Effect of Multilayer Methanol and Water in Methanol Photochemistry

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Effect of Multilayer Methanol and Water in Methanol Photochemistry on TiO Wenshao Yang, Zhenhua Geng, Qing Guo, Dongxu Dai, and Xueming Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04224 • Publication Date (Web): 24 Jul 2017 Downloaded from http://pubs.acs.org on July 30, 2017

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The Journal of Physical Chemistry

Effect of Multilayer Methanol and Water in Methanol Photochemistry on TiO2 Wenshao Yang1), Zhenhua Geng2), Qing Guo2),*), Dongxu Dai2), Xueming Yang1),2),*) 1) Hangzhou Institute of Advanced Studies, Zhejiang Normal University, 1108 Gengwen Road, Hangzhou 311231, Zhejiang, P. R. CHINA 2) State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, Liaoning, P. R. CHINA

*) To whom all correspondence should be addressed.

Email addresses: [email protected] and

[email protected] (XY).

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ACS Paragon Plus Environment


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ABSTRACT We have conducted a series of temperature programmed desorption (TPD) experiments to study the photo-induced reactions of methanol (CH3OH) on a rutile(R)-TiO2(110) surface with 355 nm light. Products, formaldehyde (CH2O) and water (H2O) have been detected. The results clearly show that the photochemistry of CH3OHTi5c is hindered by the coadsorbed multilayer CH3OH or H2O. For multilayer CH3OH adsorption, CH3OH molecules hydrogen-bonded to the bridge bonded oxygen sites inhibits the CH3OHTi5c photochemistry. Combined with previous theoretical studies (Sci. China Chem., 2015, 58, 614-619), CH3OH molecules hydrogen bonded to the BBO sites increases the energy barrier of C-H bond dissociation and decreases the energy barrier of the reverse reaction, resulting in lowering the efficiency of CH3OHTi5c photolysis. As the coverage of CH3OH keeps increasing to multilayer, the efficiency of the CH3OHTi5c photolysis does not decrease any more. However, with the coadsorbed H2O molecules, the efficiency of CH3OHTi5c photolysis decreases continually with increasing H2O coverage, which is likely due to the decrease of the chemisorbed CH3OHTi5c molecules via molecule exchange between the H2O films and the chemisorbed CH3OH layer that occurs at ~100 K and the complicated hydrogen bonds formed between H2O and CH3OHTi5c molecules.

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INTRODUCTION TiO2 is a particularly important catalyst with a wide variety of applications, including potential use in photocatalytic water (H2O) splitting to produce hydrogen

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photocatalytic degradation of organic pollutants.2 The enormous applications of TiO2 have stimulated extensive research activities on the catalytic and photocatalytic reactions on single crystals, particles, and aqueous suspensions.3 Methanol (CH3OH), as a commonly used chemical reagents, plays a very important role in H2O splitting to hydrogen production with TiO2 photocatalysts.4 In the last decades, the chemistry of CH3OH on rutile(R)-TiO2(110), as a prototype, have been extensively studied by a series of techniques. 5-26 From scanning tunneling microscopy (STM) study,7 it has been well established that CH3OH molecules molecularly adsorb on the five coordinated Ti4+ sites (Ti5c, CH3OHTi5c) of the R-TiO2(110) surface and dissociatively adsorb at bridge bonded oxygen (BBO) vacancy sites for sub-monolayer adsorption, which is consistent with temperature programmed desorption (TPD) results obtained by Henderson and coworkers.6 For multilayer adsorption,6 CH3OH molecules adsorb on BBO rows or CH3OH layers via intermolecular hydrogen bonds. Later, using two photon photoemission (2PPE) technique, Zhou and workers20 observed a new electronic state appeared and increased significantly along with the laser irradiation time on the CH3OH adsorbed R-TiO2(110) surface. Combination with 2PPE and STM studies, these authors20 proposed that the new state was attributed to CH3OHTi5c photolysis on the R-TiO2(110) surface. Recently, Guo and coworkers21 has carried out a systematic 3



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investigation of CH3OH photolysis on R-TiO2(110) using TPD method, they found that the photolysis of CH3OHTi5c to formaldehyde (CH2O) occurred in a stepwise manner mechanism on R-TiO2(110). Meanwhile, photolysis of methoxy (CH3O) on R-TiO2(110) to form CH2O was also observed by Henderson and coworkers.17,23 Further, cross coupling reaction of CH2O and CH3O to form methyl formate initiated by CH3OH photocatalysis on R-TiO2(110) was also confirmed by three different groups almost at the same time.22,24,25 Despite intensive scrutiny, some fundamental processes of CH3OH photolysis on TiO2 surfaces still remain to be addressed. For example, it is well-known that photocatalytic reactions take place at the interface and all the studies mentioned above emphasize on CH3OH photolysis with sub-monolayer coverage of CH3OH on the Ti5c sites of R-TiO2(110) surface, however, the photochemistry of multilayer CH3OH, which is much closer to mimic the practical situation in heterogeneous catalysis, has never been reminded. Henderson and coworkers17 found that coadsorbed H2O on the Ti5c sites (H2OTi5c) did not promote either molecular CH3OHTi5c photolysis or thermal dissociation of CH3OHTi5c to CH3O. While, H2OTi5c molecules on the Ti5c sites (H2OTi5c) had no influence on CH3O photochemistry, and physisorbed H2O molecules on the BBO sites via hydrogen bonding inhibited CH3O photodecomposition to CH2O. However, detailed information about the influence of coadsorbed multilayer CH3OH or H2O on CH3OHTi5c photochemistry is still lacking. In this work, we have carried out a series of experiments to investigate the influence of coadsorbed multilayer CH3OH or H2O on the photochemistry of CH3OHTi5c on the RTiO2(110) surface by TPD experiments. Multilayer CH3OH and H2O show different 4


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inhibition behavior on CH3OHTi5c photochemistry. EXPERIMENTAL METHODS Experiments were performed with an ultrahigh vacuum (UHV) chamber. Details of the TPD apparatus have been described elsewhere.21 The R-TiO2(110) single crystal (10×10×1 mm3), purchased from Princeton Scientific Corp was cleaned by cycles of Ar+ sputtering and UHV annealing. After this preparation procedure, a BBO vacancy coverage of 0.05 ML (1 ML = 5.2 × 1014 molecules/cm2, which refers to the number of Ti5c sites in 1 cm2 on R-TiO2(110).) was obtained, as gauged by H2O TPD. CH3OH (> 99.9%) and H2O (> 99.9%) purchased from Sigma-Aldrich were further purified by a few freeze-pump-thaw cycles. CH3OH and H2O were dosed to reduced R-TiO2(110) surfaces with a home-built, calibrated, molecular beam doser at a surface temperature of 100 K. All the TPD experiments were performed with a ramping rate of 2 K/s. The 355 nm laser used for CH3OH photolysis on R-TiO2(110) is generated by the third harmonic of a diode-pumped, solid state (DPSS), Q-switched 1064 nm laser (SpectraPhysics). The laser pulse duration was about 12 ns and the laser repetition rate was 50 kHz. About 40 mW of laser light (1.3×1017 photons cm-2 s-1) was used for all the experiments, in an effort to minimize the temperature increase of the surface resulting from laser irradiation. The TiO2 surface temperature rose by about 1-2 K with the surface temperature at ~100 K. RESULTS AND DISCUSSION Figure 1 shows TPD spectra collected at a mass-to-charge ratios (m/z) of 31 (CH2OH+) 5



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m/z = 31 (CH3O ) (A) 0.67 ML

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TPD signal / arb.unit

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(B) 1 ML Irradiation time 0 min 0.5 3 20

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multilayer

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2nd layer 100

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Figure 1. Typical TPD spectra collected at m/z = 31 (CH2OH+) with accumulating coverage of CH3OH dosed on the surface following different laser irradiation time at 355 nm. CH2OH+ is the most of the cracking pattern of the desorbed parent CH3OH molecule in the electron-bombardment ionizer.

after adsorbing various coverages of CH3OH (0.67 ML, 1 ML, 1.4 ML, 2.2 ML) on RTiO2(110) as a function of laser irradiation time. As the coverage of CH3OH increases to 2.2 ML, four prominent features at 139, 165, 300 and 500 K have been observed without irradiation (Figure 1D). The 139 K peak and 165 K shoulder are assigned to the multilayer 6


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and the second layer desorption of CH3OH molecules, the feature at 300 K is assigned to the desorption of molecularly adsorbed CH3OHTi5c molecules, and the broad tail around 500 K is attributed to the recombinative desorption of some CH3O groups, resulting from either dissociation at vacancy or nonvacancy sites.6

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m/z = 30 (CH2O )

(A) 0.67 ML

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(B) 1.0 ML 20

TPD signal / arb.unit.

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Irradiation time 0 min 0.5 3 20

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(C) 1.4 ML 20

10

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(D) 2.2 ML

30 20 10 0

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Figure 2. Typical TPD spectra collected at m/z = 30 (CH2O+) with accumulating coverage of CH3OH dosed on the surface following different laser irradiation time at 355 nm. The m/z = 30 signal has two components: the parent ion signal of photocatalytic product formaldehyde and the ion-fragment signal 7



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of the parent CH3OH molecule.

As the irradiation time increases, the broad tail about 500 K decreases significantly on the 0.67 ML of CH3OH covered TiO2(110) surfaces (Figure 1A), and the 300 K peak shifts to lower temperature and becomes narrower and higher, which is due to an increased concentration of H atoms at BBO sites from CH3OH photolysis.21 As the coverage of CH3OH increases, the change trends of the 300 K peak are the same along with irradiation time (Figure 1B-1D). However, the shift and the height raise of the 300 K peak become slower and slower for the same irradiation time, suggesting that the photolysis of CH3OHTi5c is slowed down as the coverage of CH3OH increase. To confirm the reaction product, TPD spectra at m/z = 30 (CH2O+) as a function of laser irradiation time were collected, as shown in Figure 2A-D. In these spectra, a new TPD peak at 265 K appears and increases prominently with increasing irradiation time, which is assigned to the desorption of CH2O product at the Ti5c sites.21 Whereas, when the coverage of CH3OH increases from 0.67 ML to 1.4 ML, the intensity of product CH2O decreases for the same irradiation time. For 20 min irradiation, the intensity of CH2O product at 1.4 ML (Figure 2C) decrease by about 20%, compared with that at 0.67 ML (Figure 2A). As the coverage of CH3OH increases to 2.2 ML (Figure 2D), the peak intensity of product CH2O is nearly the same as that at the coverage of 1.4 ML. Unfortunately, CH2O may be photo desorbed during laser irradiation.22 While, because of the overlap between TPD peaks of CH2O and CH3OH at m/z = 30, the yield of CH2O estimated from direct TPD measurement of CH2O product is not very accurate. Another major product from CH3OHTi5c photolysis 8


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on R-TiO2(110) is the H2O product (desorbing at ~500 K), which is formed via recombination of dissociated H atoms on the BBO sites and BBO atoms, leaving behind an BBO vacancy (BBOv) upon heating. When the surface temperature is about 100 K, the dissociated H atoms do not desorb by light. While, the

+

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m/z = 18 (H2O )

(A) 0.67 ML

Irradiation time

(B) 1.0 ML

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0 10

TPD signal / arb.unit

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0 min 0.5 3 20

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(C) 1.4 ML

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(D) 2.2 ML

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Figure 3. Typical TPD spectra collected at m/z = 18 (H2O+) with accumulating coverage of CH3OH dosed on the surface following different laser irradiation times at 355nm. The m/z = 18 signal has three components: the small impurity in the CH3OH sample ,the ion-fragment signal of the parent CH3OH 9



molecule, as well as the parent ion signal of H2O stem from H atoms adsorbed on the BBO sites.

effect of fragmentation of CH3OH at m/z = 18 is very small. Therefore, monitoring dissociated H atoms production by the desorption of H2O is the preferential way to measure the product formation. We have therefore collected TPD spectra at m/z = 18 as a function of laser irradiation time with different coverages of CH3OH on R-TiO2(110). As shown in Figure 3A-D, the 500 K TPD peak, which is due to recombinative desorption of H2O from dissociated H atoms on the BBO sites, shows a distinct rise as the laser irradiation time increases at different coverages of CH3OH on the surface.

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H2O yield / ML

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0.04 0.67 ML 1.0 ML 1.4 ML 2.2 ML 5.1 ML 10.3 ML

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0.00

0

5

10

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Irradiation time / min Figure 4. Yield of H2O produced from photolysis of different coverage of CH3OH as a function of laser irradiation time derived from Figure 3 while the solid lines are exponential functions that appear to follow the data very well.

To evaluate the effect of CH3OH coverage on CH3OHTi5c photolysis on R-TiO2(110) the yield of H2O product from photolysis of different coverages of CH3OH as a function 10


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of laser irradiation time has been calculated (Figure 4). Taking into account that the TPD traces of m/z = 18 at 5.1 ML and 10.3 ML coverage of CH3OH are similar to that collected at 2.2 ML coverage with different irradiation times, the TPD spectra m/z = 18 at 5.1 ML and 10.3 ML coverage are not shown. A significant drop in the yield of H2O product is observed as the coverage increases from 0.67 ML to 1.4 ML for the same irradiation time. However, as the coverage of CH3OH keeps increasing, the yield of H2O product is the same as that at 1.4 ML. As now it is unambiguous for us to draw the conclusion that the second layer CH3OH inhibits the photo reaction of CH3OHTi5c remarkably. However, the multilayer CH3OH does not affect the efficiency any more. Based on previous studies on the role of H2O in CH3OH photochemistry on R-TiO2(110),17 H2O molecules hydrogenbonded to BBO sites inhibit CH3O photodecomposition by blocking the ability of these sites to accept H atoms from CH3O decomposition. For CH3OH adsorption on R-TiO2(110), 0.67 ML is close to the saturation coverage of the first layer on Ti5c sites, and 1.4 ML is close to the saturation coverage of the second layer on the BBO sites. Meanwhile, CH3OH molecules adsorb on the BBO sites by forming hydrogen bonds,6 which is similar to H2O adsorption on the BBO sites. Thus, the second layer CH3OH molecules hydrogen-bonded to BBO sites may also inhibit CH3O photodecomposition in the same manner, resulting in the depression of CH3OHTi5c photolysis by the second layer CH3OH adsorption. On the basis of previous study of CH3OH photolysis by Henderson and coworkers,15 the broad TPD state from 400 K to 600 K (Figure 1) is attributed to dissociatively adsorbed CH3OH molecules at vacancy or non-vacancy sites, and the CH3O groups show much high 11



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reactivity than molecularly adsorbed CH3OH on the Ti5c sites. At 0.67 ML coverage of CH3OH (Figure 1A), the 500 K broad tail nearly disappears after 3 min irradiation. As the CH3OH coverage increases, the depletion of the 500 K broad tail also decreases for the same irradiation time, indicating that multilayer CH3OH also inhibits CH3O photolysis in the same manner. Recently, a theoretical work done by Fan and coworkers27 found that the saturated coverage of the first layer CH3OH on a stoichiometric R-TiO2(110) surface is 0.75 ML, which is in accordance with previous TPD measurements that the coverage of CH3OH for a saturated first layer on R-TiO2(110) has been estimated at about 0.67-0.77 ML, varying to a small extent depending on the BBOv coverage.15 More importantly, the theoretical results show that dissociation of CH3OHTi5c on a multilayer CH3OH covered R-TiO2(110) surface to CH2O occurs through a stepwise pathway, with easy O-H bond dissociation and rate-determining C-H bond dissociation. As the coverage of CH3OH increases to two or three layers, CH3OH molecules form a hydrogen-bonded network with each other, the ratedetermining barriers (the barrier of C-H bond dissociation) for two and three layers of CH3OH will rise to 1.79 and 1.76 eV, respectively. In comparison, the C-H bond dissociation barrier for sub-layer CH3OH adsorption is only about 1.57 eV.22 Thus, the ratedetermining barriers for multilayer CH3OH adsorption are higher than that for sub-layer CH3OH adsorption by 0.19-0.21 eV, which is in the range of the strength for a hydrogen bond, suggesting that the hydrogen bonds between the second layer CH3OH molecules and the BBO sites, or between the second layer CH3OH molecules and the CH3OHTi5c 12


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molecules inhibit the dissociation of CH3OHTi5c molecules.

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m/z = 31 (CH3O ) (A) 0.67 ML

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(B) 0.67 ML+ 0.5 ML

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TPD signal / arb. unit.

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0

(C) 0.67 ML+ 1.0 ML 60

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Figure 5. TPD spectra collected at m/z = 31 (CH2OH+) with accumulating coverage of H2O dosed on 0.67 ML CH3OH covered R-TiO2(110) surface following different laser irradiation time at 355 nm.

In addition, Feng et al.28 found that the CH2O yield of CH3OH photolysis from the TPD measurements is much smaller by a factor of 2/3 than the amount of dissociated CH3OH from the STM measurements at 80 K. These authors then assigned their observation to the reverse reaction during the TPD measurement. While, according to Fan’s 13



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results,27 the reverse barriers of the second step dissociation (C-H bond cleavage) for two and three layers of CH3OH are lower than that for sub-monolayer of CH3OH by 0.16-0.25 eV, suggesting that the reverse reaction will occur much easier during the TPD measurement for two and three layers of CH3OH adsorption, leading to decrease the efficiency of CH3OH photolysis. To get a further understanding of the inhibition behavior for CH3OH photolysis, another experiment was done with the multilayer H2O and sub-monolayer CH3OHTi5c coadsorbed R-TiO2(110) surfaces. The surfaces were prepared by dosing 0.67 ML methanol first and then subsequently water with different coverages at 100 K. Figure 5 shows the TPD spectra collected at m/z = 31 (CH2OH+) with different coverages of H2O (0 ML, 0.5 ML, 1 ML, 3.9 ML) dosed on the 0.67 ML CH3OH pre-covered R-TiO2(110) surface following different laser irradiation time at 355 nm. Before irradiation, with increasing H2O coverage, CH3OH TPD spectra were separated into two parts: part of CH3OH keeps adsorbing at the Ti5c sites, and the other part of CH3OH physisorbs to the surface (Figure 5A-D), which is in accordance with previous result.6 While, the chemisorbed CH3OH becomes less and less as the coverage of H2O increases. Conversely, the physisorbed CH3OH becomes more and more. As the H2O coverage increases to 3.9 ML (Figure 5D), two desorption peaks at 180 K and 157 K are observed for the physisorbed CH3OH, which are the same as the desorption temperature of the second layer and multilayer of H2O on R-TiO2(110) (Figure 6D). However, as shown in Figure 1, the second layer and multilayer peak of CH3OH are at 165 K and 139 K for multilayer CH3OH 14


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Figure 6. TPD spectra collected at m/z = 18 (H2O+) with accumulating coverage of H2O dosed on 0.67 ML CH3OH covered R-TiO2(110) surface following different laser irradiation time at 355 nm.

adsorption. Thus, the desorption temperature of the physisorbed CH3OH in Figure 5D suggests that the physisorbed CH3OH may be solved into H2O to form a super cool solution, leading to the decreased coverage of CH3OHTi5c molecules. As the coverage of H2O increases, the temperature shift of the 300 K peak becomes less and less after irradiating the surface for the same time (Figure 5A-D), similar to the 15



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multilayer CH3OH photochemistry (Figure 1D). Especially for 3.9 ML H2O coadsorption, it is hard to see any difference the TPD traces after irradiation for 0 and 20 min, suggesting that only a small portion of CH3OHTi5c molecules are dissociated. To measure the yield of CH3OHTi5c photolysis, the TPD spectra at m/z = 18 (H2O+) as a function of irradiation time were also collected (Figure 6A-D). Unlike the multilayer CH3OH situation (Figure 3D), the product H2O peak (~ 500 K) keeps decreasing as the H2O coverage increases upon the same irradiation time, indicating that the increase of the H2O coverage depresses the efficiency of CH3OHTi5c photolysis continually. Meanwhile, the yield of H2O produced from CH3OH photolysis on the different coverages of H2O and 0.67 ML CH3OH coadsorbed R-TiO2(110) surface as a function of laser irradiation time has been calculated, as shown in Figure 7. For the same time irradiation, the yield of H2O continues to fall with increasing H2O coverage, indicating that the influence of multilayer H2O molecules on the 0.67 ML CH3OHTi5c covered R-TiO2(110) surface is more complicated than multilayer CH3OH covered R-TiO2(110) surface. Although a saturated first layer of CH3OH adsorbs on R-TiO2(110), H2O molecules can still have opportunity to adsorb on the unoccupied Ti5c sites. Meanwhile, as the coverage of H2O increases, molecular exchange of the second layer H2O and the CH3OHTi5c layer will occur. From early study of CH3OH adsorption on the 2 ML H2O pre-covered RTiO2(110) surface by Henderson and coworkers,6 molecular exchange between the multilayer CH3OH and the second and first layers of H2O occurs on adsorption at 135 K. While, based on the work done by Petrik and coworkers, 29 the molecular exchange 16


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between different H2O isotopes adsorbed on the Ti5c sites and BBO rows occur with a distribution of activation energies and is surprisingly efficient at the surface temperature > 70 K. Thus, the molecular exchange between CH3OH adsorbed on the Ti5c sites and H2O

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H2O yield / ML

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0.06

Exposure of methonal & water 0.67 ML 0.67 ML +0.5 ML 0.67 ML + 1.0 ML 0.67 ML + 1.9 ML 0.67 ML + 3.9 ML

0.03

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Irradiation time / min Figure 7. Yield of H2O produced from photolysis of different coverage of H2O dosed on 0.67 ML CH3OH covered R-TiO2(110) surface as a function of laser irradiation time derived from Figure 6.

adsorbed on BBO rows maybe occurs at ~100 K in several tens of seconds after the adsorption procedure. As a result, the amount of CH3OH on the Ti5c sites will decreases with increasing H2O coverage, and then the yield of H2O from CH3OHTi5c photolysis will decrease as well. With increasing H2O coverage, the amount of H2O molecules hydrogen-bonded to BBO sites gradually increases. As we know, one CH3OH can only form one hydrogen bond with BBO atoms. However, for one H2O molecule, it can form two hydrogen bonds with 17



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BBO atoms. While, the saturated second layer of CH3OH and H2O are about 0.7 and 1.0 ML respectively, and more hydrogen bonds will be formed between the mixed second layer (H2O and CH3OH) and BBO sites than that between the pure second layer CH3OH and BBO sites. Thus, as the H2O coverage increases, less and less CH3OH will stay on the Ti5c sites, meanwhile, more and more H2O molecules will bond to the BBO atoms via hydrogen bonds, which will further inhibit CH3OH photolysis. Meanwhile, as reported by Li and coworkers, 30 the saturation coverage value for CH3OHTi5c corresponds to approximately three molecules adsorbed per four Ti5c sites, indicating that the methyl radicals create a steric hindrance that blocks approximately a quarter of the Ti5c sites. More importantly, the result suggests that CH3OH clusters formed on the Ti5c rows via intermolecular hydrogen bonds should only contain less than three CH3OHTi5c molecules. However, for H2O molecules on the Ti5c rows, the higher the H2O coverage the much longer H2OTi5c clusters formed via intermolecular hydrogen bonds there is.31,32 As a result, the efficiency of H2OTi5c photolysis decreases significantly as the size of H2OTi5c clusters increases.32 In this work, the increase of the coverage of H2O will lead to the decrease of chemisorbed CH3OHTi5c molecules very fast, namely, the chemisorbed H2OTi5c molecules will increase rapidly. Thus, with increasing H2O coverage, it is possible for the chemisorbed H2OTi5c and CH3OHTi5c molecules to form long mixed chains through intermolecular hydrogen bonds, resulting in lowering the efficiency of CH3OHTi5c photolysis by the intermolecular hydrogen bonds. CONCLUSION 18


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In summary, with systematic TPD experiments, we have investigated the effect of multilayer CH3OH/H2O on the photochemistry of CH3OHTi5c. The results clearly show that the photochemistry of the first layer CH3OH is hindered by the multilayer CH3OH and H2O. For multilayer CH3OH/ H2O adsorption, CH3OH/H2O molecules hydrogen bonded to the BBO sites are mostly likely to increase the energy barrier of C-H bond dissociation, resulting in lowering the efficiency of CH3OHTi5c photolysis. For multilayer H2O coadsorption on the CH3OHTi5c layer, the decrease of the chemisorbed CH3OHTi5c molecules via molecule exchange between the H2O films and the chemisorbed CH3OH layer that occurs at ~100 K will further lower the yield of CH3OH photolysis. These results provide new fundamental understanding into how multilayer CH3OH and H2O influences the CH3OH photochemistry. Avoiding strong hydrogen bonds between CH3OH/H2O and BBO sites should be a crucial issue in developing efficient photocatalysts for CH3OH splitting.

ACKNOWLEDGMENT This work was supported by the National Science Foundation of China (21403224), and the Chinese Ministry of Science and Technology (2013CB834605), the Youth Innovation Promotion Association CAS, and the Key Research Program of the Chinese Academy of Sciences. Notes The authors declare no competing financial interest. 19



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Effect of Multilayer Methanol and Water in Methanol Photochemistry

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