Dehydrogenation of ammonia borane (AB) is a promising approach for the production and use of hydrogen for industrial and fuel cell applications. The development of low-cost and highly active catalysts is critical for these practical applications. In this study, low-cost Co 3 O 4 /CuMoO 4 hybrid microflowers composed of nanorods with rich particle boundaries were synthesized. Co 3 O 4 /CuMoO 4 was used as a catalyst for the dehydrogenation of AB and showed a high catalytic activity with a

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... r frequency (TOF) of 129.15 mol hydrogen mol cat -1 min -1 at room temperature. The apparent activation energy (Ea) of the catalyst was found to be as low as 23.2 kJ mol -1 . It was revealed that the synergistic effect between Co 3 O 4 and CuMoO 4 played a critical role in improving the catalytic activity. Co 3 O 4 is relatively active, but a long induction time is needed when it acts as a catalyst in AB hydrolysis. In contrast, CuMoO 4 is less active, but it can immediately catalytically initiate the hydrolytic reaction. When these two compound are combined as a hybrid catalyst, its catalytic performance is significantly improved. These findings can provide some new insight for those who are trying to design some noble-metal-free hybrid catalyst with high catalytic activity towards AB hydrolysis. production in most cases. Thus, it is highly desirable to develop other types of catalysts with both low cost and high performance towards AB. Over the past several years, nanocomposites of different transitional metal oxides or hydroxides emerge as a novel type of heterogeneous catalysts to AB hydrolysis. For example, Feng et al. have reported CuO-CoO nanoparticles deposited on graphene oxide as hybrid catalysts for dehydrogenation of AB, which exhibit an initial turnover frequency (TOF) of 70.0 mol hydrogen mol cat -1 min -1 . 21 Yamada et al. have designed Cu 2 O-Co 3 O 4 composites by decorating Cu 2 O particles with Co 3 O 4 nanoparticles, which can protect Cu 2 O particles from agglomeration, resulting in the high catalytic performance. 22 CuO-NiO nanocomposites have been proved by Yen et al. to be a robust catalyst towards AB hydrolysis with the TOF of 60 mol hydrogen mol Cu -1 min -1 . Very recently, Peng et al. have tested the catalytic activity of the Cu(OH) 2 /Fe(OH) 3 nanocomposite in AB hydrolysis and they find the corresponding TOF can reach 50.3 mol hydrogen mol cat -1 min -1 , which is superior to those of previously reported Fe or Cu based systems. 24 It seems to be a feasible way to improve the catalytic activity by compositing different transitional metal oxides or hydroxides. However, what is the role of the each component of the composite in AB hydrolysis? Is there any synergetic effect between the different transitional metal-based compounds in AB hydrolysis? To address these issues, further investigation is still necessary. Very recently, we have synthesized yolk-shell Co 3 O 4 microspheres, which exhibit high catalytic activity in AB hydrolysis when they are decorated with Cu (Ⅱ). 25 On the other hand, Mo oxide species can serve as Lewis acid sites, which is favorable for the hydrolytic reaction of AB. 26 Thus, it is likely that the composites of Co 3 O 4 and CuMoO 4 exhibit high catalytic activity in AB hydrolysis. Motivated by this idea, Co 3 O 4 /CuMoO 4 hybrid microflowers composed of nanorods are prepared and acted as catalysts for the hydrolysis of AB in this work. As far as we know, such catalysts towards AB hydrolysis for hydrogen production have not been reported yet. It is found that the nanorods in the Co 3 O 4 /CuMoO 4 hybrid microflowers contain rich grain boundaries. More importantly, there is a significant synergistic effect between Co 3 O 4 and CuMoO 4 . The as-obtained CuMoO 4 hybrid materials exhibited high catalytic activity with a TOF value of 129.15 mol hydrogen mol cat -1 min -1 for the hydrolysis of AB, which was found to be superior to most of noble-metal-free catalysts in the literature. Experimental Preparation of catalysts All reagents were of analytic grade (AR) and directly used in all experiments without any further purification. The detail procedure of preparing Co 3 O 4 /CuMoO 4 was as follows: Co(NO 3 ) 2 · 6H 2 O (4.5 mmol), C 6 H 12 N 4 (2.25 mmol) and Na 3 C 6 H 5 O 7 · 2H 2 O (1.5 mmol) were dissolved in 35 mL of water. The obtained solution was transferred into a Teflon-lined stainless-steel autoclave and heated to 100 o C for 24 h. The obtained product was filtered out and washed with water, and then dried in a vacuum oven at 40 o C for 12 h. 4 samples of the dried products (90 mg for each) were weighed and transferred in 4 different beakers containing 40 mL water in each of them. Subsequently, 0.25, 0.5, 1.0, 2.0 mmol of CuCl 2 and H 2 MoO 4 (molar ratio is 1:1) mixtures were added into the above 4 solutions, and then 2.5, 5.0, 10.0, 20.0 mmol of urea were dissolved in the 40 mL ultrapure water, they were added into these 4 solutions respectively. The obtained solutions were stirred for 30 min, transferred into Teflon-lined stainless-steel autoclaves and then heated at 160 o C for 8 h. After the autoclaves were cooled to room temperature, the obtained products were filtered out, rinsed with water and dried in vacuum oven at 40 o C. The dried products were heated at 500 o C for 2 h (heating temperature rate of 2 o C min -1 ). The final products were obtained after the heat-treatment and labelled as Co/Cu-X (X: represents the concentration in mmol of the mixtures of CuCl 2 and H 2 MoO 4 ). Characterization X-Ray diffraction (XRD) patterns were recorded using a PANalytical B.V. Empyean X-ray diffractometer with CuK radiation (λ = 1.5406 Å). The surface morphology of the film catalyst was studied using a Carl Zeiss Ultra Plus scanning electron microscope (SEM). The specific surface in these nanorods. The obtained Co 3 O 4 /CuMoO 4 hybrid exhibited outstanding catalytic activity towards the AB hydrolysis due to the synergistic effect between Co 3 O 4 and CuMoO 4 . Among all the as-prepared Co/Cu samples, Co/Cu-0.5 exhibited the best catalytic activity with a TOF value of 129.15 mol hydrogen mol cat -1 min -1 , which is one of the best noble-mental-free catalysts for AB hydrolysis in class. Our findings also showed that the obtained Co/Cu-0.5 could be a promising catalyst of AB hydrolysis for practical hydrogen production. ASSOCIATED CONTENT Supporting Information N 2 adsorption-desorption isotherms of different samples can be seen in the support information. This material is available free of charge via the Internet at powder and thin film catalysts for hydrogen production by hydrolysis of sodium borohydride. Int. J. [11] Liu, H.; Yu, Y.; Yang, W.; Lei, W.; Gao, M.; Guo, S. High-density defects on PdAg nanowire networks as catalytic hot spots for efficient dehydrogenation of formic acid and reduction of nitrate. Nanoscale 2017, 9, 9305-9309. Yang, W.; Guo, S. Enhanced electron transfer and light absorption on imino polymer capped PdAg nanowire networks for efficient room-temperature dehydrogenation of formic acid.