Metal–organic frameworks (MOFs) feature a great possibility for a broad spectrum of applications. Hollow MOF structures with tunable porosity and multifunctionality at the nanoscale with beneficial properties are desired as hosts for catalytically active species. Herein, we demonstrate the formation of well-defined hollow Zn/Co-based zeolitic imidazolate frameworks (ZIFs) by use of epitaxial growth of Zn-MOF (ZIF-8) on preformed Co-MOF (ZIF-67) nanocrystals that involve in situ self-sacrifice/excavation of the Co-MOF. Moreover, any type of metal nanoparticles can be accommodated in Zn/Co-ZIF shells to generate yolk–shell metal@ZIF structures. Transmission electron microscopy and tomography studies revealed the inclusion of these nanoparticles within hollow Zn/Co-ZIF with dominance of the Zn-MOF as shell. Our findings lead to a generalization of such hollow systems that are working effectively to other types of ZIFs.

Pd nanoparticles supported on carbon nanotubes were applied in the selective oxidation of ethanol in the liquid phase. The characterization of the surface and bulk properties combined with the catalytic tests indicated the dissolution and redeposition of Pd under the reaction conditions. A dynamic interplay within the Pd life cycle was identified to be responsible for the overall reactivity. Nitrogen-doped carbon nanotubes were found to act as an excellent support for the Pd catalyst system by efficiently stabilizing and recapturing the Pd species, which resulted in high activity and selectivity to acetic acid.

The notion of metal-free catalysts is used to refer to carbon materials modified with nonmetallic elements. However, some claimed metal-free catalysts are prepared using metal-containing precursors. It is highly contested that metal residues in nitrogen-doped carbon (NC) catalysts play a crucial role in the oxygen reduction reaction (ORR). In an attempt to reconcile divergent views, a definition for truly metal-free catalysts is proposed and the differences between NC and M-N_x/C catalysts are discussed. Metal impurities at levels usually undetectable by techniques such as XPS, XRD, and EDX significantly promote the ORR. Poisoning tests to mask the metal ions reveal the involvement of metal residues as active sites or as modifiers of the electronic structure of the active sites in NC. The unique merits of both M-N_x/C and NC catalysts are discussed to inspire the development of more advanced nonprecious-metal catalysts for the ORR.

Der Begriff des metallfreien Katalysators bezeichnet Kohlenstoffmaterialien, die mit nichtmetallischen Elementen modifiziert sind. Manche Katalysatoren, die als metallfrei gelten, werden jedoch unter Verwendung von metallhaltigen Vorstufen hergestellt. Spuren von Metallen in stickstoffdotiertem Kohlenstoff (“nitrogen-doped carbon”, NC) spielen eine umstrittene Rolle bei der Sauerstoffreduktionsreaktion (ORR). Es wird eine Definition für tatsächlich metallfreie Katalysatoren vorgeschlagen, und der Unterschied zwischen stickstoffdotiertem Kohlenstoff und M-N_x/C wird herausgearbeitet. Metallverunreinigungen, die typischerweise unterhalb der Nachweisgrenze von XPS, XRD und EDX liegen, beschleunigen deutlich die ORR. Durch Vergiftungstests, bei denen die Metallionen maskiert werden, wird deutlich, dass Metallreste die elektronische Struktur des NC verändern können oder selbst als aktive Zentren agieren. Die Vorzüge von sowohl M-N_x/C- als auch NC-Systemen werden diskutiert, um die Weiterentwicklung von Nicht-Edelmetall-Katalysatoren für die ORR voranzutreiben.

A multi-functional flow set-up was developed for the rate- and temperature-controlled reduction of copper catalysts, their application in high-pressure methanol synthesis and the determination of the copper surface area by N₂O frontal chromatography. The influence of constant-rate reduction on the catalytic properties of a ternary Cu/ZnO/Al₂O₃ catalyst was investigated. The temperature during the constant-rate reduction was found to decrease, indicating autocatalytic kinetics, but no significant catalytic effect of the milder reduction conditions was observed compared with a slow linear heating ramp.

Reversible interconversion of water into H₂ and O₂, and the recombination of H₂ and O₂ to H₂O thereby harnessing the energy of the reaction provides a completely green cycle for sustainable energy conversion and storage. The realization of this goal is however hampered by the lack of efficient catalysts for water splitting and oxygen reduction. We report exceptionally active bifunctional catalysts for oxygen electrodes comprising Mn₃O₄ and Co₃O₄ nanoparticles embedded in nitrogen-doped carbon, obtained by selective pyrolysis and subsequent mild calcination of manganese and cobalt N₄ macrocyclic complexes. Intimate interaction was observed between the metals and nitrogen suggesting residual M–N_x coordination in the catalysts. The catalysts afford remarkably lower reversible overpotentials in KOH (0.1 M) than those for RuO₂, IrO₂, Pt, NiO, Mn₃O₄, and Co₃O₄, thus placing them among the best non-precious-metal catalysts for reversible oxygen electrodes reported to date.

Die reversible Umwandlung von Wasser in H₂ und O₂ sowie die Rekombination von H₂ und O₂ zu H₂O unter Gewinnung von Energie ist die Basis für einen vollständig nachhaltigen Zyklus der Energiekonversion und -speicherung. Die Realisierung dieses Ziels wird jedoch von der Nichtverfügbarkeit effizienter Katalysatoren für die Wasserspaltung und Sauerstoffreduktion behindert. Wir berichten über hochaktive bifunktionale Katalysatoren für Sauerstoffelektroden, die aus Mn₃O₄- und Co₃O₄-Nanopartikeln, eingeschlossen in N-dotierten Kohlenstoff, bestehen und durch selektive Pyrolyse und nachfolgende milde Kalzinierung von Mn- oder Co-N₄-macrocyclischen Komplexen erhalten wurden. Eine sehr starke Wechselwirkung zwischen den Metallzentren und Stickstoff-haltigen Resten wurde beobachtet, was auf eine M-N_x-Koordination schließen lässt. Die Katalysatoren zeigen eine deutlich verringerte reversible Überspannung in KOH (0.1 M) gegenüber der von RuO₂, IrO₂, Pt, NiO, Mn₃O₄ und Co₃O₄. Damit gehören sie zu den besten Nichtedelmetall-Katalysatoren für reversible Sauerstoffelektroden.

The catalytic performance of a Ni/MgAlO_x catalyst was investigated in the high temperature CO₂ reforming of CH₄. The catalyst was developed using a Ni, Mg, Al hydrotalcite-like precursor obtained by co-precipitation. Despite the high Ni loading of 55 wt%, the synthesized Ni/MgAlO_x catalyst possessed a thermally stable microstructure up to 900 °C with Ni nanoparticles of 9 nm. This stability is attributed to the embedding nature of the oxide matrix, and allows increasing the reaction temperature without losing active Ni surface area. To evaluate the effect of the reaction temperature on the reforming performance and the coking behavior, two different reaction temperatures (800 and 900 °C) were investigated. At both temperatures the prepared catalyst showed high rates of CH₄ consumption. The higher temperature promotes the stability of the catalyst performance due to mitigation of the carbon formation.

High-temperature Fischer–Tropsch synthesis for the production of short-chain olefins over iron catalysts supported on multiwalled carbon nanotubes (CNTs) was investigated under industrially relevant conditions (340 °C, 25 bar, H₂/CO=1) to elucidate the influence of nitrogen and oxygen functionalization of the CNTs on the activity, selectivity, and long-term stability. Surface functionalization of the CNTs was achieved by means of a gas-phase treatment using nitric acid vapor at 200 °C for oxygen functionalization (O-CNTs) and ammonia at 400 °C for the subsequent nitrogen doping (N-CNTs). Ammonium iron citrate impregnation followed by calcination was applied for the deposition of iron nanoparticles with particle sizes below 9 nm. Subsequent to reduction in pure H₂ at 380 °C, the Fe/N-CNT and Fe/O-CNT catalysts were applied in Fischer–Tropsch synthesis, in which they showed comparable initial conversion values with an excellent olefin selectivity [S(C₃–C₆)>85 %] and low chain growth probability (α≤0.5). TEM analysis of the used catalysts detected particle sizes of 23 and 26 nm on O-CNTs and N-CNTs, respectively, and Fe₅C₂ was identified as the major phase by using XRD, with only traces of Fe₃O₄. After 50 h time on stream under steady-state conditions, an almost twofold higher activity compared to the Fe/O-CNT catalysts had been maintained by the Fe/N-CNT catalysts, which are considered excellent Fischer–Tropsch catalysts for the production of short-chain olefins owing to their high activity, high selectivity to olefins, low chain growth probability, and superior long-term stability.

A high-yielding exfoliation of graphene at high concentrations in aqueous solutions is critical for both fundamental study and future applications. Herein, we demonstrate the formation of stable aqueous dispersions of pristine graphene by using the surfactant sodium taurodeoxycholate under tip sonication at concentrations of up to 7.1 mg mL⁻¹. TEM showed that about 8 % of the graphene flakes consisted of monolayers and 82 % of the flakes consisted of less than five layers. The dispersions were stable regardless of freezing (−20 °C) or heat treatment (80 °C) for 24 h. The concentration could be significantly improved to about 12 mg mL⁻¹ by vacuum-evaporation of the dispersions at ambient temperature. The as-prepared graphene dispersions were readily cast into conductive films and were also processed to prepare Pt/graphene nanocomposites that were used as highly active electrocatalysts for the oxygen-reduction reaction.

The deposition of hydrogen evolution sites on photocatalysts is a crucial step in the multistep process of synthesizing a catalyst that is active for overall photocatalytic water splitting. An alternative approach to conventional photodeposition was developed, applying the photocatalytic reforming of aqueous methanol solutions to deposit metal particles on semiconductor materials such as Ga₂O₃ and (Ga_0.6Zn_0.4)(N_0.6O_0.4). The method allows optimizing the loading of the co-catalysts based on the stepwise addition of their precursors and the continuous online monitoring of the evolved hydrogen. Moreover, a synergetic effect between different co-catalysts can be directly established.

MoO₃/γ-Al₂O₃ composites are synthesized by CVD under atmospheric pressure using Mo(CO)₆ as the precursor and porous γ-Al₂O₃ particles in a horizontal, rotating, hot-wall reactor, which is also used for calcination in air. The composites are characterized by N₂ physisorption, atomic absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), and laser Raman spectroscopy (LRS). The synthesized samples exhibit excellent porosity, even at high Mo loadings. A much higher Mo yield is achieved when applying sublimation-adsorption in static air instead of using flowing N₂. A high degree of Mo dispersion on alumina is confirmed by XRD, LRS, and TEM; with a Mo surface density as high as 5.2 atoms nm⁻², the sample is X-ray amorphous, there are no polymeric molybdate species detectable by LRS, and the island size of the molybdate species is about 1 nm according to TEM. The XPS analysis shows that exclusively Mo^VI species are present on all synthesized samples. Thus, the applied rotating, hot-wall reactor achieves efficient mixing and homogeneous deposition.

Zyklische temperaturprogrammierte Reduktions- und Reoxidationsexperimente können dem Mars-van-Krevelen-Oxidationsmechanismus entsprechen, sofern als Reduktionsmittel ein Kohlenwasserstoff eingesetzt wird. Damit ermöglichen diese Redoxzyklen die schnelle Analyse der Aktivität und Selektivität von Multikomponenten-Mischoxid-Katalysatoren für die selektive Oxidation von Kohlenwasserstoffen wie Propen oder Buten. Mittels dieser Technik wurde quantitativ zwischen redoxaktiven Bismutmolybdaten und inaktiven Bismutwolframaten (Bi₂M_xO_3x+3, M = Mo, W) unterschieden, die als Komponenten von technischen Katalysatoren für die Selektivoxidation von Propen zu Acrolein fungieren. Die Trägerphase Fe₃Co₇Mo₁₂O₄₆ wies aufgrund ihrer relativ hohen Oberfläche nur eine geringe spezifische Reduzierbarkeit auf.

Cyclic temperature-programmed reduction (TPR) and reoxidation (TPO) experiments can mimic the redox mechanism suggested by Mars and van Krevelen when using a hydrocarbon as reductant. The redox cycles enable fast probing of the activity and selectivity of multi-component oxide catalysts applied in the selective oxidation of short hydrocarbons. The technique was applied to quantitatively assess the redox properties of active bismuth molybdates and inactive bismuth tungstates (Bi₂M_xO_3x+3, M = Mo, W), which are components of industrial acrolein synthesis catalysts. The specific reducibility of the supporting complex oxide Fe₃Co₇Mo₁₂O₄₆ is found to be low due to its comparably high surface area.

A novel continuous method for the preparation of a ternary Cu/ZnO/Al₂O₃ catalyst based on a cascade of micromixers and a tubular aging reactor is presented as a promising alternative route to the conventional batch process. Its application, in combination with immediate spray drying, enables monitoring of the formation of the final precursor by exchange reactions between initially separated phases during the aging step. These exchange reactions were successfully simulated by consecutive precipitation by using micromixers in series as analytical tool. After 60 min of continuous aging, calcination, and reduction, a catalyst is produced that exhibits an almost equal mass-related activity in methanol synthesis compared to a commercial catalyst and an area-related activity that is about 50 % higher.

Highly loaded copper catalysts supported on alumina are synthesized applying the cyclic two-step CVD of the precursor copper(II)diethylamino-2-propoxide in a fluidized-bed reactor. Copper/zinc oxide/alumina composites are synthesized by either the CVD of the precursor bis[bis(trimethylsilyl)amido]zinc on Cu/Al₂O₃, or the CVD of the Cu precursor on Zn-pretreated alumina, impregnating with diethyl zinc in addition. The composites are extensively characterized by atomic absorption spectroscopy (AAS), elemental analysis (EA), mass spectrometry (MS), N₂ physisorption, N₂O reactive frontal chromatography (RFC), and X-ray diffraction (XRD). The Cu and ZnO nanoparticles originating from the efficient two-step procedure, consisting of adsorption and subsequent decomposition of the adsorbed species in two separated steps, are highly dispersed, X-ray amorphous, and, in the case of the Cu-containing catalysts, have high specific Cu surface areas. The catalytic activities are determined both in methanol synthesis, to judge the contact between the deposited Cu and ZnO nanoparticles, and in the steam reforming of methanol (SRM) to probe the stability of the Cu particles. The turn-over frequencies (TOF) in methanol synthesis of these Cu/ZnO/Al₂O₃ catalysts are higher than that of a commercial ternary catalyst. The varied sequence of the CVD of Cu and ZnO on alumina leads to catalysts with similar activities in the case of similar specific Cu areas.

The adsorption of methanol on pure ZnO and Au-decorated ZnO nanoparticles and its thermal decomposition monitored by temperature-programmed desorption (TPD) experiments and by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), both applied under continuous flow conditions in fixed bed reactors, is reported. Two distinguishable methoxy species are formed during methanol adsorption on ZnO differing in the CO stretching bands. During the subsequent TPD experiments two different H₂ peaks are observed, indicating the conversion of methoxy into formate species. By applying different heating rates, activation energies of 109 kJ mol⁻¹ and 127 kJmol⁻¹ for the selective oxidation of the two methoxy species are derived. Correspondingly, the methoxy decomposition results in two distinguishable formate species, which are identified by the asymmetric and symmetric OCO stretching bands on pure ZnO and Au/ZnO. Based on the decreased intensities of the OH bands during methanol adsorption, which are specific for the various ZnO single crystal surfaces, on the different reactivities of these surfaces, and on the formate FTIR bands observed on ZnO single crystal surfaces, the two methoxy and the corresponding formate species are identified to be adsorbed on the exposed less reactive non-polar ZnO() surface and on the highly reactive polar ZnO() surface. The simultaneous formation of H₂, CO, and CO₂ at about 550–600 K during the TPD experiments indicate the decomposition of adsorbed formate species. The CO/CO₂ ratio decreases with increasing Au loading, and a broad band due to electronic transitions from donor sites to the conduction band is observed in the DRIFT spectra for the Au-decorated ZnO nanoparticles. Thus, the presence of the Au nanoparticles results in an enhanced reducibility of ZnO facilitating the generation of oxygen vacancies.

Metal stearate-stabilized Cu nanoparticles, synthesized by an efficient one-step process, were applied in the continuous liquid-phase synthesis of methanol. After optimizing the reduction procedure, twofold higher rates of methanol formation were found for Cu–Zn colloids, compared to the conventional ternary Cu/ZnO/Al₂O₃ catalyst applied as fine powder in the liquid phase. Structural changes were investigated as a function of time on stream; after reduction in H₂, spherical, well-separated 5–10 nm Cu particles stabilized by a Zn stearate shell were found. Under catalytic high-pressure conditions Zn stearate was hydrolyzed forming ZnO. High-resolution transmission electron microscopy revealed the presence of triangular ZnO prisms with truncated edges. Applying optimized synthesis conditions these triangularly shaped ZnO particles were found to be mostly attached to the spherical Cu particles. The catalytic results and the structural and spectroscopic characterization suggest that these ZnO particles act as a reservoir, releasing ZnO_x species, which diffuse onto the Cu particles and promote the catalytic activity.

High surface area ZnO nanoparticles are synthesized by applying a novel continuous precipitation method using a micromixer coupled directly to a bench-top spray dryer. The polycrystalline material is obtained by fast turbulent precipitation from aqueous zinc nitrate solutions with either sodium or potassium carbonate followed by immediate quenching of the aging due to the rapid water removal. Specific surface areas up to 98 m² g⁻¹ are obtained, depending on the precipitant and the sequence of unit operations applied after precipitation.