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.

The direct dehydrogenation of hydrocarbons to olefins under mild conditions is an atom-economical but challenging route. Here, we have investigated photocatalytic ethylbenzene dehydrogenation into styrene on rutile­(R)-TiO2(110) using the temperature-programmed desorption (TPD) method. The results demonstrate that photocatalytic ethylbenzene dehydrogenation into styrene occurs on R-TiO2(110) in a stepwise manner, in which the initial α-C–H bond cleavage occurs facilely under UV irradiation via a possible homolytic hydrogen atom transfer process and then is followed by the second C–H bond cleavage induced by either photocatalysis at ∼120 K or thermocatalysis at >400 K. With preadsorbed oxygen atoms to eliminate hydrogen atoms from ethylbenzene dehydrogenation and excess electrons on the surface, the yield of styrene is largely enhanced by about 4 times. The results not only demonstrate a photocatalytic route for ethylbenzene dehydrogenation into styrene on R-TiO2(110) but also advance our understanding of the photocatalytic activation of the saturated C–H bond with TiO2.

Understanding the mechanism of ethanol (EtOH) photochemistry is of significance for photocatalytic H 2 production. Here, we reported a systematical study of EtOH photochemistry on rutile (R)−TiO 2 (110), aiming to illustrate how photogenerated holes and electrons are involved in bond breaking. We found that the yields of aldehyde from the ethoxy group and EtOH photooxidation on R− TiO 2 (110) are proportional to the square root of the photon flux, demonstrating that one hole can induce molecular EtOH decomposition to aldehyde. The initial O−H bond cleavage occurs mainly via a proton-coupled hole transfer process, and the C−H bond cleavage is a hole-mediated process, leaving two electrons on the surface, in agreement with the "current doubling effect". In addition, the rate of aldehyde formation from the ethoxy group is about 3 orders of magnitude faster than that from EtOH, suggesting that the O−H bond cleavage determines the rate of EtOH photochemistry. The results may considerably broaden our understanding of TiO 2 photocatalysis.

The low-temperature C−H bond activation of alkanes remains a big challenge in alkane dehydrogenation. In this work, ethylbenzene (EB) oxidative dehydrogenation has been investigated on rutile(R)-TiO 2 (110) under both ultrahigh vacuum (UHV) and ambient conditions. Under UHV conditions, styrene is produced with nearly 100% selectivity in a stepwise manner, in which the first C−H bond dissociation of EB occurs at <285 K with the help of surface O 2 2− species, followed by the second C−H bond dissociation at about 400 K. However, styrene, acetophenone, and 2,3-diphenylbutane products are produced from EB oxidative dehydrogenation under ambient conditions, suggesting that α-H dissociation is the initial step of EB oxidative dehydrogenation. This may be also possible for EB oxidative dehydrogenation on R-TiO 2 (110) under UHV conditions. The different pathways of EB oxidative dehydrogenation under UHV and ambient conditions may originate from different intermediates and O 2 concentrations. This work provides new insight into the fundamental understandings of the low-temperature C−H bond activation of alkyl chains of aromatic hydrocarbons, which may promote the development of new catalysts for efficient styrene production from EB oxidative dehydrogenation under mild conditions.

Excess electrons play a crucial role in surface reactions on transition metal oxides. However, the role of excess electrons in the selectivity of bond cleavage in photocatalytic reactions is rarely studied. Using ethylene glycol (EG) as a probe, the photochemistry of EG on reduced and O2 pre-dosed rutile-TiO2(110) surfaces has been investigated using the temperature-programmed desorption (TPD) method, which illustrates that excess electrons play an important role in determining the bond cleavage selectivity in EG photochemistry. Acetaldehyde (CH3CHO) formation via the C–O bond cleavage dominates on the reduced surface at high EG coverage. However, formaldehyde (CH2O) formation via the C–C bond cleavage is largely enhanced with increasing O2 exposure; conversely, the yield of the CH3CHO product remains nearly constant. By decreasing EG coverage to minimize the thermal reaction between EG and O2, enhanced CH2O production is still observed at 0.08 ML EG coverage, which is most likely due to the hindrance of excess electrons to the hole-mediated half-reaction of CH2O formation. This may be common for other hole-mediated half-reactions on oxides with excess electrons.