Christoph Mahr

Nanoporous gold (npAu) has garnered significant attention as a highly active and selective oxidation catalyst, particularly at low temperatures. While most studies have focused on npAu derived from AuAg alloys, the incorporation of other less noble metals (LNMs), such as Cu, was shown to be a viable opportunity for tuning and enhancing its catalytic properties. The interplay between multiple LNMs present at the same time remains underexplored though. While trimetallic systems like npAu(CuAg) offer the chance for broadening the catalytic scope of npAu even further, a key challenge in utilizing them lies in understanding whether synergistic interactions or a combination of effects exerted by the individual LNMs determine their catalytic performance. To address this, we developed a reliable and reproducible method for synthesizing npAu(CuAg) and compared the trimetallic catalysts to their bimetallic counterparts npAu(Ag) and npAu(Cu) with respect to their catalytic activity for aerobic CO oxidation. Using controlled dealloying, we synthesized npAu(CuAg) with ∼20 nm ligaments, exhibiting an Ag‐rich surface, and a Cu‐enriched bulk. These trimetallic catalysts showed high conversion over a broad temperature regime ranging from room temerature to 250 °C. Electron microscopy and X‐ray photoelectron spectroscopy revealed that during activation under reaction conditions Cu segregates to the surface at elevated temperatures so that finally Ag as well as Cu sites are available enabling the activation of oxygen – on the former at low temperatures and on the latter in the high temperature region >120 °C. In this way, an overall temperature window for catalytic activity is achieved which encompasses the range of npAu(Ag) as well as that of npAu(Cu). At the same time, the thermal stability of the nanoporous network is improved, demonstrating in essence that the ternary alloy outperforms the binary systems by unifying catalytically relevant features of both LNMs.

In this publication, we study strain in κ-(InxGa1−x)2O3/κ-Ga2O3/Al2O3 heterostructures grown by plasma-assisted molecular beam epitaxy using a combination of density functional theory (DFT), x-ray diffraction (XRD), nanobeam electron diffraction (NBED), and energy-dispersive x-ray spectroscopy (EDX) for layers with low (2%) and high (18%) In concentration. Lattice parameters and elastic moduli were computed using the density functional theory formalism. The strain between κ-(InxGa1−x)2O3 and κ-Ga2O3 as well as Al2O3 and κ-Ga2O3 was measured in two different zone axes by NBED. Due to the quasi-hexagonality of the κ-phase, this yields an approximation for the full strain tensor. The obtained NBED results are supported by reciprocal space maps from XRD measurements. The relaxation of the layers was analyzed by comparing the measured strain data with those computed from the DFT lattice parameters and elastic moduli using the element concentrations measured by EDX, thus allowing for direct validation of the theoretical calculations by experimental data. This analysis results in satisfactory agreement and reveals that the layer with a low In concentration is fully strained (measured εzz=0.0037, computed εzz=0.0039), whereas the layer with high In concentration is fully relaxed (measured εzz=0.0222, computed εzz=0.0219). NBED, XRD, and theoretical predictions agree within their respective error margins for the strain between κ-(InxGa1−x)2O3 and κ-Ga2O3. Overall, the absolute value of the strain between κ-Ga2O3 and Al2O3 was measured to be smaller by NBED (measured εzz=−0.0628) and XRD than it was computed from literature lattice parameters (εzz=−0.0716).

We investigate the composition of α-phase intermediate layers at epitaxial Ga2O3/Al2O3 interfaces using high angle annular dark field scanning transmission electron microscopy. Their presence is considered a general phenomenon as they are observed independent of the growth technique [Schewski et al., Appl. Phys. Exp. 8, 011101]. Samples were grown by plasma assisted molecular beam epitaxy using different growth conditions. Almost independent of these, the quantitative evaluation of the measured intensities gave Ga concentrations of ∼25%. We show that the previously published model, based on a pure α-Ga2O3 interlayer, fails if it is adapted to the measured composition. Density functional theory (DFT) computations were used to overcome the approximations made in this model and suggest that a stabilization of the layer is possible due to the low Ga concentration (≤35%) at which the α-phase is the most stable. Our surface model computations suggest an exchange of Ga atoms at the surface with Al atoms from the underlying substrate as a possible formation mechanism.

Dry reforming of methane (DRM) is an attractive reaction for synthesis gas production, since it converts two greenhouse gases into valuable chemical feedstock. Silica supported bimetallic catalysts with constant metal loading but varying Ni/Co ratios (x_Ni=1, 0.8, 0.6, 0.5, 0.4, 0.2, 0) were produced via spray‐drying for the application in DRM, aiming at the identification of compositions with superior activity. In this study, building block particles refer to an inert framework, supporting the catalyst. The elemental distribution of Si, Ni, and Co within such building block particles was evaluated using scanning transmission electron microscopy. Additionally, X‐ray diffraction and temperature‐programmed reduction experiments confirmed that Ni and Co initially exist in an oxidic state within the SiO₂ framework after production and later transform into an alloyed metallic state upon reduction with hydrogen, as confirmed by X‐ray photoelectron spectroscopy. A systematic comparison of the activity over a temperature range (323–1150 K) was performed using temperature scanning measurements. The highest intrinsic activity was found with the bimetallic Ni₄₀Co₆₀ particles.

Nanoporous gold (npAu) attracted increasing attention over the last 20 years as a highly active and selective oxidation catalyst in particular at low temperatures. Previous research mainly focused on npAu that was fabricated by corrosive dealloying of AuAg parent alloys. Yet, the use of other binary alloys, such as AuCu, promises interesting variations of the catalytic properties, when considering that residual amounts of the less noble metal were shown to be co‐catalytically involved. Aiming at providing a platform for systematic studies in this direction for Cu, we not only dealt with strategies for a reliable and reproducible preparation of npAu(Cu) catalysts from AuCu, but also with their potential for CO oxidation in comparison to npAu(Ag). We were able to develop an approach based on thermally quenched Au_0.3Cu_0.7 alloys, providing distinct synthetic advantages as a starting material for the catalyst fabrication versus the thermodynamically more stable AuCu₃ intermetallic compound. Using PCD (potentiostatically controlled dealloying), well‐defined pore structures with ligament diameters of ∼40 nm and variable residual Cu concentrations in the range between ∼0.6 at % and ∼1.2 at % could be straightforwardly obtained. After activating such catalysts at 150 °C, they reproducibly showed catalytic activity for aerobic CO oxidation in a broad temperature window between 40 °C and 250 °C. As opposed to npAu(Ag), the activity increased with decreasing residual Cu content, outperforming the former at temperatures above ∼60 °C not only with respect to CO₂ formation rates but also with respect to thermal stability. Based on X‐ray photoelectron spectroscopic and transmission electron microscopic results, it was possible to conclude that Cu segregates to the surface and, with rising Cu bulk content, increasingly occurs in form of Cu²⁺ species at the surface. While the latter are expected to be catalytically inactive, Cu and Cu⁺ species are likely candidates for the activation of oxygen being not possible on pure Au.

Dealloying of Ag–Au alloy nanoparticles (NPs) strongly differs from the corresponding bulk alloy materials. Here, we have investigated the effects of potentiodynamic and potentiostatic dealloying on structure and distribution of residual Ag atoms for Au rich NPs. Two different sizes of Ag rich alloy NPs, 77 ± 26 nm Ag₇₇Au₂₃ and 12 ± 5 nm Ag₈₆Au₁₄, were prepared. 77 nm Ag₇₇Au₂₃ NPs form a homogeneous alloy, while 12 nm Ag₈₆Au₁₄ NPs show an Ag rich shell–Au rich core arrangement. The two groups of as-prepared NPs were dealloyed either under potentiodynamic (0.2–1.3 V_RHE) or potentiostatic (0.9, 1.2, and 1.6 V_RHE) conditions in 0.1 M HClO₄. For the initial 77 nm Ag₇₇Au₂₃ NPs, both dealloying protocols lead to pore evolution. Interestingly, instead of homogenous Ag distribution, numerous Ag rich regions form and locate near the pores and particle edges. The critical dealloying potential also differs by ∼500 mV depending on the dealloying method. The initial 12 nm Ag₈₆Au₁₄ NPs remain dense and solid, but Ag distribution and thickness of the Au passivation layer vary between both dealloying protocols. When the Au passivation layer is very thin, the residual Ag atoms tend to segregate to the particle surface after dealloying. Due to the size effect, small NPs are less electrochemically stable and show a lower critical dealloying potential. In this systematic study, we demonstrate that the mobility of Au surface atoms and dealloying conditions control the structure and residual Ag distribution within dealloyed NPs.

Formation of hetero‐contacts between particles of different materials in nanoparticle hetero‐aggregates can lead to new functional properties. Improvement of the functional behavior requires a detailed characterization of mixing between the two types of particles, in order to correlate different mixing with the performance of the material. Scanning transmission electron microscopy (STEM) is an option for this task. To obtain statistically relevant results, STEM‐images of many hetero‐aggregates have to be acquired and evaluated. This can be time‐consuming if it is done manually. In the present work, the applicability of convolutional neural networks for the automated analysis of STEM‐images acquired from TiO–WO nanoparticle hetero‐aggregates is investigated. Hetero‐aggregates are obtained in a double flame spray pyrolysis (DFSP) setup, in which a variation of setup parameters is expected to affect the mixing of TiO and WO. Mixing is investigated by a measurement of cluster sizes (the number of connected particles of the same material within an aggregate) and coordination numbers (the number of particle contacts with particles of the same or the different material). Results show that the distribution of measured values is wide for both quantities, rendering it challenging to correlate mixing with parameters varied in the DFSP setup.

Hetero‐contacts are interfaces between different materials at the nanoscale leading to novel functional properties. In hetero‐aggregates, primary particles of at least two different materials are mixed at primary particle or cluster level. Double flame spray pyrolysis (DFSP) is a versatile technique for the controlled synthesis of such materials. Characterization of hetero‐aggregates by scanning transmission electron microscopy (STEM) requires acquisition and evaluation of many aggregate images in order to derive statistically significant results. Usually, STEM energy dispersive X‐ray spectroscopy (EDXS) is used to acquire elemental maps providing the material distribution of the primary particles within hetero‐aggregates. However, the acquisition of a single EDXS map takes up to several minutes. For this reason, determination of material types of primary particles from the intensity in high‐angle annular dark field STEM images alone is desirable. These images can be acquired within a couple of seconds. In the present work, a method is suggested which allows for achieving this objective. It can be applied to distinguish materials with a significant difference in their atomic number and hence sufficient material contrast in the STEM images.

Nanoporous gold (NPG) is characterized by a bicontinuous network of nanometer-sized metallic struts and interconnected pores formed spontaneously by oxidative dissolution of the less noble element from gold alloys. The resulting material exhibits decent catalytic activity for low-temperature, aerobic total as well as partial oxidation reactions, the oxidative coupling of methanol to methyl formate being the prototypical example. This review not only provides a critical discussion of ways to tune the morphology and composition of this material and its implication for catalysis and electrocatalysis, but will also exemplarily review the current mechanistic understanding of the partial oxidation of methanol using information from quantum chemical studies, model studies on single-crystal surfaces, gas phase catalysis, aerobic liquid phase oxidation, and electrocatalysis. In this respect, a particular focus will be on mechanistic aspects not well understood, yet. Apart from the mechanistic aspects of catalysis, best practice examples with respect to material preparation and characterization will be discussed. These can improve the reproducibility of the materials property such as the catalytic activity and selectivity as well as the scope of reactions being identified as the main challenges for a broader application of NPG in target-oriented organic synthesis.