Manfred Erwin Schuster

Dissolved Carbon Controls the Initial Stages of Nanocarbon Growth

Carbon is a versatile material that, depending on its hybrid-ization and assembly in one-, two-, or three-dimensionalnetworks, exhibits important electronic and chemical proper-ties with countless practical applications. For example, it isfound in printer inks, pencils, water purification systems,thermal isolation, and antistatic materials.

Soot structure and reactivity analysis by Raman microspectroscopy, temperature-programmed oxidation, and high-resolution transmission electron microscopy.

Raman microspectroscopy (RM), temperature-programmed oxidation (TPO), high-resolution transmission electron microscopy (HRTEM), and electron energy loss spectroscopy (EELS) were combined to get comprehensive information on the relationship between structure and reactivity of soot in samples of spark discharge (GfG), heavy duty engine diesel (EURO VI and IV) soot, and graphite powder upon oxidation by oxygen at increasing temperatures. GfG soot and graphite powder represent the higher and lower reactivity limits. Raman microspectroscopic analysis was conducted by determination of spectral parameters using a five band fitting procedure (G, D1-D4) as well as by evaluation of the dispersive character of the D mode. The analysis of spectral parameters shows a higher degree of disorder and a higher amount of molecular carbon for untreated GfG soot samples than for samples of untreated EURO VI and EURO IV soot. The structural analysis based on the dispersive character of the D mode revealed substantial differences in ordering descending from graphite powder, EURO IV, VI to GfG soot. HRTEM images and EELS analysis of EURO IV and VI samples indicated a different morphology and a higher structural order as compared to GfG soot in full agreement with the Raman analysis. These findings are also confirmed by the reactivity of soot during oxidation (TPO), where GfG soot was found to be the most reactive and EURO IV and VI soot samples exhibited a moderate reactivity.

The Haber–Bosch Process Revisited: On the Real Structure and Stability of “Ammonia Iron” under Working Conditions†

Ammonia synthesis is one of the largest processes in chemical industries. It was first operated at BASF one hundred years ago based on the fundamental work of Fritz Haber and process engineering by Carl Bosch. Haber combined feed gas recycling with application of high pressure (P = 200 bar) and a Ruthenium catalyst to achieve sufficiently high conversions of nitrogen according to N2 + 3 H2 .2 NH3. This success enabled the large scale production of artificial fertilizers, which was a prerequisite to face the world’s increase in population and is known as the “extraction of air from bread” – a term that was coined later by Max von Laue. Today, contrary to the generation of syngas for ammonia, only little has changed in the industrial process for the actual synthesis of ammonia.The process is operated at typical temperatures of 500 °C and pressures around 200 bar, resulting in ammonia concentrations in the exhaust gas of up to 17 vol.%. Approximately 80% of the worldwide ammonia output of 136 Mtons (2011) is used for the production of fertilizers. A key development for the modern Haber-Bosch process, however, has been the catalyst development at BASF that was led by Alwin Mittasch in the early 20 century. After testing 22 000 different formulations in a gigantic effort, the work was concluded in 1922 with the identification of a very unique catalyst synthesis. To achieve a highly active iron catalyst, magnetite, Fe3O4, was promoted by fusing it together with irreducible oxides (K2O, Al2O3, later also CaO) in an oxide melt at temperatures around 1000 °C. The fused magnetite is mechanically granulated and its reduction need to be conducted with great care in the syngas feed to finally give the active α-Fe catalyst. This special synthesis leads to certain crucial properties of the resulting α-Fe phase, which is commonly termed “ammonia iron”. In addition to its outstanding economic relevance, ammonia synthesis acts as a “drosophila reaction” for catalysis research and has always been a test case for the maturity of catalysis science in the context of a technologically mature application. Today, due to the enormous efforts in surface science, physical and theoretical chemistry, and chemical engineering a consistent picture of the reaction mechanism and the role of the Fe catalyst and its promoters has emerged. Key contributions to the modern understanding of the ammonia synthesis reactions came from the teams lead by Gerhard Ertl, Michel Boudart, Gabor Somorjai, Haldor Topsoe and Jens K. Norskov, just to mention a few. However, even after 100 years of application and research there still is scientific interest in the Haber-Bosch process, mainly because of two aspects. Firstly, catalysts with improved lowtemperature activity, higher specific surface area and higher tolerance against poisons and on-off operations are generally desirable. Also the development of a more elegant synthesis route for the Fe-based catalyst without the melting step and the extremely critical activation procedure could foster the potential application of ammonia as an energy storage molecule. Secondly, there still is a gap between the model studies conducted with well-defined simplified materials with clean surfaces at low pressures to elaborate the current knowledge of ammonia synthesis and the industrial process. These so-called pressure and materials gaps often prevent straightforward extrapolation of model studies to real industrial processes. Thus, the question of a dynamical change of the catalyst under true reaction conditions remains to be studied and calls for in situ experimentation. This point requires special attention in case of the ammonia synthesis over iron catalysts, because it is well known and has been studied for decades in the context of steel hardening and catalytic ammonia decomposition that iron can be easily nitrided by ammonia. Ertl and co-workers described the reaction mechanism of ammonia synthesis. 14] He and other authors showed that the reaction is structure sensitive. The dissociative chemisorption of di-nitrogen on the iron surface is the rate limiting step in ammonia synthesis and opens possibilities for sub-surface diffusion of the atomic nitrogen. Ertl et al. proposed the surface dissolution of nitrogen into iron forming a surface nitride of the approximate composition Fe2N and the presence of in-situ formed metastable γFe4N. [6a] Thus, for experimental conditions remote from the HaberBosch process, participation of stoichiometric bulk nitrides like FeN has been excluded. Instead, Herzog et al. proposed formation of [∗] Timur Kandemir, Dr. Manfred.E. Schuster, Dr. Malte Behrens, Prof. Dr. Robert Schlogl Department of Inorganic Chemistry Fritz-Haber-Institut der Max-Planck-Gesellschaft Faradayweg 4-6, D-14195 Berlin, Germany Fax: (+)49-(0)30-8413-4401 E-mail: behrens@fhi-berlin.mpg.de, acsek@fhi-berlin.mpg.de

In Situ Study of the Gas-Phase Electrolysis of Water on Platinum by NAP-XPS

Chasing down the active state: Near-ambient-pressure X-ray photoelectron spectroscopy was used to study the surface of a Pt electrode during the oxygen evolution reaction (OER). A hydrated Pt metal phase with dissolved oxygen in the near-surface region is OER-active and considered to be the precursor of the analytically detected PtO2 , which is in fact the deactivation product of the electrode.

Dynamics of Palladium on Nanocarbon in the Direct Synthesis of H2O2

This work aims to clarify the nanostructural transformation accompanying the loss of activity and selectivity for the hydrogen peroxide synthesis of palladium and gold-palladium nanoparticles supported on N-functionalized carbon nanotubes. High-resolution X-ray photoemission spectroscopy (XPS) allows the discrimination of metallic palladium, electronically modified metallic palladium hosting impurities, and cationic palladium. This is paralleled by the morphological heterogeneity observed by high-resolution TEM, in which nanoparticles with an average size of 2 nm coexisted with very small palladium clusters. The morphological distribution of palladium is modified after reaction through sintering and dissolution/redeposition pathways. The loss of selectivity is correlated to the extent to which these processes occur as a result of the instability of the particle at the carbon surface. We assign beneficial activity in the selective hydrogenation of oxygen to palladium clusters with a modified electronic structure compared with palladium metal or palladium oxides. These beneficial species are formed and stabilized on carbons modified with nitrogen atoms in substitutional positions. The formation of larger metallic palladium particles not only reduces the number of active sites for the synthesis, but also enhances the activity for deep hydrogenation to water. The structural instability of the active species is thus detrimental in a dual way. Minimizing the chance of sintering of palladium clusters by all means is thus the key to better performing catalysts.

A Green Chemistry of Graphene: Photochemical Reduction towards Monolayer Graphene Sheets and the Role of Water Adlayers

Clean sheets: Stable aqueous dispersions of graphene sheets (GSs) are obtained by exposing graphene oxide to irradiation with light at room temperature, without using any chemical additives. The photochemical reduction method is sustainable and scalable, repairs a majority of defects in the graphene layers, and can be used to fine-tune surface functional groups. Interestingly, the aqueous GS dispersions are stable without any added surfactant. The existence of a water layer that is strongly bound to GS is evidenced.

Morphology and Microstructure of Li/MgO Catalysts for the Oxidative Coupling of Methane

A series of catalysts for the oxidative coupling of methane (OCM) based on MgO with a varying content of Li have been synthesized by the gel‐combustion method. The resulting catalytically active systems are studied by a combination of TEM and SEM methods. Samples with a low abundance of Li exhibit a hierarchical pore system built from tubular structures made from primary MgO particles. Upon calcination at 1073 K, these particles undergo a change in shape from cubic via truncated octahedral to platelet morphologies, depending on the Li content of the precursor. Morphological indications have been found for the role of Li as flux in this transformation. The modification of the primary particle morphology leads to a drastic change in secondary structure from open sponges to compact sintered plates upon addition of Li at loadings above 10 wt %, with respect to the precursor. The microstructure of the primary particles reveals two families of high‐energy structures, namely edge‐and‐step structures and protrusions on flat terraces. A relation was found between catalytic function in OCM and the transformation from cubic to complex‐ terminated particles. Based on these findings, it is suggested that sites active for the coupling reaction of methane are related to the protrusions arising from segregation of oxygen vacancies to the surface of MgO.

Comprehensive evaluation of in vitro toxicity of three large-scale produced carbon nanotubes on human Jurkat T cells and a comparison to crocidolite asbestos

Abstract This study has evaluated the effects of three industrially relevant multi-walled carbon nanotubes (MWNTs) on human Jurkat T cells and compared them to those of crocidolite asbestos. No overt acute toxicity was observed for all MWNTs tested although signs of oxidative stress were evident. MWNTs did not activate resting Jurkat cells and only slightly stimulated the release of the cytokine interleukin-2 (IL-2) in activated cells. Similar to MWNTs, crocidolite had little toxic effects on Jurkat cells but neither induced the formation of reactive oxygen species nor changes in IL-2 signaling. These findings suggest that, in contrast to many other cell types, T cells are relatively resistant to stress induced by high-aspect ratio particles.

New Insights from Microcalorimetry on the FeOx/CNT‐Based Electrocatalysts Active in the Conversion of CO2 to Fuels

Fe oxide nanoparticles show enhanced electrocatalytic performance in the reduction of CO(2) to isopropanol when deposited on an N-functionalized carbon nanotube (CNT) support rather than on a pristine or oxidized CNT support. XRD and high-resolution TEM were used to investigate the nanostructure of the electrocatalysts, and CO(2) adsorptive microcalorimetry was used to study the chemical nature of the interaction of CO(2) with the surface sites. Although the particles always present the same Fe(3)O(4) phase, their structural anisotropy and size inhomogeneity are consequences of the preparation method of the carbon surface. Two types of chemisorption sites have been determined by using microcalorimetry: irreversible sites (280 kJ mol(-1)) at the uncoordinated sites of the facets and reversible sites (120 kJ mol(-1)) at the hydrated oxide surface of the small nanoparticles. N-Functionalization of the carbon support is advantageous, as it causes the formation of small nanoparticles, which are highly populated by reversible chemisorbing sites. These characteristic features correlate with a higher electrocatalytic performance.

On the Nature of Selective Palladium-Based Nanoparticles on Nitrogen-Doped Carbon Nanotubes for the Direct Synthesis of H2O2

Catalysts based on Pd and Pd–Au nanoparticles supported on N‐doped carbon nanotubes (N‐CNTs) are studied in the direct synthesis of H2O2. The initial selectivity in H2O2 formation is rather high (>95 %); however, there is a fast initial decrease during the first hour of time on stream. This was due to the initial presence of an organic capping agent (polyvinyl alcohol, which is used in the catalyst synthesis to obtain a high dispersion of metal particles). The removal of this capping agent during the reaction leads to a high mobility of metal nanoparticles. The high initial selectivity, when the capping agent is present, is due to small Pd terraces fully covered with chemisorbed O2 and limited H2 chemisorbed sites that consecutively hydrogenate the formed H2O2. The alloying of Pd with Au decreases the intrinsic reaction rate (per mg of Pd) and increases the selectivity in H2O2 formation, whereas Au alone is inactive. Au also has a minor effect on the consecutive conversion of H2O2 in both the decomposition and hydrogenolysis (in the presence of H2 only) reactions. These results suggest that Au does not block the unselective sites of H2O2 conversion but mainly creates isolated small terraces of Pd that can limit H2 chemisorption sites, which thus leads to higher selectivity to H2O2 under given reaction conditions.