Theoretical investigation of catalyst stability and deactivation

Edelmetall Nanopartikel auf Oxid-Trägern werden in der Industrie als heterogene Katalysatoren genutzt, zum Beispiel in Auto-Abgaskatalysatoren. Es ist wichtig, die Lebenszeit dieser Nanopartikel zu erhöhen, um Umweltausbeutung zu verhindern. Die Katalysatorlebenszeit ist allerdings begrenzt, da durch Sintern eine Katalysator-Deaktivierung hervorgerufen wird. In dieser Doktorarbeit wird sowohl die thermodynamische Stabilität der Nanopartikel auf Trägern untersucht, als auch die Sinterungskinetik. Im ersten Teil wird die thermodynamische Stabilität von reinen Edelmetall-Nanopartikeln untersucht. Um die Stabilität von kuboktaedrischen, oktaedrischen und kubischen Nanopartikeln, bestehend aus Übergangsmetallen und Al und Mg zu untersuchen, wird die Dichtefunktionaltheorie (DFT) genutzt. Es wird ein einfaches Modell entwickelt, welches nicht nur die Beiträge der Oberflächen berücksichtigt, wie in Wulff\u27s Konstruktion, sondern auch die von Ecken und Kanten. Besonders ist, dass nur die Kohäsionsenergie und die Oberflächenenergien der fcc(111) und fcc(100) Oberflächen notwendig sind, um auch Oberflächenatome mit niedriger Koordinationszahl zu beschreiben. Mit dem entwickelten Modell kann die Stabilität von Nanopartikeln mit einem mittleren absoluten Fehler von 0.09~eV/Atom berechnet werden. Um zu verstehen, wie die Stabilität der Nanopartikel zusätzlich durch den Oxid-Träger beeinflusst wird, wurden zahlreiche Metall/Oxid Grenzflächen mit Hilfe von DFT untersucht. Wird nur ein einzelnes Oxid betrachtet, so kann die Abweichung in der Adhäsionsenergie verschiedener Metalle mit der Adsorptionsenergie von Sauerstoff auf den Metalloberflächen beschrieben werden. Dies ist ähnlich zu Skalierungsbeziehungen, welche für die Beschreibung von Adsorbaten auf Metalloberflächen verwendet werden. Die verschiedenen Oxide unterscheiden sich in ihrer Sauerstoffatomanzahl auf der Oberfläche. Dies kann als Deskriptor genutzt und von reinen Oxidoberflächen bestimmt werden. Insofern kann die Adhäsion der untersuchten Grenzflächen, welche von Metall-Sauerstoff-Bindungen dominiert werden, durch eine universelle Skalierungsrelation beschrieben werden, mit einem Deskriptor für das Oxid und einem für das Metall. Um die Kinetik von Sinterung zu beschreiben, wurde ein auf Kinetik Monte Carlo (kMC) basierendes Modell entwickelt. Dieses beschreibt die Teilchenbewegung und -verschmelzung am Beispiel von Pt Nanopartikeln auf Quarzoberflächen mit Defektstellen. Die notwendigen Diffusionskonstanten wurden aus der Literatur entnommen und basieren auf einem 3D-Gitter DFT/kMC Modell. Untersucht wurde der Einfluss von Temperatur, Teilchengröße, Teilchenkonzentration und Oberflächendefekten. Um die konkurrierenden Sinterungsmechanismen Teilchenbewegung und -verschmelzung sowie Ostwald-Reifung zu beschreiben, wurde ein adaptives kMC Modell entwickelt, in dem die Ostwald-Reifung mittels eines Mean Field Modells beschrieben wird. Bei hohen Temperaturen und Sauerstoff in der Gasphase ist Ostwald-Reifung, bei der über die Gasphase PtO$_2$ zwischen den Pt Nanopartikeln übertragen wird, der dominante Sinterungsmechanismus für das untersuchte Pt/Quarz System. Das bisher genutzte Mean Field Modell für Ostwald-Reifung basiert auf Annahmen, welche für die makro-skopische Diffusion gültig sind. Im Falle von Nanopartikeln ist die Längenskala allerdings deutlich kleiner. Daher wurde ein kMC-Modell entwickelt, mit dessen Hilfe die Diffusion von einzelnen Atomen oder Molekülen in der Gasphase unter Berücksichtigung von Kollisionen mit dem Hintergrundgas simuliert werden kann. Dieses Modell reproduziert die Eigenschaften eines idealen Gases, wie dessen Diffusionskonstante. Um das kMC-Modell zu testen, wird die Diffusion zwischen zwei Oberflächen abhängig von deren Abstand untersucht. Ist der Abstand kleiner als der mittlere freie Weg des untersuchten Atoms in der Gasphase, dann kommt es zu Unterschieden zwischen dem kMC-Modell und Ergebnissen, berechnet mit Kontinuitätsgleichungen basierend auf den Fickschen Gesetzen. Im Anschluss wurde das kMC-Modell weiterentwickelt, um Ostwald-Reifung über die Gasphase zu simulieren und um die Limitierungen des bisher genutzten Mean Field Modells einschätzen zu können. Im Allgemeinen stimmen die Ergebnisse, welche mit den beiden Modellen erhalten werden überein. Nur in Grenzfällen kommt es zu Abweichungen, zum Beispiel bei gleichgroßen Nanopartikeln

Structure-Dependent Strain Effects

Density functional theory calculations of atomic and molecular adsorption on (111) and (100) metal surfaces reveal marked surface and structure dependent effects of strain. Adsorption in three-fold hollow sites is found to be destabilized by compressive strain whereas the reversed trend is commonly valid for adsorption in four-fold sites. The effects, which are qualitatively explained using a simple two-orbital model, provide insights on how to modify chemical properties by strain design

Ensemble Effects in Adsorbate-Adsorbate Interactions in Microkinetic Modeling

Adsorbates on a surface experience lateral interactions that result in a distribution of adsorption energies. The adsorbate-adsorbate interactions are known to affect the kinetics of surface reactions, which motivates efforts to develop models that accurately account for the interactions. Here, we use density functional theory (DFT) calculations combined with Monte Carlo simulations to investigate how the distribution of adsorbates affects adsorption and desorption of CO from Pt(111). We find that the mean of the average adsorption energy determines the adsorption process, whereas the desorption process can be described by the low energy part of the adsorbate stability distribution. The simulated results are in very good agreement with calorimetry and temperature-programmed desorption experiments and provide a guideline of how to include adsorbate-adsorbate interactions in DFT-based mean-field kinetic models

Seespiegelschwankungen und assoziierte Sedimentationsumgebungen am Donggi Cona See, nordöstliches Tibet-Plateau, China

The world’s largest mountain plateau, the Tibetan Plateau, not only plays an important role in supplying water to more than a third of world’s population, but its unique characteristics also influence global climate. Different circulation systems interact there, including the Asian monsoons and the westerlies, though the past variability of the circulation patterns is still under debate. Sediments and landforms of different process-environments along a catchment-wide sediment cascade can be used to infer landscape evolution and past environmental conditions, e.g., climatic changes. More than 1000 lake catchments on the Tibetan Plateau receive water mainly from Asian summer monsoon precipitation and from glacial melt. Although the extent of past glaciers is still under discussion, it can be assumed that most Tibetan catchments were affected by glaciers in the past, and may still be today. At the glacial/periglacial zone, sediment transfer starts via the fluvial and the aeolian system towards the local end of a sediment cascade: the lake. However, sedimentation and geomorphological dynamics are not always triggered by climate alone. Tectonic activity is a further important driver of landscape evolution, as uplift and accommodation of the Indo-Eurasian collision is still ongoing and many lakes have formed in extensional basins. Human impact also started to significantly affect sedimentation dynamics via grazing during at least the last 2000 years. This study focuses on the reconstruction of past hydrological changes at Lake Donggi Cona (35°18’N, 98°32’E, 4090 m a.s.l.), in Qinghai province, on the north-eastern Tibetan Plateau. The lake is situated along the Kunlun fault, at the northern limit of the Asian summer monsoons. Lake level changes were assessed from landforms and sediments along the littoral zone of the lake. Basin morphology and depositional stratigraphy below the present lake level were studied with echo depth sounding and shallow seismic sub-bottom profiling. A flexible end-member modelling algorithm based on eigenspace analysis was developed and applied to the detrital, multi-modal grain-size components of lacustrine surface sediment samples to quantify transport processes that contribute sediments to the lake. High-stand sediments in nine sections from lake terraces above the present lake level (a.p.l.l.) were studied stratigraphically and end-member modelling was applied to their grain size distributions. Lake level changes were interpreted from submersed terraces and delta sediments, as well as from lake shorelines and high-stand sediments. The sensitivity of the lake basin and littoral zone to climatic change depends on the basin and shore morphology, as well as on the contribution of sediment and water from the catchment. Finally, the involved driving forces, i.e., climate, geomorphological processes, tectonics and human impact are discussed in a broader context.Das größte Gebirgsplateau der Welt, das Tibet-Plateau, spielt nicht nur eine wichtige Rolle für die Versorgung von über einem Drittel der Weltbevölkerung mit Wasser, sondern es beeinflusst mit seinen Eigenheiten auch das globale Klima. Verschiedene Zirkulationssysteme interagieren dort: die Asiatischen Monsune und die Westwindzirkulation, deren Muster und Variabilität in der Vergangenheit noch immer diskutiert werden. Sedimente und geomorphologische Formen aus verschiedenen Prozessregimen entlang einer Sedimentkaskade können verwendet werden um die Landschaftsentwicklung und frühere Umwelt- und insbesondere Klimabedingungen abzuleiten. Über 1000 tibetische Seeeinzugsgebiete erhalten Wasser vorwiegend aus dem Niederschlag der Asiatischen Sommermonsune und von Gletscherschmelze. Obwohl die damalige Ausdehnung der Gletscher noch diskutiert wird, kann davon ausgegangen werden, dass die meisten tibetischen Einzugsgebiete von Gletschern beeinflusst wurden und werden. In der glazialen und periglazialen Zone beginnt der Sedimenttransfer über das fluviale und äolische System in Richtung des lokalen Endglieds einer Sedimentkaskade: dem See. Jedoch werden sedimentologische und geomorphologische Prozesse nicht allein vom Klima bestimmt. Tektonische Aktivität ist eine weitere wichtige Steuergröße für die Landschaftsentwicklung, da die Hebung und Anpassung an die Indo-Eurasische Kollision anhält und viele Seen Extensionsbecken füllen. Menschlicher Einfluss, d.h. Beweidung, begann zudem die Sedimentationsdynamiken seit mindestens den letzten 2000 Jahren zu beeinflussen. Diese Arbeit konzentriert sich auf die Rekonstruktion der hydrologischen Veränderungen am Donggi Cona See (35°18’N, 98°32’E, 4090 m ü.d.M.), in der chinesischen Provinz Qinghai, auf dem nordöstlichen Tibet Plateau. Der See liegt an der Kunlunverwerfung und im Bereich der nördlichen Grenze der Asiatischen Sommermonsune. Seespiegelschwankungen wurden an geomorphologischen Formen und Sedimenten entlang der litoralen Zone des Sees abgeleitet. Die Beckenmorphologie und Sedimentstratigraphie unterhalb des aktuellen Seespiegels wurde über Echolotungen und Flachseismik untersucht. Um Transportprozesse zu quantifizieren, die zur Sedimentation im See beitragen wurde ein flexibler Endmember-Modellierungsalgorithmus entwickelt, basierend auf den Prinzipien der Eigenraumanalyse, und auf die detritischen, multi-modalen Korngrößenverteilungen lakustriner Oberflächensedimentproben angewendet. Hochstandssedimente von neun Profilen in Seeterrassen über dem aktuellen Seespiegel (a.p.l.l.) wurden stratigraphisch und durch Endmember-Modellierung ihrer Korngrößenverteilungen untersucht. Seespiegelschwankungen wurden aus subaquatischen Terrassen und Deltasedimenten sowie aus Strandlinien und Hochstandssedimenten abgeleitet. Die Sensitivität des Seebeckens und des Strandbereichs gegenüber klimatischen Veränderungen ist sowohl abhängig von der Becken- und Strandmorphologie, als auch von Sediment- und Wasserzustrom aus dem Einzugsgebiet. Die beeinflussenden Steuergrößen, Klima, geomorphologische Prozesse, Tektonik und Mensch, wurden schließlich in einem größeren Kontext diskutiert

Surface steps dominate the water formation on Pd(111) surfaces

Water formation is relevant in many technological processes and is also an important model reaction. Although water formation over Pd surfaces is widely studied, questions regarding the active site and the main reaction path (OH* + OH*) or (OH* + H*) are still open. Combining first-principles density functional theory calculations and kinetic Monte Carlo simulations, we find that the reaction rate is dominated by surface steps and point defects over a wide range of conditions. The main reaction path is found to be temperature dependent where the OH* + OH* path dominates at low temperatures, whereas the OH* + H* path is the main path at high temperatures. Steps facilitate the OH* formation, which is the rate limiting step under all conditions. OH* is formed via O* + H* association or OOH* splitting at low temperatures, whereas OH* is exclusively formed via O* + H* association at high temperatures. The results of the first-principles-based kinetic model are in excellent agreement with experimental observations at high and low temperatures as well as different gas-phase compositions

Late Holocene fire history documented at Lake Khamra, SW Yakutia (Eastern Siberia)

Recent large-scale fire events in Siberia have drawn increased attention to boreal forest fire history. Boreal forests contain about 25% of all global biomass and act as an enormous carbon storage. Fire events are important ecological disturbances connected to the overarching environmental changes that face the Arctic and Subarctic, like vegetation dynamics, permafrost degradation, changes in soil nutrient cycling and global warming, and act as the dominant driver behind boreal forest’s landscape carbon balance. By looking into past fire regimes we can learn about fire frequency and potential linkages to other environmental factors, e.g. fuel types, reconstructed temperature/humidity or geomorphologic landscape dynamics. Unfortunately, fire history data is still very sparse in large parts of Siberia, a region strongly influenced by climate change. The Global Charcoal Database (www.paleofire.org) lists only a handful of continuous charcoal records for all of Siberia, with only three of those featuring published data from macroscopic charcoal as opposed to microscopic charcoal from pollen slides. We aim to reconstruct the late Holocene fire history using lacustrine sediments of Lake Khamra (SW Yakutia at N 59.99°, E 112.98°). It covers an area of c. 4.6 km² with about 22 m maximum water depth, located within the zone of transition from summer-green and larch-dominated to evergreen boreal forest. We present the first continuous, high-resolution (c. 10 years/sample) macroscopic charcoal record (> 150 μm) including information on particle size and morphology for the past c. 2200 years. We compare this to complementary information from microscopic charcoal in pollen slides, a pollen and non-pollen palynomorph record as well as μXRF data. This multi-proxy approach adds valuable data about fire activity in the region and allows a comparison of different prevalent fire reconstruction methods. As the first record of its kind from Siberia, it provides a long-term context for current fire activity in central Siberian boreal forests and enables a better understanding of the environmental interactions occurring in the changing subarctic landscape

Theoretical Investigation of the Size Effect on the Oxygen Adsorption Energy of Coinage Metal Nanoparticles

This study evaluates the finite size effect on the oxygen adsorption energy of coinage metal (Cu, Ag and Au) cuboctahedral nanoparticles in the size range of 13 to 1415 atoms (0.7–3.5\ua0nm in diameter). Trends in particle size effects are well described with single point calculations, in which the metal atoms are frozen in their bulk position and the oxygen atom is added in a location determined from periodic surface calculations. This is shown explicitly for Cu nanoparticles, for which full geometry optimization only leads to a constant offset between relaxed and unrelaxed adsorption energies that is independent of particle size. With increasing cluster size, the adsorption energy converges systematically to the limit of the (211) extended surface. The 55-atomic cluster is an outlier for all of the coinage metals and all three materials show similar behavior with respect to particle size

The genesis of Yedoma Ice Complex permafrost – grain-size endmember modeling analysis from Siberia and Alaska

The late Pleistocene Yedoma Ice Complex is an ice-rich and organic-bearing type of permafrost deposit widely distributed across Beringia and is assumed to be especially prone to deep degradation with warming temperature, which is a potential tipping point of the climate system. To better understand Yedoma formation, its local characteristics, and its regional sedimentological composition, we compiled the grain-size distributions (GSDs) of 771 samples from 23 Yedoma locations across the Arctic; samples from sites located close together were pooled to form 17 study sites. In addition, we studied 160 samples from three non-Yedoma ice-wedge polygon and floodplain sites for the comparison of Yedoma samples with Holocene depositional environments. The multimodal GSDs indicate that a variety of sediment production, transport, and depositional processes were involved in Yedoma formation. To disentangle these processes, a robust endmember modeling analysis (rEMMA) was performed. Nine robust grain-size endmembers (rEMs) characterize Yedoma deposits across Beringia. The study sites of Yedoma deposits were finally classified using cluster analysis. The resulting four clusters consisted of two to five sites that are distributed randomly across northeastern Siberia and Alaska, suggesting that the differences are associated with rather local conditions. In contrast to prior studies suggesting a largely aeolian contribution to Yedoma sedimentation, the wide range of rEMs indicates that aeolian sedimentation processes cannot explain the entire variability found in GSDs of Yedoma deposits. Instead, Yedoma sedimentation is controlled by local conditions such as source rocks and weathering processes, nearby paleotopography, and diverse sediment transport processes. Our findings support the hypothesis of a polygenetic Yedoma origin involving alluvial, fluvial, and niveo-aeolian transport; accumulation in ponding waters; and in situ frost weathering as well as postdepositional processes of solifluction, cryoturbation, and pedogenesis. The characteristic rEM composition of the Yedoma clusters will help to improve how grain-size-dependent parameters in permafrost models and soil carbon budgets are considered. Our results show the characteristic properties of ice-rich Yedoma deposits in the terrestrial Arctic. Characterizing and quantifying site-specific past depositional processes is crucial for elucidating and understanding the trajectories of this unique kind of ice-rich permafrost in a warmer future

The polygenetic hypothesis of Yedoma origin – comparing grain-size distributions of Alaskan and Siberian Yedoma

The formation of late Pleistocene Yedoma in western Beringia (Siberia) is still widely debated. Moreover, different geological and cryostratigraphical views on Yedoma exist between researchers focusing on western or eastern Beringia (Alaska and Northwest Canada). These differences largely concern the prominence of the role of eolian processes. In particular, previous studies on Yedoma in the Yukon Territories and Alaska interpret these deposits as being largely loess or re-transported loess (muck). In contrast, several hypotheses have emerged over decades of research in the extensive Siberian Yedoma region, including (1) alluvial genesis, (2) ice-sheet-dammed basin sediments, (3) deltaic formation, (4) proluvial and slope deposits, (5) cryogenic-aeolian deposits, (6) nival deposits, and (7) polygenetic origins. Characteristics that most studies agree on include the dominance of large syngenetic ice wedges, mainly allochthonous silty to sandy sediment deposition in low-center polygons in combination with deposition of mainly autochthonous organic remnants from plants and animals, very harsh continental, glacial climate conditions. In terms of landscape and relief characteristics, various Yedoma types seem to exist across the extensive region where Yedoma does occur, ranging from spatially confined Yedoma valley fills including slopes to vast accumulation plains on Arctic lowlands and shelves. Accordingly, we here support the notion that Yedoma may have different depositional properties and genetic origins under a common frame of similar environmental and climatic conditions during the Late Pleistocene. This hypothesis is known as polygenetic formation of Yedoma. An important aspect of Yedoma is the dominating presence of excess ground ice. Ice wedges and segregated intra-sedimentary ice constitute the majority of this deposit by volume (50-80%) in most Yedoma regions and are thus one of the most critical factors in deposit genesis in contrast to accumulations of silty to sandy deposits in temperate regions. The Yedoma Ice Complex formation includes cryogenic processes such as cryogenic weathering, ice segregation, syngenetic ice wedge formation and growth, secondary sediment deformation and reworking due to ground ice, and cryosol formation (including phases dominated by orthels, turbels, or histels). All these processes were promoted by long-lasting harsh continental climate conditions. Furthermore, the formation of large polygon ice-wedge nets and thick continuous sequences of frozen deposits is closely related to the persistence of stable, poorly drained, low topographic gradient accumulation areas. A comprehensive cryolithogenic concept of polygenetic Yedoma formation combines cryogenic weathering, periglacial material transport and accumulation, and relief shaping under cold-arid climate conditions and considers two general formation processes: (1) the primary accumulation in low-centered ice-wedge polygons and (2) the syngenetic freezing and ice-wedge growth in non-glaciated Arctic lowlands under cold-arid climate of the late Pleistocene. Following this concept Yedoma represents a specific periglacial facies whose formation is controlled by the interaction of several climate, landscape and geological preconditions typical for non-glaciated Arctic and sub-Arctic lowlands and foothills. In contrast to the pure aeolian (loess) or glacial hypotheses, the proposed cryolithogenic concept integrates several previous formation concepts and in particular takes the important role of ground ice in the deposit formation process into account. Generally, this corresponds to the polygenetic character of Yedoma formation. It also includes the potential for several sediment sources, weathering processes, and pathways by which sediments in typical periglacial landscapes can build up the Ice Complex horizon. Generally, Yedoma consists of often poorly sorted sediments with maxima in the silt and fine sand, but also coarse sand and gravels can be included. Grain-size characteristics differ with study sites and within horizons (Schirrmeister et al. 2011). To understand the local characteristics as well as the regional variation in the sedimentology of the late Pleistocene Yedoma deposits, we analyzed the grain-size distribution (GSD) of hundreds of samples from dozens of Yedoma sites: The multi-modal GSD of two examples from the Bykovsky Peninsula (Siberia) and the Colville River (Alaska) already indicate a variety of sediment production, transport and depositional processes (Fig. 1). To disentangle these processes a robust end-member modeling analysis (EMMA) was performed on Yedoma sediments of the two sites following Dietze et al. (2012) and Dietze et al. (2014). Multiple robust grain-size end-members (rEM) were unmixed (Fig. 1). The average robust model explains 85.6 % of the total grain-size variability in Colville and only 53.5 % of the Bykovsky Yedoma grain-size variability, the latter being indicative for very poor sorting and high heterogeneity of the Bykovsky sediments in contrast to Colville sediments. Both sites were composed of five robust end-members with modes at 4, 17, 36, 210 and 340 m that explain 25, 28, 36, 8.2 and 2.8 % of the variance of Colville sediments. The Bykovsky rEM had modes at 5, 27, 120, 210 and 310 m comparable to Colville rEMs, each of them explaining 8.9, 23, 22, 21 and 25 % of the grain-size fractions that can be explained by EMMA. The various grain-size end-members supports the hypothesis of polygenetic Yedoma origin involving multiple transport and depositional processes. Although both sites were dominated by silt-transporting and depositing processes, an important amount of finer fractions were deposited at Colville, whereas rather high amounts of coarse- to fine-sandy deposits composed the Bykovsky Yedoma. Developing a site-specific interpretation of past depositional processes helps understanding the formation conditions of thaw susceptible Yedoma deposits in the terrestrial Arctic and could be crucial for understanding the future trajectories of this unique kind of permafrost in a warming Arctic

Human-induced fire regime shifts during 19th century industrialization: A robust fire regime reconstruction using northern Polish lake sediments

Fire regime shifts are driven by climate and natural vegetation changes, but can be strongly affected by human land management. Yet, it is poorly known how humans have influenced fire regimes prior to active wildfire suppression. Among the last 250 years, the human contribution to the global increase in fire occurrence during the mid-19th century is especially unclear, as data sources are limited. Here, we test the extent to which forest management has driven fire regime shifts in a temperate forest landscape. We combine multiple fire proxies (macroscopic charcoal and fire-related biomarkers) derived from highly resolved lake sediments (i.e., 3–5 years per sample), and apply a new statistical approach to classify source area- and temperature-specific fire regimes (biomass burnt, fire episodes). We compare these records with independent climate and vegetation reconstructions. We find two prominent fire regime shifts during the 19th and 20th centuries, driven by an adaptive socio-ecological cycle in human forest management. Although individual fire episodes were triggered mainly by arson (as described in historical documents) during dry summers, the biomass burnt increased unintentionally during the mid-19th century due to the plantation of flammable, fast-growing pine tree monocultures needed for industrialization. State forest management reacted with active fire management and suppression during the 20th century. However, pine cover has been increasing since the 1990s and climate projections predict increasingly dry conditions, suggesting a renewed need for adaptations to reduce the increasing fire risk. © 2019 Dietze et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited