Abstract
The contribution of the reverse spillover effect to hydrogen generation reactions is still controversial. Herein, the promotion functions for reverse spillover in the ammonia borane hydrolysis reaction are proven by constructing a spatially separated NiO/Al2O3/Pt bicomponent catalyst via atomic layer deposition and performing in situ quick X-ray absorption near-edge structure (XANES) characterization. For the NiO/Al2O3/Pt catalyst, NiO and Pt nanoparticles are attached to the outer and inner surfaces of Al2O3 nanotubes, respectively. In situ XANES results reveal that for ammonia borane hydrolysis, the H species generated at NiO sites spill across the support to the Pt sites reversely. The reverse spillover effects account for enhanced H2 generation rates for NiO/Al2O3/Pt. For the CoOx/Al2O3/Pt and NiO/TiO2/Pt catalysts, reverse spillover effects are also confirmed. We believe that an in-depth understanding of the reverse effects will be helpful to clarify the catalytic mechanisms and provide a guide for designing highly efficient catalysts for hydrogen generation reactions.
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Introduction
The ever-increasing global energy demand and the detrimental effect of the CO2 product of fossil fuels have triggered a widespread search for alternative energy sources, which are effective and renewable and do not cause further environmental issues1. Because of its high energy density and renewability, H2 has been regarded as an attractive green fuel and a promising energy carrier for the future to meet increasing energy and environmental challenges2. Catalytic H2 generation from hydrogen storage materials is considered a potential method of H2 production if they can be effectively catalyzed3,4. The search for efficient catalytic systems would be greatly facilitated by a clearer understanding of the underlying chemical process.
Noble metal catalysts, such as Pt, Pd, and Ru, have been recognized as important classes of catalysts for hydrogen generation, due to their high catalytic activity and durability5,6,7,8,9,10,11. It is noted that coupling metal catalysts with secondary metals12,13,14,15 and/or transition metal oxides16,17,18,19,20,21,22,23,24,25 is an encouraging strategy to further enhance catalytic performance. In the past, various theories (e.g., the metal-oxide interfacial sites, electron interactions, or hydrogen reverse spillover effect) have been offered to explain the enhancement of H2 generation when different components are combined in a catalyst. For example, Francisco Zaera and coworkers argued that in the photocatalytic production of H2 from water with semiconductor catalysts, the role of metal additives is a reverse spillover effect, not to trap excited electrons26. Hydrogen reverse spillover, as a form of spillover, involves the migration of adsorbed hydrogen atoms from an oxide (or other nonmetal surface) to a metal, where they recombine to molecular hydrogen27,28,29. However, due to the lack of well-defined catalysts with clearly separated functional components and the difficulties in performing in situ characterization technologies, researchers have not formed an agreement on the enhancement mechanism. It is still a challenging issue to reveal the promotion effects of reverse spillover in H2 generation reactions.
In this work, taking the ammonia borane (NH3·BH3, AB) hydrolysis reaction as an example, the promotion functions of reverse spillover in this reaction are proven using a spatially separated NiO/Al2O3/Pt catalyst as a model catalyst, in combination with in situ quick XANES characterization. The NiO/Al2O3/Pt catalyst was prepared by a facile and general template-assisted atomic layer deposition (ALD) method30,31,1d, e) for NiO/Al2O3/Pt show that Ni and Pt are distributed on the outer and inner surfaces of Al2O3 nanotubes, respectively. The STEM image, EDX map**, and line-scanning profile for a cross-sectional specimen prepared by focused ion beam milling along the vertical direction of the Al2O3 nanotubes (Supplementary Fig. 3) further demonstrate the separated structure of NiO/Al2O3/Pt. TEM images of the Al2O3/Pt and NiO/Al2O3 catalysts are shown in Fig. 1b, c. There are no Pt particles on the outer surfaces of Al2O3 nanotubes for Al2O3/Pt (Supplementary Fig. 4). Due to the small size and low contrast of NiO nanoparticles, it is not straightforward to distinguish NiO nanoparticles in Fig. 1a, c. From the HRTEM image of NiO/Al2O3 (inset in Fig. 1c), NiO nanoparticles can be clearly observed. The Pt content in the catalysts was measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES) to be 3.65 and 4.23% for NiO/Al2O3/Pt and Al2O3/Pt, and the Ni content was measured to be 8.05 and 8.71% for NiO/Al2O3/Pt and NiO/Al2O3, respectively. The N2 sorption isotherms for the NiO/Al2O3, Al2O3/Pt, and NiO/Al2O3/Pt catalysts almost overlap (Fig. 1f). The Brunauer–Emmett–Teller (BET) surface areas for the NiO/Al2O3, Al2O3/Pt, and NiO/Al2O3/Pt catalysts were calculated to be 95.4, 93.6, and 98.0 m2 g–1, respectively. Their pore volumes were 0.34, 0.34, and 0.39 cm3 g–1, respectively. The Barrett–Joiner–Halenda (BJH) pore size distribution curves (Fig. 1g) deduced from desorption branches of the N2 sorption isotherms confirm that NiO/Al2O3, Al2O3/Pt, and NiO/Al2O3/Pt samples are made up of pores with average sizes centred at 54.9, 54.4, and 57.0 nm, respectively. The pore sizes of these catalysts, i.e., the inner diameters of the Al2O3 nanotubes, correspond to the diameters of the CNC sacrificial templates. These results show that all the catalysts possess similar pore structures.
TEM images of a NiO/Al2O3/Pt, b Al2O3/Pt, and c NiO/Al2O3 catalysts. Inset in c shows a HRTEM image of NiO/Al2O3. d HAADF-STEM image and e EDX elemental map** of the NiO/Al2O3/Pt catalyst. f N2 adsorption−desorption isotherms and g the corresponding pore size distributions of the catalysts. h XPS Ni 2p analysis of NiO/Al2O3 and NiO/Al2O3/Pt. i H2-TPR profiles of NiO/Al2O3, Al2O3/Pt, and NiO/Al2O3/Pt.
The X-ray photoelectron spectroscopy (XPS) results reveal the existence of Ni2+ species in NiO/Al2O3 and NiO/Al2O3/Pt (Fig. 1h). The XPS peaks for the two catalysts are similar. The XPS peaks located at binding energies of 856.1 and 874.0 eV are attributed to Ni 2p3/2 and Ni 2p1/2, respectively, and the peaks located at binding energies of 861.8 and 879.7 eV are attributed to satellite peaks. From the X-ray diffraction (XRD) patterns for the Al2O3/Pt and NiO/Al2O3/Pt catalysts (Supplementary Fig. 5), the presence of Pt nanoparticles can be confirmed. No diffraction peak assigned to NiO is detected from the XRD patterns for NiO/Al2O3 and NiO/Al2O3/Pt, which can be ascribed to the high dispersion of ALD-prepared nanoparticles. Hydrogen temperature programmed reduction (H2-TPR) was used to study the redox properties of the catalysts (Fig. 1i). The profile obtained for Al2O3/Pt displays a principal reduction peak at 385 °C, which can be attributed to Pt interacting with Al2O337. The NiO/Al2O3 catalyst exhibits a small shoulder peak at approximately 349 °C and a strong peak centred at 431 °C, corresponding to the reductions of bulk NiO and the NiO interacting with Al2O3. In contrast, for NiO/Al2O3/Pt, the first H2 consumption peak (corresponding to the reduction of bulk NiO) shifts from 349 to 326 °C and becomes obvious, and a broadened peak centred at 408 °C (corresponding to the reductions of Pt and NiO interacting with Al2O3) can be observed. Quantification of the H2-TPR curves (Supplementary Table 1) shows that the hydrogen consumed by NiO/Al2O3/Pt (1.86 mmol H2 g–1) is greater than the sum of the hydrogen consumed by Al2O3/Pt (0.31 mmol H2 g–1) and NiO/Al2O3 (1.29 mmol H2 g–1). These results demonstrate that the reduction of NiO species is promoted after Pt addition, which can be attributed to the hydrogen spillover effect38,39,56. The crystal structure of γ-Al2O3 proposed by Gutiérrez et al.57 was adopted in our model system. The most stable (100) surface of γ-Al2O3 with three alumina layers and a Ni4O4 cluster adsorbed onto it were used for the NiO/γ-Al2O3(100) slab model. The two bottom layers of the slab were kept fixed. The thickness of the vacuum region was 20 Å. A Monkhorst-Pack grid was used for Brillouin-zone integrations with 1 × 1 × 1 k-mesh (gamma point) sampling. The solvation effect was included with an implicit solvation solvent of water using the VASPsol tool58. The free energies at room temperature (298.15 K) were obtained by adding to the DFT electronic energy (E), the zero-point energy, enthalpy, and entropy contribution from the vibrational modes. The transition states (TS) were calculated using the climbing image nudged elastic band method59, and frequency analysis was confirmed to verify the TS.
Data availability
The data that support the findings of this study are available from the corresponding authors upon reasonable request.
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Acknowledgements
We acknowledge the financial support from the National Natural Science Foundation of China (21773282, Z.G.; U1932131, Z.G.; and U1832208, Y.Q.), National Science Fund for Distinguished Young Scholars (21825204, Y.Q.), the National Key R&D Programme of China (2017YFA0700101, Y.Q.; and 2020YFA0210902, Y.Q.), and Youth Innovation Promotion Association of the Chinese Academy of Sciences (2018208, Z.G.). We are grateful to all staff at the 1W1B beamline of the Bei**g Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, and at BL14W1 and BL11B beamlines of the Shanghai Synchrotron Radiation Facility, Shanghai Advanced Research Institute, Chinese Academy of Sciences.
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Z.G. synthesized the catalysts and performed the activity tests. G.W., Z.L., M.X., J.M., and Z.J. helped to perform or provide the XANES measurement. T.L. performed the theoretical calculation. L.W. and S.X. assisted in the synthesis and characterizations of the catalysts. Z.G. and Y.Q. conceived the idea, supervised the work, and wrote the manuscript. All authors contributed to the manuscript.
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Gao, Z., Wang, G., Lei, T. et al. Enhanced hydrogen generation by reverse spillover effects over bicomponent catalysts. Nat Commun 13, 118 (2022). https://doi.org/10.1038/s41467-021-27785-5
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DOI: https://doi.org/10.1038/s41467-021-27785-5
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