Mechanism study on direct synthesis of glycerol carbonate from CO₂ and glycerol over shaped CeO₂ model catalysts

Direct synthesis of glycerol carbonate (GC) from CO₂ and glycerol (a byproduct of biodiesel production) is a route to obtain a high-value chemical from waste and low-cost byproducts but has not yet industrialized due to the lack of efficient catalysts. Ceria (CeO₂) exhibits the highest catalytic activity and GC selectivity among the heterogeneous catalysts studied so far. However, the mechanism of this reaction over CeO₂ catalysts has not been studied in detail. Herein, we synthesized CeO₂ nanocrystals with different morphologies as model catalysts that can predominantly expose {111}, {110}, and {100} facets, and their surface acid-base properties were characterized using high-sensitivity temperature-programmed desorption of NH₃ and CO₂ with quadrupole mass spectrometry as detector (NH₃-TPD-QMS and CO₂-TPD-QMS). We found that the catalytic performance (GC formation rate) is strictly linearly dependent on the density of basic sites, which is relevant to the adsorption and activation of CO₂. In addition, to illustrate a more microscopic reaction mechanisms underlying the formation of GC from CO₂ and glycerol on all three low-index surfaces (111), (110) and (100), we also performed comprehensive first principles calculations. A three-step Langmuir–Hinshelwood (LH) mechanism was identified in which the annulation reaction is the rate-limiting step. The CeO₂ (111) surface exhibits the lowest overall activation energy, which agrees well with the catalytic performance that the CeO₂ nano-octahedra, predominantly exposing {111} facets, have the highest GC formation rate. This work is the first to combine experiments on shaped CeO₂ model catalysts with first-principles calculations to gain insight into the mechanism of direct synthesis of GC from CO₂ and glycerol, and will aid in the development of catalysts with improved performance.