Henrik Grönbeck

Copper exchanged chabazite (Cu-CHA) is the preferred choice for abatement of NOx from lean burn engines. The low-temperature activity depends critically on the presence of paired [Cu(NH3)2]2 + complexes, which are required for O2 adsorption. The pairing of [Cu(NH3)2]2 + complexes has so far been studied by ab initio molecular dynamics or more recently molecular dynamics using machine learning force-fields. An alternative to molecular dynamics is kinetic Monte Carlo (kMC) simulations, which enable simulations over longer time scales. In this study, an on-the-fly kMC model is developed where the rates for [Cu(NH3)2]2 + diffusion are evaluated on the fly using a machine learning force field (ML-FF). We find that the fraction of paired complexes depends sensitively on the zeolite composition and the aluminum distribution. The results reveal a large distribution of Cu pair stabilities and that only few Cu ions are paired, demonstrating the heterogeneous nature of Cu-CHA. A mean field model based on the fraction of paired complexes is used to explore how changes in zeolite composition affect the catalytic activity. The methodology of combining kMC and ML-FF to account for dynamic changes in catalyst structure during reaction conditions is general and provide a possibility to go beyond the often used static approximation when evaluating reaction rates.

SO ₂ poisoning is independent of hydrothermal aging in deactivation of Cu–CHA catalysts for NH ₃ -SCR of NO _x .

Skeletal editing enables precise structural modifications of molecules at late stages of a synthetic sequence. We recently demonstrated skeletal editing on a single-molecule scale. Voltage pulses applied by the tip of a scanning probe microscope to an oxygen-containing seven-membered heterocycle led to both oxygen deletion and ring-contraction rearrangement reactions. An open question is whether selective skeletal editing of a heterocyclic core can be achieved by the choice of heteroatom. Here we show that tip-induced reactions of an analogous sulfur-containing seven-membered ring mainly result in sulfur deletion. Our results demonstrate that the combination of tip-induced chemistry and heteroatom selection in the molecular design is a powerful strategy for single-molecule skeletal editing, potentially enabling diverse structural transformations of heterocyclic frameworks.

Charge transfer across the oxide/metal interface modifies catalytic sites.

In this study, the nature of ammonia storage sites in H-SSZ-13 and Cu-SSZ-13 with different Si/Al and Cu/Al molar ratios is characterized by NH ₃ sorption experiments using in situ DRIFT spectroscopy, and by NH ₃ -TPD flow reactor experiments.

Liquid organic hydrogen carriers (LOHCs) enable storage and transport of H₂ using existing liquid fuel infrastructure, through reversible catalytic hydrogenation and dehydrogenation of organic molecules. A viable, commercially deployed, LOHC system is the methylcylohexane (C₇H₁₄)/toluene (C₇H₈) system. However, the Pt-based catalysts, which are used for both hydrogenation of C₇H₈ and dehydrogenation of C₇H₁₄, also promote unwanted C-C bond scission, leading to demethylation and LOHC degradation. One route to prevent C-C bond scission over Pt catalysts is to mildly poison the catalyst with sulfur. Using density functional theory (DFT) calculations and mean-field kinetic modeling, we show that the presence of sulfur, besides suppressing demethylation, also improves the C₇H₁₄ dehydrogenation activity. The enhanced performance is a result of sulfur destabilizing neighboring co-adsorbates, which promotes C₇H₈ desorption. Our computational results for dehydrogenation over Pt surfaces reproduce experimental trends for alumina-supported Pt nanoparticles, suggesting that controlled, moderate sulfur poisoning is a universal means to improve the overall performance of Pt catalysts for LOHC dehydrogenation.

The oxygen reduction reaction (ORR) on platinum is limited by a substantial overpotential, which hampers the efficiency of fuel cell technologies. While adsorbate binding energies have been widely used to explain ORR kinetics, we here illustrate a more complex role of platinum surface oxides, which are often ambiguously defined in the literature. We use operando total reflection X-ray absorption fine structure spectroscopy (RefleXAFS), supported by X-ray photoelectron spectroscopy, density functional theory, and microkinetic modeling, to resolve the surface oxides on polycrystalline platinum and their impact on ORR. We identify the formation of a surface oxide as early as 1 V_RHE in 0.1 M HClO₄ and demonstrate that platinum spontaneously oxidizes at the open-circuit potential (OCP) under O₂ saturation. Furthermore, we show that the oxide coverage increases with upper vertex potential, slower scan rates, and extended hold times at OCP, illustrating how oxides inhibit ORR during fuel cell start-up. Crucially, we demonstrate that the ORR onset is delayed until these oxides are reduced, establishing a direct, negative relationship between oxide coverage and ORR activity. This reveals a revised mechanism in which the potential-determining step is the reduction of surface oxides, and the slow kinetics of this restructuring ultimately determine when surface sites become catalytically available.

Hydrothermal aging of Cu-CHA for NH ₃ -Selective Catalytic Reduction (NH ₃ -SCR) follows two types of deactivation mechanisms.

Composite nanomaterials are crucial in a wide range of applications with the industrial Cu/ZnO catalyst used to convert CO ₂ into methanol being an important example. The reaction rate scales with the Cu surface area ^1,2 implying that the rate-limiting part of the reaction occurs on the metal surface. However, the turnover frequency (TOF, rate per Cu surface atom) is one order of magnitude higher for Cu supported on ZnO compared to pure Cu samples ^3–6 . This materials synergy, which is responsible for 90% of the catalytic activity, is still poorly understood ^7–9 , and the copper in Cu/ZnO is sometimes described ^10,11 as an element with properties radically different from the pristine metal. An understanding of this synergy thus has a direct impact on Power-to-X processes to produce methanol for storage of renewable energy and on heterogeneous catalysis in general. Here we establish the mechanism for the hugely important conversion of CO2 into methanol and how it is accelerated on the composite nanomaterial. We show that charge from donor states created by ad-/absorbed H in the ZnO transfers to the metal and distributes across the metal surface. The surface charging influences the bonding of the adsorbates on the metal and lowers the energy barrier for HCOOH dissociation, which we identify as the rate-limiting step in methanol formation. The lower barrier accounts for the order of magnitude increase in the methanol synthesis rate. The charge transfer phenomenon is general in nature and found to be essential for understanding catalytic phenomena and thus for the rational development of improved future catalysts.