2D Vanadium Sulfides: Synthesis, Atomic Structure Engineering, and Charge Density Waves.

Two ultimately thin vanadium-rich 2D materials based on VS2 are created via molecular beam epitaxy and investigated using scanning tunneling microscopy, X-ray photoemission spectroscopy, and density functional theory (DFT) calculations. The controlled synthesis of stoichiometric single-layer VS2 or either of the two vanadium-rich materials is achieved by varying the sample coverage and sulfur pressure during annealing. Through annealing of small stoichiometric single-layer VS2 islands without S pressure, S-vacancies spontaneously order in 1D arrays, giving rise to patterned adsorption. Via the comparison of DFT calculations with scanning tunneling microscopy data, the atomic structure of the S-depleted phase, with a stoichiometry of V4S7, is determined. By depositing larger amounts of vanadium and sulfur, which are subsequently annealed in a S-rich atmosphere, self-intercalated ultimately thin V5S8-derived layers are obtained, which host 2 × 2 V-layers between sheets of VS2. We provide atomic models for the thinnest V5S8-derived structures. Finally, we use scanning tunneling spectroscopy to investigate the charge density wave observed in the 2D V5S8-derived islands.

Temperature-Dependent Selectivity and Detection of Hidden Carbon Deposition in Methane Oxidation.

Reaction products in heterogeneous catalysis can be detected either on the catalyst surface or in the gas phase after desorption. However, if atoms are dissolved in the catalyst bulk, then reaction channels can become hidden. This is the case if the dissolution rate of the deposits is faster than their formation rate. This might lead to the underestimation or even overlooking of reaction channels such as, e.g., carbon deposition during hydrocarbon oxidation reactions, which is problematic as carbon can have a significant influence on the catalytic activity. Here, we demonstrate how such hidden deposition channels can be uncovered by carefully measuring the product formation rates in the local gas phase just above the catalyst surface with time-resolved ambient pressure X-ray photoelectron spectroscopy. As a case study, we investigate methane oxidation on a polycrystalline Pd catalyst in an oxygen-lean environment at a few millibar pressure. By ramping the temperature between 350 and 525 °C, we follow the time evolution of the different reaction pathways. Only in the oxygen mass-transfer limit do we observe CO production, while our data suggests that carbon deposition also happens outside this limit.

Graphene as an Adsorption Template for Studying Double Bond Activation in Catalysis.

Hydrogenated graphene (H-Gr) is an extensively studied system not only because of its capabilities as a simplified model system for hydrocarbon chemistry but also because hydrogenation is a compelling method for Gr functionalization. However, knowledge of how H-Gr interacts with molecules at higher pressures and ambient conditions is lacking. Here we present experimental and theoretical evidence that room temperature O2 exposure at millibar pressures leads to preferential removal of H dimers on H-functionalized graphene, leaving H clusters on the surface. Our density functional theory (DFT) analysis shows that the removal of H dimers is the result of water or hydrogen peroxide formation. For water formation, we show that the two H atoms in the dimer motif attack one end of the physisorbed O2 molecule. Moreover, by comparing the reaction pathways in a vacuum with the ones on free-standing graphene and on the graphene/Ir(111) system, we find that the main role of graphene is to arrange the H atoms in geometrical positions, which facilitates the activation of the O=O double bond.

Water Chemistry beneath Graphene: Condensation of a Dense OH-H2O Phase under Graphene.

Room temperature oxygen hydrogenation below graphene flakes supported by Ir(111) is investigated through a combination of X-ray photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory calculations using an evolutionary search algorithm. We demonstrate how the graphene cover and its doping level can be used to trap and characterize dense mixed O-OH-H2O phases that otherwise would not exist. Our study of these graphene-stabilized phases and their response to oxygen or hydrogen exposure reveals that additional oxygen can be dissolved into them at room temperature creating mixed O-OH-H2O phases with an increased areal coverage underneath graphene. In contrast, additional hydrogen exposure converts the mixed O-OH-H2O phases back to pure OH-H2O with a reduced areal coverage underneath graphene.

Stroboscopic operando spectroscopy of the dynamics in heterogeneous catalysis by event-averaging.

Heterogeneous catalyst surfaces are dynamic entities that respond rapidly to changes in their local gas environment, and the dynamics of the response is a decisive factor for the catalysts' action and activity. Few probes are able to map catalyst structure and local gas environment simultaneously under reaction conditions at the timescales of the dynamic changes. Here we use the CO oxidation reaction and a Pd(100) model catalyst to demonstrate how such studies can be performed by time-resolved ambient pressure photoelectron spectroscopy. Central elements of the method are cyclic gas pulsing and software-based event-averaging by image recognition of spectral features. A key finding is that at 3.2 mbar total pressure a metallic, predominantly CO-covered metallic surface turns highly active for a few seconds once the O2:CO ratio becomes high enough to lift the CO poisoning effect before mass transport limitations triggers formation of a √5 oxide.

Gas Pulse-X-Ray Probe Ambient Pressure Photoelectron Spectroscopy with Submillisecond Time Resolution.

A setup capable of conducting gas pulse-X-ray probe ambient pressure photoelectron spectroscopy with high time resolution is presented. The setup makes use of a fast valve that creates gas pulses with an internal pressure in the mbar range and a rising edge of few hundreds of microseconds. A gated detector based on a fast camera is synchronized with the valve operation to measure X-ray photoemission spectra with up to 20 µs time resolution. The setup is characterized in several experiments in which the N2 gas is pulsed either into vacuum or a constant flow of another gas. The observed width of the pulse rising edge is 80 µs, and the maximum internal pulse pressure is ∼1 mbar. The CO oxidation reaction over Pt (111) was used to demonstrate the capability of the setup to correlate the gas phase composition with that of the surface during transient supply of CO gas into an O2 stream. Thus, formation of both chemisorbed and oxide oxygen species was observed prior to CO gas perturbation. Also, the data indicated that both the Langmuir-Hinshelwood and Mars-van-Krevelen mechanisms play an important role in the oxidation of carbon monoxide under ambient conditions.

Upgrade of the SPECIES beamline at the MAX IV Laboratory.

The SPECIES beamline has been transferred to the new 1.5 GeV storage ring at the MAX IV Laboratory. Several improvements have been made to the beamline and its endstations during the transfer. Together the Ambient Pressure X-ray Photoelectron Spectroscopy and Resonant Inelastic X-ray Scattering endstations are capable of conducting photoelectron spectroscopy in elevated pressure regimes with enhanced time-resolution and flux and X-ray scattering experiments with improved resolution and flux. Both endstations offer a unique capability for experiments at low photon energies in the vacuum ultraviolet and soft X-ray range. In this paper, the upgrades on the endstations and current performance of the beamline are reported.

Cluster Superlattice Membranes.

Cluster superlattice membranes consist of a two-dimensional hexagonal lattice of similar-sized nanoclusters sandwiched between single-crystal graphene and an amorphous carbon matrix. The fabrication process involves three main steps, the templated self-organization of a metal cluster superlattice on epitaxial graphene on Ir(111), conformal embedding in an amorphous carbon matrix, and subsequent lift-off from the Ir(111) substrate. The mechanical stability provided by the carbon-graphene matrix makes the membrane stable as a free-standing material and enables transfer to other substrates. The fabrication procedure can be applied to a wide variety of cluster materials and cluster sizes from the single-atom limit to clusters of a few hundred atoms, as well as other two-dimensional layer/host matrix combinations. The versatility of the membrane composition, its mechanical stability, and the simplicity of the transfer procedure make cluster superlattice membranes a promising material in catalysis, magnetism, energy conversion, and optoelectronics.

Present and new frontiers in materials research by ambient pressure x-ray photoelectron spectroscopy.

In this topical review we catagorise all ambient pressure x-ray photoelectron spectroscopy publications that have appeared between the 1970s and the end of 2018 according to their scientific field. We find that catalysis, surface science and materials science are predominant, while, for example, electrocatalysis and thin film growth are emerging. All catalysis publications that we could identify are cited, and selected case stories with increasing complexity in terms of surface structure or chemical reaction are discussed. For thin film growth we discuss recent examples from chemical vapour deposition and atomic layer deposition. Finally, we also discuss current frontiers of ambient pressure x-ray photoelectron spectroscopy research, indicating some directions of future development of the field.

Extended fed-batch fermentation of a C5/C6 optimised yeast strain on wheat straw hydrolysate using an online refractive index sensor to measure the relative fermentation rate.

In the production of 2nd generation ethanol, using Saccharomyces cerevisiae, the highest productivity obtained using C5/C6 fermenting yeast is in the co-fermentation phase, in which xylose and glucose are fermented simultaneously. Extending this phase in a fed-batch process increases the yield, rate and additionally reduces needed yeast amount for pitching. Extending this phase, as long as possible, would further enhance yield and economy of the process. To realise the concept a fermentation monitoring technique was developed and applied. Based on online measured refractive index an optimal residual sugar concentration could be maintained in the primary fermentor during the feed phase, requiring little knowledge of the nature of the substrate. The system was able to run stably for at least five fermentor volumes giving an ethanol yield >90% throughout the run. This was achieved with addition of only urea to the wheat straw hydrolysate and with an initial yeast pitch of 0.2 g/L total of finished broth. It has the potential to improve the fermentation technology used in fuel ethanol plants, which could help to meet the growing demand for more sustainable fuels.

A five-axis parallel kinematic mirror unit for soft X-ray beamlines at MAX†IV.

With the introduction of the multi-bend achromats in the new fourth-generation storage rings the emittance has decreased by an order of magnitude resulting in increased brightness. However, the higher brightness comes with smaller beam sizes and narrower radiation cones. As a consequence, the requirements on mechanical stability regarding the beamline components increases. Here an innovative five-axis parallel kinematic mirror unit for use with soft X-ray beamlines using off-axis grazing-incidence optics is presented. Using simulations and measurements from the HIPPIE beamline at the MAX IV Laboratory it is shown that it has no Eigen frequencies below 90 Hz. Its positioning accuracy is better than 25 nm linearly and 17-35 µrad angularly depending on the mirror chamber dimensions.

Self-cleaning and surface chemical reactions during hafnium dioxide atomic layer deposition on indium arsenide.

Atomic layer deposition (ALD) enables the ultrathin high-quality oxide layers that are central to all modern metal-oxide-semiconductor circuits. Crucial to achieving superior device performance are the chemical reactions during the first deposition cycle, which could ultimately result in atomic-scale perfection of the semiconductor-oxide interface. Here, we directly observe the chemical reactions at the surface during the first cycle of hafnium dioxide deposition on indium arsenide under realistic synthesis conditions using photoelectron spectroscopy. We find that the widely used ligand exchange model of the ALD process for the removal of native oxide on the semiconductor and the simultaneous formation of the first hafnium dioxide layer must be significantly revised. Our study provides substantial evidence that the efficiency of the self-cleaning process and the quality of the resulting semiconductor-oxide interface can be controlled by the molecular adsorption process of the ALD precursors, rather than the subsequent oxide formation.

Adsorption of CO on the Fe3O4(001) Surface.

The interaction of CO with the Fe3O4(001)-(√2 × âˆš2)R45° surface was studied using temperature-programmed desorption (TPD), scanning tunneling microscopy (STM), and X-ray photoelectron spectroscopy (XPS), the latter both under ultrahigh vacuum (UHV) conditions and in CO pressures up to 1 mbar. In general, the CO-Fe3O4 interaction is found to be weak. The strongest adsorption occurs at surface defects, leading to small TPD peaks at 115, 130, and 190 K. Desorption from the regular surface occurs in two distinct regimes. For coverages up to two CO molecules per (√2 × âˆš2)R45° unit cell, the desorption maximum shows a large shift with increasing coverage, from initially 105 to 70 K. For coverages between 2 and 4 molecules per (√2 × âˆš2)R45° unit cell, a much sharper desorption feature emerges at ∼65 K. Thermodynamic analysis of the TPD data suggests a phase transition from a dilute 2D gas into an ordered overlayer with CO molecules bound to surface Fe3+ sites. XPS data acquired at 45 K in UHV are consistent with physisorption. Some carbon-containing species are observed in the near-ambient-pressure XPS experiments at room temperature but are attributed to contamination and/or reaction with CO with water from the residual gas. No evidence was found for surface reduction or carburization by CO molecules.

Exciting H2 Molecules for Graphene Functionalization.

Hydrogen functionalization of graphene by exposure to vibrationally excited H2 molecules is investigated by combined scanning tunneling microscopy, high-resolution electron energy loss spectroscopy, X-ray photoelectron spectroscopy measurements, and density functional theory calculations. The measurements reveal that vibrationally excited H2 molecules dissociatively adsorb on graphene on Ir(111) resulting in nanopatterned hydrogen functionalization structures. Calculations demonstrate that the presence of the Ir surface below the graphene lowers the H2 dissociative adsorption barrier and allows for the adsorption reaction at energies well below the dissociation threshold of the H-H bond. The first reacting H2 molecule must contain considerable vibrational energy to overcome the dissociative adsorption barrier. However, this initial adsorption further activates the surface resulting in reduced barriers for dissociative adsorption of subsequent H2 molecules. This enables functionalization by H2 molecules with lower vibrational energy, yielding an avalanche effect for the hydrogenation reaction. These results provide an example of a catalytically active graphene-coated surface and additionally set the stage for a re-interpretation of previous experimental work involving elevated H2 background gas pressures in the presence of hot filaments.

From Permeation to Cluster Arrays: Graphene on Ir(111) Exposed to Carbon Vapor.

Our scanning tunneling microscopy and X-ray photoelectron spectroscopy experiments along with first-principles calculations uncover the rich phenomenology and enable a coherent understanding of carbon vapor interaction with graphene on Ir(111). At high temperatures, carbon vapor not only permeates to the metal surface but also densifies the graphene cover. Thereby, in addition to underlayer graphene growth, upon cool down also severe wrinkling of the densified graphene cover is observed. In contrast, at low temperatures the adsorbed carbon largely remains on top and self-organizes into a regular array of fullerene-like, thermally highly stable clusters that are covalently bonded to the underlying graphene sheet. Thus, a new type of predominantly sp2-hybridized nanostructured and ultrathin carbon material emerges, which may be useful to encage or stably bind metal in finely dispersed form.

De novo biosynthesis of trans-cinnamic acid derivatives in Saccharomyces cerevisiae.

The production of natural aroma compounds is an expanding field within the branch of white biotechnology. Three aromatic compounds of interest are cinnamaldehyde, the typical cinnamon aroma that has applications in agriculture and medical sciences, as well as cinnamyl alcohol and hydrocinnamyl alcohol, which have applications in the cosmetic industry. Current production methods, which rely on extraction from plant materials or chemical synthesis, are associated with drawbacks regarding scalability, production time, and environmental impact. These considerations make the development of a sustainable microbial-based production highly desirable. Through steps of rational metabolic engineering, we engineered the yeast Saccharomyces cerevisiae as a microbial host to produce trans-cinnamic acid derivatives cinnamaldehyde, cinnamyl alcohol, and hydrocinnamyl alcohol, from externally added trans-cinnamic acid or de novo from glucose as a carbon source. We show that the desired products can be de novo synthesized in S. cerevisiae via the heterologous overexpression of the genes encoding phenylalanine ammonia lyase 2 from Arabidopsis thaliana (AtPAL2), aryl carboxylic acid reductase (acar) from Nocardia sp., and phosphopantetheinyl transferase (entD) from Escherichia coli, together with endogenous alcohol dehydrogenases. This study provides a proof of concept and a strain that can be further optimized for production of high-value aromatic compounds.

The SPECIES beamline at the MAX IV Laboratory: a facility for soft X-ray RIXS and APXPS.

SPECIES is an undulator-based soft X-ray beamline that replaced the old I511 beamline at the MAX II storage ring. SPECIES is aimed at high-resolution ambient-pressure X-ray photoelectron spectroscopy (APXPS), near-edge X-ray absorption fine-structure (NEXAFS), X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) experiments. The beamline has two branches that use a common elliptically polarizing undulator and monochromator. The beam is switched between the two branches by changing the focusing optics after the monochromator. Both branches have separate exit slits, refocusing optics and dedicated permanent endstations. This allows very fast switching between two types of experiments and offers a unique combination of the surface-sensitive XPS and bulk-sensitive RIXS techniques both in UHV and at elevated ambient-pressure conditions on a single beamline. Another unique property of the beamline is that it reaches energies down to approximately 27 eV, which is not obtainable on other current APXPS beamlines. This allows, for instance, valence band studies under ambient-pressure conditions. In this article the main properties and performance of the beamline are presented, together with selected showcase experiments performed on the new setup.

Symmetry-Driven Band Gap Engineering in Hydrogen Functionalized Graphene.

Band gap engineering in hydrogen functionalized graphene is demonstrated by changing the symmetry of the functionalization structures. Small differences in hydrogen adsorbate binding energies on graphene on Ir(111) allow tailoring of highly periodic functionalization structures favoring one distinct region of the moiré supercell. Scanning tunneling microscopy and X-ray photoelectron spectroscopy measurements show that a highly periodic hydrogen functionalized graphene sheet can thus be prepared by controlling the sample temperature (Ts) during hydrogen functionalization. At deposition temperatures of Ts = 645 K and above, hydrogen adsorbs exclusively on the HCP regions of the graphene/Ir(111) moiré structure. This finding is rationalized in terms of a slight preference for hydrogen clusters in the HCP regions over the FCC regions, as found by density functional theory calculations. Angle-resolved photoemission spectroscopy measurements demonstrate that the preferential functionalization of just one region of the moiré supercell results in a band gap opening with very limited associated band broadening. Thus, hydrogenation at elevated sample temperatures provides a pathway to efficient band gap engineering in graphene via the selective functionalization of specific regions of the moiré structure.

Iron phthalocyanine on Cu(111): Coverage-dependent assembly and symmetry breaking, temperature-induced homocoupling, and modification of the adsorbate-surface interaction by annealing.

We have examined the geometric and electronic structures of iron phthalocyanine assemblies on a Cu(111) surface at different sub- to mono-layer coverages and the changes induced by thermal annealing at temperatures between 250 and 320 °C by scanning tunneling microscopy, x-ray photoelectron spectroscopy, and x-ray absorption spectroscopy. The symmetry breaking observed in scanning tunneling microscopy images is found to be coverage dependent and to persist upon annealing. Further, we find that annealing to temperatures between 300 and 320 °C leads to both desorption of iron phthalocyanine molecules from the surface and their agglomeration. We see clear evidence of temperature-induced homocoupling reactions of the iron phthalocyanine molecules following dehydrogenation of their isoindole rings, similar to what has been observed for related tetrapyrroles on transition metal surfaces. Finally, spectroscopy indicates a modified substrate-adsorbate interaction upon annealing with a shortened bond distance. This finding could potentially explain a changed reactivity of Cu-supported iron phthalocyanine in comparison to that of the pristine compound.

Increased availability of NADH in metabolically engineered baker's yeast improves transaminase-oxidoreductase coupled asymmetric whole-cell bioconversion.

BACKGROUND: Saccharomyces cerevisiae can be engineered to perform a multitude of different chemical reactions that are not programmed in its original genetic code. It has a large potential to function as whole-cell biocatalyst for one-pot multistep synthesis of various organic molecules, and it may thus serve as a powerful alternative or complement to traditional organic synthetic routes for new chemical entities (NCEs). However, although the selectivity in many cases is high, the catalytic activity is often low which results in low space-time-yields. In the case for NADH-dependent heterologous reductive reactions, a possible constraint is the availability of cytosolic NADH, which may be limited due to competition with native oxidative enzymes that act to maintain redox homeostasis. In this study, the effect of increasing the availability of cytosolic NADH on the catalytic activity of engineered yeast for transamination-reduction coupled asymmetric one-pot conversion was investigated. RESULTS: A series of active whole-cell biocatalysts were constructed by over-expressing the (S)-selective ω-transaminase (VAMT) from Capsicum chinense together with the NADH-dependent (S)-selective alcohol dehydrogenase (SADH) originating from Rhodococcus erythropolis in strains with or without deletion of glycerol-3-phosphate dehydrogenases 1 and 2 (GPD1 and GPD2). The yeast strains were evaluated as catalysts for simultaneous: (a) kinetic resolution of the racemic mixture to (R)-1-phenylethylamine, and (b) reduction of the produced acetophenone to (S)-1-phenylethanol. For the gpd1Δgpd2Δ strain, cell metabolism was effectively used for the supply of both amine acceptors and the co-factor pyridoxal-5'-phosphate (PLP) for the ω-transaminase, as well as for regenerating NADH for the reduction. In contrast, there was nearly no formation of (S)-1-phenylethanol when using the control strain with intact GPDs and over-expressing the VAMT-SADH coupling. It was found that a gpd1Δgpd2Δ strain over-expressing SADH had a 3-fold higher reduction rate and a 3-fold lower glucose requirement than the strain with intact GPDs over-expressing SADH. CONCLUSIONS: Overall the results demonstrate that the deletion of the GPD1 and GPD2 genes significantly increases activity of the whole-cell biocatalyst, and at the same time reduces the co-substrate demand in a process configuration where only yeast and sugar is added to drive the reactions, i.e. without addition of external co-factors or prosthetic groups.