(3go) Polyoxometalate-Supported Single-Atom Catalysts
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Single-atom catalysts (SACs) comprising atomically dispersed metals on suitable supports have attracted tremendous amount of interest lately. One approach of stabilizing noble-metal single atom catalysts on polyoxometalates (POMs) â small, molecularly defined metal-oxo clusters â has recently been developed in our group making use of the highly symmetric Keggin structures. DFT calculations identified only one adsorption site to be capable to stabilizing SACs. Those molecularly defined catalytic sites are ideal systems for in situ spectroscopic investigations. A suite of in situ XAS, XPS and DRIFTS together with ab initio XANES fitting revealed the Mars-van Krevelen type mechanism of a Rh SAC during the CO oxidation reaction. We identified the support reoxidation (refill of the generated oxygen vacancy) as the rate-determining step and thus hypothesized that a support with lower oxidation potential should enhance CO oxidation activity. Indeed, when comparing Rh SACs on 4 different Keggin-type POMs with slightly different molecular composition but the same active site structure, the light-off temperature can be reduced by 130 °C. Besides fine-tuning the support redox properties, we also assessed the impact of varying the atomically dispersed metal in the active site. For 15 different metals (3d, 4d, and 5d), we observed light-off temperature differences of ~400 °C while the catalysts all exhibited the same morphology and active site structure. Integrating DFT calculations and microkinetic modelling, we were able to establish sought-after scaling relations for single-atom catalyzed reactions. We found that only linear scaling relations involving the thermodynamics of the support appropriately predict observed activity trends in an approach possibly extendable to other catalytic reactions in the interface between metal species and support.
Another benefit of using molecularly defined POMs as supports for SACs is their solubility in polar solvents. A soft ionization-based mass spectrometry method like ESI-MS allows the investigation of catalyst-bound reaction intermediates with unprecedented precision. We were able to identify the exact composition of active sites and their evolution in the catalytic cycle during CO and alcohol oxidation reactions performed in the liquid phase. Critical information on metal-dependent reaction mechanisms, the key intermediates, the dynamics of active sites and even the stepwise activation barriers were obtained, which would be challenging to gather via prevailingly adopted techniques in SAC research.
Driven by the maximum possible interface between metal active sites and extraordinarily strong Brønsted acid sites inherent to tungsten-based POMs, we envisioned our POM-supported SACs to be good catalysts for the hydrodeoxygenation (HDO) reaction. For the HDO of acetophenone as a model compound, our POM-supported SAC exhibited excellent activity and selectivity under very mild conditions (30 °C, 1 MPa H2). In contrast to our initial hypothesis, isotope labeling together with NMR and MS revealed that the reaction did not follow a commonly observed alcohol dehydration, hydrogenation pathway but the direct hydrogenolysis of the alcohol intermediate. In situ DRIFTS, inelastic neutron scattering and DFT calculations revealed the synergistic effects of facile heterolytic H2 splitting, the active involvement of water as well as the participation of support oxygen vacancies. Beyond the HDO of acetophenone, some lignin-model compounds as well as furfural can be deoxygenated at unprecedentedly mild reaction conditions.
In situ spectroscopy
Thorough structure-property-activity correlation investigations
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