Low-Temperature C–H Bond Activation via Photocatalysis: Highly Efficient Ethylbenzene Dehydrogenation into Styrene on Rutile TiO₂(110)

Abstract

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.

Low-Temperature Ethylbenzene Conversion on Rutile TiO2(100) via Photocatalysis: The Strong Photon Energy Dependence

“…The yield of the 380 K peak is about 10 times higher than that of LT C 6 H 5 CHCH 2 , indicating that the first C–H bond cleavage of C 6 H 5 C 2 H 5 occurs much more facilely than the second C–H bond cleavage on R–TiO 2 (100) under 343 nm irradiation. Similar to previous studies of C 6 H 5 C 2 H 5 conversion on R–TiO 2 (110), , the HT C 6 H 5 CHCH 2 product is formed from the dehydrogenation of C 6 H 5 C 2 H 4 groups during the TPD process and the LT C 6 H 5 CHCH 2 product formation from C 6 H 5 C 2 H 4 moiety dehydrogenation occurs via a photochemical process. The desorption temperature is nearly 60 K lower than that on R–TiO 2 (110), indicating that R–TiO 2 (100) may be more suitable for photocatalytic C 6 H 5 C 2 H 5 dehydrogenation.…”

“…On the basis of previous work, 26 the low yield of LT C 6 H 5 CHCH 2 formation may be due to two reasons. The first reason is the accumulation of OH groups on the O b sites (OH b ) formed via the initial C−H bond cleavage of C 6 H 5 C 2 H 5 , which can lower the efficiency of the hole-mediated C−H bond cleavage process by inhibiting the hole transfer from the surface to adsorbates.…”

“…26 Actually, the energy of photogenerated charge carriers in R−TiO 2 (110) (≥3.0 eV) 27,28 is much higher than the barrier of the β-C−H bond cleavage of C 6 H 5 C 2 H 5 ; thus, realizing the β-C−H bond cleavage should be very easy. However, the efficiency of lowtemperature C 6 H 5 CHCH 2 formation on R−TiO 2 (110) under 355 nm irradiation (3.55 eV) is very low, 26 indicating that the energy of photogenerated charge carriers is not efficiently involved in the β-C−H bond cleavage of C 6 H 5 C 2 H 5 and the efficiency of β-C−H bond cleavage may be enhanced with higher energy charge carriers. In addition, the dynamic information on how photogenerated charge carriers drive the reaction is still lacking.…”

“…The first reason is the accumulation of OH groups on the O b sites (OH b ) formed via the initial C−H bond cleavage of C 6 H 5 C 2 H 5 , which can lower the efficiency of the hole-mediated C−H bond cleavage process by inhibiting the hole transfer from the surface to adsorbates. 31−33 However, the yield of LT C 6 H 5 CHCH 2 formation on R−TiO 2 (110) does not show a significant increase when the OH b groups from C 6 H 5 C 2 H 5 dissociation are eliminated by surface Ti 5c -bound O atoms (O Ti ), 26 suggesting that the accumulation of OH b is unlikely the main reason for the low yield of LT C 6 H 5 CHCH 2 formation.…”

“…In comparison to the various thermocatalytic C 6 H 5 C 2 H 5 conversion techniques (DDH and ODH), photocatalysis, as a potential alternative, may achieve low-temperature C 6 H 5 C 2 H 5 conversion. Recently, photocatalysis has been successfully applied for the conversion of light alkanes [such as methane (CH 4 ), ethane (C 2 H 6 ), and propane (C 3 H 8 )] with TiO 2 -based catalysts under mild conditions. − Further, photocatalytic C 6 H 5 C 2 H 5 conversion on the model rutile (R)–TiO 2 (110) surface was also systematically investigated. , The results suggested that photocatalytic C 6 H 5 C 2 H 5 conversion into C 6 H 5 CHCH 2 occurs via a stepwise manner, in which the α-C–H bond is dissociated first with a very low energy barrier (∼0.3–0.4 eV) and is then followed by the β-C–H bond cleavage with a high energy barrier (∼1.2 eV) . Actually, the energy of photogenerated charge carriers in R–TiO 2 (110) (≥3.0 eV) , is much higher than the barrier of the β-C–H bond cleavage of C 6 H 5 C 2 H 5 ; thus, realizing the β-C–H bond cleavage should be very easy.…”