Xiao
Li
,
Yucong
Yan
,
Yi
Jiang
,
Xingqiao
Wu
,
Shi
Li
,
Jingbo
Huang
,
Junjie
Li
,
Yangfan
Lin
,
Deren
Yang
and
Hui
Zhang
*
State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, P. R. China. E-mail: msezhanghui@zju.edu.cn
First published on 21st August 2019
Hydrolysis of ammonia borane (AB) is a safe and convenient means of H2 production when efficient catalysts are used. Here we report a facile one-pot solvothermal method to synthesize Rh/WO3−x hybrid nanowires. Ultra-small Rh nanoparticles with an average size of ∼1.7 nm were tightly anchored on WO3−x nanowires. Rh/WO3−x catalysts exhibited substantially enhanced activity for hydrolytic dehydrogenation of AB under both dark and visible light irradiation conditions relative to mixed Rh nanoparticles and WO3−x nanowires (Rh + WO3−x), and Rh/C and WO3−x nanowires. X-ray photoelectron spectroscopy (XPS) analysis indicated that the synergistic effect between Rh nanoparticles and WO3−x nanowires was responsible for such an enhancement in activity. Specifically, Rh/WO3−x achieved the highest turnover frequency (TOF) with a value of 805.0 molH2 molRh−1 min−1 at room temperature under visible light irradiation. The H2 release rate as a function of reaction time exhibited a volcano plot under visible light irradiation, indicating that a self-activation process occurred in the hydrolytic dehydrogenation of AB due to additional oxygen vacancies arising from in situ reduction of WO3−x nanowires by AB, and thus an enhanced localized surface plasmon resonance (LSPR). Such a self-activation process was responsible for the enhanced catalytic activity under visible light irradiation relative to that under dark conditions, which was supported by the lower activation energy (45.2 vs. 50.5 kJ mol−1). In addition, Rh/WO3−x catalysts were relatively stable with only little loss in activity after five cycles due to the tight attachment between two components.
In heterogeneous catalysis, supports in a nanocatalyst might play a key role throughout the process.27,29–32 On one hand, supports could be utilized to disperse and downsize metal nanoparticles during the synthesis, and thus expose more active sites and lead to the enhancement in catalytic performances.15,25,26 On the other hand, coupling metal nanoparticles with supports could introduce additional synergistic effects, which intrinsically change the physical and chemical nature of the interface between them.13,19,33–35 Therefore, the catalytic activity and selectivity could be tuned by varying different supports. As a kind of typical transition metal oxide, tungsten oxide-based materials display fascinating performances, which result from the unique feature of the outer d valence electrons.24 Sub-stoichiometric tungsten oxides (WO3−x) with rich oxygen vacancies have been widely investigated in various fields, such as gas sensing,36 electrocatalysis,37 photocatalysis,38 electrochromism,39 lithium-ion batteries,40etc. In addition, some studies show that the catalytic performances of the metal nanoparticles could be enhanced when coupling them with WO3−x supports.41–43 Although there are some interesting reports about photocatalytic hydrogen evolution from AB, where other photoactive semiconductors are used to support metal nanoparticles,44,45 there are few reports on the preparation of noble metal/WO3−x hybrid nanocomposites and their application in this field.
Herein, we present a simple one-pot solvothermal method to synthesize Rh/WO3−x hybrid nanowires. The as-obtained Rh/WO3−x hybrid nanocatalysts display remarkable catalytic activity for AB hydrolytic dehydrogenation in the dark with further enhancement in catalytic activity under visible light irradiation. In addition, we find an interesting self-activation process during the catalytic reaction, which might be due to additional oxygen vacancies arising from the reduction of WO3−x nanowires by AB.
The loading of Rh in the hybrid nanowires was tuned by varying the amount of Rh2(OAc)4 to 3 and 12 mg. From the TEM images in Fig. S2a and c,† both samples (Rh/WO3−x-1 and Rh/WO3−x-3) were observed to exhibit a wire-like shape with well-dispersed nanoparticles being anchored on the surface. Obviously, the density of Rh nanoparticles on the hybrid nanowires increased with the amount of Rh precursors fed in the synthesis. The size statistics (Fig. S2b and d†) indicates that the average diameters of Rh nanoparticles in these two samples (Rh/WO3−x-1 and Rh/WO3−x-3) are close to 1.7 nm, which are similar to that of Rh nanoparticles in Rh/WO3−x-2. The size of WO3−x nanowires was also similar in the above-mentioned three samples. In addition, ICP-AES analysis showed that the Rh loading of Rh/WO3−x-1 and Rh/WO3−x-3 was 0.58 wt% and 2.55 wt%, respectively.
XPS measurements were carried out to investigate the electronic interaction between Rh nanoparticles and WO3−x nanowires. For comparison, Rh nanoparticles (Fig. S3†) and WO3−x nanowires (Fig. S4†) were synthesized (see the Experimental section for details). From the TEM image in Fig. S3,† the size of Rh nanoparticles reached 1.7 nm, which was similar to that of Rh nanoparticles in Rh/WO3−x. As shown in Fig. S4,† the as-synthesized WO3−x nanowires were the same as those in Rh/WO3−x both in morphology (Fig. S4a†) and crystal structure (Fig. S4b†). After that, we simply mixed the as-prepared Rh nanoparticles with a carbon support and WO3−x nanowires to form Rh/C and Rh + WO3−x catalysts, respectively. The morphological (Fig. S5a and S6a†) and structural (Fig. S5b and S6b†) characterization indicates that Rh nanoparticles are uniformly dispersed on the supports with lattice fringes of the Rh {111} planes being clearly observed. Due to the similar structure of Rh/WO3−x-2 to Rh/WO3−x-1 and Rh/WO3−x-3, only the XPS spectra of Rh/WO3−x-2 are shown here for simplicity. Compared to the Rh 3d XPS spectrum of Rh/C, the Rh 3d peak for Rh/WO3−x-2 shifts to a lower binding energy by 0.2 eV, as shown in Fig. 2a. However, there is almost no shift of the Rh 3d peak for Rh + WO3−x catalysts. This result indicates the increase in the surface electron density on Rh nanoparticles once they are coupled with WO3−x nanowires by direct synthesis instead of a simple mixture.46 On the other hand, the binding energy of W 4f in Rh/WO3−x-2 is positively shifted (Fig. 2b) compared with that of the WO3−x nanowires and Rh + WO3−x, further confirming the strong electronic interactions between Rh nanoparticles and WO3−x nanowires in Rh/WO3−x-2. In addition, the W 4f XPS spectra demonstrated that W species showed two oxidation states and the W 4f peaks could be fitted into two doublets: W5+ and W6+ species. WO3−x (WO2.72) and Rh have different work functions, which are 4.50 and 4.98 eV, respectively.49 So when Rh nanoparticles were coupled with WO3−x nanowires, electrons would transfer from WO3−x to Rh, leading to a negative charge state on Rh nanoparticles. Therefore, such an electron-rich state will benefit the electron transfer from Rh to the antibonding orbital of H2O molecules, and thus accelerate hydrolytic dehydrogenation of AB by forming activated transition state complexes.50
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Fig. 2 XPS spectra of (a) Rh 3d orbitals for Rh/C, Rh + WO3−x, and Rh/WO3−x-2 catalysts and (b) W 4f orbitals for WO3−x nanowires, Rh + WO3−x, and Rh/WO3−x-2 catalysts. |
Rh/WO3−x hybrid nanowires with different Rh loadings were then evaluated as catalysts for hydrolytic dehydrogenation of AB with WO3−x nanowires, Rh + WO3−x and Rh/C as the references. Rh/WO3−x-1, Rh/WO3−x-2 and Rh/WO3−x-3 were initially tested for the hydrolysis of AB under dark conditions. As shown in Fig. S7,† these three catalysts exhibited high catalytic activities for the dehydrogenation of AB with a similar TOF approaching 500 molH2 molRh−1 min−1. This result could be attributed to the same amount of Rh being involved in the catalytic reaction. So we chose Rh/WO3−x-2 with an appropriate Rh loading as the representative example for the following research on the catalytic properties and then we called it Rh/WO3−x for simplification. Fig. 3 shows a comparison of the catalytic properties of Rh/WO3−x, Rh + WO3−x, Rh/C, and WO3−x nanowires in the dark and under visible light irradiation (λ > 400 nm), respectively. Obviously, WO3−x nanowires were inactive for the dehydrogenation of AB under both conditions. As can be seen from Fig. 3a and c, Rh/WO3−x exhibited a much rapider H2 release rate and higher TOF value compared to those of Rh + WO3−x and Rh/C in the dark, implying higher activity for hydrolytic dehydrogenation of AB. Simultaneously, the total amount of produced H2 for Rh/WO3−x was ∼3.0 equivalent per mole of AB added, suggesting complete H2 conversion in the catalytic reaction. However, the dehydrogenation of AB catalyzed by Rh + WO3−x or Rh/C was incomplete since the amount of produced H2 was less than ∼3.0 equivalent per mole of AB. The incomplete dehydrogenation may arise from B-containing species poisoning during the catalytic process. According to XPS analysis (Fig. 2), Rh nanoparticles would be negatively charged in Rh/WO3−x compared with those in Rh/C and Rh + WO3−x. Such an electron-rich state not only accelerates the formation of activated transition state complexes, but also may promote the desorption of BO2− anions that were often regarded as poisoning active sites.51 However, Rh nanoparticles in Rh/C and Rh + WO3−x are vulnerable to BO2− anions, which may cause incomplete dehydrogenation.
It is well-known that the photocatalytic efficiency is highly related to light absorption of catalysts. As such, the UV-vis-NIR absorption spectra of Rh/WO3−x and WO3−x nanowires were measured and are shown in Fig. S8.† In addition to strong valence-to-conduction band transition absorption in the UV region, WO3−x nanowires also showed broad absorption in the visible and near infrared regions due to the presence of oxygen vacancies in the crystal structure.52,53 After deposition of Rh nanoparticles, Rh/WO3−x still exhibited strong adsorption in the detected regions, especially in the UV and visible regions due to localized surface plasmon resonance (LSPR) associated with Rh nanoparticles and WO3−x nanowires as well as strong coupling between them. This distinct adsorption feature endowed Rh/WO3−x with possibly enhanced catalytic properties for hydrolytic dehydrogenation of AB with the help of visible light irradiation. Fig. 3b shows H2 generation from the hydrolysis of AB catalyzed by such catalysts as a function of reaction time under visible light irradiation. It can be seen that Rh/WO3−x still showed a much higher H2 release rate compared to Rh + WO3−x and Rh/C. Interestingly, both Rh/WO3−x and Rh + WO3−x displayed improved catalytic activities for hydrolytic dehydrogenation of AB under visible light irradiation relative to that under dark conditions. However, there was no obvious enhancement by using Rh/C as catalysts in the absence of WO3−x, indicating the important role of WO3−x in photo-assisted hydrolytic dehydrogenation of AB. Specifically, the TOF value of Rh/WO3−x under visible light irradiation reached 805.0 molH2 molRh−1 min−1, which is remarkably higher than that of Rh + WO3−x (356.9 molH2 molRh−1 min−1) and Rh/C (169.2 molH2 molRh−1 min−1).
The temperature-dependent reaction kinetics for hydrolytic dehydrogenation of AB catalyzed by Rh/WO3−x was further investigated in the range of 298–328 K under dark conditions and visible light irradiation, as shown in Fig. S9a and c,† respectively. Clearly, the hydrogen generation rate increased with the reaction temperature under both conditions. Fig. S9b and d† display the corresponding Arrhenius plots of lnTOF vs. 1/T, from which the activation energy Ea could be calculated to be 50.5 kJ mol−1 in the dark and 45.2 kJ mol−1 under visible light irradiation, respectively. This decrease in Ea under visible light irradiation suggested that visible light irradiation might provide a favorable transition state with a lower energy barrier. To further understand the enhancement in the photocatalytic activities of the catalysts, the dependence of the light intensity on the catalytic H2 evolution was investigated. We adjusted the power of the Xe lamp from 33.3% to 66.7% and 100%. The results showed that increasing the light intensity led to an increase in the H2 evolution rate of AB and the corresponding TOF values (Fig. S10a and b†). These results indicated that more photogenerated electrons were excited from WO3−x nanowires under visible-light irradiation with a higher intensity and then injected into Rh nanoparticles, which resulted in the enhanced electron density of the supported Rh nanoparticles and the corresponding enhanced catalytic activities. Scheme 1 shows the possible reaction mechanism for the enhancement in catalytic activity towards AB hydrolytic dehydrogenation under visible light irradiation. When visible light is used to irradiate Rh/WO3−x catalysts, the WO3−x nanowires would generate hot electrons through LSPR. Thanks to the tight attachment of Rh nanoparticles to WO3−x nanowires, the hot electrons would rapidly transfer to the Rh nanoparticles through the interface between them. Lots of new discrete energy levels below the conduction band of WO3−x semiconductor nanowires were generated by the introduction of oxygen vacancies,54 and metallic Rh nanoparticles were the electron trapping centers, which effectively decreased the recombination of hot electrons. As such, visible light irradiation could bring more electrons to the surface of Rh nanoparticles, and thus facilitate hydrolytic dehydrogenation of AB.
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Scheme 1 Schematic illustration of the Rh/WO3−x hybrid nanowire catalysts for visible-light-enhanced hydrogen evolution. |
Interestingly, we noticed that the color of the Rh/WO3−x suspension changed from yellow-green to dark blue during the hydrolytic dehydrogenation of AB (Fig. S11a and b†). After the catalytic reaction finished, the color of the Rh/WO3−x suspension would remain dark blue if the flask was kept sealed. Once the Rh/WO3−x suspension was exposed to the air for a little while, its color would turn back to yellow-green quickly, as shown in Fig. S11c.† According to a previous report,55 the color of substoichiometric WO3−x gradually turned more blue with the increase of oxygen vacancies. As such, we can speculate that WO3−x nanowires might be in situ reduced by AB during the hydrolytic dehydrogenation, leading to the increase in oxygen vacancies. In addition, the differential of the H2 production volume V to H2 generation time t (dV/dt) was calculated and the corresponding plot of dV/dt vs. t is shown in Fig. S12.† In this plot, dV/dt can be considered as the H2 release rate. As observed, the H2 release rate remains stable for the first few minutes and then gradually decreases under dark conditions, which can be attributed to the decrease in the concentration of AB over reaction time. Upon visible light irradiation, however, the H2 release rate rises first and then falls, showing a volcano plot. This result indicates that a self-activation process occurred during hydrolytic dehydrogenation of AB only under visible light irradiation. The color change of the Rh/WO3−x suspension demonstrated the generation of additional oxygen vacancies during the catalytic reaction, leading to the formation of more discrete energy levels below the conduction band of WO3−x and thus the enhanced LSPR intensity.56,57 As such, much more hot electrons were generated from LSPR under visible light irradiation, and then injected into tightly coupled Rh nanoparticles. Such additional hot electrons further facilitated the hydrolytic dehydrogenation of AB, causing the self-activation process.
The stability of the Rh/WO3−x catalyst was also evaluated since it is another critical parameter for practical application. Reusability tests were conducted under visible light irradiation at room temperature by continuous addition of a new proportion of AB aqueous solution when the previous run was completed. As shown in Fig. 4, the catalyst can be reused at least five times and the total time for completing the hydrolytic dehydrogenation of AB gradually increases slightly. Then we further prolonged the reusability tests to ten times, and the corresponding time vs. volume plots and TOF values of each cycle are shown in Fig. S13a and b.† After five cycles, the Rh/WO3−x catalysts retain 29.7% of the initial TOF value, which is similar to a previous report,13 but only 19.1% of the initial TOF value was retained after ten cycles. There are some possible reasons for such a decrease in the activity during cycling: (1) reactants were diluted in solution with the increase of the cycling number, (2) the viscidity of the solution also increased with the number of cycles,58 and (3) the catalyst surface was covered with the hydrolysis product metaborate that had the deactivation effect.29,51 In addition, the catalyst was characterized after the fifth run by TEM (Fig. S14a†) and XRD (Fig. S15a†), showing unchanged morphology and phase of the catalyst. The corresponding size distribution of Rh nanoparticles (Fig. S14b†) indicated the slight increase of their size with the number of cycles. XPS analysis (Fig. S15b†) showed that the Rh 3d peak shifts to a higher binding energy by 0.1 eV after the fifth run, which indicated the decrease in the surface electrons on Rh nanoparticles and weakness of the interaction between Rh nanoparticles and WO3−x. It might be part of the reasons for activity degradation.
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Fig. 4 Plots of time versus volume of hydrogen generated from the hydrolysis of AB catalyzed by Rh/WO3−x for five cycles. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9na00424f |
This journal is © The Royal Society of Chemistry 2019 |