Boosting Hydrogen Evolution Activities by Strong ... - ACS Publications

Article pubs.acs.org/JPCC

Boosting Hydrogen Evolution Activities by Strong Interfacial Electronic Interaction in ZnO@Bi(NO3)3 Core−Shell Structures Shihui Zou,† Juanjuan Liu,†,‡ Hisayoshi Kobayashi,*,§ Changlei Chen,† Peisheng Qiao,† Renhong Li,† Liping Xiao,† and Jie Fan*,† †

Key Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University, Hangzhou, China College of Materials & Environmental Engineering, Hangzhou Dianzi University, Hangzhou, China § Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Kyoto, Japan ‡

S Supporting Information *

ABSTRACT: Base-free hydrogen evolution from formaldehyde solution represents one of the most important reactions in the fuel cell based hydrogen economy. However, limited progresses have been made in the rational design of cheap and efficient heterogeneous catalysts for this reaction. Here, we for the first time propose a Lewis acid−base combination strategy to design efficient heterogeneous catalysts for HER from HCHO/H2O. By utilizing the Lewis acid/base properties of Bi(NO3)3·5H2O/ZnO, we successfully fabricated core−shell structured ZnO@Bi(NO3)3 composites. A strong interfacial electronic interaction between ZnO and Bi(NO3)3·5H2O is evidenced by the unprecedented 3.3 eV upshift of Zn 2p and 0.5 eV downshift of Bi 4f, which boosts the HER activities of inert ZnO and Bi(NO3)3·5H2O. Destroying the interfacial electronic interaction leads to a fast deactivation while increasing interfacial sites proportionally enhances the activity, indicating that interfacial sites are real active sites. DFT calculations confirm that ZnO@Bi(NO3)3 composites greatly lower the activation barrier of H2 formation from two adsorbed H atoms and thus promote the H2 production. The Lewis acid−base combination strategy also applies to the TiO2@Bi(NO3)3 system, further highlighting the importance of salt−metal oxide interface in heterogeneous catalysis.

1. INTRODUCTION Hydrogen is expected to play a significant role in the future energy system due to its high energy density (142 MJ kg−1), renewable source, and environment-friendliness.1−5 However, the large-scale replacement of fossil fuels by hydrogen is still restricted owing to the technical barriers in controllable storage and release of hydrogen.3,6 To overcome this problem, great efforts have been devoted to develop advanced catalysts for selective hydrogen production from H2-rich chemicals, such as methanol,7,8 ammonia borane,9−13 formic acid,14−19 formaldehyde,20−27 and water.28−39 Among them, formaldehyde aqueous solution (HCHO/H2O), generally in the form of methanediol,40 is potentially interesting for mobile applications because of its long-term stability/recyclability, nonflammability, and considerable theoretic H2 weight efficiency (8.4 wt %).20,21 Nevertheless, traditional catalytic systems for dehydrogenation of formaldehyde solution are always limited by the low H2 production efficiency, unclear catalytic mechanism, the addition of high concentration soluble bases (e.g., NaOH), and the use of noble metal nanoparticles/compounds as the major active components.21−27,41,42 From the “H2-economy” perspective,43 it is of great interest to develop cheap and efficient catalysts for base-free hydrogen evolution from formaldehyde solution at mild conditions. © XXXX American Chemical Society

Base-free hydrogen evolution from HCHO/H2O has recently been reported in homogeneous catalysis,20 but the design of efficient heterogeneous catalysts still remains great challenge. Heim et al. proposed that [(Ru(p-cymene))2(μ-Cl)2Cl2] complex could catalyze hydrogen evolution from paraformaldehyde or formaldehyde aqueous solution in the absence ofbase additives.20 Interestingly, they found that nanoparticles and supported metal catalysts are inactive for this reaction under the same conditions, suggesting that the unique structure of the Ru complex plays a significant role in HER. From the perspective of Lewis acid−base theory, [(Ru(p-cymene))2(μ-Cl)2Cl2] is a Lewis acid−base complex made by the combination of corresponding Lewis acid (i.e., Ru salts) and base (i.e., organic ligands). It is likely that the Lewis acid−base combination between Ru salts and organic ligands brings in some coordinative-unsaturated sites which are responsible for HER. Similar results have been reported in homogeneous catalytic dehydrogenation of formic acid as well.15,44,45 For example, Boddien et al. discovered the combination of cationic iron(II) source (Lewis acid) and the tetradentate ligand (Lewis base) as a Received: December 8, 2016 Revised: January 21, 2017 Published: February 8, 2017 A

DOI: 10.1021/acs.jpcc.6b12346 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. (A) TEM, (B) HRTEM, (C) EDX elemental mapping, and (D) line-scanning profile of BN-ZnO-2%.

Synthesis of BN-ZnO-X: Typically, 1 mmol of ZnO (Sinopharm Chemicals, >99%) and X mmol of Bi(NO3)3·5H2O (Sinopharm Chemicals, >99%) were mixed in 10 mL of deionized water. After 30 min stirring at 25 °C, the precipitates were collected by centrifuging and drying, denoted as BN-ZnO-X (X is the molar ratio of Bi/Zn). For the synthesis of BN-ZnONPs-X, ZnO was replaced by the as-prepared ZnONPs. Synthesis of BN-ZnO-X-550 and Bi2O3: BN-ZnO-X-550 and Bi2O3 were obtained by annealing BN-ZnO-X and Bi(NO3)3· 5H2O, respectively, at 550 °C for 5 h with a ramp rate of 5 °C min−1. Synthesis of control groups of ZnO + H2O and Bi(NO3)3 + H2O: All control samples were synthesized by similar procedures of BN-ZnO-X. ZnO + H2O was synthesized without the addition of Bi(NO3)3·5H2O while Bi(NO3)3 + H2O was synthesized without the addition of ZnO. 2.2. Characterizations. Wide-angle XRD patterns were recorded on a Rigaku Ultimate IV diffractometer using Cu Kα radiation (40 kV, 40 mA, 10° min−1 from 10 to 80°). TEM, HRTEM, EDX mapping, and line-scanning analysis were recorded on the FEI TITAN Cs-corrected ChemiSTEM equipped with an energy dispersive X-ray (EDX) spectroscope, operating at 200 kV. XPS measurements were performed in a VG Scientific ESCALAB Mark II spectrometer equipped with two ultrahigh vacuum (UHV) chambers. All binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon to correct the shift caused by charge effect. UV/vis adsorption spectra were measured with a Shimadzu UV2450 spectrophotometer in the diffuse/reflectance mode. The solid electron paramagnetic resonance (EPR) spectra were recorded at 77 K with a Bruker EPR A-300 spectrometer working in the X-band (9.86 GHz). 2.3. Catalytic Test. H2 evolution reactions were performed under inert atmosphere (argon) with exclusion of air. 1 mmol of catalyst was transferred into a 55 mL Pyrex text tube containing 10 mL of 4% HCHO/H2O solution under constant stirring (600

highly active catalyst system for the liberation of H2 from FA, affording turnover frequencies up to 9425 h−1.15 It is thus reasonable to hypothesize that efficient heterogeneous catalysts could be designed by Lewis acid−base combination of desired materials. ZnO is well-known as support and cocatalyst for methanol/ formaldehyde steam reforming.46−49 Meanwhile, the amphiprotic nature makes it a good candidate for Lewis acid−base combination. In the present study, ZnO was employed as Lewis base to react with the strong Lewis acid, Bi(NO3)3·5H2O.50 Core−shell structured ZnO@Bi(NO3)3 composites were successfully fabricated via surface Lewis acid−base combination. Interestingly, we found that ZnO has a strong interfacial electronic interaction with Bi(NO3)3·5H2O, accompanied by an unprecedented charge transfer from Zn to Bi. Contrary to the negligible activity on bare ZnO and Bi(NO3)3, the presence of the unique interfacial structure of ZnO@Bi(NO3)3 boosts the HER activity. The energy and structural changes along the whole reaction were investigated by the DFT calculations with periodic models. The excellent HER performance of ZnO@Bi(NO3)3 composites was ascribed to the greatly lowered H2 formation barrier.

2. EXPERIMENTAL SECTION 2.1. Materials Synthesis. Synthesis of ZnO nanoparticles: ZnO nanoparticles (ZnONPs) were synthesized according to the reported literature with some modifications.51 In a typical synthesis, 1 mmol of zinc acetate dehydrate (Sinopharm Chemicals, >99%) was dissolved in 100 mL of 2-propanol under vigorous stirring at 50 °C. 110 mg of PVP (Alfa-Aesar) was then added to the transparent solution at room temperature under constant stirring, followed by the addition of 50 mL of 0.04 mol L−1 NaOH/2-propanol solution. After 1 h ultrasonication, powder samples were obtained by removing the solvent using a rotary evaporator, followed by annealing at 350 °C for 5 h. B


Article


Figure 2. (A) XRD patterns for ZnO and BN-ZnO-10%, (B) Zn 2p, (C) Bi 4f XPS spectra, and (D) UV−vis diffuse reflectance spectra for different samples.

± 10 rpm) for 30 min to get a white suspension. The mixture was then heated to the desired temperature (25, 40, 50, and 60 °C) to start the reaction. Gas volumes of 400 μL were extracted from the test tubes using a microliter syringe at regular intervals, and GCTCD was employed for evaluating the gas evolution amount, including H2, O2, CO2, and CO. For photocatalytic test, visible light was supplied by a 300 W Hg lamp with a 400 nm cut-on filter. 2.4. DFT Calculations. DFT calculations with the periodic boundary conditions were carried out using a plane wave based program, Castep.52,53 The Perdew−Burke−Ernzerhof (PBE) functional54,55 was used together with the ultrasoft-core potentials.56 The basis set cutoff energies were set to 300 eV. The electron configurations of the atoms were H: 1s1, C: 2s22p2, N: 2s22p3, O: 2s22p4, Zn: 3d104s2, and Bi: 6s26p3. The lattice parameters were a = 9.748 Å, c = 10.411 Å, and α = β = γ = 90°. The surface normal was taken in the direction b, and b = 40 Å including the vacuum region. The catalyst was modeled by a (ZnO)36 slab representing the (0001) face of wurtzite structure and a (BiO)6(OH)2(NO3)4 cluster overlaid. Hereinafter, an abbreviation “BiO” is used for the (BiO)6(OH)2(NO3)4 cluster. Two H2O molecules are dealt with a part of the reactants as well as H2CO. The optimized configurations of catalyst and substrate molecules for local minimum (LM) and transition state (TS), which are not covered by Figures 7−9, are shown in Figures S10−S13. Geometry optimization was carried out with respect to all atomic coordinates, and the lattice constants were fixed.

scanning profiles (Figure 1B−D). The lattice fringes (0.314 nm) of Bi6(NO3)4(OH)2O6·2H2O appear in the edge while that (0.248 nm) of ZnO dominates the core of the particle, indicating a ZnO@Bi(NO3)3 core−shell structure. The presence of Bi6(NO3)4(OH)2O6·2H2O rather than Bi(NO3)3·5H2O is likely due to the partially hydrolysis of Bi(NO3)3·5H2O on the surface of ZnO,57 which is also confirmed by X-ray diffraction (Figure 2A). Consistent with HRTEM results, EDX mapping images show enrichment of Zn signals in the core and homogeneous distribution of Bi signal in the shell. Meanwhile, line scanning exhibits a broad peak for Zn located at the center of the profile and two intensive peaks for Bi on both sides, further confirming the core−shell configuration. The ZnO@Bi(NO3)3 core−shell structure is expected because only the surface of ZnO is accessible to Bi(NO3)3·5H2O, where Lewis acid−base combination takes place. Interestingly, the wide-ranging EDX mapping of BN-ZnO-2% acquired in SEM mode shows homogeneous distribution of both Bi and Zn throughout the whole sample, suggesting a good dispersion of Bi(NO3)3·5H2O on the surface of ZnO. It is likely that the strong Lewis acidity of Bi(NO3)3· 5H2O drives itself to react with more base sites (ZnO) and thus achieves a good dispersion. In line with the ZnO@Bi(NO3)3 core−shell structure, the XRD pattern for BN-ZnO-10% consist of two sets Bragg peaks: ZnO and Bi6(NO3)4(OH)2O6·2H2O. No obvious differences between ZnO and BN-ZnO-10% in the intensities and widths for the characteristic diffraction peaks of ZnO were observed (JCPDS card number: 36-1451), suggesting that the addition of Bi(NO3)3·5H2O did not change the bulk structures of ZnO. Meanwhile, due to the partially hydrolysis of Bi(NO3)3·5H2O on the surface of ZnO, the characteristic diffraction peaks of Bi6(NO3)4(OH)2O6·2H2O (JCPDS card number: 28-0654) rather than Bi(NO3)3·5H2O were obtained, coinciding with the HRTEM images. Unlike the bulk structures, there are significant differences in surface electronic structures between ZnO and BN-ZnO-10%. As

3. RESULTS AND DISCUSSION 3.1. Catalyst Preparation and Characterization. The microstructures of BN-ZnO-X were characterized by TEM and SEM. As can be seen from Figure 1A and Figure S1, the surface of BN-ZnO-2% roughens as comparing to ZnO, implying the successful deposition of Bi(NO3)3·5H2O on the surface of ZnO. The fine structures of typical BN-ZnO-2% samples were further examined by HRTEM, EDX elemental mapping, and lineC


Article

The Journal of Physical Chemistry C can be seen from Figure 2B, the binding energies of Zn 2p3/2 for pure ZnO and ZnO + H2O were both at 1021.6 eV,58 suggesting that the surface electronic structure of ZnO is not affected by H2O. However, after interacting with Bi(NO3)3·5H2O, the main part (70%) of the peak upshifts significantly (3.3 eV) to the higher binding energy side (1024.9 eV). To the best of our knowledge, such a significant upshift is unprecedented. Interestingly, no obvious shift in O 1s signal was observed (Figure S2). Instead, downshifts were observed in Bi 4f signals (Figure 2C). The binding energy of Bi 4f7/2 for pure Bi(NO3)3· 5H2O locates at 159.7 eV. After dispersing in H2O, a 0.3 eV downshift emerges due to the partially hydrolysis of Bi(NO3)3· 5H2O. Notably, it further decreases to 158.9 eV after interacting with ZnO, indicating a strong electronic interaction between ZnO and Bi(NO3)3·5H2O. The 3.3 eV upshift of Zn 2p and 0.5 eV downshift of Bi 4f clearly show that electrons are readily transferred from ZnO to Bi(NO3)3·5H2O, in good accord with their Lewis base and acid properties. Meanwhile, the solid EPR spectra (Figure S3) reveal that a signal at g = 1.957 appears after the combination of ZnO and Bi(NO3)3·5H2O. Such a signal is commonly assigned to singly ionized oxygen vacancy (Vo•),59 which might be formed in the BN-ZnO system due to the electronic interaction between Bi and Zn. In addition, we notice that the band structure of BN-ZnO-10% is also different from that of ZnO and Bi(NO3)3·5H2O. Interestingly, both ZnO and Bi(NO3)3·5H2O show no visible light absorption while BNZnO-10% exhibits a detectable absorption shoulder in the visible region (400−500 nm, Figure 2D). The band gap energies calculated from the Tauc plots were 3.24 and 3.66 eV for ZnO and Bi(NO3)3·5H2O, respectively (Figure S4). The potentials of the conduction-band (CB) and valence-band (VB) edges of Bi(NO3)3·5H2O and ZnO were estimated by Mulliken electronegativity theory:60

Figure 3. (A) Time course of evolved H2 at 50 °C on ZnO, Bi(NO3)3· 5H2O, and BN-ZnO-X. (B) Arrhenius plots for BN-ZnO-X (X = 2%, 10%, 20%, and 40%).

further increase of Bi/Zn ratio to 40% leads to a decrease in activity. Kinetics study (Figure 3B), however, claims that all BNZnO-X samples share the similar apparent activation energy (64.4−79.7 kJ mol−1), indicating that the variation in Bi/Zn ratio mainly influences the effective number of active sites rather than changes their inherent reactivity. Combining the surface electronic structure and HER performance, it is reasonable to hypothesize that the unique interface between ZnO and Bi(NO3)3·5H2O is the real active site. The increase in Bi/Zn molar ratio leads to the incremental interface sites and thus the active sites for HER. Bringing in too much Bi(NO3)3·5H2O might block the interfacial sites and inhibit the exposure of interfacial sites to HCHO/H2O, which in turn decreases the HER activity. Further studies to directly identify the perimeter sites would be useful to confirm these hypotheses. Similar results have been systematically studied and modeled by Bowker et al. for catalysis at the metal−support interface.63 It is worth mentioning that in all cases no CO is detected and the pH of solution remains constant, excluding the considerable production of CO and HCOOH. The CO2 detected in gas phase, however, is less than expected (half the amount of H2), likely due to the partial adsorption on ZnO. 3.3. Influence of the Interfacial Interaction on the Catalytic Activity. The importance of the interface between support and metal has long been recognized.63−71 Bowker et al. reported that the photocatalytic reforming of methanol take place at the boundary between Pd and TiO2. The larger the extent of this boundary, the higher the reaction rate.63 Mudiyanselage et al. investigated the water-gas shift reaction on CeOx/Cu(111) by ambient-pressure X-ray photoelectron spectroscopy.69 They proposed that the presence of the oxide− metal interface activates the more efficient associative mechanism pathway and leads to an increase by more than 1 order of magnitude in the activity of the CeOx/Cu(111) system relative

E VB = X − E e + 0.5Eg

where Ee is the energy of free electrons on the hydrogen scale (ca. 4.5 eV) and X is the absolute electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms. The CB can be determined by ECB = EVB − Eg. The energy band structure diagram of Bi(NO3)3·5H2O and ZnO is schematically illustrated in Figure S4. When the p-type Bi(NO3)3·5H2O is in contact with n-type ZnO, it is able to form a p−n junction61,62 where the electrons diffuse from ZnO into Bi(NO3)3·5H2O and holes migrate from Bi(NO3)3·5H2O to ZnO. These results lead us to the hypothesis that the unique interface between ZnO and Bi(NO3)3·5H2O could significantly influence their properties. It is expected that BN-ZnO-X will exhibit distinguished catalytic properties. 3.2. Hydrogen Evolution Performance. Figure 3A shows the comparison of the HER performance of samples with different Zn/Bi ratio at 50 °C with 4% HCHO/H2O as reactants. Control experiments indicate that no noticeable H2 evolution are detected on bare ZnO and Bi(NO3)3·5H2O. However, once put them together, BN-ZnO-X samples exhibit appreciable H2 evolution, suggesting that the active sites are formed by the combination of ZnO and Bi(NO3)3·5H2O. Interestingly, the activity is closely related to the Bi/Zn molar ratio. When Bi/Zn molar ratio is less than 20%, the H2-evolution rate increases along with the Bi content. It is important to highlight that by the addition of a small amount of Bi(NO3)3·5H2O (BN-ZnO-2%), the H2 produced in 24 h is significantly improved from ca. 0.3 to 7.8 μmol. The highest H2 evolution is obtained on BN-ZnO-20% (12.4 μmol), which is ca. 40-fold higher than that of bare ZnO. A D


Article


interfacial sites (Zn2+δ) play a critical role in HER, likely being the active sites. Another possible route to confirm the influence of the interfacial interaction is realized by annealing BN-ZnO-10% at 550 °C. As shown in Figure 5A, BN-ZnO-10%-550 consists of ZnO and Bi2O3, indicating the decomposition of Bi(NO3)3· 5H2O into Bi2O3. More importantly, BN-ZnO-10%-550 shares the same Zn 2p signal with ZnO and the 3.3 eV upshift disappears, suggesting that the interfacial electronic interaction between ZnO and Bi(NO3)3·5H2O is destroyed (Figure 5B). Notably, destroying the electronic interaction has a detrimental influence on the catalytic performance. As can be seen from Figure 5C,D, the HER performance of BN-ZnO-10%-550 is similar to that of Bi2O3. Plausible H2 evolution is observed only in the first 12 h due to the instability of Bi2O3, which is confirmed by XRD patterns of used-Bi2O3 catalyst (Figure S5). BN-ZnO-10%, on the contrary, exhibits steady H2 evolution within 60 h, suggesting a better durability. ICP-MS measurements exclude the leaching of Bi into the reaction solution, confirming the good stability of BN-ZnO-10%. The structures of catalysts after hydrogen evolution reaction (used catalysts) are also examined by TEM, EDX-mapping, and line-scanning profiles (Figure S6). The similar morphology and elemental distribution of fresh and used catalysts confirm the structural stability of BN-ZnO-X. We note that the H2 evolution rate in the first 12 h is higher than the rest, which is likely due to the amorphization of Bi(NO3)3·5H2O during the first 12 h reaction (Figure S7A). However, as suggested by XPS (Figure S7B), strong electronic interaction still remains after 12 h reaction, which is responsible for the steady H2 evolution. Besides, the gradually increased HER performance of BN-ZnO-10%-550 from 12−60 h is likely caused by the asgenerated Bi salts−ZnO interface. In contrast, no interfacial interaction is generated in bare Bi2O3 catalyst, leading to a complete deactivation. More interestingly, the BN-ZnO-10%550 sample can also fall into the quasi-linear plot of H2 production vs Zn2+δ concentration shown in Figure 4B. These results clearly demonstrate the importance of the interfacial

to that of Cu(111). Somorjai et al. have also published a bunch of nice papers dealing with the role of electronic excitation in catalysis at the oxide−metal interface.65,71,72 Nevertheless, interfaces mentioned in those studies are mainly formed by physical contact of metals and oxides, whereas interfaces between metal oxides and salts are less discussed. In the present study, the most distinguished feature is the strong interfacial interaction between ZnO and Bi(NO3)3·5H2O. To correlate the catalytic activity of BN-ZnO-X to the unique interface between ZnO and Bi(NO3)3·5H2O, three experiments were performed. First of all, we evaluated the percentage of upshifted Zn 2p (i.e., Zn2+δ) and treated it as a descriptor for the number of interfacial sites in different BN-ZnO-X samples (Figure 4A). Figure 4B plots the amount of H2 produced within

Figure 4. (A) Zn 2p XPS of ZnO, BN-ZnO-0.5%, BN-ZnO-2%, and BN-ZnO-10%. (B) Amount of H2 produced within 12−24 h at 50 °C as a function of the surface Zn2+δ concentration.

12−24 h as a function of the surface Zn2+δ concentration. The quasi-linear relationship confirms that (1) the variation in Bi/Zn ratio greatly influences the number of interfacial sites and (2)

Figure 5. (A) XRD patterns of Bi2O3 and BN-ZnO-10%-550; (B) Zn 2p XPS of ZnO, BN-ZnO-10%, and BN-ZnO-10%-550; (C) HER activities of Bi2O3, BN-ZnO-10%-550, and BN-ZnO-10; and (D) H2 produced in every 12 h shown in panel C. E


Article

The Journal of Physical Chemistry C electronic interaction between ZnO and Bi species. Additionally, other Bi salts such as Bi2(SO4)3 are also able to interact with ZnO to produce efficient interfaces for HER, which provide more possibilities to functionalize the interface. Given that the Lewis acid−base combination only take place at the surface of ZnO, the number of interfacial sites is dominated by the surface area.73 It is reasonable to expect that more interfacial sites will be generated by increasing the surface area of ZnO. On the other hand, when Bi(NO3)3·5H2O contacts with ZnO to form a p−n junction, the presence of visible light may further enhance the interfacial electronic transfer between ZnO and Bi(NO3)3·5H2O. Assuming that the interface is the real active site, enhancement will be observed in HER performance as well. As shown in Figure 6, with the presence of visible light, the

Figure 7. Optimized structure of (ZnO)36-Bi6O6(OH)2(NO3)4. Purple: Bi; blue: N; red: O; gray: Zn; white: H.

→ H2 + HCOOH reaction proceeded on BN-ZnO (red line) and BiO moiety only (black line) model catalysts. Individual optimized structures for LM’s are described in the Supporting Information as well as other reaction paths investigated (Figures S10 and S11). The four-step reaction on BN-ZnO is initiated by the dissociative adsorption of H2CO molecule, with the formed HCO and H species bonded to Bi and O atoms, respectively. (Bi)−OCH subsequently reacts with H2O to form HCOOH and H, and the latter is bonded to O as the second (O)−H species on the surface. Thereafter, an H atom migrates from (O)−H to (Bi)−H. This step possesses the highest activation energy (124.4 kJ mol−1), serving as the rate-determining step. The final step is a combination of (O)−H and (Bi)−H to produce H2 molecule. Interestingly, the relative energy for TS4 on BN-ZnO is only 55.1 kJ mol−1. On the contrary, the H2 formation from two adsorbed H atoms on both BiO slab and ZnO slab (Figure S12) encounter very high energies (286.0 and 210.3 kJ mol−1, respectively). These results confirm that the hybridization effect between ZnO and Bi6O6(OH)2(NO3)4 could facilitate the formation and desorption of H2, revealing the exact role of the ZnO@Bi(NO3)3 interface in HER. On the other hand, Figure 9 shows the energy changes and transition states for HCOOH → H2 + CO2 reaction proceeded on BN-ZnO (red line), BiO moiety only (black line), and ZnO moiety only (blue line) model catalysts. Even if the energy barrier is a little bit higher as comparing to the H2CO + H2O → H2 + HCOOH reaction, the energy barrier on BN-ZnO (223 kJ mol−1) is still lower than that on ZnO (269 kJ mol−1) and BiO moiety (246 kJ mol−1), suggesting a positive effect of the combination. Taking all together, the calculations have led to the belief that the hybridization effect between ZnO and Bi6O6(OH)2(NO3)4 mainly facilitates the formation and desorption of H2.

Figure 6. BET surface area of ZnO and ZnONPs; amounts of H2 produced within 24 h at 25 °C on BN-ZnO-20% and BN-ZnONPs-20% with/without visible light irradiation.

HER activity of BN-ZnO-20% increases ca. 5 times (0.68 vs 3.52 μmol), suggesting a visible-light enhancement. Moreover, once we replace ZnO (3.03 m2 g−1) by ZnONPs (15.05 m2 g−1), the HER activity increases proportionally to the surface area, which further elucidates the critical role of interfacial electronic interaction in HER.74,75 Inspired by these results, we further apply the Lewis acid−base combination strategy to the BN-TiO2 system (Figure S8). The superior visible-light-induced photocatalytic H2 evolution activity on BN-TiO2 over bare TiO2 and Bi(NO3)3·5H2O, coupled with the electronic interaction between TiO2 and Bi(NO3)3·5H2O, again proves the feasibility of this strategy. We note that the visible-light-induced H2 evolution on BN-ZnO and BN-TiO2 merits further studies. Leveraging new tools such as electrochemical impedance spectroscopy to study the charge transfer is likely to further improve our understanding on the kinetics. 3.4. DFT Calculations. We explored potential mechanistic roles of the BN−ZnO interface with DFT calculations using a (ZnO)36-Bi6O6(OH)2(NO3)4 model catalyst (Figure 7). Additional calculations using catalyst models with BiO moiety only and ZnO slab only were also conducted for comparison. Atomic charges estimated by Mulliken population show that positive charges on Bi atoms decrease from +1.57 (BiO moiety only) to +1.51 (BN-ZnO) by acid−base combination while work functions show that the VBM for BN-ZnO is lower that for ZnO by 0.356 eV, confirming the electronic transfer from Zn to Bi (Figure S9). To calculate the possible reaction pathway, the whole dehydrogenation process are divided into two parts: H2CO + H2O → H2 + HCOOH and HCOOH → H2 + CO2. Figure 8 shows the energy changes and transition states for H2CO + H2O

4. CONCLUSIONS In summary, we have demonstrated an efficient strategy to design cheap heterogeneous catalysts for HER from HCHO/H2O. The surface Lewis acid−base combination between Bi(NO3)3·5H2O and ZnO produces a ZnO@Bi(NO3)3 core−shell structure. It is interesting to note that ZnO has a strong interfacial electronic interaction with Bi(NO3)3·5H2O. The unprecedented 3.3 eV upshift of Zn 2p and 0.5 eV downshift of Bi 4f indicate that electrons are readily transferred from Zn to Bi. Base-free F


Article


Figure 8. (A) Energy changes along the H2CO + H2O → H2 + HCOOH reaction occurred on BiO slab (black line) and BN-ZnO slab (red line). The configurations of four transition states (TS) based on (B) BiO and (C) BN-ZnO slab.

interfacial electronic interaction leads to a fast deactivation while increasing interfacial sites proportionally enhances the activity, indicating interfacial sites as active sites. DFT calculations further confirm that the hybridization effect between ZnO and Bi6O6(OH)2(NO3)4 could facilitate the formation and desorption of H2, revealing the exact role of ZnO@Bi(NO3)3 in HER. Leveraging new tools to study the interface is likely to further improve our understanding on the important role of salt−metal oxide interface in heterogeneous catalysis.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b12346. SEM images, O 1s XPS spectra, solid EPR spectra, extra XRD patterns, and DFT calculations (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

Figure 9. Energy changes along the HCOOH → H2 + CO2 reaction occurred on BiO (black line), ZnO (blue line), and BN-ZnO slab (red line). The configurations of TS and LM are listed in the right and Figure S13, respectively.

*E-mail [email protected] (J.F.). *E-mail [email protected] (H.K.). ORCID

Jie Fan: 0000-0002-8380-6338 Author Contributions

hydrogen evolution measurements show that BN-ZnO-X are much active than bare ZnO and Bi(NO3)3·5H2O. Destroying the

S.Z. and J.L. contributed equally. G


Article

The Journal of Physical Chemistry C Notes

(19) Tedsree, K.; Li, T.; Jones, S.; Chan, C. W. A.; Yu, K. M. K.; Bagot, P. A. J.; Marquis, E. A.; Smith, G. D. W.; Tsang, S. C. E. Hydrogen production from formic acid decomposition at room temperature using a Ag-Pd core-shell nanocatalyst. Nat. Nanotechnol. 2011, 6, 302−307. (20) Heim, L. E.; Schlörer, N. E.; Choi, J.-H.; Prechtl, M. H. G. Selective and mild hydrogen production using water and formaldehyde. Nat. Commun. 2014, 5, 3621. (21) Hu, H.; Jiao, Z.; Ye, J.; Lu, G.; Bi, Y. Highly efficient hydrogen production from alkaline aldehyde solutions facilitated by palladium nanotubes. Nano Energy 2014, 8, 103−109. (22) Suenobu, T.; Isaka, Y.; Shibata, S.; Fukuzumi, S. Catalytic hydrogen production from paraformaldehyde and water using an organoiridium complex. Chem. Commun. 2015, 51, 1670−1672. (23) Bi, Y.; Hu, H.; Li, Q.; Lu, G. Efficient generation of hydrogen from biomass without carbon monoxide at room temperature − Formaldehyde to hydrogen catalyzed by Ag nanocrystals. Int. J. Hydrogen Energy 2010, 35, 7177−7182. (24) Bi, Y.; Lu, G. Nano-Cu catalyze hydrogen production from formaldehyde solution at room temperature. Int. J. Hydrogen Energy 2008, 33, 2225−2232. (25) Li, Y.; Chen, T.; Wang, T.; Zhang, Y.; Lu, G.; Bi, Y. Highly efficient hydrogen production from formaldehyde over Ag/g-Al2O3 catalyst at room temperature. Int. J. Hydrogen Energy 2014, 39, 9114−9120. (26) Pan, X.; Wang, L.; Ling, F.; Li, Y.; Han, D.; Pang, Q.; Jia, L. A novel biomass assisted synthesis of Au−SrTiO3 as a catalyst for direct hydrogen generation from formaldehyde aqueous solution at low temperature. Int. J. Hydrogen Energy 2015, 40, 1752−1759. (27) Preti, D.; Squarcialupi, S.; Fachinetti, G. Aerobic, coppermediated oxidation of alkaline formaldehyde to fuel-cell grade hydrogen and formate: Mechanism and applications. Angew. Chem., Int. Ed. 2009, 48, 4763−4766. (28) Fujishima, A.; Honda, K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972, 238, 37−38. (29) Kohl, S. W.; Weiner, L.; Schwartsburd, L.; Konstantinovski, L.; Shimon, L. J. W.; Ben-David, Y.; Iron, M. A.; Milstein, D. Consecutive thermal H2 and light-induced O2 evolution from water promoted by a metal complex. Science 2009, 324, 74−77. (30) Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H. MoS2 nanoparticles grown on graphene: an advanced catalyst for the hydrogen evolution reaction. J. Am. Chem. Soc. 2011, 133, 7296−7299. (31) Kiwi, J.; Gratzel, M. Hydrogen evolution from water induced by visible light mediated by redox catalysis. Nature 1979, 281, 657−658. (32) Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M. A metal-free polymeric photocatalyst for hydrogen production from water under visible light. Nat. Mater. 2009, 8, 76−80. (33) Greeley, J.; Jaramillo, T. F.; Bonde, J.; Chorkendorff, I.; Norskov, J. K. Computational high-throughput screening of electrocatalytic materials for hydrogen evolution. Nat. Mater. 2006, 5, 909−913. (34) Maeda, K.; Teramura, K.; Lu, D.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. Photocatalyst releasing hydrogen from water. Nature 2006, 440, 295−295. (35) Zou, Z.; Ye, J.; Sayama, K.; Arakawa, H. Direct splitting of water under visible light irradiation with an oxide semiconductor photocatalyst. Nature 2001, 414, 625−627. (36) Chen, X.; Shen, S.; Guo, L.; Mao, S. S. Semiconductor-based photocatalytic hydrogen generation. Chem. Rev. 2010, 110, 6503−6570. (37) Kudo, A.; Miseki, Y. Heterogeneous photocatalyst materials for water splitting. Chem. Soc. Rev. 2009, 38, 253−278. (38) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar water splitting cells. Chem. Rev. 2010, 110, 6446−6473. (39) Maeda, K.; Takata, T.; Hara, M.; Saito, N.; Inoue, Y.; Kobayashi, H.; Domen, K. GaN:ZnO solid solution as a photocatalyst for visiblelight-driven overall water splitting. J. Am. Chem. Soc. 2005, 127, 8286− 8287. (40) Zou, S.; Liu, J.; Kobayashi, H.; Hu, X.; Xiao, L.; Fan, J. A new reaction between common compounds: lead oxide reacts with formaldehyde. Chem. Commun. 2014, 50, 6316−6318.

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (21271153, 21373181, 21222307, U1402233), Major Research Plan of National Natural Science Foundation of China (91545113), Shell Global Solutions International B. V. (PT37712), Fok Ying Tung Education Foundation (131015), and the Fundamental Research Funds for the Central Universities (2014XZZX003-02).

■

REFERENCES

(1) Schlapbach, L.; Zuttel, A. Hydrogen-storage materials for mobile applications. Nature 2001, 414, 353−358. (2) Muradov, N. Z.; Veziroğlu, T. N. From hydrocarbon to hydrogen− carbon to hydrogen economy. Int. J. Hydrogen Energy 2005, 30, 225− 237. (3) Eberle, U.; Felderhoff, M.; Schüth, F. Chemical and physical solutions for hydrogen storage. Angew. Chem., Int. Ed. 2009, 48, 6608− 6630. (4) Yadav, M.; Xu, Q. Liquid-phase chemical hydrogen storage materials. Energy Environ. Sci. 2012, 5, 9698−9725. (5) Turner, J. A. A realizable renewable energy future. Science 1999, 285, 687−689. (6) van den Berg, A. W. C.; Arean, C. O. Materials for hydrogen storage: current research trends and perspectives. Chem. Commun. 2008, 668−681. (7) Olah, G. A. Beyond oil and gas: the methanol economy. Angew. Chem., Int. Ed. 2005, 44, 2636−2639. (8) Nielsen, M.; Alberico, E.; Baumann, W.; Drexler, H.-J.; Junge, H.; Gladiali, S.; Beller, M. Low-temperature aqueous-phase methanol dehydrogenation to hydrogen and carbon dioxide. Nature 2013, 495, 85−89. (9) Sutton, A. D.; Burrell, A. K.; Dixon, D. A.; Garner, E. B.; Gordon, J. C.; Nakagawa, T.; Ott, K. C.; Robinson, J. P.; Vasiliu, M. Regeneration of ammonia borane spent fuel by direct reaction with hydrazine and liquid ammonia. Science 2011, 331, 1426−1429. (10) Singh, S. K.; Zhang, X.-B.; Xu, Q. Room-temperature hydrogen generation from hydrous hydrazine for chemical hydrogen storage. J. Am. Chem. Soc. 2009, 131, 9894−9895. (11) Singh, S. K.; Xu, Q. Complete conversion of hydrous hydrazine to hydrogen at room temperature for chemical hydrogen storage. J. Am. Chem. Soc. 2009, 131, 18032−18033. (12) Hannauer, J.; Akdim, O.; Demirci, U. B.; Geantet, C.; Herrmann, J.-M.; Miele, P.; Xu, Q. High-extent dehydrogenation of hydrazine borane N2H4BH3 by hydrolysis of BH3 and decomposition of N2H4. Energy Environ. Sci. 2011, 4, 3355−3358. (13) Hamilton, C. W.; Baker, R. T.; Staubitz, A.; Manners, I. B-N compounds for chemical hydrogen storage. Chem. Soc. Rev. 2009, 38, 279−293. (14) Coffey, R. S. The decomposition of formic acid catalysed by soluble metal complexes. Chem. Commun. 1967, 923b−924. (15) Boddien, A.; Mellmann, D.; Gärtner, F.; Jackstell, R.; Junge, H.; Dyson, P. J.; Laurenczy, G.; Ludwig, R.; Beller, M. Efficient dehydrogenation of formic acid using an iron catalyst. Science 2011, 333, 1733−1736. (16) Grasemann, M.; Laurenczy, G. Formic acid as a hydrogen source recent developments and future trends. Energy Environ. Sci. 2012, 5, 8171−8181. (17) Bi, Q.-Y.; Du, X.-L.; Liu, Y.-M.; Cao, Y.; He, H.-Y.; Fan, K.-N. Efficient subnanometric gold-vatalyzed hydrogen generation via formic acid decomposition under ambient conditions. J. Am. Chem. Soc. 2012, 134, 8926−8933. (18) Tedsree, K.; Chan, C. W. A.; Jones, S.; Cuan, Q.; Li, W.-K.; Gong, X.-Q.; Tsang, S. C. E. 13C NMR guides rational design of nanocatalysts via chemisorption evaluation in liquid phase. Science 2011, 332, 224− 228. H


Article


visible-light photocatalytic activity. J. Phys. Chem. C 2011, 115, 20555−20564. (61) Xie, T.; Xu, L.; Liu, C.; Yang, J.; Wang, M. Magnetic composite BiOCl-SrFe12O19: a novel p-n type heterojunction with enhanced photocatalytic activity. Dalton Trans. 2014, 43, 2211−2220. (62) Xie, T.; Liu, C.; Xu, L.; Yang, J.; Zhou, W. Novel heterojunction Bi2O3/SrFe12O19 magnetic photocatalyst with highly enhanced photocatalytic activity. J. Phys. Chem. C 2013, 117, 24601−24610. (63) Bowker, M.; James, D.; Stone, P.; Bennett, R.; Perkins, N.; Millard, L.; Greaves, J.; Dickinson, A. Catalysis at the metal-support interface: exemplified by the photocatalytic reforming of methanol on Pd/TiO2. J. Catal. 2003, 217, 427−433. (64) Boffa, A.; Lin, C.; Bell, A. T.; Somorjai, G. A. Promotion of CO and CO2 hydrogenation over Rh by metal oxides: The influence of oxide Lewis acidity and reducibility. J. Catal. 1994, 149, 149−158. (65) Park, J. Y.; Renzas, J. R.; Hsu, B. B.; Somorjai, G. A. Interfacial and chemical properties of Pt/TiO2, Pd/TiO2, and Pt/GaN catalytic nanodiodes influencing hot electron flow. J. Phys. Chem. C 2007, 111, 15331−15336. (66) Rupprechter, G.; Seeber, G.; Goller, H.; Hayek, K. Structure− activity correlations on Rh/Al2O3 and Rh/TiO2 thin film model catalysts after oxidation and reduction. J. Catal. 1999, 186, 201−213. (67) Schwab, G. M. Boundary-layer catalysis. Angew. Chem., Int. Ed. Engl. 1967, 6, 375−375. (68) Haruta, M. Size- and support-dependency in the catalysis of gold. Catal. Today 1997, 36, 153−166. (69) Mudiyanselage, K.; Senanayake, S. D.; Feria, L.; Kundu, S.; Baber, A. E.; Graciani, J.; Vidal, A. B.; Agnoli, S.; Evans, J.; Chang, R.; et al. Importance of the metal−oxide interface in catalysis: In situ studies of the water−gas shift reaction by ambient-pressure X-ray photoelectron spectroscopy. Angew. Chem., Int. Ed. 2013, 52, 5101−5105. (70) An, K.; Alayoglu, S.; Musselwhite, N.; Plamthottam, S.; Melaet, G.; Lindeman, A. E.; Somorjai, G. A. Enhanced CO oxidation rates at the interface of mesoporous oxides and Pt nanoparticles. J. Am. Chem. Soc. 2013, 135, 16689−16696. (71) Yamada, Y.; Tsung, C.-K.; Huang, W.; Huo, Z.; Habas, S. E.; Soejima, T.; Aliaga, C. E.; Somorjai, G. A.; Yang, P. Nanocrystal bilayer for tandem catalysis. Nat. Chem. 2011, 3, 372−376. (72) Ji, X.; Zuppero, A.; Gidwani, J. M.; Somorjai, G. A. Electron flow generated by gas phase exothermic catalytic reactions using a platinum− gallium nitride nanodiode. J. Am. Chem. Soc. 2005, 127, 5792−5793. (73) Siller, C. W. Zinc oxide pigments. The surface area and catalytic activity. J. Am. Chem. Soc. 1943, 65, 431−434. (74) Contreras, A. M.; Grunes, J.; Yan, X.-M.; Liddle, A.; Somorjai, G. A. Fabrication of platinum nanoparticles and nanowires by electron beam lithography (EBL) and nanoimprint lithography (NIL): comparison of ethylene hydrogenation kinetics. Catal. Lett. 2005, 100, 115−124. (75) Hwang, K. S.; Yang, M.; Zhu, J.; Grunes, J.; Somorjai, G. A. The molecular mechanism of the poisoning of platinum and rhodium catalyzed ethylene hydrogenation by carbon monoxide. J. Mol. Catal. A: Chem. 2003, 204−205, 499−507.

(41) Ashby, E. C.; Doctorovich, F.; Liotta, C. L.; Neumann, H. M.; Barefield, E. K.; Konda, A.; Zhang, K.; Hurley, J.; Siemer, D. D. Concerning the formation of hydrogen in nuclear waste. Quantitative generation of hydrogen via a Cannizzaro intermediate. J. Am. Chem. Soc. 1993, 115, 1171−1173. (42) Swain, C. G.; Powell, A. L.; Sheppard, W. A.; Morgan, C. R. Mechanism of the cannizzaro reaction. J. Am. Chem. Soc. 1979, 101, 3576−3583. (43) Ogden, J. M. Prospects for building a hydrogen energy infrastructure. Annu. Rev. Energy Environ. 1999, 24, 227−279. (44) Hull, J. F.; Himeda, Y.; Wang, W.-H.; Hashiguchi, B.; Periana, R.; Szalda, D. J.; Muckerman, J. T.; Fujita, E. Reversible hydrogen storage using CO2 and a proton-switchable iridium catalyst in aqueous media under mild temperatures and pressures. Nat. Chem. 2012, 4, 383−388. (45) Loges, B.; Boddien, A.; Junge, H.; Beller, M. Controlled generation of hydrogen from formic acid amine adducts at room temperature and application in H2/O2 fuel cells. Angew. Chem., Int. Ed. 2008, 47, 3962−3965. (46) Jeroro, E.; Vohs, J. M. Zn modification of the reactivity of Pd(111) toward methanol and formaldehyde. J. Am. Chem. Soc. 2008, 130, 10199−10207. (47) Friedrich, M.; Penner, S.; Heggen, M.; Armbrüster, M. High CO2 selectivity in methanol steam reforming through ZnPd/ZnO teamwork. Angew. Chem., Int. Ed. 2013, 52, 4389−4392. (48) Rameshan, C.; Stadlmayr, W.; Penner, S.; Lorenz, H.; Memmel, N.; Hävecker, M.; Blume, R.; Teschner, D.; Rocha, T.; Zemlyanov, D.; et al. Hydrogen production by methanol steam reforming on copper boosted by zinc-assisted water activation. Angew. Chem., Int. Ed. 2012, 51, 3002−3006. (49) Lin, Y.-G.; Hsu, Y.-K.; Chen, S.-Y.; Lin, Y.-K.; Chen, L.-C.; Chen, K.-H. Nanostructured zinc oxide nanorods with copper nanoparticles as a microreformation catalyst. Angew. Chem., Int. Ed. 2009, 48, 7586− 7590. (50) Tanabe, K. Solid Acids and Bases: Their Catalytic Properties; Elsevier: 2012. (51) Guo, L.; Yang, S.; Yang, C.; Yu, P.; Wang, J.; Ge, W.; Wong, G. K. L. Synthesis and characterization of poly(vinylpyrrolidone)-modified zinc oxide nanoparticles. Chem. Mater. 2000, 12, 2268−2274. (52) Payne, M. C.; Teter, M. P.; Allan, D. C.; Arias, T. A.; Joannopoulos, J. D. Iterative minimization techniques for ab initio total-energy calculations: molecular dynamics and conjugate gradients. Rev. Mod. Phys. 1992, 64, 1045−1097. (53) Milman, V.; Winkler, B.; White, J. A.; Pickard, C. J.; Payne, M. C.; Akhmatskaya, E. V.; Nobes, R. H. Electronic structure, properties, and phase stability of inorganic crystals: A pseudopotential plane-wave study. Int. J. Quantum Chem. 2000, 77, 895−910. (54) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (55) Perdew, J. P.; Burke, K.; Ernzerhof, M. Emission in symmetric heavy ion reactions at subthreshold energies. Phys. Rev. Lett. 1997, 78, 1396. (56) Vanderbilt, D. Soft self-consistent pseudopotentials in a generalized eigenvalue formalism. Phys. Rev. B: Condens. Matter Mater. Phys. 1990, 41, 7892−7895. (57) Miersch, L.; Rüffer, T.; Schlesinger, M.; Lang, H.; Mehring, M. Hydrolysis studies on bismuth nitrate: Synthesis and crystallization of four novel polynuclear basic bismuth nitrates. Inorg. Chem. 2012, 51, 9376−9384. (58) Chavillon, B.; Cario, L.; Renaud, A.; Tessier, F.; Cheviré, F.; Boujtita, M.; Pellegrin, Y.; Blart, E.; Smeigh, A.; Hammarström, L.; et al. P-type nitrogen-doped ZnO nanoparticles stable under ambient conditions. J. Am. Chem. Soc. 2012, 134, 464−470. (59) Liu, X.; Liu, M.-H.; Luo, Y.-C.; Mou, C.-Y.; Lin, S. D.; Cheng, H.; Chen, J.-M.; Lee, J.-F.; Lin, T.-S. Strong metal−support interactions between gold nanoparticles and ZnO nanorods in CO oxidation. J. Am. Chem. Soc. 2012, 134, 10251−10258. (60) Jiang, J.; Zhang, X.; Sun, P.; Zhang, L. ZnO/BiOI heterostructures: Photoinduced charge-transfer property and enhanced I


Boosting Hydrogen Evolution Activities by Strong ... - ACS Publications

Recommend Documents