Martin Muhler

Co@Co3O4 Encapsulated in Carbon Nanotube‐Grafted Nitrogen‐Doped Carbon Polyhedra as an Advanced Bifunctional Oxygen Electrode

Efficient reversible oxygen electrodes for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are vitally important for various energy conversion devices, such as regenerative fuel cells and metal-air batteries. However, realization of such electrodes is impeded by insufficient activity and instability of electrocatalysts for both water splitting and oxygen reduction. We report highly active bifunctional electrocatalysts for oxygen electrodes comprising core-shell Co@Co3O4 nanoparticles embedded in CNT-grafted N-doped carbon-polyhedra obtained by the pyrolysis of cobalt metal-organic framework (ZIF-67) in a reductive H2 atmosphere and subsequent controlled oxidative calcination. The catalysts afford 0.85 V reversible overvoltage in 0.1 m KOH, surpassing Pt/C, IrO2 , and RuO2 and thus ranking them among one of the best non-precious-metal electrocatalysts for reversible oxygen electrodes.

MnxOy/NC and CoxOy/NC Nanoparticles Embedded in a Nitrogen-Doped Carbon Matrix for High-Performance Bifunctional Oxygen Electrodes†

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.

The Nature of the Iron Based Catalyst for Dehydrogenation of Ethylbenzene to Styrene : 2: Surface Chemistry of the Active Phase

Abstract The combination of an XPS/UPS surface analysis instrument with a microreactor allowed the investigation of the surface composition of catalysts characterized by varying activities and selectivities. The active surface is a potassium iron oxide with a 1 : I atomic ratio of K : Fe, whereby iron is only in its trivalent state. Conversion of oxidic oxygen to OH groups is detrimental to the activity. No significant amount of promotor additives is present in the active surface. The process of regeneration with steam removes carbonaceous deposits but cannot reoxidize iron from Fe 2+ to Fe 3+ . A constant but small amount of potassium carbonate that cannot be increased by addition of CO 2 to the feed of the working catalyst is present at the surface. Catalysts are precursors, active materials, and irreversibly deactivated samples were studied by SEM and TEM. The surface morphology as well as the microstructure clearly indicates a solid as the active phase. This phase is generated and maintained through solid-state reactions during operation. A potassium-rich liquid film with a thickness exceeding one monolayer can be ruled out for the catalyst performance. Formation of droplets of KOH in certain regions of the catalyst signals bulk structural desintegration of the active material.

On the Role of Metals in Nitrogen-Doped Carbon Electrocatalysts for Oxygen Reduction

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-Nx /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-Nx /C and NC catalysts are discussed to inspire the development of more advanced nonprecious-metal catalysts for the ORR.

Implication of the microstructure of binary Cu/ZnO catalysts for their catalytic activity in methanol synthesis

Binary Cu/ZnO catalysts with varying molar ratios (90/10 through 10/90) were studied under methanol synthesis conditions at 493 K and at atmospheric pressure. The methanol synthesis activity of the catalysts was correlated to their specific Cu surface area (N2O reactive frontal chromatography, N2O RFC) after reduction in 2 vol% H2 at 513 K. Activity data were supplemented with a detailed analysis of the microstructure, i.e., crystallite size and strain of the reduced Cu and the ZnO phases after reduction using X-ray diffraction line profile analysis. The estimated copper surface area based on a spherical shape of the copper crystallites is in good agreement with data determined by N2O RFC. A positive correlation of the turnover frequency for methanol production with the observed microstrain of copper in the Cu/ZnO system was found. The results indicate a mutual structural interaction of both components (copper and zinc oxide) in the sense that strained copper particles are stabilized by the unstrained state of the zinc oxide microcrystallites. The observed structural deformation of ZnO in samples with higher Cu loading can originate, for instance, from epitaxial bonding of the oxide lattice to the copper metal, insufficient reduction or residual carbonate due to incomplete thermal decomposition during reduction. Additional EXAFS measurements at the Cu K and the Zn K edge show that about 5% ZnO are dissolved in the CuO matrix of the calcined precursors. Furthermore, it is shown that the microstructural changes (e.g., size and strain) of copper can be traced back to the phase composition of the corresponding hydroxycarbonate precursors.

Spinel Mn–Co Oxide in N-Doped Carbon Nanotubes as a Bifunctional Electrocatalyst Synthesized by Oxidative Cutting

The notorious instability of non-precious-metal catalysts for oxygen reduction and evolution is by far the single unresolved impediment for their practical applications. We have designed highly stable and active bifunctional catalysts for reversible oxygen electrodes by oxidative thermal scission, where we concurrently rupture nitrogen-doped carbon nanotubes and oxidize Co and Mn nanoparticles buried inside them to form spinel Mn-Co oxide nanoparticles partially embedded in the nanotubes. Impressively high dual activity for oxygen reduction and evolution is achieved using these catalysts, surpassing those of Pt/C, RuO2, and IrO2 and thus raising the prospect of functional low-cost, non-precious-metal bifunctional catalysts in metal-air batteries and reversible fuel cells, among others, for a sustainable and green energy future.

Ruthenium catalysts for ammonia synthesis at high pressures: Preparation, characterization, and power-law kinetics

Abstract Supported Ru catalysts for NH3 synthesis were prepared from Ru3(CO)12 and high-purity MgO and Al2O3. In addition to aqueous impregnation with alkali nitrates, two non-aqueous methods based on alkali carbonates were used to achieve alkali promotion resulting in long-term and high-temperature stable catalysts. For the reliable determination of the Ru particle size, the combined application of H2 chemisorption, TEM and XRD was found to be necessary. The power-law rate expressions were derived at atmospheric pressure and at 20 bar which were shown to be efficient tools to investigate the degree of interaction of the alkali promoter with the Ru metal particles. The following sequence with respect to the turnover frequency (TOF) of NH3 formation was found: Cs 2 CO 3 Ru MgO > CsNO 3 Ru MgO > Ru MgO > Ru KAl 2 O 3 > Ru Al 2 O 3 . The Cs-promoted Ru MgO catalysts turned out to be more active than a multiply-promoted Fe catalyst at atmospheric pressure with an initial TOF of about 10−2 s−1 for the non-aqueously prepared Cs 2 CO 3 Ru MgO catalyst at 588 K. The strong inhibition by H2 was found to require a lower molar H2:N2 ratio in the feed gas than 3:1 in order to achieve a high effluent NH3 mole fraction. The optimum ratio for Cs 2 CO 3 Ru MgO at 50 bar was determined to be about 3:2, resulting in an effluent NH3 mole fraction which was just a few percent lower than that of a multiply-promoted Fe catalyst operated at 107 bar and at roughly the same temperature and space velocity. Thus, alkali-promoted Ru catalysts are an alternative to the conventionally used Fe catalysts for NH3 synthesis also at high pressure.

On the nature of the active state of silver during catalytic oxidation of methanol

Under the applied high reaction temperatures (∼900 K) the Ag surface is restructured and a tightly held oxygen species is formed on the surface (Oγ) apart from O atoms dissolved in the bulk (Oβ). Methanol oxidation to formaldehyde proceeds through this Oγ species as demonstrated by application of a variety of spectroscopic techniques.

The nature of the iron oxide-based catalyst for dehydrogenation of ethylbenzene to styrene. I, Solid-state chemistry and bulk characterization

Abstract The active catalyst for the dehydrogenation of ethylbenzene is generated from a precursor material consisting of hematite and potassium hydroxide (with additional promotors) during the initial phase of catalyst operation at 873 K in a steam atmosphere. The active phase is a thin layer of KFeO 2 supported on a solid solution of K 2 Fe 22 O 34 in Fe 3 O 4 . The ternary K 2 Fe 22 O 34 phase acts as storage medium from which the active surface is continuously supplied with a near-monolayer coverage of potassium ions in an environment of Fe 3+ ions. The catalyst undergoes a continuous solid-state transformation caused by the migration of potassium ions. This requires a certain degree of imperfection in the matrix lattice which originates from the catalyst preparation and from the addition of promotors which act on the iron oxide lattice rather than on the surface chemistry. The identity of the active phase with KFeO 2 was confirmed by independent synthesis of this phase and comparison of its catalytic activity with that of the technical catalyst.

Deactivation of Supported Copper Catalysts for Methanol Synthesis

Binary Cu/ZnO and Cu/Al2O3 as well as ternary Cu/ZnO/Al2O3 catalysts were investigated with respect to their catalytic activity and stability in methanol synthesis. In a rapid aging test, activity measurements were carried out in combination with the determination of the specific Cu surface area. A close correlation between the loss of catalytic activity and the decrease in specific Cu surface area was found due to sintering of the Cu particles. Differences in the deactivation behavior and the area-activity relationship of each catalyst system imply that the catalysts should be grouped in different classes.