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. 2022 Oct 27;2(11):2607–2616. doi: 10.1021/jacsau.2c00512

Photocatalytic Oxidative Dehydrogenation of Propane for Selective Propene Production with TiO2

Fangliang Li ‡, Binli Wang §, Xiao Chen ‡, Yuemiao Lai ‡, Tao Wang ‡, Hongjun Fan ∥,*, Xueming Yang ‡,∥,⊥, Qing Guo ‡,*
PMCID: PMC9709955  PMID: 36465539

Abstract

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Oxidative dehydrogenation of propane (ODHP) as an exothermic process is a promising method to produce propene (C3H6) with lower energy consumption in chemical industry. However, the selectivity of the C3H6 product is always poor because of overoxidation. Herein, the ODHP reaction into C3H6 on a model rutile(R)-TiO2(110) surface at low temperature via photocatalysis has been realized successfully. The results illustrate that photocatalytic oxidative dehydrogenation of propane (C3H8) into C3H6 can occur efficiently on R-TiO2(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 C3H6 product. An exceptional selectivity of ∼90% for C3H6 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 C3H6 under mild conditions.

Keywords: photocatalysis, TiO2, ODHP, propene, reaction mechanism

Introduction

The recent large-scale exploitation of shale gas that contains abundant light alkanes has emerged as a way to effectively supplement traditional fossil fuels, such as naphtha and heavy oil.1−3 Light olefins from the conversion of saturated alkanes are important building blocks for many chemicals, such as plastics, fibers, and oxygenates.1−7 However, because of the stability of alkanes resulting from high symmetry, weak polarizability, and high bond energy of the sp3 C–H bond, the commercial processes for light olefins production from alkanes are always conducted at high temperatures and pressures.1−7 The severe reaction conditions result in a large amount of energy consumption, catalyst deactivation, environmental pollution, and safety issues.

Propane (C3H8) is one of the main components of shale gas, and its dehydrogenation product, propene (C3H6), is a very important industrial raw materials.1−6 The ongoing discovery of shale gas leads to the development of a direct C3H8 dehydrogenation technology to produce C3H6. Thus, a lot of scientific research has also been focused on this subject.1−6 Compared to the nonoxidative dehydrogenation of C3H8 (DHP) process, the oxidative dehydrogenation of C3H8 (ODHP) is considered as an emerging crafts for C3H6 production because of its thermodynamic preference and the hindrance of coke formation.1−6 However, the ODHP process is still a highly energy-intensive process, which is usually conducted in mixed high temperature steam,2,3,8−13 It inevitably causes the overoxidation of intermediates and products, lowering the selectivity of desired C3H6 product. Consequently, reducing the reaction temperature and preventing the facile overoxidation of C3H6 product is still the key scientific challenge for the ODHP process.

After decades of research, various catalysts have been developed for the ODHP process. Among them, emerging boron-containing catalysts (e.g., BN, supported boron oxide) can lower the reaction temperature efficiently and successfully suppress the overoxidation of olefin products to a certain extent.2,5,14−25 Although a very high olefin molecular selectivity (C3H6 and ethene) can be achieved with boron-containing catalysts, a high reaction temperature (usually in excess of 400 °C) is still required.2,5,14,15,19 However, the C3H6 selectivity is still not entirely satisfactory. Therefore, alternative catalysts and reaction pathways with the ability to better control undesired C–C bond cleavage and overoxidation need to be explored.

Photocatalysis is an emerging technology that utilizes light energy instead of high-temperature thermal energy to drive reactions under mild conditions.26 Recently, the photocatalytic oxidative dehydrogenation of small alkanes under mild conditions was successfully achieved over TiO2 based catalysts,27−29 indicating that TiO2-based photocatalysis technology may be a promising way for selective C3H6 production from C3H8 dehydrogenation under mild conditions. Herein, we report on photocatalytic ODHP to C3H6 on rutile(R)-TiO2(110) via the hole coupled C–H bond cleavage process at 90 K. A high selectivity of about 90% for the C3H6 product is achieved at about 13% C3H8 conversion. Moreover, a clear mechanistic insight into photocatalytic ODHP to C3H6 on R-TiO2(110) has also been provided.

Results and Discussion

Figure 1 shows the typical temperature-programmed desorption (TPD) spectra of mass-to-charge ratios (m/z) of 18 (H2O+), 42 (C3H6+), and 43 (C3H7+) collected on the reduced and oxidized R-TiO2(110) surfaces after adsorbing 0.36 ML (1 ML = 5.2 × 1014 atoms/cm2) of C3H8 followed by 355 nm irradiation for 0 (black line) and 5 min (red line). The oxidized surfaces were prepared by exposing the reduced R-TiO2(110) surfaces to 200 L of O2 at 300 K, resulting in major adsorption of O atoms on the Ti5c sites (OTi) of the surfaces and healing of the bridging oxygen vacancy sites (Ov).30,31 Before irradiation, only one desorption peak at 137 K appears in the TPD traces of m/z = 42 and 43 on the reduced and oxidized surfaces, which is attributed to the desorption of C3H8 on the Ti5c sites (C3H8(Ti))·32 No observation of new products during the TPD process illustrates that no thermochemistry of C3H8 occurs on both the oxidized and reduced surfaces. Likewise, no products are detected on the C3H8 covered reduced R-TiO2(110) surface after 355 nm light irradiation, indicating that reduced R-TiO2(110) is photo-inactive for the conversion of C3H8.

Figure 1.

Figure 1

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Typical TPD spectra acquired at m/z = 18 (H2O+), 42 (C3H6+), and 43 (C3H7+) after adsorbing 0.36 ML of C3H8 on the (a) reduced and (b) oxidized R-TiO2(110) surfaces at 90 K with (5 min, red lines) and without (0 min, black lines) 355 nm irradiation. The oxidized R-TiO2(110) surfaces were prepared by exposing reduced surfaces to 200 L of O2 at 300 K. The photon flux of 355 nm light is 1.6 × 1016 photons cm–2 s–1.

However, two new desorption features at 190 K (m/z = 42) and 310 K (m/z = 18) are observed on the 0.36 ML C3H8 covered oxidized R-TiO2(110) surface after irradiation for 5 min. Here, 190 K is determined for the desorption of C3H6 molecules on the Ti5c sites (C3H6(Ti)) (Figures S1 and S2 in the Supporting Information, SI). The 310 K peak can be attributed to the desorption of H2O molecules on the Ti5c sites (H2OTi) or the recombinative desorption of terminal OH groups on the Ti5c sites (OHTi).33 In addition, no recombinative desorption signal of H2O from the OH groups on the bridging oxygen rows (OHb)30 suggests that H atoms produced from C3H8 dehydrogenation are mostly likely to be abstracted by OTi atoms directly rather than bridging oxygen atoms (Ob) during the photocatalytic process. Otherwise, the C3H6 product should also be detected on the reduced R-TiO2(110) surface. To further identify the role of OTi atoms in photocatalytic ODPH on R-TiO2(110), an isotope labeling experiment with 18O2 was carried out. On the 18O2 oxidized R-TiO2(110) surfaces, only the H218OTi product is detected (Figure S3), illustrating that only 18OTi atoms are involved in photocatalytic ODHP. Therefore, photocatalytic ODHP occurs on oxidized R-TiO2(110) at 90 K following reaction 1:

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No obvious signals of other products (Figure S2) demonstrate that the C3H6 production on oxidized R-TiO2(110) via photocatalytic ODHP takes place with high selectivity.

In addition to the selectivity of C3H6 product, the conversion efficiency of C3H8 is also important. As shown in Figure 2, the formation of C3H6 and H2O products and the decay of C3H8 were monitored by the TPD traces of m/z = 18, 29, and 42 collected from the 0.36 ML C3H6 covered oxidized R-TiO2(110) surfaces at different irradiation times. With increasing irradiation time, the C3H8(Ti) peak at 137 K (m/z = 29 and 42) is gradually depleted. Concomitant to the decrease of the C3H8(Ti) peak, both H2OTi and C3H6(Ti) products increase obviously and then reach saturation. A shoulder peak at 105 K appears in the TPD spectra of m/z = 29 and 42 with irradiation time, which is attributed to C3H8 adsorption on Ob atoms.32 The appearance of the shoulder peak is due to the competitive adsorption between C3H8(Ti) and C3H6(Ti), H2OTi products, leading to part of the less strongly adsorbed C3H8(Ti) to be repelled to Ob atoms.32 In addition, a small peak at 300 K (m/z = 29, the inset of Figure 2b) appears and increases with irradiation time, as well as the small peak at 340 K (m/z = 42, the inset of Figure 2c). Referring to the OPDH process on IrO2(110),34 the former may be assigned to the desorption of C3H8 via recombination of the C3H7 groups and H atoms on the surface, and the latter is likely due to the C3H6 product produced via thermocatalytic dehydrogenation of the C3H7 groups (see more details in Figure S2).

Figure 2.

Figure 2

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Typical spectra acquired at m/z = (a) 18 (H2O+), (b) 29 (C2H5+), and (c) 42 (C3H6+) on the 0.36 ML C3H8 covered oxidized R-TiO2(110) surfaces as a function of irradiation time.

From Figure 2a and c, the yields of H2O and C3H6 products on oxidized R-TiO2(110) as a function of ultraviolet (UV) irradiation time can be obtained (Figure 3a). With increasing irradiation time, the yields of both products grow gradually and slowly reach saturation at 180 min (∼0.047 ML H2O and ∼0.043 ML C3H6). And the selectivity of the C3H6 product (SC3H6 = yield of C3H6(Ti) at 190 K/yield of H2O at 310 K) and the conversion efficiency of C3H8 (XC3H8 = yield of H2O at 310 K/initial coverage of C3H8) are also depicted in Figure 3b. As the irradiation time increases, XC3H8 increases rapidly and reaches a plateau (about 13%) while SC3H6 decreases slightly from ∼95% to ∼90%. Therefore, photocatalytic OPDH into C3H6 can be achieved on oxidized R-TiO2(110) at 90 K with high selectivity and moderate C3H8 conversion efficiency. In addition, the yield of the C3H6 product has a quasi-linear relationship with irradiation time when the irradiation time is ≤60 s (inset of Figure 3a). The quantum yield is defined as “yield of C3H8/number of incident photons”; thus, the initial quantum yield of C3H6 production can be roughly estimated to be 1.1 × 10–5.

Figure 3.

Figure 3

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(a) Yields of H2O (blue triangle) and C3H6 (red star), and (b) selectivity of C3H6 product (SC3H6, blue circle) and conversion efficiency of C3H8 (XC3H8, red square) for photocatalytic C3H8 dehydrogenation on oxidized R-TiO2(110) as a function of irradiation time, obtained from Figure 2a and c. The inset of (a) shows a quasi-linear relationship between the yield of C3H6 product and irradiation time when the irradiation time is ≤60 s. All the plotted lines are only to guide the eye.

As shown in Figure 1, no observation of product signal from C3H8 dehydrogenation on both the reduced and oxidized R-TiO2(110) surfaces suggests that R-TiO2(110) has no thermocatalytic dehydrogenation ability of C3H8. In contrast, PdO(101), RuO2(110), and IrO2(110) surfaces that have strong oxidability for C3H8 dehydrogenation, in which the C3H8 molecule has a strong interaction with the substrate by forming a molecular σ-complex precursor at first and then breaks the C–H bond as the surface temperature increases.34−40 C3H8 only weakly adsorbs on R-TiO2(110), and surface oxidation does not enhance its adsorption. Therefore, even if the C–H bond activation of C3H8 occurs on R-TiO2(110) either via thermocatalysis or photocatalysis, the weak adsorption of C3H8 cannot satisfy the conditions of the precursor mediated mechanism.25

However, photocatalytic ODHP occurs smoothly on oxidized R-TiO2(110), indicating that the process is achieved via another mechanism. Recently, a gas-phase reaction mechanism has been proposed for photocatalytic C–H bond activation of hydrocarbons on R-TiO2(110), where the O– centers derived from Ob atoms under UV excitation abstract the H atoms of hydrocarbons to form corresponding radicals.41,42 A similar gas-phase radical mechanism was also proposed in the process of ODHP of BN-based catalysts.5,25 Here, although only OTi atoms are active species for photocatalytic ODHP on R-TiO2(110), the underlying mechanism may be similar. Following a similar mechanism, the C3H7• radical will be produced first during the photocatalytic ODHP process. Then, most of the radicals will undergo further dehydrogenation into C3H6 directly (the dominating C3H6(Ti) product at 190 K), instead of entering the gas phase like the case of methane CH4 on R-TiO2(110),41 or binding to the surface via a radical-rebound process like the cases of toluene (C6H5CH3) on R-TiO2(110)42 and C3H8 on BN catalysts.5 Otherwise, compared with 0.047 ML H2O production, tiny carbon-containing products should be observed, or most of the carbon-containing products should appear at high temperature (>300 K) (small peaks of C3H6 and C3H8 produced via thermal processes at >300 K, Figure S2).

To provide a more detailed mechanistic model for photocatalytic C–H bond activation in the OPDH process on R-TiO2(110), theoretical calculations for the adsorption and photocatalytic dehydrogenation of C3H8 on the R-TiO2(110) surface were performed (theoretical details are depicted in the SI). Referring to the adsorption structure of C3H8 on PdO(101), the p-2η1 adsorption structure of C3H8 (C3H8 coordinating with the surface by forming two H–Pd dative bonds, one at each CH3 group) on R-TiO2(110) is the most stable one with an adsorption energy (Eads) of 0.60 eV (Figure S4). However, the Eads values of the other two structures (p-η2, C3H8 binding through two H atoms of one CH3 group, Eads = 0.49 eV; s-η2, C3H8 binding through two H atoms of the CH2 group, Eads = 0.52 eV, Figure S4) are very close to that of the p-2η1 structure. In addition, the existence of OTi atoms has a minimal effect on the adsorption of C3H8 (Figure S4), in agreement with the TPD result. Thus, different from exclusive 1° C–H bond cleavage resulting from the p-2η1 σ-complex precursor formation on PdO(101),36 the initial C–H cleavage of 1° and 2° C–H bonds should be taken into consideration, which produces a propyl group (−CH2CH2CH3–) or isopropyl group (−CH3CHCH3–) intermediate, respectively.

As shown in Figure 4, the energy profiles of the initial C–H bond activation of C3H8 via thermocatalysis on both the stoichiometric and oxidized R-TiO2(110) surfaces are described. On the stoichiometric R-TiO2(110) surface, the cleavage of the 1° C–H bond is endothermic by 1.09 eV with a barrier of 1.45 eV, and the cleavage of 2° C–H bond is endothermic by 1.06 eV with a barrier of 1.41 eV, both of which are endothermic processes with high energy barriers. After surface oxidation, the endothermal value for the initial C–H bond activation is largely reduced. The cleavage of the 1° C–H bond is endothermic by 0.56 eV with a barrier of 1.49 eV, and the cleavage of 2° C–H bond is endothermic by 0.60 eV with a barrier of 1.44 eV. However, the energy barriers of the initial C–H bond cleavage are nearly unchanged, which still far exceed the Eads of C3H8(Ti) (0.49–0.60 eV, Figure S4). Therefore, the initial C–H bond activation via thermocatalytic processes on both stoichiometric and oxidized R-TiO2(110) is thermodynamically infeasible. In addition, following the 1° C–H bond cleavage pathway on the oxidized surface, further dehydrogenation of the propyl intermediate into C3H6 as a prototype has also been investigated, which is exothermic by 1.44 eV with an energy barrier of 0.34 eV. Thus, the initial C–H bond cleavage is the rate-determining step for the OPDH process on R-TiO2(110), which prohibits the activation of C3H8 on the OTi atom covered R-TiO2(110) surfaces thermodynamically.

Figure 4.

Figure 4

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Energy profiles of the initial C–H cleavage of C3H8 on the (a) stoichiometric and (b) oxidized R-TiO2(110) surfaces. The cleavage of the primary (1°) and secondary (2°) C–H bonds leads to a propyl group (−CH2CH2CH3–) and isopropyl group (−CH3CHCH3–) intermediate, respectively. Following the 1° C–H bond cleavage pathway on the oxidized surface, the second dehydrogenation of the intermediate into C3H6 as a prototype is also investigated.

By introducing a hole into the OTi atom covered R-TiO2(110) surface, photocatalytic ODHP on R-TiO2(110) was performed. According to recent theoretical results about C–H bond activation of CH4 on R-TiO2(110),41 the C–H bond activation of CH4 on R-TiO2(110) under photoexcitation includes not only the energy transfer process but also the hole transfer process. Similarly, the process of hole transfer along with the 1° C–H bond cleavage pathway as a prototype is shown in the form of spin density distribution in Figure 5. The larger electron accumulation on OTi than Ob atom (Bader charge is −1.01 for OTi, while −0.87 for Ob) makes the former a better site for hole trapping. Thus, initially, the hole will be trapped by an OTi atom rather than an Ob atom (Figure 5a) due to its stronger ability of hole capture. Subsequently, the hole will be located at both the 1° C atom of C3H8 and the OTi atom (p-TS1 in Figure 5b), indicating that the photocatalytic cleavage of 1° C–H bond on oxidized R-TiO2(110) is more likely to be a homolytic process.41,42 Because the alkyl group shows a better hole capture ability than OHTi groups,43 the hole will be localized at the C atom of the C3H7 moiety to form a propyl radical after the 1° C–H bond cleavage, and the dissociated H atom transfers to an adjacent OTi atom, forming an OHTi group (Figure 5c). Likewise, the 2° C–H bond cleavage also follows the similar mechanism to form an isopropyl radical, which is no longer discussed in detail here.

Figure 5.

Figure 5

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Spin density distribution of (a) molecular state, (b) first transition state (TS1) and (c) dissociation state on the OTi atoms covered R-TiO2(110) surface in the process of the 1° C–H cleavage of C3H8, respectively. O atoms (red), titanium atoms (blue), carbon atoms (black) and hydrogen atoms (white) are represented by small balls of different colors, respectively. The yellow area represents the spin density of electron clouds.

With these calculations, the energy profiles of photocatalytic ODHP on the OTi atom covered R-TiO2(110) surface can be obtained (Figure 6). For the 1° C–H bond cleavage pathway, the initial 1° C–H bond cleavage of C3H8 to propyl radical is exothermic by 0.93 eV with a barrier of 0.53 eV. Compared with the thermocatalytic case (Figure 4), such a photocatalytic homolytic C–H cleavage is much more feasible for the formation of CH3CH2CTiH2 radical and HOTi. Then the α-H in the propyl radical intermediate will transfer to HOTi directly to form C3H6 and H2OTi easily, and this step is exothermic by 1.41 eV with an energy barrier of 0.51 eV. Similarly, for the 2° C–H bond cleavage pathway, the stepwise dehydrogenation processes are exothermic by 1.14 and 1.20 eV with the energy barriers of 0.18 and 0.22 eV, respectively. Although the latter pathway is more favorable because of the lower energy barriers for each step, the possibility of the 1° C–H bond cleavage pathway cannot be completely ruled out because of its strongly exothermic effect with moderate barriers. The two competitive pathways are more likely to occur simultaneously in a certain proportion, similar to the reaction in gas-phase systems,44,45 rather than undergoing highly regioselective 1° or 2° C–H bond cleavage.

Figure 6.

Figure 6

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Energy profiles of the ODHP process on OTi atom covered R-TiO2(110) surface under photoexcitation. The two pathways represent the initial 1° (up) and 2° (below) C–H cleavage.

Generally, the thermocatalytic C–H bond activation of alkanes on high-valent metal oxo compounds usually follows the proton coupled electron transfer (PCET) mechanism.41,46,47 For instance, on the PdO(101), RuO2(110), and IrO2(110) surfaces,39,48−50 theoretical calculations proposed that the empty dz2 orbital of the surface metal atom (Pd, Ru, Ir) accepts the electron donated from the C–H σ-bond, accompanied by a filled dxy orbital for the back-bonding to the C–H antibonding orbital, leading to the formation of C3H8 σ-complex on the surfaces (mainly staggered p-2η1 complex). As a result, metal centers acted as Lewis acidic sites lower the energy barrier of C–H bond cleavage significantly, and lattice O atoms acted as Lewis basic sites extract protons via the PCET process, synergistically resulting in heterolytic C–H bond cleavage to form stable C3H7 groups on the surfaces at low temperature.34−40 The weak interaction between C3H8 and the Ti5c atom of R-TiO2(110) inhibits the formation of the C3H8 σ-complex, similar to the case of CH4 on R-TiO2(110).32,41,50 The thermocatalytic C–H bond activation of C3H8 has a high barrier (>1.0 eV, Figure 4) on both the reduced and oxidized R-TiO2(110) surfaces, which far exceeds the Eads of C3H8. Thus, C3H8 molecules prefer to desorb from the surface rather than break the C–H bonds on both the reduced and oxidized R-TiO2(110) surfaces during the TPD process. Otherwise, different states of C3H8 desorption should be detected in the TPD spectra, similar to the result of C3H8 desorption on IrO2(110) and RuO2(110).34,40

Although the weak interaction between C3H8 and the Ti5c atoms of R-TiO2(110) inhibits the thermocatalytic OPDH process, photocatalytic OPDH into C3H6 can occur smoothly on the oxidized surface at 90 K, demonstrating that photocatalysis could alter the reaction pathway significantly. The corresponding theoretical result (Figure 6) shows that both photocatalytic 1° and 2° C–H bond cleavages of C3H8 are strongly exothermic processes, which have much lower energy barriers than those of the thermocatalytic processes. Because of the weak interaction between C3H8 and the Ti5c atom, the initial C–H bond cleavage in photocatalytic OPDH on R-TiO2(110) is unlikely to occur via the stable precursor-mediated mechanism. The theoretical result of spin density distribution (Figure 5) suggests that the photocatalytic C–H bond cleavage of C3H8 on oxidized R-TiO2(110) follows a homolytic hydrogen atom transfer (HAT) mechanism to form C3H7• radical rather than a heterolytic C–H bond cleavage process, which is close to the gas-phase C3H7• radical formation pathway of the ODHP process.25,51 However, the energy barriers for the initial C–H bond cleavage have a noticeable difference. In the gas phase, the homolytic bond dissociation energies are 410 kJ/mol (∼4.2 eV) for the 1° C–H bond and 398 kJ/mol (∼4.1 eV) for the 2° C–H bond.36 Here, the energy barriers for photocatalytic 1° and 2° C–H bond cleavage decrease to only about 0.53 and 0.18 eV on oxidized R-TiO2(110), respectively. Such small energy barriers imply that photocatalytic OPDH could occur efficiently on R-TiO2(110).

Similarly, theoretical works36 about photocatalytic CH4 dehydrogenation on R-TiO2(110) also proposed that the initial C–H bond activation is exothermic by 0.23 eV with a barrier of 0.29 eV and the hole trapped Ob atoms (Ob– centers) play a crucial role in C–H bond activation. Moreover, according to the experimental work about photocatalytic C–H bond activation of toluene on reduced R-TiO2(110),42 Ob– centers show a very high activity for photocatalytic C–H bond activation of toluene. Similar results of photocatalytic C–H bond activation of small alkanes with the assistance of O– centers on different surfaces have also been observed.52,53 Unlike direct dissociative adsorption resulted from strong interactions between C3H8 and IrO2(110),34 photocatalytic ODHP on R-TiO2(110) follows the oxygen-mediated Eley–Rideal adsorption pathway,25 in which O atoms are preabsorbed and C3H8 weakly adsorbs and facilely migrates on the surface. The dehydrogenation reaction is likely to occur when mobile C3H8 molecules efficiently collide with reactive OTi– centers.

Because the C3H8 molecule is less symmetrical and more polar than CH4, the C–H bond activation of C3H8 with the Ob– center should be much easier than CH4 on R-TiO2(110) theoretically. However, the C–H bond activation of C3H8 is only detected on the oxidized R-TiO2(110) surface, in which the hole trapped OTi atom (OTi– center) plays an important role in C–H bond cleavage (Figure 5). The big difference between theoretical predictions and experimental results may be attributed to two possible reasons. First, although both Ob– and OTi– can be generated as active sites under UV light irradiation, Ob– can only activate the active α-H of toluene42 but cannot activate the inert C–H bond of C3H8. This suggests that the dehydrogenation ability of the OTi– center may be stronger than that of the Ob– center. In addition, the observation of the reversible switch between OTi2– and OTi– by a combination of noncontact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFS) at 78 K54−57 indicates that OTi– is likely a long-lived species on R-TiO2(110) at low surface temperature, which will be suitable for low-temperature photocatalytic ODHP.

Thus, once OTi– centers are formed on R-TiO2(110) via trapping photogenerated holes, the abstraction of a H atom from C3H8 by the hole trapped OTi– center can occur to produce the C3H7• radical (propyl radicals or isopropyl radicals) via the HAT process (Figure 5). According to the work on photocatalytic C–H bond activation of toluene on R-TiO2(110),42 the formation of stable benzyl intermediates (or benzyl anions) via a radical rebound process during the UV irradiation process will result in an obvious recombination peak of toluene in the TPD spectra. In other words, R-TiO2(110) may act as a radical quenching agent, similar to the role of hexagonal boron nitride (h-BN) catalysts in the ODHP process.58 However, only a tiny recombination peak for C3H8 is observed in Figure 2, which is likely due to the recombination of stable C3H7 groups and Hb atoms.34,40 The tiny recombinative desorption peak of C3H8 demonstrates that dissociated C3H7• radicals prefer to dehydrogenate into C3H6 rather than convert into stable C3H7 groups, which is due to the low energy barrier of the second C–H bond cleavage (especially 2° C–H cleavage, 0.22 eV in Figure 6), leading to efficient C3H6 production.

Recently, Hermans et al. also obtained a high olefin selectivity (79% C3H6 and 12% ethene) over h-BN and boron nitride nanotubes (BNNTs).5 They proposed that the high olefin selectivity is largely enhanced due to the rebound step of the C3H7• radical on the nitroxyl-radical site, avoiding the ejection of C3H7• radical into gas phase or the migration of C3H7• radical on the surface for further reaction with oxidant, then resulting in a high selectivity of C3H6 product.5 For ODHP on R-TiO2(110), the C–Ti bond between the C3H7 intermediate (C3H7–) and the Ti5c site on the OTi atom covered surface is 2.09 Å after the heterolytic C–H bond cleavage (Figure S5). However, upon 355 nm irradiation, the C–Ti bond between the hole-trapped C3H7• radical and the Ti5c site is 2.76 Å, suggesting that C3H7• radicals physically adsorb on R-TiO2(110), similar to the case on Ga2O3(100).59 When the hole-trapped C3H7• radical rebounds to the R-TiO2(110) surface via a de-excitation process as in the case of toluene,42 the stable C3H7 intermediate will be formed. The small recombinative desorption peak of C3H8 at >300 K (Figure S2) suggests that the rebound step of the C3H7• radical is just a minor channel. Otherwise, a big desorption peak will be detected, similar to that of photocatalytic C–H bond activation of C6H5CH3 on R-TiO2(110).42 Therefore, once C3H7• radicals are produced via photocatalytic C–H bond activation of C3H8 on R-TiO2(110), the radicals will preferentially dehydrogenate into C3H6 rather than rebound to the surface, which is different from thermocatalytic C3H8 dehydrogenation on h-BN and boron nitride nanotubes.5

In addition, according to previous STM results, after exposing the reduced R-TiO2(110) surface to 200 L O2 at 300 K, the concentration of OTi atoms is almost the same as that of Ov (about 5%). As a result, OTi atoms are highly dispersed on Ti rows.30,31 As shown in Figure 6, the DFT results indicate that both the initial and second dehydrogenation are strongly facilitated by the isolated OTi– center, resulting in efficient ODHP into C3H6. In contrast, in the thermocatalytic dehydrogenation reaction of hydrocarbons, fully exposed metal clusters (e.g., Pd3,60 Pt3,61,62 Pd4.4,63 and so on) can simultaneously balance the adsorption of the reactants, the activation of C–H bonds, and the desorption of olefins via the synergy of adjacent sites, resulting in olefin formation with extremely high yield and selectivity. However, highly dispersed single-atom sites catalysts (SACs) encounter low catalytic efficiency and sintering due to the lack of synergistically successive dehydrogenation from neighboring sites. Therefore, although the efficiency of photocatalysis is less than that of thermocatalysis, it is a promising technology for hydrocarbon dehydrogenation with low energy consumption and possibly high atom-utilization in the future.

Conclusions

In summary, model studies of photocatalytic ODHP on R-TiO2(110) demonstrates that the process can occur on the R-TiO2(110) surface with an exceptional selectivity of >90% for C3H6 production. Further mechanistic analysis determines that OTi– centers rather than Ob– centers produced via trapping photogenerated holes play a vital role in the initial C–H bond activation of C3H8 via the HAT process, resulting in the high selectivity of C3H6 production at low surface temperature. The findings not only provide a novel mechanistic insight into photocatalytic C–H bond activation by means of the experiment and the theory, but also offer new opportunities for the rational design of highly selective ODHP pathways under mild conditions.

Methods

Experimental Section

All TPD experiments were performed with a home-built apparatus, which has been described in detail elsewhere.64 The preparation of well-ordered R-TiO2(110) crystal surfaces (Princeton Scientific Corp., 10 mm × 10 mm × 1 mm) was accomplished by cycles of Ar+ sputtering and annealing at 850 K in ultrahigh vacuum (UHV). The characterization of the ordering and cleanness of the R-TiO2(110) surfaces was conducted by low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES), respectively. The density of Ov on the surface was determined by about 5%, as gauged by H2O TPD. The purity of the C3H8, O2, and isotopic O218 gas used in the experiment was 99.99%. The 355 nm light was produced by the third harmonic output from a diode pumped, solid state (DPSS), Q-switched 1064 nm laser (Spectra-Physics). The pulse time and repetition rate of the UV laser were 12 ns and 50 kHz, respectively. The power of the laser was only 5 mW with a flux of about 1.6 × 1016 photons cm–2 s–1, and the increase of surface temperature resulting from laser irradiation can be ignored.

Computational Section

Theoretical calculations were performed with the Vienna ab initio simulation package (VASP) code65,66 and plane augmented wave potential.67 The wave function was expanded by the plane wave, with a basis cutoff of 400 eV. The spin-polarized Perdew–Burke–Ernzerhof functional68 was used. Van der Walls correction (DFT-D3)69 was applied for the dispersion interactions. The constrained minimization and climbing-image nudged elastic band methods (CI-NEB)70 were used to search the transition states and calculate the energy barriers. The Bader charge was applied to analyze the charge distribution.

The surface model was cut out of a TiO2 crystal to expose the (110) surface, with a six-layer slab and 4 × 2 surface unit cell. A 15 Å vacuum region was used to decouple the surface slab. A Monkhorst–Pack grid71 of (1 × 1 × 1) k-point mesh was used in the calculations.

The C–H bond cleavage on three kinds of R-TiO2(110) surfaces were included: stoichiometric surface, oxidized surface, and photoexcited oxidized surface. The oxidized surface was simulated with an OTi atom on the top of the slab and two H atoms adsorbed on the bridge oxygen (HOb) on the bottom of the slab, which were applied to neutralize the unpaired electrons. According to previous studies, a hydroxyl can be used as an electron-withdrawing group to inject a hole to the surface.72−74 Similarly, an OTi atom on the top of the slab and a HOb on the bottom of the slab were used to introduce a photogenerated hole to the oxidized surface in the simulation.

Acknowledgments

This work was supported by the National Key R&D Program of China (Grant No. 2018YFE0203002), the National Natural Science Foundation of China (Grant No. 22173041, 22103033, 22103031, 22173042, 21973037, NSFC Center for Chemical Dynamics), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB17000000), the Shenzhen Science and Technology Innovation Committee (Grant No. JCYJ2019080914021660, ZDSYS20200421111001787), the Guangdong Innovative & Entrepreneurial Research Team Program (Grant No. 2019ZT08L455, 2019JC01X091), the International Partnership Program of Chinese Academy of Science (Grant No. 121421KYSB20170012), and Innovation Program for Quantum Science and Technology (Grant No. 2021ZD0303304).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.2c00512.

  • Typical spectra acquired at m/z = 39 (C3H3+), 40 (C3H4+), 41 (C3H5+), and 42 (C3H6+) on the 0.40 ML C3H6 covered R-TiO2(110) surface; typical spectra acquired at different masses on the 0.36 ML C3H8 covered oxidized R-TiO2(110) surfaces followed by irradiating the surface for 0 and 5 min at 90 K, respectively; TPD traces of different masses collected on the 0.36 ML C3H8 covered R-TiO2(110) surfaces oxidized by 18O2 at 300 K followed by irradiating the surface for 0 and 5 min at 90 K; different adsorption configurations and adsorption energy of C3H8 on the stoichiometric and oxidized R-TiO2(110) surface; adsorption structure of dissociated C3H7 intermediate; and C–Ti bond and distance calculated with a van der Waals correction (PDF)

Author Contributions

† F.L. and B.W. contributed equally to this work.

The authors declare no competing financial interest.

Supplementary Material

au2c00512_si_001.pdf (841KB, pdf)

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