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Identifying the Role of Photo-generated Holes in Photocatalytic Methanol Dissociation on Rutile TiO2(110) Jiawei Zhang, Chao Peng, Haifeng Wang, and Peijun Hu ACS Catal., Just Accepted Manuscript • Publication Date (Web): 16 Feb 2017 Downloaded from http://pubs.acs.org on February 16, 2017
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ACS Catalysis
Identifying the Role of Photo-generated Holes in Photocatalytic Methanol Dissociation on Rutile TiO2(110) Jiawei Zhang,†# Chao Peng,†# Haifeng Wang,†* and P. Hu†§* †
Key Laboratory of Advanced Materials, Research Institute of Industrial Catalysis and Center for Computational Chemistry, East China University of Science and Technology, Shanghai 200237, China §
School of Chemistry and Chemical Engineering, Queen’s University Belfast, Belfast BT9 54AG, UK
ABSTRACT: As an important model reaction, the photocatalytic methanol dissociation on rutile TiO2(110) has drawn much attention, but its reaction mechanism remains elusive. Using DFT+U calculations, we investigate the whole dissociation process of methanol into formaldehyde with and without photo-generated holes, aiming to illustrate how the hole is involved in the dissociation. We find that the O-H dissociation of methanol is a heterolytic cleavage process and is likely to be thermally driven; the presence of hole has no promotion on the barrier and enthalpy change. In contrast, the subsequent C−H bond cleavage follows the homolytic cleavage mode and is likely to be photochemically driven; great enhancement can be made in both kinetics and thermodynamics when introducing holes. The essential roles of holes for promoting the C-H dissociation are identified, and what kind of catalytic reactions can or cannot be facilitated by holes is discussed. Our findings may considerably broaden the understanding of photocatalytic chemistry.
KEYWORDS: Photocatalysis, Methanol dissociation, Density functional theory, Role of photo-generated holes, TiO2
INTRODUCTION Photo-generated holes play a key role in photocatalytic reaction, as they have strong oxidability and are unavoidable in almost any system under UV irradiation.1-8 An efficient separation of hole and electron and a deep understanding of their functions during the photocatalytic redox in heterogeneous catalysis is of paramount importance.9 The radicals, forming via the trapping of photo-generated holes, are widely considered as key players in many photo-oxidations.10 Interestingly, as a typical model reaction, methanol dissociation in the presence of photo-generated holes has not been conclusive and the underlying detail is highly desired.11-23 Specifically, the conventional reaction pathways, i.e. the ground state reactions (heat driven reactions), have been clarified, but the holeassisted pathways, which have a profound influence on the photocatalytic reactions, are rarely reported. Therefore, it is of fundamental interest to understand the photocatalytic conversion process of methanol dissociation. It is very desirable to address whether photo-generated holes can assist a surface reaction and why. Titanium dioxide (TiO2) still prevails among the best materials for photo-catalysis and solar energy conversion and several studies have examined photocatalytic methanol dissociation on rutile TiO2(110) by experimental methods.24-26 Starting from partially deuterated methanol (CD3OH), Yang and coworkers have detected photocatalytic products such as CD2O on Ti5c sites, H and D atoms on bridge oxygen (Obr) sites, which demonstrated clearly that the methanol dissociation is a two-step reaction.27 However, the details regarding the role of holes in methanol dissociation have led to much debate and several mechanisms have been proposed: (i) a two-step mechanism in which both of O-H bond and C-H bond dissociations
are initiated by heat; (ii) only the C-H bond cleavage is a photo oxidation process and the O-H bond cleavage is a thermal decomposition; (iii) both of the two bond dissociations are driven by light.28-30 Apparently, the dispute of these mechanisms focuses on one issue: whether the two-step dissociation is directly facilitated by photo holes or driven by heat. Very recently, Yang and co-workers proposed an extended model of photo-catalysis, which involves non-adiabatic conversion processes and ground-state surface reaction for methanol dissociation, which is a sophisticated model describing the C-H thermal dissociation.31 Currently, the preferable pathway in the presence of holes is not unraveled, and the question of how holes participate in methanol dissociation remains to be answered. Thus, it is of great significance to obtain a full understanding of the two-step dissociation process of methanol and clarify the role of holes. Herein we report a systematic investigation to uncover the nature of CH3OH dissociation on rutile TiO2(110) and give an insight into the photocatalytic mechanism at the atomic level using first principles calculations, which may shed some lights on the methanol promotion on the photocatalytic H2 production. 32-39
COMPUTATIONAL DETAILS All calculations were performed using density functional theory (DFT) within the plane-wave pseudo-potential as implemented in VASP code with a cut-off energy of 450 eV.40 The photo-generated holes usually tend to delocalize in GGA calculations due to the self-interaction error. To overcome this issue, there are usually two methods: hybrid functional and DFT+U methods. Here, for the reason that hybrid functional is quite time-consuming, we employ DFT+U method26 as a ma-
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jor tool, where the Hubbard-type correction was applied on Ti’s 3d orbitals and O’s 2p orbitals. U=6.3 eV for O’s 2p and U=4.2 eV for Ti’s 3d orbitals were chosen based on the previous work which can give a correct description of the polaronic states of the TiO2.3,41 To ascertain the accuracy by DFT+U, HSE 06 functional42 was tested on examining some key reaction barriers, which give the same trend as DFT+U (see Table S1). The atomic positions were relaxed until the force on each atom is less than 0.05 eV/Å. The TiO2(110) surface was modelled by 3×1 periodically repeated slabs. As a compromise of accuracy and computation time, a four-layer model is used. There is a ~10 Å vacuum between slabs and its periodic replicas along the [110] direction to simulate the open surface. The transition states (TSs) were search by a constrained optimization scheme.43-45 To simulate a photo-generated hole in the consideration of simplifying the complicated photo-excited system, we remove an electron from the periodic system or introduce an OH group as an electron acceptor on the bottom surface of the slab. Similar approaches were used in previous work.3,46 A MonkhorstPack grid of 2×3×1 k points was used for all DFT calculations. The localization of the hole is further confirmed by the electronic structural analysis and the Bader charge analysis. The detailed descriptions can be found in the Supporting Information.
RESULTS AND DISSCUSSION CH3O-H Bond Breaking
We firstly investigated the adsorption and dissociation of CH3OH on TiO2(110) without photo-generated hole. It is found that CH3OH can adsorb on the five-coordinated Ti (Ti5c4+) with a moderate adsorption energy of -0.92 eV and the Ti-O bond length of 2.22 Å. The O-H bond dissociation of CH3OH could proceed with the H binding to Obr, forming the CH3O species on Ti5c (reaction I): Ti4+···O(H)CH3+Obr2–→Ti4+−O–CH3+HObr– (I) The activation barrier was calculated to be 0.17 eV and the enthalpy change is 0.01 eV, which agrees well with previous theoretical calculations.27 Furthermore, it is worth noting that the formed CH3O species appears to bond strongly to Ti5c as a monodentate methoxy anion (CH3O−, carrying 0.51 electron Table 1. The adsorption energies of CH3O(H) species (including CH3OH, CH3O- and CH3O• radical), the optimized average Ti-O bond length (dTi-O) and the corresponding Bader charge of bridge oxygen and oxygen in CH3OH or CH3O, obtained in the presence of different number of holes using DFT+U. The negative value means exothermic. Bond length unit: Å; energy unit: eV. Hole
O-H
0
C-H
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Trapped
Ead
Site
CH3OH CH3O
Ti4+··CH3OH+Obr-I2−+Obr-II–•→Ti4+··O−CH3+HObr-I−+Obr-II–•(II) To understand these results and the role of hole, an electronic structure analysis was carried out. The results are shown in Figure 1(e, h and k) and Table S4, from which it can be seen that the charges of hydrogen (approaching to Obr) in the TSs of
Charge Obr-I
Obr-II
OCH3O
/
−0.92
/
2.22
−1.15
−1.10
−1.68
−0.88
/
2.22
−0.72
−1.06
−1.67
Obr-II
−1.04
/
2.18
−1.11
−0.70
−1.65
0
/
/
−1.89 1.80
−1.09
−1.09
−1.26
1
OCH3O
/
−0.60 2.25
−1.11
−1.11
−0.87
OCH3O, Obr-I
/
−0.68 2.24
−0.68
−1.06
−0.87
OCH3O, Obr-II
/
−0.73 2.21
−1.08
−0.68
−0.86
2
according to Bader charge analyses) with a high adsorption energy of -1.89 eV and the Ti-O bond length between CH3O− and surface Ti5c4+ is 1.80 Å, which is shorter than 2.22 Å for Ti-O(H)CH3. We also investigated O-H bond dissociation of CH3OH in the presence of photo-generated holes. Firstly, we examined the possibility of a trapped-hole at O of CH3OH (O ). Based on Bader charge analyses (Table 1), the charges of O with and without hole are similar, indicating that the hole could not be trapped at O and has little influence on the adsorption of CH3OH. Secondly, we explored the possible trapping states of the hole on the surface, which could provide an oxidative center for the O-H bond scission. Typically, three possible sites around the adsorbed CH3OH, named Obr-I, Obr-II and Osub, are candidates and hence were carefully investigated in our work (see Figure 1a-c). Although Obr-I is the only H+ accepter, our calculations show that the hole trapped at the Obr–• II (O br-II ), which is the next nearest bridge oxygen to CH 3OH*, is the most stable and the energy is lower than Obr-I–• and Osub–• by 0.16 eV and 0.39 eV, respectively. With the hole trapped at the bridge oxygen site, Obr2− becomes Obr–• radical and Ti-Obr bond is stretched to 2.05 Å from 1.84 Å. Considering that the energy of the IS with Osub–• is the highest in these three sites, which is relatively unstable, it was not further considered and we focused primarily on the other two sites. The O-H dissociation with Obr-II–• was first investigated and the corresponding energy profiles are illustrated in Figure 2a. Interestingly, it is found that the energy barrier for the O-H dissociation in the presence of Obr-II–• is 0.32 eV (see reaction II) and comparing with that of O-H dissociation in the absence of hole (0.17 eV), the presence of hole does not reduce the barrier as one may expect.
dTi-O
Obr-I
1
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Figure 1. The spin density plots (top view) with an iso-value of 0.005 for trapped-hole at different sites: (a) Obr-I site, (b) Obr-II site and (c) Osub site. The middle panel d, e and f are initial state (IS), transition state (TS) and final state (FS) for CH3 OH dissociation in the absence of hole, respectively. The other two bottom panels, (g-i) and (j-l), respectively correspond to the CH3OH dissociation process (from IS to FS) with Obr-I–• and Obr-II–• radical.
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O-H dissociation are +0.64, +1.00 and +0.63 in the absence of the hole, in the presence of Obr-I–• and Obr-II–•, respectively, indicating that the H transfer in the O-H dissociation of CH3OH is a deprotonated process (a heterotypic cleavage mode). From the spin charge distribution of trapped-hole states at Obr-II site (Obr-II–•, see Figure 1j-l), we can see that the hole does not participate in the O-H dissociation, being evidenced from the basically similar charges of Obr-II–• (IS: -0.70; TS: -0.67; FS: -0.67). Besides, when H approaches to bridge oxygen in the presence of Obr-II–• in the TS, the Coulombic attraction between H and bridge oxygen is smaller because of less negatively charged Obr-II–• (-0.67 e) compared to Obr (1.10 e), leading to a higher energy barrier (0.32 eV vs 0.21 eV). Secondly, in order to fully understand whether the hole participates directly in the O-H bond breaking, the O-H bond cleavage in the presence of Obr-I–• that is the H+ accepter was considered (see Figure 1g-i and Figure 2a). It is found that the barrier for the O-H dissociation is even higher (0.60 eV, see the energy profile in Figure 2a), which can be well explained by the spin charge density in Figure 1g-h. Specifically, when H (possessing high positive charge up to +1.00 in the TS) attacks the Obr-I–•, the H that is effectively a proton would strongly repel the hole trapped at Obr-I–•, giving rise to a charge-dispersed state (see Figure 1h). Thus, the stronger repulsion between the positive charges of H+ and the hole in Obr–• I than that in reaction II due to the shorter distance leads to the higher barrier (0.60 eV). On the whole, owing to the additional Coulomb repulsion, the O-H bond dissociation of CH3-
(a)
OH in the presence of Obr–• radical is kinetically less favored than that without hole involved (i.e. the direct thermal process). Thermodynamically, the enthalpy changes for the O-H dissociation under the assistance of hole are -0.05 and 0.12 eV for Obr-I–• and Obr-II–•, respectively, which are very close to the dissociation without hole (0.01 eV). Therefore, together with the above kinetic discussions, we suggest that the O-H dissociation is a thermally driven process and photo-generated holes are not inclined to participate in the reaction (see Figure 2a). C-H Bond Cleavage
In the photo-oxidation of methanol to formaldehyde, the second step is C-H bond cleavage of CH3O species. Similar to the O-H bond cleavage, the heat-driven process (no hole involved, reaction III) was firstly studied. Our calculations show that the barrier for the step without hole is as high as 1.57 eV and the enthalpy change reaches to 0.60 eV (see Figure 2b), which indicates that the C-H bond cleavage hardly occurs both kinetically and thermodynamically. Ti4++Ti4+−O−CH3+Obr2−→2Ti3++CH2O+HObr−
In the presence of bulk hole, the hole is found to be easily trapped at the surface CH3O– species (the O-H dissociation product) with an energy release of 0.81 eV, indicating that CH3O− is an efficient hole scavenger.29 Namely, after the O-H bond cleavage, the photo-generated hole could transfer to the forming CH3O− and oxidize it to yield CH3O• radical at Ti5c4+, resulting from the recombination of the electron and trapped hole (see Figure S2). Simultaneously, the original strong Ti-O bond (1.80 Å) between the surface and CH3O is stretched to 2.22 Å (see Figure 2b). Very interestingly, once the hole is trapped at CH3O–, the dissociation of C-H bond (reaction IV) can readily occur with a barrier as low as 0.21 eV and the enthalpy change of -0.60 eV. In other words, comparing to CH3O−, the formation of CH3O• can significantly decrease the reaction barrier and promote considerably the enthalpy change for the C-H dissociation. Ti4+····•OCH3+Obr2−→Ti3++CH2O+HObr−
(b)
Figure 2. Energetic diagrams for (a) first O-H dissociation with Obr-I–•, Obr-II–•, and no hole and (b) the whole dissociation of CH3OH into CH2O following two different pathways with and without hole. Note: in (b) the process that hole traps at CH3O– uses the state of bulk trapped hole as reference.
(III)
(IV)
Why is there such a large difference with and without hole for the C-H dissociation? After careful analyses, we find that the type of CH3O adsorption state is a crucial factor. From Table 1 and the ISs in Figure 3a and 3b, it can be seen that in the absence of hole the adsorption energy of CH3O– on Ti site is -1.89 eV with the Ti-O bond of 1.80 Å. The strong bonding between O and Ti5c4+ would give rise to the difficult dissociation of C-H bond and it can essentially be understood in the following: The surface Ti5c4+, which bonds directly to CH3O species, acting as an electron acceptor, receives an electron from the dissociating H atom and could further couple with an unpaired electron from O . This leads to a strong bonding between Ti5c4+ and O ; while, as the C-H breaks, the forming CH2O can only bind with Ti5c4+ very weakly with an evidently stretched Ti-O bond (2.37 Å), which accounts for the high barrier for the C-H dissociation according to the BEP relation.47-49 Additionally, the low energy of CH3O adsorption state (i.e. the stable IS) also results in a positive enthalpy change for 0.60 eV. On the contrary, once the hole-electron separation occurred upon UV irradiation, the photo-generated hole would diffuse to surface and be trapped at CH3O– with an energy release of 0.81 eV. In other words, the bulk hole could activate CH3O– species into CH3O• (see Figure 3), evidenced by the weaker adsorption energy of -0.60 eV versus that of CH3O– (-1.89 eV) and the stretched O-Ti bond. Accordingly,
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Scheme 1. Illustration of various reaction pathways for
Figure 3. Illustration of spin charge density plots with an iso-value of 0.005 for C-H dissociation. (a) C-H dissociation without holes; (b) C-H dissociation for the one hole trapped at CH3O–. (c) C-H dissociation for two holes: one is trapped at CH3O- and the other is trapped at Obr-I. the C-H dissociation barrier decreases evidently to 0.21 eV (versus 1.57 eV for CH3O–). Moreover, the formation of weakly bounded CH3O• radical (high-energy state) also explains the considerably enthalpy change (from 0.60 eV to -0.60 eV). In particular, the difference of ∆H correlates well with the difference of adsorption energies between CH3O• and CH3O− (-0.60 eV and -1.89 eV). Thus, it can be seen that a key factor to the C-H dissociation is to weaken the strong Ti-O bond. It should be noted that in real systems, the concentration of surface-reaching holes (10-9) is very low,23 which may lead to the low concentration of CH3O• and it would significantly reduce the possibility of photocatalytic C-H dissociation. Therefore, it is important to compare the kinetics of C-H dissociation with and without hole, which is shown in the following kinetics equations: (1) •
the dissociation of methanol as investigated in this work. The pathways marked in blue are the most preferable pathways. route, and the hole plays a crucial role in the reaction. In principle, another hole could be trapped at Obr, evidenced by the increased charge of Obr in the IS (-0.68, see Table S5). With the aim of obtaining a comprehensive understanding of the effects of hole, we also extended our investigation to the second hole involved in the system. Namely, a possible twohole pathway of the dissociation of C-H bond is shown as follows (also see Scheme 1): Ti4+−•OCH3+Obr−•→Ti4++CH2O+HObr−
(V) −
•
in which the first hole is trapped at CH3O forming OCH3 radical and the other is trapped at Obr2r yielding Obr−•. In conjunction with above results (the O-H bond dissociation), we mainly analyzed the nearest trapped-hole site (Obr-I−•). One can see from Figure 3 large spin charge density accumulation around H at the TSs, indicating that the H transfer is a homolytic cleavage process. In addition, Bader charge analyses of H for the TS are +0.34, +0.32 and +0.19, corresponding to nohole, one hole (CH3O•) and two holes (CH3O• and Obr-I−•), respectively, which demonstrates clearly the homolytic cleavage
(2)
where kheat and kphoto are the rate constants for reactions III and • and IV, respectively, and , are the cover– • 2– ages of CH3O , CH3O and Obr species, respectively. Then, it can be written as: •/ / / ∗ (3) Following the transition state theory, kphoto/kheat=exp(∆Ea/RT) gives rise to an order of 1022 at T=300 K with ∆Ea being the barrier difference of 1.36 eV of the two routes. On the other hand, considering the strong hole trapping capacity of CH3O– (CH3O– + h+⇔ CH3O•, the enthalpy change is -0.81 eV), the • and ratio of is estimated at ~10-9 (see derivation in SI). Then, based on the calculated ratio rphoto/rheat, the C-H dissociation with a hole is about 1013 orders of magnitude faster than the thermal dissociation (without the hole), illustrating a large difference between the thermal and photocatalytic disociations. These results demonstrate firmly that the C-H cleavage of CH3O species is preferred through the photo-assisted
Figure 4. Energy Scheme of photo induced C-H dissocia-
tion of CH3O– species and the comparison with the heatdriven one, which shows the importance of activation of CH3O– species by trapping hole.
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Figure 5. Illustration of hole effects on heterolytic and homolytic cleavage, in which role I and role II are defined in the main text. Role I and role II do not affect the O-H dissociation, while they can have considerable influences on the C-H cleavage if the concentration of holes is considered to be reasonable high. Taking the extremely low concentration of holes into account, role I can dramatically promote the C-H cleavage, while role II is not important, as discussed in the main text. process. Our DFT result shows that energy barriers of C-H accomplishing the catalytic cycle (CH3OH +2h+ → CH2O + −• bond dissociation with the assistance of Obr-I is 0.11 eV. 2H+). Comparing with the activation barrier (0.21 eV) of C-H bond Finally, based on the above explorations, two roles of holes breaking in the absence of Obr−•, it is worth noting that the are emerging: Role I is that a hole is trapped at a reactant or energy barrier with Obr-I−• decreases further by 0.10 eV, which intermediate and thus activates the species, i.e. increasing the is entirely different from the case for the OH dissociation (the energy of the IS (see schematic diagram in Figure 4), which presence of Obr−• increases the barrier). Without Obr−•, the gives rise to a low energy barrier. Role II is subtler; a hole is charges of Obr-I keep invariant from the IS to the TS. Differenttrapped by lattice O near the reactant, forming an active lattice −• ly, the charges of Obr-I with Obr from the IS to the TS deO−• radical which is a stronger oxidative center that can facilicrease by 0.10 (-0.68 to -0.78), which could easily be extate the homolytic dissociation. For example, the homolytic Cplained by the partial combination between Obr-I−• and electron H cleavage can be promoted by O−• radical through the combifrom CH3O group, thus leading to the barrier decrease for the nation of H (electron-richer compared to H+ in the heterolytic C-H dissociation by 0.10 eV. Likewise, by comparing the cleavage) the O−• radical (more electron-deficit compared to charges of Obr-I before and after the dissociation with Obr−• (O2−). The two roles of holes for catalytic reactions are illus0.68 to -1.64, see Table S5), there is almost a complete comtrated in Figure 5. It can be seen that a heterolytic cleavage bination between the electron and the radical. Therefore, in (e.g. the O-H dissociation of CH3OH) cannot be influenced by terms of thermodynamics, there is an obvious promotion upon both role I and role II because (i) CH3OH cannot trap a hole −• the assistance of Obr and no matter where the hole is trapped and hence cannot be activated; and (ii) if a hole is trapped at a at, the enthalpy change drops significantly to approximately nearby lattice oxygen, yielding O−•, the O−• radical can induce 2.60 eV, resulting in the reverse reaction being extremely difan extra repulsion between H+ (electron-deficit) from the hetficult. erolytic cleavage and O−• radical (electron-deficit). In contrast, Noteworthily, as a homolytic cleavage process, the C-H both role I and role II can promote homolytic cleavages (e.g. breaking of CH3O• could be facilitated with Obr-I−• (i.e. the the C-H dissociation of CH3OH): (i) CH3O− anion could readisecond trapped-hole) by lowering the barrier. However, rely trap a hole and thus be activated by the hole, reducing the garding the extremely low concentration of Obr−• (10-15), we barrier of C-H dissociation (role I); and (ii) if a second hole is calculated the C-H dissociation rate with and without Obr−• and trapped by a lattice O− nearby, resulting in O−• radical that found that the pathway for the C-H dissociation with another would facilitate the homolytic C-H bond dissociation, as dishole is unlikely to occur (see details in SI). cussed before.50 It should be pointed out that for the overall reaction rate one must take the concentration of holes into After the cleavages of O-H and C-H bonds, protons are account, as argued above. generated and adsorb on Obr sites (see FSs in Figure 3b). We also explored the possibility of these protons’ departure (reacCONCLUSIONS tion VI). It is found that the barrier for the proton desorption into the water solution is 0.29 eV and the corresponding enIn this work, we investigated systematically the methanol disthalpy change is -0.28 eV. This indicates that the adsorbed H+ sociation with and without holes from theoretical perspective on Obr is inclined to deprotonate and transfer into the solution. and address the role of holes in the process. The overall reac– 2– + tions of heat and hole-assisted processes are presented in reacObr H→Obr +H (aq) (VI) tions VII and VIII, in which the first step is the deprotonation Having investigated these possible pathways, we are in the from OH of CH3OH and the second step is the C-H breaking. position to draw a general picture for the oxidation of metha(VII) CH3OH+2Obr+2Ti4+→CH2O+2H+Obr+2Ti3+ nol to formaldehyde with and without holes, which is shown in Scheme 1. For O-H dissociation, the preferable pathway is CH3OH+2h+→CH2O+2H+(aq) (VIII) that the adsorbed CH3OH dissociates directly into CH3O− and An important finding was that photo-generated hole has lita proton at Obr. Then a hole is trapped at CH3O−, giving rise to tle influence on the step of O-H dissociation (a heterlytic • CH3O radical, which could easily dissociate with a low barrier cleavage process) but considerably reduces the activation barby transferring H to the lattice Obr-I; meanwhile, the dissociatrier for the C-H breaking (a homolytic cleavage process); the ed H is transferred to solution, releasing an electron, which first dissociation may be driven by heat and the second dissocould readily combine with a second photo-generated hole,
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ciation is driven by light. Furthermore, the key of holefacilitated C-H cleavage was identified; it is due to the weaker adsorption of CH3O• radical resulting from the activation of CH3O– through trapping a hole, which leads to the decrease of Ti-OCH3 binding strength thereby reducing the energy barrier. A preferable pathway for the C-H dissociation was determined, as shown in Scheme 1 (marked in blue). It was demonstrated that holes can play two roles, the effects of which on heterlytic and homolytic dissociations were clearly analyzed. This work may offer new significant insights into general photocatalytic reactions.
ASSOCIATED CONTENT Supporting Information Computation details, optimized structures, radical’s formation and concentration estimation, kinetic analysis and Bader charge analysis for C-H and O-H dissociation. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author
[email protected] [email protected] Author Contributions #
These authors contributed equally.
Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT This project is support by National Key Basic Research Program of China (2013CB933201), National Natural Science Foundation of China (21303052, 21333003, 21622305), Chen-Guang project (13CG24), Young Elite Scientist Sponsorship Program by CAST, Fundamental Research Funds for the Central Universities. We also gratefully thank Prof. Xueming Yang for the helpful discussions.
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