Low-Temperature Hydrogen Production via Water Conversion on Pt

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

Low Temperature Hydrogen Production via Water Conversion on Pt/TiO Zhenhua Geng, Xianchi Jin, Ruimin Wang, Xiao Chen, Qing Guo, Zhibo Ma, Dongxu Dai, Hongjun Fan, and Xueming Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02945 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 1, 2018

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The Journal of Physical Chemistry

Low Temperature Hydrogen Production via Water Conversion on Pt/TiO2

Zhenhua Geng1), 2), a), Xianchi Jin1), 2), a), Ruimin Wang1), a), Xiao Chen1), 2), Qing Guo1), *), Zhibo Ma1), *), Dongxu Dai1), Hongjun Fan1), *), Xueming Yang1), *) 1

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, 457 Zhongshan

Road, Dalian 116023, Liaoning, P. R. China 2.

University of Chinese Academy of Sciences, 19A Yuquan Road, Shijingshan District, Beijing 100049, P. R.

China

a)

Authors who made similar contributions to this work.

*) To whom all correspondence should be addressed.

Email addresses: [email protected]

(QG), [email protected] (ZM), [email protected] (HF) and [email protected] (XY).

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ABSTRACT Pt supported TiO2 catalysts have proved to be good catalysts for efficient hydrogen (H2) production from photocatalytic water (H2O) splitting and water-gas shift reactions. However, its origin for efficient H2O conversion remains poorly understood. Here we report a systematic study of low temperature H2 formation via H2O conversion on Pt deposited rutile-TiO2(110) surfaces by means of spectroscopic and microscopic techniques in combination with first-principles calculations. We show that the low temperature H2 formation can occur facilely at ~200 K via H2O conversion on the Pt clusters/rutile-TiO2(110) surfaces, which is initiated by H2O dissociation at the surface defects, metal-oxide interfaces, and probably regular Ti4+ sites. More importantly, H2O assisted multistep H atoms diffusion plays a crucial role for transferring H atoms toward Pt clusters, resulting in efficient H2 evolution. These results enrich the understanding of H2 formation via H2O conversion on Pt/TiO2 catalysts, which is helpful for understanding this reaction on other metal-oxide catalysts.


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INTRODUCTION Since the discovery of the water (H2O) splitting to produce hydrogen (H2) with TiO2 and Pt electrodes by Fujishima and Honda in 1972,1 enormous efforts have been devoted to the study of H2 production from H2O conversion with TiO2 based catalysts. Up to now, TiO2-based catalysts have drawn a lot of attention to produce H2 from H2O via different pathways, such as photocatalysis under suitable light irradiation2 or via the water-gas shift (WGS) reaction.3 However, pure TiO2 is not active for photocatalytic H2 production from H2O,4 loading proper reduction cocatalysts on TiO2 based catalysts can be quite efficient for photocatalytic H2O reduction.5-8 Among various kinds of reduction cocatalysts, Pt is usually regarded as the best proton reduction cocatalyst,9-11 which not only serve as electron sinks, but also provides effective proton reductions sites, hence dramatically facilitates proton reduction reaction. Similarly, Pt clusters supported TiO2 catalysts (Pt/TiO2) have proved to be good catalysts for the low temperature WGS.12-15 During the WGS reaction, the oxygen vacancy structure at the metal-oxide interface is proposed to play an essential role in promoting H2O adsorption and dissociation, then facilitate proton transferring to Pt clusters for H2 production. Considering that Pt/TiO2 catalysts are very active for H2 production via H2O conversion, there is still a relatively scarce amount of fundamental studies on Pt/TiO2 surfaces. More research is required for the better understanding of the roles of surface oxygen vacancies, metal-oxide interfaces, and surface H2O molecules in H2O dissociation, proton



transfer and H2 production, which are essential to elucidate how efficient H2 production via H2O conversion is realized on Pt/TiO2. Due to the stability and controllable preparation of rutile(r)-TiO2(110), this surface has served as a prototypical model for catalytic and photocatalytic studies. Fundamental studies of the interaction of H2O with r-TiO2(110) 16 - 27 have been investigated both experimentally and theoretically. It is well-established that H2O molecules spontaneously dissociate at the bridge bonded oxygen (BBO) vacancy sites of the r-TiO2(110) surface to form hydroxyls on BBO rows (OHBBO).16 Upon heating to 500 K, most of the OHBBO groups recombine to produce H2O again, and only a small part of H atoms on the BBO sites (HBBO) will recombine to H2.28 Corresponding theoretical works demonstrate that the energy barrier for recombinative H2 desorption from HBBO atoms on r-TiO2(110) is considerably higher than that for recombinative H2O formation from HBBO and BBO atoms on the surface.29,30 Meanwhile, a series of investigations about H2O photolysis on r-TiO2(110) 31 - 33 illustrated that the H2O dissociation reaction occurs via transferring an H atom to a BBO site nearby and ejecting the left OH radical into the gas phase upon UV irradiation, and no H2 is formed. These results demonstrate that pure TiO2 is not active for H2 production from H2O splitting, similar to the results obtained in solution.4 From these studies, the H2O dissociation and H2 production on r-TiO2(110) are well understood. However, at a very fundamental level, it is questionable that how the pathway and efficiency for H2 production on r-TiO2(110) are dramatically changed after loading Pt clusters. Thus,


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there is a clear need for further experimental insight to the fundamental processes of H2O dissociation and H2 production on Pt/r-TiO2(110). In this work, we simultaneously apply temperature-programmed desorption (TPD) and scanning tunneling microscopy (STM) to study the fundamental processes of H2 production via H2O conversion on Pt/r-TiO2(110). Such a combination of techniques, provides a clear way to experimentally demonstrate that the H2 production can occur on the Pt clusters covered r-TiO2(110) surfaces efficiently at low temperature, which is initiated by H2O dissociation at the surface defects, metal-oxide interfaces, and probably regular Ti4+ (Ti5c) sites. Density functional theory (DFT) calculations show that Pt cocatalysts can promote H2O dissociation at the metal-oxide interfaces, and lower the activation energy of H2 production. In addition, H2O assisted multistep H diffusion plays a crucial role for efficient H2 production by transferring H atoms toward Pt clusters.

EXPERIMENTAL AND COMPUTATIONAL METHODS Experimental methods The surface photocatalysis-TPD apparatus used in this work has been described previously in detail.33 The TPD experiments were performed in an ultra-high vacuum (UHV) chamber which has a base pressure in the low 10-11 Torr range. An extremely high vacuum of 1.5×10-12 Torr in the electron-impact ionization region was maintained during the experiment. The rutile (r)-TiO2(110) surface sample (Princeton Scientific Co.) used in this work has a size of 10 × 10 × 1 mm3. A clean and well-ordered surface was prepared by many cycles of Ar+ sputtering and UHV annealing until impurities 5 ACS Paragon Plus Environment


were below the detecting limit of Auger electron spectrometer (AES) and a sharp (1 × 1) low energy electron diffraction (LEED) pattern was observed. After this preparation procedure, a BBO vacancy (BBOv) population of 7% on the surface remained, as measured by H2O TPD. Daily cleaning was accomplished by annealing the crystal to 850 K for 30 min in UHV. The clean R-TiO2(110) surface was then dosed with H2O (99.9% purity, Sigma-Aldrich) to a certain surface coverage with surface temperature at 100 K. TPD signals were collected after irradiation with a heating rate of 2 K/s. Pt was evaporated at room temperature from a multi-element miniature evaporator (Unisoku) with high-purity Pt wire (99.9% Pt). The deposition rate was calibrated using a quartz crystal microbalance. The STM experiments were performed with a low temperature scanning tunneling microscope (Matrix, Omicron) with a base pressure better than 410-11 mbar. The rTiO2(110) (Princeton Scientific, 1051 mm3) sample was cleaned by repeated cycles of Ar+ ion sputtering (1 kV, 1.5 µA, 10 min) and UHV annealing (850 K, 20 min). A clean reduced R-TiO2(110)-(11) with a BBOv population of 7% was obtained and checked by STM. We used an electrochemically etched tungsten tip for all the scanning during the experiments. The STM worked in constant current mode with a tunneling voltage of +1.25 V and a tunneling current of 100 pA. The clean R-TiO2(110) surface was then dosed with H2O (99.9% purity, Sigma-Aldrich) to a certain surface coverage with surface temperature at 80 K. We performed the experiments at 80 K. The tip was retracted back about 20 µm from the surface during H2O exposure. Pt was evaporated at room temperature from a multi-element miniature evaporator (Unisoku) with high6 ACS Paragon Plus Environment

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purity wire (99.9% Pt). The deposition rate was calibrated by counting the number of Pt atoms on the surface. Computational methods All calculations were performed using the Vienna ab initio simulation package code34,35 and plane augmented wave potential.36 The wave function was expanded by the plane wave, with a kinetic cut-off of 400 eV and density cut-off of 650 eV. The generalized gradient approximation with the spin-polarized Perdew−Burke−Ernzerhof functional37 was used to determine the optimized molecular structures of TiO2(110). Our surface model was cut out of a six-layer slab TiO2 crystal to expose the (110) surface.33,38 All Ti5c sites on the bottom layer were saturated with water molecules to maintain the bulk coordination environment.33,38 The periodically repeated slabs on the surface were decoupled by 15 Å vacuum gaps. A Monkhorst−Pack grid39 of (2 × 1 × 2) k-points was used for the 4 × 2 / 6 × 2 surface unit cell. All transition states (TS) were located by the force reversed method40 and climbing-image nudged elastic band (CI-NEB) method.41,42

RESULTS AND DISSCUSION The interaction of Pt with r-TiO2(110) has been studied systematically by Sasahara and coworkers using noncontact atomic force microscopy (NC-AFM), these authors have identified that Pt can adsorb on the Ti5c rows, BBO rows, and in BBOv sites mainly in the form of single atoms after adsorbing low coverage of Pt at room temperature.43 Similar to the work done by these authors,43 the interaction of Pt single atoms with r-TiO2(110) has also been investigated with STM after depositing 0.006 7 ACS Paragon Plus Environment


ML (1 ML = 5.2 x 1014 molecules/cm2) of Pt atoms on a reduced r-TiO2(110) surface at room temperature (Figure 1a). The bright and dark lines in the image are assigned to the Ti5c and BBO rows, respectively, with some vacancies on the BBO rows. While, many big bright spots (marked by sold green circles) appear on BBO and Ti5c rows after Pt deposition, and the sizes of these bright spots are significantly smaller than the size of Pt3 cluster. 44 Combination with the results obtained by Sasahara and coworkers,43 we could conclude that these bright spots are Pt single atoms. In addition, there are also some less bright spots on the BBO rows, which can be attributed to OH groups and OH pairs31 formed by dissociative adsorption of background H2O molecules on the BBOv sites during the Pt deposition procedure.

Figure 1. STM and TPD results for H2O dissociation on Pt1/r-TiO2(110). (a) STM image of the 0.006 ML of Pt single atoms covered r-TiO2(110) surface (acquired at bias voltage of + 1.25 V and set point current of 100 pA)), image size: 20 x 20 nm2. The big bright spots appear on BBO and Ti5c rows after Pt deposition marked by sold green circles are single Pt atoms. (b) Typical TPD spectra collected at m/z = 18 (H2O+) after dosing 1 ML of H2O on the clean and the 0.006 ML of Pt atoms covered r-TiO2(110) surfaces.

It is therefore interesting to find out whether Pt single atoms on r-TiO2(110) could promote H2O dissociation on the Ti5c sites (H2OTi), leading to catalytic H2OTi 8 ACS Paragon Plus Environment

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conversion. Then, TPD measurements were performed after adsorbing 1ML of H2O on the clean and Pt single atoms covered surfaces (Figure 1b). On the clean r-TiO2(110) surface, two peaks at 265 K and 500 K appear in the TPD spectra of H2O (m/z = 18), which can be assigned to the molecularly adsorbed H2OTi and the dissociatively adsorbed H2O at the BBOv sites (H2Ov), respectively. When Pt single atoms are deposited, the TPD profile of H2O is nearly the same with that on the clean r-TiO2(110) surface, while, no H2 product at the TPD spectrum of m/z = 2 is detected (not shown), indicating that Pt single atoms on r-TiO2(110) are not active for H2OTi dissociation, or the activation energy for H2 production on Pt single atoms is very high. However, on the Pt clusters covered r-TiO2(110) surfaces, which were prepared by annealing the Pt single atoms covered r-TiO2(110) surfaces to 850 K for 1 minute, the TPD spectra of H2O after 1ML of H2O adsorption change significantly (Figure 2a). As the Pt coverage increases, molecular adsorption peak of H2OTi around 265 K broadens and shifts to higher temperature, and the intensity of this peak also decreases significantly. Meanwhile, the recombinative peak of H2Ov at ~500 K decreases and disappears eventually at high Pt coverage. The maximum coverage of Pt single atoms on r-TiO2(110) is 0.006 ML, thus, the concentration of Pt clusters on the surface is much less than that of Ti5c and BBOv sites, demonstrating that the changes of TPD spectra cannot be attributed to the occupation of Ti5c and BBOv sites by Pt atoms or Pt clusters. In addition, the disappearance of the 500 K peak indicates that surface BBOv sites are healed by surface oxygen species during the TPD process, which is a sign of H2O conversion on the Pt clusters covered surfaces. While, the result suggests 9 ACS Paragon Plus Environment


that the existence of Pt clusters on r-TiO2(110) changes the reaction channel of H2O significantly.

Figure 2. TPD results for H2O dissociation on Pt cluster/r-TiO2(110). The Pt cluster/r-TiO2(110) surfaces were prepared by annealing to 850 K for 1 min after deposition of Pt atoms and before TPD measurements. (a) Typical TPD spectra collected at m/z = 18 (H2O+) on the 1 ML of H2O covered Pt-cluster/r-TiO2(110) surfaces as a function of Pt coverage. (b) Typical TPD spectra collected at m/z = 2 (H2+) on the 1 ML of H2O covered Pt-cluster/r-TiO2(110) surfaces as a function of Pt coverage. The inlet shows the yields of H2 production and H2O decrease as a function of Pt coverage, derived from Figure 2a&b.

Then, other products were measured. In the TPD spectra of m/z = 2 (H2+) (Figure 2b), a new TPD peak appears at about 240 K at 0.0005 ML of Pt coverage, corresponding to H2 desorption, strongly demonstrating that molecular H2 is produced from H2O conversion on the Pt clusters covered r-TiO2(110) surface during the TPD process. At the highest Pt coverage (0.006 ML), H2 formation starts at the temperature 10 ACS Paragon Plus Environment

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as low as 150 K with its peak at ~220 K. This demonstrates that H2 formation via H2O conversion can occur easily on the Pt clusters covered r-TiO2(110) surface. Our previous work demonstrated that H2 production on the HBBO atoms covered r-TiO2(110) surface starts at about 400 K,28 whereas, H2 formation on the H atoms covered Pt(111) surface 45 occurs in the temperature range of 200 K ~ 400 K. Therefore, the low temperature H2 product is most likely to be formed on the Pt clusters. Interestingly, as the coverage of Pt increases (from 0.0005 to 0.006 ML), the TPD peak of H2 increases significantly and shifts to lower temperature. While, the initial desorption temperature of H2 shifts from 200 K at 0.0005 ML of Pt coverage to 150 K at 0.006 ML of Pt coverage, indicating that the formation of H2 at high Pt coverage becomes easier, namely, the activation energy for H2 formation becomes lower. This may be due to the formation of smaller Pt cluster after sintering at low Pt coverage than that at high Pt coverage. Our theoretical calculation (details in below) shows that the H2 desorption on Pt1/r-TiO2(110) is much harder than that on Pt10/r-TiO2(110). While, as shown in Figure 1b, no H2 is formed on the single Pt atoms covered rTiO2(110) surface. Thus, in a certain cluster size range, the energy barrier of H2 formation on small Pt clusters are expected to be higher than that on big clusters, leading to the desorption of H2 at higher temperature when the Pt coverage is very low. In addition, as shown in Figure 2a, part of HBBO atoms are still detected by H2Ov desorption in the TPD spectrum of H2O at 0.0005 ML of Pt coverage, demonstrating that these HBBO atoms do not reach Pt clusters during the TPD process. As a result, the yield of H2 is low. We speculate that the intensity of Pt clusters on r-TiO2(110) is low 11 ACS Paragon Plus Environment


at low Pt coverage, H atoms created by H2Ov dissociation need to diffuse, on average, a longer way to reach the Pt clusters. Thus, more time is needed for the H atoms diffusion to reach Pt clusters. To further confirm this idea, the r-TiO2(110) surfaces after depositing 0.002 ML of Pt followed by sintering at 850 K for 1 min were used to do the similar H2 evolution experiments with different heating rates in TPD measurements. Indeed, we found that the lower heating rate results in a higher H2 yield (see in Figure S1), demonstrating that the diffusion of H atoms is very important for H2 production in our experimental condition. However, even in the low heating rate, the yield of H2 at low Pt coverage is still low, suggesting that there are some other reasons. We notice that at low Pt coverage, the initial desorption temperature of H2 is nearly the same as that for H2OTi, and thus the desorption of H2O and H2 are in competing. The desorption of H2OTi molecules leaves bare Ti5c sites on the surface, which maybe hinders the H atoms diffusion toward Pt clusters, and therefore hinders the H2 formation. As a comparison, the desorption temperature of H2 is significantly lower at high Pt coverage than that of H2O. Thus, before H2OTi desorption, H2OTi molecules maybe help H atoms diffusion toward Pt clusters. To understand the relative importance of H2 formation from low temperature H2O conversion, the yields of H2 and H2O as a function of Pt coverage have been measured quantitatively (Figure 2b inlet). The result verifies that the H2 formation is strongly dependent on the coverage of Pt. More importantly, the amount of H2 formation is nearly the same with that of H2O depletion at different Pt coverages, indicating that 12 ACS Paragon Plus Environment

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one H2O can produce one H2. While, as shown in Figure 2a, the recombinative peak of H2Ov at ~500 K disappears at high Pt coverage, suggesting that H2Ov dissociation can contribute to H2 formation. At the highest Pt coverage (0.006 ML), about 0.17 ML of H2 is produced. This value is considerably larger than the coverage of BBOv on the surface (7%), suggesting that part of H2 production comes from H2OTi conversion.

Figure 3. STM results for H2O dissociation on Pt cluster/r-TiO2(110). (a) STM image of a 0.006 ML of Pt deposited r-TiO2(110) surface after sintering at 850 K for 1 min (acquired at bias voltage of + 1.25 V and set point current of 100 pA, 10 x 10 nm2). (b) STM image of the Pt deposited rTiO2(110) surface dosed with H2O at 100 K followed by heating to room temperature for 1 min (10 x 10 nm2).

Based on our previous workers,33 the molecular and dissociative states of H2OTi are almost isoenergitic with a small barrier of 0.22 eV. However, the energy barrier of 13 ACS Paragon Plus Environment


the second O-H bond dissociation in H2OTi is 0.45 eV with a reverse barrier of 0.11 eV. Thus, it is nearly impossible for one H2OTi to produce one H2, leaving behind an O adatom on the Ti5c site (OTi). Even OTi adatom is produced after H2 desorption, OTi can facilitate H2O dissociation and proton transfer to form a terminal OH pair (OHTi), positioned along or across the Ti5c row, respectively. 46 To unravel the remaining product on the surface after H2O conversion, STM studies were carried out on the Pt clusters covered r-TiO2(110) surface, which was prepared by sintering the 0.006 ML of Pt adsorbed surface at 850 K for 1 min. Figure 3a shows that the STM image of the Pt clusters covered surface. A big bright spot is observed, which is due to the Pt cluster. From the size of the bright spot (marked by yellow circle), we estimated that the clusters have a mean size of 8-12 atoms. Then, 1 ML of H2O was introduced onto this surface at 100 K, followed by warming the surface to room temperature for 1 min to remove most of the H2OTi molecules, and then cooled down to 100 K for STM measurements (Figure 3b). It is obvious that the BBOvs are totally healed, which is consistent with the TPD’s result. While, many bright spots appear on the Ti5c sites. The big bright spot marked by yellow circle is Pt cluster. Only five H2OTi molecules are observed (marked by blue circles).19,46,47 Eighty seven less bright spots on the Ti5c rows show fuzzy features, which are marked by dash green circles. According to previous work done by Tan and coworkers,31 the species can be assigned to OHTi. In addition, six elliptical bright spots on the Ti5c rows marked by royal ovals also show fuzzy features. The features are totally different from that of H2O dimmer, trimer and longer H2O chains on the Ti5c rows,32 we guess the features may be due to the 14 ACS Paragon Plus Environment

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adsorption of several OHTi on the neighbouring Ti5c sites. The results suggest that the remaining product on the surface is OHTi after H2 production. Only a small number of OHTi are around the Pt cluster, indicating that H2OTi can dissociate at the metal-oxide interface. More importantly, most of the OHTi are randomly distributed on the surface, irrelevant to the position of the Pt cluster. Some of the OHTi are observed far away from the Pt cluster, demonstrating that the H2OTi molecules far away from Pt clusters also contribute to H2 production. From these experimental results, the low temperature H2 production on the Pt clusters covered r-TiO2(110) surface may be concluded to occur in three steps: 1) H2O molecules dissociate at the metal-oxide interface, BBOv sites, and probably at the Ti5c sites far away from the clusters to produce H atoms; 2) The H atoms transfer to the Pt clusters via multi-step diffusion processes, leaving behind OHTi and healing BBOvs; 3) molecular H2 product is produced on the Pt clusters when the surface is heated up. It is worth noting that only a portion of H atoms could transfer to the Pt clusters via multi-step diffusion processes at low Pt coverage during the TPD process. In order to gain further insights into the low temperature H2 formation via H2O conversion on Pt/r-TiO2(110), we have carried out theoretical calculations (PBE method, six O-Ti-O layers model, VASP program) to study the H2 production on Pt/rTiO2(110). The H2 production on the 1 ML of H2O covered Pt/r-TiO2(110) surface has been calculated using a six layers 6 × 2 surface unit cell. The Pt cluster was modeled by a Pt10 cluster, similar to the size of Pt clusters in the STM experiment, which adsorbs on a BBOv site. The original structure of Pt10 was designed based on previous 15 ACS Paragon Plus Environment


works on Pt8.14 On the Pt10 cluster, H atoms can adsorb either monodentately (1-H) or bidentately (2-H). When one H atom of an H2OTi molecule transfers to the Pt cluster, the formation of 1-H at the bottom layer of Pt10 is thermal neutral or slightly endothermic (up to 0.43 eV, as shown in Figure S2), and the formation of 1-H or 2H at the other Pt sites are exothermic. Among various H adsorption structures on Pt10, the 1-H at the second layer of Pt10 is the most stable, and the diffusion of a 1-H from an H2OTi molecule to the second layer of Pt10 is exothermic by -0.32 eV. When two H atoms transfer from two H2OTi molecules to the Pt10 cluster, 1-H and 2-H at a few positions are endothermic (Figure S2). While, there are also some positions on the Pt10 cluster for two H atoms diffusion from two H2OTi molecules are exothermic (up to 0.63 eV) (Figure S3). These results demonstrate that the H atom transferring from H2OTi to Pt10 cluster, namely, the dissociation of H2OTi at the metal-oxide interface to produce OHTi and 1-H or 2-H on the Pt10 cluster, is quite feasible thermodynamically, which is much easier than the H2OTi molecule dissociation on the clean r-TiO2(110) surface.33 Furthermore, the reaction profile of H transferring from H2OTi to Pt10 cluster and the dehydrogenation on the Pt10 cluster was modeled. As shown in Figure 4, at the start point, 1 ML of H2OTi are around to the Pt10 cluster (a), then one H atom of H2OTi transfers to the adjacent Pt atom to form 1-H (c). This process is thermoneutral by 0.002 eV, with a barrier of 0.34 eV (transition state (TS): b, Figure S4). In the next step, the 1-H atom transfers to a more stable adsorption structure (2-H) (e), with a


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barrier of 0.22 eV (TS: d, Figure S4). The 2-H can also easily migrate to the second layer of the Pt10 cluster to form the 1-H (f-h, Figure S4). The second H atom diffusion

Figure 4. The reaction profiles for H atom diffusion and H2 formation on H2O/Pt10/r-TiO2(110). The side views of structures shown in the figure are expressed by simplified schematic diagrams.

can also occur, followed by the relaxation of the two H atoms to their more stable sites (i). Finally, H2 is produced by the coupling of the two H atoms. The H2 desorption energy is 0.74 eV. The overall barrier is 0.99 eV, which is much lower than that for the recombinative H2 desorption from HBBO atoms on pure r-TiO2(110) (2.2 eV in Ref. 38), suggesting that Pt cluster can indeed catalyze H2 formation via H2O conversion on Pt/r-TiO2(110) at low temperature. In Figure 4 and Figure S4, the TS(b) is located by the force reversed method,40 and the others are located by CI-NEB method.41,42 To evaluate the entropy effect in the dehydrogenation reaction, we also calculated the free energies of all intermediates and transition states in Figure 4 with the method used in Ref. 48. The free energy profile is shown in Figure S4. We found that the entropy 17 ACS Paragon Plus Environment


corrections are in the range of 0.04~0.05 eV, demonstrating that the entropy does not play an important role in the H atom transfer and H2 formation on Pt10/r-TiO2(110). As shown in Figure 1b, Pt single atoms on r-TiO2(110) are not active for H2 production. We also investigated the H2 formation on Pt1/r-TiO2(110), as shown in Figure S5. All the transition states are located by CI-NEB method. The Pt single atom adsorbs on a BBOv site. The overall energy barrier and free energy for H2 production are 1.86 eV and 1.89 eV, respectively, which are much higher than those for H2 production on Pt10/r-TiO2(110), but they are a little lower than that of H2 production on pure r-TiO2(110),39 suggesting that the Pt single atoms on r-TiO2(110) cannot catalyze H2 formation via H2O conversion in our experimental condition. Based on experimental results, the HBBO atoms from the dissociation of H2Ov molecules are far away from the Pt clusters, and they need to diffuse to the Pt clusters first. Previous studies have shown that multi-step H atom diffusion processes among H2OTi molecules is possible.19,46,47 Here, the H atom transfer along the [001] and [110] directions among H2OTi, OHTi and HBBO on the 1 ML of H2O covered r-TiO2(110) surface have also been investigated. For the [001] direction diffusion, the HBBO atom (S6-1) transferring to the adjacent OHTi is exothermic by -0.19 eV, with a barrier of 0.11 eV (S6-TS2). Moreover, the H atom transferring from H2OTi (S6-3) to the adjacent OHTi is thermally neutral, with a barrier of 0.13 eV (S6-TS4) (Figure S6). As the comparison, the barrier for the diffusion of a single HBBO atom along the BBO row is about 1.24 eV, which is similar to the result reported in Ref. 19. For the [110] direction diffusion, the most facile pathway for the H atom of H2OTi diffusion would 18 ACS Paragon Plus Environment

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be the dissociation of H2OTi to generate OHTi and HBBO, followed by the inversion of HBBO atom toward the adjacent row, and the HBBO atom reacts with the OHTi on the adjacent row to form a new H2OTi (Figure S7). The overall barrier is 0.21 eV, which is slightly lower than the estimated barrier of ~0.4 eV on the 1/6 ML of H2O covered rTiO2(110) surface.19 While, the HBBO atom diffusion along the [110] direction shares the similar diffusion pathway of the H atom of H2OTi. All the TSs in Figure S6 and S7 are located by the force reversed method.40 For all the cases, the barriers and energy changes are quite small, and the H atom diffusion is facile both kinetically and thermodynamically. This suggests that HBBO atoms from the dissociation of H2Ov molecules far away from Pt clusters can transfer to OHTi produced at the metal-oxide interface to form H2OTi molecules very easily, while, the OHTi produced at the metaloxide interface can also transfer to the Ti5c sites far away from the clusters. Then H2OTi molecules unceasingly dissociate at the metal-oxide interface, leading to efficient H2 production when the surface is heated up.

CONCLUSION In summary, our experimental investigation provides strong evidence that small Pt clusters deposited r-TiO2(110) surfaces are very active for efficient H2 formation via low temperature H2O conversion. All of H2O molecules dissociated at BBOv sites, metal-oxide interfaces, and probably Ti5c sites can contribute to H2 production with proper coverage of Pt clusters on r-TiO2(110). DFT calculations indicate that the small Pt clusters can significantly lower the barriers of H2O dissociation and H2 production, making low temperature H2O conversion possible. More importantly, H2O assisted 19 ACS Paragon Plus Environment


multistep H atom diffusion plays a crucial role for efficient H2 production. These findings provides a general mechanism for H2 production on TiO2-based catalysts, which can be helpful for understanding and solving the challenging problems in catalytic H2O splitting.


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ASSOCIATED CONTENT Supporting Information TPD spectra collected at m/z = 2 at different ramping rates (Figure S1). Calculated energies for the H atoms diffusion from H2OTi to Pt10-cluster (Figure S2, S3). The reaction profiles for H2 formation at Pt10-cluster (Figure S4). The reaction profiles for H2 formation at single Pt atom (Figure S5). Calculated energies for multi-step H atom diffusion between HBBO, OHTi and H2OTi along the [001] and [110] directions (Figure S6, S7). This information is available free of charge via the internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors [email protected], [email protected], [email protected], [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (21673235, 21403228, 21403224, 21503223, 21673224), the Strategic pilot science and technology project of the Chinese Academy of Sciences (XDB17010200), the Chinese Ministry of Science and Technology (2013CB834605), the Youth Innovation Promotion Association CAS (2016169).



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TOC Graphic


Low-Temperature Hydrogen Production via Water Conversion on Pt

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