Enhanced Hydrogen Production from Methanol Photolysis on a

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Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

Enhanced Hydrogen Production from Methanol Photolysis on a Formate-Modified Rutile-TiO2(110) Surface Chenbiao Xu,†,∥ Ruimin Wang,†,∥ Fei Xu,‡,∥ Qing Guo,*,† Xing’an Wang,‡ Dongxu Dai,† Hongjun Fan,*,† and Xueming Yang*,† †

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, 457 Zhongshan Road, Dalian 116023, Liaoning, P. R. China ‡ Center for Advanced Chemical Physics and Department of Chemical Physics, School of Chemistry and Materials Science, University of Science and Technology of China, Jinzhai Road 96, Hefei 230026, Anhui Province, P. R. China S Supporting Information *

ABSTRACT: We have investigated deuterium (D2) formation from the photolysis of fully deuterated methanol (CD3OD) on the clean and formate (DCOO−) modified rutile (R)-TiO2(110) surfaces at 266 nm using temperature-programmed desorption (TPD) and density functional theory (DFT) methods. Products, D2O and D2, have been detected on both surfaces during the TPD process. About 18.5% of the dissociated D atoms from CD3OD photolysis contribute to D2 formation on the DCOO−-modified RTiO2(110) surfaces. The value is much higher than that on the clean R-TiO2(110) surfaces, suggesting that surface DCOO− can enhance D2 production from CD3OD photolysis on R-TiO2(110). Further DFT calculation suggests that the BBO-CH-O-Ti5c structure of HCOO− on the surface can largely enhance the BBOvmediated H2 formation by lowering the barrier of recombinative H2 desorption, leading to efficient H2 production.

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INTRODUCTION Titanium dioxide has attracted extensive interest in photocatalysis, heterogeneous catalysis, solar energy devices, etc.1−10 Since photoelectrocatalytic water (H2O) splitting to hydrogen (H2) production was observed on TiO2 by Honda and Fujishima in 1972,11 the field has been widely investigated because of its potential applications in clean H2 production. In early studies,12 CH3OH is used as a sacrificial reagent and plays an important role in enhanced H2 production for photocatalytic H2O splitting. Nowadays, the photocatalytic reforming of CH3 OH has been a potential technology for clean H 2 production.13 However, the efficiency of photocatalytic H2 production from CH3OH on pure TiO2 is very low. Usually, surface modification of TiO2 by loading proper reduction and/ or oxidation cocatalysts is quite efficient for H2 production. Among various kinds of cocatalysts, noble metals are usually regarded as the proton reduction cocatalysts.5,12,13 Except for noble metal cocatalysts, it is questionable whether the surface modification of TiO2 with cheap organics could also improve the efficiency of H2 production from the photolysis of CH3OH. Due to the crucial role of CH3OH in H2 production, fundamental studies of CH3OH thermal chemistry and photochemistry on TiO2 have been extensively carried out in the past decade.14−22 Previously, our group23 has found that the photolysis of CH3OH to produce formaldehyde (CH2O) on rutile (R)-TiO2(110) occurs via transferring dissociated H atoms to neighboring bridge-bonded oxygen sites (BBO), and no H2 product is observed during irradiation, while the © XXXX American Chemical Society

photolysis of methoxy (CH3O) on R-TiO2(110) to produce CH2O and H atoms on the BBO sites (BBO-H) is also observed by Henderson and co-workers.24,25 Further, Xu and co-workers26 found that the BBO−D atoms from the photolysis of fully deuterated methanol (CD3OD) on the R-TiO2(110) surface can recombine to form D2 product with a very low efficiency during the TPD process. Xu and co-workers26 also proposed that the more BBO vacancies (BBOv) on the surface, the higher the efficiency of D2 production. Recently, Wang and co-workers27 have shown that the recombinative H2 formation on the BBO-H atom covered R-TiO2(110) surface undergoes a BBOv-mediated mechanism. Either BBO-H atoms or BBOv’s will strongly decrease the barrier of H2 desorption. Therefore, a higher yield of H2 formation on R-TiO2(110) is expected to be achieved by modifying the R-TiO2(110) surface with BBO-H atoms or BBOv’s. However, a high concentration of BBOv’s on the surface will lead to surface reconstruction, and BBO-H atoms move easily at high temperature on the surface, which may hinder the building of a stably modified R-TiO2(110) surface. Thus, stably adsorbed species on R-TiO2(110) with similar properties of BBO-H atoms or BBOv’s may enhance H2 formation from CH3OH photolysis. Special Issue: Prashant V. Kamat Festschrift Received: January 23, 2018 Revised: February 12, 2018 Published: February 12, 2018 A

DOI: 10.1021/acs.jpcc.8b00724 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C In this work, fully deuterated formate (DCOO−) produced from the photolysis of fully deuterated acetaldehyde (CD3CDO), which has similar electron density to that of the BBO-H atom, is used to modify the clean R-TiO2(110) surface. Then, D2 production from CD3OD photolysis was investigated systematically on the DCOO−-modified R-TiO2(110) surfaces. The results clearly show that the DCOO−-modified RTiO2(110) surface can largely enhance the efficiency of D2 production, compared with that on the clean R-TiO2(110) surface. Theoretical results demonstrate that the BBO-CH-OTi5c structure of HCOO− on the R-TiO2(110) surface can strongly lower the barrier of H2 desorption, leading to efficient H2 production.

The adsorption energy (ΔEadsorption), the reaction energy (ΔEreaction), the H2O desorption energy (ΔEH2O_desorption), and the H2 desorption energy (ΔEH2_desorption) are defined as follows ΔEadsorption = − (Eslab + adsorbate − Eadsorbate)

(1)

ΔEreaction = Efin state − E init state

(2)

ΔE H2O_desorption = E H2O + Eslab + BBOv + (n − 2)H − Eslab + n × H (3)

ΔE H2_desorption = E H2 + Eslab + (n − 2) × H − Eslab + n × H

(4)

Here, Eslab+adsorbate is the total energy of the six-layer slab with adsorbate on the surface. Efin state and Einit state refer to final state energy and initial state energy, respectively. Eslab+n×H or Eslab+(n−2)×H are the total energies of the six-layer slab with n or (n − 2) HBBO atoms on the surfaces, respectively. Eslab+BBOv+(n−2)H is the total energy of the six-layer slab with (n − 2) HBBO atoms on the surface with one BBOv. Eadsorbate, EH2O, and EH2 are the total energies of the isolated adsorbate, H2O, and H2 species, respectively.

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EXPERIMENTAL AND THEREOTICAL METHODS The TPD apparatus used in this work has been described previously in detail.23,28 The R-TiO2(110) single crystal with a dimension of 10 × 10 × 1 mm3 (Princeton Scientific Corp.) was cleaned by several cycles of Ar+ sputtering and ultrahigh vacuum (UHV) annealing. After this surface preparation procedure, an oxygen vacancy population of ∼5% remained on the surface, as determined by H2O TPD.29 CD3CDO (Aldrich, 99+%) and CD3OD (Aldrich, 99+%) were further purified by several freeze−pump−thaw cycles and then were introduced onto the R-TiO2(110) surface with a calibrated molecular beam doser at 120 K. Based on previous work of CH3OH photolysis at 355 and 266 nm,30 the initial rate of CH3OH photolysis to produce CH2O and BBO-H atoms was found to be strongly dependent on photon energy, with the initial rate being about 100 times higher at 266 nm than that at 355 nm. Thus, a 266 nm laser light, produced from a frequency tripled Ti:sapphire femtosecond laser (repetition rate 1 kHz, pulse duration ∼100 fs), was used for DCOO− production and CD3OD photolysis. The DCOO−-modified R-TiO2(110) surfaces were prepared by irradiating the 0.3 ML CD3CDO adsorbed R-TiO2(110) surfaces for different times at 120 K followed by flashing to 400 K to remove CD3CDO molecules. Then, the surfaces were recooled to 120 K for CD3OD adsorption. TPD spectra were measured with a ramping rate of 2 K/s. The average power of the laser beam was about 20 mW, corresponding to a flux of 4.7 × 1016 photons cm−2 s−1. As shown in our previous studies,23,27 all of our calculations were performed using the Vienna ab initio simulation package code31,32 and plane augmented wave potential.33 The wave function was expanded by the plane wave, with a kinetic cutoff of 400 eV and density cutoff of 650 eV. The generalized gradient approximation with the spin-polarized Perdew− Burke−Ernzerhof functional34 was used to determine the optimized molecular structures of R-TiO2(110). An efficient force reversed method35 was used to locate the transition state (TS). Our surface model was cut out of a six-layer slab TiO2 crystal to expose the (110) surface,23,27 and the bottom two layers of atoms were fixed. All five-coordinated Ti4+ (Ti5c) sites on the bottom layer were saturated with H2O molecules to maintain the bulk coordination environment. The periodically repeated slabs on the surface were decoupled by 15 Å vacuum gaps. A Monkhorst−Pack grid36 of (2 × 1 × 2) k-points was used for the 6 × 2 surface unit cell. Isolated gas-phase molecules were optimized in a (15 × 15 × 15) unit cell with a single k-point. We performed a Bader charge analysis37 on the electron density.

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RESULTS AND DISSCUSION Similar to the work of D2 formation from CD3OD photolysis on R-TiO2(110) done by Xu and co-workers at 400 nm,26 CD3OD photolysis on the R-TiO2(110) surface with 266 nm was first performed in this work. Figure 1A shows typical TPD spectra collected at a mass-to-charge ratio (m/z) of 20 (D2O+) after adsorbing 0.5 ML CD3OD on the clean R-TiO2(110) surfaces followed by laser irradiation for various durations. Before irradiation, two desorption peaks are observed. In combination with previous studies,24 the 250 K peak (marked

Figure 1. (A) Typical TPD spectra collected at m/z = 20 (D2O+) on the 0.5 ML CD3OD adsorbed R-TiO2(110) surfaces following different laser irradiation times. (B) Typical TPD spectra collected at m/z = 4 (D2+) on the 0.5 ML CD3OD adsorbed R-TiO2(110) surfaces following different laser irradiation times. B

DOI: 10.1021/acs.jpcc.8b00724 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C with *) is due to a small impurity of D2O in CD3OD and the fragment ion from parent CD3OD molecules in the ionizer. The 570 K peak is the result of the recombinative desorption of D2O made from two OD groups on BBO rows, which are produced by spontaneous dissociation of CD3OD molecules at the BBOv sites. With increasing irradiation time, the 570 K peak increases significantly and shifts to lower temperature by about 140 K, indicating that a large amount of BBO−D atoms are produced via the photolysis of CD3OD. After D2O desorption, BBOv sites are created on the surface. In addition, the intensity of the 250 K peak also increases a little along with irradiation time. This peak can only be associated with D2O molecules on Ti5c sites, which may be due to the following thermally driven exchange reaction CD3OD(Ti5c) + D−OBBO heat,R‐TiO2 (110)

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CD3−OBBO + D2 O(Ti5c)

(5) 38

This is similar to previous experimental observations. TPD spectra at m/z = 4 (D2+) were also collected for monitoring D2 formation (Figure 1B). Before irradiation, only a 285 K peak (marked with *) is observed, which is due to the fragment ion from parent CD3OD molecules in the ionizer. With increasing irradiation time, a broad peak at 460 K peak appears and increases obviously. There are three different m/z = 4 (D2+) sources for the broad 460 K peak. The first two contributions come from fragment ions of recombinative desorbed D2O molecules, and a CD3 radical formed by dissociatively adsorbed CD3OD on BBOv sites (at about 600 K) in the ionizer. These two sources are very small and can be neglected. The third D2+ source is the molecular D2 formed via recombinative desorption of BBO−D atoms, given a peak at 460 K in the TPD spectra after irradiation. After 300 s irradiation, 0.14 ML D2O and 0.01 ML D2 are produced during the TPD process (Figure 2); namely, 0.3 ML BBO−D atoms are formed on the surface, whereas only ∼7% of BBO−D atoms (r = yield D2/yield (D2O + D2)) are recombined to produce D2 (the inlet of Figure 2A). The values are nearly the same as that obtained in Xu’s work with 400 nm irradiation,26 indicating that irradiation wavelength does not affect the efficiency of D2 production. When 0.44 ML BBO−D atoms are produced via CD3OD photolysis after 30 min irradiation, more BBOv sites can be produced on the surface before D 2 desorption during the TPD process, and about 8% of BBO− D atoms are recombined to form D2 molecules (the inlet of Figure 2A), demonstrating that the more BBOv sites on the surface, the higher the efficiency of D2 production.26 However, the efficiency of D2 formation on the clean R-TiO2(110) surface is still very low. To enhance the efficiency of D2 formation on R-TiO2(110), stably adsorbed DCCO− was used to modify the clean RTiO2(110) surface. In light of recent studies done by Xu and co-workers,39 BBO atoms are intimately involved in the photoinduced decomposition of CH3CHO on TiO2(110) in the absence of O2, and products HCOO− and acetate (CH3COO−) mainly adsorb on the surface bidentately with the carbonyl O atom bound to the Ti5c site and the carbonyl C atom bound to a nearby BBO atom. Thus, surface DCOO− was prepared by irradiating the CD3CDO-covered R-TiO2(110) surfaces with 266 nm followed by heating the surface to 400 K to remove the remaining CD3CDO molecules. Unlike the result of CH3CHO photolysis on R-TiO2(110)39 that the productions of both HCOO− and CH3COO− are the main reaction

Figure 2. Yields of D2O and D2 as a function of irradiation time, derived from Figure 1A and B. The inlet of Figure 2A shows the D2 formation fraction as a function of irradiation time. The yields of D2O and D2 formation before irradiation have been subtracted. The solid lines are to guide the eyes only. The vertical error bars give the standard deviation of the data in three repeat measurements.

channels, photoinduced decomposition of CD3CDO on RTiO2(110) mainly produces DCOO−, and the CD3COO− formation is only a minor channel (Figure S1), which may be due to the isotopic effect. By controlling the irradiation time, the R-TiO2(110) surfaces with different DCOO− coverages could be prepared (Figure S1). After irradiating the 0.3 ML CD3CDO covered R-TiO2(110) surface with 266 nm for 180 s followed by flashing to 400 K to remove remaining CD3CDO molecules, about 0.05 ML DCOO− can be produced. Then, we performed CD3OD photolysis on the 0.05 ML DCOO− modified R-TiO2(110) surfaces at 120 K. Before irradiation, the broad peak at 570 K in Figure 3A is slightly bigger than that on the 0.5 ML CD3OD covered R-TiO2(110) surface. However, the 570 K in Figure 3A has two sources. The first is recombinative desorbed D2O molecules made from two OD groups on BBO rows. The OD groups are produced by spontaneous dissociation of CD3OD molecules at the BBOv sites and photodecomposition of CD3CDO on the Ti5c sites. The other source comes from the decomposition of DCOO− during the TPD process, which desorbs at about 600 K (Figure S2A). The contribution of this source is very small. Therefore, the coverage of BBO−D atoms is nearly the same on the clean and DCOO−-modified surfaces after CD3OD adsorption, while a 500 K peak appears in the TPD spectrum of m/z = 4 after adsorbing 0.5 ML CD3OD on the 0.05 ML DCOO− modified R-TiO2(110) surface (Figure 3B). Conversely, the 500 K peak is not obviously detected on the 0.5 ML CD3OD covered R-TiO2(110) surface before irradiation. The desorption of CD3OD, D2O, and DCOO− on the DCOO−-modified surface does not occur at 500 K. Thus, the 500 K peak can be only due to the recombinative desorption of D2 made from BBO−D atoms, suggesting that C

DOI: 10.1021/acs.jpcc.8b00724 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 3. (A) Typical TPD spectra collected at m/z = 20 (D2O+) on the 0.05 ML DCOO− precovered R-TiO2(110) surfaces with 0.5 ML CD3OD adsorption following different laser irradiation times. (B) Typical TPD spectra collected at m/z = 4 (D2+) on the 0.05 ML DCOO− precovered R-TiO2(110) surfaces with 0.5 ML CD3OD adsorption following different laser irradiation times.

Figure 4. Yields of D2O and D2 as a function of irradiation time, derived from Figure 3A and B. The inlet of Figure 4B shows the D2 formation fraction as a function of irradiation time. The yields of D2O and D2 formation before irradiation have been subtracted. The solid lines are to guide the eyes only. The vertical error bars give the standard deviation of the data in three repeat measurements.

the existence of DCOO− on the surface seems to enhance recombinative D2 formation. As the irradiation time increases, the recombinative desorption peaks of D2O and D2 increase obviously and shift to lower temperature. For 30 min irradiation, the recombinative desorption peak of D2O on the DCOO−-modified R-TiO2(110) surface is only slightly higher than that on the clean R-TiO2(110) surface. However, the recombinative desorption peak of D2 on the modified surface is about 3 times higher than that on the clean surface, indicating that the efficiency of CD3OD photolysis to produce BBO−D atoms on the DCOO−-modified surfaces is improved. More importantly, the D2 formation on the DCOO−-modified surfaces is also largely enhanced. To evaluate the importance of D2 formation channel on the DCCO−-modified R-TiO2(110) surfaces, the yields of D2O and D2 have also been calculated (Figure 4A and B). As shown in Figure S3, the coverage of DCOO− on the CD3OD and DCOO− coadsorbed R-TiO2(110) surfaces is not depleted upon irradiation. Thus, the amount of D2O and D2 from DCOO− decomposition at different irradiation times will be the same and can been subtracted using the values obtained before CD3OD adsorption (Figure S2). As the irradiation time increases, the yields of D2O and D2 increase significantly (Figure 4). The ratio of dissociated BBO−D atoms for D2 production can also be derived from the yields of D2O and D2, as shown in the inlet of Figure 4B. It is worth noting that the ratio nearly keeps constant when the surfaces are covered with different coverages of BBO−D atoms. About 18.5% of dissociated BBO−D atoms are recombined to produce D2 molecules, which is much higher than that (7−8%) on the clean R-TiO2(110) surface, suggesting that the surface DCOO− on

R-TiO2(110) significantly enhances the efficiency of D2 production from CD3OD photolysis. In addition, the yield of D2 increases relatively faster than that of D2O as laser irradiation time increases on the clean R-TiO2(110) surface, whereas the increase of D2 and D2O yields nearly keeps the same trend as laser irradiation time increases on the DCOO−covered surface, suggesting that there are some other reasons for the enhanced D2 formation on the DCOO−-covered surface. As shown in Figure 3, D2 starts to desorb from the surface at 360 K, and there is nearly no molecularly adsorbed CD3OD on the DCOO−-covered surfaces at this temperature. When we calculated the TPD area of D2O from 300 to 360 K at different irradiation times, the yield of D2O is less than 0.02 ML, indicating that the coverage of BBOv on the surfaces is very low when D2 starts to desorb at 360 K. However, as surface temperature increases to 425 K, the TPD area of D2O from 300 to 425 K increases very quickly. About 40%−50% of BBO−D atoms are desorbed in the form of D2O, leaving behind BBOv sites on the surfaces. The yield of D2 calculated from the TPD area of D2 in the temperature range of 300−425 K is very small. For example, 0.5 ML BBO−D atoms are produced after irradiating the CD3OD and DCOO− coadsorbed surface for 1200 s, and about 0.10 ML BBOv and 0.007 ML D2 are formed as the surface temperature rises to 425 K, whereas about 0.045 ML D2 is produced during the whole TPD process. The results demonstrate that a large amount of BBOv sites are formed on the surface before D2 is largely produced. Based on previous studies,27 the BBOv sites can lower the barrier of D2 formation via the direct coupling of two BBO−D atoms or the BBOvmediated D2 desorption, leading to enhanced D2 production, while the energy barrier for the direct coupling of two BBO−D D

DOI: 10.1021/acs.jpcc.8b00724 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C atoms near the BBOv site and the BBOv-mediated D2 desorption are 2.04 and 1.90 eV, respectively. However, BBOv sites can migrate along BBO rows with an experimental activation energy of 1.15 eV and a theoretical activation energy of 1.03 eV.40 Also, the hopping rate increases exponentially with increasing temperature, demonstrating that the diffusion of BBOv sites occurs easily when D2 starts to desorb from the surface.40 As a result, the stable BBOv sites adjacent to BBO−D atoms do not possibly exist when D2 starts to desorb at 360 K, while the energy barrier for direct coupling of BBO−D atoms for D2 formation is very high. Thus, BBO−D atoms prefer to desorb in the form of D2O as the coverage of BBOv sites is low. This is consistent with our previous observations.26 Only when the coverage of BBOv sites on the surface increases, the probability for BBOv sites near the desorption sites of D2 will be higher, and then the D2 production could be enhanced by lowering the desorption barrier with the help of BBOv.27 However, on the DCOO−-covered surfaces, the desorption temperature of DCOO− is as high as 575 K. This suggests that DCOO− adsorbs on the surface stably, and the diffusion of the species will be very hard. Therefore, the stably adsorbed DCOO− may inhibit the diffusion of part of the BBOv sites or fix part of the BBOv sites on some thermodynamically stable sites near the DCOO− to form weakly moved BBOv sites when BBOv sites are formed and diffuse along BBO rows. Previous results41 demonstrate that BBO-H atoms can diffuse on the BBO rows at the surface temperature higher than 300 K with experimental activation energies of 0.74−0.85 eV and theoretical activation energies of 1.04−1.22 eV. The higher temperature, the higher the diffusion rate of the BBO-H atom. Thus, the BBO−D atoms may distribute uniformly on the surface due to the diffusion at high temperature, and then BBO−D atoms in a certain range around the weakly moved BBOv sites near the DCOO− could always find them easily. In addition, compared with D2 desorption on the BBOv-covered surface, the incorporation of DCOO− and BBOv on the surface may further decrease the energy barrier of D2 desorption. Thus, stable and enhanced D2 production on the DCOO−-covered surface will be achieved. In order to gain further insights into the role of DCOO− for enhanced D2 production, theoretical studies were carried out to investigate the mechanism of D2 production on the DCOO−adsorbed R-TiO2(110) surface. On the basis of previous results,39 the DCOO− produced from the decomposition of CD3CDO on R-TiO2(110) mainly adsorbs bidentately with the carbonyl O atom bound to the Ti5c site and the carbonyl C atom bound to a nearby BBO atom. Here, DCOO− and D atoms are treated as HCOO− and H atoms, respectively, and a large 6 × 2 slab is used in the calculation for better accommodating the coadsorbed HCOO− and BBO-H atom. Two optimized adsorption structures of HCOO− on the RTiO2(110) surface are considered, as shown in Figure 5. First, HCOO− can bind to the surface bidentately on one Ti5c site and one BBOv site (Figure 5A, denoted as BBO-CH-O-Ti5c), and the calculated adsorption energy is 3.47 eV, which is much higher than that of H2O and BBO-H atoms.27 Alternatively, HCOO− can also adsorb bidentately on two adjacent Ti5c sites (Figure 5B, denoted as Ti5c-O-CH-O-Ti5c). The calculated adsorption energy for this structure is 3.33 eV, and the barrier for isomerization to form BBO-CH-O-Ti5c is 0.73 eV, indicating that the isomerization is facile. Because the formation of H2 at the BBO sites near BBOv is much easier than that at the BBO sites far away from BBOv,27

Figure 5. Side view of optimized adsorption structures for HCOO− on the R-TiO2(110) surface. Left: (A) BBO-CH-O-Ti5c. Right: (B) Ti5cO-CH-O-Ti5c.

we do not consider the H2 formation at the sites far away from BBOv in this work. While the energy barrier of direct coupling of BBO-H atoms near the BBOv site to form H2 is much higher than that of H2 formation via the BBOv-mediated mechanism,27 we no longer consider it as well. In the BBOv-mediated mechanism, the H2 desorption starts with an H atom migrating from BBO to BBOv, followed by the coupling of BBO-H and BBOv-H atoms (H atom adsorbs on BBOv site) to produce H2. The transition state is featured by a BBO−Hδ+···Hδ−−BBOvtype structure. As mentioned above, on the 0.5 ML BBO−D atoms and 0.05 ML DCOO− coadsorbed R-TiO2(110) surface, about 0.1 ML BBOv sites can be formed via D2O desorption before D2 is largely produced at 425 K, while there are still about 0.3 ML BBO−D atoms on the surface at this temperature. As shown in previous works,27 the adsorption of BBOv-H is only about 0.1 eV higher than BBO-H. Thus, the formation of BBOv-D atoms is very possible at this temperature. We first focused on the BBO-CH-O-Ti5c structure of HCOO− on the R-TiO2(110) surface. In our experiments, a large amount of BBO-H atoms adsorb on the surface, and BBOv sites rarely exist at the beginning of H2O desorption. The calculated barriers for H2O desorption are 1.08−1.19 eV (depending on the relative positions of BBO-CH-O-Ti5c and BBO-H) for the 1/12 ML BBO-CH-O-Ti5c and 1/6 ML BBOH atom coadsorbed R-TiO2(110) surface, which is similar to the barrier of H2O desorption on the 1/4 ML or 1/2 ML BBOH atom adsorbed R-TiO2(110) surfaces.27,42 The result suggests that the adsorption of BBO-CH-O-Ti5c does not affect H2O desorption significantly. Because the desorption temperature of D2 is about 40 K higher than that of D2O, a large amount of BBOv’s will exist on the R-TiO2(110) surface at the beginning of D2 desorption. Thus, the desorption of H2 was studied on a model surface with 1/12 ML BBO-CH-O-Ti5c, 1/12 ML BBOv, and 1/6 ML BBOH atoms. Several relative positions of the BBOv and BBO-CHO-Ti5c are considered because they may affect the barrier of H2 desorption. In a typical and most simple case, the BBOv and BBO-CH-O-Ti5c are adjacent (Figure 6). For this HCOO− adsorption structure, the H atom transferring from the BBO site (6−1) to the BBOv site (6−3) is endothermic by 0.10 eV, with a barrier of 1.77 eV (TS6−2). The other H atom transferring from the BBO site (6−3) to the adjacent BBO site (6−5) is almost thermal neutral with a barrier of 0.99 eV (TS6−4). The barrier for the coupling of BBO-H and BBOv-H atoms to produce H2 (TS6−6) is 0.88 eV, and the H2 desorption energy is 0.11 eV. The rate-determining step is the transfer of the H atom from the BBO site to the BBOv site, and the overall barrier for H2 desorption is 1.77 eV. When the BBOv and BBO-CH-O-Ti5c are separated by one BBO and two BBO sites, the barriers of H2 desorption are 1.75 and 1.69 eV respectively, suggesting that the relative positions of the BBOv E

DOI: 10.1021/acs.jpcc.8b00724 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 6. Reaction profiles for H2 formation on the model where one BBOv is on the adjacent site of BBO-CH-O-Ti5c. The side views of selected intermediates and transition states are shown in the figure with simplified schematic diagrams.

Figure 7. Reaction profiles for H2 formation on the model where one BBOv is on the adjacent site of Ti5c-O-CH-O-Ti5c. The side views of selected intermediates and transition states are shown in the figure with simplified schematic diagrams.

the efficiency of H2 production is not observed in our experimental conditions. In addition, when a stoichiometric R-TiO2(110) surface is covered with 1/4 ML BBO-H atoms,27 the desorption of H2 via direct coupling of two BBO-H atoms is very hard with a barrier of 2.2 eV. As the coverage of BBO-H atoms on the RTiO2(110) surface increases to 1/2 ML, two BBO-H atoms are adjacent to the desorption sites of H2 in the model, and the energy barrier for H2 desorption via direct coupling of two BBO-H atoms decreases to 1.84 eV. Thus, with one BBO-H atom close to the desorption sites of H2, the energy barrier for direct H2 desorption will be much higher than 1.84 eV. Because BBO-H and BBO-CH-O-Ti5c have similar charge effects on the surface, BBO-CH-O-Ti5c can also decrease the barrier of H2 formation via direct coupling of two BBO-H atoms. If we replace the BBO-H atom with a BBO-CH-O-Ti5c, the energy barrier of direct H2 desorption will also be much higher than 1.84 eV. Therefore, when a stable BBO-CH-O-Ti5c adsorbs near the BBOv site, the energy barrier for the BBOv-mediated H2 desorption is still the lowest. Furthermore, we also calculated the reaction profiles of H2 desorption on a model surface with 1/12 ML Ti5c-O-CH-OTi5c, 1/12 ML BBOv, and 1/6 ML BBO-H atoms. When the

and BBO-CH-O-Ti5c have a slight effect on the H2 desorption. For comparison, the barrier of H2 desorption via the BBOvmediated mechanism is 1.93 eV (Figure S4) when no BBOCH-O-Ti5c adsorbs adjacent to the BBOv site. Therefore, the BBO-CH-O-Ti5c adsorption on R-TiO2(110) indeed makes the H2 desorption easier. To understand the effect of HCOO− on the H2 desorption, we have replaced the BBO-CH-O-Ti5c with a BBO-H atom in our model and fixed the BBO-H atom on the adsorption site adjacent to the BBOv site. We found that the barrier of H2 desorption is 1.82 eV (Figure S5), which is pretty similar to a previous result.27 Further analysis shows that the charges on the BBO sites are −1.15e for BBO-CH-O-Ti5c and −1.18e for the BBO-H atom, which are also very similar. Therefore, BBO-H and BBO-CH-O-Ti5c have similar charge effects on the surface, and they both make the desorption of H2 easier. However, previous results41 demonstrate that BBO-H atoms can diffuse on the BBO rows at the surface temperature higher than 300 K. Thus, the stably adsorbed BBO-H atoms adjacent to the BBOv sites do not exist when H2 starts to desorb at 360 K, while BBO-H atoms prefer to desorb in the form of H2O during the TPD process. As a result, that the BBO-H atoms could enhance F

DOI: 10.1021/acs.jpcc.8b00724 J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (21673235, 21503223, 21673224), the China Postdoctoral Science Foundation Grant (2015T80274), the Doctoral Scientific Research Foundation of LiaoNing Province (201502628), and the Youth Innovation Promotion Association CAS.

BBOv and Ti5c-O-CH-O-Ti5c are separated by one Ti5c site (Figure 7), the H atom transferring from the BBO site (7−1) to the BBOv site (7−3) is endothermic by 0.58 eV, with a barrier of 2.32 eV (TS7−2). The barrier for the coupling of BBO-H and BBOv-H atoms to produce H2 (TS7−4) is 0.79 eV, and the desorption energy of H2 is 0.59 eV. The ratedetermining step is the transfer of the H atom from the BBO site to the BBOv site, and the overall barrier for H2 desorption is 2.32 eV. In addition, several relative positions of the BBOv and Ti5c-O-CH-O-Ti5c were also considered. Whether the BBOv and Ti5c-O-CH-O-Ti5c are separated by two Ti5c sites or Ti5c-O-CH-O-Ti5c adsorbs on the Ti5c site adjacent to the BBOv, the barriers for H2 desorption are all about 2.3 eV, which are higher than that on the BBO-CH-O-Ti5c covered surface by 0.53−0.61 eV. This is even higher than the barrier of H2 desorption via the BBOv-mediated mechanism (1.93 eV) on the clean R-TiO2(110) surface, suggesting that the existence of Ti5c-O-CH-O-Ti5c makes the H2 formation via the BBOvmediated mechanism harder.

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SUMMARY In summary, our experimental investigation provides evidence that the DCOO−-modified R-TiO2(110) surface can largely enhance the efficiency of D2 production from CD3OD photolysis on this surface. Theoretical results demonstrate that the BBO-CH-O-Ti5c structure of HCOO− on the RTiO2(110) surface can largely enhance the BBOv-mediated H2 formation by lowering the barrier of recombinative H2 desorption, whereas the Ti5c-O-CH-O-Ti5c structure of HCOO− on the surface makes the BBOv-mediated H2 formation harder. The results presumably provide a possible method to improve the hydrogen production efficiency on TiO2 with cheap organics. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b00724. TPD spectra for DCOO−, CD3COO−, D2O, and D2 products on the 0.3 ML CD3CDO-covered R-TiO2(110) after irradiating for different times (Figure S1, S3). TPD spectra collected at m/z = 28 on the 0.05 ML DCOO− precovered R-TiO2(110) surfaces with 0.5 ML CD3OD adsorption as a function of irradiation time (Figure S2). Calculated H2 formation pathway (Figures S4, S5) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Qing Guo: 0000-0003-0265-1184 Xing’an Wang: 0000-0002-1206-7021 Author Contributions ∥

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Notes

The authors declare no competing financial interest. G

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