Discrimination of active species in liquid-phase hydrogenation on supported noble metal catalyst: An operando spectroscopic study on the asymmetric hydrogenation of ketopantolactone on Pt/Al2O3 and Pt/C modified by cinchonidine

- Operando spectroscopy of catalytic solid-liquid-gas reaction systems is achieved.
- Choice of the support material of a powder catalyst is crucial for spectroscopic analysis.
- Chiral modifier surface coverage can be correlated to enantioselectivity.
- Cinchonidine adsorbed to Pt with a tilted quinoline ring is relevant for enantioselection.

An often encountered problem in in situ or operando spectroscopic studies on supported metal catalysts is the discrimination between adsorbates on the support and on the active metal. Here, we show that comparative studies involving differently supported catalysts can help discriminate between species adsorbed on the active metal and the support. The adsorption of the chiral modifier cinchonidine (CD) on Pt/Al2O3 and Pt/C catalysts was investigated by means of operando attenuated total reflection infrared (ATR-IR) spectroscopy. The choice of the support material was found to be crucial, as the strong signal contributions from adsorbates on alumina significantly hampered the spectroscopic analysis of the Pt/Al2O3 catalyst. Instead, the use of a corresponding carbon supported Pt catalyst facilitated the discrimination of adsorbates on the noble metal, due to the virtual absence of adsorbed modifier species on the carbon support. Under reaction conditions of the asymmetric hydrogenation of ketopantolactone, Pt nanoparticles were predominantly covered by CD species anchored via a tilted quinoline ring with respect to the metal surface. Our operando ATR-IR spectroscopic study indicates a strong correlation between the intensity of the signal associated to tilted CD species and the obtained enantioselectivity.

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