Oxidative dehydrogenation of propane (ODHP) as an exothermic process is a promising method to produce propene (C 3 H 6 ) with lower energy consumption in chemical industry. However, the selectivity of the C 3 H 6 product is always poor because of overoxidation. Herein, the ODHP reaction into C 3 H 6 on a model rutile(R)-TiO 2 (110) surface at low temperature via photocatalysis has been realized successfully. The results illustrate that photocatalytic oxidative dehydrogenation of propane (C 3 H 8 ) into C 3 H 6 can occur efficiently on R-TiO 2 (110) at 90 K via a stepwise manner, in which the initial C−H cleavage occurs via the hole coupled C−H bond cleavage pathway followed by a radical mediated C−H cleavage to the C 3 H 6 product. An exceptional selectivity of ∼90% for C 3 H 6 production is achieved at about 13% propane conversion. The mechanistic model constructed in this study not only advances our understanding of C−H bond activation but also provides a new pathway for highly selective ODHP into C 3 H 6 under mild conditions.
Methanol has long been used as a hole scavenger in photocatalysis to improve charge separation. Although the prototypical methanol/TiO2(110) system shows salient adsorption-state-dependent photoactivity, the underlying reason still remains unclear. Through surface-sensitive ultraviolet photoelectron spectroscopy measurements, we found that methoxy anions and methanol strengthen and flatten the originally upward band bending of TiO2(110), respectively. Intensified upward band bending leads to charge separation in the depletion region and hole accumulation at the surface. Furthermore, density functional theory calculations show that the hole transfer at the CH3O–/TiO2 interface is thermodynamically allowed and that on CH3OH/TiO2 is unfavorable. The improved charge separation together with the allowed interfacial hole transfer is found to be responsible for the superior photoactivity of the methoxy anion over methanol on TiO2. These results reconcile the existing contradiction in the understanding of the charge transfer at the CH3O–/TiO2 interface based on the photoemission measured energy levels. Our work suggests that the redox potential level, rather than the vertical energy (for example, the HOMO of adsorbates) measured by spectroscopy, should be used to evaluate the possibility of the heterogeneous interfacial charge transfer.
Understanding the structure–activity relationship of an active site is of great significance toward the rational design of highly active catalysts. Herein, we present a combined experimental and theoretical study on water oxidation catalysis of mononuclear Co catalysts with CoN4Cl, CoCN3Cl, and CoC4Cl motifs incorporated into a graphene matrix. We found that the catalyst with the CoCN3Cl structure exhibits an overpotential of 359 mV at 10 mA/cm2 for the oxygen evolution reaction (OER), much lower than those of catalysts with CoC4Cl (396 mV) and CoN4Cl structure (>500 mV). By introducing the binding strength between the Co site and reaction intermediates (OH*, O* and OOH*) as the reaction descriptor, we revealed that the binding strength for CoO* in these structures is getting stronger when N is replaced by the C atom, which plays a crucial role in the rate-determining step (RDS) and water oxidation performance. The Co site distinctively coordinated with the CN3Cl structure gives rise to the most suitable binding strength of CoO* and consequently the highest OER performance (RDS: Co–OH* → CoO*), much better than that coordinated by C4Cl with a strong binding strength (RDS: CoO* → Co-OOH*) and N4Cl with a weak binding strength (RDS: Co–OH* → CoO*). This work further demonstrates the importance of the coordination environment of the metal nucleus in catalysts for RDS manipulation and activity optimization in OER through modulating the binding strength between active site and reaction intermediates.
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