ZUSAMMENFASSUNG Einzelatom‐Katalysatoren („Single‐Atom Catalysts“, SACs) bieten durch die Stabilisierung isolierter Metallatome auf oxidischen Trägermaterialien maximale Effizienz. Platin auf Ceroxid (CeO 2 ) ist ein wichtiges SAC‐System, in dem adsorbiertes CO aufgrund der Rückdonation von Pt typischerweise rotverschobene Schwingungsmoden aufweist. Mithilfe der polarisationsaufgelösten IR‐Spektroskopie an Pt‐beschichteten CeO 2 (111)‐Einkristalloberflächen beobachten wir bei niedriger Bedeckung keine CO‐Schwingungsbanden unterhalb von 2140 cm −1 , was darauf hindeutet, dass oberflächengebundene Pt‐Atome nicht in nachweisbaren Mengen vorliegen. DFT‐Berechnungen zeigen, dass diese unerwartete Beobachtung einen starken Hinweis darauf liefert, dass Pt‐Atome vergrabene Zwischengitterplätze besetzen. Solche unter der Oberfläche liegenden Positionen, wurden in früheren Studien nicht berücksichtigt, sind aber bei niedriger Bedeckung thermodynamisch begünstigt, nehmen einen ungewöhnlichen Oxidationszustand an und sind für eine direkte CO‐Bindung unzugänglich. Unsere Ergebnisse stellen die bislang vorherrschende Annahme, dass Einzelatome an der Oberfläche gebunden bleiben, infrage, und unterstreichen die entscheidende Rolle von Subsurface‐Zwischengitterspezies. Dieser Befund erfordert ein Umdenken bei der Stabilisierung aktiver Zentren in Einzelatomkatalysatoren auf reduzierbaren Oxiden.

The transformation of CO(2 )and green hydrogen into methanol presents a sustainable route for chemical and fuel production. Conventional methanol synthesis catalysts, such as Cu/ZnO/Al2O(3), employ Al2O(3) as a structural promoter, while Ga(2)O(3 )has recently emerged as a promising alternative. This study compares Cu-based catalysts supported on Al2O(3 )(CA) and Ga2O(3 )(CG), prepared via coprecipitation of layered double hydroxide precursors with identical molar Cu:M (M = Al or Ga) ratio of 70:30. Using in situ and operando X-ray absorption spectroscopy and X-ray powder diffraction, we investigate the structural and redox dynamics of Ga during activation and CO2 hydrogenation. Gallium from its precursor state undergoes several phase transitions. At elevated temperatures, Ga exhibits redox activity, transitioning from Ga(3+ )to metallic Ga0 and forming CuxGay alloys at 480 degrees C, followed by de-alloying and re-oxidation at even higher temperatures. Our results suggest that the beneficial role of Ga reported in literature arises from metal-oxide interfacial effects rather than bulk alloying. Excess Ga2O(3) leads to low conversion levels and pronounced deactivation compared to the Al2O3-supported Cu catalyst and thus should be prevented. These findings highlight the importance of controlling promoter loading and dynamic behavior in catalyst design to optimize activity, stability, and selectivity for CO2-to-methanol conversion.

We report a nickel-catalyzed reductive cross-coupling of xanthate esters and aryl and alkenyl iodides. Utilizing xanthate esters with an S-allyl substituent leads to higher yields compared to the more commonly used S-Me xanthate esters. The reaction is tolerant toward a wide array of functional groups and allows for the conversion of xanthate esters derived from pharmaceuticals to diarylmethanes. Electronic properties of the reaction partners have a strong influence on the reaction. Mechanistic investigations revealed that the benzylic radical is generated from the xanthate ester, presumably by single electron transfer by Ni(I).

For increasing the noble metal efficiency and developing more robust emission control catalysts, an accurate understanding of their structural dynamics during operation is essential. This study systematically investigates the manifold interactions of Rh with various catalyst components in alumina/ceria-zirconia based washcoats by advanced in situ/operando characterization. Pronounced structural changes, which drastically affect the catalyst performance, were observed during short fuel-cut steps. Operando X-ray absorption spectroscopy and electron microscopy investigations revealed the formation of mobile metallic Rh particles under stoichiometric/rich reaction conditions. In contrast, Rh oxidation, volatilization and diffusion into the alumina surface/subsurface occurred above 950 degrees C during O2-rich steps. While the first phenomenon contributed to the redistribution of Rh on both supports, the partial relocation of Rh in low-loaded washcoats under lean reaction conditions was exclusively observed to occur from CeO2-ZrO2 to gamma-Al2O3. Overall, the intimate interaction between the washcoat components prevented the loss of the noble metal due to volatilization.

Pt through the gas phase in the form of volatile PtO2 has become a topic of interest within recent years due to its application in the production of single atom catalysts. It is furthermore important for oxidation reactions that take place at high temperatures, e.g., CH4 oxidation in order to track noble metal loss. Here, platinum migration is observed on the nanometer scale for mixed Pt/Al2O3 and CeO2 nanoparticles as grinded powders. Furthermore, Pt migration within a reactor in a dual bed of Pt/Al2O3 followed by a bed of sieved CeO2 particles is tracked in situ on the millimeter scale via time and spatially resolved X-ray absorption spectroscopy. It is observed that gaseous PtO2 is first captured at the beginning of the CeO2 bed. When the beginning of the bed appears saturated, PtO2 adsorbs further downstream. Such adsorption behavior has to our knowledge not yet been reported in the literature since it requires time and spatially resolved in situ tracking. Furthermore, preferential adsorption sites of Pt on CeO2 were identified using experimental extended X-ray absorption fine structure data and fitting based on models from Density Functional Theory calculations. They point to geometries as, for example, found in 4-fold hollow sites on CeO2 (110) with additional ligands for stabilization.

Power-to-liquid processes are considered a crucial route to substitute petroleum-based processes for the production of valuable chemicals and liquid fuels, opening the possibility for the production of sustainable aviation fuels. Two selected catalysts, 20wt.%Co/Al2O3 (CA) and 20wt.%Co/TiO2-SiO2 (CTS), were tested in the Fischer-Tropsch synthesis (FTS) reaction under industrially relevant conditions. The CTS catalyst showed 10% higher activity, improved C5+ selectivity, and lower methane formation than the CA catalyst. The components of these complex catalysts were characterized, and the structural evolution of cobalt during activation and reaction was monitored by various in-situ and operando methods. H2 temperature-programmed reduction, X-ray diffraction, magnetometry, and X-ray absorption spectroscopy revealed a decrease in the temperature of stepwise cobalt reduction in CTS catalysts compared to CA. The reduction behavior of Co during activation was dependent on the support nature and strength of the metal-support interaction. The formation of composites with the support was proposed based on the results obtained by bulk probing XAS and more surface-sensitive NEXAFS methods. XRD, magnetometry, and XAS revealed that the size of the activated Co particles was dependent on the support chosen. The DFT modeling results confirmed the higher affinity of cobalt for titania, which might explain the smaller Co crystallite size and higher stability observed for the CTS catalyst. The difference in activity between the two commercially relevant cobalt-based catalysts on different supports could be linked to the difference in pore sizes and the strength of the metal-support interaction.

Bridging the gap between theoretically predicted and measured metal oxides charges is crucial when using synergistically computational and experimental techniques to ensure reliability of the models. Manganese oxide, with its wide range of oxidation states, is an ideal candidate for probing this relationship. Here, we conducted a magnetic moment and Bader charge analysis using two different exchange correlation functionals, i.e. RPBE and BEEF-vdW, both with DFT (U = 0 eV) and DFT+U (U = 2.5 eV), on several bulk and cluster manganese oxides with formal Mn oxidation states ranging between +1 and +7. We found that the relationship between Mn formal oxidation states and magnetic moments in both bulk and molecular structures can be described accurately by a quadratic fit. In comparison, the relationship between formal oxidation states and Bader charge is more uncertain, and could be fit by a single hyperbolic function only upon correction of the formal oxidation state via the Madelung constant. Finally, we employed the derived correlations to predict the formal oxidation state of Mn in different MnxOyHz clusters on fcc-Co(111). Both methods predict that the Mn oxidation states largely do not align with the stoichiometry of the clusters. While the magnetic moments correlation always yielded Mn oxidation states of +2, the results obtained from the Bader charges were more dependent on the cluster stoichiometry.

Single-atom catalysts (SACs) offer maximal efficiency by stabilizing isolated metal atoms on oxidic supports. Platinum on cerium oxide (CeO2) is a key SAC system, where adsorbed CO typically exhibits red-shifted vibrational modes due to Pt back-donation. Using polarization-resolved IR spectroscopy on Pt-deposited CeO2(111) single-crystal surfaces, we do not observe CO vibrational bands below 2140 cm-1 at low coverages, indicating that the surface-bound Pt atoms are not present in detectable amounts. DFT calculations demonstrate that this unexpected observation is consistent with Pt atoms occupying buried interstitial sites. Such subsurface single-atom sites, not considered in previous studies, are thermodynamically favored at low coverages, adopt an unusual oxidation state, and are inaccessible to direct CO binding. Our findings challenge the prevailing assumption that single atoms remain surface-bound and highlight the critical role of subsurface interstitial species, prompting a rethinking of how active sites in single-atom catalysts are stabilized on reducible oxides.

In this work, we have performed a quantum chemical investigation for the selective oxidation of propane toward acrylic acid on the M1 phase of a mixed metal oxide (MMO) catalyst, consisting of Mo-Te-Nb-O. The M1 phase of the catalyst has a complex surface structure, which involves different arrangements of metal sites with variable oxidation states. This complexity makes it inherently difficult to understand its activity and selectivity in catalytic reactions. We have used a multilayer cluster model of the main catalytically active site of M1 and a hybrid DFT methodology to establish the minimum energy pathways for the propane oxidation to acrylic acid via propylene, allyl alcohol, and acrolein as the key intermediates. In addition, the reactivity of propyl radicals toward the formation of isopropanol, which leads the reaction toward an unselective path of CO/CO2 generation instead of acrylic acid production, has also been depicted. We show that the formation of isopropanol has rather a low activation barrier and is therefore competing with the formation of propylene from the propyl radical after C-H activation of propane. Once propylene has formed, the allyl position can easily be activated to form acrolein, which can be further oxidized to acrylic acid. In addition, we have developed a more general linear scaling relation for C-H activation chemistry to estimate activation barriers on M1 catalysts only based on four key energetic descriptors, which are the hydrogen binding energy (E H) on the surface site, the C-H bond dissociation energy (E BDE) of the reactant molecule in the gas phase, the interaction energy at transition state structure (E int TS), and the interaction energy between metal site and the oxygen atom of oxygenated gas molecules (E MO).

The utilization of renewable hydrogen that was transported and stored as green ammonia relies on a highly efficient ammonia decomposition catalyst. Industrial demand for high-purity hydrogen requires operation at high temperature of at least 500 °C due to the endothermicity of the reaction. At these conditions, abundant Ni/MgO catalysts with nanoparticles larger than 5 nm are not limited by the widely studied N-N recombination, but the rate-determining step (RDS) has changed to ammonia dehydrogenation. A combined experimental and theoretical study on size-controlled Ni/MgO catalysts explains this behavior in a dual-site / dual-RDS model by the interplay of a size-induced lack of edge sites that are active in ammonia dehydrogenation, and an increase in the free energy barrier of this step relative to the N-N recombination. Catalyst design thus needs to aim at a stabilization of an optimal Ni nanoparticle size at high reaction temperatures to balance the two possible kinetic limitations by an optimal relative abundance of the edge sites active in ammonia dehydrogenation and the B5 sites for N-N recombination. Such stabilization is demonstrated for co-precipitated and ex-solved Ni/MgO catalysts where the optimal size is determined to be 5-6 nm. Future catalyst development for high-temperature ammonia decomposition should target the promotion of the ammonia dehydrogenation sites on larger particles and /or the modification of smaller particles to facilitate the N-N recombination step.

Single-atom catalysts (SACs) provide isolated, well-defined metal sites that are suited for mechanistic modeling in porous materials such as metal-organic frameworks (MOFs). However, the influence of framework topology and mass transport on catalytic outcomes remains poorly understood. Here we develop a multiscale kinetic model for ethylene oligomerization in Ni-grafted NU-1000 that combines density functional theory (DFT)-derived free-energy barriers with adsorption and diffusion descriptors. The framework predicts product distributions under realistic reaction conditions. The simulations show that flow-mode operation favors selective C4H8 formation across a temperature range. This selectivity window progressively narrows with increasing effective diffusion length and catalytic-site density, as longer residence times enhance chain growth beyond dimerization. In contrast, batch-mode operation shifts the product distribution toward heavier olefins. These trends provide practical guidance for tuning operating conditions and material properties to achieve desired selective Ni-MOF catalysts.

The conversion of methane to methanol by the enzyme particulate methane monooxygenase (pMMO) represents an important sink in the global methane cycle. However, the exact molecular mechanism of the enzymatic reaction is unknown. An inorganic model system of pMMO is Cu/ZSM-5. There is experimental and theoretical evidence that the CH4-to-CH3OH conversion proceeds at a mono μ-oxo dicopper site in the latter system. We have prepared a discrete molecular complex exhibiting a Cu2O core and found that it catalyzes the monooxygenation of aliphatic substrates with bond dissociation energies (BDEs) up to 82 kcal/mol. Herein, we show by theoretical calculations that this system exhibits an energy profile for the activation of C-H bonds that is comparable to that of Cu/ZSM-5 catalysts.

Aromatics play a key role in the methanol-to-olefins process, both as a co-catalyst for the production of short chain olefins and as a key reaction intermediate towards coke formation, but mechanistic details on how aromatics form are still scarce.

Single-atom catalysts (SACs) on reducible oxides are generally assumed to consist of isolated metal atoms located at the surface and directly accessible to reactants. Recent studies on Pt/CeO 2 (111), however, revealed that Pt preferentially occupies subsurface interstitial sites. Here, we investigate whether this behavior is unique to Pt or represents a more general phenomenon. Using density functional theory, we systematically compare the stability of surface and subsurface configurations for transition-metal atoms from Fe to Au on stoichiometric CeO 2 (111). Pronounced periodic trends emerge: the tendency toward subsurface incorporation increases from the 3d to the 5d series and from the Fe group toward the Ni group, with Pt and Pd exhibiting the strongest preference. Several metals also induce oxygen migration and surface reconstruction, generating additional stable configurations. Calculated CO vibrational frequencies provide spectroscopic fingerprints for experimental identification. These results demonstrate that subsurface incorporation is a general structural motif in SACs on reducible oxides, challenging the conventional view of surface-bound single-atom catalysts.

Hydrodeoxygenation (HDO) is a pivotal process in the efficient utilization of biomass, with ruthenium (Ru) emerging as a highly effective catalyst for this reaction. A dimer model compound, more representative of bio-oil oligomers than monomers, was used to explore the HDO mechanism over a Ru catalyst through both density functional theory (DFT) calculations and experimental studies. Initially, the adsorption of 2-Phenylethyl phenyl ether (PPE) was examined through DFT, leading to the determination of an optimized structure. Subsequent calculations of the HDO reaction pathways on the Ru (0001) surface revealed that the beta-O-4 linkage cleavage occurred with significantly low activation energy. For the experimental study, a Ru/Nb2O5 catalyst was synthesized using wet impregnation method. Characterization of this catalyst through scanning electron microscopy (SEM) and X-ray diffraction (XRD) confirmed its congruence with the DFT model. The catalytic performance of Ru/Nb2O5 was evaluated in the PPE HDO process, where it demonstrated high efficiency. The applicability of the Ru/Nb2O5 catalyst was extended to a real lignin bio-oil so as to further assess its effectiveness. This research provides a systematic study on PPE HDO over a Ru catalyst, illustrating the potential of using dimer model compounds in HDO mechanism investigations and the promising capabilities of Ru-based catalysts.

In this work, we have employed a combined density functional theory (DFT)-Monte Carlo (MC) approach to produce structural models of Co nanoparticles (NPs), widely employed in the Fischer-Tropsch (FT) synthesis for the production of sustainable aviation fuels (SAFs), in the 2-10 nm size range including the effects of temperature and metal-support interactions (MSI). We make use of a lattice model where the energy of Co atoms is estimated based on their first-shell coordination number (CN), an approach that was validated via DFT calculations. We report a marked increase in step and kink sites at the expense of terraces with increasing particle size, which we linked to the experimentally observed increase in turnover frequency (TOF). Increasing MSI led to a flattening of the NPs on the support as well as to decreasing Co dispersion but hardly affected the site distribution, suggesting that they do not alter the NPs intrinsic activity. We additionally report the size-dependent surface energies and chemical potentials of Co NPs, which are both shown to decrease fast in the 2-6 nm size range and approach convergence afterward. Our models provide a description of these quantities accounting simultaneously for particle size, nonideality of surface morphologies, temperature, and MSI and thus overcome several approximations that previous studies had to rely on.

The activation of methane under mild conditions is a challenging but rewarding goal; the underlying key parameters, however, remain elusive. In this study on isolated tantalum Ta2 + compounds exposed to methane in a ring-electrode ion trap, strong changes in the reactivity are observed depending on the compound's degree of oxidation. While the general reaction behavior is presented for species ranging from Ta2 + to Ta2O6 + based on experimental kinetic studies, we focus in more detail on the dehydrogenation reactions occurring on Ta2O2 + and the hydrogen atom transfer (HAT) on Ta2O5 +, for which density functional theory calculations were performed. In the first part, we elucidate the role of Ta-C-Ta bridging motifs in product structures as driving forces for the dehydrogenation of methane on Ta2O2 +; in the second part, we investigate the origins of the HAT - a hitherto unknown reaction scheme for binary tantalum oxides. For the latter, we show that the reactivity originates from the spin density on oxygen atoms, which is a typical characteristic of the reaction on other metal oxides. This reflects a change in the reactivity from oxidized metallic systems to metal oxides and demonstrates that chemical modifications of tantalum compounds can achieve different methane activation schemes.