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.

Data-Centric Heterogeneous Catalysis: Identifying Rules and Materials Genes of Alkane Selective Oxidation.

Artificial intelligence (AI) can accelerate catalyst design by identifying key physicochemical descriptive parameters correlated with the underlying processes triggering, favoring, or hindering the performance. In analogy to genes in biology, these parameters might be called "materials genes" of heterogeneous catalysis. However, widely used AI methods require big data, and only the smallest part of the available data 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, to measure 55 physicochemical parameters as well as the reactivity of 12 catalysts toward ethane, propane, and n-butane oxidation reactions. These materials are based on vanadium or manganese redox-active elements and present diverse phase compositions, crystallinities, and catalytic behaviors. By applying the sure-independence-screening-and-sparsifying-operator symbolic-regression approach to the consistent data set, we identify nonlinear property-function relationships depending on several key parameters and reflecting the intricate interplay of processes that govern the formation of olefins and oxygenates: local transport, site isolation, surface redox activity, adsorption, and the material dynamical restructuring under reaction conditions. These processes are captured by parameters derived from N2 adsorption, X-ray photoelectron spectroscopy (XPS), and near-ambient-pressure in situ XPS. The data-centric approach indicates the most relevant characterization techniques to be used for catalyst design and provides "rules" on how the catalyst properties may be tuned in order to achieve the desired performance.

X-ray Absorption Near-Edge Structure (XANES) at the O K-Edge of Bulk Co3O4: Experimental and Theoretical Studies.

We combine theoretical and experimental X-ray absorption near-edge spectroscopy (XANES) to probe the local environment around cationic sites of bulk spinel cobalt tetraoxide (Co3O4). Specifically, we analyse the oxygen K-edge spectrum. We find an excellent agreement between our calculated spectra based on the density functional theory and the projector augmented wave method, previous calculations as well as with the experiment. The oxygen K-edge spectrum shows a strong pre-edge peak which can be ascribed to dipole transitions from O 1s to O 2p states hybridized with the unoccupied 3d states of cobalt atoms. Also, since Co3O4 contains two types of Co atoms, i.e., Co3+ and Co2+, we find that contribution of Co2+ ions to the pre-edge peak is solely due to single spin-polarized t2g orbitals (dxz, dyz, and dxy) while that of Co3+ ions is due to spin-up and spin-down polarized eg orbitals (dx2-y2 and dz2). Furthermore, we deduce the magnetic moments on the Co3+ and Co2+ to be zero and 3.00 µB respectively. This is consistent with an earlier experimental study which found that the magnetic structure of Co3O4 consists of antiferromagnetically ordered Co2+ spins, each of which is surrounded by four nearest neighbours of oppositely directed spins.

Ultrathin 2D Fe-Nanosheets Stabilized by 2D Mesoporous Silica: Synthesis and Application in Ammonia Synthesis.

Developing high-performance Fe-based ammonia catalysts through simple and cost-efficient methods has received an increased level of attention. Herein, we report for the first time, the synthesis of two-dimensional (2D) FeOOH nanoflakes encapsulated by mesoporous SiO2 (mSiO2) via a simple solution-based method for ammonia synthesis. Due to the sticking of the mSiO2 coating layers and the limited spaces in between, the Fe after reduction retains the 2D morphology, showing high resistance against the sintering in the harsh Haber-Bosch process. Compared to supported Fe particles dispersed on mSiO2 spheres, the coated catalyst shows a significantly improved catalytic activity by 50% at 425 °C. Thermal desorption spectroscopy (TDS) reveals the existence of a higher density of reactive sites for N2 activation in the 2D Fe catalyst, which is possibly coupled to a larger density of surface defect sites (kinks, steps, point defects) that are generally considered as active centers in ammonia synthesis. Besides the structural impact of the coating on the 2D Fe, the electronic one is elucidated by partially substituting Si with Al in the coating, confirmed by 29Si and 27Al magic-angle spinning nuclear magnetic resonance (MAS NMR). An increased apparent activation energy (Ea) of the Al-containing catalyst evidences an influence on the nature of the active site. The herein-developed stable 2D Fe nanostructures can serve as an example of a 2D material applied in catalysis, offering the chance of a rational catalyst design based on a stepwise introduction of various promoters, in the coating and on the metal, maintaining the spatial control of the active centers.

Materials genes of heterogeneous catalysis from clean experiments and artificial intelligence.

ABSTRACT: The performance in heterogeneous catalysis is an example of a complex materials function, governed by an intricate interplay of several processes (e.g., the different surface chemical reactions, and the dynamic restructuring of the catalyst material at reaction conditions). Modeling the full catalytic progression via first-principles statistical mechanics is impractical, if not impossible. Instead, we show here how a tailored artificial-intelligence approach can be applied, even to a small number of materials, to model catalysis and determine the key descriptive parameters ("materials genes") reflecting the processes that trigger, facilitate, or hinder catalyst performance. We start from a consistent experimental set of "clean data," containing nine vanadium-based oxidation catalysts. These materials were synthesized, fully characterized, and tested according to standardized protocols. By applying the symbolic-regression SISSO approach, we identify correlations between the few most relevant materials properties and their reactivity. This approach highlights the underlying physicochemical processes, and accelerates catalyst design. IMPACT STATEMENT: Artificial intelligence (AI) accepts that there are relationships or correlations that cannot be expressed in terms of a closed mathematical form or an easy-to-do numerical simulation. For the function of materials, for example, catalysis, AI may well capture the behavior better than the theory of the past. However, currently the flexibility of AI comes together with a lack of interpretability, and AI can only predict aspects that were included in the training. The approach proposed and demonstrated in this IMPACT article is interpretable. It combines detailed experimental data (called "clean data") and symbolic regression for the identification of the key descriptive parameters (called "materials genes") that are correlated with the materials function. The approach demonstrated here for the catalytic oxidation of propane will accelerate the discovery of improved or novel materials while also enhancing physical understanding. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1557/s43577-021-00165-6.

Transition from 2D to 3D SBA-15 by High-Temperature Fluoride Addition and its Impact on the Surface Reactivity Probed by Isopropanol Conversion.

A systematic variation of the SBA-15 synthesis conditions and their impact on the structural and chemical characteristics are reported. An incremental alteration of the hydrothermal aging temperature and time was used to induce changes of the highly ordered SBA-15 structure. Any effects on the total surface area, mesopores size, micropore contributions, and pore connectivity are amplified by a combined incremental increase of the NH4 F concentration. Based on changes of the unit-cell parameter as a function of the mesopore size, and a feature in the low-angle XRD pattern, useful descriptors for the disorder of the corresponding SBA-15 are identified. An additional analysis of the Brunauer-Emmett-Teller (BET) surface area and pore size distributions enables investigations of the structural integrity of the material. This systematic approach allows the identification of coherencies between the evolution of physical SBA-15 properties. The obtained correlations of the surface and structural characteristics allow the discrimination between highly ordered 2D SBA-15, disordered 3D SBA-15, and highly nonuniform silica fractions with mainly amorphous character. The fluoride-induced disintegration of the silica structure under hydrothermal conditions was also verified by TEM. A direct influence of the structural adaption on the chemical properties of the surface was demonstrated by isopropanol conversion and H/D exchange monitored by FTIR analysis as sensitive probes for acid and redox active surface sites.

Fluctuating Storage of the Active Phase in a Mn-Na2 WO4 /SiO2 Catalyst for the Oxidative Coupling of Methane.

Structural dynamics of a Mn-Na2 WO4 /SiO2 catalyst were detected directly under reaction conditions during the oxidative coupling of methane via in situ XRD and operando Raman spectroscopy. A new concept of fluctuating storage and release of an active phase in heterogeneous catalysis is proposed that involves the transient generation of active sodium oxide species via a reversible reaction of Na2 WO4 with Mn7 SiO12 . The process is enabled by phase transitions and melting at the high reaction temperatures that are typically applied.

F-doping of nanostructured ZnO: a way to modify structural, electronic, and surface properties.

Polycrystalline ZnO is a material often used in heterogeneous catalysis. Its properties can be altered by the addition of dopants. We used gaseous fluorine (F2(g)) as direct way to incorporate fluoride in ZnO as anionic dopants. Here, the consequences of this treatment on the structural and electronic properties, as well as on the acidic/basic sites of the surface, are investigated. It is shown that the amount of F incorporation into the structure can be controlled by the synthesis parameters (t, T, p). While the surface of ZnO was altered as shown by, e.g., IR spectroscopy, XPS, and STEM/EDX measurements, the F2 treatment also influenced the electronic properties (optical band gap, conductivity) of ZnO. Furthermore, the Lewis acidity/basicity of the surface was affected which is evidenced by using, e.g., different probe molecules (CO2, NH3). In situ investigations of the fluorination process offer valuable insights on the fluorination process itself.

How to control selectivity in alkane oxidation?

The well-defined particle morphology of crystalline MnWO4 catalysts investigated in the present study facilitates obtaining insight into the origin of selectivity limitations in alkane oxidation. Hydrothermal synthesis at variable pH values granted access to a series of phase-pure MnWO4 catalysts with particles ranging from cube-like (aspect ratio 1.5) to rod- or needle-like (aspect ratio 6.8) shapes. Kinetic studies reveal a strong dependence of the propane consumption rate on the particle shape. The true origin of the structure sensitivity was unraveled by comprehensive bulk and surface analysis using nitrogen adsorption, XRD, SEM, ADF-STEM, STEM-EELS, XPS, multi-laser excitation Raman and DRIFT/operando FTIR spectroscopies, temperature-programmed oxidation (TPO), in situ NEXAFS, and DFT calculations. The active phase is composed of a thin manganese oxy-hydroxide layer formed on the surface of crystalline MnWO4. The differences in catalytic performance within the series clearly illustrate that the structural motif as the most popular descriptor in oxidation catalysis is not essential, since all MnWO4 catalysts in the series under study exhibit the same bulk crystal structure and bulk chemical composition and are phase pure and homogenous. The variable particle shape serves as a proxy that reflects the formation of varying abundance of redox active Mn2+/Mn3+ surface sites, which correlates with catalytic activity. Operando FTIR spectroscopy directly confirms the formation of Mn-OH surface species by abstraction of hydrogen atoms from the propane molecule on nucleophilic oxygen atoms and suggests that active site regeneration occurs via oxidative dehydrogenation of Mn-OH species indicating a single-site nature of the active sites that does not allow four-electron reduction of molecular oxygen. Instead, intermediates are created that cause side reactions and lower the selectivity. The findings highlight fundamental design criteria that may be applied to advance the development of new alkane oxidation catalysts with improved selectivity.

Ni Single Atom Catalysts for CO2 Activation.

We report on the activation of CO2 on Ni single-atom catalysts. These catalysts were synthesized using a solid solution approach by controlled substitution of 1-10 atom % of Mg2+ by Ni2+ inside the MgO structure. The Ni atoms are preferentially located on the surface of the MgO and, as predicted by hybrid-functional calculations, favor low-coordinated sites. The isolated Ni atoms are active for CO2 conversion through the reverse water-gas shift (rWGS) but are unable to conduct its further hydrogenation to CH4 (or MeOH), for which Ni clusters are needed. The CO formation rates correlate linearly with the concentration of Ni on the surface evidenced by XPS and microcalorimetry. The calculations show that the substitution of Mg atoms by Ni atoms on the surface of the oxide structure reduces the strength of the CO2 binding at low-coordinated sites and also promotes H2 dissociation. Astonishingly, the single-atom catalysts stayed stable over 100 h on stream, after which no clusters or particle formation could be detected. Upon catalysis, a surface carbonate adsorbate-layer was formed, of which the decompositions appear to be directly linked to the aggregation of Ni. This study on atomically dispersed Ni species brings new fundamental understanding of Ni active sites for reactions involving CO2 and clearly evidence the limits of single-atom catalysis for complex reactions.

Investigations of Cu/Zn Oxalates from Aqueous Solution: Single-Phase Precursors and Beyond.

The existence of a limited solid-solution series in the Cu/Zn binary metal oxalate system is reported. Coprecipitation was applied for the preparation of a comprehensive set of mixed Cu/Zn oxalates. Rietveld refinement of the XRD data revealed the formation of mixed-metal oxalate single phases at the compositional peripheries. Accordingly, the isomorphous substitution of ZnII into CuII oxalate takes place at Zn contents of ≤6.6 and ≥79.1 atom %. Zn incorporation leads to a pronounced unit-cell contraction accompanied by Vegard-type trends for the lattice parameters. Morphologically, both solid solutions show close resemblance to the corresponding pure single-metal oxalates, and thus distinct differences are identified (SEM). The successful formation of solid solutions was further evidenced by thermal analysis. The decomposition temperature of the oxalate was taken as an approximation for ZnII incorporation into the CuII oxalate structure. Single decomposition events are observed within the stated compositional boundaries and shift to higher temperature with increasing Zn content, whereas multiple events are present near Cu/Zn parity. Moreover, these findings are supported by IR and Raman spectroscopic investigations. This study on the Cu/Zn mixed-metal oxalate system sheds light on the important prerequisites for solid-solution formation and identifies the structural limitations that predefine its application as catalyst precursor.

Cobalt-Bridged Ionic Liquid Polymer on a Carbon Nanotube for Enhanced Oxygen Evolution Reaction Activity.

By taking inspiration from the catalytic properties of single-site catalysts and the enhancement of performance through ionic liquids on metal catalysts, we exploited a scalable way to place single cobalt ions on a carbon-nanotube surface bridged by polymerized ionic liquid. Single dispersed cobalt ions coordinated by ionic liquid are used as heterogeneous catalysts for the oxygen evolution reaction (OER). Performance data reveals high activity and stable operation without chemical instability.

Bridging the Time Gap: A Copper/Zinc Oxide/Aluminum Oxide Catalyst for Methanol Synthesis Studied under Industrially Relevant Conditions and Time Scales.

Long-term stability of catalysts is an important factor in the chemical industry. This factor is often underestimated in academic testing methods, which may lead to a time gap in the field of catalytic research. The deactivation behavior of an industrially relevant Cu/ZnO/Al2 O3 catalyst for the synthesis of methanol is reported over a period of 148 days time-on-stream (TOS). The process was investigated by a combination of quasi in situ and ex situ analysis techniques. The results show that ZnO is the most dynamic species in the catalyst, whereas only slight changes can be observed in the Cu nanoparticles. Thus, the deactivation of this catalyst is driven by the changes in the ZnO moieties. Our findings indicate that methanol synthesis is an interfacially mediated process between Cu and ZnO.

Hydrothermal synthesis of bi-functional nanostructured manganese tungstate catalysts for selective oxidation.

The mechanism of C-H activation in selective oxidation reactions of short-chain alkane molecules over transition metal oxides is critically affected by the balance of acid-base and redox sites at the surface of the catalyst. Using the example of manganese tungstate we discuss how the relative abundance of these sites can be controlled via synthetic techniques. Phase-pure catalysts composed of the thermodynamic stable monoclinic MnWO4 phase have been prepared using hydrothermal synthesis. Variation of the initial pH value resulted in rod-shaped nano-crystalline MnWO4 catalysts composed of particles with varying aspect ratio. The synthesis products have been analysed using transmission electron microscopy, X-ray diffraction, infrared, and photoelectron spectroscopy. In situ Raman spectroscopy was used to investigate the dissolution-re-crystallization processes occurring under hydrothermal conditions. Ethanol oxidation was applied to probe the surface functionalities in terms of acid-base and redox properties. Changes in the aspect ratio of the primary catalyst particles are reflected in the product distribution induced by altering the fraction of acid-base and redox sites exposed at the surface of the catalysts in agreement with the proposed mechanism of particle growth by re-crystallization during ageing under hydrothermal conditions.

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4 Nanorods.

The electronic and structural properties of vanadium-containing phases govern the formation of isolated active sites at the surface of these catalysts for selective alkane oxidation. This concept is not restricted to vanadium oxide. The deliberate use of hydrothermal techniques can turn the typical combustion catalyst manganese oxide into a selective catalyst for oxidative propane dehydrogenation. Nanostructured, crystalline MnWO4 serves as the support that stabilizes a defect-rich MnOx surface phase. Oxygen defects can be reversibly replenished and depleted at the reaction temperature. Terminating MnOx zigzag chains on the (010) crystal planes are suspected to bear structurally site-isolated oxygen defects that account for the unexpectedly good performance of the catalyst in propane activation.

Direct imaging of octahedral distortion in a complex molybdenum vanadium mixed oxide.

Complex Mo,V-based mixed oxides that crystallize in the orthorhombic M1-type structure are promising candidates for the selective oxidation of small alkanes. The oxygen sublattice of such a complex oxide has been studied by annular bright field scanning transmission electron microscopy. The recorded micrographs directly display the local distortion in the metal oxygen octahedra. From the degree of distortion we are able to draw conclusions on the distribution of oxidation states in the cation columns at different sites. The results are supported by X-ray diffraction and electron paramagnetic resonance measurements that provide integral details about the crystal structure and spin coupling, respectively.

Single crystalline wurtzite ZnO/zinc blende ZnS coaxial heterojunctions and hollow zinc blende ZnS nanotubes: synthesis, structural characterization and optical properties.

Synthesis of ZnO/ZnS heterostructures under thermodynamic conditions generally results in the wurtzite (WZ) structure of the ZnS component because its WZ phase is thermodynamically more stable than its zinc blende (ZB) phase. In this report, we demonstrate for the first time the preparation of ZnO/ZnS coaxial nanocables composed of single crystalline ZB structured ZnS epitaxially grown on WZ ZnO via a two-step thermal evaporation method. The deposition temperature is believed to play a crucial role in determining the crystalline phase of ZnS. Through a systematic structural analysis, the ZnO core and the ZnS shell are found to have an orientation relationship of (0002)ZnO(WZ)//(002)ZnS(ZB) and [01-10]ZnO(WZ)//[2-20]ZnS(ZB). Observation of the coaxial nanocables in cross-section reveals the formation of voids between the ZnO core and the ZnS shell during the coating process, which is probably associated with the nanoscale Kirkendall effect known to result in porosity. Furthermore, by immersing the ZnO/ZnS nanocable heterojunctions in an acetic acid solution to etch away the inner ZnO cores, single crystalline ZnS nanotubes orientated along the [001] direction of the ZB structure were also achieved for the first time. Finally, optical properties of the hollow ZnS tubes were investigated and discussed in detail. We believe that our study could provide some insights into the controlled fabrication of one dimensional (1D) semiconductors with desired morphology, structure and composition at the nanoscale, and the synthesized WZ ZnO/ZB ZnS nanocables as well as ZB ZnS nanotubes could be ideal candidates for the study of optoelectronics based on II-VI semiconductors.

On the structural relations of malachite. II. The brochantite MDO polytypes.

The structural relation between malachite and the brochantite MDO (maximum degree of order) polytypes is discussed. It is demonstrated that the same building blocks which form the basis of brochantite polytypism also occur in malachite. The different arrangements of these building blocks in the two mineral structures are rationalized as a result of the different coordination geometries required by the respective non-metal atoms acting as linkers. The compound stoichiometries are discussed in light of a common structured formula scheme, in which pairs of H atoms can play a similar role as single non-H atoms. An overview on the occurrence of malachite-like building blocks in several other crystal structures is given.