in methane pyrolysis reactors needs to be understood for technology scale-up calling for appropriate soot growth models. A monodisperse model is coupled with a plug flow reactor model and elementary-step reaction mechanisms to numerically simulate processes in methane pyrolysis reactors, namely, the chemical conversion of methane to hydrogen, formation of C-C coupling products and polycyclic aromatic hydrocarbons, and growth of soot particles. The soot growth model accounts for the effective structure of the aggregates by calculating the coagulation frequency from the free-molecular regime to the continuum regime. It predicts the soot mass, particle number, area, and volume concentration, along with the particle size distribution. For comparison, experiments on methane pyrolysis are carried out at different temperatures and collected soot samples are characterized using Raman spectroscopy, transmission electron microscopy (TEM), and dynamic light scattering (DLS).

Probing the Nature of Zinc in Copper-Zinc-Zirconium Catalysts by Operando Spectroscopies for CO2 Hydrogenation to Methanol.

Active Zn species in Cu-based methanol synthesis catalysts have not been clearly identified yet due to their complex nature and dynamic structural changes during reactions. Herein, atomically dispersed Zn on ZrO2 support is established in Cu-based catalysts by separating Zn and Zr components from Cu (Cu-ZnZr) via the double-nozzle flame spray pyrolysis (DFSP) method. It exhibits superiority in methanol selectivity and yield compared to those with Cu-ZnO interface and isolated ZnO nanoparticles. Operando X-ray absorption spectroscopy (XAS) reveals that the atomically dispersed Zn species are induced during the reaction due to the strengthened Zn-Zr interaction. They can suppress formate decomposition to CO and decrease the H2 dissociation energy, shifting the reaction to methanol production. This work enlightens the rational design of unique Zn species by regulating coordination environments and offers a new perspective for exploring complex interactions in multi-component catalysts.

Design of Single-Atom Catalysts and Tracking Their Fate Using Operando and Advanced X-ray Spectroscopic Tools.

The potential of operando X-ray techniques for following the structure, fate, and active site of single-atom catalysts (SACs) is highlighted with emphasis on a synergetic approach of both topics. X-ray absorption spectroscopy (XAS) and related X-ray techniques have become fascinating tools to characterize solids and they can be applied to almost all the transition metals deriving information about the symmetry, oxidation state, local coordination, and many more structural and electronic properties. SACs, a newly coined concept, recently gained much attention in the field of heterogeneous catalysis. In this way, one can achieve a minimum use of the metal, theoretically highest efficiency, and the design of only one active site-so-called single site catalysts. While single sites are not easy to characterize especially under operating conditions, XAS as local probe together with complementary methods (infrared spectroscopy, electron microscopy) is ideal in this research area to prove the structure of these sites and the dynamic changes during reaction. In this review, starting from their fundamentals, various techniques related to conventional XAS and X-ray photon in/out techniques applied to single sites are discussed with detailed mechanistic and in situ/operando studies. We systematically summarize the design strategies of SACs and outline their exploration with XAS supported by density functional theory (DFT) calculations and recent machine learning tools.

In Situ Investigations on Structural Evolutions during the Facile Synthesis of Cubic α-MoC1-x Catalysts.

Cubic α-phase molybdenum carbides (α-MoC1-x) exhibit great potential in hydrogen production at low temperatures due to their excellent activity in water dissociation. However, the design strategies of α-MoC1-x are severely restricted by the harsh synthesis conditions, which involve multistep ammonification and carburization or the utilization of a significant amount of noble metals. Herein, high-purity α-MoC1-x synthesis in a one-step carburization process was achieved with the assistance of a trace amount of Rh (0.02%). The structural evolution of Mo species during phase transition was monitored via qualitative and quantitative analysis by in situ X-ray diffraction (XRD) and in situ X-ray absorption spectroscopy (XAS), respectively. Environmental transmission electron microscopy (ETEM) was used to follow the visual changes. We reveal that the reduction of monoclinic MoO3 to cubic oxygen-deficient Mo oxide (MoOx) at low temperatures owing to the promoted H2 activation on Rh sites is vital to the following carbon atom insertion and transformation to α-MoC1-x, making the carburization follow the topological route. The systematic analysis of the relationship between the reduction behavior and the structural evolution supplies a feasible strategy for the α-MoC1-x synthesis, and in situ characterizations shed light on controlling the phase transformation during carburization.

Substitution of Copper by Magnesium in Malachite: Insights into the Synthesis and Structural Effects.

The phase width of the copper hydroxycarbonate malachite, Cu2CO3(OH)2, upon substitution with magnesium has been studied in detail. In extension of a previous study on amorphous precursors, the introduction of a hydrothermal aging step allowed the retrieval of crystalline hydroxycarbonate samples with up to 37 atom % Mg (metal content) that are suitable candidates as precursors to Cu/MgO catalysts for CO hydrogenation. Simultaneous refinements of X-ray powder diffraction and pair distribution function (PDF) data as well as complementary spectroscopic insight (X-ray absorption and infrared spectroscopy) revealed that samples with up to 18 atom % Mg are phase-pure magnesian malachites but the magnesium content can be increased beyond this threshold when mcguinnessite (CuMgCO3(OH)2) is accepted as a side phase. In a complementary study, a continuous increase of the magnesium fraction was found during aging and the corresponding structural evolution was studied by means of PDF. These findings add significant insight into the aging chemistry of crystalline Cu,Mg hydroxycarbonates. Furthermore, both phase-pure magnesian malachite and mcguinnessite-containing samples with up to 37 atom % Mg have been examined by thermogravimetry, X-ray powder diffraction, and N2 physisorption and were found to be promising candidates for use as precursors for the preparation of Cu/MgO catalysts.

Microkinetic Analysis of the Oxygen Evolution Performance at Different Stages of Iridium Oxide Degradation.

The microkinetics of the electrocatalytic oxygen evolution reaction substantially determines the performance in proton-exchange membrane water electrolysis. State-of-the-art nanoparticulated rutile IrO2 electrocatalysts present an excellent trade-off between activity and stability due to the efficient formation of intermediate surface species. To reveal and analyze the interaction of individual surface processes, a detailed dynamic microkinetic model approach is established and validated using cyclic voltammetry. We show that the interaction of three different processes, which are the adsorption of water, one potential-driven deprotonation step, and the detachment of oxygen, limits the overall reaction turnover. During the reaction, the active IrO2 surface is covered mainly by *O, *OOH, and *OO adsorbed species with a share dependent on the applied potential and of 44, 28, and 20% at an overpotential of 350 mV, respectively. In contrast to state-of-the-art calculations of ideal catalyst surfaces, this novel model-based methodology allows for experimental identification of the microkinetics as well as thermodynamic energy values of real pristine and degraded nanoparticles. We show that the loss in electrocatalytic activity during degradation is correlated to an increase in the activation energy of deprotonation processes, whereas reaction energies were marginally affected. As the effect of electrolyte-related parameters does not cause such a decrease, the model-based analysis demonstrates that material changes trigger the performance loss. These insights into the degradation of IrO2 and its effect on the surface processes provide the basis for a deeper understanding of degrading active sites for the optimization of the oxygen evolution performance.

Challenges of Green Production of 2,5-Furandicarboxylic Acid from Bio-Derived 5-Hydroxymethylfurfural: Overcoming Deactivation by Concomitant Amino Acids.

The oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) is highly attractive as FDCA is considered as substitute for the petrochemically derived terephthalic acid. There are only few reports on the direct use of unrefined HMF solutions from biomass resources and the influence of remaining constituents on the catalytic processes. In this work, the oxidation of HMF in a solution as obtained from hydrolysis and dehydration of saccharides in chicory roots was investigated without intermediate purification steps. The amount of base added to the solution was critical to increase the FDCA yield. Catalyst deactivation occurred and was attributed to poisoning by amino acids from the bio-source. A strong influence of amino acids on the catalytic activity was found for all supported Au, Pt, Pd, and Ru catalysts. A supported AuPd(2 : 1)/C alloy catalyst exhibited both superior catalytic activity and higher stability against deactivation by the critical amino acids.