Deuterium Kinetic Isotope Effect in the Photocatalyzed Dissociation of

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Deuterium Kinetic Isotope Effect in the Photocatalyzed Dissociation of Methanol on TiO2(110) Tianjun Wang, Qunqing Hao, Zhiqiang Wang, Xinchun Mao, Zhibo Ma, Zefeng Ren, Dongxu Dai, Chuanyao Zhou, and Xueming Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09077 • Publication Date (Web): 30 Oct 2018 Downloaded from http://pubs.acs.org on November 5, 2018

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The Journal of Physical Chemistry

Deuterium Kinetic Isotope Effect in the Photocatalyzed Dissociation of Methanol on TiO2(110) Tianjun Wang,1,2,7 Qunqing Hao,3 Zhiqiang Wang,4 Xinchun Mao,5 Zhibo Ma,2 Zefeng Ren,2 Dongxu Dai, 2 Chuanyao Zhou,2, *) Xueming Yang 2,6 *) 1. Shanghai Advanced Research Institute, Chinese Academy of Sciences, No.99 Haike Road, Zhangjiang Hi-Tech Park, Pudong, 201210, Shanghai, P. R. China 2. State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, 457 Zhongshan Road, Dalian, 116023, Liaoning, P. R. China 3. Science and Technology on Surface Physics and Chemistry Laboratory, P.O. Box No. 9-35, Huafengxincun, Jiangyou, 621908, Sichuan, P. R. China 4. School of Physics and Optoelectronic Engineering, Xidian University, Xi’an, 710071, Shaanxi, P.R. China 5. Institute of Materials, China Academy of Engineering Physics, 596 Yinhe Road 7th Section, Shuangliu, Chengdu, 610200, Sichuan, P. R. China 6. Department of Chemistry, Southern University of Science and Technology, 1088 Xueyuan Road, Shenzhen, Guangdong, 518055, P. R. China 7. University of Chinese Academy of Sciences, No.19A Yuquan Road, Shijingshan District, 100049, Beijing, P. R. China *) To whom all correspondence should be addressed.

Email address:

[email protected], [email protected] Tel: +86-411-84379701 Fax: +86-411-84675584 1

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Abstract Deuterium kinetic isotope effect (KIE) in the photochemistry of methanol on TiO2(110) has been studied to find the rate-determining step (RDS) and understand the reaction mechanism using two-photon photoemission spectroscopy (2PPE). Deuterium substitution of the methyl hydrogen has little effect on the kinetics of this reaction, suggesting that neither the break of the C-H(D) bond nor the transfer of H(D) atoms to the bridging sites is the RDS in the transformation of methanol into formaldehyde. In contrast, the reaction rate of MeOH is ~1.3 times of that of MeOD, suggesting that the cleavage of O-H(D) is the RDS in the photocatalyzed dissociation of methanol on TiO2(110). The results contradict with the common fact that C-H(D) is more difficult to break than O-H(D)

based on ground state energetics, implying the involvement of

photogenerated charge carriers in the reaction of C-H break whereas the cleavage of O-H is likely a thermal reaction. Difference in the activation energy of O-H and O-D dissociation reaction in the methanol/TiO2(110) system has been calculated based on the KIE measurements. Our work is consistent with the fact that methoxy is photocatalytically more reactive than methanol, and suggests that the conversion of methanol into methoxy is crucial in the photochemistry of methanol on TiO2(110) and probably other metal oxide semiconductor surfaces.

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Introduction Titanium dioxide is an attractive semiconductor in both technological and scientific fields. For example, it is widely used in pigments, sunscreen creams, self-cleaning surfaces, gas sensors, biocompatible materials, and so on.1 Particularly, owing to its capability of realizing solar to chemical2 and solar to electricity3 conversion, TiO2 is of great relevance to energy and environment, which are two main issues the human being are facing. Many of its features, such as the abundance, low cost, stability, nontoxicity and relatively high solar conversion efficiency, have enabled TiO2 in various applications in the field of clean energy. Although photo-catalyzed water splitting by TiO2 has been discovered,2 the overall hydrogen production efficiency on pure TiO2 photocatalyst is extremely low4 due to the rapid electron-hole recombination and a large overpotential. To enhance the charge separation efficiency, a lot of methods, such as loading TiO2 with noble metals, coupling TiO2 to other semiconductors and adding sacrificial reagents to the reaction solution, have been developed.5-6 As a hole scavenger,7 methanol can greatly enhance the photocatalytic hydrogen production over TiO2-based photocatalysts when it is added into the solution.8 Such remarkable effect has motivated the extensive investigation of the chemical and photochemical properties of methanol on TiO2 surfaces,1, 8-14 with materials ranging from nanoparticles to well-ordered single crystals and environments varying from vacuum to solution condition. In solution, methanol is photocatalycally oxidized into formaldehyde, formic acid and eventually carbon dioxide.8 In the vacuum condition, adsorbed species such as CH2O, CH2OO and HCOO have been identified by infrared spectroscopy on Pt loaded TiO2 powder.14 Specially, the combination of ultrahigh vacuum (UHV) based 3



surface science techniques and well-ordered single crystal surfaces provides the opportunity to understand the chemistry of methanol on TiO2 surfaces at the molecular level. The adsorption of methanol on (110), the most stable low index surface of rutile TiO2, is at the center of the study of the interaction between methanol and TiO2 surface at the beginning.15-23 Briefly, methanol prefers molecular and dissociative adsorption at the fivefold coordinated Ti sites (Ti5c) and the bridging oxygen vacancies (Ov), respectively. In 2010, we performed the first mechanistic study of the photochemistry of methanol on a TiO2 single crystal surface, i.e., rutile TiO2(110), via a combination of two-photon photoemission spectroscopy (2PPE), scanning tunneling microscopy (STM) and density functional theory (DFT) calculations.24 The ultraviolet (UV) light illumination dependent excited state at 2.4 eV above the Fermi level (EF) of submonolayer methanol-covered, stoichiometric TiO2(110) in 2PPE is associated with the dissociation of Ti5c bounded methanol rather than the previously reported wet electron state.18 It is now clear that the photocatalyzed dissociation of methanol at Ti5c sites of TiO2 produces formaldehyde and bridging hydroxyls (ObH),25-27 leading to the reduction of the TiO2 surfaces. Reduction from Ti4+ to Ti3+ increases the density of states (DOS) of both the band gap states and the unoccupied states in the conduction band which are formed by the Jahn-Teller splitting of the 3d orbitals in the TiO2 field, resulting in the enhanced localized d-d transition in Ti3+ ions as observed in 2PPE measurements (Figure 1).28-30 Therefore, the excited resonance in 2PPE provides the fingerprint to follow the reaction kinetics of the photocatalyzed dissociation of methanol on TiO2(110).12-13, 24, 31-33 Conversion of methanol into formaldehyde requires the cleavage of both the O-H 4


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and C-H bonds. According to the DFT calculations based on ground state reactions, the molecular and dissociative adsorption of methanol at Ti5c sites of TiO2(110) is nearly degenerate with the molecular form slightly more stable by several tens meV, and the reaction barrier is several hundreds meV.23,

25, 34-35

However, the production of

formaldehyde from methoxy is endothermic by about 1 eV with a barrier as high as 1.6 eV.25,

34-35

As a result, moderate change of conditions for example temperature and

coverage will affect the equilibrium between methanol and methoxy,36-38 while subsequent generation of formaldehyde requires intensive external forces. Such energetics in the methanol/TiO2(110) system has been justified by a combined 2PPE/STM study.33 The stepwise photocatalyzed dissociation of methanol on TiO2(110) where the cleavage of O-H precedes that of C-H bond has been reported (Reactions 1 and 2).25

CH 3OH Ti Ob  hv / heat  CH 3OTi  Ob H

(1)

CH 3O Ti Ob  hv  CH 2OTi  Ob H

(2)

Where the subscript “Ti” stands for the adsorption sites, i.e., Ti5c. The much higher activation barrier (1.6 eV) of Reaction 2 according to the above energetics suggests that this reaction should be the rate-determining step (RDS) in the dissociation of methanol into formaldehyde if it is a thermal reaction. To gain more insight into the elementary steps of photocatalyzed dissociation of methanol on TiO2(110), i.e., the RDS, we herein have chosen deuterium substitution of both the methyl and hydroxyl hydrogens to investigate the kinetic isotope effect (KIE). By monitoring the evolution of the excited resonance in 2PPE, we measured the kinetics of the photocatalyzed dissociation of four methanol isotopologues (CH3OH, CH3OD, 5



CD3OH and CD3OD) on TiO2(110). Different from the hypothesis based on thermal chemistry, deuterium substitution of the hydroxyl rather than the methyl hydrogen lowers the reaction rate of methanol photochemistry on TiO2, suggesting the involvement of photogenerated charge carriers in the C-H bond break reaction, while the O-H cleavage is likely a thermal reaction. Since the break of O-H occurs before the cleavage of C-H, the two reactions do take place from different surface species, i.e., Ti5c bounded methanol and methoxy, respectively. The observed isotope effect is consistent with the finding that Ti5c bounded methoxy is photocatalytically more reactive than methanol.39 Therefore, conversion of methanol into methoxy is crucial in the photochemistry of methanol on TiO2(110) and probably other metal oxide semiconductor surfaces. Experimental Details Time-dependent 2PPE, which is capable of monitoring the kinetics of photocatalyzed dissociation of methanol on TiO2(110),32 was carried out in a UHV apparatus (base pressure 1 suggests E1 should be significantly larger than E2. According to the calculations in reference 35, E1 and E2 are comparable. More work is needed to measure the accurate energetics in the photocatalyzed dissociation of methanol on TiO2(110). The current experimental results may provide some clues for future DFT calculations. In a bond cleavage event in the ground electronic state, the KIE arises from the difference of the zero-point energy (ZPE) between the related bonds both in reactants (ZPER) and the transition state (ZPETS) (Figure 4). According to the harmonic oscillator model, the angular frequency (ω) and the quantized stretching vibration energy (Ev) can be written as follows: 12


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

K mr

(8)

mr 

m1m2 m1  m2

(9)

1 Ev  (v  )h 2

(10)

Where K is the bond force constant and mr is the reduced mass. The fundamental vibration of O-H at the methanol/TiO2 interface appears at 3245 cm-1 according to a high-resolution electron energy loss spectroscopy (HREELS).16 That of O-D will be about 2295 cm-1 by taking into account of the reduced mass. Therefore, the difference in the ZPE of O-H and O-D in the methanol/TiO2 system is 475 cm-1, which is about 59.4 meV (ZPER-MeO-H- ZPER-MeO-D). As discussed in the above text, O-H bond break, a thermal reaction, is the RDS in the complete photocatalyzed dissociation of methanol on TiO2(110). The KIE of about 1.3 at 120 K yields a 4.2-meV difference in the activation energy (AE) of MeO-H and MeO-D bond cleavage reactions, i.e., (ZPETS-MeO-DZPER-MeO-D)-(ZPETS-MeO-H- ZPER-MeO-H) =4.2 meV, according to the Arrhenius rate equation. The involvement of holes in the reaction of methoxy on TiO2(110) greatly changes the energetics and consequently the reaction kinetics. Then the break of O-H bond becomes the RDS in the complete photocatalyzed dissociation of methanol on TiO2(110). Therefore, conversion of methanol into methoxy is crucial in the photochemistry of methanol on this surface and maybe other metal oxide semiconductor surfaces. Oxygen species and Au clusters are able to convert methanol to methoxy on TiO2(110).39, 47 Most 13



recently, we have found Pt loading onto TiO2(110) could also promote O-H dissociation to generate methoxy, accelerating the photocatalyzed dissociation of methanol to formaldehyde. Though some theoretical work suggests the higher lying highest occupied molecular orbitals (HOMO) of methoxy is responsible for the superior hole capturing ability over methanol on TiO2(110),48 comparative experimental work, for example, electronic structure and ultrafast charge carrier dynamics, are needed to understand the underlying mechanism. Conclusions Time-dependent 2PPE has been adopted to study the isotope effect in the photocatalyzed dissociation of methanol on TiO2(110) in order to investigate the properties of the elementary reactions. KIE has been detected in the deuterium substitution of O-H instead of C-H bond, which is different from the energetics of thermal reactions on ground electronic state, suggesting that the involvement of photogenerated charge carriers in C-H bond break reaction and O-H cleavage is likely a thermal reaction. Our results are consistent with previous studies which suggest the break of O-H and C-H bond are driven by heat and photon, respectively. Difference in the activation energy of O-H and O-D break reaction in the methanol/TiO2(110) system has been calculated based on the KIE measurements. Our work further suggests the crucial role of conversion methanol into methoxy in the photochemistry of methanol on TiO2(110), and probably other metal oxide semiconductor surfaces. Acknowledgements This work was supported by the National Natural Science Foundation of China 14


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(21573225, 21688102 and 21703164), the National Key Research and Development Program of China (2016YFA0200600 and 2018YFA0208700), the Strategic Pilot Science and Technology Project (XDB17000000) and the Youth Innovation Promotion Association of CAS (2017224).

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Figures and Captions

Figure 1. Schematic diagram showing the band gap excitation of TiO2 and the detection of photocatalyzed dissociation of methanol on TiO2 by monitoring the d-d transition in Ti3+ ions using two-photon photoemission spectroscopy. Ti3+ ions can be produced by surface hydroxylation after photoinduced methanol dissociation on TiO2(110). VB: valence band. CB: conduction band.

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Figure 2. (A) Experimental configuration showing the polarization of the laser beam and the azimuth of the substrate. (B) Typical 2PPE spectra for the bare and CH3OD covered TiO2(110) surface after the excited resonance signal has reached the maximum. P and S stand for the horizontal and vertical polarization respectively of the laser beam used for 2PPE measurements. P-NS represents the pure excited resonance signal which is obtained by subtracting the normalized s-polarized data (NS) from the p-polarized data. (C) Selected 2PPE spectra for CH3OD/TiO2(110) interface acquired at indicated illumination time. (D) Integrated excited resonance signal as a function of illumination time and the fractal kinetic model fitting.

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1.6 0.100

CH3OH

1.2

CD3OH

0.075

CH3OD CD3OD kMeOH/kMeOD

0.050

0.8

kMeOH/kMeOD

Reaction Rate (/s)

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


0.4

0.025

0.000

0

2

4

10

100

0.0 1000

Time (s)

Figure 3. Time-dependent reaction rate (left axis) of the four methanol isotopologues according to fractal kinetics model fitting and the ratio of the reaction rate kMeOH/kMeOD (right axis) which shows the kinetic isotope effect. Please note the horizontal axis is linear and logarithm before and after the break, respectively.

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Figure 4. Schematic diagram of the activation energy of MeO-H and MeO-D bond break on TiO2(110). The difference between the activation energy results in the kinetic isotope effect in this reaction.

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TOC Graphic

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Deuterium Kinetic Isotope Effect in the Photocatalyzed Dissociation of

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